Surface modification of α-Al2O3 with dicarboxylic acids for the preparation of UV-curable ceramic suspensions

Stereolithography of UV-curable ceramic suspensions can benefit from the preparation of stable, low viscosity and high solid loading ceramic suspensions without yield stress. Appropriately adding dispersants could optimize the rheological behavior to meet the requirements of stereolithography. In this work, short-chain dicarboxylic acids were utilized to modify the alumina particles and achieve well dispersed ceramic suspensions. The maximum adsorption capacities of dicarboxylic acids were determined by the method of High Performance Liquid Chromatography and the mechanism of surface modification and dispersion was also discussed. Dicarboxylic acids’ influence on the rheology behavior was systematically studied. When doses of dicarboxylic acids reach their maximum adsorption capacities, the alumina suspensions would achieve their lowest viscosities and yield stresses. 45 vol% alumina suspension with a viscosity ˂2 Pa s at shear rate 30 s⁻¹ was successfully formulated. A sintering density of 96.5% can be achieved for the sebacic acid-modified alumina UV-curable suspension.     

Rapid and uniform in-situ solidification of alumina suspension via a non-contamination DCC-HVCI method using MgO sintering additive as coagulating agent

A novel rapid, uniform and non-contamination in-situ solidification method for alumina suspension by DCC-HVCI method using MgO sintering additive as coagulating agent was reported. MgO was used to release Mg²⁺ in suspensions via reaction with acetic acid generated from glycerol diacetate (GDA) at elevated temperature as well as to improve density and suppress grain growth of alumina ceramics during sintering. Influence of adding 0.7 wt% MgO with 2.0 vol% GDA in alumina suspension on coagulation process and properties of green bodies and sintered samples were investigated. It was indicated that the controlled coagulation of the suspension could be achieved after treating at 70 °C for 10 min. Homogeneous composition distribution of Mg element in EDS result indicated the uniform solidification of suspensions. Compressive strength of wet-coagulated bodies is 2.09±0.25 MPa. Dense alumina ceramics with relative density of 99.2% and flexural strength of 354±16 MPa sintered at 1650 °C for 4 h present homogeneous microstructure. The result indicated that the novel DCC-HVCI method via a sintering additive reaction with no contamination, short coagulation time and uniform in-situ solidification is a promising colloidal forming method for preparing high-performance ceramic components with complex shape.     

Mullite coatings on ceramic substrates: Stabilisation of Al2O3–SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying

The present work deals with the preparation of stable alumina + silica suspensions with high solid loading for the production of spray-dried composite powders. These composite powders are to be used for reactive plasma spraying whereby the formation of mullite and the coating on a ceramic substrate are achieved in a single step process. Electrostatic stabilisation of alumina and silica suspensions has been studied as a function of pH. Silica suspensions are most stable at basic pH whereas alumina suspensions are stable at acidic pH. The addition of ammonium polymethacrylate (APMA) makes it possible to stabilise alumina and prepare a stable 50 wt% alumina + silica suspension at pH 10. The optimum amounts of dispersant and binder have been determined by zeta potential, viscosity and sedimentation measurements. Spray drying of the suspension yields composite powders whose morphology, size distribution and flowability have been characterized before realizing reactive plasma spraying tests.     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

Complex ceramic architectures by directed assembly of ‘responsive’ particles

Surface functionalization of alumina powders with a responsive surfactant (BCS) leads to particles that react to a chemical switch. These ‘responsive’ building blocks are capable of assembling into macroscopic and complex ceramic structures. The aggregation follows a bottom up approach and can be easily controlled. The directed assembly of concentrated suspensions leads to highly dense (∼99%) ceramic components with average 4-point bending strength of ∼200 MPa. On the other hand, the emulsification of suspensions with concentrations from 7 to 43 vol% and 50 vol% decane results in emulsions with different properties (stability, droplet size and distribution). The oil droplets provide a soft template confining the alumina particles in the continuous phase and at the oil/water interfaces. Aggregation of these emulsions followed by drying and sintering leads to macroporous (pore sizes ranging from 30 to 4 μm) alumina structures with complex shapes and a wide range of microstructures, from closed cell structures to highly interconnected foams with total porosities up to 83%. Alumina scaffolds with ∼55% porosity can reach crushing strength values above 300 MPa in compression and ∼50 MPa in 4-point bending.     

Yttria nettings by colloidal processing

Porous ceramic burners have been shown as a promising technology to produce heat and lighting by burning low calorific fuels like modern biomass. Among ceramics, yttria (Y₂O₃) presents considerable luminescent proprieties for gas burner technology. By colloidal processing of yttria, this work aims to produce luminescence ceramic nettings with potential to be used as gas burners. Processing parameters such as mean particle size, zeta potential and flow behavior were evaluated in order to prepare suitable suspensions for replica method. Yttria nanoparticles presented light emission with λ = 550 nm when being thermal stimulated at 150 °C. Besides, the nano sized powders d₅₀ = 113.8 nm and specific surface area of 13.6 m² g⁻¹ could be highly stabilized at pH 10.5. Suspensions with 30 vol% of solids, pH 10.5, 1 wt% of dispersant and 0.3 wt% of binder presented shear thinning behavior and thixotropy suitable for replica method. As a result, samples sintered at 1600 °C/1 h showed homogeneous morphology of struts and porous microstructure desirable for gas recirculation and burning process.     

Room-temperature injection molding of aqueous alumina-polyvinylpyrrolidone suspensions

Room-temperature injection molding, a novel, environmentally benign ceramic processing method, produced dense, near-net shape alumina rings by utilizing unique flow properties of aqueous, highly loaded (>50 vol.%) ceramic suspensions with ≤5 vol.% polyvinylpyrrolidone (PVP) dispersed using Darvan 821A. The rheological behavior of suspensions along with microstructural and mechanical properties of resulting specimens were evaluated by varying PVP content to determine the optimal composition for forming. Parallel-plate rheometry revealed that suspensions containing ≤5 vol.% PVP were yield pseudoplastic at room temperature, which facilitated processing without heating or complex chemical reactions. Alumina rings with high green densities (>60% true density (TD)) were machined before binder removal, and increasing PVP content was observed to enhance green machinability. After binder burnout and sintering, bulk densities were ∼98%TD with <16% linear shrinkage. Scanning electron microscopy revealed minimal pore formation within specimens. Ultimate strength of samples was determined using ASTM C1323-10, and a maximum C-strength of 261 ± 57.6 MPa was obtained.     

Fabrication and characterization of anorthite-based ceramic using mineral raw materials

The purpose of this study is to design a novel single crystalline phase ceramic based on anorthite whose properties fulfill the tableware market requirements such as high appearance quality, strength and thermal shock resistance. To obtain the single phase anorthite ceramic, ball clay, quartz, calcite, feldspar and alumina were used as raw materials. The single phase anorthite ceramic was fabricated by slip casting and sintering at 1230 °C for 1 h. It has a high flexural strength of 103 MPa, which is higher than that of the conventional porcelain. The single phase anorthite ceramic had relatively low (4.9 × 10^?6 K^?1) thermal expansion coefficient which can be matched with applicable glaze easily. Furthermore, the single phase anorthite ceramic had high degree of whiteness (L* = 94) and excellent translucency behavior which could achieve a high-quality decorative effect.     

Gelcasting of alumina suspensions containing nanoparticles with glycerol monoacrylate

An acrylic monomer of low toxicity containing two hydroxyl groups has been synthesized and used for gelcasting in water. The results have been compared to those achieved with the use of a commercially available monomer (2-hydroxyethyl acrylate). Due to the chemical structure of the synthesized monomer, no addition of the crosslinking agent was necessary for gelation and similar results in terms of rheology of suspensions, density and microstructure of the bodies were obtained with respect to those obtained with the commercial monomer. However, higher time for gelation was observed.Two alumina powders with very different particle sizes were used in this study: a commercial submicron-sized powder (d₅₀ = 0.35 μm) and a nanometer-sized alumina obtained by freeze-drying from aluminum sulphate solutions. The rheological behavior of concentrated suspensions was studied in order to establish their stability and to analyse the effect of the different monomers used in the process. Once the suspensions were optimized, the influence of the size of the powder on the gelation process was studied. The sintered density of submicrometer-sized alumina was higher (99%) than that measured when the bimodal suspension was used due to the difficulty to obtain highly concentrated suspensions from nanometric powder.     

Preparation and characterization of porous alumina ceramics through starch consolidation casting technique

This work aims at studying the influence of thermal treatment on the microstructure, resistivity and technological properties of porous alumina ceramics prepared via starch consolidation casting (SCC) technique. Colloidal suspensions were prepared with three different contents of alumina solid loading (55, 60 and 65 mass%) and corn starch (3, 8 and 13 mass%). The sintered samples at 1400, 1500, 1600 and 1700 °C, show open porosity between 46 and 64%, depending on the starch content in the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM. The effect of corn starch content on the apparent porosity, pore size distribution, linear shrinkage and electrical resistivity as well as cold crushing strength of the sintered porous alumina ceramics was also measured. These porous alumina ceramics are promising porous ceramic materials for using in a wide range of thermal, electrical and bioceramics applications as well as filters/membranes and gas burners, due to their excellent combination properties.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

Investigating the effect of porosity level and pore former type on the mechanical and corrosion resistance properties of agro-waste shaped porous alumina ceramics

The strength integrity and chemical stability of porous alumina ceramics operating under extreme service conditions are of major importance in understanding their service behavior if they are to stand the test of time. In the present study, the effect of porosity and different pore former type on the mechanical strength and corrosion resistance properties of porous alumina ceramics have been studied. Given the potential of agricultural wastes as pore-forming agents (PFAs), a series of porous alumina ceramics (Al₂O₃-xPFA; x=5, 10, 15 and 20 wt%) were successfully prepared from rice husk (RH) and sugarcane bagasse (SCB) through the powder metallurgy technique. Experimental results showed that the porosity (44–67%) and the pore size (70–178 µm) of porous alumina samples maintained a linear relationship with the PFA loading. Comprehensive mechanical strength characterization of the porous alumina samples was conducted not just as a function of porosity but also as a function of the different PFA type used. Overall, the mechanical properties showed an inverse relationship with the porosity as the developed porous alumina samples exhibited tensile and compressive strengths of 20.4–1.5 MPa and 179.5–10.9 MPa respectively. Moreover, higher strengths were observed in the SCB shaped samples up to the 15 wt% PFA mark, while beyond this point, the silica peak observed in the XRD pattern of the RH shaped samples favored their relatively high strength. The corrosion resistance characterization of the porous alumina samples in hot 10 wt% NaOH and 20 wt% H₂SO₄ solutions was also investigated by considering sample formulations with 5–15 wt% PFA addition. With increasing porosity, the mass loss range in RH and SCB shaped samples after corrosion in NaOH solution for 8 h were 1.25–3.6% and 0.44–2.9% respectively; on the other hand, after corrosion in H₂SO₄ solution for 8 h, the mass loss range in RH and SCB shaped samples were 0.62–1.5% and 0.68–3.3% respectively.     

Synthesis of SiC–TiO2 hybrid aerogel via supercritical drying combined PDCs route

The SiC-TiO₂ hybrid aerogels were obtained from polycarbosilane (PCS) and tetrabutyltitanate (TBT) as precursors using supercritical drying and polymer derived ceramics route. The polymer to ceramic conversion and the crystallization behavior were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM), suggesting the preceramic aerogels converted to the SiC-TiO₂ ceramic aerogels through pyrolysis process at different temperatures. At 1200 °C, the ceramic aerogels were homogeneous with well-distributed element components, composed of rutile TiO₂ and the β-SiC crystalline phases. The results show that the SiC-TiO₂ ceramic aerogels with netwoks structure have 23.36 nm average pore size, high surface area (58 m²/g) and pore volume (0.22 cm³/g).     

Wear behavior of graphene/alumina composite

In the present work, the dry sliding behavior of a graphene/alumina composite material was studied against alumina in air. The tests were carried out in a reciprocating wear tester with an applied load of 20 N, a sliding speed of 0.06 m s⁻¹ and a sliding distance of up to 10 km. Under the testing conditions, the graphene/ceramic composite showed approximately half the wear rate and a 10% lower friction coefficient than the monolithic alumina. It has been found that this behavior is related to the presence of graphene platelets adhered to the surface of friction that form a self-lubricating layer which provides enough lubrication in order to reduce both wear rate and friction coefficient, as compared to the alumina/alumina tribological system.     

Mechanical properties of hybrid composites prepared by ice-templating of alumina

Ceramic-polymer hybrid composites are often designed for its high strength and low density. Ice-templating (freeze casting) is a promising method for preparation of such composites. However, the most of the reported mechanical properties were gained from a small volume of material. In this work 70 cm³ of the lamellar composite with lamella length, up to 70 mm was prepared by ice-templating followed by polymer infiltration. The volume of ceramic (alumina) in starting suspensions was varied from 25 to 45 vol.% and the same manufacturing process was applied. The fracture toughness and flexural strength were determined on prepared beams from plates by loading in bending. The fractographic analysis conducted on the fracture surfaces and obtained mechanical properties demonstrated that an optimal strength/density ratio lies between 51 and 55% of alumina volume fraction. The density ranking from 2.6 to 2.8 cm⁻³ of these composites results in values of Weibull strength above 110 MPa.     

Preparation and characterization of glazes from combinations of different industrial wastes

The work reported here involves the preparation and characterization of ceramic glazes made from combinations of different industrial wastes. The wastes were float glass, granite and lime shale (a raw material waste from the oil shale industry in São Mateus do Sul, state of Paraná, Brazil), which were used to replace natural raw materials in a proportion of up to 50% in weight. The compositions were formulated using the Seger method and prepared by conventional ceramic processing. The stabilized suspensions were applied in commercial wall tile and porcelain stoneware tile, which were sintered at temperatures of 1080 °C and 1150 °C, respectively, using two different heating cycle. Three compositions were developed, two of which yielded opaque glazes and one a transparent glaze. Linear thermal expansion coefficients (α) of 80.10^?7 °C^?1 to 100.10^?7 °C^?1, and glaze softening temperatures of 600–700 °C were characterized by dilatometric analysis. The glaze compositions showed chemical resistance against acid and alkaline attack after 96 h, showing a mass loss of less than 0.1% in weight. The surface hardness of the glazes determined by the Mohs scale, Vickers microhardness and abrasion resistance (PEI indices) were between 6–7, 3–3.7 GPa and 3–4, respectively. These properties are compatible with those of commercial glazes for wall tiles and porcelain stoneware ceramics.     

Alumina fiber-reinforced silica matrix composites with improved mechanical properties prepared by a novel DCC-HVCI method

A novel direct coagulation casting via controlled release of high valence counter ions (DCC-HVCI) method was applied to prepare the alumina fiber-reinforced silica matrix composites with improved mechanical properties. In this method, the silica suspension could be rapidly coagulated via controlled release of calcium ions from calcium iodate and pH shift by hydrolysis of glycerol diacetate (GDA) at an elevated temperature. The influence of tetramethylammonium hydroxide (TMAOH) dispersant amount, volume fraction and calcium iodate concentration on the rheological properties of suspensions was investigated. Additionally, the effect of alumina fiber contents on the mechanical properties of alumina fiber-reinforced silica matrix composites was studied systematically. It was found that the stable suspension of 50 vol% solid loading could be prepared by adding 2.5 wt% TMAOH at room temperature. The addition of 0–15 wt% alumina fibers had no obvious effect on the viscosity of the silica suspension. The controlled coagulation of the suspension could be achieved by adding 6.5 g L⁻¹ calcium iodate and 1.0 wt% GDA after treating at 70 °C for 30 min. Compressive strength of green bodies with homogeneous microstructure was in the range of 2.1–3.1 MPa. Due to the fiber pull-out and fracture behaviors, the mechanical properties of alumina fiber-reinforced composites improved remarkably. The flexural strength of the composite with 10 wt% alumina fibers sintered at 1350 °C was about 7 times of that without fibers. The results indicate that this approach could provide a promising route to prepare complex-shaped fiber-reinforced ceramic matrix composites with uniform microstructure and high mechanical properties.     

Influence of anionic stabilization of alumina particles in 2-propanol medium on the electrophoretic deposition and mechanical properties of deposits

The ceramic dispersions were prepared using 0.85, 1.70, 4.25, 12.75 or 21.25 wt.% of monochloroacetic, dichloroacetic or trichoroacetic acid, 15 wt.% alumina and 2-propanol. The mechanism of anionic stabilization in 2-propanolic media was described. Alumina green bodies were prepared from the stable dispersion via electrophoretic deposition (EPD). It was found that increasing dispersion conductivity significantly influenced the EPD yields. The most effective electrophoretic depositions were performed from dispersions with conductivity in range 4.0–5.3 × 10⁻⁴ S m⁻¹. Deposits with the highest green density were prepared from the dispersion stabilized by trichloroacetic acid. This behavior was explained by low voltage drop during deposition. The surface roughness was high at low dispersion conductivity and with increasing acid concentration in dispersion the surface of deposits was smoother. The mechanism of particle arrangement in deposit was discussed. Influence of stabilizer amount in the dispersion on the hardness and fracture toughness was described.     

Heat transport enhancement of thermal energy storage material using graphene/ceramic composites

Graphene/ceramic composites are proposed by directly depositing graphene on the insulating Al₂O₃ particles by chemical vapor deposition without any metal catalysts. Carbothermic reduction occurring at the Al₂O₃ surface is vital during the initial stage of graphene nucleation and the graphene sheet can connect with neighboring sheets to completely cover Al₂O₃ particles. The quality and layer number of graphene on Al₂O₃ can be finely tailored by changing the growth temperature and gas ratio. Graphene coated Al₂O₃ (G-Al₂O₃) composites are used as effective fillers of stearic acid (SA) to increase the thermal transport property. By the optimization of the layer number of graphene, size of Al₂O₃ particles and ratio of G-Al₂O₃/SA in a quantitative, their thermal conductivities significantly increase up to 11 folds from 0.15 to 1.65 W m⁻¹ K⁻¹. The great improvement is attributed to the high thermal transfer performance of graphene and excellent wettability between graphene and SA. When the G-Al₂O₃/SA composites with the graphene coated porous Al₂O₃ foam, the thermal conductivity further reaches to 2.39 W m⁻¹ K⁻¹, and the corresponding latent heat is 38 J g⁻¹. It demonstrates the potential applications of graphene in thermal transport and thermal energy storage devices.     

Preparation and characterization of porous MgAl2O4 spinel ceramic supports from bauxite and magnesite

Porous magnesium aluminate spinel (MgAl₂O₄) ceramic supports were fabricated by reactive sintering from low-cost bauxite and magnesite at different temperatures ranging from 1100 to 1400 °C and their sintering behavior and phase evolution were evaluated. The effects of sintering temperature on the pore structure, size and distribution as well as on the main properties of spinel ceramic supports such as flexural strength, nitrogen permeation flux and chemical resistance were investigated. The supports prepared at 1300 °C showed a homogeneous pore structure with the average pore size of 4.42 μm, and exhibited high flexural strength (35.6 MPa), high gas permeability (with nitrogen gas flux of 3057 m³ m⁻² h⁻¹ under a trans-membrane pressure of 0.1 MPa) and excellent chemical resistance.