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.     

A hot isostatic process for fabricating porous materials

Hot isostatic processing (HIP) is the latest method to produce open porous materials, such as ceramics or metals. In this review paper, features and applications of HIPed porous materials are described. Sintering behavior under high pressure gas is also discussed in order to understand how porous materials are obtained by this method. HIPed porous materials have higher open porosity, higher mechanical strength, narrower pore size distribution and higher fluid permeability than conventionally sintered ones. These excellent properties of HIPed porous materials are due to the enhanced surface diffusion caused by the high gas pressure. By applying these excellent properties of HIPed porous materials, filters, grinding wheels, and porous electrodes for electrochemical analysis can be produced with better properties than products made by conventional sintering.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Porous ceramics with controllable properties prepared by protein foaming-consolidation method

A new protein foaming-consolidation method for preparing porous alumina was developed using egg yolk both as consolidating and foaming agent. This method allows the control of properties of porous alumina not only by varying alumina-to-yolk ratio but also by managing the foaming process. After drying, the green bodies were burned at 600 °C for 1 h to remove the pore creating agent, followed by sintering at 1,550 °C for 2 h. The porous alumina ceramics with pore sizes of 25–1,000 μm and relative density of 29–50% were obtained. The compressive strength of the sintered samples varied within the range of 1.1–5.7 MPa, corresponding to porosity of 40–71%. The addition of dispersant with different concentration into alumina slurries shifted the rheological properties from shear thinning behavior to a Newtonian fluid, which resulted in changes in the pore sizes of the resulting ceramics. The main advantages of the process are the simplicity of the process and the low-cost processing equipment/materials needed. These results have opened a novel preparative way for porous ceramics especially alumina-based porous materials designed for biomedical applications.     

Measuring the Young’s modulus of polystyrene-based composites by tensile test and pulse-echo method

In this study, the pure polystyrenes (PS) with different molecular weights (3.5 × 10⁵ and 5.0 × 10⁵) have been modified by the chemical modification with succinic anhydride (SA), maleic anhydride (MA), and phthalic anhydride (PhA). The modified polystyrenes (MPS) have been mixed with the pure PS with the molecular weight of 2.3 × 10⁵ in weight % ratio 90:10, 80:20, and 70:30. Young’s modulus of obtained composites has been measured mechanically by the tensile test and ultrasonic method at frequency of 5 MHz. Further, the values of Young’s modulus measured by both methods have been compared with each other. From the results, a significant difference has not been found between the values of Young’s modulus of both methods. As a result it can be stated that measuring the Young’s modulus of these materials by the ultrasonic methods is more sensitive and economical than the mechanical methods.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.     

The preparation and characterization of porous alumina ceramics using an eco-friendly pore-forming agent

This work presents the preparation of porous alumina ceramics through the sacrificial phase method, using an eco-friendly material, namely waste coffee grounds, as a pore-forming agent. The effects of coffee grounds content in the green ceramic bodies on the linear and volumetric shrinkage, as well as the total and open porosity of the sintered product, were evaluated. The influence of the resulting porosity on mechanical properties of the prepared porous alumina was determined using Brazilian disk compression test for the determination of the indirect tensile strength of the prepared samples. Microstructure and pores morphology were characterized by scanning electron microscopy. Porosities in the range 35-54 vol% were achieved, by varying the coffee grounds content from 0 to 50 wt% in the green bodies. The indirect tensile strength of the final obtained porous alumina ceramic decreased accordingly from 57.4 MPa to 17.7 MPa.     

Mechanical properties of sintered meso-porous silicon: a numerical model

Because of its optical and electrical properties, large surfaces, and compatibility with standard silicon processes, porous silicon is a very interesting material in photovoltaic and microelectromechanical systems technology. In some applications, porous silicon is annealed at high temperature and, consequently, the cylindrical pores that are generated by anodization or stain etching reorganize into randomly distributed closed sphere-like pores. Although the design of devices which involve this material needs an accurate evaluation of its mechanical properties, only few researchers have studied the mechanical properties of porous silicon, and no data are nowadays available on the mechanical properties of sintered porous silicon. In this work we propose a finite element model to estimate the mechanical properties of sintered meso-porous silicon. The model has been employed to study the dependence of the Young’s modulus and the shear modulus (upper and lower bounds) on the porosity for porosities between 0% to 40%. Interpolation functions for the Young’s modulus and shear modulus have been obtained, and the results show good agreement with the data reported for other porous media. A Monte Carlo simulation has also been employed to study the effect of the actual microstructure on the mechanical properties.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

Effect of CTAB as a foaming agent on the properties of alumina ceramic membranes

Alumina-ceramic membranes were prepared by gelcasting process using CTAB as a foaming agent. To increase the fineness, the starting alumina powder was milled for 1 h in a ball mill before the casting process. Particle size distribution and surface area measurements of the as-received and milled alumina powder were examined. The casted alumina membranes were sintered at 1500 °C. Sintering parameters in terms of bulk density (BD) and apparent porosity (AP) were determined by the Archimedes method. Pore size distribution of the sintered porous alumina membranes was measured using mercury porosimeter. Microstructure of sintered membranes was investigated by scanning electron microscope (SEM). Cold crushing strength (CCS) of the sintered specimens was also evaluated. The result revealed that the properties of porous ceramics such as porosity, average pore size, pore size distribution and cold crushing strength could be controlled by adjusting the preparation conditions e.g. solid loading, sintering temperature and foaming agent. The open porosity, cold crushing strength and average pore size of the alumina ceramics sintered at 1500 °C were around 58.35%, 18 MPa and178 nm, respectively.     

Mechanical strength and damage tolerance of highly porous alumina ceramics produced from sintered particle stabilized foams

Highly porous alumina particle stabilized foams were prepared by combining the concepts of particle stabilized foams and gelcasting, using sulfonate surfactants and poly vinyl alcohol (PVA) as the gelcasting polymer. The ceramic samples sintered at 1500 °C for 2 h had porosities from 65% to 93%, with pore sizes in two categories: “big pore” around 300 μm and “small pore”, around 100–150 μm, depending on the type and amount of surfactant added. The mechanical behaviour of the foams (axial and diametral compression) depended on the overall porosity and pore size. On average, tensile and compressive strengths around 5 and 16 MPa respectively were measured for samples with bigger pore sizes and larger porosities. Samples with smaller pore sizes and lower porosities produced average values of 12 and 57 MPa for tensile and compressive strengths, respectively. The elastic modulus reached a maximum around 3GPa for “small pore” size samples. The effect of increasing amount of PVA in the samples had a strong effect on the green mechanical strength, but it did not significantly affect the mechanical response of the sintered alumina foams. Large and complex shape sintered components produced using this route showed a remarkable damage tolerance, due to crack tip blunting.     

Mechanical strength and defect distributions in flash sintered 3YSZ

The influence of preexisting defects and the generation of new ones during flash sintering was studied in 3YSZ. Weibull statistics was used to analyse mechanical testing data from flash and conventionally sintered specimens with equal densification. The obtained values for Weibull modulus and characteristic strength are 6.09 and 371 MPa for the conventionally sintered samples, respectively, and 5.92 and 506 MPa for the flash sintered samples. It can be inferred that flash sintering does not alter significantly the defect distribution on the studied material. Preexisting defects were manufactured with rice starch as a pore forming agent, in 5, 10 and 15 vol%. Those samples were flash sintered in isothermal and constant heating rate experiments. The experiments show that there is no noticeable difference on the incubation time and the onset temperature for flash sintering.     

Mechanical properties of porous methyl silsesquioxane and nanoclustering silica films using atomic force microscope

Mechanical properties of porous methyl silsesquioxane samples with dielectric constant 2.4 and 2.0 and a recently developed nanoclustering silica film samples with dielectric constants 2.3 and 2.0 were evaluated using an atomic force microscope based nanoindentation. It was found that the Young’s modulus and the hardness decrease while the fracture toughness increases with a decrease in the dielectric constant in the same type of material. Moreover, the Young’s modulus and the hardness of the nanoclustering silica films were observed to be at least twice and fracture toughness values ~1.3–1.5 higher than those for methyl silsesquioxane films with similar dielectric constants. The high resolution topographic imaging capability of atomic force microscope was shown to be particularly useful in the measurement of cracks generated by the ultra-low indentation loads, and the evaluation of the fracture toughness of the nanoscale volumes of materials.     

Preparation and characterization of three-dimensional nanostructured macroporous bacterial cellulose/agarose scaffold for tissue engineering

We fabricated a three-dimensional nanostructured macroporous bacterial cellulose scaffold (3D BC scaffold) and a three-dimensional nanostructured macroporous bacterial cellulose/agarose scaffold (3D BC/A). Results of scanning electron microscopy (SEM) and mercury intrusion porosimeter showed that both the 3D BC and the 3D BC/A have interconnected macropores characterized by nanofibrous pore walls (The diameter of the dominant pores was about 100 μm and ranges from <1 μm to >1,000 μm). 3D BC/A also has high surface area (80 ± 5 m²/g) and sufficient porosity (88.5 ± 0.4%) compare with 3D BC (surface area: 92.81 ± 2.02 m²/g; porosity 90.42 ± 0.24%). 3D BC/A do support C5.18 cell and hBMSC attachment, proliferation evaluated with SEM, confocal microscopy and cell proliferation assay. Furthermore, 3D/ABC has enhanced mechanical property (ultimate compressive strength: 26.26 ± 4.6 kPa, Young’s modulus: 39.26 ± 5.72 kPa)) than that 3D/BC has (ultimate compressive strength: 7.04 ± 2.34 kPa, Young’s modulus: 10.76 ± 3.52 kPa). Overall, the 3D BC/A scaffold had more potential than 3D BC scaffold for using as a scaffold for tissue engineering and tissue repair applications.     

Elastic Properties of 4–6 nm-thick Glassy Carbon Thin Films

Glassy carbon is a disordered, nanoporous form of carbon with superior thermal and chemical stability in extreme environments. Freestanding glassy carbon specimens with 4–6 nm thickness and 0.5 nm average pore size were synthesized and fabricated from polyfurfuryl alcohol precursors. Elastic properties of the specimens were measured in situ inside a scanning electron microscope using a custom-built micro-electro-mechanical system. The Young’s modulus, fracture stress and strain values were measured to be about 62 GPa, 870 MPa and 1.3%, respectively; showing strong size effects compared to a modulus value of 30 GPa at the bulk scale. This size effect is explained on the basis of the increased significance of surface elastic properties at the nanometer length-scale.     

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm². The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.     

Analyses of Young's modulus and thermal expansion coefficient of sintered porous alumina compacts

This paper reports the derivation of Young's modulus (E) and thermal expansion coefficient (TEC, β) for a sintered porous structure with open pores. The theoretical E is affected by the number of grains and the grain boundary area in sintered ceramics. The measured E–porosity relationship for porous alumina compacts were compared with the theoretical E values that were derived for the present open-pore structure and also for the dispersed (closed)-pore structure treated previously. With decreasing porosity (50% → 10%), the scattered E values showed a gradually increasing tendency, which were located between two theoretical curves for the open-pore structure. The sudden increase of E values in the porosity range from 10% to 0% was well explained by the theoretical dependence of E on porosity for the open- or closed-pore structure. The β values for the porous alumina structures were independent of porosity and close to the β values reported for fully dense alumina compacts. This result was in accordance with the theoretical β–porosity relationships for the open-pore and closed-pore structures.     

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.     

Preparation and properties of porous alumina with inter-locked platelets structure

Porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering of AlOF mesophase gas by the reaction of HF and Al₂O₃. The effect of heating treatment temperatures on porosity, the formation of inter-locked platelets structure and compressive strength of porous alumina ceramics was determined by Archimedes' method, XRD, SEM and compressive tests. The results indicated that after heating at temperatures from 1300 °C to 1600 °C, the porosity of alumina ceramics decreased from 61.6% to 48.4%. Increasing the heating treatment temperature was beneficial to form inter-locked structure between alumina platelets. The maximum compressive strength of porous ceramics with porosity of 48.4% can reach 29.8 MPa heated at 1600 °C; this strength was attributed to the strong bonding between the alumina platelets.     

Flexural strength of a conventionally processed and additively manufactured debased 94% alumina

Mechanical strength of a 94 wt% debased alumina was measured using ASTM-C1161 specimens fabricated via conventional and lithography-based ceramic manufacturing (LCM) methods. The effects of build orientation and a 1500°C wet hydrogen fire added to the LCM firing sequence on strength were evaluated. A Weibull fit to the conventional flexural specimen data yielded 20 and 356 MPa for the modulus and characteristic strength, respectively. Weibull fits of the data from the LCM specimens yielded moduli between 7.5 and 11.3 and characteristics strengths between 333 and 339 MPa. A Weibull fit to data from LCM specimens subjected to the wet hydrogen fire yielded 14.2 and 376 MPa for the modulus and characteristic strength, respectively. The 95% confidence intervals for all Weibull parameters are reported. Average Archimedes bulk densities of LCM and conventional specimens were 3.732 and 3.730 g/cm³, respectively. Process dependent differences in surface morphology were observed in scanning electron microscope (SEM) images of specimen surfaces. SEM images of LCM specimen cross-sections showed alumina grain texture dependent on build direction, but no evidence of porosity concentrated in planes between printed layers. Fracture surfaces of LCM and conventionally processed specimens revealed hackle lines and mirror regions indicative of fracture initiation at the sample surface rather than the interior.