Materials conductance/capacitance sensors

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

Meyer et al. (2006) developed a three-layer structure capacitance sensor consisting of two conductive textile layers sandwiching a compressible spacer material (foam or textile) for motion and muscle activity detection. As the body moves, pressure is applied to the capacitance sensor, more specifically onto the spacer layer, and capacitance changes accordingly. The sensors were shown to be able to detect arm movement when placed on the biceps and triceps. 

A chemicapacitive sensor is a capacitor that has a selectively absorbing material such as a polymer as a dielectric As chemicals are absorbed into the dielectric, effective permittivity of the capacitive sensor is changed. This leads a change of the capacitance. A cross-secti(Mial view of the basic structure of micromachined parallel plates is shown in Fig. 5 [9]. The conductance of the polymer depends on the types and quantities of biochemicals. 

Other sensors are composed of nanostructured materials such as carbon nanotubes and metal oxide nanowires. These materials have a controlled pore size and an increased adsorptive capacity due to the large surface area enabling the selective uptake of gaseous species. The absorption can lead to a measurable change in some specific properties, e.g., conductance, capacitance, etc. (Li et al. 2003). 

Frequency-dependent measurements of the materials dielectric impedance as characterized by its equivalent capacitance, C, and conductance, G, are used to calculate the complex permitivity, e = d — id, where co = 2nf, f is the measurement frequency, and C0 is the equivalent air replacement capacitance of the sensor. 

Especially in rnicromachined sensors, these parasitics are often comparable to or larger than the sense capacitance and can significantly reduce the sensor performance. The relatively small variation of the relative permeability compared with the range of conductance of different materials is the reason why parasitics tend to be much more important in sensors with capacitive rather than (piezo)-resistive interfaces. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

Barometry measures a broad variety of pressures using an equally broad variety of measurement techniques, including liquid column methods, elastic element methods, and electrical sensors. Electrical sensors include resistance strain gauges, capacitances, piezoresistive instruments, and piezoelectric devices. The technologies range from those developed by French mathematician Blaise Pascal, Greek mathematician Archimedes, and Torricelli to early twenty-first century MEMS sensors and those used to conduct nanoscale materials science. 

Capacitive humidity sensors commonly contain layers of hydrophilic inorganic oxides which act as a dielectric. Absorption of polar water molecules has a strong effect on the dielectric constant of the material. The magnitude of this effect increases with a large inner surface which can accept large amounts of water. An example of this type of dielectric is porous j8-alumina. Colloidal ferric oxide, certain semiconductors, perowskites and certain polymers have also been used. /1-alumina is characterized by ionic conductance. Materials of this type can be characterized by a complex resistance composed of real (ohmic) as well as capacitive terms. The behaviour of such solids can be symbolized by a model and an associated equivalent circuit as given in Fig. 5.8. 

Conductive textiles that change their electrical properties as a result of the environmental impact can be used as sensors. Smart textiles possess the properties of conventional textile materials and carry additive functional values. Typical examples are textiles that react to deformations such as pressure sensors, stretch sensors, and breathing sensors. Different physical principles are adopted to reach the same purpose, such as capacitive or resistive behavior of the textile sensor. On the other hand, biochemical, optical, temperature, humidity, and biopotential sensors can be made with smart textiles. 

The possibility of utilizing polymer-nanocomposites for the manufacture of gas and vapor sensors was studied in our laboratory in 1992 [40]. The specificity of nanocomposites, which makes them attractive as a gas sensor material, is the existence of a hopping conductivity between the nanoparticles through the polymer in a range of concentrations close to the percolation threshold (see Sect. 2.3). Since the absorption and partial dissolution of the gas or vapor molecules in polymer matrix changes its properties, it is possible to monitor these changes measuring electrophysical characteristics of the composites such as conductance and capacitance. The important point here... 

---