Spectroscopy Diagnostics On The Electric-Arc Characteristics And The Formation Mechanism Of Erosion For Silver-Based Conductive Ceramic Composite Contact Materials

As the essential components of many electric types of equipment,electric contacts are widely used in industry and daily life.However,the property of the contact material is the dominant factor determining the quality of the devices.Many researchers in the developed countries around the world have devoted to the investigation of contact materials because of their importance.Ag/CdO contact materials were well developed and applied as electrical contacts used in electromechanical devices such as relays and connectors operating in low-power conditions where high reliability and long service life are required.In the past,Ag/CdO has been long recognized as the preferred material in make-and-break switching for its outstanding performance.However, because it generates the poison cadmium gas during the service,which is hazardous to human health,Ag/CdO has been prohibited in many countries.Now Ag/SnO₂ is often used as the materials of the electric contacts for their anti-welding and corrosion resisting properties.As the best replacement of Ag/CdO,Ag/SnO₂ contact material has been greatly developed nowadays,especially in European countries.However its contact resistance and temperature rise was higher.The defect microstructure with coarse grains and oxide clusters degraded the deformation abilities and electrical properties.So it is still on the way to real practical application.According to the present study of contact materials and the existing problems in these fields,a new notion was put forward to work on those materials and turn out a kind of Ag-based conductive ceramic contact materials to meet the application demands in low-voltage transformer industry.In this thesis,different series of metalloid conductive ceramics were prepared through conventional technique of ceramic preparation,and then mixed with silver powders to form electrical composite contact materials.We mainly developed two kinds of contact materials,Ag/La₂NiO₄ materials and Ag/LaFe_1-xNi_xO₃ conductive ceramic contact materials with the weight percent from 8%to 12%.The experiment results prove that the materials have merits of good electrical properties and mechanical properties.In a word,it is very possible for the new conductive ceramic materials to replace of Ag/CdO and also they will become a strong challenger to Ag/SnO₂ in the future.The conductive ceramic has particular network microstructure.The electrical contact materials which were mixed up with this kind of ceramics and silver had fine mechanical characteristic,nice corrosion-resisting performance and obvious chemical stability.All the above-mentioned performances made this kind of contact materials nearly satisfy most kinds of demands in electrical contact fields.One of the important investigation subjects of the thesis is to study and discuss the capabilities of contact materials because of their importance.Electrical contacts are widely used in electromechanieal devices such as air switches,relays and connectors. During their performance,ares come into being between the opening contacts during a "break" operation and during the contact bounce period in the "make" operation, when switching low voltage systems.The arc behavior during the closing and opening of electrical contacts not only influences the surface morphology,but also causes the erosion of contact material.Arcs can result in mass transfer,welding loss of contact and the early end of lifetime.To understand the interaction of arc discharge and erosion of contact materials is of great importance.The interaction of arc and erosion of the contact material is an interesting field that attracts much theoretical and experimental attention.Breaking ares quench and establish for a very short period of time,and besides,temperature of arc plasmas even exceed the value 6000K, so that the arc can boil away everything that contact to it directly.For this reason,an indirect and fast spectroscopic diagnostic of electric arc plasma have gained in importance.But the erosion mechanism is very complex and it covers so many contents that it is impossible to include all the aspects in a dissertation.So here we just focus some interesting subjects to discuss,such as spectroscopy diagnostics of arc plasma.Employing with the plasma spectroscopic diagnostic method,we investigated the properties of the two composite materials.The results showed that conventional testing methods of materials associate with spectroscopy diagnostic system can represent the general characteristics of contact materials.In addition,it offered an effective monitoring means for the design and performance comparisons of the compositions of contact materials.The thesis focused on the study of the systematic techniques and the properties of several new Ag-based conductive ceramics contact materials.The results showed that the two series of Ag/La₂NiO₄ and Ag/LaFe_1-xNi_xO₃ composite materials were new conductive ceramics contact materials,owing to their superior chemical and temperature stabilities,ease for fabrication and excellent anti-arc erosion properties. Both of the composite materials demonstrated their superiority over traditionally used contact materials,accompanied with non-toxicity,environmental friendliness and long service life time.The innovations in the thesis are illustrated in 5 aspects as following:1.Investigations of the new Ag-based conductive ceramic contact materialsTwo series of Ag-based conductive ceramic contact materials Ag/La₂NiO₄ and Ag/LaFe_1-xNi_xO₃,combining Ag with conductive ceramics with electric resistance in mΩ·cm scale,were studied.The electric resistances of both composite materials were about 2.1μΩ·cm,and their heat capacities had excellent temperature and chemical stability.The later was confirmed by arc ablation tests.X ray diffraction patterns showed that,under the influence of arc plasma,the ceramic grains were tightly bound to Ag substrate with layer section,which would ease the materials to be fabricated. These properties demonstrated superiority of both of the composite materials over Ag/CdO and Ag/SnO₂ as new contact materials,2.Studies on erosion mechanism of contact arc with time-resolved spectrumA spectroscopy diagnostic system was designed to obtain the time-resolved photoelectric spectroscopy.The system can monitor the motions of Ag atoms and ions during arc ablation,and predict the real conditions of the material erosions based on intensities variations of the spectral lines of different materials.With this system,the time-resolved photoelectric spectra of Ag/La₂NiO₄ and Ag/LaFe_1-xNi_xO₃ were studied, and the comparisons of experimental results with other contact materials were provided.The results showed that the two composite materials had excellent anti-arc erosion properties.In addition,this spectroscopy diagnostic system was an analytical means for anti-arc erosion of materials,as well as helpful for designs of new material.3.Measurements on contact arc behaviors under the real working conditions Contact point life simulation system was used to simulate the real working conditions of the contact point and study the behavior of contact arc plasma of the two composite materials.The results showed that the average ablation time of the arc plasma was in the range of 4 ms-4.3 ms,shorter than that of normally used contact materials.The shortening of ablation time could improve the resistivity of the materials to arc erosion,decrease the amounts of materials being eroded and translocated,and thus prolong the service time of the contact materials.There are mainly two mechanisms of arc erosion of metal oxides:one is the degradation and sublimation of oxides, which consumes a large amount of energy to diminish the erosion;and the other is to increase the surface tension and viscosity of the contact surface melting cell.During the arc plasma ablation,the metal dioxides would suspend in the surface in the form of micro-particles,with increased surface tension,metal viscosity and wettability of Ag to the contact,thus diminishing the lost of Ag due to its spray.The analysis of contact service life showed that addition of metal oxides could drastically improve arc plasma dynamic properties of the electric materials.4.Diagnostics on contact arc plasma through multi-wavelength spectrumThe design was equipped with a CCD multi-channel spectrometer,employing the plasma spectroscopic diagnostic method to the study of the properties of contact materials,and offered an effective monitoring means for the design and performance comparisons of the compositions of contact materials.Spectroscopic analysis of arc plasma with spectrometer with respect to various stoichiometrically composed Ag/La₂NiO₄ and Ag/LaFe_0.25Ni_0.75O₃ composite ceramic materials were implemented. The results showed that moderate increase of the fraction of Ni of the third metal phase in the Ag-based conductive metal composite materials can facilitate the enhancement of resistivity of the materials to arc erosions;with the same amount of injection,the ceramic materials obtained by adding La₂NiO₄ has better performance than which by adding LaFe_0.25Ni_0.75O₃.Results showed that spectrometer was highly instructive to the fabrication and use of metal materials.5.Curve fitting of spectrum lines and calculation on parameters of plasma We measured the electron temperature of arc plasma by methods of two-lines and slope respectively and analyzed the changes of spectrum lines' broadening as well as their mechanism.The results indicate Stark Broadening is the main reason for brodening of Ag I 546.5nm spectrum line from Ag/LaFe0.25Ni0.7503 contacting arc. Then we fit the spectrum lines by Lorentzian fitting,which is bettor than Gaussian fitting.As a result,we achieved FWHM,intensity of integral,central wavelength and etc.which can be used to deduce relevant characteristics parameters of plasma,such as electron temperature.Broadenings of spectrum lines approximately linearly increase with the increase of inputting energy.It shows electron density of plasma become larger as the energy enhancing.Using both methods of two-lines and slope, we respectively calculated electron temperature of Ag/LaFe_0.25Ni_0.75O₃ contacting arc in a 24V30A circuit.More exact electron temperature,1.1×10⁴ K,was achieved by the method of slope.