Electrochemical synthesis of hafnium carbide powder in molten chloride bath and its densification

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Journal of the European Ceramic Society 32 (2012) 4481–4487

Electrochemical synthesis of hafnium carbide powder in molten chloride bath and its densification Amr M. Abdelkader a,∗ , Derek J. Fray b a

b

School of Materials, University of Manchester, Grosvenor Street, Manchester M1 7HS, UK Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Received 7 January 2012; received in revised form 4 June 2012; accepted 9 July 2012 Available online 25 July 2012

Abstract Nanocrystalline powder of hafnium-rich-HfC has been successfully synthesised by the electro-deoxidation of HfO2 –carbon precursors in molten chloride. The progress of the solid state reduction was monitored ex situ by analysing partially reduced samples using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It has been shown that the reduction started by converting HfO2 to CaHfO3 and an oxycarbide phase of the form HfCx O2(1−x). The CaHfO3 phase then also reduced to give HfCx O2(1−x) , which subsequently reduced to HfC by ionising oxygen. The morphological analysis indicated almost no growth in the grain size occurred during the course of the electro-deoxidation. This investigation showed some loss of carbon during the electro-deoxidation resulted in metallic rich HfC. The synthesised powder exhibited better sinterability than the commercial HfC powder. Using the synthesised powder, fully dense monolithic HfC ceramics were produced by pressureless sintering at 1973 K with average grain size of about 3 ␮m. © 2012 Elsevier Ltd. All rights reserved. Keywords: Hafnium carbide; Nanoparticles; Electro-deoxidation

1. Introduction Production of hafnium carbide, or composites based thereon, is of interest for use in environments that will experience extreme thermal and chemical environments because of its high melting point (∼4173 K), solid-state phase stability, and good thermomechanical and thermochemical properties.1,2 Hafnium carbides are also promising materials for cutting tools,3 thermophotovoltaic radiators,4 and field emitter tips and arrays5 due to their wear resistance, high emissivity, and high current capacity at elevated temperatures.6 Conventional methods for preparing hafnium carbide include: (1) carbothermic reduction of metal oxides,7 (2) reaction between mixtures of carbon and hafnium metal or hydride,8 and (3) polymer pyrolysis of organometallic precursors.9 In cases where HfC coatings are desired, CVD processes starting from hafnium chloride (HfCl4 ), methane and hydrogen are

∗

Corresponding author. Tel.: +44 01613063549. E-mail addresses: [email protected], amr [email protected] (A.M. Abdelkader). 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.07.010

used.10,11 Amongst the current available preparation method, the solid state carbothermic reduction of HfO2 is the primary route of commercial production of HfC powder. Due to the high temperature nature of the carbothermic process, the produced carbide has relatively large particle sizes. Hence, hot pressing is usually necessary to produce bulk objects with high relative densities even though the products mostly have noticeable residual micropores.6,12 The processing routine could be markedly simplified and the sinterability would be further improved if the HfC particles size was reduced down to nanoscale. Although, wet chemical methods were reported recently to produce nano-sized particles of HfC, the solution-based methods use expensive not readily available compounds that make the scale up of the process not feasible.13–16 In addition, very little information on the densification behaviour of these materials has been presented and it has not been reported if bulk samples could be fabricated by pressureless sintering. A simple, scalable, low temperature technique is thus needed to synthesis nano-sized HfC powder. Recently, scientists at the University of Cambridge have succeeded in producing different metals and alloys directly from their oxides using a novel electrochemical technique known as the electro-deoxidation

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process.17–19 The effort of the present work is directed towards applying the electro-deoxidation concept for bulk production of hafnium carbide nanopowder or nanoparticles using the cheapest hafnium-bearing compound, HfO2 , as a starting material. The results of cathodically reducing HfO2 , in the presence of carbon, in a molten salt electrolysis cell at 1173 K is presented in this paper along with the mechanism of HfC formation. Pressureless sintering of the produced HfC powder was also investigated. 2. Experimental work Pure hafnium dioxide (Alfa-Aesar 99.99% with Zr <100 ppm) and graphite powder (Aldrich with average particles size 1–2 ␮m) were used in the present work to form the cathode precursor. General grade graphite rods (EC17) with 10 mm diameter available from Tokai Carbon Ltd. were used as anodes. Calcium chloride was chosen as the electrolyte in the present work due to its proven capability as an oxygen ion carrier. High purity CaCl2 was produced by vacuum drying of calcium chloride dihydrate CaCl2 ·2H2 O (Aldrich 99% purity). CaO, produced by the decomposition of calcium carbonate (Aldrich 99.99%) at 1100 ◦ C for 12 h, was added to the melt to facilitate oxygen transport. Details of preparing the electrolyte can be found elsewhere.20 The hafnium oxide was first milled for 24 h in ethanol in order to reduce the particle size. The average particle size was determined using the particle sizing analyser (Malvern, Mastersizer 2000) to be about 150 nm. The graphite powder and 2 wt.% of the binder (PVB/PVA (poly vinyl butyral-co-vinyl alcohol-covinyl acetate)) was then added to the oxide and the powder was milled again for 4 h. About 3 g of the powder was then uniaxially pressed using hydrostatic press at about 50 MPa with a space die diameter of 20 mm. The green pellets, about 2–3 mm thickness, were held under a continuous flow of argon gas for 4 h before being heated in four stages: ramping from room temperature to 423 K at 5 K/min, holding at 423 K for 8 h, ramping to 1173 K at 2 K/min, and then holding at that temperature for 2 h. The samples were allowed to cool under argon in the furnace before it were subjected to characterisation process and then stored in a humid-free desiccator. The electrochemical experiments were conducted in a molten salt reactor consisting of a vertical tubular Inconel® vessel with 70 mm inside diameter and 500 mm height placed inside a Vecstar® ceramic-lined 100 mm vertical tube furnace. The cathode was assembled by placing the oxide-carbon pellet in a stainless steel cup (25 mm outside diameter, 21 mm inside diameter, 7 mm height, and 3 mm thick with many 2 mm holes). Further details about the electrochemical setup can be obtained from our previous work.20 Calcium chloride containing 1 mol% CaO was packed in an alumina crucible and heated to 573 K and then introduced into the reaction vessel. The vessel was then sealed and argon gas was flushed into the reactor for 5 h to purge the system of atmospheric air. The furnace temperature was then ramped to 573 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. Once the maximum temperature was achieved, two carbon electrodes were lowered and the salt was subjected to pre-electrolysis process at 1.5 V for 12 h to remove metallic impurities and any remaining moisture. After finishing the pre-electrolysis, a fresh carbon electrode and the cathode assembly were lowered into the salt. The electrochemical experiments were conducted at 3 V constant voltage using PSS-210-GW INSTEK programmable power supply equipped with Instek PSU software. After reduction, the samples were removed from the reactor, ultrasonically washed with distilled water and then leached with dilute acetic acid and HCl assisted by vacuum impregnation to remove the attached solidified CaCl2 and carbon residue. The samples were then dried under vacuum at about 333 K. The existing phases of the samples were identify by Xray diffraction (XRD) analysis using a Philips PW1710 X-ray diffractometer under Cu K␣ radiation with the incidence beam angle of 2◦ in the range of 10–80◦ . The phases present were analysed by means of the software Highscore® . The microstructure of the samples was investigated by scanning electron microscopy (SEM) using JEOL JSM-5800LV, or JEOL 6340F FEGSEM for high resolution images. Oxygen and carbon content of the samples were quantitatively determined using an Eltra ONH-2000 analyzer and Exeter analytical CE-440 elemental analyzer. The carbon content was double checked using LECO CHN-628. Densification of the end-product powder into compact monolithic samples was achieved via a two-step process. In the first step, the powder was uniaxially pressed at 50 MPa and then cold isostatically pressed at 250 MPa. The resulting green pellet was sintered under a flow of argon using an Astro® graphite-resistant furnace. The furnace was ramped to the required temperature at 20 K/min, and kept there for 1 h. The samples were allowed to cool under argon in the furnace and, after cooling, the final density of the sintered samples was measured using the Archimedes’s method with deionised water. 3. Results and discussion 3.1. Stability investigation of HfO2 –C mixture in argon atmosphere and molten salt at high temperature In order to understand the mechanism of reduction, one must be certain of the starting material. To that end two preliminary investigations were conducted to determine the effects of sintering the green HfO2 –carbon pellet in argon and the effect of submerging the pellets in molten salt. Fig. 1a shows the XRD pattern for the sample containing 7 wt.% after heating under a flow of argon at 1173 K for 2 h. Clearly, the sample shows exactly the same phases as found in the green pellet. This finding was constant with the thermodynamic data that expected no reaction between HfO2 and carbon at the testing temperature. HfO2 + 3C = HfC + 2CO(g)

G1173 ◦ = 60.166 kCal (1)

However, after immersing the sintered pellet in the molten salt for 30 min, the XRD analysis detected CaHfO3 . This phase

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Fig. 2. Chronoamperograms for nominally identical HfO2 pellets with 7 wt.% graphite at 1173 K and constant voltage of 3 V.

Fig. 1. XRD spectra of HfO2 after (a) 1 h sintering in Ar at 1173 K, (b) submerged in the electrolyte for 30 min and 0 min electro-deoxidation, and (c) 30 min, (d) 1 h, (e) 2 h, (f) 3 h, (g) 4 h, (h) 5 h, (i) 6 h, (j) 7 h, (k) 8 h, (l) 16 h of constant voltage (3 V) electro-deoxidation at 1173 K.

results from the reaction between HfO2 and the dissolved CaO (reaction (2)) due to the high activity of the later in the molten salt. HfO2 + CaO(l) = CaHfO3

G1173 ◦ = −22.409 kCal

complete the reduction is 17,545 C, calculated assuming only the electro-deoxidation of HfO2 , the recorded charge passed indicated that full reduction of the oxide phase could not be achieved without carbon addition under the applied conditions. In order to determine the mechanism of the reduction of hafnium oxide and the role of carbon during the course of electro-deoxidation, a series of interrupted constant voltage experiments were conducted. Pellets were lowered into the electrolyte for 30 min prior to the electro-deoxidation in order to allow thermal equilibration and wetting of the pellet by the molten salt electrolyte. A 3 V constant voltage was then applied for different intervals, the products were then washed and analysed using XRD to determine the phases present. In order to gain a representative phase analysis the pellets were ground and the powder analysed, the results of which are shown in Fig. 1. According to Fig. 1, the reduction process can be divided into three main stages:

(2)

3.2. Constant voltage chronoamperometry The chronoamperogram of the HfO2 pellet that contained 7 wt.% carbon and was cathodically polarised at 3 V versus graphite anode is displayed in Fig. 2. The chronoamperogram shows that the output current declines in the first 8 h following a parabolic scheme. The current for the last 6 h of the run is featureless and almost constant at a value of 0.5 A. It is interesting to note that the chronoamperogram exhibits different features to that reported for the electro-deoxidation of HfO2 without carbon addition.21 When no carbon was added to the pellet, the current dropped rapidly due to the fast reduction of the surface layer and then increased to a plateau due to the slow progression in the core of the pellet.21 The disappearance of the high current plateau clearly shows the role of carbon on providing a conductive phase within the pellet which speeds up the progress of the reduction in the interior part of the pellet. More interestingly, the charge passed was much higher when carbon was added to the pellet. The charge passed was 18,340 C and 6048 C with and without carbon respectively. As the theoretical charge required to

Stage I: during the first 3 h of reduction, the peaks of monoclinic HfO2 and carbon diminished and peaks of CaHfO3 and HfC started to appear which increased with time. These results suggest that both CaHfO3 and HfC were formed at the expense of HfO2 and C. The peaks of cubic HfC were shifted towards the large angles. The lattice parameter of the carbide ˚ during this phase was almost constant at a value of 4.623 A stage of reduction as can be seen from Fig. 3. This value is markedly smaller than that reported for stoichiometric HfC ˚ 22 ). This is a characteristic of the presence of oxy(4.643 A gen in the carbide lattice forming a cubic oxycarbide phase of the form HfCx O2(1−x) .23–26 The constant value of the lattice parameter suggested that oxygen/carbon ratio was unchanged during this stage of reduction. Stage II: this stage takes place between 3 and 7 h of the electrodeoxidation and it is characterised by the decrease of the CaHfO3 peaks and the increase of the HfCx O2(1−x) peaks. By the end of this stage, there was no detectable CaHfO3 or carbon, suggesting that all the tetravalent hafnium compounds have been reduced to HfCx O2(1−x) phase. The value of the cell parameter for the phase HfCx O2(1−x) remained almost

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Based on these results, one can summarise the reaction sequence at the cathode as represented by Eqs. (4)–(6) in which the stoichiometric HfOC was chosen as a represented of all the oxycarbide phases 2HfO2 + Ca2+ + 2e− + C = CaHfO3 + HfOC

(4)

CaHfO3 + 4e− + C = HfOC + Ca2+ + 2O2−

(5)

−

HfOC + 2e = HfC + O

Fig. 3. changes of the lattice parameter of the phase HfCx O2(1−x) with time of electro-deoxidation.

unchanged during this stage. It is only when CaHfO3 became the minor phase that the lattice parameter value started to increase slightly. Stage III: This stage continued from 7 h until the end of the electro-deoxidation run and it is characterised basically by the reduction of the HfCx O2(1−x) to HfC as can be perceived from the gradually growth of the cell parameter value. The oxygen content of the produced powder, presented in Table 1, reduced from 5.7 wt.% after 6 h to 6500 ppm after 16 h of the electrodeoxidation. These findings addressed the conditions at which the phase HfCx O2(1−x) was formed and then reduced. Although the oxycarbide phase was reported during the solid state carbothermic reduction of HfO2 and ZrO2 by reaction (3),27,28 the operating temperature in the current work is far too low to form such compound. Also, the currentless experiments (Section 3.1 and Fig. 1a and b) showed no such phases existed after immersing the cathode pellet in the molten salt. Comparing the results of the first stage of electro-deoxidation of HfO2 with and without carbon addition, the mechanism of the oxycarbide phase formation can be elucidated. The electro-deoxidation of pure HfO2 started by the interaction with the salt leading to the formation of CaHfO3 and the hypothesised high temperature phase HfO.21 The reduction mechanism in the presence of carbon is similar, assuming that the hypothesised high temperature phase HfO was stabilised by carbon. HfO2 (s) + 3xC(s) = HfCx O2(1−x) (s) + 2xCO(g)

(3)

Table 1 Oxygen and carbon content of the cathode material after different time of electrodeoxidation. Time (h)

Oxygen content (wt.%)

Carbon content (wt.%)

6 8 10 12 14 16

5.75 4.37 2.44 1.13 0.82 0.65

6.92 6.74 6.51 5.83 5.57 5.12

2−

(6)

Interestingly, the measured carbon content in the samples electro-deoxidised for 12 and 16 h was 5.8 and 5.1 wt.%, respectively, whereas stoichiometric HfC contains 6.3 wt.% C. This loss of carbon can be explained by three hypothesis: (I) physical loss of carbon from the pellet into the electrolyte, (II) dissociation of the HfCx O2(1−x) giving off CO/CO2 gases as shown by Eq. (7), and (III) dissolution of carbon by forming soluble ions either in the melt or in the leaching solution during the washing step. The first possibility can be dismissed because losing around 27 wt.% of the initial carbon (around 26% of the total solid volume of the pellet) would make the remaining powder loosely joined and the pellet would lose its integrity. The second hypothesis is also not possible since the dissociation reaction is expected to happen at temperature higher than the operating temperature. Hence, more focus will be given to the third hypothesis. HfOC + C = HfC + CO(g)

(7)

It was said before that carbonate ions CO3 2− form in the melt as a result of dissolving CO2 that produced by the anode reaction.29 It was also reported that these carbonate ions could find their way to the cathode and react with the cathode material to give carbide phase.20,29 In the current work, CO3 2− ions could form at the cathode by the reaction between carbon and oxygen in the oxycarbide and the oxygen ions CO + O2− = CO3 2−

(8)

To confirm this hypothesis, the electro-deoxidation for 14 and 16 h was repeated and the resulting pellets were washed only with distilled water. The XRD results showed some traces of CaCO3 in both samples, giving some evidence that carbon could have dissolved from the cathode mixture through the formation of carbonate ions. As described by Eqs. (1)–(3), the formation of HfC phase passes through intermediate steps, all of which involve rearrangement of atoms and/or ions into new phases. It is important therefore to investigate the solid-state transformation from one phase to another in order to gain clear insights into the kinetic of the process. The partially reduced sample was subjected to microstructural analysis using electron microscope. Fig. 4 shows the SEM for samples before and after different intervals of the electro-deoxidation. As soon as the conversion of hafnia into CaHfO3 started, the smooth rounded surface of the particles were destroyed and flake-like grains, believed to be HfCx O2(1−x) , appeared as can be seen from Fig. 4b and c. The particle sizes were in general equal or smaller than the initial powder. The relatively large flake-like particles were limited

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Fig. 4. Micrographs of various stages of electro-deoxidation. (a) HfO2 powder before electro-deoxidation, (b) the oxide phase after 45 min of the electro-deoxidation, (c) and (d) overview of the existing morphologies after 2 and 3 h of electro-deoxidation, (e) the round particles of the oxycarbide phase after 8 h of electro-deoxidation and (f) the HfC powder after 16 h of electro-deoxidation.

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3.3. Pressureless sintering of the produced HfC powder Consolidation of the produced HfC powder is required for practical applications. The nanocrystalline powder of HfC that resulted from 16 h of electro-deoxidation was cold pressed using uniaxial isostatic press at 50 and 250 MPa respectively. The green compact has a relative density of 70% as measured by the Archimedes principle with an average grain size of 150 nm. Sintering this compact at 1773 K produced monolithic samples with a relative density of 98.7% and average grain size less than 1 ␮m (Fig. 5a). Increasing the sintering temperature to 1973 produces an almost fully dense sample as can be detected from the SEM image (Fig. 5b). However, the grains have grown in size to be nearly 2 ␮m. It was reported before that the commercially available HfC powder (about 0.7 ␮m) consolidated by spark plasma sintering at 2673 K produces a ceramic with an increase in the grain size to about 19 ␮m.30 Also, pressureless sintering of HfC powder with a particle size of about 225 nm at a temperature 2673 K was reported to produce compacts with a relative density of 98.4% and average grain size of 4 ␮m.27 The obvious increase in the sinterability of HfC powder produced by the electro-deoxidation process can be explained by two reasons. First, the particle size is relatively smaller than the previous work, and second, the carbon content is lower than stoichiometric making the material slightly closer to metallic. 4. Conclusions

Fig. 5. SEM micrograph for the surface of HfC sintered at (a) 1173 K and (b) 2073 K using the powder synthesised by electro-deoxidation for 16 h.

only for the first 6 h of electro-deoxidation, and might be attributed to a HfCx O2(1−x) phase rich in oxygen. Interestingly, the long-prism grains of CaHfO3 that were observed during the electro-deoxidation of pure HfO2 were rarely detected in the current case. After the conversion of CaHfO3 to HfCx O2(1−x) and reducing the oxygen content of the latter, the particles regained their smooth surface and even lightly sintered into 5–10 particles agglomerates. The role of carbon in the electro-deoxidation process can be now discussed. Carbon as the conventional reducing agent formed a more reducing atmosphere in the cathode area and, by forming HfC, made the electro-deoxidation more favourable from the thermodynamic point of view than metallising the HfO2 pellet. However, carbon also played a very important role in accelerating the rate of the process first by introducing a conductive phase throughout the cathode pellet, secondly by stabilising the high temperature phase HfO and preventing its back oxidation. Moreover, the presence of carbon hindered the growth of the CaHfO3 into blocking crystals in the core of the pellet as can be revealed from the microstructure results. However, carbon has detrimental effect on the current efficiency due to introducing many parasitic reactions, such as that occurring by dissolving CO3 2− ions in the melt.

Nanometer sized powder of HfC was prepared using electrodeoxidation in molten chloride bath. A precursor of HfO2 and carbon was used as the cathode and a potential of 3 V was applied between this cathode and a graphite anode. A series of interrupted experiments were conducted and the mechanism of reduction was found to be split into three steps: (1) the interaction of HfO2 with the electrolyte in the presence of carbon to form CaHfO3 and HfCx O2(1−x) simultaneously, (2) the reduction of CaHfO3 to the oxycarbide phase HfCx O2(1−x) , (3) the removal of oxygen from HfCx O2(1−x) to form HfC. Compared with the electro-deoxidation of pure HfO2 , the addition of graphite powder made the reduction faster. With the exception of the first formed HfCx O2(1−x) , the SEM analysis indicated that particles size of the intermediate and the final phases were always smaller or equal to that of the initial HfO2 . The percentage of carbon measured in the synthesised HfC powder was found to be less than the stoichiometric and this drop of carbon in the target material was associated by forming CaCO3 on the cathode. The produced powder showed better sinterability than that reported for commercial HfC powder. Pressureless sintering at 1973 K for the powder synthesised after 16 h of electro-deoxidation showed fully dense monolithic HfC with grain size of about 2 ␮m. References 1. Perry AJ. Refractories HfC and HfN – a survey. Powder Metall Int 1987;19:29. 2. Weimer AW. Carbide, nitride and boride materials synthesis and processing; 1997.

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