Gas sensing experiments

This chapter includes two different sensor system architectures for monolithic gas sensing systems. Section 5.1 describes a mixed-signal architecture. This is an improved version of the first analog implementation [81,91], which was used to develop a first sensor array (see Sect. 6.1). Based on the experience with these analog devices, a complete sensor system with advanced control, readout and interface circuit was devised. This system includes the circular microhotplate that has been described and characterized in Sect. 4.1. Additionally to the fabrication process, a prototype packaging concept was developed that will be presented in Sect. 5.1.6. A microhotplate with a Pt-temperature sensor requires a different system architecture as will be described in Sect. 5.2. A fully differential analog architecture will be presented, which enables operating temperatures up to 500 °C. 

The usefulness of the LSER approach hinges on the similarity of the partitioning coefficients obtained from the sensing experiments (Ks) and the gas chromatographic experiments (Kqc)- In other words, it is assumed that the relationship Ks Kgc holds. This is how LSER is used for evaluation of a new sensing material. First, the coefficient Kqc is obtained from the tabulated database or experimentally. Second, the multiple linear regression technique (see Chapter 10) is used to obtain the best fit for the sensor test data, and the individual coefficients in (2.3) are evaluated. This approach has been used successfully in evaluation of multiple materials for gas sensors (Abraham et al., 1995 Grate et al., 1996). 

Contrast the difference in interaction of the mixture of vapors with the solid phase used in a gas chromatographic experiment and in a direct sensing application. 

Abstract We have achieved selective gas sensing based on different size semiconductor nanocrystals incorporated into rationally selected polymer matrices. From the high-throughput screening experiments, we have found that when CdSe nanocrystals of different size (2.8 and 5.6nm diameter) were incorporated into different types of polymer fdms, the photoluminescence (PL) response patterns upon laser excitation at 407-nm and exposure to polar and nonpolar solvent vapors were dependent on the nature of polymer. We analyzed the spectral PL response from both sizes of CdSe nanocrystals using multivariate analysis tools. Results of this multivariate analysis demonstrate that a single film with different size CdSe nanocrystals serves as a selective sensor. The stability of PL response to vapors was evaluated upon 16h of continuous exposure to laser excitation. 

The second approach is based on addition of a precious metal or oxide catalyst (Fig. 12.1b). The design of a catalyst usually requires a large number of experiments because various catalyst materials are available and their concentrations should be optimized. In this respect, the combinatorial method is advantageous. Indeed, there are reports on the design of catalysts or surface doping elements that promote the selective gas sensing reaction.48"50... 

Not much literature is available on p-phase and we speculate that its formation in our experiments is an artefact of the deposition technique. Moreover, there is no reported evidence of the gas sensing behaviour of P-Ti02 in the published literature therefore, evaluation of sensing characteristics of textured rutile phase obtained after reduction for 8h at 700°C in 5% H2/Ar stream appears to be a more adequate protocol, so that the uncertainty arising due to the role of the former can be ruled out. 

Erunning a gas chromatograpliy experiment consists of the following steps the carrier gas which flows at constant rate takes over the sample from the injector and carries its components through the column at different rates which depend on the extent of interaction with the stationary phase. Jf the stationary phase is suitably chosen and the length of the column is sufficient, the sample components are completely separated at the column outlet. The gas stream passes from the column into the detector, which senses the components and feeds the signals into the recorder. 

Graphene-based composite materials have been studied for gas sensors as well. For example, Pt/ RGO/SiC-based devices were fabricated for hydrogen gas sensing (Shaflei et al. 2010). Experiments have shown that the flexible gas sensor can also be designed on the basis of RGO (Dua et al. 2010). 

As was shown before, conventional gas-sensing metal oxides, such as SnO, In Oj, WO3, ZnO, as a rule are used in conductometric gas sensors operated at high temperatures. However, during experiments carried out recently (last decade) it was established that conductometric gas sensors based on these metal oxides can operate at room temperatures as well. Examples of these sensors are presented in Table 2.19. 

A major advantage of this method is that it is a noncontact and nondestructive way of measuring. Additionally, it is not expected to influence the electrical or chemical structure of the material. The technique is very simple, but fast, accurate, and versatile it can, in principle, be used in many environments. However, experience has shown that for practical sensor applications Kelvin probes are not suitable due to the necessary instrumentation and the limits to miniaturization. Most probes are rather macroscopic with a reference electrode the size of a few millimeters or centimeters. In addition, this method of measurement is complicated in comparison with other methods used in gas sensing and requires specific conditions and equipment. As a result, these devices are not available on the sensor... 

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