8. Intrinsic and Related Forms of Breakdown in Solids

8.1 Introduction Homogeneous materials, free from faults and from leakage currents which might cause excessive heating, give results which conflict in at least four ways with the idea of a single strength characteristic of the material: the voltage withstood is more nearly proportional to (thickness)1/2 than to thickness; if good contact is secured between the electrodes and the material, failure occurs outside the electrode area, where the stress in the material is relatively low, and at a distance from the electrode which precludes explanation in terms of the electrostatic stress-concentration at its edge; the result depends on the time taken to reach the breakdown voltage; it depends even more upon the immersion medium, and can be increased greatly by increasing the conductivity of the medium

8.1 Introduction Stress concentration Local intrinsic breakdown In tests by the plane slab method: the strength is dependent upon thickness because the stress concentration where a discharge impinges on the material is a function of the ratio(streamer diameter/specimen thickness); The failure occurs outside the electrodes because only there can discharges impinge, if the electrodes make good contact; The results depend upon time because a breakdown channel can be formed either by a few discharges acting at a high stress, or by many more acting at a lower stress; The conductivity of the medium is important because high conductivity reduces the stress in the medium, and so postpones the onset of discharges to higher applied voltage.

8.2 The definition of intrinsic strength Intrinsic breakdown strength Homogeneous material Breakdown cannot occur through interstices electrical weak point Mechanical strength : mechanically strong enough to withstand the large forces set up by the field, which may exceed 100kg/cm2. conductivity : low enough that suitable conditions of temperature and frequency can be chosen to eliminate thermal instability at fields below the intrinsic value. plastic pressed from powder

8.3 Theories of intrinsic strength Intrinsic breakdown mechanism Calculate the conditions for electron multiplication by collision Calculate the collective stability of the whole population of conduction electrons The earliest theory of the second type is due to Von Hippel, who calculated the field necessary to render the normal thermal distribution of electron energies unstable, so that a thermal electron might be idenfinitely accelerated in the field. This is referred to as the ‘low energy’ criterion, since an electron of low energy may be accelerated. The Von Hippel criterion leads to too high a breakdown field, on the grounds that electrons of more than average energy are more easily accelerated, and are always present in sufficient numbers to cause instability.

8.3 Theories of intrinsic strength The rate of energy loss from an electron by collision with lattice vibrations F1 Fc F2

8.4 The measurement of intrinsic strength Only two methods of measurement need be seriously considered, the ‘recessed specimen’ and McKeown`s technique. Liquid or gaseous immersion methods are no longer used, since they are more difficult and less reliable than the others. Liquid electrodes have been used for special reasons, but are inconvenient. There are two limitations to the method. The cooling of the stressed area is not very good, even if the specimen is tested in an insulating ambient liquid, and thermal failures are obtained unless the specimen has very low electrical conductivity. The second difficulty is that the thin apex of the specimen is mechanically unsupported, and tends to buckle and become thinner under the compressive force from the electrodes. 1cm 30-100um

8.4 The measurement of intrinsic strength The Mckeown technique uses as electrodes two ball-bearings, located within a cylindrical hole in a disc of any convenient plastic such as Perspex. The lower ball is cemented into position with an epoxy resin, and the hole filled with the same resin in liquid form, which is gently heated in vacuo until it is degassed. The sample, in the form of a thin disc, is placed in the hole, on top of the ball, and the liquid degassed again, The top ball is then inserted, and the whole cured until solid. This technique has proved extremely satisfactory at room temperature for measuring the intrinsic strength of low-loss polymers, but for reasons which are not yet understood it gives results which are consistently higher than those given by the ‘recessed’ specimens.

8.4 The measurement of intrinsic strength Mckeown technique recessed specimen Ionic bombardment Electrolysis in the ambient liquid Fatigue form the mechanical distortion discharge breakdown

8.5 Comparison of theory with experiment

8.6 Current problems in the measurement of intrinsic strength The cause of the extreme variability in simple ionic crystals. The role of mechanical constraint and of the epoxy resin in the ‘McKeown’ type of test. The role of mechanical distortion in soft materials under electrostatic forces, and of possible mechanical fracture by the same forces in brittle materials. The role of field-enhanced conductivity in causing thermal failure. The apparent dependence of electric strength on the area tested. The role of intercystalline of inter-particle surfaces in causing electrical weakness.