Sintering Mechanism and Microstructure of TaC/SiC Composites Consolidated by plasma-activated sintering

TaC/SiC composites with 5 wt% SiC addition were densified by plasma-activated sintering (PAS) at 1500–1800 °C for 5 min under 30 MPa. The effects of plasma-activated sintering on microstructures, densification and mechanical properties of the composites were investigated. The results showed that TaC/SiC composites achieved a relative density more than 99% of the theoretical density at 1600 °C. A low eutectic liquid phase generated by the oxide on the particle surface was observed in the composite to realize a relatively low temperature sintering densification. While the TaC particle size decreased insignificantly with increasing sintering temperature, the transformation of morphology of SiC particles changing from equiaxed to elongated grain was activated, accompanying with a slight particle size decreasing of the SiC phase, thus promoting a relatively high flexural strength of 550 MPa under 1800 °C. Besides, some ultra-fine 2 nm Ta₂Si was observed in the glassy pockets, strengthening the amorphous phase and thus increasing the flexural strength.     

Role of MgO on densification and mechanical properties in spark plasma sintered Os0.9Re0.1B2 ceramic

To promote the densification and therefore the mechanical properties of boride-based ceramics, MgO was added as sintering aid into Os_0.9Re_0.1B₂ powders for densification by using spark plasma sintering (SPS). The Os_0.9Re_0.1B₂ powders were synthesized by mechanochemical method from powder mixture of Os, Re and amorphous B. The role of MgO on densification, phase composition, microstructure and mechanical properties (hardness, fracture toughness and wear behavior) were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS), micro indentation and ball-on-disk tribometer. The results show that, with the introduction of MgO as sintering aid, the relative density of the Os_0.9Re_0.1B₂ ceramic samples increased. When the MgO content reached 9 wt%, the as-sintered sample is almost fully dense. No obvious regularity was found from the samples with the addition of different content of MgO. Vickers hardness values of the samples with 0, 3 wt% and 9 wt% MgO are found to be very close with each other within the experimental error (~30 GPa), while the sample with the addition of 6 wt% MgO exhibits the highest hardness of ~35 GPa. The fracture toughness of the samples is decreased slightly with the addition of MgO. The friction coefficient and wear rate of the sample with the addition of 6 wt% MgO was also found to be the lowest among all samples, which indicate best wear resistance. As a whole, with the addition content of 6 wt% MgO, the Os_0.9Re_0.1B₂ ceramic sample performs relatively excellent mechanical properties among four groups of samples.     

Microstructure,ionic conductivity and mechanical properties of tape-cast Li1.5Al0.5Ti1.5P3O12 electrolyte sheets

Free-standing Li_1.5Al_0.5Ti_1.5P₃O₁₂ electrolyte sheets with a thickness of 50–150 μm were prepared by tape casting followed by sintering at 850–1000 °C in air. While a sintering temperature of 850 °C was too low to achieve appreciable densification and grain growth, a peak relative density of 95% was obtained at 920 °C. At higher sintering temperatures, the microstructure changed from a bimodal grain size distribution towards exclusively large grains (> 10 μm), accompanied by a decrease in relative density (down to 86% at 1000 °C). In contrast, ionic conductivity increased with increasing sintering temperature, from 0.1 mS/cm at 920 °C to 0.3 mS/cm at 1000 °C. Sintering behavior was improved by adding 1.5% of amorphous silica to the slurry. In this way, almost full densification (99.8%) and an ionic conductivity of 0.2 mS/cm was achieved at 920 °C.Mechanical characterization was carried out on the almost fully densified material, yielding elastic modulus and hardness values of 109 and 8.7 GPa, respectively. The fracture strength and Weibull modulus were also characterized. The results confirm that densification and reduction of grain size improve the mechanical properties.     

Biomineralization,strength and cytocompatibility improvement of bredigite scaffolds through doping/coating

Coprecipitation-derived, sacrificial polymeric (urethane) foam-fabricated bredigite (Ca₇MgSi₄O₁₆) scaffolds were processed by individual and combined treatments of fluoride doping and poly (lactic-co-glycolic acid) (PLGA) coating and then studied in terms of structure, mechanical strength, bioactivity and cell biocompatibility in vitro. According to scanning electron microscopy and Archimedes porosimetry, the geometrical characteristics of pores for all the scaffolds are in the appropriate range for hard tissue regeneration applications. The apatite-formation ability of the samples immersed in a simulated body fluid is improved by doping for both the bare and coated conditions, based on microscopic and energy-dispersive X-ray spectroscopic analyses. Both the treatments advantageously buffer physiological pH changes imposed due to the fast bioresorption of the ceramic. Also, the biodegradable PLGA coating typically enhances the compressive strength of the scaffolds, which is critical for bone tissue engineering. In accordance with the MTT assay on osteoblast-like cells (MG-63) cultures, both the processes individually enhance the cell viability, while the highest improvement is obtained for the combined application of them. It is finally concluded that fluoride doping and PLGA coating are impressive approaches to improve the bioperformance of bredigite-based scaffolds.     

High performance lead-free Na0.5K0.5NbO3 piezoelectric ceramics obtained via oscillatory hot-pressing

Lead-free Na_0.5K_0.5NbO₃ (KNN) piezoelectric ceramics is regarded as a potential candidate for PZT material, while high performance is difficult to be obtained due to its poor sinterability and non-stoichiometric component. In this work, oscillatory pressure-assisted hot pressing (OPAHP) is utilized to fabricate KNN ceramics with high density. The KNN ceramics sintered at 860 °C exhibits superior performance with piezoelectric parameter (d₃₃) of 142 pC/N, electromechanical coupling factors (k_p) of 0.41, and relative permittivity (ε^T₃₃/ε₀) of 472–620. Additionally, hardness and flexural strength are measured as 3.55 GPa and 99.13 MPa, respectively. This work indicates that OPAHP technique is effective for fabricating KNN piezoelectric ceramics with high performance.     

Influence of carbonation on the low-temperature consolidation by Spark Plasma Sintering of carbonated calcium phosphate bioceramics

Calcium phosphates (CaP) such as biomimetic nanocrystalline apatite or amorphous calcium phosphate are hydrated bioactive compounds particularly suitable for bone repair applications due to their similarity with bone mineral. However, their consolidation in ceramic parts deserves special attention as they are thermodynamically metastable and can decompose into less bioactive phases upon heating. Adapted strategies are needed to obtain bulk bioceramics. Spark Plasma Sintering (SPS) has been shown to allow cold sintering of such compounds at temperatures like 150 °C while preserving the hydrated character and nanosized dimensions of the precursor powders. To this date, however, the role of the degree of carbonation of these precursors on the densification of CO₃-bearing CaP compounds via SPS has not been explored despite the natural carbonation of bone. In this work, several carbonated CaP hydrated compounds were prepared and consolidated by SPS and the characteristics of the obtained ceramics was scrutinized with respect to the starting powders. Two carbonation routes were carried out: via volume carbonation during powder synthesis or via subsequent surface ion exchange. All samples tested led to apatitic compounds after SPS, including amorphous CaP. We show that the degree of carbonation negatively affects the densification rate and propose possible hypotheses explaining this behavior. Evolution in the nature of the carbonate sites (apatitic A-, B-types and labile surface carbonates) before and after SPS is also noticed and commented. The consolidation of such compounds is however proven possible, and gives rise to bone-like apatitic compounds with great potential as bioactive resorbable ceramics for bone regeneration.     

Fabrication of porous titania sheet via tape casting: Microstructure and water permeability study

In this article, we report the effects of slurry formulation and sintering conditions on the microstructure and permeability of porous titania sheets prepared by tape casting. It was found that solid concentration and binder content in the titania slurry play a vital role in the porosity and microstructure of the sintered titania sheets. Solid concentration and binder content were optimized based on the green tape quality and open porosity of the sintered titania sheets. The optimum solid concentration with the lowest surface roughness was obtained at 0.61 g/cm³. The effects of temperature and sintering time on the open porosity and crystal structure of the final product were also investigated. Increasing the sintering temperature from 1000 to 1100 °C resulted in increasing the pore size from 170 to 264 nm and decreasing the open porosity. Finally, water permeability of the porous titania sheets was studied to evaluate the permeation flux and maximum operating pressure. The results revealed that the permeability of the porous titania sheet is increased not only by increasing the open porosity but also by increasing the pore size.     

Application of the cold sintering process to the electrolyte material BaCe0.8Zr0.1Y0.1O3-δ

This paper describes and discusses the application of the original sintering process named cold sintering to the electrolyte material BaCe_0.8Zr_0.1Y_0.1O_3-δ to enhance its densification at a temperature below that needed in a conventional sintering. This new technique enables the acceleration of the densification resulting in a more compacted microstructure with an unexpected high relative density of 83 % at only 180 °C. A subsequent annealing at 1200 °C further enhances the densification which reaches 94 %. The electrochemical performance of CSP sintered ceramics was investigated and optimized by varying different process parameters. The comparison with the conventional sintered material reveals an increase of the total conductivity by mostly increasing the grain boundary one. This result emphasizes the benefits of CSP to not only reduce the sintering temperature but also to enhance the electrochemical properties.     

Highly textured and strongly anisotropic TiB2 ceramics prepared using magnetic field alignment (9T)

Highly textured TiB₂ ceramics were prepared by slip casting an aqueous suspension in a magnetic field of 9 T, followed by sintering using Field Assisted Sintering Technology (FAST). Particle size refinement by ball milling improved both the degree of texturing and densification of the material (RD > 98 %). The sintered material exhibited a Lotgering orientation factor of 0.90, with the c-axis of TiB₂ oriented parallel to the magnetic field and FAST pressing direction. The texturing effect induced by the uniaxial pressing was negligible. The textured TiB₂ material exhibited a significant anisotropy in mechanical properties; the values of hardness and indentation elastic modulus measured along directions transverse to the c-axis of TiB₂ were 37 % and 13 % higher than the ones measured along the c-axis direction. Moreover, the specific wear rate of a surface of textured TiB₂ parallel to the field was one order of magnitude lower than a surface perpendicular to the field.     

Synthesis,microstructure and electromechanical properties of undoped (K0·5Na0·5)NbO3

Conventional solid state mixed oxide route using manual and ball milling is investigated for the preparation of K_0·5Na_0·5NbO₃ (KNN) ceramics. Microstructure engineering was made using two milling methods and sintering techniques, and the crystal growth; then electromechanical properties were investigated as a function of sintering temperature, densification and grain size. The sintering conditions were set at 920°C/5 min for spark plasma sintering and 1090–1120°C/10 and 48 h for classical sintering. KNN crystal was grown using floating zone technique under nitrogen gas, where the translation and rotation speeds were fixed at 3 mm h^?1 and 20 rev min^?1 respectively. Piezoelectric and dielectric performances were measured and related to the microstructure. High k_t (33 to 48%), k_p of 18 to 48% and d₃₃ of 127–140 pC N^?1 were reached for relative densities of 84 to 96%. KNN ceramics are now available for the design of ultrasonic sensors.