ハフニウム

外観

灰色, 粉末又は塊

溶解性

ふっ化水素酸に溶け、王水に徐々に溶けるが、塩酸、硫酸及び硝酸にほとんど溶けない。

主な性質

1. 単体ハフニウムは灰色の重い金属で延展性に富んでいる
2. ハフニウムは空気中で表面に丈夫な薄酸化膜ができる（耐食性に非常に優れる）
3. ハフニウムは通常の酸には溶けない（酸化力のある酸には溶ける）
4. ハフニウムは機械的性質に優れ、耐熱性にも優れている
5. ハフニウムは中性子吸収率が極めて大きく、中性子照射に強い（原子力の制御棒として用いられる）。ジルコニウムは中性子吸収率が低く、制御棒の被覆に使われているのと好対象

解説

ハフニウム,Hf．原子番号72の元素．電子配置[Xe]4f 145d26s2の周期表4族遷移金属元素．原子量178.49(2)．天然同位体存在比は 174Hf 0.16(1)％，176Hf 5.26(7)％，177Hf 18.60(9)％，178Hf 27.28(7)％，179Hf 13.62(2)％，180Hf 35.08(16)％．質量数153～188までの放射性核種がつくられている．1923年，G. Hevesy(ヘベシー)とD. CosterがX線分光分析法によってジルコニウム鉱物(ジルコン)中から発見した．元素名は発見の実験が行われたデンマークのコペンハーゲンのラテン語名“Hafnia”から．元素記号も名称から．

用途

白熱電球のフィラメント、X線管球の陰極、高圧放電管の極、TI-HF合金の形でのゲッター、整流器

用途

原子力の制御棒。X線管球の陰極。高圧放電管の極。合金材料。

主な用途

1. 原子炉制御棒（原子炉部品）
2. Ｘ線電流管、電圧放電管（電子、光学機器）
3. 超強力耐熱合金（航空機用ジェットエンジン部品）
4. 高速切削バイト（機械加工工具）

存在

地殻中の存在度3 ppm．固有の鉱物はなく，親縁元素のジルコニウムの鉱物，ジルコン，バッデレイ石中につねに存在する．ジルコンがハフニウムの主要鉱物でジルコニウムの1～4％ 存在する．主要資源国は，ジルコニウムの資源国と同じで，オーストラリア，南アフリカで世界の全埋蔵量の80％ を占める．日本は全量，金属を輸入している．同族のジルコニウムとの分離は困難で，Hevesyらは，フッ化カリウムまたはフッ化アンモニウムとの複塩の再結晶の繰り返しによって成功した．現在はイオン交換クロマトグラフィー，チオシアナト錯塩の溶媒抽出で分離する．工業的には，溶媒抽出法，四塩化物のKCl-AlCl3溶融塩溶液からの蒸留などの方法がある．金属は，歴史的にはvan Arkelとde Boerによって四ヨウ化物の蒸気をタングステンフィラメント上で還元して得られたが，現在は四塩化物のマグネシウムによる還元で得られる(Kroll法)．

性質

純粋な金属は六方晶系構造で，銀白色光沢を示し，軟らかい．密度13.310 g cm－3(6.506 g cm－3)(ともに20 ℃)．融点2230 ℃(1852 ℃)，沸点5197 ℃(4377 ℃)．第一イオン化エネルギー654.1 kJ mol－1(659.9 kJ mol－1)．原子半径0.156 nm(0.159 nm)．イオン半径0.085 nm(六配位)(0.086 nm)．(　)内はジルコニウム．ジルコニウムとの分離が困難な理由は，ランタノイド収縮により半径がほぼ等しく，化学的性質もよく似ているためである．高温で酸素，窒素，硫黄と反応する．700 ℃ 以上で水素を吸蔵してHf H1.86 の化合物をつくる．加熱するとハロゲンと反応して四ハロゲン化物をつくる．通常は酸化数2～4．最高酸化数6．冷鉱酸，アルカリ水溶液には侵されない．常温で水と反応しない．

化学的特性

Hafnium is a refractory metal which occurs in nature in zirconium minerals.

物理的性質

Hafnium is a ductile metal that looks and feels much like stainless steel, but it is significantlyheavier than steel. When freshly cut, metallic hafnium has a bright silvery shine. Whenthe fresh surface is exposed to air, it rapidly forms a protective oxidized coating on its surface.Therefore, once oxidized, hafnium resists corrosion, as do most transition metals, whenexposed to the air. Chemically and physically, hafnium is very similar to zirconium, whichis located just above it in group 4 on the periodic table. In fact, they are so similar that it isalmost impossible to secure a pure sample of either one without a small percentage of theother. Each will contain a small amount of the other metal after final refining.
Hafnium’s melting point is 2,227°C, its boiling point varies from about 2,500°C to5,000°C depending on its purity, and its density is 13.29 g/cm3. The compound hafniumnitride (HfN) has the highest melting point (over 3,300°C) of any two-element compound.

同位体

There are 44 known isotopes for hafnium. Five are stable and one of the unstableisotopes has such a long half-life (Hf-174 with a 2.0×10⁺¹⁵ years) that it is includedas contributing 0.16% to the amount of hafnium found in the Earth’s crust. The percentagecontributions of the 5 stable isotopes to the element’s natural existence on Earth areas follows: Hf-176 = 5.26%, Hf-177 = 18.60%, Hf-178 = 27.28%, Hf-179 = 13.62%,and Hf-180 = 35.08%.

名前の由来

Named after Hafnia, the Latin name for the city of Copenhagen, Denmark.

天然物の起源

Hafnium is the 47th most abundant element on Earth. Thus, it is more abundant thaneither gold or silver. Because hafnium and zirconium are always found together in nature, bothmetals are refined and produced by the Kroll process. Pure samples of either hafnium or zirconiumare almost impossible to separate by the Kroll or other refining processes. Baddeleyite(ZrO₂), a zirconium ore, and zircon (ZrSiO₄) are treated with chlorine along with a carboncatalyst that produces a mixture of zirconium and hafnium tetrachlorides. These are reducedby using sodium or magnesium, resulting in the production of both metals. The molten metalsare separated by the process known as fractionation, which depends on their different meltingpoints and densities. As the mixture of the two metals cools during the fractionation process,the denser solidified hafnium sinks to the bottom of the vessel while the less dense zirconium(with a higher melting point than hafnium) floats on top.

特性

As the first element in the third series of the transition elements, hafnium’s atomic number(₇₂Hf ) follows the lanthanide series of rare-earths. The lanthanide series is separated out ofthe normal position of sequenced atomic numbers and is placed below the third series on theperiodic table (₅₇La to ₇₁Li). This rearrangement of the table allowed the positioning of elementsof the third series within groups more related to similar chemical and physical characteristics—for example, the triads of Ti, Zr, and Hf; V, Nb, and Ta; and Cu, Ag, and Au.

使用

Hafnium has a great affinity for absorbing slow neutrons. This attribute, along with itsstrength and resistance to corrosion, makes it superior to cadmium, which is also used formaking control rods for nuclear reactors. This use is of particular importance for the type ofnuclear reactors used aboard submarines. By moving the control rods in and out of a nuclearreactor, the fission chain reaction can be controlled as the neutrons are absorbed in the metalof the rods. The drawback to hafnium control rods is their expense: it costs approximately onemillion dollars for several dozen rods for use in a single nuclear reactor.
In vacuum tubes and other applications that must have gases removed, hafnium is used asa “getter” to absorb any trace oxygen or nitrogen in the tube, thus extending the life of thevacuum tube. Hafnium’s qualities also make it ideal for filaments in light bulbs and, whenmixed with rare-earth metals, as a “sparking” misch metal. Hafnium is also used to a lesserextent as an alloying agent for several other metals, including iron, titanium, and niobium.

定義

hafnium: Symbol Hf. A silvery lustrousmetallic transition element;a.n. 72; r.a.m. 178.49; r.d. 13.3; m.p.2227±20°C; b.p. 4602°C. The elementis found with zirconium and is extractedby formation of the chlorideand reduction by the Kroll process. Itis used in tungsten alloys in filamentsand electrodes and as a neutron absorber.The metal forms a passiveoxide layer in air. Most of its compoundsare hafnium(IV) complexes;less stable hafnium(III) complexesalso exist. The element was first reportedby Urbain in 1911, and its existencewas finally established by Dirk Coster (1889–1950) and Georgede Hevesey (1885–1966) in 1923.

用途

用途は，熱中性子吸収断面積がいちじるしく大きい(Zrの0.185 barn に対して105 barn，1 barn ＝ 10－28 m2)ので，原子炉の制御棒に用いられる．ジェットエンジンや発電用ガスタービンのブレードに用いられる超強力耐熱合金や，高温用セラミックの材料，酸素，窒素のゲッターなどとしても用いられる．炭化ハフニウムはプラズマ電極，高圧放電管電極材料，切削用工具などに使われる．超合金用の需要がもっとも多い．「ハフニウム及びその化合物」は，労働安全衛生法第57条2第1項に定める「名称等を通知すべき有害物」である．

一般的な説明

HAFNIUM, is a grayish metallic colored powder. Dust from dry powder may be ignited by static electricity. The dry powder reacts with moisture to produce hydrogen, a flammable gas. The heat from this reaction may be sufficient to ignite the hydrogen. HAFNIUM does not appreciably react with large quantities of water.

空気と水の反応

Highly flammable. The dry powder reacts with moisture to produce hydrogen, a flammable gas. The heat from this reaction may be sufficient to ignite the hydrogen. HAFNIUM does not appreciably react with large quantities of water.

反応プロフィール

Metals, such as HAFNIUM METAL(reactivity similar to zirconium), are reducing agents and tend to react with oxidizing agents. Their reactivity is strongly influenced by their state of subdivision: in bulk they often resist chemical combination; in powdered form they may react very rapidly. Thus, as a bulk metal HAFNIUM is somewhat unreactive, but finely divided material may be pyrophoric. The metal reacts exothermically with compounds having active hydrogen atoms (such as acids and water) to form flammable hydrogen gas and caustic products. The reactions are less vigorous than the similar reactions of alkali metals, but the released heat can still ignite the released hydrogen. Materials in this group may react with azo/diazo compounds to form explosive products. These metals and the products of their corrosion by air and water can catalyze polymerization reactions in several classes of organic compounds; these polymerizations sometimes proceed rapidly or even explosively. Some metals in this group form explosive products with halogenated hydrocarbons.

危険性

Although the metal hafnium is not harmful, its powder and dust are both toxic if inhaledand explosive even when wet.

健康ハザード

Fire will produce irritating, corrosive and/or toxic gases. Inhalation of decomposition products may cause severe injury or death. Contact with substance may cause severe burns to skin and eyes. Runoff from fire control may cause pollution.

火災危険

Flammable/combustible material. May ignite on contact with moist air or moisture. May burn rapidly with flare-burning effect. Some react vigorously or explosively on contact with water. Some may decompose explosively when heated or involved in a fire. May re-ignite after fire is extinguished. Runoff may create fire or explosion hazard. Containers may explode when heated.

工業用途

Pure hafnium is a lustrous, silvery metal that is not so ductile nor so easily worked as zirconium; nevertheless, hafnium can be hot- and cold-rolled on the same equipment and with similar techniques as those used for zirconium. All zirconium chemicals and alloys may contain some hafnium, and hafnium metal usually contains about 2% zirconium.The metal has a closepacked hexagonal structure. The electric conductivity is about 6% that of copper. It has excellent resistance to a wide range of corrosive environments.
Because of the startling similarity in their chemical properties, zirconium and hafnium always occur together in nature. In their respective ability to absorb neutrons, however, they differ greatly, and this difference has led to their use in surprisingly different ways in nuclear reactors. Zirconium, with a low neutron-absorption cross section (0.18 barn), is highly desirable as a structural material in water-cooled nuclear reactor cores. Hafnium, on the other hand, because of its high neutron-absorption cross section (105 barns), can be used as a neutron-absorbing control material in the same nuclear reactor cores. Thus, the two elements, which occur together so intimately in nature that they are very difficult to separate, are used as individual and important but contrasting components in the cores of nuclear reactors.

職業ばく露

Hafnium metal has been used as a control rod material in nuclear reactors. Thus, those engaged in fabrication and machining of such rods may be exposed.

輸送方法

UN1326 Hafnium powder, wetted with not <,25% water (a visible excess of water must be present) (1) mechanically produced, particle size<53 μm; (2) chemically produced, particle size<840 μm, Hazard Class: 4.1; Labels: 4.1-Flammable solid. UN2545 Hafnium pow der, dry, Hazard Class: 4.1; Labels: 4.1-Flammable solid. UN1346 Hafnium powder, wetted with not less than 25% water (a visible excess of water must be present) (1) mechanically produced, particle size less than 53 μm; (2) chemically produced, particle size less than 840 μm, Hazard Class: 4.1; Labels: 4.1-Flammable solid.

不和合性

Fine powder or dust may form explosive mixture in air. The powder is highly flammable and a strong reducing agent. The powder or dust reacts with moisture forming flammable hydrogen gas; may spontaneously ignite on contact with moist air; and at higher temperatures, with nitrogen, phosphorous, oxygen, halogens, and sulfur; contact with hot nitric acid; heat, shock, friction, strong oxidizers; or ignition sources may cause explosions.

廃棄物の処理

Recovery. Consider recycling, otherwise, this chemical must be disposed of in compliance with existing federal and local regulations.