Cryocrystallography: effect of cooling medium on sample cooling rate

The rates of cooling of small samples with cold N(2) gas (100 K), liquid N(2) (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 N(2) 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 N(2) 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%).