Vacuum sintered lunar regolith simulant: Pore-forming and thermal conductivity

Lunar regolith molding technologies are receiving an increasing attention in lunar exploration program. Most studies are carried out in the air on earth, ignored the effects of high vacuum (~?10^-12 mbar) on the lunar surface. This paper presents the results of a study aimed at assessing the effect of vacuum on the sintering of low Ti basalt type lunar simulant CLRS-1. The results show that porous sample with density of 1.19?g?cm^-3 can be obtained by sintering at 1100?°C under vacuum, which has much lower thermal conductivity (0.265?W?m^-1 K^-1) than concrete and other lunar resource derived structural materials. It could potentially be applied as the thermal insulation material for the Moon base construction. The pore-forming mechanism in vacuum was investigated. It was found that evaporation of the products of solid dissolved mineral crystals led to a great deal of weight loss and inhibited the densification during sintering.     

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.     

Spark plasma sintering of a lunar regolith simulant: effects of parameters on microstructure evolution,phase transformation,and mechanical properties

The spark plasma sintering (SPS) process is a potentially effective in-situ resource utilization (ISRU) technology for consolidating lunar regolith in order to produce structural components for future space exploration. This study examined the fundamental mechanisms of the effects of SPS conditions on microstructure evolution, phase transformation, and mechanical properties. For this purpose, a lunar regolith simulant (FJS-1) was selected and sintered for a total of 16 cases based on four primary SPS testing parameters: temperature, applied external pressure, dwell time, and heating rate. The Taguchi design method was used to examine the effects and sensitivity of each testing parameter. Laboratory tests were conducted in multiple length scales, including density, porosity, optical microscopy, scanning electron microscopy aided by energy-dispersive spectroscopy, transmission electron microscopy, nanoindentation, and strength testing (in both compressive and flexural). Taguchi analysis results of SPS parameters and sintering mechanism discussion indicated that the sintering temperature is the dominant factor changing microstructure heterogeneity and densification during the SPS process. The contribution of applied pressure to the surface and the grain boundary diffusion rate and the nucleation rate indicated that the applied pressure may have enhanced both phase transformation and homogeneity during the sintering process. Strength of the sintered samples were approximately 10 times greater than those of a typical plain concrete. The collective results indicate that the SPS technology, a potentially viable ISRU method, can be used to produce property-specific and application-targeted building components on the lunar surface.     

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.     

Digital light processing of lunar regolith structures with high mechanical properties

It is highly desirable to establish an extraterrestrial base on the moon due to its practicality and scientific significance in the future space explorations, which promotes aerospace scientists to propose many conceivable fabrication methods. Herein, we fabricated architectural and functional structures with lunar regolith simulants via digital light processing (DLP) technology, followed by sintering. The printing slurry was prepared by mixing CLRS-2 lunar regolith simulant powders with photocurable resins, and it exhibits excellent print-ability. The microstructures, chemical compositions, particle size distribution and thermal-gravimetric characteristic of the simulants were analyzed, respectively. The average compressive strength and flexure strength of the sintered samples are 428.1?MPa and 129.5?MPa respectively, which are higher than those reported in previous researches. These improved mechanical properties could be due to the small average diameter of pores and the chemical compositions.     

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.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

In situ hot pressing/reaction synthesis,mechanical and thermal properties of Lu2SiO5

Lu₂SiO₅ is a promising candidate of environmental barrier coatings (EBC) for silicon based ceramics due to its excellent high temperature stability. However, little information is available for the mechanical and thermal properties of Lu₂SiO₅, which frustrated evaluation of its performances for EBC applications. In this paper, dense Lu₂SiO₅ ceramic is successfully fabricated from Lu₂O₃ and SiO₂ powders by in situ hot pressing/reaction sintering at 1500 °C. Mechanical properties, including Young's modulus, bulk modulus, shear modulus, Poisson's ratio, fracture toughness, Vickers hardness, and bending strength are reported for the first time. Lu₂SiO₅ possesses excellent high temperature mechanical properties up to at least 1300 °C. Thermal stress for the case of Lu₂SiO₅ or Y₂SiO₅ coating on silicon bond coat and thermal stress resistance parameter are also estimated based on the experimental mechanical and thermal properties. The present results suggest that Lu₂SiO₅ has better reliability than Y₂SiO₅ in harsh thermal environment.     

Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic

The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 °C and 1350 °C for 15 min on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13–14.6 GPa) and fracture toughness (~3 MPam^1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 °C and reduced significantly at 800 °C thus resulting in the enhancement of electrical conduction.     

Plastic deformation-induced improved mechanical and thermal properties in hot-forged SiC-TiC composite

A novel SiC-20 vol% TiC composite prepared via a two-step sintering technique using 6.5 vol% Y₂O₃-Sc₂O₃-MgO exhibited high deformation (60 %) on hot forging attributed to the high-temperature plasticity of TiC (ductile to brittle transition temperature ～800 °C) and fine-grained microstructure (～276 nm). The newly developed SiC-TiC composite exhibited a ～2-fold increase in nominal strain as compared to that of monolithic SiC. The plastic deformation caused by grain-boundary sliding in monolithic SiC was supplemented by the plastic deformation of TiC in the SiC-TiC composite. The hot-forged composite exhibited anisotropy in its microstructure and mechanical and thermal properties due to the preferred alignment of α-SiC platelets formed in situ. The relative density, flexural strength, fracture toughness, and thermal conductivity of the composite increased from 98.4 %, 608 MPa, 5.1 MPa?m^1/2, and 34.6 Wm^?1 K^?1 in the as-sintered specimen to 99.9 %, 718–777 MPa, 6.9–7.8 MPa?m^1/2, and 54.8–74.7 Wm^?1 K^?1, respectively, on hot forging.     

Microstructure,luminescence, and dielectric properties of microwave-sintered Ce:LuAG nano-ceramics

Herein, phase pure and highly crystalline Ce:LuAG nano-ceramics were fabricated using a novel, ultra-fast microwave sintering approach. The influence of microwave sintering on the microstructural, photoluminescence, and dielectric characteristics of Ce:LuAG nano-ceramic powders was examined. Microwave-assisted sintering of Ce:LuAG nano-ceramic powders yielded high crystallinity, low lattice strain, and reduced grain size. The process also improved the sintering kinetics and enhanced the surface diffusion between the grains, resulting in enhanced luminescence and dielectric properties. The Cole-Cole impedance plots showed single semicircular arcs, indicating non-Debye relaxation and a high dielectric constant in the microwave-sintered Ce:LuAG nano-ceramic and highlighting its potential for use in optoelectronics.     

Effect of ZrB2 powders on densification,microstructure, mechanical properties and thermal conductivity of ZrB2-SiC ceramics

With the view to improve the densification behaviour and mechanical properties of ZrB₂-SiC ceramics, three synthesis routes were investigated for the production of ZrB₂, prior to the fabrication of ZrB₂-20 vol. % SiC via spark plasma sintering (SPS). Two borothermal reduction routes, modified with a water-washing stage (BRW) and partial solid solution of Ti (BRS), were utilised, alongside a boro/carbothermal mechanism (BRCR) were utilised to synthesise ZrB₂, as a precursor material for the production of ZrB₂-SiC. It was determined that reduction in the primary ZrB₂ particle size, alongside a diminished oxygen content, was capable of improving densification. ZrB₂-SiC ceramics, with ZrB₂ derived from BRW synthesis, exhibited a favorable combination of high relative density (98.6%), promoting a marked increase in Vickers hardness (21.4 ± 1.7 GPa) and improved thermal conductivity (68.7 W·m^-1K^-1).     

Preparation and mechanical properties of SiCw-reinforced WC-10Ni3Al cemented carbide by microwave sintering

SiC_w-reinforced WC-10Ni₃Al cemented carbide was prepared by microwave sintering method, and the effects of the sintering temperature and SiC_w content on the microstructure and mechanical properties of WC-10Ni₃Al cemented carbide were investigated; the promotion effect and strengthening mechanism of SiC_w were then analysed. The experimental results showed that the relative density, hardness, flexural strength and fracture toughness of WC-10Ni₃Al cemented carbide increased and then decreased with increasing SiC_w addition and sintering temperature. When the sintering temperature was 1500 °C and the content of SiC_w was 0.3 wt%, the sample reached the highest mechanical properties and had a relative density of 96.5%, hardness of 1570 HV, flexural strength of 1275 MPa and fracture toughness of 13.1 MPa mm^1/2, which were 4.0%, 23.1%, 12.5% and 8.1% higher than those of the sample without SiC_w, respectively. During microwave sintering of WC-Ni₃Al, the addition of an appropriate SiC_w content can increase the microwave absorption of the sample, and produce many micro-high-temperature regions within the sample, which can accelerate the generation of the Ni₃Al liquid phase. This promotes liquid phase flow to fill pores and rearrange the WC grains, thereby improving density and mechanical properties of the sample. The strengthening mechanisms of SiC_w on microwave sintered WC-Ni₃Al consist of promoting densification enhancement, fine-grained strengthening, and solid solution strengthening of Ni₃Al by Si atoms.     

Crystal structure,mechanical and thermal properties of Yb4Al2O9: A combination of experimental and theoretical investigations

The key requirements for a successful thermal and environmental barrier coating (T/EBC) material include stability in high temperature water vapor, low Young's modulus, close thermal expansion coefficient (TEC) with mullite, low thermal conductivity and weak mechanical anisotropy. The current prime candidates for top coat are ytterbium silicates (Yb₂SiO₅ and Yb₂Si₂O₇). A major weakness of these two silicates is the severe anisotropy in mechanical properties and thermal expansion that would lead to cracking of the coating. Thus, searching for new materials with weak mechanical and thermal anisotropy is of signification. In this work, the crystal structure, mechanical and thermal properties of a promising T/EBC candidate, Yb₄Al₂O₉, are investigated theoretically and experimentally. Good ductility, low shear deformation resistance, low Young's modulus (151 GPa) and low thermal conductivity (0.78 W m⁻¹ K⁻¹) is underpinned by heterogeneous bonding characteristic and distortion of the structure. Close TEC (6.27 × 10⁻⁶ K⁻¹) with mullite and weak mechanical anisotropy highlight the suitability of Yb₄Al₂O₉ as a prospective T/EBC.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering

Hydroxyapatite (HAP) possesses excellent bioactivity/osteointegration properties. Nevertheless, its inferior flexural strength and fracture toughness limit its use in human weight-bearing parts. We investigated a microwave sintering technology which can be effectively used to develop titanium dioxide-hydroxyapatite (TiO₂-HAP) ceramics with different amounts of TiO₂ (0.8,1.6,2.4,3.2,4.0,4.8,5.6 and 6.4 wt%), which contribute to extremely high flexural strength (90–130 MPa) along with a good combination of elastic modulus and fracture toughness. The results of the Rietveld refinement show that multiphase bioceramics (HAP, β-TCP) can be achieved by doping nano-TiO₂ under microwave sintering. Despite the fact that the main phases of the sintered TiO₂-HAP ceramics are HAP and β-TCP, X-ray diffraction confirms the formation of the CaTiO₃ and CaTi₂O₄(OH)₂ phases. Furthermore, the sintering reactions to form these phases are discussed and the results show that an appropriate amount of nano-TiO₂ can not only effectively inhibit the growth of grain, but also change the fracture mode and increase the relative density. Finally, it is found that doping nano-TiO₂ by microwave heating is an effective technique for producing HAP/β-TCP composite load-bearing implants in clinical applications without coarsening the size of grain.     

Cost effective preparation of Si3N4 ceramics with improved thermal conductivity and mechanical properties

High-purity silicon powder is used as the starting material for cost-effective preparation of silicon nitride ceramics with both high thermal conductivity and excellent mechanical properties using RE₂O₃ (RE=Y, La or Er) and MgO as sintering additives. Nitridation is a key procedure that would affect the properties of green bodies and the sintered samples. The β: (α+β) ratio can be increased as the samples nitrided at 1450ºC and a large amount of long rod-like β-Si₃N₄ grains were developed in the samples. It was found that the addition of Er₂O₃-MgO could help to improve the mechanical properties of the sintered Si₃N₄ ceramics, the thermal conductivity, flexural strength and fracture toughness of the sample were 90 W/(m∙K), 953±28.3 MPa and 10.64±0.61 MPa·m^1/2, respectively. The RE³⁺ species with larger ionic radius tended to increase the oxygen of nitrided samples and decrease N/O ratio (triangle grain boundary) of sintered samples.     

Effects of cross-scale h-BN grains and orientation degree on the mechanical and thermal properties of BN-matrix textured ceramics

h-BN is a two-dimensional ceramic material with a lamellar structure, known for its typical orientation characteristics on mechanical and thermal properties. By optimizing the size and arrangement of h-BN grains in the matrix, the anisotropic characteristics of h-BN ceramics can be fully utilized to obtain ceramic materials with high thermal conductivity or high strength. In order to study the effect of grain orientation distribution on the mechanical and thermal properties of materials, the index of orientation distribution (IOP) was used to quantitatively characterize the orientation degree of h-BN grains and analyzed the effect of h-BN grain size on material properties. The results show when the initial h-BN size is 13.50 μm, the ceramic has the highest orientation degree/-507, and the mechanical and thermal properties show obvious anisotropy. While the related properties of BN-YAG ceramics varies significantly with the decrease of initial h-BN grain size.