Hiroshi Nakatsugawa

Electrical transport properties in LiMn2O4, Li0.95Mn2O4, and LiMn1.95B0.05O4 (B=Al or Ga) around room temperature

In order to identify the carrier responsible for the electrical transport at room temperature in LiMn2O4 from the viewpoint of practical applications as a cathode material, the bulk conductivity measurements by complex-plane impedance analyses have been carried out on LiMn2O4, Li0.95Mn2O4, and LiMn1.95B0.05O4 (B=Al3+ or Ga3+) together with the measurements of four-probe dc conductivities and dielectric relaxation processes, because these are two candidates for the carrier, a Li ion or a nonadiabatic small polaron of an eg electron on Mn3+. The comparison of the ionic conductivity estimated numerically from the parameters obtained experimentally for the Li-diffusion in LiMn2O4 with the bulk conductivity indicates that the Li-diffusion seems difficult to play the primary role in the electrical conduction. Instead, a hopping-process of nonadiabatic small polarons of eg electrons is likely to dominate predominantly the electrical transport properties. The dielectric relaxation process, and the activation ener...

The origin of the change in type of the majority carrier in

Electrical transport properties of polycrystalline ceramic specimens of the system (x = 0.05, 0.10, and 0.15) have been investigated as functions of temperature by means of complex-plane impedance analysis, dielectric properties, four-probe dc conductivity, Seebeck coefficients, and magnetic susceptibilities. The type of the majority carrier changes from electrons to holes when x increases from 0.10 to 0.15, because the Seebeck coefficient changes from negative to positive with x increasing from 0.10 to 0.15. The complex-plane impedance analysis distinguishes the bulk conduction from the conduction across grain boundaries. In a specimen with x = 0.15, the activation energy in the bulk conduction is nearly equal to that for the dielectric relaxation. This implies that the hopping process of small polarons of holes dominates the transport properties in this specimen above 190 K. The temperature dependencies of the magnetic susceptibilities indicate that the number of high-spin ions (S = 2) decreases with increasing x. The expansion of the lattice parameters and the increment of the ratio with increasing x suggest that ions are preferentially substituted for low-spin , which relaxes the rhombohedral structural distortion in the system. The change in the type of the charge carrier has been discussed in terms of the electronic structures deduced from these experimental results.

Electrical transport in below 60 K

The electrical transport properties of have been investigated using complex-plane impedance analysis and dielectric measurements with measurements of four-probe dc conductivities and magnetic susceptibilities. The temperature dependencies of the bulk conductivities obtained by impedance analysis, and dielectric relaxation processes provide evidence of the hopping conduction below 60 K, which is remarkably different from the hopping conduction in , despite the degree of doping with Sr being very small. The features in the low-doping range are: (i) the emergence of the hopping conduction at markedly lower temperatures in comparison with the case for non-doped ; (ii) the abrupt reduction of the activation energy required for the conduction, 0.042 eV at x = 0.03 to 0.021 eV at x = 0.07, from 0.34 eV for ; and (iii) a progressive decrease in the activation energy with increasing x. The difference in conduction behaviour between and is due to the alternation of the spin state of caused by the Sr doping. The results have been discussed in terms of a process of hopping of small polarons localized by the electron-lattice interaction in tandem with the electron-magnon interaction.

Electronic structures and magnetic properties in Sr1-xLaxRuO3 (0.0 ≤ x ≤ 0.5)

In order to find out more about the suppression of ferromagnetic (FM) interactions in Sr1-xLaxRuO3, electronic structures and magnetic properties have been investigated upon changing x from 0.0 to 0.5, using an XRD method with Rietveld analysis, a SQUID magnetometer and a DV-Xα computational method. In comparison with magnetic properties in Sr1-xCaxRuO3, FM interactions in Sr1-xLaxRuO3 are found to be suppressed very rapidly against x. Neither structural distortion nor cation-size disorder can account for such rapid suppression. Instead, this may be attributed to the effect of La-O hybridization created by La substitution for Sr. This hybridization effect weakens the FM order around Ru ions and, as a result, the long-range FM states are suppressed even if x is small. The DV-Xα cluster method was employed to estimate the energy difference between the up and down spin density of states in SrRuO3 and Sr0.5La0.5RuO3. This calculation predicts that Sr1-xLaxRuO3 contains La-O hybridization which suppresses FM interaction even at small x.

Small polarons in La2/3TiO3−δ

The thermoelectric power and dc conductivity of La2/3TiO3−δ (δ=0.057, 0.07, and 0.16) were investigated. The thermoelectric power is negative between 80 and 350 K. The measured thermoelectric power of La2/3TiO3−δ increases linearly with temperature, in agreement with the model proposed by C. Wood and D. Emin [Phys. Rev. B 29, 4582 (1984)], and represented by A+BT. This temperature dependence indicates that the charge carrier in this material is a small polaron. There exists a linear relation between log(σT) and 1/T in the range of 200–300 K, the activation energies for small polaron hopping were 0.15, 0.21, and 0.24 eV for δ=0.16, 0.07, and 0.057, respectively. These properties are discussed in terms of a hopping process involving small polarons. This conclusion is confirmed theoretically. Based upon polaron energies obtained experimentally, several parameters relevant to small polaron transport in this material are estimated.The thermoelectric power and dc conductivity of La2/3TiO3−δ (δ=0.057, 0.07, and 0.16) were investigated. The thermoelectric power is negative between 80 and 350 K. The measured thermoelectric power of La2/3TiO3−δ increases linearly with temperature, in agreement with the model proposed by C. Wood and D. Emin [Phys. Rev. B 29, 4582 (1984)], and represented by A+BT. This temperature dependence indicates that the charge carrier in this material is a small polaron. There exists a linear relation between log(σT) and 1/T in the range of 200–300 K, the activation energies for small polaron hopping were 0.15, 0.21, and 0.24 eV for δ=0.16, 0.07, and 0.057, respectively. These properties are discussed in terms of a hopping process involving small polarons. This conclusion is confirmed theoretically. Based upon polaron energies obtained experimentally, several parameters relevant to small polaron transport in this material are estimated.

Correlation between hopping conduction and transferred exchange interaction in La2NiO4+δ below 300 K

In order to investigate the correlation between electrical transport properties and the transferred exchange interaction between Ni2+ and La3+ in La2NiO4+δ, bulk conductivities and dielectric properties of La2NiO4.02 and La2NiO4.125 have been measured as a function of temperature up to 300 K. The complex-plane impedance analyses determined bulk conductivities. Temperature dependencies of bulk conductivities, the self-consistent agreement of the energies obtained in the bulk conduction and dielectric relaxation processes, and the electron transferred integrals determined in dielectric measurements are indicative of the nonadiabatic hopping conduction, although the conductivity in La2NiO4.125 is very high with a very low hopping energy in comparison with La2NiO4.02. These results have been discussed in terms of the transferred exchange interaction between Ni2+3dz2 orbital and La3+6s orbital through 2pz orbital of apical O2−. With increasing the content of excess oxygen, the decrease in the number of Ni2+3dz2 orbitals, which play an important role in the transferred exchange interaction, changes remarkably the conduction behaviour in La2NiO4.125 from La2NiO4.02.

Thermoelectric Properties of Single-Crystalline SiC and Dense Sintered SiC for Self-Cooling Devices

We investigated the thermoelectric properties of 4H-SiC substrates and dense sintered SiC, which are expected to be candidate materials for use in self-cooling devices because of their high Seebeck coefficient, low electrical resistivity, and high thermal conductivity. The carrier concentration of 4H-SiC samples doped with nitrogen is in the range of 1016 to 1019 cm-3. The sintered SiC samples of the α-type and β-type contain less than 1000 ppm of cation impurities and have a relative density higher than 98% with respect to single-crystalline SiC. 4H-SiC with a carrier concentration of 1019 cm-3 has the highest power factor of 2.7 ×10-3 WK-2m-1 and a high thermal conductivity of 260 WK-1m-1 at room temperature. One-dimensional calculations for heat distribution indicate that a Si chip in a self-cooling device, which consists of 4H-SiC with a carrier concentration of 1019 cm-3, could be refrigerated more strongly than one on a copper plate under specific operating conditions.

Thermoelectric properties in Bi2-xPbxSr3-yYyCo2O9-δ ceramics

Thermoelectric (TE) properties (resistivity and thermopower) of polycrystalline ceramics in the Bi2-xPbxSr3-yYyCo2O9-δ system prepared by a solid-state reaction in air have been investigated as a function of temperature up to about 300 K, changing the combination of x and y. These oxides are chemically very stable, and TE properties and the lattice structures do not change even after they are heated at high temperatures. Y doping has a profound effect in reducing resistivity. When y (the amount of Y) is small, ligand holes created by O 2p-Co 3d hybridization are mainly responsible for insulating conduction. With increasing y, resistivity decreases rapidly and insulating conduction transfers to metallic conduction. This is due to the shrinkage in the lattice constant by the Y doping, which leads to band crossing of the O 2p and Co 3d levels. Measurements of TE properties including thermal conductivity have been carried out on the material with x = y = 0.5 in the high-temperature range up to 800 K. The TE figure of merit increases from 2×10-5 K-1 to 7×10-5 K-1 as the temperature increases from 400 K to 800 K, which indicates that this compound has high potential as a good TE material at high temperatures.

The NiO(001) surface structure calculated by a two-dimensional polarizable point-ion shell model

Abstract Using a two-dimensional polarizable point-ion shell model and the effective plane-wise summation technique for evaluations of two-dimensional Madelung potentials, the NiO (001) surface structure, in which a two-dimensional translation operation is possible, has been calculated. The calculations result in a rumpling of the surface layer; anions at the top layer move outwards along the direction normal to (001) surface relative to cations. The general features obtained by EELFS and LEED experiments are interpretable in terms of the results calculated, though there are some deviations from the experimental results. Such deviations are accounted for by the electronic polarizability of O 2− ion on the top layer, which is expected to be larger than bulk value 1.98 A 3 .

Electric Current Dependence of a Self-Cooling Device Consisting of Silicon Wafers Connected to a Power MOSFET

A self-cooling device has been developed by combining a commercial n-channel power metal–oxide–semiconductor field-effect transistor (MOSFET) and single-crystalline Sb-doped n-type or B-doped p-type silicon wafers in order to improve the heat removal or cooling quantitatively. The electric current dependence of the temperature distribution in the self-cooling device and the voltage between the source and drain electrodes have been measured to estimate the Peltier heat flux. We found that the average temperature is decreased for a power MOSFET in which an electric current of 50 A flows. In particular, the average temperature of the power MOSFET was decreased by 2.7°C with the n-type Si wafer and by 3.5°C with the p-type Si wafer, although an electric current of 40 A makes little difference. This certainly warrants further work with improved measurement conditions. Nonetheless, the results strongly indicate that such n-type or p-type silicon wafers are candidate materials for use in self-cooling devices.