SnO2 Thick Film Research Articles

Effects of previous treatments on the electrical response of SnO 2-thick films exposed to a CO atmosphere

The influence of CO on the electrical conductivity on SnO 2 thick films is studied. Possible sensing mechanisms are discerned by measuring the conductivity as a function of temperature during heating and cooling, and the transient responses to step changes in CO pressure at constant temperature. Studies of samples with prior treatments confirm the proposed mechanisms responsible for the observed film responses. The film conductivity is affected by CO adsorption and by reaction with previously adsorbed oxygen. Also, the oxygen vacancy concentration in the grains can be altered and, as a consequence, the sample resistivity.

Effect of Ni doping on thick film SnO 2 gas sensor

Ni- and/or Al-doped and undoped SnO 2 thick film gas sensors were prepared using screen printing technique and tested for their LPG gas sensitivity. Tin oxide powder was prepared using a chemical precipitation technique. The sensitivity, optimum working temperature and response time were investigated in relation to dopants as well as preparation route. The results show that the gas sensitivity is affected not only by the additive but the way it is added into the sensor material. The results indicated a reduction in grain size on Al and Ni doping. The results on resistance, response and recovery time were explained in terms of n–p junction formation between SnO 2 and NiO, which increases the depletion barrier height.

Effect of Simultaneous Modification with Metal Loading and Mesoporous Layer on H2 Sensing Properties of SnO2 Thick Film Sensors

Effect of Simultaneous Modification with Metal Loading and Mesoporous Layer on H2 Sensing Properties of SnO2 Thick Film Sensors

Selective filter for SnO 2-based gas sensor: application to hydrogen trace detection

The main drawback of the SnO 2-based gas sensors is their low selectivity. In this study, we present a highly selective hydrogen sensor with minimum cross sensitivity to ethanol, methane, carbon monoxide and hydrogen sulfide. Thick film SnO 2 sensors are treated by hexamethyldisiloxane (HMDS) at high temperature (500–600 °C). After this treatment, the electrical properties of sensors are greatly modified: the sensitivity towards hydrogen is markedly increased and a high selectivity to hydrogen is achieved. In the meantime, transient responses curves to H 2 trace are altered: the response and recovery times are longer for treated sensors than for the untreated one. Nevertheless, if the temperature of the sensor is increasing from 450 °C to 550 °C, the response time notably decreases and the detection of hydrogen trace as low as 250 ppb (v/v) is quite possible with a good stability. HMDS-treated material has been characterized with different analytical methods: SiO 2 formation is confirmed by Fourier transform infrared transmission spectroscopy (FTIR) and scanning electron microscopy (SEM) equipped with X probe. Moreover, temperature-programmed desorption (TPD) experiments reveal that surface hydroxyl groups are involved in the HMDS–SnO 2 interaction. It could be supposed that a weakly porous Si-based coating film overlaps the treated SnO 2 material and acts as a molecular sieve. Hydrogen can pass easily through the dense filter whereas the diffusion of other gases, and especially oxygen, is considerably reduced. The increase in sensitivity is explained with a simplified model based on an inversely proportional relation between the conductance of the SnO 2 material and the concentration of adsorbed oxygen ions.

CO-sensing properties of doped SnO2 sensors in H2-rich gases

Differently doped thick film SnO2 sensors were investigated in reducing atmospheres with high H2 content. Such atmospheres appear in reforming processes frequently used for the production of hydrogen for the operation of fuel cells. The thermodynamic stability of SnO2 against reduction to the metal limits the operation temperature to temperatures below 320°C. Pt-, Pd-, and Au-doped SnO2-sensors showed sensitivity to CO. A maximum sensitivity was found for SnO2 sensors doped with 2wt.% Pt at around 220°C. The cross-sensitivities to hydrogen, carbon dioxide, and water are much lower. Typical response time (τ63) to changes in the CO content of about 2–3s were found. diffuse reflectance infrared Fourier transform (DRIFT)-spectroscopy indicates that the surface of SnO2 is covered mainly with formate, OH, and methoxy (OCH3) groups. A simple model describing the resistance changes upon changes in CO concentration is proposed.

Metal oxide-based monolithic complementary metal oxide semiconductor gas sensor microsystem.

A fully integrated gas sensor microsystem is presented, which comprises for the first time a micro hot plate as well as advanced analog and digital circuitry on a single chip. The micro hot plate is coated with a nanocrystalline SnO2 thick film. The sensor chip is produced in an industrial 0.8-microm CMOS process with subsequent micromachining steps. A novel circular micro hot plate, which is 500 x 500 microm(2) in size, features an excellent temperature homogeneity of +/-2% over the heated area (300-microm diameter) and a high thermal efficiency of 6.0 degrees C/mW. A robust prototype package was developed, which relies on standard microelectronic packaging methods. Apart from a microcontroller board for managing chip communication and providing power supply and reference signals, no additional measurement equipment is needed. The on-chip digital temperature controller can accurately adjust the membrane temperature between 170 and 300 degrees C with an error of +/-2 degrees C. The on-chip logarithmic converter covers a wide measurement range between 1 kOmega and 10 MOmega. CO concentrations in the sub-parts-per-million range are detectable, and a resolution of +/-0.1 ppm CO was achieved, which renders the sensor capable of measuring CO concentrations at threshold levels.

Model of the thickness effect of SnO2 thick film on the detection properties

The variations of the electrical response of SnO2 thick film sensors (10–80μm) prepared by screen-printing technology versus their thickness depend on the studied gases: at 500°C, the ratio Ggas/Gair for CO (300ppm) presents a maximum while the ratio for ethanol (100ppm) continuously decreases with increasing thickness. A simplified geometrical model based on a resistivity distribution within SnO2 depleted area, depending on oxygen consumption by the reducing gas, is presented. This model takes into account the role of the metal (gold electrodes) which enhances the creation of space charge area thanks to the three boundary points “gas–metal–oxide”. Calculation of conductance (G) using the proposed model allows to simulate the influence of the thickness on the response for both studied gases.

Mesoporous catalytic filters for semiconductor gas sensors

An effective way to improve sensor selectivity and stability is the use of catalytic filters to block interfering and poisoning gas molecules from reaching the sensor surface. Mesoporous silica with high resistivity and hugh surface areas are ideally suited as a base material for this application. When impregnated with proper catalysts, mesoporous silica has a great potential to eliminate responses to undesired gases even of thin-film and/or micro-machined sensors. In this paper, we report our initial results on thick film SnO 2-based gas sensors covered with a catalytic filter consisting of Pd and Pt loaded mesoporous silica. Results indicate that selective oxidation of CO in the catalytic filter leads to the elimination of CO interference to a CH 4 sensor with no perceptible deterioration in sensing performance.

Influence of particle size on the conductance of SnO2 thick films

Fil: Ponce, Miguel Adolfo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnologia de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingenieria. Instituto de Investigaciones en Ciencia y Tecnologia de Materiales; Argentina

The gas-sensing potential of nanocrystalline SnO 2 produced by a mechanochemical milling via centrifugal action

In this work, the synthesis of undoped nanocrystalline tin dioxide powders and the subsequent preparation of SnO2 thick-films were studied. An initial mixture of SnCl2 and Ca(OH)2 was sealed in a vial for milling in an air atmosphere. Heat treatment of the milled powder resulted in the formation of tetragonal and orthorhombic SnO2 phases, which was confirmed by X-ray diffraction (XRD) analysis. It was found that crystallite size could be controlled by varying the milling time, the rotation speed and the temperature used for the heat treatment. Crystallite sizes in the range 20 to 30 nm (determined by XRD measurements) were obtained. The total pore volume was 0.22 ml/g for a measured particle size of 37 m2/g. No contamination of the powder during milling was found. The response of the prepared thick-films to H2S gas in the concentration range 0.5 to 10 ppm in air was investigated as a function of the preparation conditions. The advantage of mechanochemical synthesis of powder is its relative simplicity, low cost and possibility of obtaining isolated, unagglomerated nanosized grains. It is shown that chemical reactions, which usually occur in the vibratory mill to produce the SnO phase, can also be initiated during a short processing time in the centrifugal mill.

Improvement in the Selectivity of Semiconducting Resistive-type NO2 Sensors Linked with Calorimetric Hydrocarbon Sensors

Improvement in the selectivity of semiconducting resistive-type NO2 sensors has been achieved by numerical compensation derived from the output of calorimetric hydrocarbon sensors. The electrical resistance of resistive-type sensors using thick-film SnO2 increases due to exposure to NO2, whereas its resistance is decreased by i-C4H10. Oxidizing gases such as O2 and NO2 which are electronegative are adsorbed on the surface of n-type semiconductors, whereas reducing gases such as hydrocarbons and carbon monoxide react with these adsorbed oxidizing gases, causing electrons to return to the semiconductor. Calorimetric sensors are based on the principle of catalytic combustion between reducing gases and atmospheric oxygen on the surface of gas-detecting materials, therefore they are sensitive to reducing gases such as hydrocarbons, but are insensitive to oxidizing gases. It was found that the resistance of NO2 sensors in the range from 0 to 100 ppm NO2 was unaffected by the co-existence of 0 to 300 ppm i-C4H10 using a semiconducting NO2 sensor, a calorimetric sensor and numerical compensation.

The influence of Rh surface doping on anomalous properties of thick-film SnO 2 gas sensors

Here there are presented new results from the investigations of SnO 2 thick-film sensor. The effects of rhodium surface doping on dc and ac properties of SnO 2 gas sensors were studied. The measurements reveal anomalous behaviour of the sensors. Below characteristic temperature, the resistance of the sensors increases in presence of reducting gases. The mechanism associated with the presence of rhodium can be connected with the increase of acceptor surface states concentration and causes the inversion of near-surface layers.

Investigation of CO/CH 4 mixture measured with differently doped SnO 2 sensors

Arrays consisting of thick-film SnO 2 sensors (based on three different materials and operated at two different temperatures) were exposed to mixtures of carbon monoxide (0, 3, 7 and 14 ppm), methane (0, 400, 1500 and 5000 ppm) and water (0, 30 and 50% RH at 25°C). It was possible to discriminate between different mixtures of CO and CH 4 in the presence of different relative humidities in air. In addition, it was possible to quantify both CH 4 and CO in the background of the other by using multivariate data analysis.

Scanning tunneling microscopy and spectroscopy of tin oxide films

Abstract SnO2 thick films have been studied with scanning tunneling microscopy and spectroscopy. Topographic images revealed grains with an average diameter of about 100 nm and roughness of 50 nm. Tunneling current-voltage characteristics measured indicate that these small grains present a non rectifying behavior. Temperature dependence of electrical conductivity during heating and cooling and resistivity transients at step isothermal changes in oxygen pressure were also investigated. Results are consistent with those of STM and confirm that oxygen adsorption and diffusion into the tin oxide grain account for the observed conductance changes.

High performance gas sensing of CO: comparative tests for (SnO 2-based) semiconducting and for electrochemical sensors

A comparison of the stability and sensitivity for two different sensor types (semiconductor SnO 2 devices and amperometric electrochemical sensors) has been performed. Sensitivities and drifts in the signal and in the background for various concentrations of CO have been studied for thick film SnO 2 sensors (undoped, Pt and Pd doped) over a period in excess of 8 months. Similar performance data have been recorded for amperometric sensors over a period in excess of 3 years. The two sensor types investigated here were also compared to well known commercial SnO 2-based sensors at similar concentrations.

Ageing and p-type conduction in SnO 2 gas sensors

A new model is proposed for conduction in thick-film SnO 2 gas sensors, which offers a good explanation for the ageing effect. In this model, electrical resistance is governed by necks and it results from three interdependent resistances of three different regions of the sample, connected in parallel: a surface resistance which is that of a gas-sensitive, predominantly n-type semiconductor, a resistance, belonging to a different region of the sample, which, due to its interaction with water vapour, behaves like a mixed n-type and p-type semiconductor, and the n-type bulk resistance of the neck, slightly affected by gases and/or water vapour.

High-Performance Gas Sensing of CO: Comparative Tests for Semiconducting (SnO2-Based) and for Amperometric Gas Sensors

A comparison of the stability and sensitivity for two different sensor types (semiconductor SnO(2) devices, amperometric electrochemical sensors) has been performed. Sensitivities and drifts in the signal and in the background for various concentrations of CO have been studied for thick-film SnO(2) sensors (Pt and Pd doped) for a period in excess of 8 months. Similar performance data have been recorded for commercial amperometric sensors for a period in excess of 4 years. The two different sensor types investigated here were also compared to the well-known commercial Figaro TGS 822 sensor at similar concentrations. An objective approach for comparing different types of sensors has been developed using the "analytical sensitivity".

Sensitivity, response and recovery time of SnO 2 based thick-film sensor array for H 2, CO, CH 4 and LPG

This paper deals with the studies carried out on sensitivity, response and recovery time of a sensor array comprising of undoped and doped (palladium, platinum and gold) thick-film SnO 2 sensors. Initially, the sensitivity of all the sensors of the array has been studied for H 2, CO, CH 4 and LPG followed by detailed analysis of transient response of Pd-doped sensors as it possesses better sensitivity for all the test gases. The response and recovery time of the Pd-doped sensor has been found to depend on type of gas and its concentration (upto 400 ppm). The response time is minimum for CO and maximum for H 2 while recovery time is maximum for LPG and approximately same for H 2 and CO. The observed results of response time have been explained on the basis of two-film theory, i.e. gas film-gas phase and solid-gas film interfaces. Further, the experimental result of the sensor array (sensitivity versus concentration and sensitivity versus response time) for all the test gases at an operating temperature of 350°C have been validated with an analytical model and the observed results are found to be in good agreement to that predicted by the model.

Correlation between XPS, Raman and TEM measurements and the gas sensitivity of Pt and Pd doped SnO 2 based gas sensors

Nanocrystalline thick-film SnO2 sensors with different dopants were fabricated by an optimized screen printing process and subsequent annealing. Powders were used as starting materials which were prepared by a wet chemical process from SnCl4. Microanalysis was performed of both, the precursors and the final sensor materials with their different annealing conditions. Gas sensing tests with CO, CH4 and NO2 in air with controlled humidity were correlated with results from X-ray photoemission spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). As an interesting result, the distribution of the transition metal dopants Pd and Pt (as deduced from TEM and XPS data) rules out the existence of metallic clusters or even atoms in the metallic state at the surface. This finding does not allow to explain the sensor effects on SnO2 based materials as usually done by means of spill-over effects or Fermi energy control.

Effects of gas diffusivity and reactivity on sensing properties of thick film SnO2-based sensors1Paper presented at the 2nd East Asia Conference on Chemical Sensors, Xi'an, P.R. China, 1995.1

Abstract Effects of gas diffusivity and reactivity on gas-sensing properties of porous thick film SnO 2 -based sensors have been investigated by employing several kinds of objective gases. The interior of a pure SnO 2 sensor was confirmed to exhibit higher sensitivity to H 2 than the surface. A reverse tendency was observed for CH 4 or C 2 H 5 OH with larger molecular size than O 2 . Excess enhancement in catalytic activity of sensor materials, for example by loading 1.0 wt% Pt or Pd, resulted in deterioration of the interior H 2 sensitivity, while the interior CH 4 or C 2 H 5 OH sensitivity was enhanced by the Pt or Pd loading. Loading of 1.0 wt% Au led to significant enhancement in both the interior and the surface H 2 sensitivity, irrespective of the small chemical sensitization effect of Au.