Showing papers on "Sintering published in 2019"

High-entropy pyrochlores with low thermal conductivity for thermal barrier coating materials

Abstract: High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE1/5)2Zr2O7 have been formed after heated at 1000°C. The (5RE1/5)2Zr2O7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE1/5)2Zr2O7 high-entropy ceramics are below 1 W·m–1·K–1 in the temperature range of 300–1200°C. The (5RE1/5)2Zr2O7 can be potential thermal barrier coating materials.

A sodium-ion sulfide solid electrolyte with unprecedented conductivity at room temperature.

Abstract: Solid electrolytes are key materials to enable solid-state rechargeable batteries, a promising technology that could address the safety and energy density issues. Here, we report a sulfide sodium-ion conductor, Na2.88Sb0.88W0.12S4, with conductivity superior to that of the benchmark electrolyte, Li10GeP2S12. Partial substitution of antimony in Na3SbS4 with tungsten introduces sodium vacancies and tetragonal to cubic phase transition, giving rise to the highest room-temperature conductivity of 32 mS cm−1 for a sintered body, Na2.88Sb0.88W0.12S4. Moreover, this sulfide possesses additional advantages including stability against humid atmosphere and densification at much lower sintering temperatures than those (>1000 °C) of typical oxide sodium-ion conductors. The discovery of the fast sodium-ion conductors boosts the ongoing research for solid-state rechargeable battery technology with high safety, cost-effectiveness, large energy and power densities. Solid-state rechargeable batteries using solid electrolytes instead of liquid ones could address the safety and energy density issues. Here the authors report a Na-ion solid electrolyte Na2.88Sb0.88W0.12S4 which exhibits record high ionic conductivity of 32 mS/cm at room temperature.

Cold Sintering: Progress, Challenges, and Future Opportunities

Abstract: Cold sintering is an unusually low-temperature process that uses a transient transport phase, which is most often liquid, and an applied uniaxial force to assist in densification of a powder compac...

Evaporation-induced sintering of liquid metal droplets with biological nanofibrils for flexible conductivity and responsive actuation

Abstract: Liquid metal (LM) droplets show the superiority in coalescing into integral liquid conductors applicable in flexible and deformable electronics. However, the large surface tension, oxide shells and poor compatibility with most other materials may prevent spontaneous coalescence of LM droplets and/or hybridisation into composites, unless external interventions (e.g., shear and laser) are applied. Here, we show that biological nanofibrils (NFs; including cellulose, silk fibroin and amyloid) enable evaporation-induced sintering of LM droplets under ambient conditions into conductive coating on diverse substrates and free-standing films. The resultants possess an insulating NFs-rich layer and a conductive LM-rich layer, offering flexibility, high reflectivity, stretchable conductivity, electromagnetic shielding, degradability and rapid actuating behaviours. Thus this sintering approach not only extends fundamental knowledge about sintering LM droplets, but also starts a new scenario of producing flexible coating and free-standing composites with flexibility, conductivity, sustainability and degradability, and applicable in microcircuits, wearable electronics and soft robotics. Providing mechanical sintering of liquid metal droplets under ambient conditions for flexible electronics remains elusive. Here, they propose biological nanofibrils for enabling evaporation-induced sintering of EGaIn droplets into conductive coating on diverse substrates and free-standing films.

Synergistic modulation of mobility and thermal conductivity in (Bi,Sb)2Te3 towards high thermoelectric performance

Abstract: Modulating microstructures in a wide range from atomic defects to microscale structures independently can partially decouple the transport of charge carriers and phonons and thus enhance the figure of merit (zT) of thermoelectric materials. High mobility requires atomic scale purity, while introducing nanoscopic inhomogeneities leads to low thermal conductivity. Through a two-step sintering process with excess Te, lower reduction of mobility and decreased thermal conductivity were simultaneously achieved in (Bi,Sb)2Te3. Grain boundaries and defects that strongly impede charge carrier transport are reduced by the two-step sintering process leading to a higher mobility compared to that of the one-step sintered bulk. At the same time, removal of Te as well as Sb-rich inhomogeneities with lattice misfit to the matrix decreased the thermal conductivity. In this way, simultaneous maintenance of high mobility and low lattice thermal conductivity was illustrated. By further optimization of carrier concentration, the produced material showed an encouraging zT value of 1.38 at 323 K. The present work demonstrates a method for synthesizing high-efficiency thermoelectric materials through simultaneous optimization of the electrical and thermal transport properties.

Fused filament fabrication, debinding and sintering as a low cost additive manufacturing method of 316L stainless steel

Abstract: By using filaments comprising metal or ceramic powders and polymer binders, solid metal and ceramic parts can be created by combining low-cost fused filament fabrication (FFF) with debinding and sintering. In this work, we explored a fabrication route using a FFF filament filled with 316 L steel powder at 55 vol.-%. We investigated the printing, debinding and sintering parameters and optimized them with respect to the mechanical properties of the final part. Special focus was placed on debinding and sintering in order to obtain components of low residual porosity. Solvent debinding of the printed green bodies created an internal network of interconnected pores and was followed by thermal debinding. Thermal debinding allowed for complete removal of the remaining binder and produced mechanically stable brown parts. Sintering at 1360 °C provided densification of the parts and generated nearly isotropic linear shrinkage of about 20%. Using optimized parameters, it was possible to fabricate 316 L steel components with a density greater than 95% via the material extrusion additive manufacturing, debinding and sintering route, with achievable deflections in a 3-point bending test similar to rolled sheet material, albeit at lower strength.

CO2 hydrogenation to methanol over Cu catalysts supported on La-modified SBA-15: The crucial role of Cu–LaOx interfaces

Abstract: The direct hydrogenation of CO2 to methanol has become a very active research field because CO2 can be prospectively recycled to mitigate greenhouse effect and store clean synthetic fuels. This reaction can be catalyzed by supported Cu catalysts and the catalysts display strong support or promoter effects. Sintering of Cu species accelerates the separation of Cu–oxide interfaces, reduces the active component, and diminishes the methanol selectivity. In this work, we report a Cu catalyst supported on La-modified SBA-15, where the Cu–LaOx interface is generated through the interaction of highly dispersed Cu nanoparticles with LaOx species bedded into the SBA-15 pore wall. The optimized Cu1La0.2/SBA-15 catalyst can achieve methanol selectivity up to 81.2% with no deterioration in activity over 100 h on stream compared with the La-free catalyst. A thorough study reveals that La species not only significantly improve the CO2 adsorption but also enhance Cu dispersion to produce well-dispersed active sites. The H/D exchange experiments show that the methanol synthesis displays a strong thermodynamic isotope effect and the Cu–LaOx interface plays a crucial role for the methanol synthesis rate in CO2/D2 feed. In situ DRIFTS studies reveal that *HCOO and *OCH3 species are the key intermediates formed during the activation of CO2 and methanol synthesis over the Cu1La0.2/SBA-15 catalyst.

Ceramic Mineral Waste-Forms for Nuclear Waste Immobilization

Abstract: Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li-Cs, Tl, Ag, Be-Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98-99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.

Hybrid Ti matrix composites with TiB2 and TiC compounds

Abstract: Spark plasma sintering method was utilized to produce monolithic Ti and four composite samples with different amount of TiB2 and TiC as additives. The Sintering conditions of 1200 °C for 5 min under 50 MPa external pressure was employed for SPS process. The effect of different amount of additives was studied on the relative density, microstructure and mechanical characterizations of as-sintered samples. Microstructural investigations, thermodynamic assessments and XRD analysis revealed the formation of in-situ TiBw phase in the Titanium matrix composites (TMCs). However, thanks to the short dwell time and existence of coarse TiB2 particles, some semi reacted titanium diboride surrounded by TiBw phase remained into the samples specially the ones with high TiB2 contents. On the other hand, Titanium also diffuse into the TiC phase and the new phase of TiC1-x was produced. The best relative density of 99.77 was obtained for monolithic sample. Adding additives decreased the UTS, elongation and bending strength of as-sintered samples whilst the hardness of specimens increased with addition of more TiB2/TiC. The fractographs shown the role of additives on obtaining a finer microstructure by hindering the extensive grain growth. Predominant fracture mode was brittle and adding more additives rose up the transgranular fractures.

Reactive spark plasma sintering of TiB2–SiC–TiN novel composite

Abstract: The effects of adding SiC as a reinforcement and TiN as an additive on TiB2-based composites fabricated by the spark plasma sintering (SPS) technique were investigated. SPS was implemented at the sintering conditions of 1900 °C temperature, 7 min holding time and 40 MPa pressure. Adding these two secondary phases had noticeable effects on the microstructure of TiB2-based composites. A relative densities of 99.9% was obtained for TiB2–SiC–TiN composite. Detection of in-situ formed phases and investigation on them were done using SEM, XRD, EDS and thermodynamic assessment. These evaluations proved the formation of in-situ phases of TiC, BN nano-platelets, TiSi and B4C in the TiB2-based composite codoped with SiC and TiN. Formation of these in-situ phases had fascinating effects on the sinterability and ultimate microstructure of titanium diboride.

Nanoindentation and nanostructural characterization of ZrB2–SiC composite doped with graphite nano-flakes

Abstract: Nano-sized graphite was used as a dopant for fabrication of ZrB2–SiC ceramic via spark plasma sintering at 1800 °C for 8 min under 35 MPa. As-sintered composite was characterized by XRD, SEM, EDS, STEM, TEM and nanoindentation in order to study the micro/nanostructure and mechanical properties of the sample. A near fully-dense ternary composite was obtained after densification process. In-situ formation of ZrC was attributed to the chemical reaction of graphite nano-flakes with ZrO2 nano-layers covered the surface of starting ZrB2 powders. Reactive role of graphite as an effective sintering aid, via removal of oxide impurities, was illustrated by TEM, as some ultrafine porosities were remained in the sintered bulk in graphite-free areas. The hardness and elastic modulus of the composite, obtained by the nanoindentation method, showed an excellent harmony with the reported data in the literature. The average hardness of 15.2, 18.3 and 10.7 GPa were achieved for ZrB2, SiC and ZrB2/SiC interface, respectively. Average Young's moduli of matrix and reinforcement phases were measured as 328 and 306 GPa, respectively, which showed favorable adaption in mechanical properties of composite components. The nano-indentational characteristics of composite, especially pop-ins in the load-displacement curves, were also discussed.

Wear and mechanical properties of Al6061/SiC/B4C hybrid composites produced with powder metallurgy

Abstract: This study investigates the production of various reinforced and non-reinforced composite materials using powder metallurgy (PM). It presents the new approach into optimize the mechanical properties of hybrid composites (Al-SiC-B4C) produced with powder extrusion process. A16061 powders are used as the matrix material and B4C and SiC powders are used as the reinforcement materials. Matrix and reinforcement materials are mixed in a three-dimensional mixer. The mixtures are then subjected to cold pressing to form metal block samples. Block samples are subjected to hot extrusion in an extrusion mold after being subjected to a sintering process. This produces samples with a cross-sectional area of 25 × 30 mm2. These extruded samples were subjected to T6 heat treatment. The composite materials produced are examined in terms of density, hardness, transverse rupture strength, tensile strength, and wear resistance. Furthermore, optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy and XRD are performed to examine the microstructure, surface fractures, and surface abrasion. In this study, high density Al6061/B4C/SiC hybrid composite materials were successfully produced. After extrusion, some micro particles were found to crack. The highest hardness occurred in 12%B4C reinforced composites. The lowest hardness was obtained in Al6061 alloy without reinforcement. The highest tensile strength occurred in 12%SiC particle reinforced composite material. The highest wear resistance was obtained for 9%B4C+3%SiC samples due to the hardness of B4C and the good adhesion properties of the matrix and SiC.

Composite NASICON (Na3Zr2Si2PO12) Solid-State Electrolyte with Enhanced Na+ Ionic Conductivity: Effect of Liquid Phase Sintering.

Abstract: NASICON-type of solid-state electrolyte, Na3Zr2Si2PO12 (NZSP), is one of the potential solid-state electrolytes for all-solid-state Na battery and Na-air battery. However, in solid-state synthesis, high sintering temperature above 1200 °C and long duration are required, which led to loss of volatile materials and formation of impurities at the grain boundaries. This hampers the total ionic conductivity of NZSP to be in the range of 10-4 S cm-1. Herein, we have reduced both the sintering temperature and time of the NZSP electrolyte by sintering the NZSP powders with different amounts of Na2SiO3 additive, which provides the liquid phase for the sintering process. The addition of 5 wt % Na2SiO3 has shown the highest total ionic conductivity of 1.45 mS cm-1 at room temperature. A systematic study of the effect of Na2SiO3 on the microstructure and electrical properties of the NZSP electrolyte is conducted by the structural study with the help of morphological and chemical observations using X-ray diffraction (XRD), scanning electron microscopy, and using focused ion-beam-time of flight-secondary ion mass spectroscopy. The XRD results revealed that cations from Na2SiO3 diffused into the bulk change the stoichiometry of NZSP, leading to an enlarged bottleneck area and hence lowering activation energy in the bulk, which contributes to the increment of the bulk ion conductivity, as indicated by the electrochemical impedance spectroscopy result. In addition, higher density and better microstructure contribute to improved grain boundary conductivity. More importantly, this study has achieved a highly ionic conductive NZSP only by facile addition of Na2SiO3 into the NZSP powder prior to the sintering stage.