Abstract
Borides, carbides, and nitrides are usually synthesized under extreme conditions. Temperature, pressure, atmospheric/surrounding environment, and heat-treatment processing are important parameters to get pure phase. Synthesis temperature range covers from a low (say 200°C) to a high temperature (2000°C). Pressure under hot/cold pressing is an important parameter in making different phases and stoichiometry. For preparation of nitrides, atmospheric/surrounding environment such as N2, NH3, and N2/H2 are required; and for preparation of carbides, methane, ethane, propane, and urea, C/CO/CO2/H2 are required; and boric acid or boron trioxide for preparation of borides has to be maintained. Heat-treatment effect such as quenching from high to low temperature and fast/slow heating is also important in production of unusual phases and tailoring of properties. Suitable precursors used during synthesis help in getting stoichiometric compounds. Their crystal structure and bonding can be characterized by many techniques including X-ray diffraction (XRD), neutron scattering/diffraction, extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Raman/infrared spectroscopy, electron energy loss spectroscopy (EELS), and X-ray fluorescence (XRF). Stoichiometry can be determined by chemical analysis and inductively coupled plasma mass spectrometry (ICP-MS). In transition metal nitrides and carbides, N/C atoms usually occupy interstitial sites of host metal lattice, whereas B atoms do not occupy interstitial sites in transition metal borides. Microstructure can be investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atomic/magnetic force microscopy (AFM/MFM), etc. Samples with different particle sizes from micron to nanometer can be produced by varying heat-treatment procedures. Usually, borides, carbides, and nitrides have very good thermal conductivity but poor electrical conductivity. Some of them show superconductivity with Tc from 1 to 39K (e.g., MgB2). Their hardness varies from soft to hard and can be comparable with diamond. The unusual high magnetic moment in Fe16N2 is observed as compared to pure iron. These materials have good thermal and chemical stability because of high melting points and covalent bonding character. They are used in many applications such as interconnector, electrical insulator, hard material, cutting tools (e.g., WC), bearing in motors, gas turbines, car engine parts, cantilevers in AFM, protective coating agents, diamagnetic sample holder (e.g., BN, SiNx) in vibrating sample magnetometer and nuclear magnetic resonance (NMR), magnetic material, superconducting material, etc. They have aesthetic applications such as appearance of gold color after coating of material with TiN. Also, they are used as catalysts or support for catalyst in catalysis. They have extraordinary stability in H2 atmosphere. In view of the high neutron absorption cross-section, the boron carbides are used in nuclear reactors as control rods, shielding, and shutdown pellets.
Published Version
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