Solid/liquid transformation

The next four sections will discuss some of the above work in detail. The first two sections concern solid state transformations with section (11.3.1) concentrating on steels while section (11.3.2) looks in more detail at the DICTRA programme. The last two sections concern liquid- solid transformations. Section (11.3.3) deals with conventional solidification while section (11.3.4) deals with rapid solidification. 

Thermoporometry. Thermoporometry is the calorimetric study of the liquid-solid transformation of a capillary condensate that saturates a porous material such as a membrane. The basic principle involved is the freezing (or melting) point depression as a result of the strong curvature of the liquid-solid interface present in small pores. The thermodynamic basis of this phenomenon has been described by Brun et al. [1973] who introduced thermoporometry as a new pore structure analysis technique. It is capable of characterizing the pore size and shape. Unlike many other methods, this technique gives the actual size of the cavities instead of the size of the openings [Eyraud. 1984]. 

Fig.1. Estimated static temperature-time-transformation (TTT) diagram of cocoa butter obtained with the FP900 apparatus. Open and filled symbols correspond to values 1 and 99% of the liquid-solid transformation achieved. Hatched domain corresponds to the melting range (MR) of Form II.

Studying the kinetics of the liquid-solid transformation shows that the polycluster-structure formation competes with crystallization and, under certain conditions, it becomes dominant and leads the liquid to transfer to the glassy state (Sect. 6.2). 

Gibbs free energy of formation Latent heat of fusion Enthalpy change at the triple point Heat of vaporization Enthalpy of vaporization Volume change for liquid-solid transformation... 

An additional great advantage is that the examinations may be carried out in the liquid state, and hence the picture is not distorted by the changes caused by the liquid-solid transformation. 

A film is deposited in a conventional chemical vapor deposition (CVD) process when the gaseous reactants are presented with a large hot support surface. Supported growth of whiskers occurs also when the gaseous reactants are presented with discrete hot metal catalyst particles located on the surface of a suitable substrate. Unsupported whisker growth occurs when hot metal catalyst particles are freely interspersed with the gaseous reactants in the vapor phase. The most common mechanism for whisker growth is a vapor-liquid-solid transformation, and the most versatile VLS process is a metal particle catalyzed chemical vapor deposition. 

The kinetics of solid-solid phase transformations tend to be much more sluggish than liquid-solid transformations. This is due to a number of factors ... 

Figure 14 Schematics of the additivity principles used for a model liquid-solid transformation. (a) The thermal path followed hy the sample is cut into small isothermal plateaus of duration 5f, where the isothermal data of the TTT diagram can be applied. ti d is the induction time, estimated for the transformation using an additivity principle. Once find has elapsed, phase transformation begins. EstimaAon of its progress is presented in (b). (b) Calculation of the evolution offs-fs is known at time step i - 1. The fictitious time tf corresponding to this solid fracAon on the curveis calculated, where /,7 (t) is the isothermal evolution of the solid fraction at the temperature T, of the sample at timestep i. The increment of sohd fracAon at time step i is then given by fs.Ti(f + 0 -fs.T,(tn...

The corresponding preparation methods may be grouped into two main streams based upon the gas-solid and liquid-solid nature of the transformations. Gas-solid transformation methodsare broadly used in the context of ultrafme oxide powder synthesis. Liquid-solid transformation methods follow a bottom-up approach. A number of methods have been developed, among which some generally used are discussed below ... 

Liquid-Solid Transformation Methods These are further grouped in to Co-precipitation method, Microemulsion technique. Solvothermal method, and Surface/Template derivatized method. 

L. D Souza, R. Richards, Synthesis of metal-oxide nanoparticles Liquid-solid transformations in synthesis, properties and applications of oxide nanoparticles (Rodriguez, J.A., Femandez-Garcia, M Eds.). N. J. Whiley, 2007. 

The theory for the rate of nucleation also follows from that derived for vapours. Becker originally proposed that the nucleation rate, I, in condensed systems, such as are involved in the liquid-solid transformation, was determined by an expression of the type... 

The classical theory of nucleation was developed by Volmer and Weber, and Becker and Doring" for the condensation of a pure vapour to form a liquid. The subsequent theory-- for the liquid-solid transformation was based on this earlier work. The theory considered homogeneous nucleation, ia the formation of one phase by the aggregation of components of another phase without change of composition and without being influenced ly impurities or external surfaces. Impurity particles and external surfaces are taken into account in heterogeneous nucleation theory (Section 2.3). Modifications to the classical theory are necessary to allow for the effects of compositional changes. 

Thermoporometry is based on the thermodynamic conditions of the liquid-solid transformation of a capillary condensate inside a porous material. 

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