Preparation of high strength lamellar porous calcium silicate ceramics by directional freeze casting

In this study, porous calcium silicate (CS) ceramics with oriented arrangement of lamellar macropore structure were prepared by directional freeze casting method. The lamellar macropores were connected by the micropores on the pore wall, which had good pore interconnectivity. The effects of solid loading of the slurry, freezing temperature, sintering additive content, and sintering temperature on the microstructures and compressive strength of the synthesized porous materials were investigated systematically. The results showed that with the increase of solid loading (≤20 vol%) and sintering additive content, the sizes of lamellar pores and pore walls increased gradually, the open porosity decreased and the compressive strength increased. The sintering temperature had little effect on the pore size of the ceramics, but increasing the sintering temperature (≤1050 °C) promoted the densification of the pore wall, reduced the porosity, and improved the strength. The decrease of freezing temperature had little effect on porosity, but it reduced the size of lamellar pore and pore wall, so as to improve the strength. Finally, porous CS ceramics with lamellar macropores of about 300–600 μm and 2–10 μm micropores on the pore wall were obtained. The porous CS ceramics had high pore interconnectivity, an open porosity of 66.25% and a compressive strength of 5.47 MPa, which was expected to be used in bone tissue engineering.     

Experimental modelling of copper foams processed through powder metallurgy route using a compressible space holder material

The present work is focused on the processing of open cellular copper foams through space holder technique without the use of binders. In this work, moderate pressures were used during the cold compaction of the powders. The main objective was to obtain dense cell walls by limiting the use of binders and use a compressible and lubricant type of space holder material. It has been shown in this study that the use of high compaction pressure helps to decrease/limit the quantity of binder required and this, in turn, yields relatively dense cell walls necessary for better mechanical strength of foams. Using 2N factorial method, mathematical models have been developed to express the final porosity and pore size as functions of the various processing parameters viz compaction pressure, sintering temperature/ time and space holder content. The most significant sintering parameters influencing the porosity and pore size of the processed foams have also been found out.     

Effects of Particle Packing Characteristics on Solid-State Sintering

Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid-state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.     

Porous ceramic supports for membranes prepared from kaolin and doloma mixtures

The present paper reports the in situ synthesis of porous ceramic supports from local kaolin and kaolin–doloma mixtures. These raw materials have been dictated by their natural abundance (low price) and their beneficial properties. In this work, four different processing routes have been presented. In addition, two support shapes are of particular interest: tubular and flat configurations, which are currently the most used supports in membrane research. Tubular configurations have been produced by extrusion method whereas flat configurations have been produced by both dry-pressing and roll pressing. The doloma addition to kaolin has a positive effect on the porosity ratio of supports compared to those prepared from kaolin alone. Moreover, the influence of the sintering temperature on the total porosity, average pore size, pore size distribution and strength of supports has been investigated. It has been found that higher sintering temperatures (1250 °C) were needed to obtain a uniform pore size distribution within total porosity ratios of 43 and, 51% when processes 3 and 2 were applied, respectively.     

Effects of nano-alumina content on the formation of interconnected pores in porous purging plug materials

The physical properties and microstructure of porous purging plug materials added with different nano-alumina contents and firing temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, air permeability, pore size distribution, mean pore size, apparent porosity, bulk density, and cold crushing strength (CCS) tests. The results showed that the addition of nano-alumina had a great effect on the physical properties and microstructure of the porous purging plug materials. With increasing nano-alumina content in the composition, the main phase was α-Al₂O₃ in all compositions and the mean pore size, apparent porosity and air permeability all increased due to the increased number of pores and pore size of the specimens which facilitated the formation of interconnected pores. When the sintering temperature was changed from 1550 °C to 1650 °C, some of the smaller pores vanished due to solid phase sintering, which reduced the apparent porosity, and some open pores connected to form interconnected pores, which promoted increased air permeability. In addition, the strength and porosity were found to follow the relationship σ = σ₀ exp (-b P). When the apparent porosity increased, the CCS decreased, and vice versa.     

Pore evolution of microporous magnesia aggregates with the introduction of nano-sized MgO

Microporous refractories applied in the working-lining of metallurgical furnaces have been rapidly developed in recent years owing to the outstanding mechanical properties, thermal insulation performance and slag resistance, the pore structure of which plays a critical role in the variation of service performance. Meanwhile, the microporous magnesia aggregates were prepared in our previous research with the introduction of nano-sized particles to overcome the shortcomings of high thermal conductivity, poor thermal shock resistance and slag penetration resistance, however, the pore evolution during sintering still remains to be investigated. Hence, in this study, the pore evolution of microporous magnesia aggregates is explored specifically and the effect of nano-sized MgO on pore structure and sintering is simultaneously discussed. The sintering model of microporous magnesia was built for analyzing the pore structure evolution process. The results revealed that a micro-nano double-scale sintering model developed by the introduction of nano-sized MgO dramatically promoted the sintering kinetic force and boundary migration velocity. The sintering pressure discrepancy and free energy change per unit mole of specimens were respectively increased by ～43 times and ～48 times, which effectively improved the closed porosity and pore distribution homogeneity, while reduced the pore size. Meanwhile, the high sintering diving force lead to the significant improvement of direct bonding degree and grain size of microporous magnesia. With the addition of 3 wt% nano-sized MgO, the optimal sintering properties with closed porosity of 6.4%, bulk density of 3.23 g/cm³ and median equivalent pores diameters of 4.07 μm were achieved. The exploration of pore evolution in microporous magnesia aggregates contributed to the fabrication and industrialization development of microporous refractories.     

Novel model for the sintering of ceramics with bimodal pore size distributions: Application to the sintering of lime

A mathematical model for the sintering of ceramics with bimodal pore size distributions at intermediate and final stages is developed. It considers the simultaneous effects of coarsening by surface diffusion, and densification by grain boundary diffusion and lattice diffusion. This model involves population balances for the pores in different zones determined by each porosimetry peak, and is able to predict the evolution of pore size distribution function, surface area, and porosity over time. The model is experimentally validated for the sintering of lime and it is reliable in predicting the so called “initial induction period” in sintering, which is due to a decrease in intra‐aggregate porosity offset by an increase inter‐aggregate porosity. In addition, a novel methodology for determination of mechanisms based on the analysis of the pore size distribution function is proposed, and with this, it was demonstrated that lattice diffusion is the controlling mechanism in the CaO sintering. © 2016 American Institute of Chemical Engineers AIChE J, 63: 893–902, 2017     

Effects of Pore Morphology and Grain Size on the Dielectric Properties and Tetragonal–Cubic Phase Transition of High-Purity Barium Titanate

The effects of pore morphology and grain size on the dielectric behavior of high-purity stoichiometric BaTiO₃ have been intensively investigated. It was found that the dielectric constant was influenced not only by grain size but also by pore morphology. Dielectric constants below the Curie temperature could be evaluated by the Maxwell relationship for specimens with fractional density >90%ρ_t and be estimated by the modified Niesel's equation, but depolarization might be involved for specimens with fractional density <90%ρ_t. Dielectric Behavior above the Curie temperature followed the Curie–Weiss low. The Curie constants could be separated into two regions depending on the pore morphology, decreasing linearly with increasing porosity at different rates. The results suggest that the tetragonal–cubic phase transition temperature of specimens with fractional density <90%ρ_t is affected by depolarization due to the presence of continous channel pores. The dissipation factor was increased with increasing porosity due to the adsorption of water. In this study, a high-density (<99%ρ_t), uniform, and fine-grained (∼1.2 μm) microstructure of high-purity stoichimetric barium titanate has been produced by using wet processing ad pressureless sintering, in which a high dielectric constant (>6100 at 25°C and 1 kHz) and a low dissipation factor (<0.025) could be achieved.     

Fabrication of porous titania sheet via tape casting: Microstructure and water permeability study

In this article, we report the effects of slurry formulation and sintering conditions on the microstructure and permeability of porous titania sheets prepared by tape casting. It was found that solid concentration and binder content in the titania slurry play a vital role in the porosity and microstructure of the sintered titania sheets. Solid concentration and binder content were optimized based on the green tape quality and open porosity of the sintered titania sheets. The optimum solid concentration with the lowest surface roughness was obtained at 0.61 g/cm³. The effects of temperature and sintering time on the open porosity and crystal structure of the final product were also investigated. Increasing the sintering temperature from 1000 to 1100 °C resulted in increasing the pore size from 170 to 264 nm and decreasing the open porosity. Finally, water permeability of the porous titania sheets was studied to evaluate the permeation flux and maximum operating pressure. The results revealed that the permeability of the porous titania sheet is increased not only by increasing the open porosity but also by increasing the pore size.     

Cellular mullite materials processed by direct foaming and protein casting

In this paper, cellular mullite bodies were developed by thermal direct-consolidation of foamed aqueous mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose suspensions, burning out (650 °C, 2 h) and sintering (1600 °C, 2 h). Some modifications to the shaping route conventionally used in protein casting were incorporated in the proposed processing to obtain bodies with controlled and homogeneous microstructures. The materials were characterized by porosity measurements, analysis of phases by XRD, and microstructural analysis by SEM and Hg-porosimetry. Characteristic parameters of cell size distributions, percentage of open and closed cells, window size and interstitial pore size distributions, and microstructural features of the mullite matrix were determined. Moreover, basic 2D cell size parameters and global 3D stereological parameters were analysed. The obtained results showed that mullite bodies with hierarchical porosity and different microstructural features were developed from the design and control of processing routes, which use BSA as a foaming and binder/consolidator agent.     

Simulation of densification behavior of nano-powder in final sintering stage: Effect of pore-size distribution

In the final sintering stage, nano-sized powder frequently forms a pore structure where most pores are surrounded by more than 5 grains. The pore structure is different from that of coarse powder. In this study, the densification behavior of nano-sized powder is modelled and simulated in the final sintering stage. The porous body has the initial size distribution of pores, represented as a Weibull function. The mechanical interaction between pores is analyzed to simulate the evolution of porosity characteristics as well as densification kinetics. The densification rate for the size-distributed pores is lower than that for single-sized ones. The experimental relationship between the densification rate and the porosity could well be reproduced by choosing appropriate pore-size distributions. The simulation also shows that the sintering stress with densification may increase or decrease depending on the size distribution, but is remarkably lower than that for single-sized pores.     

Preparation and Sintering of Colloidal Silica-Potassium Silicate Gels

Porous silica gelled from colloidal silica and potassium silicate mixtures as developed by Shoup has been investigated. Various gels exhibit an average pore size from 50 to 250 nm and a porosity up to 85%. Dealkalization by solution leaching is found to depend on the average pore size and is easier for large pore sizes. A domain model has been proposed for the gel networks to account for the observation of both macropores and mesopores as measured by Hg porosimetry and N₂ desorption, respectively. The pore coarsening observed during sintering is explained by the material transport from areas around interdomain pores to those around intradomain pores in order to minimize the total surface energy.     

Macro-porous ceramic supports for membranes prepared from quartz sand and calcite mixtures

A new type of porous ceramic supports for membranes has been designed. The new supports have been fabricated from polycrystalline quartz sand and calcite raw materials. In this work, two configurations of support (tubular and flat) have been produced using extrusion method. The open porosity, the pore size distribution, the average pore size (APS), the strength and the permeability of sintered supports have been found to depend mainly on the weight ratio of calcite (CaCO₃) additive. The results showed that with the addition of 15–35 wt.% of calcite and sintering temperature of about 1375 °C for 1 h the best characteristics of sintered supports could be obtained. The developed tubular ceramic supports with the APS 6.3–12 μm, open porosity 42–55%, the water permeability (16–68 m³/h m² bar) and flexural strength 8–18 MPa hopefully offer many perspectives for a wide use in membranes technology.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

Fabrication and characterisation of cellular alumina articles produced via radiation curable dispersions

A general and versatile method for the production of cellular materials from radiation curable solvent-free colloidal ceramic dispersions containing pore formers has been developed. By this technique cellular ceramic articles with a precisely controlled porosity, cell size and shape are obtained for compositions containing solid pore formers. Monolithic bulk samples are obtained by thermal curing, whereas thin films and multi-layered articles are advantageously produced by UV curing. In this work the influence of three different spherical pore former types, PE, PS and PMMA, on the processing and final properties of the porous materials using alumina as model material is studied. The effect of pore former type and concentration on rheology, curing behaviour, debinding and sintering steps as well as thermal conductivity and mechanical strength of the sintered cellular materials is presented. It is also shown that the choice of pore former type modifies the sintering behaviour and resulting 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.     

Fabrication of Highly Porous Chromium Carbide with Multiple Pore Structure

A unique process, which consists of camphene‐based freeze‐casting and reactive sintering, has been developed to fabricate highly porous Cr₃C₂ ceramics, which were featured with a well‐defined multisize porous structure, consisting of large pore channels throughout the sample and small pores on the porous struts. The porous structure was found to be affected by the solid loading content and the casting temperature. Decreasing the solid loading content or increasing the freezing temperature increased the size of large pore channels. However, the size of small pore was not significantly influenced by the variation in the freeze‐casting parameters. Compared with other techniques reported for the fabrication of porous ceramics, high porosity can easily be achieved by this method. The sintering mechanism of the formation of the porous structure was also discussed in this study.     

Application of partial sintering of waste glasses for preparation of porous glass bodies

Porous glasses were prepared by partial sintering of waste glasses. Polyvinyl alcohol was added as a binder to the glass powder and the mixture was un-axially cold pressed under two different forces, followed by sintering at 700 °C. The effects of thermal history, particle size of glass powder, binder content and applied pressing forces on pore size and total porosity of fabricated porous glasses were investigated and final products with the porosity of 15–32 % were prepared. The average pore size of the specimens was determined using mercury porosimetry. The morphology of the porous glasses was observed by scanning electron microscopy. These produced porous glasses can be used for selective and accurate filtration.     

Characterization of the compaction and sintering of hydroxyapatite powders by mercury porosimetry

Mercury porosimetry was used to measure the bulk and real densities, pore volumes and pore size distributions of compacts of hydroxyapatite before and after sintering. The hydroxyapatites were prepared by two different methods and had widely different surface areas. The properties were determined as a function of compaction force and sintering temperature. Densities from porosimetry were in good agreement with geometric densities. A linear relation was found between pore volume and log of the applied force. There was also a linear relationship between bulk volume and pore volume of the compacts. A bimodal pore size distribution was observed for the high surface area hydroxyapatite which disappeared with increasing compaction loads. Pressurization and depressurization measurements indicated that the main body of the pores in the compacts attained a more regular “spherical” shape with increasing compaction force than did the “necks”. The pore volume, percent porosity, and bulk density of the compacts remained unchanged up to 600°C; however, the surface area and the average pore diameter changed at 400°C. The distribution of pores became more uniform, narrower in distribution, and larger in size as the sintering temperature increased. The change in pore area with pore volume indicated that two mechanisms were operating during sintering. The pore area proved to be the most sensitive indicator of changes during sintering.     

A simple pathway in preparation of controlled porosity of biphasic calcium phosphate scaffold for dentin regeneration

A simple pathway in preparation of biphasic calcium phosphate scaffold of hydroxyapatite/beta-tricalcium phosphate with controlled pore size, shape and porosity using phosphoric acid and calcium carbonate was successfully developed. Microporosity was controlled by adjusting temperature and soaking time of the sintering process while macroporosity was obtained through addition of polyethylene spherical particles. The advantage of this method is that a highly pure biphasic calcium phosphate scaffold consisting of hydroxyapatite/beta-tricalcium phosphate in a controlled ratio of 20/80 with a mean pore size of 300 μm and 65% porosity can be produced. These properties of scaffold are of high potential for use in dentin regeneration.