A. Scharmann

Steady and oscillatory thermocapillary convection in liquid columns with free cylindrical surface

In liquid columns (Prandtl number 8·9) with free cylindrical surface heated from above, strong thermocapillary convection (TC) has been observed. Stationary thermocapillary convection exists in the form of a single axially symmetric roll bound to the free surface. For aspect ratios l/a The influence of buoyant forces due to horizontal temperature gradients in the experiments was also studied. Buoyant forces become obvious for a contaminated free surface and in bulk regions far from the cylinder surface. The thermocapillary convection shows a transition to time-dependent oscillatory motion when a critical Marangoni number Ma c is exceeded. A unique Ma c = 7 × 10 3 has been found for zones with lengths l Ma c ) of the distortion has been found. The non-dimensional frequency of the temperature oscillations is independent of zone size if the aspect ratio is held constant. However, the non-dimensional frequency of temperature oscillations increases linearly with the aspect ratio of the zone. This result is interpreted as a dependence of the phase velocity of the running disturbance on the aspect ratio.

Experiments on surface tension driven flow in floating zone melting

Abstract Surface tension driven flow in a NaNO 3 floating zone has been observed. It is comparable to natural convection and probably also important in many crystal growth techniques with a free melt surface. In experiments, related to floating zones, temperature oscillations have been found to be induced by oscillations of the surface tension driven flow which might be instable at higher driving forces. The frequency of these oscillations is proportional to the average temperature gradient imposed upon the free surface. This dependence might be used to identify oscillations of this kind. Surface tension driven flow induced by the concentration gradient of a surface-active impurity has been observed to be not negligible. A further mechanism for growth instability is discussed, which is based on surface tension gradients due to the concentration gradient of a surface-active impurity in front of a growing crystal.

Some evidence for the existence and magnitude of a critical marangoni number for the onset of oscillatory flow in crystal growth melts

Abstract Experiments on floating half zones of NaNO3 heated from above show the onset of oscillatory Marangoni convection at a critical Marangoni number Ma(T)c ∼ 104. The dependence of Ma(T)c on zone length is investigated and discussed. Literature on the onset of temperature oscillations is reviewed. It gives Ma(T)c of the same order of magnitude for other melts. Effects of oscillatory Marangoni convection in crystal growth from melt are discussed. A description of the state of oscillatory Marangoni convection in this geometry is given.

Tungsten and fluorine co-doping of VO2 films

Abstract Fluorine- and tungsten-doped vanadium dioxide (VO2) is a promising coating material for applications as energy-conserving windows. We prepared VO2 films simultaneously co-doped with fluorine and tungsten and report on the results of optical measurements and photoelectron spectrometry. A comparison is given to single-element doping. Clear evidence for an interaction of fluorine and tungsten in VO2 is found in the switching behaviour at the semiconductor-to-metal phase transition. An explanation is given based on two different effects of fluorine incorporation observed in the ultraviolet photoelectron spectrometry results. Concerning other film properties, the two elements act independently of each other.

Dependence of the electrical and optical behaviour of ITO–silver–ITO multilayers on the silver properties

Abstract ITO–metal–ITO (IMI) multilayers and ITO single layers were prepared by DC-magnetron sputtering with different oxygen concentrations of the sputtering atmosphere during ITO deposition, and different substrate temperatures during a subsequent annealing treatment in vacuum. The intermediate metal layer consisted of 10 or 30 nm silver. Electron concentration and mobility of the Ag partial layer were determined from measurements of the electrical resistivity and by evaluating the complex dielectric function, obtained from optical transmission and reflectance. Microstructure and purity were studied with X-ray diffraction (XRD) and with X-ray photo electron spectroscopy (XPS) depth profiles. With a 10-nm intermediate silver layer an IMI sheet resistance of 4.7 Ω/□. was achieved. The Ag films had almost bulk electron concentrations but reduced electron mobilities. The ITO crystallinity in IMI systems influenced the microstructure and purity of the Ag layer. Minimum Ag resistivity and optimum Ag purity were obtained by ITO deposition under stoichiometric conditions and annealing in vacuum at 300°C. An increase of the ITO lattice constant is indicative that as-deposited IMI multilayers prepared with ITO deposition at high oxygen concentration or under stoichiometric conditions are associated with stress. The presence of the Ag layer impedes the lattice relaxation upon annealing.

W- and F-doped VO2 films studied by photoelectron spectrometry

Thermochromic tungsten- and fluorine-doped vanadium dioxide films, which are in discussion as intelligent window coatings, were deposited by reactive sputtering. Results of optical measurements and photoelectron spectrometry (XPS, UPS) at low doping levels (≤2.6%) are presented, and together with structural properties they can be well correlated. By applying antireflective coatings the transmittance of films in the visible spectral range may be enhanced to more than 60% with fairly good switching characteristics at room temperature in the case of tungsten doping.

Marangoni convection in open boat and crucible

Abstract Thermocapillary or Marangoni Convection (MC) has been experimentally studied in open boat and Czochralski growth model experiments with an oxide melt. The streamlines and the flow velocities of MC have been visualized and measured. MC has been identified to be a significant flow phenomenon for heat and mass transport in melt growth of oxide crystals from medium size noble metal crucibles. In this case MC will dominate the fluid flow in a layer of 10 mm thickness near the free melt surface. The experiments are discussed with application to melt growth. It is pointed out that the surface phenomenon MC is sensitive to numerous kinds of surface perturbations thus resulting in unsteady MC. Another reason for time dependent MC might be the high flow velocity in meniscus regions which are thermal boundary regions as well. Time dependent MC will generate time dependence of the buoyant flow in the whole melt-volume due to intimate coupling of both phenomena.

Ion induced electron emission from metal surfaces

Abstract The total electron emission yield γ of sputter-cleaned Al, Cu, Ag, W, and Au surfaces has been measured under bombardment with H + , H 2 + , H 3 + , He + , Ne + , and Ar + ions in the energy range from 80 keV to 1 MeV. The yield γ is found to be proportional to the electronic stopping power in the case of proton impact. Deviations from a strict proportionality are reported for the heavier ions. The yields induced by the molecular ions H 2 + and H 3 + are compared with the proton yields. It is found that the additional electron(s) of the molecular ions directly contribute to the secondary electron yield at the highest velocities of this experiment.

Luminescent sites in CaWO4 and CaWO4 : Pb crystals

Abstract The transmission and emission spectra of CaWO 4 and CaWO 4 /(1 × 10 -4 ) Pb crystals are measured. In Pb-doped CaWO 4 an additional relatively sharp absorption and broad emission band are observed. The absorption band is attributed to electronic transitions within the Pb 2+ -ion, the emission band to transitions in WO 2- 4 -complexes stabilized by neighbouring Pb 2+ -ions. Quenching of CaWO 4 crystals produces new emission bands which are ascribed to higher tungstate complexes. The optical transitions are discussed in a MO-diagram of tungstate complexes.

Verification of the oscillatory state of thermocapillary convection in a floating zone under low gravity

Abstract The oscillatory thermocapillary convection in a floating zone has been investigated in a sounding rocket experiment (TEXUS IIIb on 30 April 1981). The thermocapillary convection became oscillatory under 10−4 g for both realized supercritical Marangoni numbers (Ma = 1.6 × 104 and Ma = 3.6 × 104). The frequency spectrum of the temperature oscillations under low gravity is very similar to that under earth gravity. Thermocapillary convection needs no interaction with buoyancy forces to become unstable.