Bong-Seo Kim

Mn-doped V2VI3 semiconductors: Single crystal growth and magnetic properties

We have grown Mn-doped V2VI3 single crystals using the temperature gradient solidification method. We report on the structural and magnetic properties of Mn-doped Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 compound semiconductors. The lattice constants of several percent Mn-doped V2VI3 were slightly smaller than those of the undoped V2VI3 due to the smaller Mn atomic radius (1.40A) than those of Bi (1.60A) and Sb (1.45A). Mn-doped Bi2Te3 and Sb2Te3 had ferromagnetic ordering at TC=10 and 17K, respectively. However, Mn-doped Bi2Se3 and Sb2Se3 showed spin glass and paramagnetic properties, respectively.

Thermoelectric properties of non-stoichiometric MnTe compounds

Non-stoichiometric MnxTe1−x (x = 0.48–0.52) has been prepared by a melt-quench process followed by spark-plasma-sintering to investigate its thermoelectric properties. Polycrystalline MnxTe1−x with x > 0.51 has a nearly single MnTe phase. The measured Seebeck coefficient and electrical conductivity show a similar trend, in which a transition occurs near 473 K with increasing temperature. The thermal conductivity of MnxTe1−x compounds shows a tendency to decrease with increasing Mn content. Along with a low thermal conductivity and a high power factor, the samples with x > 0.51 have a high figure of merit, which reaches 0.41 at 773 K. The results indicate that production of a homogenous MnTe single phase is an effective way to improve the thermoelectric properties of p-type non-stoichiometric MnxTe1−x compounds.

Prediction of the band structures of Bi2Te3-related binary and Sb/Se-doped ternary thermoelectric materials

Density functional calculations are performed to study the band structures of Bi2Te3-related binary (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and Sb/Se-doped ternary compounds [(Bi1−xSbx)2Te3 and Bi2(Te1−ySey)3]. The band gap was found to be increased by Sb doping and to be monotonically increased by Se doping. In ternary compounds, the change in the conduction band structure is more significant as compared to the change in the valence band structure. The band degeneracy of the valence band maximum is maintained at 6 in binaries and ternaries. However, when going from Bi2Te3 to Sb2Te3 (Bi2Se3), the degeneracy of the conduction band minimum is reduced from 6 to 2(1). Based on the results for the band structures, we suggest suitable stoichiometries of ternary compounds for high thermoelectric performance.

Magnetic properties of very high permeability, low coercivity, and high electrical resistivity in Fe87Zr7B5Ag1 amorphous alloy

The magnetic properties of an Fe87Zr7B5Ag1 (at. %) amorphous alloy, which contains an additional insoluble element (Ag) in a small amount, have been investigated as a function of annealing temperatures in order to know its potential applicability as a core material used at high‐frequency. As a result, a new excellent soft magnetic material with very high resistivity was developed. The amorphous alloy annealed at a relatively low temperature of Ta=300 °C exhibited a very high initial permeability μi of 146 000 at 1 kHz and 2 mOe, resistivity ρRT of 4.5 μΩ m, and very low coercivity Hc of 20 mOe, respectively. The values obtained are the best ones among various kinds of Fe‐based soft amorphous materials reported up to now. Furthermore, the amorphous ribbon heat treated at Ta=300 °C still retained good ductility enough to endure 180° bending, which is very important from the viewpoint of processing for mass production. The phenomenon of good soft magnetic properties presumably arises from the homogenous form...

Formation mechanism of C/SiC/C multi-layers during self-propagating high-temperature synthesis

Abstract In order to study the formation mechanism of a silicon carbide layer during the self-propagating high-temperature synthesis (SHS) reaction between graphite and silicon layers, multi-layers of carbon–silicon carbide–carbon on UO 2 pellets were prepared and analyzed. The carbon and silicon were deposited by thermal decomposition of propane in a chemical vapor deposition (CVD) unit and a microwave-pulsed electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) unit using silane and propane gases, respectively. The activation energy for the formation of silicon carbide determined by differential scanning calorimetry (DSC) depended on the initial carbon precursors. The final product layer was a fine, crystalline beta-silicon carbide. The numerically estimated value of the combustion limit was 0.06 at an initial temperature of 1200 °C, which supported the finding that pre-heating above the temperature was required for the formation of the silicon carbide. From thermal analysis and microstructure observations, the formation mechanism of the silicon carbide layer included carbon diffusion at the interface between liquid silicon and silicon carbide.

Enhancement of thermoelectric properties of Mg2Si compounds with Bi doping through carrier concentration tuning

AbstractThe Bi-doped Mg2Si powder was fabricated with solid state reaction method and consolidated with hot pressing method and then its thermoelectric properties were investigated. The n-type transport properties were measured in all samples and temperature dependence of the electrical properties shows a behavior of degenerate semiconductors for Bi-doped samples. The electrical resistivity and the Seebeck coefficient were greatly reduced with Bi, which was mainly due to the increment of the carrier concentration. The samples have maximum carrier concentration of 8.2 × 1018 cm−3. The largest ZT value of 0.61 was achieve at 873 K for Mg2.04SiBi0.02. The Bi-doping was found to be an effective n-type dopant to adjust carrier concentration.

Kinetics of Fe-30% Ni-12.5% Co invar alloy during high temperature oxidation

The oxidation behavior of Fe-30% Ni-12.5%Co Invar alloy possessing low thermal expansion-high strength has been studied by exposing it in temperature ranges of 1000–1200 in an air atmosphere. The formed oxide scale consisted of two layers, an outer layer and an inner layer, and the oxidation mechanism showed uniform growth for all oxidation conditions. The growth rate of the scale had a parabolic relationship with oxidation time, and the estimated activation energy for the growth of the whole oxide layer was approximately 19.84 kcal/mol. The outer scale consisted of five oxide layers, whose outermost scale consisted of major phase CoFe2O4 containing a particulate Fe2O3 phase. The oxide scale of Fe-30 Ni-12.5% Co had different compositions and phases from the Fe-30%Ni alloy investigated in previous studies. Especaally, when the alloy was exposed for a longer oxidation time and at a higher temperature, the volume fraction ratio of CoFe2O4 to Fe2O3 was found to increase

Enhanced thermoelectric properties and development of nanotwins in Na-doped Bi0.5Sb1.5Te3 alloy

We found that Na is a good source to develop twin structures in the Bi-Te system, such as Ag as noted in a previous study. The twin boundaries had a considerable influence on reductions of the lattice thermal conductivity due to phonon scattering by the nano-ordered layers and on reductions of the electrical resistivity owing to the defects generated by the substitution of Na into the cation sites. Here, we report the enhanced thermoelectric properties of a Na-doped p-type Bi0.5Sb1.5Te3 alloy. Measurements show that the electrical resistivity and the Seebeck coefficient decrease with Na doping due to an increase in the free carrier (hole) concentration and that the lattice thermal conductivity decreases with Na doping. The achieved maximum ZT value was 1.20 at 423 K, which is approximately 20% higher than that of Bi0.5Sb1.5Te3 under the same fabrication conditions. These results were achievable by controlling the morphology of the twin structure and the carrier concentration by means of Na doping.

Defects responsible for abnormal n-type conductivity in Ag-excess doped PbTe thermoelectrics

Density functional calculations have been performed to investigate the role of Ag defects in PbTe thermoelectric materials. Ag-defects can be either donor, acceptor, or isovalent neutral defect. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect at Pb-site is formed and the environment changes to the Pb-rich/Te-poor condition. Under Pb-rich condition, the ionized Ag-interstitial defect (Ag I +) becomes the major donor. The formation energy of Ag I + is smaller than other native and Ag-related defects. Also it is found that Ag I + is an effective dopant. There is no additional impurity state near the band gap and the conduction band minimum. The charge state of Ag I + defect is maintained even when the Fermi level is located above the conduction band minimum. The diffusion constant of Ag I + is calculated based on the temperature dependent Fermi level, formation energy, and migration energy. When T > 550 K, the diffusion length of Ag within a few minutes is comparable to the grain size of the polycrystalline PbTe, implying that Ag is dissolved into PbTe and this donor defect is distributed over the whole lattice in Ag-excess doped polycrystalline PbTe. The predicted solubility of Ag I + well explains the increased electron carrier concentration and electrical conductivity reported in Ag-excess doped polycrystalline PbTe at T = 450–750 K [Pei et al., Adv. Energy Mater. 1, 291 (2011)]. In addition, we suggest that this abnormal doping behavior is also found for Au-doped PbTe.

Single Crystal Growth and Magnetic Properties of Mn-doped Bi 2 Se 3 and Sb 2 Se 3

We have grown Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals using the temperature gradient solidification method. We report on the structural and magnetic propertis of Mn-doped Bi₂Se₃ and Sb₂Se₃ compound semiconductors. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the un-doped samples due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin glass and paramagnetic properties, respectively.