JPS6228118B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a single crystal of alumina from a solid phase body of alumina all at once without passing through the melt state. Conventionally, in order to make alumina single crystal products, the alumina raw material was first melted, a homogeneous single crystal nucleus was added to the melt, and the surrounding area was gradually solidified to create a single crystal block, which was then machined. It was then made into a product. However, in this method, the raw material must be heated above its melting point, resulting in large energy losses.In addition, there are problems such as severe wear and tear on the melt container and heating furnace materials, and product contamination from the furnace materials and heaters. It was hot. In addition, the processing of the obtained single crystal lump was also subject to various constraints in terms of shape. For example, in the case of α-alumina single crystal (hereinafter referred to as "saphire"), conventional methods for melting and solidifying the alumina raw material include the Bernoulli method, the Czyocholarski method, the Schmidt-Wuichnicki method, The Bagdazarov method, the Taiko method, etc. are known. Since all of these manufacturing methods produce a melt at a temperature above the melting point of alumina (2050°C), they are subject to the various restrictions and conditions described above. That is,
Thermal efficiency is poor, temperature control is difficult, and the containers and heater materials used for molybdenum, tungsten, etc. are subject to significant wear and tear, and product contamination from the jigs inside these furnaces has become a problem. In addition, except for the tubes and ribbons produced by the Taiko method, all others are block or rod-shaped, and depending on the final product, further diamond processing is essential, which also makes the product price extremely high. . Furthermore, shapes such as crucibles and test tubes cannot be manufactured as a practical matter using conventional methods. This invention solves all of the conventional problems at once, and involves molding a raw material for alumina fine powder containing magnesia as a crystal growth inhibitor into a predetermined shape, and then high-density sintering with a porosity of 5% or less. This high-density sintered body is fired at a temperature below the melting point and at least 2/3 of the melting point expressed in absolute temperature, and the growth of crystal grains is caused to start from only one place in the sintered body. This is a method for producing an alumina single crystal, which is characterized in that the alumina single crystal is heat-treated to form a single crystal.
This invention will be further explained below. In the present invention, first, a raw material of fine alumina powder containing magnesia as a crystal growth inhibitor is formed into a predetermined shape and then fired into a high-density sintered body having a porosity of 5% or less. The method of molding the raw material of fine alumina powder and sintering it to make it densified may be the same as the conventional method. The point to be careful is to suppress abnormal growth of particles and increase the density. High density here is 95% of the theoretical density
The above refers to the porosity of the sintered body, which is 5%.
This means the following. If the density of the sintered body is lower than this, when the sintered body is then heat-treated to become a single crystal, air bubbles will be present inside the body, thereby detracting from the substantial merit of being a single crystal. This kind of high-density sintered body is then heat treated to become a single crystal, but this heat treatment
The sintering process may be continued in the same furnace in which the high-density sintered body was obtained, or the sintered body may be taken out of this furnace and placed in another furnace. It may also be carried out under heating conditions. These should be determined in consideration of the degree of difference between the conditions for obtaining a high-density sintered body and the conditions for heat treatment for further converting it into a single crystal, economic efficiency, and the like. The heat treatment temperature is below the melting point of the high-density sintered body and at least 2/3 of the melting point expressed in absolute temperature. If the temperature exceeds the melting point, the purpose of the present invention to obtain a single crystal in a solid state will not be achieved, and the melting point expressed in absolute temperature will not be achieved.
This is because if the temperature is lower than 2/3, single crystallization will be insufficient or it will take a long time. When a high-density sintered body is heat-treated to form a single crystal, it is necessary to cause grain growth to start from only one place in the sintered body. When the growth of crystal grains occurs at multiple locations rather than at just one location, the sintered body becomes an aggregate of many coarse crystals and does not become a single single crystal. When heat treating a sintered body, in order to start the growth of crystal grains from only one place, it is possible to create a temperature gradient in the furnace during the heat treatment, or to make a part of the sintered body thin. , a method such as allowing a crystal inducer to be included in a part of the sintered body in advance may be taken. Next, the single crystallization process will be explained in more detail using sapphire as an example. It has been known that when alumina porcelain is treated at high temperatures, so-called abnormal particle growth occurs and the alumina crystals become coarser, making the alumina porcelain defective as a ceramic material. In such cases, there are usually a large number of nuclei that cause abnormal particle growth, and therefore the material has the appearance of a large number of coarse crystals colliding with each other. In addition, crystal growth occurs before sintering is completed, that is, while there are many pores in the material, the crystal particles grow while incorporating the pores, so the presence of many pores within the crystal. Become. Therefore, what was obtained here was a material far from saphire. Therefore, the present invention attempts to prevent the sintered body from becoming an aggregate of coarse crystals as described above and from the presence of many pores within the crystals. That is, by including magnesia as a crystal growth inhibitor in the raw material, abnormal grain growth is prevented in obtaining a high-density sintered body, which is then turned into a single crystal in the next step. . It is thought that single crystallization of the high-density sintered body begins with the volatilization of magnesia MgO as a crystal growth inhibitor in the sintered body, and progresses further. Therefore, from the standpoint of promoting single crystallization, it is preferable that the amount of MgO added is very small unless abnormal particle growth occurs during sintering. According to the inventor's experiments, if the amount of MgO added is less than 30 ppm, abnormal particle growth tends to occur during sintering, and if it is more than 0.15%, single crystallization is difficult to proceed.
Even after heat treatment for 20 hours, a good single crystal could not be obtained. Once a high-density sintered body with suppressed abnormal grain growth is obtained, it is then heat-treated to form a single crystal. α―
In order to induce single crystallization from one end of an alumina sintered body by heat treatment, MgO in the sintered body must be
It is effective to remove by volatilization, and there are various methods for doing so. For example, making the local area where the sintered body is going to start single crystallization thinner;
Another method is to form an acute-angled ridge on the end face of the sintered body. Furthermore, a similar effect can be expected by providing a temperature gradient in the heating furnace, or by increasing or decreasing the concentration of MgO in the sintered body in a certain direction. In either case, it is necessary to start crystal growth from only one place in the sintered body, and once crystal grain growth begins here and grows into a giant crystal, fine grains adjacent to this giant crystal grain will grow. The crystal is absorbed by the giant particle due to the difference in interfacial energy, and the giant particle grows further. In addition, there is a method that takes advantage of this phenomenon and encapsulates and molds one giant particle in advance during the forming stage of the raw material powder, sinters it, and then turns the whole into a single crystal using the giant particle as a core. It is effectively employed in alumina. The attached photograph in Figure 1 captures the process in which alumina polycrystals transition to single crystals when the present invention is adopted.The right half of the figure becomes a single crystal, and the left half polycrystal grains are squeezed to form a single crystal. This shows a situation in which the government is shifting from the right to the left. According to the present invention described above, a single crystal of alumina can be obtained all at once from a solid phase body without passing through a melt state. Therefore, not only is there no loss of thermal energy for obtaining the melt, no need for a container for the melt and a heating furnace for this purpose, but also the contamination of the product is greatly reduced. It became.
Furthermore, according to the present invention, by forming the raw material into the required shape during molding, there is no need for processing after it becomes a single crystal, and complex shapes can now be easily obtained. The single crystal obtained by the present invention can be used for various purposes, and one of the applications that is currently expected to be used is the arc tube of a high-pressure sodium lamp. Until now, polycrystalline translucent alumina sintered bodies have been used in this arc tube, but the grain boundaries cause light scattering, resulting in a decrease in lamp efficiency. Furthermore, the sodium vapor sealed in the arc tube selectively corrodes grain boundaries, resulting in a shortened lamp life. According to the present invention, sapphire tubes can be obtained with almost the same ease as in the past, and in addition, for example, the
Even more transparent sapphire tubes can be provided by chemical polishing as described in No. 69354. Example 1 (Saphire) High purity alumina powder (99.9%) 100 parts Magnesium nitrate hexahydrate 0.06 parts Polyvinyl alcohol 1 part Distilled water 100 parts The above raw materials were mixed to form a slip, which was then granulated using a spray dryer. This granulated powder was molded into a tube shape using an isostatic press at a pressure of 2 tons/cm ² . The outer surface of this tube was machined using a lathe. The obtained processed body has an outer diameter of approximately 11.2 mm,
It had an inner diameter of 8.8mmφ and a length of 200mm. I got 50 similar tubes here. Each of these was calcined in air to 1200°C to burn off the polyvinyl alcohol. 20 tubes in this tube are placed in a hydrogen atmosphere.
The temperature was raised at 100°C/Hr and held at 1900°C for 3 hours to simultaneously perform high-density sintering and subsequent single crystallization.
As a result, the outer diameter is approximately 9 mmφ, the inner diameter is 7 mmφ, and the length is approximately 9 mmφ.
A 160 mm single crystal tube was obtained. The remaining 30 tubes are partially single-crystalline, but when you observe them, you can see that the crystals have grown from the liquid corner at the bottom of the tube. Example 2 (Saphire) Using the same granulated powder as in Example 1, 10 tubes each having a length of 100 mm and a wall thickness of 2 mm as shown in (1) to (8) below were molded. Next, this tube is machined on a lathe to have the wall thickness shown in the table below, and the No.
1, No. 2, No. 4, and No. 6 were machined from the one shown in Figure 2 A to the one shown in Figure 2 B so that the tips were at 45 degrees.

[Table] Next, this was calcined in air, and then fired at 170°C for 10 hours in a dry hydrogen atmosphere. The obtained tube was heated to 5°C/cm in an argon atmosphere, with the tube processed at a high temperature and the unprocessed part at a low temperature.
The mixture was heated at 1870°C for 10 hours with a temperature gradient of . A comparison of the single crystals obtained was as shown in the following table.

[Table] From the above results, it was found that the wall thickness and the shape of the single crystallization initiation part have a large effect, and that the temperature gradient in the furnace is also sufficiently effective. Example 3 (Saphire) The α-alumina tube sintered at 1700°C described in Example 2 was heated in a vacuum continuous furnace for 10 minutes while keeping the inside of the furnace at 1900°C.
A temperature gradient of 20° C./cm was applied at a portion with a speed difference of cm/hour. Through this treatment, it was possible to obtain a tube whose entire structure was single crystallized. Example 4 (β-Alumina) A 0.2 mm β-alumina was added to one end of a 15 mm x 15 mm x 0.5 mm pellet of β-alumina fine powder consisting of 90% Al ₂ O ₃ , 8% Na ₂ O, and 2% MgO. It was molded to include alumina single crystal flakes, and then heated to 1700°C for 50 minutes.
After sintering in an alkaline atmosphere for a period of time, a single crystal pellet having the same orientation as the original β-alumina single crystal buried over almost the entire surface was obtained.

[Brief explanation of the drawing]

Figure 1 is a micrograph showing the state in which polycrystalline alumina becomes a single crystal, and Figure 2 is a cross-sectional view of an alumina tube, where A is before tip processing and B is after tip processing. be.

Claims (1)

[Claims] 1. A raw material for fine alumina powder containing magnesia as a crystal growth inhibitor is formed into a predetermined shape and then fired into a high-density sintered body with a porosity of 5% or less, and then this high-density sintered body is It is characterized by heat-treating the sintered body at a temperature below the melting point and at least 2/3 of the melting point expressed in absolute temperature, and in such a way that the growth of crystal grains starts from only one place in the sintered body to form a single crystal. A method for producing an alumina single crystal. 2. A method for producing an alumina single crystal according to claim 1, characterized in that the high-density sintered body is heat-treated to form a single crystal by providing a temperature gradient in a furnace. 3. A method for producing an alumina single crystal according to claim 1, characterized in that a part of the high-density sintered body is thinned and heat treated to form a single crystal. 4. A method for producing an alumina single crystal according to claim 1, characterized in that a crystal growth inducer is preliminarily present in a part of the high-density sintered body and then heat-treated to form a single crystal. 5. The alumina monomer according to claim 1, wherein the high-density sintered body is α-alumina, and the alumina monocrystalline body is heat-treated to form a single crystal by providing a concentration gradient in the amount of magnesia added therein. Method for producing crystals.