Yttria-stabilized Zirconia Films Research Articles

Improved Reversibility of Thin-Film Solid Oxide Cells at 500 °C by Tailoring Sputtering Processes for Depositing Yttria-Stabilized Zirconia Electrolyte.

The present study investigates the impact of sputtering configurations on the microstructure and crystallinity of thin-film yttria-stabilized zirconia electrolytes for anodized aluminum oxide-supported all-sputtered thin-film reversible solid oxide cells. Employing various sputtering parameters, such as target-substrate distance and substrate rotation speed, the present study reveals distinct surface characteristics and crystalline structures of thin-film yttria-stabilized zirconia. The microstructure analysis includes scanning electron microscopy and atomic force microscopy examinations, uncovering the influence of the process parameters on the surface morphology, roughness, and grain size. X-ray diffraction data illustrate the texture preferences and crystallite characteristics. The electrochemical characterization of the reversible solid oxide cells demonstrates that the optimized sputtering configuration significantly outperforms the others in both SOFC and SOEC modes, showing exceptional current densities of 964 mA/cm2 at 1.3 V in electrolysis mode at 500 °C. Electrochemical impedance spectroscopy further reveals improved charge transfer reactions at the interface of the electrolyte. The enhanced electrochemical performance is attributed to the unique microstructure and crystallinity of the thin film of yttria-stabilized zirconia. The record-breaking electrolysis performance of this work at 500 °C underscores the potential of tailored sputtering parameters in optimizing the reversible solid oxide cell performance.

Highly crystalline yttria‐stabilized zirconia and titania thin films on plastic substrates via sol‐gel transfer process

AbstractWe prepared yttria‐stabilized zirconia (YSZ) and titania crystalline thin films on polycarbonate (PC) substrates by the sol‐gel transfer technique that we have developed recently. Precursor gel films were deposited by spin‐coating on a polyimide/polyvinylpyrrolidone mixture layer on Si(100) substrates, and then were calcined at 600°C. The spin‐coating and the calcination were cycled, followed by final firing at 600–1000°C, to obtain 140–190 nm thick, YSZ and titania crystalline thin films. The film densification progressed as the final firing temperature increased, where the porosity calculated from the refractive index decreased down to a few percent. The film thus obtained was heated at 190°C on a hot plate, and a PC substrate was pressed on the film to transfer it to the PC substrate. The transferability quantified by image analysis of the film area was found to be significantly different between the YSZ and titania films. In the case of YSZ films, the area of silicon that was fractured and adhered to the transferred film increased significantly with increasing firing temperature, leading to a decrease down to 0% in the fraction of the successful transfer area without such a damage. On the other hand, the fraction of the successful transfer area remained around 90% irrespective of the firing temperature for the titania films. The surface of the titania films that were fired at 800–1000°C and transferred to PC substrates had voids of several tens nm scale, which may have been formed during anatase‐to‐rutile phase transformation. We thought that the reduced titania/Si(100) contact area due to such void formation facilitated the delamination of the titania films from Si(100) substrates. High temperature firing, which promotes crystallization and densification of films, thus affects differently the film transferability to plastic substrates, depending on the type of films. The work also suggested possible use of titania films as release‐assisting layers.

The Effect of Deposition Temperature on Structural, Morphological, and Dielectric Properties of Yttria-Doped Zirconia Thin Films

The aim of this study was to investigate the effect of deposition temperature on the structural, optical, morphological, and dielectric properties of yttria-stabilised zirconia (YSZ) films prepared by sol-gel spin-coating method. X-ray diffraction (XRD) measurements of YSZ films showed that the peaks of the cubic phase were prominent and the peak intensities increased with deposition temperature. The crystallite size, dislocation density, and microstrain of the thin films were identified by XRD. It was observed that the crystal size of the YSZ thin films increased from 16 nm to 22 nm with the deposition temperature. The surface roughness of the thin films was found to have changed as revealed by Atomic Force Microscopy (AFM) measurements. The roughness increased from 7.72 nm to 11.92 nm with increasing temperature. The optical transmittance of the YSZ thin films was investigated in the wavelength range 200-900 nm and was found to increase slightly with increasing deposition temperature. Metal-Oxide-Semiconductor (MOS) devices were fabricated from these YSZ materials for dielectric characterization. The dielectric properties of the Ag/YSZ/n-Si MOS structure were investigated. It was found that the capacitance, conductivity and other dielectric parameters of these structures are strongly frequency dependent.

Mixed-Potential Ammonia Sensor Based on a Dense Yttria-Stabilized Zirconia Film Manufactured at Room Temperature by Powder Aerosol Deposition

Powder aerosol deposition (often abbreviated as PAD, PADM, or ADM) is a coating method used to obtain dense ceramic films at room temperature. The suitability of this method to obtain ammonia mixed-potential sensors based on an yttria-stabilized zirconia (YSZ) electrolyte that is manufactured using PAD and a V2O5–WO3–TiO2 (VWT)-covered electrode is investigated in this study. The sensor characteristics are compared with data from sensors with screen-printed YSZ solid electrolytes. The PAD sensors outperform those in terms of sensitivity with 117 mV/decade NH3 compared to 88 mV/decade. A variation in the sensor temperature shows that the NH3 sensitivity strongly depends on the sensor temperature and decreases with higher sensor temperature. Above 560 °C, the characteristic curve shifts from exponential to linear dependency. Variations in the water and the oxygen content in the base gas (usually 10% oxygen, 2% water vapor in nitrogen) reveal a strong dependence of the characteristic curve on the oxygen content. Water vapor concentration variations barely affect the sensor signal.

Fabrication of YSZ ceramic thin films with sol-gel method for mixed potential-type zirconia-based NO2 sensor

In this work, yttria-stabilized zirconia (YSZ) thin films were prepared by sol-gel and spin-coating methods, which are simple and inexpensive, with good film quality and excellent electrical conductivity. Using this film, a YSZ film mixed-potential type NO2 gas sensor with NiO as the sensitive electrode material was fabricated. The films had the best dielectric properties when spin-coated four times, and the sensors had the highest response (48 mV–100 ppm NO2) as well as the best sensitivity (14.8 mV/decade for 5–20 ppm NO2 and 42 mV/decade for 20–500 ppm NO2). The YSZ thin-film gas sensor operates at a lower temperature, with an optimum operating temperature of 550 °C, and the power consumption of the sensor has been reduced by approximately 46 % or more compared to conventional bulk YSZ NO2 gas sensors. This provides a very effective direction for the low-power design of high-temperature devices such as the YSZ gas sensor. In addition, thin-film devices are more conducive to integrating sensors into microelectronic circuits, greatly improving the application scenarios for such devices. This sensor has excellent repeatability, very good selectivity, and outstanding long-term stability over 30 days of continuous measurement. In conclusion, based on the excellent sensing performance and low power consumption of the YSZ thin-film gas sensor, the sensor has a promising future for in-situ monitoring of NOx.

(Digital Presentation) Improvement of the Compatibility between the Current-Independent Constant Voltage Loss in Wagner’s Equation for SOFCs Using Sm-Doped Ceria Electrolytes and Jarzynski’s Equality

A solid oxide fuel cell (SOFC) converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. Yttria-stabilized zirconia (YSZ) films are often used as electrolytes, in which only oxygen ions are carriers. In such cases, the open-circuit voltage (OCV = 1.15 V at 1073 K) is equal to the Nernst voltage (Vth = 1.15 V at 1073 K).Samaria-doped ceria (SDC) has higher ionic conductivity than YSZ. Therefore, SDC films are possible electrolyte candidates. However, when SDC electrolytes are used in SOFCs, the OCV at 1073 K is only 0.80 V. The voltage loss has been explained by Wagner’s equation.Numerous subsequent models have been created based on Wagner’s equation. The current-voltage relationship can be calculated with the cathode and anode polarization voltage losses using the Riess model [1]. Furthermore, Duncan and Wachsman created a nonlinear model [2]. This model can also be used to explain the equilibration process. According to the Riess model, the OCV should decrease during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. The change in the equilibration of thick SDC electrolytes in response to a change in the anode gas has never been explained using the model of Duncan and Wachsman. Experimentally, when a very thick (6.6 mm) SDC electrolyte is used, the OCV can reach 0.80 V in only 5 minutes [4]. According to Weppner, the corresponding delay in the electron diffusion current should be more than 2080 minutes [5].We proposed a current-independent anode voltage loss (0.35 V = 1.15 V- 0.80 V) [6]. Since we disproved the existence of large leakage currents in SDC electrolytes, our explanations seem to disregard and disprove Wagner’s equation. However, we noted that the dismissal of the idea of large leakage currents in SDC electrolytes is only a side effect. When SDC electrolytes experience a large anode voltage loss (0.35 V), the leakage currents are very small. This phenomenon is called the “anode shielding effect” [7].The ionic activation energy (Ea) of SDC electrolytes is 0.7 eV (= 0.35 V ×2e). During ion hopping processes in SDC electrolytes, the work done by ions on the surrounding lattice structure is 0.7 eV. The ions should regain 0.7 eV after hopping. However, when there are many electrons in the hopping path, the hopping processes are very complex, and calculating the work loss is challenging [8].Jarzynski’s equality is very useful for calculating the Gibbs energy loss at the nanoscale. One ion hopping process in SDC electrolytes is at the nanoscale. However, the number of oxygen ions in SDC electrolytes is on the order of 1021, which is near Avogadro’s number. The number of oxygen ions that can participate in the hopping process is on the order of 1016 at 1073 K. Furthermore, the average frequency of the oxygen ions is 1013 Hz. Consequently, we are discussing an average Gibbs energy loss on the order of 1029 in one second. This means that our model is much more statistical as follows. Jarzynski’s equality ⊇ Our model (OCV = Vth -Ea /2e) (1) Fortunately, the canonical ensemble for the ion hopping process follows Boltzmann's distribution, which is an exponential distribution and determined only by the temperature of the environment. We try to improve the compatibility between the current-independent constant voltage loss in Wagner’s equation for SOFCs using SDC electrolytes and Jarzynski’s equality.

XPS investigation of monoatomic and cluster argon sputtering of zirconium dioxide

The surfaces of zirconium dioxide and yttria-stabilized zirconia (YSZ) have been analyzed using x-ray photoelectron spectroscopy after ion sputtering with monoatomic Ar+ or an argon gas cluster ion beam (GCIB). The O/Z ratio and new components in the Zr 3d lines show reduction to lower oxidation states when sputtered with monoatomic Ar+, but significantly less damage is observed when GCIB sputtering is used. The damaged surface layer caused by Ar+ sputtering can be removed by subsequent GCIB sputtering. However, the depth resolution observed in depth profiles of thin YSZ films was significantly better when Ar+ sputtering is used. Differences in the Sn content in the oxidized Zr-4 specimen were also observed when comparing Ar+ and GCIB sputtering, suggesting preferential sputtering.

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

AbstractZirconia 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.

Effect of calcination temperature on sensing performance of YSZ based electrochemical H2S gas sensor with a NiFe2O4 electrode

Electrochemical gas sensors based on yttria-stabilized zirconia (YSZ) film have attracted ascending attention for detecting H2S. Due to the poor sinterability of YSZ film, high-temperature calcination is usually required in the preparation procedure to ensure the good contact between YSZ and sensing electrode, which could particularly limit the triple phase boundary (TPB) length at the interface of H2S|YSZ|sensing electrode as well as the porosity of sensing electrode. Therefore, the calcination temperature would significantly affect the sensing performance, which has been ignored in reported studies. In this work, YSZ based electrochemical gas sensors with NiFe2O4 (NFO) sensing electrode are prepared to detect H2S at ppb-level. The effect of the calcination temperature of NFO sensing electrode on the sensing performance is investigated. The sensor with an optimal calcination temperature of 1000 °C delivers a response value of − 42 mV for 500 ppb H2S at 460 °C with a detection limitation of 10 ppb H2S. Good linear slopes with a sensitivity of − 13.1 mV/decade toward 20–100 ppb H2S and a sensitivity of − 46.9 mV/decade toward 100–500 ppb H2S are obtained. Meanwhile, the sensor also displays attracting stability and short response/recovery time. The good sensing performance is attributed to enhanced TPB length, good porosity of NFO electrode and limited interaction between the NFO and YSZ film by optimizing the calcination temperature of NFO sensing electrode.

Nanosecond laser irradiation of Yttria stabilized Zirconia: Pulsed laser ablation and surface treatment

Nanosecond pulsed laser irradiation resulting in material ablation and surface micro structuring of Yttria Stabilized Zirconia (YSZ) targets, has been investigated. Our study aims to evaluate suitable ablation conditions enabling either, creation of desired YSZ surfaces having micro porosity for directed use, or generation of YSZ vapors for deposition of homogenous, dense and particle free YSZ films via conventional Pulsed Laser Deposition (PLD) technique. The work being reported here involves a thermal model based theoretical simulation describing heat transport, melting and vaporization of a laser irradiated YSZ target. Our simulation has been validated against our experimental data on nanosecond laser ablation from a sample target of ceramic YSZ. Surface porosity generated on laser irradiated YSZ targets for low laser fluence levels ∼ 1 ​J/cm2 has been explained in terms of pore formation via entrapment of gas bubbles in the solidifying interface on laser melting and re-solidification thereafter, of the YSZ target. To understand surface crater formation on laser irradiated YSZ targets at high laser fluence levels ∼10 ​J/cm2 maximum temperature reached by the laser irradiated YSZ target has been compared with the thermodynamic critical temperature of YSZ. Our study confirms that judicious choice of laser fluence enables either, creation of surfaces having micro porosity of desired nature on YSZ target, or controlled generation of YSZ vapors for deposition of good quality YSZ films and coatings using laser deposition technique.

Epitaxial growth of high-quality yttria-stabilized zirconia films with uniform thickness on silicon by the combination of PLD and RF sputtering

High-quality (100)-oriented yttria-stabilized zirconia (YSZ) films with uniform thickness were epitaxially grown on silicon substrates by a two-step process of pulsed laser deposition (PLD) and radio-frequency magnetron sputtering (RFS). First, we fabricated a YSZ-seed layer on (001)-oriented silicon substrates using a PLD apparatus, and then transferred the YSZ-seed substrate to an RFS equipment to complete the growth of a thicker YSZ film. The dependence of YSZ crystal quality on RFS-growth temperature was investigated in the range of 200 to 850 °C. All the resulting YSZ films exhibited pure (100) orientation, and the full width at half maximum of the YSZ (200) X-ray rocking curve was as low as 0.64° for the YSZ film grown at 850 °C, and increased to 1.02° with the RFS-growth temperature declining to 200 °C. The results evidenced that the YSZ film can be epitaxially grown at 200 °C in the presence of a YSZ-seed layer. However, the splitting of the YSZ (200) X-ray diffraction peak appeared at 500 °C and 200 °C, which may be attributed to a transformation of the intrinsic stress in the YSZ films from compressive to tensile stress with decreasing temperature. In addition, the YSZ film had a uniform thickness distribution with an inhomogeneity of less than 5 % and a very flat and dense surface with root-mean-square surface roughness below 1 nm.

Tribological behavior of plasma-sprayed Yttria-stabilized zirconia thermal barrier coatings on 316L stainless steel under high-temperature conditions

Yttria Stabilized Zirconia (YSZ) coatings by plasma spray coating were performed to investigate the behavior of 316L Stainless Steel (SS) under high temperatures. The structural and mechanical properties of YSZ films were examined by successful scanning electron microscopy and Vickers microhardness. A tribotester determined the friction and wear properties at 500 and 1000 °C temperature conditions. The results showed that the surface hardness values of the YSZ-coated samples were significantly improved due to obtained ceramic layer compared with untreated 316L samples. Although the abrasive wear mechanism was observed in both the coated and uncoated samples, it was observed that higher temperatures produced smoother wear tracks. Also, increased wear test temperatures resulted in micro cracks and structural degradations on the surface of untreated stainless steel samples.

Preparation of an yttria-stabilized zirconia electrolyte on a porous Ni-based cermet substrate by laser chemical vapor deposition

This work investigated the use of laser chemical vapor deposition (LCVD) to apply an yttria-stabilized zirconia (YSZ) thin film electrolyte to a porous Ni metal/YSZ substrate. Using this process, a dense YSZ film having a columnar structure and small grain sizes was successfully fabricated on a porous cermet substrate at a relatively low temperature of approximately 700 °C. A 15 µm thick film was obtained following a deposition time of only 20 min. Solid oxide fuel cell (SOFC) power generation tests were performed using the sample on which YSZ was deposited and stable power generation was observed. However, the open circuit voltages were lower than the theoretical values and the ohmic loss of the specimen was significantly higher than that estimated from the conductivity of YSZ. These inferior performance parameters can possibly be attributed to the formation of cracks in the electrolyte film during the SOFC test, and thus enhanced performance could be obtained by suppressing crack formation. In addition, the reason for the crack formation was also discussed in this work.

(Digital Presentation) Additional Voltage Loss in Terms of Electromagnetic Potential Related to Jarzynski’s Equality Using Sm-Doped Ceria Electrolytes in Wagner’s Equation for SOFCs

A solid oxide fuel cell (SOFC) converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. Yttria-stabilized zirconia (YSZ) films are often used as electrolytes in which only oxygen ions are carriers. In such cases, the open circuit voltage (OCV = 1.15 V at 1073 K) is equal to the Nernst voltage (Vth = 1.15 V at 1073 K).Samaria-doped ceria (SDC) have higher ionic conductivity. Therefore, SDC films are possible electrolyte candidates. However, when SDC electrolytes are used in SOFCs, the OCV is only 0.80 V at 1073 K. The voltage loss has been explained by Wagner’s equation.Numerous subsequent models have been created based on Wagner’s equation. Using the Riess model, the current-voltage relationship can be calculated with the cathode and anode polarization voltage losses [1]. Furthermore, a nonlinear model was created by Duncan and Wachsman [2]. This model can also be used to explain the equilibrium process. According to the Riess model, the OCV should decrease during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. The change in the equilibration of thick SDC electrolytes in response to a change in the anode gas has never been explained using the model defined by Duncan and Wachsman. Experimentally, when a very thick (6.6 mm) SDC electrolyte is used, the OCV can reach 0.80 V in only 5 minutes [4]. According to Weppner, the corresponding delay in the electron diffusion current should be more than 2080 minutes [5].We proposed a current-independent anode voltage loss (0.35 V = 1.15 V- 0.80 V) [6]. Since we disproved the existence of large leakage currents in SDC electrolytes, our explanations seem to disregard and disprove Wagner’s equation. However, we noticed that the dismissal of this idea regarding large leakage currents in SDC electrolytes is nothing more than a side effect. When SDC electrolytes experience a large anode voltage loss (0.35 V), the leakage currents are very small. This is called the “anode shielding effect” [7].The ionic activation energy (Ea) of SDC electrolytes is 0.7 eV (= 0.35 V ×2e). During ion hopping processes in SDC electrolytes, the work done by ions on the surrounding lattice structure is 0.7 eV. The ions should regain the 0.7 eV after hopping. However, when there are many electrons in the hopping path, hopping processes are very complex, and calculating the loss of work is challenging. Jarzynski’s equality is a very useful method of calculating the loss of work only from the first equilibrium state to the second equilibrium state. Because the distribution of hopping ions always should be a canonical ensemble, the loss of work can be calculated. When there are enough electrons, the ions cannot regain the 0.7 eV, and voltage loss (0.35 V) occurs [8].To assist the above explanation, we use the electromagnetic potential to explain the voltage loss that occurs during ion hopping. We noticed that the potential of ions during hopping can be calculated from the Lorenz gauge condition.Reference:[1] I. Riess, J. Phys. Chem. Solids., 47(2), 129 (1986). doi: 10.1016/0022-3697(86)90121-6.[2] K. L. Duncan and E. D. Wachsman, J. Electrochem. Soc., 156, B1030 (2009). doi:10.1149/1.315851.[3] T. Miyashita, J. Mater. Sci., 41(10), 3183 (2006). doi: 10.1007/s10853-006-6371-8.[4] T. Miyashita, J. Electrochem. Soc., 164(11) 3190 (2017). doi: 10.1149/2.0251711jes[5] J. Liu and W. Weppner, Ionics, 5(1–2), 115 (1999). doi: 10.1007/BF02375914.[6] T. Miyashita, J. Mater. Sci., 40(22), 6027 (2005). doi: 10.1007/s10855-005-4560-2.[7] B. Dalslet, P. Blennow, P. V. Hendriksen, N. Bonanos, D. Lybye, and M. Mogensen, J. Solid State Electrochem., 10(8), 547 (2006). doi: 10.1007/s10008-006-0135-x.[8] T. Miyashita, Miyashita, ECSarXiv (2020) “Open-Circuit Voltage Anomalies in Yttria-Stabilized Zirconia and Samaria-Doped Ceria Bilayered Electrolytes”, https://ecsarxiv.org/xhn73/

Resonant Activation of Resistance Switching in Memristors Based on YSZ Films with Au Nanoparticles

The resonant activation of resistance switching (RS) of a memristor based on an yttria stabilized zirconia (YSZ) film with embedded Au nanoparticles (NPs) was investigated. The switching was made by triangular pulses with high frequency (HF) sinusoid added. A non-monotonous dependence a ratio of the electric current through the memristor in the low resistive state to the current in the high resistive one on the HF sinusoid frequency was found. The effect was explained by a finite electron tunneling time between the Pt electrode and Au NPs. This conclusion was supported by measuring a dependence of HF memristor capacitance on the probing signal frequency.

Solution-processed thin film transistors incorporating YSZ gate dielectrics processed at 400 °C

This work investigates a solution process for yttria-stabilized zirconia (YSZ) thin film deposition involving the addition of yttria nanoparticles, at 400 °C, in air. Different yttrium doping levels in the YSZ were studied and a wide range of optical, structural, surface, dielectric, and electronic transport properties were also investigated. An optimum yttrium doping level of 5% mol. resulted in the smoothest films (RRMS ∼ 0.5 nm), a wide bandgap (∼5.96 eV), a dielectric constant in excess of 26, and a leakage current of ∼0.3 nA cm−2 at 2 MV/cm. The solution-processed YSZ films were incorporated as gate dielectrics in thin films transistors with solution-processed In2O3 semiconducting channels. Excellent operational characteristics, such as negligible hysteresis, low operational voltages (5 V), electron mobility in excess of 36 cm2 V−1 s−1, high on/off current modulation ratio on the order of 107, and low interfacial trap density states (<1012 cm−2), were demonstrated. In addition, excellent film homogeneity was achieved over a large area (16 × 16 cm2), with both film thickness and capacitance deviation of <1.2%.

Design of Ordered Mesoporous CeO2-YSZ Nanocomposite Thin Films with Mixed Ionic/Electronic Conductivity via Surface Engineering.

Mixed ionic and electronic conductors represent a technologically relevant materials system for electrochemical device applications in the field of energy storage and conversion. Here, we report about the design of mixed-conducting nanocomposites by facile surface modification using atomic layer deposition (ALD). ALD is the method of choice, as it allows coating of even complex surfaces. Thermally stable mesoporous thin films of 8 mol-% yttria-stabilized zirconia (YSZ) with different pore sizes of 17, 24, and 40 nm were prepared through an evaporation-induced self-assembly process. The free surface of the YSZ films was uniformly coated via ALD with a ceria layer of either 3 or 7 nm thickness. Electrochemical impedance spectroscopy was utilized to probe the influence of the coating on the charge-transport properties. Interestingly, the porosity is found to have no effect at all. In contrast, the thickness of the ceria surface layer plays an important role. While the nanocomposites with a 7 nm coating only show ionic conductivity, those with a 3 nm coating exhibit mixed conductivity. The results highlight the possibility of tailoring the electrical transport properties by varying the coating thickness, thereby providing innovative design principles for the next-generation electrochemical devices.

Influence of substrate temperature on the microstructure of YSZ films and their application as the insulating layer of thin film sensors for harsh temperature environments

Thin film sensors are employed to monitor the health of hot-section components of aeroengine intelligence (for instance, blades), and electrical insulating layers are needed between the metal components and thin film sensors. For this purpose, the electrical insulation characteristics of an yttria-stabilized zirconia (YSZ)/Al2O3 multilayer insulating structure were investigated. First, YSZ thin films were deposited by DC reactive sputtering at various substrate temperatures, and the microstructural features were investigated by scanning electron microscopy and X-ray diffraction. The results indicate that the micromorphology of the YSZ thin film gradually became denser with increasing substrate temperature, and no new phases appeared. The compact and uniform topography of the YSZ thin film improved the insulation properties of the multilayer insulating structure and enhanced the adhesion of the thin film sensors. In addition, the electrical insulation properties of the YSZ/Al2O3 multilayer insulating structure were evaluated via insulation resistance tests from 25 to 800 °C, in which the YSZ thin film was deposited at 550 °C. The results show that the insulation resistance of the multilayer structure increased by an order of magnitude compared with that of the conventional Al2O3 insulating layer, reaching 135 kΩ (5.1 × 10−6 S/m) at 800 °C. Notably, the insulation resistance was still greater than 75 kΩ after annealing at 800 °C for 5 h. Finally, the shunt effect of the YSZ/Al2O3 multilayer insulating structure was estimated using a PdCr thin film strain gauge. The relative resistance error was 0.24%, which demonstrates that the YSZ/Al2O3 multilayer insulating structure is suitable for thin film sensors.

Influence of the Sputtering Technique and Thermal Annealing on YSZ Thin Films for Oxygen Sensing Applications

Yttria-stabilized zirconia (YSZ) thin films were deposited using direct current (reactive and metallic) and radio frequency magnetron sputtering. The effect of the deposition technique and annealing treatment on the microstructure and crystallinity of the thin films was assessed. Using the films produced in this work, oxygen gas sensors were built and their performance under vacuum conditions was evaluated. All the films exhibited a cubic crystalline structure after a post-deposition thermal treatment, regardless of the sputtering technique. When the annealing treatment surpassed 1000 °C, impurities were detected on the thin film surface. The oxygen gas sensors employing the reactive and oxide-sputtered YSZ thin films displayed a proportional increase in the sensor current as the oxygen partial pressure was increased in the evaluated pressure range (5 × 10−6 to 2 × 10−3 mbar). The sensors which employed the metallic-deposited YSZ films suffered from electronic conductivity at low partial pressures.

Influence of Yttria Segregation and Platinum Nanoparticles on the Ionic Conductivity of Yttria-Stabilized Zirconia (YSZ) Films Under Hydrogen Exposure

Yttria-stabilized zirconia (YSZ) films have high ionic conductivities over the range 400-800oC which make them useful for solid oxide fuels cells, oxygen sensors, and other sensing applications. Adding catalytic particles to the YSZ surface can induce reactivities and associated conductivity changes for gases other than oxygen. In this work, the surface reactivity towards hydrogen was investigated on thin films with a 8-YSZ composition (8at.% Y2O3 - 92at.% ZrO2) that were deposited onto Langasite (La3Ga5SiO14) substrates using RF-magnetron sputtering in a 5% O2 - 95% Ar gas mixture. Films were grown to thicknesses up to 200 nm using different growth temperatures (30 - 7000C), deposition rates (0.03 - 0.07 nm/s), and substrate bias (-300 - +300 V). To create Pt nanoparticles on the YSZ film surface, Pt was deposited in-situ via e-beam evaporation to equivalent Pt thicknesses of 1 - 10 nm with the YSZ film held at either 25oC or 400oC and then subsequently annealed in an air furnace up to 1000oC to cause Pt agglomeration and nanoparticle formation. To assess steady-state surface reactivity to hydrogen in the range 400 - 600oC after the 1000oC treatments, film ionic conductivities were measured with electrochemical impedance spectroscopy (EIS) using an impedance/gain-phase analyzer after long annealing times in air, in pure N2, or in a 4% H2 - 96% N2 mixture within the furnace tube. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also used to analyze the characteristics of the as-deposited and annealed YSZ and Pt/YSZ films.YSZ films were synthesized with a variety of crystallographic textures, nanomorphology, and composition using the deposition parameters discussed above. Films deposited at 25, 400, or 700oC all have (111) cubic YSZ preferred texture with random in-plane orientation on the Langasite substrate. The smoothest films were produced using a rate of 0.07 nm/s at 400oC. However, films deposited using the other growth conditions contain significant nano-roughness and hillocks generated by film stress as evidenced by Williamson Hall XRD measurements. In films deposited at 25oC there is also a minor (200) grain population in addition to (111). To create Pt nanoparticles on these various film surfaces, Pt was evaporated with the YSZ film surface being held at either 25oC or 400oC. Pt layers (1 - 10 nm) were uniform at 25oC whereas the Pt started to agglomerate during deposition at 4000C.Following deposition, all the YSZ and Pt/YSZ films were annealed in air up to 1000oC and held for 1 hour prior to measuring conductivity and hydrogen reactivity over the range 400-600oC. This 1000oC annealing treatment was performed to (i) promote Pt agglomeration at high temperature to form individual Pt islands, (ii) stabilize the YSZ nanostructure in terms of film stress and grain size, and (iii) obtain high temperature ionic conductivity measurements over a range of temperatures. XPS indicates that post-deposition air annealing causes significant yttria segregation to the YSZ surface region as evidenced by increases in the Y3d/Zr3d photoelectron peak area ratio upon annealing up to 1000oC. This yttria enrichment decreases the film ionic conductivity and decreases the activation energy for ion transport compared to the as-deposited films. EIS experiments performed in air, N2 or 4%H2-N2 between 400-600oC indicate that the YSZ films without Pt that are most sensitive to hydrogen are those deposited at 400-700oC using a rate of 0.03 nm/s which contain rough surfaces, a population of (200) grains, and minimal yttria surface enrichment. The EIS results also show that YSZ films decorated with Pt nanoparticles all exhibit varying baseline conductivities and sensitivities to hydrogen with a magnitude dependent on the Pt nanoparticle distribution as well as film nanostructure. These YSZ and Pt/YSZ films hold promise as hydrogen sensing films that can be incorporated onto a variety of sensing platforms for H2 gas detection and management. Figure 1