Thermal conductivity of a diamond magnetite composite fluid under the effect of a uniform magnetic field

https://doi.org/10.1016/j.diamond.2015.01.008Get rights and content

Highlights

  • Magnetic fields induce chain-like structuring of diamond magnetite composite fluid.

  • A thermal conductivity enhancement is observed when the magnetic field is applied.

  • Thermal enhancement due to magnetic fields strongly depends on diamond concentration.

Abstract

We study the thermal diffusion through a fluid made of diamond microparticles immersed in a ferrofluid. A thermal conductivity enhancement is observed when a magnetic field is applied to the fluid; this phenomenon is due to a better heat propagation through a tridimensional chain-like structuring formed by the diamond particles in the direction of the field; additionally the thermal conductivity of the material could be controlled in a switchable way and moreover with the intensity and direction of the field. This thermal improvement shows a strong dependence on the diamond volume fraction, presenting a maximum of 76% when the diamond volume concentration is 15% and as the concentration increases, the relative thermal conductivity enlargement decreases, becoming zero at the maximum diamond volume fraction. This complex behavior could be modeled with the use of the Lewis–Nielsen effective thermal conductivity model, with a form factor that depends on the diamond particle concentration.

Introduction

One of the great challenges in materials science is to devise working materials whose thermal properties can be tuned at will in a certain range. This is hard to achieve with homogeneous materials; however, this objective could be reached when working with composites [1], [2].

In particular, electronics, computing and systems for energy saving and generation are among the most demanding and require the development of flexible and versatile composites which confront the challenges of controlling the dissipation of heat, not only at the micro- but also at large scale [3]. New kinds of materials are needed in order to solve the increasing demands in diverse applications. An innovative way of improving the thermal conductivities of working media is to suspend ultrafine metallic or nonmetallic solid powders in traditional fluids, taking into account that the thermal conductivities of most solid materials are higher than those of liquids [4], [5], [6]. Those kinds of materials can exploit the physical properties of the liquids, which are their high heat capacity, the tendency to take the form of the system of interest, and those that can be a good option when a suitable thermal contact is desirable.

Among the most promising composite materials are smart fluids, which are those whose properties can be changed by applying external stimuli (electric or magnetic fields, heat, mechanical force, light, etc.) [7], [8]. Ferrofluids are colloidal stable suspension of ferromagnetic particles of tenths of nanometers in a liquid carrier. The nanoparticles are coated with adsorbed surfactant layers to keep the suspension state stable. In the absence of magnetic field, the particles are randomly oriented and the fluid has no net magnetization. However, in the presence of an external magnetic field, the magnetic nanoparticles become aligned in the direction of the field, and thus forming chain-like structures [9], [10], [11]. The analysis of the thermal properties and the viscosity of ferrofluids among other features have attracted greatest attention of diverse research groups in the last years [12], [13], [14], [15], [16]. On the other hand, it is well known that diamond has excellent physical properties. Its thermal conductivity is higher than any of the metals, and offers a wide variety of applications when large heat transfer rates are required [17], [18].

In this work, the ability of a magnetic field to induce the formation of chain-like structures in a suspension of a non-magnetic diamond microparticles in a carrier ferrofluid, called negative magnetophoresis [19], [20], is used to control heat transport. The thermal analysis of the samples was performed using the thermal wave resonator cavity (TWRC), which has become, in the last few years, an attractive technique, in the thermal characterization of liquids and gases, due to its simplicity, versatility and accuracy [21], [22], [23]. This technique provided the thermal diffusivity of the samples, which is the physical property that measures how fast a material is heated or cooled, and rules the heat transfer under dynamic conditions. Additionally, the thermal wave cavity has been successfully used to study heat transfer in mixtures of liquids [24]. In this work we are going farther proposing to use the TWRC to measure stable mixtures of solid particles in liquids under the influence of magnetic fields.

Section snippets

Preparation of samples

Diamond microparticles with a Gaussian size distribution with 3.2 μm mean diameter and standard deviation of 0.6 μm were purchased from Diamond Tech Quality; and ferrofluid (Ferrotec EMG 900 @ 3% magnetite volume fraction, Gaussian particle size distribution with 10 nm mean diameter and standard deviation of 1 nm) was used to obtain the samples. The fluids were prepared by a one-step technique, which consists in the mixing of the diamond microparticles with the ferrofluid matrix, at different

Results and discussion

Samples of ferrofluid at different volume concentrations of diamond particles were obtained. To examine the formation of chain-like structures, a sample of a low diamond concentration (1% diamond particles and 3% of magnetite) under a magnetic field was observed with the use of a microscope objective (50 × Edmund Optics, #59-881) mounted into a digital camera; images are shown in Fig. 6. For the case of nonmagnetic particles suspended in a magnetic carrier fluid there is an arrangement to form

Conclusions

The usefulness of the negative magnetophoresis phenomena to develop magnetically sensitive smart fluids to control its thermal conductivity is shown. In particular when the magnetic field is parallel to the heat flux, a thermal conductivity enhancement is observed in comparison with the case in which the magnetic field has been turned off. The thermal conductivity enhancement shows a strong dependence on the diamond volume fraction, showing a maximum enhancement of 76% when the diamond

Prime novelty statement

We found a complex thermal behavior for a diamond-magnetite composite fluid under the effect of a magnetic filed. An important thermal conductivity enhancement is observed when the field is applied due to a tridimensional chain-like structuring formed by the diamond particles in the direction of the field, which promotes an easy thermal transport over these high thermal conductivity roads.

Additionally the thermal conductivity of the material could be controlled in a switchable way and moreover

Acknowledgments

R. A. Medina-Esquivel acknowledges support from the Secretariat of Public Education and Council on Science and Technology (SEP-CONACYT) under Grant No. 135131. M. A. Zambrano-Arjona acknowledges support from the SEP-CONACYT under Grant No. 182982.

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