An Electromagnet-Based Magnetically-Activated Thermal Switch WithoutMoving Parts

C. Rodrigues1 , M. M. Dias1 , L. Martins1 , J. P. Araujo1 , J. C.R.E Oliveira2 , A. M. Pereira1 , J. Ventura1

1IFIMUP and IN-Institute of Nanoscience and Nanotechnology, Departamento de Fısica e Astronomia da Faculdade deCiencias da Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal

2CFP, Department of Engineering Physics, FEUP, R. Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

Abstract

With the ever increasing power dissipation in electrical devices, new thermal management solutions arein high demand to maintain an optimal operating temperature and efficient performance. In particular,recently developed magnetically-activated thermal switches (MATSs) provide an alternative to existingdevices, using the magnetic and thermal properties of superparamagnetic nanofluids to dissipate heat in acontrolled manner. However, the presence of moving parts is a major drawback in those systems that muststill be addressed. Herein, we present a compact and automatized MATS composed by an encapsulatedsuperparamagnetic nanofluid and an electromagnet allowing to activate the MATS without any movingpart. We investigate the effect of different temperature gradients (40, 26 and 10 ◦C) and powers applied tothe coil (6.5, 15, 25 and 39 W) on the performance of this novel MATS. The results show that the highest(44.4%) and fastest (0.6 ◦C/s) temperature variation occur for the highest studied temperature gradient. Onthe other hand, with increasing power, there is also an increase in the efficiency of the heat exchange processbetween the two surfaces. These results remove one of the main barriers preventing the actual applicationof magnetic thermal switches and opens new venues for the design of efficient thermal management devices.

Keywords:Thermal management, thermal switches, ferrofluids.

1. Introduction

The scavening for new and improved ways of controlling a system’s temperature and managing heattransfer between objects is an important topic, with the thermal management market growing 7.6% annu-ally [1]. With the development of novel microelectromechanical systems (MEMS) arrives the need for thermalmanagement solutions capable of adapting to different operational modes and changes in the environment’stemperature. One of the solutions for this issue lies on the use of a thermal switch (TS) [2, 3, 4, 5]. A TSallows controlling the transfer of heat between two surfaces by establishing and breaking a thermal connec-tion between them. This ON/OFF capability allows the thermal switch to adapt and perform differentlydepending on the desired operating mode and design.

Typically, TSs can be divided into three main categories: active solid-state, passive solid-state and fluidicthermal switches [6]. Active thermal switches need to be externally activated by applying a voltage [7, 8],magnetic [9, 10, 11, 12, 13, 14] or electric fields [15], among other [16]. This generates or breaks contactbetween the heat source and the cold surface. This is also their main disadvantage, as the thermal contactresistance between surfaces can lead to significant heat losses.

Passive solid-state thermal switches, or thermal diodes, function with materials that transfer heat asym-metrically, meaning that, for the same temperature, the heat flow for one direction is larger than for theopposite direction. This eliminates the need for an external activation mechanism and can be achieved bypairing two materials with different thermal conductivities, one that varies with temperature and the second

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Figure 1: Magnetically-Activated Thermal Switch prototype. (a) In the OFF state the nanofluid captures heat from the heatsource and (b) transports it to the cold side as the magnetic field is applied.

nearly constant for the same temperature range. These pairs can be made of polymers [17], metals [18, 19]or other materials [20].

However, the interface between two solids offers limitations to thermal transport usually caused bysurface roughness that reduces the actual contact area [21]. Although interface engineering, such as theuse of carbons nanotubes [22], can mitigate the issue, the main problem still remains. Fluids remove thisproblem altogether: by adapting to the contact surface, the interface resistance is decreased. Thus, fluidicthermal switches have been widely reported in the literature [23, 24, 25, 26, 27]. Liquid metal coolantshave been used taken advantage of the electrowetting effect [28]. Nanofluids [29], made of high thermalconductivity nanoparticles suspended in a fluid, display enhanced thermal properties and are more effectivein heat removal. In particular, magnetic nanofluids, besides offering enhanced thermal conductivities, canbe externally redirected to specific areas on demand by just applying small magnetic fields [2, 27]. Thus,they can be moved into localized hot spots where they can absorb heat that is then released at a far awaylocation after the field is removed. However, up to now, magnetic nanofluids have been primarily used as asecondary transport mechanism rather than a primary mean of heat transportation [30, 31].

In a previous work [2], the authors reported a novel magnetically-actuated thermal switch (MATS)relying on the mechanical motion of permanent magnets to move a magnetic nanofluid between two surfacesat different temperatures. Such apparatus allowed the realization of heat transference between hot and coldsides with a remote operation (Fig. 1). However, and although with a reliable and efficient performance, thenecessity for moving parts is a majority disadvantage for the majority of actual applications, preventing itsuse in a broad range of systems. Taking this into consideration, we idealized and developed a new design inwhich all mobile parts are removed and that relies instead on the magnetization/demagnetization processesof a soft magnetic material. With the reported results, we provide the groundwork for future applicationsof the MATS. This device can also provide a future alternative to the more commonly used phase changematerials [32, 33, 34, 35] in thermal management applications.

2. Experimental Details

The MATS consists on a poly(methyl methacrylate) (PMMA) cubic container, with 3 cm of side lengthand a cylindrical cavity in its center with 1.5 cm of diameter (Fig. 1) partially filled with ferrofluid (ap-proximately 4.42 cm3) and sealed with two copper plates. A small air spacer of approximately 0.5 cm is leftbetween the fluid and the top contact. The top copper plate is originally kept at room temperature whilethe bottom is heated with a Peltier element (Multicomp), creating a controllable temperature gradient.

2

Figure 2: Experimental set-up used to test the thermal switch concept without mechanical parts.

An experimental set-up without movable parts was designed to test the thermal switch concept for variousfrequencies, ranging from 0.5 to 18 Hz. An electromagnet with a half horseshoe shape was assembled byplacing a soft ferrite core partially inside a coil consisting of insulated copper wires (Fig. 2). When anelectrical current flows through the copper coil, the ferrite material is magnetized and an external magneticfield (H ) is created. When the current is turned on, the fluid rises to the upper contact due to the gradientof the magnitude of the applied magnetic field, according to [36]:

~F = ∇(µ0~H. ~M), (1)

where ~F is the resulting force, µ0 is the magnetic permeability of vacuum, ~H is the external field and ~Mthe magnetization of the particles. Then, when the current is turned off, the ferrofluid returns to its originalposition at the bottom of the container.

The current is applied to the coil in pulses using a waveform generator and a power supply. To controlthe applied current, a MOSFET was used as switch. The temperatures on both cold and hot sides arerecorded using two thermocouples. Each of the thermocouples was attached to a copper plate using a thinsheet of Indium and GE Varnish (GE-7031), to ensure an optimal thermal contact. The data is collectedusing a data logger (Pico Technology) and a custom made LabView program.

The FER-01 nanofluid used in the experiments with different temperature gradients is a commerciallyacquired nanofluid (Supermagnet) with a concentration of 4.4 vol.% of 10 nm Fe3O4 nanoparticles dispersedin a poly-α-olefin oil.

3. Experimental Results

3.1. MATS Operation

The MATS was designed with the goal of removing heat from the hot side to the cold side by applying amagnetic field in a coil with a given frequency and without movable parts. At the start of the experiment,the temperature gradient across the MATS is stable at approximately 20 ◦C [Fig. 3(a)]. Then, at the 0s mark, the magnetic field is applied. As current flows through the coil, magnetizing the ferrite core, the

3

Figure 3: Experimental results for the FER-01 comercial nanofluid at ∆T = 20 ◦C for various (a) frequencies of appliedmagnetic field at 25 W and (b) powers applied in the coil at 5Hz.

magnetic nanofluid is pulled from the hot to the cold side, increasing the temperature of the later due to thetransfer of heat from the hotter fluid to the colder surface. At the same time, the hot side also suffers froma temperature increase due to having less fluid in contact with the surface. When the current (and thusthe magnetic field) is removed and the fluid returns to its original position, the temperature of the hot sidesuddenly decreases as the now colder fluid absorbs the heat generated by the Peltier. This process repeatsitself with each cycle of the magnetic field leading to an overall decrease in temperature in the hot side andincrease in the cold side.

Figure 3(a) shows these results for the FER-01 commercial nanofluid for different frequencies. Generallyspeaking, the larger changes in temperature occur when the magnetic field is turned on. At low frequencies,the change in temperature for each individual cycle is small. For higher frequencies (between 5 and 12Hz), the results show a higher decrease of the temperature gradient with the cold side experiencing a largerchange (up to 6 ◦C) than the hot side (up to 2 ◦C), i.e, the higher frequency of the magnetic field allows ahigher number of heat exchange cycles between the two surfaces. However, further increasing the frequencyto 18 Hz leads to a negligible decrease of the temperature gradient, as the time the nanofluid spends in thehot and cold sides is insufficient for a proper heat transfer to occur.

Figure 3(b) shows how different applied electrical powers affect the heat exchange between the hot andcold surface, with a fixed frequency of 5 Hz. One can see that, with the increase of the power applied to thecoil, the heat exchanges between the two surfaces become more efficient and the temperature gradient tendsto decrease. For an applied power of 6.5 W one obtains a ∆T of 19 ◦C, while for 39 W, ∆T decreases toonly 10 ◦C [Fig. 3(b)]. However, such efficiency increase occurs at the cost of increased power consumptionand thus a trade-off may be necessary for some applications.

To better evaluate the performance of the TS one must calculate how much the temperature gradientchanges with the ON/OFF cycling of the magnetic field. For this we define the temperature span betweenthe initial temperature gradient and the gradient at a given point in time (T%) as:

T% = (1− TtT0

)× 100%, (2)

where Tt is the temperature difference at t seconds and T0 is the temperature difference at t = 0 s. A largerT% indicates a larger decrease of the temperature gradient when compared to the initial gradient. Thisequation was used to evaluate the temperature span at i) t = 350 s where, depending on the frequency,the temperature just entered or is already in the stable regime, and ii) when Tt is minimum [Fig. 4(a)].In Figure 4(a), we can see that the temperature span increases with increasing frequency until it reaches a

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Figure 4: Temperature span for the FER-01 nanofluid at t = 350 s and for Tt minimum for the temperature gradients of 20◦C: (a) for different frequencies (the inset is the ∆T rate for the FER-01 nanofluid over time at 5 Hz for ∆T = 20◦C.) and (b)various applied powers at 5 Hz (the inset is the magnetic field intensity of different positions of the cage for different appliedpowers).

maximum between 5 and 10 Hz, decreasing then for the higher frequencies. As the trends of the calculatedtemperature spans for each Tt minimum and t = 350 s are similar, in the following we will only consider theresults of the former.

The largest changes in temperature occur just when the MATS starts operation. From the results, onecan see that this change occurs at different rates (∆T/∆t) for different experimental parameters. The insetof Fig. 4(a) shows the variation of temperature over time (∆T rate) for the FER-01 ferrofluid at 5 Hz foran initial ∆T = 20 ◦C. When the magnetic field is applied (at t = 0 s), there is a peak in the ∆T variation,with the temperature gradient decreasing at a rate of 0.26 ◦C/s, followed by a gradual ∆T/∆t decrease tovalues close to zero.

On the other hand, we can see that the temperature span increases with increasing power applied tothe coil [Fig. 4(b)]. By increasing the applied power, the magnetic force increases and consequently thenanofluid is more rapidly accelerated. This way, the higher the generated force, the larger the amount ofnanoparticles that participate in the heat transport, thus making the MATS more efficient. The inset ofFig. 4(b) shows the intensity of the magnetic field measured at different MATS positions. As expected,when applying the current pulse there is a magnetic field gradient along the MATS cage. The H-magnitudeis maximum near the half horseshoe shape (540 Oe for an applied power of 39 W), while at the bottom sideof the device H is minimum (75 Oe for 39 W).

3.2. Influence of ∆T on the Thermal Switch performance

Previous experiments with ∆T = 35 ◦C showed that frequency plays a major role in the performanceof the TS [2]. However, several applications work at lower temperature gradients and we thus varied thetemperature gradient to evaluate its effects on the TS performance. Figure 5 shows the temperature spanas a function of frequency for Tt minimum for three different temperature spans, ∆T = 10, 26 and 40 ◦C.All show that the temperature span increases rapidly with frequency until it reaches a maximum between 5and 10 Hz. For a frequency of 18 Hz the temperature span decreases significantly for the three temperaturegradients, as the fluid does not have time at each side of the TS to capture and release enough heat tomaintain the performance. The MATS performance is overall best for the largest temperature gradient,∆T = 40 ◦C. The maximum values obtained were of 43.6% for ∆T = 10 ◦C at 5 Hz, 43.5% for ∆T = 26◦C at 5 Hz and 44.4% for ∆T = 40 ◦C at 10 Hz. However, the proximity of the temperature spanvalues between temperature gradients in the low frequency region shows that the difference of operationaltemperatures does not greatly influence the performance of the MATS. Comparing the ∆T/∆t results for the

5

Figure 5: (a) Temperature span for the FER-01 nanofluid for Tt minimum for the temperature gradients of 10, 26 and 40 ◦C.(b) Maximum ∆T rate for the three temperature gradients as a function of frequency.

three temperature gradient at different frequencies, one can see that, with increasing temperature gradients,the maximum ∆T/∆t magnitude also increases [Fig. 5(b)]. Also, while ∆T = 10 ◦C and ∆T = 26 ◦Cshowed a small rate variation with frequency, ∆T = 40 ◦C presents larger changes. For ∆T = 40 ◦C, the∆T rate increases significantly and presents a maximum for 5 Hz.

4. Conclusion

We presented a novel MATS without moving parts that opens a new range of applications for this typeof systems. We showed that the magnetic field frequency, operation power and temperature gradient affectthe performance of the MATS prototype. For different temperature gradients, the MATS presents a slightlyhigher performance for ∆T = 40 ◦C (maximum T% = 44.4%). We also show that the temperature gradientrate increases with the initial ∆T , from a maximum of 0.25 ◦C/s at ∆T = 10 ◦C to 0.6 ◦C/s at ∆T = 40◦C. It was found that, by increasing the magnetic force gradient, heat transport by the nanofluid becomesmore efficient. Therefore, the temperature span was maximum when the pulsed current was applied witha frequency of 10 Hz and 25 W. With these results we conclude that the use of magnetic nanofluids in athermal switch is a viable option, showing a high performance even in early stages of development, leadingto a broad range of applications as in magneto-caloric refrigeration [37, 38].

Acknowledgments

The authors acknowledge funding from FEDER and ON2 through project Norte-070124-FEDER-000070and from FCT through the Associated Laboratory - IN. J. V. acknowledges financial support throughFSE/POPH and project PTDC/CTM-NAN/3146/2014. This work was funded through project EXPL/EMS-ENE/2315/2013 from Fundacao para a Ciencia e Tecnologia (FCT), Portugal. C. Rodrigues thanks ina-noEnergy for the research grant. B. D. Bordalo acknowledges FCT for grant PD/BD/114455/2016.

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