Phase stability and transformation

Diefenbacher et al. (2005) Optical spectroscopies Serpentine Phase stability and transformation - + + C02 sequestration... 

Finnegan, M.P., Zhang, H., and Banfield, J.F., Phase stability and transformation in titania nanoparticles in aqueous solutions dominated by surface energy, J. Phys. Chem. C, 111, 1962, 2007. 

Nucleation and Growth (Round 1). Phase transformations, such as the solidification of a solid from a liquid phase, or the transformation of one solid crystal form to another (remember allotropy ), are important for many industrial processes. We have investigated the thermodynamics that lead to phase stability and the establishment of equilibrium between phases in Chapter 2, but we now turn our attention toward determining what factors influence the rate at which transformations occur. In this section, we will simply look at the phase transformation kinetics from an overall rate standpoint. In Section 3.2.1, we will look at the fundamental principles involved in creating ordered, solid particles from a disordered, solid phase, termed crystallization or devitrification. 

Turan, S. and Knowles, K.M., (1996b), Effect of boron nitride on the phase stability and phase transformations in silicon carbide , J. Am. Ceram. Soc., 79 (12), 3325-3328. 

The form that silicon carbide takes depends on many factors including thermal history, impurity type and level, and environment. The p form is generally felt to be the stable phase at low temperatures, whereas the a form is the high-temperature form. There are many exceptions to the rule, as the conversion to a from /3 and the converse have been reported. The stability and transformations of the various polytypes vary among themselves and constitute a subject that is too broad for this effort. The basic a and p descriptors will be used for the remainder of this section. 

Table 7.1 shows the infrared assignments for some of the common metal carbonates. Significant amounts of impurities can cause the infrared peaks to shift noticeably, so X-ray diffraction is still the most reliable analytical tool for crystal form assignment. Additional information on infrared absorption of carbonates [15], relative thermodynamic stabilities and transformation between phases has been published [16, 17]. 

Hitherto we have discussed the formation of amorphous covalent ceramics only on the basis of polymer derived materials. In Sects. 2.2 and 4.2.2.2, thin amorphous, hydrogen stabilized SiC layers (a-SiC H) are also considered which are formed, first of all, by gas phase processes (CVD, PVD). They represent another type of amorphous covalent ceramics. And though it is not expected that properties of such layers agree completely with those of the polymer derived ACC, the basic ideas of stability and transformability of the ACC state discussed above should be transferable to this type of amorphous covalent ceramics, too. 

Hai] Haidemenopoulos, G.N., Grajicic, M., Olson, G.B., Cohen, M., Thermodynamies Based Alloy Design Criteria for Austenite Stabilization and Transformation Toughening in the Fe-Ni-Co System , J. Alloys Compd., 220, 142-147 (1995) (Calculation, Experimental, Phase Relations, 15)... 

It is shown that the stabilities of solids can be related to Parr s physical hardness parameter for solids, and that this is proportional to Pearson s chemical hardness parameter for molecules. For sp-bonded metals, the bulk moduli correlate with the chemical hardness density (CffD), and for covalently bonded crystals, the octahedral shear moduli correlate with CHD. By analogy with molecules, the chemical hardness is related to the gap in the spectrum of bonding energies. This is verified for the Group IV elements and the isoelec-tronic III-V compounds. Since polarization requires excitation of the valence electrons, polarizability is related to band-gaps, and thence to chemical hardness and elastic moduli. Another measure of stability is indentation hardness, and it is shown that this correlates linearly with reciprocal polarizability. Finally, it is shown that theoretical values of critical transformation pressures correlate linearly with indentation hardness numbers, so the latter are a good measure of phase stability. 

Based on the reversibility of their phase transformation behavior, polymorphs can easily be classified as being either enantiotropic (interchange reversibly with temperature) or monotropic (irreversible phase transformation). Enantiotropic polymorphs are each characterized by phase stability over well-defined temperature ranges. In the monotropic system, one polymorph will be stable at all temperatures, and the other is only metastable. Ostwald formulated the rule of successive reactions, which states that the phase that will crystallize out of a melt will be the state that can be reached with the minimum loss of free... 

Calculation, thermodynamic optimization of phase diagrams. The knowledge of phase equilibria, phase stability, phase transformations is an important reference point in the description and understanding of the fundamental properties of the alloys and of their possible technological applications. This interest has promoted a multi-disciplinary and multi-national effort dedicated not only to experimental methods, but also to techniques of optimization, calculation and prediction of... 

Figure 5.26. Iron binary alloys. Examples of the effects produced by the addition of different metals on the stability of the yFe (cF4-Cu type) field are shown. In the Fe-Ge and Fe-Cr systems the 7 field forms a closed loop surrounded by the a-j two-phase field and, around it, by the a field. Notice in the Fe-Cr diagram a minimum in the a-7 transformation temperature. The iron-rich region of the Fe-Ru diagram shows a different behaviour the 7 field is bounded by several, mutually intersecting, two (and three) phase equilibria. The Fe-Ir alloys are characterized, in certain temperature ranges, by the formation of a continuous fee solid solution between Ir and yFe. Compare with Fig. 5.27 where an indication is given of the effects produced by the different elements of the Periodic Table on the stability and extension of the yFe field.

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