Piezoresistive devices

B-doped p-type diamond films are known to exhibit piezoresistivity. Namely, the resistance increases under compressive stress [418]. The piezoresistivity is expressed by the gauge factor K, which is defined by  

The gauge factor K can also be calculated as follows assuming that the biaxial stress a is present in the film, it is evaluated from the shift of the Raman peak position Au (in units of cm ) from the corresponding peak position of natural diamond at 1333cm by the following relationship [420]  

The - and + signs correspond to the compressive and tensile stresses, respectively. The strain e can be given by  

In Ref. [421], an HOD film of about 6-pm thickness were deposited on p-type Si by the two-step process using a custom-made CVD reactor under the conditions given in Table H.4. Then, B ions were implanted at —76°C, which was followed by a rapid thermal annealing. The B concentration was 10 /cm, the hole mobility was 80cm /V s, and the hole concentration was lO /cm at 20°C. The gauge factor K was as high as 1200 at lOOp strain at room temperature, which is far superior to 6 for polycrystalline diamond films and 550 for homoepitaxial film. 

It was demonstrated by Kohn et al. that HOD films are useful for sensor and MEMS applications because of the high mechanical strength, the high thermal conductivity, the low thermal expansion coefficient, and the chemical inertness of 

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The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

A quite important parameter for the operation of washing machines is their load. There are several ways to measure the wash load, e.g the force required to rotate the drum/tub/pulsator could be measured and the required amount of water worked out accordingly, or the machine load could be measured by a remote pressure-sensing device with piezoresistive transducers and wireless communication... 

Electric sensing devices (strain gauges, piezoresistive transducers, and piezoelectric transducers)... 

Erincipal advantages of piezoresistive transducers are a much higher ridge voltage output and smaller size. Full-scale output voltages of 50 to 100 mV/V of excitation are typical. Some newer devices provide digital rather than analog output. 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

There exist a number of readout techniques based on optical beam deflection, variation in capacitance, piezoresistance, and piezoelectricity. Piezoelectricity is more suited for a detection method based on resonance frequency than the method based on cantilever bending. The capacitive method is not suitable for liquid-based applications. The piezoresistive readout has many advantages, and it is ideally suited for handheld devices. 

As is evident from equation 4, Nip can be made zero by making both (<4 — and (rrxx — ffyy) zero. Fig. 7.3.19 shows the result of FEM stress analysis by sensor device simulation. With the piezoresistive gages arranged concentrically with the inner circumference of the metal stem, both n xx — other hand, since TNS is proportional to the temperature characteristics of (o, /x — [Pg.328]

Fiber optic intravascular pressure sensors can be made in sizes comparable to piezoresistive ones, but at a lower cost. The fiber optic device determines the diaphragm displacement optically by measuring the varying reflection of light from the back of the deflecting diaphragm. These optical devices are inherently safer electrically, but lack a convenient way to measure relative pressure without an additional lumen either atmospherically vented or connected to a second pressure sensor. 

Piezoresistive sensors automatically normalized to ambient pressure are used as clinical pressure sensors today. They have to be protected against water by use of a filter in the sensing line. The installation is set up in the sidestream by means of a thin hose as the sensing line in which only a negligible flow occurs. Electronic ventilators utilize additional pressure sensors for device control. 

Figure 10.1 Respiratory-monitoring shirt with integrated piezoresistive sensors. Data are acquired and wirelessly transmitted using a Shimmer device.

A number of sensitive magnetic field detection devices have been developed as biosensors giant magnetoresistive (GMR) sensors [4], piezoresistive cantilevers [5], inductive sensors [6], superconducting quantum interference devices (SQUIDs) [7, 8], anisotropic magnetoresistive (AMR) rings [9], and miniature Hall crosses [10]. 

A low-cost, low-power-consuming micromachined flow and pressure sensing device has also been reported. This sensor can be used for monitoring pressure and flow rate in clean fluids without particles and without the tendency to coat the channel. The pressure is measured with capacitive or piezoresistive pressure sensors and the flow rate is computed from the pressure drop over a well-defined, hydraulic resistance. Although this device is yet to be used for process analysis in industrial stream, it will gain more use in this area in the future. 

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