Development of mullite/zirconia composites from a mixture of aluminum dross and zircon

In order to obtain mullite/zirconia composites, mixtures of aluminum dross and zircon were sintered. Aluminum dross was collected and purified by a milling, sieving and washing process. Stoichiometric mixtures of aluminum dross and zircon were sintered at several temperatures (1400, 1450 and 1500 °C) for several periods of time (2, 4 and 6 h). After the purifying treatment the dross contained mainly Al₂O₃, AlN, MgAl₂O₄, SiO₂ and metallic Al. A homogeneous mullite matrix with small zirconia particles was obtained by sintering the aluminum dross–zircon samples at 1500 °C for 6 h.     

Dense mullite zirconia composites obtained from the reaction sintering of milled stoichiometric alumina zircon mixtures by SPS

The main objective of this article is to obtain dense (porosity under 0.5%) polyphasic ceramics belonging to the Al₂O₃–SiO₂–ZrO₂ system by SPS sintering of high energy powders milled drily; the stoichiometric (54.45:45.54 zircon–alumina, weight basis) mixture was explored in this work. Particularly the principal sintering variables: sintering temperature and dwell time were investigated. The textural, structural and microstructural changes were evaluated together with the hardness and toughness of the obtained ceramics and their microstructure. The effect of the mechanical pre-treatment was carried out by X-ray diffraction and particle distribution evaluation. Due to the rapid heating process an incomplete reaction was achieved in several cases, as a consequence multiphasic ceramics with different alumina, mullite, zircon and zirconia contents were obtained.     

In situ formation of zirconia–alumina–spinel–mullite ceramic composites

A study was made to prepare and characterize multicomposite containing zirconia–mullite–spinel [ZrO₂–Al₆Si₂O₁₃–xMgAl₂O₄ (0.25 ≤ x ≤ 4)]. This multicomposite was prepared by reaction sintering of relatively pure commercial grades of zircon, alumina and magnesium carbonate. Spinel and mullite could be in situ formed, while ZrO₂ evolved mainly in monoclinic polymorph. The sintering properties in terms of bulk density (BD) and apparent porosity (AP) were measured. The formed phases were identified by X-ray diffraction pattern (XRD). Cold crushing strength (CCS) of the fired and thermally shocked bodies was evaluated. The fractured surfaces of the fired compacts were scanned by SEM. The results showed great dependence on the firing temperature as well as zircon content, type and amount of glassy phase. XRD exhibited that all samples contain m-ZrO₂ and Al₂O₃ and with decreasing the amount of MgO and increasing zircon, the spinel phase decreases while mullite begins to form at high zircon contents. Batches containing mullite, ZrO₂ and Al₂O₃ gave higher mechanical properties. Also, all samples retained more than 90% of their original strength after being subjected to 20 cycles of thermal shock (Δt = 1000 °C).     

Effect of rare earth oxides on the microstructure and properties of mullite/hBN composites

Mullite/hBN composites were fabricated with different rare earth oxides additives (ReO: Er₂O₃, CeO₂, La₂O₃, Lu₂O₃) by pressureless sintering at 1600 °C for 4 h. The impacts of ReO on the phase composition, microstructure, mechanical, dielectric and tribological properties of the composites were investigated. XRD results showed that all the ReO additives were beneficial to the formation of mullite phase. With the decrease in the ionic radius of the ReO, the mullite grains of the composite were refined while their mechanical properties were increased. The sample sintered with Lu₂O₃ showed the smallest grain size and the most excellent mechanical properties, e.g., its relative density, flexure strength, fracture toughness and hardness reached 93.7%, 222 MPa, 3.2 MPa m^1/2 and 6.02 GPa, respectively. Due to the different porosity and phase composition of the composites, the sample sintered with La₂O₃ had the lowest dielectric constant while the sample sintered with Er₂O₃ exhibited the lowest dielectric loss. Attributing to the highest hardness and fracture toughness, the sample sintered with Lu₂O₃ showed the best tribological properties.     

莫来石加入量对SiC-莫来石材料性能的影响

以SiC颗粒(2～1mm、≤1mm)、SiC细粉(≤0.066mm)、莫来石细粉(≤0.074mm)和SiO2微粉(≤0.045mm)为原料,在w(SiC细粉)=40%的基础配方中分别以质量分数为5%、10%、15%、20%和25%的莫来石细粉取代相应量的SiC细粉,并外加2%的SiO2微粉,成型后分别在1300℃和1500℃的大气气氛中煅烧3h,测定试样煅烧前后的质量变化率以及烧后试样的显气孔率、常温抗折强度和抗热震性,并用扫描电镜观察试样的显微结构。结果表明:在SiC自保护氧化的作用下,试样的抗氧化性较强,而且随着莫来石加入量的增大,1300℃烧后试样的氧化程度减小,1500℃烧后试样的氧化程度先减小,至15%后又增大;1300℃烧后试样比1500℃具有较高的抗热震性,并且随着试样中莫来石加入量的增大,烧后试样的抗热震性提高。     

Effect of MgO addition and sintering parameters on mullite formation through reaction sintering kaolin and alumina

Abstract

In the present work, the influence of MgO addition and sintering parameters on the formation and densification of mullite was investigated. The morphology of powders and the microstructure of the sintered samples were characterised by means of a scanning electron microscope. X-ray diffraction was used to characterise phases formed in sintered samples. The density of sintered samples was measured using a densimeter and quantified according to the Archimedes principle. MgO was added at 1, 2, 3, 4, 5 and 6 wt-% to kaolin and alumina and the powders were ball milled for 5 h then uniaxially compacted at 75 MPa and finally sintered at 1500, 1550, 1600 and 1650°C for 2, 4, 6 and 8 h. It was found that addition of MgO not only affected mullite formation but also promoted grain growth. For samples containing 0, 1 and 2 wt-%MgO only mullite was formed. While, in addition to mullite, Al₂O₃ was present in sample containing 3 wt-%MgO. At higher MgO content (4, 5 and 6 wt-%), three phases, i.e. mullite, Al₂O₃ and spinel, were formed. Addition of 1 wt-%MgO increased the density of all samples for all sintering times and higher densities corresponded to higher sintering temperatures. At higher MgO content, higher temperatures led to lower densities and lower temperatures led to higher densities for almost all sintering times.     

Highly refractory lightweights from zirconia and zircon

Summary Using a simple technology and the method of foaming, it is possible to make zirconia and zircon lightweight refractories possessing excellent heat-insulating properties. Their high refractoriness means they can be used in high-temperature furnaces; this applies especially to zirconia lightweight.     

Effect of glass-ceramics and sillimanite sand additions on microstructure and properties of porcelain

Both quartz and feldspar were progressively replaced by ground sillimanite sand and glass-ceramics in a classical porcelain composition and the effect of these replacements on the vitrification behaviour and mechanical and thermal properties in relation to the microstructure of specimens has been investigated. Progressive additions of sillimanite sand and crystallising glasses leading to the complete replacement of quartz and feldspar increased the strength, toughness and modulus significantly, and drastically reduced the per cent thermal expansion in the composition. The improvement in the properties is attributed to dramatic changes in the microstructural features as a result of significant reduction in the content of glassy phase and the simultaneous increase in crystalline phases such as mullite, sillimanite and cordierite.     

Effect of In-doping on the microstructure and CH4-sensing ability of porous CaZrO3/MgO composites

Porous In-doped CaZrO₃/MgO composites (nominally, CaZr_0.95In_0.05O_2.975/MgO and CaZr_0.90In_0.10O_2.95/MgO) were prepared by pressureless reactive sintering of CaMg(CO₃)₂, ZrO₂ and In₂O₃ with 0.5 mass% LiF additive. The In-doped composites had a uniform open-porous microstructure, similarly to the porous CaZrO₃/MgO composites as reported before, but their pore-size became slightly larger. The CH₄-sensing properties both in argon and air atmospheres were investigated at 600–900 °C. It was found that the In-doping was effective to increase the CH₄-sensitibity in air for all temperature range, whereas it decreased the CH₄-sensitibity in argon. These results can be qualitatively explained by the increase in oxygen vacancies induced by the In-doping.     

Temperature-induced changes in morphology and microstructure of electrospun mullite nanofibers fabricated from a hybrid mullite sol

Mullite nanofibers with small diameter and high surface area are an ideal candidate as the reinforcements in composite materials, and have promising applications in the fields of catalysis, filtration, thermal storage and so forth. In this work, electrospun mullite nanofibers were successfully synthesized using a hybrid mullite sol. The morphology and microstructure of fibers calcined at different temperatures were investigated. The morphology of fibers synthesized at 900 °C is porous with coarse surface, and after crystallization it becomes compact with smooth surface. The densities of fibers increase with the increasing temperatures. At 1200 °C the surface of fibers becomes coarse again, as a result of the grain growth of mullite. The crystallization path of fibers was revealed that the Al-rich mullite (4Al₂O₃·SiO₂) together with amorphous silica formed at 1000 °C, changed into mullite with higher silica contents as temperature further increased, and finally transformed into a stable 3Al₂O₃·2SiO₂ phase at 1200 °C. During this crystallization process, the flow of amorphous silica phase and the formation of mullite crystal structure benefit the densification of fibers, leading to the resultant fibers with fine and compact microstructure. The present findings can provide a guideline for the preparation of the promising high-mechanical mullite nanofibers and the synthesized nanofibers display great potential as reinforcements in structural ceramic composites.     

Synthesis of high-purity zircon,zirconia, and silica nanopowders from local zircon sand

High-purity zircon (ZrSiO₄) nanopowder was successfully produced from Indonesian natural zircon sand using a low-cost purification approach via magnetic separation, immersion in HCl, and reaction with NaOH, followed by a top-down nanosizing process using wet ball-milling for 10?h and annealing at 200?°C for 2?h. Furthermore, polymorph zirconia (ZrO₂ – amorphous, tetragonal, and monoclinic) and silica (SiO₂ – amorphous and cristobalite) nanopowders were also successfully derived from the purified zircon powder using a bottom-up method via alkali fusion and co-precipitation processes followed by calcination. The crystallite size of the powders was estimated from X-ray diffraction (XRD) data analysis to give 40, 31, 61, and 149?nm, respectively, for the zircon, tetragonal- and monoclinic-zirconia, and cristobalite. Microstructural characteristics of the zircon, silica, and zirconia nanopowders were revealed in transmission electron microscopy (TEM) images which confirmed that the average sizes of the particles were in a good agreement with the XRD estimated values.     

Effect of seeds on the formation of sol-gel mullite

Mullite precursor gels have been prepared from a mixture of particulate boehmite sol and tetraethoxysilane at a pH of 4. The sol has been seeded with submicron size -Al₂O₃, γ-Al₂O₃ and mullite limited to two percent by weight of the mullite phase. These gels have been invesitgated with respect to phase formation, densification and microstructural development. Dilatometric studies on the seeded precursor gels indicate marked shrinkage in their profiles. Of the different seed nuclei, fine mullite particles have shown excellent influence in the early ceramic phase formation as well as densification. Further they induce complete transformation of the precursor gel to the high temperature phase as well as a uniform fine-grained microstructure.     

杂质相对MgO润湿行为和显微结构的影响

为了研究不同钙硅比的硅酸盐杂质相对MgO润湿行为及镁质耐火材料显微结构的影响,采用座滴法研究了C/S比(即CaO与SiO2物质的量比,下同)分别为0. 6、1、1. 5和1. 8的硅酸盐杂质相与MgO基板的润湿行为,并用烧结试样验证润湿性与显微结构的关系,用扫描电子显微镜(SEM)观察烧后试样的显微结构。结果表明:1)伴随温度的升高,随着C/S比由小到大,杂质相在MgO基板上由润湿到不润湿; 2)在烧结试样的显微结构中,直接结合程度随着C/S比增大而增大。     

Sintering and microstructural study of mullite prepared from kaolinite and reactive alumina: Effect of MgO and TiO2

Mullite ceramic was prepared using kaolinite and synthesized alumina (combustion route) by solid-state interaction process. The influence of TiO₂ and MgO additives in phase formation, microstructural evolution, densification, and mechanical strengthening was evaluated in this work. TiO₂ and MgO were used as sintering additives. According to the stoichiometric composition of mullite (3Al₂O₃·2SiO₂), the raw materials, ie kaolinite, synthesized alumina, and different wt% of additives were wet mixed, dried, and uniaxially pressed followed by sintering at different temperature. 1600°C sintered samples from each batch exhibit enhanced properties. The 1 wt% TiO₂ addition shows bulk density up to 2.96 g/cm³ with a maximum strength of 156.3 MPa. The addition of MgO up to 1 wt% favored the growth of mullite by obtaining a density and strength matching with the batch containing 1 wt% TiO₂. These additives have shown a positive effect on mullite phase formation by reducing the temperature for complete mullitization by 100°C. Both additives promote sintering by liquid phase formation. However, the grain growth, compact microstructure, and larger elongated mullite crystals in MgO containing batch enhance its hardness properties.     

Sintering and characterization of flyash-based mullite with MgO addition

Mullite ceramics were fabricated at relatively low sintering temperatures (1500-1550 °C) from recycled flyash and bauxite with MgO addition as raw materials. The densification behavior was investigated as function of magnesia content and sintering temperature. The results of thermal analysis, bulk density and pore structure indicate that MgO addition effectively promoted sintering, especially above 1450 °C. Due to the presence of large interlocked elongated mullite crystals above 1450 °C, associated with enhanced densification, an improvement in mechanical strength was obtained for the samples containing magnesia. The addition of magnesia slightly decreases the LTEC at 1300 °C due to the formation of low-expansion α-cordierite, but slightly increases the LTEC above 1400 °C due to the formation of high expansion corundum and MgAl₂O₄ spinel.