Advances in Applied Ceramics: Structural,Functional and Bioceramics

Yttria-stabilised zirconia (Y-TZP) ceramics are widely used for dental and prosthesis applications; however, they are susceptible to low-temperature degradation (LTD). Despite several explanations of the LTD mechanism, it is not fully understood yet. Commercial TZ-3Y-E grade powder was used to further study the LTD before sintering it. Hydrothermal ageing treatment was applied to samples at 134°C for 5 h. STA analysis confirmed that the powder is binderfree. SEM and XRD analyses show homogeneous particle size and tetragonal as a major phase and monoclinic as a minor phase, respectively. BET method analysis shows a slight change in the pore size, pore volume and surface area of the powder samples, before and after heating at 400°C. Particle size distribution (SD) calculated from SEM images shows ∼ 40–50 nm particle size range of the powders. The results show that LTD was not observed in the powder after hydrothermal ageing treatment.

ZrSi₂ was fabricated via reactive hot-pressing technique for 45 min at 1250°C and 1350°C, respectively. The hot-pressed sample exhibited Vickers hardness of 10.93 ± 0.32 GPa and indentation fracture toughness of 3.16 ± 0.54 MPa.m^1/2. Thermal conductivity (RT-2.99 W (m.K)⁻¹) and diffusivity (RT–18.2 mm² s⁻¹) of the sample decreased with increasing temperature due to phonon scattering. Oxidation studies were carried out for the hot-pressed samples from 1000°C to 1200°C for 5, 48 and 100 h in air. The oxidation test results showed that ZrSi₂ followed selective oxidation with Si diffusing into the oxide layers at high temperatures. At 1200°C, the surface of ZrSi₂ oxidised completely to form a high-temperature stable phase, zircon (ZrSiO₄). The autoclave test at 250°C and 250 bar shown negligible mass gain (∼0.022 g) which suggests the usage of ZrSi₂ for nuclear fuel cladding applications.

Zirconium diboride (ZrB₂) is a typical ultra-high temperature ceramic material (UHTC). In this work, the preparation of ZrB₂ by reducing zirconia (ZrO₂) with calcium hexaboride (CaB₆) was studied in detail. With the assistance of molten salt, the reduction rate was dramatically enhanced. Additionally, the results showed that ZrB₂ products with different particle shapes were prepared by changing the type of molten salt (NaCl, KCl, MgCl₂, and CaCl₂). However, the particle size of products could not be properly refined when nano-ZrO₂ was used as the raw material. During the process of reducing ZrO₂ by CaB₆, B₂O₃ could be formed, which would cause the loss of B source. To decrease the consumption of CaB₆, Ca-assisted reduction was performed, and monophase ZrB₂ was obtained. The varied morphologies of ZrB₂ particles obtained by Ca-assisted reduction were the result of complex reaction mechanisms.

ZrO₂-reinforced porous mullite ceramics were fabricated by a foam-gel casting approach using zircon (ZrSiO₄) powders and alumina (Al₂O₃) powders as raw materials. The influence of firing temperature on the phase composition of porous samples was analysed. Based on the analysis results, the effects of a blowing agent on the physical properties, microstructure, mechanical strength and heat conductivity of ZrO₂-reinforced porous mullite samples fired at 1600°C were investigated. In the present study, linear shrinkage, porosity and mean pore size of ZrO₂-reinforced porous mullite ceramics increased, and volume density, mechanical strength and heat conductivity decreased. When the dosage of the blowing agent was 1.0 vol.-%, the flexural strength and compressive strength of ZrO₂-reinforced porous mullite sample with 77.4% porosity respectively reached 3.0 and 10.9 MPa, and their heat conductivity was kept at 0.181 W m⁻¹ K⁻¹.

Carbon fibre has excellent mechanical properties at high temperatures but weak oxidation resistance. The paper prepared the anti-oxidative silica coating on the surface of needled carbon fibre felt by the sol–gel method and discussed the effects of sol content and sintering temperature on the coatings’ performance. The phase composition, density, thermal conductivity and thermal stability of derived composites were characterised, and the structure and morphology of SiO₂ coatings were also analysed by scanning electron microscopy. The results indicated that a uniform and complete coating could be formed on the fibre surface when the sol content exceeds 20%. Ceramic particles sintered below 1200°C were amorphous silica, while those sintered at 1350°C were cristobalite. Compared with other processes, 20% silica sol content or sintering at 1350°C could provide better antioxidant protection. The new coating process is significant for retarding fibre oxidation in a high-temperature environment.

Anti-oxidation coatings of MoSi₂-SiC-Si-ZrB₂ were prepared on the graphite surface by laser cladding. The XRD, scratching bonding force, SEM and isothermal oxidation tests were used to investigate the microstructure and properties of the coatings. The coating with 40 wt% ZrB₂ added (MoSi₂:SiC:Si:ZrB₂= 16:4:5:16.6) has the lowest degree of oxidation, the best oxidation resistance and the highest bonding strength between coating and matrix. The scratching bonding force still maintained at about 9.9 N after laser ablation, and there were no obvious destructive defects but some pores and cracks on the coating. After a Si-enriched amorphous layer was formed, the relative oxygen permeability was significantly decreased and the oxidation of the coating was passivated, which prevented the matrix from further oxidation.

Pressure Slip Casting (PSC) using polymer moulds offers several advantages over Conventional Slip Casting (CSC) of ceramics such as enhanced productivity in combination with higher green density, homogeneity and low rejections. PSC is currently practiced in table-ware industries however, application to the technical ceramics is limited owing to the collapse of cast part while de-moulding during pressure cast cycle under pneumatic pressure. Current study focuses on this key issue and demonstrated pressure casting process successfully for the fabrication of alumina parts. Slips of a mixture of alumina with different particle sizes in the various proportions and solid loadings were prepared. Slip under PSC resulted in effective interlocking of the particles retaining the shape while de-moulding and achieved a sintered density of 98.6% of theoretical density (TD). Slurry on CSC exhibited a lower sintered density of 97% of TD. Selection of particles with sizes in optimised proportion for PSC results in effective interlocking of particles in green parts as well as grains on sintering as revealed by the microstructure. This leads to higher density and mechanical properties. Slip thus optimised were shaped into solid spheres of ϕ 60 mm by PSC targeting grinding applications.

Carbon fibre-reinforced carbon aerogel composites (C/CAs) were prepared by using polyacrylonitrile (PAN) and phenolic (PH) carbon precursor fibres as reinforcements, respectively, then impregnated with resorcinol-formaldehyde sol, aging, solvent exchanging, CO₂ supercritical drying and carbonization at 1000°C. The physical properties, structural morphology and mechanical and thermal insulation behaviour of the two kinds of composites were further compared. Pore structure analysis shows that the PH-based C/CAs have a larger specific surface area (438.92 m²·g⁻¹) and richer mesoporous volume than the PAN-based C/CAs. Mechanical behaviour of the C/CAs has been changed, but the stronger enhancement effects belong to PAN-based C/CAs (compressive strength in xy and z-directions are 1.63±0.10 MPa and 1.22±0.19 MPa, bending strength is 3.62±0.50 MPa). However, the PH-based C/CA composites have relatively low thermal conductivities, ranging from 0.027 to 0.164 W·m⁻¹·K⁻¹ between 25 and 1800°C.

The binder jetting (BJ) process shows significant superiority in manufacturing ceramic parts with a complex structure. The performance of the green part is highly reliant on the features of raw materials. A kind of grading zirconia powder M0 was designed, its intrinsic characteristics including microstructure, size and size distribution, bulk density, porosity, hall velocity and spread performance were systematically evaluated. The design principles of M0 were illustrated in detail. Meanwhile, a kind of organic epoxy binder was prepared and its properties including density, viscosity and surface tension were estimated. The jet ability of binder was evaluated quantitatively. The compatibility and bonding strength between binder and powder were assessed through contact angle, infiltration time and microstructure of M0. As a result, M0 and epoxy binder can be successfully applied in the printer, the zirconia structure with good dimensional accuracy (shrinkage rate ≤ ±2%) and flexible strength (1.74 ± 0.28 MPa) were achieved.

Garnet-type solid-state electrolytes are promising candidates for solid-state lithium batteries, nevertheless their ionic conductivity is still not enough for commercial applications. On the other hand, doping still is the common way to improve the ionic conductivities of these solid electrolytes. In this study, mono and dual-doped garnet-type solid electrolytes were synthesised by substituting indium (In), gallium (Ga), indium-titanium (In-Ti) and gallium-titanium (Ga-Ti) to the Li₇La₃Zr₂O₁₂ structure by a solid-state reaction method. The contribution of substitutions to the formation of crystal phases was investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). On the other hand, morphological analyses were done by scanning electron microscope (SEM) and the ionic conductivities of the solid electrolytes were determined by electrochemical impedance spectroscopy (EIS). The study showed that while Li_7-3xIn_xLa₃Zr₂O₁₂ (for x = 0.05, 0.10, 0.15, 0.20) and Li_7-3xGa_xLa₃Zr₂O₁₂ (for x = 0.05) samples were formed in tetragonal phase with a space group of I41/acd:2, dual substituted Li_7-3xIn_xLa₃Zr_1.8Ti_0.2O₁₂ and Li_7-3xGa_xLa₃Zr_1.8Ti_0.2O₁₂ solid electrolytes for all x values were formed in cubic phase with a space group of I-43d. The highest conductivity is reached for Li_6.85Ga_0.05La₃Zr_1.8Ti_0.2O₁₂. The radial distribution function studies showed that when more In and Ga atoms take place in the sites of Li atoms, more O atoms take place in the vicinity of both substituted In and Ga atoms within the Li₇La₃Zr_1.8Ti_0.2O₁₂ (LLZTO) crystal framework which can eventuate in a change in the conduction mechanism.

From a new perspective, polymorphism is now often considered as a functional property that extends the applications of many materials. BiFeO₃ is still an interesting material from both fundamental and applied points of view. A unique characteristic of BiFeO₃ film, the polymorphism was stabilised only using the epitaxial strain caused by the substrate of the film. Here we report the hydrothermal synthesis of biphasic BiFeO₃ ceramics using low NaOH concentration and temperature. The ability of BiFeO₃ ceramics to morph into tetragonal and rhombohedral allotropic phases was demonstrated, confirming the theoretical prediction that the polymorphism can be stabilised even in powder form. Based on our experimental study NaOH(aq) is proposed as responsible for the strain on the structure of the rhombohedral phase. In addition, the in-depth characterisation of this biphasic ceramic opens up new opportunities for the technological applications of BiFeO₃ material.