Electrical Properties of Ag-C Contact Materials Containing Different Allotropes of Carbon
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Electro-mechanical properties and welding characteristics of Ag/MoS2, Ag/WS2, Ag/CNTs and Ag/CdO materials for high-DC current contact applications
Abstract — The optimisation of contact materials emerges as a cornerstone for reliable functionality of high-speed DC circuit breaker. Ag/CdO is widely used despite its RoHS non-conformity. To overcome this issue new materials are requested. We have prepared Ag/MoS 2 , Ag/WS 2 , Ag/Carbon-nanotubes composite materials and investigated their structural, mechanical, electrical and thermal properties. These materials have been used in switching operation tests on a laboratory-scale fully instrumented model of circuit breaker, under a maximum current load of 12 kA. Welding and sticking tendencies were examined. A relationship between the microscopic variables, the occurrence of welding events and their strengths was established. Keywords—Elecrical contact materials, electro-thermal characteristics, welding tests I. I NTRODUCTION Electrical contact materials (CM) are a key component in DC circuit breakers. Most of the operation time, CMs are in contact (strong mechanical force) in order to establish a low resistance path for electrical current without rising their temperature due to Joule heating. When a short circuit appears in the network, within a few milliseconds, contacts are forced to separate from each other leaving an air gap between them, which act as an insulating dielectric. Under these circumstances an electrical arc appears. This results in the dissipation of about 1kJ energy in less than 5 ms through the contacts causing local melting, and possible sticking or welding of the contacts. Sticking or welding can also occur without the presence of any arc, if the melting voltage of the contact material is reached [1]. Based on the conditions CMs are exposed to, their most important specifications could be defined as: i) High electrical and thermal conductivity ii) Optimal mechanical properties iii) Resistance to welding Although pure metals meet the first two requirements they show high affinity for welding, which ruled them out for this application. Metal based composite material emerged as an optimal solution. Ag/CdO has been extensively used in industry as the best contact material since more than 50 years [2, 3]. Its shadow side, however, is that cadmium, as other toxic elements, is forbidden for public use by the European Union directive "RoHS" (Restriction of the use, of certain Hazardous Substances in electrical and electronic equipment) [4]. An important effort has thus been invested to develop a new composite by replacing CdO with other oxides (SnO
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.