Thermal expansion behavior

One of the biggest challenges in this industry is the wide variety of substrates that can be encountered for any given application. Not only can the materials be substantially different in their chemical make up, but they may also be quite different in surface roughness, surface curvature and thermal expansion behavior. To help adhesion to these substrates, preparation of the surface to be bonded may be critical. This preparation may be as simple as a cleaning step, but may also include chemical priming and sanding of the surface. 

Mori M and Hiei Y. Thermal expansion behavior or titanium-doped La(Sr)Cr03 solid oxide fuel cell interconnects. J. Am. Ceram. Soc. 2001 84 2573-2578. 

Tests of a graphite-reinforced polyimide composite (C6000/PMR15) did not show any effect of radiation exposure (1 MEV electrons 6x109 rad total dose) on the thermal expansion behavior (14). DMA curves for unirradiated and irradiated composites were essentially identical over the temperature range of the thermal expansion measurements. 

To assess the effect of elastomer degradation on composite performance, additional composites were fabricated with the same 121°C cure epoxy without any addition of the elastomer (211. The expansion behavior of the modified epoxy composite was similar to the toughened material. For electron doses less than 10 rads the CTE of the toughened and untoughened composites were essentially the same which suggests that the epoxy matrix and not the elastomeric component controls the thermal expansion behavior. 

Tschatifeser, P. and Parker, S.C. (1995) Thermal expansion behavior of zeolites and AIPO4S. J. Phys. Chem., 99, 10609-10615. 

Because of the structural variety possible with polyimides, many smdies have sought to understand their structure-property relationships, often focusing on one specific target property. Such studies have included those aimed at understanding thermal expansion behavior, optical properties electronic structure, dielectric constant and loss, PTIR analysis, adhesion, water absorption, and molecular ordering, as well as others. The references cited... 

The CTE of 6FDA/TFDB is a little higher than that of PMDA/ODA. The former has a high CTE because the main chains contain bent hexafluoroisopro-pylidene units. We discuss the thermal expansion behavior of GFDA/TFDB in the next section, comparing it with the low-thermal-expansion fluorinated polytmide PMDA/TFDB. In addition, the refractive index of GFDA/TFDB, 1.556 at... 

A thermally induced residual stress. The origin of the residual stresses is a mismatch of thermal expansion behaviors among the components. Rigid connection of each component with different thermal expansion coefficients causes residual stresses. For example, the electrolyte and electrodes are fabricated and connected at a high temperature. If the thermal expansion behaviors are not identical among the components, residual stresses will occur in the cell at room temperature. For stacks, similar residual stresses will occur by a mismatch of thermal expansion behavior among cells and other stack components. 

Typical materials for the electrolyte are YSZ, samaria doped ceria (SDC), and LaGaC>3. The intrinsic property of the thermal expansion behavior of an electrolyte depends only on the material species. However, the other mechanical properties (Young s modulus, Poisson s ratio, and strength) depend on the morphology through the manufacturing processes. Accordingly, the reported mechanical properties are not unique. The reported thermal expansion coefficient (TEC) and other mechanical properties for the electrolyte materials are listed in Table 10.1. 

The origin of chemically induced stresses is a change of the lattice distance of materials due to changes in the physical state. For example, it is known that the lattice constant of LaCrCL increases under a reducing atmosphere [3], On the basis of the analogy between the thermal expansion and chemical expansion, the chemical expansion behavior can be simulated along with the thermal expansion behavior. 

Thus Equation (10.33) is solved as the new equilibrium equation. To calculate the thermal expansion behavior of the model, the thermal expansion coefficient is necessary as the calculating parameter. If the temperature of the model is even, the initial temperature and the final temperature are used as just a calculating parameter. If a temperature distribution exists in the model, the temperature distribution data is dispensable for the stress calculation. The temperature is firstly calculated by a CFD and the calculated data is used as the boundary condition in the stress calculation. If the thermal expansion coefficient is temperature dependent, the temperature dependence must be considered in the calculation. Here the temperature data at the nodes is transferred from STAR-CD to the ABAQUS. 

In addition to the occurrence of the residual stress, cells warp toward the cathode side because of the mismatch of the thermal expansion behavior between the electrolyte and anode. Thus, the deformations of cells at room temperature are also... 

If this hypothesis is right, the specific volumes that characterize the RAF and MAF have to be essentially different below the crystallization temperature. Figure 17 exhibits a sketch to illustrate this point. This sketch basically shows a hypothetical thermal-expansion behavior associated with the RAF and MAF for PET, crystallized at some arbitrary crystallization temperature, Tc. Above Tc, in the equilibrium melt, only one phase occurs, i.e. the specific volumes for the RAF and MAF are the same. If vitrification of the RAF occurs at Tc, the slope of specific volume versus temperature for this fraction should change at Tc, and become characteristic of the glassy state in the temperature interval below Tc. In the same manner for the MAF, the slope of specific volume versus temperature, below Tc, should continue to be the same as for the equilibrium melt and change only at the real Tg. Therefore, if room temperature (25 °C) is considered as the reference, the specific volume for the RAF at 25 °C must be larger than that for the MAF. The same reasoning would lead to the anticipation that the specific volume of the RAF will be a direct function of Tc. 

Glasses and amorphous polymers have a characteristic thermal expansion behavior, an example of which is shown in Figure 7.9. These materials pass through a glass transition tem-... 

Figure 7.9 Thermal expansion behavior of reheated soda-lime-silica glass. The decrease in slope just before Tg implies the thermally induced relaxation of a rapidly quenched glass.

Fig. 25. Thermal expansion behavior of Fi-berite 934 epoxies as influenced by aging history...

Figure 26 shows the thermal expansion behavior of requenched epoxies. Upon reaging, the densification process was again measurable. Data shown in Figure 26, therefore, supports the notion that physical aging processes are thermoreversible. 

Table 12. Thermal expansion behavior and fractional free volume at Tg for epoxy resin systems before and after crosslinking...

The effects of heat treatment temperatures on thermal conductivity, thermal conductivity at high temperatures and thermal expansion behavior have been studied. At room temperature, the value of thermal conductivity for unidirectional (UD) carbon-carbon composites is 700 W/m K. In the case of three-dimensional (3D) carbon-carbon composites, this value is determined by the volume of the fiber arrangements. On the other hand, the thermal expansion of carbon-carbon composites in the fiber axial direction is chiefly governed by the thermal expansion of the fiber. 

Carbon fibers show negative thermal expansion behavior at temperatures between 20 and around 500°C as shown in figure 4. This behavior depends on each fiber s grade. By utilization of this negative behavior, materials whose coefficient of thermal expansion is zero can be created when quasi-isotropic laminates are controlled with an optimum fiber volume fraction and are incorporated with matrices which have positive coefficient of thermal expansion. In practice, those materials can be used in satellites or space telescopes which demand severe thermal environment resistance. 

Figure 8 shows thermal expansion behavior of UD-C/C with the heat treatment at 3000 . There are large differences in the thermal expansion between the fiber axial direction and the fiber vertical direction. 

---