Ferroelectric glass-ceramics
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electric glass-ceramics—An overview The formation of glass-ceramics has advantages when compared to ceramics and single crystals, through the possibility of controlling physical properties, namely, electrical and optical properties, by controlling the volume fraction of the active phase dispersed in the glass matrix (please see the Introductory article in this issue). For example, to maintain optical transparency, the process of nucleation and crystal growth requires great control when the size of crystals dispersed in the glass matrix are not large enough to cause light scattering. However, for most electrical applications, it is necessary that the crystals have a size sufficient to present, for example, a ferroelectric response. These two attributes represent a difficult tradeoff. One solution to ensure optical transparency exploits the small difference between the refractive indices of crystals and glass matrix. If this difference is negligible, it allows, regardless of the crystal size, maintenance of the optical transparency of the glass-ceramic.
Ferroelectricity—General principles Interest in ferroelectric materials results from its broad spectrum of technological applications. Ferroelectricity was discovered in the 17th century by E. Seignette, in La Rochelle, France, during the study of NaKC4H4O6.4H2O, known as Rochelle salt. In 1824, D. Brewster found that this material responded electrically to thermal stimuli,
a phenomenon known as pyroelectricity. Later, in 1880, Pierre and Paul Curie characterized the response of this salt, and other crystals such as quartz, sphalerite (ZnS), and tourmaline ((Ca, K, Na, [])(Al, Fe, Li, Mg, Mn)3(Al, Cr, Fe, V)6(BO3)3 (Si, Al, B)6O18(OH, F)4), to the effect of pressure (mechanical stimulation) to generate electrical polarization, resulting in the discovery of piezoelectricity.1 In 1918, W. Anderson and J. Cady2,3 observed an anomaly in the piezoelectric characteristics of Rochelle salt, indicating that the linearity between polarization and external field becomes invalid for high electric fields. In 1921, J. Valasek noticed that the spontaneous polarization presented by Rochelle salt could be reversed by the action of an external electric field.4 It was described, for the first time, as the phenomenon of ferroelectricity, a term adopted from ferromagnetism in reference to the hysteresis behavior they share. With the discovery of ferroelectricity in barium titanate (BaTiO3) single crystals in 1940, scientific research grew exponentially in this field due to promising applications in the electronics industry, particularly in the production of computer components. This discovery led to a better understanding of the phenomenon because of the simplicity of the barium titanate structure, a perovskite. In the 1950s, a new class of ferroelectric materials was discovered: Ba(Ti, Sn)O3 compounds, called relaxors. This anomalous behavior was measured.5
Manuel Pedro Fernandes Graça, Institute of Nanostructures, Nanofabrication, and Nanomodeling, Universidade de Aveiro, Portugal; [email protected]
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