Limiting-current type surface modified gas sensor based on β″-alumina fast solid ionic conductor

A new type of limiting current solid state ionic gas sensor is presented. The sensor is based on the reaction of the gaseous species with the electroactive component under the condition of limited access of the gas by a kinetic barrier. This allows one to employ the available fast solid alkali metal, silver and copper ionic conductors at ambient, and moderately increased temperatures. Theoretical considerations are presented and compared to experimental results of this type of limiting-current surface modified solid state electrochemical chlorine gas sensors. A linear relationship between the electrical current and the gas concentration is observed. The device may be operated at low temperatures, is sensitive to low partial gas pressures and may also overcome problems of cross-sensitivity. Sensors for gases other than chlorine may be based on the same principles.     

Studies on solid electrolyte gas cells with high-temperature-type proton conductor and oxide ion conductor

Various types of gas cells are studied using high-temperature-type proton and oxide ion conductors as the solid electrolyte. Steam and hydrogen concentration cells could be constructed using the SrCeO₃-based proton conductive solid electrolyte. Using the oxide ion conductor, YSZ, the steam concentration cell could also be constructed in hydrogen atmosphere. Some characteristics of these cells are discussed.     

Investigation of solid sodium reference electrodes for solid-state electrochemical gas sensors

Several types of solid reference electrodes for potentiometric gas sensors based on solid sodium ion conductors (e.g., Na⁺-/-Alumina or NASICON) are described and the observed experimental results are discussed in view of theoretical predictions. Defined sodium activities are established by appropriate equilibria of binary and ternary phases. The binary systems Na-Sb and Na-Bi, and the ternary systems Na-M-O (M is a transition-metal, Co or Ni) were found to show excellent kinetic performance and are capable to fix the chemical potential of sodium well defined over extended periods of time. Some of the sodium reference electrodes are even stable in air. The results are compared with the application of elemental sodium as reference electrode.     

Characterization of solid oxide fuel cells based on solid electrolytes or mixed ionic electronic conductors

The relation between cell voltage (V_cell), applied chemical potential difference (Δμ(O₂)) and cell current (I_t) for solid oxide fuel cells (SOFC) based on mixed ionic electronic conductors is derived by considering also the effect of electrode impedance. Four-probe measurements, combined with current interruption analysis, are considered to yield the relation between ionic current (I_i) and overpotential (η). The theoretical relations are used to analyze experiments on fuel cells with Ce_0.8Sm_0.2O_1.9 and Ce_0.8Gd_0.2O_1.9 electrolytes with La_0.8Sr_0.16CoO₃ or Pt as the cathode and Ni/Ce_0.9Ca_0.1O_1.9−xor Pt as the anode. The electrode overpotentials of these cells, determined by current interruption measurements, are discussed assuming different models including impeded mass transport in the gas phase for molecular and monoatomic oxygen and Butler-Volmer type charge transfer overpotential.     

The effect of water vapor on the electrical properties and sensitivity of thin-film gas sensors based on tin dioxide

The results of theoretical and experimental studies into the effect of water vapor on the electrical conductance of a gas sensor and the sensor response to hydrogen action are discussed. A relation describing the dependence of electrical conductance G₀ on absolute humidity in the pure air is derived using a hypothesis of the presence of space-charge regions depleted of electrons between the SnO ₂ grains in a polycrystalline tin dioxide film. Due to dissociative chemisorption of water molecules, the energy-band bending at the SnO ₂ grain interfaces decreases and the oxygen-vacancy concentration in the grains increases, resuling in an increase in G₀. An equation for the sensor response to hydrogen action is derived (the G₁/G₀, ratio, where G₁ is the sensor conductance in a gas mixture containing molecular hydrogen). The expression describes the dependence of G₁/G₀ on the hydrogen concentration in the interval 50–6·10³ ppm, band bending at the SnO ₂ grain interface, and sensor temperature. The dependences of the sensor conductance, highest possible conductance, and energy-band bending on temperature and absolute humidity resulting from processing of the experimental data are in good agreement with the theoretical predictions. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 50–56, August, 2008.     

Dependence of an electrochemical cell performance upon the thickness of the ionic conductor

The thickness of the ionic conductor in an electrochemical cell is shown to be among the governing parameters of the cell's performance. Internal field, I-V characteristics and p-n ‘junctions’ formed upon voltage application are investigated through a computer model based on fundamental charge transport equations including also Poisson's equation.     

Influence of calcium substitution on the phase transition and ionic conductivity in BICAVOX oxide ion conductor

Samples of Bi₄Ca _x V_{2? x} O_{11?(3 x /2)?δ} in the composition range 0.07 ≤ x ≤ 0.30 were prepared by conventional solid state reactions. The stability of different phases as a function of composition was analysed by X-ray powder diffraction, FT-IR spectra, differential thermal analysis and AC impedance spectroscopy. For the compositions x ≤ 0.10, monoclinic α-phase structure is retained at room temperature. For x = 0.13, orthorhombic β-phase is observed, whereas for x ≥ 0.17, high O^2?conducting tetragonal γ-phase is stabilised. However, the highest ionic conductivity σ_300°C = 3.27 × 10^?4 S cm^?1 was observed for x = 0.17. This higher value of conductivity of the substituted compound as compared to the parent compound can be attributed to the increased oxygen ion vacancies generated as a result of cation doping. AC impedance spectroscopy reveals the fact that this ionic conductivity is mainly due to the grain contribution.     

Investigation on the thermal radiation properties of antimony doped tin oxide particles

This paper reports the preparation of antimony doped tin oxide crystalline powders by chemical coprecipitation method. The influence of sintering temperature and the sintering retention time on the thermal infrared emissivity is analysed. The thermal infrared reflectivity is measured and the optimum doping concentration is proposed.     

Indium and tin oxide polycrystalline thin films as NO gas sensors produced by reactive pulsed laser ablation and deposition

Pulsed laser ablation is a very interesting method of depositing thin films of several materials and compounds, such as oxides, nitrides, insulators, semiconductors, and superconductors. Indium and tin oxide polycrystalline thin films have been grown on silicon (100) substrates by reactive PLD from two metallic targets of indium and tin by multilayered deposition, in the presence of oxygen, using a frequency-doubled Nd-YAG laser (5=532 nm). The films produced have been studied to evaluate their use as NO gas sensors, and the best performance has been found by varying some important parameters, such as the substrate temperature and the pressure of oxygen in the deposition chamber. X-ray diffraction analysis of the deposited films shows that they are polycrystalline with a preferential (400) orientation. Electrical resistivity measurements, performed by using a four-point probe technique, show a sharp increase in resistivity when the films are exposed to NO. The electrical responses of tin oxide-indium oxide multilayered thin films are reported.     

Strain sensors based on carbon nanotubes-cuprous oxide composite

This paper presents the design, fabrication and experimental results of carbon nanotubes-cuprous oxide (CNTs-Cu₂O) composite based strain sensors. The press-tablets were fabricated from the blend of CNTs (25 wt%) and Cu₂O (75 wt%) at a pressure of 353 MPa. The diameter of multiwalled carbon nanotubes (MWCNTs) varied between 10 and 30 nm. The sizes of Cu₂O micro-particles were in the range of 3-4 μm. The thickness of the press-tablets was 1 mm. The samples were installed on the polymer elastic beam by glue. The electric contacts to the samples were made by silver paste. The inter-electrodes distance (length) and diameter of the surface-type samples were in the range of 6-8 mm and 10 mm, respectively. The DC resistance of the strain sensors increases under tension and decreases under compression, while the average strain sensitivities are in the range of 44-46 and 24-28 for tension and compression, respectively. The simulation is in good agreement with the experimental results.     

Electrochemical sensors based on transition metal oxide bronzes

Tungsten and molybdenum oxide bronzes of the general formulas A_xMO₃ and A_xM₆O₁₇ (A=K, Li, Na; M=W, Mo) are well known as electrode materials with good response to pH changes. However, there is also a high cross sensitivity to other ions, e.g. Na⁺ or Li⁺. Currently, the synthesis of tailored materials for special applications is the priority task. Preparation routes such as fused salt electrolysis, solid state reactions in vacuum or hydrothermal synthesis were studied. New compounds of the general formula Li_xMo_1−yW_yO₃ and also A_xWO₃ (A = bi- or trivalent) have a high technological potential. Preliminary experiments have shown that some of these new materials possess properties of reference electrodes within a restricted pH range. To prepare electrodes, usually single crystals are required. This is adverse because of high operating expense of single crystal synthesis. In some cases it is also possible to use polycrystalline materials for the preparation. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Oxygen self-diffusion and ionic conductivity of oxide solid electrolytes

The self-diffusion coefficient for a stochastically nonuniform thermodynamic system is represented as the mean value of the transition rates. A model for the ionic transport in solid oxide electrolytes is proposed. The existence of percolation cluster of the doping cations is taken into account in the model. The maximum of the concentration dependence of the ionic conductivity is explained by the blocking effect and random distribution of traps. The problem of inconsistency of theoretical and experimental values for the pre-exponential factor is discussed and an approach is proposed to overcome this disagreement.     

The effect of local inhomogeneities on the performance of solid ionic devices

Application of voltages to mixed conductors can lead to the formation of local inhomogeneities. As a result, high local field values may appear with an impact on the device performance. The effect of voltage application is investigated through a computer model based on Maxwell's third equation and fundamental equations governing charge transport. Results are in agreement with experimental ones reported in the literature. The formation of p-n junctions is also clearly demonstrated.     

Room-temperature gas sensors based on gallium nitride nanoparticles

Room-temperature sensing characteristics for H₂, ethanol, NH₃, H₂S and water have been investigated with thick-film sensors based on GaN nanoparticles, prepared by a simple chemical route. In general, GaN nanoparticles exhibit satisfactory sensor properties for these gases and vapors even at room temperature. The sensitivity for ethanol is found to be highest, the sensitivity and recovery times being smallest. Gas sensor properties of GaN seem to be related to intrinsic defects, which act as sorption sites for the gas molecules.     

Structure and conductivity characterization of new type ionic conductor stabilized bismuth oxide ternary systems

In this study, the new type electrolyte (Yb₂O₃)_x(Dy₂O₃)_y(Bi₂O₃)_1-x-y ternary compounds were synthesized with different stoichiometric ratios by the solid-state reaction method at different annealing treatment and also their microstructural and electrical properties were analysed. X-ray powder diffraction results showed that the high temperature δ-phase of pure monoclinic Bi₂O₃ has been synthesized by doping of Yb₂O₃. Grain size and grain form of pellet formed samples was compared from their surface images taken by the scanning electron microscopy. The grain size has been varying between ～17–37 µm, and degrading with the increasing dopant concentrations. The relationships between the structural parameters (e.g. lattice parameters, crystallite size and the lattice microstrain) and structural properties (e.g. ionic radii of dopant cations and heat treatment procedure) were particularly discussed. Total conductivity values were calculated by Nyquistic complex impedance plot. Impedance measurement revelaed that total conductivity values of the samples increase with the increasing Yb dopant ratio. The activation energies calculated by the Arrhenius approach are measured at around 1?eV. In addition, activation energies and pre-exponential terms decrease with the increasing Yb cation dopant rate for the same ambient temperature.     

New solid electrolytes based on bismuth oxide

The oxide ion conducting systems Bi₂O₃-Y₂O₃-Pr₆O₁₁ and Bi₂O₃-Y₂O₃-ZrO₂ have been prepared and studied in order to combine the advantages of stabilized Bi₂O₃ and ZrO₂ solid electrolytes. Coprecipitation of high purity oxides was used for preparation. The formation of the fluorite-type cubic structure was confirmed by X-ray powder diffraction analysis. The relative contributions of ions and electrons to the total conductivity were measured by the concentration cell method. Using Ultraviolet Photoemission Spectroscopy (UPS) we have determined the work function Φ of electrons, the position of the Fermi level in relation to the valence band edge (E_F-E_V) and the change of the ionization potential (I) as a function of temperature. For bismuth oxide-based solid electrolytes, we used an Fe/FeO mixture as reference contact to establish a defined oxygen partial pressure in the solid electrolyte sample. Oxygen isotope exchange experiments were performed in an exchange cell with a gas regulating system and mass spectrometer to determine the oxygen surface exchange rate. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HC's) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga₂O₃ were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C₃H₆, O₂ gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C₃H₆. The optimal composition is found to be at 20 mass-% Ga₂O₃. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Sensing behaviour of tin doped chromium oxide gas sensor toward ethanol

In this paper, the enhancement of gas sensing response due to addition of tin into Cr₂O₃ has been reported. Sn-doped Cr₂O₃ nanoparticles have been prepared by a co-precipitation method and characterised by X-ray diffraction, field emission scanning electron microscopy and energy dispersive X-ray analysis. X-ray diffraction studies revealed the substitution of Cr³⁺ ions by Sn⁴⁺ ions. Field emission scanning electron microscopy images exhibited presence of clusters and agglomerates on the surface. The concentration of tin, used as dopant, was varied from 1 to 5?wt.% and its effect on gas sensing response has been studied. Synthesised powders were applied as thick film onto alumina substrate and tested for ethanol sensing at different operating temperatures and all the sensors gave an optimum response at 250?^°C. The activation energy of conduction for all the samples was estimated using Arrhenius plots and it was observed that the sample doped with 4?wt.% Sn possesses minimum activation energy, and interestingly this sample gave the best sensing response in the lot.     

Instability controlled synthesis of tin oxide nanofibers and their gas sensing properties

Instability dependent electrospinning process has been controlled to obtain tin oxide nanofibers with morphological variation. The effect of spinning parameters such as viscosity, conductivity, flow rate, distance and applied voltage on growth rate of different instabilities was simulated and different deposition conditions were defined from the simulation results. The structural morphology was analyzed using X-Ray Diffraction (XRD) and Scanning Electron microscope (SEM). The sensing behavior of different structures was investigated. The branched structure obtained due to axisymmetric instabilities exhibited best sensing performance owing to high surface to volume ratio.