U.S. patent number 3,666,666 [Application Number 04/885,789] was granted by the patent office on 1972-05-30 for ferroelectric ceramic materials.
Invention is credited to Gene H. Haertling.
United States Patent |
3,666,666 |
Haertling |
May 30, 1972 |
FERROELECTRIC CERAMIC MATERIALS
Abstract
An electrooptic ferroelectric ceramic material of a lead
lanthanum zirconate titanate solid solution having about 5 to 25
atom percent lanthanum with the ratio of zirconium to titanium
varying from about 5/95 to about 95/5, hot-pressed, having an
optical transmittance throughout the visible spectrum of about 100
percent for optically polished plates about 0.25 millimeters thick,
with an effective birefringence of from about -0.003 to -0.03 at
saturation remanence polarization to near zero as the remanent
polarization is switched to electrical zero, and an effective
electrooptic coefficient at saturation remanence from about 1
.times. 10.sup..sup.-2 to 5 .times. 10.sup..sup.-2 m.sup.2 /C, and
for memory applications a coercive field from about 2 to
10kV/cm.
Inventors: |
Haertling; Gene H.
(Albuquerque, NM) |
Assignee: |
|
Family
ID: |
25387702 |
Appl.
No.: |
04/885,789 |
Filed: |
December 17, 1969 |
Current U.S.
Class: |
252/62.9PZ;
359/323; 501/134; 501/152 |
Current CPC
Class: |
C04B
35/48 (20130101); C04B 35/46 (20130101); G02F
1/0027 (20130101); C04B 35/51 (20130101); C04B
35/50 (20130101); H01G 7/026 (20130101) |
Current International
Class: |
H01G
7/00 (20060101); C04B 35/51 (20060101); C04B
35/48 (20060101); C04B 35/50 (20060101); C04B
35/46 (20060101); G02F 1/00 (20060101); H01G
7/02 (20060101); C04b 035/46 (); C04b 035/48 ();
C04b 035/50 (); G02f 001/26 () |
Field of
Search: |
;252/62.9 ;106/39
;350/150 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
haertling, Am. Ceram. Soc. Bull., Vol. 47, P. 389 (1968).
|
Primary Examiner: Levow; Tobias E.
Assistant Examiner: Cooper; J.
Claims
What is claimed is:
1. An optical material having electrically variable birefringence
consisting essentially of a ferroelectric ceramic of
Pb.sub.1.sub.-x La.sub.x (Zr.sub.y Ti.sub.z).sub.1.sub.-x/4 0.sub.3
falling within the area ABCD of FIG. 2 where x is between about 5
and 25 atom percent with a ratio of y/z from about 5/95 to about
95/5 and prepared by hot-pressing at temperatures from about
800.degree. C. to about 1,300.degree. C. for about 1 to 64 hours at
a pressure of about 500 to 20,000 psi, said ceramic having an
optical transmittance in at least a portion of the visible spectrum
of about 100 percent after correction for reflection losses with
material about 0.25 millimeters thick.
2. The material of claim 1 having about 100 percent transmittance
after correction for reflection losses in the electromagnetic
spectrum from about 0.7 to about 7 microns.
3. The material of claim 1 having an effective birefringence of
from about -0.003 to -0.03 at saturation remanence polarization
varying to near zero as remanent polarization is switched to
electrical zero and having an effective electrooptic coefficient at
saturation remanence from about 1 .times. 10.sup..sup.-2 to about 5
.times. 10.sup..sup.-2 m.sup.2 /C.
4. The material of claim 3 falling within the area EFG and having a
coercive field from about 2 to 10kV/cm and having a plurality of
stable polarization states between zero polarization and saturation
remanent polarization.
5. The material of claim 3 falling within the area FBHG of FIG. 2
and having a coercive field from about 10 to 40kV/cm.
6. The material of claim 3 falling within the area AEHCD and having
a coercive field of about zero.
7. The material of claim 3 falling within the area AEHCI and having
a coercive field of about zero.
Description
BACKGROUND OF INVENTION
There is increasing demand for improved electrooptic materials for
optical communications, information processing, memory and display
systems, that is, materials which exhibit the conventional Pockels
or Kerr effects, i.e., reversible variation of birefringence by an
applied electric field. Electrooptic materials which exhibit
non-volatile memory capabilities, i.e., materials which may be
switched from one birefringence value to another and retain the new
birefringence when the switching field is removed, are required for
optical memories and controlled persistence displays. Also, for
memories and controlled persistence displays, it is important that
large numbers of small discrete areas be switchable independently
in order to achieve a high density of storage sites in a given
piece of electrooptic material.
Birefringent, optically uniaxial materials are characterized by two
refractive indexes: n.sub.e, the refractive index parallel to the
optic axis, and n.sub.o, the refractive index perpendicular to the
optic axis. The birefringence of these materials is defined as the
difference, n.sub.e -n.sub.o, of the two refractive indexes.
The propagation velocity of light in birefringent materials depends
on the orientation of the optical electric vector, i.e., on the
light polarization condition. In uniaxially birefringent materials,
incident light which is linearly polarized parallel to the optic
axis propagates with a velocity c/n.sub. e, and incident light
which is linearly polarized perpendicular to the optic axis
propagates with a different velocity c/n.sub. o.
Linearly polarized light with its plane of polarization at some
angle other that 0.degree. or 90.degree. to the optic axis is
resolved into two perpendicular linearly polarized components when
it enters the birefringent material. The polarization plane of one
compound is parallel to the optic axis; the plane of the other
component is perpendicular to the optic axis. Since the propagation
velocities of the two components are different, a phase difference
develops between the two components as they travel through the
birefringent material. This phase difference increases as the light
progresses through the material. The total phase difference of the
components as they emerge from the material is called the
retardation .GAMMA.. The retardation obviously depends on the
birefringence n.sub.e -n.sub.o (which determines the difference in
component velocities) and the material thickness t:
.GAMMA. = (n.sub.e -n.sub.o)t = .DELTA. n t.
The polarization condition of the light emerging from the material
depends on the retardation. As the two phase-displaced components
emerge, they recombine (interfere) to produce elliptically
polarized light. The elliptical polarization may vary from circular
to linear depending on the retardation. Assuming the incident
linearly polarized light is monochromatic with wavelength .lambda.
(in air), light emerging from the opposite surface of the material
is circularly polarized if .GAMMA. is an odd multiple of .lambda./
4, and the emerging light is linearly polarized if .GAMMA. is an
even multiple of .lambda./ 4. If .GAMMA. is an integral multiple of
.lambda., the polarization planes of the incident and emergent
light are parallel. If .GAMMA. is an odd multiple of .lambda./ 2
the polarization plane of the emergent light is rotated with
respect to that of the incident light by an angle 2.rho., where
.rho. is the angle between the polarization plane of the incident
light and the optic axis.
Electrooptic materials commonly used in the past have been
ferroelectric single crystals. These materials exhibit the
conventional Pockels or Kerr electrooptic effects. However, with
two possible exceptions i.e., gadolinium molybdate and bismuth
titanate, single crystals do not exhibit non-volatile optical
memory capabilities. The exceptions, gadolinium molybdate and
bismuth titanate, have only binary memory capabilities. Also,
single crystals have poor localized switching capabilities. When
switched in small localized areas, the locally switched areas are
surrounded by wide partially switched fringes thus preventing high
density localized switching. Other limitations on single crystal
electrooptic materials are high cost and difficulties in growing
large homogeneous crystals.
Some of the limitations of ferroelectric single crystals in
electrooptic applications have been overcome by the discovery of
the electrooptic properties of fine-grained, hot-pressed lead
zirconate-lead titanate ferroelectric ceramics (Land and Thacher,
Proc. IEEE, Vol. 57, No. 5, pp. 751-768, May 1969). For example,
these materials become uniaxially birefringent on a macroscopic
scale when they are electrically poled (polarized by an external
field). Furthermore, their effective birefringence is electrically
variable by either applying an external biasing field (the
conventional electrooptic effect) or by partially switching the
ferroelectric polarization. Variation of the effective
birefringence by partial or incremental switching is a property
unique to ferroelectric ceramics. Locally switched domains in
ceramics remain in their switched orientation after the switching
field is removed and the locally switched areas can be "erased" by
switching them back to their original orientation. Locally switched
areas have narrow fringes, usually only 5 to 10 grain diameters in
width, which permits a high density of storage sites on a ceramic
plate. Ferroelectric ceramics may be hot-pressed in virtually any
size and shape and are relatively inexpensive compared to single
crystals. The optic axis orientation in ferroelectric ceramics
depends on the direction of the electric poling or switching field,
hence it can be switched in any direction. This is not possible in
single crystals.
Ferroelectric ceramics may be prepared by sintering at atmospheric
pressure as well as by sintering at high pressures (i.e., pressure
sintering or hot-pressing). Materials sintered at atmospheric
pressure, regardless of material composition and sintering
parameters, are inhomogeneous and have relatively high porosity (2
to 6 percent) and, as such, are incapable of use as electrooptic
elements or devices due to large and uncontrolled internal light
scattering caused by inhomogeneity and porosity. Many ferroelectric
ceramics sintered at atmospheric pressure have a generally
yellowish, opaque appearance regardless of size and plate
thickness. The optical transmittance of these materials may be
typically less than one percent (neglecting reflection losses) for
plates 0.05 mm thick and any light which is transmitted will
generally be completely depolarized due to internal scattering.
This depolarization and low optical transmittance is inherent in
all commonly available atmospheric-pressure sintered ferroelectric
ceramics, including lead zirconate-lead titanate compositions
previously known in the art and including those having rare earth
element additives of any amount such as those having 10 atom
percent or less.
Ferroelectric ceramics produced by high pressure sintering
techniques (i.e., hot-pressing techniques) may have significantly
increased homogeneity and decreased porosity (essentially zero)
from that of ferroelectric ceramics sintered at atmospheric
pressure. Hot-pressed ferroelectric ceramics may exhibit higher
optical transmittance as well as the additional capabilities of
non-volatile optical memory, high density localized switching and
the like as previously described for fine-grained, hot-pressed lead
zirconate-lead titanate ceramics (Land and Thacher, IEEE Proc.,
Vol. 57 No. 5, pp. 751-768, May 1969).
For the purposes of this invention, optical transmittance may be
defined as the ratio of light intensity transmitted by an optical
material or device into a specific detector, to the light intensity
incident on the optical material or device measured by the same
detector. This ratio is always expressed as a percentage in this
application.
Conventional hot-pressed ceramics including fine-grained lead
zirconate-lead titanate solid solutions, exhibit undesirable
scattering of transmitted light which limits their optical
transmittance and requires that they be used in thin plates
(thickness typically 0.1 mm or less). For example, optically
polished plates 0.25 mm thick may have a maximum optical
transmittance of about 5 percent (neglecting reflection losses);
plates 0.05 mm thick may have a maximum optical transmittance of
about 60 percent (neglecting reflection losses). These maximum
transmittances occur at the red end of the visible light spectrum;
scattering dispersion reduces the optical transmittances to
essentially zero at the violet end of the light spectrum.
In many applications it is highly desirable to use plates thicker
than 0.1 mm for mechanical strength and rigidity and to increase
the range of electrically variable retardation (product of
birefringence and plate thickness). In other applications it is
desirable to use optical networks comprised of two or more
electrooptic devices in series. Therefore, ferroelectric ceramic
electrooptic materials with greatly increased transmittance
(reduced light scattering) are required for these applications and
desirable for all electrooptic applications.
The birefringence of fine-grained, hot-pressed lead zirconate-lead
titanate solid solutions may be about -0.02 at saturation remanence
and it may vary to about -0.01 at zero remanent polarization. This
means that the birefringence may be varied over a range of about 50
percent of its value at saturation remanence by partial or
incremental switching of the remanent polarization. It is highly
desirable for optical memory and controlled persistence display
applications that the range of birefringence variation as a
function of remanent polarization be increased. In this material
the effective electrooptic coefficients (r.sub.eff
/.gamma.T.sub.33.sup.T) may be about 1 .times. 10.sup..sup.-2
m.sup.2 /C and the coercive field (field at which the polarization
can be switched from saturation remanence to zero) may be about
12kV/cm.
SUMMARY OF INVENTION
In view of the above, it is an object of this invention to provide
a ferroelectric ceramic composition having optical transmittance
throughout the visible spectrum.
It is a further object of this invention to provide a ferroelectric
ceramic composition capable of having optical transmittance
throughout the visible spectrum of about 100 percent.
It is a further object of this invention to provide a ferroelectric
ceramic composition having an effective birefringence dependent on
remanent polarization with a range extending from some maximum at
saturation remanence to about zero near zero remanence.
It is a further object of this invention to provide a ferroelectric
ceramic composition having a high effective electrooptic
coefficient and a relatively low coercive field.
It is a further object of this invention to provide a ferroelectric
ceramic composition having all of the above properties as well as
the desirable properties of conventional hot-pressed ferroelectric
ceramics.
Various other objects and advantages will appear from the following
description of the invention, and most novel features will be
particularly pointed out hereinafter in connection with the
appended claims.
It will be understood that various changes in the details and
materials, which have been herein described and illustrated in
order to explain the nature of the invention, may be made by those
skilled in the art.
The invention comprises a hot-pressed ferroelectric ceramic solid
solution composed of lead lanthanum zirconate titanate with about 5
to 25 atom percent lanthanum substituted for the lead and with the
zirconium to titanium ratio varying from about 5/95 to about
95/5.
DESCRIPTION OF DRAWING
Various characteristics of the ferroelectric material of the
present invention are shown in the accompanied drawing wherein:
FIG. 1 is a perspective and somewhat schematic view of a
ferroelectric ceramic optical system;
FIG. 2 is a partial phase diagram of the lead lanthum zirconate
titanate solid solution system;
FIGS. 3a and 3b are graphs of polarization versus applied electric
field for ferroelectric ceramic materials of this invention;
FIG. 4 is a graph of effective birefringence versus remanent
polarization for different grain sizes of a ferroelectric material
of this invention with a hysteresis loop as shown in FIG. 3a;
FIG. 5a is a graph of effective birefringence versus electric field
for different grain sizes of the material used in FIG. 4;
FIG. 5a is a graph of effective birefringence versus electric field
for ferroelectric material as shown in FIG. 3b; and
FIGS. 6a and 6b are graphs of typical transmittance versus
wavelength for materials of this invention.
DETAILED DESCRIPTION
A typical electrooptic device 10 is shown in FIG. 1 in somewhat
simplified and diagrammatic form with exaggerated dimensions.
Electrooptical device 10 may include a ferroelectric ceramic plate
or member 12 composed of the material of this invention and
prepared as described below.
Plate 12 may have any convenient electrode arrangement or pattern
to provide a desired optical output, such as those described in the
Land and Thacher article referred to hereinabove, for example, a
pair of electrodes 14 and 16. Electrodes 14 and 16 may be disposed
on a surface of plate 12 on opposite sides of a polarization area
or information location 18. An electric field may be produced
between electrodes 14 and 16 in location 18 of plate 12 by an
appropriate power source or pulse generator 20. Pulse generator 20
may be any appropriate electrical pulse source which may produce
pulses of a desired polarity and amplitude and switch domains of
plate 12 disposed between electrodes 14 and 16 at location 18 in
one or more directions. A source of light 21 may be positioned near
plate 12 so as to impinge light, such as shown by arrow 22, against
location 18 and through plate 12. The light source may be any
conventional ordinary or white light source, such as an
incandescent or mercury arc lamp, or for certain applications a
monochromatic or narrow band light source, such as a laser or
filtered light source, which is capable of projecting a desired
beam of light against location 18. The light source may also
include standard collimating means including special lens or fiber
optic systems. The light source preferably includes a linear
polarizer element 23 between the light source and location 18 so as
to polarize the light impinging on location 18 some prescribed
direction. A linear analyzer 25 and a suitable photosensitive
device 26 may be aligned with the light beam emerging from plate
12, such as shown by arrow 24, to sense the amplitude of the light
beam emerging from plate 12 and polarized in the direction of the
linear analyzer. Linear analyzer 25 is generally positioned so as
to have its polarization axis at right angles to that of linear
polarizer 23. The electrooptical device 10 may thus effectively
control the color or intensity of light from a white light source
or intensity of light from a monochromatic source as the emerging
beam impinges upon photosensitive device 26.
Plate 12 is an optically uniaxial ferroelectric material having a
multiplicity of grains with uniform nominal grain diameters
typically about 10 microns or less, a relative density greater than
about 99 percent theoretical, and maximum homogeneity, light
transmittance, surface smoothness. The grain diameter needed to
achieve the desired electrically controlled optical properties may
be dependent upon the particular ferroelectric composition and the
hot-pressing parameters used. It has been found that in order to
achieve these properties, the ferroelectric material must be
prepared by hot-pressing or pressure sintering techniques.
An optically uniaxial ferroelectric ceramic for purposes of this
invention, is one in which the poled or polarized ceramic is
effectively optically uniaxial, i.e., it exhibits the macroscopic
symmetry properties of an optically uniaxial, birefringent crystal.
The individual grains or crystallites of an optically uniaxial
ceramic may exhibit either uniaxial (tetragonal, rhombohedral and
hexagonal) symmetry or the generally biaxial (orthrhombic,
monoclinic, and triclinic) symmetry. A poled ferroelectric ceramic
is generally optically birefringent. With the individual
crystallites exhibiting negative birefringence, the electrical
polar direction is the fast axis of the ceramic. The effective
birefringence in a ferroelectric ceramic plate depends upon the
degree or magnitude of electrical poling in a given direction,
i.e., whether the ceramic is fully or only partially poled in a
particular direction. The orientation of the optic axis depends
upon the direction of electrical poling in the ceramic. It has been
found that electric control of the light transmission properties of
device 10 may be effected by varying the magnitude of the
ferroelectric polarization at location 18 in plate 12 by the
application of an external electric field by pulse generator 20.
The pulse amplitude and pulse width may be adjusted to produce
partial or incremental switching of the ceramic polarization. The
pulse amplitude is adjusted to produce the required switching
speed; the pulse width is adjusted to produce the desired change in
polarization. Incremental switching of the ceramic polarization
produces corresponding incremental changes in the effective
birefringence of the ceramic plate. Typical pulse widths and pulse
amplitudes may vary from about 0.1 microsecond to 100 microseconds
and about 0 to about 30 kilovolts per centimeter depending on the
electrode separation distance, plate thickness and composition.
Ferroelectric ceramic plate 12, in accordance with this invention,
is a ferroelectric ceramic Pb.sub.1.sub.-x La.sub.x (Zr.sub.y
Ti.sub.z).sub.1.sub.-x/4 O.sub.3, where x is between about 5 and
about 25 atom percent with a ratio of y/z from about 5/95 to about
95/5. This compositional series is a lead lanthanum zirconate
titanate (hereinafter referred to as PLZT) solid solution having
lanthanum substituted for lead in the prescribed amounts.
Compositions may also be prepared according to an alternate
formula; i.e., Pb.sub.1.sub.-3x/2 La.sub.x (Zr.sub.y Ti.sub.z)
O.sub.3. However, where this formula is used, an additional
quantity of lead oxide, ranging from 0.1 to 8 weight percent, must
be added to the original batch weight.
The phase diagram of the (Pb, La)(Zr, Ti) O.sub.3 system is given
in FIG. 2. Compositions or solid solutions covered by this
invention are included within the rectangular area ABCD.
Compositions which are ferroelectric tetragonal phase and fall
within the area EFG may exhibit good memory material
characteristics while those which fall within the area FBHG may
exhibit good "hard" (high coercivity) conventional electrooptic
material characteristics. Paraelectric cubic phase as well as mixed
paraelectric-ferroelectric compositions (having a coercive field of
about 0) falling in the remaining area, AEHCD, and principally
those in area AEHCI may exhibit good Kerr effect characteristics.
As the lanthanum substituent increases, the magnitude of the Kerr
effect may decrease.
For example, ferroelectric ceramic material prepared in accordance
with this invention, having a Zr/Ti ratio of 65/35 and having a
lanthanum substitute of from about 5 to 8 atom percent may exhibit
a polarization hysteresis curve similar to that shown in FIG. 3a
while a material having lanthanum substituted at greater than about
9 atom percent may have a polarization hysteresis curve similar to
that shown in FIG. 3b. As the lanthanum substitute is increased,
the hysteresis curve of FIG. 3a becomes more slanted with respect
to the polarization axis and becomes narrower until reaching the
condition shown in FIG. 3b. The materials having lanthanum
substituted up to about 8 atom percent exhibit a plurality of
stable, polarization states between the remanent polarization
states 27 and 28. These states are shown by way of example as 30,
32 and 34, 32 being at zero polarization. Many materials may
exhibit 10 or more stable polarization states between saturation
remanent and zero polarization. As the lanthanum substitute is
increased, the saturation remanent polarization decreases in
amplitude and approaches zero polarization. The group of materials
having a hysteresis characteristic similar to that shown in FIG. 3a
may have coercive fields varying from about 2 to 10 kilovolts per
centimeter.
Ferroelectric ceramic material prepared in accordance with this
invention, having a Zr/Ti ratio of from 55/45 to 5/95 and having a
lanthanum substitute of from 12 to 20 atom percent respectively
(within the area FBHG of FIG. 2), may exhibit a polarization
hysteresis loop curve similar to that shown in FIG. 3a with
increased values of coercive field ranging from 10kV/cm to 40kV/cm.
Materials in this compositional range are not readily switched by
an electrical pulse from one polarization state to another (such as
envisioned in a memory device) and hence may be utilized in a
conventional electrooptic mode of operation after the material is
initially and uniformly poled.
The above materials may exhibit effective birefringences varying
from 0 to as much as about -0.003 to -0.03 (depending upon
composition and hot-pressing parameters), the latter birefringence
being at saturation remanence polarization with the former being at
zero polarization. The variation of effective birefringence with
varying remanent polarization is illustrated in a typical example
in FIG. 4 for the composition PLZT 8/65/35 (where 8/65/35
designates atom percents of La, Zr and Ti respectively). Generally,
the maximum effective birefringence at a given polarization
increases with decreasing lanthanum substitute. The three curves of
FIG. 4 apply to three different grain sizes, 2.mu., 3.mu.and
10.mu., of the PLZT-8/65/35 composition. As the grain size
increases, the effective birefringence at saturation remanent
polarization (normalized remanent polarization of 1.0 and -1.0)
also increases. Also, the minimum birefringence (near zero remanent
polarization) increases with increasing grain size. The minimum
birefringence of the 2.mu.grain size material is zero which results
in a 100 percent range of birefringence variation.
The dependence of the effective birefringence of the three samples
of FIG. 4 on bias electric field E is shown in FIG. 5a. The ceramic
plates were first poled to saturation remanent polarization (+1.0
of FIG. 4), and then the bias field was applied in the saturation
(positive) direction. It is apparent that the birefringence
increases with increasing bias field and to fields as high as
10kV/cm the increase is approximately a linear function of the
applied field. The electrooptic coefficients (r.sub.eff
/.epsilon..sub.33.sup.T) are all larger than any previously
measured for fine-grained lead zirconate-lead titanate solid
solutions.
The curves of FIG. 5(b) apply to two compositions each having a
polarization hysteresis curve similar to that of FIG. 3b. Curve A
of FIG. 5b is a plot of effective birefringence vs. bias electric
field for the composition PLZT-9/65/35; curve B is a similar plot
for the composition PLZT-11/65/35. Note that the composition
PLZT-9/65/35 falls on the FE tetragonal-PE cubic phase boundary of
FIG. 2. For this reason, one would expect the birefringence
variation with electric field to be greater for this material than
for the PLZT-11/65/35 composition which falls well inside the PE
cubic phase of FIG. 2.
A ferroelectric ceramic plate made of a material within the above
compositional ranges may exhibit an optical transmittance
throughout the visible spectrum of about 100 percent (after
correcting for reflection losses) with optically polished surfaces
and a plate of about 0.25 millimeters or less. As the thickness of
the plate increases the transmittance may decrease, for example,
with a plate 1.5 millimeters thick the transmittance may be about
50 percent. Some materials may exhibit a 100 percent transmittance
with plates slightly greater or less than 0.25 millimeters thick,
depending upon the composition and hot-pressing parameters,
however, this variation in transmittance with plate thickness may
be minimal. FIGS. 6a and 6b illustrate the transmittance of a
typical ferroelectric ceramic (PLZT 8/65/35 ) within the
compositional range noted above over the visible light spectrum and
infrared spectrums respectively.
These ferroelectric ceramic compositions may be prepared by (1)
weighing lead oxide, zirconia, titania and lanthana powders, (2)
wet mixing the powders in a suitable liquid medium such as
distilled water, (3) drying the wet mixed powders, (4) calcining
the dried powder mixture at a temperature of about 900.degree. C.
for about 1 hour, (5) granulating or wet ball milling of the
calcine to break down the partially sintered particle aggregates,
(6) drying the wet milled calcine, and (7) compressing the
resulting powder into a slug. As stated previously, it has been
found that in order for the finished ceramic plate to exhibit the
above noted properties, that the slug must then be further
processed by hot-pressing at a temperature from about 800.degree.
C. to about 1,300.degree. C. for about 1 to 64 hours at a pressure
of from about 500 to 20,000 psi in an appropriate hot-pressing
apparatus. The grain size may be controlled by selecting raw
materials oxide powders which are of high chemical purity
(generally greater than about 99.2 percent) and by the proper
selection of hot-pressing conditions of temperature, time and
pressure. After hot-pressing, it is desirable that the finished
slug be sliced into thin wafers or plates and the major surfaces
polished to an optical quality finish. The plates may then be
annealed at from about 500 to 700.degree. C. for about 15 minutes,
cooled to room temperature, electrodes positioned or plated thereon
and the plate polarized to a desired uniform initial
polarization.
Ferroelectric ceramic materials thus formed and within the
compositional ranges noted above exhibit the high optical
transmittance, wide range of variable effective birefringence, high
effective electrooptic coefficients and low coercive fields desired
in electrooptic devices. The properties exhibited by these
materials may be orders of magnitude better than any previously
known ferroelectric ceramic electrooptical material. The uniform
optical transmittance over the visible spectrum and the wide range
of effective birefringence permits use of such ferroelectric
material in optical displays requiring the production of color over
the entire visible spectrum. This latter feature is further
enhanced by the high level of optical transmittance with plate
thicknesses many orders of magnitude greater than previous
materials.
Commercially available oxide raw materials having chemical purity
as noted above may commonly include iron in quantities greater than
500 parts per million. It has been discovered that the optical
clarity of the hot-pressed ferroelectric ceramic materials may be
improved by insuring that the iron content of the raw material
oxides is below about 300 parts per million.
Examples of typical hot-pressed lead lanthanum zirconate titanate
materials prepared in accordance with this invention are listed in
the following table illustrating some of their electrical and
optical characteristics. All materials listed exhibited about 100
percent optical transmittance except PLZT 2/65/35 which exhibited
about 25 percent transmittance for samples 0.25 millimeters
thick.
Somp- E.sub.c .epsilon..sub.33.sup.T /.epsilon..sub.o K.sub.p
P.sub.r Tan .delta. osition (Poled) (PLZT) (kV/cm) (.mu.C/cm.sup.2)
(%)
__________________________________________________________________________
2/65/35 13.5 652 0.450 39.3 2.8 6/65/35 7 1210 0.525 32.0 2.5
8/65/35 5 3380 0.647 31.0 2.4 9/65/35 4 4050 low 12.0 5.3 11/65/35
<2 3900 0.0 2.0 5.6 12/65/35 <2 2200 0.0 0.0 4.6 14/65/35
<2 1450 0.0 0.0 2.3 8/10/90 36 355 0.210 29.0 1.0 18/10/90 16
866 0.320 24.0 1.1 16/20/80 16 890 0.325 24.4 1.2 14/30/70 15.5
1025 0.352 25.2 1.1 12/40/60 15 1284 0.382 25.2 1.2 8/40/60 21 884
0.413 30.5 1.2 8/53/47 16 2020 0.488 27.7 1.5 9/60/40 7 2200 0.430
24.8 3.8 6/80/20 9 832 31.8 2.0 8/80/20 4 2.0 8/70/30 6 4050 0.446
26.0 4.7
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