Abstract
The role of grain size and porosity in the piezoelectric and elastic properties of SiO 2 -based materials was investigated using resonant piezoelectric spectroscopy (RPS) and resonant ultrasound spectroscopy (RUS). RPS performed on agate revealed a piezoelectric effect comparable in magnitude to that in single-crystal quartz. The observed strong piezoelectricity in agate requires preferential orientation of SiO 2 during crystal growth. Similarly, in novaculite and sandstone finite (but weak) RPS signals were evident, suggesting that the expected randomization of the piezoelectric quartz grains is incomplete. On the other hand, Vycor, a silica glass with a porosity of 40, showed no evidence of the piezoelectric effect. According to temperature-dependent RPS and RUS measurements, the α-β transition temperature in quartz does not change in polycrystalline samples. Finally, the temperature dependence under heating of the elastic constants is reversible in quartz and agate and irreversible in sandstone and Vycor.
Highlights
Many SiO2-based materials form crystal structures containing SiO4 tetrahedra that are linked to form three-dimensional frameworks
No resonant piezoelectric spectroscopy (RPS) signals were observed for Vycor at room temperature
The second harmonic generation (SHG) signal for quartz shows the same trend as RPS (Ríos et al 2001)
Summary
Many SiO2-based materials form crystal structures containing SiO4 tetrahedra that are linked to form three-dimensional frameworks. The phase transition is displacive and transforms as the piezoelectric coefficient d14 (Salje et al 1992). The α phase, space group P3121, is piezoelectric with the two active coefficients d11 and d14 (and their symmetry equivalent parameters). The transition from the α phase to the (high-temperature) β phase, space group P6222, maintains d14 piezoelectricity. The transition β → α near 847 K is co-elastic with a large decrease of the molar volume and other non-symmetry-breaking spontaneous strains under cooling (Carpenter et al 1998). The non-symmetry breaking strains are e1+e2 and e3, namely the contraction in the plane perpendicular to the crystallographic c-axis and the contraction along the c-axis (Carpenter et al 1998). All shear spontaneous strains ei (i > 3) are strictly zero
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