Sintering interval

FIG. 183, Temperature dependence of the liquid phase content for mixes with a narrow (a) and a broad (b) sintering interval. 

The composition of talc is situated close to the eutectic, so that the amount of liquid phase suddenly formed would deform the product. However, in a talc-day mixture, one may choose a proportion of components providing an amount of meJt just suitable for satisfactory sintering (about 30%) and for inhibition of the proto-enstatite-clinoenstatite inversion. This amount corresponds to approximately 10% clay. The composition of such a mix which roughly corresponds to commercial steatite compositions is indicated in Fig. 189 by point A. The diagram then allows equilibrium amounts of the melt to be evaluated for various temperatures, as shown in the diagram in Fig. 190. The shape of curve A indicates that about 30 % of melt is formed rapidly at 1345 °C. This is the temperature of the tridymite-enstatite-cordierite eutectic. The amount of melt increases with temperature, so that the sintering interval is comparatively narrow. 

With ware sintering without the liquid phase, the upper limit of the sintering interval is given by the temperature of excessive crystal growth. 

Ceramics with a wide sintering interval include, for instance, porcelain for which temperature variations within limits of about 20 ""C have little effect on the final properties. In contrast, for example, with cordierite ceramics a change in temperature of several degrees may damage the ware seriously as a result of excessive melt formation. 

Within the sintering interval, we can expect an exponential relationship between temperature and sintering rate, or the necessary time of firing. The relationship has a logarithmic form... 

Hard porcelain finds chemical and electrotechnical applications, as well as in tableware, etc. Soft porcelain is used in the manufacture of decorative ware and tableware. Old Asian porcelains also belong to the soft types. One special type of porcelain contains Li20 and can be fired below 1200 °C. The firing temperature is also reduced by divalent oxides — hence the presence of CaC03 in the formulae. Such measures also narrow the sintering interval, since the melts arising show a steeper viscosity curve and their content increases rapidly with increasing temperature. 

The presence of impurities in raw materials reduces to some extent the temperature of formation of eutectic melts, simultaneously broadening the sintering interval. Addition of feldspar is very effective in this direction. Feldspar is used in raw material mixes for those steatite products which do not have to meet particularly stringent requirements for electrical properties. 

It is manufactured from limestone (CaC03) and bauxite (mainly AI2O3). The melting point of mixtures of limestone and bauxite is in the range ca. 1500 to 1600°C and the sintering interval for rendering the batch liquid is limited to a very narrow temperature range. 

A clear solution can be obtained more quickly, and incidentally the transfer can be made more satisfactorily, by rapidly filtering the solution through a sintered glass funnel with exclusion of carbon dioxide with the aid of the apparatus shown in Fig. 10.9. It is advisable to calibrate the test-tube in approximately 5 mL intervals and to put the graduations on a thin slip of paper gummed to the outside of the tube. 

Precipitate about 0.1 g potassium (present as potassium chloride in 50 mL water) with 40 mL of the sodium TPB solution added slowly and with constant stirring. Allow to stand for 30 minutes, filter through a sintered-glass filtering crucible, wash with distilled water, and dry for 1 hour at 120 °C. Shake 20-25 mg of the dry precipitate with 200 mL distilled water in a stoppered bottle at 5-minute intervals during 1 hour. Filter through a Whatman No. 40 filter paper, and use the filtrate as the wash liquid. 

These salts decompose [39] to the carbonates in the temperature intervals Li, 811-826 K Na, 737-814 K and K, 754-798 K (from DTA measurements, 5 K min-1). The reaction of lithium oxalate [98] (742— 765 K) obeyed the contracting volume equation [eqn. (7), n = 3] with E = 223 13 kJ mole-1. A marked increase in surface area during the initial stages of decomposition was later followed by extensive sintering. 

Consequently, the pressed ZnO sample possesses intercrystalline barriers characterized by a wide spread with respect to the height which can be considered as a specific type of intercrystalline contacts. At the same time a rigorous compliance with the Ohm law over the whole interval of applied fields is observed for a ZnO film sintered under vacuum conditions. This result and the fact that the estimation of the average value of the voltage drop per contact is about 0.2 eV kT 0.025 eV... 

The phase composition from the surface to the interior of the samples was determined by X-ray diffractometry (XRD) through successive grinding of the surface at 100 pm intervals. The microstructural characterization of the sintered specimens was achieved by scanning electron microscopy (SEM) in backscattered mode. The hardness change from the surface to the interior of the sample was measured by the Vickers indentation method at 19.6 N load. 

Stirring is maintained as the mixture is allowed to cool to room temperature, and 600 mL of petroleum ether (Note 7) is added in 100-mL portions at 5-min intervals (Note 9). The stirrer is disconnected from its motor, and the solution is poured into a sintered-glass funnel containing 50 g of Florisil (approximately 7 cm in... 

The kinetics of diamond powder infiltration with cobalt of VK15 sintered carbide and Co-Mo and Co-Ti melts was studied experimentally at 8 GPa (Fig, 1). Confidence intervals for T and k values, the reliability being a= 0,95, do not exceed 8 %. According to [3], the limit of WC solubility in Co attains 10 mass % or 3.2 at. %. The additive contents of Co-Mo and Co-Ti alloys was 10 mass % (accordingly, the atomic portions were 0.12 Ti and 0,064 % Mo). Samples of alloys were sintered from mixtures of cobalt-molibdenum and cobalt-titanium hydride powders in a vacuum furnace at 1000 °C. 

The three sequences shown in Fig. 19 demonstrate that the decrease of the specific surface area is accompanied by a dramatic decrease in intensity of the spectra and by a drastic spectral simplification. In particular, all bands in the 2120-1100 cm interval, attributed to species formed at very reactive three-fold and four-fold coordinated oxygen sites located on edges and steps (O4J) and on corners (Of ) are strongly affected by sintering and are nearly totally absent on MgO smoke (Fig. 19c). A similar effect was discussed in Section III.C for the H2 adsorption and splitting on the same MgO samples (Fig. 11) and on the chemistry of CO adsorbed on Mg sites (spectral region not reported in Fig. 19) (12,26,177). 

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