Direct foaming of macroporous ceramics containing colloidal alumina

Liquid foams containing Al₂O₃ nanoparticles were obtained after direct foaming of a colloidal alumina suspension with ammonium stearate. These systems were stable for at least 24 h and were comprised by small cells (<35 μm). Up to 10 wt% of these foams were added to an ultrastable Al₂O₃-stabilised one and resulted in macroporous samples with high total porosity (>70%). Their green mechanical strength was proportional to the amount of colloidal alumina added, but lower than a composition with calcium aluminate cement. When compared with compositions prepared with colloidal alumina suspension, the colloidal foams resulted in samples with a higher number of small pores (<30 μm) and lower linear shrinkage after firing at 1600 °C for 5 h (~9%). Thus, colloidal alumina foams can be used for processing macroporous refractory ceramics with smaller pores, lower dimensional changes after firing and higher porosity.     

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

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

Non-shrinkage porous ceramics by In-situ hollow sphere method based on the Kirkendall effect of Al particle

The significant shrinkage of porous ceramics after sintering has produced a number of issues with their use and development. As a result, we proposed an in-situ hollow sphere method for producing non-shrinkage alumina porous ceramics. The obtained green samples were made up of Al₂O₃ and Al powders, with pores emerging inside the materials due to the Kirkendall effect of Al particles after sintering. The expansion of hollowing particles exactly offsets the shrinkage generated by sintering throughout the process. When 50 vol. % Al powder (10 µm) is added, the linear shrinkage rate of the sample after sintering at 1500 °C can reach −3.47 %, and its apparent porosity and flexural strength are 30.69 % and 44.03 MPa, respectively. According to approximate calculations, the pores formed by the oxidation of Al powder are smaller than the initial size of Al powder. This method suggests a novel approach for producing controlled shrinkage porous ceramics.     

Preparation and properties of porous mullite-based ceramics fabricated by solid state reaction

In this study, the effect of the alumina particle size on the formation of mullite using a silica gel powder and micro- and nano-scale Al₂O₃ powders as raw materials was investigated. The optimized Al₂O₃ source was then reacted with the silica gel to prepare porous mullite-based ceramics. The results revealed that the highly reactive nano-Al₂O₃ powder could form mullite at a relatively low firing temperature. Therefore, the nano-Al₂O₃ powder was used to prepare porous mullite-based ceramics by firing at 1600 °C, 1650 °C and 1700 °C. The pore size of the prepared porous mullite-based ceramics ranges from tens to hundreds of micrometres, with the apparent porosity being 42.8–58.0%. Further, the mullite content in the samples increased with increasing firing temperature, and a higher firing temperature promoted sintering, resulting in improved strength of the sample. After calcination at 1700 °C, the mullite content in the sample reached 81.8%, and the sample showed excellent thermal shock resistance. The strengths of the samples before and after thermal shock were found to be 23.6 and 15.58 MPa, with the residual strength ratio being 66%.     

Preparation of near net size porous alumina-calcium aluminate ceramics by gelcasting-pore-forming agent processs

In the processing of porous ceramics, shrinkage from green body to sintered compact during drying and sintering is one of the key concerns which affect microstructure and properties of porous ceramics. Through releasing gases from the burning of the pore forming agents, and volume expansion from the formation of low density resultants during sintering, the sintering shrinkage can be effectively compensated and near net size preparation can be achieved. Herein, near net size porous alumina-calcium aluminate ceramics with controllable shrinkage have been prepared using a combination of gelcasting and pore-forming agent process by adjusting the amount of CaCO₃ and polymethyl methacrylate (PMMA) microspheres added. Al₂O₃ and CaCO₃ were used as raw materials, PMMA microspheres were used as pore-forming agent, isobutylene/maleic anhydride copolymer (Isobam104) was used as gelling agent and dispersing agent. The effects of the addition amounts of CaCO₃ and PMMA in the slurry on the phase composition, shrinkage, porosity, and strength of porous alumina-calcium aluminate ceramics were investigated. The results show that as the CaCO₃ addition amount increases from 0 to 20 wt%, the shrinkage of the samples gradually decreases from 7.3% to −1.4%, and the consequent porosity increases from 58% to 66%, while the compressive strength increases from 5.9 to 15.5 MPa. When PMMA content increases from 10 to 50 wt%, the shrinkage of the samples decreases first and then increases, the porosity increases from 51% to 74%, and the compressive strength decreases from 12.5 to 5.3 MPa. The mechanisms for controlling shrinkages during preparation of porous alumina-calcium aluminate ceramics can be attributed to the following aspects: on one hand, gas release from burning of PMMA and decomposition of CaCO₃ during sintering; on the other hand, volume expansion due to the formation of lower density calcium aluminates which come from the reactions between CaO and Al₂O₃. The near net size preparation technique is of great significance for the manufacture of porous ceramics since the subsequent machining cost can be effectively reduced.     

Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

Ultra-low shrinkage and high-strength porous TiB/TiB2 ceramics via low temperature in-situ reaction sintering combined with the freeze-drying method

Ultra-low shrinkage porous TiB₂-based ceramics reinforced by the TiB whiskers are firstly fabricated through the in-situ reaction between TiB₂ and Ti at a low temperature (1450 °C). The growth of TiB whiskers with a high aspect ratio at pore channels is achieved through a vapor-solid growth mechanism, while low aspect ratio TiB whiskers at pore walls are dominated by a solid-state reaction diffusion growth, forming bimodal distribution whiskers in porous TiB₂-based ceramics. The overlapping TiB whiskers with low-speed growth at particle contact points can significantly inhibit the shrinkage and improve the strength of porous TiB₂-based ceramics. When the solid content is fixed at 20 vol% and target TiB content changes from 0 to 80 vol%, the porous ceramics show slight sintering shrinkage (from 1.1 to 4.7%) and high porosity (from 79.3 to 73.7%) while keeping high compressive strength of 1.8–18.2 MPa, which is higher than most reported porous ceramics at the same porosity.     

Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)₂Te₃ composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)₂Te₃ porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)₂Te₃ porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm^?1K^?1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)₂Te₃ bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.     

The effect of natural dolomite admixtures on calcium zirconate–periclase materials microstructure evolution

The reaction sintering mechanism of dolomite–zirconia mixtures was investigated using fine grounded dolomite raw material and zirconium powder. The used dolomite raw materials differed by the content of impurities (SiO₂, Al₂O₃ and Fe₂O₃ oxides). The microstructure evolution of MgO–CaZrO₃ and CaZrO₃ sintered materials was presented as a temperature function. One- and two-step firing processes of calcium raw materials powder mixed with chemically pure zirconium oxide were applied. The kinetics of reaction of CaZrO₃ synthesis was estimated by determining the “free” calcium oxide by chemical and XRD analysis. The densification process was evaluated by firing shrinkage, apparent density, pore diameter and pore size distribution measurements. The microstructure of sintered materials was observed by SEM. It was observed that CaZrO₃ synthesis was definitely finished at temperature of 1500 °C in the both applied ways of the synthesis (one- or two-step process). The only phase present in the model material synthesized from chemically pure reagents (CaCO₃ and ZrO₂) after firing at temperature of 1500 °C was calcium zirconate.In the materials synthesized from natural dolomites and ZrO₂ two main phases were present—calcium zirconate and periclase. During firing of CaZrO₃–MgO materials at lower temperatures the presence of transient phases was detected (mainly ferrites and calcium aluminates, 4CaO·Al₂O₃·Fe₂O₃ or 2CaO·Fe₂O₃). These phases disappeared at higher temperatures. This is probably related to the dissolution of impurities in the main phases of CaZrO₃–MgO.The material obtained from the mixture of zirconium oxide and natural dolomite with the high impurities content has the highest densification level (～95% theoretical density of CaZrO₃–MgO) at 1500 and 1600 °C.     

Effect of firing temperature and atmosphere on sintering of ceramics made from Bayer process bauxite residue

Bauxite residue, the principal waste from the Bayer process, was dried, pressed and studied for its thermal and sintering behaviour under different atmospheres, up to 1100 °C. For sintering in air and N₂, shrinkage begins at 800 °C and ranges from 2.6% to 13.9%, after firing at 1000–1100 °C. Bulk density varies from 1.7 to 2.3 g/cm³ whereas water absorption from 31.5% to 17.7%. The main crystalline phases identified on firing in air were hematite (Fe₂O₃), gehlenite (Ca₂Al₂SiO₇) and perovskite (CaTiO₃) whereas magnetite (Fe₃O₄) was also found on firing in N₂. Microstructures are characterised by irregularly shaped, <20 μm Feret diameter, pores in a ceramic matrix with interconnected porosity. The average pore size is greater in samples fired in N₂. On sintering in 4%H₂/Ar, shrinkage begins at 710 °C. After firing at 1100 °C, shrinkage is 20.1% and water absorption 1%. The main crystalline phases are magnetite, wustite (FeO), gehlenite and perovskite. Microstructures are characterised by a compact heterogeneous matrix, with isolated <15 μm Feret diameter, closed pores. The grains have reacted with the adjacent phase and their shape is rounded with no sharp facets. Increased sintering temperature results in improved physical properties for all atmospheres tested and in higher average pore size when sintering takes place in air and N₂. The use of magnetite-reducing sintering conditions can potentially assist in the production of a variety of ceramic compositions containing bauxite residue.     

Preparation and properties of porous Al5BO9 for high-temperature wave-transparent and thermal insulating applications

Porous Al₅BO₉ is a promising high-temperature wave-transparent material. However, method for the preparation of this material is not readily available. Herein, porous Al₅BO₉ ceramics with controlled porosity and small volume shrinkage are successfully prepared by using Al₂O₃ and B₂O₃ as starting materials without pore formers. The SEM and pore size distribution studies show that the as-prepared porous Al₅BO₉ ceramics exhibit a uniform pore structure and a narrow pore size distribution. Intriguingly, simply adjusting the densities of the green bodies, the density and porosity of the porous Al₅BO₉ ceramics can be controlled. The pore-forming mechanism is presumed to be a combination of boron oxide volatilization during the high-temperature synthesis and lap of elongated grains. Porous Al₅BO₉ ceramics have good high-temperature stability, which can maintain dimensional and composition stability up to 1673 K. The compressive strength can reach 211 MPa at 32.4% porosity and the dielectric constant can be as low as 3.02 at 43.2% porosity. In addition, the dielectric constant and loss tangent keep almost unchanged with temperature.     

Recycled granulate obtained from waste alumina-rich refractory powder by the cold bonding process

Within the scope of the present study the cold-bonding process was used for the recycling of waste filter powder which was mixed with two different binders in different concentrations; alumino-silicate cement and potassium water glass, and combinations of these two materials, and hardened at room temperature. Selected samples were also fired at 1200 °C. Tests to determine tensile and compressive strength, density and porosity, as well as dilatometry and SEM analyses, were performed. As expected, compressive strength increased as a function of the concentration of the potassium water glass. When combinations of both binders were used, compressive strengths were higher, but a significant increase in strength was also achieved by firing the samples. The compressive strengths of the non-fired samples were in the range from 0.8 to 2.4 MPa, whereas after firing strengths of up to 36 MPa were obtained. During the firing density increased, and porosity was reduced, while the average pore size increased. The results of dilatometric analysis showed that the granulate produced with cement shrink upon firing up to 300 °C, but then start slowly to expand, whereas the granulate produced by water glass first expanded on firing up to 800 °C, and then began to shrink swiftly. In the case of combinations of the two binders, shrinkage as well as expansion on firing was less pronounced. Selected granulate prepared using potassium water glass were also tested in a refractory concrete matrix in order to verify their usability.     

Thermal behaviour of geopolymers prepared using class F fly ash and elevated temperature curing

This article reports a study of thermal stability of properties upon firing at 800-1200 °C of geopolymer materials prepared using class F fly ash and Na and K alkaline activators. Compressive strength and shrinkage measurements, XRD, SEM (BEI), TGA and MIP were utilised in these studies. The materials were prepared at water/binder ratios in a range of 0.09-0.35, using compaction pressures up to 10 MPa and curing temperatures 80 and 100 °C. Thermal stability of the studied geopolymer materials was rather low. In the samples prepared using sodium-containing activators rapid deterioration of strength at 800 °C was observed, which was connected to a dramatic increase of the average pore size. Initially amorphous structures were replaced by the crystalline Na-feldspars. In materials prepared using fly ash and potassium silicate compressive strength was significantly increased on heating, deterioration of strength started at 1000 °C. After firing these materials remained amorphous with reduced average pore size and significantly increased compressive strength. Compaction at 1-10 MPa reduced shrinkage on firing in all materials. Geopolymer materials prepared using class F fly ash and alkaline activators showed high shrinkage as well as large changes in compressive strength with increasing fired temperature in the range of 800-1200 °C. Thus the materials were found unsuitable for refractory insulation applications.     

Correlations between pore structure parameters and gas permeability of corundum porous materials

Corundum porous materials with different contents of calcium hexaluminate formed in situ were prepared using pure calcium aluminate cement as the calcium source. The surface fractal dimensions of the porous materials were calculated based on the experimental data of mercury intrusion. Correlations between pore structural parameters and the permeability coefficients k₁ and k₂ of the porous materials were then studied based on the grey system theory. The results showed that pores in the corundum porous materials have great fractal characteristics. The surface fractal dimension was a significant pore structural parameter that reflected the complexity of pore shape, pore surface, and pore-size distribution, which had the maximum correlation coefficient with the permeability of this type of porous materials. The apparent porosity and pore-size distribution had relatively high correlation coefficients to the permeability as well. Increasing the apparent porosity and the volume percentage of larger pores, and decreasing the volume percentage of smaller pores all benefited the permeability of the porous materials. In addition, the mean pore size and median pore size showed lower correlation coefficients to the permeability—especially for porous materials with a wide pore-size distribution.     

Influence of mineralogy and firing temperature on the porosity of bricks

The changes in brick porosity upon firing (700 up to 1100 °C) and its relation to the mineralogical composition are examined. Two types of raw clay with a composition representative of that used in brick-making industry were selected to manufacture the bricks: one contains notable amounts of carbonates, with a grain size of under 1 mm, and the other is predominantly quartzitic and lacking in carbonates. We demonstrate that the presence or absence of carbonates strongly influences the porosity development and, therefore, the brick texture and physical-mechanical properties. The carbonates in the raw clay promote the formation of fissures and of pores under 1 μm in size when the bricks are fired between 800 and 1000 °C. The absence of carbonates results in a continuous reduction in porosity and a significant increase in the pore fraction with a radius (r)>1 μm as the firing temperature rises and smaller pores coalesce. Porosity and pore size distribution results obtained from the combined use of hydric tests (HT), mercury intrusion porosimetry (MIP) and digital image analysis (DIA) of scanning electron microscopy photomicrographs are compared. A clear correlation between the water absorption and drying behaviour of the bricks and the porosity plus pore size distribution is observed. DIA discloses the evolution of size, shape and connectivity of macropores (r> 1 μm) and evidences that MIP results underestimate the macropore content. Conversely, MIP gives a good estimate of the open porosity and of the distribution of pores with r<1 μm. It is concluded that the combined use of these complementary techniques helps to fully characterise the pore system of bricks. These results as well as the study of the evolution of the speed of ultrasound waves vs. time yield useful information to evaluate the bricks physical–mechanical behaviour and durability. The relevance of these findings in the conservation of historic buildings is discussed.     

Application of mullite-zeolite-alumina microfiltration membranes coated by SiO2 nanoparticles for separation of oil-in-water emulsions

The main objective of this work is the manipulation of hydrophilic materials in support, intermediate and selective layer to synthesize a novel nano-tubular ceramic membrane for treatment of oily wastewater. First, porous mullite-alumina-zeolite composite membranes were prepared by an extrusion method. Changes in porosity, pore size, shrinkage, and mechanical strength of the support membranes were investigated as function of percentage composition and sintering temperature in order to obtain the optimal conditions. According to the results, the most favorable condition set was determined to be a support membrane with a weight percent of 50, 30, and 20 for mullite, alumina, and zeolite, respectively, and a porosity of 38%, a pore size of 0.39 µm, and a shrinkage of 10.2% sintered at 1250 °C and with good mechanical properties at 24.6 MPa. The cross-flow filtration technique was employed to coat the natural zeolite on the inner surface of the support membrane to achieve a narrower pore size distribution. Finally, a thick layer of nano-SiO₂ was coated on the membrane by utilizing the dip-coating method to develop a hydrophilic membrane while avoiding defects. Moreover, scanning electron microscopic (SEM) analysis of the SiO₂ membranes showed that the natural zeolite and nano-SiO₂ layer is homogeneous and demonstrates high adhesion to the support membrane. Besides, the result of COD rejection showed that the SiO₂ membranes have an undeniable capability in rejection of oil droplets with a reasonable permeation flux. Therefore, the obtained membranes are highly promising for practical applications and environmental remediation in sensitive Persian Gulf zone.     

New multifunctional porous Yb2SiO5 ceramics prepared by freeze casting

New porous Yb₂SiO₅ ceramics were prepared by a water-based freeze casting technique using synthesized Yb₂SiO₅ powders. The prepared porous Yb₂SiO₅ ceramics exhibit multiple pore structures, including lamellar channel pores and small pores, in its skeleton. The effects of the solid content and sintering temperature on the pore structure, porosity, dielectric and mechanical properties of the porous Yb₂SiO₅ ceramics were investigated. The sample with 20 vol% solids content prepared at 1550 °C exhibited an ultra-low linear shrinkage (i.e. 4.5%), a high porosity (i.e. 79.1%), a high compressive strength (i.e. 4.9 MPa), a low dielectric constant (i.e. 2.38) and low thermal conductivity (i.e. 0.168 W/(m K)). These results indicate that porous Yb₂SiO₅ ceramics are good candidates for ultra-high temperature broadband radome structures and thermal insulator materials.