Microstructural observation and reaction mechanism of ZnAI2O4 formation in the presence of various fluorides

The rates of ZnAI₂O₄ formation in the presence of various fluorides were controlled by the nucleation process. The rate constants of the nucleation process for various fluorides were in the order LiF > NaF and MgF₂ > CaF₂ < SrF₂ < BaF₂. This trend corresponds to the order of the promotion of ZnAI₂O₄ formation by these fluorides. In order to interpret the dependence of the kinetic constant of the nucleation of ZnAI₂O₄ from the intermediate phase on the kinds of counter cation of fluorides, it was assumed that the activated state for promotive formation of ZnAI₂O₄ under the presence of fluoride is similar to that of the following reaction: $$ZnO + Al_2 O_3 + 2MF\left( {M'F_2 } \right) = ZnF_2 + M_2 Al_2 O_4 \left( {M'Al_2 O_4 } \right).$$ The order of the rate constant obtained in this study showed agreement with the order of the standard heats of formation for the above reaction. This relation was interpreted by applying the linear free energy relationship (LIFER).     

Laser glazing of an Fe-C-Sn alloy

Morphology and geometry of melted zones, cooling rates, microstructure and microhardness in the laser-glazed Fe-4%C-10%Sn alloy have been investigated. The computer simulation on the basis of the moving gaussian source model was used successfully to predict the maximum width and depth of the melted zone and the cooling rate. The microstructure from the surface to the bottom of the laser-melted zone is a non-crystalline phase, dendritic grains and a microcrystalline zone successively. Values of the averaged-spacing of the non-crystalline phase are 0.205⁶ and 0.121⁹nm, respectively; twinned martensites, having an axial ratioc/a of 1.128, existed in dendritic grains, and carbides of Fe₃ C at the interdendritic regions; the microcrystalline zone was composed of -Fe and a new bet (a=0.415 nm,c=0.955 nm) phase. The different microstructure in the melted zone can be explained by the results of the heat flow calculation. A fine eutectic structure (-Fe + Fe₃C) was observed in heat-affected zones. Microhardness of the eutectic structure can be predicted by the empirical relation of fracture stress to the interlamellar spacing of pearlite.     

Equations of state for hard-sphere fluids

Equations of state and contact values of hard-sphere radial distribution functions (rdf's) which are given by a linear combination of the Percus— Yevick and scaled-particle virial expressions are considered. In the one-component case the mixing coefficient() is, in general, a function of the volume fraction. In mixtures the coefficient( _i ,d _i ), in general, depends upon the volume fraction _i , and diameterd _i , of each species,i andj. For the contact valuesY _ij of the rdf's, the mixing coefficients _ij ( _k ) also depend on speciesi andj. Density expansions for the exact for the one-component hard-sphere fluid are obtained and compared with several approximations made in earlier works and in our own work, as well as with simulations. For a mixture, it turns out that one cannot obtain the exact fourth virial coefficient by using a linear combination of the Percus-Yevick and scaled-particle virial expressions forY _ij unless one allows _ij to depend on mole fractionsx _i even at the zeroth order of its density expansion. We also find that _ij must depend on particle speciesi andj in order to satisfy the exact limits obtained earlier by Sung and Stell. A new equation of state for the binary hard-sphere mixture which satisfies all the exact limits we have considered is suggested.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Thermodynamic properties of iron doped beta″-alumina by e.m.f. measurements with beta-alumina electrolytes

Thermodynamic studies of the non-stoichiometric iron doped beta-alumina (ID) phase were carried out by electrochemical measurements coupled with coulometric titration using the cell Na_liq/Li-alumina/ID. Hot pressing and glass sealing techniques were developed and employed to obtain a suitable and stable Li-alumina/ID interface. The equilibrium e.m.f. of the cell was determined as a function of sodium concentration over the temperature range 444 to 523 K. The range of sodium concentrations over which the ID phase is stable was also determined. The relative partial molar thermodynamic quantities of sodium, , , and in ID alumina as a function of sodium concentration were obtained from cell e.m.f. data.     

Observation of large diamagnetism in La-Sr-Nb-O films up to room temperature

Large diamagnetic transitions along with sharp resistive transitions were observed in the La-Sr-Nb-O system near room temperature (290 K). The amplitude of the diamagnetism reaches 35% of that of a pure Nb sheet. In addition, a behavior similar to weak magnetic spin ordering was observed for some samples at a temperature of about 290 K, over a temperature range of 30 K. The diamagnetism reappears above this temperature and continues up toT 320 K. It is not clear what composition ratio of La-Sr-Nb-O is responsible for this large diamagnetism and the high critical temperature. The yield probability of these samples is around 50%. The characteristics of the samples having not passed through many thermal cycles remain stable for about 1 month.     

Heme pigment content in meat as affected by the addition of curing agents

The effects of curing agents on the content of heme pigments (HP) in meat were examined. Minced porcine skeletal muscle was treated with NaCl, NaNO(2) and sodium ascorbate (NaAsA), separately or in combination, and stored at 2-3°C for 7 days. HP content decreased with increase in NaCl concentration and the decrease was about 50% and 80% at NaCl concentrations of 2% and 10%, respectively. Two percent NaCl prevented HP destruction, when previously mixed with 100 ppm NaNO(2) or 0·1% NaAsA. Although some decrease in HP content was noted following application of a mixture of NaCl, NaNO(2) and NaAsA, it was essentially the same as that of the control during 7 days of refrigerated storage. In a model solution containing the same curing agents as those applied to the meat. NaCl had no effect and myoglobin (Mb) content remained constant during storage. From the present results, endogenous muscle constituents appear to act in concert with NaCl to bring about a decrease in HP content.     

Collagen and its rational content in meat products: Part 1. Analytical studies

Computer-assisted simulation has been used to study the effect of collagen content on the biological value of meat proteins. It has been shown that an increase in the collagen content from 2·5% to 15-20% of the total amount of proteins contained in minced meat tangibly enhances protein utilization for tissue synthesis. Hence, the above collagen content in meat products heightens their nutritional and biological value and renders them more suited for human metabolism.     

Precipitation in solids

The diffusional decomposition mechanism in solid solutions is discussed. The mechanism, despite the great variety of its morphological and kinetic peculiarities for different alloys, remains the same. Fundamental properties of large collections of particles are revealed and the physical aspects of the decomposition process are reported. Therefore, the division of phase transitions in solids into first and second orders, and the division of phase transitions of the first order into those decomposing by spinodal or nucleation-growth mechanisms are, to a certain extent, artificial and do not have adequate experimental confirmation. Proof is given that solid solutions decompose by the spinodal mechanism only, and thus they do not need to overcome the fluctuating energy barrier, as was supposed in nucleation-growth theory. The results given testify to the narrowness and lack of grounding of the generally assumed linear concepts of dynamics of phase segregation based on an exponential increase of small fluctuations; they also show that the incubation period before precipitation of a new phase in the solid is a process of searching by the system for a correlation distance (that is, the stochastic process being submitted to the laws of dynamic scaling).     

Hot ductility of simulated stainless-steel weld metals

Simulation of stainless-steel weld metals was performed using a Gleeble-1500 thermomechanical simulator. Two classes of materials were investigated, including both fully austenitic and austenitic-ferritic stainless steels. The niobium content varied within each class. The simulation comprised heating to melting point, melting for a short time, and cooling to a number of temperatures, at which point the samples were fractured under a tensile load. The hot ductility, in terms of reduction of area, was measured. Metallographic examinations were performed using both optical and electron microscopy. The hot ductilities of the austenitic-ferritic weld metals investigated were superior to those of fully austenitic weld metals of corresponding niobium content. The beneficial effects of ferrite were found to decrease with increasing niobium content. The effect of niobium on hot ductility was detrimental, i.e. an increase in niobium content resulted in a decrease in hot ductility which was attributed to the formation of (FeCrNi)₂Nb-, a low melting eutectic, along the austenitic grain boundaries. The criterion of hot ductility by simulation of the weld metals was also found to be reliable for evaluating susceptibility to solidification cracking.     

Design and development of the d.c. plasma reactor for the synthesis of ultrafine powders

After analysing different plasma processes, we have designed and developed a 14kW d.c. plasma unit for generating ultrafine powders. Experiments to characterize the torch demonstrated that it is possible to operate the plasma unit over a wide range of conditions without losing its discharge stability. Temperature distribution profiles were determined radially as well as axially in the reaction chamber, and the efficiency of the torch was calculated for a fixed power input. Ultrafine powders of Ta, Mo and W were synthesized, and size and size distribution obtained. The particle size was found to be 0.03–0.1 m, with a perfect spherical shape. The analysis of the samples is discussed.