Preparation of Ag/Cu/Ti Nanofluids by Spark Discharge System and Its Control Parameters Study

This study selected silver, copper, and titanium as the research objects to explore the relationship between nanofluids properties and electrical dischargemachining (EDM)processes. Regarding the products,UV-visible spectroscopy (UV-Vis)was applied tomeasure the concentration distribution of nanofluids; zeta-size analysis is applied for measuring nanometal particles’ Zeta-Potential and the size distribution ofmetallic particles in the fluid. Finally, various instruments, including scanning electronmicroscope (SEM), were applied to observe the shape, size, and composition ratio of metal particles after processing. According to the experimental results, the control of the discharge pulse time, in addition to affecting the concentration of metallic liquid and temperature in the process, affects the size of the metal particles after the process. As the resistivity of silver and copper is very low, at about 15 × 10Ω⋅m, if TON is set to between 10∼50μs, good preparation efficiency can be obtained. The resistivity of titanium is 420 × 10 −9 Ω⋅m, which is much larger than that of silver or copper. Hence, TON should be set to approximately 100μs to achieve a good discharge success rate.


Introduction
Previously, in the preparation of metal nanoparticles, nanoparticle size or concentration was controlled chemically by adding suspensions control [1].In the method proposed by this study, the Electric Spark Discharge Method (ESDM) of EDM is used to crack metal materials into nanometer sized particles.The preparation process requires only deionized water (DW) or an alcohol solution as the medium and setting the parameters including EDM voltage and current to prepare metal nanoparticles suspending in the dielectric liquid.
In the electrical discharge method proposed in this study, the electrical discharge generates an arc discharge between electrodes in the insulating liquid, which then produces a thermal effect on the surfaces of two electrodes.Besides the heat generated by the arc discharge, the processed liquid generates a gasification popping phenomenon due to the discharge pressure and thermal effect, and the melted electrode portion is thus dissolved in the processing liquid with the vaporization of liquid phenomenon [2].As the electrical discharge is not subject to the hardness, strength, or toughness of the material, any conductive material can be processed.Studies on EDM preparation of nanoparticles in recent years EDM focused on analysis of the characteristics and applications of nanoparticles in dielectric fluid or fluid [3][4][5].For example, through arc discharge, the preparation of nanoparticles made of titanium has an excellent corrosion resistance and high strength-weight ratio [6].Nanoparticles of silver with a good effect on medical sterilization have been successfully developed by EDM methods [7][8][9][10].In the same way, gold nanoparticle fluid is prepared in 95% alcohol [11,12], with a particle size of about 15-30 nm and absorption peak wavelength of about 525 nm.This study has found that the suspension of gold nanoparticles in alcohol can be converted into aqueous phase gold nanoparticles by adding the same amount of DW after vaporization of alcohol.After phase transformation, the gold particles remain suspended in DW; however, the gold particle size has changed.In the research of CuO nanoparticles [13] without a suspending agent, CuO fluid with good stability of suspension can be obtained [14].The above methods are improved, and the dry vacuum extraction method is applied to prepare Al 2 O 3 nanoparticles by the vaporization of metal electrodes in the plasma state, combined with a cooling system for water collection [15].For EDM processing, a set of continuous preparation methods has been developed [16][17][18], which can effectively be applied to mass production and further developed toward nanotechnology applications.

Electric Spark Discharge Method.
The main materials used in this study were metals (conductors and semiconductors).A variety of parameter settings (voltage/current/onoff duration) were used to observe the spark generation of various metals, in order to determine the characteristic relationships between the processing parameters of three types of metals (silver, titanium, and copper) and nanometal fluids.The purity of the three metals is silver with 99.9%, copper with 99.9%, and titanium with 99.5%.The preparation of nanometal fluids by EDM was performed in the dielectric fluid, which uses a discharge system to pulse power  ON - OFF in order to generate spark discharge between the electrodes of a very small space interval.The heat generated by EDM melts the surface of the electrode into metal particles or charged ions and then sputters and dissolves them in the dielectric fluid [14].During the discharge machining process, a magnetic stirrer and stirring bar continuously operated at the bottom, thus allowing the nanoparticles in the fluid and temperature to be evenly distributed.The schematic diagram for preparing nanofluids using the electrical discharge method is as shown in Figure 1.

Dielectric
Liquid.Discharge machining fluid (dielectric fluid) affects the degree of insulation of EDM at work.If the insulation of the dielectric fluid is poor, even if the electrode gap is considerably wide, it would break the insulation and discharge.When processing liquid contains conductive particles, it results in a decline of insulation in the processing liquid and causes an increased electrode gap.Different dielectric fluids [19] directly affect the electrode material removal rate (MRR), surface roughness [20], and electrode loss rate.Choosing pure dielectric fluid can help the preservation of nanofluids and convenience of analysis.If the dielectric fluid has suspended particles, it indirectly causes decreased surface roughness, thus, the surface to be machined must be relatively smoother.In other words, the amount of melted processing electrode by a single arc decreases and then lowers the removal rate of the material to be machined.However, if there are no suspended particles in the dielectric fluid, at the beginning of discharge, the resulting electrode discharge surface is processed roughly in order to generate many sophisticated salient points, thus causing endpoint discharge to lead to an increased material removal rate [21].This study selected DW as the dielectric fluid.After electrical discharge machining, the molten metal particles from electrode and ions processing were suspended in the dielectric fluid.

Discharge Duration and Duty.
In EDM machining processing, there are many discharge scenarios, including successful discharges, discharge difficulties, short circuit, and open circuit.Moreover, when changing the  ON - OFF period,   (current), or adding capacitance settings to turn on the -axis settings, discharge waveform changes of EDM may occur, thus indirectly affecting the metallic properties of the fluid.During the discharge process of EDM, the power frequency provided by the system is very high, and discharge frequency ranges between 1 k Hz∼1 M Hz (period is 1 ms∼ 1 s).If the discharge conditions are good, the servo control system maintains electrode spacing of about 30 m, due to the stable current feedback signals.However, when the electrode spacing is too large, difficulties in arc discharge may result.When the -axis begins operation, the electrode spacing changes become more obvious.Figure 2 shows a schematic diagram of the discharge waveform.

Parameters
Setting.EDM has many control parameters; however, during the preparation process of nanofluids, the main control parameters are as follows: (a) discharge pulse on-off duration ( ON - OFF ) for setting the time of discharge ( ON ) and stop ( OFF ), which affects the accuracy of the EDM electrode consumption rate; (b) current   setting, if the   is greater, the discharge current is also greater and the processing speed is faster; however, it lowers the accuracy rate, resulting in electrode surface roughness, and the general finishing uses smaller   settings; (c) capacitance value setting (capacitor) affects the instantaneous current at the state of discharge pulse  ON and is suitable for materials of poor conductivity and discharge difficulties; however, in this study, adding the capacitance may cause poor precision of materials and many deposits at the bottom of the prepared nanofluids; (d) the vertical control knob (machining -axis) controls the up and down time of the -axis.If the setting is turned off, EDM can continuously process the anode; (e) high voltage switch (HV), the switch for the power supplies of 120 V and 240 V.The experiment is carried under the normal temperature which is 25 ± 1 ∘ C.

Characterization of Silver Particles.
The silver fluid with geometric  ON - OFF setting had the same UV absorption peak at 398 nm, as shown in Figure 3.The silver ion absorption peak was 198 nm, the particle size distribution was of approximately 1∼10 nm, and an improved material removal rate was obtained at 30 : 30 because it can get the highest absorption.The concentration of silver fluid declined with increasing proportions.As shown in Figure 3(b), when the  ON parameter was set, the UV absorption peak resulted in a red shift to 404 nm and the particle size distribution range became slightly wider.With the  ON setting, the concentration of silver fluid declined.When the  OFF parameter was set, the UV absorption peak resulted in a red shift to 404 nm, and concentration declined with the setting of  OFF .However, the setting of 30 : 50 could obtain the best silver fluid concentration and smaller silver particles (particle size less than 1 nm).As shown in Figure 4, silver particles' Zeta-Potential was in the range of −30 to −50 mV.Under the condition of no surface active agent, it still achieved good suspension stability.The fluid color changed according to particle size.The color was partially orange in the case of small particles, as compared to silver in the case of large particles.In an environment with an applied electric field, the dissociation of silver ions was exacerbated.When the concentration of charged ions increased, the DW degree of insulation declined, resulting in a higher success rate of electrode discharge.Discharge concentration led to rough and larger particle size; hence, the appropriate turnoff time ( OFF ) allowed the silver ions to be more smoothly discharged between the electrodes, in order to generate nanosilver fluids with high concentration and small particle size.
In SEM analysis, carbon gel was used as a carrier, where oxygen is accompanied by carbon glue, and C and O are found in the analysis.The SEM-EDX analysis results are as shown in Table 1 and Figure 5. EDX analysis had measurement of trace elements.As shown in Table 1, Nb, Cl, and Al in silver fluid were minor elements in the analysis.

Characterization of Copper Particles.
Copper particles are shaped like rice grains in the SEM photograph, and most particle sizes are about 100∼200 nm.Under the setting parameters of all ratios of  ON - OFF , various samples have the same UV absorption peaks of approximately 278 nm, as shown in Figure 6.The fluid was copper yellow and did not change with the different settings of  ON - OFF .The particle size results of zeta-size analysis, as shown in Figure 7, suggested that the copper particle size distribution is similar.Hence, the change of particle size can only be realized by changing the current (  ) of the process parameters.
Among the geometric parameters, the 10 : 10 setting resulted in better material removal rate.The concentration of the copper particles in the fluid was relatively high, and copper fluid concentration gradually decreased with rising proportions.The Zeta-Potential was concentrated around −30 mV, indicating good suspension stability.When the parameter setting of  ON increased, in a given process time, the copper fluid also increased with the increasing  ON , and the Zeta-Potential of copper particles was evenly distributed.For example, at a setting of 300 : 10, the Zeta-Potential was approximately −30 mV to achieve good suspension stability.When the  OFF parameter setting increased, the copper fluid concentration decreased with  OFF .By reducing working frequency and increasing turnoff time, the copper fluid concentration fell accordingly.The Zeta-Potential of copper particles was smaller and close to −20 mV.Therefore, suspension stability was poor and resulted in deposition after one day.It is inferred that the times of successful discharges decreased due to lowering working frequency.As the ion concentration decreased due to discharge, the particle electric double layer was affected, resulting in a lower measurement of Zeta-Potential.
In EDX analysis, due to carbon glue, the analysis results of the copper fluid may have minor elements of Ta, Nb, and Zr.57% O, and copper in the fluid seemed to react with oxygen during the discharge process to generate the copperoxygen compound.The copper particle may be CuO.Figure 8 and Table 2 illustrate the SEM and EDX analysis results of nanocopper particles.

Characterization of Titanium Particles.
Under the settings of all proportions of  ON - OFF , nanotitanium fluid had the same UV absorption peak of approximately 245 nm, as shown in Figure 9. Without obvious wavelength shift, the particle size distribution of titanium particles was observed as very similar in the zeta-size analysis.Particle size was distributed in the case of three groups of parameters of approximately 100 nm, as shown in Figure 10.The optimal process parameters of titanium fluid are  ON set at 100 s and  ON set as greater or lower than 100 s, and the titanium fluid concentration gradually decreased.When the  OFF was greater, the turnoff time was longer, the concentration of titanium particles was lower, and the color of the titanium fluid was uniformly dark blue with fluorescent colors.Under the condition of a given current, the characteristics of the titanium fluid by the changes in  ON - OFF parameter settings were similar to copper.In other words, the distribution of particles had no significant difference.Changing particle size was realized by current (  ) setting, controlled discharge temperature, or adding surface active agent.Under the given current condition, the Zeta-Potential of titanium particles of various proportions of  ON - OFF was similar.The Zeta-Potential was negative at −40 mV, thus, the suspension characteristics were good.
To sum up, the characteristics of titanium particles prepared in the case of various  ON - OFF proportions were similar; however, the concentration of the fluid varied.The hardness of titanium material was very high, and its resistivity was relatively higher than other metals.It can be observed from the settings of discharge that better fluid concentration can only be obtained by a longer  ON setting.Under the conditions of medium and high frequency (s grade), the resistivity of metal electrodes affects the discharge of the process.SEM-EDX analysis results of the titanium fluid are as shown in Table 3.Besides the minor elements (Au and Al), it can be found that oxygen accounts for about 1/3 of the particle.The analysis results are similar to those of the copper articles.It can be concluded that titanium reacts with oxygen during the discharge process to generate compounds, and the product of the fluid is very likely to be TiO 2 [22].SEM analysis results are as shown in Figure 11.

Results and Discussion
. When using EDM electrosparking for the preparation of fluid, the distribution of metal particles in the fluid was very wide.Unlike the chemical method to accurately control particle size, this method can produce large amounts with high efficiency.Metals have different characteristics.The results indicated that metal characteristics have great influence on the preparation of metal fluids using the electrosparking method.The temperature of arc is very high in order to melt the metal and vaporize it into nanometer particles.The discharge frequency is very high and is similar to a pulse discharge.In the environment of DW discharge, the conductivity of metal materials affects the sparking discharge, and the characteristics of the prepared fluids differ accordingly.The resistivity of silver and copper is very low, at about 15 × 10 −9 Ω⋅m.If  ON is set in the range of 10∼50 s, a good discharge success rate could be obtained, and the titanium resistivity is 420 × 10 −9 Ω⋅m, which is far greater than silver or copper.Hence, a good discharge success rate can be obtained when  ON is set at approximately 100 s.This study found that pulse setting ( ON - OFF ) mainly affects fluid concentration.However, the current setting and ambient temperature affect particle size, while  ON - OFF affects particle size distribution range.Silver has obvious UV shifts with  ON - OFF settings.The minimum particle distribution can be obtained at the proportion of 30 : 50.The UV absorption wavelength shift is not apparent in the case of copper and titanium, as compared with silver.In the particle size distribution analysis, it can be confirmed that the particle size distribution of copper and silver has no significant difference in the various settings of  ON - OFF .
The results of UV-Vis analysis suggest that fluid concentration changes with  ON - OFF settings.In zeta-size analysis, the distribution of various metal particles in DW can be analyzed.It is confirmed that the Zeta-Potential can be maintained around −30 mV without adding a surface active agent, indicating that the particles in the fluid have very good suspension stability.Zeta-Potential changes with different settings of  ON - OFF .The particle distribution peak value and Zeta-Potential peak value of metals, in the cases of different discharge parameters, are as shown in Table 4.  explored the changes of the characteristics of the nanofluids of various metals in cases of different proportions of  ON - OFF .The parameter settings were discussed according to parameter settings of geometric proportion, increased discharge time, and increased turnoff time, in order to summarize the law between parameters and fluid characteristics.

Conclusions
Based on the current findings, our future study will attempt to plan electrosparking process parameters that can achieve the optimum process efficiency for the mass production of nanofluids according to the metal properties.The conclusions of this study are as follows.
(1) In an environment of room temperature and atmospheric pressure, metal nanofluids were prepared (2) Silver's resistivity was low (15 × 10 −9 Ω⋅m); thus, when the  ON - OFF ratio was set to 30 : 50.A nanosilver fluid with the best concentration and smallest particle size was prepared.In the electric field, the dissociation of silver was very good, and nanosilver fluids of high concentration and small particle size were easily prepared.
(3) In electrosparking of copper, current (  ) was set fixed, and in the case of various proportions of  ON - OFF , the UV absorption peak and particle size distribution of the resulting copper fluids differed.Changes in fluid density and  ON - OFF were consistent, and the Zeta-Potential of copper particles was generally lower.(4) The resistivity of titanium was greater (420 × 10 −9 Ω⋅m); thus, good discharge was obtained when the  ON setting was 100 s.The example of titanium suggested that it takes a longer discharge time to set the material removal rate in the case of metals with poor conductivity rates.
(5) The results of UV-Vis analysis suggested that the discharge pulse ( ON - OFF ) setting is the main factor influencing fluid concentration.Zeta-Potential changed with different settings of  ON - OFF proportions.The analysis results of zeta-size indicated that without adding a surface active agent, the Zeta-Potential could be maintained around −30 mV, or better, indicating that the particles in the fluid have very good suspension stability.
(6) The oxygen and hydrogen are electrolyzed from the water when the discharge is happening.The metal will combine some of the oxygen.Besides, when analyzing the samples with SEM-EDX, the process of drying will also cause the oxidation phenomenon.The carbon came from the carbon conductive tape when doing the analysis of SEM-EDX.The minor elements will be  detected because the purity of Ag, Cu, and Ti is only up to 99.9%.

Figure 1 :
Figure 1: Schematic diagram of preparing nanofluids using the electrical discharge method.

Figure 2 :
Figure 2: Schematic diagram with considerations of discharge difficulties, capacitor settings, and -axis settings.

Figure 3 :
Figure 3: Silver fluid UV-V analysis results: (a)  ON - OFF in geometric proportion; (b)  ON in progressive increase; (c)  OFF in progressive increase.

Figure 6 :
Figure 6: Copper fluid UV-Vis analysis results; (a)  ON - OFF in geometric proportion; (b)  ON in progressive increase; (c)  OFF in progressive increase.

Table 4 :
Particle distribution peak value and Zeta-Potential peak value of metals in the case of different discharge parameters.usingelectrosparking, and the particles stably suspended in DW without adding any surface suspending agent.

Figure 9 :
Figure 9: Titanium fluid UV-Vis analysis results.(a)  ON - OFF in geometric proportion; (b)  ON in progressive increase; (c)  OFF in progressive increase.
The proportion of C and O in carbon glue was Advances in Materials Science and Engineering