Hiroyasu Kido

Thermoelectric Properties of P-doped Mg2Si Semiconductors

The thermoelectric properties of P-doped Mg2Si [Mg2Si:P=1:x (0.0005x0.03)] fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), the Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. P-doped Mg2Si samples are n-type in the measured temperature range. The electron concentration of P-doped Mg2Si at 300 K ranges from 6.7×1018 for the P concentration of x=0.0005, to 3.1×1019 cm-3 for x=0.03. First-principles calculation revealed that P atoms are expected to be primarily located at the Si sites in Mg2Si. ρ and S are strongly affected by x, but κ does not depend on x. The sample x=0.03 shows a maximum value of the figure of merit ZT of 0.33 at 865 K.

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr1−xMnx)O3+δ (0.0⩽x⩽1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr1−xMnx)O3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn4+ ion content increases monotonically in the range 0.2⩽x⩽1.0. The magnetic measurement of La(Cr1−xMnx)O3+δ indicates that a Mn3+ ion is a high-spin state with (de)3(dγ)1. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr3+, the Mn3+, the Mn4+ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr1−xMnx)O3+δ is a p-type semiconductor and exhibits the hopping conductivity.

Increase of the thermoelectric power factor in Cu∕Bi∕Cu,Ni∕Bi∕Ni, and Cu∕Bi∕Ni composite materials

The resultant thermoelectric power factors P of three types of Cu∕Bi∕Cu,Ni∕Bi∕Ni, and Cu∕Bi∕Ni composite materials welded with Bi were measured at 298 K as a function of relative thickness of Bi and compared with P values calculated by treating these devices as an electrical and thermal circuit. It was first demonstrated experimentally that the observed P values of composite devices have a local maximum at an optimum volume fraction (corresponding to the thickness) of Bi, as predicted theoretically. The maximum P values of composite materials were several times higher than those of pure Ni and Bi and were about 100 times larger than that of pure Cu. The dependence of P on the thickness of Bi was found to be explained roughly by introducing the modified thermal conductivity and an enhancement factor in the Seebeck coefficient to our simple model in which a device was treated as an electrical and thermal circuit.

Electronic structure of β-Fe(Si2−xGex) ternaries

Abstract The geometrical and electronic structures of β-Fe(Si2−xGex) ternaries were investigated using first principles pseudopotential calculations based on generalized gradient approximation density function theory. These compounds are indirect-bandgap semiconductors with theoretical energy band gaps ranging from 0.51 to 0.79 eV . Substitution of Ge for Si in β-FeSi2 leads to a reduction of the gap width and an increase in the volume of the unit cell with almost the same shape of the last valence band and the first conduction band but for the Y point. The analyses of bond overlap population and Mulliken population revealed that the covalency between Fe and Si atoms are strengthened by increasing Ge content and that the electron transfer occurs from Ge to Fe or Si.

Geometrical and electronic structures of β-FeSi1.875X0.125 (X = B, N, Al or P)

The geometrical and electronic structures of β-FeSi1.875X0.125 (X = B, N, Al or P) were investigated using first principles pseudopotential calculations based on generalized gradient approximation (GGA) density function theory. The calculated structural parameters depend strongly on the kinds of dopants and sites. The total energy calculations for substitution of dopants at the SiI and the SiII sites revealed that Al and P prefer the SiI sites, whereas B and N prefer the SiII sites. The calculations predict that β-FeSi1.875B0.125 and β-FeSi1.875Al0.125 show p-type conduction, while β-FeSi1.875P0.125 shows n-type. The prediction of the carrier type of β-FeSi1.875B0.125 does not agree with the experimental results. The experimental results are theoretically interpreted on the assumption that B atoms are located not at the Si sites but at the FeI sites in B-doped β-FeSi2.

Designing Pore-Size in Silica Gels:H20-TMOS System,

Abstract : A series of microporous silica gels were synthesized by hydrolyzing tetramethylorthosilicate (TMOS) with varying amounts of water. No catalyst or alcohol was added. The gels were characterized by water sorption isotherms, the BET nitrogen surface area measurements, and differential thermal analysis. Although the gelation time for all H20/TMOS molar ratios (R) was approximately the same (about 3 hr), the pore-size (pore diameter) and sorption capacity were found to be minimum in the R range 4.96-5.37 but increased with both increasing and decreasing R. Water sorption isotherms measured at 25 deg C after degassing at 200 deg C of gels prepared with R = 4.96-5.37 exhibited moderate Type I (Brunauer classification) isotherm which changed to intermediate between Type I and Type IV with decreasing or increasing R. These results indicated that the amount of organic or water which was not utilized in the reaction and the degree of cross-linking apparently played a role for the observed variations in pore-size or shape of water isotherms.