Heon Phil Ha

Thermoelectric properties of 25%Bi2Te3-75%Sb2Te3 solid solution prepared by hot-pressing method

Abstract The Seebeck coefficient and electrical conductivity of the 25%Bi 2 Te 3 -75%Sb 2 Te 3 solid solution prepared by hot-pressing method were measured and the effects of particle size, oxidation, hot-pressing temperature and time on the Seebeck coefficient of the polycrystalline solid solution were examined in detail. It has been found that the mechanical deformation during pulverization or pressing processes is very important in controlling the Seebeck coefficient. The role of the anion vacancies formed by the mechanical deformation as well as the change of the antistructure defect concentration were discussed. By optimizing particle size and hot-pressing temperature, the figure of merit about 2.9 x 10 −3 K −1 could be obtained for the P -type polycrystalline 25%Bi 2 Te 3 -75%Sb 2 Te 3 solid solutions without excess Te addition.

The effect of excess tellurium on the thermo electric properties of Bi2Te3-Sb2Te3 solid solutions

Abstract The thermoelectric properties of Bi 2 Te 3 -Sb 2 Te 3 solid solutions are closely related to excess Te in the melt during growth of ingots. The role of excess Te in connection with various process parameters which affect the thermoelectric properties of p -type Bi 2 Te 3 -Sb 2 Te 3 solid solutions have been studied using the zone melting and Bridgman methods. The growth speed and amount of excess Te are important in optimizing the figure of merit. When the process parameters were optimized, a maximum figure of merit of 2.95 × 10 −3 K −1 was obtained by the zone melting method at a rate of 0.1 mm min −1 for a composition 22.5%Bi 2 Te 3 -77.5%Sb 2 Te 3 with 5 wt% excess Te in the melt. The effect of annealing of as-grown ingots on the thermoelectric properties is also reported.

Liquid miscibility gap in the Al–Pb–Sn system

Abstract The liquid miscibility gap boundary of two immiscible liquids, Al- and Pb-rich, has been measured in a temperature range between 1073 and 1293 K and a ternary liquid thermodynamic parameter has been evaluated based on experimental data measured in the present work together with phase equilibrium data available in the literature. From these, the phase diagram of the Al–Pb–Sn system including the liquid miscibility gap has been calculated using the CALPHAD method. The calculated results are in good agreement with the experimental data.

Enhancement of the yield of high-quality ingots in the zone-melting growth of p-type bismuth telluride alloys

The influences of zone melting growth speed on the thermoelectric properties of p-type bismuth telluride alloys were investigated. When the growth speed was fast, thermoelectric properties were uniform along ingots, as the composition of the molten zone did not change significantly. On the other hand, when growth speed was slow, thermoelectric properties varied along ingots due to the change of the solidified composition determined by the composition of a molten zone. The maximum figure of merit appeared in the middle of an ingot grown at slow growth speed. In the present study, the molten zone leveling method was developed as an attempt to extend theregion with high figure of merit. The basic idea of the method was to preserve a uniform composition of liquid at the solid-liquid interface during growth of an ingot so as to obtain a uniform composition of precipitated solids. Ingots composed of two sections with different compositions were used to control the accumulation of Te within the molten zone. As a result, high figure of merit could be obtained over more than 90% of ingot when zone melting growth speed was slow.

Cs promoted Ag/Al2O3 catalysts for selective catalytic reduction of NOx by methane: effect of SO2 and H2O

Highly active alumina supported silver nano-particles were evaluated for SCR of NOx using methane in an excess oxygen atmosphere. Cs promoted Ag/Al2O3 catalysts showed high NO reduction activity. In addition, the prepared materials exhibited promotional effects in the presence of SO2 and H2O. The X-ray diffraction profiles of fresh and used catalysts indicated that the prepared materials are thermally stable up to 70 h of time-on-stream reaction. Activity results well correlated with the characterization data, which revealed that the high deNOx ability was due to superior NO adsorption properties as well as the presence of an appropriate amount of Ag+ and metallic Ag particles.

An Ab Initio Study of the Energies of Coherent Interfaces Formed between bcc Iron and Carbides or Nitrides of Transition Metals

An ab initio study was carried out on interface energies at coherent interfaces between bcc Fe and MXs (NaCl structure, M = Ti, Zr, Hf, V, Nb, Ta, X=C, N). The interface energies have positive values for carbides and nitrides of group IVB metals (Ti, Zr, Hf), while they have negative values for carbides and nitrides of group VB metals (V, Nb, Ta). Influence of bond energy was estimated using the discrete lattice plane/nearest neighbor broken bond (DLP/NNBB) model. It was found that the dependence of interface energy on the type of carbides and nitrides was closely related to changes of the bond energies between Fe, M and X(=C, N) atoms before and after formation of the interfaces Fe/MX.

In situ-DRIFTS Study of Sb–V–CeO2/TiO2 Catalyst Under Standard and Fast NH3-SCR Conditions

In this study, the NOx conversion under standard and fast SCR conditions on the Sb–V–CeO2/TiO2 catalyst has been conducted in the temperature range of 150–400 °C. The Sb–V–CeO2/TiO2 catalyst, under both fast and standard NH3-SCR conditions, showed higher NOx conversion than the commercial V2O5/WO3–TiO2 catalyst at low temperatures. In comparison to the V2O5/WO3–TiO2 catalyst, the NO oxidation to NO2 was promoted for the Sb–V–CeO2/TiO2 catalyst at the temperatures between 150 and 400 °C. The in-situ DRIFTS studies at 200 °C on the Sb–V–CeO2/TiO2 catalyst surface indicated the IR bands assigned to NO2, nitrate species and ammonia adsorbed species assigned to Brønsted and Lewis acid sites under both standard and fast SCR conditions. Furthermore, in-situ DRIFTS results of Sb–V–CeO2/TiO2 catalyst revealed that the formation of surface adsorbed NO2, bridged nitrate and bidentate nitrate species. The reduction of nitrates to nitrites is induced by NO2, which reacts with the adsorbed ammonium species forming intermediate NH4NO2. Eventually, this is converted to N2 and H2O in both SCR conditions.

Rational Selection of Fe2V4O13 over FeVO4 as a Preferred Active Site on Sb-Promoted TiO2 for Catalytic NOX Reduction with NH3

FeVO4 (Fe1) is a particular class of metal vanadate that has recently been highly profiled as an active site to selectively reduce NOX with NH3 (NH3-SCR). This primarily results from NOX/NH3-accessible VO43− anions and an electronic inductive effect between the Fe and V species, leading to the formation of abundant catalytic defects available for NOX turnover. Motivated by a structural inspection of the vanadates reported to date, this study detailed the use of Fe2V4O13 (Fe2) as a novel active site deposited on anatase (TiO2) for NH3-SCR. While providing the aforementioned structural benefits, Fe2/TiO2 also enhanced the redox character as well as the number of sites accessible to NOX/NH3 over Fe1/TiO2 because of the greater electronic inductive effect of Fe2. Therefore, Fe2/TiO2 converted NOX better than Fe1/TiO2 in the presence of H2O. To further improve the NH3-SCR performance of Fe2/TiO2, its catalytic surface was modified via two steps. The first step was to incorporate 1.9 wt% Sb into Fe2/TiO2. Sb could promote the redox feature of Fe2/TiO2 and help its surface to preferentially interact with NH3/NOX, thereby making the resulting Fe2–Sb1.9/TiO2 outperform Fe2/TiO2 during NH3-SCR in the presence of H2O. The second step was to functionalize the Fe2–Sb1.9/TiO2 surface with SO32−/SO42− species. The resulting Fe2–Sb1.9/TiO2 (S) was validated to further increase redox cycling of Fe2–Sb1.9/TiO2, favor NO2 production from NO oxidation for fast NH3-SCR, and hamper surface interplay with SO2. Fe2–Sb1.9/TiO2 (S), therefore, showed higher NOX conversions than a control simulating a commercial catalyst during NH3-SCR feeding H2O and SO2. Fe2–Sb1.9/TiO2 (S) also showed greater durability than the control because of its enhanced resistance to SO2, ammonium (bi)sulfates, and alkali metals.

Synthesis of spherical and size-controlled Pt dendrimer-encapsulated nanoparticles for SCR of NOx

Highly active and selective alumina-supported Pt dendrimer-encapsulated nanocomposite materials were successfully prepared and evaluated for the selective catalytic reduction (SCR) of NOx using methane as reductant. The physicochemical properties of the prepared samples were investigated with X-ray diffraction (XRD), Brunauer Emmett Teller (BET) surface area, inductively coupled plasma mass spectrometry (ICP-MS), and transmission electron microscopy-energy-dispersive X-ray analysis (EDX) techniques. The resulted metallic Pt40 particles are spherical and well separated from each other. As derived from the transmission electron microscopy images, the average particle diameter of dendrimer-encapsulated nanoparticles (DENs) is 2.4 nm. The diffraction profiles of fresh and used catalysts indicated that the prepared materials are highly stable after 40 h of time-on-stream reaction. For DENs Pt/Al2O3 sample, the distribution of platinum particle size ranged between 2 and 20 nm. In the present investigation, the N2 selectivity is attained more than 85% at 400 °C. In addition, the time-on-stream results revealed that the DENs Pt/Al2O3 catalysts are very effective on the reduction of NOx under wide temperature window.

The influence of process parameters on the carrier generation during the hot pressing of Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ solid solutions

Hot pressed Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ alloys show different thermoelectric properties compared to alloys grown by the ordinary zone melting growth method. When specimens were hot pressed, thermoelectric properties changed according to different particle size, pressing time and hot pressing temperature. The effects of these individual parameters on the thermoelectric properties of hot pressed materials were examined. Special emphasis was put on carrier generation mechanisms related to the following parameters; oxidation, mechanical deformation during pulverization and the hot pressing temperature. It was found that all processing parameters influence the generation of electrically active defects. Defects induced by the mechanical deformation and oxygen cause generation of donors. Defect concentration is also altered with different hot pressing temperatures depending on the amount of previously received mechanical deformation. A figure of merit of 2.4/spl times/10/sup -3/ K/sup -1/ could be obtained at an optimal process condition.