Metal oxides, thermal expansion coefficients

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

Under thermal cycling conditions, the principal source of stress within the oxide scale is the temperature change . Christl et have noted that, when cooling 2.25%Cr-l%Mo steel from 600°C in air, compressive stresses build up in the haematite, whilst tensile stresses build up in the magnetite and spinel layers. This arises because the thermal expansion coefficients of the individual oxide layers increase in the order a metal < a spinel < a magnetite < a haematite . ... 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

Naturally the final size of the metallic reinforcing particles is limited by the initial size of the oxide particles. This is rather important, since there is a critical maximum particle size which can lead to degradation of the composite properties via thermal stress-induced cracking.2 The critical maximum particle size was evaluated by Kolhe et al.,n both experimentally and via finite element analysis. Assuming perfectly spherical particles, the difference in thermal expansion coefficients between Ni and a-Al203 resulted in a critical particle size for an isolated Ni particle of 3.0 pm. Experimental observations... 

Table 1.1. Abundance of the metal in the earths s crust, optical band gap Es (d direct i indirect) [23,24], crystal structure and lattice parameters a and c [23,24], density, thermal conductivity k, thermal expansion coefficient at room temperature a [25-27], piezoelectric stress ea, e3i, eis and strain d33, dn, dig coefficients [28], electromechanical coupling factors IC33, ksi, fcis [29], static e(0) and optical e(oo) dielectric constants [23,30,31] (see also Sect. 3.3, Table 3.3), melting temperature of the compound Tm and of the metal Tm(metal), temperature Tvp at which the metal has a vapor pressure of 10 3 Pa, heat of formation AH per formula unit [32] of zinc oxide in comparison to other TCOs and to silicon...

This section covers some other heterometallic rare earth oxides, including Al, Ti, Zr, Sn, Mo, W, Mn, Fe, Co, Ni, and Cu complex oxides, while certain well-known oxysalts, Y-Ba-Cu-O, for example, will not be specifically discussed. For these heterometallic compounds, due to their relatively complex compositions, it is usually difficult to obtain phase-pure products, especially when some dopant ions are added. At elevated temperatures, some of these oxides undergo phase transitions, which may significantly change their physical and chemical properties such as thermal expansion coefficient and ionic conductivity. And for fhose oxides with variable metal valencies, different nonstoichiometric compositions may also result in distinct functionalities in magnetism and catalysis. 

The choice of permeation barrier materials selected here for coatings is reasonable based on permeation resistance, but external coatings of ceramics on metals are difficult to perfect since many metals have high thermal expansion coefficients and most ceramics have low ones. This causes large thermal stresses in the coatings to develop, which leads to defect formation in the coatings and lowers permeation resistance. A better technique, which will be discussed in the next section, relies on the formation of intrinsic oxide films on the surface of the metals, either by direct oxidation or by alloying followed by oxidation." ... 

The deposition of a suitable catalyst in the intimate body of the above-described filters is controlled primarily by the structure of the filter itself, but it is also influenced by the nature of its constituent material. In fact, shear stresses may arise at the interface between this material and the deposited catalyst, owing to thermal expansion mismatch between the two phases. Since most catalyst supports are based on inorganic oxides, this problem would be particularly serious for metal-based filters, owing to their much higher thermal expansion coefficients. However, in some metal alloys, such as the FeCrAlloy, a thin surface layer of a metal oxide (e.g., ALOO is formed at high temperatures, which improves their thermal resistance and allows a proper basis for catalyst anchoring. 

Heating in the course of annealing precursors that were cathodically deposited on metal substrates can cause cracking of deposits [204] as a result of different thermal expansion coefficients. However, as a result of the high uniformity of component distribution, the time and, sometimes also, the temperature of the thermal treatment can be substantially reduced. The last factor is particularly essential for thallium systems because it allows one to carry out annealing without any additional sources of thallium oxide vapor [194,195,199]. 

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