X-Message-Number: 2007 Date: Mon, 22 Mar 1993 12:18:44 MST From: "Richard Schroeppel" <> Subject: CRYONICS Alternative cooling: Solid-liquid phase transition Current cryonics practice uses the liquid-gas phase transition of LN2 as a thermal buffer to maintain the patient at a constant temperature. The patient is partly immersed in the LN2. The temperature of the system is (roughly) constant as long as any liquid remains to evaporate; heat that leaks into the dewar goes into the liquid->gas phase change, and not into warming the patient. We've been discussing changing the storage temperature to around -130C. One nuisance is that there are no convenient substances which liquefy at that temperature. ("Convenient" abbreviates cost, toxicity, environmental, & handling considerations.) I suggest we examine using another phase transition to supply the thermal buffer: Solid->liquid. The patient would still be immersed in the liquid, but solid lumps of the coolant would be in the bottom of the dewar. There would be a smallish air space above the liquid level. Incoming heat would go to melting the solid. To "top up" the coolant, we could either drain liquid & add solid, or pour LN2 into the dewar to refreeze some liquid. Advantages: Another set of cooling substances to choose from. Less coolant is released to the outside world, so the environmental constraints can be loosened, and more expensive coolants are feasible. Liquid-solid freezing points can be varied over a range by mixing two or three substances. We can synthesize whatever value we choose. The venting system on the dewar might be simpler. Disadvantages: Solids are harder to handle than fluids: details of the topping-up process need to be worked out. The solid-liquid phase transition generally takes fewer calories per gram (water is 80 to melt, but 500 to boil), so the required amount of buffer may be larger. A spill of LN2 is "self-cleaning". The replacement coolant won't be. The patient may have to be wrapped more carefully, if the coolant is a contact toxin. Measuring the remaining cooling capacity in the system won't be as easy as looking at the level of LN2 in the dewar. Rich Schroeppel Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2007