U.S. patent application number 10/994575 was filed with the patent office on 2005-04-07 for single crystals of lead magnesium niobate-lead titanate.
This patent application is currently assigned to II-VI Incorporated. Invention is credited to Eissler, Elgin E..
Application Number | 20050072352 10/994575 |
Document ID | / |
Family ID | 22870051 |
Filed Date | 2005-04-07 |
United States Patent
Application |
20050072352 |
Kind Code |
A1 |
Eissler, Elgin E. |
April 7, 2005 |
Single crystals of lead magnesium niobate-lead titanate
Abstract
A method of making a single crystals of lead magnesium
niobate-lead titanate (PMN-PT). The method includes providing a
crucible containing PMN-PT starting material, and placing the
crucible into a furnace having at least two temperature zones,
which is subsequently pressured with an inert gas. The crucible
then descends into the temperature zones of the vertical. The
temperature zones of the furnace include a first temperature zone
having a temperature higher than a melting temperature of the
PMN-PT material and a second temperature zone having a temperature
less than the melting temperature of the PMN-PT material. The
single crystals of PMN-PT prepared using the above-described method
exhibits a [110] crystal orientation and may be used in
electroacoustic transducers. The electroacoustic transducers are
useful in devices that detect or generate acoustic waves.
Inventors: |
Eissler, Elgin E.; (Renfrew,
PA) |
Correspondence
Address: |
WEBB ZIESENHEIM LOGSDON ORKIN & HANSON, P.C.
700 KOPPERS BUILDING
436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
II-VI Incorporated
|
Family ID: |
22870051 |
Appl. No.: |
10/994575 |
Filed: |
November 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10994575 |
Nov 22, 2004 |
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10380232 |
Mar 11, 2003 |
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10380232 |
Mar 11, 2003 |
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PCT/US01/28592 |
Sep 12, 2001 |
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60231635 |
Sep 11, 2000 |
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Current U.S.
Class: |
117/8 |
Current CPC
Class: |
C30B 29/32 20130101;
C30B 29/30 20130101; C30B 11/00 20130101 |
Class at
Publication: |
117/008 |
International
Class: |
C30B 001/00; C30B
003/00; C30B 005/00; C30B 028/02 |
Claims
I claim:
1. A method of making a single crystal of lead magnesium
niobate-lead titanate (PMN-PT) comprising the steps of: (a)
providing a crucible containing PMN-PT starting material; (b)
placing the crucible containing the PMN-PT starting material into a
furnace having at least two temperature zones and pressurizing the
interior of said furnace with an inert gas; (c) establishing at
least: (i) a first temperature zone having a temperature higher
than a melting temperature of the PMN-PT material; and (ii) a
second temperature zone having a temperature less than the melting
temperature of the PMN-PT material; and (d) exposing the crucible
and PMN-PT starting material sequentially to said first temperature
zone and then said second temperature zone.
2. The method of claim 1, wherein the PMN-PT starting material
comprises high purity powders of Pb.sub.3O.sub.4, MgCO.sub.3,
Nb.sub.2O.sub.5, and TiO.sub.2.
3. The method of claim 1, wherein the crucible is covered with a
loosely fitted alumina disk.
4. The method of claim 1, wherein the pressure in the furnace is
from 10 to 500 atmospheres.
5. The method of claim 1, wherein the inert gas is one or more
selected from the group consisting of helium, neon, argon, krypton,
and xenon.
6. The method of claim 1, wherein the inert gas is argon.
7. The method of claim 1, wherein the temperature of the first
temperature zone is greater than 1,300.degree. C. and the
temperature of the second temperature zone is not more than
1,300.degree. C.
8. The method of claim 1, wherein the furnace is a vertical furnace
and wherein the exposing step (d) includes descending the crucible
in said vertical furnace at a rate of from 0.2 mm/hr to 12
mm/hr.
9. The method of claim 1, wherein the temperature of the first
temperature zone and the temperature of the second temperature zone
create a temperature gradient in the furnace and said temperature
gradient is from 10.degree. C./cm to 50.degree. C./cm.
10. The method of claim 1, wherein in addition to the inert gas, a
small partial pressure of oxygen is used to pressurize the
furnace.
11. The method of claim 1, wherein the single crystal of PMN-PT is
a single grain crystal of a (n-1)PMN-(n)PT solid solution
exhibiting [110] crystal orientation.
12. A single grain crystal of PMN-PT prepared using the method of
claim 1.
13. An electroacoustic transducer that includes at least one single
crystal of PMN-PT of claim 12.
14. A device that includes the electroacoustic transducer of claim
13 to detect or generate acoustic waves.
15. The device of claim 14, wherein the device is selected from the
group consisting of sonar devices, mine detection devices,
hydrophone devices, ultrasonic therapy devices, ultrasonic body fat
measuring devices, ultrasonic imaging devices, ultrasonic devices
for assessing the structural properties of wooden members,
ultrasonic devices for measuring animal backfat, ultrasonic devices
for fill level monitoring, ultrasonic devices for flow rate
measurement and ultrasonic devices for inspecting rods used in the
core of nuclear reactors.
16. A method of making a single grain crystal of lead magnesium
niobate-lead titanate ((n-1)PMN-(n)PT) exhibiting a crystal
orientation substantially in a [110] direction by crucible
interface seeding comprising the steps of: (a) providing a crucible
containing lead magnesium niobate-lead titanate (PMN-PT) starting
material comprising Pb.sub.3O.sub.4, MgCO.sub.3, Nb.sub.2O.sub.5,
and TiO.sub.2; (b) placing the crucible containing the PMN-PT
starting material into a furnace having at least two temperature
zones and pressurizing the interior of the furnace to a pressure of
from 75 to 100 atmospheres with an inert gas; (c) establishing at
least: (i) a first temperature zone having a temperature higher
than a melting temperature of the PMN-PT material; and (ii) a
second temperature zone having a temperature less than the melting
temperature of the PMN-PT material; and (d) exposing the crucible
and PMN-PT starting material sequentially to said first temperature
zone and then said second temperature zone.
17. The method of claim 16, wherein the crucible is covered with a
loosely fitted alumina disk.
18. The method of claim 16, wherein the temperature of the first
temperature zone is greater than 1,300.degree. C. and the
temperature of the second temperature zone is not more than
1,300.degree. C.
19. The method of claim 16, wherein the crucible descends at a rate
of from 0.2 mm/hr to 12 mm/hr.
20. The method of claim 16, wherein the temperature of the first
temperature zone and the temperature of the second temperature zone
create a temperature gradient in the furnace and said temperature
gradient is from 10.degree. C./cm to 50.degree. C./cm.
21. The method of claim 16, wherein in addition to the inert gas, a
small partial pressure of oxygen is used to pressurize the
furnace.
22. A single grain crystal of PMN-PT prepared using the method of
claim 16.
23. An electroacoustic transducer that includes at least one single
crystal of PMN-PT of claim 22.
24. A device that includes the electroacoustic transducer of claim
23 to detect or generate acoustic waves.
25. The device of claim 24, wherein the device is selected from the
group consisting of sonar devices, mine detection devices,
hydrophone devices, ultrasonic therapy devices, ultrasonic body fat
measuring devices, ultrasonic imaging devices, ultrasonic devices
for assessing the structural properties of wooden members,
ultrasonic devices for measuring animal backfat, ultrasonic devices
for fill level monitoring, ultrasonic devices for flow rate
measurement, and ultrasonic devices for inspecting rods used in the
core of nuclear reactors.
26. A single grain crystal of a lead magnesium niobate-lead
titanate (PMN-PT) comprising a (n-1)PMN-(n)PT solid solution with a
single crystal orientation substantially in a [110] direction.
27. An electroacoustic transducer that includes at least one single
crystal of PMN-PT of claim 26.
28. A device that includes the electroacoustic transducer of claim
27 to detect or generate acoustic waves.
29. The device of claim 28, wherein the device is selected from the
group consisting of sonar devices, mine detection devices,
hydrophone devices, ultrasonic therapy devices, ultrasonic body fat
measuring devices, ultrasonic imaging devices, ultrasonic devices
for assessing the structural properties of wooden members,
ultrasonic devices for measuring animal backfat, ultrasonic devices
for fill level monitoring, ultrasonic devices for flow rate
measurement, and ultrasonic devices for inspecting rods used in the
core of nuclear reactors.
30. The method of claim 1, wherein the exposing step (d) includes
establishing sequentially controlled first and second temperature
zones wherein the zones define a temperature gradient therebetween
and wherein the temperature gradient moves relative to said
crucible.
31. The method of claim 1, wherein the furnace is a vertical
Bridgman furnace.
32. The method of claim 16, wherein the exposing step (d) includes
establishing sequentially controlled first and second temperature
zones wherein the zones define a temperature gradient therebetween
and wherein the temperature gradient moves relative to said
crucible.
33. The method of claim 16, wherein the furnace is a vertical
Bridgman furnace.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/380,232, filed Mar. 11, 2003, which is the national phase of
PCT Application No. PCT/US01/28592, filed Sep. 12, 2001, and
designating inter alia the United States, which claims priority to
U.S. Provisional Application No. 60/231,635, filed Sep. 11,
2000.
BACKGROUND OF INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to the growth of single crystals of
lead magnesium niobate-lead titanate (PMN-PT) grown near the
morphotropic phase boundary. These ferro-relaxor single crystals
are useful in various military, medical, and industrial sonic
transduction applications.
[0004] 2. Brief Description of the Prior Art
[0005] Piezoelectric materials are materials that expand and/or
contract when an electric field is applied to them. Conversely,
they also will produce an electric field across themselves if a
mechanical force is applied to them. Common uses for materials
possessing the piezoelectric effect are in gas lighters, high
frequency speakers, weighing devices, and micro-positioners, to
mention a few. The piezoelectric effect occurs in materials with an
asymmetric crystal structure. When an external force is applied,
the charge centers of the crystal structure separate creating
electric charges on the surface of the crystal. As mentioned above,
the effect is also reversible. Electric charges on the crystal will
cause a mechanical deformation of the crystal. Quartz, turmalin,
and seignette are common natural piezoelectric materials.
[0006] One area of emphasis in the art of piezoelectric materials
has been directed to making polycrystalline ceramic piezoelectrics
because physical properties can be tailored to a given application.
Common ceramic piezoelectric materials are lead-zirconate-titanate
(PZT) and lead-magnesium-niobate (PMN). Piezoelectric materials
typically deform linearly in response to an applied electric field.
Because the strains in piezoelectrics are small, piezoelectric
actuators are used mainly in speakers or precision
micro-positioning applications where small and precise motion is
needed.
[0007] Piezoelectric ceramics are currently the material of choice
for ultrasonic transducer applications, offering relatively high
coupling, a wide range of dielectric constants, and low dielectric
loss. These merits translate into transducer performance in the
form of relatively high sensitivity, broad bandwidth, impedance
matching, and minimal thermal heating.
[0008] Relaxor-ferroelectrics are similar to piezoelectrics except
that strain is produced by a second order electrostrictive effect
as opposed to the first order effect. The most widely studied
relaxor material is the PMN-PT solid solution system. This material
is among the most widely used so-called "smart" materials.
Achieving high quality single crystals of solid solution lead
magnesium niobate-lead titanate (PMN-PT) greatly enhances the
electromechanical properties of the PMN-PT as compared to the
ceramic form. Obtaining high quality single crystals of PMN-PT is a
significant objective in the art of piezoelectric materials.
[0009] PMN-PT has been grown by several techniques including flux,
top seeded flux, and vertical Bridgman methods. To date, the
vertical Bridgman method has been the most successful in producing
large single crystals of PMN-PT. As shown in FIG. 1, a crucible
system 10 with a flat bottom section 11 may used in the vertical
Bridgman method. In this example, a piece of an oriented crystal
seed 16 is placed at the bottom 11 of a platinum crucible 12 with a
hermetically sealed lid 18 to seed crystal growth. During crystal
growth, a crystal 14 forms from solidification of the melt, and the
orientation of the seed crystal is propagated throughout the entire
crystal. An important feature of the vertical Bridgman method is
that the crystal shape is controlled by the crucible shape.
However, the Bridgman method has several disadvantages, including
crucible-melt compatibility (chemical and thermal), the presence of
mechanical stress (crucible/crystal contraction), compositional
segregation (stoichiometry, impurities), crucible cost, and removal
of the crystal from the crucible.
[0010] Another approach to grow single crystal of PMN-PT is shown
in FIG. 2, which shows a crucible system 20 with a conical bottom
section 21. In this case, the use of a seed is optional. As the
melt solidifies at the bottom 21 of the inverted cone shape of a
platinum crucible 22, several grains of various orientations form
simultaneously. One of the grains will grow faster than the others
and eventually choke off the growth of the other grains as the
solidified crystal propagates upward in the inverted conical
section of the crucible. The result is the formation of a crystal
having a major grain 24 and one or more minor grains 26. This
process is known as grain selection.
[0011] The grain selection method has several disadvantages. The
seed is usually made of the same material of the crystal being
grown and, therefore, has the same melting point. Special
precautions and crucible design must be employed to prevent the
melting of the seed during the initial period of the growth. With a
conical crucible bottom 21, a large portion of the total length of
the grown crystal is taken up by the conical section rather than
the desired full diameter cylindrical section. In addition, a
portion of the conical section consists of several grains before
the point of a single crystal is obtained.
[0012] In current practice, crucibles are made from very thin
platinum metal. After the starting materials are placed in the
crucible, a platinum lid is welded to the crucible to form a
hermetically sealed container. The sealed crucible prevents lead
oxide vapor from escaping from the crucible during growth. The wall
of the crucible is thin because the coefficient of thermal
expansion of the platinum is greater than that of the PMN-PT. As
the crystal cools, the contraction of the crucible is greater than
that of the crystal resulting in the creation of compressive forces
within the crystal. The thin crucible wall partially relieves these
stresses.
[0013] Such methods for producing PMN-PT single crystals are
disclosed in U.S. Pat. No. 5,998,910 to Park et al., the disclosure
of which is herein incorporated by reference.
[0014] There remains a need for a method to produce PMN-PT single
crystals that does not rely on seeds or a conical bottomed
crucible, but achieves a single crystal of specific orientation
from a flat-bottomed crucible in such a way that it is easily
removed from the crucible.
[0015] These and other advantages of the present invention will be
clarified in the description of the preferred embodiment taken
together with the attached drawings in which like reference
numerals represent like elements throughout.
SUMMARY OF THE INVENTION
[0016] The present invention is directed to a method of making a
single grain crystal of lead magnesium niobate-lead titanate
(PMN-PT). The method includes:
[0017] (a) providing a crucible containing PMN-PT starting
materials;
[0018] (b) placing the crucible containing the PNM-PT starting
material into a furnace having at least two temperature zones and
pressurizing the interior of the furnace with an inert gas;
[0019] (c) establishing at least:
[0020] (i) a first temperature zone having a temperature higher
than the melting temperature of the PMN-PT material; and
[0021] (ii) a second temperature zone having a temperature less
than the melting temperature of the PMN-PT material; and
[0022] (d) exposing the crucible and PMN-PT starting material
sequentially to said first temperature zone and then said second
temperature zone.
[0023] The present invention is further directed to single grain
single crystals of PMN-PT prepared using the above-described
method.
[0024] The present invention is also directed to electroacoustic
transducers that include the single crystal of PMN-PT described
above. In addition, the present invention is directed to devices
that include such an electroacoustic transducer to detect or
generate acoustic waves.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows a cross section of a prior art flat bottomed
crucible using a seed of the prior art;
[0026] FIG. 2 shows a cross section of a prior art conical-shaped
crucible using grain selection; and
[0027] FIG. 3 shows a cross section of a crucible configuration of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0028] Unless otherwise indicated, all numbers or expressions
referring to quantities of ingredients, reaction conditions, etc.
used herein are to be understood as modified in all instances by
the term "about."
[0029] Various numerical ranges are disclosed in this patent
application. Because these ranges are continuous, they include
every value between the minimum and maximum values. Unless
expressly indicated otherwise, the various numerical ranges
specified in this application are approximations.
[0030] The method of making a single grain PMN-PT crystal of the
present invention is unique in that it does not rely on seeds or a
conical-bottomed crucible. The present method achieves a single
crystal of specific orientation from a flat-bottomed crucible. The
starting materials used to make PMN-PT crystals include high purity
(>99.9%) powders of Pb3O4, MgCO3, Nb2O5, and TiO2. Raw powders
of the starting materials are weighed in desired molar ratios with
excess Pb3O4 as a flux. The powder is dry mixed for a desired
period of time using a tumbling mill. In accordance with the
present invention, the PMN-PT starting materials are placed in a
flat-bottomed crucible system 30 as shown in FIG. 3. An open
topped, cylindrically shaped iridium crucible 34 having a flat
bottom 35, substantially vertical sides 37 is covered with a
loosely fitted alumina disk 32 and placed in a high-pressure
vertical Bridgman furnace. The furnace is pressurized with an inert
gas, optionally with a small partial pressure of oxygen. The
contents of the crucible are heated, melted, and solidified by
moving the crucible downward through and out of the heated zones of
the furnace. The process provides a single crystal 34, which does
not exhibit a visible region of grain selection as would be
expected toward the bottom of the crucible in prior art methods.
The purpose of the high pressure is to suppress the evaporation of
lead oxide vapors. This feature eliminates the inconvenient
requirement that the crucible be welded with a hermetic seal as in
prior art methods.
[0031] Any suitable pressure may be used in the present method of
making PMN-PT single crystals. The pressure in the vertical
Bridgman furnace may be at least 10 atmospheres, in many cases, at
least 50 atmospheres, and typically, at least 75 atmospheres. When
the pressure is too low, the evaporation of lead oxide vapors may
not be adequately suppressed. The pressure in the vertical Bridgman
furnace may be up to 500 atmospheres, in many cases, up to 250
atmospheres, and typically, up to 100 atmospheres. The upper limit
of pressure is determined by the pressure rating of the vertical
Bridgman furnace. The pressure applied in the present method of
making PMN-PT single crystals may vary in any range of values
inclusive of those stated above.
[0032] Any suitable inert gas may be used to pressurize the
vertical Bridgman furnace in the present invention. Examples of
suitable inert gases include, but are not limited to, helium, neon,
argon, krypton, and xenon. In one presently preferred embodiment of
the present invention, the inert gas is argon.
[0033] The temperature ranges in the vertical Bridgman furnace are
selected such that, as the flat-bottomed crucible 34 containing
PMN-PT starting materials descends the vertical length of the
vertical Bridgman furnace, the starting materials melt and
crystallize as a PMN-PT single crystal. The first temperature zone
encountered, which is the temperature zone nearest the top of the
vertical Bridgman furnace, exceeds the melting point of the PMN-PT
starting materials. Thus, when the starting material laden crucible
34 encounters the first temperature zone, the starting materials
melt. As the crucible continues to descend, it is exposed to
successive temperature zones. At least one of which, a second
temperature zone, is lower than the melting point of the PMN-PT
starting materials. The bottom 35 of the crucible is first exposed
to the second temperature zone, and crystallization of the PMN-PT
begins at the bottom of the crucible. The single crystal continues
to grow toward the top of the crucible as the crucible continues to
descend the length of the vertical Bridgman furnace, until the
entire crucible is exposed to the temperatures of the second
temperature zone or lower temperatures.
[0034] The temperature of first temperature zone of the vertical
Bridgman furnace is greater than 1,300.degree. C., in many cases at
least 1,350.degree. C., typically at least 1,375.degree. C. and in
some cases at least 1,400.degree. C. When the temperature of the
first temperature zone is too low, the PMN-PT materials may not
melt completely, precluding formation of the desired PMN-PT single
crystal material. The temperature of the first temperature zone of
the vertical Bridgman furnace is not more than 1,800.degree. C., in
many cases not more than 1,700.degree. C., typically not more than
1,600.degree. C. and in some cases not more than 1,500.degree. C.
The materials of construction of the vertical Bridgman furnace
limit how high the temperature of the first temperature zone may
be. When it is too high, the vertical Bridgman furnace can become
damaged. In a presently preferred embodiment, the temperature of
the first temperature zone is 1,420.degree. C. The temperature of
the first temperature zone may vary in any range of values
inclusive of those stated above.
[0035] The temperature of the second temperature zone of the
vertical Bridgman furnace is not more than 1,300.degree. C., in
many cases not more than 1,290.degree. C., typically not more than
1,275.degree. C. and in some cases not more than 1,250.degree. C.
When the temperature of the second temperature zone is too high,
the PMN-PT materials may not crystallize completely, precluding
formation of the desired PMN-PT single crystal material. The
temperature of the second temperature zone of the vertical Bridgman
furnace is not less than 1,000.degree. C., in many cases not less
than 1,100.degree. C., typically not less than 1,150.degree. C. and
in some cases not less than 1,200.degree. C. When the temperature
of the second temperature zone is too low, the desired temperature
gradient cannot be achieved and a polycrystalline material or
multi-grain crystal may result. In a presently preferred
embodiment, the temperature of the second temperature zone is less
than 1,300.degree. C. The temperature of the second temperature
zone may vary in any range of values inclusive of those stated
above.
[0036] The temperature of the first temperature zone and the
temperature of the second temperature zone create a temperature
gradient in the vertical Bridgman furnace. The temperature gradient
in the vertical Bridgman furnace is at least 10.degree. C./cm, in
many cases at least 15.degree. C./cm, typically at least
17.5.degree. C./cm and in some cases at least 20.degree. C./cm.
When the temperature gradient is too low, a polycrystalline
material or multi grain crystal may result and not the desired
PMN-PT single crystal material. The temperature gradient of the
vertical Bridgman furnace is not more than 50.degree. C./cm, in
many cases not more than 45.degree. C./cm, typically not more than
40.degree. C./cm and in some cases not more than 35.degree. C./cm.
When the temperature gradient is too high, a polycrystalline
material or multi grain crystal may result and not the desired
PMN-PT single crystal material. In a presently preferred
embodiment, the temperature gradient is 25.degree. C./cm. The
temperature gradient of the vertical Bridgman furnace may vary in
any range of values inclusive of those stated above.
[0037] The rate at which the crucible is moved downward through the
vertical Bridgman furnace is at least 0.2 mm/hour, often times at
least 0.3 mm/hour and typically 0.4 mm/hour. When the descent of
the crucible is too slow, the production of the PMN-PT single
crystal may not be economical on a commercial scale. The rate at
which the crucible is moved downward through the vertical Bridgman
furnace is not faster than 12 mm/hour, often not faster than 10
mm/hour, typically not faster than 8 mm/hour and in some cases not
faster than 5 mm/hour. When the descent of the crucible is too
fast, a polycrystalline material or multi-grain crystal may result
and not the desired PMN-PT single crystal material. In a presently
preferred embodiment, the rate of descent is 0.4 mm/hr. The rate of
descent of the crucible in the vertical Bridgman furnace may vary
in any range of values inclusive of those stated above.
[0038] Of course, as will readily occur to persons skilled in the
art, rather than moving the crucible through stationary first and
second temperature zones as described herein, the crucible may,
alternatively, be stationary while the temperature gradient
produced by the first and second temperature zones is movable
relative to the crucible. This could be accomplished by way of a
suitable, known furnace control system to sequentially regulate the
heat input along the vertical height of the stationary crucible so
as to duplicate the moving temperature gradient of the first and
second temperature zones as described hereinabove.
[0039] The PMN-PT single crystal produced by the present method is
at least 0.1 inches, in some cases at least 0.25 inches, typically
at least 0.5 inches and in some cases at least 1 inch in diameter.
The diameter of the PMN-PT single crystal will be dictated by its
end use application. The PMN-PT single crystal is not more than 8
inches, in some cases not more than 6 inches, typically not more
than 5 inches and in some cases not more than 4 inches in diameter.
When the diameter of the forming single crystal is too large, a
proper gradient and freeze front may not be maintained in the
PMN-PT material and a polycrystalline material or multi grain
crystal may result and not the desired PMN-PT single crystal
material. The diameter of the PMN-PT single crystal may vary in any
range of values inclusive of those stated above.
[0040] The thickness of the PMN-PT single crystal produced by the
present method is at least 0.1 inches, in some cases at least 0.2
inches, typically at least 0.3 inches and in some cases at least
0.4 inches. The thickness of the PMN-PT single crystal will be
dictated by its end use application. The thickness of the PMN-PT
single crystal is not more than 5 inches, in some cases not more
than 4 inches, typically not more than 3 inches and in some cases
not more than 2 inches. When the thickness of the forming single
crystal is too large, a proper gradient may not be obtainable due
to the size of the vertical Bridgman furnace employed. The
thickness of the PMN-PT single crystal may vary in any range of
values inclusive of those stated above.
[0041] In a presently preferred embodiment, the PMN-PT single
crystal is 2 inches in diameter and 0.5 inches thick.
[0042] The crystals produced using the method of the present
invention are compositions and crystallographic orientations of
single crystals of (n-I)PMN-(n)PT solid solutions, which obtain
ultrahigh electromechanical coupling and avoid induced phase
transitions during transducer fabrication and driving. The
resulting crystals display a single crystal of orientation
substantially in the [110] direction. Grain selection occurs very
rapidly, and a region of grain selection is not apparent from the
first to solidify material in contact with the bottom of the
crucible in the present method. This is an unexpected finding, as
crystals grown under these conditions typically grow
polycrystalline, and if large grains are achieved, they occur after
a period of grain selection where dominant grains grow at the
expense of smaller ones. For example, prior art processes leave a
region where the dominant grain growth can be observed and the
complete grain may be only 2 inches in diameter and 0.5 inches in
length. Thus, the resulting crystal is typically a single grain
PMN-PT crystal.
[0043] Not wishing to be limited to any single theory, it is
believed that the properties of the iridium material of crucible 34
may provide an explanation for the unexpected advantages of the
present method. Iridium (Ir) and lead (Pb) may form an Ir-Pb phase,
which is a solid at and above the melting point of the PMN-PT
material. The lead oxide in PMN-PT tends to decompose, leaving
elemental lead to react with the iridium in the crucible to form a
thin layer of Ir-Pb. Assuming that the Ir-Pb layer forms with an
orientation of [100], the b and c axes would lie in the layer
forming a 4 angstrom by 5.6 angstrom lattice. PMN-PT growing in
orientation [110] has planes normal to that direction which also
have the lattice constants of 4 angstroms.times.5.6 angstroms.
Thus, the Ir--Pb layer forms a hetero-seeding layer with matched
crystal lattice dimensions to the PMN-PT. This process is referred
to as Crucible Interface Seeding.
[0044] The addition of PT, which has a Curie point of 490.degree.
C., to pure PMN, shifts the Curie point, Tc, of the composition
toward higher temperatures. A region of the phase diagram of PMN-PT
(temperature versus ratio of PMN to PT) includes a morphotropic
phase boundary (MPB). The MPB lies between 0.32-0.35 fractional PT
composition and is piezoelectric in nature. Materials with this
composition are excellent candidates for piezoelectric transducers,
due to optimization of their physical properties.
[0045] Compositionally, the PMN-PT single crystals made using the
method of the present invention lie near the MPB between the
tetragonal and rhombohedral phases. MPB compositions have
anomalously high dielectric and piezoelectric properties as a
result of enhanced polarizability arising from coupling between two
equivalent energy states, i.e., the tetragonal and rhombohedral
phases, allowing optimum domain reorientation during the poling
process. Further modifications using acceptor and donor dopants
give a wide range of piezoelectric compositions. The
electro-mechanical properties of the PMN-PT single crystal are
enhanced when it is grown near the MPB.
[0046] The present invention is also directed to electroacoustic
transducers that include the single crystal PMN-PT prepared using
the method described above.
[0047] In addition, the present invention is directed to devices
that include such electroacoustic transducers to detect or generate
acoustic waves. Such devices are typically utilized in various
military, medical, and industrial sonic transduction
applications.
[0048] Examples of military sonic transduction applications
include, but are not limited to, sonar devices, mine detection
devices, and hydrophone devices. Examples of medical sonic
transduction applications include, but are not limited to,
ultrasonic therapy devices, ultrasonic body fat measuring devices,
and ultrasonic imaging devices. Examples of industrial sonic
transduction applications include, but are not limited to,
ultrasonic devices for assessing the structural properties of
wooden members, measuring animal backfat, fill level monitoring,
flow rate measurement, and for inspecting rods used in the core of
nuclear reactors.
[0049] The present invention has been described with reference to
specific details of particular embodiments thereof. It is not
intended that such details be regarded as limitations upon the
scope of the invention except insofar as and to the extent that
they are included in the accompanying claims.
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