2.1. Preparation of Nanosilver Colloid by Electrical Spark Discharge Method (ESDM)
There are two ways to fabricate nanoparticles: one is physical, and the other is a chemical method. The chemical way uses the “bottom-up” method, which is similar to the “self-assembly” technology of substances and functional substances produced by nature. These methods are all constructed from the level of the molecule to the nanoscale. The physical preparation method is generally a “top-down” etching technique. Mechanical pulverization and grinding use higher-hardness materials as a medium to grind particles by shear force, friction force, and impact force to break down the particles to a small size.
The electrical discharge machine (EDM) is traditionally used for processing metal materials with high toughness and hardness [
10,
11]. Its principle is that, when the switch is turned on, a spark is generated to consume the metallic contact surface. However, contact will become loose if the consumption occurs over time [
12,
13,
14]. Therefore, the EDM takes advantage of the principle to put the processing electrode and the object in an insulation liquid [
15]. When the two electrodes are placed at a very small distance (μm), the electrons will flow from the cathode to the anode and damage the insulation state of the insulation liquid. The electrons continuously impact on the surface of the object [
16]. This will generate a high spark with a high temperature. The spark temperature is around 6000 °C to 10,000 °C, and it will melt the surface into micro- or nanoparticles [
17,
18]. Using the principle of an EDM to fabricate the nanoparticle is called the Electrical Spark Discharge Method (ESDM).
The proposed EDM system mainly consists of the following sub-systems:
Power system: 100 DC volts and above;
Servo control system: controls the z-axis motor to maintain the two electrodes at a distance of micrometers;
Parameter control panel: adjusts the discharge cycle or z-axis speed, etc.
The EDM monitoring system has traditionally used an oscillator to observe the voltage and current wavelengths between electrodes, as shown in
Figure 1 [
19,
20,
21]. T
on is the discharging time, when the electrodes will successfully carry out spark discharge; T
off is the stopping time. The period (T
on + T
off) will continuously repeat during the processing time, for example, 20 min and 40 min in this study. When the discharge is successful, the period T
on when the pulse voltage appears is composed of a period of spark discharge ignition delay time and a period of spark discharge time. When the discharge is successful, the initial stage of the pulse voltage is the ignition delay time, and the electric field strength provided by the pulse voltage during this period is not enough to break down the dielectric of the electrode gap. Therefore, the voltage between the gap (V
IEG) will be maintained at the open circuit voltage, and the current between the gap (I
IEG) will be 0A, as shown in
Figure 1a. The dielectric strength of the electrode gap gradually decreases during the ignition delay of the spark discharge. The period after the dielectric in the electrode gap is broken down by the electric field strength is the spark discharge period. During the spark discharge, the electrode gap presents a low-resistance state, so I
IEG rapidly rises to a maximum value, and V
IEG drops to a very low voltage value, during which spark discharge occurs between the tool and the workpiece, as shown in
Figure 1b. During a successful spark discharge time, the spark will be generated, as shown in
Figure 1c. When the power pulse voltage is turned off during T
off, the electrode gap will immediately end the spark discharge state because no pulse voltage provides the energy required for discharge. During the T
off period, I
IEG and V
IEG both drop to zero, as shown in
Figure 1d. During T
off, the electrode gap will gradually recover the insulation, and the insulation degree of the electrode gap at the end of T
off will affect the ignition delay time of the next cycle of discharge. In addition, after a successful electrode discharge, the deionized water (DW) will be restored to a state of insulation in order to facilitate the next cycle of discharge and eliminate the metallic particles between the electrodes. Therefore, the T
on-T
off settings affect processing efficiency and quality [
22]. There are three scenarios of discharge [
23,
24,
25] as shown in the following:
Discharge failure (situation 1): There is voltage without a current. This is the same as with an open circuit.
Discharge failure (situation 2): As the two electrodes are very close, it is likely to cause a short circuit, similar to a current without voltage.
Discharge success: The spark damages the DW insulation and successfully generates a spark discharge. In addition, there are voltage and a current at the same time.
Nanometal colloids, such as nanogold, nanosilver, nano-copper, nano-aluminum, and nano-titanium, can be made with any method able to conduct electricity using an EDM [
26,
27,
28,
29,
30]. Nanosilver colloids have been applied in biomedical research, and silver ions can resist bacteria growth. For example, an enzyme with silver ion cannot ferment [
31,
32].
The nanometal colloids can be analyzed using a spectrophotometer, which can measure two indicative parameters:
Absorption peak: The instrument will emit UV and visible light to radiate the object under analysis. Through the sensor receiving the light, it can measure the absorption peak of each wavelength of the colloid. Higher absorption peaks suggest higher concentrations of colloid at the wavelength. On the contrary, a lower absorption rate suggests a lower concentration of colloid [
33,
34,
35].
Wavelength of the highest absorption peak: Nanoparticles will radiate under the stimulation of certain wavelengths. For example, nanosilver has the highest absorption value around 400 nm [
36], while nano-copper is around 280 nm [
37].
As an EDM during discharge has no cyclic changes, and the frequency is very high, the discharge feature cannot be observed using an oscillator [
38]; however, it can be observed by capturing an instant image of it. The disadvantage of the traditional method is that it cannot accumulate the successful discharge times. In other words, using an oscillator to observe the efficiency of the EDM is very inconvenient. This study proposed a monitoring system that can realize real-time discharge monitoring and obtain the statistics for successful discharge times, electrode energy consumption, and discharge success rate.
The EDM vaporizes the silver wire into nano-sized silver particles. The working electrodes and workpiece in this study are distinguished as electrodes. The electrode connected to positive electricity is the anode, while that connected to negative electricity is the cathode. The nanosilver colloid is prepared by ESDM in this study. The silver wire (with a diameter of 1 mm) is ground into nano-sized (1~100 nm) silver nanoparticles by the high temperature of the ESDM [
39]. The discharge parameter setting panel is used to adjust the process parameters of the EDM, and the setting panel provides a voltmeter and ammeter for measuring the mean values of the voltage and current between the electrodes.
Figure 2a is a diagram of the EDM and its discharge parameter setting panel. The anode and cathode connection connects the two electrodes in the beaker on the platform. The electromagnetic heating stirrer is used to stir the deionized water inside the beaker. The reason is that the electrode gap will gradually recover the insulation, and the insulation degree of the electrode gap at the end of T
off will affect the ignition delay time of the next cycle of discharge, so the stirring moves the nanoparticles away from the gap between the electrodes to make the gap recover the insulation more quickly. The oscilloscope is used to observe the voltage and current of the gap between the electrodes. In
Figure 2b, Z-Axis is used to control the rise/fall of the
Z-axis. CAPACITOR is used to set the capacitance value and is proportional to the transient current of discharge pulse at T
on. T
on and T
off control the discharge and off time of the electrodes. Polarity adjusts the polarity of the upper and lower electrodes. SERVO controls the servo motor speed and is proportional to the motor speed. Stabilizing controls the sensitivity of the feedback circuit and is also proportional to the motor speed. HV switches the DC power supply from 140 V to 240 V. In addition, Ip is proportional to the discharge current and processing rate, and inversely proportional to precision.
This study integrated multi-field technology research and successfully developed a micro-EDM set as shown in
Figure 3. This micro-EDM was composed of an EDM jig mechanism, a hardware circuit system, and a software monitoring interface. In terms of design, the application of 3D printing and PLA materials can greatly reduce the manufacturing cost and the size of the equipment. The 3D-printed fix jig can hold the electrodes in the beaker. The hardware circuit part is designed from the circuit design of a PCB board combined with electronic components to achieve a large electric discharge machine. The PCB boards are the following: 1. motor control circuit and discharge feedback circuit (for controlling the motor to move forward and backward according to the feedback of the voltage); 2. discharge control circuit (for giving the output of the 100V DC pulse wave at the electrodes); and 3. discharge success rate circuit (for calculating the discharge success times, rate, and energy consumption with the software) [
40,
41,
42]. The electrical spark discharge function and signal feedback are realized by VisSim software 6.0 (Visual Solution Inc., Pleasant Prairie, the USA) and an RT-DAC4/PCI interface card. Through this interface card, the software can be used to replace local electronic circuits to achieve the goals of a smaller circuit size and lower cost. Unlike the EDM, which needs an oscilloscope to observe the voltage and current of the gap between the electrodes and is not likely to count the discharge success times, the micro-EDM can accomplish these jobs with VisSim software. Because the micro-EDM is self-made, any repair is clear and easy. Although both an EDM and a micro-EDM can fabricate a nanosilver colloid by ESDM, the micro-EDM can make smaller-sized particles and better suspension colloids than the EDM does.