Showing papers in "Journal of The European Ceramic Society in 2004"

Ceramic materials for thermal barrier coatings

Abstract: This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia, other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

Influence of mineralogy and firing temperature on the porosity of bricks

Abstract: The changes in brick porosity upon firing (700 up to 1100 � C) and its relation to the mineralogical composition are examined. Two types of raw clay with a composition representative of that used in brick-making industry were selected to manufacture the bricks: one contains notable amounts of carbonates, with a grain size of under 1 mm, and the other is predominantly quartzitic and lacking in carbonates. We demonstrate that the presence or absence of carbonates strongly influences the porosity development and, therefore, the brick texture and physical-mechanical properties. The carbonates in the raw clay promote the formation of fissures and of pores under 1 mm in size when the bricks are fired between 800 and 1000 � C. The absence of carbonates results in a continuous reduction in porosity and a significant increase in the pore fraction with a radius (r) > 1 mm as the firing temperature rises and smaller pores coalesce. Porosity and pore size distribution results obtained from the combined use of hydric tests (HT), mercury intrusion porosimetry (MIP) and digital image analysis (DIA) of scanning electron microscopy photomicrographs are compared. A clear correlation between the water absorption and drying behaviour of the bricks and the porosity plus pore size distribution is observed. DIA discloses the evolution of size, shape and connectivity of macropores (r > 1 mm) and evidences that MIP results underestimate the macropore content. Conversely, MIP gives a good estimate of the open porosity and of the distribution of pores with r <1 mm. It is concluded that the combined use of these complementary techniques helps to fully characterise the pore system of bricks. These results as well as the study of the evolution of the speed of ultrasound waves vs. time yield useful information to evaluate the bricks physical–mechanical behaviour and durability. The relevance of these findings in the conservation of historic buildings is discussed. # 2003 Elsevier Ltd. All rights reserved.

Phase transformation and growth of mullite in kaolin ceramics

Abstract: The phase transformation and growth of mullite (3Al 2 O 3 ·2SiO 2 ) in kaolin ceramics have been investigated using X-ray diffractometer, transmission electron microscope, select area electron diffractometer, energy dispersion spectrometer and differential thermal analysis. The mullite which was transformed from kaolin appears at 1050 °C by XRD and tallies with DTA. The initial mullite crystal showed a plate-like morphology. The Al 2 O 3 content in mullite crystal increased from 49.57 to 71.37 wt.% but the lattice parameters of a , b and c axes decreased from 8.085, 8.106 and 3.215 A to 7.882, 7.974 and 2.946 A, respectively, with the grain width increasing from 20 to 70 nm when the kaolin was sintered at 1300 °C for 30 min. The nonisothermal activation energy of mullite crystallization in kaolin ceramics was 1182.3 kJ mol −1 . The growth morphology parameters n and m were both about 2.0, indicating that the bulk nucleation was dominant in mullitization and the crystal growth was controlled by diffusion. Seemingly, this study has been attempted to provide an integrative presentation of the thermal–structural characterization together with detailed kinetic and mechanistic interpretations.

Novel high capacitance materials:- BaTiO3:La and CaCu3Ti4O12

Abstract: A review is given of the origins of high permittivity in two groups of materials, La-doped BaTiO3 and a new barrier layer capacitor material, CaCu3Ti4O12. Factors that influence permittivity include: dopant, doping mechanism, processing conditions and grain size. La-doped BaTiO3 has high permittivity due to its ferroelectric nature at low temperatures and a novel doping mechanism: A-site substitution linked to the creation of B-site vacancies for charge compensation. Permittivities of 25,000 have been achieved, which can be increased further to ∼36,000 by additional doping with Zr. The value of impedance spectroscopy to characterize materials that have heterogeneous electrical microstructures is illustrated with the example of CaCu3Ti4O12; the high permittivity is not a bulk effect, as widely stated in the literature, but is a thin layer effect typical of a barrier layer capacitor. By attention to processing conditions to achieve large grain sizes, effective permittivities as high as 300,000 have been obtained.

Band gap energy of pure and Al-doped ZnO thin films

Abstract: Pulsed laser deposition (PLD) technique is used to deposit pure and Al-doped ZnO thin films at different temperatures on glass substrates. From the transmission data from optical spectroscopy the band gap energy Eg of the films is derived. The dependences of Eg on the deposition temperatures of the pure and Al-doped ZnO films are different. The band gap energy of the pure ZnO increases and saturates with temperature. However, Eg of Al-doped ZnO shows an exponential decrease. Refractive indices of 1.9–2.1 in the VIS are determined by the spectroscopic ellipsometry (SE). Photoluminescence (PL) data reveal the strong near band emission by increasing the deposition temperature. # 2003 Elsevier Ltd. All rights reserved.

Amorphous Si(Al)OC ceramic from polysiloxanes: bulk ceramic processing, crystallization behavior and applications

Abstract: Here we report on bulk Si–Al–O–C ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors. Prior to the pyrolysis the precursors were cross-linked with a catalyst, or modified by the sol-gel-technique with an Al-containing alkoxide compound, namely alumatrane. This particular procedure yields amorphous ceramics with various compositions (Si1.00O1.60C0.80, Si1.00Al0.04O1.70C0.48, Si1.00Al0.07O1.80C0.49, and Si1.00Al0.11O1.90C0.49) after thermal decomposition at 1100 °C in Ar depending on the amount of Al-alkoxide used in the polymer reaction synthesis. The as-produced ceramics are amorphous and remain so up to 1300 °C. Phase separation accompanied by densification (1300–1500 °C) and formation of mullite at T > 1600 °C are the stages during heat-treatment. Bulk SiAlOC ceramics are characterized in terms of microstructure and crystallization in the temperature regime ranging from 1100 to 1700 °C. Aluminum-free SiOC forms SiC along with cracking of the bulk compacts. In contrast, the presence of Al in the SiOC matrix forms SiC and mullite and prevents micro cracking at elevated temperatures due to transient viscous sintering. The nano-crystals formed are embedded in an amorphous Si(Al)OC matrix in both cases. Potential application of polysiloxane derived SiOC ceramic in the field of ceramic micro electro mechanical systems (MEMS) is reported.

Synthesis of nanocrystalline yttria powder and fabrication of transparent YAG ceramics

Abstract: Synthesis of nanocrystalline yttria powder from Y(NO 3 ) 3 solution and ammonia water was investigated. It was found that the precursor precipitate is Y 2 (OH) 5 (NO 3 ).H 2 O. The addition of small amount of ammonia sulfate in yttrium nitrate solution can reduce the agglomeration and particle size of the produced yttria powders. Nanocrystalline yttria powder (60 nm in average size) was obtained by calcining the precursor at 1100 °C for 4 h. Using this yttria powder and a commercial ultrafine Al 2 O 3 powder, fully transparent YAG ceramics was fabricated by vacuum sintering at 1700 °C for 4 h through a solid-state reaction method. It was found that the addition of 0.5 wt.% tetraethyl orthosilicate (TEOS) is suitable for the fabrication of transparent YAG ceramics. If the amount of TEOS is less than 0.05 wt.%, abnormal grain growth occurs, and pores are entrapped in the grains. If the amount of TEOS is more than 3 wt.%, large amount of liquid phase is yielded, leaving some residual inclusions at grain boundaries. These are detrimental for the transparency of YAG ceramics.

Sodium-bismuth titanate based lead-free ferroelectric materials

Abstract: In search of lead-free ferroelectric ceramics with improved properties, an investigation was carried out on barium and strontium-modified Na0.5Bi0.5TiO3 (NBT) ceramics. Many compositions among the Ba and Sr-modified ceramic materials exhibit diffuse phase transition and are characterised by a strong temperature and frequency dispersion of the permittivity which would be connected with the cation disorder in the A site of the perovskite unit cell.

Permeability of ceramic foams to compressible and incompressible flow

Abstract: Ceramic foams constitute a new structural material characterized by a high porosity and high superficial area, formed by megapores interconnected by filaments. This results in a structure with low resistance to fluid flow, and to their use as filter. This work investigates the influence of several structural parameters, such as porosity, tortuosity, surface area and pore diameter, in predicting the permeability of ceramic foams. Foams with different pore densities were used as porous media. The permeability was measured utilizing air and water as flowing fluids, the former taken as an ideal gas and the latter as incompressible. No existing correlation was able to predict the permeability in the whole range studied. An Ergun-type correlation was fitted to the data, and represented very well the permeability of the media for all the foams, flowing fluids and operational range studied. Pore diameter was the structural parameter that best represented the media.

The ball on three balls test for strength testing of brittle discs: Part II: analysis of possible errors in the strength determination

Abstract: Biaxial strength testing of brittle materials using the ball on three balls test (B3B test) is a useful tool for direct strength testing of disc specimens offering some advantages compared with the testing of bended beams: there are less problems with the alignment of the specimens in the fixture, in many cases the production of the specimens is easier, the area of maximum tensile stress amplitudes is far from the edges of the specimen (were often machining defects exist) and the testing procedure can easier be miniaturised. In a preceding paper, a proposal for a suitable testing set up was made, the stress fields in the disc specimens were analysed and first testing results were reported. A simple solution for the maximum tensile stress amplitude in the discs was given for a range of parameters concerning the geometry of the specimen, the supporting situation during the test and the material properties. In this paper, possible sources for deviations of this stress from the reported idealised solution (which would result in measuring errors) are discussed. For example, the not accounted influence of friction forces between the disc and the support balls, the influence of a possible buckling of the disc and the influence of geometric inaccuracies are analysed. It is shown, that the thickness of the disc is the most sensitive parameter determining the maximum tensile stress. Therefore it has to be determined with high accuracy.

Structural, elastic, thermophysical and dielectric properties of zinc aluminate (ZnAl2O4)

Abstract: Dense zinc aluminate (gahnite) ceramics have been prepared at different sinter temperatures ranging from 1200 to 1600 °C from zinc aluminate powder prepared via the solid-state synthesis. A maximum achieved relative density of 93% was achieved. Several bulk properties like Young's modulus, heat capacity, thermal diffusivity and conductivity have been determined and estimations of the bulk properties at 100% density are made. Furthermore, in spinel-type materials like zinc aluminate the process of cation inversion occurs, which is in general not taken into account in computer simulations for the prediction of bulk properties. In order to determine whether the amount of cation inversion can be influenced by the preparation method resulting in different bulk properties, zinc aluminate powders were synthesised using solid-state synthesis, co-precipitation and a sol-gel method at different temperatures. The resulting powders were zinc deficient due to the volatile nature of zinc at the calcining temperatures. The cation inversion of these powders was investigated using solid-state MAS 27A1 NMR indicating that the cation inversion is very small for pure zinc aluminate irrespective of the preparation method.

Modelling of thermal conductivity of porous materials: application to thick thermal barrier coatings

Abstract: Modelling of thermal conductivity of two and three phase composite materials is used to determine the thermal conductivity of thick porous zirconia based thermal barrier coatings for use in high temperature applications. These coatings, depending on the deposition technique and process parameters exhibit different degrees of porosity. The porosity of the coating has an affect on thermal properties in completely different ways depending on the morphology and the orientation of the pores dispersed within a continuous matrix. In this work air plasma sprayed coatings have been considered. The experimental results were successfully compared to the modelled thermal conductivities. In the model the effects of porosity were taken into account considering the shape, orientation and volumetric percentage of pores. Image analysis and mercury porosimetry was used in experimental porosity determination.

On the description of indentation size effect in hardness testing for ceramics: Analysis of the nanoindentation data

Abstract: The nanoindentation hardnesses of a commercially available soda-lime glass, a tetragonal ZrO 2 polycrystalline and a hot-pressed Si 3 N 4 were measured in the peak load range from 7.5 to 500 mN. The experimental results revealed that, for each material, the measured hardness exhibits a peak-load- dependence, i.e., indentation size effect (ISE). Such a peak-load-dependence was then analyzed using the Meyer’ law, the Hay–Kendall approach, the proportional specimen resistance (PSR) model, the elastic recovery model and the modified PSR model. The analyses revealed that: (1) Meyer's law provides a satisfactory description for the experimental data for each material but cannot provide any knowledge of the origin of the observed ISE; (2) the Hays–Kendall approach, the elastic recovery model and the PSR model yield meaningless values of the parameters included in the corresponding equations, invalidating the applicability of these models in analyzing the ISE in the nanoindentation region; (3) the modified PSR model is sufficient for describing the observed ISE but the physical meaning of this model seems to be more complex than those proposed originally. For each material, the true hardness was also determined based on the PSR model, the elastic recovery model and the modified PSR model, respectively. It was found that the true hardness values deduced based on different models are similar with each other and this similarity was attributed to the similarity between the empirical equations adopted in these models.

The polarization of mixed conducting SOFC cathodes: Effects of surface reaction coefficient, ionic conductivity and geometry

Abstract: Multi-dimensional finite element simulations of current distributions in mixed ionic and electronic conducting cathodes (MIEC) are presented for the case that the cathodic oxygen incorporation into an electrolyte takes place through the bulk of the electrode. The effects of the ionic conductivity and the surface reaction coefficient on the overall process are analyzed. Depending on these material parameters different parts of the cathode are involved in the oxide ion transport to the electrolyte (from a very small region close to the three phase boundary for a fast surface reaction up to the entire cathode for a very slow surface reaction). The calculations also reveal which combinations of ionic conductivity and surface reaction coefficient are appropriate to achieve acceptable polarization resistances. The influence of the particle size is discussed and interpolation formulae are given to estimate the cathodic polarization in porous MIECs.

Synthesis and in vitro bioactivity of dicalcium silicate powders

Abstract: In the present investigation, β-dicalcium silicate (β-Ca 2 SiO 4 ) and γ-dicalcium silicate (γ-Ca 2 SiO 4 ) powders were synthesized by sol-gel process and hydrothermal synthesis, respectively. The in vitro bioactivity of both β-, and γ-Ca 2 SiO 4 was investigated by soaking the powders in simulated body fluid (SBF) for various time periods to analyze the nucleation and growth of hydroxyapatite (HAp) on the surface of the powders. After soaked in SBF for 6 h, HAp appeared on the surface of β-, and γ-Ca 2 SiO 4 particles, and uniform lathlike aggregates with typical morphology of HAp crystals formed after 5 days. The β-Ca 2 SiO 4 showed strong hydration when soaked in SBF and the hydration was favorable for formation of carbonate-containing hydroxyapatite on the surface of the materials. The γ-Ca 2 SiO 4 was not hydrated in SBF solution and showed a slower formation of carbonate-containing hydroxyapatite on the surface when compared with β-Ca 2 SiO 4 . In addition, the Si concentration in the SBF increased quickly up to 5 days and remained invariable with increased soaking time. The results obtained indicate that hydroxyapatite nuclei can form and grow on the β-, and γ-Ca 2 SiO 4 particles, and these two dicalcium silicates are potential candidates as biomaterials for hard tissue repair.

Use of soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix

Abstract: The study was directed towards determining the feasibility of using soda-lime scrap-glass as a fluxing agent in a porcelain stoneware tile mix. Both laboratory and industrial scale tests were carried out. Starting from a reference mix composition, different amounts (5–20 wt.%) of sodium feldspar were replaced with the same amounts of soda-lime scrap-glass. The soda-lime scrap-glass was added in the form of powder, prepared by wet grinding in a centrifugal ball mill. The rheological study of the resulting slips showed that increasing the amount of glass in the mix composition increased the viscosity but decreased the yield stress. Both the laboratory experiments and results of industrial trials showed that the only mix in which the soda-lime glass acted as a good fluxing agent, i.e. lowered the firing temperature, was the mix in which the scrap-glass replaced 10 wt.% of the sodium feldspar. This mix also showed better mechanical characteristics, attributed to enhanced microstructural homogeneity.

Preparation of interpenetrating ceramic–metal composites

Abstract: Ceramic reinforced metals are attractive because of their enhanced elastic modulus, high strength, tribological properties and low thermal expansion. Most work in this sector has focused on particle- or fiber-reinforced composites where the ceramic phase is not continuous. This work presents aluminium–alumina composites where both phases are interpenetrating throughout the microstructure. Ceramic preforms were produced with sacrificial pore forming agents leading to porosities between 50% and 67%. Pore wall microstructure was varied by changing the sintering temperature. Permeability and strength was measured for the porous preforms and infiltration results were compared with theoretical predictions based on capillary law and Darcian flow. A direct squeeze-casting process was used to infiltrate the preforms with aluminium resulting in an interpenetrating microstructure on both macropore and micropore scale.

Effects of dopants and hydrogen on the electrical conductivity of ZnO

Abstract: The effects of dopants on the electrical conductivity of ZnO were investigated through the ac impedance spectroscopy. The doping of Al increased the electrical conductivity of ZnO greatly, whereas the doping of Li decreased it both in the grain and in the grain boundary. The doping of the 3d transition metals (Co, Mn, and Cu) made the grain boundary more resistive, but the doping effect on the electrical conductivity inside the grain was varied depending on the doping elements. The doping of Co had no significant effects on the electrical conductivity of the grain, and the doping of Mn made the grain a little more resistive. The doping of Cu made the grain much more resistive. In addition, hydrogen was introduced into ZnO by the ion implantation method. The electrical conductivity in the hydrogen-implanted ZnO layer increased by four orders of magnitude. The mechanisms for the doping effects were discussed in this investigation.

Preparation of porous SiC ceramic with a woodlike microstructure by sol-gel and carbothermal reduction processing

Abstract: Highly porous silicon carbide ceramic with a woodlike microstructure has been prepared by carbothermal reduction reaction of charcoal/silica composites in static argon atmosphere, which were fabricated by infiltrating silica sol into a porous biocarbon template from tilia amurensis wood using a vacuum/pressure infiltration process. The morphology of resulting porous SiC ceramic, as well as the conversion mechanism of wood to porous SiC ceramic, have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. Experimental results show that the biomorphic cellular morphology of tilia amurensis wood charcoal tissue is remained in the porous SiC ceramic with high precision which consists of β-SiC with traces of α-SiC. The strut thickness of resulting SiC ceramic becomes thinner, even partially disappears with the increase of silica amount contained in the charcoal/silica composites. The morphology of the SiC ceramic reveals that solid–solid, gas–solid and gas–gas reactions occurred during the charcoal-to-ceramic conversion.

Printing parameters and ink components affecting ultra-fine-line gravure-offset printing for electronics applications

Abstract: The gravure offset printing method was examined for the manufacture of thick-film conductors on alumina, using Ag based inks. In this research, a roller type gravure offset manufacturing process was developed to give conductor lines and spaces down to 20 μm, together with a low square resistance. Novel hydrocarbon resin based inks have been used and compared to traditional ethyl cellulose based inks normally used in thick-film technology. The inks had a high viscosity compared to graphical inks due to their high solid content, which was necessary to enable a high printed mass. Different printing parameters were studied and printed sample properties were statistically compared. The results show that the cross sectional area of the lines is the best correlating measured property to describe the quality of the printing. 100% of the ink was transferred from the blanket to the substrate after 30 seconds. Prints from ink containing 85 wt.% of silver with optimised parameters resulted in a square resistance of 5 mΩ/sq. for 300 μm wide and 17 μm thick lines produced from a single print. These promising results are important requirements for high throughput electronics manufacture.

Incorporation of granite cutting sludge in industrial porcelain tile formulations

Abstract: Granite wastes in the form of sludge, obtained from granite cutting industry, were incorporated in the batch formulations of porcelain tiles. The maximum possible substitution of sludge for feldspar was investigated. Samples of different formulations, in the form of pellets or extruded bars, were produced at both laboratory and pilot-plant scales and characterized throughout all the stages of the production process. The experimental results and their theoretical interpretation show that suitable incorporation of granite sludge can result in porcelain tiles with superior properties, in terms of water absorption (0.07%) and bending strength (>50 MPa). Sludge incorporation had negligible effect on density, shrinkage and plasticity during all stages of tile-production process, anticipating no modifications in the industrial production line.

A novel process for the synthesis of cordierite (Mg2Al4Si5O18) powders from rice husk ash and other sources of silica and their comparative study

Abstract: Cordierite (Mg2Al4Si5O18) powders were synthesised by utilising agro-based ‘waste’ material-rice husk ash and two other sources of silica, i.e. tetraethylorthosilicate (TEOS) and fumed silica, for a comparative study following sol–gel technique. The gel and calcined powders obtained from different silica sources were characterised by thermogravimetry analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD) study, Fourier transformed infrared (FTIR) spectroscopy, 27 Al and 29 Si solid state MAS NMR spectroscopy, scanning electron microscopy (SEM) and particle size analysis. For rice husk ash as the source material, DTA and XRD results confirmed that α-cordierite was formed at 1365 °C through the intermediate phases of cristobalite and MgAl2O4 spinel while in case of TEOS and fumed silica sources, α-cordierite formed at 1320 and 1360 °C respectively through the intermediate phases of μ-cordierite and magnesium aluminate (MgAl2O4) spinel for the former and via the MgAl2O4 spinel for the latter. FTIR studies showed the vibration bands of SiO4 tetrahedra, AlO6 octahedra and AlO4 tetrahedra for different sources of silica calcined at different temperatures. The 27 Al solid state NMR confirmed the highly abundance of AlO6 octahedra while 29 Si solid state NMR revealed the Q4 state for SiO4 tetrahedra for the sample obtained from rice husk ash after calcination at 800 °C. The SEM showed the irregular morphology of the calcined powders (1400 °C) for all the samples and comparatively wide size distribution of the particles was found for the sample obtained from rice husk ash.

Densification and grain growth in pulse electric current sintering of alumina

Abstract: Pulse electric current sintering (PECS) method was used to sinter an ultrafine and high purity α-Al2O3 powder. The powder compact was either slowly heated (50 °C/min) or rapidly heated (300 °C/min) to temperatures ranging between 1000 and 1400 °C and then rapidly cooled down without holding. The densification and grain growth behaviors at different stages of the PECS process were investigated. At the earlier stages, fast heating greatly promoted the formation of necking between particles and enhanced both densification and grain growth; at the late stage, faster heating resulted in smaller grain sizes.

Effect of SrTiO3 concentration and sintering temperature on microstructure and dielectric constant of Ba1−xSrxTiO3

Abstract: The Ba 1− x Sr x TiO 3 materials have received increased attention as one of the most important materials for electroceramic components, such as high dielectric ceramic capacitors, tunable phase shifters and PTCR. In this paper, the effect of SrTiO 3 concentration and sintering temperature on the microstructure and dielectric constant of Ba 1− x Sr x TiO 3 materials at the Curie temperature have been investigated. When Ba 1− x Sr x TiO 3 materials were sintered at 1350 °C, the peak value of the dielectric constant, ϵ max , monotonically decreased with increasing SrTiO 3 concentration. At the sintering temperature of 1400 °C the dielectric constant maximum at the T C increased with an increase in the x value, reaching the highest value at around x =0.4 and then decreased. As sintering temperature increased to 1450 °C, ϵ max increased with increasing SrTiO 3 concentration up to x =0.6. The dielectric properties of Ba 1− x Sr x TiO 3 materials were discussed in terms of SrTiO 3 concentration and microstructure.

Combustion synthesis of lanthanum chromite starting from water solutions: Investigation of process mechanism by DTA–TGA–MS

Abstract: The mechanism of combustion synthesis of lanthanum chromite was investigated by carrying out simultaneous differential thermal analysis (DTA), thermal-gravimetric analysis (TGA) and quadrupole mass spectrometry measurements (MS). The whole process was found to involve several phenomena: urea and nitrates thermal decomposition, exothermal reactions occurring directly between nitrates and urea as well as between their decomposition products, final reaction between solid oxides. In order to better understand the complex calorimetric, thermal-gravimetric and mass spectrometric curves, the behaviour on heating of each single reagent as well as that of binary mixtures, made by combining urea with each single nitrate, were also studied. Combustion synthesis was performed either under an oxidizing atmosphere or under an inert one. Chemical reactions possibly occurring during this combustion synthesis have been proposed on the basis of MS analysis of gaseous reaction products. The effect of using an excess of sacrificial fuel jointly with the addition of ammonium nitrate was also investigated.

Transparent and conductive ZnO:Al thin films prepared by sol-gel dip-coating

Abstract: Aluminum doped zinc oxide polycrystalline thin films (AZO) were prepared by sol-gel dip-coating process. The sol was prepared from an ethanolic solution of zinc acetate using lithium hydroxide or succinic acid as hydrolytic catalyst. The quantity of aluminum in the sol was varied from 1 to 10 mol%. The structural characteristics studied by X-ray diffractometry were complemented by resistivity measurements and UV–Vis-NIR spectroscopy. The films are transparent from the near ultraviolet to the near infrared, presenting an absorption cut-off at almost 290 nm, irrespective of the nature of the catalyst and doping level. The best conductors were obtained for the AZO films containing 3 mol% of aluminum, prepared under acidic and basic catalysis and sintered at 450 °C. Their optical band-gap of 4.4 eV calculated from the absorption cut-off is larger than the values for band-gap widening predicted by the standard model for polar semiconductors. These polycrystalline films are textured with preferential orientation of grains along the wurtzite c -axis or the (100) direction.

Synthesis of boron carbide powder by a carbothermic reduction method

Abstract: Carbothermic reduction process is an economic method to produce boron carbide powder. In the present article this method was utilized to produce a boron carbide powder using commercial purity raw materials. Boric acid as a source of boron, and carbon active and petroleum coke as reducing agents were used. Mixtures of boric acid and carbon bearing material with a particle size of less than 44 μm were placed in a graphite crucible and heated under a flow of argon atmosphere in a tube furnace to 1400–1550 °C for 1–5 h. This resulted in the formation of boron carbide powder with or without un-reacted starting raw materials. It was found that the optimum weight ratio of boric acid to carbon bearing material was 3.5 and 3.3 for petroleum coke and carbon active, respectively. Heat treatment of these blends at 1470 °C for 5 h resulted in the synthesis of boron carbide powders, which contained 0.82 and 0.59 weight percent free carbon, respectively.

Effect of substitution of fly ash for quartz in triaxial kaolin–quartz–feldspar system

Abstract: Quartz was progressively replaced by 5, 10 and 15 wt.% fly ash (a byproduct of Indian thermal power plant) in a traditional triaxial porcelain composition consisting of kaolinitic clay, quartz and feldspar. The effects of this substitution upon firing at different temperatures (1150–1300 °C) were investigated by measuring the linear shrinkage, bulk density, porosity and flexural strength. Quantitative estimation of mullite and residual quartz content was done by XRD and microstructures of some selected samples were studied using SEM. The results show that fly ash containing samples achieved higher densities (max. 2.46 g cm −3 ) in the entire temperature range (1150–1300 °C) of firing and matured early than a traditional porcelain composition. The maximum flexural strength (70.5 MPa) was obtained at 1300 °C in the 15 wt.% fly ash containing sample compared with (61.1 MPa) obtained in the traditional porcelain sample. XRD studies reveal higher mullitization in fly ash containing samples. SEM photomicrographs of the polished and etched specimens of 1300 °C heated samples show the presence of quartz grains and secondary mullite needles embedded in feldspathic glassy matrix. Quartz grains (25–50 μm) are associated with circumferential cracks around them.

Effect of firing conditions, filler grain size and quartz content on bending strength and physical properties of sanitaryware porcelain

Abstract: The effect of filler grain size, quartz content in the filler and firing conditions (sintering temperature, firing time) on the physical and mechanical properties of a sanitary-ware porcelain has been studied in the narrow range of values used by industrial practice The investigation has been carried out using the Taguchi method for experimental design Quartz grain size is the most important factor regarding the physical properties and dominates bending strength in two ways, directly by inducing compressive stresses to the vitreous phase and indirectly through the development of a favorable microstructure The characteristics of the last are discussed in detail The optimum grain size of quartz was found to be 5–20 μm This results in 20–30% increase of bending strength compared to the reference porcelain The results confirm the “Matrix Reinforcement” theory, however, the positive effect of mullite content on bending strength was not observed

Ceramic TiO2-foams: characterisation of a potential scaffold

Abstract: The Schwartzwalder process was chosen for the production of ceramic TiO 2 scaffolds and showed a fully open structure with a permeability for water of 39%. The window sizes were 445 μm (45 ppi foams) and 380 μm for the 60 ppi foams. The porosity of all foams was above 78% ( n =8). It was shown that scaffolds can be produced with defined pore sizes, shape and architecture, which is a requirement for scaffold production. The macro- and microarchitecture was reproducible. Hence a reproducible ceramic scaffold processing method has been established. The interconnectivity of the pores in the scaffold was tested with a novel method. For the tests a new device was constructed where the permeability was linked to the degree of interconnectivity. Results from the permeability measurements in the mercury intrusion meter and permeability tester show that increasing pore size increases the rate of permeability. The tortuosity, which was measured in the mercury intrusion meter, was several factors higher for 60 ppi foams compared to 45 ppi and therefore also understates the lower permeability. An initial cell culture test showed that fibroblasts adhere on the foam's surface.