Flame synthesis of mixed tin‐silver‐copper nanopowders and conductive coatings

The single‐step direct synthesis of tin‐silver‐copper nanopowders and nanostructured coatings using the flame‐based high‐temperature reducing jet (HTRJ) process is reported. Nanostructured coatings were deposited and sintered within the HTRJ reactor to study the effect of reductive sintering temperature on their electrical conductivity and surface morphology. Although the ultimate application of these nanoparticles is in printed electronics, which requires dispersing them as stable inks before depositing and sintering them, our approach of direct deposition from the gas phase provides an upper limit on the conductivity achievable with a given composition. The directly deposited coatings had high electrical conductivity, including a value of 2 × 10⁶ S/m for 36 wt % Cu‐40 wt % Ag‐24 wt % Sn sintered at 200°C. This is twice the conductivity of a pure silver coating prepared under similar conditions. Moreover, similarly high electrical conductivity was achieved using only 20% Ag with sintering at 300°C. © 2015 American Institute of Chemical Engineers AIChE J, 62: 408–414, 2016     

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.     

Sintering effect on microstructural evolution and mechanical properties of spark plasma sintered Ti matrix composites reinforced by reduced graphene oxides

Ti matrix composites reinforced with 0.6?wt% reduced graphene oxide (rGO) sheets were fabricated using spark plasma sintering (SPS) technology at different sintering temperatures from 800?°C to 1100?°C. Effects of SPS sintering temperature on microstructural evolution and mechanical properties of rGO/Ti composites were studied. Results showed that with an increase in the sintering temperature, the relative density and densification of the composites were improved. The Ti grains were apparently refined owing to the presence of rGO. The optimum sintering temperature was found to be 1000?°C with a duration of 5?min under a pressure of 45?MPa in vacuum, and the structure of rGO was retained. At the same time, the reaction between Ti matrix and rGO at such high sintering temperatures resulted in uniform distribution of micro/nano TiC particle inside the rGO/Ti composites. The sintered rGO/Ti composites exhibited the best mechanical properties at the sintering temperature of 1000?°C, obtaining the values of micro-hardness, ultimate tensile strength, 0.2% yield strength of 224 HV, 535?MPa and 446?MPa, respectively. These are much higher than the composites sintered at the temperature of 900?°C. The fracture mode of the composites was found to change from a predominate trans-granular mode at low sintering temperatures to a ductile fracture mode with quasi-cleavage at higher temperatures, which is consistent with the theoretical calculations.     

Water-assisted cold isostatic pressing to enhance sinterability of alumina ceramics

Water-Assisted cold isostatic Pressing (WAP) enabled pressure-less sintering of fine grained (0.6 μm) and nearly full dense (99.3%) weakly translucent alumina ceramics. As a term of comparison, Dry Pressed (DP) samples, prepared under identical processing conditions, maintained an opaque appearance, and their relative density did not exceed 97.7%. The enhanced compaction of WAP allowed to lower the sintering temperature down to 1350°C, which is approximately 100–150°C lower than that of DP powder. WAP samples resulted in an apparent activation energy for sintering (640 kJ/mol) matching literature values, confirming that the enhanced sinterability was attributed to an improved green density rather than a change in sintering mechanisms.     

Microstructure and properties of high alumina castables containing calcium alumino‐titanate

Calcium alumino‐titanate (CAT)‐containing high alumina castables were prepared using bauxite, CAT, and α‐Al₂O₃ as starting materials, and subsequently heat treated at various temperatures ranging from 1400°C to 1600°C. The thermo‐mechanical properties of the specimens as a function of the temperature were characterized in terms of linear shrinkage, bulk density, apparent porosity, cold crushing strength (CCS), modulus of rupture (MOR), residual ratio of MOR, and coefficient of thermal expansion (CTE). X‐ray diffraction (XRD) and scanning electron microscopy (SEM) were used to determine the phase composition and microstructural evolution, respectively. Sintering at temperatures between 1450 and 1500°C promoted normal grain growth, with the formation of a granular surface contact, and abnormal grain growth occurred at higher temperatures (≥ 1550°C). The cold strength of the specimen initially increased and subsequently decreased with temperature, whereas the residual ratio of MOR initially decreased with increasing temperature to 1550°C, then increased above 1550°C. In addition, the CTE of the specimen was only minimally influenced by the firing temperature.     

Comparison between microwave and conventional sintering on the properties and microstructural evolution of tetragonal zirconia

In this research, the comparison between microwave sintering and conventional sintering on the mechanical properties and microstructural evolution of 3?mol% yttria-stabilised zirconia were studied. Green bodies were compacted and sintered at various temperatures ranging from 1200?°C to 1500?°C. The results showed that microwave assisted sintering was beneficial in enhancing the densification and mechanical properties of zirconia, particularly when sintered at 1200?°C. It was revealed that as the sintering temperature was increased to 1400?°C and beyond, the grain size and mechanical properties for both microwave- and conventional-sintered ceramics were comparable thus suggesting that the sintering temperature where densification mechanism was activated, grain size was strongly influenced by the sintering temperature and not the sintering mode.     

Effect of sintering temperature on the thermal expansion behavior of ZrMgMo3O12p/2024Al composite

A 2024Al metal matrix composite with 10?vol% negative expansion ceramic ZrMgMo₃O₁₂ was fabricated by vacuum hot pressing, and the influence of sintering temperature on the microstructure and thermal expansion coefficient (CTE) of alloys was investigated. Experimental results showed that all ZrMgMo₃O_12p/2024Al composites sintered at 500–530?°C had a similar reticular structure and exhibited different linear expansion coefficients at 40–150?°C and 150–300?°C. The addition of 10?vol% ZrMgMo₃O₁₂ decreased the CTEs of 2024Al by ~ 16% at 40–150?°C and by ~ 7% at 150–300?°C. This addition also increased the hardness of 2024Al by ~ 23%. The density of the composites and the content of Al₂Cu in ZrMgMo₃O_12p/2024Al increased as the sintering temperature increased. The CTEs of the composites decreased, whereas hardness increased. Thermal cycling from 40?°C to 300?°C caused the CTEs of the composites to decrease gradually and reach a stable value after seven cycles. The lowest CTEs of 15.4?×?10^?6 °C^?1 at 40–150?°C and 20.1?×?10^?6 °C^?1 at 150–300?°C were obtained after 10 thermal cycles and were reduced by ~ 32% and ~ 17%, respectively, compared with the CTE of the 2024Al. Among the current reinforcements, ZrMgMo₃O₁₂ negative expansion ceramics showed the highest efficiency to decrease the CTE of Al matrix composites.     

Microstructural evolution and sintering properties of palygorskite nanofibers

In this study, the morphological evolution and sintering properties of the palygorskite nanofibers were studied along with the increase of temperature, using raw palygorskite as materials. The palygorskite powder was calcined at different temperatures in the range of 100°C-1200°C, and the microstructural evolution of the palygorskite nanofibers was investigated by thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscope (HRTEM). Furthermore, the palygorskite powder was shaped to bars by dry pressing and sintered from 700°C to 1200°C. The properties of the sintered palygorskite were characterized by bending strength, mercury intrusion porosimeter (MIP), and stepwise isothermal dilatometry (SID). The results showed that the morphology of palygorskite nanofibers maintained unchanged till 1000°C. The palygorskite nanofibers molted to bind each other and formed a solid interwoven network structure at 1100°C. Correspondingly, it was shown from the sharply decrease of the sintered palygorskite porosity from 45.46% at 1000°C to 1.82% at 1100°C that the dense sintering of palygorskite started at 1100°C. With the sintering proceeding, some closed micropores fused each other to form bigger opening pores, resulting in a slight increase of porosity at 1200°C. However, the pore size distribution got more uniform and the density of the sintered body increased. So the bending strength of the sintered body reached the maximum of 176.67 Mpa and finally the main crystalline phases of the sintered sample changed to quartz, enstatite, and kyanite. The sintering activation energy of the palygorskite was measured by means of SID with a value of 906.46 kJ·mol⁻¹.     

Mullite (Nextel™ 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties

A mullite matrix containing homogeneously distributed ultra-fine (70–350 nm) pores was reinforced with NdPO₄-coated woven mullite fibre mats (Nextel™ 720) leading to damage-tolerant composites with good high temperature (1300 °C) strength and thermal cycling resistance. Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities. After sintering at 1200 °C for 3 h, the compacts had a density of 86.4% of theoretical density and showed damage-tolerant behaviour up to 1300 °C, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300 °C, respectively. No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150 °C for up to 300 cycles. The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1) the presence of a dense NdPO₄ interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores (<350 nm) within the mullite matrix.     

Rapid manufacturing of silica glass parts with complex structures through stereolithography and pressureless spark plasma sintering

This paper reports a method for preparing silica glass by pressureless spark plasma sintering (PL-SPS), which can rapidly manufacture silica glass parts with complex structures by coupling with stereolithography 3D printing technology. The rapid sintering process and microstructure evolution of silica glass prepared by PL-SPS were mainly investigated. The experimental results showed that the sintering temperature and dwelling time were the main factors affecting the PL-SPS of silica glass. The microstructure evolution indicated that the densification rate of the sample was very fast from 1250 °C to 1300 °C, and the interlayer defects caused by the printed layer thickness could be healed in the final stage of densification. Like conventional pressureless sintering, silica glass with a relative density of more than 99% and a visible-light transmittance of more than 90% could also be obtained through PL-SPS, but the entire working time was shortened from 22.53 h to 0.49 h.     

Sintering parameters study of a biphasic calcium phosphate bioceramic synthesized by alcoholic sol-gel technique

In the sintering of biphasic calcium phosphate bioceramics (BCP) can occur phases transformations accompanied by a sudden thermal expansion due to different coefficients of thermal expansion of each phase, which generates internal stress concentrations inducing undesired cracks within the sample. Therefore, this work aimed to study the sintering parameters of a BCP, composed of hydroxyapatite (HAp) and β-tricalcium phosphate (β-TCP), synthesized by the alcoholic sol-gel technique, in order to evaluate the better conditions for avoiding defect generation. BCP powders were uniaxially cold-pressed at 300 MPa and air-sintered at 1070 °C/2 h (BCP1070 sample) and 1130 °C/2 h (BCP1130 sample), with heating rates of 10 °C/min and 5 °C/min, respectively. Samples were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, relative density determination, Vickers hardness test, and nanohardness tests. The results displayed that the assessed sintering parameters were suitable for promoting a satisfactory particle consolidation. The transformation from HAp to β-TCP occurred simultaneously with grain growth and material densification under all conditions. The mechanical tests revealed that BCP1070 and BCP1130 samples have different behaviour when analyzed micro or nanostructurally. But, the heating rate (10 °C/min) combined with the sintering temperature (1070 °C) anabled to obtain a suitable sinterability with samples presenting smaller grain size and without defects. Further, it kept the microporosity which is an essential property for application in bioceramic materials.     

Investigating the effect of SPS‎ parameters on densification ‎and fracture toughness of ZrB2-SiC nanocomposite‎

In this study, the effect of sintering parameters on densification and fracture toughness of spark plasma sintering ZrB₂-SiC nanocomposites was evaluated. For this purpose, ZrB₂-??30?vol% SiC nanocomposites in the conditions of ?1600?°C-4?min, 1700?°C-4?min, 1800?°C-4?min, 1800?°C-8?min, 1800?°C-12?min? were sintered.? Scanning Electron Microscopy (SEM) was used in order to investigate the ?microstructural variations. The bulk density was measured accoring to ASTM C 373–88. Single edge notch beam (SENB) method was used to ?determine the fracture toughness of samples. Microstructural observations showed that ?an increase in sintering temperature led to slight ?increase in SiC grains size but no sensitive variation in ZrB₂. However, increasing the sintering time resulted to increase both ZrB₂ and SiC grain size. Also, it was found, temperature and time ascent always increases the relative density. In addition, it was concluded that optimal temperature and time to reach the highest fracture toughness are 1800?°C and 8?min, respectively. Investigation of SEM images of the Vickers indent and their path propagation showed that the deviation and branching of crack are the most important toughening ?mechanisms in ZrB₂-SiC nanocomposites.?     

Preparation and properties of porous ceramic aggregates using electrical insulators waste

In this paper, porous ceramic aggregates were prepared by electrical insulators waste (EIW). Effects of sintering temperature and content of EIW on the aggregates’ properties such as bulk density, and apparent porosity, total porosity, and cold crushing strength were investigated. With increasing sintering temperature and content of EIW, bulk density and cold crushing strength of the aggregates increased, apparent porosity and total porosity decreased. Based on these results, total porosity of specimens in group B sintered at 1200 °C is 62.0%, cold crushing strength is 35.3 N, and thermal conductivity is 0.165 W/(m K) at 300 °C. Comprehensive properties of specimens can be optimized by adjusting sintering temperature. Meanwhile, strength variation resulted from the combined effects of phase transformation and matrix densification under different sintering temperatures.     

Glass–Ceramic Foams from Borosilicate Glass Waste

In this study, we reported the studies on a glass–ceramic foam with wollastonite and cristobalite micrometric crystals prepared by sintering a borosilicate glass waste with organic binder as foaming agent. The waste glass, coming from the dismantling of washing machine, was characterized by high CaO content and low-temperature sinterability. The effect of the temperature on the sinter-crystallization ability of the borosilicate glass waste was followed with thermal analysis, heating microscopy, and electron scanning microscopy (ESEM) observations. Additionally, the effect of temperature on the evolution of crystalline phases and density variation was monitored with XRD and density measurements. The softening started at 800°C and crystallization at 845°C to be completed at 900°C with a linear expansion of 38–40% in the range 850–900°C. Wollastonite and cristobalite were identified as crystalline phases in variable proportions dependently upon temperature. No crack evidence was found at high ESEM magnification even though cristobalite crystals were present. The final products showed a total porosity around 78–79% and an apparent density of about 0.5 g/cm³, in line with common porous closed-cell glass foams used for thermal insulation.     

Preparation study for the low thermal expansion spodumene/mullite composites

In this work, spodumene/mullite ceramics with low thermal expansion were successfully prepared from spodumene, quartz, talc, and clay. The effects of spodumene content and sintering temperature on the mechanical properties of spodumene/mullite ceramics were investigated. The formed phases were then detected by X-ray diffraction analysis and the microstructures of the sintered bodies were determined by scanning electron microscopy. The interaction effects of the spodumene content and sintering temperature on the apparent porosity and bulk density were studied by response surface methodology. The results demonstrate that an appropriate sintering temperature and spodumene content can promote densification, improve the mechanical properties, and reduce the coefficient of thermal expansion (CTE) of spodumene/mullite ceramics. At the spodumene content of 40 wt.%, the sintering temperature of 1270°C, and the holding time of 90 min, the bending strength was 60.45 MPa, the CTE was 1.73 × 10^–6/°C (α_{[25–650°C]} < 2 × 10^–6/°C), the bulk density was 2.28 g cm^-3, and the apparent porosity was 0.43%. Therefore, this study was of guiding significance for reducing the production cost of spodumene low thermal expansion ceramics and improving product quality.     

Mechanical properties of refractory castables bonded with hydratable magnesium carboxylate-boric acid

Hydratable magnesium carboxylate (HMC), which is similar to the properties of cement, can be used as a potential binder for refractory castables. However, its decomposition may lead to poor mechanical properties at medium temperatures (300 °C–1100 °C). This work investigated the effects of boric acid on the mechanical properties and microstructural evolution of castables bonded with hydratable magnesium carboxylate. The mechanical strength, bulk density, apparent porosity, thermal shock resistance, and sintering properties of the castables were evaluated. The results showed that the mechanical properties of HMC-bonded castables (HMCC) at various temperatures can be improved by adding boric acid. Boric acid reacts with HMC to form magnesium carboxylate borate ester (MCBE), which improves the bonding strength between HMC molecules. Thus, the cold modulus of rupture of HMCC containing boric acid dried at 110 °C are higher than that of calcium aluminate cement-bonded refractory castables (CACC). The decomposition temperature of MCBE is 77 °C higher than that of HMC, so MCBE can endow castables with better mechanical properties at 110 °C–500 °C. The B₄C obtained by MCBE pyrolysis could form a boron-rich liquid phase, which can accelerate the structural densification of castables via transient liquid phase sintering, thus improving the mechanical properties of castables at 500 °C–1100 °C. Moreover, boric acid can improve the thermal shock resistance of HMCC. The residual strength rate first increases and then decreases with an increasing boric acid, and reaches a maximum value of 29.7% (1 wt% boric acid is added), which is 2.3 times that of the CACC. The nanoindentation test showed that the microcracks in the matrix of 1 wt% boric acid castables are easy to initiate but difficult to propagate, so the microcracks are many and wavy.     

Prediction and control of microstructure evolution for sub-microscale α-Al2O3 during low-heating-rate sintering based on the master sintering curve theory

The master sintering curve (MSC) can sometimes be used for analyzing the shrinkage behaviour of ceramics. Densification of α-Al₂O₃ with the mean particle size of 350 nm was continuously recorded during heating at 0.5, 2 and 5 °C/min. A MSC was successfully constructed using dilatometry data with the help of combined-stage sintering model. The validity of the MSC has been verified by a few experimental runs. The microstructural evolution with densification during different heating-rate sintering was explored. The sintered microstructure is a function of the time–temperature sintering conditions, and it is verified that there exists a link between sintered density and microstructure. The MSC can be used to predict and control microstructure evolution during sintering of α-Al₂O₃ ceramics.     

Preparation of mullite by the reaction sintering of kaolinite and alumina

In the present study, mullite specimens and mullite/alumina composites are prepared by reaction sintering kaolinite and alumina at a temperature above 1000°C. The phase and microstructural evolution of the specimens and their mechanical properties are investigated. Primary mullite appears at a temperature around 1200°C. The alumina particles are inert to the formation of primary mullite. Alumina starts to react with the silica in glassy phase to form secondary mullite above 1300°C. The formation of secondary mullite decreases the amount of glassy phase. Furthermore, the addition of alumina reduces the size of mullite grains and their aspect ratio. The strength and toughness of the resulting mullite increase with the increase of alumina content; however, the mechanical properties of the mullite and mullite/alumina composites are lower than those of alumina for their relatively low density.     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Flash sintering of additively manufactured 3YSZ gears

We report on the fabrication of a complex gear-shaped 3 mol.% yttria-stabilized zirconia via a combination of additive manufacturing and flash sintering. The gear was printed via direct ink printing technique. Flash sintering was performed at an electric field of 150 V/cm and the specimens were rapidly densified within a few seconds at a furnace temperature of 1200°C. The flash-sintered gear has a 95% relative density and shows no obvious warping or cracking after the rapid densification. Microstructure analysis and hardness measurement revealed grain size gradients and hardness variation along both thickness and lateral directions of the flash-sintered gear. This study demonstrates a possible route to produce dense complex-shaped parts using the flash sintering technique.