METHOD FOR MAKING ULTRA-FINE CERAMIC PARTICLES

Abstract: A method for making ultra-fine ceramic particles, in which metal powder constituting a portion of the ultra-fine ceramic particles intended for production is injected into a plasma jet so that the metal powder is vaporized. The vaporized metal powder is then mixed with a reactive gas, which includes an element constituting the other portion of the ultra-fine ceramic particles, filled in the surrounding area of the plasma jet, and thereby the vaporized metal powder and the reactive gas produce a synthetic reaction. The ultra-fine ceramic particles are produced continu­ ously because of the reaction flame.

Description: This invention relates to a method for making ultra-fine ceramic particles by the application of plasma. Ultra-fine ceramic particles with a particle diameter less than 100 nm can be sintered easily at relatively low temperatures due to a large contribution of the surface energy thereof. The catalytic activities of the particles are also enhanced at relatively low temperatures for the same reason. Accordingly, the mass-production of ultra-fine ceramic particles at a low cost is widely desired. The vacuum deposition (metallizing) method, for example, is a well-known procedure for making ultra-fine ceramic particles. This technique makes use of a slow deposition of the particles on a base at reduced pressures. This technique, however, is not suitable for mass production. Because it utilizes reduced pressures to effectively evaporate raw materials and consumes a large quantity of thermal energy, and because the formation rate of the particles is slow and the production rate of particles is low. Another technique with an improved formation rate of ultra-fine ceramic particles utilizes a plasma jet. Metal powder is injected into a plasma jet to vaporize the metal in the technique. The metal vapor is then mixed with a reactive gas such as nitrogen in order to produce ultra-fine ceramic particles. Plasma Chemistry and its application (Japanese title: Purazuma Kagaku to Sono Oyo ) published by Shoka-bo in Japan on September 25, 1971 discloses a technique for making ultra-fine ceramic particles of TiN and Mg₃N₂. In this technique, 200-mesh titanium or magnesium powder is injected into nitrogen plasma by a nitrogen carrier gas at a flow rate of 5 L/min, an electrical input of 12 KW, and a powder injection rate of 0.5 g/min. The extraction rates thereof were 30 % for TiN and 40 % for Mg₃N₂ according to the book. DE-OS 2 157 723 discloses a method for producing submicron-metal-oxides consisting of zinc oxide and antimon oxide, wherein zinc, zinc oxide, antimon or antimon oxide are introduced into a plasma gas stream having a temperature and an energy high enough for vaporization of the said metals or metal oxides. After vaporization a second gas is added by mixing, whereby the metal oxide is diluted and cooled down. At the same time the oxidizing component of the said second gas is adjusted in such a way, that a complete oxidation to the corresponding metal oxide is effected. US-PS 3 848 068 discloses a method for producing a particulate metal oxide. By this method solid metal particles or atomized molten metals are introduced into a plasma gas stream containing up to 100 % of oxygen on a free-element basis. The plasma gas stream is enveloped with a gas also containing up to 100 % of oxygen on a free-element basis. The metal is introduced into said plasma gas stream in particulate form. Then the particulate metal is vaporized and the metal vapors are caused to chemically react with the reactant gas. The resultant compound is quenched by a stream of gas containing up to 100 % of oxygen on a free-element basis and a metal oxide is being obtained in finely divided form. However, it is considered to be difficult to employ these techniques on a profitable basis. Although this technique of utilizing plasma as its heat source is more productive than the vacuum deposition method, this technique still produces a relatively small quantity of ultra-fine ceramic particles, because a relatively small quantity of metal powder is injected from a point of view that all the metal powder is vaporized. Accordingly, it is the primary object of the present invention to provide a method for producing ultra-fine ceramic particles, which overcomes the above-mentioned disadvanteges. It is a further object of the present invention to provide a method for efficiently making ultra-fine ceramic particles. It is a still further object of the present invention to provide a method for continuously making ultra-fine ceramic particles. It is another object of the present invention to provide a method for making ultra-fine ceramic particles at a low cost. This invention achieves this object by the following means: Metal powder contituting a portion of an intended ultra-fine ceramic particles is injected into a plasma jet and thereby the metal powder is vaporized. The vaporized metal powder is then mixed with a reactive gas, which includes an element constituting the other portion of the intended ultra-fine ceramic particles, filled in the surrounding area of the plasma jet. Thereby the vaporized metal powder and the reactive gas make a synthetic reaction. Because this reaction is exothermic reaction and the injection rate for the metal powder is such that a reaction flame develops and can be maintained, the ultra-fine ceramic particles are produced continuously and can be gathered. In this reaction, almost all kinds of metal can be employed for the metal powder constituting a portion of the intended ultra-fine ceramic particles. Metals which can be employed for the metal powder include silicon, titanium, zinc, magnesium, manganese, chromium, vanadium, aluminum, and the like. The metal powder is injected into a plasma jet, for example, by means of a carrier gas. The reaction flame is difficult to maintain if the injection rate is less than 70 g/min. The reaction flame becomes larger and achieves stability when the injection rates equal or exceed 180 g/min. Accordingly, it may be necessary that the injection rate is not less than 70 g/min. And it is desirable that the injection rate is not less than 180 g/min. Metal powder having particle sized not more than 400 µm is preferred for obtaining the ultra-fine ceramic particles. The carrier gas may be used for injecting metal powder into the plasma jet. Either inert gas or reactive gas can be utilized as the carrier gas. Preferably, the metal powder should be continuously injected into the plasma jet. However, it may be injected intermittently. The intermittent injection should be done at intervals not exceeding 1 second. The reaction flame can achieve prolonged stability at intervals shorter than 1 second. The ambience around the plasma jet is filled with a reactive gas which includes an element constituting the other portion of the ultra-fine ceramic particles intended for production. The element constituting the other portion of the ultra-fine ceramic particles can be oxygen, nitrogen, carbon and the like. Reactive gases including these elements are gaseous oxygen, gaseous nitrogen, gaseous ammonia, hydrocarbon, carbon monoxide, and the like. Oxides, nitrides, carbides or the likes can be obtained according to the type of a reactive gas employed. In the process of this invention, a metal powder injected into a plasma jet is then vaporized. The vaporized metal powder then reacts with a reactive gas, thereby producing ultra-fine ceramic particles. This synthetic reaction generates heat. Accordingly, the reaction of vaporized metal powder and the reactive gas is promoted by the heat, thereby making a reaction flame around the plasma jet. Once the reaction flame is made, it achieves stability and the ultra-fine particles are produced continuously by the effects of the plasma jet and the heat generated by the synthesis of the ultra-fine ceramic particles. An advantage of this invention is that the ultra-fine ceramic particles can be produced at a low cost. Plasma is the only thermal energy necessary for the synthesis of ultra-fine ceramic particles by this technique. The vaporization of metal powder is achieved by the heat generated by the reaction of the vaporized metal powder and a reactive gas. The thermal efficiency of this method is extremely high. Another advantage of this invention is that homogeneous ultra-fine ceramic particles can be mass-produced continuously with high efficiency due to the stable reaction flame. A further advantage of this invention is that cost-efficient production of ultra-fine ceramic particles can be achieved. Apparatuses applying this technique are simpler than apparatuses used in conventional physical production methods such as the vacuum deposition method because this invention can be applied at atmospheric pressures. Figure 1 is a schematic structural view of an apparatus for making ultra-fine ceramic particles employed in the examples according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENT A technique for making ultra-fine silicon dioxide particles are described hereunder as an example of the method for making ultra-fine ceramic particles by this invention. With reference to the drawing, a nozzle 1 of a plasma-injecting apparatus comprises a tungsten cathode 2 and a copper anode 3. Plasma gas is injected through the passage 4 formed by the tungsten cathode 2 and the copper anode 3. The passage of the injection pipe 5 is connected with the copper anode 3. The passage of the injection pipe 5 is extended through the copper anode 3 to form an opening at a portion of the passage 4. A metal powder 6 and the carrier gas are injected through the injection pipe 5. An end of the nozzle 1 is connected with a closed vessel 7. The passage 4 is extended into the inside of the closed vessel 7. A reactive gas injecting pipe 8 and the exhaust pipe 9 are respectively jointed to the closed vessel 7. An apparatus for making ultra-fine ceramic particles as composed above was employed in producing ultra-fine silicon dioxide particles. First, the closed vessel 7 was filled with air containing oxygen. Then, a mixture of a primary gas and a secondary gas was injected through the passage 4 formed by the tungsten cathode 2 and the copper anode 3. More specifically, 54 L/min of argon gas (the primary gas) and 5 L/min of hydrogen gas (the secondary gas) were injected through the passage 4. Thereupon, an electric current of approximately 50 V 600 A was conducted between the tungsten cathode 2 and the copper anode 3 to generate plasma. The plasma spurted up into the closed vessel 7 in a form of a plasma jet 10. Then, silicon powder with an average particle diameter of 100 µm was injected into the closed vessel 7 by means of a nitrogen carrier gas. The nitrogen gas was injected at 0.9 L/min and the silicon powder was injected at 180 g/min. Then, the silicon powder and the oxygen reacted and produced a large reaction flame 11 around the plasma jet 10, thereby producing the ultra-fine silicon dioxide particles. This reaction is exothermic reaction. Thereby the evaporation of the silicon powder was enhanced by the heat generated by this thermal reaction. And the reaction flame 11 was maintained by the heat of the plasma jet 10 and the thermal energy of the reaction. The vapor of the ultra-fine silicon dioxide particles thus produced was then cooled by the surrounding air, and thereby the particle growth was inhibited. The ultra-fine silicon dioxide thus made deposited on the surface of water-cooled copper pipes (not shown), which were later extracted. As a result of this production method, ultra-fine silicon dioxide particles with an average particle diameter of 50 nm were obtained at a composition rate of 97 %. The production rate was 390 g/min which is approximately 100 times faster than conventional techniques for making ultra-fine ceramic particles. The production rate for ultra-fine ceramic particles by the technique described in this invention far exceeds the production rate of other conventional techniques.

Claims: A method for producing ultra-fine ceramic particles comprising the steps of forming a plasma jet in a reaction gas atmosphere containing an element constituting a portion of the intended ultra-fine ceramic particles, introducing a metal powder constituting the other portion of said intended ultra-fine ceramic particles into said plasma jet to vaporize said metal powder by the heat of said plasma jet and to react the resulting said metal with said element contained in said reaction gas atmosphere for synthesizing said intended ultra-fine ceramic particles under the condition, that the injection rate for the metal powder is such that a reaction flame develops and can be maintained, and gathering the resulting ultra-fine ceramic particles. A method according to claim 1, wherein the metal forming said metal powder is one selected from the group consisting of silicon, titanium, zinc, magnesium, manganese, chromium, vanadium, aluminum and a mixture thereof. A method according to claim 1, wherein the particle size of said metal powder is not more than 400 µm. A method according to claim 1, wherein said element is one selected from the group consisting oxygen, nitrogen and carbon. A method according to claim 1, wherein said reaction gas is one selected from the group consisting of gaseous oxygen, gaseous nitrogen, gaseous ammonium, gaseous hydrocarbon and a mixture thereof. A method according to claim 1, wherein the rate of injecting said metal powder into said plasma jet is not less than 70 grams per minute. A method according to claim 1, wherein said metal powder is injected by means of a carrier gas. A method according to claim 7, wherein said carrier gas is one selected from the group consisting of reaction gas and inert gas and a mixture thereof. A method according to claim 1, wherein said metal powder is continuously injected into said plasma jet. A method according to claim 1, wherein said metal powder is intermittently injected into said plasma jet at intervals not exceeding 1 second.

Inventors: ABE, SUSUMU, OGAWA, MASAHIRO
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1985
Main classification: C01B13
IPC codes: C01B21, C01G1, C01B13, B01J19, C01B31, B01J8, C01F5, B01J12, C01F7, C01B33