J.P. Tu

Tribological application of carbon nanotubes in a metal-based composite coating and composites

Abstract Ni–P–carbon nanotube (CNT) composite coating and carbon nanotube/copper matrix composites were prepared by electroless plating and powder metallurgy techniques, respectively. The effects of CNTs on the tribological properties of these composites were evaluated. The results demonstrated that the Ni–P–CNT electroless composite coating exhibited higher wear resistance and lower friction coefficient than Ni–P–SiC and Ni–P–graphite composite coatings. After annealing at 673 K for 2 h, the wear resistance of the Ni–P–CNT composite coating was improved. Carbon nanotube/copper matrix composites revealed a lower wear rate and friction coefficient compared with pure copper, and their wear rates and friction coefficients showed a decreasing trend with increasing volume fraction of CNTs within the range from 0 to 12 vol.% due to the effects of the reinforcement and reduced friction of CNTs. The favorable effects of CNTs on the tribological properties are attributed to improved mechanical properties and unique topological structure of the hollow nanotubes.

Syntheses and thermoelectric properties of Bi2Te3∕Sb2Te3 bulk nanocomposites with laminated nanostructure

Nanocomposites with constituent sizes of <50nm are considered as a promising approach to enhance the figure of merit of bulk thermoelectric materials. A simple route involving hydrothermal synthesis and hot pressing was used in this work to prepare Bi2Te3∕Sb2Te3 bulk nanocomposites. It is shown that the composites have a laminated structure composed of Bi2Te3 and Sb2Te3 nanolayers with the thickness varying alternately between 5 and 50nm. The transport measurements indicate that the nanoscale laminated structure improves the thermoelectric performance with the maximal dimensionless figure of merit of 1.47 for the nanocomposite hot pressed from Bi2Te3 and Sb2Te3 nanopowders.

In Situ Synthesis and Properties of Carbon-Coated LiFePO4 as Li-Ion Battery Cathodes

An in situ synthesis method for carbon-coated LiFePO 4 powders has been investigated in detail using inexpensive FePO 4 as an iron source and polypropylene as a reductive agent and carbon source. Thermogravimetric and differential thermal analysis of the precursor mixture indicated that the pyrolysis of polypropylene and the combination reaction of LiFePO 4 could be processed synchronously at a synthesis temperature between 500 and 800°C. X-ray diffraction analyses and scanning electron microscopy observations showed that LiFePO 4 /C composites with fine particle sizes and homogeneous carbon coating could be directly synthesized by the in situ method. The electrochemical performances of the carbon-coated LiFePO 4 powder synthesized at 700°C were evaluated using an electrochemical model cell by galvanostatic charge/discharge and cyclic voltammetry measurements. The in situ synthesized LiFePO 4 /C composite had a high electrochemical capacity of 164 mA h g - 1 at the 0.1C rate, and possessed a favorable capacity cycling maintenance at the 0.3 and 0.5C rates. The good electrochemical properties of the LiFePO 4 /C composite are suggested to originate from the good crystallinity, the fine particle sizes, and the efficient electronic conductive coating layer of the material.

Continuous Ni-layer on multiwall carbon nanotubes by an electroless plating method

Abstract Electroless plating has been successfully applied for nickel coatings on multiwall carbon nanotubes (MWNTs) grown by chemical vapor deposition (CVD). The samples before and after coating were checked using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results showed that the coating process can be divided into two steps: nickel was first deposited as nanoparticles at the activated sites on the pre-treated surface of carbon nanotubes in the initial stage; it was then thickened later, as the reaction time increased and eventually formed a continuous layer. Finally a uniform Ni-layer on individual tubes with thickness of 20–40 nm can be obtained after coating. A simple model for the mechanism of the coating is also discussed.

Tribological properties of Ni-P-multi-walled carbon nanotubes electroless composite coating

Abstract Carbon nanotubes (CNTs) have super-strong mechanical characteristics and unique hollow nanotube structure and are believed as very ideal materials for fabricating the excellent composites. In this paper, tribological properties of Ni–P-CNTs composite coating prepared by electroless technique were investigated using a ring-on-block test rig. The results indicated that addition of CNTs would result in an increase in hardness of the composite coating. Due to reinforcement and reduced friction, the tribological properties of Ni–P-CNTs composite coating were greatly improved. The Ni–P-CNTs composite coating exhibited not only higher wear resistance but also lower friction coefficient than the Ni–P–SiC and Ni–P-graphite coatings. After treatment at 400 °C for 2 h, the wear resistance of the Ni–P-CNTs electroless composite coating improved because of the Ni3P precipitation in Ni-matrix.

Improved thermoelectric figure of merit in n-type Cosb3 based nanocomposites

Nanocomposites offer a promising approach to incorporate nanostructured constituents to bulk thermoelectric materials. n-type CoSb3 nanocomposites were prepared by hot pressing the mixture of nanoscale and microsized CoSb3 powders synthesized by solvothermal method and melting, respectively. Microstructure analysis shows that the bulk materials are composed of nano- and micrograins. The nanocomposite structures are effective in reducing thermal conductivity more than electrical conductivity, hence in improving the thermoelectric performance. A dimensionless figure of merit of 0.71 is obtained for the nanocomposite with 40wt% nanopowder inclusions, about 54% increase of that without nanopowder inclusions.

Thermoelectric properties of Yb0.15Co4Sb12 based nanocomposites with CoSb3 nano-inclusion

Bulk skutterudite nanocomposites of Yb filled cobalt antimonide, Yb0.15Co4Sb12, with different contents of CoSb3 nano-inclusion were prepared by spark plasma sintering. The nanocrystals in the bulk composites are less than 100 nm. The Seebeck coefficient is enhanced by potential barrier scattering with the incorporation of a small number of CoSb3 nanoparticles into the bulk materials, but is reduced with a further increase in the content of nanoparticles. Though the nano-inclusion is the unfilled CoSb3, it is effective in reducing thermal conductivity. The figure of merit reaches a maximum of 0.9 for both the nanocomposites with 5% and 10% CoSb3 nano-inclusions, an increase of about 10% compared with the Yb0.15Co4Sb12 matrix. It still exhibits good thermoelectric performance after long-time annealing at 773 K for the nanocomposite due to the improved phase purity and almost unchanged nanostructure.

In-situ investigation and effect of additives on low temperature aqueous chemical synthesis of Bi2Te3 nanocapsules

Bi2Te3 nanocapsules with hollow structure have been successfully synthesized by a low temperature aqueous chemical route in an open system at 65 °C. The nanocapsules are 50–200 nm in diameter and 100–800 nm in length. The tube walls have a thickness of about 6 nm with the normal parallel to the c-axis of the Bi2Te3 crystal lattice. Chemical reactions during synthesis and the formation of the nanocapsules were investigated in-situ.

One-step Solid-state Synthesis and Electrochemical Performance of Nb-doped LiFePO4/C

Abstract Nb-doped LiFePO4/C was synthesized using one-step solid-state method. The effect of Nb contents on the electrochemical performance was investigated. The results showed that the electrochemical performances were remarkably promoted. The discharge capacities of Li0.96Nb0.008FePO4/C cathode materials were 161, 148, and 132 mAh·g−1 at the charge and discharge current rates of 0.5C, 1C, and 2C, respectively. The results of cyclic voltammetry and EIS analyses showed that the resistance and polarization of the LiFePO4/C composite electrode could be effectively decreased by Nb doping, which would improve the electronic conductivity of LiFePO4.

Formation of pyrite thin films by sulfidation annealing of iron films

Abstract Iron films deposited by magnetron sputtering on glass substrates were converted into FeS 2 films by sulfidation during isothermal annealing at 400°C. The formation of FeS 2 microcrystalline has been investigated under different annealing times. The conversion of Fe→FeS 2 may occur during annealing for 1 h. The grain size and the amount of FeS 2 phase significantly increase with the annealing time. However, the crystallization rate indicates an insignificant rise when the annealing time is longer than 10 h. The iron films can be substantially converted into FeS 2 thin films which have an approximate stoichiometric composition under the condition of sulfidation annealing from 20 h to 30 h.