Electric field-assisted synthesis/sintering cerium oxide: 5 wt.% gadolinium oxide

Gadolinium oxide ceramic powders were mixed to cerium oxide ceramic powders, pressed to pellets, and sintered either at 1450 °C or applying 200 V cm⁻¹ electric field at 800 °C, 900 °C and 1000 °C. The structural phases and the microstructure of the sintered pellets were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The formation of substitutional solid solution was followed by monitoring the increase of the electrical conductivity by impedance spectroscopy and X-ray diffraction. The main results show that Joule heating due to the flow through the pellets of the electric current, which was produced by the application of the electric field, allows for promoting partial solid solution as well as partial sintering the ceria-gadolinia pellets. Moreover, grain growth that occurred in the high temperature sintered pellets was inhibited in the electric field-assisted synthesized/sintered pellets, being an alternative technique for producing cerium oxide-gadolinium oxide solid solutions.     

Two-step sinter-crystallization of K2O–CaO–P2O5–SiO2 (45S5-K) bioactive glass

Bioactive glasses and glass-ceramics (GCs) effectively regenerate bone tissue, however most GCs show improved mechanical properties. In this work, we developed and tested a rarely studied bioactive glass composition (24.4K₂O-26.9CaO-46.1SiO₂-2.6P₂O₅ mol%, identified as 45S5-K) with different particle sizes and heating rates to obtain a sintered GC that combines good fracture strength, low elastic modulus, and bioactivity. We analyzed the influence of the sintering processing conditions in the elastic modulus, Vickers microhardness, density, and crystal phase formation in the GC. The best GC shows improved properties compared with its parent glass. This glass achieves a good densification degree with a two-step viscous flow sintering approach and the resulting GC shows as high bioactivity as that of the standard 45S5 Bioglass®. Furthermore, the GC elastic modulus (56 GPa) is relatively low, minimizing stress shielding. Therefore, we unveiled the glass sintering behavior with concurrent crystallization of this complex bioactive glass composition and developed a potential GC for bone regeneration.     

Transient liquid phase sintering of high-purity mullite for high-temperature structural ceramics

High-purity mullite ceramics, promising engineering ceramics for high-temperature applications, were fabricated using transient liquid phase sintering to improve their high-temperature mechanical properties. Small amounts of ultrafine alumina or silica powders were uniformly mixed with the mullite precursor depending on the silica-alumina ratio of the resulting ceramics to allow for the formation of a transient liquid phase during sintering, thus, enhancing densification at the early stage of sintering and mullite formation by the reaction between additional alumina and the residual glassy phase (mullitization) at the final stage of sintering. The addition of alumina powder to the silica-rich mullite precursor resulted in a reaction between the glassy silica and alumina phases during sintering, thereby forming a mullite phase without inhibiting densification. The addition of fine silica powder to the mullite single-phase precursor led to densification with an abnormal grain growth of mullite, whereas some of the added silica remained as a glassy phase after sintering. The resulting mullite ceramics prepared using different powder compositions showed different sintering behaviors, depending on the amount of alumina added. Upon selecting an optimum process and the amount of alumina to be added, the pure mullite ceramics obtained via transient liquid phase sintering exhibited high density (approximately 99%) and excellent high-temperature flexural strength (approximately 320 MPa) at 1500 °C in air. These results clearly demonstrate that pure mullite ceramics fabricated via transient liquid phase sintering with compositions close to those of stoichiometric mullite could be a promising process for the fabrication of high-temperature structural ceramics used in an ambient atmosphere. The transient liquid phase sintering process proposed in this study could be a powerful processing tool that allows for the preparation of superior high-temperature structural ceramics used in the ambient processing atmosphere.     

CO2 laser post-treatment of sol-gel-derived Y3Al5O12:Ce phosphor ceramic powders

Cerium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Ce, YAG:Ce) was prepared using a sol-gel method and then fired for CO₂ laser post-treatments. Phase transformations and formation of impurities were not observed in YAG:Ce after CO₂ laser sintering. The shift of the diffraction peak and the appearance of another Raman peak indicate a more homogeneous distribution of Ce activators and enhanced crystallinity in laser-sintered YAG hosts. Larger spheres (100–200 μm) with tiny crystallites (<10 μm) were observed on the smoother surface in the laser-sintered YAG:Ce, unlike the irregular, porous, and layered powders in the sol-gel-derived YAG:Ce (1–100 μm). Photoluminescence (PL) measurements revealed an emission increase of 180% and a red shift of the emission peak for the laser-sintered YAG:Ce powders compared with the sol-gel-derived powders. Both have comparable thermal PL quenching behavior; however, the YAG:Ce powders with CO₂ laser treatment exhibited a PL efficiency improvement of approximately 4%.     

Ion migration behavior and its interaction with sintering environment in cation conductor during flash sintering

Flash sintering has been reported in various ceramics. Nevertheless, anion and cation conductors exhibit different flash-sintering behaviors, and the interaction mechanism between the conductive species and the sintering environment has remained unclear. Herein, we report the flash-sintering phenomena of a typical cation conductor, Na₃Zr₂(SiO₄)₂(PO₄) with anode region surrounded by air and NaNO₃ environments. The results prove that the ionic behavior and joule heating distribution can be controlled by changing the electrode environment. Four possible scenarios describing the ion migration behavior and interaction with the environment are proposed for providing a guidance for controlling the ion interaction behavior during flash sintering.     

Highly transparent Yb:Y2O3 ceramics obtained by solid-state reaction and combined sintering procedures

(Y_0.87-xLa_0.1Zr_0.03Yb_x)₂O₃ (x?=?0.02, 0.04, 0.05) transparent ceramics were obtained by solid-state reaction and combined sintering procedures with La₂O₃ and ZrO₂ as sintering additives. A method based on two-step intermediate sintering in air followed by vacuum sintering was applied in order to control the densification and grain growth of the samples during the final sintering process. The results indicate that La₂O₃ and ZrO₂ co-additives can improve the microstructure and optical properties of Yb:Y₂O₃ ceramics at relatively low sintering temperature. On the other hand, the addition of Zr⁴⁺ ions leads to the formation of dispersed scattering volumes in the ceramic bodies. Transmittance of 78.8% was measured for the 2.0?at% Yb:Y₂O₃ ceramic sample at the wavelength of 1100?nm. The spectroscopic properties of Yb:Y₂O₃ ceramics were investigated at room temperature. The obtained results show that the absorption cross-section at 978?nm is in the range of 2.08?×?10^–20 to 2.36?×?10^–20 cm², whereas the emission cross-section at 1032?nm is ~1.0?×?10^–20 cm².     

Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi₂O₃ sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi₂O₃ sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi₂O₃ was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi₂O₃/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).     

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.     

The cold sintering process: A review on processing features,densification mechanisms and perspectives

Since its first introduction in 2016, cold sintering process (CSP) has gained worldwide interest from the scientific community as green and innovative fabrication route due to the dramatic reduction of processing time, energy, and costs. Cold sintering resembles the geological formation of rocks where a ceramic powder is densified with the aid of a liquid phase under an intense external pressure and limited heating conditions (below 350 °C). Up to date, tens of different materials, including composites, have been successfully processed through CSP and extraordinary results in terms of densification, microstructure and final properties have been achieved. In the present review, processing features and variables, possible densification mechanisms and issues also for the realization of ceramic-based composites are explored. Advantages with respect to existing techniques are analysed and current challenges are described to lay the ground for new processing opportunities to be faced in the near future.     

Layered ceramics based on InGaO3(ZnO)2: Preparation and experimental investigation of high-temperature heat capacity and thermal conductivity

This paper reports on a method for producing ceramics from a high-purity, submicron InGaO₃(ZnO)₂ powder synthesised using a PVA-assisted gel combustion method, as well as an experimental study of the thermophysical properties of the ceramic materials obtained. The platelet-like crystallites of the InGaO₃(ZnO)₂ obtained were several microns long and up to several hundred nanometres thick. Layered ceramics obtained by sintering compacted InGaO₃(ZnO)₂ powders at temperatures of 1373–1773 K had a bulk density that was 68–96 % of the theoretical density. The temperature dependence of heat capacity in the range 306–1346 K was studied experimentally for InGaO₃(ZnO)₂ using the DSC method. It was found that, in the range 323–1173 K, layered InGaO₃(ZnO)₂ ceramics had a low thermal conductivity, which decreased from 2.0–1.3 W/(m K. The results obtained make it possible to consider this material as a promising thermal barrier coating.