JPH025720B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing rutile (TiO ₂ ) single crystal.

[Problems to be solved by the prior art and the invention]

Calcite, which has a large birefringence and is used in high-precision optical equipment parts such as polarizing prisms, is a natural product because the synthesis technology has not been fully established. However, high-quality natural calcite has recently been in short supply, and its low hardness causes problems such as inconvenience in handling.

Therefore, rutile (TiO ₂ ), which has a high birefringence index of 0.3, has recently attracted attention and is expected to be a substitute for calcite. However, the rutile currently on the market is a product grown using the Bernoulli method, and has drawbacks such as large optical distortion due to thermal distortion and many products containing bubbles, making it unsuitable as a component for high-precision optical equipment. It is something.

Attempts have also been made to grow crystals using other melt growth methods, such as the pulling method. Aggregated structures often appear, and only crystals of inferior quality to the Bernoulli method crystals are obtained.

Therefore, the present invention seeks to propose a method for producing an optically undistorted and bubble-free rutile single crystal that can also be used as a component of high-precision opto-mechanical equipment.

[Reason why good quality crystals could not be obtained by conventional methods]

In titanium oxide, the existence of a series of independent crystal phases with similar crystal structures, expressed as TinO ₂ n _-1 (n is an integer of 1 or more), is known to exist due to the coexistence of Ti ⁴⁺ and Ti ³⁺ . It is being Each phase exists stably only under a specific range of oxygen partial pressure. Therefore, when a crystal solidifies from a melt, the composition of the solidified crystal, that is, the crystal structure, depends on the oxygen partial pressure in the atmosphere. That is, when the oxygen partial pressure is high, a crystal with a large n is obtained, and when the oxygen partial pressure is low, a crystal phase with a small n is obtained. Depending on the value of the oxygen partial pressure, two types of crystal phases having adjacent compositions may crystallize simultaneously. In such a case, it is obvious that a single crystal cannot be obtained even if ordinary crystal growth techniques from melt are applied.

The crystal phase of the composition TinO ₂ n _-1 is, when n is large,
That is, when the composition is very close to TiO ₂ , it easily changes to a rutile structure while remaining a single crystal by slow cooling or annealing. This is because the structure of TinO ₂ n _-1 has oxygen-deficient lattice defects introduced into the rutile structure in an orderly manner, and these are easily removed by oxygen diffusion at low temperatures. Furthermore, under the same oxygen partial pressure, the lower the temperature, the lower the allowable content of Ti ³⁺ . Therefore,
When the crystals solidified in the above-mentioned two-phase coexistence state are slowly cooled to room temperature, they contain a subgrain structure even though they all have a rutile structure. This is the reason why good quality crystals have not been obtained by conventional melt growth methods.

Therefore, the means to obtain high-quality rutile crystals without subgrain structure is to supply oxygen partial pressure such that only one type of crystal phase is crystallized during solidification.

[Means for solving problems]

The present invention is based on the above-mentioned concept, and is designed to grow a single crystal of titanium oxide from a melt while maintaining the oxygen partial pressure in the atmosphere within a range of 3 x 10 ^-2 atmospheres or less. .

The present invention has revealed for the first time that the oxygen partial pressure range in which only one type of titanium oxide crystallizes falls within this range.

The method for growing crystals from a melt applied in the present invention may be any method such as the pulling method, Bernoulli method, Brigeman method, or floating zone method, but since it is easy to achieve high quality grown crystals, The zone method is preferred, and the floating zone method, which is free from impurity contamination from the container, is particularly preferred.

TiO ₂ as a starting material used in the present invention may be a commercially available special grade reagent, but as a raw material for optical crystals, the higher the purity, the better. In addition, depending on the use, physical
Fe, Ni, Co,
Mn, Cr, Li, Mg, Cu, Zn, Cd, Al, Ga, V,
Small amounts of elements such as Nb, Ta, Si, Ge, Zr, Hf, Mo, and W can be added.

The crystal growth rate in the present invention is 0.1 to 300 mm/
time, preferably 0.5 to 15 mm/hour, particularly preferably 3 to 8 mm/hour.

In the present invention, in order to achieve an oxygen partial pressure range of 3×10 ^-2 atmospheres or less in the crystal growth atmosphere, a predetermined amount of free oxygen must be released by thermal equilibrium reaction or thermal decomposition reaction at a high temperature near the crystal growth region _. Gases such as NOx and NOx are used. In order to correctly set the oxygen partial pressure range, the higher the purity of these gases, the better.

Further, in order to realize the oxygen partial pressure range, a mixed gas consisting of oxygen and an inert gas may be used.
The inert gas is a rare gas element such as Ar or nitrogen. It is preferable that these inert gases have higher purity in order to correctly set the oxygen partial pressure range.

The crystals obtained by the present invention have a composition of approximately TiO _2.0 during the cooling process to room temperature, but they contain Ti ³⁺ to an extent that is difficult to detect by analysis, and _their transparency may be insufficient. be. In such cases, the crystal
Transparency can be increased by annealing in air or oxygen at temperatures below 1000°C. Annealing temperature is 600~
The temperature is 1000°C, preferably 700-900°C. The longer the annealing temperature, the better, but it is important to increase production efficiency.
3 to 100 hours to ensure sufficient transparency.
Preferably it is 5 to 70 hours.

Example 1 Commercially available TiO ₂ (99.98%) powder was rubber press molded into a rod shape under a hydrostatic pressure of 1 ton/cm ² and sintered in air at 1400°C. This was loaded as a raw material rod into a condensing floating zone method single crystal manufacturing apparatus using a spheroidal mirror, and a separately prepared rutile single crystal was loaded as a seed crystal. In order to control the oxygen partial pressure in the atmosphere, CO ₂ was introduced into the crystal growth chamber and continued to flow at a flow rate of 2/min until the crystal growth was completed. Black crystals were obtained.

The growth conditions are that the rotation speed of the raw material rod and the seed crystal is 25 times/min in opposite directions, and the crystal growth rate is 5 mm/min.
It was hot at the time. The obtained crystal was annealed in air at 800°C for 48 hours to obtain a slightly yellowish transparent crystal. A sample with planes parallel to the growth direction and the direction perpendicular to it is cut out from this crystal, and after optical polishing and examination using a polarizing microscope, it is a high-quality rutile single crystal with no detectable distortion, bubbles, or subgrain structure. has become clear.

Example 2 In exactly the same operation as Example 1, ultra-high purity Ar and O ₂ were used to control the oxygen partial pressure in the atmosphere.
A mixed gas mixed at a ratio of 100:3 was introduced into the crystal growth chamber and continued to flow at a flow rate of 2/min until the end of growth. Other operations were performed in exactly the same manner as in Example 1,
A high-quality rutile single crystal with almost no detectable distortion, bubbles, subgrain structure, etc. was obtained.

〔Effect of the invention〕

As is clear from the above examples, the feature of the present invention is that when growing a rutile single crystal from a melt,
The purpose is to crystallize only one type of crystal phase by maintaining the oxygen partial pressure in the atmosphere within a range of 3 x 10 ^-2 atmospheres or less, and to obtain a high-quality rutile single crystal without subgrain structure.

On the other hand, if oxygen, air, etc. are supplied to make the oxygen partial pressure higher than the range limited in the present invention, or if ultra-high purity Ar is supplied to make the oxygen partial pressure almost zero, crystals will crystallize. contains many subgrain structures, and high quality crystals cannot be obtained.
Although it is difficult to determine the lower limit of oxygen partial pressure, ultra-high purity Ar and Ar containing 100ppm _O2 are used.
When an atmosphere with an oxygen partial pressure of 10 ^-6 atmospheres was created by mixing at a ratio of 100:1 and crystals were grown in exactly the same manner as in Example 1, subgrains were locally detected, but mostly good quality rutile was grown. A single crystal was obtained.

Claims (1)

[Claims] 1. In a method of growing crystals from a melt obtained by melting TiO ₂ at high temperature, the oxygen partial pressure in the atmosphere is
A method for producing a rutile single crystal characterized by maintaining the temperature within a range of ×10 ^-2 atmospheres or less. 2. Claims characterized in that the oxygen partial pressure in the atmosphere is controlled to a range of 3×10 ^-2 atmospheres or less by introducing CO ₂ or NOx that releases a small amount of oxygen into the vicinity of the melt and the growing crystal. 2. The method for producing a rutile single crystal according to item 1. 3 Introducing a mixed gas of oxygen and inert gas near the melt and growing crystal to increase the oxygen partial pressure in the atmosphere by 3
The method for producing a rutile single crystal according to claim 1, characterized in that the pressure is controlled to a range of 10 ^-2 atmospheres or less.