Framework structures, thermal expansion

Gibbs energy minimization has also predicted negative isobaric expansion coefficients for certain crystalline zeolite framework structures, which subsequently were confirmed experimentally [6], Many solids show negative thermal expansion at very low temperatures, including even some alkali halides (Barron and White (Further reading)). Many other solids on heating expand in some directions and contract in others. 

Many aluminosilicate glass ceramics are based on framework structures of AIO4 tetrahedra, which, when crystallized, posseses low thermal expansions. This gives the glass ceramics based on them near zero expansions and thus excellent dimensional stability, thermal shock resistance, and mechanical strength. Aluminosilicate glass ceramics are used commercially as telescope mirrors, thermally stable structures for satellites and space probes, gyroscope components, heat-resistant windows, stove tops, and cookware. 

The structures with D = 3 (frameworks) are usually most stable mechanically, thermally, and chemically. The layered (D = 2) and chain (D = 1) structures, and especially structures with finite anions (D = 0) with similar bond strength and polarity, are more labile many of these may absorb water into the interlayer or interchain space, or hydrolyze to produce amorphous products or even be water-soluble. In addition, structures with D = 1 or 2 usually display strong anisotropy of the conductivity and/or thermal expansion (although some exceptions are possible). Multiple examples with D = 0, 2, and 3 are listed in Sections 7.4—7.6. 

This structural chassis of box-like architecture is the framework designed to accomodate all MART-LIME subsystems. It is fabricated from a carbon fibre laminate of high elastic modulus and low coefficient of thermal expansion, internal to which is a conducting mesh for both electromagnetic insulation and grounding. 

A series of phosphate ceramics of the formula A M2(P04)3 [A=Ca, Sr M = Ti, Zr, Hf and Sn] belonging to the calcium titanium phosphate family have been synthesised and their thermal expansion behaviour investigated. These compounds are candidate matrices for nuclear waste immobilization. The thermal expansivity and expansion anisotropy of the compotmds in the temperature range of 300 - 1273 K have been found to depend on the nature of the metal ions in the interstitial space and structural framework. The observed expansion behaviour has been explained in terms of the crystal chemistry of the compounds. 

High-temperature structural determinations on a large variety of crystals have revealed that coefficients of thermal expansion of a given type of cation-anion bond are fairly constant and depend inversely on the electrostatic bond strength. Tetrahedral (Al,Si)-0 bonds in framework silicates, in particular, typically have a slightly negative or zero expansion coefficient. 

Both the high-quartz and the keatite structure contain more than one possible site for the non-framework cations. Size and oxygen coordination preferences of the cations determine site occupations, which in turn exert significant influences on the framework geometry and thermal expansion. 

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