U.S. patent application number 09/747561 was filed with the patent office on 2001-09-13 for optical device with negative thermal expansion substrate and uses therefor.
This patent application is currently assigned to Corning Incorporated.. Invention is credited to Merkel, Gregory A..
Application Number | 20010021292 09/747561 |
Document ID | / |
Family ID | 22195939 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021292 |
Kind Code |
A1 |
Merkel, Gregory A. |
September 13, 2001 |
Optical device with negative thermal expansion substrate and uses
therefor
Abstract
Negative thermal expansion materials, methods of preparation and
uses therefor are disclosed. The materials are useful for negative
thermal expansion substrates, such as those used for optical fiber
gratings.
Inventors: |
Merkel, Gregory A.; (Big
Flats, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Assignee: |
Corning Incorporated.
|
Family ID: |
22195939 |
Appl. No.: |
09/747561 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09747561 |
Dec 22, 2000 |
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09305763 |
May 5, 1999 |
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6187700 |
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60086053 |
May 19, 1998 |
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Current U.S.
Class: |
385/37 ;
385/141 |
Current CPC
Class: |
C04B 35/495 20130101;
G02B 6/12 20130101; G02B 6/0218 20130101; C04B 35/01 20130101 |
Class at
Publication: |
385/37 ;
385/141 |
International
Class: |
G02B 006/34 |
Claims
What is claimed is:
1. An optical device comprising: a negative expansion substrate
having a composition comprising two phases having negative
coefficients of thermal expansion, one of the phases having a room
temperature coefficient of thermal expansion more negative than
-50.times.10.sup.-7.degree. C..sup.-1; and a thermally sensitive,
positive expansion optical component affixed to the substrate.
2. The optical device of claim 1, wherein one phase has the
composition M.sub.2B.sub.3O.sub.12 where M is selected from the
group including aluminum, scandium, indium, yttrium, the lanthanide
metals, zirconium, and hafnium, and where B is selected from the
group consisting of tungsten, molybdenum, and phosphorus, and where
M and B are selected such that the compound M.sub.2B.sub.3O.sub.12
has a negative CTE, and wherein the second phase has the
composition AX.sub.2O.sub.8, where A is selected from the group
consisting of zirconium and hafnium, and X is selected from the
group consisting of tungsten and molybdenum.
3. The optical device of claim 1, wherein the first material is
AW.sub.2O.sub.8 and the second material is A.sub.2P.sub.2WO.sub.12,
wherein A is selected from the group consisting of Zr and Hf.
4. The optical device of claim 3, wherein the substrate does not
exhibit microcracking.
5. The optical device of claim 3, wherein the substrate further
includes a crystalline or non-crystalline oxide phase, including a
glassy phase, which contains a metal selected from the group
consisting of alkaline earth metals, alkali metals, manganese,
iron, cobalt, copper, zinc, aluminum, gallium, bismuth, yttrium,
lanthanide metals, scandium, niobium, titanium and nickel.
6. The optical device of claim 4, wherein, on a weight percent
basis, the AW.sub.2O.sub.8 is present in an amount ranging from
about 50% to 95%, A.sub.2P.sub.2WO.sub.12 is present in amount of
about 5% to 50%.
7. The optical device of claim 1, wherein the optical component is
an optical fiber grating.
8. The optical device of claim 4, wherein the substrate has a mean
linear coefficient of thermal expansion of about
-40.times.10.sup.-7.degree. C..sup.-1 to -88.times.10.sup.31
7.degree. C..sup.-1 over a temperature range of about -40.degree.
C. to 85.degree. C.
9. The optical device of claim 3, wherein the device is not
hermetically sealed.
10. An optical device comprising: a negative expansion substrate
having a composition comprising two phases having negative
coefficients of thermal expansion, one of the phases having a room
temperature coefficient of thermal expansion more negative than
-50.times.10.sup.-7.degree. C..sup.-1; and a fiber Bragg grating
having a Bragg wavelength affixed to the substrate, wherein the
absolute value of the average temperature dependence of the Bragg
wavelength between 0C and 70.degree. C. is not more than about
0.0025 nm/.degree. C.
11. The optical device of claim 10, wherein one phase has the
composition M.sub.2B.sub.3O.sub.12 where M is selected from the
group including aluminum, scandium, indium, yttrium, the lanthanide
metals, zirconium, and hafnium, and where B is selected from the
group consisting of tungsten, molybdenum, and phosphorus, and where
M and B are selected such that the compound M.sub.2B.sub.3O.sub.12
has a negative CTE, and wherein the second phase has the
composition AX.sub.2O.sub.8, where A is selected from the group
consisting of zirconium and hafnium, and X is selected from the
group consisting of tungsten and molybdenum.
12. The optical device of claim 10, wherein the first material is
AW.sub.2O.sub.8 and the second material is A.sub.2P.sub.2WO.sub.12,
wherein A is selected from the group consisting of Zr and Hf.
13. The optical device of claim 12, wherein the substrate does not
exhibit microcracking.
Description
[0001] This application claims priority to U.S. application Ser.
No. 09/305763, filed on May 5, 1999, which claims priority to
Provisional U.S. Application No. 60/086,053, filed on May 19, 1998,
the contents of which are herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The invention pertains to negative thermal expansion
materials, devices made therefrom, and methods of making the
materials. More particularly, the present invention concerns
compositions including zirconium phosphate tungstates, which can be
used to make substrates for athermalized optical fiber reflective
grating devices.
BACKGROUND OF THE INVENTION
[0003] Negative thermal expansion (contraction with increasing
temperature) is an unusual and potentially useful property for a
solid material, and very few crystalline materials possess strongly
negative expansions over an extended temperature range. Materials
that exhibit a negative expansion due to extensive microcracking,
by virtue of a negative coefficient of thermal expansion ("CTE")
along at least one crystallographic axis and a substantially
different CTE along at least one other axis, include some lithium
aluminosilicates, "NZPs" (compounds with crystal structures similar
to that of NaZr.sub.2P.sub.3O.sub.12) , and
Ta.sub.2O.sub.5-WO.sub.3 compounds. On the other hand, materials
having negative mean lattice expansions that do not require
microcracking for negative bulk ceramic CTEs are even more limited,
and include certain synthetic alkali-free zeolites having low
aluminum contents, ZrW.sub.2O.sub.8, HfW.sub.2O.sub.8,
ZrV.sub.2-xP.sub.xO.sub.7 (above about 100.degree. C.), and
NbZrP.sub.3O.sub.12 (an NZP type compound).
[0004] Of the compounds that have negative mean lattice expansions,
zeolite expansions from 25 to 100.degree. C. are in the range
-20.times.10.sup.-7.degree. C..sup.-1 to
-40.times.10.sup.-7.degree. C..sup.-1, but have the disadvantages
of being dependent upon the amount of adsorbed water in the
zeolite, and their CTE curves can exhibit considerable hysteresis.
The CTE of NbZrP.sub.3O.sub.12 below 100.degree. C. is about
-27.times.10.sup.-7.degree. C..sup.-1, while that of
ZrW.sub.2O.sub.8 and HfW.sub.2O.sub.8 is about
-90.times.10.sup.-7.degree- . C..sup.-1. Thus, ZrW.sub.2O.sub.8 and
HfW.sub.2O.sub.8 are presently unique as materials that display a
CTE more negative than -40.times.10.sup.-7.degree. C..sup.-1
without microcracking.
[0005] Martinek and Hummel (1960, J. Amer. Ceram. Soc., 53,
159-161) first reported the existence of Zr.sub.2P.sub.2WO.sub.12
in their study of the phase relations in the
ZrO.sub.2-WO.sub.3-P.sub.2O.sub.5 system at 1125.degree. C. An XRD
powder pattern was presented for this new compound, which
reportedly has a melting point above 1750.degree. C., although
extensive volatilization occurs in air at 1600.degree. C. Synthesis
of Zr.sub.2P.sub.2WO.sub.12 was achieved by calcining a mixture of
hydrous zirconium carbonate, tungstic acid, and mono-hydrogen
ammonium phosphate.
[0006] Tsvigunov and Sirotinkin (1990, Russ. Jour.of Inorg. Chem.,
35, 1740) subsequently reported a more complete and precise powder
XRD pattern for this compound, which they synthesized from a
mixture of ZrO.sub.2, WO.sub.3, and NH.sub.4H.sub.2PO.sub.4. Evans
et al. (1995, Jour. Solid State Chem., 120, 101-104) have shown
that the structure of Zr.sub.2P.sub.2WO.sub.12 (also referred to as
Zr.sub.2(WO.sub.4)(PO.sub.4- ).sub.2) is comprised of ZrO.sub.6
octahedra sharing corners with WO.sub.4 and PO.sub.4 tetrahedra.
Those workers report that dilatometric and variable temperature
X-ray diffractometry studies indicate that Zr.sub.2P.sub.2WO.sub.12
exhibits a negative thermal expansion over a broad temperature
range. More recently, Evans et al. (1997, Journal Solid State
Chem., 133, 580-83) have reported that Zr.sub.2P.sub.2WO.sub.12 has
a mean lattice CTE of about -30.times.10.sup.-7.degree.
C..sup.-1.
[0007] The Zr.sub.2P.sub.2WO.sub.12 bodies synthesized according to
the methods reported in the above literature have porosities
greater than about 25%, typically greater than 30%. Such high
porosity bodies generally are not useful for industrial
applications. Thus, it would be useful to provide a composition
having a low, preferably a negative thermal expansion, comprised of
Zr.sub.2P.sub.2WO.sub.12, or analogues thereof in which Hf is fully
or partially substituted for Zr, having a porosity less than about
20%, preferably less than about 10%, and more preferably less than
about 5%.
[0008] Bodies having a highly negative CTE, such as -30 to
-100.times.10.sup.-7.degree. C..sup.-1, can find use as substrates
for athermalization of fiber Bragg gratings (FBGs). In the latter
application, a FBG is mounted in tension on the negative expansion
substrate. Applications of FBGs include passive wavelength division
multiplexing and filtering in dense WDM systems, as well as
distributed fiber sensors for smart systems to monitor bridges,
structures, and highways.
[0009] For such applications, variation of the center wavelength of
fiber Bragg gratings (FBGs) with respect to temperature, due to
thermal expansion of the fiber and variation of the refractive
index of the glass, must be minimized. For example, at a Bragg
wavelength of 1550 nm, thermal variation of .lambda..sub.B is
expected to be 0.012 nm/.degree. C., whereas a value less than
0.002 nm/.degree. C. is desired. Variation in .lambda..sub.B with
temperature can be reduced to well below 0.002 nm/.degree. C. by
mounting the FBG in tension on a substrate having a negative
thermal expansion of about -70 to -85.times.10.sup.-7.degree.
C..sup.-1 within that range also -70 to -80.times.10.sup.-7.degree.
C..sup.-1, -75 to -82.times.10.sup.-7.degree. C..sup.-1. The
reduction in tension with increasing temperature associated with
the contraction of the substrate partially or entirely offsets the
contribution to increased optical path length resulting from the
thermal expansion and change in refractive index of the glass.
[0010] P-eucryptite based ceramics formed by controlled
devitrification of sintered lithium aluminosilicate glass are being
studied as FBG substrates and are disclosed in international patent
application no. PCT/US/13062, Beall et al., entitled, "Athermal
Optical Device." The attainment of CTEs of -70 to
-85.times.10.sup.-7.degree. C..sup.-1 in .beta.-eucryptite bodies
requires extensive microcracking; an unmicrocracked
.beta.-eucryptite exhibits a CTE near -5.times.10.sup.-7.degree.
C..sup.-1. This microcracking results from internal stresses
associated with the large difference in CTE along the c and a axes
of the crystals (approximately -176 and +78.times.10.sup.-7.degree.
C..sup.-1, respectively), coupled with the coarse grain size of the
crystals.
[0011] U.S. Pat. No. 5,694,503, issued to Fleming et al., discloses
using the negative coefficient of thermal expansion material
ZrW.sub.2O.sub.8 to form substrates for temperature compensated
fiber Bragg gratings. Since the coefficient of thermal expansion of
ZrW.sub.2O.sub.8 may be too negative to provide temperature
compensation for Bragg gratings, the Fleming et al. patent suggests
mixing ZrW.sub.2O.sub.8 with a positive coefficient of thermal
expansion material such as alumina, silica, zirconia, magnesia,
calcia, or yttria in an amount to raise the coefficient of thermal
expansion.
[0012] The mixtures of ZrW.sub.2O.sub.8 with positive coefficient
of thermal expansion materials suggested in the Fleming et al.
patent, however, have several disadvantages. Large relative
differences in the thermal expansion coefficients of
ZrW.sub.2O.sub.8 and the positive CTE materials can cause
microcracking in the composite material upon heating and cooling of
the material. Such microcracking can result in hysteresis in the
thermal expansion curve or dimensional change of the body with
changes in humidity, characteristics that are undesirable in a
fiber Bragg grating substrate. Furthermore, many of the positive
CTE components recommended in the Fleming et al. patent react with
the ZrW.sub.2O.sub.8 during sintering to form copious amounts of
liquid. Such reactions and liquid formation tend to cause the body
to slump during firing. Alternatively, some of the positive CTE
components recommended in the Fleming et al. patent react with the
ZrW.sub.2O.sub.8 to form other high CTE crystalline phases so that
the ceramic body does not have the desired strongly negative CTE
after firing. In addition, ceramics comprised of ZrW.sub.2O.sub.8
and ZrO.sub.2 undergo a length change having an absolute value
greater than 500 parts per million over 700 hours at 85% relative
humidity and 85 .degree. C., which is undesirably large.
[0013] The presence of microcracking in a FBG substrate requires
that the fiber/substrate package be hermetically sealed to prevent
dimensional drift of the substrate due to opening and closing of
the microcracks resulting from variations in humidity. Hermetic
sealing adds significantly to the cost of the assembly, and the
reliability of the device becomes dependent upon long-term
reliability of the hermetic seal.
[0014] Thus, it would be desirable to provide an unmicrocracked
material having a porosity less than about 25%, preferably less
than about 10% and more preferably less than about 5%. Further,
there is a need for a body having a CTE of about
-70.times.10.sup.-7.degree. C..sup.-1 to
-85.times.10.sup.-7.degree. C..sup.-1 to provide temperature
compensation for the gratings of current interest which could be
used to make FBG substrates because hermetic sealing would not be
required for long-term stability. In addition, it would be
desirable to provide a material that has a length change having an
absolute value less than 500 ppm over 700 hours at 85.degree. C.
and 85% relative humidity.
SUMMARY OF INVENTION
[0015] The present invention provides a low-porosity body
containing at least one phase having a negative thermal expansion,
a method of making the phase, and devices made from the phase. In
one embodiment, the body is comprised of the compound
Zr.sub.2P.sub.2WO.sub.12 which exhibits a room-temperature CTE of
about -40.times.10.sup.-7.degree. C..sup.-1, and a CTE from about
25 to 800.degree. C. of about -25.times.10.sup.-7.degree- .
C..sup.-1. Hafnium may be partially or entirely substituted for
zirconium. The body further comprises a crystalline or
non-crystalline oxide phase, which may include a glassy phase,
which contains a metal selected from the group consisting of
alkaline earth metals, alkali metals, manganese, iron, cobalt,
copper, zinc, aluminum, gallium, and bismuth. The oxide phase may
also contain one or more of the metals selected from the group
zirconium, tungsten and phosphorous.
[0016] In another embodiment, this invention comprises a ceramic
body comprised of two negative CTE phases, preferably wherein at
least one of the phases has a room temperature CTE more negative
than -50.times.10.sup.-7C.sup.-1. In an exemplary embodiment, one
phase has the composition M.sub.2B.sub.3O.sub.12 where M is
selected from the group including aluminum, scandium, indium,
yttrium, the lanthanide metals, zirconium, and hafnium, and where B
is selected from the group consisting of tungsten, molybdenum, and
phosphorus, and where M and B are selected such that the compound
M.sub.2B.sub.3O.sub.12 has a negative CTE, and comprising a second
phase of the composition AX.sub.2O.sub.8, where A is selected from
the group consisting of zirconium and hafnium, and X is selected
from the group consisting of tungsten and molybdenum.
[0017] For example, the ceramic body may comprise a mixture of
ZrW.sub.2O.sub.8 and Zr.sub.2P.sub.2WO.sub.12, wherein hafnium may
be partially or fully substituted for zirconium in
Zr.sub.2P.sub.2WO.sub.12 and ZrW.sub.2O.sub.8. In a preferred
embodiment, the ceramic body is unmicrocracked. The ceramic body
may further include a crystalline or non-crystalline oxide phase,
which may include a glassy phase, which contains a metal selected
from the group consisting of alkaline earth metals, alkali metals,
lanthanum group metals, niobium, titanium, manganese, iron, cobalt,
nickel, copper, zinc, yttrium, scandium, aluminum, gallium, and
bismuth. The oxide phase may also contain one or more of the metals
selected from the group zirconium, tungsten and phosphorous.
[0018] The invention also includes a method of making ceramic
bodies of the present invention comprising mixing together powders
of Zr.sub.2P.sub.2WO.sub.12 or precursor powders of
Zr.sub.2P.sub.2WO.sub.12 or analogues of these powders in which Hf
is substituted for Zr, or mixtures thereof, and at least one oxide
or oxide precursor of metals selected from the group consisting of
alkaline earth metals, alkali metals, manganese, iron, cobalt,
copper, zinc, aluminum, gallium, and bismuth.
[0019] Another embodiment of this invention involves a method of
raising and making the negative thermal expansion of a ceramic body
less negative comprising a phase having a room temperature
coefficient of thermal expansion more negative than
50.times.10.sup.-7.degree. C..sup.-1 comprising mixing together the
first phase with at least a second phase or precursors of the first
phase and at least the second phase, the second phase having a
negative thermal expansion less negative than
-50.times.10.sup.-7.degree. C..sup.-1. A body made according to
this method preferably will not exhibit microcracking.
Alternatively, the method includes forming a ceramic body
comprising two negative CTE phases, preferably wherein the room
temperature CTE of one of the phases is more negative than
-50.times.10.sup.-7C.sup.-1 and the other phase is less negative
than -50.times.10.sup.-7C.sup.-1, to provide a body having a CTE
less negative than -50.times.10.sup.-7C.sup.-1.
[0020] In an exemplary embodiment, one phase has the composition
M.sub.2B.sub.3O.sub.12 where M is selected from the group including
aluminum, scandium, indium, yttrium, the lanthanide metals,
zirconium, and hafnium, and where B is selected from the group
consisting of tungsten, molybdenum, and phosphorus, and where M and
B are selected such that the compound M.sub.2B.sub.3O.sub.12 has a
negative CTE, and comprising a second phase of the composition
AX.sub.2O.sub.8, where A is selected from the group consisting of
zirconium and hafnium, and X is selected from the group consisting
of tungsten and molybdenum. In another exemplary embodiment, the
method comprises mixing ZrW.sub.2O.sub.8 or precursors of
ZrW.sub.2O.sub.8, or analogues of these materials in which Hf is
substituted for Zr, with Zr.sub.2P.sub.2WO.sub.12 or precursor
powders of Zr.sub.2P.sub.2WO.sub.12 or analogues of these powders
in which Hf is substituted for Zr. Optionally, these may also be
mixed with at least one oxide or oxide precursor of metals selected
from the group consisting of alkaline earth metals, alkali metals,
manganese, iron, cobalt, copper, zinc, aluminum, gallium, and
bismuth. The mixed powders are consolidated together using a
ceramic forming method and heated to sinter the ceramic body.
Preferably, the heating occurs at a temperature of about
1050.degree. C. to 1300.degree. C., more preferably 1120.degree. C.
to 1230.degree. C. for about 1 minute to 10 hours. When the
ZrW.sub.2O.sub.8 phase is desired to be present in the ceramic
body, preferably heating occurs at a temperature of about
1150.degree. C. to 1230.degree. C.
[0021] Another aspect of the invention involves an optical device
comprising a negative expansion substrate having a composition
comprising two negative CTE phases, preferably wherein at least one
of the phases has a room temperature CTE more negative than
-50.times.10.sup.-7.degree. C..sup.-1, and one of the phases has a
thermal expansion less negative than -50.times.10.sup.-7.degree.
C..sup.-1. For example, the substrate composition may comprise a
mixture of ZrW.sub.2O.sub.8 and Zr.sub.2P.sub.2WO.sub.12. Hafnium
may be partially or fully substituted for zirconium in
Zr.sub.2P.sub.2WO.sub.12 and ZrW.sub.2O.sub.8. In a preferred
embodiment, the substrate is unmicrocracked. The substrate may
further include a crystalline or non-crystalline oxide phase, which
may include a glassy phase, which contains a metal selected from
the group consisting of alkaline earth metals, alkali metals,
lanthanum group metals, niobium, titanium, manganese, iron, cobalt,
nickel, copper, zinc, yttrium, scandium, aluminum, gallium, and
bismuth. The oxide phase may also contain one or more of the metals
selected from the group zirconium, tungsten and phosphorous.
[0022] The device further comprises a thermally sensitive, positive
expansion optical component affixed to the substrate. In one
embodiment, the optical component is an optical fiber grating and
the substrate has a mean linear coefficient of thermal expansion of
about -40.times.10.sup.-7.degree. C..sup.-1 to
-85.times.10.sup.-7.degree. C..sup.-1 over a temperature range of
about -40.degree. C. to 85.degree. C. Another aspect of the
invention relates to a negative expansion substrate having a
composition comprising Zr.sub.2P.sub.2WO.sub.12 and a crystalline
or non-crystalline oxide phase, which may include a glassy phase,
which contains a metal selected from the group consisting of
alkaline earth metals, alkali metals, manganese, iron, cobalt,
copper, zinc, aluminum, gallium, and bismuth. Hafnium may be fully
or partially substituted for zirconium. The oxide phase may also
contain one or more of the metals selected from the group
zirconium, tungsten and phosphorous.
[0023] Thus the present invention generally provides a novel
ceramic body comprised of phase having a negative CTE, such as
Zr.sub.2P.sub.2WO.sub.1- 2 or Hf.sub.2P.sub.2WO.sub.12 or mixtures
thereof, which exhibits a negative coefficient of thermal expansion
(CTE) at all temperatures from at least as low as 25.degree. C. to
at least as high as 500.degree. C. The mean CTE near room
temperature is about -40.times.10.sup.-7.degree. C..sup.-1, while
the mean CTE from 25 to 800.degree. C. is about
-25.times.10.sup.-7.degree. C..sup.-1. The Zr.sub.2P.sub.2WO.sub.12
or Hf.sub.2P.sub.2WO.sub.12 phase is stable at all temperatures
from at least as low as -50.degree. C. to at least as high as
1150.degree. C.
[0024] Also disclosed is a method for fabricating the sintered
ceramic body which, in some embodiments, entails the addition of
small amounts of additives which function as sintering aids to
powders of Zr.sub.2P.sub.2WO.sub.12 or Hf.sub.2P.sub.2WO.sub.12 or
mixtures thereof or their precursors. These additives include the
compounds of lithium, sodium, potassium, magnesium, calcium,
barium, manganese, iron, copper, and zinc. Aluminum compounds may
also be used as sintering aids, but are not as effective. Compounds
of rubidium, cesium, and strontium would also likely be effective
for densification.
[0025] The present invention also includes a ceramic body comprised
mainly of the phases ZrW.sub.2O.sub.8 and Zr.sub.2P.sub.2WO.sub.12
and their hafnium analogues and mixtures thereof, having a mean
linear coefficient of thermal expansion of about -40 to
-85.times.10.sup.-7.degree. C..sup.-1 over the temperature range
-40.degree. C. to +85.degree. C. These bodies also exhibit a
negative CTE to higher temperatures as well. Preferred embodiments
of the invention have less than 10% total porosity, especially less
than 5% porosity. In one embodiment, achievement of low porosity is
enhanced by the addition of small amounts (0.01 to 5.0 wt %) of
certain sintering additives, such as the oxides or oxide-forming
compounds of alkali (group IA) metals, alkaline earth (group IIA)
metals, manganese, iron, cobalt, nickel, copper, zinc, yttrium,
scandium, lanthanide metals, niobium, titanium, aluminum, gallium,
and bismuth. Many of these materials have the desirable properties
of having excellent dimensional stability at 85.degree. C. and 85%
relative humidity and possessing no microcracking, and thus exhibit
no hysteresis in their thermal expansion curves. Such ceramics are
suitable as athermalizing substrates for fiber Bragg gratings.
[0026] Additional features and advantages of the invention will be
set forth in the description which follows. It is to be understood
that both the foregoing general description and the following
detailed description are exemplary and explanatory and are intended
to provide further explanation of the invention as claimed.
[0027] The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate one embodiment
of the invention and together with the description serve to explain
the principles of the invention. In the drawings, like reference
characters denote similar elements throughout the several views. It
is to be understood that various elements of the drawings are not
intended to be drawn to scale, but instead are sometimes purposely
distorted for the purposes of illustrating the invention. +PG
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 shows the relationship of the dependence of the
coefficient of thermal expansion at 25.degree. C. on the weight
percent of Zr.sub.2P.sub.2WO.sub.12 in a ceramic body comprised
mainly of Zr.sub.2P.sub.2WO.sub.12 and ZrW.sub.2O.sub.8;
[0029] FIG. 2 shows the dimensional change of sample bars of
ZrW.sub.2O.sub.8 with 15 weight percent ZrO.sub.2 at 85.degree. C.
and 85% relative humidity compared with the dimensional changes of
inventive bodies under the same conditions;
[0030] FIG. 3 is a schematic drawing of an embodiment of an
athermal optical fiber grating device;
[0031] FIG. 4 is a schematic drawing of an embodiment of an
athermal optical fiber grating device;
[0032] FIG. 5 is a schematic drawing an embodiment of an athermal
optical fiber grating device;
[0033] FIG. 6 is an enlarged view of the affixing channel shown in
FIG. 5;
[0034] FIG. 7 is a schematic drawing of an embodiment of an
athermal fiber grating device;
[0035] FIG. 8 is a schematic drawing of an embodiment of an
athermal optical fiber fused coupler device;
[0036] FIG. 9 is a schematic drawing of an embodiment of an
athermal planar waveguide device; and
[0037] FIG. 10 is a plot of center wavelength (nm)versus
temperature (.degree. C.) of an embodiment of an athermal fiber
Bragg grating device.
DETAILED DESCRIPTION
[0038] Reference will now be made in detail to the present
preferred embodiment of the invention, an example of which is
illustrated in the accompanying drawings.
[0039] According to one embodiment of the present invention, small
amounts of additives, which function as sintering aids, are mixed
with Zr.sub.2P.sub.2WO.sub.12 powder or to a mixture of precursor
compounds that can form Zr.sub.2P.sub.2WO.sub.12 by reaction. The
precursors may include, for example, ZrO.sub.2,
Zr(NO.sub.3).sub.4.xH.sub.2O, ZrOCl.sub.2.xH.sub.2O,
Zr(SO.sub.4).sub.2.xH.sub.2O, WO.sub.3, H.sub.2WO.sub.4.xH.sub.2O,
P.sub.2O.sub.5, ZrP.sub.2O.sub.7, Zr.sub.2P.sub.2O.sub.9,
Zr(HPO.sub.4).sub.2.xH.sub.2O, ZrW.sub.2O.sub.8,
NH.sub.4H.sub.2PO.sub.4, (NH.sub.4).sub.2HPO.sub.4,
(NH.sub.4).sub.3PO.sub.4.3H.sub.2O, phospho-tungstic acid, etc.
Hafnium may be partially or fully substituted for zirconium. The
mixtures may be dry mixed or blended using an appropriate organic
or inorganic liquid medium. Water may be used when the precursors
have low aqueous solubility. If mixed in a fluid medium, the
precursors may also be milled by any of the methods known to the
art (e.g., ball milling, attrition milling, vibratory milling,
etc.). When some or all of the precursors are soluble in water,
they may be dissolved in water and subsequently coprecipitated to
form an even more intimate mixture.
[0040] The mixture may optionally be dried and calcined, followed
by milling, or may be directly "green" formed into the desired
shape. Organic binders such as a polyethylene glycol or methyl
cellulose may optionally be added to provide strength to the green
body. The body is then raised to and held at a temperature
sufficient for densification to take place, such as at 1150.degree.
C. for 4 hours.
[0041] In another aspect of the invention, ceramic bodies are
comprised predominately of two materials having a negative CTE,
such as ZrW.sub.2O.sub.8 and Zr.sub.2P.sub.2WO.sub.12 or their
hafnium analogues or mixtures thereof, and the bodies exhibit CTEs
of -40 to -85.times.10.sup.-7.degree. C..sup.-1 between -40.degree.
C. and +85.degree. C. In one embodiment one of the materials has a
room temperature CTE more negative than about
-50.times.10.sup.-7.degree. C..sup.-1. Depending upon the ratio of
the two negative CTE materials, it is possible to achieve these
CTEs in the body without microcracking. Ceramic bodies having
negative CTEs within this range without relying upon microcracking
have not previously been reported in the literature, and represent
a unique invention.
[0042] In a two-phase ceramic containing ZrW.sub.2O.sub.8, the
second phase must either be in chemical equilibrium with
ZrW.sub.2O.sub.8 at the sintering temperature, or else reaction
between the second phase and ZrW.sub.2O.sub.8 must be sufficiently
slow that most of the second phase is retained through the firing
process. Also, to avoid microcracking, the difference between the
CTEs of the two phases, ACTE, must be minimized (while still being
sufficient to achieve the desired bulk CTE of about
-70.times.10.sup.-7.degree. C..sup.-1 to
-85.times.10.sup.-7.degree. C..sup.-1), and the mean grain size of
the components must be finer than some critical value which is
dictated by the value of .DELTA.CTE. The term "grain size" is meant
to include either the size of a primary crystal, or the size of a
cluster or aggregate of two or more adjacent crystals of the same
phase.
[0043] Addition of chemically compatible phases with positive CTEs,
such as ZrSiO.sub.4, ZrO.sub.2, or WO.sub.3, to a
ZrW.sub.2O.sub.8-based ceramic could be employed to create
materials having expansions less negative than that of pure
ZrW.sub.2O.sub.8, however, the large disparity in CTE between the
two components would result in microcracking unless the grains of
the two phases were maintained at less than about 1 micron during
sintering.
[0044] The present invention involves the discovery of an
alternative to prevent microcracking. Applicant has discovered that
the combination of a material having a strongly negative CTE, for
example, a first material such as ZrW.sub.2O.sub.8 having a room
temperature CTE more negative than -50.times.10.sup.-7.degree.
C..sup.-1, with a second phase whose CTE is also negative, but not
as strongly negative as that of the first material, results in
lower stresses from the CTE mismatch between the phases. Thus, the
present invention provides a body that avoids microcracking for
grain sizes less than about 15 microns, preferably less than about
10 microns. In an additional alternative embodiment, the grain
sizes are greater than 1 micron.
[0045] Applicant has recently measured the room temperature CTE of
the compound Zr.sub.2P.sub.2WO.sub.12 to be about
-40.times.10.sup.-7.degree. C..sup.-1. Applicant has discovered
that because the CTE of this compound is not extremely different
from that of ZrW.sub.2O.sub.8, a ceramic body containing both
phases can exhibit a CTE of intermediate value without undergoing
microcracking, provided that the grain size of the two components
is less than about 10 microns. This restriction on grain size can
easily be met for an appropriate choice of starting materials and
sintering conditions.
[0046] According to an exemplary embodiment of the present
invention, powders of ZrW.sub.2O.sub.8 and
Zr.sub.2P.sub.2WO.sub.12, or precursors that form ZrW.sub.2O.sub.8
and Zr.sub.2P.sub.2WO.sub.12 by reaction upon firing, or their
hafnium analogues or mixtures thereof, are mixed together and
formed into the desired shape by ceramic processes known in the
art, e.g., dry pressing, injection molding, extrusion, slip
casting, etc. Mixing may be performed on the dry powders, or the
powders may be mixed with a liquid and may optionally be further
reduced in particle size by ball milling, attrition milling,
vibratory milling, etc. An organic binder may be added to the
powders to increase handling strength in the green (pre-fired)
state. Optionally, inorganic or organometallic compounds which
serve as densification aids may also be added in small quantities
to the mixture of starting materials. It has been found that oxides
or oxide-forming compounds of alkali (group IA) metals, alkaline
earth (group IIA) metals, manganese, iron, cobalt, nickel, copper,
zinc, yttrium, scandium, lanthanide metals, niobium, titanium,
aluminum, gallium, and bismuth are especially effective at reducing
porosity of the ceramic body during firing. The quantity of
sintering aids is preferably the minimum amount required for
densification, as it has been found that larger additions of such
additives tend to produce large-scale cracking of the body during
firing.
[0047] The body may be placed upon a glass or ceramic setter pallet
or in a partially enclosed container of a glass or ceramic material
and heated to a maximum temperature of between about 1120 and
1230.degree. C., preferably between about 1150 to 1200.degree. C.,
and held for a period of time sufficient for reaction and sintering
to take place, such as 1 minute to 10 hours. A glass or ceramic
powder or "sand" may be placed between the body and the pallet to
reduce drag during shrinkage of the body during firing. Zirconium
oxide or zircon powder or silica powder or sand are especially
preferred in this role. After holding at peak temperature, the body
is rapidly cooled to a temperature below about 500.degree. C. in a
few minutes to minimize decomposition of the ZrW.sub.2O.sub.8
phase. ZrW.sub.2O.sub.8 is unstable below about 1140.degree. C.,
but decomposition proceeds only very slowly at temperatures less
than about 800.degree. C. The fired body may optionally undergo
surface grinding or other machining if desired.
[0048] The present invention is illustrated by the following
non-limiting examples.
EXAMPLES 1 to 18
[0049] Examples 1-18 were prepared by mixing together powders of
zirconium oxide, tungsten oxide, and acid zirconium phosphate,
Zr(HPO.sub.4).sub.2.0.93H.sub.2O, in the proportions required to
yield the compound Zr.sub.2P.sub.2WO.sub.12 after firing. Mixtures
for Examples 2-18 also contained 1 weight percent addition of a
metal oxide, or metal oxide forming source. Powders were mixed with
a sufficient amount of isopropanol to form a slurry, and the slurry
was milled in a vibratory mill for approximately sixteen hours
using zirconium oxide milling media. The slurry was subsequently
dried in a dish at about 85.degree. C., repulverized, and the
powder pressed into 1.2 cm diameter, 0.4 cm thick pills, or 7.6
cm.times.1.3 cm.times.0.4 cm bars in a steel mold at a pressure of
about 70 Mpa (10,000 pounds/inch.sup.2). The pills and bars were
placed on coarse zirconium oxide sand in covered aluminum oxide
setter boxes inside of an electrically heated furnace. Temperature
of the furnace was raised at a rate of about 30.degree. C./hr to
350.degree. C. to allow volatilization of the water from the acid
zirconium phosphate, then heated at a rate of about 100 to
200.degree. C./hr to a maximum temperature of 1150.degree. C. After
holding at 1150.degree. C. for 4 hours, power to the furnace was
shut off and the samples gradually cooled to room temperature.
[0050] The fired bars were cut to a 5 cm length for measurement of
thermal expansion. Porosity was measured by the Archimedes method,
and phases present after firing were determined by powder x-ray
diffractometry (XRD). Coefficients of thermal expansion were
measured by dilatometry.
[0051] Example 1 shows that, in the absence of a sintering
additive, the Zr.sub.2P.sub.2WO.sub.12 ceramic was poorly
densified, containing about 32 percent porosity. The mean
coefficient of thermal expansion (CTE) from 25 to 50.degree. C. was
-32.times.10.sup.-7.degree. C..sup.-1, and from 25 to 500.degree.
C. was -30.times.10.sup.-7.degree. C..sup.-1. This specimen
consisted entirely of the desired Zr.sub.2P.sub.2WO.sub.12
phase.
[0052] Examples 2-4 demonstrate that the addition of only 1% of the
carbonates of the alkali (Group IA) metals is highly effective in
promoting densification of the Zr.sub.2P.sub.2WO.sub.12 ceramics.
Samples contain essentially no open porosity, and total porosities
are reduced to approximately 1-5%. Thermal expansions remain
strongly negative at room temperature. XRD showed that these
ceramic bodies contain mostly Zr.sub.2P.sub.2WO.sub.12 with minor
amounts of LiZr.sub.2P.sub.3O.sub.12, NaZr.sub.2P.sub.3O.sub.12,
and KZr.sub.2P.sub.3O.sub.12, in samples 2-4, respectively. These
secondary phases are responsible for the somewhat less negative CTE
from 25 to 500.degree. C. for Examples 2 and 3.
[0053] Examples 5-7 illustrate that the addition of only 1% of the
oxides or carbonates of the alkaline earth (Group IIA) metals is
also very effective in reducing the porosity of
Zr.sub.2P.sub.2WO.sub.12 ceramics. Total porosities are less than
3%, and there is virtually no open porosity. Fired bodies contained
predominately Zr.sub.2P.sub.2WO.sub.12. Example 5 contained a very
minor amount of MgWO.sub.4, Example 6 contained very minor amounts
of CaZr.sub.4P.sub.6O.sub.24 and CaWO.sub.4, and Example 7 had a
very small amount of BaZr.sub.4P.sub.6O.sub.24. CTEs were very
negative from 25 to 50.degree. C. (-27 to
-35.times.10.sup.-7.degree. C..sup.-1) and 25 to 500.degree. C.
(-24 to -26.times.10.sup.-7.degree. C..sup.-1).
1TABLE 1 Experimental data for Zr.sub.2P.sub.2WO.sub.12 ceramics.
Mean Coefficient Measured on pills Measured on bars of Thermal
Weight Bulk Bulk Expansion (10.sup.-7 cC.sup.-1) Example % Weight %
Density % Open % Total Density % Open % Total -40 to 25 to 25 to
Number Zr.sub.2P.sub.2WO.sub.12 Additive Additive (g/cm.sup.3)
Porosity Porosity (g/cm.sup.3) Porosity Porosity +85.degree. C.
50.degree. C. 500.degree. C. 1 100 0 none 2.71 28.5 30.5 2.65 31.1
32.1 -32 -30 2 99 1 Li.sub.2CO.sub.3 3.85 0.1 1.3 3.87 0.3 0.8 -25
-15 3 99 1 Na.sub.2CO.sub.3 3.81 0.0 2.3 3.84 0.1 1.5 -27 -18 4 99
1 K.sub.2CO.sub.3 3.69 0.4 5.4 3.70 0.5 5.1 -39 -35 -27 5 99 1 MgO
3.87 0.0 0.8 3.87 0.1 0.8 -35 -24 6 99 1 CaCO.sub.3 3.72 0.3 4.6
3.81 0.2 2.3 -34 -26 7 99 1 BaCO.sub.3 3.82 0.1 2.1 3.82 0.2 2.1
-27 -24 8 99 1 MnCO.sub.3 3.84 0.0 1.5 3.86 0.0 1.0 -35 -28 9 99 1
Fe.sub.2O.sub.3 3.82 0.2 2.1 3.80 0.4 2.6 -29 -26 10 99 1 CuO 3.76
0.2 3.6 3.75 0.2 3.8 -36 -27 11 99 1 ZnO 3.87 0.0 0.8 3.88 0.1 0.5
-39 -38 -26 12 99 1 Al.sub.2O.sub.3 3.14 16.5 19.5 13 99 1
TiO.sub.2 2.59 31.4 33.6 14 99 1 NiO 2.47 37.1 36.7 15 99 1
Y.sub.2O.sub.3 2.52 32.8 35.4 16 99 1 SiO.sub.2 2.36 37.8 39.5 17
99 1 SnO 2.66 29.6 31.8 18 99 1 Nb.sub.2O.sub.5 2.59 33.7 33.6
[0054] Examples 8-11 show that the addition of only 1% of the
oxides or carbonates of manganese, iron, copper, and zinc is highly
effective in densifying Zr.sub.2P.sub.2WO.sub.12 ceramics to less
than 4% total porosity, with essentially no open porosity. Example
8 contained a trace of MnWO.sub.4, and Example 9 contained a very
minor amount of an unidentified phase. Examples 10 and 11 exhibited
only Zr.sub.2P.sub.2WO.sub.12 in their XRD patterns. All samples
had highly negative CTEs of -29 to -38.times.10.sup.-7.degree.
C..sup.-1 from 25 to 50.degree. C., and -26 to
-28.times.10.sup.-7.degree. C..sup.-1 from 25 to 500.degree. C.
[0055] Example 12 demonstrates that aluminum oxide also serves to
lower the porosity of Zr.sub.2P.sub.2WO.sub.12 ceramics; however,
total porosity is still about 20%. XRD indicated the presence only
of Zr.sub.2P.sub.2WO.sub.12. Thermal expansion was not
measured.
[0056] Examples 13-18 prove that not all oxides are effective as
sintering aids for Zr.sub.2P.sub.2WO.sub.12 ceramics. Thus, the
addition of 1% of the oxides of titanium, nickel, yttrium, silicon,
tin, and niobium, for example, yield a ceramic body containing
greater than 30% porosity.
EXAMPLES 19 to 28
[0057] Powder preparation for Examples 19 and 21-28 was conducted
in the same manner as Examples 1-18, with the proportions of the
starting materials adjusted to yield Zr.sub.2P.sub.2WO.sub.12 and
ZrW.sub.2O.sub.8 in the ratios stated in Table 2. In these
examples, the weight percentages of Zr.sub.2P.sub.2WO.sub.12 and
ZrW.sub.2O.sub.8 are relative to the sum of the weights of the
Zr.sub.2P.sub.2WO.sub.12 and ZrW.sub.2O.sub.8 components only.
Weight percent of additive is relative to total weight percent of
starting material.
2TABLE 2 Experimental data for Zr.sub.2P.sub.2WO.sub.12 ceramics
with various amounts of ZrW.sub.2O.sub.8. Heating rate Weight
Weight above Example % % Weight % Sample 900.degree. C. Soak Soak
Number Zr.sub.2P.sub.2WO.sub.12 ZrW.sub.2O.sub.8 Additive Additive
Geometry (.degree. C./hr) Temp Time 19 100 0 0.00 none 7.6 cm 200
1150 4 bar 20 50 50 0.00 none 7.6 cm 100 1150 4 bar 21 49 51 0.00
none 7.6 cm 200 1150 4 bar 22 32 68 0.00 none 7.6 cm 100 1150 4 bar
23 32 68 0.00 none 7.6 cm 100 1150 4 bar 24 32 68 0.00 none 7.6 cm
25 1150 4 bar 25 24 76 0.00 none 7.6 cm 200 1150 4 bar 26 10 91
0.00 none 7.6 cm 200 1150 4 bar 27 5 95 0.00 none 7.6 cm 200 1150 4
bar 28 0 100 0.00 none 7.6 cm 200 1150 4 bar 29 32 68 0.25 AlOOH
7.6 cm 100 1150 4 bar 30 32 68 0.25 TiO.sub.2 7.6 cm 100 1150 4 bar
31 32 68 0.25 Nb.sub.2O.sub.5 7.6 cm 100 1150 4 bar 32 32 68 0.25
SiO.sub.2 7.6 cm 100 1150 4 bar 33 32 68 0.05 Li.sub.2CO.sub.3 7.6
cm 100 1150 4 bar 34 32 68 0.10 Na.sub.2CO.sub.3 7.6 cm 100 1150 4
bar 35 32 68 0.05 Na.sub.2CO.sub.3 7.6 cm 100 1150 4 bar 36 32 68
0.01 Na.sub.2CO.sub.3 7.6 cm 100 1150 4 bar 37 32 68 0.25
K.sub.2CO.sub.3 7.6 cm 100 1150 4 bar 38 32 68 0.25 K.sub.2CO.sub.3
7.6 cm 25 1150 4 bar 39 32 68 0.05 K.sub.2CO.sub.3 7.6 cm 100 1150
4 bar 40 32 68 0.25 CaCO.sub.3 7.6 cm 100 1150 4 bar 41 32 68 0.25
CaCO.sub.3 7.6 cm 25 1150 4 bar 42 32 68 0.05 CaCO.sub.3 7.6 cm 100
1150 4 bar 43 32 68 0.25 BaCO.sub.3 7.6 cm 100 1150 4 bar 44 32 68
0.25 BaCO.sub.3 7.6 cm 25 1150 4 bar 45 32 68 0.10 BaCO.sub.3 7.6
cm 100 1150 4 bar 46 32 68 1.00 MgO 1.3 cm 100 1150 4 disc 47 32 68
0.25 MgO 7.6 cm 100 1150 4 bar 48 32 68 0.25 MgO 7.6 cm 25 1150 4
bar 49 32 68 0.05 MgO 7.6 cm 100 1150 4 bar 50 32 68 1.00 CuO 1.3
cm 100 1150 4 disc 51 32 68 0.25 CuO 7.6 cm 100 1150 4 bar 52 32 68
0.05 CuO 7.6 cm 100 1150 4 bar 53 32 68 1.00 ZnO 1.3 cm 100 1150 4
disc 54 32 68 1.00 ZnO 7.6 cm 100 1150 4 bar 55 32 68 0.25 ZnO 7.6
cm 100 1150 4 bar 56 32 68 0.25 ZnO 7.6 cm 25 1150 4 bar 57 32 68
0.10 ZnO 7.6 cm 100 1150 4 bar 58 32 68 1.00 MnCO.sub.3 1.3 cm 100
1150 4 disc 59 32 68 1.00 MnCO.sub.3 7.6 cm 100 1150 4 bar 60 32 68
0.25 MnCO.sub.3 7.6 cm 100 1150 4 bar 61 32 68 0.25 MnCO.sub.3 7.6
cm 25 1150 4 bar 62 32 68 0.05 MnCO.sub.3 7.6 cm 100 1150 4 bar 63
32 68 1.00 Fe.sub.2O.sub.3 1.3 cm 100 1150 4 disc 64 32 68 1.00
Fe.sub.2O.sub.3 7.6 cm 100 1150 4 bar 65 32 68 0.25 Fe.sub.2O.sub.3
7.6 cm 100 1150 4 bar 66 32 68 0.50 Y.sub.2O.sub.3 11.4 cm 100 1150
4 bar 67 32 68 0.25 Y.sub.2O.sub.3 7.6 cm 100 1150 4 bar 68 32 68
0.25 Y.sub.2O.sub.3 11.4 cm 100 1150 4 bar 69 32 68 0.25
Y.sub.2O.sub.3 11.4 cm 100 1150 4 bar 70 32 68 0.12 Y.sub.2O.sub.3
11.4 cm 100 1150 4 bar 71 32 68 0.05 Y.sub.2O.sub.3 11.4 cm 100
1150 4 bar 72 32 68 0.05 Y.sub.2O.sub.3 11.4 cm 100 1150 4 bar 73
32 68 0.05 Y.sub.2O.sub.3 11.4 cm 100 1150 4 bar 74 25 75 0.05
Y.sub.2O.sub.3 11.4 cm 100 1150 4 bar 75 25 75 0.05 Y.sub.2O.sub.3
11.4 cm 450 1175 4 bar 76 25 75 0.05 Y.sub.2O.sub.3 11.4 cm 450
1175 8 bar 77 15 85 0.05 Y.sub.2O.sub.3 11.4 cm 450 1175 8 bar 78
10 88* 0.05 Y.sub.2O.sub.3 11.4 cm 450 1175 6 bar 79 32 68 0.25
La.sub.2O.sub.3 7.6 cm 100 1160 4 bar 80 32 68 0.25 CeO.sub.2 7.6
cm 100 1160 4 bar 81 32 68 0.25 MoO.sub.3 7.6 cm 100 1160 4 bar 82
32 68 0.25 Bi.sub.2O.sub.3 7.6 cm 100 1160 4 bar 83 32 68 0.25
B.sub.2O.sub.3 7.6 cm 100 1160 4 bar 84 32 68 0.25 Ga.sub.2O.sub.3
7.6 cm 100 1160 4 bar 85 32 68 0.25 In.sub.2O.sub.3 7.6 cm 100 1160
4 bar 86 32 68 0.25 SnO 7.6 cm 100 1160 4 bar 87 32 68 0.25
Cr.sub.2O.sub.3 7.6 cm 100 1160 4 bar 88 32 68 0.25 NiO 7.6 cm 100
1160 4 bar *Example 78 also contains 2% tungsten oxide as an
additional phase.
[0058] Example 20 was prepared by precipitation from aqueous
solutions of ZrOCl.sub.2.8H.sub.2O, NH.sub.4H.sub.2PO.sub.4, and
H.sub.2WO.sub.4. According to this method, 79.18 grams of zirconyl
chloride (previously assayed to be 2.739.times.10.sup.-3 moles Zr
per gram of salt) was dissolved in 161 grams of water to form the
first solution. A second solution was formed by dissolving 16.63
grams of ammonium dihydrogen phosphate in 73 grams of water. A
third solution was made by dissolving 54.19 grams of tungstic acid
in 152 grams of water and 54 grams of 14.8 N ammonium hydroxide
solution, and heating to 95.degree. C. The tungsten and phosphate
solutions were mixed together and then added to the zirconium
solution. More ammonium hydroxide was added to the mixture to
deflocculate the suspension. The slurry was stirred and heated to
85.degree. C. until dry. The solid was then calcined at 900.degree.
C. for 4 hours and subsequently crushed and vibratory milled in
isopropanol for 14 hours with zirconia media.
[0059] Mixed powders for Examples 19-28 were pressed into 7.6 cm
long bars at 70 Mpa. Samples were set on zirconia sand in covered
alumina boxes and fired at about 30.degree. C./hr to 350.degree.
C., then heated at rate ranging from 25.degree. C./hr to
200.degree. C./hr to 1150.degree. C. and held at that temperature
for 4 hours. After 4 hours, the alumina boxes were removed from the
furnace and the samples immediately taken from the boxes and
transferred to a zirconia sand-covered alumina pallet which was at
room temperature. The rapid cooling of the samples prevented the
decomposition of the metastable ZrW.sub.2O.sub.8 phase into
ZrO.sub.2+WO.sub.3 below about 1140.degree. C.
[0060] After firing, samples were inspected for appearance and, in
some cases, further characterized for porosity, phase assemblage,
and CTE. Properties of fired samples are reported in Table 3.
3TABLE 3 Experimental data for Zr.sub.2P.sub.2WO.sub.12 ceramics
with various amounts of ZrW.sub.2O.sub.8. Mean Coefficient of
Thermal Theo- Expansion Bulk reti- (10.sup.-7.degree. C..sup.-1)
Example Density cal % Total -40 to Number Appearance (g/cm.sup.3)
Density Porosity +85.degree. C. 19 no cracks 2.47 3.90 36.7 -40 20
no cracks -61 * 21 no cracks 3.24 4.43 26.8 -66 * 22 no cracks -72
23 no cracks 3.12 4.63 32.5 -72 24 no cracks 25 no cracks 3.66 4.74
22.7 -76 26 no cracks 4.03 4.94 18.4 -83 27 no cracks 4.27 5.01
14.7 -85 28 no cracks 4.43 5.08 12.8 -90 29 no cracks 4.42 4.63 4.6
-71 * 30 no cracks 4.06 4.63 12.3 -73 * 31 no cracks 3.97 4.63 14.3
-80 32 no cracks 3.52 4.63 24.0 -79 33 scattered 4.07 4.63 12.1 34
no cracks 4.35 4.63 6.0 35 no cracks 3.81 4.63 17.8 36 no cracks
3.16 4.63 31.8 37 extremely 4.38 4.63 5.4 38 highly cracked 39 no
cracks 3.81 4.63 17.7 40 extremely 4.49 4.63 3.1 41 extremely 42 no
cracks 4.30 4.63 7.1 43 very few 4.54 4.63 2.0 -70 44 scattered 45
no cracks 4.45 4.63 3.9 -72 46 slumped 4.35 4.63 6.1 47 extremely
4.56 4.63 1.5 48 scattered 49 no cracks 4.51 4.63 2.6 50 no cracks
4.46 4.63 3.7 51 scattered 4.58 4.63 1.1 52 no cracks 4.20 4.63 9.4
53 open cracks 4.43 4.63 4.4 54 crazed 55 extremely 4.55 4.63 1.8
56 no cracks -71 * 57 no cracks 4.51 4.63 2.6 -70 58 open cracks
4.47 4.63 3.5 59 open cracks 60 highly 4.54 4.63 2.0 cracked 61
highly cracked 62 no cracks 4.15 4.63 10.4 63 open cracks 4.46 4.63
3.7 64 open cracks 65 no cracks 4.51 4.63 2.6 -75 66 moderately 67
no cracks 4.38 4.63 5.4 -77 68 no cracks -67 69 no cracks -70 70 no
cracks 4.27 4.63 7.8 -66 * 71 no cracks 3.79 4.63 18.1 -68 * 72 no
cracks 4.44 4.63 4.1 -68 * 73 no cracks -69 74 no cracks -68 75 no
cracks -70 76 no cracks -74 77 no cracks -79 78 no cracks 4.79 4.96
3.6 -84 79 scattered 4.51 4.63 2.6 80 very few 4.52 4.63 2.3 81 no
cracks 3.01 4.63 35.0 82 no cracks 4.52 4.63 2.4 83 no cracks 3.22
4.63 30.4 84 very few 4.51 4.63 2.7 85 no cracks 3.14 4.63 32.3 86
no cracks 3.19 4.63 31.1 87 no cracks 3.32 4.63 28.3 88 extremely
4.54 4.63 2.0 * mean CTE from -40.degree. to +40.degree. C.
[0061] With increasing addition of ZrW.sub.2O.sub.8 to
Zr.sub.2P.sub.2WO.sub.12, the amount of porosity in the ceramic
body decreases, and the thermal expansion becomes progressively
more negative, reaching -90.times.10.sup.-7.degree. C..sup.-1 for
100% ZrW.sub.2O.sub.8 as shown in FIG. 1. For the preferred range
of CTE between -70 and -85.times.10.sup.-7.degree. C..sup.-1
desired for athermalization of fiber Bragg gratings, the amount of
Zr.sub.2P.sub.2WO.sub.12 is between about 5% and 40%, and the
amount of ZrW.sub.2O.sub.8 is between about 60% and 95%. For the
preferred range of CTE between -65 and -80.times.10.sup.-7.degree.
C..sup.-1 desired for athermalization of fiber Bragg gratings, the
amount of Zr.sub.2P.sub.2WO.sub.12 is between about 10% and 50%,
and the amount of ZrW.sub.2O.sub.8 is between about 50% and
90%.
EXAMPLES 29 to 88
[0062] Powder preparation for Examples 29 to 88 was conducted in
the same manner as Examples 1-18, with the proportions of the
starting materials adjusted to yield Zr.sub.2P.sub.2WO.sub.12 and
ZrW.sub.2O.sub.8 in the ratios stated in Table 2, with the
following exceptions: Example 68 was prepared using ZrO.sub.2,
WO.sub.3, and a ZrP.sub.2O.sub.7 powder prepared by calcination of
acid zirconium phosphate at 1050.degree. C. for 4 hours, and
Examples 69 and 73-78 were prepared from ZrO.sub.2, WO.sub.3, and
pre-reacted Zr.sub.2P.sub.2WO.sub.12. The pre-reacted
Zr.sub.2P.sub.2WO.sub.12 was formed by dry ball milling a mixture
of ZrO.sub.2, WO.sub.3 and acid zirconium phosphate in the
appropriate ratios and calcining the mixture at 1050.degree. C. for
4 hours. Also, the powders for Examples 72 to 78 were milled in
water instead of isopropanol. Examples 29 to 88 were formulated to
yield ceramics with coefficients of thermal expansion between -65
and -85.times.10.sup.-7.deg- ree. C..sup.-1.
[0063] In these examples, the weight percentages of
Zr.sub.2P.sub.2WO.sub.12 and ZrW.sub.2O.sub.8 are relative to the
sum of the weights of the Zr.sub.2P.sub.2WO.sub.12 and
ZrW.sub.2O.sub.8 components only. Weight percent of additive is
relative to total weight percent of starting material.
[0064] Example 29 shows that 0.25% aluminum monohydrate is an
effective sintering aid in reducing the porosity of a
68%ZrW.sub.2O.sub.8-32%Zr.sub- .2P.sub.2WO.sub.12 ceramic to 4.6%.
The fired sample exhibited good integrity, with no cracking.
[0065] Example 30 shows that 0.25% addition of titania reduces
porosity of a 68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12
ceramic to about 12% with no cracking.
[0066] Example 31 illustrates that 0.25% addition of niobium oxide
reduces the porosity of a
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramic to about
14% with no cracking.
[0067] Example 32 demonstrates that 0.25% silica addition is not
effective as a sintering aid for
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramics.
[0068] Example 33 shows that although 0.05% lithium carbonate
(equivalent to 0.02% Li.sub.2O) reduces porosity of a
68%ZrW.sub.2O.sub.8-32%Zr.sub.2- P.sub.2WO.sub.12 ceramic, this
amount of additive results in scattered cracks.
[0069] Example 34 illustrates that 0.10% sodium carbonate
(equivalent to 0.06% Na.sub.2O) reduces the porosity of the
68%ZrW.sub.2O.sub.8-32%Zr.su- b.2P.sub.2WO.sub.12 composition to
only 6%, without cracking. Example 35 shows that the amount of
Na.sub.2CO.sub.3 can be reduced to 0.05% (0.03% Na.sub.2O) and
still reduce porosity to 17.8%. However, Example 36 shows that
0.01% sodium carbonate (0.006% Na.sub.2O) is not sufficient to
reduce the porosity of this ceramic composition.
[0070] Example 37 illustrates that although 0.25% potassium
carbonate (equivalent to 0.17% K.sub.2O) is a very effective
sintering aid for the
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 composition,
reducing porosity to 5.4%, the ceramic body exhibits extreme
cracking after firing. Example 38 shows that slowing the heating
rate to 25.degree. C./hour above 1100.degree. C. does not eliminate
the cracking of this composition. Thus, the amount of K.sub.2O
present in a 68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12
ceramic must be less than 0.17% to avoid cracking. Example 39
demonstrates that even 0.05% K.sub.2CO.sub.3 (0.034% K.sub.2O) is
effective as a densification aid for these ceramics.
[0071] Examples 40 and 41 demonstrate that addition of 0.25%
calcium carbonate (0.14% CaO) is also very effective in reducing
porosity; however, samples are extremely cracked after firing.
Thus, the amount of CaO in a
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramic must be
less than 0.14% to avoid cracking. Example 42 shows that even only
0.05% CaCO.sub.3 (0.03% CaO) is sufficient to substantially reduce
the porosity in these ceramics.
[0072] Examples 43 and 44 show that the addition of 0.25% barium
carbonate (0.19% BaO) is effective in reducing porosity to 2.0%;
however, the sintered body exhibits a small amount of cracking.
Example 45 shows that reduction of the amount of barium carbonate
to 0.1% (0.08% BaO) is still effective in lowering porosity to less
than 4%, and results in a crack-free body. Thus, the amount of BaO
should be less than about 0.19% to avoid cracking.
[0073] Example 46 demonstrates that the addition of 1.0% magnesium
oxide, MgO, to the 68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12
composition results in the formation of excessive liquid, so that
the body underwent considerable slumping. Examples 47 and 48 show
that reduction in the MgO concentration to 0.25% yielded a cracked
body that was otherwise well densified. Example 49 demonstrates
that addition of only 0.05% MgO still yields a low-porosity body
which is also free of cracks. Thus, the amount of MgO must be less
than 0.25% to avoid cracking.
[0074] Examples 50 and 51 show that 1.0% or 0.25% cupric oxide,
CuO, is effective in lowering porosity of the
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.su- b.2WO.sub.12 composition.
However, XRD revealed considerable amounts of zirconium oxide and
tungsten oxide in the fired body which are undesirable for strongly
negative CTE. However, 0.05% CuO addition (Example 52) still yields
a low-porosity ceramic, while maintaining the ZrW.sub.2O.sub.8 and
Zr.sub.2P.sub.2WO.sub.12 phases with essentially no secondary
zirconium or tungsten oxides. Thus, the amount of CuO should be
less than about 0.25% to prevent the formation of excessive amounts
of ZrO.sub.2 and WO.sub.3 in the fired ceramic.
[0075] Examples 53 and 54 illustrate that addition of 1.0% zinc
oxide, ZnO, to the 68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12
composition is effective in reducing porosity, but results in
cracking of the body. Examples 55 and 56 show that reduction of the
amount of ZnO to 0.25% is still useful in densifying the ceramic,
and can result in a crack-free body when the heating rate above
1100.degree. C. is less than 100.degree. C./hour. Further lowering
of the amount of ZnO to 0.1% results in a dense, crack-free body
even for heating rates of 100 .degree. C./hour (Example 57).
[0076] Examples 58 and 59 demonstrate that 1.0% manganese carbonate
(yielding 0.62% MnO) is effective in densifying
68%ZrW.sub.2O.sub.8-32%Zr- .sub.2P.sub.2WO.sub.12 ceramics, but
produces cracking in the fired body. Examples 60 and 61 show that
0.25% MnCO.sub.3 (0.15% MnO) still reduces porosity to low levels,
but cracking is present. Example 62 shows that 0.05% MnCO.sub.3
(0.03% MnO) is still effective as a sintering aid, and does not
produce cracking. Thus, the amount of MnO in the
%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramic must be less
than about 0.15% to prevent crack formation during sintering.
[0077] Examples 63 and 64 illustrate that 1.0% ferric oxide,
Fe.sub.2O.sub.3, is effective as a sintering aid for
%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramics, but results
in cracking. Example 65 shows that 0.25% Fe.sub.2O.sub.3 yields a
low-porosity, uncracked body. Thus, the amount of Fe.sub.2O.sub.3
to be used as a sintering aid for the
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.- sub.12 composition is
preferably less than 1.0% to avoid cracking.
[0078] Example 66 shows that addition of 0.50% yttrium oxide,
Y.sub.2O.sub.3, yields a cracked body of
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P- .sub.2WO.sub.12 composition.
Example 67 shows that reducing the yttria to 0.25% eliminates
cracking and still reduces porosity to 5.4%. Example 68
demonstrates that the phosphorus can be supplied as ZrP.sub.2O
powder. Example 69 shows that the phosphorus can be supplied as
pre-reacted Zr.sub.2P.sub.2WO.sub.12 powder. Example 70 illustrates
that the amount of yttria can be reduced to 0.12% and still yield a
68%ZrW.sub.2O.sub.8-32%Zr.sub.2P.sub.2WO.sub.12 ceramic with low
porosity without cracking. Example 71 shows that reduction of the
amount of yttria to only 0.05% results in an increase in porosity
to 18.1% when the powders are milled in isopropanol. However,
Example 72 demonstrates that 0.05% yttria is sufficient to serve as
an effective densification aid when the powders are milled in
water. Example 73 shows that phosphorus may be supplied as
pre-reacted Zr.sub.2P.sub.2WO.sub.12 powder to the raw material
mixture.
[0079] Example 74 demonstrates that a ceramic comprised of 75%
ZrW.sub.2O.sub.8 and 25% Zr.sub.2P.sub.2WO.sub.12 with 0.05% yttria
additive is uncracked and possesses a CTE of
-68.times.10.sup.-7.degree. C..sup.-1. Examples 75 and 76 show that
this composition may be fired at 1175.degree. C. for 4 to 8
hours.
[0080] Example 77 shows that the amount of ZrW.sub.2O.sub.8 can be
increased to 85 weight % with 0.05% yttria as a sintering aid to
yield a crack-free sample with a CTE of -79.times.10.sup.-7.degree.
C..sup.-1.
[0081] Example 78 shows that the amount of ZrW.sub.2O.sub.8 can be
further increased to 88 weight % with 0.05% yttria as a sintering
aid to yield a crack-free, low-porosity ceramic with a CTE of
-84.times.10.sup.-7.degree- . C..sup.-1. Example 78 also contains
an addition of about 2 weight % excess tungsten oxide as a separate
phase, in addition to the 10 weight % Zr.sub.2P.sub.2WO.sub.12.
[0082] Examples 79 and 80 show that addition of 0.25% of the oxides
of the rare earth metals lanthanum and cerium are very effective at
reducing the porosity of 68% ZrW.sub.2O.sub.8 and 32%
Zr.sub.2P.sub.2WO.sub.12 bodies to less than 3%. Slight cracking of
these compositions shows that the amount of La.sub.2O.sub.3 or
CeO.sub.2 additive is preferably less than 0.25%.
[0083] Example 81 demonstrates that the addition of 0.25% MoO.sub.3
is not effective as a sintering aid for 68% ZrW.sub.2O.sub.8 and
32% Zr.sub.2P.sub.2WO.sub.12 ceramics.
[0084] Example 82 illustrates that 0.25% bismuth oxide greatly
lowers the porosity of 68% ZrW.sub.2O.sub.8 and 32%
Zr.sub.2P.sub.2WO.sub.12 bodies, and that no cracking is
present.
[0085] Example 83 shows that addition of 0.25% B.sub.2O.sub.3 to a
68% ZrW.sub.2O.sub.8 and 32% Zr.sub.2P.sub.2WO.sub.12 body does not
significantly reduce porosity.
[0086] Example 84 demonstrates that 0.25% gallium oxide is highly
effective in lowering the porosity of 68% ZrW.sub.2O.sub.8 and 32%
Zr.sub.2P.sub.2WO.sub.12 ceramics. However, amounts less than 0.25
weight % are preferred to avoid cracking.
[0087] Examples 85, 86, and 87 illustrate that 0.25% addition of
the oxides of indium, or tin, or chromium is not greatly effective
in reducing the porosity of 68% ZrW.sub.2O.sub.8 and 32%
Zr.sub.2P.sub.2WO.sub.12 bodies.
[0088] Example 88 shows that addition of 0.25% nickel oxide to the
68% ZrW.sub.2O.sub.8 and 32% Zr.sub.2P.sub.2WO.sub.12 composition
reduces porosity to very low levels; however, the amount of NiO is
preferably less than 0.25% to prevent cracking of the ceramic.
COMPARATIVE EXAMPLES
[0089] Although the thermal expansion coefficient of zirconium
tungstate is more strongly negative than that which is required for
athermalization of a fiber Bragg grating, ceramic bodies having the
desired thermal expansion can be prepared from mixtures of
ZrW.sub.2O.sub.8 with one or more additional phases whose CTEs are
either positive or less negative than that of zirconium tungstate.
The amount of additional phases that would be required to yield a
ceramic having a specific CTE can be estimated from the
relation
.alpha..sub.m=V.sub.1.alpha..sub.1+V.sub.2.alpha..sub.2+V.sub.3.alpha..sub-
.3+ . . . +V.sub.n.alpha..sub.n (Equation 1)
[0090] in which .alpha..sub.m is the coefficient of thermal
expansion of the mixture, .alpha..sub.1 is the CTE of component 1,
chosen to be ZrW.sub.2O.sub.8, .alpha..sub.2 is the CTE of
component 2, etc. for n phases, and V.sub.1, V.sub.2, . . . V.sub.n
are the volume fractions of components 1 (ZrW.sub.2O.sub.8), 2,
etc. Equation 1 is only approximate, because it does not take into
account the effect of the different elastic moduli of the various
phases, or the possibility of microcracking.
[0091] The terms in Equation 1 refer to the actual volume fractions
of the phases that are present in the fired ceramic. For a given
mixture of starting materials, one must understand the phase
relations for that system at the temperature to which the body is
fired. Addition of a metal oxide to the ZrW.sub.2O.sub.8 precursor
(such as ZrO.sub.2+WO.sub.3) does not ensure that the fired ceramic
will consist only of ZrW.sub.2O.sub.8 plus that metal oxide. In
many instances, the metal oxide additive will react with the
zirconium tungstate to form two or more new phases. In order to
achieve the desired CTE in the fired ceramic, it is necessary to
know the phases that will form, the volume fractions of those
phases, and their CTEs, so that the amount of metal oxide
additive(s) can be judiciously selected.
[0092] In the comparative examples described below, oxide compounds
having positive coefficients of thermal expansion were added to
zirconium and tungsten oxides in amounts that nominally would yield
ZrW.sub.2O.sub.8-based ceramics having mean CTEs of about
-75.times.10.sup.-7.degree. C..sup.-1 if the oxide additives
remained unreacted during firing of the ceramic. The amount of
metal oxide second phase that would be required to yield a ceramic
having this CTE was computed from the relation
.alpha..sub.m=-75.times.10.sup.-7.degree.
C..sup.-1=V.sub.zw.alpha..sub.zw- +V.sub.mo.alpha..sub.mo (Equation
2)
[0093] in which .alpha..sub.m is the mean coefficient of thermal
expansion of the mixture near 25.degree. C., chosen to be
-75.times.10.sup.-7.degre- e. C..sup.-1, .alpha..sub.zw is the mean
CTE of ZrW.sub.2O.sub.8 near 25.degree. C., which is about
-90.times.10.sup.-7.degree. C..sup.-1, .alpha..sub.mo is the mean
CTE of the metal oxide added as the oxide or added as its
precursor, and V.sub.zw and V.sub.mo are the volume fractions of
ZrW.sub.2O.sub.8 and metal oxide, respectively.
[0094] The compositions of the examples are provided in Table 4.
Table 5 lists the appearances of the samples and their phase
compositions after firing as determined by X-ray diffractometry.
CTE was measured by dilatometry.
4TABLE 4 Experimental data for ZrW.sub.2O.sub.3 ceramics with
various amounts of second phase additives Heating Volume % Nominal
CTE rate above Soak Example Weight % Volume % Weight % Additive of
Oxide Sample 1100.degree. C. Temperature Soak Time Number
ZrW.sub.2O.sub.8 ZrW.sub.2O.sub.8 Additive Additive as Oxide
(25-100.degree. C.) Geometry (.degree. C./hr) (.degree. C.) (hours)
C1 90.4 88 Al.sub.2O.sub.3 9.6 12 70 11.4 cm bar 100 1150 4 C2 90.4
88 Al.sub.2O.sub.3 9.6 12 70 11.4 cm bar 450 1200 8 C3 93.5 91 MgO
6.5 9 120 11.4 cm bar 100 1150 4 C4 89.7 91 CaCO.sub.3 10.3 9 130
11.4 cm bar 100 1150 4 C5 88.1 88 Y.sub.2O.sub.3 11.9 12 70 11.4 cm
bar 100 1150 4 C6 88.1 88 Y.sub.2O.sub.3 11.9 12 70 11.4 cm bar 450
1200 8 C7 86.3 88 ZrO.sub.2 13.7 12 70 11.4 cm bar 100 1150 4 C8
86.3 88 ZrO.sub.2 13.7 12 70 11.4 cm bar 450 1200 8 C9 90.8 81
Fused SiO.sub.2 9.2 19 5 11.4 cm bar 100 1150 4 C10 90.8 81 Fused
SiO.sub.2 9.2 19 5 11.4 cm bar 450 1200 8 C11 95.1 91 Quartz
SiO.sub.2 4.9 9 120 11.4 cm bar 100 1150 4 C12 95.1 91 Quartz
SiO.sub.2 4.9 9 120 11.4 cm bar 450 1200 8 C13 85.0 83.6
ZrSiO.sub.4 15 16.4 40 7.6 cm bar 100 1175 8.5
[0095]
5TABLE 5 Experimental data for ZrW.sub.2O.sub.8 ceramics with
various amounts of second phase additives Mean CTE Coefficient
predicted by of Thermal weighted Expansion average Crystalline
(10.sup.-7.degree. C..sup.-1) of starting Example Phases -40 to
oxides Number Appearance (Powder XRD) +85.degree. C.
(10.sup.-7.degree. C..sup.-1) C1 Sintered bar
Al.sub.2(WO.sub.4).sub.3, +100 -75 ZrO.sub.2 C2 Severely
Al.sub.2(WO.sub.4).sub.3, ** -75 slumped bar; ZrO.sub.2,
Al.sub.2O.sub.3 severe sticking C3 Slumped bar; MgWO.sub.4,
ZrO.sub.2, ** -76 severe sticking unidentified phase(s) C4 Severely
CaWO.sub.4, ZrO.sub.2, ** -75 slumped bar; unidentified severe
sticking phase(s) C5 Concave ZrO.sub.2, +101 -75 surfaces;
unidentified severe sticking phase(s) C6 Deformed, ZrO.sub.2, -75
tapered bar; unidentified sticking phase(s) C7 Chalky surface
ZrW.sub.2O.sub.8, ZrO.sub.2 -75 C8 Chalky surface ZrW.sub.2O.sub.8,
ZrO.sub.2 -68 -75 C9 Sintered bar, ZrSiO.sub.4, WO.sub.3, -76
slightly chalky cristobalite, surface ZrO.sub.2 C10 Sintered bar,
ZrSiO.sub.4, WO.sub.3, +64 -76 slightly chalky cristobalite surface
C11 Sintered bar, ZrW.sub.2O.sub.8, -76 slightly chalky
ZrSiO.sub.4, WO.sub.3, surface cristobalite, ZrO.sub.2 C12 Sintered
bar, ZrW.sub.2O.sub.8, -61 -76 hard surface ZrSiO.sub.4, WO.sub.3
C13 Sintered bar, ZrW.sub.2O.sub.8, -51 -69 hard surface
ZrSiO.sub.4, trace WO.sub.3 ** Too deformed for CTE measurement
[0096] Samples were prepared by vibratory milling the powders in
water for 16 hours, adding a polyethylene glycol binder, drying the
slurry, granulating the dried cake, and uniaxially pressing bars of
11.4 cm length, except for Example C13, which was pressed into a
bar of 7.6 cm length. Bars were set on zirconium oxide "sand" in an
alumina box and fired at 1150.degree. C. or 1175.degree. C., or
were set on zircon "sand" on an aluminosilicate pallet and fired at
1200.degree. C. (Table 4).
[0097] Examples C1 and C2 were prepared to yield a ceramic with a
nominal composition of 88 volume % ZrW.sub.2O.sub.8+12 volume %
(9.6 weight %) Al.sub.2O.sub.3. However, the results in Table 5
demonstrate that ZrW.sub.2O.sub.8 is unstable when fired with
alumina, and reacts to form Al.sub.2(WO.sub.4).sub.3+ZrO.sub.2.
This results in a positive CTE for this material of
+100.times.10.sup.-7.degree. C..sup.-1. Thus, the concept of
forming a ZrW.sub.2O.sub.8+Al.sub.2O.sub.3 ceramic is untenable,
because no such ceramic can be produced due to reaction of alumina
with zirconium tungstate. However, it is contemplated that a
ceramic article consisting of ZrW.sub.2O.sub.8 with small amounts
of Al.sub.2(WO.sub.4).sub.3 could be fabricated by addition of
aluminum and tungsten oxides to the ZrW.sub.2O.sub.8 precursor
powders, and the amount of Al.sub.2(WO.sub.4).sub.3 in such a
ceramic could be selected so as to yield a body with a CTE that is
less negative than -90.times.10.sup.-7.degree. C..sup.-1. Also, it
is contemplated that a body comprised of
ZrW.sub.2O.sub.8+Al.sub.2(WO.sub.4).sub.3+ZrO.sub.2 having a CTE
less negative than -90.times.10.sup.-7.degree. C..sup.-1 could be
prepared by adding only alumina, provided that the alumina addition
is much less than 9.6 weight %.
[0098] Example C3 was prepared to yield a ceramic with a nominal
composition of 91 volume % ZrW.sub.2O.sub.8+9 volume % (6.5 weight
%) MgO. However, as seen in Table 5, this composition, even when
fired at only 1150.degree. C. (just above the lower thermal
stability limit of ZrW.sub.2O.sub.8) , undergoes reaction between
ZrW.sub.2O.sub.8 and MgO to form MgWO.sub.4+ZrO.sub.2+unidentified
phases. No ZrW.sub.2O.sub.8 remains in the body, and the remaining
phases all have large positive CTEs, making this ceramic
inappropriate for use as a substrate for a fiber Bragg grating.
Furthermore, the ceramic body underwent slumping and sticking to
the zirconia sand due to extensive formation of liquid during
firing. Thus, the formation of a ZrW.sub.2O.sub.8+MgO ceramic is
not feasible, because no such ceramic can be produced. Instead, MgO
reacts with ZrW.sub.2O.sub.8 to form phases other than MgO.
Addition of MgO to ZrW.sub.2O.sub.8 in amounts of more than a few
weight percent is undesirable.
[0099] Example C4 was prepared to yield a ceramic with a nominal
composition of 91 volume % ZrW.sub.2O.sub.8+9 volume % Cao, in
which the CaO was provided as 10.3 weight % CaCO.sub.3. However, as
seen in Table 5, this composition, even when fired at only
1150.degree. C., undergoes reaction between ZrW.sub.2O.sub.8 and
CaO to form CaWO.sub.4+ZrO.sub.2+un- identified phases. No
ZrW.sub.2O.sub.8 remains in the body, and the remaining phases all
have large positive CTEs (the CTE of CaWO.sub.4 is about
100.times.10.sup.-7.degree. C..sup.-1), making this ceramic
inappropriate for use as a substrate for a fiber Bragg grating.
Furthermore, the ceramic body underwent severe slumping and
sticking to the zirconia sand due to extensive formation of liquid
during firing. Thus, the formation of a ZrW.sub.2O.sub.8+CaO
ceramic is not feasible, because no such ceramic can be produced.
Instead, CaO reacts with ZrW.sub.2O.sub.8 to form phases other than
CaO. Addition of CaO, or a CaO source such as CaCO.sub.3, to
ZrW.sub.2O.sub.8 in amounts of more than a few weight percent is
undesirable.
[0100] Examples C5 and C6 were formulated to yield a ceramic with a
nominal composition of 88 volume % ZrW.sub.2O.sub.8+12 volume %
(11.9 weight %) Y.sub.2O.sub.3. However, Table 5 shows that, after
firing, the ceramic contained no ZrW.sub.2O.sub.8. Instead,
reaction of the starting materials yielded ZrO.sub.2, and several
unidentified phases. The high CTE resulting from this reaction,
+101.times.10.sup.-7.degree. C..sup.-1, demonstrates that the
amount of yttria in a zirconium tungstate ceramic should be kept to
less than a few percent.
[0101] Examples C7 and C8 were prepared to yield a ceramic with a
nominal composition of 88 volume % ZrW.sub.2O.sub.8+12 volume %
(13.7 weight %) ZrO.sub.2. X-ray diffractometry of the fired
samples (Table 5) shows that these two phases do, in fact, coexist.
The fired bar possessed a very powdery surface after firing, which
was found to consist almost entirely of zirconium oxide. Such a
surface would be unsuitable for direct attachment of a fiber
grating, and would require machining to expose the low-porosity
interior of the sample. The CTE of the sample, after removal of the
surface layer, was measured to be -68.times.10.sup.-7.degree.
C..sup.-1, only slightly less negative than the predicted value of
-75.times.10.sup.-7.degree. C..sup.-1. The dimensional stability of
a similar ceramic of ZrW.sub.2O.sub.8 with 15 weight % ZrO.sub.2
was determined by monitoring the lengths of two sintered bars as a
function of time in an 85 .degree. C., 85% relative humidity
environment. The dimensional changes of these bars is provided in
Table 6 (examples D4 and D5) and depicted in FIG. 2.
6TABLE 6 Change in length expressed as .DELTA.L/L, in parts per
million, of ceramic bars exposed to 85.degree. C., 85% relative
humidity, for indicated durations. Example D1 Example D2 Example D3
68% ZrW.sub.2O.sub.8 68% ZrW.sub.2O.sub.8 68% ZrW.sub.2O.sub.8
Example D4 Example D5 Time 32% Zr.sub.2P.sub.2WO.sub.12 32%
Zr.sub.2P.sub.2WO.sub.12 Time 32% Zr.sub.2P.sub.2WO.sub.12 Time 85%
ZrW.sub.2O.sub.8 85% ZrW.sub.2O.sub.8 (hours) 0.25% Y.sub.2O.sub.3
0.25% Y.sub.2O.sub.3 (hours) 0.10% BaCO.sub.3 (hours) 15% ZrO.sub.2
15% ZrO.sub.2 0 0 0 0 0 0 0 0 160 -15 -5 178 -13 162 -222 -191 322
0 10 338 -15 301 -476 -458 482 -13 -2 498 -10 462 -1044 -1123 642
-10 7 658 -13 623 -2279 -2800 802 -10 7 800 -13 783 -4376 -5441
[0102] The severe shrinkage of these ceramics over a relatively
short span of time renders them inappropriate for use as a fiber
grating substrate. For comparison, bars of the inventive
compositions 68%ZrW.sub.2O.sub.8+32% Zr.sub.2P.sub.2WO.sub.12+0.25%
Y.sub.2O.sub.3 (Examples D1 and D2) and the inventive composition
68%ZrW.sub.2O.sub.8+32% Zr.sub.2P.sub.2WO.sub.12+0.10% BaCO.sub.3
(Example D3) undergo negligible dimensional change with time under
the same conditions of temperature and humidity.
[0103] Examples C9 and C10 were prepared to yield a ceramic with a
nominal composition of 81 volume % ZrW.sub.2O.sub.8+19 volume %
(9.2 weight %) SiO.sub.2 in which the SiO.sub.2 is added as fused
silica. Table 5 shows that this combination results in reaction of
the ZrW.sub.2O.sub.8 with SiO.sub.2 to form zircon
(ZrSiO.sub.4)+tungsten oxide, which have CTEs at 25.degree. C. of
about 40.times.10.sup.-7.degree. C..sup.-1 and
160.times.10.sup.-7.degree. C..sup.-1, respectively. Consequently,
the CTE of this ceramic body is highly positive,
64.times.10.sup.-7.degree. C..sup.-1. The presence of a small
amount of cristobalite, a crystalline form of silica, indicates
that the reaction did not proceed to completion during the time
allowed. Thus, the formation of a ZrW.sub.2O.sub.8+fused SiO.sub.2
ceramic is not possible, because no such ceramic can be produced
due to reaction of silica with the ZrW.sub.2O.sub.8.
[0104] Examples C11 and C12 were formulated to yield a ceramic
having a nominal composition of 91 volume % ZrW.sub.2O.sub.8+9
volume % (4.9 weight %) SiO.sub.2 in which the SiO.sub.2 is added
as quartz, a crystalline form of silica. Less volume percent of
this form of silica was added than for the case where the SiO.sub.2
was added as fused silica because quartz has a much higher thermal
expansion than fused silica, so less is required to compensate the
CTE of the mixture by Equation 2. As seen in Table 5, the quartz
also reacts with ZrW.sub.2O.sub.8 to form zircon and tungsten
oxide, although in these two examples the lower amount of silica
allows for some ZrW.sub.2O.sub.8 to remain in the body after
firing. However, the CTE of this mixed phase ceramic is only
-61.times.10.sup.-7.degree. C..sup.-1, less negative than desired,
and less negative than predicted from Equation 2 because of the
reaction of silica with ZrW.sub.2O.sub.8.
[0105] Example C13 was formulated to yield a ceramic consisting of
85 weight % (83.6 volume %) ZrW.sub.2O.sub.8+15 weight % (16.5
volume %) ZrSiO.sub.4, in which the silicate was added as a very
fine zircon powder. After firing, the ceramic consisted of
ZrW.sub.2O.sub.8+ZrSiO.sub- .4, with only trace amounts of residual
ZrO.sub.2 and WO.sub.3. The nominal CTE predicted for this
composition is -69.times.10.sub.-7.degree. C..sup.-1; however, the
measured expansion from -40.degree. to +40.degree. C. was
-51.times.10.sup.-7C..sup.-1. Furthermore, the dilatometric CTE
curve exhibited a hysteresis upon heating to +90.degree. C. and
cooling back to 20.degree. C., with an increase in sample length
corresponding to a .DELTA.L/L of over 400 parts per million.
Examination of the sample by scanning electron microscopy showed
that the sample had pervasive microcracking throughout the
ZrW.sub.2O.sub.8 matrix. The dimensional instability of the sample
with thermal cycling above room temperature renders this
composition inappropriate for application as a fiber grating
substrate.
[0106] The materials and methods of the present invention can be
utilized for a variety of applications requiring negative thermal
expansion materials, such as providing temperature compensation for
optical devices such as gratings. The materials of the present
invention could be used to make support members for such
devices.
[0107] For example, referring to FIG. 3, there is illustrated a
first exemplary embodiment of the invention. The optical fiber
reflective grating device 20 has a substrate 22 formed from a flat
block of a negative expansion material, such as the materials of
the present invention. An optical fiber 24 having at least one
UV-induced reflective grating 26 written therein is mounted on the
surface 28 and attached at either end of the surface at points 30
and 32. It is important that the fiber is straight and not subject
to compression as a result of the negative expansion and thus the
fiber is usually mounted under tension. Before attachment the fiber
is placed under a controlled tension, as shown schematically by the
use of a weight 34. The proper choice of tension assures that the
fiber is not under compression at all anticipated use temperatures.
However, the fiber can be under tension at all anticipated use
temperatures. The required degree of tension to compensate for the
negative expansion in a particular application can readily be
calculated by those with skill in this art.
[0108] The attachment material could be an organic polymer, for
example an epoxy cement, an inorganic frit, for example ground
glass, ceramic or glass-ceramic material, or a metal. In one
embodiment the fiber is tacked to the substrate with a UV-cured
epoxy adhesive. Mechanical means for attaching the fiber can also
be used.
[0109] Generally the optical fiber reflective grating is supplied
with a coating material surrounding the fiber. In the preferred
packaging approach the coating in the grating region of the fiber
is left intact while it is removed in the substrate attachment
region at each end of the grating. However, the device can have the
coating completely removed between the attachment locations.
Removal of the coating can be accomplished by one of two methods :
a non-contact, non-chemical stripping mechanism or by conventional
chemical stripping.
[0110] In another embodiment shown in FIG. 4, the fiber is not
attached directly to the substrate. Bonding pads 40, 42 made from a
material differing from the substrate, for example a glass or a
ceramic, are attached to the substrate at either end. The fiber 26
is mounted to the pads at points 44, 46. These pads afford better
attachment properties of the pad to the fiber than could be
achieved from the substrate directly to the fiber because of the
large thermal expansion mismatch. Suitable pad materials have a
coefficient of thermal expansion intermediate between that of the
fiber and the substrate, for example, between -50 and
+5.times.10.sup.-7, preferably about -20.times.10.sup.-7.
Alternatively the pad could be a fused silica with a coefficient of
expansion closely matching that of the fiber. The pad allows the
stress of this joint induced by both the thermal mismatch and the
tension of the fiber, to be spread out over a wider area, lessening
the chances of cracking and detachment. The attachment materials
for the fiber and pad connections are similar to those used for
mounting the fiber directly to the substrate, for example, an epoxy
cement, an inorganic frit, for example ground glass, ceramic or
glass-ceramic material, or a metal.
[0111] In another embodiment shown in FIG. 5, the negative
expansion of the substrate material 22 is used to create a clamping
force on the fiber. The attachment feature, which might be a hole
or channel 50, 52 in a raised portion 54, 56 of the substrate, is
formed in the substrate at room temperature with a gap that is very
slightly smaller than the fiber. Referring to FIG. 6, by lowering
the temperature to a point lower than any anticipated use
temperature, the substrate expands and allows the insertion of the
fiber 24 into the channel 50. Warming of the substrate then causes
substrate contraction and creates a clamping force for holding the
fiber in the channel.
[0112] In another embodiment, FIG. 7, the fiber 24 is attached to
the substrate at points 30, 32 and the intermediate fiber length 60
is cushioned by a low modulus damping material 62. This low-modulus
material, for example a silicone rubber coating surrounding the
fiber or a pad of a silicone rubber, a natural or synthetic rubber
or mixtures thereof, between the fiber and the substrate protects
the fiber reflective grating against external perturbations such as
mechanical shock or vibration. Bowing of the fiber is also
minimized. In one embodiment the low modulus material is adhesively
attached to the fiber and the substrate.
[0113] Mounting the fiber under tension will alter the optical
properties of the device (for example, the center wavelength of a
grating). This can be addressed by biasing the device with a
reflective grating written therein to account for the tension, or
it can be done by mounting a fiber, for example a germania doped
silica fiber, without a reflective grating written therein under
tension and then exposing the fiber to UV light in order to
fabricate the grating in the device in situ.
[0114] In a typical embodiment of the invention, the temperature
sensitivity of the center wavelength is about 0.0125 nm/.degree. C.
for the uncompensated grating, the stress sensitivity of the center
wavelength is 0.125 nm shift for 9 g of tension, the bare fiber has
a diameter of 125 microns, a coated fiber has a diameter of 250
microns. The strength of the fiber is greater than 200 kpsi, and
therefore has a very high reliability.
[0115] Although this invention has been described for UV photo
induced gratings it can also be applied to the packaging of other
thermally sensitive devices. For instance, optical fiber couplers
and optical waveguides could be athermalized by attachment to a
negative expansion substrate.
[0116] An optical fiber fused coupler has two or more fibers fused
together at one or more points along their length and is mounted on
a substrate. Such couplers are thermally sensitive which results in
a certain amount of thermal instability. Especially sensitive are
biconically tapered couplers in which interferometric effects are
used, for example a Mach-Zehnder interferometer. Such couplers can
be athermalized by mounting the coupler to a negative expansion
substrate. FIG. 8 illustrates a fused biconical coupler device 70
which includes a negative expansion substrate 72 to which are
mounted two fibers 74, 76. The fibers are fused together at regions
78, 80. The fibers are attached to the substrate near the ends at
locations 82, 84 in the same manner as described above for the
optical fiber reflective grating.
[0117] Waveguides can be defined, for example, in optical fibers or
planar substrates. Such waveguides are thermally sensitive which
results in a certain amount of thermal instability. Such waveguides
can be athermalized by mounting the waveguide to a negative
expansion substrate. FIG. 9 illustrates a planar waveguide device
90 which includes a negative expansion substrate 92 on which is
adhesively mounted a layer of material 94 in which a planar
waveguide 96 is fabricated by methods well known to those skilled
in this art. The waveguide material can be, for example, a doped
silica such as a germania silicate, other suitable glass
compositions, polymers and semiconductors, including semiconductors
with gain, such as laser diodes.
[0118] A fiber Bragg grating device with greatly reduced
temperature dependence was fabricated by attaching a fiber Bragg
grating under tension to a substrate of Example 78 of Tables 2 and
3 using a tin zinc phosphate glass frit containing 45 weight % of a
magnesium cobalt pyrophosphate filler. FIG. 10 shows the beneficial
athermalization properties of the invention with the center
wavelength of this fiber Bragg grating device plotted against
temperature of the device. At -5.degree. C. the center wavelength
of the grating was 1531.353 nm, and at 75.degree. C. the center
wavelength was 1531.277 nm. Thus, over this range in temperature,
the device exhibited a variation in wavelength versus temperature
of -0.00095 nm/.degree. C., compared with +0.012 nm/.degree. C. for
an unattached grating. In a preferred embodiment, the device
includes a negative expansion substrate having a composition
comprising two phases having negative coefficients of thermal
expansion, one of the phases having a room temperature coefficient
of thermal expansion more negative than -50.times.10.sup.-7.degree.
C..sup.-1 and a fiber Bragg grating affixed to the substrate,
wherein the absolute value of the average temperature dependence of
the Bragg wavelength between 0.degree. C. and 70.degree. C. is not
more than about 0.0025 nm/.degree. C. In a further embodiment, the
device includes a negative expansion substrate having a composition
comprising two phases having negative coefficients of thermal
expansion, one of the phases having a room temperature coefficient
of thermal expansion more negative than -50.times.10.sup.-7.degree.
C..sup.-1 and a fiber Bragg grating affixed to the substrate,
wherein the absolute value of the average temperature dependence of
the Bragg wavelength between 0.degree. C. and 70.degree. C. is not
more than about 0.001 nm/.degree. C.
[0119] It will be understood that while the exemplary device
embodiments included flat substrates, this invention is not limited
to a particularly shaped substrate. For example, it is contemplated
that the ceramic bodies of the present invention could be utilized
to make tubular and cylindrical substrates, or substrates that
contain a V-shaped, U-shaped, or rectangular trough or groove.
[0120] The device of this invention is a completely passive system
and mechanically simple, and demonstrates athermalization. The
method of producing the device is advantageous because it provides
temperature compensated optical devices which tolerate shock and
vibration and are thermally stable.
[0121] It will be apparent to those skilled in the art that various
modifications and variations can be made in the of the present
invention without departing from the spirit or scope of the
invention. Thus, it is intended that the present invention cover
the modifications and variations of this invention provided they
come within the scope of the appended claims and their
equivalents.
* * * * *