Capacitive-type relative humidity sensors have been widely used. However, it is known that the output of the sensor gradually increases to a greater or lesser degree by aging of the sensor in a hot and humid atmosphere. [1][2][3][4][5] While the magnitude of such drift depends on the type of sensing polymers used, the origin of the drift has not yet been made clear. An understanding of this drift phenomenon and its implications is essential for preparing a stable and reliable capacitive-type humidity sensor.This type of sensor employs a simple sensing mechanism. The water sorption and desorption of the sensing polymer changes its effective permittivity of dielectrics. This phenomenon causes a sensor-capacitance change. Many factors are thought to contribute to a sensor's long-term lack of stability. Among them, in terms of the sensing mechanism, changes in the sensing polymer's water sorption characteristics over time can lead to changes in the sensor's dielectric properties. In the case of some linear polyimides, it has been suggested that the lack of long-term stability arises from a hydrolysis of the film in the presence of moisture and heat, which changes the polarizability and water-sorption properties of the polyimide. 4 These changes modify the humidity-sensing characteristics of the film. The chemical stability of the materials in a hot and humid atmosphere is important. In addition, the swelling and/or deformation of the film in such an atmosphere may also lead to changes in the water-sorption ability of the sensing films. An exact understanding of this phenomenon makes it easier to choose the appropriate sensing materials and to prepare reliable sensors. In this study, several polymers with different chemical structures and hydrophilicity were chosen for the model sensing materials. The drift phenomenon in a hot and humid atmosphere was analyzed based on dielectric, water sorption ability, and X-ray photoelectron spectroscopy (XPS) measurements as well as infrared (IR) analysis.
Laser ablation of transparent materials is induced by non-linear absorption and some laser-induced damages are introduced in the bulk as well as on the surface. This process is used in laser marking and other applications such as refractive index modification of optical materials and 3-D data storage. We have observed the laser ablation dynamics in inside of bulk transparent materials by nanosecond time-resolved imaging technique. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at inside of bulk PMMA and soda glass. Second harmonic radiation (532nm) from the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter at 532 nm. Series of images were taken at different intervals between the fundamental and the second harmonic light, which was controlled by optical delay line. In observation at longer intervals than 5Ons, another laser was used as illuminating source. When the laser was focused at inside of the bulk PMMA, damages occurred simultaneously at several independent points without the ablation at the surface. They located along laser incident axis. Propagation of shock waves, which started from these points, was clearly observed in the bulk. In the glass, absorbing point in the bulk formed a continuous line and its end-point moved from inside outward to the surface along the laser beam. Laser induced damages (cracks) continued to develop until some microseconds after laser pulse in PMMA.
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