Ceramic thermal expansion coefficient

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

Because of the high functional values that polyimides can provide, a small-scale custom synthesis by users or toU producers is often economically viable despite high cost, especially for aerospace and microelectronic appHcations. For the majority of iudustrial appHcations, the yellow color generally associated with polyimides is quite acceptable. However, transparency or low absorbance is an essential requirement iu some appHcations such as multilayer thermal iusulation blankets for satellites and protective coatings for solar cells and other space components (93). For iutedayer dielectric appHcations iu semiconductor devices, polyimides having low and controlled thermal expansion coefficients are required to match those of substrate materials such as metals, ceramics, and semiconductors usediu those devices (94). 

Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Estimate the thermal shock resistance AT for the ceramics listed in Table 15.7. Use the data for Young s modulus E, modulus of rupture c, and thermal expansion coefficient a given in Table 15.7. How well do your calculated estimates of AT agree with the values given for AT in Table 15.7 ... 

The thermal expansion coefficient of ceramic material can vary from 2 X 10 to 7 X 10 mm/mm°C. The values obtained for bodies normally supplied are about 5 x 10 to 6 x 10 mm/mm°C values outside this range will be obtained only from special bodies. 

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). 

Thermal expansion differences exist between the tooth and the polymer as well as between the polymer and the filler. The tooth has a thermal expansion coefficient of 11 x 10-6/°C while conventional filled composites are 2-4 times greater [63, 252], Stresses arise as a result of these differences, and a breakdown between the junction of the restoration and the cavity margin may result. The breakdown leads to subsequent leakage of oral fluids down the resulting marginal gap and the potential for further decay. Ideal materials would have nearly identical thermal expansion of resin, filler, and tooth structure. Presently, the coefficients of thermal expansion in dental restorative resins are controlled and reduced by the amount and size of the ceramic filler particles in the resin. The microfilled composites with the lower filler loading have greater coefficient of thermal expansions that can be 5-7 times that of tooth structure. Acrylic resin systems without ceramic filler have coefficients of thermal expansion that are 9 times that of tooth structure [202-204, 253],... 

For use over a wide temperature range, it is necessary to match the thermal expansion coefficients of electrode and insulation sheath. RRDEs of glassy carbon embedded in borosilicate glass for use up to 450° C [123] and gold sputtered on to a chromium or titanium substrate on a Macor ceramic cylinder for use up to at least 125°C [124] are examples. 

Shrinking is a so-called mechanical bonding technique. Two hot objects are fitted together. When they cool the outer part attaches itself to the inner one. For this to be possible the outer part must have a bigger thermal expansion coefficient than the inner one. In this way ceramics and metals can be joined. 

Aluminum nitride is one of the few materials that is both a good thermal conductor and a good electrical insulator. It is also a high-temperature ceramic, that has a low thermal expansion coefficient, and a low dielectric constant. It is also stable to molten metals such as aluminum, has good wear resistance, and good thermal shock resistance. 

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