Amorphous exothermic heat

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

Both the quantities AH and AE are relatively easily accessible to experimental determination. The magnitude of the exothermic heat effect observed when an amorphous alloy is heated in a calorimeter up to its crystallization temperature T ) is a direct measure of AH. It can be derived from the area under the exothermic DSC peak. The activation energy AE can be obtained, for instance, from an Arrhenius-type plot when the crystallization temperature is studied as a function of the heating rate (see section 4.1),... 

Manning and Briscoe offer two possible explanations for the difference in superconducting properties between vapour-quenched and liquid-quenched alloys. The first explanation is based on the assumption that the liquid-quenched alloys are microcrystalUne rather than amorphous. The observation of a strongly exothermic heat effect (T ) observed by Buschow in the differential scanning calorimetry experiments does not corroborate this view. The second explanation is based on the assumption of an atomic arrangement in the vapour-quenched alloy, which is much more uniformly disordered than in the liquid-quenched alloy. The rather low transition widths A 7 seem to be in favour of this explanation, although even lower ATq values were found in liquid-quenched La .AUj alloys by Johnson and Tsuei (1976). 

The quantitative application of IMC for the rate of enthalpy relaxation of a pure amorphous APIs and excipients are explicitly documented but unfortunately few examples are available hitherto for ASD (Caron et al. 2010). Exothermic heat flow is detected in TAM below Tg as a consequence of relaxation. During IMC measurement, the early data points are often excluded during data analysis to avoid noise resulting from sample positioning (Kawakami and Pikal 2005). However, IMC records more temporal data points during enthalpy relaxation and hence yields relaxation parameters from a single run when compared to DSC. Thus, the power-time profiles obtained in TAM can be directly treated with the power equations of relaxation models viz., KWW (Eq. 14.3) or MSE equation with respect to time. The derivative form of the MSE equation can describe the experimental relaxation data measured by IMC more consistent, especially those recorded at lower annealing temperature (Kawakami and... 

Finally, DSC is also well suited for the characterization of amorphous forms. At the glass transition temperature, where the amorphous form converts from the glassy to the rubbery state, a jump in the heat capacity will be observed. It may also happen that the amorphous substance crystallizes when it becomes mbbery. This will manifest itself in the DSC by an exothermic heat of crystallization and subsequently the crystalline solid will melt (Figure 8.7). 

Polyvinyl chloride (PVC) has peculiar material characteristics because of its high level of syndiotactic vinyl chloride units. The polymer as a result has a low to modest level of crystallinity (-10%). The uncrystaUized regions in PVC are subject to dissolution by polar solvents. The solutions formed have exothermic heats of mixing and negative deviations from ideality. Non-volatile polar liquids, which dissolve amorphous PVC, are called plasticizers . PVC now becomes an elastomeric mixture of small crystalline regions and a solution phase. These materials are called plasticized polyvinyl chloride. 

The interaction of cellulose with various liquids is accompanied by exothermic heat effects (loelovich, 2011). The interaction enthalpy of cellulose with low-polar liquids, e.g., alcohols having numbers of C-atoms S2, is not function of amorphicity degree, and depends on specific surface of the pores. Some other liquids. 

To confirm that the matrix is amorphous following primary solidification, isothermal dsc experiments can be performed. The character of the isothermal transformation kinetics makes it possible to distinguish a microcrystalline stmcture from an amorphous stmcture assuming that the rate of heat released, dH/dt in an exothermic transformation is proportional to the transformation rate, dxjdt where H is the enthalpy and x(t) is the transformed volume fraction at time t. If microcrystals do exist in a grain growth process, the isothermal calorimetric signal dUldt s proportional to, where ris... 

Wood Hill (1991b) induced phase-separation in the clear glasses by heating them at temperatures above their transition temperatures. They found evidence for amorphous phase-separation (APS) prior to the formation of crystallites. Below the first exotherm, APS appeared to take place by spinodal decomposition so that the glass had an intercoimected structure (Cahn, 1961). At higher temperatures the microstructure consisted of distinct droplets in a matrix phase. 

Amorphous or crystalline silicon both react exothermally when heated with alkali-metal carbonates, attaining incandescence and evolving carbon monoxide. 

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

The monochloride is soluble in various inert organic liquids, more particularly in benzene, chloroform, carbon tetrachloride and carbon disulphide, without undergoing chemical change. It is an exothermic compound, its heat of formation from gaseous chlorine and the amorphous modification of selenium being 22-1 Cals.1 Water causes a gradual decomposition of the chloride, selenium dioxide and selenium being formed 2... 

Fig. 14. DSC-traces of glasses of a macromolecule with a mesogen in the side-chain. Amorphous glass (top curve) and the corresponding LC-glass (bottom curve) drawn after data from Ref.21). Heating rates 50 K/min. The exotherm indicates the irreversible transition to the mesophase...

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