Nanocomposites and nanomaterials

Influence of vacuum annealing on the dispersion of thin double niobium-palladium films deposited onto oxide ceramic materials
Stetsyuk T.V.1, Fesenko O.M.2, Shakhnin D.B.3, Gab A.I.3 
1 Frantsevich Institute for Materials Science Problems of NAS of Ukraine, 
3 Krzhyzhanovskogo st., Kyiv, 03142, Ukraine.
E-mail: tvst@ukr.net
2 Transfer Technology Department, Institute of Physics of the NAS of Ukraine. Prospect Nauki, 46, Kyiv-03039, Ukraine. 
3 University "Ukraine", 23 L’vivska st., Kyiv, 03115, Ukraine. 

Permanent vacuum-tight precision ceramic joints with an ultra-thin seam can be made by brazing or pressure welding of metallized ceramic parts. This purpose can be achieved by application onto ceramic surfaces of double metal films, one part of which is 100–200 nm thick and consists of an adhesion-active metal (e.g. Hf, Nb etc.), and the other layer is slightly thicker (1–3 μm) and serves as a solder (e.g. Pd, Ag, Ni etc.) ensuring joining of metallized ceramic materials during brazing or pressure welding with a fine brazed seam (2–4 μm thickness). In this work, specimens made of leucosapphire, alumina and dioxide-zirconium ceramics were used metallized with a 150 nm Nb nanofilm and then by 1,5 μm Pd film  atop serving as a soldering metal for ceramic samples joining. 
As a result of annealing at 1000 °C for 5 min, the films remained almost unchanged; after 10 min, the number of defects in the film on all oxides increased, the film began to swell and even crack; and after 20 min, the number of defects in the film on all oxides increased even more. With annealing temperature up to 1100 °C, the film underwent significant changes after 5 min of exposure; after 20 min, the number of defects on all oxides increased. Annealing at 1200 °C accelerated sharply the dispersion of the film which began to deteriorate significantly after 10 min of exposure; 20 min of annealing led not only to significant decay of the film, but also to the interaction of Pd with Nb, with film residues still covering 70-80% of the substrate surface area, which made it possible to use double Nb-Pd films to join oxide ceramics at temperatures up to 1200 °C.
According to our study results, kinetic curves of the investigated thin double metallic films decomposition have been built, from which the basic process parameters (temperature and exposition time at this temperature) for brazing or pressure welding of ceramic materials can be determined. Using the data obtained, process regimes for joining by brazing and pressure welding were selected. According to these regimes, joints’ prototypes with seams up to 2 μm thick were made, which strength reached 150 MPa.