Preparation and characterization of novel ceramic membranes for micro-filtration applications

Porous microfiltration range ceramic membranes were prepared using kaolin and other suitable materials like feldspar, quartz, boric acid, activated carbon, sodium metasilicate and titanium dioxide following standard paste casting route. The membranes were casted as circular disks of 40 mm ID and 5 mm thickness. They were characterized using thermo gravimetric analysis (TGA), particle size distribution (PSD), X-ray diffraction (XRD) and scanning electron microscope (SEM) to evaluate the effect of maximum sintering temperature on the structure, porosity and mechanical integrity. The prepared membranes were initially dried at 120 °C and 250 °C for 24 h each and finally sintered at 850 °C, 900 °C and 950 °C for 6 h. Morphological parameters viz. pore size distribution, porosity, average pore size of the prepared membranes were determined and the membrane performance were evaluated by carrying out the permeation experiment with pure water. Results show that the average pore size of the membranes increases from 1.59 µm to 2.56 µm and porosity of the membrane supports decreases from 18.88% to 5.59% with increase in sintering temperature from 850 °C to 950 °C. The membrane corrosion resistance was also tested using acid and base and it is observed that there is no significant weight loss in the process. Based on market price of the inorganic precursors, the membrane cost was estimated to be $92/m² which can be considered low cost in the microfiltration range for industrial applications.     

Fundamental study on the preparation of insulating ceramics via the phase reconstruction of phosphate tailings

Insulating ceramics with anorthite and diopside as the main crystal phases were manufactured using phosphate tailings, coal gangue, and quartz as raw materials, which were fired at 1160–1190 °C for 2 h. The structure and micromorphology of the samples were investigated using X-ray diffraction, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. Dense ceramic samples exhibited an apparent porosity of less than 0.3%, densities of 2.64–2.86 g/cm³, bending strength of 127.71–172.73 MPa, dielectric constants of 6.98–7.79, and dielectric losses of 0.0086–0.0024 at 20 °C and frequency of 1 MHz. The effects of impurity elements on ceramic properties are determined. The excellent electrical properties can be attributed to the solid solution of iron in the crystalline phases, such as diopside, which reduce leakage conductance in ceramics and alleviate the influence of relaxation and space charge polarization on the electrical properties of ceramics. This study provides a new strategy for preparing high-value insulating ceramics from solid waste.     

Sintering behaviors and properties of porous ceramics derived from artificially cultured diatom frustules

Bimodal porous ceramics with high strength have been fabricated by conventional powder metallurgy utilizing artificially cultured diatom frustules (DFs). The effect of sintering temperature on thermal behaviors, phase transition, and pore structures features of DFs-based porous ceramics is investigated between 800 and 1200°C. The phase evolution of DFs powders is investigated with thermal analysis (DIL and DSC-TG). Phase transition behaviors analyzed with XRD, Raman, and FT-IR spectra confirm the transformation of quartz into cristobalite phases occurs under 1050°C. Sintering under 950°C could bind DFs powders tightly into high strength porous ceramics while maintain the multilayer pore structures simultaneously, having porosity of 56.4%, compressive strength of 15.0 MPa and surface area of 50.9 m²/g, respectively. Slit-shaped microstructures and mesopores (2-50 nm) are observed in DFs-based porous ceramics sintered under 1050°C. Collapse and blockage of pore structures as well as partial fusion of DFs particles happened at the temperature of 1100°C, indicating the presence of diminished multilayers and particle agglomeration.     

Reaction-bonded robust SiC ceramic membranes sintered at low temperature with coal gangue

Herein, we report an in-situ reaction-bonded SiC membrane sintered at low temperature using a solid waste (i.e. coal gangue) as the sintering aid to form strong neck connections. The effects of sintering temperature and coal gangue proportion on their properties regarding pore size, open porosity, bending strength and pure water permeability were investigated. The single-channel tubular SiC membrane sintered at 1300 °C with a coal gangue proportion of 12 wt% was optimal, exhibiting an average pore size of 2.78 μm, a open porosity of 47.08%, a bending strength of 34.01 ± 1.3 MPa and a high water permeability of 83967 L m⁻² h⁻¹ bar⁻¹. The membrane could completely reject D₅₀ = 0.87 μm SiC solids and presented a steady-state water permeability of 458 L m⁻² h⁻¹·bar⁻¹. The SiC membrane could be regenerated through ultrasonication and its steady-state water permeability was almost unchanged for 3 cycles, proving its mechanical robustness. This work may appeal to the practical low-cost production of high-performance SiC membranes.     

Porous mullite ceramics with enhanced compressive strength from fly ash-based ceramic microspheres: Facile synthesis,structure, and performance

Porous mullite ceramics are widely used in heat insulation owing to their high temperature and corrosion resistant properties. Reducing the thermal conductivity by increasing porosity, while ensuring a high compressive strength, is vital for the synthesis of high-strength and lightweight porous mullite ceramics. In this study, ceramic microspheres are initially prepared from pre-treated high-alumina fly ash by spray drying, and then used to successfully prepare porous mullite ceramics with enhanced compressive strength via a simple direct stacking and sintering approach. The influence of sintering temperature and time on the microstructure and properties of porous mullite ceramics was evaluated, and the corresponding formation mechanism was elucidated. Results show that the porous mullite ceramics, calcined at 1550 °C for 3 h, possess a porosity of 47%, compressive strength of 31.4 MPa, and thermal conductivity of 0.775 W/(m?K) (at 25 °C), similar to mullite ceramics prepared from pure raw materials. The uniform pore size distribution and sintered neck between the microspheres contribute to the high compressive strength of mullite ceramics, while maintaining high porosity.     

Preparation,characterization and self-foaming mechanism of total-tailings-based foamed glass-ceramics

A kind of foamed glass-ceramics (FGC) that can be used in the construction field was synthesized by self-foaming technology using untreated phosphorus tailings (PT) and coal gangue (CG) as raw materials in this study. The effects of PT addition and sintering temperature on the phase transformation, microstructure, and properties of FGC were explored, also its self-foaming mechanism during the sintering process was revealed. The results showed that FGC with 80 wt.% PT addition had the best comprehensive performances when treated at 1150 °C, whose porosity was 61.6%, bulk density was 1.04 g/cm³, and compressive strength was 10.11 MPa. Based on the theoretical calculation, CaO and K₂O included in the tailings acted as sintering aids, promoting the formation of a glassy phase, which increased the pore size and solidified Ba²⁺ and Ni²⁺ within FGC. The pore formation mechanism included in-situ pore formation caused by the combustion of C and the gas released by the reduction of Fe₂O₃ was wrapped by the glassy phase. This self-foaming synthesized FGC from total tailings provided a novel approach to the high-value utilization of tailings and the development of novel environmentally friendly construction materials.     

Physicochemical properties of binder gel in alkali-activated fly ash/slag exposed to high temperatures

The present study investigated the physicochemical properties of binder gel in alkali-activated fly ash/slag exposed to high temperatures. Strength test results showed that the strength increased until exposure to 400 °C and thereafter started to decrease. The strength increase below 400 °C was attributed to the binder gel which formed after exposure, decreasing the porosity. The dehydration of C-A-S-H and the formation of N-A-S-H simultaneously occurred, inducing the transformation of pore structure from microporous to mesoporous state. The crystallization of the binder gel resulted in an increase in the porosity, thereby inducing a decrease in the strength above 400 °C and is responsible for the transformation of the pore structure from mesoporous to macroporous state. The porosity under high temperatures had an inverse relationship with the strength, and was significantly altered as the binder gel underwent additional formation, dehydration and crystallization.     

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m³ m⁻² h⁻¹ bar⁻¹, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.     

Effect of preparation method on the mechanism for oxidation of C/C-BN composites

C/C-BN composites and C_f/BN/PyC composites exhibiting different structures for pyrolytic carbon (PyC) and boron nitride (BN) were studied comparatively to determine their oxidation behavior. This study used five types of samples. Porous C/C composites were modified with silane coupling agents (APS) and then fully impregnated in water-based slurry of hexagonal boron nitride (h-BN); the resulting C/C-BN preforms were densified by depositing PyC by chemical vapor infiltration (CVI), resulting in three types of C/C-BN composites. The other two C_f/BN/PyC composites were obtained by depositing a BN interphase and PyC in carbon fiber preforms by CVI; one was treated with heat, and the other was not. This study was focused on determining how the PyC deposition mechanism, morphology and pore structure were affected by the method of BN introduction. In the 600–900 °C temperature range, the C_f/BN/PyC composites and C/C composites underwent oxidation via a mixed diffusion/reaction mode. The C/C-BN composites had a different pore structure due to the formation of nodules comprising h-BN particles; both interfacial debonding and cracking were reduced, resulting in higher resistance to gas diffusion, lower oxidation rate and larger activation energy (Ea) in the temperature range 600–800 °C. In addition, the mechanism for oxidation of C/C-BN composites gradually exhibited diffusion control at 800–900 °C because the formation of h-BN oxidation products healed the defects. The oxidation mechanism was more dependent on pore structure than on BN structure or content.     

Mechanical,structural and oxidation resistance enhancement of carbon foam by in situ grown SiC nanowires

A method for processing carbon foams containing both silicon carbide (SiC) nanowires and bulk SiC and silicon nitride (Si3N4) phases has been developed by reaction of powder mixtures containing precursors for carbon, sacrificial template, silicon (Si), short carbon fibers (SCF) and activated carbon (AC). In situ growth of Si nanowires during pyrolysis of the foam at 1000 °C under N2 changed the foam׳s microstructure by covering the porous skeleton inside and out. In situ-grown SiC nanowires were found smoothly curved with diameters ranging around two main modes at 30 and 500 nm while their lengths were up to several tens of micrometers. SCF were found effectively mixed and well-bonded to pore walls. Following density, porosity and pore size distribution analyses, the heat-treated (HT) foam was densified using a chemical vapor infiltration (CVI) process. Thereafter, density increased from 0.62 to 1.30 g/cm³ while flexural strength increased from 29.3 to 49.1 MPa. The latter increase was attributed to the densification process as well as to low surface defects, presence of SCF and coating, by SiC nanowires, of the entire SiC matrix porous structure. The foam׳s oxidation resistance improved significantly from 58 to 84 wt% residual mass of the heat treated and densified sample. The growth mechanism of Si nanowires was supported by the vapor–liquid–solid mechanism developed under pyrolysis conditions of novolac and reducing environment of coal cover.     

Effects of heating rate on microstructure and transparency of spark-plasma-sintered alumina

Commercial alumina powder was densified by spark plasma sintering (SPS) at 1150 °C. During SPS processing, the effects of the heating rate were examined on microstructure and transparency. With decreasing heating rate, the grain size and the residual porosity decreased, while the transparency increased. At a heating rate of 2 °C/min, the grain size was 0.29 μm, and the in-line transmission was 46% for a wavelength of 640 nm. The mechanisms for the fine microstructure and low porosity at slow heating, which are conflicting with some existing results, were explained by considering the role of defect concentration and grain-boundary diffusion during densification.     

Experimental study on foam glass prepared by hydrothermal hot pressing-calcination technique using waste glass and fly ash

Foam glass is a lightweight and high-strength building and decoration material with superior performance in heat insulation, sound absorption, moisture resistance and fire protection. The use of waste glass powder and fly ash to prepare foam glass is one of the most important ways to utilize solid waste as a resource. In this study, waste glass powder and fly ash were used as raw materials to prepare foam glass by a hydrothermal hot pressing–calcination method. The effects of fly ash content (0 wt%, 10 wt%, 20 wt%, 30 wt%), heating rate (1 °C/min, 3 °C/min, 5 °C/min, 8 °C/min, 10 °C/min) and calcination temperature (600 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C) on the microscopic morphology, density, compressive strength, porosity and other properties of the foam glass samples were studied. Their microstructure and morphology were analyzed by thermogravimetric analysis–mass spectrometry, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. At a fly ash content of 10 wt%, the heating rate was 5 °C/min, the calcination temperature was 800 °C, the foam glass density was 0.3 g/cm³, the compressive strength was 1.65 MPa, the total porosity was 75.5%, and the effective thermal conductivity was 0.206 W/m·K. The effective thermal conductivity models of the composite materials were used to verify the experimental data. The relationship between the thermal conductivity of foam glass materials and the related influencing factors was investigated.     

Porous SiC ceramics prepared via freeze-casting and solid state sintering

Porous SiC ceramics were prepared by freeze-casting process. In order to enhance the mechanical properties of the porous SiC, poly(vinyl alcohol) (PVA) was added as binder and pore morphology controller in this work. The results indicated that high porosity (>60%) SiC ceramics was obtained although the sintering temperature was over 2000 °C. The pore structure could be divided into two kinds: macropores generated by sublimation of large ice crystals, and micropores in the ceramic matrix caused by sublimating of small ice crystals, stacking of SiC particles, and burning out of PVA. With the increase of the sintering temperature, the specimens exhibited higher density, thus resulted in higher strength. Porous SiC ceramics sintered at 2100 °C showed a good flexural strength of 11.25 MPa with an open porosity as high as 66.46%.     

The wetting-to-nonwetting transition of CVD BN coatings deposited at different temperatures

Boron nitride (BN) coatings (thickness 20–40 μm) were prepared on graphite substrates by chemical vapor deposition, with precursors of BCl₃ and NH₃ (ratio of 1:4) and pressure of 500 ± 50 Pa. The influence of the deposition temperature (650°C–1250°C) on the wettability of BN coatings with deionized water was studied. The wetting angle rapidly increases at 1100°C–1250°C, and the wetting-to-nonwetting transition occurs. The crystal structure and surface morphology of the BN coatings were characterized by a stylus instrument, scanning electron microscopy, and transmission electron microscopy. Research shows that the contact angle or nonwettability increases with a higher degree of crystallinity and a lower surface roughness, which were both under the control of the deposition temperature since the pressure and gas flows were kept constant in this study. At a deposition temperature of 650°C–950°C, the increase in the degree of crystallinity dominates; at 950°C–1100°C, the increase in surface roughness takes over. At 1100°C–1250°C, the degree of crystallinity continues to increase, while the surface roughness decreases due to the advantage of nucleation and the breakage of large surface clusters into smaller clusters. This results in increases (650°C–950°C), then decreases (950°C–1100°C) and again fast increases (1100°C–1250°C) in the wetting angle between the BN coating and deionized water and finally in the wetting-to-nonwetting transition (1100°C–1250°C).     

A new type of microwave dielectric ceramic based on K2O–SrO–P2O5 composition with high quality factor and low sintering temperature

A new type of microwave dielectric ceramics with low dielectric loss was fabricated through a traditional solid-phase method. X-ray diffraction and density tests showed that KSrPO₄ ceramics with a single orthorhombic phase could be synthesized and densified at 950 °C, and the crystal structure of KSrPO₄ was further confirmed by Rietveld refinement analysis. The densification temperature of KSrPO₄ was lower than 961 °C, indicating the ceramics could be used in LTCC devices. Additionally, based on the complex chemical bond theory, some internal parameters of KSrPO₄ ceramics were calculated and the effects of these parameters on the properties of KSrPO₄ were systematically analyzed for the first time. Furthermore, the composite dielectric constant and loss of KSrPO₄ ceramics were analyzed by infrared reflectance spectroscopy, and the theoretical loss and the actual loss were compared. Finally, a vector network analyzer was employed to measure the microwave dielectric properties of all samples. The results showed that KSrPO₄ sintered at 950 °C obtain the best microwave dielectric properties, including ε_r = 7.85, Q·f = 34,527 GHz (at 10.43 GHz) and τ_f = ?14.82 ppm/°C.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.     

Sintering and characterization of in situ formed porous cordierite ceramics with nano boehmite additions

In this study, we investigated the effect of sintering temperature and nano boehmite additions on the phase composition, densification, and mechanical properties of porous cordierite ceramics. Ceramic samples were sintered at temperatures ranging from 1200 to 1400°C. Carbon powder was used as a pore forming agent to improve the porosity of the ceramic structure. Nano boehmite and carbon additions significantly enhanced ceramic porosity and average pore size in sintered samples. The bulk density and apparent porosity of the sintered samples were found to be 0.96–1.53 g/cm³ and 42.3%–65.6%, respectively. Sintered samples had cold crushing strengths of 1.5–14.3 MPa. The microstructure obtained by scanning electron microscopy was used to measure average pore size in sintered samples and was found to be 41.93 µm for stoichiometric composition (SC), 67.72 µm for SC and nano boehmite, and 102.98 µm for SC, nano boehmite, and carbon. The microstructure of the sintered samples revealed that the crystallinity of the in situ formed phases increased with the increase in nano boehmite additions.     

Excellent electrostrictive properties of low temperature sintered PZT ceramics with high concentration LiBiO2 sintering aid

The effect of LiBiO₂ (LBO) additive on the sintering of Pb_0.97La_0.03(Zr_0.53Ti_0.47)_0.9925O₃ (PLZT) ceramics was carefully investigated. 6.0 wt% LBO added PLZT powders could be fully densified to 98% relative density at a temperature as low as 950 °C. It is worthy to notice that there are distinct enhancements in piezoelectric and electrostrictive properties by increasing the soaking time from 2 h to 7 h, which could mainly originate from the improvement of crystallinity and grain size of PLZT ceramics. By controlling the soaking time and concentration of LBO addition, PLZT ceramics sintered at 950 °C could exhibit high curie temperature of 240 °C and very high S₁₁ of 0.22% under 3.0 kV/mm, which is even better than that of traditionally sintered PZT-5, PMN–PZT, and this is very promising for actuators designed in multilayer structure in high temperature environment.     

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al₂O₃–NaCl composite with bulk density of 2.92 g/cm³ was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al₂O₃ particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al₂O₃ structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.