- Email: [email protected]
Decoration of zinc oxide nanoparticles onto carbon fibers as composite filaments for infrared heaters
Surfaces and Interfaces 6 (2017) 98–102
Contents lists available at ScienceDirect
Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Decoration of zinc oxide nanoparticles onto carbon fibers as composite filaments for infrared heaters Chien-Te Hsieh a,∗, Dong-Ying Tzou a, Ze-Shien Huang a, Jo-Pei Hsu a, Chi-Yuan Lee b a b
Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 32003, Taiwan Department of Mechanical Engineering, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 7 June 2016 Revised 17 October 2016 Accepted 7 December 2016 Available online 24 December 2016 Keywords: Carbon fiber Nano-structures Thermal properties Thermal analysis
a b s t r a c t This study examines the thermal properties of ZnO-coated carbon fiber (CF) filaments for highperformance infrared (IR) heaters. An efficient pulse microwave (PM) method is applied to deposit different morphologies of ZnO crystals at various pH values of Zn2+ solutions, i.e., fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). Under the applied voltage of 25 V, the CF composite filament, prepared by the PM route at pH = 10, offers the highest thermal radiation power and superior heat-storage capability among these IR heaters. Its saturation temperature and heating rate of ZnO-coated CF heater can reach to 184 °C and 28 °C/min, respectively. The enhanced thermal performance can be ascribed to the facts (i) the ZnO crystals create more emissive surface area and (ii) the composite filament illuminate more homogeneous spectral IR rays, showing the synergetic effect. © 2016 Published by Elsevier B.V.
1. Introduction Infrared (IR) heaters have been recognized as a promising heating source in thermal treatment and drying operation, such as dehydration of fibrils and paints [1], synthesis of metallic nanoparticles [2], ignition of chemical reaction on microchips [3], sintering of cathode materials for Li-ion batteries [4], inactivation of bacterial spores [5], and so on. Theoretically, IR ray is an electromagnetic radiation with wavelength between visible light and microwave radiation [6]. It is generally known that the IR ray can be divided into three regions: IR-A (0.7–1.4 μm), IR-B (1.4–3 μm), and IR-C (3–10 0 0 μm), based on its wavelength. The wavelength range and thermal radiation efficiency of IR heaters basically depend on the filament of emitting elements. So far, two commonlyused types of IR filaments dominate the practical applications, i.e., ceramic and carbon fiber (CF). The IR heaters equipped with ceramic (e.g., alumina) illuminate the IR ray with a wavelength region of 3–18 μm, whereas the wavelength of IR heaters fabricated with CFs falls into the IR-B region. Therefore, the IR radiation of CF heaters is ideally matched to the spectrum of water absorption. This finding reveals that the IR radiation is capable of penetrating the skin to subcutaneous tissues, transforming the light energy into thermal energy. This heat transfer induces an improvement in blood circulation and a reduction of toxins from the human body
∗
Corresponding author. E-mail address: [email protected] (C.-T. Hsieh).
http://dx.doi.org/10.1016/j.surfin.2016.12.001 2468-0230/© 2016 Published by Elsevier B.V.
by sweat production [6]. Accordingly, the CF heater emits rays that are beneficial for human’s health and healing. However, improving the thermal efficiency of CF filaments is still a challenge and rarely discussed in literatures. Traditionally, commercial IR heaters are fabricated by using CF as filament, which is carefully sealed in a quartz tube after vacuum suction process. The power consumption of IR heaters strongly depends on the heater size and applied voltage. One strategy concerning an improved thermal efficiency is to directly modify the CF filaments. The CF filaments are typically composed of thousands of carbon fibers, having an average diameter of 8–10 μm. The purpose of this work is to deposit metal oxide nanoparticles (i.e., zinc oxide (ZnO)) onto the surface of CFs. It is expected that (i) the ZnO-coated CF filament emit irradiation with a wider wavelength range and (ii) the decoration of ZnO nanostructures facilitates heating surface area, thus, imparting an improved thermal efficiency of IR heaters. In fact, ZnO materials have received considerable attentions in a variety of applications such as photocatalysts, gas sensors, solar cells, transparent conductors, and piezoeletronic materials due to their excellent electronic, mechanical, and optical performance [7–10]. Many efforts have been made to synthesize ZnO structures such as spray pyrolysis [11], electrochemical deposition [12–15], hydrothermal method [16], and template electrosynthesis [17]. More recently, our previous work has proposed a novel perspective to rapidly synthesize ZnO crystals, using a pulse microwave (PM) method [18]. The PM method displays a great potential to deposit ZnO crystals onto different carbon supports (e.g., carbon nanotubes and carbon black) at low
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
99
temperatures (e.g., 80 °C). The PM method only takes a short period of 8–12 min, showing a commercial feasibility. Within the above scope, this present work adopts the PM method to coat ZnO layers onto the CFs, forming a composite filament. The pH value of Zn2+ solution was chosen as a controlling factor in affecting the morphology of ZnO structures. The composite IR filaments were assembled as IR heaters, operated at a fixed applied potential. The heating rate and maximal heating temperature were systematically investigated. This work would shed some lights on how the introduction of ZnO crystals on CF filament enhances the thermal radiation efficiency of IR heaters. 2. Experimental In the present work, commercial CF, made from polyacrylonitrile (PAN) precursor, served as heating filament for IR heaters. Each CF filament consisted of a bundle of individual fiber, and total number of fiber was approximately 30 0 0. Each CF filament was carefully cut into a length of 20 cm. Before the deposition of ZnO, the CF filaments were impregnated in distilled water and placed in an ultrasonic bath for 0.5 h. Then the wetted CF filaments were dried at 105 °C in a vacuum oven overnight. Herein an aqueous Zn2+ solution was prepared for the ZnO deposition by the PM method, in which an optimal synthesis conditions have been reported previously [18]. The Zn2+ solution was composed of 2 M (CH3 COO)2 Zn·2H2 O solution and distilled water. The pH value of the solution was adjusted to pH = 9, 10, 11, and 12, respectively, by using 0.5 M KOH. Afterward, the cleaned CF was impregnated into the Zn-containing solution (volume: 50 mL) at ambient temperature for 2 h. Then the CF slurries were placed in the center of household microwave oven (Tatung Co., 900 W, 2.45 GHz). The microwave oven was equipped with a thermocouple and temperature controller. The reaction temperature program started from 25 to 80 °C with a heating rate of 5 °C/min. The PM deposition temperature was maintained at 80 °C for 8 min. The power-on and power-off periods were set at 3 s and 3 s, respectively. After the PM deposition, the treated CF filaments were dehydrated at 105 °C in a vacuum oven overnight. The microstructural observation of the resulting ZnO deposits onto CF samples was characterized by a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6701F). The crystalline structure of ZnO crystals was examined by X-ray diffraction (XRD) with Cu-Kα radiation, using an automated X-ray diffractometer (Shimadzu Labx XRD-60 0 0). The thermal efficiency of IR heaters was investigated in a quartz tube with an inner diameter of 3 cm and a length of 20 cm. The CF filaments could be fixed in the center of quartz tube by using fixtures. One thermal couple (Ktype) was also equipped in the center of quartz tube to detect the real temperature of IR heaters. A galvanostat-potentiastat instrument was adopted to apply stable potential difference between both ends of CF filaments. The potential differences were set at 25 V in the present work. To avoid any oxidation on CFs, a vacuum pump was used to ensure the low-pressure operation of IR heaters, i.e., the operating pressure <0.001 torr. 3. Results and discussion The crystallographic structure of ZnO-coated CFs was inspected using XRD analysis, as shown in Fig. 1. The XRD patterns confirm the presence of well-crystalline ZnO, which is good agreement with wurtzite structure [19,20]. The result reflects that the PM method is capable of growing ZnO wurzite crystals at low temperature of 80 °C. Our preliminary studies also confirm the lowtemperature growth of ZnO crystals on graphene sheets [21] and metal oxides [22] under microwave irradiation. This can be attributed to the fact that PM synthesis induces a dipole change in
Fig. 1. Typical XRD patterns for ZnO-coated CF filaments prepared by the PM method at different pH values.
Fig. 2. FE-SEM image of original CFs.
polar molecules (e.g., water and hydration molecules) with uniform temperature distribution, thus leading to highly crystalline ZnO [23]. SEM photo of original CF sample in Fig. 2 reflects that an individual fiber possesses a smooth surface. Each fiber has an average diameter of 8 μm. To inspect the dispersion of ZnO over CF filaments, SEM was also adopted to observe the morphology of ZnOcoated CF samples, as depicted in Fig. 3(a)−(d). The SEM images clearly show the CFs decorated with different topographies of ZnO crystals, indicating the importance of pH value on the formation of ZnO crystals, e.g., fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). This finding reveals that the pH value plays a vital role in determining the shape formation of ZnO crystals during the PM process. A growth mechanism of ZnO crystals onto CFs under microwave irradiation has
100
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
Fig. 3. FE-SEM images for ZnO-coated CF filaments prepared the PM method at different pH values: (a) 9, (b) 10, (c) 11, and (d) 12.
been proposed in literature [19]. First of all, the Zn2+ ions would be physically adsorbed on the surface of CFs, originated from an interaction between ions and oxygen groups (e.g., carboxylic group). Pioneering study has pointed out the chemical reactions of ZnO crystals synthesized from a zinc precursor as follows [20]:
Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2 O
(R1)
Zn2+ + 4OH− → Zn(OH)4 2+ → ZnO + H2 O + 2OH−
(R2)
Herein the chemical reaction (R1) takes place in the presence of KOH solution at initial stage. The intermediates, Zn(OH)2 species, can be formed, and followed by fast growth of ZnO crystals on CFs with the aid of microwave irradiation. It is worth noting that the reaction (R2) could initialize to occur at higher pH value because the Zn(OH)4 2+ species are able to be formed in the presence of excessive OH− ions [20,24,25]. Accordingly, the PM route at high pH value (e.g., pH = 12) could produce a large number of nuclei, thus, forming ZnO nanoparticles over the surface of CFs. To clarify, the as-prepared ZnO-coated fibers are designated to ZnO–CF9, ZnO–CF10, ZnO–CF11, and ZnO–CF12, according to the pH value of Zn2+ solutions (i.e., pH = 9, 10, 11, and 12), respectively. The variation of temperature with time for all IR heaters is illustrated in Fig. 4. It can be seen that all IR heaters reach their saturation temperatures after 8 min. However, the IR heaters possess various thermodynamic performances such as heating rates and maximal temperatures. The thermal-equilibrium temperature shows an order as 184 °C (ZnO–CF10) >175 °C (ZnO–CF11) >170 °C (ZnO–CF12) >132 °C (ZnO–CF9) >109 °C (original CF). The ZnO–CF10 heater also exhibits the highest heating rate of 28 °C/min, among these CF heaters. Under this operating condition, there is an increase of ∼70% in the heating rate as compared with the CFs without the decoration of ZnO crystals. The influences of pH value of Zn2+ solution on both the maximal temperature and
Fig. 4. The temperature kinetic curves of temperature versus time for all IR heaters.
the heating rate are depicted in Fig. 5. As observed from this figure, the ZnO–CF10 heater is capable of showing the thermal conversion efficiency, i.e., the maximal temperature and the highest heating rate. This finding reveals two facts: (i) the deposition of ZnO crystals shows the positive effect on the thermal efficiency of IR heaters and (ii) the morphology of ZnO crystals on the CFs is very vital in affecting the thermal efficiency of ZnO-coated CF
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
101
Fig. 7. The thermal-imaging photographs of (a) original CF and (b) ZnO–CF10 filaments.
Fig. 5. The saturation temperature and heating rate as a function of pH value of Zn2+ solution.
Fig. 8. The emissive power and heat capacity as a function of pH value of Zn2+ solution.
Fig. 6. The saturation temperature as a function of heating/cooling cycle number for all IR heaters.
filaments. For ZnO–CF10 heater, an increase of 69% in the maximal surface temperature can be observed as compared with original CF heater. This improved thermal emissive efficiency can be attributed to the fact that the addition of ZnO crystals over the CFs creates more emissive surface sites, facilitating not only the maximal temperature but also the heating rate. To evaluate the durability, a heating/cooling cyclic test was carried out on the ZnOcoated CF composite filaments. The thermal-equilibrium temperature as a function of cycle number is illustrated in Fig. 6. The operating conditions of the stability test are identical with the experimental section. As observed from this figure, the ZnO-coated CF filaments basically show a stable performance after cycling, implying good adhesion between the ZnO deposits and the CF matrix. Fig. 7 shows the thermal-imaging photographs of original CF
and ZnO–CF10 filaments while the IR heaters reach their saturation temperatures, showing different temperature distributions. The photos reflect that the ZnO–CF10 filament exhibits higher emissive intensity than original CF one, indicating different spectral characters. This finding reveals that the deposition of ZnO is able to improve the radiation ability of CF filament, originated from the spectral-combined (or synergistic) effect between CF (IR-B ray) and ZnO deposits (IR-C ray). To characterize the thermal efficiency of CF heaters, the radiation power (Qir ) for all CF heaters could be evaluated based on Stefan-Boltzmann law [25].
Qir = A ε σ T 4
(1)
where A is the surface area of radiating body (i.e., the CF filaments in this case), ε is the emissivity of radiating body, σ is the StefanBoltzmann constant (= 5.6704 × 10−8 W/m2 K), and T is surface absolute temperature. The emissive power of IR heaters, Qir , is basically proportional to the fourth power of its temperature, according to Eq. (1). Herein the ε value of CF filaments is approximately 0.98, i.e., grey body. The calculated Qir values as a function of pH value of Zn2+ solution is illustrated in Fig. 8. Under the operat-
102
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
ing condition, the ZnO–CF10 heater exhibits the highest Qir value of 47.1 W among the IR heaters. The presence of ZnO crystals on the IR heaters enhances the Qir value by a factor of two, as compared with the pristine CF heater, i.e., 23.0 W. This enhancement can be attributed to two reasons. First, the introduction of ZnO crystals provides large emissive surface area to irradiate IR light. Second, the CFs and ZnO crystals could illuminate different spectral IR lights (i.e., IR-B and IR-C region), showing the coupling effect. It is worth noting that the ZnO–CF10 filament is composed of core-shell composite fibers, capable of illuminating higher intensity IR ray than the other ZnO morphologies such as fragments (Qir : 29.1 W), bulky islands (Qir : 43.5 W), and nanoparticles (Qir : 41.6 W). This is presumably due to one reason that the core-shell composite filament is able to radiate IR light uniformly with a wider spectral region, whereas the other three IR heaters could not illuminate more homogeneous IR rays from the CF filaments with an incomplete ZnO coating. The heat capacity (Cp ) of CF filaments could be estimated by using the following formula:
Cp = Qir /m T
(2)
where m and T represent the weight of CF filament and the temperature difference between initial and saturation temperature, respectively. The variation of Cp value with the pH value of Zn2+ solution is also depicted in Fig. 8. Again, the core-shell CF filament (i.e., pH = 10) enables the highest heat capacity of 2.73 J/g K among the CF heaters. This result reflects that the core-shell filament displays superior heat-storage capability. The presence of ZnO thin film is believed to accumulate heat and to prevent heat dissipation from the CF surface, thus raising the heat-storage ability. On the basis of the experimental results, the ZnO–CF10 heater exhibits a commercial feasibility for high-performance IR devices due to its high heating rate, high irradiation power, low energy consumption, and excellent heat-storage capability. 4. Conclusions The IR heaters fabricated with CF filaments decorated with different morphologies of ZnO crystals have been confirmed to exhibit high heating rate, high thermal radiation power and low energy consumption. The PM method was adopted to deposit ZnO crystals onto the surface of CFs at low temperature. The pH value of Zn2+ solution was picked up as the controlling factor in affecting the morphology of ZnO crystals, including fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). Based on the operating condition, the saturation temperature and heating rate of ZnO–CF10 heater could attain as high as 184 °C and 28 °C/min, respectively. The enhanced thermal performance could be attributed to the facts (i) the ZnO crystals provide large emissive surface area and (ii) the composite filament illuminate more homogeneous spectral IR rays, resulting from the coupling effect. The CF composite filament, prepared by the PM route at pH = 10, exhibited the highest thermal emissive power and superior heat-storage capability. This improved performance was presumably due to the formation of core-shell structured filaments, capable of accumulating heat and preventing the heat dissipation from the CF surface. Accordingly, the robust design of ZnO-coated CF composite could serve as a potential filament for IR heaters in industrial applications because of its low price, high performance, and easy fabrication.
Acknowledgments The authors are very grateful for the financial support from the Ministry of Science and Technology (Taiwan) under the contracts MOST 105-2218-E-155-007 and MOST 105-2622-E-155-011-CC2. References [1] M. Pettersson, S. Stenström, Modeling of an electric IR heater at transient and steady state conditions Part II: modelling a paper dryer, Int. J. Heat Mass Transfer 43 (20 0 0) 1223–1232. [2] J.H. Jung, H.C. Oh, H.S. Noh, J.H. Ji, S.S. Kim, Metal nanoparticle generation using a small ceramic heater with a local heating area, J. Aerosol Sci. 37 (2006) 1662–1670. [3] M.N. Slyadnev, Y. Tanaka, M. Tokeshi, T. Kitamori, Photothermal temperature control of a chemical reaction on a microchip using an infrared diode laser, Anal. Chem. 73 (2001) 4037–4044. [4] C.T. Hsieh, Y.F. Chen, C.T. Pai, C.Y. Mo, Synthesis of lithium nickel cobalt manganese oxide cathode materials by infrared induction heating, J. Power Sources 269 (2014) 31–36. [5] D. Hamanaka, T. Uchino, N. Furuse, W. Han, S.I. Tanaka, Effect of the wavelength of infrared heaters on the inactivation of bacterial spores at various water activities, Int. J. Food Microbiol. 108 (2006) 281–285. [6] K. Witek, T. Piotrowski, A. Skwarek, Analysis of polymer foil heaters as infrared radiation sources, Mater. Sci. Eng. B 177 (2012) 1373–1377. [7] M. Guo, P. Diao, S. Cai, Highly hydrophilic and superhydrophobic ZnO nanorod array films, Thin Solid Films 515 (2007) 7162–7166. [8] L. Huang, S.P. Lau, H.Y. Yang, E.S.P. Leong, S.F. Yu, Stable superhydrophobic surface via carbon nanotubes coated with a ZnO thin film, J. Phys. Chem. B 109 (2005) 7746–7748. [9] B. Xu, Z. Cai, Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic, Appl. Surf. Sci. 254 (2008) 5899–5904. [10] B. Ingham, B.N. Illy, M.P. Ryan, In situ synchrotron studies of ZnO nanostructures during electrochemical deposition, Curr. Appl. Phys. 8 (2008) 455–458. [11] R.D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, Photoinduced surface wettability conversion of ZnO and TiO2 thin films, J. Phys. Chem. B 105 (2001) 1984–1990. [12] X. Gan, X. Gao, J. Qiu, X. Li, Growth and characterization of ZnO-SDS hybrid thin films prepared by electrochemical self-assembly method, Appl. Surf. Sci. 254 (2008) 3839–3844. [13] S. Wei, J. Lian, X. Chen, Q. Jiang, Effects of seed layer on the structure and property of zinc oxide thin films electrochemically deposited on ITO-coated glass, Appl. Surf. Sci. 254 (2008) 6605–6610. [14] J.S. Wellings, N.B. Chaure, S.N. Heavens, I.M. Dharmadasa, Growth and characterization of electrodeposited ZnO thin films, Thin Solid Films 516 (2008) 3893–3898. [15] C Tazlaoanu, L Ion, I Enculescu, M Sima, M Enculescu, E Matei, R Neumann, R Bazavan, D Bazavan, S Antohe, Transport properties of electrodeposited ZnO nanowires, Phys. E 40 (2008) 2504–2507. [16] D.C. Olson, S.E. Shaheen, R.T. Collins, D.S. Ginley, The effect of atmosphere and ZnO morphology on the performance of hybrid poly(3-hexylthiophene)/ZnO nanofiber photovoltaic devices, J. Phys. Chem. C 111 (2007) 16670–16678. [17] M. Lai, D.J. Riley, Templated electrosynthesis of zinc oxide nanorods, Chem. Mater. 18 (2006) 2233–2237. [18] C.T. Hsieh, J.S. Lin, Y.F. Chen, C.Y. Lin, W.Y. Li, Graphene sheets anchored with ZnO nanocrystals as electrode materials for electrochemical capacitors, Mater. Chem. Phys. 143 (2014) 853–859. [19] N.F. Hamedani, A.R. Mahjoub, A.A. Khodadadi, Y. Mortazavi, Microwave assisted fast synthesis of various ZnO morphologies for selective detection of CO, CH4 and ethanol, Sens. Actuators B 156 (2011) 737–742. [20] X. Pu, D. Zhang, X. Yi, X. Shao, W. Li, M. Sun, L. Li, X. Qian, Rapid chemical synthesis and optical properties of ZnO ellipsoidal nanostructures, Adv. Powder Technol. 21 (2010) 344–349. [21] C.T. Hsieh, C.Y. Lin, Y.F. Chen, J.S. Lin, Synthesis of ZnO@graphene composites as anode materials for lithium ion batteries, Electrochim. Acta 111 (2013) 359–365. [22] C.T. Hsieh, B.S. Chang, J.Y. Lin, R.S. Juang, Improvement of rate capability of spinel lithium titanate anodes using microwave-assisted zinc nanocoating, J. Alloys Compd. 513 (2012) 393–398. [23] C.T. Hsieh, D.Y. Tzou, C.C. Chen, C.Y. Lai, Y.Y. Liu, Deposition and super liquid repellency of fluorinated ZnO nanoparticles on carbon fabrics, Surf. Coat. Technol. 22 (2012) 4639–4644. [24] Y. Cao, B. Liu, R. Huang, Z. Xia, S. Ge, Flash synthesis of flower-like ZnO nanostructures by microwave-induced combustion process, Mater. Lett. 65 (2011) 160–163. [25] R. Sharma, K.K. Kar, Carbon nanotube coated carbon fiber based composite filaments for luminescent bulbs, Mater. Lett. 137 (2014) 150–152.
Contents lists available at ScienceDirect
Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Decoration of zinc oxide nanoparticles onto carbon fibers as composite filaments for infrared heaters Chien-Te Hsieh a,∗, Dong-Ying Tzou a, Ze-Shien Huang a, Jo-Pei Hsu a, Chi-Yuan Lee b a b
Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan 32003, Taiwan Department of Mechanical Engineering, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 7 June 2016 Revised 17 October 2016 Accepted 7 December 2016 Available online 24 December 2016 Keywords: Carbon fiber Nano-structures Thermal properties Thermal analysis
a b s t r a c t This study examines the thermal properties of ZnO-coated carbon fiber (CF) filaments for highperformance infrared (IR) heaters. An efficient pulse microwave (PM) method is applied to deposit different morphologies of ZnO crystals at various pH values of Zn2+ solutions, i.e., fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). Under the applied voltage of 25 V, the CF composite filament, prepared by the PM route at pH = 10, offers the highest thermal radiation power and superior heat-storage capability among these IR heaters. Its saturation temperature and heating rate of ZnO-coated CF heater can reach to 184 °C and 28 °C/min, respectively. The enhanced thermal performance can be ascribed to the facts (i) the ZnO crystals create more emissive surface area and (ii) the composite filament illuminate more homogeneous spectral IR rays, showing the synergetic effect. © 2016 Published by Elsevier B.V.
1. Introduction Infrared (IR) heaters have been recognized as a promising heating source in thermal treatment and drying operation, such as dehydration of fibrils and paints [1], synthesis of metallic nanoparticles [2], ignition of chemical reaction on microchips [3], sintering of cathode materials for Li-ion batteries [4], inactivation of bacterial spores [5], and so on. Theoretically, IR ray is an electromagnetic radiation with wavelength between visible light and microwave radiation [6]. It is generally known that the IR ray can be divided into three regions: IR-A (0.7–1.4 μm), IR-B (1.4–3 μm), and IR-C (3–10 0 0 μm), based on its wavelength. The wavelength range and thermal radiation efficiency of IR heaters basically depend on the filament of emitting elements. So far, two commonlyused types of IR filaments dominate the practical applications, i.e., ceramic and carbon fiber (CF). The IR heaters equipped with ceramic (e.g., alumina) illuminate the IR ray with a wavelength region of 3–18 μm, whereas the wavelength of IR heaters fabricated with CFs falls into the IR-B region. Therefore, the IR radiation of CF heaters is ideally matched to the spectrum of water absorption. This finding reveals that the IR radiation is capable of penetrating the skin to subcutaneous tissues, transforming the light energy into thermal energy. This heat transfer induces an improvement in blood circulation and a reduction of toxins from the human body
∗
Corresponding author. E-mail address: [email protected] (C.-T. Hsieh).
http://dx.doi.org/10.1016/j.surfin.2016.12.001 2468-0230/© 2016 Published by Elsevier B.V.
by sweat production [6]. Accordingly, the CF heater emits rays that are beneficial for human’s health and healing. However, improving the thermal efficiency of CF filaments is still a challenge and rarely discussed in literatures. Traditionally, commercial IR heaters are fabricated by using CF as filament, which is carefully sealed in a quartz tube after vacuum suction process. The power consumption of IR heaters strongly depends on the heater size and applied voltage. One strategy concerning an improved thermal efficiency is to directly modify the CF filaments. The CF filaments are typically composed of thousands of carbon fibers, having an average diameter of 8–10 μm. The purpose of this work is to deposit metal oxide nanoparticles (i.e., zinc oxide (ZnO)) onto the surface of CFs. It is expected that (i) the ZnO-coated CF filament emit irradiation with a wider wavelength range and (ii) the decoration of ZnO nanostructures facilitates heating surface area, thus, imparting an improved thermal efficiency of IR heaters. In fact, ZnO materials have received considerable attentions in a variety of applications such as photocatalysts, gas sensors, solar cells, transparent conductors, and piezoeletronic materials due to their excellent electronic, mechanical, and optical performance [7–10]. Many efforts have been made to synthesize ZnO structures such as spray pyrolysis [11], electrochemical deposition [12–15], hydrothermal method [16], and template electrosynthesis [17]. More recently, our previous work has proposed a novel perspective to rapidly synthesize ZnO crystals, using a pulse microwave (PM) method [18]. The PM method displays a great potential to deposit ZnO crystals onto different carbon supports (e.g., carbon nanotubes and carbon black) at low
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
99
temperatures (e.g., 80 °C). The PM method only takes a short period of 8–12 min, showing a commercial feasibility. Within the above scope, this present work adopts the PM method to coat ZnO layers onto the CFs, forming a composite filament. The pH value of Zn2+ solution was chosen as a controlling factor in affecting the morphology of ZnO structures. The composite IR filaments were assembled as IR heaters, operated at a fixed applied potential. The heating rate and maximal heating temperature were systematically investigated. This work would shed some lights on how the introduction of ZnO crystals on CF filament enhances the thermal radiation efficiency of IR heaters. 2. Experimental In the present work, commercial CF, made from polyacrylonitrile (PAN) precursor, served as heating filament for IR heaters. Each CF filament consisted of a bundle of individual fiber, and total number of fiber was approximately 30 0 0. Each CF filament was carefully cut into a length of 20 cm. Before the deposition of ZnO, the CF filaments were impregnated in distilled water and placed in an ultrasonic bath for 0.5 h. Then the wetted CF filaments were dried at 105 °C in a vacuum oven overnight. Herein an aqueous Zn2+ solution was prepared for the ZnO deposition by the PM method, in which an optimal synthesis conditions have been reported previously [18]. The Zn2+ solution was composed of 2 M (CH3 COO)2 Zn·2H2 O solution and distilled water. The pH value of the solution was adjusted to pH = 9, 10, 11, and 12, respectively, by using 0.5 M KOH. Afterward, the cleaned CF was impregnated into the Zn-containing solution (volume: 50 mL) at ambient temperature for 2 h. Then the CF slurries were placed in the center of household microwave oven (Tatung Co., 900 W, 2.45 GHz). The microwave oven was equipped with a thermocouple and temperature controller. The reaction temperature program started from 25 to 80 °C with a heating rate of 5 °C/min. The PM deposition temperature was maintained at 80 °C for 8 min. The power-on and power-off periods were set at 3 s and 3 s, respectively. After the PM deposition, the treated CF filaments were dehydrated at 105 °C in a vacuum oven overnight. The microstructural observation of the resulting ZnO deposits onto CF samples was characterized by a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6701F). The crystalline structure of ZnO crystals was examined by X-ray diffraction (XRD) with Cu-Kα radiation, using an automated X-ray diffractometer (Shimadzu Labx XRD-60 0 0). The thermal efficiency of IR heaters was investigated in a quartz tube with an inner diameter of 3 cm and a length of 20 cm. The CF filaments could be fixed in the center of quartz tube by using fixtures. One thermal couple (Ktype) was also equipped in the center of quartz tube to detect the real temperature of IR heaters. A galvanostat-potentiastat instrument was adopted to apply stable potential difference between both ends of CF filaments. The potential differences were set at 25 V in the present work. To avoid any oxidation on CFs, a vacuum pump was used to ensure the low-pressure operation of IR heaters, i.e., the operating pressure <0.001 torr. 3. Results and discussion The crystallographic structure of ZnO-coated CFs was inspected using XRD analysis, as shown in Fig. 1. The XRD patterns confirm the presence of well-crystalline ZnO, which is good agreement with wurtzite structure [19,20]. The result reflects that the PM method is capable of growing ZnO wurzite crystals at low temperature of 80 °C. Our preliminary studies also confirm the lowtemperature growth of ZnO crystals on graphene sheets [21] and metal oxides [22] under microwave irradiation. This can be attributed to the fact that PM synthesis induces a dipole change in
Fig. 1. Typical XRD patterns for ZnO-coated CF filaments prepared by the PM method at different pH values.
Fig. 2. FE-SEM image of original CFs.
polar molecules (e.g., water and hydration molecules) with uniform temperature distribution, thus leading to highly crystalline ZnO [23]. SEM photo of original CF sample in Fig. 2 reflects that an individual fiber possesses a smooth surface. Each fiber has an average diameter of 8 μm. To inspect the dispersion of ZnO over CF filaments, SEM was also adopted to observe the morphology of ZnOcoated CF samples, as depicted in Fig. 3(a)−(d). The SEM images clearly show the CFs decorated with different topographies of ZnO crystals, indicating the importance of pH value on the formation of ZnO crystals, e.g., fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). This finding reveals that the pH value plays a vital role in determining the shape formation of ZnO crystals during the PM process. A growth mechanism of ZnO crystals onto CFs under microwave irradiation has
100
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
Fig. 3. FE-SEM images for ZnO-coated CF filaments prepared the PM method at different pH values: (a) 9, (b) 10, (c) 11, and (d) 12.
been proposed in literature [19]. First of all, the Zn2+ ions would be physically adsorbed on the surface of CFs, originated from an interaction between ions and oxygen groups (e.g., carboxylic group). Pioneering study has pointed out the chemical reactions of ZnO crystals synthesized from a zinc precursor as follows [20]:
Zn2+ + 2OH− → Zn(OH)2 → ZnO + H2 O
(R1)
Zn2+ + 4OH− → Zn(OH)4 2+ → ZnO + H2 O + 2OH−
(R2)
Herein the chemical reaction (R1) takes place in the presence of KOH solution at initial stage. The intermediates, Zn(OH)2 species, can be formed, and followed by fast growth of ZnO crystals on CFs with the aid of microwave irradiation. It is worth noting that the reaction (R2) could initialize to occur at higher pH value because the Zn(OH)4 2+ species are able to be formed in the presence of excessive OH− ions [20,24,25]. Accordingly, the PM route at high pH value (e.g., pH = 12) could produce a large number of nuclei, thus, forming ZnO nanoparticles over the surface of CFs. To clarify, the as-prepared ZnO-coated fibers are designated to ZnO–CF9, ZnO–CF10, ZnO–CF11, and ZnO–CF12, according to the pH value of Zn2+ solutions (i.e., pH = 9, 10, 11, and 12), respectively. The variation of temperature with time for all IR heaters is illustrated in Fig. 4. It can be seen that all IR heaters reach their saturation temperatures after 8 min. However, the IR heaters possess various thermodynamic performances such as heating rates and maximal temperatures. The thermal-equilibrium temperature shows an order as 184 °C (ZnO–CF10) >175 °C (ZnO–CF11) >170 °C (ZnO–CF12) >132 °C (ZnO–CF9) >109 °C (original CF). The ZnO–CF10 heater also exhibits the highest heating rate of 28 °C/min, among these CF heaters. Under this operating condition, there is an increase of ∼70% in the heating rate as compared with the CFs without the decoration of ZnO crystals. The influences of pH value of Zn2+ solution on both the maximal temperature and
Fig. 4. The temperature kinetic curves of temperature versus time for all IR heaters.
the heating rate are depicted in Fig. 5. As observed from this figure, the ZnO–CF10 heater is capable of showing the thermal conversion efficiency, i.e., the maximal temperature and the highest heating rate. This finding reveals two facts: (i) the deposition of ZnO crystals shows the positive effect on the thermal efficiency of IR heaters and (ii) the morphology of ZnO crystals on the CFs is very vital in affecting the thermal efficiency of ZnO-coated CF
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
101
Fig. 7. The thermal-imaging photographs of (a) original CF and (b) ZnO–CF10 filaments.
Fig. 5. The saturation temperature and heating rate as a function of pH value of Zn2+ solution.
Fig. 8. The emissive power and heat capacity as a function of pH value of Zn2+ solution.
Fig. 6. The saturation temperature as a function of heating/cooling cycle number for all IR heaters.
filaments. For ZnO–CF10 heater, an increase of 69% in the maximal surface temperature can be observed as compared with original CF heater. This improved thermal emissive efficiency can be attributed to the fact that the addition of ZnO crystals over the CFs creates more emissive surface sites, facilitating not only the maximal temperature but also the heating rate. To evaluate the durability, a heating/cooling cyclic test was carried out on the ZnOcoated CF composite filaments. The thermal-equilibrium temperature as a function of cycle number is illustrated in Fig. 6. The operating conditions of the stability test are identical with the experimental section. As observed from this figure, the ZnO-coated CF filaments basically show a stable performance after cycling, implying good adhesion between the ZnO deposits and the CF matrix. Fig. 7 shows the thermal-imaging photographs of original CF
and ZnO–CF10 filaments while the IR heaters reach their saturation temperatures, showing different temperature distributions. The photos reflect that the ZnO–CF10 filament exhibits higher emissive intensity than original CF one, indicating different spectral characters. This finding reveals that the deposition of ZnO is able to improve the radiation ability of CF filament, originated from the spectral-combined (or synergistic) effect between CF (IR-B ray) and ZnO deposits (IR-C ray). To characterize the thermal efficiency of CF heaters, the radiation power (Qir ) for all CF heaters could be evaluated based on Stefan-Boltzmann law [25].
Qir = A ε σ T 4
(1)
where A is the surface area of radiating body (i.e., the CF filaments in this case), ε is the emissivity of radiating body, σ is the StefanBoltzmann constant (= 5.6704 × 10−8 W/m2 K), and T is surface absolute temperature. The emissive power of IR heaters, Qir , is basically proportional to the fourth power of its temperature, according to Eq. (1). Herein the ε value of CF filaments is approximately 0.98, i.e., grey body. The calculated Qir values as a function of pH value of Zn2+ solution is illustrated in Fig. 8. Under the operat-
102
C.-T. Hsieh et al. / Surfaces and Interfaces 6 (2017) 98–102
ing condition, the ZnO–CF10 heater exhibits the highest Qir value of 47.1 W among the IR heaters. The presence of ZnO crystals on the IR heaters enhances the Qir value by a factor of two, as compared with the pristine CF heater, i.e., 23.0 W. This enhancement can be attributed to two reasons. First, the introduction of ZnO crystals provides large emissive surface area to irradiate IR light. Second, the CFs and ZnO crystals could illuminate different spectral IR lights (i.e., IR-B and IR-C region), showing the coupling effect. It is worth noting that the ZnO–CF10 filament is composed of core-shell composite fibers, capable of illuminating higher intensity IR ray than the other ZnO morphologies such as fragments (Qir : 29.1 W), bulky islands (Qir : 43.5 W), and nanoparticles (Qir : 41.6 W). This is presumably due to one reason that the core-shell composite filament is able to radiate IR light uniformly with a wider spectral region, whereas the other three IR heaters could not illuminate more homogeneous IR rays from the CF filaments with an incomplete ZnO coating. The heat capacity (Cp ) of CF filaments could be estimated by using the following formula:
Cp = Qir /m T
(2)
where m and T represent the weight of CF filament and the temperature difference between initial and saturation temperature, respectively. The variation of Cp value with the pH value of Zn2+ solution is also depicted in Fig. 8. Again, the core-shell CF filament (i.e., pH = 10) enables the highest heat capacity of 2.73 J/g K among the CF heaters. This result reflects that the core-shell filament displays superior heat-storage capability. The presence of ZnO thin film is believed to accumulate heat and to prevent heat dissipation from the CF surface, thus raising the heat-storage ability. On the basis of the experimental results, the ZnO–CF10 heater exhibits a commercial feasibility for high-performance IR devices due to its high heating rate, high irradiation power, low energy consumption, and excellent heat-storage capability. 4. Conclusions The IR heaters fabricated with CF filaments decorated with different morphologies of ZnO crystals have been confirmed to exhibit high heating rate, high thermal radiation power and low energy consumption. The PM method was adopted to deposit ZnO crystals onto the surface of CFs at low temperature. The pH value of Zn2+ solution was picked up as the controlling factor in affecting the morphology of ZnO crystals, including fragments (pH = 9), thin films (pH = 10), bulky islands (pH = 11), and nanoparticles (pH = 12). Based on the operating condition, the saturation temperature and heating rate of ZnO–CF10 heater could attain as high as 184 °C and 28 °C/min, respectively. The enhanced thermal performance could be attributed to the facts (i) the ZnO crystals provide large emissive surface area and (ii) the composite filament illuminate more homogeneous spectral IR rays, resulting from the coupling effect. The CF composite filament, prepared by the PM route at pH = 10, exhibited the highest thermal emissive power and superior heat-storage capability. This improved performance was presumably due to the formation of core-shell structured filaments, capable of accumulating heat and preventing the heat dissipation from the CF surface. Accordingly, the robust design of ZnO-coated CF composite could serve as a potential filament for IR heaters in industrial applications because of its low price, high performance, and easy fabrication.
Acknowledgments The authors are very grateful for the financial support from the Ministry of Science and Technology (Taiwan) under the contracts MOST 105-2218-E-155-007 and MOST 105-2622-E-155-011-CC2. References [1] M. Pettersson, S. Stenström, Modeling of an electric IR heater at transient and steady state conditions Part II: modelling a paper dryer, Int. J. Heat Mass Transfer 43 (20 0 0) 1223–1232. [2] J.H. Jung, H.C. Oh, H.S. Noh, J.H. Ji, S.S. Kim, Metal nanoparticle generation using a small ceramic heater with a local heating area, J. Aerosol Sci. 37 (2006) 1662–1670. [3] M.N. Slyadnev, Y. Tanaka, M. Tokeshi, T. Kitamori, Photothermal temperature control of a chemical reaction on a microchip using an infrared diode laser, Anal. Chem. 73 (2001) 4037–4044. [4] C.T. Hsieh, Y.F. Chen, C.T. Pai, C.Y. Mo, Synthesis of lithium nickel cobalt manganese oxide cathode materials by infrared induction heating, J. Power Sources 269 (2014) 31–36. [5] D. Hamanaka, T. Uchino, N. Furuse, W. Han, S.I. Tanaka, Effect of the wavelength of infrared heaters on the inactivation of bacterial spores at various water activities, Int. J. Food Microbiol. 108 (2006) 281–285. [6] K. Witek, T. Piotrowski, A. Skwarek, Analysis of polymer foil heaters as infrared radiation sources, Mater. Sci. Eng. B 177 (2012) 1373–1377. [7] M. Guo, P. Diao, S. Cai, Highly hydrophilic and superhydrophobic ZnO nanorod array films, Thin Solid Films 515 (2007) 7162–7166. [8] L. Huang, S.P. Lau, H.Y. Yang, E.S.P. Leong, S.F. Yu, Stable superhydrophobic surface via carbon nanotubes coated with a ZnO thin film, J. Phys. Chem. B 109 (2005) 7746–7748. [9] B. Xu, Z. Cai, Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic, Appl. Surf. Sci. 254 (2008) 5899–5904. [10] B. Ingham, B.N. Illy, M.P. Ryan, In situ synchrotron studies of ZnO nanostructures during electrochemical deposition, Curr. Appl. Phys. 8 (2008) 455–458. [11] R.D. Sun, A. Nakajima, A. Fujishima, T. Watanabe, K. Hashimoto, Photoinduced surface wettability conversion of ZnO and TiO2 thin films, J. Phys. Chem. B 105 (2001) 1984–1990. [12] X. Gan, X. Gao, J. Qiu, X. Li, Growth and characterization of ZnO-SDS hybrid thin films prepared by electrochemical self-assembly method, Appl. Surf. Sci. 254 (2008) 3839–3844. [13] S. Wei, J. Lian, X. Chen, Q. Jiang, Effects of seed layer on the structure and property of zinc oxide thin films electrochemically deposited on ITO-coated glass, Appl. Surf. Sci. 254 (2008) 6605–6610. [14] J.S. Wellings, N.B. Chaure, S.N. Heavens, I.M. Dharmadasa, Growth and characterization of electrodeposited ZnO thin films, Thin Solid Films 516 (2008) 3893–3898. [15] C Tazlaoanu, L Ion, I Enculescu, M Sima, M Enculescu, E Matei, R Neumann, R Bazavan, D Bazavan, S Antohe, Transport properties of electrodeposited ZnO nanowires, Phys. E 40 (2008) 2504–2507. [16] D.C. Olson, S.E. Shaheen, R.T. Collins, D.S. Ginley, The effect of atmosphere and ZnO morphology on the performance of hybrid poly(3-hexylthiophene)/ZnO nanofiber photovoltaic devices, J. Phys. Chem. C 111 (2007) 16670–16678. [17] M. Lai, D.J. Riley, Templated electrosynthesis of zinc oxide nanorods, Chem. Mater. 18 (2006) 2233–2237. [18] C.T. Hsieh, J.S. Lin, Y.F. Chen, C.Y. Lin, W.Y. Li, Graphene sheets anchored with ZnO nanocrystals as electrode materials for electrochemical capacitors, Mater. Chem. Phys. 143 (2014) 853–859. [19] N.F. Hamedani, A.R. Mahjoub, A.A. Khodadadi, Y. Mortazavi, Microwave assisted fast synthesis of various ZnO morphologies for selective detection of CO, CH4 and ethanol, Sens. Actuators B 156 (2011) 737–742. [20] X. Pu, D. Zhang, X. Yi, X. Shao, W. Li, M. Sun, L. Li, X. Qian, Rapid chemical synthesis and optical properties of ZnO ellipsoidal nanostructures, Adv. Powder Technol. 21 (2010) 344–349. [21] C.T. Hsieh, C.Y. Lin, Y.F. Chen, J.S. Lin, Synthesis of ZnO@graphene composites as anode materials for lithium ion batteries, Electrochim. Acta 111 (2013) 359–365. [22] C.T. Hsieh, B.S. Chang, J.Y. Lin, R.S. Juang, Improvement of rate capability of spinel lithium titanate anodes using microwave-assisted zinc nanocoating, J. Alloys Compd. 513 (2012) 393–398. [23] C.T. Hsieh, D.Y. Tzou, C.C. Chen, C.Y. Lai, Y.Y. Liu, Deposition and super liquid repellency of fluorinated ZnO nanoparticles on carbon fabrics, Surf. Coat. Technol. 22 (2012) 4639–4644. [24] Y. Cao, B. Liu, R. Huang, Z. Xia, S. Ge, Flash synthesis of flower-like ZnO nanostructures by microwave-induced combustion process, Mater. Lett. 65 (2011) 160–163. [25] R. Sharma, K.K. Kar, Carbon nanotube coated carbon fiber based composite filaments for luminescent bulbs, Mater. Lett. 137 (2014) 150–152.