A feedthrough assembly includes: a ferrule; an insulating structure; and a seal fixedly securing the insulating structure within the ferrule, the seal comprising a glass and single-phase particulate dispersed therein; wherein the glass includes: 25% to 40% B2O3; 0 to 25% CaO; 0 to 25% MgO; 0 to 25% SrO; 0 to 10% La2O3; 5% to 15% SiO2; and 10% to 20% Al2O3; wherein all percentages are mole percentages of the glass.