Impurities in ceramics

Table 3 Methods for Bulk Chemical and Impurity Analysis of Metallic and Nonmetallic Impurities in Ceramic Powders...

The concentration of impurities in ceramics is usually much greater than that of intrinsic defects. 

Lastly, high temperature heat treatments (sintering, plasma spraying) tend to reduce the strength of volatile impurities in ceramics, particularly mercury. It can also help the segregation of certain impurities at grain boundaries or on the surface and increase their bioavailability. 

Fig. 4.4. Stages in zone refining o bar of impure silicon (a) We start with a bar that has a uniform concentration of impurity, Q. (b) The left-hand end of the bar is melted by o small electric tube furnace, making a liquid zone. The bar is encapsulated in a ceramic tube to stop the liquid running away. ( ) The furnace is moved off to the right, pulling the zone with it. (d) As the zone moves, it takes in more impurity from the melted solid on the right than it leaves behind in the freshly frozen solid on the left. The surplus pushes up the concentration of impurity in the zone, which in turn pushes up the concentration of impurity in the next layer of solid frozen from it. (e) Eventually we reach steady state, (f) When the zone gets to the end of the bar the concentrations in both solid and liquid increase rapidly, (g) How we set up eqn. (4.1).

An EDX spectrum typical of thin-film analysis in TEM/(S)TEM is shown in Eig. 4.26. It was obtained from a polycrystalline TiC/Zr02 ceramic by use of an Si(Li) detector at 100 keV primary electron energy. Eor spectrum recording the electron probe of approximately 1 nm in diameter was focused on the triple junction between the grains in the STEM mode (Eig. 4.26a). Besides the elements expected for the material under investigation, viz. Ti and Zr, Si, Ee, and Co were also detected, hinting at the presence of a (Ee, Co) silicide as an impurity. Eor ceramic materials it is known that... 

Chemical analysis of niobium oxide indicated that the purity of the final product depends strongly on the purity of the initial solution. Account should be taken of about 0.02-0.03% wt. cationic impurities, introduced due to interactions with metal parts of the equipment. The main added impurities are Fe, Ni, Cr, which originate mostly from the stainless steel filter. The purity of the final product can be significantly increased by using a filter made of niobium or other appropriate material. Nevertheless, the material obtained using a stainless steel filter is sufficient for use in ceramic applications or as an initial material for carbide manufacture. 

FIGURE 55 Earthenware. Large earthenware vessels from the sixth-fourth centuries B.C.E., recovered from under the Mediterranean Sea, at Caesarea Maritima, Israel. Earthenware one of the simplest types of ceramic material, is highly porous and permeable. It is made from clay and a variety of additives fired at about 950°C. Iron oxides impurities in the clay usually make earthenware buff, red, or brown. Most earthenware, like the vessels shown, was not glazed but, if required, sometimes was waterproofed or decorated with a layer of glaze. 

Particulate composites, 26 754-755 ceramics processing, 5 653-654 extensional modulus of, 26 777 fabrication of, 26 766 Particulate emission limits, 13 183 Particulate emissions, reducing, 11 689 Particulate fillers, ll 302t, 303 for rubber, 21 772, 773 Particulate fluidized-bed regime, 11 801 Particulate impurities in gases, 13 464 in high purity gases, 13 466 Particulate matter (PM), 1 798-801 ... 

The source unit must vaporize and excite a portion of the sample, which is generally used as one of the electrodes between which the electric discharge takes place. No single excitation source is ideally suited for all applications of emission spectrochemistry. Trace impurities in metals, alloying constituents in high concentrations, biological substances, ceramics, slags, oils, nonconductors, refractories—all may require different excitation techniques and sample preparation procedures. Table 1 summarizes the important characteristics of the commonly used spectrochemical source units. 

It was noticed in some instances that alkali-metal impurities in alumina, or in the ceramic honeycombs, adversely influence the catalyst activity. For example, spodumene (lithium aluminum silicate) was found unsuitable for use as catalyst support, although it had good thermal properties. 

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