NaCl-KCl-NdCl3-AlCl3熔盐体系电解制备Nd-Al合金及机理

在700℃的NaCl-KCl体系中,采用循环伏安法、方波伏安法和开路计时电位法等电化学测试方法研究了钨电极上Nd(Ⅲ)的电化学行为以及与Al(Ⅲ)共沉积的电化学过程。结果表明:在低扫描速率(0.1～0.5V·s~(-1))下,NaCl-KCl-NdCl_3熔盐中Nd(Ⅲ)的还原是一步交换3个电子的准可逆过程,该还原过程由扩散步骤控制,扩散系数约为1.2×10~(-5) cm~2·s~(-1)。将AlCl_3引入NaCl-KCl-NdCl_3熔体后,循环伏安、方波伏安以及开路计时电位测试结果表明,电解过程中形成了3种Nd-Al金属间化合物(Nd_3Al_(11)、NdAl_3和NdAl_2),其形成机理为Nd在预先沉积的Al上发生欠电位沉积。通过电动势法,计算了700℃时Nd-Al金属间化合物的热力学参数,如两相共存下Nd的活度和相对偏摩尔吉布斯自由能以及Nd-Al金属间化合物的标准吉布斯自由能。在-2.3 V下进行恒电位电解5 h制备了Nd-Al合金,所得合金由Nd_3Al_(11)、NdAl_3、NdAl_2和Al4种物相组成。

Preparation and Reaction Mechanism of Nd-Al Alloys by Electrolysis in Molten NaCl-KCl-NdCl3-AlCl3

_{: The electrochemical behaviors of Nd(III) on tungsten electrode and its co-deposition with Al(III) in NaCl-KCl salts at 700 °C were investigated by a series of electrochemical techniques such as cyclic voltammetry, square wave voltammetryand open-circuit chronopotentiometry, and thus Nd-Al alloy was prepared via direct co-deposition of Nd and Al on the inert electrode. The results show thatthe reduction of Nd(III) in NaCl-KCl-NdCl3 melts isa one-step process with three electrons exchanged, and the reaction is anirreversible processcontrolled by diffusionat low scan rate (0.1~0.5V/s) with the calculated diffusion coefficient is approximately 7.6×10^-5cm²/s. After AlCl3introduced into the NaCl-KCl-NdCl3 melts, the curves of cyclic voltammetry, square wave voltammetryand open-circuit chronopotentiometry indicate that three kinds of Nd-Al intermetallic compounds (Nd11Al3, NdAl3 and NdAl2) were formed. It can be deduced that the underpotential deposition of Nd occurs on the tungsten electrode covered with the pre-deposited metal Al from the open circuit chronopotentiometriy. Based on the results of open circuit chronopotentiometriy, the thermodynamic properties of Nd-Al intermetallic compounds were determined by electromotive force measurement at 700 °C, such as the activity of Nd in two-phase coexisting state and relative partial molar Gibbs energies, as well as the standard Gibbs energies ofNd-Al intermetallic compound. Finally, potentiostatic electrolysis was carried out at -2.3Vfor 5h to obtain a product alloys. X-ray diffraction (XRD) and scan electron micrograph (SEM)-energy dispersive spectrometry (EDS) analysis indicated that the obtained alloys were mainly composed of Nd11Al3, NdAl3, NdAl2 and Al phases.}