Solid transformations

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

Secondary processing does not always lead to phase transformations, as was shown during studies of the polymorphs of ranitidine hydrochloride [92]. No solid-solid transformation could be detected during either the grinding or compression of metastable Form I, stable Form II, or of a 1 1 mixture of these forms. The dissolution rates of both forms were found to be equivalent, and the solution-mediated transformation of Form I to Form II was observed to be slow. 

As a further example of a solid-solid transformation associated with reorganization of bonds, we take the process described by Bergman and co-workers (81). These investigators have found that solid methyl p-dimethylaminobenzene-sulfonate (36a) is cleanly converted to the p-trimethylammonium benezenesul-fonate zwitterion (36b) on standing at room temperature, and more rapidly so at higher temperatures, whereas even the most concentrated solutions of this compound remain unchanged at these temperatures. Crystal structure analysis... 

The solid-solid transformation of 2-amino-3-hydroxy-6-phenylazopyridine, 57a, to 57b proceeds through two intermediate phases (119). X-Ray and IR studies of the former, low-temperature, and the latter, high-temperature phase show that they are the phenolazo and quinone hydrazone forms, respectively. This solid-state tautomerism can be accounted for by a cooperative intermolecular shift of protons across the various hydrogen bonds. However, because of the complexity of the hydrogen-bond network, the actual pathway of the proton shift has not been uniquely defined. 

In most of the early studies 9> of H20(as) the vapor was condensed on metal surfaces in the temperature range 77 K diffraction data, supplemented by new experimental studies, convinced Olander and Rice that most deposits obtained at or above 77 K are likely contaminated with crystalline ice. They established conditions for the deposition of pure H20(as) on a variety of substrates 10>. Briefly put, the temperature of the substrate should be low, preferably below 55 K, and the rate of deposition very small (a few mg/hour). There is evidence that H20(as) can be deposited on a substrate at 77 K if the deposition is slow enough. The use of high deposition rates at 77 K leads to polycrystalline ice Ic mixed with H20(as). A sample of pure H20(as) is stable indefinitely long (at least several months) if maintained below 20 K. At about 135 K, with some variation from sample to sample, the amorphous solid transforms spontaneously and irreversibly to ice Ic. 

It Is known that many carboxyl acid crystals have polymorphs (3) and that the crystals suspended In saturated solution sometimes display a solid-transformation (2). If a-form crystal or 3-form crystal In saturated solution changes Into other crystal form within experimental time scale, solubilities for a-form crystal or 3-form crystal do not have substantial meaning. So stability of a-form crystal or 3-form crystal suspended In the saturated solution was Investigated. 

It Is unknown whether suspended a-form crystal or 3-form crystal In the saturated solution displays a solid-solid transformation or not. The reason for above experimental results may be assumed as follows In the standpoint of Industrial crystallization. Since 3-form crystal Is less soluble than a-form crystal at high temperature above 284K. The state that a-form crystal Is suspended In the saturated solution Is considered to be supersaturated for 3-form crystal. So the state has the potential to take place primary nucleatlon and crystal growth for the 3-form. In this way, 3-form crystal may be produced In the suspension of a form crystal. Rewarding to the formation of 3-form crystal, a part of a-form crystal suspended may be dissolved. Opposite phenomena may take place at low temperature below 284K. 

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. 

For solidification described by the lever rule and assuming linear liquidus and solidus lines, the composition of the solid C, as a function of the fraction solid transformed (/,) is given by the equation... 

IL3.3.I Using the Scheil solidification model Although it is realised that some back-diffusion will occur, results on the calculation of solidification using the Scheil simulation have proved to be successful in a number of cases. For Al-alloys it appears to be particularly successful, allowing not only very accurate predictions for fiaction solid transformed (/,) as a function of temperature but also for predicting the phases which appear during solidification, which in Al-alloys, is a complex phenomenon. 

As with solid-to-gas transformations, solid-to-solid transformations are... 

We need to describe the conversion of the sohd from reactant to product. We can use concentrations in the solid to describe the solid transformations. However, the concentrations are varying within each particle, while we want the overall conversion to describe the transformation. Therefore, we wiU use a variable describing the solid conversion to describe the conversion of a sohd, in analogy with the conversion X of homogeneous fluids used previously, so that Xj goes from 0 to 1 as the reaction proceeds. 

Another important example of a solid transformation where growth requires diffusion through the product film is the transformation of solid spheres. The principles of this process are similar to those for planar films, but now the concentration profile is not linear, and the expression one obtains for the transformation and the solid conversion is more complex. 

At particular critical points (Tq, Pc) on the phase diagram of a substance, two phases can be found in thermodynamic equilibrium. Therefore, upon application of a pressure or a temperature gradient, a transformation occurs from one phase into the other. This is a phase transition, in many aspects similar to a transformation implying the change of aggregation state. However, the extent of the changes in a solid to solid transformation is much smaller. For example, latent heat or latent volumes associated with the transformations are quite small, sometimes even difficult to detect. 

P.G. Shewmon. Interfacial stability in solid-solid transformations. Trans. TMS-AIME, 233 736-748, 1965. 

There is a large class of industrially important heterogeneous reactions in which a gas or a liquid is brought into contact with a solid and reacts with the solid transforming it into a product. Among the most important are the reduction of iron oxide to metallic iron in a blast furnace the combustion of coal particles in a pulverised fuel boiler and the incineration of solid wastes. These examples also happen to be some of the most complex chemically. Further simple examples are the roasting of sulphide ores such as zinc blende ... 

---