Forsterite thermal expansion

Incidentally, it must be noted that, although most olivine compounds are stable (at P = 1 bar) up to the melting temperature, in some compounds polymorphic transitions are observed at temperatures below the melting point (for instance, Ca2Si04 has a transition to a monoclinic polymorph at 1120 K). Forsterite at 1 bar melts at 7) = 2163 K, whereas fayalite melts at a much lower temperature Tf = 1490 K). The significance of this difference, as shown by Hazen (1977), must be ascribed to different polyhedral thermal expansions for polyhedra containing Mg and Fe cations. Hazen (1977) pointed out that, if the curve of volumes of the members of the (Mg,Fe)2Si04 mixture is extrapolated at various... 

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. 

Forsterite can be used for high frequency and high-temperature applications. Its high thermal expansion coefficient and concomitant poor thermal shock resistance are drawbacks to its use, although the high expansion can be used to advantage when an insulator must be bonded to a high-thermal-expansion metal. ... 

The high thermal expansion coefficient makes forsterite suitable for ceramic-to-metal seals, but it also causes the material to have poor thermal shock resistance. Table 2.3 lists the physical properties of forsterite. 

Forsterite, 2MgO Si02, ceramics are easier to fire than steatite or cordierite and have low dielectric losses, even to microwave frequencies. Their high thermal expansion limits their use as substrate materials and results in poor thermal shock resistance. Forsterite ceramics are used in high-power applications such as ceramic vacuum tube envelopes. 

With the use of the thermal expansion data of Skinner (1966) for diopside, enstatite, spinel, pyrope and forsterite, the volume change at lOOO C is -7.61 cm. Thus, fortunately, the differential thermal expansion may be ignored. 

In olivenes, FeO is an impurity. This impurity tends to reduce the refractoriness of forsterite bricks. The iron content is adjusted to get a refractoriness of 1750°C. The RUL value minimum is 1550°C. Thermal expansion is low, which results in a moderately good spalling resistance. Fosterite is resistant to both acid and basic slags. Its resistance to acid slag is not as much as that of silica, and its resistance to basic slag is not as much as that of chrome-magnesite. 

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