Cristobalite thermal expansion

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

In many cases, more than one crystalline phase forms. Often, phases that form initially transform into another phase as heat treatment progresses. For example, crystalline phases of /5-quartz solid solutions (e.g., (S-eucryptite), which are not stable at high temperatures, convert to more stable phases (e.g., 6-spodumene, cristobalite, sapphirine, or lithium disilicate). The stable high temperature phase may have different properties— in this case, higher thermal expansion. Therefore, glass ceramics with the same chemical composition may have very different properties, depending on the heat treatment. 

Swainson IP, Dove MT (1995b) On the thermal expansion of p-cristobalite. Phys Chem Minerals 22 61-65 Tantz FS, Heine V, Dove MT, Chen X (1991) Rigid unit modes in the molecular dynamics simulation of qnartz and the incommensurate phase transition. Phys Chem Minerals 18 326-336 Teznka Y, Shin S, Ishigame M (1991) Observation of the silent soft phonon in p-quartz by means of hyper-raman scattering. Phys Rev Lett 66 2356-2359... 

One of most striking examples of the importance of atomic packing on a is silica. Vitreous silica has an extremely low a, whereas quartz and cristobalite have much higher thermal expansion coefficients, as shown in Fig. 4.5. 

The variation in the coefficient of thermal expansion of quartz and cristobalite with temperature reveals very strikingly the a-P inversions, and is best seen by plotting the percentage volume expansion for each mineral against the temperature (Figure 9). Owing to the difficulty of obtaining pure tridymite its expansion... 

When colloidal silica is embedded in a matrix of sodium silicate it tends to crystallize and form cristobalite at the temperatures the cores reach when metals are cast. Due to the difference in thermal expansion coefficient, the expansions and contractions of the cristobalite crystals embedded in the glass matrix tend to crack the binder film surrounding the sand grains therefore weakening the mold or core. This weakening effect has to be added to the... 

The phase transition temperature between low and high modifications of cristobalite does not appear to be constant, but a typical temperature is around 215°C. The transition is accompanied by large changes in thermal expansion. The z- and c-axis of a-cristobalite increase rapidly at rates of 9.3 X 10 and 3.5 X 10 A K respectively whereas in p-cristobalite, a expands at only 2.1 X 10 A This behavior translates into very large, spontaneous strains of -1% along /z-axis and -2.2% along c-axis during inversion. 

It is relatively easy to apply a glaze to these materials, as richterite glass-ceramics exhibit a high coefficient of thermal expansion of 115 x 10 (0°(D-300°C) due to the formation of a cristobalite secondary phase. Therefore, a glaze with a lower coefficient of thermal expansion can be applied to produce compressive strain in the surface of the material. As a result, the material is strengthened as described in Section 2.3.3. 

---