On the thermal conductivity of sintered metallic fibre structures

The present paper investigates the anisotropic thermal conductivity of a novel sintered metallic fibre structure with different porosities (i.e. 51.9%, 74.9%, 81.4%). Different methodologies are applied: numerical calculations (i.e. Finite Element and Lattice Monte Carlo methods) based on micro-computed tomography images, experimental tests (i.e. steady-state plate method) and analytical modelling. Good agreement between the numerical methods and experimental measurements is obtained for high porosity models (i.e. porosity > 72.5%). Furthermore, the thermal conductivity decreases with increasing porosity. A distinct thermal anisotropy is found where maximum values are in the parallel direction and minimum values in the transverse direction to the fibres.     

All electrochemical layer deposition for crystalline silicon solar cell manufacturing: Experiments and interpretation

A manufacturing process for crystalline silicon solar cells is presented which consists mainly of electrochemical steps. The deposition of doping glass layers for the front side emitter as well as the back surface field is performed anodically onto the etched and cleaned wafers. The doping atoms, phosphorus or boron, are diffused into the silicon crystal in a furnace at 950 °C in an atmosphere of simply clean air. After the diffusion process the front side doping glass has a blue colour and is suitable to serve as an antireflection coating with a very low surface recombination velocity. For this reason, the doping glass is not etched away on the sun exposed regions of the solar cell. The masking technology for all electrochemical processes provides inherently an edge exclusion and, therefore, no additional processing for preventing shorts on the wafer edge is necessary. For the metallization a reusable rubber mask defines the pattern. First, the mask is used for the doping glass patterning by wet chemical etching. Then, on both sides first nickel is deposited electrolytically directly onto silicon, and in a second step copper electroplating onto the nickel barrier is performed. All three steps, etching, nickel and copper deposition are self adjusting through said rubber mask. A short forming gas anneal finishes the solar cell processing. During all electrochemical processing the wafer is electrically contacted on the opposite surface on a stainless steel plate by the force of vacuum clamping. With this low cost processing 12.5% cell efficiency has been achieved on multi-crystalline 156 mm wafers, which originally have a minority carrier lifetime of 4 μs measured after damage etch and thermal oxidation. In this paper, experiments, surface analysis and physical interpretations are presented.     

Deposition and electrical characteristics of S-doped boron nitride thin films

cBN/Si n–p heterojunctions have been fabricated and characterized. n-type cBN films were grown on p-type Si wafers using RF reactive sputter, and the n-type cBN films were obtained by adding S (sulfur) into working gas. The I–V (current–voltage) characteristics have obvious rectification. The fitting results show that the current transporting model for the cBN/Si n–p heterojunctions is the same as Anderson’s transporting model. The C–V (capacitance–voltage) characteristics are close to that of ideal heterojunctions. The built-in potential in the cBN/Si n–p heterojunctions was determined from C–V measurements at 4.71 V. The dopant concentration in the n-type cBN films was also determined from the C–V measurements at 6.50 × 10¹⁴/cm³.     

Microwave-assisted hydrothermal synthesis of zinc oxide particles starting from chloride precursor

Zinc oxide (ZnO) was synthesized using a microwave assisted hydrothermal (MAH) process based on chloride/urea/water solution and under 800 W irradiation for 5 min. In the bath, Zn²⁺ ions reacted with the complex carbonate and hydroxide ions to form zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)₆·H₂O), and the conversion from Zn₄CO₃(OH)₆·H₂O to ZnO was synchronously achieved by a MAH process. The as-prepared ZnO has a sponge-like morphology. However, the initial sponge-like morphology of ZnO could change to a net-like structure after thermal treatment, and compact nano-scale ZnO particles were finally obtained when the period of thermal treatment increased to 30 min. Pure ZnO nanoparticles was obtained from calcination of loose sponge-like ZnO particles at 500 °C. The analysis of optical properties of these ZnO nanoparticles showed that the intensity of 393 nm emission increased with the calcination temperature because the defects were reduced and the crystallinity was improved.     

Single-crystal oxoborate (Pb3O)2(BO3)2WO4: Growth and characterization

An oxoborate, (Pb₃O)₂(BO₃)₂WO₄, has been prepared by solid-state reaction methods below 620 °C. Single-crystal XRD analysis shows that it crystallizes in the orthorhombic group Cmcm with a = 18.480(4) Å, b = 6.3567(13) Å, c = 11.672(2) Å, Z = 4. The crystal structure is composed of one-dimensional 1/∞ [Pb₃O]⁴⁺ chains formed by corner-sharing OPb₄ tetrahedra. BO₃ and WO₄ groups are located around the chains to hold them together via PbO bonds. The IR spectra further confirmed the presence of BO₃ groups. Furthermore we have performed theoretical calculations by employing the all-electron full potential linearized augmented plane wave (FP-LAPW) method to solve the Kohn Sham equations. Starting from our XRD data we have optimized the atomic positions by minimizing the forces. These are used to calculate the electronic band structure, the atomic site-decomposed density of states, electron charge density and the chemical bonding features. The calculated electronic band structure and densities of states suggest that this oxoborate possesses a wide energy band gap. The valence band maxima and the conduction band minima are located at Y point in the Brillouin zone resulting in a direct energy band gap of 2.3 eV using the local density approximation and 2.6 eV for the Engel–Vosko generalized gradient approximation. This compares well with our experimentally measured energy band gap of 2.9 eV. From our calculated electron charge density distribution, we obtain an image of the electron clouds that surround the molecules in the unit cell of the crystal. The chemical bonding features were analyzed and the substantial covalent interactions are observed between Pb and O, B and O and W and O atoms.     

Characterizations and electrochemical performance of pure and metal-doped Li4Ti5O12 for anode materials of lithium-ion batteries

Pure and metal (Cu, Al, Sn, and V)-doped Li₄Ti₅O₁₂ powders are prepared with solid-state reaction method. The effects of dopants on the physical and electrochemical properties are characterized by using TGA, XRD, and SEM. Compared with pure Li₄Ti₅O₁₂, metal-doped Li₄Ti₅O₁₂ powders show structural stability and enhanced lithium ion diffusivity brought by doped metal ions. Voltage characteristics and initial charge–discharge characteristics according to the C rates in pure and metal-doped Li₄Ti₅O₁₂ electrode materials are studied. Pure Li₄Ti₅O₁₂ powder shows a relatively good discharge capacity of 164 mAh/g at a rate 0.2C, and some of metal-doped Li₄Ti₅O₁₂ powders show higher discharge capacities. Metal-doped Li₄Ti₅O₁₂ powders are promising candidates as anode materials for lithium-ion batteries.     

Electrochemical properties of magnesium doped LiFePO4 cathode material prepared by sol–gel method

Magnesium doped Li_1?2xMg_xFePO₄/C (x = 0.00, 0.01, 0.03, 0.05) cathode materials were synthesized by sol–gel method, and the effect of magnesium doping as well as its content on the electrochemical properties for lithium batteries was also investigated_. Their morphology was studied with field emission scanning electron microscope and Li_1?2xMg_xFePO₄ materials showed the olivine phase without impurities. The thin carbon layer of Li_1?2xMg_xFePO₄/C was confirmed by high resolution transmission electron microscopy. The magnesium doped Li_1?2xMg_xFePO₄/C particles were smaller than those undoped. The Li_1?2xMg_xFePO₄/C materials showed better cycling behavior than undoped LiFePO₄, especially at high C-rate in which Li_0.94Mg_0.03FePO₄/C composition exhibited the best electrochemical properties.