You can convert work to heat completely. But not heat into work completely. In fact the latter has been understood properly since the 1840s. And it doesn`t matter what materials you choose, the only thing that matters is the temperature difference. You will max out at about 30% for anything in the practical range we humans operate within, for more details look up Carnot’s theorem. It’s another form of the laws of thermodynamics. Note that efficiency says nothing about heat capacity
