Zhifeng Ren

Bio: Zhifeng Ren is an academic researcher from Texas Center for Superconductivity. The author has contributed to research in topics: Thermoelectric effect & Thermoelectric materials. The author has an hindex of 122, co-authored 695 publications receiving 71212 citations. Previous affiliations of Zhifeng Ren include Massachusetts Institute of Technology & University of Cincinnati.

Fast phase formation of double-filled p-type skutterudites by ball-milling and hot-pressing

Abstract: Filled-skutterudites are promising mid-temperature thermoelectric materials for heat-to-electricity conversion. Traditional preparation methods need a very long time annealing (7 to 14 days) to form the right skutterudite phase. Annealing is especially critical for p-type filled-skutterudites, since Fe4Sb12 needs filler atoms to enter the cage to form a stable phase. In this work, we prepared Ce and Nd double-filled p-type skutterudite materials by directly ball-milling the quenched ingot without annealing followed by hot-pressing. The results showed that with appropriate ball-milling time, a pure p-type filled-skutterudite phase can be obtained in just 5 minutes by hot-pressing. The samples prepared in this way have the same quality as those prepared by traditional long time annealing methods, and showed ZT values above 1 between 700 and 800 K. This simple and efficient method is very useful for the preparation of many other materials that are kinetically difficult to make.

Zintl-phase Eu2ZnSb2: A promising thermoelectric material with ultralow thermal conductivity.

Abstract: Zintl compounds are considered to be potential thermoelectric materials due to their “phonon glass electron crystal” (PGEC) structure. A promising Zintl-phase thermoelectric material, 2-1-2–type Eu2ZnSb2 (P63/mmc), was prepared and investigated. The extremely low lattice thermal conductivity is attributed to the external Eu atomic layers inserted in the [Zn2Sb2]2- network in the structure of 1-2-2–type EuZn2Sb2 ( P 3 ¯ m 1 ) , as well as the abundant inversion domain boundary. By regulating the Zn deficiency, the electrical properties are significantly enhanced, and the maximum ZT value reaches ∼1.0 at 823 K for Eu2Zn0.98Sb2. Our discovery provides a class of Zintl thermoelectric materials applicable in the medium-temperature range.

Skutterudite unicouple characterization for energy harvesting applications

Abstract: Skutterudites are promising thermoelectric materials because of their high figure of merit, ZT, and good thermomechanical properties. This work reports the effective figure of merit, ZTeff, and the efficiency of skutterudite legs and a unicouple working under a large temperature difference. The p- and n-type legs are fabricated with electrodes sintered directly to the skutterudite during a hot pressing process. CoSi2 is used as the electrode for the n-type skutterudite (Yb0.35Co4Sb12) and Co2Si for the p-type skutterudite (NdFe3.5Co0.5Sb12). A technique is developed to measure the ZTeff of individual legs and the efficiency of a unicouple. An ZTeff of 0.74 is determined for the n-type legs operating between 52 and 595 °C, and an ZTeff of 0.51 for the p-type legs operating between 77 and 600 °C. The efficiency of the p–n unicouple is determined to be 9.1% operating between ∼70 and 550 °C.

New insight into the material parameter B to understand the enhanced thermoelectric performance of Mg2Sn1−x−yGexSby

Abstract: Historically, a material parameter B incorporating weighted mobility and lattice thermal conductivity has guided the exploration of novel thermoelectric materials. However, the conventional definition of B neglects the bipolar effect which can dramatically change the thermoelectric energy conversion efficiency at high temperatures. In this paper, a generalized material parameter B* is derived, which connects weighted mobility, lattice thermal conductivity, and the band gap. Based on the new parameter B*, we explain the successful tuning of the electron and phonon transport in Mg2Sn1−x−yGexSby, with an improved ZT value from 0.6 in Mg2Sn0.99Sb0.01 to 1.4 in Mg2Sn0.73Ge0.25Sb0.02. We uncover that the Ge alloying approach simultaneously improves all the key variables in the material parameter B*, with an ∼25% enhancement in the weighted mobility, ∼27% band gap widening, and ∼50% reduction in the lattice thermal conductivity. We show that a higher generalized parameter B* leads to a higher optimized ZT in Mg2Sn0.73Ge0.25Sb0.02, and some common thermoelectric materials. The new parameter B* provides a better characterization of material's thermoelectric transport, particularly at high temperatures, and therefore can facilitate the search for good thermoelectric materials.

Efficient Alkaline Water/Seawater Hydrogen Evolution by a Nanorod‐Nanoparticle‐Structured Ni‐MoN Catalyst with Fast Water‐Dissociation Kinetics

Abstract: Achieving efficient and durable nonprecious hydrogen evolution reaction (HER) catalysts for scaling up alkaline water/seawater electrolysis is desirable but remains a significant challenge. Here, a heterogeneous Ni‐MoN catalyst consisting of Ni and MoN nanoparticles on amorphous MoN nanorods that can sustain large‐current‐density HER with outstanding performance is demonstrated. The hierarchical nanorod–nanoparticle structure, along with a large surface area and multidimensional boundaries/defects endows the catalyst with abundant active sites. The hydrophilic surface helps to achieve accelerated gas‐release capabilities and is effective in preventing catalyst degradation during water electrolysis. Theoretical calculations further prove that the combination of Ni and MoN effectively modulates the electron redistribution at their interface and promotes the sluggish water‐dissociation kinetics at the Mo sites. Consequently, this Ni‐MoN catalyst requires low overpotentials of 61 and 136 mV to drive current densities of 100 and 1000 mA cm−2, respectively, in 1 m KOH and remains stable during operation for 200 h at a constant current density of 100 or 500 mA cm−2. This good HER catalyst also works well in alkaline seawater electrolyte and shows outstanding performance toward overall seawater electrolysis with ultralow cell voltages.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Carbon Nanotubes--the Route Toward Applications

Abstract: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.