The radioactive decay of strontium-90 generates a significant amount of heat, 0.95 W/g in the form of pure strontium metal or approximately 0.460 W/g as strontium titanatePu. It is used as a heat source in many Russian/Soviet radioisotope thermoelectric generators, usually in the form of strontium titanate. It was also used in the US "Sentinel" series of RTGs.  Startup company Zeno Power is developing RTGs that use strontium-90 from the DOD, and is aiming to ship product by 2026.