Piezoresistive/piezoresistivity materials

The size, cost, and accuracy requirements for successful sensors place a substantial burden on the processes used to manufacture them. Typically, today s sensors are manufactured by processes that are specific to one sensor or, at best, to a limited class of sensors. This process specificity has occurred because each type of sensor is usually made of unique materials in unique configurations that best convert the quantities to be measured into electrical signals. For example, pressure transducers may use piezoelectric or piezoresistive materials on thin diaphragms, whereas ion-selective electrodes use ion-conductive glasses or polymers around electrodes. Unfortunately, this situation implies large developmental costs. 

The sensor arrays considered in this paper are fabricated using a piezoresistive material. Piezoresistive materials are materials which exhibit a change in its resistivity upon an application of a mechanical stress on its surface. This phenomenon is called piezoresistivity. It has been found that the contact resistance could be described by the following equation [10] ... 

Where R is the resistivity of the material and F is the stress force applied while K is function of the roughness and elasticity of the material [10]. A gauge factor to characterise the sensitivity of the piezoresistive material to the applied stress force has been reported [10] which could be described using the following equation ... 

Respiratory rate is measured by techniques based on either measuring thoracic expansion or based on measuring changes in skin impedance. For the former technique, most systems use strain gauges made from piezoresistive material combined with textile structures. Hertleer et al. reported a fabric sensor made of SS yam knitted in spandex belt [18]. For the latter technique, noninvasive skin electrodes are placed on the thorax, and the variation of the electrical impedance can be detected during respiration cycles. 

Measurements from stress gauges, assuming equal accuracy and time resolution, are equivalent to measurements from particle velocity gauges in exploring a material s equation of state. Both piezoresistive and piezoelectric techniques have been used extensively in shock-compression science. 

Detectors that have been widely used for materials studies can be conveniently categorized by the physical phenomena utilized in the measurement as piezoelectric, piezoresistant, electromagnetic, or optical. In each, the phenomena are capable of providing a signal in a time short compared to changes in characteristics of wave profiles. Given the desired time resolution of one nanosecond, the detectors must be massless. ... 

In bulk material, the resistivity is independent of crystal orientation because silicon is cubic. However, if the carriers are constrained to travel in a very thin sheet, eg, in an inversion layer, the mobility, and thus the resistivity, become anisotropic (18). Mobility is also sensitive to both hydrostatic pressure and uniaxial tension and compression, which gives rise to a substantial piezoresistive effect. Because of crystal symmetry, however, there is no piezoelectric effect. The resistivity gradually decreases as hydrostatic pressure is increased, and then abrupdy drops several orders of magnitude at ca 11 GPa (160,000 psi), where a phase transformation occurs and silicon becomes a metal (35). The longitudinal piezoresistive coefficient varies with the direction of stress, the impurity concentration, and the temperature. At about 25°C, given stress in a (100) direction and resistivities of a few hundredths of an O-cm, the coefficient values are 500—600 m2/N (50—60 cm2/dyn). 

The principle of the resistance strain gauge 2 0 is that the electrical resistance of a conductor will change when it is stretched or compressed due to the consequent variation in its physical dimensions. There is an additional effect called the piezoresistance which is the relation between the resistivity p of the material and the mechanical strain. The resistance R of a conductor of area of cross-section A and length x is given by ... 

Semiconductor strain gauges have a much larger piezoresistance effect leading to gauge factors of between 100 and 175 for P type material and between -100 and -140 for N type material. These consequently are much more sensitive to changes in strain than the metal resistance types. On the other hand, they are affected to a greater extent by variations in temperature. 

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. 

Kato et al. [26] directly observed the shock-pressure history by using an in-material piezoresistive carbon gauge to study the ice Hugoniot in detail below 1 GPa. They found that the HEL (Hugoniot Elastic Limit) of ice was between... 

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

Fig. 7.3.1 shows the principle of the piezoresistive sensor. Diffused resistors (gages) are formed on the thin-walled section called the diaphragm. An applied pressure is detected via the piezoresistive effect, which is the change in electrical resistance when a stress is applied to the diaphragm. The sensitivity is determined by the material, diameter, and thickness of the diaphragm. The thin-film piezoresistive sensor offers low sensitivity because the piezoresistive coefficient of thin-film silicon is less than one-third of that of single-crystal silicon. 

Piezoresistive sensors. To measure the pressure, the resistance change to stress (the piezoresistance effect) may be employed. When silicon is stressed, the resulting strain breaks the cubic symmetry of the underlying crystal structure. The band structure of silicon is very sensitive to its crystal structure and, as a result, the consequent modification causes changes in the resistivity of the material (holes in the case of p " material). This change is... 

Piezoresistive monitoring assesses the thoracic circumference and measures the resistance of a sensor made of conductive material when it is stretched (Merrit, 2008 Al-Khalidi et al., 2011). 

Interest has been shown in the piezoresistance properties of Magnus Green Salt. As in the case for Ir(CO)2(acac), a high hydrostatic gauge factor is observed (25). Materials with a smaller metal-metal interaction, for example [M(NHs)4] [M CU] (M, M = Pd, Pt), exhibit smaller pressure effects. 

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... 

The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. Polymers that incorporate conductive fillers in the form of micro/nanoparticles or nanotubes can act as an insulator, semiconductor or conductor material, depending on the amount of the conductive fillers. Fig. 14.1 shows the evolution of logarithm of resistivity p with volume fraction of conductive fillers (f) for a nonconductive polymer matrix charged with conductive fillers. The resistivity-volume fraction plot can be divided into two distinctive zones ... 

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 ... 

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