Edison effect is a famous American scientistthomas edison It was discovered on May 13, 1883, but it starts from 1877.After Edison invented the carbon filament lamp that year, the problem of too short life appeared soon after its application:Carbon filamentIt is difficult to withstand the high temperature of electric fire, and it will be "evaporated" soon after use,Light bulb:The life of is over.Edison tried every means to improve it. In 1883, he suddenly had the idea of sealing another bulbCopper wire, which may prevent the carbon filament from evaporating and prolong the life of the bulb.After repeated tests, although the carbon wire evaporated as usual, he found a strange phenomenon from this failed test, that is, after the carbon wire was heated, there was a weak current on the copper wire.The copper wire is not connected to the carbon wire. Where did it come fromelectric current？Can't the current fly through the air?At that time, it was an incredible thing. Sensitive Edison was sure that it was a new discovery and thought that it might be made according to this discoveryGalvanometer、voltmeterAnd other practical appliances.He applied for itpatent, named "Edison Effect".
Edison effect is a kind of emission through thermal excitationcarrierThe way.The reason for this phenomenon is thatcarrierThermal energy enables them to overcome the bound potential energy (in metallic materials, this bound potential energy is also calledwork functionOr escape work).The carriers generated by thermal emission may beElectronicsperhapsIons。After the carrier is emitted, a carrier with the same size and opposite polarity will be generated in the original region.However, if the emitter is connected to the battery, the charge generated on the object will be immediately neutralized by the carrier provided by the battery, and finally the emitter will reach the electrical balance and return to the previous state.The thermal emission of electrons is called thermionic emission.
British physicistFlemingBut according to the Edison effect, he invented the electronic tube (diode).Later, people made it on the basis of Fleming diodetriode, contributing to the world's firstRadio stationIn 1921 in the United StatesPittsburghCity establishmentradioCommunication wildfire quickly appeared around the world[1]。
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In 1883, when Edison was developing and improving the electric bulb, he indirectly connected the galvanometer to the heated filament and its nearby anti pollution metal sheet, and observed that there was current passing through the galvanometer. This accidental discovery was called the Edison effect.The effect is that the heated filament emits hot evaporating electrons to the surrounding space, thus forming a current between the filament and the metal sheet.This effect is of great significance in the history of science because it is closely related to the later discovery of electrons, the discovery of hot electron emission, and the invention of vacuum diodes.
When high intensity pulsed laser irradiation failsTransparent materialDue to the strong absorption of light by the material, heating, melting, evaporation and sputtering will occur instantaneously, leading to local damage of the material.In association with the Edison effect, the electrical properties of projectiles produced by pulsed laser irradiation on the target were studied, and the spatial pulse electrical signals were observed in the adjacent area in front of the target.Studying this signal will help us understand the mechanism of laser damage to materials andLaser processing technologyIt is of great significance.
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When the pulsed laser focus shines on the metal surfacepower density Very large, the metal absorbs light and heats rapidly. When its temperature reaches the melting point, it continues to be heated in the form of molten liquid until the temperature rises to the vaporization point and is vaporized.In this heating process, there are hot electron emission and hot ion emission.Since the escape work of the electron is much smaller than that of the ion, the electron starts to emit at a lower temperature, while the ion only emits at a much higher temperature.Electron emission comes first, and the electron mass is far less than that of positive ions. In the evaporation stream generated by heating, electrons fly away from the metal target at a higher speed than positive ions.After the metal is melted, due to the extrusion force of the rapidly expanding molten metal and the impact of the steam, the molten liquid produces sputtering.There are not only charged particles, but also neutral particles and small droplets in the sputter.Due to the low electron mass, this sputtering effect causes electrons to be farther away from the metal target.This space separation of positive and negative charges will inevitably cause a space potential difference in the adjacent space in front of the target, so that electrical signals can be observed in the experiment.Because the factors involved in a series of processes such as heating, melting, evaporation, sputtering and so on produced by laser pulse irradiation target are extremely complex, it is difficult to use a mathematical formula to describe the spatial distribution form of the generated potential difference in theory.
Obviously, the strength and waveform of the observed electrical signal are closely related to the strength and waveform of the pulsed laser.The heating, melting, evaporation and sputtering effects generated when the light intensity is applied are strong, so the generated electron emission, ion emission and sputtering effects are stronger, and the generated spatial signals are correspondingly increased.Because ordinary pulse laser is composed of a large number of small peak pulses of relaxation oscillation, the generated electrical signals have a similar structure.
Similarly, the observed electrical signals are closely related to the optical, thermal and electrical properties of the target material.The action process of light on metal involves the absorption coefficient of metal to light, the specific heat, specific gravity, melting point, boiling point, melting heat, heat of vaporization and thermal conductivity of metal, as well as comprehensive parametersThermal diffusivity, evaporation specific energy, electron work and ion power.For example, the thermal diffusion coefficient, specific energy of evaporation, work of electron escape, work of ion escape, melting point and boiling point of copper are larger than those of aluminum, so under the same laser pulse irradiation, the electrical signal generated by copper target is weaker than that of aluminum target[2]。
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When the target is irradiated by ordinary pulsed laser, strong electron emission, ion emission and sputtering of charged particles are generated due to the local rapid and intense heating of metal materials, and the space separation of positive and negative charges occurs in front of the target, thus generating space electrical signals.Due to the small action area of focused pulse laser and high optical power density, the generated electrical signal is strong.This electrical signal is closely related to the heating and destruction of materials, so it can be used in the application fields related to laser destruction of materials.As long as appropriate target materials are selected, under the action of focused pulse laser, it is possible to make a strong micro electron emission source or ion emission source[3]。
