TiO2 one-dimensional photonic crystal with low infrared-emissivity in the 8–14 μm band

Journal Pre-proof Preparation and characterization of Ge/TiO2 one-dimensional photonic crystal with low infrared-emissivity in the 8-14 ␮m band Weigang Zhang, Dandan Lv

PII:

S0025-5408(19)31965-8

DOI:

https://doi.org/10.1016/j.materresbull.2019.110747

Reference:

MRB 110747

To appear in:

Materials Research Bulletin

Received Date:

31 July 2019

Revised Date:

12 December 2019

Accepted Date:

18 December 2019

Please cite this article as: Zhang W, Lv D, Preparation and characterization of Ge/TiO2 one-dimensional photonic crystal with low infrared-emissivity in the 8-14 ␮m band, Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110747

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Preparation and characterization of Ge/TiO2 one-dimensional photonic crystal with low infrared-emissivity in the 8-14 μm band

Weigang Zhang∗ [email protected], Dandan Lv

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College of Materials and Chemical Engineering, Chuzhou University, Hui feng Road 1, Chuzhou 239000, China

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Corresponding author. Tel.: +86 550 3511055.

Optical coating technology

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Graphical Abstract

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∗

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Highlights

Ge/TiO2 1DPC was successfully prepared by optical coating technology.

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Infrared emissivity in the 8-14 µm band of the 1DPC can be as low as 0.202.

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Low emissivity materials can be prepared by using non-infrared transparent materials.

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ABSTRACT: Ge/TiO2 one-dimensional photonic crystals (1DPC), with low

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infrared-emissivity in the 8-14 µm band, were successfully designed and prepared by alternating the Ge and TiO2 layers on the quartz substrate using optical coating

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technology. The microstructure and spectral emissivity of the as-prepared 1DPC were characterized using scanning electron microscopy (SEM) and Fourier-transform infrared spectrometry (FTIR), respectively. The microstructural analysis indicates that the as-prepared 1DPC has clear structural multilayer characteristics, and the thicknesses of Ge layer and TiO2 layer are very close or identical with the target

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values. The spectral emissivity shows the as-prepared 1DPC has low infrared emissivity in the 8-14 µm band. In fact, the average emissivity can be as low as 0.202, and meets the requirement of our design. The results of this paper show that low infrared emissivity materials can be prepared successfully using materials, which are not transparent for infrared light, via a suitable one-dimensional photonic structural design. This result increases the selection range of raw materials needed

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to prepare low infrared-emissivity materials.

Keywords: Low infrared-emissivity; One-dimensional photonic crystal; Optical

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coating technology; Microstructure

1. Introduction

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Low infrared-emissivity coatings, which consist of a resin matrix and

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low-emissivity functional pigments, have attracted attention from researchers worldwide because of possible applications in the field of infrared stealth [1-4]. In particular, resin/metal composite coatings have many practical application due to their low emissivity, simple preparation, convenient production, low cost, and their application is not limited by the target geometry [5,6]. Unfortunately, more than 50 wt% of the resin/metal composite coating is organic resin [7,8], which is difficult to 3

apply to very hot components such as tail nozzles of aircraft engine. Therefore, the development of low infrared-emissivity materials, which use inorganic ingredients, has important practical significance. In addition, it is assumed that raw materials, which are suitable to prepare low-infrared emissivity materials, must have good infrared transparency [9,10] – a condition that greatly limits the range of candidate materials. Therefore, the development of a new method to prepare low

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infrared-emissivity materials using non infrared-transparent materials will greatly expand the selection range of raw materials for low infrared emissivity materials. Photonic crystals are low-dimensional metamaterials with a periodic

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structure, which were theoretically discovered by Veselago in 1968 [11]. The

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presence of a photonic bandgap in the photonic crystal produces a selective strong reflection of the incident light. One-dimensional photonic crystals (1DPCs)

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are formed by stacking two dielectric materials with different refractive indices.

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They have the structural characteristics of multi-layer films. 1DPCs can produce selective strong reflections that depend on the specific design of optical materials

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[12-15]. In addition, according to the Kirchhoff’s law [16] and the principle of conservation of energy, the reflectivity of non-transparent materials such as 1DPCs

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has a decisive effect on the emissivity: the higher the reflectivity, the lower the emissivity [17,18]. Therefore, by designing a one-dimensional photonic structure to produce strong reflection in the 8-14 μm band, we can obtain low emissivity in the 8-14 μm band of this material. To the best of our knowledge, the published studies on low infrared emissivity photonic crystals were focused on their theoretical design,

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and the materials used in these designs were mainly dielectric materials with good infrared transparency [19,20]. It seems that the application of non infrared-transparent dielectric materials for the design and preparation of low infrared-emissivity photonic crystals has not yet been reported. In this paper, Ge/TiO2 1DPC, with low infrared emissivity in the 8-14 μm band, was designed and prepared using optical coating technology. The effect of the number

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of periods on the infrared emissivity of 1DPC was studied. The device structure and spectral emissivity of as-prepared 1DHPC was also systematically investigated.

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2. Experimental

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2.1. Materials

Ge particles (purity 99.999 wt%, refractive index 4.0 (1.7~23 μm) ) and TiO2

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particles (purity 99.99 wt%, refractive index 2.1 (0.4~10 μm) ) were purchased from Nanjing Chemical Reagent Limited Company, China. All reagents were analytical

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grade and used as-received without additional treatment.

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2.2. Preparation of the 1DPC

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The quartz substrate (surface roughness 10 nm, diameter 5 cm, thickness 1 mm) were thoroughly cleaned with an ultrasonic bath. Ge and TiO2 were alternatingly deposited on the quartz substrate using an optical coating machine (OTFC-900), aiming to fabricate the 1DPC with low emissivity at 8~14 μm. Pure Ge particles and TiO2 particles were pressed into pellets as targets. The target-to-substrate distance was 60 cm. During the whole deposition process, the deposition rates of Ge layer and 5

TiO2 layer were 0.4 nm/s and 0.6 nm/s, respectively, and the substrate temperature was maintained at 250℃, the chamber pressure was 0.9×10-3 Pa, the accelerating voltage and current were 6 kV and 24 mA, respectively. 2.3. Characterization The Photographs of the quartz substrate and 1DPC were recorded with a Nikon

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digital camera (COOLPIXL22). The morphology, microstructure, and energy spectrum of the 1DPC were characterized using field-emission scanning electron microscopy (S-4800). The sample was sputtered with a thin layer of Au prior to

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imaging. The normal spectral emissivity of the 1DPC, at wavelengths ranging from 7-14 µm, was measured using a Fourier transform infrared spectrometer (JASCO

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FTIR-6100) at 100℃. A Landcal R1500T blackbody furnace was used as

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near-blackbody source. The normal spectral emissivity is the ratio of the radiance of a sample to that of a blackbody at the same temperature and for the same spectral and

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directional conditions.

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3. Results and discussion

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3.1. Theoretical analysis and design of the 1DPC A characteristic matrix method for 1DPC was used to calculate the reflection

spectra of Ge/TiO2 1DPC using the software Translight 3.01b [21]. According to Kirchhoff’s law and the principle of conservation of energy [16], the relationship between infrared emissivity (ε) and reflectivity (r) in a particular band of a non-transparent material such as a 1DPC can be expressed as: 6

ε=1－r

(1)

As shown in Equation (1), reflectivity has a strong effect on the infrared emissivity of the 1DPC. Hence, the infrared emissivity of a particular band of a 1DPC can be calculated when the average reflectivity of a certain band is known, for example from a simulated reflection spectrum. To ensure the lowest emissivity in a photonic crystal, it must have the

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maximum reflectivity. To obtain the strongest reflectivity the 1DPC has to achieve the lowest emissivity under the same conditions. The optical thicknesses (the product of the physical thickness and refractive index of the dielectric layer) of the two

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dielectric layers (Ge layer and TiO2 layer) are designed to be a quarter of the center

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wavelengths (11 µm) of the reflection peak in the 8~14 μm band [21]. Then, the thicknesses of the Ge (lGe=0.688 µm) and TiO2 layers (lTiO2=1.31 µm) can be

(2)

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lGe=11/(4×nGe)

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calculated using the following formula:

lTiO2=11/(4×nTiO2)

(3)

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where, nGe and nTiO2 are the refractive indices of Ge and TiO2, respectively. To study the effect of the number of periods of the photonic crystals on the

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reflection intensity, we calculated the reflection spectra for Ge/TiO2 1DPCs with 2-6 periods – see Fig. 1. With increasing number of periods, the 1DPCs reveals clear reflection peaks in the 8-14 μm band, and the intensity is enhanced significantly. Once the number of periods reaches 5 periods, the intensity of the reflection peak tends to be stable [20,22]. The average reflectivity in the 8-14 µm band is

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significantly increased from 0.583 to 0.923, when the number of periods increased from 2 to 5. If the number of periods is further increased, the average reflectivity in the 8-14 µm band will no longer increase significantly. This produces an average emissivity decrease, for the 8-14 µm band, from 0.417 to 0.077, which then tends to be stable (Table 1). Considering the cost and complexity of the preparation of the 1DPC, we selected five as the number of periods for the

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Ge/TiO2 1DPC. The device-structure of the Ge/TiO2 1DPC is shown in Fig. 2. It was designed to achieve low infrared-emissivity in the 8-14 µm band.

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3.2. Microstructure and infrared emissivity of the 1DPC

The Ge/TiO2 1DPC, according to the structural model of Fig. 2, was successfully

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prepared using optical coating methods (Fig. 3(b)) [13]. The 1DPC, with its smooth

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surface, has a color that can change significantly with the observation angle, which is the main optical feature of a one-dimensional photonic crystal [23]. This indicates

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that the as-prepared Ge/TiO2 1DPC has indeed one-dimensional photonic structural properties. Fig. 4(a) shows the cross-sectional SEM image of the Ge/TiO2 1DPC

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according to the structural model. The 1DPC has clear structural multilayer film

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characteristics: it is periodically stacked using different dielectric materials, the different dielectric layers have a uniform thickness, and the overall structural characteristics are consistent with our design. The EDS patterns (Fig. 4(c), (d)) of points A and B in Fig. 4(b) illustrate that the corresponding dielectric layers are Ge layers and TiO2 layers, respectively. The as-prepared 1DPC shows a uniform thickness for the Ge- and TiO2 - layers, the average thicknesses of the two layers 8

are 0.647 µm and 1.231 µm, respectively. The measured results are practically the same as the thicknesses of Ge layer and TiO2 layer in the structural model, which facilitates low emissivity performance - consistent with our design. Fig. 5 shows the normal spectral emissivity for the wavelengths 7-14 µm, of Ge/TiO2 1DPC, according to the structural model. The as-prepared 1DPC has ultra-low infrared emissivity in the 8-10 µm band, and the emissivity can be as low as

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0.1 or less. In the 10-14 µm band, the as-prepared 1DPC shows low infrared

emissivity, which can be as low as 0.4 or less. The average emissivity in the 8-14 µm band can be as low as 0.202. The difference in emissivity performance between the

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different bands is mainly related to the infrared transparency of the dielectric layer.

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The transparent band of the Ge layer is the 1.7-23 µm band, which covers the entire 8-14 µm band. However, the transparent band of the TiO2 layer is 0.4-10 µm, which

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does not cover the entire 8-14 µm band. Therefore, the as-prepared 1 DPC shows

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weak absorption of infrared light and ultra-low emissivity for the 8-10 µm band. On the other hand, the as-prepared 1 DPC has a relatively high emissivity due to poor

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infrared transparency in the 10-14 µm band. Nevertheless, since the overall thickness of the TiO2 layers is only a few microns, the absorption of infrared light is limited.

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Hence, the as-prepared 1 DPC has still a low average emissivity (0.202) in the 8-14 µm band. In other words, the common expectation for low infrared emissivity materials to have good infrared transparency appears inaccurate. It is clearly possible to achieve low infrared emissivity with a well-designed one-dimensional photonic structure, regardless of its infrared transparency. This is a new approach

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for the design of low infrared emissivity materials. 4. Conclusions In summary, a Ge/TiO2 1DPC, with low infrared emissivity in the 8-14 µm band, was successfully designed and prepared using optical coating technology. The as-prepared 1DPC shows a typical structural characteristic of a multilayer device,

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which is consistent with the device-design. The spectral emissivity measurements shows that the as-prepared 1DPC has low infrared emissivity in the 8-14 µm band, and the average emissivity can be as low as 0.202, which can fully meet the

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requirements for many low infrared emissivity applications. In addition, the results

of this paper indicate that, with a well-designed one-dimensional photonic structure,

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regardless of its infrared transparency, one can achieve low infrared emissivity in

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the 8-14 µm band. These results can aid the development and understanding of

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low infrared emissivity materials and devices.

Author statement

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We declare that this manuscript entitled “Preparation and characterization of Ge/TiO2 one-dimensional photonic crystal with low infrared-emissivity in the 8-14 μm band” is original, has not been published before and is not currently being considered for publication elsewhere. We would like to draw the attention of the Editor to the following publications of one or more of us that refer to aspects of the manuscript presently being submitted. Where relevant copies of such publications are attached. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the Corresponding Author is the sole contact for the Editorial process. He is responsible for communicating with the other authors about progress, 10

submissions of revisions and final approval of proofs. Signed by all authors as follows: Weigang Zhang, Dandan Lv

Conflict of interest The authors declared that they have no conflicts of interest to this work.

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Acknowledgements

This work was financially supported by the National Natural Science Foundation

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of China (61705029), Natural Science Foundation of Anhui Province

(1808085MF187) and Demonstration Experiment and Training Center of Anhui

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Province (2017sxzx33). We appreciate Andrew L. Reynolds to share the software of

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translight.

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Figure and Table captions

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ro of -p re lP na ur Jo Fig. 1. Calculated reflection spectra of the Ge/TiO2 1DPCs with different numbers of

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periods.

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Fig. 2. Device structure of the 1DPC with low infrared emissivity in the 8-14 µm

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band.

Fig. 3. Photographs of the quartz substrate (a) and the Ge/TiO2 1DPC according to the device-structure (b).

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Fig. 4. (a) Cross-sectional SEM image of Ge/TiO2 1DPC with the designed device

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structure. (b) Partially magnified image of (a). (c) EDS pattern of point A in (b). (d) EDS pattern of point B in (b).

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Table 1 Calculated infrared emissivity at the wavelength of 8-14 µm of the Ge/TiO2 1DPCs with different numbers of periods. Average reflectivity

Average emissivity

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0.583

0.417

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0.823

0.173

4

0.884

0.116

5

0.923

6

0.927

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Number of periods

0.077

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0.073

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