Fabrication of bimodal porous alumina ceramics

An alumina foam with two kinds of pores was prepared by combining the sponge method and the pore-former method. The large pores were provided by the sponge method, and their sizes were in the range of 1-2 mm. The small pores were produced by the pore-former method with size in the micrometer range. The large pores offered a high porosity while the small pores offered a large surface area. The strength of samples sintered at different temperatures was measured, and the effect of sintering temperature on foam strength was analyzed by discussing porosity and grain bonding area. The sample sintered at 1550 °C has a compressive strength of 1.3 MPa and a porosity of 86%.     

Microstructure and mechanical properties of small amounts of In2O3 reinforced Pb(ZrxTi1−x)O3 ceramics

Piezoelectric Pb(Zr_xTi_1−x)O₃ (PZT) ceramics with small amount (0.5-2.0 wt.%) of In₂O₃ are prepared by conventional sintering method. Based on X-ray diffraction analysis, the tetragonality of PZT matrix decreases with In₂O₃ content, indicating that In₂O₃ diffuses into PZT matrix. The microstructure of PZT matrix is significantly refined by doping small amounts of In₂O₃. The grain size reduction and the matrix grain boundary reinforcement are the probable mechanism responsible for the high strength and hardness in the PZT/In₂O₃ materials. The enhancement in Young’s modulus is attributed to In³⁺ substitution. The decreased tetragonality with In₂O₃ addition results in less crack energy absorption by domain switching and, hence, causes the small reduction in fracture toughness.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

Improvement of SiCf/SiC density by slurry infiltration and tape stacking

SiC fiber-reinforced SiC–matrix ceramic composites (SiC_f/SiC) were fabricated by vacuum infiltration of a SiC slurry into Tyranno™-SA grade-3 fabrics coated with a 200 nm-thick pyrolytic carbon (PyC) layer followed by hot pressing using a transient eutectic-phase. The density of the composite was improved using a special infiltration apparatus with a pressure gradient and alternating tape insertion between fabrics. Their overall properties were compared with those of monolithic SiC and composite containing chopped fibers. Although the density of the composites decreased with increasing fiber fraction, SiC_f/SiC containing 50 vol.% fibers had a density of 3.13 g/cm³, which is the highest reported thus far. The composites containing continuous fibers had a maximum flexural strength of 607 MPa and a step increase in the stress–displacement behavior during the three-point bending test due to fiber reinforcement, which was not observed in the monolith.     

One component metal sintering additive for β-SiC based on thermodynamic calculation and experimental observations

Various types of metals were examined as sintering additives for β-SiC by considering the standard Gibbs formation free energy and vapor pressure under hot pressing conditions (1973-2123 K), particularly for applications in nuclear reactors. Metallic elements having the low long-term activation under neutron irradiation condition, such as Cr, Fe, Ta, Ti, V and W, as well as widely used elements, Al, Mg and B, were considered. The conclusions drawn from thermodynamic considerations were compared with the experimental observations. Al and Mg were found to be effective sintering additives, whereas the others were not due to the formation of metal carbides or silicides from the decomposition of SiC under hot pressing conditions.     

Effects of complex substitution of La and Nd for Bi on the microwave dielectric properties of BiNbO4 ceramics

La₂O₃ and Nd₂O₃ were used to substitute Bi₂O₃ and the effects of complex substitution on the sintering behavior and the microwave dielectric properties of BiNbO₄ ceramics were studied. With 0.5 wt.% CuO-V₂O₅ mixtures addition, all of the Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics could be densified below 920 °C. The triclinic phases are identified in Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics with x=0.01 sintered at 820 °C and the triclinic intensities increase with increasing the x value and sintering temperature. The saturated bulk density slightly decreases from 7.17 to 7.13 g/cm³ and the ε_r value from 44.24 to 42.76 with increasing x from 0 to 0.07 for Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics. The saturated Q×f value is between 10,300 and 12,400 GHz depending on the x value. The τ_f values of dense Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics decrease from 28.32 to 12.79 ppm/°C with x varying from 0 to 0.01 and remain almost unchanged with further increasing x.     

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.     

Influence of cobalt phase on thermal shock resistance of Al2O3-TiC composites evaluated by indentation technique

Cobalt-coated Al₂O₃ and TiC powders were prepared using an electroless method to improve resistance to thermal shock. The mixture of cobalt-coated Al₂O₃ and TiC powders (about 70 wt.% Al₂O₃-Co + 30 wt.% TiC-Co) was hot-pressed into an Al₂O₃-TiC-Co composite. The thermal shock properties of the composite were evaluated by indentation technique and compared with the traditional Al₂O₃-TiC composite. The composites containing 3.96 vol.% cobalt exhibited better resistance to crack propagation, cyclic thermal shock and higher critical temperature difference (ΔT_c). The calculation of thermal shock resistance parameters (R parameters) shows that the incorporation of cobalt improves the resistance to thermal shock fracture and thermal shock damage. The thermal physic parameters are changed very little but the flexure strength and fracture toughness of the composites are improved greatly by introducing cobalt into Al₂O₃-TiC (AT) composites. The better thermal shock resistance of the composites should be attributed to the higher flexure strength and fracture toughness.     

The effect of carbon nanotubes microstructures on reinforcing properties of SWNTs/alumina composite

Alumina reinforced with 1 wt% single-wall carbon nanotubes (SWNTs) was fabricated by hot-pressing. The fracture toughness of SWNTs/Al₂O₃ composite reaches 6.40 ± 0.3 MPa m^1/2, which is twice as high as that of unreinforced alumina. Nanoindentation introduced controlled cracks and the damage were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SWNTs reinforcing mechanisms including CNT pullout, CNT fracture, CNT bridging and crack deflection were directly observed, and the relationship between carbon nanotubes microstructures in the matrix and mechanical properties was also discussed in detailed.     

Preparation of graphene nanosheet/alumina composites by spark plasma sintering

Graphene nanosheet/alumina composite has been prepared by spark plasma sintering. A homogeneous distribution of nanosheets in an alumina matrix could be obtained by the electrostatic attraction between graphite oxide and alumina particles and their subsequent reduction. The introduction of graphene nanosheet leads to refinement of grain size of alumina after hot pressing. The experimental results have shown that the fracture toughness and conductivity of the graphene nanosheet/alumina composite are about 53% and 13 orders of magnitude higher than those of unreinforced alumina material, respectively.     

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.     

Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si₃N₄ green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si₂N₂O was formed through reaction between Si₃N₄ and silica sol at a temperature above 1550 °C. Si₃N₄/Si₂N₂O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180 MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39 GHz) were obtained by varying infiltration cycle and sintering temperature.     

Controlling the oriented growth of Ti2SnC grains with carbon fiber as a reactive template in the Ti-Sn-C system

Carbon short fibers with a length of ∼3 mm used as a reactive template were mixed with Ti and Sn powders. The mixture was pressurelessly sintered at 1200 °C for 2 h in a vacuum atmosphere. The microstructure shows that Ti₂SnC grains with a plate-like shape grow along a preferred direction, forming a ‘string’ structure in which Ti₂SnC platelets pack themselves closely with a top-bottom-top-bottom sequence. With increasing the length of the C fibers, the length of the ‘string’ structures increases. A large amount of long and aligned ‘string’ structures have formed after sintering the samples prepared by infiltration of the carbon short fibers with a length of ∼50 mm in a Ti-Sn slurry and then stacking and cold pressing. X-ray diffraction and scanning electron microscopy, respectively were used to analyze and observe the phase composition and microstructure.     

Fabrication and characterization of highly porous mullite ceramics

Highly porous mullite ceramics were fabricated by a reaction-bonding technique from a powder mixture of Al₂O₃ and SiC, with graphite particles as the pore-forming agent. The effects of sintering temperature on porosity and strength as well as pore size and surface area were investigated. It has been shown that the strength and pore size increase but the porosity and surface area decrease with the increase in sintering temperature. Due to the formation of a fine-grained microstructure with well-developed necks, an average strength up to 106 MPa was achieved at a porosity of 32.4%. On the other hand, a relatively high surface area of 12.4 m² g⁻¹ was obtained for a 61% porous mullite ceramic, which was observed to have a good thermal-shock resistance to crack propagation.     

Shape deformation and texture development of consolidated Ca α-SiAlON ceramics prepared by hot-forging

Consolidated Ca α-SiAlON ceramics with gradually varying microstructure and property was prepared by hot-forging. The shape deformation as well as texture development of the sample during forging was studied in detail. It was found that the forging process promoted the growth of elongated α-SiAlON grains and enhanced their preferred orientation with the longitude perpendicular to the pressing force. The strong texture offered the hot-forged sample an increased fracture toughness of 7.9 MPa m^1/2, which was primarily attributed to the pull-out and debonding behaviors of elongated grains.     

Sintering kinetic studies in nonstoichiometric strontium titanate ceramics

The effect of nonstoichiometry on the densification of SrTiO₃ ceramics with Sr/Ti ratios from 0.997 to 1.02 was systematically addressed. The kinetics of densification was studied by dilatometric analysis. X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for crystallographic and microstructure characterization. Ti excess enhanced matter transport during sintering whereas Sr excess decreased it. The shrinkage rate and average grain size increased with the decrease of Sr/Ti ratio. Close values of the activation energy for the initial densification and the near constant onset temperature for densification suggest that identical transport mechanisms control the densification of all the compositions. Small excesses of TiO₂ and SrO were mostly incorporated into the perovskite lattice inducing alterations in the defect chemistry of the material and the mass transport during sintering is controlled by Sr vacancies. Very small stoichiometric variations have a strong influence on the sintering kinetics and resulting microstructure of ST ceramics.     

Preparation and characterization of Ag-doped BaTiO3 based X7R ceramics

Ag-doped BaTiO₃ based X7R (temperature coefficient of capacitance within the range of ±15% between −55 and +125 °C) ceramics with different amounts of silver (0.0-20.0 mol%) were prepared in this paper. The X-ray diffraction analysis indicated that no phases other than BaTiO₃ and silver were observed in the ceramics. The energy dispersive X-ray spectroscopy analysis showed that the silver particles presented homogeneous distribution in the BaTiO₃ ceramics. The dielectric properties of Ag-doped ceramics were investigated. A small amount of silver (<0.5 mol%) and a large amount of silver (>2.0 mol%) could both improve the sintered density and permittivity, but more content of silver (0.5-2.0 mol%) would decrease the relative density and permittivity. Specially, the temperature coefficient of capacitors of the ceramics doped with 20 mol% silver still met the X7R characteristics, and the room temperature permittivity of the ceramics was 6823, which was the highest dielectric constant in the BaTiO₃ based X7R ceramics.     

Thermal shock resistance of the porous Al2O3/ZrO2 ceramics prepared by gelcasting

Porous Al₂O₃/ZrO₂ ceramics with porosity varying from 6% to 50% were fabricated by gelcasting using polystyrene (PS) as pore-forming agent. The effects of sintering temperature on porosity, strength as well as pore size were investigated. The flexural strength of these porous ceramics at room temperature significantly decreases as the porosity increases. Thermal shock resistance of these ceramics was improved by increasing the porosity. Both the critical difference temperature (ΔT_c) and residual strength of high porosity ceramics were higher than those of low porosity ceramics. These improvements can be attributed to the pores in the specimens which relax the thermal shock stress and arrest the propagation of microcracks effectively, which is confirmed by XRD analysis of specimens which encountered different thermal shock temperature difference.     

Preparation of ZnAl2O4-based microwave dielectric ceramics and GPS antenna by aqueous gelcasting

A novel forming method-aqueous gelcasting was used to prepare 90 wt.% (0.75ZnAl₂O₄-0.25TiO₂)-10 wt.% MgTiO₃ (ZTM) microwave dielectric ceramics and GPS antenna. The effects of aqueous gelcasting and dry pressing on the phase compositions, microstructures and microwave dielectric properties of ZTM ceramics were investigated. The samples’ cracking problem happening in the process of drying and binder removal was successfully overcome. The phase compositions are completely the same no matter what forming method is adopted, but the ZTM ceramics prepared by aqueous gelcasting are denser than that prepared by dry pressing. Fewer pores and more uniform microstructures are observed in the ZTM ceramics prepared by aqueous gelcasting. Therefore, much better microwave dielectric properties are obtained in the ZTM ceramics prepared by aqueous gelcasting. Finally, a GPS antenna was successfully fabricated by aqueous gelcasting using the ZTM material, which meets the requirements of GPS application.     

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.