Zero thermal expansion materials

Materials that do not expand at all as the temperature rises, zero thermal expansion (ZTE) materials, would be of value for many purposes, especially in microelectronic devices that may become warm due to power consumption. For this reason, materials that show ZTE are being actively sought. In the past, a ZTE solid could be constructed by using a composite of two materials, one showing thermal contraction and one showing thermal expansion. However, composite materials often have accompanying drawbacks in use, and a number of solids have now been fabricated that show virtually no expansion at all as the temperature rises. 

The previous two sections have shown that some oxide perovskites have a marked thermal contraction and that this property can be modified by the traditional ceramic 

---

Lightweight and isotropic near-zero thermal expansion materials can be obtained by adding metallic particles to a negative thermal expansion glass-ceramic matrix [95]. These materials find applications in mirrors and general optics, sensors, microwave components and antennae because of their impressive thermal and dimensional stability. 

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. 

Li20-Si02 (LAS). The trade names of such glass-ceramic matrix materials are Corningware, Zerodur and Ceran. This type of glass-ceramic matrix material has nearly zero thermal expansion and high thermal shock resistance. It is used for the production of optical and telescopic mirrors. 

Sol-gel methods are used to prepare a heterogeneous, nanometer composite of calcium zirconium phosphate and strontium zirconium phosphate, Cao.sSro.sZrsOn. In this composite, individual domains both expand and contract upon heating, leading to a material with virtually zero thermal expansion over the range 0-500 °C. 

Usefiil zero thermal expansion composites are made by combining materials that show the unusual property of negative thermal expansion (i.e. contraction) with normal (positive) expansion materials. Examples of phosphates showing negative thermal expansion are the diphosphate-divanadate solid solutions ZrP2- V Oy and the microporous aluminophosphate AIPO-17 which shows a particularly large effect. ... 

Note that in general the nitrides and carbides of Si, with their lower thermal expansion coefficients, are more resistant to thermal shock than oxides. In theory, a material with zero thermal expansion would not be susceptible to thermal shock. In practice, a number of such materials do actually exist commercially, including some glass-ceramics that have been developed which, as a result of thermal expansion anisotropy, have extremely low a s (see Ch. 4). Another good example is fused silica which also has an extremely low a and thus is not prone to thermal shock. 

The expansion of chemical bonds with increasing temperature leads the vast majority of known solids to expand with heating (positive thermal expansion, PTE), a property once thought to be an immutable law of nature. A relatively small number of materials are known that defy this expectation and contract upon heating i.e. display negative thermal expansion, NTE) or are temperature-invariant i.e. display zero thermal expansion, ZTE). These novel behaviours arise due to a range of physical mechanisms that include magnetostriction and, most... 

Glass matrix composites, in particular with carbon fibre reinforcement, have been also proposed for a variety of applications which require thermal dimensional stability, i.e. materials with multidimensional near zero thermal expansion coefficients such as support structures for laser mirrors [21, 22]. 

Zirconyl pyrophosphate, (ZrO)2P207, mp = 2070°C, p = 3.88 g/cc, is stable up to about 1600 C, and calcium orthophosphate, Ca3(P04)2 which melts at 1820°C, can be used for some purposes at temperatures up to 1600°C. The zirconyl compound, which loses P2O5 above 1600°C, has potential use as a low-expansion ceramic, although its strength is rather limited [3]. Up to llOO C the net expansion is <1.7 x 10 C (Table 12.27). Zirconyl pyrophosphate is a good electrical insulator and its dielectric constant is similar to that of alumina [4]. Solid solutions of composition Ca Sr,. Zr4(P04)g and Zr 2-Nx i show almost zero thermal expansion over a considerable temperature range, and are promising materials for future application (see below). 

Kim I. J., Kim H. C.(2004) Zero level thermal expansion materials based on ZrTi04-Al2TiQ5 ceramics synthesized by reaction sintering. / Ceram Proc Res 5 308-12. 

As a further advantage, composites make effective use of some materials that are otherwise unable to stand alone, such as mineral fibers or wood flour. When incorporated into polymers— in particular those such as unsaturated polyesters or phenolics—particles can reduce manufacturing shrinkage and yield a more usable product. In service, zero thermal expansion coefficients can be achieved by a suitable choice of starting materials. 

After rough machining (and the working out of faults in the glass, if required), the blanks are converted from the glassy condition into the semi-crystalline condition through a thermal treatment. Due to that treatment the material also receives the property of zero thermal expansion. 

Its virtually zero thermal expansion coefficient made this material interesting for other applications, too. The idea of using Zerodur for cookware soon came to mind. Trademarks, such as Pyroflam , Jena 2000 , Ceradur and Corning Vision produced by the main suppliers Corning Glassworks and Schott, are well-known cookware articles. At the end of the 1960s this product benefit led to the development of Ceran cooktop panels. 

Thermal Stress Sudden temperature changes of the ambient (thermal shocks) cause an inhomogeneous temperature distribution and thus thermal stress in materials. These stresses, especially tensile ones,can bring about brittle fracture. A simple model, shown in Fig. 2.14, will be used to explain this. A very thin plate (with negligible thickness in direction z) is a model of external layers of a hot material, which become cold due to a sudden temperature decrease in the ambient. For a non-zero thermal expansion coefficient a of the material, the plate dimensions tend to decrease on cooling. The model plate, like the external layers of a material, is fixed in direction x to a rigid substrate which is a model of the bulk of the material which does not initially change its temperature and dimensions. 

---