Material Considerations

MATERIALS CONSIDERATIONS

Materials that are insulators.  There are a large number of materials that are insulators, and they can be either inorganic or organic.  Practically all organic materials that are used for insulation purposes are polymers. 

Inorganic materials.  The insulating materials for aircraft wiring are almost always organic.  The exceptions would be where fiber glass is used along with a polymer, e.g., with Poly (vinyl chloride) and silicon rubber.  However, their usage is relatively limited currently in most aircraft.  Also, inorganic fillers sometimes are used with a polymer.  In some other wiring applications, ceramics are used for insulation.

Organic materials.  As indicated above, organic wiring insulation used in aircraft is polymeric.  The definition of a polymer is “a long molecule which contains a chain of atoms”.[i]  This definition is very broad and encompasses living material that is life itself to highly sophisticated synthetic materials.  The “chain of atoms” actually is composed of small groups of atoms that have a periodic structure that is repeated many times.  While a few naturally occurring polymers are employed for insulation purposes, most insulating materials used today for wiring are synthetic.  Organic polymers, due to their structure and composition, almost always are insulators, i.e., they have a band gap of approximately three or more eV- see the discussion below on the physical basis of insulators for details.

There are several classes of polymers that have been employed for wire insulation: hydrocarbons, halogenated polymers, nitrogen containing polymers, polyesters, silicones, cellulose, and varnish. The last two categories often are from natural sources.  Only a selected few materials from the above classes have been used for wire insulation in aircraft.  The reasons for this involve both specific chemical and physical properties of a material as it relates to aircraft usage.  Such factors as weight, thickness, resistance to attack, both from chemical and physical sources, etc., are considered when choosing materials for aircraft wire insulation.  Other applications where electrical insulation is required make greater use of more common polymers.  See the individual material pages for more information.

What is an insulator?  Perhaps the way to answer this question is to consider the difference between an insulator and conductor.  One approach is to consider resistivity, which is defined as resistance to the passage of electrical current through a material.  This quantity, in units of Ohm-cm or Ohm-m (keeping to MKS units) can vary well over twenty five orders of magnitude, e.g., some metals have a value of  approximately 10^-8 Ohms-m to about 10¹⁷ Ohms-m for quartz.  Semiconductors, e.g., silicon, have values that fall midway between those typical for conductors and insulators.  This range of values is one of the largest for any physical material property. 

            Another approach to describe an insulator is to examine the electronic structure of materials.  The outermost electrons for an element (in terms of atomic structure) are in the valence band.  These electrons participate in bonds with neighboring atoms in a material.  Above the valence band is the conduction band.  Nominally, there are no electrons in the conduction band.  If the energy difference between the valence and conduction bands is zero, or even overlaps, then the material is a metallic conductor.  In these instances, some electrons go easily between the valence band and the conduction band.  However, if there is a definite energy difference between the valence and conduction bands, then electrons will not go between the two bands unless there is an external energy source.  Note also that the electrons are forbidden to be between the valence and conduction bands if there is an energy difference.  The usual unit to describe this energy difference (called the band gap) is the electron-Volt (eV)[ii].  For semiconductors, the band gap ranges from about 0.5 to 3 eV (this upper limit is not precise); the value for silicon, probably the most widely used semiconductor, is about 1.1 eV.   The upper value for band gaps with insulating materials is approximately 9 eV.  Figure 1 illustrates the energy levels for a semiconductor or insulator.

Figure 1.  Energy level diagram for an insulator or semi-conductor; the widths of the various levels are not scaled to actual energy values.