Abstract. Thermal diffusivities of synthetic quartz single crystals have been
measured between −120 and 800 ∘C using a laser flash method.
At −120 ∘C, the lattice thermal diffusivities are D[001]=15.7(8) mm2 s−1
and D[100]=8.0(4) mm2 s−1 in the [001] and [100]
directions, respectively. Between −80 and 560 ∘C, the temperature
dependence is well approximated by a D(T)=1/Tn dependency (with n=1.824(29) and n=1.590(21) for the [001] and [100]
directions), whereas for lower temperatures measured thermal diffusivities
show smaller values. The anisotropy of the thermal diffusivity
D[001]∕D[100] decreases linearly over T in α-
and β-quartz, with a discontinuity at the α–β
phase transition at Tα,β=573 ∘C. In the measured signal–time curves
of α-quartz, an unusual radiative heat transfer is observed,
which can be linked to the phase transition. However, the effect is
already observed far below the actual transition temperature. The standard
evaluation procedure insufficiently describes the behaviour and leads to an underestimation of the thermal diffusivity
of ≥20 %. Applying
a new semi-empirical model of radiation absorption and re-emission
reproduces well the observed radiative heat transfer originating in
the phase transition. In the β-quartz region, the radiative heat transfer is not
influenced by the phase transition effect observed in α-quartz
and for the thermal diffusivity evaluation common models for (semi)transparent
samples can be used.
Experimental characterization of the coupled conductive and radiative heat transfer in ceramic foams with a flash method at high temperature
