Technological features of casting zirconia parts

Conclusions Alkaline slips made of zirconium dioxide stabilized with calcium oxide, which have been obtained by grinding the materials in a rubber-lined zirconium-ball mill, exhibit poor casting properties and a tendency to stratification. The green ware obtained shows a low bulk density of the order of 2.5 g/cm³. The inadvisability of preparing material for casting in this way is further confirmed by the need to restore a large number of the grinding balls which quickly wear out during operation. In order to obtain a uniform degree of fineness of the material, the grinding time for zirconium balls, as opposed to the process in metal mills, should be increased.A sharp improvement in the casting properties of slip, an increase in the porosity of the green ware and a reduction in shrinkage during firing can be obtained by casting acidic slips treated with hydrochloric acid. This determines the technological advisability of washing the stabilized ZrO₂ with acid after grinding, and the possibility, therefore, of grinding for a shorter period in steel metal-ball mills.The most favorable conditions for casting slip made of stabilized ZrO₂ treated with hydrochloric acid are a moisture content of about 30% and a pH value of 1.5–2. Here the densityof the casting is 2.8 g/cm³ and the density of the fired part 5.45 g/cm³. The casting properties of slip are improved and the density of the green ware is raised by 0. 2 g/cm³ if the materials are left to stand for 2 to 4 days after treatment.     

New method of determining the slag-resistance of refractories

Conclusions A new method is proposed for determining the slag resistance of refractories with lever apparatus which is qualitative.The new technique permits the specific features of the destruction of refractories by slags to be determined, and explains the mechanism of this process by mineralogical analysis and by describing the external appearance of the changed specimens.Data on the changes in the specimens during tests and the compositions of the zones will be given in the next article.Data obtained by the new method show that the slag destruction of refractories accelerates with time.     

Early soot formation stages in low-pressure diffusional counterflow hydrocarbon flames

Early soot formation stages were investigated in low-pressure counterflow hydrocarbon flames. A mechanism is proposed for the formation of aggregates and discrete spherical soot particles. It is proposed that the formation of the physical surface is preceded by the emergence of a skeleton structure consisting of carbon molecule chains as a result of their polarizing and dipole interaction with the charged soot nuclei.Kiev. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 97–100, May–June, 1993.     

Environment-Sensitive Hardness and Machinability of Al2O3

The influences of water, toluene, and n -alcohols on the pendulum hardness of Al₂O₃ monocrystals and on the ease of drilling of mono- and polycrystalline Al₂O₃ were studied. Adsorption-induced increases in pendulum hardness were produced by certain alcohol environments; these increases resulted in rates of drilling with diamond-studded core bits of up to 10 times those measured under water. A correlation between pendulum hardness, machinability, and surface charge (as indicated by ζ-potential measurements) was established and revealed that Al₂O₃ is hardest and most effectively drilled with a diamond core bit at its zero point of charge. The possible use of this correlation in developing more cost-effective and ecologically viable cutting fluids for the machining of Al₂O₃ is discussed.     

Mechanosynthesis of complex oxides and preparation of mixed conducting nanocomposites for catalytic membrane reactors

Nanopowders of LaGaO₃- and LaMnO₃-based complex perovskites (P) and ceria-based fluorites (F) were prepared by mechanosynthesis. Compatible nanocomposites F + P and P + P with mixed ion and electron conducting (MIEC) properties were prepared and sintered at moderate temperatures up to dense ceramics. The obtained materials were studied by means of XRD, SEM, TEM, electrical conductivity measurements, temperature programmed (TP) reduction/oxidation and preliminary estimations of permeability were obtained. A new strategy based on the advantages of the mechanochemical ceramic approach is proposed to design multilayer ceramic membranes for CMR. Casting technology and one-step sintering were used for the production of thin film membranes with MIEC properties on porous substrates. The coarse fraction of as-milled powders from agglomerates with density 70% was used for the porous substrate, and fine fractions of aggregates with sizes <1 μm were used in preparation of composites for thin dense films. Ceria-based composites prepared by the Pechini route and/or mechanochemical method are proposed as materials for protecting thin films.     

Effect of phosphate esters on PVC flammability from a multi-test viewpoint

Many tests have been developed to provide relative flame/smoke data. In spite of the proliferation of such tests and test equipment, many chemical and polymer producers, as well as fabricators, use only one technique to evaluate the flame/smoke properties of a polymer system. This paper demonstrates the importance of multi-method testing to rate the efect of phosphate esters and various fillers on PVC flammability and smoke generation.     

Terpolymers. II. Mechanical properties and transition temperatures of terpolymers of vinyl stearate,vinyl acetate,and vinyl chloride

Vinyl stearate was studied as a major internal plasticizer in terpolymers containing vinyl acetate and vinyl chloride. The terpolymers were prepared by systematically replacing vinyl acetate by close increments of vinyl stearate starting with combinations of vinyl acetate and vinyl chloride, in increments, over all compositions. For comparison of properties, a complete range of copolymers of vinyl stearate and vinyl chloride, as well as mixtures of poly(vinyl chloride) and di-2-ethylhexyl phthalate (DOP) were also made. The external plasticizer was more efficient in reducing the glass temperature than was vinyl stearate. The decline in T_g with weight fraction of plasticizer was linear for the copolymers and terpolymers but concave downward with the liquid diluent. The linear decline was shown to involve mere additivity of the free volume contributed by each side-chain methylene (or methyl) group in both vinyl esters to reducing T_g. The mechanism of the diluent system was more complex. However, the magnitude of the reduction of tensile modulus at a given weight fraction of DOP could be equaled or exceeded by the same amount of vinyl stearate, by increasing the vinyl acetate content of the base copolymer to 40 mole-% or more. Unfortunately, the ultimate strengths and elongations of internally plasticized systems were reduced more than those of the mixtures at comparable compositions. Vinyl stearate was found to markedly retard photolytic degradation compared to both vinyl acetate and the external plasticizer in unstabilized samples having nearly the same thermal treatment. The effect was greater than could be ascribed to dilution by the long alkyl group. The production of a stearoyl radical more stable than the radicals initiating dehydrochlorination is suggested as a possible mechanism.     

Morphology and microstructure of autoclaved clinker and slag-lime pastes in presence and in absence of silica sand

Development of microstructure in four hydrothermal reactions has been undertaken using scanning electron microscopy. These are clinker, clinker-sand, slag-lime and slag-lime-sand hydrothermal reactions. The microstructure of clinker hydration products displayed crumpled foils and tabular masses of calcium silicate hydrates; few cubic crystals of hydrogarnet appeared only during the initial stage of the reaction. In clinker-sand mixture the C-S-H phase was the only product identified. In slag-lime hydration the microstructure displayed both of the hydrogarnet crystals and the C-S-H phase. The hydration of slag-lime-sand mixture (an optimum composition) was associated with the formation of ill-crystallized tobermorite and crystalline 11A tobermorite as the main products.     

Molecular weight distribution of the graft copolymer comprising mixtures of styrene and acrylamiae on cellulose acetate film by means of gel permeation chromatography

Styrene portion of the radiation-induced graft copolymer comprising styrene and acrylamide was separated by acid hydrolysis and the effects of various grafting parameters (e.g., reaction time, reaction temperature, solvents, monomer composition, etc.) on molecular weight distribution were evaluated by means of gel permeation chromatography. When a single monomer or mixture of two monomers are grafted, the molecular weights is found to increase, but polymer dispersity decreases with the increase of reaction time or reaction temperature except at a higher reaction time due to the continuous enlargment of the growing chain through increased swelling and molecular motion of the trapped radicals. At higher reaction time the degradation of the graft chains lead to lower molecular weight and higher polymer dispersity. Effects of solvents (e.g., methanol, ethanol, and t-butanol) on the molecular weight and molecular weight distribution were discussed on the basis of swelling property and chain transfer constants of the solvents. Styrene-type graft radical being long lived compared to acrylamide type, gave long-chain styrene graft with the increase of styrene content in the reaction mixture. A comparison of the effect of one-and two-component systems on a molecular weight distribution is also discussed.