Materials Science and Engineering C 33 (2013) 99–102
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Nacre biomimetic design—A possible approach to prepare low infrared emissivity composite coatings Weigang Zhang ⁎, Guoyue Xu, Ruya Ding, Kaige Duan, Jialiang Qiao College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiang Jun Road 29, Nanjing 211106, China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 12 July 2012 Accepted 9 August 2012 Available online 18 August 2012 Keywords: Nacre biomimetics Composite coatings Infrared emissivity One-dimensional photonic structure
a b s t r a c t Mimicking the highly organized brick-and-mortar structure of nacre, a kind of nacre-like organic–inorganic composite material of polyurethane (PU)/flaky bronze composite coatings with low infrared emissivity was successfully designed and prepared by using PU and flaky bronze powders as adhesives and pigments, respectively. The infrared emissivity and microstructure of the coatings were systematically investigated by infrared emissometer and scanning electron microscopy, respectively, and the cause of low infrared emissivity of the coatings was discussed by using the theories of one-dimensional photonic structure. The results show that the infrared emissivity of the nacre-like PU/flaky bronze composite coatings can be as low as 0.206 at the bronze content of 60 wt. %, and it is significantly lower than the value of PU/sphere bronze composite coatings. Microstructure observation illustrated that the nacre-like PU/flaky bronze composite coatings have similar one-dimensional photonic structural characteristics. The low infrared emissivity of PU/flaky bronze composite coatings is derived from the similar one-dimensional photonic structure in the coatings. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bivalve nacre, a biological organic–inorganic composite material, has received considerable attention due to its special microstructure, excellent mechanical and optical properties [1–3]. Mimicking the highly organized brick-and-mortar structure of nacre, many kinds of nacre-like composite materials such as bulk nano-laminar composites, montmorrillonite/polyimide nano-composites, α-zirconium hydrogen phosphate hydrate/chitosan multi-layered films, and TiC/metal multilayers with excellent mechanical properties were fabricated [4–7]. However, from our knowledge, nacre-like organic–inorganic composite coatings with low infrared emissivity have not been reported. Nacre is periodic stacked by aragonite platelets and organic matrix with different refractive index. It is confirmed that nacre is a onedimensional biological photonic crystal [8], and it shows a strong reflection peak in a specific wavelength range [8,9]. These characteristics of nacre may help us design nacre-like, low infrared emissivity composite coatings. Low infrared emissivity coatings have received extensive attention due to their civil and military applications such as solar thermal collectors and infrared stealth materials in recent years [10–13]. According to the Kirchhoff's law [14] and Principle of Conservation of Energy, the relationship between the infrared emissivity (ε) and
reflectivity (r) of non-transparent material such as organic–inorganic composite coatings can be expressed as: ε ¼ 1−r:
ð1Þ
As shown in Eq. (1), the reflectivity has a decisive effect on the infrared emissivity of organic–inorganic composite coatings. According to these features, low infrared emissivity composite coatings can be designed inspired from the structure of nacre. In this paper, nacre-like PU/flaky bronze composite coatings with low infrared emissivity were designed and prepared. The infrared emissivity of the coatings was systematically studied, and the cause of low infrared emissivity of the coatings was discussed by using the theories of one-dimensional photonic structure. 2. Experimental 2.1. Materials Bronze powders (purity 99 wt. %) and PU (liquid, solid content 70 wt. %) were purchased from Nanjing Chemical Agent Limited Company, China. All reagents were analytical grade and were used as received without further treatment. 2.2. Preparation of composite coatings
⁎ Corresponding author. Tel.: +86 25 84892903; fax: +86 25 84892951. E-mail address: [email protected] (W. Zhang). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.012
Steel substrate (10 cm × 5 cm; thickness, 0.3 mm), properly cleaned by ethanol and distilled water, respectively, was used as the
100
W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102
coating substrate to measure the emissivity changes of various coatings. Firstly, fixed amounts of PU and bronze powders with different ratios (8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2.5:7.5, W/W) were mixed together under continuous ultrasonication for 10 min. Then the mixture was painted onto the substrates by spray technique (98PSI, Japan) using an accurate speed motor (980 r/min) and appropriate pressure (0.4–0.5 MPa). The spray gun is perpendicular to the substrate, and the distance between spray gun and substrate is 20–25 cm. During spraying, the ambient temperature is 20–25 °C, the carrier gas is air and the humidity of the air is 40–50%. The coating thickness is controlled about 40 μm. The different thickness of pure PU coating can be controlled by the spraying times. Finally the coatings were solidified completely after curing for 24 h at 50 °C and kept for further characterization. In all cases, duplicate experiments were carried out to ensure reproducibility.
2.3. Characterization Infrared emissivity values at the wavelength of 8–14 μm were measured by IR-2 infrared emissometer (Shanghai Institute of Technological Physics, China). The morphology and microstructure of the coatings were observed by scanning electron microscopy (JSM-6360LV). The thickness of coatings was measured by using a digital magnetism thickness instrument (China). The water covering area of flaky bronze powders was defined by water covered area per gram of bronze powders and measured by DIN standard 55923 [15], then the thickness of flaky bronze would be calculated as 0.452 μm by the formula of 1/(bronze density 8.8 g/cm3 * water covering area) [16].
3. Results and discussion 3.1. Effect of bronze content on the emissivity of PU/bronze composite coatings Fig. 1 shows the relationship between bronze content and infrared emissivity of PU/bronze composite coatings. It can be seen that the emissivity of PU/flaky bronze composite coatings is first significantly decreased and then slightly increased with increasing bronze content, and the emissivity approaches the lowest value of 0.206 when the bronze content is about 60 wt. %. Also we can see that the emissivity of PU/flaky bronze composite coatings is apparently lower than the emissivity of PU/sphere bronze composite coatings at any bronze content.
Fig. 1. The emissivity dependence on bronze content of PU/bronze composite coatings.
3.2. Microstructure of PU/bronze composite coatings Fig. 2 shows the SEM images of bronze powders and PU/bronze composite coatings. We can clearly see the differences between flaky bronze powders and sphere bronze powders from Fig. 2a and b. The image of Fig. 2c clearly shows that sphere bronze particles in PU/sphere bronze composite coatings are discretely distributed with relatively large inter-particle distances, and there are many interspaces between the particles, which will contribute to low spectral reflection and high spectral absorption; this may be the main aspect leading to PU/sphere bronze composite coatings with higher infrared emissivity. Also it can be seen from Fig. 2d and e that PU/flaky bronze composite coatings are stacked by PU coated flaky bronze powders, indicating that the coatings have obvious nacre-like multilayered structural characteristics. According to the observed results, the microstructure of PU/flaky bronze composite coatings is confirmed as Fig. 3. It can be seen that the coatings are periodic stacked by PU and flaky bronze powders with different refractive index, revealing that the coatings have similar one-dimensional photonic structural characteristics [8,17]. 3.3. Cause of low infrared emissivity of PU/flaky bronze composite coatings According to the microstructure and interface characteristics of PU/flaky bronze composite coatings, we speculate that the reflection of infrared radiation from the coatings includes three parts (Fig. 3): (1) the reflection from the interface of surface PU layer and air (r1), (2) the reflection from the surface of top flaky bronze layer (r2), and (3) the reflection from the one-dimensional photonic structure in the coatings (r3). In addition, resin has always a high emissivity, so surface resin layer has important influence on the emissivity of the composite coatings, and it will be significantly increased with increasing thickness of the surface resin layer. Then, according to Eq. (1), the infrared emissivity of PU/flaky bronze composite coatings can be expressed as: ε ¼ 1−ðr 1 þ r 2 þ r3 Þ þ Δε
ð2Þ
where Δε is the emissivity increment caused by the surface resin layer of the composite coatings. According to Fresnel's law, r1 can be expressed as: r 1 ðθ ¼ θi Þ ¼
1 n1 cosθi −n2 cosθr 2 1 n2 cosθi −n1 cosθr 2 þ 2 n1 cosθi þ n2 cosθr 2 n2 cosθi þ n1 cosθr
ð3Þ
where n1 and n2 are the refractive indices of air (1.0) and PU (1.52), respectively, θi is the angle of incidence, and θr is the angle of reflection. We use normal emissivity in this paper, so θi = θr = 0°. Then, the value of r1 can be calculated as 0.043. According to Eq. (1) and infrared emissivity (0.44) of pure bronze powders coating, the value of r2 can be calculated as 0.56. In this work, we approximately think that the PU/flaky bronze composite coatings have completely regular one-dimensional photonic structural characteristics for ease of calculation, so we can introduce the photonic bandgap calculation software (Translight) to calculate the reflection spectra for the one-dimensional photonic structure in PU/flaky bronze composite coatings. Then, the value of r3 can be obtained from the reflection spectra. Fig. 4 shows the relationship between the thickness and infrared emissivity of pure PU coating. It can be seen that when the thickness is increased from 0 μm to 40 μm, the emissivity of pure PU coating increased significantly from 0.230 (the emissivity of steel substrate) to 0.907. With successively increasing thickness from 40 μm to 74 μm, the emissivity of pure PU coating increased slightly from 0.907 to 0.934, and always kept at a high value. In order to
W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102
101
Fig. 2. SEM images of bronze powders and PU/bronze composite coatings (bronze content 60 wt. %), (a) flaky bronze powders, (b) sphere bronze powders, (c) PU/sphere bronze composite coatings, (d) surface of PU/flaky bronze composite coatings, and (e) cross-section of PU/flaky bronze composite coatings.
quantitative calculation, the relationship between the emissivity and thickness of pure PU coating was fitted by the method of S-type curve fitting (Fig. 4), and the fitting function can be expressed as: εPU ¼ A2 þ
ðA1 −A2 Þ 1 þ expððx−x0 Þ=dxÞ
ð4Þ
where εPU is the emissivity of pure PU coating, x is the thickness of pure PU coating, parameters A1 = 0.16949, A2 = 0.91881, x0 = 10.46309, and dx= 4.29556. According to Eq. (4), the emissivity increment (Δε) caused by the different thickness of the surface PU layer of PU/flaky bronze composite coatings can be obtained as: Δε ¼ A2 þ
ðA1 −A2 Þ −0:23 1 þ expððl2 −x0 Þ=dxÞ
Fig. 3. Microstructure model of PU/flaky bronze composite coatings.
ð5Þ
where l2 is the thickness of surface PU layer of PU/flaky bronze composite coatings in different bronze content, it can be calculated as the following equation: l1 sρ1 ¼ yl2 sρ2
ð6Þ
where s is the surface area of the composite coatings, y is the ratio of bronze content to PU content, l1 is the thickness of flaky bronze, ρ1 (bronze density 8.8 g/cm3), and ρ2 (PU density 1.073 g/cm3). Fig. 5 shows the calculated reflection spectra of one-dimensional photonic structure in PU/flaky bronze composite coatings (bronze content 60 wt. %). It can be seen with a strong reflection peak at the wavelength of 8–14 μm. According to Fig. 5, the value of r3 at the wavelength of 8–14 μm for PU/flaky bronze composite coatings can be calculated as 0.252.
Fig. 4. The emissivity dependence on thickness of pure PU coating.
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W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102 Table 1 Calculated and measured emissivity of PU/flaky bronze composite coatings with different bronze content.
Fig. 5. Calculated reflection spectra of one-dimensional photonic structure in PU/flaky bronze composite coatings (bronze content 60 wt. %).
When bronze content is 60 wt. %, l2 can be calculated as 2.472 μm according to Eq. (6). Then, according to Eq. (5), Δε (bronze content 60 wt. %) can be calculated as 0.04. At last, according to Eq. (2) and the values of r1 (0.043), r2 (0.56), r3 (0.252) and Δε (0.04), the infrared emissivity of PU/flaky bronze composite coatings (bronze content 60 wt.%) can be calculated as 0.185. Using the same method as described above, the infrared emissivity of PU/flaky bronze composite coatings of other bronze content are calculated too (Table 1). It can be seen that the calculated emissivity is almost consistent with the measured values (Table 1). In addition, Table 1 indicates that the surface PU layer has an important influence on the emissivity of the composite coatings. When the bronze content decreased from 40 wt. % to 20 wt. %, the thickness of surface PU layer will be increased from 5.561 μm to 14.83 μm, leading to a sharp increase of Δε from 0.121 to 0.49 (Table 1), so we can see a sharp decrease of emissivity in PU/flake bronze composite coatings at first three points (Fig. 1). Furthermore, Table 1 can easily explain that the phenomenon of the emissivity of PU/flaky bronze composite coatings is first significantly decreased and then slightly increased with increasing bronze content (Fig. 1). First decrease mainly derived from the sharp decrease of Δε from 0.49 to 0.04 with bronze content increased from 20 wt. % to 60 wt. % (Table 1). Decrease and then increase mainly derived from r2 first increased from 0.183 to 0.252 with bronze content increased from 50 wt. % to 60 wt. %, and then decreased from 0.252 to 0.166 with bronze content increased from 60 wt. % to 70 wt. % (Table 1). 4. Conclusions In summary, nacre-like PU/flaky bronze composite coatings with low infrared emissivity were successfully designed and prepared.
Bronze content
r3
Δε
Calculated emissivity
Measured emissivity
20 30 40 50 60 70 75
0.149 0.151 0.192 0.183 0.252 0.166 0.152
0.490 0.236 0.121 0.068 0.040 0.024 0.018
0.738 0.482 0.326 0.282 0.185 0.255 0.263
0.762 0.450 0.299 0.246 0.206 0.225 0.231
wt. % wt. % wt. % wt. % wt. % wt. % wt. %
Infrared emissivity test results show that the infrared emissivity of nacre-like PU/flaky bronze composite coatings is significantly lower than PU/sphere bronze composite coatings. The low infrared emissivity of PU/flaky bronze composite coatings is derived from the similar one-dimensional photonic structure in the coatings. At last, we confirmed that nacre biomimetic design may be a promising method to prepare low infrared emissivity composite coatings.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China under Grant No. 51173079. We appreciate Andrew L. Reynolds to share the software of translight.
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Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Nacre biomimetic design—A possible approach to prepare low infrared emissivity composite coatings Weigang Zhang ⁎, Guoyue Xu, Ruya Ding, Kaige Duan, Jialiang Qiao College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Jiang Jun Road 29, Nanjing 211106, China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 12 July 2012 Accepted 9 August 2012 Available online 18 August 2012 Keywords: Nacre biomimetics Composite coatings Infrared emissivity One-dimensional photonic structure
a b s t r a c t Mimicking the highly organized brick-and-mortar structure of nacre, a kind of nacre-like organic–inorganic composite material of polyurethane (PU)/flaky bronze composite coatings with low infrared emissivity was successfully designed and prepared by using PU and flaky bronze powders as adhesives and pigments, respectively. The infrared emissivity and microstructure of the coatings were systematically investigated by infrared emissometer and scanning electron microscopy, respectively, and the cause of low infrared emissivity of the coatings was discussed by using the theories of one-dimensional photonic structure. The results show that the infrared emissivity of the nacre-like PU/flaky bronze composite coatings can be as low as 0.206 at the bronze content of 60 wt. %, and it is significantly lower than the value of PU/sphere bronze composite coatings. Microstructure observation illustrated that the nacre-like PU/flaky bronze composite coatings have similar one-dimensional photonic structural characteristics. The low infrared emissivity of PU/flaky bronze composite coatings is derived from the similar one-dimensional photonic structure in the coatings. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Bivalve nacre, a biological organic–inorganic composite material, has received considerable attention due to its special microstructure, excellent mechanical and optical properties [1–3]. Mimicking the highly organized brick-and-mortar structure of nacre, many kinds of nacre-like composite materials such as bulk nano-laminar composites, montmorrillonite/polyimide nano-composites, α-zirconium hydrogen phosphate hydrate/chitosan multi-layered films, and TiC/metal multilayers with excellent mechanical properties were fabricated [4–7]. However, from our knowledge, nacre-like organic–inorganic composite coatings with low infrared emissivity have not been reported. Nacre is periodic stacked by aragonite platelets and organic matrix with different refractive index. It is confirmed that nacre is a onedimensional biological photonic crystal [8], and it shows a strong reflection peak in a specific wavelength range [8,9]. These characteristics of nacre may help us design nacre-like, low infrared emissivity composite coatings. Low infrared emissivity coatings have received extensive attention due to their civil and military applications such as solar thermal collectors and infrared stealth materials in recent years [10–13]. According to the Kirchhoff's law [14] and Principle of Conservation of Energy, the relationship between the infrared emissivity (ε) and
reflectivity (r) of non-transparent material such as organic–inorganic composite coatings can be expressed as: ε ¼ 1−r:
ð1Þ
As shown in Eq. (1), the reflectivity has a decisive effect on the infrared emissivity of organic–inorganic composite coatings. According to these features, low infrared emissivity composite coatings can be designed inspired from the structure of nacre. In this paper, nacre-like PU/flaky bronze composite coatings with low infrared emissivity were designed and prepared. The infrared emissivity of the coatings was systematically studied, and the cause of low infrared emissivity of the coatings was discussed by using the theories of one-dimensional photonic structure. 2. Experimental 2.1. Materials Bronze powders (purity 99 wt. %) and PU (liquid, solid content 70 wt. %) were purchased from Nanjing Chemical Agent Limited Company, China. All reagents were analytical grade and were used as received without further treatment. 2.2. Preparation of composite coatings
⁎ Corresponding author. Tel.: +86 25 84892903; fax: +86 25 84892951. E-mail address: [email protected] (W. Zhang). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.08.012
Steel substrate (10 cm × 5 cm; thickness, 0.3 mm), properly cleaned by ethanol and distilled water, respectively, was used as the
100
W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102
coating substrate to measure the emissivity changes of various coatings. Firstly, fixed amounts of PU and bronze powders with different ratios (8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2.5:7.5, W/W) were mixed together under continuous ultrasonication for 10 min. Then the mixture was painted onto the substrates by spray technique (98PSI, Japan) using an accurate speed motor (980 r/min) and appropriate pressure (0.4–0.5 MPa). The spray gun is perpendicular to the substrate, and the distance between spray gun and substrate is 20–25 cm. During spraying, the ambient temperature is 20–25 °C, the carrier gas is air and the humidity of the air is 40–50%. The coating thickness is controlled about 40 μm. The different thickness of pure PU coating can be controlled by the spraying times. Finally the coatings were solidified completely after curing for 24 h at 50 °C and kept for further characterization. In all cases, duplicate experiments were carried out to ensure reproducibility.
2.3. Characterization Infrared emissivity values at the wavelength of 8–14 μm were measured by IR-2 infrared emissometer (Shanghai Institute of Technological Physics, China). The morphology and microstructure of the coatings were observed by scanning electron microscopy (JSM-6360LV). The thickness of coatings was measured by using a digital magnetism thickness instrument (China). The water covering area of flaky bronze powders was defined by water covered area per gram of bronze powders and measured by DIN standard 55923 [15], then the thickness of flaky bronze would be calculated as 0.452 μm by the formula of 1/(bronze density 8.8 g/cm3 * water covering area) [16].
3. Results and discussion 3.1. Effect of bronze content on the emissivity of PU/bronze composite coatings Fig. 1 shows the relationship between bronze content and infrared emissivity of PU/bronze composite coatings. It can be seen that the emissivity of PU/flaky bronze composite coatings is first significantly decreased and then slightly increased with increasing bronze content, and the emissivity approaches the lowest value of 0.206 when the bronze content is about 60 wt. %. Also we can see that the emissivity of PU/flaky bronze composite coatings is apparently lower than the emissivity of PU/sphere bronze composite coatings at any bronze content.
Fig. 1. The emissivity dependence on bronze content of PU/bronze composite coatings.
3.2. Microstructure of PU/bronze composite coatings Fig. 2 shows the SEM images of bronze powders and PU/bronze composite coatings. We can clearly see the differences between flaky bronze powders and sphere bronze powders from Fig. 2a and b. The image of Fig. 2c clearly shows that sphere bronze particles in PU/sphere bronze composite coatings are discretely distributed with relatively large inter-particle distances, and there are many interspaces between the particles, which will contribute to low spectral reflection and high spectral absorption; this may be the main aspect leading to PU/sphere bronze composite coatings with higher infrared emissivity. Also it can be seen from Fig. 2d and e that PU/flaky bronze composite coatings are stacked by PU coated flaky bronze powders, indicating that the coatings have obvious nacre-like multilayered structural characteristics. According to the observed results, the microstructure of PU/flaky bronze composite coatings is confirmed as Fig. 3. It can be seen that the coatings are periodic stacked by PU and flaky bronze powders with different refractive index, revealing that the coatings have similar one-dimensional photonic structural characteristics [8,17]. 3.3. Cause of low infrared emissivity of PU/flaky bronze composite coatings According to the microstructure and interface characteristics of PU/flaky bronze composite coatings, we speculate that the reflection of infrared radiation from the coatings includes three parts (Fig. 3): (1) the reflection from the interface of surface PU layer and air (r1), (2) the reflection from the surface of top flaky bronze layer (r2), and (3) the reflection from the one-dimensional photonic structure in the coatings (r3). In addition, resin has always a high emissivity, so surface resin layer has important influence on the emissivity of the composite coatings, and it will be significantly increased with increasing thickness of the surface resin layer. Then, according to Eq. (1), the infrared emissivity of PU/flaky bronze composite coatings can be expressed as: ε ¼ 1−ðr 1 þ r 2 þ r3 Þ þ Δε
ð2Þ
where Δε is the emissivity increment caused by the surface resin layer of the composite coatings. According to Fresnel's law, r1 can be expressed as: r 1 ðθ ¼ θi Þ ¼
1 n1 cosθi −n2 cosθr 2 1 n2 cosθi −n1 cosθr 2 þ 2 n1 cosθi þ n2 cosθr 2 n2 cosθi þ n1 cosθr
ð3Þ
where n1 and n2 are the refractive indices of air (1.0) and PU (1.52), respectively, θi is the angle of incidence, and θr is the angle of reflection. We use normal emissivity in this paper, so θi = θr = 0°. Then, the value of r1 can be calculated as 0.043. According to Eq. (1) and infrared emissivity (0.44) of pure bronze powders coating, the value of r2 can be calculated as 0.56. In this work, we approximately think that the PU/flaky bronze composite coatings have completely regular one-dimensional photonic structural characteristics for ease of calculation, so we can introduce the photonic bandgap calculation software (Translight) to calculate the reflection spectra for the one-dimensional photonic structure in PU/flaky bronze composite coatings. Then, the value of r3 can be obtained from the reflection spectra. Fig. 4 shows the relationship between the thickness and infrared emissivity of pure PU coating. It can be seen that when the thickness is increased from 0 μm to 40 μm, the emissivity of pure PU coating increased significantly from 0.230 (the emissivity of steel substrate) to 0.907. With successively increasing thickness from 40 μm to 74 μm, the emissivity of pure PU coating increased slightly from 0.907 to 0.934, and always kept at a high value. In order to
W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102
101
Fig. 2. SEM images of bronze powders and PU/bronze composite coatings (bronze content 60 wt. %), (a) flaky bronze powders, (b) sphere bronze powders, (c) PU/sphere bronze composite coatings, (d) surface of PU/flaky bronze composite coatings, and (e) cross-section of PU/flaky bronze composite coatings.
quantitative calculation, the relationship between the emissivity and thickness of pure PU coating was fitted by the method of S-type curve fitting (Fig. 4), and the fitting function can be expressed as: εPU ¼ A2 þ
ðA1 −A2 Þ 1 þ expððx−x0 Þ=dxÞ
ð4Þ
where εPU is the emissivity of pure PU coating, x is the thickness of pure PU coating, parameters A1 = 0.16949, A2 = 0.91881, x0 = 10.46309, and dx= 4.29556. According to Eq. (4), the emissivity increment (Δε) caused by the different thickness of the surface PU layer of PU/flaky bronze composite coatings can be obtained as: Δε ¼ A2 þ
ðA1 −A2 Þ −0:23 1 þ expððl2 −x0 Þ=dxÞ
Fig. 3. Microstructure model of PU/flaky bronze composite coatings.
ð5Þ
where l2 is the thickness of surface PU layer of PU/flaky bronze composite coatings in different bronze content, it can be calculated as the following equation: l1 sρ1 ¼ yl2 sρ2
ð6Þ
where s is the surface area of the composite coatings, y is the ratio of bronze content to PU content, l1 is the thickness of flaky bronze, ρ1 (bronze density 8.8 g/cm3), and ρ2 (PU density 1.073 g/cm3). Fig. 5 shows the calculated reflection spectra of one-dimensional photonic structure in PU/flaky bronze composite coatings (bronze content 60 wt. %). It can be seen with a strong reflection peak at the wavelength of 8–14 μm. According to Fig. 5, the value of r3 at the wavelength of 8–14 μm for PU/flaky bronze composite coatings can be calculated as 0.252.
Fig. 4. The emissivity dependence on thickness of pure PU coating.
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W. Zhang et al. / Materials Science and Engineering C 33 (2013) 99–102 Table 1 Calculated and measured emissivity of PU/flaky bronze composite coatings with different bronze content.
Fig. 5. Calculated reflection spectra of one-dimensional photonic structure in PU/flaky bronze composite coatings (bronze content 60 wt. %).
When bronze content is 60 wt. %, l2 can be calculated as 2.472 μm according to Eq. (6). Then, according to Eq. (5), Δε (bronze content 60 wt. %) can be calculated as 0.04. At last, according to Eq. (2) and the values of r1 (0.043), r2 (0.56), r3 (0.252) and Δε (0.04), the infrared emissivity of PU/flaky bronze composite coatings (bronze content 60 wt.%) can be calculated as 0.185. Using the same method as described above, the infrared emissivity of PU/flaky bronze composite coatings of other bronze content are calculated too (Table 1). It can be seen that the calculated emissivity is almost consistent with the measured values (Table 1). In addition, Table 1 indicates that the surface PU layer has an important influence on the emissivity of the composite coatings. When the bronze content decreased from 40 wt. % to 20 wt. %, the thickness of surface PU layer will be increased from 5.561 μm to 14.83 μm, leading to a sharp increase of Δε from 0.121 to 0.49 (Table 1), so we can see a sharp decrease of emissivity in PU/flake bronze composite coatings at first three points (Fig. 1). Furthermore, Table 1 can easily explain that the phenomenon of the emissivity of PU/flaky bronze composite coatings is first significantly decreased and then slightly increased with increasing bronze content (Fig. 1). First decrease mainly derived from the sharp decrease of Δε from 0.49 to 0.04 with bronze content increased from 20 wt. % to 60 wt. % (Table 1). Decrease and then increase mainly derived from r2 first increased from 0.183 to 0.252 with bronze content increased from 50 wt. % to 60 wt. %, and then decreased from 0.252 to 0.166 with bronze content increased from 60 wt. % to 70 wt. % (Table 1). 4. Conclusions In summary, nacre-like PU/flaky bronze composite coatings with low infrared emissivity were successfully designed and prepared.
Bronze content
r3
Δε
Calculated emissivity
Measured emissivity
20 30 40 50 60 70 75
0.149 0.151 0.192 0.183 0.252 0.166 0.152
0.490 0.236 0.121 0.068 0.040 0.024 0.018
0.738 0.482 0.326 0.282 0.185 0.255 0.263
0.762 0.450 0.299 0.246 0.206 0.225 0.231
wt. % wt. % wt. % wt. % wt. % wt. % wt. %
Infrared emissivity test results show that the infrared emissivity of nacre-like PU/flaky bronze composite coatings is significantly lower than PU/sphere bronze composite coatings. The low infrared emissivity of PU/flaky bronze composite coatings is derived from the similar one-dimensional photonic structure in the coatings. At last, we confirmed that nacre biomimetic design may be a promising method to prepare low infrared emissivity composite coatings.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China under Grant No. 51173079. We appreciate Andrew L. Reynolds to share the software of translight.
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