FLASH sintering of porcelain stoneware: Effect of composition modifications

In this work, we have exploited FLASH sintering as an alternative sintering process in the production of porcelain stoneware. FLASH sintering of porcelain stoneware occurred at temperatures ∼ 1020 °C, for 500 V cm^-1, 2 mA mm^-2 in 30 s. These conditions are significantly less severe than those typically applied in its conventional sintering, 1150–1250 °C for 1 h. Despite the reduction of time and sintering temperature, FLASH sintered samples exhibit heterogeneous microstructure and elemental distribution, with localized glassy phase formed on the positive pole. By decreasing the feldspar content, less localized glassy phase and more uniform microstructures were obtained, being of relevance the highest density and microstructure uniformity attained in the composition without feldspar. These results extend the FLASH sintering applicability and illustrate its importance for the development of alternative sintering technologies in traditional ceramic industry, that in addition may benefit from the reduction of feldspar in the porcelain stoneware production.     

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.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

Evolution of the microstructure and mechanical properties of stereolithography formed alumina cores sintered in vacuum

Stereolithography (SL) was used to form alumina ceramic cores. The effect of sintering temperature on the microstructure and mechanical properties of the alumina ceramics are investigated, which were sintered in vacuum. The results indicate that, as the sintering temperature increased the particle size of alumina slightly increased, and the interlayer spacing first decreased and then increased. The open porosity of alumina ceramics significantly decreased as the sintering temperature in vacuum increased. The flexural strength and hardness increased as the sintering temperature increased. When sintered at 1150 °C, the flexural strength was found to be 33.7 MPa, the shrinkage was 2.3 %, 2.4 %, and 5.3 % in the X, Y, and Z directions, respectively, and the open porosity was 37.9 %. These results are similar to those found from sintering at 1280 °C in air.     

On the physico-mechanical,electrical and dielectric properties of mullite-glass composites

Mullite-glass composites were obtained by solid-state reactive sintering of kaolinite clay and kaolin waste mixtures with waste additions up to 100 wt%. The structural and microstructural analysis of starting powders and sintered samples were evaluated by X-ray diffractometry (XRD) and field-emission scanning electron microscopy (FESEM). The mechanical properties were evaluated by measuring the flexural strength of sintered bodies. Electrical properties of the composites were assessed by impedance spectroscopy (at 30 °C and from 400 to 700 °C) in air. A viscous flux mechanism resulting from the glassy phase filled up the open porosity and increased the mechanical strength. Electrical conductivity, dielectric constant and dielectric loss were strongly dependent on the microstructural features, namely glassy phase and porosity. The activation energies (0.89–0.99 eV) for electrical conduction were lower than typical literature values of mullite-based materials. The results indicated that the herein synthesized mullite-glass composites with up to 53.6 wt% mullite are promising low-cost materials for electronics-related applications.     

Sintering behaviour and properties of magnesium orthosilicate-hydroxyapatite ceramic

The effect of pressureless sintering on the properties of magnesium orthosilicate-hydroxyapatite (MO-HA) ceramic has been studied. The amount of MO composition in the green body was varied from 10 wt% to 50 wt% through mechanical ball milling and was subsequently sintered at varying temperatures in air atmosphere from 1000 °C to 1300 °C for 2 h. The magnesium orthosilicate phase was stable during sintering but the hydroxyapatite phase decomposed to tricalcium phosphate. The MO-HA composites generally exhibited lower mechanical properties across all the investigated composition. Nevertheless, a high fracture toughness of 2.5 MPam^1/2 was recorded for sintered body that contained 20 wt% MO. This finding indicates the potential of this ceramic composite to be used for biomedical applications.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.     

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.     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

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.     

Evaluating the suitability of fast sintering techniques for the consolidation of calcium phosphate scaffolds produced by DLP

Porous scaffolds were fabricated via Digital Light Processing (DLP) from β-TCP powder and sintered by conventional sintering in air (CSA), rapid sintering in air (RSA) and pressure-less spark plasma sintering in vacuum (pl-SPS), at four different temperatures: 1200, 1300, 1400 and 1500 ºC. Each sintering strategy resulted in scaffolds with different phase composition, microstructure and mechanical properties. Long dwell times or high temperatures were required to achieve a complete β→α transformation, and rapid cooling rates avoided the reverse transformation. The presence of graphite in the sintering chamber played a crucial role in stabilising the α-TCP phase, phase prevailing in SPS-treated scaffolds, hindered their densification and avoided the generation of transformation-induced cracks. All scaffolds exhibited compressive strengths within the range of cancellous bone, with the highest average value of 22 ± 4 MPa achieved by the RSA scaffolds sintered at 1300 ºC, thanks to their greater densification and fine microstructure.     

Processing of Yttria powders derived from hydroxide precursors

Roles of calcination and sintering schedules on the final sintered density and microstructure of Y₂O₃ compacts were studied. Optimum calcination procedure appeared to be at 1000°C and for one hour in air atmosphere. Final sintered density of 95% theoretical was achieved around 1450° and nearly pore free (>99% theoretical) pellets were obtained at 1650°C for one hour sintering in vacuum. High heating/cooling rates when coupled with high sintering temperature caused discontinuous grain growth and large internal cracks.     

Sintering behaviour of fluorapatite–silicate composites produced from natural fluorapatite and quartz

In this work, the sintering behaviour of fluorapatite (FAp)–silicate composites prepared by mixing variable amounts of natural quartz (2.5 wt% to 20 wt%) and FAp was studied. The composites were pressureless sintered in air at temperatures from 1000 °C to 1350 °C. The effects of temperatures on the densification, phase formation, chemical bonding and Vickers hardness of the composites were evaluated. All the samples exhibited mixed phase, comprising FAp and francolite as the major constituents along with some minor phases of cristobalite, wollastonite, dicalcium silicate and/or whitlockite dependent on the quartz content and sintering temperature. The composite containing 2.5 wt% quartz exhibited the best sintering properties. The highest bulk density of 3 g/cm³ and a Vickers hardness of >4.2 GPa were obtained for the 2.5 wt% quartz–FAp composite when sintered at 1100 °C. The addition of quartz was found to alter the microstructure of the composites, where it exhibited a rod-like morphology when sintered at 1000 °C and a regular rounded grain structure when sintered at 1350 °C. A wetted grain surface was observed for composites containing high quartz content and was believed to be associated with a transient liquid phase sintering.     

二次烧结温度对牙科氧化锆陶瓷组织和力学性能的影响

研究二次烧结温度对氧化锆牙科陶瓷微观组织和力学性能的影响。方法：氧化锆亚微米粉经过干压、冷等静压成型后在1050ºC预烧结,然后将预烧结牙科氧化锆瓷块在1300ºC-1600ºC进行二次烧结。对不同二次烧结温度下材料的线收缩率、烧结密度、物相、三点抗弯强度进行测量分析,并通过扫描电镜观察试样的断面形貌。结果：结果表明随着二次烧结温度提高,氧化锆的密度、弯曲强度呈上升趋势。在1350ºC时体积密度达到6.10g/cm³,1500ºC时的机械性能最优,三点弯曲强度为852MPa,主晶相为四方相。结论：亚微米氧化锆粉体烧结活性高,力学性能优良,能够满足口腔全瓷修复材料的要求。     

Porous ceramic monoliths based on diatomite

Porous silica ceramics were obtained at low forming pressure and low sintering temperature by using diatomaceous earth as a silica source and boric acid as an inexpensive additive. The starting raw material, diatomite from surface coal mine Kolubara, Serbia, was purified from organic and inorganic impurities by using heat and chemical treatment. Boric acid was used as binding and sintering aid up to 2 wt%. Powder was compacted by using different pressures of 40, 60 and 80 MPa. The pressed samples were sintered at 850, 1000, 1150, and 1300 °C for 4 h in air. A relatively high porosity in the range of 60–70% is obtained for the samples pressed at 40, 60 and 80 MPa and sintered at 1000 °C. Median pore size diameters are in the range of macroporous up to 2 μm in the samples sintered at 1150 and 1300 °C. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scaning electron microscopy (SEM) and mercury porosimetry measurements were employed to characterize the phases, functional groups, microstructure and pore size distribution of the obtained samples. In addition, measurements of densities and open porosities by immersion technique, according to Archimedes principle, were used. The relations between mechanical properties (Young modulus, Poisson ratio, and compressive strength) versus content of boric acid in the investigated samples were studied and disscussed.     

Mechanical and Thermal Properties of Pressureless Sintered Silicon Carbide Ceramics with Alumina–Yttria–Calcia

The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with Al₂O₃–Y₂O₃–CaO without applied pressure was investigated. SiC ceramics containing A₂O₃–Y₂O₃–CaO as sintering additives can be sintered to >97% theoretical density at temperatures between 1750°C and 1900°C without applied pressure. A toughened microstructure, consisting of relatively large elongated grains and relatively small equiaxed grains, has been obtained when sintered at temperatures as low as 1800°C for 2 h in an argon atmosphere without applied pressure. The achievement of toughened microstructures under such mild conditions is the result of the additive composition. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature because of the decrease in the lattice oxygen content of the SiC grains. Typical sintered density, flexural strength, fracture toughness, hardness, and thermal conductivity of the 1850°C‐sintered SiC, which consisted of 62.2% 4H, 35.7% 6H, and 2.1% 3C, were 99.0%, 628 MPa, 5.3 MPa·m^1/2, 29.1 GPa, and 80 W·(m·K)^?1, respectively.     

Sintering behavior and growth mechanism of β-TCP in nanocrystalline hydroxyapatite synthesized by mechanical alloying

Nanocrystalline carbonated HAp powder has been synthesized successfully within 2 h by mechanical alloying the stoichiometric mixture of CaCO₃, CaHPO₄·2H₂O at room temperature under open air. To observe the sintering behavior of HAp the as-milled sample is sintered at different temperatures. The amorphous HAp phase (~14 vol%) in as-synthesized sample transforms completely to crystalline HAp after sintering at 700 °C and after sintering the sample at 800 °C, the crystalline HAp partially transforms to β-TCP phase. Presence of low content of β-TCP phase in HAp powder could be useful in artificial hard tissue applications. Increase in sintering temperature up to 1000 °C results in enhancement of decomposition rate of HAp into β-TCP phase. Microstructure characterization in terms of lattice imperfections and relative phase abundances in non-sintered and all sintered samples are made both by analyzing the respective XRD patterns using Rietveld's structure refinement method as well as TEM images. The growth mechanism of β-TCP from crystalline HAp phase has been proposed based on structure and microstructure characterizations of sintered samples.     

Porous cordierite-based ceramics processed by starch consolidation casting – Microstructure and high-temperature mechanical behavior

Porous cordierite-based ceramics with different microstructural features and mechanical behavior were formed by starch consolidation casting (SCC) using native potato and corn starches and sintered at 1275, 1300 and 1330 °C. The composition and microstructure of the ceramic materials were investigated via quantitative phase analysis using X-ray diffraction (with Rietveld refinement), the Archimedes method, mercury porosimetry, scanning electron microscopy and optical microscopy with stereology-based image analysis. The mechanical behavior of samples was evaluated by diametral compression tests at room temperature, 1000 and 1100 °C. The type of starch used and the sintering temperatures were the main factors determining the characteristics of the developed porous microstructures. Materials prepared with corn starch achieved the lowest porosity and the lowest values of mean chord length, mean pore distance and pore throat size. Because of these features, these materials thus presented, in general, higher values of apparent Young's modulus, elastic limit and mechanical strength than those prepared with potato starch. Despite the presence of a silicate glassy phase, both porous materials, mainly those prepared with corn starch, still enhanced the basic mechanical properties at high temperature, in particular, the mechanical strength and the apparent Young's modulus due to the special combination of the porous microstructure features.     

Dense zircon (ZrSiO4) ceramics by high energy ball milling and spark plasma sintering

The addition of sintering additives has always been detrimental to the mechanical properties of sintered ceramics; therefore, methods to reduce or, as in this case, eliminate sintering additives are usually relevant. In this paper, dense zircon ceramics were obtained starting from mechanically activated powder compacted by spark plasma sintering without employing sintering additives.The high energy ball milling (HEBM) of starting powder was effective to enhance the sintering kinetics. The structural changes of the zircon powder introduced by the HEBM were evaluated. The phase composition and the microstructure of bulk zircon material were analyzed by SEM (EDAX) and XRD. The Vickers hardness and the fracture toughness were evaluated as well.Fully dense materials were obtained at 1400 °C with a heating rate of 100 °C/min, 10 min soaking time and 100 MPa uniaxial pressure. The zircon samples sintered at temperatures above 1400 °C were dissociated in monoclinic zirconia and amorphous silica. The dissociation was detrimental for the mechanical properties. Unlike conventional sintering methods (hot pressing, pressureless sintering) SPS permitted to overcome the dissociation of the zircon material and to obtain additive free, fully dense zircon ceramic with outstanding mechanical properties.     

Fine-Grained Silicon Carbide Ceramics with Oxynitride Glass

By using an oxynitride glass composition from the Y-Mg-Si-Al-O-N system as a sintering additive, the effect of atmosphere on densification was investigated during the liquid-phase sintering of SiC, and the resulting microstructure and mechanical properties of the sintered and subsequently annealed materials were investigated. SiC ceramics that were densified with 10 wt% oxynitride glass showed higher sinterability in a nitrogen atmosphere. Oxynitride glass enlarged the stability region of β-SiC and suppressed β→ alpha phase transformation, which resulted in an equiaxed microstructure. Grain growth of fine-grained SiC in some extent (up to ∼300 nm) was beneficial in improving both room-temperature strength and toughness. The best results were obtained when the ceramics were hot-pressed at 1800°C for 1 h in a nitrogen atmosphere and subsequently annealed at 1900°C for 3 h in an argon atmosphere. The room-temperature flexural strength and fracture toughness of the material were 847 MPa and 3.5 MPa·m^1/2, respectively.