| Summary: | Glass-ceramics containing pyroelectric Sr<sub>2</sub>TiSi<sub>2</sub>O<sub>8</sub> (STS) crystals are produced from parent glasses of compositions STS + 0.8 SiO<sub>2</sub> + (0.2 − x) Na<sub>2</sub>O + x CaO, with x = 0; 0.05; 0.10; and 0.15. The aim of this work is to investigate the effect of the additions to the stoichiometric STS composition on the crystallization mechanisms and on the piezoelectric properties of the glass-ceramic. The DSC analyses evidence that the glass transition temperatures T<sub>g</sub>, the onset temperature of the crystallization peak T<sub>o</sub> and the temperature T<sub>c</sub> of the maximum of this peak increase with the CaO/Na<sub>2</sub>O ratio. On the basis of the DSC data, the crystallization of the parent glass samples was operated by thermal treatment. The observation of the cross-sections of the heat-treated samples highlights that the competition between the surface and volume crystallization mechanisms is influenced by the CaO/Na<sub>2</sub>O ratio and the temperature. For all the samples, the XRD analyses performed on the surfaces as obtained after the crystallization treatment evidenced a preferential orientation of the STS crystals with the plans (00l) parallel to the surface. The XRD analyses performed after grinding the surface show that only the surface crystallized layers are preferentially oriented. Moreover, changes in preferential orientation to plans (202) or (201) are observed over the depth, depending on the composition and the temperature of the thermal treatment. These changes influence the polarization of the surface crystallized layer and, consequently, its piezoelectric charge coefficient d<sub>33</sub>. The highest values of d<sub>33</sub> were measured on the glass-ceramic samples exhibiting mainly a (202) preferential orientation over their thickness.
|
|---|