Toshitada Shimozaki

Measurement of the Impurity Diffusivity of Cu in Fe by Laser Induced Breakdown Spectrometry

The impurity diffusion coefficients of Cu in Fe have been determined in the temperature range of 1073 - 1163 K by means of Laser Induced Breakdown Spectrometry (LIBS). The volume diffusion coefficients for Cu impurity diffusion in a-iron found in this work are in good agreement with the previously published result. The grain boundary diffusion coefficient gb D s d was also calculated using the volume diffusivity and processing the tails of the measured profiles. The values of the activation energy for volume and grain boundary diffusion were approximately 280 and 161 kJmol-1, respectively. This indicates the possibility of a monovacancy diffusion mechanism in case of volume diffusion. The results for the diffusion coefficients are Dv= 2.2 ×10-2exp(-280 kJmol-1/RT) m2s-1 and gb D s d = 2.6 ×10-11exp(-161 kJmol-1/RT) m3s-1.

Thermal equilibrium concentrations and diffusivities of intrinsic point defects in silicon

Recently, inherent defects in silicon crystal cause a serious problem resulting from the trend toward small scale and highly integrated circuits used in semiconductor devices. There is no doubt that intrinsic point defects in silicon are closely related to the formation mechanisms of these defects. Understanding the behavior of the point defects is thus extremely important. The equations for dopant diffusions and oxidation stacking faults in silicon, considering the intrinsic point defect concentrations as unknowns, are solved by mathematically self-consistent method and also the mechanisms of silicon self- and dopant-diffusions are determined. The thermal equilibrium concentrations and diffusivities of self-interstitials and vacancies in silicon, ∗CI, ∗CV, DI and DV are obtained as functions of the absolute temperature T. Consequently, the relations of ∗CI<(∗CI∗CV)1/2<∗CV and ∂Tln(DV)<∂Tln(DI) are derived for an arbitrary temperature.

Power generation characteristics of oxide thermoelectric modules incorporating nanostructured ZnO sintered materials

A practical oxide thermoelectric module consisting of 12 pairs of p-NaCo2O4 and nanostructured n-ZnO sintered materials was successfully fabricated by using a diffusion welding technique. By applying a nanostructure using a void forming agency (VFA), the electrical conductivity of the n-ZnO sintered materials were drastically enhanced, leading that the power factor of the materials were dramatically improved. It is revealed that the oxide module can be operated up to 930 K on the hot side temperature. A maximum power output of 52.5 mW was achieved at the temperature condition of TH/TL=934/479 K, where TH and TL are temperatures at the hot and cold sides of the module, respectively. The details of the power generation characteristics of the present oxide module will be presented and the performance will be discussed with those of our previous and other oxide modules.

Analytical Solutions of the Boltzmann Transformation Equation

The so-called continuity equation derived by Fick is one of the most fundamental and extremely important equations in physics and/or in materials science. As is well known, this partial differential equation is also called the diffusion equation or the heat conduction equation and is applicable to physical phenomena of the conservation system. Incorporating the parabolic law relevant to a random movement into it, Boltzmann obtained the ordinary differential equation (B-equation). Matano then applied the B-equation to the analysis of the nonlinear problem for the interdiffusion experiment. The empirical Boltzmann-Matano (B-M) method has been successful in the metallurgical field. However, the nonlinear B-equation was not mathematically solved for a long time. Recently, the analytical solutions of the B-equation were obtained in accordance with the results of the B-M method. In the present study, an applicable limitation of the B-equation to the interdiffusion problems is investigated from a mathematical point of view.

Determination of Intrinsic Diffusion Coefficients in Binary Alloys with Variable Molar Volume by the M-M Method

The Multiple-Marker (M-M) method is useful because it enables the determination of the intrinsic diffusion coefficients not only at the Kirkendall marker position but also at places where the M-M are located. However, the analysis is not applicable to the alloys with variable molar volume. In this work, a new graphical method that is applicable to the alloys with variable molar volume is proposed.

Numerical Analysis for the Behavior of Multiple Markers in Multiple Phase Diffusion Couples

The movement of multiple markers (M-M) embedded in a multiple phases diffusion couple (M-couple) has been numerically analyzed for binary two phases models taking the molar volume change effect to the diffusion direction into account. From the results obtained by this analysis the places where vacancies are annihilated or generated can be visualized. It has been clarified that a part of M-M is necessarily shown by a linear line due to parabolic movement of the inter-phase interface. Some other interesting results obtained in this study will be reported.

Fabrication and power generation characteristics of p-NaCo2O4/n-ZnO oxide thermoelectric modules

A prototype of thermoelectric module using oxide materials has been successfully fabricated, and the power generation characteristics have been examined in the high temperature region around 773 K (500 degC). Twelve couples of sintered p-NaCo2O4 and n-ZnO materials were connected via silver conducting strips with a planer arrangement jointed by using a diffusion welding technique under 16 MPa at 1023 K in Ar. A maximum power output of 58 mW was achieved at a temperature condition of TH/TL = 839/377 K. The details of the power generation characteristics of the oxide module will be presented and the performance will be discussed with other oxide modules

Interdiffusion in Fe/Pt Multilayer Thin Films

Interdiffusion in Fe/Pt multilayer thin films has been studied. [Fe(1nm)/Pt(1.5nm)]20 multilayers were prepared by DC magnetron sputtering technique and subsequently annealed at temperatures of 543 - 633K in vacuum lower than 10-6 torr. X-ray diffraction (XRD) studies on these multilayer systems revealed the interdiffusion coefficients from slope of the best straight line fit of first peak intensity versus annealing time. The temperature dependence of interdiffusion in the range of 543 - 633K can be described by D=4.98×10-24 exp (0.88eV/kT) m2S-1. The coercivity, measured by Vibrating Sample Magnetometer, of the multilayer with annealing time at 603K increased, which is believed to the increase of surface roughness by interdiffusion at the interfaces of Fe and Pt multilayers, enhancement of composition gradient; and/or formation of Fe-Pt reaction phase at the interface of Fe and Pt.

A Limited Condition for Bifurcate or Trifurcate Kirkendall Planes in Multiple Phase Diffusion Couples

In general, only one Kirkendall plane can be seen in a diffusion couple. However, bifurcate or trifurcate Kirkendall planes have been reported in Ti/TiAl3 or Co/CoSi2 multi-phase diffusion couples (M-couple) [1,2]. The authors [3] have previously shown a numerical technique to analyze the movement of multiple markers (M-M) embedded in a M-couple taking the molar volume change effect to the diffusion direction into account. Using this technique, one can visualize the places where vacancies (lattice planes) are annihilated or generated in the couple. Here, we try to demonstrate the bifurcate or trifurcate Kirkendall planes in the M-couple and clarify the limited conditions of bifurcate or trifurcate Kirkendall planes by using this numerical technique.

Oxidation Behavior of TiAl3 Formed in Ti/Al Diffusion Couple and Reaction Diffusion in Ti/TiAl3 Multi-Phase Diffusion Couple

Oxidation resistance of TiAl3, one of the candidates of coating materials for high temperature structural materials such as Ti3Al and TiAl, has been studied. Specimens were prepared by forming TiAl3 in Al/Ti/Al reaction diffusion couples at 923 K and then TiAl3 layer was exposed to air by dissolving Al plate in a 1N NaOH solution. The obtained TiAl3/Ti/TiAl3 couples were annealed in air in the temperature range from 1173 K to 1468 K. The oxidation rate was compared with that determined by using bulk TiAl3. The present data show a bend on the Arrhenius plot of parabolic phase growth rate constant, k2, at 1323 K. Above 1323K, the constant coincides well with the extrapolated values of bulk data while the value in the lower temperature range is larger than that of bulk specimens. During the oxidation experiments, intermetallic compounds Ti3Al, TiAl and TiAl2 were formed between Ti and TiAl3. Interdiffusion coefficients in the Ti3Al, TiAl phases determined from these diffusion couples are more than one order of magnitude larger than the interdiffusion coefficients determined by previous investigators from single-phase diffusion couples but coincide with the coefficients determined from multi-phase diffusion couples. This difference between interdiffusion coefficients has been discussed and explained by the effect of boundary diffusion in the diffusion layers formed in the multi-phase diffusion couples.