Incongruent phase transformation

What is the principal difference between congruent and incongruent phase transformations ... 

Figure 10.15 shows a model of thermally induced phase transformation in a spherical HAp powder particle, considering a paraboUc temperature gradient according to Fourier s law. During the short residence time of the particle in the plasma jet, the innermost core wiU remain at a temperature well below 1550°C owing to the low thermal diffusivity, showing HAp and OAp as stable phases (see Steps 1 and 2 in Scheme 10.1). The second shell has been heated to a temperature above the incongruent melting point of hydroxyapatite (1570 °C), and consists of...

Non-thermal deposition methods are defined as those ones carried out at temperatures much below the incongruent melting point of hydroxyapatite, in particular at or near ambient temperature. However, frequently post-depositional heat treatment must be applied to either crystallise amorphous calcium phosphate (ACP), transform (dehydrated/dehydroxylated) precursor phases such as octacal-cium phosphate (OCP) to hydroxyapatite, and/or to remove organic compounds used in coating preparation, for example during sol-gel, dip coating, electrochemical and EPD. 

Silicon carbide (SiC) is the only solid compound known in this system. It occurs with two modifications, cubic jS-SiC (referred to as low temperature modification) and hexagonal a-SiC with numerous polytypes where 4H, 6H and 15R are the most common ones. The a-/) -SiC transformation temperature is not well known. Kistler-de Coppi (1985) [77] studied in detail the transformation kinetics at temperatures between 1973 K and 2573 K. According to these results (metastable) ) -SiC transforms at temperatures above 2273 K to a-SiC. SiC melts incongruently forming a sihcon-rich liquid phase and graphite. 

The solid is said to melt incongruently. Many intermediate compounds transform into a liquid at a peritectic point. At a peritectic point, three phases coexist. The point is thus an invariant point, and the reaction is an invariant reaction. 

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