Publications Scientifiques
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Item Work function measurements of contact materials for industrial use(IOP, 1998) Akbi, Mohamed; Lefort, AndréThe nature of the contact material is important for the characteristics of electric arcs and particularly for the electronic emission. Work functions of new industrial materials made with silver alloys and silver oxide alloys are not known at present. An experimental set-up is described which allows work function measurements from room temperature up to 700 K. The Fowler method was used for the measurement of the work function by the photoelectric effect. As a first application of the experimental device, work functions of metals (Ag, Cu, Ni, Sn and Zn) were determined. Furthermore, the influence of industrial surface treatment such as the application of electric arcs and repeated mechanical shocks on the Ag contact work surface was studied by SEM to observe their effect on electron work function. Breaking arcs in air cause a remarkable increase in the work function of the silver contact material by progressive formation of silver oxidesItem Quantitative analysis of deuterium in a-C:D layers, a Round Robin experiment(2000) Behrisch, R.; Mayer, M.; Jacob, W.; Assmann, W.; Dollinger, G.; Bergmaier, A.; Kreissig, U.; Friedrich, M.; Sun, G.Y.; Hildebrandt, D.; Akbi, Mohamed; Schneider, W.; Schleubner, D.; Knapp, W.; Edelmann, C.The absolute amount of deuterium in amorphous deuterated carbon (a-C:D) layers has been measured by six laboratories with di erent techniques, such as MeV ion beam analysis, secondary ion mass spectrometry (SIMS), and thermal desorption spectrometry (TDS). The a-C:D layers have been deposited from a CD 4 glow discharge plasma onto carbon and silicon substrates. The results for the absolute numbers obtained with the di erent analysing techniques show a scatter of up to about 35% around the average value. These deviations are larger than the errors stated by the experimentalists and indicate possible systematic uncertainties in some of the measurementsItem A method for measuring the photoelectric work function of contact materials versus temperature(IEEE, 2014) Akbi, MohamedThe nature of the contact material plays a key role to determine the characteristics of electrical arcs and particularly those related to electronic emission. Mechanical and thermodynamic properties as well as electronic emission of such complicated alloys present a lack of reliable and accurate experimental data. The purpose of this paper is to present the development of a method for measuring photoelectric work functions of contact materials. Also reported in this paper are the results of experimental work whose purpose has been the buildup of a reliable photoelectric system and associated monochromatic ultraviolet radiations source, and the photoelectric measurement of the EWF of contact materials. As a first test of the experimental Ultra High Vacuum setup, the electron work functions (EWF) of silver contacts, namely pure polycrystalline metals that are actually used in relays, were measured photoelectrically, using both Fowler’s method of isothermal curves and linearized Fowler plots. Ultrahigh vacuum techniques were employed to obtain residual gas pressure of about 5×10−9 mbar that allows accurate and reliable photoelectric work function measurements. The EWF measured at room temperature of polycrystalline Ag contact (as commercially available) increased from 3.70 eV before heating, to 4.30 eV, and finally, stabilized at the vicinity of 4.26 eV after several vacuum heat treatments at 533 and 773 K. The EWF value obtained, i.e., 4.26 ± 0.03 eV, is in good agreement with the results found in the literature. This shows that the experimental method is valid and the experimental setup is usable. Furthermore, new photoelectric measurements versus temperature have shown a linear decrease of the EWF of silver contacts with increasing temperature, i.e., the temperature coefficient α = dφ/dt is constant and negative α = −4.58 · 10−4 eV/K in the experiment temperature range 300–780 KItem Experimental determination of work function of silver alloys(1994) Lefort, Andre; Akbi, Mohamed; Parizet, Marie-JoseModels about arc roots need a good knowledge of physical constants characterizing contact material. With pure metal, all the constants are well known; but for alloys some are not known. In this paper we relate how we have measured work function of silver alloys. Measuring method used is the Fowler one where photoelectronic current produced by different ultraviolet radiations is measured in high vacuum. Experimental set-up have been tested for pure materials and then, have been applied to some silver alloys. Results about pure metals show an evolution of the work function with temperature; it decreases when temperature increases. Results about silver-nickel alloys show a large dependence of obtained results with the preparation of contact surface. For example, polishing and cleaning induce, because of atom migration, a large modification of the ratio of the two components on surface; this phenomenon induces a variation of work function comparatively with obtained values with prLtary product. By heating alloy contact in high vacuum we have observed large variations of work function, which result from material component vaporisation by sheets. Surface analyses by electron microscope produce proves of the phenomenon. An empirical relation is given to calculate work function of binary alloy when this constant is known for each alloy componentItem Effects of vacuum heat treatment on the photoelectric work functionand surface morphology of multilayered silver–metal electrical contacts(Elsevier, 2014) Akbi, Mohamed; Bouchou, Aissa; Zouache, NoureddineContact materials used for electrical breakers are often made with silver alloys. Mechanical and thermodynamical properties as well as electron emission of such complicated alloys present a lack of reliable and accurate experimental data. This paper deals mainly with electron work function (EWF) measurements about silver–metal (Ag–Me) electrical contacts (Ag–Ni (60/40) and Ag–W (50/50)), before and after surface heat treatments at 513 K–873 K, under UHV conditions (residual gas pressure of 1.4 × 10−7 mbar). The electron work function (EWF) of silver alloyed contacts was measured photoelectrically, using both Fowler's method of isothermal curves and linearized Fowler plots. An interesting fact brought to light by this investigation is that after vacuum heat treatments, the diffusion and/or evaporation phenomena, affecting the atomic composition of the alloy surface, somehow confine the EWF of the silver–nickel alloy, Φ(Ag–Ni), determined at room temperature in interval]Φ(Ag), Φ(Ni) [=] 4.26 eV, 4.51 eV[. Surface analysis of two specimens before and after heating showed a significant increase of tungsten atomic proportion on the contact surface for Ag–W contacts after VH treatments. A multilayer model, taking into account the strong intergranular and volume segregation gives a good interpretation of the obtained resultsItem Effects of surface treatments on photoelectric work function of silver-nickel alloys(2014) Akbi, Mohamed; Bouchou, Aissa; Ferhat-Taleb, MahdjoubTheoretical models of arc roots need a good knowledge of physical constants characterizing contact material. With pure metals, all the constants are well known, whereas for the new industrial materials made with silver alloys some of these parameters are still not known. The purpose of this paper is to get a better understanding of emission of electrons occurring in an alloyed cathode submitted to several vacuum outgassing cycles at room temperature and residual gas pressure of 1.4 × 10−7 mbar. The electron work function (EWF) of silver alloyed contacts, Ag–Ni (70/30), was measured photoelectrically, using Fowler's method of isothermal curves. Experimental results about silver–nickel alloys show a large dependence of obtained results with the preparation of contact surface. The EWF of the contact pastille made with silver alloys Ag–Ni varies with surface cleaning by vacuum outgassing cycles. For an unpolished contact, the EWF varies between 4.34 eV and 4.51 eV (the EWF of Nickel), after 7 cleaning cycles and cleaning time of 9 days. On the other hand, for a polished contact, the EWF varies between the EWF of the two components, namely from 4.26 eV for Ag to 4.51 eV for Ni, at room temperature, after 22 cleaning cycles and a cleaning time of 16 days. The error in determining EWF was ±0.03 eV. A multilayer model, taking into account the strong intergranular and volume segregation gives a good interpretation of the obtained results. In addition, a change of order of 0.1 eV was observed for silver alloys Ag–Ni (60/40) EWF after polishing. Afterwards, the microstructure of the contact surface was analyzed with scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDS). The analyzes of the cathode surfaces before and after polishing enabled us to have evidence about the decrease of the electron work function for polished samplesItem Effects of arcing in air on the microstructure, morphology and photoelectric work function of ag- Ni (60/40) Contact materials(World academy of science, engineering and technology, 2015) Akbi, MohamedThe present work aims to throw light on the effects of arcing in air on the surface state of contact pastilles made of silvernickel Ag-Ni (60/40). Also, the photoelectric emission from these electrical contacts has been investigated in the spectral range of 196- 256 nm. In order to study the effects of arcing on the EWF, the metallic samples were subjected to electrical arcs in air, at atmospheric pressure and room temperature, after that, they have been introduced into the vacuum chamber of an experimental UHV set-up for EWF measurements. Both Fowler method of isothermal curves and linearized Fowler plots were used for the measurement of the EWF by the photoelectric effect. It has been found that the EWF varies with the number of applied arcs. Thus, after 500 arcs in air, the observed EWF increasing is probably due to progressive inclusion of oxide on alloy surface. Microscopic examination is necessary to get better understandings on EWF of silver alloys, for both virgin and arced electrical contactsItem Effect of arcing in air on the photoelectric work function of silver-based contacts(IEEE Transactions, 2015) Akbi, MohamedIn this paper, we report new photoelectric measurements that throw light on the effect of arcing in air on the electron work function (EWF) of contact pastilles made of pure silver [Ag (99.999%)], silver-nickel alloy [Ag-Ni (60/40)], silver-tungsten alloy [Ag-W (50/50)], and silver-tungsten carbide [Ag-WC (50/50)]. The influence of industrial treatment (polishing, mechanical shocks, and electrical arcs in air) of pure metals and silver-metal alloys on their EWF is experimentally investigated. To study the effects of arcing on the EWF, the metallic samples were subjected to electrical arcs in air, at atmospheric pressure, and room temperature, after that, they have been introduced into the vacuum chamber of an experimental ultrahigh vacuum setup for EWF measurements. Fowler's method of isothermal curves was used for the measurement of the EWF by the photoelectric effect. Contacts were mounted in a contactor working repetitively in air (laboratory atmosphere). A cycle of arcing consisted in opening contacts on charging, with production of an electric arc with adjustable duration, and closing a few seconds after arc extinction at zero voltage. When subjected to 500 arcs, the EWF for Ag contacts is $4.50 ± 0.03$ eV at room temperature, while for virgin Ag contacts, it was 4.30 ± 0.03 eV. The increase in the EWF is due to the progressive inclusion of silver oxide in the Ag contact surface during arcing in air. The conditioned (500 arcs) silver--metal alloys studied in this paper exhibit the same electronic emission behavior, namely, arcing in air increases their EWF. We demonstrated that the electromechanical conditioning by successive electrical arcs affects the photo-EWFItem Deuterium trapping in divertor tiles of ASDEX-Upgrade(Elsevier, 1999) Hildebrandt, D.; Akbi, Mohamed; Juttner, B.; Schneider, W.The deuterium inventory of divertor target tiles used in ASDEX-Upgrade for up to 2000 discharges has been analyzed by thermodesorption spectrometry. In addition, surface analysis techniques as auger electron spectrometry, secondary ion mass spectrometry, nuclear reaction analysis, electron microscopy and optical surface pro®lometry have been applied for investigating erosion and deposition phenomena. The original plasma facing surfaces were graphite (EK98) and plasma sprayed tungsten, respectively. The total deuterium inventories have been found to vary between 4 ´ 1021 D-atoms/m2 and 3 ´ 1023 D-atoms/m2. The deuterium is contained in a deposit on the surface of the graphite and tungsten tiles consisting mainly of carbon, boron and the hydrogen isotopes. There is strong indication that morphological e ects in¯uence impurity deposition, deuterium trapping and re-erosion of the contaminationItem On the temperature dependence of the photoelectric work function of contact materials(2014) Akbi, MohamedThe nature of the contact material plays a key role to determine the characteristics of the electric arcs and particularly those of the electronic emission. Mechanical and thermodynamic properties as well as electron emission of such complicated alloys present a lack of reliable and accurate experimental data. The purpose of this paper is to present the development of a method for measuring photoelectric work function of pure contact materials that are actually used in relays. Also reported in this manuscript are the results of experimental work whose purpose has been the buildup of a reliable photoelectric system and associated monochromatic ultra-violet radiations source, and the photoelectric measurement of the electron work functions (EWF) of contact materials. In order to study the influence of temperature on the EWF, a vacuum furnace was used for heating the metallic samples up to 700 K. The Fowler’s method of isothermal curves was used for the photoelectric measurement of the EWF. As a first test of the experimental UHV set-up, the EWF of the silver contact materials, namely pure polycrystalline metals (Ag, Cu, Ni, Sn and Zn) were investigated to study the influence of surface cathode temperature on the EWF. In the present study, the photoelectric measurements about silver contacts have shown a linear decrease of the EWF with increasing temperature, i.e. the temperature coefficient dt d is constant and negative: 4.58104eV / K in the experiment temperature range [300 K 780 K]