Conduction mechanism, humidity sensors

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

In order to increase the electric p-type conductivity, several rare-earth cations were doped into the Ce site of the SrCeOs electrolyte. The highest conductivity was observed for the cerate with Yb substitution (Iwahara et al. 1981). Of the three rare-earth cations, Yb3+, Y +, and Sc +, Yb has the closest ionic radius to Ce. It is easily concluded that the best candidate dopant is Yb. Since the hole concentration produced by the substitution is closely related to the humidity sensing mechanism, the hole concentration in the solid electrolyte directly influences the applicability for the humidity sensor. The... 

Fu et al described fast humidity sensors based on ceria nanowires prepared by a hydrothermal method. Sensitivity increased gradually as the humidity increased and resistance decreased exponentially with increasing humidity. The humidity sensing mechanism was ion-type conductivity, and a model based on the morphology and surface energy of the nanowires was used to explain these results (Fig. 6.21). 

The transduction mechanisms of these sensors are based on the conduction of semiconductors such as tin oxide [16], or polymers such as polypyrrole [17]. More sensitive are sensors that weigh impinging molecules [18] and more sensitive still is the biological nose. Recently there has been a renewal of interest in optical sensors incorporating fluorescent molecules [19]. Typically a device will have 3 to 30 sensors, the output of each being a voltage. This may be measured at the steady state, or the time development of the voltages may be monitored. Humidity and temperature control is important for many sensors. 

Most textile sensors are based on the use of conductive materials. Indeed, the electrical resistance is dependent on other factors (Castano and Flatau, 2014) such as mechanical strain, temperature or humidity. When a mechanical strain is applied on conductive elements, the resulting change in electrical resistance is called piezoresistivity (Carmona et al., 1987) and can be estimated by deriving Ohm s law ... 

The main types of instruments measuring humidity in gases are various types of hygrometers (gravimetric, mechanical, condensation, infrared absorbance detector, electric sensor, thermal conductivity, Al203/sili-con, P2O5) and psychrometers. 

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