Porous Ceramic Structures Research Articles

Defect structure transformation during substitution in quadruple perovskite CaCu3-Ti4-Fe2O12 studied by positron annihilation spectroscopy

Positron annihilation spectroscopic studies have been performed to investigate the defect structural evolution in polycrystalline CaCu3-xTi4-xFe2xO12 (x = 0.0, 0.1, 0.3, 0.5 and 0.7) cubic perovskite samples. The positron lifetimes, relative intensities and Doppler-broadened annihilation lineshape parameters all indicate about the annulment of crystalline vacancies from the structure and attending porous ceramic structure at higher concentration of substitution. The compositional dependence of the intensity of the defect specific positron lifetime component clearly suggests that, in the initial stages of substitution, positron annihilates in vacancy-type defects. But, for higher concentrations of Fe3+, the materials are devoid of such defects and positron trapping takes place within the pores. The results of coincidence Doppler broadening spectroscopy measurements support such transformation in which the positron trapping sites are relocated to the ceramic pores leading to the modification of the material properties.

Additive‐Manufactured Ordered Porous Structures Made of Ceria for Concentrating Solar Applications

Porous structures made of redox active ceria are attractive for high‐temperature concentrating solar applications and particularly for the thermochemical splitting of H2O and CO2 as their enhanced heat and mass transport properties lead to fast reaction rates, especially with regard to the absorption of concentrated solar radiation during the endothermic reduction step. Hierarchically ordered porous structures, fabricated by the Schwartzwald replica method on 3D‐printed polymer scaffolds, are experimentally assessed for their ability to volumetrically absorb high‐flux irradiation of up to 670 suns. Temperature distributions across the porosity‐gradient path are measured (peak 1724 K) and compared with that obtained for a reticulated porous ceramic (RPC) structure with a uniform porosity. To assist the analysis, a Monte Carlo ray‐tracing model is developed for pore‐level numerical simulations of the ordered geometries and applied to analyze the absorbing–emitting–scattering exchange and determine the radiation attenuation and the temperature distribution at a radiative equilibrium. In contrast to the Bouguer's law exponential‐decay attenuation of incident radiation observed for the RPC, the ordered structures with a porosity gradient exhibit a step‐wise radiative attenuation that leads to a more uniform temperature distribution across the structure. This in turn predicts a superior redox performance.

Effect of graphite and common rubber plasticizers on properties and performance of ceramizable styrene\u2013butadiene rubber-based composites

Ceramizable composites are highly filled polymer dispersion composites which create stiff porous and durable ceramic structure when exposed to fire or elevated temperature. However, the incorporation of large amounts of mineral fillers into the composites strongly decreases their processing performance. In order to improve extrusion properties of these composites, plasticizers like triethylamine, ethylene glycol, naphthalene, dibutyl phthalate and graphite were used. Extrudability of the composite mixes was examined as an indicator of their processing performance. After the vulcanization, mechanical properties of the composites were tested. In order to check the micromorphology of the samples scanning electron microscopy was performed. Because of the significant flammability of the plasticizers, it was also important to examine how these additives change combustion behavior of the composites by cone calorimetry. Additionally, composites were ceramized in three different thermal conditions and their compression strength was measured. The incorporation of graphite platelets resulted in optimum balance between enhancing extrudability and preserving satisfactory mechanical properties and ceramization performance. The obtained results showed that ceramizable composites are susceptible to plasticizing and their mechanical and combustibility properties can be preserved like before the plasticizers addition.

Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes

Abstract The frozen moulds, including homogeneous, unidirectional and bidirectional freezing, were designed using materials with different thermal conductivities, and the temperature variations of the moulds and samples during the freezing process were simulated by finite element analysis. Highly porous SiC ceramics with significant differences in pore structure were fabricated by using the SiC/water slurries prepared via uniform or oriented freeze casting with various freezing modes, and porosity and compressive strength of the as-fabricated ceramics were investigated. The results showed that the pore structure of ceramics prepared by homogeneous freezing was relatively intricate and inconsistent, and had a higher compressive strength. In contrast, the pore structure of ceramics fabricated using bidirectional freezing mode was more ordered and higher porosity was observed. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the bottom to hundreds of micrometers in the top.

Effects of CAD/CAM titanium alloy surface treatment and resin luting on shear bond strength and durability of composite resin

To assess the effects of two surface treatments (sandblasting, SB; microarc-oxidation, MAO) and resin luting on shear bond strength and durability of titanium alloy and composite-resin. Eighty cylindrical titanium alloy specimens with a diameter of 10 mm and a height of 8 mm were fabricated by CAD/CAM technique. It was divided into two groups according to the surface treatment methods: sandblasting with Al2O3 particles on the surface of SB specimens; porous ceramic film structure could be formed on the surface of MAO specimens after surface treatment. Each group was classified into SB-resin luting-N group (not used), SB-resin luting-Y group (used), MAO-resin luting-N group (not used), MAO-resin luting-Y group (used) depending on whether or not resin luting was applied. Each specimen was bonded and cured with the Cemerage resin, and the shear bond strength after 0 and 5 000 thermocycling was tested. The results were statistically analyzed. The surface morphology of titanium alloy specimens before and after the shear bond strength test was observed by scanning electron microscopy (SEM). The shear bond strength between titanium alloy and composite-resin was the highest in the SB combined with resin luting group after 0 thermocycling (16.2±1.8) MPa; was the lowest in MAO group after 5 000 thermocycling (8.9±1.5) MPa. The shear bond strength of SB and MAO surface treatment methods combined without resin luting group after 5 000 thermocycling were (10.7±2.2) MPa and (8.9±1.5) MPa, which were statistically lower than those in the thermocycling 0 (P=0.000 and P=0.001). The shear bond strength of SB and MAO surface treatment methods combined with resin luting group after 5 000 thermocycling were (15.5±2.1) MPa and (11.7±1.3) MPa, respectively, which were lower than those in the thermocycling 0 group, but there was no statistical significance (P=0.087 and P=0.234). Both the surface treatment methods of SB and MAO combined with resin luting can improve the shear bond strength and durability of titanium alloy and composite-resin. The SB combined with resin luting is more significant. At present, the effect of SB is better than that of MAO due to the limitation of technical parameters of micro-arc oxidation.

Fabrication of Porous Materials by Spark Plasma Sintering: A Review.

Spark plasma sintering (SPS), a sintering method that uses the action of pulsed direct current and pressure, has received a lot of attention due to its capability of exerting control over the microstructure of the sintered material and flexibility in terms of the heating rate and heating mode. Historically, SPS was developed in search of ways to preserve a fine-grained structure of the sintered material while eliminating porosity and reaching a high relative density. These goals have, therefore, been pursued in the majority of studies on the behavior of materials during SPS. Recently, the potential of SPS for the fabrication of porous materials has been recognized. This article is the first review to focus on the achievements in this area. The major approaches to the formation of porous materials by SPS are described: partial densification of powders (under low pressures, in pressureless sintering processes or at low temperatures), sintering of hollow particles/spheres, sintering of porous particles, and sintering with removable space holders or pore formers. In the case of conductive materials processed by SPS using the first approach, the formation of inter-particle contacts may be associated with local melting and non-conventional mechanisms of mass transfer. Studies of the morphology and microstructure of the inter-particle contacts as well as modeling of the processes occurring at the inter-particle contacts help gain insights into the physics of the initial stage of SPS. For pre-consolidated specimens, an SPS device can be used as a furnace to heat the materials at a high rate, which can also be beneficial for controlling the formation of porous structures. In sintering with space holders, SPS processing allows controlling the structure of the pore walls. In this article, using the literature data and our own research results, we have discussed the formation and structure of porous metals, intermetallics, ceramics, and carbon materials obtained by SPS.

Porous polysilazane-derived ceramic structures generated through photopolymerization-assisted solidification templating

A novel approach to the preparation of porous polysilazane-derived ceramics is presented, combining non-aqueous solidification templating (freeze casting) of a liquid preceramic polymer with a photo-induced thiol-ene “click” reaction at temperatures below −10 °C. Upon directional solidification of the reaction mixture consisting of the preceramic polymer, a quaternary thiol, a photoinitiator, and camphene as pore-structuring agent, low-temperature photopolymerization of the preceramic polymer was achieved by irradiation with visible light, its feasibility demonstrated by photorheology. Effects of the polymer/structuring agent ratio and the solidification rate on porosity and pore morphology were evaluated using porosimetry and tomography techniques. The structural features were set in relation to mechanical properties, showing compressive strengths up to 74 MPa at porosities between 58% and 76%.This new processing technique facilitates the generation of polysilazane-derived ceramics with highly tailorable pore structures, with prospective uses in processes ranging from membrane-based separation to catalysis.

Radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina RPC structures as integrated solar thermochemical reactor

This paper investigated radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina reticulated porous ceramic structures as integrated solar thermochemical reactor. High-flux solar radiation absorption and axial temperature distribution in the ceramic foams reactor were analyzed by adopting surface-to-surface radiation model coupled to the P1 approximation for radiation heat transfer. The radiative heat transfer and thermal characteristics of different foam-type RPC structures, including SiC, CeO2, FeAl2O4, NiFeAlO4, Fe3O4/SiC, and NiFe2O4/SiC were evaluated. The mass flow rate and foam structural parameters, including the permeability, pore mean cell size, and extinction coefficients have significantly affected the axial temperature distribution, pressure drop, heat transfer, and fluid flow. Integrated porous structure to the solar receiver could maximize the incorporation of redox powder in the reacting medium, lower the pressure drop, and enhance the thermal performance of the thermochemical reacting system. SiC structure was the candidate materials in the case where more heat flux and high axial temperature distribution is needed. However, Fe-based oxide coated Al2O3 structure could be considered regarding the heat transfer enhancement along with the catalyst activity of oxygen carriers for solar thermochemical reacting system performance.

The thermal shock resistance prediction of porous ceramic sandwich structures with temperature-dependent material properties

A general numerical model to predict the thermal shock resistance of porous ceramic sandwich (PCS) structures with temperature-dependent material properties is developed. Knowledge of the temperature distribution and associated thermal stress in PCS panel is determined by the finite element method of coupled thermoelasticity. The present work considers the hot/cold shock induced center/edge cracks and measures the time-varied thermal stress intensity factors at the crack tip area. The roles of crack length, relative density of foam core, thermal shock load and geometric parameters of the PCS structures are examined. Moreover, fracture failure analysis of the whole PCS structures is carried out and crack propagation manners are detected. The thermal shock resistance curves of the structures are provided and the critical thermal shock temperatures are estimated for any selected characteristic materials. Results reveal that the thermal shock resistance of the PCS structures will be dramatically underestimated with the ignorance of temperature-dependent material properties. The analysis model of this paper provides a rapid prediction of thermal shock behavior of PCS structures at arbitrary temperatures.

Preceramic core-shell particles for the preparation of hybrid colloidal crystal films by melt-shear organization and conversion into porous ceramics

In this work, the preparation of porous hybrid particle-based films by core-shell particle design and convenient film preparation is reported. Monodisperse core particles consisting of poly(methyl methacrylate‑co‑allyl methacrylate) (P(MMA‑co‑ALMA)) were synthesized by starved-feed emulsion polymerization followed by the introduction of an initiator-containing monomer (inimer) for subsequent atom transfer radical polymerization (ATRP). The inimer shell allowed for the introduction of allylhydrido polycarbosilane (SMP-10) under ATRP conditions by grafting to the core particles. The functionalization of the prepared core-shell particles was investigated by IR spectroscopy (FTIR), scanning transmission electron microscopy (STEM) and solid-state NMR combined with dynamic nuclear polarization (DNP). The obtained hard core/soft preceramic shell particles were subjected to the melt-shear organization technique, enabling a convenient alignment into a colloidal crystal structure in one single step without the presence of a dispersion medium or solvent for the designed particles. Moreover, the hybrid particle-based films were converted into a porous ceramic structure upon thermal treatment. As a result, freestanding ceramic porous films have been obtained after degradation of the organic template core particles. Noteworthy, the conversion of the matrix material consisting of SMP-10 into the ceramic occurred with preservation of the pristine colloidal crystal template structure. Herein, the first example of core-shell particle preparation by combining different polymerization methodologies and application of the convenient melt-shear organization technique is shown, paving a new way to ceramic materials with tailored morphology and porosity.

Effect of alumina fiber content on pore structure and properties of porous ceramics

AbstractPorous Al2O3 ceramics with different contents of alumina fibers were prepared by gel‐casting process. The effects of Al2O3 fiber content on pore size distribution, porosity, compressive strength, and load‐displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al2O3 fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber‐reinforced porous ceramics changed from particles to fibers.

On the quenching of stainless steel rods with a honeycomb porous plate on a nanoparticle deposited surface in saturated water

Quenching of a stainless-steel rod with a porous ceramic structure, i.e. honeycomb porous plate (HPP), attached to its lower surface was investigated in distilled water under saturated conditions at atmospheric pressure. The experiments were performed on bare surface (BS) and on a TiO2 nanoparticle-deposited surface (NPDS). When the HPP was attached, the quenching rate increased significantly on both tested surfaces. The quench time for the NPDS with the HPP was 28-times shorter than that for the bare stainless-steel surface. The results suggested that the combination of the HPP ability to transport water to the heat-transfer surface by capillary action, and the increase of surface roughness, capillarity and wettability properties by the deposited nanoparticle layer were responsible for the enhancement obtained.

The modified Mori-Tanaka scheme for the prediction of the effective elastic properties of highly porous ceramics

In this paper, two modifications are proposed to be applied to the well-known Mori-Tanaka (MT) scheme to improve its performance in the estimation of the mechanical properties of highly porous ceramic structures containing complicated agglomerates of merged and open-cell spherical pores of different radii. In the first modification, the effect of the merged pores is considered by estimating their number with the theory of geometrical probabilities and treating them as corresponding ellipsoids of the same volume. In the second modification, porous structures containing open pores are treated as a damaged material with reduced load-carrying capacity and the formulations are modified to consider the effect of the open pores. In order to investigate the reliability of the analytical estimations, different groups of artificial porous structures with porosity values ranging from 10% to 90% are constructed by random positioning of the spherical voids of different radii in a representative volume element (RVE) and their effective elastic properties are obtained by means of the finite element method (FEM). For each level of porosity, a total of 30 random structures are examined to assess the variations caused by the statistical nature of the microstructure. Comparison between the findings of the statistical FEM and the analytical results show that the proposed modifications considerably increase the precision of the MT scheme in the estimation of the effective elastic moduli of highly porous materials. Furthermore, unlike the classical MT method, the modified formulations are capable of demonstrating of the probable anisotropy in the effective elastic properties of the porous structures. Good agreement is also observed between the results obtained from the developed formulations and published numerical and experimental observations for ceramic structures.

Modeling of Bone Tissue Structure and Porous Ceramics

The results of computer modeling of the structure and mechanical behavior of porous ceramics, microvolumes of compact bone tissue and mesovolumes of bone containing compact and spongy bone tissue are presented. The structure and mineral content of bone tissue fragments are determined in which the most uniform distribution of deformations is realized under axial tension and compression. It was found that the geometry of the porous space of ceramic samples significantly influences the character of the distribution of the regions in which microfractures are possible.

PZT/PLZT - elastomer composites with improved piezoelectric voltage coefficient

Lead Zirconate Titanate (PZT) and Lanthanum-modified Lead Zirconate Titanate (PLZT) ceramic sensor materials are widely used because of their excellent piezoelectric coefficients. These materials are brittle, high density and have low achievable piezoelectric voltage coefficients. The density of the sintered ceramics shall be reduced by burnable polymeric sponge method. The achievable porosity level in this case is nearly 60 – 90%. However, the porous ceramic structure with 3-3 connectivity produced by this method is very fragile in nature. The strength of the porous structure is improved with Sylgard®-184 (silicone elastomer) by vacuum impregnation method maintaining the dynamic vacuum level in the range of -650 mm Hg. The elastomer Sylgard®-184 is having low density, low dielectric constant and high compliance (as a resultant stiffness of the composites is increased). To obtain a net dipole moment, the impregnated ceramic composites were subjected to poling treatment with varying conditions of D.C. field and temperature. The properties of the poled PZT/PLZT - elastomer composites were characterized with LCR meter for measuring the dielectric constant values (k), d33 meter used for measuring piezo-electric charge coefficient values (d33) and piezo-electric voltage coefficient (g33) values which were derived from d33 values. The voltage coefficient (g33) values of these composites are increased by 10 fold as compared to the conventional solid ceramics demonstrates that it is possible to fabricate a conformable detector.

Characterization and modelling of structure and transport properties of porous ceramics

Characterization and modelling of structure and transport properties of porous ceramics

Obtainment of Porous Ceramic Structures: A Comparison among Different Compositions and Methods of Conformation

This paper presents comparative results concerning to the obtaining of porous ceramic structures obtained by pressing, slip casting and polymeric sponge method. Three compositions were prepared, using calcium carbonate as pore-generating agent and characterized by X-ray fluorescence, thermogravimetric analysis and determination of sintering behavior. Each formulation was wet mixed and dried. Then, each formulation was formed by each one of the investigated methods. The obtained samples were dried and heat treated with the appropriate heating rate for calcium carbonate degasification and sintered at 900, 1100 and 1180 °C. The sintered samples were characterized by determination of porosity, crystalline phases formed, compressive strength and permeability. Results showed that porosity and permeability depends strongly on the composition and used conformation method.

Membrane-spacer assembly for flow-electrode capacitive deionization

Abstract Flow-electrode capacitive deionization (FCDI) is a desalination process designed to overcome the limited desalination capacity of conventional CDI systems due to their fixed electrodes. Such a FCDI cell system is comprised of a current collector, freestanding ion-exchange membrane (IEM), gasket, and spacer for flowing saline water. To simplify the cell system, in this study we combined the membrane and spacer into a single unit, by coating the IEM on a porous ceramic structure that acts as the spacer. The combination of membrane with the porous structure avoids the use of costly freestanding IEM. Furthermore, the FCDI system can be readily scaled up by simply inserting the IEM-coated porous structures in between the channels for flow electrodes. However, coating the IEM on such porous ceramic structures can cause a sudden drop in the treatment capacity, if the coated IEM penetrates the ceramic pores and prevents these pores from acting as saline flow channels. To address this issue, we blocked the larger microscale pores on the outer surface with SiO2 and polymeric multilayers. Thus, the IEM is coated only onto the top surface of the porous structure, while the internal pores remain empty to function as water channels.

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.

Enhance photocatalysis of TiO2 and ZnO ceramics by addition of fused silica as a UV guiding medium

Abstract We report a facile method to prepare porous TiO2 and ZnO photo-catalytic ceramics in which colloidal silica was added to yield nano fused silica (FS). The colloidal silica forms continuous media within the porous ceramic structure, and when calcined at 550 °C, it converted to fused silica phase which is an excellent UV wave-guiding material. With light wave guiding effect of FS and capillary effect of reaction solution in the porous channels, the photo-catalytic reactions could occur at the vast active sites at interface inside the porous bulks, as well as at the external surfaces. Photo-catalysis experiments and kinetics were investigated and analyzed. The photo-catalytic efficiencies tested for methylene blue were enhanced by a factor of 4.1 for TiO2/FS system and, astonishingly, by a factor of 34.6 for ZnO/FS system, when compared with pure TiO2 and ZnO under identical testing conditions. A model of UV-waveguide ceramic systems was proposed and discussed. This study proposes a practical approach to construct UV-waveguide porous structures, of which the principle is also potentially applicable to other types of photo-catalytic materials, including visible light active photo-catalysts.