Alumina Sensor Research Articles

High-Performance Humidity Sensor Capable for Monitoring Low-Moisture Levels in Energy Industries

Humidity sensors are extensively used in industrial processing and environmental control. There are two categories of humidity sensors: High-humidity (relative humidity) measurement and Low-humidity (moisture) measurement. The proper units for low-humidity measurement are dew point and parts per million in volume (ppmv) or parts per billion in volume (ppbv). It is very important to monitor moisture levels in natural gas during processing and transportation in pipelines. It is of critical importance to monitor moisture levels in lithium-ion battery manufacturing. In these cases, the moisture levels are limited to be less than -50°C dew point or 40 ppmv. The conventional relative humidity sensors cannot be used because of their low sensitivity. The dew point sensors on the market are polymer-based and gamma-phase-alumina based. The polymer-based sensors have low sensitivity, which can only accurately detect moisture levels of -60°C dew point and higher. The gamma-phase alumina sensors suffer from long-term drift due to phase change.We present novel high-performance humidity sensors based on porous alpha-phase alumina and silicon dioxide nanostructures, which are capable of detecting moisture levels as low as -100°C dew point or 13 ppbv with excellent long-term stability. The porous alpha-phase alumina films were produced by anodic-spark-deposition on aluminum substrates in which electric sparks were produced due to high-voltage anodization (>120V). Local high-temperature (about 2200°C) at the points of sparks converted the amorphous alumina into alpha-phase alumina (Sapphire). It provides an extremely stable structure for humidity sensors. Hydrophilic silicon dioxide films deposited by atomic layer deposition was used as sensing materials. The sensors’ sensitivity, temperature coefficient, response speed, and long-term stability were studied in details. The sensors have great promise for monitoring moisture levels in natural gas industry and lithium-ion battery manufacturing.

Dual-functional Au-porous anodic alumina (PAA) sensors for enrichment and label-free detection of airborne virus with surface-enhanced Raman scattering

The development of a rapid and sensitive method for the enrichment and direct detection of airborne viruses has become urgently needed to prevent their spread. We have developed a dual-functional, label-free platform for the enrichment and optical identification of airborne viruses. This platform allows for the on-site enrichment and identification of airborne viruses through a virus enrichment component combined with surface-enhanced Raman scattering (SERS). In this work, we created an Au-porous anodic alumina (PAA) composite film, by ion sputtering Au nanoparticles (Au NPs) on a porous structure. The Au-PAA serves as a dual-functional sensor: it combines ultrafiltration through the vertical nanopores in the PAA for virus enrichment and constitutes a plasmonic substrate due to the surface plasmon near-field enhancements within the nanochannel structure. This sensor demonstrates an extraordinary enrichment efficiency (about 98 %) of obtained bioaerosols and enables simultaneous, sensitive detection at the single-virus level. Finite-difference time-domain (FDTD) simulations evidenced the electric field intensity and charge distribution of the Au-PAA. This Au-PAA composite film offers a powerful system for on-site, label-free viral particle enrichment and SERS detection. Contaminated air containing viruses can be collected in a few minutes and analyzed immediately by our platform, which is particularly beneficial for timely monitoring of viruses in the air of large public spaces. Additionally, this platform can be applied to evaluate the indoor air quality.

Vibration Sensors on Flexible Substrates Based on Nanoparticle Films Grown by Physical Vapor Deposition.

Flexible electronics have gained a lot of attention in recent years due to their compatibility with soft robotics, artificial arms, and many other applications. Meanwhile, the detection of acoustic frequencies is a very useful tool for applications ranging from voice recognition to machine condition monitoring. In this work, the dynamic response of Pt nanoparticles (Pt NPs)-based strain sensors on flexible substrates is investigated. the nanoparticles were grown in a vacuum by magnetron-sputtering inert-gas condensation. Nanoparticle sensors made on cracked alumina deposited by atomic layer deposition on the flexible substrate and reference nanoparticle sensors, without the alumina layer, were first characterized by their response to strain. The sensors were then characterized by their dynamic response to acoustic frequency vibrations between 20 Hz and 6250 Hz. The results show that alumina sensors outperformed the reference sensors in terms of voltage amplitude. Sensors on the alumina layer could accurately detect frequencies up to 6250 Hz, compared with the reference sensors, which were sensitive to frequencies up to 4250 Hz, while they could distinguish between two neighboring frequencies with a difference of no more than 2 Hz.

Long-lasting stability and low-concentration SO2 gas detection aptitude of Sn-doped alumina sensors

The Sn-doping effect on the SO2 gas sensing properties has been investigated by a resistance-based measurement method. During structural analysis, the undoped aluminium oxide (Al2O3) nanoparticles display a rhombohedral phase and the Sn-doped nanoparticles reveal the orthorhombic phase which causes a diffraction shift to a higher angle. From the surface analysis, particles expose dense microstructure, which is further enhanced by doping. High-resolution scanning microscopy images confirm a non-identical orientation of particles, suggesting polycrystalline nature. Besides, the particles are interconnected through one-by-one approach by forming a necklace of bead-like structure. The optical spectroscopy measurement endows a decrease in absorption intensity followed by an increase in bandgap with increasing Sn-doping concentration (SnxAl2-xO3). In sensor investigation, Sn-doped sensor senses the SO2 gas more efficiently even at low and higher concentrations i.e. from 10 to 300 ppm. The sensor adduces an elevated sensitivity of 78.14% at 300 ppm. In addition, the sensor adsorbs the gas molecules within 17 s, depicting a fast response time. As a result of the greater sensitivity, assistive technology such as pre-concentration is no longer required. The Sn-doped sensor bestows 96.83% reproducibility, showing a practical importance.

Design and Research of Wireless Passive High-Temperature Sensor Based on SIW Resonance

The temperature of advanced components in aviation and aerospace fields is difficult to obtain timely. In this study, we aimed to investigate microwave backscattering technology combined with the theory of substrate integrated waveguide and resonant cavity to design a wireless passive temperature sensor and explore its potential in this field. We employed silicon carbide and aluminum ceramic as the substrate to make sensors. The interrogation antenna was designed to test the sensor, which could completely cover the working frequency of the sensor and had good radiation characteristics. Based on the test results, the silicon carbide sensor was capable of bearing a temperature limit of about 1000 °C compared to the alumina sensor. From 25 °C to 500 °C, its sensitivity was 73.68 kHz/°C. Furthermore, the sensitivity was 440 kHz/°C in the range of 501 °C to 1000 °C. Moreover, we observed the surface of this sensor by using the scanning electron microscope, and the results showed that the damage to the sensor surface film structure caused by long-term high temperature is the major reason for the failure of the sensor. In conclusion, the performance of the silicon carbide sensor is superior to the alumina sensor.

Anodic alumina sensor for water activity monitoring in food materials

Water activity is a parameter that characterizes the hydration state of a particular substance and the extent of water binding therein. Many food system technology aspects are related to food system hydration, hence water activity (Aw) measurements can become widely used in technological application. However, widespread implementation of water activity monitoring is hampered by the lack of available measuring instruments. This article provides the examples of potential water activity use in food industry and proposes a method of water activity measurement with a piezoquartz sensor coated with a porous alumina layer.

Fiber-optic Fabry-Perot pressure sensor based on low-temperature co-fired ceramic technology for high-temperature applications.

In this study, a novel batch-producible fiber-optic Fabry-Perot (FP) pressure sensor based on a low-temperature co-fired ceramic technology is proposed and experimentally demonstrated for high-temperature applications. The sensor is fabricated by inserting a well-cut single-mode fiber (SMF) into a zirconia fiber ferrule, followed by insertion of the overall structure into an alumina sensor head. The FP cavity in the sensor is formed by placing the end face of the SMF in parallel to the diaphragm. The external pressure can be detected by demodulating the FP cavity length of the sensor. A theoretical analysis indicates that the pressure sensitivity can be designed flexibly by adjusting the parameters of the ceramic diaphragm, radius, and thickness. Experimental results demonstrate that the pressure sensor exhibits a high linear sensitivity of approximately 0.1μm/kPa at room temperature in the pressure range up to 160kPa. The repeatability error and nonlinear error of three repeatable experiments are approximately 2.60% and smaller than 0.101%, respectively. The temperature coefficient and coefficient of the pressure-sensitivity changes with temperature are 0.023μm/°C and 0.205nm/(kPa°C) in the temperature range of 20°C-300°C.

Synthesis, characterization and hydrogen sensing properties of nanosized colloidal rhodium oxides prepared by Pulsed Laser Ablation in water

Nanosized colloidal rhodium oxide particles in water were obtained by using a nanosecond pulsed excitation, starting from a high purity rhodium target. The as-synthesized colloids show an almost spherical shape and size in the range between 2 and 12 nm. Characterization results indicate that colloidal particles are composed mainly of oxidized RhxOy phases, whose content increase after annealing at 200 °C in air and decrease under hydrogen treatment in mild conditions, suggesting a reversible behavior. The hydrogen sensing characteristics of thin films deposited by drop casting from the colloidal solution onto alumina sensor substrates provided with platinum interdigitated electrodes, were also investigated. The developed conductometric hydrogen sensor shows high response to low concentration of the target gas in air at low operating temperature and full reversibility with fast dynamics. Further, a good selectivity towards hydrogen was demonstrated, testing other simple molecules such as CO, CO2, NO2 and O2. To our knowledge, this is the first report on the use of nanosized rhodium/rhodium oxide colloids in conductometric gas sensors.

Solutions for thermally mismatched brazing operations for ceramic tokamak magnetic sensor

Abstract To monitor high-frequency fluctuations of the equilibrium magnetic field in tokamaks, a 3D magnetic sensor has been developed. The sensor, which is positioned inside the vacuum vessel behind the protective tiles of the tokamak and is exposed to potential temperatures up to 400°C, is based on thick-film and LTCC (low-temperature co-fired ceramic) technology. To connect the sensor to the cabling that runs inside the vacuum vessel, mineral-insulated cables have to be brazed to the sensor to ensure electrical connection together with mechanical robustness and sufficient thermal stability. As the brazing temperature is about 600°C, direct brazing to the alumina sensor substrate can cause failure by cracking induced by thermal stresses. It arises both by temperature gradients stemming from the localised heating and by the high thermal mismatch of alumina with the braze and wire materials. In previous work, high stresses from temperature gradients were efficiently decoupled by brazing indirectly to alumina beams attached to the main substrate, and local thermal stresses between alumina and braze/wire by using a porous metallisation. However, as the slender alumina beams protruding out of the substrate are somewhat cumbersome and fragile, three alternatives were studied in the present work: 1) testing shorter and more robust beams, 2) replacing the alumina beam by a silver wire, and 3) depositing a porous temperature- and stress-decoupling dielectric to enable direct brazing on the main alumina substrate. These solutions are characterised with respect to their mechanical robustness and of the degree of thermal decoupling with the substrate they provide.

Fabrication of an aluminum-porous alumina sensor by in situ monitoring anodization of thin aluminum films

The aim of the present work is to optimize a well-known plasmon-based aluminum/porous anodic alumina sensor. This kind of sensors is produced by partial electrochemical anodization of an aluminum film, while the remaining, non-anodized metallic film is used for supporting the propagation of surface plasmons. The anodized porous alumina is preferable against a flat solid surface as it presents much larger sensing area and thus enhanced detecting efficiency. In this work, a novel method for controlling the remaining aluminum film thickness is reported, based on a simple optical reflectance measurement during the electrochemical anodization of the initial metallic film.

A Sensitive Digital Moisture Detector For Nanostructured Thin Film Sensor

abstract A digital moisture measuring instrument based on phase angle measuring technique with porous silicon (PSi) or porous alumina (PA) as capacitive moisture sensor is proposed. The proposed technique can measure digitally the phase angle change of capacitive impedance of porous silicon or porous alumina sensor due to change in moisture concentration in terms of clock pulses. Analysis shows that the proposed circuit leads to higher precision by minimizing the errors caused by parasitic earth capacitance as well as offset voltage in the circuit. Simulation and experimental results are reported to confirm the effectiveness of the technique.

Probing Adsorption of Polyacrylamide-Based Polymers on Anisotropic Basal Planes of Kaolinite Using Quartz Crystal Microbalance

Quartz crystal microbalance with dissipation (QCM-D) was applied to investigate the adsorption characteristics of polyacrylamide-based polymers (PAMs) on anisotropic basal planes of kaolinite. Kaolinite basal planes were differentiated by depositing kaolinite nanoparticles (KNPs) on silica and alumina sensors in solutions of controlled pH values. Adsorption of an in-house synthesized organic-inorganic Al(OH)3-PAM (Al-PAM) as an example of cationic hybrid PAM and a commercially available partially hydrolyzed polyacrylamide (MF1011) as an example of anionic PAM was studied. Cationic Al-PAM was found to adsorb irreversibly and preferentially on tetrahedral silica basal planes of kaolinite. In contrast, anionic MF1011 adsorbed strongly on aluminum-hydroxy basal planes, while its adsorption on tetrahedral silica basal planes was weak and reversible. Adsorption study revealed that both electrostatic attraction and hydrogen-bonding mechanisms contribute to adsorption of PAMs on kaolinite. The adsorbed Al-PAM layer was able to release trapped water overtime and became more compact, while MF1011 film became more dissipative as backbones stretched out from kaolinite surface with minimal overlapping. Experimental results obtained from this study provide clear insights into the phenomenon that governs flocculation-based solid-liquid separation processes using multicomponent flocculants of anionic and cationic nature.

Electrochemical DO sensor based on sub-micron ZnO-doped RuO2 sensing electrode: Influence of sintering temperature on sensing performance

Electrochemical dissolved oxygen (DO) sensors based on thick-film sub-micron 20mol% ZnO-doped RuO2 sensing electrode (SE) were fabricated on the alumina sensor substrates and were annealed at different sintering temperatures (STs) during the preparation. Their structural and electrochemical properties were closely studied and analysed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and cyclic voltammetry (CV). STs for this research were designated to be 750°C, 800°C, 850°C and 900°C, in order to examine the influence STs on SE characteristics. SEM images demonstrated contrasting surface morphology for different STs. Furthermore, both FTIR spectra and CV plots obtained also signify the variations of ST on SEs. The relationship between the DO and the sensor's response at STs of 800°C and between pH and SE's response was found to be relatively linear with high DO sensitivity for sub-micron 20mol% ZnO-doped RuO2-SE. It has been experimentally established that the optimum results have been obtained when the SEs were sintered at 800°C. Moreover, morphology of the “thick” SE revealed both the presence of “inner” active surfaces within SE and the developed open porosity of SE. Lastly, the pH dependence was quantified and verified in order to determine the effect of various sintering temperatures on the characteristics on the SEs.

Development of antifouling of electrochemical solid-state dissolved oxygen sensors based on nanostructured Cu0.4Ru3.4O7 + RuO2 sensing electrodes

Tailoring nanostructured sensing electrode materials to high antifouling resistance has been one of the main priorities of the development of water quality sensors in the 21st century. Nanostructured Cu0.4Ru3.4O7+RuO2-SEs have been developed to address the bio-fouling problem. The change in Cu0.4Ru3.4O7+RuO2 structural development being promoted by advances in nano- and micro-scale pattering. Nanostructured Cu0.4Ru3.4O7+RuO2-SEs with different mol% of Cu2O were screen-printed on alumina sensor substrates and were consequently subjected to a 3-month field trial at the Water Treatment Plant. Their structural and electrochemical properties before and after the experiment were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical cyclic voltammerty (CV) techniques. The relationship between dissolved oxygen (DO) and the sensor's potential difference was found to be relatively linear, with the maximum sensitivity of −46mV per decade being achieved at 20mol% Cu2O at 7.27 pH. Moreover, a 3-month field trial in the sewerage environment has shown that Cu0.4Ru3.4O7+RuO2-SE with 20mol% of Cu2O possesses much higher defences against bio-fouling than the same SE with only 10mol% of Cu2O. The super-hydrophobic property of the developed Cu0.4Ru3.4O7+RuO2 complex oxide has been considered as one of the essential pre-requisites for high antifouling resistance. Multiple antifouling defence strategies from biomimetic to bio-inspired must be incorporated in further development of nanostructured oxide SE to solve problems of bio-fouling on the sensor's SE.

Investigation of Electrochemical Properties of La2O3-RuO2 Thin-Film Sensing Electrodes Used in Sensors for the Analysis of Complex Solutions

Lanthanum oxide (La2O3)-doped ruthenium oxide (RuO2) thin-film sensing electrodes (SEs) have been prepared by screen-printing for the development of integrated ceramic water quality sensors. Their chemical and structural modifications were investigated by X-ray diffraction (XRD), scanning electrode microscopy, X-ray photoelectron microscopy (XPS), and energy-dispersive X-ray analysis (EDX) techniques. The La2O3–RuO2 SE applied onto an alumina sensor substrate has displayed a linear response to dissolved oxygen in water from 0.5 to 8.0 ppm within a temperature range of 9–35°C. A Nernstian slope of −49.3 mV per decade at 7.48 pH was observed. Selectivity measurements revealed that the presence of Ca2+, Mg2+, Li+, Br−, NO3−, PO43−, and Cl− ions has no significant effect on the sensor's electromotive-force (emf) response. This sensor overcomes the poor selectivity problem commonly observed in semiconductor-based water sensors. This work has also shown that these water sensors based on La2O3–RuO2 SE have potential utilization in instruments for the simultaneous qualitative analysis of complex liquid media based on the principles of an “electronic tongue” device.

Potentiometric solid-state sensor for DO measurement in water using sub-micron Cu 0.4Ru 3.4O 7 + RuO 2 sensing electrode

The dissolved oxygen (DO) sensing electrode (SE) concept utilizing sub-micron-sized ruthenium oxide (RuO 2), doped with other nanostructured oxides, has been extended to investigate the possibility of employing copper (II) oxide (Cu 2O) as a dopant in order to improve sensor's characteristics and meet long term antifouling needs for SEs. In this work, a thin-film SE made of RuO 2 was constructed on the alumina sensor substrate, and a range of dopants and their concentrations was added to it in order to optimize SE properties. The Cu 2O-doped RuO 2 SE had shown a linear response to DO between 0.5 and 8.0 ppm at various temperatures, with two sensitivity maxima of 47.4 and 46.0 mV per decade for Cu 2O concentrations of 10 and 20 mol%, respectively. The maximum sensitivity for Cu 0.4Ru 3.4O 7 + RuO 2-SE was obtained at a dopant concentration of 10%. Selectivity measurements revealed that the presence of Ca 2+, Mg 2+, Li +, Na +, NO 3−, PO 4 3−, SO 4 2−, F −, K + and Cl − in the solution had no significant effect on the sensor's emf. The sensor allows overcoming the problem of an insufficient selectivity of semiconductor-based water sensors. It was also found that the doping of RuO 2-SE by Cu 2O allowed it to function at full capacity in a natural outdoor water body with no obvious effects of biofouling.

Detection of Cyclic Volatile Organic Compounds Using Single-Step Anodized Nanoporous Alumina Sensors

Conventional methods of fabricating anodic aluminum oxide (AAO) materials have utilized a multistep anodization process to manufacture sensor substrates and templates for nanostructured materials. The multistep anodization produces structured, highly ordered hexagonal nanopores. In this paper, it is demonstrated that a single-step anodization process employed in the manufacturing of moisture sensors produces a nanoporous AAO material suitable for the detection and discrimination of low molecular weight volatile organic compounds. Electrical impedance methods have been employed to analyze the electrical response of the single-step anodized AAO materials in the presence of cyclic organic vapors. The sensor exhibits an impedance response that discriminates cyclohexane, cyclohexene, benzene, toluene, and the three isomers of xylene. The impedance measurements have a direct correlation with the relative permittivity, polarizability, and dipole moments of the organic analytes.

Potentiometric DO detection in water by ceramic sensor based on sub-micron RuO2 sensing electrode

An alumina sensor using sub-micron RuO2 sensing electrode (SE) was fabricated and examined for potentiometric dissolved oxygen (DO) detection in water at a temperature range of 9–35 °C. The electromotive force (emf) response at these temperatures was linear to the logarithm of DO concentration in the range from 0.6 to 8.0 ppm (log[O2], −4.71 to −3.59). RuO2-SE displays a Nernstian slope of −41 mV per decade at pH 8.0. It was also found that the response/recovery time to DO changes were sluggish as the water temperature cools down. Response time T90 to DO changes increased from 8 min at a temperature of 23 °C to about 30 min at a temperature of 9 °C. The proton conductivity of hydrous RuO2 appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru (III,IV) cluster ions. In strong alkaline solutions, the sensor’s emf exhibited a mixed potential of fast and slow electrochemical reactions involving DO, RuO42− and OH− ions. The results also revealed that as pH of the solution increases to pH 10.0–13.0, the response/recovery rate becomes faster, stabilizing more or less quickly depending upon the solution alkalinity. Scanning electron microscopy, energy dispersive X-ray-analysis and impedance spectroscopy techniques were used to examine respectively the morphology, crystalline structure and electrochemical behaviour of sub-micron RuO2 oxides.

Impedance properties of polypyrrolic sensors prepared by MAPLE technology

Thin sensitive layers of polypyrrole (PPY) are deposited onto alumina sensor substrates by matrix-assisted pulsed laser evaporation (MAPLE) technology. The depositions are carried out with the KrF excimer laser from dimethylsulfoxide (DMSO) matrix. After 1-month “ageing” period the complex impedance of prepared sensors is measured in the frequency range from 15 Hz to 10 MHz in following atmospheres: “pure” synthetic air and synthetic air with variable (23% or 90%) relative humidity. The general character of Nyquist diagrams, plots of complex admittance versus frequency, parameters of sensor equivalent circuit and phase-angle sensitivities are then evaluated from the obtained impedance data. While equivalent circuit capacity ( C) of PPY-based sensors remains from 1.7 to 2.0 pF in all the above-mentioned atmospheres, equivalent circuit resistance ( R) varies widely from 6 to 140 MΩ, mostly in dependence on surrounding atmosphere humidity. The differences in charge-transport mechanisms between PPY and tin dioxide sensitive layers and also influence of surrounding atmosphere to electric properties of PPY are discussed.

Morphology of Pt-doped nanofabricated RuO 2 sensing electrodes and their properties in water quality monitoring sensors

Integrated water quality monitoring sensors were fabricated using micro-fabrication technologies, nanostructured Pt-doped RuO 2 sensing electrode (SE) and an Ag/AgCl reference electrode (RE). The properties of RuO 2–SE were evaluated for potentiometric detection of pH and dissolved oxygen (DO) at temperatures from 9 to 30 °C. Pt-doped nanostructured RuO 2–SEs were prepared on alumina sensor substrates. Their morphology was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron dispersion spectroscopy (EDX), thermo-gravimetric/differential thermal analysis (TGA/DTA) and Fourier transform infrared spectroscopy (FTIR). It was found that morphology and surface compositions of nanostructured RuO 2–SEs are sensitive to superoxide oxygen ions (O 2 −). Sensors with Pt-doped RuO 2–SE showed Nernstian response to pH from 2.0 to 13.0 and were also capable of measuring DO in the range of 0.6–8.0 ppm. The measurement results show very good linearity and an excellent reproducibility was obtained during the 8 months of testing so far. Nernstian slope was 58 mV/pH at a temperature of 23 °C. Although RuO 2–SEs have exhibited very good response time to pH changes, which was within 1–2 s at a temperature of 23 °C, as water temperature cooled down, the sensor response time increased and was about 8–10 min at working temperature of 9 °C. Influence of the hydrogen ion (H +) diffusion in RuO 2 films on the output emf drift at pH measurements was also investigated.