A comparative investigation on the corrosion degradation of plasma sprayed YSZ and LnMgAl11O19 (Ln = Nd,Sm, Gd) coatings exposed to the molten V2O5 + Na2SO4 salt mixture at 1100 °C

Corrosive attack of the molten 50 wt. % V₂O₅ + 50 wt.% Na₂SO₄ salt mixture has been comparably studied for the APS YSZ and LnMgAl₁₁O₁₉ (LnMA, Ln = Nd, Sm, Gd) thermal barrier coatings upon a 10 h anneal at 1100 °C in air. The YSZ coating suffered from a deepest infiltration of the molten salt along its thickness direction through the open and connected pores as well as inter-lamellae microcracks. A large number of newly formed voids were widely distributed in the YSZ coating due to the corrosion degradation followed by the t, t’ to m-ZrO₂ phase transformation. While, a relative thin corrosion layer mainly consisting of α-Al₂O₃ and LnVO₄ were present for the corroded LnMA coatings. The much reduced number of open pores and connected microcracks together with the rapid chemical reaction between the molten salt and LnMA coatings, especially for the NdMA coating, preventing further infiltration of the molten salt, was beneficial to mitigate further attacks to the inner coating at the expense of sacrificing a thinner top layer. The presences of amorphous phases were thought to further accelerate the corrosion reaction and strengthen such a corrosion protection mode for all the APS LnMA coatings.     

Magnesium borohydride hydrolysis with kinetics controlled by ammoniate formation

The possibility of hydrogen generation by hydrolysis of magnesium borohydride and its ammoniates was explored. Results show that catalyst-free Mg(BH₄)₂ can generate 1700 mL (H₂)·g⁻¹ in 1 min, 2760 mL (H₂)·g⁻¹ in 2 h, and 3004 mL (H₂)·g⁻¹ in 5 h without any diborane (B₂H₆) emission. Mg(BH₄)₂ presents the highest hydrogen yield reported to date. However, the hydrogen generation rate of Mg(BH₄)₂ may be too fast to be controllable in some hydrogen production cases. Therefore, NH₃ was added to form ammoniates to further regulate the hydrogen supply kinetics of Mg(BH₄)₂. The hydrogen yields of Mg(BH₄)₂·0.5NH₃, Mg(BH₄)₂·NH₃, Mg(BH₄)₂·2NH₃, Mg(BH₄)₂·3NH₃ and Mg(BH₄)₂·6NH₃ are 2376, 2029, 1780, 1665 and 1180 mL (H₂) g⁻¹, respectively, which demonstrates a well-controlled hydrogen supply rate. These results indicate that catalyst-free Mg(BH₄)₂ and its ammoniates have good hydrolysis performance and show promise as convenient high-density hydrogen generation materials.     

The mechanism of flame propagation affected by flow/shock wave in a confined combustion chamber equipped with a perforated plate

The whole evolution of flame propagation in a confined combustion chamber was firstly experimentally observed in a newly designed experimental apparatus equipped with a perforated plate. The effect of the flame-flow/acoustic/shock wave interaction on the flame propagation was studied. The experiment was conducted with a stoichiometric hydrogen-air mixture. According to the flame morphology and the flame tip velocity, the whole evolution of flame propagation in the experimental apparatus was classified into the following three stages: laminar flame, jet flame and turbulent flame. In the present work, different flame propagation modes were obtained in different conditions. Depending on the initial pressure, three different flame propagation modes were observed. At an initial pressure of 1 bar, the flame propagation after perforated plate was mainly controlled by the interactions of the flame and combustion-generated flow ahead of the flame front. As initial pressures went up to 3 bar and 5 bar, shock waves were clearly observed ahead of the flame, which played a significant role on the flame propagation. The flame decelerated sharply and even propagated backwards, induced by the flame-shock wave interactions. Depending on the intensity of the shock wave, the backward-propagation velocity was higher at 5 bar with a stronger shock wave. In addition, the pressure oscillation at different initial pressures was discussed.     

挂轨巡检机器人绳索式剪叉升降机构的设计研究

针对目前挂轨巡检机器人升降机构的特点和不足,从运动稳定性和展开性能出发,设计了一种绳索式剪叉升降机构.对比分析了剪叉升降机构不同驱动方式的优、缺点,给出了剪叉升降机构具体的结构设计方案,对所设计的剪叉升降机构进行了运动学仿真分析,验证了方案的可行性.试制样机试验结果证明所设计的剪叉升降机构运动平稳,展开性能优异,能够满...     

热喷涂金属基防滑耐磨涂层的研究进展

采用热喷涂技术制备的金属基防滑涂层能有效增强材料表面间的摩擦阻力,同时在耐磨及防腐等方面表现出良好的性能,在工业生产、海洋平台及船舶甲板等领域有广泛应用。相对于高分子防滑涂层,金属基涂层具有使用寿命长、摩擦系数稳定、不使用有毒溶剂等优点。首先介绍了金属基防滑涂层的防滑原理、制备工艺及特点,分别阐述了四类金属基(Al基、Fe基、NiCr基和Co基)防滑涂层的研究现状及其应用背景,分析喷涂工艺参数、粉末参数、服役环境等因素对涂层摩擦学行为的影响规律。作为防滑涂层,Al基涂层具有良好的耐腐蚀性能且价格相对低廉;Fe基、NiCr基均表现出较好的抗高温磨损性能及耐蚀性能,但Fe基非晶涂层成本偏高;Co基涂层的低温耐磨性较优越。在实际应用中需要根据服役条件选择防滑材料,再根据材料本身特性选取相应的喷涂工艺。最后提出新型防滑材料、涂层制备技术及先进表征技术应是今后金属基防滑涂层的重点研究方向。     

压电分流阻尼电路设计的极点配置方法

针对作为被动控制的压电分流阻尼电路提出极点配置方法。对压电分流阻尼系统建立机电耦合方程,考虑模态位移为输出量,得到系统极点的特征方程。运用主导极点与虚轴的距离对系统动态响应衰减有着关键作用这一特性,确立优化目标,然后求解出压电分流阻尼电路参数的最优值。最后通过数值仿真对所设计的优化方法进行检验,并与传统传递函数优化法进行比较。仿真结果表明,运用极点配置方法设计的压电分流电路有着良好的抑制振动的效果,这验证了极点配置方法的有效性。     

Experimental investigation and thermodynamic modeling of the Mo–Co–B ternary system

Among the ternary borides, the Mo–Co–B system is of great interest because of its excellent hardness, toughness, and stability performance. Eight samples with 60 at.% Co were designed to investigate the isothermal section of Mo–Co–B system at 1073 K in the Co-rich portion. Scanning electron microscopy, energy-dispersion spectroscopy, electron probe microanalysis, differential scanning calorimetry, and X-ray diffraction were used to investigate the phase equilibria of the samples. The formation enthalpies of the ternary borides were obtained by first-principles calculations to serve as key information for thermodynamic assessment. By coupling the reviewed experimental data from the literature, the presently determined phase equilibria, and the calculated formation enthalpies of the compounds, the thermodynamic parameters for the Mo–Co–B ternary system were optimized and used to calculate the isothermal sections, vertical section, and liquidus projection of the system. Comprehensive comparisons showed that the calculated results are in reasonable agreement with the reported phase diagram and thermodynamic data.     

Degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2

Degradation of the hydrogen absorbing capacity in cyclically hydrogenated TiMn₂ (Ti–60 at.% Mn) with Laves structure was studied by measurement of pressure–composition–isotherm (PCT) curves, X-ray diffraction and microstructure observation. Characteristic changes with cyclic hydrogenation are observed as follows. The hydrogen absorbing capacity remarkably decreases, the lattice constant increases, the amount of hydrogen retained in TiMn₂ increases and it approaches almost zero by dehydrogenation at 673 K in vacuum. Nano-sized regions with Moiré patterns are produced in TiMn₂ with hydrogenation and Debye rings corresponding to titanium hydride δ-TiH appear in the diffraction pattern. Based on these observations it is concluded that the degradation of hydrogen absorbing capacity after cyclic hydrogenation is attributable to the introduction of retained hydrogen, heterogeneous strain and/or nano-sized regions.     

The numerical solution of gaseous slip flows in microtubes

The gaseous flow in a circular microtube is studied by using of a finite-difference technique. The influences of gas compressibility, rarefaction effect and temperature difference between inlet gas and tube wall on the mass flow rate are comprehensively described. The numerical results indicate that rarefaction effect decreases friction factor and increases mass flow rate due to velocity slip at tube wall. Although inlet Mach number in a microtube is low, gaseous compressibility effect may be significant, because in such a flow problem the length-diameter ratio of tube is usually large so that flow Reynolds number may change much more along the flow path.     

Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system

Liquid desiccant cooling systems are being explored as an alternative to the conventional vapor compression systems. The dehumidifier and the regenerator form the heart of these systems. A falling film tubular absorber and a falling film plate regenerator have been studied experimentally. The experimental results have been compared with the predictions from theoretical models. Two wetness factors to account for the improper wetting of exchanger surfaces have been defined and estimated. After taking these factors into account, the theoretical models predict the experimental data within ±30%.