Solid polymer electrolytes are widely used in batteries and fuel cells because of the high ionic conductivity that can be achieved at room temperature. The ions are usually Li or protons, although other ions can be shown to conduct in these polymer films. There has been very little published work on solid polymer electrolyte films used as chemical sensors. We have found that thin films of polymers like polyethylene oxide ͑PEO͒ are very sensitive to low concentrations of volatile organic compounds ͑VOCs͒ such as common solvents. Evidence of a new sensing mechanism involving the percolation of ions through narrow channels of amorphous polymer is presented. We will present impedance spectroscopy of PEO films in the frequency range 0.0001 Hz to 1 MHz for different concentrations of VOCs and relative humidity. We find that the measurement frequency is important for distinguishing ionic conductivity from the double layer capacitance and the parasitic capacitance.There is a rich literature on the electrical properties of polyethylene oxide ͑PEO͒ because of its importance as an electrolyte in lithium batteries. 1,2 However, there have been relatively few reports of its use as a sensor material. 3 Much of the reported work on PEO as an electrolyte involves attempts to increase the ionic mobility without the use of water because of the incompatibility with lithium. Plasticizing agents and nonaqueous electrolytes like propylene carbonate are often used to increase the room temperature ionic conductivity. 1 In this paper, we report on the chemical sensing characteristics and mechanism of thin films of PEO doped with LiClO 4 deposited on planar arrays of interdigitated electrodes. The planar configuration allows easy integration with sensing electronics and fast response to vapor phase analytes. An unusual pattern of relative responses to vapors with different solubility parameter values, 4 makes PEO a useful addition to arrays of chemiresistors used in pattern recognition of vapors and mixtures of vapors. We discuss the issues of the mixed crystalline and amorphous polymer phases, as well as, the temperature dependence of the chemical sensor responses.
ExperimentalThe PEO was purchased from Polysciences and had a nominal molecular weight ͑Mw͒ of 4,000,000 g/mol. The LiClO 4 was purchased from Aldrich Chemical and both components were dissolved in acetonitrile, mixed and deposited onto planar interdigitated electrode ͑IDE͒ arrays. Polyethylene glycol ͑short chain PEO͒ in two different molecular weights ͑PEG68 has molecular weight of 6800 g/mol and PEG34, 3400 g/mol͒ was purchased from Scientific Polymer Products, Inc. Carbon loaded PEG films were also fabricated as described in Ref. 4 to make chemiresistors for direct comparison to sensing by ion conductivity in the same type of polymer film.The 50 pair electrode arrays had 5 m wide gold lines separated by 10 m gaps on quartz substrates. A photomicrograph is shown in Fig. 1 of part of the array. The electrode lengths are 0.16 cm. Since the nominal thickness of the polymer fil...
Measurements of the performance of a miniature, portable 12-mm-diameter, 57-mm-length low-temperature cofired ceramic (LTCC) ion mobility spectrometer drift tube were undertaken to verify models of ion transport and determine the physical shape of the ion "swarms" in the LTCC tube. Simplified two-dimensional Gaussian models of ion swarm shape were fit to measured data to extract geometrical shape parameters. Results indicate that tube-transfer function effects that produce asymmetric ion swarms are minimized in the tube reducing temporal dispersion. Data are presented that illustrate the swarm shape as a function of gate time, electric field magnitude, and total charge in the ion swarm. Characterization and understanding of the ion transport mechanisms and effects that limit the resolution and other performance parameters of miniature IMS drift tubes is essential to the development of practical, robust, portable systems for "first responder" and homeland security missions.
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