Fabrication and evaluation of NiO/Y2O3-stabilized-ZrO2 hollow fibers for anode-supported micro-tubular solid oxide fuel cells

In this work NiO/3 mol% Y₂O₃–ZrO₂ (3YSZ) and NiO/8 mol% Y₂O₃–ZrO₂ (8YSZ) hollow fibers were prepared by phase-inversion. The effect of different kinds of YSZ (3YSZ and 8YSZ) on the porosity, electrical conductivity, shrinkage and flexural strength of the hollow fibers were systematically evaluated. When compared with Ni–8YSZ the porosity and shrinkage of Ni–3YSZ hollow fibers increases while the electrical conductivity decreases, while at the same time also exhibiting enhanced flexural strength. Single cells with Ni–3YSZ and Ni–8YSZ hollow fibers as the supported anode were successfully fabricated showing maximum power densities of 0.53 and 0.67 W cm⁻² at 800 °C, respectively. Furthermore, in order to improve the cell performance, a Ni–8YSZ anode functional layer was added between the electrolyte and Ni–YSZ hollow fiber. Here enhanced peak power densities of 0.79 and 0.73 W cm⁻² were achieved at 800 °C for single cells with Ni–3YSZ and Ni–8YSZ hollow fibers, respectively.     

Hydrogen storage characteristics of melt spun Mg–23.5Ni–5Cu alloys mixed with LaNi5 and/or Nb2O5

Mg-based materials have a high potential for commercialization due to the large hydrogen storage capacity, low cost and abundance in the Earth's crust. Despite these advantages, reaction kinetics of Mg with hydrogen is slow. Various investigations to increase the hydriding kinetics of Mg were performed. Although LaNi₅ has a low hydrogen storage capacity, it can store hydrogen at a low temperature. In this study, a Mg–23.5Ni–5Cu alloy was prepared by gravity casting method in a large quantity (approximately 7.5 kg), and then melt-spun and heat-treated. LaNi₅, Nb₂O₅ or LaNi₅ + Nb₂O₅ were added to the Mg–23.5Ni–5Cu alloy by reactive mechanical grinding in a planetary mill. The hydrogen storage capacity and hydriding behavior of the Mg–23.5Ni–5Cu alloys with LaNi₅, Nb₂O₅ or LaNi₅ + Nb₂O₅ added were then investigated.     

Interface structure and mechanical properties of the brazed joint of TiC cermet and steel

A commercially available Ag–Cu braze alloy foil with Zn was used to join TiC cermet and steel. According to the experimental observations, the interface structure is (Cu, Ni)/Ag (s.s.) + Cu (s.s.)/(Cu, Ni)/(Cu, Ni) + (Fe, Ni) from TiC cermet to steel side. With increased brazing temperature or time, the amounts of (Cu, Ni) near the base metals/Ag–31Cu–23Zn interface and (Cu, Ni) + (Fe, Ni) near the Ag–31Cu–23Zn/steel interface increase, while the amount of Ag (s.s.) + Cu (s.s.) in the middle of the braze alloy decreases. The whole joining process consists of diffusion and solution among atoms of the braze alloy foil and base metals. The maximum shear strength is 120.7 MPa for the joint brazed at 850 °C for 10 min.     

Catalytic hydrogen production from 2-propanol in supercritical water: Comparison of some metal catalysts

The gasification of organics in supercritical water is a promising method for the direct production of hydrogen at high pressures, and in order to improve the hydrogen yield or selectivity, activities of various catalysts are evaluated. In this study, hydrogen production from 2-propanol over Ni/Al₂O₃ and Fe–Cr catalysts was investigated in supercritical water. The experiments were carried out in the temperature range of 400–600 °C and in the reaction time range of 10–30 s, under a pressure of 25 MPa. The hydrogen yields and selectivities of Ni/Al₂O₃ and Fe–Cr used in this study, and those of Pt/Al₂O₃ and Ru/Al₂O₃ used in our previous work were compared. The hydrogen contents of the gaseous products obtained by using Ni/Al₂O₃ and Fe–Cr were measured as 62 mol% and 70 mol%, respectively, at low temperatures and reaction times. However, the hydrogen yields remained in low levels when compared with that of Pt/Al₂O₃ used in previous study. Pt/Al₂O₃ was established to be the most effective and selective catalyst for hydrogen production. During the catalytic gasification of a 0.5 M solution of 2-propanol, hydrogen content up to 96 mol% and hydrogen yield of 1.05 mol/mol 2-propanol were obtained.     

Characterization of silicon carbide joints fabricated using SiC particulate-reinforced Ag–Cu–Ti alloys

CVD silicon carbide was brazed to itself using two Ag–Cu–Ti braze alloys reinforced with SiC particulates to control braze thermal expansion and enhance joint strength. Powders of the braze alloys, Ticusil (composition in wt%: Ag–26.7Cu–4.5Ti, T_L: 900 °C) and Cusil-ABA (Ag–35.3Cu–1.75Ti, T_L: 815 °C) were pre-mixed with 5, 10 and 15 wt% SiC particulates (～20–30 μm) using glycerin to create braze pastes that were applied to the surfaces to be joined. Joints were vacuum brazed and examined using optical microscopy (OM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and the Knoop hardness test. The SiC particles were randomly distributed in the braze matrix and bonded to it via reaction with the titanium from the braze alloy. Titanium together with Si and C segregated at the particle/braze interface, and promoted nucleation and precipitation of the Cu-rich secondary phase on particle surfaces. The Si–Ti–C-rich reaction layers also formed at the interface between CVD SiC substrate and the braze alloy. The loss of Ti in the reaction with SiC particulates did not impair either the bond quality or the thickness of the reaction layer on the CVD SiC substrate. Microhardness measurements showed that the dispersed SiC particulates lowered the braze hardness by depleting the braze matrix of Ti. Theoretical calculations indicated the CTE of the braze to decrease by nearly 45–60% with the incorporation of about 45 vol% SiC.     

The role of trace Fe in Fe–N-doped amorphous carbon with excellent electrocatalytic performance for oxygen reduction reaction

Synthesized by dripping iron acetate into the N-doped carbon film enriched with pyridinic N and followed by annealing at 800 °C, Fe–N-doped amorphous carbon (d_Fe–N-C) with an Fe content of 0.2 at.% showed excellent electrocatalytic activity, stability and methanol tolerance via a four-electron pathway for oxygen reduction reaction (ORR), which outperformed commercial Pt/C catalyst. More importantly, by tuning the Fe content and annealing temperature, the trace Fe in d_Fe–N-C was supposed to form high active FeN₄ sites with pyridinic N and played an important role in the excellent electrocatalytic performance for ORR.     

The efficient synthesis of carbon nano-onions using chemical vapor deposition on an unsupported Ni–Fe alloy catalyst

Well graphitized carbon nano-onions (CNOs) with large yield have been synthesized by the catalytic decomposition of methane over an unsupported Ni–Fe catalyst at 850 °C. The unsupported Ni–Fe catalyst was prepared by a reduction-substitution method. In the Ni–Fe alloy particle, α-Fe(Ni) phase (kamacite) transforms to γ-Fe–Ni phase at the high temperature for hydrogen reduction and chemical vapor deposition. The synthesized CNOs contain either a Fe_0.64Ni_0.36 particle or a hollow core with thick graphitic layers and a polyhedral shape. Based on the characterization, we believe that the catalyst involved in the synthesis of carbon products is Fe–Ni–C austenite rather than the γ-Fe–Ni phase (Fe_0.64Ni_0.36). A growth mechanism for the CNOs is proposed.     

Preparation,characterization and performance of Pd/SiO2 catalyst for benzene catalytic hydrogenation

Palladium nanoparticles supported on silica were prepared by hydrazine reduction in aqueous medium at room temperature. They were characterized by XRD, TEM, EDX, H₂-adsorption, and H₂-TPD. The catalytic properties were evaluated in the gas-phase hydrogenation of benzene in the temperature range of 75–250 °C. Metal particles with a size range of 4.0–25.8 nm were obtained. The metal surface area and hydrogen storage increase with decreasing metal particle size. The H₂-TPD profiles exhibited a main peak appeared at 540 °C with two shoulders at lower (445 °C) and higher (605 °C) temperatures. These peaks were ascribed to strongly adsorbed hydrogen on the surface catalyst. The catalytic activity of the catalysts strongly depends on the metal loading. It increases with decreasing Pd loading. This is ascribed to metal surface area, which increases with decreasing Pd content.     

Bi-metallic catalysts of mesoporous Al2O3 supported on Fe,Ni and Mn for methane decomposition: Effect of activation temperature

Methane decomposition reaction has been studied at three different activation temperatures (500 °C, 800 °C and 950 °C) over mesoporous alumina supported Ni–Fe and Mn–Fe based bimetallic catalysts. On co-impregnation of Ni on Fe/Al₂O₃ the activity of the catalyst was retained even at the high activation temperature at 950 °C and up to 180 min. The Ni promotion enhanced the reducibility of Fe/Al₂O₃ oxides showing higher catalytic activity with a hydrogen yield of 69%. The reactivity of bimetallic Mn and Fe over Al₂O₃ catalyst decreased at 800 °C and 950 °C activation temperatures. Regeneration studies revealed that the catalyst could be effectively recycled up to 9 times. The addition of O₂ (1 ml, 2 ml, 4 ml) in the feed enhanced substantially CH₄ conversion, the yield of hydrogen and the stability of the catalyst.     

Low temperature growth mechanisms of vertically aligned carbon nanofibers and nanotubes by radio frequency-plasma enhanced chemical vapor deposition

Using radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD), carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were synthesized at low temperature. Base growth vertical turbostratic CNFs were grown using a sputtered 8 nm Ni thin film catalyst on Si substrates at 140 °C. Tip growth vertical platelet nanofibers were grown using a Ni nanocatalyst in 8 nm Ni films on TiN/Si at 180 °C. Using a Ni catalyst on glass substrate at 180 °C a transformation of the structure from CNFs to CNTs was observed. By adding hydrogen, tip growth vertical multi-walled carbon nanotubes were produced at 180 °C using FeNi nanocatalyst in 8 nm FeNi films on glass substrates. Compared to the most widely used thermal CVD method, in which the synthesis temperature was 400–850 °C, RF-PECVD had a huge advantage in low temperature growth and control of other deposition parameters. Despite significant progress in CNT synthesis by PECVD, the low temperature growth mechanisms are not clearly understood. Here, low temperature growth mechanisms of CNFs and CNTs in RF-PECVD are discussed based on plasma physics and chemistry, catalyst, substrate characteristics, temperature, and type of gas.     

Synthesis and characterization of magnesium–carbon compounds for hydrogen storage

Magnesium (Mg) and carbon (C) compounds were synthesized by ball-milling a mixture of Mg and different graphites with different crystallinities. The materials were characterized by X-ray diffraction, X-ray absorption spectroscopy, and X-ray total scattering techniques. Hydrogen storage properties were also investigated. In the case of the material using low-crystalline graphite, a Mg and C compound was formed as main phase, and its chemical bonding state was similar to that of magnesium carbide (Mg₂C₃). The hydrogen absorption reaction of the Mg–C compound occurred at around 400 °C under 3 MPa of hydrogen pressure to form magnesium hydride (MgH₂) and the C–H bonds in the carbon material. The hydrogenated Mg–C material desorbed about 3.7 mass% of hydrogen below 420 °C with two processes, which were the decomposition of MgH₂ and the subsequent reaction of the generated Mg and the C–H bonds. From the results, it is concluded that the Mg–C compound absorb and desorb about 3.7 mass% of hydrogen below 420 °C.     

Single-source-precursor synthesis of soft magnetic Fe3Si- and Fe5Si3-containing SiOC ceramic nanocomposites

We present here the single-source-precursor synthesis of Fe₃Si and Fe₅Si₃-containing SiOC ceramic nanocomposites and investigation of their magnetic properties. The materials were prepared upon chemical modification of a hydroxy- and ethoxy-substituted polymethylsilsesquioxane with iron (III) acetylacetonate (Fe(acac)₃) in different amounts (5, 15, 30 and 50 wt%), followed by cross-linking at 180 °C and pyrolysis in argon at temperatures ranging from 1000 °C to 1500 °C. The polymer-to-ceramic transformation of the iron-modified polysilsesquioxane and the evolution at high temperatures of the synthesized SiFeOC-based nanocomposite were studied by means of thermogravimetric analysis (TGA) coupled with evolved gas analysis (EGA) as well as X-ray diffraction (XRD). Upon pyrolysis at 1100 °C, the non-modified polysilsesquioxane converts into an amorphous SiOC ceramic; whereas the iron-modified precursors lead to Fe₃Si/SiOC nanocomposites. Annealing of Fe₃Si/SiOC at temperatures exceeding 1300 °C induced the crystallization of Fe₅Si₃ and β-SiC. The crystallization of the different iron-containing phases at different temperatures is considered to be a consequence of the in situ generation of a Fe–C–Si alloy within the materials during pyrolysis. Depending on the Fe and Si content in the alloy, either Fe₃Si and graphitic carbon (at 1000–1200 °C) or Fe₅Si₃ and β-SiC (at T > 1300 °C) crystallize. All SiFeOC-based ceramic samples were found to exhibit soft magnetic properties. Magnetization versus applied field measurements of the samples show a saturation magnetization up to 26.0 emu/g, depending on the Fe content within the SiFeOC-based samples as well as on the crystalline iron silicide phases formed during pyrolysis.     

Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (T_MAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp³-bonded carbon phase to the sp²-bonded carbon phase with increasing T_MAX and its almost full completion for T_MAX = 1600 °C.     

Preparation and characterization of BaCe0.95Tb0.05O3−α hollow fibre membranes for hydrogen permeation

BaCe_0.95Tb_0.05O_3?α (BCTb) perovskite hollow fibre membranes were fabricated by spinning the slurry mixture containing 66.67 wt% BCTb powder, 6.67 wt% polyethersulphone (PESf) and 26.67 wt% N-methyl-2-pyrrolidone (NMP) followed by sintering at elevated temperatures. The influence of sintering temperature on the membrane properties was investigated in terms of crystal phase, morphology, porosity and mechanical strength. In order to obtain gas-tight hollow fibres with sufficient mechanical strength, the sintering temperature should be controlled between 1350 and 1450 °C. Hydrogen permeation through the BCTb hollow fibre membranes was carried out between 700 and 1000 °C using 50% H₂–He mixture as feed on the shell side and N₂ as sweep gas in the fibre lumen. The measured hydrogen permeation flux through the BCTb hollow fibre membranes reached up to 0.422 μmol cm^?2 s^?1 at 1000 °C when the flow rates of the H₂–He feed and the nitrogen sweep were 40 mL min^?1 and 30 mL min^?1, respectively.     

Hydrogenation of carbon monoxide to methane over mesoporous nickel-M-alumina (M = Fe,Ni, Co,Ce, and La) xerogel catalysts

Mesoporous nickel(30 wt%)-M(10 wt%)-alumina xerogel (30Ni10MAX) catalysts with different second metal (M = Fe, Ni, Co, Ce, and La) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. In the methanation reaction, yield for CH₄ decreased in the order of 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. Experimental results revealed that CO dissociation energy of the catalyst and H₂ adsorption ability of the catalyst played a key role in determining the catalytic performance of 30Ni10MAX catalyst in the methanation reaction. Optimal CO dissociation energy of the catalyst and large H₂ adsorption ability of the catalyst were favorable for methane production. Among the catalysts tested, 30Ni10FeAX catalyst with the most optimal CO dissociation energy and the largest H₂ adsorption ability exhibited the best catalytic performance in terms of conversion of CO and yield for CH₄ in the methanation reaction. The enhanced catalytic performance of 30Ni10FeAX was also due to a formation of nickel–iron alloy and a facile reduction.     

Chemical hydrogen storage and release properties using redox reaction over the Cu-added Fe/Ce/Zr mixed oxide medium

The chemical hydrogen storage (hydrogen reduction) and release (water-splitting oxidation) properties of the Cu-added Fe/Ce/Zr mixed oxide medium were investigated. The media with Cu content ranging from 0 to 5 wt% were prepared by a co-precipitation method using urea as a precipitant. The hydrogen reduction and the water-splitting oxidation on the medium were tested by temperature programmed reduction/oxidation (TPR/TPO) and repeated isothermal redox cycles at 550 °C for reduction and 350 °C for oxidation. The initial reduction rates and oxidation rates of the media increased with increasing the amount of the Cu additive. In addition, the reactivity of the medium for water-splitting oxidation was enhanced as the CeO₂/ZrO₂ ratio increased. Especially, the Fe-based mixed oxide medium with Cu/CeO₂/ZrO₂ contents of 3/30/10 wt% (Cu(3%)-Fe-CeO₂/ZrO₂(3/1)) showed superior performance in chemical hydrogen storage and release. As the results of isothermal redox cycles using the medium, the total amount of hydrogen evolved in water-splitting oxidation was maintained at ca. 8.5 mmol g^?1-medium (ca. 1.8 wt% hydrogen storage amounts on the basis of the total medium) over 15 repeated redox cycles.     

Platinum–tin bimetallic nanoparticles for methanol tolerant oxygen-reduction activity

Carbon-supported Pt–Sn/C bimetallic nanoparticle electrocatalysts were prepared by the simple reduction of the metal precursors using ethylene glycol. The catalysts heat-treated under argon atmosphere to improve alloying of platinum with tin. As-prepared Pt–Sn bimetallic nanoparticles exhibit a single-phase fcc structure of Pt and heat-treatment leading to fcc Pt₇₅Sn₂₅ phase and hexagonal alloy structure of the Pt₅₀Sn₅₀ phase. Transmission electron microscopy image of the as-prepared Pt–Sn/C catalyst reveals a mean particle diameter of ca. 5.8 nm with a relatively narrow size distribution and the particle size increased to ca. 20 nm when heat-treated at 500 °C due to agglomeration. The electrocatalytic activity of oxygen reduction assessed using rotating ring disk electrode technique (hydrodynamic voltammetry) indicated the order of electrocatalytic activity to be: Pt–Sn/C (as-prepared) > Pt–Sn/C (250 °C) > Pt–Sn/C (500 °C) > Pt–Sn/C (600 °C) > Pt–Sn/C (800 °C). Kinetic analysis reveals that the oxygen reduction reaction on Pt–Sn/C catalysts follows a four-electron process leading to water. Moreover, the Pt–Sn/C catalyst exhibited much higher methanol tolerance during the oxygen reduction reaction than the Pt/C catalyst, assessing that the present Pt–Sn/C bimetallic catalyst may function as a methanol-tolerant cathode catalyst in a direct methanol fuel cell.     

Preparation of supported skeletal Ni catalyst and its catalytic performance on dicyclopentadiene hydrogenation

Preparation and catalytic performance of skeletal Ni catalysts supported on Al₂O₃ were studied. The effects of alloy powder/pseudo-boehmite powder mass ratios and calcination temperatures of precursors on surface properties, compressive strength and catalytic performance were investigated. It was found that catalysts prepared by precursors which were molded with alloy powder/pseudo-boehmite powder mass ratio of 4/6 and calcinated at 860 °C in air atmosphere exhibited excellent compressive strength (16.11 N/mm), high dicyclopentadiene conversion (> 95%) and appropriate THDCPD selectivity (> 50%) during 1000-hour evaluation. The operational conditions were obtained as following: T = 120 °C, P = 2.0 MPa, LSHV = 2.0 h^− 1 and hydrogen–oil ratio = 300:1.     

The interfacial and tribological properties of Ni–Cr alloy/ceramic tribo-couples under dry sliding contact

In this paper, the tribological behaviors of Ni–Cr alloy sliding against Si₃N₄ and WC–Co at 20 °C and 600 °C were investigated on a tribometer with a ball-on-disk configuration. The experimental results indicated that Ni–Cr alloy sliding against WC–Co exhibited higher wear resistance than that sliding against Si₃N₄. From the viewpoints of the interfacial interactions between metal and ceramic (chemical reaction, wetting, adhesion, transference), the wear mechanisms were elucidated. The tribological behaviors of Ni–Cr alloy/ceramic tribo-couples were well correlated with the interfacial characteristics, namely the reactive interface and the non-reactive interface. Ni–Cr alloy/Si₃N₄ tribo-couple showed severe adhesive wear as a result of the interfacial reaction between Ni and Si₃N₄, while the non-reactivity of Ni/WC interface is the most important factor corresponding to the moderate adhesive wear in Ni–Cr alloy sliding against WC–Co. Finally, the relations among the interfacial characteristics, wear behavior, and temperature were discussed. The results may provide some experimental evidences on the design and optimization of metal/ceramic tribo-couples.     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.