Effect of boric acid on the porosity of clay and diatomite monoliths

Porous silica ceramics were obtained at low forming pressure (40–80 MPa) and low sintering temperature (850–1300 °C) for 4 h in air. Boric acid was used as a low-cost additive, in the amount of 2 wt%. Relatively high porosities of nearly 40% and 65% are obtained for the samples of clay and diatomite pressed at 40 MPa, and sintered at 1000 °C, respectively. The samples sintered at 1150 °C and 1300 °C have the average pore size diameters in the range of macroporous for clay 0.2–10 μm and for diatomite 0.2–5 μm. X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and mercury porosimetry measurements were employed to characterize of the obtained samples. Measurements of densities and open porosities by immersion technique were used, according to the Archimedes principle. The relations between mechanical characteristics of the samples formed by using different pressures and sintered at different temperature, were discussed.     

The fabrication and characterization of sintered diatomite for potential microfiltration applications

Porous ceramic membranes have lately become a subject of special interest due to their outstanding thermal and chemical stability. We investigated whether a sintered diatomite support layer could also serve as a separation layer to minimize any processing difficulties, and investigated whether the support layer and the separation layer could be made from the same material to avoid a thermal mismatch during a high-temperature sintering process. We prepared sintered diatomite as a porous ceramic membrane for microfiltration, as diatomite particles are inherently porous and irregular. The pore characteristics of the sintered diatomite specimens were studied by scanning electron micrography, mercury porosimetry, and capillary flow porosimetry.     

Fabrication of microporous ceramics from ceramic powders of quartz–natural zeolite mixtures

The possibility of producing ceramic powders suitable for the fabrication of microporous filters was investigated through the thermal treatment of the powder mixtures of a high-purity (99.09% SiO₂) quartz and clinoptilolite type of natural zeolite. The quartz and zeolite, mixed in the ratio of 3:1 by weight, was wet ground in a ball mill, the powder was sieved on a 90-μm screen, and the undersize was dried and sintered in the powder form at the temperatures of 1000, 1100 and 1200 °C for 7 h in an air furnace. The powder sinter products were deagglomerated by gentle crushing in an agate mortar and then characterized by phase composition, density, and specific surface area measurements. The added zeolite facilitated the transformation of quartz to cristobalite. The phase transformation of quartz to cristobalite first appeared at around 1000 °C, and, at 1200 °C, led to a ceramic powder sufficiently high in cristobalite content for the fabrication of the microporous ceramic bodies. Re-sintering at 1200 °C of the pressed forms of the ceramic powder resulted in microporous (0.5–3 μm) ceramics with a high porosity of 48.5%, and a three-point bend strength of 140 kg/cm². The ceramics obtained may have potential for filter applications.     

A study of the sintering of diatomaceous earth to produce porous ceramic monoliths with bimodal porosity and high strength

Diatomite powder, a naturally occurring porous raw material, was used to fabricate ceramic materials with bimodal porosity and high strength. The effect of the sintering temperature on the density and porosity of dry pressed diatomite green bodies was evaluated using mercury porosimetry and water immersion measurements. It was found that the intrinsic porosity of the diatomite particles with a pore size around 0.2 µm was lost at sintering temperatures above 1200 °C. Maintaining the sintering temperature at around 1000 °C resulted in highly porous materials that also displayed a high compressive strength. Microstructural studies by scanning electron microscopy and energy-dispersive X-ray analysis suggested that the pore collapse was facilitated by the presence of low melting impurities like Na₂O and K₂O.     

Characterization of basaltic tuffs and their applications for the production of ceramic and glass–ceramic materials

Alkaline basaltic tuffs, from Southern Turkey were characterized and employed to obtain ceramic and glass–ceramic materials by combined sintering and crystallization process. The chemical and mineralogical compositions were analyzed by X-ray fluorescence spectrometry and X-ray diffraction analyses, respectively. The phase formation and the sintering behaviour were investigated by DTA, differential dilatometer and hot-stage microscopy. The micro-structure and residual porosity of the sintered samples were observed by SEM and evaluated by pycnometric techniques. Ceramic material, based on 50% basaltic tuff and 50% clay, was obtained at 1150 °C with 13% total porosity and 4% water absorption. Glass–ceramic materials were synthesized directly using the milled basaltic tuff by mean of the sinter-crystallization technique, in the range 900–1150 °C. The investigation has showed that, due to the high porosity and low crystallinity, alkaline tuffs could be a suitable raw material for ceramic application.     

The recycling of MSWI bottom ash in silicate based ceramic

The possibility of recycling the Municipal Solid Wastes Incinerated (MSWI) bottom ash by its incorporation in ceramic tiles was investigated. The MSWI bottom ash was introduced both in untreated (previous deironization) and vitrified condition. The sintering of the different products was investigated by determining water absorption and linear shrinkage. To evaluate possible variations due to the presence of different amount of bottom ash, mineralogical and microstructural examinations by quantitative X-ray diffraction and scanning electron microscopy, were performed. While the untreated bottom ash, till an amount of 5 wt%, did not seem to affect the above characteristics of selected materials, the vitrified bottom ash, 5–10 wt%, as strong fluxing agent, promoted the sintering of porcelain stoneware. Leaching results allow to assert that the fired samples are not dangerous.     

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe₂O₃ powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe₂O₃-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe₂O₃. It is suggested that the Fe₂O₃ dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe₂O₃-YSZ hollow fiber membranes sintered at 1150 °C show a 3-fold increase of the permeate flux of pure water (F) (743 L m⁻² h⁻¹) along with comparable bending strength (152 MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.     

Effects of sintering temperature and holding time on porosity and shrinkage of glass tubes

The influences of sintering temperature and holding time on porosity and shrinkage of glass tubes have been studied by optical microscope. It is evident that there exists three stages for the sintering process of glass. At the first stage, both increasing temperature and prolonging the holding time contribute to lowering the porosity and to intensifying the shrinkage greatly. At the second stage, the glass further densifies and the voids among particles become smaller and less. Finally, at the third stage the shrinkage rate almost keeps unchanged to sintering temperature and holding time.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Young׳s modulus,hardness and thermal expansion of sintered Al2W3O12 with different porosity fractions

Owing to their unusual thermal expansion properties ceramic phases from A₂M₃O₁₂ family have potential for applications as thermal expansion controlling fillers inside soft matrices or as materials with high thermal shock resistance when prepared in monolithic forms. In spite of this, the consolidation routes for achieving bulk forms with adequate microstructure and their mechanical and thermal properties are scarcely known and rarely studied. A prelaminar study on sinterability of Al₂W₃O₁₂, a low thermal expansion phase, was accomplished for the temperature range between 850 °C and 1000 °C. Sintered samples with the porosity fraction between 0.1 and 0.25 were produced and their Young׳s moduli, hardness and thermal expansion studied through nanoidentation and dilatometry. Acoustic emission was employed for studying of microcrack formation during heating and cooling of sintered samples. Sintering study showed that the temperatures higher than 1150 °C may lead to the decomposition of tungstate due to WO₃ evaporation, while the sintering at the temperature of 850 °C provokes only small changes over grain size distribution. Hardness and Young׳s modulus decrease linearly in porosity range between 0.1 and 0.25. Young׳s modulus for fully dense Al₂W₃O₁₂ was calculated to be 70 GPa, illustrating that the phases from A₂M₃O₁₂ family are considerable softer than traditional ceramics. Microcrack formation was observed on cooling and heating, as well, causing the discrepancy between the intrinsic coefficient of thermal expansion (CTE), measured in powder form, and the CTE measured in bulk form. The key feature for future development of A₂M₃O₁₂ phases for thermal shock resistance applications is the better understanding of sintering processes in order to improve microstructure and reduce influence of microcracks over mechanical and thermal properties.     

Short fiber reinforced 3d printed ceramic composite with shear induced alignment

This paper accounts for utilization of shear induced alignment method during ceramic stereolithography. Lateral oscillation mechanism, combined with 3d printed wall pattern, was employed to generate necessary shear to align fiber in desired direction. First, semicircular channel pattern was printed to assess the effect of difference between wall direction and oscillation direction on the fiber alignment. Then, flexural strength of ceramic matrix was tested with nickel coated carbon fiber and ceramic fiber reinforcements. The results demonstrated that the shear induced alignment further improves the flexural strength compare to randomly distributed samples. Flexural strength of aligned samples with 1.0 wt% carbon fiber loading was improved by ~90% compared to randomly orientated samples and by ~333% compared to unreinforced samples. Finally, fracture surface morphology of the flexural strength test specimens was evaluated. The main fracture mechanism was observed as fiber pull-out.     

硅藻土基多孔陶瓷的制备

以硅藻土为主要原料,加入天然有机细粉为造孔剂,水玻璃为高温粘合剂,轻质碳酸钙和轻质碳酸镁为添加剂,经半干压成型,常规烧成,制备出了性能优良的硅藻土多孔陶瓷。随着造孔剂的加入量和烧成制度的改变,制品的气孔率、体积密度和抗压强度等性能也随着改变。     

Structural studies of porous Ni/YSZ cermets fabricated by the solid-state reaction method

Carbon black-added NiO/YSZ composites exhibited a highly porous structure due to the enhanced evolution of CO and CO₂ gases associated with exothermic reactions during sintering. The addition of carbon black resulted in a decrease in the density and grain size of NiO/YSZ composites. The porosity of NiO/YSZ composites increased with decreasing the sintering temperature and increasing the NiO content. Additionally, highly porous Ni/YSZ cermets were fabricated by reducing the NiO/YSZ composites in (Ar+6% H₂), which was attributed to the change of NiO to Ni and then the extraction of oxygen. The carbon black used as a pore-former was highly effective for preparing porous Ni/YSZ cermets.     

Sintering behaviour of cobalt ferrite ceramic

Pure cobalt ferrite ceramic powder was prepared using standard solid-state ceramic processing. Uniaxially pressed pure cobalt ferrite discs, sintered under isothermal ramp rate and single dwell time conditions, yielded a maximum theoretical density (%D_th) of <90%. Discs made from finer particle size powder yielded a %D_th of 91.5%. Based on dilatometry analysis, a sintering profile comprising non-isothermal sintering, and two-step sintering was devised, yielding discs with %D_th of 96%. Cylindrical rods of pure cobalt ferrite were cold iso-statically pressed and sintered according to the revised sintering profile. Pycnometry analysis was used to quantify the percentages of open and closed pores in the rods after sintering.     

The use of egg shells to produce Cathode Ray Tube (CRT) glass foams

Cleaned Cathode Ray Tube (CRT) (panel and funnel) waste glasses produced from dismantling TV and PC colour kinescopes were used to prepare glass foams by a simple and economic processing route, consisting of a direct heating of glass powders at relatively low temperatures (600–800 °C). This study reports on the feasibility of producing glass foams using waste egg shells as an alternative calcium carbonate-based (95 wt%) foaming agent derived from food industry. The foaming process was found to depend on a combination of composition, processing temperature and mixture of raw materials (glass wastes). Hot stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize foams and evaluate the foaming ability and the sintering process. The experimental compositions allowed producing well sintered glass foams with suitable properties for some functional applications with environmental benefits such as: (1) reduced energy consumption because of the low heat treatment temperatures used; and (2) materials produced exclusively from residues.     

Pore structure of carbons coated on ceramic particles

Pore structure of carbon coated on ceramic particles by carbonization of precursor in a powder mixture at 900 °C was studied by focusing on the effects of substrate ceramics (MgO, TiO₂ and various phases of Al₂O₃) and of carbon precursor (poly(vinyl alcohol) (PVA), poly(ethylene terephthalate) (PET) and hydroxyl propyl cellulose (HPC)). By dissolving substrate MgO particles, carbon coated was found to have a high BET surface area, more than 1000 m²/g, which was almost the same as the value estimated from apparent surface area measured on carbon-coated MgO particles under the assumption of zero surface area of the substrate. The carbon separated was found to be rich in micropores from the analyses by DFT method and α_s plot. The dependence of the BET surface area on the amount of carbon coated on TiO₂ with a high surface area was the same for three carbon precursors, although the carbon yields from the precursors were slightly different. Porous Al₂O₃ substrates, γ-Al₂O₃as-received and that formed from Al(OH)₃ during carbonization, gave a high BET surface area, but dense Al₂O₃, α-Al₂O₃, gave a low surface area.     

Porous bone implants

Porous conical implants made by sintering micro-spheres made from bioactive and bio-compatible glasses have been tested mechanically as well as in vivo by inserting the implant through the cortical bone into the bone marrow. The behaviour is compared to a reference implant made by sintering micro-spheres of metallic titanium. Due to capillary forces the implant pores were filled with bone marrow fluid when inserted. An extended bone ingrowth occurred in the cones of bioactive glass, in the titanium ones only in the cortical area. Further, the factors influencing the formation of a chemical bond between glass and living tissue are discussed.     

Cleaner production of porcelain tile powders. Fired compact properties

A new ceramic powder preparation process, the droplet-powder granulation process (DPGP), was recently proposed for the cleaner production of ceramic tiles. The DPGP granules and resulting pressed compacts were characterized and compared with the granules and compacts obtained by spray-drying (SD) and dry granulation (G) processes in a previous paper. The results showed the feasibility of using the DPGP in the pre-firing stage of porcelain tile manufacture.This study compares the firing behaviour and fired properties of compacts pressed from three different types of granulated powders: DPGP, SD, and G and from a non-granulated powder (NG) obtained by dry milling. The evolution of compact microstructure (porosity and pore size distribution) with firing temperature was monitored and the fired compact properties (bulk density, water absorption, and stain resistance) were determined.The study shows that the DPGP improved the sintering behaviour and final properties of the resulting porcelain tiles with respect to those obtained by the G process. However, the fired compacts prepared from the DPGP powder exhibited a higher porosity and pore size compared with those of the compacts obtained from the SD granules at the same pressing pressure. The results obtained open up the possibility of manufacturing glazed porcelain tiles with a more eco-friendly process. However, the results also indicate that polished porcelain tile manufacture by the DPGP requires further research in order to improve granule characteristics, in particular green granule deformability, which is the critical factor in porcelain tile densification and vitrification during firing.     

Investigation of biphasic calcium phosphate/gelatin nanocomposite scaffolds as a bone tissue engineering

The porous scaffold of nanobiphasic calcium phosphate (n-BCP) and gelatin from bovine skin type B was prepared by freeze-drying method. The porogen which used was Naphthalene. EDC (N-(3-dimethyl aminopropyl)-N′-ethyl carbodiimide hydrochloride) for stabilization of gelatin by cross-linking method was used. The scaffold was characterized by SEM, XRD and FTIR. As a result, a biocompatible scaffold with good cell attachment, facility in formation in desired shapes and simplicity in production were prepared for bone tissue engineering.