M K ! I . i DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH ' . RADIO RESEARCH i Special Report No. 11 i , ; THERMIONIC EMISSION ! A SURVEY OF EXISTING i • KNOWLEDGE WITH PARTICULAR i REFERENCE TO THE FILAMENTS OF RADIO VALVES. I .>■ PRICE2s.6d.NET "V ! f V : . /• * . SV p .V" r. i :v : .?•; ’i ■ t: ■ ■ ‘ -v • . .* DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH RADIO RESEARCH Special Report No. 11 THERMIONIC EMISSION A SURVEY OF EXISTING KNOWLEDGE WITH PARTICULAR REFERENCE TO THE FILAMENTS OF RADIO VALVES BY W. S. STILES, Ph.D. Crown Copyright Reserved LONDON: PUBLISHED BY HIS MAJESTY’S STATIONERY OFFICE 1932 47—29—11 ii RADIO RESEARCH BOARD Lieut.-Col. A. G. Lee, O.B.E., M.C., M.I.E.E. (Chairman). Colonel A. S. Angwin, D.S.O., M.C. {representing the Post Office). Professor E. V. Appleton, D.Sc., F.R.S. N. Ashbridge, Esq. Captain J. W. S. Dorling, R.N. (representing the Admiralty). Professor C. L. Fortescue, O.B.E. Colonel A. C. Fuller, O.B.E. (representing the War Office). Sir Joseph E. Petavel, K.B.E., D.Sc., F.R.S. G. C. Simpson, Esq., C.B., C.B.E., D.Sc., LL.D., F.R.S. H. E. Wimperis, Esq., C.B.E., F.R.Ae.S., M.I.E.E. (representing the Air Ministry). COMMITTEE ON THERMIONIC VALVES E. H. Rayner, Esq., Sc.D. (Chairman). Professor E. V. Appleton, D.Sc., F.R.S. S. Brydon, Esq., D.Sc. Professor C. L. Fortescue, O.B.E. N. Hecht, Esq. Professor F. Horton, D.Sc., F.R.S. H. G. Hughes, Esq., M.Sc. G. W. C. Kaye, Esq., O.B.E., D.Sc. R. A. Watson Watt, Esq. Professor R. Whiddington, D.Sc., F.R.S. V, • . iii PREFATORY NOTE rTTHE subject of thermionics has developed in a few years from A a method of studying the electrical properties of matter to being the basis of one of the world's widest commercial applications of physical science. The phenomena involved are fundamental, being those on which are based theories of the constitution of matter as we at present perceive it. The literature on the subject is widespread, and much of it inevitably abstruse. It was decided by the Radio Research Board on the advice of the Thermionics Committee, that for the furtherance of knowledge, for facilitating research and technical developments, a critical survey of the literature of the subject would be of very real value. The compilation of this survey was entrusted to W. S. Stiles, Ph.D., of the National Physical Laboratory. An endeavour has been made to include the most important papers up to December, 1930. It will be observed that in the Bibliography which accom­ panies the Survey a Decimal System of Notation has been employed, and that gaps in the numbering have been left to enable the reader to insert later references approximately in their proper position. E. H. RAYNER, Chairman, Thermionic Committee, Radio Research Board. Department of Scientific and Industrial Research, 16 Old Queen Street, London, S.W.l. January, 1932. iv CONTENTS PAGE Section 0. General Outline 1 Section 1. The Theory of the Temperature Emission of Electrons .. 12 Section 2. Variation with Temperature of Specific Electron Emission in Vacuo and Values of the Richardson Constants .. 44 Section 3. Heat Effects in Thermionic Emission 59 Section 4. The Distribution of Velocities of Thermionic Electrons .. 65 Section 5. Effect of Applied Electric Field at the surface of the Emitter (Schottky Effect) 70 Section 6. The Photo-thermionic Effect 78 Section 7. Thoriated Filaments and other Thin Film Emitters 81 Section 8. Oxide-coated Filaments 97 Bibliography 107 Author Index 114 SECTION 0 GENERAL OUTLINE PHENOMENA of a thermionic character had been observed 1 many years before the nature of the effects was realised. The systematic study of the subject may be said to commence with Richardson’s paper On the Negative Radiation from Hot Platinum (0.099) in 1901, and his comprehensive memoir, The Electrical Con­ ductivity imparted to a Vacuum by Hot Conductors (0.100) in 1903. The attitude is here definitely adopted that the negative leak from a hot body to an auxiliary electrode in vacuo represents the simplest case of conductivity produced by matter at high tem­ peratures. It is assumed that electrons, normally retained in the substance by a potential discontinuity at the surface, are able, with rise of temperature and consequent increase in the velocities of thermal agitation, to escape through the surface and constitute a current between the hot body and the auxiliary electrode. If a Maxwellian distribution of velocities of the electrons within the metal be assumed, and if there is a potential discontinuity O at the surface, then the number of electrons which pass out from unit area of the surface per second is given by kT N 2^exp{-®e/AT}*, where k = gas constant for a single electron. T = absolute temperature. e = charge of an electron. m = mass of an electron. N = number of electrons per unit volume in the metal. If all the electrons which are emitted are driven across to an auxiliary electrode, the anode, by means of a sufficiently intense electric field, the saturation current between the anode and the emitter so obtained will amount to SN exp { — O e/kT}, where S is the area of the emitting surface. The experimental test and verification of the formula relating saturation current and temperature was carried out by Richardson for platinum, carbon and sodium. He found that the conductivity * To avoid confusion between the base of natural logarithms and the electronic charge, ex is written exp (x). 2 THERMIONIC EMISSION in vacuo between an electrically-heated filament of Pt or C and a cold auxiliary electrode was, in fact, essentially unipolar, a current being obtained only when the applied potential was such as to drive electrons away from the emitter. For sodium, secondary effects gave rise to a conductivity in the opposite direction, but this never exceeded a twentieth of the " negative ” conductivity. The values of the constants in the formula Saturation current per unit area ^ = AT* exp (— b/T) in amps./cm.2 derived as the final results, are as follows :— A b Pt .. 1-6 x 107 4-93 X 104 C .. 1-6 x 1015 7-8 X 104 Na .. 1-6 X 1012 3-16 X 104 Richardson points out in the same paper that the product of b and the cube root of the atomic volume is nearly a constant for these three elements. The hypothesis that the negative thermionic current is to be attributed to electrons ejected from within the substance of the emitter was almost immediately called in question by Wilson, On the Discharge of Electricity from Hot Platinum (0.110), who in an investigation of the effects of gases on the negative leak from platinum wires found that treatment of the Pt wire with nitric acid very considerably reduced the saturation current, the admission of a little hydrogen, however, bringing the current back to its former value. Although the introduction of nitrogen and water vapour gave the same current as obtained in a vacuum, provided ionisation by collision did not occur, for hydrogen a considerably larger current was obtained. Wilson drew the conclusion that the thermionic emission of platinum in vacuo is due to traces of hydrogen occluded on the surface of the platinum. Wilson further showed that a thermo-dynamic proof of the Richardson formula i = AT* exp (— b/T) could be given, based on the analogy between the emission of negative ions by the hot body and the evaporation of molecules from a fluid. This proof did not require the electrons to come from within the metal, as assumed by Richardson, and hence if the thermionic current were due to some surface action producing electrons, the formula relating measured saturation current and temperature would still have the form of the Richardson result. While the emission from metals occupied the attention of English physicists, a number of investigations were being carried out by Wehnelt, On the Discharge of Negative Ions by Glowing Metallic Oxides and AUied Phenomena (0.119), on the discharge of negative ions by glowing metallic oxides. The results obtained by him are summarised in a paper of 1905 (0.120). Metallic oxides were found to give a negative emission similar to that from metals, which I GENERAL OUTLINE 3 varied with temperature in the way required by Richardson's formula. Wehnelt did not give actual values of the emission constants, but for BaO and CaO Richardson calculated them from Wehnelt’s results :— A b BaO 1 • 15 x 108 4-49 X 104 CaO 0-72 x 108 4-28 X 104 A fairly extensive investigation of the current from a Nemst filament, including the variation with temperature of the negative leak in vacuum, was carried out by Owen, On the Discharge of Electricity from a Nemst Filament (0.121), in 1904, and again, Richardson calculated the emission constants which were not given explicitly by Owen :— Nemst filament. A = 104; b = 4*62 X 104. The paper by Richardson, The Emission of Negative Electricity by Hot Bodies (0.130), in 1904, summarises the state of the subject at the time and includes a table of emission constants (Richardson, Wilson, Owen, Wehnelt), with a critical discussion of these. At the time of this review it was still questionable whether or not the emission from oxide-coated metals consisted merely of a secondary effect due to a lowering of the exit work for the metal by the oxide layer.