Titanate glass-ceramics

Lynch and Shelby (1984) examined the reasons for the high dielectric properties of lead titanate glass-ceramics. They defined the matrix phase around the ferroelectric crystals. They also determined a special effect they called clamping. This phenomenon occurs when the glass matrix contracts and produces compressive stress within the ferroelectric particles, causing paraelectric-to-ferroelectric phase transition. As a result, the crystal size must be controlled for electro-optic applications. The crj tals should preferably be smaller than 0.1 pm. 

Lynch S.M. and Shelby, "Crystal Clamping in Lead Titanate Glass-Ceramics," J. Am. Ceram. Soc., 67, 424-27 (1984). 

McCauley D., Newnham R.E., and Randall, "Intrinsic Size Effects in a Barium Titanate Glass-Ceramic,"/. Am. Ceram. Soc., 81, 979-87 (1998). 

Yao K, Zhang LY, Yao X, Zhu WG (1997) Preparation and properties of barium titanate glass-ceramics sintered from sol-gel-derived powders. J Mater Sci 32(14) 3659-3665... 

McCauley D, Newnham RE, Randall CA (1998) Intrinsic size effects in a Barium Titanate glass-ceramic. J Am Ceram Soc 81(4) 979-987... 

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Most glass-ceramics have low dielectric constants, typically 6—7 at 1 MHz and 20°C. Glass-ceramics comprised primarily of network formers can have dielectric constants as low as 4, with even lower values (K < 3) possible in microporous glass-ceramics (13). On the other hand, very high dielectric constants (over 1000) can be obtained from relatively depolymerized glasses with crystals of high dielectric constant, such as lead or alkaline earth titanate (11,14). 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Myhra, S., Smart, R. St. C. Turner, P. S. 1988fc. The surfaces of titanate minerals, ceramics and silicate glasses surface analytical and electron microscope studies. Scanning Microscopy, 2, 715-734. 

Aluminium titanate (tielite, Al2Ti05) has excellent thermal shock resistance but poor mechanical strength which can, however, be improved by reinforcing with whiskers of a related phase such as potassium hollandite (K2Al2Ti60i6). Such composites can be formed by thermal decomposition of sol-gel precursors, reaction sintering of the two phases or by thermal treatment of an appropriate glass-ceramic material. Al MS NMR has been used to study the co-formation of these two phases during thermal treatment, and indicates that hollandite crystallises as whiskers within the tielite matrix (Kohn and Jansen 1998). 

Crystalline dispersions of several niobate crystals or barium titanate (up to 70 vol %) in a silicate glass phase form transparent ferroelectric glass ceramics. The small size of the crystals ( 500 A) accounts for transparency. 

Figure 1.14 The microstructure of a commercial cordierite glass ceramic (Coming Code 9606). The predominant phase with somewhat rounded grains is cordierite and the angular, needle-shaped crystals are magnesium aluminum titanate. (From Ref. 85.)...

Kokubo et al. (1969) studied the crystallization mechanism of the above glass-ceramic in detail. They found that a metastable benitoite-type crystal, BaTiSi Og, was formed at 800°C, according to a mechanism of surface crystallization. Beginning at 950 C, the crystal was progressively transformed into the stable barium titanate and hexacelsian. The hexacelsian crystal grew anisotropically and parallel to the surface of the glass-ceramic. The preferred orientation of hexacelsian was attributed to the metastable benitoite-type crystals. Moreover, volume crystallization took place at the same time as surface crystallization. In volume crystaUization, however, barium titanate and hexacelsian were formed as primary crystals. 

Kokubo T, Sakka S., Sako W., and Ikejiri S., "Preparation of Glass-Ceramic Containing Crystalline Apatite and Magnesium Titanate for Dental Crown,"/. Ceram. Soc. Jpn. Int. Ed., 97, 236-40 (1989). 

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