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.

Efficiency and output power of thermoelectric module by taking into account corrected Joule and Thomson heat

Abstract: The maximum conversion efficiency of a thermoelectric module composed of p- and n-type materials has been widely calculated using a constant property model since the 1950s, but this conventional model is only valid in limited conditions and no Thomson heat is accounted for. Since Thomson heat causes the efficiency under- or over-rated depending on the temperature dependence of Seebeck coefficient, it cannot be ignored especially in large temperature difference between the hot and cold sides. In addition, incorrect Joule heat is taken into consideration for heat flux evaluation of a thermoelectric module at thermal boundaries due to the assumption of constant properties in the conventional model. For this reason, more practical predictions for efficiency and output power and its corresponding optimum conditions of p- and n-type materials need to be revisited. In this study, generic formulae are derived based on a cumulative temperature dependence model including Thomson effect. The formulae reliably predict the maximum efficiency and output power of a thermoelectric module at a large temperature.

Ultrasensitive chemical detection using a nanocoax sensor.

Abstract: We report on the design, fabrication, and performance of a nanoporous, coaxial array capacitive detector for highly sensitive chemical detection Composed of an array of vertically aligned nanoscale coaxial electrodes constructed with porous dielectric coax annuli around carbon nanotube cores, this sensor is shown to achieve parts per billion level detection sensitivity, at room temperature, to a broad class of organic molecules The nanoscale, 3D architecture and microscale array pitch of the sensor enable rapid access of target molecules and chip-based multiplexing capabilities, respectively

Nanothermometer using single crystal silver nanospheres.

Abstract: Conventional thermometers have been widely employed in scientific researches and industrial applications. Thermometers with nanometer scaled spatial resolution attract more and more attentions recently with the rapid development of nanotechnology and nanoengineering. Many kinds of nanothermometers have been designed by duplicating the conventional thermometers at nanoscale through decreasing the geometrical size of the conventional thermometers. For example, nanoscale thermocouples are fabricated from nano-junctions based on Seebeck effect, liquid-in-tube nanothermometers from nanotubes based on temperature-dependent thermal expansion of liquid, nanosized fluorescence thermometers from nanoparticles based on temperature-dependent photoluminescence, nanoscale infrared thermometers from metal nanoparticles based on blackbody radiation, Coulomb Blockade nanothermometers from nanosized superconductor-insulator-metal tunnel junctions based on the Coulomb blockade of tunneling, and complex structured nanothermometers from MicroElectro-Mechanical-Systems based on temperature-dependent resonator quality factor or Fermi-level shift, etc. In all of these nanothermometers, the physical properties such as the voltage in nanoscale thermocouples, volume of liquid in liquid-in-tube nanothermometers, or photoluminescence spectrum in nanosized fluorescence thermometers are restored to their original state at room temperature after the temperature drops from high temperature. These nanothermometers are usually employed in real-time and in situ temperature detection, but not for recording and readout later, which is not useful for the situation where real-time readout is not possible like in the case of explosion, but the open-ended gallium-filled carbon nanotube thermometers can in principle also be a readout device after the event. Here another kind of nanothermometers, ex situ nanothermometers, which can record the temperature they were exposed to and be read later when the event is over, are demonstrated to measure temperature based on temperature-dependent size distribution and areal density of metal nanospheres. Compared with the reported nanothermometers, the nanothermometers made of nanospheres with a nanometer scaled spatial resolution, described in this paper, can record the highest temperature in the event and be read at a later time after the event is over. Figure 1a shows the deposited silver nanoparticles before being heated. Silver nanoparticles aggregate on the carbon supporting film coated on TEM grid. The shape of the nanoparticles is irregular and the size varies from 10nm to 100 nm. After heating to a certain temperature, smaller silver nanoparticles nucleate and grow on the whole carbon film (Fig. 1b). It is speculated that the surface diffusion causes the nucleation and growth of the new nanoparticles. Statistical analysis shows that all the nucleated nanoparticles are spherical with an average circularity M1⁄4 0.82–0.85. So these nucleated nanoparticles are named as nanospheres. The spherical morphology of the nanospheres should come from the surface melting because of melting point depression. The inset in Figure 1c shows a high-resolution TEM (HRTEM) image of a nanosphere with diameter of 16 nm observed at 500 8C. The surface of the silver nanosphere melts at 500 8Cwhile the melting point of bulk silver is 961 8C, a direct observation of the significant melting point reduction of nanosized silver particles. Themelting liquid layer covers the nanospheres, conceals the lowest-energy growth facets, and forms perfect spheres because of surface tension. When the surface-melted nanospheres cool down quickly from heating temperature, the main spherical shape is kept. High magnification TEM images (Fig. 1c) show that the nucleated nanospheres distribute uniformly on the carbon film and the size distribution of the nanospheres is narrow. HRTEM image of the nanospheres (Fig. 1d) indicates that the nanospheres are single crystalline at room temperature. EDS spectrum (Fig. 1e) and selected area electron diffraction of the nanospheres (Fig. 1f) confirm that the nucleated nanospheres are silver with face-centered cubic structure. Figure 2 shows the room temperature TEM images, size distribution, and the average diameter of the nanospheres after heating at different annealing temperatures and cooling down. Each sample was heated at a certain heating temperature for 5min and then cooled down to room temperature for TEM examination. TEM images (Fig. 2a) show that the nucleated nanospheres are smaller with heating at 300 8C than those at 500 8C. Histogram of the nanospheres (Fig. 2b) indicates that the diameter of most nanospheres is 4 nm after heating at 300 8C and 14 nm at 700 8C. The average diameter is systematically larger with higher heating temperature (Fig. 2c) because of coalescence.

Visible light diffraction studies on periodically aligned arrays of carbon nanotubes: Experimental and theoretical comparison

Abstract: We have investigated visible light diffraction on honeycomb arrays of aligned carbon nanotubes grown on nickel nanoparticles prepared using the nanosphere lithography. A monolayer of 980nm polystyrene spheres was used as the mask for the deposition of nickel nanoparticles from which carbon nanotubes of 100nm in diameter and up to a couple of microns in length were grown. We show that a standard theory of diffraction from point scatterers explains all the observed diffraction features including Bragg’s law and the strong enhancement of the second and fifth order diffraction spots.

Enhanced Thermoelectric Figure-of-Merit in p-type Nanostructured Bismuth Antimony Tellurium Alloys Made from Elemental Chunks

Abstract: By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 °C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 °C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

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.