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Zirconium hydride describes an alloy made by combining zirconium and hydrogen. Hydrogen acts as a hardening agent, preventing dislocations in the zirconium atom crystal lattice from sliding past one another. Varying the amount of hydrogen and the form of its presence in the zirconium hydride (precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting zirconium hydride. Zirconium hydride with increased hydrogen content can be made harder and stronger than zirconium, but such zirconium hydride is also less ductile than zirconium. ==Material properties== Zirconium is found in the Earth's crust only in the form of an ore, usually a zirconium silicate, such as zircon. Zirconium is extracted from zirconium ore by removing the oxygen and silica. This process, known as the Kroll process, was first applied to titanium. The Kroll process results in an alloy containing hafnium. The hafnium and other impurities are removed in a subsequent step. Zirconium hydride is created by combining refined zirconium with hydrogen. Like titanium, solid zirconium dissolves hydrogen quite readily. The density of zirconium hydride varies based the hydrogen and ranges between 5.56 and 6.52 g cm−3. Even in the narrow range of concentrations which make up zirconium hydride, mixtures of hydrogen and zirconium can form a number of different structures, with very different properties. Understanding such properties is essential to making quality zirconium hydride. At room temperature, the most stable form of zirconium is the hexagonal close-packed (HCP) structure α-zirconium. It is a fairly soft metallic material that can dissolve only a small concentration of hydrogen, no more than 0.069 wt% at 550 °C. If zirconium hydride contains more than 0.069% hydrogen at zirconium hydride making temperatures then it transforms into a body-centred cubic (BCC) structure called β-zirconium. It can dissolve considerably more hydrogen, more than 1.2% hydrogen above 900 °C. When zirconium hydrides with less than 0.7% hydrogen, known as hypoeutectoid zirconium hydride, are cooled from the β phase the mixture attempts to revert to the α phase, resulting in an excess of hydrogen. Another polymorphic form is the γ phase, is generally accepted to be a metastable phase. Zirconium hydrides are odorless, dark gray to black metallic powders.〔(Occupational Safety and Health Guideline for Zirconium & Compounds ), Occupational Safety & Health Administration, U.S. Department of Labor〕 They behave as usual metals in terms of electrical conductivity and magnetic properties (paramagnetic, unless contaminated with ferromagnetic impurities). Their structure and composition is stable at ambient conditions. Similar to other metal hydrides, different crystalline phases of zirconium hydrides are conventionally labeled with Greek letters, and α is reserved for the metal. The known ZrHx phases are γ (''x'' = 1), δ (''x'' = 1.5–1.65) and ε (''x'' = 1.75–2). Fractional ''x'' values often correspond to mixtures, so the compositions with ''x'' = 0.8–1.5 usually contain a mixture of α, γ and δ phases, and δ and ε phases coexist for ''x'' = 1.65–1.75. As a function of increasing ''x'', the transition between δ-Zr and ε-Zr is observed as a gradual distortion of the face-centered cubic δ (fluorite-type) to face-centered tetragonal ε lattice. This distortion is accompanied by a rapid decrease in Vickers hardness, which is constant at 260 HV for ''x'' < 1.6, linearly decreases to 160 HV for 1.6 < ''x'' < 1.75 and stabilizes at about 160 HV for 1.75 < ''x'' < 2.0. This hardness decrease is accompanied by the decrease in magnetic susceptibility.〔〔 The mass density behaves differently with the increasing hydrogen content: it decreases linearly from 6.52 to 5.66 g/cm3 for ''x'' = 0–1.6 and changes little for ''x'' = 1.6–2.0.〔 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「Zirconium hydride」の詳細全文を読む スポンサード リンク
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