Synthesis of spherical silica/multiwall carbon nanotubes hybrid nanostructures and investigation of thermal conductivity of related nanofluids

Abstract

In this study, a hybrid of silica nanosphere/multiwall carbon nanotube (MWCNT) has been synthesized by wet chemical method at room temperature. The effect of MWCNTs, silica nanospheres and hybrid nanostructures (80% silica nanosphere/20% MWCNT and 50% silica nanosphere/50% MWCNT) on the thermal conductivity of distilled water has been investigated. SDBS was used as the dispersant to stabilize nanomaterials in the aqueous suspension and its concentration was 1.5 times of the concentration of nanomaterials. As results show, by increasing the concentration of nanomaterials, effective thermal conductivity of nanofluids increased. The most and the least enhancement in the effective thermal conductivity of the fluids were associated with MWCNTs (23.3%) and silica nanospheres (8.8%), while the enhancement for the hybrid nanomaterial was a value between MWCNT and silica nanoparticles. Furthermore, the hybrid consisting of higher percentage of MWCNTs showed more increase in effective thermal conductivity of the nanofluid, compared with the other hybrid.

Introduction

Carbon nanotubes have unique properties among one-dimensional nanostructures. Physical strength, chemical stability, mechanical resistance, very high electrical and thermal conductivity are some of the exceptional characteristics of them, and these characteristics have attracted many researchers to carbon nanotubes from early 1990s. Development of carbon nanotubes has lead to a new category of hybrid nanomaterials, consisting of a composite of CNTs¹ with metallic, semi-conductive or non-conductive nanoparticles.

A hybrid material is a substance which combines physical and chemical properties of different materials simultaneously. In fact, a hybrid substance provides properties of its components in a homogenous phase. Researchers have done a lot of studies on optical, mechanical, electrical and thermal properties of these composites [1]. As well, such composites with the base of carbon nanotubes have been used in electrochemical-sensors, bio-sensors, nanocatalysts, etc. [2], but to the authors’ knowledge the application of these hybrid nanomaterials inside the fluids for preparing nanofluids, has not developed yet, as much as other applications.

Nanofluids are fluids with the base of nanotechnology which are prepared by stable dispersion of nanoparticles (particles with the average size of 1–100 nm) in the base fluids which are usually liquid [3], [4], [5], [6], [7]. Choi was the first person who called such suspensions as “nanofluids” [8] and mentioned two main characteristics of these fluids as following:

• Proper dispersion of particles inside the fluids

• High thermal conductivity

Nanofluids are known as suitable candidates for the future generation of heat exchanging fluids. These fluids provide high thermal conductivity with an appropriate stability of the particles and they have least negative effects on the walls of the heat exchangers in comparison with fluids containing larger particles. So they can decrease the dimensions of the heat exchangers, leading to a decrease in costs of the utility used in a process. There has been a lot of research to study the effect of nanoparticles in heat transfer improvement of the base fluid. For instance, Masuda reported that Al₂O₃ particles with the dimension of 13 nm and at 0.75 vol.% in water would increase thermal conductivity of base fluid as much as 30% [9]. Eastman showed an enhancement of about 60% in thermal conductivity of water containing 5 vol.% of CuO nanoparticles [10]. Lee and Choi did study on four nanofluids including water/Al₂O₃, water/CuO, ethylene glycol/Al₂O₃ and ethylene glycol/CuO. In their research at least 20% increase in thermal conductivity of base fluids was obtained at 4 vol.% of nanoparticles [11]. Xuan and Li showed that effective thermal conductivity (k_eff) of Cu nanofluid would increase from 1.24 to 1.78 by increasing the amount of nanoparticles from 2.5% to 7.5 vol.% [12]. Tzeng et al. used nanofluids in engine oil at cooling systems [13]. Xuan and Roetzel presented some formulae for investigation of convection inside the tubes [14]. Li and Xuan showed Nusselt number would increase 12% by adding 1 vol.% of Cu nanoparticles to water inside the tube that is exposed to constant heat flux [15]. According to the results of the study of Choi and Zhang, carbon nanotubes increase thermal conductivity of engine oil more than other nanoparticles investigated by them [16]. Keblinski studied parameters which influence thermal conductivity of nanofluids theoretically [17].

The main purpose of our research team is to make an improvement in the thermal and rheological properties of water based drilling fluids. Some functions of such fluids are: carrying drilled cuttings from the bottom of the well to the surface, cooling and cleaning the drill bit during drilling and suspending the drill cuttings while drilling is stopped [18], [19]. From an economic point of view, making an improvement in the thermal and rheological properties of drilling fluids would be of great importance. In the previous work [20], it has been shown that spherical silica nanoparticles influence rheological properties of drilling cement slurry in a positive way. Beside this positive effect, silica nanoparticles have different features such as easy, direct and cheap synthesis at room temperature [3], suitable yield, high mechanical strength and nontoxic nature. But in applications like drilling fluids we need to have a nanostructure with better thermal properties than silica nanoparticles, while it owns suitable rheological properties like silica nanospheres. Exceptional thermal and mechanical properties of carbon nanotubes gave us the idea of synthesizing a hybrid nanostructure of silica nanosphere/MWCNT² to reach our goal. In fact, our purpose is to develop a nanostructure that possesses some feature of both silica nanospheres (suitable rheological properties) and carbon nanotubes (suitable thermal properties) in a homogenous phase. Researchers synthesized hybrid of silica nanoparticles and carbon nanotubes by sol–gel [21], [22], combination of sonication and sol–gel techniques [23] and chemical vapor deposition [24] methods.

Therefore in this research, a hybrid nanostructure of silica nanosphere/MWCNT has been synthesized by wet chemical method for the first time. Also, thermal conductivity of nanofluids consisting of distilled water as base fluid and MWCNTs, silica nanospheres and hybrid of them as suspended particles has been investigated. The effect of this hybrid nanostructure on the rheological properties of water will be reported elsewhere [25].