Molten-phase sintering

Sintered deposits form at the furnace exit at lower gas temperatures and in zones subject to rapid changes in direction. The deposit is composed of spheroidal particles, <40p, bound together by a molten substance. In those cases where substantial quantities of coarse pyrites are liberated from the pulverized coal, the spheroids are nearly pure FeaOa, as shown in Figure 11. The matrix contained silica, alumina, iron, and potassium, and has an initial deformation temperature of 1832°C, as determined by differential thermal analysis. The heavier pure iron spheroids deposit as a result of inertial impact. The mineral source of the molten phase is most likely illite. 

The formation of liquid phase during the liquid-phase sintering is schematically shown in Fig. 5.28 [73]. It is started from a mixture of two powders, i.e., the major component and an additive. The additive is molten to liquid phase or reacts with the... 

As with silicon nitride, solid forms of silicon carbide are made by one of three processes sintered, hot-pressed, or reaction-sintered. Sintering requires use of an additive such as alumina, carbon, or boron to promote liquid-phase sintering. Hot pressing is done at 2000°C and results in a very hard, dense material. Reaction sintering occurs when a mixture of SiC powder and carbon is heated in contact with molten silicon. The reaction results in a nearly complete conversion to SiC, although according to Schwartz, most materials made by this process contain an excess of carhon or silicon. ... 

Resistant to acids, including HF and molten metals not resistant to alkalis, molten sodium sulfate SiC produced by sintering is prone to oxidation and corrosion while single phase SiC is resistant oxidation results in formation of SiC>2 SiC is more sensitive to hot corrosion than Si3N4... 

As mentioned in Section 3.7.1.2, there is a considerable scatter of solubility product values obtained in the molten KCl-NaCl eutectic using different methods of solubility determination. This disagreement in the solubility parameters may be explained by differences in the sizes of oxide particles whose solubility is to be determined. The difference in size causes the scatter of the solubility data according to the Ostwald-Freundlich equation and the employment of the isothermal saturation method, which implies the use of commercial powders (often pressed and sintered), leads to values which are considerably lower than those obtained by the potentiometric titration technique where the metal-oxides are formed in situ. Owing to this fact, the regularities connected with the effect of physico-chemical parameters of the oxides or the oxide cations should be derived only from solubility data obtained under the same or similar experimental conditions. However, this does not concern the dissociation constants of the oxides, since homogeneous acid-base equilibria are not sensitive to the properties of the solid phase of... 

For ill-designed composite membranes, for example, formed by depositing palladium onto substrates which it does not wet, surface tension will force the thin film to contract and ball up if the palladium atoms acquire sufficient surface mobility. Pinholes may form as a prelude to complete de-wetting, or pinholes may remain from the initial fabrication if the palladium did not fully wet its substrate. Kinetics of de-wetting is accelerated at elevated temperature and in the presence of adsorbates such as CO, which increase surface mobility of Pd. If molten metals do not wet ceramics, they will be expelled from ceramic pores. During sintering of cermets, Pd and other metals will not adhere to the ceramic phase, if the metal and ceramic do not wet. 

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