Electron Emission from Solids (II): Applications

Electron Emission from Solids (II): Applications

Certain solids can emit electrons when an electric field of an appropriate size and strength is applied to them, or when they are subjected to heat or radiation. Electron microscopes, electron emitters, CRT terminals, and television receivers are some applications of the theory of electron emission from solids.
The applications of electron emissions are considered in the following text. This is the continuation of Electron emissions from solids (I):History and Theory.

Thermionic emission forms the basis for tube circuit elements, X-ray tubes, and thermionic energy conversion systems. In 1904, John Ambrose Fleming invented the two electrode rectifying tube (diode), and in 1907, Lee de Forest placed a current carrying grid between the cathode and anode in a thermionic emitter tube, creating the triode. Diodes and triodes are the basic building blocks of tube based electronic devices. In a triode, a weak current on the grid controls the modulation of a stronger current passing as an electron beam between the cathode and anode, the whole system acting as a switch and amplifier. In a tube radio, for example, the modulation of a weak signal picked up by the antenna and propagated across the grid control a stronger household current which runs the actual sound producing apparatus. A thermionic tube needs to warm up and requires energy for heating; this energy requirement and the relatively large size, cost, and complexity of thermionic tubes led to their replacement by transistors in most applications after 1960.

X-rays are produced when high energy electrons in an electron beam strike a solid target; they were discovered accidentally by Wilhelm Conrad Rontgen during experiments with cathode ray tubes. A gas X-ray tube employs emitted electrons to create ions, which in turn collide with a target to produce rays. In a high vacuum X-ray tube, electrons produced by thermionic emission produce X-rays by striking a metallic anode. Potential differences that are great enough to cause direct field emission are undesirable in an X-ray apparatus. Therefore, a series of electromagnetic accelerators is used in X-ray tubes, producing very high energy X rays. X-rays can also be an unwanted by-product of any electron beam apparatus, such as a welding device, which relies on the deceleration of high energy electrons.

Thermionic energy conversion involves creating a sufficiently high temperature differential between circuit terminals of suitable composition to cause current to flow. It has been investigated as a method of converting nuclear and solar energy to electrical energy without parts such as turbines. As an alternative to the semiconductor based photovoltaic cell, thermionic energy converters have never achieved significant commercial success.

Photoemission formed the basis for V.K.Zworykin's ionoscope and its successor, the image orthocon, which was the earlier version of the television camera. The image orthocon operated by focusing an image onto a photocathode plate placed parallel to a positively charged metal grid. Electrons emitted from the photocathode impinged on the grid, creating an electrostatic image which was reinforced by photomultiplication at a glass plate immediately behind the grid. An electron beam scanned the glass plate. Variations in the plate charge caused variations in the reflected beam signal. Variations in the return beam translated the image into electronic signal. Modern television cameras are simpler in design and employ semiconductors.

Photoemission also formed the basis for the photomultiplier, a device used beginning in the 1930s' to detect low level electromagnetic radiation. In a photomultiplier tube, light impinging on a photocathode produce electrons, which, in turn, produce secondary electrons through interactions with a series of metal oxide dynodes. Photomultiplier tubes are capable of detecting a candle at a distance of 10 kilometers and of magnifying the signal with very less added 'noise'. They have been used as scintillation counters and as detectors of faint light in the region of the visible spectrum in astronomy. The wavelength sensitivity of a photomultiplier tube is dependent on the substrate from which the cathode is constructed.

Field emission is the principal means of generating electron beams for use in television receivers, CRT terminals, and transmission and scanning electron microscopes. And electron gun based on field emission consists of a fine wire cathode which emits electrons from its tip, and a plane or concave anode with a hole in its center through which the electron beam passes. A focusing electrode between the cathode and anode adjusts the radiated electrons into a narrow diameter beam.

The choice of an emitter for an electron microscope is more critical than for a CRT device. An electron microscope requires maximum brightness to achieve adequate magnification and a precise emitter which emits uniformly in all directions in order for the electronic lens to perform without aberration. The stability of emitters is of importance to electron microscopy to ensure both the longevity of instrument components, and the reproducibility of results. The lifetime of emitters is compromised by the poisoning of emitter centers by foreign substances in an imperfect vacuum and by evaporation of the cathode material itself. Tungsten and tungsten coated with cesium are favored emitter substances; carbon sheathed in tungsten, and lanthanum and calcium boride have used in electron microscopes.

Field emission microscopy and field ion microscopy employ field emission to study the structure of the emitter directly. In field emission microscopy, the anode incorporates a phosphorous screen, which converts variations in the radiation cone into visible patterns.

Because of their small diameter, emitter tips typically consist of a single crystal. The normal orientation of atoms within the crystal lattice leads to small variations in the work function at the surface of the crystal, which translate into light and dark spots in the visible image. Lattice defects and impurities also become visible. The field ion microscope, which is capable of a considerably higher level of resolution, also relies on field emission, but produces an image from a gas ionized near the surface by emitted electrons rather than from the other electrons themselves. The field ion microscope is the only microscope that allows scientists to view individual atoms directly. It is useful for studying spatial relationships at the surface of metallic solids, and it is a valuable tool for designing more effective emitters.

References

1) Introduction to Electron Beam Technology by Bakish, Robert A.
2) Photoemission in Solids by Cardona, Manuel, and Lothar Ley.
3) Electronics Inventions 1745-1976 by Dummler G.W.A.
4) Field Ion Microscopy by Hren, John J. and Srinivasa Ranganathan