Key Engineering Materials Vol. 616

Abstract: In the present study, a new method for preparing zirconium phosphate bonded silicon nitride (Si₃N₄) ceramic coatings with low porosity was developed using cold spray and pressureless sintering technique. Zirconium phosphate (ZrP₂O₇) was used as a binder material instead of the traditional organic materials, in order to prevent the residual carbon which was severe to the dielectric properties of the Si₃N₄ ceramics. Firstly, aqueous Si₃N₄ slurries with 0~40 wt.% solid loading were prepared, using deionized water as the liquid medium. Then the Si₃N₄ slurries were cold sprayed on the Si₃N₄ porous ceramic substrates and finally the samples were presurelessly sintered at 1000 ^oC to achieve the Si₃N₄ ceramic coatings. The rheological properties and viscosity of Si₃N₄ slurries were investigated in detail, as a function of solid loading. The optimum cold spray parameters were spray distance 21 cm and spray time 20 s. The porosity of the obtained Si₃N₄ ceramic coating was ~5 % and the interface between the coating and substrate was well bonded, without microcracks, indicating a good thermal physical match between the coating and substrate.

Abstract: Titanium carbide (TiC) was consolidated with 20 mol% zirconium carbide (ZrC) by spark plasma sintering in the temperature range of 1773–2473 K, and the phase formation, microstructure, relative density and mechanical properties were investigated. The composite consisted of Ti-rich (Ti, Zr)C and Zr-rich (Zr, Ti)C solid solutions at 1773–2373 K, and was single-phase (Ti, Zr)C at 2473 K. The relative density of the composite was over 98% above 2073 K. The composite prepared at 2273 K exhibited the maximum H_V of 29.7 GPa with the K_IC of 3.76 MPa m^1/2.

Abstract: WC-8wt%Co nanopowder was consolidated by spark plasma sintering at process temperatures (T_SPS) from 1100 to 1400 °C. The nanoindentation hardness and Young’s modulus of the consolidated specimens were measured under different peak load levels (P_max). The hardnesses and modulus of WC-8wt% Co shows a clear dependence on the microstructures and peak load levels. At 1200 and 1300 °C, the hardness and modulus were higher than those at 1100 and 1400 °C due to the higher relative density and fine grain size. The relationship of stiffness (S) and contact depth (h_c) of nanoindentation was discussed.

Abstract: High-performance copper-coated tungsten composite powders were successfully prepared using electroless plating at an appropriate pH in the plating bath. The effect of pH value in the plating bath on the microstructure and purity of the coated Cu layer was studied systematically. With the increase of pH in the bath, the surface roughness, particle size and average grain size of coated Cu increased, and the particle shape gradually changed from round into square block. At pH 12–12.5, coated Cu was highly pure with very little oxygen content (less than 0.09 wt.%). The promotion effect of the increased pH on both main electroless plating reaction and side reactions causes the changes of surface morphology, average grain size of Cu and the oxygen content in the composite powders.

Abstract: In this paper, the W-Cu functionally graded material (FGM) was prepared by using the non-aqueous tape-casting technique combined with vacuum hot-pressing sintering. The graded composite material with high density, uniform transition and graded component was designed by 7 layers with the copper content range from 40 to 100 wt. %. Then the structures and properties of the composite were characterized. The scanning acoustic microscope (SAM) results for the W-Cu graded material showed that the interface between different layers was of high smoothness and parallel. The SEM-EDS results of cross section show that the W and Cu content changed gradually along the laminating direction after sintering. The equivalent electrical conductivity and the equivalent thermal conductivity of the W-Cu graded material were 0.3976×10⁸ S/m and 323.5 W/(m·K), respectively, which were much higher than that of the W-40 wt. % Cu homogeneous composite. The Vickers hardness of the high tungsten content surface and the high copper surface were 163 HV and 80 HV, respectively, which were same with that of the homogeneous material.

Abstract: Uni-directionally arranged mesoporous titanium oxide powders were prepared for titanium sheet through electrochemical anodization, one-step anodization with mixture of HNO₃ and H₂SO₄ or two-steps anodization with HNO₃ after H₂SO₄. The characterization of novel titanium oxide was carried out by XRD, XPS, SEM and TEM. The doping of S and N was confirmed with XPS evaluations during the anodization process in mixture of HNO₃ and H₂SO₄. Furthermore, the S and N co-doped titanium oxide possessed the unique mesoporous structure.

Abstract: The flip-flop property is an attractive phenomenon on the surface of Nafion^® thin film. The sulfonic groups which are hydrophilic can change the wettability of the surface by lift up or curling down. In this study, an applied voltage makes the control of sulfonic groups become available. It has been proved that when a positive voltage is loading to the film through a conductive droplet, the flip-flop property can be enhanced. Being the opposite, it also can be inhibited by a negative voltage.

Abstract: Grafting reactions between an n-decoxy derivative of ion-exchangeable layered perovskite HLaNb₂O₇·xH₂O and polyethylene glycols (PEGs) with various molecular masses were investigated. X-ray diffraction analysis showed that a reflection corresponding to the interlayer distance of the n-decoxy derivative of HLaNb₂O₇·xH₂O (2.73 nm) disappeared and that new diffraction lines ascribable to smaller interlayer distances, which increased with increases in the molecular mass of PEGs, appeared. The solid-state ¹³C CP/MAS NMR spectra of the products indicated that ethylene oxide chains were covalently bound to the interlayer surface of HLaNb₂O₇∙xH₂O. The signal assignable the carbons bound to OH groups suggests that the grafting reactions for the formation of Nb-O-C bonds typically involve one of the terminal OH groups in PEGs.

Abstract: Carbon-doped Al₂O₃ is used as a material for dosimeter using thermoluminescence (TL) technique or optically stimulated luminescence (OSL) technique, and the emission bands are originated from defect emission. We have an original crystal growth named micro-pulling down (μ-PD) method which had higher temperature graduation during the crystal growth. Thus, crystal defect is easy to be formed without some dopant using the μ-PD method. We grew pure Al₂O₃ samples with different heating condition, and different peaks in TL spectra were obtained among the pure Al₂O₃.

Abstract: We evaluated optical and scintillation properties of a Cr 0.5%:Y₃Ga₅O₁₂ (YGG) and Cr 0.5%:Y₃Al₅O₁₂ (YAG) crystals grown by the micro-pulling down method for infrared emission scintillators. In the photoluminescence and X-ray induced radioluminescence measurements, the emissions peaks of Cr:YGG were located in the longer wavelength than those of Cr:YAG. Radioluminescene spectrum of Cr:YGG had a broad peak at 716 nm related to Cr^{3+ 2}E→⁴A₂ and ⁴T→⁴A₂ transitions.