High temperature wettability studies for development of unmatched glass-metal joints in solar receiver tube

This paper presents the high-temperature wettability studies carried out during glass-metal joining for parabolic trough solar receiver tube. The bonding characteristics of the glass and metal directly depend upon the wettablity measured in terms of (contact angle) the types of oxide formed at the metal surface and the microstructural changes during the fabrication process. In this research work three different substrate materials, namely, Austenitic Stainless Steel SS304, copper and low carbon steel (AISI 1018) were polished against different grades of emery paper in order to generate variations in their surface roughness. Borosilicate glass pallet was melted over these specimens and high-temperature wettablity (contact angle) of glass over candidate material substrate specimen was examined. X-ray diffraction was used to detect the types of oxide formed on the substrate during the wetability study. Microstructural and macroscopic examination of the substrates revealed the morphological changes on substrate surfaces at different length scales. The wettablity of borosilicate glass over Austenitic Stainless steel SS 304 substrate was better as compared to copper and low carbon steel substrates. In the case of low carbon steel, metal specimen got distorted at joining temperature because of poor thermal stability of low carbon steel and no bonding was observed.