Evaporation cryostats

Fig. 5.4. Evaporation cryostat for temperatures T > 4.2 K. A mixture of gaseous and liquid 4He is transferred from the storage dewar to the cryostat by means of a transfer tube. The mixture is pumped through a spiral tube which is first thermally anchored to the experiment chamber. The temperature of the experiment can be...

Fig. 12. Cryogeiiic laser dilatometer with evaporator cryostat.

The heat of evaporation of 4He is about 2.6 kJ/1 at its normal boiling temperature of 4.2 K. The enthalpy of helium gas between 4.2 and 77 K (300K) is 64kM (200kJ/l). Hence, it is important to make use of the enthalpy, letting the gas leave the cryostat to a temperature close to room temperature. 

It is more convenient to use the enthalpy of the cold gas evaporating from the LHe in a continuous gas flow cryostat (Fig. 5.4). The flux and hence the temperature of the experiment can be regulated by a heater or a needle valve. 

Referring to Fig. 5.6, the 3He refrigerator [25] contains a pump P and an evaporator E. They are connected by a stainless steel tube T internal to the copper support C. The latter is in good thermal contact with the working plane B of a pumped 4He cryostat (for example that of Fig. 5.3) not shown in figure. The tube is connected to a charcoal cryopump P linked to the 4lie bath by a thermal connection L. A thermometer Th monitors the temperature of the pump. A thermal shield (not shown), at the temperature of the 4He bath, surrounds the refrigerator. 

Liquid nitrogen is stored in large vacuum vessels (cryostats). The liquid nitrogen is evaporated through a heat exchanger and the gas used in the factory. The units are normally supplied and maintained by the gas company. The cryostats can be easily topped up by a road tanker. This system is more economic in medium to large units. 

After evaporating the aluminum counter electrode and prior to dielectric measurements, a second annealing step is performed for several hours at temperatures above Tg under nitrogen flow in the cryostat of the dielectric spectrometer. 

To separate the floccules from the KH150BS under low viscosity conditions conveniently, a method of solvent dilution and filtration was employed at low temperature. The detailed procedures were as follows (1) The sample was diluted with an equal mass of a solvent made of a 1 1 mixture of butanone and toluene. (2) One container of the above solution was set in each of four cryostats whose temperature was previously set at -5°C, -10°C, -15°Cand -20°C. (3) The samples were separated into filtrated residue substances and filtrates with a vacuum pump, a grit filter (G4, aperture 5pm - 6pm) and a filter flask. (4) The solvents in the filtrates were again obtained with a rotary evaporator at negative pressure and a small amount of solvent in the filtrated residues was vaporized on a water bath. Finally, the floccules and filtrated oils were obtained by a series of above procedures. 

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