Chang Sun Park

The oxygen-deficiency-dependent Seebeck coefficient and electrical properties of mesoporous La0.7Sr0.3MnO3−x films

The thermoelectric power factor of La0.7Sr0.3MnO3 (LSMO) is closely related to its oxygen-deficient nature. In this study, the oxygen content of mesoporous LSMO films was controlled using various annealing atmospheres (oxygen, nitrogen, air, and argon) to investigate the relationship between oxygen deficiency and the power factor. The effect of the mesoporous structure on the power factor of the prepared LSMO films was also studied through analyses of structural and electrical properties. The oxygen-deficient state induces an increase in the lattice parameter of LSMO, which results in an increase in the electrical resistivity and an enhanced Seebeck coefficient. This phenomenon was emphasized upon the introduction of a pore structure in LSMO because of an increase of the unstable surface area. The oxygen-deficient state results in an increase in the amount of manganese in the Mn3+ valence state and in an increase of the lattice parameter; these effects were confirmed through photoemission spectroscopic analysis. As a result, LSMO can be applied as a thermoelectric material because of the successful preparation of LSMO films with a mesoporous structure and because of the enhancement of the power factor as a consequence of increased oxygen deficiency.

Effect of thermal treatment on the textural properties and thermal stability of surface modified zirconia aerogel powders

In the present investigation, surface modified zirconia aerogel powders were heated at various temperatures to study the effect of thermal treatment on their textural properties and thermal stability. Zirconia aerogel powders were prepared using a sol-gel technique and ambient pressure drying method followed by thermal treatment at various temperatures ranging between 300-800°C under an air atmosphere for 1 h. The synthesised zirconia aerogel powders were characterised by N2 adsorption-desorption analysis, X-ray diffraction (XRD), field-emission scanning electron microscopy, Fourier-transform infra-red spectroscopy, and thermogravimetric differential scanning calorimetry. Thermal treatment causes phase transformation, crystallite growth and agglomeration in zirconia, strongly affecting the pore structure and surface area of zirconia aerogels. From N2 adsorption-desorption analysis, it was observed that the surface area of zirconia aerogel powder suddenly decreased until 500°C, was maintained to 600°C, and decreased again with increasing temperature up to 800°C. The results of XRD plots revealed the amorphous nature of zirconia up to 500°C, which then transformed to a tetragonal phase (at about 600°C), which has large surface area and fairly high thermal stability. The observed surface area of 153 m2/g for the 600°C-annealed zirconia aerogel powder with a tetragonal structure is high enough for catalysis applications.

A study of the annealing atmosphere effects on the electrical properties of graphene-incorporated direct-patternable ZnO thin films

An improvement in the electrical properties of the films is absolutely required for the application of ZnO thin film in electrical devices. And the conventional dry etching process is accompanied by the generation of physical defects, degradation of the properties. The electrical property of direct-patternable, graphene-incorporated ZnO thin film prepared by photochemical solution deposition was improved by N2 atmosphere annealing. Direct-patterning of graphene-incorporated ZnO thin films by photochemical solution deposition was performed without a photoresist or etching process. The transmittance of graphene-incorporated ZnO thin films was similar to ZnO thin film. The transmittance of ZnO thin film was unaffected by annealing atmosphere such as air or N2. The incorporation of graphene slightly decreased the crystallinity of the ZnO thin films and annealing atmosphere did not affect to the crystallinity of ZnO thin films. The resistivity was decreased due to the mobility enhancement because of the π-bond nature of the graphene surface. In addition, N2 atmosphere annealing decreased the resistivity of ZnO thin film than O2 atmosphere annealing due to decreasing of oxidation on the surface. These results suggest that a micro-patterned system can be simply fabricated at low cost and the electrical properties of ZnO thin films can be improved by graphene incorporation and N2 atmosphere annealing.

Mesoporous Thin Films: Thermoelectric Application

The efficiency of a thermoelectric device depends on material properties through the figure of merit, Z = σS2/κ, where σ, S, and κ are electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. To maximize the thermoelectric figure of merit of a material, high electrical conductivity, high Seebeck coefficient, and low thermal conductivity are required. This work has focused on the synthesis of a mesoporous titania films for its application in thermoelectric generation. The mesoporous titania film was synthesized with titanium tetraisopropoxide. The triblock copolymer, Pluronic P-123 (EO20 PO 70EO 20) was used as surfactant in 1-propanol. As a result, an improvement of electrical conductivity and reduced annealing with a lowering of thermal conductivity by distributions of pores were found to be effective to enhance the thermoelectric property.

Effect of Pt doping in mesoporous TiO2 thin films on their electrical property

Pt-doped mesoporous TiO2 thin films were synthesised by using sol-gel process. In this work, Pt was doped to enhance the electrical conductivity of matrix film material. Although Pt concentration was increased, pore structure and crystallisation of matrix TiO2 were not changed significantly because Pt was uniformly doped in TiO2. Porosity of the films was slightly decreased from 30.2% to 25% with increasing Pt concentration up to 6 at.% while the electrical conductivity of Pt doped TiO2 film was increased about 10 times. From these results, it can be expected that thermoelectric property could be enhanced owing to this significantly enhanced electrical conductivity and maintaining mesoporous structure of TiO2 film.

A Study on the Resistive Switching of La0.7Sr0.3MnO3 Film Using Spectromicroscopy

Resistive random access memory (ReRAM) is a promising candidate for next generation nonvolatile memory. La0.7Sr0.3.MnO3 (LSMO) of perovskite manganite family has a great deal of attention for ReRAM material because it makes resistive switching (RS) of interface type without a “forming process”. However, the full understanding of the electronic structure and RS mechanism of LSMO remains a challenging problem. Therefore, this study performed spectromicroscopic analysis to understand the relation between the change of electronic structure and RS characteristic. The results demonstrated the electron occupation by field-induced oxygen vacancies and strong correlation effects.

New approaches for high doping and high crystal fraction in the mixed phase nano-crystal silicon thin film with low angle laterally grown grain by near room temperature deposition process with neutral beam assisted CVD

The NBaCVD system can control the crystalline phase and the doping efficiency simultaneously by the energy of impinge neutral particles. During the deposition process, energetic H-neutral atoms transport their energy to the surface of depositing film to enhance crystallization (crystal volume fraction (Xc) ~85%) and dopant activation (~1*1020 #/cm3, ~30 cm2/Vs) with low H ratio at near room temperature on the substrate. Also the increase of H enhance transport path (mobility incensement) which is deduced from transition of crystal orientation from [111] to [311] at constant Xc. The various analysis data of the thin films (CAFM, GIWAXS) represent the evidence of obvious flat-type nc embedded pm phase, and grain dominant charge transport characteristics.

Effect of surface modification of SiO2 nanoparticles on the mechanical properties of ambient pressure dried silica aerogels

In this study, we synthesised silica aerogel composites using SiO2 nanoparticles and studied the effects of surface modification of silica nanoparticles on the properties of silica aerogel composites. The surface condition of the silica nanoparticles was controlled to be either hydrophobic or hydrophilic. Accordingly, the effect of the surface chemical state of silica nanoparticles on the network structure of hybridised silica aerogels and finally on their mechanical properties could be investigated. The microstructural and mechanical properties of silica aerogels were strongly dependent on the nature of the surface of hybridised silica nanoparticles.