Slip casting molds produced by hot isostatic process

Slip casting can produce large green bodies of fairly complex shape with high homogeneity. In this work, porous alumina produced by hot isostatic process (HIP) is evaluated to apply as slip casting molds. HIPed porous alumina molds have higher water suction rate than conventionally sintered ones with a same open porosity. The high water suction rate of HIPed molds is due to the low specific surface area of HIPed porous materials. The high water suction rate of HIPed porous alumina molds allows high casting rates.     

Restraining of calcium contamination in near‐net shape alumina ceramics during slip casting

The traditional method for shaping ceramics is by slip casting on gypsum molds; however, its application for near‐net shaping of ceramic components is limited due to contamination by calcium ions. The focus of this study is the modification of the mold to limit Ca²⁺ contamination and to maintain favorable sucking properties. Cement was added to a standard gypsum mold to suppress its erosion, and a decrease in the sucking rate was observed due to its reduced macroporosity. The highest values of green densities were obtained at gypsum/cement weight ratios of 90/10 and 70/30. The microstructure analysis showed that alumina blocks prepared from the molds containing higher quantities of cement (30 or 50 wt%) were resistant to abnormal grain growth caused by Ca²⁺ contamination from the gypsum. The gypsum/cement mixtures for making molds for slip casting significantly limit mold erosion due to a lower sucking rate and abnormal grain growth of the slip cast samples because of the decreased diffusivity of Ca²⁺ ions. Therefore, the present modification of the mold renders the slip casting method more suitable for the near‐net shaping of ceramics.     

脱硫石膏热处理制备注浆成型模具

石灰石-石膏湿法烟气脱硫技术以石灰石为原料吸收烟气中的二氧化硫,得到石膏晶体。在控制二氧化硫排放的同时,脱硫石膏的处理与综合利用也非常重要。以脱硫石膏为原料,分析脱硫石膏的成分,通过热处理使其脱水,并表征热处理过程对石膏物理性能的影响。将热处理后的石膏用于石膏模具的制作,并最终制备固体氧化物燃料电池阳极支撑体。燃料电池测试的结果显示,脱硫石膏可以用于管式固体氧化物燃料电池的制备。     

Pressure filtration assisted gel casting in translucent alumina ceramics fabrication

High density and homogeneous green body is important for fabricating transparent ceramics. Inspired by the improved samples’ homogeneity in slip casting, we introduced low pressure filtration to gel casting to further increase the relative density and homogeneity of the green body and to shorten the drying time. The effect of pressure filtration on drying characteristics, bulk density and porosity of the pre-sintered bodies, and the microstructure and optical transmittance of the resultant ceramics were intensively investigated. The results showed that pressure filtration can reduce the drying time by 27.3%, improve the density of the pre-sintered bodies and reduce the residual pores of the sintered ceramics. The in-line transmittance of the translucent alumina prepared by the pressure filtration assisted gel casting was 32.7% at 600?nm with a thickness of 1?mm, which is about 6% higher than that without pressure filtration. In principle, this process can be applied to fabricate any other high-performance ceramic.     

Pressure slip casting of coarse grain oxide ceramics

This contribution investigates the pressure slip casting of large coarse grain oxide ceramic bodies with a water soluble organic additive system. This organic additive system allows the preparation of a stable and pumpable slip containing alumina rich magnesia aluminate spinel of a size of up to 3 mm and an easy demolding of crack free, dimensionally stable bodies with negligible gradients due to sedimentation. Cut out samples of fired bodies are examined on apparent porosity, dynamic elastic modulus, modulus of rupture, and pore size distribution. Computer tomography showed very homogenous and dense bodies. The effects of different maximum grain sizes as well as possible sedimentation and segregation of the slip on the mechanical properties and microstructure are evaluated by using the Student's t-test. The most promising results of this study indicate that it is possible to reproducible fabricate coarse grain ceramics for refractory and other high temperature applications by pressure slip casting.     

Manufacture of carbon-bonded alumina based on a lactose-tannin binder system via slip casting

Binders play an important role in the manufacture of carbon-bonded alumina (Al₂O₃-C) refractory materials. To diminish the currently used coal tar pitch binder Carbores® P, which could release hazardous polycyclic aromatic hydrocarbons during operations, environmental-friendly lactose and tannin are applied as the main binders in this study. Compact cylinders of fine-grained Al₂O₃-C based on the lactose-tannin (16 wt%) binder system with significantly reduced pitch content (4 wt%) are successfully prepared via slip casting for the first time. The lactose to tannin mass ratio is systematically varied from 5:1 to 1:5 in the raw material formulation. The specimens based on the new binder system are less dense, more porous and contain less residual carbon than the pitch-based reference. Mechanical tests show that the specimens with lactose-tannin ratio of 5:1, 4:1 and 3:1 have considerable cold crushing strengths, although the splitting tensile strengths are low. In contrast to the well-established pressing route, the slip casting route for compact Al₂O₃-C is more comparable to the replica route for Al₂O₃-C foam filter due to the pressureless shaping process.     

Study on the sintering behavior and characterization of the IGZO ceramics by slip casting

High-purity In₂O₃, Ga₂O₃ and ZnO (In:Ga:Zn = 1:1:1) mixtures were employed to fabricate the IGZO (In-Ga-Zn-O) ceramics by slip casting. The rheological properties and zeta potential of IGZO slurry were analyzed in details. The optimum amount of dispersant used in the IGZO slurry is studied in details. This study investigated the relationship between the phase composition of sintered IGZO ceramics and sintering temperatures. The IGZO ceramics with high density and low resistivity are successfully synthesized in an air atmosphere, and their grain size is distributed uniformly.     

Fabrication of high-density NiFe2O4 ceramics by slip casting and pressureless sintering

High-density NiFe₂O₄ ceramics with homogeneous microstructure were produced by slip casting and pressureless sintering. The slurry stability, sintering behavior, and microstructure of NiFe₂O₄ ceramics were investigated. A stable slurry can be obtained by adding 12.5 wt% NiFe₂O₄ nanoparticle and 5 wt% nano-binder at a slurry pH around 11.0. The linear shrinkage and linear shrinkage rate for both NiFe₂O₄ ceramic green bodies shaped by cold press molding and slip casting showed nearly the same trends. The temperature associated with the maximum linear shrinkage rate of slip casted green body was 1263.5°C, which was lower than that of cold press molded sample (1272.0°C). The sintering activation energy of slip casted green body was also lower than that of cold press molded sample (279.18 vs 288.47 kJ mol⁻¹), owing to high density and homogeneity of slip-casted green compact. A high-density NiFe₂O₄ ceramics with uniform grain size distribution can be produced by slip casting and pressureless sintering at 1350°C for 6 hours, attributed to the ability of slip casting to minimize agglomerates and micropores. It demonstrated that slip casting was more suitable to prepare high-density NiFe₂O₄ ceramics with good homogeneity.     

Direct coarse powder aqueous slip casting and pressureless sintering of highly transparent AlON ceramics

Direct coarse powder aqueous slip casting was proposed to prepare AlON green body for pressureless sintering of highly transparent AlON ceramics. It was reported that anti-hydrolysis treatment to AlON powder before aqueous slip casting was essential due to hydrolysis reaction between AlON and water. However, both XRD of hydrolyzed powder and pH value of aqueous slurry indicate that the hydrolyzate amount is positively correlating with hydrolysis reaction area, i.e., surface area of powder. Therefore, replacing fine particle with coarse one to reduce surface area of powder is an efficient hydrolyzates reduction scheme by lessening hydrolysis reaction sites. As a result, the hydrolyzate in green body of direct slip casting coarse AlON powder (91 vol% of particles have a size of >1.0 μm) was successfully reduced to an insignificant amount so that the sintered ceramics had a high transmittance (84.1% at ~3750 nm), which also verified the effectiveness of the proposed hydrolyzate-reduction scheme.     

Cellular ceramics by slip casting of emulsified suspensions

Cellular ceramics were processed by slip casting of alumina suspensions emulsified with sunflower oil. A model behavior was derived on assuming that the templating dispersed phase and its droplet size distribution are retained as porosity in the resulting cellular ceramics, whereas the continuous ceramic suspension evolves to dense struts and thin inter-pore walls. Representative values of solid load and oil to suspension ratio were selected to seek close packed spatial distribution of nearly spherical. Stirring rate and additions of surfactant were also varied for greater flexibility in adjusting droplet size and rheology of the corresponding emulsified suspensions; this also contributes to minimize undue losses of porosity, relative to the model behavior, and determines microstructural features such as size distributions and average cell size.     

Fabrication of porous fibrous alumina ceramics by direct coagulation casting combined with 3D printing

A novel forming method for preparing porous alumina ceramics using alumina fibers as raw materials by direct coagulation casting (DCC) combined with 3D printing was proposed. Porous fibrous alumina ceramics were fabricated through temperature induced coagulation of aqueous-based DCC process using sodium tripolyphosphate (STPP) as dispersant and adding K₂SO₄ as removable sintering additives. The sacrificial coated sand molds was fabricated by 3D printing technology, followed by the infiltration of silica sol solution for the subsequent suspension casting. Stable alumina suspension of 40 vol% solid loading was obtained by adding 2.0 wt% STPP and 40 wt% K₂SO₄. The controlled coagulation of the suspension could be realized after heating at 90 °C for about 35 min. The ceramic sample sintered at 1450 °C for 2 h showed the highest compressive strength of 24.33 MPa with porosity of 57.38%. All samples sintered at 1300–1450 °C had uniform pore size distributions with average pore size of 7.2 µm, which indicated the good structure stability when sintered at high temperature.     

Al2O3添加剂对ZrO2陶瓷性能影响的探讨

本研究采用注浆成型法,通过添加Al_2O_3,探讨了Al_2O_3的加入量对ZrO_2陶瓷性能的影响。     

ZnO semiconductors obtained by slip casting: Application and reuse in photocatalysis

This work described the acquisition of immobilized ZnO semiconductors using the slip casting technique, for application as reusable photocatalysts in the degradation of Rhodamine B. The influence of the heat treatment temperature (800°C, 900°C, and 1000°C) on the physical, thermal, microstructural, and photocatalytic properties was investigated. All samples presented the wurtzite crystal structure, and the surface was completely absent of organic matter residues. The samples presented band gap values around 3.2 eV. The ones heat treated at 800°C showed lower density (3.40 g/cm³, corresponding to 60% of the ZnO theoretical density), smaller average grain size, in addition to higher apparent porosity (around 40%). These characteristics provide better photocatalytic activity to the sample heat treated at 800°C, since it promoted 92.2% dye degradation, while samples heat treated at 900°C and 1000°C promoted 81.8% and 54.2% dye degradation, respectively. The integrity of all samples was maintained after the photocatalytic tests. Thus, the reuse capability of the sample with the best photocatalytic performance, that is, the sample heat treated at 800°C, was evaluated in six cycles of photocatalysis. The sample proved to be reusable, promoting degradation of practically 100% of the dye after the third cycle of reuse.     

Effect of K2SO4 additions on properties of porous fibrous alumina ceramics prepared by DCC and lost‐mold method

Flawless porous fibrous alumina ceramics with high performance were fabricated via a novel approach involving direct coagulation casting and lost‐mold method. Stable alumina suspensions were prepared by adding sodium tri‐polyphosphate as dispersant using alumina fibers as raw material and K₂SO₄ as sintering aid. Resin‐coated sand molds with designed shapes for suspension casting were fabricated through 3D printing with subsequent post‐treatments. Alumina green bodies were obtained by in situ coagulation of the suspension after treating at 90°C within 40 minutes. Porous alumina ceramics were obtained after direct furnace sintering of green bodies without demolding, in which the molds would collapse automatically at around 650°C with less exhaust emission. The effect of various K₂SO₄ contents and sintering temperatures on mechanical properties of porous ceramics was investigated. The SEM results showed that the fibers interconnected with obvious interfacial bondings on junctions when sintered at 1450°C. The XRD patterns showed that the sample sintered with various K₂SO₄ additions consisted of different phases, mainly including aluminosilicates. Porosity of ceramic samples increased slightly whereas the compressive strength enhanced significantly with increasing K₂SO₄ addition. The density of sintered samples with different K₂SO₄ contents was in accord with the porosity variation tendency. The ceramic samples had uniform pore size distribution with average size from 3.18 to 7.24 μm as increasing K₂SO₄ addition to 40 wt%. This approach may provide a convenient and general route to fabricate various dense and porous advanced ceramics with complex shapes and good composition homogeneity.     

泡沫注凝法制备刚玉-莫来石多孔陶瓷

以T60板状刚玉粉(d50=10.74μm)、ZTP-200氧化铝微粉(d50=4.96μm)、RG4000煅烧氧化铝微粉(d50=1.46μm)和NS-30型硅溶胶为原料,采用泡沫注凝法制备了刚玉-莫来石多孔陶瓷,主要研究了粉料粒度级配(d50分别为1.46、4.96、10.74μm的三种氧化铝粉料的质量分数分别为44%、26%、30%,36%、28%、36%,30%、29%、41%,21%、29%、50%)和浆料中硅溶胶加入量(质量分数分别为0、2%、4%)对坯体的烧后收缩及烧后试样的致密度、常温耐压强度、热导率、相组成和显微结构的影响。结果表明:1)d50=10.74μm的粗颗粒加入量增加,试样的烧后线收缩率、致密度和常温耐压强度减小;2)硅溶胶加入量增加,试样的烧后线收缩率和体积密度减小,但由于SiO2与Al2O3反应生成莫来石,试样的常温耐压强度变化较复杂;3)泡沫注凝法制备的刚玉-莫来石复相多孔陶瓷具有球形孔结构,平均孔径约为140μm,其性能如下:烧后线收缩率9.29%,体积密度0.78 g·cm-3,显气孔率79.8%,常温耐压强度25.3 MPa,1 000℃(热面温度)下的热导率为0.481W·m-1·K-1。     

Porous asymmetric microfiltration membranes shaped by combined alumina freeze and tape casting

An assembled asymmetric alumina microfiltration membrane with high performance was prepared by combining freeze and tape casting techniques followed by two sintering steps. Freeze casting was used for manufacturing of the porous support layer with a highly interconnected pore network. Tape casting was applied on the top layer to form a pre-membrane with smaller pore size and controlled thickness, which was set on the sintered support. Morphology influences were investigated for different solid loadings, additives content and the assembled layer membrane structures. No delamination among the layers was observed. The assembled ceramic membrane had an average pore size between 30 and 50 μm together with a top surface layer around 0.35 μm, which is suitable to the microfiltration separation process. Porosity in the range of 26–50 % and water flux of 11–32 m³ m^?2 h^?1 bar^?1 were reached for samples prepared with two sintering steps at 1600 and 1300 °C for 2 h.     

Cardanol as a dispersant plasticizer for an alumina/toluene tape casting slip

Cardanol, a naturally occurring C₁₅ alkyl chain substituted phenol obtained from cashew nut shell liquid (CNSL), is used as a dispersant and plasticizer for PMMA binder based alumina tape casting slips in toluene medium. The best dispersion of an alumina powder with BET surface area of 10.4 m²/g occurs at a cardanol concentration 2 wt.% of the powder at which the cardanol molecules form a monolayer on the particles with an end-on-adsorbed configuration through phenolic hydroxyl anchored on the surface. Cardanol added in excess of the dispersing agent acts as a plasticizer for PMMA, as revealed by significant decrease in the T_g of the polymer, reduction in the slurry viscosity and increase in the tape flexibility. Green tapes with good flexibility (failure strain, 10–20%) and strength (5.7–7 MPa) were obtained using a binder content 12 wt.% of alumina and employing a plasticizer to binder ratio in the range 0.66–0.82 by weight. The green tapes were thermally debinded with or without extraction of the plasticizer with methanol. The solvent extraction of the plasticizer prior to thermal debinding, however, did not show any effect on the density (97% TD) of the tapes sintered at 1500°C.     

Development and testing of carbon-bonded alumina foam filters for continuous casting of steel

Carbon-bonded alumina filters with special design for continuous casting were produced by the replication technique using different coating procedures. The filters were investigated regarding filter strut diameter, porosity and cold crushing strength. The potential filter capacity and the mechanical load expected during the immersion in a continuous casting tundish were calculated. The best compromise between mechanical strength and structural features was achieved by combining the advantages of centrifugation, dip coating and spray coating. This filter type was analyzed by computed tomography and withstood the contact with molten steel in a casting test. After melt contact, the filter material exhibited near-surface decarburization and alumina-based in-situ layer formation. The observed layers were more pronounced in zones with potentially lower melt flow velocity. The results of the study indicated that the produced filters are able to withstand the severe conditions in continuous casting.     

In situ vibration enhanced pressure slip casting of submicrometer alumina powders

This paper presents a novel method for improvement of particle packing in consolidation of submicrometer alumina powders by pressure slip casting. In this method, filtration cell is subjected to a mechanical vibration field with constant frequency of 50 Hz and vibration amplitudes ranging from 0 (no vibration) to 2 mm. Filtration rate, thickness and green density of the fabricated samples were measured to investigate the influence of vibration on filtration characteristics. It was revealed that employment of vibration can significantly increase filtration rate. Furthermore, there is an optimum vibration amplitude which results in the structure with the highest packing density. This value is shifted to higher vibration amplitudes as more concentrated alumina slurries is used. As the available formulation based on Darcy's law could not predict the results of the present investigation, a “Correction Factor” was utilized in order to increase the accuracy of the prediction in the presence of a vibration field.     

Effect of additives on porosity of alumina materials obtained by freeze casting

The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35?vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10?wt-% with respect to solids. In the case of PVA, contents of 2?wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.