Tadashi Sasamoto

Electronic conductivity, Seebeck coefficient, defect and electronic structure of nonstoichiometric La1-xSrxMnO3

Abstract In order to elucidate the relationship between the electrical properties and composition ( d and x ) of La 1− x Sr x MnO 3+ d , precise measurements were made on the conductivity, σ , and Seebeck coefficient, Q , for the oxide with 0≤ x ≤0.7 as a function of T and P (O 2 ) up to 1273 K. Analysis was made for the high-temperature paramagnetic state using the nonstoichiometry data and defect and electronic structure models reported by the present authors. It was shown that σ and Q in the oxygen excess La 1− x Sr x MnO 3+ d ( d >0) are fixed to the value at those of the stoichiometric oxygen content, d =0. In the oxygen deficient La 1− x Sr x MnO 3+ d , they are essentially determined by the mean Mn valence and temperature. The predominant electrical conduction was found to take place by the electron hopping on the e g ↑ level of Mn. In La 1− x Sr x MnO 3+ d ( d ≤0) under the condition of z = x +2 d ≤1/3, σ is given by: σ=(2.8×10 6 /T){2 (−z 2 −z+6)(6−18z)/(17−z) 2 +(−z 2 −z+6) (z 2 +18z+5)/(17−z) 2 } exp {(−Ea/(kT)} where the activation energy Ea=−0.59(3+z)+2.00 eV . For z ≥1/3, it is given by: σ=(2.8×10 6 /T) z(1−z) exp {(−Ea/(kT)} where Ea=−0.036(3+z)+0.16 eV . Q is also described essentially by this model. However, the effect of minority carrier conduction is clearly found in Q in addition to the major conduction on e g ↑ level. The major carrier conduction is p-type and the minor carrier is n-type for z ≤0.5 and vice versa for z ≥0.5.

Nonstoichiometry and thermochemical stability of the perovskite-type La1−xSrxMnO3−δ

In order to make clear the extent of nonstoichiometry and decomposition P(O2) of the perovskite-type La1-xSrxM nO3−δ, measurements are made by high temperature gravimetry, coulometric titration and iodometry on the oxides with x=0.2 and 0.4. The decomposition P(O2) was found to increase with increasing x. The decomposition products for x=0.4 were (La0.6Sr0.4 )2MnO4 and MnO, and mixtures of (La1−xSrx)2MnO4, MnO, and La2O3 for x=0.2. For x=0.4, the plot of 3−δ versus log P(O2) showed plateaus at °=0 and 0.0225, while no plateau was observed corresponding to the Mn3+ state at °=x2(=0.2). By calculation for oxygen partial molar enthalpy and entropy, it was found that oxygen vacancies distribute randomly on the oxygen sublattice and electronic state is essentially metallic for La0.6Sr0.4MnO3−° with °>0.0225. In the region of °< 0.0225, oxygen vacancies seemed to distribute on limited sites. However, details of the defect equilibrium for this region was not clarified.

Change in antibacterial characteristics with doping amount of ZnO in MgO–ZnO solid solution

Abstract Antibacterial activity for MgO–ZnO solid solution was studied by measuring the change in electrical conductivity with bacterial growth. MgO–ZnO solid solution powders were prepared by heating at 1400°C for 3 h in air. A single phase with cubic type structure was obtained in the weight ratio range (MgO/ZnO) of 4.0 and 1.5, but the ratio of 0.67 resulted in a ZnO phase in addition to solid solution. After milling the solid solution powders by planetary ball mill, the average particle size and the specific surface area of these powders became 0.1 μm and 26 m 2 /g, respectively, which were used in the test of antibacterial activity. From the results of antibacterial tests, the activity increased with increasing the powder concentration in the medium. With increasing the doping amount of ZnO in MgO–ZnO solid solution, it was found to show a decrease in the antibacterial activity against Escherichia coli and Staphylococcus aureus . The pH value in physiological saline at the powder concentration of 2.5 mg/ml showed the alkali region above 10.0, and decreased with the increase of ZnO amount in solid solution. The decrease in antibacterial activity, therefore, was associated with the decrease of pH value in medium

Adsorption and growth inhibition of bacteria on carbon materials containing zinc oxide

An ion-exchange resin with particle size of 0.5 mm was treated for 24 h by either an aqueous solution of [Zn(NH3)4]2+ complex or 29% aqueous ammonia. The resins treated were carbonized for 10 min in nitrogen gas at 500, 700 and 1000°C to prepare carbons with and without ZnO. The adsorption and the growth inhibition of bacteria on their carbon samples were studied. The adsorbed amount of bacteria on samples without ZnO increased with the increase of carbonization temperature and the amount of carbon powder. The adsorbed amount of Staphylococcus aureus was smaller than that of Escherichia coli. However, it was found that all bacteria in the suspension were adsorbed onto the samples containing ZnO, irrespective of the carbonization temperature and the amount of carbon powder. The antibacterial activity on the carbon samples containing ZnO increased with the increase of the amount of ZnO in samples and decreased with the increase of carbonization temperature. The antibacterial activity for Staphylococcus aureus was found to be stronger than that for Escherichia coli. No activity of the carbon samples without ZnO was observed. The occurrence of antibacterial activity was supposed to be due to the generation of hydrogen peroxide from ZnO in carbon samples.

Preparation and Electrical Properties of Sintered Bodies Composed of Monophase Spinel Mn(2-X)Co2XNi(1-X)O4 ( 0 \LeqX \Leq 1) Derived from Rock-Salt-Type Oxides

Preparation of spinel-type oxides, Mn(2-X )Co2X Ni(1-X )O4 (0X1), and their electrical properties were investigated. Die-pressed oxides (25 mm diam. ×4 mm), containing metals with desired molar ratios, were heated to 1400° C, and held at that temperature for 3 h in nitrogen. The sintered specimens were quenched to 800° C and then oxidized for 95 h in air to completely convert them into spinel-type oxides. The lattice constants of the spinels decreased with increasing X. Two turning points, at X=0.5 and 0.84, were observed. Based on these results, the cation distributions were estimated. Calculation of the Seebeck coefficient proved that the carriers of the oxides with X=0 and 0.25 are electrons and those of the oxides with X=0.5 to 1 are holes. Activation energies calculated from mobility were 0.34 to 0.36 eV. The conduction was considered to be due to the small polaron hopping mechanism.

Antioxidation of carbon-carbon composites by SiC concentration gradient and zircon overcoating

Abstract After the formation of a concentration gradient of SiC within a carbon-carbon composite, a zircon (ZrSiO4) thin film was coated on the surface. Oxidation-resistance tests of the treated CC composites were then carried out. A concentration gradient of SiC was produced by heating the CC composite in silicon powder at 1450 °C for 3 hours. The concentration of SiC along the direction of the carbon fiber sheets decreased rapidly up to 0.25 mm and, in the perpendicular direction, decreased steeply in a range of 0.4–0.6 mm. By the overcoating of zircon with the thickness of 1.5 μm, a remarkable improvement in the oxidation resistance was observed; the weight loss was almost negligible at 1000 °C and one hundredth of the original composite at 1400 °C.

Preparation of crystalline CdSe particles by chemical bath deposition

Crystalline CdSe particles were prepared by keeping the precursor solutions at temperatures above 60 °C. It was essential to use sodium sulfite as a stabilizing agent for selenium ions and sodium dicarboxylate as a complexing agent for cadmium in the precursor solution. The principal crystalline phase of the samples obtained at 60 °C was a cubic zincblende-type phase, but those prepared at 80 °C coexisted with a hexagonal wurtzite-type phase. The ratio of cadmium to selenium in the samples decreased with an increase of the concentration of selenourea in the precursor solutions, irrespective of the kind of complex agents and the keeping time of the precursor solutions. The band-gap energy of CdSe with an atomic ratio (Cd/Se) of 1 showed a value of 1.74 eV, but that with the ratio of 2.3 gave a slightly smaller value of 1.42 eV.

Indium tin oxide thin films prepared by thermal decomposition of ethylene glycol solution

Indium tin oxide (ITO) films were prepared on a glass substrate by thermal decomposition of ethylene glycol (EG) solutions which were prepared by dissolving indium and tin chlorides into ethylene glycol and then heating the solution at 200 °C. Homogeneous ITO film with a thickness of 1 μm without any cracks was obtained from the EG solution with a viscosity of about 50 cSt by spin coating, with the rotation of the substrate at 2000 rpm. The crystallization of the film thus obtained occurred above 400 °C. A well-crystallized ITO film was obtained by heat-treating at 600 °C. The resistivity of the ITO film obtained on a glass substrate showed a minimum value of 1.1 × 10 −4 ohm cm at about 7 at.% Sn.

Preparation and oxidation of Al4SiC4

Single-phase Al 4 SiC 4 was successfully synthesized by heating the powder mixture of aluminum, silicon, and carbon black in the molar ratio corresponding to the chemical composition of Al 4 SiC 4 with a small amount of triethanolamine (TEA) above 1200 °C in argon gas. Without TEA, two phases of Al 4 C 3 and SiC were formed, irrespective of the heating temperature. The marked mass gain by oxidation of Al 4 SiC 4 was observed with forming Al 2 O 3 and SiO 2 in the range of 850 to 1150 °C, which slightly increased above 1150 °C. At 16(10 °C, mullite was formed by the reaction between Al 2 O 3 and SiO 2 .

Preparation of carbon material with SiC-concentration gradient by silicon impregnation and its oxidation behaviour

Abstract A carbon material with a SiC-concentration gradient was prepared by a silicon impregnation process. In order to prepare the carbon material with a SiC-concentration gradient without any adhesion of metallic silicon, the weight of silicon powder per physical surface area of the carbon substrate was in the range of 3·9-4·1 g cm −2 and then heating the carbon substrate in silicon powder at 1450°C for 3 h, but the amount of silicon powder beyond this range resulted in the adhesion of metallic silicon. The concentration of SiC along the thickness in the sample obtained decreased rapidly up to 0·5 mm and gradually in a range of 0·5-1·0 mm. A remarkable improvement in the oxidation resistance was observed on the sample with a SiC-concentration gradient at 1400°C, which was due to the formation of a protective film of SiO 2 glass on the surface. In the results of the thermal shock test in the sample, no destructions and no cracking of the samples were observed.