A feedthrough includes a sheet 938 having a hole 936, where the sheet includes a first material 916 that is a ceramic comprising alumina. The feedthrough further includes a second material 940 substantially filling the hole, where the second material includes a platinum powder mixture and an alumina additive. The platinum powder mixture includes a first platinum powder having a median particle size of between approximately 3 and 10 micrometers and a second platinum powder that is coarser than the first platinum powder and has a median particle size of between approximately 5 and 20 micrometers. The platinum powder mixture includes between approximately 50 and 80 percent by weight of the first platinum powder and between approximately 20 and 50 percent by weight of the second platinum powder. The first and second materials have a co-fired bond therebetween that hermetically seals the hole.
