Ceramics oxides

Ceramic oxides represent the most extensive group of ceramic materials produced today. Traditionally, but rather artificially, the oxide ceramics are divided into traditional and advanced groups. The traditional ceramics include mostly silica-based products prepared from natural raw materials (clays), including building parts (bricks, tiles), pottery, sanitaryware, and porcelain, but also ceramics with other main components (e.g., alumina, magnesia), which are applied in the field of electroceramics (insulators), or industrial refractories. 

In the following sections, an attempt is made to address the questions of recent developments in the field of ceramic oxides. As this topic cannot be covered fully within the space available, oxides have been selected which are considered to be the 

Ceramics Science and Technology Volume 2 Properties. Edited by Ralf Riedel and I-Wei Chen Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31156-9 

From the point of view of the volume of production, polycrystalline alumina is the material most frequently used as ceramics for structural applications. However, in comparison with for example, silicon nitride, where the influence of various additives on microstructure and properties has been well characterized and understood, and despite several decades of lasting research effort, alumina remains a material with many unknown factors yet to be revealed. Alumina-based materials can be divided roughly into three groups  

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Metal to ceramic (oxide) adhesion is very important to the microelectronics industry. An electron transfer model by Burlitch and co-workers [75] shows the importance of electron donating capability in enhancing adhesion. Their calculations are able to explain the enhancement in adhesion when a NiPt layer is added to a Pt-NiO interface. 

Most talc sold to paper, ceramics, and other industrial customers is manufactured to specifications agreed to between the producer and consumer. In paper, properties such as color, abrasion, surface area, and tint ate most important, whereas in ceramics, oxide chemistry, fired color, pressing characteristics, and alkaH metal content ate mote important. There ate some military specifications for talc used in corrosive coatings (6) and for cosmetic talc products used for cleaning of personnel in chemical warfare zones (7). 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Fig. 6. Catalyst inhibition mechanisms where ( ) are active catalyst sites the catalyst carrier and the catalytic support (a) masking of catalyst (b) poisoning of catalyst (c) thermal aging of catalyst and (d) attrition of ceramic oxide metal substrate monolith system, which causes the loss of active catalytic material resulting in less catalyst in the reactor unit and eventual loss in performance.

The ceramic oxide carrier is bonded to the monolith by both chemical and physical means. The bonding differs for a ceramic monolith and a metallic monolith. Attrition is a physical loss of the carrier from the monolith from the surface shear effects caused by the exhaust gas, a sudden start-up or shutdown causing a thermal shock as a result of different coefficients of thermal expansion at the boundary between the carrier and the monolith, physical vibration of the cataly2ed honeycomb, or abrasion from particulates in the exhaust air (21) (see Fig. 6d). 

Electronics High-grade ceramics Oxides for electric components Ferrites for permanent magnets Audio Video coatings... 

Solid Oxide Fuel Cell In SOF(7s the electrolyte is a ceramic oxide ion conductor, such as vttriurn-doped zirconium oxide. The conduetKity of this material is 0.1 S/ern at 1273 K (1832°F) it decreases to 0.01 S/ern at 1073 K (1472°F), and by another order of magnitude at 773 K (932°F). Because the resistive losses need to be kept below about 50 rn, the operating temperature of the... 

It is to be expected that tire conduction data for ceramic oxides would follow the same trends as those found in semiconductors, i.e. the more ionic the metal-oxygen bond, the more the oxides behave like insulators or solid elee-trolytes having a large band gap between the valence electrons and holes, and... 

Table 7.2 Calculated surface energies of ceramic oxides...

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

I.J. Hastings, (ed.). Fission Product Behavior in Ceramic Oxide Fuel. Adv. in Ceramics 17, Amer. Ceram. Soc. (1986). 

Above 1000°C Refractory metals Mo, W, Ta Alloys of Nb, Mo, W, Ta Ceramics Oxides AI2O3, MgO etc. Nitrides, Carbides SiaN., SiC Special furnaces Experimental turbines... 

Early work on superconductors concentrated on metals or metal mixtures (alloys). Niobium alloys are particularly good superconductors, and in 1973 a niobium alloy, Nb3Ge, was found to have Tc — 23 K, the highest known value for a metal superconductor. In 1986, a ceramic oxide with formula La2- Ba CuOq was found to show superconductivity at 30 K. Through intense research efforts on ceramic oxides, YBa2 C U3 Oj-, with Tc — 93 K, was discovered in 1987. 

Ceramic oxide superconductors have distinct atomic layers. The Cu-containing superconductors contain planes of copper and oxygen atoms, as the molecular view shows. These planes alternate with layers containing oxygen and the other metals that make up the superconductor. Superconductivity takes place in the Cu—O planes. 

The record for the highest superconducting temperature in the year 2004 is 138 K, held by a nonstoichiometric ceramic oxide, HgQ g Tig 2 Ba2 Ca2 C1I3 Og 33. This is still far below room temperature, but research continues. 

C. N. R. Rao, B. Raveau, Transition Metal Oxides Structures, Properies, and Synthesis of Ceramic Oxides. Wiley, 1998. 

An idea to use polybasic hydroxy carboxylic acids in syntheses of oxides goes back to Pechini [3], Evaporating solutions of metal salts in citric acid at presence of ethylene glycol he obtained a polymeric resin as a precursor of target oxides. Then this process was extensively used to manufacture various ceramic oxide powders in several publications [4-8],... 

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