Processing,microstructure and performance of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramics prepared by laser floating zone melting with ultra-high temperature gradient

Directionally solidified Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary eutectic/off-eutectic composite ceramics with high density, homogeneous microstructures, well-oriented growth have been prepared by laser floating zone melting at different solidification rates from 4 to 400 µm/s. Uniform and stable melting zone is obtained by optimizing temperature field distribution to keep continuous and stable eutectic growth and prevent from cracks and defects. The as-solidified composite ceramic exhibits complexly irregular eutectic structure, in which the eutectic spacing is rapidly refined but dotted ZrO₂ number inside Al₂O₃ phase is decreased as increasing the solidification rate. The formation mechanism of ZrO₂ distributed inside Al₂O₃ matrix is revealed by examining the depression of solid/liquid interface. Furthermore, after heat exposure 1500 °C for 200 h, the eutectic microstructure only shows tiny coarsening, which indicates it has excellent microstructural stability. As increasing the ZrO₂ content, the fracture toughness can be improved up to 3.5 MPa m^1/2 at 20.6 mol% ZrO₂.     

Structural,microstructural, ferroelectric and photoluminescent properties of praseodymium modified Ba0.98Ca0.02Zr0.02Ti0.98O3 ceramics

The ‘x’ wt% (x = 0, 0.02, 0.04 and 0.06) Pr₆O₁₁ modified Ba_0.98Ca_0.02Zr_0.02Ti_0.98O₃ (BCZT – x Pr) piezoelectric ceramics have been fabricated by the solid state reaction method with sintering at 1450 °C (x = 0) and 1350 °C (0.02 ≤ x ≤ 0.06) for 2 h. The impact of Pr concentration on the structural, microstructural, photoluminescence and ferroelectric properties has been systematically investigated. The x-ray diffraction (XRD) patterns revealed the co-existence of tetragonal and orthorhombic phases at room temperature upto x = 0.04 Pr concentration. The grain size was found to decrease upto x = 0.04 Pr content. Room temperature Raman spectroscopy results were consistent with the XRD results. The photoluminescence (PL) spectra showed significant emissions consisting of strong blue (489 nm), green (528 nm) and red (649 nm) wavelengths. The emission intensities of PL spectrum were strongly Pr concentration dependent and a maximum value was obtained for 0.04 Pr modified BCZT ceramic. Further, a large remnant polarization (2P_r ~ 13 µC/cm²) and low coercive field (E_C ~ 22 V/cm) were obtained for BCZT – 0.04 Pr ceramic. The crystal structure and microstructure affect the photoluminescence and ferroelectric properties. Such properties of 0.04 Pr modified BCZT ceramic make it the potential candidate for novel integrated and multifunctional devices.     

Strengthening and toughening of TiN-based and TiB2-based ceramic tool materials with HfC additive

Effects of HfC addition on the microstructures and mechanical properties of TiN-based and TiB₂-based ceramic tool materials have been investigated. Their pore number decreased gradually and relative densities increased progressively when the HfC content increased from 15 wt% to 25 wt%. The achieved high relative densities to some extent derived from the high sintering pressure and the metal phases. HfC grains of about 1 µm evenly dispersed in these materials. Both TiN and TiB₂ grains become smaller with increasing HfC content from 15 wt% to 25 wt%, which indicated that HfC additive can inhibit TiN grain and TiB₂ grain growth, leading to the formation of a fine microstructure advantageous to improve flexural strength. Especially, TiB₂-HfC ceramics exhibited the typical core-rim structure that can enhance flexural strength and fracture toughness. The toughening mechanisms of TiB₂-HfC ceramics mainly included the pullout of HfC grain, crack deflection, crack bridging, transgranular fracture and the core-rim structure, while the toughening mechanisms of TiN-HfC ceramics mainly included pullout of HfC grain, fine grain, crack deflection and crack bridging. Besides, HfC hardness had an important influence on the hardness of these materials. Higher HfC content increased Vickers hardness of TiN-HfC composite, but lowered Vickers hardness of TiB₂-HfC composite, being HfC hardness higher than for TiN while HfC hardness is lower than for TiB₂. The decrease of fracture toughness of TiN-HfC ceramic tool materials with the increase of HfC content was attributed to the formation of a weaker interface strength.     

Sintering,microstructure and mechanical properties of Al2O3–Y2O3–ZrO2 (AYZ) eutectic composition ceramic microcomposites

We report on how the mechanical properties of sintered ceramics (i.e., a random mixture of equiaxed grains) with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition compare with those of rapidly or directionally solidified Al₂O₃–Y₂O₃–ZrO₂ eutectic melts. Ceramic microcomposites with the Al₂O₃–Y₂O₃–ZrO₂ eutectic composition were fabricated by sintering in air at 1400–1500 °C, or hot pressing at 1300–1400 °C. Fully dense, three phase composites of Al₂O₃, Y₂O₃-stabilized ZrO₂ and YAG with grain sizes ranging from 0.4 to 0.8 μm were obtained. The grain size of the three phases was controlled by the size of the initial powders. Annealing at 1500 °C for 96 h resulted in grain sizes of 0.5–1.8 μm. The finest scale microcomposite had a maximum hardness of 19 GPa and a four-point bend strength of 282 MPa. The fracture toughness, as determined by Vickers indentation and indented four-point bending methods, ranged from 2.3 to 4.7 MPa m^1/2. Although strengths and fracture toughnesses are lower than some directionally or rapidly solidified eutectic composites, the intergranular fracture patterns in the sintered ceramic suggest that ceramic microcomposites have the potential to be tailored to yield stronger, tougher composites that may be comparable with melt solidified eutectic composites.     

A facile solvothermal method for high-quality Gd2Zr2O7 nanopowder preparation

A facile solvothermal method has been developed to synthesize Gd₂Zr₂O₇ nanopowders with well-controlled particle size and dispersion. The nanopowders were consolidated into dense ceramic pellets by pressureless sintering, and effects of the solvothermal method in microstructure control are demonstrated by comparison with conventional precipitation approach. X-ray diffraction, electron microscopy, gas adsorption, and a standard operating procedure (SOP) in Malvern Instruments were employed to investigate crystallite/grain sizes and structural morphology evolutions of nanopowders and ceramics. Well-crystallized Gd₂Zr₂O₇ nanopowders with little agglomeration and narrow size distribution synthesized by this method have an average crystallite size of 3.5 nm and high surface area of 162 m²/g. This method also allows the formation of homogeneous and dense Gd₂Zr₂O₇ ceramic with bulk density over 94% of the theoretical value achieved after sintering at 1500 °C for 6 h.     

Preparation,microstructure and mechanical properties of ZrB2–ZrO2 ceramics

ZrB₂–ZrO₂ ceramics with ZrO₂ content varied from 15 to 30 vol.% were prepared by hot pressing. The content of ZrO₂ was found to have an evident effect on the preparation, phase constitution, microstructure as well as the mechanical properties of ZrB₂–ZrO₂ ceramics. ZrB₂–30 vol.% ZrO₂ provided the optimal combination of dense microstructure (2.6 μm, as the average grain size) and excellent properties, including the flexural strength of 803 MPa, and the hardness of 22.7 GPa tested under 9.8 N. The highest t-ZrO₂ transformability of 35.2 vol.% during fracture for ZrB₂–30 vol.% ZrO₂ brought the best toughness of 6.5 MPa m^1/2 compared with any other ceramic. In addition, the dependence of toughness on the test method as well as the hardness on the indentation load was also investigated.     

Densification,mechanical and thermal properties of ZrC1 − x ceramics fabricated by two-step reactive hot pressing of ZrC and ZrH2 powders

Densification behavior, mechanical and thermal properties of ZrC_1 ? x ceramics with various C/Zr ratios of 0.6–1.0 have been investigated by two-step reactive hot pressing of ZrC and ZrH₂ powders at 30 MPa and 1500–2100 °C. The two-step reactive hot pressed ZrC_1 ? x ceramic has a higher relative density (> 95.3%) than that (91.9%) of stoichiometric ZrC sintered at 2100 °C. A cubic Zr₂C-type ordered phase forms in the ZrC_1 ? x sample obtained at a ZrC/ZrH₂ molar ratio of 0.6 at a relatively low temperature of 1100 °C. The decrease in C/Zr ratio is beneficial to densification of ZrC_1 ? x ceramic, however, excess grain growth occurs after sintering above densification temperature. The elastic modulus and Vickers hardness decrease with decreasing the C/Zr ratio. With decreasing the C/Zr ratio, both thermal conductivity and specific heat decrease due to the enhanced scattering of conducting phonons and electrons by carbon vacancies.     

Effect of composition and grain size on dielectric,ferroelectric and induced strain behavior of PLZT/ZrO2 composites

Lead lanthanum zirconate titanate ceramics (PLZT) are well known for their excellent dielectric, piezoelectric and ferroelectric properties. In this study, PLZT 9/70/30, 9/65/35 and 9/60/40 ceramics were prepared by vibro-milling mixed-oxide method. All compositions of powders were uniaxial pressed in pellets and sintered at the temperatures of 1200–1275 °C with various soaking times of 2, 4 and 6 h. The X-ray diffraction (XRD) patterns confirmed that all the PLZT samples had perovskite structure with ZrO₂ as a second phase and PLZT/ZrO₂ composite structure was formed. Dielectric behavior at the frequency of 1 kHz showed broad peak indicating relaxor ferroelectric behavior and the difference of the temperature at maximum dielectric at different frequencies increased when Zr:Ti ratio increased. Polarization with electric field (P-E loop) at room temperature showed that when Zr:Ti ratio increased, the coercive field decreased resulting from crystal structure change from tetragonal to rhombohedral. Induced strain with electric field depended on microstructure where the value of S_max/E_max tended to decrease with increasing grain size. It can be concluded that dielectric and ferroelectric behavior predominantly depended on composition of PLZT ceramics and induced strain behavior predominantly depended on grain size of PLZT ceramics.     

Effect of different preparation methods on the microstructure and mechanical properties of Si3N4 ceramic composites

In this study, Si₃N₄ ceramic composites were fabricated by using ball-milling, titration preparation and urea preparation methods, respectively. The effect of different preparation methods on microstructure and mechanical properties of the Si₃N₄ ceramic composites was investigated. Obviously, the Si₃N₄ ceramic composite prepared by the urea preparation method (U-SN sample) showed better sintering behavior and higher mechanical properties than that prepared by the other two methods. Compared with the Si₃N₄ ceramic composite by the titration preparation method (T-SN sample), we could avoid the complex titration process or uncontrollable pH value during the preparation process of the U-SN sample. Meanwhile, the coated Y-Al precursor layer in thickness of nanometers was more homogeneous than that prepared by the traditional titration method. B-SN represented the Si₃N₄ ceramic composite prepared by the ball-milling method. These samples were all sintered from room temperature to 1750 °C via hot-pressing sintering. The U-SN specimen showed the optimal flexural strength and fracture toughness of being 817 MPa and 6.90 MPa/m², respectively, which could be attributed to its smallest grain size (0.46 µm) among these three samples.     

High-strength ZrB2-based ceramics prepared by reactive pulsed electric current sintering of ZrB2–ZrH2 powders

Fully densified ZrB₂-based ceramic composites were produced by reactive pulsed electric current sintering (PECS) of ZrB₂–ZrH₂ powders within a total thermal cycle time of only 35 min. The composition of the final composite was directly influenced by the initial ZrH₂ content in the starting powder batch. With increasing ZrH₂ content, ZrB₂–ZrO₂, ZrB₂–ZrB–ZrO₂ and ZrB₂–ZrB–Zr₃O composites were obtained. The ZrB₂–ZrB–ZrO₂ composite derived from a 9.8 wt% ZrH₂ starting powder exhibited an excellent flexural strength of 1382 MPa combined with a Vickers hardness of 17.1 GPa and a fracture toughness of 5.0 MPa m^1/2. The high strength was attributed to a fine grain size and the removal of B₂O₃ through reaction with Zr. Higher ZrH₂ content starting powders were densified through solution-reprecipitation resulting in the formation of coarser angular ZrB₂–ZrB composites with a Zr₃O grain boundary phase with a fracture toughness of 5.0 MPa m^1/2 and an acceptable strength in the 852–939 MPa range.     

The control of abnormal grain growth in low-voltage SnO2 varistors by microseed addition

In this research, the addition effects of three different quantities of micron-sized seeds (microseeds) to a SnO₂ varistor prepared from nanomaterials on the microstructure and electrical properties were studied. Moreover, surge-withstanding capability of low-voltage SnO₂ varistors was investigated. The X-ray diffraction pattern disclosed a single phase SnO₂ for microseed grains. The morphological features of samples were characterized using scanning electron microscopy. The abnormal distribution of grain size with elongated grains of SnO₂ in fine grains matrix was observed in sintered samples without microseeds. The low content of microseed addition (0.3 wt%) had not controlled abnormal grain growth, however, it increased mean grain size to 37 µm. Although the high content of microseeds (7.5 wt%) stopped abnormal grain growth, it had a negative effect on relative density and mean grain size. The normal grain size distribution with maximum mean grain size (45 µm) was obtained in samples containing 1.5 wt% microseeds. These samples showed the lowest breakdown field (240 V/cm) and the highest surge-withstanding capability (1.5 kA/cm²). Furthermore, the standard deviation of the electrical parameters of these samples was improved due to normal grain-size distribution.     

Al alloy/Ti3SiC2 composites fabricated by pressureless infiltration with melt-spun Al alloy ribbons

Al alloy/Ti₃SiC₂ composites with compressive strengths ranging from 743 to 932 MPa have been successfully fabricated by a new two-step pressureless infiltration method. 6061 Al alloy ribbons prepared by melt spinning were employed as the Al alloy matrix for melt infiltration. Shifts in phase constitution and reaction mechanisms of Ti₃SiC₂ preforms in molten Al at 950 °C were investigated, and the compression performance of Al alloy/Ti₃SiC₂ composites was tested. The Vickers hardness of the composites was enhanced to a maximum of 751 HV by increasing the Al content.     

Temporal stability of oxygen-ion conductivity in 1Nb2O5-10Sc2O3-89ZrO2

A slight Nb₂O₅ co-doping in 11Sc₂O₃-89ZrO₂ was earlier reported to stabilize the high-symmetry cubic phase completely and enhances the conductivity significantly. The present work looked at the temporal stability of conductivity in 1Nb₂O₅-10Sc₂O₃-89ZrO₂ (1Nb10ScSZ) for the electrolyte application in solid oxide fuel cells. In-situ conductivity measurement was done using impedance spectroscopy at 650 °C in the air for 2000 h. A substantial conductivity loss (29%) was observed in the first 1000 h. Following which, conductivity remained relatively stable for the next 1000 h. Impedance analysis showed that the main contribution to the conductivity degradation was from grain conductivity. Phase analysis performed using XRD, TEM and Raman spectroscopy revealed that both the unaged and aged 1Nb10ScSZ samples consisted of metastable t″-phase. However, the extent of tetragonality was found to increase after ageing. The formation of low-symmetry phase was suggested to be the reason for the grain conductivity loss in 1Nb10ScSZ.     

Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering

The influence of sintering temperature on the microstructure and mechanical properties of Al₂O₃?20 wt% ZrO₂ composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m^1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

Microstructure design and preparation of Al2O3/TiC/TiN micro-nano-composite ceramic tool materials based on properties prediction with finite element method

The objective of this paper was to improve the accuracy of semi-empirical method used to design ceramic cutting tool materials. The mechanical properties were predicted by employing finite element model of material microstructure, so as to design microstructure and prepare new ceramic materials. Based on the Voronoi and randomness method, the microstructure model representing the complexity and randomness of micro-nano-composite ceramic material microstructure was established. Combining the representative volume element (RVE) of ceramic material microstructure with mechanical tests, the simulations of mechanical tests were conducted to acquire the flexure strength, fracture toughness and hardness of materials. The microstructure models with various parameters were designed and the material properties were predicted to determine the optimal microstructure parameters. Then, The ceramic cutting tool materials possessing the optimal microstructure parameters were developed for machining ultra-high strength steels. The results showed that the mechanical properties of ceramic materials first improved and then declined as the nano-scale TiC volume fraction increased. To obtain the best comprehensive mechanical properties, the contents of micro-scale TiN, TiC and nano-scale TiC were set as 20%, 10% and 10%, respectively. The prepared ceramic materials possessed the flexure strength of 881.4 MPa, the fracture toughness of 7.8 MPa m^1/2, and the Vickers hardness of 20.8 GPa. This research is beneficial to the development of cutting tool design theory and the improvement of the tool life.     

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO₂-SiO₂ aerogels into the porous Si₃N₄ ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3 W m^?1 K^?1. Meanwhile, the density, mechanical and dielectric properties of the porous Si₃N₄ ceramics could not be affected after introducing ZrO₂-SiO₂ aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86 MPa) and low dielectric constant (2.86). The ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramics composite shows great potential as radome materials applied in the fields of aerospace.     

Carbon-coated Li2S cathode for improving the electrochemical properties of an all-solid-state lithium-sulfur battery using Li2S-P2S5 solid electrolyte

Li₂S is coated with carbon to improve the electrical conductivity of the composite cathode in all-solid-state lithium-sulfur batteries. Carbon is applied by thermal evaporation from a polyacrylonitrile (PAN) source at 600 °C for 5 h. It is shown that the carbon coating is impurity free, and the crystallinity of Li₂S is well maintained. The electronic conductivity of Li₂S is dramatically improved from 9.21 × 10^?9 S cm^?1 to 2.39 × 10^?2 S cm^?1 upon carbon coating. An all-solid-state battery prepared with the carbon-coated Li₂S shows a high initial capacity of 585 mAh g^?1 (g of Li₂S) that increases up to 730 mAh g^?1 (g of carbon-coated Li₂S) by the 10th cycle. This high capacity is stable throughout the 25 cycles tested, with an excellent coulombic efficiency of 99%. Carbon-coated Li₂S is advantageous for all-solid-state batteries due to the increased electrical conductivity, while allowing a reduction of the total carbon content present in the composite cathode.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

Performance and stability in H2S of SrFe0.75Mo0.25O3-δ as electrode in proton ceramic fuel cells

The H₂S-tolerance of SrFe_0.75Mo_0.25O_3-δ (SFM) electrodes has been investigated in symmetric proton ceramic fuel cells (PCFC) with BaZr_0.8Ce_0.1Y_0.1O_3-δ (BZCY81) electrolyte. The ionic conductivity of the electrolyte under wet reducing conditions was found to be insignificantly affected in the presence of up to 5000 ppm H₂S. The fuel cell exhibited an OCV of about 0.9 V at 700 °C, which dropped to about 0.6 V and 0.4 V upon exposure to 500 and 5000 ppm H₂S, respectively, on the fuel side. Post characterization of the fuel cell revealed significant degradation of the anode in terms of microstructure and chemical composition due to formation of sulfides such as SrS, MoS₂ and Fe₃S₄. Nevertheless, the fuel cell was still functional due to the sufficient electronic conductivity of some of these sulfides.     

Electrically conductive ZrO2–TiN composites

1.75 mol% Y₂O₃-stabilized ZrO₂–TiN composites could be fully densified by hot pressing for 1 h at 1550 °C in vacuum under a mechanical pressure of 28 MPa. Composites with 35–95 vol% TiN were investigated and the best mechanical properties, i.e., a Vickers hardness of 14.7 GPa, an indentation toughness of 5.9 MPa m^1/2 and an excellent bending strength of 1674 MPa were obtained with 40 vol% TiN. The active toughening mechanisms were identified and their contribution to the overall composite toughness as function of the TiN content was modelled, experimentally verified and discussed. Transformation toughening was found to be the primary toughening mechanism. The TiN grain size was found to increase with increasing TiN content, resulting in a decreasing hardness and strength. A maximum strength was obtained at 40 vol% TiN. The electrical resistivity of the composites decreases exponentially with increasing TiN content and correlates well with the Polder-Van Santen mixture rule. Thus at around 40 vol% TiN, the conductivity is high enough to allow EDM machining of the composite, therefore avoiding the expensive grinding operation for final shaping and surface finishing of components.