A gypsum-based composite structure is prepared from a slurry comprising a mixture of calcium sulfate hemihydrate, a cellulose ether additive other than CMC and an amount of water that is sufficient to form a slurry. The apparatus has a central axis and an inflatable outer dumbbell-shaped balloon. A first semiconductor device is attached to the first heat sink in thermal and electrical communication therewith and electrically insulated from the second heat sink.