Dielectric perovskites materials

Another important group of oxide materials with a very low electrical conductivity is the oxide dielectrics. A number of these are based upon the perovskites, MXO3 or M0 X02. The archetype of these materials is BaTiC>3, which has a high dielectric constant, or relative permittivity to vacuum, the value at room temperature being 1600, and commercial use is made of the isostructural PbTi(>3 and ZrTi03 which form solid solutions, the PZT dielectrics. These materials lose their dielectric properties as the temperature... 

A recent report presented Continuous Composition Spread (CCS) approaches (37) to identify thin film dielectrics with varying composition among the system Zr-Sn-Ti-O (78). A structural-driven approach was applied to the discovery of a high dielectric perovskite polymorphic material (79). 

It has been reported that the dielectric constant of thin BST films decreases with decreasing film thickness when metal electrodes are adopted even without any interfacial non-perovskite material. This is due to the intrinsic interfacial low dielectric layer that originates from the termination of the chemical bonding of the perovskite structure at the interfaced. The large dielectric, polarizability of the perovskite can not penetrate into the metal layer due to the extremely high carrier concentration of the metal. 

Chapterb- The perovskite family of materials is of considerable technological importance for its excellent temperature stable microwave properties for dielectric resonator based filters, oscillators and antenna applications. In this chapter author review the preparation, characterization and the microwave dielectric properties of Ca[(Lii/3A2/3)i-xMx]03.5 [A=Nb, Ta and M=Ti, Zr, Sn] dielectric ceramics. This family of perovskite materials shows relative permittivity in the range 20 to 56 with a quality factor up to 45000 GHz and temperature coefficient of resonant frequency (xf) in the range from -21 to +83 ppm/°C. The Xf can be tailored by adjusting the titanium content. The sintering temperature can be lowered below 950 °C to suit LTCC by the addition of low melting glasses. 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

The compositions of most dielectric materials used for ceramic capacitors are based on ferroelectric barium titanate. As discussed in detail in Pragraph 1.3 the permittivity of ferroelectric perovskites shows marked changes with temperature, particularly close to the phase transition. From the device point of view a high dielectric permittivity with stable properties over a wide temperature range is required. There are various specifications which have to be fulfilled (e.g. X7R AC/C(T = 25°C) < 0.15 in a range between -55°C and 125°C). 

The first ingredient can be taken for granted, since rl behavior in these materials does not occur in the absence of disorder. The third ingredient is also an experimental fact in that RL behavior occurs in ABO3 oxides with very large dielectric permittivity. The second ingredient is manifested in many experimental observations common to all perovskite rls, as will be discussed later. 

Metal-metal eutectics have been studied for many years as a result of their excellent mechanical properties. Recently, oxide-oxide eutectics were identified as materials with potential use in photonic crystals. For example, rod-like micrometer-scaled microstructures of terbium-scandium-aluminum garnet terbium-scandium perovskite eutectics have been solidihed by the micro-pulling-down method (Pawlak et al., 2006). If the phases are etched away, a pseudohexagonally packed dielectric periodic array of pillars or periodic array of pseudohexagonally packed holes in the dielectric materials is left. 

Barium titanate (BaTiOj), a perovskite-type electro-ceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. The wide applications of barium titanates include multiplayer capacitors in electronic circuits, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensivity to gas adsorption. 

With the progress in microwave telecommunication technology, dielectric materials have come to play an important role in the miniaturization and compactness of microwave passive components. The dielectric materials available for micro-wave devices are required to have predictable properties with respect to a high dielectric constant (K), high quality factor (Qf), and small temperature coefficient of resonant frequency (TCP). Numerous microwave dielectric materials have been prepared and investigated for their microwave dielectric properties and for satisfying these requirements. In particular, complex perovskite compounds A(B,B )03... 

Several kinds of dielectric materials have been widely investigated to improve their properties and to meet the requisites of high dielectric constant (K), low dielectric loss (Qf), and low TCF. Based on these requisites, complex perovskite compound, A(B,B )03, was extensively studied from the viewpoint of the compositional and structural dependence on their microwave dielectric properties. Among them, much attention has been paid to lead-based ceramics with complex perovskite structures because of their superior dielectric properties required for microwave devices. 

Extensive studies have been carried out on the microwave dielectric materials with high K and thermal stability for the miniaturization of microwave passive component. In particular, CaTi03-based materials have attracted considerable interest due to their high K. These titanates can be easily combined with other perovskite compounds to form solid solutions. However, they have a large TCF for practical apphcations. Various attempts have been made to control the TCFs of CaTiOj-based materials. However, most of them are mainly empirical, such as the addition of the materials with negative TCF values. 

The ferroelectric Pb(Mgy3Nb2/3)03 (PMN) ceramic has been the snbject of extensive investigations due to its high dielectric coefficient and high electrostrictive coefficient, which renders it suitable for use in capacitors and electrostrictive actuators. However, the successful exploitation of this material is limited by the difficulty of producing a single-phase material with the perovskite structnre. Conventional solid state synthesis techniques invariably resnlt in the formation of one or more pyrochlore phases, which exhibit poor dielectric properties. 

BaTi03, which has a perovskite crystal lattice, is a ferroelectric material i.e. a material in which the change in polarization P with varying applied electric field E traces a dielectric hysteresis loop analogous to the hysteresis loop exhibited by ferromagnetic materials. 

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