| 翻訳と辞書 |
| Helium cryogenics ： ウィキペディア英語版 |
Helium cryogenics
In the field of cryogenics, helium () is utilized for a variety of reasons. The combination of helium’s extremely low molecular weight and weak interatomic reactions yield interesting properties when helium is cooled below its critical temperature of 5.2 K to form a liquid. Even at absolute zero (0K), helium does not condense to form a solid. In this state, the zero point vibrational energies of helium are comparable to very weak interatomic binding interactions, thus preventing lattice formation and giving helium its fluid characteristics.〔Yang, Shengfu, and Andrew M. Ellis. "Helium Droplets: A Chemistry Perspective." Chemical Society Reviews 42.2 (2012): 472-84. Print.〕 Within this liquid state, helium has two phases referred to as helium I and helium II. Helium I displays thermodynamic and hydrodynamic properties of classical fluids, along with quantum characteristics. However, below its lambda point of 2.17 K, helium transitions to He II and becomes a quantum superfluid with zero viscosity.〔Woods, A. D B, and R. A. Cowley. "Structure and Excitations of Liquid Helium." Reports on Progress in Physics 36.9 (1973): 1135-231. Print.〕
Under extreme conditions such as when cooled beyond Tλ, helium has the ability to form a new state of matter, known as a Bose–Einstein condensate (BEC), in which the atoms virtually lose all their energy. Without energy to transfer between molecules, the atoms begin to aggregate creating an area of equivalent density and energy.〔Penrose, Oliver, and Lars Onsager. "Bose–Einstein Condensation and Liquid Helium." Physical Review 104.3 (1956): 576-84. Print.〕 From observations, liquid helium only exhibits super-fluidity because it contains isolated islands of BECs, which have well-defined magnitude and phase, as well as well-defined phonon–roton (P-R) modes.〔Haussmann, R. "Properties of a Fermi Liquid at the Superfluid Transition in the Crossover Region between BCS Superconductivity and Bose–Einstein Condensation." Physical Review B 49.18 (1994): 12975-2983. Print.〕 A phonon refers to a quantum of energy associated with a compressional wave such as the vibration of a crystal lattice while a roton refers to an elementary excitation in superfluid helium. In the BEC’s, the P-R modes have the same energy, which explains the zero point vibrational energies of helium in preventing lattice formation.〔Bossy, Jacques, Jonathan Pearce, Helmut Schober, and Henry Glyde. "Phonon–Roton Modes and Localized Bose–Einstein Condensation in Liquid Helium under Pressure in Nanoporous Media." Physical Review Letters 101.2 (2008): n. pag. Print.〕
When helium is below Tλ, the surface of the liquid becomes smoother, indicating the transition from liquid to superfluid.〔Charlton, T. R., R. M. Dalgliesh, O. Kirichek, S. Langridge, A. Ganshin, and P. V. E. Mcclintock. "Neutron Reflection from a Liquid Helium Surface." Low Temperature Physics 34.4 (2008): 316-19. Print.〕 Experiments involving neutron bombardment correlate with the existence of BEC’s, thereby confirming the source of liquid helium’s unique properties such as super-fluidity and heat transfer.〔〔Tsipenyuk, Yu. M., O. Kirichek, and O. Petrenko. "Small-angle Scattering of Neutrons on Normal and Superfluid Liquid Helium." Low Temperature Physics 39.9 (2013): 777. Print.〕
Though seemingly paradoxical, cryogenic helium systems can move heat from an area of relatively low temperature to an area of relatively high temperature. Though this phenomenon appears to violate the second law of thermodynamics, experiments have shown this to prevail in systems where the area of low temperature is constantly heated, and the area of high temperature is constantly cooled. It is believed this phenomenon is related to the heat associated with the phase change between liquid and gaseous helium.〔
==Applications==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
■ウィキペディアで「Helium cryogenics」の詳細全文を読む
スポンサード リンク
| 翻訳と辞書 : 翻訳のためのインターネットリソース |
Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.
|
|