Enhanced Transparency of Rough Surface Sapphire by Surface

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Enhanced Transparency of Rough Surface Sapphire by Surface Vitrifaction Process Qinhua Wei, Jia Lin, Hongsheng Shi,* Gao Tang, Wenxiang Chai, and Laishun Qin College of Materials Science &Engineering, China Jiliang University, Hangzhou 310018, China S Supporting Information *

ABSTRACT: A novel, simple, and low-cost in situ surface vitrification method has been effectively developed to enhance the optical transparency of rough surface sapphire at UV− visible−IR regions. This method is to obtain a glass layer on the sapphire surface through vitrifaction process. The thickness, refractive index, components and transition temperature of the glass layer have been investigated and discussed respectively by XRD, DSC, SEM and EDS elemental analysis. The experimental results show that the vitrified sapphire has high transparency even after 1000 °C annealing at UV−visibleIR regions. KEYWORDS: sapphire, surface vitrification process, optical property, lanthanide glass, spin coating method

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ransparent substrate materials including crystal, transparent ceramic, quartz glass, polymer, and sapphire have been generally used for various optical components and optoelectronic devices.1−5 Because of the excellent thermostability, wide optical transparency (from ultraviolet (UV) to mid-infrared (IR)), good chemical stability,6−8 and extraordinary mechanical hardness, sapphire has been widely used in touch screen panels, safety lenses,9 glasses display windows,10 and some extreme environment of military, metallurgy, aerospace in the high-temperature window (such as aircraft radome, high power laser window, satellite, etc.). But sapphire is one of the hardest materials next to diamond (i.e., Moh’s hardness scale of ∼9), and can be scratched only by a few expensive substances such as cubic boron nitride and diamond. The traditional polishing methods are expensive and difficult for machining and polish of sapphire. Besides, because of the high refraction index (1.75−1.78), the sapphire exhibits lower transmission compared to the other transparent substrates such as glasses, polymers, and quartzes.11−13 Therefore, enhancing transparency and reducing machining cost are important for sapphire to extend the application fields. Many novel methods were developed, such as antireflection coatings (ARCs) of multiple dielectric layers,14 nanoscale patterned sapphire substrate (NNPS)15 and so on. Those methods can increase the transmittance from 87 to 96% or decrease the reflectivity from 20 to 5%.16 But it is helpless for the unpolished or single-sides polished sapphire (transmittance is below 40%). Additional, the application fields of ARCs and NNPS are also limited for a high-temperature environment. On the basis of that, a novel idea is carried out: sapphire surface vitrifaction. In consideration of furnace cooling condition (low cooling rate) for vitrification process, the glass component is the key factor and should meet the requirements as following: © XXXX American Chemical Society

1. High glass-forming ability: the component easily forms glass in a broad temperature range. 2. High crystallization temperature: devitrification must be prevented during furnace cooling. 3. Excellent optical transparency in UV−visible− IR regions. As many previous reports, the RE-based aluminosilicate glasses show relatively large glass-forming regions (>850 °C), high crystallization temperature, low refraction index and excellent optical transparency at UV−visible−IR regions.17,18 Those glasses are promising candidate materials or replacing sapphire in some applications.19−21 Therefore, RE-based aluminosilicate glasses should also be suitable for sapphire surface vitrifaction process. In this letter, the in situ sapphire surface vitrifaction has been accomplished through joining La2O3−Gd2O3−SiO2−Al2O3 (La−Gd−Si−Al) compositions glass layer on the sapphire surface. The glass layer was directly synthesized in air by melting the precursor film at temperature higher than 1500 °C in an electric furnace and then furnace cooling. The glass precursor film was prepared by the sol−gel and spin coating technique (Figure 1a, the inset is the picture of samples). Details of this method, are presented in the Supporting Information. The rough surface sapphire with low transparency just needs simple spin coating and heat treatment process and becomes highly transparent. Compared to the current polishing technology for the commercial sapphire, the in situ vitrifaction method has significant advantage in the reduction of processing cost and difficulty. Besides, the Received: December 18, 2017 Accepted: February 20, 2018 Published: February 20, 2018 A

DOI: 10.1021/acsami.7b19191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Procedure for the surface vitrifaction process; inset is the picture of samples. (b) SEM image and EDS of vitrified sample.

refraction index of the surface will also become lower than that of sapphire, which is beneficial to minimizing the optical loss of transmitted light. So far, the studies on sapphire surface vitrifaction have seldom been reported. The amorphous glass layer was confirmed by XRD observation (Figure S1), indicates that the surface vitrifaction process was successfully achieved by spin coating and heat treatment process. The microstructure and glass component of vitrified sapphire sample were confirmed and discussed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) (Figure 1b). The interface of glass layer and sapphire substrate is clear. And the thickness of finally glass layer is estimated to about 500 μm while the sapphire substrate is 850 μm (original substrate is 1000 μm). The EDS presents that the glass layer mainly includes La, Gd, Si, Al, and O elements, whereas the sapphire substrate (Al2O3) comprises Al and O elements (Figure S3). It can be concluded that the sapphire substrate was participated in this process and supplied the Al atomic to glass layer. The DSC curve of vitrified sapphire (Figure S2) shows that the glass transition (Tg) temperature and crystallization temperature (Tp) are respectively located at 950 and 1120 °C, which indicates that the glass layer has a good thermal stability (details are presented in the Supporting Information). The refraction index of glass layer is about 1.57 at wavelengths of 980 nm. Generally, the refraction index strongly depends on the glass component.22 Therefore, it is absolutely procurable that the refraction index of glass layer is tunable by substituting La with other RE cations elements (Sc3+, Y3+, Lu3+) to formulate a favors index.22 In other words, the refraction index of sapphire surface can be tuned by surface vitrifaction process. In theory, the lower refraction index of surface glass layer can reduce the optical reflectance of sapphire and enhance the transparency, like as antireflection coatings.23 To evaluate the transmittance properties of vitrified sapphire. The UV−visible (200 nm-1000 nm) and IR (1.2−9 μm) transmittance spectra of different samples were measured (Figure 2a, b). The inset is the picture of different samples. The

Figure 2. Transmittance spectra of vitrified sample, double-sides polishing (DSP) and single-sides polishing (SSP) sapphire. (a) UV− visible transmittance spectra. (b) IR transmittance spectra. The inset is the picture of different samples. (c, d) Schematic diagram of antireflection effect.

max transmittance of UV−visible light is about 83, 85, and 32%, corresponding to vitrified sapphire, commercial DSP and SSP sapphire, respectively. The transmittance of SSP sapphire has been enhanced greatly through vitrifaction process. Meantime the absorption edge is shifted from 220 to 336 nm, which should be ascribed to the intrinsic absorption of La−Gd−Si−Al glass layer. The maximum IR transmittance of vitrified sapphire almost reach 85% while the transmittance of SSP sapphire is just only 40% between 1.2 and 9 μm. The transparency of the vitrified sapphire is at the same level with the commercial DSP sapphire (85%). However, an absorption peak of 2.8 μm is appeared. As the previous reported,24 the 2.8 μm absorption band should be attributed to traces of water that presented in the glass batch and subsequently bound into the glass structure as metal-hydroxyl chemical bond. Therefore, the transmittance of the vitrified sapphire should be further improved by polishing the surface and removing of traces of OH from the B

DOI: 10.1021/acsami.7b19191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

temperature of the glass basically agreed with the DSC result. Hence, the vitrified sapphire keeps good thermal stability. In conclusion, surface vitrifaction process was successfully accomplished through joining La−Gd−Si−Al glass layer (thickness is about 500 μm) on sapphire rough surface. The sapphire transmittance of UV−visible and IR were improved highly from 32% to 83%. The much smoother surface and lower refraction index of La−Gd−Si−Al glass layer eliminate the refraction of uneven surface in different directions. The glass layer with a high glass transition ranging roughly between 900 and 1000 °C leads to the transparent vitrified sapphire even after 1000 °C annealing, which can be used in some severe fields as sapphire.

glass layer. Because of the natural characteristic of aluminosilicate glass, the transmittance of vitrified sapphire starts to deviate under 4 μm and decreases to 50% in the range from 4.5 to 5.1 μm. The absorption edge is about 5.5 μm. Compared to commercial sapphire (6 μm), there is a blue shift of 0.5 μm. On the basis of those results, the surface vitrifaction has great effect on the transmittance of rough surface sapphire and gets the same transparent level as the commercial sapphire. The main reasons of antireflection was discussed as following: 1. A lot of visible or IR light was wasteful because of the reflection and refraction of light in different directions on the rough surface (Figure 2c). In this case, the precursor film was melted and atomic interdiffusion was occurred between the sapphire and glass layer under high temperature. The much smoother La−Gd−Si−Al glass layer was then synthesized on the rough surface, eliminating the refraction of sapphire uneven surface in different directions (Figure 2d). Finally, the transmittance of sapphire was improved from 32 to 83% at UV−visible-IR regions. 2. The surface reflectivity is strong depend on the refractive index of material. And the reflectivity can be given by the well-known expression eq 1

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19191. Experimental details; XRD patterns; DSC curve; EDS results (PDF)

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⎛ n − 1⎞ ⎜ ⎟ ⎝ n + 1⎠ (1) where n is refraction index and R is reflectivity. On the basis of eq 1, the reflectance of glass and sapphire remained 4.9% and 7.8%, respectively. The lower refractive index of La−Gd−Si-Al glass layer (ng = 1.57 < ns) can greatly decrease the reflectivity and improve the transmittance. Sapphire has excellent thermostability and can be widely used in many extreme environment. Therefore, the thermostability of vitrified sapphire is also important forapplication. The UV−visible and IR transmittance spectra of different samples (Figure 3) indicate that the vitrified sapphire (inset)

AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

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ORCID

Hongsheng Shi: 0000-0002-9498-1932 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial supported by the National Natural Science Foundation of China (11575170, 11605194, and 61505193), Zhejiang Provincial Department of Education Project (Y201534219).

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REFERENCES

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Figure 3. UV−visible and IR transmittance spectra of samples at 900, 1000, and 1100 °C annealing; inset is the picture of samples.

still keeps high transmittance of 80% after 900 and 1000 °C annealing, whereas it is nontransparent (about 8%) after 1100 °C annealing. The XRD results show that (Figure S4) the glass layer is still amorphous after 1000 °C annealing, whereas it begins to crystallize at 1100 °C. Obviously, the generation and growth of crystalline grain leads to the optical loss increasing greatly and the layer being nontransparent. The crystallization C

DOI: 10.1021/acsami.7b19191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b19191 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX