Ceramic materials, preparation techniques

Mass transport processes in ceramics are of interest due to their importance in materials preparation techniques. The phenomenon of sintering by diffusion is reasonably well understood for metallic systems.For 2-component ionic systems two additional features must be considered. (i) In order to produce an overall transfer of material a net flux of each component will occur, the components having in general unequal mobilities. These fluxes are interdependent since, locally, certain concentration ratios must be maintained. (ii) The concentration of defects will vary with distance from the free surface. As an example of the incorporation of these effects we may consider the changes in morphology of a nearly planar surface to which Mullins theory of mass transport may be applied. For the two component case, eg. NaCl, the equation describing the surface evolution by volume diffusion processes may be written as( )... 

In the early days of TEM, sample preparation was divided into two categories, one for thin films and one for bulk materials. Thin-films, particularly metal layers, were often deposited on substrates and later removed by some sort of technique involving dissolution of the substrate. Bulk materials were cut and polished into thin slabs, which were then either electropolished (metals) or ion-milled (ceramics). The latter technique uses a focused ion beam (typically Ar+) of high-energy, which sputters the surface of the thinned slab. These techniques produce so-called plan-view thin foils. 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

Inductively coupled plasma-mass spectrometry (ICP-MS) has been utilized as a bulk technique for the analysis of obsidian, chert and ceramic compositional analyses 12-14). However, due to the high level of spatial variation of ceramic materials, increased sample preparation is necessary with volatile acids coupled with microwave digestion (MD-ICP-MS) to properly represent the variability of ceramic assemblages IS, 16). Due to the increased sample preparation and exposure to volatile chemicals, researchers have continued to utilize neutron activation analysis (INAA) as the preferred method of chemical characterization of archaeological ceramics (77). 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

One of the most important of these new experimental tools has been the development and application of the vacuum microbalance technique in which the sensitive microbalance operates directly in the vacuum or reaction system. The success of the method depends upon the coordination of a number of different experimental as well as theoretical disciplines. Thus, from an experimental point of view precise weighing techniques on properly prepared specimens must be coordinated with high vacuum techniques and the use of ceramic materials at high temperatures. From a theoretical viewpoint thermodynamic calculations must be made for all of the reactions involved and the results interpreted in terms of diffusion process for gas-solid reactions in which a film is formed or the gas diffuses into the solid, or in terms of the absolute reaction rate theory or its equivalent for gas reactions on solids including catalytic reactions and the combustion of fuels. 

Thus this book presents current developments and concepts in the chemical techniques for production and characterization of state-of-the-art ceramic materials in a truly interdisciplinary fashion. The 27 chapters are divided into five parts reflecting topical groups. The first part discusses the starting materials—how to prepare and modify them in the nanoscale range. Powders are the most heavily used form of starting ceramic materials. The synthesis, characterization, and behavior of ceramic powders are presented in parts I and II. In the third part, processing of ceramic films via the sol-gel technique is discussed. Fabrication of... 

Solution preparation of ceramic powders is often thought to be a speciality technique for small lots of high purity ceramic materials. High quality powders can be (obtained, and solution technique are chosen to produce many of the powders used in the ceramic industry in large quantities. A major example is AI2O3, most of which is produced when the mineral bauxite is dissolved in an alkaline solution and hydrated oxides are precipitated from the solution ... 

Thermal decomposition also plays an important role in the conventional ceramic powder process, which makes use of starting materials such as metal carbonates that decompose during calcining, exposing reactive surfaces, which in turn aid in phase formation and chemical homogenization. Thermal decomposition is an important final step in solution preparation techniques. Precipitated hydroxides, carbonates, or oxalates are decomposed to oxides. Freeze-drying yields dried salts such as sulfates, which must be thermally decomposed to oxides. Similarly, many solution techniques use nitrates, which are decomposed as part of the process or in a separate step. 

The following sections discuss Pb(Zr, TijOa ferroelectric ceramics. However, the preparation techniques are applicable to other materials. Ferroelectric single crystals are not included. 

There are four principal advantages to using sol-gel processing techniques over traditional methods of preparing ceramic materials ... 

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