稀释制冷机为什么能够抵达毫开尔文温区，其中氦-3溶入超流氦-4的过程为何伴随吸热？
How does a dilution refrigerator reach the millikelvin regime, and why does dissolving helium-3 into superfluid helium-4 absorb heat?
即使在接近绝对零度时氦-3仍能溶入超流氦-4，但溶解态的氦-3准粒子所含焓高于纯氦-3液体中的对应值。氦-3原子跨越相界面进入稀释相时必须从环境吸取这份焓差，效果等同于蒸发制冷，却不受蒸气压随温度骤降至零的制约。把稀释相不断抽出、纯化并回注循环，机器便能连续泵热，稳定停留在几毫开尔文以下。
Helium-3 stays soluble in superfluid helium-4 even close to absolute zero, yet each dissolved atom carries more enthalpy than an atom in the concentrated helium-3 liquid. Crossing the phase boundary therefore draws the enthalpy difference out of the surroundings, mimicking evaporative cooling without suffering from vapor pressure that collapses toward zero at low temperature. Continuously extracting, purifying, and returning the dilute stream lets the machine pump heat steadily down to a few millikelvin.
难度 专业 · 适合 高中(15-18)
CC BY 4.0 · 已核验 · Verified 2026-10-04
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F. Pobell 所著《Matter and Methods at Low Temperatures》第3版，Springer
F. Pobell, Matter and Methods at Low Temperatures, 3rd ed., Springer
独立核验依据 · Independent verification: 核验依据 1 ISBN 9783540537519 (2007)
#氦-3/氦-4混合物 #稀释制冷
只有机械泵和一池液氦时,为什么对杜瓦减压抽气就能降到 4.2 K 以下,却怎么也冻不出固态氦?
With just a mechanical pump and a helium bath, why does pumping on the dewar get below 4.2 K, yet solid helium never appears?
减压抽气降低了液面上方的饱和蒸气压,液体便在更低温度下沸腾蒸发,蒸发潜热持续带走热量,温度可降至 1 K 量级。⁴He 即使接近绝对零度仍保持液态,只有外加约 25 个大气压才会固化,所以沿自身饱和蒸气压曲线降温永远得不到固态氦。同样的原理让 ³He 减压制冷能进一步到达约 0.3 K。
Pumping lowers the saturated vapor pressure above the bath, so helium boils at lower temperature and the latent heat of evaporation carries heat away until roughly 1 K. ⁴He stays liquid even near absolute zero and solidifies only under about 25 bar of applied pressure, so cooling along its own vapor-pressure curve can never freeze it. The same principle lets pumped ³He refrigerators reach about 0.3 K.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-10-04
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F. 波贝尔《低温物理与方法》减压液池章节实验任务
F. Pobell, Matter and Methods at Low Temperatures, pumped bath task
独立核验依据 · Independent verification: 核验依据 1 ISBN 9783540537519 (2007)
#低温制冷 #减压液氦降温
超导块材为什么能稳稳悬在磁轨上方,普通抗磁性材料却做不到?
Why can a bulk superconductor sit stably above a magnet track while ordinary diamagnets cannot?
迈斯纳效应使超导体内部磁感应为零,表面屏蔽电流排斥外磁场从而产生悬浮升力。真正让它锁在原位的是磁通钉扎:第二类超导体中的缺陷钉住量子化磁通线,使悬浮体在竖直和水平方向偏离后都受到回复力。普通抗磁体只有微弱感生磁化,升力不足且没有自稳机制,只能被磁场轻微推开。这种自稳定性正是超导磁浮演示和高温超导轴承的基础。
The Meissner effect keeps the superconductor interior field-free, and surface screening currents repel the magnet to provide lift. What actually locks the levitation is flux pinning: defects in a type-II superconductor pin quantized flux lines, giving restoring forces against vertical or horizontal displacement. An ordinary diamagnet has only weak induced magnetization—too little lift and no self-stability—and is merely pushed aside. This self-stability underlies superconducting maglev demonstrations and HTS bearings.
难度 进阶 · 适合 大众
CC BY 4.0 · 已核验 · Verified 2026-11-02
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张裕恒《超导物理》第二类超导体磁化章节任务
Zhang Yuheng, Superconducting Physics, type-II superconductor magnetization task
独立核验依据 · Independent verification: 核验依据 1 ISBN 9787312021770（豆瓣书目条目: 超导物理）
#超导磁悬浮 #磁通钉扎
液氦-4越过λ点进入超流态后，为什么热量不再主要依靠普通对流输运，而是以温度波的形式传播，即所谓第二声？
Once liquid helium-4 crosses the lambda point into the superfluid state, why does heat stop being carried by ordinary convection and instead propagate as a temperature wave known as second sound?
λ点以下的氦-4可视为正常组分与超流组分的叠加，熵只由正常组分携带。局部受热时超流成分流向热源、正常成分反向退开，这种相对流动把能量以波的方式输送出去。因此第二声本质上是熵波而非压强波，其传播速度强烈依赖温度，并在约1.8 K附近出现明显峰值。
Below the lambda point helium-4 behaves as interpenetrating normal and superfluid components, with entropy carried only by the normal part. A local temperature rise sends the superfluid component toward the heat source while the normal component recedes, and this counterflow transports energy as a wave. Second sound is thus an entropy wave rather than a pressure wave, with a velocity that depends strongly on temperature and peaks near 1.8 K.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-10-04
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J. Wilks 与 D. S. Betts 合著《An Introduction to Liquid Helium》第2版，牛津大学出版社
J. Wilks and D. S. Betts, An Introduction to Liquid Helium, 2nd ed., Oxford University Press
独立核验依据 · Independent verification: 核验依据 1 ISBN 9780198514206 (1987)
#二流体模型 #第二声
给盛放超流氦的容器局部加热，液氦会沿细管逆着重力爬升形成喷泉——是什么样的内部结构让温差变成了驱动力？
Heat one spot of a vessel holding superfluid helium and the liquid climbs a capillary against gravity into a fountain—what internal structure converts a temperature difference into driving force?
Tisza与Landau的二流体图景把He II看作正常分量与零熵超流分量的相互贯穿混合物。极窄的毛细管只允许无黏性的超流组分通过，受热端正常组分占比升高时，超流组分会穿过超漏涌向热端以恢复局域配比，结果热端质量堆积、静水压上升。定量关系Δp = ρSΔT 把温差换算成压差，这就是热机械效应，也正是喷泉的泵送机制。
In the two-fluid picture of Tisza and Landau, He II behaves as interpenetrating normal and entropy-free superfluid components. Very narrow capillaries pass only the inviscid superfluid fraction, so whenever heating raises the local normal fraction, superfluid streams through the superleak into the heated region to restore the local composition, piling up mass and raising the static pressure at the hot end. The quantitative law Δp = ρSΔT turns temperature difference into mechanical head—the thermomechanical effect that actually pumps the fountain.
难度 进阶 · 适合 初中(12-15)
CC BY 4.0 · 已核验 · Verified 2026-10-04
来源 · Source
J. Wilks，《The Properties of Liquid and Solid Helium》，Oxford University Press（1967）
J. Wilks, The Properties of Liquid and Solid Helium, Oxford University Press (1967)
独立核验依据 · Independent verification: 核验依据 1 DOI 10.1016/0041-624x(68)90026-7
#二流体模型 #热机械效应