Thin mass-flow sensors

For discrete micromechanical devices that do not include integrated electronics, the situation is different Platinum is an excellent choice for sensors based on a thermal measurement principle and is used in very high volumes, for example, in the air mass flow sensor chip produced at Bosch. Here, a platinum thin film serves as both a heater and a temperature sensor on a thin dielectric membrane consisting of silicon oxide and silicon nitride. The advantages of platinum as a thin film for thermal sensors compared to, for example, polysilicon are as follows ... 

From the vantage point of microfluidics, the structures developed by Petersen et al [33] are the most appropriate. More recently, Baltes and coworkers combined CMOS circuitry with the microfabrication of sensors to construct a thermal mass flow system based on thin-film pyrometers [66]. As free standing mass flow sensors, they have attractive features. However, all of these silicon-based devices operate at relatively high temperatures in the 100-200 °C range. This elevated temperature limits their potential application in more complex microfluidic systems. The ideal flow sensor would be a very-low-temperature element that could be used on the walls of the microchannel. 

It provides a more direct approach for temperature and pressnre compensation than other presently-available mass flow sensors requiring measurement of temperature and pressure. For some gas mixtures of varying composition, mass flow is indicated accurately (e.g. CO2 and He) without calibration corrections. Because it can be fabricated by conventional thin film deposition and silicon processing techniques. It offers the possibility of lower cost and broader applications than present conunercially available gas flow sensors. 

In order for both mass and heat-flow sensors to operate, the thin-film sample must adhere to the top surface of the QCM and be of uniform thickness. The mechanical behaviour of films on the quartz microbalance has been modeled by Kanazawa(12), who examined the amplitude of the shear displacement in the quartz crystal and in the overlying film for several cases. For a 1 volt peak RF applied voltage typical of the Stanford Research Systems oscillator driver, the amplitude of the shear wave of a bare crystal is 132 nm. Mecca [29] has calculated the inertial acceleration at the centre of a similar quartz resonator, and finds that it is roughly 10 g, where g is the gravitational constant. At these extremely high accelerations, powder or polycrystalline samples do not follow the transverse motion of the QCM surface and cannot be used without being physically bound to the surface with a thin adhesive layer. 

The recent work of Albano et aL illustrates a particularly innovative ap-phcation of molecular imprinting [148]. In the presence of sodium dodecyl sulfate (SDS), a thin layer of polypyrrole was electrodeposited onto the surface of a quartz crystal Removal of the SDS by washing with water afforded a molecularly imprinted sensor element which could be used to monitor levels of pollutant siuTactants in river water. Thus, the piezoelectric quartz sensor was contained within a flow cell so that the MlP-coated side of the crystal remained in contact with analyte solution while the electrodes of the sensor were connected to an oscillator circuit. A significant drop in the quartz crystal oscillation frequency occmred when the sensor was in contact with SDS. This is consistent with an increase in mass on the surface of the sensor, suggesting that SDS molecules had bound to the layer of imprinted polypyrrole. A reference polypyrrole-coated sensor, prepared in parallel to the MlP-sensor but in the absence of SDS, showed only a minimal response to the target analyte. 

A numerical heat transfer model of thin fibrous materials under high heat flux eonditions (bench-top burner) was developed by Torvi and Dale [37]. The model is applicable to two common, flame resistant fabrics, Nomex IIIA and Kevlar /PBI. A fabric-air gap-test sensor system (Figure 12.4) is used in which heat transfer is assmned to be one-dimensional. The fabric s thermal properties represent the average thermal property values of the fibrous stmcture. Mass transfer, hot gas flow and fabrie stmctural changes are not considered. The fabric s thermal properties are taken as fimetions of temperature only. The authors use energy balance equations and models of heat transfer modes to develop a differential equation (equation 12.26), and initial and boimdary conditions ... 

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