Browsing by Author "Bouchou, Aissa"

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Effects of surface treatments on photoelectric work function of silver-nickel alloys
(2014) Akbi, Mohamed; Bouchou, Aissa; Ferhat-Taleb, Mahdjoub
Theoretical 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 samples
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, Noureddine
Contact 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 results