Thermal mismatch

Thermal expansion mismatch between the reinforcement and the matrix is an important consideration. Thermal mismatch is something that is difficult to avoid ia any composite, however, the overall thermal expansion characteristics of a composite can be controlled by controlling the proportion of reinforcement and matrix and the distribution of the reinforcement ia the matrix. Many models have been proposed to predict the coefficients of thermal expansion of composites, determine these coefficients experimentally, and analy2e the general thermal expansion characteristics of metal-matrix composites (29-33). 

The successful operation of SOFCs requires individual cell components that are thermally compatible so that stable interfaces are established at 1000°C (1832°F), i.e., thermal expansion coefficients for cell components must be closely matched to reduce stresses arising from differential thermal expansion between components. Fortunately, the electrolyte, interconnection, and cathode listed in Table 8-1 have reasonably close thermal expansion coefficients [i.e., 10 cm/cm°C from room temperature to 1000°C (1832°F)]. An anode made of 100 mol% nickel would have excellent electrical conductivity. However, the thermal expansion coefficient of 100 mol% nickel would be 50% greater than the ceramic electrolyte, or the cathode tube, which causes a thermal mismatch. This thermal mismatch has been resolved by mixing ceramic powders with Ni or NiO. The trade-off of the amount of Ni (to achieve high conductivity) and amount of ceramic (to better match the other component thermal coefficients of expansion) is Ni/YSZ 30/70, by volume (1). 

BN coating on SiC fibers or Al203 fibers reduces the thermal mismatch between fiber and matrix and also reduces fiber-matrix interfacial shear strength in ceramic matrix composites leading to higher overall strength and toughness. 

Micro-crack induced by non-uniform distribution of carbon fibres and thermal mismatch between Cf and sialon in the Cf/a-sialon composite (reprinted from J. Eur. Ceram. Soc., 22(2), Yu Z B, Thompson D P and Bhatti A R, Synergistic roles of carbon fibres and Zr02 particles in strengthening and toughening Li-a-sialon composites, 225-235 (2002). Copyright 2002, with permission of Elsevier). 

From equation (18.5), the stress caused by thermal mismatch in the matrix is proportional to the volume fraction of the fibres. Therefore, large thermal stresses in some local regions, where the distribution of carbon fibres is not uniform, will result in some cracks in the composite. 

Thermal mismatch between SiC fibres and a sialon matrix is also an issue which can cause poor mechanical performance, even though there may be excellent chemical and physical compatibility between the fibres and the... 

Therefore, in SiC fibre-reinforced sialon composites, thermal treatment of fibres, thermal mismatch and chemical reactions between the fibre and the sialon matrix are significant factors affecting the interfacial bonding in these composites. The sintering additive plays an important role in controlling the nature of the interface and requires careful selection. 

A detector layer of an imager presented in US-A-S264699 is thinned to allow the detector to act like a flexible membrane and to elastically respond to thermal mismatch resulting from different coefficients of thermal expansion between the detector and a semiconductor read-out circuit. 

From the foregoing analysis 1t is clear that a process modification that helps eliminate or minimize the irreversibility associated with the reactor feed preparation will lead to a major reduction in the thermal mismatch, reduce the exergy dependence on the power plant, and increase the overall energy efficiency. In the author s opinion, this conclusion would not be evident as readily without the thermodynamic analysis of process irreversibilities, which attests to the value of such exergy analyses. 

Knowing all the parameters except the thermal conductivity of the fiber, we can use these expressions to obtain thermal conductivity. It should be noted that in real composites, it is likely that the thermal contact at the fiber/matrix interface will be less than perfect because of thermal mismatch between the fiber and the matrix. 

J. D. Thompson, J. S. Brown, and P. W. Carr, Dependence of thermal mismatch broadening on column diameter in high-speed liquid chromatography at elevated temperatures, A aZ. Chem. 73 (2001), 3340-3347. 

Negative pressure specifically. With subscripts c, e, i, m, P, ST, TH, oo craze traction, Mises equivalent, one of three principal stresses, maximum level of craze traction where cavitation in PB begins, negative pressure in particle, negative pressure due to one of three principal stresses, negative pressure due to thermal mismatch, uniaxial applied stress at the borders With subscripts xx, yy, zz etc. for components of the local stress tensor Ratio of slope of the falling to the rising part of the traction cavitation law Craze dilatation Time constant... 

The negative pressure a.pjj induced in the PB spheres due to the thermal mismatch between them and the composite surroundings is... 

The peak traction can now be obtained from the condition that at this site the PB domains will cavitate under the action of the negative pressures induced by the three principal stresses a , CT99, a , and the added negative pressure ct h. due to the thermal mismatch by reaching the cavitational strength of PB that we have estimated earlier to be 60 MPa at room temperature. Thus,... 

In the first cycle the shape of the heating period (oxide formation) is different from the cooling period (thermal mismatch between layer and support). After the first cycle, the shape of the subsequent peaks are identical and all processes seem to be reversible. 

The maximum stress is obtained after each cycle and so no stress relaxation occurs. Note that the deflection measurements start with a dried membrane which already shows a certain deflection which is equivalent to a tensile stress level of 30 0 MPa. It is not clear at the moment whether it is allowed to sum up these two contributions or that the drying stress relaxes during heating and is replaced by stresses originating in the phase transformation/thermal mismatch processes. In any case when summing up is allowed the final stress in the y-alumina after cooling down is not greater than 30 MPa in the other case it is zero. 

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