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Welding Journal | January 2014

C D duce the risk of oxygen enrichment (Refs. 15, 30). In Fig. 10, data on BaTiO3 wetting by silver and silver-copper alloy are presented. Oxygen dissolved in Ag-Cu alloy works as a strong adhesive element. Contact angle for pure silver in vacuum at 1250 K was equal to 129 deg, the same value in air was 96 deg, and in pure oxygen –75 deg. Copper addition to silver melt (~10% (at.)) leads to considerable contact angle decrease to 45–47 deg (in air) and to almost complete spreading of alloy (θ ≈ 5–10 deg) in the pure oxygen atmosphere. According to Ref. 14, oxygen is an adhesion active and surface active element. In the liquid metal, oxygen exists in the form of O–2 ions and can form complex metal-oxygen particles with metal ions in melt (Me2+ – O-2). Such complex particles have a positive pole at metal ion and negative pole at oxygen ion. Positive metallic ion of the complex is adsorbed on negatively charged oxygen ions, forming the surface of BaTiO3. Localization of external electrons for metal ion at oxygen ion must weaken the metallic bond intensity with other metal ions. The bond between the metallic ion of metal-oxygen complex and another metal atom must be weaker than metallic bond atoms with each other. When the bond energy complex-Me is less than bond energy Me-Me, it is the condition for metal-oxygen complex surface activity. Adsorption of metal ions on the negative charged oxide surface results in high adhesion. The temperature increase intensifies the wetting process; contact angle decreases to 25–30 deg in air (with 10 % (at.) of Cu in liquid Ag). For pure oxygen, full spreading can be reached with Cu content at about 6–7% (at.). The high capillary activity of alloys in a pure oxygen atmosphere is caused by a high equilibrium concentration of oxygen in melt under high oxygen partial pressure (1 atm). Oxygen partial pressure for the air is 0.21 atm. Concentration of oxygen that saturates metal melt is described by Siverts’s law (in many cases) (Refs. 31, 32), liquid metal 2 2 (2) ⎡⎣⎢ ⎤⎦⎥ O k p O = ( ) where p(O2) is oxygen partial pressure and k is constant. The concentration of oxygen dissolved in the silver melt in air is equal: Ag ⎡⎣⎢ ⎤⎦⎥ O k air = 0, 21 (3) JANUARY 2014, VOL. 93 12-s WELDING RESEARCH Fig. 11 — Microstructure of Ag-Cu-O, BaTiO3 interface at characteristic emission of elements (top part – ceramic, low part – alloy). A — ×600; B — barium; C — titanium; D — silver; E — copper. B–E — ×1000). A E B


Welding Journal | January 2014
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