C precursors in molten calcium chloride

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Direct electrochemical synthesis of zirconium carbide from zirconia/C precursors in molten calcium chloride Lei Dai, Xianyu Wang, Huizhu Zhou, Yao Yu, Jing Zhu, Yuehua Li, Ling Wang

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Cite this article as: Lei Dai, Xianyu Wang, Huizhu Zhou, Yao Yu, Jing Zhu, Yuehua Li, Ling Wang, Direct electrochemical synthesis of zirconium carbide from zirconia/C precursors in molten calcium chloride, Ceramics International, http://dx.doi.org/ 10.1016/j.ceramint.2014.12.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Direct electrochemical synthesis of zirconium carbide from zirconia/C precursors in molten calcium chloride Lei Daia,b, Xianyu Wanga, Huizhu Zhoua, Yao Yua, Jing Zhua, Yuehua Lia, Ling Wanga,b* a College of Chemical Engineering, Hebei United University, Tangshan, Hebei 063009, P. R. China b Hebei Province Key Laboratory of Inorganic and Non-Metal Materials, Tangshan, Hebei 063009, P. R. China

*Corresponding author: Tel.: +86 315 2592170; Fax: +86 315 2592170. E-mail address: [email protected]

Abstract ZrC was successfully synthesized by direct electrolysis of non-sintering ZrO2/C mixture in molten CaCl2 electrolyte. The process of the electrochemical solid state reduction was investigated by characterizing the composition and microstructure of the products under different experimental conditions, in conjunction with analyzing the voltammetric features of initial powders in molten CaCl2. The results showed that the pure ZrC powder could be obtained by direct electrolysis of non-sintering ZrO2/C mixture in 1123 K CaCl2 melt at 3.1 V for 7 h when carbon content in mixture was over stoichiometric ratio of ZrC. Besides the role of carbon source, carbon also promoted the electrochemical reduction of ZrO2. The reaction process involved the electrochemical reduction of ZrO2 in the presence of carbon to form ZrC with CaZrO3 as intermediate. 1

Keywords: Zirconium carbide; Electrochemical reduction; CaCl2 molten salt; ZrO2; Carbon

1. Introduction Zirconium carbide (ZrC) which is one of the refractory transition metal carbides, has many attractive properties, such as high melting temperature, great hardness, excellent mechanical stability, relatively lower density compared with other refractory carbides, high emissivity and current capacity at elevated temperatures [1-2]. As an important

technological

material

， it

can

be

applied

in

cutting

tools,

thermophotovoltaic radiators, coating of nuclear particle fuels and field emitter tips [3]. Currently, several typical processes have been developed for the preparation of ZrC, such as carbothermal reduction, direct reaction of mechanically activated Zr and C in Ar, preceramic polymer method, sol–gel process, chemical vapor deposition and Mg-thermite method [4-9]. Among the available synthesis methods, the most commonly chosen route for the production of ZrC is carbothermal reduction due to the simple process and suitability for production at industrial scale. However, high temperature (>2073 K) and long production period are usually required in this process [10, 11], which means high energy consumption and low production efficiency. Therefore, it is significant to improve the production process by lowering reaction temperature and shortening reaction time. In the decade, direct electrochemical reduction of solid metal oxides into metals 2

in molten salts has been widely investigated, not only for the extraction of a lot of reactive metals such as Ti and Cr, but also for the synthesis of many functional alloys and inorganic materials [12-16]. More recently, TiC and HfC have been prepared by electrochemical reduction of TiO2 or HfO2 in the presence of carbon due to the strong affinities between formed metals and carbon [17, 18]. In their works, oxide/C mixture pellets were firstly sintered at high temperature in inert atmosphere, and then electrolyzed as cathode below 1273 K to obtain the desired products. In addition, carbon as additive was also used to accelerate electrochemical reduction process by increasing the porosity and O2− vacancies of the samples during the sintering process [19, 20]. The purpose of the present work is to directly electrolyze ZrO2/C precursors in molten CaCl2 electrolyte for the preparation of zirconium carbide at relatively low temperature. In order to simplify the process, ZrO2/C mixture was directly preformed and electrolyzed without sintering step. Reliable results were obtained for the direct electrolysis of ZrO2/C mixture to form ZrC at 1123 K and the formation mechanism of ZrC was also investigated. 2. Experimental 2.1. Fabrication of ZrO2/C mixture cathode Pure ZrO2 powder (99.9%) and carbon powder were used to form the cathode precursor in the present work. ZrO2 and carbon powder were weighed and mixed with different molar ratios. The mixture was then thoroughly milled in a ball-milling machine for 4 h. After drying, the mixture was pressed into pellets of 10 mm in 3

diameter and 3 mm in thickness, and then assembled into cathode by wrapping the non-sintering pellets with Kanthal wire mesh. In this way, the contact area between the current collector-Kanthal wire and the pellets was increased. Meanwhile, the powdering and falling down of the pellets could be avoided. 2.2 Constant voltage electrolysis The electrolysis experiments were conducted in a molten salt reactor which consisted of a vertical tubular stainless steel vessel placed inside a vertical tube furnace. An alumina crucible filled with about 800 g of dehydrated CaCl2 was placed at the bottom of the stainless steel vessel. Argon gas was continuously introduced into the reactor. After air in the reactor was completely replaced by Ar, the furnace was heated, reached and kept at the pre-set temperature. When CaCl2 was melted, a pre-electrolysis was conducted under a constant voltage of 2.5 V for 0.5 h to remove the moisture and impurities. The ZrO2/C mixture cathode and high purity graphite rod anode were then inserted into CaCl2 melt, and the electrolysis was performed at a constant voltage. The experiment data was collected by a PC computer aid system. After electrolysis, the specimen was lifted from the molten salt and naturally cooled with the furnace in a stream of argon before removal from the vertical vessel, and then was immediately washed in distilled water. The phase composition of the samples was determined through X-ray diffraction (XRD, D/MAX2500PC) analysis under Cu-Kα radiation with the incidence beam angle of 2° in the range of 10∼90°. The microstructure of the samples was characterized by field emission scanning electron microscopy (SEM, S-4800).

4

2.3 Cyclic voltammetry The CV experiment was conducted in a three-electrode configuration. The working electrode is a metallic cavity electrode (MCE), which consists of a molybdenum foil (width: 10 mm, thickness: 0.5 mm, length 20 mm, purity: 99.9%) with one circular hole (1.0 mm diameter) filled in sample powder by repeatedly finger-pressing. High purity graphite and Kanthal wire were used as the counter electrode and the pseudo-reference electrode, respectively. The cyclic voltammetry (CV) measurements were carried out in 1123 K CaCl2 melt in argon atmosphere using electrochemical work station (ZAHNER IM6e). 3. Results and discussion 3.1 Morphology of ZrO2/C mixture before electrolysis In order to achieve uniform dispersion of carbon and ZrO2 powder, the mixture with a certain ratio of ZrO2/C was ball-milled for 4 h. Based upon the SEM micrographs and EDS analysis for sample pellet with the molar ratio of 1:2 before electrolysis (Fig.1), it was clearly seen that carbon particles were flake-like with a size of micron level, while the ZrO2 particles were sphere-like with a size less than 200 nm. The significant difference in the particles size and morphology resulted in insufficient contact between ZrO2 particles and carbon flakes, although the carbon content in the mixture was doubled in comparison with ZrC stoichiometry. In addition, the pellet was porous, which was beneficial for the diffusion of molten salt into the pellet interior.

5

3.2 The possibility of carbothermal reaction of ZrO2 and C in molten salt Theoretically, carbothermal reaction of ZrO2 and carbon can occur following reaction (1) ~ (3) under certain conditions. However, based on thermodynamic calculation, the operating temperature (1123 K) in present work was too low for the reactions to take place. In order to verify the thermodynamic calculation results, a preliminary investigation was conducted to determine the effect of immersion of the sample pellet in CaCl2 molten at 1123 K on its chemical composition. Fig. 2 showed the XRD pattern of the sample immersed in the CaCl2 molten at 1123 K for 7 h. It was clearly shown that the phase composition of the sample pellet after and before the immersion experiment had not any change, which was consistent with the theoretical calculation. ZrO2 + C = ZrC + O2 (g) ZrO2 + 3C = ZrC + 2CO (g) ZrO2 +2C = ZrC + CO2 (g)

△G △G △G

1123 K

1123 K

1123 K

= 699.387 kJ/mol = 277.947 kJ/mol = 303.430 kJ/mol

(1) (2) (3)

3.3 Constant voltage electrolysis of the samples with different ZrO2/C molar ratios The electrolysis experiments were carried out at 3.1 V (versus graphite anode). The chronoamperograms of the sample pellets with different ZrO2/C molar ratios were recorded and displayed in Fig.3. For the sample with ZrO2/C molar ratio of 1:1, the chronoamperogram showed that the output current dropped rapidly in the first 50 min due to the fast reduction of the surface layer and then slowly decreased to a plateau due to the slow progression in the core of the pellet. It was interesting to note that the chronoamperograms of samples with higher carbon content exhibited 6

different features to the sample with ZrO2/C molar ratio of 1:1. With the increase of carbon content in the sample, the output current dropped more slowly and then kept at a higher current level. The appearance of the high current plateau indicated that the role of carbon could speed up the process of the reduction in the interior part of the pellet. In order to further determine the role of carbon during the electrolysis process, the samples with different ZrO2/C molar ratios were electrolyzed at 3.1 V for 7 h and then the products were analyzed using XRD to determine the phases presented, as shown in Fig.4. For the sample with ZrO2/C molar ratio of 1:1, the diffraction peaks of ZrC phase in the product were detected. However, the main phase was CaZrO3 and there was no ZrO2 or Zr phase appeared in the product. Based on prior reports [16, 21, 22], O2− from the partial reduction of ZrO2 in the electrolysis process firstly reacted with Ca2+ to form CaO. And then the remaining ZrO2 reacted with CaO following reaction (4) to form CaZrO3. The reaction was a spontaneous process according to the

△G

1123K

of reaction (4).

ZrO2 + CaO = CaZrO3

△G

1123 K

= -38.828 kJ/mol

(4)

When the ZrO2/C molar ratio was 1:1.5, the obtained product mainly consisted of ZrC and only trace of CaZrO3 existed except for carbon residue. To further increase carbon content up to ZrO2/C molar ratios of 1:2, the product after electrolysis for 7 h only contained ZrC and remaining carbon. Compared with the sample with low carbon content, the samples with higher carbon content displayed higher reduction rate and purity of the product. The possible 7

reason was as follow. The electro-reduction of a solid insulator compound-ZrO2 should take place at the conductor/insulator/electrolyte three phase interlines (3PIs). The propagation rate of the 3PIs into the solid phase determined the overall reduction speed. The process involved consecutive transfer and transportation steps: (1) electron transportation in the electronic conducting and transfer at the solid/solid interface; (2) ion transfer at the solid/liquid interface and ion diffusion in the ionic conducting liquid. Obviously, difficulty in any step of these transfer and transportation would affect the overall reduction speed. For all these samples in the process, the ion transfer and diffusion were the same. Therefore, the charge transportation and transfer should be the limiting steps in comparison with other steps. According to this understanding, the slower reduction kinetics and incomplete reduction of the sample with lower carbon content might be related with two factors: (1) low 3PI length due to the lower dispersion of carbon in ZrO2, resulting from the significant difference of particle size and morphology (as shown in Fig.1); (2) no formation of adequate electronic network due to lower carbon content, which in turn slowed down the 3PI propagation rate. Considering the electronic conduction function of carbon and the difference of two kind of particles in size and morphology, more carbon was needed to achieve the complete electro-reduction and carbide of ZrO2 under this experimental condition. 3.4 Influence of cell voltage For determining the influence of cell voltage on the electrolysis process, the standard Gibbs free energy changes (∆G) and decomposition voltages (∆E) of possible referred reactions ((5)-(9)) at 1123 K were calculated based on 8

thermodynamic data. In the meanwhile, the samples with ZrO2/C molar ratios of 1:2 were electrolyzed at different voltages for 7 h in 1123 K CaCl2 melt and then phase compositions of products after electrolysis were analyzed using XRD. The experimental results were shown in Fig.5. ZrO2 = Zr + O2

∆G1123K =885.705 kJ/mol ∆E1123K=2.29 V

(5)

CaZrO3 = Zr + CaO + O2

∆G1123K=924.533 kJ/mol ∆E1123K=2.40 V

(6)

Zr + C = ZrC ZrO2 + C = ZrC + O2

∆G1123K=-186.318 kJ/mol

(7)

∆G1123K=699.386 kJ/mol ∆E1123K=1.81 V

(8)

CaZrO3 + C = ZrC + CaO + O2 ∆G1123K=738.215 kJ/mol ∆E1123K=1.91 V

(9)

When a voltage of 2.0 V that was lower than the theoretical decomposition voltage of ZrO2 (2.29 V) was applied, the product showed the same phases as the raw materials (ZrO2 and carbon) in the pellet, which indicated that no reaction occurred during the electrolysis process. Although, the conversion of ZrO2 to CaZrO3 with the participation of Ca2+ could occur spontaneously due to the negative

△G

1123 K

value,

this reaction needed the participation of O2- supplied by the reduction of ZrO2. Therefore, there was no detectable CaZrO3 phase in the product. It was notable that due to the spontaneous reaction between Zr and carbon to form ZrC, the

△G

1123 K

values and decomposition voltages of reaction between ZrO2 (or CaZrO3) and carbon to form ZrC became lower than ones of the decomposition of ZrO2 (or CaZrO3). However, the reactions (8) and (9) did not occur with the voltage of 2.0 V, which might be related to some kinetic complications such as reaction irreversibility. According to the electrochemistry polarization theory, the actual decomposition voltage always deviates from the theoretical decomposition voltage for an irreversible 9

reaction due to the existence of overpotential. In terms of the electrochemical reductions, a polarization potential of the cathode, which is more negative than the equilibrium potential, is needed to effectively start a reduction. With the voltage increasing to 2.6 and 2.8 V, the peaks of ZrO2 and C diminished and ZrC started to appear in the products. These results suggested that ZrC was formed at the expense of ZrO2 and C. CaZrO3 was also formed due to the supply of O2- from the reduction of ZrO2 with the participation of Ca2+. It was notable that there was no Zr detected, indicating that the reaction between Zr and C to form ZrC was preferable. With the voltage increasing to 3.1 V, pure ZrC could be obtained. Fig.6 showed the SEM images of products obtained after electrolysis at different voltages for 7 h. It could be seen that the morphology of product electrolyzed at 2.0 V was similar to the one before electrolysis. This was consistent with the result of XRD, that is, no reaction occurred at 2.0 V. With the voltage up to 2.6 V, the particles seemed to be lightly connected and tend to agglomerate. Compared with the initial powders before electrolysis, the surfaces of the particles of product electrolyzed at 3.1 V became rough and the big particles were converted into smaller particles agglomerates (Fig.6D). 3.5 Voltammetric features of the initial powders in molten CaCl2 The voltammetric behaviors of carbon, ZrO2 and ZrO2/C powders on the MCE in molten CaCl2 at 1123 K were investigated, aiming to further understand the electro-reduction process of the initial powders. Fig.7A showed the CVs of raw powders of the first potential cycle at the scan rate of 10 mV/s. Each of these CVs was 10

recorded on a new electrode under the same condition. For bare MCE (Mo) electrode, the CV showed reduction peaks c1 and c2 formed between -1.3 and -1.5 V until the potential scan was reversed, which could be attributed to the stepwise reduction of CaCl2 [15, 23]. The voltammetric features of the carbon powder (Mo-C) electrode were similar to the ones of the Mo electrode, except for a little larger current peaks c1 and c2, which was attributed to the improved conductivity of the electrode with carbon as a conductive phase. Meanwhile, this phenomenon also indicated that there was no reaction carbon powders participated in. A tiny oxidation peak, a1, appeared on all the CVs of the Mo and Mo-C electrode, which was obviously due to the re-oxidation of the deposited Ca metal. Compared with the Mo and Mo-C electrode, the CV of ZrO2 powder (Mo-ZrO2) electrode showed larger current from -0.65 V with the forward (negative) potential scan and a shoulder, c3, was seen before the reduction current increased continuously to the current peak, c2. With the reverse (positive) scan, a very small but noticeable oxidation peak, a2, was formed at -0.4 V, which was absent on the CVs of the Mo and Mo-C electrode. There features could be attributed to the redox reaction (10) on the Mo-ZrO2 electrode. ZrO2 + 4e = Zr + 2O2-

(10)

Compared with Mo-ZrO2 electrode, on the CV of ZrO2/C powder (Mo-ZrO2/C) electrode (molar ratio of 1:1), the current began to increase from a more positive potential (-0.45 V) and the current peak c3 became larger, indicating that carbon participated in and promoted the reduction of ZrO2. These features were consistent 11

with the thermodynamic analysis above, that is, the

△G

1123 K

values of reaction

between ZrO2 (or CaZrO3) and carbon to form ZrC was lower than ones of the direct decomposition of ZrO2 (or CaZrO3), which meant that the reactions for the formation of ZrC were more apt to occur. It was noteworthy that an oxidation peak, a3, appeared at -0.6 V with the reverse (positive) scan, which was absent on the CV of Mo-ZrO2 electrode and could be attributed to the re-oxidation of ZrC. In addition, carbon introduced into the sample was an electronic conductor. Therefore, the contact area between conductor and CaCl2 would expand, which was conducive to charge transfer in the reduction process of Ca2+ ions [14], resulting in the increase of the reduction current peaks (c1 and c2). The consecutive CVs of the Mo-ZrO2-C electrode at the scan rate of 10 mV/s were shown in Fig.7B. It was clearly seen from the CVs that the reduction current gradually decreased and the current value in the (n + 1) th cycle was smaller than that in the nth cycle, which meant the gradual depletion of ZrO2 and C during the repeated cycle. This was a typical voltammetric feature for the reduction of solid metal oxides in molten CaCl2 as observed previously [21, 24]. It also was a strong evidence of the irreversibility of the reaction due to the low inherent oxide concentration and irrecoverable escape of the ionized oxygen into molten CaCl2. 4. Conclusions ZrC powder was prepared by direct electrochemical reduction of non-sintering ZrO2/C mixture in 1123 K CaCl2 melt. A series of electrolysis experiments were carried out using ZrO2/C mixture sample as cathode and graphite as anode in order to 12

optimize preparation conditions, such as, ZrO2/C molar ratios and electrolysis voltages. The immersion experiment and cyclic voltammetry, in combination with thermodynamic calculation were conducted to investigate the mechanism of the electrolysis process. It was found that pure ZrC powder could be obtained under the optimal conditions by direct reduction of non-sintering ZrO2/C mixture and the presence of carbon could promote the electrolysis process. Based on the theoretical analysis and experimental results, the electro-reduction process could be split into two steps: (1) the partial reduction of ZrO2 and the interaction of the unreacted ZrO2, the formed O2+ ion and electrolyte to form CaZrO3; (2) the direct electrochemical reduction of ZrO2 and CaZrO3 in the presence of carbon to form ZrC. Acknowledgements The authors are grateful to financial support from Natural Science Foundation of China (51201058, 51272067, 51472073) and Hebei Province Iron and Steel United Foundation of China (E2014209009)

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Figure 1 SEM images of (A) the ZrO2/C mixture pellet with molar ratio of 1:2 before electrolysis, (B) partial enlargement of (A) and (C) EDS Figure 2 XRD patterns of the ZrO2/C mixture with molar ratio of 1:2 after immersed in the molten CaCl2 salt at 1123 K for 7 h Figure 3 Typical current–time plots recorded during electrolysis of the ZrO2/C mixture with different molar ratios in 1123 K CaCl2 melt at 3.1 V Figure 4 XRD patterns of the products with different molar ratios in 1123 K CaCl2 melt after electrolysis at 3.1 V for 7 h Figure 5 XRD patterns of the products with molar ratio of 1:2 in 1123 K CaCl2 melt after electrolysis at different cell voltages for 7 h Figure 6 SEM images of the products with molar ratio of 1:2 in 1123 K CaCl2 molten after electrolysis at different cell voltages for 7 h. (A) 2.0 V; (B) 2.6 V; (C) 3.1V; (D) partial enlargement of (C) Figure 7 CVs of (A) different powder electrode and (B) Mo-ZrO2/C electrode on consecutive potential scan in 1123 K CaCl2 melt at the scan rate of 10 mV/s

17

Fig.1

18

50000

1-C 2-ZrO2

1

Intensity/a.u.

40000

30000

2

20000

2

10000

2

2

2

22 2 2 2 22 2 21 2 22 2 2

0 10

20

30

40

50

60 ο

2θ/ Fig.2

19

2 2 2 222 70

80

90

1.8 1.6

Current/A

1.4 1.2 1.0

Zr:C=1:2 0.8

Zr:C=1:1.5 0.6

Zr:C=1:1 0.4 0

100

200

300

Time/min Fig.3

20

400

500

2 1

Intensity/a.u.

2

3

1-ZrC 2-C 3-CaZrO3

1

2

2

1

1 21 2

2 2

2

31

1

3

3

1:2 1

1

1

1

1:1.5

3 3

1

2

2 1

2

10

20

30

40

3

1 33 3

50

2θ/ Fig.4

1 3 60

ο

21

1

1 70

3

1:1 80

90

2

1-ZrC 2-C 3-CaZrO3 4-ZrO2 4

4

Intensity/a.u.

4 3 2

24 4 4 4 44 444 244 4 1 13 3 1 4 1 2 3

4

2.6V

2 2

1 4 4

1

2

3 1

41 1

2.8V

2

1

1 1

1 3.1V

12 1 22 10

20

30

40

50

2θ/ Fig.5

22

2.0V

3 1

60 ο

70

80

90

Fig.6

23

0.050 0.025

A

a3

a2

0.000

a1

Current/A

-0.025 -0.050

c3

-0.075 -0.100 -0.125

Blank C ZrO2 ZrO2/C

-0.150 -0.175

c1

-0.200 -1.6

c2 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential/V

0.050 0.025

B

0.000

Current/A

-0.025 -0.050 -0.075 -0.100 -0.125

cycle 5

-0.150 -0.175

cycle 1

-0.200 -1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

Potential/V Fig.7

24

-0.4

-0.2

0.0