Xuetong Zhang

Preparation and photoluminescence of highly ordered TiO2 nanowire arrays

Highly ordered TiO2 nanowire (TN) arrays were prepared in anodic alumina membranes (AAMs) by a sol-gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence (PL) measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. A model is also presented to explain the PL intensity drop-down of the TN arrays embedded in AAMs: the blue PL band of AAMs arises from the F+ centers on the pore walls, and the TNs first form in the center area of the pores and then extend to the pore walls.

Elastic, Conductive, Polymeric Hydrogels and Sponges

As a result of inherent rigidity of the conjugated macromolecular chains resulted from the delocalized π-electron system along the polymer backbone, it has been a huge challenge to make conducting polymer hydrogels elastic by far. Herein elastic and conductive polypyrrole hydrogels with only conducting polymer as the continuous phase have been simply synthesized in the indispensable conditions of 1) mixed solvent, 2) deficient oxidant, and 3) monthly secondary growth. The elastic mechanism and oxidative polymerization mechanism on the resulting PPy hydrogels have been discussed. The resulting hydrogels show some novel properties, e.g., shape memory elasticity, fast functionalization with various guest objects, and fast removal of organic infectants from aqueous solutions, all of which cannot be observed from traditional non-elastic conducting polymer counterparts. Whats more, light-weight, elastic, and conductive organic sponges with excellent stress-sensing behavior have been successfully achieved via using the resulting polypyrrole hydrogels as precursors.

Influence of pressures on the crystallization process of an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Amorphous Fe73.5Cu1Nb3Si13.5B9 alloys, prepared by a melt-spinning technique, were annealed at a temperature of 823 K under pressures in the range of 1–5 GPa and ambient pressure. The high pressure experiments were carried out in a belt-type pressure apparatus. The microstructure of the annealed alloys has been investigated by x-ray diffraction, electron diffraction, and transmission electron microscopy. Experimental results show that the initial crystalline phase in these annealed alloys is α-Fe solid solution (named as α-Fe phase below), and high pressure has a great influence on the crystallization process of the α-Fe phase. The grain size of the α-Fe phase decreases with the increase of pressure (P). The volume fraction of the α-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (P>2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy and latent applications of high-pressure anne...

Polypyrrole/Silver Coaxial Nanowire Aero-Sponges for Temperature-Independent Stress Sensing and Stress-Triggered Joule Heating

To obtain ideal sensing materials with nearly zero temperature coefficient resistance (TCR) for self-temperature-compensated pressure sensors, we proposed an Incipient Network Conformal Growth (INCG) technology to prepare hybrid and elastic porous materials: the nanoparticles (NPs) are first dispersed in solvent to form an incipient network, another component is then introduced to coat the incipient network conformally via wet chemical route. The conformal coatings not only endow NPs with high stability but also offer them additional structural elasticity, meeting requirements for future generations of portable, compressive and flexible devices. The resultant polypyrrole/silver coaxial nanowire hybrid aero-sponges prepared via INCG technology have been processed into a piezoresistive sensor with highly sensing stability (low TCR 0.86 × 10(-3)/°C), sensitivity (0.33 kPa(-1)), short response time (1 ms), minimum detectable pressure (4.93 Pa) after suffering repeated stimuli, temperature change and electric heating. Moreover, a stress-triggered Joule heater can be also fabricated mainly by the PPy-Ag NW hybrid aero-sponges with nearly zero temperature coefficient.

From anisotropic graphene aerogels to electron- and photo-driven phase change composites

To overcome fatal shortcomings of organic phase change materials (PCMs), such as leakage during work, low thermal conductivity and shortage of multiple driving ways, we propose a novel strategy to synthesize structurally, mechanically, electrically and optically anisotropic graphene aerogels (AN-GAs) by using gaseous hydrogen chloride to in situ solidify ordered graphene oxide liquid crystals followed by chemical reduction, supercritical fluid drying and annealing in an Ar atmosphere in sequence. The confined pore space and aligned wall structure of the resulting AN-GAs have benefited crystallization of organic phase change molecules and thus highly efficient phase change composites (PCCs) are fabricated with long durability and good strength. The resulting PCCs can also be driven either by applying a small voltage (1–3 V) with high electro-heat efficiency (up to 85%) or by irradiating with weak sunlight (0.8–1.0 sun) with high photo-heat efficiency (up to 77%).

Template synthesis of high-density carbon nanotube arrays

Carbon nanotube arrays with densities up to 10 12 cm ˇ2 , have been successfully synthesized by pyrolysis of acetylene within high ordered nanochannel arrays in an alumina template. Transmission electron microscopy and Raman spectrum analysis show that the carbon nanotubes are well graphitized. The influence of preparation conditions on the structure of carbon nanotubes within the alumina template is discussed. # 2001 Elsevier Science B.V. All rights reserved.

Carbon nanotubes filled partially or completely with nickel

We report the catalytic synthesis of carbon nanotubes filled with Ni by chemical vapor deposition over the Raney-Ni catalyst. Straight and two types of bamboo-shaped carbon nanotubes have been discovered under TEM. Further investigation with TEM shows that an obvious tendency exists, i.e. the straight nanotube encapsulating Ni completely; as for the compartments of bamboo-shaped nanotube, either every one filled with a small Ni particle or only the end is capped with a needle-shaped Ni, which suggests different growth mechanism.

Smart and flexible supercapacitor based on a porous carbon nanotube film and polyaniline hydrogel

Smart and flexible supercapacitors are of ever increasing importance for energy-storage devices. Herein, a simple and effective strategy to make a supercapacitor in a “flexible” and “electrochromic” way was based on the direct and convenient immobilization of a polyaniline (PANI) hydrogel onto a porous carbon nanotube (CNT) film. The resultant symmetric all-solid-state supercapacitor showed a favorable specific capacitance of 315 F g−1 and a relatively high cycling stability (92% capacitance retention after 1500 cycles). The bending test indicated that the obtained supercapacitor was flexible and its performance was only decreased by less than 7.0% after 150 bending cycles with a bending angle of 180°. Furthermore, the supercapacitor displayed a rapid and reversible chromatic transition between different working stages, and its level of stored energy could be quickly determined by the naked eye in a predictable and noticeable manner. This work may open a simple and convenient avenue for the fabrication of smart, flexible and all-solid-state supercapacitors.

A versatile ambient pressure drying approach to synthesize silica-based composite aerogels

A versatile ambient pressure drying (APD) method for synthesizing silica aerogels, as well as silica-based composite aerogels with different colors or functions, has been developed in this work. The presented polyethoxydisiloxane (PEDS) was held under basic conditions in ethanol and alcogels were obtained, which was followed by hexane solvent-exchange and pore surface modification, and then the wet gels were dried under ambient pressure at temperatures ranging from 25 to 150 °C to produce silica aerogels with high BET surface areas and low thermal conductivities. Various guests were introduced, e.g., dyes, Fe3O4 nanoparticles and Ag nano-wires in the sol–gel process, and colored, magnetic or bio-functional silica aerogels were synthesized by the APD method. All the products were of high BET surface areas varying from 500 to 1000 m2 g−1 and of average pore sizes in the range of 8–20 nm, regardless of the drying temperature and the presence of dye molecules or embedded nano-particles. The tapping densities of the aerogels were in the range of 0.03 to 0.2 g cm−3, and the thermal conductivities were found varying from 0.022 to 0.040 W m−1 K−1. Moreover, the structures and morphologies of the aerogels were investigated by FTIR and SEM, respectively. The stability of the colored aerogels was studied by UV-vis spectrometry, and the results indicate that the dyes were embedded in the aerogels and may be gradually released. Furthermore, the capacity of the magnetic aerogels for wastewater treatment was investigated. In general, an APD method, involving a single hexane-exchange step, has been developed, and the preparation of various silica-based composite aerogels prepared by APD is achieved. This APD method shows a great potential to synthesize silica aerogels and its hybrid aerogels on an industrial scale.

Evaluation of effective mass transport coefficients through comparison of solidification on the ground and on board a satellite

Mass transport caused by buoyancy convection in front of the solid–liquid interface was evaluated in terms of measurements of primary dendritic spacing combining with separation of the effective (or integral) mass transport coefficient DL. It was shown that DL in normal gravity (1g) condition was 1.64 times as high as that in microgravity (μg) condition at the cooling rate (v) of 0.056 K/s for Pd40Ni40P20 alloy. The higher DL value is due to the contribution of buoyancy convection on the ground.