Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Dual effect synergistically triggered Ce:(Y,Tb)3(Al,Mn)5O12 transparent ceramics enabling a high color-rendering index and excellent thermal stability for white LEDs

Ce:Y₃Al₅O₁₂ transparent ceramics (TCs) with appropriate emission light proportion and high thermal stability are significant to construct white light emitting diode devices with excellent chromaticity parameters. In this work, strategies of controlling crystal-field splitting around Ce³⁺ ion and doping orange-red emitting ion, were adopted to fabricate Ce:(Y,Tb)₃(Al,Mn)₅O₁₂ TCs via vacuum sintering technique. Notably, 85.4 % of the room-temperature luminescence intensity of the TC was retained at 150 °C, and the color rendering index was as high as 79.8. Furthermore, a 12 nm red shift and a 16.2 % increase of full width at half maximum were achieved owing to the synergistic effects of Tb³⁺ and Mn²⁺ ions. By combining TCs with a 460 nm blue chip, a warm white light with a low correlated color temperature of 4155 K was acquired. Meanwhile, the action mechanism of Tb³⁺ ion and the energy transfer between Ce³⁺ and Mn²⁺ ions were verified in prepared TCs.     

Mechanical properties and thermal shock in thin ZrO2–Y2O3–Al2O3 films obtained by the sol-gel method

Thin multilayer coatings of ZrO₂–Y₂O₃–Al₂O₃ were prepared using the sol-gel method and dip-coating technique in order to advance in the study of what influence the incorporation of Al₂O₃ has on films of Y₂O₃-doped ZrO₂, investigating its role in the synthesis of the solutions and in the characteristics and properties of the coatings. After the characterization of the solutions used in the process, the microstructure of the films was studied and their mechanical behaviour and resistance to thermal shock were determined so as to optimize the characteristics and functionality of these coatings. With increased alumina content, 3YSZ-Al₂O₃ (20 mol%), the cubic phase of the zirconia disappeared completely at the sintering temperature used (700 °C), resulting in the tetragonal phase with Al in solution. There was also a decrease in the coatings' hardness and Young's modulus, and an increase in toughness and resistance to thermal shock. These results allow guidelines to be established for the design of multilayer structures that are, tougher, more resistant, and have improved surface properties.     

Performance of Al2O3 particle reinforced glass-based seals in planar solid oxide fuel cells

Glass-based materials are usually considered as excellent seals for jointing adjacent components in planar solid oxide fuel cells, but the uncontrollable crystallization in the glass may cause delamination and micro-cracks in such seals. To solve this problem, Al2O3 ceramic particles were added to a BaO–CaO–Al2O3–B2O3–SiO2 glass system to reduce negative effects caused by crystalline phase on the gas tightness and the joint strength in the seals. At an operating temperature of 750 °C, the glass-based seals with 20 wt% Al2O3 addition (GA80) exhibited extremely low leakage rates (~0.002 sccm/cm under an input gas pressure of 13.6 kPa) and higher shear strength (3.31 MPa). The Al2O3 ceramic addition and the crystalline phase BaAl2Si2O8 reinforced the glass matrix. Further thermal cycle analyses indicated that leakage rates for the GA80 seals remained at around 0.0025 sccm/cm after 10 thermal cycles, which was consistent with minor microstructural change and good interface bonding. Single cell testing with of GA80 seals was performed and the results demonstrated stable electrochemical performance through 6 thermal cycles at an open circuit voltage of 1.16–1.18 V, as well as a power density above 546 mW/cm2 at a current density of 925 mA/cm2. These results showed the high thermal cycle stability of the glass/Al2O3 composite seals in intermediate temperature planar solid oxide fuel cells.     

LCD-SLA 3D printing of BaTiO3 piezoelectric ceramics

Stereolithography-based 3D printing is a promising method to produce complex shapes from piezoceramic materials. In this study, LCD-SLA 3D printing was used to create lead-free piezoceramics based on barium titanate (BaTiO₃, BT). Three types of BT powders (micron, submicron and nanoscale) were tested in LCD-SLA 3D printing, and a technique for the preparation of a ceramic slurry suitable for LCD-SLA printing has been developed. Using TGA-DSC analysis, the thermal debinding parameters to obtain crack-free samples were determined, followed by further sintering and the study of the piezoelectric properties (ε_r = 1965, d₃₃ = 200 pC/N, tan = 1,7 %). The results of the study demonstrate high potential for the production of complex piezoceramic elements that can be used in aviation, in particular, aviation radio equipment; in the marine industry for transceiver modules of hydroacoustic antennas; and in the nuclear industry for pressure control sensors in the steam–water path.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

Enhanced thermal shrinkage behavior of phenolic-derived carbon aerogel-reinforced by HNTs with superior compressive strength performance

A novel carbon/m-HNTs composite aerogel was synthesized by introducing the modified halloysite nanotubes (m-HNTs) into phenolic (PR) aerogels through chemical grafting, followed with carbonization treatment. In order to explore the best proportion of HNTs to phenolic, the micromorphology of PR/m-HNTs were investigated by SEM before carbonization, confirming 10 wt% of m-HNTs is most beneficial to the porous network of aerogels. The interaction between PR and HNTs was studied by FTIR spectra, and microstructure evolution of the target product-carbon/m-HNTs composite aerogel were illustrated by SEM and TEM techniques. SEM patterns indicated that the carbon/m-HNTs aerogels maintain a stable porous structure at 1000 °C (carbonization temperature), while a ~20 nm carbon layer was formed around m-HNTs generating an integral unit through TEM analysis. Specific surface area and pore size distribution of composite aerogels were analyzed based on mercury intrusion porosimetry and N₂ adsorption–desorption method, the obtained results stayed around 500 m²g^?1 and 1.00 cm³g^?1 (pore volume) without significant discrepancy, compared with pure aerogel, showing the uniformity of pore size. The weight loss rate (26.76%) decreased greatly compared with pure aerogel, at the same time, the best volumetric shrinkage rate was only 30.83%, contributed by the existence of HNTs supporting the neighbor structure to avoid over-shrinking. The highest compressive strength reached to 4.43 MPa, while the data of pure aerogel was only 1.52 MPa, demonstrating the excellent mechanical property of carbon/m-HNTs aerogels.     

Effect of suspension characteristics on the performance of thermal barrier coatings deposited by suspension plasma spray

This paper investigates the influence of suspension characteristics on microstructure and performance of suspensions plasma sprayed (SPS) thermal barrier coatings (TBCs). Five suspensions were produced using various suspension characteristics, namely, type of solvent and solid load content, and the resultant suspensions were utilized to deposit five different TBCs under identical processing conditions. The produced TBCs were evaluated for their performance i.e. thermal conductivity, thermal cyclic fatigue (TCF) and thermal shock (TS) lifetime. This experimental study revealed that the differences in the microstructure of SPS TBCs produced using varied suspensions resulted in a wide-ranging overall TBC performance. All TBCs exhibited thermal conductivity lower than 1 W/(m. K) except water-ethanol mixed suspension produced TBC. The TS lifetime was also affected to a large extent where 10 wt % solid loaded ethanol and 25 wt % solid loaded water suspensions produced TBCs exhibited the highest and the lowest lifetime, respectively. On the contrary, TCF lifetime was not as significantly affected as thermal conductivity and TS lifetime, and all ethanol suspensions showed marginally better TCF lifetime than water and ethanol-water mixed suspensions deposited TBCs.     

Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.