observation of convection phenomenon by high performance transparent heater based on pt decorated ni micromesh

Observation of convection phenomenonby high-performance transparent heater based on Pt-decorated Ni micromesh Han-Jung Kim,1Dong-Ik Kim,1Sam-Soo Kim,2Young-You Kim,3 Sung-Eun Park,2Gyuse

Observation of convection phenomenon by high-performance transparent heater based on Pt-decorated Ni micromesh

Han-Jung Kim, Dong-Ik Kim, Sam-Soo Kim, Young-You Kim, Sung-Eun Park, Gyuseok Choi, Dong Wook Lee, and Yoonkap Kim

Citation: AIP Advances 7, 025112 (2017); doi: 10.1063/1.4977021

View online: http://dx.doi.org/10.1063/1.4977021

View Table of Contents: http://aip.scitation.org/toc/adv/7/2

Published by the American Institute of Physics

Observation of convection phenomenon

by high-performance transparent heater

based on Pt-decorated Ni micromesh

Han-Jung Kim,1Dong-Ik Kim,1Sam-Soo Kim,2Young-You Kim,3

Sung-Eun Park,2Gyuseok Choi,2Dong Wook Lee,4and Yoonkap Kim2, a

1Center for Integrated Smart Sensors (CISS), Korea Advanced Institute of Science

and Technology (KAIST), Daejeon 34141, South Korea

2Convergence Materials & Parts Technology Research Center, Gumi Electronics

& Information Technology Research Institute (GERI), Gumi 39171, South Korea

3Department of Physics, Kongju National University, Gongju 32588, South Korea

4National NanoFab Center (NNFC), Korea Advanced Institute of Science

and Technology (KAIST), Daejeon 34141, South Korea

(Received 16 January 2017; accepted 8 February 2017; published online 22 February 2017)

In this study, we report for the first time on the convection phenomenon for the consistent and sensitive detection of target materials (particulate matter (PM) or gases) with a high-performance transparent heater The high-performance transpar-ent heater, based on Pt-decorated Ni micromesh, was fabricated by a combination

of transfer printing process and Pt sputtering The resulting transparent heater exhib-ited excellent mechanical durability, adhesion with substrates, flexibility, and heat-generating performance We monitored the changes in the PM concentration and temperature in an airtight chamber while operating the heater The temperature in the chamber was increased slightly, and the PM2.5concentration was increased by approximately 50 times relative to the initial state which PM is deposed in the chamber We anticipate that our experimental findings will aid in the development and application of heaters for sensors and actuators as well as transparent

elec-trodes and heating devices © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4977021]

INTRODUCTION

In recent years, transparent electronics have been studied to develop and commercialize opto-electronic appliances, such as transparent sensors,1 3 solar cells,4,5displays,6,7and speakers.8,9 For application in transparent and flexible electronic devices, necessary components such as electrodes, resistors, and memories should be both transparent and flexible with excellent electrical and mechan-ical properties So far, most studies on optoelectronic components have focused on the fabrication and performance of flexible transparent electrodes.10–15However, studies on the production, char-acterization, and application of other components such as transparent flexible resistors, heaters, and memories are rare The development of transparent and flexible heaters with high performances is necessary for fabricating high-quality transparent flexible sensors for gases and particulate matters (PM).1,2In general, the heaters in such sensors are the main technical factors for enhancing the selec-tivity, sensiselec-tivity, and response and recovery speeds of the sensors, or the adsorption and desorption the target materials (gas or PM).1 , 2 , 16 – 19

In this study, a high-performance transparent heater based on Pt-decorated Ni micromesh was fabricated by a combination of transfer printing process and Pt sputtering.11 , 20The resulting

Ni micromesh-based heater with Pt decoration exhibited excellent mechanical durability, such as adhesion to substrates and flexibility, and heat-generating performance The electrical and

a Author to whom correspondence should be addressed Electronic mail: yoonkap@geri.re.kr

2158-3226/2017/7(2)/025112/7 7, 025112-1 © Author(s) 2017

025112-2 Kim et al. AIP Advances 7, 025112 (2017)

heat-generating performances of the heater were enhanced with increasing the degree of Pt decora-tion In addition, for the first time, change in PM concentration in an airtight chamber was observed

by convective heat transfer from the Pt-decorated Ni micromesh transparent heater at low voltages This clearly demonstrates the potential of our transparent heater as a high-performance alternative heater (or fan) for sensors and actuators

RESULTS AND DISCUSSIONS

Figure 1(a) shows a photograph, optical micrograph, and field emission scanning electron microscopy (FE-SEM) image of the transfer-printed pure Ni micromesh on a glass substrate We recently reported on the fabrication of uniform metal mesh structures on various substrates of glass and flexible plastic films in our previous studies.11,20As shown in Fig.1(a), a large-scale (100 mm ×

100 mm) transparent Ni micromesh structure is successfully fabricated on the glass substrate, which has a width of 1.8 µm and a pitch of 100 µm

Figure1(b)shows the optical transmittance spectra of the 150 nm-thick Ni mesh on glass and commercial transparent heaters (indium tin oxide (ITO)-coated glass and fluorine-doped tin oxide (FTO)-coated glass) All transmittance spectra were obtained using a UV-VIS-NIR spectrophotome-ter (SolidSpec-3700, Shimadzu Scientific Instruments) with air as a reference As shown in Fig.1(b), commercial FTO glass has light transmittances inferior to those of ITO glass throughout the visi-ble range The transfer-printed Ni micromesh on glass shows an optical transmittance of 89.5% at

550 nm and 87.2% throughout the visible range This value is similar to that of the commercial ITO glass

The electrical conductivity, or more correctly, the sheet resistance is another important parameter

of transparent heaters The electrical property was measured using a four-point probe-type sheet resistance meter (FPP-1000, DASOL ENG)

The transmittance at 550 nm, sheet resistance, and figure of merit (FoM) of the commercial trans-parent heaters and Ni micromeshes with different thickness of 150, 300, and 600 nm are summarized

FIG 1 (a) Photograph and micrographs (optical micrograph and FE-SEM) of the transfer-printed Ni micromesh on a glass substrate with of 1.8 µm and pitch of 100 µm (b) Optical transmittance spectra of the Ni micromesh and commercial transparent

in Table S1 Here, we calculate the FoM value as the ratio of the electrical conductance to the optical conductance (σdc/σopt) The expression for σdc/σoptis as follows:20,21

T= 1+ Z0

2RS

σopt

σdc

!−2

(1)

where RSand T are the measured sheet resistance and transmittance at 550 nm, respectively, and Z0is the impedance of free space (377 Ω) As shown in Table S1 insupplementary material, the optical and electrical properties of the Ni micromesh depend on the mesh thickness In other words, the optical transmittance and sheet resistance decrease with increasing mesh thickness A trade-off between the optical transparency and electrical conductivity of the Ni micromesh is observed However, the FoM value increases with increasing micromesh thickness As shown in Table S1 insupplementary material, the FoM values of the 150-, 300-, and 600-nm-thick Ni micromesh on glass are 44.0, 89.0, and 172.3, respectively In general, the minimum FoM value necessary for transparent electronic applications is 35.10 , 22 , 23Therefore, we expect that the transfer-printed Ni micromesh could be used

as a transparent electronic electrode, heater, or sensor in various optoelectronic devices These results also indicate that the optical transmittance, sheet resistance, and FoM value of the metal mesh can be controlled by changing the mesh thickness In this study, we performed additional experiments using the 600 nm-thick Ni mesh that had the highest FoM value of 172.3

The adhesion of a transfer-printed Ni micromesh structure to the underlying substrate is an important consideration for practical applications In order to evaluate the adhesive characteristics of the transfer-printed Ni micromesh, the Ni micromesh on glass was subjected to ultra-sonication in deionized (DI) water for 30 min For comparison, the spin-coated Ag nanowire (AgNW) networks on glass and commercial ITO glass were tested under the same conditions The experimental results are shown in Fig S1 insupplementary material As seen in Fig S1 insupplementary material, the sheet resistance of the AgNW network is increased significantly because of the poor adhesion between the AgNW network and the glass during the adhesion test In contrast, the sheet resistance of the transfer-printed Ni micromesh on glass and commercial ITO glass remains almost unchanged during this test This means the adhesion between the transfer-printed Ni mesh and glass substrate is strong enough for practical use

The mechanical flexibility of transparent electronic components is necessary for use in wearable optoelectronic devices To evaluate the mechanical flexibility of the transfer-printed Ni micromesh on

a flexible substrate, we measured the change in the sheet resistance of the transfer-printed micromesh

on a polyethylene terephthalate (PET) substrate during inner and outer bending tests Figure S2 and S3

insupplementary materialshow the experimental results of the inner and outer bending tests of the Ni micromesh/PET, ITO/PET, and AgNW network/PET specimens with decreasing bending radii In the graphs, the change in the sheet resistance of the sample caused by the physical bending of the flexible

substrate can be expressed as (R R0)/R0, where R0is the measured initial sheet resistance, and R is

the measured sheet resistance in the bent state In general, the poor mechanical flexibility of ITO on flexible substrates is widely known.22Meanwhile, previous studies have shown that AgNW networks

on flexible substrates have superior flexibility compared to ITO/flexible substrate.10 , 23As expected,

as shown in Fig S2 and S3 insupplementary material, the ITO/PET shows less mechanical flexibility than the AgNW network/PET at bending radii of 10 mm or less However, the Ni micromesh/PET possesses high mechanical flexibility, comparable to that of AgNW network/PET The inner/outer bending test results show that the Ni micromesh/PET maintains a constant sheet resistance until a bending radius of 4 mm (inner bending) and 6 mm (outer bending) These results clearly indicate that the mechanical stability (adhesion and flexibility) of the transfer-printed Ni micromesh is better than those of the commercial ITO film and AgNW networks

The surface of the transfer-printed Ni micromesh was decorated with Pt in order to improve the durability and performance of the transparent heater The Pt-decorated Ni micromesh structures on a transparent substrate were prepared using a table-top sputter coater (EM SCD005, Leica) Here, the thickness of the Pt was controlled by changing the sputtering time The inset of Fig.2(a)shows the Pt-decorated 600 nm-thick Ni micromesh structures on glass with increasing Pt sputtering times (0,

60, 120, and 180 s) The Pt-decorated Ni micromesh structures become darker in color with increas-ing Pt sputterincreas-ing time The Pt-decorated Ni micromesh structures also exhibit the high mechanical

025112-4 Kim et al. AIP Advances 7, 025112 (2017)

FIG 2 (a) Electrical properties of based micromesh films decorated with Pt; inset: photograph of the Pt-decorated Ni-based micromesh films (b) Temperature profile of Ni-Ni-based micromesh films with varied Pt decoration as functions of time

at applied voltage of 3 V.

stability (adhesion and flexibility) of the Ni micromesh structure Figure S4 in supplementary materialshows the optical transmittance spectra of the Pt-decorated Ni mesh structures as a function

of Pt sputtering time As shown in the inset of Fig.2(a)and Figure S4 insupplementary material, the optical transparency of the Pt-decorated Ni mesh structures decreases with increases in the Pt sputtering time

The transmittance at 550 nm, sheet resistance, and FoM value of the Pt-decorated Ni micromesh with different Pt sputtering times are summarized in TableI As shown in TableI, the conductivity of the Pt-decorated Ni mesh structures is increased slightly with increases in Pt sputtering time However, the FoM value decreases significantly because of the decreased light transmittance Regardless of the

TABLE I The transmittances at 550 nm, sheet resistances, and FoM values of pure Ni micromesh and Pt-decorated Ni micromeshes with different Pt sputtering times of 60, 120, and 180 s.

Sample Transmittance at 550 nm (%) Sheet resistance (Ω/sq) Figure of merit

decrease, the FoM value of the Pt-decorated Ni mesh structures remains above the minimum value

of 35 for applications

The Ni micromesh film was prepared in a two-terminal side-contact configuration to assess the performance of the film as a transparent heater.20As shown in Fig.2(a), each Pt-decorated Ni micromesh film creates an Ohmic contact which decreases in electrical resistance as the Pt decoration

is increased on the surface of the Ni micromesh film

A direct current (DC) voltage of 3 V was applied to the Pt-decorated Ni micromesh film through

Ag side-contact electrodes to obtain the temperature variation profile, using direct measurements from a thermocouple mounted on the back side of the film At the constant DC voltage of 3 V, the steady-state temperature of the Ni-based micromesh films is gradually increased with increases in the Pt decorations, as exhibited in Figure2(b) The Ni-based micromesh film without Pt-decoration reaches the steady-state temperature of 160◦C Meanwhile, the steady-state temperature of the Ni-based micromesh film decorated with Pt for 180 s reaches to 200◦C After the metallic mesh film is heated by Joule heating, the heat dissipates by conduction through the substrate, and by convection and radiation to the air.24The heat lost by conduction and radiation is negligible compared to the convective heat loss, because of the low thermal-conductivity substrate and the low emissivity of the electrode material Thus, it is presumed that air convection is the main path of heat dissipation for the Ni-based micromesh films.24 , 25The convective heat power loss is expressed by24 , 25

Q c = hA(T s−T i)=V2

where h is the convective heat transfer coefficient, A is the surface area, V is the input DC voltage, R

is the resistance, and T s and T iare the steady-state and initial temperatures, respectively

The steady-state temperature could be obtained from Eq (2) as Eq (3):

T s=V2∆t

From Eq (3), it is obvious that the steady-state temperature is mainly determined by the DC voltage, resistance, and surface area For a high-performance heater at a low input DC voltage, decreased sheet resistance and surface area are required The sheet resistance of the Ni-based micromesh film decreases with increased Pt decorating time, as listed in TableI This implies that more electrical connections are formed and more efficient transduction of electric energy occurs in the Ni-based micromesh film because of the increase in the surface area with Pt decoration

Figure3shows that the difference in the steady-state temperatures of the Ni mesh films with and without Pt decoration gradually increases with increasing input voltage This means that the electric energy in the heater can be efficiently used for Joule heating with Pt decoration

FIG 3 Steady-state temperatures of Pt-decorated Ni-based transparent heaters with increasing input voltage.

025112-6 Kim et al. AIP Advances 7, 025112 (2017)

FIG 4 Changes in the PM2.5 concentration and temperature in an airtight chamber when operating the Pt-decorated Ni-based transparent heater(3V applied; inset: photograph of measurement system of PM concentration and temperature in the chamber.

In order to confirm the natural convection by heat generated from the Pt-decorated transparent heater, we monitored the changes in the concentration of PM2.5 (PM smaller than 2.5 µm) and temperature in an airtight chamber while operating the transparent heater, as shown in the inset of Figure4 For this test, the Pt-decorated Ni micromesh (10 mm × 10 mm) was used with an applied voltage of 3V, and temperature of the heater was 175◦C

The volume of the chamber used in this experiment was 6,000 cm3 The PM was generated by burning incense and contained particles of sizes from <0.3 µm to >10 µm.26,27The PM2.5 concen-tration data in the chamber was obtained using an air-quality monitor (BR-SMART 126, Bramc) Temperature data was measured using a digital thermometer (Testo 174T, TESTO) We first mon-itored the changes in PM2.5concentration without the heater Figure S5 insupplementary material

shows the change in PM2.5concentration as a function of time at room temperature As shown, the

PM2.5concentration in the chamber gradually decreases over time, attributed to PM deposition in the chamber by gravity and Brownian motion.28 – 30According to previous studies, the highest deposition rates occur for the largest particles of ∼1–10 µm, which are governed mostly by gravity and tend

to settle on horizontal surfaces, and for the smallest particles of ∼0.01–0.1 µm, which are mostly governed by Brownian motion and tend to diffuse and collide with the floor, walls, or ceiling of the chamber Fig.4shows the changes in PM2.5concentration and temperature in the chamber during operation of the Pt-decorated Ni micromesh-based transparent heater As shown in Fig.4, the tem-perature in the chamber is increased by about 1.5◦C in ∼10 min The PM2.5concentration in the chamber is increased by ∼50 times compared to the initial state, which PM is deposed in the chamber,

of ∼20 µg/m3 These experimental results clearly indicate that the movement of the deposited PM

is reactivated by convective heat transfer, driven by the Pt-decorated transparent heater Therefore, the Pt-decorated transparent heater can contribute to the detection or filtration of PM or gases in real time at a low power density

Lastly, Figure S6 insupplementary materialshows an infrared (IR) image of the Pt-decorated

Ni micromesh-based flexible transparent heater (40 mm × 20 mm) while bent; high-temperature heat generation reaching 190◦C is clearly demonstrated on the polyethersulfone film without local heating, as is the stable mechanical flexibility of the heater

CONCLUSIONS

In this letter, we reported on convective heat transfer.by a high-performance transparent heater based on Pt-decorated Ni micromesh The mechanically stable transparent heater was fabricated simply The electrical and heat-generating performances of the heater were enhanced with increased

Pt decoration In addition, changes in PM2.5concentration and temperature in an airtight chamber

while the PM2.5concentration was increased by ∼50 times relative to the initial state which PM is deposed in the chamber Based on these results, we believe that these Pt-decorated Ni mesh-based transparent heaters could be widely used as alternatives to heaters (or fans) in gas sensors, PM detectors, convective heating actuators, incubator, and transparent electrodes

SUPPLEMENTARY MATERIAL

Seesupplementary materialfor the optical and mechanical properties of Ni-based micromesh film

ACKNOWLEDGMENTS

This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project (CISS-2011-0031870) and a grant (The core project-02) from Gumi Core Components and Materials Technology Development Program of the Gumi Regional Government, 2016

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CONCLUSIONS

In this letter, we reported on convective heat transfer .by a high- performance transparent heater based on Pt- decorated Ni micromesh. .. chamber.

In order to confirm the natural convection by heat generated from the Pt- decorated transparent heater, we monitored the changes in the concentration of PM2.5 (PM