Versatile Measurement System for the Characterization of Gas Sensing Materials

Tommaso Addabbo, Francesco Bertocci, Ada Fort, Marco Mugnaini, Valerio Vignoli, Luay Shahin, Santina Rocchi Department of Information Engineering and Mathematical Sciences

University of Siena Siena, Italy

ada@dii.unisi.it

Abstract—A measurement system designed for the development of novel conductometric gas sensors and of new gas sensing materials is described. The system allows to simultaneously characterize up to 8 sensors. Prototype sensors can be easily realized thanks to an ad hoc structure based on an alumina substrate equipped with electrodes, a heater and an accurate temperature sensor, on this structure the studied material can be deposited by screen printing, spin coating or dip coating. The system is designed to study the behavior of the sensors by accurately setting the operating conditions in terms of chemical environment composition, gas flow, humidity and temperature. The system is fully programmable and it individually controls the film temperatures or measures them with a resolution lower than 0.1 °C. Both chemical transients and thermal transients can be studied. These features make the system suitable for determining the principal performance indexes of a gas sensing device (e.g., sensitivity, stability, selectivity, response/recovery times, etc.) as functions of various combinations of measurement conditions (e.g., gas concentrations, temperature, humidity, flow). The proposed measurement system will find also useful applications in sensor model validation.

Keywords—conductometric gas sensors, metal oxide, instrumentation, sensor characterization

I. INTRODUCTION In response to the need for measuring gas concentrations in

many different fields of application, from the food quality assessment, to the combustion process control and monitoring, or to the environmental monitoring, a lot of attention has been paid to the development of solid state gas sensors. In this context a special consideration goes to the development of conductometric gas sensors based on metal oxide sensing materials. These latter, having many attractive characteristics such as low cost production processes and requirements of simple readout electronics, seem the ideal candidates for the development of low cost systems. Some semiconductor metal oxides, as SnO2 or ZnO, have been thoroughly studied in the last 4 decades and they are already used also in commercial devices. Nevertheless, some issues related to their usage are still open so that, also in the recent years, many research efforts have been devoted to the preparation and to the study of different or new metal oxides. It should be underlined that to decide if a new material is suitable or not for gas sensing, and to characterize the performance of a novel sensor based on it, is a challenging task, that is mostly performed on the basis of

measurement results. It must be noted, in fact, that many phenomena contribute to the sensing mechanism, which, in case of metal oxide conductometric sensors, are very complex: they comprise chemical solid-gas reactions and physical phenomena related to electronic conduction. Especially the first ones are very difficult to be observed and often remain the subject of hypotheses difficult to assess with independent measurements.

In addition, the chemical-physical behavior of the sensor depends, of course, on the material in terms of composition and on bulk and surface structure., but it is also heavily affected by the crystal bulk and surface defect population and finally by the sensor micro- and macro-structure. All these aspects that are crucial for the electronic conduction in the sensors [1]-[3] are almost impossible to be a-priori determined.

As a last remark, it must be stressed that the sensor performance is strongly dependent on its working conditions, in terms of temperature, humidity, and presence of interfering gases. For instance the temperature of the film has usually a dramatic influence on the sensor performance (in most cases there is an exponential dependence of the sensor conductance on the temperature). For this reason the characterization of the sensor should be performed in experimental conditions that grant for a very accurate knowledge of the working conditions.

A further issue that makes the experimental characterization of this kind of gas sensing material a really hard task is their very limited response speed. For this reason very long measurements are required, in order to get meaningful information in terms of reproducibility. For this reason, an experimental set-up working on an array of prototype sensors is very recommendable.

In this paper a fully programmable and versatile system for the simultaneous characterization of up to 8 novel conductometric gas sensors is presented. The system individually controls the film temperatures or measures them with a resolution of about 0.1 °C. Both chemical transients and thermal transients can be studied. These features make the system suitable both for determining the performance indexes of a gas sensing device (e.g., sensitivity, stability, selectivity, response/recovery times, etc.) as functions of various combinations of measurement conditions (e.g., gas concentrations, temperature, humidity, flow), and for validating sensor models.

II. SYSTEM ARCHITECTURE

A. Sensor Structure A proper structure for the facile development of

conductometric gas sensors was designed (Fig. 1). This structure allows for testing new materials prepared as micro- or nano-structured powders. On one side of a 0.2 mm thick alumina substrate an electrode pair for the chemical resistor as well as a platinum based Resistive Temperature Device (RTD) have been designed and built through a screen printing technique; on the other side a heater resistor has been deposited in the same way.

The realization of RTD Pt-based sensor close to the gas sensing film allows to obtain an accurate sensing of the film average temperature, which is of the utmost importance for gas sensors characterization as already underlined in the introduction.

Fig. 1. Sensor structure: a) heater; b1) temperature sensor, b2) chemical sensor

A sensing layer can then be deposited on top of the two electrodes by screen printing, spin-, drop- or dip-coating, dried and fired. A simulation study was performed in order to understand how the average temperature of the sensing film is related to the temperature sensed by the RTD sensor. In figure 2 the results of a 2-D simulation are shown, in particular the temperature distribution on the substrate, is color coded in figure 2a) and contour plotted in figure 2b). Figure 2d) shows the temperature of the middle line (maximum temperature, continuous line plot) and on the edge of the substrate (minimum temperature, dashed line plot). It can be seen that an uniform temperature is expected in the region where the sensing film is deposited (temperature is distributed within 5 °C). Finally figure 2c) shows the average temperature of the chemical sensing film area, and the average temperature of the RTD area, which is the actual sensed temperature. From these simulation results it appears that in the range 150°C-400° the temperature difference between the sensed temperature and chemical film temperature is lower than 2°C.

B. Measurement System Overview The system is composed by three main blocks: the chemical

sampling system, the conditioning and acquisition electronics and the host computer which manages the overall experiment and store data (Fig. 3).

The chemical sampling system consists of up to 9 digitally-controlled flow-meters (operating in the range from 5 mL/min to 500 mL/min) that are used to set the flows of the reference

gases coming from the gas reservoirs. One of these is usually dedicated to a carrier gas (nitrogen or synthetic dry air) the other ones to reference mixtures of the carrier gas and target gases at known concentrations.

Fig. 2. Temperature distribution on the sensor surface obtained from 2-D

simulations accounting for convection and heat conduction in the test chamber (Tchamber=45°C, heat transfer coefficient=20 W/(Km2), thermal

conductivity=50 W/(Km))

The flows are mixed in order to obtain a mixture with selected concentrations of the target gases. The humidity level is set by a water bubbler, kept at a fixed and known temperature by a thermostatic bath, fed by the carrier gas.

PCLabview

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controller

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Fig. 3. System overview

During the measurements the total flow entering the chamber is kept constant whereas its composition can be varied by changing the flow-meter set points. This allows to test the sensors under chemical transients or under different static chemical conditions. An important part of the chemical sampling system is the measurement chamber: the one used in

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the presented system is a cylindrical stainless steel chamber that can host up to 8 sensors placed in a circular array. The symmetry of the chamber grants very close chemical testing conditions for all the sensors.

The conditioning and acquisition electronics comprises: eight front-end boards, one board containing the heater drivers, and a real time PXI system equipped with one 8 analog output board, one 16 single ended analog input board, and one controller for real time operations.

The eight front-end boards are mounted through connectors on a circular support-board placed in the chamber. On the front end boards, connectors tailored to the sensor pin-out allow for easy placing and substitution of the sensors. Each front-end board hosts the conditioning electronics for the measurement signals. As it can be seen in figure 3, a R-V converter based on a constant, accurate and stable current generator and on an instrumentation amplifier is used for the read-out of the Pt-RTD value. Being this circuit very close to the sensor pins, noise and interference are minimized, and this allows for obtaining highly accurate temperature measurements. The chemical sensing film resistance is converted into voltage by a voltage divider with selectable high precision reference resistances. This technique was chosen due to the large measurement range required for the sensing layer resistance which, being exponentially related to the temperature and often also to the charged adsorbed chemical species density, usually varies of some orders of magnitude during a test.

The conditioning circuits needed for heater driving are placed on a different external board, connected via a flat cable to the front-end boards.

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Fig. 4. Front end board and power circuit (one per sensor)

The analog measurement signals are fed to the sixteen 16-bit A/D converters of a commercial acquisition board (board 1 in Fig. 3, National Instrument PXI-6351) hosted in a PXI rack (NI PXIe 1071). The excitation signal for the heaters are generated by a DAC board with 8 Analog outputs (board 2 in Fig. 3, National Instruments PXI 6713) in the same PXI rack. Finally, a controller board provides the management of acquired data: it executes a real time program with a 5 kHz timing, which performs sensor signal reading and a low level PID control loop for temperature management. The system exploits the temperature reading from the Pt-RTDs to set the power dissipated by the sensor heaters. Being the relationship

'dissipated power – sensor temperature' substantially linear within the temperature range of interest (see Fig. 2), the control loop design and parameter tuning are fast and simple.

C. Film Temperature Measurement and Control System Using a Pt-based RTD as a sensor and the selected

architecture of the front-end, the voltage output, VRTD (see Fig. 4), is linearly related to the film temperature, and this is highly desirable since the characterization of the measurement chain becomes simple. In particular we have:

RTDRTD TIRV Δ=Δ α0 (1)

where α is the temperature coefficient of the RTD, R0 its value at 0 °C, TRTD is the average RTD temperature, whereas I is the current defined in Fig. 4.

With the adopted design parameters, the VLSB of the ADC dedicated to the temperature reading corresponds to a resolution of 0.03 °C, that is the resolution obtainable if no other source of noise is present. In any case, from the measurements it can be seen that the system resolution is about 0.1 °C.

The thermal system can be represented by this simplified linear (see Fig. 2) lumped parameter model:

ENVTHTHTH TGPdt

dTCTG +=+ (2)

where GTH represents the thermal conductance (convection), CTH the thermal capacitance, and TENV the environment temperature, whereas T is the sensor average temperature.

The control loop implements a simple PI controller as follows:

HEATERkk,HEATER

kkkckk

RPV

)ee(etPP

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where γ and β are the control loop parameters, tc is the loop period, ek represents the difference between the measured temperature and the desired one at the k-th step, and Pk is the power dissipated on the heater. The PI parameters were set using classical techniques after having characterized the thermal systems. The obtained performance are satisfactory: for instance, the open loop thermal system has a time constant of about 10 s whereas, when avoiding saturation of the heater voltage, the closed loop grants a rise time of about 1s (see Fig. 5). This ensures, in many conditions, a thermal transient faster than the chemical one, induced by temperature changes, and allows to apply transient temperature testing [4]-[7].

D. System Software The system is completed by a software consisting of two

VIs (LabView). The first VI, exploiting the real-time LabView software, is a real time program executing data acquisition and the above described control loop with a fast timing of 200 μs. The second one is a VI running on a host PC, which sets all the measurement parameters. In particular, at the beginning, the host transmits to the real time system the desired temperature profiles, the control parameters and the parameters

characterizing each sensor and front-end module (gains, ranges and so on). This enables for working with different sensors and easily replacing both sensors or front-end modules. The host starts the real-time program and controls the chemical sampling systems during the measurement. Measurement data are sent to the host with a lower sampling rate, processed and displayed. In Fig. 6 the front panel of the instrument is shown.

Fig. 5. Two example of closed-loop controlled temperature.

Fig. 6. Front panel of the VI running on the host PC.

III. EXPERIMENTAL RESULTS In this section some examples of the possible test results are

shown. Note that the developed VI allows for: • performing tests in transient chemical conditions at

fixed temperatures. The measurements can be repeated at each temperature (in order to assess stability and repeatability). This allows to study the sensor response dependence on temperature and on gas concentration;

• performing tests in fixed chemical conditions but variable temperature, this is useful for model validation [4][8].

In Fig. 7 an example of results obtained with the first measurement technique is shown: here the characterization of the response in terms of both temperature and concentration dependence is obtained by a fully automatic measurement campaign. In particular, figure 7a) shows raw data, i.e. the sensing film resistance values, whereas figure 7b, 7c and7d

show processed data. The total gas flow is fixed and equal to 200 mL/min. During measurement synthetic air is used as a carrier gas, and it is injected for 4 min, than a mixture of air and 1000 ppm of CO is fed to the chamber for 4 min followed by air for 8 min, a mixture of air and 400 ppm CO for 4 min, air for 8 min, a mixture of air and 200 ppm CO for 4 min, and finally air for 4 min. This allows to evaluate the response of the sensor to different concentrations of the test gas, in this example CO. In figure 7b the sensor responses are reported. The responses are obtained by normalizing the sensor resistance values, evaluating (R-R0)/R0 x 100, where R0 is the baseline resistance value, obtained at the considered temperature in the carrier gas. These values at the end of the gas test are automatically evaluated and plotted as a function of the gas concentration (figure 7c) and of the temperature (figure 7b).

Note that the systems allows to work in open loop so that the VHEATER is set at a constant value and the temperature is measured. This allows for monitoring the very small changes in the film temperature related to chemical reactions, which can be observed thanks to the high resolution provided by this temperature measurement system. The simultaneous monitoring of the film temperature and resistance could be very promising and lead to further understanding of the sensing mechanism.

Fig. 7. Automatic characterization of a sensor: (a) raw data and (b,c,d) processed data. Each measurement is performed at a fixed temperature,

providing a fixed gas flow of 200 mL/min (synthetic air for 4 min, a mixture of air + 1000 ppm CO for 4 min, 8 min air, 4 min mixture of air + 400 ppm

CO, 8 min air, 4 min mixture of air + 200 ppm CO, 4 min air). The protocol and the duration of the measuremment are fully programmable.

IV. CONCLUSIONS A versatile system for automatic characterization of novel

gas sensing materials has been presented. Being fully programmable and capable of simultaneously testing 8 sensors, the systems allows for assessment of sensor performance also

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Fig. 8. Example of temperature variation during chemical transients.

The system provides an accurate measurement of the sensing film temperature, with a resolution of 0.1 °C, and a control of this temperature, which is fast (settling time less than 1 s) and accurate (uncertainty of less than 3 °C). It provides an accurate control of the gas mixture flow (accuracy 1% Full Scale), and composition. An efficient measurement system of the chemical film resistance value (measurement range from 200 Ω to 100 MΩ, accuracy up to 1% ) is also provided.

REFERENCES [1] A.Fort, S. Rocchi, M. B. Serrano-Santos, R. Spinicci, V. Vignoli,

“Surface State Model for Conductance Responses During Thermal-Modulation of SnO2-Based Thick Film Sensors: Part I—Model Derivation”, IEEE Transaction on Instrumentation and Measurement,Vol. 55, n. 6, pp. 2102 – 2106, 2006.

[2] G. Korotcenkov, “Metal oxides for solid-state gas sensors: What determines our choice?”, Mater. Sci. Eng., B, vol. 139, pp. 1–23, 2007.

[3] A.Fort, M.Mugnaini, S.Rocchi, and V.Vignoli, “Surface state models for conductance response of metal oxide gas sensors during thermal transients,” in Chemical Sensors: Simulation and Modeling, vol. 2: Conductometric-Type Sensors, G. Korotcenkov Ed., NJ: Momentum Press, 2012.

[4] A.Fort, M. Mugnaini, S. Rocchi, M. B. Serrano-Santos, R. Spinicci, V. Vignoli, “Surface State Model for Conductance Responses During Thermal-Modulation of SnO2-Based Thick Film Sensors: Part II—Experimental Verification”, IEEE Transaction on Instrumentation and Measurement,Vol. 55, n. 6, pp. 2107 – 2117, 2006.

[5] A.Burresi, A. Fort, S. Rocchi, M.B. Serrano Santos, N. Ulivieri, V. Vignoli, “Temperature profile investigation of SnO2 sensors for CO detection enhancement”, IEEE Trans. on Instrumentation and Measurement, vol. 54, no. 1, pp. 79–86, 2005.

[6] S. Bicelli, A. Depari, G. Faglia, A. Flammini, A. Fort, M. Mugnaini, A. Ponzoni, V. Vignoli, S. Rocchi, “Model and experimental characterization of dynamic behavior of low power Carbon Monoxide MOX sensors operated with pulsed temperature profiles”, IEEE Transaction on Instrumentation and Measurement, Vol. 58, n. 5, pp. 1324 – 1332, 2009.

[7] A. Burresi, A. Fort, S. Rocchi, B. Serrano, N. Ulivieri, V. Vignoli, “Dynamic CO recognition in presence of interfering gases by using one MOX sensor and a selected temperature profile”, Sensors and Actuators B, vol. 106, pp. 40-43, 2005.

[8] A.Fort, M. Mugnaini, S. Rocchi, V. Vignoli, E. Comini, G. Faglia, A.Ponzoni, “Metal-Oxide nanowire sensors for CO detection: Characterization and Modeling”, Sensors and Actuators B, Vol. 148, n. 1, pp. 283-291, 2010

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