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Welding Journal | January 2014

located in octahedral cavities formed by oxygen ions and have enough “room” for displacement within the BTO elementary cell (a = 4.011 for cubic lattice). It is an explanation of essential mobility for titanium ions oscillating freely within the octahedral environment of oxygen ions. It determines high polarizability of barium titanate under electric field action (Ref. 19). Such increased mobility of titanium ions (according to Weyl’s scheme (Ref. 20)) leads to some more of their displacement into crystal bulk after surface formation. So, for BTO, we can neglect the interaction of liquid metal phase with titanium cations. However, beside oxygen, BTO surface also contains large-sized barium ions (Ba2+). It makes this situation more complex. On the base of ions size data (r(Ba2+) = 0.135 nm, r(O2–) = 0.140 nm) and structure of BTO crystal lattice, we can see that only a quarter of the ceramic’s surface is occupied by Ba2+ ions, and the rest by oxygen ions. Overall, we can guess that main regularities of interactions in BaTiO3 liquid metal systems have to be similar to regularity for “classical” oxide (Al2O3) metal systems. Nevertheless, the interaction of certain liquid metal phases with barium ions at a BaTiO3 surface should be considered. Free formation energies of chemical compounds for the metals under investigation with barium are within 167–250 kJ/mol (for comparison, heat of formation for the oxides is ΔH(Al2O3) = –1675 kJ/mol, ΔH(SiO2) = –911 kJ/mol) (Ref. 21). Only a silicon compound with barium (BaSi3) is formed with significant heat release (ΔH(BaSi3) = –544 kJ/mol). But pure silicon can only moderately wet the BaTiO3 surface, and its adhesion is lower than the same value for aluminum, though Al-Ba compounds are considerably less stable thermodynamically according to phase diagrams data (Ref. 22). Thus, wetting and adhesion in BTO metal systems is evidently not determined by Ba-Me interaction to a significant degree. An inactive matrix of Cu-Ga, Ag-Cu, and Cu-Sn alloys (Fig. 3) does not wet the BTO surface (θ ∼ 120 ÷ 130 deg). A titanium addition reduces contact angles down to 20–70 deg for titanium concentration up to 10–25% (at.). It has been assumed that wetting the BTO surface is, first of all, a result of interaction between liquid metal (titanium) and oxygen of solid phase, as in the case of “classical” oxide materials (Al2O3 and MgO). Formation of titanium oxide (TiO) having metal-like properties in a BaTiO3/Ti-containing alloy system is the reason of high adhesion in this case. Titanium, as a transition metal, is characterized by its ability to participate simultaneously in several chemical bonding interactions of different types — ionic one with BaTiO3 surface and metallic with liquid metal phase. In other words, titanium from a liquid phase can become a bridge connecting the solid BaTiO3 phase with molten metal. Our SEM research of contact boundary BaTiO3/titanium-containing alloy has shown the presence of a transitive zone 5–7 μm wide, which obviously is a product of interphase reaction. Figure 4 shows the structure of cooled drop (Cu-8.6 Sn)-20Ti on the BaTiO3 substrate. Analysis of the BaTiO3/liquid metal interface shows the character of elements distribution in the direction perpendicular to the interface — Fig. 4B. Chemical composition for the BaTiO3 phase in volume is reproduced precisely as ~20% (at.) of barium and titanium, and ~60% (at.) of oxygen. This ratio remains unchanged to the BaTiO3/metal interface. Barium concentration is insignificant at the interface. That is why metal interaction with barium is possible only as a monolayer adsorption at the BaTiO3 surface. Titanium concentration increases from 20% (at.) in the transition zone up to ~50–60% (at.) in the contact zone. Oxygen concentration in this zone is about 18% (at.). In Fig. 4C, the layer (new phase) with high titanium concentration is clearly visible. Titanium segregation from the melt at interface is the main reason for high wettability of BaTiO3 by Ti-containing alloys. The metal chemistry studies by M. V. Nevitt (Ref. 23) show that oxygen stabilizes intermetallic compounds like Ti2Cu; the Cu2-3Ti3-4Ophase has been identified. A special investigation of the processes occurred at different temperatures in the contact zone by a high-temperature, X-ray diffraction method of pressed mixture with barium titanate, copper, and titanium powders carried out as well (Fig. 5). Two new phases with TiO and Cu3Ti3O structures were identified in this system. Both substances can be responsible for wetting. However, the Cu3Ti3O pattern disappears at 1370 K (Fig. 5B); probably, this compound is not stable. Just TiO can WELDING JOURNAL 9-s WELDING RESEARCH Table 1 — The Results of Wetting of Semiconducting Barium Titanate by Some Pure Metals Metal Temperature, К Contact Angle θ, deg Work of Adhesion, Our Data Literature MJ/M2 Cu 1373 122±3 102 9 610 Ag 1253 136±3 90 10 260 1273 132±1 139 9 310 1373 129±2 134 9 345 Au 1353 127±1 114 10; 445 124 9 1423 124±2 119 9 490 Ge 1273 113±3 — 375 Sn 873 120±1 138 9 285 973 115±2 — 330 In 673 152±1 — 70 773 132±2 — 195 873 117±2 — 325 Pb 673 145±2 143 9 80 773 134±4 — 140 873 118±3 — 250 973 109±4 138 9 320 Pd 1860 116±3 — 845 Ni 1743 113±1 — 1030 Fe 1823 96±2 — 1350 Si 1733 84±1 — 830 Al 1073 140±1 — 210 1173 136±2 — 250 1273 129±3 — 340 1373 93±1 — 880 1423 85±2 — 1015 1473 78±2 — 1140 Co 1793 108±2 — 1245


Welding Journal | January 2014
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