**2. Cu–Ni MMCs in MEMS and Electronics**

A survey of the literature for the incorporation of particles into Cu–Ni coatings is shown in Table 1. The composite coatings described in Table 1 are for electrodeposited processing only. Other Cu–Ni composites have been made using different techniques, but fall outside the scope

This chapter will cover the electrodeposition of Cu–Ni alloys onto steels and other substrates to improve corrosion resistance and mechanical properties. The influence of the deposition parameters will be covered as well as the electrodeposition mechanism. The resulting me‐

**Reference Possible Applications Deposition Conditions Composite**

1.0 M NiSO4⋅6H2O 0.04 M CuSO4⋅5H2O 0.3M Na3C6H5O7⋅2H2O 3.125–12.5 g/L Al2O3 *j* = 2–50 mA/cm<sup>2</sup>

200–250 g/L CuSO4⋅5H2O 45–90 g/L H2SO4 Plating rate: ~0.2 μm/min 40°C

Alkaline noncyanide-based copper-plating solution 2–8.5 g/L Ni 40°C Stir rate: 250 rpm

> 120 g/L Na3C6H5O7⋅2H2O 25 g/L H3BO3 12 g/L NiCl⋅7H2O 100 g/L NiSO4⋅7H2O 5–25 g/L CuSO4⋅5H2O *j* = 0.5–2 A/cm<sup>2</sup>

150 g/L CuSO4⋅5H2O 10 g/L H2SO4 Ni 0.1–20 mg/mL *j* = 1–100 mA/cm<sup>2</sup>

25 g/dm<sup>3</sup>

50 g/dm<sup>3</sup>

40 g/dm<sup>3</sup>

5–25 g/dm<sup>3</sup>

0–20 g/dm<sup>3</sup>

35°C

Na3C6H5O7⋅2H2O

H3BO3

Na2SO4⋅10H2O

NiSO4⋅7H2O

*j* = 0.33–1.33 A/dm<sup>2</sup>

CuSO4⋅5H2O

Al2O3 and TiO2

26°C

Cu–Ni alloy incorporated with γAl2O3 (~30 nm)

Cu incorporated with Ni nanopowder (~50 nm)

Cu incorporated with Ni nanoparticles

Cu–Ni incorporated with Cr nanoparticles (~40 nm)

Cu incorporated with Ni nanoparticles (~100 nm)

Cu–Ni incorporated with Al2O3 and TiO2

chanical and corrosion properties will also be discussed in the chapter.

Recessed microelectrodes for MEMS devices

Magnetic microactuators for MEMS Devices

Electrical and electronics (speakers)

Electrical and electronics (MEMS devices)

Mechanical and Young's modulus

Mechanical and hardness 50 g/dm<sup>3</sup>

of this chapter.

86 Electrodeposition of Composite Materials

**Panda et al. [5]**

**Huang et al. [10]**

**Chen et al. [6]**

**Huang et al. [8, 9]**

**Chrobak et al. [21]**

**Fawzy et al. [37]**

Cu–Ni composites have shown to be beneficial in the area of MEMS devices, microactuators, and electronics. Al2O3, Ni nanoparticles, and Cr particles have been incorporated into the metal matrix to produce improved mechanical properties, magnetic properties, and chromia scale for oxidation resistance [5, 6, 8–10].

Panda et al. [5] evaluated the electrodeposition of graded Ni–Cu alloys and Ni–Cu–γAl2O3 composites into deeply depressed electrodes made using X-ray lithography for use in MEMS devices. The rotating cylinder experiment showed that the current efficiency was below 100% for the deposition over a diverse range of current densities. With the inclusion of 12.5 g/L of alumina at a rotation rate of 1,000 rpm, it was found that the current efficiency was drastically lowered by the incorporation of the nanoparticles below 20 mA/cm2 but produced deposits with higher copper content which was preferred. At the greater current densities (30–50 mA/ cm2 ), the current efficiency was less effected but lead to coatings with higher nickel content. The Cu weight ratio was studied at different heights on the micropost for pure Cu–Ni and Cu– Ni–Al2O3 at current densities of 10 and 15 mA/cm2 , seen in Figures 2 and 3, respectively. A rise in the concentration of Cu along the post was expected due to the reduction of boundary layer thickness because of the diffusion-controlled reaction mechanism of copper. The composite micropost showed a sharp increase in the Cu concentration starting at about a height of 300 μm with the incorporation of alumina, which suggests that the incorporated nanoparticles into the plating bath helped to improve the mass transport at the site of the recess.

**Figure 2.** The copper weight ratio versus the height of the micropost with and without alumina for the current density of 10 mA/cm2 and a duty cycle of 0.125. "Reproduced by permission of The Electrochemical Society." [5].

**Figure 3.** The copper weight ratio versus the height of the micropost with and without alumina for the current density of 15 mA/cm2 and a duty cycle of 0.125. "Reproduced by permission of The Electrochemical Society." [5].

Huang et al. [10] experimented with a Cu–Ni composite from a plating bath consisting of an alkaline copper solution incorporated with 2–5 g/L of ~50 nm nickel nanoparticles to increase the performance in magnetic microactuators for MEMS. The superconducting quantum interference device (SQUID) measurements for magnetism seen in Figure 4 demonstrates that with the inclusion of ferromagnetic Ni particles into the copper matrix, the film shifts from diamagnetic to ferromagnetic in nature as the curve becomes larger. The vertical displacement of the magnetic microactuator was measured for the actuator coils fabricated from copper and Cu–Ni composite materials. Figure 5 indicates that under the same experimental conditions, the Cu–Ni composite coil possessed a greater vertical displacement versus the pure Cu film, leading to about a 9% increase in the actuation enlargement performance.

The Cu weight ratio was studied at different heights on the micropost for pure Cu–Ni and Cu–

in the concentration of Cu along the post was expected due to the reduction of boundary layer thickness because of the diffusion-controlled reaction mechanism of copper. The composite micropost showed a sharp increase in the Cu concentration starting at about a height of 300 μm with the incorporation of alumina, which suggests that the incorporated nanoparticles into

**Figure 2.** The copper weight ratio versus the height of the micropost with and without alumina for the current density

and a duty cycle of 0.125. "Reproduced by permission of The Electrochemical Society." [5].

**Figure 3.** The copper weight ratio versus the height of the micropost with and without alumina for the current density

and a duty cycle of 0.125. "Reproduced by permission of The Electrochemical Society." [5].

Huang et al. [10] experimented with a Cu–Ni composite from a plating bath consisting of an alkaline copper solution incorporated with 2–5 g/L of ~50 nm nickel nanoparticles to increase the performance in magnetic microactuators for MEMS. The superconducting quantum interference device (SQUID) measurements for magnetism seen in Figure 4 demonstrates that with the inclusion of ferromagnetic Ni particles into the copper matrix, the film shifts from

the plating bath helped to improve the mass transport at the site of the recess.

, seen in Figures 2 and 3, respectively. A rise

Ni–Al2O3 at current densities of 10 and 15 mA/cm2

88 Electrodeposition of Composite Materials

of 10 mA/cm2

of 15 mA/cm2

**Figure 4.** The SQUID measurements of copper versus the Cu–Ni composite electrodeposited from a bath that con‐ tained 2–5 g/L of nickel nanopowder. "Reprinted with permission from [10]. Copyright [2007], AIP Publishing LLC."

**Figure 5.** The vertical displacement results of the diaphragms compared to the normalized input experienced by the driving coils, which consists of the copper and Cu–Ni composite."Reprinted with permission from [10]. Copyright [2007], AIP Publishing LLC."

Chen et al. [6] proposed a new coil material for a reduced power electromagnetic microactu‐ ation device using Cu–Ni nanocomposites from an alkaline Cu-plating bath integrated with 2–8.5 g/L 100 nm nickel nanoparticles. Different coil widths ranging from 10–500 μm were examined with SQUID and resistivity measurements to determine the optimal power-saving microspeaker device. The SQUID and resistivity analysis found that the 200 μm wide inductive coil from the bath containing 2 g/L of Ni nanoparticles had optimal ferromagnetic character‐ istics with low resistivity. The sound pressure level was then evaluated for the pure copper and optimal Cu–Ni nanocomposite wire for a frequency ranging from 1–6 kHz, which resulted in a 40% power savings for the Cu–Ni nanocomposite versus the pure copper coil [6].

**Figure 6.** The stacked X-ray diffraction patterns of the Cr2O3 scale generated on four different samples after oxidation in atmosphere at 800 °C for 20 h: [a] commercial grade Cu–30Ni–20Cr, [b] commercial grade Cu–50Ni–20Cr, [c] electro‐ deposited Cu–30Ni–20Cr composite, and [d] electrodeposited Cu–50Ni–20Cr composite [9]."Reprinted with the per‐ mission of Cambridge University Press."

A crucial problem for extending the applications of Cu–Cr MMCs is poor resistance to oxidation, specifically in high-temperature environments for the electrical and electronic industries [8, 9]. This is due to the fact that at high temperatures chromium has a low solubility into copper, which greatly decreases the chromium amount diffusing to the surface of the copper matrix, preventing growth of the protective Cr2O3 scale. Nickel can be added into the matrix to increase the solubility of Cr into the composite to help increase the resistance to oxidation [9]. Huang et al. [8] discussed the electrodeposition of Cu–Ni–Cr, where copper and nickel were at a 1:1 ratio. They found that incorporating 15–20 wt.% Cr nanoparticles (~40 nm) into the Cu–Ni matrix allowed for a good chromia scale to form during the oxidation process in atmosphere at 800°C. It was found that incorporating less than 15 wt.% Cr lead to nonuniform growths of the chromia scale. Huang et al. [9] also examined the electrodeposition of Cu–30Ni–20Cr and Cu–50Ni–20Cr for the formationof the Cr2O3 protective scale. Figure 6 displays an X-ray diffraction pattern for the two electrodeposited coatings plus two commer‐ cially available alloys of the same composition after oxidation in atmosphere at 800°C. The result shows that the commercial alloys mainly consist of more NiO, Cu2O, and CuO after oxidation, whereas the electrochemically prepared coatings consist primarily of Cr2O3 and NiCr2O4. Figure 7 shows SEM pictures of the Cr2O3 scale created from the oxidation process at 800°C. It was discovered that for the electrodeposited samples the Cr2O3 scale quickly formed in the initial stage of growth and continued through the steady-state stage, where the coating acts as a reservoir for Cr to maintain the chromia scale growth. The commercially available sample Cu–50Ni–20Cr showed a similar result to the electrodeposited coatings but the Cu– 30Ni–20Cr showed virtually no growth of the Cr2O3 scale, which showed that the commercial grade relied on an increase in Ni content to grow the chromia scale [9].

Chen et al. [6] proposed a new coil material for a reduced power electromagnetic microactu‐ ation device using Cu–Ni nanocomposites from an alkaline Cu-plating bath integrated with 2–8.5 g/L 100 nm nickel nanoparticles. Different coil widths ranging from 10–500 μm were examined with SQUID and resistivity measurements to determine the optimal power-saving microspeaker device. The SQUID and resistivity analysis found that the 200 μm wide inductive coil from the bath containing 2 g/L of Ni nanoparticles had optimal ferromagnetic character‐ istics with low resistivity. The sound pressure level was then evaluated for the pure copper and optimal Cu–Ni nanocomposite wire for a frequency ranging from 1–6 kHz, which resulted

in a 40% power savings for the Cu–Ni nanocomposite versus the pure copper coil [6].

**Figure 6.** The stacked X-ray diffraction patterns of the Cr2O3 scale generated on four different samples after oxidation in atmosphere at 800 °C for 20 h: [a] commercial grade Cu–30Ni–20Cr, [b] commercial grade Cu–50Ni–20Cr, [c] electro‐ deposited Cu–30Ni–20Cr composite, and [d] electrodeposited Cu–50Ni–20Cr composite [9]."Reprinted with the per‐

A crucial problem for extending the applications of Cu–Cr MMCs is poor resistance to oxidation, specifically in high-temperature environments for the electrical and electronic industries [8, 9]. This is due to the fact that at high temperatures chromium has a low solubility into copper, which greatly decreases the chromium amount diffusing to the surface of the copper matrix, preventing growth of the protective Cr2O3 scale. Nickel can be added into the matrix to increase the solubility of Cr into the composite to help increase the resistance to oxidation [9]. Huang et al. [8] discussed the electrodeposition of Cu–Ni–Cr, where copper and nickel were at a 1:1 ratio. They found that incorporating 15–20 wt.% Cr nanoparticles (~40 nm) into the Cu–Ni matrix allowed for a good chromia scale to form during the oxidation process in atmosphere at 800°C. It was found that incorporating less than 15 wt.% Cr lead to nonuniform growths of the chromia scale. Huang et al. [9] also examined the electrodeposition of Cu–30Ni–20Cr and Cu–50Ni–20Cr for the formationof the Cr2O3 protective scale. Figure 6 displays an X-ray diffraction pattern for the two electrodeposited coatings plus two commer‐ cially available alloys of the same composition after oxidation in atmosphere at 800°C. The

mission of Cambridge University Press."

90 Electrodeposition of Composite Materials

**Figure 7.** The cross-sectional SEM of the Cr2O3 scales formed on the surface of the Cu–Ni–Cr alloys of [a] Cu–30Ni– 20Cr and [b] Cu–50Ni–20Cr, which was oxidized for 20 h in atmosphere at 800°C and the inset displays the structure of the different oxides at the surface, which are designated by arrows [9]."Reprinted with the permission of Cambridge University Press."
