Trans. Nonferrous Met. Soc. China 31(2021) 1475-1483

Thermodynamic simulation of metal behaviors in Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O system

Ze-lin MIAO¹, Jing ZHAN^1,2, Zi-wei XU¹

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. National Engineering Laboratory for High Efficiency Recovery of Refractory Nonferrous Metals Resources,

Central South University, Changsha 410083, China

Received 5 June 2020; accepted 28 November 2020

Abstract: To investigate the behaviors of Cu²⁺ and Ni²⁺ with the change of conditions in Cu²⁺-Ni²⁺-NH₃-NH₄⁺- C₂O₄^2--H₂O reaction system, mathematical models of thermodynamics based on the principle of mass conservation were established. The simulation results indicate that the precipitation of metal ions from the aqueous phase is a complicated dynamic equilibrium process, during which the coordination reactions of Cu²⁺ and Ni²⁺ with NH₃ forming [Cu(NH₃)_n]²⁺ (n=3-5) and [Ni(NH₃)_m]²⁺ (m=3-6) are predominant under high pH conditions, respectively. The pH ranges for the simultaneous precipitation of Cu²⁺ and Ni²⁺ are 2.0-6.5 and 2.0-5.5 when [NH₃]_T equals 0.6 and 4.2 mol/L, respectively, with the prefixed [C₂O₄^2-]_T of 0.6 mol/L. Due to the fractional precipitation of Cu²⁺ and Ni²⁺, Cu-Ni composite is obtained after the thermal decomposition of Cu-Ni oxalate complex salts prepared in a pure water system when pH>7.0. By applying the mixed solvent (water/ethanol) as the precipitation medium, the Cu-Ni alloy rods can be finally fabricated with high purity and crystallinity.

Key words: coordination-precipitation equilibrium; thermodynamic mathematical models; Cu-Ni oxalate complex salts; Cu-Ni alloy

1 Introduction

Expanding the fabrication methodologies for alloy materials has attracted substantially practical and theoretical research interest due to the significantly modified physical and chemical properties compared with their monometallic counterparts [1]. Benefiting from the intrinsically good magnetic, thermal-conductive, mechanical, and electronic properties [2-4], Cu-Ni alloy has earned great attention in the past few decades and was employed in the electromagnetic wave absorbing [5] and magnetocaloric fields [6], and also served as heterogeneous catalysts for many important reactions such as the evolution of hydrogen [7] and oxygen [8], decomposition of organic compounds [9,10], and the oxidation of alcohols [11,12]. Nickel has an identical face- centered-cubic (fcc) crystal structure and similar lattice parameters with copper (3.51 and 3.61 for Cu and Ni, respectively), making it possible to form the substitutional alloy by simply replacing the copper fraction with nickel atoms [13,14]. Currently, considerable efforts have been made to explore the fabrication approaches for Cu-Ni alloy, including mechanical alloying [6,15], direct-reduction [16], electrodeposition [17], spark discharge/laser ablation [18,19], hydrothermal [5,20], and ultra- sonically spray pyrolysis [21]. However, issues such as the relatively high cost, imprecise particle size and morphology control, extreme processing temperatures and usage of toxic organic solvents in some cases significantly restrict their further industrialization.

Liquid-phase precipitation, as a bottom-up strategy, combined with the post-annealing process at the moderate temperature could not only positively reduce the fabrication cost but also enable the rational tuning of metallic components at the near-atomic level and optimization of morphology and dimensions of the final products [22]. In our previous study, highly dispersed Cu and Ni powders were respectively fabricated through the thermal decomposition of the oxalate complex salts with a fibrous morphology synthesized in the corresponding liquid-phase system using oxalate acid as the precipitant and NH₃ as the morphology controlling reagent [23,24]. These attempts could pave a new avenue for the large-scale fabrication of Cu-Ni alloy. Additionally, such NH₃-induced one-dimensional (1D) morphology on the precursors coupled with the porous structure aroused by the removal of volatiles endows the metallic products with attractive properties such as the significantly increased specific surface area and the enhanced electronic conductivity [25,26]. However, due to the different thermodynamic properties of liquid-phase reactions involving Cu²⁺ and Ni²⁺ [27,28], their homogeneous precipitation and uniform distribution in oxalate complex salts would be the significant issues that affect the quality of the final products. Thus, a deep and comprehensive understanding of the behaviors of metal ions, i.e., Cu²⁺ and Ni²⁺, in this reaction system is urgently needed to evaluate their possibility of the simultaneous precipitation and to determine the corresponding pH range.

In the present study, the mathematical models of thermodynamics for the Cu²⁺-Ni²⁺-NH₃-NH₄⁺- C₂O₄^2--H₂O reaction system will be established based on the mass conservation principle. With the change of pH, the total concentrations of Cu²⁺ and Ni²⁺ and their potential species in pure water solvent will be investigated. To achieve the maintenance of the predetermined molar ratio of metal ions in the raw materials at high pH conditions, the modification of the precipitation medium by using the water-ethanol mixture will be proposed to prepare the Cu-Ni oxalate complex salts and further thermal decomposition will be applied to obtaining Cu-Ni alloy powders with the accurate Ni to Cu molar ratio.

2 Mathematical models of coordination-precipitation equilibrium in Cu²⁺- Ni²⁺-NH₃- NH₄⁺-C₂O₄^2--H₂O system

In the Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O reaction system, Cu²⁺ and Ni²⁺ could coordinate with various ligands including NH₃, OH^-, and C₂O₄^2- in the solution phase to form a series of metal complexes with different coordination numbers. Their cumulative formation constants (β) at 298 K [29] are listed in Table 1.

Table 1 Cumulative formation constants (β) of Cu/Ni complexes with different ligands (298 K)

There also occurs the dissociation of weak acids and bases including H₂C₂O₄, NH₄⁺, and H₂O, whose equilibrium constants (K) [29] are listed in Table 2.

Based on the mass balance, the mathematical models of the coordination-precipitation process in the Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O system are established, where [Me²⁺]_T (Me=Cu, Ni), [NH₃]_T, [C₂O₄^2-]_T represent the total concentration of metal ions, ammonia, and oxalate with all possible forms coexisting in the liquid phase, respectively.

Table 2 Dissociation reactions of acids and bases with corresponding equilibrium constants (K)

The total concentrations of metal ions [Me²⁺]_T are calculated by the sum of the concentrations of free metal ions ([Cu²⁺] and [Ni²⁺]) and their three types of metal complexes:

[Cu²⁺]_T=[Cu²⁺]+[Cu(NH₃)_n²⁺]_T+[Cu(OH)_n^2-n]_T+

[Cu(C₂O₄) _n^2-2n]_T=[Cu²⁺]{1+10^4.31[NH₃]+

10^7.98[NH₃]²+10^11.02[NH₃]³+10^13.32 [NH₃]⁴+

10^12.86[NH₃]⁵+10^pH-7.00+

10^2pH-14.32+10^3pH-25.00+10^4pH-37.5+

10^6.16[C₂O₄^2-]+10^8.50[C₂O₄^2-]²} (1)

[Ni²⁺]_T=[Ni²⁺]+[Ni(NH₃)_m²⁺]_T+[Ni(OH)_m^2-m]_T+

[Ni(C₂O₄)_m^2-2m]_T=[Ni²⁺]{1+10^2.80[NH₃]+

10^5.04[NH₃]²+10^6.77[NH₃]³+10^7.96[NH₃]⁴+

10^8.71 [NH₃]⁵+10^8.74[NH₃]⁶+10^pH-9.03+

10^2pH-19.45+10^3pH-30.67+10^5.30[C₂O₄^2-]+

10^7.64[C₂O₄^2-]²+10^8.50[C₂O₄^2-]³} (2)

Meanwhile, [NH₃]_T and [C₂O₄^2-]_T can be obtained according to the following equations:

[NH₃]_T=[NH₃]+[NH₄⁺]+∑n[Cu(NH₃)_n²⁺]+

∑m[Ni(NH₃) _m ²⁺]=[NH₃](1+10^9.24-pH)+

[Cu²⁺]{10^4.31[NH₃]+2×10^7.98[NH₃]²+

3×10^11.02 [NH₃]³+4×10^13.32[NH₃]⁴+

5×10^12.86 [NH₃]⁵}+[Ni²⁺]{10^2.80[NH₃]+

2×10^5.04[NH₃]²+3×10^6.77[NH₃]³+

4×10^7.96 [NH₃]⁴+5×10^8.71 [NH₃]⁵+

6×10^8.74[NH₃]⁶} (3)

[C₂O₄^2-]_T=[C₂O₄^2-]+[HC₂O₄^-]+[H₂C₂O₄]+

∑n[Cu(C₂O₄)_n^2-2n]+∑m[Ni(C₂O₄) _m ^2-2m]=

[C₂O₄^2-](1+10^4.27-pH+10^5.54-2pH)+

[Cu²⁺]{10^6.16[C₂O₄^2-]+2×10^8.50[C₂O₄^2-]²}+

[Ni²⁺]{10^5.30 [C₂O₄^2-]+2×10^7.64 [C₂O₄^2-]²+

3×10^8.50[C₂O₄^2-]³} (4)

Besides, Cu²⁺ and Ni²⁺ can form the precipitates with C₂O₄^2- and OH^-, and their solubility product constants (K_sp) [29] are listed in Table 3. Therefore, the concentrations of free metal ions, namely [Cu²⁺] and [Ni²⁺], are determined by

[Cu²⁺]=min{10^-7.64/[C₂O₄^2-], 10^-19.66/[OH^-]²} (5)

[Ni²⁺]=min{10^-9.40/[C₂O₄^2-], 10^-14.70/[OH^-]²} (6)

Table 3 Solubility product constants (K_sp) of MeC₂O₄ and Me(OH)₂ (Me=Cu, Ni)

3 Thermodynamic simulation results

According to the thermodynamic mathematical models established based on the reactions above, the values of [Cu²⁺]_T, [Ni²⁺]_T, and the concentrations of metal complexes with the change of pH can be calculated through Eqs. (1)-(6) with the constant values of [NH₃]_T and [C₂O₄^2-]_T.

3.1 Relationship between lg[Cu²⁺]_T and pH

The curves of lg[Cu²⁺]_T vs pH in Cu²⁺-Ni²⁺- NH₃-NH₄⁺-C₂O₄^2--H₂O reaction system with [NH₃]_T ranging from 0.6 to 4.2 mol/L are plotted in Fig. 1. The [C₂O₄^2-]_T is predefined to be 0.6 mol/L. It can be observed that when pH<4.0, these curves overlap and show the same declining trends with the increase of pH. Combined with Fig. 2 that displays the concentrations of various copper species at different pH values, the dominance of [Cu(C₂O₄)_n]^2-2n complex indicates the interaction between Cu²⁺ and C₂O₄^2- and the negligible influence of NH₃ in pH range of 0-4.0. However, when pH=4.0-9.0, the lg[Cu²⁺]_T dramatically increases, which can be ascribed to the prominent coordination effect of NH₃. It can also be noticed from Fig. 2 that [Cu(NH₃)_n]²⁺ gradually increases at this time, especially for those copper ammonia complexes with the higher coordination numbers (n=3-5), parallel to the continuous decrease of free Cu²⁺. At the higher pH values (pH>10.0), Cu²⁺ is more likely to be precipitated as Cu(OH)₂, leading to a decrease of lg[Cu²⁺]_T again. When pH is larger than 13.0, the total concentrations of [Cu(OH)_n]^2-n complexes will exceed those of other species and are predominant in the system. Meanwhile, the coordination reaction between Cu²⁺ and OH^- results in a slight increase of [Cu²⁺]_T (pH>12.0) as shown in Fig. 1.

Fig. 1 Relationship between lg[Cu²⁺]_T and pH with different [NH₃]_T ([C₂O₄^2-]_T=0.6 mol/L)

Fig. 2 Concentrations of various Cu²⁺ species in Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O system ([C₂O₄^2-]_T= 0.6 mol/L, [NH₃]_T=3.0 mol/L)

3.2 Relationship between lg[Ni²⁺]_T and pH

Figure 3 shows the relationships between lg[Ni²⁺]_T and pH with the predefined [C₂O₄^2-]_T of 0.6 mol/L and different [NH₃]_T, and Fig. 4 displays the corresponding change of concentration of Ni²⁺ species coexisting in Cu²⁺-Ni²⁺-NH₃-NH₄⁺- C₂O₄^2--H₂O reaction system. The total concentration of Ni²⁺ decreases initially at the low pH values due to the formation of NiC₂O₄. This kind of oxalate salt was reported to be likely dissolved in the extremely acidic conditions [30], resulting in the highest concentration of free Ni²⁺ at pH=0, as shown in Fig. 4. It can also be found that [Ni(C₂O₄)_m]^2-2m complexes are the main species of nickel ions in the pH range of 0-8.5. With the continuous increase of pH, the capability of Ni²⁺ coordinating with NH₃ gradually becomes prominent and even more significant than that with C₂O₄^2- when pH>9.0, leading to the sharp increase of lg[Ni²⁺]_T (Fig. 3). Nickel ammonia complexes turn to be dominant at this point, especially [Ni(NH₃)_m]²⁺ (m=3-6). Similarly, due to the formation of Ni(OH)₂, lg[Ni²⁺]_T decreases when pH is larger than 11.0. However, in Fig. 3, the increase of lg[Ni²⁺] when pH>12.0 can only be observed at the low values of [NH₃]_T (0.6 and 1.2 mol/L), which is slightly different from the behaviors of copper ions in the same reaction system.

Fig. 3 Relationship between lg[Ni²⁺]_T and pH with different [NH₃]_T ([C₂O₄^2-]_T = 0.6 mol/L)

Fig. 4 Concentrations of various Ni²⁺ species in Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O system ([C₂O₄^2-]_T= 0.6 mol/L, [NH₃]_T=3.0 mol/L)

3.3 Coprecipitation pH ranges of Cu²⁺ and Ni²⁺

According to the above thermodynamic simulation results, it can be concluded that the precipitation of Cu²⁺ and Ni²⁺ into oxalate complex salts is a complicated dynamic equilibrium process associated with the competition of various reactions in the system, during which the precipitating rates and precipitation sequences of metal ions are significantly influenced by their interactions with NH₃/NH₄⁺, OH^-, and C₂O₄^2-, etc. To determine the coprecipitating pH ranges, the curves of lg[Me²⁺] (Me=Cu, Ni) vs pH with the minimum and maximum [NH₃]_T (0.6 and 4.2 mol/L) and the fixed [C₂O₄^2-]_T of 0.6 mol/L are plotted in Fig. 5. Due to the larger cumulative formation constants (β) of copper complexes compared to those of nickel species, the total concentration of Cu²⁺ in solution is constantly higher than that of Ni²⁺ until pH reaches 11.0. The simultaneous precipitation of metal ions and Cu/Ni molar ratio control could only be achieved at pH=2.0-5.5 and 2.0-6.5 for [NH₃]_T= 4.2 mol/L and 0.6 mol/L, respectively, where both of [Cu²⁺]_T and [Ni²⁺]_T are lower than 10^-5 mol/L. However, it should be noted that the fibrous morphology cannot be formed in this pH range based on our previous reports [22-24]. When pH>7.0, the coprecipitation of Ni²⁺ and Cu²⁺ with oxalate ions would be thermodynamically unfavorable to control, leading to the change of the predefined molar ratio of Cu/Ni in the precipitates and the high possibility of the formation of Cu-Ni composites as the final products instead of the substitutional alloys after the annealing process. Therefore, to obtain Cu-Ni alloy with the maintained metallic molar ratio and the fibrous morphology, a modified solvent of the water-ethanol mixture will be applied due to the reduced solubility of oxalate precipitates and metal complexes in a solvent with a lower surface tension than water, and also its successfully practical utilization in the fabrication of Ni-Co fibrous alloy powders [22].

Fig. 5 Coprecipitation pH ranges of Cu²⁺ and Ni²⁺ with [NH₃]_T =0.6 and 4.2 mol/L ([C₂O₄^2-]_T=0.6 mol/L)

4 Preparation of Cu-Ni alloy powders

4.1 Experimental and characterization

All of the reagents including nickel(II) chloride hexahydrate (NiCl₂·6H₂O), copper(II) chloride dihydrate (CuCl₂·2H₂O), oxalic acid dihydrate (H₂C₂O₄·2H₂O), ammonium hydroxide (NH₃·H₂O), polyvinylpyrrolidone (PVP), and ethanol were of analytical grade and directly used without further purification.

In a typical experiment, a 100 mL solution dissolving CuCl₂·2H₂O (7.842 g), NiCl₂·6H₂O (9.702 g) and the surfactant PVP (0.5 g) was slowly dripped into 100 mL H₂C₂O₄ (13.010 g) solution at the rate of 2.22 mL/min, during which the pH was adjusted to 7.6 by the dropwise addition of NH₃·H₂O. The system was maintained at a constant temperature of 323 K in a water bath and vigorously stirred for 2 h after feeding. Precipitated solids were collected, rinsed by deionized water and ethanol, and dried in the vacuum oven at 353 K for 12 h. The pure water solvent was replaced by a mixture of water and ethanol (1/1, volume ratio) as the modified system. Final products were obtained via thermal decomposition of the prepared oxalate complex salts in Ar/H₂ (95/5, volume ratio) at 673 K for 30 min.

Field-emission scanning electron microscopy (FE-SEM, TESCAN MIRA3 LMU) and transmission electron microscopy (TEM, Tecnai G2 20ST) images were obtained to show the morphology and particle sizes of the prepared powders. Energy-dispersive X-ray spectroscopy (EDX, GENESIS 60S) measurement was performed to detect the molar ratio of Cu to Ni at the selected area of the oxalate complex salts. X-ray diffractometer (XRD, Rigaku-TTR III) with the radiation of Cu K_α (λ=0.154056 nm) was applied to determining the phase and composition of the final products.

4.2 Results and discussion

Fig. 6 XRD patterns of Cu-Ni oxalate complex salts prepared at pH=7.6 and without NH₃·H₂O in modified solvent system [32]

It has been reported that the morphology and particle size of oxalate complex salts can be simply controlled by adjusting the volume of NH₃·H₂O added into the suspension, and the quasi-1D morphology will be formed at high pH values due to the shape-induced effect of NH₃, for example, 8.0-8.2 for Cu [23], 8.4-8.8 for Ni [24], 8.0-8.4 for Ni-Co [22], and 8.0 for Co-Zn system [31], etc. Thus, the alkaline environment of the Cu-Ni system is expected to be favorable for the precipitation of Cu-Ni alloy precursors with the aforementioned 1D morphology. The XRD patterns of Cu-Ni oxalate complex salts prepared at pH=7.6 and without NH₃·H₂O are both recorded and displayed in Fig. 6. The Cu-Ni alloy precursors obtained at pH=7.6 is suggested to be a novel solid solution containing NH₃ instead of the mechanical mixture of MeC₂O₄ (Me=Cu and Ni), which has been illustrated in detail according to the recently published work [32].

Figure 7(a) shows the SEM images of Cu-Ni oxalate complex salts obtained at pH=7.6 in the modified reaction system. It can be observed that the prepared precursors display a fibrous morphology with a length of 10-20 μm and a diameter of 0.4-0.6 μm. The EDX results (Fig. 7(b)) indicate the equivalent atomic proportion of Cu and Ni in the precipitates, which is the same as the molar ratio of metal sources in the feedstock. By releasing the volatile components including water of crystallization, coordinated NH₃, and CO₂ originated from the thermal decomposition of oxalate salts, a special porous Cu-Ni alloy powders was formed (Fig. 7(c)), which can be clearly observed from the TEM image (Fig. 7(d)) by showing the numerous interconnected nanoparticles with the existence of pores at the nanoscales.

Fig. 7 SEM image (a) and EDX result (b) of Cu-Ni oxalate complex salts prepared in modified system, SEM (c) and TEM (d) images of corresponding final products

The crystal phase and composition of final products obtained by thermal decomposition of precursors prepared in the pure water and mixed solvent system were identified and compared by the XRD technique. The diffraction patterns indicate that the product obtained from the pure water system (Fig. 8(a)) has the mixed phases of Cu and Ni. The diffraction peaks located at 2θ=43.3°, 50.4°, and 74.1° are ascribed to the (111), (200) and (220) crystal facets of the fcc structured Cu (JCPDS No.04-0836) [33,34] and the peaks at 2θ of 44.5°, 51.8°, and 76.4°correspond to the (111), (200), and (220) facets of Ni (JCPDS No. 04-0850) [35]. However, when using a ethanol-H₂O mixture as the solvent, Cu-Ni alloy was finally obtained (Fig. 8(b)), and the three diffraction peaks at 2θ=43.9°, 51.2°, and 75.3° can be well-indexed to the (111), (200), and (220) facets of Cu-Ni alloy (JCPDS No. 65-9047). No peaks of Cu, Ni, or any other substances were detected, indicating its high purity and thus ruling out the fractional precipitation of Cu²⁺ and Ni²⁺ during the coordination-coprecipitation process.

Fig. 8 XRD patterns of Cu-Ni composite (a) and Cu-Ni alloy (b)

5 Conclusions

(1) The thermodynamic mathematical models of Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O system were established according to the mass conservation principle, and the curves of lg[Me²⁺] (Me=Cu, Ni) vs pH were plotted to illustrate the trends of concentration of metal ions with the change of pH during the precipitation process.

(2) At the low pH values, free Me²⁺ and Cu(C₂O₄)_n^2-2n are predominant during which the precipitation of Me²⁺ by C₂O₄^2- mainly occurs. While at high pH conditions, the dominance of [Cu(NH₃)_n]²⁺ (n=3-5) and [Ni(NH₃)_m]²⁺ (m=3-6) in solution indicates the preferable coordination of metal ions with NH₃ than other ligands. With the predefined [C₂O₄^2-]_T of 0.6 mol/L, the coprecipitation pH ranges of Cu²⁺ and Ni²⁺ with [NH₃]_T of 0.6 mol/L and 4.2 mol/L are 2.0-6.5 and 2.0-5.5, respectively.

(3) Experimental results confirmed the thermodynamically favorable fractional precipitation of Cu²⁺ and Ni²⁺ when preparing the oxalate complex salts at pH=7.6, leading to the formation of Cu-Ni composite after the annealing process. By applying the water-ethanol mixture as the modified precipitation medium, Cu-Ni alloy powders with a quasi-1D morphology and the porous structure were finally obtained.

Acknowledgments

The authors are grateful for the financial supports from Natural Science Foundation of Hunan Province, China (No. 2020JJ4735), Science and Technology Department of Hunan Province Tackling Key Scientific and Technological Problems and Transformation of Major Scientific and Technological Achievements, China (No. 2018GK4001), and the Hunan Key Laboratory for Rare Earth Functional Materials, China (No. 2017TP1031).

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Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O体系金属离子沉淀行为热力学分析

苗泽林¹，湛 菁^1,2，徐子伟¹

1. 中南大学 冶金与环境学院，长沙 410083；

2. 中南大学 难冶有色金属资源高效利用国家工程实验室，长沙 410083

摘 要：针对反应体系Cu²⁺-Ni²⁺-NH₃-NH₄⁺-C₂O₄^2--H₂O中Cu²⁺和Ni²⁺的反应行为，根据物质守恒原理，建立热力学数学模型。模拟计算结果表明，该体系中金属离子的沉淀是一个复杂的动态平衡过程；在高pH条件下，Cu²⁺和Ni²⁺与NH₃配合分别形成[Cu(NH₃)_n]²⁺ (n=3-5)和 [Ni(NH₃)_m]²⁺ (m=3-6)的反应占主导地位。当[C₂O₄^2-]_T=0.6 mol/L时，Cu²⁺和Ni²⁺的共沉淀pH范围在[NH₃]_T为0.6 mol/L和4.2 mol/L时分别为2.0-6.5和2.0-5.5。在pH>7.0的纯水体系中，由于Cu²⁺和Ni²⁺的分步沉淀，Cu-Ni草酸复盐热分解后得到的产物为Cu-Ni复合物。改变沉淀介质为混合溶剂(水/乙醇)可最终制备具有高纯度和良好结晶度的棒状Cu-Ni合金粉末。

关键词：配位-沉淀平衡；热力学数学模型；Cu-Ni 草酸复盐；Cu-Ni合金

(Edited by Xiang-qun LI)

Corresponding author: Jing ZHAN, Tel: +86-13975147556, E-mail: zhanjing@csu.edu.cn

DOI: 10.1016/S1003-6326(21)65591-7

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