Removal of metallic impurities from zirconium by hydrogen plasma arc melting

Abstract

Zr has found a new application as a lining material of nuclear fuel element [1] and has also used prospective materials (Zr-based oxides) as high-k gate dielectrics for advanced MOS FETs [2]. It is widely regarded that the decrease of defect concentration is very important to control material properties in the field of nano-scale film technology. Since metallic impurities in the dielectric films can affect the defect formation and, consequently, to deteriorate the interface quality and reliability, the removal of the impurities plays an important role to determine the properties of gate dielectrics. Then, it is needed to utilize a high-purity source to obtain an excellent gate oxide layer with uniformity and lower defect density. Despite of requiring a high-purity source for the film deposition, to our knowledge, detailed impurity analysis and purification of Zr metal have not been carried out. It has been found that hydrogen plasma arc melting (HPAM) enables the removal of non-metallic impurities from Fe, Mo, and Ta and also metallic impurities from Zr alloy and Nb [3–6]. For example, Mimura et al. [6] have revealed that Fe content in a Zr-1 mass% Fe alloy was reduced from 1 mass% to a few mass ppm level after 50 vol% H2–Ar plasma arc melting for 180 min at a reduced pressure, while the reduction of Fe was very little when Ar plasma gas was only used. Furthermore, the impurity elimination was found to be more effective for higher H2 content in the plasma gas. Therefore, in the present study, removal of metallic impurities from Zr by HPAM has been carried. Experiments were carried out using a laboratoryscale plasma arc furnace equipped with a transferred arc type plasma torch, the detailed constitution of which has been described in elsewhere [6]. The specimen was set on a water-cooled copper crucible of 45 mm in diameter and 4 mm in depth. After the specimen (about 30 g) was melted down by Ar plasma arc heating, hydrogen was added to the Ar plasma gas to obtain the desired content under atmospheric pressure. The distance between the tip of the plasma torch and the melted specimen was about 20 mm. High purity argon gas (>99.9995%) and hydrogen gas (>99.9999%) were mixed and introduced into the plasma torch at a flow rate 5 l/min. The hydrogen content in the plasma gas was varied from 0 to 20% at atmospheric pressure. The specimen after first melting was melted again turning upside down for uniform refining. The GDMS (VG ELEMENTAL: VG9000)