Abstract:
A method of continuously producing a non-oxide ceramic formed of a metal constituent and a non-metal constituent. A salt of the metal constituent and a compound of the non-metal constituent and a compound of the non-metal constituent are introduced into a liquid alkali metal or a liquid alkaline earth metal or mixtures to react the constituents substantially submerged in the liquid metal to form ceramic particles. The liquid metal is present in excess of the stoichiometric amount necessary to convert the constituents into ceramic particles to absorb the heat of reaction to maintain the temperature of the ceramic particles below the sintering temperature. Ceramic particles made by the method are part of the invention.

Description:
RELATED APPLICATIONS 
   This is a continuation of application Ser. No. 10/238,297 pending and application Ser. No. 10/238,791 filed Sep. 10, 2002 pending which was a continuation of application Ser. No. 10/125,988 filed Apr. 20, 2002 pending and application Ser. No. 10/125,942 filed Apr. 19, 2002, pending which were continuations of application Ser. No. 09/264,577 filed Mar. 8, 1999, U.S. Pat. No. 6,409,797 which was a continuation-in-part of application Ser. No. 08/782,816, filed Jan. 13, 1997, U.S. Pat. No. 5,958,106, which was a continuation-in-part of application Ser. No. 08/691,423, Aug. 2, 1996, U.S. Pat. No. 5,779,761, which was a continuation of application Ser. No. 08/283,358, Aug. 1, 1994, abandoned, the entire disclosures of which are incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   This invention relates to the direct production of non-oxide ceramic powders of carbides, nitrides, silicides, sulfides, borides or mixtures thereof from volatile compounds in a liquid metal reactor. Particular interest exists for liquid sodium as the reacting metal and the present invention will be described with particular reference to sodium but is applicable to other alkali and alkaline earth metal reducing media. Elemental materials have been successfully made by the Armstrong Process described in U.S. Pat. Nos. 5,779,761, 5,958,106 and 6,409,797, but those patents describe the use of one or more halide feed streams, whereas the present invention is related to ceramics using different feed streams than previously disclosed. For the purposes of this invention the feedstocks include those metals that have volatile liquid compounds such as, by way of example only, the halides of titanium, tantalum, zirconium, aluminum, vanadium and silicon. 
   SUMMARY OF THE INVENTION 
   The proposed process is an innovative low-temperature approach to producing non-oxide ceramic powders. The process uses the reaction of liquid metal sodium (or other alkali or alkaline earth metal) and two or more inorganic feedstocks, mixed together and introduced through a sonic nozzle submerged in the liquid sodium, to produce sodium salts of the anions of the feedstocks and the ceramic compound of the cationic constituents of the inorganic reactants. By way of example, a representative reaction to make titanium carbide is as follows:
 
8Na+TiCl 4 +CCl 4 →TiC+8NaCl
 
   The reaction products, TiC and NaCl are both solids in the temperature ranges at which the process operates and can be separated from liquid sodium by standard techniques leaving a water soluble salt and a ceramic powder. A mixed ceramic such as TiC/TiN can be made by introducing both carbon and nitrogen compounds into the sodium along with the volatile titanium compound. This may be accomplished by either a mixture of, for example, CCl 4  and N 2  or the single compound C 2 N 2  as follows:
 
16Na+4TiCl 4 +C 2 N 2 →2TiC+2TiN+16NaCl
 
   Using N 2  permits different ratios of the ceramics to be produced, while using C 2 N 2  results in equal molar quantities of each ceramic. 
   Accordingly, an object of the present invention is to provide a low-temperature and continuous ceramic production process, with short residence times, eliminating the excessive energy consumption requirements of current production technology, while providing great latitude in the product produced. 
   The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated. 
       FIG. 1  is a schematic representation of a continuous process for carrying out the invention; 
       FIG. 2  is a representation of a sonic nozzle and reactor for carrying out the invention; and 
       FIG. 3  is a flow diagram of a process for carrying out the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The liquid metal serves both as a reactant and a heat transfer medium. A quantity of each of the inorganic feeds is boiled in one or more inorganic supply tanks at a pressure greater than that required for sonic flow through the submerged injection nozzle. The mixed inorganic vapors flow through the nozzle beneath the liquid metal surface and react with the liquid metal to produce salts of the anionic parts of the feedstocks and ceramic powder. Other methods of obtaining predetermined vapor mixtures are intended to be included in this invention, the mixing chambers described above being only one. 
   The process begins with the injection of a vapor mixture through a sonic flow nozzle. Key to the process is the submerged reaction of the inorganic vapor in a liquid metal. The reaction process is controlled through the use of a choked-flow (sonic, or critical-flow) nozzle, described at page 5-14, of Perry&#39;s Chemical Engineers&#39; Handbook, sixth edition, McGraw-Hill Book Company, 1984, which is incorporated herein by reference. A choke-flow nozzle is a vapor injection nozzle that achieves sonic velocity of the vapor at the nozzle throat. Then, any change in downstream conditions that causes a pressure change cannot propagate upstream to affect the discharge. The downstream pressure may then be reduced indefinitely without increasing or decreasing the discharge. Under choke-flow conditions, only the upstream conditions need to be controlled to control the flow rate. The choke-flow nozzle serves two purposes: (1) it isolates the vapor generator from the liquid metal system, precluding the possibility of liquid metal backing up into the inorganic feed system and causing potentially dangerous contact with the liquid inorganic feedstock, and (2) it delivers the vapor at a fixed rate, independent of temperature and pressure fluctuations in the reaction zone, allowing easy and absolute control of the reaction kinetics. 
   The liquid metal stream also has multiple functional uses: (1) it rapidly chills the reaction products, forming solid powder products (2) it transports the chilled reaction products to a separator, and (3) it serves as a heat transfer medium allowing control of the reaction temperature. A liquid metal flow rate is selected to control the temperature increase through the reactor, preferably at less than about 200° C. to about 600° C. Liquid metal containing the reaction products, for instance, sodium chloride and titanium carbide, exits the reactor and enters a separator where most of the liquid metal is removed and recycled to the reactor. 
   The process of the invention may be practiced with the use of any alkali metal including mixtures thereof, or alkaline earth metal including mixtures thereof depending on the feed stock to be reduced. By way of example, sodium will be chosen, not for purposes of limitation but merely for purposes of illustration, because it is cheapest and preferred. Titanium tetrachloride will be chosen by way of example as one of the reacting inorganic materials and carbon tetrachloride as the other as they are readily available. Other gases useful in the invention include the halides of Ta, Zr, Si, and Al, as well as gaseous compounds of C, B, N, and Si. Mixtures of the gases can be used to manufacture mixed ceramics, such as, by way of example TiC/TiN. Si appears as both a metal constituent and a non-metal constituent, since Si can form ceramics with metals or non-metals. 
   Referring to  FIG. 1  there is shown a schematic block diagram representation of the invention in which a carbon tetrachloride boiler  20  is connected via line  23  to a mixing chamber  45 . The mixing chamber  45  receives titanium tetrachloride from a boiler  40  through a line  43 . In the mixing chamber  45 , an apparatus such as a fan (not shown) mixes the two gases which then exit the mixing chamber through a line  46  and enter a reactor  50 . The mixing chamber  45  may not require a fan or other means for obtaining a predetermined ratio of gases may be used. In the reactor  50  there is present liquid sodium which enters the reactor through a line  58  and a sonic nozzle  70 , see  FIG. 2 , hereinafter described, which introduces the mixed vapor of titanium tetrachloride and carbon tetrachloride submerged in a liquid sodium environment. 
   The reaction is exothermic and produces ceramic titanium carbide powder. All products leave the reactor  50  through an exit or outlet line  54 . The reacted materials, along with unreacted and excess liquid sodium, enter a separator  80  such as a filter to separate the solids including the reaction products of titanium carbide and sodium chloride from liquid sodium, the liquid sodium leaving the separator  80  via a line  91  and the solids leaving the separator  80  through a line  85  which leads to a vacuum still  140 . The vacuum still  140  draws off any entrained liquid sodium and recycles it via line  142  to a sodium pump  65  while the product or combined solids of sodium chloride and titanium carbide are transferred via line  141  to a wash station  145 . The product is washed with water thereby dissolving the sodium chloride leaving behind the ceramic titanium carbide powder to exit through a line  146  as product. The wash water containing the dissolved sodium chloride can be disposed of as required. 
   Referring to  FIG. 3  there is shown a more detailed engineering diagram in which a carbon tetrachloride supply  10  is connected via line  11  to a metering pump  15  and to the carbon tetrachloride boiler  20 . The carbon tetrachloride boiler  20  is fundamentally a heat exchanger  22  operated at a sufficient temperature to flash carbon tetrachloride from a liquid to a gas. A line  23  transports the gaseous carbon tetrachloride from the heat exchanger or flash boiler  20  to the mixing chamber  45 . Similarly, a titanium tetrachloride supply  30  is connected via line  31  to a metering pump  35  which transports the liquid titanium tetrachloride to a flash boiler  40  which is also a heat exchanger  42  operating at a temperature sufficient to convert the liquid titanium tetrachloride to vapor. The gaseous titanium tetrachloride is conducted via line  43  to the mixing chamber  45 , previously described, for mixing the gaseous carbon tetrachloride and the gaseous titanium tetrachloride therein, the combined gases leaving through the line  46  and being transported through a valve  47  to the reaction chamber  50 . 
   The reaction chamber  50  consists of a conduit  51  having a sodium inlet  52  and an outlet  53 . As shown in the Figure, a sonic nozzle  70  receives the mixed gas from the line  46  leaving the mixing chamber  45  and introduces the gas into the reaction chamber  50 , at conditions hereinafter to be described. 
   Sodium is provided from a source thereof  55 , illustrated as a sodium fill sump having an exit line  56  which passes through a valve  57  and becomes line  58  passing through a sodium flow meter  60  and then through a sodium pump  65  into the reaction chamber  50 . In the reaction chamber  50 , as shown in more detail in  FIG. 2 , there is a reaction zone  72  wherein the mixed gases from the sonic nozzle  70  (and mixing chamber  45 ) enter the liquid sodium environment which acts as a heat sink  75 . Downstream from the reaction zone  72  is a quench zone  74  denoted in broken lines. It is in the area of the reaction zone  72  that the reaction is believed to take place in the gaseous state, that is the reduction of the mixed titanium tetrachloride and carbon tetrachloride gases to the ceramic titanium carbide occurs at a very elevated temperature due to the exothermic nature of the reaction. The presence of greater than stoichiometric quantities of sodium necessary to reduce the gases to the corresponding elemental material provides a sufficient heat sink  75  and quench zone  74  such that the reaction products are quickly cooled below the sintering temperature of the titanium carbide formed and below the melting temperature of the sodium chloride formed during reaction. The rapid cooling of the reaction products below the melting point of the salt and the sintering temperature of the ceramic are important features of the present invention. 
   Another important feature of the present invention is that the entire system can be operated at relatively low temperatures, depending on the reducing metal employed. It is only required that the temperature of the system be greater than the melting point of the reducing metal. In the case of sodium, the system operating temperature must be greater than about 97° C., which is the approximate melting temperature of sodium. For safety sake, of course, the system will be operated at a sufficiently greater temperature than 97° C. to accommodate usual engineering fluctuations which occur in operating any system, preferably about 400° C. 
   Very high temperatures are not needed and are preferably avoided. This low temperature operation is also made possible by the fact that the mixed gases entering subsurface to a liquid sodium environment react completely and are reduced at the relatively low temperatures of the inventive system. In prior art systems, the constituents have to be heated to a much higher temperature to initiate a reaction, but for some reason, introducing the titanium tetrachloride and carbon tetrachloride vapor in a completely liquid sodium environment initiates the reaction at lower than expected temperatures providing all the advantages previously described. 
   After the reaction occurs, the reaction products are titanium carbide which is a solid particulate material at the temperatures at which the system is operated and sodium chloride which is also a particulate solid at the temperature at which the system operates and unreacted liquid sodium present in greater than stoichiometric quantities. This mixture of material leaves the reaction chamber  50  via a line  54  and is transported to a solids separator  80 . The solids separator  80  has a housing  81  and contains a series of baffles  82  which permit the solids to settle out while the liquid sodium is transported to a pair of filters  90   a ,  90   b . The solids exit the separator  80  through a line  85 , and may be transferred to a wash station  145  as previously described. The sodium in the normal operation of the loop exits the solids separator  80  through the filter  90   a  via line  91  and passes through a heat exchanger  95  wherein heat is removed. As before mentioned the reaction to produce the titanium carbide is exothermic thereby adding a significant quantity of heat to the liquid sodium. It is at the heat exchanger  95  that some of this heat is removed. The cooled sodium exiting the heat exchanger  95  through line  91  passes through valves  97  and  99  and joins with the line  56  to be recycled through line  58  to the sodium flow meter  60  and the pump  65 . 
   A cold trap  100  is provided with a housing  101  and a heat exchanger  102 . The cold trap  100  is operated intermittently and is used to precipitate any oxygen which may find its way into the sodium since sodium oxide is a corrosive and unwanted material. The heat exchanger  102  is operated at parameters sufficient to reduce the temperature of the liquid sodium to the point where sodium oxide precipitates. The cold trap  100  has a valve  104  intermediate line  91  and the cold trap inlet  105 . A valve  107  intermediate cold trap outlet  106  and line  91  isolates the cold trap  100  from the normal recycle of sodium through the reactor  50  and the solids separator  80 . 
   The filter  90 ( b ) in the solids separator  80  is used only intermittently and when the system is to be drained. The filter has an outlet line  111  leading to a valve  112 . Line  111  joins line  109  between a valve  108  and inlet  116  of a drain sump  115 . The drain sump  115  is sued, as is obvious to those of ordinary skill in the art, to drain the system for clean-up and maintenance. A vacuum pump  120  is connected through line  118  and valves  122  to drain sump  115 , thereby providing the necessary vacuum to the system. The vacuum  120  is also connected through valves  123 ,  134  and  137  to a line  131  connecting a source of inert gas  130  such as helium or argon and the fill sump  55 . Line  131  passes through a flow meter  132 . A valve  133  in line  136  connects line  131  with line  118  exiting from the drain sump  115 . Accordingly, the fill sump  55  which stores make-up sodium for the system is under an inert gas atmosphere maintained at a pressure determined by the vacuum pump  120 . 
   The operation of the system has been described in general but the reactor  50  may be a 10 cm stainless steel vessel while the heat exchanger  22  and  42  may be constructed of materials and of a design known to persons of ordinary skill in the art. The mixed gas which flows from the mixer  45  into the reactor  50  by means of the sonic nozzle  70  flows at a pressure which is controlled by the boiler temperatures heating the feeds until the vapor pressures of the inorganic feeds exceed the critical pressure ratio as determined by the ambient pressure of the reactor  50 . The ambient pressure of the reactor  50  is determined by the vacuum pump  120 . The nozzle  70  may have (by way of example only) a diameter of 1 cm and positioned within a 5 cm diameter reaction vessel)  50  so that the gas is completely submerged within the liquid sodium medium. All parts of the apparatus in contact with the inorganic vapors from boilers  20  and  40  must be maintained above the temperature which gives a vapor pressure of both inorganic feeds greater than the critical pressure ratio by suitable heat tracing of the various connecting pipes and control valves, all as is well known in the engineering art. It is imperative, for safety purposes, that the mixed gas flowing through the nozzle  70  into the liquid sodium environment is at greater than sonic velocity. 
   Accordingly, it is preferred that the vapor be between two atmospheres and about ten atmospheres of pressure in order to ensure that the velocity of the mixed gas is greater than the sonic velocity. By operating at greater than sonic velocity, it is ensured that the sodium does not back up into the nozzle  70  thereby possibly clogging the nozzle but also possibly creating a dangerous environment in which an explosion could occur. 
   Preferably the sodium is maintained at less than about 600° C. It is understood that because of the exothermic nature of the reaction, that the reaction zone  72  and portions of the quench zone  74  will be at temperatures greater than the operating temperatures of the system. Nevertheless, because of the excess of liquid sodium (or other reducing metal) with respect to the stoichiometric quantities necessary to reduce the titanium tetrachloride and carbon tetrachloride, the quenching action is so rapid that the formed titanium carbide particles do not sinter. This is not to say that the formed titanium carbide particles do not at some instant in time have a temperature greater than the sintering temperature, that is particles may be formed at a temperature in excess of the titanium carbide sintering temperature but they are rapidly quenched to a temperature below the sintering temperature while at the same time being transported downstream due to the flow of sodium past the nozzle  70 . It is the combination of this excess of sodium having a high heat capacity along with the flow and mixing generated by the reaction which prevents the titanium carbide particles from sintering to any significant degree which is a major aspect of the present invention. The reaction zone  72  and the quench zone  74  are shown for purposes of illustration only and do not represent an accurate determination of how far downstream the zones extend. 
   Although the example given refers to titanium carbide as the product, it should be understood that a variety of ceramics can be made by the subject invention. Most preferably, volatile liquid compounds such as the halides of titanium, tantalum, zirconium, silicon and aluminum may be combined with one or more of various halides of carbon, boron, silicon, sulfur and/or various other gaseous compounds, such as C 2 N 2  or others in order to provide single or mixed ceramics. As before stated, mixed compounds such as cyanogen (C 2 N 2 ) may be used as a feedstock alone or in combination with other sources of non-metallics to provide mixed carbide and nitride ceramics, while other non-metal constituents, such as boron, silicon and sulfur, may be used to provide a variety of ceramics. In separating the produced salt, distillation is probably preferred to water washing, but this is within the skill of the relevant art. 
   While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.