Vacuum brazing of alumina ceramic to titanium for biomedical implants using pure gold as the filler metal

One of the many promising applications of metal/ceramic joining is in biomedical implantable devices. This work is focused on vacuum brazing of C.P titanium to 96% alumina ceramic using pure gold as the filler metal. A novel method of brazing is developed where resistance heating of C.P titanium is done inside a thermal evaporator using a Ta heating electrode. The design of electrode is optimized using Ansys resistive heating simulations. The materials chosen in this study are biocompatible and have prior history in implantable devices approved by FDA. This research is part of Boston Retinal implant project to make a biocompatible implantable device (www.bostonretina.org). ^ Pure gold braze has been used in the construction of single terminal feedthrough in low density hermetic packages utilizing a single platinum pin brazed to an alumina or sapphire ceramic donut (brazed to a titanium case or ferrule for many years in implantable pacemakers. Pure gold (99.99%) brazing of 96% alumina ceramic with CP titanium has been performed and evaluated in this dissertation. Brazing has been done by using electrical resistance heating. The 96% alumina ceramic disk was manufactured by high temperature cofired ceramic (HTCC) processing while the Ti ferrule and gold performs were purchased from outside. Hermetic joints having leak rate of the order of 1.6 × 10-8 atm-cc/ sec on a helium leak detector were measured. ^ Alumina ceramics made by HTCC processing were centreless grounded utilizing 800 grit diamond wheel to provide a smooth surface for sputtering of a thin film of Nb. Since pure alumina demonstrates no adhesion or wetting to gold, an adhesion layer must be used on the alumina surface. Niobium (Nb), Tantalum (Ta) and Tungsten (W) were chosen for evaluation since all are refractory (less dissolution into molten gold), all form stable oxides (necessary for adhesion to alumina) and all are readily thin film deposited as metals. Wetting studies are also performed to determine the wetting angle of pure gold to Ti, Ta, Nb and W substrates. Nano tribological scratch testing of thin film of Nb (which demonstrated the best wetting properties towards gold) on polished 96% alumina ceramic is performed to determine the adhesion strength of thin film to the substrate. The wetting studies also determined the thickness of the intermetallic compounds layers formed between Ti and gold, reaction microstructure and the dissolution of the metal into the molten gold.^

Developing a high density platinum/alumina hermetic feedthrough

Typically, hermetic feedthroughs for implantable devices, such as pacemakers, use a alumina ceramic insulator brazed to a platinum wire pin. This combination of material has a long history in implantable devices and has been approved by the FDA for implantable hermetic feedthroughs. The growing demand for increased input/output (I/O) hermetic feedthroughs for implantable neural stimulator applications could be addressed by developing a new, cofired platinum/alumina multilayer ceramic technology in a configuration that supports 300 plus I/Os, which is not commercially available. Seven platinum powders with different particle sizes were used to develop different conductive cofire inks to control the densification mismatch between platinum and alumina. Firing profile (ramp rate, burn- out and holding times) and firing atmosphere and concentrations (hydrogen (wet/dry), air, neutral, vacuum) were also optimized. Platinum and alumina exhibit the alloy formation reaction in a reduced atmosphere. Formation of any compound can increase the bonding of the metal/ceramic interface, resulting in enhanced hermeticity. The feedthrough fabricated in a reduced atmosphere demonstrated significantly superior performance than that of other atmospheres. A composite structure of tungsten/platinum ratios graded thru the via structure (pure W, 50/50 W/Pt, 80/20 Pt/W and pure Pt) exhibited the best performance in comparison to the performance of other materials used for ink metallization. Studies on the high temperature reaction of platinum and alumina, previously unreported, showed that, at low temperatures in reduced atmosphere, Pt 3Al or Pt8Al21 with a tetragonal structure would be formed. Cubic Pt3Al is formed upon heating the sample to temperatures above 1350 °C. This cubic structure is the equilibrium state of Pt-Al alloy at high temperatures. The alumina dissolves into the platinum ink and is redeposited as a surface coating. This was observed on both cofired samples and pure platinum thin films coated on a 99.6 Wt% alumina and fired at 1550 °C. Different mechanisms are proposed to describe this behavior based on the size of the platinum particle