Dielectric properties moisture effects

The induced dipole moment of a polymer in an electric field is proportional to the strength of the field, and the proportionality constant is related to the polarizability of the atoms in the polymer. The dielectric properties of polymers are affected adversely by the presence of moisture, and this effect is greater in hydrophilic than in hydrophobic polymers. 

The dielectric constant is affected in different ways by a number of different mechanisms. Some of these mechanisms are interdependent and so it is difficult to ascertain their individual dependencies on fluorine incorporation. For instance, the increased hydrophobicity caused by fluorination decreases the ambient dielectric constant by elimination of water from the polymer, while the incorporation of fluorine also affects the intrinsic properties of the polymer irrespective of the moisture effect. Most published dielectric data were measured under ambient conditions and thus the distinction between these effects is lost. 

Some general conclusion from these studies are (1) Cu/PI TFML structures have excellent thermal and mechanical stability under extremes of temperature, humidity, and radiation (2) the adhesion of polyimide is highly dependent on interface chemistry and surface preparation (3) PI rapidly absorbs and desorbs water, which has an appreciable effect on its dielectric properties and thus the electrical charactersitics of TFML interconnections the electrical design tolerances must accommodate these variations or the package must be hermetically sealed (4) properly baked and sealed TFML packages can maintain MIL-STD internal moisture levels of less than 5000 ppm at 100°C. 

The calculation of the effect of a liquid on the dielectric properties of foams requires repre ntation of the structure as a polymeric matrix with two-layer ellipsoidal inclusions, one of the layers being a film of sorbed moisture. 

Temperature also has a similar effect on the dielectric properties of cheese. The effect of temperature, however, depends on the moisture content of the cheese being processed. Medium-moisture content cheese exhibits a decrease in e for temperatures between 5°C and 55°C. Further increase in processing temperature results in an increase in e. The increase in e value reversed again at 65°C. The decrease in s value between 65°C and 85°C is common for both medium- and low-moisture cheese. The dielectric property changes at higher temperatures are similar to that of soy protein. Higher processing temperatures result in protein denaturation. 

The dielectric properties of these foams are not only influenced by the foam density but also by moisture absorption and temperature effects. A single scouting experiment showed, that the increase of the dielectric constant due to heating to 100°C is small in comparison with the density dependency (a 45.7 kg/m3 foam sample was heated from 22°C to 100°C, the dielectric constant at 1 kHz. increased from 1.065 up to 1.071). ... 

Two basic contributions are expected to the variation of dielectric properties of a hydrated material with respect to those of a dry one that of the polar water molecules themselves and the second one due to the modification of the various polarization and relaxation mechanisms of the matrix material itself by water [37]. In the low frequency region of measurements, there is a third contribution, often ignored in works dealing with high frequency measurements, which arises from the influence of moisture on conductivity and conductivity effects. The increase of electrical conductivity of the sample is the major effect present in wet samples dielectric response is often masked by conductivity, and it superposes the dielectric processes in the loss spectra and demands a conductivity correction of the dielectric loss spectra [9]. This dc conductivity strongly affects the modifled loss factor, e". In this case, it can be expressed as shown in the following equation ... 

Shevchenko, The effect of moisture absorption on mechanical and dielectric properties of epoxy resins and composites , Acta Polymerica 1992 43(3) 191-192. 

Effect of fiber size The dielectric constant of alkali-treated fiber composites changed slightly with fiber size, however untreated fiber composites did not show variation with fiber size [39]. The effective dielectric constant decreases with increasing filler size due to increased interface volume when filler of less particle size was used for a given volume fraction of filler [31]. Also at a given volume fraction of filler, the smaller particle size has more polarization in the interface surface as a result of increased moisture absorption for small size fillers due to increased surface area [78]. Water has unfavorable dielectric properties, which increases the dielectric constant. 

Khanna VK (2012) Detection mechanisms and physico-chemical models of solid-state humidity sensors. In Korotcenkov G (ed) Chemical sensors simulation and modelling, Vol. 3 Solid state devices. Momentum, New York Khanna VK, Nahar RK (1984) Effect of moisture on the dielectric properties of porous alumina films. Sens Actuators 5 187-198... 

Mathews DA (1963) Review of the lithium chloride radiosonde hygrometer. In Proceedings of the conference on humidity and moisture, vol VI, Washington, DC, pp 219-227 Matsuguchi M, Sadaoka Y, Sakai Y, Kuroiwa T, Ito A (1991) A capacitive-type humidity sensor using cross-linked poly(methylmethacrylate) thin films. J Electrochem Soc 138 1862-1865 Matsuguchi M, Sadaoka Y, Nosaka K, Ishibashi M, Sakai Y, Kuroiwa T, Ito A (1993) Effect of sorbed water on the dielectric properties of acetylene-terminated polyimide resins and their application to a humidity sensor. J Electrochem Soc 140 825-829... 

Reid JD, Lawrence WH, Buck RP. Dielectric properties of an epoxy resin and its composite 1. Moisture effects on dipole relaxation. J Appl Polym Sci 1986 30 1771. 

Keating, M., Evaluation of Moisture Effects on the Dielectric Properties of Polymer Films, Thermal Analysis Application Brief Number TA-124, TA Instmments... 

Concerted emphasis has been given to the variation of the dielectric properties with moisture content the ensuing data being used for the effective optimisation of applicator designs intended for industrial drying applications. Typical variations of with moisture content (on dry basis) and electric field orientation established within the material Is given In Figure 18. 

The moisture content of a plastic affects such conditions as electrical insulation resistance, dielectric losses, mechanical properties, dimensions, and appearances. The effect on the properties due to moisture content depends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a product test under extreme conditions of usage in which critical dimensions and needed properties are verified. Plastics with very low water-moisture absorption rates tend to have better dimensional stability. 

When water is used as the immersion liquid, the test is essentially the ASTM Standard Test Method (D570) for Water Absorption of Plastics.( ) Determinations of the relative rate of water absorption are important in evaluating the effects of moisture exposure on such properties as mechanical strength, electrical resistivity, dielectric... 

The absorption of moisture critically affects other important resin properties, particularly those associated with low-dielectric and thermomechanical applications. Results of a 96-h boiling water immersion test are presented in Table 2.2. The moisture absorbed decreased substantially with fluoromethylene chain length from n = 3 to n = 6, followed by only modest decreases for n = 8 and 10. This latter behavior was somewhat unexpected and may be the effect of decreased cross-link density counteracting the increased fluorine content. These 100°C measurements are just above the glass transition and the situation may be different at room temperature. These measurements are in progress. 

All electrical property values are strongly dependent on water content for water, the dielectric constant is approximately 81 and resistivity is about 106 2 cm. The dielectric constant has been used as a measure of moisture in coal (Speight, 1994, and references cited therein). However, it should be noted that the effect is not considered to be additive due to the different electrical properties of physically and chemically bound water. With an increase in moisture content, electrical conductivity and dielectric constant increase, whereas resistivity and dielectric strength decrease. Hence, except for special purposes (e.g., dielectric strength measurements of underground coal blocks), electrical measurements require the meticulous drying of coal prior to experiments. 

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