GB1028351A - Field-emission cathode - Google Patents

Abstract

<PICT:1028351/C6-C7/1> <PICT:1028351/C6-C7/2> In the production of a field-emission cathode, one end of a thin rod 3 cut from a zone-melted bar of crystalline lanthanum hexaboride is supported from a rhenium loop 4 and is treated in an electrolytic bath to obtain a fine polished needle-like point by immersing the end of the rod as anode into a capillary tube having an annular platinum cap cathode 11 and upwardly through which is fed sulphuric acid. Twenty pulses of square wave D.C. potential are applied directly, the current passed changing progressively from the form (b) to (c) to (d) as shown in Fig.6, E indicating an etching or shaping action and P a polishing effect, and then three pulses are applied successively from each of three condensers C3, C2, C1 of 8, 4, 2 m F to effect final polishing. By selecting the acid flow rate a waist may be produced adjacent the cathode and by withdrawing the rod a desired shape may be formed. For a needle point of small cone angle a waist is formed near the end of a thin needle until the weight of the depending portion causes fracture and the broken end is then polished. A bar with more than one point may be formed by first cutting the end longitudinally e.g. with two orthogonal cuts. An initial cleaning of the bar may be effected by passing a heating current through the loop 4.ALSO:In the production of a field-emission cathode, a bar cut from a sintered disc of lanthanum hexaboride is zone-melted. A thin rod is then cut from the bar and one end treated in an electrolytic etching bath to form a fine needle-like point. A plurality of points (Fig. 8, not shown) may be obtained by first cutting the end into a plurality of parts (Fig. 7, not shown). The rod diameter may be 0.1 mm. and the tip diameter may be 1 micron.ALSO:In the production of a field-emission cathode, a bar cut from a sintered disc of lanthanum hexaboride is zone-melted.