1. Field of the Invention
The present invention relates to: a heat treatment method for heat treating a substance or particles by means of a thermal plasma; the substance thus heat treated through the heat treatment method; a thermochemical synthesis method for producing a synthetic material by the use of thermal plasma heating; the synthetic material produced through the thermochemical synthesis method; a sphering method for forming each of particles into a spherical shape through the thermal plasma heating; each of the particles thus spherically-shaped through the sphering method; a surface coating method for coating each of particles with a surface material produced by melting a surface of each of the particles or an added material on the surface of each of the particles by means of the thermal plasma; each of the particles coated with the surface material; and, a thermal plasma heat treatment apparatus for carrying out the above methods.
2. Description of the Prior Art
For example, polycrystalline silicon of the silicon wafers for IC's is produced through the following chemical reaction: SiCl.sub.4 +2H.sub.2 =Si+4HCl.
Heretofore, this chemical reaction has been realized through a conventional method such as Siemens method, Union Carbide method and the like using electric furnaces, flames and like heating means. The above chemical reaction requires a high temperature atmosphere having a temperature of more than 1000.degree. C. Higher atmosphere is required when a faster reaction rate is demanded. Consequently, the conventional method is disadvantageous in cost. Further, in the conventional method, when the flame is used as its heating means, there is a fear that a product produced through the above chemical reation is contaminated with the flame. Consequently, a thermal plasma is a preferable heating means to realize such high temperature atmosphere required in the above chemical reaction, since the thermal plasma is free from any of the above-mentioned problems inherent in the conventional method.
On the other hand, each of particles of a powder produced through the chemical reaction assumes a non-spherical shape. However, in order to increase in density of a sintered product or in brightness of a fluorescent substance (i.e., phosphor), it is desirable for each of particles of the powder to assume a spherical shape. Consequently, heretofore, a so-called sphering treatment is conducted, in which treatment each of the particles as a starting material is heated to a temperature slightly higher than its melting point so that each of the particles assumes a spherical shape under the effect of its melted surface tension. Such sphering treatment conducted in a radio-frequency induction heating method using a thermal plasma is known (see: a paper presented by Tadahiro SAKUTA at a meeting of the switch protection research division of the Japanese Electric Society, 1993; and, Japanese Patent laid-Open No. Hei 6-25717).
In the above chemical reaction, when the atmosphere temperature thereof is too low, its reaction rate decreases. On the other hand, when the atmosphere temperature is too high, a product obtained is decomposed, which makes it impossible to obtain a product having a predetermined composition and physical properties. Further, in the above-mentioned sphering treatment in which each of the particles as a starting material should be heated to a temperature slightly higher than its melting point using the radio-frequency induction thermal plasma so that each of the particles assumes a spherical shape under the effect of its melted surface tension, when each of the particles is not heated to the temperature slightly higher than its melting point, each of the particles fails to assume a spherical shape since its surface is not melted. On the other hand, when each of the particles is heated to a temperature excessively higher than its boiling point, each of the particles changes in construction or composition by evaporation of its components or ingredients. Consequently, when the thermal plasma is used as heating means in the above sphering treatment, it is necessary to keep constant a temperature of a heating portion in the thermal plasma. When each of the particles is made of a substance having a high melting point and a boiling point slightly higher than the melting point, the heating portion of the thermal plasma must be kept at a high temperature within a relatively narrow range. The radio-frequency induction thermal plasma (hereinafter referred to as the radio-frequency plasma) has a benefit to make it easy to obtain a high temperature necessary for the above sphering treatment. Further, the radio-frequency plasma has the advantage over a so-called electrode-type thermal plasma that it is free from any contaminants. Conseqeuntly, the radio-frequency plasma is a preferable heating means for the sphering treatment and for the synthesis of the substance.
In order to protect the surface of each of the spherical particles from attacks by moisture and harmful radiation, only the surface of each of the particles can be melted so that each of the particles is coated with a layer of the thus melted and solidified surface. One of zinc sulfide phosphors, for example, a phosphor ZnS:Cu is used an luminescent material in an electroluminescence display unit and like units. However, this zinc sulfide phosphor is poor in water-resisting properties. Due to its poor water resistance, this luminescent material reacts with moisture contained in the air and loses its luminescent properties when used solely in the units. Consequently, until now, in many applications, the particles of the zinc sulfide phosphor (ZnS:Cu) are sandwiched in between a pair of protective films to isolate themselves from the air. However, this raises the manufacturing cost of the display unit. Under such circumstances, considered as a hint to the present invention is a finding that: when the individual particles of the zinc sulfide phosphor (ZnS:Cu) are coated with individual protection means to isolate themselves from the air, it is possible to eliminate the use of the protective films, which may reduce the manufacturing cost of the display unit.
Based on the above finding, as shown in FIG. 7(a), each of the particles of the zinc sulfide phosphor (ZnS:Cu) is coated with silica powder, and then heated so that the silica powder deposited on the surface of each of the particles is melted to form a silica layer covering each of the particles, whereby each of the particles is improved in water resistence by means of the silica layer after completion of solidification thereof. On the other hand, a melting point of silica is high (i.e., more than or equal to 1000 degrees centigrade), whereas, the zinc sulfide phosphor (ZnS:Cu) to be coated with the silica layer is badly damaged when subjected to a temperature of more than or equal to 300 degrees centigrade. Consequently, in order to coat the particle of the zinc sulfide phosphor (ZnS:Cu) with the silica layer, it is necessary to melt only the silica on the surface of each of the particles without heating the zinc sulfide phosphor (ZnS:Cu) to be coated with the silica.
However, in FIG. 6, a temperature of a vortex stream flame center 2 of the radio-frequency plasma described above reaches 10,000 K. Even a temperature of a front-end portion 3 of the flame reaches 5,000 K. Consequently, in a conventional heat treatment apparatus in which the conventional radio-frequency plasma issued from a plasma torch is directly applied to a workpiece, though it is possible to conduct the sphering treatment with respect to a material having a simple composition, as for materials having complicated compositions with low melting points and intremetallic compounds, the heat of the radio-frequency plasma is too high in temperature so that these materials decompose when subjected to the heat, which makes it impossible to obtain the product particles having the same properties as those of the starting material. Further, in the sphering treatment of a material having a low melting point, the temperature of the plasma is too high so that a large amount of ultrafine powder is produced, which impairs the yield of usable particles having a predetermined grain size. In addition, classifying operation of such ultrafine powder is costly, and somtimes hard to accomplish. In general, a temperature necessary for the sphering treatment is within a range of from 773 to 3273 K. Consequently, it is necessary to heat the particles in a restricted area of the front-end portion of the plasma. In this case, it is possible to increase the range of the predetermined temperature of the plasma flame when the temperature of the vortex stream flame center 2 in the plasma is decreased. However, when the temperature of the plasma is decreased unduly, the plasma flame becomes unstable or ceases to exist. Consequently, in the conventional heat treatment apparatus, the amount of its radio-frequency input energy, types of plasma gases and their flow rates used in the apparatus are appropriately selected to stabilize the radio-frequency plasma flame (see Japanese Patent Laid-Open No. Hei 8-109375).
In case that the particles of the starting material, i.e., zinc sulfide phosphor (ZnS:Cu) is coated with silica, a direct heat treatment method using the conventional radio-frequency plasma suffers from an ultrahigh temperature of the plasma flame, in which a predetermined temperature is available only in a narrow flame range or area. Due to such narrow flame area, each of the particles subjected to the conventional heat treatment method is heated to an undue high temperature even in its inner portion, which damages the zinc sulfide phosphor (ZnS:Cu) so that the thus treated particle product often fails to emit light in the display unit.
On the other hand, as for a so-called carbon fiber, it is known that the carbon fiber could be improved in tensile strength and in elastic modulus when subjected to a high temperature heat treatment. If a pitch-based carbon fiber is improved in strength so as to have the same strength as that of a polyacrylonitrile-, i.e., PAN-based carbon fiber through the high temperature heat treatment, it becomes possible to reduce the manufactureing cost of the desired carbon fiber. In this case, it is necessary to conduct such heat treatment at a temperature of 3000 degrees centigrade in an inert-gas atmosphere free from any impurity contamination. Although the heat treatment may be conducted in an electric furnace, it is preferable to conduct the heat treatment using the radio-frequency induction thermal plasma. However, in the conventional heat treatment method in which the plasma flame is directly applied to the workpiece, it is hard to control its plasma flame in temperature, which makes it difficult to uniformly heat the workpiece, and, therefore makes it difficult to improve the workpiece in material properties.