Publications Scientifiques
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Item 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 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 contacts