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Electro-deoxidation of hafnium dioxide and niobia-doped hafnium dioxide in molten calcium chloride
Electrochimica Acta 64 (2012) 10–16
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electro-deoxidation of hafnium dioxide and niobia-doped hafnium dioxide in molten calcium chloride Amr M. Abdelkader ∗ , Derek J. Fray Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 23 November 2011 Accepted 24 November 2011 Available online 6 December 2011 Keywords: Hafnium oxide Molten salt Electro-deoxidation
a b s t r a c t This paper investigates the capability of the FFC-Cambridge process of producing metallic hafnium powder that can meet commercial standards. The results show that the reduction in the core of the pellet is hindered in an intermediate step associated with the formation of CaHfO3 . Doping the initial hafnium oxide with niobium oxides in the initial pellet prevents this blocking. The charge passed is much higher than that observed with undoped HfO2 . The powder obtained after 36 h of electro-deoxidation is an Hf–Nb alloy cubes with 0.8 wt.% oxygen content and 5–20 m size. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Hafnium was known for years as a control rod material in the nuclear reactors due to its high thermal neutron absorption cross section, excellent mechanical properties, and good corrosionresistance [1]. However, a large part of the world production is currently consumed in superalloys. Hafnium has a variety of functions in superalloys: it improves the mechanical properties particularly at high temperature either by solid solution strengthening, or by the formation of a fine dispersion of oxides, carbides or nitrides. It, also, improves the high temperature oxidation resistance and the corrosion resistance by forming a thin tenacious layer of oxide [1–3]. Furthermore, hafnium is also used in gas-filled and incandescent lamps, and is an efficient getter for scavenging oxygen and nitrogen [4]. Commercially, the ingenious process developed by Kroll for producing zirconium and titanium also produces hafnium. In this process, hafnium tetrachloride is reduced by high purity molten magnesium according to the following overall reaction: HfCl4 + 2Mg → 2MgCl2 + Hf
G◦ 1373 = −280.557 kJ
(1)
The process is a batch-type and extremely intensive in term of equipments, labour, and maintenance which, coupling with the high cost of hafnium raw materials and the limited supply of both raw materials and final products, make hafnium an expensive metal ($472 per kg in 2009) [5]. Many other methods, mostly
∗ Corresponding author. E-mail address: amr [email protected] (A.M. Abdelkader). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.11.107
metallothermic in nature, were reported to successfully prepare hafnium on the lab scale, but none of them found way to industry. The FFC-Cambridge process could introduce an affordable alternative to Kroll process [6]. In fact, the production of hafnium powder through the FFC-Cambridge process was reported in the early stage of its development [7]. However, there is no published work so far describing the quality of the product or the reduction mechanism. In addition, zirconium dioxide, which has chemical and conductivity properties very close to that of hafnium, was reported to reduce only partially through the FFC-Cambridge process [8]. The purpose of the present work is therefore to investigate the possibility of using the FFC-Cambridge process to produce hafnium metal and identify the reactions pathway that occur as the HfO2 is reduced to hafnium metal. 2. Experimental Oxide pellets were prepared by mixing hafnium oxide (AlfaAesar, catalogue number 35666, 99.99% with Zr < 0.5%), 1 wt.% of the binder and 10 wt.% graphite powder (Aldrich catalogue number 282863 with average particles size 1–2 m). The binder used in all the current experiments was PVB/PVA (poly-vinyl butyral-co-vinyl alcohol-co-vinyl acetate) 80% by weight vinyl butyral. The niobium doped samples were prepared by adding 5 wt.% of niobium pentoxide (Aldrich, catalogue number 203920, 99.99% purity) to the initial mixture. The as-received materials were ultrasonically mixed in isopropyl alcohol using ultrasonic probe (Branson 450 sonifier) for about 40 min to breakdown any agglomerates and ensure homogeneity. The produced slurry was then partially dried in air and then in a vacuum oven until all the alcohol had vaporized. The dry mass
A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16
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Fig. 1. The HfO2 O-ring green pellet used as a cathode precursor in the present work.
was further mixed in some cases using mortar and pestle. About 3 g of the powder was then uniaxially pressed using a hydrolytic press at about 50 MPa with a space die diameter of 20 mm in an O-ring shape (Fig. 1). This shape has many advantageous; it gives higher surface area which facilitates the diffusion of the salt inside the pellet, and it reduces the internal stress that might be caused by further thermal treatments. To prepare the O-ring pellet, a steel cylinder having an outside diameter of 8 mm was fixed vertically in the centre of the die. The pellets that were produced ranged from 3 to 5 mm thickness. The green pellets were then sintered in air at 1273 K for 3 h in a GD-1700 muffle furnace using a Eurotherm® 114 temperature controller at a heating rate of 3 K/min. When carbon burns in air, pores that have the same size of the carbon particles are produced. The electrochemical experiments described in this work were conducted in an Inconel® reaction vessel, housed in a programmable furnace (Lenton), the vessel had a cooler region near the lid where the temperature was below 473 K. The electrolyte used was 1 mol% CaO–CaCl2 prepared from dihydrate CaCl2 (Aldrich C/1500/65, 99% purity), which was heated gradually up to 473 K over 5 days under vacuum, and CaO, which produced by the thermal decomposition of calcium carbonate (Aldrich catalogue number 239216, 99.99%) at 1373 K for 12 h. Details on the assemblage used in the electrochemical experiments and the procedure used to prepare the electrolyte have been described in details in previous studies [9]. About 420 g of the electrolyte was packed in the alumina crucible (Almath CL96). The crucible was then lowered into the bottom of the reactor vessel. The vessel was then sealed by clamping the lid, which carried the electrodes, and argon gas was flushed into the reactor for 3 h to purge the system of atmospheric air. The salt temperature was then ramped to 523 K by 2 K/min and kept at this temperature for 12 h before it was ramped again to 1173 K at a rate of 5 K/min, kept for an hour and finally heated to 1223 K. Once the maximum temperature was achieved, two carbon electrodes (Tokai Carbon HK0 10 mm × 100 mm with a 10 mm M3 thread at one end) were then lowered and the salt was subjected to the preelectrolysis process at 1.3 V for 12 h to remove metallic impurities and any remaining moisture. After finishing the pre-electrolysis, one of the carbon electrodes was lifted to the upper part of the reactor, while the cathode assemblage was lowered into the melt. The cathode assemblage consists of a stainless steel cup (30 mm outer
diameter, 20 mm inside diameter, and 7 mm height with 2 mm holes drilled in the side and bottom walls) holding the oxide pellet. Graphite rod was lowered as pseudo reference electrode in the some experiments. After the electro-deoxidation, the pellet was lifted out from the melt to the cooling zone where it was allowed to cool under a continuous flow of argon gas. The electrochemical experiments were conducted under constant potential difference of 3 V using a PSS-210-GW INSTEK programmable power supply. This device was equipped with Instek PSU software to record the variation of the current versus time as well as the potential. An Agilent 34970A Data Acquisition Switch Unit was used as a high impedance voltmeter to measure the potential between the cathode, anode and a graphite pseudo-reference electrode. After removing from the reactor, the samples were ultrasonically washed with distilled water to remove the attached solidified CaCl2 and carbon residue, filtered, and dried under vacuum at about 333 K. The samples were treated successively with dilute acetic acid, 1 N hydrochloric acid assisted by vacuum impregnation to remove any residual CaCl2 , CaO, or excess Ca. Finally, the samples were rinsed by ethyl alcohol, then with acetone before drying them again in a vacuum oven. Different present phases of the samples were identified by X-ray diffraction (XRD) analysis. XRD analysis was carried out with Philips PW1710 X-Ray Diffractometer under Cu K␣ radiation with the incidence beam angle of 2◦ . The diffraction angle range was between 10◦ and 80◦ with a step increment of 0.02◦ and a count time of 1 s. The phases present were analysed by means of the software Highscore® which using the standard spectra patterns supplied by JCPDS-International Centre for Diffraction Data. Microstructure and chemical composition of the samples were investigated by JEOL JSM-5800LV, CamScan MX2600 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analyser (EDX) at an accelerating voltage of 15 kV. Non-conductive samples were sputter-coated with gold using an Emitech K550 sputter coater. Some of the produced hafnium powder was imaged by the JEOL 6340F FEGSEM, which allowed for high resolution SEM images. Oxygen content of the samples was quantitatively determined in a destructive technique using an Eltra ONH-2000 analysers. This machine uses the hot extraction technique and infrared detection of the carbon monoxide evolved.
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A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16 Table 1 Sequence of cathodic reaction steps based on the calculated standard electrode potentials (versus Ca/Ca2+ at 1173 K).a Stage
E◦ (V)
Reaction +2
I II III
−2
2HfO2 + Ca + O = CaHfO3 + Hf CaHfO3 + 4e− = Hf+ Ca+2 + 3O2− 2HfO2 + Ca+2 + 2e− = CaHfO3 + HfO CaHfO3 + 2e− = HfO + Ca+2 + 2O2− HfO + 2e− = Hf + O2− (4) (5) HfO2 + 4e− = Hf + 2O2−
(2) (3)
+0.2 V >0.55 >0.2 Unknown +0.272
a The standard potentials was calculated based on the thermodynamic date available from HSC chemistry, Version 6.0.
Fig. 2. Current–time curve for HfO2 pellet electro-deoxidised at constant potential difference of 3 V in molten CaCl2 – 1 mol% CaO melt.
3. Results and discussion 3.1. Electro-deoxidation of pure hafnium oxide Upon applying a constant potential difference in any twoterminal cell, the cell response adjusts according to the electrochemical reactions that take place on the surface of the two electrodes. The response, usually recorded as the variation of current over time, is therefore a useful tool to understand the reactions. In the present work, the anodic and cathodic potential are recorded versus graphite pseudo reference. To ensure a stable reference electrode, the graphite pseudo reference was replaced every 12 h. A typical time-current plot, recorded when 3 V constant potential difference was applied between a cup cathode holding a 3 g pellet of HfO2 and a graphite anode, is illustrated in Fig. 2. The key features of the current–time curve are: (1) sharp decline of the current in the first 7 min (up to point A), (2) current-shoulder in the 7–70 min range (feature B), (3) decrease linearly between 1 and 3 h and parabolicaly between 3 and 5 h (features C and D), and (4) a steady time-invariant current starting from about 8 h until the end of the run (feature E). 3.1.1. Feature (A) The sample collected after 7 min of the electro-deoxidation has a dark grey layer on the bottom surface and on the area around the wire collector in the top part. The XRD trace in Fig. 3a indicated it is mainly ␣-Hf with some traces of HfO2 . The fast reduction of the surface layer is a well-documented phenomenon and was mathematically described through the thin-layer 3PIs model [10]. Using the cup-cathode in the present work accelerates the surface metallisation by increasing the number of contact points. The thin-layer 3PI is always reflected in the cell response by quick drop in the current, which agrees well with Fig. 2. Also, the high resistance oxide on the surface caused short-lived iR potential drop that diminishes as the reduction starts to propagate in the surface layer. 3.2. Feature (B and C) Once the surface layer metallised, the reduction starts to propagate into the core of the pellet and follows what is known as the penetration 3PI model [11]. The rate of the progression in the depth direction is slow and allows the intermediate steps of the reduction to be distinguished. The first reaction to take place is expected from the thermodynamic date (Table 1) to be the interaction of HfO2 with electrolyte forming calcium hafnate with the emergence of hafnium monoxide. The later is a high temperature
phase that dissociates at lower temperature to HfO2 and Hf when the sample cools down [12]. The XRD analysis agrees well with the thermodynamic calculations. The XRD pattern of the sample electro-deoxidised for 15 min detected CaHfO3 for the first time (Fig. 3b). As the pellet is electrolysed for longer time, i.e. 0.5 and 1.5 h, the intensity of CaHfO3 increases while that of HfO2 decreases (Fig. 3 c and d). However, the potential value at the first 4 h (+0.2 V versus Ca/Ca2+ ) is more negative than that expected from the thermodynamic calculation of reaction (2). By analogy with the niobium oxide electro-deoxidation [13], reduction of HfO2 on the surface layer to hafnium metal through quick steps of forming calcium hafnate and hafnium monoxide (represented by the overall reaction (5)) could be kinetically more favourable than forming CaHfO3 in the core of the pellet which explains the high negative potential. After 3 h of the electro-deoxidation, the total amount of hafnium is detectable by the XRD measurements (Fig. 3e), indicating the metallisation of hafnium is not limited to the surface layer but for some micrometers beneath it. The maximum thickness of the metallised surface crust is about 500 m for the sample electro-deoxidised for 4 h (Fig. 4). On the other hand, the formation of perovskite CaHfO3 is still taking place in the core of the pellet as the sample terminated after 4 h shows HfO2 , and CaHfO3 as the main components of the interior part (Fig. 3f). Converting HfO2 to perovskite phase in the core of the pellet is thermodynamically possible either electrochemically through reaction (2) or chemically in the presence of oxygen (resulted from metallising the surface curst) through reaction (6). The SEM Images of the partially reduced sample gives a good idea about the mechanism of forming CaHfO3 : HfO2 + Ca+2 + O2− = CaHfO3 ,
G◦ 1173 = −93.758 kJ
(6)
Fig. 5 shows the SEM image of the sample after 2 h of the electrodeoxidation. Layered structure of the perovskite phase starts to form by consuming irregular small particles of HfO2 . The transformation from the monoclinic HfO2 to the cubic NaCl crystal structure of the perovskite needs a complete reconstruction of the lattice starting by destroying the crystal of HfO2 , which explain the irregularity of the HfO2 particles. These particles have high surface area which speeds up the reactions with calcium ions to readily form the crystalline CaHfO3 . The layered nucleation pattern of the perovskite gives more support for the electrochemical route of forming CaHfO3 as such kind of directional crystallographic orientations is common in the electrochemical processes [14,15]. Growth of the layers continued in one preferred direction forming wedge-like structure of CaHfO3 as can be deduced from the sample electro-deoxidized for 3 h (Fig. 6). 3.2.1. Feature (D) The XRD analysis of the samples taken after 5 and 6 h, i.e. during feature D shows almost no changes from that after 3 h. However, the cores of the pellets are stronger and harder to break. The pellet was ground using a ball mill for an hour in order to analyse the
A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16
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Fig. 3. XRD traces of the HfO2 electro-deoxidized in molten CaO–CaCl2 at 3 V measured (a) at the surface of the pellet after 7 min; and measured for the whole pellet (b) after 15 min, (c) after 30 min, (d) after 1.5 h, (e) after 3 h, (f) after 4 h, (g) 6 h, and (h) after 16 h.
obtained powder by SEM. The SEM images show prisms with welldefined edges indicating the CaHfO3 phase is still in its growing stage (Fig. 7). Indeed, insertation of calcium oxide to the hafnium oxide increases the total volume of the crystal and produces more dense structure in the core of the pellets. The non-porous core
explains not only the hard-to-break pellets, but also the slow kinetic of the electro-deoxidation of the interior parts which approaches almost zero at the end of feature D. The oxygen analysis of the samples taken at the edges of period D shows only 4% reduction in the total oxygen content during feature D.
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A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16
Fig. 4. Optical microscope images for the HfO2 pellet after (a) 15 min, (b) 60 min, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h of the electro-deoxidation (the pellet thinness is 3 mm).
3.2.2. Feature (E) The invariation of the value of the current observed during this period suggests no change in the process that takes place during this stage. The SEM results agree with this suggestion; the images of the sample taken from the core of the pellets electrodeoxidised for 10 and 14 h show well-defined-edges prisms of CaHfO3 (Figs. 8 and 9). The strength of the pellets increased and longer time is now needed to grind the core (3 h). In addition, the width of the calcium hafnate peaks in the XRD pattern shrinks for
longer times of the electro-deoxidation indicating an increase in the phase crystallinity. Therefore, the process represented by feature E is the crystallisation of the hafnate phase due to annealing the pellet in the high temperature molten salt. This crystallisation blocks the core of the pellet from further reduction not only by reducing the porosity, but also by reducing the disordering in the lattice. The stopping of the electro-deoxidation reactions is discussed further as follow: The electro-deoxidation of an oxide needs (1) energy enough to break the oxygen-cations bond and (2) substantial oxide ion
Fig. 5. SEM Image for the perovskite during its growth taken after 2 h of the electrodeoxidation of HfO2 pellet.
Fig. 7. The CaHfO3 prism after 5 h of electro-deoxidation, growing in size and forming sharp edges, which was strong enough to sustain 1 h of milling without damaging the edges.
Fig. 6. Long prism of CaHfO3 still in its growing stage after 3 h of electro-deoxidation.
Fig. 8. SEM image showing the dense core of the pellet after manually removing the metal surface crust.
A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16
Fig. 9. The perovskite prisms produced by electro-deoxidation a pellet of HfO2 for 8 h after ball milling the powder for 3 h.
conduction in the oxide phase, in the oxide/electrolyte interface, and in the electrolyte. Since the migration of oxide ion is known to be fast in the CaO–CaCl2 system, the last reason is excluded from the current discussion. From the energy point of view, the thermodynamic calculation shows that CaHfO3 is a very stable compound and reducing it through the electro-deoxidation process might not be possible without depositing calcium on the cathode. The chemical stability of the calcium hafnate is due to the nature of the bonds in the compounds. It has been reported that interaction of charges between O and Hf due to O 2p and Hf 5d hybridisation makes the Hf-O bond covalent while the Ca-O bond is ionic [16]. The energy required to break these mixed ionic-covalent bonds is high, which hinders the electro-deoxidation of perovskite hafnate at moderated temperature unless high negative potential is applied. The present study, however, is limited to 3.0 V to ensure no chlorine evolution on the anode. The second reason that stops the electro-deoxidation of HfO2 at the stage of forming CaHfO3 is kinetic in nature. It is well known that the migration of oxide ion in the perovskite hafnate is a function of the oxygen vacancies in the lattice [17]. Reducing the disordering in a large crystal similar to that shown in Fig. 6 makes the movement of the oxide ion (and consequently the deoxidation
15
Fig. 10. Current–time diagram for the HfO2 doped by 5 wt.% Nb2 O5 pellet electrodeoxidized for 24 at constant potential difference of 3 V in CaO–CaCl2 .
process) a very slow process. In addition to the slow migration inside the crystal, the oxide ion have to diffuse through the intergranular boundaries to another grain and then to the metallic surface crust as the solid/electrolyte interface is limited only to the pellet surface due to the extinction of interior pores. It should be mentioned here that, although the applied potential difference is theoretically enough to deposit calcium from the CaO in the electrolyte, no calcium deposits were observed on the current collector. Also the cathodic potential never shifted to a negative value that might be claimed to be for calcium deposition (about 1.35 V versus graphite pseudo reference [18]). The increase in the overall cell resistance due to the presence of the extremely high electric resistance perovskite on the cathode is believed to be responsible of preventing calcium deposition by increasing the iR loss and lowering the potential at the cathode. 3.3. Electro-deoxidation of hafnium oxide doped by niobium oxide The formation of the well crystalline perovskite CaHfO3 is believed from the previous section to delay the electro-deoxidation process due to the stability of CaHfO3 and the slow migration of the oxide ion in the lattice and in the oxide intergranular boundaries. The oxide ion migration in perovskite-type oxides depends
Fig. 11. XRD traces for the HfO2 doped by 5 wt.% Nb2 O5 pellet electro-deoxidized for (a) 24 h and (b) 36 h at constant potential difference of 3 V in CaO–CaCl2 .
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A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16
Fig. 12. The cubes of Hf–Nb alloys produced after 36 h of electro-oxidation of HfO2 doped by 5 wt.% Nb2 O5 pellet at constant potential difference of 3 V in CaO–CaCl2 .
on the formation of oxygen vacancies [19]. It is well known that these vacancies, and consequently the oxide ion conductivity of perovskites, ABO3 , increases significantly by doping of different valence cations into the A or the B sites [20]. Recently, niobium has been reported to be one of the best B-site dopants for improving the oxygen conductivity [21,22]. Therefore, a series of experiments was conducted in order to study the effect of niobium doping on the electro-deoxidation of hafnium dioxide. Fig. 10 shows the electrochemical response of a cell when 3 V constant potential difference was applied between carbon anode and a stainless steel cup cathode containing a pellet of HfO2 doped by 5 wt.% Nb2 O5 . The recorded current is higher than that observed with pure hafnium oxide which by itself indicates more reduction in the cathode pellet (2040 C was passed compared with 6048 C passed in the pure hafnium cathode). The pellet taken after 24 h of electro-deoxidation was very porous and easy to grind manually to give very fine black powder. The XRD analysis shows the co-existence of Hf–Nb cubic alloy with some calcium hafnate as a minor phase (Fig. 11). However, the SEM images shows no large prisms or any other form of well-crystalline phases indicating the reduction is not hindered in the core of the pellet. By running the electro-deoxidation for longer time (36 h), only the Hf–Nb alloy is detected by the XRD (Fig. 11). The oxygen content of the produced powder is 0.8 wt.%.
Fig. 13. SEM image of the hafnium particles formed on the surface of the pellet after 16 h of electro-deoxidation.
The SEM image (Fig. 12) of the sample electro-deoxidised for 36 h shows cubes of the powder reflecting high crystallinity which means the metallic phase has sufficient time to crystalline after its formation. Comparing the alloyed hafnium powder (Fig. 12) to that of the pure hafnium (Fig. 13) proves that the dopant materials is not only useful for facilitating the reduction of the perovskite phase, but in producing morphology that can resist back oxidation during handling and kinetically stabilised against self-ignition in air. These results created an opportunity for more investigations on the electro-deoxidation of hafnium oxide. It is now clear that producing pure hafnium powder with quality meeting the commercial standards is not possible via the FFC-Cambridge process. However, doping the initial oxide precursor with another oxide of different oxidation states was proved to facilitate the oxide ion migration in the calcium hafnate intermediate phase by forming oxygen vacancies. Moreover, the reduction is expected to proceed even faster if the perovskite structure of CaHfO3 double doped in the Ca-site and Hf-site. By doping the Hf-site with Nb ions, the FFCCambridge process was reported to produce Hf–Nb alloy with low oxygen. 4. Conclusions The concept of the electro-deoxidation was tested for HfO2 in molten calcium chloride. The results obtained from the electrodeoxidation of a HfO2 pellet under constant potential difference indicated that the reduction was blocked in the core of the pellet associated with the formation of CaHfO3 . The recorded cathodic potential shifted to less negative values during the formation of the perovskite phase, which was related to the switch from the metallisation of HfO2 on the surface to perovskitisation of HfO2 in the core of the pellet. An interesting finding of the present work is the effect of mixing HfO2 with Nb2 O5 in the initial pellet. The charge passed was higher than that observed with undoped HfO2 . The pellet obtained after 36 h of electro-deoxidation was an Hf–Nb alloy with 0.8 wt.% oxygen content. The obtained powder was well crystallised in cubic morphology with particle sizes ranging between 5 and 20 m which protected the powder from back oxidation upon air exposure or during the washing step. References [1] J.H. Schemel, ASTM Manual on Zirconium and Hafnium, American Society for Testing and Materials, Philadelphia, 1977. [2] V.M. Beglov, B.K. Pisarev, G.G. Reznikova, Metal Sci. Heat Treatment 34 (1992) 251. [3] C. Ribaudo, J. Mazumder, J. Metals 40 (1988) 120. [4] S. Ramakrishnan, M.W. Rogozinski, J. Phys. D: Appl. Phys. 30 (1997) 636. [5] J. Gambogi, United States Geological Survey, Retrieved 2008-10-27, 2009. [6] G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. [7] G.Z. Chen, D.J. Fray, Understanding the electro-reduction of metal oxides in molten salts, in: A.T. Tabereaux (Ed.), Light Metals 2004, Warrendale, Minerals, Metals & Materials Soc, 2004, p. 881. [8] K.S. Mohandas, D.J. Fray, Metall. Mater. Trans. B 40 (2009) 685. [9] A.M. Abdelkader, D.J. Fray, Electrochim. Acta 55 (2010) 2924. [10] W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, X.H. Hu, G.Z. Chen, Chemphyschem 7 (2006) 1750. [11] W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, G.Z. Chen, Chem. Eur. J. 13 (2007) 604. [12] M.B. Panish, R. Liane, J. Chem. Phys. 38 (1963) 253. [13] A.M. Abdelkader, PhD thesis, University of Cambridge, 2011. [14] M.R. Rahman, T. Okajima, T. Ohsaka, Chem. Commun. 46 (2010) 5172. [15] W. Ye, J. Yan, Q. Ye, F. Zhou, J. Phys. Chem. C 114 (2010) 15617. [16] D. Cherrad, D. Maouche, M. Reffas, A. Benamrani, Solid State Commun. 150 (2010) 350. [17] V.V. Kharton, A.A. Yaremchenko, E.N. Naumovich, F.M.B. Marques, J. Solid State Electrochem. 4 (2000) 243. [18] C. Schwandt, D.J. Fray, Z. Naturforsch. A 62 (2007) 655. [19] L.G. Tejuca, J.L.G. Fierro, Properties and Applications of Perovskite-type Oxides, Dekker, New York, NY, 1993. [20] T. Ishihara, Perovskite Oxide for Solid Oxide Fuel Cells, Springer, Dordrecht/London, 2009. [21] K. Zhang, R. Ran, L. Ge, Z. Shao, W. Jin, N. Xu, J. Membr. Sci. 323 (2008) 436. [22] T. Nagai, W. Ito, T. Sakon, Solid State Ionics 177 (2007) 3433.
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electro-deoxidation of hafnium dioxide and niobia-doped hafnium dioxide in molten calcium chloride Amr M. Abdelkader ∗ , Derek J. Fray Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK
a r t i c l e
i n f o
Article history: Received 8 July 2011 Received in revised form 23 November 2011 Accepted 24 November 2011 Available online 6 December 2011 Keywords: Hafnium oxide Molten salt Electro-deoxidation
a b s t r a c t This paper investigates the capability of the FFC-Cambridge process of producing metallic hafnium powder that can meet commercial standards. The results show that the reduction in the core of the pellet is hindered in an intermediate step associated with the formation of CaHfO3 . Doping the initial hafnium oxide with niobium oxides in the initial pellet prevents this blocking. The charge passed is much higher than that observed with undoped HfO2 . The powder obtained after 36 h of electro-deoxidation is an Hf–Nb alloy cubes with 0.8 wt.% oxygen content and 5–20 m size. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Hafnium was known for years as a control rod material in the nuclear reactors due to its high thermal neutron absorption cross section, excellent mechanical properties, and good corrosionresistance [1]. However, a large part of the world production is currently consumed in superalloys. Hafnium has a variety of functions in superalloys: it improves the mechanical properties particularly at high temperature either by solid solution strengthening, or by the formation of a fine dispersion of oxides, carbides or nitrides. It, also, improves the high temperature oxidation resistance and the corrosion resistance by forming a thin tenacious layer of oxide [1–3]. Furthermore, hafnium is also used in gas-filled and incandescent lamps, and is an efficient getter for scavenging oxygen and nitrogen [4]. Commercially, the ingenious process developed by Kroll for producing zirconium and titanium also produces hafnium. In this process, hafnium tetrachloride is reduced by high purity molten magnesium according to the following overall reaction: HfCl4 + 2Mg → 2MgCl2 + Hf
G◦ 1373 = −280.557 kJ
(1)
The process is a batch-type and extremely intensive in term of equipments, labour, and maintenance which, coupling with the high cost of hafnium raw materials and the limited supply of both raw materials and final products, make hafnium an expensive metal ($472 per kg in 2009) [5]. Many other methods, mostly
∗ Corresponding author. E-mail address: amr [email protected] (A.M. Abdelkader). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.11.107
metallothermic in nature, were reported to successfully prepare hafnium on the lab scale, but none of them found way to industry. The FFC-Cambridge process could introduce an affordable alternative to Kroll process [6]. In fact, the production of hafnium powder through the FFC-Cambridge process was reported in the early stage of its development [7]. However, there is no published work so far describing the quality of the product or the reduction mechanism. In addition, zirconium dioxide, which has chemical and conductivity properties very close to that of hafnium, was reported to reduce only partially through the FFC-Cambridge process [8]. The purpose of the present work is therefore to investigate the possibility of using the FFC-Cambridge process to produce hafnium metal and identify the reactions pathway that occur as the HfO2 is reduced to hafnium metal. 2. Experimental Oxide pellets were prepared by mixing hafnium oxide (AlfaAesar, catalogue number 35666, 99.99% with Zr < 0.5%), 1 wt.% of the binder and 10 wt.% graphite powder (Aldrich catalogue number 282863 with average particles size 1–2 m). The binder used in all the current experiments was PVB/PVA (poly-vinyl butyral-co-vinyl alcohol-co-vinyl acetate) 80% by weight vinyl butyral. The niobium doped samples were prepared by adding 5 wt.% of niobium pentoxide (Aldrich, catalogue number 203920, 99.99% purity) to the initial mixture. The as-received materials were ultrasonically mixed in isopropyl alcohol using ultrasonic probe (Branson 450 sonifier) for about 40 min to breakdown any agglomerates and ensure homogeneity. The produced slurry was then partially dried in air and then in a vacuum oven until all the alcohol had vaporized. The dry mass
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Fig. 1. The HfO2 O-ring green pellet used as a cathode precursor in the present work.
was further mixed in some cases using mortar and pestle. About 3 g of the powder was then uniaxially pressed using a hydrolytic press at about 50 MPa with a space die diameter of 20 mm in an O-ring shape (Fig. 1). This shape has many advantageous; it gives higher surface area which facilitates the diffusion of the salt inside the pellet, and it reduces the internal stress that might be caused by further thermal treatments. To prepare the O-ring pellet, a steel cylinder having an outside diameter of 8 mm was fixed vertically in the centre of the die. The pellets that were produced ranged from 3 to 5 mm thickness. The green pellets were then sintered in air at 1273 K for 3 h in a GD-1700 muffle furnace using a Eurotherm® 114 temperature controller at a heating rate of 3 K/min. When carbon burns in air, pores that have the same size of the carbon particles are produced. The electrochemical experiments described in this work were conducted in an Inconel® reaction vessel, housed in a programmable furnace (Lenton), the vessel had a cooler region near the lid where the temperature was below 473 K. The electrolyte used was 1 mol% CaO–CaCl2 prepared from dihydrate CaCl2 (Aldrich C/1500/65, 99% purity), which was heated gradually up to 473 K over 5 days under vacuum, and CaO, which produced by the thermal decomposition of calcium carbonate (Aldrich catalogue number 239216, 99.99%) at 1373 K for 12 h. Details on the assemblage used in the electrochemical experiments and the procedure used to prepare the electrolyte have been described in details in previous studies [9]. About 420 g of the electrolyte was packed in the alumina crucible (Almath CL96). The crucible was then lowered into the bottom of the reactor vessel. The vessel was then sealed by clamping the lid, which carried the electrodes, and argon gas was flushed into the reactor for 3 h to purge the system of atmospheric air. The salt temperature was then ramped to 523 K by 2 K/min and kept at this temperature for 12 h before it was ramped again to 1173 K at a rate of 5 K/min, kept for an hour and finally heated to 1223 K. Once the maximum temperature was achieved, two carbon electrodes (Tokai Carbon HK0 10 mm × 100 mm with a 10 mm M3 thread at one end) were then lowered and the salt was subjected to the preelectrolysis process at 1.3 V for 12 h to remove metallic impurities and any remaining moisture. After finishing the pre-electrolysis, one of the carbon electrodes was lifted to the upper part of the reactor, while the cathode assemblage was lowered into the melt. The cathode assemblage consists of a stainless steel cup (30 mm outer
diameter, 20 mm inside diameter, and 7 mm height with 2 mm holes drilled in the side and bottom walls) holding the oxide pellet. Graphite rod was lowered as pseudo reference electrode in the some experiments. After the electro-deoxidation, the pellet was lifted out from the melt to the cooling zone where it was allowed to cool under a continuous flow of argon gas. The electrochemical experiments were conducted under constant potential difference of 3 V using a PSS-210-GW INSTEK programmable power supply. This device was equipped with Instek PSU software to record the variation of the current versus time as well as the potential. An Agilent 34970A Data Acquisition Switch Unit was used as a high impedance voltmeter to measure the potential between the cathode, anode and a graphite pseudo-reference electrode. After removing from the reactor, the samples were ultrasonically washed with distilled water to remove the attached solidified CaCl2 and carbon residue, filtered, and dried under vacuum at about 333 K. The samples were treated successively with dilute acetic acid, 1 N hydrochloric acid assisted by vacuum impregnation to remove any residual CaCl2 , CaO, or excess Ca. Finally, the samples were rinsed by ethyl alcohol, then with acetone before drying them again in a vacuum oven. Different present phases of the samples were identified by X-ray diffraction (XRD) analysis. XRD analysis was carried out with Philips PW1710 X-Ray Diffractometer under Cu K␣ radiation with the incidence beam angle of 2◦ . The diffraction angle range was between 10◦ and 80◦ with a step increment of 0.02◦ and a count time of 1 s. The phases present were analysed by means of the software Highscore® which using the standard spectra patterns supplied by JCPDS-International Centre for Diffraction Data. Microstructure and chemical composition of the samples were investigated by JEOL JSM-5800LV, CamScan MX2600 scanning electron microscopy (SEM) equipped with an energy dispersive X-ray analyser (EDX) at an accelerating voltage of 15 kV. Non-conductive samples were sputter-coated with gold using an Emitech K550 sputter coater. Some of the produced hafnium powder was imaged by the JEOL 6340F FEGSEM, which allowed for high resolution SEM images. Oxygen content of the samples was quantitatively determined in a destructive technique using an Eltra ONH-2000 analysers. This machine uses the hot extraction technique and infrared detection of the carbon monoxide evolved.
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A.M. Abdelkader, D.J. Fray / Electrochimica Acta 64 (2012) 10–16 Table 1 Sequence of cathodic reaction steps based on the calculated standard electrode potentials (versus Ca/Ca2+ at 1173 K).a Stage
E◦ (V)
Reaction +2
I II III
−2
2HfO2 + Ca + O = CaHfO3 + Hf CaHfO3 + 4e− = Hf+ Ca+2 + 3O2− 2HfO2 + Ca+2 + 2e− = CaHfO3 + HfO CaHfO3 + 2e− = HfO + Ca+2 + 2O2− HfO + 2e− = Hf + O2− (4) (5) HfO2 + 4e− = Hf + 2O2−
(2) (3)
+0.2 V >0.55 >0.2 Unknown +0.272
a The standard potentials was calculated based on the thermodynamic date available from HSC chemistry, Version 6.0.
Fig. 2. Current–time curve for HfO2 pellet electro-deoxidised at constant potential difference of 3 V in molten CaCl2 – 1 mol% CaO melt.
3. Results and discussion 3.1. Electro-deoxidation of pure hafnium oxide Upon applying a constant potential difference in any twoterminal cell, the cell response adjusts according to the electrochemical reactions that take place on the surface of the two electrodes. The response, usually recorded as the variation of current over time, is therefore a useful tool to understand the reactions. In the present work, the anodic and cathodic potential are recorded versus graphite pseudo reference. To ensure a stable reference electrode, the graphite pseudo reference was replaced every 12 h. A typical time-current plot, recorded when 3 V constant potential difference was applied between a cup cathode holding a 3 g pellet of HfO2 and a graphite anode, is illustrated in Fig. 2. The key features of the current–time curve are: (1) sharp decline of the current in the first 7 min (up to point A), (2) current-shoulder in the 7–70 min range (feature B), (3) decrease linearly between 1 and 3 h and parabolicaly between 3 and 5 h (features C and D), and (4) a steady time-invariant current starting from about 8 h until the end of the run (feature E). 3.1.1. Feature (A) The sample collected after 7 min of the electro-deoxidation has a dark grey layer on the bottom surface and on the area around the wire collector in the top part. The XRD trace in Fig. 3a indicated it is mainly ␣-Hf with some traces of HfO2 . The fast reduction of the surface layer is a well-documented phenomenon and was mathematically described through the thin-layer 3PIs model [10]. Using the cup-cathode in the present work accelerates the surface metallisation by increasing the number of contact points. The thin-layer 3PI is always reflected in the cell response by quick drop in the current, which agrees well with Fig. 2. Also, the high resistance oxide on the surface caused short-lived iR potential drop that diminishes as the reduction starts to propagate in the surface layer. 3.2. Feature (B and C) Once the surface layer metallised, the reduction starts to propagate into the core of the pellet and follows what is known as the penetration 3PI model [11]. The rate of the progression in the depth direction is slow and allows the intermediate steps of the reduction to be distinguished. The first reaction to take place is expected from the thermodynamic date (Table 1) to be the interaction of HfO2 with electrolyte forming calcium hafnate with the emergence of hafnium monoxide. The later is a high temperature
phase that dissociates at lower temperature to HfO2 and Hf when the sample cools down [12]. The XRD analysis agrees well with the thermodynamic calculations. The XRD pattern of the sample electro-deoxidised for 15 min detected CaHfO3 for the first time (Fig. 3b). As the pellet is electrolysed for longer time, i.e. 0.5 and 1.5 h, the intensity of CaHfO3 increases while that of HfO2 decreases (Fig. 3 c and d). However, the potential value at the first 4 h (+0.2 V versus Ca/Ca2+ ) is more negative than that expected from the thermodynamic calculation of reaction (2). By analogy with the niobium oxide electro-deoxidation [13], reduction of HfO2 on the surface layer to hafnium metal through quick steps of forming calcium hafnate and hafnium monoxide (represented by the overall reaction (5)) could be kinetically more favourable than forming CaHfO3 in the core of the pellet which explains the high negative potential. After 3 h of the electro-deoxidation, the total amount of hafnium is detectable by the XRD measurements (Fig. 3e), indicating the metallisation of hafnium is not limited to the surface layer but for some micrometers beneath it. The maximum thickness of the metallised surface crust is about 500 m for the sample electro-deoxidised for 4 h (Fig. 4). On the other hand, the formation of perovskite CaHfO3 is still taking place in the core of the pellet as the sample terminated after 4 h shows HfO2 , and CaHfO3 as the main components of the interior part (Fig. 3f). Converting HfO2 to perovskite phase in the core of the pellet is thermodynamically possible either electrochemically through reaction (2) or chemically in the presence of oxygen (resulted from metallising the surface curst) through reaction (6). The SEM Images of the partially reduced sample gives a good idea about the mechanism of forming CaHfO3 : HfO2 + Ca+2 + O2− = CaHfO3 ,
G◦ 1173 = −93.758 kJ
(6)
Fig. 5 shows the SEM image of the sample after 2 h of the electrodeoxidation. Layered structure of the perovskite phase starts to form by consuming irregular small particles of HfO2 . The transformation from the monoclinic HfO2 to the cubic NaCl crystal structure of the perovskite needs a complete reconstruction of the lattice starting by destroying the crystal of HfO2 , which explain the irregularity of the HfO2 particles. These particles have high surface area which speeds up the reactions with calcium ions to readily form the crystalline CaHfO3 . The layered nucleation pattern of the perovskite gives more support for the electrochemical route of forming CaHfO3 as such kind of directional crystallographic orientations is common in the electrochemical processes [14,15]. Growth of the layers continued in one preferred direction forming wedge-like structure of CaHfO3 as can be deduced from the sample electro-deoxidized for 3 h (Fig. 6). 3.2.1. Feature (D) The XRD analysis of the samples taken after 5 and 6 h, i.e. during feature D shows almost no changes from that after 3 h. However, the cores of the pellets are stronger and harder to break. The pellet was ground using a ball mill for an hour in order to analyse the
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Fig. 3. XRD traces of the HfO2 electro-deoxidized in molten CaO–CaCl2 at 3 V measured (a) at the surface of the pellet after 7 min; and measured for the whole pellet (b) after 15 min, (c) after 30 min, (d) after 1.5 h, (e) after 3 h, (f) after 4 h, (g) 6 h, and (h) after 16 h.
obtained powder by SEM. The SEM images show prisms with welldefined edges indicating the CaHfO3 phase is still in its growing stage (Fig. 7). Indeed, insertation of calcium oxide to the hafnium oxide increases the total volume of the crystal and produces more dense structure in the core of the pellets. The non-porous core
explains not only the hard-to-break pellets, but also the slow kinetic of the electro-deoxidation of the interior parts which approaches almost zero at the end of feature D. The oxygen analysis of the samples taken at the edges of period D shows only 4% reduction in the total oxygen content during feature D.
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Fig. 4. Optical microscope images for the HfO2 pellet after (a) 15 min, (b) 60 min, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h of the electro-deoxidation (the pellet thinness is 3 mm).
3.2.2. Feature (E) The invariation of the value of the current observed during this period suggests no change in the process that takes place during this stage. The SEM results agree with this suggestion; the images of the sample taken from the core of the pellets electrodeoxidised for 10 and 14 h show well-defined-edges prisms of CaHfO3 (Figs. 8 and 9). The strength of the pellets increased and longer time is now needed to grind the core (3 h). In addition, the width of the calcium hafnate peaks in the XRD pattern shrinks for
longer times of the electro-deoxidation indicating an increase in the phase crystallinity. Therefore, the process represented by feature E is the crystallisation of the hafnate phase due to annealing the pellet in the high temperature molten salt. This crystallisation blocks the core of the pellet from further reduction not only by reducing the porosity, but also by reducing the disordering in the lattice. The stopping of the electro-deoxidation reactions is discussed further as follow: The electro-deoxidation of an oxide needs (1) energy enough to break the oxygen-cations bond and (2) substantial oxide ion
Fig. 5. SEM Image for the perovskite during its growth taken after 2 h of the electrodeoxidation of HfO2 pellet.
Fig. 7. The CaHfO3 prism after 5 h of electro-deoxidation, growing in size and forming sharp edges, which was strong enough to sustain 1 h of milling without damaging the edges.
Fig. 6. Long prism of CaHfO3 still in its growing stage after 3 h of electro-deoxidation.
Fig. 8. SEM image showing the dense core of the pellet after manually removing the metal surface crust.
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Fig. 9. The perovskite prisms produced by electro-deoxidation a pellet of HfO2 for 8 h after ball milling the powder for 3 h.
conduction in the oxide phase, in the oxide/electrolyte interface, and in the electrolyte. Since the migration of oxide ion is known to be fast in the CaO–CaCl2 system, the last reason is excluded from the current discussion. From the energy point of view, the thermodynamic calculation shows that CaHfO3 is a very stable compound and reducing it through the electro-deoxidation process might not be possible without depositing calcium on the cathode. The chemical stability of the calcium hafnate is due to the nature of the bonds in the compounds. It has been reported that interaction of charges between O and Hf due to O 2p and Hf 5d hybridisation makes the Hf-O bond covalent while the Ca-O bond is ionic [16]. The energy required to break these mixed ionic-covalent bonds is high, which hinders the electro-deoxidation of perovskite hafnate at moderated temperature unless high negative potential is applied. The present study, however, is limited to 3.0 V to ensure no chlorine evolution on the anode. The second reason that stops the electro-deoxidation of HfO2 at the stage of forming CaHfO3 is kinetic in nature. It is well known that the migration of oxide ion in the perovskite hafnate is a function of the oxygen vacancies in the lattice [17]. Reducing the disordering in a large crystal similar to that shown in Fig. 6 makes the movement of the oxide ion (and consequently the deoxidation
15
Fig. 10. Current–time diagram for the HfO2 doped by 5 wt.% Nb2 O5 pellet electrodeoxidized for 24 at constant potential difference of 3 V in CaO–CaCl2 .
process) a very slow process. In addition to the slow migration inside the crystal, the oxide ion have to diffuse through the intergranular boundaries to another grain and then to the metallic surface crust as the solid/electrolyte interface is limited only to the pellet surface due to the extinction of interior pores. It should be mentioned here that, although the applied potential difference is theoretically enough to deposit calcium from the CaO in the electrolyte, no calcium deposits were observed on the current collector. Also the cathodic potential never shifted to a negative value that might be claimed to be for calcium deposition (about 1.35 V versus graphite pseudo reference [18]). The increase in the overall cell resistance due to the presence of the extremely high electric resistance perovskite on the cathode is believed to be responsible of preventing calcium deposition by increasing the iR loss and lowering the potential at the cathode. 3.3. Electro-deoxidation of hafnium oxide doped by niobium oxide The formation of the well crystalline perovskite CaHfO3 is believed from the previous section to delay the electro-deoxidation process due to the stability of CaHfO3 and the slow migration of the oxide ion in the lattice and in the oxide intergranular boundaries. The oxide ion migration in perovskite-type oxides depends
Fig. 11. XRD traces for the HfO2 doped by 5 wt.% Nb2 O5 pellet electro-deoxidized for (a) 24 h and (b) 36 h at constant potential difference of 3 V in CaO–CaCl2 .
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Fig. 12. The cubes of Hf–Nb alloys produced after 36 h of electro-oxidation of HfO2 doped by 5 wt.% Nb2 O5 pellet at constant potential difference of 3 V in CaO–CaCl2 .
on the formation of oxygen vacancies [19]. It is well known that these vacancies, and consequently the oxide ion conductivity of perovskites, ABO3 , increases significantly by doping of different valence cations into the A or the B sites [20]. Recently, niobium has been reported to be one of the best B-site dopants for improving the oxygen conductivity [21,22]. Therefore, a series of experiments was conducted in order to study the effect of niobium doping on the electro-deoxidation of hafnium dioxide. Fig. 10 shows the electrochemical response of a cell when 3 V constant potential difference was applied between carbon anode and a stainless steel cup cathode containing a pellet of HfO2 doped by 5 wt.% Nb2 O5 . The recorded current is higher than that observed with pure hafnium oxide which by itself indicates more reduction in the cathode pellet (2040 C was passed compared with 6048 C passed in the pure hafnium cathode). The pellet taken after 24 h of electro-deoxidation was very porous and easy to grind manually to give very fine black powder. The XRD analysis shows the co-existence of Hf–Nb cubic alloy with some calcium hafnate as a minor phase (Fig. 11). However, the SEM images shows no large prisms or any other form of well-crystalline phases indicating the reduction is not hindered in the core of the pellet. By running the electro-deoxidation for longer time (36 h), only the Hf–Nb alloy is detected by the XRD (Fig. 11). The oxygen content of the produced powder is 0.8 wt.%.
Fig. 13. SEM image of the hafnium particles formed on the surface of the pellet after 16 h of electro-deoxidation.
The SEM image (Fig. 12) of the sample electro-deoxidised for 36 h shows cubes of the powder reflecting high crystallinity which means the metallic phase has sufficient time to crystalline after its formation. Comparing the alloyed hafnium powder (Fig. 12) to that of the pure hafnium (Fig. 13) proves that the dopant materials is not only useful for facilitating the reduction of the perovskite phase, but in producing morphology that can resist back oxidation during handling and kinetically stabilised against self-ignition in air. These results created an opportunity for more investigations on the electro-deoxidation of hafnium oxide. It is now clear that producing pure hafnium powder with quality meeting the commercial standards is not possible via the FFC-Cambridge process. However, doping the initial oxide precursor with another oxide of different oxidation states was proved to facilitate the oxide ion migration in the calcium hafnate intermediate phase by forming oxygen vacancies. Moreover, the reduction is expected to proceed even faster if the perovskite structure of CaHfO3 double doped in the Ca-site and Hf-site. By doping the Hf-site with Nb ions, the FFCCambridge process was reported to produce Hf–Nb alloy with low oxygen. 4. Conclusions The concept of the electro-deoxidation was tested for HfO2 in molten calcium chloride. The results obtained from the electrodeoxidation of a HfO2 pellet under constant potential difference indicated that the reduction was blocked in the core of the pellet associated with the formation of CaHfO3 . The recorded cathodic potential shifted to less negative values during the formation of the perovskite phase, which was related to the switch from the metallisation of HfO2 on the surface to perovskitisation of HfO2 in the core of the pellet. An interesting finding of the present work is the effect of mixing HfO2 with Nb2 O5 in the initial pellet. The charge passed was higher than that observed with undoped HfO2 . The pellet obtained after 36 h of electro-deoxidation was an Hf–Nb alloy with 0.8 wt.% oxygen content. The obtained powder was well crystallised in cubic morphology with particle sizes ranging between 5 and 20 m which protected the powder from back oxidation upon air exposure or during the washing step. References [1] J.H. Schemel, ASTM Manual on Zirconium and Hafnium, American Society for Testing and Materials, Philadelphia, 1977. [2] V.M. Beglov, B.K. Pisarev, G.G. Reznikova, Metal Sci. Heat Treatment 34 (1992) 251. [3] C. Ribaudo, J. Mazumder, J. Metals 40 (1988) 120. [4] S. Ramakrishnan, M.W. Rogozinski, J. Phys. D: Appl. Phys. 30 (1997) 636. [5] J. Gambogi, United States Geological Survey, Retrieved 2008-10-27, 2009. [6] G.Z. Chen, D.J. Fray, T.W. Farthing, Nature 407 (2000) 361. [7] G.Z. Chen, D.J. Fray, Understanding the electro-reduction of metal oxides in molten salts, in: A.T. Tabereaux (Ed.), Light Metals 2004, Warrendale, Minerals, Metals & Materials Soc, 2004, p. 881. [8] K.S. Mohandas, D.J. Fray, Metall. Mater. Trans. B 40 (2009) 685. [9] A.M. Abdelkader, D.J. Fray, Electrochim. Acta 55 (2010) 2924. [10] W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, X.H. Hu, G.Z. Chen, Chemphyschem 7 (2006) 1750. [11] W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, G.Z. Chen, Chem. Eur. J. 13 (2007) 604. [12] M.B. Panish, R. Liane, J. Chem. Phys. 38 (1963) 253. [13] A.M. Abdelkader, PhD thesis, University of Cambridge, 2011. [14] M.R. Rahman, T. Okajima, T. Ohsaka, Chem. Commun. 46 (2010) 5172. [15] W. Ye, J. Yan, Q. Ye, F. Zhou, J. Phys. Chem. C 114 (2010) 15617. [16] D. Cherrad, D. Maouche, M. Reffas, A. Benamrani, Solid State Commun. 150 (2010) 350. [17] V.V. Kharton, A.A. Yaremchenko, E.N. Naumovich, F.M.B. Marques, J. Solid State Electrochem. 4 (2000) 243. [18] C. Schwandt, D.J. Fray, Z. Naturforsch. A 62 (2007) 655. [19] L.G. Tejuca, J.L.G. Fierro, Properties and Applications of Perovskite-type Oxides, Dekker, New York, NY, 1993. [20] T. Ishihara, Perovskite Oxide for Solid Oxide Fuel Cells, Springer, Dordrecht/London, 2009. [21] K. Zhang, R. Ran, L. Ge, Z. Shao, W. Jin, N. Xu, J. Membr. Sci. 323 (2008) 436. [22] T. Nagai, W. Ito, T. Sakon, Solid State Ionics 177 (2007) 3433.