Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE₂O₃, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y₂O₃, was used as sintering additive of hot-pressed Si₃N₄ ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 °C, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 °C, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol⁻¹, corresponding to the heat of solution of the Si₃N₄ in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y₂O₃-Al₂O₃-doped-Si₃N₄ ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the β-Si₃N₄ grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE₂O₃ can be a cheap alternative in the fabrication of Si₃N₄ ceramics, resulting in promising mechanical properties.     

Reaction sintering fabrication of (AlN, TiN)-Al2O3 composite

The (AlN, TiN)-Al₂O₃ composites were fabricated by reaction sintering powder mixtures containing 10-30 wt.% (Al, Ti)-Al₂O₃ at 1420-1520°C in nitrogen. It was found that the densification and mechanical properties of the sintered composites depended strongly on the Al, Ti contents of the starting powder and hot pressing parameters. Reaction sintering 20 wt.% (Al, Ti)-Al₂O₃ powder in nitrogen in 1520°C for 30 min yields (AlN, TiN)-Al₂O₃ composites with the best mechanical properties, with a hardness HRA of 94.1, bending strength of 687 MPa, and fracture toughness of 6.5 MPa m^1/2. Microstructure analysis indicated that TiN is present as well dispersed particulates within a matrix of Al₂O₃. The AlN identified by XRD was not directly observed, but probably resides at the Al₂O₃ grain boundary. The fracture mode of these composites was observed to be transgranular.     

Synthesis of BaTi4O9 ceramics by reaction-sintering process

Synthesis of BaTi₄O₉ ceramics by a reaction-sintering process was investigated. The mixture of raw materials for stoichiometric BaTi₄O₉ were pressed and sintered into ceramics without any calcination stage involved. Pure BaTi₄O₉ phases were obtained at 1150-1280 °C. High-sintered density, 98.2-99.5% of theoretical value (4.533 g/cm³), can be obtained for pellets sintered at 1200-1280 °C for 2-6 h. Some rod-shaped grains 3-7 μm in the longitudinal axis appear in pellets sintered at 1230 °C. Both the size and the amount of these rod-shaped grains increase at higher sintering temperature.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.     

Effect of the sintering temperature on the properties of Ce0.85La0.10Ca0.05O2−δ electrolyte material

Rare earth and alkaline earth co-doped Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte material with the powder obtained by solid-state reaction method was sintered at 1300, 1400, 1500 and 1600 °C respectively. The results showed that the ionic conductivity of the sample sintered at 1400 °C was slightly lower compared to that sintered at 1500 °C in the temperature range of 300-550 °C, while the sample sintered at 1400 °C showed the highest ionic conductivity in all the samples above 550 °C. The ionic conductivity of ∼0.021 S/cm at 600 °C and the relative density of 98.2% were observed for the sample sintered at 1400 °C. In addition, the highest flexural strength with 145 MPa was also obtained for the sample sintered at 1400 °C. It suggested that the sintering temperature for Ce_0.85La_0.10Ca_0.05O_2−δ electrolyte may be reduced to as low as 1400 °C with desired properties.     

Low-temperature sintering of temperature-stable LaNbO4 microwave dielectric ceramics

We demonstrate the correlation between sintering behavior and microstructural observations in low-temperature sintered, LaNbO₄ microwave ceramics. Small CuO additions to LaNbO₄ significantly lowered the sintering temperature from 1250 to 950 °C. To elucidate the sintering mechanism, the internal microstructure of the sample manipulated by a focused ion beam (FIB) was investigated using transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). LaNbO₄ with 3 wt% CuO sintered at 950 °C for 2 h possessed the following excellent microwave dielectric properties: a quality factor (Qxf) of 49,000 GHz, relative dielectric constant (?_r) of 19.5, and temperature coefficient of resonant frequency (τ_f) of 1 ppm/°C. The ferroelastic phase transformation was also investigated using in situ X-ray diffraction (XRD) to explain the variation of τ_f in low-temperature sintered LaNbO₄ as a function of CuO content.     

Sintering and crystallization of cordierite glass ceramics for high frequency multilayer chip inductors

Cordierite-based ceramics were developed by sintering a glass selected from MgO-Al₂O₃-SiO₂ system with an aim to use the material as high frequency chip inductors. A small amount of B₂O₃ and P₂O₅ were added to optimize the preparation conditions. The glass powder and sintered bodies were characterized by different analytical techniques such as TG-DTA analysis, X-ray diffraction, transmission and scanning electron microscopy. Pellets uniaxially pressed from the glass power could be sintered well at 950 °C having a density of above 99% theoretically, dielectric constant of 5.5, dielectric loss of 0.001 and thermal expansion coefficient of 26.7×10⁻⁷ °C⁻¹ (20-400 °C). Crystalline phases in this sintered sample are predominantly α-cordierite (hexagonal high cordierite) and trace amount of μ-cordierite. SEM depicted a uniformly dense microstructure with crystals of granular habit in the sintered sample.     

Fabrication and mechanical properties of monolithic MoSi2 by spark plasma sintering

Fine MoSi₂ powders containing a small amount of Mo₅Si₃ have been prepared by self-propagating high-temperature synthesis (SHS), followed by spark plasma sintering (SPS) for 10 min at 1200-1500°C and 30 MPa. Dense MoSi₂ materials, in which the grain size is ∼7.5 μm, have been fabricated at 1300°C. They exhibit excellent mechanical properties: Vicker’s hardness H_v (10.6 GPa), fracture toughness K_IC (4.5 MPa m^1/2), and bending strength σ_b (560 MPa). The strength of 325 MPa can be retained up to 1000°C.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO₂ ceramics doped with 0.75 mol% Ca and 2.5 mol% Ta were sintered at different temperatures ranging from 1300 to 1450°C. The effects of sintering temperature on the microstructure, nonlinear electrical behavior, and dielectric properties of the ceramics were studied. The sample sintered at 1300°C exhibits the highest nonlinear coefficient (5.5) and a comparatively lower relative dielectric constant.     

Preparation of nanostructured TiO2 ceramics by spark plasma sintering

The effect of spark plasma sintering (SPS) on the densification of TiO₂ ceramics was investigated using a nanocrystalline TiO₂ powder. A fully-dense TiO₂ specimen with an average grain size of ∼200 nm was obtained by SPS at 700 °C for 1 h. In contrast, a theoretical density specimen could only be obtained using conventional sintering above 900 °C for 1 h with an average grain size of 1-2 μm.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

Study on properties of CaO-SiO2-B2O3 system glass-ceramic

Low temperature co-fired ceramic (LTCC) is prepared by sintering a glass selected from CaO-SiO₂-B₂O₃ system, and its sintered bodies are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It is found that the optimal sintering temperature for this glass-ceramic is 820 °C for 15 min, and the major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. The glass-ceramic possesses excellent dielectric properties: ?_r = 6.5, tan δ < 2 × 10⁻³ at 10 MHz, temperature coefficient of dielectric constant about −51 × 10⁻⁶ °C⁻¹ and coefficient of thermal expansion about 8 × 10⁻⁶ °C⁻¹ at 20-400 °C. Thus, this material is supposed to be suitable for the tape casting process and be compatible with Ag electrode, which could be used as the LTCC materials for the application in wireless communications.     

Far infrared reflectance of sintered nickel manganite samples for negative temperature coefficient thermistors

Single phase complex spinel (Mn, Ni, Co, Fe)₃O₄ samples were sintered at 1050, 1200 and 1300 °C for 30 min and at 1200 °C for 120 min. Morphological changes of the obtained samples with the sintering temperature and time were analyzed by X-ray diffraction and scanning electron microscope (SEM). Room temperature far infrared reflectivity spectra for all samples were measured in the frequency range between 50 and 1200 cm⁻¹. The obtained spectra for all samples showed the presence of the same oscillators, but their intensities increased with the sintering temperature and time in correlation with the increase in sample density and microstructure changes during sintering. The measured spectra were numerically analyzed using the Kramers-Krönig method and the four-parameter model of coupled oscillators. Optical modes were calculated for six observed ionic oscillators belonging to the spinel structure of (Mn, Ni, Co, Fe)₃O₄ of which four were strong and two were weak.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

Microstructure and mechanical properties of titanium carbonitride whisker reinforced β-sialon matrix composites

The microstructure, hardness, fracture toughness and thermal shock resistance were investigated for 15 vol.% TiC_0.3N_0.7 whisker reinforced β-sialon (Si_6−zAl_zO₂N_8−z with z=0.6) composites with additions of three different volume fractions 2, 5 and 20 vol.%, of an yttrium-containing glass oxynitride phase. The composites were prepared by hot pressing at 1750°C for 90 min under a uniaxial pressure of 30 MPa in nitrogen atmosphere. The TiC_0.3N_0.7 whiskers were found to survive without deteriorating in morphology or reacting with the β-sialon matrix and/or the glass phase. The TiC_0.3N_0.7 whiskers had no obvious influence on the matrix microstructure, but their presence improved both the hardness and the fracture toughness of the composites. The highest hardness was obtained for the whisker composite with 2 vol.% glass phase (H_v=18.6 GPa). The fracture toughness and thermal shock resistance improved with increasing glass content. The whisker reinforced composite containing 20 vol.% glass showed the highest fracture toughness (K_1C=6.8 MPa m^1/2). No unstable crack extension occurred during the thermal shock test of the obtained composites in the temperature interval 90-700°C, but above 700°C severe oxidation of the whiskers precludes further evaluation of thermal shock properties by the indentation-quench method applied.     

Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si₃N₄) with lithium yttrium oxide (LiYO₂) and zirconia (ZrO₂) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si₃N₄ grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si₃N₄ grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.