Provided are silicon fine particles that are effectively prevented from being oxidized and have a crystallite diameter close to that of an amorphous substance. The silicon fine particles of the present invention have an average diameter of primary particles of 30 to 900 nm, a crystallite diameter of less than 10 nm, a chlorine concentration of 1 to 10% by mass, and a ratio (C.sub.o/S) of an oxygen concentration (C.sub.o: % by mass) to a specific surface area (S: m.sup.2/g) of less than 0.05. The method for producing silicon fine particles of the present invention includes: heating a gas containing trichlorosilane to a temperature of 600 to 950° C. in a reactor and thermally decomposing the trichlorosilane to produce a silicon fine particle precursor containing chlorine, then collecting the silicon fine particle precursor, and then heating and dechlorinating the collected silicon fine particle precursor at a temperature of 750 to 900° C. under supply of an inert gas or under reduced pressure.

Provided are a method and apparatus capable of producing fine particles with favorable particle size distribution. In a production method in which feedstock for fine particle production is supplied intermittently into a modulated induction thermal plasma flame, the feedstock is vaporized to form a gas phase mixture, and the mixture is cooled to produce the fine particles: a modulated induction thermal plasma flame in which the temperature state is time-modulated is generated; the modulated induction thermal plasma flame is switched between a high temperature state and a low temperature state; and when the modulated induction thermal plasma flame is in the high temperature state, the feedstock is supplied together with a carrier gas, and when the modulated induction thermal plasma flame is in the low temperature state, supply of the feedstock is suspended and a gas of the same type as the carrier gas is supplied.

Provided are a method and apparatus capable of producing fine particles with favorable particle size distribution. In a production method in which feedstock for fine particle production is supplied intermittently into a modulated induction thermal plasma flame, the feedstock is vaporized to form a gas phase mixture, and the mixture is cooled to produce the fine particles: a modulated induction thermal plasma flame in which the temperature state is time-modulated is generated; the modulated induction thermal plasma flame is switched between a high temperature state and a low temperature state; and when the modulated induction thermal plasma flame is in the high temperature state, the feedstock is supplied together with a carrier gas, and when the modulated induction thermal plasma flame is in the low temperature state, supply of the feedstock is suspended and a gas of the same type as the carrier gas is supplied.

A metal powder having a BET specific surface area of 5 to 250 m.sup.2/g is obtained by contacting and mixing together a gas of a metal chloride (metal source gas) and a reducing gas (e.g., hydrogen gas) that have been separately heated so as to instantaneously form fine metal particles based on the gas phase reduction reaction thereof, and collecting the fine metal particles from the gas stream after the reaction.

A metal powder having a BET specific surface area of 5 to 250 m.sup.2/g is obtained by contacting and mixing together a gas of a metal chloride (metal source gas) and a reducing gas (e.g., hydrogen gas) that have been separately heated so as to instantaneously form fine metal particles based on the gas phase reduction reaction thereof, and collecting the fine metal particles from the gas stream after the reaction.

The fluidized bed process for preparing polysilicon by chemical vapor deposition is improved by positioning at least one Laval nozzle upstream from a gas inlet into the reactor.

The fluidized bed process for preparing polysilicon by chemical vapor deposition is improved by positioning at least one Laval nozzle upstream from a gas inlet into the reactor.

A process for removal of impurities, in particular of dopants, from chlorosilanes which includes the following steps: (a) heating a deposition surface (3); (b) contacting the heated deposition surface (3) with at least one gaseous chlorosilane mixture, the gaseous chlorosilane mixture including at least one chlorosilane and at least one impurity, in particular at least one dopant; (c) at least partially removing the impurity, in particular the dopant, by forming polycrystalline silicon depositions on the deposition surface (3), the polycrystalline silicon depositions being enriched with the impurity, in particular with the dopant; (d) discharging the purified gaseous chlorosilane mixture; (e) contacting the heated deposition surface (3) with an etching gas to return the polycrystalline silicon depositions and the impurity, in particular the dopant, into the gas phase to form a gaseous etching gas mixture; and (f) discharging the gaseous etching gas mixture.

A process for removal of impurities, in particular of dopants, from chlorosilanes which includes the following steps: (a) heating a deposition surface (3); (b) contacting the heated deposition surface (3) with at least one gaseous chlorosilane mixture, the gaseous chlorosilane mixture including at least one chlorosilane and at least one impurity, in particular at least one dopant; (c) at least partially removing the impurity, in particular the dopant, by forming polycrystalline silicon depositions on the deposition surface (3), the polycrystalline silicon depositions being enriched with the impurity, in particular with the dopant; (d) discharging the purified gaseous chlorosilane mixture; (e) contacting the heated deposition surface (3) with an etching gas to return the polycrystalline silicon depositions and the impurity, in particular the dopant, into the gas phase to form a gaseous etching gas mixture; and (f) discharging the gaseous etching gas mixture.

A submicron sized Si based powder having an average primary particle size between 20 nm and 200 nm, wherein the powder has a surface layer comprising SiO.sub.x, with 0<x<2, the surface layer having an average thickness between 0.5 nm and 10 nm, and wherein the powder has a total oxygen content equal or less than 3% by weight at room temperature. The method for making the powder comprises a step where a Si precursor is vaporized in a gas stream at high temperature, after which the gas stream is quenched to obtain Si particles, and the Si particles are quenched at low temperature in an oxygen containing gas.