Capacitor Dielectric Constants

Capacitors. The outstandingly low dielectric loss of parylenes make them superior candidates for dielectrics in high quality capacitors. Furthermore, their dielectric constant and loss remain constant over a wide temperature range. In addition, they can be easily formed as thin, pinhole-free films. Kemet Flatkaps are fabricated by coating thin aluminum foil with Parylene N on both sides and winding the coated foils in pairs (62). 

In addition to the obvious effect of high dielectric constant glasses on the capacitance of the circuit elements iato which they enter, their dielectric strengths maybe more important. Siace the amount of energy a capacitor can store varies as the first power of the dielectric constant and the second power of the voltage, a glass with twice the dielectric strength is as effective as one with four times the dielectric constant. 

Electronic and Electrical Applications. Sulfolane has been tested quite extensively as the solvent in batteries (qv), particularly for lithium batteries. This is because of its high dielectric constant, low volatUity, exceUent solubilizing characteristics, and aprotic nature. These batteries usuaUy consist of anode, cathode polymeric material, aprotic solvent (sulfolane), and ionizable salt (145—156). Sulfolane has also been patented for use in a wide variety of other electronic and electrical appHcations, eg, as a coil-insulating component, solvent in electronic display devices, as capacitor impregnants, and as a solvent in electroplating baths (157—161). 

Other. Insoluble alkaline-earth metal and heavy metal stannates are prepared by the metathetic reaction of a soluble salt of the metal with a soluble alkah—metal stannate. They are used as additives to ceramic dielectric bodies (32). The use of bismuth stannate [12777-45-6] Bi2(Sn02)3 5H20, with barium titanate produces a ceramic capacitor body of uniform dielectric constant over a substantial temperature range (33). Ceramic and dielectric properties of individual stannates are given in Reference 34. Other typical commercially available stannates are barium stannate [12009-18-6] BaSnO calcium stannate [12013 6-6] CaSnO magnesium stannate [12032-29-0], MgSnO and strontium stannate [12143-34-9], SrSnO. ... 

The main electroceramic apphcations of titanium dioxide derive from its high dielectric constant (see Table 6). Rutile itself can be used as a dielectric iu multilayer capacitors, but it is much more common to use Ti02 for the manufacture of alkaline-earth titanates, eg, by the cocalciuation of barium carbonate and anatase. The electrical properties of these dielectrics are extremely sensitive to the presence of small (<20 ppm) quantities of impurities, and high performance titanates require consistently pure (eg, >99.9%) Ti02- Typical products are made by the hydrolysis of high purity titanium tetrachloride. 

Barium titanate has widespread use ia the electronics iadustry. Its high dielectric constant and the ease with which its electrical properties can be modified by combination with other materials make it exceptionally suitable for a variety of items, ie, miniature capacitors (see Ceramics as electrical materials). 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

There is also growing iaterest ia thin-film dielectric capacitors. For example, through the use of processiag techniques such as sol—gel solution deposition, thin (--- 0.25 fim) ceramic layers having dielectric constants ranging from 500 to 2000 ia the PZT, Pb(Zr,Ti)03, and PMN—PT, Pb(Mn2 3Nb2 3)03-PbTi03, compositional families respectively, have been prepared (3). 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This propei ty is useful in process control for polymers, ceramic materials, and semiconduc tors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). TEe value for water is higher than for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

Since the interface behaves like a capacitor, Helmholtz described it as two rigid charged planes of opposite sign [2]. For a more quantitative description Gouy and Chapman introduced a model for the electrolyte at a microscopic level [2]. In the Gouy-Chapman approach the interfacial properties are related to ionic distributions at the interface, the solvent is a dielectric medium of dielectric constant e filling the solution half-space up to the perfect charged plane—the wall. The ionic solution is considered as formed... 

Dielectric constant/loss The value of the dielectric constant is important in the wire because of the effect that it has in coupling currents in one set of wires into another set of wires. The higher the dielectric constant, the higher the value capacitor that is formed between two wires. The capacitor thus formed is a signal carrying device at the frequencies used in communications and a signal can be capacitively coupled from one circuit to another. PE is the preferred choice for insulation of communication wire because of its low dielectric constant that minimizes the intercircuit coupling effect usually referred to as cross-talk. 

Capacitor There are several applications for plastics in electrical devices that use the intrinsic characteristics of the plastics for the effect on the electrical circuit. The most obvious of these is the use of plastics particularly in the form of thin films as the dielectric in capacitors. TP polyester films such as Mylar are especially useful for this type of application because of the high dielectric strength in conjunction with a good dielectric constant. 

The dielectric constant is the ratio of the capacity of a condenser made with a particular dielectric to the capacity of the same condenser with air as the dielectric. For a material used to support and insulate components of an electrical network from each other and ground, it is generally desirable to have a low level of dielectric constant. For a material to function as the dielectric of a capacitor, on the other hand, it is desirable to have a high value of dielectric constant, so that the capacitor may be physically as small as possible. 

Capacitive Sensors. This device usually consists of a capacitor which is formed either from two concentric cylinders or from a pair of parallel plates. The solid sample to be analyzed for moisture content is passed between these plates. Since w has a large dielectric constant, the w content of the sample causes a significant change in the dielectric constant of the solid, which is measured using bridge or frequency techniques. 

Following the concepts of H. Helmholtz (1853), the EDL has a rigid structnre, and all excess charges on the solntion side are packed against the interface. Thus, the EDL is likened to a capacitor with plates separated by a distance 5, which is that of the closest approach of an ion s center to the surface. The EDL capacitance depends on 5 and on the value of the dielectric constant s for the medium between the plates. Adopting a value of 5 of 10 to 20 nm and a value of s = 4.5 (the water molecules in the layer between the plates are oriented, and the value of e is much lower than that in the bulk solution), we obtain C = 20 to 40 jjE/cm, which corresponds to the values observed. However, this model has a defect, in that the values of capacitance calculated depend neither on concentration nor on potential, which is at variance with experience (the model disregards thermal motion of the ions). 

In storage capacitors it is important that we keep the dielectric constant to a minimum. This is not just a question of selecting the most appropriate polymer we must also ensure that contaminants that could raise the dielectric constant are kept to a minimum. Polymer manufacturers sell special grades of polypropylene (which is invariably the polymer of choice) that they describe as "ultraclean . These resins are made with catalysts that are extremely active and thus leave very little residue in the polymer. Such resins typically contain little or no anti-oxidant and are extruded under conditions and using equipment that are designed to minimize oxidation. 

The dipole moment of a molecule can be obtained from a measurement of the variation with temperature of the dielectric constant of a pure liquid or gaseous substance. In an electric field, as between the electrostatically charged plates of a capacitor, polar molecules tend to orient themselves, each one pointing its positive end toward the negative plate and its negative end toward the positive plate. This orientation of the molecules partially neutralizes the applied field and thus increases the capacity of the capacitor, an effect described by saying that the substance has a dielectric constant greater than unity (80 for liquid water at 20°C). The dipole moments of some simple molecules can also be determined very accurately by microwave spectroscopy. 

The electric field or ionic term corresponds to an ideal parallel-plate capacitor, with potential drop g (ion) = qMd/4ire. Itincludes a contribution from the polarizability of the electrolyte, since the dielectric constant is included in the expression. The distance d between the layers of charge is often taken to be from the outer Helmholtz plane (distance of closest approach of ions in solution to the metal in the absence of specific adsorption) to the position of the image charge in the metal a model for the metal is required to define this position properly. The capacitance per unit area of the ideal capacitor is a constant, e/Aird, often written as Klon. The contribution to 1/C is 1 /Klon this term is much less important in the sum (larger capacitance) than the other two contributions.2... 

Any theory which includes an infinite barrier to metal electrons at the interface will make the reciprocal capacitance too large because it makes the effective interplanar spacing of the inner-layer capacitor too large.76 This is why Rice s early5 results (see below) were so incorrect. The fact that the electron tail penetrates a region of higher dielectric constant further reduces the calculated inverse capacity.30,77... 

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