6-s WELDING RESEARCH Wettability by Liquid Metals, Metalization, and Brazing of Barium Titanate Ceramics Introduction A study investigates the contact interaction and wetting of BaTiO3 perovskite ceramics by liquid metals The perovskite-type structure ceramic materials, such as barium titanate, play a major role in modern electronics and electrical engineering. Barium titanate (BTO) is widely used for creating multilayer ceramic capacitors (MLCCs), embedded decoupling capacitors (EDC), electrical ceramic filters, and other piezoelectric and ferroelectric components (Refs. 1–3). Ferroelectric ceramics are ideal for use in supersonic equipment. On the base of these materials, a ferroelectric memory device was created (Refs. 4, 5). Barium titanate is also an excellent photorefractive material (Ref. 6). Oxygen release that occurs during the annealing of BaTiO3 in high vacuum and at sufficiently high temperatures leads to the transformation of dielectric ferroelectric BaTiO3 into semiconducting ceramics (Ref. 7). As a semiconductor, BaTiO3 particularly exhibits a positive temperature coefficient of resistivity (PTCR). It means that at a certain temperature (Curie temperature), this material exhibits strong resistivity increase (typically by several orders of magnitude) (Ref. 6); due to its PTCR properties, barium titanate is often used as thermistors material in the thermal switches. The great importance for all such ceramics is creation of strong contacts (including electric ones) in combinations BaTiO3/metals and BaTiO3/ BaTiO3. Strength of metal-oxide contact and uniformity of metal coating are determined essentially by a wettability degree of ceramic materials (BTO) surface by liquid metals. High adhesion of liquid metals to ceramics surface is a crucial factor for creating mechanically strong contact. According to Ref. 8, the degree of perovskite type ceramics wettability by liquid metal, and intensity of interaction between liquid metal and solid phase, can also determine some electric properties of the contact, e.g., ohmic or nonohmic one, p-n-transition (in the case of semiconducting ceramics), and Schottky barrier height. Thus, creating a strong adherent metal coating on the BaTiO3 surface for its joining to metals and metalization of perovskite type ceramics is a perspective direction of investigation. Recently, scientific and technical interest in this problem was increased considerably. Up to now, there are only a few published works concerning research of contact properties of some metals in relation to perovskite-type ceramics, in particular to barium titanate (Refs. 9–11). The experimental data on wetting of perovskite ceramics by some pure metals in these works contradict each other sometimes. In addition, in Ref. 10 the conditions of experiments are described only qualitatively, e.g., as atmosphere with “high” and “low” oxygen pressure, so these data need verification. Regularities of strong adherent contact formations are studied little; scientific bases of these processes are practically absent. The present work aims to systematically investigate the details of phenomena for wettability, adhesion, and interaction intensity of BaTiO3 perovskite-type ceramics in different forms — ferroelectric BY T. V. SYDORENKO AND YU. V. NAIDICH KEYWORDS Wetting Metalization Brazing Barium Titanate Ceramics T. V. SYDORENKO and YU. V. NAIDICH (tvsid@ukr.net) are with the I.M. Frantsevich Institute of Problems of Materials Science NAS of Ukraine, Kyiv, Ukraine. ABSTRACT Detailed investigations, including wetting studies by liquid metals and metal coatings deposition onto ceramic surfaces and brazing processes, were carried out for semiconducting and ferroelectric perovskite ceramic states of barium titanate (BaTiO3). Pure metals (Cu, Ag, Au, Ge, Sn, Pb, Ga, In, Al, Si, Ni, Co, Fe, Pd) and Ti-containing alloys based on In and binary Cu-Sn, Cu-Ga, and Cu-Ag systems were investigated under high vacuum for the semiconducting BaTiO3-x surface. The degree of wettability correlates approximately with the chemical affinity of the liquid metal phase to oxygen (wetting increases when the liquid metal affinity to oxygen increases). Addition of Ti to the liquid metal phase increases capillary properties and adhesion in the system under investigation. Investigations of the wettability of ferroelectric BaTiO3 ceramics were performed in air gaseous media by alloys Ag-Cu-O. Oxygen in the gaseous media preserves stoichiometric composition of barium titanate and being dissolved in Ag-Cu alloys promotes BaTiO3 wetting. For the first time, wettability experiments in liquid metal/ceramic material systems (BaTiO3 in this case) were carried out when the gaseous phase was pure oxygen. At greater oxygen partial pressure (1 atm for O2 comparing to 1/5 atm for air), wettability further increases significantly. Vacuum brazing technology for semiconducting materials and joining processes in air or pure oxygen atmosphere for ferroelectric ceramic materials based on BaTiO3 have been developed. Various detailed brazing models of BaTiO3 and BaTiO3-x were created. JANUARY 2014, VOL. 93
Welding Journal | January 2014
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