Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Editor's Note

Journal of Materials Synthesis and Processing -     

Kinetic analysis of the combustion synthesis of molybdenum and titanium silicides

The temperature profiles associated with the passage of self-propagating combustion waves during the synthesis of MoSi₂ and Ti₅Si₃ were determined. From these profiles, kinetic analyses of the combustion synthesis process for these two silicides were made. The synthesis is associated with high heating rates: 1.3 × 104 and 4.9 × 104 K·s₋₁ for MoSi₂ and Ti₅Si₃, respectively. The width of the combustion zone was determined as 1.3 and 1.8 mm for the silicides of Mo and Ti, respectively. The degree of conversion, η, and its spatial distribution and the conversion rate, ∂η/∂t, were determined. However, because of the inherent characteristics of wave propagation in MoSi₂, only in the case of Ti₅Si₃ could the activation energy be calculated. An average value of 190 kJ μ mol₋1 was determined for titanium suicide.     

Plane strain analytical solutions for a functionally graded elastic–plastic pressurized tube

Plane strain analytical solutions to functionally graded elastic and elastic–plastic pressurized tube problems are obtained in the framework of small deformation theory. The modulus of elasticity and the uniaxial yield limit of the tube material are assumed to vary radially according to two parametric parabolic forms. The analytical plastic model is based on Tresca's yield criterion, its associated flow rule and ideally plastic material behaviour. Elastic, partially plastic and fully plastic stress states are investigated. It is shown that the elastoplastic response of the functionally graded pressurized tube is affected significantly by the material nonhomogeneity. Different modes of plasticization may take place unlike the homogeneous case. It is also shown mathematically that the nonhomogeneous elastoplastic solution presented here reduces to that of a homogeneous one by appropriate choice of the material parameters.     

The combustion synthesis of refractory nitrides

The self-sustaining combustion of niobium in a nitrogen atmosphere was investigated. The effect of initial porosity, nitrogen pressure, and the amount of added diluent on the combustion process and on the nature of the products of combustion was determined. Auxiliary investigations on the interaction between niobium and nitrogen were made under isothermal conditions. Results of X-ray analyses of these investigations are compared to those obtained under self-propagating reaction conditions.     

Morphology relations during bulk-transport sintering

Based on a minimum neck surface area, geometric relationships describing the morphology of sintering spheres have been mathematically established. Calculations resulting from this model give the relative shrinkage and surface area reduction as functions of both neck size and particle coordination. The specific surface area is shown to decrease monotonically with neck growth and to be a sensitive measure of sintering progression. An empirical relationship between neck size, coordination and surface area is derived. Previously published shrinkage results are found to be in good agreement with those predicted by the proposed model. Prediction of the kinetics of surface area reduction and assessment of the relative contribution of surface and bulk-transport processes are now possible through the relationships generated from this model.     

Simultaneous Synthesis and Densification of MoSi2 by Field-Activated Combustion

A method to simultaneously synthesize and consolidate MoSi₂ from powders of Mo and Si was investigated. Combustion synthesis was carried out under the combined effect of an electric field and mechanical pressure. Highly dense molybdenum silicide up to (99.2%) was produced from elemental powders in one step. Minor amounts of Mo₅Si₃ were present at the boundaries of MoSi₂ grains in the interior of samples made from stoichiometric reactants. The addition of 2.5 mol% Si excess, however, resulted in Mo₅Si₃-free, dense MoSi₂ products.     

Correlation of the phase transformation accompanying the hydration of calcium aluminate with ultrasonic wave behaviour

Observations of changes in the amplitude and velocity of the ultrasonic wave transmitted through a hydrating high alumina cement were found to have no corresponding changes in X-ray phase analyses. A continuous X-ray scan of an hydrating sample showed no evidence for the completion of hydration. X-ray patterns from a sample aged for two months and from one aged for 24 hours were basically identical.     

Phytic acid interactions in food systems

Phytic acid is present in many plant systems, constituting about 1 to 5% by weight of many cereals and legumes. Concern about its presence in food arises from evidence that it decreases the bioavailability of many essential minerals by interacting with multivalent cations and/or proteins to form complexes that may be insoluble or otherwise unavailable under physiologic conditions. The precise structure of phytic acid and its salts is still a matter of controversy and lack of a good method of analysis is also a problem. It forms fairly stable chelates with almost all multivalent cations which are insoluble above pH 6 to 7, although pH, type, and concentration of cation have a tremendous influence on their solubility characteristics. In addition, at low pH and low cation concentration, phytate‐protein complexes are formed due to direct electrostatic interaction, while at pH >6 to 7, a ternary phytic acid‐mineral‐protein complex is formed which dissociates at high Na concentrations. These complexes appear to be responsible for the decreased bioavailability of the complexed minerals and are also more resistant to proteolytic digestion at low pH. Development of methods for producing low‐phytate food products must take into account the nature and extent of the interactions between phytic acid and other food components. Simple mechanical treatment, such as milling, is useful for those seeds in which phytic acid tends to be localized in specific regions. Enzyme treatment, either directly with phytase or indirectly through the action of microorganisms, such as yeast during bread‐making, is quite effective, provided pH and other environmental conditions are favorable. It is also possible to produce low‐phytate products by taking advantage of some specific interactions. For example, adjustment of pH and/or ionic strength so as to dissociate phytate‐protein complexes and then using centrifugation or ultrafiltration (UF) has been shown to be useful. Phytic acid can also influence certain functional properties, such as pH‐solubility profiles of the proteins and the cookability of the seeds.