U.S. patent application number 09/835570 was filed with the patent office on 2001-10-18 for isotropic negative thermal expansion ceramics and process for making.
This patent application is currently assigned to Agere Systems Guardian Corp.. Invention is credited to Fleming, Debra Anne, Johnson, David Wilfred, Kowach, Glen Robert, Lemaire, Paul Joseph.
Application Number | 20010031692 09/835570 |
Document ID | / |
Family ID | 22517753 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031692 |
Kind Code |
A1 |
Fleming, Debra Anne ; et
al. |
October 18, 2001 |
Isotropic negative thermal expansion ceramics and process for
making
Abstract
Ceramic monoliths are described which exhibit tunable
coefficients of thermal expansion from about -5 to
-11.times.10.sup.-6.degree. C..sup.-1 near ambient temperatures.
These two-phase ceramics, which are fabricated, for example, by
reactive sintering of WO.sub.3 and ZrO.sub.2, consists of a matrix
of ZrW.sub.2O.sub.8 with inclusions of ZrO.sub.2 having diameters
less than 10 .mu.m. Additives may increase the density of the
monoliths to greater than 98% of the calculated density. Green body
densities, pre-sintered particle size distribution, sintering
atmosphere, microstructure, and mechanical properties are
discussed. These ceramics may be used as substrates for thermally
compensating fiber Bragg gratings.
Inventors: |
Fleming, Debra Anne;
(Berkeley Heights, NJ) ; Johnson, David Wilfred;
(Bedminster, NJ) ; Kowach, Glen Robert; (Edison,
NJ) ; Lemaire, Paul Joseph; (Madison, NJ) |
Correspondence
Address: |
Keith G. Haddaway
VENABLE
P.O. Box 34385
Washington
DC
20043-9998
US
|
Assignee: |
Agere Systems Guardian
Corp.
|
Family ID: |
22517753 |
Appl. No.: |
09/835570 |
Filed: |
April 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09835570 |
Apr 17, 2001 |
|
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09146518 |
Sep 3, 1998 |
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6258743 |
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Current U.S.
Class: |
501/103 ;
501/1 |
Current CPC
Class: |
C04B 35/486 20130101;
C04B 35/495 20130101; G02B 6/0218 20130101 |
Class at
Publication: |
501/103 ;
501/1 |
International
Class: |
C04B 035/48; C04B
035/00 |
Claims
What is claimed is:
1. A method of making a ceramic body having an isotropic negative
coefficient of thermal expansion, the body having a major
constituent selected from the group consisting of ZrW.sub.2O.sub.8,
HfW.sub.2O.sub.8, ZrV.sub.2O.sub.7 and HfV.sub.2O.sub.7, comprising
the steps of, a) preparing a powder mixture comprising a first and
a second oxide precursor powder, the first oxide precursor powder
selected from the group consisting of ZrO.sub.2 powder and
HfO.sub.2 powder, and the second oxide precursor powder selected
from the group consisting of WO.sub.3 powder and V.sub.2O.sub.5
powder; b) forming a green body comprising said powder mixture; and
c) heat treating the green body at a temperature below the melting
temperature of the selected major constituent, and forming the
selected major constituent from the heat treated green body by
reactive sintering.
2. The method according to claim 1, wherein the powder mixture
contains a non-stoichiometric amount of one of the first and second
precursor powders, such that said ceramic body further comprises a
second constituent, the non-stoichiometric amount selected to
provide the ceramic body with a predetermined value of the negative
coefficient of thermal expansion.
3. The method according to claim 1, wherein the powder mixture
further comprises a sintering aid selected to yield an increased
density of the ceramic body.
4. The method according to claim 3, wherein the sintering aid is
selected from the group consisting of Y.sub.2O.sub.3,
Bi.sub.2O.sub.3, Al.sub.2O.sub.3, ZnO, TiO.sub.2, SnO.sub.2 and
combinations thereof.
5. The method according to claim 1, wherein said powder mixture is
prepared by vibrational milling of said first and second oxide
precursor powders in a milling solvent to form a slurry, adding a
binder to said slurry, and evaporating said milling solvent to form
a powder mixture.
6. The method according to claim 1, wherein said green body is
formed by sieving said powder mixture to form a sieved powder, and
pressing said sieved powder to form a green powder.
7. The method according to claim 1, wherein said heat treatment
comprises firing said green body in an oxygen atmosphere, slow
heating said fired body to about 250.degree. C., subsequently
heating to about 1150.degree. to 1200.degree. and soaking said body
for about 5 hours, and quenching said heated body to room
temperature to form said ceramic body having an isotropic negative
coefficient of thermal expansion.
8. A composition consisting essentially of the ceramic body
produced by the process of claim 1.
9. A two-phase composition comprising a first phase consisting
essentially of the ceramic body produced by the process of claim 2,
and a second phase comprising at least one oxide selected from the
group consisting of WO.sub.3, V.sub.2O.sub.5, ZrO.sub.2 and
HfO.sub.2.
10. The two-phase composition according to claim 9, wherein said
second phase consists essentially of WO.sub.3.
11. The two-phase composition according to claim 9, wherein said
second phase consists essentially of ZrO.sub.2.
12. The two-phase composition according to claim 9, wherein said
second phase consists essentially of HfO.sub.2.
13. The two-phase composition according to claim 9, wherein said
second phase consists essentially of V.sub.2O.sub.5.
14. The two-phase composition according to claim 10, containing up
to about 35 weight % WO.sub.3, compared to the weight of the first
phase.
15. The two-phase composition according to claim 11, containing up
to about 35 weight % ZrO.sub.2, compared to the weight of the first
phase.
16. The two-phase composition according to claim 12, containing up
to about 35 weight % HfO.sub.2, compared to the weight of the first
phase.
17. The two-phase composition according to claim 13, containing up
to about 35 weight % V.sub.2O.sub.5, compared to the weight of the
first phase.
18. The two-phase composition according to claim 9 having a thermal
expansion coefficient of about -9.+-.2.5.times.10.sup.-6.degree.
C..sup.-1.
19. The two-phase composition according to claim 9 having a thermal
expansion coefficient of about -9.+-.1.times.10.sup.-6.degree.
C..sup.-1.
20. The two-phase composition according to claim 9 having a thermal
expansion coefficient of about -9.+-.0.25.times.10.sup.-6.degree.
C..sup.-1.
21. An article comprising an optical fiber attached to a support,
said support comprising a two-phase composition according to claim
9.
22. An article comprising an optical fiber attached to a support,
said support comprising a two-phase composition according to claim
11.
23. An article comprising an optical fiber attached to a support,
said support comprising a two-phase composition according to claim
15.
24. An article comprising an optical fiber attached to a support,
said support comprising a two-phase composition according to claim
18.
Description
FIELD OF THE INVENTION
[0001] This invention pertains to isotropic, negative thermal
expansion ceramics, and to a process for preparing isotropic,
negative thermal expansion ceramics which may, for instance, be
used in temperature compensating grating packages.
TECHNOLOGY REVIEW
[0002] As described in U.S. Pat. No. 5,694,503, incorporated herein
by reference, it is possible to package a fiber grating on or in a
negative expansion material so that the package and grating
dimensions decrease with an increase in temperature, resulting in
minimal variation of the reflection wavelength with
temperature.
[0003] In order for this approach to be practical the value for the
thermal expansion of the package/substrate material must lie within
an acceptable range. The ideal expansion coefficient is given by
the expression 1 ideal = fiber + - A ( 1 - P e ) nom
[0004] where .alpha..sub.fiber is the thermal expansion of the
fiber grating (for instance, 0.55.times.10.sup.-6.degree.
C..sup.-1), A is the temperature sensitivity of the unpackaged
grating (e.g., 0.0115 nm .degree.C..sup.-1 for a particular 1550 nm
grating), P.sub.e is the photoelastic constant (typically 0.22) and
.lambda..sub.nom is the nominal grating wavelength (.about.1550 nm
in many cases). For a particular 1550 nm grating an "ideal" package
material would have a thermal expansion coefficient (CTE) of
-8.96.times.10.sup.-6.degree. C..sup.-1. A package material will
still be beneficial even if its thermal expansion is not exactly
equal to the ideal value. For the above assumptions a factor of
about 20 improvement in temperature sensitivity would be achieved
if the package material's thermal expansion coefficient were within
0.47.times.10.sup.-6.degree. C..sup.-1 of the ideal value. Such
improvement in thermal stability of a fiber grating would be of
commercial importance.
[0005] It is known that ZrW.sub.2O.sub.8 is metastable at room
temperature, with the lower limit of stability being at
1105.+-.3.degree. C., below which ZrW.sub.2O.sub.8 decomposes into
ZrO.sub.2 and WO.sub.3. See, for instance, J. Graham et al., J.
American Ceramics Society, Volume 42, page 570 (1959), and L. L. Y.
Chang et al., J. American Ceramics Society, Volume 50, page 211
(1967). It is also known that ZrW.sub.2O.sub.8 has a relatively
high and isotropic negative coefficient of thermal expansion (CTE)
over an extensive range of temperatures that includes room
temperature. See, for instance, C. Martinek et al., J. American
Ceramics Society, Volume 51, page 227 (1968) and T. A. Mary et al.,
Science, Volume 272, page 90 (1996). Specifically, the CTE is
substantially constant from near absolute zero temperature to
150.degree. C., with a value near -10.times.10.sup.-6.degree.
C..sup.-1. The material exhibits an order-disorder transition at
150.degree. C., after which the CTE drops to
-5.times.10.sup.-6.degree. C..sup.-1. This value of the CTE is
maintained until the decomposition of ZrW.sub.2O.sub.8 which occurs
at a relatively high rate near 800.degree. C.
[0006] In view of this complex behavior of ZrW.sub.2O.sub.8, it is
not surprising that earlier attempts to produce mechanically strong
monolithic bodies of ZrW.sub.2O.sub.8, that have a predetermined
negative CTE, did not yield fully satisfactory results. For
instance, C. Verdon et al., Scripta Materialia, Volume 36, page
1075 (1997) report that their attempts to form electrically
conducting bodies from ZrW.sub.2O.sub.8 and Cu with a low CTE
resulted in decomposition of the ZrW.sub.2O.sub.8 and formation of
Cu.sub.2O along with other compounds. Such decomposition is
generally undesirable and hinders the production of suitable
bodies.
[0007] To the best of our knowledge, prior art efforts to make
ZrW.sub.2O.sub.8 ceramic bodies used the conventional technique of
sintering ZrW.sub.2O.sub.8 powder. Thus produced bodies have
densities less than 90% of the theoretical density and exhibit
relatively poor mechanical properties, specifically, a low modulus
of rupture. Additional prior art describes the preparation of
ZrW.sub.2O.sub.8 powder from oxide precursors to be an incomplete
reaction.
[0008] In view of the importance, for instance, a reduction of the
temperature dependence of the reflection wavelength of optical
fiber gratings, it would be highly desirable to be able to
reproducibly make mechanically strong ceramic bodies having tunable
negative CTE values, the ceramic bodies being useful, for example,
for packaging of fiber gratings. This application discloses a
method for making such bodies.
SUMMARY OF THE INVENTION
[0009] We have made the surprising discovery that reactive
sintering of appropriate percursor powders (e.g., ZrO.sub.2 and
WO.sub.3) can result in (negative CTE) bodies (e.g.,
ZrW.sub.2O.sub.8) with substantially improved properties, as
compared to analogous bodies produced by the prior art sintering
techniques.
[0010] To the best of our knowledge, the prior art does not provide
any suggestion that the use of reactive sintering could provide the
observed improved results. By "reactive sintering" we mean herein
compacting the unsintered body of the precursor oxides, rather than
the powder of the desired final phase, and then forming the desired
phase and densifying the body in a single heat treatment step.
[0011] In a broad aspect of the invention is embodied in a method
of making negative CTE ceramic bodies, and in bodies produced by
the method.
[0012] More specifically, the invention is embodied in a method of
making a ceramic body having isotropic negative thermal expansion,
the body having a major constituent selected from the group
consisting of ZrW.sub.2O.sub.8, HfW.sub.2O.sub.8, ZrV.sub.2O.sub.7
and HfV.sub.2O.sub.7. The method comprises the steps of providing a
powder mixture, forming a "green" body that comprises the powder
mixture, and heat treating the green body. The powder mixture
comprises a first and a second oxide precursor powder, selected
respectively from the group consisting of ZrO.sub.2 powder and
HfO.sub.2 powder, and the group consisting of WO.sub.3 powder and
V.sub.2O.sub.5 powder. The heat treatment of the green body
includes heating the body to a temperature below the melting
temperature of the selected constituent, such that the selected
major constituent is formed from the green body by reactive
sintering.
[0013] By a "green" body we mean herein the compacted precursor
powders as a monolithic body prior to sintering.
[0014] In a preferred embodiment the powder mixture has a
non-stoichiometric composition (e.g., excess ZrO.sub.2), and the
heat treatment results in formation of a 2-phase material, e.g.,
ZrW.sub.2O.sub.8 majority phase, with ZrO.sub.2 inclusions
dispersed in the majority phase. This embodiment allows tailoring
of the CTE of the body.
[0015] In a further preferred embodiment the powder mixture
comprises a minor amount of a sintering aid (e.g., Y.sub.2O.sub.3,
Bi.sub.2O.sub.3, Al.sub.2O.sub.3, ZnO, TiO.sub.2, SnO.sub.2),
whereby the density of the sintered body is substantially
increased.
[0016] Important note for processing
[0017] Due to the unusually narrow stability region of
ZrW.sub.2O.sub.8, standard techniques for synthesizing and
densifying composite ceramics containing ZrW.sub.2O.sub.8 produce
monoliths with inadequate physical properties. High purity
ZrW.sub.2O.sub.8 is thermally stable between 1105 and 1260.degree.
C. Above 1260.degree. C. ZrW.sub.2O.sub.8 peritectically decomposes
into a Liquid phase, and below 1105.degree. C. it decomposes into
ZrO.sub.2 and WO.sub.3. Additives significantly alter the stability
region of ZrW.sub.2O.sub.8; this study demonstrates a working
temperature range of 1140 to 1180.degree. C. for Y.sub.2O.sub.3
doped ZrW.sub.2O.sub.8.
BRIEF DESCRIPTION OF THE DRAWING
[0018] FIG. 1. Percent densities of ZrW.sub.2O.sub.8.xZrO.sub.2
monoliths with Y.sub.2O.sub.3 additive plotted with respect to
weight percent of additional ZrO.sub.2.
[0019] FIG. 2. SEM micrograph of a ZrW.sub.2O.sub.8.18 wt. %
ZrO.sub.2 monolith viewed with secondary electron imaging.
ZrO.sub.2 inclusions with various particle sizes having a maximum
diameter of approximately 10 .mu.m appear with darker contrast.
[0020] FIG. 3. Relative thermal expansion of a ZrW.sub.2O.sub.8.9.5
wt. % ZrO.sub.2 monolith taken over an extensive temperature range.
The coefficient of thermal expansion (CTE) for the range -100 to
100.degree. C. and 200 to 300.degree. C. is -10.times.10.sup.-6 and
3.times.10.sup.-6.degree. C..sup.-1, respectively. A reversible
order-disorder transition takes place near 140.degree. C. which
does not compromise the mechanical strength of the monolith.
[0021] FIG. 4. Relative thermal expansion over ambient working
temperatures for several ZrW.sub.2O.sub.8.xZrO.sub.2 monoliths. The
following compositions are presented (label, wt. % excess
ZrO.sub.2, volume fraction ZrO.sub.2): a, 37.0%, 0.337; b, 19.5%,
0.174; c, 9.5%, 0.084, d, 0%, 0. Nonlinearity of the thermal
expansion is enhanced as the zirconia content increases.
[0022] FIG. 5 illustrates the dependence of the Coefficient of
Thermal Expansion (CTE) between 0 and 100.degree. C. of two-phase
ZrW.sub.2O.sub.8.xZrO.sub.2 ceramics as weight percent of
additional ZrO.sub.2. The change in CTE demonstrates a linear
relationship to the relative amount of ZrO.sub.2 inclusions.
[0023] FIG. 6. Reflection wavelength of a fiber Bragg grating as a
function of temperature is compensated by a ZrW.sub.2O.sub.8 9.5
wt. % ZrO.sub.2 substrate throughout ambient temperature range.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The synthesis of ZrW.sub.2O.sub.8 has been described as
challenging due its metastability. However, the fabrication of
monoliths via reactive sintering is straightforward and
reproducible upon consideration of the following points: additives
(including impurities), particle size, and sintering atmosphere.
Similarly, reactive sintering may be used to prepare
HfW.sub.2O.sub.8, ZrV.sub.2O.sub.7, and HfV.sub.2O.sub.7.
[0025] Additives
[0026] In phase equilibria, additives which form a eutectic may be
utilized as a liquid phase sintering aid, thereby significantly
increasing the density of a ceramic. Liquid phases accelerate
material transport during sintering by serving as a solvent for the
solid phase with subsequent precipitation to yield dense ceramics.
Monoliths of ZrW.sub.2O.sub.8.xZrO.sub.2 prepared with a
Y.sub.2O.sub.3 additive have achieved, densities greater than 99%
the calculated theoretical density (FIG. 1). The density of a high
purity ZrW.sub.2O.sub.8 monolith is only 92%. Several oxides
typically used as sintering aids including refractory oxides were
investigated (Table 1). Increased densities were observed without
decomposition of ZrW.sub.2O.sub.8 for small amounts (0.1 wt. %) of
additives. In order to easily observe enhanced differences in
density, the samples were milled for only two hours (vide infra).
Of the additives which were surveyed, all increased the density of
the monoliths with exception of SiO.sub.2. The following series
illustrates the relative effectiveness of the additives:
Y.sub.2O.sub.3>Bi.sub.2O.sub.3>Al.s-
ub.2O.sub.3.about.ZnO>TiO.sub.2.
[0027] Densification via liquid phase sintering has been
demonstrated in several ceramic systems and is extensively employed
in the ceramic industry in order to decrease sintering times and
temperatures. For effective liquid phase sintering, only 1 vol. %
of liquid within grain boundaries is necessary to aid
densification. Increasing the concentration of Al.sub.2O.sub.3,
TiO.sub.2, or Y.sub.2O.sub.3, to a few weight percent leads to
complete decomposition of ZrW.sub.2O.sub.8 and melting of the
monolith at the sintering temperature. This suggests that a liquid
phase is present at 1180.degree. C. even at low concentrations of
additives. Similar equilibria have been observed in the alkali
metal-WO.sub.3--ZrO.sub.2 systems with eutectic temperatures below
600.degree. C. Consequently, liquid phase sintering is a preferred
mechanism for enhanced densification.
[0028] Due to the impact of additives on the sintered density, it
is important to characterize the impurities in the starting
materials and grinding media. An important consideration is the
selection of appropriate grinding media based on composition in
order to prevent undesired impurities, such as Al.sub.2O.sub.3 and
calcium stabilized ZrO.sub.2, from incorporation into the
monoliths.
[0029] Pre-sintering Particle Size
[0030] The particle size, particle size distribution, and
uniformity of particle packing are important factors in the
densification process. In this study of the preparation of
ZrW.sub.2O.sub.8.xZrO.sub.2 monoliths, these factors have the
greatest impact on sintered density. In general, smaller particles
are more reactive and tend to densify at lower temperatures. Coarse
particles have less surface energy which diminishes the driving
force for consolidation. For conventional powder processing, the
optimum particle diameter is approximately 1 .mu.m. Particles
significantly smaller than this are difficult to pack uniformly and
densely due to agglomeration and undergo excessive shrinkage with
entrapped porosity during firing. In addition, a range of particle
sizes is advantageous towards achieving maximum packing density in
the green body. However, non-uniformities in particle packing may
result in voids which are difficult to eliminate during the
densification process. Variations in green body densities produced
during the forming process can also result in warpage during
sintering.
[0031] The dependence of sintered density on green body density was
accomplished by dry-pressing samples at applied pressures of 1000
psi to 20,000 psi. All the samples were pressed in the same 1/4 in.
cylindrical die and therefore had the same green diameter.
Information in Table 2 expounds the effects of green body density
on linear shrinkage and the aspect ratio of the pellets after
sintering. Samples pressed at lower pressures had lower green
densities and underwent greater linear shrinkage during firing. All
samples consolidated to a similar density of approximately 4.45
g/cm.sup.3, independent of their initial green densities. The
initial particle size or surface area, rather than initial green
density of the compact had a greater impact on the final sintered
density. These data suggest that the primary driving force for
densification of ZrW.sub.2O.sub.8 is the reduction of surface free
energy. However, complexities due to reactive sintering with the
aid of additives obscure the sole effect of surface energy
minimization.
[0032] The as-received mixture of powders had a bimodal particle
size distribution around 600 .mu.m and 50 .mu.m. Monoliths
fabricated from this coarse, unreactive material actually decreased
in density after sintering. It was necessary to comminute the
starting materials by vibratory milling. As the milling time
increased the particle size initially decreased significantly to
approximately 1 .mu.m diameter. Milling times were limited to 16 h
due to concerns of contamination from the grinding media.
[0033] Sintering Atmosphere
[0034] Loss of oxygen in ZrW.sub.2O.sub.8 has been observed at
temperatures as low as 500.degree. C. in vacuum, while retaining
the crystal structure. Another study found that oxygen scavengers
(e.g. Cu) promote decomposition of ZrW.sub.2O.sub.8 at low
temperatures (600.degree. C.) through the formation of Cu.sub.2O.
It has been found that monoliths sintered in air at 1180.degree. C.
partially decompose on the surface, followed by volatilization of
WO.sub.3. Heating ZrW.sub.2O.sub.8 in a N.sub.2 atmosphere at the
sintering temperature leads to complete decomposition into
ZrO.sub.2 and WO.sub.3, whereas sintering in pure oxygen produced
single phase ZrW.sub.2O.sub.8 with minimal surface decomposition.
In addition, monoliths sintered in oxygen demonstrated reproducible
physical properties.
[0035] Microstructure
[0036] The microstructure of a nearly stoichiometric
ZrW.sub.2O.sub.8 monolith reveals large grains of ZrW.sub.2O.sub.8
having approximately a 20 .mu.m diameter which form the matrix of
the ceramic. The uniform distribution of pores and grain sizes is
indicative of a homogeneously sintered material. A SEM micrograph
of a ZrW.sub.2O.sub.8.18 wt. % ZrO.sub.2 monolith is presented in
FIG. 2 in which excess zirconia is observed as nearly spherical
inclusions having various diameters. Individual ZrW.sub.2O.sub.8
grains cannot be distinguished in the SEM image.
[0037] For the ZrW.sub.2O.sub.8.xZrO.sub.2 compositions surveyed,
the ZrO.sub.2 volume fraction was below the percolation limit such
that a 0-3 connectivity (isolated inclusions of ZrO.sub.2 in a
matrix of ZrW.sub.2O.sub.8) was preserved. The percolation limit
for a second phase with monodisperse diameters has been calculated
to exist at a critical volume fraction of 0.183. Although we have
prepared samples with a greater volume fraction of zirconia, all
evidence indicates isolated inclusions. Thus significantly higher
volume fractions are necessary to reach the percolation limit which
is due to the distribution of diameters of the inclusions. Beyond
the percolation limit the properties of a monolith with two
interpenetrating 3-dimensional matrices (3-3 connectivity) may
demonstrate anomalies. In addition, it is possible that the stress
placed on a 3-3 ceramic due to the tetragonal to monoclinic phase
transformation of the ZrO.sub.2 matrix would lead to catastrophic
failure of the monoliths.
[0038] Powder X-ray diffraction indicates the presence of only the
.alpha.--ZrW.sub.2O.sub.8 phase (Powder Diffraction File, Joint
Committee on Powder Diffraction Standards, JCPDS, Swarthmore, Pa.,
card number 13-557) and monoclinic Baddeleyite ZrO.sub.2 phase
(JCPDS card number 37-1484). However, small amounts of additional
phases may be present. The lower detectable limit of other phases
by X-ray diffraction is estimated to be 1%.
[0039] Thermal Expansivity
[0040] ZrW.sub.2O.sub.8 has demonstrated a negative thermal
expansion from 0.3 to 1050 K. The temperature dependence of the
thermal expansion of a monolith of ZrW.sub.2O.sub.8 with 9.5 wt. %
excess ZrO.sub.2 is shown in FIG. 3. The order-disorder phase
transition at 150.degree. C. is reversible and does not compromise
the mechanical strength of the monoliths. In addition, cracking due
to the large difference in thermal expansion of ZrW.sub.2O.sub.8
and ZrO.sub.2 was not observed over the temperature range of
-100.degree. to 300.degree. C. The linear negative thermal
expansion of the composite facilitates its utilization in
applications.
[0041] The coefficient of thermal expansion of diphasic ceramic
monoliths can be tuned by compensating the large negative thermal
expansion of ZrW.sub.2O.sub.8 with a material having a positive CTE
such as ZrO.sub.2. Zirconia was chosen as the second phase for its
thermodynamic stability in the presence of ZrW.sub.2O.sub.8 and for
the ease of processing via the reactive sintering technique.
Several other refractories react with ZrW.sub.2O.sub.8 at the
sintering temperature and therefore lead to irreproducible results
due to decomposition. In addition to stability, the thermal
expansion of ZrO.sub.2 is roughly linear from 20 to 100.degree. C.
with a CTE of 8.times.10.sup.-6.degree. C..sup.-1. This is
advantageous since the compensation of the thermal expansion leads
to an averaged CTE which is also linear over ambient temperatures
(FIG. 4). These diphasic ceramics can be tuned from -5 to
-11.times.10.sup.-6.degre- e. C..sup.-1 for dense monoliths. A
linear correlation between the CTE and the volume fraction of
ZrO.sub.2 occurs over a broad range of compositions (FIG. 5). A
simple model, based on an additive relationship of thermal
expansions for a monolith without zirconia with a CTE of
-11.times.10.sup.-6.degree. C..sup.-1 and pure ZrO.sub.2 with a CTE
of 8.times.10.sup.-6.degree. C..sup.-1, would have a slope of 19.
The linear fit to our data has a slope of 17.6 which is in close
agreement for the compositions surveyed. This approach, which
neglects any anomalies at the percolation limit, suggests that a
monolith with a volume fraction of ZrO.sub.2 of 0.58 would have a
zero CTE.
[0042] The thermal expansion was measured along three directions
normal to the faces of the ceramic bars. Although ZrW.sub.2O.sub.8
crystallizes in a cubic space group, the monoliths are prepared by
uniaxially compressing the pre-sintered powder mixture. This
processing could introduce anisotropic macroscopic properties.
However, the thermal expansion of the bars demonstrates isotropic
behavior. Heating rates up to 20.degree. C./min to temperatures of
400.degree. C. with a TMA analyzer did not reveal any decomposition
or cracking which would be distinguished as irreproducibility or
discontinuities in the CTE, respectively. Degradation of the
ceramics was not observed through several heating and cooling
cycles.
[0043] Compensating the CTE with excess WO.sub.3 has been
accomplished, but the monoliths have extremely high porosity at the
surface to a depth of approximately 120 .mu.m. In order to
fabricate mechanically robust monoliths, extensive machining is
necessary to remove the porous layer.
[0044] Fiber Grating Package
[0045] The Bragg wavelength (.lambda.) in a vacuum is given by
.lambda.=2n.sub.eff.LAMBDA.
[0046] where n.sub.eff is the effective refractive index for the
guided mode in the fiber, and .LAMBDA. is the period of the index
modulations of the fiber (.about.0.5 .mu.m for a particular 1550 nm
grating). The Bragg wavelength of a fiber Bragg grating is
temperature dependent primarily due to the temperature dependence
of the refractive index of the silica based glass. In addition, the
Bragg wavelength is strain dependent by altering the fringe
spacing. As the temperature increases the refractive index of glass
increases and vice versa. Also, due to the positive thermal
expansion of silica (CTE.about.0.5.times.10.sup.-6.degree.
C..sup.-1), the fringe spacing increases slightly with temperature.
The wavelength shift that corresponds to refractive index (n)
changes due to temperature (T) variations and thermal expansion of
silica glass (.alpha..sub.thermal) can be calculated as follows: 2
= 2 n T = 2 n T + 2 n T = 2 n n n T T + 2 n T T = ( 1 n n T +
thermal ) T where CTE = thermal = 1 T
[0047] For a 100.degree. C. temperature change the shift in
wavelength for a particular 1550 nm grating is measured to be about
1.15 nm. The portion of this shift due to the CTE of the silica
fiber (.alpha..sub.thermal) is .about.0.08 nm, which is less than
8% of the total shift.
[0048] As mentioned before, the grating wavelength is also
sensitive to strain. If a grating is stretched, then the grating
wavelength will increase. The strain and wavelength relationship
can be presented as follows: 3 = l l ( 1 - P e )
[0049] where .DELTA.l/l=.epsilon., the strain, and P.sub.e is the
effective photo-elastic constant (.about.0.22) Due to this effect,
it is possible to package a fiber grating on or in a negative
expansion material so that package and grating dimensions decrease
with an increase in temperature, resulting in a value of .LAMBDA.
that falls as the temperature increases. By choosing the
appropriate negative expansion coefficient
(.about.-9.times.10.sup.-6.degree. C..sup.-1) that maintains a
constant n.sub.eff.LAMBDA. product, a grating whose reflecting
wavelength shows minimal variation with temperature can be
achieved.
[0050] The dependence of the reflection wavelength of both a
compensated and an uncompensated fiber Bragg grating as a function
of temperature is shown in FIG. 6. The grating that is compensated
by a ZrW.sub.2O.sub.8.9.5 wt. % ZrO.sub.2 monolith demonstrates a
0.05 nm deviation from -40 to 80.degree. C., which is nearly ideal.
This substrate can be utilized to fabricate a thermally compensated
package suitable for WDM (wavelength division multiplex)
applications.
[0051] Mechanical Strength
[0052] Four-point bending tests were performed on bars of
ZrW.sub.2O.sub.8. Preliminary tests revealed a low modulus of
elasticity (0.5.times.10.sup.6 psi) for the
ZrW.sub.2O.sub.8.xZrO.sub.2 monoliths. The modulus of rupture was
determined to be 3000 psi. With regards to the grating package, an
applied force of 89 N is required to break the monolith which is
much greater than the maximum tensile force which is exerted by the
fiber (2 N).
[0053] The monoliths demonstrate brittle failure having an
approximate conchoidal fracture surface. The mechanism proceeds via
intergranular fracture around ZrO.sub.2 inclusions and
intragranular throughout the ZrW.sub.2O.sub.8 matrix. Therefore
shearing at boundaries between ZrW.sub.2O.sub.8 grains does not
occur at room temperature.
[0054] Conclusion
[0055] Reactive sintering of WO.sub.3 and ZrO.sub.2 powders
produces dense monoliths with adequate strengths. The reactive
sintering technique circumvents the inherent metastability of
ZrW.sub.2O.sub.8 in the early stages of densification, thereby
yielding reproducible fabrication conditions. Monoliths containing
zirconia inclusions demonstrate a range of thermal expansion
coefficients linearly related to the volume fraction of ZrO.sub.2.
A monolith of ZrW.sub.2O.sub.8.xZrO.sub.2, which exhibits a
negative thermal expansion in the desired range, has been
successfully prepared and shown to thermally compensate a fiber
Bragg grating.
1TABLE 1 Effect of Sintering Aids on the Density of Monoliths
Additive (0.1 wt. %) Density (g/cm.sup.3) None 3.25 SiO.sub.2 3.10
TiO.sub.2 3.82 ZnO 3.94 Al.sub.2O.sub.3 3.95 Bi.sub.2O.sub.3 4.05
Y.sub.2O.sub.3 4.21
[0056]
2TABLE 2 Effect of Green Body Density on Shrinkage Applied Sintered
Linear Pressure Diameter Shrinkage Aspect Density (psi) (mm)
(%).sup.a Ratio.sup.b (g/cm.sup.3) 1000 5.94 6.45 1.13 4.49 5000
6.02 5.20 1.17 4.42 10000 6.03 4.25 1.24 4.51 20000 6.20 2.36 1.32
4.41 .sup.aLinear shrinkage determined along diameter. .sup.bAspect
ratio defined as diameter/height of cylindrical monolith after
sintering.
EXAMPLES
[0057] The following examples are presented to assist those skilled
in this technology to understand and practice the invention,
without in any way intending to limit the invention to the
exemplified embodiments.
Example 1
[0058] Preparation of ZrW.sub.2O.sub.8 composite ceramics via
reactive sintering technique utilizing methyl ethyl ketone (MEK) as
the milling solvent.
[0059] Milling of ZrO.sub.2 and WO.sub.3 mixture in methyl ethyl
ketone (MEK) for 10-20 hours utilizing stabilized ZrO.sub.2
grinding media.
[0060] Addition of approximately 2 wt. % of an organic binder
(QPAC-40, PAC Polymers Inc.) which is soluble in MEK to above
mixture,
[0061] Evaporation of MEK during continuous stirring of
mixture,
[0062] Sieving of dried mixture through 30-100 mesh screen,
[0063] Pressing of powder mixtures to form green body,
[0064] Firing in an oxygen atmosphere on a bed of coarse
ZrW.sub.2O.sub.8 grains on Pt foil,
[0065] Slowly heating said green body around 50.degree. C. per hour
to around 250.degree. C.,
[0066] Subsequent heating of said green body around 500.degree. C.
per hour to around 1150 to 1200.degree. C. with an optimal
sintering temperature of 1180.degree. C.,
[0067] Holding said green body at 1180.degree. C. for around 5
hours,
[0068] Cooling quickly to room temperature by withdrawing the
sintered monolith from the furnace.
Example 2
[0069] Preparation of ZrW.sub.2O.sub.8 composite ceramics via
reactive sintering technique utilizing water as the milling
solvent.
[0070] Milling of ZrO.sub.2 and WO.sub.3 mixutre in water for 10-20
hours utilizing stabilized ZrO.sub.2 grinding media,
[0071] Addition of approximately 5-10 wt. % of an organic binder
(polyvinyl alcohol) which is soluble in water to mixture,
[0072] Evaporation of water during continuous stirring of
mixture,
[0073] Sieving of dried mixture through 30-100 mesh screen,
[0074] Pressing of powder mixture to form green body,
[0075] Firing in an oxygen atmosphere on a bed of coarse
ZrW.sub.2O.sub.8 grains on Pt foil,
[0076] Slowly heating said green body around 50.degree. C. per hour
to around 250.degree. C.,
[0077] Subsequent heating of said green body around 500.degree. C.
per hour to around 1150 to 1200.degree. C. with an optimal
sintering temperature of 1180.degree. C.,
[0078] Holding said green body at 1180.degree. C. for around 5
hours,
[0079] Cooling quicly to room temperature by withdrawing the
sintered monolith from the furnace.
Example 3
[0080] Extrusion of pre-reacted powders.
[0081] Milling of ZrO.sub.2 and WO.sub.3 mixture in methyl ethyl
ketone (MEK) for 10-20 hours utilizing stabilized ZrO.sub.2
grinding media,
[0082] Evaporation of MEK,
[0083] Sieving of dried mixture through 30-100 mesh screen,
[0084] Addition of approximately 0.3 wt. % plasticizer and
lubricant (Union Carbide, PEG-400) to powder mixture.
[0085] Addition of approximately 1.0 wt. % dispersant (Angus
Chemical Co., AMP-95) to powder mixture,
[0086] Addition of approximately 4.5 wt. % binder (Rohm and Haas,
B-1051) to powder mixture,
[0087] Addition of approximately 2.4 wt. % binder (Rohm and Haas,
B-1052) to powder mixture,
[0088] Addition of approximately 6.1 wt. % water to powder
mixture.
[0089] Blended under low shear conditions,
[0090] Extruded through die at room temperature.
Example 4
[0091] Preparation of ZrV.sub.2O.sub.7 composite ceramics via
reactive sintering technique utilizing water as the milling
solvent.
[0092] Milling of ZrO.sub.2 and V.sub.2O.sub.5 mixture in water for
10-20 hours utilizing stabilized ZrO.sub.2 grinding media,
[0093] Addition of approximately 5-10 wt. % of an organic binder
(polyvinyl alcohol) which is soluble in water to mixture,
[0094] Evaporation of water during continuous stirring of
mixture,
[0095] Sieving of dried mixture through 30-100 mesh screen,
[0096] Pressing of powder mixture to form green body,
[0097] Firing in an oxygen atmosphere on a bed of coarse
ZrV.sub.2O.sub.7 grains on Pt foil,
[0098] Slowly heating said green body around 50.degree. C. per hour
to around 250.degree. C.,
[0099] Holding said green body at 850-900.degree. C. for around 5
hours,
[0100] Cooling quickly to room temperature by withdrawing the
sintered monolith from the furnace.
[0101] It is understood that various other modifications will be
apparent to and can readily be made by those skilled in the art
without departing from the scope and spirit of this invention.
Accordingly, it is not intended that the scope of the claims
appended hereto be limited to the description as set forth herein,
but rather that the claims be construed as encompassing all the
features of patentable novelty that reside in the present
invention, including all features that would be treated as
equivalents thereof by those skilled in the art to which this
invention pertains.
* * * * *