Orthogonal gas sensor arrays with intelligent algorithms for early warning of electrical fires

The feasibility of utilizing chemical sensor arrays and multivariable analyses as the basis for an early-warning combustion alarm for electrical fires was evaluated. During the pre-combustion phase of electrical fires, electronic components will heat up, resulting in an out-gassing of chemical vapors, which generally will precede the formation of smoke, scorching and fire. A variety of materials (PVC, Teflon^®, Kapton^®, and silicone rubber) that are frequently used as wire insulation were subjected to electrically induced thermal excursions, thereby simulating an electrical failure and possible pre-combustion condition. The off-gassing vapors from the various coatings can serve as chemical signatures for a pending fire and were detected by an array of chemical sensors (e.g., an electronic nose). Principal component analyses and KNN identification algorithms applied to the sensor response patterns successfully identified the various vapor sources. A 20-sensor array including electrochemical sensors, quartz microbalance (QMB) sensors with different polymer coatings, and heated metal oxide sensors (MOXs) was evaluated and the optimal performance was obtained using the electrochemical and MOXs. The use of heterogeneous orthogonal sensors increased the information content of sensor array signals and a diminutive array can still identify fire materials and extent of damage. The small, lightweight, inexpensive and low power sensors used to detect vapors during pre-fire conditions were ideal for space or commercial aircraft applications.     

Vapor recognition with an integrated array of polymer-coated flexural plate wave sensors

Preliminary testing of a prototype instrument employing an integrated array of six polymer-coated flexural plate wave (FPW) sensors and an adsorbent preconcentrator is described. Responses to thermally desorbed samples of individual organic solvent vapors and binary and ternary vapor mixtures are linear with concentration, and mixture responses are equivalent to the sums of the responses of the component vapors, which co-elute from the preconcentrator in most cases. Limits of detection as low as 0.3 ppm are achieved from a 60-s (34 cm³) air sample and peak widths at half-maximum range from 1 to 4 s. Tests at different flow rates suggest that the kinetics of vapor sorption in the sensor coating films may limit responses at higher flow rates, however, low data acquisition rates may also be contributory. Assessments of array performance using independent test data and Monte Carlo simulations with pattern recognition indicate that individual vapors and certain binary and ternary mixtures can be recognized/discriminated with very low error. More complex mixtures, and those containing homologous vapors, are problematic. This is the first report demonstrating multi-vapor analysis with an integrated FPW sensor array.     

Composites of carboxylate-capped TiO2 nanoparticles and carbon black as chemiresistive vapor sensors

Titanium (IV) dioxide (TiO₂) nanoparticles (NPs) with a 1-5 nm diameter were synthesized by a sol-gel method, functionalized with carboxylate ligands, and combined with carbon black (CB) to produce chemiresistive chemical vapor sensor films. The TiO₂ acted as an inorganic support phase for the swellable, organic capping groups of the NPs, and the CB imparted electrical conductivity to the film. Such sensor composite films exhibited a reproducible, reversible change in relative differential resistance upon exposure to a series of organic test vapors. The response of such chemiresistive composites was comparable to, but generally somewhat smaller than, that of thiol-capped Au NPs. For a given analyte, the resistance response and signal-to-noise ratio of the capped TiO₂-NP/CB composites varied with the identity of the capping ligand. Hence, an array of TiO₂-NP/CB composites, with each film having a compositionally different carboxylate capping ligand, provided good vapor discrimination and quantification when exposed to a series of organic vapors. Principal components analysis of the relative differential resistance response of the sensor array revealed a clear clustering of the response for each analyte tested. This approach expands the options for composite-based chemiresistive vapor sensing, from use of organic monomeric or polymeric sorbent phases, to use of electrically insulating capped inorganic NPs as the nonconductive phase of chemiresistive composite vapor sensors.     

Ionic liquids used as QCM coating materials for the detection of alcohols

Six imidazolium-based ionic liquids (ILs) were synthesized and employed as sensing materials coated on quartz crystal microbalance for the detection of organic vapors. Acetone, ethanol, dichloromethane, benzene, toluene and hexane were selected as representatives for common environmental pollutants, and good linear responses from 0 to 100% of concentrations were observed. The halogen-anion-containing imidazolium ILs-coated sensors showed fast response, excellent reversibility, and considerable sensitivity and selectivity towards alcohols, and the selective factors were up to 30 times for ethanol versus other VOCs. The existence of water vapor reduced the frequency response of the sensor, but a good linear relationship remained.     

Chemo-sensitivity of latex-based films containing segregated networks of carbon nanotubes

In contrast to conventional hydrophobic Conductive Polymer nanoComposites (CPCs) used to design vapor sensors, which are mostly soluble in organic solvents, monodispersed acrylate copolymer latexes present the double advantage of being more sensitive and selective towards polar vapors such as water. A hierarchically structured latex based CPC film was obtained by co-dispersion of an aqueous acrylic emulsion with multiwalled carbon nanotubes (CNTs), followed by spray layer by layer (sLbL) assembly. The analysis of CPC films morphology by AFM and TEM show that a segregated network of CNT as been achieved by partial coalescence of latex nanoparticles and homogeneously assembled in 3D. Transducer sensitivity was investigated as a function of CNT content, latex glass transition temperature (T_g), organic vapor nature and vapor concentration. The source of the high sensitivity and selectivity observed for these latex-based composites towards water vapor is assumed to mainly result from ionic interaction of SDS with water molecules offering interesting perspectives of development. The different diffusion regimes through the CPC transducer are visualized, modeled and interpreted with the Langmuir-Henry-Clustering (LHC) model, showing that only water is reaching a clustering mode at high vapor concentration. Finally it is believed that the unique hierarchical architecture of BA latex-CNT sensors is responsible for their quick, stable and reproducible responses to vapors.     

Non-solvent induced phase separation as a method for making high-performance chemiresistors based on conductive polymer nanocomposites

The fabrication of novel porous conductive composite vapor sensors characterized by different porosities and specific surface areas is described in this study. These samples were obtained by the dry-cast non-solvent induced phase separation (NIPS) method. Porous composite structures have been studied by the SEM, BET and water evaporation methods. Testing different concentrations of several organic vapors, the porous sensors showed improved sensitivities and response times as compared to their dense counterpart. Improved characteristics of the sensor response were correlated to better sorption and diffusion properties of sensing film due to increased porosity and specific surface area obtained by this method of film fabrication. A competition theory was proposed that describes the optimum porosity and thickness of sensing films in which the highest sensitivities were observed.     

Effect of soft-hard segment structure on vapor responsiveness of polyurethane conducting composite thin films loaded with multi-walled carbon nanotubes

A series of multi-walled carbon nanotubes/polyurethane (MWNTs/PU) composite conducting dispersoids were prepared via an in situ coupling reaction among linear hydroxyl-terminated polymer diols, 1,6-hexamethylene diisocyanate (HDI) and various chain extenders. The composite conducting thin films were formed by spin-coating and depositing the dispersoids onto comb-like electrode substrates. The resulting structure and the dispersion quality of MWNTs in the dispersoids were examined by means of FTIR, XRD, TEM, SEM and UV-vis analyses. The response of the as-prepared films toward some volatile organic solvent vapors such as benzene, anhydrous ether, acetone and chloroform was evaluated. The experimental results indicated that the composite conducting films constructed by hydroxyl-terminated poly(butadiene-co-acrylonitrile), trimethylolpropane, and MWNTs-OH bear better vapor responsiveness. The dispersion behavior of MWNTs in the dispersoids, types of MWNTs and soft-hard segmental compositions are believed to be closely related with the sensing properties of the films. In particular, the chemical linkage of MWNT-OH with HDI in the PU matrix is expected to improve the dispersivity and further to enhance the sensing properties of the composite sensors. The vapor sensing properties well reveal that these materials have a possibility as a candidate of volatile organic solvent vapor sensors.     

Optical parameters of calix[4]arene films and their response to volatile organic vapors

The Langmuir-Blodgett (LB) technique was employed to produce thin LB films using an amphiphilic calix-4-resorcinarene onto different substrates such as quartz, gold coated glass and quartz crystals. The characteristics of the calix LB films are assessed by UV-visible, quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) measurements. UV-vis and QCM measurements indicated that this material deposited very well onto the solid substrates with a transfer ratio of >0.95. Using SPR data, the thickness and refractive index of this LB film are determined to be 1.14 nm/deposited layer and 1.6 respectively. The sensing application of calixarene LB films towards volatile organic vapors such as chloroform, benzene, toluene and ethanol vapors is studied by the SPR technique. The response of this LB film to saturated chloroform vapor is much larger than for the other vapors. The response is fast and fully recoverable. It can be proposed that this sensing material deposited onto gold coated glass substrates has a good sensitivity and selectivity for chloroform vapor. This material may also find potential applications in the development of room temperature organic vapor sensing devices.     

Modeling Carbon-Black/Polymer Composite Sensors

Conductive polymer composite sensors have shown great potential in identifying gaseous analytes. To more thoroughly understand the physical and chemical mechanisms of this type of sensor, a mathematical model was developed by combining two sub-models: a conductivity model and a thermodynamic model, which gives a relationship between the vapor concentration of analyte(s) and the change of the sensor signals. In this work, 64 chemiresistors representing eight different carbon concentrations (8-60 vol% carbon) were constructed by depositing thin films of a carbon-black/polyisobutylene composite onto concentric spiral platinum electrodes on a silicon chip. The responses of the sensors were measured in dry air and at various vapor pressures of toluene and trichloroethylene. Three parameters in the conductivity model were determined by fitting the experimental data. It was shown that by applying this model, the sensor responses can be adequately predicted for given vapor pressures; furthermore the analyte vapor concentrations can be estimated based on the sensor responses. This model will guide the improvement of the design and fabrication of conductive polymer composite sensors for detecting and identifying mixtures of organic vapors.     

Active gas/odor sensing system using automatically controlled gas blender and numerical optimization technique

As measurement of a vapor mixture composition is a difficult technique, no method using a sensing system has yet been established in spite of great effort by many researchers. In this paper, the authors propose a new gas/odor sensing system using a gas blender and a nonlinear numerical optimization algorithm by which the concentration of each component in an unknown vapor can be quantified. The component vapors are internally blended and the mixture ratio is modified by the system so that the sensor array output pattern of the blended vapor can be made equal to that of the unknown one. After several iterations, convergence is obtained and the vapor concentration of each component is determined from the mixture composition of the blended vapor. Although the conventional system is passive, this system is considered as an active one as it performs exploratory behavior prior to recognition. Here, gasoline vapor concentration is measured under the condition that one or two interference vapors exist together. Gasoline vapor has been adopted as an example of odors in the passenger compartment of a car, since it sometimes smells unpleasant. The measurement is essential for designing a car in order to keep it comfortable for passengers. The sensors used here are three semiconductor gas sensors and two electrochemical sensors, which are chosen in order to obtain high sensitivity to gasoline. The nonlinear numerical optimization techniques used are the simplex method and the gradient descent method and these two methods are compared here. It is found that the quantification error is within ten ppm for two- or three-component vapors.     

Effects of morphologies on acetone-sensing properties of tungsten trioxide nanocrystals

In this work, triclinic WO₃ nanoplates and WO₃ nanoparticles were comparatively investigated as sensing materials to detect acetone vapors. Single-crystalline WO₃ nanoplates with large side-to-thickness ratios were synthesized via a topochemical conversion from tungstate-based inorganic-organic hybrid nanobelts, and the WO₃ nanoparticles were obtained by calcining commercial H₂WO₄ powders at 550 °C. The acetone-sensing properties were evaluated by measuring the change in electrical resistance of the WO₃ sensors before and after exposure to acetone vapors with various concentrations. The WO₃ nanoplate sensors showed a high and stable sensitive response to acetone vapors with a concentration range of 2-1000 ppm, and the sensitivity was up to 42 for 1000 ppm of acetone vapor operating at 300 °C. The response and recovery times were as short as 3-10 s and 12-13 s, respectively, for the WO₃ nanoplate sensors when operating at 300 °C. The acetone-sensing performance of the WO₃ nanoplate sensors was more excellent than that of the WO₃ nanoparticle sensors under a similar operating condition. The enhancement of the WO₃ nanoplate sensors in the acetone-sensing property was attributed to the poriferous textures, single-crystalline microstructures and high surface areas of the aggregates consisting of WO₃ nanoplates, which were more favorable in rapid and efficient diffusion of acetone vapors than the WO₃ nanoparticles.     

炭黑填充聚乙烯导电复合材料气敏性能研究

采用溶液共混法成功制备出炭黑/聚乙烯导电气敏复合材料,研究了在苯、甲苯、二甲苯三种有机溶剂蒸汽中的电阻变化.实验结果表明,该复合材料在二甲苯蒸汽中电阻变化最大,在苯蒸汽中电阻变化最小.进一步研究了其在100 ppm至800 ppm(ppm=10-6)二甲苯蒸汽中的气敏响应性,实验结果表明:复合材料的灵敏度从0.04增大至0.11.     

Electrical response of electrospun PEDOT-PSSA nanofibers to organic and inorganic gases

Electrospun isolated nanofibers of poly(3,4-ethylenedioxythiophene) doped with (poly styrene sulfonic acid)-PEDOT-PSSA were used to sense vapors of several aliphatic alcohols. Due to the large surface to volume ratio and small quantity of active material used in their fabrication, these sensors have a similar or faster response time when compared to alcohol sensors based on PEDOT. Increasing the size of the alcohol molecule leads to longer response times, which is attributed to slower diffusion of the larger molecule into the polymer. The sensors were annealed in air at 70 °C and used to sense NH₃, HCl and NO₂ gas. The response time for NH₃ was faster than HCl, and the sensors showed a large initial response to NO₂ at room temperature which is very desirable, as some NO₂ gas sensors only operate at elevated temperatures. Electrospinning is a simple and inexpensive method of preparing PEDOT-PSSA nanofibers making it an attractive technique to fabricate polymer based low cost, rapid response and reusable gas sensors.     

Improvement of gas sensing performance of carbon black/waterborne polyurethane composites: Effect of crosslinking treatment

When carbon black (CB) filled waterborne polyurethane (WPU) composites are exposed to organic solvent vapors, electrical resistance of the materials increases rapidly. They can thus serve as gas sensors. To improve the composites’ performance for practical applications, crosslinking agent was added to the composite latexes, forming intra-molecular crosslinked networks among the matrix polymer of the composites. The method greatly increased the filler/matrix interfacial interaction and reduced the mobility of CB particles. In the composites that had absorbed solvent vapors, reconstruction of conduction paths through re-aggregation of the disconnected filler particulates became difficult. As a result, the unwanted negative vapor coefficient (NVC) effect was significantly weakened, while the gas sensitivity and the performance reproducibility were enhanced as well.     

柔性高灵敏单壁碳纳米管气体传感器研究

在柔性聚对二甲苯C基底上制作了基于单壁碳纳米管的小型化、高灵敏、反应快速的气体传感器。使用介质电泳集成碳管束,并利用单链脱氧核糖核酸修饰增强器件灵敏度。当传感器暴露在甲醇蒸气中时,会出现明显可重复的反应,它可以检测含量低至4.3×10-6的甲醇,并且在相当宽的体积分数范围有清晰的分辨能力。对于4.3×10-6的含量,未修饰的传感器对应电阻变化率是4.8%;经过脱氧核糖核酸修饰,电阻变化率增加到了12.3%。此外,该传感器还显示了很快的响应速度和很好的测试复验性。研究表明:这种柔性气体传感器在未来环境监测应用中有很好的前景。     

Thin film characterization and vapor sensing properties of a novel perylenediimide material

A novel N,N′-(glycine tert-butylester)-3,4,9,10-perylenediimide was chosen for the study of Langmuir–Blodgett (LB) thin film characterization and the sensing properties against selected volatile organic vapors. Different number of LB film layer was deposited onto a glass and quartz crystal substrate. The thin film fabrication process was monitored with UV–vis and quartz crystal microbalance (QCM) measurement techniques. The results indicated that absorbance increased linearly with the number of the layers on film. A similar linear relationship between frequency shift and number of the layers was observed by the QCM measurement. It can be concluded that high quality and uniform LB films were produced by using this novel perylenediimide material. Chloroform, toluene, benzene, ethyl alcohol and isopropyl alcohol vapors were selected to test this material's applicability in room temperature as a vapor sensor. This novel material showed a fast, large and reproducible response to chloroform and isopropyl alcohol vapor.     

A distributed Bragg reflector porous silicon layer for optical interferometric sensing of organic vapor

In this study, we successfully demonstrated the rapid, sensitive, and reversible sensing of organic vapor using a distributed Bragg reflector (DBR) porous silicon (PS) layer. We fabricated the DBR PS layer on a p⁺-type silicon substrate and investigated its reflectance spectra before, during, and after exposure to the different concentrations of various organic vapors. When the DBR PS layer sample was exposed to methanol, acetone, ethanol, and isopropanol vapors, the maximum reflectance peak promptly shifted toward longer wavelengths by about 4.5, 23.2, 26.0, and 38.2 nm, respectively. We determined that the red-shift in the reflectance spectrum could be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the DBR PS layer. The DBR PS layer showed excellent sensing ability under the different concentrations and types of organic vapors. In addition, a slight hysteresis of the red-shift was observed during repeated exposure to organic vapors at different concentrations. After removing the organic vapors, the reflectance spectrum promptly returned to its original state.     

基于QPSO的震动传感器片上相位补偿器设计方法

震动传感器的系统相位非一致性会对地震波到时时差提取产生很大的误差,严重影响震源定位精度;针对这一问题,提出了一种基于量子粒子群优化算法（QPSO）的震动传感器片上相位补偿器设计方法。首先对震动传感器进行相位标定,获得传感器与参考传感器的相位差;其次设计基于QPSO算法的相位补偿滤波器对相位差进行修正,使其无限趋近于0;最后,将相位补偿滤波器封装成FPGA软核部署于FPGA上,完成对震动传感器的相位片上实时补偿。为了验证该方法的性能,将相位补偿滤波器部署于自研的多通道震动信号采集系统上,对8个相同型号震动传感器进行相位一致性校准。试验结果表明,在震动传感器频响范围内,该方法可以将2.5°内的传感器相位差实时修正至0.0044°以下,实现了震动传感器阵列的相位一致性实时校准。该成果在地下浅层震源定位领域具有较强的应用价值。     

Comparison of odor detection thresholds and odor discriminablities of a conducting polymer composite electronic nose versus mammalian olfaction

Response data from an array of conducting polymer composite vapor detectors that form an electronic nose were collected for the purpose of comparing selected, quantitatively measurable, phenomena in odor detection and classification to the olfactory characteristics of monkeys and humans. Odor detection thresholds and discriminability between structurally similar pairs of odorants were the two primary quantities evaluated for this comparison. Comparisons were only made for volatile organic vapors as opposed to aroma active odorant vapors. Electronic nose detection thresholds for a homologous series of n-alkane and 1-alcohol odorants were determined and the results were compared to literature values for the mean olfactory detection thresholds observed in psychophysical experiments on humans exposed to these same vapors. The trends in odor detection thresholds of the electronic nose towards the tested analytes were very similar to those exhibited by humans. The discrimination performance of the electronic nose for distinguishing between pairs of odorants within incrementally varying series of esters, carboxylic acids and alcohols were also compared to the published data of Laska and co-workers on the psychophysical performance of humans and monkeys for these same odorant pairs. Similar trends were generally observed between the humans, monkeys, and the electronic nose in that discrimination performance increased as the compounds of an odorant pair became more structurally dissimilar. With use of the Fisher linear discriminant algorithm for classification of these test pairs of odorants, the electronic nose exhibited significantly better discriminability than humans or monkeys for the odorant pairs evaluated in this work under the test conditions for which the discriminability was evaluated.     

Mimicking the sense of olfaction: A conducting-polymer-based electronic nose

Abstract— We describe herein the construction of a simple, low-power, broadly responsive vapor sensor. Carbon-black-organic-polymer composites have been shown to swell reversibly upon exposure to vapors. Thin films of carbon-black-organic-polymer composites have been deposited across two metallic leads, with swelling-induced resistance changes of the films signaling the presence of vapors. To identify and classify vapors, arrays of such vapor-sensing elements have been constructed, with each element containing a different organic polymer as the insulating phase. The differing gas-solid partition coefficients for the various polymers of the sensor array produce a pattern of resistance changes that can be used to classify vapors and vapor mixtures. This type of sensor array has been shown to resolve all organic vapors that have been analyzed, and can even resolve H²O from D²O.