Thermogelling polysaccharides for aqueous gelcasting—part II: influence of gelling additives on rheological properties and gelcasting of alumina

In the present work the influence of the addition of aqueous solutions of agar, agarose and carrageenan on the rheological properties of alumina slurries is studied. Precursor binder solutions were prepared to concentrations between 1.5 and 4 wt.% in water by heating at 92 °C. Alumina suspensions were prepared at a solid loading of 50 vol.% and heated to 60 °C. At this temperature different amounts of the starting precursor solutions of each gelling binder were added so that the final concentrations of binder were 0.25, 0.5 and 0.75 wt.% referred to dry solids. For concentrations of the precursor solution ⩽2 wt.%, the viscosity of the slurry tends to decrease when the total amount of gel increases, while for concentrations of the precursor solution ⩾3 wt.% viscosity increases with the amount of gel. This defines a yield concentration of precursor solution where the suspension properties change. Gelcasting performance is a function of slurry viscosity, gelling behaviour and time, and body deformation during drying. In general, carrageenan leads to higher shape distortion in the body. Better shape retention is achieved with precursor solutions with ⩾3 wt.% additive and total content of gel of ⩾0.5 wt.%.     

Physical,mechanical and microstructural characterization of TiC–ZrN ceramics

This study aims to investigate the impact of zirconium nitride (ZrN) additive on the microstructural features and physical-mechanical characteristics of TiC. For this objective, two different samples, namely monolithic TiC and TiC-5 wt% ZrN, were produced by spark plasma sintering method at 1900 °C for 10 min under 40 MPa. X-ray diffraction, field emission scanning electron microscopy, and thermodynamical evaluations confirmed the formation of a single solid solution of (Ti,Zr)(C,N), along with a carbon-rich secondary phase in the doped ceramic. The monolithic TiC provided a higher relative density (95.5%) than the ZrN-doped sample. The fractographical assessment revealed a change in the fracture mode of TiC from transgranular to intergranular with introducing the ZrN additive. Reinforcing TiC with ZrN resulted in a Vickers hardness of 2640 HV_0.1 kg, a flexural strength of 444 MPa, and a thermal conductivity of 14.9 W/mK. Furthermore, the TiC–ZrN sample presented a higher coefficient of friction (0.37 on average) compared to the monolithic TiC (0.34 on average).     

Proton-conducting ceramics as electrode/electrolyte materials for SOFC's—part I: preparation,mechanical and thermal properties of sintered bodies

The aim of these investigations was to prepare and to examine compounds of a high temperature solid oxide fuel cell with a proton conducting electrolyte in view of the mechanical and thermal properties. The powders were made by the conventional solid reaction of carbonates and oxides. The stoichiometry of the electrolyte Ba,Ca niobate (BCN) was varied with x=0, x=0.12 and x=0.18. As potential cathode material SrCeO₃ and SrZrO₃ stabilised with 5% Yb was prepared, and as anode material cermet of BCN and Ni with 50:50 wt.% was synthesised. The mechanical properties like bending strength (room and high temperature), Young modulus (E), modulus of rigidity (G), Poison's ratio, micro hardness and fracture toughness were measured on sintered samples. The highest values for bending strength, E and G could be found for BCN12 (156 MPa, 160 GPa, 63 GPa) and the cerate (175 MPa, 145 GPa, 56 GPa), the lowest for the cermet BCN/Ni (72 MPa, 68 GPa, 29 GPa). The investigation of the thermal properties of the bulk material showed a thermal stability to a temperature of 1400 °C. The thermal expansion coefficient measured at 1000 °C was found to be in the range of 10–12×10⁻⁶/K. Further investigations with respect to the mechanical and thermal properties have to be made for the whole system of cathode–electrolyte–anode.     

Plastic forming and microstructural development of α-alumina ceramics from highly compacted green bodies using extrusion

High density (>99% TD) and microstructurally controlled α-alumina ceramics were produced from seeded nano-size boehmite (γ-AlOOH) sols with a very fine crystallite size (2–3 nm). A totally wet processing technique comprising vacuum filtering and pressure filtration (PF) was applied in order to increase the solids-loading of the sol and hence form an extrudable paste suitable for plastic forming using extrusion. High packing densities (>68% TD in the green state) are achieved by PF starting from the slurry state resulting in the formation of a consolidated paste which is further consolidated by extrusion. This combined processing technique was successfully applied, in an attempt to reduce the γ-Al₂O₃ formation temperature, and hence lower the θ- to α-Al₂O₃ transition temperature. The microstructure of dense α-Al₂O₃ bodies derived from seeded boehmite sol contains very fine (250 nm) alumina grains after sintering at 1200 °C for 2 h. Although the DTA evidence points to a θ- to α-Al₂O₃ transition temperature of 1208 °C for a seeded (with 30 nm TiO₂) sample, X-ray analysis indicates that a seeded, pressure filtrated and extruded sample is transformed to α-Al₂O₃ phase after sintering at 1100 °C for 2 h.     

The main role of silica—Based cement free binders on the microstructural evolution and mechanical behaviour of high alumina castables

One of the major innovations for monolithic refractories in the past decade has been the development of cement-free binders, no-cement castables (NCCs). NCCs with colloidal silica suspensions as binders are extended at industrial scale. Microsilica-based powder binders are valuable alternative to colloidal silica to avoid installation and transport difficulties.The success of NCCs depends on the high temperature microstructure developed. Phase equilibrium diagrams are a powerful tool for the understanding of such development.This work deals with the study of alumina castables fabricated using both silica gel technologies with the aim of establishing the main factors that determine the high temperature (1300–1400?°C, 30–300?min) microstructure and how it affects the mechanical behaviour. Deformation and fracture are determined by the formation of mullite in the matrix. Reaction kinetics is highly dependent of temperature, time and the alumina - silica relative amount at local level, determined by the composition of the binder.     

Processing of Al2O3/SiC nanocomposites—part 2: green body formation and sintering

Pressure filtration was used to form green compacts from aqueous slurries of alumina with 5 vol.% silicon carbide. Green densities of 64%TD were achieved for slurries containing a 50 vol.% solids loading. Lower green densities were obtained for a very fine alumina due to the practical limits on maximum slurry solids loading when using finer powders. The samples were dried in a purposely built humidity cabinet to limit sample cracking. It was found that a higher consolidated layer permeability gave a higher initial drying rate. Near fully dense (99% TD) nanocomposites were produced, via pressureless sintering at 1900 °C. Poor sintered densities were obtained in the case of the fine alumina because of localised sintering of these low green density compacts. The required intra/inter-granular nanocomposite microstructures have been obtained for several different systems, with an average grain size of approximately 5 μm. Abnormal grain growth was noted for samples containing the larger particle size silicon carbide. This shows that a maximum particle size limit exists when selecting the powders for a 5 vol.% nanocomposite.     

Synthesis and mechanical and tribological characterization of alumina–yttria stabilized zirconia (YSZ) nanocomposites with YSZ synthesized by means of an aqueous solution–gel method or a hydrothermal route

In the present study, yttria stabilized zirconia (YSZ) nanoparticles, prepared by means of an aqueous solution–gel method or a hydrothermal route, are incorporated in a matrix of submicron alumina particles by wet mechanical milling. The microstructural characteristics and the mechanical and tribological properties of the obtained alumina–YSZ nanocomposites are evaluated as a function of different processing conditions like milling time, YSZ amount, sintering procedure and synthesis method of YSZ.     

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al₂O₃@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl₃ and the commercial α-Al₂O₃ nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al₂O₃ phases after the calcination process and the shell material was completely converted to γ-Al₂O₃ at 1000?°C. However, for the α-Al₂O₃@amorphous alumina core-shell nanoparticles were completely converted to α-Al₂O₃ at 1000?°C. It can be concluded that α-Al₂O₃ core particles, as the seed crystalline, help to transforming of γ-Al₂O₃ phase as the shell material directly without forming transitional phases to α-Al₂O₃. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al₂O₃@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ～99.8% and the in-line transmittance value is ～80% within the IR range.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: III. Cerium oxide

Three series of cerium-promoted nickel—alumina catalysts with different nickel-to-aluminium ratios each containing different amounts of cerium have been prepared and characterized. The calcination and reduction behaviour were found not to be altered by the presence of cerium. Part of the promoter was found to separate during the precipitation process as poorly crystalline CeO₂, the amount of which was largely determined by the drying temperature. This phase separation process was accompanied by a partial change in the valence state of the cerium. The effect of cerium on the nickel particle sizes was very small. Cerium enhances the activity of coprecipitated nickel—alumina catalysts in the carbon monoxide methanation reaction. This enhancement is accompanied by an increased apparent activation energy. Cerium- and lanthanum-promoted materials are compared with one another and it is concluded that although both promoters behave differently in determining the catalyst structure, their behaviour in the carbon monoxide methanation reaction is very similar and the specific activities of both types of material are nearly equal.     

Reactive SPS for sol–gel alumina samples: Structure,sintering behavior,and mechanical properties

This work presents a fast and direct controlled routine for the fabrication of fully dense alumina based on the reactive spark plasma sintering (reactive-SPS) of boehmite (γ-AlOOH) nano-powders obtained by the sol–gel technique. The evolution of the transition aluminas during sintering has been studied. Some boehmite powders were seeded with α-Al₂O₃ particles prior to the gelation. Boehmite seeded powders exhibited a direct transition to α-Al₂O₃ at 1070 °C, enhancing the transformation kinetics and lowering the required temperature by more than 100 °C. For comparison, other samples were prepared by previously annealing the seeded and unseeded boehmite powders. Thus, α-Al₂O₃ powders were obtained and were sintered by standard-SPS. A detailed structural and mechanical characterization is presented, comparing the hardness and indentation fracture resistance for different grain sizes and porosities. Both the reactive-SPSed samples and the standard-SPSed samples showed a high hardness (18–20 GPa), whereas the reactive-SPSed samples exhibited a lower indentation fracture resistance due to a large grain size (～10 μm). Improvements of this procedure for obtaining smaller grain size are discussed. In summary, the presented technique brings a revolutionary fast method for the fabrication of fully dense alumina, as this process reduces the time and temperature required for alumina densification.     

Comparison of different alumina powders for the aqueous processing and pressureless sintering of Al2O3–SiC nanocomposites

Four different alumina powders, from European and Japanese sources having similar particle size (350–700 nm) were used for the fabrication of nanocomposites. They were compared in terms of green properties, sintering behaviour, microstructure and mechanical properties. The processing route used (attrition milling and freeze-drying) leads to a reduction in green density of the processed aluminas and composites compared to the as-received alumina. All powders had similar green properties except one, which contained a binder from the manufacturer. The presence of this binder led to the formation of hard agglomerates. In this case the pressing did not eliminate, totally, the inter-agglomerate pores, leading to an incomplete sintering. Calcining the powder to remove the binder resulted in similar pressing and sintering behaviour to the other powders and densities >99% were achieved at 1750 °C by pressureless sintering. All the composites exhibited similar microstructures (matrix grain size ∼3 μm) and elastic properties, hardness and fracture toughness. A finer matrix microstructure could be obtained with one of the European powders which achieved ∼99% density at 1700 °C. The presence of 5 vol.% SiC resulted in a mean grain size of ∼2 μm for the alumina matrix compared with 13.9 μm for a monolithic alumina prepared under identical conditions.     

Thermodynamical evaluation,microstructural characterization and mechanical properties of B4C–TiB2 nanocomposite produced by in-situ reaction of Nano-TiO2

This work discusses the pressureless sintering of a boron carbide-titanium diboride (B₄C– TiB₂) nanocomposite via in-situ reaction of the boron carbide/titanium dioxide/carbon system. Attempting to sinter pure boron carbide leads to poor mechanical properties. In this work, the effect of adding TiO₂ to B₄C on mechanical properties of the boron carbide was investigated. Thermodynamic simulations were performed with HSC chemistry software to determine the phases which were most likely to form during the sintering process. The reaction thermodynamics suggested that during the sintering process, formation of TiB₂ occurs preferentially over formation of TiC. For examination of the microstructural evolution of the samples, Scanning Electron Microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized. The density, porosity, Young's modulus, microhardness and fracture toughness of the specimens were compared. Optimum properties were achieved by adding 10 wt% TiO₂. In the sample possessing 10 wt% TiO₂, the relative density, Young's modulus, hardness and fracture toughness were 94.26%, 428 GPa, 23.04 GPa and 5.19 MPa m^0.5, respectively, and the porosity was decreased to 5.73%. Furthermore, phase analysis via XRD confirmed that the final product was free of unreacted TiO₂ or carbon.     

Processing of Al2O3/SiC nanocomposites—part 1: aqueous colloidal processing

An aqueous sol preparation route was investigated for the production of Al₂O₃/SiC nanocomposites. Different alumina and silicon carbide powders were used as well as two polyelectrolyte dispersing agents. The alumina powders were dispersed first, via wet ball milling, with silicon carbide being added during a second milling stage. Dispersed sols were prepared with silicon carbide powders having an average particle size ranging from 0.5 μm down to 50 nm. The rheological characteristics of the sols were determined and used together with the results from TEM studies of the sols to ascertain the spatial arrangement of sol particles or ‘sol architecture’. It was found that whilst a polydispersion approach is suitable for the coarser SiC powders, this cannot be applied to very fine particles. Rather, the pH of the sol has to be lowered to encourage heterocoagulation.     

Synergistic promoting effect of ball milling and KF–alumina support as a green tool for solvent-free synthesis of 2-arylidene-benzothiazinones

A solvent-free procedure is developed for the reaction of benzothiazinones with benzaldehyde derivatives, where the solid KF–Al₂O₃ support and ball milling synergistically leads to a green and efficient synthesis of several 2-arylidene-benzothiazinones. Therefore, a cooperative effect of the solid support and ball milling leads to excellent yields of the target dienes, while the catalyst can be recycled for subsequent reactions without significant loss of its activity.     

Deposition and characterization of nanocrystalline and amorphous Ni–W coatings with embedded alumina nanoparticles

Nanocrystalline and amorphous Ni–W coatings containing Al₂O₃ nanoparticles were electrodeposited from three different ammoniacal citrate baths by direct current (DC) method. The effects of nanoparticles on compositional, structural and morphological features of Ni–W coatings were investigated. The effects of bath chemical composition and current density on codeposition behavior of nanoparticles were also studied. Guglielmi model for particle deposition was applied to identify the kinetics of particle deposition. The presence of nanoparticles may affect on coating grain size, tungsten content and the rate of metal deposition. In addition, nanoparticles can result in more compact coatings with fewer defects. The extent of these effects depends on bath chemical composition and may be influenced by the synergistic effect of Ni on deposition of W. It was also found that the kinetics of particle deposition and the effect of current density on codeposition behavior of nanoparticles are highly dependent on bath chemical composition.     

Synthesis and characterization of alumina supported sub-nanoporous SiO2–10 wt%TiO2 membrane for nitrogen separation

Membrane of SiO₂–10TiO₂ was produced by dip coating on mesoporous-titania-coated macroporous alumina support. The amorphous SiO₂–10TiO₂ top layer with thickness of 0.9 μm was obtained as a homogenous and defect free surface. Gas permeation tests show a decrease in permeation in order of CH₄ > N₂ for the support, and N₂ > CH₄ for the multilayer membrane. The membrane delivering promised single gas separation factor for N₂/CH₄ (1.75). It was also observed that the membrane separation factor in binary gas mixture was 12–15% of the single gas separation factor, while the permeation revealed a decrease in gas mixture.     

Development and Application of Spinel and High—alumina／Spinel Castables

The development and application of domestic spinel and high-alumian/spinel(including corun-dum/spinel and calcined-bauxite/spinel) castables were disucssed in this paper,It expounded the tech-nical characters and virtues of high grade and medi-um grade spinel and the importance of the develop-met and successful application of calcined-bauxite/spinel castalbes.Because of the technical and eco-nomical feasibility,the developement and application of spinel and high-alumina/spinel castables,in which domestic refractory workers play an important role ,have been flurishing.     

Sorption of dissolved organics from aqueous solution by polystyrene resins—I. Resin characterization and sorption equilibrium

The results of sorption equilibrium studies indicate that adsorption is only one of the three mechanisms involved in removing organics from aqueous solution by polystyrene resins. The other two mechanisms are associated with the resin's capability to incorporate organics into their polymer matrix while swelling. To describe the overall sorption behaviour, it is not sufficient to characterize the resin structure by estimating pore size distribution and internal surface area of the dry polymers. The amount of swelling in different organic solvents must also be evaluated as an important structural parameter of polystyrene resins. The role of the different mechanisms depends on both the resin structure and the physical and chemical properties of the solute. The solute's affinity for the polystyrene surface can be correlated with the solute's benzene-water partition coefficient. A heuristic model is presented which relates the sorption capacity to both the resin and solute properties.     

Microstructural and mechanical characterization of sol gel-derived Si–O–C glasses

The mechanical properties of three silicon oxycarbide glasses pyrolysed under inert (Ar) atmosphere were studied as a function of the pyrolysis temperature. The silicon oxycarbide glasses were prepared from various alkyl substituted alkoxysilanes such as HSi(OEt)₃ and HMeSi(OEt)₂ in different ratios by using the sol-gel method. The Si–O–C-glasses obtained were respectively: (i) silicon oxycarbide network with excess carbon, (ii) stoichometric SiC_xO_2(1−x) where x=0.30 and (iii) silicon oxycarbide matrix with an excess of Si. Si–C bonds introduced in the starting silica gel network can be partially retained in the final glass after pyrolysis under inert atmosphere. After pyrolysis at temperatures between 600–1500 °C, the presence of tetracoordinated C atoms in the silica network results in an improvement of mechanical properties and thermal stability compared with silica glass. By using elemental analysis, density, SEM, BET and XRD (combined with Rietveld-analysis), the glass characterization was performed. Flexural strength (MOR), elastic modulus (E) and Vickers hardness (H_V) were measured and will be discussed in terms of glass composition and microstructure.     

Optical and mechanical characterization of transparent α-alumina fabricated by spark plasma sintering

The aim of this work was the analysis of the experimental results of a transparent alumina (BMA15) ceramic which was fabricated by Spark Plasma Sintering (SPS) from nanopowder (BMA15, Baikowski Chimie, France), at different temperatures (1200°C, 1250°C, 1300°C). With the application of a maximum uniaxial pressure of 73 MPa during all the fabrication-cycle (more than 3 hours). We sought an optimal sintering temperature combining better optical and mechanical properties of our pellets. The sintered alumina (BMA15) has a crystalline and dense microstructure. The samples sintered at 1200°C exhibit the best optical properties, in particular: good real inline transmission (RIT) and an optical gap greater than those of the samples sintered at 1250°C and 1300°C. Due to their low density, the Young modulus of alumina sintered at 1200 °C, deduced by ultrasound, has a low value which is about 385 GPa. Similarly, its small grain size gives it a better Vickers hardness ~ 21 GPa. Therefore, the value of the coefficient of friction μ stabilizes around the mean value of 0.21.