Thermal penetration coefficient

It is usual here to introduce the material dependent thermal penetration coefficient... 

It increases quickly at first and then slower as time t goes on. With large thermal penetration coefficients b the material can swallow large heat flows, so that the surface temperature increases more slowly than in a body with small b. 

The assumption of a time independent contact temperature m was therefore appropriate. Its position depends on the thermal penetration coefficients... 

The time independent contact temperature lies in the vicinity of the initial temperature of the body with the larger thermal penetration coefficient. Equation (2.147) explains why different solids at the same temperature feel as if they were warmed to differing degrees when touched by the hand or foot. 

As a consequence, the overall penetrant uptake cannot be used to get direct informations on the degree of plasticization, due to the multiplicity of the polymer-diluent interactions. The same amount of sorbed water may differently depress the glass transition temperature of systems having different thermal expansion coefficients, hydrogen bond capacity or characterized by a nodular structure that can be easily crazed in presence of sorbed water. The sorption modes, the models used to describe them and the mechanisms of plasticization are presented in the following discussion. 

Sometimes filled adhesives will show better resistance to moisture resistance than unfilled adhesives simply because incorporating inert fillers into the adhesive lowers the organic volume that can be affected by moisture. Aluminum powder seems to be particularly effective, especially on aluminum substrates. The filler can provide a reduction of shrinkage on cure, a reduction of the thermal expansion coefficient, and a reduction of the permeability to water and other penetrants. However, fillers do not always produce more durable bonds. 

The surface of most porcelain ware is glazed. The main requirement is suitable adjustment of the thermal expansion coefficients of body and glaze, in order to produce a low compression stress in the glaze after cooling. Adhesion of the glaze to the body is very satisfactory as a result of perfect wetting which follows from the similarity of the two materials. Penetration of glaze into the body pores also has positive effects. 

The TMA technique can be used for Tg-value determinations, resin cure studies, penetration experiments or orientation effect determinations. The most important application is thought to be the linear thermal expansion coefficient (l.e.c.) determination of engineering polymers. An example of this application is given in chapter 3.1.2. The results of a polymer shrinkage experiment monitored by TMA are described in chapter 3.1.3. 

Parameter improvements (reduced rock mass permeability and rock mass thermal expansion by the KTH/SKI team, and increased thermal expansion coefficient and reduced swelling pressure constant of the buffer by JNC team) -Inclusion of the sealing of rock fractures by penetrating bentonite by the KTH/SKI team, which can explain the uniform (axisymmetric) wetting of the bentonite. 

TMA consists of a quartz probe which rests on top of a flat sample (a few mm square) in a temperature controlled chamber. When set up in neutral buoyancy then as the temperature is increased the probe rises in direct response to the expansion of the sample yielding thermal expansion coefficient versus temperature scans. Alternatively, with a penetration probe under dead loading a thermal softening profile is obtained (penetration distance versus temperature). Although this is a simple and versatile experiment, it gives only a semi-quantitative indication of mechanical modulus versus temperature. 

In PMC, the polymer matrix is expected to wet and bond to the second (reinforcing) constituent, and it is expected to flow easily for complete penetration and elimination of voids in the system. It must be elastic enough with low shrinkage and low thermal expansion coefficients (TEC) it must be easily processable, must have proper chemical resistance, in addition to low and high temperature capabilities, dimensional stability and so on. 

A feature of the instrument is a rotating analysis head, which can be oriented through 180° to adjust the analysis configuration for different test types and sample geometries. In addition to operation in the dynamic mechanical mode, the DMA 8000 operates in a constant-force (TMA) mode versus time or temperature. Applications such as thermal expansion coefficient, softening and penetration, or extension or contraction in the tension geometry provide data equivalent to those obtained by many conunerdal standalone TMA instruments. 

When thermal expansion or penetration (modulus) is being measured, the sample is placed on a platform of a quartz sample tube. The thermal expansion coefficient of quartz is small (about 0.6 x 10 K ) compared to the polymer materials. The quartz tube is connected to the armature of a linear variable... 

Figure 10.7 Schematic presentation of the Perkin-Elmer thermal mechanical analyser (top). Bottom left expansion probe for measurement of linear thermal expansion coefficient. Bottom right penetration probe for measurement of stiffness.

Thermal expansion coefficients are determined by thermomechanical analysis using a dilatation-penetration probe for adhesive pastes and an extension probe for self-standing films. The output of the thermal analyzer equipped with a dilatation probe is a curve plotting the variation of adhesive thickness as a function of the temperature. 

Blondiaux et al. (50)(53) showed that copper, nickel and cobalt diffuse into silicon samples. At 240° only a very small penetration of nickel is noticed after a 3 h irradiation. At 560 and 800°C an important diffusion of cobalt, nickel and copper was observed, namely several hundred m. Diffusion in a furnace at identical temperatures and during identical periods of time led to the same results. It was thus shown that in the cases studied, radiation enhanced diffusion is negligible compared to the thermal one. Deep thermal diffusion can lead to analytical errors in spite of etching after irradiation. It thus seems necessary to take the thermal diffusion coefficients in account in order to evaluate possible errors and to limit the temperature of the sample by adequate cooling. 

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