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 AIP ADVANCES 7, 025112 (2017) Observation of convection phenomenon by high-performance transparent heater based on Pt-decorated Ni micromesh Han-Jung Kim,1 Dong-Ik Kim,1 Sam-Soo Kim,2 Young-You Kim,3 Sung-Eun Park,2 Gyuseok Choi,2 Dong Wook Lee,4 and Yoonkap Kim2,a Center for Integrated Smart Sensors (CISS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea Convergence Materials & Parts Technology Research Center, Gumi Electronics & Information Technology Research Institute (GERI), Gumi 39171, South Korea Department of Physics, Kongju National University, Gongju 32588, South Korea National NanoFab Center (NNFC), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea (Received 16 January 2017; accepted 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 transparent heater, based on Pt-decorated Ni micromesh, was fabricated by a combination of transfer printing process and Pt sputtering The resulting transparent heater exhibited excellent mechanical durability, adhesion with substrates, flexibility, and heatgenerating 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 PM2.5 concentration 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 electrodes 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 optoelectronic appliances, such as transparent sensors,1–3 solar cells,4,5 displays,6,7 and 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 mechanical properties So far, most studies on optoelectronic components have focused on the fabrication and performance of flexible transparent electrodes.10–15 However, studies on the production, characterization, 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).1,2 In general, the heaters in such sensors are the main technical factors for enhancing the selectivity, sensitivity, 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.11,20 The 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 decoration 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.11,20 As 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 Figure 1(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 spectrophotometer (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 visible 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 transparent heaters and Ni micromeshes with different thickness of 150, 300, and 600 nm are summarized FIG (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 conductive ITO and FTO films 025112-3 Kim et al AIP Advances 7, 025112 (2017) 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 /σopt is as follows:20,21 T= 1+ Z0 σopt 2RS σdc −2 (1) where RS and T are the measured sheet resistance and transmittance at 550 nm, respectively, and Z is the impedance of free space (377 Ω) As shown in Table S1 in supplementary 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 in supplementary 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 35.10,22,23 Therefore, 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 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 in supplementary material As seen in Fig S1 in supplementary 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 transferprinted 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 in supplementary material show 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 R0 is 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.22 Meanwhile, previous studies have shown that AgNW networks on flexible substrates have superior flexibility compared to ITO/flexible substrate.10,23 As expected, as shown in Fig S2 and S3 in supplementary 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 mm (inner bending) and 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 increasing Pt sputtering time The Pt-decorated Ni micromesh structures also exhibit the high mechanical 025112-4 Kim et al AIP Advances 7, 025112 (2017) FIG (a) Electrical properties of Ni-based micromesh films decorated with Pt; inset: photograph of the Pt-decorated Nibased micromesh films (b) Temperature profile of Ni-based micromesh films with varied Pt decoration as functions of time at applied voltage of V stability (adhesion and flexibility) of the Ni micromesh structure Figure S4 in supplementary material shows 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 in supplementary 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 Table I As shown in Table I, 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 Pure Ni micromesh Pt (60 s)/Ni micromesh Pt (120 s)/Ni micromesh Pt (180 s)/Ni micromesh Transmittance at 550 nm (%) Sheet resistance (Ω/sq) Figure of merit 86.1 82.1 73.3 62.5 14.2 11.4 10.1 9.2 172.3 160.5 111.1 77.6 025112-5 Kim et al AIP Advances 7, 025112 (2017) 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.20 As 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 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 V, the steady-state temperature of the Ni-based micromesh films is gradually increased with increases in the Pt decorations, as exhibited in Figure 2(b) The Ni-based micromesh film without Pt-decoration reaches the steady-state temperature of 160 ◦ C Meanwhile, the steady-state temperature of the Nibased 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.24 The 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.24,25 The convective heat power loss is expressed by24,25 V2 ∆t (2) R 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 i are the steady-state and initial temperatures, respectively The steady-state temperature could be obtained from Eq (2) as Eq (3): Qc = hA(Ts − Ti ) = V ∆t + Ti (3) RhA 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 Table I 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 Figure shows 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 Ts = FIG 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 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 Figure 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 10 µm.26,27 The PM2.5 concentration 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 monitored the changes in PM2.5 concentration without the heater Figure S5 in supplementary material shows the change in PM2.5 concentration as a function of time at room temperature As shown, the PM2.5 concentration in the chamber gradually decreases over time, attributed to PM deposition in the chamber by gravity and Brownian motion.28–30 According 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 shows the changes in PM2.5 concentration and temperature in the chamber during operation of the Pt-decorated Ni micromesh-based transparent heater As shown in Fig 4, the temperature in the chamber is increased by about 1.5 ◦ C in ∼10 The PM2.5 concentration 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 in supplementary material shows 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 PM2.5 concentration and temperature in an airtight chamber were observed during operation of the heater The temperature in the chamber was increased slightly, 025112-7 Kim et al AIP Advances 7, 025112 (2017) while the PM2.5 concentration 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 See supplementary material for 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 H Choi, J S Choi, J.-S Kim, J.-H Choe, K H Chung, J.-W Shin, J T Kim, D.-H Youn, K.-C Kim, J.-I Lee, S.-Y Choi, P Kim, C.-G Choi, and Y.-J Yu, Small 10, 3685 (2014) H Kim, S J Kim, Y.-J Kim, Y.-S Shim, S Y Kim, B H Hong, and H W Jang, ACS Nano 9, 10453 (2015) B Meng, W Tang, Z.-H Too, X Zhang, M Han, W Liu, and H Zhang, Energy Environ Sci 6, 3235 (2013) C.-C Chen, L Dou, R Zhu, C.-H Chung, T.-B Song, Y B Zheng, S Hawks, G Li, P S Weiss, and Y Yang, ACS Nano 6, 7185 (2012) R R Lunt and V Bulovic, Appl Phys Lett 98, 113305 (2011) C W Hsu, B Zhen, W Qiu, O Shapira, B G DeLacy, J D Joannopoulos, and M Soljaˇ ci´c, Nat Commun 5, 3152 (2014) T.-J Ha and A Dodabalapur, Appl Phys Lett 102, 123506 (2013) S C Xu, B Y Man, S Z Jiang, C S Chen, C Yang, M Liu, X G Gao, Z C Sun, and C Zhang, Appl Phys Lett 102, 151902 (2013) S Xu, B Man, S Jiang, M Liu, C Yang, C Chen, and C Zhang, Cryst Eng Comm 16, 3532 (2014) 10 H.-J Kim, S.-H Lee, J Lee, E.-S Lee, J.-H Choi, J.-H Jung, J.-Y Jung, and D.-G Choi, Small 10, 3767 (2014) 11 H.-J Kim, M Song, J.-H Jeong, C S Kim, S Surabhi, J.-R Jeong, D.-H Kim, and D.-G Choi, J Power Sources 331, 22 (2016) 12 H Kang, S Jung, S Jeong, G Kim, and K Lee, Nat Commun 6, 6503 (2014) 13 H Wu, D Kong, Z Ruan, P.-C Hsu, S Wang, Z Yu, T J Carney, L Hu, S Fan, and Y Cui, Nat Nanotechnol 8, 421 (2013) 14 J.-A Jeong and H.-K Kim, Sol Energ Mat Sol Cells 93, 1801 (2009) 15 T T Larsen-Olsen, R R Søndergaard, K Norrman, M Jørgensen, and F C Krebs, Energy Environ Sci 5, 9467 (2012) 16 S E Moon, N.-J Choi, H.-K Lee, J Lee, and W S Yang, ETRI J 35, 617 (2013) 17 C.-Y Lee, C.-M Chiang, Y.-H Wang, and R.-H Ma, Sens Actuators B 122, 503 (2007) 18 W.-J Hwang, K.-S Shin, J.-H Roh, D.-S Lee, and S.-H Choa, Sensors 11, 2580, (2011) 19 J F Creemer, D Briand, H W Zandbergen, W van der Vlist, C R de Boer, N F de Rooij, and P M Sarro, Sens Actuators A 148, 416 (2008) 20 H.-J Kim, Y Kim, J.-H Jeong, J.-H Choi, J Lee, and D.-G Choi, J Mater Chem A 3, 16621 (2015) 21 P Li, J Ma, H Xu, X Xue, and Y Liu, J Mater Chem C 4, 3581 (2016) 22 M Vosgueritchian, D J Lipomi, and Z Bao, Adv Funct Mater 22, 421 (2012) 23 M Song, D S You, K Lim, S Park, S Jung, C S Kim, D.-H Kim, D.-G Kim, J.-K Kim, J Park, Y.-C Kang, J Heo, S.-H Jin, J H Park, and J.-W Kang, Adv Funct Mater 23, 4177 (2013) 24 J J Bae, S C Lim, G H Han, Y W Jo, D L Doung, E S Kim, S J Chae, T Q Huy, N V Luan, and Y H Lee, Adv Funct Mater 22, 4819 (2012) 25 S.-H Park, S.-M Lee, E.-H Ko, T.-H Kim, Y.-C Nah, S.-J Lee, J H Lee, and H.-K Kim, Sci Rep 6, 33868 (2016) 26 R Zhang, C Liu, P.-C Hsu, C Zhang, N Liu, J Zhang, H R Lee, Y Lu, Y Qiu, S Chu, and Y Cui, Nano Lett 16, 3642 (2016) 27 C Liu, P.-C Hsu, H.-W Lee, M Ye, G Zheng, N Liu, W Li, and Y Cui, Nat Commun 6, 6205 (2015) 28 W W Nazaroff, Indoor Air 14, 175 (2004) 29 T Hussein, A Hruˇska, P Doh´ anyosov´a, L Dˇzumbov´a, J Hemerka, M Kulmala, and J Smol´ık, Atmos Environ 43, 905 (2009) 30 J Grau-Bov´ e and M Strliˇc, Herit Sci 1, (2013) Y ...AIP ADVANCES 7, 025112 (2017) Observation of convection phenomenon by high- performance transparent heater based on Pt- decorated Ni micromesh Han-Jung Kim,1 Dong-Ik Kim,1 Sam-Soo Kim,2 Young-You... properties of Ni -based micromesh films decorated with Pt; inset: photograph of the Pt- decorated Nibased micromesh films (b) Temperature profile of Ni -based micromesh films with varied Pt decoration as... 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