Experimental investigation of thermo-physical properties of platelet mesoporous SBA-15 silica particles dispersed in ethylene glycol and water mixture

This paper presents an experimental investigation of thermophysical properties of platelet mesoporous SBA-15 particles dispersed in 60:40 (v/v) ethylene glycol:water mixture. The effect of weight fraction of particles and temperature is studied on density, viscosity and thermal conductivity of nanofluids. The maximum measured thermal conductivity enhancement reaches up to 22% for the nanofluids containing 5 wt% of SBA-15 at 60 °C. The SBA-15 nanofluids show Newtonian behavior in the tested temperature range. Also, the relative density increases between 0.4% and 2.2% when the weight percent of the nanoparticles varies between 1 and 5 at 60 °C. Structural and morphological characterization of synthesized SBA-15 have been carried out using Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and N₂ adsorption–desorption isotherms methods.     

Synthesis of mesoporous silica particles with controlled pore structure

Silica particles, with controllable porosity, were synthesized using two different precursors, tetraethylortosilicate (TEOS) and sodium silicate, but without the addition of template. Characteristics of silica particles (aggregates) prepared by these two methods were compared. The pore structure was tuned only by changing the processing parameters, such as precursor concentration, base concentration, temperature and reaction time. The pore structure of prepared silica particles (aggregates) is strongly influenced by processing conditions and easy controllable in broad range of the specific surface area, pore size, size distribution and pore volume. However, the silica particles synthesized from TEOS have very low total pore volume (ranging from 0.06 to 0.2 cm³/g) and a large portion of pores smaller than 4 nm. On the other side, the silica particles prepared from sodium silicate can be defined as a mesoporous silica with the average pore size up to 20 nm and much higher total pore volume (ranging from 0.8 to 1.5 cm³/g), which are important advantages for their application in encapsulation of enzymes.     

Low hydrothermal temperature synthesis of porous calcium silicate hydrate with enhanced reactivity SiO2

This study presents a novel approach for the synthesis of porous calcium silicate hydrate (CSH) at a low hydrothermal temperature of 110 °C based on enhanced reactivity SiO₂ (i.e. silica/polyethylene glycol (PEG₂₀₀₀) composites) as the source silica material. The as-prepared CSH materials exhibited a porous microstructure with a large number of small mesopores. The porosity formation mechanism of CSH was apparent that cavitation, resulting from sonication, enabled PEG₂₀₀₀ (via intercalation on silica) to break apart Si–O–Si structural units, thereby enhancing SiO₂ reactivity at a low hydrothermal temperature. In addition, the presence of PEG₂₀₀₀ effectively prevented the aggregation of particles during the formation process of the porous CSH solid. The low temperature synthesis proposed herein represents a viable and effective method for the further development of porous CSH as a functional ceramic material.     

Preparation and characterization of SiO2/polydopamine/Ag nanocomposites with long-term antibacterial activity

Silver nanoparticles (Ag NPs) with diameter of approximately 10 nm were prepared by the reduction of silver nitrate using green synthesis, an eco-friendly approach. The synthesized Ag NPs were homogeneously deposited on silicon dioxide (SiO₂) particles modified with dopamine, leading to the formation of SiO₂/polydopamine (PD)/Ag nanocomposites (NCs) with a core–shell–satellite structure investigated by transmission electron microscopy. The Ag content of SiO₂/PD/Ag NCs determined by inductively coupled plasma optical emission spectrometry was approximately 5.92 wt%. The antibacterial properties of both Ag NPs and SiO₂/PD/Ag NCs against Vibrio natriegens (V. natriegens) and Erythrobacter pelagi sp. nov. (E. pelagi) were investigated by bacterial growth curves and inhibition zone. Compared to Ag NPs, the SiO₂/PD/Ag NCs exhibited superior long-term antibacterial activity, attributed to its controlled release of Ag⁺ ions.     

Comparison of freeze drying and spray drying to obtain porous nanostructured granules from nanosized suspensions

This work studies the spray drying and freeze drying of different nanosized ceramic materials and the physicochemical characteristics of the obtained granules. Colloidal suspensions of alumina, titania, and a 87/13 mixture were studied. The influence of temperature, pressure, nozzle diameter, and solids loading on the morphology and characteristics of dried granules were evaluated. It was demonstrated that these processing parameters have practically no influence, and the only parameter determining the granules characteristics is the solids content of the suspensions, in both processes. Spray drying leads to a monomodal distribution with higher granule size, while freeze drying produces more porous granules, with a bimodal intragranular distribution. The flowability of spray-dried powder is better than that of the freeze-dried powder and suit better the requirements of a feedstock targeted to obtain coatings by plasma thermal spraying whereas freeze drying can produce high porosity, softer granules.     

Manufacturing porous multi-channel ceramics by laser gelling

This article describes a novel layer manufacturing process for forming a ceramic part with porous multi-channel architecture by laser gelling under low laser energy. The process involves bonding silica powder by gelled silica sol after exposure by a CO₂ laser. Lower laser energy density of 0.8 J/mm² is required to produce ceramic parts by “gelling effect”. Therefore, the geometrical deflection and thermal distortion can be reduced after laser scanning. The inner porous structures were supported by ceramic slurries to prevent sagged deflection and to enhance dimensional accuracy due to optimal slurry suspension. The flexural strength of the green specimen was 4.7 MPa, while that of the gelled specimen was 12.5 MPa after heat-treatment at 1200 °C for 1.5 h. The proposed process has potential for fabricating complex interconnected porous ceramics for tissue engineering applications.     

Joining of alumina with a porous alumina interlayer

In the joining of structural ceramics, a porous interlayer is generally believed to deteriorate the mechanical properties of the joint. This paper, however, shows that a porous interlayer can sustain high adhesion strength when cavities or interfacial cracks are eliminated. The characteristic of the new slurry approach, described in this work, is that a pure alumina slurry interlayer is dried between two adjoining dense alumina plates and sintered with a negligible external pressure to form the porous interlayer. The effect of slurry concentration was studied to optimize the microstructure of interlayer. By controlling the interlayer microstructure and nature of the flaws, it was possible to fabricate high-strength bonds. The new slurry approach opens up the possibility of pure diffusion bonding which requires neither high pressure during heat treatments nor flat surfaces.     

The role of zircon particle size distribution, surface area and contamination on the properties of silica-zircon ceramic materials

Zircon is used as an additive to silica ceramics for use in investment casting to improve their high temperature properties. However, little is known about the mechanisms by which this occurs. To investigate the effect of zircon addition to a silica ceramic a number of silica-zircon formulations were created utilising three different batches of zircon with different particle size distributions (PSDs), surface areas and contaminant inclusions. The contaminant inclusion of the zircon, present in the zircon from the ball-milling stage of manufacture, appeared to have a large effect on the room temperature flexural strength, high temperature flexural strength and high temperature creep properties. It is also suggested that any increase in post-fired cristobalite content and any change to crystal growth morphology was due to the inherent contaminant inclusions and not because of the zircon itself. Hence, use of silica-zircon materials in ceramics for investment casting should account for variation in the contaminant inclusion of the zircon in order to maintain the specific material properties required.     

Preparation and characterization of a nigerian mesoporous clay-based membrane for uranium removal from underground water

This study reports the removal of uranium in underground wastewater using a Nigerian clay-based membrane. The clay and sintered clay were characterized using XRD, XRF, TGA/DTA, FESEM and PSD. The raw clay was mixed with cassava starch (10, 15, 20 and 25 wt%) and sintered at a temperature of 1300 °C. A multi-point BET analysis of the produced clay-based membranes was conducted to determine the surface area, pore volume and average pore size. Sintering characteristics were determined by apparent porosity, bulk density and flexural strength. The radioactivity of the feed and the permeated water was counted using a gamma spectrometer with an HPGe detector. From the XRD, TGA and FESEM, 1300 °C was found to be optimum for the mullite formation from the clay. The average pore sizes of the produced membranes from the BET results were observed to be in the range from 51 to 70 Å and with a steady state flux range of the tested membranes in the range 1.92×10⁻⁵–2.09×10⁻⁴ m³ m⁻² s⁻¹. The permeation flux produced is of high quality with a rejection in the range of 1.78–2.56 Bq/l of the uranium activity by the tested membranes. This low-cost membrane will have an application for the treatment of uranium-containing wastewater from fracking, oil exploration and phosphate mining industries.     

Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics

Porous yttria-stabilized zirconia (YSZ) ceramics were fabricated by tert-butyl alcohol (TBA)-based gel-casting method for potential applications in heat-insulation materials. The effect of sintering temperature on compressive strength of porous YSZ ceramics was investigated on the basis of measurements linear shrinkage, porosity and pore size. As the sintering temperature increased from 1350 to 1550 °C, a decrease of porosity from 77 to 65%, a decrease of average pore size from and an increase of linear shrinkage from 15.4 to 31.8% were observed. The compressive strength increased remarkably from 3 to 27 MPa with increasing sintering temperature from 1350 to 1550 °C, which was related to the corresponding change of linear shrinkage, porosity, pore size and microstructure. A remarkable decrease of compressive strength with increasing porosity was observed. The compressive strength decreased also with increasing pore size.     

Influence of erbia or europia doping on crystal structure and microstructure of ceria–zirconia (CZ) solid solutions

In this study we have prepared three zirconia–ceria compositions, namely 12, 50 and 80 mole% CeO₂. Along with each pure ceria–zirconia composition we have prepared two parallel erbia- or europia-doped materials in which 0.5 mole% of each of the two starting oxides is replaced by 1 mole erbia or europia.     

Functionally graded porous ceramics with dense surface layer produced by freeze-casting

Graded and porous Al₂O₃ ceramics with dense surface layer were fabricated by camphene-based freeze-casting process. The Al₂O₃/camphene/dispersant slurries were prepared by ball-milling at 60 °C for 24 h, before pouring into silicone rubber die. The warm slurry was cast into a mold at 25 °C, where the top surface of the cast body was exposed to air to allow for the controlled evaporation of molten camphene, and the bottom was attached to a copper plate before being placed on the top of a metal plate immersed in a water bath which can be cooled by ice-water (°C) and liquid nitrogen (−196 °C). This processing method can produce a graded porosity and pore structure distribution. A typical four pore structure, that is surface dense layer, transition layer, aligned pore distribution region and inner porous region is formed between the top surface and bottom surface. This technique is considered potentially useful in fabricating novel porous ceramics with special structure.     

Fabrication of glassy membrane filters and characterization of its hydrophilic nature by thin layer wicking approach

Contact angle measurement of porous ceramics is difficult by goniometry application and it is impossible to determine where the water drops are immediately absorbed by the filter pores if the porous sample is of capillary type. The dynamic wicking approach is a potential technique for the contact angle measurement of capillary materials. In this study, different microstructures of glassy membrane filters were prepared and the water contact angle was determined by using thin layer wicking approach. The success of wicking results was examined. The glassy filters were shaped by two techniques (slip casting and pressing) and they were sintered at different temperatures (900, 950, 1000 and 1050 °C). Results indicated that both the shaping techniques and sintering temperature produced different microstructures and the contact angles obtained by the wicking approach were consistent with the hydrophilic nature of the filter materials. Low temperature sintering did not produce a sufficiently good glassy dispersion and high temperature led to crystallization. The best glassy filter was obtained at moderate temperatures: the sintering temperature being at 900 °C and 1000 °C for the material shaped by pressing and slip casting, respectively. At these temperatures, the glassy pore wall microstructure without crystallization could be obtained. The wicking results have been correlated with the hydrophilic nature of the filters so that the measured water contact angle was equal to or lower than 11°. Normally, the crystallization of the glassy material decreases the hydrophilic nature of the filters and the obtained wicking results are consistent with the phenomena and indicated higher contact angles.     

Fabrication of alumina parts by electrophoretic deposition from ethanol and aqueous suspensions

This paper discusses the effect of the properties of alumina suspensions in ethanol and in water, on green and sintered ceramic parts formed by electrophoretic deposition. The results of the study demonstrate that a small amount of water present in ethanol suspensions as a hidden additive due to the hygroscopicity of alumina powder and ethanol can detrimentally affect the behaviour of the suspension, thus lowering the reliability of the process. Electrophoretic deposition from aqueous suspensions appears to be advantageous over ethanol, from a reliability standpoint, and due to higher achievable green and sintered densities of the deposits and higher deposition rates. Dolapix CE64 appears to be superior surfactant in water as it results in deposits with the lowest green and sintered porosities.     

Thermodynamics of densification of powder compact

This paper established a necessary condition for the sintering of powder compacts by examining the total free energy balance in terms of the particle size, neck size and contact number. The thermodynamic analysis of the proposed model clarifies the relation of shrinkage (q) of powder compact-contact angle ()-relative density at a given dihedral angle (_e) of a grain boundary. Faster densification proceeds in the region with a larger coordination number (n) of particles at a small q value. A large shrinkage is needed to eliminate the large pores formed in the structure of small n value. Full density can be achieved in the range of 117° < _e < _c, where _c is the critical dihedral angle allowing the shrinkage required for full densification. The derived concepts are effective to interpret the densification of hierarchical particle clusters. The relative density of ceria powder compact approached nonlinearly unity with decreasing ratio of pore size (r(P)) to grain size (r) and this tendency was well expressed by the present densification model. The influence of grain growth on the densification of powder compact and size of large pore isolated in a dense matrix are also quantitatively discussed.     

Preparation and mechanical characterization of dense and porous zirconia produced by gel casting with gelatin as a gelling agent

A modified gel casting procedure based on a natural gelatin for food industry and commercial polyethylene spheres as pore formers was successfully exploited to produce dense and porous ceramic bodies made of yttria stabilized tetragonal zirconia polycrystal (Y-TZP). Vickers and Knoop microhardness, elastic modulus and fracture toughness measurements on dense samples obtained by experimental investigation closely matched results found in the literature for similar materials. However, after a careful analysis of obtained results, no indentation size effect and a lower scattering of experimental data from low load indentations were observed, in comparison with literature.     

Approach of phase transformations and densification in nanostructured and (Bi2O3, ZnO,TiO2) doped silica ceramics

The low temperature sintering of silica is studied under the influence of sintering aids and nanosized powders using X-Ray Diffractometry (XRD) and high-temperature environmental scanning electron microscopy analyses (HT-ESEM). Two particular aids were chosen to conduct this study, [Bi₂O₃–ZnO]_eutectic and titania. We report a lowering of the crystallization temperature when the former compound is introduced in the silica while a raise is observed when the latter is used. Moreover, the amorphous silica crystallization into cristobalite inhibits drastically the kinetics of densification of silica-based materials.     

Flow behavior of polymerizable ceramic suspensions as function of ceramic volume fraction and temperature

The effect of volume fraction and temperature on flow behavior is reported for suspensions of coarse silica powders in two non-aqueous polymerizable solutions. The concentration dependence of the viscosity at temperatures 25-75 °C can be reduced to a single Krieger-Dougherty curve for all suspensions. The temperature dependence of viscosity for suspensions with 60 vol% silica could be fit to an Arrhenius equation. The suspensions had a larger apparent activation energy than the suspension medium. This could be explained in terms of thermal dilution, where the higher thermal expansion of the liquid reduces the solids loading for very concentrated suspensions.     

Improvement of oxidation resistance of unidirectional Cf/SiO2 composites by the addition of SiCp

Unidirectional carbon fiber reinforced fused silica composites (uni-C_f/SiO₂) with addition of different contents of SiC particle (SiC_p) were prepared by slurry infiltrating and hot-pressing. The model of oxygen infiltrating into the composite was supposed according to the characterization of fiber/matrix interface observed by transmission electronic microscope (TEM). The oxidation process of the composite was analyzed by thermo-gravimetry and differential scanning calorimeter (TG-DSC) method and the oxidation resistance was evaluated by the residual flexural strength and the fracture surface of the composite after heat treatment at elevated temperatures method. The results showed that the oxidation of carbon fiber started at 480 °C and ended at 800 °C and the oxidation of SiC_p started at above 1000 °C in the composite. The addition of 20 wt.% SiC_p had a better oxidation resistance. According to the characterization of fiber/matrix interface observed by TEM, gaps existed at the fiber/matrix interface which resulted from the CTE mismatch of carbon fiber and SiO₂ matrix. While the CTE mismatch between SiC_p and SiO₂ matrix could also result in the pre-existing gaps in the matrix. The oxygen penetrated along the gaps and simultaneously reacted with carbon fiber ends and SiC_p, which filled the gaps at the fiber/matrix interface and the pre-existing gaps in the matrix and subsequently prevented oxygen from infiltrating inward.