Ceramic capacitors perovskite

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 perovskite structure is, of course, of special significance in the electroceramics context since the ferroelectric perovskites are dominant in the ceramic capacitor, PTC thermistor and electromechanical transducer industries. The structure favours the existence of soft modes (low frequency phonons) as evidenced by its tendency to instability, for example the ferroelectric-paraelectric transition. Instability is evident in the case of the T23 compound which exhibits a tetragonal-orthorhombic transition in the region of 700 °C (the exact temperature depends on the oxygen content). Extensive twinning, very reminiscent of ferroelectric domain structures, is observed. 

Piezoceramics are currently no longer manufactured from BaTi03, but from lead titanate zirconate, Pb(Ti,Zr)03, which aLso crystallizes in a perovskite lattice. Ceramic capacitors to a value of 4.1 10 DM were produced in 1995, ca. 50% of the turnover for functional ceramics, in which the three region USA, Japan and Europe have equal shares. 

Barium titanate, strontium titanate and other perovskite ceramics Capacitors, sensors, embedded electronics, security inks... 

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. 

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. 

The importance of perovskites became apparent with the discovery of the valuable dielectric and ferroelectric properties of barium titanate, BaTiOj, in the 1940s. This material was rapidly employed in electronics in the form of capacitors and transducers. In the decades that followed, attempts to improve the material properties of BaTiOj lead to intensive research on the structure - property relations of a large number of nominally ionic ceramic perovskite-related phases with overall compositions ABOj, with a result that vast numbers of new phases were synthesised. 

Relatively few applications have utilized the ferroelectric effect in ceramics. Ferroelectric ceramics have been widely employed because of the other properties that they display, however. Their dielectric, piezoelectric, and pyroelectric properties have led to their use in capacitor, actuator and other piezoelectric applications, and infrared detection devices. Again, the most widely used materials are the lead-based ABO perovskite compounds. 

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