Microwave sintering of cordierite precursor green bodies prepared by starch consolidation

This paper reports on a study of the microwave sintering behavior of green disks prepared by the starch consolidation forming method to produce cordierite-based porous materials. Green disks were formed by thermogelling the aqueous suspensions of talc, kaolin and alumina (29.6 vol.%) and potato starch (11.5 vol.%) at 75 and 85 °C for 4 h, drying and calcining. They were characterized by bulk density and apparent porosity measurements, and SEM. Microwave sintering was carried out at 1300 and 1330 °C for 15, 20 and 25 min, applying 50 °C/min. For purposes of comparison, an analysis of green disks prepared and calcined in the same conditions and conventionally sintered (1330 °C for 4 h) was also made. The materials were characterized by XRD, bulk density and apparent porosity measurements, and microstructurally analyzed SEM. The results were analyzed considering the behavior of starch in aqueous suspension at varying temperatures, and the experimental conditions of consolidation and sintering.     

Porous mullite ceramics with low thermal conductivity prepared by foaming and starch consolidation

Porous mullite ceramics were prepared from an industrial grade mullite powder by foaming and starch consolidation. The viscosities of the original suspensions and the foamed ones with solid loading of 62.5 and 67.5 wt% were measured. After the steps of forming and drying, the green bodies were sintered under different temperatures from 1,200 to 1,600 °C for 2 h. The influence of solid loading of suspension and sintering temperature on the porosity and compressive strength was evaluated. The sintered mullite ceramics, with porosity from 86 to 73 vol% and corresponding compressive strength from 1 to 22 MPa, contained a multi-modal microstructure with large spherical pores and small pores on internal walls. Thermal conductivity measurement carried out by the transient plane source technique at room temperature resulted in values as low as 0.09 W/mK. In addition, the relationship between thermal conductivity and porosity was discussed in detail.     

Two alternative synthesis routes for MnZn ferrites using mechanochemical treatments

Two different methodologies to synthesise Mn_xZn_1−xFe₂O₄ (x = 0.5, 0.65 and 0.85) are compared. In the first method, mechanochemically activated mixtures of elemental oxides were thermally treated at 1100 and 1200 °C under N₂ atmosphere. In the second, previously obtained MnFe₂O₄ and ZnFe₂O₄ were well-mixed and treated at 1100 °C under N₂ atmosphere. Both series of materials were characterised by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), electron probe microanalysis and density measurements. Through a two-step processing, both methods allowed to obtain MnZn ferrites of several compositions, which exhibited high saturation magnetisations and very low coercivities. In this way, in order to overcome some limitations of the conventional preparation technique, alternative synthesis routes for these ferrites are proposed in this study.     

Mechanical evaluation of cordierite precursor green bodies obtained by starch thermogelling

The mechanical behavior of green bodies (porous cordierite precursors) obtained from suspensions of kaolin, talc and alumina powders consolidated by starch thermogelling was studied. Different starches were employed as consolidator/binders: potato, cassava, corn or modified cassava.Aqueous suspensions of the powders (29.6 vol.%) with 11.7 vol.% of starches were prepared by intensive mechanical mixing, homogenization and vacuum degasification. Disks were prepared by thermogelling the suspensions for 4 h at 75–85 °C and additional drying. Green bodies were characterized by bulk density and apparent porosity measurements and microstructural analysis by SEM/EDAX.The mechanical evaluation was carried out by diametral compression in displacement control. Apparent stress–strain relations were obtained from load–displacement curves and several mechanical parameters were determined: mechanical strength, apparent Young modulus and yield stress. Crack patterns were analyzed together with fractographic analysis by SEM. Mechanical results were related to the behavior of the starches in aqueous suspension and the properties of the formed gels.     

沸石分子筛的绿色经济合成研究进展

沸石分子筛具有独特的孔道结构和催化活性,是目前应用较为广泛的催化材料之一。沸石分子筛一般在有机模板剂体系中采用水热晶化法合成,导致合成成本高、生产效率低和环境污染等问题。因此,沸石分子筛的绿色经济合成具有重要意义。主要从合成体系模板剂、合成原料和合成方法重点阐述国内外绿色合成的新进展及目前存在的主要问题。     

玉米淀粉造孔剂对富铝煤矸石合成轻质莫来石料的影响

为了综合利用煤矸石,以天然富铝煤矸石矿为原料,聚乙烯醇(PVA)为结合剂,玉米淀粉为造孔剂,于1 500℃保温3 h反应烧结合成轻质莫来石料,研究了造孔剂加入量(其质量分数分别为10%、20%、30%、40%、50%)对轻质莫来石烧结性能以及显微结构的影响。结果表明:随着玉米淀粉加入量的增大,合成的轻质莫来石料体积密度明显降低,显气孔率和线收缩率显著增大,玉米淀粉的添加量以40%(w)为宜,此时合成的轻质莫来石料的体积密度和显气孔率分别为1.21 g·cm-3和62%;合成的轻质莫来石料的主晶相为莫来石,次晶相为刚玉,莫来石晶体呈柱状发育,刚玉晶粒被莫来石晶粒包裹,构成了疏松的空间网络结构。     

Porous mullite honeycomb by hydrothermal treatment of fired kaolin bodies in NaOH

A conventional hydrothermal treatment with various concentrations of NaOH was used at 150° and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin body. The effect of hydrothermal treatment time on the dissolution of the glass was examined. At 150°C, the dissolution of glass was almost complete after treatment for 8 hrs in 5N-NaOH solution and about 40–43 wt% of the glass was removed from the fired kaolin body leading to porous mullite. However, when the fired kaolin body was treated for more than 5 hrs in 5N-NaOH at 190°C, a composite of mullite and a nonporous crystalline phase of unknown symmetry resulted. These crystals formed from the dissolution and recrystallization of the glass. After the dissolution of glass in 2N-NaOH solution at 190°C for 5 hrs, a porous mullite body of 52.8% porosity with an average pore diameter of 0.57 m could be obtained, and this was only composed of mullite whiskers. Growth of unidentified nonporous crystals in the body which was treated in 5N-NaOH solution at 190°C led to a decrease in specific surface area and therefore, these crystals should be avoided.     

Macroporous mullite materials prepared by novel shaping strategies based on starch thermogelation for thermal insulation

A rigorous microstructural analysis of porous mullite materials developed using novel shaping strategies based on the starch consolidation casting, and their thermal properties in relation to the processing and starch type were accomplished in view of their use as thermal insulators. In order to characterize the size and morphology of pore, basic size and 2D shape factors, and global 3D stereological parameters were determined using microscopy techniques. Results indicated that the porosity volume, pore connectivity degree, and mean free path were the determining factors of the lowest heat transfer by conduction registered in materials prepared with cassava starch. This material is the best candidate to be used in thermal insulation.     

Properties of sintered mullite and cordierite pressed bodies manufactured using Al-rich anodising sludge

Mullite and cordierite based refractory ceramic materials were produced from formulations based on an industrial Al-rich sludge derived from the wastewater treatment of the aluminium anodising process. Some common ceramic raw materials, like ball clay, kaolin, diatomite and talc, were also added. Cylindrical samples processed by uniaxial dry pressing were sintered at different temperatures (between 1300 and 1650 °C, 1 h soak at each temperature) to study the ceramic properties.     

固相含量对淀粉原位凝固成型氧化铝陶瓷工艺中浆料流变性能及坯体性能的影响

探讨了固相含量对淀粉-Al2O3悬浮浆料的流变特性、固化行为、成型坯体显微结构和性能的影响。通过酯化淀粉原位凝固成型工艺可成型出结构完好、致密度较高的陶瓷坯体。研究表明浆料表观黏度随固相含量的增加而增加,但即使是固相体积分数为58%的高浓悬浮浆料,表观黏度仍小于1mPa·s,易于注模;随固相含量的增加,坯体的线收缩率和干燥强度下降,而坯体的相对密度不断增加;固相含量对坯体的微观结构和气孔分布也有显著影响。     

Thermal conductivity of ceramic green bodies during drying

The thermal conductivity of alumina and kaolin green bodies has been studied as a function of the water loss during drying. Experimental measurements show strong variations with 3 distinct regimes. In the first regime, thermal conductivity increases during shrinkage. When shrinkage stops, a decrease in thermal conductivity with water loss is observed which becomes even stronger during the last phase of drying. This can be explained by the variations in the volume fractions of each phase and the effective thermal contacts between grains. Using analytical relations, the thermal resistance of an equivalent plane of small area grain-grain contacts is shown to increase strongly at the end of drying due to the removal of water. Finally, in certain drying conditions, if a portion of the heat required for drying, is supplied by conduction through the green body, then the rate of water evaporation increases with higher thermal conductivity.     

Preparation of homogeneous mullite‐based fibrous ceramics by starch consolidation

Silica sol is one of the frequently used binders in high‐temperature resisting fibrous porous ceramics, but in the drying process, it can diffuse with water and influence the uniformity of ceramics. To solve this diffusion problem and fabricate homogeneous fibrous porous ceramics, cationic starch was firstly introduced in mullite fibrous system. The effects of starch content and high‐temperature binder content on microstructure, physical, and mechanical properties were also investigated. The results indicate that starch consolidated mullite fibrous ceramics owned a homogeneous 3D skeleton structure, since the introduced starch can absorb both water and silica particles by gelatinization and ensure the even distribution of binders. Compared with the mullite fibrous ceramics fabricated without starch addition, starch consolidated ceramics owned better microstructure and higher interior compression strength. Furthermore, both the starch and silica sol content had great impact on the microstructure, density, porosity, thermal conductivity, and compressive strength of the fibrous ceramic.     

Preparation and characterization of porous alumina ceramics through starch consolidation casting technique

This work aims at studying the influence of thermal treatment on the microstructure, resistivity and technological properties of porous alumina ceramics prepared via starch consolidation casting (SCC) technique. Colloidal suspensions were prepared with three different contents of alumina solid loading (55, 60 and 65 mass%) and corn starch (3, 8 and 13 mass%). The sintered samples at 1400, 1500, 1600 and 1700 °C, show open porosity between 46 and 64%, depending on the starch content in the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM. The effect of corn starch content on the apparent porosity, pore size distribution, linear shrinkage and electrical resistivity as well as cold crushing strength of the sintered porous alumina ceramics was also measured. These porous alumina ceramics are promising porous ceramic materials for using in a wide range of thermal, electrical and bioceramics applications as well as filters/membranes and gas burners, due to their excellent combination properties.     

Distribution of water in ceramic green bodies during drying

In order to investigate drying mechanisms at different stages, the distribution of water within the ceramic green bodies at different scales has been examined. The experimental measurements, using a simple weighing technique and Magnetic Resonance Imaging (MRI), show that during the first stage of drying involving shrinkage the material is constituted of uniquely solid and water with no gradient in water content within the sample. Then, during the second stage of drying, significant differences of water content as a function of position appear. As a complement, at the grain scale, observations using environmental scanning electron microscopy were made giving useful information on the solid–liquid–gas interfaces in the near surface part of the green body. Finally, the gradients in the water distribution were exploited to make a simple estimate of the diffusion coefficient of water with its dependence on the moisture content.     

Development of mullite fibers and novel zirconia-toughened mullite fibers for high temperature applications

Polycrystalline mullite fibers and novel zirconia-toughened mullite (ZTM) fibers with average diameters between 9.7 and 10.3 μm containing 3, 7 and 15 wt.-% tetragonal ZrO₂ (ZTM3, ZTM7, ZTM15) in the final ceramic were prepared via dry spinning followed by continuous calcination and sintering in air. A shift in the formation of transient alumina phases and tetragonal ZrO₂ to higher temperatures with increasing amounts of ZrO₂ was observed. Concomitantly, the mullite formation temperature was lowered to 1229 °C for ZTM15 fibers. X-ray diffraction revealed formation of the desired tetragonal crystal structure of ZrO₂ directly from the amorphous precursor. Room temperature Weibull strengths of 1320, 1390 and 1740 MPa and Weibull moduli of 9.5, 7.1 and 9.0 were determined for mullite, ZTM3 and ZTM15 fibers, respectively. Average Young’s moduli ranged from 190 to 220 GPa. SEM images revealed crack-free fiber surfaces and compact microstructures independent of the amount of ZrO₂.     

Process control and optimized preparation of porous alumina ceramics by starch consolidation casting

This work concerns details of porosity and pore size control in starch consolidation casting of alumina ceramics using corn starch. In particular, the influence of the solids loading (68-78 wt.% alumina in suspensions with nominal starch contents of 20-50 vol.%) on the porosity, bulk density and shrinkage of alumina ceramics is studied. The results indicate a linear decrease of the linear shrinkage and the bulk density (and a corresponding increase in porosity) as the alumina concentration increases, with slopes that are independent of the starch content. The pore size is characterized via microscopic image analysis, the pore throat size via mercury porosimetry. Relations between the volumetric shrinkage, porosity and the volume fractions of starch and water in the suspensions are discussed, and a new concept, called “affine limit porosity” is proposed to explain the apparently paradoxical finding that the porosity increases with increasing alumina content in the suspension.     

Preparation of porous Al2O3 ceramics by starch consolidation casting method

Porous alumina ceramics were fabricated by starch consolidation casting using corn starch as a curing agent while their microstructure, mechanical properties, pore size distribution, and corrosion resistance were examined. Results showed that the porous alumina ceramics with the flexural strength of about 44.31MPa, apparent porosity of about 47.67% and pore size distribution in the range of 1‐4 μm could be obtained with 3wt% SiO₂ and 3wt% MgO additives. Corrosion resistance results showed mass losses: hot H₂SO₄ solution and NaOH solution for 10 hours were 0.77% and 2.19%, which showed that these porous alumina ceramics may offer better corrosion resistance in acidic conditions.     

Influence of cassava starch content and sintering temperature on the alumina consolidation technique

The influence of starch content and sintering temperature on the preparation of alumina bodies were studied. The process was water-based and cassava starch was used as consolidator, binder and pore former. Colloidal suspensions were prepared with three different starch concentrations and the ideal dispersant content and gel point were determined by rheological analysis. The wet samples were demolded after consolidation in silicone mold at 60 °C for 2 h. After the drying step the samples were sintered at 1200, 1400 and 1600 °C, showing open porosities between 13 and 55%, depending on the starch content on the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM (scanning electron microscopy) and Hg porosimetry. Basically, the pore structures are dominated by large spherically shaped pores left by the starch particles, which are connected through small pore channels.     

Conformable green bodies: Plastic forming of robocasted advanced ceramics

Robocasting, or the additive manufacturing of ceramics by continuous extrusion of a ceramic paste, has limited capabilities when printing complex unsupported structures such as overhangs or free standing thin artefacts. In this paper we address this limitation using a new type of paste, which allows for shaping of the green bodies after printing. To illustrate the flexibility of the paste, it was used to produce both alumina and silicon carbide parts. The paste consists of a solution of phenolic resin in methyl ethyl ketone and ceramic powders. Fabricated parts can be cut, bent, folded and draped over various objects. Once dry and fully solid, the parts become rigid and can be processed further by slow pyrolysis and sintering. Sintered samples exhibit flexural strength comparable to both conventionally produced and robocasted ceramics and shaping of the green bodies after printing does not affect the mechanical strength of the sintered parts.     

Gel casting of mullite for radome applications

Gel-casting method was used to fabricate mullite ceramics because this method has not been applied to fabricate dense mullite in literature. Gel-casting parameters such as monomer types (acrylamide and metyhlenebisacrylamide), effect of initiator (ammonium persulfate), and catalyst (tetramethylenediamine) were studied. All samples reached to relative densities of 97% to 98.2% after sintering at 1540°C for 2 hours. Backscattered SEM images revealed anisotropically grown mullite grains, glassy phase located at triple junctions, hexagonally-shaped and faceted Al₂O₃ grains, and white-color grains consisting of ZrO₂ and undissolved TiO₂. A Weibull modulus of 12.33 (ie, indicating tough ceramics) and characteristic strength of 186 MPa were calculated, below which no failure was expected. A critical quenching temperature was found as about 400°C (ΔT ~ 380°C) by indentation-quench method. In addition, a thermal shock resistance parameter (eg, R in °C) of 142.4°C was calculated, which was higher than commonly used Al₂O₃ ceramics. Lower dielectric constant and loss are desired for wide frequency band application and shorter signal transmission delay time in radomes. The dielectric constant was found as nearly 7.5 up to 10 GHz and loss tangent was 0.0031 at 5 MHz. Dielectric and thermomechanical results suggest that mullite is a suitable candidate ceramic for radome applications.