Measurement of Thermal Expansion

An initial attempt was made to correlate the structural properties of the kl-(BEDT-TTF)2M(CF3)4(TCE) salts with their superconducting transition temperature [31]. The relationship between Tc and any single unit cell parameter failed to show any discemable trend. The best correlation was obtained by plotting Tc as a function of the b/c ratio, where b is the interlayer and c is an intralayer direction. A similar conclusion was reached through the determination of uniaxial pressure coefficients of p -(BEDT-TTF)2SF5CH2CF2S03 and k-(BEDT-TTF)2Cu(NCS)2 through the measurement of thermal expansion [32]. These results also indicated that expansion of the interlayer direction and compression of an intralayer direction... 

Phase transitions Differential thermal analysis (DTA) or differential scanning calorimetry (DSC), light scattering and various spectroscopic techniques, diffraction methods (especially X-ray diffraction), measurement of thermal expansion and any other property changing with the transition... 

The constants ai/(l + a0) and a2/(l + do) are given as the result of experiments on compressibility. If d0 is known from measurements of thermal expansion, we can then find d and a2 directly from experiment. 

Bradshaw A. L. and Schleicher K. E. (1970) Direct measurement of thermal expansion of seawater under pressure. Deep-Sea Res. 17, 691-706. 

Fig. 8 SEM photographs of Be/Cu sintered compacts used for the measurement of thermal expansion coefficient. The measurement were performed at RT to 700 "C. (The arrows indicate cracks.)...

There is a design in which an A1203 coating is to be applied with an insert layer to mitigate the thermal stress. The measurement of thermal expansion coefficient of relevant materials is of considerable value for the development of a protective coating system. Results of the measurements of the coefficient are summarized in Fig. 13 [67], However, successful coating system was not developed yet. 

Measurements of thermal expansion coefficients are useful in assessing the compatibility of different materials for fabrication into components. Mismatches in behaviour can cause stresses to build up when temperature changes occur resulting in eventual weakening and failure of the structure. Many crystalline materials can exist in a number of polymorphic forms which are stable at different temperatures. The transition between crystal structures is usually accompanied by a change in density and thermal expansion coefficient which can be detected by TMA. 

Figure 3.8 Measurement of thermal expansion behavior of (a) the SMP and (b) the SMP based syntactic foam...

Dilatometry. The measurement of thermal expansion, strictly, as a function of temperature. In particular, a continuous record can reveal phase changes in materials. 

Tremendous interest has arisen in recent years regarding die issues of chain mobility and glass transition at polymer surfaces (1-4). Keddie et al used ellipsometric measurements of thermal expansivity to study Tg of polystyrene on silicon as a function of film thickness (1). The observed Tg asymptotically approached the bulk value of 100 C as thickness was increased, but for thinner films lO s of nm), Tg was significantly depressed. Clearly, such effects are of critical importance in the application of ultrathin polymer films as lubricants and protective coatings in magnetic data storage devices and micromechanical systems (5). Until recently, however, direct mechanical measurements of polymer behavior at the nanoscopic scale (which is necessary for mechanical analysis of ultrathin films) and at elevated temperatures has not been possible. 

As in the case of heat capacity, anomalies can also occur in the thermal expansivity of materials. Various transformations that occur in the solid state can alter the interatomic forces of the material and lead to changes in its overall dimensions. For example, the ferromagnetic Curie point of dysprosium at low temperatures leads to an irregularity in the expansivity. Other examples are KH2PO4 with a ferro-electric Curie point at 122 K, and the glass transitions of soft rubbers. In fact, the measurement of thermal expansion is a useful method by which solid-state transitions can be investigated. Table 3.8 gives values of thermal expansion for some structural materials at low temperatures. 

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