**2. Synthesis of nanostructured carbides — TaC**

Refractory, ultrathin and nanostructured metal carbides (TaC, NbC, MoC and WC) have been produced at low temperature from ammonium oxalate complexes via gas-solid reactions with suitable characteristics for various purposes: high surface area (38 to 58m2 /g), size of crystallites (18-20 nm), extremely porous particles and high acidity.

Recent discoveries aimed at facilitating the routes for obtaining these carbides with high surface area have led to a rapid growth in the application of these materials in catalysis, composite alloys (Medeiros, 2002; Oyama, 1992, and Ledoux, 1990) and metal-ceramic composites (Upadhyaya, 1998 and Matthews, 1994).

Nanosized refractory metal carbides, particularly TaC, are produced at low temperature and short reaction time through gas-solid reaction in fixed bed reactor using tris (oxalate) hydrated ammonium oxytantalate-(NH4)3TaO(C2O4)3nH2O-precursor of fine grain size (ultrafine particles with high surface area). However, for successful precursor carburizing process, certain parameters such as temperature, composition of the gas mixture (H2 and CH4), methane concentration, reaction time, precursor cost and heating rate must be taken into account.

The system used for carrying out gas-solid reactions is composed of a horizontal tubular resistive furnace (alumina tube) with temperature controller, fixed bed reactor (slip casting), gas inlet and outlet system, valves and flow meters. The gas composition determines reduction and carburization, which is characteristic of atmosphere. The amount of CH4 in the mixture cannot be too excessive, since much carbon would precipitate on the surface of particles, hindering the reaction and producing free carbon in the product, which hinders the diffusion process. In the case of low CH4 concentration, carburization does not occur. The complete carbide reaction occurs at temperature below 1000°C with CH4 concentration ranging from 3 to 5%. Under some conditions, carburization is completed within 2 hours (Medeiros, 2002).

In recent work (Lima, 2013) Fig.1, TaC synthesized revealed the formation of pure phase with characteristics different from those conventionally obtained for the product, with average crystallite size of the order of nanometers, approximately 12.05 nm, and surface area of 19 m2 / g. These characteristics allow its use not only as reinforcement for MMC composites and solid inserts but also as catalysts.

of the reinforcing phase added to steel provide homogeneous dispersion of these steel carbides into the matrix, thus providing uniformity of properties and allowing compounds to be used

In the production of metal composites reinforced with carbides, powder metallurgy offers some economic and technological advantages in relation to other competing processes such as low cost of raw material processing and relatively low temperatures involved in the process. As the microstructure of the sintered steel is the result of process parameters (time, milling speed, compaction pressure, sintering time and temperature, sintering in solid or liquid state) and also the characteristics of starting powders (size and particle size distribution, compres‐ sibility and chemical purity); any changes in these parameters also affect the sintering kinetics, with wide variations in their microstructure and consequently their performance in relation to specific application. The use of the technique Powder Metallurgy (PM) in the manufacture

Refractory, ultrathin and nanostructured metal carbides (TaC, NbC, MoC and WC) have been produced at low temperature from ammonium oxalate complexes via gas-solid reactions with

Recent discoveries aimed at facilitating the routes for obtaining these carbides with high surface area have led to a rapid growth in the application of these materials in catalysis, composite alloys (Medeiros, 2002; Oyama, 1992, and Ledoux, 1990) and metal-ceramic

Nanosized refractory metal carbides, particularly TaC, are produced at low temperature and short reaction time through gas-solid reaction in fixed bed reactor using tris (oxalate) hydrated ammonium oxytantalate-(NH4)3TaO(C2O4)3nH2O-precursor of fine grain size (ultrafine particles with high surface area). However, for successful precursor carburizing process, certain parameters such as temperature, composition of the gas mixture (H2 and CH4), methane concentration, reaction time, precursor cost and heating rate must be taken into account.

The system used for carrying out gas-solid reactions is composed of a horizontal tubular resistive furnace (alumina tube) with temperature controller, fixed bed reactor (slip casting), gas inlet and outlet system, valves and flow meters. The gas composition determines reduction and carburization, which is characteristic of atmosphere. The amount of CH4 in the mixture cannot be too excessive, since much carbon would precipitate on the surface of particles, hindering the reaction and producing free carbon in the product, which hinders the diffusion process. In the case of low CH4 concentration, carburization does not occur. The complete carbide reaction occurs at temperature below 1000°C with CH4 concentration ranging from 3 to 5%. Under some conditions, carburization is completed within 2 hours (Medeiros, 2002).

/g), size of crystallites

in a variety of applications.

108 Sintering Techniques of Materials

of MMC composites is increasing.

**2. Synthesis of nanostructured carbides — TaC**

(18-20 nm), extremely porous particles and high acidity.

composites (Upadhyaya, 1998 and Matthews, 1994).

suitable characteristics for various purposes: high surface area (38 to 58m2

**Figure 1.** Electron microscopy of tantalum carbide (Lima, 2013). Magnification of 7000x.
