Gaojie Xu

Thermoelectric properties of the Bi- and Na- substituted Ca3Co4O9 system

Bi- and Na-substituted Ca3Co4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of Bi3+ and Na+ for Ca2+, as well as Bi3+ and Na+ double substitution, cause both the electrical conductivity (σ) and thermoelectric power (S) to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit ZT (=S2σT/κ) reaches 0.32 at 1000 K in the double-substituted sample.

Strategies for improving the cyclability and thermo-stability of LiMn2O4-based batteries at elevated temperatures

Lithium manganese oxide (LiMn2O4) is one of the most suitable cathode materials for widespread large-scale applications of lithium ion batteries due to its advantages of high thermal stability, guaranteed safety, low cost, environmental friendliness, relatively good power density and acceptable energy density. However, LiMn2O4-based batteries with LiPF6-based carbonate electrolytes always suffer from severe capacity deterioration and poor thermostability, especially at elevated temperatures. Hence, it is necessary to systematically and comprehensively summarize the progress in understanding and modifying LiMn2O4-based batteries from various aspects. In this review, different reported possible fading mechanisms of LiMn2O4-based batteries are first discussed in detail. Then, the various proposed strategies to improve the cyclability and thermostability of LiMn2O4-based batteries at elevated temperatures, including electrolyte optimization, element doping, surface coating, development of functional separators/gel electrolytes and binders, control of special morphologies etc., are covered. Finally, several perspectives are summarized based on the elaborated progress and our own experimental evaluations, aiming to provide some possible attractive strategies and research directions for future upgrading of LiMn2O4-based batteries with improved high temperature performances.

High-temperature transport properties of Nb and Ta substituted CaMnO3 system

Abstract Polycrystalline samples of CaMn 1− x M x O 3 (M=Nb and Ta, 0≤ x ≤0.30) were studied by means of X-ray diffraction, electrical resistivity ( ρ ), thermoelectric power ( S ) and thermal conductivity ( κ ) at high temperature. Both Nb and Ta substitutions lead to increasing lattice parameters a and b and decreasing c . The Nb-doped samples possess lower ρ and | S | than that of Ta-doped samples, which is suggested to be related to the effective d – p hybridization between Nb and O. A good thermoelectric performance with the figure of merit Z =0.8×10 −4 K −1 at 1000 K was obtained for CaMn 0.96 Nb 0.04 O 3 .

Enhanced thermoelectric performance in p-type BiSbTe bulk alloy with nanoinclusion of ZnAlO

p-type BiSbTe/x wt % ZnAlO (x=0, 0.25, 0.5, 0.75, and 1.0) composites were prepared by zone melting method. A peak thermoelectric figure of merit (ZT) of 1.33 at 370 K was achieved for the sample with x=0.75, about 34% higher than that of BiSbTe at the same temperature, making these composites more attractive for commercial applications. This enhancement of ZT can be mainly ascribed to a remarkable increase of the electrical conductivity and a simultaneous decrease of the lattice thermal conductivity by the introduction of ZnAlO nanopowder.

Facile Approach to Large-Scale Synthesis of 1D Calcium and Titanium Precipitate (CTP) with High Electrorheological Activity

Nanorods of calcium and titanium precipitate (CTP) were prepared via a simple precipitation route in an ethanol/water mixed solution system under mild conditions. The obtained rodlike particles were highly uniform in width (23 ± 3 nm), and its length could be tuned by adjusting the concentration of oxalic acid reactant. The nanorods materials show a promising electrorheological activity, of which ER efficiency was about 4 times higher that of granular CTP suspensions. The facile synthesis route may be regarded as a green chemistry method, and its novelty relies on the large-scale production of 1D CTP giant ER materials.

Preparation of uniform titania microspheres with good electrorheological performance and their size effect

Titania is a potential high-performance electrorheological (ER) material due to its relatively high dielectric constant and polarization ability. The size and shape of titania particles are critical factors influencing ER activity. In this paper, we have developed an acetic acid (AA) assisted sol–gel strategy for the synthesis of uniform titania microspheres with good ER effect. The diameter of the titania microspheres (from 420 to 240 nm) can be tuned by changing the amount of AA added. The facile route has the great potential in bulk synthesis of the titania based ER materials. Results from surface area, rheological, conductivity and dielectric properties measurements indicate that the enhanced ER performance is due to the combined effect of the increased polar molecules content and reduced particle size.

Fabrication of uniform core–shell structural calcium and titanium precipitation particles and enhanced electrorheological activities

A simple co-precipitation route was developed to synthesize uniform core-shell structured calcium and titanium precipitation (CTP) particles with ideal morphology and no aggregation. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), and interface tension/contact angle (CA) measurement were utilized to characterize the components, structure, morphology, and wettability of the SiO(2)-CTP materials. The obtained core-shell structural SiO(2)-CTP particles were well dispersed spherical nanoparticles with a narrow size distribution. The electrorheological (ER) properties were studied by the shear stress under various electric fields. The SiO(2) (2.3 wt%)-CTP ER fluid showed notable ER activity with a shear stress of about 109 kPa (at 5 kV mm(-1)), which outclassed the shear stress (65 kPa) of the CTP ER fluid. The ER properties of samples can be tuned by a few factors in the experimental process, such as the concentration of SiO(2) particles and citric acid, pointing out the great potential for application of this route in bulk synthesis of many other types of ER materials.

Thermoelectric properties of Bi2.2−xPbxSr2Co2Oy system

Polycrystalline samples of Bi2.2−xPbxSr2Co2Oy (x=0, 0.4, and 0.8) with a layered structure have been prepared using a hot pressing technique and their thermoelectric properties methodically investigated. It is found that the substitution of Pb for Bi is a fairly effective means of improving the thermoelectric properties of the Bi2.2−xPbxSr2Co2Oy system owing to resistivity (ρ) decreasing and thermoelectric power (S) increasing simultaneously. The thermal conductivity (κ) at high temperature also decreases markedly with Pb substitution. As a result, for the x=0.8 sample the dimensionless figure of merit, ZT(=S2T/ρκ), increases with temperature and reaches 0.26 at 1000 K, which indicates that this material is a strong candidate as a high-temperature thermoelectric material.

Size-controllable synthesis of monodispersed colloidal silica nanoparticles via hydrolysis of elemental silicon

A new method is presented for preparing monodisperse and uniform-size silica nanoparticles using a two-stage hydrolysis of silicon powder in aqueous medium. The influence of synthesis conditions including solution composition and temperature on the formation of silica nanoparticles were systematically investigated. The structure and morphology of the silica particles were characterized via transmission electron microscopy (TEM) and dynamic light scattering (DLS). Various-sized particles in the range 10-100 nm were synthesized. The size of the nanoparticles can be precisely controlled by using a facile regrowth procedure in the same reaction media.

The influence of high dielectric constant core on the activity of core–shell structure electrorheological fluid

The core-shell structural dielectric particles are applied widely in the electrorheological (ER) fluids. The properties of the dielectric core are critical factors influencing their ER activity. In this paper, we successfully synthesized two kinds of core-shell hydroxyl titanium oxalate (TOC) particles with SiO(2) and TiO(2) as core, respectively. The obtained core-shell structural SiO(2)-TOC and TiO(2)-TOC particles were well-dispersed spherical nanoparticles with ideal morphology and a narrow size distribution. Under DC electric fields, the TiO(2)-TOC ER fluid showed notable ER activity with a yield stress of about 96 kPa (at 4 kV/mm), which is 3 times of that SiO(2)-TOC ER fluid and outclassed the yield stress of the TOC ER fluid. The dielectric spectra indicated that the higher dielectric constant of TiO(2) core induces the stronger interaction between the neighboring particles, which contribute to the enhancement of ER activity.