Investigation of the Effect of Double-Filler Atoms on the Thermoelectric Properties of Ce-YbCo4Sb12.

Skutterudite compounds have been studied as potential thermoelectric materials due to their high thermoelectric efficiency, which makes them attractive candidates for applications in thermoelectric power generation. In this study, the effects of double-filling on the thermoelectric properties of the CexYb0.2-xCo4Sb12 skutterudite material system were investigated through the process of melt spinning and spark plasma sintering (SPS). By replacing Yb with Ce, the carrier concentration was compensated for by the extra electron from Ce donors, leading to optimized electrical conductivity, Seebeck coefficient, and power factor of the CexYb0.2-xCo4Sb12 system. However, at high temperatures, the power factor showed a downturn due to bipolar conduction in the intrinsic conduction regime. The lattice thermal conductivity of the CexYb0.2-xCo4Sb12 skutterudite system was clearly suppressed in the range between 0.025 and 0.1 for Ce content, due to the introduction of the dual phonon scattering center from Ce and Yb fillers. The highest ZT value of 1.15 at 750 K was achieved for the Ce0.05Yb0.15Co4Sb12 sample. The thermoelectric properties could be further improved by controlling the secondary phase formation of CoSb2 in this double-filled skutterudite system.

Contradicting Influence of Zn Alloying on Electronic and Thermal Properties of a YbCd2 Sb2 -Based Zintl Phase at 700†K.

Zintl compounds are promising thermoelectric materials for power generation as their electronic and thermal transport properties can be simultaneously engineered with anion/cation alloying. Recently, a peak thermoelectric figure-of-merit, zT, of 1.4 was achieved in a (Yb0.9 Mg0.1 )Cd1.2 Mg0.4 Zn0.4 Sb2 Zintl phase at 700 K. Although the effects of alloying Zn in lattice thermal conductivity had been studied thoroughly, how the Zn alloying affects its electronic transport properties has not yet been fully investigated. This study evaluates how the Zn alloying at Cd sites alters the band parameters of (Yb0.9 Mg0.1 )Cd1.6-x Mg0.4 Znx Sb2 (x=0-0.6) using the Single Parabolic Band model at 700 K. The Zn alloying increased the density-of-states effective mass (md * ) from 0.87 to 0.97 m0 . Among Zn-alloyed samples, the md * of the x=0.4 sample was the lowest (0.93 m0 ). The Zn alloying decreased the non-degenerate mobility (µ0 ) from 71 to 57 cm2 s-1 V-1 . Regardless of Zn alloying content, the µ0 of the Zn-alloyed samples were similar (∼57 cm2 s-1 V-1 ). Consequently, the x=0.4 with the highest zT exhibited the lowest weighted mobility (µW ). The lowest µW represents the lowest theoretical electronic transport properties among other x. The highest zT at x=0.4 despite the lowest µW was explained with a significant lattice thermal conductivity reduction achieved with Zn alloying with x=0.4, which outweighed the deteriorated electronic transport properties also due to the alloying.

Fabrication of Large-Area Mullite-Cordierite Composite Substrates for Semiconductor Probe Cards and Enhancement of Their Reliability.

This work aims to fabricate a large-area ceramic substrate for the application of probe cards. Mullite (M) and cordierite (C), which both have a low thermal expansion coefficient, excellent resistance to thermal shock, and high durability, were selected as starting powders. The mullite-cordierite composites were produced through different composition ratios of starting powders (M:C = 100:0, M:C = 90:10, M:C = 70:30, M:C = 50:50, M:C = 30:70, and M:C = 0:100). The effects of composition ratio and sintering temperature on the density, porosity, thermal expansion coefficient, and flexural strength of the mullite-cordierite composite pellets were investigated. The results showed that the mullite-cordierite composite pellet containing 70 wt% mullite and 30 wt% cordierite sintered at 1350 °C performed exceptionally well. Based on these findings, a large-area mullite-cordierite composite substrate with a diameter of 320 mm for use in semiconductor probe cards was successfully fabricated. Additionally, the changes in sheet resistance and flexural strength were measured to determine the effect of the environmental tests on the large-area substrate such as damp heat and thermal shock. The results indicated that the mullite-cordierite composite substrate was extremely reliable and durable.

Thermoelectric Properties of Cu2Te Nanoparticle Incorporated N-Type Bi2Te2.7Se0.3.

To develop highly efficient thermoelectric materials, the generation of homogeneous heterostructures in a matrix is considered to mitigate the interdependency of the thermoelectric compartments. In this study, Cu2Te nanoparticles were introduced onto Bi2Te2.7Se0.3 n-type materials and their thermoelectric properties were investigated in terms of the amount of Cu2Te nanoparticles. A homogeneous dispersion of Cu2Te nanoparticles was obtained up to 0.4 wt.% Cu2Te, whereas the Cu2Te nanoparticles tended to agglomerate with each other at greater than 0.6 wt.% Cu2Te. The highest power factor was obtained under the optimal dispersion conditions (0.4 wt.% Cu2Te incorporation), which was considered to originate from the potential barrier on the interface between Cu2Te and Bi2Te2.7Se0.3. The Cu2Te incorporation also reduced the lattice thermal conductivity, and the dimensionless figure of merit ZT was increased to 0.75 at 374 K for 0.4 wt.% Cu2Te incorporation compared with that of 0.65 at 425 K for pristine Bi2Te2.7Se0.3. This approach could also be an effective means of controlling the temperature dependence of ZT, which could be modulated against target applications.

Enhanced Production of Bacterial Cellulose from Miscanthus as Sustainable Feedstock through Statistical Optimization of Culture Conditions.

Biorefineries are attracting attention as an alternative to the petroleum industry to reduce carbon emissions and achieve sustainable development. In particular, because forests play an important role in potentially reducing greenhouse gas emissions to net zero, alternatives to cellulose produced by plants are required. Bacterial cellulose (BC) can prevent deforestation and has a high potential for use as a biomaterial in various industries such as food, cosmetics, and pharmaceuticals. This study aimed to improve BC production from lignocellulose, a sustainable feedstock, and to optimize the culture conditions for Gluconacetobacter xylinus using Miscanthus hydrolysates as a medium. The productivity of BC was improved using statistical optimization of the major culture parameters which were as follows: temperature, 29 °C; initial pH, 5.1; and sodium alginate concentration, 0.09% (w/v). The predicted and actual values of BC production in the optimal conditions were 14.07 g/L and 14.88 g/L, respectively, confirming that our prediction model was statistically significant. Additionally, BC production using Miscanthus hydrolysates was 1.12-fold higher than in the control group (commercial glucose). Our result indicate that lignocellulose can be used in the BC production processes in the near future.

Development of Colorimetric Whole-Cell Biosensor for Detection of Heavy Metals in Environment for Public Health.

Heavy metals cause various fetal diseases in humans. Heavy metals from factory wastewater can contaminate drinking water, fish, and crops. Inductively coupled plasma-mass spectrometry (ICP-MS) and atomic absorption spectrometry (AAS) are commonly used to analyze heavy metal contents; however, these methods require pre-treatment processes and are expensive and complex. To overcome these limitations, three metal-sensing materials using a whole-cell biosensor in Escherichia coli (E. coli) were developed. Strains were engineered to harbor three kinds of plasmids containing the copA, zntA, and mer promoters for sensing copper, cadmium, and mercury, respectively. The luciferase (lux) gene was inserted as a reporter into the plasmid, which was later replaced with a fused protein sequence containing OmpA (1-159) and mCherry for optical detection. The constructed strains could detect mercury, cadmium, and copper at 0.1-0.75 ppm, 0.2-0.75 ppm, and 2-7.5 ppm, respectively, with linearity values of 0.99030, 0.99676, and 0.95933, respectively. The immobilization linearity value was 0.99765. Notably, these three heavy metals could be detected by visual analysis of the strains. Overall, these findings establish this novel sensor as a potential approach for heavy metal detection in biological samples and foods.

Investigation of Phase Segregation in p-Type Bi0.5Sb1.5Te3 Thermoelectric Alloys by In Situ Melt Spinning to Determine Possible Carrier Filtering Effect.

One means of enhancing the performance of thermoelectric materials is to generate secondary nanoprecipitates of metallic or semiconducting properties in a thermoelectric matrix, to form proper band bending and, in turn, to induce a low-energy carrier filtering effect. However, forming nanocomposites is challenging, and proper band bending relationships with secondary phases are largely unknown. Herein, we investigate the in situ phase segregation behavior during melt spinning with various metal elements, including Ti, V, Nb, Mo, W, Ni, Pd, and Cu, in p-type Bi0.5Sb1.5Te3 (BST) thermoelectric alloys. The results showed that various metal chalcogenides were formed, which were related to the added metal elements as secondary phases. The electrical conductivity, Seebeck coefficient, and thermal conductivity of the BST composite with various secondary phases were measured and compared with those of pristine BST alloys. Possible band alignments with the secondary phases are introduced, which could be utilized for further investigation of a possible carrier filtering effect when forming nanocomposites.

Improved Electrical Characteristics of Gallium Oxide/P-Epi Silicon Carbide Static Induction Transistors with UV/Ozone Treatment Fabricated by RF Sputter.

In this study, static induction transistors (SITs) with beta gallium oxide (ß-Ga2O3) channels are grown on a p-epi silicon carbide (SiC) layer via radio frequency sputtering. The Ga2O3 films are subjected to UV/ozone treatment, which results in reduced oxygen vacancies in the X-ray photoelectron spectroscopy data, lower surface roughness (3.51 nm) and resistivity (319 Ω·cm), and higher mobility (4.01 cm2V-1s-1). The gate leakage current is as low as 1.0 × 10-11 A at VGS = 10 V by the depletion layer formed between n-Ga2O3 and p-epi SiC at the gate region with a PN heterojunction. The UV/O3-treated SITs exhibit higher (approximately 1.64 × 102 times) drain current (VDS = 12 V) and on/off ratio (4.32 × 105) than non-treated control devices.

Silver Nanowire Network Hybridized with Silver Nanoparticle-Anchored Ruthenium Oxide Nanosheets for Foldable Transparent Conductive Electrodes.

Facile strategies in flexible transparent conductive electrode materials that can sustain their electrical conductivities under 1 mm-scale radius of curvature are required for wider applications such as foldable devices. We propose a rational design as well as a fabrication process for a silver nanowire-based transparent conductive electrode with low sheet resistance and high transmittance even after prolonged cyclic bending. The electrode is fabricated on a poly(ethylene terephthalate) film through the hybridization of silver nanowires with silver nanoparticles-anchored RuO2 nanosheets. This hybridization significantly improves the performance of the silver nanowire network under severe bending strain and creates an electrically percolative structure between silver nanowires and RuO2 nanosheets in the presence of anchored silver nanoparticles on the surface of RuO2 nanosheets. The resistance change of this hybrid transparent conductive electrode is 8.8% after 200,000 bending cycles at a curvature radius of 1 mm, making it feasible for use in foldable devices.

Effect of ZnO and SnO2 Nanolayers at Grain Boundaries on Thermoelectric Properties of Polycrystalline Skutterudites.

Nanostructuring is considered one of the key approaches to achieve highly efficient thermoelectric alloys by reducing thermal conductivity. In this study, we investigated the effect of oxide (ZnO and SnO2) nanolayers at the grain boundaries of polycrystalline In0.2Yb0.1Co4Sb12 skutterudites on their electrical and thermal transport properties. Skutterudite powders with oxide nanolayers were prepared by atomic layer deposition method, and the number of deposition cycles was varied to control the coating thickness. The coated powders were consolidated by spark plasma sintering. With increasing number of deposition cycle, the electrical conductivity gradually decreased, while the Seebeck coefficient changed insignificantly; this indicates that the carrier mobility decreased due to the oxide nanolayers. In contrast, the lattice thermal conductivity increased with an increase in the number of deposition cycles, demonstrating the reduction in phonon scattering by grain boundaries owing to the oxide nanolayers. Thus, we could easily control the thermoelectric properties of skutterudite materials through adjusting the oxide nanolayer by atomic layer deposition method.

Two Steps to Improve the Thermoelectric Performance of the Ca5-xYbxAl2-yInySb6 System.

A series of quaternary and quinary Zintl phase thermoelectric (TE) compounds, Ca5-xYbxAl2-yInySb6 (3.07(1) ≤ x ≤ 4.88(2); 0.16(2) ≤ y ≤ 2.00), containing Al/In mixed sites as well as Ca/Yb mixed sites has been successfully synthesized by a direct arc-melting method, and the X-ray diffraction analyses indicated that the products initially adopted an orthorhombic Ba5Al2Bi6-type structure (space group Pbam, Z = 2). However, after a postannealing process at 973 K for 1 month, the particular Yb rich compounds underwent a transformation of the original structure type to a Ca5Ga2Sb6-type phase regardless of the In substitution for Al. The noticeable site preference of cationic Ca and Yb in the three available cationic sites could be understood on the basis of a size match between the central cation and the volume of the anionic polyhedra. The observed phase transition was nicely explained by DFT calculations, proving that the Ca5Ga2Sb6-type phase was energetically more favorable than the Ba5Al2Sb6-type phase for the particular Yb-rich compound. Moreover, this energy difference between the two title phases was originally the result of both the site energy in the Ca site and the bond energies in the [(Al/In)2Sb8] anionic building blocks. A series of thermoelectric property data indicated that a two-step process involving a partial/full In substitution for Al and a phase transition from the Ba5Al2Sb6-type to the Ca5Ga2Sb6-type phase successfully enhanced the electrical conductivities and the Seebeck coefficients of the title compounds. This kind of combined effect eventually resulted in a ZT improvement for the quinary compound Ca1.14(2)Yb3.86Al1.68(1)In0.32Sb6 by approximately 4 times in comparison to its quaternary predecessor Ca1.55(1)Yb3.45Al2Sb6.

Enzymatic Synthesis of Formate Ester through Immobilized Lipase and Its Reuse.

Octyl formate is an important substance used in the perfume industry in products such as cosmetics, perfumes, and flavoring. Octyl formate is mostly produced by chemical catalysts. However, using enzymes as catalysts has gathered increasing interest due to their environment-friendly proprieties. In the present study, we aimed to identify the optimal conditions for the synthesis of octyl formate through immobilized enzyme-mediated esterification. We investigated the effects of enzymatic reaction parameters including the type of immobilized enzyme, enzyme concentration, molar ratio of reactants, reaction temperature, and type of solvent using the optimization method of one factor at a time (OFAT). The maximum conversion achieved was 96.51% with Novozym 435 (15 g/L), a 1:7 formic acid to octanol ratio, a reaction temperature of 40 °C, and with 1,2-dichloroethane as solvent. Moreover, we demonstrated that the Novozym 435 can be reused under the optimal conditions without affecting the octyl formate yield, which could help reduce the economic burden associated with enzymatic synthesis.

Novel Hybrid Conductor of Irregularly Patterned Graphene Mesh and Silver Nanowire Networks.

We prepared the hybrid conductor of the Ag nanowire (NW) network and irregularly patterned graphene (GP) mesh with enhanced optical transmittance (~98.5%) and mechano-electric stability (ΔR/Ro: ~42.4% at 200,000 (200k) cycles) under 6.7% strain. Irregularly patterned GP meshes were prepared with a bottom-side etching method using chemical etchant (HNO3). The GP mesh pattern was judiciously and easily tuned by the regulation of treatment time (0-180 min) and concentration (0-20 M) of chemical etchants. As-formed hybrid conductor of Ag NW and GP mesh exhibit enhanced/controllable electrical-optical properties and mechano-electric stabilities; hybrid conductor exhibits enhanced optical transmittance (TT = 98.5%) and improved conductivity (ΔRs: 22%) compared with that of a conventional hybrid conductor at similar TT. It is also noteworthy that our hybrid conductor shows far superior mechano-electric stability (ΔR/Ro: ~42.4% at 200k cycles; TT: ~98.5%) to that of controls (Ag NW (ΔR/Ro: ~293% at 200k cycles), Ag NW-pristine GP hybrid (ΔR/Ro: ~121% at 200k cycles)) ascribed to our unique hybrid structure.

Effect of Flash Light Sintering on Silver Nanowire Electrode Networks.

We investigated the flash light sintering process to effectively reduce electrical resistance in silver nanowire networks. The optimum condition of the flash light sintering process reduces the electrical resistance by ~20%, while the effect of the conventional thermal annealing processes is rather limited for silver nanowire networks. After flash light sintering, the morphology of the junction between the silver nanowires changes to a mixed-phase structure of the two individual nanowires. This facile and fast process for silver nanowire welding could be highly advantageous to the mass production of silver nanowire networks.

Influence of Pd Doping on Electrical and Thermal Properties of n-Type Cu0.008Bi2Te2.7Se0.3 Alloys.

Doping is known as an effective way to modify both electrical and thermal transport properties of thermoelectric alloys to enhance their energy conversion efficiency. In this project, we report the effect of Pd doping on the electrical and thermal properties of n-type Cu0.008Bi2Te2.7Se0.3 alloys. Pd doping was found to increase the electrical conductivity along with the electron carrier concentration. As a result, the effective mass and power factors also increased upon the Pd doping. While the bipolar thermal conductivity was reduced with the Pd doping due to the increased carrier concentration, the contribution of Pd to point defect phonon scattering on the lattice thermal conductivity was found to be very small. Consequently, Pd doping resulted in an enhanced thermoelectric figure of merit, zT, at a high temperature, due to the enhanced power factor and the reduced bipolar thermal conductivity.

Grain Boundary Interfaces Controlled by Reduced Graphene Oxide in Nonstoichiometric SrTiO3-δ Thermoelectrics.

Point defect or doping in Strontium titanium oxide (STO) largely determines the thermoelectric (TE) properties. So far, insufficient knowledge exists on the impact of double Schottky barrier on the TE performance. Herein, we report a drastic effect of double Schottky barrier on the TE performance in undoped STO. It demonstrates that incorporation of Reduced Graphene Oxide (RGO) into undoped STO weakens the double Schottky barrier and thereby results in a simultaneous increase in both carrier concentration and mobility of undoped STO. The enhanced mobility exhibits single crystal-like behavior. This increase in the carrier concentration and mobility boosts the electrical conductivity and power factor of undoped STO, which is attributed to the reduction of the double Schottky barrier height and/or the band alignment of STO and RGO that allow the charge transfer through the interface at grain boundaries. Furthermore, this STO/RGO interface also enhances the phonon scattering, which results in low thermal conductivity. This strategy significantly increases the ratio of σ/κ, resulting in an enhancement in ZT as compared with pure undoped STO. This study opens a new window to optimize the TE properties of many candidate materials.

Effect of C and N Addition on Thermoelectric Properties of TiNiSn Half-Heusler Compounds.

We investigated the thermoelectric properties of the ternary half-Heusler compound, TiNiSn, when introducing C and N. The addition of C or N to TiNiSn leads to an enhanced power factor and a decreasing lattice thermal conductivity by point defect phonon scattering. The thermoelectric performances of TiNiSn alloys are significantly improved by adding 1 at. % TiN, TiC, and figure of merit (ZT) values of 0.43 and 0.34, respectively, can be obtained at 723 K. This increase in thermoelectric performance is very helpful in the commercialization of thermoelectric power generation in the mid-temperature range.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation.

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi0.4Sb1.6Te3-8 mol % FeTe2, which is a 43% enhancement in ZT compared to the pristine Bi0.4Sb1.6Te3. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

Correction: Direct characterization of graphene doping state by in situ photoemission spectroscopy with Ar gas cluster ion beam sputtering.

Correction for 'Direct characterization of graphene doping state by in situ photoemission spectroscopy with Ar gas cluster ion beam sputtering' by Dong-Jin Yun et al., Phys. Chem. Chem. Phys., 2018, 20, 615-622.

Direct characterization of graphene doping state by in situ photoemission spectroscopy with Ar gas cluster ion beam sputtering.

On the basis of an in situ photoemission spectroscopy (PES) system, we propose a novel, direct diagnosis method for the characterization of graphene (Gr) doping states at organic semiconductor (OSC)/electrode interfaces. Our in situ PES system enables ultraviolet/X-ray photoelectron spectroscopy (UPS/XPS) measurements during the OSC growth or removal process. We directly deposit C60 films on three different p-type dopants-gold chloride (AuCl3), (trifluoromethyl-sulfonyl)imide (TFSI), and nitric acid (HNO3). We periodically characterize the chemical/electronic state changes of the C60/Gr structures during their aging processes under ambient conditions. Depositing the OSC on the p-type doped Gr also prevents severe degradation of the electrical properties, with almost negligible transition over one month, while the p-type doped Gr without an OSC changes a lot following one month of aging. Our results indicate that the chemical/electronic structures of the Gr layer are completely reflected in the energy level alignments at the C60/Gr interfaces. Therefore, we strongly believe that the variation of energy level alignments at the OSC/graphene interface is a key standard for determining the doping state of graphene after a certain period of aging.