Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Enhanced thermal stability in K<Subscript>2</Subscript>O-metakaolin-based geopolymer concretes by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and SiO<Subscript>2</Subscript> fillers addition

Based on the principle of stability of geopolymer gel as refractory binder, a geopolymeric paste in the K₂O–Al₂O₃–SiO₂ system was developed and used to produce refractory concretes by adding various amount of α-quartz sand (grain size in the range 0.1 μm to 1 mm) and fine powder alumina (grain size in the range 0.1–100 μm). The consolidated samples were characterized before and after sintering using optical dilatometer, DSC, XRD and SEM. The total shrinkage in the range of 25–900 °C was less than 3%, reduced with respect to the most diffused potassium or sodium based geopolymer systems, which generally records a >5% shrinkage. The maximum shrinkage of the basic geopolymer composition was recorded at 1000 °C with a 17% shrinkage which is reduced to 12% by alumina addition. The temperature of maximum densification was shifted from 1000 °C to 1150 or 1200 °C by adding 75 wt% α-quartz sand or fine powder alumina respectively. The sequences of sintering of geopolymer concretes could be resumed as dehydration, dehydroxylation, densification and finally plastic deformation due to the importance of liquid phase. The geopolymer formulations developed in this study appeared as promising candidates for high-temperature applications: refractory, fire resistant or insulating materials.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Low-temperature sintering and microwave dielectric properties of Mg<Subscript>3</Subscript>B<Subscript>2</Subscript>O<Subscript>6</Subscript>-LMZBS composites

The Mg₃B₂O₆ ceramics with lithium magnesium zinc borosilicate (LMZBS) glass were prepared at a lower sintering temperature. The effects of the glass addition on the densification, phase development, microstructure and microwave dielectric properties of the Mg₃B₂O₆ ceramics were investigated. The addition of LMZBS glass improved the densification and lowered the sintering temperature of Mg₃B₂O₆ ceramics from 1,300 to 950 °C. X-ray diffraction patterns showed that Mg₃B₂O₆ transformed into Mg₂B₂O₅ and a new phase, Li₂ZnSiO₄, crystallized from the glass phase. Because of the high dielectric performance of these phases, Mg₃B₂O₆ mixed with 55 wt% LMZBS sintered at 950 °C for 3 h had ε_r = 6.8, Q × f = 50,000 GHz, and τ_f = ?64 ppm/°C at 7.28 GHz. The chemical compatibility of ceramic-glass composites with Ag was also investigated for LTCC.     

Microstructure and electrical properties of Pr<Subscript>6</Subscript>O<Subscript>11</Subscript> doped ZnO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-based varistors

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped by Pr₆O₁₁ in the content range of 0–5.49 wt% were investigated at different sintering temperatures (1,100, 1,150, 1,175, 1,200 °C). The increase of sintering temperature leads to more dense ceramics, which increases the nonlinear property, whereas it decreases the voltage-gradient and leakage current. With increasing Pr₆O₁₁ content, the breakdown voltage increases because of the decreases of ZnO grain size. The improvement of non linear coefficient together with the decrease of leakage current are related to the uniformly distribution of secondary phases along the grain boundaries of the ZnO. The varistors sintered at 1,175 °C with the 3.37 wt% Pr₆O₁₁ doping possess the best electrical properties: the varistor voltage, nonlinear coefficient, and leakage current are 340 V/mm, 46 and 0.63 μA, respectively.     

Phase stability,microstructure and high-temperature properties of NbSi<Subscript>2</Subscript>- and TaSi<Subscript>2</Subscript>-oxide conducting ceramic composites

NbSi₂- and TaSi₂-based electroconductive ceramic composites with the addition of 40–70 vol% Al₂O₃ and ZrO₂ particles were fabricated by high-temperature sintering (1400–1600 °C) under argon. Their phase stability, microstructural evolution, oxidation kinetics and electrical properties were studied at high temperatures. The densification of the composites was improved by increasing the oxide phase content and sintering temperature. The interaction of the starting metal disilicides with residual oxygen sources resulted in the formation of the hexagonal-structured 5–3 metal silicide (Nb₅Si₃ and Ta₅Si₃) phases. The increasing sintering temperature and volume percentage of the oxide phase reduced the pest oxidation, particularly for the silicide–alumina composites, which exhibited lower oxidation-induced mass changes than their dense monolithic metal silicides. Depending on the silicide–oxide volume percentage, their electrical conductivities ranged from 5.3 to 111.3 S/cm at 900 °C. Their phase stability, reduced oxidation rates and high electrical conductivities at high temperatures show promise for future high-temperature applications in advanced sensing.     

Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

High temperature flexural strength and fracture toughness of AlN with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic

The effects of temperature on the fast fracture behavior of aluminum nitride with 5 wt% Y₂O₃ ceramic were investigated. Four-point flexural strength and fracture toughness were measured in air at several temperatures (30–1,300 °C). The flexural strength gradually decreased with the increase of temperature up to 1,000 °C due to the change in the fracture mode from transgranular to intergranular, and then became almost constant up to 1,300 °C. Two main flaw types as fracture origin were identified: small surface flaw and large pores. The volume fraction of the large pores was only 0.01%; however, they limited the strength on about 50% of the specimens. The fracture toughness decreased slightly up to 800 °C controlled by the elastic modulus change, and then decreased significantly at 1,000 °C due to the decrease in the grain-boundary toughness. Above 1,000 °C, the fracture toughness increased significantly, and at 1,300 °C, its value was close to that measured at room temperature.     

Hollow biomorphic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> fibers produced from sisal

Al₂O₃ fibers with a hollow morphology were produced by Al-vapor infiltration-reaction and subsequent oxidation from pyrolysed fibers of natural sisal. Following pyrolysis, the bio-fiber template was reacted with gaseous Al at 1,400 °C–1,600 °C in vacuum to form Al₄C₃. After an oxidation/sintering process at 1,550 °C, the biomorphic Al₄C₃ fibers were fully converted into Al₂O₃, maintaining the microstructural features of the native sisal. Phase and microstructural characterization during processing were evaluated by high temperature X-ray diffractometry and scanning electron microscopy, respectively. Thermo-analyses were performed in the Al₄C₃ samples in order to estimate the reactions and the weight change during the oxidation step.     

Low temperature preparation of the Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript> ceramics with the addition of BaO and B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Zn₂SiO₄ ceramics with the addition of BaO and B₂O₃ are fabricated by traditional solid-state preparation process at a sintering temperature of 900 °C. The introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is achieved by adding 10 and 20 wt. % flux BB to the mixed ZnO-SiO₂ ceramic powders pre-sintered at 1,100 °C, respectively. The chemical composition of the flux BB (50 wt.%BaO-50 wt.% B₂O₃) is located at a liquid phase zone with a temperature range of about 869–900 °C in the binary diagram BaO-B₂O₃. In addition, the introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is also achieved by the means of a chemical combination of H₂SiO₃, H₃BO₃, ZnO and Ba(OH)₂·8H₂O, which can result in the formation of the hydrated barium borates with low melting characteristics. In turn, by the liquid sintering aid of the barium borate melts, the preparation process of the Zn₂SiO₄ ceramics can be further simplified. In the two preparation methods, the Zn₂SiO₄ ceramics with the 1.5–2.0 ZnO/SiO₂ molar ratios and the addition of a 10 wt. % flux BB can show good dielectric properties whereas the bending strength mainly depends on the microstructure of the Zn₂SiO₄ ceramics and SiO₂ content in the composition of the specimen.     

Effect of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on formation and superconductivity of bulk MgB<Subscript>2</Subscript>

Bulk materials of MgB₂ have been prepared with the stoichiometry of MgB₂(Al₂O₃) _x (x = 0, 2, 5, 10 and 20% nano-Al₂O₃ powders), by using solid-state reaction route. All samples were sintered at 750 °C for 30 min in a calorimeter to monitor the sintering reaction process. It is found that the onset temperatures of reaction between Mg and B powders increase significantly with increasing the amount of Al₂O₃. However, the reaction time is shortened for the nano-Al₂O₃ powders can effectively activate the reaction as a catalyst. The critical transition temperature decreases from 38.5 to 31.6 K, and the corresponding temperature window becomes narrow (less than 2.6 K). Furthermore, the amount of MgO impurity was found to increase with the increase of Al₂O₃, which probably indicates that partial Mg was replaced by Al.     

Intense blue and violet luminescence in the (B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>):Eu<Superscript>2+</Superscript> system

Glassy-crystalline samples of compositions (B₂O₃-Al₂O₃-SiO₂):Eu²⁺ (3 at %) and (B₂O₃-2SiO₂):Eu²⁺ (3 at %) were obtained by sintering the initial powdered mixtures at 1300°C in air. Being excited by laser radiation at a wavelength of 325 nm, the former samples exhibit intense blue photoluminescence with a maximum at 434–448 nm, while the latter samples emit in the violet spectral interval with a maximum at 409 nm. An increase in the content of B₂O₃ leads to a shift of the emission maximum toward a shorter wavelength, while additional annealing at 1300°C C in vacuum shifts the spectrum toward longer wavelengths.     

Crack propagation in layered Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> composites prepared by electrophoretic deposition

Crack propagation through layered Al₂O₃/ZrO₂ composites was studied. The specimens were prepared via electrophoretic deposition of alumina and zirconia powders from suspensions with monochloroacetic acid and isopropanol. The kinetics of electrophoretic deposition could be described fully if the electrophoretic mobility and conductivity of suspensions were known. The conductivity of suspensions increased in the course of deposition. Adjusting to properly controlled kinetics of deposition and sintering, deposits were prepared with strongly bonded layers of different pre-defined thicknesses and, consequently, with different magnitudes of residual stress. Cracks, produced by an indentation technique, propagated askew towards layer interfaces deflected towards the interface in the Al₂O₃ layers and away from the interface in the ZrO₂ layers. Changes in the direction of crack propagation were described for the whole range of angles of incidence (0°–90°). The biggest change in the crack propagation was observed for the angle of incidence 45° and was ca. 15°, irrespective of the magnitude of residual stress in the layers.     

Preparation of apatite-type La<Subscript>9.33</Subscript>(SiO<Subscript>4</Subscript>)<Subscript>6</Subscript>O<Subscript>2</Subscript> electrolyte by urea-nitrates combustion

Apatite-type La_9.33(SiO₄)₆O₂ powders have been prepared by urea-nitrates combustion at low temperature. Process parameters of combustion and characteristics of electrolyte were studied and optimized. Gelation time of precursor has been shortened distinctly by introducing an appropriate solvent system. Molar ratio of nitric acid to lanthanum oxide dependence of the nature of the phases has first been characterized. Well-crystallized La_9.33(SiO₄)₆O₂ powders with an average size of 30.5 nm were obtained at a calcining temperature as low as 800°C for 12h. Dense ceramic with a relative density of 96% was prepared by sintering the green compact of these nanopowders at 1400°C for 3 h. The sintering body exhibited a high ionic conductivity of 4.38 × 10⁻³ S/cm at 700°C.     

Mechanical properties and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/mullite composite

An Al₂O₃/5 vol.% mullite composite was synthesized by using reaction sintering of Al₂O₃/0.78 wt.% SiC at 1,600 °C for 2 h in air. The phase analysis of the Al₂O₃/mullite composite was carried out using X-ray diffraction (XRD). There were two kinds of mullite in alumina/mullite composite, namely, 3Al₂O₃·2SiO₂ and Al_5.65Si_0.35O_9.175. The microstructure of the Al₂O₃/mullite composite was investigated using scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties such as Young’s modulus, Poisson’s ratio, hardness, toughness and strength of the Al₂O₃/mullite composite were investigated. The influence of mullite on the composite is discussed.     

Varistor properties and aging behavior of ZnO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> ceramics with sintering process

The microstructure, electrical properties, and DC-accelerated aging behavior of the ZnO-V₂O₅-Mn₃O₄ ceramics were investigated at different sintering temperatures of 850–925°C. The microstructure of the ZnO-V₂O₅-Mn₃O₄ ceramics consisted of ZnO grain as a primary phase, and Zn₃(VO₄)₂ which acts as a liquid-phase sintering aid, in addition to Mn-rich phase as secondary phases. The maximum value (3,172 V/cm) and minimum value (977 V/cm) of breakdown field were obtained at sintering temperature of 850 and 900°C, respectively. The nonlinear coefficient exhibited the highest value, reaching 30 at 925°C and the lowest value, reaching 4 at 850°C. The optimum sintering temperature was 900°C, which exhibited not only high nonlinearity with 24 in nonlinear coefficient, but also the high stability, with %ΔE_1mA = −0.9% and %∆α = −12.5% for DC-accelerated aging stress of 0.85 E_1mA/85°C/24 h.     

Investigation of Li<Subscript>2</Subscript>Zn<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>-based temperature stable dielectric ceramics for LTCC applications

A low temperature co-fired ceramic (LTCC) was fabricated at 910 °C /2 h from the powder mixture of Li₂Zn₃Ti₄O₁₂, TiO₂ and a B₂O₃–La₂O₃–MgO–TiO₂ glass (BLMT), and the influence of TiO₂ on microstructure and dielectric properties of the composite was investigated in the composition range (wt%) of 20BLMT–(80???x)Li₂Zn₃Ti₄O₁₂–xTiO₂ (x?=?0, 2.5, 5, 7.5, 9 and 10). The results showed that all samples consisted of Li₂Zn₃Ti₄O₁₂, TiO₂, LaBO₃ and LaMgB₅O₁₀ phase. And LaBO₃, LaMgB₅O₁₀ and a small amounts of TiO₂ were crystallized from BLMT glass during sintering process. As x increases, dielectric constant and temperature coefficient of resonance frequency of the composites demonstrated gradually increase, whereas the quality factor of the sample of x?=?0 wt% was about 41,500 GHz and the ones maintained stable at a high level of 49,000–51,000 GHz for other samples. The composite with x?=?9 wt% had an optimal microwave dielectric properties with the dielectric constant of 20.2, quality factor of 50,000 GHz and temperature coefficient of resonant frequency of ??0.33 ppm/°C.     

Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> co-stabilized ZrO<Subscript>2</Subscript>-WC composites

Y₂O₃ + Nd₂O₃ co-stabilized ZrO₂-based composites with 40 vol% WC were fully densified by pulsed electric current sintering (PECS) at 1350 °C and 1450 °C. The influence of the PECS temperature and Nd₂O₃ co-stabilizer content on the densification, hardness, fracture toughness and bending strength of the composites was investigated. The best combination of properties was obtained for a 1 mol% Y₂O₃ and 0.75 mol% Nd₂O₃ co-stabilized composite densified for 2 min at 1450 °C under a pressure of 62 MPa, resulting in a hardness of 15.5 ± 0.2 GPa, an excellent toughness of 9.6 ± 0.4 MPa.m^0.5 and an impressive 3-point bending strength of 2.04 ± 0.08 GPa. The hydrothermal stability of the 1 mol% Y₂O₃ + 1 mol% Nd₂O₃ co-stabilized ZrO₂-WC (60/40) composites was compared with that of the equivalent 2 mol% Y₂O₃ stabilized ceramic. The double stabilized composite did not degrade in 1.5 MPa steam at 200 °C after 4000 min, whereas the yttria stabilized composite degraded after less than 2000 min. Moreover, the (1Y,1Nd) ZrO₂-WC composites have a substantially higher toughness (~9 MPa.m^0.5) than their 2Y stabilized equivalents (~7 MPa.m^0.5).     

High-temperature oxidation of MoSi<Subscript>2</Subscript>

Oxidation behavior of MoSi₂ was investigated in air over the temperature range of 1400–1700 °C. Spallation of the SiO₂ scale did not occur at any temperature, and Mo₅Si₃ formation did not happen below 1700 °C. A change in the rate-controlling mechanism was detected within the temperature range of this study. Activation energy for oxidation of MoSi₂ at high temperatures was determined to be 204 kJ/mol. This value is less than the value of activation energy for oxidation of MoSi₂ controlled by diffusion of O₂ through amorphous SiO₂ layer reported at lower temperatures. The decrease in activation energy is attributed to the increased degree of crystallization of amorphous silica to β-cristobalite at high temperatures resulting in enhanced O₂ diffusion through SiO₄⁻⁴ tetrahedral structure.