Ching-Wu Chu

Field-driven hysteretic and reversible resistive switch at the Ag-Pr0.7Ca0.3MnO3 interface

The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag–Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical migration is the cause for the switch.

Relationship between thermoelectric figure of merit and energy conversion efficiency

Significance Thermoelectric materials generate electricity from temperature gradients. The dimensionless figure of merit, ZT = S2ρ−1κ−1T, is calculated from the Seebeck coefficient (S), electrical resistivity (ρ), and thermal conductivity (κ). The calculated efficiency based on ZT using the conventional formula is not reliable in some cases due to the assumption of temperature-independent S, ρ, and κ. We established a new efficiency formula by introducing an engineering figure of merit (ZT)eng and an engineering power factor (PF)eng to predict reliably and accurately the efficiency of materials at a large temperature difference between the hot and cold sides, unlike the conventional ZT and PF providing performance only at specific temperatures. These new formulas will profoundly impact the search for new thermoelectric materials. The formula for maximum efficiency (ηmax) of heat conversion into electricity by a thermoelectric device in terms of the dimensionless figure of merit (ZT) has been widely used to assess the desirability of thermoelectric materials for devices. Unfortunately, the ηmax values vary greatly depending on how the average ZT values are used, raising questions about the applicability of ZT in the case of a large temperature difference between the hot and cold sides due to the neglect of the temperature dependences of the material properties that affect ZT. To avoid the complex numerical simulation that gives accurate efficiency, we have defined an engineering dimensionless figure of merit (ZT)eng and an engineering power factor (PF)eng as functions of the temperature difference between the cold and hot sides to predict reliably and accurately the practical conversion efficiency and output power, respectively, overcoming the reporting of unrealistic efficiency using average ZT values.

n-type thermoelectric material Mg2Sn0.75Ge0.25 for high power generation

Significance Thermoelectric materials have been extensively studied for applications in conversion of waste heat into electricity. The efficiency is related to the figure-of-merit, ZT = (S2σ/κ)T, where S, σ, and κ are the Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively. Pursuing higher ZT for higher efficiency has been the focus by mainly reducing the thermal conductivity. In this paper, we point out, for a given ZT, higher power factor (S2σ) should be pursued for achieving more power because power is determined by (Th − Tc)2(S2σ)/L, where Th, Tc, and L are the hot and cold side temperatures, and leg length, respectively. We found a new material, Mg2Sn0.75Ge0.25, having both high ZT and high power factor. Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg2Sn-based material, Mg2Sn0.75Ge0.25, is a good example to meet the dual requirements in efficiency and output power. It was found that Mg2Sn0.75Ge0.25 has an average ZT of 0.9 and PF of 52 μW⋅cm−1⋅K−2 over the temperature range of 25–450 °C, a peak ZT of 1.4 at 450 °C, and peak PF of 55 μW⋅cm−1⋅K−2 at 350 °C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with Th = 400 °C and Tc = 50 °C to be of 10.5% and 6.6 W⋅cm−2 under a temperature gradient of 150 °C⋅mm−1, respectively.

Superconductivity in the C32 intermetallic compounds AAl2−xSix, with A=Ca and Sr; and 0.6<x<1.2

Abstract The intermetallic compounds AAl 2− x Si x , where A=Ca, Sr or Ba, crystallize in the C32 structure, same as the recently discovered MgB 2 with a high superconducting transition temperature of 39 K. For x =1, superconductivity has been observed in AAlSi with A=Ca and Sr, but not with A=Ba. The transition temperatures are 7.8 and 5.1 K, respectively for CaAlSi and SrAlSi. The CaAl 2− x Si x compound system display a T c -peak at x =1, a possible x -induced electronic transition at x ∼0.75 and a possible miscibility gap near x ∼1.1 which results in a very broad superconducting transition. The Seebeck coefficients of AAlSi indicate that their carriers are predominantly electrons in nature, in contrast to the holes in MgB 2 .

Achieving high power factor and output power density in p-type half-Heuslers Nb1-xTixFeSb.

Significance Thermoelectric technology can boost energy consumption efficiency by converting some of the waste heat into useful electricity. Heat-to-power conversion efficiency optimization is mainly achieved by decreasing the thermal conductivity in many materials. In comparison, there has been much less success in increasing the power factor. We report successful power factor enhancement by improving the carrier mobility. Our successful approach could suggest methods to improve the power factor in other materials. Using our approach, the highest power factor reaches ∼106 μW⋅cm−1⋅K−2 at room temperature. Such a high power factor further yields a record output power density in a single-leg device tested between 293 K and 868 K, thus demonstrating the importance of high power factor for power generation applications. Improvements in thermoelectric material performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor has been less successful in comparison. In this work, a peak power factor of ∼106 μW⋅cm−1⋅K−2 is achieved by increasing the hot pressing temperature up to 1,373 K in the p-type half-Heusler Nb0.95Ti0.05FeSb. The high power factor subsequently yields a record output power density of ∼22 W⋅cm−2 based on a single-leg device operating at between 293 K and 868 K. Such a high-output power density can be beneficial for large-scale power generation applications.

Highly active catalyst derived from a 3D foam of Fe(PO3)2/Ni2P for extremely efficient water oxidation

Significance The oxygen evolution reaction (OER) is a sluggish reaction with poor catalytic efficiency, which is one of the major bottlenecks in realizing water splitting, CO2 reduction, and rechargeable metal–air batteries. In particular, the commercial utilization of water electrolyzers requires an exceptional electrocatalyst that has the capacity of delivering ultra-high oxidative current densities above 500 mA/cm2 at an overpotential below 300 mV with long-term durability. Few catalysts can satisfy such strict criteria. Here we report a promising oxygen-evolving catalyst with superior catalytic performance and long-term durability; to the best of our knowledge, it is one of the most active OER catalysts reported thus far that satisfies the criteria for large-scale commercialization of water–alkali electrolyzers. Commercial hydrogen production by electrocatalytic water splitting will benefit from the realization of more efficient and less expensive catalysts compared with noble metal catalysts, especially for the oxygen evolution reaction, which requires a current density of 500 mA/cm2 at an overpotential below 300 mV with long-term stability. Here we report a robust oxygen-evolving electrocatalyst consisting of ferrous metaphosphate on self-supported conductive nickel foam that is commercially available in large scale. We find that this catalyst, which may be associated with the in situ generated nickel–iron oxide/hydroxide and iron oxyhydroxide catalysts at the surface, yields current densities of 10 mA/cm2 at an overpotential of 177 mV, 500 mA/cm2 at only 265 mV, and 1,705 mA/cm2 at 300 mV, with high durability in alkaline electrolyte of 1 M KOH even after 10,000 cycles, representing activity enhancement by a factor of 49 in boosting water oxidation at 300 mV relative to the state-of-the-art IrO2 catalyst.

Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes

Significance Fatigue is a deadly disease for metals. Fatigue often happens under cyclic loading even if the strain level is low. However, a stretchable transparent electrode, which can be made of metal and is a key element in stretchable electronics, needs high stability at large strains. Here we show that Au nanomesh on a slippery substrate is fatigue-free when cyclically stretched to large strains (>100%). Moreover, cells can grow on the Au nanomesh. A metal mesh that conducts electricity, is biocompatible, and is completely free of fatigue will be an ideal electrode not only for flexible electronics, but also for implantable electronics. Next-generation flexible electronics require highly stretchable and transparent electrodes. Few electronic conductors are both transparent and stretchable, and even fewer can be cyclically stretched to a large strain without causing fatigue. Fatigue, which is often an issue of strained materials causing failure at low strain levels of cyclic loading, is detrimental to materials under repeated loads in practical applications. Here we show that optimizing topology and/or tuning adhesion of metal nanomeshes can significantly improve stretchability and eliminate strain fatigue. The ligaments in an Au nanomesh on a slippery substrate can locally shift to relax stress upon stretching and return to the original configuration when stress is removed. The Au nanomesh keeps a low sheet resistance and high transparency, comparable to those of strain-free indium tin oxide films, when the nanomesh is stretched to a strain of 300%, or shows no fatigue after 50,000 stretches to a strain up to 150%. Moreover, the Au nanomesh is biocompatible and penetrable to biomacromolecules in fluid. The superstretchable transparent conductors are highly desirable for stretchable photoelectronics, electronic skins, and implantable electronics.

Synthesis and physical properties of superconducting RuSr2EuCu2O8

Abstract RuSr 2 EuCu 2 O 8 has been synthesized and characterized by X-ray diffraction, magnetization, and electrical transport measurements. Similar to RuSr 2 GdCu 2 O 8 , this compound exhibits ferromagnetic (below 130 K) and superconducting (below 30 K) order.

Robust ferroelectric state in multiferroic Mn 1 − x Zn x WO 4

We report on the remarkably robust ferroelectric state in the multiferroic compound

Higher thermoelectric performance of Zintl phases (Eu0.5Yb0.5)1-xCaxMg2Bi2 by band engineering and strain fluctuation.

{mathrm{Mn}}_{1ensuremath{-}x}{mathrm{Zn}}_{x}{mathrm{WO}}_{4}