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This 5 page Class Notes was uploaded by Mrs. Linda Wiegand on Friday September 18, 2015. The Class Notes belongs to PHY 4550 at University of Florida taught by Staff in Fall. Since its upload, it has received 6 views. For similar materials see /class/206780/phy-4550-university-of-florida in Physics 2 at University of Florida.
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Date Created: 09/18/15
Properties of Materials at Low Temperatures J G We Srmrtorrl Linanr Acaclemmr Cciilcl Introduction Many materials are unsuitable for low temperature use Material selection must always be done carefully A signi cant amount of material data at low temperatures is avai a e u Testing may be required Some Unsuitable Materials for Low Temperature Use a see sea V77a iee rt Manensitic stainless steels Undergoes ductile to brittle transition wlien cooled dow I l Carbon steels also becomes brittle Rubber Teflon and most plastics 77v hv fi Oigtiiwik Introduction Ase awkaewes Weigh a Material properties change sign antly with em cralurc 0 quot55 r 2 elhe variaii n orerrorm a simple average between known values a Kelvin K c 273 Cryogenics is generally de ned as lt lzo K Some Suitable Materials for ow Temperature Use b aeieiew Thermal Contraction a Large amounts ol39contractiaii can occur when materials are cooled to cryogenic temperatures onevclopment of interferences or gaps duc to dissimilar materials Olnci39eased strain and possible failure olmpnet on wiring OMost contraction occurs above 77 K mm 20m ra Warsml Why does fiber glass not shrink as much as a pure plastic on cooHng Integral w Thermal Contraction A It has no plastic in it B Glass shrinks very lime C It is m gic w Strength Of Materials a 77v ki v if a Tends to increase at low temperatures as long as there is no ductile to brittle transition 300 K values are more conservative 1 Slight improvement in fatigue properties at cryogenic temperatures Speci c Heat of Solids C QInAT 39139f In general at cryogenic temperatures C 5 decreases rapidly with decreasing temperature This has 2 important effects oSystems cool down faster as they get colder oAt cryogenic temperatures small heat leaks may cause large temperature rises Thermal Conductivity vrv v geist k Varies signi cantly with temperature Temperature dependence must be considered when calculating heat transfer rates mm m n s m u Heat Flow Fatalquot IMF Temperature um area grm 39mr Harm er 7 LEE MA 4 Ax T em condnnivigv Thermal Conductivity Mean Mrlirrlt39 anfm l 1 pm Penicil r uni I39ld it vul ill v deimn n Cu 3 cvdquot mgmm 39 a 1mm Ni 7 Amsmfr 39J rerdupl irily umber Thermal Conductivity 7 Solutioanse thermal cnnduclh nlegrais Conduction heat transfer may be written as Q G92 e where the geometry Factor G is de ned by w Thermal Conductivity 777477V4770 g 7777 7 The them39lal conductivity integrals are then e med as Ti 9 i l K T IT Advantages 0 Simple a Only end point temperatures are important The actual temperature distribution is not a 5 Wm wt 20 m Electrical Resistivity i i iva k Drups with temperature due tn reduc phonon electran scattering RRR resistivity at 300 K resistivity at 42 K An indication of material purity Superconductivity a separate course Me mini mi in39 Electrical Resistivity of Copper 5 Wm i Ebc onsasaldealGas w nmu C omiuwivixiay or quotv M and c my A n10quot T termini Electriml W b t i u l i m z 4k T Islinhet aimm39tlpe 511 0 E23 Wh cmmmilx mnzmw a mva m i WedemannanzLorentz Law ir menmi camim39vity r Seatnualnmducliv y L Lurenz rumba With quantum treatment a 245 x ID SWQ 1K1 Real values
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