Joining of SiCf/SiC using a layered Ti3SiC2-SiCw and TiC gradient filler

A layered filler consisting of Ti₃SiC₂-SiC whiskers and TiC transition layer was used to join SiC_f/SiC. The effects of SiC_w reinforcement in Ti₃SiC₂ filler were examined after joining at 1400 or 1500 °C in terms of the microstructural evolution, joining strength, and oxidation/chemical resistances. The TiC transition layer formed by an in-situ reaction of Ti coating resulted in a decrease in thermal expansion mismatch between SiC_f/SiC and Ti₃SiC₂, revealing a sound joint without cracks formation. However, SiC_f/SiC joint without TiC layer showed formation of cracks and low joining strength. The incorporation of SiC_w in Ti₃SiC₂ filler showed an increase in joining strength, oxidation, and chemical etching resistance due to the strengthening effect. The Ti₃SiC₂ filler containing 10 wt.% SiC_w along with the formation of TiC was the optimal condition for joining of SiC_f/SiC at 1400 °C, showing the highest joining strength of 198 MPa as well as improved oxidation and chemical resistance.     

Synthesis and thermal expansion of chalcogenide MAX phase Hf2SeC

Thermal expansion of MAX phases along different directions tends to be different because of the anisotropy of hexagonal crystals. Herein, a new Hf₂SeC phase was synthesized and confirmed to be relatively isotropic, and the coefficients of thermal expansion (CTEs) along a and c directions were determined to be 9.73 μK^?1 and 10.18 μK^?1, respectively. The strong MS bond endowed Hf₂SC and Zr₂SC lower CTEs than those of Hf₂SeC and Zr₂SeC. The relationship between the thermal expansion anisotropy and the ratio of elastic stiffness constant c₁₁ and c₃₃ was established. This straightforward approximation can be used to roughly predict the thermal expansion anisotropy of MAX phases.     

Investigation on synthesis,characterization and hydrogenation behaviour of hydrogen storage material: Fe1−xZrxTi1.3 (x = 0.2)

The present study is aimed at investigations on the structural and microstructural characterization and hydrogenation behaviour of Zr substituted Fe_1−xZr_xTi_1.3 (x = 0.2) alloys. This storage alloy has been synthesized using R.F.induction melting under an argon atmosphere in a previously outgassed graphite crucible. The structural characterization (XRD) has revealed that the as-synthesized sample is multiphasic in nature and embodies the phases FeTi, Fe₂Ti, FeTi₂ and Ti. Microstructural evaluations (SEM) exhibited the presence of interfaces and cracking. The P–C isotherm was determined and it showed the storage capacity of ∼1.20 wt.% at 200°C. The activation as well as desorption kinetics (75% desorption of H₂ in 6 min) were found to be better for Zr-substituted alloy (Fe_0.8Zr_0.2Ti_1.3) as compared to FeTi_1.3. The improved activation and desorption kinetics are thought to be due to presence of several interfaces e.g. Fe(Zr)Ti⧸ Fe(Zr)Ti₂, Fe(Zr)Ti⧸{Fe(Zr)}₂Ti and volume expansion induced cracking of Ti.     

Hydrogen generation via the reaction of an activated aluminum slurry with water

Presented here is the formulation and characterization of a stable aluminum slurry fuel that reacts readily and exothermically with liquid water to produce hydrogen gas and AlOOH (boehmite). Bulk pure aluminum is first surface-treated with a gallium-indium eutectic, then ground into a powder and suspended in mineral oil containing 4–8 wt% fumed silica as a shear-thinning agent. Aluminum mass fractions of up to 65 wt% are shown here. This formulation results in a slurry fuel that can be pumped continuously at low power while remaining in suspension for over 2 months. The fuel is also shown here to exhibit a high degree of reaction completion (93.4%) with a total measured energy density of 28.7 MJ/L and specific energy of 17.5 MJ/kg. Finally, to show the feasibility of using this fuel for on-demand hydrogen production in a representative power system, a prototype continuous-flow reactor was developed, and validation experiments were performed, the results of which are presented here.     

Performance of the titanium sublimation trap in relatively poor vacuum systems

The properties of small sublimation traps designed for use in general vacuum systems, as an alternative to the conventional liquid nitrogen trap, are described in this paper. Particular attention is paid to traps suitable for interposing between sensitive electronic measuring equipment and manifolds where pressures are in the range 10⁻³–10⁻⁴ mbar. It is shown that simple systems can be built and operated where the active titanium surface is maintained by slow but continuous sublimation onto a stainless steel substrate from a titanium molybdenum alloy wire. Provided care is taken to outgas all internal surfaces effective operation can be maintained at trap temperatures above 200°C. This is a particularly valuable feature for systems where neither liquid nitrogen nor cooling water is available. Applications are in conventional leak detection systems where the feature of trapping gases such as oxygen and water vapour in combination with being completely inert to helium and argon is particularly attractive. It is shown that the background pressure in a small mass spectrometer can be kept below 10⁻⁶ mbar even with pressures in the main manifold of the order of 10⁻³ mbar.     

Determination of concentration of surface states at the illuminated semiconductor—electrolyte interface

Surface states at the semiconductor—electrolyte interface under illumination have been determined. The faradaic reaction involved at the (CdTe and GaP) interface is the photo-electrochemical reduction of carbon dioxide to carbon monoxide in dimethylformamide—water mixtures. A new equivalent circuit has been proposed to account for the impedance data. Surface states act as faradaic mediators for the reduction of carbon dioxide. Surface state density at a given bias potential has been calculated to be of the order of 10¹⁴ cm⁻². Adsorbed ions induce surface states. Surface state density induced by tetraalkylammonium ions decrease with increase of alkyl chain length. The number of surface states increases with water concentration. The resistance for the surface states in the Tafel region is larger than the resistance of those in the space—charge region indicating that the rate determining step lies on the solution side of the interface. However, in the limiting current density region, the space charge resistance is larger than the surface state resistance.     

Preparation and properties of boron nitride nanosheets/cellulose nanofiber shear-oriented films with high thermal conductivity

A highly thermally conductive boron nitride nanosheets/cellulose nanofiber (BNNS/CNF) oriented film was prepared by doctor blading method via mechanical shear-induced orientation. The SEM images for cross-sectional parts showed that BNNS were well aligned within the film, forming a good layered structure. The XRD results further confirmed the high orientation effect of BNNS. Due to the excellent thermal stability of BNNS and its good physical barrier effect on the matrix after the orientation treatment, the thermal stability of shear-oriented films was largely improved. Resulting from the shear-induced orientation, BNNS were closely contacted with each other, forming a good thermally conductive pathway within the CNF matrix. Thus the influence of the interfacial thermal resistance was dramatically reduced, and the thermal conductivity of shear-oriented films increased in proportion to filler loading. With 50 wt% BNNS, the thermal conductivity of the shear-oriented film reached 24.66 W/(m·K), which exhibited 1106% enhancement compared to the pure CNF.     

Synthesis,structure and photoelectric properties of selenide composites with in situ constructed Sb2Se3/NaSbSe2 heterojunction

Inorganic semiconductor heterojunctions have attracted extensive attention for technological applications owing to interface phenomena. In this work, selenide composites with in situ constructed Sb₂Se₃/NaSbSe₂ heterojunctions with similar band gap width have been synthesized by conventional melt-quenching method. The microstructure and photoelectric properties of the composites were investigated at different NaSbSe₂ contents. The study revealed that NaSbSe₂ content has direct influences on the photoelectric performance of the composites by modifying the heterojunction size and the grain boundary resistance, as evidenced by scanning electron microscopy and impedance spectroscopy, respectively. The photocurrent density of Sb₂Se₃ with 2.5 and 5 mol% NaSbSe₂ was approximately 180 times larger than that of the pure Sb₂Se₃ at a bias voltage of -1.5 V and under an illumination of 7 mW/cm², due to an effective separation of photogenerated carriers through the interface electric field. Additionally, the composites exhibited an excellent stability even in 0.5 M LiClO₄ aqueous electrolyte.     

Characterization of metal hydrides for thermal applications in vehicles below 0 °C

Metal hydrides promise great potential for thermal applications in vehicles due to their fast reaction rates even at low temperature. However, almost no detailed data is known in literature about thermochemical equilibria and reaction rates of metal hydrides below 0 °C, which, though, is crucial for the low working temperature levels in vehicle applications.Therefore, this work presents a precise experimental set-up to measure characteristics of metal hydrides in the temperature range of −30 to 200 °C and a pressure range of 0.1 mbar–100 bar. LaNi_4.85Al_0.15 and Hydralloy C5 were characterized. The first pressure concentration-isotherms for both materials below 0 °C are published. LaNi_4.85Al_0.15 shows an equilibrium pressure down to 55 mbar for desorption and 120 mbar for absorption at mid-plateau and −20 °C. C5 reacts between 580 mbar for desorption and 1.6 bar for absorption at −30 °C at mid-plateau.For LaNi_4.85Al_0.15, additionally reaction rate coefficients down to −20 °C were measured and compared to values of LaNi₅ for the effect of Al-substitution. The reaction rate coefficient of LaNi_4.85Al_0.15 at −20 °C is 0.0018 s⁻¹. The obtained data is discussed against the background of preheating applications in fuel cell and conventional vehicles.