010s.pdf

Welding Journal | January 2014

A Fig. 5 — X-ray diffraction patterns of the BaTiO3 (Cu-28 Ti) system. A — 1280 K; B — 1370 K. A be accounted for the explanation of a wetting in system studied at high temperatures. The same data were published, for example, in Ref. 24 for Al2O3 systems. Varying wetting effects of titanium in different systems studied — Cu-Ag, Cu-Ga, Cu-Sn (Fig. 3) — can be explained by several reasons, in particular by various thermodynamic activity of titanium in alloys. Ag-Cu-Ti alloys demonstrate some peculiarity. A eutectic system with about 60% (at.) of silver can dissolve in about 2% (at.) of titanium at 1270 K. A titanium content increase leads to arising the second equilibrium liquid phase that contains 64% (at.) of Cu, 28% (at.) of Ti, and 8% (at.) of Ag (Ref. 14). The mechanism details of such interface processes in a complex BaTiO3 – (Ag–Cu–Ti)phase I – (Ag– Cu–Ti)phase II system requires special consideration. It is possible now to note only that this process can be useful for improving adhesive bonding of BaTiO3 to metal (arising of second liquid phase with a high Ti concentration). The temperature dependence of contact angle for indium-titanium alloys on the BaTiO3 surface has been investigated as well. The contact angle of a In-Ti melt drops significantly at a low temperature (in interval, ∼770–870 K). Almost full spreading of the In-Ti melt on a BTO surface occurs at 830–870 K. It can be used for brazing not only semiconducting barium titanate (BaTiO3–x) but the ferroelectric one (BaTiO3) as well. Brazing Alloys and Technological Conditions for Semiconducting BTO Joining Metalization of materials using liquid metal film is a perspective method. However, a high degree of wetting for solid surfaces by such metal is required. Theoretically (Ref. 14), for producing continuous film of liquid metal, spreading factor (K) has to be positive (Equation 1). K = WA – WC (1) where WA is work of adhesion and WC is work of cohesion. For brazing and metalization of perovskite, compounds were chosen for brazed compositions that are well wetted for the surface of such materials. Braze alloys based on titaniumcontaining systems (Cu-Sn-Ti, Ag-Cu-Ti, In-Ti) for joining and metalization of perovskite BaTiO3 ceramic were used for creating uniform coatings and strong brazed samples — Fig. 6A. The shear strength of brazed ceramic/ceramic butt joints was measured (Table 2, Figs. 7, 8). It is shown that the strength of brazed perovskite samples obtained using Cu- Sn-Ti alloys equaled 42 MPa. It is about 80% of the average strength of monolithic samples. Beside basic requirements (particularly sufficiently high wetting) for materials to be joined, compliance of their coefficients for thermal expansion is important, because stresses caused by JANUARY 2014, VOL. 93 10-s WELDING RESEARCH B B Fig. 6 — The samples of the semiconducting BaTiO3 ceramics metalized and brazed in vacuum using the following: A — In-Ti alloys; B — Ag-Cu-Ti paste. Table 2 — The Result of Measurements of the Shear Strength of Brazing Barium Titanate Ceramic Samples Shear Strength of Brazing Ceramic Samples, MPa Ag–Cu–O Ag–Cu–Pt–O In–Ti Cu–Sn–Ti Ag–Cu–Pb–Ti 46±4 28±3 20±2 42±3 41±2


Welding Journal | January 2014
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