Suspension characterization and electrophoretic deposition of Yttria-stabilized Zirconia nanoparticles on an iron-nickel based superalloy

Yttria-Stabilized Zirconia (YSZ) is the most common material for thermal barrier coatings. Suspensions of 3 mol% YSZ nanoparticles in acetone medium have been prepared in presence of different amounts of iodine as dispersant. Size distribution of particles in the suspensions and zeta potential were measured as a function of dispersant concentration. Adding 1.2 g/l iodine was found to be effective for the dispersion of YSZ nanoparticles in acetone. The stability of YSZ suspension in acetone increased with iodine content increasing until reached 1.2 g/l. Mean diameter of particles and zeta potential of the YSZ suspension in acetone were 912 nm and 2.4 mV respectively, and with addition of 1.2 g/l iodine shifted to 111.6 nm and 50.2 mV respectively. Electrophoretic deposition (EPD) process has been carried out from this suspension at different applied voltages and deposition times. A uniform green coating was obtained at voltage of 6 V and deposition time of 2 min the thickness of the green coating is measured about 25 µm.     

Electrophoretic Deposition of Y2O3-Stabilized ZrO2 Electrolyte Films in Solid Oxide Fuel Cells

An electrophoretic deposition (EPD) method was applied for the preparation of yttria-stabilized zirconia (YSZ) films for solid oxide fuel cell (SOFC) applications. Dense YSZ films with uniform thickness can be readily prepared with the EPD method by using acetylacetone or acetone as a solvent. The open-circuit voltages of SOFC, for which the YSZ films were prepared by the EPD method, increased with increasing repetitions of deposition and sintering. It was found that the open-circuit voltage exceeded 1.0 V after five repetitions. When the planar SOFC was fabricated using La_0.6Sr_0.4MnO₃ as a cathode, and electroless plating Pt as an anode, the open-circuit voltage and the maximum power density attained were 1.03 V and 1.84 W·cm⁻², respectively. Consequently, it became evident that the electrophoretic deposition was a suitable processing route for the formation of gas-tight YSZ films with thickness less than 10 μm.     

Nanomechanical behaviour of Ni – YSZ nanocomposite coatings on superalloy 690 as diffusion barrier coatings for nuclear applications

Ni-YSZ nanocomposite coatings with different Ni content in the range of 0–50 wt.% were developed on Inconel 690 substrates using an electron beam physical vapour deposition method to optimise the Ni concentration in order to enhance the durability of coatings for nuclear applications. X ray diffraction confirmed the formation of cubic phases of Ni and YSZ in the Ni-YSZ nanocomposite coatings. Increased addition of Ni was found to increase the crystallite size of Ni which resulted in lower strain. FESEM analysis of cross-sectional view of the compositionally graded Ni-YSZ coating with low concentration of Ni showed dense columnar structure and exhibited increased porosity along the columnar boundaries with higher concentration of Ni. FESEM and XRD analyses suggest that the grains of the columnar growth could be along (111) plane of the YSZ phase. The elemental composition of individual layers constituting the compositionally graded Ni-YSZ was confirmed by Energy Dispersive Spectroscopy analysis. The nanoindentation analysis of the as deposited coatings showed an increase in the hardness from 1.7 to 9.1 GPa, reduced Young’s modulus from 48 to 168 GPa, elastic recovery from 15.03% to 32.78% and resistance to plastic deformation from 0.0021 to 0.027 with the increased Ni content. Scratch test confirmed superior adhesion of the Ni-YSZ (50 wt.%: 50 wt.%) nanocomposite coating with the substrate. Also, investigation on the scratch track of Ni-YSZ (50 wt.%:50 wt.%) coating did not reveal chipping or spallation of the coating throughout the scratch track indicating a good adherence of coating with the substrate. The structural and the nanomechanical properties of Ni-YSZ (50 wt.%:50 wt.%) nanocomposite coating suggest that it could be used as diffusion barrier coating in the components of nuclear vitrification furnaces which are operated at higher temperatures.     

Tribological properties of YSZ and YSZ/Ni-YSZ nanocomposite coatings prepared by electron beam physical vapour deposition

Metal-ceramic nanocomposite coatings have been applied to many industrial applications owing to their remarkable properties such as wear, corrosion and high temperature oxidation resistance than that of metals and alloys in high temperature environments. In this study, YSZ and Ni-YSZ nanocomposite coatings deposited by electron beam physical vapour deposition (EBPVD) for high temperature environments have been investigated. Initially friction and wear behaviour of YSZ coatings deposited at various substrate temperature were studied. Then the effect on wear response of Ni-YSZ nanocomposites with different Ni content were investigated using a ball-on-disc micro tribometer. The structural and tribochemical changes that occurred in the wear tracks of YSZ and Ni-YSZ coatings were investigated using field emission scanning electron microscopy and Raman spectroscopy. The results obtained on sliding wear and friction behaviour of these nanocomposite coatings suggest that 50 wt.% of Ni in YSZ nanocomposite provides good wear resistance behaviour than that of other coatings. Such an improvement in tribomechanical and wear performance of the nanocomposite coating could be attributed to the optimum amount of Ni which promotes the formation of NiO from Ni due to the frictional heat between nanocomposite coating and the sliding counter body in wear track as confirmed by Raman analysis.     

Metal-ceramic diffusion barrier nanocomposite coatings on nickel based superalloys for corrosion and high temperature oxidation resistance

Ceramic diffusion barrier coatings are inevitable in high temperature aerospace and nuclear applications to protect superalloys from oxidation and hot corrosion. Compositionally graded coating (CGC) of Ni-YSZ with five layers was deposited on an Inconel-690 substrate through electron beam physical vapor deposition (EBPVD) method. After heat treatment, the phase formation and crystallite determination in each layer of the CGC were studied by X-ray diffraction. Pulsed radio frequency glow discharge optical emission spectroscopy (RF-GDOES) showed outward diffusion of Ni towards the surface of the CGC. High resolution transmission electron microscopy (HRTEM) studies of the cross-sectional region of the heat-treated coating at 1273 K revealed no secondary phases within the coating as well as at substrate-coating interface. The corrosion behavior of Ni-YSZ coating under 3 M HNO₃ medium showed that the heat-treated CGC of Ni-YSZ exhibited better corrosion resistance due to the formation of NiO than as-deposited Ni-YSZ coating.     

Suppression of Ni agglomeration in PLD fabricated Ni-YSZ composite for surface modification of SOFC anode

The suppression of Ni agglomeration in Ni-yttria stabilized zirconia (Ni-YSZ) nano-composite thin films deposited by pulsed laser deposition (PLD) has been investigated by varying post-annealing temperatures at a range of 800–1200 °C. Grain growth to a certain extent appears to be necessary to obtain a stable Ni-YSZ composite microstructure by suppressing massive Ni agglomeration. The microstructurally stable and uniform nano-porous Ni-YSZ thin film was obtained by 1200 °C post-annealing and reduction of the NiO–YSZ thin film, and it was applied as the surface modification layer of the bulk anode support used in conventional solid oxide fuel cells (SOFCs). By this approach, we were able to successfully realize a thin film electrolyte SOFC exhibiting the open cell voltage (OCV) higher than 1 V with a 1-μm thick film electrolyte on the porous anode support.     

Fabrication of wormhole-like YSZ and Ni-YSZ by the novel soft-hard template CTAB/NaCl-assisted route. Suppressing Ni coalescence in SOFC

A novel one-pot synthesis route leading to the formation of a wormhole-like structure was developed for the successful fabrication of porous YSZ and Ni-YSZ systems. This method involved co-precipitation in the presence of the micelle-forming agents CTAB/Pluronic P123 and crystallising NaCl. The obtained skeletons were mechanically stable and presented almost 50% uniform, open porosity without using any additional pore-formers. The fabricated 0.3 M CTAB/NaCl Ni-YSZ showed better long-term electrical stability in hydrogen than a traditional Ni-YSZ cermet. It resulted from the suppression of Ni structural changes throughout the anode scaffold. Moreover, higher electrochemical activity of this novel anode is expected due to the smaller particle sizes of Ni/YSZ, high homogeneity, highly developed TPB, and better interfacial interaction between the Ni and YSZ. Therefore, the novel soft-hard templating method is recognised as a promising route for the fabrication of the YSZ or Ni-YSZ with a highly developed microstructure and improved stability.     

Preparation of Yttria-Stabilized Zirconia Thin Films on Strontium-Doped LaMnO3 Cathode Substrates via Electrophoretic Deposition for Solid Oxide Fuel Cells

A yttria-stabilized zirconia (YSZ) thin film on an La_0.8Sr_0.2MnO₃ porous cathode substrate was prepared, using electrophoretic deposition (EPD) to fabricate a solid oxide fuel cell (SOFC). The electrical conductivity of an La_0.8Sr_0.2MnO₃ substrate is satisfactorily high at room temperature; therefore, YSZ powder could be deposited electrophoretically onto an La_0.8Sr_0.2MnO₃ substrate without any extra surface treatment, such as a metal coating. Successive repetition of EPD and sintering was required to obtain a film without gas leakage, because of the thermal expansion coefficient mismatch between the YSZ and the La_0.8Sr_0.2MnO₃ substrate. On the other hand, the electromotive force of the oxygen concentration in the cell that used YSZ film prepared via EPD increased and attained the theoretical value when the number of deposition and calcination cycles was increased. Six or more successive repetitions were required to obtain a YSZ film without gas leakage. A planar-type SOFC was fabricated, using nickel as the anode and YSZ film (∼10 μm thick) that had been deposited onto the La_0.8Sr_0.2MnO₃ substrate as the electrolyte and cathode. The cell exhibited an open circuit voltage of 1.0 V and a maximum power density of 1.5 W/cm². Thus, the EPD method could be used as a colloidal process to prepare YSZ thin-film electrolytes for SOFCs.     

Fabrication of ceramic membranes on porous ceramic supports by electrophoretic deposition

Abstract

Nanoporous alumina membrane and continuous zeolite L membrane were fabricated on the inner surface of microporous alumina tubes. In the former case, an electrophoretic deposition (EPD) technique was used for the deposition of bimodal alumina particles for the subsequent low temperature sintering. In the latter case, the EPD was used for the seeding process of zeolite L particles for the subsequent hydrothermal synthesis. A thin layer of polypyrrole was synthesised on the inside wall of the porous tubes by the chemical polymerisation of pyrrole to give the wall electric conduction for the EPD electrode. The thickness of the coating layers was controlled by altering the applied voltage and deposition time. The interfacial connection of the alumina or zeolite coated layer and the substrate was evaluated by SEM observations before and after the thermal treatment. The nanoporous structure of the alumina membrane was also characterised by a pore size analyser.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Fabrication of ceramic composite coatings using electrophoretic deposition,reaction bonding and low temperature sintering

We have developed a novel combination of electrophoretic deposition (EPD), reaction bonding and low temperature sintering techniques for the fabrication of yttria stablised zirconia (YSZ)/alumina composite coatings on Fecralloys. A mixture of ethanol and acetylacetone solvent was found to be an effective medium for YSZ and aluminium particle suspension. With the particle size of YSZ and aluminium being significantly reduced during ball milling. By using the EPD process, uniform green form coatings containing YSZ and aluminium particles were produced on Fecralloys. After oxidation of aluminium at 500°C and sintering at 1200°C, a dense and adherent YSZ/Al₂O₃ coating was produced. The presence of aluminium in the green form coatings not only contribute to the bonding between the coating and the metal substrate, but also compensate for the volume shrinkage of the coatings during sintering by the volume expansion arising from oxidation of aluminium to alumina.     

Kinetics of Electrophoretic Deposition for Nanocrystalline Zinc Oxide Coatings

An integrated process combining the preparation of ZnO nanoparticles and the formation of ZnO coatings using electrophoretic deposition (EPD) is reported. The work focuses on the deposition kinetics of nanocrystalline ZnO coatings on copper electrodes during EPD by direct measurement of the thickness of the deposited layer. The experimental results show that the EPD process is a powerful route to fabricate uniform coatings with desired thickness and excellent surface smoothness, which might be attributed to small particle size and narrow size distribution. On the other hand, the deposition kinetics changes with applied voltage and deposition time. The deposition thickness increases with increasing applied voltage and deposition time. In a short deposition time, the deviation of deposition rate between the theoretical and experimental values is caused by voltage drops during deposition, and the discrepancy increases with the applied voltage. Moreover, the increasing voltage drop and depletion of the suspension lead to decreasing current and lower deposition rate after longer deposition time. The critical transition time of deposition kinetics is found to exponentially decrease with increasing applied voltage.     

Effect of HCl on electrophoretic deposition of yttria stabilized zirconia particles in organic solvents

This study investigates the electrophoretic deposition (EPD) of YSZ particles onto a metal substrate from an organic solvent, the conductivity of which was manipulated by HCl additions. The green density is dependent on electrical conductivity and deposition time. It was found that a uniform coating with up to 67% relative green density could be produced after 10 min deposition from a 20 g/L suspension with electrical conductivity in the range of 10–15 μS/cm (0.5–0.7 mM HCl concentration). Direct measurements of the green YSZ coating density were supported by micro-indentation data using a spherical indenter.     

Model on thermal conductivity prediction of quasi-columnar structured coating by plasma spray physical vapor deposition

Firstly, the yttria-stabilized zirconia (YSZ) coating and gadolinium zirconate (GZO) coating with the quasi-columnar structure were manufactured by plasma spray physical vapor deposition. At the same time, a novel three-dimensional geometrical model was established that could satisfactorily reflect such quasi-columnar structural characteristics. Then, based on this model, the three-dimensional spatial distribution of pores and porosity of coatings and the thermal resistance behaviors of the quasi-columnar structured coating were analyzed. Later on, the thermodynamic model was established to estimate the thermal conductivity of the quasi-columnar structured coatings at different temperatures. Finally, a model for predicting the effective thermal conductivity of the GZO/YSZ double-layer coating with quasi-columnar structure was validated to account for the effects of the variable thickness ratios of GZO top layer to YSZ inner layer.     

Enhancement of mechanical properties of hydroxyapatite coating prepared by electrophoretic deposition method

The addition of bio-inert ceramics such as alumina and zirconia can significantly improve the mechanical properties of hydroxyapatite bioactive coatings and increase their biocompatibility. In the present study, the surface of a titanium substrate was coated by the electrophoretic deposition method (EPD). Moreover, the reaction bonding process has been used to precipitate the nanocomposite containing the hydroxyapatite (HA), alumina, yitteria-stabilized zirconia (YSZ). The coating process was performed by an electrical power supply and a suspension of hydroxyapatite, aluminum, and YSZ nanopowders. For preparing a suspension consisting of 50% isopropanol and 50% acetone, 0.6 g/L of iodine was used as a stabilizer. Green and sintered coatings were analyzed by FE-SEM and XRD. In addition, the mechanical properties such as bonding strength, hardness, and toughness were measured. The hardness, bonding strength, and toughness of the HA coating were 107 ± 10.3 HV, 10.8 ± 3.2MPa, and 0.72MPa√m, respectively, while those of the HA-Al₂O₃-YSZ nanocomposite coating were 213 ± 1.8 HV, 35 ± 1.6MPa, and 1.6MPa√m, respectively.     

Electrophoretic deposition of functionally-graded NiO–YSZ composite films

Functionally-graded NiO–8 mol % YSZ composite films were prepared by a controlled voltage-decay electophoretic deposition (EPD) process. The films consisted of three layers with varying NiO concentrations and porosities. Effects of different parameters including the type of the organic media, solid concentration, NiO:YSZ ratio, and iodine on the stability of EPD suspensions and deposition kinetics were studied. A stable NiO–YSZ suspension was attained in isopropanol with NiO–YSZ ratio of 60:40 and iodine concentration of 0.5 mM. The composite film contained varying NiO concentration from 46 wt.% near the substrate to 32 wt.% close to the electrolyte with 42 wt% NiO in the intermediate region. The thickness of each layer is about 10, 44 and 68 μm, respectively. The prepared anode could be promising for solid oxide full cells as it compromises good contact to the electrode with higher corrosion resistance and active reaction zone with the electrolyte.     

Fabrication of BSCF-based mixed oxide ionic-electronic conducting multi-layered membrane by sequential electrophoretic deposition process

The Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF)-based multi-layered oxygen separation membrane was fabricated by the sequential electrophoretic deposition (EPD) process. A thin porous/dense bi-layer of BSCF was formed on a thick porous support of BSCF. The porous support prepared by a sacrificial template method using BSCF powder mixed with wheat starch (30 wt%) as a pore-forming agent, followed by uniaxial pressing and low-temperature sintering, was directly used as an EPD electrode. A thin BSCF layer was first formed on the porous support, and then a thin BSCF + PMMA (polymethyl methacrylate) layer was sequentially formed on the thin BSCF layer using a bimodal suspension of BSCF and PMMA. A 30-μm thin porous/dense bi-layer of BSCF of which the total thickness was obtained by optimizing the processes of EPD and subsequent co-sintering. The oxygen separation performance of 3.7 ml (STP) min^?1 cm^?2 at 860 °C was achieved for the BSCF-based multi-layered oxygen separation membrane.     

Electrophoretic deposition studies of Ba(Zr-Ce-Y)O3 ceramic coating

Electrophoretic deposition (EPD) is now a well established colloidal processing technique which uses electrophoresis mechanism for the movement of suspended charged particles in a suspension in the presence of an electric field. In this work, electrophoretic deposition of BaZr_0.4Ce_0.4Y_0.2O_3-δ (BZCY) in ethanol medium was performed under different conditions on both conducting and non-conducting (porous anode) substrate without using any external additives in a suspension bath. Process parameters such as deposition time, voltage, and rate of deposition of suspended particles were studied under various conditions. Green coating deposited under different potential (30, 50, and 70V) was uniform and crack free, even at extended time of deposition. Surface roughnesses have also been evaluated to correlate it with deposition conditions. It is also found that the rate of deposition on conducting substrate was higher as compared to that on non-conducting substrate (anode). XRD studies of the calcined powder and coating exhibit an expected simple cubic perovskite structure. The deposition yield increases linearly with voltage for each deposition time for both conducting and non-conducting substrates. The coating on non-conducting porous anode heat treated at 1500°C for 2 hours was dense and well adherent to the anode substrate. A film thickness of about 13 μm was obtained at 70V. Such dense BZCY electrolyte coating on BZCY+NiO anode (Half cell) could be well utilized for fabrication of proton conducting SOFC single cell by applying suitable cathode layer on electrolyte film.     

Electrophoretic deposition of ethanol steam-reforming catalysts on metal plates for the development of catalytic-wall reactors

A procedure based on electrophoretic deposition (EPD) was developed to coat metal plates with powder catalysts. The method was tested on stainless-steel plates with three Ni-based catalysts for the steam reforming of ethanol. The catalysts (Ni/La₂O₃/γ-Al₂O₃) contained 15% Ni and 8% La, and were prepared using three types of γ-alumina with different textural properties. The powder catalysts were suspended in isopropanol, and EPD deposition was performed with a voltage of 100 V and a distance between electrodes of 2 cm. Deposition time was varied between 3 and 7 min, which gave a thickness of the catalyst layer from around 30 to 100 μm. The morphology of the catalyst layer was dependent on the textural characteristics of the γ-Al₂O₃ used to prepare the catalyst. The activity of the catalyst plates was tested at 773 K using a steam to carbon molar ratio of 4. Significant differences in the selectivity towards ethanol dehydrogenation, reforming, and dehydration to ethylene could be observed between the three catalysts. Carbon deposition on the surface of the plates could be easily determined by SEM/ESEM.     

Formation of MUA (mercaptoundeconic acid)-capped CDSe nanoparticle films by electrophoretic deposition

Electrophoretic deposition (EPD) has gained increasing interest for the deposition of materials such as TiO₂, carbon nanotubes and trioctylphosphine oxide (TOPO)-capped CdSe nanoparticles. In this study, a mercaptoundecanoic acid (MUA) CdSe nanoparticle film was formed by electrophoretic deposition. A colloidal suspension of TOPO-capped CdSe nanoparticles was prepared by the hot injection method, followed by ligand exchange to produce MUA-capped CdSe nanoparticles. As-prepared MUA-capped CdSe nanoparticles were washed using ethyl acetate and ethyl ether. Then, the washed nanoparticles were resuspended in ethanol and immediately used for EPD. A CdSe nanoparticle film measuring 2.75 µm in thickness was deposited at an applied voltage of 5 V and deposition time of 5 min.