Liquid nitrogen is liquid nitrogen with molecular weight of 28.0 13, relative density of 0.808 1(- 195.8℃), melting point of -209.86℃, boiling point of-195.8℃ and critical temperature of-/kloc. It is a colorless, transparent, odorless and non-toxic liquid, which has no heat conduction, no electricity conduction, no spontaneous combustion, no combustion-supporting, and stable chemical properties. 1 unit volume of liquid nitrogen can produce about 650 times the volume of nitrogen, which is a by-product of oxygen production. Most medical refrigeration instruments at home and abroad use liquid nitrogen as refrigerant, which is widely used in clinical departments. However, liquid nitrogen is easy to vaporize at room temperature, difficult to preserve and troublesome to transport, so its application in areas where there is no liquid nitrogen production is limited.