Design of Lu2O3-reinforced Cf/SiC-ZrB2-ZrC ultra-high temperature ceramic matrix composites: Wetting and interfacial reactivity by ZrSi2 based alloys

The wettability and infiltration of molten ZrSi₂ and ZrSi₂-Lu₂O₃ alloys into C_f/SiC and B₄C-infiltrated C_f/SiC composites were investigated to understand the interfacial interactions that occur during the development of C_f/SiC-ZrC and C_f/SiC-ZrB₂-ZrC-Lu₂O₃ materials. A significant evaporation of Si from the liquid affected the wetting behaviour of the alloy when tested in a vacuum at 1670 °C. The better wetting and spreading of the alloy over the surface was observed for the composites with lower overall porosity (12 %). On the other hand, the formation of an outer dense layer, followed up by the uniform infiltrated region up to ～ 1 mm was observed for the C_f/SiC with higher porosity (21 %). The infiltrated alloy reacted with SiC matrix to form ZrC or with B₄C-infiltrated SiC matrix to form ZrB₂-ZrC-SiC. The Lu₂O₃ particles were not wetted by the melt, and were pushed away of the reaction zone by the solidification front.     

颗粒增强钢铁基表面复合材料铸渗技术的研究与发展

从材料的选择、铸渗工艺、及铸渗应用等方面,介绍了国内外运用铸渗法制备钢铁基表面复合材料的新进展.分析了复合材料中增强相和基体间的相互作用和界面问题.提出了今后研究工作中值得重视的几个问题.     

Brazing temperature-dependent interfacial reaction layer features between CBN and Cu-Sn-Ti active filler metal

CBN/Cu-Sn-Ti (CBN: cubic boron nitride) composites are prepared by active brazing sintering at 1123 K, 1173 K, 1223 K and 1273 K, respectively. The effects of brazing temperature on the wettability, interfacial characteristics, and elemental distribution variations are fully investigated. When the brazing temperature is below 1223 K, completely uncoated and/or partially coated CBN particles with sharp edges can still be observed, and the reaction layer, mainly composed of TiN and TiB2, appears to be thin and uneven. When the brazing temperature is 1223 K, all CBN particles are completely coated, suggesting that adequate wetting has taken place. Besides, as Ti diffuses thoroughly and enriches the interface, the reaction layer, filled primarily by TiN, TiB₂ and TiB, becomes thicker (about 1.30 μm), more uniform, stable and continuous. Further increasing the temperature to 1273 K is unnecessary or even harmful as the reaction layer thickness undergoes negligible change yet some tiny micro-cracks appear on the interface, which may likely deteriorate the grinding capability of the final brazing products.     

Comparing sensitivities of differently oriented multi-walled carbon nanotubes integrated on silicon wafer for electrochemical biosensors

In this study, we report on multi-walled carbon nanotubes fabricated on silicon substrate with four different orientations via chemical vapor deposition. It is well-known that chemical treatments improve the nanotube electrochemical reactivity by creating edge-like defects on their exposed sidewalls. Before use, we performed an acid treatment on carbon nanotubes. To prove the effect of the treatment on these nanostructured electrodes, contact angles were measured. Then, sensitivities and detection limits were evaluated performing cyclic voltammetry. Two target molecules were used: potassium ferricyanide, an inorganic electroactive molecule, and hydrogen peroxide that is a product of reactions catalyzed by many enzymes, such as oxidases and peroxidases. Carbon nanotubes with tilted tips become hydrophilic after the treatment showing a contact angle of 22° ± 2°. This kind of electrode has shown also the best electrochemical performance. Sensitivity and detection limit values are 110.0 ± 0.5 μA/(mM cm²) and 8 μM for potassium ferricyanide solutions and 16.4 ± 0.1 μA/(mM cm²) and 24 μM using hydrogen peroxide as target compound. Considering the results of wettability and voltammetric measurements, nanotubes with tilted tips-based electrodes are found to be the most promising for future biosensing applications.     

Tribological properties of fluorinated graphene reinforced polyimide composite coatings under different lubricated conditions

The tribological properties of polyimide (PI) and PI/fluorinated graphene (FG) nanocomposites, as a new class of graphene reinforced polymer, are investigated using a ball-on-disk configuration under different lubricated conditions of dry sliding, water lubrication and oil lubrication. Experimental results reveal that single incorporation of FG can effectively improve the tribological performance of PI under all the three conditions. In addition, compared to the results under dry sliding, the phenomenon that the friction coefficient decreases while the wear rate increases under water lubrication condition is observed and researched in detail. The worst anti-wear performance under water-lubricated condition can be ascribed to the fact that the water can be adsorbed by the polar imide radicals of the PI and PI/FG nanocomposite, therefore leading to the property deterioration of the PI and PI/FG nanocomposite coatings.     

Investigation of coalesced droplet vertical jumping and horizontal moving on textured surface using the lattice Boltzmann method

The coalesced droplet vertical jumping and horizontal moving on conical posts textured surface are numerically studied using the three-dimensional (3D) multi-relaxation-time (MRT) pseudopotential lattice Boltzmann model. The influences of wettability gradient and roughness gradient are investigated systematically. It is found that the coalesced droplet on the flat and conical posts textured surfaces can move horizontally from superhydrophobic bend to hydrophobic bend without the roughness gradient. Moreover, the coalesced droplet is able to spontaneously move from lower conical post density region to higher conical post density region without the wettability gradient. Specifically, the in-line array textured surface is more beneficial to the coalesced droplet horizontal moving than the staggered array at the same wettability parameter. However, the staggered array textured surface is more beneficial to the coalesced droplet vertical jumping than the in-line array. The hybrid effect of wettability gradient and roughness gradient plays critical roles in coalesced droplet vertical jumping and horizontal moving. The present work demonstrates that the dropwise condensation heat transfer can be enhanced in a self-sustained manner if the wettability and roughness of the textured surface are properly designed. It is also confirmed that the 3D MRT pseudopotential lattice Boltzmann model is of potential to simulate coalesced droplet behaviors on textured surface.     

Growth kinetics and surface properties of single-crystalline aluminum-doped zinc oxide nanowires on silicon substrates

We present here the results from a systematic investigation on the growth kinetics and surface properties of Al-doped ZnO (AZO) nanowires synthesized on (0 0 1)Si substrates under different hydrothermal conditions. The as-synthesized vertical AZO nanowires exhibited a hydrophilic characteristic and their crystal structures were determined to be perfectly single crystalline with the axis of the wire parallel to the [0 0 0 1] direction. TEM and EDS results revealed that the as-synthesized AZO nanowires have tapered tips, and the Al-doped concentration in the AZO nanowires was about 1.6 at%. After a series of SEM examinations, the average length of AZO nanowires synthesized at each temperature studied was found to follow a linear relationship with the reaction time, indicating that the hydrothermal growth of AZO nanowires was a reaction-controlled process. The activation energy for linear growth of AZO nanowires on Si substrate, as obtained from an Arrhenius plot, was found to be about 46 kJ/mol. From UV–vis spectroscopic measurements, it was found that the Si substrate coated with vertically-aligned AZO nanowire arrays exhibited remarkably reduced reflectance (10–12%) over a wide range of visible wavelengths (400–800 nm) and angles of light incidence (8–60°). The good broadband and omnidirectional antireflection characteristics can be attributed to the light trapping effect and the graded refractive index resulting from the tapered AZO nanowire structures.     

Photoinitiator system and water effects on C=C conversion and solubility of experimental etch-and-rinse dental adhesives

The purpose of this study was to determine the influence of the photoinitiator system and moisture condition on the degree of C=C conversion (DC), the water sorption (W_sp), and the solubility (W_sl) of experimental two-step etch-and-rinse dental adhesives. Different photoinitiator systems were added at 0.5 mol% to an experimental adhesive blend (55:45 wt% Bis-GMA:HEMA), defining the experimental groups: camphorquinone (CQ)+ethyl-4-dimethylaminobenzoate (EDMAB), 9,10-phenanthrenequinone (PQ), PQ+EDMAB, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO). The adhesives were tested in two moisture conditions: neat and wet (with the addition of 10 wt% D₂O). The DC (n=6) was evaluated by Fourier-transformed infrared spectroscopy (FTIR). W_sp and W_sl were determined (n=10) after successive weighting procedures. Data were submitted to two-way ANOVA and Tukey׳s post hoc test (α=0.05). Pearson׳s correlation tests were used to analyze the correlation between DC and W_sp or W_sl. TPO and BAPO presented the highest DC in the neat condition while CQ+EDMAB presented the highest in the wet condition. W_sp and W_sl were both dependent on the photoinitiator system and moisture condition. PQ–based materials presented the highest W_sp and W_sl in both neat and wet conditions. Pearson׳s tests were not able to detect any significant correlation between DC and W_sp or DC and W_sl. Within the limitations of the present study, it can be concluded that the photoinitiator system and moisture condition influenced the DC, W_sp, and W_sl of experimental two-step etch-and-rinse adhesives.     

Synthesis and plasma treatment of nitrogen-doped graphene fibers for high-performance supercapacitors

Graphene fiber-based supercapacitor has aroused great interest as a flexible power source in future wearable electronics. However, the low electrochemical performance of graphene fibers (GFs) usually causes the serious limitation of use in practical applications due to the material stacking, hydrophobicity and fabrication process complexity. In this work, a facile and effective plasma-assisted strategy is put forward to increase specific surface area, tune hierarchically porous structure and promote wettability of nitrogen-doped graphene fibers (NGFs), resulting in the improvement of electrochemical performance. The supercapacitor assembled from plasma-treated NGFs shows superior capacitance (878 mF/cm² at 0.1 mA/cm² current density) and high energy density (19.5 μW h/cm² at 40 mW/cm² power density), which is 23.7% and 131.4% higher than that of NGFs and GFs, respectively. Additionally, the fiber-based supercapacitor based on plasma-treated NGFs exhibits high rate capability of 59.8% and excellent cyclic performance (95.8% retention over 10,000 cycles). These plasma-treated NGFs can be promising candidates for high-performance and flexible power sources in future wearable electronics.     

Wetting of YSZ by molten Sn–8Zr,Sn–4Zr–4Ti,and Sn–8Ti alloys at 800–900 °C

The wetting of 3% yttria-stabilized zirconia (YSZ) by Sn–8Zr, Sn–4Zr–4Ti, and Sn–8Ti alloys was studied at 800–900 °C. Both Zr and Ti improve the wettability via the formation of reaction products and adsorption. In the systems containing Zr additives in the alloys, ZrO_2-x precipitates preferentially, and the wettability is dominated by interface adsorption. An anomalous temperature dependence was found in the final wettability of these systems owing to the decrease in adsorption with an increase in the temperature. The spreading dynamics are controlled by the dissolution of Zr, followed by the formation of a wetting ridge. The wettability of the Sn–8Ti/YSZ system is dominated by the precipitation of reaction products (Ti₂O₃ and Ti_11.31Sn₃O₁₀). The reaction kinetics is the limiting factor for spreading in Sn–8Ti/YSZ, and the adsorption at the interface significantly decreased the energy barrier for wetting.