Porous mullite–ZrO2 composites from reaction sintering of zircon and aluminum

Mullite–zirconia porous bodies were prepared by reaction sintering of zircon and alumina derived from oxidation reaction of Al at sintering temperatures between 1200 and 1600 °C. The results show that the incorporation of TiO₂ improves the oxidation reaction of Al, dissociation of zircon subsequently formation of mullite and zirconia. Composites containing TiO₂ obtain a high tetragonal concentration at 1500 °C, which reduces by increasing sintering temperature to 1600 °C. No tetragonal zirconia phase was detected at 1500 °C in TiO₂-free composites while tetragonal concentration was increased over this temperature. The major oxidation reaction of Al proceeds with a liquid–gas mechanism that is suitable for producing low dense ceramics. In spite of the higher porosity of the composites containing TiO₂, they possess almost the same flexural strength values as obtained from the TiO₂-free composites.     

Synthesis and densification of magnesium aluminate spinel: effect of MgO reactivity

Stoichiometric magnesium aluminate spinel was synthesized by reaction sintering of alumina with caustic and sintered magnesia. The volume expansion of 5–7% during MgAl₂O₄ formation was utilized to identify the starting temperature of spinel formation and densification by high temperature dilatometry. The magnesia reactivity was determined by measurement of crystallite size and specific surface area. Caustic magnesia and sintered magnesia behave differently vis-à-vis phase formation and densification of spinel. Densification of stoichiometric Mag-Al spinel was carried out between 1650 and 1750 °C. Attempts were made to correlate the MgO reactivity with microstructure and densification of spinel.     

Improvement in performance of MgO–CaO refractories by addition of nano-sized ZrO2

MgO–CaO refractories added with different sized ZrO₂ powders were sintered at 1600 °C, the effect of the ZrO₂ powders on performance of MgO–CaO refractories was investigated. The results showed that the densification of the MgO–CaO refractories was appreciably promoted when a small amount of ZrO₂ was added owing to the formation of small size CaZrO₃ facilitated to sintering, and the densification was promoted further with increasing the amount of ZrO₂ due to the volume expansion caused by the reaction of the added ZrO₂ and CaO to form CaZrO₃ in the refractories, and the addition of nano-sized ZrO₂ was more effective. The thermal shock resistance of the MgO–CaO refractories was improved by modification of the microstructure due to the formed CaZrO₃ particles that predominately located on the grain boundaries and triple points in the whole microstructure, and the addition of nano-sized ZrO₂ was more effective attributed to its well dispersion and the critical addition amount was effectively decreased to 6%. The slaking resistance of the MgO–CaO refractories was appreciably improved by addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of microstructure, the nano-sized ZrO₂ addition was more effective due to its higher activity. The slag corrosion resistance of the MgO–CaO refractories was enhanced by addition of ZrO₂ due to the increase of the viscosity of the liquid phase and thus inhibited further penetration of slag at elevated temperatures.     

Low-temperature reaction-sintering of mullite ceramics with an Y2O3 addition

Mullite ceramics were fabricated at relatively low temperatures from powder mixtures of -Al₂O₃ and quartz, with an Y₂O₃ addition. The mullitization process was analyzed by X-ray diffraction. The densification behavior was investigated as a function of the Y₂O₃ content, sintering temperature and holding time as well as mullite seeds. It has been shown that mullitization occurs via a nucleation and growth mechanism within an yttrious aluminosilicate glass, but lattice and grain-boundary diffusion becomes important during the densification process. Moreover, the incorporation of mullite seeds was observed to enhance both mullitization and densification. At 1400°C for 5 h or 1450°C for 2 h, 15 mol% Y₂O₃-doped and 5 mol% mullite-seeded specimens can be sintered to almost full density.     

Phase change and electrical resistivity of Zn–Mn–Ni–O-based NTC thermistors produced using IZC powder recycled from used dry batteries

Zn–Mn–Ni–Oxide-based NTC thermistors with variable Ni/Mn ratios were fabricated from powder mixtures of recycled IZC, and commercial MnCO₃ and NiCO₃. Solid phases and electrical resistivity of each sintered sample were studied as a function of Ni/Mn ratio, sintering temperature and sintering time. At 1200 °C for 2 h, samples with the Ni/Mn ratios of 0.38 and higher were found to consist of cubic spinel as a major phase. After sintering at 1250 °C for 10 h, densification proceeded with a phase change from cubic spinel to tetragonal one. The electrical resistivity of the samples obtained at 1200 °C for 2 h progressively decreased with an increasing Ni/Mn ratio up to 0.38, at which the value became the lowest (4.2 × 10³ Ω cm at room temperature) of all the samples fabricated.     

Piezoelectric properties of Pb(Mn1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics with sintering aid of 2CaO–Fe2O3 compound

Since the electromechanical devices move towards enhanced power density, high mechanical quality factor (Q_m) and electromechanical coupling factor (k_p) are commonly needed for the high powered piezoelectric transformer with Q_m≥2000 and k_p=0.60. Although Pb(Mn_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMnN–PZ–PT) ceramic system has potential for piezoelectric transformer application, further improvements of Q_m and k_p are needed. Addition of 2CaO–Fe₂O₃ has been proved to have many beneficial effects on Pb(Zr,Ti)O₃ ceramics. Therefore, 2CaO–Fe₂O₃ is used as additive in order to improve the piezoelectric properties in this study. The piezoelectric properties, density and microstructures of 0.07Pb(Mn_1/3Nb_2/3)O₃–0.468PbZrO₃–0.462PbTiO₃ (PMnN–PZ–PT) piezoelectric ceramics with 2CaO–Fe₂O₃ additive sintered at 1100 and 1250 °C have been studied. When sintering temperature is 1250 °C, Q_m has the maximum 2150 with 0.3 wt.% 2CaO–Fe₂O₃ addition. The k_p more than 0.6 is observed for samples sintered at 1100 °C. The addition of 2CaO–Fe₂O₃ can significantly enhance the densification of PMnN–PZ–PT ceramics when the sintering temperature is 1250 °C. The grain growth occurred with the amount of 2CaO–Fe₂O₃ at both sintering temperatures.     

Design of 7 wt.% Y2O3–ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance

Plasma-sprayed stand-alone coatings of 7 wt.% Y₂O₃–ZrO₂ (YSZ), nominally 74 wt.% Al₂O₃–26 wt.% SiO₂ mullite, and a 46:54 volume ratio composite of YSZ to mullite were examined using X-ray diffraction, dilatometry, and compression creep. X-ray diffraction and dilatometer results showed that the as-sprayed predominantly amorphous mullite crystallized at 970 °C. Creep tests were conducted on all three coating types in the as-sprayed condition at stresses from 40 to 80 MPa and temperatures of 1000–1200 °C. The primary deformation mechanism in coatings made from all three materials was stress-assisted densification of the porous coating. While the creep behavior of YSZ/mullite composite specimens was between that of pure YSZ and pure mullite specimens for all combinations of temperature and stress tested, the creep response of the composite was more similar to that of pure mullite for all cases tested, consistent with mullite being the continuous phase in the composite.     

Effect of atmosphere on the reaction sintering of Si2N2O

The reaction sintering of Si₂N₂O from an equimolar mixture of Si₃N₄ and SiO₂ with 5 wt% Al₂O₃ addition was investigated in 98 or 980 kPa N₂ at 1600–1850°C. At the initial stage, Si₃N₄ densification occurred through a liquid phase of SiO₂---Al₂O₃ system. Further densification was observed together with the formation and exaggerated grain growth of Si₂N₂O. High N₂ pressure was useful for the prevention of thermal decomposition of Si₂N₂O and bloating of the compact. Among various packing powders, which have various SiO partial pressures, an equimolar mixture of Si₃N₂O and SiO₂ was the most effective for the densification. The effect of N₂ and packing powder on reaction sintering of Si₂N₂O was discussed in relation to observed kinetics and thermodynamic calculations. Bending strength of sintered materials was 310–320 MPa.     

Densification and phase transformation of Si3N4 in the presence of mechanically activated BaCO3–Al2O3–SiO2 mixture

Densification as well as the →β phase transformation of Si₃N₄ were monitored as a function of activation time of the BaCO₃–Al₂O₃–SiO₂ additive mixture. The composition of the ternary mixture corresponded to celsian (BaAl₂Si₂O₈—BAS). Previously, mechanically activated powder mixtures for various lengths of time were added to Si₃N₄ in the amount of 10–30%. Sintering was performed at 1650–1700°C in nitrogen atmosphere up to 8 h. The changes in densification degree, as well as phase composition, were followed as a function of heating time and the time of mechanical activation of the additive mixture. The results obtained showed that mechanical activation retarded densification in samples heated up to 1700°C. On the other hand, for the constant sintering time, →β transformation of Si₃N₄ was enhanced with increasing activation time, and the amount of additives.     

Minimum amount of nano-sized nickel particles to enhance the strength of alumina

Previous studies demonstrated that the strength of alumina could be enhanced by incorporating >5 vol.% nano-sized nickel particles. In the present study, the possibility of using a much smaller amount, <0.5 vol.%, of nano-sized Ni particles to improve the mechanical properties of Al₂O₃ is explored. As the Ni content is low, the densification of Al₂O₃ is affected little and the Ni inclusions remain small after pressureless sintering at 1600 °C. The presence of Ni inclusions can refine the matrix grains; the strength of Al₂O₃ matrix is consequently enhanced.     

Sintering of tricalcium phosphate–fluorapatite composites with zirconia

Zirconia (ZrO₂) addition effects on densification and microstructure of tricalcium phosphate–26.52 wt% fluorapatite composites were investigated, using X-ray diffraction, scanning electron microscopy and by analysis using ³¹P nuclear magnetic resonance. The tricalcium phosphate–26.52 wt% fluorapatite–zirconia composites densification increases versus temperature. At 1300 °C, the composites apparent porosity reaches 9% with 5 wt% zirconia. XRD analysis of the composites reveals the presence of tricalcium phosphate, fluorapatite and zirconia without any other structures. Above 1300 °C, the densification was hindered by grain growth and the formation of both intragranular porosity and new compounds. The ³¹P MAS-NMR analysis of composites sintered at various temperatures or with different percentages of zirconia reveals the presence of tetrahedral P sites. At 1400 °C, XRD analysis of the tricalcium phosphate–26.52 wt% fluorapatite–20 wt% zirconia composites shows the presence of calcium zirconate and tetracalcium phosphate. This result indicated that partial decomposition of tricalcium phosphate during sintering process of composites when 20 wt% or less ZrO₂ was added. Thus, zirconia reacts with tricalcium phosphate forming calcium zirconate and tetracalcium phosphate.     

Effects of precursor pH and sintering temperature on synthesizing and morphology of sol–gel processed mullite

Needlelike mullite, which shows high aspect ratios, has been synthesized by coprecipitation from aluminum nitrate enneahydrate and colloidal silica sols. The effects of precursor pH and sintering temperature on the synthesizing behavior and the morphology of the needlelike mullite have been investigated. The equilibrium mullite phase appeared in the samples, which had been sintered at and above 1200 °C for each precursor pH. Crystallization of cristobalite from excessive SiO₂ occurred during sintering from 1200 °C, and the amount of cristobalite increased with sintering temperature up to 1400 °C. However, resorption of cristobalite into mullite decreased the amount of the phase from 1500 °C. The mullite synthesized with precursor pH2 (acidic sample) displayed high aspect ratios at high sintering temperature. However, morphology of the mullite synthesized with precursor pH8 (basic sample) transformed to rodlike or granular shape with increasing sintering temperature.     

Colloidal processing of a mullite matrix material suitable for infiltrating woven fibre preforms using electrophoretic deposition

Commercially available alumina and silica precursors for the preparation of mullite ceramic via colloidal processing and viscous transient sintering have been identified, including fumed nanosize powders and colloidal suspensions. These materials were chosen due to the fact that they can be used in the form of a sol, as mullite matrix precursors, to infiltrate woven fibre preforms using electrophoretic deposition. The sintered density of the mullite matrices sintered for 2 h, at the upper temperature for fabricating SiC-fibre reinforced composites (1300 °C) is only ≈ 90% of theoretical. However, by exploiting a viscous flow densification mechanism, it is envisaged that hot-pressing can be used to produce fully dense mullite matrix composites at the required temperatures. Additionally. using a simple pressureless sintering route, almost fully dense (98% of theoretical density) monolithic mullite has been obtained from the pre-mullite powders. A very homogeneous and fine microstructure was achieved by sintering for 5 h at a temperature of ≈ 1450 °C.     

Alumina ceramics based on seeded boehmite and electrophoretic deposition

It is shown that seeding a commercial boehmite sol with crystallographically suitable seeds reduces both the crystallization temperature for the final -A1₂O₃ phase and the sintering densification temperature. The seeding component was a tailored combination of δ- and -alumina particles in nanometre and micrometer ranges, respectively, dispersed in water. Electrophoretic deposition (EPD) provided a quick, simple and cost-effective processing route to prepare dense monolithic alumina ceramics from the seeded boehmite suspensions. EPD-formed green bodies could be sintered and densified at temperatures as low as 1250 °C for 2 h.     

Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Catalytic activity of ZrO₂ supported PdO catalysts for methane combustion has been investigated in comparison with Al₂O₃ supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO₂ support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al₂O₃ catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO₂ catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO₂ catalyst. In order to confirm the formation of the solid solution of PdO and ZrO₂, X-ray diffraction (XRD) analyses of the mixtures of ZrO₂ and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO₂ in the mixture was larger than that of ZrO₂. Furthermore, the Pd thin film on ZrO₂ substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO₂ support might be stabilized by the formation of the solid solution of PdO and ZrO₂.     

Effects of additives on sintering of CVD-MgO powders

The effects of a large number of sintering aids for the densification of magnesia were examined. Al₂O₃, BaO, Fe₂O₃, P₂O₅, SiO₂, TiO₂, Y₂O₃ and ZrO₂ are effective for the sintering of CVD-MgO powders at low doping levels. The effects of TiO₂ and ZrO₂ are significant. Heavy doping is harmful for densification. The eight oxides above are also effective for the sintering of seawater MgO, but the degree of effectiveness is smaller than for CVD-MgO. In the doping of BaO, P₂O₅, SiO₂ and TiO₂, which form eutectic liquids with MgO below 1600°C, there is an optimum firing temperature for densification.

Vickers hardness of doped MgO is proportional to the relative density and is unaffected by doping. Corrosion resistance of MgO ceramics for liquid PbO is also unaffected by dopants, except for P₂O₅.     

Application of pulse discharge sintering (PDS) technique to rapid synthesis of Ti3SiC2 from Ti/Si/C powders

To synthesize Ti₃SiC₂ samples, pulse discharge sintering (PDS) technique was utilized to sinter elemental powders of Ti/Si/C with stoichiometric and off-stoichiometric ratios in a temperature range of 1200–1500 °C. The results showed that high purity Ti₃SiC₂ could not be obtained from the Ti/Si/C powder with molar ratio of 3:1:2, and Ti₃SiC₂ preferred to form at relatively low sintering temperature for a short time. When 5Ti/2Si/3C and 3Ti/1.5Si/2C powders were sintered for 15 min, the TiC content was respectively decreased to 6.4 and 10 wt.% at 1250–1300 °C. The corresponding relative density of the samples sintered from 5Ti/2Si/3C powder was calculated to be as high as 99% at the temperature above 1300 °C. It is suggested that low-temperature rapid synthesis of Ti₃SiC₂ would be possible through the PDS technique, provided that the composition of the starting powders should be adjusted to be off-stoichiometric ratio from 3:1:2.     

Extrusion of alumina fibers using zirconia sol as binder

Zirconia sol was used as an extrusion aid for the preparation of alumina fibers. The amount of zirconia sol required for getting an extrudable slurry has been optimised. The only phase present in the dried and sintered fibers is -Al₂O₃. Sintering studies showed that a dense microstructure is formed at 1600 °C. SEM micrographs revealed intergranular fracture to be the predominant fracture mode. Tensile strength is the highest for fibers sintered at 1600 °C.     

Role of wollastonite additive on density, microstructure and mechanical properties of alumina

Wollastonite is a low fusing material and has a considerable effect on the densification of alumina bodies at sintering temperatures ranging from 1300 to 1450 °C. The wollastonite acts as a flux and accelerates the liquid forming process leading to lower temperature densification. The addition of 1.0 wt.% wollastonite alters the microstructure to form well-defined grains with an average grain size in the range of 3.0–5.0 μm. The grains are distributed uniformly and the fracture occurs along the grain boundaries and the microstructure reveals the presence of highly dense and sintered grains. The hardness and bend strength for 0–5.0 wt.% wollastonite increases from 8.5 to 14 GPa and from 119 to 249 MPa respectively at a sintering temperature of 1400 °C.     

Phase diagram of the system: Al2O3–ZrO2

The system Al₂O₃–ZrO₂ was studied by differential thermal analysis in inert atmosphere and in vacuum. The eutectic was located at 1866°C and 40% mass of ZrO₂. Zirconia solid solution at the eutectic temperature is up to 1.1±0.3% mass of Al₂O₃. Enthalpy of melting of this eutectic is 1080±90 J/g. Pure ZrO₂ transforms from monoclinic to tetragonal at 1162±7°C, but the saturated solid solution of ZrO₂, with 0.7±0.2% mass Al₂O₃ at this temperature, transforms at 1085±5°C. Inverse transitions occur with hysteresis correspondingly at 1055±5 and 995±5°C. Enthalpy of transformation of pure ZrO₂ from monoclinic to tetragonal phase is 42±5 J/g (5.2±0.6 J/mol) but only 30±5 J/g for a ZrO₂ saturated solid solution.