The influence of powder properties and sintering treatments on the development of microstructure in sintered UO2 pellets was studied. The nature and type of porosity and grain morphology are influenced by the calcination temperature and sintering treatment and these influences persist even at a high sintering temperature.

The investigation is aimed at a study of the relations between powder properties, sintering treatments and the densification and microstructure development in urania pellets. Urania powders for this study were prepared from ammonium diuranate by decomposition and reduction at various temperatures, namely, 400°, 600°, 900°, 1200° and 1500°C. Pellets prepared from these powders were sintered at temperatures in the range of 1200° to 1700°C with different soaking periods.The influence of (i) sintering temperatures, (ii) soaking periods at different temperatures, (iii) intermediate soaks at varying time periods at various temperatures, (iv) reheating of prepared sintered pellets and (v) rates of heating during sintering, on the density and microstructure development of pellets was studied in this investigation.The study revealed that urania pellets of specified density and grain development can be prepared by first attempting to achieve densification and then continuing the thermal treatments to attain the desired grain size and microstructure.

High density (95% theoretical) single-phase uranium monocarbide has been fabricated by warm pressing a mixture of suitably sized dicarbide and uranium metal powders at 800°C at 675 kg/cm2 for 5 minutes and by subsequent sintering at 600°-1000°C. The high density and the single phase structure could be retained upto 1800°C, though some grain growth occurs during this treatment (from 15 to 50 microns). The relatively low temperature reaction sintering technique enables production of high density pellets with controlled microstructure, free from other phases.

The influence of amount of calcia addition and calcination temperature on the sintering characteristics of zirconia was studied. The extent of stabilization was studied by X-ray diffraction. It was noticed that the optimum amount of calcia addition to obtain high density without cracking was 0.50 to 0.75 wt%. Higher amount of calcia addition and higher calcination temperature resulted in lowered density.Stabilization of zirconia with calcia addition, i.e. the formation of cubic phase, is found to be a function of amount of calcia added. The cubic solid solution formation took place mainly between 1000° and 1200°c. With the addition of 5.0 wt% of CaO, about 85% of cubic phase is formed at 1200°C and 100% at 1700°C.

Studies on the influence of various fabrication parameters viz., calcination temperature, grinding period, sintering temperature and soaking time, on the initial a.c. permeability and Q-factor of Nickel-Zinc-Ferrite (Ni0.3Zn0.7 Fe2O4 samples, were made with a view to develop ferrite cores, for use at high frequencies. A higher calcination temperature, a lower sintering temperature and a shorter soaking period have been found desirable to attain reasonably high density coupled with a low initial permeability and high Q. Repeated calcinations interspaced with wet grinding resulted in further improvement of the properties.

UO2 powders prepared by calcination of ADU at 500°C are very active and have a low optimum sintering temperature. However, these powders are fluffy and have poor flow and packing characteristics. With a view to optimising the packing properties while still retaining the sinterability, the powders were heated for a very short duration (1 sec to 5 min) in the temperature range of 900°-1400°C, so that essentially only the surface characteristics were changed without any significant effect on the crystallinity of the powders. Sintering studies were carried out with these flash heated powders in the temperature range of 1200°-1600°C in hydrogen atmosphere, and compared with those of the untreated powders. It was found that flash heated powders retain their sinterability and have better flow and packing characteristics.

The phase transformations in zirconia take place between 900°-1000° and 1200°c while heating, and between 1000° and 800°–600°C while cooling causing material movement. Hence, effect of amount of CaO and CaF2 additions and calcination temperature on densification and stabilization was studied by thermal cycling between 1200° and 700°C (lower range) and 1400° and 700°c (higher range) up to 25 thermal cycles.It was observed that higher densification can be obtained by thermal cycling at lower temperature compared to the temperature required in normal sintering process, particularly so, with CaO additions. High density was attained with CaO additions up to 2 % compared to equivalent amount of CaF2 additions in the lower temperature range. In general, uncalcined compositions attained high density which decreased with increasing calcination temperature. Thermal cycling promoted stabilization. The reaction was almost completed within 5 thermal cycles.

Influence of 1% addition of minor constituents such as MgO, TiO2, Fe2O3 on the sintering behaviour of beryllium oxide, both in air and in vacuum, has been studied at different sintering temperatures from 1100°-1500°C and for soaking periods of 2, 4, and 16 hrs. It was noticed that additions promote densification, more so, in air sintering and at lower temperatures (1100°-1300°C). At higher temperatures (over 1300° C) the influence was not significantly noticeable as the density of the pellets of pure BeO was fairly high—especially so in vacuum sintering. A typical value of the density attained by BeO with iron oxide addition is 2.84 gm/cc at temperatures above 1200°C in air or in vacuum. The samples with magnesia additions tend to be translucent and those with titania and iron oxide additions coloured, the colour depending on the atmosphere. An attempt has been made to explain the role of the additions in promoting the sinter ability.

Beryllia of high density and controlled microstructure is necessary for application as moderator in various reactor systems. Normally, high density beryllia is prepared by hot pressing at temperatures beyond 1700°C. The sintering behaviour, either in cold pressing and sintering or in hot pressing is influenced by the powder characteristics of the raw material. In this investigation, the sintering behaviour during hot pressing of beryllia powders of divergent characteristics (i.e. prepared from different compounds and subjected to different calcination treatments) has been studied. It was found that powders derived from the sulphate by calcination at 750°C, attained a densification of over 99% theoretical density (2.99 gm/cc; theoretical density 3.01 gm/cc) at a temperature of 1400°C. Attainment of densification at lower temperatures results in little grain development and thus offers the chance to obtain samples sintered to a high density and controlled microstructure. Attempts are made to relate the densification behaviour and microstructure development to the characteristics of the raw powders.

An attempt was made to study the influence of origin and calcination treatments on the development of crystallinity, surface area and the morphology of magnesia powders. Magnesia powders were prepared by thermal decomposition/calcination of different parent compounds namely, magnesium hydroxide, oxalate, carbonate, salicylate and succinate. Calcination temperatures upto 900°C in either air or vacuum and soaking time from half to 16 hrs were used. The development of crystallinity in the resultant magnesia powders was studied at room temperatures by X-ray line broadening techniques. The surface areas of the magnesia powders were determined by the BET method. A limited study was made on the morphology of the powders using electron microscope.It was found that at the decomposition temperature of the parent compound, the crystallinity of the resultant magnesium oxide was very poor, and the surface area extremely high. With an increase in calcination temperature, the crystallinity of the oxide powders improved ...

Bauxites from the different profiles in Western India have been studied for the first time with a view to examining their potential as a refractory material. The results show that the sintering behaviour of these Gibbsite—Boehmite—Halloysite (GBH) assemblages proceeded in two stages with (i) a decomposition stage at 400°–1200°C and (ii) mullite formation stage at 1200°–1600°C. Microstructures of the raw materials and sintered pellets were studied to evaluate the possibility of utilizing them in the ceramic industry.

X-ray line profile analysis of magnesia powders derived from three parent compounds—hydroxide, carbonate and oxalate—at various calcination temperatures ranging between 400° and 1200°C was carried out using Warren and Averbach's method to evaluate strain and effective domain size. The percentage degree of crystallinity was calculated using crystallinity index method of Wakelin, Virgin and Crystal. It has been found that residual microstrains are absent in these magnesia powders and the line broadening was only due to the small crystallite size. The parent compound influences the changes occurring during calcination and ultimately the powder properties such as degree of crystallinity, the crystallite size and the particle size etc which in turn influence the sinterability of these powders.

Influence of minor additions of MgO, TiO2 and Fe2O3, on sintering of BeO powders has been studied. Beryllia powders calcined in the presence of co-precipitated additives at 900°–1100°C were sintered at temperatures 1100°–1500°C. Significant effect of additives is noticed on sintering behaviour of beryllia. Influence of calcination temperature and particle properties on sintering behaviour of BeO powders and distribution of additives in the sintered compacts are briefly discussed.

Alumina by virtue of its various desirable properties finds various applications. It is widely used as refractory, as a base for substrates and radomes and other specialized applications. It is used as an abrasive, in cutting tools, and the usefulness of this material as grinding and polishing powders is well established. The properties of alumina ceramics are governed by the composition, fabrication parameters and sintering treatments. By a judicious control of the various parameters, bodies can be "tailor made" to meet any functional requirements.

Sintering diagrams suggested by Ashby delineate, in terms of variables, the regions in which a mechanism of sintering is predominantly operative. We have suggested modifications in the method of constructing such diagrams and devised a computer program which gives what we call Relative Contribution Diagrams (RCD) directly as an output. The additional advantages of RCD have been reported earlier.An important problem in constructing these diagrams is that a very large number of rate equations and physical properties of the materials have to be contended with. These, it has been suspected, would communicate their inaccuracies to the diagrams. RCD of UO2 were used to identify the properties which have a significant effect on the diagrams. The reciprocal result of this exercise, which determines the level of inaccuracies that can be tolerated in the remaining properties, is also very useful in increasing the confidence with which sintering diagrams may be used.

Influence of minor additions of MgO, TiO2 and Fe2O2 on BeO powders prepared by calcination of the hydroxide has been studied. Minor additions have no significant influence on the decomposition characteristics of beryllium hydroxide. Depending on the temperature of calcination, the minor additions have varying degree of influence on surface area, crystallite size, particle size and particle morphology. Reduction in surface area and increase in crystallite size with increase in calcination temperature were dependent on the type of the additive present. The crystallite size and particle size were of the same order of magnitude at lower calcination temperature. With increase in calcination temperature the particle growth was much more than the crystallite growth.

Considerable work has been done on determining the effects of sintering variables and powder properties on sintering behaviour of BeO. However, there is little to be found in literature on the effect of these variables on the activation energy of sintering.The usual approach to determining the sintering mechanism has been to establish it on the basis of the activation energy of sintering. It has been reported that factors like purity, green density, porosity, density range, temperature range etc can be shown to wield influence on the value of activation energy.Since the possibility of more than one mechanism operating under a set of conditions cannot be ruled out, there is confusion in establishing the operative mechanism (s). Especially, as there is considerable variation in the reported values of activation energy. The situation is even more unsatisfactory when sintering is considered over a wide range of variables, in which the operative mechanisms are likely to change.