Fabrication Of Porous Ceramics Research Articles

Preparation of cost-effective and self-reinforced porous mullite ceramics for advanced heat insulation and sound absorption applications

Self-reinforcing porous mullite ceramics have attracted considerable attention in the field of insulation and sound absorption materials, due to their precise and controllable porous structure, high temperature and corrosion resistance, and low thermal conductivity. However, the high cost and relatively low performance obstruct their practical applications. In this work, self-reinforcing porous mullite ceramics were fabricated by utilizing construction waste via foam-gel casting and freeze-drying methods, and the impact of blowing agent dosage and sintering temperatures on the microstructures, sound absorption, and thermal insulation properties were systematically investigated. In addition, the related mechanism was also discussed in detail. The results show that, at a sintering temperature of 1000 °C and blowing agent dosage of 0.10 wt%, the self-reinforcing porous mullite ceramics can be successfully obtained, which exhibited a bulk density as low as 0.24 g/cm³ and porosity more than 90 %. Consequently, at a frequency of 2000 Hz, its absorption coefficient reached about 0.81. Furthermore, its thermal conductivity also decreased to about 0.06266 W/(m·K). This development not only unveils innovative avenues for the cost-effective fabrication of porous ceramics but also significantly broadens their potential applications within high-performance thermal insulation and sound absorption domains.

A REVIEW OF THE DEVELOPMENT OF THE GEL CASTING METHOD FOR POROUS CERAMIC FABRICATION

The objective of this paper is to present an overview of the development of gel casting methods on synthesis techniques and the characteristics of gel casting porous ceramics and its applications. The gel casting method is a colloidal ceramic synthesis method that requires a short period for the ceramic body to develop. This review provides the latest developments in the gel casting process of synthesizing porous ceramics. At the beginning of the gel casting method development, the acrylamide (AM) system was the most commonly utilized system. Over time, the use of the AM system began to be minimized because of its toxicity. Researchers began to develop natural polymers as pore templates and natural minerals as raw materials in the gel casting method. In addition to polymers and raw materials, the gel casting process also requires additives such as gelling agents and dispersants. At the end of this review, a mapping of how to select the additives in the gel casting process, for AM systems and non-AM systems for the fabrication of porous ceramics is presented. The characteristic of porous ceramics produced by AM and non-AM systems are almost similar. Porous ceramic applications have been used as bone tissue supports, energy storage, filters, and catalyst support. This review will be a reference for future research related to the process of fabrication of advanced porous ceramics using the environmentally friendly gel casting method.

Porous Ceramics via Processable Pickering Emulsion Stabilized by Oppositely Charged Colloids.

We report a method to produce processable Pickering emulsions in which electrostatic heteroaggregation of oppositely charged fumed silica and alumina particles is exploited. The ability of weakly charged heteroaggregates to adsorb to the oil-water interface favors the formation of highly stable emulsions. A control over the microstructure and rheology of the Pickering emulsion is demonstrated by tuning parameters such as composition, pH, and total concentration of particles. The Pickering emulsions formed under optimal conditions are found to be gel-like and are excellent templates for the fabrication of porous ceramics. Furthermore, the porous mullite ceramic structure which has exceptional thermal and chemical resistance under harsh environments is obtained by drying and sintering of the emulsion template. The phase evolution, microstructure, and porosity of the resulting ceramic are characterized.

Fabrication of porous (Ba,Sr)(Co,Fe)O3-δ (BSCF) ceramics using gelatinization and retrogradation phenomena of starch as pore-forming agent

Porous (Ba,Sr)(Co,Fe)O3-δ (BSCF) ceramics with high open porosity and good electrical conductivity was fabricated using Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), which shows a high mixed ionic-electronic conductivity. In general, during the fabrication of porous ceramics by the sacrificial template method using pore former particles, closed pores are easily formed unless sufficient pore former particles are added. In this study, we have devised a method using the gelatinization-retrogradation phenomena of starch for producing a porous body with an excellent percolated pore network structure. By dispersing BSCF and starch in an aqueous slurry (0–50% by weight) and heating, gelatinization of the starch occurred and the starch particles adhered to each other. Furthermore, in order to retain the percolated structure, the water solvent was removed by freeze-drying without heating to obtain a dried green body. The sintering behavior of the porous BSCF bodies prepared under various conditions was characterized by microstructural observations and relative density measurements. By optimizing the process conditions of the gelatinization and retrogradation, a porous body having an open porosity of 48.3%, and with 99% of the total pores open, was obtained. The matrix was also well connected and showed a sufficiently high conductivity which was similar to the porous bodies made by the traditional sacrificial template method.

Fabrication of Porous Ceramics using Tailings Mud from Huzhou City as Raw Material

The comprehensive utilization of tailings mud has become a common concern of the society. The properties of tailings mud produced from Huzhou city were analyzed in this work. Moreover, porous ceramics with high strength were prepared by using the tailings mud as main raw material and adding a small amount of foaming agent. The results show that the foaming agent contents have a certain effect on the porosity and compressive strength of the porous ceramic samples. These results provided a certain theoretical basis for the comprehensive utilization of tailings mud and solve the pollution of tailings mud to the environment.

Fabrication of porous alumina by directional freeze casting and annealing in TBA–water–sucrose system

The volume change difference between water and tert-butyl alcohol (TBA) hydrate was approximately 10.1% when ceramic slurry with water–TBA was frozen for fabrication of porous ceramics. Residual stress may be produced at the pore walls and led to low compressive strength of the ceramics. In this regard, sucrose was used in the ceramic slurry. Several water–TBA crystals were observed in the sucrose amorphous bodies after directional freezing. Water and TBA in the sucrose amorphous bodies were recrystallized during subsequent annealing. Sucrose was used as a protective agent and annealing additive in the TBA–water–sucrose system to eliminate residual stress and change the pore structure from snowflakes and dendrites into tiny cylinders. Adding sucrose and annealing process exerted minimal effect on the open porosity of porous ceramics but remarkably improved their compressive strength. When TBA and sucrose contents were 30 and 20 wt%, respectively, in the ceramic slurry with 20 vol% alumina content, the compressive strength of porous ceramics was 49.9 MPa after annealing at − 15 °C for 2 h. The value was 3.6-fold higher than that of the ceramic without annealing. Results provide a new method for improving the mechanical properties of porous ceramics prepared by freeze casting.

Fabrication of porous ceramics with double-pore structure by stepwise freeze casting using water/diphenyl methane emulsion

Porous alumina with double-pore structure was fabricated by stepwise freeze casting using water/diphenyl methane (DPM) emulsion as freezing medium. The effects of DPM content in the alumina slurry on the pore structure, mechanical properties, and specific surface area of porous ceramics were investigated. The porosity of porous ceramics was not significantly affected by DPM content in the alumina slurry due to their same solid contents. The lamellar pore size decreased, and the spherical pore size increased in porous alumina with increasing DPM content in the slurry. Closed spherical pores were formed in lamellar pore walls when the DPM content was 10vol%. The compressive strength slightly increased, and the specific surface area decreased in the samples with DPM compared with those in porous ceramics with pure water as freezing medium. The specific surface area of porous alumina was 0.566m2/g, and the interception rate for titan yellow was 44.16% when the DPM content reached 25vol% because the large spherical pore was connected to the outside. Hence, the fabricated porous alumina with DPM exhibited potential as adsorption and filtration materials.

Freeze cast fabrication of porous ceramics using tert-butyl alcohol–water crystals as template

Tert-butyl alcohol (TBA)–water (H2O) crystals are used as template to fabricate aligned porous alumina with special pore structures. Alumina slurries were prepared using TBA–H2O mixtures, which could undergo continuous crystallization by unidirectional freezing. The pore shape of porous alumina constantly changed from lamellar to snowflake, dendritic, needle-like, and hexagonal with the increase in the TBA content in alumina slurries. The hexagonal porous structure obtained by pure TBA slurry exhibited the best mechanical properties, whereas the mixture structure with irregular pore walls showed the worst, exhibiting inferior properties to lamellar pore structures.

Wollastonite ceramics with bimodal porous structures prepared by sol–gel and SPS techniques

In this work, we suggest a novel synthetic route combining sol–gel and SPS for the fabrication of porous ceramics based on wollastonite with bimodal pore size distributions (100–500 nm and 1–500 μm) and high mechanical strength (Young's modulus from 72.5 to 172 MPa). We studied peculiarities in the formation of the biporous silicate framework using two types of pore forming additives (templates) of various natures, shapes and sizes, introduced during different stages of sol–gel and SPS synthesis: organo-nonorganic (polymer latex of “core–shell” type) and nonorganic (carbonaceous filler). The influence of carbonaceous template content (5% or 25%) on the structural and mechanical properties of ceramics synthesized by spark plasma current was described. The original approach presented here provides formation of wollastonite ceramics with high mechanical strength and a biporous structure that is similar to the texture of bone tissue and is capable of executing its main functions. The obtained ceramics meet all the requirements of the bioceramics class demanded by modern medicine.

Fabrication of porous ceramics as clay/glass composite

<p>Nowadays porous ceramics are widely researched, becoming an increasingly marketable material in the world, mainly due to the wide possibilities of usage in different technical and technology industries. Porous ceramics are successfully used in the filtration and has a high potential of usage also in the production of heat insulation materials thus obtaining the material which combine high resistance that can compete with other heat insulation and constructive materials.</p><p class="R-AbstractKeywords">Article reports a study of porous ceramics, which are produced using foamglass pellets as melting fillers, despite the fact that these additives are not frequently used as filler in traditional ceramic materials. The basis of this method is mixing fire resistant material with hard and melting substance.</p><p class="R-AbstractKeywords">For the production of porous ceramics clay, hard filler, water and various sized foamglass pellets were used, thus allowing to determine optimal size of melting filler and thereby ensuring the necessary physical and mechanical properties of the obtained porous ceramic samples and required amount, size and division of pores. Compressive strength tests were performed, as well as density and water absorption of the samples was determined.</p><p class="R-AbstractKeywords">Obtained results of the study shows that ceramic materials, obtained within the research, have great potential of application for load-bearing constructions as constructive building materials, as well as insulation materials. Production of porous ceramics materials, where foamglass pellets are used as melting filler, allows to produce more effective ceramics, creating high added value for the final product. </p>

Fabrication of Porous Ceramics by Direct Foaming

Porous ceramics have been extensively studied during the last two decades because of their application potentials in various fields including thermal insulators, radome materials, gas or molten metal filters, catalytic supports and biomedical substitutes for bone. This paper gives a brief review on the recent developments of preparation of porous ceramics by direct foaming method, it shows that the direct foaming method is a more effective way for preparation of high performance porous ceramics with high porosity, high mechanical strength and an even pore size distribution compared with conventional methods.

Porous Ceramics, Ceramic/Polymer, and Metal‐Doped Ceramic/Polymer Nanocomposites via Freeze Casting of Photo‐Curable Colloidal Fluids

A novel and general process for the fabrication of porous ceramics and metal/ceramic/polymer nanocomposites by freeze‐photocuring‐casting (FPC) is presented. The homogenous fluid containing water, curable monomers, and colloidal particles is frozen, and the phase separated microstructure is fixed by photo‐initiated polymerization. At the relatively high resin/particle ratio explored here, the structure is sufficiently strong to withstand the capillary forces during ambient drying; however, it is capable of transforming to macroscopic ceramics such as Al2O3 and SiO2 by conventional debinding and sintering. The use of FPC fluids enables producing porous macroscopic shapes such as microspheres, films, and multilayer coatings having an ordered, interconnected pore network and pore sizes down to the low sub‐micrometer range. The pore structure is found to be dependent on the radius of curvature of the macroscopic structure. Ag–TiO2/polyacrylate porous nanocomposites are produced by co‐shaping of FPC containing sterically stable colloids, soluble metal salts, and monomers. Ag nanoparticles form in situ by reduction of Ag+ ions on the TiO2 surface. This triggers the photo‐polymerization of the resin without the need of a photoinitiator. It is found that if the cured layers in a multilayer stack are frozen before the newly deposited layer, the ice crystal structure formed is linked via epitaxial growth.

Recent developments in gelcasting of ceramics

Gelcasting is a well-established colloidal processing method with a short forming time, high yields, high green capacity and low-cost machining, and has been used to prepare high-quality and complex-shaped dense/porous ceramic parts. In this article, we reviewed recent developments in gelcasting technology for ceramic preparation. For environmental pollution reduction during ceramic preparation by gelcasting, the development of low-toxic and nontoxic gelcasting systems is discussed. The occurrence and control of inner stress and surface-exfoliation in ceramic green bodies prepared by gelcasting are analyzed, and then some methods to control and eliminate the inner stress and surface-exfoliation in ceramic gelcast green bodies, especially for colloidal injection molding of ceramics (CIMC) are proposed. Finally, the applications of gelcasting for the fabrication of porous ceramics and complex-shaped ceramics (e.g., microbeads, rutile capacitor, thin-wall rutile tube, refractory nozzle) are summarized.

Application of Mechano-chemical Process for Fabrication of Porous Ceramics from Waste Materials

The influence of the mechano-chemical activation on the fabrication of porous ceramics from black paper sludge ash (BPSA, waste materials) was studied using planetary ball mill to obtain activated powder. Mechano-chemical process is used to modify properties of materials, enhance the reactivity of materials and produce advanced materials. After mixing activated powder with alkaline solution (KOH) and kept until completely solidification in steam oven, so that porous ceramics were formed by generation of gas. The physical properties of the porous ceramics were investigated using SEM, X-ray CT and mercury porosimeter. Based on the results, mean particle size and the visible pore size of solidified specimen were decreased with increasing of rotation speed and treatment time. The porous structure can be controlled by generation of gas. These were used to explain the formation of the graded porosity.

Novel fabrication route for porous ceramics using waste materials by non-firing process

We have successfully fabricated non-firing porous ceramics from Black paper sludge ash (BPSA, waste materials). It is an alternative ecological and economic waste recovery technique for waste pulp materials from pulp and paper industries. It has been gradually affecting our environment. In this study, black paper sludge ash (solid waste, BPSA obtained from a waste material from pulp and paper industries) was investigated for the fabrication of porous ceramics. The BPSA was activated with mechano-chemical process. Then, KOH solution was added and reacted to the paste (waste composite) until completely solidified. The structural properties of the porous ceramics were investigated using X-ray CT and mercury porosimeter. Results show increasing the concentration of KOH solution and reaction temperature created an observable pore size of specimen and a number of noticeable pores in the specimen increased.

Gelcasting of Strontium Doped Lanthanum Manganite for Solid Oxide Fuel Cell Applications

Solid oxide fuel cells (SOFCs) are devices that offer high efficiency power output with negligible emissions. Cathode supported tubular SOFCs consist of porous cathode tubes made up of strontium doped lanthanum manganite, La1-x-SrxMnO3 (LSM) that work as functional component as well as structural support for the rest of the cell components. Gelcasting is one of the most suitable methods for the fabrication of porous ceramics. This paper describes the fabrication of porous LSM cathode by gelcasting process. Gelcasting parameters such as monomer concentration, powder volume fraction, pH of the slurry, etc were optimized. Slow drying of green specimens minimized warpage and cracking. Sintered specimens with controlled porosity were obtained by the use of suitable pore-forming agents. The coefficient of thermal expansion (CTE) of sintered specimens was measured, which was found matching with the CTE values of cell components reported in the literature. These results have shown the suitability of the gelcast generated LSM cathodes for SOFC applications.

Purification of diatomite powder by acid leaching for use in fabrication of porous ceramics

This study presents an investigation of the purification of a raw diatomite by acid leaching conducted in hot solution (75 °C and 5 M HCl) for long periods up to 240 h, evaluates the use of these purified powders in the fabrication of porous materials, and examines their porosities. The treatment for 1 h increased the specific surface area of diatomite from 189 m 2/g to 222 m 2/g, but the SiO 2 content of 78.66% was still low. Material leached for a relatively short time shows an amorphous phase with quartz and plagioclase phases and the plagioclase is removable at longer leaching times during which no disruptive effect on the fine microscopic pores of the particles was observed. Thus, the diatomite based powders have potential use in the fabrication of porous coating as a membrane-separating layer: sintering at a relatively high temperature (1300 °C) still showed a significantly high apparent porosity of 48%.

Opportunities of porous ceramics fabricated by gelcasting in mitigating environmental issues

Porous ceramics fabricated by gelcasting bring many unique capabilities in mitigating environmental issues, particularly waste recycling and neutralization of hazardous emissions. Potential opportunities for using gelcasting technique in setting mechanically foamed slurry can be described by adaptable designing and modification of the foamed slurry characteristics with a goal of preserving the environment. In this paper, we describe three directions of porous shaping routes through gelcasting in an attempt to mitigate environmental issues. Firstly, the fabrication of porous ceramics with inclusion of non-through holes and double-layered porous structure was described as an effective means of reducing pressure drop during filtration. Secondly, the non-toxic gel-former has been successfully applied to the gelling of waste loaded foamed slurry under ambient condition. The process has great benefit not only for the fabrication of lightweight ceramics with multifunction purposes, but also for the development of endless recycling system. Lastly, the pyrolysis of gelcast dried porous ceramics under oxygen-free atmosphere has provided a promising material as a filter having a heating function for oxidation of hazardous emissions.

Gelcasting fabrication of porous ceramics using a continuous process

A continuous process for gelcasting fabrication of porous ceramics is reported. The key characteristic of this method is the mixing of slurry and N 2 bubbles, formed from a separated canister containing a surfactant prior to foaming of slurry, in a spiral mixer. The microstructure of the resulting porous ceramic is compared with that of sample fabricated by the conventional discontinuous process with N 2-gas controlling system. Porous ceramic produced using the new method displays lower density, higher open and total porosities, and broad pore size distribution. The obtained porous ceramics by the two routes are further investigated by sound absorption and heat insulation tests. Porous ceramic produced using the new method shows higher value of sound absorption coefficients, which suggests that the large porosity augment the sound absorbing performance of ceramics sample. The pore structure could also affect the thermal conductivity of the porous ceramics. With high porosity, sample fabricated with new method displayed lower thermal conductivity.

Fabrication of porous ceramics with controlled pore size by colloidal processing

Well-defined macroporous ceramics consisting of TiO2 and ZrO2 have been fabricated by two methods. One is via a template-assisted colloidal processing technique and the other is via a hetero-coagulation of template/ceramic particle colloidal processing. The former technique is as follows. Close-packed polymer spheres were first prepared asa template using centrifugation or gravitational sedimentation, followed by infiltration with alkoxide precursors. Then the removal of the template beads was achieved by calcination of the organic–inorganic hybrids at appropriate temperatures, yielding well-ordered macroporous ceramics. The latter technique is as follows. Core–shell composites of polymer/ceramic were obtained by mixing the oppositely charged two suspensions via electrostatic attraction following by filtration and calcination to produce macroporous ceramic materials. SEM images revealed that macroporous TiO2 and ZrO2 with ordered and uniform macropores have been obtained by both procedures.