Nano Particle Preparation by Sonic at Ion

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Int J Adv Manuf Technol (2005) 26: 552–558 DOI 10.1007/s00170-003-2029-8 ORIGINAL ARTICLE H. Chang · T.T. Tsung · L.C. Chen · Y.C. Yang · H.M. Lin · C.K. Lin · C.S. Jwo Nanoparticle suspension preparation using the arc spray nanoparticle synthesis system combined with ultrasonic vibration and rotating electrode Received: 4 September 2003 / Accepted: 12 November 2003 / Published online: 24 November 2004 © Springer-Verlag London Limited 2004 Abstract This study aims to investigate the use of a
  DOI 10.1007/s00170-003-2029-8 ORIGINAL ARTICLE Int J Adv Manuf Technol (2005) 26: 552–558 H. Chang · T.T. Tsung · L.C. Chen · Y.C. Yang · H.M. Lin · C.K. Lin · C.S. Jwo Nanoparticle suspensionpreparationusingthearcspraynanoparticlesynthesissystemcombined with ultrasonicvibration androtating electrode Received: 4 September 2003 / Accepted: 12 November 2003 / Published online: 24 November 2004 © Springer-Verlag London Limited 2004 Abstract This study aims to investigate the use of a newnanoparticle preparation method, i.e., the arc spray nanoparticlesynthesis system (ASNSS) combined with ultrasonic vibrationand rotating electrode, to prepare TiO 2 nanoparticle suspension.For the proposed new method of nanoparticle suspension prep-aration, this study has designed four different process modes of experimentation for comparison, in order to obtain suspendednanoparticles with smaller particle size and relatively good dis-persion. This study discusses the process modes with differ-ent settings of variables including peak current, pulse duration,breakdown voltage, temperature of the dielectric fluid and am-plitude of ultrasonic vibration, in order to determine the betterconditions for the preparation of TiO 2 nanoparticles suspension.The Transition Electron Microscope (TEM) image of the ex-periment result shows that TiO 2 nanoparticles prepared by theultrasonic vibration assisted vacuum arc spray process has anaverage particle size of less than 10nm. H. Chang ( u ) · T.T. Tsung · Y.C. YangDepartment of Mechanical Engineering,National Taipei University of Technology,Taiwan, R.O.C.E-mail: 886-2-27712171 ext. 2035Fax: 886-2-27317191L.C. ChenInstitute of Automation Technology,National Taipei University of Technology,Taiwan, R.O.C.H.M. LinDepartment of Materials Engineering,Tatung University,Taiwan, R.O.C.C.K. LinDepartment of Material Science,Feng Chia University,Taiwan, R.O.C.C.S. JwoDepartment of Air-Conditioning and Refrigeration Engineering,National Taipei University of Technology,Taiwan, R.O.C. Keywords Nanoparticles suspension · Rotating electrode · Ultrasonic vibration · Vacuum arc spray process 1 Introduction Nanomaterial is composed of particles with a grain size rang-ing from 1 to 100 nm. Because of the quantum size effect, smallsize effect, surface effect, and macroscopic quantum tunnelingeffect in itself, nanomaterials can present many unique qualities.Since nano-TiO 2 possesses such characteristics as strong reduc-tion and oxidation capability, high chemical stability, harmlessto the environment, and low prices, it can be used as a new typeof photocatalyst [1], and has many applications in areas suchas bacteria and fungus resistance, emission purification, air pu-rification, water purification, self-cleaning, and photosynthesis.Hence, TiO 2 nanoparticle suspension has a great potential of be-coming one of the star industries in the future.Nanoparticle preparation methods are generally divided intogas phase method, liquid phase method, and mechanical ballmilling method, which can be further divided into vacuum con-densation method, gas condensation synthesis method, electronbeam or lasergas phase deposition method, sol-gel method, elec-trochemical deposition method, etc. [2,3].The gas condensation method, however, can often result inthe loss of the various distinctive qualities of nanoparticles be-cause the condensation process itself can cause nanoparticles toaggregate. Using the basic principles of the gas condensationmethod, this study has developed the vacuum arc spray nanopar-ticle synthesis system (ASNSS) combined with ultrasonic vibra-tion and rotating electrode, which includes four different typesof process modes, i.e., conventional vacuum arc spray, ultrasonicvibration assisted, revolving electrode assisted, and vibro-rotaryassisted processes. In the experiment, the differences betweenthe TiO 2 nanoparticles prepared by the four different processmodes are compared and the influence of various process vari-ables on the preparation of TiO 2 nanoparticles suspension is alsodiscussed. The experiment device mainly comprises a heatingsystem, an ultrasonic vibration system, a pressure control sys-  553 tem, and a temperature control system [4]. The particles thusproduced are dispersed in deionized water, which helps reducethe aggregation phenomenon upon particle collection. 2 Theoryanalysis The major principle of the nanoparticle synthesis system de-veloped by this study is to use the rod for producing materialas electrodes and to place the two electrodes into the insulationdielectric liquid, which is made of deionized water. A DC volt-age ranging from tens to hundreds of volts is then provided be-tween the electrodes. When the gap between the two electrodesbecomes sufficiently small, themagnitude of the electricfield be-tween the electrodes will surpass the insulation endurance of thedielectric liquid, which will then lead to an insulation breakdownand a plasma channel will thus be formed. Under high plasmatemperature, the insulation fluid surrounding the electrodes willgasify and dilate, which will generate an ultra-high pressure, of which the energy density can reach as high as 3J / mm 3 , a tem-perature up to 40000 K, and a pressure of 3000 bar.Of all the processing capacity of the electric arc, 18% is usedto melt the cathode, 8% is used to melt the anode, and the re-maining 74% is used to grow the electric arc. Throughout theprocess, the ratio between energy density and electron numberdensity is held constant and is mainly subject to the followingtwo factors: (1) breakdown voltage, and (2) the thermal prop-erty of the dielectric liquid. The energy density equation can beexpressed as follows [5]: ρ  H n e = U  6 . 317 × 10 18 =  H   m i  y i  y e . (1)In Eq. 1, ρ  H  (J / cm 3 ) denotes energy density, n e (electrons / cm 3 ) the electron number density, U  the mean breakdown volt-age, m i the particle mass, and y i the fraction of particle. As Eq. 1shows, the energy density is proportional to the electron num-ber density and mean breakdown voltage. Hence, it requires highmean breakdown voltage to generate great energy density.The growth of the nucleus determines the desired size of the nanoparticles. The growth rate of the nucleus is controlledprimarily by the concentration of the vaporized metal and thetemperature of deionized water [6]. The temperature has a morecritical influence on the sizing of nanoparticles since the growthrate of the nucleus is significantly affected by the metal trans-fer frequency from the gaseous state ( β ) to solid state ( α ) on theinterface. The relationship of these parameters is shown in Eq. 2:  f  β → α = ν exp  − ∆ gkT   , (2)where ν is the vibration frequency of the atom, and ∆ G is theactivation energy of diffusion.Equation 2 indicates that when temperature drops, the growthrate of the crystal nucleus will decrease rapidly. So, the mosteffective way to check the growth of grains and acquire tinynanoparticles is a quick cooling. Table1. Adjustable range of ultrasonic system parametersUltrasonic frequency (kHz) Amplitude ( µ m) Rotary speed (rpm)21 . 7 ∼ 18 . 5 0 ∼ 10 0 ∼ 500 This study integrates the ultrasonic vibration system withelectrode by tightening the screw that locks the end of the axis of the ultrasonic system with the titanium rod. The oscillator usedin the ultrasonic system is a Lengevin-type piezoelectrical trans-ducer with an input working power of 300 W. A motor-driventiming belt is used to make sure that the axis of the ultrasonic vi-bration and the electrode rotate at the set speed. Since the inputvoltageoftheultrasonicvibrationsystemisindirectproportiontoits amplitude, the amplitude of ultrasonic vibration can be alteredthrough voltage adjustment. In order for the ultrasonic vibrationto enhance the process of the vacuum electric arc, the end spot of the titanium electrode has to be the resonance point. By adjust-ing thefrequency oftheultrasonic system,the endof thetitaniumelectrode can be made at exactly the position of half-wave, andsince it is the resonance point, the ultrasonic microjet can injectinto the cavitation bubble at a speed of approximately 110 m / s.This generates ahigh-pressure injection effect, thereby achievingthe best result of ultrasonic vibration [7,8]. The adjustable rangeof the ultrasonic system variables is shown in Table 1. 3 Experiment design The experiment deviceof the combined vacuum arc spray systemin this study is illustrated in Fig. 1. The main technique involvedin the process includes the use of the titanium bulk to be pro-duced as the electrode and the integration of the device with anultrasonic vibrator, of which the electrode can revolve aroundsimultaneously. The experiment device mainly comprises a heat-ing system, an ultrasonic system, a pressure control system, anda temperature control system. Of which, the heating device canprovide a stable electric arc to serve as the needed heat source fornanoparticle preparation. In addition, the system allows differ-ent settings for important process variables such as peak current,voltage, pulse duration, off time, and gap voltage. The ultra-sonic system allows different settings of frequency, amplitude,and rotation speed. With the help of the ultrasonic vibration,the disturbance of the dielectric liquid can be increased and thenanoparticles thus produced can quickly come out of the fusionzone. In the meantime, the gasified metal can be quickly cooleddown. The pressure control system is used to maintain an ap-propriate vacuum pressure inside the vacuum chamber, and usesdeionized water as the dielectric liquid. The temperature con-trol system allows different cooling temperature settings, whichhelps grain nucleation and prohibits the growth of grains. Hence,smaller sized nanoparticle suspensions can be obtained.For the preparation of finer and well-dispersed nanoparticles,the experiment used four different process modes for compari-son, asshown inFig. 2.Thefour modes usedaretheconventional  554 Fig.1. Schematic diagram of the ASNSScombined with the ultrasonic vibration androtating electrode for nanomaterial produc-tion Fig.2. The four different process modes adopted vacuum arc spray process, ultrasonic vibration assisted vacuumarc spray process, rotating electrode assisted vacuum arc sprayprocess, and vibro-rotary assisted vacuum arc spray process.The influence of such process parameters as breakdown volt-age, pulse duration, and temperature of the dielectric fluid, aswell as the amplitude of the ultrasonic system on the nanopar-ticles prepared, is analyzed and compared through experimenta-tion in order to identify the process conditions conducive to theproduction of nanoparticles with smaller mean particle sizes. 4 Results and discussion The particle size distribution of the TiO 2 nanoparticle suspen-sions prepared by different process modes, as measured byHORIBA LB-500 particle size distribution analyzer, is shownin Fig. 3. Figures 3a and 3b illustrate the relationship betweenthe peak current and the mean particle size under a breakdownvoltage of 220 V and 90 V, respectively. Under a preparationcondition with a breakdown voltage of 220 V, the size of the par-ticles prepared by the vacuum arc spray (conventional) processwill increase as the current rises. Contrarily, the size of particlesprepared by the ultrasonic vibration assisted process decreases asthe current drops, but the particle size will begin to grow withthe increase of current as it reaches around 6A. As Fig. 3a indi-cates, the particles produced by the ultrasonic vibration assistedprocess at a current of around 5 . 5A are the smallest ( ∼ 50 nm) of the four different process modes, while the mean particle sizes of theparticles prepared by theother threeprocess modes all exceed100 nm. The size of the particles prepared by the revolving elec-trode assisted process (250 rpm) grows slowly with the increaseof current. But the influence of current on particle size is signifi-cantly smaller by comparison. On the contrary, the size of theparticles prepared by the vibro-rotary assisted process slowly de-creases as current rises, but will gradually increase as the currentgoes up to 7A. Under a preparation condition with a breakdownvoltage of 90V, the vacuum arc spray (conventional) processunder a current of below 6A cannot produce stably suspendedparticles, and the mean particle size is the largest among the fourdifferent process modes. The size of the particles prepared by theultrasonic vibration assisted process will decrease as the currentincreases, but as the current reaches 6A, the particle size willstart to grow with the increase of current. Both of the sizes of the particles prepared by the revolving electrode assisted process(250 rpm) and the vibro-rotary assisted process slowly decreaseas the current increases. In addition, all three processes of the ul-trasonic vibration assisted process, revolving electrode assistedprocess, and vibro-rotary assisted process under a current below3A, fail to produce stably suspended particles. As Fig. 3b indi-cates, under a breakdown voltage of 90 V, all four process modesproduce particles with a mean size exceeding 150 nm. The com-parison of Figs. 3a with 3b shows that finer TiO 2 nanoparticlescan be obtained under greater breakdown voltages. Furthermore,the Field Emission Scanning Electron Microscope (FESEM) is  555 Fig.3. Relationship between the current and particle size under differentprocess modes with breakdown used to observe the morphology of the particles produced by theultrasonic vibration assisted process under a current of 6 A. Fig-ures 4a and 4b are the FESEM images under a current of 6 A andwith a breakdown voltage of 220 V and 90 V, respectively. AsFig. 4a indicates, the particles are more or less in a round shapewith a mean size of 60 nm. In Fig. 4b, the mean particle size isapproximately 200 nm.From the above experiment results and discussions, it is clearthat the ultrasonic vibration assisted vacuum arc spray processcan effectively enhance the capability of the vacuum arc spray Fig.4a,b. The FESEM image of ultrasonic vibration assisted process with acurrent of 6 A and a breakdown voltage of  a 220 V and b 90 V system for the preparation of nano-level TiO 2 nanoparticle sus-pension. Ultrasonic vibration produces alternating pressure vari-ation, which leads to increased hydrostatic pressure variation.When the gap is widened, a large pressure drop causes moremolten metal to be removed from the crater [9,10]. Since theparticles can be effectively removed from the crater, the proba-bility of carbon deposition is reduced while the stability of thearc is enhanced [11,12]. In addition, the energy of the ultrasonicvibration can generate minute disturbance and impact on the dis-charge fusion zone, making the gasified metal easier to moveaway from the fusion zone and quickly cooled down by the low-temperature coolant surrounding it. In the meantime, this canretard the growth stage of nanoparticles after nucleation whenthe metal is solidifying. Hence, we will be able to obtain betternano-level particles and better dispersed suspension.Among the process variables, it is found that the pulse du-ration can affect the arc temperature and crater radius. In orderto investigate the influence of pulse duration on the preparation
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