Effects of pore structure on thermal conductivity and strength of alumina porous ceramics using carbon black as pore-forming agent

In the present work, carbon black (CB) works as a pore-forming agent in the preparation of alumina porous ceramics. The pore structures (i.e. mean pore size, pore size distribution and various pores size proportions) were characterized by means of Micro-image Analysis and Process System (MIAPS) software and mercury intrusion porosimetry. Then their correlation and thermal conductivity as well as strength were determined using grey relation theory. The results showed that the porosity and mean pore size increased against the amount of CB, whereas the thermal conductivity, cold crushing strength and cold modulus of rupture reduced. The <2 μm pores were helpful for enhancing the strength and decreasing the thermal conductivity whereas the >14 μm pores had the opposite effects.     

The membrane properties of alumina-coated alumina support layers and alumina-coated diatomite–kaolin composite support layers

ABSTRACT

Porous ceramic membranes are a current research focus because of their outstanding thermal and chemical stability. Recent research has utilised inexpensive natural materials such as diatomite to reduce the expense of these porous ceramic membranes. However, insufficient data exist for microfiltration applications using the diatomite-based membranes. The measured membrane properties of alumina-coated alumina support layers and alumina-coated diatomite–kaolin composite support layers have been compared. These experiments have been used to determine whether the average pore size could be reduced effectively by controlling the thickness of the alumina coating layer, while maintaining acceptable water permeability. The membrane properties of the alumina-coated alumina support layers and the alumina-coated diatomite–kaolin composite support layers were examined using the scanning electron microscopy, mercury porosimetry, and a dead-end microfiltration system.     

Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR)

Nuclear magnetic resonance (NMR) has been widely used in petrophysical characterization of sandstones and carbonates, but little attention has been paid in the use of this technique to study petrophysical properties of coals, which is essential for evaluating coalbed methane reservoir. In this study, two sets of NMR experiments were designed to study the pore types, pore structures, porosity and permeability of coals. Results show that NMR transverse relaxation (T₂) distributions strongly relate to the coal pore structure and coal rank. Three T₂ spectrum peaks identified by the relaxation time at 0.5-2.5 ms, 20-50 ms and >100 ms correspond to pores of <0.1 μm, >0.1 μm and cleats, respectively, which is consistent with results from computed tomography scan and mercury intrusion porosimetry. Based on calculated producible and irreducible porosities through a T₂ cutoff time method, we propose a new NMR-based permeability model that better estimates the permeability of coals. In combination with mercury intrusion porosimetry, we also propose a NMR-based pore structure model that efficiently estimates the pore size distribution of coals. The new experiments and modeling prove the applicability of NMR in petrophysical characterization of intact coal samples, which has potential applications for NMR well logging in coalbed methane exploration.     

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.     

Microstructure and electrical properties of zirconia and composite nanostructured ceramics sintered by different methods

The aim of this study is the preparation and characterization of dense cubic zirconia ceramics and zirconia nanocomposites (reinforced with 5 wt% alumina). The powders were obtained through sol–gel methods and densified using classical sintering and spark plasma sintering (SPS) methods. The obtained ceramics were characterized through X-ray diffraction, scanning electron microscopy and impedance spectroscopy at room and high temperature. The average grain size of cubic zirconia particles was found to be approximately 8 and 2.5 μm for the classical sintering and 99 nm for SPS. The alumina particles in composites have an average grain size of 0.7 μm for classical sintering and 53 nm for SPS ones. The total conductivity for nanocomposites sintered through both methods was also determined.     

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.     

Effect of dispersant on paste rheology in preparation of porous alumina with oriented pores by extrusion method

The effect of additives on paste rheology was investigated for preparation of porous ceramics with unidirectionally aligned cylindrical pores. Ammonium poly-carboxylic acid (APA) used as a dispersant and it was adsorbed on alumina powder surface. The adsorption isotherm of APA was fitted by Langmuir equation. The saturated monolayer adsorption was 5.9 mg/g. The apparent viscosity became a minimum at 0.8 mass % of APA corresponding to 71.2 mPa⋅s. This APA amount of 5.6 mg/g, is in good agreement with the observed APA amount. Since the nylon 66 fibers (0–35 vol. %) mixed with the alumina powder have a strong interaction with each other, they became twisted and agglomerated. This agglomeration increased with increasing fiber content but decreased by adding oleic acid. The pastes with added oleic acid were capable of being extruded at higher pressure. The obtained porous alumina ceramics showed highly oriented cylindrical pores parallel to the extrusion direction. The pore orientation was higher in the oleic acid added pastes than those without oleic acid. The added nylon 66 fibers are mostly converted to pores while maintaining the original shape after sintering. The pore size distribution of the obtained porous ceramics measured by mercury porosimetry method showed a peak at about 4 μm which is apparently smaller than that observed in the SEM photographs and the obtained result is considered to be corresponding to the necks formed by fiber contacts.     

Highly Porous Geopolymers Through Templating and Surface Interactions

The natural porosity of geopolymers and the simplicity of geopolymer synthesis make them a potential candidate for the formation of highly porous ceramics. Here, the synthesis of highly porous (≈70 vol% or more) metakaolin geopolymer is demonstrated using a novel emulsion preparation with one‐pot curing and hydrophobization of the interior pores with alkylalkoxysilanes. Using mercury intrusion porosimetry, tailoring of the characteristic percolation pore size is demonstrated over a range of ≈200 nm to 10 μm. Using powder X‐ray diffraction, reactivity was shown to be decreased versus typical geopolymers, but substantial geopolymerization still occurred and the samples formed cohesive monoliths. Optional calcination under inert conditions allowed for formation of a glassy ceramic with a notable SiC phase, as well as further increasing the porosity by removing the hydrophobic pore coatings.     

Effects of carbonation on the pore structure of non-hydraulic lime mortars

The pore structures of carbonated non-hydraulic lime mortars made with a range of different aggregates and concentrations of lime have been determined using mercury intrusion porosimetry (MIP). MIP data have been correlated with scanning electron microscopy images and other porosity data. During carbonation there is an increase in pore volume in the ∼ 0.1 μm pore diameter range across all mortar types which is attributed to the transformation of portlandite to calcite. Also there is a monotonic increase in the volumes of pores with diameters below 0.03 μm. A model is proposed for the changes in pore structure caused by carbonation. This attributes the increase in the volume of sub 0.03 μm pores to the attachment of calcite crystals to the surface of aggregate particles, and in some cases to the surface of portlandite crystals. This phenomenon may explain the continuing presence of portlandite in mortars that, apparently, have fully carbonated.     

Fabrication of CaSiO3 bioceramics with open and unidirectional macro-channels using an ice/fiber-templated method

Porous CaSiO₃ bioceramics with open and unidirectional macro-channels of pore size more than 200 μm are of particular interest for biomedical applications. An ice/fiber-templated method was employed for the fabrication of CaSiO₃ bioceramics with interconnected lamellar pores and macro-channels of pore size more than 200 μm. The pores formed by ice crystals transformed from cellular to lamellar, while the pores formed by fibers were aligned macro-channels, which were also in alignment with the lamellar pores. Keeping the initial slurry concentration constant and increasing the packing density of fibers, the volume fraction of macro-channels and open porosity increased, and the compressive strength decreased. Maintaining the packing density of fibers and increasing the initial slurry concentration, the pore sizes of lamellar pores and open porosity decreased, and the compressive strength increased. The results indicated that it was possible to manufacture porous CaSiO₃ bioceramics with the macro-channels of 250–350 μm, lamellae spacing of 50–100 μm, open porosity of 71.12–83.94% and compressive strength of 0.87–3.59 MPa, indicating the suitability for tissue engineering.     

Effect of zirconia content and particle size on the properties of 3D-printed alumina-based ceramic cores

Alumina-based ceramic cores are used to manufacture the internal structures of hollow alloy blades, requiring both high precision and moderate properties. In this work, zirconia is regarded as a promoter to improve the mechanical properties of sintered ceramic. The effect of zirconia content and particle size on the microstructure and mechanical properties of ceramics was evaluated. The results indicate that the flexural strength of sintered ceramics reached the maximum of 14.5 ± 0.5 MPa when 20 wt% micron-sized (10 μm) zirconia (agglomerate size, consistent with the alumina particle size) was added, and 26.5±2.5 MPa when 15 wt% 0.3 μm zirconia was added. Zirconia with submicron-sized (0.3 μm) particles effectively filled the pores between alumina particles, thus leading to the maximum flexural strength with a relatively low content. The corresponding sintered ceramics had a bulk density of 2.0 g/cm³ and open porosity of 59.6%.     

Preparation of a new ceramic microfiltration membrane from mineral coal fly ash: Application to the treatment of the textile dying effluents

New microfiltration membranes from mineral coal fly-ash material are obtained using ceramic method. Paste from mineral coal fly ash (obtained by calcinations at 800 °C of non-grinded mineral coal) is extruded to elaborate a porous tubular configuration used as supports. The support heated at 1125 °C, shows an average pore diameter and porosity of about 4.5 μm and 51%, respectively. The properties in terms of mechanical and corrosion resistances are very interesting. The elaboration of the layer based on fly-ash powder (obtained by sintering at 700 °C of a finely grinded mineral coal) is performed by slip-casting method. The heating treatment at 800 °C leads to an average pore size of 0.25 μm. The water permeability determined of this membrane is 475 L/h m² bar. This membrane can be used for crossflow microfiltration. The application to the treatment of the dying effluents generated by the washing baths in the textile industry shows an important decrease of turbidity (inferior to 1 NTU), of chemical oxygen demand (COD) values (retention rate of about 75%) and a total color removal. The performances in term of permeate flux and efficiency were determined and compared to those obtained using a commercial alumina microfiltration membrane. Almost the same stabilised permeate flux was obtained (about 100 L h⁻¹ m⁻²). So, it seems that the prepared membrane is suitable for such wastewater treatment.     

Synthesis and Characterization of Suspension-Derived, Porous Ion-Conducting Ceramic Membranes

Porous, oxygen-ion-conducting ceramic membranes can have applications as supports for fuel cells, sensors, and thin membrane films, or as filters for membrane filtration. This paper reports on the preparation of unsupported and supported yttria-stabilized zirconia (YSZ) and yttria-doped bismuth oxide (BY) membranes with submicrometer pore sizes. Fluorite-structured BY powder that has been synthesized using the citrate method and commercial YSZ powder have been used to prepare stable aqueous suspensions. Unsupported and supported YSZ and BY membranes have been prepared from the stable suspensions of YSZ and BY. The supported BY membranes are crack free but contain small defects. Defect-free YSZ membranes that are supported on porous alumina have been prepared under controlled conditions. The average pore size is 100 nm, with a porosity of 57%, for an unsupported YSZ membrane (measured by mercury porosimetry), and 114 nm for a supported membrane (as estimated via helium permeation). The ionic conductivity of the YSZ membranes is 0.00044–0.01 S/cm in the temperature range of 600°–900°C, which is lower than that of dense YSZ disks.     

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.     

Process control and optimized preparation of porous alumina ceramics by starch consolidation casting

This work concerns details of porosity and pore size control in starch consolidation casting of alumina ceramics using corn starch. In particular, the influence of the solids loading (68-78 wt.% alumina in suspensions with nominal starch contents of 20-50 vol.%) on the porosity, bulk density and shrinkage of alumina ceramics is studied. The results indicate a linear decrease of the linear shrinkage and the bulk density (and a corresponding increase in porosity) as the alumina concentration increases, with slopes that are independent of the starch content. The pore size is characterized via microscopic image analysis, the pore throat size via mercury porosimetry. Relations between the volumetric shrinkage, porosity and the volume fractions of starch and water in the suspensions are discussed, and a new concept, called “affine limit porosity” is proposed to explain the apparently paradoxical finding that the porosity increases with increasing alumina content in the suspension.     

Pore size and shape in mortar by thermoporometry

The pore structure of mortar (w/c = 0.55) was examined using thermoporometry (TPM), nitrogen adsorption/desorption (NAD), and mercury intrusion porosimetry (MIP). The TPM measurements were calibrated by comparison to NAD and MIP measurements on porous glass; similar comparisons were made on dried and resaturated mortars. For undried mortars, TPM provides the size of pore entries (from the freezing cycle) and interiors (from the melting cycle). In keeping with previous studies, we find that there is an unfrozen layer of water between the ice and the pore wall in porous glass that is about 0.8 nm thick; when lime-saturated water is used, the thickness of that layer increases by about 10%. In mortar, the unfrozen layer is about 1.0-1.2 nm thick, so no freezing occurs in pores with diameters ≤ 4.5 nm, at least down to − 40 °C (where the radius of the crystal/liquid interface is ∼ 1.5 nm). Based on the hysteresis in the freezing and melting curves, the larger mesopores in mortar were found to be rather spheroidal, while the smaller ones were more cylindrical.     

Comparison of X-ray micro-tomography measurements of densities and porosity principally to values measured by mercury porosimetry for carbon-carbon composites

X-ray Micro-Tomography (μCT) applied to carbon-carbon composites is shown to be able to quantify the amount, shape, and distribution in three dimensions of both open and closed porosity with a minimum dimension greater than 10 μm. Being a non-destructive technique, it is also able to track these values following each densification cycle. It is also demonstrated that μCT is able to obtain bulk density values for non-uniform samples as well as the same results for skeletal density as other techniques used conventionally. Furthermore, values for open porosity comparable to those obtained by mercury porosimetry can be obtained by X-ray Micro-Tomography if the value obtained by the mercury porosimeter is truncated below the resolution of the μCT. Finally, it is shown that in conjunction with data from the mercury porosimetry, μCT is also able to demonstrate the presence of “bottle-neck” pores i.e. open pores with restricted pore access dimensions.     

Processing and properties of low cost macroporous alumina ceramics with tailored porosity and pore size fabricated using rice husk and sucrose

The present study demonstrates a cost effective way to fabricate porous ceramics with tailored microstructures using rice husk (RH) of various range of particle sizes as a pore former and sucrose as a binder as well as a pore former. Sample microstructures reveal randomly oriented elongated coarse pores and fine pores (avg. size 4 μm) created due to burnout of RH and sucrose, respectively. Porous alumina ceramics with 20–66 vol% porosity and 50–516 μm avg. pore size (length), having isolated and/or interconnected pores, were fabricated using this process. Mechanical properties of the porous samples were determined as a function of porosity and pore microstructure. The obtained porous ceramics exhibited flexural strength of 207.6–22.3 MPa, compressive strength of 180–9.18 MPa, elastic modulus of 250–18 GPa and hardness of 149–18 HRD. Suggested application area includes thermal, filtration, gas purging etc.     

Effect of dispersant on paste rheology in preparation of porous alumina with oriented pores by extrusion method

The effect of additives on paste rheology was investigated for preparation of porous ceramics with unidirectionally aligned cylindrical pores. Ammonium poly-carboxylic acid (APA) used as a dispersant and it was adsorbed on alumina powder surface. The adsorption isotherm of APA was fitted by Langmuir equation. The saturated monolayer adsorption was 5.9 mg/g. The apparent viscosity became a minimum at 0.8 mass % of APA corresponding to 71.2 mPa?s. This APA amount of 5.6 mg/g, is in good agreement with the observed APA amount. Since the nylon 66 fibers (0–35 vol. %) mixed with the alumina powder have a strong interaction with each other, they became twisted and agglomerated. This agglomeration increased with increasing fiber content but decreased by adding oleic acid. The pastes with added oleic acid were capable of being extruded at higher pressure. The obtained porous alumina ceramics showed highly oriented cylindrical pores parallel to the extrusion direction. The pore orientation was higher in the oleic acid added pastes than those without oleic acid. The added nylon 66 fibers are mostly converted to pores while maintaining the original shape after sintering. The pore size distribution of the obtained porous ceramics measured by mercury porosimetry method showed a peak at about 4 μm which is apparently smaller than that observed in the SEM photographs and the obtained result is considered to be corresponding to the necks formed by fiber contacts.