The vacuum knowledge(3)
Third, the main factors affecting the level of vacuum insulation
Vacuum insulation is a very complex physical process and the mechanism is so far no clear conclusion. From a practical point of view, mainly in the following aspects:
1, the geometry of the electrodes
Geometry of the electrodes has a great influence on the distribution of the electric field, are often not appropriate due to the geometry, causing localized excessive concentration of the electric field caused by the breakdown, which is particularly prominent in the high voltage vacuum products.
Radius of curvature of the edges of the electrodes is an important factor. In general, large radius of curvature the capacity electrode is smaller than the breakdown voltage of the large radius of curvature.
In addition, the breakdown voltage is also inversely proportional to the electrode area, and that with the increase of the electrode area is reduced. The main area is increased resulting in a reduced withstand voltage because the discharge probability increases.
2, the gap distance
Vacuum breakdown voltage and the gap distance has a relatively clear relationship. Tests showed that when the gap distance is small (≤5mm), the breakdown voltage of the gap distance increases linearly increase, but with the further increase in the gap distance, the breakdown voltage of the slowdown, i.e. vacuum gap breakdown electric field strength increases gap distance decreases. When the gap reached a certain length (≥20mm), increasing the gap distance alone raise the pressure level has been very difficult, but this time using more favorable than single fracture fracture.
Generally believed that electrical short gap breakdown under field emission is mainly caused by the long gap between the lower and the electrical breakdown of the particle effects are mainly due.
3, the electrode material
Vacuum switches operate in a high vacuum over 10-2Pa, because this time is very rare gas molecules, gas molecules collide against the free breakdown has no effect, so the breakdown voltage and electrode materials exhibit a strong correlation.
Breakdown voltage of the vacuum gap varies with different electrode materials, the researchers found that the breakdown voltage and the hardness and mechanical strength of the material. In general, the hardness and high mechanical strength material, tend to have a higher dielectric strength. For example, steel electrodes hardness after quenching, before hardening than the breakdown voltage can be increased by 80%.
In addition, the breakdown voltage and also a cathode material physical constants such as melting point, specific heat and density are related, i.e., a material whose melting point higher breakdown voltage is higher. Specific heat and density in terms versa. The essence of the problem is in the same action of thermal energy, the greater the probability of occurrence of the molten material, the lower the breakdown voltage.
4, the degree of vacuum
Figure 1 shows the relationship between the breakdown voltage and the gas pressure in the gap between. Can be seen from FIG vacuum higher than 10-2Pa (10-4 Torr), the breakdown voltage is substantially longer with decreasing gas pressure increases, the gas molecules collide because the phenomenon is no longer free to function. When the gas pressure is gradually increased from l0-2Pa (vacuum reduction), the breakdown strength is gradually decreased, while close to 1 Torr (approximately 102Pa) minimum, and later with increased pressure and increased. As can be seen from the curve when the degree of vacuum higher than 10-2Pa its compressive strength is substantially unchanged. This indicates that the degree of vacuum interrupter when 10-2Pa more fully able to meet the needs of normal use.
5, the electrode surface condition
Condition of the electrode surface breakdown voltage greater impact on the vacuum gap. Oxide electrode surface, impurities and metal particles will cause the breakdown voltage of the vacuum gap decreased.
In addition, regardless of the electrode surface in a vacuum interrupter manufacturing process was how large current breaking will involve electrode surface becomes uneven, which will also make the breakdown voltage decreases.
6. Aging effect
Aging electrode voltage and current Burn Burn two kinds.
When a new test vacuum gap, the breakdown voltage tends to lower the first few times. With the increase of the number of trials of the breakdown voltage is gradually increased, eventually stabilize at a certain value. This breakdown voltage with frequency increasing breakdown voltage phenomenon is the role of the old mill.
Aging is the voltage on the electrode surface by microscopic projections discharge elimination of impurities and defects. So that a small current through the discharge point of the surface of microscopic projections seizure, evaporation, smooth surface of the electrode, a local electric field enhancement effect is reduced, improving the breakdown voltage. Aging purification effect on the electrode surface is also very important. Since the electron emission electrode surface prone to lower the work function of the place where the impurities, also makes the discharge breakdown melting and volatile impurities, can also increase the breakdown voltage of the gap. Aging process, if at the same time the vacuum pump pumping, the evaporating gaseous substance pumped, the better. Aging voltage only suitable for use in a vacuum gap breakdown voltage increase, the vacuum interrupter contacts gap breakdown voltage increase will not have much effect. Arc on the contact surface of the burning effect will make the voltage experienced all fail.
