M. Drechsler’s research while affiliated with French National Centre for Scientific Research and other places

If a conical metal tip is heated in ultra-high-vacuum, the curvature radius at the apex increases continuously. A measurement of this phenomenon permits to determine the surface self-diffusion coefficient using Herring's equation and Nichols and Mullins numerical data. The increase of the curvature radius at the apex is determined in situ by measuring the field electron current as a function of voltage and the final radius and profile are determined by scanning electron microscopy. The logarithm of the surface-self-diffusion coefficient of tungsten without adsorbed layer versus the reciprocal temperatures, varies linearly in the experimental region (D=3.2.10-8 cm2/sec at 2100 K and D=3.6.10-5 cm2/sec at 2900 K). As activation energy a value of 3.08 eV/atom is found.

A field emission technique is described to measure the influence of a definite adsorption layer on surface self-diffusion. The rate determining activation energy for this diffusion is determined in a field electron microscope by measuring time and temperature of the transformation of a build-up form into an annealed form of an emitter crystal.If a clean tungsten surface is covered with one tenth or less of a monolayer of carbon, the surface self-diffusion remains unchanged. However, coverage of one half to one monolayer of carbon leads to an increase of the surface self-diffusion energy from 3.1 to 8.7 eV. If oxygen is adsorbed on clean tungsten an increase of the surface self-diffusion energy from 3.1 to 4.2 eV is found. These results imply some earlier self-diffusion data on other metals may be influenced by undetected adsorption layers.

To determine the temperature of a heated crystal in a field electron or ion microscope, not only the temperature of the heated loop, but also the temperature gradient along the emitter tip due to heat conduction and radiation must be considered. A method is described to calculate the temperature distribution along the tip. Typical numerical results are given for tips of tungsten and some other metals at temperatures between 1000 and 2900 °K. Considered are conical tips of different length (half cone angle between 0.5° and 6°) and some non-conical tips. The temperature gradient near the apex (which influences the shape change by surface diffusion) has a maximum for a particular angle of the conical tip. Very high gradients occur at tip constrictions.

The morphological evolution of conical tungsten tips of cone angles between 1 and 15° at temperatures between 1800° and 3000°K is studied inside a scanning electron microscope. The experimental results confirm the theory of Nichols and Mullins: existence of a critical cone angle α = 3°, formation of solid drops for α < 3°, formation of characteristic steady-state shapes for α > 3°. For a temperature in the order of 2900 °K (clean surface) the curvature radius at the tip apex increases with time according to the law. For temperatures below 2700°K discrepancies are found, which are due to an evaporation of tungsten compounds produced at the surface by a chemical reaction with the residual gas (10−5 torr). The measurements at 2900°K have enabled the determination of the surface diffusion coefficient of tungsten: D = 2.4 ×106cm2/sec.

An ultra high vacuum field emission shadow microscope is described, which shall be used to study shape changes of solids in situ. The principle of focusing and the variation of magnification are studied as a function of the potentials and the distances between emitter, object and screen. For good adjustement a relation between potentials and distances is given. Possibilities and limits of this type of microscope given by the magnification, the resolution and the image distorsion are discussed. Un microscope à ombre à effet de champ travaillant en ultra-vide est décrit ; il est construit pour l'étude des déformations morphologiques de pointes métalliques in situ. Le principe de mise au point et la variation du grossissement ont été étudiés en fonction des potentiels et des distances entre projecteur, objet et écran. On établit une relation entre les potentiels et les distances pour une bonne mise au point. On envisage les possibilités et les limites de ce microscope données par le grossissement, la résolution et la distorsion de l'image.

To determine the temperature of a heated crystal in a field electron or ion microscope, not only the temperature of the heated loop, but also the temperature gradient along the emitter tip due to heat conduction and radiation must be considered. A method is described to calculate the temperature distribution along the tip. Typical numerical results are given for tips of tungsten and some other metals at temperatures between 1000 and 2900 °K. Considered are conical tips of different length (half cone angle between 0.5° and 6°) and some non-conical tips. The temperature gradient near the apex (which influences the shape change by surface diffusion) has a maximum for a particular angle of the conical tip. Very high gradients occur at tip constrictions.

An apparatus is described to transform the end of a wire (tungsten in the vacuum into a fine tip by electron bombardment. Curvature radii at the tip apex down to 103 × are obtained. The tips are controlled by (I) an optical microscope (2) an electron microscope and (3) a field electron microscope. The interpretation of the process of tip sharpening is based on evaporation, surface diffusion and surface energy in the presence of a temperature gradient caused by the electron bombardment. It seems possible to use the method for the production and the regeneration of point cathodes in electron microscopes as well as a field emitter in field emission microscopes.

Résumé. 2014 On décrit des applications des pointes métalliques fines pour lesquelles le profil, en particulier la valeur de l'angle du cône doit être important. Un meilleur contrôle du profil est obtenu en utilisant un microscope électronique à balayage. On décrit une méthode électrolytique d'obten-tion de pointes coniques de différents angles en agissant sur la durée des impulsions de tension, l'angle de plongée et la viscosité du bain. Abstract. 2014 Applications of fine metal points are mentionned of which the profile, in particular the cone angle, should be important. The point profile is better controled, if a scanning electron microscope is used. An electrolytic method of obtaining conical points of different angles is described, whereby the duration of tension pulses, the dipping angle and the bath viscosity is varied.