Wenlan Li

Electroless silver coating on fine copper powder and its effects on oxidation resistance

Electroless silver coating on fine copper powder (3.4 μm) and its effects on oxidation resistance were investigated by varying the silver contents. As-coated copper powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the uniformity of silver coating was improved with silver content. When the silver content reached 20 wt.%, silver was homogeneously distributed around the copper particles and few free silver particles were detected. As a result, the sheet resistances of metal films reduced with the silver content (presenting the improvement of oxidation resistance), and at the level of 20 wt.% silver content, it had a minimum value and hardly increased with the increasing oxidation time.

Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering

Abstract SPS (spark plasma sintering) process was used to sinter nanocrystalline ZrO 2 (3Y). It was found to be different with the usual rapid sintering method, the density of the samples kept increasing with the rising of the sintering temperature. A higher density could be reached at a lower temperature and shorter dwelling time than that by hot-pressing under the similar pressures. In contrast to the samples with a differential densification from edge to center prepared by a rapid hot-pressing, no obvious densification gradient could be found in the samples sintered by SPS. The grain sizes of the Y-TZP obtained by SPS were smaller than those by the pressureless sintering method, while the grain growth speed was much higher under SPS conditions. All these unique sintering behaviors were explained by the special sintering process of SPS.

Combustion synthesis of network silicon nitride porous ceramics

A porous silicon nitride ceramics, composed of a network interconnected β-Si3N4 rods, was prepared by using the combustion synthesis method. Si powder with more than 20 wt.% diluent α-Si3N4 was fully nitrided under high nitrogen pressure. The interconnected open porosity of the obtained combustion products was as high as 64.40%. In the combustion products, rod-like β-Si3N4 grains were formed to build up the network silicon nitride porous ceramics.

Nano ZrO2 (Y2O3) particles processing by heating of ethanol-aqueous salt solutions

Abstract A new method of heating a ZrOCl 2 and Y(NO 3 ) 3 solution with an ethanol–water mixture as a solvent was used to synthesize ZrO 2 (Y 2 O 3 ) nanoparticles. The reaction mechanism, the influence of heating temperature and time on synthesizing process and final powder characteristics were investigated. It was revealed that during the heating period, the hydrolysis reaction of Y(NO 3 ) 3 was strongly suppressed and no Y(NO 3 ) 3 would precipitate. The composition of the precipitate was similar to when only ZrOCl 2 ·8H 2 O was heated in the alcohol–aqueous solution. Only when the heating temperature was high enough that the dielectric constant decreased to

Study of Ni-Cr-Co-W-Mo-B interlayer alloy and its bonding behaviour for a Ni-base single crystal superalloy

A new Ni-Cr-Co-W-Mo-B alloy was selected as an interlayer alloy for transient liquid phase (TLP) bonding of a Ni-base single crystal superalloy. The microstructure and wettability on the substrate of the interlayer alloy were investigated. The microstructure characteristics in the bond during TLP process were evaluated. Single crystal could be achieved during TLP bonding if the crystallographic orientation is maintained in both mating halves

Improving thermal conductivity of Sm2O3-doped AlN ceramics by changing sintering conditions

Abstract The effects of sintering conditions including soaking time and packing powder on the thermal conductivity of Aluminum nitride (AlN) ceramics with Sm 2 O 3 additives were investigated. The thermal conductivity values of AlN ceramics sintered were improved after increasing soaking time, but not considerably due to the interconnected secondary phases. While the specimen sintered without packing powder showed much higher thermal conductivity by 58 W·m −1 ·K −1 than the specimen sintered with packing powder. It is due to the removal of the secondary phase with poor thermal conductivity to a fairly low level, and the decreasing of the oxygen content via carbothermal reduction.

Microwave sintering of α/β-Si3N4

Abstract Dense silicon nitride from α- and β-Si 3 N 4 powders with 6 wt% (Y 2 O 3 + Al 2 O 3 ) additives were fabricated in a TE 103 single mode cavity microwave sintering system operating at 2.45 GHz. Packing powders were used. The sintering behaviour was investigated with special interest in the evolution of the α- to β-Si 3 N 4 phase transformation, densification and microstructure development. Densities of about 96.7% TD and 95% TD were achieved by microwave sintering of α- and β-Si 3 N 4 powders; the α to β conversion is more rapid than densification in the microwave heating of α-Si 3 N 4 .

Preparation and growth mechanism of β-Si3N4 rod-like crystals by combustion synthesis

Abstract This paper presents the preparation of β-Si 3 N 4 rod-like crystals that have an average aspect ratio of ∼10 by combustion synthesis of the mixture of Si, α-Si 3 N 4 and Y 2 O 3 under 5 MPa nitrogen pressure. By using a quenching device, the growth morphology of β-Si 3 N 4 rod-like crystals was observed and it is concluded that the growth mechanisms result from the synergy of VLS (vapor–liquid–solid) and VS (vapor–solid).

Mechanism of the formation of β–Sialon by self-propagating high-temperature synthesis

β–Sialon has been prepared by SHS (Self-propagating High-temperature Synthesis) with Si, Al, Al 2 O 3 , and β–Si 3 N 4 under high nitrogen pressure. Rod-like crystals of b–Sialon with an aspect ratio of 4–15 and whiskers with various aspect ratios were obtained in the process. Considering the morphologies of the products, it is concluded that VLS (vapor-liquid-solid) is the predominant mechanism of the formation of rod-like crystals and whiskers.

Combustion synthesis of tungsten carbides under electric fieldI. Field activated combustion synthesis

Abstract The activation of self-propagating combustion reactions in the W–C system was achieved by using an electric field. Self-sustaining combustion in elemental reactants with the composition corresponding to WC can be activated only when the imposed field is above the threshold value of 1V cm −1 . The nature of the combustion products depended on the magnitude of the field. The product consisted of two kinds of tungsten carbides, WC and W 2 C. The effects of tungsten particle sizes and the relative densities of the reactant compacts on the synthesis of tungsten carbide were also investigated. Finally, the reaction mechanism of tungsten carbide was proposed. X-ray and microscopic analysis of the quenched combustion front suggested that the synthesis of WC is a process involving the solid diffusion of carbon into a carbide layer. W 2 C is the intermediate phase between WC and reactants (W and C).