High Temperature Materials and Processes, VoL 9, Nos. 2-4, 1990 Purification of Refractory Metals by Solid-State Refining O.N. Carlson Ames Laboratory-Department of Energy and. Iowa State University, Ames, LA 50011, USA CONTENTS in ultra-high vacuum. The advantages and limita- tions of electrotransport purification are discussed, Page and the degree of purity attainable by this technique is summarized for various metals. The potential of ABSTRACT 125 thermotransport purification is also evaluated. 1. INTRODUCTION 125 2. EXTERNAL GETTERING 125 2.1. By Gases 126 2.2. By Alkaline Metals 126 1. INTRODUCTION 2.3. By Reactive Metals 126 3. VACUUM DEGASSING 128 Some of the purest metals in the world have 3.1. Deoxygenation 128 been prepared by solid-state refining techniques. 3.2. Denitrification 129 Since these techniques involve diffusion in the solid, 3.3. Dehydrogenation 129 long times at high temperatures are required. This 3.4. Decarburization 129 limits the size of the specimens (at least in one PURIFI- 4. ELECTROTRANSPORT dimension) to small rods, wires, thin plates or CATION 129 sheets. 4.1. Environmental Contamina- tion 130 The most widely used solid-state refining proces- 4.2. Application of Electrotransport ses are: external gettering, vacuum degassing and Purification 130 electrotransport. Purification by one or more of 5. PURIFICATION BY THERMO- these techniques has found increasing usage in the TRANSPORT 131 preparation of high purity refractory and reactive 6. CONCLUSIONS 132 metals in recent years. It is the purpose of this REFERENCES 133 paper to evaluate the efficacy of these processes and to review the underlying principles of each. ABSTRACT Different solid-state refining techniques can be 2. EXTERNAL GETTERING used to decrease the interstitial impurity level of refractory and other high melting metals. Purifica- Interstitial impurities can often be removed tion by external gettering is particularly effective for rather completely by reaction with a chemically carbon, oxygen and nitrogen in vanadium and active solid, liquid or gaseous element in contact niobium. Oxygen and nitrogen can also be readily with the surfaces of the solid. This is referred to as removed from niobium and tantalum by degassing external gettering. 125 VoL 9, Nos. 2-4,1990 Purification of Refractory Metals by Solid-State Refining 2.1. By Gases the getter to form a stable oxide that can be re- moved either chemically or mechanically. The A familiar example of external gettering of a metal deoxygenation of a metal by one of the alkaline by a gas is the decarburization of steel during earth metals was first demonstrated by Allen et al. heating in an oxygen-containing atmosphere. A /3/, who showed that it is possible to predict the similar technique has been used to decarburize degree of purification from thermodynamic data. niobium and tantalum. According to Schulze /l/, Peterson and coworkers /4/ calculated the equi- carbon can be reduced to extremely low levels (< 1 librium oxygen concentration in vanadium after atm ppm) in niobium by heating at 2000°C in an equilibration with calcium, barium and magnesium, oxygen partial pressure of 10"3 Pa. The residual assuming Henry's law of behavior in the low oxygen oxygen is then removed by annealing in an ultra- range (see Table 1). Deoxygenation occurs by the high vacuum, details of which are given in a later reaction section. Stein et al. /2/ used a somewhat similar techni- 1200°C que to decarburize iron in a continuously recycled V[0](s) + Ca(ji) » CaO(s) + V(s) hydrogen atmosphere by the reaction: and the oxygen content of the base metal is de- 850°C creased until its partial pressure equals the dissocia- Fe[C](s) + 2H2(g) » Fe(s) + CH4 tion pressure of CaO. From the standard free energy data the equilibrium 02 pressure of CaO at The methane was removed from the flowing gas by 1200°C can be shown to be 7.4 χ 10"33 Pa. The recirculating it over zirconium turnings. The free solubility of oxygen in vanadium obeys Henry's law energy change for the reaction /5/ and is described by the equation 850°C log c0(atm% O) = Valog P02(MPa) - Zr + CH4 » + 2H2 2.75 + 22,050/T 6 is -173.4 kJ, and the calculated equilibrium con- Hence at 1200°C, c0 = 4.7 χ 20" atm% Ο = -0.05 centration of carbon is extremely low (10"6 atm atm ppm. Note that this calculated value is more ppm). Carbon concentrations reportedly as low as than 2 orders of magnitude lower than the experi- 0.02 atm ppm in iron were obtained experimentally mentally observed value of 8 atm ppm. Peterson et by this method. al. /4/ attributed this failure to reach the calculated The deoxygenation of molybdenum or tungsten oxygen level to a slow approach to equilibrium. by hydrogen has not actually been reported, but it Yoshinari et al. /6/, on the other hand, ascribed it is certainly thermodynamically feasible for these to trapping of oxygen by calcium that had diffused metals. They are both prepared commercially in into the vanadium. powder form by the reduction of their respective oxides with oxygen. Thus under conditions at which oxygen can diffuse readily, it should react with 2.3. By Reactive Metals hydrogen at the surface to form H20 gas. A major drawback to gettering with calcium or magnesium is the lack of any significant change in 2.2. By Alkaline Earth Metals the carbon or nitrogen contents of the base metal, as is seen from the data in Table 1. For this reason A method that has found increasing application as metals with a greater affinity for these interstitial a deoxygenation technique during the past decade impurities (e.g., titanium, zirconium or yttrium) are is external gettering by one of the alkaline earth used as getters. Table 1 shows that vanadium coated metals. with a thin layer of titanium and heated to 1000°C The base metal is heated in direct contact with exhibits a dramatic decrease in the carbon and the more active metal in liquid or vapor form. The nitrogen contents to below analyzable limits, i.e., <9 oxygen atoms diffuse to the surface and react with and <4 atm ppm, respectively. The experimentally 126 O.N. Carlson High Temperature Materials and Processes TABLE 1 Purification of Vanadium by External Gettering with Different Metals Atm. ppm Ο Atm. ppm C Atm. ppm Ν Temp. Getter °C Init. Calc. Expt. Init. Expt. Init. Expt. Ref. Ca 1200 225 0.05 8 - - 4 Ca 900 5000 45 - - - 6 Ca 1000 1.6% 110 85 65 235 125 7 Mg 1200 225 0.7 6 - - - 4 Ba 1200 225 0.5 14 - - - 4 Tia 1000 925 20.0 65 210 10 7 4 8 b Ti 1170 5000 25 - - 10 6 b Zr 1070 735 6 - - - 9 aVapor deposited layer bFoil wrapper determined oxygen content is only slightly higher Shibiata and Koiwa /13/ report that both oxygen than the calculated equilibrium value (60 vs 20 atm and hydrogen can be reduced to very low levels by ppm). Yoshinari and coworkers /6/ demonstrated annealing niobium wires wrapped with zirconium that titanium foil wrapped around a vanadium rod foils at 1070°C. By this treatment the RRR was and sealed in a quartz capsule is equally effective increased from 166 to 630 (see Table 2), which in reducing the oxygen level (from 5000 to 25 atm corresponds to a decrease in oxygen content from ppm), but no results were reported for carbon or 600 to 20 atm ppm. nitrogen. Padamsee /14/ has developed a solid-state Zirconium foil is also effective as an oxygen gettering technique that is applicable to niobium getter, as is shown in Table 1. Vanadium containing microwave cavity production using yttrium foils as 735 atm ppm oxygen is deoxidized to a level of 6 the getter. During heating, yttrium vapor is deposi- atm ppm by zirconium at 1070°C /8/. Uz and ted on the niobium surface, forming a layer of Carlson /10/ decreased the carbon content of a V- several micrometers thick. The data in Table 2 20% Ti alloy from 1340 to 250 atm ppm by the show an increase in the RRR from 68 to 175. This zirconium foil technique. corresponds to a decrease in oxygen from 255 to External gettering with a reactive metal has also 100 atm ppm and in nitrogen from 115 to 80 atm been applied extensively to niobium. The use of ppm. Padamsee obtained niobium with an RRR as niobium for superconducting accelerator microwave high as 540 starting with metal with an RRR of cavities has resulted in a major emphasis on obtain- 165. This corresponds to a total interstitial content ing large quantities of the metal in high purity. of -60 atm ppm. Interstitially dissolved impurities, particularly oxygen, Kneisel /15/ purified niobium by heating it for adversely affect the thermal conductivity of niobium 10 hr at 1300°C in titanium foil. Following this in the superconducting temperature range /ll, 12/. treatment the Ti02/TiN and Ti-rich diffusion layers It is, therefore, extremely critical that these im- were removed by chemical etching. Studies were purities be held to the low ppm range for this conducted on niobium from different commercial application. A residual resisitivity ratio (RRR) of sources, the results of which are given in Table 2. 200, corresponding to a total O + C + N content of RRR values consistently greater than 300 were -185 atm ppm /l/, is the minimum specification obtained for the different materials with the excep- for use as a cavity material. tion of the metal obtained from Kawecki.