Binary carbides/nitrides

TABLE 4 Selected Binary Carbides, Nitrides, and Borides with Transition Temperature. 

The representative binary carbide, nitride, and oxide systems displaying metallic properties are summarized in Figure 1, where they are arranged in conformity with the... 

The deposition of a binary compound can be achieved by a coreduction reaction. In this manner, ceramic materials such as oxides, carbides, nitrides, borides, and silicides can be produced readily and usually more readily than the parent metal. A common example is the deposition of titanium diboride ... 

Plutonium forms several binary compounds that are of interest because of their refractory character and stability 1 at high lemperatures, These include the carbide, nitride, silicide. and sulfide of the element. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

Binary compounds such as carbides, nitrides, phosphides, and sulfides are commonly made by direct interaction of the elements. 

For Ti, Zr, and Hf carbonitrides, including also binary carbides and nitrides, the Young s, shear, and bulk modulus as well as the Poisson number have been measured by ultrasonic measurements as a function of the C/N ratio. The data are given in Figure 10. 

Metallic carbides, nitrides, and oxides are used industrially in many applications their physical properties are also of intrinsic interest. This section pinpoints various preparative techniques and reviews methods of crystal growth for this group of compounds. More detailed discussion is found in the reviews cited and in the references therein. The discussion is confined to binary compounds, M Xi, (M is a cation X = C, N, or O a and b are simple integers) that display metallic properties the very numerous ternaries MoMcXj, (M, M being different cations) cannot be described in this brief presentation. 

Table 1. Binary Oxide, Nitride, and Carbide Systems Displaying Metallic Properties ...

Other Binary Compounds. Various borides, sulfides, carbides, nitrides, etc., have been obtained by direct interaction of the elements at elevated temperatures. Like other actinide and lanthanide metals, thorium also reacts at elevated temperatures with hydrogen. Products with a range of compositions can be obtained, but two definite phases, ThH2 and Th4H15, have been characterized. 

A large number of binary ceramics such as carbides, nitrides, and oxides have been obtained by the decomposition of metal-organic precursors. The following text presents some selected examples to demonstrate the promises and limitations of molecular precursor approach. A complementary account of the synthesis of metal oxide nanoparticles from solution-based methods is given in Chapter 12.03. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

Besides diamond and cBN, the well known boron carbide B4C is among the hardest materials and has been comprehensively reviewed by F. Thevenot [103], In the present chapter, the latest developments concerning the binary and ternary systems B-N, boron carbide nitrides (B-C-N), and boron suboxides are discussed. Other hard materials based on boron are described by R. Telle et al. in Part III. 

Bulk moduli for ambient conditions (with index o ) are listed in Tables 10.7 for elements, SlO.l for compounds MX, S10.2 for MX2, S10.3 for MX3, S10.4 for binary oxides, S10.5 for binary nitrides, S10.6 for binary borides, S10.7 for binary carbides and silicides, S10.8 for binary phosphides and arsenides, S10.9 for ternary oxides and coordination compounds, SIO.IO for molecular substances and polymers, SlO.l 1 for characteristics of polymorphous modifications of elements and the MX compounds, S10.12 for various phases ofMX2 crystals. 

Also some ternary carbides, nitrides or oxides if necessary for the understanding of the respective binary systems these phases are treated together with the binary systems or follow them. ) Auch einige temare Carbide, Nitride oder Oxide soweit erforderlich zum Verstandnis der entsprechenden binaren Systeme diese Phasen sind mit den binaren Systemen zusammen oder unmittelbar anschliefiend behandelt. 

Thermodynamic Data on Alloys , by Kubaschewski and Catterall, critically surveys data on some 400 binary and ternary metallic systems, with the term alloy interpreted to include transition-metal oxides, sulphides, carbides, nitrides, and phosphides. When available, values are given for the integral enthalpy and entropy of formation of the alloy, the relative partial molar enthalpy and entropy of one component, and the volume change on mixing. 

In this chapter we describe the results of theoretical studies of the electronic structure and properties of refractory nitrides, as compared with the properties of binary carbides. We shall consider here only those compounds with ideal crystal lattices, without any defects or impurities. The electronic structure and chemical bonding in nonstoichiometric nitrides will be discussed in Chapter 4. 

While most of the binary carbides and nitrides considered above form unlimited homogeneous solid solutions, some other 5 and p elements (B, Be, Al, Mg, etc.) have only a low solubility in these phases. As their content in carbides and nitrides increases, ternary compounds with very specific crystal structures are formed which were reviewed by Alyamovsky et al (1981) and Goldschmidt (1967). It is well known (see Samsonov, Serebryakov and Neronov (1975)) that B or transition metal borides do not form unlimited solid solutions, when interacting with MX phases (X = C, N) and single-phase TiNjBj, compounds exist over a narrow composition range for example, when z + y = 0.62-0.94, y < 0.03 (see Alyamovsky, Zainulin and Shveikin (1976)). As the B/N ratio increases. 

The problems outlined above determined the structure of the book we start with the simplest compounds - binary carbides and nitrides (Chapters 2, 3) and proceed gradually to more complicated systems (Chapters 3-8), demonstrating some capabilities (and at the same time limitations) of the computational methods used. The book therefore acquires a certain reference character, which is somewhat unusual for publications on quantum chemistry of solid-phase materials. Apart from the properties of the particular substances, the authors have tried to discuss the methods used in the calculation of some experimental characteristics (primarily the thermomechanical characteristics, which are most interesting for this series of compounds) which have not been adequately addressed until recently (Chapter 1). The last statement can be applied as well to Chapters 4-8, where we make an attempt to develop a unified approach to the available data on the electronic energy spectra of ternary... 

Despite having outstanding and desirable properties, MAX phases are very prone to thermal decomposition in vacuum or inert atmosphere when exposed to elevated temperatures. Above 1400 °C, MAX phases decomposed to binary carbide(e.g.,TiC ) or binary nitride (e.g., TiN ), primarily through the sublimation of A-elements such as A1 or Si, which results in a porous surface layer of MX being formed. Positive activation energies were determined for the decomposition of MAX phases except for TijAlCj where negative activation energy of 71.9 kJ moF was obtained due to... 

Hafnium has been successfully alloyed with iron, titanium, niobium, tantalum, and other metals. Hafnium carbide is the most refractory binary composition known, and the nitride is the most refractory of all known metal nitrides (m.p. 3310C). At 700 degrees C hafnium rapidly absorbs hydrogen to form the composition HfHl.86. 

Many of the binary compounds of the lanthanides, such as oxides, nitrides, and carbides, can exist as non stoichiometric compounds. These form crystals where some of the anions ate missing from the sites the anions normally occupy. 

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