Optimization of electrophoretic deposition of alumina onto steel substrates from its suspension in iso-propanol using statistical design of experiments

Statistical design of experiments was used to investigate the effect the process parameters on electrophoretic deposition (EPD) of alumina onto steel substrates from its suspension in iso-propanol. The process parameters considered were (i) concentration of particles in the suspension (solid loading), (ii) electrode separation, (iii) applied potential, and (iv) deposition time on the quantity of ceramic particles electrophoretically deposited. A 2⁴ full factorial matrix, with four repetitions of the center point, was used to develop the predictive regression equation for deposition of alumina per unit area of the electrode in the design space. The results show that particle concentration has the most dominant effect with more than 50% contribution to the deposited amount. A good correlation was obtained between predicted and experimental values suggesting that the model can predict data accurately in the experimental matrix.     

Application of alginate gelation to aqueous tape casting technology

This work deals with a novel forming process for manufacturing ceramic tapes in an aqueous medium, that combines tape casting and gel-casting technologies. Aqueous tape casting suspensions of Al₂O₃ are prepared to a solid loading of 57 vol.% (84 wt.%) by adding 15 wt.% of a binder system consisting of acrylic emulsions and a small concentration (≤0.5 wt.%) of alginate. The as-cast tape is immersed in a CaCl₂ solution and the alginate undergoes gelation. This allows releasing of the tape from the carrier film so that no sticking occurs and crack formation is avoided. Sintered densities are ∼97% of theoretical, similar to those obtained by conventional aqueous tape casting.     

Dielectric properties of barium strontium titanate thick films prepared by electrophoretic deposition

Ba_xSr_1−xTiO₃ (x = 0.5, 0.6, 0.7) thick films were prepared by electrophoretic deposition (EPD) technique on platinum metallic foils using BaTiO₃ and SrTiO₃ nanoparticles with different molar proportion of 1:1, 3:2 and 7:3, respectively. An isostatic pressure method was used to increase density of the thick films before high temperature sintering. Microstructures of the deposited films were examined with XRD and SEM techniques. Porosity of the thick films decreased after the isostatic pressure process. The Ba_0.5Sr_0.5TiO₃ thick films of 10 μm, 15 μm and 20 μm showed a tunability of 28.8%, 33.3% and 33.9%, respectively, at room temperature and at a biasing field of 2 kV/mm. The dielectric constant was from 2138 to 3446 and dielectric loss was from 0.016 to 0.011 at zero bias field at 10 kHz. The temperature dependence of dielectric constant was also measured and the effect of porosity and thickness on the electrical performance of the thick films was discussed.     

Preparation of a crack-free rough titania coating on stainless steel mesh by electrophoretic deposition

TiO₂ (anatase) coating was prepared on stainless mesh by electrophoretic (EPD) process utilizing an isopropyl alcohol (IPA)-based suspension with submicron TiO₂ powder. When the deposition time was 30 s, a smooth thin coating was obtained. It remained crack-free even after sintering. Coating surface morphology was roughened by UV pre-illumination of the suspension. Photocatalytic decomposition of IPA to acetone and resultant electrochemical reaction at cathode during EPD provides heterogeneous deposition.     

Preparation and super-hydrophilic properties of TiO2/SnO2 composite thin films

SnO₂-TiO₂ composite thin films were fabricated on soda-lime glass with sol-gel technology. By measuring the contact angle of the film surface and the degradation of methyl orange, we studied the influence of SnO₂ doping concentration, heat-treatment temperature and film thickness on the super-hydrophilicity and photocatalytic activity of the composite films. The results indicate that the doping of SnO₂ into TiO₂ can improve their hydrophilicity and photocatalytic activity, and the composite film with 1-5 mol% SnO₂ and heat-treated at 450°C is of super-hydrophilicity. The optimal SnO₂ concentration for the photocatalytic activity is 10 mol% and larger film thickness is helpful to reduce the contact angle of the composite films.     

MAO-preparation of nanocrystalline hydroxyapatite-titania composite films: Formation stages and effect of the growth time

The HAp-TiO₂ nanostructured layers with a porous morphology were fabricated through micro arc oxidation technique for different times, i.e. 3, 6, and 10 min. Compositional studies, performed by XRD and XPS techniques, showed that the layers consisted of hydroxyapatite, anatase, calcium titanate, and α-TCP phases. Based on the XRD results, it was observed that the hydroxyapatite relative content reached its maximum value at intermediate times. As an important achievement, it was figured out that the HAp phase gradually grows on a titania inner layer which forms within preliminary stages of the MAO treatment. Considering the XRD patterns, the hydroxyapatite crystalline size was determined as ∼48 nm which is favorable for bio-applications. Based on the SEM and AFM observations, the layers exhibited a porous morphology with a rough surface where the pores size increased with time. The highest porosity and roughness was achieved at intermediate times. The layers fabricated for intermediate growth times exhibited the most appropriate physical and chemical properties for bio-applications.     

Preparation of Al-Cu-Fe thin films by vapor deposition technique from a single source

In this paper, we report the formation of stable icosahedral Al-Cu-Fe quasicrystalline thin films by thermal vapor deposition techniques (indirect heating and e-beam heating) from a single source. Deposition of these films by a single-source indirect heating method, in the stable icosahedral phase, is reported for the first time. A direct comparison between the two different heating methods has been made. The final compositions of the prepared films with desired properties were found to be Al_62.9Cu_24.6Fe_12.5 (indirect heating method) and Al_63.1Cu_24.5Fe_12.4 (e-beam method), respectively. The resistivities of the films prepared by both methods were ∼2000 μΩ cm at room temperature and ∼4000 μΩ cm at 10 K.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Pulsed laser deposition of alumina coating for corrosion protection against liquid uranium

Alumina coatings find wide applications as tribological coatings and as corrosion protective coatings for structural materials against chemical attack. We have investigated alumina coatings deposited on Stainless Steel (SS) substrates via pulsed laser deposition (PLD) technique. Characterization tests performed on these coatings including their compatibility with liquid uranium suggests alumina to be a potential candidate as a coating material for handling and containment of liquid uranium. We present here results of our detailed parametric study including dependence of average mass removal rate on laser fluence and ablation geometry and average deposition efficiency during PLD. These measurements provide vital inputs facilitating proper choice of process parameters for PLD runs. Deposited coatings have been characterized in terms of their microstructure, surface profile, adhesion to substrate, crystalline phase and corrosion resistance against liquid uranium. Our PLD based alumina coatings have shown a high degree of compaction and excellent corrosion resistance to molten uranium even upto a temperature of 1165 °C.     

Preparation of hydroxyapatite-containing titania coating on titanium substrate by micro-arc oxidation

Hydroxyapatite-containing titania coatings on titanium substrates were formed by micro-arc oxidation (MAO) in electrolyte containing calcium acetate monohydrate (CH₃COO)₂Ca·H₂O) and sodium phosphate monobasic dihydrate (NaH₂PO₄·2H₂O) using a pulse power supply. Scanning electron microscopy (SEM) with Energy dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD) were employed to characterize the microstructure, elemental composition and phase components of the coatings. The coatings were rough and porous, without apparent interface to the titanium substrates. All the oxidized coatings contained Ca and P as well as Ti and O, and the porous coatings were made up of anatase, rutile and hydroxyapatite. Such MAO films are expected to have significant applications as artificial bone joints and dental implants.     

Characterisation and photocatalytic activity of structure-controlled spherical granules of an anatase/hydroxyapatite composite

Materials that can purify the environments are desirable. Anatase (TiO₂) has received attention because it is stable and can decompose organic substances because of its photocatalytic activity. To make use of anatase effectively, we deposited nano-sized anatase particles on porous hydroxyapatite (HA) ceramics composed of rod-shaped particles. Spherical porous HA granules composed of rod-shaped HA particles were prepared using a hydrothermal process. The granules were soaked in a solution containing a water-soluble titanium complex and then hydrothermally treated. Nano-sized anatase particles were deposited on each rod-shaped HA particle. The anatase/HA granules composed of rod-shaped HA particles showed higher photocatalytic activity than those composed of globular HA particles. The granules are expected to be useful as an environment-purifying material with high manageability and photocatalytic activity.     

Dielectric constant and mixing model of BaTiO3 composite thick films

BaTiO₃ (BT) composite thick films of X7R BT particles with different BT gel fractions were prepared by using an aqueous BT sol. The dielectric constant versus different BT gel volume fractions showed a sigmoidal behavior as the BT gel phase filled the interstitials of the X7R BT particle compact. To explain and predict the effect of the BT gel as a second phase based on the experimental results, various models such as series, parallel, cubic, Lichtenecker’s model, and Hashin-Shtrikman bounds were considered. None of the existing theoretical models fit the experimental results. An empirical sigmoidal fitting function was proposed to fit the experimental data.     

A new alternative representation of impedance data using the derivative of the tangent of the phase angle: Application to the YSZ system and composites

The aim of this work is to propose a new alternative representation of impedance data using the derivative of the tangent of the phase angle, which allows enhanced discrimination between processes with relaxation frequencies that are very close. The new representation allows discrimination between overlapped processes within a factor of 2 in their relaxation frequencies for process with similar strength. Equations for the simplified behaviour of the impedance data have been proposed to obtain all the parameters of the processes involved in the impedance spectrum. This new alternative representation has been applied to bulk and grain boundary responses of YSZ with very satisfactory results. It has also been applied to the qualitative study of impedance data of a CuO composite showing the usefulness of this representation to discriminate different electrode processes. This approach provides an ab initio method of identify the contributing components to an electrochemical impedance spectrum with quite remarkable resolution. It is suggested that if this method is applied to provide starting parameters for non-linear least squares fitting using constant phase elements, then problems due to correlation of parameters and identification of components can be minimised.     

Optimization of the chemical vapor deposition process for fabrication of carbon nanotube/Al composite powders

In order to optimize the chemical vapor deposition process for fabrication of carbon nanotube/Al composite powders, the effect of different reaction conditions (such as reaction temperature, reaction time, and reaction gas ratio) on the morphological and structural development of the powder and dispersion of CNTs in Al powder was investigated using transmission electron microscope. The results showed that low temperatures (500-550 °C) give rise to herringbone-type carbon nanofibers and high temperatures (600-630 °C) lead to multi-walled CNTs. Long reaction times broaden the CNT size distribution and increase the CNT yield. Appropriate nitrogen flow is preferred for CNT growth, but high and low nitrogen flow result in carbon nanospheres and CNTs with coarse surfaces, respectively. Above results show that appropriate parameters are effective in dispersing the nanotubes in the Al powder which simultaneously protects the nanotubes from damage.     

Flexible ceramic-reinforced polyurethane composite coatings on synthetic fibres: Process of continuous liquid film coating and its influence on the coating thickness

A concept for the realisation of composite layers on the surface of synthetic fibres by continuous liquid film coating of spread roving or monofilaments with a slurry containing ceramic particles and a polymeric binder is successfully demonstrated. Polyamide 6 monofilaments were coated with alumina particles and a polyurethane elastomer based binder system to achieve higher abrasion resistance. Under frictional load, the coating is abraded in small units consisting of some particles and the surrounding polyurethane matrix, which predetermines it to be used as sacrificial layer. The slurries showed shear thinning behaviour. Increasing the content of the rheological agent (carboxymethylcellulose [CMC]) led to an exponential increase of the viscosity, which therefore can be tailored by varying the CMC content. An increase of fibre radius, slurry viscosity or withdrawal speed led to an increase of the coating thickness, which is in accordance to published results of Newtonian or non-Newtonian coating fluids.     

Experimental evidence of enhanced ferroelectricity in Ca doped BiFeO3

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), polarization and piezoelectric measurements. Structural studies by XRD and TEM reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ where enhanced ferroelectric and piezoelectric properties are produced by internal strain. Resistive switching is observed in BFO and Ca-doped BFO which are affected by the barrier contact and work function of multiferroic materials and Pt electrodes. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Piezoelectric properties are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain. This observation introduces magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom which are already present in the multiferroic BiFeO₃.     

The fabrication and corrosion behavior of electroless Ni-P-carbon nanotube composite coatings

Ni-P-carbon nanotube (CNT) composite coatings were fabricated successfully from a suspension of CNT in an electroless bath. The microhardness and corrosion behavior of the composite coatings were investigated. The electrochemical properties of the composite coatings were studied using electrochemical workstation system. The corrosion behavior of the amorphous Ni-P-CNT composite coatings was evaluated by polarization curves and electrochemical impedance spectroscopy in 0.1 mol/l NaCl solution at room temperature. It was noted that the amorphous Ni-P-CNT composite coatings provided higher corrosion resistance than the amorphous Ni-P coating. The mechanism of improvement of the electrochemical properties of the electroless composite coatings was also discussed.     

Effect of liquid-forming additives on the sintering and mechanical properties of Al2O3/3Y-TZP (30 vol.%) composite

Al₂O₃/3Y-TZP (30 vol.%) composite was pressurelessly sintered with addition of TiO₂MnO₂ and/or CaOAl₂O₃SiO₂ glass. It was found that TiO₂MnO₂ addition greatly enhanced the densification of the composite by the formation of a low-viscosity liquid at sintering temperature. In contrast, the high-viscosity liquid formed by CaOAl₂O₃SiO₂ glass improved mechanical properties because of its repressing effect on grain growth. The composite could be obtained at a temperature as low as 1400°C by co-doping with TiO₂MnO₂ and CAS glass. Bending strength of 552±64 MPa and fracture toughness of 6.03±0.22 MPa m^1/2 were obtained with a doping level of 2 wt.% TiO₂MnO₂ and 2 wt.% CAS glass.     

On the green density, sintering behavior and electrical property of tape cast Ce0.9Gd0.1O1.95 electrolyte films

Gadolinia doped ceria (Ce_0.9Gd_0.1O_1.95, GDC) electrolyte films were tape cast from oxalate coprecipitated GDC powders, gelcast GDC powders and their mixtures, respectively, to evaluate the effects of the original particle size and distribution on the properties of the green and sintered GDC cast tapes. The apparent density of different original powders, as well as the green density, sintered behavior, and electrical conductivity of tapes cast from the various starting powders were investigated. Mixing the coprecipitated and the gelcast GDC powders not only results in a higher packing efficiency of particles in the loose powders, but also results in higher green and sintered densities of cast tapes. Furthermore, tapes cast from the 50/50 powder mixtures can be sintered to 96.2% of theoretical density at relatively low sintering temperature of 1400°C, whereas those from the oxalate coprecipitated and from the gelcast powders were only 89.7 and 94.1% dense, respectively. The ac impedance measurement shows that GDC films cast from the 50/50 powder mixture exhibit good electrical conductivity (4.2 and 6.0 S m⁻¹ at 700 and 800°C in air, respectively). The test results have revealed that high-density GDC films can be fabricated by tape casting technique at relatively low sintering temperature by optimizing the particle size distribution of the starting powders.     

Characterization of electrochemically deposited Cu-Ni black coatings

Electrochemically deposited Cu-Ni black coatings on molybdenum substrate from ethylenediaminetetraacetic acid (EDTA) bath solution are shown to exhibit good optical properties (α=0.94, ε = 0.09). The deposit is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Cu is present in metallic and +2 oxidation states in the as-prepared Cu-Ni black coating, whereas Ni²⁺ as well as Ni³⁺ species are observed in the same coating. Cu and Ni are observed in their metallic state after 10 and 20 min sputtering. X-ray initiated Auger electron spectroscopy (XAES) of Cu and Ni also agrees well with XPS investigations.