Composite ceramic-metal coatings by means of combined electrophoretic deposition and galvanic methods

A method based on the combination of electrophoretic and galvanic deposition techniques has been developed to fabricate metal-ceramic composite coatings on metallic substrates. A ZrO₂-Ni composite coating with interpenetrating microstructure was produced on stainless steel plates. For electrophoretic deposition of the ceramic component, a non-aqueous suspension consisting of zirconia nanoparticles, ethanol and addition of 4-hydroxybenzoic acid was optimised by electrokinetic sonic amplitude (ESA) measurements. The zirconia deposits were partially sintered to create an open porous structure (porosity = 40–50%), which was subsequently filled with Ni by galvanic deposition. The bonding strength between the composite coating and the stainless steel substrate was improved by a final heat-treatement at 950°C for 3 h which promoted the diffusion of Ni into the steel substrate and the formation of a diffusion interlayer. The high adhesion strength of the composite coating to the stainless steel substrate after the diffusion bonding heat-treatment was confirmed by 3-point flexural strength tests. The coating exhibited a homogeneous interpenetrating microstructure with hardness values >6 GPa.     

Electrophoretic deposition and photoluminescent properties of Eu-doped BaTiO3 thin film from a suspension of monodispersed nanocrystallites

Monodispersed Eu-doped BaTiO₃ nanocrystallites with a pseudo-cubic perovskite phase were successfully prepared using highly concentrated alkoxide solution under hydrothermal conditions at temperatures <100 °C. The obtained nanoparticles had a very narrow particle size distribution and the average value was about 10 nm. By dispersing a piece of BaTiO₃ bulk gel into mixed solvent of 2-methoxyethonal and acetylacetone, we obtained well-dispersed and stable suspensions of Eu-doped BaTiO₃ nanocrystallites with a high transmittance. Using these suspensions, nanostructured thin films with a low porosity and a smooth surface were synthesized on Pt/Ti/SiO₂/Si substrate by electrophoretic deposition. The obtained Eu-doped BaTiO₃ thin films showed strong, visible room temperature photoluminescence (PL) associated with Eu ions. The PL spectra had typical ones for Eu³⁺ band corresponding to ⁵D_o–⁷F_j electronic transitions with a maximum intensity at 613 nm.     

Electrophoretic deposition and photoluminescent properties of Eu-doped BaTiO3 thin film from a suspension of monodispersed nanocrystallites

Monodispersed Eu-doped BaTiO₃ nanocrystallites with a pseudo-cubic perovskite phase were successfully prepared using highly concentrated alkoxide solution under hydrothermal conditions at temperatures <100 °C. The obtained nanoparticles had a very narrow particle size distribution and the average value was about 10 nm. By dispersing a piece of BaTiO₃ bulk gel into mixed solvent of 2-methoxyethonal and acetylacetone, we obtained well-dispersed and stable suspensions of Eu-doped BaTiO₃ nanocrystallites with a high transmittance. Using these suspensions, nanostructured thin films with a low porosity and a smooth surface were synthesized on Pt/Ti/SiO₂/Si substrate by electrophoretic deposition. The obtained Eu-doped BaTiO₃ thin films showed strong, visible room temperature photoluminescence (PL) associated with Eu ions. The PL spectra had typical ones for Eu³⁺ band corresponding to ⁵D_o–⁷F_j electronic transitions with a maximum intensity at 613 nm.     

Surface reactivity and electrophoretic deposition of ZrO<Subscript>2</Subscript>–MgO mechanical mixture

Electrophoretic deposition (EPD) is a precision technique useful for obtaining high quality ceramic bodies with controlled dimensions and smooth coatings. The electrophoretic deposition rate is highly dependent on the surface chemistry of the powders, especially when dealing with multi-component systems. The objective of this work is to study the surface reactivity of both ZrO₂ and MgO in ethanol suspension to provide experimental benchmarks to control EPD of a ZrO₂–3 wt% MgO mechanical mixture (Z3M) in ethanol. Infrared spectroscopy (FTIR) showed that ZrO₂ surface spontaneously reacts with ethanol, generating negative electrophoretic mobility of the particles (−0.07 × 10⁻⁸ V⁻¹ s⁻¹) measured by Electroacoustic Sonic Amplitude (ESA). MgO surface also spontaneously reacted with ethanol, but a positive electrophoretic mobility was observed in this case (0.26 × 10⁻⁸ V⁻¹ s⁻¹). Scanning Electron Microscopy of Z3M dried from ethanol suspension showed that MgO particles were located around the ZrO₂ particles, forming composite agglomerates, probably due to the electrostatic attraction between MgO and ZrO₂ particles. Homogeneous deposits could be obtained from EPD of Z3M ethanol suspensions. Mercury intrusion porosimetry showed that the ZrO₂–MgO green deposited bodies using different voltages had similar pores diameters distributions, indicating that the ZrO₂–MgO agglomerates are not affected by the increasing deposition rates.     

Formation of tubular BaTiO3 nanoparticle assembly through the Kirkendall effect using Na2Ti3O7 nanowires as template

We report the solvothermal synthesis of tubular BaTiO₃ nanoparticle assembly in ethanol/water mixed solvent system using Na₂Ti₃O₇ nanowires as template. The obtained tubular BaTiO₃ nanoparticle assembly, with a straight interior shell, is constructed by nanoparticles with diameters of 150–400 nm on the outer surface. The tubular BaTiO₃ nanoparticle assembly retains the one-dimensional characteristic inherited from the precursor Na₂Ti₃O₇ nanowires during solvothermal process. The formation mechanism of the hollow interior of tubular BaTiO₃ nanoparticle assembly is deriving from nanoscale Kirkendall effect, and the growth of nanoparticles on the outer surface with different sizes is controlled by the Ostwald ripening process, resulting in larger meaning outer diameter.     

Structural and chemical characterization of BaTiO3 nanorods

An electron-microscopy investigation was performed on BaTiO₃ nanorods that were processed by sol-gel electrophoretic deposition (EPD) into anodic aluminium oxide (AAO) membranes. The BaTiO₃ nanorods grown within the template membranes had diameters ranging from 150 to 200 nm, with an average length of 10-50 μm. By using various electron-microscopy techniques we showed that the processed BaTiO₃ nanorods were homogeneous in their chemical composition. The BaTiO₃ nanorods were always polycrystalline and were composed of well-crystallized, defect-free, pseudo-cubic BaTiO₃ grains, ranging from 10 to 30 nm. No intergranular phases were observed between the BaTiO₃ grains. A low-temperature hexagonal polymorph that is coherently intergrown with the BaTiO₃ perovskite matrix was also observed as a minor phase. When annealing the AAO templates containing the BaTiO₃ sol in an oxygen atmosphere the presence of the hexagonal polymorph was diminished.     

Electrophoretic coagulation behavior of ferroelectric barium titanate powders in mixed solutions of alcohol and acetylacetone

Although the deposition of BaTiO₃, known as a ferroelectric compound with spontaneous dipole moments, has never occurred in either propanol and acetylacetone, these mixtures were found to enhance the deposition of the powders on a cathode. The present study carried out the systematic investigation of the solution-mixing effect of propanol and acetylacetone on the electrophoretic deposition of BaTiO₃. The optimum mixing ratio for the deposition was 50–70 vol% of propanol, while a depressed migration of BaTiO₃ powders was observed in the mixed solutions. Therefore, it was considered that the enhanced deposition was attributed to the coagulation of the BaTiO₃ powders on the cathode. The deposition characteristics varied with the kind of alcohol used showed that a more highly enhanced deposition was achieved with alcohols having longer side-chains such as octanol. An intertwinement between the side-chains in the alcohols was considered to have an effect on the coagulation of the BaTiO₃ powders.     

Low-temperature hydrothermal-galvanic couple synthesis of BaTiO3 thin films on Ti-coated silicon substrates

Crystalline BaTiO₃ films were synthesized on Ti-coated silicon substrates at temperatures below 100 °C using a novel hydrothermal-galvanic couple method: Ti/Si was employed as the working electrode and platinum as the counter electrode without applying any external voltages or currents. Using this method, forming BaTiO₃ is possible at temperatures as low as 50 °C over a period of 2 h, which is not possible with the conventional hydrothermal method. The growth rate of BaTiO₃ is also much faster when prepared by such a novel method. The growth kinetics can be fitted rather well by a modified Johnson-Mehl-Avrami-Erofe'ev equation. The obtained activation energy for forming BaTiO₃ on Ti/Si is 97 ± 9 kJ mol^− 1.     

Effect of oxygen injection on synthesizing barium titanate nanoparticles by plasma chemical vapor deposition

Nanoparticles of barium titanate were prepared by plasma chemical vapor deposition using an inductively coupled plasma technique that has a high potential for preparing nanoparticles. The present paper describes the details of the reaction control achieved by accelerating the reactions between Ba atoms and oxygen using direct injection of oxygen into the tail of the plasma flame. According to our previous reports, the crystalline phases of the powder products were different at each collection area in the CVD apparatus. This was due to the remaining active Ba atoms without a reaction in the plasma tail flame, as observed from the optical emission spectroscopy. After the reactions between Ba atoms and oxygen occurred by direct oxygen injection, BaTiO₃ perovskite phases were produced as a major phase in each collection area. The optimal oxygen injecting conditions for obtaining perovskite BaTiO₃ single phase are summarized as follows: the temperature of the plasma tail flame at oxygen-injecting position = approximately 1000 K and molar ratio of oxygen to the reactants (O₂/(Ba+Ti)) = 4000. Furthermore, BaTiO₃ nanoparticles with average particle size under 10 nm were prepared by oxygen injection (the average particle size without oxygen injection is 15.4 nm). Well-crystallized BaTiO₃ nanoparticles with spherical shape were observed by TEM.     

Bioactive ceramic coatings containing carbon nanotubes on metallic substrates by electrophoretic deposition

A range of potentially bioactive ceramic coatings, based on combinations of either hydroxyapatite (HA) or titanium oxide nanoparticles with carbon nanotubes (CNTs), have been deposited on metallic substrates, using electrophoretic deposition (EPD). Sol–gel derived, ultrafine HA powders (10–70 nm) were dispersed in multi-wall nanotube-containing ethanol suspensions maintained at pH = ∼3.5 and successfully coated onto Ti alloy wires at 20 V for 1–3 min For TiO₂/CNT coatings, commercially available titania nanopowders and surface-treated CNTs in aqueous suspensions were co-deposited on stainless steel planar substrates. A field strength of 20 V/cm and deposition time of 4 min were used working at pH = 5. Although the co-deposition mechanism was not investigated in detail, the evidence suggests that co-deposition occurs due to the opposite signs of the surface charges (zeta potentials) of the particles, at the working pH. Electrostatic attraction between CNTs and TiO₂ particles leads to the creation of composite particles in suspension, consisting of TiO₂ particles homogenously attached onto the surface of individual CNTs. Under the applied electric field, these net negatively charged “composite TiO₂/CNT” elements migrate to and deposit on the anode (working electrode). The process of EPD at constant voltage conditions was optimised in both systems to achieve homogeneous and reasonably adhered deposits of varying thicknesses on the metallic substrates_.     

Preparation of ZrC-SiC composite coatings by chemical vapor deposition and study of co-deposition mechanism

In this work, the ZrC-SiC composite coatings were co-deposited by chemical vapor deposition (CVD) using ZrCl₄, MTS, CH₄ and H₂ as raw materials. The morphologies, compositions and phases of the composite coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that the morphologies, compositions and phases of the composite coatings were related to the deposition temperature, the flow rate of the carrier H₂ gas, and the ratio of C/Zr. Moreover, the co-deposition mechanism of the composite coatings was also studied. It was found that different deposition temperatures resulted in different deposition mechanisms. At temperatures in the range of 1150–1250 °C, the ZrC-SiC co-deposition was controlled by the surface kinetic process. At temperatures in the range of 1250–1400 °C, the ZrC-SiC co-deposition was controlled by the mass transport process.     

Preparation and dielectric properties of polycrystalline films with dense nano-structured BaTiO3 by chemical vapor deposition using inductively coupled plasma

To clarify the dielectric properties of BaTiO₃ with nanometer size region, it is necessary to fabricate the dense structure composed of BaTiO₃ nanoparticles. In the present study, BaTiO₃ nanoparticles were directly deposited on Pt/Al₂O₃/SiO₂/Si substrate by introducing Ba(DPM)₂ and Ti(OiPr)₄ into an inductively coupled plasma (ICP). The optimal condition for preparing dense structure of BaTiO₃ nanoparticles was investigated by changing the substrate temperature. Single phase BaTiO₃ of perovskite structure was obtained at the substrate temperatures between 773 and 1173 K. The dense structure of BaTiO₃ nanoparticles with particle sizes of about 30 nm was successfully obtained at the substrate temperature of 773 K. At the substrate temperature>873 K, the deposited nanoparticles sintered to be the columnar structure. The ε_r and tan δ of the BaTiO₃ nanoparticles were estimated to be 285 and 6.6%, respectively (1 kHz and 100 mV). The phase of the BaTiO₃ nanoparticles were found to be paraelectric by the measurement of C-V curves. The breakdown field of the dense structure of BaTiO₃ nanoparticles was estimated to be 649 kV/cm according to I-V curves. These features are favorable for applying the structure to the dielectric layer of multilayer capacitors.     

Impartation of hydroxyapatite formation ability to ultra‐high molecular weight polyethylene by deposition of apatite nuclei

The authors aimed to impart hydroxyapatite formation ability to ultra‐high molecular weight polyethylene (UHMWPE) by deposition of apatite nuclei (ApN) by the following two methods. The first method was electrophoretic deposition (EPD). A porous UHMWPE was placed between electrodes in the ApN‐dispersed ethanol and constant voltage was applied. By this treatment, the ApN were migrated from anode‐side surface to the cathode one through the pores by an electric field in the pores of the UHMWPE and deposited inside the pores. The second method was direct precipitation (DP) of the ApN. A porous UHMWPE was soaked in a simulated body fluid (1.0SBF) with higher pH than the physiological one and subsequently, its temperature was raised. By this treatment, the ApN were precipitated in the pores of the UHMWPE directly in the reaction solution. For both methods, the ApN‐deposited UHMWPE showed HAp formation ability not only on the top surface but also inside the pores near the surface of the porous UHMWPE in 1.0SBF although the adhesion strength of thus‐formed HAp layer was higher in the case of the EPD in comparison with the DP, oxygen plasma treatment before the DP enabled to achieve a similar level of the HAp layer adhesion to the EPD.Inspec keywords: calcium compounds, adhesives, porous materials, precipitation, plasma materials processing, pH, adhesion, polymers, phosphorus compounds, electrophoretic coatings, electrochemical electrodesOther keywords: hydroxyapatite formation ability, ultrahigh molecular weight polyethylene, apatite nuclei, electrophoretic deposition, EPD, porous UHMWPE, ApN‐dispersed ethanol, ApN‐deposited UHMWPE, HAp formation ability, oxygen plasma treatment, anodes, cathodes, electric field, direct precipitation, pH value, reaction solution, adhesion strength, HAp layer adhesion, Ca₁₀ (PO₄)₆ (OH)₂      

Plant-mediated biosynthesis of silver nanoparticles by leaf extracts of <Emphasis Type="Italic">Lasienthra africanum</Emphasis> and a study of the influence of kinetic parameters

Lasienthra africanum (LA) leaf extract was employed for nano-silver synthesis. The reducing effect of the plant extract was investigated at different times, pH, temperatures and concentrations. The effect of various kinetic parameters was studied using UV–vis spectroscopy. Blue-shifted surface plasmon bands indicating smaller sized nanoparticles were obtained at neutral pH (6.8–7.0), temperature of 65°C and concentration ratio of 1:10 (leaf extract: AgNO₃) with increasing reaction times under the reaction conditions. The kinetics of the reaction followed pseudo-first- and -second-order rate equations, and was thermodynamically favoured at higher time. Spherically shaped nanoparticles were obtained at different reaction conditions.     

Effect of temperature and time on solvothermal synthesis of tetragonal BaTiO<Subscript>3</Subscript>

Tetragonal BaTiO₃ nanoparticles are synthesized via solvothermal route in an ethanol water mixture. Ba(OH)₂·8H₂O is used as Ba precursor and TiO₂ (P25 Degussa ∼25 nm, 30% anatase, 70% rutile) is used as Ti precursor in the Ba : Ti molar ratio 2 : 1. Effect of temperature and time study on solvothermal synthesis of BaTiO₃ revealed that a moderate reaction temperature i.e. 185°C and longer reaction time favour tetragonal phase stabilization. Dissolution–precipitation appears to be the transformation mechanism for the crystallization of BaTiO₃ from particulate TiO₂ precursor.     

Preparation of the BaTiO3–SiO2 hybrid particles for the catalyst of methane steam reforming process

BaTiO₃ nano-coated SiO₂ (BaTiO₃–SiO₂) hybrid particles were prepared by liquid phase deposition and sol–gel process. The obtained BaTiO₃–SiO₂ hybrid particles have relatively high surface area (20 m² g⁻¹) at 600 °C annealing temperature. Ni component was impregnated to the obtained BaTiO₃–SiO₂ hybrid particles, and the obtained catalyst was used for the methane steam reforming process to consider the effect of the surface area on the catalytic activity. The catalytic activity of the Ni/BaTiO₃–SiO₂ catalyst was approximately three times as large as that of the reported Ni/BaTiO₃ catalyst, even in the lower process temperature. However, the limitation temperature for methane steam reforming process of this hybrid material was 600 °C, because of the diffusion of the Ba component.     

Preparation of barium titanate nanocube particles by solvothermal method and their characterization

Barium titanate (BaTiO₃) nanocube particles below 20 nm were prepared by solvothermal method. A selection of organic solvent and inorganic materials of Ba and Ti sources was most important for the preparation of nanocubes. A nucleation and particle growth of BaTiO₃ nanoparticles led to a formation of the BaTiO₃ nanocubes with a size of 10–15 nm at temperatures above 200 °C.     

Dielectric response of Ag/BaTiO<Subscript>3</Subscript>/epoxy nanocomposites

Dielectric properties and relaxation phenomena of hybrid material (functionalized nanosilver/BaTiO₃/epoxy) were studied as a function of ceramic content. Nanoparticles were obtained through chemical reduction in ethanol and triethylenetetramine. Epoxy resin, functionalized Ag and BaTiO₃ were mixed and composites were prepared onto glass substrates by dipping technique. Samples containing various amounts of ceramic filler were examined by thermal and SEM analysis. Dielectric measurements were performed at different frequencies and temperatures. It was found that hybrid materials had high permittivities and their relaxation processes were influenced by the epoxy resin near its T _g, while metallic and ceramic content modified the real permittivity values.     

Electrophoretic deposition of alkaline earth coatings from EDTA complexes

A novel process route combining electrophoretic deposition (EPD) and ethylene-diamine-tetra-acetic acid (EDTA) complexation has been developed and used to produce crack-free coatings of alkaline earth oxides with thickness up to 8 m after heat treatment at 700°C. Aqueous systems were found to be unsuitable for the EPD process due to their high conductivity and the effect of liberated gas evolved on the electrodes, and a non-aqueous based alternative formulation was developed based on methanol (solvent/carrier) and ethanediol (stabiliser) in conjunction with the EDTA complexes to allow the EPD process to be successfully used to deposit porous coatings. The coating process has been investigated by electrochemical impedance spectroscopy (EIS), and is thought to involve a combination of the migration of complexed ions in solution, their precipitation as charged colloidal particles in the vicinity of the anode, followed by their aggregation at, and adhesion to the anode surface. It is believed the process may have application for the low cost deposition of a wide range of porous coatings.     

Electrophoretic deposition of porous CaO–MgO–SiO<Subscript>2</Subscript> glass–ceramic coatings with B<Subscript>2</Subscript>O<Subscript>3</Subscript> as additive on Ti–6Al–4V alloy

The sub-micron glass–ceramic powders in CaO–MgO–SiO₂ system with 10 wt% B₂O₃ additive were synthesized by sol–gel process. Then bioactive porous CaO–MgO–SiO₂ glass–ceramic coatings on Ti–6Al–4V alloy substrates were fabricated using electrophoretic deposition (EPD) technique. After being calcined at 850°C, the above coatings with thickness of 10–150 μm were uniform and crack-free, possessing porous structure with sub-micron and micron size connected pores. Ethanol was employed as the most suitable solvent to prepare the suspension for EPD. The coating porous appearance and porosity distribution could be controlled by adjusting the suspension concentration, applied voltage and deposition time. The heat-treated coatings possessed high crystalline and was mainly composed of diopside, akermanite, merwinite, calcium silicate and calcium borate silicate. Bonelike apatite was formed on the coatings after 7 days of soaking in simulated body fluid (SBF). The bonding strength of the coatings was needed to be further improved.