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.

Effect of filler mass and binding on thermal conductivity of fully filled skutterudites

Abstract: The relations between the thermal conductivity of cagelike structures and their crystal parameters are investigated using a two-dimensional toy model. The model consists of host atoms on a rectangular lattice with fillers at the center of each rectangle. The effect of mass and size of the filler on thermal conductivity is investigated using equilibrium molecular-dynamics simulations. We show that the thermal conductivity decreases with increasing atomic displacement parameter while it has local minima versus the filler to host mass ratio. Similar trends were observed in experiments on filled skutterudites. The trends are explained by analyzing the effect of the filler on the phonon dispersion and relaxation times of the host material.

Glucose sensors made of novel carbon nanotube-gold nanoparticle composites

Abstract: Carbon nanotube and metal particle composites have been exploited to fabricate high performance electrochemical devices. However, the physical and chemical procedures to synthesize the composites are labor intensive and inefficient. Our study reveals an one-step wet chemistry method to accomplish fast and controllable production of gold nanoparticle (AuNP) and carbon naotube (CNT) composites. Such a process is sensitive to the surface charge. Especially, when functionalized with carboxyl groups, the CNTs carried negative charges and showed low level association with negatively charged AuNPs. Thermal treatment was employed to decompose the carboxyl groups and render each CNT a charge-free surface thereby achieving a high level AuNP-CNT association. The fabricated glucose sensors demonstrated dependence of their sensitivities to the amount of AuNPs on the CNTs. The enhancement of sensitivity can be attributed to an accelerated electron transfer rate from glucose oxidase Gox to the electrode. The Michaelis-Menten kinetics also indicated improved performance in the glucose sensor made of AuNP-CNTs. Therefore, our research revealed a novel approach to produce metallic nanoparticle and CNT composite for fabricating high performance electrochemical sensors.

Transport properties of Ni, Co, Fe, Mn doped Cu0.01Bi2Te2.7Se0.3 for thermoelectric device applications

Abstract: Bi2Te3 based thermoelectric devices typically use a nickel layer as a diffusion barrier to block the diffusion of solder or copper atoms from the electrode into the thermoelectric material. Previous studies have shown degradation in the efficiency of these thermoelectric devices may be due to the diffusion of the barrier layer into the thermoelectric material. In this work, Ni, Co, Fe, and Mn are intentionally doped into Cu0.01Bi2Te2.7Se0.3 in order to understand their effects on the thermoelectric material. Thermoelectric transport properties including the Seebeck coefficient, thermal conductivity, electrical resistivity, carrier concentration, and carrier mobility of Cu0.01Bi2Te2.7Se0.3 doped with 2 atomic percent M (M = Ni, Co, Fe, Mn) as Cu0.01Bi2Te2.7Se0.3M0.02 are studied in a temperature range of 5-525 K. It is seen that the introduction of Ni, Co, Fe, or Mn does not affect the overall figure of merit, and therefore demonstrates that the diffusion barrier is not leading to device degradation as pre...

Nanostructured YbAgCu4 for potentially cryogenic thermoelectric cooling.

Abstract: We have studied the thermoelectric properties of nanostructured YbAgCu4 materials. A high power factor of ∼131 μW cm(-1) K(-2) has been obtained at 22 K for nanostructured samples prepared by ball milling the arc melted ingot into nanopowder and hot pressing the nanopowder. The implementation of nanostructuring method decreased the thermal conductivity at 42 K by 30-50% through boundary scattering comparing with the previously reported value of polycrystalline YbAgCu4. A peak dimensionless thermoelectric figure-of-merit, ZT, of 0.11 has been achieved at 42 K, which may find potential applications for cryogenic cooling below 77 K. The nanostructuring approach can be extended to other heavy Fermion materials to achieve high power factor and low thermal conductivity and ultimately higher ZT.

Superconducting epitaxial Tl2Ba2CuO6 + δ thin films on SrTiO3 with tetragonal lattice and continuously adjustable critical temperature

Abstract: Epitaxial Tl 2 Ba 2 CuO 6 + ° (Tl-2201) thin films on single crytalline SrTiO 3 substrate have been synthesized by RF magnetron sputtering followed by a two-step annealing process. X-ray diffraction data show that the film is c -axis oriented with full width at half maximum of rocking curve (FWHM) of 0.265° and alignment in the ab -plane. The zero-resistance T c of the same film can be increased or decreased continually and reversibly in the 11–80 K range by annealing in argon or air, with no change in tetragonal lattice symmetry.

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.