Unary structures

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

Much of what we need to know abont the thermodynamics of composites has been described in the previous sections. For example, if the composite matrix is composed of a metal, ceramic, or polymer, its phase stability behavior will be dictated by the free energy considerations of the preceding sections. Unary, binary, ternary, and even higher-order phase diagrams can be employed as appropriate to describe the phase behavior of both the reinforcement or matrix component of the composite system. At this level of discussion on composites, there is really only one topic that needs some further elaboration a thermodynamic description of the interphase. As we did back in Chapter 1, we will reserve the term interphase for a phase consisting of three-dimensional structure (e.g., with a characteristic thickness) and will use the term interface for a two-dimensional surface. Once this topic has been addressed, we will briefly describe how composite phase diagrams differ from those of the metal, ceramic, and polymer constituents that we have studied so far. 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

All structural data for unary, binary and ternary phases have been indicated in Table 2. 

Selected theoretical values of y and a Broughton and Gilmer [ 14] used molecular dynamics to calculate the surface energy and surface stress for a unary system with a P-T diagram of the type in Figure 1.4 for which the atoms were assumed to interact according to a Lennard-Jones potential. The solid phase was assumed to take the fee structure. The results from this simulation for temperatures and pressures near... 

FIG. 7 The unary C12MG phase diagram manifold. The temperature scale is to the left (outside the diagram). Each arm corresponds to a particular crystal structure the left arm is the equilibrium diagram. The transformations that occur among polymorphs are indicated by dashed arrows. 

The equilibrium and nonequilibrium aspects of the phase behavior of C12MG are displayed in the phase manifold in Fig. 7. This manifold graphically depicts both equilibrium and nonequilibrium phase behavior in a unary system. It has several arms, each corresponding to a particular phase structure. The equilibrium phase behavior is depicted to the left, which shows the two phase transformations of XI that occur on heating. The behavior of X2 and X3 is shown in their respective arms. Path directions (heating or cooling) are very important to phase reaction kinetics and are indicated by arrowheads. 

Perhaps because of its polyfunctionality and complex molecular structure, C12MG (and probably also its analogs) displays a high level of complexity in its physical behavior. Polymorphism exists in the dry material three different crystalline polymorphs that vary widely in their kinetic stabilities have been discovered. Nonequuilibrium phenomena exist in unary and in binary aqueous systems. The likelihood that similar complexity also exists in ternary systems is very high. Unfortunately, one cannot be sure that this behavior is unique to these molecules because few other systems have been studied in the depth that C12MG has been. It would have been desirable to have studied in depth one or two other analogs, but time did not allow this. 

Crystallographic data of the unary, binary and ternary phases are listed in Table 2. The solubilities of the third component in each of the binary B-Fe, B-Mo and Fe-Mo phases were found to be small. Two ternary phases with the crystal structures different from any of the unary and binary phases were found, namely T2-Mo2FeB2 and T4-Moi+ cFe2 xB4. The Ti-Mo2Fei3B5 phase forms from the melt at 1100°C and decomposes below 1000°C [1966Hasl],... 

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