Hydrogen reduction behavior and microstructural characteristics of WO3 and WO3-NiO powders

The characterization and understanding of the hydrogen reduction and sintering behavior of powder mixtures prepared from WO₃ and WO₃-NiO have been investigated. The nano-sized W and W-Ni powders were prepared by ball milling and hydrogen reduction of oxide powders. The reduction behavior is analyzed by temperature-programmed reduction method with different heating rates in Ar-10% H₂ atmosphere. X-ray diffractometry analysis revealed that the oxide powders are changed to W and W-Ni powders with an average particle size of about 100 nm by hydrogen reduction at 800 °C for 1 h. The hydrogen reduction kinetics was evaluated by the amount of peak shift with heating rates. The activation energies for the reduction of pure WO₃ and WO₃-NiO, estimated by the slope of the Kissinger plot, were measured as 87.4–117.4 kJ/mol depending on reduction steps. The consolidated W-Ni by spark plasma sintering has relatively dense and large grains with neck growth by enhanced mass transport due to the addition of Ni. These results are help to optimize the powder synthesis process and to understand the hydrogen reduction behavior and Ni addition effect related to microstructure of powders and sintered bodies.     

粉末注射成形钛合金粘结剂体系的研究进展

钛及钛合金具有低比重、高比强度、优异的生物相容性以及良好的耐腐蚀性等特点,在航空航天、生物医疗、化工、船舶、汽车等领域极具应用潜力。钛合金粉末注射成形技术（powder injection molding,PIM）提高了材料的利用率,实现了中小型复杂形状钛产品的大批量、低成本制备,显著地推动了钛及钛合金产品的生产及应用。目前关于粉末注射成形钛合金粘结剂体系的相关文献报道十分有限,新型粉末注射成形钛合金粘结剂体系的开发处于停滞不前的状态。本文分析总结了不同粉末注射成形钛合金粘结剂体系的研究现状,并针对目前存在的问题提出改进措施。     

Direct ink writing of cordierite ceramics with low thermal expansion coefficient

Since the application of cordierite ceramics is limited by the disadvantages of traditional preparation techniques, 3D printing technology provides the only choice for the rapid preparation of cordierite ceramics with highly complex structures. In this work, the fabrication of cordierite ceramics with complex structures was achieved by direct ink writing. The near-net-shape of cordierite ceramics was realized by the volume expansion caused by the phase transformation. A cordierite ceramic with an average shrinkage rate of 1.58 % was obtained at 1400 °C. The low shrinkage avoids design and manufacturing procedures carried out for dimensional and alignment errors. In addition, the coefficient of thermal expansion was as low as 1.69 × 10^?6 °C^?1. The effect of configuration on the thermal behavior of cordierite ceramics is understood by analyzing the phase composition and microstructure. The cordierites ink reported in this work offers additional possibilities for the production of novel complex structures.     

Role of zirconia phase transformation,interface processes,and Ta2O5 doping on the blistering phenomenon of molten glass in contact with zirconia-based refractories

Bubble formation and removal within the molten glass is an important issue in glass industry. Various sources of bubbles have been identified in glass manufacturing: decomposition of the glass components, air trapping, oxidation/reduction reactions, precipitation resulting from insufficient refining, etc. It has been demonstrated in a previous paper that the blistering phenomenon at the interface between a molten glass and a zirconia-based refractory can be ascribed to the oxygen semipermeability through the zirconia phase. The objective of this study is to clarify the role of temperature on the blistering process, and especially, below and above the phase transition temperature of zirconia (monoclinic/tetragonal transformation) and to evaluate the role of zirconia doping on the blistering level. The influence of the kinetics of the surface processes at the glass/refractory interface is emphasized. Quantitative measurement of the slight blistering ascribed to the so-called “redox shock” is also given.     

3D printing of porous low-temperature in-situ mullite ceramic using waste rice husk ash-derived silica

The in-situ mullite (3Al₂O₃·2SiO₂) foams are fabricated by 3D printing (direct ink writing (DIW)) technique and utilize waste rice husk ash (RHA). The Al₂O₃-SiO₂ inks are prepared using an aqueous binder with α-alumina and two different silica sources, i.e., RHA extracted biogenic nano-silica (NS) and commercial silica (CS). The ink rheological features are first designed in terms of solid-to-liquid ratio and dispersant, and found that a higher amount of dispersant is needed for functionalization of NS-containing ink than CS (micro-sized) consisting of ink. Secondly, the DIW log-pile structures are fired at different temperatures (1200?1500 °C), and NS containing samples exhibited remarkable enhanced properties at a lower firing temperature than CS. At 1400 °C, alumina and RHA nano-silica entirely transformed into mullite and retained ～75 % porosity, ～8 MPa cold compressive strength, and thermal conductivity ～0.173 W/m·k that designate a simple and effective way to fabricate of mullite foamy structure.     

Relaxor antiferroelectric-like characteristic boosting enhanced energy storage performance in eco-friendly (Bi0.5Na0.5)TiO3-based ceramics

Ideal relaxor antiferroelectrics (RAFEs) have high field-induced polarization, low remnant polarization and very slim hysteresis, which can generate high recoverable energy storage W_rec and high energy storage efficiency η, thus attracting much attention for energy storage applications. True RAFEs, on the other hand, are extremely rare, and the majority of them contain environmentally hazardous lead. In this work, we use a viscous polymer rolling process to synthesize a novel and eco-friendly 0.65Bi_0.5Na_0.4K_0.1TiO₃-0.35[2/3SrTiO₃-1/3Bi(Mg_2/3Nb_1/3)O₃] (BNKT-ST-BMN) dielectric material, which possesses a very typical RAFE-like characteristic. As a result, this material has a high W_rec of 4.43 J/cm³ and a η of 86% at an electric felid of 290 kV/cm, as well as a high thermal stability of W_rec (>3 J/cm³) over a wide range of 30–140 °C at 250 kV/cm. Our findings suggest that the BNKT-ST-BMN material could be a potential candidate for use in energy storage pulse capacitors.     

Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li_6.5La₃Zr_1.5(Ta/Nb)_0.5O₁₂, typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm^?1. However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li_6.5La₃Zr_1.5Ta_(0.5?x)Nb_xO₁₂, where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li_6.5La₃Zr_1.5Ta_0.1Nb_0.4O₁₂ (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10^?3 S cm^?1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.     

Ultrafast high-temperature sintering of barium titanate ceramics with colossal dielectric constants

Application of Ultrafast High-temperature Sintering (UHS) technique to rapidly densify barium titanate ceramics has been explored for the first time. Bulk ceramic with ~94% density was obtained by UHS at ~1340 °C for 60 seconds. The densification process was accompanied with progressive sample discolouration from light to dark grey. Further analysis indicates that oxygen vacancy and its associated Ti-rich phase Ba₄Ti₁₂O₂₇ are present in the ceramics. Their roles in ultrafast densification and sample discoloration are discussed. Due to the presence of oxygen vacancies, the UHSed ceramics generally exhibit a colossal dielectric constant of ~ 15–30k at 1 kHz, with dielectric loss of ~0.07–0.10, while the ceramics without oxygen vacancy retain a dielectric constant of ~3000–6000 and dielectric loss of ~ 0.06 at 1 kHz which are comparable to that of the conventionally sintered ceramics. Furthermore, the challenges in applying UHS to sinter thick BT ceramics are discussed, aided by thermal simulations.     

Thermoelectric properties and thermal stability of metal-doped cuprous sulfide thermoelectrics

Mn doping and S-evaporation are strategies used to improve the thermoelectric properties and thermal stability of cuprous sulfide thermoelectric materials. Cu_1.8S and Mn-alloyed Cu_1.8S powders were prepared via ball milling, and different samples were obtained via current-assisted sintering at different times. It was found that Mn and S-evaporation optimized the carrier concentration and thus improved the figure of merit (ZT) of the samples. The introduction of pore defects induced by S-evaporation also improved the ZT. The maximum ZT of the optimized sample reached 0.89 at 500 °C. Mn in the samples reacted with oxygen to form an oxide film on the surface of the block, which inhibited the kinetic process of Cu_1.8S decomposition and improved the thermal stability of the samples. However, the reaction between Mn and oxygen led to a continuous loss of metal cations in the material, resulting in changes in the thermoelectric properties.     

Structural,dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Zn0.5Ti0.5)O3 thin films prepared by sol–gel method

Lead free (1−x)(0.8Bi_0.5Na_0.5Ti_0.5O₃–0.2Bi_0.5K_0.5TiO₃)–xBiZn_0.5Ti_0.5O₃ (x=0–0.06) (BNT–BKT–BZT) thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel processing technique. The effects of BZT content on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT–BKT–BZT thin films were investigated systematically. The BNT–BKT–BZT thin films undergo a transition from ferroelectric to relaxor phase with increasing temperature. The phase transition temperature decreases with the increase of BZT content. The BNT–BKT–BZT thin film with x=0.04 exhibits the best ferroelectric properties (P_max=40 µC/cm² and P_r=10 µC/cm²), largest dielectric constant (ε=560) and piezoelectric constant (d₃₃=40 pm/V). This finding demonstrates that the BNT–BKT–BZT thin film has an excellent potential for demanding high piezoelectric properties in lead free films.