Passive pyrolytic graphite heat switch for sub-Kelvin coolers

We describe a passive heat switch based on a commercial pyrolytic graphite sheet. Measurements of the thermal conductivity of the graphite are presented, confirming a large difference between room temperature and ≈4 K. The implementation of a graphite heat switch in a cryostat operating a 3 He/ 4 He sorption refrigerator is demonstrated. • Flexible pyrolytic graphite sheet is ideal for a passive heat switch. • The graphite's thermal conductivity is metal-like at room temperature but drops sharply at ≈4 K. • The pyrolytic graphite's thermal conductivity is ideal for use as a passive heat switch. • Graphite heat switches are easy to implement and use a widely available and affordable material.