Silicon carbide-based foams from direct blowing of polycarbosilane

Macro-cellular porous silicon carbide foams were produced using a polycarbosilane preceramic polymer and a chemical blowing agent (azodicarbonamide). Polycarbosilane (PCS) was mixed with a blowing agent and the mixture was foamed close to the melting point of PCS at 250-260 °C, under nitrogen in order to avoid cross-linking by oxidation. The foamed PCS was then cured under air at 200 °C and pyrolyzed at 1000 °C, leading to the formation of open macro-cellular ceramic components. Porosity ranged from 59 to 85 vol%, and the cell size ranged from 416 to 1455 μm; these values could be modulated by changing the content of blowing agent and foaming temperature. This process is a simple and efficient way to produce silicon carbide-based foam with tailored pore architecture and porosity.     

Fabrication and characterization of porous silver powder prepared by spray drying and calcining technology

High porosity porous silver powder with about 100 µm average size and 5 µm pore size was fabricated by spray drying and calcining technology. Effects of calcining temperature and process of spray-dried powder on the phases, grain size, particle morphology and pore microstructure of silver powder were investigated. The results showed that porous silver with approximately spherical shape and via hole structure was obtained using 0.25 mol Ag₂CO₃ solution of ammonia water, which was spray-dried at 200 °C and calcined at 400 °C for 30 min with heat treatment technology curve of gradient temperature in air. And there were not Ag₂CO₃, Ag₂O and AgO phases existing in the porous silver. However, using 0.25 mol Ag₂CO₃ solution of ammonia water, the porous silver powder could not be fabricated by spray pyrolysis technology with a solution feed rate of 300 mL/h, flux of carrier gas of 0.30 MPa, and 640 °C furnace set temperature.     

Manufacturing of highly porous calcium phosphate bioceramics via gel-casting using agarose

There is a technological need for highly porous bioceramics to be produced in an environmentally friendly manner. Gel-casting of highly porous HAp-(α-TCP) (CaP) foams using agarose as a gelling agent was investigated. Foaming of gel-cast suspension was performed at the temperature of 60 °C followed by transformation of the foams from a liquid state to a gelled state by cooling them to 15 °C. The sintered (1250 °C, 2 h soaking time) foams were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption isoterm and Hg porosimetry. XRD study revealed that additives used in the gel-casting process did not influence the phase composition of the investigated materials. The macroporous microstructure of HAp-(α-TCP) foams was typically composed of approximately spherical pores (cells) interconnected by circular windows. The foams exhibited a broad pore size distribution with cells and windows ranging from 250 to 900 μm, and from 25 to 250 μm, respectively. The mode for spherical pore size was approximately 500 μm while the above value for window was ∼100 μm. Additionally, the small amount of wall microporosity in the range of 0.2–0.9 μm was confirmed by SEM and Hg porosimetry. The obtained porous (P = 90%) HAp-(α-TCP) scaffolds with interesting two types of macropores and a small amount of micropores seem to be a promising bone substitution material.     

Porosity control of cold consolidated geomaterial foam: Temperature effect

Porous K-geopolymers with mutli-scale porosity were synthetized, based on the production of molecular hydrogen due to the oxidation of free silicon associated with polycondensation reactions. The various drying steps at low temperatures influencing the foams morphology were evaluated in function of parameters like mass effect, mold dimensions and drying cycles. The results obtained evidenced the possibility to perform reproducible foams with a control of their porosity. These foams are in agreement with the environmental demand with their recycling properties and their low cost of production due to the cheap materials used and the low temperature of synthesis. Some other advantages like their fire resistance, acid/base resistance, their good mechanical properties and their good thermal conductivity is a good point for future applications. As an example a homogeneous foam with a pore size of 1.5 mm at 50 °C (9 days for an achieved drying but consolidated before) can be prepared. Moreover the porosity can be controlled with various temperature cycles decreasing the time of synthesis. From the cycles tested (70-55 °C and 70-23 °C), some homogeneous samples were obtained with pores sizes varying from 0.5 to 1.5 mm. Then the work was extended to larger surfaces exchange, what evidences the importance of drying and mass effects upon the porosity as well as the mechanical properties of the mold used during synthesis.     

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.     

Formation and characterization of porous spherical biphasic calcium phosphate (BCP) granules using PCL

We developed a process to fabricate porous spherical BCP granules using different volumetric ratios of polycaprolactone (PCL). This technique takes advantage of the formation of spherical drop shapes that occur for any liquid. The method to produce porous granules is based on the liquid immiscibility effect of a PCL solution and distilled water. Granules were sintered at 1300 °C and 1400 °C using conventional and microwave sintering techniques. Using this approach, the granules showed a good pore-interconnected relation and granules with a porosity that ranged from 36.12% to 59.8%, inner pore sizes that ranged from 30 μm to 250 μm, and a granular size of about 1.5 mm could be obtained. Granules were characterized for microstructure, phase composition and porosity. Using this novel approach, we were able to achieve desirable porous characteristics that simulate natural bone structure.     

Porous alkali-activated material from hypergolic coal gangue by microwave foaming for methylene blue removal

Overall, 100% hypergolic coal gangue (HCG)-based geopolymer foams were produced by a novel saponification-microwave foaming combined route. Microwave foaming with and without expired vegetable oil was first used to produce CG-based geopolymer foams. Macropores were mainly generated by microwave foaming, and mesopores were mainly obtained by the addition of expired soybean oil that underwent a saponification reaction. The effects of the oil content on the density, porosity, pore morphology, compression strength, and methylene blue adsorption properties were studied. High total porosity (85.9–89.0 vol%) and acceptable compression strength (0.46–1.1 MPa) HCG-based geopolymer foams were produced. Foams with 12.59 wt% oil exhibited the best adsorption properties, with an adsorption capacity up to 9.4 mg/g and high removal efficiency of about 95.3%. These solid-waste-based porous components are promising monolithic adsorbents for wastewater treatment.     

Effect of high temperature on the mechanical properties of hierarchical porous cenosphere/geopolymer composite foams

A hierarchical porous cenosphere/geopolymer composite foam (FHCs/KGP) was fabricated by the simultaneous incorporation of O₂ pore from hydrogen peroxide and cenosphere filler addition. Effects of both H₂O₂ content and high-temperature treatment on the microstructure, porosity and strength of porous FHCs/KGP foams were investigated systematically. The obtained FHCs/KGP foams showed typical amorphous structure and desirable porosity from 65 to 82%. The composites could crystallize in situ to FHCs/leucite foams above 1000℃. Compression strength of the FHCs/leucite foams showed a maximum value of 5 ± 0.3 MPa when treated at 1000°C. The improvement of mechanical properties for the composite foams was attributed to crack deflection, fractured microspheres and the good bond between the FHCs and matrix. This study could open opportunities to employ cellular foams as alternatives in structure and filtration applications.     

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part II. Foaming of poly(?-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone fluid mixtures and formation of tubular foams via solution extrusion

This paper reports on the foaming of poly(?-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone mixtures. Experiments were carried out in specially designed molds with porous metal surfaces and fluid circulation features to generate foams with uniform dimensions at 60, 70 and 80 °C at pressures in the range 7-28 MPa. Depending upon the conditions, foams with pores in the range from 5 to 200 μm were generated. Adding acetone to carbon dioxide improved the uniformity of the pores compared to foams formed by carbon dioxide alone. In addition, a unique high-pressure solution extrusion system was designed and used to form porous tubular constructs by piston-extrusion of a solution from a high-pressure dissolution chamber through an annular die into a second chamber maintained at controlled pressure/temperature and fluid conditions. Long uniform porous tubular constructs with 6 mm ID and 1 mm wall thickness were generated with glassy polymers like poly(methyl methacrylate) by extruding solutions composed of 50 wt% polymer + 50 wt% acetone, or 25 wt% polymer + 10% acetone + 65% carbon dioxide at 70 °C and 28 MPa. Pores were in the 50 μm range. The feasibility of forming similar tubular constructs were demonstrated with poly(?-caprolactone-co-lactide) as well. Tubular foams of the copolymer with interconnected pores with pore sizes in the 50 μm range were generated by extrusion of the copolymer solution composed of 25 wt% polymer + 10 wt% acetone + 65 wt% carbon dioxide at 70 °C and 28 MPa. Reducing the acetone content in the solution led to a reduction of pore sizes. Comparisons with the foaming behavior of the homopolymer poly(?-caprolactone) that were carried out in the molds with porous metal plates show that the foaming behavior of the copolymer is more akin to the foaming behavior of the caprolactone homopolymer component.     

Effect of elevated temperatures on geopolymer paste, mortar and concrete

Geopolymers are generally believed to provide good fire resistance due to their ceramic-like properties. Previous experimental studies on geopolymer under elevated temperatures have mainly focused on metakaolin-based geopolymers. This paper presents the results of a study on the effect of elevated temperature on geopolymer paste, mortar and concrete made using fly ash as a precursor. The geopolymer was synthesized with sodium silicate and potassium hydroxide solutions. Various experimental parameters have been examined such as specimen sizing, aggregate sizing, aggregate type and superplasticizer type. The study identifies specimen size and aggregate size as the two main factors that govern geopolymer behavior at elevated temperatures (800 °C). Aggregate sizes larger than 10 mm resulted in good strength performances in both ambient and elevated temperatures. Strength loss in geopolymer concrete at elevated temperatures is attributed to the thermal mismatch between the geopolymer matrix and the aggregates.     

Preparation and characterization of foams from sheet glass and fly ash using carbonates as foaming agents

Glass foams were produced using sheet glass cullet and fly ashes from thermal power plant with added carbonates (commercial dolomite- and calcite-based sludges) as foaming agents. The influence of type and amount of carbonates as well as of the sintering temperature on the apparent density, compressive strength, microstructure and crystalline phases was evaluated. The experimental results showed that homogenous microstructures of large pores could be obtained by adding just 1–2 wt.% carbonates and using low sintering temperature (850 °C), leading to foams presenting apparent density and compressive strength values of about 0.36–0.41 g/cm³ and 2.40–2.80 MPa, respectively. Good correlations between compressive strength, apparent density and microstructure (pore size, struts’ thickness and internal porosity) were observed.     

Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gel-casting method for potential applications in heat-insulation materials. The effect of sintering temperature on compressive strength of porous YSZ ceramics was investigated on the basis of measurements linear shrinkage, porosity and pore size. As the sintering temperature increased from 1350 to 1550 °C, a decrease of porosity from 77 to 65%, a decrease of average pore size from and an increase of linear shrinkage from 15.4 to 31.8% were observed. The compressive strength increased remarkably from 3 to 27 MPa with increasing sintering temperature from 1350 to 1550 °C, which was related to the corresponding change of linear shrinkage, porosity, pore size and microstructure. A remarkable decrease of compressive strength with increasing porosity was observed. The compressive strength decreased also with increasing pore size.     

Low-temperature fabrication and characterization of porous SiC ceramics using silicone resin as binder

Commercially available silicone resin and silicon carbide (SiC) powders were adopted as the starting materials for the fabrication of porous SiC ceramics. During the heat treatment process, silicone resin experienced an organic–inorganic transformation and acted as the bonding material between SiC particles at a low temperature of 1000 °C. The mean particle size of starting SiC powders and silicone resin content can control the pore size, open porosity and fracture strength. The flexural strength of porous SiC ceramics increases with increasing silicone resin content and decreasing mean particle size of SiC powders. Larger pores can be obtained with coarser starting SiC powders and higher silicone resin content. The fracture surface of porous SiC ceramics was observed.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Poly(l-lactic acid) short fibers prepared by solvent evaporation using sodium tripolyphosphate

A porous material consisting of biodegradable polymer fibers may be one of the best candidates for implants used in the regeneration of damaged tissue, because it has a continuous pore structure that would allow ingrowth of nutriments, tissues, blood vessels or cells. In the present work, short fibers of biodegradable poly(l-lactic acid) (PLLA) were successfully prepared by the dropwise addition of PLLA dissolved in methylene chloride to a poly(vinyl alcohol) (PVA) solution containing sodium tripolyphosphate with stirring. It was suggested that droplets of the PLLA solution form spheres coated with PVA, which are then deformed into fibrous shapes due to stirring. The length of fibers was 200-800 μm and was controlled by the stirring rate, the PLLA concentration of the droplets and the PVA concentration. A PLLA porous block could be easily prepared by sintering the PLLA fibers at 173 °C for 10 min. The material had a continuous pore structure with the average pore size of approximately 40 μm and porosity of about 80%.     

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.     

Formation of rod-like Si3N4 grains in porous SRBSN bodies using 6Y2O3–2MgO sintering additives

The porous reaction-bonded silicon nitride (RBSN) bodies using (6 wt.% Y₂O₃–2 wt.% MgO) 6Y2M were fabricated by nitridation process at 1350 °C for 8 h. The porous gas pressure sintered (GPSed)-RBSN bodies post-sintered at 1550–1850 °C for 6 h show a microstructure with low aspect ratios having high porosity. The compressive strength of samples sintered at 1650 °C, 1750 °C and 1850 °C were about 146 MPa, 251 MPa and 285 MPa, respectively. The duration time for sintering had a significant effect on the microstructure and grain morphology of the GPSed-RBSN bodies. Even though the GPSed-RBSN was carried out at the comparatively low temperature (1550 °C) for 9 h, high aspect ratio of rod-like Si₃N₄ grains with about 9 was observed. The material properties of samples such as porosity, phase ratio (β/(α + β)) and compressive strength of sample sintered at 9 h were about 43.2%, 99% and 141 MPa, respectively.     

Mechanical activation of fly ash: Effect on reaction, structure and properties of resulting geopolymer

Geopolymerisation of mechanically activated fly ash was studied at ambient (27 °C) and elevated (60 °C) temperatures by isothermal conduction calorimeter. Under both the conditions, mechanical activation enhanced the rate and decreased time of reaction. It was interesting to observe that in the samples milled for 45 min (median size ∼5 μm), a broad peak corresponding to geopolymerisation initiated at 27 °C after 32 h. The rate maxima at 60 °C, a measure of fly ash reactivity, showed a non-linear dependence on particle size and increased rapidly when the median size was reduced to less than 5-7 μm. Improvement in strength properties is correlated with median particle size, and reactivity of fly ash. The characterisation of the geopolymer samples by SEM-EDS, XRD and FTIR revealed that mechanical activation leads to microstructure and structural variations which can be invoked to explain the variation in the properties.     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

Processing and microstructure characterization of porous corundum–spinel ceramics prepared by in situ decomposition pore-forming technique

Porous corundum–spinel ceramics were prepared from Al(OH)₃ and basic magnesium carbonate by an in situ decomposition pore-forming technique. Apparent porosity was detected by Archimedes’ Principle with water as medium. Pore size distribution and the volume percentage of micropores were measured by mercury intrusion porosimetry, and the microstructure was analyzed by SEM. The apparent porosity of the sintered sample decreased with increasing the Al(OH)₃ content in the raw mixture. With increasing temperature from 1200 °C to 1300 °C the porosity of the sample increased rapidly, from 1300 °C to 1500 °C the apparent porosity increased slightly, while it decreased rapidly when the temperature increased from 1500 °C to 1600 °C. The pores in the samples consist of two groups. One group is composed of micropores whose diameter is mostly in the range from 150 nm to 300 nm while the other is composed of bigger pores whose diameter is in the range from 0.5 μm to 1 μm. It was found that the composition of the starting powders and the sintering temperature are responsible for the apparent porosity and the pore size distribution of the samples. However the spinel formation and sintering play a more important role on porosity and pore size distribution.