I was just reading about this, and two statements I found to be interesting and pertinent:

"...Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature." (≈ Callen, pp. 189â€“190)

and...

"...It is impossible by any procedure to reduce the temperature of a system to zero in a finite number of operations." (≈ Guggenheim, p. 157)

an adiabatic process is one in which no energy is transferred.

"...Consequently no adiabatic process initiated at nonzero temperature can lead to zero temperature." (≈ Callen, pp. 189â€“190)

and...

"...It is impossible by any procedure to reduce the temperature of a system to zero in a finite number of operations." (≈ Guggenheim, p. 157)

an adiabatic process is one in which no energy is transferred.