Increasing Sintering Temperature Research Articles

Microwave absorption, thermal and mechanical properties of amorphous nano carbon doped aluminium nitride composite ceramics

Low-cost amorphous nano-carbon (ANC) was employed as an absorbent to modify the microwave performance of AlN ceramics (AlN-4 wt% Y2O3-x wt% C, x = 0, 2.0, 3.8, 3.9, 4.0) via hot-pressing sintering, and the thermal and mechanical properties were also investigated. With the increase of ANC content from 0 to 4.0 wt%, the relative density of AlN composite decreases from 3.28 g·cm-3 to 3.16 g·cm-3 when sintered at 1800 °C for 2 h, and with the increase of sintering temperature from 1700 °C, the density of the 4.0 wt% ANC doped AlN composite presents trend of increase and reaches the maximum value of 3.16 g·cm-3 at 1800 °C and then decreases. The XRD pattern shows that the composites are composed of AlN and Y-Al-O (YAG, YAP, YAM et al.) phases. SEM results indicate that average grain sizes for AlN and secondary phases gradually decrease with increasing carbon content, suggesting inhibited grain growth due to ANC addition. No obvious changes in phase constitution nor intensity changes were detected with the increase in carbon content and sintering temperature. As the carbon content increases from 0 to 4.0 wt%, the dielectric constant gradually increases from ∼9 to 13.89 at 10 GHz, the dielectric loss gradually increases from the vicinity of 10-3 to 0.38 at 10 GHz, and the reflective loss decreases from -27.92 dB to -49.40 dB, the thermal conductivity of the AlN-ANC composite decreases from 116.63 W·m-1·K-1 to 50.13 W·m-1·K-1. In addition, with 4.0 wt% ANC addition, the dielectric constant, dielectric loss, reflective loss, hardness, elastic modulus, bending strength, and fracture toughness are 13.89, 0.38, 49.40 dB, 6.4±0.4 GPa, 262 GPa, 268.78 MPa, and 1.4±0.1 MPa m1/2, respectively. Its ability to efficiently absorb microwave energy enhances the performance of devices including radar systems, communication equipment, and microwave heating systems, making it a promising candidate for microwave vacuum electronic applications.

Effect of sintering temperature on microstructure and electrical properties of Na0.5Bi0.47Sr0.02TiO3-δ ceramics

In this investigation, a series of Na0.5Bi0.47Sr0.02TiO3-δ samples, sintered from 1000 to 1100 °C, were synthesized utilizing the solid-state method. The XRD results indicate that all samples exhibit a perovskite phase, with the exception of those sintered at 1100 °C, which display no discernible impurity peaks. The SEM results indicate that the average grain size exhibits an increase proportionate to the sintering temperature, which attributes to the variations in the actual Na/Bi ratio. From the study of grain conductivity we can obtain that the dominant factor affecting grain conductivity is the activation energy. At 1100 °C, the oxygen vacancy concentration emerges as the primary determinant of grain conductivity. It should be noticed that the phase transition temperature of the sample increases (350 °C→400 °C) with the increase of sintering temperature, which is due to the local A-site disorder aggravated by high temperature sintering. Therefore, reducing the sintering temperature is beneficial to improve the grain conductivity. The grain boundary conductivity initially rises, then diminishes with increasing sintering temperature. It is evident that the enhanced macroscopic grain boundary conductivity stems from a marked reduction in space charge potential within the 1000–1025 °C temperature range. Within the 1050 °C−1100 °C temperature range, the space charge potential barely changed, the significant decrease of intrinsic grain boundary conductivity can be attributed to the surge in the coverage ratio of grain boundary impurities and the number of grain boundary impurities increased caused by the rapid reduction of grain boundary area and thermal evaporation. The research on electrical performance of NBT-based oxygen-ion conductors will promote the application of NBT-based oxygen ion conductors in solid oxide fuel cells, oxygen sensors and other fields.

Effect of nitrogen addition on sintering behavior of high-entropy alloy CrMnFeCoNi powder

The effect of nitrogen addition on sintering behavior of high-entropy alloy CrMnFeCoNi powder has been investigated. CrMnFeCoNi alloy powders were nitrided to add nitrogen and then consolidated at three different temperatures. The microstructure of the nitrided CrMnFeCoNi alloy powders and sintered compacts was characterized using electron probe microanalysis (EPMA) system. In the compact sintered at 800 °C, a high-nitrogen-content region formed a continuous network structure around the low-nitrogen-content substrate. The hardness of the sintered compacts with nitrogen was higher than that of the compacts without nitrogen. The high-nitrogen-content region expanded into the substrate region. The Vickers hardness of the sintered compacts increased with an increase of sintering temperature up to 950 °C. Sintering at 1100 °C did not completely finish; the vacuum pressure in the sintering chamber rapidly increased because chromium nitride (CrN) decomposed. The results indicate that high-temperature sintering is ineffective for nitrogen-added CrMnFeCoNi alloy powder.

The effect of degree of sintering on the structural and mechanical properties of Si3N4–SiO2 glass ceramics

Silica-based glass ceramics have been extensively used in biomedical applications due to their superior biocompatibility and controllable properties. However, their low mechanical strength limits their application. This could be addressed by optimizing the crystalline phase which determines their final properties. Silicon nitride has attracted attention due to its combination of good mechanical and biological properties. Therefore, to combine the advantage of silica-based glass and silicon nitride ceramic, this study developed a silicon nitride-silicon dioxide (Si3N4–SiO2) glass ceramics. The effects of spark plasma sintering parameters on the structural and mechanical properties of Si3N4–SiO2 glass ceramics were investigated. Full densification was reached at a sintering temperature of 1300 °C, a holding time of 10 min and an applied pressure of 80 MPa (relative density = 99.19 %). No silicon oxynitride (Si2N2O) crystalline phase was formed in the sintered glass ceramics, as confirmed by XRD. The interface between the β-Si3N4 crystalline and amorphous SiO2 was investigated by HRTEM, and the results indicated that an amorphous interfacial oxide was formed at the interface. The mechanical properties increased with increasing sintering temperature, as a result of the increased density. The Si3N4–SiO2 glass ceramics sintered at 1500 °C exhibited the highest value of toughness and flexural strength, at 4.6 ± 0.28 MPa m1/2 and 360 ± 27 MPa. The indentation cracks observed by SEM showed that the large β-Si3N4 grains promoted crack deflection, while the equiaxed Si3N4 grains with a lower aspect ratio led to transgranular fracture. The mechanical properties of these Si3N4–SiO2 glass ceramics are comparable to commercial glass ceramics, indicating their promising aspects in biomedical applications.

Effect of Various Sintering Temperatures on the Friction and Mechanical Properties of PEEK/PTFE Blends

Polytetrafluoroethylene (PTFE) has a very low coefficient of friction and outstanding corrosion resistance, but its limited use is due to high wear and tear and low hardness. Polyetheretherketone (PEEK) offers low wear, high temperature resistance and a slightly higher coefficient of friction. In the research we describe here we examined the mechanical and tribological properties of PEEK/PTFE blends manufactured at various sintering temperatures (350 °C–390 °C). The experimental data demonstrated that the PEEK/PTFE blends exhibited the highest tensile strength, 22.97 Mpa, when sintered at 380 °C, making them the most resistant to breaking or deforming. The elongation at break increased with the increase of sintering temperature, and the highest value of elongation at break for the PEEK/PTFE blends was 429.16% when sintered at 390 °C, and the lowest value was 283.37% when sintered at 350 °C. The PEEK/PTFE blends exhibited the lowest coefficient of friction, measuring 0.174, when sintered at 350 °C. The lowest mass abrasion rate for the PEEK/PTFE blends was 6.04 × 10−7 mm3/N·m when sintered at 360 °C.

Influence of sintering temperature on electrocaloric and energy storage properties of sol-gel derived lead-free BaHf0.20Ti0.80O3 ferroelectric ceramics for sustainable energy solutions

Ferroelectric materials have emerged as intriguing contenders for a variety of applications in the ongoing effort to look for sustainable energy solutions, including energy storage and solid-state cooling systems. In this work, a systematic investigation has been conducted on the microstructural, dielectric, electrocaloric, and energy storage characteristics of sol-gel elaborated BaHf0.20Ti0.80O3 (BHT) ferroelectric ceramics. The structural investigation by Raman and X-ray diffraction indicated that when the sintering temperature increases, the tetragonality of BHT ceramics reduces, and the evolution of the cubic phase increases. The evolution of the microstructure in BHT ceramics is found to be significantly reliant on the sintering temperature. With increasing temperature of sintering, the bulk density of BHT ceramics increases. The improved dielectric and ferroelectric properties were attained in BHT ceramics at higher sintering temperatures. The BHT ceramic sintered at 1350 °C exhibited a maximum electrocaloric temperature change of ΔT ∼0.6 °C with a corresponding efficiency ΔT/ΔE of 0.3 K mm/kV near ambient temperature under a low electric field of 20 kV/cm. The BHT ceramic sintered at 1450 °C exhibited excellent room temperature recoverable energy density of JRec ∼67.84 10−3J/cm3 and high energy storage efficiency of η ∼75.13 % under a low electric field of 20 kV/cm. The highest JRec ∼73.22 10−3J/cm3 with corresponding η of 64.60 % were obtained in BHT ceramic sintered at temperature 1350 °C. These findings broaden the possibility of exploring BHT-based systems as efficient lead-free ferroelectrics for both solid-state refrigeration and energy storage applications.

Effect of phase evolution and pyroplastic formation behavior on glass-ceramic foam derived from silicomanganese slag and feldspar tailings

Utilizing solid waste to produce foam material has become a significant direction in resource recycling. This study focuses solely on using silicomanganese slag and feldspar tailings as raw materials, utilizing a powder sintering process to produce autocatalytic foaming glass–ceramic foam. We formulated four mix ratios based on the compositional characteristics of the silicomanganese slag and feldspar tailings. The study then explored the evolution of phase formation and macroscopic pore morphology under varying preparation temperatures. Moreover, through multi-phase co-melting experiments, this work unveiled the high-temperature molten mass generation mechanism between silicomanganese slag and feldspar tailings. Our findings reveal that the glass melt generated from silicomanganese slag particles and the 'solid–liquid erosion behavior' of feldspar particles play pivotal roles in the formation of high-temperature co-melt substances. As the sintering temperature increases, the mixed powder exhibits three different states of high-temperature molten mass, which are the reasons for the evolution of the macroscopic pore structure. It is suggested that the addition of silicomanganese slag should be less than 50 %, and the heat treatment temperature should be below 1145 °C in preparing glass–ceramic foam with silicomanganese slag and feldspar tailings. The sample with 40 % silicomanganese slag and 60 % feldspar tailings, treated at 1140 °C, yielded glass–ceramic foam with a compressive strength of 1.68 MPa, apparent density of 0.279 g/cm3, porosity of 81.7 %, and thermal conductivity of 0.074 W·m−1·K−1. In summary, this study provides a novel approach for the high-value utilization of silicomanganese slag and offers an economical route for developing lightweight, eco-friendly structural materials.

Isostatic hot-pressed tungsten radiation shields against gamma radiation

An isostatic hot-pressing technique for the fabrication of dense W samples' is presented. The samples’ microstructure and chemical content were analyzed. The most compact samples were received at 5 GPa pressure and 1500 and 2000 °C temperatures, with densities of 18.50 and 19.07 g/cm3, respectively. It has been established that high temperature exposure has a positive effect on both the microstructure and density. It is shown that as the sintering temperature increases, the porosity of the samples decreases from 32.31% to 0.94%, and their relative density increases from 67.69% to 99.06%. The crystal structure investigation revealed that all the samples contain the main body-centered cubic W phase, and the availability of the WO2 phase is observed just after the temperature of sintering increases above 1000 °C, which is confirmed by the appearance of 111 and 22-2 reflections. A study of the shielding effectiveness against gamma-radiation was carried out using the Phy-X/PSD software. A gamma-ray source of Co60 with energies of 0.8–2.5 MeV was applied for the calculations. The results were calculated for the sample with the highest density (19.07 g/cm3) and compared with the calculations for Pb. The W shielding effectiveness main parameters are determined: linear attenuation coefficient (at 0.8 MeV–1.46 cm-1), mean free path (at 2.5 MeV–1.2 cm) and half value layer (at 2.5 MeV–0.86 cm). These values turned out to be higher than for Pb, which makes shields based on W promising for use in the field of radiation protection.

Research on preparation and related properties of macro-micro porous mullite ceramic skeletons via twice pore-forming technology.

A lot of solid waste coal gangue is produced every year in the process of coal mining and coal washing, which poses a great threat to human health. How to deal with coal gangue properly is still a serious problem. In this study the macro-micro composite porous mullite ceramic skeletons were successfully prepared using solid waste coal gangue and α-Al2O3 as main raw materials via twice pore-forming technology. The main phase composition of the porous ceramic skeletons was mullite tested by X-ray Diffractometer (XRD). The morphology and microstructure of the porous ceramic skeletons were analyzed by Scanning Electron Microscope (SEM). The results show that the microstructure of porous ceramic skeletons was mainly composed of mullite whiskers. With the increase of sintering temperature from 1200 °C to 1350 °C, the maximum length of mullite whiskers grew up from 2.68 μm to 8.10 μm and their average length grew up from 0.78 μm to 2.98 μm. The maximum compressive strength of the porous ceramic skeletons with 30 PPI and 45 PPI were 1.25 MPa and 1.54 MPa tested by Universal Testing Machine (UTM) at the sintering temperature of 1250 °C, respectively. The linear shrinkage, bulk density and pore stem density of the porous ceramic skeletons became larger with the rising of sintering temperatures from 1150 °C to 1350 °C. However, the corresponding performance values of 45 PPI porous ceramic skeletons were greater than that of 30 PPI at the same sintering temperature. The prepared porous ceramic skeletons will be used in ceramic-metal wear-resistant composites for the later research and the study provides a new idea for coal gangue on the comprehensive utilization with high added value and brings both good environmental and economic benefits.

Elastoplastic nanoindentation behaviors of sintered nano-silver under various sintering parameters

Purpose This study aims to characterize the mechanical properties of sintered nano-silver under various sintering processes by nano-indentation tests. Design/methodology/approach Through microstructure observations and characterization, the influences of sintering process on the microstructure evolutions of sintered nano-silver were presented. And, the indentation load, indentation displacement curves of sintered silver under various sintering processes were measured by using nano-indentation test. Based on the nano-indentation test, a reverse analysis of the finite element calculation was used to determine the yielding stress and hardening exponent. Findings The porosity decreases with the increase of the sintering temperature, while the average particle size of sintered nano-silver increases with the increase of sintering temperature and sintering time. In addition, the porosity reduced from 34.88%, 30.52%, to 25.04% if the ramp rate was decreased from 25°C/min, 15°C/min, to 5°C/min, respectively. The particle size appears more frequently within 1 µm and 2 µm under the lower ramp rate. With reverse analysis, the strain hardening exponent gradually heightened with the increase of temperature, while the yielding stress value decreased significantly with the increase of temperature. When the sintering time increased, the strain hardening exponent increased slightly. Practical implications The mechanical properties of sintered nano-silver under different sintering processes are clearly understood. Originality/value This paper could provide a novel perspective on understanding the sintering process effects on the mechanical properties of sintered nano-silver.

Tailoring tactics for preparation of superior thermal insulation materials from blast furnace ferronickel slag: Control of sintering temperature

Blast furnace ferronickel slag (BFFS) was converted to thermal insulation materials by sintering of its mixture with 15 wt% fly ash cenosphere (FAC) from 1000 °C to 1200 °C. The influence of sintering temperature on the phase compositions, microstructures, and properties of the materials was investigated based on both thermodynamic and experimental analyses. It was demonstrated that increasing sintering temperature from 1000 °C to 1200 °C led to linear increases of contents of diopside (CaMgSi2O6; y = 0.039T – 21.5, where y and T denote the content and temperature, respectively), anorthite (CaAl2Si2O8; y = 0.0504T – 46.54), and magnesium aluminate spinel (MgAl2O4; y = 0.043T – 30.86) in the thermal insulation materials. Meanwhile, there existed linear decreases of contents of akermanite (Ca2MgSi2O7; y = -0.0352T + 66.18) and gehlenite (Ca2Al2SiO7; y = -0.0914T + 129.96), which could form a solid solution above 1100 °C, suppressing the expansion associated with the generation of magnesium aluminate spinel. Along with the phase transformation, the macroporous structure was also identified. By sintering at 1100 °C for 2 h, an optimal high-quality thermal insulation material with apparent porosity of 46.04%, water absorption of 42.12%, bulk density of 1.35 g/cm3, linear shrinkage of 0.82%, compressive strength of 6.83 MPa, thermal conductivity of 0.31 W/(m·K) at 25 °C and 0.42 W/(m·K) at 800 °C, and refractoriness of 1259 °C could be obtained.

Preparation and properties of in situ TiC/corundum–mullite electrically conductive ceramic

AbstractIn situ TiC/corundum–mullite electrically conductive ceramics were prepared by carbon‐bed sintering method using bauxite, clay and Ti3SiC2 powders as raw materials. The phase composition, microstructure, shrinkage, apparent porosity, bulk density, compressive strength, and resistivity of samples were investigated. The X‐ray diffraction results indicate that the TiC is completely generated by desilicidation reaction of the Ti3SiC2 at 1 300°C. When the sintering temperature is 1 300°C and Ti3SiC2 powder addition is 22 wt.%, the properties of the sample are optimum, with a longitudinal shrinkage of 2.04%, a radial shrinkage of 2.56%, an apparent porosity of 19.01%, a bulk density of 2.53 g/cm3, a compressive strength of 181 MPa, and a resistivity of 267 Ω·cm. The electrically conductive phase TiC is connected into a complete electrically conductive network. The shrinkages of samples decrease with increasing sintering temperature, which indicates that the samples expand due to secondary mullitization. The continuous electrically conductive network of TiC is destroyed. As the Ti3SiC2 powder addition increases from 14 to 20 wt.%, the resistivity of the samples decreases significantly, but the further increase in the Ti3SiC2 powder addition has no significant effect on the resistivity.

Examining the effects of sintering temperature on double perovskite La2NiTiO6: Analysis of structural, optical, electrical properties, and leakage current characteristics

This study explores the influence of varying sintering temperatures on the structural, optical, electrical, and leakage current features of La2NiTiO6 (LNTO), a double perovskite material synthesized through the citrate auto-ignition method. The X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray (EDX) analysis were employed to investigate the structure, microstructure, and compositional distribution of the samples. The samples sintered at various temperatures showed the monoclinic structure with space group P21/n and were confirmed by Rietveld's refinement of the XRD patterns. The average grain size increased with higher sintering temperatures, which is consistent with the particle size estimated from the histogram plot of the FE-SEM data. The UV–visible (UV–Vis) spectroscopy showed that the energy band gap decreased as sintering temperatures increased. Impedance spectroscopy analysis revealed non-Debye type relaxation of charge carriers and confirmed the negative temperature coefficient of resistance (NTCR) behavior. Electrical analysis affirmed that the direct current (DC) conductivity is thermally activated, following the Arrhenius equation with an inverse temperature dependency. As the sintering temperature increases, the leakage current density decreases, and the leakage current conforms to the ohmic conduction mechanism.

Investigation of physical and metallographic properties of nickel matrix composite by ultrasonic method

Ultrasonic testing (UT) is one of the most widely used non-destructive testing techniques for the characterization and evaluation of materials. In this study, the material properties of AstCrM-BN-Cr-Ti-Ni composites produced by mechanical mixing and electroless nickel plating technique were evaluated using UT. Changes in ultrasonic wave velocity, Young's modulus, density, porosity and hardness are observed in the heat-treated material at different temperatures. Samples analyzed by Scanning Electron Microscopy (SEM) and X-Ray Diffractometry (XRD) exhibited changes in grain structure at different sintering temperatures. It has been observed that the change in grain size also affects mechanical and physical properties according to ultrasonic properties. In conclusion, the experimental results showed a linear relationship between the mean grain size and the UT parameters (ultrasonic longitudinal and transverse velocity, Young's modulus), physical (sintering temperature and density) and mechanical properties (hardness and porosity) of the material. Grain growth occurs with increasing sintering temperature, porosity content decreases, hardness, Young's modulus and ultrasonic wave velocity values also increase.

Novel ceramic supports for catalyst with hierarchical pore structures fabricated via additive manufacturing-direct ink writing

The use of direct ink writing technology, combined with in-situ grown whiskers, has facilitated the development of 3D-printed ceramic catalyst supports-aluminum borate porous ceramics (ABPCs) with hierarchical pore structures. The optimal slurry consists of 0.2 wt% polycarboxylate, 19 wt% water addition, and 1.2 wt% sodium alginate. The most effective printing parameters are determined to be a 1.0 mm nozzle diameter, 10 mm/s printing speed, 4.0 kPa air pressure, and 0.4 mm layer height. After sintering at 1100 ℃, pure-phase aluminum borate (Al18B4O33) ceramics are achieved, leading to the formation of needle-like aluminum borate whiskers. The aspect ratio of whiskers, flexural strength, BET-specific surface area, and photocatalytic activity of ABPCs increase initially and then decrease with increasing sintering temperature. The ABPCs coated with a titanium oxide layer display a photocatalytic degradation rate of 83.92% for methyl orange solution. These results underscore the immense potential of ABPCs in high-temperature catalyst support applications.

Investigation on structural, electrical and magnetic properties of ErFeO3: Sintered at different temperature

This investigation reports the effect, on structural, electrical, and magnetic properties of ErFeO3 orthoferrites, sintered at different temperatures: 800 °C, 1000 °C, and 1200 °C. XRD analysis confirms that all the samples exhibit a single-phase orthorhombic structure. Raman studies indicate no significant variation in the observed Raman modes with increasing sintering temperature, suggesting that, the absence of crystal symmetry changes. Field-FESEM images reveal that the porosity of the samples is influenced by the shape of the grains present within them. Specifically, the sample sintered at 1000 °C exhibits higher porosity, resulting in higher leakage current density (J-E) compared to the other samples. The dielectric and magnetization parameters show an increase for the samples sintered at 1000 °C, while a decrease is observed for the samples sintered at 1200 °C. These variations are attributed to the bond length and bond angle variation of the Fe–O in the studied samples. Overall, this investigation aims, to establish a relationship between the physical properties and microstructural parameters, provide valuable insights into the magnetic interactions present in the samples and the comprehensive behavior of ErFeO3 orthoferrites samples sintered at different sintering temperatures will be discussed in this paper.

Effects of (KMg)3+ co-doping and sintering temperature on the negative thermal expansion property of Sc2W3O12 ceramics

(KMg)3+ co-doped Sc2W3O12 ceramics have been prepared via co-precipitation method. Effects of (KMg)3+ co-doping amount and sintering temperature on the phase constitution, microstructure, morphology, and negative thermal expansion (NTE) performance of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1.25) ceramics were studied. Results indicate that: Sc3+ cations can be substituted by (KMg)3+ dual-cations and a composition-induced structural phase transition was observed in KxMgxSc2-xW3O12. For x ≤ 0.25, orthorhombic Sc2W3O12 gradually changed into hexagonal KxMgxSc2-xW3O12 and coexisted. Single phase KxMgxSc2-xW3O12 with hexagonal symmetry can be prepared for 0.25 < x ≤ 1. When x ≥ 1.25, (KMg)3+ substitution reaches its solid solution limit in Sc2W3O12, and impurity MgWO4 formed in the samples. As co-doping amount x increased, the NTE performance of the KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics gradually enhanced. The coefficient of thermal expansion (CTE) of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics enhanced from −2.14 × 10−6 °C−1 to −19.68 × 10−6 °C−1 in 30–700 °C. With the increase of sintering temperature from 800 to 1100 °C, the grains grew up and the density of the K0.75Mg0.75Sc1.25W3O12 ceramics increased as well, and the NTE performance was also improved. The linear CTE of K0.75Mg0.75Sc1.25W3O12 ceramics enhances from −2.15 × 10−6 °C−1 to −14.70 × 10−6 °C−1 in 30–700 °C.

Silica Sol as a Filler in Process of Materializing Mullite Fiber (SiO2.Al2O3)

Research has been carried out on the effect of adding silica sol as a filler in making mullite fiber (SiO2.Al2O3). The process of making mullite fiber is carried out using the cast molding method. Sample preparation of (I)silica sol and alumina fiber. (II) silica sol with alumina fiber and kaolin. (III) alumina fiber with kaolin and distilled water. Then the samples were mixed until homogeneous in their respective containers. Then printed in a PVC pipe measuring 2.81 cm in diameter and 1.5 cm high. Mullite fiber samples were dried at a temperature of 110°C for 1 hour, then sintered at a temperature of 1200°C, 1300°C, 1400°C, 1500°C. The density of mullite fiber increases linearly, the porosity decreases linearly, and the compressive strength of mullite fiber increases linearly as the sintering temperature increases. XRD characterization shows that in the three mullite fiber samples the same phase appears, namely mullite, sillimanite has an orthorhombic structure, and cristobalite has a tetragonal structure.

Effect of sintering temperature on microstructure and mechanical properties of Ti2AlNb-based composites

Ti2AlNb-based composites are a great choice for achieving high-temperature properties by introducing reinforcement to meet engineering application requirements. In this study, Ti–22Al–25Nb (at.%) alloys and nano-Al2O3/Ti–22Al–25Nb composites were prepared by mechanical alloying (MA) and hot pressing (HP). The effects of sintering temperature on the microstructure and mechanical properties of Ti–22Al–25Nb alloys and Ti2AlNb-based composites were investigated. The sintering temperature effectively regulated the morphology and size of the primary O phase. The addition of Al2O3 particles refined the grain size of the matrix and precipitates, and it induced the phase transformation. When the sintering temperature reached 1350 °C, Ti–22Al–25Nb alloys and Ti2AlNb-based composites obtained a fine and uniform equiaxed microstructure. The hardness and ultimate compressive strength of the composite first increased and then decreased with increasing sintering temperature. Since the addition of Al2O3 particles promoted the precipitation of the ultrafine acicular O phase from the B2 phase, the composite sintered at 1350 °C had the optimal mechanical properties. The hardness of the Ti–22Al–25Nb alloy and Ti2AlNb-based composite was 4.12 GPa and 5.1 GPa, respectively. The ultimate compressive strength of the composite at room temperature (RT) and 650 °C was 2642.3 MPa and 2462.5 MPa, which was 25.8% and 9.6% higher than that of the Ti2AlNb alloy. The strengthening mechanism of the composites mainly includes dispersion strengthening and Orowan strengthening of the nano-Al2O3 particles, as well as precipitation strengthening of the acicular O phase. This study provides a theoretical guidance for designing Ti2AlNb-based composites with excellent high-temperature properties.

Synthesis, microstructure, and dielectric properties of novel dual-phase high-entropy (Ba0.2Ca0.2Sr0.2Na0.2Bi0.2)WO4 ceramics for LTCC applications

Novel dual-phase high-entropy (Ba0.2Sr0.2Ca0.2Na0.2Bi0.2)WO4 ceramics were designed to reduce the sintering temperature and optimize the frequencytemperature coefficients of scheelite-type AWO4 ceramics. XRD and Rietveld refinement analyses revealed that the ceramic contained dual phases of BaWO4 and Na0.5Bi0.5WO4. The phases had a tetragonal structure with the I41/a space group, as verified by TEM. With increasing sintering temperature, the ɛr exhibited a decreasing trend owing to the combined influence of ionic polarizability and density. In addition, the variation trend of the Q × f value was similar to that of the density, and its intrinsic influence was investigated by analyzing the FWHM of the Raman vibrational peak. The τf was well optimized and exhibited a variation trend similar to those of the lattice distortion and microstrain. Finally, the (Ba0.2Sr0.2Ca0.2Na0.2Bi0.2)WO4 sintered at 950 °C had the outstanding properties: ɛr of 13.8, Q×f of 33,110 GHz and τf of −34 ppm/°C at 9 GHz.