Search

This paper describes the process for the hydrolysed aluminium treatment (HA) on alumina surface and its influence on the rheological characteristics of alumina slurries. Three different commercial grade aluminas were provided with the surface treatment with hydrolysed aluminium using aluminium nitrate and hexamethylenetetramine (HMTA) under controlled conditions. The HA treatment increased the concentration of Al-OH surface sites resulting in higher H+ adsorption on the alumina surface. A highly concentrated (>55 vol%) electrostatically stabilized alumina slurry was prepared from HA treated alumina powders. The rheology of such slurry was studied and the results on the viscosity and yield stress are presented. The alumina slurries followed the Casson Model flow behaviour. The ‘wet’ and ‘flow’ behaviour of the alumina with and without HA treatment was also studied and the results are compared. The surface treatment showed the advantage of maintaining low viscosity and yield stress of alumina slurries even at higher solids loading (>55 vol%) that are prepared in the acidic aqueous medium. The results on viscosity and yield stress were compared with that of the polyelectrolyte dispersed system.

Alumina–aluminium titanate composite was fabricated through paste extrusion using boehmite gel as a binder. The rheological characteristics of alumina paste containing various amounts of boehmite binder were studied at different shear rates. The results of investigations on the development of torque, yield stress, viscosity and fluidity are presented. In general, Bingham type plastic behavior was observed. Alumina pastes containing 18.5 vol.% of boehmite exhibits adequate plasticity and fluidity for extrusion. The torque value for this composition is 68 kPa at shear rate 28.5 s−1. The viscosity curves of alumina pastes showed “shear thinning” flow behavior. The mixture of 54.15 vol.% alumina and 18.5 vol.% boehmite resulted in 61% of theoretical green density on extrusion. These conditions were extended to synthesis of alumina–aluminium titanate composites, and the extrusion was carried out using 18.5 vol.% boehmite. Aluminium titanate was synthesized through sol–gel method and dispersed uniformly in alumina matrix through colloidal process. The composites could be sintered at 1400°C to 98% of its theoretical value. The SEM fractograph of sintered composite showed reasonably dense, uniformly distributed grains in the order of 2–3 μm. The present study shows that boehmite binder phase yields good workability for extrusion and contributes high green and sintered densities to the composites.

The present work reports the extrusion characteristics of alumina pastes involving boehmite gel as a binder. Submicron sized alumina (average particle size 0.35 μm) is dispersed in boehmite sol and converted to form a thick alumina paste by controlled flocculation and centrifugation. The paste is further subjected to extrusion for fabricating alumina shapes such as rods and tubes. The paste rheology, workability, green strength and sintered properties are studied and reported. The paste rheology was studied by torque rheometer and the results on the development of torque, viscosity and the yield stress are presented. The alumina paste containing 18 vol.% boehmite binder resulted in 53.4 vol.% solids loading with suitable consistency for low pressure extrusion. The system exhibits a low torque value of 51.85 KPa at shear rate 28.5 s−1 and showed plastic behavior. The apparent viscosity of the system showed a shear dependent flow behavior. Alumina–18 vol.% boehmite resulted in a theoretical green density value of 61.4% and a green strength of about 1.05 N/mm2 on extrusion. The extruded alumina on sintering has a theoretical sintered density of about 98% at 1500°C. The microstructure of sintered alumina showed an average grain size approx. 2 μm. The combination of colloidal processing and gel assisted extrusion yields highly dense, fine grained ceramics. Boehmite ge1 as a binder has advantages such as matrix compatibility, good workability and better green strength.

This work reports the use of monohydroxy aluminium oxide particulate sol [boehmite, AlOOH], as a dispersing medium for slurry compaction of submicron-sized alumina powders. Nanoparticulate boehmite sol, at controlled pH conditions, resulted in homogeneously dispersed alumina suspension which could be compressed into a definite shape by applying simple pressure. The partially flocculated boehmite sol at pH 6.2 acts as good dispersant and resulted in alumina–boehmite solids loading >80 wt.%. The alumina dispersed in boehmite sol containing 17.24 vol.% boehmite concentration showed maximum green and sintered density. The maximum theoretical green density was 63% and the maximum linear drying shrinkage was

The process for alumina gel casting was developed using an inorganic binder The monohydroxy aluminium oxide (boehmite, AlOOH) was incorporated with ultrafine alumina of particles (0·5 μ m) and the slurry rheology was studied and presented. The effect of boehmite in slurry viscosity was observed with respect to different amounts of boehmite and time. The alumina 54 vol% slurry with 10 wt% boehmite showed the viscosity of 880 mPa s at 93 s −1 . An external coagulating agent, HMTA, was incorporated with alumina–boehmite slurry and the effective change in slurry viscosity with respect to concentration and time was studied. The addition of HMTA results in faster gelation and the optimum concentration was determined as 0·21 mol L −1 . The alumina gelcast body was dried under humidity conditions at 40°C, RH 70%. The defect free dried green body was obtained and the total linear drying shrinkage was calculated as 3·2% and the green density observed was 59·3% of theoretical value. The sintered density of 98% (TD) was achieved at 1450°C in 2 h. The mechanical hardness of sintered alumina measured as 2286 kg mm −2 . The sintered ceramic showed an extremely fine grained microstructure with an average grain size μ m. The boehmite acts as an excellent binder and sintering aid for alumina ceramics.

High temperature densification and associated microstructural features of alumina derived through sintering of microwave dehydroxylated boehmite are presented. α-alumina has been obtained through a series of transitional alumina phases within 60 min of exposure of boehmite gel to microwaves of 2.45 GHz and 600 W power. The phase formation has been identified by XRD. The surface area measurements on various gel samples exposed to microwaves at intermediate time intervals are also reported.The densification characteristics of microwave dehydroxylated gel precursors when sintered by conventional route resulted 95% of theoretical density at 1400°C and an average grain size of 2 μm. On the other hand, part of the same gel precursor conventionally dehydroxylated and sintered at 1400°C resulted 97% theoretical density with very fine microstructure. Thus, the present study indicates the possibility of using low power microwaves for dehydroxylation of boehmite precursor gels to active phases which further can be sintered under conventional heating to dense, fine grained alumina.

Silica gel has been prepared by the hydrolysis of TEOS under three different pH conditions. These gels have been subjected to drying in an air oven, under humidity, and also by exposure to microwaves, followed by further calcination at 500°C under slow heating schedule (3°C/min). Silica samples thus obtained are characterised by B.E.T. specific surface area and pore volume data. The thermal decomposition behaviour of the precursor gels are also reported. The adsorption isotherms of these samples indicate different behaviour related to the method of synthesis. Increase in pH of hydrolysis of the TEOS from 3 to 8 results in increase in specific surface area in tune with the earlier reports. However more significant observation is the variation in pore size distribution as evidenced from adsorption isotherms related to method of drying. Silica gels prepared at pH 3 show Type I behaviour irrespective of the method of drying. However gel prepared at pH 6 shows Type II behaviour when dried under microwave with specific surface area as high as 635 m2/g and pore volume 0.9733 cc/g. The precursor gels prepared at still higher pH exhibit Type IV behaviour when subjected to microwave drying. The pH conditions of synthesis of precursor gels along with drying techniques appear to affect not only the surface areas and porosities but also the resultant adsorption isotherms, in sol-gel silica.

Al2O3-SiC composite precursor has been made by dispersing fine SiC particles in a boehmite (AlOOH) matrix at pH 3.5 and slowly flocculating to pH 7.5 by dropwise addition of dilute NH4OH solution while the whole mixture was kept under constant stirring. The gel thus obtained is further dried under humid conditions and thermally dehydroxylated below 600°C. This precursor has been further subjected to microwaves having frequency 2.45 GHz and 600 W power under a “Picket Fencing” arrangement and sintered to reasonably higher densities. In a similar way, identical samples have been sintered in an electrical furnace up to 1450°C and the sintered properties are compared.

A rapid microwave decomposition method has been reported for the preparation of ZnO varistor precursors which on subsequent heat treatment for short period results in non-ohmic properties. Thus a mixture of nitrates of zinc and praseodymium along with minor additives such as cobalt, chromium and potassium when subjected to microwaves of 2.45 GHz in a domestic oven for 8 min, resulted in the decomposition of nitrates and formation of the precursor oxide mixtures. The precursor powder consists of easily friable agglomerates of size about 5 μm, having the major phase as ZnO. Such powder on further compaction at about 150 MPa and sintering in the range of 1200–1350 °C, resulted in high density (> 95%) varistors. The electrical measurements of the varistor sample showed nonlinearity coefficient (∝) above 30. Electrical field ( E ) vs current density ( J ) measurements showed breakdown fields of 135 V/mm for Pr 2 O 3 -doped varistors. The fractograph of the sintered varistor sample indicates an average grain size of 6–8 μm.

Mechanical properties of gadolinium-doped ceria (Ce0.9Gd0.1O1.95, 10GDC) green tape prepared by aqueous-based tape casting process were characterized by tensile test and shear punch test (SPT). SPT was found to be a useful method for characterizing mechanical properties of green tapes. Microstructures and mechanical properties such as flexural modulus, bending strength, and microhardness of tapes sintered at 1,300–1,500 °C have been evaluated. Indentation fracture toughness was also determined by the method of Palmqvist cracks at different applied loads for tapes sintered at 1,500 °C. Grain size measurements showed that excessive grain growth occurred during sintering despite using 10GDC nanopowders as the starting material. However, mechanical properties of sintered tapes improved by increasing sintering temperature and the results are comparable with those reported for 10GDC in literature.

Composites of polyvinylidene fluoride (PVDF) with micron and nano sized BaTiO3 powders were developed for electromagnetic interference (EMI) shielding applications in the X band. PVDF-nano BaTiO3 composites show better shielding property compared to PVDF-micron sized BaTiO3 composites. The composite of PVDF with 40 vol% of nano BaTiO3 showed the best EMI shielding effectiveness and is about 9 dB. The contributions from reflection and absorption to the total EMI shielding effectiveness is same for the PVDF-BaTiO3 composites. Addition of small amount of silver particles improved the shielding properties of these composites due to the increased conductivity. An EMI shielding effectiveness of about 26 dB is obtained in the measured frequency range for the PVDF-20 vol% nano BaTiO 3-10 vol% Ag composite of thickness 1.2 mm. Novel three phase composite combining the advantages of metal, nano ceramic and polymer is obtained with the potential for effective EMI shielding applications. � 2013 Elsevier Ltd.

Growth of multiwalled carbon nanotube (CNT) assemblies by chemical vapour decomposition (CVD) technique was achieved through decomposition of acetylene using iron impregnated montmorillonite (MM) catalysts. Various amounts of iron loaded montmorillonite catalysts were prepared by wet impregnation method and calcined at 450 °C. The catalysts were subjected to X-ray diffraction (XRD) and surface area analyses. Acetylene decomposition at a feed ratio of N2:H2:C2H2 = 1:1:0.18 was conducted in the presence of iron impregnated montmorillonite catalysts in the CVD reactor for the growth of CNT structures. The role of Fe-activated clay catalyst on the formation of CNT structures has been systematically examined at various temperatures and correlated with the morphological features of CNTs. Catalyst assisted acetylene decomposition results the formation of different carbon nanostructures such as nanotubes, nanofibres and nanoflakes. These clay–CNT products were characterised for their morphological, thermal, qualitative and quantitative analyses. The morphological variations of CNT assemblies reveal Fe-montmorillonite catalysts have high selectivity at given reaction conditions. Thermogravimetric and Raman spectral analyses prove that the CNTs contain a good crystallanity and less structural defects.

Nanocrystalline samarium doped ceria electrolyte [Ce0.9Sm0.1O1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10−2 S cm−1 at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10−6 m2 s−1 and 1.2 W m−1 K−1, respectively.

Gadolinia-doped ceria (Ce 0.9 Gd 0.1 O 1.95 , GDC) electrolyte was fabricated by aqueous-based tape casting method for solid oxide fuel cells (SOFCs). The ceramic powder prepared by combustion synthesis was used with poly acrylic acid (PAA), poly vinyl alcohol (PVA), poly ethylene glycol (PEG), Octanol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate and double distilled water as dispersant, binder, plasticizer, defoamer, surfactant and solvent respectively, to prepare stable GDC slurry. The conditions for preparing stable GDC slurries were studied and optimized by sedimentation, zeta potential and viscosity measurements. Green tapes with smooth surface, flexibility, thickness in the range of 0.35–0.4 mm and 45% relative green density were prepared. Conventional and flash sintering techniques were used and compared for densification which demonstrated the possibility of surpassing sintering at high temperatures and retarding related grain growth.

Combustion synthesized nanocrystalline Co and Al co-doped ZnO powders [(Zn1−x−yCoxAlyO; x=0.04, 0.03, 0.02; y=0.01, 0.02, 0.03)] were fabricated into cylindrical discs by uni-axial pressing and sintered intentionally at 1000 °C for 2 h to assess the mechanical performance. The crystallinity of the pure and doped ZnO was confirmed by X-ray diffraction analysis. The microstructures of the sintered samples were investigated by scanning electron microscopy (SEM) to examine the density, porosity, grain size and its distribution. Grains of 0.5–3 μm were observed for the samples sintered at 1000 °C. The mechanical properties such as micro-hardness, fracture toughness and strain hardening co-efficient were investigated by the Vickers indentation method. It was found that the crack mode observed during the indentation on the samples belongs to median cracks under a load of 19.6 N. Also, the hardness was enhanced with increasing mol% of Co, while the trend was reversed with the increase of Al content. In addition, the strain hardening coefficient and fracture toughness were calculated using the indentation data.

Nickel-zinc ferrites of different compositions Ni 1 - x Zn x Fe 2 O 4 (x = 0.2, 0.3, 0.4 and 0.5) are prepared using flash combustion technique. The effect of frequency and composition on the ac-resistivity is studied for Ni-Zn ferrite samples sintered in the temperature range 1150° - 1350°C. The physical properties, such as bulk density and porosity, are also studied for sintered ferrite samples. The experimental results indicate that the ac-resistivity (p a c ) decreases with increasing frequency. The ac-resistivity of the order of 10 5 Ω cm is obtained at 10 kHz in room temperature. These high values of resistivity make these Ni-Zn ferrites suitable for high frequency applications.

The transformation of ZnO morphologies in an in situ thermohydrolysis technique using hexamethylene tetramine is studied with and without surfactants. The photocatalytic and photoluminescence properties of these morphologically tuned ZnO morphologies are studied and the results presented. In the absence of any surfactants, the HMTA assisted in situ hydrolysis resulted in microtube and multipod morphologies. The addition of nonionic [Span-80] and cationic [CTAB] surfactants transforms the morphologies to microspheres, microdiscs and nanorods. The photoluminescence analysis shows a red luminescence in nanorods and green-indigo and blue-green emissions in microtubes and other morphologies. Photocatalytic reaction efficiency in UV light follows the order microtubes > nanorods > microdiscs > microspheres>multipods.

Sintered ZnO ceramics containing additions such as Bi203, Pr6Oll and SrO with minor dopants of Co304, Cr203 and MnO2 have shown non-linear current-voltage properties, and hence have found wide use in electronic circuits to protect solid-state components against transient overvoltages [1]. The electrical characteristics, such as breakdown voltage (VB) and non-linearity coefficient (0l), of these sintered polycrystalline ceramics are determined primarily by the additives and the microstructure. The microstructure of ZnO varistors comprises semiconducting n-type ZnO grains surrounded by insulating barriers at the ZnO grain boundaries. The electrical conduction through the grain boundary barrier is explained by a double Schottky barrier (DSB) defects model [2]. During sintering, the presence of secondary phase particles forms a liquid phase between grain-grain junctions. The processing and sintering of the ZnO compositions sometimes results in inhomogeneous grain growth, which in turn leads to random grain size distribution. A microstructure having large uniformly distributed grains is essential for low voltage varistors [3]. Different approaches have been taken to control the grain growth in zinc oxide to result in uniform size. Titanium oxide has been introduced up to 0.5 mol % in the varistor composition, with marginal success [4]. Ready-grown ZnO grains prepared by calcining ZnO particles above 1300 °C, usually denoted by the term "seed", have been added to varistor composition and such seeds can arrest the non-uniform growth in bulk ZnO. A similar seeding technique was successfully used to control the grain growth in BaTiO3 ceramics, essentially for suppression of duplex microstructures [5]. Seed grains are generally monocrystalline particles which are slightly larger than those of the original powder [6]. An attempt has been made here to study the effect of such calcined zinc oxide grains in a varistor composition such as ZnO-SrO, which is less extensively studied. Details of the preparation of the varistors, their non-ohmic behaviour and microstructural features are reported. Calcined zinc oxide grains (CZO) were prepared by calcination of ZnO powders at near sintering temperature (1100 °C/2 h) in an alumina crucible. The pre-fired powders were gently crushed in an agate mortar and subsequently fractionated by sieving. The resultant grains consist of crystalline fragments with a grain size about 10-15/xm, which

The present work encompasses the strategic design, synthesis and development of high energy field ZnO varistors. The key aspect of the study involves the chemical synthesis of ZnO based nanorod nanofillers by a pH selective precipitation and reflux method. High field varistors were developed out of these crystalline nanofillers by employing them as fillers to the micron sized commercial varistor powder. A systematic investigation of the microstructural changes of the nanofiller added varistor samples were carried out under step-sintering and the normal sintering conditions. Comparative studies were also performed with the nano and micro counterparts. The influence of nanofillers in the varistor powder packing, grain structure refinement, grain size reduction and on the I–V properties were analyzed under the step-sintering conditions. Based on the performance analysis, the current study foresees ample opportunities for miniaturizing varistors with at least half the size of currently available varistors.