U.S. patent application number 09/780824 was filed with the patent office on 2001-09-06 for process for producing a raw material powder to grow a single crystal and the single crystal.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Honda, Akihiko, Imaeda, Minoru, Imai, Katsuhiro, Murasato, Masahiro.
Application Number | 20010018887 09/780824 |
Document ID | / |
Family ID | 26585271 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010018887 |
Kind Code |
A1 |
Murasato, Masahiro ; et
al. |
September 6, 2001 |
Process for producing a raw material powder to grow a single
crystal and the single crystal
Abstract
In a process for producing a raw material powder comprising
lithium potassium niobate for growing a single crystal of lithium
potassium niobate, starting raw materials comprising lithium
carbonate powder, potassium carbonate powder and niobium pentoxide
powder are mixed in a solvent, lithium carbonate powder and
potassium carbonate powder are entirely dissolved into the solvent,
lithium carbonate and potassium carbonate are deposited around
niobium pentoxide powder by spray-drying the mixture to obtain
granulated powder, and then the granulated powder is thermally
treated to produce the raw material powder.
Inventors: |
Murasato, Masahiro; (Chita
City, JP) ; Honda, Akihiko; (Aichi Gun, JP) ;
Imai, Katsuhiro; (Nagoya City, JP) ; Imaeda,
Minoru; (Nagoya City, JP) |
Correspondence
Address: |
Stephen P. Burr
BURR & BROWN
P.O. Box 7068
Syracuse
NY
13261-7068
US
|
Assignee: |
NGK Insulators, Ltd.
|
Family ID: |
26585271 |
Appl. No.: |
09/780824 |
Filed: |
February 9, 2001 |
Current U.S.
Class: |
117/13 |
Current CPC
Class: |
C30B 15/00 20130101;
C30B 29/30 20130101; C30B 15/00 20130101; C30B 29/30 20130101 |
Class at
Publication: |
117/13 |
International
Class: |
C30B 015/00; C30B
021/06; C30B 027/02; C30B 028/10; C30B 030/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2000 |
JP |
2000-34,764 |
Dec 18, 2000 |
JP |
2000-383,098 |
Claims
What is claimed is:
1. A process for producing a raw material powder comprising lithium
potassium niobate to grow a single crystal of lithium potassium
niobate, said process comprising the steps of mixing starting raw
materials comprising lithium carbonate powder, potassium carbonate
powder and niobium pentoxide powder in a solvent, entirely
dissolving the lithium carbonate powder and the potassium carbonate
powder into the solvent, then depositing lithium carbonate and
potassium carbonate around said niobium pentoxide powder by
spray-drying the mixture to obtain granulated powder, and thermally
treating the granulated powder to produce said raw material
powder.
2. A process for producing a raw material powder comprising lithium
potassium niobate to grow a single crystal of lithium potassium
niobate, said process comprising the steps of mixing starting raw
materials comprising lithium carbonate powder, potassium carbonate
powder and niobium pentoxide powder in a solvent, dissolving all of
the potassium carbonate powder and a part of the lithium carbonate
powder into the solvent, then depositing lithium carbonate and
potassium carbonate around said niobium pentoxide powder by
spray-drying the mixture to obtain granulated powder, and thermally
treating the granulated powder to produce said raw material powder,
wherein secondary particles are substantially excluded from said
raw material powder.
3. A process for producing a raw material powder comprising lithium
potassium niobate-lithium potassium tantalate solid solution to
grow a single crystal of lithium potassium niobate-lithium
potassium tantalate solid solution, said process comprising the
steps of mixing starting raw materials comprising lithium carbonate
powder, potassium carbonate powder, niobium pentoxide powder and
tantalum pentoxide powder in a solvent, entirely dissolving the
lithium carbonate powder and the potassium carbonate powder into
said solvent, then depositing lithium carbonate and potassium
carbonate around said niobium pentoxide powder and said tantalum
pentoxide powder by spray-drying the mixture to obtain granulated
powder, and thermally treating the granulated powder to produce
said raw material powder.
4. A process for producing a raw material powder comprising lithium
potassium niobate-lithium potassium tantalate solid solution to
grow a single crystal of lithium potassium niobate-lithium
potassium tantalate solid solution, said process comprising the
steps of mixing starting raw materials comprising lithium carbonate
powder, potassium carbonate powder, niobium pentoxide powder and
tantalum pentoxide powder in a solvent, dissolving all of the
potassium carbonate powder and a part of the lithium carbonate
powder into said solvent, then depositing lithium carbonate and
potassium carbonate around said niobium pentoxide powder and said
tantalum pentoxide powder by spray-drying the mixture to obtain
granulated powder, and thermally treating the granulated powder to
produce said raw material powder, wherein secondary particles are
substantially excluded from said raw material powder.
5. A process for producing a raw material powder according to claim
2 or 4, wherein secondary particles of said lithium carbonate
powder having the particle diameters of 10 .mu.m or more are
excluded from said starting raw materials by sorting said lithium
carbonate powder in dry state.
6. A process for producing a raw material powder according to claim
2 or 4, wherein secondary particles of lithium carbonate powder
having the diameter of 10 .mu.m or more are crushed in the step of
mixing.
7. A process for producing a raw material powder according to any
one of claim 1 to 4, wherein an organic binder is included in said
solvent.
8. A process for producing a raw material powder according to claim
5, wherein an organic binder is included in said solvent.
9. A process for producing a raw material powder according to claim
6, wherein an organic binder is included in said solvent.
10. A process for producing a raw material powder according to any
one of claims 1 to 4, wherein said solvent is water.
11. A process for producing a raw material powder according to
claim 5, wherein said solvent is water.
12. A process for producing a raw material powder according to
claim 6, wherein said solvent is water.
13. A process for producing a raw material powder according to
claim 7, wherein said solvent is water.
14. A process for producing a raw material powder according to
claim 8, wherein said solvent is water.
15. A process for producing a raw material powder according to
claim 9, wherein said solvent is water.
16. A process for producing a single crystal of lithium potassium
niobate, said process comprising the steps of mixing starting raw
materials comprising lithium carbonate powder, potassium carbonate
powder and niobium pentoxide powder in a solvent, entirely
dissolving the lithium carbonate powder and the potassium carbonate
powder into said solvent, then depositing lithium carbonate and
potassium carbonate around said niobium pentoxide powder by
spray-drying the mixture to obtain granulated powder, thermally
treating the granulated powder to produce a raw material powder
comprising lithium potassium niobate, then placing the raw material
powder in a crucible, melting the powder, contacting a seed crystal
against the melt, and growing a single crystal of lithium potassium
niobate by pulling down the seed crystal from the crucible
downwardly.
17. A process for producing a single crystal of lithium potassium
niobate, said process comprising the steps of mixing starting raw
materials comprising lithium carbonate powder, potassium carbonate
powder and niobium pentoxide powder in a solvent, dissolving all of
said potassium carbonate powder and a part of said lithium
carbonate powder into said solvent, then depositing lithium
carbonate and potassium carbonate around said niobium pentoxide
powder by spray-drying the mixture to obtain granulated powder,
thermally treating the granulated powder to produce a raw material
powder comprising lithium potassium niobate, substantially
excluding secondary particles of the lithium carbonate powder from
the raw material powder, then placing the raw material powder in a
crucible, melting the powder, contacting a seed crystal against the
melt, and growing a single crystal of lithium potassium niobate by
pulling down the seed crystal from the crucible downwardly.
18. A process for producing a single crystal of lithium potassium
niobate-lithium potassium tantalate solid solution, said process
comprising the steps of mixing starting materials comprising
lithium carbonate powder, potassium carbonate powder, niobium
pentoxide powder and tantalum pentoxide powder in a solvent,
entirely dissolving said lithium carbonate powder and said
potassium carbonate powder into said solvent, then depositing
lithium carbonate and potassium carbonate around said niobium
pentoxide powder and said tantalum pentoxide powder by spray-drying
the mixture to obtain granulated powder, thermally treating the
granulated to produce a raw material powder comprising lithium
potassium niobate-lithium potassium tantalate solid solution, then
placing the raw material powder in a crucible, melting the powder,
contacting a seed crystal against the melt, and growing a single
crystal of lithium potassium niobate-lithium potassium tantalate
solid solution by pulling down the seed crystal from the crucible
downwardly.
19. A process for producing a single crystal of lithium potassium
niobate-lithium potassium tantalate solid solution, said process
comprising the steps of mixing starting materials comprising
lithium carbonate powder, potassium carbonate powder, niobium
pentoxide powder and tantalum pentoxide powder in a solvent,
dissolving all of said potassium carbonate powder and a part of
said lithium carbonate powder into said solvent, then depositing
lithium carbonate and potassium carbonate around said niobium
pentoxide powder and said tantalum pentoxide powder by spray-drying
the mixture to obtain granulated powder, thermally treating the
granulated powder to produce a raw material powder comprising
lithium potassium niobate-lithium potassium tantalate solid
solution, substantially excluding secondary particles of the
lithium carbonate powder, then placing the raw material powder in a
crucible, melting the powder, contacting a seed crystal against the
melt, and growing a single crystal of lithium potassium
niobate-lithium potassium tantalate solid solution by pulling down
the seed crystal from the crucible downwardly.
20. A process for producing a single crystal according to claim 17
or 19, wherein the secondary particles of the lithium carbonate
powder having the diameter of 10 .mu.m or more are excluded from
said starting raw materials by sorting said lithium carbonate
powder in dry state.
21. A process for producing a single crystal according to claim 17
or 19, wherein the secondary particles of lithium carbonate having
the diameter of 10 .mu.m or more are crushed in the step of
mixing.
22. A process for producing a single crystal according to any one
of claims 16 to 19, wherein an organic binder is included in said
solvent.
23. A process for producing a single crystal according to claim 20,
wherein an organic binder is included in said solvent.
24. A process for producing a single crystal according to claim 21,
wherein an organic binder is included in said solvent.
25. A process for producing a single crystal according to any one
of claims 16 to 19, wherein said solvent is water.
26. A process for producing a single crystal according to claim 20,
wherein said solvent is water.
27. A process for producing a single crystal according to claim 21,
wherein said solvent is water.
28. A process for producing a single crystal according to claim 22,
wherein said solvent is water.
29. A process for producing a single crystal according to claim 23,
wherein said solvent is water.
30. A process for producing a single crystal according to claim 24,
wherein said solvent is water.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to processes for producing raw
material powders for growing single crystals of lithium potassium
niobate or lithium potassium niobate-lithium potassium tantalate
solid solution, and processes for producing the single crystals
using said raw material powders.
[0003] 2. Description of the Related Art
[0004] A single crystal of lithium potassium niobate and a single
crystal of lithium potassium niobate-lithium potassium tantalate
solid solution are remarked especially as single crystals for a
blue light second harmonic generation (SHG) device for a
semiconductor laser. The device can emit even the ultraviolet
lights having the wavelengths of 390 nm or so, thus the crystals
can be suitable for wide applications such as optical disk memory,
medicine and photochemical fields, and various optical measurements
by using such short-wavelength lights. Since the above single
crystals have a large electro-optic effect, they can be also
applied to optical memory devices using the photo-refractive
effect.
[0005] However, for an application of a second harmonic generation
device, for example, even a small fluctuation in a composition of
the single crystal may affect the wavelength of the second harmonic
wave generated by the device. Therefore, the specification of the
range of the composition required for said single crystals is
severe, and the fluctuation in the composition should be suppressed
in a narrow range. However, since the composition consists of as
many as three or four components, growing a single crystal at a
high rate is extremely difficult to achieve while controlling the
proportions of the components to be constant.
[0006] NGK Insulators, Ltd. suggested a micro pulling-down method
for growing the above single crystal with the constant
compositional proportions, for example, in JP-A-8-319191. In this
method, a raw material comprising lithium potassium niobate is put
into a platinum crucible and melted, and then the melt is pulled
down gradually and continuously through a nozzle attached to the
bottom of the crucible. An infrared irradiator and a detector are
installed underneath the nozzle. Infrared rays are irradiated to
the single crystal being pulled out through the nozzle, and the
wavelength of the second harmonic wave oscillated from the single
crystal is measured to control the composition of the single
crystal constant.
[0007] Such a micro pulling-down method is useful for growing the
above oxide single crystal having many components. However, for the
widespread uses of the above single crystal, increased industrial
productivity is indispensable. For this purpose, the capacity of
the crucible needs to be increased to grow as many single crystals
as possible.
[0008] The inventors further examined this technique and found
that, in case of mass production by enlarging the capacity of the
crucible, the fluctuation in composition, development of cracks,
inclusions and/or, in some cases, the formation of other nuclei, or
crystalline deterioration accompanying anomalous growing rate,
which hardly occurred in small-scale experiments, might be caused
depending on the particle size of the raw material powder and the
shape of the crucibles, which sometimes resulted in a yield
loss.
SUMMARY OF THE INVENTION
[0009] An object of the invention is to prevent the fluctuation in
the single crystal composition, the development of cracks or
inclusions, and the crystalline deteriorations on growing a single
crystal of lithium potassium niobate or a single crystal of lithium
potassium niobate-lithium potassium tantalate solid solution.
[0010] A first aspect of the invention relates to a process for
producing a raw material powder comprising lithium potassium
niobate to grow a single crystal of lithium potassium niobate, said
process comprising the steps of mixing starting raw materials
comprising lithium carbonate powder, potassium carbonate powder and
niobium pentoxide powder in a solvent, entirely dissolving the
lithium carbonate powder and potassium carbonate powder into the
solvent, then depositing lithium carbonate and potassium carbonate
around said niobium pentoxide powder by spray-drying the mixture to
obtain granulated powder, and thermally treating the granulated
powder to produce the raw material powder.
[0011] According to the invention, a single crystal of lithium
potassium niobate can be also grown by placing and melting the
above raw material powder in a crucible, contacting a seed crystal
against the melt, and pulling down the seed crystal from the
crucible downwardly.
[0012] The invention is also related to a process for producing a
raw material powder comprising lithium potassium niobate-lithium
potassium tantalate solid solution to grow a single crystal of
lithium potassium niobate-lithium potassium tantalate solid
solution, said process comprising the steps of mixing starting raw
materials comprising lithium carbonate powder, potassium carbonate
powder, niobium pentoxide powder and tantalum pentoxide powder in a
solvent, entirely dissolving the lithium carbonate powder and
potassium carbonate powder into the solvent, then depositing
lithium carbonate and potassium carbonate around said niobium
pentoxide powder and said tantalum pentoxide powder by spray-drying
the mixture to obtain granulated powder, and thermally treating the
granulated powder to produce the raw material powder.
[0013] According to the invention, a single crystal of lithium
potassium niobate-lithium potassium tantalate solid solution can be
also grown by placing and melting the above raw material powder in
a crucible, contacting a seed crystal against the melt, and pulling
down the seed crystal from the crucible downwardly.
[0014] In the first aspect of the invention, lithium carbonate and
potassium carbonate depositing around the niobium pentoxide powder
(and tantalum pentoxide powder, if necessary) are considered to be
in fine powder forms.
[0015] The inventors found that the above fluctuation in the
composition, cracks, inclusions and crystalline deteriorations were
caused in accordance with the state change of the raw material
powder, for example, when the capacity of the crucible was
enlarged.
[0016] For example, when the raw material powder is supplied to the
crucible, the raw material powder is melted and convected in the
crucible, and then the melt is pulled out from the crucible.
However, if the raw material powder has a heterogeneous portion,
the composition become heterogeneous topically or temporary, which
causes the fluctuation in the composition of the single crystal
pulled out from the crucible. Development of cracks, inclusions or
crystalline deteriorations may also occur topically depending on
the fluctuation in the composition.
[0017] The existence of secondary particles in the potassium
carbonate powder or the lithium carbonate powder as a starting raw
material is considered as a cause of occurrence of the above
heterogeneous portions in the raw material powder. Such starting
material powders have hydroscopic property so that they tend to
absorb moisture in air during the storage and aggregate to form the
secondary coarse particles having particle diameters from 10 to 50
.mu.m. As the secondary coarse particles cannot sufficiently react
with other components in the stage of heat treatment, they remain
as heterogeneous portions in the raw material powder.
[0018] It is found that the raw material powder of lithium
potassium niobate or lithium potassium niobate-lithium potassium
tantalate solid solution produced by the heat treatment of the
above granulated powder according to the invention can
substantially eliminate the above heterogeneous portions to
homogenize the composition of the single crystal pulled out from
the crucible and to prevent cracks, inclusions and the crystalline
deteriorations.
[0019] That is, the raw material powder is obtained in such manner
that among the starting powders, the potassium carbonate powder and
the lithium carbonate powder are dissolved and then deposited again
on the surface of the granulated powder by spray-drying and that
the granulated powder is then subjected to the heat treatment to
react potassium, lithium and niobium (and tantalum, if necessary).
It is found that the obtained raw material powder is extremely well
suited for growing the single crystal of the above systems.
[0020] From the further examination, the inventors found that, in
the step of mixing the start materials, even when a part of the
lithium carbonate powder was dissolved without entirely dissolving
the lithium carbonate powder, while the rest was left undissolved
in the solvent, the above heterogeneous portions were also
substantially eliminated to homogenize the composition of the
single crystal pulled out from the crucible and to prevent cracks,
inclusions and the crystalline deteriorations.
[0021] That is, when the lithium carbonate powder is not entirely
dissolved into the solvent in the mixing step, the dissolved
lithium carbonate deposits on the niobium pentoxide power (and
tantalum pentoxide powder, if necessary) in the granulation step.
On the other hand, the powder undissolved in the solvent during the
mixing step remains in situ in the granulated powder and, even
after the heat treatment, in the raw material powder.
[0022] Herein, the inventors found that the residual secondary
particle of the lithium carbonate powder having the particle
diameters of 10 .mu.m or more in the raw material powder might
cause the above fluctuation in the composition, cracks and
inclusions. In other words, the inventors ascertained that, in this
case, the above fluctuation in the composition, cracks and
inclusions had their cause in the formation of the secondary
particles by aggregating the lithium carbonate powder.
[0023] If such secondary particles are not substantially observed
at least at the stage of the raw material powder, the occurrences
of the above fluctuation in the composition, cracks and inclusions
can be prevented without dissolving the entire amount of lithium
carbonate powder into the solvent.
[0024] Since potassium carbonate has a high solubility to a
solvent, a small amount of the solvent can dissolve the entire
amount of potassium carbonate. In contrast, since lithium carbonate
has a lower solubility than that of potassium carbonate, a great
amount of the solvent is required to dissolve the entire amount of
lithium carbonate. Therefore, if the throughput of the raw
materials is high on dissolving the entire amount of lithium
carbonate powder, a spray-drying operation will take a longer time
and an operation efficiency will be decreased. On the other hand,
by dissolving a part of lithium carbonate powder while leaving the
rest undissolved in the solvent, the required amount of the solvent
is relatively decreased and a spray-drying operation can be also
done within a shorter period of time.
[0025] At the stage of the raw material powder, the presence of the
secondary particles of the lithium carbonate powder having the
particle diameters of 10 .mu.m or more is inspected with an
electron microscope. When ten view fields of at least 80
.mu.m.times.120 .mu.m are observed with an electron microscope
(1000 magnification), observing no or little (preferably one or
less) secondary particles of the lithium carbonate powder having
the particle diameter of 10 .mu.m or more satisfies the above
requirement.
[0026] To exclude the secondary particles of the lithium carbonate
powder having the particle diameters of 10 .mu.m or more at the
stage of the starting material powder, for example, the lithium
carbonate powder is sorted in the dry state at the stage of
measuring and preparing the starting material powder. By doing so,
when the secondary coarse particles exist in the lithium carbonate
powder, such secondary particles are excluded so that the secondary
particles of the lithium carbonate powder having the particle
diameter of 10 .mu.m or more may not be included in the starting
materials. Such a sorting method itself is commonly known.
[0027] Moreover, even when the secondary particles having the
particle diameters of 10 .mu.m or more comprising lithium carbonate
powder exist in the starting materials, such particles can be
crushed on mixing. More specifically, by increasing the hardness of
balls or agitating media, enhancing the agitation rate or
elongating the period of agitation time in a ball mill or a media
agitating mill, a condition can be set to crush the secondary
particles.
[0028] The invention can be favorably applied for producing not
only single crystal fibers, but also plates comprising the single
crystal.
[0029] When the potassium carbonate powder and the lithium
carbonate powder are entirely dissolved, the average particle
diameter of each of the potassium carbonate powder and lithium
carbonate powder is not particularly limited. Although the average
diameter of each of the niobium pentoxide powder and the tantalum
pentoxide powder is not particularly limited, it is preferably 10
.mu.m or less from the view of acquiring the homogeneous raw
material powder by the subsequent heat treatment, and preferably
0.1 .mu.m or more from the view of handling easiness.
[0030] The proportions of the constituting elements are not
particularly limited as far as the single crystal can be eventually
grown. However, when a single crystal of lithium potassium niobate
is produced, the proportions of the lithium carbonate powder, the
potassium carbonate powder and the niobium pentoxide powder are
preferably 17-27 mol %, 28-32 mol % and 43-53 mol %, respectively.
Moreover, when a single crystal of lithium potassium
niobate-lithium potassium tantalate solid solution is produced, the
proportions of the lithium carbonate powder, the potassium
carbonate powder, the niobium pentoxide powder and the tantalum
pentoxide powder are preferably 17-27 mol %, 28-32 mol % 38.7-52.5
mol % and 0.2-5.3 mol %, respectively.
[0031] The single crystal of lithium potassium niobate and that of
lithium potassium niobate-lithium potassium tantalate solid
solution may be replaced by other elements other than K, Li, Nb, Ta
and O within a range of taking a tungsten bronze structure
consisting of K, Li, Nb and, if necessary, Ta and O. For example,
Na and Rb can substitute K and Li. In this case, the substitution
rate is preferably 10 atomic % or less, when the proportion of
potassium or lithium is taken as 100 atomic %. A laser-generating
doping substance comprising such as Cr or a rare earth series
elements of Er or Nd can also be added.
[0032] According to an embodiment of the invention, the lithium
carbonate powder and the potassium carbonate powder are entirely
dissolved into a solvent. In this case, whether each powder can be
entirely dissolved depends on the solubility of that powder to the
solvent and the volume of the solvent. Therefore, a sufficient
amount of the solvent needs to be used to dissolve each powder, and
adequate agitation and mixing are preferable.
[0033] The solvent is not particularly limited, but water and acids
are especially preferable in that the solubilities of the potassium
carbonate powder and the lithium carbonate powder thereto are high.
Lithium carbonate powder has high solubilities to acids.
[0034] Also, by adding an organic binder into the solvent,
potassium carbonate powder and lithium carbonate powder
precipitated in water can easily attach to the surface of niobium
pentoxide or tantalum pentoxide in the subsequent spray-drying.
Such binders that are usable in an alkaline solution in which
potassium carbonate and lithium carbonate are dissolved, acrylic
binder, for example, are preferably used as the organic binder. The
weight ratio of the organic binder per 100 parts by weight of the
solvent is preferably 0.1-0.3 wt. %.
[0035] Such conditions as temperature, period of time and
atmosphere for the heat treatment of the granulated powder are not
particularly limited as far as each components of the granulated
powder adequately react to produce the respective compounds.
However, in general, the temperature is preferably
800-1,000.degree. C. and the holding time is at least 2 hours.
[0036] FIG. 1 is a schematic sectional view of a manufacturing
apparatus according to one example of the present invention for the
growth of single crystals.
[0037] A crucible 7 is placed in a furnace housing. An upper
furnace unit 1 is arranged to surround the crucible 7 and an upper
space 5 thereof, and has a heater 2 buried therein. A nozzle 13
extends downwardly from a bottom part of the crucible 7. An opening
13a is formed at a lower end of the nozzle 13. A lower furnace unit
3 is arranged to surround the nozzle 13 and a surrounding space 6
thereof, and has a heater 4 buried therein. Such configuration of
the heating furnace itself may be obviously varied in various ways.
The crucible 7 and the nozzle 13 are both formed of a
corrosion-resistant conductive material.
[0038] One electrode of a power source 10A is connected to a point
A of the crucible 7 with an electric cable 9, and the other
electrode of the power source 10A is connected to a lower bent B of
the crucible 7. One electrode of a power source 10B is connected to
a point C of the nozzle 13 with the electric cable 9, and the other
electrode of the power source 10B is connected to a lower end D of
the nozzle 13. These current-carrying systems are isolated from
each other and configured to control their voltage
independently.
[0039] An after-heater 12 is further located in the space 6 to
surround the nozzle 13 with a distance. An intake tube 11 extends
upward in the crucible 7 and an intake opening 22 is provided at
the upper end of the intake tube 11. The intake opening 22
protrudes from a bottom portion of a melt.
[0040] The upper furnace unit 1, the lower furnace unit 3 and the
after-heater 12 are allowed to heat for setting an appropriate
temperature distribution for each of the space 5 and space 6. Then
a raw material of the melt is supplied into the crucible 7 and the
electricity is supplied to the crucible 7 and the nozzle 13 for
heating. In this condition, at a single crystal-growing part 23
located at the bottom end of the nozzle 13, the melt 8 slightly
protrudes from the opening 13a and a relatively flat surface is
formed there. A reference numeral 30 denotes the liquid level of
the melt.
[0041] The gravity applied to the melt 8 in the nozzle 13 is
greatly reduced due to the melt contacting against the inner
surface of the nozzle 13. Especially, a uniform solid phase-liquid
phase interface can be formed by setting the internal diameter of
the nozzle 13 to 0.5 mm or less.
[0042] In this condition, a seed crystal is moved upward and the
end face of the seed crystal is contacted with the surface of the
melt. Then, the seed crystal is pulled downward. Hereupon, a
uniform solid phase-liquid phase interface (meniscus) is formed
between the upper end of the seed crystal and the melt being pulled
out from the nozzle.
[0043] As a result, a single crystal fiber 14 is continuously
formed on the upper side of the seed crystal and pulled downward.
According to this embodiment, a roller 28, which is a drive unit,
carries the single crystal fiber 14.
[0044] When the single crystal is continuously pulled out
downwardly, a laser beam having a wavelength of near
2.lambda..sub.0 is radiated from a laser source 27 to the single
crystal 14 as shown by an arrow R in FIG. 1 indicating, and a
output light S from the single crystal having a wavelength of near
a second harmonic wave.lambda..sub.0 is received by a
light-receiving unit 26 through a long wavelength cut filter 41 to
detect the intensity thereof. A signal from the light-receiving
unit 26 is transmitted to a control unit 33 through a signal line
25, wherein the signal is processed. When the measured intensity of
the output light deviates from a predetermined value, the control
unit transmits a certain signal to a raw material supplying
apparatus 24 as a feedback. In this case, a raw material powder
having a slightly different composition from the raw material
powder thrown into the crucible at first may be further thrown into
the crucible.
[0045] For more accurate control, a part of the long wavelength
light of near 2.lambda..sub.0 is measured by a combination of a
reflecting mirror 29 and the light-receiving unit 26, and the
resulting signal is transmitted to the control apparatus 33 through
the signal line 25.
[0046] A single crystal plate can be formed by changing the shape
of the nozzle 13.
[0047] Hereafter, further detailed experimental results are
described.
EXAMPLE 1
[0048] A single crystal manufacturing apparatus as shown in FIG. 1
was used to produce a single crystal plate of lithium potassium
niobate according to the invention.
[0049] Lithium carbonate powder, potassium carbonate powder and
niobium pentoxide powder were weighed to give a ratio of 19 mol
%:30 mol %:51 mol % as calculated in molar ratio of lithium atoms,
potassium atoms and niobium atoms, respectively, and then each
powder was added to water. In this case, the weight ratio of water
and the starting materials was set to 6:1 to completely dissolve
the lithium carbonate powder and the potassium carbonate powder
into water. Then, 0.3 parts by weight of an acrylic binder was
added per 100 parts by weight of water. Each powder was crushed in
a pot mill for 15 hours, then mixed and slurry was obtained. The
slurry was treated by a spray-dryer to make granulated powder.
[0050] A differential thermal analysis on a portion taken from this
granulated powder showed the reaction temperature of 690.degree. C.
Then, the granulated powder was subjected to the heat treatment in
an electric furnace at 900.degree. C. for 4 hours to give an
intended raw material powder. An electron microscope analysis on a
portion taken from this raw material power showed that the average
particle diameter of the powder was about 5 .mu.m and no secondary
coarse particles having particle diameters of 10 .mu.m or more
could be observed. A chemical composition analysis on randomly
selected particles by using an X-ray analyzer attached to the
electron microscope showed that such particles gave almost the same
analyzed values. Thus, it was suggested that lithium carbonate and
potassium carbonate precipitated around the niobium pentoxide
particles.
[0051] Then, 0.1 g of the above raw material powder was charged
into a platinum crucible which was preliminarily filled with a
molten single crystal of lithium potassium niobate, and its melted
state was examined. As a result, the powder started melting at
1050.degree. C. and entirely melted at 1050.degree. C. Also, a
chemical composition analysis on a portion taken from the above raw
material by an inductively coupled plasma method and an X-ray
fluorescence method showed that a ratio of lithium, potassium and
niobium were 19 mol %:30 mol %:51 mol %, respectively within a
range of errors by these measuring methods (the ratio agreed with
the formulated composition of the above raw materials).
[0052] A single crystal plate was formed by the micro pulling-down
method with the above raw material powder and the apparatus
previously described referring to FIG. 1. Specifically, the
temperature of whole furnace was controlled by the upper furnace
unit 1 and the lower furnace unit 3. The apparatus was configured
to control a temperature gradient near a single crystal-growing
part 23 by the electric power supply for a nozzle 13 and the heat
generation of an after-heater 12. A single crystal pulling-down
mechanism was mounted to pull down the single crystalline plate in
vertical direction, while controlling the pulling-down velocity
evenly within a range from 2 to 100 mm/hour.
[0053] Then, a growth-initiating raw material was prepared in the
same manner as the above raw material powder except that the first
mixture ratio was adjusted to 25:30:45 for lithium, potassium and
niobium, respectively, in the growth-initiating raw material
powder. The growth-initiating raw material powder was filled in a
platinum crucible 7, and then the crucible 7 was located in place.
The temperature of a space 5 in an upper furnace unit 1 was
adjusted to a range from 1100 to 1200.degree. C. to melt the raw
materials in the crucible 7. The temperature of a space 6 in a
lower furnace unit 3 was controlled evenly within
500-1000.degree.C. Predetermined electricity was supplied to the
crucible 7, the nozzle 13 and the after-heater 12 to grow single
crystals. In this case, the temperature of the single
crystal-growing part could be 980-1150.degree. C. and the
temperature gradient at the single crystal growing part could be
controlled to 10-150.degree. C./mm.
[0054] The size of an opening of the nozzle 13 was set to 1
mm.times.50 mm, and the length of the nozzle was set to 10 mm. The
plane shape of the crucible 7 was of an elliptical shape with a
major axis of 50 mm, a minor axis of 10 mm and a height of 10 mm.
In this condition, a single crystal plate having a rectangular
cross section (30 mm.times.1 mm) was pulled down at a rate of 20
mm/hour.
[0055] The raw material powder was continuously fed from a raw
material feeder 24 to the crucible 7 at a rate of 2.66 g/hour,
while the single crystal was growing. The fed raw material powder
was immediately melted in whole, and fluctuation in the
composition, cracks or inclusions which would be caused by melting
residue of the raw material powder or varied melting rates did not
occur. In addition, compositions of various parts of the single
crystal grown were evaluated by the c axis length of the crystal
lattice to give the ratio of 19 mol %:30 mol %:51 mol % for
lithium, potassium and niobium, respectively, which agreed with the
composition of the raw material powder.
COMPARATIVE EXAMPLE 1
[0056] A granulated powder was made in the same manner as in
Example 1. However, unlike Example 1, since water and the starting
raw material were mixed at a weight ratio of 6:4, the whole amount
of potassium carbonate powder was dissolved after mixing, but
lithium carbonate powder was partly undissolved to remain as
residue in the water.
[0057] A differential thermal analysis on a portion taken from the
acquired granulated powder showed the reaction temperature of
690.degree. C. The granulated powder was also subjected to the heat
treatment according to Example 1 to give a raw material powder. An
electron microscope analysis on the raw material powder showed the
presence of secondary particles of lithium carbonate having the
diameter of at least 50 .mu.m. The melting state of the raw
material powder was examined according to Example 1. The raw
material powder started melting at 1050.degree. C. and entirely
melted at 1050.degree. C. A chemical composition analysis on the
raw material powder according to Example 1 also showed the ratio of
17 mol %:31 mol %:52 mol % for lithium, potassium and niobium,
respectively, which was lacking in lithium as compared to the
composition of the mixture.
[0058] A single crystal plate was formed according to Example 1 by
using the raw material powder. The raw material powder being fed to
the crucible immediately melted in whole, and fluctuation in the
composition or cracks which would be caused by the melting residue
of the raw material powder did not occur. In addition, compositions
of the single crystal after the growing were evaluated by the c
axis length of the crystal lattice to give the ratio of 17 mol %:30
mol %:53 mol % for lithium, potassium and niobium,
respectively.
COMPARATIVE EXAMPLE 2
[0059] Starting materials were weighed according to Example 1.
Ethanol was used as a solvent. Ethanol and the starting materials
were mixed at a ratio of 6:4. The starting materials were insoluble
to ethanol. The starting materials and ethanol were crushed in a
pot mill for 15 hours, then mixed and slurry was obtained. The
slurry was put into a vat and dried in a fan dryer at 180.degree.
C. for 8 hours to produce granulated powder.
[0060] A differential thermal analysis on a portion taken from the
granulated powder showed the reaction temperature of 790.degree.
C.
[0061] Then, the granulated powder was subjected to the heat
treatment according to Example 1 to give a raw material powder. An
electron microscope analysis on the raw material powder showed the
presence of secondary particles of potassium carbonate having the
particle diameter of 100 m or more. The melted state of the raw
material powder was also examined according to Example 1. The raw
material powder started melting at 950.degree. C. and entirely
melted at 1200.degree. C. A chemical composition analysis on the
raw material powder according to Example 1 also showed that the
composition of the powder agreed with the prepared composition
within a range of errors by these measuring methods.
[0062] A single crystal plate was formed according to Example 1 by
using the above raw material powder. The fed raw material powder
was not completely melted, and fluctuation in the composition or
cracks which would be caused by the melting residue of the raw
material powder occurred.
EXAMPLE 2
[0063] Lithium carbonate powder, potassium carbonate powder and
niobium pentoxide powder were weighed according to Example 1,
except that the lithium carbonate powder was sorted in the dry
state and did not include the secondary particles of 10 .mu.m or
more. Starting materials were mixed according to Example 1, except
that the weight ratio of water and the starting materials was 6:4.
The whole amount of the potassium carbonate powder was dissolved
into water after mixing, but the lithium carbonate powder was
partly dissolved and the rest remained as a residue in the water.
Then, an acrylic binder was added to the water to give slurry via
crushing and mixing. The slurry was spray-dried to produce
granulated powder.
[0064] A differential thermal analysis according to Example 1 on a
portion taken from the granulated powder showed the reaction
temperature of 690.degree. C.
[0065] Then, the granulated powder was subjected to the heat
treatment in an electric furnace at 900.degree. C. for 4 hours to
give an intended raw material powder. An electron microscope
analysis on a portion taken from this raw material power showed
that the average diameter of the powder was about 5 .mu.m and no
secondary coarse particles having particle diameters of 10 .mu.m or
more could be observed. A chemical composition analysis on randomly
selected particles by using an X-ray analyzer attached to the
electron microscope showed that such particles gave almost the same
analyzed values. Thus, it was suggested that lithium carbonate and
potassium carbonate precipitated around the niobium pentoxide
particles.
[0066] The melted state of the raw material powder was also
examined according to Example 1. As a result, the powder started
melting at 1050.degree. C. and entirely melted at 1050.degree. C.
Also, a chemical composition analysis on a portion taken from the
raw material by an inductively coupled plasma method and an X-ray
fluorescence method showed the same results as in Example 1.
[0067] A single crystal plate was formed by the micro pulling-down
method with the above raw material powder according to Example 1.
As a result, a single crystal plate without any fluctuation in
composition, cracks or inclusions could be obtained. Compositions
of various parts of the single crystal plate were evaluated by the
c axis length of the crystal lattice to give the composition ratio
of lithium, potassium and niobium agreed with the composition of
the raw material powder.
[0068] As described hereinabove, according to the invention, when a
single crystal of lithium potassium niobate or lithium potassium
niobate-lithium potassium tantalate solid solution are grown,
fluctuation in composition of the single crystal, cracks,
inclusions and deterioration of crystallinity can be prevented.
* * * * *