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.

Experimental Studies on Anisotropic Thermoelectric Properties and Structures of

Abstract: The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi2Te3-based n-type single crystals is about 0.85 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 1.65 W m -1 K -1 even though the power factor is 47 × 10 -4 Wm -1 K -2 . In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT. Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 0.85 to 1.04 at about 125 °C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity. Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller.

Enhancing the Scratch Resistance by Introducing Chemical Bonding in Highly Stretchable and Transparent Electrodes.

Abstract: Stretchable transparent electrodes are key elements in flexible electronics and e-skins. However, existing stretchable transparent electrodes, including graphene sheets, carbon nanotube, and metal nanowire networks, weakly adheres to the substrate by van der Waals forces. Such electrodes suffer from poor scratch-resistance or poor durability, and this issue has been one of the biggest problems for their applications in industry. Here we show that, by introducing a Au-S bond between a Au nanomesh (AuNM) and the underlying elastomeric substrate, the AuNM strongly adheres to the substrate and can withstand scratches of a pressure of several megapascals. We find that the strong chemical bond, on the other hand, leads to a stiffening effect and localized rupture of the AuNM upon stretching; thus the stretchability is poor. A prestraining process is applied to suppress the localized rupture and has successfully improved the stretchability: electrical resistance of the prestrained AuNM exhibits modest change by one-time stretching to 160%, or repeated stretching to 50% for 25 000 cycles. This conductor is an ideal platform for robust stretchable photoelectronics. The idea of introducing a covalent bond to improve the scratch-resistance may also be applied to other systems including Ag nanowire films, carbon nanotube films, graphene, and so forth.

Robust and selective electrochemical reduction of CO2: the case of integrated 3D TiO2@MoS2 architectures and Ti–S bonding effects

Abstract: Developing efficient and affordable catalysts toward electrochemical reduction of CO2 to valuable chemicals is of great significance for energy and environmental sustainability, which can be efficaciously achieved by catalyst structure steering. In this study, we propose an ingenious strategy to modulate MoS2 favorable for CO2 reduction by fabricating an integrated three-dimensional (3D) TiO2@MoS2 architecture containing Ti–S bonds, the formation of which, revealed by density functional theory calculations, has changed the electric properties of the MoS2 layer and the adsorption characters of Mo exposed edges. The modulated MoS2 is vigorous for CO2 reduction due to the decrease of both the binding energy of CO2 and the energy barriers of CO2 reduction reaction pathways. Experimentally, the integrated 3D TiO2@MoS2 architectures can act as efficient and stable catalysts for selective reduction of CO2 to CO. The optimized composite showed a negligible onset overpotential of 100 mV for CO formation in KHCO3 solution, and a maximum faradaic efficiency of ∼82% for CO at −0.7 V vs. RHE with a large partial current density for CO of 68 mA cm−2. Additionally, the integrated 3D electrodes exhibited superior stability during CO2 reduction. This study will shed light on the modification of electrocatalysts for efficient CO2 reduction through structure steering.

Investigating the thermoelectric properties of p-type half-Heusler Hfx(ZrTi)1−xCoSb0.8Sn0.2 by reducing Hf concentration for power generation

Abstract: Based on the fact that Hf is much more expensive than other commonly used elements in HfCoSb-based half-Heusler materials, we studied the thermoelectric properties of the p-type half-Heusler Hfx(ZrTi)1−xCoSb0.8Sn0.2 by reducing Hf concentration. A peak ZT of ∼1.0 was achieved at 700 °C with the composition of Hf0.19Zr0.76Ti0.05CoSb0.8Sn0.2 by keeping the Hf/Zr ratio at 1/4 and Hf/Ti at 4/1. This composition has much reduced cost and similar thermoelectric performance compared with our previously reported best p-type half-Heusler: Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. Due to the decreased usage of Hf, it is more favorable for consideration in applications. In addition, a higher output power is expected because of the higher power factor even though the conversion efficiency is the same due to the same ZT.

Field emission of silicon nanowires grown on carbon cloth

Abstract: A low operating electric field has been achieved on silicon nanowires grown on carbon cloth. The silicon nanowires were grown on carbon cloth via the vapor-liquid-solid reaction using silane gas as the silicon source and gold as catalyst from the decomposition of hydrogen gold tetrachloride. An emission current density of 1mA∕cm2 was obtained at an operating electric field of 0.7V∕μm. Such low field is resulted from a high field enhancement factor of 6.1×104 due to the combined effects of the high intrinsic aspect ratio of silicon nanowires and the woven geometry of carbon cloth. Such results may lead silicon nanowire field emitters to practical applications in vacuum microelectronic devices including microwave devices.

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.