Alumina thermal expansion coefficient

Moreover, since also the mismatch of palladium and alumina thermal expansion coefficients is reduced, high temperature delamination and defects formation in the membrane are also reduced. It is also possible to control the palladium thickness (to about 5 pm) by increasing the hydrogen flux and so minimizing the cost of the composite membrane. 

Cyclic Oxidation In many industrial applications it is particularly important for the component to be resistant to thermal shock for example, resistance-heating wires or blading for gas turbines. Chromia, and especially alumina, scales that form on nickel-base alloys are prone to spalling when thermally cycled as a result of the stress build-up arising from the mismatch in the thermal expansion coefficients of the oxide and the alloy as well as that derived from the growth process. A very useful compilation of data on the cyclic oxidation of about 40 superalloys in the temperature range 1 000-1 I50°C has been made by Barrett et... 

For the cathode seal material, there is a criterion that the thermal expansion coefficient of the metal component must be lower than that of the a-alumina header. A nickel-cobalt-iron alloy (NiloK) with a... 

For most particulate composites the mismatch between the particles and the matrix is more important than the anisotropy of either component (though alumina/aluminium titanate composites provide a notable exception and are described below). The main features of the stresses can therefore be understood in terms of a simple elastic model assuming thermoelastic isotropy and consisting of a spherical particle in a concentric spherical shell of matrix with dimensions chosen to give the appropriate volume fractions. The particles are predicted to be under a uniform hydrostatic stress, ap after cooling. If the particles have a larger thermal expansion coefficient than the matrix, this stress is tensile, and vice versa. For small particle volume fractions the stress... 

LGMs of the AT/alumina and AT/ZTA displayed some very interesting properties which include excellent machinability, low thermal expansion coefficient, improved thermal shock resistance, low hardness (about 5 GPa), low Young s modulus (E) (250 GPa) and excellent flaw tolerance [Pratapa, 1997 Pratapa Low, 1998 Skala, 2000 Manurung, 2001], These materials appeared to display a large degree of near-surface quasi-plasticity under the Hertzian or the Vickers indenter which effectively inhibits the formation and propagation of cracks. The ductile behaviour of these materials was... 

For most metal-reinforced nanocomposites the thermal expansion coefficient of the metal phase will be larger than that of the matrix, reversing the expected stress fields compared to SiC-reinforced alumina. Thus while the tensile radial stresses surrounding occluded particles may induce transgranular cracking, the compressive hoop stresses may inhibit crack propagation if the particles are located at grain boundaries. Macrostresses in sub-micron Ni... 

In forsterite ceramics the mineral forsterite (Mg2Si04) crystallizes. They have excellent low-dielectric-loss characteristics but a high thermal expansion coefficient which imparts poor thermal shock resistance. During the 1960s they were manufactured for parts of rather specialized high-power devices constructed from titanium and forsterite and for which the operating temperature precluded the use of a glass-metal construction. The close match between the thermal expansion coefficients of titanium and forsterite made this possible. Today alumina-metal constructions have completely replaced those based on titanium-forsterite and the ceramic is now manufactured only to meet the occasional special request. 

They have a very high thermal expansion coefficient at ambient temperatures. This creates a large potential thermal mis-match in contact with silicon, alumina, and other surfaces. This expansion mis-match can cause wire breakage and other damage during temperature cycling. 

Shrinkage of the samples during sintering was measured using a dual-push-rod dilatometer with alumina as a reference material. The thermal expansion coefficient was determined from the slope of... 

Estimate the thermal expansion coefficient of alumina from Fig. 4.4. Does your answer depend on the temperature range over which you carry out the calculation Explain. 

Addition of alkali oxides to germania initially reduces the thermal expansion coefficient, which passes through a minimum at 2 to 5 mol% alkali oxide. Further additions of alkali oxides result in a continuous increase in the thermal expansion coefficient out to the limit of glass formation. The position of the minimum in thermal expansion coefficient is near the low alkali germanate anomaly in viscosity and glass transformation temperature, which occurs at 2 mol% alkali oxide. No unusual behavior in the thermal expansion coefficient is found in the 15 to 20 mol% alkali oxide region where the traditional germanate anomaly in density and refractive index occurs. Replacement of alkali oxides by alumina reduces the thermal expansion coefficient, but has little effect on the shape of the thermal expansion coefficient versus composition curve, which still displays a minimum at 2 to 5 mol% alkali oxide. 

The number of circuits which can be placed on one silicon "chip" is restricted by the difficulty of keeping the wafer of silicon bonded to its ceramic substrate as it heats up. At present, the most frequently used packaging material is alumina which has a thermal expansion coefficient, nearly twice that of silicon. Differential expansion between the silicon and its substrate is containable in current packages but the manufacturers want to move to larger circuits, for faster computers and to reduce size and cost of the overall equipment, so differences in expansion set a limit on how large the circuits can be. The semiconductor companies have a real need for a substrate material with thermal expansion similar to that of silicon. (Table 3)... 

FIGURE 13. Thermal expansion coefficient (/°C) as a function of SiC whisker content in alumina. 

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