nickel particle composite with induction heating

Accepted Manuscript Title: A phase change microactuator based on paraffin wax/expanded graphite/nickel particle composite with induction heating Authors: Bendong Liu, Jiechao Yang, Zhen Zhang, Jiahui Yang, Desheng Li PII: DOI: Reference:

S0924-4247(17)31914-3 https://doi.org/10.1016/j.sna.2018.04.006 SNA 10719

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

31-10-2017 4-4-2018 6-4-2018

Please cite this article as: Liu B, Yang J, Zhang Z, Yang J, Li D, A phase change microactuator based on paraffin wax/expanded graphite/nickel particle composite with induction heating, Sensors and Actuators: A. Physical (2010), https://doi.org/10.1016/j.sna.2018.04.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A phase change microactuator based on paraffin wax/expanded graphite/nickel particle composite with induction heating Bendong Liua,*, Jiechao Yanga, Zhen Zhanga, Jiahui Yanga,b, Desheng Lia a

College of Mechanical Engineering and Applied Electronics Technology, Beijing

University of Technology, Beijing, China



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Highlights

A novel microactuator using a phase change material (PCM) composite and induction heating was introduced.

The ratio of paraffin wax in the electrical conductive PCM composite is higher than 95%.

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

The electrical conductive PCM composite has a larger expansion. 

Eddy current and Joule heating are generated in the PCM composite. The microactuator has no wire or electrodes to connect with the conductive PCM composite.

The new microactuator has some advantages such as ease of fabrication, small-sized,

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

Beijing Vocational College of Agriculture, Beijing, 102208, China

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b

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low cost, and ease of integration into Micro-Electro-Mechanical Systems (MEMS).

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Abstract

This paper presents a new microactuator using a phase change material (PCM)

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composite with induction heating. The PCM composite is consisting of paraffin wax, expanded graphite and nickel particle, which was used as an electrical conductive

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phase change material with improved magnetic permeability. Expanded graphite was used as an electrical conductive porous matrix absorbed with paraffin wax and nickel

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particle. This work relies on induction heating generated in the PCM composite with eddy current, rather than using external resistive heater. Experiments show that about 140 μm actuation height was realized in 5 second with the input power of 1.42 W at

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excitation frequency of 1 kHz. Since an induction heating method is adopted with electrical conductive PCM composite, which means the new microactuator has no wire or electrodes to connect with the conductive PCM composite. The new PCMcomposite-based microactuator has some advantages, such as ease of fabrication,

*Corresponding author. Tel: +86-010-67396187. E-mail: [email protected](Bendong

Liu)

small-sized, low cost, and eases of integration into Micro-Electro-Mechanical Systems (MEMS). Keywords: Microactuator; Composite PCM; Expanded graphite; Induction heating;

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Thermal actuator

1. Introduction Paraffin wax as phase change material (PCM) typically expands of up to 15% when changing from solid to liquid [1]. Generally, this expansion can produce both large driving force and large displacement, which are essential for microactuators. Other merits of paraffin wax are easy in fabrication, low-cost, low driving voltage and

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simple to operate, hence, these PCMs have good reasons to be considered for microactuators [2,3].

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In recent years, thermal microactuator using paraffin wax has been demonstrated in a wide applications in the field of microelectromechanical systems (MEMS) [4],

such as microvalve [5,6,7], micropump [8], microgripper [9] and microswitch [10,11].

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It also has been used as microactuaors in Braille cell arrays [12] and a deep-sea

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sampler [13].

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Heat energy is required when paraffin wax changing from solid to liquid.

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Generally, a metal meandered resistive heater is adopted in paraffin-based microactuators. For example, Lee et al presented an electrothermally controlled

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microactuator with a gold resistive heater for a refreshable Braille cell [12]. The gold microheater was located at the bottom of the container and the dot height could reach

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500 μm in 60 s. An aluminium film microheater was adopted by Dubois in a paraffin– PDMS composite thermal microactuator [14]. Ogden et al developed a latchable

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microactuator actuated by phase change of paraffin with copper resistive microheaters [15].

However, paraffin wax has very low thermal conductivity, which necessitates high

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power consumption and long actuation times—unfavorable features for an effective actuator [16]. Goldschmidtböing et al. added nano carbon black to make the paraffin conductive [17]. This approach has the advantage that the heat is generated in the phase change material. The experiment turned out that the conductive paraffin actuator is slightly more efficient than actuators with buried resistive microheaters. Similarly, in order to reduce the actuation time and power required, Sant et al added

Graphite in to paraffin wax to create an in situ heater that utilizes resistive heating as a voltage is applied across the graphite–paraffin wax mixture[18]. Even so, the electrical connection between the electrodes of power supply and electrical conductive paraffin wax inside the PCM chamber is required. Aim to eliminating the complicated wire connections between the power supply and the embedded microheater, Baek et al. reported a novel wireless induction heating microvalve using

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metal disc as heating elements[19].

Oh and Ahn introduced a novel microvalve using a paraffin-based ferrofluid

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(Ferro-wax) plug actuated with a permanent magnet[20]. The Ferro-wax plug changes the phase from solid to liquid by an on-chip resistive heating and moves remotely in a channel by a magnetic actuation. An optically actuated restrictor microvalve based on

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phase change composites of paraffin and graphite is demonstrated by Kolari et al [21].

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This paper describes a new paraffin-based microactuator with induction heating

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using of PCM doping with higher electrical conductivity material (expanded graphite)

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and higher magnetic permeability material (nickel particle). Eddy current and Joule heating are generated in the PCM composite when high frequency current is applied

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to an excitation coil under the PCM chamber. Unlike the traditional phase change microactuators with resistive heater, the heat is generated inside the paraffin

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composite with induction heating. Since an induction heating method has no need physical contact between the heater and the external power supply circuit, which

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means the microactuator has no wire or electrodes to connect with the conductive PCM composite. As a result of the structure and the fabrication process of the new

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PCM-composite-based microactuator is simpler. 2. Design As shown in Fig. 1, the microactuator developed in this work consists of a polydimethylsiloxane (PDMS) diaphragm, a column of PCM composite, a glass PCM chamber, a glass substrate and an excitation coil. The PDMS diaphragm is used as an elastic membrane, which can deform readily under expansion pressure when the

paraffin wax changes from solid to liquid. Upon cooling, the same level of shrinkage occurs and the PDMS diaphragm recovers the initial flat state. The PCM composite is a mixer of expanded graphite (EG), paraffin wax (PW) and nickel particle (NP). The EG is generally produced by using H2SO4-graphite intercalation composite which can yield hundreds times expansion in volume during

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heat treatment [22]. During expansion, EG retains the same structural layer as natural graphite flakes, but produces different sizes of pores with very large specific surface area [23]. Scanning electron microscope shows that their microstructure is roughly

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that of a flattened irregular honeycomb network of graphite [24]. EG (mesh 80, from

Qingdao Tianhe Graphite Co. Ltd, China), whose SEM (COXEM-30) image is shown in Fig. 2, was used as a porous matrix of the PCM composite in this study. Paraffin

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wax can be absorbed in the pores of EG, which was classified into three categories by

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Inagaki et al. [25], namely large spaces among voluble worm-like EG particles,

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crevice-like pores and net-like pores inside or on the surface of EG particle. The EG

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also retains high electrical conductivity and high thermal conductivity similar to that of prior to expended [26]. Compact EG networks are formed gradually with proper

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mass fraction of EG in the PCM composite. These networks provided electrical conduction and thermal conduction paths when the PCM composite is placed in

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alternating magnetic field. In order to improve the magnetic induction, micro nickel particles with high magnetic permittivity are added to the PCM composite. As

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mentioned above, the EG has multilayer architecture, it can host nickel particles between EG sheets. Hence, nickel particles can be regarded as suspend in the PCM composite.

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The working principle of the PCM-composite-based microactuator is shown in

Fig. 3. The initial state of the PCM-composite-based microactuator is shown in Fig. 3 (a), the paraffin wax is in solid state and the PDMS diaphragm is flat. Alternating magnetic field is generated around the excitation coil when high frequency Alternating Current is applied to the coil. And then circular eddy current is generated in the electrical conductive PCM composite along with the EG matrix according to

Faraday's law of electromagnetic induction. Thus, Joules heat can be easily generated in the PCM composite, which ensures that every portion of the PCM composite can be uniformly heated. Melting of the solid paraffin wax and subsequent expansion pushes the thin PDMS diaphragm to accomplish actuation as shown in Fig. 3 (b). At the stage of deformation, the PDMS diaphragm generates an elastic force that tends to cause the membrane to recover to the undeformed condition. Therefore, the

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diaphragm will return to its original state passively as the paraffin wax shrinkage to solid state in case of power off the excitation coil.

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According to Faraday’s Law of electromagnetic induction, electrical potential

is equal to the negative of the time rate of change of the magnetic flux enclosed by the

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eddy current circuit as shown in equation (1)

d𝛷𝐵 d𝑡

(1)

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𝜀=−

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Where 𝜀 is the electrical potential, 𝛷𝐵 is the magnetic flux through a single loop.

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If the magnetic flux through an infinitesimal area element dS, where we may

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consider the field to be constant, the magnetic flux can be described as equation (2) d𝛷B = 𝐵 ⋅ d𝑆

(2)

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where B is Magnetic induction with units of Wb⁄m2 , B=𝜇H, 𝜇 is the magnetic permeability which is frequency-independent. Magnetic field strength is H with units

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of A⁄m.

In terms of ampere’s law, the H-field can be calculated with equation (3) (3)

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∮ 𝐻 ∙ d𝑙 = 𝐼𝑓

where 𝐼𝑓 represents the ‘free current’ enclosed by the loop. According to equation (1)-(3), if we increase the magnetic permeability of the PCM composite, the electrical potential will be improved. Accordingly, the induction heating energy will be increased. That is why we added nickel particles into the PCM composite.

The skin depth has an important impact on the induction heating efficiency. In order to achieve a better heating effect, the skin depth must be less than or comparable to the depth of PCM chamber. The formula for calculating the skin depth is shown in equation (4) [27]: 𝛿 = √𝜌/(𝜋𝑓𝜇)

(4)

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Where 𝛿 is the skin depth, 𝜌 is resistivity of the conductor, 𝜇 is magnetic permeability of the conductor, and 𝑓 is the frequency of current.

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A better induction heating frequency f can be calculated with equation (4) based on the specific material and the specific dimension of the micro induction heater.

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3.1 Glass substrate and glass PCM chamber.

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3. Fabrication

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A piece of glass with dimension of 6 mm×6 mm×0.15 mm was used as the

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substrate of the PCM-composite-based microactuator. The PCM chamber was also made in a piece of glass with dimension of 6 mm×6 mm×1 mm. A through hole with

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a diameter of 1 mm was drilled with an ultrasonic machine (JR17-25K, Beijing Jingrui Ultrasound Equipment Co. Ltd, China) at frequency of 17 kHz, which was

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used as the PCM chamber filled with the PCM composite.

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3.2 PCM composite.

In order to reduce power dissipation, the melting point of paraffin needs to be as

low as possible. However, more importantly, to prevent a meltdown of the paraffin

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during the normal storage, the melting point of paraffin wax should be higher than ambient temperature. Hence, the paraffin wax (Huabao, Shanghai Huayong paraffin Co. Ltd, China) with melting point of 52 ℃ was chosen as the phase change material. The volume expansion is about 15% when transform from solid to liquid. Expanded graphite particles (Qingdao Tianhe Graphite Co. Ltd, China) with an average expansion factor of 280 were adopted as the electrical conductive porous matrix in

this study. The mixtures were made in a glass Petri dish, which was placed on hot plate at 60 °C. The paraffin wax was put in a glass Petri dish, after which the liquid paraffin wax was impregnated into EG and the mixture was stirred using a roll mixer. Then, nickel particles (mesh 2000, from Shandong Boao metal material Co. Ltd, China) with average diameter of 6 μm were added to the mixture. After the combination was thoroughly mixed, the mixture was then removed from the hot plate

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and allowed to solidify at room temperature.

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3.3 PDMS membrane.

A prepolymer of PDMS (Sylgard 184, Dow Corning) and curing agent was thoroughly mixed at a ratio of 10:1 (wt/wt). The mixed PDMS was degassed in a

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vacuum chamber to remove air bubbles for 30 minutes. After that, the PDMS mixture was carefully spin coated on a glass substrate at 500 r/min for 8 s and then 1600 r/min

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for 20 s with a spin coater(SC-1B, Beijing Jinshengweina Technology Co. Ltd,

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China), subsequently, cured on hot plate (BP-2B, Beijing Shengweina New

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Technology Institute, China) at 60℃ for 15 minutes. After the PDMS membrane was cured, we peeled off the PDMS membrane from the glass substrate and cut it into

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pieces with dimension of 6 mm×6 mm. Although the thicknesses of the membranes were not identical, the average thickness was approximately 90 μm.

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3.4 Bonding and Assembly.

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The assembly starts with the bonding of PCM chamber and glass substrate utilizing a transparent epoxy (Gleihow 568, Gleihow New Materials Co. Ltd, China). The epoxy adhesive was thinly spread both on the upward surface of the glass

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substrate and on the bottom surface of the glass PCM chamber. Then the epoxy adhesive pasted layers were pressed together for 30 min in the air at room temperature. In order to fill the PCM composite into the PCM chamber, the PCM composite were reheated to 50 ℃ on a hotplate and a small portion of the PCM composite was pressed into the chamber. The PCM chamber with glass substrate were also heated to 50 ℃ to prevent the PCM composite from solidifying too quickly.

Once the PCM chamber is filled with PCM composite, the prototype was allowed to cool to room temperature. After solidifying, the almost filled PCM chamber was toped-up using very small volumes of solid PCM composite. A piece of PDMS diaphragm and the glass PCM chamber were irreversible bonded after treated with oxygen plasma. Planar excitation coils were enwound with

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enameled copper wire with a diameter of 80 μm. The outer diameter of the planar coil is about 1.6 mm and the turns of the coil are 10. Finally, we pasted the planar

excitation coil at bottom of the glass substrate with transparent epoxy (Gleihow 568,

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Gleihow New Materials Co. Ltd, China). Fig. 4 shows a picture of a prototype of a PCM-composite-based microactuator.

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4. Experimental Setup

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4.1 physical characteristics of the PCM composite

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In order to measure the electrical conductivity of the PCM composite, a

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columnar PCM composite was fabricated. After being heated to the molten state, same volumes of the PCM composites were placed into a Teflon tube with the inner

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diameter of 5 mm. In order to improve the electrical connection, both surfaces of the columnar PCM composite were connected with a copper electrode, respectively. The

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resistance (𝑅) of a columnar PCM composite between the two electrodes was measured with a Precision LCR Meter (TH2817A Precision LCR Meter, Changzhou

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Tonghui Electronic Co., Ltd, China) at room temperature. The resistivity of the PCM composite can be calculated with equation (5). 𝜋𝑟 2 𝑅 𝐿

(5)

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𝜌=

where 𝜌 is the resistivity of the PCM composite, 𝑟 is the radius of the columnar PCM composite, 𝐿 is the length of the columnar PCM composite. For the purpose of measuring the magnetic permeability of the PCM composite, a ring of PCM composite was fabricated. The molten PCM composites were placed

into a Teflon tube with an inner diameter of 5 mm. After that, the Teflon tube with the PCM composite was bent into a ring with two end faces pressed together, which were fixed together by bound tapes. Source coil with 45 turns and induction coil with 5 turns were evenly winded around the ring, respectively. Then, the magnetic permeability of the PCM composite was measured with an auto test system of magnetic materials (MATS-2010SD, Hunan Linkjoin Technology CO., LTD, Hunan,

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China).

In order to characterize the heat capacitance of the composite, a cylindrical-

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shaped composite sample was measured with a differential scanning calorimeter (STA 449C, NETZSCH Scientific Instruments CO., LTD, Germany). A sample was sealed into an aluminum oxide pan with the diameter of 1mm and the height of 3 mm.

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Temperature was scanned from 20 ℃ to 70 ℃ at a heating rate of 5 ℃ per minute.

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The thermal expansion of the composite was measured with a glass capillary

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dilatometer (1894, Jiangshan Kanghui Glass CO., LTD, Zhejiang, China) with

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resolution of 1%. The glass container filled with the composite sample was placed in a thermostat water bath (HH-1, Jintan Science Analysis Instrument Co., LTD,

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Jiangsu, China) with accuracy of 0.3 ℃, where the temperature was increased from 20 ℃ to 70 ℃ in steps of 3 ℃.

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4.2 Measurement of microactuator performance

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The experimental setup consisted of a Polarizing optical microscope (6XD-3, Shanghai Optical instrument, China) with a CCD camera (Panasonic Super Dynamic Ⅱ WV-CP460) attached, a computer, two multimeters (Keithley 2000), a high

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frequency pulse generator (SP1631A) and a switch, as shown in Fig. 5. The prototype of PCM-composite-based microactuator was placed on the X-Y stage padded with one thick glass. The excitation coil of the microactuator was connected with the pulse generator with multimeter 1 in parallel measuring the voltage of the microactuator and multimeter 2 in series measuring the current applied to the microactuator. Sinusoidal alternating current with various frequencies and various amplitude were adopted in

our experiments. The optical microscope with a CCD camera attached was used to record continuous video during the characterization experiments. The data was transmitted to a computer and processed by the computer. The actuation height reported in this paper is the center deflection measure with a Surface Scanning Laser Confocal Displacement (MeterLT-9000) with resolution of 0.3 μm.

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Throughout the experiment, the PCM-composite-based microactuators were tested in an open atmosphere at 25 ℃. The total input power can be calculated with

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equation (6): 𝑃 = 𝑈𝐼

(6)

Where 𝑃 is the total input power, 𝑈 is the voltage applied to the PCM-composite-

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based microactuator, 𝐼 is the current applied to the PCM-composite-based

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microactuator.

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5.1 PCM composite

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5. Result and discussion

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In the present work, firstly, EG/PW composite with mass fraction of EG varying from 0 to 3 wt% were prepared and characterized. The PCM composite were

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investigated with a POM (6XD-3) and the POMs images are shown in Fig. 6. The darker regions represent the saturated EG, whereas the lighter regions represent the

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superfluous PW. For the EG(1)/PW(99) composite as shown in Fig. 6(a), the EG particles have almost no chance of making contact with one another, hence, electrical conduction paths may not be formed in the PCM composite. For the EG(2)/PW(98)

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composite as shown in Fig. 6(b), some EG particles connected with other EG particles, thus, the electrical conduction paths were formed even though the path was not a straight line. For the EG(3)/PW(97) composite as shown in Fig. 6(c), the superfluous paraffin are rarely seen in the POM images because most of paraffin wax were absorbed in EG particles. More straight line electrical paths were available from point to point in the PCM composite. It is obvious that the expansion will decrease as

the fraction of EG increases in the PCM composite. The EG(2)/PW(98) composite was considered as a compromise to balance the expansion and the electrical conduction requirements of a induction heating system. In order to improve the magnetic permeability, NP with average diameter of 6 μm were added to the EG(2)/PW(98) composite. The mass fractions of NP varying

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from 0 to 10 wt% were prepared. Generally, NP would deposit at the bottom of the container because of the higher density of NP compared with EG and PW. Due to the micro size of NP and the porous structure of EG, NP can be stored into the porous

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matrix of EG. When the mass fraction of NP increased to 3 wt%, the NP could hardly be seen in the POM image of the composite at the bottom of the Petri dish.

Furthermore, when over 4 wt% NP was added into the PCM composite, the obvious

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presence of NP could be observed from the bottom of the Petri dish, which signified

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the saturation absorption of NP in the porous matrix of EG. Finally, as shown in Fig.

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6(d), 3 wt% of NP and 97 wt% of the EG(2)/PW(98) composite was considered as a

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compromise with the aim of enhancing the magnetic permeability of the PCM composite. As shown in Fig. 6(d), black particles in red circles are nickel particles and

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the lighter regions represent the superfluous paraffin wax.

500 μm

(a)

(b) 500 μm

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500 μm

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10 μm

(d)

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(c)

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Fig. 6. Micrograph of PCM composite. (a) EG(1)/PW(99) composite, (b) EG(2)/PW(98) composite, (c) EG(3)/PW(97) composite, (d) 3 wt% of NP and 97 wt%

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of the EG(2)/PW(98) .

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5.2 Physical properties of the PCM composite The magnetic permeability 𝜇 of the PCM composite was 0.001 H/m measured

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with an auto test system of magnetic materials (MATS-2010SD). After the resistance of PCM composite was measured with a LCR meter, the resistivity of the PCM composite was 1.12 Ω ∙ m calculated with equation (5). Suppose that the skin depth 𝛿

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is 2/3 of the height of the PCM composite pillar, the optimal frequency of Alternating Current is about 932 Hz calculated with equation (4) and the measured parameters of the PCM composite. As show in Fig. 7, the volume expansion of the composite was about 14% at 70 ℃ measured with a glass capillary dilatometer. The onset temperature for melting,

as obtained from the dilatometer, was about 52 ℃, and complete melting was reached at about 60 ℃. Fig. 8 shows the heat capacitance of the PCM composite respect to the temperature measured with a differential scanning calorimetry (DSC). The solid blue line stands for the amount of heat per gram of sample needs to be absorbed per

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second, and the dotted red line stands for the temperature of sample. The trough of solid blue line from 52 ℃ to 62 ℃ refers to the phase transition of the paraffin in

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the composite.

5.3 Actuation performance of the PCM-composite-based microactuator

The completely assembled microactuator was positioned on an X-Y stage of

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an optical microscope. In order to eliminate the induction influence of the meal stage,

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the PCM-composite-based microactuator and the stage were separated with a glass

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slide of 2 mm in thickness. To depict the actuation characteristics of PCM-compositebased microactuator, 10 prototypes were fabricated and tested. The resistance of the

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PCM-composite-based microactuator was 0.47Ω and the inductance was 0.54 μH

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measured with a Precision LCR Meter (TH2817A) at the frequency of 1 kHz. According to the calculation of the optimal frequency mentioned above, AC power of 1 kHz was adopted firstly with the input power varies from 0 W to 1.42 W. Obvious

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actuation height was observed when the input power was 0.24 W with the AC

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frequency of 1 kHz. The actuation process was recorded with a video camera. Fig. 9 displays micrographs extracted from a video with the total input power of 0.84 W. It should be noted that deflection of the PDMS diaphragm seems like symmetric

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images, because the glass PCM chamber may lead mirror reflection.

10 s

20 s

30 s

100μm

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0s

Fig. 9. Images of the actuation for a microactutor with different actuation time (0 s, 10 s, 20 s and 30 s). The total input power was 0.84 W with AC frequency of 1 kHz.

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The actuator's work density 𝑊𝑑 can be calculated with equation (7): 𝐹max 𝑑max

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𝑊𝑑 =

2𝑉

(7)

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V is the volume of the actuation material.

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Where 𝐹max is the maximum force delivered, 𝑑max is the maximum actuation height,

𝜂=

𝐹max 𝑑max 2𝑃in 𝑡

(8)

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And the energy efficiency 𝜂 can be calculated with equation (8):

Where 𝑃in is the input power and t is the actuation time.

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With a contact load of 0.4 N, about 34 μm actuation height was reached in 6

second with the input power of 0.97 W. The actuator's work density is 8662 J⁄m3 calculated with equation (7), and the energy efficiency is 1.16 × 10−6 calculated with

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equation (8).

In order to examine the optimal AC frequency, AC frequency with range of 100

Hz to 10 kHz were tested with our prototypes. As shown in Fig. 10, the actuation height of the PCM-composite-based microactuator is plotted against time with the total input power of 0.58W for various AC frequencies. It can be seen from Fig. 10

that the AC frequency of 1 kHz has the highest actuation height, which is in agreement with the theoretical predictions. Fig. 11 shows the actuation height of PCM-composite-based microactuator respect to the actuation time with various total input powers at frequency of 1 kHz. The first clear observation is that the higher input power leads to the higher actuation

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height with the same actuation period. This is to be expected because higher power produces more energy per unit time, causing a larger volume expansion. Notice that it is an approximately linear relationship between actuation height and actuation time at

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initial actuation period. After that, the actuation height rising is slower and the height

increase rate is nonlinear. For example, the actuation height reached about 280 μm in the initial 15 seconds with the input power of 1.09 W, whereas, the actuation height

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only increased about 50 μm in the next 15 second. The reason is that some heat

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energy is stored in the molten PW as the heating was going on, instead of shifting the

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PW from solid phase to liquid phase. The energy mainly caused temperatures

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increasing in PW, has little effect on volume expansion. The linear relationship between actuation height and actuation time is preferable. Because the actuation

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height is a liner function of the input power and actuation time, one can control the actuation height by adjusting the power setting and the actuation period. As seen in

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Fig. 11, about 140 μm actuation height is realized in 5 second with the input power of 1.42 W. Hence, the PCM-composite-based microactuator with induction heating has a

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relatively fast response time. Table 1 shows the important performance parameters of the existing paraffin

based actuators. It can be seen from Table 1 that the microactuator developed in this

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work required shorter actuation time to raise the dot height of about 150 μm compared with the actuators using resistive heater. It is noticed that all actuators [2, 3, 12, 14, 15, 17] with resistive heater in table 1 adopted embedded heater, which was surrounded by paraffin wax. It can be seen that the in situ heater actuator [18] has the shortest actuation time because the electric current was applied to the graphiteparaffin directly. However, due to electrodes must be connected with the paraffin wax

and the paraffin with two electrically conductive rings must to be sealed in a chamber, the fabrication process of the in situ actuator is complex than that of this work. The mass ratio of graphite in the graphite-paraffin mixture of the in situ heater actuator ranged from 20% to 50% [18], which is much higher than that in this work. The expended graphite only occupied about 2% weight of the conductive phase change composite. The less of conductive material in the mixture, the more phase change

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material in the mixture, the composite can cause larger amount of expansion accordingly.

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More than 20 W was adopted in the wireless actuated microvalve system [19]

using a copper wound solenoid coil with a diameter of 6.0 mm and three turns. The maximum induction heating power was only about 1.42W and the actuator volume

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can be less than 2 mm×2 mm×0.5 mm in this work. The induction heating

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microactuator introduced in this work is more ease for the integration into MEMS

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systems.

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It is noticed that the membrane of PDMS is permeable to paraffin, however, we have not discovered obvious seepage in about one hundred cycles experiments. For

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long term stability, other material membrane is studied underway. No obvious separation was observed in our experiments, because the expanded graphite has good

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absorbability with paraffin and most of the nickel particles can stay in the holes of the

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expanded graphite.

Table 1. Comparison list of the performance parameters for actuators with PCM. Power(W)

Actuation time(s)

PCM volume（ μL）

Lehto et al [2]

37

0.23

17

1.88

resistive heating

Sharma et al [3]

60

0.6

3

1.27

resistive heating

Lee et al [12]

150

0.21

60

4.29

resistive heating

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Dot height(μm)

Heating method

90

0.11

40

0.63

resistive heating

Ogden et al [15]

50

2.8

1

2.57

resistive heating

Goldschmidtbo ng et al [17]

14

1.9

2.5

1.62

resistive heating

Sant et al [18]

150

0.56

3

2.82

in situ heating

in this work

140

1.42

5

0.85

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Dubois et al [14]

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induction heating

6. Conclusion

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A new PCM-composite-based microactuator was successfully fabricated and

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characterized based on the electrical conductive PCM composite and induction heating. The electrical conductive PCM composite consists of paraffin wax, expanded

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graphite and nickel particle, in which, porous expanded graphite network provided

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electrical conduction paths and nickel particle improved the magnetic permeability of the PCM composite. The electrical resistivity and the magnetic permeability of PCM

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composites are measured. Experiments show that about 140 μm actuation height is realized in 5 second with the input power of 1.42 W at excitation frequency of 1 kHz.

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The new PCM-composite-based microactuator has some advantages over many other existing PCM-based microactuators, including ease of fabrication, small-sized, low

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cost, and ease of integration into MEMS systems. Moreover, the PCM-compositebased microactuator is also attractive for braille display, microswitch, micropump and

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microvalve in microfluidics. Acknowledgments Bendong Liu received the Ph.D. degree in mechanical design and theory from Beijing University of technology, Beijing, China, in 2007. Currently, he is an associate professor in Beijing University of technology. He has published about 30 journal and conference papers. His current research interests include Micro Electro Mechanical Systems (MEMS), micro actuators, micro sensor and micro electromagnetic devices.

Jiechao Yang received the bachelor’s degree of mechanical engineering and automation from Beijing University of Technology in 2014. He is currently pursuing the Master degree at Beijing University of Technology. His current research interests include Micro Thermal Actuator and MEMS fabrication technologies. Zhen Zhang received the bachelor’s degree of mechanical engineering and automation from Beijing University of Technology in 2014. He is currently pursuing the Master degree at Beijing University of Technology. His current research interests include Micro Pump and

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MEMS fabrication technologies. Jiahui Yang received the Master degree in mechanical design and theory from Beijing

University of technology, Beijing, China, in 2006. Currently, she is a teacher in Beijing

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Vocational College of Agriculture and is also pursuing a PhD on mechanics engineering at Beijing University of technology.

Desheng Li received the Ph.D. degree in engineering system and technology Saga University, Japan in 1996. He is currently a Professor in Beijing University of Technology. His current

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research interests include MEMS technology, automatic control technology.

Project of National Natural Science Foundation of China No 51105011 and Project of Beijing

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Board of education No KM201210005015 supported this research.

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References

References

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[1] M. Freund, G. Mózes, Paraffin Products Properties, Technologies, Applications, Dev. Pet. Sci. 23 (7) (1983) 275-276.

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[2] M. Lehto, R. Boden, U. Simu, K. Hjort, A Polymeric Paraffin microactuator, J. Microelectromech. Syst. 17 (5) (2008) 1172-1177. [3] G. Sharma, L. Klintberg, K. Hjort, Viton-based fluoroelastomer microfluidics, J. Micromech. Microeng. 21 (2) (2011) 243-253.

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[4] S. Ogden , L. Klintberg , G. Thornell , K. Hjort , R. Bodén, Review on miniaturized paraffin phase change actuators, valves, and pumps, Microfluid. Nanofluid. 17 (1) (2014) 5371. [5] B. Yang, Q. Lin, A latchable microvalve using phase change of paraffin wax. Sens. Actuators A 134(1) (2007) 194-200. [6] S. Ogden, J. G. Jonsson, Thornell, K. Hjort, A latchable high-pressure thermohydraulic valve actuator, Sens. Actuators A 188 (12) (2012) 292-297.

[7] P. Selvaganapathy, E. T. Carlen, C. H. Mastrangelo, Electrothermally actuated inline microfluidic valve, Sens. Actuators A 104 (3) (2003) 275-282. [8] R. Bodén, M. Lehto, U. Simu, A polymeric paraffin actuated high-pressure micropump, Sens. Actuators A 127 (1) (2006) 88-93. [9] J.S. Lee, S. Lucyszyn, Design and pressure analysis for bulk-micromachined electrothermal hydraulic microactuators using a PCM, Sens. Actuators A 133 (2) (2007) 294300.

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[10] J.Y. Choi, J. Ruan, F. Coccetti, Three-Dimensional RF MEMS Switch for Power Applications, IEEE T. Ind. Electron. 56 (4) (2009) 1031-1039.

[11] W. Zhang, M. Weng, P. Zhou, L. Chen, Z. Huang, & L. Zhang, et al. Transparency-

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switchable actuator based on aligned carbon nanotube and paraffin-polydimethylsiloxane composite. Carbon 116 (2017) 625-632.

[12] J.S. Lee, S. Lucyszyn, A Micromachined Refreshable Braille Cell, J. Microelectromech.

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Syst. 14 (4) (2005) 673-682.

[13] J. Jonsson, S. Ogden, L. Johansson, Acoustically enriching, large-depth aquatic sampler,

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Lab Chip 12 (9) (2012) 1619-1628.

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[14] P. Dubois, E. Vela, S Koster, D. Briand, H.R. Shea, Paraffin-PDMS composite thermo

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microactuator with large vertical displacement capability, Proc. 10th Int. Conf. New Actuators, Bremen, Germany (2006) 215–218.

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[15] S. Ogden, R. Bodén, K. Hjort, A latchable valve for high-pressure microfluidics, J. Microelectromech. Syst. 19 (2010) 396–401. [16] A. Malfliet, G. Deferme, L. Stappers, and J. Fransaer, Synthesis and characterization of

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composite coatings for thermal actuation, J. Electrochem. Soc. 154 (1) (2007) D50–D56. [17] F. Goldschmidtbong, P. Katus, A. Geipel, P. Woias, A novel self-heating paraffin

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membrane micro-actuator, Int. Conf. Micro Electro Mech. Syst. (2008) 531-534. [18] H.-J. Sant, T. Ho, B.K. Gale, An in situ heater for a phase-change-material-based actuation system, J. Micromech. Microeng. 20 (8) (2010) 085039. [19] S.-K. Baek, Y.-K. Yoon, H.-S. Jeon, S. Seo, J.-H. Park, A wireless sequentially actuated

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microvalve system, J. Micromech. Microeng. 23 (2013) 045006. [20] K.W. Oh, K. Namkoong, P. Chinsung, A phase change microvlve using a meltable magnetic material: ferro-wax, Micro Total Anal. Syst. 1 (2005) 554-556. [21] K. Kolari, T. Havia, I. Stuns, K. Hjort, Flow restrictor silicon membrane microvalve actuated by optically controlled paraffin phase transition, J. Micromech. Microeng. 24 (2014) 084003.

[22] S. Kim, L.-T. Drzal, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 93 (1) (2009) 136–142. [23] Y.-F. Zhao, M. Xiao, S.-J. Wang, X.-C. Ge, Y.-Z. Meng, Preparation and properties of electrically conductive PPS/expanded graphite nanocomposites, Compos. Sci. Technol. 67 (11-12) (2007) 2528–2534. [24] G. Zheng, J. Wu, W. Wang, C. Pan, Characterizations of expanded graphite/polymer

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composites prepared by in situ polymerization, Carbon 42 (2004) 2839-2847. [25] M. Inagaki, T. Nagata, T. Suwa, M. Toyoda, Sorption kinetics of various oils onto exfoliated graphite, New Carbon Mater. 21 (2) (2006) 98–102.

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[26] D.-D.-L Chung, Exfoliation of graphite, J. Mater. Sci. 22 (12) (1987) 4190-4198.

PDMS diaphragm

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[27] D. John, Induction heating handbook, McGraw Hill Book Co, 1979.

PCM composite

Glass PCM chamber

Glass substrate

Excitation coil

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Fig. 1. Exploded view of the PCM-composite-based microactuator

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Fig. 2 SEM images of expanded graphite (a) expanded graphite particle (×500),

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(b) porous structure of expanded graphite (×2500)

Magnetic flux

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PDMS diaphragm

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EG

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PW

Excitation coil

（a）

Alternating currnet

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Glass PCM chamber

Glass substrate （b）

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Fig. 3. Working principle of the PCM-composite-based microactuator with induction heating. (a) The initial state of the microactuator when the paraffin wax is solid. (b)

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The actuation state when the paraffin wax melts to liquid state. The black network

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matrix represents expanded graphite, green dots means nickel particles and the white pores of network are filled with paraffin wax.

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CCD camera

Multimeter 1

Multimeter 2

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Computer

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Fig. 4. Top view of the PCM-composite-based microactuator

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Microactuator

Switch

Pulse generator

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Fig. 5. Schematic of the experimental setup for testing the PCM-composite-based

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microactuator

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Volume expansion (%)

14 12 10 8 6 4 2 0

20

30

40

Temperature (℃)

50

60

70

Fig. 7. Volume expansion versus temperature of the composite.

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DSC(mW/mg)

Tempeture (℃)

DSC(mW/mg) tempeture/℃

Time (min)

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120 105 90 75 60 45 30 15 0

100 Hz

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1 kHz

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5 kHz 10 kHz

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Actuation height (μm)

Fig. 8. Heat capacitance of the PCM composite respect to the temperature

5

10

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0

15

20

25

30

Actuation time (s)

Actuation height (μm)

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Fig. 10. Actuation height versus actuation time with various AC frequencies

400 350 300 250 200 150 100 50 0

1.42 W 1.09 W

0.84 W 0.58 W

0

10

20

30

Actuation time (s)

40

50

Fig. 11. Actuation height versus actuation time with various input power at frequency

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of 1 kHz