4.3.1 Deposition system
Due to the small size of the chips, a specific system for electrophoretic deposition has been established as show in Figure 4.8. A vacuum pump was used to generate the vacuum
environment, which connects to the metal tube through a conical flask, thereby providing the attracting force for the tiny chip. Meanwhile, the metal connecting with the chip also established the current path between a DC power (PL330 fabricated by Thurlby Thandar Instrument) and chip. The conical flask is the extra safety device to protect the pump from any corruption damage if there is any possible leakage between the chip and metal tip. A feedback and inspect system was built with a multi meter and computer. An ampere meter (Model: MS8226T manufactured by Huayi Ltd.) was connected to this system in series, enabling an in situ current measurement. The computer was used to record the testing data of the ampere meter during the electrophoretic deposition process. A 50 ml Pyrex beaker (Fisher Scientific limited) was used to provide the suitable electrophoretic cell for the deposition of the particles. The anode was well polished aluminium with submerged dimensions of 25mm×20mm×1mm, which was connected by alligator clip. The cathode was the target chip. The circuit consisted of mutually parallel electrodes at a fixed separation distance of 14 mm, connected to a DC programmable power supply.
Figure 4.6 Schematic of the electrophoretic deposition system, and (b) detail of the vacuum tip which holds the chips while providing electrical connection.
4.3.2 Acid Charging Method
In order to perform the electrophoretic deposition, the particles should get charged in advance.
Previous study has shown that Ni/Au particles cannot be deposited electrophorectically after immersed in the water solution, however can get charged after being immersed into the hydrochloric acid (HCl) solution and then successfully used in the electrophoretic deposition [148]. However the mechanism of this charging process is not clear and the effect of acid immersion on the particles has not been fully estimated.
The EPD process of the particles in this work was carried out using organic solvent isopropyl alcohol (IPA) to avoid the potential electrolysis. Although these particles (1.79 g/cm3)
are significantly denser than water (1 g/cm3) or IPA solvent (0.78 g/cm3), because of their small size they can remain suspended in such solvents for approximately 25 minutes thereby allowing their processing and deposition sufficient time to take place. In this work 30 mg 9.8 μm Ni/Au coated particles were prepared to allow a rapid EPD process by being immersed into 40 ml HCl solution (32% wt%, Fisher Scientific Ltd.) for 30 minutes. Then the suspension was rinsed with deionized water for three times, and subsequently transferred into isopropyl alcohol (IPA) for the following electrophoretic deposition. The ultrasonic bath was used to agitate the suspension for 2 minutes before electrophoretic deposition.
A DC potential of 15 V was applied between the electrodes resulting in an electric field strength of 10.7 V/cm. The deposition time was ranged from 5 to 10 minutes. After the EPD process, the particles left in the suspension were recycled by employing a permanent magnet at the bottom of the beaker thus allowing the separate the particles that were attracted at the bottom from the suspension. The deposition results were observed using FEGSEM, and the effects of acid immersion on the particles were analysed along with the deposition efficiency.
4.3.3 Zeta Potential Study
Zeta potential is an essential parameter in EPD, which plays significantly effects on the suspension stability and particle mobility. It is usually of the order of a few tens of millivolts, and quantifies the charge developed at the interface between a solid surface and liquid medium [16]. Figure 4.7 illustrates the double electrical layers that are believed to form around a charged particle and identifies the zeta potential. In the first (Stern) layer there is a relatively high concentration of counter ions, while in the outer and more diffuse layer, a balance between the electrostatic force and random thermal motion determines the distribution of ions, and the potential decays gradually with the distance from the particle surface, until it reaches that of the bulk solution (which is conventionally taken to be zero). When the suspended particle is placed within an electric field it will move, and the zeta potential is defined as the potential at an imaginary “slipping plane” within which the particle and any of its surrounding ions move through the solution as a unit.
However zeta potential is very difficult to measure directly and it is usually calculated from the mobility of the particle within an electric field using the Henry equation [18]:
2 ( )
where
U
E is electrophoretic mobility, which can be directly measured by Zetamaster,Z
is zeta potential, is dielectric constant, and
is the viscosity. The unit of , termed the Debyelength, is reciprocal length of
1 which is often taken as measurement of the “thickness” of the electrical double layer. The parameter “a” is the radius of the particle, therefore a refers the ratio of the particle radius to electrical double layer thickness. (f a) is 1.5, as referred to the Smoluchowski approximation, when the radius of the particles is larger than 0.2 microns dispersed in electrolytes containing more than 10-3 molar salt.Figure 4.7 Schematic image showing the double layer distribution of ions surrounding a charged particle and the evolution of the electric potential from the surface potential to zero
far from the particle.
Four groups of suspensions at different concentrations of the Ni2+ ions ranging from 10-5 mol/L, 10-4 mol/L, 10-3 mol/L, to 10-2 mol/L were prepared. For each group, 30 mg particles were well dispersed into 40 ml deionized water, and the only difference is the dissolved mess of NiCl2·6H2O. The pH of each suspension was tested immediately after the preparation as well as the zeta potential of the particles in each suspension. Two group suspensions (10-4 mol/L and 10-2 mol/L) are chosen to study the influence of different pH on the zeta potential of the particles, and each group was divided into two separate 20 ml waiting for being added into HCl and NaOH solution respectively for the following zeta potential test. Electrophoretic depositions at different pH values were also carried out using suspensions with two different concentrations of Ni2+ ions (10-4 and 10-2 mol/L) under the same applied voltage of 20 V for 2 minutes in order to study the deposition quality as a function of pH.
4.3.4 Electrophoretic Deposition Process
Even 0.95 mg NiCl2·6H2O cannot be fully dissolved in 40 ml pure isopropyl alcohol (IPA), so that different mass of NiCl2·6H2O was dissolved in 1ml deionized water first, and then filled with 39 ml IPA forming the final suspension for electrophoretic deposition. Three groups of suspensions with the different concentrations of Ni2+ ions (10-4 mol/L, 10-3 mol/L and 10-2 mol/L) were prepared for the subsequent EPD process. Different parameters were applied to perform electrophoretic deposition experiments including different voltage ranging from 10V to 30V and deposition time 10s to 2 minutes, and the current was measured in Ampere Meter and recorded in the connected laptop. After the deposition, the chips were transferred into deionized water immediately and rinsed three times. The deposition morphology was observed by FEG-SEM, and the cross section was cut using Focused Ion Beam (FIB) along with the EDS analysis.