Subject borides

In the past, there was quite a lot of confusion regarding the true nature of some alkaline-earth compounds. Several boride, nitride, and carbide compounds, as well as some mixed species, especially of calcium, were in doubt or at least subject to some ongoing discussion. For example, the phase relationships and structures of CaC2 phases I-IV were not well understood. The three nitride... 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

This article covers only a part of the chemistry of boron. Boron-carbon compounds are covered in other articles in this volume see Boron Organoboranes Boron Metallacarbaboranes, and Boron Polyhedral Carboranes). The main subject of the latter two articles, and the separate one on Boron Hydrides is the extensive chemistry of the multicenter bonded boron-hydride systems. This area has been a major focus of boron research for the past 60 years. There is some direct overlap between the two articles Borides Solid-state Chemistry and Borates Solid-state Chemistry, and this more general one covering the inorganic chemistry of boron. Boron-Nitrogen Compounds are also covered separately. These articles should be consulted for more detailed discussions of the structure, bonding, and properties of borides, solid-state borates, and boron-nitrogen compounds. 

The metal azides are, by common experience, brittle when subjected to mechanical stress, they shatter before appreciable plastic deformation takes place. This arises because, as with most inorganic materials, dislocation densities are low, grown-in dislocations are usually immobile, and slip can take place only on a limited number of planes. However, with the possible exception of diamond and certain borides and nitrides, few materials are ideally brittle, and some plastic deformation is possible, the amount depending upon the temperature and the rate of strain low temperatures and high rates of strain both favor brittle behavior. 

Since the early work of Moissan and Stock (1900) [42] on the synthesis of the silicon borides SiB3 and SiBg, the Si-B system has been the subject of numerous investigations. Experimental phase diagram data available on this system are summarized in Table 3 [27, 43-61]. 

The subject of Raney nickel desulfurization has been exhaustively dealt with in CHEC-I. A few further interesting applications of this procedure will be presented in this section, along with the use of some nickel(O) complexes and nickel boride for this purpose. The major goal of understanding the mechanism of hydrodesulfurization by studying metal complexes as models is briefly dealt with in Section 2.10.6. 

When zirconium silicate (ZrSi04) or a mixture of Z1O3 and SiOj is reacted with aluminum in the presence of aluminum oxide and then rdieated, zirconium silicide (ZrSi ) becomes the major product. Titanium dioxide (TiOs) and boron (111) oxide (BgO,) with aluminum similarly form titanium boride (TiBs). If the reduction of the oxides such as TiOg or Si02 with aluminum is performed in the presence of carbon black, the carbides TiC and SiC are formed embedded in aluminum oxide. This subject is also treated in a British patent titled Autothermic Fired Ceramics. ... 

Schick s work includes the study of borides, carbides, nitrides, and oxides of some elements in Groups IIA, IIIB, IVA, IVB, VB, VIIB, and VIII as well as selected rare earths and actinides. As far as possible, the tables have been made compatible with the JANAF tables. Among the subjects treated are phase diagrams, heat capacities, enthalpies, entropies, enthalpies of phase transformation, formation, and reaction, melting temperatures, triple points, free energies of formation, vapour pressures, compositions of vapour species, ionization and appearance potentials, e.m.f. of cells, and enthalpies of solution and dilution. Volume 1 summarizes the techniques used to analyse data and cites the data analysed, and Volume 2 gives tables of values produced by this study. 

Ceramics are studied and used for such places that are subject to high temperatures, but many others have a variety of nses. Ceramics is dehned as an inorganic, nonmetallic material processed or consolidated at high temperatnres. Ceramics includes silicates, oxides, carbides, nitrides, snlhdes, and borides of metal or metalloid. The traditional ceramics are mostly silicates as discnssed earlier and nsed as a pot or similar purposes. But today ceramics are pursued as material for high-temperature, electric properties such as ferroelectricity, piezoelectricity, magnetic properties, high mechanical properties, and optical properties. In a word, they are pnrsned as HIGH-TECH material. 

YG Tkachenko, SS Ordanyan, VK Yulyugin, DS Yurchenko, VI Unrod. The role of the boride phase in MeC-MeBj alloys when subjected to wear in a vacuum. J Less Common Met 67 437, 1979. 

Car] Plates of AISI 420 steel were subjected to boriding at 850°C for 15 h in different boriding media and then examined with metallography, XRD and Mossbauer spectroscopy. Structure of surface, phase constituents and role of Cr in formation of coatings at boriding were reported. 

Bad] Samples from Cr-Fe alloys containing Fe-0.5 mass% Cr and Fe-4 mass% Cr were subjected to boriding at 1000°C. Boride layers of Fc2B and FeB were found and their textine was studied. 

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