System and method for atomization of liquid metal
CROSS-REFERENCE TO PRIOR APPLICATION 
     This is a continuation-in-part of U.S. patent application Ser. No. 07/780,924, filed Oct. 22, 1991 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the field of atomization of liquid metals, to produce metallic powders. The invention also relates to the field of cryogenic gases, and provides a system and method for producing a stream of cold gas, the temperature and pressure of the stream being very precisely regulated. 
     Metal powders are useful in various applications. For example, in the manufacture of printed circuit boards, conductive layers are applied to a substrate in the form of metal powder. If the particles of the powder are too coarse, conductors of the circuit pattern may become short-circuited. To maximize the line density, and to increase the efficiency and yield of the manufacturing process, one needs a metal powder having small, fine, spherical particles. 
     Metal powders are also useful in applying a uniform metallic coating to a surface, such as by flame spraying or welding. As in the case of printed circuit boards, a uniform coating requires small, spherical, and uniform particles. 
     Still another application of metal powders is in metal injection molding. In this process, metal powder is mixed with a plastic material and is formed into a shaped article, the particles of the powder becoming fused together with the application of heat. Again, the results of this type of process are most favorable when the particles are small, spherical, and uniform. 
     Metal powders can also be used for other purposes, such as for soldering and sintering. 
     Methods of making metal powders have been known in the prior art. A metal powder can be made by directing a pressurized gas, at ambient temperature, towards a liquid metal. The liquid metal is atomized by the gas, and cools to form a powder. The gas is preferably inert, or relatively inert, to prevent oxidation of the metal. The preferred gas is nitrogen, which remains substantially inert throughout a wide range of temperatures. 
     It has also been known to use a cryogenic liquid, instead of a gas, as the agent which atomizes the liquid metal. 
     The present invention uses a cold gas to atomize the liquid metal, to form a metal powder. A major problem with such use of cold gas is in the need to control accurately the pressure and temperature of the gas. Such control is necessary to allow precise control of the distribution of particle sizes, and to control the configuration of the particles. It has been found necessary that the pressure fluctuations be less than about 1 psi, and the temperature fluctuations should be less than about ±2° F. 
     Although cryogenic fluid delivery systems have been known for a long time, it has proven difficult to provide a cold gas stream having the above degree of consistency. Examples of dispensing systems of the prior art are shown in U.S. Pat. Nos. 4,909,038, 4,715,187, 4,336,689, 4,961,325, and 4,570,578. Other systems of the prior art include heaters which vaporize specific volumes of liquefied gas, and which use additional trim heaters to achieve desired gas temperatures. None of the above-mentioned systems provides the precision of control of temperature and pressure required in the liquid metal atomization process. 
     Another problem in the production of metal powders is the appearance of multiple &#34;phases&#34;. That is, when a two-component alloy is melted and then slowly cooled, one component may solidify first, causing localized regions of increased concentration of that component. The separated components may manifest themselves as streaks, or dendrites, in the particles of the finished powder. This effect makes the particles less spherical and less homogeneous, and should therefore be minimized. 
     The present invention solves the above-described problems by providing an apparatus and method which produces a consistent cold gas stream, and which can be used to atomize liquid metals. The apparatus is simple, economical, and reliable, and provides a stream of gas which fulfills the temperature and pressure criteria specified above. The invention is not limited to use in liquid metal atomization, but can be used in any system or process which requires a consistent cold gas stream. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a cold gas stream is used to atomize a liquid metal, thereby producing metal particles forming a powder. The cold gas not only atomizes the liquid metal, but also cools the resulting metal particles, and yields a clean and shiny powder. The metal particles are cooled very rapidly by the cold gas, and the result is a very fine and uniform powder. The above-described method also has a high throughput rate. 
     The invention also includes a method and apparatus for producing the cold gas stream. This cold gas stream originates from two separate streams, one cold and one relatively warm. The cold stream is preferably obtained by subcooling a liquefied gas stream to obtain a liquid having a constant temperature of -320° F., regardless of its pressure. The warm gas stream is at ambient temperature. The cold and warm streams are passed through pressure regulators, so that they have the same pressure. When the cold and warm streams are combined, the liquid stream vaporizes. The initial liquid gas stream and warm gas streams are combined in proportions chosen such that the combined cold gas stream has a desired temperature. 
     The combined stream then passes into an insulated container. The container defines an interior region having a volume significantly greater than the volume of the conduits leading to the chamber. Thus, the container acts as a buffer to reduce fluctuations in gas pressure. 
     Disposed within the container is a finned-tube heat exchanger coil, through which the gas stream passes. One end of the coil opens to the interior of the container, the other end of the coil being connected to an outlet line. If the coil is sufficiently long, the gas flowing through the coil comes into temperature equilibrium with the gas in the interior of the container. Thus, the gas appearing at the outlet line has an essentially constant temperature. The gas at the outlet line also has a constant pressure, due to the buffering effect of the chamber. The temperature of the output stream can be varied by adjusting the proportions of the initial cold and warm gas streams used to make the mixture. 
     It is therefore an object of the present invention to provide an improved method and apparatus for making metal powders. 
     It is another object of the present invention to provide a system and method of providing a consistent cold gas stream, such as can be used to atomize liquid metals. 
     It is another object to provide a cold gas stream in which the pressure variations in the stream are not more than about 1 psi, and wherein the temperature fluctuations are less than about ±2° F. 
     It is another object to provide a cold gas stream, the temperature of which can be determined in advance. 
     It is another object to produce a consistent cold gas stream in an efficient and economical manner. 
     It is another object to enhance the efficiency and reliability of a liquid metal atomization process, so as to produce metal powders having particles of desired size and uniformity. 
     It is another object to provide a cold gas stream which originates from two separate streams, one in gaseous form and one in liquid form. 
    
    
     Other objects and advantages of the invention will be apparent to those skilled in the art, from a reading of the following brief description of the drawing, the detailed description of the invention, and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a schematic diagram showing the system made according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a system and method for producing a metal powder. The invention also includes an apparatus and method for providing a consistent cold gas stream, which can be used to atomize a liquid metal. The gas stream is typically nitrogen, and the invention will be described with respect to nitrogen. However, it is understood that other gases, especially inert or relatively inert gases, could be used instead of nitrogen, according to the same principles. 
     As used herein, the term &#34;cold gas&#34; means a gas whose temperature is lower than ambient temperature, but higher than the temperature at which the gas becomes a liquid. When used for atomizing a molten metal, the temperature range of interest lies between about -50° F. and about -250° F., but the term &#34;cold gas&#34; is intended to include the broader definition given above. 
     In the FIGURE, liquid nitrogen is provided from a tank (not shown) and is conveyed, through conduit 1, into subcooler 2. The liquid nitrogen is cooled, in the subcooler, to a temperature of -320° F., regardless of the inlet pressure. The subcooled liquid nitrogen then passes to pressure regulator 3. 
     The subcooler can be constructed according to the teachings of U.S. Pat. No. 4,510,760, entitled &#34;Compact Integrated Gas Phase Separator and Subcooler and Process&#34;, the disclosure of which is incorporated by reference herein. Other subcooler structures can also be used. Also, one can practice the invention without a subcooler. However, use of the subcooler is preferred because it produces a liquid nitrogen stream which is consistent in temperature, regardless of liquid pressure, and because it eliminates all gaseous components from the liquid supply. 
     Meanwhile, a source (not shown) of gaseous nitrogen, preferably at ambient temperature, is connected to supply conduit 4. The gaseous nitrogen passes through pressure regulator 5. Pressure regulators 3 and 5 are set such that the pressure in the gaseous line 4 equals the pressure in the liquid line. The liquid and gas streams are applied to three-way proportional control valve 6, in which the streams are blended, in a desired ratio, to produce a cold gas having a desired predetermined temperature. Thus, the liquid nitrogen is vaporized in valve 6, when the liquid is mixed with the warm gas, to produce a cold gas in conduit 7. 
     The cold gas mixture then passes, through conduit 7, to a vacuum-insulated surge vessel 8. The vessel defines an interior region 9 which acts as a pressure surge buffering chamber, and which is sufficiently insulated so that heat does not infiltrate into the cold gas stream. The pressure in region 9 is monitored by gauge 12. The volume of region 9 is significantly larger than the effective volume of the conduits leading from the sources of liquid and gaseous nitrogen. As illustrated in the FIGURE, the volume of region 9 is at least one order of magnitude, and preferably several orders of magnitude, greater than the effective volume of the conduits. Due to this difference in volume, pressure fluctuations in the line are damped by the greater volume of gas in the chamber, and the pressure of the gas in the chamber therefore remains substantially constant. 
     The cold gas in the chamber passes through temperature equalization coil 10. As shown in the FIGURE, one end of the coil is open to region 9, i.e. the interior of the coil is fluidly connected to the interior of the chamber. The coil is connected to outlet line 16. Gauge 13 measures the pressure of the gas leaving the vessel, and pressure regulator 14 can be used to reduce the pressure further, if necessary, to the level required for a specific application. The final output pressure can be monitored with gauge 15. 
     The coil is preferably of sufficient length to allow the cold gas within the coil to come into thermal equilibrium with the interior region 9, but not so long as to create an appreciable pressure drop within the coil. Because the cold gas in the coil is made to come into thermal equilibrium with the cold gas outside the coil, in region 9, the temperature of the cold gas in the coil is very stable. Thus, the temperature of the cold gas leaving the coil, through outlet line 16, is also essentially constant. 
     Coil 10 is preferably constructed as a finned-tube heat exchanger, but it can also assume other forms. In general, it is necessary only that the gas in the chamber pass through an elongated conduit, disposed within the chamber, so that the gas can come into thermal equilibrium with the gas in the region outside the conduit. 
     The temperature of the cold gas stream is regulated by temperature controller 11 and control valve 6. Controller 11 is connected to outlet line 16, and monitors the temperature of the gas in the line. In response to changes in the temperature of the cold gas stream, controller 11 adjusts the setting of valve 6, to change the proportion of liquid and gaseous nitrogen components in the original mixture. If the temperature in line 16 is too high, controller 11 causes valve 6 to admit more liquid nitrogen from subcooler 2. If the temperature in line 16 is too low, controller 11 causes valve 6 to reduce the amount of liquid nitrogen from subcooler 2. 
     The cold gas which is withdrawn from line 16 is therefore consistent in both pressure and temperature, and is substantially free of surges of pressure, temperature, or flow rate. 
     The present invention also includes a method for making a metal powder. According to this method, one directs a stream of cold gas through an atomizing nozzle and towards a stream of liquid metal, thereby atomizing and cooling the liquid metal, and producing the metal powder. In the preferred embodiment, one obtains the cold gas stream from the apparatus described above. The resulting metal powder contains small, fine, spherical particles. The powder is substantially homogeneous, and free of multiple phases, described above. 
     In practicing the above-described method for making a lead solder powder, for example, experiments have produced optimum results when the temperature of the cold gas entering the nozzle is in the range of about -140° F. to about -200° F., with the preferred temperature being about -150° F., and when the pressure of the cold gas is in the range of about 30-40 psig. The lower the pressure, the greater the percentage of larger particles in the resulting powder. Conversely, higher pressures produce a greater percentage of smaller particles. Thus, the pressure directly affects the size distribution of particles in the powder. Powders having predominantly large particles and powders having mainly small particles both have utility, in varying applications. 
     The apparatus used for performing the atomization is essentially similar to that used in prior art atomization processes. The only major differences are that in the present invention, one may need to insulate the conduit carrying cold gas to the atomizing nozzle, and that one must physically separate the equipment for cooling the atomizing gas from the equipment which melts the metal to be atomized. It is an important feature of the present invention that one can achieve superior results by passing a cold gas, as defined above, through a conventional atomizing nozzle. 
     While the invention has been described with respect to the particular embodiment shown in the FIGURE, it is understood that the physical arrangement may be modified, within the scope of the invention. The initial sources of liquid and gas can be varied, as can the shape of the pressure surge chamber and temperature equalization coil. The arrangement of valves and gauges can be varied. As noted above, the invention can be practiced with gases other than nitrogen. Also, it is intended that the gas in conduit 4 be the same substance as the liquid in conduit 1 (such as nitrogen), but it is possible to use different substances in these different conduits. These and other similar modifications should be considered within the spirit and scope of the following claims.