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氦 2He
氫（非金屬） 氦（惰性氣體）
鋰（鹼金屬） 鈹（鹼土金屬） 硼（類金屬） 碳（非金屬） 氮（非金屬） 氧（非金屬） 氟（鹵素） 氖（惰性氣體）
鈉（鹼金屬） 鎂（鹼土金屬） 鋁（貧金屬） 矽（類金屬） 磷（非金屬） 硫（非金屬） 氯（鹵素） 氬（惰性氣體）
鉀（鹼金屬） 鈣（鹼土金屬） 鈧（過渡金屬） 鈦（過渡金屬） 釩（過渡金屬） 鉻（過渡金屬） 錳（過渡金屬） 鐵（過渡金屬） 鈷（過渡金屬） 鎳（過渡金屬） 銅（過渡金屬） 鋅（過渡金屬） 鎵（貧金屬） 鍺（類金屬） 砷（類金屬） 硒（非金屬） 溴（鹵素） 氪（惰性氣體）
銣（鹼金屬） 鍶（鹼土金屬） 釔（過渡金屬） 鋯（過渡金屬） 鈮（過渡金屬） 鉬（過渡金屬） 鎝（過渡金屬） 釕（過渡金屬） 銠（過渡金屬） 鈀（過渡金屬） 銀（過渡金屬） 鎘（過渡金屬） 銦（貧金屬） 錫（貧金屬） 銻（類金屬） 碲（類金屬） 碘（鹵素） 氙（惰性氣體）
銫（鹼金屬） 鋇（鹼土金屬） 鑭（鑭系元素） 鈰（鑭系元素） 鐠（鑭系元素） 釹（鑭系元素） 鉕（鑭系元素） 釤（鑭系元素） 銪（鑭系元素） 釓（鑭系元素） 鋱（鑭系元素） 鏑（鑭系元素） 鈥（鑭系元素） 鉺（鑭系元素） 銩（鑭系元素） 鐿（鑭系元素） 鎦（鑭系元素） 鉿（過渡金屬） 鉭（過渡金屬） 鎢（過渡金屬） 錸（過渡金屬） 鋨（過渡金屬） 銥（過渡金屬） 鉑（過渡金屬） 金（過渡金屬） 汞（過渡金屬） 鉈（貧金屬） 鉛（貧金屬） 鉍（貧金屬） 釙（貧金屬） 砈（類金屬） 氡（惰性氣體）
鍅（鹼金屬） 鐳（鹼土金屬） 錒（錒系元素） 釷（錒系元素） 鏷（錒系元素） 鈾（錒系元素） 錼（錒系元素） 鈽（錒系元素） 鋂（錒系元素） 鋦（錒系元素） 鉳（錒系元素） 鉲（錒系元素） 鑀（錒系元素） 鐨（錒系元素） 鍆（錒系元素） 鍩（錒系元素） 鐒（錒系元素） 鑪（過渡金屬） 𨧀（過渡金屬） 𨭎（過渡金屬） 𨨏（過渡金屬） 𨭆（過渡金屬） 䥑（預測為過渡金屬） 鐽（預測為過渡金屬） 錀（預測為過渡金屬） 鎶（過渡金屬） 鉨（預測為貧金屬） 鈇（貧金屬） 鏌（預測為貧金屬） 鉝（預測為貧金屬） 鿬（預測為鹵素） 鿫（預測為惰性氣體）
-
↑
氦
↓
氖
氫 ← 氦 → 鋰
外觀
無色氣體，高壓電場下發橙紅色光


氦的譜線
概況
名稱·符號·序數氦（helium）·He·2
元素類別稀有氣體
族·週期·區18·1·s
標準原子質量4.002602(2)
電子排布1s2
2
氦的電子層（2）
氦的電子層（2）
歷史
發現皮埃爾·讓森, 約瑟夫·諾曼·洛克耶（1868年）
分離威廉·拉姆齊，皮·特奧多爾·克利夫（英語：Per Teodor Cleve），尼爾斯·朗勒特（英語：Abraham Langlet）（1895年）
物理性質
物態氣態
密度（0 °C, 101.325 kPa）
0.1786 g/L
熔點時液體密度0.145 g·cm−3
沸點時液體密度0.125 g·cm−3
熔點(at 2.5 MPa) 0.95 K，−272.20 °C，−457.96 °F
沸點4.222 K，−268.928 °C，−452.070 °F
三相點2.177 K（−271 °C），5.043 kPa
臨界點5.1953 K，0.22746 MPa
熔化熱0.0138 kJ·mol−1
汽化熱0.0829 kJ·mol−1
比熱容5R/2 = 20.786 J·mol−1·K−1
蒸氣壓（(由ITS-90定義)）
壓/Pa 1 10 100 1 k 10 k 100 k
溫/K     1.23 1.67 2.48 4.21
原子性質
氧化態0
電負性無數據（鮑林標度）
電離能第一：2372.3 kJ·mol−1
第二：5250.5 kJ·mol−1
共價半徑28 pm
范德華半徑1 E-10 m pm
氦的原子譜線
雜項
晶體結構六方最密堆積
磁序抗磁性[1]
熱導率0.1513 W·m−1·K−1
聲速972 m·s−1
CAS號7440-59-7
同位素
主條目：氦的同位素
同位素 豐度 半衰期​（t1/2） 衰變
方式 能量​（MeV） 產物
3He 0.000137%* 穩定，帶1粒中子
4He 99.999863%* 穩定，帶2粒中子
  • 為大氣層中的數值；其它地方可能有所不同。

氦是一種化學元素，元素符號為He，原子序數為2. 它是一種無色、無臭、無味、無毒、惰性（英語：Inert）的單原子氣體（英語：monatomic gas），在元素周期表中位於稀有氣體的最上方。氦的熔點和沸點是所有元素中最低的，因此除某些極端情況外，氦均以氣體形態存在。

氦是原子量第二低的元素，也是已知宇宙中豐度第二高的元素。宇宙中氦的質量占宇宙元素總質量的24%，相當於所有原子序數更高的元素的總質量的12倍多；它在太陽和木星中的豐度和在宇宙中的豐度相近。氦-4每個核子的平均核束縛能遠高過排在氦之後的三個元素（鋰、鈹、硼），因此擁有很高的豐度。氦的高束縛能也可以解釋為什麼它既是核聚變產物也是放射性衰變的產物。宇宙中大多數的氦是氦-4，由宇宙大爆炸和恆星中的氫核聚變生成。

氦的英文名稱Helium 來自希臘神話中的太陽神赫利俄斯。法國天文學家皮埃爾·讓森在1868年一次日食時，在太陽光中發現了未知的黃色譜線，首次檢測到這種元素。英國的約瑟夫·諾曼·洛克耶同樣在這次日食中觀測到了這條黃色譜線，提出這條譜線來自一種新的元素，並為該元素命名。氦的正式發現是在1895年：兩位瑞典化學家皮·特奧多爾·克利夫（英語：Per Teodor Cleve）和尼爾斯·朗勒特（英語：Abraham Langlet）發現釔鈾礦（英語：cleveite）釋放了氦氣。1903年，美國部分地區的天然氣田中發現了大量的氦；美國至今仍是全球最大的氦生產國。

氦在低溫物理學中的使用（特別是用於超導磁體的冷卻）約占氦產量的四分之一，是氦最主要的一種用途；最主要的商業應用是在MRI掃描儀中。氦在工業上同樣有廣泛用途：氦可以用做增壓氣和充換氣，也可以做電弧焊或生長硅晶圓等過程中的保護氣。這些工業用途總共消耗氦產量的一半。氦可充在氣球和飛艇中作為上升氣，這種一種用途所占比例較少但較為知名。[2]由於氦的密度和空氣不同，吸入少量的氦會引發人聲的頻率和品質變化。在科研中，氦的兩種液相（氦I和氦II）的行為對量子力學的研究（尤其是超流體方面的研究）十分重要。關於接近絕對零度時的物質性質（例如超導）的研究同樣要使用氦。

氦在地球上相對稀有，僅占大氣層體積的0.00052%.今天地層中大多數氦是由重元素（例如鈾、釷）的天然α衰變產生的。衰變產生的α粒子就是氦-4的原子核，被天然氣捕獲。天然氣中的氦最多可占總體積的7%，可通過低溫分餾方法將其分離出來。氦是不可再生能源，如果將氦釋放到大氣層中，它會逃逸至太空。[3][4][5]

History 編輯

科學發現 編輯

首個證明氦存在的證據是太陽色球的發射光譜中的一條亮黃色譜線。1868年8月18日，法國天文學家皮埃爾·讓森在印度的貢土爾觀測日全食時，發現了這條波長為587.49 nm的譜線。[6][7]起初人們推測這條譜線來自鈉。同年10月20日，英國天文學家約瑟夫·諾曼·洛克耶在太陽光譜中發現了一條黃線。由於這條譜線的波長和夫朗和斐譜線中鈉產生的D1 線和 D2的波長相似，洛克耶將其命名為D3線。[8]他還提出這條譜線來自太陽上的一種尚未在地球上發現的元素。洛克耶和英國化學家愛德華·弗蘭克蘭以希臘語中的ἥλιος (helios，意為「太陽」)一詞，將這一元素命名為Helium.[9][10][11]

 
氦的譜線

1882年，意大利物理學家路易吉·帕爾米耶里（英語：Luigi Palmieri）在分析維蘇威火山的岩漿時發現了氦的D3線，這是氦在地球上的首次發現紀錄。[12]

 
地層氦的發現者威廉·拉姆齊爵士

1895年3月26日，蘇格蘭化學家威廉·拉姆齊爵士將釔鈾礦（英語：cleveite）（一種瀝青鈾礦，其質量的10%為 稀土元素）用酸處理，首次在地球上分離出氦。拉姆齊當時在尋找氬，他用硫酸處理礦物，分離出釋放出的氣體中的氮和氧。在剩下的氣體中，他發現了一條和太陽光譜中的 D3譜線吻合的黃色譜線。[8][13][14][15]洛克耶和英國物理學家威廉·克魯克斯鑑定了這一氣體樣品，證明了它是氦氣。同一年，兩位化學家皮·特奧多爾·克利夫（英語：Per Teodor Cleve）和尼爾斯·朗勒特（英語：Abraham Langlet）在瑞典烏普薩拉獨立從釔鈾礦中分離出氦；他們收集的氦足以測定這一元素的原子量。[7][16][17]在拉姆齊分離氦之前，美國地質化學家威廉·弗朗西斯·希爾布蘭德（英語：William Francis Hillebrand）同樣注意到一份瀝青鈾礦樣品中的一條不尋常的譜線，並從中分離出氦；但他認為這些譜線來自氮氣。他致以拉姆齊的賀信是科學史上「發現」和「鄰近發現」的一個有趣的例子。[18]

1907年，歐內斯特·盧瑟福與托馬斯·羅伊茲（英語：Thomas Royds）讓α粒子穿透玻璃壁進入真空管，向管中放電後觀察管內氣體的發射光譜，證明α粒子就是氦核。1908年，荷蘭物理學家海克·卡末林·昂內斯將氦冷卻至不到1K的低溫，從而首次製得液態氦。[19]他還試着將氦固化，但是氦沒有固、液、氣三相平衡的三相點，因此他的嘗試沒有成功。1926年，昂內斯的學生威廉·亨德里克·科索姆（英語：Willem Hendrik Keesom）在低溫下向氦加壓，製得了1 cm3的固態氦。[20]

1938年，蘇聯物理學家彼得·列昂尼多維奇·卡皮察發現氦-4在接近絕對零度時幾乎沒有粘度，從而發現了今天所說的超流體。[21]這一現象和玻色-愛因斯坦凝聚有關。1972年，美國物理學家道格拉斯·奧謝羅夫、戴維·李、以及羅伯特·科爾曼·理查森發現氦-3也有超流體現象，但所需的溫度比氦-4低得多。氦-3的超流體現象被認為和氦-3費米子配對形成玻色子有關，這種配對和超導體中電子形成的庫珀對類似。[22]

提取和使用 編輯

1903年，美國德克薩斯州德克斯特（英語：Dexter, Kansas）的一次鑽探開採出了一口無法燃燒的氣井。堪薩斯州的地質學家伊拉斯謨·霍沃思（英語：Erasmus Haworth）將取自這口井的氣體樣品帶回堪薩斯大學分析，在化學家漢密爾頓·凱迪（英語：Hamilton Cady）和大衛·麥克法蘭的幫助下，他發現該氣體中含有72%(體積分數，下同)氮氣、15%甲烷、1%氫氣和12%無法鑑定的氣體。[7][23]凱迪和麥克法蘭進一步分析後確認樣品中含有1.84%的氦。[24][25]這說明儘管氦在地球上十分稀少，在北美大平原地下卻有大量集中的儲藏，可作為天然氣的副產品提取出來。[26]

美國因此成為全球最大的氦生產國。在理查德·斯瑞弗（英語：Richard Threlfall）的建議下，美國海軍在一戰期間贊助了三家實驗性的小型制氦廠，以便給防空氣球提供不可燃、比空氣輕的提升氣。此前全世界製得的氦氣總共不到1立方米；而這三家工廠總共生產了5,700 m3 （200,000 立方英尺）純度為92%的氦。[8]其中一部分氣體用來填充美國海軍的C-7飛艇。1921年12月1日，C-7飛艇進行了處女航，從弗吉尼亞州的漢普頓錨地到華盛頓特區的伯林菲爾德（英語：Bolling Field），成為世界第一艘氦氣填充的飛艇。[27]

儘管一戰時還沒有用低溫氣體液化來純化氦氣的技術，氦氣的生產依舊持續。 一戰時期，氦氣主要作為浮升器中的提升氣使用；二戰時這方面的需求提升了。此外，二戰時對電弧焊所用的保護氦氣的需求也有提高。氦質譜儀（英語：helium mass spectrometer）在美國製造原子彈的曼哈頓計劃中起到了重要作用。[28]

1930年到1945年生產的氦氣純度約為98.3%(其餘為氮氣)，這一純度對飛艇而言已經足夠。1945年有少量的99.9%純氦氣被製備用於電弧焊。到1949年，已經可獲得商業使用量級的99.95%純氦氣了。[29]

1925年，美國政府在德克薩斯州阿馬里洛建立了美國國家氦氣儲備庫（英語：National Helium Reserve），以便為軍用飛艇和商業飛艇提供氦氣。[8]而當時德國的齊柏林飛艇（如興登堡號）不得不使用易燃的氫氣做為提升氣，有如下原因：氦氣的價格昂貴，美國壟斷了氦氣生產，而美國1927年通過的《氦氣管控法案》禁止美國出口氦氣。二戰後的氦氣需求有所收縮，但美國在50年代擴充了氦氣儲備庫，以保障太空競賽和冷戰期間生產液氫/液氧火箭燃料所需的液氦冷凍劑供應。1965年美國的氦氣使用量是戰時最高使用量的的八倍多。[30]

《氦氣管控法案1960年修正案》(公共法 86–777)通過後，美國礦業局（英語：United States Bureau of Mines）安排五家私有工廠從天然氣中收集氦。礦業局為這一「氦氣轉化」項目建造了一條長425-英里（684-公里）的輸氣管道，從堪薩斯州布什頓（英語：Bushton, Kansas）出發，將這些工廠和已經部分耗竭的克利夫賽德氣田（為政府所有，位於阿馬里洛附近）連接起來。工廠提取的氦-氮混合氣儲存在克利夫賽德氣田中，當需要時再提取出來進行純化。[31]

到了1995年，美國政府儲備了10億立方米的這種混合氣，為此欠下了14億美元的債務，促使美國議會在1996年決定逐步清空這一儲備。[7][32]這一決定的結果是《1996年氦氣私人化法》[33] (公共法 104–273) ，它指導美國內政部自2005年起出售儲備氣體，以清空儲備庫。[34]截至2012年，美國國家氦氣儲備庫（英語：National Helium Reserve）仍擁有全世界30%的氦氣。[35]儘管美國參議院通過的一項法案能讓儲備庫繼續出售氦氣，[36]預計到2018年，庫內的儲備才會耗盡。[35]其它的一些大的氦氣儲備位於美國堪薩斯州的雨果頓氣田（英語：Hugoton Natural Gas Area），以及位於該氣田附近，分布於德克薩斯州與俄克拉荷馬州的鍋柄（英語：panhandles）和堪薩斯州的天然氣田。

很長一段時間內，美國生產了全世界商業用氦氣的90%，其餘的10%由加拿大、波蘭、俄羅斯等國的工廠提供。20世紀90年代中葉，阿爾及利亞阿爾澤開辦了一家新的氦生產廠，產量達1700萬立方米，可滿足整個歐洲的需求。而到了2000年，美國國內的氦氣消費量已上漲至每年1.5萬噸。[37]2004-2006年，卡塔爾的拉斯拉凡港和阿爾及利亞的斯基克達建立了兩家新的氦廠。阿爾及利亞從此成為世界第二大氦生產國。[38]而這次，氦氣的消費量和生產成本都上漲了。[39] In the 2002 to 2007 period helium prices doubled.[40]

中國唯一的氦氣生產裝置位於四川威遠氣田，於20世紀70年代建成；氣田中的氦氣含量為0.2%.[41]

Characteristics 編輯

The helium atom 編輯

主條目：Helium atom
 
The helium atom. Depicted are the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.

Helium in quantum mechanics 編輯

In the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom. Helium is composed of two electrons in atomic orbitals surrounding a nucleus containing two protons along with some neutrons. As in Newtonian mechanics, no system consisting of more than two particles can be solved with an exact analytical mathematical approach (see 3-body problem) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Such computational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps.[42] In such models it is found that each electron in helium partly screens the nucleus from the other, so that the effective nuclear charge Z which each electron sees, is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.

The related stability of the helium-4 nucleus and electron shell 編輯

The nucleus of the helium-4 atom is identical with an alpha particle. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. Adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable). This arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature.

For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements.

In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions involving both heavy-particle emission, and fusion. Some stable helium-3 is produced in fusion reactions from hydrogen, but it is a very small fraction, compared with the highly favorable helium-4. The stability of helium-4 is the reason hydrogen is converted to helium-4 (not deuterium or helium-3 or heavier elements) in the Sun.[可疑] It is also partly responsible for the fact that the alpha particle is by far the most common type of baryonic particle to be ejected from atomic nuclei; in other words, alpha decay is far more common than cluster decay.

 
Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.

The unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the Big Bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. So tight was helium-4 binding that helium-4 production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and also leaving few to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus no energetic drive was available, once helium had been formed, to make elements 3, 4 and 5. It was barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon. However, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process). There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.

All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.

Gas and plasma phases 編輯

 
Helium discharge tube shaped like the element's atomic symbol

Helium is the second least reactive noble gas, after neon, and thus the second least reactive of all elements.[43] It is inert and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. For similar reasons, and also due to the small size of helium atoms, helium's diffusion rate through solids is three times that of air and around 65% that of hydrogen.[8]

Helium is the least water soluble monatomic gas,[44] and one of the least water soluble of any gas (CF4, SF6, and C4F8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5),[45] and helium's index of refraction is closer to unity than that of any other gas.[46] Helium has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its Joule-Thomson inversion temperature (of about 32 to 50 K at 1 atmosphere) does it cool upon free expansion.[8] Once precooled below this temperature, helium can be liquefied through expansion cooling.

Most extraterrestrial helium is found in a plasma state, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the solar wind together with ionized hydrogen, the particles interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora.[47]

Solid, liquid, and superfluid phases 編輯

Template:Sync

 
Liquefied helium. This helium is not only liquid, but has been cooled to the point of superfluidity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium. See superfluid.
主條目：Liquid helium

Unlike any other element, helium will remain liquid down to absolute zero at normal pressures. This is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about −272 °C or −457 °F) and about 25 bar (2.5 MPa) of pressure.[48] It is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same. The solid has a sharp melting point and has a crystalline structure, but it is highly compressible; applying pressure in a laboratory can decrease its volume by more than 30%.[49] With a bulk modulus of about 27 MPa[50] it is ~100 times more compressible than water. Solid helium has a density of 0.214 ± 0.006 g/cm3 at 1.15 K and 66 atm; the projected density at 0 K and 25 bar (2.5 MPa) is 0.187 ± 0.009 g/cm3.[51]

Helium I state 編輯

Below its boiling point of 4.22 kelvins and above the lambda point of 2.1768 kelvins, the isotope helium-4 exists in a normal colorless liquid state, called helium I.[8] Like other cryogenic liquids, helium I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further.

Helium I has a gas-like index of refraction of 1.026 which makes its surface so hard to see that floats of styrofoam are often used to show where the surface is.[8] This colorless liquid has a very low viscosity and a density of 0.145–0.125 g/mL (between about 0 and 4 K),[52] which is only one-fourth the value expected from classical physics.[8] Quantum mechanics is needed to explain this property and thus both types of liquid helium are called quantum fluids, meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion (thermal energy) from masking the atomic properties.[8]

Helium II state 編輯

Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. When helium II boils, due to its high thermal conductivity it does not bubble but rather evaporates directly from its surface. Helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope.[8]

 
Unlike ordinary liquids, helium II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. The Rollin film also covers the interior of the larger container; if it were not sealed, the helium II would creep out and escape.[8]

Helium II is a superfluid, a quantum mechanical state (see: macroscopic quantum phenomena) of matter with strange properties . For example, when it flows through capillaries as thin as 10−7 to 10−8 m it has no measurable viscosity.[7] However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the two-fluid model for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.[53]

In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.[54]

The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper.[8] This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8 K through helium II as waves in a phenomenon known as second sound.[8]

Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin.[8][55][56] As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the van der Waals force.[57] These waves are known as third sound.[58]

Isotopes 編輯

主條目：Isotopes of helium

There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth's atmosphere, there is one 3
He atom for every million 4
He atoms.[7] Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.[59]

Helium-3 is present on Earth only in trace amounts; most of it since Earth's formation, though some falls to Earth trapped in cosmic dust.[60] Trace amounts are also produced by the beta decay of tritium.[61] Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's mantle.[60] 3
He is much more abundant in stars, as a product of nuclear fusion. Thus in the interstellar medium, the proportion of 3
He to 4
He is around 100 times higher than on Earth.[62] Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon's surface contains helium-3 at concentrations on the order of 0.01 ppm, much higher than the ca. 5 ppt found in the Earth's atmosphere.[63][64] A number of people, starting with Gerald Kulcinski in 1986,[65] have proposed to explore the moon, mine lunar regolith and use the helium-3 for fusion.

Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling in a 1-K pot. Similar cooling of helium-3, which has a lower boiling point, can achieve about 0.2 kelvin in a helium-3 refrigerator. Equal mixtures of liquid 3
He and 4
He below 0.8 K separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: helium-4 atoms are bosons while helium-3 atoms are fermions).[8] Dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins.

It is possible to produce exotic helium isotopes, which rapidly decay into other substances. The shortest-lived heavy helium isotope is helium-5 with a half-life of 7.6×10−22 s. Helium-6 decays by emitting a beta particle and has a half-life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. Helium-7 and helium-8 are created in certain nuclear reactions.[8] Helium-6 and helium-8 are known to exhibit a nuclear halo.[8]

Compounds 編輯

參見：Noble gas compound
 
Structure of the helium hydride ion, HHe+.
 
Structure of the suspected fluoroheliate anion, OHeF-.

Helium has a valence of zero and is chemically unreactive under all normal conditions.[49] It is an electrical insulator unless ionized. As with the other noble gases, helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential.[8] Helium can form unstable compounds, known as excimers, with tungsten, iodine, fluorine, sulfur and phosphorus when it is subjected to a glow discharge, to electron bombardment, or else is a plasma for another reason. The molecular compounds HeNe, HgHe10, and WHe2, and the molecular ions He+
2, He2+
2, HeH+
, and HeD+
 have been created this way.[66] HeH+ is also stable in its ground state, but is extremely reactive—it is the strongest Brønsted acid known, and therefore can exist only in isolation, as it will protonate any molecule or counteranion it comes into contact with. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently held together only by polarization forces.[8] Theoretically, other true compounds may also be possible, such as helium fluorohydride (HHeF) which would be analogous to HArF, discovered in 2000.[67] Calculations show that two new compounds containing a helium-oxygen bond could be stable.[68] Two new molecular species, predicted using theory, CsFHeO and N(CH3)4FHeO, are derivatives of a metastable [F– HeO] anion first theorized in 2005 by a group from Taiwan. If confirmed by experiment, the only remaining element with no known stable compounds would be neon.[69]

Helium has been put inside the hollow carbon cage molecules (the fullerenes) by heating under high pressure. The endohedral fullerene molecules formed are stable up to high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside.[70] If helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy.[71] Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.

Occurrence and production 編輯

Natural abundance 編輯

Although it is rare on Earth, Helium is the second most abundant element in the known Universe (after hydrogen), constituting 23% of its baryonic mass.[7] The vast majority of helium was formed by Big Bang nucleosynthesis one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models. In stars, it is formed by the nuclear fusion of hydrogen in proton-proton chain reactions and the CNO cycle, part of stellar nucleosynthesis.[59]

In the Earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million.[72][73] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes.[74][75][76] In the Earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.

Most helium on Earth is a result of radioactive decay. Helium is found in large amounts in minerals of uranium and thorium, including cleveite, pitchblende, carnotite and monazite, because they emit alpha particles (helium nuclei, He2+) to which electrons immediately combine as soon as the particle is stopped by the rock. In this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere.[77][78][79] In the Earth's crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. Because helium is trapped in the subsurface under conditions that also trap natural gas, the greatest natural concentrations of helium on the planet are found in natural gas, from which most commercial helium is extracted. The concentration varies in a broad range from a few ppm up to over 7% in a small gas field in San Juan County, New Mexico.[80][81]

Modern extraction and distribution 編輯

For large-scale use, helium is extracted by fractional distillation from natural gas, which can contain up to 7% helium.[82] Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure Grade-A helium.[8] The principal impurity in Grade-A helium is neon. In a final production step, most of the helium that is produced is liquefied via a cryogenic process. This is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation, as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers.[38][83]

In 2008, approximately 169 million standard cubic meters (SCM) of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78% from the United States, 10% from Algeria, and most of the remainder from Russia, Poland and Qatar.[84] In the United States, most helium is extracted from natural gas of the Hugoton and nearby gas fields in Kansas, Oklahoma, and the Panhandle Field in Texas.[85][38] Much of this gas was once sent by pipeline to the National Helium Reserve, but since 2005 this reserve is presently being depleted and sold off.

Diffusion of crude natural gas through special semipermeable membranes and other barriers is another method to recover and purify helium.[86] In 1996, the U.S. had proven helium reserves, in such gas well complexes, of about 147 billion standard cubic feet (4.2 billion SCM).[87] At rates of use at that time (72 million SCM per year in the U.S.; see pie chart below) this is enough helium for about 58 years of U.S. use, and less than this (perhaps 80% of the time) at world use rates, although factors in saving and processing impact effective reserve numbers. It is estimated that the resource base for yet-unproven helium in natural gas in the U.S. is 31–53 trillion SCM, about 1000 times the proven reserves.[88]

Helium must be extracted from natural gas because it is present in air at only a fraction of that of neon, yet the demand for it is far higher. It is estimated that if all neon production were retooled to save helium, that 0.1% of the world's helium demands would be satisfied. Similarly, only 1% of the world's helium demands could be satisfied by re-tooling all air distillation plants.[89] Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this process is a completely uneconomic method of production.[90]

Helium is commercially available in either liquid or gaseous form. As a liquid, it can be supplied in small insulated containers called dewars which hold up to 1,000 liters of helium, or in large ISO containers which have nominal capacities as large as 42 m3 (around 11,000 U.S. gallons). In gaseous form, small quantities of helium are supplied in high-pressure cylinders holding up to 8 m3 (approx. 282 standard cubic feet), while large quantities of high-pressure gas are supplied in tube trailers which have capacities of up to 4,860 m3 (approx. 172,000 standard cubic feet).

Conservation advocates 編輯

According to helium conservationists like Robert Coleman Richardson, the free market price of helium has contributed to "wasteful" usage (e.g. for helium balloons). Prices in the 2000s have been lowered by U.S. Congress' decision to sell off the country's large helium stockpile by 2015.[91] According to Richardson, the current price needs to be multiplied by 20 to eliminate the excessive wasting of helium. In their book, the Future of helium as a natural resource (Routledge, 2012), Nuttall, Clarke & Glowacki (2012) also proposed to create an International Helium Agency (IHA) to build a sustainable market for this precious commodity.[92]

Applications 編輯

 
The largest single use of liquid helium is to cool the superconducting magnets in modern MRI scanners.
File:HeliumUsePieChart1996.jpg
Estimated U.S. fractional helium use by category, by the United States Geological Survey, in 1996. Most of the cryogenic use is for superconducting MRI magnets.

While balloons are perhaps the most well-known use of helium, they are a minor part of all helium use.[32] Helium is used for many purposes that require some of its unique properties, such as its low boiling point, low density, low solubility, high thermal conductivity, or inertness. Of the 2008 world helium total production of about 32 million kg (193 million standard cubic meters) helium per year, the largest use (about 22% of the total in 2008) is in cryogenic applications, most of which involves cooling the superconducting magnets in medical MRI scanners.[93] Other major uses (totalling to about 60% of use in 1996) were pressurizing and purging systems, maintenance of controlled atmospheres, welding, and leak detection. Other uses by category were relatively minor fractions.[94]

Controlled atmospheres 編輯

Helium is used as a protective gas in growing silicon and germanium crystals, in titanium and zirconium production, and in gas chromatography,[49] because it is inert. Because of its inertness, thermally and calorically perfect nature, high speed of sound, and high value of the heat capacity ratio, it is also useful in supersonic wind tunnels[95] and impulse facilities.[96]

Gas tungsten arc welding 編輯

主條目：gas tungsten arc welding

Helium is used as a shielding gas in arc welding processes on materials that at welding temperatures are contaminated and weakened by air or nitrogen.[7] A number of inert shielding gases are used in gas tungsten arc welding, but helium is used instead of cheaper argon especially for welding materials that have higher heat conductivity, like aluminium or copper.

Minor uses 編輯

Industrial leak detection 編輯

 
A dual chamber helium leak detection machine

One industrial application for helium is leak detection. Because helium diffuses through solids three times faster than air, it is used as a tracer gas to detect leaks in high-vacuum equipment (such as cryogenic tanks) and high-pressure containers.[97] The tested object is placed in a chamber, which is then evacuated and filled with helium. The helium that escapes through the leaks is detected by a sensitive device (helium mass spectrometer), even at the leak rates as small as 10−9 mbar·L/s (10−10 Pa·m3/s). The measurement procedure is normally automatic and is called helium integral test. A simpler procedure is to fill the tested object with helium and to manually search for leaks with a hand-held device.[98]

Helium leaks through cracks should not be confused with gas permeation through a bulk material. While helium has documented permeation constants (thus a calculable permeation rate) through glasses, ceramics, and synthetic materials, inert gases such as helium will not permeate most bulk metals.[99]

Flight 編輯

 
Because of its low density and incombustibility, helium is the gas of choice to fill airships such as the Goodyear blimp.

Because it is lighter than air, airships and balloons are inflated with helium for lift. While hydrogen gas is approximately 7% more buoyant,[來源請求] helium has the advantage of being non-flammable (in addition to being fire retardant). Another minor use is in rocketry, where helium is used as an ullage medium to displace fuel and oxidizers in storage tanks and to condense hydrogen and oxygen to make rocket fuel. It is also used to purge fuel and oxidizer from ground support equipment prior to launch and to pre-cool liquid hydrogen in space vehicles. For example, the Saturn V booster used in the Apollo program needed about 370,000 m3 (13 million cubic feet) of helium to launch.[49]

Minor commercial and recreational uses 編輯

For its low solubility in nervous tissue, helium mixtures such as trimix, heliox and heliair are used for deep diving to reduce the effects of narcosis.[100][101] At depths below 150公尺（490英尺） small amounts of hydrogen[來源請求] are added to a helium-oxygen mixture to counter the effects of high-pressure nervous syndrome.[102] At these depths the low density of helium is found to considerably reduce the effort of breathing.[103]

Helium-neon lasers, a type of low-powered gas laser producing a red beam, had various practical applications which included barcode readers and laser pointers, before they were almost universally replaced by cheaper diode lasers.[7]

For its inertness and high thermal conductivity, neutron transparency, and because it does not form radioactive isotopes under reactor conditions, helium is used as a heat-transfer medium in some gas-cooled nuclear reactors.[97]

Helium, mixed with a heavier gas such as xenon, is useful for thermoacoustic refrigeration due to the resulting high heat capacity ratio and low Prandtl number.[104] The inertness of helium has environmental advantages over conventional refrigeration systems which contribute to ozone depletion or global warming.[105]

Scientific uses 編輯

The use of helium reduces the distorting effects of temperature variations in the space between lenses in some telescopes, due to its extremely low index of refraction.[8] This method is especially used in solar telescopes where a vacuum tight telescope tube would be too heavy.[106][107]

Helium is a commonly used carrier gas for gas chromatography.

The age of rocks and minerals that contain uranium and thorium can be estimated by measuring the level of helium with a process known as helium dating.[7][8]

Helium at low temperatures is used in cryogenics, and in certain cryogenics applications. As examples of applications, liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging. The Large Hadron Collider at CERN uses 96 metric tons of liquid helium to maintain the temperature at 1.9 kelvin.[108]

Inhalation and safety 編輯

Neutral helium at standard conditions is non-toxic, plays no biological role and is found in trace amounts in human blood.

 
Effect of helium on a human voice
The effect of helium on a human voice
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The speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract.[7][109] The fundamental frequency (sometimes called pitch) does not change, since this is produced by direct vibration of the vocal folds, which is unchanged.[110] However, the higher resonant frequencies cause a change in timbre, resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon.

Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration.[7][111] Breathing pure helium continuously causes death by asphyxiation within minutes. This fact is utilized in the design of suicide bags.

Inhaling helium directly from pressurized cylinders is extremely dangerous, as the high flow rate can result in barotrauma, fatally rupturing lung tissue.[111][112] However, death caused by helium is rare, with only two fatalities reported between 2000 and 2004 in the United States.[112] However, there were two cases in 2010, one in the USA[113] in January and another in Northern Ireland in November.[114] An Oregon girl died in 2012 from barotrauma,[115] and an another girl from hypoxia later in the year.[116]

The safety issues for cryogenic helium are similar to those of liquid nitrogen; its extremely low temperatures can result in cold burns and the liquid-to-gas expansion ratio can cause explosions if no pressure-relief devices are installed. Containers of helium gas at 5 to 10 K should be handled as if they contain liquid helium due to the rapid and significant thermal expansion that occurs when helium gas at less than 10 K is warmed to room temperature.[49]

At high pressures (more than about 20 atm or two MPa), a mixture of helium and oxygen (heliox) can lead to high-pressure nervous syndrome, a sort of reverse-anesthetic effect; adding a small amount of nitrogen to the mixture can alleviate the problem.[117][118]

See also 編輯

References 編輯

  1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
  2. ^ Helium: Up, Up and Away? Melinda Rose, Photonics Spectra, Oct. 2008. Accessed Feb 27, 2010.
  3. ^ Connor, Steve. Why the world is running out of helium – Science – News. The Independent. 2010-08-23 [2013-09-16]. 
  4. ^ Posted by Ethan on December 12, 2012. Why the World Will Run Out of Helium – Starts With A Bang. Scienceblogs.com. 2012-12-12 [2013-09-16]. 
  5. ^ Witchalls, Clint (18 August 2010) Nobel prizewinner: We are running out of helium. New Scientist. 
  6. ^ Kochhar, R. K. French astronomers in India during the 17th – 19th centuries. Journal of the British Astronomical Association. 1991, 101 (2): 95–100. Bibcode:1991JBAA..101...95K. 
  7. ^ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 Emsley, John. Nature's Building Blocks. Oxford: Oxford University Press. 2001: 175–179. ISBN 0-19-850341-5. 
  8. ^ 8.00 8.01 8.02 8.03 8.04 8.05 8.06 8.07 8.08 8.09 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 8.21 8.22 Clifford A. Hampel. The Encyclopedia of the Chemical Elements. New York: Van Nostrand Reinhold. 1968: 256–268. ISBN 0-442-15598-0. 
  9. ^ Sir Norman Lockyer – discovery of the element that he named helium" Balloon Professional Magazine, 7 August 2009.
  10. ^ Helium. Oxford English Dictionary. 2008 [2008-07-20]. 
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