Functional Ceramics

Unlike silicate ceramics, raw materials used for the preparation of non-silicate ceramics are generally synthetic powders and not mixtures of crashed rocks. But these synthetic powders can result from natural products, which the English terminology makes easy to understand by distinguishing between starting materials (for example, alumina powders) and raw materials (bauxite rocks, in this case, whose treatment by the Bayer process yields the alumina powders) [CAS 90], 

Ceramic compositions offer in general a simple chemistiy, but microstractural parameters are complex. 

Alumina AI2O3 is by far the foremost basic compound for technical ceramics , because alumina exhibits exceptional versatility abrasion, cut, friction and wear, refractory uses, electricity and electronics, optics, biomedical, jewelry and the list can go on and on [CAS 90]. 

Silica Si02 is also a basic compoimd both for ceramists and glassmakers the alumina-silica diagram has for ceramists the same importance as the iron-carbon diagram has for metallurgists. 

Magnesia (MgO) and spinel (MgAl204) are primarily used as refractory materials in the iron and steel industry. 

The electrical properties of the perovskites are strongly coupled to their crystal structure. Perovskites are mixed oxides with cations that do not fit well together in the cubic anion lattice, some of them being slightly too small for their lattice sites. As a result these mixed compounds display a unique combination of electrical, thermal, and electromechanical properties. Perovskites are the active components of many active materials and transducers. 

The relationship between the mechanical stresses and strains and electrical surface charges and polarization is given by the piezoelectric coefficients d and e. These parameters are defined in terms of the stress r, the strain t, the elasticity modulus (or = Ex), the electric field strength O (in Vm ) and polarization P (dipole moment per unit volume in C/m )  

The piezoelectric generator coefficient d indicates the mechanical strains that are induced by electric potentials and the piezoelectric motorcoefficient e is a measure of the dipole moments that are created by strains. These two parameters are similar in nature and proportional to one another. The electromechanical coupling coefficient k is proportional to both d and e, its square giving the efficiency of conversion between mechanical and electrical energy that can be achieved with the ferroelectric 

Doping PZT with lanthanum ions (La replaces Pb ) shifts the morphotropic boundary to higher x-values and lowers the Curie temperature of the oxidic alloy. At 5% La, the Curie temperature can even drop below room temperature if X is high enough. The solid has become cubic and paraelectric and no longer has a permanent electric dipole moment at room temperature but is very polarizable. This material is called PLZT (short for lead lanthanum zirconate titanate) and is used for its nonlinear optical properties, e.g., in doubling the frequency of laser radiation. 

Ferroelectricity in perovskites provides a good example of how the crystal structure of a solid affects its electrical properties and uses.  

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The removal of pores and internal cracks is also of value where functional ceramics are concerned. Dielectrics such as are used in capacitors in enormous quantities, alumina in particular, have long been made with special attention to removing any pores because these considerably lower the breakdown field and therefore the potential difference that the capacitors can withstand. 

Improved characterization of the morphological/microstructural properties of porous solids, and the associated transport properties of fluids imbibed into these materials, is crucial to the development of new porous materials, such as ceramics. Of particular interest is the fabrication of so-called functionalized ceramics, which contain a pore structure tailored to a specific biomedical or industrial application (e.g., molecular filters, catalysts, gas storage cells, drug delivery devices, tissue scaffolds) [1-3]. Functionalization of ceramics can involve the use of graded or layered pore microstructure, morphology or chemical composition. 

Fabrication processing of these materials is highly complex, particularly for materials created to have interfaces in morphology or a microstructure [4—5], for example in co-fired multi-layer ceramics. In addition, there is both a scientific and a practical interest in studying the influence of a particular pore microstructure on the motional behavior of fluids imbibed into these materials [6-9]. This is due to the fact that the actual use of functionalized ceramics in industrial and biomedical applications often involves the movement of one or more fluids through the material. Research in this area is therefore bi-directional one must characterize both how the spatial microstructure (e.g., pore size, surface chemistry, surface area, connectivity) of the material evolves during processing, and how this microstructure affects the motional properties (e.g., molecular diffusion, adsorption coefficients, thermodynamic constants) of fluids contained within it. 

The inherent sensitivity of NMR signals to the fluid-substrate interactions via a large number of mechanisms provides a direct connection between the NMR measurables, the pore structure and the motional characteristics of the imbibed fluid. While the large number of potential NMR variables makes the experimental design and analysis complex, it also provides the potential for a measurement method capable of measuring and spatially resolving the parameters of interest to functionalized ceramics. 

Full evaluation of functionalized ceramics requires the ability to characterize the spatial variations in structure and morphology. Using NMRI, it is possible to map the underlying structure on a spatial scale of hundreds of microns. 

Institute of Structural and Functional Ceramics, University of Leoben, A8700 Leoben, Austria. Department of Chemistry, University of New Mexico, Albuquerque, NM 87131. 

What new developments the future holds nobody knows for certain. In electronics people will try to miniaturise electric equipment even further. Non-functional, ceramic packaging will be converted into functional components. For this new ceramic materials are necessary, as are ways to process them. High-temperature superconductors will lead to magnetic levitation craft, cheap electricity and improved MRI (magnetic resonance imaging). 

Ferroelectric ceramics and single crystals have found wide applications in many electronic, acoustoptic and piezoelectric devices [1,2], Perovskites represent one of the most important classes of inorganic powders that are of great interest in functional ceramics used for electronic components among them BaTiOj is a typical and most frequently used representative. 

Functional ceramic thin films have been used in a variety of electronic devices. In particular, Si02 thin films are widely used for device passivation and protection of magnetic and optical disks. The synthesis of Si02 thin films can be conveifiently... 

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. 

Forsgren, J. (2010) Functional ceramics in biomedical applications. On the use of ceramics for controlled drug release and targeted cell stimulation. PhD dissertation. Uppsala University, Sweden, Acta University Upsaleiensis, Uppsala, ISBN 978-91-554-7930-5. 

From Metal-Organic Precursors to Functional Ceramics and Related Nanoscale Materials... 

Functional Ceramics Chemical Composition, Function and Form... 

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