1Shao Fu Chang, 1Mann Juin Kao, 1Chin Guo Kuo, 2Ker Jer Huang, 3Chien Chon Chen*
1Department of Industrial Education, National Taiwan Normal University, Taipei, Taiwan
2Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan
3Department of Energy Engineering, National United University, Miaoli, Taiwan
Abstract
This research provides a systematic way to study and better understand the relationship among AAO process parameters (such as voltage, time, and solution), performance measure (such as pore size and thickness) and thermal insulation effectiveness.
In addition, the use of vacuum and coating of AAO film for thermal insulation is an unexplored and promising area. Design of anodization molds and fabrication process receipts for industry scale AAO film production is novel. This research measure heat transfer coefficients at the nano scale, which is an important and relatively unexplored problem in the area of nano heat transfer research.
Introduction
Recent estimates by several research institutes suggest that by the year 2015, $1 trillion worth of products worldwide will incorporate nanotechnology in key functional components. Roco [1] identifies four generations of nanotechnology products: passive nanostructures (circa 2000), active nanostructures (circa 2005), systems of nanosystems (circa 2010), and molecular nanosystems (circa 2015-2020).
During the first generation, the emphasis was more on discovery and production of nanostructures. In the second generation, the focus has shifted towards devices and complex nanosystems. Nanostructures play a fundamental role in nanotechnology research, especially in the development of nanosystems. In addition, nanostructures such as nanotubes and nano film have also played an important role in the applied research community. For example, TiO2 film has
Ceramics are suitable for applications requiring wear resistance, high temperature strength, electrical or thermal insulation or other specialized characteristics
[6-8]. Due to these characteristics, AAO has been used primarily as a template to fabricate other nanostructures, including nanotubes [9] nanorods [10], nanowires [11], wiskers [12], and nanospheres [13]. However, AAO also has excellent potential for more general use as thermal insulation material.
According to statistics from the U.S. Department of Energy, the energy spent annually on buildings is estimated to be 38% of the U.S. total energy consumption. A typical U.S. family spends $1,300 a year on home energy, of which about half is wasted.
An effective solution to reduce heating and cooling costs while also making the home more comfortable is to properly envelop the house with insulation materials, which will provide home resistance to heat flow [14]. The ideal insulation material should be environmentally friendly and biocompatible in terms of both its manufacture and the material itself. It should also be lightweight, moisture-resistant, and heat resistant. Nanotube-based insulation materials are a promising alternative to existing insulation materials. They are extremely lightweight, even in large volumes, and are very moisture and heat resistant. If filled with a vacuum, they can provide excellent insulation characteristics. Based on the unique thermal insulation properties of fullerenes, as well as proprietary deposition techniques using thin layers of reflective material as a support, a novel type of multi-layer vacuum insulation based on carbon nanomaterials has been proposed [15]. Fabricated samples of fullerene-based insulation were shown to possess R-values of 36 to 40 per inch of thickness at cryogenic temperatures, which considerably exceeds those of commonly available insulation materials such as polyurethane (R6.3), expanded polystyrene (R3.8), and even vacuum insulated panels (R9-24).
Application of such insulation could result in significant size and weight reduction while maintaining cost-effectiveness. However, carbon nanotubes are not biocompatible due to their lung toxicity and cytotoxicity properties [16]. In contrast, anodic aluminum oxide (AAO)-based nanotubes are biocompatible and non-toxic. AAO (also known as Al2O3) is a kind of ceramic that has a high melting
75 point and hardness. Ceramics are suitable for applications requiring wear resistance, high temperature strength, electrical or thermal insulation or other specialized characteristics [6]. Fan et al. [17]
have shown that different average alumina particle sizes can affect the thermal conductivities of polymer-alumina composite samples. At a fixed loading level of 50% wt, the thermal conductivity kept increasing with increased particle size. When half of the larger alumina particles (AlO μm and AlO mesh) were replaced with the same weight of nano-sized alumina (AlO nm), the overall thermal conductivity of the composite samples was reduced, especially for the largest particles studied (AlO mesh). This can be attributed to fewer numbers of larger size particles as well as many more interfaces introduced along the conduction paths, resulting in a decrease in effective thermal conductance. The above research suggests that AAO-based nanotubes would make excellent insulation materials. The issue, then, is how to develop sufficient quantities of AAO-based insulation for industrial applications.
Experimental procedural
Aluminum foil (99.7%) was put inside the electrochemical holder forming AAO through electrolyte polishing and anodization in electrochemical bath. As described in Chen [11-13], AAO film can be fabricated using an anodization process. The fabrication processes for 15 nm AAO template consists of the following steps:
(i) Polish the aluminum (Al) substrate (99.7%); then anneal in an air furnace at 550℃.
(ii) Electro-polish the substrate in a bath consisting of HClO4, C2H6O, and CH3(CH2)3OCH2CH2OH with 42 V DC for 10 minutes.
(iii) First anodization – Polish the Al substrate with 18 V DC in H2SO4 solution for 20 minutes.
(iv) Remove the first anodization film by soaking in a solution of CrO3 and H3PO4 for 40 minutes.
(v) Second anodization – Repeat anodization using the solution from the first anodization, but for a longer time (several hours) to form AAO films of varying thickness.
(vi) Remove Al substrate by soaking in a solution of CuCl2 and HCl for 30 minutes. widening time is 200 min (step vii), that 300 nm pore
size of AAO can be formed.
This section describes a series of experiments that will be used to evaluate insulator efficiency and heat transfer coefficient in the nanotube. The effectiveness of an insulator is indicated by its R (resistance) value R=d/k, where k is conduction coefficient, d is the thickness of the insulator, and the unit of R is K·m²/W. When heat transfer occurs in a vacuum through radiation, the heat radiation can be expressed by the Stefan-Boltzmann law:
P=eσA(T4-TC4), where P is net radiated power, e is emissivity (=1 for ideal radiator), σ is the Stefan constant (5.6703 × 10-8 W．m-2．K-4), A is radiating area, T is the temperature of the radiator, and TC is the surrounding temperature. For the proposed research, we designed a simple apparatus to measure insulator efficiency and heat transfer coefficient within a nanotube (shown in Figure 1). A “heating” apparatus will be used to control cooling temperature, and insulators will prevent temperature from being conducted to the chamber walls; so that a temperature gradient will form on the sides of the test sample.
Figure 1 schematic diagram of a simple apparatus to measure insulator efficiency and heat transfer coefficient
Results and Discussion
Figure 2 showed AAO SEM images when Al foil (#1070) through anodization (1vol. % H3PO4，1
℃，195V). the morphology of AAO top view with 450 nm pore diameter, 50 nm pore wall thickness, 4.5×108 pore‧cm-2 pore density, and 62% porosity (Fig. 2(a))，bottom view after remove barrier layer has 400 nm pore diameter (Fig. 2(b)), and a straight
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(a)
(b)
(c)
tube with semi-cup struccture of barrier layer on the bottom (Fig. 2(c)).
Figure 2 SEM images of AAO (a) top view, (b) bottom vied, and (c) side view.
Heat transfer occurs through conduction, convection, or radiation. Since heat conduction and heat convection efficiency decreases as vacuum values increase, vacuum-filled tubes may have heat insulation characteristics. Thermal insulators are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both.
Radiant barriers are materials that reflect radiation and therefore reduce the flow of heat from radiation sources. In order to decrease the electromagnetic radiation in the vacuum tube, the inner pore wall is usually coated with a metal film for shining and smoothing the wall surface. Based on the above concepts, AAO may serve as a suitable insulating material for heat transfer when nanotubes are placed in a vacuum and covered with an oxide film on the pore surface.
Based on previous experiments, we know that AAO thickness can be fabricated from 1 to 150 μm.
When AAO samples of various thickness are inserted in the equipment shown in Figure 1, a conduction coefficient (k) can be obtained. Furthermore, to control vacuum values in an AAO nanotube, the heat radiation in the vacuum nanotube can be evaluated.
Since the AAO is a ceramic thin film, it can be combined with a glass plate or epoxy to form a composite material. The compound resistance in series is evaluated as:
) radiant barrier of metal film. Because AAO has small pore diameters and a high aspect ratio, coating the inner wall with a metal film using a vaporization process can be difficult. However, coating may be achieved by using an electroless deposition process. There are some metals—for example, silver (Ag), copper (Cu), and nickel (Ni)—that are used to coat metal, glass, plastic, or ceramic substrates for decoration or conduction applications in industry.
Because Ni has lower thermal conductivity (91 W/mk) than Ag (430 W/mk) and Cu (400 W/mk), it is suitable for use as a radiant barrier by an electroless deposition process.
Acknowledgment
The authors gratefully appreciate the financial support of the National Science Council of ROC under the contract No.101-2627-M-239-001-, 101-3113-S-262-001-, and Chung-Shan Institute of Science and Technology (CSIST) under the contract No.(CSIST-442-V202).
References
[1] M.C. Roco, Journal of Nanoparticle Research 7 (2005)707.
[2] C.C. Chen, W.D. Jehng, L.L. Li, .W.G. Diau, Journal of The Electrochemical Society, 156 (2009)C304-C312.
[3] C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, C.M. Lan, S.F. Chen, L. Luo, C.S. Hung, W.G.
Diau, J. Phys. Chem. C 112 (2008)19151.
[4] P.D. Cojocaru, C.S. Xavier, L.S. Hoon, Digest of Technical Papers - SID International Symposium 38 (2007)1409.
[5] J.F. Novak, R.L. Fink, Z.Yaniv, Proceedings of the 12th International Display Workshops in Conjunction with Asia Display 2005 (2005) 257.
77 Acta Metallurgica Sinica 18 (2005) 249.
[9] Z. Fang, Y. Wang, X. Peng X. Liu; C. Zhen, Materials Letters. 57, 4187 (2003).
[10] J.P. Tu,; C.X. Jiang,; S.Y. Guo,; L.P. Zhu,; F.M.
Fu,; X.B. Zhao, Surface and Coatings Technology, 198 (2005) 464.
[11] C.C. Chen, Y. Bisrat, Z.P. Luo, R.E. Schaak, C.G.
Chao, D.C. Lagoudas, Nanotechnology, 17 (2006) 367.
[12] W.C. Say, C.C. Chen, Jpn. J. Appl. Phys., 46 (2007) 7577.
[13] S.H. Chen, C.C. Chen, Z.P. Luo, C.G. Chao, Materials Letter, 63 (2009) 1665.
[14] U.S. Department of Energy web site (9/15/2007):
http://www.eere.energy.gov/consumer/your_hom e/insulation_airsealing/index.cfm/mytopic=1132 0.
[15] E. M. Wexler, R. O. Loutfy, 2004 Inter Society Conference on Thermal Phenomena, (2004) 570.
[16] S.K. Smart, A.I. Cassady, G.Q.Lua, D.J.Martina, Carbon 44 (2006) 1034.
[17] L. Fan, B. Su, J. Qu, C.P. Wong, 9th Int’l Symposium on Advanced Packaging Materials, (2004) 193.
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