METHOD FOR MAKING A FINE POWDER OF A METAL COMPOUND HAVING CERAMIC COATINGS THEREON

Abstract: A method for making fine powder of a metallic com­ pound coated with ceramics is described. In the method, a gaseous mixture of at least one metal vapor constituting one component of a metallic compound and a carrier gas for the at least one metal vapor is passed at a speed of 50 to 2000 m/second into another element constituting the other ele­ ment of the metallic compound or in an atmosphere of a gas containing the other element, which is heated to a tempera­ ture of not larger than 0.8 time as low as a melting point of the at least one metal so that the metal vapor and another element are reacted with each other while rapidly cooling, thereby obtaining a fine powder of a metallic compound. The metallic compound fine powder is further passed into other metal vapor or other metallic compound vapor to cover the metallic compound fine powder with the film of the other metal or metallic compound.

Description: This invention relates to a method of making a fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof. The fine powder of a metal containing compound obtained by the method of the invention is suitably used as a sintering material. As is well known in the art, sintering of a metal containing compound powder such as silicon nitride powder does not smoothly proceed when using the metal containing compound powder alone. It is the usual practice to add, to metal containing compound powder, sintering aids such as yttrium oxide, aluminium oxide and the like. In order to cause the sintering to proceed smoothly, it is preferred that a metal containing compound powder and a sintering aid are uniformly mixed to make a larger contact area thereof. However, since the metal containing compound powder and the sintering aid are both in the form of powders, it is difficult to mix the powders substantially uniformly, with an attendant problem that the strength of the sintered product may not be satisfactory depending on the degree of uniformity in the mixing. Further, the particle size of the metal containing compound powder is too large to obtain the strong sintered product. EP-A-0 091 109 discloses a method of making a ceramic-metallic composite fine powder material composed of fine powder particles each having a metallic core and a ceramic surface layer which is a compound of the metal composing said core and another element, the average value of the ratio of the thickness of the surface layer of a powder particle to the radius of the particle being substantially greater than 0.05, wherein said metal in a gaseous form is mixed with said another element in the gaseous state, the resulting mixture being then passed through a convergent-divergent nozzle and being thereby rapidly cooled by adiabatic expansion. The speed of the vapor spouted from the divergent nozzle is approximately as fast as that of the sound. Consequently, the vapor is cooled at an extremely high speed and produces metal particles without reacting with the gas. Thus, a fine metal powder is first produced which afterwards reacts with the other element and forms a ceramic layer on the surface of the fine metal powder. For said reason, the known ceramic layer essentially contains as component the core metal. It is an object of the invention to provide a method for making fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof which overcomes the problems of the prior art. It is another object of the invention to provide a method for making fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof, which has a very fine particle size and on which a ceramic film is uniformly coated on individual particles of the metal containing compound fine powder. It is a further object of the invention to provide a method for making fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof, which can be produced in a high preparatory efficiency. The above objects can be accomplished, according to the present invention, by a method for making a fine powder comprising a metal containing compound coated with a ceramic film on the surfaces thereof, which comprises the steps of: vaporizing a first metal constituting one element of said metal containing compound; blowing out the vapor of said first metal with a carrier gas into a gaseous mixture containing another element of said metal containing compound heated to a temperature of not larger than 0.8 times as low as the melting point of said first metal at a speed of 50 to 2000 m/sec so that said vapor of said first metal and said other element are reacted with each other while cooling, thereby obtaining a metal containing compound in a fine powder; coating a second metal onto the surface of said metal containing compound fine powder by heating said metal containing compound fine powder at a temperature of 800 to 1100 o C and by passing the resulting metal containing compound fine powder through the vapor of said second metal; and oxidizing said second metal coated on said metal containing compound fine powder to form a ceramic film on the surfaces of said metal containing compound fine powder. Fig.1 is a sectional view of a fine nozzle used in the method of the invention; Fig.2 is a sectional view of a Laval nozzle used in the method of the invention; Fig.3 is a reaction state diagram showing a region where silicon nitride is stable and a region where metallic silicon vapor and nitrogen gas are stable as gases in case where the partial pressure of nitrogen gas is 1013 Pa (10⁻² atmospheric pressure); and Fig.4 is a schematic structural view showing an apparatus of making a metal containing compound fine powder used in Examples 1 - 3 of the present invention. The first metal and the other element which constitute a metal containing compound produced in accordance with the method of the invention should be a combination of an arbitrary metal and another element which are chemically combined together. In particular, preferable metals include silicon, aluminium, titanium, zirconium and mixtures thereof. The other element includes nitrogen, carbon, oxygen and another metallic element (in which case the resulting metal containing compound is a metal-metal compound). In this connection, the other element which constitutes a part of the gaseous mixture may be not only a gas of the element itself, but also an element produced from a compound containing the element and capable of releasing the element. For instance, when the other element is nitrogen, there can be used not only nitrogen gas, but also compounds such as ammonia. If the other element is carbon, there may be used lower aliphatic hydrocarbons such as methane, ethane, propane and the like. The second metal which is coated on the surface of the metal containing compound fine powder is expected to serve as sintering aids and include, yttrium, aluminium, and magnesium. In the practice of the invention, a carrier gas is used to smoothly carry the vapor of the first metal and to control the pressure within a furnace at a predetermined level. The carrier gas includes an inert gas such as argon or the like. The abovementioned other gaseous element or a gas containing the other element therein may be also used as the carrier gas. It will be noted that if the other element or a gas containing the other element therein is used, such an element reacts with some metals, so that the rate of synthesis can be increased. Into a chamber in which the metal containing compound fine powder is coated with the second metal inert gases such as argon should preferably be charged in order to keep the temperature and pressure at predetermined levels. The gaseous mixture comprising a vapor of the first metal and a carrier gas prior to rapid cooling should preferably have a temperature of not lower than 200 °C and a pressure of not larger than 10108 Pa (10⁻¹ atmospheric pressure) when fine powder of silicon nitride is made. This is because the temperature and pressure should be so controlled that the first metal is stably present as a vapor. The gaseous mixture is blown out into the other element, as a gas, constituting an intended metal containing compound or an atmosphere of a gas containing the other element, thereby causing the first metal and the other element to react while rapidly cooling. In the practice of the invention, the rapid cooling of the gaseous mixture should be effected in a gas of the other element or a gas containing the other element. The shape of a nozzle from which the gaseous mixture is blown out greatly contributes to the degree of rapid cooling. More particularly, if a nozzle used is a divergent nozzle, the effect of the rapid cooling is great. In order to effect the rapid cooling in the other element or in a gas containing the other element, it is necessary that the temperature of the other element or the gas comprising the other element should be lower than the temperature of the gaseous mixture. In this case, the temperature (°C) is below 0.8 time as low as the melting point (°C) of said first metal. For the reaction between the first metal and the other element under rapid cooling conditions, the gaseous mixture should be blown out into the other element or an atmosphere of a gas containing the other element at a speed of 50 to 2000 m/second. If higher speeds are used, the reaction does not proceed satisfactorily. In order to attain an optimum rate of synthesis or an optimum yield, the blowing-out speed should preferably be in the range of 50 to 800 m/second. Subsequently, the resulting metal containing compound fine powder is coated with a second metal. To this end, the metal containing compound fine powder is heated at a temperature of 800 to 1100°C. At lower temperatures, no coating layer is formed. Reference is now made to how the rapid cooling of the gaseous mixture is effective in providing a metal containing compound fine powder with regard to the production of a silicon nitride fine powder from metallic silicon vapor and nitrogen gas. Metallic silicon is rather difficult to vaporize and has such a low equilibrium vapor pressure as 66,5 Pa (0.5 Torr. (6.6 x 10⁻⁴ atm.)) even at a temperature of 2000 °C. The method of making a fine powder of silicon nitride by the use of metallic silicon vapor obtained by heating metallic silicon at temperatures over 2000 °C is not better from the standpoint of thermal energy consumption than conventional methods. However, according to the method of the invention, metallic silicon can be quickly converted the vaporized state to the solid state when metallic silicon vapor is rapidly cooled, so that the method according to the invention can produce a metal containing compound fine powder which has an extremely fine particle size of the order of 0.01 µm and also has substantially uniform particle size. In contrast, in known methods utilizing gas phase reactions, such a fine and uniform metal containing compound powder cannot be obtained. Metallic silicon and nitrogen gas react with each other at a relatively low temperature of around 900 to 1700°C, thereby forming silicon nitride which is stable in the above temperature range. In this connection, however, if silicon nitride is prepared using a gas phase reaction between metallic silicon-generating gas [silicon gas (SiH₄ )] and nitrogen-generating gas [ammonia gas (NH₃ )] at a temperature of 900 to 1700°C, the reaction readily advances in the direction where metallic silicon and nitrogen gas are chemically combined to give solid silicon nitride, with the result that within a very short time period, silicon nitride particles grow up to provide coarse particles. Reference is then made to choice of a cooling means for rapidly cooling the gaseous mixture. In the practice of the invention, the temperature of the other element or a gas containing the other element greatly contributes to the rapid cooling. The gaseous mixture is cooled by means of the other element or the gas containing the other element. In addition, the gaseous mixture is also rapidly cooled by blowing out through a nozzle, utilizing a pressure difference, into the other element or the gas containing the other element prior to contact of the gaseous mixture with the other element or the gas containing the other element. A great difference in cooling rate is produced between a case where a fine nozzle as shown in Fig.1 is used and a case where a divergent nozzle (Laval nozzle) as shown in Fig.2 is used. More specifically, when the fine nozzle is used, the blowing speed is from 50 to 880 m/second. Accordingly, the divergent nozzle has a greater effect with regard to rapid cooling, but if the cooling rate is too great in the method of the invention, the reaction between the first metal and the other element or a gas containing the other element does not proceed satisfactorily. Where, for example, silicon nitride fine powder is made, the fine nozzle is more effective in rate of synthesis than the divergent nozzle. The temperature and pressure conditions before or after the rapid cooling or adiabatic expansion of the gaseous mixture are described using production of silicon nitride fine powder. The temperature and pressure conditions at which silicon nitride either breaks up or stays in the reacted state are determined by the second law of thermodynamics. In other words, the chemical reaction between metallic silicon vapor and nitrogen gas can be expressed in the following formula (1): 3(Si) + 2(N₂) = (Si₃N₄) (1) The change of free energy ΔF in the above reaction (1) may be expressed by the following formula (2) or (3): in which, ΔF o : reference free energy change R: gas constant T: temperature (°K) P si : partial pressure of metallic silicon vapor In this formula (3), silicon nitride is stable as a solid when ΔF is negative, while metallic silicon vapor and nitrogen gas are stable as gases when F is positive. In Fig.3, there is shown a reaction state diagram indicating a region in which silicon nitride is stable and a region in which metallic silicon vapor and nitrogen gas are stable as gases when the partial pressure of nitrogen gas is 1013 Pa [10⁻² atmospheres (7.6 Torr)]. From Fig.2, it can be seen that metallic silicon vapor and nitrogen gas are more stable than solid silicon nitride at a pressure of around 1013 Pa (10⁻² atmospheres) when the temperature is greater than about 2000°C. Therefore, in the method of making fine powder according to the invention, it will be clear that it is preferable to mix the vapor of the first metal and the carrier gas in the region where the vapor of the first metal is stable, to rapidly cool the mixture gas by blowing out the mixture into the other element in the gaseous state or a gas containing the other element of lower temperatures in which the reaction is carried out, and to continue the cooling without promoting further growth of particles. In the practice of the invention, a gaseous mixture of the first metal and the carrier gas is blown out into the other element or a gas containing the other element at a predetermined speed through a nozzle, by which the first metal and the other element are cooled to a temperature range, where they are reacted, by adiabatic expansion caused by the passage through the nozzle and direct contact of the mixture with the other element or the gas containing the other element. Thereafter, the reaction system can be rapidly cooled to a region in which the metal containing compound is kept stable. The resulting metal containing compound fine powder is heated to a temperature of 800 to 1100°C at which it is not decomposed, after which the fine powder is passed through the vapor of a second metal and rapidly cooled by passing it through a fine nozzle or Laval nozzle while being deposited with the second metal on the surfaces thereof, followed by oxidation and stabilization on the surfaces thereof. The present invention has the features and advantages summarized below. (a) The vapor of the first metal constituting the gaseous mixture is rapidly cooled by the adiabatic expansion upon passage through a nozzle and direct contact with the other element to give a fine solid powder having such a size as in the vaporized state. At the same time, the vapor of the first metal reacts with the other element constituting the metal containing compound. As a result, there can be obtained a fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof which has a very fine and substantially uniform particle size. (b) Vapors can be readily and continuously converted from the vaporized state to the solid state by passage through a nozzle and contact with the other element or a gas containing the other element, which leads to efficient and continuous production of said fine powder of high purity. (c) When the gaseous mixture is passed into the other element or a gas containing the other element at an appropriate cooling rate, the rate of synthesis of said fine powder can be remarkably improved. (d) The final fine powder has a ceramic film formed on the surfaces of the metal containing compound, which prevents the inner metal containing compound from deteriorating in nature and ensures easy handling. (f) The final fine powder has a sintering aid uniformly deposited on the surface of the metal containing compound, and thus a sintered product having good sintering ability and high strength is obtained. Examples of the present invention are described with reference to the accompanying drawings. Example 1 Fig.4 shows a schematic structural view of an apparatus for making a fine powder of a metal containing compound coated with a ceramic film on the surfaces thereof used in Example 1 according to the invention. In the figure, the reference numeral 1 indicates a furnace shell which is substantially formed as a closed container and the shell 1 is vertically divided by partition walls 2, 3 and 4 into four chambers 5, 6, 7 and 8. In the first chamber 5 is provided a first crucible 9. The first crucible 9 has a gas-preheating chamber 11 having a carrier gas introduction port 10 and a mixing chamber 12 communicating with the gas-preheating chamber 11. Around the first crucible 9 is provided a first heater 13 for keeping the gas-preheating chamber 11 and the mixing chamber 12 at a predetermined temperature, T₁. A first metal which is charged into the mixing chamber 12 is molten by means of the first heater 13 to provide a molten first metal 14, which will be vaporized as vapor of a first metal. The first crucible 9 has a bottom wall, through which extends a first nozzle or tube 15 having a diameter of 7 mm. The first tube 15 is passed through the partition wall 2 and communicates with the mixing chamber 12 and the second chamber 6. The second chamber 6 has a reaction gas introduction tube 16, which intersects at right angles with the first tube 15, for introducing another element or a gas containing the other element constituting an intended metal containing compound. The introduction tube 16 is connected to a bomb not shown. The second chamber 6 and the third chamber 7 communicate with each other through a second fine tube 17. Beneath the second fine tube 17 is provided a second crucible 18, in which a second metal is placed. Around the second fine tube 17 and the second crucible 18 are provided a second heater 19 and a third heater 20, which are used to heat the second fine tube 17 and the second crucible 18, respectively. The third chamber 7 has an inert gas introduction tube 21 through which an inert gas such as nitrogen or argon is introduced and which is connected to a bomb not shown. The bottom of the second crucible 18 and the fourth chamber 8 communicate with each other through a fine tube 22. A collector pipe 23 for collecting a synthesized fine powder of a metal containing compound coated with a ceramic film is provided at the bottom of the fourth chamber 8 and below the third fine tube 22. The collector tube 23 is made of a copper pipe, through which cooling water is passed. Part of the fourth chamber 8 is connected to an evacuation means such as a vacuum pump not shown. The vacuum pump is operated to evacuate the second chamber 6 and the mixing chamber 12 to predetermined pressures, P₂ and P₁, respectively. It will be noted that the third chamber 7 is set, using an inert gas, to a pressure lower by 399 to 665 Pa (3 to 5 Torr). than the second chamber 6. The apparatus of the type as described above is used to make silicon nitride fine powder coated with an yttrium oxide film in the following manner. Metallic silicon is charged into the mixing chamber 12 and the furnace shell 1 is reduced, by means of a vacuum pump, to an extent of 1,3 Pa [10⁻² Torr. (1.3 x 10⁻⁵ atmospheres)]. Thereafter, nitrogen gas is charged from the carrier gas introduction port 10 through the gas-preheating chamber 11 into the mixing chamber 12. At the same time, the first crucible 9 in the furnace shell 1 is rapidly heated by the use of the first heater 13 until the temperature, T₁, in the mixing chamber 12 reaches about 2200°C. As a result, the metallic silicon melts to provide molten silicon 14. The introduction rate of nitrogen gas is controlled to be 6 liters/minute so that the pressure, P₁, within the mixing chamber 8 is adjusted to about 1995 Pa [15 Torr. (2 x 10⁻² atmospheres)]. On the other hand, ammonia gas (NH₃) of a normal temperature is charged, at a rate of 12 liters/minute, into the second chamber 6 through the tube 16 in the direction intersecting at right angles with the first fine tube 15, so that the inner pressure in the lower chamber 6 is about 1064 Pa [8 Torr. (1.1 x 10⁻² atmospheres)]. Yttrium chloride powder is charged into the second crucible 18 and is heated to 800 to 1100°C by means of the third heater 20 and vaporized. Argon gas heated to about 1000 °C is introduced from the inert gas introduction tube 21 so that the third chamber 7 is at a pressure lower by 665 Pa (5 Torr.) than the chamber 6. To the fourth chamber 8 is fed oxygen from the oxygen introduction tube 24. As a result, the gaseous mixture of the silicon vapor and the nitrogen gas formed in the mixing chamber 12 is passed into the second chamber 6 through the first fine tube 15 by virtue of the pressure difference between the mixing chamber 12 and the second chamber 6. The passed gaseous mixture comprises about 1 g/minute of the silicon vapor and is cooled down to 1800°C immediately after the passage. This gaseous mixture is further rapidly cooled by contact with ammonia gas and converted into silicon nitride in the form of fine powder, followed by continuing further cooling with the ammonia gas. The silicon nitride fine powder is blown out into the third chamber 7 through the second fine tube 17 while heating with the second heater 19. In the third chamber 7, yttrium vapor produced by decomposition is deposited on the silicon nitride fine powder. The silicon nitride fine powder on which yttrium is deposited is passed through the third fine tube 22 into the fourth chamber 8 in which it is deposited on the collector tube 23 while oxidizing with oxygen and stabilizing. After completion of the reaction, the synthesized powder deposited on the collector tube 23 is sampled to check the content thereof. As a result, it is found that 70 % of the powder consists of yttrium oxide film-deposited silicon nitride. The rate of synthesis or yield is 70 %. The resulting silicon nitride fine powder has an average particle size of 10 nm (100 angstrom) and a uniform yttrium oxide film thereof. Example 2 The same apparatus as used in Example 1 is used to make an aluminium oxide film-deposited silicon nitride fine powder. In this example, the fine powder is made under the same conditions as in Example 1 except that aluminium chloride is used instead of yttrium chloride. The resulting silicon nitride fine powder has an average particle size of 10 nm (100 angstrom) with a rate of synthesis being 70 % and it also has a uniform deposition of aluminium oxide on the surface thereof. Example 3 The same apparatus as used in Example 1 is used to make a magnesium oxide film-deposited silicon nitride fine powder. In this example, the magnesium oxide film-deposited silicon nitride fine powder is made under the same conditions as in Example 1, except that metallic magnesium is used instead of yttrium chloride. As a result, the magnesium oxide film-deposited silicon nitride fine powder has an average particle size of 10 nm (100 angstrom) with a rate of synthesis being 68 % and it has a uniform deposition of magnesium oxide on the surface thereof. In the above examples, specific examples relating to the production of ceramic coated silicon nitride fine powder coated with a ceramic film according to the invention are described, but the present invention should not be construed as limiting to these examples. For instance, the method of the invention is also applicable to deposition of a ceramic film on fine powders of various metal containing compounds such as silicon carbide, aluminium nitride, and titanium carbide.

Claims: A method for making a fine powder comprising a metal containing compound coated with a ceramic film on the surfaces thereof, which comprises the steps of: vaporizing a first metal constituting one element of said metal containing compound; blowing out the vapor of said first metal with a carrier gas into a gaseous mixture containing another element of said metal containing compound heated to a temperature of not larger than 0.8 times as low as the melting point of said first metal at a speed of 50 to 2000 m/sec so that said vapor of said first metal and said other element are reacted with each other while cooling, thereby obtaining a metal containing compound in a fine powder; coating a second metal onto the surface of said metal containing compound fine powder by heating said metal containing compound fine powder at a temperature of 800 to 1100 o C and by passing the resulting metal containing compound fine powder through the vapor of said second metal; and oxidizing said second metal coated on said metal containing compound fine powder to form a ceramic film on the surfaces of said metal containing compound fine powder. A method according to claim 1, wherein said first metal is selected from the group consisting of silicon, aluminium, titanium and zirconium. A method according to claim 1, wherein said second metal is selected from the group consisting of yttrium, aluminium, and magnesium. A method according to claim 1, wherein said first metal is silicon, said gaseous mixture is ammonia gas, said metal containing compound is silicon nitride, said second metal is yttrium, and said ceramic film is yttrium oxide.

Inventors: ABE, SUSUMU, OGAWA, MASAHIRO
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1985
Main classification: B22F1
IPC codes: C01B21, C04B35, C23C14, B22F9, B22F1