Pulsed Temperature Mode Research Articles

No2 Sensing with SWCNT Decorated by Nanoparticles in Temperature Pulsed Mode: Modeling and Characterization.

In this paper, NO2 sensing by means of single-wall carbon nanotubes (SWCNT) networks, decorated with nanoparticles of TiO2 and Au, is proposed. In particular, it is shown that the performance of these materials can be enhanced using pulsed temperature mode. This sensing strategy effectiveness is theoretically and experimentally assessed. In this paper, in fact, a dynamic model for conductive gas sensors formed by networks of nanowires, considering the junctions between different wires as the main contribution to sensor conductance, and in the presence of the target gas, is presented and validated. The model accounts for variable temperature and gas concentration and sheds some light on the mechanisms leading to the sensor response improvement related to temperature pulsed working mode. It is also shown how the addition of a different material can be modeled through different surface adsorption kinetics.

H2S sensing mechanism of SnO2-CuWO4 operated under pulsed temperature modulation

This paper reports the sensing mechanism of SnO2-CuWO4 mixed oxides towards H2S detection revealed through Pulsed Temperature Mode (PTM). After thick film deposition onto commercial substrates, the final annealing temperature enhances the material reliability through the changes occurred during the experimental conditions. PTM approach represents a tradeoff in terms of sensing mechanism versus sensing performance, in order to get insights about the fundamental aspects. The measurements have been done under dynamic gas flow conditions, in the presence of 50% relative humidity (RH). The gas sensors based on SnO2-CuWO4 exhibit sensor signals higher than 100 to 10 ppm H2S, low cross-sensitivity to common gaseous interfering agents (CO, CH4, NH3, SO2), average selective sensitivity of 8.64 towards the main interfering NO2 gas, moderate response (4 min)/recovery (5 min) transients and power consumption.In order to emphasize the role of moisture towards H2S detection mechanism, the photoacoustic spectroscopic insights were correlated with Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy results. Such approach has shown promising aspects towards further operational sensitive devices.

NH3 sensing with self-assembled ZnO-nanowire μHP sensors in isothermal and temperature-pulsed mode

Dielectrophoretic alignment is found to be a simple and efficient method to deposit the solution prepared ZnO nanowires onto micro hot plate substrates. Due to the strong surface effects, positive temperature coefficient for resistance was encountered with ZnO nanowires in the high temperature range (>250°C). The response to ammonia (NH3) was evaluated in isothermal and temperature-pulsed operation mode; the relative higher response observed in the latter case demonstrates that the use of this methodology is a good strategy to improve the performance of metal oxide sensors based on nanomaterials. Here, we evaluate the response to NH3 and qualitatively describe the sensing mechanism in temperature-pulsed mode, highlighting the main differences compared to the standard isothermal methodology.

Room temperature continuous wave operation of λ ∼ 3–3.2 μm quantum cascade lasers

We demonstrate quantum cascade lasers emitting at wavelengths of 3–3.2 μm in the InP-based material system. The laser core consists of GaInAs/AlInAs using strain balancing technique. In room temperature pulsed mode operation, threshold current densities of 1.66 kA/cm2 and 1.97 kA/cm2, and characteristic temperatures (T0) of 108 K and 102 K, are obtained for the devices emitting at 3.2 μm and 3 μm, respectively. Room temperature continuous wave operation is achieved at both wavelengths.

High power broad area quantum cascade lasers

Broad area quantum cascade lasers (QCLs) are studied with ridge widths up to 400 μm, in room temperature pulsed mode operation at an emission wavelength around 4.45 μm. The peak output power scales linearly with the ridge width. A maximum total peak output power of 120 W is obtained from a single 400-μm-wide device with a cavity length of 3 mm. A stable far field emission characteristic is observed with dual lobes at ±38° for all tested devices, which suggests that these broad area QCLs are highly resistant to filamentation.

Detection of CO and NO using low power metal oxide sensors

AbstractIn this work we present the fabrication and characterization of low power SnO2 gas sensors based on Porous Silicon micro‐hotplates for the detection of toxic gases, like CO and NO. The response of the SnO2 gas sensors was measured for various gas concentrations (100‐500 ppm). Analysis was performed in isothermal operation mode, by keeping constant the micro‐hotplate temperature and in pulsed temperature mode, by applying voltage pulses to the heater. In both cases, the response of the sensors increased as the temperature and the concentration of the detecting gases increased. Comparison between the two different operation modes, shows that the sensors exhibit higher sensitivity in pulsed temperature mode. In addition, with this technique, a significant reduction of power consumption can be achieved. Moreover, the sensors show significant selectivity in detecting NO, so discrimination between the two gases can be achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser-ablated nanocrystalline SnO 2 material for low-level CO detection

This paper presents results from a study, which shows that laser-ablated nanocrystalline SnO 2 can be used for low-level CO detection such as in environmental monitoring applications. Laser ablation has the advantage of producing nanocrystalline material free of surface contamination that can be detrimental for the detection capabilities of the sensors. Initially, the performance of this material was evaluated with respect to doping agents, operating temperature and film thickness. Subsequently, the sensors were operated in a pulsed-temperature mode. The sensors produced show superior sensitivities to CO concentrations at single-figure ppm levels at high resolution and with very low cross sensitivity to common interference gases.

Thermally isolated MOSFET for gas sensing application

This work reports on thermally isolated electronic components for gas sensing applications. The device is composed of an array of 4 MOSFETs, a diode and a semiconductor resistor integrated on a micro-hotplate, which is fabricated using bulk micromachining of silicon. The thermal efficiency of the device is 2/spl deg/C/mW with a thermal constant less than 100 ms. Holes are made in the passivation film over the gates of the MOSFET and gas sensitive films deposited on top of the gate insulator. The low thermal mass device realized allows new modes of operation for MOSFET gas sensors such as a combination of the field and thermal effects and a pulsed temperature mode of operation.

Gas-sensitive p-GaAs field effect device with catalytic gate

A new gas-sensitive catalytic gate p-channel GaAs field effect device with additional sidegate has been developed. With SnO 2 as catalytic gate, the device resembles a JFET. The n-type SnO 2 gate forms a hetero pn-junction to the p-type channel. The gas-induced change of the gate to channel electric field affects this junction and thus modulates the conductivity of the channel. Gas measurements were carried out at constant temperature and in temperature pulsed mode with test gases like ammonia, methane, carbon monoxide and nitric dioxide in synthetic air with 50% humidity. SnO 2-catalysed GaAs–FETs show NO 2 and NH 3 sensitivity. The Pd-catalysed device reacts only to NO 2. CO and CH 4 sensitivity of the device was not observed.

Design and fabrication of high-temperature micro-hotplates for drop-coated gas sensors

This paper reports on the optimisation of micromachined hotplates for gas-sensing applications designed to stand high-temperature coating processes and modes of operation. Different thin film materials, geometry and dimensions have been investigated regarding their power consumption, temperature distribution over the sensing area and robustness when annealed at high temperature. In comparison with oxide films, the micro-hotplates made of nitride were the most robust. They withstood annealing temperatures of up to 700°C and a pulsed temperature mode of operation. Their robustness allowed the sensing area to be heated up to a high temperature (550–700°C). These micro-hotplates are therefore suitable for the annealing “on-chip” of the gas-sensitive materials deposited in a drop-coating procedure. Sensors coated with a Pd-doped tin oxide drop and annealed using the integrated heater have been tested under different concentrations of CO and CH 4 in air.

Permeated Porous Silicon Suspended Membrane as Sub-ppm Benzene Sensor for Air Quality Monitoring

To realise advanced microsensors for a reliable monitoring of very low concentrations of pollutant species such as NOx, SO2, CO, O3 and aromatic hydrocarbons, the use of porous silicon (PS) permeated with semiconducting oxides has been explored. To reduce the power consumption and to make feasible the device to operate in a fast pulsed temperature mode, a novel sensor architecture has been designed. The main feature of the device is represented by a permeated suspended macroporous Si membrane, few tens of microns thick. In this paper the porous silicon formation through a suspended silicon membrane and the morphological characterization of the PS layer are reported. Moreover, the performance of a C6H6 gas sensor based on the suspended macroporous Si membrane (≅30 μm thick), permeated with the chemical precursor of Sn oxide is presented. The results have demonstrated the feasibility to realize a macroporous silicon suspended membrane with high specific surface area, efficient electrical insulation and negligible warpage. Furthermore, the permeation of the oxidized macroporous silicon membrane with SnO2 has been proved to be a valuable approach to fabricate gas sensors suitable to detect aromatic hydrocarbons in a sub-ppm range.

Micromachined thin film SnO 2 gas sensors in temperature-pulsed operation mode

Gas detection measurements based on a micromachined SnO 2 gas sensor with periodically pulsed heater voltage are presented. Additionally, the field-effect-induced changes in resistivity of the sensitive layer caused by the heater voltage were investigated. The combination of both results leads to an improved design for low power SnO 2 gas sensors. In temperature-pulsed mode, the sensor resistances were measured at constant delays after the pulse edges. The measurements were carried out with the common test gases carbon monoxide and nitric dioxide in synthetic air with 50% humidity. In the cold pulse phase, the CO sensor response is higher and shows only a slow decrease with increasing pulse duration. The sensor sensitivity is related to the pulsed heated mode, on the one hand, and the continuously heated, on the other. The comparison of the measurement results reveals that the temperature-pulsed operation mode (TPOM) caused a significant reduction of power consumption and higher sensitivity.

Optimization of temperature programmed sensing for gas identification using micro-hotplate sensors

Micro-hotplate chemical gas sensors, such as those being developed at the National Institute of Standards and Technology (NIST) by micromachining Si, can be operated in a temperature-pulsed mode, due to their small size and mass. In the temperature-pulsed mode of operation, different gases give different dynamic responses (i.e. signatures) depending on the temperature program used. In this paper a new methodology is presented to optimize the operation of micro-hotplate gas sensors for discriminating volatile organic compounds at a fixed concentration, while minimizing the detection time. The extension of the methodology to cases where concentrations vary is currently under investigation. The Wavelet Network method is applied to accurately predict the sensor’s response for a given temperature profile. Once a dynamic model is obtained, it is used for off-line optimization of the temperature profile, i.e. the maximization of the difference between two gas signatures. The difference between two response curves was initially measured by a metric based on the Euclidean distance. This metric was then modified using the Haar wavelet transformation. The methodology was implemented in a case study in which either methanol or ethanol had to be detected in air, but the methodology is generic, and it can be applied to any two gases.

Equilibrium and non-equilibrium conductance response of sintered SnO 2 samples to CO

Conductance responses of some sintered samples of SnO 2 prepared in the form of thick films have been studied in the CO concentration range from one ppm upwards in dry synthetic air and also in ambients containing different amounts of humidity. The experiments have been carried out both at equilibrium conditions, by allowing the samples to stabilize at different constant temperatures, and at non-equilibrium conditions, by continuously cycling the sensors between two temperatures. The effects of different electrode materials and of some additives like antimony in SnO 2 on the conductance response to CO under equilibrium and non-equilibrium conditions have been studied. In this paper the emphasis is to describe some measurements concerning a peculiar effect of electrode materials (between Au and Pt) and Sb doping on the conductance response to CO in the temperature-pulsed mode of sensor operation. A drastic difference in the non-equilibrium conductance response to CO in ambients containing humidity is found between Sb-doped SnO 2 sensors with Au and Pt as electrode materials.

Effect of water vapour on the CO response behaviour of tin dioxide sensors in constant temperature and temperature-pulsed modes of operation

Tin dioxide-based semiconductor gas sensors are operated both in the constant temperature and temperature-pulsed modes and are subjected to various concentrations of carbon monoxide and water vapour simultaneously. Certain features in the conductance versus time curves obtained in the temperature-pulsed mode are shown to be dependent on the carbon monoxide concentration. Numerical values for some of the characteristic features are determined from the experimental results. Based on these values and results obtained in the constant temperature measurements, the interfering effect of water vapour on the determination of the carbon monoxide concentration in the two modes of operation is compared.