Advanced Materials Research Vol. 412

Abstract: The micro-sized silver used for solar cell grid electrode was obtained from silver nitrate via chemical reduction under ultrasonic wave and alkaline environment using polyethylene glycol (PEG-6000) as dispersant and alcohol as surfactant, and after agitation, the sol was filtered, washed and dried at constant temperature. The PEG and alcohol protective mechanism in the preparing process of spherical silver particles was studied. Effects of the amount of dispersant, pH, dosage of alcohol, were investigated. The silver powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Results show that with 0.35mol/LAgNO₃ solution, 25mL ethanol, T = 40°C, pH = 8 and 10min ultrasonic wave, spherical silver power about 1.38 ~ 2.11μm with narrow particle size distribution was fabricated. The paste prepared with ready-made silver powder was printed on the silicon and the contact resistance is very low, indicating the electrical property of the prepared silver is qualified for solar cells.

Abstract: Flake copper powder is coated with silver by using a solution containing [Ag (NH₃)]⁺. Factors such as copper particle size distribution, [Ag (NH₃)]⁺ concentrations and the dispersing agent are studied and correlated with the silver content of coated copper and electric conductivity. At the condition of silver concentration of 0.8 mol/L and dispersing agent of 1.0g/L a powder of copper coated with silver with electric conductivity of 0.8×10^-3Ω·cm is obtained. XRD patterns indicate that the powder consists of only the Cu and Ag phases without other intermediate phases. SEM results with different silver contents reveal that when Ag content rises, its surface topography transforms through the steps: individual particles → small islands → continuous 2D layers → multi-layers.

Abstract: A novel glycine-nitrate process (GNP)-reduction method has been developed to fabricate ultrafine tungsten heavy alloy powders, with ammonium metatungstate (AMT), iron nitrate nonahydrate (Fe (NO₃)₃·9H₂O), nickel nitrate hexahydrate (Ni (NO₃)₂·6H₂O) as raw materials and gylcine as a complexant and incendiary agent. Precursor powders were obtained by self-propagation reaction in a suspension containing above materials. The precursor powders were then hydrogen-reduced to obtain composite powders with 90W-7Ni-3Fe composition (wt.%). Phase constitution and morphology of the precursor powders and the reduced powders were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). Chemical composition of resultant powders was analyzed by energy dispersive spectrometer (EDS) analysis. It was shown that the GNP-reduction method produces ultrafine 90W-7Ni-3Fe powders with particle size of about 200 nm and highly dispersion of the composition.

Abstract: Cu-Zn alloy was prepared by high energy ball milling of elemental copper and zinc by the Simoloyer attrition mill, the different parameters such as milling time, ball-to-powder ratio and rotational speeds were analyzed. The results show that the different Cu-Zn alloy phase can be produced by different ball milling parameters, It has been found that milling time is highly significant to refining process, and the ratios of ball to powder are also benefited to the new phase form.

Abstract: The higher mechanical strength of Al₈₇Ce₃Ni_8.5Mn_1.5 nanophase amorphous composites has been obtained with two methods. The first nanophase amorphous composites are directly produced by the single roller spin quenching technology. The method taken for the second nanophase amorphous composites is at first to obtain amorphous single-phase alloy, followed by annealed at different temperatures .The formative condition, the microstructure, the particle size, the volume fraction of α-Al phase and microhardness of nanophase amorphous composites etc have been investigated and compared by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The microstructure of composites produced by the second method is higher than the former, the fabricated material structure of the system is more uniform and the process is easier to control.

Abstract: The dispersion behavior of Li_1.075Nb_0.625Ti_0.45O₃ (LNT) powders in aqueous media was studied in this paper. The effect of pH, concentration of ammonium polyacrylate (PAA-NH₄) dispersant and solids loading on the dispersion of LNT powders was reported. Optimum dispersing conditions were investigated in terms of zeta potential, sedimentation, mean particle size (d₅₀), and rheology measurements. After adsorption of 0.6 wt% PAA-NH₄, the absolute zeta potential reached the maximum value of 90 mV. The computed specific interaction energy (-ΔG⁰_SP=8.6 kcal/mol) between particles and dispersant, indicated strong adsorption of PAA-NH₄ on the LNT powder surface. Good agreement between zeta potential, mean particle size, sedimentation, and rheological test was found.

Abstract: In order to prepare ultrafine La₃NbO₇ powder, a potential material for thermal barrier coatings, the calcination process of La₃NbO₇ was studied in this paper.The precursor of La₃NbO₇ was synthesized by using a citric acid complex method. A calcination process had been systematically investigated. The reaction temperature was determined by differential scanning calorimetry (DSC). The phase composition of powders was characterized by X-ray diffraction (XRD), and the morphology was obtained by scanning electron microscope (SEM). The results revealed that the single-phase La₃NbO₇ powder could be successfully prepared while the calcination temperature exceeded 800°C and a better morphology could be maintained at 800°C for 4 hours. Considering all above, an optimum calcination scheme was adopted at 800°C for 4 hours. The as-prepared La₃NbO₇ powders had a grain size of about 50nm and an average particle size of about 300nm.

Abstract: La-doped barium titanate ceramics with 0~1.0 at.% La₂O₃ were prepared by conventional solid state reaction method. The crystal structure and dielectric properties of La-doped barium titanate ceramics were investigated. Lanthanum ions have entered the unit cell and substituted for Ba²⁺ ions on the A sites of perovskite structure. The a-axis lattice constant increases and the c-axis lattice constant decreases as lanthanum content increases. Addition of lanthanum leads to the fall of the Curie temperature of La-doped BTO ceramics. There are two different doping mechanisms for La-doped BTO ceramics. The dielectric loss of BTO ceramics with 0.3 at.% La₂O₃ is higher than that of pure BTO ceramics. When lanthanum content is above 0.5 at.%, the dielectric loss of La-doped BTO ceramics decrease with the increasing of lanthanum content. The result of dielectric constant as a function of temperature at different frequencies indicates that La-doped BTO ceramics are not relaxor ferroelectrics but ferroelectrics with diffuse phase transition.

Abstract: The effect of CaO-B₂O₃-SiO₂ glass on the densification, microstructure and microwave dielectric properties of Ba₄Nd_9.33Ti₁₈O₅₄ ceramics was investigated in this work. It was found that the addition of a few amount of glass (≤2%) could effectively decrease the sintering temperature owing to the viscous sintering. However, locally porous structures were formed when overmuch glass was added because of enwrapped air bubbles and rapid grain growth. In addition, the amount of remaining glass after sintering could be decreased due to the re-crystallization of CaO-B₂O₃-SiO₂ glass. Sol-gel derived Ba₄Nd_9.33Ti₁₈O₅₄ ceramics containing 2 wt% glass can be well densified at 950°C and exhibit good microwave dielectric properties of ε_r=68 and Q×f=7400 GHz.

Abstract: First, (K,Na)NbO₃ (abbreviated as KNN) powders were synthesized by microwave hydrothermal method using Nb₂O₅, NaOH and KOH as raw materials. The effects of NaOH/KOH mole ratio and reaction temperature on the structure of KNN powders were studied systematically. Near spherical KNN powders of about 800 nm in diameter can be obtained at the optimized processing parameters as follows: NaOH/KOH mole ratio was 1.40/4.60, reaction temperature was 200 °C. Second, the ceramics were successfully prepared from the microwave hydrothermal synthesized KNN powders under 1090 °C for 2 h. This ceramic sample showed the enhanced piezoelectric properties such as piezoelectric constant d₃₃=142 pC/N and planar electromechanical coupling coefficient k_p=38%, in addition to other good properties as relative dielectric constant ε_r=426, Curie temperature T_c=410 °C, remnant polarization P_r=17.45μC/cm² and coercive field E_c=1.41 kV/mm, indicating that microwave hydrothermal method can improve the properties of KNN ceramics high efficiently.