Carbide and nitride syntheses

Table I Summary of intermediate phases, temperatures and surface areas in carbide and nitride syntheses...

Thin films of carbides and nitrides of Group 6 metals were synthesized by reaction of a metal film with a reactive gas at high temperature and by reactive sputtering. The phases obtained depended on the experimental conditions. High temperatures metastable phases (/i-WC, v and 6-MoC]. ) were obtained by reactive sputter deposition of films. The carbon concentration in such films depended on the temperature of the substrate and on the pressure. In some cases ordered sublattices of carbon and nitrogen were observed and epitaxial relationships between the deposit and the substrate were studied. 

The purpose of this chapter is to present systematic experimental procedures to deposit thin films of carbides and nitrides. Strong relationships between synthesis methods and conditions, and product structures, crystallinity, purity and phase were observed. Thin films of W, Cr, and Mo carbides and nitrides were synthesized in two different ways (1) chemical... 

Synthesis of carbide and nitride films by reactive sputtering. Table 14.3 presents a summary of the experimental conditions for the formation of carbides and nitrides. The phases formed are different from those in Table 14.2. As sputtering is a non-equilibrium technique, it was possible to synthesize the 6-MoC and the fi-WCi x carbides and the... 

The thermodynamic considerations above are useful guides to the preparation of carbides and nitrides, but are not limiting as the syntheses can be carried out at nonequilibrium conditions. For example, in the carburization or nitridation of oxides it is possible to lower the temperature of reaction by sweeping away the product gases [23] ... 

Butterfly clusters are still not common, and, at present, few catalytic processes based on them are known. They have, however, been considered as surface analogs for Fischer-Tropsch, nitrogen fixation, and isocyanite chemistry (26, 163). There is growing evidence that small ligands such as carbide and nitride coordinated within the cavity of a butterfly framework exhibit unusual patterns of chemical reactivity. The two tetraosmium nitride isomers have also been synthesized as follows (90) ... 

Temperature-Programmed Reactions. The Boudart group developed temperature-programmed reaction (TPRe) methods to synthesize carbides and nitrides of molybdenum and tungsten with very high surface areas suitable for... 

Unlike many ceramic materials such as oxides which can be produced fiom raw materials found in nature, the refiactory carbides and nitrides generally do not exist in the natural state. A few minor exceptions include a form of silicon carbide found in the meteorite field of Canyon Diablo in Arizona ) and titanium nitride detected in some silicate meteorites as the mineral osbomite.I ) These exceptions can be considered as curiosities and the refi tory carbides and nitrides must be synthesized for scientific or conunercial use. 

Carbide and nitride powders are conventionally prepared by the carboreduction of oxide powder with subsequent nitridation in a nitrogen atmosphere or carborization in an inert gas. Diboride powders are synthesized by carbothermal reduction, where boron needs to form the boride and carbon aids in the removal of oxygen. These processes require a high temperature and a long heating time. Fine powders of these compounds are prepared by the thermite method by reduction with Mg. 

Comparison of the Syntheses of Vanadium, Niobium, and Molybdenum Carbides and Nitrides by Temperature-Programmed Reaction... 

The carbides and nitrides of the early transition metals, vanadium, niobium, and molybdenum, are known to possess good catalytic properties. The compounds are synthesized by a temperature programmed reaction (TPR) method where a reactive gas is reacted with a precursor oxide as the temperature is uniformly increased. Results under similar reaction conditions are presented to compare the progress of the reaction, the formation of intermediate phases, and the development of surface areas. The increase in surface area is influenced by the phenomena of pseudomorphism and topotaxy. It is believed that pseudomorphism, found in all of the above syntheses, is associated with the development of internal pores, while topotaxy, found in some of the nitrides, maximizes this process to yield hi surface area products. 

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

In the first experiments in which chemistry of metal clusters was demonstrated, the reactant was present in the carrier gas. The problem with this approach is that the reactant is also decomposed during the high-temperature vaporization process, and thus reactive radical fragments are present which may participate in the cluster growth process. Although not exploited, this may be a good method to synthesize metal cluster carbides, oxides, nitrides, sulfides, and hydrides, to name a few. It is not a viable mode of operation for the measurement of reactivity toward molecular species, but has been used to examine the stoichiometry of metal cluster compounds. ... 

Other synthesis methods have also been developed but may not be practiced on a large commercial scale as compared to those indicated above. Examples of syntheses that begin from liquid phase precursors include the sol-gel, hydrothermal, and Pechini methods. In the course of these reaction schemes, polycondensation or precipitation occurs, and the volatile components are removed, often by thermal methods. Other synthesis routes to ceramic materials include the nitridation of metals to form metal nitrides and the carbothermal reduction of oxides to form carbides and borides. 

Bulk as well as nanocrystalline phases of metal oxides, nitrides and carbides are often synthesized by employing carbothermal reactions. For example, carbon (activated carbon or carbon nanotubes) mixed with an oxide produces sub-oxide or metal vapour species that react with C, 0, or NHj to produce the desired product. Thus, heating a mixture of Ga Oj and carbon in or NHj produces GaN. Carbothermal reactions generally involve the following steps ... 

In this book, we briefly examine the different types of reactions and methods employed in the synthesis of inorganic solid materials. Besides the traditional ceramic procedures, we discuss precursor methods, combustion method, topochemical reactions, intercalation reactions, ion-exchange reactions, alkali-flux method, sol-gel method, mechanochemical synthesis, microwave synthesis, electrochemical methods, pyrosol process, arc and skull methods and high-pressure methods. Hydrothermal and solvothermal syntheses are discussed separately and also in sections dealing with specific materials. Superconducting cuprates and intergrowth structures are discussed in separate sections. Synthesis of nanomaterials is dealt with in some detail. Synthetic methods for metal borides, carbides, nitrides, fluorides, sili-cides, phosphides and chalcogenides are also outlined. 

Silicon carbide and alumina still dominate the abrasive industry at the present time. However their performance in the grinding of superalloys, ceramics, reinforced plastics, and other hard materials is generally unsatisfactory. This has led to the development of new abrasives such as synthetic diamond and cubic boron nitride. Cubic boron nitride was first synthesized in 1957 and has been available commercially since the 1970 s. Although not as hard as diamond, c-BN does not react with carbide formers such as Fe, Co. Ni, Al, Ta, and B at 1000 (while diamond does). However, it reacts with aluminum at 1050°C, with Fe and Ni alloys containing Al above 12S0"C, and with water and water-soluble oils.1 1... 

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