Rheological properties of montmorillonitic clay suspensions: Effect of firing and interlayer cations

The effect of a prior firing of three montmorillonite clays, exhibiting different nature of interlayer cations, on the rheological behaviour of related aqueous suspensions (5 and 10 mass% of solid content) was examined. Calcinations were performed at 150 °C, 250 °C, 300 °C or 450 °C for 30 min. The rheological properties were characterized at 25 °C in the flow mode using the Herschel–Bulkley model.The alkaline interlayer cation (Na) tended to increase the yield stress of montmorillonite suspensions in comparison with earth-alkaline ones (Ca, Mg). As expected, increasing solid content led to increasing yield stress.For calcinations until 200 °C, the relevant suspensions exhibited an increasing yield stress due to a gel-like behaviour in relation with a card-house-like structure. Furthermore, calcination above 300 °C favoured the decrease of the corresponding yield stress. This behaviour seemed to be related to the modification of the surface properties of the clay platelets, more precisely to the beginning of clay dehydroxylation.     

Investigation of calcium zirconate ceramic synthesized by slip casting and calcination

In order to provide large scale bodies composed of synthesized calcium zirconate ceramic, the slip casting technology as well as calcination was applied. Optimized suspensions with high solid content and low viscosity were obtained by adding a dispersant agent and subsequently slip cast into plaster moulds to produce bars. Densification as well as material properties were studied at temperatures ranging from 1200 to 1500 °C. Due to their good properties, bars sintered at 1500 °C were selected for crushing to get coarse and fine grains for calcium zirconate rich refractory materials. To evaluate the applicability of the slip casting route, the mechanical properties of the obtained coarse grains were compared with commercial fused-cast product.     

Fabrication of patterned Ba0.71Sr0.29TiO3 thick film on Si substrate by tape casting method

Patterned barium strontium titanate (BST) thick films are fabricated in the grooved silicon substrate using tape casting method and sintered from 800 to 1250 °C for 2 h. The slurry used for the tape casting is from sol-precipitation method, and the crystallization of the as precipitated BST powders is improved by hydrothermal treatment at 200 °C for 5 h. The patterned BST thick films have a size of 800 × 300 μm and homogeneous thickness of 30 μm. After sintering below 1000 °C, the obtained BST thick films have a dielectric abnormality at about 30 °C and the dielectric loss is about 0.02.     

Far infrared and microstructural studies of mechanically activated nickel manganite

Nickel manganite powder synthesized by calcination of a stoichiometric mixture of manganese (MnO Aldrich 99.9%) and nickel oxide (NiO Merck, 99.5%) containing 0.5 wt% CoO and Fe₂O₃, was additionally mechanically activated in a high energy planetary ball mill for 5–60 min. The resulting powders were uniaxially pressed into disc shape pellets and then sintered for 60 min at 900 °C, 1050 °C and 1200 °C. Morphological changes of the obtained nickel manganite ceramics induced by mechanical activation were monitored using scanning electron microscopy, while changes in structural characteristics were followed using X-ray powder diffraction. Room temperature far infrared reflectivity spectra for all sintered samples were recorded in the frequency range between 50 cm^?1and 1200 cm^?1. The observed spectra for all samples showed the presence of the same oscillators, but their intensities depended on the sintering temperature and the time of mechanical acivation. Transversal and longitudinal optical modes were calculated for six ionic oscillators (four strong, and two shoulders) belonging to the nickel manganite partially inverse spinel structure.     

A novel route for manufacturing asymmetric BSCF-based perovskite structures by a combined tape and freeze casting method

In this work, we present an innovative route for the preparation of asymmetric structures by combining the tape and freeze casting methods A perovskite oxide ceramic based on BSCF (Ba_0.5S_0.5Co_0.8Fe_0.2O_3-δ) was chosen as the mother material. BSCF freeze cast substrates showed highly ordered and interconnected pores networks with open porosity from 66% to 79%. The top layer for the tape cast was altered by doping BSCF with Zr, thus forming a slightly different compound BSCFZ. The BSCFZ slurries containing 1.0 wt% dispersant and pH >6 were characterised by pseudoplastic behaviour, ideal for the preparation of tape cast layers. Coupling the BSCF freeze cast substrate with the BCSFZ tape cast green top layer to make asymmetric structures was achieved by the pre-sintering of the substrate between 900 and 1050 °C. This allowed for the densification and coupling of the BSCFZ top layer, resulting in suitable adhesion with no cracks or delamination. Higher pre-sintering temperatures than 1050 °C limited the densification of the BSCFZ top layer as the coefficient of expansion between the substrate and top layer did not match, leading to the formation of surfaces with high porosity.     

Characteristics of samaria-doped ceria nanoparticles prepared by spray pyrolysis

Samaria-doped ceria (SDC) nanoparticles were prepared by spray pyrolysis. The means sizes of the samaria-doped ceria nanoparticles were controlled from 21 to 150 nm by changing the calcination temperatures between 700 and 1200 °C. The pellets formed from the SDC particles calcined at temperatures between 700 and 1000 °C had similar grain sizes between 0.75 and 0.82 μm. However, pellet formed from the SDC particles calcined at a temperature of 1200 °C had large grain size of 1.22 μm. The pellet formed from the SDC particles calcined at a temperature of 1000 °C had slightly smaller resistance of grain-boundary than those of the pellets formed from the SDC particles calcined at temperatures between 700 and 900 °C. However, the pellet formed from the SDC particles calcined at a temperature of 1200 °C had low resistance of grain-boundary. The pellet formed from the SDC particles calcined at a temperature of 1200 °C had conductivity of 44.65 × 10^?3 S cm^?1 at a measuring temperature of 700 °C that more twice than those of the pellets formed from the SDC calcined below 1000 °C.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

New synthesis route for lead zirconate titanate powder

Phase pure lead zirconate titanate (PZT) powder was produced via a new aqueous coprecipitation method. A suite of characterization techniques, including FTIR, Raman, X-ray diffraction, SEM as well as nitrogen sorption were employed to investigate the structural evolution of the synthesized and calcined powder. The dried precipitate formed in aqueous phase yielded approximately 80 wt% final product after calcination. The PZT perovskite structure was obtained after calcination at 550 °C for 3 h. Milling of the calcined powder reduced the mean particle size from approximately 10 µm to 2 µm. With increasing calcination temperature from 550 °C to 700 °C, both surface area and pore volume decreased while pore size increased from 3.4 nm to 9.8 nm. The bulk density of pelletized samples increased from 4.83 to 7.57 g/cm³ with increasing sintering temperature from 800 °C to 1200 °C. Powder processing using this aqueous route is simple and reproducible leading to a method that is readily scalable for industrial applications.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Effect of pre-oxidation on the microstructure,mechanical and dielectric properties of highly porous silicon nitride ceramics

Highly porous Si₃N₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si₃N₄ ceramics were investigated. As the pre-oxidation temperature increased from 1200 °C to 1400 °C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 °C, and then decreased to 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si₃N₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.     

Adsorption of methylene blue and zinc ions on raw and acid-activated bentonite from Morocco

Adsorption of methylene blue (BM) and zinc ions on raw and acid-activated Moroccan bentonite composed of montmorillonite (88 mass%), a mixture of quartz and K-feldspar (9 mass%), calcite (3 mass%) and insignificant amounts of organic matter was evaluated at 25 °C using UV–visible and atomic absorption spectrometry. The adsorption capacity of MB and Zn ions by raw bentonite were about 2.2 and 1.1 mmol/(g of bentonite) and the best-fit isotherm models were those of Harkins–Jura and Langmuir. Acid-activation of the bentonite reduced the maximum uptake of MB and Zn ions by 30 and 95% and the best-fit models of the isotherms were Freundlich and Dubinin–Radushkevich for MB and Zn ions respectively. The reduced adsorption was associated with partial collapse of the montmorillonite particles and the formation of amorphous silica.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Crystallization behavior and sintering of cordierite synthesized by an aqueous sol–gel route

The purpose of the research was to investigate crystallization behavior and sintering of cordierite synthesized by a low-price aqueous sol–gel route starting from silicic acid and magnesium and aluminum salts. Viscous sintering of the gel occurred in the temperature range of 800–850 °C, followed by μ-cordierite crystallization at about 900 °C, which proves the homogeneity of the gel. Decreasing of μ-cordierite crystallinity in a wide temperature range prior to commencing of α-cordierite crystallization at about 1200 °C indicates reconstructive type of μ- → α-cordierite transformation. The transformation was fully completed at 1350 °C. The value of the Avrami parameter indicates that μ-cordierite crystallization was controlled by surface or interface nucleation, which implies that viscous sintering occurred in the primary gel particles, which leads to shrinkage, and thereafter nucleation occurred on the surface or interface of the particles. The overall activation energy of μ-cordierite crystallization was 382.0 kJ/mol. The sinterability of the powder obtained by calcination at 1300 °C, where well-crystallized α-cordierite was formed, was better than that of the powder obtained by calcination at 850 °C, where the most intensive shrinkage occurred before the onset of crystallization of μ-cordierite.     

Preparation of cerium titanate brannerite by solution combustion,and phase transformation during heat treatment

An aqueous solution route was employed to prepare cerium titanate oxide brannerite. Thermal analysis, X-ray diffraction, Raman spectroscopy, transmission and scanning electron spectroscopy, were used to investigate the brannerite structure formation and bulk properties. Mixed metal oxides (TiO₂, CeO₂ and brannerite CeTi₂O₆) were formed upon calcination at 800 °C for 12 h. The amount of brannerite phase decreased to form the constituent oxides with increasing calcination temperature and only pure TiO₂ and CeO₂ were present after 1200 °C calcination. The brannerite CeTi₂O₆ phase reformed at 1300 °C, and its relative amount was increased with dwell time. After 48 h calcination at 1300 °C, brannerite with only minor metal oxide impurities (<1%) was observed. The sample melted at 1400 °C which led to the collapse of brannerite back to its constituent oxides. Further, the phase formation was influenced by pelletization of the powders, which may be explained by a molar volume increase during brannerite formation. Attempts to confirm this hypothesis using Pu brannerite were inconclusive.     

Preparation and characterisation of mixed matrix Al2O3- and TiO2-based ceramic membranes prepared using polymeric synthesis route

Al₂O₃- and TiO₂-based ceramic membranes prepared using polymeric synthesis route were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and gas permeability tests. The influence of the final calcination temperature and the systematic investigation of the properties of the membranes are provided. The calcination temperature affected morphological, structural and chemical properties, as well as the gas permeability of the ceramic membranes. XRD analysis revealed rhombohedral and tetragonal structures of Al₂O₃ and TiO₂-based ceramic, respectively, prepared at calcination temperatures of 1100 and 1200 °C. The TiO₂-based ceramic matrix calcined at temperatures of 1100 and 1200 °C exhibited a well-defined crystalline microstructure with the grains increasing in size as a function of temperature. FTIR analysis revealed that phosphorus additives in orthoclase clay tend to form phosphonate groups during the calcination process. The decomposition of organic source was not fulfilled as tested at calcination temperatures of 1000, 1100 and 1200 °C.     

Aqueous tape casting of reaction bonded aluminium oxide (RBAO)

Reaction bonding of alumina (RBAO) has been proved to be a successful route to make high-performance alumina. However, the aluminium metal in the starting mixtures prevents using aqueous processing techniques in the initial phase. Here a route is presented for aqueous tape casting of RBAO by hydrophobization of the starting powder and preparing a suitable tape cast suspension. Thermal analysis (TGA/DSC) showed that it is possible to combust the organic phase prior to oxidation of the aluminium, opening routes to laminated ceramic-metallic composites. TGA/DSC combined with imaging (SEM) and phase analysis (XRD) also showed that the oxidation of the aluminium in RBAO occurs in two steps. Sintering experiments showed anisotropic grain growth at 1550 °C, resulting in a lower density than sintering at 1500 °C or 1600 °C. The bending strength of the sintered tape increased with temperature, resulting in a bending strength of 290 MPa after sintering at 1600 °C.     

Al2O3-3YTZP-Graphene multilayers produced by tape casting and spark plasma sintering

This work aims to establish a colloidal route to obtain laminates of alumina–zirconia combining layers with and without graphene. Green tapes of alumina, alumina with 5 vol.% of 3Y-TZP and alumina with 5 vol.% of 3Y-TZP and graphene-oxide (2 vol.%) were obtained by aqueous tape casting. It is possible to design materials for different structural applications with a controlled microstructure with a high number of different layers. The tapes were punched into 20-mm discs, joined to form laminates alternating up to 18-layers, and sintered in one-step by spark plasma sintering (SPS) at 1400 °C. It has demonstrated that there is a significant graphite diffusion provoked by the required graphite holders into the SPS-furnace. Dense laminates with layer thicknesses ∼100 μm and good cohesion between layers were obtained. Nanoindentation results showed that hardness and elastic modulus values were higher than 27 GPa and 300 GPa, respectively, and similar for all layers.     

Crystalline phases and physical properties of modified stoneware body with glaze sludge

Ceramic stoneware body has been modified with ceramic and glass industrial wastes by replacing 25–100% as flux in the formula. The effects of solid wastes added to the bodies were studied after firing in the temperature range 950–1280 °C. The physical properties of linear shrinkage, bulk density, apparent porosity, water absorption and 3-point bending strength were determined. A composition which related to the general stoneware properties was found when using soda-lime cullet and glaze sludge. It had a firing range lowered to 1050–1100 °C. SEM results demonstrated the sintered microstructure increased in density with increase in solid waste in the modified body. XRD results after firing showed the crystalline phases comprised of mullite, albite calcian and quartz. Thermal expansion was measured in the range 6.53–6.67 × 10^?6 K^?1 at 30–500 °C. The modified bodies were capable of forming prototype products by slip casting and jiggering.     

Microstructure and mechanical properties of ceramics obtained from chemically co-precipitated Al2O3-GdAlO3 nano-powders with eutectic composition

An efficient and flexible chemical co-precipitation method has been used to synthesize nanoscale Al₂O₃-GdAlO₃ powders with eutectic composition. The as-synthesized powders exhibit a highly dispersive and homogeneous distribution with an average particle size of 50 nm. The phase transition in the resulting powders strongly depends upon the calcination temperature. GdAlO₃ undergoes complete crystallization after calcination at 1050 °C, however, the diffraction peaks of α-Al₂O₃ are found at a relatively high calcination temperature of at least 1300 °C. The fully-densified Al₂O₃-GdAlO₃ ceramic with eutectic composition obtained by hot pressing the nanoscale powders at 1500 °C exhibits a room temperature flexural strength of 556 MPa, a Vickers hardness of 17.3 GPa and a fracture toughness of 7.5 MPa m^1/2. The high temperature flexural strength of the as-sintered Al₂O₃-GdAlO₃ ceramic is measured to be 515 MPa after bending tests at 1000 °C.     

The phase transformation and microstructure of TiAl/Ti2AlC composites caused by hot pressing

Two series of raw materials were adopted to form TiAl/Ti₂AlC composites: Ti/Al/TiC and Ti/Al/C. Differential thermal analysis (DTA) of starting powers and X-ray diffraction (XRD) of samples sintered at different temperatures from 600 °C to 1300 °C by hot pressing were utilized to analyze the phase transformation and the mechanism of synthesis. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of the products. The experimental results showed that Ti reacted with Al to form TiAl intermetallics below 900 °C; and above 900 °C, TiAl reacted with TiC to produce dense TiAl/Ti₂AlC composites. The products sintered at 1200 °C had fine crystals and dense fibres, and the distribution of Ti₂AlC particles in TiAl matrix was homogeneous.