Second law of thermodynamics says- it is impossible to fully convert heat into work. My question is- "Is it possible to convert heat totally into non-usable form of energy?"
Thanks in advance.
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Unless I am mistaken if the temperature is exactly uniform everywhere, i.e., the spacetime has the heat energy (meaning some distribution of matter or energy with uniform density and uniform average kinetic energy) the same everywhere, and it was static, that'd be the case. Eventually, though, in a region large enough, any small perturbation could grow, eg by gravitation. So it's unstable.
Anyway your question could be ambiguous, or rather not constraining enough. If you imagine setting an isolated/insulated system of neutral particles at a uniform temperature, with temperatures well below any interaction thresholds, no part of it can exercise any macroscopic work on any other. But practically you can't do a perfect isolation forever.
Another case is a very cold and very large universe, say 100 trillion years from now after it's expanded so the energy/matter densities are uniform and very low, and the universe continues expanding, it is called the heat death of the universe. The expansion would be driven by the cosmological constant which may be thought of as the energy of vacuum. Macroscopically no work gets done on anything, and it is a high entropy state. Unless the vacuum is unstable somehow over the long run (since we don't know about the quantum gravity vacuum yet enough).
In all those cases the time period when the region or isolated system, or spacetime was in essence high entropy and unable to do any work on anything, could last a long time. It you want more definitive answers you might need to constrain things more.