Ceramic powders specific surface area

Physical adsorption is the basis for the various techniques to measure surface area of ceramic powders. The surface area is determined in terms of the amount of the gas adsorbed by a given mass of solid powder at a given temperature, under different gas pressures p. In practice, gases with a fixed volume are used for the powder, so that the amount of gas adsorbed can be identified according to the decrease in pressure of the gas. The amount of gas adsorbed versus p, or p/po, when the gas is at pressures below its saturation vapor pressure po, can be plotted as a graph, which is known as the adsorption isotherm. Figure 4.3 shows the types of these isotherms, according to Brunauer, Emmett and Teller (BET) classification [35-38]. The Type VI isotherm is called stepped isotherm, which is relatively rarely observed, but has special theoretical interest. This isotherm offers the possibility to determine the monolayer capacity of a solid, which is defined as the amount of gas that is required to cover the surface of the unit mass of the solid with a monolayer, so as to calculate the specific surface area of the solid. 

To characterize a ceramic powder, a representative sample must be taken. Methods of sampling and their errors therefore are discussed. Powder characteristics, including shape, size, size distribution, pore size distribution, density, and specific surface area, are discussed. Emphasis is placed on particle size distribution, using log-normal distributions, because of its importance in ceramic powder processing. A quantitative method for the comparison of two particle size distributions is presented, in addition to equations describing the blending of several powders to reach a particular size distribution. 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

The aspect for the extrusion is frequently concerned with plasticity of the ceramic dough. This is because ceramics are non plastic material when mixed with water therefore, it is necessary to add some additives to improve plasticity. High plasticity could enhance the workability of the mixture. However, the excess quantity of additive could obstruct the high sinter density of final products thus led to an attempt to reduce specific surface area of the ceramic powder by calcination . In this work, the porous ceramic is desirable because of its use as a substrate for palladium therefore large amount of additives were become benefit to the tubular support. The various amounts of additive and water used in the producing of tubular ceramic dough were investigated to succeed the extrusion. 

To conclude, the LuaOstEu (1 at. %) nanopowders were prepared by co-precipitation method using ammonium hydracarbonate as precipitant. It was shown that Lu203 Eu low-agglomerated monodispersed spherical powders with specific surface area of S=14 m /g can be obtained by precursor calcination at T=1000 °C. It was determined, that the resultant powders can be used for production of Lu203 Eu translucent ceramics with average crystalline size of 18-20 mkm, nearly full density (99 %), and in-line transmittance coefficient up to 20 % even if the uniaxial pressure method is used for nanopowder compaction. 

The major physical properties of ceramic powders constitute size distribution of primary particles and agglomerates, specific surface area, density, porosity, and morphology (e.g., shape, texture, and angularity). 

In contrast to conventional phosphor powders, ceramic powder synthesis aims at the generation of powders with highly sinteractive surfaces with particle sizes down to the submicron range and specific surface areas of up to 50 mVg. In addition, homogeneous doping on a molecular. scale is of substantial importance. 

Abstract High specific surface area metallic, ceramic and cermet powders may be active... 

P-a phase transformation of SiC powder was proportional to the dimension of the specific surface area of the powder.The present result suggested that the size of plate-like SiC grains grown in the porous SiC ceramics depended on the starting p-SiC particle size. However, the coarse P-SiC particles did not transform into a-phase easily and did not grow into the plate-Uke shape under the present sintering condition, and this result was consistent with their report... 

In crystallized glass type LTCCs, since one kind of ceramic powder is used, it is not necessary to consider its charging characteristics. However, in order to achieve a homogenous structure, it is desirable to have a sharp distribution of particle size, and it is necessary to decide the powder particle size and specific surface area taking into account formability and sinterability in the same way as with glass/ceramic composites. 

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