Solid state, phase changes

Driving force for solid-state phase change A J-f... 

From a practical point of view, creep becomes an important natural phenomenon when the temperature at which a metal is loaded lies above about 0,4 to 0,5 of its melting point on an absolute scale. In some metals such as zirconium, which undergo a solid state phase change, creep becomes an important effect above about one-half of the temperature of the phase transformation. In many metals such as steel, creep is almost nonexistent at room temperatures if the metal is not loaded above its annealed yield strength. However, at 900"F (482"C> steel can creep readily al very small stresses, and equipment such as boilers and tubes for petroleum cracking stills, intended to operate at high temperatures for long periods... 

The behavior of shape memory alloys can be explained on the basis of solid state phase changes that occur within the material. All SMAs exist in one of two phases, known as martensite and austenite, shown in the diagram on page 132. Austenite is the "parent ... 

Caution. When large quantities of B5H9 are stored at low temperature in a glass container, a solid-state phase change involving expansion can rupture the container. 

When I was young, solid-state phase changes and chemical reactions were regarded more as a nuisance than as an area worthy of serious study and attention. It was the topochemical approach to solid-state chemical reactions, pioneered by Gerhard Schmidt, that transformed the subject for me and for many others. The textbook example is the photochemical dimerization of (fj-cinnamic acids in solution such compounds yield mixtures of the various possible stereoisomeric products, but irradiation of a particular crystal leads to a single product, or to no reaction, depending on the crystal structure [40]. Thus, one can determine the relative positions of the atoms before the reaction and after it, and haice deduce the metrical relationships that needed to be satisfied for reaction to proceed. In the meantime we have learned that not all chemical reactions in solids are topochemical. Some proceed not in the ordered bulk of the crystal but at defects, on the surface, or at other irregularities. 

The effect of low-doses gamma radiation on the relative proportions of a and 3 forms is evident from Fig. 2, where k decreases with absorbed dose. Namely, k tends to zero when no 3 form is present and to unity when a form is absent. As it is well known (5), the 3 form is as stable as the a form, and the 3 -)ct solid-state phase change does not take place. Because of that, the decrease of total crystallinity (Fig. 1) and k (Fig. 2) with... 

The choice of material was restricted to those having first-order solid-state phase changes in the appropriate temperature ranges. Melting points were excluded because several types of specimen holders in common use could not retain liquids. A number of recognized melting point Standards (e.g. Pb, Zn) may contaminate thermocouples and sample holders through alloy formation, and decomposition processes were considered to be unsuitable because the atmosphere could not be controlled in many types of apparatus. 

The ordinary drop calorimeter is widely in use - considering the limited number of substances to which it can be applied, rather too widely. Its application should really be limited to metals and stoichiometric compounds. If the substance undergoes a phase transformation in the temperature range studied or another solid-state phase change, the rapid cooling may produce undefined end states in the specimen. However, the resulting enthalpies may still be useful for approximate conversions of enthalpies of reaction from one temperature to another but should not be differentiated to obtain true heat capacities. 

Son Change, Moreshouse H L and Jeffrey H, An experimental investigation of solid-state phase-change materials for solar thermal storage , J. Sol. Energy Eng., 1991, 113(4), 244-9. 

There is no simple explanation other than to say that the free energies of each of the elements are determined by the temperature, pressure, and the distance between the ion cores and their electronic environment. Subjecting elements to high pressure alters their intemuclear distance and makes a P-1 contribution to the free energy which can produce solid-state phase changes. The only simple explanation seems to be that those phases that happen to have the lowest free energies under ambient conditions are the ones that appear in Figure 5.3. 

When equipped with a hot stage, the powder method is abo useful for observing solid-state phase changes or peritectic reactions in which a solid reacts with a liquid to form a new solid. 

Debye— Scherrer Monochromatic Fine powder Lattice parameters Material identification Solid-state phase change... 

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