Thermomechanical effects

Three types of measurements were used for investigation of reversible thermomechanical effects in glassy polymers. They include (i) thermoelastic temperature changes at elevated hydrostatic pressure 65,66), (fi) thermoelastic temperature changes... 

The postulate of local thermodynamic equilibrium in a discontinuous system is replaced by the requirement that the intensive properties change very slowly in each part, so that the parts are in thermodynamic equilibrium at every instant. The intensive properties are a function of time only, and they are discontinuous at the interface and may change by jumps. In the following sections, thermomechanical effects and thermoelectricity are summarized. 

Here Ju is the rate of energy density transfer across unit cross-section in unit time arising from the flux in moles of species 1 across unit-cross section in unit time. This ratio is clearly the energy transported under isothermal conditions per mole of species 1, denoted by Uj in Eq. (6.6.3). We see then that a thermomechanical effect is predicted for a fixed pressure difference across the junction, AP, and at constant temperature, a particle flux J1 gives rise to a proportional energy transport - Ujjx. This is a very sensible conclusion. 

Like the thermoelectric Seebeck effect, the thermomechanical effect implies the appearance of a pressure difference Ap = p2 - pi in the capillary connected vessels filled with a mobile substance—a hquid or gas— when the vessels are maintained at different temperatures with the temperature difference AT = T2 — Tj. The case of the vessels separated by a porous partition rather than one capillary is called thermoosmosis. The inverse phenomenon— the appearance of a temperature difference as a result of the pressure difference in the vessels—is called the mechanocaloric effect. 

Consider the reason for the appearance of the thermomechanical effect and its expected value. Let us say that two vessels, 1 and 2, are filled with some identical fluid (hquid or gas) and connected by a capillary, the fluids being held at preset constant temperatures T and T + dT. Let Jq desig nate the heat flux that passes through the capillary between the vessels, while Jg designates a potential fluid flux that diffuses through the same cap iUary (Figure 2.3). In accordance to the preceding deduced relationships (also see Section 1.5), the thermodynamic forces that initiate the fluxes are determined by the formula... 

Figure 2.3 On initiation of thermomechanical effect. The connecting capillary between two vessels is drawn schematically.

Here, the thermomechanical effect is seen only when the molar enthalpy of the fluid is unequal to ratio Li2/L22-... 

Thus, no thermomechanical effect is observed in the case of the large holes. 

Other types of damage may be produced through thermomechanical effects. For example, when being annealed at 450°C a CVD aluminum film on a Si substrate is subjected to compressive thermoelastic stresses owing to the considerable difference between the thermal expansion coefficients of aluminum (a = 23 x 10 °C 0 and the silicon substrate (a. = 3.5 x 10 °C 0-When cooling, the film may therefore contract by as much as 1%. Due to the combined action... 

The differences between the various carbon samples (Table VIII) may represent differences in the nature or in the treatment of the samples. The apparent erratic behavior of zinc oxide is probably attributable to causes other than the radiation, particularly since the doses were so low. The surprising increase in the area of copper oxide occurs at an unreasonably low dose, and is perhaps to be ascribed to thermomechanical effects in the sample. The dose rate was high enough to raise the temperature by 50-100° in 10 seconds. 

Conduction Heat Transfer with Thermomechanical Effects... 

Ohman, J., Antikainen, J, Niemi, A. 2003. Thermomechanical effects of hydraulic conductivity in a nuclear waste repository setting. In the Proceedings of this conference. 

It has been found by Allen S and Jones that at low temperatures in helium, when a temperature difference is produced, a pressure difference arises. This phenomenon is known as the fountain effect, or thermomechanical effect [30]. The superfluid is believed to carry no entropy. 

The equation above shows that the thermomechanical effect is dependent on two factors. One is proportional to the ratio L12/L11, and represents a coupling between the flow of the substance and the flow of entropy (heat). The other is proportional to the partial molar entropy S, since the difference in temperamre causes a difference in chemical potential AjLtj = —S T + FAP. 

At high temperatures, the clay masonry presents thermomechanical effects (such as thermal expansion of the piece and the mortar, spallings, and loss of strength) as well as variation of the material properties due to the degradation. This chapter describes the characteristics of the thermal and mechanical properties that are of particular relevance to the variation of the temperature and then particularized to the piece and the mortar are described. The independent behavior of these elements will greatly influence the subsequent response of the masonry wall as a single homogeneous material. 

When two vessels A and B containing Hell are joined by a very fine capillary the liquid level in both vessels is the same when the temperatures and pressures are the same. If an excess pressure is applied to the surface of the liquid in vessel A, say, liquid flows through the capillary into the vessel B, as expected, but the temperature of A rises while that of B falls. This is the thermomechanical effect. When the excess pressure is removed the system returns to its original state. The converse effect (the mechanocalorie effect) also occurs. If the levels in A and B are initially the same and the temperature of one vessel is raised, the liquid level rises in that vessel and falls in the other. 

To resolve the viscosity paradox, assume that at the lambda point all the fluid is normal fluid with a normal viscosity, and at absolute zero all the fluid is superfluid with zero viscosity. In the thin channel experiment described above, only the superfluid atoms, which have zero entropy and do not interact, can flow through the slit. On the other hand, the oscillating disk is damped by the normal fluid and thus accounts for the shape of the viscosity curve below the lambda point. The flow of helium II through very thin channels is accompanied by two very interesting thermal effects called the thermomechanical effect and the mechanocaloric effect. ... 

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