Rapid communication Effects of CaO on crystallization properties in BaO–SrO–TiO 2 –SiO 2 glass–ceramics Fei Duan Division of Thermal and Fluids Engineering, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore abstract article info Article history: Received 29 October 2010 Received in revised form 31 December 2010 Available online 31 January 2011 Keywords: Glass–ceramics; Optical properties; Piezoelectric properties; Grain growth The role of calcium oxide (CaO) was investigated in the crystallization of BaO–SrO–TiO 2 –SiO 2 glass. The CaO dopant altered the oriented growth facets of crystal unit cells in the glass–ceramics. The crystallites are acicular in micron scale in the samples having as small as 1% CaO dopant, but are long granular in the nanometer scale in the sample without CaO. The glass–ceramics which have nano crystallites are observed with a less crystallization density qualitatively, a higher transmissivity in the wavelength range of 200– 2600 nm, and 30% lower piezoelectric coefficient, d 33 , at 9.5 ± 0.3 pC/N than those glass–ceramics having CaO in the study. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Piezoelectric materials have attracted more attention recently due to their piezoelectric, pyroelectric, nonlinear optical, and dielectric properties [1–11]. The growth technology of the piezoelectric single crystals helps to develop the new piezoelectric glass–ceramics in the systems of LiNbO 3 , Ba 2 TiSi 2 O 8 , and Sr 2 TiSi 2 O 8 . These non-ferroelectric glass–ceramics with the crystalline phase in the residual glass matrix have a low dielectric coefficient, and high temperature and pressure stabilities. The glass–ceramic materials are easily fabricated into a large area, and do not have aging and depolarization problems [7–10]. Thus, the piezo-composites show a good potential for the applications in such sensors as hydrophone [7,12]. However, from the point of view of both fundamental study and application, glass forming and post-thermal crystallization treatment have challenging problems because the forming of the crystal materials has little glass forming ability normally [13]. In addition, the influence of the calcium cation as a network modifier in the glass forming can give rise to modify the properties such as viscosity, diffusivity in the vitreous network, transparency, etc. Although there are studies published [14–16], the non-network forming cations like calcium in glass are still under study. In this paper, the effects of calcium oxide (CaO) on the crystallization of the glass in the BaO–SrO–TiO 2 –SiO 2 systems are introduced by investigating the crystalline phase, the microstructure, and the properties of the piezoelectric glass–ceramics. 2. Experimental In order to obtain a crystalline phase of Ba 0.2 Sr 1.8 TiSi 2 O 8 from the base glass [7], reagent grade chemicals of 47.51% SrCO 3 , 7.06% BaCO 3 , 14.28% TiO 2 , and 31.15% SiO 2 in mass ratio were prepared and mixed. To study the effects of CaO, 1.79 mass% or 2.68 mass% amount of CaCO 3 was added into the base glass recipe, listed in Table 1. Each batch was then melted at 1520 °C within a crucible in an electric furnace. Afterwards, the melt was quenched to form a fully transparent glass cube with a light brown color, which was annealed at 600 °C. A small piece of the glass was ground into fine powders for Differential Thermal Analysis (DTA, Parkin Elmer DTA7) at a heating rate of 20 °C/min in order to determine the transition point (T g ) and crystallization temperature (T c ), also listed in Table 1. The original glass bulks of S1, S2 and S3 were then cut into small cubes. After each surface of the glass cubes was polished, the glass bulk specimens were treated in an electric furnace by preheating to 760 °C slowly, further increasing a temperature quickly to 960 °C, and maintaining there for 3 h to crystallize the glass into glass–ceramics. The reason of setting the bulk crystallizing temperature at about 60 °C above T c was to produce the good oriented crystallites in the residual glass [17]. The crystalline phases of the glass–ceramics were determined from the ground glass–ceramic powder of S1, S2 and S3 by using X-ray diffraction method (XRD and Philips PW1820) in the angle of 10–70°. The glass–ceramic cubes were then separated into two equal pieces, and machined toward the original surface of the cubes in parallel until the thickness reached 0.8 mm. As shown in the insert of Fig. 1 for the glass–ceramic samples S1 (right) and S3 (left), both the front surface (top surface) and backside (bottom) were polished for the transmis- sivity measurement in a wavelength range of 200–2600 nm under a UV–VIS-IR spectrometer (Shimadzu UV3101PC) firstly. The surface and backside of the polished glass–ceramics were also scanned in XRD to determine the oriented growth facets of crystallites. The similar method was applied to detect the thin film coating [18]. The macroscopic piezoelectric constant, d 33 , was then measured in a quasi-static d 33 meter (ZJ-2), recorded in Table 1. The uncertainty of d 33 there comes from the standard deviation of six different positions Journal of Non-Crystalline Solids 357 (2011) 1494–1497 E-mail address: feiduan@ntu.edu.sg. 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.01.011 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol