Microstructure and thermal properties of nanostructured gadolinia doped yttria-stabilized zirconia thermal barrier coatings produced by air plasma spraying

In this article, the nanostructured 2 mol% Gd₂O₃-4.5 mol% Y₂O₃-ZrO₂(2GdYSZ) coating was developed by the atmospheric plasma spraying technique. And the microstructure and thermal properties of plasma-sprayed 2GdYSZ coating were investigated. The result from the investigation indicates that the as-sprayed coating is characterized by typical microstructure consisting of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks. The 2GdYSZ coating shows a lower resistance to destabilization of the metastable tetragonal (t′) phase compared to the yttria stabilized zirconia(YSZ). The thermal diffusivity and thermal conductivity of the nano-2GdYSZ coating at room temperature are 0.431 mm² s⁻¹ and 1.042 W/m K, respectively. Addition of gadolinia to the nano-YSZ can significantly reduce the thermal conductivity compared to the nano-YSZ and the conventional YSZ. The reduction is mainly attributed to the synergetic effect of gadolinia doping along with nanostructure.     

Thermal conductivity of yttria-stabilized zirconia thin films grown by plasma-enhanced atomic layer deposition

Zirconia doped with yttrium, widely known as yttria-stabilized zirconia (YSZ), has found recent applications in advanced electronic and energy devices, particularly when deposited in thin film form by atomic layer deposition (ALD). Although ample studies reported the thermal conductivity of YSZ films and coatings, these data were typically limited to Y₂O₃ concentrations around 8 mol% and thicknesses greater than 1 μm, which were primarily targeted for thermal barrier coating applications. Here, we present the first experimental report of the thermal conductivity of YSZ thin films (∼50 nm), deposited by plasma-enhanced ALD (PEALD), with variable Y₂O₃ content (0–36.9 mol%). Time-domain thermoreflectance measures the effective thermal conductivity of the film and its interfaces, independently confirmed with frequency-domain thermoreflectance. The effective thermal conductivity decreases from 1.85 to 1.22 W m⁻¹ K⁻¹ with increasing Y₂O₃ doping concentration from 0 to 7.7 mol%, predominantly due to increased phonon scattering by oxygen vacancies, and exhibits relatively weak concentration dependence above 7.7 mol%. The effective thermal conductivities of our PEALD YSZ films are higher by ∼15%–128% than those reported previously for thermal ALD YSZ films with similar composition. We attribute this to the relatively larger grain sizes (∼23–27 nm) of our films.     

Synthesis gas production in methane conversion using the P|yttria-stabilized zirconia|Ag electrochemical membrane system

The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H₂). The Pd|YSZ|Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H₂ production strongly depended on the oxidation state of the Pd anode surface. Namely, the H₂ treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H₂ production, while air treatment enhanced the H₂ production rate. From the results of the partial oxidation of CH₄ with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH₄ to synthesis gas was the Pd–YSZ–gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdO_ad, for the selective oxidation of CH₄ to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH₄ oxidation with the adsorbed oxygen. The difference with respect to the H₂ formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical compatibility of rare earth apatite with yttria-stabilized zirconia

The chemical compatibility of a series of rare earth apatite (RE-apatite), with Y₂O₃-stabilized ZrO₂ (YSZ) has been investigated. Three types of RE-apatite powders with different ionic radius (RE = Gd, Nd and La) were prepared, and bulks prepared from the powder mixtures of RE-apatite and YSZ were heat-treated at 1300 °C up to 100 h in this study. It was found that Gd-apatite reacted with YSZ and formed a reaction layer (Gd₂Si₂O₇) at the Gd-apatite/YSZ interface. Meanwhile, the intense Gd³⁺ diffusion resulted in the formation of Gd solid solutions in YSZ and much YSZ phase transformation. In contrary, as for Nd- or La-apatite/YSZ composite, which has larger ionic radius, no reaction product was observed at interface and there was less RE diffusion into YSZ as well as YSZ transformation. These results clearly indicated that large ionic radius of RE³⁺ could enhance the chemical compatibility of RE-apatite with YSZ.     

Production of Dense Yttria-Stabilized Zirconia Thin Films by Dip-Coating for IT-SOFC Application

Yttria-stabilized zirconia (YSZ) thin films were prepared by conventional and modified dip-coating techniques followed by heating to an appropriate temperature in air. Scanning electron microscopy showed that films of the thickness ranging from 20 to 30 m were dense and crack-free. The electrical properties of the films were investigated by ac impedance spectroscopy. La_0.8Sr_0.2MnO₃ paste was printed on to a YSZ electrolyte/anode assembly to create single fuel cells which were tested in the temperature range 650–800 °C. The results showed that both open circuit voltage (OCV) and maximum power density values of the cells with electrolytes produced by the modified dip coating were higher than those fabricated by conventional processing. At 800 °C, the OCV reached 0.98 V and a maximum power density of 190 mW cm^–2 was attained, demonstrating that the modified dip coating process is a simple and cost-effective fabrication technique for IT-SOFCs, though further improvement is necessary.     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

Fabrication of YSZ electrolyte using electrostatic spray deposition (ESD):I – a comprehensive parametric study

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer (3 m thickness) yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. Reducing the thickness of a state-of-the-art electrolyte, and thereby reducing the cell internal IR drop, is a promising strategy to make the intermediate temperature SOFC (ITSOFC) operating at 600–800 °C possible. About 8 mol% YSZ colloidal solution in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 17–6 h, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests on the cell were carried out at 800 °C to evaluate the electrolyte layer formed by ESD. With a 97 H₂/3 H₂O mixture and air as fuel and oxidant gas, respectively, open circuit voltage (OCV) was found to be close to the theoretical value.     

Optical properties of gadolinia-doped cubic yttria stabilized zirconia single crystals

Cubic zirconia single crystals stabilized with yttria and doped with Gd₂O₃ (0.10–5.00 mol%) were prepared by the optical floating zone method, and characterized by a combination of X-ray diffraction (XRD), and Raman, electron paramagnetic resonance (EPR), ultraviolet–visible (UV–Vis), photoluminescence excitation (PLE) and photoluminescence (PL) spectroscopic techniques. XRD and Raman spectroscopy showed that the crystal samples were all in the cubic phase, whereas the ceramic sample consisted of a mixture of monoclinic and cubic phases. The absorption spectrum showed four peaks at 245, 273, 308, and 314 nm in the ultraviolet region, and the optical band gap differed between samples with ≤3.00 mol% and those with >3.00 mol% Gd₂O₃. The emission spectrum showed a weak peak at 308 nm and a strong peak at 314 nm, which are attributed to the ⁶P_5/2 → ⁸S_7/2 and ⁶P_7/2 → ⁸S_7/2 transitions of Gd³⁺, respectively. The intensities of the peaks in the excitation and emission spectra increased with Gd³⁺ concentration, reached a maximum at 2.00 mol%, then decreased with higher concentrations. This quenching is considered to be the result of the electric dipole-dipole interactions, and this interpretation is supported by the Gd³⁺ EPR spectra, which showed progressive broadening with increasing Gd³⁺ concentration throughout the concentration range investigated.     

Improvements to the sintering of yttria-stabilized zirconia by the addition of Ni

We have studied the effect of nickel oxide (NiO) on the sintering of yttria-stabilized zirconia (YSZ) at temperatures from 1100 to 1400 °C. Differences in the densification behaviour were observed between the direct use of NiO powders and Ni metal as precursor. Our results show that with the addition of Ni into YSZ, sintering was completed at 1300 °C instead of 1400 °C, a 100 °C reduction. The addition of Ni also increased the shrinkage rate at 1200 °C from −0.29×10⁻⁶ s⁻¹ to −0.46×10⁻⁶ s⁻¹. Young's modulus of the samples heat treated at 1200 °C measured by microindentation also increased from 26 GPa for YSZ to 65 or 191 GPa for YSZ plus NiO or Ni, respectively. Addition of NiO or Ni also stabilised the cubic phase and promoted grain growth in YSZ during sintering.     

Tailoring the properties of yttria-stabilized zirconia powders prepared by the sol-gel method for potential use in solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) powders have been prepared by the sol-gel method, following two alternative procedures: a series of powders was obtained by drying the sol-gel solutions in air at 100 °C until dry residue, and another series of powders was obtained by scratching the thin films deposited on cylindrical wide flat glassy surfaces after evaporating to dryness in air at 100 °C for 2 h. Samples were characterized by Scanning Electron Microscopy (SEM), nitrogen adsorption at −196 °C and Fourier Transform Infrared (FT-IR) spectroscopy. In general, a noticeable contraction of the pores is observed as the molecular size of the alcohols used grows. Powders prepared by conventional drying of sol-gel solutions at 100 °C exhibit remarkably high values of specific surface area (up to 148 m² g^− 1). On the contrary, samples prepared by scratching of the deposited thin films show a noticeable decrease in their specific surface area. Values of fractal dimension follow the same trend and indicate that, in general, the texture of the samples is mainly microporous for the first series of samples and more ordered for the second one. Finally, in order to investigate the effect of the calcination temperature on the morphological and textural properties of 3 mol% yttria-stabilized zirconia powders, once the 3YSZ powders were dried at 100 °C they were subjected to calcination at different temperatures. The experimental results suggest that the removal of residual water and alcohol occluded within the powder particles as well as the elimination of gases produced during the calcination stage play a very important role in the development of the porosity and surface area of the samples.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Bismuth oxide-coated (La,Sr)MnO3 cathodes for intermediate temperature solid oxide fuel cells with yttria-stabilized zirconia electrolytes

Taking Y₂O₃ stabilized Bi₂O₃ (YSB) as an example, bismuth oxide-added (La,Sr)MnO₃ (LSM) is evaluated as a cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) with 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) electrolytes. YSB was added to LSM cathodes using an impregnation method, dramatically improving the electrode performance. The interfacial polarization resistance R_p, at 700 °C for the electrode coated with 50 wt.% of YSB is 0.14 Ω cm², which is only 0.2% of the value for a pure LSM electrode. The high oxygen ionic conductivity and the catalytic activity of YSB, as well as the favorable electrode microstructure are likely reasons for the dramatic reduction of R_p. The YSB-added LSM cathodes also exhibited lower overpotential and higher exchange current density than the pure LSM cathode. Moreover, these electrodes show much lower R_p than that of parallel-fabricated LSM electrodes with samaria-doped-CeO₂ as well as other LSM-based electrodes reported in the literature, demonstrating the superiority of the of YSB as the ionic conduction component in composite LSM electrodes. The superior performance of the single cell further demonstrates that the bismuth oxide-added LSM cathode is an excellent candidate for IT-SOFCs.     

Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with different columnar morphologies by varying the spray distance. Although similar quasi-columnar structures were formed at the spray distances of 600 mm and 1400 mm, the formation mechanisms of particles in the coatings were different. Besides, an electron beam physical vapor deposition (EB-PVD) like columnar coating out of pure vapor was deposited at a spray distance of 1000 mm and the columnar consisted of elongated nano-sized secondary columns. The hardness and Young׳s modulus of the coatings were investigated. Compared to the other two quasi-columnar structures, the EB-PVD like columnar coating exhibited higher hardness (~9.0 GPa ) and Young׳s modulus (~110.9 GPa), mainly due to its low porosity and defect.     

A pH sensor based on the TiO2 nanotube array modified Ti electrode

In this paper, a novel solid state pH sensor was fabricated by anodization of titanium substrate electrode. The relationship between pH sensitivity and hydrophilicity or surface morphology of TiO₂ film was investigated. Amorphous TiO₂ nanotube has better pH response than anatase TiO₂ nanotube. After being irradiated by ultraviolet light (UV), the potential response of the electrode modified by amorphous TiO₂ nanotube was close to Nernst equation (59 mV/pH). SEM, XRD, and XPS were used to characterize electrodes. Possible mechanism was discussed by analyzing surface hydroxyl groups, crystal structure and hydrophilicity of the electrodes. The electrode has been used to detect some kinds of soft drinks and shows good response.     

Luminescence performance of yttrium-stabilized zirconia ceramics doped with Eu3+ ions fabricated by Spark Plasma Sintering technique

Yttrium stabilized zirconia (YSZ) ceramics doped with Eu³⁺ ions have been successfully fabricated by Spark Plasma Sintering (SPS) technique. The influence of the europium concentration and post-annealing process on the structural, optical, and luminescent properties of the ceramics has been studied. It is shown that an increase in the europium concentration from 0.1 to 3 wt% does not lead to significant changes in the transmission spectra. However, annealing in air atmosphere at temperature from 700 °C to 1300 °C significantly affects the transmission spectrum, as a possible consequenceofthe formation of oxygen vacancy defects. The analysis of the photoexcitation and photoluminescence spectra showed that the main excitation bands are determined by direct excitation of the ⁷F₀ ground state of Eu³⁺ions to the higher 4f energy levels with further radiation transitionsfrom these states. Moreover, the europium ion in the obtained ceramics occupy low-symmetry sites without inversion center.The luminescence decay kineticsare described by a doubleexponential function with decay time τ₁ ~ 20 ns and τ₂~ 90 ns for intrinsic emission centers and millisecond (τ ~ 1.4 ms) for Eu³⁺emission, for all investigated ceramics. The luminescence spectra in nanosecond time region are characteristic for yttrium-stabilized zirconia and are caused by oxygen vacancies in the presence of heavy cations (Y³⁺ and Eu³⁺).     

Surface modification of p-Si by a polyethylenimine coating: influence of the surface pre-treatment. Application to a potentiometric transducer as pH sensor

p-Si electrodes coated with linear polyethylenimine (L-PEI) allow the fabrication of a pH sensitive film for potentiometric transducers. The coating is realized in one step through the anodic oxidation of pure ethylenediamine (EDA) charged with 0.1 M LiCF₃SO₃ (Lithium Triflate). Such an electrochemical procedure leads to the thickness control of the coating. The best silicon surface pre-treatment before any coating is obtained with potassium dichromate in sulfuric acid, which leads to OH-terminated p-Si. This pre-treatment allows a uniform thin coating. In this work, the thickness is 2.6 nm. The pH response is high and close to 50 mV per pH unit.     

Analytical modelling of monolayer graphene-based ion-sensitive FET to pH changes

Graphene has attracted great interest because of unique properties such as high sensitivity, high mobility, and biocompatibility. It is also known as a superior candidate for pH sensing. Graphene-based ion-sensitive field-effect transistor (ISFET) is currently getting much attention as a novel material with organic nature and ionic liquid gate that is intrinsically sensitive to pH changes. pH is an important factor in enzyme stabilities which can affect the enzymatic reaction and broaden the number of enzyme applications. More accurate and consistent results of enzymes must be optimized to realize their full potential as catalysts accordingly. In this paper, a monolayer graphene-based ISFET pH sensor is studied by simulating its electrical measurement of buffer solutions for different pH values. Electrical detection model of each pH value is suggested by conductance modelling of monolayer graphene. Hydrogen ion (H⁺) concentration as a function of carrier concentration is proposed, and the control parameter (Ƥ) is defined based on the electro-active ions absorbed by the surface of the graphene with different pH values. Finally, the proposed new analytical model is compared with experimental data and shows good overall agreement.     

Comparing the deposition mechanisms in suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) deposition of yttria-stabilised zirconia (YSZ)

Thermal spraying using liquid feedstock has emerged as a promising technology for the deposition of finely structured ceramic coatings. In order to provide a comparative assessment of the deposition mechanisms occurring when spraying suspension or solution feedstock, suspensions of 300 nm-sized ZrO₂–4.5 mol.% Y₂O₃ particles dispersed in water and in ethanol and solutions of zirconium and yttrium salts, corresponding to ZrO₂–4.5 mol.% Y₂O₃ and ZrO₂–8 mol.% Y₂O₃ stoichiometries, were processed by plasma spraying using different parameter settings. In-flight diagnostics of sprayed droplets, together with the morphological, microstructural and phase analysis of individual lamellae collected onto polished substrates, performed by SEM, FIB, AFM and micro-Raman spectroscopy, led to the identification of deposition mechanisms, which were subsequently verified through the characterisation of complete coating layers.     

Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin‐layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flow‐induced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified Hoffman–Lauritzen equation. Injection‐molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 502–523, 2005     

Advancing the fabrication of YSZ-inverse photonic glasses for broadband omnidirectional reflector films

A single-step and all-colloidal deposition method to fabricate yttrium-stabilized zirconia (YSZ)-inverse photonic glasses with 3 μm pores was developed. The process is based on electrostatic attraction and repulsion in suspension, controlled by surface charge of polystyrene (PS) microspheres and YSZ nanoparticles, used as pore templates and matrix material, respectively. The pH was used as a tool to change surface charges and particle-particle interactions. Photonic glass films with 3 μm pores yielded broadband omnidirectional reflection over the wavelengths of 1–5 μm, relevant for thermal radiation at temperatures around 1200 °C. These highly porous materials maintained their structural stability and reflectance after being annealed at 1200 °C for 120 h.