Dielectric sensors

Figure 4.12 The lay up of a thick TGDDM epoxy (Hercules 3501-6) graphite laminate autoclave run showing the positions of the dielectric sensors and thermocouples...

Hinrichs and Thuen [28] used ultrasonic attenuation to determine the proper time for pressure application during an otherwise traditional pre-established cure cycle. Because dielectric is an electrical property, it is influenced by moisture content and temperature as well as viscosity, so it may vary quantitatively. Ultrasonic measurements are also affected by other parameters (i.e., void content), but they are a mechanical measurement rather than an electric one. The ultrasonic sensors used by Hinrichs unfortunately were less reliable than the dielectric sensors. 

Solid-state conductivity (chemiresistance) sensors Dielectrometric sensors Dielectric sensors Absorptivity elements Index of refraction elements... 

A thermodielectric profile can be obtained with a thermodielectric analyzer [15,108-110], This device registers the relationship between the linearly scanned temperature (T) (30°C rectified output voltage (0 V test sample, in the form of a powder (particle size 0.1-0.2mm) and a reference (calcined A1203 powder) (see Figures 4.41 and 4.42 for further information). 

FIGURE 4.42 Inside view of the specimen holder for TDA. 1, electric conductors to the dielectric sensor 2, ceramic specimen holder for a TDA 3, nickel made concentric cylindrical electrodes 4, void where is located a platinum-rhodium thermocouple 5, hollow place where is located the samples under test in the form of powders (about 1 g of sample). 

Atahigh temperature (r>400°C), the dielectric sensor behaves as apure resistive circuit because of the decrease of the internal resistance of the capacitors, that is, Rx and R, (see Figure 4.41). In this instance, the following approximation obtained from Equation 4.33 is valid [15] ... 

Figure 4.43a shows the block diagram of the dielectric differential thermal analyzer, where the dielectric sensor circuit and the capacitors formed by the sample and reference (Cr) and specimen holders (Cx) are schematically represented, and a photo of the home-made equipment is shown in Figure 4.43b [89,109,110],... 

The powdered zeolite (grain size between 0.2 and 0.5 mm) to be tested is placed between the electrodes of the capacitor, which is also the sample holder. The dielectric sensor circuit, which is fed with an alternating voltage (amplitude 0.1 V 30Hz standard commercial capacitor of known capacitance (Cs = 1 lOpF) in place of the reference capacitor (Cr) (see Figure 4.49c) [15,16,111-113,120], For the calculation of the real part of the relative permittivity, at different frequencies and temperatures, the following relation was used [15,120]... 

A thin layer (approximately 10 mil thick) of RTV silicone was coated on the Micromet mini-dielectric sensor. The RTV silicone was first cured at room temperature, then in an oven at 120 C for a period of time, similar to certain RTV production cure schedules. The permittivity and loss factor of the RTV silicone were recorded during this cure cycle. 

Particle size analysis using non-invasive dielectric sensors... 

Dielectric sensors Embed Electrical permittivity Intermediate Poor... 

Interest in the synthesis and processing of mesoporous silica materials has grown extensively since their discovery in 1992, and the exciting potential that these films hold in low-k dielectrics, sensors, nanowire fabrication, catalysis, membrane separations, and many other applications will continue to fuel academic and industrial interest in these films. While there are many new synthesis routes for processing mesoporous silica thin films, spin coating and dip coating remain the most facile methods available. These methods deliver high quality reproducible films that can be used for any of the variety of applications. 

Block ID, Chan LL, Cunningham BT (2006) Photonic crystal optical biosensor incorporating structured low-index porous dielectric. Sensors Actuators B Chem 120 187-193... 

As is noted in the following sections, dielectric sensors have usually been regarded as providing local viscosity data (Senturia and Sheppard, 1986), but detailed studies over a wide frequency range have shown that the absolute value of the static dielectric constant (so) may also provide data on conversion comparable to those obtained by DSC (Casalini et al, 1997). However, to achieve this it is noted that So must be measured to high accuracy (a relative error of 10 ) for this to be a viable technique. 

Prohling or imaging of a physical variable, e.g., monitoring of moisture diffusion process through an organic material is readily accomplished with low frequency dielectric sensors. Alternatively, a microwave probe can be used, but the sample penetration depth of the microwave probe would be limited, in many cases, to a few microns, whereas the penetration depth of low frequency dielectric measurements can be controlled by changing the separation between the sensor head electrodes. 

Dielectric spectroscopy techniques are promising for numerous applications that require non-invasive, non-destructive, non-contact, and real-time measurements. Non-invasive measurements with gas, liquid, solid, and mixed samples are possible on distance scales from nanometers to meters and a frequency of excitation from microhertz to terahertz. The main advantages of fringing electric field dielectric sensors include one-side access to material under test, convenience of application... 

BaTi03 Dielectric Sensors dielectric amplifiers memory devices... 

In such a study, test panels of window film were mounted on glass plates for accelerated ageing. A small hole was cut in the plastic film and an interdigitated single surface dielectric sensor applied to the exposed surface of the adhesive after chilling the plate in a freezer to aid removal of the backing. Measurements of dielectric loss and permittivity were carried out from 0.1 to 100 kHz in decade steps at ambient temperature before and after 600 and 1200h exposure. 

A dielectric sensor is attached to the inner wall of a cylindrical sample container made of aluminum. A stirring rod is inserted through a lid at the top, and the sample container is immersed in a thermostated waterbath. This experimental set-up (Fig. 2) allow dielectric measurements during hydrate formation (11). 

Yeniknez, B. and Sozer, E. M., A grid of dielectric sensors to monitor mold filling and resin cure in resin transfer molding . Composites Part A Applied Science and Manufacturing, 40,476-489, 2009. 

Hegg, M. C., Ogale, A., Mescher, A., Mamishev, A. V. and Minaie, B., Remote monitoring of resin transfer molding processes by distributed dielectric sensors . Journal of Composite Materials, 39(17), 1519-1539, 2005. 

Skordos, A. A., Karkanas, P. I. and Partridge, I. K., A dielectric sensor for measuring flow in resin transfer moulding . Measurement Science and Technology, 11(1), 25-31, 2000. 

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