Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2001-02-09
2002-06-11
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S081000, C117S083000, C252S06290R, C427S255120
Reexamination Certificate
active
06402835
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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 ultraviolet light having wavelengths of 390 nm or so, thus the crystals can be suitable for wide applications such as for optical disk memory, medicine and photochemical fields, and various optical measurements by using such short-wavelength light. 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.
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 the 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.
NGK Insulators, Ltd. suggested a micro pulling-down method for growing the above single crystal with 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.
Such a micro pulling-down method is useful for growing the above oxide single crystal having many components. However, for 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.
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
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.
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. The process comprises the steps of mixing raw starting 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 the niobium pentoxide powder by spray-drying the mixture to obtain granulated powder, and thermally treating the granulated powder to produce the raw material powder.
According to the invention, a single crystal of lithium potassium niobate can also be 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.
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. The process comprises the steps of mixing raw starting 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.
According to the invention, a single crystal of lithium potassium niobatelithium potassium tantalate solid solution can also be 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.
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.
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.
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 becomes 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.
The existence of secondary particles in the potassium carbonate powder or the lithium carbonate powder as a raw starting material is considered as a cause of occurrence of the above heterogeneous portions in the raw material powder. Such starting material powders are hydroscopic, 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 &mgr;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.
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.
That is, the raw material powder is obtained in such a 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 a heat treatment to react potassiun, lithium and niob
Honda Akihiko
Imaeda Minoru
Imai Katsuhiro
Murasato Masahiro
Burr & Brown
Hiteshew Felisa
NGK Insulators Ltd.
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