1. Introduction
Pure nickel is used widely in the preparation of various metal microstructures because of its high ductility, strength, and fatigue and corrosion resistances and superior magnetoelasticity [
1,
2]. Various types of pure-nickel microstructures have been used successfully in micromachines and microsystems, such as the microscopic coil springs of semiconductor devices [
3], the microgear reducer of a microscopic transmission system [
4], the microrotor of a microgyroscope [
5], and the microcantilevers of hydrogen sensors [
6]. Recently, the integral fabrication of a high-working-frequency terahertz rectangular waveguide cavity was reported, and this novel process depends on the manufacture of a pure-nickel sacrificial rectangular mandrel and its selective chemical dissolution [
7,
8]. The transmission of terahertz signals can be improved significantly through such fabrication of such a cavity, and so a pure-nickel sacrificial rectangular mandrel with controllable size and good surface roughness and fillet radius has great application potential in the manufacturing of terahertz microcavity components.
Various technologies are currently available for machining pure-nickel microstructures. Song et al. studied wire electrical-discharge machining experimentally and manufactured complex pure-nickel parts at microscale and mesoscale using the optimal combination of machining parameters [
9]. Hendijanifard et al. studied the Marangoni flow and phase explosion during the laser micromachining of pure nickel and machined microholes in pure-nickel films [
10]. However, the influences of heat-affected zones, residual stresses, and melting layers mean that those two methods inevitably have some drawbacks. Cormier et al. fabricated pure-nickel pyramidal fin arrays using cold-spray additive manufacturing, but that approach fell short of achieving high dimensional accuracy and good surface accuracy [
11]. Bi et al. obtained a pure-nickel rectangular mandrel with controllable size, high dimensional accuracy, good surface roughness and fillet radius by wire electrochemical micromachining, but the low machining efficiency of that method is not conducive to the mass production of pure-nickel rectangular mandrels [
8]. Therefore, it is necessary to explore other types of micromachining technology for pure-nickel microstructures.
Electrochemical deposition (ECD) is a typical additive micromachining technology in which the product is formed layer by layer, and ECD technology based on an aqueous solution generally has the advantages of a wide range of application materials, low operating temperature, coordinated control of microstructure, morphology, and properties, and flexible application form, among others [
12,
13]. Theoretically, when the metal atoms or grains formed by the reduction reaction are stacked in a controlled manner as designed, metal-based structures and parts of any shape can be fabricated by ECD [
14]. With increasing application requirements in the fields of microelectromechanical systems and terahertz devices, ECD has gradually been recognized as a mature micromachining technology to meet these high-precision requirements [
15,
16].
In this paper, an ECD method is proposed for fabricating pure-nickel microstructures with controllable size, high dimensional accuracy, and good surface roughness and edge radius. Taking the example of fabricating a pure-nickel rectangular mandrel that corresponds to the size of the end face of a 1.7-THz rectangular waveguide cavity, the manufacturing methods are described in detail together with the corresponding experimental investigations.
4. Conclusions
An ECD machining process for pure-nickel microstructures with controllable size, high dimensional accuracy, and good surface roughness and edge radius was investigated and discussed systematically, the following conclusions were obtained.
In the process of preparing the mask with rectangular grooves, an exposure time of 70 s and a development time of 720 s were considered to be optimal choices in the present study.
During the electrochemical deposition, the temperature of the solution was kept at 45 °C to give the best reaction rate with a current density of 1.0 A/dm2. The pH of the electrochemical deposition solution was controlled to be in the range of 3.5–4.5.
The width of the final prepared rectangular mandrel is consistent with the mask. The measurement results show that the bottom surface roughness is less than 0.1 μm, the side roughness is less than 0.2 μm, and the edge radius is less than 9.2 μm.
Because the specific size is controllable and the dimensional accuracy, surface roughness, and edge radius are good, the proposed method can be used to manufacture various types of high-quality pure-nickel microstructures.