Sintered products

Dehydroxylation of goethite produces the ferrite reds - extremely colour fast and pure hematite. With low temperature calcination the acicular shape of the goethite precursor is retained, whereas high temperatures lead to a sintered product. 

The sintered product is cut into the required shape and the active material is infused using one of a number of techniques - e.g. for the negative electrode by impregnating the sinter with concentrated aqueous cadmium nitrate, followed by thermal decomposition, or by cathodic polarization in molten Cd(NC>3)2 baths, etc. The plates are washed and the impregnation cycle is repeated up to 5-10 times until the required loading is attained. Finally the plates are formed by a sequence of carefully controlled charge/discharge cycles. Safety precautions are very important in the manufacture of cadmium-based electrodes because of the health hazards associated with this material. 

A special type of such compacts is a mixture where diamond is the second phase. The products contain >90 vol.% c-BN and 2-10 vol.% diamond (particle size 0.5-2 pm). After mixing the powders, HP-HT sintering without any binder phases follows (<100 kbar and <1800 °C). The sintered products show high density [257]. 

Several operations for the production of iron and steel, including sinter production, coke production, and electric arc furnaces, have been identified as potential emission sources of PCDD/Fs. China is the largest producer of steel in the world. According to 2005 statistical data, the iron and steel production of China was 349 million tons. It is reasonable to assume that the iron and steel industry could be a major source of PCDD/ F emission to air in China, but data are not available for an assessment of the emissions from this source. 

The sintered product usually has a density higher than 95% of theoretical and a crystallite size in the 5-30 /rm range. Both an excess and a deficiency of PbO result in inferior piezoelectric properties so that careful control of all aspects of the manufacturing process is essential. 

Sinter returns. This is usually the minus in. (0.6 cm) or minus f in. (1 cm) material separated from the sintered product and recycled to the feed end of the machine. An optimum value of sinter returns rate exists to maximize bed permeability. In current practice 20% to 30% of the machine discharge is recycled. 

Impermeable silicon carbides of both types, sintered and reaction bonded, perform generally better than the permeable refractories as shown in Table 19-3 Both reaction bonded and sintered products can be exposed to higher temperature for longer periods of time with lower weight loss than the oxide, Si3N4 or Si20N2 bonded refractories. This is due to the lower surface area available for reaction and to the greater relative inertness of their bond phases. 

For impermeable materials such as the reaction bonded and sintered products, true corrosion rates can usually be calculated, as the reactions generally occur only on the surface of the parts. 

The minimum porosity is attained at the ratio where the voids between the coarse particles are completely occupied by the finer partices. As follow s from Fig. 153, this is the case at about 70% of the coarse fraction. A sliarp minimum would be attained at a very high ratio of particle sizes. In fact, the behaviour of volume moves along the indicated curve, since the ratio of particle sizes attainable docs not usually exceed 1 10 in practice. With binary mixes, it is possible practically to attain a porosity of 25%, and for ternary mixes, 22%. This corresponds to a volume shrinkage of 22 — 40%, i.e. linear shrinkage 7—13 i - for sintered products. Theoretically, it should... 

The initial mix for the manufacture of rutile ceramics usually contains about 90 % T1O2 in the form of anatase or rutile. Anatase is converted above 900 °C to rutile, which is then stable over the entire temperature range. Only rutile is therefore present in a sintered product. The polymorphic inversion involves a considerable volume contraction, since the density of anatase is 3.9 while that of rutile is 4.25 g/cm. This is why cracks occur on firing in anatase-containing bodies. This drawback can be eliminated by pre-calcining at least some of the Ti02 at temperatures exceeding 1000 C, to convert it to rutile. 

Since, under economically acceptable conditions, carbon neither melts nor is sintered, production has to be carried out in such a way that a molded article is formed suitable for its intended application, which only needs further mechanical working. The raw materials, solids and binders are thus mixed, molded into the required form, fired to the carbon article and, if required, finally graphitized. 

The most extensively used method is the carbothermic reaction of UO2 with carbon under vacuum at 1600-2000°C. The pelletized mixture of UO2 and carbon is heated in vacuum in a furnace that allows semicontinuous production (Fig. 1) The optimum reduction temperature is 1600°C for material that is to be pressed and sintered to form pellets. The residual oxygen contents are in the range 0.35-0.63%. Higher temperatures result in a less sinterable product, while lower temperatures give unacceptably high residual oxygen contents. 

Nevertheless, today s advanced granulation and pressing technology have made it possible to achieve very rigid dimensional tolerances of the sintered product. It is easy to understand that a prerequisite is to keep the properties of the raw materials, as well as the conditions during grade powder preparation and granulation, strictly reproducible. 

Solid residence time 2,500 to 20,000 s gas residence time <1 s solid particle diameter 7 pm to 20 mm. Sinter product product diameter 80 to 150 mm capacity 15 to 300 kg/s product crush strength >10 kPa. 

Initial reduction is carried out at lower temperatures to prevent liquefaction of uranium (IV) chloride. After sintering can no longer occur, a higher temperature increases the reaction rate. A sintered product may contain unreacted and occluded uranium(IV) chloride. 

In this case, the "fine" particles and the "coarse" particles were separated so that the difference in size between individual particles was minimized. That is, most of the individual particles in each fraction were almost the same size. Both the fine and coarse particles have a sintering slope of 1/2 but it is the coarse particles which sinter to form a solid having a density closest to theoretical density. This is an excellent example of the effect of pore volume, or void formation, and its effect upon the final density of a solid formed by powder compaction and sintering techniques. Quite obviously, the fine particles give rise to many more voids than the coarser particles so that the attained density of the final sintered solid is much less than for the solid prepared using coarser particles. It is also clear that if one wishes to obtain a sintered product with a density close to the theoretical density, one needs to start with a particle size distribution having particles of varied diameters so that void volume is minimized. 

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