Large-area Detectors and Sensors

In this chapter we review recent progress of organic transistors for sensor applications. Emphasis is put on large-area, flexible pressure sensors suitable for electronic artificial skin and for photodetectors suitable for sheet image scanners. We also describe future prospects of large-area sensors and the other issues. 

Large-area pressure sensor sheets are mechanically flexible, as shown in Fig. 16.1, and can therefore be wrapped around fine cylindrical bars, for example robot fingers. A sense of touch for humanoid robots is far behind the senses of sight and hearing. This is mainly because a flexible, large-area pressure sensor matrix has not been manufactured at reasonable cost. Flexible pressure sensors have been made from polymers or rubber. With increasing number of sensors in the matrix, however, problems associated with wiring cannot be overlooked this makes it impossible to increase the density or total number of sensors to that comparable with human skin. 

In this scheme we have overcome the above problem by introducing organic transistor integrated circuits as a flexible active matrix to read out pressure images, or distribution of pressure. As a result, we have successfully developed large-area, flexible pressure sensors with the number of pressure sensors exceeding 1,000. As shown in Fig. 16.2, the device is manufactured by laminating four different functional films  

a base plastic film with organic transistors, 

a film suspending copper electrodes for power supply. 

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Dynamic explosion detectors use a piezoresistive pressure sensor installed behind the large-area, gas-tight, welded membrane. To ensure optimum pressure transference from the membrane to the active sensor element, the space between the membrane and the sensor is filled with a special, highly elastic oil. The construc tion is such that the dynamic explosion detec tor can withstand overpressures of 10 bar without any damage or effect on its setup characteristic. The operational range is adjustable between 0 and 5 bar abs. Dynamic explo-... 

A widely used technique is IR monitoring, which is utilized for both point and area (open-path) measurement applications but cannot detect H2. The previously discussed detectors were point sensors. To monitor a large area, one would have to locate many monitors (points). In contrast, open-path IR combustibles monitors project their beams in a path that is typically 10-200 m in length and monitor all of the combustibles in that path. 

Both thick- and thin-film versions of a solid state, resistive hydrogen sensor were designed and fabricated at ORNL [69, 70], Both versions of the sensors (25 mm x 25 mm x 0.6 mm) are small enough to be incorporated into hand-held leak detectors or distributed sensor systems for safety monitoring throughout large areas. 

It was clearly demonstrated that the composite BN semiconductor polycrystalline bulk detectors with BN grains embedded in a polymer matrix operate as an effective detector of thermal neutrons even if they contain natural boron only (Uher et al. 2007). A reasonable signal-to-noise ratio was achieved with detector thickness of about 1 mm. A Monte Carlo simulation of neutron thermal reactions in the BN detector was done to estimate the detection efficiency and compare with widely used He-based detectors to prove advantages of BN detectors. They are found to be promising for neutron imaging and for large area sensors. 

They have two disadvantages. First they are only capable of sensing a flammable gas at a single point. If the position of the sensor is unfavorable in relation to the origin of the flammable gas release and the pattern of air flow and ventilation in the hazardous area, then the gas detector will not detect a dangerous release of gas until it is too late to take effective action. Generally point gas detectors can only provide adequate protection at a facility if deployed in large numbers. 

SAW-based detectors can draw vapor directly or throngh a sample concentrator before analysis by the sensor array. Sample concentrators are snbstances with high surface areas to sorb large amounts of incoming molecules from an air sample. Charcoal is commonly used to sorb targeted chenficals from the vapor stream. Charcoal sorbs CWAs and TICs strongly, striping them from the air and, thus. 

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