Z. F. Ren

Multiferroicity: the coupling between magnetic and polarization orders

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Carbon nanotube/carbon fiber hybrid multiscale composites

Carbon nanotubes were grown directly on carbon fibers using chemical vapor deposition. When embedded in a polymer matrix, the change in length scale of carbon nanotubes relative to carbon fibers results in a multiscale composite, where individual carbon fibers are surrounded by a sheath of nanocomposite reinforcement. Single-fiber composites were fabricated to examine the influence of local nanotube reinforcement on load transfer at the fiber/matrix interface. Results of the single-fiber composite tests indicate that the nanocomposite reinforcement improves interfacial load transfer. Selective reinforcement by nanotubes at the fiber/matrix interface likely results in local stiffening of the polymer matrix near the fiber/matrix interface, thus, improving load transfer.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot

Patterned growth of freestanding carbon nanotube(s) on submicron nickel dot(s) on silicon has been achieved by plasma-enhanced-hot-filament-chemical-vapor deposition (PE-HF-CVD). A thin film nickel grid was fabricated on a silicon wafer by standard microlithographic techniques, and the PE-HF-CVD was done using acetylene (C2H2) gas as the carbon source and ammonia (NH3) as a catalyst and dilution gas. Well separated, single carbon nanotubes were observed to grow on the grid. The structures had rounded base diameters of approximately 150 nm, heights ranging from 0.1 to 5 μm, and sharp pointed tips. Transmission electron microscopy cross-sectional image clearly showed that the structures are indeed hollow nanotubes. The diameter and height depend on the nickel dot size and growth time, respectively. This nanotube growth process is compatible with silicon integrated circuit processing. Using this method, devices requiring freestanding vertical carbon nanotube(s) such as scanning probe microscopy, field emissi...

Experimental Studies on Anisotropic Thermoelectric Properties and Structures of n-Type Bi2Te2.7Se0.3

The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi(2)Te(3)-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 x 10(-4) W m(-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 degrees 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.

Electrochemical characterization of carbon nanotubes as electrode in electrochemical double-layer capacitors

Abstract Carbon nanotubes uniformly 50 nm in diameter were directly grown on graphite foil. Cyclic voltammetry (CV) shows that the carbon nanotube/graphite foil electrode has a high specific capacitance (115.7 F/g at a scan rate of 100 mV/s) and exhibits typical double-layer behavior. A rectangular-shaped CV curve persists even at a scan rate of 100 mV/s in 1.0 M H 2 SO 4 aqueous solution, which suggests that the carbon nanotube electrode could be an excellent candidate as the electrode in electrochemical double-layer capacitors. In addition, the influence of the potential scan rate, aging, and the electrolyte solution on the specific capacitance of nanotube electrodes was also studied.

Enhanced thermal conductivity and viscosity of copper nanoparticles in ethylene glycol nanofluid

This study investigates the thermal conductivity and viscosity of copper nanoparticles in ethylene glycol. The nanofluid was prepared by synthesizing copper nanoparticles using a chemical reduction method, with water as the solvent, and then dispersing them in ethylene glycol using a sonicator. Volume loadings of up to 2% were prepared. The measured increase in thermal conductivity was twice the value predicted by the Maxwell effective medium theory. The increase in viscosity was about four times of that predicted by the Einstein law of viscosity. Analytical calculations suggest that this nanofluid would not be beneficial as a coolant in heat exchangers without changing the tube diameter. However, increasing the tube diameter to exploit the increased thermal conductivity of the nanofluid can lead to better thermal performance.

Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers

Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

Receiving and transmitting light-like radio waves: Antenna effect in arrays of aligned carbon nanotubes

We present optical measurements of random arrays of aligned carbon nanotubes, and show that the response is consistent with conventional radio antenna theory. We first demonstrate the polarization effect, the suppression of the reflected signal when the electric field of the incoming radiation is polarized perpendicular to the nanotube axis. Next, we observe the interference colors of the reflected light from an array, and show that they result from the length matching antenna effect. This antenna effect could be used in a variety of optoelectronic devices, including THz and IR detectors.

Effect of length and spacing of vertically aligned carbon nanotubes on field emission properties

The length and the spacing of carbon nanotube (CNT) films are varied independently to investigate their effect on the field-emission characteristics of the vertically aligned CNT films grown by plasma-enhanced hot filament chemical vapor deposition using pulsed-current electrochemically deposited catalyst particles. It is shown that, in general, the macroscopic electric field Emac,1, defined as the electric field when the emission current density reaches 1 mA/cm2, can be reduced by increasing the length and the spacing of CNTs. However, for the very-high-density CNT films, the increase of length increases Emac,1 slightly, whereas for the very short CNT films, the increase of spacing does not effectively reduce Emac,1.