炭化ハフニウム

解説

炭化ハフニウム，タンカハフニウム，HfCx．xは1以下の非化学量論相で，xとともに融点が変化する．灰色の金属様結晶．融点3887 ℃．炭化ハフニウム，炭化タンタルを1：4で混合して不活性ガス中で熱した混晶は，融点4215 ℃ であり，最高の融点といわれている．原子炉の制御棒に用いられる．

化学的特性

dark, gray, brittle solid; fcc, a=0.4640 nm; high cross section for absorption of thermal neutrons; resistivity 8.8μohm· cm; most refractory binary material known; hardness 2300kgf/mm2; used in control rods of nuclear reactors; can be prepared by heating HfO2 with lampblack under H2 at 1900°C–2300°C; used in crucible form for melting hafnium oxide, other oxides [KIR80] [HAW93] [CER91]

物理的性質

Dark gray brittle solid, most refractory binary material known. Controls rods in nuclear reactors, crucible container for melting HfO and other oxides. 2 Corrosion resistant to liquid metals such as Nb, Ta, Mo, and W. Severe oxidation in air above 1100–1400°C and stable up to 2000°C in helium.

使用

Hafnium carbide (HfC) with a melting point over 3890°C is a refractory binary compound. HfC and NbC can also be used as refractory coatings in nuclear reactors. The mixed carbide Ta4HfC5 possesses the highest melting point of any currently known compound at 4215°C.

調製方法

Hafnium carbide is a dark gray, brittle solid. It can be prepared by heating a mixture of the elements or by reacting hafnium tetrachloride with methane at 2100 C. Sufficient quantities of hafnium oxide or hafnium metal sponge are obtained during the large-scale production of pure zirconium for nuclear reactors. On an industrial scale, hafnium carbide can be produced from the hydrided hafnium sponge at 1500–1700 ℃ or from hafnium oxide at 2000–2200 ℃ by carburization in vacuo in the presence of hydrogen. The resulting carbide contains almost the theoretical quantity of carbon (6.30% °C) and a maximum of 0.1% free carbon (219). The hafnium carbide obtained is not a true stoichiometric compound as much as a solution of carbon at specific interstitial sites of a face-centered cubic hafnium lattice.
Hafnium carbide is inert to most reagents at room temperature but is dissolved by hydrofluoric acid solutions. Hafnium carbide reacts exothermally with halogens at 250–500 ℃to form hafnium tetrahalide, and to form hafnium oxide with oxygen above 500℃. In the presence of hydrogen, hafnium carbide slowly loses some of its carbon at higher temperatures.