Ultrafast high-temperature sintering of lanthanum-chromite-based ceramics

Conventional sintering of lanthanum-chromite-based ceramics typically requires a long isothermal duration, which leads to severe loss of volatile components. In this study, we prepared dense LaCrO₃ (LCO), La_0.8Ca_0.2CrO₃ (LCCO), and La_0.8(Mg_0.05Ca_0.05Sr_0.05Ba_0.05)CrO₃ (4LCO) ceramics with stable single-phase structures through ultrafast high-temperature sintering (UHS), and the total sintering period was shorter than 8 min. An investigation of the effects of sintering parameters and alkaline earth (AE) metal dopants on the density showed that doping with AE metal promoted the densification of LaCrO₃ through the liquid-phase-assisted sintering mechanism. The hardness and conductivity of the ceramics were in the order of LCCO>4LCO>LCO because of the effect of lattice distortion and the relative densities of the pellets. This work presents a compositional-design-based method to obtain high-performance perovskite-type oxides, and it is expected to broaden the use of UHS for the densification of novel ceramics.     

Ultrafast high-temperature synthesis and densification of high-entropy carbides

Recently, high-entropy carbides have attracted great attention due to their remarkable component complexity and excellent properties. However, the high melting points and low self-diffusion coefficients of carbides lead to the difficulties in forming solid solution and sintering densification. In this work, six dense multicomponent carbides (containing 5–8 cations) were prepared by a novel ultrafast high-temperature sintering (UHS) technique within a full period of 6 min, and three of them formed a single-phase high-entropy solid solution. The solid solubility of the UHSed multicomponent carbides was highly sensitive to the compositional variation. The presence of Cr₃C₂ liquid had significant contributions to the formation of solid solution and the densification of multicomponent carbides. All UHSed multicomponent carbides exhibited high hardness, which, unexpectedly, did not simply increase with increasing number of the components. The highest nanohardness with a value of 36.6 ± 1.5 GPa was achieved in the (Ti_1/5Cr_1/5Nb_1/5Ta_1/5V_1/5)C_x high-entropy carbide. This work is expected to expedite the development of high-entropy carbides and broaden the application of UHS in the synthesis and densification of advanced ceramics.     

Ultrafast high-temperature sintering of gadolinia-doped ceria

Ultrafast high-temperature sintering (UHS) is a rapidly growing research area of material science and engineering. Herein we present UHS of gadolinia-doped ceria (GDC) powders in single and multi-step approaches. The sintered ceramics were characterized from a physical and electrochemical point of view. When the power is applied gradually during the multistep UHS process crack-free GDC ceramics can be obtained with 95 % bulk density using commercial powder. Oxalate converted GDC powder gave 86 % bulk density with the same multistep sintering process. Additionally, it is shown that multistep UHS is also suitable for multilayer co-sintering necessary for solid oxide fuel cells (SOFC), as demonstrated by the production of dense GDC electrolyte in tight contact with porous electrodes.     

Ultrafast high-temperature sintering of silicon nitride: A comparison with the state-of-the-art techniques

Ultrafast High-temperature Sintering (UHS) has been successfully applied to fabricate the silicon nitride (Si₃N₄) bulks, as the first attempt of ultra-rapid consolidation of a non-oxide ceramics. At a heating rate of 875 °C/min, the bulk Si₃N₄ ceramic with a relative density greater than 96 % and an α-β phase transformation degree above 80 % could be obtained within 300 s. The effects of ultrafast heating on the liquid phase sintering (LPS) were also comparatively studied. Results showed that, the ultrafast heating rate and high temperature under UHS might promote the LPS system evolving to a nonequilibrium state. By comparing with other pressureless sintering processes with much lower heating rates, UHS apart from reducing the processing time, and it is also an effective method to form a bimodal microstructure composed of interlocked rod-like β-Si₃N₄ grains.     

Effect of second phase addition of zirconia on the mechanical response of textured alumina ceramics

The effect of second phase addition of zirconia on the mechanical response of textured alumina was analysed. Highly textured monolithic tape-casted alumina was obtained through templated grain growth. Compositions containing 1, 2, 5 and 10 vol% of (i) non-stabilised and (ii) 3 mol% yttria-stabilised zirconia, respectively, were investigated. XRD analyses revealed that the texture degree decreased with increasing second phase content. Microstructural analysis showed zirconia grains inside the textured alumina grains for contents ≤ 5 vol%, affecting the mode of fracture. Fracture toughness of textured alumina significantly decreased with the addition of a second phase. In the case of non-stabilised zirconia, the constraint of the alumina matrix and the small grain size led to a lower fracture toughness in comparison to monolithic textured alumina (K_Ic = 5.1 MPa m^1/2). The fracture toughness of textured alumina with 3 mol% yttria-stabilised zirconia was comparable to equiaxed alumina, independent of the content ratio (K_Ic = 3.5 MPa m^1/2).     

Ultrafast high-temperature sintering of (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 high-entropy ceramics with defective fluorite structure

An entropy-stabilized rare earth hafnate (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ (5RH) with defective fluorite structure was successfully prepared by the emerging ultrafast high-temperature sintering (UHS) in less than six minutes. The 5RH ceramic possessed a higher thermal expansion coefficient (11.23 ×10^?6/K, 1500 °C) and extremely low thermal conductivity (0.94 W/(m·k), 1300 ℃) owing to the larger lattice distortion of high-entropy materials. After high-temperature annealing at 1500 ℃, the 5RH showed extremely sluggish grain growth characteristics and excellent high-temperature phase stability, mainly attributed to the non-equilibrium sintering characteristic of the UHS and the sluggish diffusion effect of high-entropy materials. Therefore, (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ has excellent potential as a next-generation thermal barrier coating material to replace traditional Y₂O₃ stabilized ZrO₂. Finally, using the UHS to prepare high-entropy ceramics provides a new technique for fast-sintering and developing next-generation thermal barrier coating materials.     

Magnetic slip casting for dense and textured ceramics: A review of current achievements and issues

Ceramic materials are ubiquitous in technologies operating under high mechanical, thermal or chemical constrains. Research in ceramic processing aims at creating ceramics with properties that are still challenging to obtain, such as toughness, transparency, conductivity, among others. Magnetic slip casting is a process where an external magnetic field is used to createcontrolled texture in ceramics. Over the past 20 years of research on magnetic slip casting, dense and textured ceramics of multiple chemistry were found to exhibit enhanced properties. This paper reviews the progress in the field of magnetic slip casting, details the processing parameters and the textures obtained for a diverse range of compositions. The structural and functional properties of the magnetically textured slip casted and sintered ceramics are presented. This overview of the magnetic slip casting process allows to identify critical directions for future advancement in advanced technical ceramics.     

Ultrafast synthesis and pressureless densification of multicomponent nitride and carbonitride ceramics

High-entropy nitride and carbonitride ceramics have received wide attention for their excellent properties such as high hardness, high melting point, and high wear resistance, but the susceptibility of nitrides to decomposition at high sintering temperatures has rendered their densification challenging. In this work, four multicomponent nitrides and five multicomponent carbonitrides (containing five to seven cations) were prepared through ultrafast high-temperature sintering. While only (Cr_1/7Zr_1/7Nb_1/7Hf_1/7Ta_1/7Ti_1/7V_1/7)N formed a single phase among the nitrides, all carbonitride systems formed a single-phase high-entropy solid solution with a relative density exceeding 92%. The polyanionic structure of carbonitrides is responsible for their high configurational entropy, which in turn results in their high solid solubility. Although carbonitrides showed higher hardness and modulus than nitrides with the same cations, their fracture toughness was lower. Among carbonitrides with different C/N ratios, the system with a C/N ratio of 5:5 showed the highest solid solubility and best overall mechanical properties.     

Effects of sintering temperature on structural and electromagnetic properties of MgCuZn ferrite prepared by microwave sintering

Nanocrystalline magnesium–copper–zinc (Mg_0.30Cu_0.20Zn_0.50Fe₂O₄) ferrites were prepared by microwave sintering technique. The effects of the sintering temperature on particle size and magnetic properties were investigated. In this article, optimum sintering temperature required for MgCuZn ferrite system for obtaining good electromagnetic properties, suitable for applications in low temperature co-fired ceramics (LTCC) chip components was studied. The grain size, initial permeability, dielectric constant and saturation magnetisations were found to increase, and dielectric loss was found to decrease with the increasing sintering temperature. Mg–Cu–Zn ferrites with a permeability of μ?=?1110 (at 1?MHz) were fully densified at the standard LTCC sintering temperature of 950°C.     

Evolution of the microstructure during the early stages of sintering barium hexaferrite nanoplatelets

Nanoparticles usually exhibit a specific structure and composition, which can influence the development of the microstructure during their sintering. Barium hexaferrite nanoplatelets have a specific, iron-rich structure defined by the termination at the surfaces with the S blocks of their SRS^*R^* hexaferrite structure (S and R represent a cubic (Fe₆O₈)²⁺ and a hexagonal (BaFe₆O₁₁)²⁻ structural block, respectively). The unsubstituted and Sc-substituted hexaferrite nanoplatelets were hydrothermally synthesized and fired at different temperatures. A combination of morpho-structural analyses (XRD, SEM, TEM, and aberration-corrected STEM) and magnetic measurements was used to reveal the evolution of the microstructure during sintering. During the initial stages of sintering the nanoplatelets thicken predominantly by the fusion of individual original nanoplatelets. Due to the Fe-rich surfaces of the nanoplatelets, the fusion growth results in an inhomogeneity that leads to the formation of planar defects in the grains and the precipitation of Fe₂O₃ as the secondary phase. In the Sc-substituted hexaferrite grains, superstructural compositional ordering was detected for the first time. The Sc substitution caused exaggerated grain growth in barium hexaferrite ceramics sintered at 1300 °C.     

Effect of components on the microstructures and properties of rare-earth zirconate ceramics prepared by ultrafast high-throughput sintering

Rare-earth zirconate ceramics are conventionally sintered at high temperatures for long durations to achieve a full density. Herein, we synthesized and densified five lanthanide-group rare-earth zirconates (Ln₂Zr₂O₇, Ln = La, Nd, Sm, Eu, Gd) containing different numbers of Ln cations in a high-throughput mode using a novel ultrafast high-temperature sintering technique within a cycle of 5 min, and investigated the effects of components on the structure and properties of the resultant ceramics. Under the high-throughput mode, the same sintering conditions make the analysis and comparison of the results more reasonable. The average grain size decreased while hardness and Young’s modulus increased with an increase in the number of the Ln components in the zirconate ceramics. The mechanisms were ascribed to the sluggish diffusion and lattice distortion effect caused by the increase in entropy.     

Rheology and processing of UV-curable textured alumina inks for additive manufacturing

A UV-curable resin containing alumina (Al₂O₃) powder and platelets was developed and characterized using a proof-of concept tape-casting system and trialed on a commercially available 3D-printing platform (Admatec Admaflex 130). The influence of solids loading and solids platelet fraction on resin viscosity and depth of cure was investigated. Resins containing up to 40 vol% solids loading at platelet fractions up to 50% were found to have sufficiently low viscosity for tape casting and cure depths ranging from 0.29 to 0.39 mm. Alumina platelets were observed to lower ink viscosity but also reduce shear thinning behavior compared to inks containing only powder. Printability of inks was assessed by layered tape casting, and verified with a trial build on an Admaflex 130. Due to the low solids loading of the resin, hot pressing at 1300°C and 66 MPa was employed to sinter specimens containing no platelets and 15% platelet fractions, yielding specimens with near theoretical densities (99.7%) and average grain sizes of 0.96 µm and 0.83 µm, respectively. Alignment before and after sintering was assessed by SEM and XRD, with lotgering factors of 0.045±0.005 and 0.114 measured in the green and sintered specimens, respectively.     

The CMAS corrosion behavior of high-entropy (Y0.2Dy0.2Er0.2Tm0.2Yb0.2)4Hf3O12 hafnate material prepared by ultrafast high-temperature sintering (UHS)

High-temperature molten calcium-magnesium-alumina-silicate (CMAS) corrosion has become a fatal factor for the failure of aero-engine thermal barrier coatings. In this study, a promising entropy-stabilized (Y_0.2Dy_0.2Er_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ (5YH) hafnate was prepared by the emerging ultrafast high-temperature sintering (UHS), and its corrosion and wetting behavior of molten CMAS were investigated. For the corrosion mechanism, the precipitation of the high-entropy apatite phase promotes the formation of the HfO₂ phase, and it can improve the density and stability of the slow-growing reaction layer, hindering the further penetration of molten CMAS. At 1300 ℃, a reaction layer with a three-layered morphology is generated, resulting from the decreased viscosity of the molten CMAS. Moreover, computational analysis shows that molten CMAS on the 5YH surface has a larger contact angle (17°) than traditional YSZ (13°), and the spreading area is about 90 % of traditional YSZ, which benefits for its good CMAS corrosion resistance.     

Suppressed grain growth in highly porous barium titanate foams by two‐step sintering

Porous barium titanate has gained significant attention in recent years for their potential use in applications such as scaffolds for bone tissue engineering, stress sensors, gas sensors, and many others. However, there is very little control over the grain size of the material during the sintering processes specially to achieve little or no growth of the starting powders. Here, using the two‐step sintering method barium titanate foams were shown to be synthesized with controlled grain size of the struts without significant differences in the pore structure of the materials. In order to evaluate the applicability of two‐step sintering for a variety of processing methods, highly porous (>80% porosity) foams synthesized through the direct polyurethane foaming method were used to create conditions furthest from bulk where two‐step sintering has shown success. Two‐step sintering parameters were identified and the processing conditions were confirmed to not alter the mechanical properties of the samples due to expected residual stresses or thermal shock resulting from the rapid heating and cooling rates employed.     

Low-temperature reactive spark plasma sintering of dense SiC-Ti3SiC2 ceramics

SiC-based ceramics are of great interest for various advanced applications. However, its fabrication requires high-temperature treatment at ～2000 – 2100 °С. In this study, we developed an approach based on low-temperature reactive spark plasma sintering to produce dense SiC-based ceramics with superior mechanical properties. It was found that an SPS temperature of 1600 °C and introduction of 10 – 15 wt% of mechanically activated non-oxide Ti–Si–C additive is required to manufacture ceramics with a theoretical density of higher than 90%. Nonetheless, employing 5 – 15 wt% of the additive mixture and an SPS temperature of 1700 °C, the maximum density of ～ 98% was achieved. The controlled formation and decomposition of the in-situ Ti₃SiC₂ MAX phase enables the fabrication of the engineering ceramics with enhanced compressive strength (550 MPa), elastic modulus (485 GPa), and microhardness (32 GPa), which are comparable to the best-reported SiC ceramics. The study has a significant potential for practical application in the production of advanced SiC-based ceramics for various purposes and could be used for further understanding and development of the high-temperature sintering methods.     

Effect of temperature and holding time on the densification of alumina obtained by two-step sintering

The two-step sintering technique is a process of controlling the sintering curve, which provides materials with higher density and smaller grain size when compared to conventional sintering. This technique was evaluated by optical dilatometry with three commercial alumina powders of different purity (92, 96 and 99 wt% of Al₂O₃) and particle size (between 0.73 and 2.16 µm). Different sintering conditions in the first (temperature, T1) and second (temperature, T2, and holding time, t2) steps were studied in order to evaluate the effect of these variables on densification and grain growth. Considering T1 as the temperature at which a relative density (D_rel) of 83% was achieved, and for the range of conditions tested, it was found that higher D_rel values and lower grain size of alumina were obtained with higher T2 and lower t2. Alumina with 99 wt% purity sintered at T1 of 1550 °C for 5 min and T2 of 1500 °C for 4 h showed the best relationship between higher densification (~96% relative density) and reduced grain size (0.94±0.15 µm). Thus, this work demonstrated that suppression of grain growth can also be obtained for commercial alumina.     

Sintering behavior,microstructural evolution,and mechanical properties of ultra-fine grained alumina synthesized via in-situ spark plasma sintering

Ultra-fine grained Al₂O₃ was fabricated by in-situ spark plasma sintering (SPS) process directly from amorphous powders. During in-situ sintering, phase transformation from amorphous to stable α-phase was completed by 1100 °C. High relative density over 99% of in-situ sintered Al₂O₃ was obtained in the sintering condition of 1400 °C under 65 MPa pressure without holding time. The grain size of in-situ sintered Al₂O₃ body was much finer (~400 nm) than that of Al₂O₃ sintered from the crystalline α-Al₂O₃ powders. For in-situ sintered Al₂O₃ from amorphous powders, we observed a characteristic microstructural feature of highly elongated grains in the ultra-fine grained matrix due to abnormal grain growth. Moreover, the properties of abnormally grown grains were controllable. Fracture toughness of in-situ sintered Al₂O₃ with the elongated grains was significantly enhanced due to the self-reinforcing effect via the crack deflection and bridging phenomena.     

Low‐temperature sintering and enhanced piezoelectric properties of random and textured PIN–PMN–PT ceramics with Li2CO3

Low‐temperature sintered random and textured 36PIN–30PMN–34PT piezoelectric ceramics were successfully synthesized at a temperature as low as 950°C using Li₂CO₃ as sintering aids. The effects of Li₂CO₃ addition on microstructure, dielectric, ferroelectric, and piezoelectric properties in 36PIN–30PMN–34PT ternary system were systematically investigated. The results showed that the grain size of the specimens increased with the addition of sintering aids. The optimum properties for the random samples were obtained at 0.5 wt% Li₂CO₃ addition, with piezoelectric constant d₃₃ of 450 pC/N, planar electromechanical coupling coefficient k_p of 49%, peak permittivity ε_max of 25 612, remanent polarization P_r of 36.3 μC/cm². Moreover, the low‐temperature‐sintered textured samples at 0.5 wt% Li₂CO₃ addition exhibited a higher piezoelectric constant d₃₃ of 560 pC/N. These results indicated that the low‐temperature‐sintered 36PIN–30PMN–34PT piezoelectric ceramics were very promising candidates for the multilayer piezoelectric applications.     

Fabrication of highly dense Si3N4 via record low-content additive system for low-temperature pressureless sintering

Dense Si₃N₄ ceramics were fabricated by pressureless sintering at a low temperature of 1650°C with a short holding period of 1 h under a nitrogen atmosphere. The role of ternary oxide additives (Y₂O₃–MgO–Al₂O₃, Y₂O₃–MgO–SiO₂, Y₂O₃–MgO–ZrO₂) on the phase, microstructure, and mechanical properties of Si₃N₄ was examined. Only 5 wt.% of Y₂O₃–MgO–Al₂O₃ additive was sufficient to achieve >98% of theoretical density with remarkably high biaxial strength (∼1200 MPa) and prominent hardness (∼15.5 GPa). Among the three additives used, Y₂O₃–MgO–Al₂O₃ displayed the finest grain diameter (0.54 μm), whereas Y₂O₃–MgO–ZrO₂ produced the largest average grain diameter (∼0.95 μm); the influence was seen on their mechanical properties. The low additive content Si₃N₄ system is expected to have superior high-temperature properties compared to the system with high additive content. This study shows a cost-effective fabrication of highly dense Si₃N₄ with excellent mechanical properties.     

Field assisted sintering of ceramic constituted by alumina and yttria stabilized zirconia

We show that a two-phase 50 vol% 3YSZ-alumina ceramic flash-sinters at a furnace temperature of 1060 °C under an electrical field of 150 V cm⁻¹. In contrast undoped, single-phase alumina remains immune to field assisted sintering at fields up to 1000 V cm⁻¹, although single-phase 3YSZ flash sinters at 750 °C (furnace temperature). The mechanisms of field assisted sintering are divided into two regimes. At low fields the sintering rate increases gradually (FAST), while at high fields sintering occurs abruptly (FLASH). Interestingly, alumina/zirconia composites show a hybrid behavior such that early sintering occurs in FAST mode, which is then followed by flash-sintering. The specimens held in the flashed state, after they had sintered to near full density, show much higher rate of grain growth than in conventional experiments. These results are in contrast to earlier work where the rate of grain growth had been shown to be slower under weak electrical fields.