Microstructure evolution during spark plasma sintering of FJS-1 lunar soil simulant

A spark plasma sintering (SPS) process has been explored to densify FJS-lunar soil simulants for structural applications in space explorations. The effect of SPS conditions, such as temperature and pressure, on the densification behavior, phase transformation, microstructural evolution, and mechanical properties of FJS-1 have been examined by conducting the X-ray diffraction analysis, electron microscopy imaging, and nano/micro indentation testing. Test analysis results were also compared to results from the FJS-1 powder and sintered samples without pressure. The FJS-1 powder was composed of sodian anorthite, augite, pigeonite, and iron titanium oxide. When FJS-lunar soil simulants were sintered without pressure, the main phase evolved from sodian anorthite to the intermediate sodian anorthite, jadeite and glass, and iron titanium oxide at 1000°C, which were further transformed into filiform and feather-shaped augite and schorlomite at 1100°C. Most densification processes in pressureless sintering occurred at 1050°C-1100°C. During the SPS process, the main phases were sodian anorthite, pigeonite, and iron titanium oxide at 900°C. These phases were transformed to sodian anorthite, glass, and feather-shaped augite at 1000°C and 1050°C, with the nucleation of dendritic schorlomite at 1050°C. Significant densification by SPS can be observed as low as 900°C, which indicates that the application of pressure can substantially lower the sintering temperature. The SPSed samples showed higher Vickers microhardness than the pressureless sintered samples. The mechanical properties of the local phases were represented by the contour maps of elastic modulus and nanohardness. Multiscale mechanical test results along with the microstructural characteristics further imply that the SPS can be considered a promising in-situ resource utilization (ISRU) method to densify lunar soils.     

Sintering of lunar regolith structures fabricated via digital light processing

Architectural and functional structures composed of lunar regolith-simulant CLRS-2 were fabricated via digital light processing and sintered at 1100 °C and 1150 °C under an air or argon atmosphere. This work is to investigate effects of atmosphere and temperature on mechanical properties, microstructure, and chemical composition of lunar regolith products. Samples sintered at 1150 °C in air underwent the highest sintering shrinkage and showed the best mechanical properties, likely due to the formation of glassy phase and dense structure following sintering. Conversely, argon-sintered samples exhibited lower density resulting from the lack of glassy phase. Phase analysis revealed varying chemical composition and therefore different underlying reaction mechanisms under two sintering atmospheres, indicating that sintering atmosphere significantly influences the microstructure and macroscopic properties of lunar regolith products.     

Microstructural,mechanical, and thermal properties of microwave-sintered KLS-1 lunar regolith simulant

KLS-1 Lunar regolith simulant was microwave sintered to explore its potential applicability in future lunar construction. The effects of sintering temperature on linear shrinkage, density, porosity, and microstructural, mechanical, and thermal properties were investigated. As the sintering temperature increased, linear shrinkage and density increased and porosity decreased. Structural evolution in the sintered samples was characterized by scanning electron microscopy and X-ray diffraction. Unconfined compressive strength testing showed that mechanical strength increased significantly with increasing sintering temperature, with 1120 °C giving the highest strength of 37.0 ± 4.8 MPa. The sintered samples exhibited a coefficient of thermal expansion of approximately 5 × 10⁻⁶ °C⁻¹, which was well-maintained even after cyclic temperature stress between −100 and 200 °C. Therefore, this microwave processing appears promising for the fabrication of building material with sufficient mechanical strength and thermal durability for lunar construction.     

Thermal properties of processed lunar regolith simulant

In this paper, the thermal characterization of lunar regolith simulant, sintered using a conventional oven under ambient and vacuum conditions is presented. Additionally, thermal characterization of samples is performed for the parts manufactured using solar, laser, and microwave processing. Samples for oven sintering are prepared using regolith bulk simulant as well as simulant pressed at 255 MPa for 10 minutes. Similar experiments are performed with a mixture of Johnson Space Center (JSC)-2A + 20 wt% of ilmenite, a common lunar mineral. Samples are characterized regarding their thermal capacity, thermal diffusivity, density, and thermal conductivity. Furthermore, sample morphology is studied using scanning electron microscopy. Lastly, processing of regolith for lunar thermal energy storage is discussed.     

Rapid Reactive Synthesis and Sintering of Submicron TiC/SiC Composites through Spark Plasma Sintering

Submicrometer TiC/SiC composites were fabricated by a rapid reactive sintering process through spark plasma sintering (SPS) technique using the carbon, titanium, and nanosized-SiC powders without any additive. It was found that the composite could be sintered in a relatively short time (8 min at 1480°C) to 97.9% of theoretical density. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction techniques and observed by scanning electron microscopy. The effect of nanosized and microsized SiC additives on the microstructure of TiC/SiC composites was investigated.     

Spark plasma sintering of SiAlON ceramics

Investigated was the spark plasma sintering (SPS) of sialon ceramics from SHS-produced powders. Experimentally established were (a) sintering temperatures that ensure a required density, phase composition, and microstructure of sintered multicomponent sialon ceramics, (b) individual stages of the SPS process, and (c) the effect of starting powder composition on the phase composition and microstructure of sintered sialon ceramics.      

Densification improvement of spark plasma sintered TiB2-based composites with micron-, submicron- and nano-sized SiC particulates

Taguchi design of experiments methodology was used to determine the most influential spark plasma sintering (SPS) parameters on densification of TiB₂–SiC ceramic composites. In this case, four processing factors (SPS temperature, soaking time, applied external pressure and SiC particle size) at three levels were examined in order to acquire the optimum conditions. The statistical analysis identified the sintering temperature as the most effective factor influencing the relative density of TiB₂–SiC ceramics. A relative density of 99.5% was achieved at the optimal SPS conditions; i.e. temperature of 1800?°C, soaking time of 15?min and pressure of 30?MPa by adding 200-nm SiC particulates to the TiB₂ matrix. The experimental measurements and predicted values for the relative density of composite fabricated at the optimum SPS conditions and reinforced with the proper SiC particle size were almost similar. The mechanisms of sintering and densification of spark plasma sintered TiB₂–SiC composites were discussed in details.     

Sintering behavior of 8YSZ-Ni cermet: Comparison between conventional,FST/SPS,and flash sintering

Cermets are ceramic metal composites. The metallic phase in the cermet typically undergoes oxidation during sintering in air. Electric field-assisted sintering processes such as field-assisted sintering technology/spark plasma sintering (FAST/SPS) and flash involves very high heating rates, short processing time and low processing temperature. The main aim of this work was to see if field-assisted sintering techniques can prevent the oxidation of the metallic phase in the cermet. Sintering behavior of 8YSZ-5 wt.% Ni cermet was studied by three different techniques namely; conventional sintering, FAST/SPS and flash sintering. Phases and microstructure were analyzed through X-ray diffraction and scanning electron microscopy, respectively. Temperature and time required for sintering the samples via FAST/SPS and flash sintering was significantly lower than that during conventional sintering. In addition, we found limited grain growth during FAST/SPS and flash sintering. During conventional sintering in reducing atmosphere (Ar and vacuum), Ni particles retained their elemental state, however the extent of densification was poor in the cermet. FAST/SPS in argon and vacuum resulted in almost complete densification (relative density > 97%) and Ni particles were retained in their elemental state in the cermet. During flash sintering in air, the samples sintered to a high densification (relative density ∼98%), however, Ni particles were completely oxidized.     

自蔓延高温合成La0.67Sr0.33MnO3粉体的烧结性能及巨磁效应的研究

利用自蔓延高温合成技术(self-propagating high temperature syntheris,SHS)合成La0.67Sr0.33MnO3粉体,探讨了自蔓延合成工艺对粉体结构及放电等离子体(spark plasma sintering,SPS)和普通烧结对La0.67Sr0.33MnO3粉体烧结性能和陶瓷显微结构的影响.经XRD,SEM,密度测试等结果表明:在自蔓延法合成出的物相为单一的钙钛矿型结构.SPS烧结与传统的固相烧结法相比:SPS快速烧结大大降低了传统固相法烧结温度,烧结后的晶粒大小基本均匀,烧结体致密度高.经过巨磁电子效应(colossal magnetoresistance,CMR)的测量得出,采用SPS放电等离子烧结的样品相对于普通烧结的样品,低温CMR效应有所增大.     

Impact of spark plasma sintering (SPS) on mullite formation in porcelains

The effect of the spark plasma sintering (SPS) process on mullite formation in porcelains was studied using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. SPS affected the kinetics and morphology of formed mullite. After sintering at 1100°C, unlike conventional sintering, SPS promoted the formation of mullite due to the combination of vacuum and applied pressure. Mullite crystal growth was altered by the atmosphere (vacuum), dwell time (0‐15 minutes), and temperature (1000‐1200°C). The applied pressure caused the mullite needles to orient perpendicular to the direction of the applied load. Depending on SPS dwell time, the mullite formed after sintering at 1100°C also had different crystal structure (tetragonal for short dwell time of 0‐5 minutes and orthorhombic for a long dwell time of 10‐15 minutes). Dissolution of mullite was observed at 1100°C by extending the dwell time by up to 15 minutes and the dissolved mullite reprecipitated on the small needles (~40 nm) and coarsened via Oswald ripening resulting in larger mullite needles (~60 nm).     

Spark plasma sintering and hot pressing of ZTA-NbC materials – A comparison of mechanical and electrical properties

Zirconia toughened alumina can be made electrically conductive and thus electric discharge machinable by addition of a percolating dispersion of niobium carbide. In order to boost the productivity of the sintering process spark plasma sintering was tested at identical temperature and pressure but shorter dwell than in hot pressing. SPS sintering parameters for ZTA-NbC are developed and spark plasma sintered ceramics are compared to the hot pressed benchmark.During SPS a percolating NbC backbone of niobium carbide grains is formed which enhances electrical conductivity but impedes densification. Identical strength at however higher sintering temperature is achieved by SPS but the fracture resistance and hardness were always superior in hot pressed samples. The monoclinic content of zirconia grains in as fired SPS samples is higher despite smaller average grain size and the transformation toughening effect is less pronounced. SPS promises economic benefits due to shorter dwell and cooling cycles.     

Synthesis of dense NiZn ferrites by spark plasma sintering

Dense NiZn ferrites were fabricated by spark plasma sintering (SPS) at 900 °C and 20 MPa in short periods. The powder was densified to 98% of the theoretical density by the SPS process. The SPS disks exhibited a higher saturation magnetization (Ms), up to 272 emu/cm³, than did the disks sintered by the conventional process. A higher coercivity (Hci) was obtained when the green bodies were sintered by the SPS process for 5 min. A modest holding time is essential to obtain fine grain and uniformity in the SPS process. Secondary crystallization, inhomogeneous microstructure and intragranular pores were found as a result of the rapid sintering and relatively longer holding time in the SPS process. Infrared (IR) spectra were also measured in the range from 350 to 700 cm⁻¹ to study the efforts of the SPS process on NiZn ferrites.     

Microstructure homogeneity control in spark plasma sintering of Al2O3 ceramics

Homogeneous microstructure control in the SPS (spark plasma sintering) sintered big size Al₂O₃ ceramic was realized by the synergy effect of grain boundary tailoring and proper pressure profile design. Two-step pressure profile itself did not show any efficient densification enhancement if no grain boundary modifier MgO added. The two-step pressure profile can effectively reduce average grain size and grain size difference over the sintered specimen, while MgO doping can reduce the average grain size in the whole sintered samples. Finally, a general strategy to overcome the intrinsic temperature gradient in SPS is suggested.     

Hot compaction of polyoxymethylene,part 1: Processing and mechanical evaluation

The hot‐compaction of polyoxymethylene powders in the solid state, otherwise pressure‐assisted sintering, is studied as an alternative way to the melt‐compression or injection molding processes. A native powder issued from suspension polymerization has been used, together with powders obtained from grinding of melt‐extruded pellets. The experimental conditions were optimized with regard to temperature, pressure, and time. Temperature is the most sensitive parameter of the process. Sintering at temperature close to the onset of the melting range turned out to be necessary for an efficient welding of the powder particles, as judged from mechanical properties. Despite a strong loss of ductility, the sintered samples have been easily machined out into strips for mechanical testing. A significant increase in crystallinity is observed for the sintered samples, as compared with the compression‐molded ones, accompanied with a nearly twofold increase of the Young's modulus. Scanning electron microscopy revealed that rupture of the sintered samples involves both inter‐ and intra‐granular fracture. Welding of the crystallites via molecular diffusion at the particle interface is suggested to be the mechanism of sintering. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1274–1284, 2006     

Synthesis of high-purity bulk Ti2AlN by spark plasma sintering (SPS)

Single-phase bulk Ti₂AlN was prepared by spark plasma sintering (SPS) at 1200 °C of Ti/Al/TiN powders in stoichiometric proportion. Investigated by X-ray diffraction (XRD) of samples and the sintering process parameter, the reaction procedure could be analyzed. Scanning electron microscopy (SEM) and electron probe micro-analysis (EPMA) coupled with energy-dispersive spectroscopy (EDS) were utilized to investigate the morphology characteristics. When sintered at 1200 °C, Ti₂AlN phase was well developed with a close and lamellated structure. The distribution of Ti₂AlN grains was homogeneous.     

Vat photopolymerization of low-titanium lunar regolith simulant for optimal mechanical performance

Recently additive manufacturing of lunar regolith to utilize in-situ resources of the Moon for deep space exploration has attracted attention. However, most previous works have been limited by low precision, inferior mechanical properties, and complex processes, such as ball grinding. Furthermore, the regional distribution difference of the lunar regolith which shows compositional diversity, demands the exploration of manufacturing of low-titanium lunar soil, which has not been comprehensively studied before. Herein, the vat photopolymerization and heat treatment of raw low-titanium lunar regolith simulant were investigated to achieve high dimensional precision and high mechanical properties. The influence of solid content and photoinitiator concentration on printability is carefully examined based on the characterization of rheological behaviors and curing depth. Then the vat photopolymerization is used to build green bodies with good interlayer bonding strength and high dimensional precision. Besides this, the effect of the debinding heating rate and sintering temperature on samples were optimized in air and nitrogen to enhance the mechanical properties of printed samples. Finally, the optimal sintered parts with a flexural strength of 108.8 MPa and compressive strength of 222.8 MPa were obtained.     

Rapid formation of α-sialon during spark plasma sintering: Its origin and implications

α-Sialons with varied chemical compositions and starting precursors were sintered in spark plasma sintering (SPS) furnace to verify the influence of electrical conduction, mechanical pressure and heat transfer efficiency. The internal temperature that the sample was exposed to and its evolution during the heating were experimentally estimated and discussed. XRD and SEM grain size analysis were carried out to evaluate the phase and microstructure evolution. Mechanical pressure was eliminated to create a free sintering condition under SPS. By free sintering and surrounding the sample with graphite wool the heat transfer was sufficiently retarded and a usual slow reaction was demonstrated. It is verified that the efficiency of heat transfer is more influential in the enhancement of α-sialon formation and grain growth by SPS than the efficiency of electric conduction.     

Influence of powder characteristics on cold sintering of nano-sized ZnO with density above 99 %

Cold sintering parameters such as, temperature, pressure, aqueous phase, heating rate and dwelling time has been widely discussed in the literature but the role of starting powder with respective microstructure development is mostly overlooked. There is a need for understanding the effect of powder agglomerates and the role of inter particle friction on the densification behavior during cold sintering process. Present study encompasses investigation and optimization of these parameters for ZnO which enabled > 99 % of relative density with grain sizes below 200 nm. Additionally, role of external atmosphere was also studied to investigate its impact on densification during the process. All cold sintering experiments were carried out in a FAST/SPS device for studying aqueous phase evaporation and ensuring the reproducibility of process parameters. Microstructure characterization (scanning and transmission electron microscopy) showed – without any post heat treatment– defect free grain boundary structure opposite to what documented by previous studies.     

Comparison of hot pressing and spark plasma sintering in the densification behavior of indium tin oxide‐borosilicate glass composites

The goal of this study was to fabricate borosilicate glass matrix composites with high optical transmittance and high conductivity by forming percolated segregated networks of indium tin oxide (ITO) in the microstructure. ITO nanoparticles and borosilicate glass microspheres were mechanically mixed with ITO concentrations varying from 0 to 2.99 vol%. The mixes were then consolidated using either hot pressing (HP) or spark plasma sintering (SPS). The effects of changing sintering methods, along with varying other processing parameters such as heating rate, maximum temperature, and applied pressure, had surprising and unanticipated effects. Ac impedance spectroscopy (IS), SEM, and EDS results indicated the successful formation of a grain‐like microstructure of the sintered glass using both HP and SPS processing, with the ITO particles segregated to the boundary regions in all samples. IS results indicated percolation threshold values between 0.154 and 0.307 vol% ITO in the HP samples and between 0.307 and 0.764 vol% ITO in the SPS samples, with resistivities as low as 29 (Ω·cm) at 2.99 vol% ITO. Optical properties were dominated by impurities and light scattering at defects such as pores. Contrary to conventional belief, it was found that samples made using SPS required far higher temperatures to fully densify, with all other processing conditions being the same, compared with HP. This behavior was confirmed through repeat tests using different SPS equipment and a wide range of processing conditions.     

Densification kinetics of boron carbide with medium entropy alloy as a sintering aid during spark plasma sintering

The high sintering temperature of pure B₄C considerably limits its widespread application, thus searching an effective sintering aid is critical. In this work, B₄C-based ceramic with 1 vol.% nonequiatomic Fe₅₀Mn₃₀Co₁₀Cr₁₀ medium entropy alloy as a sintering aid were fabricated at 1900-2000°C by spark plasma sintering (SPS) under applied pressure, and their mechanical properties were examined and compared with pure B₄C ceramic sintered at same condition. The maximal flexural strength of 255.59 MPa, microhardness of 2297.6 Hv_0.2 and fracture toughness of 3.62 MPa m^1/2 could be obtained at optimized SPS pressure of 50 MPa, which were all higher than those of pure B₄C ceramic. To better understand the densification kinetics mechanisms, the densification ratio as a function of SPS temperature and pressure was theoretically analyzed using steady creep model. It was found that densification controlled by grain-boundary sliding at lower pressure transferred to power law creep regime at higher pressure, which were proved by the dislocation net shown in transmission electron microscopy image.