Abstract:
Methods of fabricating cermet materials and methods of utilizing the same such as in filtering particulate and gaseous pollutants from internal combustion engines having intermetallic and ceramic phases. The cermet material may be made from a transition metal aluminide phase and an aluminia phase. The mixture may be pressed to form a green compact body and then heated in a nitrogen-containing atmosphere so as to melt aluminum particles and form the cermet. Filler materials may be added to increase the porosity or tailor the catalytic properties of the cermet material. Additionally, the cermet material may be reinforced with fibers or screens. The cermet material may also be formed so as to pass an electrical current therethrough to heat the material during use.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 10/213,120 entitled CERMET MATERIALS, SELF-CLEANING CERMET FILTERS, APPARATUS AND SYSTEMS EMPLOYING SAME, filed on even date herewith. 
    
    
     STATEMENT OF GOVERNMENT RIGHTS 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC07-99ID13727 and Contract No. DE-AC07-05ID14517 between the United States Department of Energy and Battelle Energy Alliance, LLC. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to cermet filters used to filter combustion products from internal combustion engines. More particularly, this invention relates to a porous, intermetallic-ceramic composite filter that is self-cleaning and can improve emissions from internal combustion engines. 
     2. State of the Art 
     Emissions from vehicles, such as heavy-duty diesel engine vehicles, contribute greatly to pollution problems of the United States of America (USA). Heavy-duty vehicle emissions produce ozone, particulate materials (PM), nitrogen oxides (NO X ), sulfur oxides (SO X ), and volatile organic compounds (VOCs). These emissions can cause adverse health effects such as premature mortality, aggravation of respiratory and cardiovascular disease, chronic bronchitis, changes to lung tissues and structures, and altered respiratory defense mechanisms, among other things. Further, ozone is known to cause crop and forestry losses and PM causes damage to materials and soiling. NO X  and PM also significantly contribute to unsightly smog and substantial visibility impairment in many parts of the USA. 
     Emissions from diesel engine heavy-duty trucks significantly contribute to these problems throughout the country. By 2007, heavy-duty vehicles will account for 29 percent of NO X  and 14 percent of PM emissions in the USA. These proportions are even higher in some urban areas. For example in Albuquerque, N. Mex., heavy-duty vehicles contribute 37 percent of NO X  and 20 percent of PM emissions. 
     The United States Environmental Protection Agency (EPA) is proposing a PM emissions standard for new heavy-duty engines of 0.01 grams per brake-horsepower-per hour (g/bhp-hr) to take full effect in 2007. The EPA is also proposing more stringent standards for NO X  and VOC emissions to be phased in between 2007 and 2010. In order to meet these rigorous new standards, new and improved filters are needed. 
     Engine and catalyst manufacturers have experimented with many catalytic converters and with a wide variety of regenerative catalytic traps. Precious metal catalytic traps are somewhat effective in oxidizing gaseous hydrocarbons and CO as well as the particulate soluble organic fraction (SOF). However, precious metal catalysts are very expensive. Base metal catalytic traps promote soot oxidation but have little effect on NO X , CO X , or SO X . 
     Intermetallic-ceramic catalyst supports have been produced in the past. For example, U.S. Pat. No. 5,951,791 to Bell et al. discloses using nickel aluminide to coat the inside of an alumina fiber preform. U.S. Pat. No. 5,774,779 to Tuchinskiy and U.S. Pat. No. 4,990,181 to Pierotti et al. disclose using nickel aluminide as a catalyst support. U.S. Pat. No. 4,992,233 to Swaroop et al. discloses using iron aluminide alloys in exhaust filter applications. Also, U.S. Pat. No. 5,496,655 to Lessing discloses using a porous NiAl or Ni 3 Al with a ceramic filler to catalyze steam reforming of hydrocarbons to power fuel cells. 
     Porous ceramic filters made from carbide and oxide materials are well known in the art. However, conventional ceramic filters used with heavy-duty diesel engines have a significant lifetime problem. These ceramic filters have a short lifetime due to severe carbon particulate plugging and structural failure due to high vehicle motion stresses and extreme thermal stresses. Typically, the filters are plugged after 500 hours of service, which makes them a financial and operational liability for companies. Further, the filters often require complicated heating and control systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention fulfills the need in the art for a strong and tough porous intermetallic-ceramic composite filter that not only filters particulates but reduces undesirable gaseous pollutants. The present invention eliminates the need for complicated systems to heat the filter and is self-cleaning so it does not plug up with particulate matter. Further, the intermetallic-ceramic composite filters of the present invention do not require coating with other materials for functioning. 
     The present invention comprises a self-cleaning filter used for filtering particulate and gaseous pollutants from internal combustion engines. The filter is made from a porous cermet-type material having an intermetallic phase and a ceramic phase. 
     An exemplary embodiment uses a porous cermet filter having a transition metal aluminide phase such as cobalt, iron, nickel, or titanium-type aluminides and an alumina phase. 
     In another exemplary embodiment, a reinforcement material such as metal fibers, ceramic fibers, or metal screens may be incorporated into the porous cermet filter for added strength. 
     In another exemplary embodiment, the porous cermet filter is electrically conductive and a current may be passed therethrough to heat it during use. 
     In another exemplary embodiment, at least one resistive heating element may be incorporated into the porous cermet filter during manufacture. An electrical current may be applied to the resistive heating element to heat the porous cermet filter during use. 
     In another exemplary embodiment, the resistive heating element has a coefficient of thermal expansion approximately the same as that of the cermet material. 
     In another exemplary embodiment, the resistive heating element may be coated with a ceramic material prior to incorporation into the cermet filter. 
     In another exemplary embodiment, the ceramic material coating the resistive heating element electrically insulates it from the cermet material. 
     In another exemplary embodiment, an external heating element may be provided to heat the porous cermet filter during use. 
     In another exemplary embodiment, the cermet may be manufactured using a combustion synthesis process by forming a green compact of a transition metal, aluminum, and alumina particles to produce a transition metal aluminide-alumina porous cermet filter. Alternatively, aluminum and thermite particles may be used to produce a nickel aluminide-alumina porous cermet filter. 
     In another exemplary embodiment, the combustion synthesis process may be performed under a nitrogen-containing atmosphere to form ammonia-producing phases in the porous cermet filter. 
     In another exemplary embodiment, the combustion synthesis process may be performed with sugar added to the green compact under a nitrogen-containing atmosphere to form ammonia-producing phases in the porous cermet filter. 
     In another exemplary embodiment, the combustion synthesis process may be performed with an alkali carbonate or an alkali bicarbonate and sugar added to the green compact under a nitrogen-containing atmosphere to form ammonia-producing phases and hydrocarbon gas-producing phases. In this embodiment, an alkali oxide is formed during the combustion synthesis process, which may either be leached out to increase porosity or left in the cermet filter because of its ability to absorb NO X . 
     In another exemplary embodiment, a sacrificial filler may be added to the green compact to increase the porosity of the cermet filter. 
     In another exemplary embodiment, a porous cermet filter may be produced with a graded porosity by layering relatively larger and smaller sacrificial filler materials in the green compact. 
     In another exemplary embodiment, electrodes may be incorporated into the porous cermet filter during the combustion synthesis process. 
     In another exemplary embodiment, a housing may be bonded to the porous cermet filter during the combustion synthesis process. 
     The disclosed invention also encompasses methods of manufacture and use of the inventive cermet filter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention: 
         FIG. 1  is sectional view of a porous cermet filter. 
         FIGS. 2A and 2B  are photomicrographs of the microstructure of the cermet material in FIG.  1 . 
         FIG. 3A  is a sectional view of a porous cermet filter produced with sugar filler materials. 
         FIG. 3B  is a sectional view of a porous cermet filter produced with human hair sacrificial filler materials. 
         FIG. 3C  is a longitudinal view of a porous cermet filter produced with human hair sacrificial filler materials. 
         FIG. 3D  is a sectional view of a porous cermet filter having a graded porosity. 
         FIGS. 3E and 3F  are views of a porous cermet filter having a graded porosity across the length thereof. 
         FIG. 4A  is a longitudinal view of a porous cermet filter with a reinforcement material. 
         FIG. 4B  is a sectional view of a porous cermet filter with a reinforcement material. 
         FIG. 4C  is a sectional view of a porous cermet filter with multiple resistive heating elements. 
         FIGS. 5A and 5B  are gas chromatography analyses of an activated cermet filter produced by a combustion synthesis process under a nitrogen atmosphere. 
         FIGS. 6A and 6B  are gas chromatography analyses of an activated cermet filter produced by a combustion synthesis process using a sodium bicarbonate and sugar filler material. 
         FIG. 7  is a view of electrodes bonded to a cermet filter. 
         FIG. 8A  is a sectional view of a cermet filter having multiple resistive heating elements. 
         FIG. 8B  is a view of the cermet filter of FIG.  9 A. 
         FIG. 8C  is a sectional view of a resistive heating element coated with a ceramic material. 
         FIG. 9  is a sectional view of a cermet filter with an external heating element. 
         FIGS. 10A and 10B  are sectional views of a cermet filter bonded to a filter housing. 
         FIGS. 11A and 11B  are flow diagrams showing a porous cermet filter coupled to an exhaust system of an internal combustion engine from a vehicle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a sectional view of filter  10  made from cermet  12  and having pores  14  is shown. For purposes of illustration, the sizes of pores  14  in relation to filter  10  are exaggerated. The porous structure of filter  10  is used for its high internal surface area in order to trap particulate matter and for chemical reactions. In an exemplary embodiment, pores  14  are irregular in shape and non-linear. In another exemplary embodiment, filter  10  may be disk or cylinder shaped. Cermet  12  in filter  10  is a metallic-ceramic composite material. Cermet  12  of filter  10  may be coating free, in that it lacks a coating of another material deposited over the inside of pores  14 . 
     Referring to the photomicrographs of  FIGS. 2A and 2B , a representative microstructure of cermet  12  is shown having a ceramic phase  16  and a metallic phase  18 . As shown in  FIGS. 2A and 2B , ceramic phase  16  may be discontinuous and bonded by metallic phase  18 .  FIG. 2A  shows a 50 wt % NiAl-50 wt % alumina cermet  12  formed from fine alumina particles approximately 5-10 μm in size, nickel particles, and aluminum particles.  FIG. 2B  shows a 50 wt % NiAl-50 wt % alumina cermet  12  formed from coarse ceramic particles approximately 100-200 μm in size. Various intermetallic compounds may be used for metallic phase  18 , such as aluminide compounds of the form AB or A 3 B. In an exemplary embodiment, the metallic phase  18  is an aluminide such as NiAl, Ni 3 Al, FeAl, Fe 3 Al, CoAl, CO 3 Al, or other transition metal aluminides due to the desirable catalytic properties of transition metals. Transition metals are defined as the elements Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Ceramic phase  16  may be oxide or non-oxide ceramics. Exemplary materials for ceramic phase  16  include alumina (Al 2 O 3 ), zirconia silicate (ZrSiO 4 ), zirconia, mullite, cordierite, or iron III oxide (Fe 2 O 3 ). In an exemplary embodiment, ceramic phase  16  may be a refractory material, allowing filter  10  to be used at very high temperatures. 
     Depending on the intended use of filter  10 , the composition of cermet  12  may be tailored to affect certain functional characteristics such as incorporating specific catalysts to perform selected catalysis or control of the coefficient of thermal expansion (CTE). An exemplary cermet  12  may exhibit properties such as high melting temperatures (greater than 1500° C.), high fracture toughness (22 MPa-ml), high strength, high oxidation resistance, controlled porosity (greater than 50%), inexpensive starting materials, an ability to be formed into net shapes, electrically conducting, light weight (less than 3 grams/cm 3 ), high heat capacity, and controllable CTE. 
     Cermet  12  may be made using conventional powder metallurgy techniques. However, an exemplary manufacturing method mixes metallic and ceramic powders and uses a combustion synthesis process in order to form filter  10  in a single step. In this process, a net shape filter  10  can be produced. In an exemplary embodiment, a mixture is formed of aluminum and transition metal powders mixed together in a 1:1 atomic ratio and alumina powder. A small amount of organic binder, such as one weight percent polyvinyl alcohol (PVA), may be used. Upon combustion synthesis, cermet  12  is formed having an alumina ceramic phase  16  and an aluminide metallic phase  18  such as NiAl, CoAl, or FeAl. In another exemplary embodiment, aluminum and transition metal powders such as nickel, cobalt, or iron powders are mixed in a ratio of 1:3 to form an aluminide metallic phase  18  such as Ni 3 Al, Fe 3 Al, or Co 3 Al. In another exemplary embodiment, a sacrificial filler such as polystyrene spheres or fibers may be added to the mixture to increase formation of pores  14 . In all cases, the green body is placed in a structurally configured mold and pressed into discs or cylinders to form a green compact. 
     The green compact is then fired in a furnace between 600° C.-700° C. or higher to set off the combustion synthesis process. At 660° C., the aluminum powder melts. The molten aluminum spreads over the surface of the transition metal particles by capillary action. Molten aluminum reacts with the transition particles in a highly exothermic reaction to form an aluminide compound. The extremely reactive molten aluminum reacts with the transition metal powder and sets off a solid flame. The alumina is not involved in the reaction. The solid flame is an intensely luminous combustion front separating the initial reactant mixture and the solid combustion product. In a pure metallic combustion synthesis process, the combustion front is white-hot and the temperature is extremely high. However, in the exemplary embodiments of the present invention, the presence of ceramic phase  16  made from alumina moderates the combustion process. The combustion process continues until the entire green compact has been reacted to form a net shape cermet  12  with an aluminide metallic phase  18  and an alumina ceramic phase  16 . As an example, the chemical reaction involved when nickel is used as the transition metal is shown below.
 
Aluminum combustion synthesis: 3Ni+3Al+Al 2 O 3 →3NiAl+Al 2 O 3   (1) 
 
     In another embodiment of the combustion synthesis process, NiAl-alumina cermets may be formed using aluminum particles and thermite (NiO) particles as starting materials. When aluminum reaches the melting temperature, it is extremely reactive. The molten aluminum reduces NiO to Ni. During this reduction step, the Al reacts with the oxygen to form alumina, and the remaining molten Al combusts with Ni to form NiAl. The energy released by alumina and NiAl formation is tremendous and would raise the overall reaction temperature higher than that from the aluminum combustion synthesis process. Net shape bulk products formed by the aluminum-thermite reaction would thus achieve higher densities because the reaction temperature is significantly higher than the reaction in the previous embodiment. The chemical reaction for the aluminum-thermite reaction is shown below.
 
Aluminum-thermite combustion synthesis: 5Al+3NiO→3NiAl+Al 2 O 3   (2) 
 
     If a sacrificial filler is added, it is burned off during this process, assisting in leaving pores  14  behind. The sacrificial filler and the volume change due to the formation of aluminide metallic phase  18  all contribute to the formation of pores  14 . In an exemplary embodiment, spherical and fiber sacrificial filler materials are used to produce a highly porous cermet  12  with non-linear pore channels. Consolidation pressure affects porosity development of cermet  12 . Higher consolidation pressures lead to a denser, less porous material. Also, the size and shape of the ceramic particles used for ceramic phase  16  control the shape and size of pores  14 . Larger ceramic particles lead to a large pore size for pores  14 . For example, when the ceramic particle size was between 5-10 μm, the average porosity of pores  14  is less than 10 μm. When the ceramic particle size was between 100-200 μm, the average porosity is greater than 50 μm. Exemplary materials for the sacrificial filler are polystyrene particles, Styrofoam® particles, sugar, corn starch, hair, and organic long, thin fibers. Referring to  FIG. 3A , a photomicrograph of a cross-section of cermet  12  produced with sugar filler materials is shown. The sizes of pores  14  in  FIG. 3A  ranged from submicron to 150 μm. 
     In another exemplary embodiment, hair fibers are frozen in liquid nitrogen and then crushed into short fragments. The hair fibers mix well with the metal and ceramic powders and are compacted into the green compact. The green compact is fired under the conditions previously discussed herein. During combustion synthesis, the hair fibers burn out and produce linear or longitudinal pore channels. These type of pores and channels facilitate good gas and fluid transport through the filter and produce a lower pressure drop across filter  10  during use. The photomicrograph in  FIG. 3B  shows a cross-section of pores  14  produced with a human hair sacrificial filler.  FIG. 3C  shows a longitudinal section of pores  14  produced with a human hair sacrificial filler. It is currently believed by the inventor that animal hairs will produce similar results. The sacrificial filler materials need not be limited to the examples herein and any suitable combustible materials may be used in the process as long as they substantially burn off to assist in formation of pores  14 . 
     Referring to  FIGS. 3D-3F , in another exemplary embodiment, a graded porosity in cermet  12  may be produced during the synthesis process. Relatively smaller sacrificial filler materials produce smaller pores  14  while relatively larger sacrificial filler materials produce larger pores  14 . Sequential layering of smaller and larger sacrificial filler materials in the powder mixture in the mold during the consolidation operation produces a graded porous cermet  12 . Referring to  FIG. 3D , the porosity may be graded in the direction from top to bottom of cermet  12 . Referring to  FIGS. 3E and 3F , the porosity may be graded across the length of cermet  12 . After combustion synthesis, pores  14  of cermet  12  will retain layers of pores reminiscent of the sizes of the initial sacrificial filler materials. 
     Referring to  FIGS. 4A-4C , in another exemplary embodiment, reinforcement material  17  can be added to the green compact prior to combustion.  FIG. 4A  is a longitudinal view of cermet  12  synthesized with a metal reinforcement material  17 .  FIG. 4B  is a sectional view of cermet  12  synthesized with a metal reinforcement material  17 .  FIG. 4C  is a sectional view of cermet  12  reinforced by multiple resistive heating elements  20 . Exemplary structures suitable for use as metal reinforcement  17  include long, thin metal fibers or metal screens which are incorporated into the green compact prior to consolidation. Exemplary metals suitable for use as reinforcement material  17  include stainless steels, nickel superalloys, cobalt alloys, titanium alloys, and combinations thereof. In another exemplary embodiment, reinforcement material  17  may be ceramic fibers such as alumina, silicon carbide, or silicon nitride. In an exemplary embodiment, reinforcement material  17  may be fused into cermet  12  during the combustion synthesis process. 
     In another exemplary embodiment of the combustion synthesis process, the manufacturing of filter  10  may be performed under a nitrogen atmosphere. Cermet  12  of filter  10  will emit ammonia after activation by water.  FIGS. 5A and 5B  show the gas chromatography (GC) analysis of the ammonia standard and the ammonia generated from a cermet filter after activation. PH paper tests further confirmed generation of ammonia from cermet  12  by turning dark blue after cermet  12  was activated, indicating the presence of ammonia. During ammonia emission, cermet  12  generates a significant amount of heat. The exothermic reaction indicates formation of new phases in cermet  12  during the combustion synthesis reaction. These new phases are currently believed by the inventor to be nitrogen-containing nitride compounds that will hydrolyze to give off ammonia. It is well known that bulk AlN and Si 3 N 4  can be synthesized from metal powder compacts by a high pressure nitrogen combustion process. It is currently believed by the inventor that the combustion synthesis processes can form other metal nitrides. These new phases formed in cermet  12  are currently believed by the inventor to be some type of metal nitrides. These nitride phases produce ammonia when they are activated by water. The ammonia emission phenomenon is also observed with cermet filters synthesized with sacrificial filler materials such as sugar. The general combustion synthesis reactions for ammonia-producing cermet filters are represented by the chemical equations below. 
         Ni   +   Al   +       Al   2     ⁢     O   3         ⁢           ⁢     →     N   2       ⁢           ⁢     NiAl   +       Al   2     ⁢     O   3       ⁢           +   Ω         
         Ni   +   Al   +       Al   2     ⁢     O   3       +   sugar     ⁢           ⁢     →     N   2       ⁢           ⁢     NiAl   +       Al   2     ⁢     O   3       ⁢           +   Ω         
         Ω=ammonia-producing phases
 
Alternatively, the aluminum-thermite combustion synthesis process may be used instead of the aluminum combustion synthesis process to produce a nickel aluminide-alumina cermet having ammonia-producing phases.
       

     In another exemplary embodiment, mixed filler materials, sodium carbonate (Na 2 CO 3 ) or sodium bicarbonate and sugar, are used in the combustion synthesis of cermet  12 . Alternatively, any other alkali carbonates or bicarbonates may be used. When the combustion synthesis is carried out under a nitrogen atmosphere, ammonia-producing phases and additional new phases are formed in cermet  12 . These additional new phases generate light hydrocarbon gases when cermet  12  is activated by water. The major hydrocarbon gas produced is methane. However, other light hydrocarbon gases have also been detected.  FIGS. 6A and 6B  show the GC analysis after activation. The primary hydrocarbon produced is methane with minor amounts of other higher hydrocarbons. These new additional phases are currently believed by the inventor to be metal carbides. Particularly, alkaline carbides such as sodium carbide will produce light hydrocarbon gases when hydrolyzed. The compositions of the ammonia- and hydrocarbon gas-producing phases have not been fully identified. This alternative formulation for ammonia- and hydrocarbon gas-producing cermet filters is shown in the following chemical equation. 
         Ni   +   Al   +       Al   2     ⁢     O   3       +   sugar   +       Na   2     ⁢     CO   3         ⁢           ⁢     →     N   2       ⁢           ⁢     NiAl   +       Al   2     ⁢     O   3       ⁢           +       Na   2     ⁢   O     +   Ω   +   Φ         
         Ω=ammonia-producing phases Φ=light hydrocarbon gas-producing phases
 
Alternatively, the aluminum-thermite combustion synthesis process may be used instead of the aluminum combustion synthesis process to produce a nickel aluminide-alumina cermet having ammonia-producing phases and hydrocarbon gas-producing phases.
       

     Ammonia and hydrocarbon gases are reducing agents for NO X , which is present in the diesel exhaust gas. The internal generation capability of reducing agents has significant implications for destruction of exhaust NO X . Further, sodium carbonate (Na 2 CO 3 ) decomposes to CO 2  gas and Na 2 O during the combustion synthesis process. If different alkali carbonates or bicarbonates are used instead of sodium carbonate or bicarbonate, other types of alkali oxides will be formed during synthesis of cermet  12 . Further, CO 2  escaping from the green compact during the combustion synthesis process produces pores  14  in cermet  12 . 
     In an exemplary embodiment, post combustion synthesis leaching of cermet  12  dissolving the Na 2 O particles further increases the porosity of cermet  12 . The leaching may be accomplished by soaking cermet  12  in hot or cold water to dissolve the sodium oxide. In another exemplary embodiment, the Na 2 O is used to absorb NO X  during use of filter  10 . 
     In an exemplary embodiment, cermet  12  is electrically conductive so that direct internal electrical heating is possible when a current is passed through cermet  12 . The temperature of cermet  12  is controlled by the magnitude of the applied current. A larger current will yield a higher temperature for cermet  12 . As an example, the battery on a vehicle may be used to provide a DC power source to accomplish this. However, uniform electrical heating depends on good contact and continuity between the portions of metallic phase  18  as well as substantial uniformity throughout cermet  12 . The electrical resistance of cermet  12  may be tailored by adding additional metal powders to the green compact. To increase the electrical resistance of cermet  12 , metals such as chromium, manganese, silicon, etc. may be added to the powder mixture before combustion. 
     Referring to  FIG. 7 , in an exemplary embodiment that uses direct internal electrical heating, electrodes  23  are incorporated into cermet  12  during the combustion synthesis process.  FIG. 7  shows a thin stainless steel ring electrode  23  that has been bonded to the surface of cermet  12  during combustion synthesis. An Electrical wire may be attached to the electrode to provide direct internal heating. During compaction, the powder mixture is sandwiched between electrodes  23  inside the mold. In an exemplary embodiment, to assist bonding of electrodes  23  to cermet  12 , nickel and aluminum fine powders may be coated on the underside of the electrodes using silver paint. However, another metal besides nickel may be used depending on the composition of metallic phase  18 . The combustion synthesis process will result in permanent bonding of electrodes  23  to cermet  12  in one single operation. 
     Referring to  FIGS. 8A-8C , another exemplary embodiment uses internal resistive heating element  20  incorporated in cermet  12 .  FIG. 8A  shows a cross-sectional view of cermet  12  containing multiple resistive heating elements  20 .  FIG. 8B  shows a frontal view of cermet  12 . Resistive heating element  20  may be made from nickel-chromium alloys (nichrome), nickel-chromium-iron alloys, molybdenum disilicide (MoSi 2 ), or any other appropriate heating element material known to one of ordinary skill in the art. In another exemplary embodiment, the coefficient of thermal expansion of resistive heating element  20  may be selected to be substantially the same as cermet  12 . 
     During combustion synthesis, resistive heating element  20  will be permanently bonded inside filter  10 . Referring to  FIG. 8C , in another exemplary embodiment, resistive heating element  20  may be coated with ceramic material  21  prior to incorporating it inside filter  10 . Examples of such ceramic materials  21  are refractory oxides such as zirconium dioxide (ZrO 2 ), alumina, magnesium oxide (MgO), silicon dioxide (SiO 2 ), or titanium dioxide (TiO 2 ). These types of ceramic materials are well known to one of ordinary skill in the art. The ceramic material  21  may be used to electrically insulate resistive heating element  20  to prevent stray currents from passing through cermet  12 . In another exemplary embodiment, resistive heating element  20  structurally strengthens cermet  12 . Referring to  FIG. 9 , in another exemplary embodiment, filter  10  may be electrically insulated with insulation  19 . An external heating source  22  may generally surround filter  10  and insulation  19  to heat filter  10 . External heating source  22  may be a source such as a resistance heating coil. Other types of external heating sources will be known to one of ordinary skill in the art. 
     Referring to  FIGS. 10A-10B , in another exemplary embodiment, filter housing  24  is reaction bonded to cermet  12  of filter  10  during the combustion synthesis process. Exemplary materials for filter housing  24  include stainless steels, nickel superalloys, and cobalt alloys. Other materials will be known to one of ordinary skill in the art. Prior to compaction of the metal and ceramic powders of cermet  12 , the interior of filter housing  24  is coated with a thin layer of aluminum and nickel powder. Another metal besides nickel may be used depending on the composition of metallic phase  18 . For instance, iron would be suitable if the metallic phase  18  is an iron aluminide. The green cermet powder is then compacted in filter housing  24  with the thin layer of aluminum and nickel powder in place. During combustion synthesis, the aluminum and nickel powder also combusts. The combustion heat from the process in-situ bonds filter  10  permanently to the wall of filter housing  24 .  FIG. 10B  shows good bonding between cermet  12  of filter  10  and filter housing  24 . This exemplary embodiment provides a single-step process to form and bond the cermet  12  of filter  10  to filter housing  24 . 
     Referring to  FIG. 11A , filter  10  is coupled to exhaust system  26  of internal combustion engine  25  on a vehicle  27 , such as a heavy-duty diesel engine truck. Carbon particles present in exhaust gases  28  will be trapped by pores  14  (not shown) of filter  10 . Filtered exhaust gases  30  then exit filter  10  containing substantially less pollutants. During engine operation, filter  10  may heat up periodically or maintain continuous heating to burn off collected carbon particles and keep pores  14  open. For instance, electrical source  34  on vehicle  27  may be used to beat filter  10 . Further, the burning off of the carbon particles reduces NO X  gases in the final exhaust gas. The carbon particles in the exhaust stream are very fine and have extremely high active surfaces. At elevated temperatures on a condensable surface such as filter  10 , the carbon particles can destroy NO X  and lower the NO X  level in the final exhaust gas. Further, under an oxidizing environment, cermet  12  can destroy CO, H 2 , and other hydrocarbon gases. Since intermetallic and oxide compositions have a profound influence on pollutant destruction, the catalytic properties of cermet  12  and the operating environment can be tailored to oxidize CO and hydrocarbons while also destroying NO X . The process by which the NO X  gases and carbon particles are destroyed is represented by the following chemical equations:
 
C (s) +2NO (g) →N 2(g) +CO 2(g)  
 
C (s) +NO 2(g) →½N 2(g) +CO 2(g)  
 
     In an exemplary embodiment, pores  14  are of sufficient concentration so that the pressure drop is less than 5 psi across filter  10 . Under a fuel lean condition, non-transient steady state driving conditions, the amount of carbon particulate generated from a diesel engine may not be enough to reduce all of the NO X  present. Thus, referring to  FIG. 11B , in another exemplary embodiment using diesel fuel, vapor from the headspace of the fuel tank  32  can be injected into filter  10  for supplemental NO X  reduction. 
     The tables below include data from tests of various intermetallic and intermetallic-oxide cermets used for filter  10 . The data in the following tables is merely illustrative and other variations on the compositions of the cermets used are fully embraced by the present invention. Tables 1-6 show the test results for several cermet filter compositions. Table 1 shows nitric oxide (NO) conversion to nitrogen for two cermet filters and a molybdenum disilicide filter under a reducing atmosphere. Table 2 shows the percent products of incomplete combustion (PICs) destroyed as a function of temperature for a specific cermet composition. Tables 3-7 show the results of catalytic steam reforming of various hydrocarbon fuels to synthesis gas (CO and H 2 ) by a 50 wt % NiAl-50 wt % alumina cermet filter. Tables 8-9 show pressure drop data with and without the presence of filtered carbon particles in a 50 wt % NiAl-50 wt % alumina cermet filter having approximately 30% porosity. 
     Table 10 shows test data that simulated the removal of exhaust gas-entrained fine carbon particles generated from an internal combustion engine such as a diesel engine. The individual carbon grain size was approximately 10 nanometers. Agglomerated particle size was approximately 0.5-1.0 μm. The carbon particles were generated by the thermal plasma decomposition of methane. An airflow rate of 60 liters/minute under high pressure (56 psi) was used. The test lasted 60 minutes. The porosity of the cermet filter used in the pressure drop tests was approximately 30 percent. Test data shows that for a filter porosity of approximately 30 percent containing approximately 0.448 in 2  filter area, the carbon filtration efficiency was 99.86 percent. No clogging of the cermet filter was observed. The weight of the trapped carbon after filtering was measured by tapping the cermet filter to remove all of the carbon particles. 
     Although the foregoing description of embodiments and test data contains many specifics, these should not be construed as limiting the scope of the present invention, but merely as providing illustrations of some exemplary embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the present invention, as disclosed herein, which fall within the meaning and scope of the claims are embraced thereby. 
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 NO (g)  Reduction for NiAl Cermets and MoSi 2   
               
               
                 Under a Reducing Atmosphere 
               
               
                 Test Condition: 5% NO + 1.67% C 2 H 4  + 93.33% He 
               
             
          
           
               
                   
                 % NO (g)  Conversion to N 2  Under a 
               
               
                   
                 Reducing Atmosphere 
               
             
          
           
               
                   
                 50 wt % NiAl + 50 
                 50 wt % NiAl + 50 
                   
               
               
                 Temperature (° C.) 
                 wt % ZrSiO 4   
                 wt % Fe 2 O 3   
                 MoSi 2   
               
               
                   
               
             
          
           
               
                 300 
                 0 
                 0 
                 0 
               
               
                 400 
                 0 
                 2.5 
                 0 
               
               
                 475 
                 0 
                 2.6 
                 2 
               
               
                 550 
                 0 
                 4.7 
                 2 
               
               
                 623 
                 3.8 
                 13.3 
                 2 
               
               
                 696 
                 11.3 
                 43.1 
                 3 
               
               
                 797 
                 38.7 
                 98.3 
                 7 
               
               
                 904 
                   
                 100 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 % Products of Incomplete Combustion Destroyed vs. Temperature 
               
               
                 for a Cermet Composition of (NiAl) 0.2 /Ni 0.02 /(ZrSiO 4 ) 0.04   
               
             
          
           
               
                   
                 Temperature (° C.) 
                 C 3 H 6   
                 CH 4   
                 CO 
               
               
                   
                   
               
             
          
           
               
                   
                 25 
                 0 
                 0 
                 0 
               
               
                   
                 101 
                 0 
                 0 
                 0 
               
               
                   
                 203 
                 2.1 
                 0 
                 5.3 
               
               
                   
                 302 
                 1.4 
                 0 
                 6.1 
               
               
                   
                 403 
                 0 
                 0 
                 8.9 
               
               
                   
                 478 
                 11.3 
                 15.9 
                 24 
               
               
                   
                 553 
                 47.8 
                 233 
                 81.2 
               
               
                   
                 627 
                 93.8 
                 25.2 
                 97.0 
               
               
                   
                 702 
                 100 
                 36.0 
                 100 
               
               
                   
                 797 
                 100 
                 52 
                 100 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Steam Reforming of Methanol Using a 50 wt % NiAl—50 wt % Al 2 O 3   
               
               
                 Test Conditions: 10 cc CH 3 OH/H 2 O = 0.5 mole ratio mixture + 5 cc He 
               
             
          
           
               
                 Temperature (° C.) 
                 CO 2  (vol. %) 
                 H 2  (vol. %) 
                 CO (vol. %) 
               
               
                   
               
             
          
           
               
                 500 
                 0.1 
                 13.0 
                 3.5 
               
               
                 600 
                 1.5 
                 43.0 
                 11.4 
               
               
                 700 
                 4.7 
                 46.0 
                 9.6 
               
               
                 800 
                 2.2 
                 47.7 
                 12.0 
               
               
                 900 
                 2.1 
                 50.0 
                 15.1 
               
               
                 967 
                 3.2 
                 50.3 
                 12.4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Steam Reforming of Unleaded Gasoline Using a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter 
               
               
                 Test Conditions: 6.4 cc gasoline + 12.8 cc H 2 O 10 cc He 
               
             
          
           
               
                 Temperature (° C.) 
                 CO 2  (vol. %) 
                 H 2  (vol. %) 
                 CO (vol. %) 
               
               
                   
               
             
          
           
               
                 788 
                 0.2 
                 5.7 
                 0.5 
               
               
                 896 
                 0.3 
                 4.6 
                 1.2 
               
               
                 1018 
                 0.4 
                 5.9 
                 0.4 
               
               
                 1034 
                 3.7 
                 22.1 
                 2.2 
               
               
                 1042 
                 4.2 
                 24.4 
                 2.7 
               
               
                 1050 
                 3.7 
                 20.2 
                 1.9 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Steam Reforming of a #1 Diesel Fuel Using a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter 
               
               
                 Test Conditions: 6.4 cc Diesel Fuel + 12.8 cc H 2 O 10 cc He 
               
             
          
           
               
                 Temperature (° C.) 
                 CO 2  (vol. %) 
                 H 2  (vol. %) 
                 CO (vol. %) 
               
               
                   
               
             
          
           
               
                 902 
                 0.3 
                 3.2 
                 3.3 
               
               
                 1000 
                 0.3 
                 25.2 
                 9.4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Steam Reforming of #2 Diesel Fuel Using a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter 
               
             
          
           
               
                   
                   
                 % Conversion = {[CO] + [H 2 ]}/ 
               
               
                   
                 Temperature (° C.) 
                 {[CO] + [H 2 ] + Σ [C x H y ]} 
               
               
                   
                   
               
             
          
           
               
                   
                 600 
                 43.7 
               
               
                   
                 700 
                 48.9 
               
               
                   
                 800 
                 53.4 
               
               
                   
                 900 
                 59.7 
               
               
                   
                 968 
                 72.6 
               
               
                   
                 1013 
                 94.2 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Steam Reforming of Methane Using a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter 
               
               
                 Test Conditions: 5.0 cc CH 4  + 12.8 cc H 2 O 35 cc He 
               
             
          
           
               
                 Temperature (° C.) 
                 CO 2  (vol. %) 
                 H 2  (vol. %) 
                 CO (vol. %) 
               
               
                   
               
             
          
           
               
                 703 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 791 
                 &lt;0.1 
                 &lt;0.1 
                 &lt;0.1 
               
               
                 905 
                 &lt;0.1 
                 1.3 
                 0.2 
               
               
                 1005 
                 &lt;0.1 
                 6.5 
                 1.4 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Pressure Drop Tests without Carbon Particles in a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter 
               
             
          
           
               
                 Temperature 
                   
                   
                 Upstream 
                 Downstream P 
               
               
                 (° C.) 
                 Flow rate (l/m) 
                 Line P (psi) 
                 (psi) 
                 (psig) 
               
               
                   
               
             
          
           
               
                 24 
                 10 
                 56 
                 7 
                 0 
               
               
                 24 
                 20 
                 56 
                 18 
                 0 
               
               
                 24 
                 30 
                 56 
                 25 
                 0 
               
               
                 24 
                 40 
                 56 
                 32 
                 0 
               
               
                 24 
                 60 
                 56 
                 47 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Pressure Drop Tests with Carbon Particles in a 50 wt % 
               
               
                 NiAl—50 wt % Al 2 O 3  Cermet Filter with 30% Porosity 
               
             
          
           
               
                 Temperature 
                   
                   
                 Upstream 
                 Downstream P 
               
               
                 (° C.) 
                 Flow rate (l/m) 
                 Line P (psi) 
                 (psi) 
                 (psig) 
               
               
                   
               
             
          
           
               
                 24 
                 10 
                 56 
                 8.5 
                 0 
               
               
                 24 
                 20 
                 56 
                 15 
                 0 
               
               
                 24 
                 30 
                 56 
                 23 
                 0 
               
               
                 24 
                 40 
                 56 
                 30 
                 0 
               
               
                 24 
                 50 
                 56 
                 37 
                 0 
               
               
                 24 
                 60 
                 56 
                 51.5 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
               
                 Fine Carbon Particulate Filtration Tests 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Initial wt of cermet filter = 
                 Wt of carbon before filtering = 
               
               
                 3.3073 g 
                 1.3846 g 
               
               
                 Final wt of cermet filter = 
                 Wt of carbon after filtering = 
               
               
                 3.3070 g 
                 1.3826 g 
               
               
                 No clogging of the filter observed 
                 Carbon filtration efficiency = 99.86% 
               
               
                 Effective cermet filter area = 
                 Filter porosity˜30% 
               
               
                 0.448 in 2 .