Cryocrystallography: effect of cooling medium on sample cooling rate

The rates of cooling of small samples with cold N₂ gas (100 K), liquid N₂ (77 K) and liquid propane (100 K) have been measured. The samples were one bare Cu-constantan thermocouple and one coated with a 0.25 mm layer of silicone rubber cement. Gas cooling yielded the lowest rate, liquid N₂ the highest. With the gas, cooling of the centers of the samples from 295 to 140 K took 0.8 and 2 s for the bare and coated samples, respectively; with liquid N₂ the times were 0.15 and 0.6 s, and with liquid propane they were 0.15-0.18 and 1.2 s, respectively (time reproducibility is within ±10%).