Study On Polar Glass-Ceramics And Thin Film In BaO-TiO₂-SiO₂ System

In conventional process for crystallization of glass, the as-produced glass-ceramics present a grain alignment without oriented structure due to the effect of nucleating reagents or other factors, such as phase separation and spinodal decomposition. However, a large mount of researches shows that a material with grain oriented structure possesses more excellent properties in many aspects, including mechanical, electrical, and optical properties. Just as Ramesh and Schlom pointed in Science, 2002, to obtain a optimal function for many functional materials, such as ferroelectrics, not only needs the grain oriented structure to be fabricated, but also needs the orientation direction of the oriented grain structure to be controlled. Up to date, the controlling the crystal growth in specific orientation that fabricate a material with certain grain oriented structure has become one of the important field in materials science and technology, and a powerful way to design a material with the desired properties Fresnoite, with chemical formula of Ba₂TiSi₂O₈, is a polar crystal belonging to tetragonal crystal system with space group C²_4V P4bm, its c axis as polar axis with axis ratio a/c of 1:0.611. In the crystal lattice, Si⁴⁺ exists in fourfold coordination with a double-tetragonal [Si₂O₇] structure; and Ti⁴⁺ has a specific fivefold coordination with pyramidal [TiO₅] and locates in the center of the pyramid; meanwhile, the double tetragonal [Si₂O₇] and square pyramidal [TiO₅] consist of the layer structure. Between the layers is Ba²⁺ which is in 10-fold coordination. Due to its specific 5-fold Ti4+coordination and the layer structure, fresnoite attracts attention of researchers since it was discovered. This mineral is a piezoelectric with odd piezoelectric coefficient: both of its d₃₃ and d₃₁ are in same sign which is different from the most of ferroelectric piezoelectrics, such as PbTiO₃, so that it has larger merit factor of hydrostatic piezoelectricity suitable for application in high-sensitive receiving transducers. It also has stable temperature dependences of dielectric and piezoelectric properties below 875K which render it potential application in high temperature piezoelectric devices. Besides, this crystal possesses excellent surface wave acoustic (SAW) feature, including low temperature coefficient of delay (TCD) and relative high coupling factor for potential application in SAW devices. In one word, fresnoite crystal attracted interesting of many researchers due to its wide application in piezoelectric and other aspects. Further, the polar –oriented glass ceramics, in which polar microcrystals are deposited from the glass matrix and connected in (1-3) configuration, also attracts attention as pointed by Prof.Cross of Pennsylvania State University, the famous physicist in ferroelectrics, that the occurrence of polar oriented glass ceramics "provides a new type of piezoelectric materials but also provide a new way to fabricate a piezoelectric". In this thesis, the fresnoite was selected as the research objective, and the fresnoite glass ceramics with polar oriented structure were fabricated through a special isothermally orientating crystallization process. The effects of the contents of glass forming oxide, process conditions, and additive oxides on the microstructure were studied. Meanwhile, the thin films with oriented structure was obtained by sputtering and sol-gel method, the influence of the preparation process on the microstructure of the thin films also researched in this thesis. All theses works on the fresnoite materials was devoted to set up a foundation for further study in application of fresnoite for devices. My main works and progress obtained includes as follows: 1. Based on the analysis on the effects of glass forming oxide on the glass forming ability, main crystallized phase, and piezoelectric properties, the basic composition of glass in BaO-TiO2-SiO2 system was determined, and the influences of additive oxide on the microstructure and piezoelectric properties were discussed. 2. Based on the crystallization theory of glass, the main factors influencing oriented crystallization were discussed, the temperature effect on the orientation structure was analysized to determine a suitable nucleating temperature. The measurement in electrical properties shows that the nucleating temperature is the key to control the orientation structure and piezoelectric properties of polar glass ceramics. 3. DTA analysis based on Johnson-Mehl-Avrami theory indicated that The activation energy for crystallization is 238kJ/mol and the Avrami index is about 3. This result means that the crystallization of BaO-TiO2-SiO2 glass is a interface-reaction controlling process with saturated nucleus quantity. 4. To explain the process of grain growth in the isothermal process for oriented...