Phase Formation Rate Research Articles

β-phase transformation and thermoelectric power in FeSi 2 and Fe 2Si 5 based alloys containing small amounts of Cu

The effects of Cu addition on the β phase formation rate and the thermoelectric power in various FeSi 2 and Fe 2Si 5 based alloys was examined. The peritectoid reaction ( a+ ε→ β) in FeSi 2 alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic ϵ was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi 2) 99− X Mn 1Cu X ( X=0–1. O at.%), (FeSi 2) 99− X Co 1Cu X ( X=0–1.0 at.%) alloys were attained after the elimination of ϵ. In the case of eutectoid reaction ( α→ β+Si). Differential thermal analysis, X-ray diffraction and microscopic observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The rate also depends on the annealing temperature and reached a maximum at about 1073 K for most alloys. The addition of only 0.1 at.% Cu was still very effective even in Mn or Co doped alloys. The thermoelectric power of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. The value of the 0.1 at.% Cu added alloy was higher than that of both the conventional p- and a-type FeSi 2 based alloys. These results suggest that the Fe 2Si 5 alloys with a small amount of Cu may be attractive as new thermoelectric materials.

β-FeSi 2 Phase formation from a unidirectionally solidified rod-type eutectic structure composed of both α and ϵ phases

Kinetics of the transformation from the α and ϵ phases to the β phase in an iron silicide thermoelectric material was examined. The specimens with a rod-type eutectic structure composed of the α and ϵ phases, prepared by a unidirectional solidification technique, were used as a diffusion couple. The β phase formation from α and ϵ consisted of two processes, depending on temperature. The first process which occurred below 1143 K proceeded in the following three stages: (1) the ring-like β phase was formed on the interface between the matrix α and rod shaped ϵ by the conventional peritectoid reaction, (2) the lamellar structure composed of the β and Si phases was formed by the eutectoid reaction of the remaining α phase after an incubation period, (3) finally, the β phase was formed by the subsequent reaction between the remaining ϵ and Si phases formed by the eutectoid reaction described in stage (2). In the second process above 1153 K, where most of the β phase was formed by the peritectoid reaction, the eutectoid decomposed slowly. A rapidly solidified specimen has a very fine eutectic structure and the β phase was mostly formed by the peritectoid reaction, irrespective of temperature. These two apparently different processes were explained by the combination of the simultaneous progress of the peritectoid and eutectoid reactions of which the kinetics depended on temperature. It was also found that a small addition of Mn remarkably decreased the β phase formation rate.

Kinetics of interlayer formation on polycrystallineα-Al2O3/copper-titanium alloy interface

Copper-titanium alloys are known to wet an alumina surface. Traditionally, contact angles are used as measurable parameters to monitor the progress of wetting. Contact angle is a measure of the thermodynamic equilibrium at the interface. For reactive wetting systems, it is well known that the interfacial properties vary as a function of time. In the present study, kinetics of reaction were studied by monitoring the rate of interfacial phase formation. An immersion apparatus was built for this purpose. The extent of reaction was measured both by a simple surface compositional analysis and by interfacial reaction layer thickness measurements. The reaction layer exhibited a parabolic growth with an associated activation energy of 180 to 230 kJ/mole. A speculative growth mechanism is proposed based on the experimental observations and the information available in the literature.

Phase evolution in silver-doped BiPbSrCaCuO(2223)/Ag superconducting composites

Silver doping into (Bi, Pb)2Sr2Ca2Cu3O10 superconducting composite tapes was found to accelerate the formation process of high-Tc (2223) phase owing to lowering the partial melting point of the samples. The differential thermal analysis (DTA) results showed that the partial melting temperature of the sample was lowered by about 10 °C from 850 °C to 840 °C by silver doping. However, with sufficient sintering both the silver-doped and undoped samples can reach a very high level of high-Tc phase fraction, suggesting that the silver doping only speeds up the rate of high-Tc phase formation, but does not change the final phase assemblage of the materials. The reaction kinetics was analyzed by using the Avrami equation, and the results indicated that the conversion process of low-Tc (2212) phase to high-Tc (2223) phase was a diffusion-controlled, two-dimensional reaction. The correlation of the phase evolution with electrical property inside the superconducting tape during the process of heat treatment was also discussed.

On a new approach for finding the non-steady state kinetic rate of phase formation

In this study an expression is obtained for the non-steady state kinetic rate of phase formation by using elements of the probability theory. Its most important feature is that it is valid for various cluster sizes larger or equal to the critical (nucleus) size. The results from this expression are compared with the numerical solution of the basic kinetic equation of phase formation, known as Zeldovich's equation, and the coincidence is remarkable. Owing to its greater simplicity, the expression proposed here can be used instead of the numerical solution in a large number of cases. It is shown that some known expressions for the non-steady state kinetic rate valid for nucleus sizes can be used for various larger sizes when the induction period τ k can be determined for these sizes with sufficient accuracy. It is shown that these known expressions for nucleus size, as well as for larger sizes are less accurate than the expression proposed in this paper in view of closeness to the numerical solution.

ニオブチューブ法Nb<SUB>3</SUB>Al極細多芯線の微視的組織と超電導特性

Microstructure of Nb tube processed Nb3Al multifilamentary wires was studied by an X-ray diffractometer, a transmission electron microscope, etc., and its effect on superconducting properties was discussed from the viewpoint of Al-core sizes, heat treatments and additive elements. Samples with various Al-core diameters were prepared by arranging the filament numbers and final composite diameters. With decreasing Al-core sizes, the volume ratio of A15 phase to other intermediate phase increased, but the lattice parameter of A15 phase decreased conversely. This was accompanied by an increase in Tc and Hc2 (4.2 K), suggesting that the small diffusion spacing between Nb and Al, which was realized by reducing the Al-core size, facilitates to form the metastable A15 phase with the composition near stoichiometry. However, both the n value of voltage-current characteristics and its magnetic field dependence were remarkably decreased when the Nb/Al composite was heavily cold-drawn into Al-core sizes below 50 nm, suggesting that the Al filament “sausaging” caused by heavy cold-drawing is one of the reasons for degradation in superconducting properties at small Al-core sizes. TEM microstructures were in good agreement with reported ones of Nb/Al multilayer thin films. Alloying Al-cores, which are essential to improve the workability of Nb/Al composites, influences the superconducting properties through the diffusion reaction. Additive elements of Ag and Mg facilitate the formation of the A15 phase with excellent superconducting properties at a low reaction temperature (below 1200 K). They enable the Nb/Al composite to be used for alternating current (ac) superconductors, since ac conductors consisting of resistive Cu alloy matrix must be heat treated below 1200 K. On the other hand, the addition of Cu, which depresses the A15 phase formation rate, is suitable for the high-field superconductors, since it makes it easy to control the high temperature heat treatments for short periods.

Pressure effect on the rate of crystalline phase formation of L-alpha-dipalmitoylphosphatidylcholines in multilamellar dispersions

Fully hydrated dipalmitoylphosphatidylcholine (DPPC) in multilamellar dispersions undergoes a subtransition between the crystalline Lc phase and the gel L beta phase. This so-called subtransition occurs only if the DPPC-H2O system has been incubated at temperatures near 0 degrees C for an extended period. We have examined the effect of pressure, up to 336 atm, on the rate of crystalline Lc phase formation of multilamellar DPPC dispersions at 0 degrees C. The hydrostatic pressure is generated by a centrifugal field; the formation of the lamellar Lc phase in the multicomponent DPPC dispersions is monitored calorimetrically at ambient pressure. Results indicate that the rate of formation of the hydrated crystalline Lc phase decreases with increasing pressure. Based on transition state theory, the retardation of the formation of the hydrated crystalline Lc phase by pressure is due to an increase in the volume of the lipid-water system when the activated state is formed. We interpret that the positive value of activation volume is attributed primarily to the dehydration of the lipid polar head group. Although the acyl chain ordering and the head group dehydration are both associated with the L beta----Lc phase transition, the observation of the pressure effects on the rate of crystalline Lc phase formation is used to show that the head group dehydration plays a predominant role in controlling the kinetics of the L beta----Lc phase transition.

熱および溶質の分布を考慮した2元共晶合金の凝固問題の解析

The solidification of alloys has been treated as a heat conduction problem involving moving boundaries by many workers. However, the previous treatments of this problem needed a simplified assumption about the solid phase distribution in the freezing region. The necessity of making such assumptions is due to the fact that the solution for the treezing region is complicated because of an interdependence of the heat flow and the rate of solid phase formation. That is, the heat flow is modified by the generation of the latent heat of solidification the amount of which is dependent on the rate of solid phase formation which in turn is governed by the rate of heat extraction from the freezing region. The purpose of this work is to present a solution to this problem by taking into consideration the coupled effect of heat flow and the rate of solidification. The numerical solution for the freezing region is matched, through a trial and error method, with the analytical solutions obtainable for completely solid or liquid regions. The application of a numerical method for the solution of the freezing region can be possible because the rate of solid phase formation is expressed as a function of temperature by the application of the assumption of complete mixing of solutes within a small element in the liquid phase; the use of Scheil’s equation. As the result of calculations, not only the temperature change for the entire region and the velocities of moving boundaries, but also the solid fraction within the freezing region and the shape of the dendrites based on it were obtained. The results of calculations showed good agreement with the experiments using the H2O-NaCl system.

Measurement of diffusion-induced strains at metal bond interfaces

Mechanical separation of contacts in Si devices is known to originate in interdiffusion effects which promote the growth of intermetallic compound phases. The structural differences in these phases suggest that interfacial strains may develop between them. Using a novel beam-bending approach, these strains have been measured and their variation during aging correlated with the conditions and rate of phase formation.