Conventional methods for producing metal powder include a water atomizing method, which provides metal powder by injecting a high pressure water jet to a flow of a molten material; a gas atomizing method, which employs spraying of N2 gas or Ar gas in place of the water jet used in the atomizing method; and a centrifugation method, in which a molten material jet is injected into cooling water present in a rotary drum rotating at high speed. Fine particles are also produced through a breakdown method such as mechanical formation employing a mill or the like and also through a buildup method such as a precipitation method or a sol-gel method.
However, in the water atomizing method and the gas atomizing method, the nozzle structure is complicated and an excessive load is imposed on nozzles, resulting in lowered durability of the nozzle, since the molten material is formed into powder form by a flow of high pressure cooling water or cooling gas. Meanwhile, in the centrifugation method, the structure of the apparatus is complicated, in order to enable high-speed rotation of the rotary drum. Furthermore, in these methods, the molten metal is pulverized on the basis of collision energy. Thus, the resulting particle size is varied, and the yield of fine particles is poor. The breakdown method employing mechanical formation or the like can produce only large particles having a minimum size of, for example, approximately 100 μm. The buildup method such as a precipitation method can produce fine particles having a maximum size of approximately 1 μm, and particles which are larger than approximately 1 μm cannot be obtained. Therefore, when conventional methods and apparatuses for producing fine particles are employed, fine particles having a size ranging from several micrometers to the order of 10 μm, particularly fine particles having a size of about 3 μm, are difficult to obtain. Also, in the breakdown method, a large portion of the molten metal cannot be converted into fine particles and remains as a lump, thereby deteriorating the yield thereof. In addition, the particle size distribution assumes a broaden profile, causing the problem that fine particles having a desired particle diameter cannot be obtained in a large amount.
Conventionally, a liquid quenching method has been known for producing amorphous metal. According to the liquid quenching method, a molten material is cooled and solidified by, for example, causing a molten metal liquid to spout into a coolant, whereby amorphous metal is produced. Even when a centrifugation method, which can attain a relatively large cooling rate, is employed in combination with the liquid quenching method, the heat flux between two liquids (i.e., molten material and coolant) is limited to the critical heat flux in the case where heat conduction is induced by cooling based on convection or a conventional boiling method. Thus, the cooling rate is limited to 104 to 105 K/s, which problematically imposes limitation on the type of metal which can be converted into an amorphous material.
Previously, the present applicant filed a patent application for a method for producing fine particles and amorphous material of molten material which includes supplying into a liquid coolant a molten material which has been formed by melting a raw material to be converted into fine particles or amorphous material, with a small difference in flow speed of the two liquids, to thereby cause boiling by spontaneous bubble nucleation and employing the resultant pressure wave for producing fine particles and amorphous material thereof (see Patent Documents: WO 01/81033 and WO 01/81032).
However, according to the method for which the present applicant previously filed a patent application, when a high-melting material having a melting point of, for example, 800° C. or higher is used, vapor film cannot be broken satisfactorily through condensation. Thus, formation of fine particles or amorphous material of molten material cannot be fully achieved.
Thus, an object of the present invention is to provide, on the basis of improvement of the previously developed technique, a method for producing fine particles, the method being capable of producing fine particles from a high-melting-point raw material and readily producing submicron fine particles which have not been readily produced through the previously developed technique. Another object of the invention is to provide an apparatus therefor.