Trans. Nonferrous Met. Soc. China 31(2021) 2442-2453

Achieving remarkable piezoelectric activity in Sb-Mn co-modified CaBi₄Ti₄O₁₅ piezoelectric ceramics

Yang LIU¹, Yang YU¹, Chen-yi YIN¹, Liang ZHENG¹, Peng ZHENG¹, Wang-feng BAI², Li-li LI¹, Fei WEN¹, Yang ZHANG¹

1. Lab for Nanoelectronics and Nanoelectronic Devices, Department of Electronics Science and Technology, Hangzhou Dianzi University, Hangzhou 310018, China;

2. College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China

Received 21 August 2021; accepted 6 April 2021

Abstract: CaBi₄Ti₄O₁₅ (CBT)-based Aurivillius high-temperature piezoceramics with different Sb-Mn co-doping amounts were synthesized via the conventional sintering technique. The influences of doping amount on the product were studied via their crystal structure, microstructure, and piezoelectric performance. It is found that an appropriate Sb-Mn co-doping amount can effectively optimize the crystal structure and decrease the oxygen vacancy concentration in CBT ceramics, leading to enhanced electrical properties. Optimized electrical performance with a high Curie temperature (T_C) of 792 °C and a remarkable piezoelectric coefficient (d₃₃) of 25 pC/N were achieved at a doping amount (x) of 0.05. Furthermore, this ceramic is found to exhibit an excellent thermal stability, with d₃₃ retaining 88% of its original value after annealing at 600 °C for 2 h. Moreover, this ceramic shows a high electrical resistivity (ρ) of 1.35×10⁸ Ω·cm with a small dielectric loss (tan δ) of 1.7% at 400 °C. Because of such outstanding piezoelectric performance, it is believed that these Sb-Mn co-doped CBT ceramics could be potential candidates for high-temperature piezoelectric applications.

Key words: CaBi₄Ti₄O₁₅; piezoelectric ceramics; piezoelectric activity; resistivity; thermal stability

1 Introduction

In recent years, numerous industries have realized improved performances for piezoelectric sensors with high operating temperatures, particularly the fields of nuclear power and gas turbine [1-3]. Aurivillius ferroelectrics with a bismuth layer structure have been extensively investigated because of their ultrahigh ferroelectric phase transition temperature and excellent thermal stability [4]. However, their reduced piezoelectric activity dramatically restrict their practical applications. Therefore, improving the piezoelectric performance of Aurivillius ferroelectrics is urgently needed.

The structure of Aurivillius ferroelectrics consists of the regular symbiosis of (Bi₂O₂)²⁺ layers and pseudo-perovskite layers, which can be described as (Bi₂O₂)²⁺(A_m–1B_mO_3m+1)^2-. Here, A corresponds to a mono-, di-, or trivalent ion, B corresponds to a tetravalent, pentavalent, or hexavalent transition element, and m represents the number of octahedral layers [5, 6]. Among various Aurivillius ferroelectrics, CaBi₄Ti₄O₁₅ (CBT) has been widely studied, because its high Curie temperature (T_C) of 790 °C makes it attractive for high-temperature applications [7]. However, similar to other Aurivillius ferroelectric ceramics, the piezoelectric activity of pure CBT ceramics fabricated via the conventional sintering approach is generally low. Because spontaneous polarization switching occurs mainly in the a-b plane [8-10]. In addition, the high electrical conductivity caused by the high oxygen vacancy concentration is also a significant obstacle to their practical applications.

Recently, the mixed-valence cations co-doping strategy at the B site has been proposed to improve the piezoelectric performance of several Aurivillius ferroelectric ceramics. For instance, LI et al [11,12] prepared Cu-Nb and Cu-Ta co-doped Bi₄Ti₃O₁₂ ceramics. These ceramics presented outstanding piezoelectric coefficient (d₃₃) values of 38 and 34 pC/N, accompanied by high T_C values of 678 and 677 °C, respectively. CHEN et al [13,14] designed W-Cr and Mo-Cr co-doped CaBi₂Nb₂O₉ ceramics. Improved d₃₃ values of 15 pC/N and good thermal stability were achieved. All of the studies mentioned above demonstrate that the mixed- valence cations co-doping strategy at the B site is an excellent method to enhance the piezoelectricity of Aurivillius ferroelectric ceramics. Notably, a high piezoelectric activity accompanied by an undeteriorated T_C value has also been reported in Nb-Mn and Nb-Mg co-doped CBT ceramics. This work further suggests that the mixed-valence cations co-doping strategy at the B site is also a feasible way to improve the piezoelectric properties of the CBT ceramics [6,15]. Nevertheless, this method of mixed-valence cations co-doping has rarely been applied to CBT-based ceramics, and the potential improvement mechanism has remained unclear.

In this study, CBT ceramics co-doped with mixed-valence Sb-Mn cations at the Ti site were prepared with different doping amounts. The influence of different doping amounts on the structure and electrical performance was thoroughly investigated. A remarkable piezoelectric coefficient and good thermal stability were achieved at a doping amount (x) of 0.05. Furthermore, an enhanced electrical resistivity and almost unaffected Curie temperature were obtained in this ceramic. The relevant mechanisms for the optimized piezoelectricity performance and resistivity were also systematically discussed.

2 Experimental

CaBi₄Ti_4-x(Sb_2/3Mn_1/3)_xO₁₅ (CBTSM-x) ceramics (x=0, 0.02, 0.04, 0.05, 0.06, and 0.1) were synthesized via the conventional sintering technique. TiO₂ (99.8%), CaCO₃ (99.99%), Bi₂O₃ (99.99%), and MnCO₃ (99.95%) were employed as raw materials. Sb₂O₃ (99.5%) was selected as the Sb source, and it is oxidized to Sb₂O₅ at high temperature [16]. The raw materials were mixed using a planetary ball mill with ethyl alcohol. Calcination was carried out at 825 °C for 4 h. The powders were then ball milled again for 12 h. Finally, these powders were mixed with polyvinyl alcohol (PVA) and pressed into discs under a pressure of 150 MPa. After removing PVA, the discs were sintered in the temperature range of 1000-1100 °C for 1 h.

The surface morphology of the ceramics was investigated using scanning electron microscopy (SEM, JEOL JSM6460-LV). The Archimedes drainage method was used for density measurements. The crystalline phases of the ceramic powders were determined via X-ray diffraction (XRD, Ultima IV). The Raman spectra were measured using a Raman spectrometer (iHR320). The dielectric performance was measured with an impedance analyzer (Keysight E4990A). The DC conduction was measured using a high-resistance meter (Keithley 2410) under a voltage of 10 V. An impedance analyzer (Keysight E4990A) was employed to measure the AC conduction in the frequency range between 100 Hz and 10 MHz. For the piezoelectric characterization, a poling treatment of the obtained ceramics was carried out under a 14-16 kV/cm electric field at 160 °C for half an hour. Subsequently, the piezoelectric constants (d₃₃) were recorded at room temperature with a piezo-d₃₃ meter (ZJ-3AN). The ferroelectric properties were measured using a ferroelectric analyzer (RADIANT Technologies Inc RT1-Premier II).

3 Results and discussion

3.1 Microstructure and phase structure characterization

Figure 1(a) shows the XRD patterns of the CBTSM-x ceramics obtained in the 2θ range from 20° to 60°. All of the ceramics exhibit a pure Aurivillius phase structure, and no other impurities can be observed according to the standard PDF card (No. 52-1640). This indicates that the co-doped ions were successfully incorporated into the CBT lattice. The highest intensity peak was found to correspond to the (119) reflection plane, which is consistent with the most intense reflections (112m+1) typical of Aurivillius ferroelectrics [17]. An additional peak can be seen on the left of the (119) peak, and its intensity increased as the doping amount increased. This peak is consistent with the Bi_1.74Ti₂O_6.624 impurity phase, according to the standard PDF card (No. 89-4732). Normally, Bi₂Ti₂O₇ can be easily formed at the early stages of the thermal process because of its excellent relative stability in the Bi₂O₃-TiO₂ system [18,19]. When the ambient temperature exceeded over 700 °C, the Bi₂Ti₂O₇ phase began to change into the CaBi₄Ti₄O₁₅ phase. However, because of the volatilization of Bi ions, a certain amount of Bi₂Ti₂O₇ probably converted into Bi_1.74Ti₂O_6.624, as its Bi/Ti ratio was lower than that of CaBi₄Ti₄O₁₅, resulting in a structural cross-over. The zoomed-in view of the (117) and (119) diffraction peaks are illustrated in Figs. 1(b) and 1(c). Both two peaks shift to higher 2θ degrees when the doping amount increases from 0 to 0.04. They further shift to lower degree with higher doping amount to 0.1. To further determine the variation of the crystal structure, the lattice parameter dependence on different doping amounts is illustrated in Fig. 2(a). The a and b values decrease as x increases from 0 to 0.05, whereas the c value increases, giving rise to an overall decreasing cell volume V. However, for x>0.05, the a and b values increase, whereas the c value decreases slightly, causing V to increase. The increasing V in the high-doping region might be caused by the larger effective ionic radius of (Sb⁵⁺_2/3Mn²⁺_1/3) (0.623 ) compared to that of Ti⁴⁺(~0.605  ). On the other hand, the lattice shrinkage in the low-doping region might be ascribed to the variation of electronegativity of the doped elements. As revealed by Pauling, Sb has a larger electronegativity (2.05) than Ti (1.54). This could lead to a larger interaction between Sb and O, thus shortening the Sb—O bond in the octahedron, which may explain the lattice shrinkage at x<0.05 [20]. In addition, the Mn²⁺ ion might be partially oxidized to the Mn⁴⁺ ion in the low-doping region [21,22], giving rise to a smaller effective ionic radius for Sb⁵⁺_2/3Mn⁴⁺_1/3 (0.577 ) compared to that of Ti⁴⁺. To assess the degree of lattice distortion, Fig. 2(b) shows the a/b ratio dependence as a function of x. The value of the a/b ratio first increased and then fell upon increasing the doping amount, reaching a maximum value at x=0.05. The result demonstrates that the tetragonality of the CBT lattice improved, whereas the anisotropy decreased with Sb-Mn co-doping. This finding may facilitate the switching of the spontaneous polarization in the a-b plane and yield a superior piezoelectric performance [23].

Fig. 1 XRD patterns of CBTSM-x ceramics (a), enlarged view of (117) (b) and (119) (c) diffraction peaks

Fig. 2 Lattice parameters a, b, c and unit cell V as function of x (a), and a/b ratio (b) of CBTSM-x ceramics as function of x

Figure 3(a) shows the Raman spectra of the CBTSM-x samples recorded at room temperature. Several sharp peaks at ~125, ~156, ~270, ~555, and ~860 cm^-1 are in agreement with the previous works [24,25]. The mode at ~125 cm^-1 can be attributed to the vibration of the Bi³⁺ ion in the (Bi₂O₂)²⁺ structure [26]. On the other hand, the vibrations of the A-site ions in the pseudo- perovskite layer are related to the mode at ~156 cm^-1 [27]. The modes at ~270 cm^-1 (B_2g and B_3g modes) can be assigned to the O—Ti—O vibration [28]. In comparison, the Raman peak at ~555 cm^-1 (A_1g mode) corresponds to the relative displacement of the oxygen atom in the TiO₆ octahedron, and the mode at 839 cm^-1 (B_1g mode) is associated with the stretching of the TiO₆ octahedron [24]. No extra peaks are observed, which further demonstrates that the co-doped Sb and Mn have been properly incorporated into the CBT lattice. Moreover, enlarged Raman peaks in various frequency regions are presented in Figs. 3(b)-(f). No clear shift can be detected in  the phonon modes, which may be attributed to the small co-doping amount of Sb and Mn.

Fig. 3 Raman spectra of CBTSM-x ceramics (a), and Raman peaks in various frequency regions (b-f)

The microstructures of the natural surface of the obtained CBTSM-x ceramics are shown in Fig. 4. As illustrated, all of the samples show similar microstructures with strongly anisotropic grains, which is a typical characteristic of the Aurivillius ceramics [28]. However, microstructures with a higher relative density (ρ₀) are observed with increased doping concentration. This behavior might be attributed to the addition of Mn, which enhances the sinterability of the ceramic by promoting the diffusion of grain boundaries [29]. Figure 5 displays the elemental mappings of the CBTSM-0.05 sample. It can be seen that each element shows a uniform distribution in the sample, further confirming the diffusion of Sb and Mn into the CBT lattice.

3.2 Dielectric behavior

Fig. 4 SEM images collected on natural surface of ceramics

Fig. 5 Elemental mappings of CBTSM-0.05 ceramic

Figure 6(a) presents the dependence of dielectric permittivity (ε_r) of the Sb-Mn co-doped CBT ceramics on temperature at 1 MHz. The dielectric permittivity of all of the ceramics increases slowly as the temperature increases to 500 °C, and then it increases sharply, with a unique transition temperature at ~790 °C. The T_C value corresponding to x is shown in Fig. 6(b). The T_C values are extremely close for all of the ceramics, lying in the range of 792-793 °C. Figure 6(c) illustrates the temperature dependence of the dielectric loss (tan δ) for all of the samples. The values of tan δ are relatively low for temperatures below 450 °C but rise sharply when the temperature is further increased. To illustrate the changes in  tan δ induced by Sb-Mn co-doping, the tan δ values obtained at 400 °C for all of the ceramics are presented in Fig. 6(d). The tanδ values first decreases and then increases upon increasing doping amount. The CBTSM-0.05 ceramic exhibits a much lower tan δ value (1.7%) than the CBTSM-0 ceramic (2.9%), which may be caused by a decrease in the oxygen vacancy concentration after co-doping [30-33]. These outstanding dielectric properties will facilitate the practical application of this ceramic in high-temperature fields.

3.3 Piezoelectric and ferroelectric properties

Figure 7(a) shows the piezoelectric constant (d₃₃) of CBT ceramics with different Sb-Mn co-doping amounts. The d₃₃ value first increases and then declines with increased Sb-Mn content, showing the highest value of 25 pC/N at x=0.05. All of the co-doped CBTSM-x ceramics have a larger d₃₃ value than the pure CBT ceramic, illustrating that co-doping with Sb-Mn efficiently enhances the piezoelectricity of the CBT ceramic. To provide a better evaluation of the piezoelectric properties exhibited by the sample at x=0.05, a comparison between the T_C and d₃₃ values measured here and the values reported in the literature is shown in Fig. 7(b) [7,9,15,29,33-39]. Most of the previous research results are located in the green region, while the CBTSM-0.05 sample clearly stands out because of its excellent piezoelectricity and high T_C.

It is thought that the piezoelectric properties often depend on ferroelectricity. The P-E loops and I-E curves of CBTSM-x ceramics measured at 10 Hz and 100 °C under an electric field of 180 kV/cm are presented in Figs. 8(a) and (b), respectively. All of the ceramics show saturated hysteresis loops with significant ferroelectric switching peaks. The remnant polarization (P_r) values are illustrated in Fig. 7(a). The P_r value first increases and then decreases with increased doping amount, reaching a maximum value at x = 0.05. The enhanced ferroelectricity may be ascribed to the improvement of the tetragonal structure [11]. It is worth noting that the variation trend of the P_r value is similar to that of the d₃₃ value, suggesting that the large piezoelectric activity may be partly attributed to the enhanced P_r.

Fig. 6 Dependence of ε_r on temperature for CBTSM-x ceramics at 1 MHz (a), correlation between T_C and x (b), tan δ values at 1 MHz for CBTSM-x ceramics as function of temperature (c), and tan δ values obtained at 400 °C as function of x (d)

Fig. 7 Dependence of piezoelectric coefficient (d₃₃) and remnant polarization (P_r) on doping amount (a), and comparison between T_C and d₃₃ values measured and those reported in previous works (b)

3.4 Resistivity and complex impedance

Fig. 8 P–E loops (a) and I–E (b) curves for CBTSM-x ceramics

Fig. 9 Variation of ρ as function of temperature for Sb-Mn co-doped CBT ceramics (The inset illustrates the correlation between ρ and x at 400 °C) (a), and comparison between d₃₃ and ρ values measured at 400 °C and those of previous works (b)

Resistivity is also a key indicator for evaluating the electrical properties of Aurivillius ferroelectric ceramics. Figure 9(a) displays the variation of the electrical resistivity with respect to temperature for the Sb-Mn co-doped CBT ceramics. The electrical resistivity ρ decreased with increasing temperature for all of the ceramics, indicating that the carriers are thermally activated. The resistivity measured at 400 °C first increases and then decreases as the Sb-Mn co-doping amount increases, as displayed in the inset of Fig. 9(a). The CBTSM-0.05 sample exhibits a ρ of 1.35×10⁸ Ω·cm, which is an order of magnitude higher than that of the undoped ceramic (1.48×10⁷ Ω·cm). It is worth noting that the CBTSM-0.05 ceramic simultaneously exhibits a significant resistivity and a remarkable d₃₃ value compared with the CBT-based ceramics investigated in previous reports [7,15,28,29,34,39-41], suggesting that Sb-Mn co-doping can comprehensively boost the electrical properties of the CBT ceramic.

To investigate the possible mechanisms behind the enhanced electrical resistivity, the complex impedances of the CBTSM-0 and CBTSM-0.05 ceramics were measured from 100 Hz to 10 MHz at different temperatures, and the results are shown in Figs. 10(a, b), respectively. Only one semicircle can be detected, indicating that grains play a dominant role in the conductivity of the ceramics [42]. An equivalent circuit model (R-CPE-C), shown in the inset of Fig. 10(a), was adopted to fit the complex impedances with the aid of Z-view software. Good fittings to the experimental data were achieved for both ceramics. The CPE-P values of the constant phase element (CPE) lie between 0.46 and 0.54, which may be attributed to the Warburg diffusion related to the oxygen migration process. The diameter of the semicircle decreases with increasing temperature, indicating a reduction in resistance. The activation energy (E_a) can be calculated using the Arrhenius law:

σ=Aexp[E_a/(kT)]                         (1)

where σ corresponds to the conductivity, A represents the pre-exponent factor, k is the Boltzmann constant, and T is the temperature. Figure 11(a) shows the relationship between conductivity and temperature for the CBTSM-x ceramics. The obtained E_a values are presented in Fig. 11(b). As seen, the calculated E_a value for the pristine CBT ceramic is 1.25 eV, indicating that the conduction is dominated by the migration of oxygen vacancies in the grains [13,33]. However, higher E_a values of 1.43-1.67 eV were acquired for the Sb-Mn co-doped ceramics, indicating intrinsic charge carrier conduction [43]. The higher E_a values suggest that the oxygen vacancy concentration is significantly reduced because of Sb-Mn co-doping.

Fig. 10 Nyquist diagrams for CBTSM-0 (a) and CBTSM-0.05 (b) at 500-750 °C

Fig. 11 Correlation between ln σ and T ^-1 for various doping amounts (a), and variation trends of E_a with various x (b)

To obtain deeper insight into the reduced oxygen vacancy concentration, a detailed electrochemical analysis is provided here. During the sintering process, Bi₂O₃ volatilization is unavoidable, and oxygen vacancies  are generated to maintain the electric neutrality (Eq. (2)) [11]. Evidently, the Sb₂O₅ donor dopant can directly reduce the oxygen vacancy concentration (Eq. (3)), which is beneficial for the improvement of the electrical resistivity. In contrast, the acceptor substitution of Mn²⁺ for Ti⁴⁺ generates lattice defects and oxygen vacancies, as expressed by Eq. (4) [31]. Generally, the extra oxygen vacancies should cause a reduction in the resistivity. However, opposite results have been obtained for the W-Mn and Sb-Cu doped BIT ceramics [22,31]. It was speculated that the movement of these extra oxygen vacancies is limited near defect centers  because of  the formation of strongly combined defect dipoles , which do not contribute to the conductive process of the ceramic, as described by Eq. (5) [31]. On the contrary, the number of intrinsic oxygen vacancies decreases to retain the overall balance for the oxygen vacancy amount. In addition, the Mn²⁺ ions in the raw oxides are partially oxidized to the Mn⁴⁺ ions in the low-doping region. The Mn⁴⁺ ions production might represent another reason for the reduction of the oxygen vacancies in the low-doping region [44, 45], which can be expressed by Eq. (6). Generally speaking, the reduced oxygen vacancy concentration in the Sb-Mn co-doped CBT ceramics might be ascribed to the combined action of the Sb donor doping and Mn acceptor doping.

Bi₂O₃→2Bi(g)↑+3/2O₂(g)↑++         (2)

              (3)

↑   (4)

→                      (5)

→                 (6)

3.5 Thermal stability

The thermal stability of the Aurivillius piezoelectric materials is an important indicator of their practical applications. To illustrate the thermal stability of CBTSM-x ceramics, thermal annealing behaviors at different temperatures are presented in Fig. 12(a). The poled samples were annealed from 25 to 800 °C in steps of 50 °C for 2 h. Subsequently, the d₃₃ values were measured at room temperature. The piezoelectric performance remains stable up to 500 ^oC. However, when the annealing temperature exceedes over 500 °C, the d₃₃ value for all of the samples decreases. This may be because depolarization occurs at high temperatures, as a consequence of the space charge movement [22]. Despite this, the d₃₃ value of the CBTSM-0.05 ceramic retains 88% of the starting value after being annealed for 2 h at 600 °C. Figure 12(b) illustrates a comparison between the thermal annealing behavior of the CBTSM-0.05 ceramic measured in this work and those reported in the Refs. [7,15,33,37,40,46,47].

Fig. 12 Thermal depolarization behavior of CBTSM-x ceramics (a), and comparison between thermal stability behavior at 600 °C reported in present work and existing literature (b)

Compared with other works, the CBTSM-0.05 ceramic shows an excellent thermal stability with remarkable piezoelectric activity. A reduced number of defects may be an important underlying reason for this behavior. Also, the thermal stability of piezoelectric ceramics is considered to depend significantly on domain wall motion. In this work, as the tetragonality of the lattice structure was optimized via Sb-Mn co-doping, the number of unstable non-180^o domain walls should be reduced, which might account for the excellent thermal stability [48].

4 Conclusions

(1) Ti-site Sb-Mn co-doped CaBi₄Ti_4-x- (Sb_2/3Mn_1/3)_xO₁₅ ceramics were synthesized via the conventional sintering method.

(2) Addition of Sb and Mn resulted in an efficient enhancement of the tetragonal distortion and in a reduction of the oxygen vacancy amount, thus promoting optimized piezoelectric activity and superior electrical resistivity.

(3) The CBTSM-0.05 ceramic exhibits the best electrical properties with a greatly enhanced piezoelectric activity (d₃₃=25 pC/N). It also presents outstanding thermal stability with the d₃₃ value retaining 88% of its original value after annealing at 600 °C for 2 h.

(4) The CBTSM-0.05 ceramic also exhibits improved resistivity (1.35×10⁸ Ω·cm at 400 °C) and a high T_C (792 °C).

(5) The Sb-Mn co-doping method can be an effective approach to improve the electrical properties of CBT-based ceramics.

Acknowledgments

The authors are grateful for the financial support from the Key Research and Development Project of Zhejiang Province, China (No. 2017C01056).

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具有优异压电活性的Sb-Mn共改性的CaBi₄Ti₄O₁₅压电陶瓷

刘 洋¹，余 洋¹，尹晨懿¹，郑 梁¹，郑 鹏¹，白王峰²，李丽丽¹，汶 飞¹，张 阳¹

1. 杭州电子科技大学 电子科学与技术系 纳米电子学与纳电子器件实验室，杭州 310018；

2. 杭州电子科技大学 材料与环境工程学院，杭州 310018

摘  要：采用常规烧结方法合成不同Sb-Mn共掺杂量的CaBi₄Ti₄O₁₅ (CBT)基Aurivillius高温压电陶瓷。从晶体结构、显微组织和压电性能等方面研究掺杂量(x)对陶瓷性能的影响。研究发现，适当的Sb-Mn掺杂量可以有效地优化CBT陶瓷的晶格结构，降低氧空位浓度，从而提高其电学性能。在x=0.05组分时获得优化的电性能，792 °C的高居里温度、25 pC/N的压电系数(d₃₃)。另外，这种陶瓷表现出良好的热稳定性，在600 °C退火2 h后d₃₃仍保持初始值的88%。此外, 在400 °C时陶瓷还显示出高电阻率(ρ=1.35×10⁸ Ω·cm)和小的介电损耗(tan δ=1.7%)。由于这些优异的压电性能，Sb-Mn共掺杂的CBT陶瓷有可能成为高温压电应用的潜在候选材料。

关键词：钛酸铋钙；压电陶瓷；压电活性；电阻率；热稳定性

(Edited by Xiang-qun LI)

Corresponding author: Liang ZHENG, Tel: +86-13175112391, E-mail: zhlbsbx@hdu.edu.cn; Peng ZHENG, E-mail: zhengpeng@hdu.edu.cn

DOI: 10.1016/S1003-6326(21)65665-0

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press