Metals thermionic emission

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

The forward current at a semiconductor-metal junction is mainly determined by a majority carrier transfer i.e. electrons for n-type, as illustrated in Fig. 1 d. In this majority carrier device the socalled thermionic emission model is applied according to which all electrons reaching the surface are transferred to the metal. In this case we have ... 

The free-electron gas was first applied to a metal by A. Sommerfeld (1928) and this application is also known as the Sommerfeld model. Although the model does not give results that are in quantitative agreement with experiments, it does predict the qualitative behavior of the electronic contribution to the heat capacity, electrical and thermal conductivity, and thermionic emission. The reason for the success of this model is that the quantum effects due to the antisymmetric character of the electronic wave function are very large and dominate the effects of the Coulombic interactions. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

The most common conventional gas source is an electron impact (El) source. This consists of a metal chamber with a volume of a few cm3, through which the sample flows in the form of a gas. Electrons produced by thermionic emission from a heated tungsten filament are passed through this gas, and accelerated by a relatively low voltage ( 100eV), causing ionization within the sample gas. A plate inside the chamber carries a low positive potential (the repeller ) which ejects the positive ions into a region which contains a series of plates (called lenses) and slits, which serve to focus, collimate, and accelerate the ion beam into the next part of the system... 

The beam of ionizing electrons is produced by thermionic emission from a resis-tively heated metal wire ox filament typically made of rhenium or tungsten. The filament reaches up to 2000 °C during operation. Some reduction of the working temperature without loss of electron emission (1-10 mA mm ) can be achieved by use of thoriated iridium or thoriated rhenium filaments. [22] There is a wide variety of filaments available from different manufactures working almost equally well, e.g., the filament can be a straight wire, a ribbon, or a small coil (Fig. 5.9). 

In practice, unless

high temperatures. Thus, when the work function is increased by an adsorption process, the electronegative film may be partly evaporated before the requisite temperature for thermionic emission has been reached. An exceptional case is the adsorption of O2 on W. However, the thermionic method has proved very useful for stud5ring the electropositive films produced by alkali metals (39). Cs, for example, reduces the work function to such an extent that thermionic measurements may be made at temperatures as low as 150°. 

In the absence of an external field, electrons in the metal are confronted by a semi-infinite potential barrier (upper solid line in Fig. la), so that escape is possible only over the barrier. The process of thermionic emission consists of boiling electrons out of the Fermi sea with kinetic energy > x + M- The presence of a field F volts/cm. at and near the surface modifies the barrier as shown. It follows from elementary electrostatics that the potential V will not be noticed by electrons sufficiently far in the interior of the metal. However, electrons approaching the surface are now confronted by a finite potential barrier, so that tunneling can occur for sufficiently low and thin barriers. 

The most popular thermionic detector (TID) is the nitrogen-phosphorus detector (NPD). The NPD is specific for compounds containing nitrogen or phosphorus. The detector uses a thermionic emission source in the form of a bead or cylinder composed of a ceramic material impregnated with an alkyl-metal. The sample impinges on the electrically heated and now molten potassium and rubidium metal salts of the active element. Samples which contain N or P are ionized and the resulting current measured. In this mode, the detector is usually operated at 600-800°C with hydrogen flows about 10 times less than those used for flame-ionization detection (FID). 

In Chap. XX, Sec. 3, we spoke about the detachment of electrons from atoms, and in Sec. 4 of that chapter we took up the resulting chemical equilibrium, similar to chemical equilibrium in gases. But electrons can be detached not only from atoms but from matter in bulk, and particularly from metals. If the detachment is produced by heat, we have thermionic emission, a process very similar to the vaporization of a solid to form a gas. The equilibrium concerned is very similar to the equilibrium in problems of vapor pressure, and the equilibrium relations can be used, along with a direct calculation of the rate of condensation, to find the rate of thermionic emission. In connection with the equilibrium of a metal and its electron gas, we can find relations between the electrical potentials near two metals in an electron gas and derive information about the so-called Volta difference of potential, or contact potential difference, between the metals. We begin by a kinetic discussion of the collisions of electrons with metallic surfaces. 

Kinetic Determination of Thermionic Emission.—We have now found the pressure of an electron gas in equilibrium with a metal, at an... 

Neglecting the No(dCp/dNy, the Vs are the latent heats. Thus we have the important statement that the contact difference of potential between two metals equals the difference of their latent heats, or approximately of their work functions. This relation is found to be verified experimentally. The contact difference of potential can be found by purely electrostatic experiments, and the work functions by thermionic emission the results obtained in these two quite different types of experiment are in agreement. The small correction terms arising from the No(dCp/dN) s lie almost within the errors of the experiments, so that we hardly need consider them in our statement of the general theorem. 

Definite layers, more than one molecule thick, can however be obtained on metals under certain special conditions. Instances of these are the monatomic layer of caesium on top of a monatomic layer of oxygen, and the similar layers of alternate oxygen and barium atoms, which are of such importance for thermionic emission (Chap. VIII, 4). 

The effect on the dissociation of hydrogen and on the emission of electrons was produced at extremely low pressures of oxygen, about lO 8 mm. of mercury, showing that the combination between the metal and the adsorbed layer of gas is extremely firm that the layer is monatomic was shown, as described previously, by the fact that the thermionic emission and hydrogen dissociation recommenced instantly, when the temperature was raised to a value at which the adsorbed film began to be removed this proved that the least removal of the screen of oxygen adsorbed left a clean surface, which could scarcely occur unless the layer were one atom thick if it were thicker, the removal of the screen and return of the properties of the clean metal surface would return in stages, not suddenly. 

The same energy, then called work function, enters the formula for the thermal emission of metals such as happens in heated filaments (direct heated kathodes) of radio valves (thermionic emission). The thermal energy of the electrons is still very small but it has to be taken into account for this phenomenon, since only those rare electrons with a kinetic energy equal to V0 — Wi can escape. In Table 26 we have used the experimental values of this quantity to calculate Vo, which is very difficult to determine. 

---