Utilization of granite sludge for production of cordierite ceramics by direct coagulation casting

In this work, an attempt to produce cordierite ceramics from granite sludge waste, talc and alumina was performed by direct coagulation casting process. To optimize the conditions for cordierite formation, three mix-compositions were firstly prepared by processing the starting materials in different conditions. The first mix was prepared by firing the mix of granite sludge, talc and alumina up to 1300 °C while the second and third mixes were fabricated by firing alumina and talc at 1300 °C or 1350 °C, respectively, then the granite sludge was added. Both batches were fired at different temperatures. According to the percentage of formed cordierite, the third mix was selected to be solidified by direct coagulation casting method followed by sintering at different temperatures. The casted cordierite was examined by thermal analysis while the sintered bodies were tested for their physical, mechanical and electrical properties. The results indicated that the pre-heating of alumina and talc at 1350 °C (third mix) enhanced the formation of cordierite and some amounts of spinel. For the casted sintered specimens, the porosity was decreased with increasing the sintering temperature. Also, there was an increase in compressive strength for the samples sintered up to 1250 °C. The dielectric constant values were varied between 4.5 and 5.89 while the dielectric loss was varied between 2 × 10^?3 and 7 × 10^?3, at room temperature.     

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.     

Mineralogical and dielectric properties of mullite and cordierite ceramics produced using wastes

In this work technical ceramics containing industrial inorganic wastes was carried out. Ceramic formulations prepared with clay, magnesium oxide and residues of kaolin and alumina as raw materials, were formed in a disk-shaped specimens using the uniaxial pressing process and sintering at temperatures from 950°C to 1400°C. The mineralogical, physical and dielectric characteristics of the fired samples were investigated. The dielectric properties, the relative dielectric constant (ε_r) and the loss tangent (tan δ) were evaluated at frequencies of 0.1, 1, 10, and 100?kHz at room temperature. Mullite and cordierite were present as major phases at the highest temperatures. Relative dielectric constant values closest to that of mullite (ε_r = ~ 5 to ~ 6) and cordierite (ε_r =?~ 4 to ~ 6) at 1?kHz. On the other hand, the lowest dielectric losses (tan δ ~ 0.06 to ~ 0.04) were observed for the formulations containing the mullite major phase, and tan δ ~ 0.009 to ~ 0.003 for formulations that showed cordierite as main phase. It was verified that an increase in temperature promoted a reduction of porosity, a property that had a direct influence on the dielectric properties of the formulations. The materials obtained from the residues presented low dielectric constants and loss tangents, which make them suitable for use in electrical and electronic systems.     

Indirect solid freeform fabrication by binder assisted slip casting

A method has been developed for the consolidation of ceramic bodies in molds prepared by solid freeform fabrication. The method is based on slip casting with an added latex binder. The latex binder makes it possible to create internal structures, such as pore channels, without the cracking that usually takes place when shrinkage is obstructed by internal mold structures. The latex binder adds plasticity to the consolidating body in the wet state. This was observed by rheological measurements during slip casting. Measurements of mechanical properties showed that the latex binder also adds plasticity and strength in the dry state.     

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.     

Fabrication and properties of cordierite / anorthite composites

The designed cordierite/anorthite batch compositions were studied to gain the favorable properties of both materials where, cordierite has low thermal expansion coffecient and high strength than anorthite.Low cost starting materials such as sugar beat filter cake and talc carbonate were used in the present study. Examination of the effect of cordierite addition (in the range of 10-25mass-%) on the physical, mechanical, electrical and thermal expansion coefficient properties as well as the phase composition and microstructure for the fabricated cordierite/anorthite composites were carried out. The results reveal that the increase in the cordierite content up to 20 mass-%, improves the sinterability and electrical properties. In addition, it increases the bending strength, hardness and decreases the thermal expansion coffecient. However, further increase in the cordierite content up to 25 mass% has negative influence on the physical, electrical and mechanical properties of the cordierite/anorthite composites.     

Studies on slip casting behavior of lanthanum strontium manganite

Dispersion conditions for slip (slurry) formulation of a powder mixture of lanthanum strontium manganite (La_0.84Sr_0.16MnO₃ - LSM) and carbon (pore former) in water was studied through detailed zeta-potential and rheological measurements. The zeta potential variation with pH for the aqueous suspensions of only LSM or carbon exhibited a maximum value in alkaline medium (−40 mV to −50 mV at a pH of 10-11), establishing the pH window for their co-dispersion for slurry formulation. A study of the viscosity variation with shear rate for the slurries with varying solid content (in the range of 45-65 wt.%) exhibited pseudo-plastic flow behavior, indicating presence of flocculates in them. The yield stress values obtained from the Casson equation reduced with decreasing solid content, indicating reduction in the flocculate strength. The slip with solid content of 50 wt.% exhibited optimum flow characteristics to form long tubes with uniform wall thickness (wall thickness 2-4 mm and length of 150-200 mm). The tubular specimens formed after controlled carbon burn out and sintering at 1400 °C for 1 h possessed about 35% open porosity. The porosity remained the same upon further sintering at 1400 °C for 8 h.     

Magnetic slip casting for dense and textured ceramics: A review of current achievements and issues

Ceramic materials are ubiquitous in technologies operating under high mechanical, thermal or chemical constrains. Research in ceramic processing aims at creating ceramics with properties that are still challenging to obtain, such as toughness, transparency, conductivity, among others. Magnetic slip casting is a process where an external magnetic field is used to createcontrolled texture in ceramics. Over the past 20 years of research on magnetic slip casting, dense and textured ceramics of multiple chemistry were found to exhibit enhanced properties. This paper reviews the progress in the field of magnetic slip casting, details the processing parameters and the textures obtained for a diverse range of compositions. The structural and functional properties of the magnetically textured slip casted and sintered ceramics are presented. This overview of the magnetic slip casting process allows to identify critical directions for future advancement in advanced technical ceramics.     

Aqueous slip casting of transparent yttrium aluminum garnet (YAG) ceramics

Transparent YAG ceramics were prepared by slip casting an aqueous dispersed mixture of commercial Al₂O₃ and Y₂O₃ powders. The powders were co-dispersed with poly(acrylic acid) and citric acid. Polyethylene glycol of 0.5 wt.% (PEG 4000) and 0.5 wt.% tetraethyl orthosilicate were added as binder and a sintering aid, respectively. Dried samples were vacuum sintered at 1800 °C for 16 h. In general, YAG ceramics cast from Newtonian suspensions were optically transparent and had optical transmittances >80% from 340 to 840 nm. Slightly flocculated dispersions, as evidenced by higher viscosity and non-Newtonian rheology, resulted in translucent samples with large pores and lower optical transmittances.     

低膨胀率堇青石陶瓷的研究

以粘土、滑石、氧化铝等为原料 ,经 1390～14 0 0℃烧结 ,制备了堇青石陶瓷。使用X射线荧光分析和X射线衍射分析确定了其化学组成和晶相组成 ,并测定了热膨胀率。根据堇青石的晶体结构和烧结机理 ,研究并讨论了陶瓷的化学组成、原料种类和质量对其晶相组成和热膨胀率的影响。     

Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

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.     

Investigation of affecting parameters on slip casting of yttria-magnesia IR-transparent bodies

Slip casting of MgOY₂O₃ suspensions has been investigated in this research. For this purpose relatively pure commercial raw materials were used. The Taguchi experiment method was used for design of experiments and selection of suitable solvents and dispersants and theirs optimum values. Preparation of suspensions was done in two stages, according to particle size distributions of powders after different milling time and Taguchi results. The best suspensions were selected based on viscosity and solid load of suspensions. Green bodies were slip casted using the optimized suspensions in order to obtain the green bodies with maximum density and a homogeneous structure in term of porosity size distribution. Results showed that using from ethanol as solvent, an optimum dispersant level of 3%wt. TEA +1%wt. DEA with a constant TEA/DEA ratio of 3/1 and mean particle size of 155 nm (milling time of 48 h) was the best choice for preparing of optimum suspensions. The highest relative green density of 65.91% was obtained for a suspension containing 35 vol % of solid particles casted under 3MPa. The dense Y₂O₃MgO submicron composite samples were sintered by SPS technique at 1250?°C under 70MPa for 6?min with a density about 99.5%.     

Fabrication and characterization of porous cordierite ceramics prepared from fly ash and natural minerals

Low cost, single-phase porous cordierite ceramics are successfully synthesized by in-situ solid-state reactions from fly ash, quartz, and magnesite. The effects of sintering temperature and magnesite content on phase transformation, open porosity, bulk density, mechanical properties, and microstructure are carefully investigated. Factsage analyses are carried out to calculate the isopleth diagrams, and the results agree well with the experimental outcomes. Thermal gravimetric and differential scanning calorimeter analyses (TG-DSC) are performed to characterize the weight loss and transient behaviors of the raw materials. Linear thermal expansion properties are also studied. The α-cordierite phase is the only phase observed in S-3 (magnesite content 25%) sintered at 1300 °C for 2 h, and its linear thermal expansion coefficient (2.71 × 10⁻⁶ K⁻¹) is close to that of typical cordierite. Both the compressive strength (72.64 MPa) and flexural strength (23.92 MPa) of the as-synthesized samples are high with an open porosity of 33.16% and a bulk density of 1.61 g/cm³.     

The phase transformation and thermal expansion properties of cordierite ceramics prepared using drift sands to replace pure quartz

Cordierite ceramics were prepared using drift sands as the raw material instead of traditional pure quartz through solid-state sintering within an enlarged temperature range between 1100 and 1300 °C. The effects of adding Na₂B₄O₇ on the phase transformation and thermal expansion coefficients were also investigated. The experimental results show that compared with pure quartz, drift sands have a lower softening temperature. Thus the sample prepared from drift sands shows a lower temperature for cordierite phase transformation, which was further reduced by adding 1.5 wt% Na₂B₄O₇. Moreover, with an increase of Na₂B₄O₇, the sintering temperature range was enlarged, and the amounts of the interphases of quartz and spinel as well as the thermal expansion coefficient for the obtained cordierite ceramics were decreased.     

Performance study of foam ceramics prepared by direct foaming method using red mud and K-feldspar washed waste

In this study, foam ceramics were prepared via a direct foaming method at high temperatures (1080–1120 °C), using red mud (RM) and K-feldspar washed waste (KFW) as the raw materials and SiC as the foaming agent, respectively. The chemical compositions and crystalline phases of the raw materials as well as the structural and mechanical properties of the foam ceramics were investigated. By adjusting the formulation and sintering process parameters, the porous structure of the foam ceramics could be effectively modulated. In addition to some residual crystalline phases in the raw materials, new phases, including rutile (TiO₂) and anorthite (CaAl₂Si₂O₈), were generated in foam ceramics. The compressive strength of the foam ceramics decreased with an increase in the KFW/RM ratio and sintering temperature, which was mainly related to the low density of the foam ceramics and the poor support of the pore walls to the structure. Among all the foam ceramics investigated, the foam ceramic with the KFW/RM ratio of 1:1, SiC content of 1 wt%, sintering temperature of 1100 °C and sintering time of 60 min showed the best overall performance with a bulk density, an apparent porosity, an average pore size and a compressive strength of 0.77 g/cm³, 61.89%, 0.52 mm, and 3.64 MPa, respectively. Its excellent porous structure and mechanical properties rendered it suitable for application as insulation materials or decorative materials for building partition walls.     

Fabrication of Si3N4 ceramics substrates with high thermal conductivity by tape casting and gas pressure sintering

In this paper, high thermal conductivity Si₃N₄ ceramics were successfully fabricated through exploring and optimizing the tape casting process. The impact of various organic additives on the rheological characteristics of Si₃N₄ slurry was explored, and the pore size distribution and microstructure of the green tapes at different solid loadings were investigated, as well as the microstructure of Si₃N₄ ceramics. Green tapes with a narrow pore size distribution, a small average pore size, and a high density of 1.88 g cm⁻³ were prepared by the investigation and optimization of the Si₃N₄ slurry formulation. After gas pressure sintering, Si₃N₄ ceramics with a density of 3.23 g cm⁻³, dimensions of 78 mm × 78 mm, and a thickness of 0.55 mm were obtained. The microstructure of the Si₃N₄ ceramics showed a bimodal distribution and a low content of glassy phases. The thermal conductivity of the Si₃N₄ ceramics was 100.5 W m⁻¹ K⁻¹, the flexural strength was 735 ± 24 MPa, and the fracture toughness was 7.17 MPa m^1/2.     

含钛添加剂对堇青石陶瓷热膨胀性能的影响

以粘土、滑石、氧化铝微粉等为原料 ,于1370～ 14 0 0℃烧成 ,合成了堇青石陶瓷。研究了添加 5 %～ 2 0 %的钛酸铝 (AT )或 5 %～ 15 %的氧化铝(A)与氧化钛 (T)的混合物 (A与T的摩尔比 1∶1)对堇青石陶瓷的相组成及热膨胀率的影响。结果表明 :添加AT或A T均能有效地降低材料的热膨胀率 ;XRD分析表明 ,在添加 15 %以上AT或添加10 %以上A T的堇青石试样中形成了莫来石与钛酸铝晶体 ;SEM显微照片表明 ,引入AT或A T均促进了堇青石的合成。从效果和节约能源考虑 ,应添加 10 %～ 15 %的A T ,烧成温度为 14 0 0℃左右。     

Porous Si3N4 ceramics prepared via slip casting of Si and reaction bonded silicon nitride

Slip casting process combined with reaction bonded silicon nitride (RBSN) was used to prepare porous Si₃N₄ ceramic with near-net and complex shape. A butyl stearate (BS) coated process was introduced to restrain the hydrolysis of Si, and ammonium polyacrylate (NH₄PAA) was used to enhance the dispersion of coated Si. The measured oxygen content showed that the hydrolysis of Si was strongly prohibited by BS coating, and relatively low viscosity was obtained with the addition of 0.25-1.5 wt% NH₄PAA to the 60 wt% solid load slurry. 40-60 wt% solid load slurries were used for slip casting in the experiment. After vacuum degassing, slip casting, debindering and nitridation, a density of 1.57-1.92 g/cm³ (porosity 50.9-40%) and a flexural strength of 47-108 MPa were obtained. The samples without vacuum degassing showed a large number of nanowires grown in the large pores.     

The effect of slip casting parameters on the green density of MgAl2O4 spinel

The aim of this paper was to investigate the effect of slip casting parameters on the green density of MgAl₂O₄ spinel. In order to obtain samples with suitable mechanical and optical properties, it is necessary to prepare bulk samples with a fine grain size along with a low level of impurity and high density. Slip casting is widely used in the processing of optical ceramics to achieve a body with high green density and low sintering temperature. In the present study, several spinel suspensions with similar solid content but different viscosities and particle sizes (90, 150, 300 and 500 nm) were prepared and shaped into a dense body. Viscosity of suspension depended on dispersant content, such that the addition of dispersant firstly caused viscosity to decrease, but it was increased by further dispersant addition, irrespective of the suspension particle size. The green density range of samples was 36–67% of the theoretical value. Rheological behaviour and green density measurements showed that powder particles smaller than 90 nm were unsuitable for slip casting because agglomeration of powder particles led to high viscosity and hence, low green density. The optimal particle size for slip casting was found to be 150 nm.