molecules Article
Fabrication of Refractive Index Tunable Coating with Moisture-Resistant Function for High-Power Laser Systems Based on Homogeneous Embedding of Surface-Modified Nanoparticles Wei Yang, Xiangyang Lei, Haohao Hui, Qinghua Zhang and Xueran Deng *
ID
Research Center of Laser Fusion, CAEP, Mianyang 621900, China;
[email protected] (W.Y.);
[email protected] (X.L.);
[email protected] (H.H.);
[email protected] (Q.Z.) * Correspondence:
[email protected]; Tel.: +86-138-8070-6602 Academic Editors: Ahmad Mehdi and Sébastien Clément Received: 23 April 2018; Accepted: 4 May 2018; Published: 7 May 2018
Abstract: Moisture-resistant silicone coatings were prepared on the surface of potassium dihydrogen phosphate (KDP) crystal by means of spin-coating, in which hydrophobic-modified SiO2 nanoparticles were embedded in a certain proportion. The refractive index of such coating can be tuned arbitrarily in the range of 1.21–1.44, which endows the KDP optical component with excellent transmission capability as well as the moisture proof effect. A dual-layer anti-reflective coating system was obtained by covering this silicone coating with a porous SiO2 coating which is specially treated to enhance the moisture resistance. Transmittance of such a dual-layer coating system could reach 99.60% and 99.62% at 1064 nm and 532 nm, respectively, by precisely matching the refractive index of both layers. Furthermore, the long-term stability of this coating system has been verified at high humidity ambient of 80% RH for 27 weeks. Keywords: moisture-resistant; refractive index tunable; high-power laser system
1. Introduction As the frequency converting optical component for the high-power laser system of the inertial confinement fusion (ICF) facility, water-soluble potassium dihydrogen phosphate (KDP) crystal needs a coating system to achieve both anti-reflective and moisture-resistant function, in order to ensure its high laser transmission efficiency and long-term stability. The high-power laser system requires KDP components to possess high transmittance at 1064 nm and 532 nm to double/triple the frequency of original incident laser, which could realize high output power to implement final ignition of the ICF facility. Therefore, a coating system with required refractive index (RI), thickness, and the moisture-resistant function should be acquired for each layer. Silica coating prepared using the sol-gel technique is a favorable candidate for the high-power laser system owing to its high laser resistant ability compared with coatings prepared by physical vapor deposition (PVD) [1–3]. The double-wavelength anti-reflective coating system for 1064 nm and 532 nm using the sol-gel process has been proposed by Thomas for the first time [4]. Subsequent studies implemented the coating preparation based on this theoretical design by fabricating the bottom layer with silicone coating (due to its dense structure and higher RI) using Si-O oligomer sol as a source [5], and the top layer with porous silica coating (owing to its porous structure and lower RI) using monodisperse colloidal silica nanoparticles (NPs) [6,7]. However, the RI of conventional synthesized silicone coating is higher than that of the design value because RI is an intrinsic characteristic of a material, which will result in transmittance reduction at desired wavelengths. Zhang et al. improved
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the method to prepare this double-wavelength anti-reflective coating system and transmittance; about 99.6% and 99.8% at 532 nm and 1064 nm were achieved, respectively [8]. The RI of top layer, however, is as low as 1.14 with a porosity about 73%. Generally, larger coating porosity means a larger possibility to absorb water vapor and contaminants in the environment [9,10], which also will lead to the transmittance reduction in long-term running of the component. Another RI modulation approach was proposed by Ye et al., in which the RI was tuned by mixing the base-catalyzed silica and acid-catalyzed silica sols in different proportions [11], but the precise control of pH is a potential challenge for this method because acid coating is harmful to the KDP substrate. Cai et al. reported a method to fill silica NPs in the silicone coating to modulate its RI, and the durability has been proven to be satisfactory [12]. However, the important moisture prevention of coating systems is not specifically discussed, which is very crucial to keep the long-term stability of KDP substrate. In summary, dense silicone coating with required RI is hard to obtain due to the intrinsic feature of raw materials, and silica NPs with moisture-resistant ability need extra treatment to function. An approach to modulate the RI of silicone coating precisely and enable both coatings with moisture-resistant ability is proposed in this study, based on the homogeneous embedding of the silicone coating and surface modification of the porous silica coating. A series of dual-layer double-wavelength broadband coating systems on KDP substrate were designed and a KDP component with excellent transmission properties and long-term stability was successfully fabricated. 2. Results and Discussion 2.1. Design of the Double-Wavelength Anti-Reflective Coatings Design of this dual-layer coating system with double-wavelength anti-reflective function on KDP substrates was performed using the thin film design software (TFCalc® ) and the optimized results are listed in Table 1. The RI was set to be variable from 1.10 to 1.44, and the starting thickness was set to be 100 nm for both layers. The optimal anti-reflective coating with a transmittance of near 100% at target wavelengths can be obtained while the RI and thickness was about 1.30 and 135 nm for the bottom layer, and around 1.14 and 154 nm for the top layer. However, the bottom layer with a low RI of 1.30 has a poor abrasive-resistance [13,14], while the top layer with such low RI of 1.14 was prone to absorb organic contaminants or water vapor from the environment [9,10], which will cause unexpected transmittance loss. To solve these problems, relatively high and compromised RI of 1.38 and 1.22 were adopted for the bottom and top layers, respectively. This pair of matched RIs can still achieve high transmittance around 99.7% at 532 nm and 1064 nm of the dual-layer coating system, while the abrasive-resistance for the bottom layer and anti-contamination for the top layer could be ensured. Table 1. Optimized design of a dual-layer coating system for double-wavelength anti-reflection. Bottom Layer
Substrates
KDP
Top Layer
n
Thickness (nm)
n
Thickness (nm)
1.26 1.29 1.30 1.32 1.34 1.34 1.36 1.38 1.38
136.7 135.9 135.0 132.1 130.6 135.0 129.1 127.6 130.5
1.12 1.14 1.14 1.15 1.16 1.22 1.17 1.17 1.22
162.5 153.8 153.6 155.0 152.7 143.4 150.6 148.6 143.4
T532 nm (%)
T1064 nm (%)
99.92 99.99 100.00 100.00 99.98 99.80 99.95 99.91 99.78
99.91 100.00 100.00 99.99 99.96 99.72 99.90 99.81 99.67
n: All the refractive index discussed in this paper is at 632 nm.
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2.2. Fabrication and Characterization of Top Layer for the Double-Wavelength Anti-Reflective Coating 2.2. Fabrication Fabrication and and Characterization Characterization of of Top TopLayer Layerfor forthe theDouble-Wavelength Double-WavelengthAnti-Reflective Anti-ReflectiveCoating CoatingSystem 2.2. System System The porous silica silica coating coating consisting consisting of of HMDS-modified HMDS-modified silica the porosity porosity The top top layer layer is is aa porous silica NPs NPs with with the The50% top layer is a porousby silica coatinghydrolysis consisting approach of HMDS-modified silica NPs with the porosity of about were fabricated a familiar of a silane [14,15]. The synthetic of about 50% were fabricated by a familiar hydrolysis approach of a silane [14,15]. The synthetic route route of the about 50% were fabricated by a familiar hydrolysis approach of a silane [14,15]. The synthetic route of of the HMDS-modified HMDS-modified silica silica NPs NPs is is presented presented in in Scheme Scheme 1. 1. Hydrolysis Hydrolysis and and condensation condensation of of TEOS TEOS is is of the HMDS-modified silica NPs is presented in Scheme 1. Hydrolysis and condensation of TEOS is performed under alkali condition using ammonia as catalyst to form Si-O oligomer. Colloidal silica performed under alkali condition using ammonia as catalyst to form Si-O oligomer. Colloidal silica performed under by alkali condition using ammoniaduring as catalyst to form Si-O oligomer. Colloidal silica NPs the the self-assembly of oligomer the aging is then introduced NPs are areproduced produced by self-assembly of oligomer during theprocess. aging HMDS process. HMDS is then NPs are produced by the self-assembly of oligomer during the aging process. HMDS is then to realize the of the hydroxyl withgroup a hydrophobic methyl group. Then, a porous top introduced tograft realize graft ofgroup hydroxyl with a hydrophobic methyl group. Then, silica a porous introduced to realize the graft of hydroxyl group with a hydrophobic methyl group. Then, a porous layer is prepared using this oligomer sol by means of spin-coating. silica top layer is prepared using this oligomer sol by means of spin-coating. silica top layer is prepared using this oligomer sol by means of spin-coating.
Scheme 1. 1. Synthetic routes of of HMDS-modified HMDS-modified silica silica oligomer oligomer sol. sol. Scheme Synthetic routes Scheme 1. Synthetic routes of HMDS-modified silica oligomer sol.
The modification of HMDS can be evaluated by calculating the grafting rate of trimethylsilyl The The modification modification of of HMDS HMDS can can be be evaluated evaluated by by calculating calculating the the grafting grafting rate rate of of trimethylsilyl trimethylsilyl (TMS, -Si(CH3)3) group via 29 MAS-NMR as shown in Figure 1. Peak 1 represents the silicon atoms 29Si (TMS, -Si(CH ) ) group via Si MAS-NMR as shown in Figure 1. Peak 1 represents the (TMS, -Si(CH33)33) group via 29Si MAS-NMR as shown in Figure 1. Peak 1 represents the silicon silicon atoms atoms that have been totally condensed with only -OSi bond. Peak 2 implies the silicon atoms bonded with that have been totally condensed with only -OSi bond. Peak 2 implies the silicon atoms bonded that have been totally condensed with only -OSi bond. Peak 2 implies the silicon atoms bonded with with -OEt without hydrolysis or -OH without condensation. Peak 3 represents the silicon atoms bonded -OEt -OEt without without hydrolysis hydrolysis or or -OH -OH without without condensation. condensation. Peak Peak 33 represents represents the the silicon silicon atoms atoms bonded bonded with -(CH3)3, indicating the effective HMDS modification with a ratio of about 20.9%. This grafting with indicating the the effective effective HMDS HMDS modification modification with with aa ratio with -(CH -(CH33))33,, indicating ratio of of about about 20.9%. 20.9%. This This grafting grafting rate is reasonable according to the result of about 14–33% from the work of Suratwala et al. [16]. rate is reasonable according to the result of about 14–33% from the work of Suratwala et al. [16]. rate is reasonable according to the result of about 14–33% from the work of Suratwala et al. [16].
Figure 1. 29Si MAS-NMR spectra of HMDS-modified silica NPs. 29Si MAS-NMR spectra of HMDS-modified silica NPs. Figure 1. 1. 29 Figure Si MAS-NMR spectra of HMDS-modified silica NPs.
Moisture-resistance of this coating is surveyed via the water contact angle measurement as Moisture-resistance of this coating is surveyed via the water contact angle measurement as shown in Figure 2, in which solidified coatings prepared as-synthesized colloidal silica sol and Moisture-resistance of this coating is surveyed via from the water contact angle measurement as shown in Figure 2, in which solidified coatings prepared from as-synthesized colloidal silica sol and HMDS-modified colloidal silica sol are coatings depictedprepared and compared. Prominent improvement of the shown in Figure 2, in which solidified from as-synthesized colloidal silica sol HMDS-modified colloidal silica sol are depicted and compared. Prominent improvement of the hydrophobicity of HMDS-modified silica coating is achievedProminent by enlarging the water contact and HMDS-modified colloidal silica porous sol are depicted and compared. improvement of the hydrophobicity of HMDS-modified porous silica coating is achieved by enlarging the water contact angle from 115.6° to 151.6°. This silica NPsilica synthesis a favorable approach for contact further hydrophobicity of HMDS-modified porous coatingprovides is achieved by enlarging the water angle from 115.6° to 151.6°. This silica NP synthesis provides a favorable approach for further embedding of◦bottom coating. angle from modification 115.6◦ to 151.6 . This silicone silica NP synthesis provides a favorable approach for further embedding modification of bottom silicone coating. embedding modification of bottom silicone coating.
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Figure 2. contact angle measurement results of solidified coatings from (a) As-synthesized Figure 2. 2. Water Water contact contact angle angle measurement measurement results results of of solidified solidified coatings coatings from from (a) (a) As-synthesized As-synthesized Figure Water colloidal silica sol; (b) HMDS-modified colloidal silica sol. colloidal silica silica sol; sol; (b) (b) HMDS-modified HMDS-modified colloidal colloidalsilica silicasol. sol. colloidal
2.3. Fabrication and Characterization of Bottom Layer for the Double-Wavelength Anti-Reflective 2.3. Fabrication Fabrication and and Characterization Characterization of of Bottom Bottom Layer Layer for for the the Double-Wavelength Double-Wavelength Anti-Reflective Anti-Reflective Coating Coating 2.3. System System System Coating The NP embedded silicone coating. As described above, the The bottom bottom layer layer in this study isisaa asilica silica NP embedded silicone coating. As As described above, the The layer in inthis thisstudy studyis silica NP embedded silicone coating. described above, embedding of silica NPs is realized by blending the pre-polymer sol (33 wt %) and the HMDSembedding of silica NPs NPs is realized by blending the pre-polymer sol (33 sol wt (33 %) and the and HMDSthe embedding of silica is realized by blending the pre-polymer wt %) the modified silica (3.3 the between RI modified colloidal colloidalcolloidal silica sol solsilica (3.3 wt wt %). Figure demonstrates the relationship relationship between coating coating RI HMDS-modified sol %). (3.3Figure wt %).33 demonstrates Figure 3 demonstrates the relationship between and mass ratio of pre-polymer in the blended sol system, indicating that the RI of the bottom layer and mass ratiomass of pre-polymer in the blended sol system, indicating that thethat RI the of the bottom layer coating RI and ratio of pre-polymer in the blended sol system, indicating RI of the bottom can tuned from 1.21 1.44 of of It seen can be becan tuned from from 1.21 to to 1.44 with increasing of the theofconcentration concentration of pre-polymer. pre-polymer. It also also can be seen layer be tuned 1.21 towith 1.44 increasing with increasing the concentration of pre-polymer. It can alsobe can be that the coating RI gradually tends to stabilize after the concentration of pre-polymer exceeds 10%. that the coating RI gradually tends to stabilize after the concentration of pre-polymer exceeds 10%. seen that the coating RI gradually tends to stabilize after the concentration of pre-polymer exceeds The sol mass ratio of of 6% (sol A) for the fabrication of The blended blended sol with with massmass ratioratio of pre-polymer pre-polymer of about about 6% 6% (sol(sol A) is is qualified forfor thethe fabrication of 10%. The blended sol with of pre-polymer of about A)qualified is qualified fabrication the bottom layer in the double-wavelength anti-reflective coating system, as the solidified coating thethe bottom layer coating of bottom layerininthe thedouble-wavelength double-wavelengthanti-reflective anti-reflectivecoating coatingsystem, system, as as the solidified coating from this sol has the desired RI of about 1.38. from this this sol sol has has the the desired desired RI RI of of about about1.38. 1.38. from
Figure mass ratio of pre-polymer. Figure 3. 3. The The relationship relationship between between the the coating coating reflective reflective index index and and mass mass ratio ratio of of pre-polymer. pre-polymer.
AFM AFM was was utilized utilized to to characterize characterize the the microstructure microstructure of of the the embedded embedded silicone silicone coating coating as as shown shown AFM was utilized toobvious characterize the microstructure of the embedded silicone coating as4a, shown inb in Figure 4, in which embedding structure of NPs can be observed. Figure in Figure 4, in which obvious embedding structure of NPs can be observed. Figure 4a, and and b Figure 4, in whichmorphology obvious embedding structure of NPs can be observed. Figure 4a, and b demonstrate demonstrate demonstrate the the morphology (in (in an an area area of of 500 500 nm nm ×× 500 500 nm) nm) of of solidified solidified coatings coatings prepared prepared from from asasthe morphology (in an area ofand 500sol nmA, × respectively. 500 nm) of solidified coatings prepared from as-synthesized synthesized pre-polymer sol The coating prepared from as-synthesized presynthesized pre-polymer sol and sol A, respectively. The coating prepared from as-synthesized prepre-polymer sol and sol A, surface respectively. surface The coating prepared from as-synthesized pre-polymerappears sol has polymer polymer sol sol has has aa smooth smooth surface with with surface roughness roughness Rq Rq of of 0.49 0.49 nm. nm. The The bump bump structure structure appears aonsmooth surface with after surface roughness Rq of 0.49 nm. The bump structure appears on the coating on the the coating coating surface surface after the the addition addition of of silica silica NPs NPs and and the the surface surface roughness roughness Rq Rq increases increases to to 2.14 2.14 surface after the addition of silica NPs and the surface roughness Rq increases to 2.14 nm, indicating nm, indicating that the silica NPs were successfully embedded in the solidified silicone coatings. nm, indicating that the silica NPs were successfully embedded in the solidified silicone coatings. that the silica NPs were successfully embedded in the solidified silicone coatings.
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Figure 4. AFM images of solidified coatings from (a) As-synthesized pre-polymer sol; (b) Sol A. Figure 4. AFM images of solidified coatings from (a) As-synthesized pre-polymer sol; (b) Sol A. Figure 4. AFM images of solidified coatings from (a) As-synthesized pre-polymer sol; (b) Sol A.
The embedding of HMDS-modified silica NPs into the bottom silicone coating is to modulate its The embedding ofHMDS-modified HMDS-modified silicaits NPs intothe thebottom bottom silicone coating iscontact to modulate of silica NPs into silicone coating is to modulate its RI toThe the embedding designed value as well as to enhance moisture-resistance capability. Water angle its RI to the designed value as well as to enhance its moisture-resistance capability. Water contact RI to the designed value well as enhance itswhich moisture-resistance capability. contact angle measurement results areasgiven in to Figure 5, in solidified coatings from Water as-synthesized preangle measurement are given in increase Figure 5,from in which solidified as-synthesized measurement results are given in Figure 5, in which solidified coatings fromfrom as-synthesized prepolymer sol and sol Aresults are compared. An 83.3° to 109.4° is coatings achieved after the embedding ◦ to 109.4◦ is achieved after the pre-polymer sol and sol A are compared. An increase from 83.3 polymer sol and sol Asilica are compared. An increase from 83.3° to 109.4° is of achieved after the embedding of HMDS-modified NPs, indicating successful enhancement moisture-resistance of the embedding of HMDS-modified NPs, indicating enhancement of moisture-resistance of HMDS-modified silica NPs,silica indicating successfulsuccessful enhancement of moisture-resistance of the bottom silicone coating. of the bottom silicone coating. bottom silicone coating.
Figure 5. Water contact angle measurement of solidified coatings from (a) As-synthesized preFigure 5. Water contact angle measurement of solidified coatings from (a) As-synthesized prepolymer (b)contact Sol A. angle Figure 5. sol; Water measurement of solidified coatings from (a) As-synthesized pre-polymer polymer sol; (b) Sol A. sol; (b) Sol A.
2.4. Fabrication and Characterization of Dual-Layer Coating System with Double-Wavelength Anti2.4. Fabrication and Characterization of Dual-Layer Coating System with Double-Wavelength AntiReflective and Moisture-Resistant Function 2.4. Fabrication and Characterization of Dual-Layer Coating System with Double-Wavelength Anti-Reflective Reflective and Moisture-Resistant Function and Moisture-Resistant HMDS-modified Function silica NP embedded silicone coating (Coating A) was spin-coated and HMDS-modified silica NP embedded coating (Coating A) was spin-coated and solidified on the KDP surface with an RI ofsilicone 1.38silicone andcoating a thickness of 130 the bottom layer. Porous HMDS-modified silica NP embedded (Coating A)nm wasas spin-coated and solidified solidified on the KDP surface with an RI of 1.38 and a thickness of 130 nm as the bottom layer. Porous silica coating (Coating B) prepared using HMDS-modified NPs sol was then spin-coated over the on the KDP surface with an RI of 1.38 and a thickness of 130 nm as the bottom layer. Porous silica silica coating (Coating B) prepared using HMDS-modified NPs sol was then spin-coated over the Coating(Coating A with an of 1.22 and thickness of 145 nm thewas topthen layer. To evaluate thethe transmission coating B) RI prepared usinga HMDS-modified NPsassol spin-coated over Coating A Coating A with an RI of 1.22 and a thickness of 145 nm as the top layer. To evaluate the transmission property of the dual-layer coating system more precisely, residual reflectance spectra is recorded with an RI of 1.22 and a thickness of 145 nm as the top layer. To evaluate the transmission property property of the dual-layer coating system more precisely, the residual reflectance spectra is recorded via a spectrometer to exclude the effect of absorption from the KDP substrate. By introducing of the dual-layer coating system more precisely, the residual reflectance spectra is recorded viathe a via a spectrometer to exclude the effect of absorption from the KDP substrate. By introducing the coefficient of the standard reflective sample, reflectance spectra of this dual-layer coating system is spectrometer to exclude the effect of absorption from the KDP substrate. By introducing the coefficient coefficient thereflective standard reflective sample, of reflectance spectra of thiscoating dual-layer coating system is depicted inofFigure 6, in which thereflectance spectrum a theoretically designed coatingsystem system also given of the standard sample, spectra of this dual-layer isisdepicted in depicted in Figure 6, in which the spectrum of a theoretically designed coating system is also given as a reference. Although the reflectance of this designed coating system slightly higher than of the Figure 6, in which the spectrum of a theoretically coatingissystem is also given as athat reference. as a reference. Although the reflectance of this coating system is slightly higher than that of the optimized design at the desired wavelengths, the transmittance still reaches as high as 99.62% and Although the reflectance of this coating system is slightly higher than that of the optimized design at optimized design at the desired wavelengths, the transmittance still reaches as high as 99.62% and 99.60% at 532 nm and 1064 nm, respectively, indicating successful modulation and exact matching of the desired wavelengths, the transmittance still reaches as high as 99.62% and 99.60% at 532 nm and 99.60% at 532 nm and 1064 nm, respectively, indicating successful modulation and exact matching of both nm, layers’ RIs. 1064 respectively, indicating successful modulation and exact matching of both layers’ RIs. both layers’ RIs.
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Figure 6. Reflectance spectra of fabricated dual-layer coating system and optimized theoretical design. Figure Figure 6. 6. Reflectance Reflectance spectra spectra of of fabricated fabricated dual-layer dual-layercoating coatingsystem systemand andoptimized optimizedtheoretical theoreticaldesign. design.
Long-term stability of KDP component is verified by measuring its spectral property under an Long-term stability of KDP KDP component component is verified by by measuring measuring its spectral property property under under an an verified spectral extremeLong-term condition stability of 80%of RH every week. is Figure 7 displays the its transmittance variation of KDP extreme condition condition of of 80% 80% RH RH every every week. week. Figure 7 displays displays the the transmittance transmittance variation variation of of KDP KDP extreme component under 80% RH over 27 weeks. Only very few transmittance decrease appears in such a component under under 80% 80% RH RH over over 27 27 weeks. weeks. Only Only very very few few transmittance transmittance decrease decrease appears appears in such such aa component longlong period, implying successful enhancement ofmoisture-resistance moisture-resistance by HMDS modification of silica period, implying implying successful successful enhancement enhancement of of moisture-resistance by HMDS HMDS modification modificationof of silica silica long period, by NPsNPs and embedding of the HMDS-modified silica NPs into the silicone coating. NPs and and embedding embedding of of the the HMDS-modified HMDS-modifiedsilica silicaNPs NPsinto intothe thesilicone siliconecoating. coating.
Figure 7. Transmittance variation of dual-layer coating system under 80% RH for 27 weeks. Figure 7. Transmittancevariation variation of dual-layer system under 80%80% RH for weeks. Figure 7. Transmittance dual-layercoating coating system under RH27 for 27 weeks. 3. Materials and Methods
3. Materials and Methods 3. Materials and Methods
3.1. Materials
3.1. Materials Tetraethylorthosilicate (TEOS) was purchased from Sinopharm Chemical Reagent Co., Ltd. 3.1. Materials
(Beijing, China). Methyltriethoxysilane Hexamethyldisiloxane (HMDS),Reagent decane, Co., and Ltd. secTetraethylorthosilicate (TEOS) was (MTES), purchased from Sinopharm Chemical Tetraethylorthosilicate (TEOS) wasAladdin purchased fromTechnology SinopharmCo., Chemical Reagent Co., Ltd. butanol was purchased from Shanghai Bio-Chem Ltd. (Shanghai, China). (Beijing, China). Methyltriethoxysilane (MTES), Hexamethyldisiloxane (HMDS), decane, and sec-butanol (Beijing, China). Methyltriethoxysilane (MTES), Hexamethyldisiloxane (HMDS), decane, and secAnhydrous ethanol, heptane, Aladdin and ammonia water were purchased from Chengdu Kelong Chemical was purchased from Shanghai Bio-Chem Technology Co., Ltd. (Shanghai, China). Anhydrous butanol washeptane, purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Co., Ltd. (Chengdu, China). The water deionized. Thefrom TEOS and MTES were distillation purified. ethanol, and ammonia waterwas were purchased Chengdu Kelong Chemical Co., Ltd. Other chemicals were used without further purification. Anhydrous ethanol, heptane, and ammonia water were purchased from Chengdu Kelong Chemical (Chengdu, China). The water was deionized. The TEOS and MTES were distillation purified. Other
Co.,chemicals Ltd. (Chengdu, China). The water was deionized. The TEOS and MTES were distillation purified. were used without further purification. 3.2. Preparation of Silicone Pre-Polymer Sol Other chemicals were used without further purification. MTES (1783 g) that had been distillation purified, anhydrous ethanol (266.8 g), and water (586.8 3.2. Preparation of Silicone Pre-Polymer Sol g) were blended and stirred at 80 °C in a 5 L glass reactor with a distillation head, which could realize the distillation or reflux process in the same equipment. Pre-mixed water (198 g) and anhydrous
MTES (1783 g) that had been distillation purified, anhydrous ethanol (266.8 g), and water (586.8 g) were blended and stirred at 80 °C in a 5 L glass reactor with a distillation head, which could realize the distillation or reflux process in the same equipment. Pre-mixed water (198 g) and anhydrous
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3.2. Preparation of Silicone Pre-Polymer Sol MTES (1783 g) that had been distillation purified, anhydrous ethanol (266.8 g), and water (586.8 g) were blended and stirred at 80 ◦ C in a 5 L glass reactor with a distillation head, which could realize the distillation or reflux process in the same equipment. Pre-mixed water (198 g) and anhydrous ethanol (184 g) mixture were added when the solution started boiling. The resultant solution was distilled and refluxed for another 20 h, and then diluted with anhydrous ethanol to prepare pre-polymer sol with the concentration of 33 wt % after cooling to room temperature. 3.3. Preparation of Colloidal Silica Sol The colloidal silica sol was prepared using TEOS (208 g) as matrix which was mixed with anhydrous ethanol (1610 g) and ammonium hydroxide aqueous solution (51.4 g, containing 30% ammonia) with stirring at 6 ◦ C for 3 h. The solution was kept still in a sealed glass container for three to five days at room temperature to implement the aging process. It was then refluxed for 10 h to remove the extra ammonia. This colloidal suspension contained about 3.3% silica by weight in ethanol and was filtered through a 0.2-µm polyvinylidene fluoride (PVDF) membrane filter prior to use. 3.4. HMDS Modification of the Colloidal Silica NPs The HMDS (180 g) was added into the refluxed colloidal silica sol (1818 g, 3.3 wt %), then stirring and aging were performed at room temperature for another 7 days. After that, decane (540 g) was added into the solution, and a sequential distillation at 120 ◦ C was applied to this solution until all the ethanol and byproducts were removed. 3.5. Embedding of the Silicone Pre-Polymer with Modified Silica NPs The silicone pre-polymer sol (33 wt %) and the HMDS-modified colloidal silica sol (3.3 wt %) were mixed in a certain proportion, and the RI of coatings prepared using this mixed sol can be were tuned according to the proportion. 3.6. Coating Preparation The silicone pre-polymer and colloidal silica mixed sol was spin-coated on the surface of KDP substrates (50 mm × 50 mm × 10 mm) to fabricate the bottom layer under a velocity from 1000 to 1500 rpm at a rated time of 60 s to ensure that its thickness was around 130 nm at different mix proportion. After solidification of this bottom layer, the top layer was spin-coated using HMDS-modified colloidal silica sol under a velocity of about 300 rpm at a rated time of 80 s. The thickness of the top layer was around 150 nm after the solidification, in order to match the theoretical design of optimized conditions for the double-wavelength anti-reflective coating system. 3.7. Characterization Ellipsometer (Sopra GES-5E, Courbevoie, France) was employed to measure the RI and thickness of coatings. The transmission spectra were recorded by a Perkin Elmer 950 UV-Vis spectrometer (Perkin Elmer, Waltham, MA, USA). Water contact angle was analyzed using Krüss DSA 100L (Krüss, Hamburg, Germany). The microstructural property of coatings was characterized using AFM (Bruker Dimension Icon, Karlsruhe, Germany). MAS NMR spectra were recorded in solid state using Varian Infinityplus 300WB (Varian, Palo Alto, CA, USA). The long-term stability verification was carried out in the BINDER KMF 115 cabinet (BINDER, Tuttlingen, Germany). 4. Conclusions HMDS-modified porous silica coating with an RI of 1.22 and HMDS-modified silica NP embedded silicone coating with an RI of 1.38 have been prepared to fabricate a dual-layer coating system with double-wavelength anti-reflective and moisture-resistant function over KDP substrate. Successful
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and precise RI modulation of the silicone coating has been achieved by controlling the embedding ratio of HMDS-modified silica NPs. Transmittance values as high as 99.60% at 532 nm and 99.62% at 1064 nm of this coating system were realized and very few transmittance reduction occurred during a 27-week-long period under extremely high humidity of 80% RH. Author Contributions: H.H. implemented the theoretical design of the coating system (Software); Q.Z. analyzed the data (Formal analysis); X.L. checked all the experiments and data (Validation); W.Y. performed the experiments and wrote the paper (Writing-original draft); X.D. direct this work and revised the paper (Supervision, Writing-review and editing). Acknowledgments: This work is supported by the National Natural Science Foundation of China (Grant No. 51505575). Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors. © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
