Effect of mechanical activation process parameters on the properties of LiFePO4 cathode material

Pure, nano-sized LiFePO₄ and carbon-coated LiFePO₄ (LiFePO₄/C) positive electrode (cathode) materials are synthesized by a mechanical activation process that consists of high-energy ball milling and firing steps. The influence of the processing parameters such as firing temperature, firing time and ball-milling time on the structure, particle size, morphology and electrochemical performance of the active material is investigated. An increase in firing temperature causes a pronounced growth in particle size, especially above 600 °C. A firing time longer than 10 h at 600 °C results in particle agglomeration; whereas, a ball milling time longer than 15 h does not further reduce the particle size. The electrochemical properties also vary considerably depending on these parameters and the highest initial discharge capacity is obtained with a LiFePO₄/C sample prepared by ball milling for 15 h and firing for 10 h at 600 °C. Comparison of the cyclic voltammograms of LiFePO₄ and LiFePO₄/C shows enhanced reaction kinetics and reversibility for the carbon-coated sample. Good cycle performance is exhibited by LiFePO₄/C in lithium batteries cycled at room temperature. At the high current density of 2C, an initial discharge capacity of 125 mAh g⁻¹ (73.5% of theoretical capacity) is obtained with a low capacity fading of 0.18% per cycle over 55 cycles.     

Effect of hydrogen treatment on solidification structures and mechanical properties of TiAl alloys

The effect of hydrogen treatment (HT) on the solidification structures and mechanical properties of Ti-47Al alloys were studied using different contents of hydrogen. Experimental results indicate that together with the refinement of lamellar structure, the grain size of Ti-47Al alloys is reduced. With the hydrogen addition increasing to 1%, the grains of Ti-47Al alloys are refined from 1000 to 100 μm, and the average lamellar spacing of Ti-47Al alloys is decreased by approximately 50%. The hardness, compressive strength and yield stress of Ti-47Al alloys are improved due to the refinement effect of HT on grains and microstructures. The refinement effect of HT on cast TiAl alloys is found to be related to the enhancement of constitutional supercooling induced by hydrogen ahead of the solid-liquid interface.     

Effect of hydrogen charging on the mechanical properties of advanced high strength steels

The present work investigates the influence of hydrogen on the mechanical properties of four multiphase high strength steels by means of tensile tests on notched samples. This was done by performing mechanical tests on both hydrogen charged and uncharged specimens at a cross-head displacement speed of 5 mm/min. A considerable hydrogen influence was observed, as the ductility dropped by 8–60%. In order to demonstrate the influence of diffusible hydrogen, some parameters in the experimental set-up were varied. After tensile tests, fractography was performed. It was found that hydrogen charging caused a change from ductile to transgranular cleavage failure near the notch with a transition zone to a fracture surface with ductile features near the centre.     

Carbon corrosion behaviors and the mechanical properties of proton exchange membrane fuel cell cathode catalyst layer

Carbon corrosion-induced catalyst layer destruction is the primary reason to the performance decay of proton exchange membrane fuel cells (PEMFCs). In this study, the accelerated stress test (AST) on carbon corrosion was conducted, and real-time CO₂ evolution was measured in-situ by non-dispersive infrared (NDIR) analysis. The performance degradation was investigated by the reduction of the current density and the loss of electrochemical active surface area (ECSA) of Pt. The loss of catalyst layer porosity and increase of mass transport resistance were investigated by the visible reduction of porosity and thickness in the cathode catalyst layer (CCL). Further mechanical tensile tests showed that the elastic modulus of CCL remained unchanged initially, and then increased probably due to the compaction of CCL. In the final step, it decreased due to the complete failure of the material. Thus, carbon corrosion was proved to alter the mechanical strength of CCL.     

机械活化预处理对蔗渣酶解产糖影响的研究

采用机械活化方法对蔗渣进行预处理,研究其对蔗渣酶解产糖的影响。用红外光谱、X-射线衍射和扫描电镜测定预处理前后蔗渣结构及表面形态的变化,并分析其作用机理。研究结果表明,机械活化用于蔗渣预处理,可明显提高预处理后蔗渣的酶解产糖率。酶解时间为48 h时,蔗渣酶解产糖率从未处理时的19.86%提高到59.34%。蔗渣酶解产糖率的提高是由于机械活化处理使得蔗渣纤维素分子间部分氢键发生断裂、结晶度下降、表面有序结构被破坏的所致。     

Hydrogen effects on the mechanical properties of nanocrystalline free-standing Palladium thin films

Pd membranes are used both in hydrogen detection and hydrogen separation devices. Their properties can be improved at the nanometer scale. We simulate atomistically the tensile properties of Pd mono and polycrystalline free-standing nanofilms of 7 and 10 nm thicknesses with different H concentrations at 300 K. We use Monte Carlo to place H atoms in the Pd membranes. H segregates to the surfaces and subsurfaces of all films and to the grain boundaries (GBs) in the poly-crystalline films. We perform molecular dynamics uniaxial tensile tests. For all pure Pd nanomembranes the Young's modulus and yield strength are substantially reduced compared with the bulk ones, and decrease with increasing H concentration. Dislocations and twin formation are the main deformation mechanisms. Polycrystalline samples also show GB deformation. Wrinkling of the surfaces increase with strain, enhanced by the presence of hydrogen.     

Effect of multi-wall carbon nanotubes supported palladium addition on hydrogen storage properties of magnesium hydride

To improve the hydrogen storage performance of magnesium hydride, multi-wall carbon nanotubes supported palladium (Pd/MWCNTs) was introduced to the magnesium-based materials. Pd/MWCNTs catalysts with different amounts of Pd (20 wt.%, 40 wt.%, 60 wt.%, 80 wt.%) were synthesized by a solution chemical reduction method. Afterwards, Mg₉₅–Pd_m/MWCNTs_5−m (m = 0, 1, 2, 3, 4, 5) were prepared for the first time by hydriding combustion synthesis (HCS) and mechanical milling (MM). It is determined by X-ray diffraction (XRD) analysis that Pd/MWCNTs can significantly increase the hydrogenation degree of magnesium during the HCS process. The microstructures of the composites obtained by transmission electron microscope (TEM) and field emission scanning electronic microscopy (FESEM) analyses show that Pd nanoparticles are well supported on the surface of carbon nanotubes and the Pd/MWCNTs are dispersed uniformly on the surface of MgH₂ particles. Moreover, it is revealed that there is a synergistic effect of MWCNTs and Pd on the hydrogen storage properties of the composites. The Mg₉₅–Pd₃/MWCNTs₂ shows the optimal hydriding/dehydriding properties, requiring only 100 s to reach its saturated hydrogen absorption capacity of 6.67 wt.% at 473 K, and desorbing 6.66 wt.% hydrogen within 1200 s at 573 K. Additionally, the dehydrogenation activation energy of MgH₂ in this system is decreased to 78.6 kJ/mol H₂, much lower than that of as-received MgH₂.     

Effect of high temperature aging on microstructure and mechanical properties of HR3C heat resistant steel

Abstract

Microstructure and mechanical properties of the HR3C austenite heat resistant steel were investigated after artificial aging at 650°C for time up to 3000 h. The results show that as the aging time increased, the room temperature tensile and impact fracture mechanisms of the HR3C steel change from trans- to intergranular fracture. M₂₃C₆ type carbides and MX type carbonitrides continuously precipitate during aging, leading to the change of the mechanical properties and fracture mode of the steel. Moreover, the dissolution of the coherent twins and the transformation from the incoherent twins to the thermodynamically stable austenite subgrains have great effects on the mechanical properties of the aged steel, too. When increasing the aging time to ≧2000 h, the microstructure and mechanical properties of the steel are nearly constant, indicating a good thermal stability of the HR3C steel at elevated temperature.     

Effect of microwave irritated Co-B-Cr catalyst on the hydrolysis of sodium borohydride

In this work, Co-B-Cr catalysts were synthesized from CoCl₂.6H₂O and Cr(NO₃)₂ 9H₂O compounds by using NaBH₄ as chemical reducing agent at temperature range of 5–8°C. The microwave irradiation method utilized depends on different gas medium (N₂, Ar, CO₂), microwave power (0–1,000 W), and microwave applying time (0–20 min) to increase the catalytic activity of Co-B-Cr catalysis used in the hydrolysis of NaBH₄. It was found that the Co-B-Cr catalyst with best catalytic activity for NaBH₄ hydrolysis was produced under microwave conditions of N₂ gases for 15 min treatment time and 500 W applying power. Hydrolysis of NaBH₄ is completed in 500 s by using Co-B-Cr catalysis treatment optimum irradiation microwave conditions and it is completed in 1,200 s in the case of non-microwave treatment of Co-B-Cr catalyst. The effect of microwave irradiation on Co-B-Cr surface was investigated by using scanning electron microscopy analysis.     

松子壳热解炭活化特性研究

以松子壳为原料,采用常规热解法得到松子壳炭,利用水蒸气活化的方法制备了微孔率较高的活性炭,并测定其吸附能力。利用红外光谱(FT-IR)、氮气吸脱附曲线、扫描电镜(SEM)对热解炭及相应活性炭进行了表征。最佳活化工艺为活化温度850℃,活化时间60 min,水蒸气流量0.3 g/min。在该条件下松子壳活性炭得率为34%,亚甲基蓝吸附值为186 mg/g,碘吸附值为1 097 mg/g,比表面积为1 094.895 m2/g,平均孔径为3.95nm。微观结构分析表明,热解炭已经具备一定的孔隙结构,活化过程中活化剂能够有效去除堵塞热解炭孔隙的杂质和不定型炭,形成丰富的微孔结构和少量的介孔、大孔。该研究为松子壳活性炭的制备提供了理论依据。     

Effects of mechanical milling on hydrogen storage properties of alloy

The hydrogen storage alloy of Ti_0.32Cr_0.43V_0.25 was prepared by arc melting and high energy ball milling. Effects of ball milling were studied for various time periods (30–300 min) at 200 rpm. The hydrogen storage capacity of the alloy decreased with the increase in milling time. The reasons for the drop in the hydrogen storage capacity are twofold: surface contamination of the alloy powder and the microstructural changes. The latter includes the increase in lattice strain, the decrease in crystallite size and the consequent increase in subgrain boundaries. Despite the microstructural changes, the BCC phase of the alloy was maintained and its lattice constant remained nearly the same.     

CdS supported on electrochemically reduced rGO for photo reduction of water to hydrogen

The paper reports synthesis of CdS supported rGO where reduction of GO to rGO was carried out electrochemically in situ. This catalyst had a better activity for photo-electrochemical dissociation of water to hydrogen. Catalysts were characterized by XRD, FTIR, DRS, XPS, EIS and Mott-Schottky analysis. It is reported that this catalyst showed formation of heterojunction at the interface of CdS and rGO by chemical interaction between the two. This resulted in a greater band bending, a higher charge carrier density, low resistance for electronic mobility and low recombination rate. A better activity of the catalyst is due to the above mentioned attributes.     

Phase stability and mechanical properties of niobium dihydride

First-principles calculation shows that the NbH_x phases (1 ≤ x ≤ 2) with face-centered cubic (FCC), orthorhombic (FCO), and tetragonal (FCT) modifications are all energetically favorable with negative heats of formation, while FCC NbH₁ and NbH_1.25 could not be formed due to their mechanical unstableness. It is also revealed that FCT and FCO could coexist in NbH₁ and NbH_1.25, FCC and FCT coexist in NbH_1.5 and NbH_1.75, while only FCC in NbH₂. Calculations also indicate that the magnitude of the elastic moduli of NbH_x phases at each H concentration is as follows: E > B > G, and the G, E, and G/B values of NbH_x phases reach a minimum when x = 1.5. Moreover, electronic structures are discussed to provide a deep understanding of various properties, and the derived results are in good agreement with similar experimental evidence in the literature.     

Fast synthesis of TiNi by mechanical alloying and its hydrogenation properties

Mechanical alloying is widely used for the synthesis of hydrogen storage materials. However, amorphization and contamination triggered by long-time milling are serious drawbacks for obtaining efficient hydrogen storage. In this work, short-time ball milling synthesis is explored for a representative hydride forming compound: TiNi. Through structural, morphological and chemical characterizations, we evidence that formation of TiNi is complete in only 20 min with minor Fe contamination (0.2 wt%). Cross-sectional analysis of powder stuck on milling balls reveals that alloy formation occurs through the interdiffusion between thin layers of co-laminated pure elements. Hydrogenation thermodynamics and kinetics of short-time mechanically alloyed TiNi are similar to those of coarse-grained compounds obtained by classical high-temperature melting. Mechanical alloying is a suitable method for fast and energy-efficient synthesis of intermetallic compounds such as TiNi.     

Effect of electron irradiation on the properties of CdTe/CdS solar cells

Polycrystalline CdTe/CdS solar cells are used in space, as well as terrestrial, applications. The results of the studies on the effect of 8 MeV electron irradiation on p-CdTe/n-CdS thin film solar cells prepared by radio frequency (RF) sputtering are presented in this article. Solar cell parameters like short circuit current (I_sc), open circuit voltage (V_oc), fill factor (FF), conversion efficiency (η), saturation current (I_s) and ideality factor (n) have been considered. CdTe thin film solar cells exhibit good stability against electron irradiation up to 100 kGy.     

Effect of Nb2O5 on MgH2 properties during mechanical milling

Recently, it was shown that hydrogen absorption–desorption kinetics in magnesium were improved by milling magnesium hydride (MgH₂) with transition metal oxides. Herein, we investigate the role of the most effective of these oxides, Nb₂O₅ when added in larger volume fraction. The effect of Nb₂O₅ on magnesium crystalline structure, particle size and (ab)desorption properties has been characterised. Moreover, we report that pure MgH₂ can also show fast hydrogen sorption kinetics after a long milling time. The effects of Nb₂O₅ on MgH₂ sorption properties are rationalised in a new approach considering Nb₂O₅ as a dispersing agent, which helps reduce MgH₂ particle size during milling.     

Role of the activation process on catalytic properties of iron supported catalyst in Fischer-Tropsch synthesis

Monometallic Fe and bimetallic Ru–Fe supported catalysts were prepared by impregnation method and tested in Fischer-Tropsch synthesis (F-T). Their physicochemical properties were characterized by BET, SEM-EDS, FTIR, XRD, H₂-TPR, TPD-NH₃ techniques. The catalytic activity tests showed that the activation process has a huge impact on the reactivity properties in the studied process. The most active system in F-T reaction was 40%Fe/Al₂O₃–Cr₂O₃ catalyst which exhibited CO conversion equal 89.9% and the selectivity towards liquid product of 66.6%. The liquid products obtained in F-T process on 40%Fe/Al₂O₃–Cr₂O₃ consisted mainly of linear (76.8%), branched (17.7%) and unsaturated (5.5%) hydrocarbons. The chain growth probability in F-T process was estimated using an Anderson-Schulz–Flory distribution of obtained liquid product for all tested catalysts. The calculated α value for the most active catalyst was 0.76. The activity results indicate that iron carbides formed during activation process affects on the catalytic activity and selectivity of the iron catalyst during F-T synthesis. The activity measurements showed that the activity of the iron supported catalysts depends strongly on the catalyst phase composition and their acidic properties. XRD results confirmed that the most active catalyst in the investigated process exhibited the highest concentration of iron carbides phases on its surface.     

Microstructural effects on the electrical and mechanical properties of Ni–YSZ cermet for SOFC anode

The electrical and mechanical properties of Ni–YSZ cermet as the anode support of solid oxide fuel cell (SOFC) are determined by the metallic and ceramic components, respectively. We used YSZ and NiO commercial powders of the average particle size from 1 to 10 μm to fabricate Ni–YSZ cermets with different microstructures. The porosity of the cermets was also modified by the amount of carbon black addition. The distribution of each phase of cermets was analyzed with scanning electron microscopy combined with energy dispersive spectroscopy. The electrical conductivity and fracture strength of the Ni–YSZ cermets were investigated and interpreted in a view of percolation phenomena. The finer particles, either NiO or YSZ, were interlinked well by sintering and the electrical and mechanical properties of Ni–YSZ cermets were enhanced by the percolation of Ni and YSZ, respectively.     

Effects of crystallization on the high-temperature mechanical properties of a glass sealant for solid oxide fuel cell

Effects of crystallization on the high-temperature mechanical properties of a newly developed silicate-based glass sealant (GC-9) are investigated for use in planar solid oxide fuel cell (pSOFC). The aged, crystallized GC-9 glass is produced by heat treatment of the original GC-9 glass at 900 °C for 3 h. Not only crystalline phases are formed but the residual glass is also changed in the aged GC-9 glass after the heat treatment. Mechanical properties of the aged GC-9 glass are determined by four-point bending technique at temperature from 25 °C to 750 °C. The glass transition temperature of the given glass is reduced but the softening temperature is increased by such a crystallization heat treatment. The aged GC-9 glass exhibits a greater flexural strength and Young's modulus than the non-aged one at temperature below 650 °C due to the existence of crystalline phases. At temperature of 700 °C and 750 °C, a greater extent of stress relaxation is found in the aged GC-9 glass such that its strength and stiffness are much lower than those of the non-aged one. The changes in the thermal and mechanical properties through the given aging treatment are favorable for application of the GC-9 glass sealant in pSOFC.