JP2004284889A - Hydrothermal synthetic method of potassium niobate crystal or crystal thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To grow a single domain large size single crystal and a single crystal film, to manufacture an engineered domain, and to provide a hydrothermal synthetic method of a potassium niobate single crystal and a single crystal film, which can be used as a functional material for a piezoelectric element, an optical element or the like. <P>SOLUTION: The hydrothermal synthetic method of the potassium niobate crystal comprises adding K<SB>2</SB>NbO<SB>3</SB>F into deionized water, then keeping the resulting solution at a high temperature and high pressure so as to crystallize KNbO<SB>3</SB>from the K<SB>2</SB>NbO<SB>3</SB>F solution, and forming and growing KNbO<SB>3</SB>. When the crystal is grown in a temperature range of 225-435°C, an orthorhombic crystal having engineered domains in the crystal is obtained, and a single domain crystal is synthesized at a growing temperature of 100-225°C. Further, in a system that a single crystal substrate or the like is immersed in deionized water, the potassium niobate crystal grows as a single crystal thin film on the substrate. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
[Industrial applications]
The present invention relates to a method for producing a crystal or a crystalline thin film of potassium niobate (KNbO ₃ ) used for a piezoelectric element, an optical element or the like by hydrothermal synthesis.
[0002]
[Prior art]
Potassium niobate (KNbO ₃ ) is a substance exhibiting ferroelectric properties, and various physical properties, crystal growth, crystal structure, and the like have been reported. Further, potassium niobate crystals have attracted attention as wavelength conversion optical crystals for blue lasers because of their nonlinear optical constants, and blue region solid-state lasers incorporating potassium niobate crystals are also commercially available. Recently, potassium niobate crystals have been reported to exhibit large electromechanical coupling coefficients in surface acoustic waves and bulk waves, and are expected as next-generation piezoelectric materials.
[0003]
In the conventional method, potassium niobate crystals are grown by crystal growth from the melt, but crystal defects are likely to be introduced during crystal growth. One of the causes of defect introduction is that potassium niobate exhibits inconsistent melting. It is difficult to efficiently grow a uniform crystal under the condition that the melt composition is different from the crystal composition during the growth, and the melt composition changes as the crystal grows. One of the causes of defect introduction is that the cubic KNbO ₃ undergoes a phase transition at 435 ° C. to a tetragonal phase at 435 ° C. during crystal growth to an orthorhombic phase at 225 ° C. Due to the cubic → tetragonal → orthorhombic phase transition, even if a single crystal can be grown, many types of domains are generated during cooling, making subsequent single domain processing difficult, and during single domain processing Cracks tend to occur.
[0004]
Cracks tend to occur as the diameter of the grown crystal increases, and when the growth diameter increases, single-domain processing without cracks can hardly be performed. Therefore, in crystal growth from the melt that passes through two phase transition points during cooling and subsequent single-domain processing, a large-diameter single-domain orthorhombic potassium niobate crystal useful as a piezoelectric element is formed. It is difficult to manufacture. Further, since the introduction of crystal defects is inevitable, the maximum diameter of the crystal growth is 20 to 30 mm.
[0005]
However, considering the use in piezoelectric applications utilizing surface acoustic waves, bulk waves, and the like, including optical applications, large-diameter single crystals of 50.8 mm (2 inches) or more are grown similarly to other piezoelectric crystals. It is necessary to produce a single-domain single crystal by a subsequent polarization treatment, or to produce a single-domain single crystal film having an area of 50.8 mm (2 inches) or more in diameter.
In addition, when many fine domains (engineered domains) appear in a crystal, piezoelectric characteristics are improved as compared with a single-domain crystal. Fabrication of engineered domains using melt-grown crystals is being studied.However, it is difficult to create engineered domains in melt-grown crystals in which the crystal under cooling has thermal strain, and it is now in a practical stage. Not reached.
[0006]
[Problems to be solved by the invention]
In hydrothermal synthesis, the crystal growth temperature can be set lower than the phase transition temperature, so that it is not affected by inconsistent dissolution or phase transition during crystal growth, etc. Suitable for creating domains. For example, it has been reported that KNbO ₃ is produced in 1 to 24 hours when KOH and Nb ₂ O ₅ are hydrothermally synthesized under conditions of about 180 ° C. and about 0.5 MPa (Lecture of the Annual Meeting of the Ceramic Society of Japan). Proceedings (2002) p. 103, Proceedings of the Autumn Meeting of the Ceramic Society of Japan, Vol. 14 (2001) p. 205).
[0007]
When the inventors repeated the reported method, 10 μm KNbO ₃ could be grown by hydrothermal synthesis for 3 days. However, when the hydrothermal synthesis time was increased to about three weeks in order to make the crystals grow larger, the grown KNbO ₃ disappeared. It is considered that the disappearance of KNbO ₃ is a result of the generated KNbO ₃ being in a metastable phase and approaching equilibrium as the hydrothermal synthesis time becomes longer and dissolved. The fact that KNbO ₃ becomes a metastable phase is also presumed from the fact that it has been reported in the proceedings of the above-mentioned lecture that the phase relationship changes when H ⁺ enters the crystal by replacing K in KNbO ₃ .
[0008]
On the other hand, the present inventors have reported a method of growing potassium niobate crystals using an aqueous solution of K ₂ NbO ₃ F (Abstracts of the Annual Meeting of the Society of Inorganic Materials, Academic Conference, Vol. 103 (2001), pp. 74-75). Materials of the 71st meeting of the 150th Committee of Technology for Acoustic Wave Devices, Japan Society for the Promotion of Science (2001) 13.1.23-24). According to the reported method, potassium niobate crystals are obtained by putting K ₂ NbO ₃ F in pure water and stirring, then evaporating the water and filtering off the precipitate. However, when grown from an aqueous solution, the crystals did not grow even after long-term growth, and satisfactory crystal quality was not obtained.
[0009]
[Means for Solving the Problems]
The present invention has been devised in order to solve such a problem. By performing hydrothermal synthesis using K ₂ NbO ₃ F, it is possible to grow single-domain large-diameter single crystals and single crystal films, and to develop engineers. It is an object of the present invention to provide a potassium niobate single crystal and a single crystal thin film which enable fabrication of an ard domain and are used as a functional material of a piezoelectric element, an optical element, and the like.
[0010]
In the hydrothermal synthesis of the present invention, in order to achieve the object, a solution in which K ₂ NbO ₃ F powder is added to pure water is maintained in a temperature range of 100 to 435 ° C. under a pressurized atmosphere, and K ₂ NbO ₃ F KNbO ₃ is crystallized from a solution in which is dissolved. When a glass plate of fused quartz or the like or a potassium niobate crystal substrate is immersed in pure water in advance, KNbO ₃ generated by hydrothermal synthesis is crystallized as a single crystal thin film on the substrate.
[0011]
[Action and Embodiment]
In general, as the solubility of a solute in a solvent increases, the crystal growth rate increases and the grown crystal diameter increases. Solute solubility increases with increasing temperature, but aqueous solutions are limited by a maximum temperature of 100 ° C. On the other hand, in the hydrothermal synthesis, since the growth temperature can be set higher, it is possible to expect an increase in the crystal growth rate and an improvement in the solubility that is effective in increasing the diameter of the grown crystal. In addition, since the potassium niobate crystal is hydrothermally grown below the phase transition temperature, the growth temperature can be controlled and a tetragonal or orthorhombic ferroelectric single crystal can be directly synthesized. The ability to directly synthesize orthorhombic crystals in the low-temperature phase without undergoing phase transition means that single-domain crystals can be produced.
[0012]
Hydrothermal synthesis is also advantageous in creating an engineered domain. That is, in the crystal grown hydrothermally, thermal distortion is small because the entire temperature gradient is small. Therefore, the nucleation density of the low-temperature phase increases when the crystal is cooled after the tetragonal crystal is first generated by controlling the crystal growth temperature. As a result, fine engineered domains are created in the potassium niobate crystal. In this regard, in the crystal growth in the aqueous solution, the generation and growth of the crystal proceeds in a temperature range of 100 ° C. or less, so that no engineered domain is generated.
[0013]
Even with hydrothermal synthesis, large-diameter potassium niobate crystals cannot be obtained by crystal growth from Nb ₂ O ₅ . For example, when hydrothermal synthesis is performed at 180 ° C. or lower using Nb ₂ O ₅ and KOH, grown KNbO ₃ disappears, and a large-diameter potassium niobate crystal cannot be obtained. It is considered that the disappearance of KNbO ₃ is caused by the fact that a part of K is replaced by H to form (K, H) NbO ₃ having high solubility.
On the other hand, in crystal growth from K ₂ NbO ₃ F, the degree of substitution of K for H is small, and it is considered that KNbO ₃ is stably present. The stable presence of KNbO ₃ is also supported by the fact that the X-ray diffraction results of the generated potassium niobate crystals agree with the potassium niobate crystals produced by melt growth. Also, it can coupled with the source in which _K 2 NbO ₃ F of KNbO ₃ is a large amount in the reaction system, loss without crystal growth of KNbO ₃ is promoted.
[0014]
In hydrothermal synthesis, an aqueous solution in which K ₂ NbO ₃ F is dissolved is obtained by maintaining a suspension in which K ₂ NbO ₃ F is added to pure water under a high-temperature and high-pressure atmosphere. The solubility of K ₂ NbO ₃ F can be changed by adjusting the temperature and pressure, and a high concentration of up to 50 g / l can be achieved. When the K ₂ NbO ₃ F aqueous solution is cooled at a slow cooling rate of 0.1 to 20 ° C./day, KNbO ₃ is crystallized, and potassium niobate crystals are grown.
[0015]
The concentration of K ₂ NbO ₃ F depends on the atmosphere, but can be changed in the range of 1 to 50 g / l, and the pressure and temperature of the atmosphere are set higher as the suspension having a higher K ₂ NbO ₃ F concentration is used. I do. The crystal growth temperature is selected from a temperature range of 235 ° C. or less for producing a single domain crystal which does not undergo a phase transition, and a temperature range of 235 to 435 ° C. for producing a potassium niobate crystal having an engineered domain. You. The atmospheric pressure is selected in the range of 0.1 to 40 MPa according to the K ₂ NbO ₃ F concentration of the suspension, the target crystal growth rate, and the grown crystal diameter.
[0016]
Embodiment 1
KNbO ₃ and KF were blended at a mass ratio of 1: 1 and the mixture was compacted and fired at 850 ° C. As a result of X-ray diffraction, the fired body was found to be a single phase of K ₂ NbO ₃ F. The fired body was mechanically pulverized to prepare a K ₂ NbO ₃ F powder.
K ₂ NbO ₃ F powder: 2 g of pure water was firstly prepared _K 2 NbO ₃ F suspension in addition to 120 ml. The solubility of K ₂ NbO ₃ F in pure water increases with the liquid temperature, and becomes a solution in which K ₂ NbO ₃ F is homogeneously dissolved at 100 ° C. or higher.
The K ₂ NbO ₃ F suspension was set in an autoclave with a volume of 200 ml. The K ₂ NbO ₃ F solution was heated and maintained at 380 ° C. in an autoclave maintained at an atmospheric pressure of 25 MPa. After heating and holding for 2 weeks, the mixture was gradually cooled, the K ₂ NbO ₃ F solution was taken out, and the product was separated by filtration. The resulting product was orthorhombic potassium niobate crystals with an engineered domain (FIG. 1). When the heating and holding were further continued, potassium niobate crystals could be grown to a large diameter that cannot be achieved by conventional melt growth.
[0017]
Embodiment 2
A K ₂ NbO ₃ F solution was prepared by adding 2 g of K ₂ NbO ₃ F prepared in the same manner as in Example 1 and 120 g of pure water together with 5 g of an orthorhombic potassium niobate seed crystal. The K ₂ NbO ₃ F solution was placed in a 200 ml autoclave, which was kept at an atmospheric pressure of 25 MPa, and heated and maintained at 385 ° C. After heating and holding for 3 weeks, seed crystals were taken out of the K ₂ NbO ₃ F solution, and potassium niobate crystals grown by hydrothermal synthesis were weighed to be 8 g.
The obtained product was a large-diameter orthorhombic potassium niobate crystal having an engineered domain. When the produced crystal was irradiated with a Nd: YAG laser, the wavelength conversion characteristics were detected, and it was confirmed that the crystal was a ferroelectric substance.
[0018]
Embodiment 3
A K ₂ NbO ₃ F solution was prepared by adding 2 g of K ₂ NbO ₃ F prepared in the same manner as in Example 1 and 120 g of pure water together with 2 g of a single-domain orthorhombic potassium niobate seed crystal. The K ₂ NbO ₃ F solution was charged into a 200 ml autoclave at an atmospheric pressure of 5 MPa, and the autoclave was heated and maintained at 180 ° C. After heating and holding for 2 weeks, single domain orthorhombic potassium niobate crystals were obtained (FIG. 2).
When the synthesized potassium niobate crystal was irradiated with a Nd: YAG laser, green light emission was observed. Green light emission indicates that wavelength conversion by piezoelectricity that changes Nd: YAG 1064 nm infrared light to 532 nm green light has occurred. From these results, it was confirmed that orthorhombic single-domain potassium niobate crystals using the orthorhombic potassium niobate crystals as seed crystals were grown.
[0019]
Embodiment 4
K ₂ NbO ₃ F was added to pure water immersed in a polished potassium niobate {100} crystal substrate, and KNbO ₃ was grown under the same conditions as in Example 3. The grown crystal was epitaxially grown on a crystal substrate as a single-crystal thin film having a thickness of 10 μm. The obtained single-crystal thin film is an orthorhombic single-domain crystal, and emits green light when irradiated with Nd: YAG laser.
[0020]
【The invention's effect】
As described above, when hydrothermal synthesis is performed using K ₂ NbO ₃ F as a raw material, it becomes possible to grow single-domain large-diameter single crystals and single-crystal films and to produce engineered domains. A potassium niobate crystal useful as a single-domain single crystal having a large diameter required for a piezoelectric element as well as an optical element can be obtained. In addition, since ferroelectric crystals are directly synthesized by hydrothermal synthesis, potassium niobate crystals that can be applied to piezoelectric applications can be manufactured at lower cost than conventional methods that require single-domain treatment after growing single crystals. Is done.
[Brief description of the drawings]
FIG. 1 is a polarizing microscope photograph showing an engineered domain of a potassium niobate crystal synthesized in Example 1. FIG. 2 is an SEM photograph showing a single domain crystal synthesized in Example 3.

Claims (2)

Pure water to which K ₂ NbO ₃ F powder is added is kept in a temperature range of 100 to 435 ° C. under a pressurized atmosphere, and KNbO ₃ is crystallized from a solution in which K ₂ NbO ₃ F is dissolved. A method for hydrothermal synthesis of potassium niobate crystals. It was added _K 2 NbO ₃ F powder in pure water by immersing the substrate, held at a temperature range of one hundred to four hundred thirty-five ° C. under a pressurized _atmosphere, a solution of elaborate dissolving K 2 NbO ₃ F a KNbO ₃ on the substrate A method for synthesizing a potassium niobate crystal thin film, characterized by crystallizing as a single crystal thin film.

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