Guanjun Qiao

Anisotropic Wetting Behavior Arising from Superhydrophobic Surfaces: Parallel Grooved Structure

It has been found experimentally that superhydrophobic surfaces exhibit strong anisotropic wetting behavior. This study reports a simple but robust thermodynamic methodology to investigate the anisotropic superhydrophobic behavior for parallel grooved surfaces. Free energy and its barrier and the corresponding contact angle and its hysteresis for various orientations of the groove structure are calculated based on the proposed thermodynamic model. It is revealed that the strong anisotropy of equilibrium contact angle (ECA) and contact angle hysteresis (CAH) is shown in the noncomposite state but almost isotropic wetting properties are exhibited in the composite state. Furthermore, for the noncomposite state, decreasing groove width and spacing or increasing groove depth can amplify the anisotropy for ECA. Meanwhile, decreasing groove width and increasing depth can amplify the anisotropy for CAH, while varying groove spacing can barely influence CAH. For the composite state, however, the surface geometry hardly leads to the anisotropic behavior. In addition, using a fitting approximation, a simple quantitative correlation between wettability and orientation can be established well, which is consistent with the numerical calculations.

Active brazing of pure alumina to Kovar alloy based on the partial transient liquid phase (PTLP) technique with Ni-Ti interlayer

Abstract Pure alumina (99.9% grade compositions) was brazed to 4J33-Kovar alloy based on PTLP technique with nickel and titanium foils interlayer. The solder formed a sandwich microstructure: an α-Ti solid solution belt at mid part and Ti 2 Ni intermetallics belts at two sides. This structure accounts for the good property of the joint. Microanalysis identified a reaction product at the alumina-braze interface as Ni 2 Ti 4 O phase. The special properties and structural compatibility of Ni 2 Ti 4 O contributed to the firm joint. The effects of brazing conditions on the joint properties were investigated. The joint shear strength showed the highest value of about 65MPa and did not monotonously increase with the brazing temperature ascending. It was shown that extending of brazing time gave thicker reaction layer and higher joint strength.

Mechanical properties and microstructure of Si/SiC materials derived from native wood

Mechanical properties and microstructure of Si/SiC ceramics derived from native wood were investigated. Three types of native natural wood (pine, birch, bamboo) and a man-made wood (MDF: medium density fiberboard) were pyrolyzed at 900°C in an inert atmosphere to prepare charcoal. The silicon melt was infiltrated into the porous charcoal while the sample was heated to 1600°C under vacuum. XRD analysis indicated that the final product prepared by this technique was a multi-phase dense solid, composed of β-SiC and free silicon. The material derived from birch showed the highest strength of 300 MPa and toughness of 4.0 MPa·√m, which increased with SiC content. These Si/SiC materials have a cell-pipe structure similar to the original natural wood. The material derived from MDF gave lower strength and toughness than that from natural wood with cell-pipe structure.

Preparation of macroporous SiC from Si and wood powder using infiltration-reaction process

Abstract A new type of porous β-SiC ceramics with good mechanical properties and high porosity was fabricated by infiltrating liquid silicon into woodceramics, which were prepared from basswood powders impregnated with phenolic resin. Microstructural observation and phase identification of resulting woodceramics and porous β-SiC were performed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Weight loss during heating of wood powder and phenolic resin in N 2 atmosphere was investigated by thermogravimetric analysis (TGA). Bending strength and bulk porosity of final SiC were also measured and calculated. Experimental results showed that woodceramics and porous β-SiC possessed topologically homogeneous pore structure, woodceramics were almost completely converted into porous β-SiC, and β-SiC possessed bending strength higher than woodceramics and high bulk porosity over 60%, and that final SiC ceramics were of pore microstructure pseudomorphous to woodceramics, and a potential candidate for absorption/separation application under severe conditions.

Wetting characteristics on hierarchical structures patterned by a femtosecond laser

Micro-scale hierarchical structures consisting of parallel grooves decorated by embossed triangle patterns are prepared by femtosecond laser irradiation on silicon (Si) wafers. The effects of surface morphology on wetting properties are investigated, and the results show that increasing the vertex angle of the triangle and groove spacing will lead to the enhancement of wettability and anisotropy, respectively. The structured surfaces also exhibit high adhesive force with droplets remaining attached to the surface even when the sample is turned upside down. Furthermore, the evaporation process of a water droplet on such an anisotropic surface is characterized to study its dynamic wetting behavior.

R-curve behavior of laminated SiC/BN ceramics

Laminated SiC/BN ceramics with a perfect layered structure was fabricated by tap-casting process and hot-pressing sintering. Damage resistance and R-curve behavior of laminated SiC/BN ceramics were evaluated using the indentation-strength-in-bending technique, and compared with those of monolithic SiC ceramics. The results showed that the indentation strengths for monolithic SiC ceramics decreased steeply with indentation load, while for laminated SiC/BN ceramics reduced slightly with indentation load, suggesting an exceptional damage resistance. Moreover, a rising R-curve behavior was demonstrated for laminated SiC/BN ceramics, a plateau R-curve behavior for monolithic SiC ceramics. The excellent damage resistance and pronounced R-curve behavior of laminated SiC/BN ceramics were attributed to crack deflection, crack branching and crack delamination at the SiC/BN weak interfaces.

Comparison between fatigue behavior of some ceramics: a new concept of intrinsic stress-corrosion exponent n0

Abstract The fatigue behavior was systematically investigated with three typical ceramic materials, including Y 2 O 3 –ZrO 2 , Si 3 N 4 and a machinable glass–ceramic. Cyclic, static and dynamic fatigue tests were conducted in three environments, i.e., moist air, distilled water and kerosene. The effects of environment and loading condition on fatigue behavior were analyzed. The difference between materials was discussed. Experimental results showed that, for all the three materials, fatigue life under cyclic load was the shortest and a low value of n (fatigue exponent) was obtained. Compared with the other materials, Si 3 N 4 ceramic had a very large n value under static load. Therefore the static fatigue for Si 3 N 4 ceramics may be ignored. Cyclic load decreases the fatigue life of transformation-toughening ceramics (Y–TZP) more seriously, while no remarkable difference was observed between cyclic and static fatigue for original glass of the glass–ceramic. A new idea about the physical meaning of n value and the concept of intrinsic stress-corrosion exponent n 0 were introduced. The exponent n can be separated into two terms, i.e., n = n 0 + n μ , where n 0 is a material constant and just related to the most basic properties, such as atom-bonds or crystalline structure, n μ reflects the contribution of microstructure toughening and is very sensitive to environments or loading conditions. This hypothesis can be used to describe and explain the experiment results successfully.

New roughness parameter for the characterization of regularly textured or ordered patterned superhydrophobic surfaces.

Surface geometry affects strongly superhydrophobic behavior. To characterize the effect, roughness as a comprehensive geometrical parameter is used, but this parameter in its general mathematic expression cannot reflect exactly such a geometrical effect, in particular, for the regularly textured or ordered patterned superhydrophobic surfaces. In this study, we propose a new parameter to mathematically describe roughness for such superhydrophobic surfaces. On the basis of this parameter, an ideal surface texture with the maximum roughness for achieving the superhydrophobicity is suggested, which is consistent with the previous experimental observations and theoretical considerations.

Fabrication of silicon nitride/ boron nitride nanocomposite powder

Si3N4/BN nanocomposite powders with the microstructure of the micro-sized α-Si3N4 particles coated with nano-sized BN particles were synthesized via the chemical reaction of boric acid, urea, and α-Si3N4powder in a hydrogen gas. The results of XRD, TEM, and selected area electron diffraction showed that amorphous BN and a little amount of turbostratic BN(t-BN) were coated on Si3N4 particles as the second phase after reaction at 1100°C. After reheating the composite powders at 1450°C in a nitrogen gas, the amorphous and turbostratic BN is transformed into h-BN. These nanocomposite powders can be used to prepare Si3N4/BN ceramic composites by hot-pressing at 1800°C, which have perfect machinability and can be drilled with normal metal tools.

Property and Microstructure of Machinable B4C/BN Nanocomposites

The B4C/BN nanocomposites were fabricated by hot-pressing sintering of the B4C/BN nanocomposite powders at 1850oC for 1h under the pressure of 30MPa. The composite powders with the microstructure of B4C particles coated with nano-sized BN particles were prepared by the chemical reaction of H3BO3 and CO(NH2)2 on the surface of B4C particles at high temperature. The microstructure investigation of the nanocomposites sintered samples showed that the nano-sized h-BN particles were homogenously distributed in the B4C matrix. With the increasing content of h-BN, the density of the B4C/BN nanocomposites decreased gradually; the fracture strength and fracture toughness of the B4C/BN nanocomposites decreased gradually, the strength and toughness of the B4C/BN nanocomposites with the h-BN content of 10wt% and 20wt% achieved high values. The Vickers hardness of the B4C/BN nanocomposites decreased remarkably with the increasing content of h-BN, while the machinability of the B4C/BN nanocomposites was significantly improved. The B4C/BN nanocomposites with the h-BN content more than 20wt% exhibited excellent machinability.