Trans. Nonferrous Met. Soc. China 28(2018) 2351-2359

Electrolytic production of Ti-Ge intermetallics from oxides in molten CaCl₂-NaCl

Yin-shuai WANG¹, Xing-li ZOU^1,2, Xiong-gang LU¹, Shang-shu LI¹, Kai ZHENG¹, Shu-juan WANG¹, Qian XU¹, Zhong-fu ZHOU^1,3

1. State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China;

2. Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, USA;

3. Institute of Mathematics and Physics, Aberystwyth University, Aberystwyth SY23 3BZ, United Kingdom

Received 16 October 2017; accepted 24 December 2017

Abstract: Titanium germanium intermetallics (Ti_xGe_y)were directly prepared from titanium oxide (TiO₂) and germanium oxide (GeO₂) powders mixture by using an electrodeoxidation process. The electrochemical experiment was carried out in a molten flux CaCl₂-NaCl at 800 °C with a potential of 3.0 V. The results show that monolithic germanide Ti₅Ge₃ intermetallic can be directly produced from TiO₂-GeO₂ or CaTiO₃-GeO₂ precursors (both molar ratios are 5:3), and the obtained Ti₅Ge₃ powders exhibit homogenous particle structure. In addition, the phase composition of the final product can be dramatically affected by the initial molar ratio of TiO₂ to GeO₂. The reaction mechanism of the electrodeoxidation process was discussed based on the experimental results. It is suggested that the electrodeoxidation process is an environmentally friendly method for the preparation of Ti-Ge intermetallics.

Key words: Ti-Ge intermetallics; oxides; electrodeoxidation; molten salt

1 Introduction

Titanium alloys are very attractive for the aerospace and chemical industries due to their lower density, high specific strength, excellent corrosion resistance, high melting point and good thermal property [1-4]. Moreover, the addition of IVA-group element into Ti-based alloys can further improve their corrosion resistance, mechanical property, and high temperature behavior [5-10]. The use of Ge alloys (i.e., germanides) for advanced applications in modern device technology is of current interest. Among these prospective applications, germanides are of interest in forming Schottky contacts on Ge [11]. FLEISCHER [7] has reported that Ti₅Ge₃ intermetallic compounds still have high strength at high temperature. ZAREMBO et al [9] and COLINET and TEDENAC [8] have studied the thermodynamic and mechanical properties of Ti-Ge system. It was suggested that Ti-Ge intermetallic compounds have excellent mechanical and thermodynamic properties. The Ti-Ge phase diagram is shown in Fig. 1 [12]. Obviously, three intermetallic compounds, i.e., Ti₅Ge₃, Ti₆Ge₅ and TiGe₂, are stable in the Ti-Ge system. Based on the critical review of MURRAY [13] and the previous work [9,10], it is also concluded that the compounds Ti₅Ge₃, Ti₆Ge₅ and TiGe₂ are stable. Generally, the conventional process used to produce metal germanides commonly involves arc melting of the ultrapure elements Ti and Ge, which always suffered from high energy costs [10], because the conventional processes used for the preparation of Ge involve the reduction of germanium oxides or sulfides by hydrogen or carbon [14], and pure Ti is mainly produced by the Kroll process [15].

Direct extraction of metals and/or alloys from compounds without the production of hazardous substance is the trend in the future [16]. Processes with potential for reducing or even avoiding CO₂ emissions such as the electrodeoxidation technology would be especially desirable [5,17,18]. In recent years, the electrodeoxidation technologies have been used to produce a large number of metals and alloys in molten salts [19]. A variety of electrodeoxidation processes such as the Fray-Farthing-Chen (FFC) process and the solid oxide membrane (SOM)-assisted electrodeoxidation process are currently being investigated [15,19-25]. It has been proved that the electrodeoxidation of solid oxides in molten salts is an efficient and rapid approach for extracting metals, alloys and semiconductors [22]. The key innovation of this process is based on the thermodynamic fact that the decomposition voltages of many solid oxides are lower than those of the inorganic molten salts at high temperatures, ranging from 600 to 900 °C [15,19]. Therefore, based on the low decomposition voltage of TiO₂ and GeO₂, it is reasonable to believe that Ti-Ge intermetallics can be directly prepared from TiO₂ and GeO₂ powders mixture at 800 °C with a potential of 3.0 V by using the molten salt electrodeoxidation process.

The aim of this work is to report the success of applying the electrodeoxidation process to the preparation of titanium-germanium compounds directly from mixed TiO₂-GeO₂ powders with different molar ratios (5:3, 6:5, 1:2) in molten CaCl₂-NaCl. The effects of the stoichiometry of the initial mixtures on the characteristics of the final products were investigated. In addition, direct electrochemical preparation of Ti₅Ge₃ from CaTiO₃ and GeO₂ was also investigated. Taking Ti₅Ge₃ as an example, the specific reaction process and the formation mechanism were also described in details.

Fig. 1 Phase diagram of Ti-Ge system [12]

2 Experimental

Commercially available powder of titanium oxide, TiO₂ (Aldrich, ≥99.99%) and germanium oxide, GeO₂ (Aldrich, ≥99.99%) were used as raw materials. The two reagent grade oxide powders were mixed at different stoichiometric ratios and ball-milled with anhydrous alcohol for 10 h to ensure the maximum homogenization of the constituents. And about 0.3 g of the mixture was pressed to form a cylindrical pellet (10 mm in diameter and 3 mm in thickness) under a pressure load of 15 MPa. The pellet was then sintered in air at 900 °C for 4 h in a muffle furnace with a heating rate of 5 °C/min to obtain adequate strength and the desired porosity for the electrodeoxidation process. The sintered oxide pellet was sandwiched by two porous nickel foils and then attached to a Fe-Cr-Al alloy wire as a cathode. The anode was prepared by connecting a 150 mm-long and 10 mm- diameter graphite rod to a Fe-Cr-Al alloy wire. High purity CaCl₂ was produced by vacuum drying of calcium chloride dihydrate CaCl₂·2H₂O (99% purity) [26]. The eutectic mixture of NaCl-CaCl₂ was prepared by ball milling the dry pure CaCl₂ and NaCl (99% purity, n(CaCl₂):n(NaCl)=1:1). About 160 g of the eutectic mixture was used as electrolyte in each experiment. Schematic illustration of the electrodeoxidation process for the electrosynthesis of Ti-Ge intermetallics from oxides precursors is illustrated in Fig. 2. The experiments were performed under an atmosphere of highly purified argon gas. Pre-electrolysis process was carried out before electrodeoxidation to remove the impurity contained in the molten salt. Then, the electrodeoxidation experiment was performed at 800 °C with a potential of 3.0 V. After the experiment, the samples were allowed to cool under a stream of argon gas in the furnace, then taken out of the furnace, washed with tap water to remove the residual solidified NaCl-CaCl₂ and dried under vacuum at about 90 °C.

Fig. 2 Schematic illustration of electrodeoxidation process for electrosynthesis of Ti-Ge intermetallics from oxides (TiO₂/GeO₂) precursors

The phase composition of the starting materials and the products was characterized by X-ray diffraction (XRD, D8 Advance, Bruker) with the incidence beam angle of 2° in the range of 10°-80°. The surface morphology and elemental composition of the products were characterized by scanning electron microscopy (SEM, JEOL JSM-6700F, Japan) and energy dispersive X-ray spectroscopy (EDX, Oxford INCA EDS system), respectively. A BioLogic HCP-803 electrochemical workstation was used to control and record the current- time curve during the electrodeoxidation process.

3 Results and discussion

3.1 Influence of sintering on raw material and current features

The influence of sintering on the raw material composition was investigated firstly to confirm whether there were new compounds generated during the sintering process. The XRD pattern of the mixed oxide pellet (n(TiO₂):n(GeO₂)=5:3) after being sintered at 900 °C for 4 h is given in Fig. 3. It is apparent that no peaks occurred other than those of the starting oxides, which indicates that the sintering process cannot influence the composition of the pellet precursor.  Figure 4 shows the current-time curve recorded during the electrolysis of 5TiO₂-3GeO₂ mixture pellet. Obviously, at the beginning of the electrodeoxidation process, the current increases with the expanding of the electrochemical three-phase interlines (3PIs) reaction area [23,27]. Then, the current decreases from approximately 1900 mA to around 400 mA after 30 min of electrolysis. After that, the reduction current gradually decreases and tends to be constant at a stable level (background current). It can be seen that the background current is approximately 200 mA. After being electrolyzed for different time, the XRD patterns of the pellet’s outer and inner parts are shown in Fig. 5. It can be seen from Fig. 5(a) that the outer part of the pellet reduced for 5 h mainly contains Ti₅Ge₃. In addition to Ti₅Ge₃, the inner part of the reduced pellet still contains CaTiO₃, Ge and Ti₆Ge₅. To completely electrolyze the inner part of the pellet to form Ti₅Ge₃, longer electrolysis time (8 h) is needed (Fig. 5(b)). It is obvious that the electrodeoxidation process for the pellet’s interior is slower than that for the pellet’s surface. Generally, the electrodeoxidation reaction occurs in the region of electronic conductor/oxide-compounds/electrolyte 3PIs and the 3PIs expand gradually from the surface to the center of the pellet [27].

Fig. 3 XRD pattern of 5TiO₂-3GeO₂ mixture pellet after being sintered at 900 °C for 4 h

Fig. 4 Typical current-time curve recorded during electrolysis of 5TiO₂-3GeO₂ mixture pellet under 3.0 V at 800 °C (The inset is initial part of current-time curve)

Fig. 5 XRD patterns of outer (a) and inner part (b) of 5TiO₂-3GeO₂ mixture pellet after being electrolyzed under  3.0 V at 800 °C for different time

3.2 Influence of reaction time and raw materials on final products

To investigate the influence of reaction time on the final products, the electrodeoxidation of mixed powders (n(TiO₂):n(GeO₂)=5:3) was carried out at 800 °C with a constant voltage of 3.0 V for 2, 4, 6, and 8 h,  respectively. Figure 6 shows the XRD patterns of the cathode pellets after being electrolyzed for different  time. Obviously, the product electrolyzed for 2 h contains Ge, but no metallic Ti phase is observed. This observation may imply that GeO₂ was firstly electrodeoxidized to Ge following reaction (1):

GeO₂+4e→Ge+2O^2-

(ΔG_{800 °C}=373.5 kJ/mol, E_{800 °C}=0.97 V)        (1)

Fig. 6 XRD patterns of 5TiO₂-3GeO₂ mixture pellet after being electrolyzed under 3.0 V at 800 °C for different time

However, during the electrodeoxidation process, TiO₂ would convert into CaTiO₃ following Reaction (2):

TiO₂+Ca²⁺+O^2-→CaTiO₃

(ΔG_{800 °C}=-86.9 kJ/mol)                           (2)

The products obtained from 2-6 h electrolysis consist of Ti₅Ge₃, Ti₆Ge₅, CaTiO₃ and Ti₂O₃ (see Fig. 6), which may suggest that Reactions (3) and (4) occurred, i.e., Ca-containing intermediate compounds (CaTiO₃), were reduced to Ti/Ti₂O₃ [20] and CaO.

CaTiO₃+4e→Ti+2O^2-+CaO

(ΔG_{800 °C}=836.7 kJ/mol, E_{800 °C} =2.17 V)       (3)

2CaTiO₃+2e→Ti₂O₃+O^2-+2CaO

(ΔG_{800 °C}=902.0 kJ/mol, E_{800 °C}=2.34 V)        (4)

Then, CaO would dissolve into molten CaCl₂-NaCl following Reaction (5), and Ti₂O₃ was further reduced to Ti following Reaction (6):

CaO→Ca²⁺+O^2-                                          (5)

Ti₂O₃+6e→2Ti+3O^2-

(ΔG_{800 °C}=1222.5 kJ/mol, E_{800 °C}=2.11 V)       (6)

Generally, two simultaneous processes are involved during the synthesis of Ti₅Ge₃, i.e., (1) Ge reacts with Ti to form Ti₅Ge₃ and Ti₆Ge₅ at the experiment temperature (Reactions (7) and (8)); (2) the formed Ti₆Ge₅ further reacts with Ti to form Ti₅Ge₃ (Reaction (9)).

5Ti+3Ge→Ti₅Ge₃

(ΔG_{800 °C}=-652.6 kJ/mol)                         (7)

6Ti+5Ge→Ti₆Ge₅

(ΔG_{800 °C}=-828.1 kJ/mol)                           (8)

3Ti₆Ge₅+7Ti→5Ti₅Ge₃

(ΔG_{800 °C}=-778.7 kJ/mol)                           (9)

Figure 7 shows the typical SEM image and its elemental maps (Ti and Ge) as well as EDS spectrum of the Ti₅Ge₃ product synthesized from a mixed powder pellet (n(TiO₂):n(GeO₂)=5:3). It can be seen that Ti and Ge elements are uniformly distributed in the product. In addition, Fig. 6 also demonstrates that CaTiO₃ occurred as an intermediate product during the synthesis of Ti₅Ge₃. Therefore, based on this observation, we try to use CaTiO₃-GeO₂ as precursors to produce Ti₅Ge₃. Figure 8 shows the current-time curve of the electrolysis of 5CaTiO₃-3GeO₂ mixture pellet. It is found that the electrolysis of 5CaTiO₃-3GeO₂ only needs 6 h. The current efficiency which can only be used as a general evaluation for the electrolysis process is defined as the ratio of the theoretical charge for reducing all oxides contained in the pellet and the total charge passed through the electrolytic cell. The current efficiency for Ti₅Ge₃ production from 5TiO₂-3GeO₂ electrolyzed for  8 h is calculated to be 37.8%. As a comparison, the current efficiency for Ti₅Ge₃ production from 5CaTiO₃-3GeO₂ electrolyzed for 6 h is calculated to be 40.2%. Figure 9(a) shows the XRD patterns of the mixed CaTiO₃ and GeO₂ powders (n(CaTiO₃):n(GeO₂)=5:3) after being electrolyzed for different time. Apparently, the reaction process of CaTiO₃-GeO₂ is similar to   that of TiO₂-GeO₂, and Ti₅Ge₃ can be obtained as final products. Figure 9(b) shows the XRD patterns of Ti₅Ge₃ obtained from different raw materials, i.e., CaTiO₃- GeO₂ and TiO₂-GeO₂. Obviously, the two Ti₅Ge₃ are similar. Figures 9(c) and (d) show the typical SEM image and EDS spectrum of the Ti₅Ge₃ synthesized from 5CaTiO₃-3GeO₂ powder. The SEM image indicates that the Ti₅Ge₃ particles possess homogenous structure, and there is no other impurity elements contained in the product (Fig. 9(d)).

Fig. 7 SEM image of completely electrolyzed product (electrolyzed for 8 h) obtained from mixed powder pellet (n(TiO₂):n(GeO₂)= 5:3) under 3.0 V at 800 °C (a), elemental maps of Ti (b) and Ge (c) corresponding to (a), and EDS spectrum (d) measured over  image (a)

Fig. 8 Current-time curve recorded during electrolysis of 5CaTiO₃-3GeO₂ mixture pellet under 3.0 V at 800 °C (The inset is initial part of current-time curve)

Fig. 9 XRD patterns of mixed 5CaTiO₃-3GeO₂ powder pellet electrolyzed for different time under 3.0 V at 800 °C (a), XRD patterns of final products obtained from different raw materials under 3.0 V at 800 °C (b), SEM image of completely electrolyzed products (electrolyzed for 6 h) from mixed 5CaTiO₃-3GeO₂ powder pellet under 3.0 V at 800 °C (c) and EDS spectrum (d) of obtained Ti₅Ge₃ product corresponding to (c)

3.3 Electrochemical synthesis of Ti_xGe_y

Electrolysis of TiO₂-GeO₂ pellets with different starting stoichiometries, i.e., molar ratios of TiO₂ to GeO₂=5:3, 6:5, 1:2, was also carried out at 800 °C for  8 h with a potential of 3.0 V. The XRD patterns and the SEM images of the completely electrolyzed products are shown in Figs. 10 and 11, respectively. A summary of germanide phases formed after electrolysis and the serial number of SEM images/XRD patterns with different starting molar ratios of TiO₂ to GeO₂ are listed in Table 1. Figure 10 reveals that a single-phase Ti₅Ge₃ intermetallic is obtained when the TiO₂/GeO₂ molar ratio is 5:3, and two germanide phases, i.e., Ti₆Ge₅ and Ti₅Ge₃, are formed as the product when TiO₂/GeO₂ molar ratio equals 6:5. The final product consists of Ti₆Ge₅ and pure metal Ge when TiO₂/GeO₂ molar ratio is 1:2. It should be noted that the XRD results cannot prove that the oxygen content has been completely removed during electrolysis, a small amount of oxygen component may still exist in the products due to the fact that the molten salt electrolysis is difficult to completely remove the oxygen element. Figure 11 reveals that all final products possess porous morphologies. Additionally, the microstructures of the products obtained from different starting stoichiometries have slight differences. As shown in Fig. 11, the single-phase Ti₅Ge₃ obtained from 5TiO₂-3GeO₂ exhibits homogenous particle structure, which shows a slight difference from the SEM images (Figs. 11(b) and (c)) of the other two products. The reason may be attributed to their different phase compositions.

Fig. 10 XRD patterns of products completely electrolyzed for  8 h under 3.0 V at 800 °C from powder mixtures with different starting molar ratios of TiO₂ to GeO₂

Fig. 11 SEM images of products completely electrolyzed for  8 h under 3.0 V at 800 °C with different starting molar ratios of TiO₂ to GeO₂

Table 1 Summary of germanide phases of electrolytic products and serial number of SEM images/XRD patterns with different starting molar ratios of TiO₂ to GeO₂ under 3.0 V at 800 °C

3.4 Formation mechanisms

Based on the experimental result, a mechanism for the formation of Ti₅Ge₃ from TiO₂/GeO₂ by the electrodeoxidation process was suggested and summarized in Fig. 12. During the electrolysis, the intermetallic Ti₅Ge₃ was formed following a multi-step process, i.e., at the beginning of the electrolysis, when the applied voltage is higher than the decomposition potential of GeO₂ (GeO₂ 0.97 V, TiO₂ 1.94 V at 800 °C [28]), GeO₂ was firstly electrodeoxidized to Ge at 800 °C. Simultaneously, CaTiO₃ was formed during the electrodeoxidation process. Subsequently, CaTiO₃ compounds were reduced to Ti-Ti₂O₃ and CaO. Then, Ti₂O₃ was reduced to Ti, the formed Ti and Ge would react to form Ti₆Ge₅ and Ti₅Ge₃. It is worth noting that there is a general trend that Ti₆Ge₅ was formed firstly during the electrolysis process, then the formed Ti₆Ge₅ would further react with Ti to form Ti₅Ge₃ if there is enough Ti. This experimental observation is supported by previous thermodynamic considerations [9,11,29]. In addition, the Gibbs free energy change of the reaction 5Ti+3Ge→Ti₅Ge₃ is negative at the electrolysis temperature  [11],  which  means  that  the electrodeoxidation-generated Ti and Ge can react to  form Ti₅Ge₃. Therefore, it is reasonable to believe that two reaction routes, i.e., Ti, Ge→Ti₅Ge₃ and Ti, Ge→Ti₆Ge₅, Ti→Ti₅Ge₃ coexist during the electro- synthesis process.

It should be noted that TiGe₂ was not produced during the electrolysis of 1TiO₂-2GeO₂, (as shown in Figs. 10(c) and 13). The foregoing study [29] suggested that if sufficient time was given for the reaction to continue, Ti₆Ge₅ would transform to TiGe₂ by the reaction with the excess Ge available according to the reaction of 7Ge+Ti₆Ge₅→6TiGe₂. However, the reaction is a very slow solid-state reaction at the electrolysis temperature. In the work of JAIN and KAO [10], it was suggested that hundreds of hours were needed to synthesize TiGe₂ at 900 °C. Therefore, it is difficult to form TiGe₂ in molten salt electrodeoxidation process because the electrolysis time is insufficient for the solid transformation from Ti₆Ge₅ to TiGe₂.

4 Conclusions

1) Ti_xGe_y intermetallic compounds were synthesized by direct electrodeoxidation of mixed TiO₂-GeO₂ powders in molten CaCl₂-NaCl at 800 °C with a potential of 3.0 V.

2) The reaction mechanism for the synthesis of Ti₅Ge₃ is a multi-step process, which generally includes the electrodeoxidation of GeO₂, the formation of  CaTiO₃ compounds, the electrodeoxidation of CaTiO₃ compounds and the formation of Ti₅Ge₃ intermetallic.

3) Single-phase Ti₅Ge₃ can be obtained from 5TiO₂-3GeO₂ and 5CaTiO₃-3GeO₂.

4) The results imply that the molten salt electrodeoxidation process is a fast and environmentally friendly process for the production of Ti_xGe_y intermetallic.

Fig. 12 Schematic illustration of formation mechanism of Ti₅Ge₃ from 5TiO₂-3GeO₂ during electrodeoxidation process under 3.0 V at 800 °C

Fig. 13 XRD patterns of 1TiO₂-2GeO₂ electrolyzed under 3.0 V at 800 °C for different time

Acknowledgments

The authors thank the Instrumental Analysis and Research Center of Shanghai University for materials characterization.

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CaCl₂-NaCl熔盐中电解氧化物制备Ti-Ge金属间化合物

王银帅¹，邹星礼^1,2，鲁雄刚¹，李尚书¹，郑 凯¹，汪淑娟¹，许 茜¹，周忠福^1,3

1. 上海大学 省部共建高品质特殊钢冶金与制备国家重点实验室，上海市钢铁冶金新技术重点实验室，材料科学与工程学院，上海 200072；

2. Center for Electrochemistry, Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, USA;

3. Institute of Mathematics and Physics, Aberystwyth University, Aberystwyth SY23 3BZ, United Kingdom

摘  要：采用电解脱氧工艺，以不同比例的TiO₂和GeO₂混合物为前驱体，在电压为3.0 V、温度为800 °C的电解条件下，制备Ti-Ge(Ti_xGe_y)金属间化合物。在同样电解条件下，以5TiO₂-3GeO₂ 或 5CaTiO₃-3GeO₂为前驱体时，均可电解得到Ti₅Ge₃金属间化合物。获得的电解产物表现出相对均匀的晶粒结构。此外，初始原料(TiO₂/GeO₂)的摩尔比对最终电解产物有非常大的影响。根据实验数据，详细阐述电解脱氧制备Ti_xGe_y金属间化合物的相关反应机理。结果表明，电解脱氧工艺是一种环境友好的制备Ti_xGe_y金属间化合物的方法。

关键词：Ti-Ge金属间化合物；氧化物；电解脱氧；熔盐

(Edited by Xiang-qun LI)

Foundation item: Project (51574164) supported by the National Natural Science Foundation of China; Project (2014CB643403) supported by the National Basic Research Program of China

Corresponding author: Xing-li ZOU, E-mail: xlzou@shu.edu.cn; Xiong-gang LU, Tel: +86-21-56334042, Fax: +86-21-56335768, E-mail: luxg@shu.edu.cn

DOI: 10.1016/S1003-6326(18)64880-0