Temperature compensating dielectrics

Masse, D.J., Purcel, R.A., Readey, D.W., Maguire, E.A., and Hartwig, C.P. (1971) New low-loss high-K temperature compensated dielectric for microwave... 

Masse D. J. A new low loss high-k temperature compensated dielectric for microwave applications. Paper presented at the Proc. IEEE. 91 )... 

The main advantages of ultrasonic transmitters are the absence of moving parts and the ability to measure the level without making physical contact with the process material. The reading can be unaffected by changes in the composition, density, moisture content, electrical conductivity, and dielectric constant of the process fluid. If temperature compensation and automatic self-calibration are included, the resulting level reading can be accurate to 0.25% of full scale. 

One of the early dielectric constant detectors was that designed by Grant [10] but the detector cell had a volume of 2-3 ml. Poppe and Kunysten (11) described a dielectric constant detector which included a reference cell for temperature compensation. The cell consisted of two stainless steel plates 2 cm x i cm X 1 mm separated by a gasket 50 pm thick. The two cells were identical and clamped back to back, sharing a common electrode. 

Rare earth oxides are used for manufacturing ceramic capacitors. Their presence extends the capacitor s lifetime and improves some properties such as the compensation temperature coefficient, dielectricity and magnetic permeability. Specifically, Ce, La, Pr and Nd help keep the dielectric constant of a capacitor virtually unchanged [9]. 

Magnesium titanate has many useful applications, for example, in dew sensors, in pigments, and in the electrical and electronic industries as a dielectric material for manufacturing on-chip capacitors, high-frequency capacitors, and temperature compensating capacitors. 

The behavior of a polar dielectric in an electric field is of the same kind. If the dielectric, is exposed to an external electric field of intensity X, and this field is reduced in intensify by an amount SX, the temperature of the dielectric will not remain constant, unless a certain amount of heat enters the substance from outside, to compensate for the cooling which would otherwise occur. Alternatively, when the field is increased in intensity by an amount SX, we have the converse effect. In ionic solutions these effects are vciy important in any process which involves a change in the intensity of the ionic fields to which the solvent is exposed—that is to say, in almost all ionic processes. When, for example, ions are removed from a dilute solution, the portion of the solvent which was adjacent to each ion becomes free and no longer subject to the intense electric field of the ion. In the solution there is, therefore, for each ion removed, a cooling effect of the kind mentioned above. If the tempera-... 

A great variety of aqueous—organic mixtures can be used. Most of them are listed in Table I with their respective freezing point and the temperature at which their bulk dielectric constant (D) equals that of pure water. These mixtures have physicochemical properties differing from those of an aqueous solution at normal temperature, but some of these differences can be compensated for. For example, the dielectric constant varies upon addition of cosolvent and cooling of the mixture in such a way that cooled mixed solvents can be prepared which keep D at is original value in water and are isodielectric with water at any selected temperature (Travers and Douzou, 1970, 1974). 

The addition of an amino acid to mixed solvents at selected temperatures can be a means to compensate even partially for the decrease of dielectric constant due to the solvent addition. Limitations are imposed by the solubility of the amino acid in such mixtures for instance, there is a salting-out effect in methanol-water 50 50 at 25°C when the concentration of glycine is about 0.5 Af (8 20). 

Fortunately, they are several species of low-loss dielectric ceramics with tailored temperature coefficient of dielectric constant, which can be made lower than 1 ppm/K for a certain temperature window around room temperature. Physically, this can be accomplished either by intrinsic compensation of the temperature dependence of thermal volume expansion V(T) and lattice polarizability a(T) via the Clausius-Mossotti relation ... 

A similarly transparent explanation is available for the effect of temperature. If T is increased (at fixed dielectric constant), the value of decreases, and hence the number 9 of condensed counterions also decreases. A smaller number of condensed counterions means that the charge density of the condensed layer is less. The free volume, which scales like the charge density of the condensed layer, compensates by contracting. A negative thermal expansion coefficient of the condensed layer (again, at fixed dielectric constant) may be interesting, but it is also easily understood. 

The compensation effect (Table 25) has been mentioned earlier. This effect can be easily explained by the concept of multidipole interaction [323]. The distance and angles Qu 6it A0if in a polyfunctional molecule and in the transition state depend on the temperature due to defor-mational vibrations. The amplitude of vibration increases with temperature and one can expect that AG decreases. The temperature dependence of AG can be supposed analogous to that of the dielectric constant (e = e0e LT), viz. 

---