Microstructural Study on Molten Marks of Fire-Causing Copper Wires

• Main Authors: Kuan-Heng Liu, 劉冠亨
• Other Authors: Guo-Ju Chen
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/12625707054911934804

博士 === 義守大學 === 材料科學與工程學系 === 104 === The electrical fires are generally the main causes of building fires, but the public is not always convinced of the critical evidence adopted by the fire investigators to identify the electrical fires. Basically, the critical evidence used for the fire investigation departments to identify an electrical fire is mainly the short-circuited arc beads of wires at the fire scene. Unfortunately, the scientific definition of the "fire-causing arc bead" (FCAB) has not yet been clear due to the lack of thorough scientific investigations. Therefore, the verification of arc bead influences significantly the public reliance of the investigations as well as the evidence weight of judicial prosecution, litigation and judgment. This verification problem is necessarily an urgent issue for the fire agencies to tackle immediately. This study analyzed the short circuit of a copper wire at ambient atmosphere. We scrutinized the microstructures of fire-causing electric wires and simulated the external environmental conditions required for the formation of FCABs. The experimental results revealed that when the copper wire was short-circuited and blown out, the molten copper liquid formed arc beads at the blown part under the effect of surface tension. In the solidification process, the FCABs can be divided into two categories (the non-oxygen-permeated arc beads and the oxygen- permeated arc beads) that exhibit two different kinds of molten marks at ambient atmosphere. In this study of the non-oxygen-permeated FCABs, our metallographic investigation showed that the primary thermal dendrites of copper at the non-oxygen-permeated FCABs grew parallel to each other, but in the opposite direction to the heat flow. We derived the relationships of the undercooling (∆T0), the growth velocity (ν), and the primary spacing (λ) of the dendrites with respect to the wire’s diameters (D). This study also explored the oxygen-permeated FCABs at ambient atmosphere, and successfully identified various phases of the oxygen-permeated FCAB. A cuprous oxide flake was formed on the surface of the molten mark during the rapid solidification process, and there were two microstructural constituents, namely Cu-κ eutectic structure and the solutal Cu dendrites. Due to the oxygen-permeated FCABs formed at atmosphere in the local equilibrium solidification process, the phases of oxygen-permeated FCABs segregated to the cuprous oxide flake, the Cu-κ eutectic and Cu phase solutal dendrites, which were the fingerprints of the oxygen-permeated FCABs.