Papers by Author: Toshiyuki Takagi

Abstract: We report on preparation of Mg₂(Si,Ge,Sn)-based thermoelectric materials by a direct induction melting method in Al₂O₃ crucible. A 40 g ingot of Mg₂Si_0.8Sn_0.1Ge_0.1 was prepared after addition to the batch 10 wt% of Mg excess. Evolution of crystal structure of the induction melted sample upon annealing and spark plasma sintering (SPS) was tracked by room-temperature X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. An evidence for the formation of Mg₂(Si,Ge,Sn) solid solution was obtained from the crystal lattice parameter of this phase which was found to be larger than that of undoped Mg₂Si. XRD and SEM indicated that alongside with the main phase of the Mg₂(Si,Ge,Sn) solid solution, an impurity phase of Mg₂Sn exists in the sample. Amount of the Mg₂Sn impurity phase is significantly reduced in spark plasma sintered sample.

Abstract: In the present work the influence of Co alloying on transport properties of Ni-Mn-Al melt-spun ribbons was investigated. Results of electrical resistivity measurements showed that Ni₅₀Mn₃₁Al₁₉ and Ni₄₅Co₅Mn₃₂Al₁₈ samples exhibit a temperature hysteresis of the electrical resistivity which was attributed to a first order martensitic transformation while no hysteresis was observed in Ni₄₀Co₁₀Mn₃₃Al₁₇ sample. Measurements of the Hall effect indicated that the Co alloying leads to an increase of carrier mobility.

Abstract: We report on structural and transport properties of Ni-Mn-In thin films. The results obtained have indicated that transport properties of the thin films strongly depend on the structural ordering of the films. Specifically, annealing converts the semiconducting-like character of the resistivity into that typical for metals. This is due to annealing-induced crystallization of as-deposited films which are in partially amorphous state. In both the as-deposited and the annealed thin films the field dependence of the resistivity is negative, which is typical for thin films of Heusler alloys and can be ascribed to the suppression of the spin disorder scattering.

Abstract: NiTi shape memory alloys (NiTi-SMA or Nitinol) have successful application thanksto their excellent and unique material properties. However, considering the practical applicationto human body, elution from the toxic Ni ions into human body should be taken into account.Many researchers have dealt with the issue and already proposed various surface treatmenttechniques including the surface oxidation and ion implantation.Our focus is to enhance thefunctionality of Nitinol. We have been developing the smart sensor utilizing metal-containingDiamond-like Carbon (Me-DLC) thin coating. Emphasis can be made on the unique propertiesof Me-DLC deposited on the Nitinol. Strong adhesive strength has been obtained in Me-DLCand Nitinol system. In this paper, we propose a novel Me-DLC coating with multifunctionalityof the suppression of the toxic Ni ions. The paper includes deposition technique, performanceof the coating concerning adhesive strength and suppression of the toxic Ni ions, and finallymultilayered structure aiming at multifunctionality.

Abstract: Systematic measurement of minor magnetic hysteresis loops of traditional hysteresis tests requires substantially lower magnetization of samples and offers higher sensitivity of detection of changes in the ferromagnetic materials’ microstructure. The keynote idea of this method, called Magnetic Adaptive Testing (MAT) is utilization of sensitive correlations between the varied microstructure of the magnetized material and the corresponding, highly sensitive modifications of some of the minor hysteresis loops. The paper presents some of the recent results of the MAT measurement performed on specially prepared series of cast iron samples. Results of the non-destructive magnetic tests were compared with the destructive mechanical measurements of Brinell hardness and linear correlation was found between them. A very good correlation was also found between magnetic descriptors and conductivity and chill/ferrite area fraction. Based on these results, Magnetic Adaptive Testing is suggested as a highly promising non-destructive method for monitoring structural changes in different types of ferromagnetic materials.

Abstract: A new type of smart sensor based on metal-containing diamond-like carbon (DLC) coatings is presented. DLC coatings are widely used as protective coatings to improve the surface properties of objective materials; for example, to increase hardness and chemical stability. With the addition of metal clusters into DLC coatings, electrical conduction appears to depend on the microstructure. Such coatings can be used in fabricating resistive sensors. In this paper, we present tungsten-containing DLC (W-DLC) as a possible strain sensor. The strain sensitivity is greatly affected by the deposition condition. We also fabricate a double-layered DLC/W-DLC coating. The double-layered structure is expected to be used as a smart coating having functionality as a sensor with a protective DLC overcoat.

Abstract: Fatigue cracks and SCC (Stress Corrosion Cracking) on a back surface were detected, and the shape of cracks were evaluated using a surface wave generated by focusing angle beam ultrasound. Internal ultrasonic images showed a little shorter length than optical images. Cross section of the fatigue crack and internal distribution of complicated shaped SCC were estimated.

Abstract: The propagation characteristics of the back-reflected Rayleigh surface wave in chemical vapor deposit (CVD) diamond coated specimens were investigated experimentally using the ultrasonic backward radiation measurement. The deliberated delaminations produced between the coating layer and the substrate decreased Rayleigh surface wave velocity. This initial experimental result obtained in the present study demonstrates the high potential of the backward radiated ultrasound as a tool for nondestructive evaluation of very thin CVD diamond coating layer.

Abstract: Although electrochemical transients can provide very sensitive indication about the onset of localized corrosion, including initiation of stress corrosion cracking (SCC), information on location of the initiation site is not available. In the present study, we proposed an estimation method of location and magnitude of SCC initiation events, where electrochemical transients associated with local anodic events are measured by multiple reference electrodes and location of the initiation sites are estimated based on relative magnitude of the electrochemical transients simultaneously measured at multiple locations.

Abstract: The conductivity of metal-carbon-silicon nanocomposite films considered as potential candidates for the application as wide-range temperature sensors for severe environmental conditions is studied. The films combine unique properties of amorphous carbons with a new functionality imparted by the presence of metal nanoclusters in host matrix. The deposition of carbon-silicon phase was performed using PECVD of siloxane vapors. Metals (W, Nb, and Cr) with concentration in the range from 12 to 40 at. % were incorporated in the carbon-silicon host matrix by DC magnetron co-sputtering. The conductivity of the films decreases with temperature in the range 80-400 K, being well described by the power-law dependence. The conductivity mechanism found satisfactory explanation in the framework of the model of inelastic tunneling of electrons between metal nanoclusters dispersed in carbon-silicon matrix. The parallel study of the influence of metal concentration increase on carbon phase microstructure was carried out using Raman spectroscopy.