Dielectric heating loss tangents

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

The heating characteristics of a particular material (for example, a solvent) under microwave irradiation conditions are dependent on the dielectric properties of the material. The ability of a specific substance to convert electromagnetic energy into heat at a given frequency and temperature is determined by the so-called loss tangent, tan <5. The loss factor is expressed as the quotient tan (3 = "/ , where e" is the dielectric loss, indicative of the efficiency with which electromagnetic radiation is... 

Although the intermediate peak and the low-temperature (-80°C) peak have similar magnitudes in a mechanical measurement, in a dielectric experiment, the intermediate peak is over twice as large (Figure 2). This seems to be due to moisture, since heating briefly to 300 C decreases the peak loss tangent substantially. 

The adhesives are suitable for use in flip chip bonding of semiconductor parts. They exhibit good shelf stability, productivity, strength properties and heat resistance. The electrical properties, such as dielectric constant and dielectric loss tangent are highly satisfactory. 

Of considerable interest is the use of silicone rubbers for insulation in electrotechnical equipment. This is accounted for by superior heat resistance of elastomers and their good dielectric properties. E.g., the dielectric permeability of polyorganosiloxane elastomers at 500 V and 60 Hz is 3.5-5.5, their electric strength at 60 Hz is 15-20 KV/mm, and the dielectric loss tangent, which characterises the losses of electric energy in insulation, at 500 V and 60 Hz amounts only to 0.001. It is very important that these characteristics are preserved in a much wider temperature range than in the case of natural and synthetic organic elastomers. 

The heating effect relies upon dielectric polarization [1], itself containing components of electronic, atomic, dipolar, and interfacial polarization, of which the last two have timescales which allow them to contribute to the overall heating effect at these frequencies. The loss tangent, tan 5, consists of two components, s, the dielectric constant, and s", the dielectric loss, where... 

The extent to which a material is heated when subjected to microwave radiation depends on two parameters the dielectric constant e and the dielectric loss factor e". The dielectric constant describes the ease with which a material is polarized by an electric field, while fhe loss factor measures fhe efficiency wifh which the electromagnetic radiation is converted into heat. The ratio of these properties gives the dielectric loss tangent or dissipation factor ... 

The product of the dielectric constant and the loss tangent (tan 5) is called the loss factorP E", and is a relative measure of how easily a material will be heated by microwave energy. 

Also, in solid materials, microwaves are used on an industrial scale for heating purposes. The ability of the solid material to absorb microwave heating depends on two properties the dielectric constant and the loss tangent. Some materials absorb the microwave energy very easily, while others are transparent or impermeable to it. 

Dielectric Loss If an organic dielectric is incorporated into an a.c. line the heat generated by dielectric loss in this material must be included in the operating costs. Unfortunately most polymers are poor dielectrics (have relatively large loss tangents) at low temperatures so this term can be comparable to the superconductor loss for lines operating near 4.2 K. 

The dielectric constant and loss tangent are the substrate variables. The loss tangent is apparently related to variables such as the molecular weight, viscosity, and conductivity of the material to be dried. Some examples will serve to illustrate the difficulty in predicting the rate at which a material will be heated. 

A convenient way to evaluate the ability of two closely related substances to convert microwave energy into heat is to compare their respective loss tangent values, where the loss tangent is defined as the tangent of the ratio of the loss factor and the dielectric properties (Equation 25.1 Table 25.3). For deeper insight into the mechanism of microwave dielectric heating, the review by Mingos et al. is recommended. 

---