First-Principles Study of the Electronic Structure and Thermoelectric Properties of Al-Doped ZnO

ZnO materials doped with elements such as Al, Ga, etc. are of great interest for high-temperature thermoelectric applications. In this work, the effects of Al doping on the electronic structure and thermoelectric properties of the ZnO system are presented. The energy band structure and density of states of Al-doped ZnO were investigated using the projector-augmented plane wave pseudopotential method within the local density approximation. The calculated energy band structure was then used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of Al-doped ZnO. The electronic structure calculation showed that the position of the Fermi level of the doped sample was shifted to a higher energy level compared with the undoped material. The conduction band near the Fermi energy was a combination of hybridized Zn sp-orbitals and Al s-orbital. The calculated thermoelectric properties were compared with the experimental results, showing some agreement. For the Al-doped ZnO system, the Seebeck coefficient was shown to be negative and its absolute value increased with temperature. The electrical conductivity and electronic thermal conductivity followed the trend of the experimental results.     

Thermoelectric Properties of Ca3Co4-x Ga x O9+δ Prepared by Thermal Hydro-decomposition

The polycrystalline samples of Ca₃Co_4-x Ga _x O_9+δ (0 ≤ x ≤ 0.15) were prepared by a simple thermal hydro-decomposition method. The high density ceramics were fabricated using a spark plasma sintering technique. The crystal structure of calcined powders was characterized by x-ray diffraction. The single phase of Ca₃Co_4-x Ga _x O_9+δ was obtained. The scanning electron micrograph illustrated the grain alignment perpendicular to the direction of the pressure in the sintering process. The evidence from x-ray absorption near edge spectra were used to confirm the oxidation state of the Ga dopant. The thermoelectric properties of the misfit-layered of Ca₃Co_4-x Ga _x O_9+δ were investigated. Seebeck coefficient tended to decrease with increasing Ga content due to the hole-doping effect. The electrical resistivity and thermal conductivity were monotonically decreased with increasing Ga content. The Ga doping of x = 0.15 showed the highest power factor of 3.99 × 10^-4 W/mK² at 1,023 K and the lowest thermal conductivity of 1.45 W/mK at 1,073 K. This resulted in the highest ZT of 0.29 at 1,073 K. From the optical absorption spectra, the electronic structure near the Fermi level show no significant change with Ga doping.     

Phase Formation,Dielectric, Ferroelectric and Magnetic Properties of Cr2O3 Doped (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 Ceramics

Abstract

In this paper, we studied the effect of Cr₂O₃ doping (0–0.8?wt%) on the phase formation, dielectric, ferroelectric and magnetic properties of (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ (BCTZ) ceramics prepared by the solid state combustion method. All samples were prepared with a calcined and sintered temperature of 1050?°C and 1450?°C, respectively, for 2?h. The results of the XRD patterns showed the coexistence between the tetragonal (T) and orthorhombic (O) phases for all samples, and the tetragonal phase increased with increasing amounts of Cr₂O₃. The dielectric constant decreased when Cr₂O₃ increased. The P-E hysteresis loops of the BCTZ ceramics doped with Cr₂O₃ between 0 and 0.2?wt% showed slim and saturated loops. By increasing Cr₂O₃ doping from 0.4 to 0.8?wt%, the P-E loops were unsaturated and a leakage current was produced. The undoped BCTZ ceramics exhibited diamagnetic behavior. The sample with 0.2?wt% Cr₂O₃ showed ferromagnetic behavior. Increasing Cr₂O₃ doping from 0.4 to 0.8?wt%, caused the ceramics to exhibit paramagnetic behavior. Doping with Cr₂O₃ led to improper ratios between the tetragonal and orthorhombic phases, decreased density and increased porosity which caused a decrease in the electric properties.     

Thermoelectric properties of Al-doped ZnO: experiment and simulation

Advancement in doping other elements, such as Ce, Dy, Ni, Sb, In and Ga in ZnO^[1], have stimulated great interest for high-temperature thermoelectric application. In this work, the effects of Al-doping in a ZnO system on the electronic structure and thermoelectric properties are presented, by experiment and calculation.Nanosized powders of Zn_1-xAl_xO (x=0, 0.01, 0.02, 0.03 and 0.06) were synthesized by hydrothermal method. From XRD results, all samples contain ZnO as the main phase and ZnAl₂O₄ (spinel phase) peaks were visible when Al additive concentrations were just 6 at%. The shape of the samples changed and the particle size decreased with increasing Al concentration. Seebeck coefficients, on the other hand, did not vary significantly. They were negative and the absolute values increased with temperature. However, the electrical resistivity decreased significantly for higher Al content.The electronic structure calculations were carried out using the open-source software package ABINIT^[2], which is based on DFT. The energy band gap, density of states of Al-doped ZnO were investigated using PAW pseudopotential method within the LDA+U. The calculated density of states was then used in combination with the Boltzmann transport equation^[3] to calculate the thermoelectric parameters of Al-doped ZnO. The electronic band structures showed that the position of the Fermi level of the doped sample was shifted upwards in comparison to the undoped one. After doping Al in ZnO, the energy band gap was decreased, Seebeck coefficient and electrical conductivity were increased.Finally, the calculated results were compared with the experimental results. The good agreement of thermoelectric properties between the calculation and the experimental results were obtained.     

Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

ABSTRACT: Ferromagnetism was observed at room-temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3.6H2O using polyvinylpyrrolidone (PVP) as a surfactant. The structure and morphology of the products were characterized by; X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using ultraviolet and visible spectroscopy (UV-Vis) and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy (XANES). The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of ~ 9-19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of ~ 100-250 nm. The samples also showed a strong UV absorption and room temperature photoluminescence. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied by Vibrating Sample Magnetometer (VSM). The samples exhibited room temperature ferromagnetism (RT-FM) with a small magnetization of ~ 0.0026-0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange and the possible mechanism of formation was discussed based on the experimental results.     

Fabrication and Magnetic Properties of Electrospun La0.7Sr0.3MnO3 Nanostructures

La_0.7Sr_0.3MnO₃ (abbreviated as LSMO) nanostructures were fabricated by a simple electrospinning using a solution that contained poly(vinylpyrrolidone) (PVP), lanthanum, strontium and manganese nitrates. The LSMO nanostructures were successfully obtained from calcination of the as-spun LSMO/PVP composite nanofibers at 500–900 °C in air for 7 h. The as-spun and calcined LSMO/PVP composite nanofibers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Analysis of phase composition by XRD revealed that all the calcined samples have a single rhombohedral LSMO phase. The SEM results showed that the crystal structure and morphology of the LSMO nanofibers were affected by the calcination temperature. Crystallite size of the nanoparticles contained in nanofibers increased with an increase in calcination temperature. The specific saturation magnetization (M _s ) values were obtained to be 1.23, 28.61, and 40.52 emu/g at 10 kOe for the LSMO samples calcined respectively at 500, 700, and 900 °C. It is found that the increase of the tendency of M _s is consistent with the enhancement of crystallinity, and the values of M _s for the calcined LSMO samples were observed to increase with increasing crystallite size. This increase in M _s for the calcined LSMO samples with increasing crystallite size may be explained by considering a magnetic domain of the samples.     

Structure and Magnetic Properties of Monodisperse Fe3+-doped CeO2 Nanospheres

Nano-Micro Letters - This work reports the study concerning the structure and magnetic properties of undoped CeO2 and Fe-doped CeO2 (Ce1−xFe x O2, 0.01 ≤ x ≤ 0.07) nanospheres...     

Thermoelectric properties of transition metals-doped Ca<Subscript>3</Subscript>Co<Subscript>3.8</Subscript>M<Subscript>0.2</Subscript>O<Subscript>9+δ</Subscript> (M = Co,Cr, Fe,Ni, Cu and Zn)

The thermoelectric properties of the Ca₃Co₄O_9+δ and the transition metals-doped Ca₃Co_3.8M_0.2O_9+δ (where M = Cr, Fe, Ni, Cu and Zn) ceramics were reported. Ca₃Co₄O_9+δ single phase was checked by using X-ray diffraction analysis performed for the Ca₃Co_3.8M_0.2O_9+δ samples. The scanning electron micrographs showed some degrees of grains alignment in the compacted direction. The resistivity of the samples measured from 100 up to 700 °C varies in magnitude for different transition metals substitution. The variation of resistivity was explained by a change of carrier concentration induced by the doped ions. The thermopower increased with increasing temperature but showed no obvious change for any transition metals doping. The thermal conductivities changed for the doped samples but were relatively independent of temperature. The ZT was calculated to be the highest for the Fe substitution for the whole measurement temperature with the maximum value of 0.12 at 700 °C.     

AIDS education and intervention trials among youths in factories: a pilot project

The project aimed to conduct a pilot study and intervention trials among youths in a factory of Khon Kaen. After contacting and obtaining agreement from owners/managers of factories, a survey using a self administered questionnaire, in-depth interview and focus group with workers to determine their level of knowledge and awareness of AIDS and high risk behaviour. A series of in-depth interviews with 16 workers and group discussion with 8 groups were conducted to find out their possible motivation for prevention and their acceptance of interventions/media. The data was used as a baseline for evaluating change after interventions and to modify the intervention education strategies and content. The study showed that the groups of factory workers which were not involved in the AIDS prevention had a different level of knowledge, attitudes, and behavior related to AIDS prevention than the groups which received the intervention and the methods used in the intervention achieved a level of success. The information we collected also showed that the best kinds of media for this purpose were videos and informational cartoons, which were also of special interest to the study group. It is hoped that the models will be adopted by relevant government and non-government agencies to be used in factories throughout the country.     

The formation mechanism of aluminium oxide tunnel barriers

The functional properties of magnetic tunnel junctions are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates the two ferromagnetic layers. Three-dimensional atom probe analysis has been used to study the chemistry of a magnetic tunnel junction structure comprising an aluminium oxide barrier formed by in situ oxidation, both in the under-oxidised and fully oxidised states and before and after annealing. Low oxidation times result in discrete oxide islands. Further oxidation leads to a more continuous, but still non-stoichiometric, barrier with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe layer. Post-deposition annealing leads to an increase in the barrier area, but only in the case of the fully oxidised and annealed structure is a continuous planar layer formed, which is close to the stoichiometric Al:O ratio of 2:3. These results are surprising, in that the planar layers are usually considered unstable with respect to breaking up into separate islands. Analysis of the various driving forces suggests that the formation of a continuous layer requires a combination of factors, including the strain energy resulting from the expansion of the oxide during internal oxidation on annealing.