Materials Transactions, Vol. 50, No. 8 (2009) pp. 2098 to 2103 #2009 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS) Ho Chang1;*1, Xin-Quan Chen1;*2, Ching-Song Jwo2 and Sih-Li Chen3 1Department of Mechanical Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R. O. China 2Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, R. O. China 3Department of Mechanical Engineering, National Taiwan University, Taipei 10617, Taiwan, R. O. China Nanoparticles can be dispersed by different methods in order to completely utilize their unique material characteristics. This study uses a self-developed nanofluid synthesis system to produce a CuO nanofluid with good suspension stability and particle dispersion without additives. The prepared CuO nanofluid has an average particle size of 60 nm and a substantially improved size distribution. The effect of the electrical double layer on the stability of the prepared CuO nanofluid was experimentally assessed by studies using different concentrations and by rheological experiments. Furthermore, the relationships between the pH value and Zeta potential, hydrodynamic radius and Uv/Vis spectra of the prepared CuO nanofluids are also discussed. The experimental results show that the CuO nanofluid still maintains its Zeta potential at above 30 mV for longer than 6 months, indicating high suspension stability. [doi:10.2320/matertrans.M2009129] (Received April 9, 2009; Accepted June 4, 2009; Published July 25, 2009) Keywords: CuO nanofluid, zeta potential, sterical stabilization, rheology 1. Introduction force and van der Waals attraction force in the suspension fluid. Finally, employing DLVO theory to calculate the total In colloidal science, the interactions between particles in a interacting energy Vtotal produced by double-layer repulsion suspension fluid include van der Waals force, electrical force and van der Waals attraction force, the study inves- double layer action and sterical action. These interactions tigates the relationship between potential energy and stability lead to the dispersion of colloids, maintain the suspension of CuO nanofluid. and dispersion for a long time, and do not flocculate sediments.1,2) If nanoparticles can stay suspended in fluid 2. Theory Analysis stably, they can be applied in many industries to exert special effects of nanomaterials. The most common method for The model used to describe the ion distribution function particles to maintain good dispersion and stable suspension in and potential in the region near the charged interface was fluid has been ultrasonic vibration, a physical method. Many developed independently by Gouy and Chapman by combin- related research papers have reported the use of ultrasonic ing the Poisson equation for the second derivative of the vibration to disperse and suspend nanoparticles in fluid, electric potential as a function of charge density with the and investigated the influence of particle size and surface Boltzmann equation for the charge density as a function of potential on suspension and dispersion.3) But currently most potential.8–10) The net volume charge density at the points of the physical dispersion methods are being replaced by where the potential is is thus expressed as: chemical methods, mainly because the addition of chemicals ze can more evenly disperse the particles in suspension fluid, ¼2zen sinh ð1Þ 0 kT and can maintain the suspension stability over a longer time. Therefore, many studies have investigated the addition of is related to in Possion’s equation, which for a flat double different acidic and saltic materials in nanofluids to enhance layer takes the form as follows: the suspension stability.4–6) d2 The study uses a self-developed arc spray nanofluid ¼ ð2Þ 2 synthesis system (ASNSS) to prepare a CuO nanofluid with dx " high suspension stability.7) This good suspension stability where " is the permittivity. does not depend on a physical or chemical method, but it can The combination of eqs. (1) and (2) is expressed as: keep CuO nanoparticles stably suspended for an extended d2 2zen ze period. This study investigates the influence of the various ¼ 0 sinh ð3Þ 2 forces in the prepared suspension fluid, observes the dx " kT suspension properties of CuO nanofluid as well as the The solution of this expression, with the boundary conditions morphological and surface potential changes of CuO nano- ( ¼ 0 when x ¼ 0; and ¼ 0, d =dx ¼ 0 when x ¼1) fluid over an extended period of time, and explores the taken into account, can be written as:11) influence of electrostatic repulsion force, steric stabilization 2kT 1 þ exp½x ¼ ln ð4Þ ze 1 À exp½x *1Corresponding author, E-mail: [email protected] *2Graduate Student, National Taipei University of Technology where Electrostatic and Sterical Stabilization of CuO Nanofluid Prepared by Vacuum Arc Spray Nanofluid Synthesis System (ASNSS) 2099 In the calculation of , the solution ionic strength, I ¼ exp½ze 0=2kTÀ1 p ¼ ð5Þ ðc z 2Þ=2, is often used. For water at 25C, ¼ 3:288 I, exp½ze =2kTþ1 i i 0 which is measured in nmÀ1 while both the ion concentration À3 and ci and the ionic strength I are measured in mol/L. In 10 M 1 1 1:1aqueous electrolyte solution 1= ¼ 9:6 nm and for the 2 2 2 2 2 8e NA 1 2e n0z = ¼ I 2 ¼ system of interest in colloid science, 1 ranges from a 1000"kT "kT fraction of a nanometer to about 100 nm.12) 1 1 DLVO theory is named after its developers, Derjaguin, 2e2N cz2 2 2F2cz2 2 ¼ A ¼ ð6Þ Landau, Verwey and Overbeek. It describes the change of "kT "RT interacting energy occurring when an electrolyte is added to a where NA is Avogadro’s constant, I is the ionic strength, colloid solution and the colloidal particles approach to each c is the concentration of electrolyte, n0 is the corresponding other, and how this influences the overall stability of the bulk concentration of each ionic species, z is the valence, colloidal solution. All the interactions among particles can be e is the charge on electron (1:602 à 10À19 C), F is the force acquired through the summing of the interacting energy of between two charges, R is the molar gas constant, and electric charge and the interacting energy of van der Waals k is Boltzmann’s constant. In the above equation, denotes force. the Debye-Huckel reciprocal length parameter and has When two atoms or molecules approach each other, mutual the unit of (length)À1, 1= is the distance at which the attraction will be produced because of the difference between potential drops to 1/e of its value at the Stern plane, and positive and negative electric charges. This kind of attraction this distance is called the double layer thickness or Debye is called van der Waals force, with its action equation length. expressed as follows:13) ÀA y y x2 þ xy þ x V ¼ þ þ 2ln ð7Þ van der Waals 12 x2 þ xy þ x x2 þ xy þ x þ y x2 þ xy þ x þ y In the equation, A denotes the Hamaker constant, x denotes H0=2a1, y denotes the a2=a1, a1 and a2 denote the particle radius, and H0 denotes the distance separating the surfaces. When two different kinds of particles are in a solution, the solution’s double-layer action equation is expressed as follows: " 2R 1 þ expðH Þ V ¼ g ð 2 þ 2 Þ Â ln 0 þ ln½1 À expð2H Þ ð8Þ double layer 12 01 02 2 0 4 1 þ R 1 À expðH0Þ À1 À1 À1 where g12 ¼ða1 þ a2 Þ and R ¼ 02= 01. absorption. Hence, we can judge the suspension stability The term g12 is a geometry factor dependent upon the radii, from the concentration change. a1 and a2, of the two particles under consideration. 01 and 02 are the surface potentials of the two particles at infinite 3. Experimental separation, and " is the dielectric constant of the solution. Suppose that there is only a single kind of particles in Using the basic principles of the gas condensation method, solution, then eqs. (7) and (8) can be simplified as: this study has developed the vacuum arc spray nanofluid ÀAa synthesis system (ASNSS) combined with ultrasonic vibra- V ¼ ð9Þ 14,15) van der Waals 12H2 tion. The experimental devices are mainly comprised 0 of an electrical power utility, a servo-positioning system, a 2 ½ þ ð Þ "a 0 ln 1 exp H0 vacuum chamber, a vacuum pump, a heating source, a Vdouble layer ¼ ð10Þ 2 cooling system, and a pressure control unit. The ultrasonic Finally, the summing of the two kinds of energy yields the system allows different settings of frequency and amplitude. total energy for the dispersion stability of particles in With the help of ultrasonic vibration, the disturbance of the solution, as shown in the following equation: dielectric liquid can be increased and the nanoparticles thus ÁV ¼ ÁVattðvanderwaalÞþÁVrepðelectronstaticÞ produced can quickly come out of the fusion zone. In the meantime, the gasified metal can be quickly cooled down. "a 2 ln½1 þ expðH Þ ð11Þ ¼Aa=12H2 þ 0 0 Ultrasonic vibration can effectively improve the state of 0 2 dispersion of nanofluids and obtain smaller nanoparticles. From this equation, the relationship among the total action After the analysis of the results of multiple experiments, a force ÁV, van der Waals attraction force ÁVatt and electro- CuO nanofluid with high suspension stability is prepared. static repulsion force ÁVrep can be seen. Besides, as known The main process parameters are peak current 4A, on-time from Lambert-Beer theory, the absorption of solution is pulse duration 50 ms, and off-time pulse duration 50 ms.