Micromachined metal oxide gas sensors: opportunities to improve sensor performance
TL;DR: In this article, a review of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects is presented, and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.
Abstract: This review deals with gas sensors combining a metal oxide based sensing layer and a substrate realized by using micromachining. It starts by giving an overview of the design principles and technology involved in the fabrication of micromachined substrates examining thermal and mechanical aspects. Both kinds of micromachined substrates, closed-membrane-type and the suspended-membrane-type, are discussed. The deposition of the sensing layer is complicated by the mechanical fragility of the micromachined substrates. Different approaches used for the formation of the sensing layer such as thin film and thick film deposition techniques are reviewed. Finally, the gas sensing function of the sensitive layer is analyzed and various ways for extracting the information are presented with respect to the improvement of sensor performance brought by this new approach.
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TL;DR: In this article, the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors have been analyzed, and it was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors.
Abstract: This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (τres), recovery time (τrec), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another.
509 citations
TL;DR: A comparison of Orthogonality versus Independence, cross-sensitivity and Diversity, and Optimization of Excitation Profiles: Results and Outlook 609.
Abstract: 6.2. Orthogonality versus Independence 584 6.3. Cross-sensitivity and Diversity 585 6.4. Multiple Roles of Redundancy 585 7. Data Preprocessing 586 7.1. Baseline Correction 586 7.2. Scaling 587 7.2.1. Global Techniques 588 7.2.2. Local Techniques 588 7.2.3. Nonlinear Transforms 588 8. Drift Compensation 588 8.1. Univariate Drift Compensation 589 8.2. Multivariate Drift Compensation 589 9. Feature Extraction from Sensor Dynamics 591 9.1. Transient Analysis 591 9.1.1. Oversampling Procedures 592 9.1.2. Ad hoc Transient Parameters 593 9.1.3. Model-Based Parameters 593 9.1.4. Comparative Studies 595 9.2. Temperature-Modulation Analysis 596 10. Multivariate Calibration 599 10.1. Multiway Analysis 599 10.2. Dynamical Models 602 11. Array Optimization 604 11.1. Sensor Selection 604 11.2. Feature Selection 605 11.3. Optimization of Excitation Profiles 607 12. Conclusion and Outlook 608 13. References 609
405 citations
TL;DR: Investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect and pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity.
Abstract: Several metal-organic framework (MOF) materials were under investigated to test their applicability as sensor materials for impedimetric gas sensors. The materials were tested in a temperature range of 120 °C - 240 °C with varying concentrations of O2, CO2, C3H8, NO, H2, ethanol and methanol in the gas atmosphere and under different test gas humidity conditions. Different sensor configurations were studied in a frequency range of 1 Hz -1 MHz and time-continuous measurements were performed at 1 Hz. The materials did not show any impedance response to O2, CO2, C3H8, NO, or H2 in the gas atmospheres, although for some materials a significant impedance decrease was induced by a change of the ethanol or methanol concentration in the gas phase. Moreover, pronounced promising and reversible changes in the electric properties of a special MOF material were monitored under varying humidity, with a linear response curve at 120 °C. Further investigations were carried out with differently doped MOF materials of this class, to evaluate the influence of special dopants on the sensor effect.
366 citations
TL;DR: In this article, single-walled carbon nanotubes were coated with crystalline tin oxide by a simple chemical-solution route and the room-temperature chemical treatment results in a nominally complete and unifor...
Abstract: Single-walled carbon nanotubes coated with crystalline tin oxide by a simple chemical-solution route are reported. The room-temperature chemical treatment results in a nominally complete and unifor...
328 citations
TL;DR: Transition metal oxides (TMOs) are a fascinating class of materials due to their wide ranging electronic, chemical and mechanical properties Additionally, they are gaining increasing attention for their thermoelectric (TE) properties due to tunable electronic and phonon transport properties and well established synthesis techniques as mentioned in this paper.
306 citations
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TL;DR: In this paper, the authors demonstrated that the sensing characteristics of a semiconductor gas sensor using SnO2 can be improved by controlling fundamental factors which affect its receptor and transducer functions.
Abstract: It is demonstrated that the sensing characteristics of a semiconductor gas sensor using SnO2 can be improved by controlling fundamental factors which affect its receptor and transducer functions. The transducer function is deeply related with the microstructure of the elements, i.e., the grain size of SnO2 (D) and the depth of the surface space-charge layer (L). The sensitivity is drastically promoted when D is made comparable to or less than 2L, either by control of D for pure SnO2 elements or by control of the Debye length for impurity-doped elements. On the other hand, the receptor function is drastically modified by the introduction of foreign receptors on the surface of SnO2. In the particular cases of Pd and Ag promoters, the oxides (PdO and Ag2O) formed in air interact with the SnO2 surface to produce an electron-deficient space-charge layer, and this contributes much to promoting the gas sensitivity. For a test gas having a specific reactivity, such specificity can be utilized for exploiting gas-selective receptors, as exemplified by CuOSnO2 and La2O3SnO2 elements, which detect H2S and ethanol gas respectively very sensitively.
1,534 citations
1,260 citations
TL;DR: A survey on the current status and future prospects in research and development of SnO2-based sensors is given in this paper, where the influence of contact geometry and crystallinity on the sensor response signal is outlined.
Abstract: A survey is given on the current status and future prospects in research and development of SnO2-based sensors. Atomistic models of molecular recognition are discussed first. They include physisorption, chemisorption, surface reaction, catalytic reaction, grain boundary reaction, bulk reaction and three-phase boundary reaction steps. The influence of contact geometry and crystallinity on the sensor response signal is outlined. A brief summary is given of the current status of sensor research and development with emphasis on ceramic, thick-film and thin-film sensors based on crystalline, polycrystalline and nanocrystalline SnO2. Three different aspects are mentioned in the outline which are expected to lead to significantly improved performances of future sensors: the improved selectivity through the modulation frequency in a.c. measurements, the improved stability through the better control of structures, and the improved selectivity and drift compensation through pattern recognition.
891 citations
TL;DR: The present review describes the reasons for this complexity and outlines unifying concepts to understand the huge amount of published, mostly empirical data, which leads to a comprehension of gas-sensing phenomena in both the application and research domains.
Abstract: Gas sensors based on semiconducting materials have become of great interest to both sensor users and researchers. In this context, a huge number of publications have appeared in the literature which deal with metal oxide gas sensors, in general, and with the prototype material SnO2, in particular. The amount of data published grows continuously and has led to a situation in which even experts in this field tend to lose an overview. The present review describes the reasons for this complexity and outlines unifying concepts to understand the huge amount of published, mostly empirical data. This leads to a comprehension of gas-sensing phenomena in both the application and research domains.
711 citations
TL;DR: Some recent advances in technology and basic scientific understanding of gas-sensitive resistors based on semiconducting oxides are described in this paper, including new materials which have greatly improved performance especially against variations of relative humidity and which have led to new applications, ozone measurement being an example.
Abstract: Some recent advances in technology and basic scientific understanding of gas-sensitive resistors based on semiconducting oxides are described. Highlights are: new materials which have greatly improved performance especially against variations of relative humidity and which have led to new applications, ozone measurement being an example; fabrication methods which have made repeatable and reliable devices available in very large numbers at very low cost; new concepts, including self-diagnostic multiple-electrode array devices and two phase, transducer–detector systems, leading to room-temperature operation; deliberate surface modification, either by gases, or by chemical grafting, or by surface segregation; a general response model which allows systematic discussion of the behaviour of a wide range of materials and separation of the different influences upon response; more detailed understanding of elementary reaction processes on the surface; and the application of computational modelling methods to support interpretations and suggest new lines of development.
668 citations