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We report for the first time transparent hydroxyapatite (HAp) ceramics densified using a cold sintering process and biomineralization. Under a uniaxial pressure of 800 MPa at 180 °C, granular HAp nanoparticles mixed with simulated body fluid were densified up to 98.5% relative density. During the densification, pseudo-biomineralization occurred among the HAp nanoparticles, leading to the precipitation of a new HAp phase. Additionally, the transformation of granular HAp nanoparticles into nearly circular grains produced fine microstructures with uniform and nanosized grains, reducing light scattering. As a result, the transmittance of 81.6% at λ = 550 nm was achieved. Transparent HAp exhibited Vickers hardness (3.88 ± 0.22 GPa), fracture toughness (0.62 ± 0.03 MPa·m1/2), biaxial flexural strength (41.69 ± 1.23 MPa), and Young's modulus (78.1 ± 2.04 GPa). These findings will pave the way for the fabrication and potential applications of transparent ceramics at low temperatures. [ABSTRACT FROM AUTHOR]

Using melt-derived LD glass powders and 5–20 M NaOH solutions, porous lithium disilicate (Li2Si2O5, LD) glass–ceramics were prepared by the cold sintering process (CSP) associated with the post-annealing technique. In this novel technique, H2O vapor originating from condensation reactions between residual Si–OH groups in cold-sintered LD glasses played the role of a foaming agent. With the increasing concentration of NaOH solutions, many more residual Si–OH groups appeared, and then rising trends in number as well as size were found for spherical pores formed in the resultant porous LD glass–ceramics. Correspondingly, the total porosities and average pore sizes varied from 25.6 ± 1.3% to 48.6 ± 1.9% and from 1.89 ± 0.68 μm to 13.40 ± 10.27 μm, respectively. Meanwhile, both the volume fractions and average aspect ratios of precipitated LD crystals within their pore walls presented progressively increasing tendencies, ranging from 55.75% to 76.85% and from 4.18 to 6.53, respectively. Young's modulus and the hardness of pore walls for resultant porous LD glass–ceramics presented remarkable enhancement from 56.9 ± 2.5 GPa to 79.1 ± 2.1 GPa and from 4.6 ± 0.9 GPa to 8.1 ± 0.8 GPa, whereas their biaxial flexural strengths dropped from 152.0 ± 6.8 MPa to 77.4 ± 5.4 MPa. Using H2O vapor as a foaming agent, this work reveals that CSP associated with the post-annealing technique is a feasible and eco-friendly methodology by which to prepare porous glass–ceramics. [ABSTRACT FROM AUTHOR]

This work aims to enhance the fracture toughness of brittle Al2O3 ceramics and apply insulated Al2O3 ceramics with electrical conductivity by dispersing second tungsten (W) metal particles. In order to investigate the effects of W dispersion on mechanical and electrical properties, Al2O3–W composites with various amounts of W (ranging from 5 vol% to 20 vol%) were fabricated by the hot‐press sintering method at various sintering temperatures. Microstructure analysis revealed submicron Al2O3 matrix grains and W particles. The existence of three phases of Al2O3, W, and AlWO4 was confirmed by X‐ray diffraction patterns. All Al2O3–W composites showed higher fracture toughness than monolithic Al2O3. The toughening mechanism was attributed to crack deflection and crack bridging. Transgranular fracture was visible in all composites. Electrical resistivity dramatically lowered from 2.9 × 1012 Ω cm of monolithic Al2O3 to 4.1 × 102 Ω cm of the composite with 20 vol% W addition. The percolation threshold is calculated as 18.5%. With the increase in sintering temperature, the amount of W particles was decreased and Al2O3 grains became large, leading to the reduced number of conductive pathways formed by the dispersed W particles. As a result, electrical conductivity was decreased. [ABSTRACT FROM AUTHOR]

Abstract: Al2O3/Ti composites containing 0‐30 vol% dispersed fine Ti particles were fabricated using a hot‐press sintering method at 1500°C from mixtures of Al2O3 and TiH2 powders. During sintering, TiH2 decomposed to form metallic Ti. The effects of the Ti content on the mechanical and electrical properties of the composites were then investigated. No Ti‐Al intermetallic compounds were detected by X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy indicated the presence of Al‐Ti‐O solid solution and Ti‐O phases. The composites showed enhanced densification; the measured densities were higher than the calculated theoretical values. Microstructural observation revealed homogeneously distributed fine Ti particles dispersed in the Al2O3 matrix. The Ti particle size ranged from submicrometer to a few micrometers depending on the Ti content. The fracture mode of the composites was primarily transgranular, in contrast to the intergranular fracture mode of monolithic Al2O3. Although the flexural strength was decreased with increase in Ti content, the composite containing 20 vol% Ti displayed the maximum fracture toughness of 4.3 MPa·cm1/2, which was 37% greater than that of monolithic Al2O3. The composites containing more than 15 vol% Ti exhibited drastic decreases in resistivity (~10−1 Ωcm), which were attributed to the formation of interconnected Ti networks at these Ti contents. The percolation threshold volume for electrical conduction in the present system was calculated to be 13.8 vol%. The results indicate that dispersing fine Ti particles into Al2O3 increased the fracture toughness and improved the conductivity of Al2O3. [ABSTRACT FROM AUTHOR]

ZrO 2 -Ti composites containing 0–30 vol% Ti were fabricated by hot-press sintering method at 1350 °C from mixtures of ZrO 2 (Here, 3 mol% yttrium-doped tetragonal-stabilized zirconia, 3Y-TZP) and TiH 2 powders. The effects of Ti content on microstructures, mechanical properties, and electrical conductivity were investigated. Surface cracks induced in ZrO 2 -Ti composites were healed using electrochemical anodization technique at room temperature to recover the degraded flexural strength. ZrO 2 -15 vol% Ti exhibited the highest fracture toughness with the value of 6.76 MPa·m1/2, which increased by 21% compared to monolithic ZrO 2. Composites containing more than 15 vol% Ti exhibited excellent electrical conductivity (∼100 Ωcm). The percolation threshold volume for electrical conduction in the present system was calculated to be 13.9 vol%. Based on the excellent electrical conductivity, electrochemical anodization process was used to heal cracks introduced on the surface of these composites. After anodization with a current density of 2 A/dm2 for 3 h at room temperature, composites containing a crack of 112 μm in length and 0.3 μm in width was completely healed with a fully recovered flexural strength. Image 1 • ZrO 2 -Ti composite was prepared by hot-press sintering method. • Fracture toughness of ZrO 2 was improved by dispersing Ti particles. • Excellent electrical conductivity was developed. • Surface crack was healed by electrochemical anodization at room temperature. • Composite with high fracture toughness showed high crack-healing efficiency. [ABSTRACT FROM AUTHOR]