thermal expansion mismatch can considerably lower the strength of the joint. Applying indium-based alloys having high plasticity is especially expedient for metalization and joining BaTiO3 ceramics. Additionally, the technology of brazing and metalization BTO was realized using capillary impregnation of low-melting braze alloys through titanium powder. For such alloy cleanliness, titanium powder was obtained from TiH dissociating into titanium and hydrogen during heating. The indium was deposition on the titanic powder layer onto a BaTiO3 ceramics surface. During heating up to 970 K, indium spread well over the whole BaTiO3 surface and filling brazing gap. The thin film of In-Ti is easily formed on the surface of perovskite ceramic in such conditions — Fig. 6B. Ferroelectric BaTiO3 Barium titanate with a stoichiometric structure having high ferro- and piezoelectric characteristics can be heated up without any changes only within a oxygencontaining environment (in air). For such materials joining, special braze alloys and technological processes are required. Oxygen being dissolved in some metals leads to a substantial increase of wetting degree and adhesion of these metals to ceramics. The oxygen effect on wetting and also on interface and surface tension of metal melts was investigated earlier in our works (Refs. 14, 25–28). It has been shown that oxygen effectively increases the adhesion of Cu, Ag, Ni, and some other metals to ionic compounds, for example, to oxides. Several systems (Cu-O-Al2O3, Cu-OMgO, Ni-O-Al2O3, Ag-O-Al2O3, Ag-Cu- O-Al2O3) were studied in detail. The Ag- Cu-O system is especially interesting. We have made the assumption that the “oxygen” technology will work for ferroelectric barium titanate as well. According to Refs. 14, 15, oxygen that has sufficient affinity to an electron, being dissolved in liquid metal, will increase the wettability of a surface for ionic or ioncovalent substances. Up to now, there are only solitary works concerning the possibility of a perovskite compound (Pb (Mg0.33Nb0.67) O3) for wetting and joining by Ag-CuO alloys (Ref. 29). Scientific background of this process is not developed; the reasons for oxygen influence on wetting are not explained in this work. Thus, the investigation of wetting ceramic ferroelectric materials based on BTO, elaboration of braze compositions and technological conditions for brazed BTO ceramic joints, and creation of strongly adherent metal coatings on the ferroelectric perovskite ceramic surfaces were the main purpose of the present work. The Ag-Cu-O system alloys were used as a braze alloys base. Experimental Data and Discussion For ferroelectric BTO ceramic, experiments and technological processes were carried out per method in air media and, for the first time, under pure oxygen atmosphere using the sessile drop method as well. For this purpose, a special device was created — Fig. 9. Experiments were carried out in oxygen flow with the partial pressure of oxygen about 1 atm at 1250, 1320, and 1370 K. Technical pure oxygen was used. But the oxygen is reactive. Pure oxygen at high pressure, such as from a cylinder, can react violently with common materials such as oil and grease. Take all reasonably practicable precautions to ensure safety to prevent oxygen enrichment by keeping oxygen equipment in good condition and taking care when using it. Good ventilation will also re- WELDING JOURNAL 11-s WELDING RESEARCH Fig. 7 — Scheme of the shear strength test for the brazed ceramic samples. Fig. 8 — The Weibull’s graph of brazed strength in vacuum semiconducting BaTiO3 ceramic samples. Fig. 10 — Dependence of contact angle for the ferroelectric BaTiO3 ceramic by Ag-Cu-O melt in air environment and oxygen on concentration of copper at 1250 K. Fig. 9 — Scheme of the apparatus for determining the contact angle of metal melts in the air (oxygen) atmosphere. The labeled numbers represent the following: silica tube (1); furnace (2); heat transparent screen (3); digital camera (4); metal refractory wire (5); and metal sample on the BaTiO3 ceramic substrate (6).
Welding Journal | January 2014
To see the actual publication please follow the link above