Trans. Nonferrous Met. Soc. China 23(2013) 3456-3461

Thermodynamics analysis of Ni²⁺-C₂H₈N₂--H₂O system and preparation of Ni microfiber

Yong-lin YAO, Chuan-fu ZHANG, Jing ZHAN, Feng-hua DING, Jian-hui WU

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

Received 27 March 2013; accepted 18 June 2013

Abstract: According to the principles of simultaneous equilibrium and mass equilibrium, the thermodynamics model of the precipitation-coordination equilibrium of Ni²⁺-C₂H₈N₂--H₂O system was established, and calculation for the relationships between concentration of each substance in solution and parameters was carried out, including pH value, concentrations of ethylenediamine and oxalate by MATLAB program. The results show that Ni exists as Ni²⁺ and [Ni(C₂O₄)_n]^2-2n mainly at pH<1 and pH=1-6, respectively. When pH> 6, the complex between Ni²⁺ and ethylenediamine is predominant. The precursor of Ni microfiber was prepared by an oxalate precipitation process using ethylenediamine as a coordination agent, and the role of ethylenediamine in the growth of the precursor fiber was discussed. The Ni microfiber can be obtained by a thermal decomposition-reduction process of the precursor in N₂ and H₂ mixed atmosphere. The diameters and aspect ratios of the obtained Ni microfibers are 0.2-1 μm and 20-30, respectively.

Key words: thermodynamics analysis; Ni microfiber; precursor; thermal decomposition; reduction

1 Introduction

As an important transition metal material, superfine nickel powder has some unique properties in electromagnetism, heat, light and chemical activity areas because of its super volume effect, surface effect, and good thermal and electrical conductivity [1,2]. Micro- and nano-nickel powders are widely used as catalyst [3-5], electronical slurry [6], microwave absorbing material [7] and so on. Up to date, some methods including carbonyl process [8], liquid reduction method [9] and mechanochemical method [10] are widely used for preparation of superfine nickel powder; however, the carbonyl process is toxic, and the shape is difficult to control by liquid reduction and mechanochemistry method.

The shape of a material has a great influence on its property. Magnetic metal fiber is a potential microwave absorbing material because of its unique shape anisotropy. The polycrystal iron fiber including Fe, Co, Ni and their alloy fibers as microwave absorbing materials were researched by many people [11-13]. Template-based approaches are widely used for the preparation of magnetic metal micro/nano fibers and wires. Many magnetic metal fibers/wires were successfully synthesized by porous anodic aluminum oxide (AAO) or polycarbonate membrane as template [14,15]. However, this method has a complicated process and a low yield which limit its wide application. The precursor of nickel microfiber can be prepared by an oxalate coordination precipitation method with ammonia as a coordination agent, and the nickel fiber can be obtained by a thermal decomposition-reduction process for the precursor [16-18]. But the pH value of the Ni²⁺-NH₃--H₂O system for obtaining precursor fibers is high (~9), which will lead to an equipment spoilage and environmental pollution. In this work, we prepared a new Ni precursor fiber at a neutral environment (pH=6.5) and a less dosage of coordination agent with ethylenediamine instead of ammonia as a coordination agent. The precipitation-coordination equilibrium of Ni²⁺-C₂H₈N₂--H₂O system was first calculated and the effects of pH, concentrations of ethylenediamine and oxalate on the precipitation process were determined. The role of ethylenediamine in the fiber growth mechanism was revealed, which can provide reference for other fiber preparation by coordination precipitation process. The precursor was decomposed and reduced in N₂ and H₂ mixed atmosphere and the nickel fiber can be obtained.

2 Precipitation-coordination equilibrium of Ni²⁺-C₂H₈N₂--H₂O system

2.1 Establishment of precipitation-coordination equilibrium model

C₂H₈N₂,  and OH^- can combine with Ni²⁺ to form complex compounds in the Ni²⁺-C₂H₈N₂- -H₂O system, and the accumulation formation constants (β) are listed in Table 1 [19].

Table 1 Cumulative formation constants of complexes

The reaction equilibrium constants (K) of the acid-base equilibrium reaction between oxalate and ethylenediamine are listed in Table 2 [19].

Table 2 Equilibrium constants of oxalate and ethylenediamine

[Ni]_T, [C₂O₄]_T, [C₂H₈N₂]_T and [OH]_T are analytical concentrations of each composition in solution, and according to mass balance and simultaneous equilibrium principle, and we can get some equations as follows:

[Ni]_T=[Ni²⁺]+[Ni(OH)⁺]+[Ni(OH)₂]+[]+[Ni(C₂O₄)]+[]+[]+

[Ni(C₂H₈N₂)²⁺]+[]+[]

[C₂O₄]_T=[Ni(C₂O₄)]+2[]+3[]+

[(C₂O₄)^2-]+[H(C₂O₄)^-]+[H₂(C₂O₄)][C₂H₈N₂]_T= [C₂H₈N₂]+[C₂H₉N₂⁺]+[]+

[Ni(C₂H₈N₂)²⁺]+2[]+3[]

[OH]_T= [NiOH⁺]+2[Ni(OH)₂]+3[]+[OH^-]

Ni²⁺ can combine with  or OH^- to form oxalate or hydroxide precipitate at different conditions and the solubilities (K_sp) of each precipitate are listed in Table 3 [19].

Table 3 Solubility products of NiC₂O₄ and Ni(OH)₂

When NiC₂O₄(s) exists in the solution, the concentration of Ni²⁺ can be calculated by [Ni²⁺]= K_sp/[]. The NiC₂O₄(s) will convert to Ni(OH)₂(s) when pH is high enough and the concentration of Ni²⁺ can be calculated by [Ni²⁺]=K_sp/[OH^-]²= K_sp×10^28-2pH. From the above, the concentration of Ni²⁺ can be described as [Ni²⁺]={10^-9.4/[], 10^13.3-2pH}_min.

The concentration of each substance in the solution can be calculated using MATLAB program according to the mass equilibrium, complex equilibrium, acid-base equilibrium and precipitate equilibrium equations. Here we only calculated the results at pH<10 to avoid the form of hydroxide precipitate at high pH values.

2.2 Calculation results

Figure 1 shows the relationships of [Ni]_T with pH and [C₂H₈N₂]_T at [C₂O₄]_T= 0.1 mol/L. Figure 2 indicates the corresponding distribution state of Ni in solution. As shown in Fig. 1, when pH<1, NiC₂O₄(s) will dissolve to a certain extent. In this case, Ni exists as Ni²⁺ mainly which is confirmed in Fig. 2. With the increase of pH, [Ni]_T decreases because the dissociation of H₂C₂O₄ and  leads to a rise of [] and the precipitation rate of NiC₂O₄(s) increases. When pH=1-6,  will combine with Ni²⁺ and [Ni(C₂O₄)_n]^2-2n is the main existing form of Ni according to Fig. 2, and so the change law of Ni²⁺ precipitation rate with pH is similar with that of . When pH>6, the complex between ethylenediamine and Ni²⁺ is superior, which leads to a further decrease of the precipitation rate. As shown in Fig. 2, the concentration of ethylenediamine does not influence the concentrations of Ni²⁺, [Ni(C₂O₄)_n]^2-2n and [Ni(OH)_n]^2-n, but is positively correlated with the concentration of . So, the precipitation rate changes hardly with the variation of ethylenediamine at pH<6 but decreases with the increase of the concentration of ethylenediamine at pH>6.

Fig. 1 Relationships among [Ni]_T and pH and [C₂H₈N₂]_T at [C₂O₄]_T= 0.1 mol/L

Fig. 2 Relationships between each Ni species and pH at [C₂O₄]_T=0.1 mol/L

Figure 3 shows the relationships of [Ni]_T with pH and [C₂O₄]_T at [C₂H₈N₂]_T=0.1 mol/L. Figure 4 indicates the corresponding distribution state of Ni in solution. When pH<1, [Ni]_T decreases with the increase of [C₂O₄]_T. Associating with Fig. 4, the concentration of [Ni(C₂O₄)_n]^2-2n has little relationship with [C₂O₄]_T in the range of pH< 1, which indicates that the precipitation action rather than the complex between  and Ni²⁺ is predominant. When pH=1-6, it can be seen that Ni exists as [Ni(C₂O₄)_n]^2-2n mainly and  will increase with the increase of [C₂O₄]_T, which leads to the same change trend for [Ni]_T shown in Fig. 3. When pH>6,  has little change and the complex between Ni²⁺ and ethylenediamine determines the precipitation rate.

It is noted that the system of Ni²⁺-C₂H₈N₂- -H₂O is complicated and the analyses above are the results in certain conditions (such as [C₂O₄]_T=0.1 mol/L, [C₂H₈N₂]_T=0.1 mol/L). But we can get the results in any condition with the above method because of the same principle. The change laws of precipitation rate and each Ni species with pH have a guiding significance for the preparation of Ni precursor fiber, which is discussed in detail in the next section.

Fig. 3 Relationships among [Ni]_T and pH and [C₂O₄]_T at [C₂H₈N₂]_T= 0.1 mol/L

Fig. 4 Relationships between each Ni species and pH at [C₂H₈N₂]_T=0.1 mol/L

3 Preparation of Ni microfiber

3.1 Experimental

All the reagents including H₂C₂O₄·2H₂O, NiCl·6H₂O, ethylenediamine, and ethanol are analytically pure. The H₂C₂O₄ and NiCl₂ solutions with a certain concentration were prepared and their pH values were adjusted by ethylenediamine. The two solutions were mixed and stirred at 60 °C for 3 h and some blue precipitates (precursors) can be obtained. The precursors were washed with ethanol and water three times and then dried at 60 °C for 4 h. The dried precursors were decomposed and reduced in N₂ and H₂ mixed atmosphere V(N₂):V(H₂)=9:1 at 420 °C for 30 min and the nickel microfibers can be obtained. X-ray diffraction (XRD, TTR III) was applied to determining the phase compositions of precursors and Ni microfibers. The morphologies of the precursors and Ni microfibers were observed with an electron scanning microscope (SEM, JSM-6360LV) and the composition of the Ni microfibers was characterized with the energy dispersive spectrometer (EDS, GENESIS) equipped on the SEM.

3.2 Results and discussion

Figures 5 (a) and (b) show the XRD patterns of precursors prepared at pH<2 (no addition of ethylenediamine) and pH=6.5, respectively. As shown in Fig. 5(a), in the case of no ethylenediamine addition, the crystal structure of precursor can be determined as NiC₂O₄·2H₂O according to the Joint Committee on Powder Diffraction Standards (JCPDS, No. 25—0581). However, there is no corresponding JCPDS card with the precursor prepared at pH=6.5. For Ni²⁺--C₂H₈N₂ system, the coordination of Ni and C₂H₈N₂ can be described as [Ni(C₂H₈N₂)(C₂O₄)] chains [20], and single crystal of Ni(C₂H₈N₂)(C₂O₄)·2H₂O has been synthesized [21]. So, we suppose that during the precipitation  process, the ethylenediamine combined Ni ions and formed a new crystal structure which can be described as [Ni(C₂H₈N₂)_y]C₂O₄·xH₂O [22] and the exact structural formula will be determined in a further study.

The SEM photographs of precursors prepared at different pH values are shown in Fig. 6. When no ethylenediamine is added, the precursors are clusters consisting of some cube particles. Under this circumstance, the direct contact of oxalate and Ni²⁺ causes a quick nucleation and many crystal nucleus bump and form an agglomeration. When ethylenediamine is added, Ni²⁺ will combine with it and the speed of nucleation and growth can be controlled, which is beneficial to obtaining separated particles, as shown in Fig. 6(b). When the concentration of ethylenediamine increases to a certain extent (pH=6), the growth pattern of precursor changes and the fibrous precursors appear, but the shape is not uniform and some breakages and branches exist. When pH=6.5, the shapes of precursors become uniform and the surfaces are smooth.

Fig. 5 XRD patterns of precursors prepared at pH<2 (no addition of ethylenediamine, NiC₂O₄·2H₂O) (a) and pH=6.5 (b)

Fig. 6 SEM images of precursors prepared at different pH values

According to the above equilibrium calculation results, the pH value should be controlled at 1-3 for a satisfactory precipitation rate. However, the pH value has an important effect on the precursor shape. The complex of ethylenediamine and Ni²⁺ is not dominant at pH<6 but it becomes superior at pH>6. This corresponds to the shape change process in which the precursor is cube-like and fiber-like at pH<6 and pH>6, respectively. In our experiment, the addition of ethylenediamine was controlled by pH, so we can conclude that ethylenediamine can change the growth pattern of precursor. Micron-sized precipitated powder is usually an agglomerate of small crystallites and the crystallites aggregate to a certain shape under the action of surface energy differences [23]. In the precipitation process, ethylenediamine can be adsorbed on a certain crystal face of the precursor and restrain its growth due to the surface energy change, which causes the final formation of fibrous precursor. The result is consistent with the phenomenon in other coordinated-precipitation process using ammonia as coordination agent [24,25]. But when pH>7, the complex of ethylenediamine and Ni²⁺ becomes serious, which may cause a bad precipitation rate even the precursor precipitate cannot be obtained, so giving consideration to both precipitation rate and precursor shape, the pH should be controlled at 6.5.

Figure 7 shows the XRD pattern of the Ni microfibers through thermal decomposition-reduction of precursor fibers. As shown in Fig. 7, the sample shows a good crystallinity and the peaks match well with those of the JCPDS (No. 04—0850) data of Ni. The XRD pattern without impurity peaks indicates that the product has a high purity.

Fig. 7 XRD pattern of Ni microfibers

Figure 8(a) shows the SEM image of the Ni microfibers from which we can see the diameters of the fibers are 0.2-1 μm, and the aspect ratios are 20-30. The surfaces are not smooth as the precursors because of the release of CO, CO₂ and ethylenediamine in the thermal decomposition process. As shown in Fig. 8(b), the EDS spectrum further indicates the purity of the Ni microfibers.

Fig. 8 SEM image (a) and EDS spectrum (b) of Ni microfibers

4 Conclusions

1) The precipitation-coordination equilibrium model of Ni²⁺-C₂H₈N₂--H₂O system was established and calculated by MATLAB program. Ni exists as Ni²⁺ and [Ni(C₂O₄)_n]^2-2n mainly at pH<1 and pH=1-6, respectively. When pH>6, the complex between Ni²⁺ and ethylenediamine is predominant. All substances except  are positively correlated with [C₂O₄]_T but only  increases with the increase of [C₂H₈N₂]_T.

2) The Ni precursor fiber was prepared at pH=6.5 by an oxalate precipitation process with ethylenediamine as a coordination agent, and the growth mechanism can be changed with the addition of ethylenediamine. The pure Ni microfiber can be obtained by a thermal decomposition-reduction of the precursor fiber at 420 °C for 30 min in N₂ and H₂ mixed atmosphere. The diameters and aspect ratios of the obtained Ni microfiber are 0.2-1 μm and 20-30, respectively.

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Ni²⁺-C₂H₈N₂--H₂O体系的热力学分析及Ni微米纤维的制备

姚永林，张传福，湛 菁，丁风华，邬建辉

中南大学 冶金与环境学院，长沙 410083

摘  要：根据同时平衡和质量平衡原理，建立了Ni²⁺-C₂H₈N₂--H₂O体系的沉淀-配合平衡热力学模型，并对该模型进行计算来揭示该体系中各物种浓度随pH值、草酸及乙二胺浓度的变化规律。结果表明，当pH<1和pH=1-6时，溶液中的Ni分别主要以游离Ni²⁺及[Ni(C₂O₄)_n]^2-2n形式存在。当pH>6时，Ni²⁺与乙二胺的配合作用占优势。当用乙二胺作配位剂，采用草酸盐沉淀法制备了Ni微米纤维前驱体，并讨论了乙二胺在前驱体生长机制中的作用。将前驱体在N₂和H₂混合气氛中热分解还原，可得到直径0.2~1 μm、长径比20~30的Ni微米纤维。

关键词：热力学分析；Ni微米纤维；前驱体；热分解；还原

(Edited by Hua YANG)

Foundation item: Project (CX2012B046) supported by Hunan Provincial Innovation Foundation for Postgraduate, China; Project (20090162120080) supported by the Doctorate Fund of Education Minister of China

Corresponding author: Jing ZHAN; Tel: +86-731-88836048; E-mail: zhanjing2001@hotmail.com

DOI: 10.1016/S1003-6326(13)62888-5