High thermal expansion

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

Unfilled Tooth Restorative Resins. UnfiUed reskis were some of the first polymer materials iatroduced to repak defects ki anterior teeth where aesthetics were of concern. They have been completely replaced by the fiUed composite reskis that have overcome the problems of poor color StabUity, low physical strength, high volume shrinkage, high thermal expansion, and low abrasion resistance commonly associated with unfiUed reskis. 

Materials using plasticizers could be a problem. Materials that exhibit substantial moisture absorption are not stable dimensionally. Many organic plastics show a high thermal expansion differential in comparison... 

The formation of barium chromate often leads to the physical separation of the sealing glass and the metal alloy due to barium chromate s high thermal expansion. Along interfacial regions where oxygen or air access is blocked, chromium or chromia can react with barium-calcium-aluminosilicate (BCAS) glass-ceramic to form a chromium-rich solid solution and a series of pores. 

Figure 1.4 Schematic representation of the relationship between the shape of the potential energy well and selected physical properties. Materials with a deep well (a) have a high melting point, high elastic modulus, and low thermal expansion coefficient. Those with a shallow well (b) have a low melting point, low elastic modulus, and high thermal expansion coefficient. Adapted from C. R. Barrett, W. D. Nix, and A. S. Tetelman, The Principles of Engineering Materials. Copyright 1973 by Prentice-Hall, Inc.

Figure 5.23 Potential energy diagrams for materials with (a) high melting point, high elastic modulus, and low thermal expansion coefficient and (b) low melting point, low elastic modulus, and high thermal expansion coefficient.

Composite materials inherently develop residual stresses during processing. This happens because the two (or more) phases that constitute the composite behave differently when subjected to nonmechanical loading. For example, consider a reinforcing phase that has low thermal expansion characteristics embedded in a matrix phase with high thermal expansion characteristics. If the material is initially stress free and the temperature is decreased, then the matrix will try to shrink more than the reinforcement. This places the reinforcement in a state of compression (i.e. a compressive residual stress). If the phases are well bonded, then models can be developed to predict the residual stress field that is induced during processing. 

Low thermal expansion material High thermal expansion material... 

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. 

PPFMs, particularly, Teflon AF and polytetrafluoro ethylene Excellent electrical Properties (Eowest dielectric constant among most polymers < 2) Poor thermal stability Adhesion problems High thermal expansion coefficients... 

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." ... 

These types of materials will probably play an important role in the further development of combustion catalysts, especially if their activity can be improved. Moreover, they can be extruded to give active monoliths directly. This, however, still generates problems with respect to mechanical strength and thermal shock properties due to the relatively high thermal expansion coefficient [64]. 

When fluxing action is needed during melting, and glass with high thermal expansion is undesirable, boric oxide is added forming borosilicate glasses, which have lower thermal expansion and improved resistance to attack by aqueous solutions because of lower sodium oxide contents. 

These compositions are for glass for every day use. The batch materials for soda-lime glasses are easily available at reasonably moderate cost. These materials readily meet at moderate temperatures. These glasses have a relatively high thermal expansion coefficient and only moderate resistance to attack by a contained product. 

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