Device for removal of solid impurities from liquid nitrogen

Research and technica/ notes 2000

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Magnetization curve for a niobium sample

at high fields the Hall probe's deviation from linearity increases (1% until 30kG, 2% until 150kG). However, at low temperatures and high magnetic fields the outputs of the Hall probes are modulated by the

Device for removal of solid impurities from liquid nitrogen J.D. Brownridge Department of Physics, Applied Physics and Astronomy, State University of New York at Binghamton, Binghamton, NY 13901, USA Received 15 April 1988

A device made of pyroelectric crystals is used to remove ice crystals and other particles from liquid nitrogen.. The method of construction is described. Objects weighing several hundred grams are attracted to and manipulated with this device at 77 K.

Keywords: nitrogen; electrostatics

pyroelectric crystals;

0011-2275/89/010070-02 $03.00 © 1989 Butterworth & Co (Publishers) Ltd

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Cryogenics 1989 Vol 29 January

Shubnikov-de-Haas oscillations, related to the filling of different Landau levels. This is the real limit in the use of our set-up at low temperatures. At 4.2 K it is possible to reach 3 T without oscillations in the output signal. We checked our set-up with a tubular niobium sample with a resistive ratio of RRR = 30. The dimensions of this sample were 13 mm external diameter, 10.4 mm internal diameter, and 26.4 mm length. The magnetization curve is shown in Figure 3. We observe diamagnetic behaviour (after correction for the demagnetization factor) until B,h= 1300G. Reasonable values for a cold worked sample of Be2 = 3500 G and Bc3 = 4500 G (related to surface effect) are observed. It should be noted that this type of set-up does not measure the magnetization but the difference between internal and external magnetic field. It has, therefore, been very suitable for measurements on the new high T~ superconducting materials which have a granular structure and give two contributions to the internal field (one related to a total body behaviour, the other related to the insulated grains). In our geometry these contributions are opposite in sign and easily distinguishable. Moreover, by varying the sample geometry or the positions of the Hall probes, it is possible, in principle, to cancel the contribution of the grains and thus obtain a clear and complete set of data. Work is in progress in this direction.

Reference 1 Kim, Y.B., Heml~tead, C.F, Strnad, A.R. Phys Rev (1963) 129 528

One of the difficulties encountered when maintaining liquid nitrogen in unpressurized dewars for long periods is the build-up of ice crystals and other solid impurities. This note describes a technique for continually collecting these particles in liquid nitrogen and their subsequent removal. Larger objects such as nylon spacers and metal washers may also be removed from liquid nitrogen dewars with this device in a manner analogous to collecting ferromagnetic materials with a magnet. -~ The device is made by encapsulating pyroelectric caesium nitrate in epoxy. Usually, pyroelectricity is manifested only when the temperature of the pyroelectric is changing and becomes electrically neutral when the temperature is held constant 1. Caesium nitrate belongs to a class of materials that will remain polarized at constant temperature in a cryogenic environment. One manifestation of the polarization is a strong electric field at each end of the polarized crystal. The polarization is spontaneous and occurs when the temperature is lowered to near 77 K. A device made from a single crystal will collect ice crystals and other particles at both ends. A polycrystalline device will collect the impurities over its entire surface. Single crystals of caesium nitrate are grown from an aqueous solution using standard crystal-growing techniques z. To make a device using a single crystal, coat a

Research and technical notes 5 mm or longer crystal with a nonconducting epoxy such as Stycast* and place it on a small sheet of polyethylene (such as Glad Cling Wrap) t to cure. To make a device out of polycrystalline material, thoroughly mix one part of epoxy with two parts of caesium nitrate. Compress the mixture between two flat plates covered with polyethylene to a thickness of approximately 1 mm. Small washers may be used as spacers. A total premixed volume of approximately 3 cm 3 will make a disc 1 mm thick and 4 cm diameter. Exact measurements are not necessary. After the epoxy has cured for 12-24 h at room temperature or 30 rain at 90°C, separate the plates and peel off the polyethylene. The pyroelectric disc may now be cut or drilled as desired. Several discs may be used to form a collector. The discs are suspended in dewars from strings to collect impurities or ice crystals as they form. The collector is withdrawn periodically to remove the impurities. The frequency of removal depends on the rate of ice build-up;

collectors have been left in place for more than three months with no noticeable deterioration of their effective-

*Trademark of Emmerson and Cuming Inc., 77 Dragon Court, Woburn, MA 01888, USA tTrademark of First Brands Corp., 39 Old Ridgebury Road, Danbury, CT 06817, USA

1 Lang, Sidney B. Sourcebook of Pyroelectricity Gordon and Breach Science Publishers, New York, USA (1974) 2 Holden, Alaa and Singer, Phylls Crystals and Crystal Growing Doubleday and Company, Inc., USA (1960)

Capillary flow impedances for the dilution refrigerator

dilution refrigerator, typical values for the impedance on the filling line of the He4 pot, for the main condense impedance on the He 3 input line just below the He4 pot and for the secondary condense impedance on the He 3 input line just below the still are 1012/cm3, I0 ~1/cm3 and 10t 2/cm3, respectively. The impedance of a capillary may be measured by pressurizing it with 1 atm of helium gas and by measuring the displaced water inside an inverted calibrated cylinder for a given time period; the displaced water results from the escaping gas bubbles leaving the end of the capillary tubing. According to Landau and LifshitzI

W. Wei and R. Rosenbaum* University of Southern California, University Park Campus, Department of Physics, Los Angeles, California, 90089-0484, USA Received 5 April 1988

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To manipulate large items in liquid nitrogen it is often convenient to use discs of 2-5 cm diameter with a small vertical dowel attached to their centres with epoxy. A 5-cm disc will lift 400-500 g of metal and several grams of a good insulator such as Teflon. To attain high lift ratios, the pyroelectric disc must make contact with a flat surface of the object after they both reach 77 K.

Acknowledgement I wish to acknowledge the many helpful discussions with Sol Raboy and Noel Yeh.

References

2 2 Pin -- Pout

A very simple technique for making capillary-wire flow impedances for the dilution refrigerator is described.

Keywords: refrigerators; helium; flow impedances Capillary impedances are used in the dilution refrigerator to restrict the flow of liquid He 4 to the evaporator (pot) or to condense the He 3 gas at the He'* pot and at the still. The impedances required are quite high (101 t/cm310t2/cm 3) and require the use of very small diameter Cu-Ni tubing. Often these capillaries become plugged with dirt particles or residual solder-flux particles or with pump oil that has diffused into the pumping lines of the refrigerator. It is not uncommon to have to replace these flow impedances every few years. For a small 150/~mol/s *Permanent address: TeI-Aviv University, School of Physics and Astronomy, Ramat-Aviv, 69978, Israel 0011-2275/89/010071-02 $03.00 © 1989 Butterworth & Co (Publishers) Ltd

(1)

Z(l/cm3) = A P'r/H=g,,Po,t where Pi, is the pressure in dynes/cm2 at the input of the impedance, Poutis the ambient room temperature pressure in dynes/cm2 at the end of the capillary, AI;' is the helium gas flow rate in cm3/s through the capillary and r/a, p, = 194 x 10 -6 dynes s/cm 2 (Poise) is the viscosity of the He 4 gas at room temperature. Wheatley et al. have given an alternative expression2: Z(l/cm 3) = 10s( 1 + A P / 2 P o , O A P ( m m / H g ) 14.5A l?(cm3/s)

(2)

where AP = Pin - Poet" Wheatley et al. have suggested that a capillary flow impedance made by passing a tightly fitting wire into the capillary channel tends to plug less often than a capillary tube with a single central channel because the annular area between the wire and tubing is more difficult to block than a single circular cross-sectional area 2. The capillary-wire flow impedance is usually fabricated

Cryogenics 1989 Vol 29 January 71