Zirconia Sensor Research Articles

Liquid-diffusion-limited growth of vanadium dioxide single crystals

Vanadium dioxide is a strongly correlated material with an ultrafast first-order phase transition between monoclinic/insulator and rutile/metallic close to room temperature. The unusual and complex properties of this transition make $\mathrm{V}{\mathrm{O}}_{2}$ one of the most heavily investigated materials in modern condensed matter physics. Consequently, high-quality single crystals are in large demand. Here we report the growth of mm-sized $\mathrm{V}{\mathrm{O}}_{2}$ crystals by thermal decomposition of liquid ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ at $\ensuremath{\sim}{1000}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$. Time-resolved zirconia sensor measurements of the oxygen release reveal that the crystal growth rate is limited by liquid-phase diffusion; the properties of the gaseous environment, which were previously assumed to be decisive, are almost insignificant. Consequently, large and stoichiometric single crystals of $\mathrm{V}{\mathrm{O}}_{2}$ can be obtained at lower temperatures and gas purities than usually applied. These results signify the role of gas-liquid diffusion in crystal growth and will simplify future research on $\mathrm{V}{\mathrm{O}}_{2}$ and related materials for applications in ultrafast electronics and thermal energy management.

Zirconia Coatings as Efficient Soil Moisture Sensors for Water Irrigation

In agriculture sector, a soil moisture sensor plays a vital role in controlling the irrigation through the adequate and timely moisture of soil essential for the crop production. Owing to the chemically inert nature of zirconia, it is found to be a good material for soil moisture sensing. Therefore, the present study reports the synthesis of sol-gel based zirconia coatings and their application as efficient soil moisture sensor for water irrigation. For this purpose, zirconia was coated on 316L stainless steel substrate using zirconium propoxide precursor through sol-gel process and spin coating technique. The obtained zirconia coatings were used to fabricate and study the zirconia soil moisture sensors. Zirconia sensors were fabricated using zirconia coatings synthesized from various concentrations of zirconium precursor. The chemical structure and crystalline nature of zirconia coatings were analyzed using FTIR spectroscopy and X- ray diffractometer. Moreover, the contact angle and roughness of these coatings have been measured. It is observed that the contact angles and roughness of the synthesized zirconia coatings increased with increase in the precursor concentrations. All zirconia coatings manifested the tetragonal crystal structure. The resistance of the zirconia soil moisture sensors was measured for different moisture content in soil (%). The zirconia sensors are tested in an active black soil moisture condition and their resistances changed for the soil moisture range of 20 to 50%. Further, zirconia soil moisture sensor monitoring system was developed using NImyDAQ and LabVIEW which is useful for the water irrigation.

Electro-thermo-mechanical modelling of automotive exhaust sensors

The development of automotive exhaust sensors is generally based on a time and resources consuming empirical approach. Building numerical models to study the behavior of the sensors is therefore an important key to reduce the development time and improve sensor quality. Zirconia sensor accuracy and response are dependent from temperature, so temperature is generally controlled in the desired range through a heating element. In the present study an electro-thermo-mechanical model of a heated zirconia oxygen sensor with planar structure has been provided. The numerical model has been used as a tool to study and compare different geometries of heaters. Several heater configurations have been modelled and compared in terms of: temperature rise in different points on the surface of the sensor both on the electrode side and heater side, average and maximum temperature, thermal stress and time for the sensor to be considered functional. The improved heater provided a lower peak temperature, but higher average temperature, more uniform temperature distribution, lower thermal stress and lower time than the base heater to reach the prescribed operational conditions.

Characterization of Electrochemical Surface Area and Porosity of Zirconia Sensors

Solid-state zirconia sensors are used in a wide variety of applications including controlling the air to fuel ratio in combustion engines and pollution monitoring. These sensors use either a layer of zirconia as a solid-state ionic electrolyte or a gas-porous ceramic as a protective layer. There is a need for quantitative methods to assess the tortuosity of these porous layers and the size of the electrode area exposed which can be performed on completed sensor devices. We demonstrate using electrochemical double layer capacitance and transport studies in aqueous potassium ferri/ferro-cyanide electrolytes that these parameters can be readily measured. The technique is demonstrated on sensors procured from ESL ElectroScience as well as sensors produced in-house using additive manufacturing. The processes that we develop can be applied as quality control to ensure sample-to-sample reproducibility of the porous layer.

Characterization of Electrochemical Surface Area and Porosity of Zirconia Sensors

The platinum electrode/yttria-stabilized zirconia metal/solid electrolyte interface is used in a wide variety of sensing applications including lambda oxygen sensors, wide range oxygen sensors, HC and NOx sensors.1–3 In many cases the electrode is protected by a porous ceramic layer to shield the device from exhaust particle impingement,1 or in the case of some types of mixed potential sensors, the electrolyte itself is porous.4The electrochemically active surface area and porosity are important factors that may control the performance of all types of zirconia-based sensors. While measuring AC impedance behavior in the gas phase is possible, the high diffusivity of gases through the porous layer makes extracting diffusion parameters difficult. Mixed potential electrochemical sensors being electro-kinetic devices, are particularly sensitive to these parameters, but few techniques have been developed to measure active surface areas and characterize transport through porous layers. We have developed sensors consisting of a yttria-stabilized zirconia (YSZ) substrate coated with an insulating ceramic onto which dense electrodes of Pt, La0.8Sr0.2CrO3, or Au0.5Pd0.5alloy are screen printed. Finally, a porous YSZ electrolyte is deposited on top of the electrodes. Measuring the effective diffusivity through the porous layer and the electrochemically active surface area of the underlying electrodes is useful in optimizing the performance of the devices. The electrochemically active surface area of the Pt electrodes beneath the porous electrolyte layer was determined by immersing the sensor in aqueous H2SO4 solution and measuring the double layer capacitance or adsorption of a monolayer of H. The oxidation and reduction of [Fe(CN)6]-4/-3 observed by cyclic voltammetry (CV) acts as a probe for the effective diffusion constant through the porous layer as the kinetics of this reaction are fast and the reaction is easily reversible. The effective diffusion coefficient is obtained by fitting to the Randles-Secvik equation for the peak CV current as a function of scan rate. Figure 1 shows the CVs obtained on (a) a Pt disk and (b) the Pt sensor electrode beneath a porous YSZ electrolyte. The effective diffusion coefficient is on the order of 10-6 cm2/s for the Pt disk and is consistent with values reported in the literature.4 The diffusion coefficient is 10-10 cm2/s for the transport through the porous layer on sensor indicating significant hindrance by the YSZ. Measuring these characteristics for both sensors produced by screen printing (ESL ElectroScience) and sensors produced by additive manufacturing in our lab will guide the optimization of sensor performance characteristics. The measurements are correlated to the device sensing and AC impedance response.

Evaluation of the in-depth temperature sensing performance of Eu- and Dy-doped YSZ in air plasma sprayed thermal barrier coatings

In-depth temperature sensing in air plasma sprayed (APS) thermal barrier coatings (TBCs) has been achieved with europium (Eu) and dysprosium (Dy) doped yttria stabilized zirconia (YSZ) sensor coatings. The luminescence properties of YSZ:Eu and YSZ:Dy, including spectrum, intensity and lifetime, were evaluated and their temperature sensing performances were compared using a lifetime-based measurement system. Both sensor TBCs display excellent temperature sensitivity in high temperature environment (400–800°C for YSZ:Eu, 500–900°C for YSZ:Dy) with a topcoat thickness up to 300μm. It was found that the upper limit of temperature sensing was determined by the ratio between luminescence intensity and background thermal radiation (signal-background-ratio). YSZ:Dy showed higher luminescent intensity than YSZ:Eu at elevated temperatures, and therefore displayed better temperature sensing performance with an increased limit. The effect of topcoat thickness on the temperature sensing performance was also evaluated, showing that the attenuation factor of YSZ increased significantly with topcoat thickness. Interestingly, it was found that the topcoat attenuation factor gradually decreased as temperature increased, which improved the temperature sensing performance of sensor TBCs at elevated temperature. The findings in the temperature sensing of APS TBCs provide basis for the future development of on-line APS TBCs temperature monitoring technology.

Variation of the redox conditions and the resultant phase assemblages during iron ore sintering

The oxygen potential prevailing during iron ore sintering was measured with a zirconia sensor in a series of sinter pot experiments. This was done to get a better indication of the redox conditions during commercial sintering. It was found that the pO2 is appreciably more oxidizing than previously assumed, with a minimum value of ~0.01atm. It is concluded that this value represents the oxygen potential of the gas phase and it is therefore a mixture of combustion gas and downdraft air.The contents of a quenched sinter pot where the reactions were interrupted with the flame front situated midway through the sinter bed were investigated. X-ray diffraction analysis, using an internal standard to quantify the amorphous slag phase, revealed that at the flame front only magnetite and slag were present. SFCA phases only formed at the top of the bed after the flame front had passed.Thermodynamic modeling of the phases at equilibrium agree qualitatively with the phase analysis and explained the extensive presence of magnetite and melt, as well as the formation of calcium ferrite phases during cooling below 1100°C.

Simple method for determining metal power oxidation kinetics with a zirconia sensor

A basic zirconia oxygen sensor was utilized to monitor the gas–solid oxidation reaction characteristics of metal/metal-oxide powder beds. The method described allowed for simple determination of chemical reaction rate constants for the oxidation reactions. The signal output of the sensor was analyzed for oxidation of the powder bed and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed, and a discussion of relevant thermodynamic and kinetic theory was provided. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor. Combining the experimental data with relevant kinetic theory, chemical reaction rate constants were calculated from plots of sensor ocv versus time for each reaction step during the metal oxidation sequences.

Measurement of non-condensable gas in a PWR small-break LOCA simulation test with LSTF for OECD/NEA ROSA Project and RELAP5 post-test analysis

An OECD/NEA ROSA Project experiment with the large scale test facility was conducted simulating a PWR 0.5% cold leg small-break loss-of-coolant accident with steam generator (SG) secondary-side depressurization as an accident management measure to clarify such complex phenomena as non-uniform flow among the SG U-tubes under natural circulation (NC) with non-condensable gas. Non-condensable gas inflow to the primary system from accumulator tanks was simulated with air. The gas concentration was measured at the SG outlet plenum by using a gas measurement device to measure oxygen concentration in steam–gas mixture that is developed employing a Zirconia sensor. Non-uniform flow behaviors were observed in the SG U-tubes with non-condensable gas ingress depending probably on gas accumulation rate as well as asymmetric NC between two loops. Air concentration in steam–gas mixture was estimated to be about 40-50% at the SG outlet plena after the U-tube downflow side became empty of liquid, which was a bit larger than the estimated value based on the gas phase temperature. Post-test analysis by JAEA-modified RELAP5/MOD3.2.1.2 code revealed that the code underpredicts the primary pressure a bit after the gas inflow with some discrepancies in the SG U-tube liquid level and the NC mass flow rate.

A bias correction method for fast fuel-to-air ratio estimation in diesel engines

λ probes in turbocharged diesel engines are usually located downstream of the turbine, exhibiting a good dynamic response but a significant delay because of the exhaust line transport and the hardware itself. With the introduction of after-treatment systems, new sensors that can measure the exhaust concentrations are required for optimal control and diagnosis. Zirconia-based potentiometric sensors permit the measurement of nitrogen oxides and oxygen with the same hardware. However, their dynamic response is slower and more filtered than that of traditional λ probes and, in addition, the sensor location downstream of the after-treatment systems increases this problem. The paper uses a Kalman filter for online dynamic estimation of the relative fuel-to-air ratio λ−1 in a turbocharged diesel engine. The combination of a fast drifted fuel-to-air ratio model with a slow but accurate zirconia sensor permits the model bias to be corrected. This bias is modelled with a look-up table depending on the engine operating point and is integrated online on the basis of the Kalman filter output. The calculation burden is alleviated by using the converged gain of the steady-state Kalman filter, precalculated offline. Finally, robustness conditions for stopping the bias updating are included in order to account for the sensor and model uncertainties. The proposed algorithm and sensor layout are successfully proved in a turbocharged diesel engine. Experimental and simulation results are included to support validation of the algorithm.

Conductivity Measurement of Calcia Stabilized Zirconia Prepared by Mechanical Route

Calcia-stabilized zirconia (CSZ) sensor elements of 8 and 16 mol% was synthesized by mixing appropriate amounts of Calcia and Zirconia using in agate mortar and pestle followed by sintering at 1500°C for 3 hours. The electrical conductivity of the stabilized zirconia was measured at different temperature using the two probe impedance analyzer in the temperature range 150°C to 500°C. It was found that the calcia influences the grain boundary thickness. The conductivity is discussed in terms of the ionic vacancies in the bulk due to calcium ion substitution and the available grain boundary path.

Measurement of pH in high-temperature nickel laterite pressure acid leach process solutions

The performance of a flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor for pH measurement in high-temperature nickel laterite pressure acid leach (PAL) process solutions was evaluated. Very good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to ternary MgSO 4–Al 2(SO 4) 3–H 2SO 4 solutions by combining all the divalent metal sulphates into one with the properties of MgSO 4. The experimental results support the postulation that the high-temperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO 4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition.

Improvement in response of resistive oxygen sensor based on ceria–zirconia thick film with Pt catalyst on surface

A resistive oxygen sensor based on ceria doped with 10 mol% zirconia can distinguish between fuel-rich and lean conditions in the case of a model exhaust gas in an equilibrium state. However, the response is unpredictable in a model exhaust gas in a nonequilibrium state. In this study, we investigated the response of an oxygen sensor in a model exhaust gas in a nonequilibrium state. The following problems were identified in relation to the ceria–zirconia sensor. First, in a nonequilibrium state, the resistive oxygen sensor indicated a propane lean condition when in fact the system was in a propane rich condition; under the same conditions, an electromotive force sensor based on an oxide ion conductor indicated a propane rich condition. Second, in an equilibrium state, the response of the resistive sensor, which was heated by a self-heater at 600 °C, was very slow in indicating a change from lean to rich conditions. In order to solve these problems, we fabricated a resistive oxygen sensor including a Pt catalyst on the surface by a simple new process and investigated its response. Through optimization of the Pt content, we were able to resolve the first response problem. That is, the sensor could distinguish between rich and lean conditions in a nonequilibrium state. Furthermore, the response speed was also improved by the Pt catalyst.

Improvement in Propene Sensing Characteristics by the Use of Additives to In2O3 Sensing Electrode of Mixed-Potential-Type Zirconia Sensor

In an effort to improve propene selectivity and long-term stability of the mixed-potential-type yttria-stabilized zirconia (YSZ)-based tubular sensor attached with sensing electrode (SE), each of the nanosized noble metals (Pt, Ru, Pd, Rh, or Ir) and YSZ powder was added to SE. It was found that the addition of nanosized Pt gave considerable improvement in selectivity, and the addition of YSZ powder exhibited great improvement in long-term stability of the sensor. Thus, the sensor attached with composite SE consisting of , Pt, and YSZ was fabricated and its sensing characteristics to were examined. As a result, the present sensor exhibited high selectivity and excellent long-term stability to at under the wet condition for the examined period of about . Based on the results obtained from the measurement of catalytic activity for CO oxidation to , the observation of morphology, and the analysis of composition of SEs, it was speculated that the added Pt nanoparticles could promote the oxidation of CO and and then improve the selectivity, while the added YSZ powder could stabilize the morphology of the SE∕YSZ interface and give better long-term stability.

Effects of Reducing Atmosphere on the Luminescence of Eu3+‐Doped Yttria‐Stabilized Zirconia Sensor Layers in Thermal Barrier Coatings

The use of Eu3+‐doped yttrium‐stabilized zirconia (YSZ) as an embedded layer in thermal barrier coatings for luminescence temperature sensing has previously been demonstrated in air. In this work the effects of exposure to reducing atmospheres on the luminescence spectra and luminescence lifetime as a function of temperature are reported. No effect of reducing atmosphere, down to oxygen partial pressures( ) of <10−11 ppm, on either the Eu3+ luminescence spectrum or the luminescence lifetime up to 900°C are seen although the overall luminescence intensity is decreased. The insensitivity of the temperature dependence of the luminescence lifetime to oxygen partial pressure indicates that Eu3+‐doped YSZ based temperature sensors can be used with confidence in reducing as well as in air atmospheres. The results also suggest that the charge transfer mechanism responsible for the temperature dependence of the luminescence lifetime is unaffected by the oxygen vacancies introduced by reduction.

Zr/ZrO2 Sensors for in Situ Measurement of pH in High-Temperature and -Pressure Aqueous Solutions

The aim of this study is to develop new pH sensors that can be used to test and monitor hydrogen ion activity in hydrothermal conditions. A Zr/ZrO2 oxidation electrode is fabricated for in situ pH measurement of high-temperature aqueous solutions. This sensor responds rapidly and precisely to pH over a wide range of temperature and pressure. The Zr/ZrO2 electrode was made by oxidizing zirconium metal wire with Na2CO3 melt, which produced a thin film of ZrO2 on its surface. Thus, an oxidation-reduction electrode was produced. The Zr/ZrO2 electrode has a good electrochemical stability over a wide range of pH in high-temperature aqueous solutions when used with a Ag/AgCl reference electrode. Measurements of the Zr/ZrO2 sensor potential against a Ag/AgCl reference electrode is shown to vary linearly with pH between temperatures 20 and 200 degrees C. The slope of the potential versus pH at high temperature is slightly below the theoretical value indicated by the Nernst equation; such deviation is attributed to the fact that the sensor is not strictly at equilibrium with the solution to be tested in a short period of time. The Zr/ZrO2 sensor can be calibrated over the conditions that exist in the natural deep-seawater. Our studies showed that the Zr/ZrO2 electrode is a suitable pH sensor for the hydrothermal systems at midocean ridge or other geothermal systems with the high-temperature environment. Yttria-stabilized zirconia sensors have also been used to investigate the pH of hydrothermal fluids in hot springs vents at midocean ridge. These sensors, however, are not sensitive below 200 degrees C. Zr/ZrO2 sensors have wider temperature range and can be severed as good alternative sensors for measuring the pH of hydrothermal fluids.

Eliminating chemical effects from thermal expansion coefficient measurements

A new approach for measuring coefficient of thermal expansion (CTE) in materials that exhibit chemical expansion as well is introduced. This allows separating the expansion that is induced by temperature changes alone from the total expansion, which is induced by temperature and chemical changes in the material. Combining this with measurements that are done at constant temperature and in controlled oxygen environment can yield better understanding of the subject of solid expansion in oxides. In order to understand whether the oxygen partial pressure has an influence on the CTE or not, measurements were carried out on three different oxide materials at several oxygen activities. A comparison between SrTiO3, SrZrO3 and Al2O3 was done by a single push-rod dilatometer equipped with a system that allows purging with different gas mixtures and a pO2 monitoring device. In order to make the experiment within a reasonable timeframe, very porous samples have been used. The experiments were done at three different temperatures: 722, 822 and 1022 °C. The pO2 was controlled by CO–CO2–Ar gas mixture, and monitored with an in house built zirconia sensor. The main results reveal that the CTE of the perovskites is influenced by the pO2 while that of alumina, as expected, is not. The CTE of SrTiO3 is more sensitive to changes in the pO2 than that of SrZrO3. In some of the measurements there exists a clear change of the behavior of the CTE vs. log(pO2) at a certain oxygen partial pressure.

High temperature sensor array for simultaneous determination of O 2, CO, and CO 2 with kernel ridge regression data analysis

A sensor array comprising of three chemical gas sensors was evaluated to predict the concentrations of O 2, CO, and CO 2 in a gas stream with the sensors at 600 °C. The data analysis involved a non-linear multivariate regression method (kernel ridge regression, KRR) along with a searching algorithm to predict gas concentrations. The sensors in the array included a resistance-based 2% CuO/10% La 2O 3/TiO 2 sensor, and two potentiometric sensors, including a yttria stabilized zirconia (YSZ) sensor with a metal/metal oxide internal reference electrode, and a lithium phosphate-based sensor. In addition, the possibility of using the KRR algorithm to predict gas concentrations beyond the training data is explored.

‘In situ’ diagnostics of solid electrolyte sensors measuring oxygen activity in melts by a developed impedance method

An impedance method for the periodic ‘in situ’ diagnostics of the solid electrolyte/liquid-metal electrode interface during the lifespan of yttria-stabilized zirconia (YSZ) based sensors measuring oxygen partial pressure in melts was developed. Polarization effects on the YSZ, resulting from the corrosive measuring environment (molten alkaline metals), may be interpreted as a blocking reaction layer on the electrolyte/liquid-metal electrode interface. The proposed impedance method allows information to be obtained about the level of polarization of the YSZ/liquid-metal electrode interface that is characterized by a second semi-arc of a hodograph of impedance. The second semi-arc represents the parameters of polarization resistance (Rf) and capacitance of the double electrical layer on the interface YSZ/liquid metal (CD). Analysis of the impedance method on the single crystal zirconia sensor measuring dissolved oxygen in molten lead at temperatures of 380–480 °C revealed that this sensor, with a Bi–Bi2O3 reference electrode, showed a negligible level of polarization effects on the electrode/electrolyte interface at temperatures as low as 380 °C. The results of the present work may be applicable for the diagnostics of oxygen sensors with more complicated applications, such as in the measurement of oxygen activity in lead–bismuth, sodium or lithium heat carriers in liquid-metal nuclear facilities.

Assessment of gas‐fluidized beds mixing and hydrodynamics by zirconia sensors

AbstractGas‐mixing and bed hydrodynamics associated with the rise of isolated bubbles in gas‐fluidized beds of two granular solids are investigated by means of a novel technique. These diagnostics consist of an array of miniaturized zirconia‐based oxygen sensors located along the axis of a hot (1123 K) incipiently fluidized bed, in which isolated bubbles of a tracer gas (nitrogen) are injected. Time‐resolved oxygen concentrations and relative pressures are recorded at several levels along the bed. It is shown how the response of the sensors array can be related to the concurrent effects of bubble motion, bubble‐emulsion phase, mass transfer and interstitial gas flow in the emulsion phase, combined with each other to give rise to the propagation of two perturbations to the basal value of oxygen concentration. Analysis of time‐resolved oxygen concentration profiles enables the assessment of: (a) the bubble rise velocity; (b) the gas velocity associated with the interstitial flow; and (c) the interphase bubble‐emulsion phase, mass‐transfer rate and coefficient. Analysis of the response of sensors located in the splash zone of the fluidization column provides indirect inference of the hydrodynamic patterns associated with bubble bursting at the surface of the bed. © 2005 American Institute of Chemical Engineers AIChE J, 2006