M. Drechsler

Temperature distribution along metal tips: (For field emission microscopy and the study of surface phenomena)

Abstract 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.

Évolutions morphologiques des pointes métalliques par traitement thermique (vérification de la théorie de nichols et mullins)

Abstract 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°. For a temperature in the order of 2900 °K (clean surface) the curvature radius at the tip apex increases with time according to the t 1 4 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 ×10 6 cm 2 /sec.

A measurement of the surface energy anisotropy of nickel by transmission electron microscopy of field emitter crystals

Abstract A combination of field emission (FEM) and transmission electron microscopy (TEM) is used to measure the anisotropy of the surface tension (γ) of a metal (nickel). The stationary form of a clean nickel tip is produced and controlled in a field electron microscope, under ultrahigh vacuum. The shape of the tip is visualized in a TEM and then analysed. It is shown that the cap around the apex of a bulbous tip closely approximates the equilibrium shape of a nickel crystal. By using the inverse Wulff construction, the anisotropy of γ can be measured as a function of crystallographic orientation. Values of γ normalized to (111) have been obtained for the 〈200〉 and the 〈220〉 zones. The maximum anisotropy is found at [026] along the 〈200〉 zone ( γ 026 γ 111 = 1.05 ). The results agree with the available experimental or theoretical data reasonably well.

Surface self-diffusion studied by microscopic measurements of crystallite profile evolutions

Dendritic gold crystallites on graphite are heated in ultra high vacuum up to less than 0.5 of the melting point (Tm). Electron microscopy shows that the gold crystallites change their shapes by surface self-diffusion. The dendritic contours round off while the crystallite remains very flat (20 to 40 A). The increase with time of the radii of dendrite tips is measured statistically. Such an evolution can be described by analogy to the blunting of either metal tips (Nichols and Mullins) or monoatomic cleavage tips (Hoche and Bethge). Using this result, a new technique to measure surface self-diffusion coefficients (D) is proposed. Test measurements have shown that this is an interesting, very sensitive method to measure D (down to 10−13 cm−2 s−1) which enables measurements to be made in an unusual low temperature range (0.25 Tm < T < 0.5Tm). In special cases the dendrites are split by the surface self-diffusion which is qualitatively in agreement wih the theory.

Une détermination du coefficient d'auto-diffusion de surface avec des pointes à émission de champ (tungstène)

Abstract To measure the surface self-diffusion coefficient, a method is described which is based on the increase of the curvature radius of a tip (Nichols and Mullins). This radius increase depending on temperature and time is measured under ultra high vacuum condition. The cleanliness of the surface is controlled by field electron microscopy. The radius variation is measured in situ by Fowler-Nordheim characteristics, and the initial and final radius by scanning electron microscopy. Measured are, for tungsten between 2100 and 2850°K, the activation energy ( 3.1 eV atome ) and the diffusivity ( 0.9 cm 2 s ) .

Adsorption, diffusion and self-diffusion on tungsten surfaces with adsorbed palladium

The influence of a Pd adsorption layer on the surface self-diffusion of tungsten has been measured using a field emitter technique. By adsorption of 1.2 × 1015 Pd atoms/cm2 — measured by high energy ion backscattering — the surface self-diffusion energy decreases from 70 to 54 kcal/mole, and the diffusivity from 0.95 to about 0.2 cm2/s, which corresponds to an increase of the diffusion coefficient by a factor 40 at 1600 K and 80 at 1400 K. Furthermore are determined (1) adsorption lifetimes in adsorption-desorption equilibria, (2) the isosteric adsorption heat (103 kcal/mole), and (3) the coefficient of Pd surface diffusion on W resulting in diffusion energies of 32 kcal/mole around (001) and 17 kcal/mole around (111).

Analysis of faces on micro-crystals

It is shown that the appearance and size of the different hkl faces on a micro-crystal can be described as a function of the surface curvature and of crystallographic parameters. The curvature versus angle data of a particle can be determined from the crystal profiles visualized in micrographs (TEM, HREM, SEM). Typical face appearance and face size data are presented. The validity of such data is checked by field ion microscopy. In a first attempt we show the determination of the faces on a particle of a supported catalyst.

Une détermination du coefficient d'autodiffusion de surface en présence d'une couche d'adsorption à l'aide de pointes àémission de champ (nickel sur tungsténe)

The tip blunting technique to measure the surface self-diffusion of clean metals (A. Piquet, Vu Thien Binh, H. Roux, R. Uzan and M. Drechsler) is extended to study the influence of an adsorption layer on diffusion. The system studied is nickel on tungsten. The increase of the apex radius is measured by means of FEM characteristics. In the temperature range used (1200–1500 K), the nickel monolayer (1.16 × 1015 atoms/cm2) is maintained by compensation of desorbed Ni atoms with a continual flux from an evaporation source. The adsorption life time between 1350 and 1500 K decreases from 850 to 16 s. The conservation of the degree of coverage leads to a method to determine the desorption activation energy of nickel (Ed = 4.56 eV/atom). The surface self-diffusion data of tungsten with a nickel monolayer are found to be D0 = 3 × 10−3cm/2s and Qs = 1.9 eV/atom, compared to the clean tungsten data D0 = 1 cm2/s and Qs = 3.1 eV/atom. The Ni monolayer increases the surface self-diffusion coefficient by a factor 160 at 1200 K and 20 at 1500 K. The results are discussed with respect to nickel activated sintering of tungsten powders.

Processus simultane d'évaporation libre et de diffusion de surface☆

Abstract If a conical metal tip is annealed in vacuum, the radius at the apex increases with time by surface diffusion. This was calculated by Nichols and Mullins and recently verified by the authors. This phenomenon is now calculated in case of a simultaneous action of evaporation and surface diffusion. By evaporation the blunting rate should be lowered until it becomes zero for a critical value of the curvature radius. Numerical data for different temperatures and cone angles are calculated for some metals (W, Mo, Pt, Ni, Cu). Measured are profile changes of Mo tips during annealing. The lowering of the blunting rate and the existence of a limit radius are confirmed. An evaporation can also occur as a consequence of a surface reaction. The measured radius changes of tungsten tips annealled in 1.5 × 10 −5 torr and 2.5 × 10 −5 torr of oxygen agree fairly well with the predictions. Surface self-diffusion measurements at high temperatures may be erroneous if evaporation is not considered, an example for Mo at 2150°K is given.

Sharpening of metal tips by heat treatment in vacuum

It is well known that a metal tip blunts if it is heated in vacuum. Sharpening may occur in the case of an evaporation of the tip material. To test this, molybdenum tips were heated in vacuum. A typical result: at 2400K, the tip radius decreases from 8 mu m to 0.3 mu m thus confirming the hypothesis. Measured and calculated final radii agree roughly. The pure material evaporation can be replaced by a surface reaction followed by the evaporation of the reaction products. This is shown on tungsten tips heated (1750K) in the presence of oxygen (0.5 mTorr). The evaporation of tungsten oxides results in a sharpening to a radius to 0.05 mu m. Under special conditions, in connection with the formation of solid drops, radii down to 0.01 mu m are obtained.