黃粉超,焦劍,程皓,王瑾,王佳
TiO2對(duì)中空硅減反射涂層硬度的影響
黃粉超1,焦劍2,程皓1,王瑾2,王佳2
(1.西安超碼科技有限公司,西安 710025;2.西北工業(yè)大學(xué),西安 710129)
提高中空硅減反射(AR)涂層的硬度。采用溶膠-凝膠法制備中空二氧化硅納米微球(HSNs)膠體溶液,通過異丙醇鈦(TTIP)的水解縮合作用,在HSNs表面沉積納米TiO2后,制備HSNs@TiO2膠體溶液。將HSNs@TiO2膠體溶液與酸性硅溶膠(ACSS)復(fù)合,制備HSNs@TiO2/ACSS減反射液,通過旋涂法在玻璃基板上制備相應(yīng)的AR涂層。通過特高分辨率場(chǎng)發(fā)射掃描電子顯微鏡、高分辨透射電子顯微鏡和原子力顯微鏡對(duì)HSNs和HSNs@TiO2納米粒子的形貌進(jìn)行分析,通過紫外-可見分光光度計(jì)和納米壓痕儀對(duì)HSNs/ACSS AR涂層和HSNs@TiO2/ACSS AR涂層的透射率、硬度和彈性模量分別進(jìn)行分析。納米TiO2沉積在HSNs表面后,減反射液中HSNs@TiO2納米粒子的粒徑較HSNs粒徑增大1~30 nm不等。由HSNs@TiO2/ACSS減反射液制備的AR涂層表面顆粒及團(tuán)簇明顯,表面粗糙度(RMS)可達(dá)9.61 nm,遠(yuǎn)高于HSNs/ACSS AR涂層的3.62 nm。含有較大粒徑HSNs@TiO2納米粒子的HSNs@TiO2/ACSS AR涂層使玻璃基板在550 nm波長(zhǎng)處的透射率增加1.3%,低于HSNs/ACSS AR涂層的增加值2.8%。納米TiO2沉積之前,HSNS/ACSS AR涂層的硬度和彈性模量分別為2.3 GPa和56.3 GPa,納米TiO2沉積之后,HSNs@TiO2/ACSS AR涂層的硬度和彈性模量分別為3.3 GPa和55.2 GPa,AR涂層的硬度顯著提高。溶膠-凝膠法在HSNs上沉積納米TiO2后,可有效提高AR涂層的硬度,因此AR涂層的環(huán)境適用性有望得到進(jìn)一步提高。
減反射涂層;溶膠-凝膠;中空SiO2;納米TiO2;硬度;透射率
隨著太陽能與電子科學(xué)技術(shù)領(lǐng)域的發(fā)展,具有高透射率、機(jī)械性能優(yōu)異、使用方便及成本低的減反射(AR)涂層在太陽能光伏[1-2]、集熱器[3-5]、探測(cè)器[6]、高功率激光[7-8]、顯示器件[9-11]等領(lǐng)域受到了國(guó)內(nèi)外學(xué)者的廣泛關(guān)注。AR涂層可以有效降低光的反射,提高光的透射[12]。在戶外惡劣環(huán)境中使用時(shí),摩擦、物體打擊等都會(huì)對(duì)AR涂層造成損傷,使AR涂層的機(jī)械性能變差,影響其使用壽命[13-14]。因此,研制一種具有優(yōu)異機(jī)械性能的高透射率AR涂層,對(duì)太陽能的有效收集利用和電子器件的性能提升都具有重要的現(xiàn)實(shí)意義。
近年來,SiO2AR涂層因其綜合性能優(yōu)異[15],受到國(guó)內(nèi)外學(xué)者的廣泛關(guān)注。其中,中空SiO2AR涂層因其低折射、高透射的特點(diǎn),已成為SiO2AR涂層研究領(lǐng)域的熱點(diǎn)之一。Tao等人[16]用甲基三乙氧基硅烷(MTES)和正硅酸乙酯(TEOS)作為前驅(qū)體,通過一步堿催化溶膠-凝膠法,制備了HSNs膠體溶液,將K9玻璃在該膠體溶液中浸漬后,得到了中空二氧化硅(HSNs)AR涂層,使玻璃的透射率達(dá)到97.65%。Guo等人[17]制備了苯乙烯-丙烯酸酯乳液@有機(jī)-無機(jī)二氧化硅前體(SA@OISP)核/殼分級(jí)納米結(jié)構(gòu),通過浸涂和煅燒處理,該SA@OISP納米球可形成中空閉孔二氧化硅減反射涂層(CHAR),CHAR在380~1100 nm波長(zhǎng)范圍內(nèi)的平均透射率為97.64%,接近理想單層AR涂層的最高透射率98.09%。
高透射的中空SiO2AR涂層實(shí)質(zhì)是由中空SiO2納米粒子在基底上密切堆積而成,與基底之間主要通過范德華力結(jié)合,機(jī)械性能較差[18-19],這在一定程度上縮短了其使用壽命,限制了其使用范圍。提高AR涂層機(jī)械性能的方法有很多[20-21],向AR涂層中加入無機(jī)納米顆粒,是一種提高AR涂層機(jī)械性能的有效方法,如添加納米TiO2[22-23]、納米SiO2[24-25]等。Guo等人[26]采用單浸漬溶膠-凝膠法,制備了二氧化硅-中空納米微球(SiO2-HNS)混合的SiO2涂層,該AR涂層的壓痕硬度約為2.0 GPa。Zhang等人[27]制備了一種由HSNs和ACSS組成的閉孔納米復(fù)合涂層,此涂層不添加無機(jī)納米顆粒,硬度約為1.6 GPa,低于添加納米顆粒的其他AR涂層。Miao等人[28]制備了雙層SiO2-TiO2涂層,第一層由雜化甲基功能化納米多孔SiO2組成,第二層是沉積在SiO2AR層頂部的超薄TiO2納米多孔層,在0.025 kg載荷下,SiO2-TiO2涂層的顯微硬度為598HV。由此可見,在具有高透射率AR涂層中引入無機(jī)納米粒子,可有效提高AR涂層的硬度。
本文針對(duì)中空硅AR涂層機(jī)械性能較差的問題,考慮向其中引入TiO2成分以提高AR涂層的硬度,創(chuàng)新性地提出了制備具有核-殼結(jié)構(gòu)的HSNs@TiO2納米粒子,并以該納米粒子作分散相制備AR涂層,希望在保留AR涂層中的大量空隙以提高其透射性能的同時(shí),利用TiO2提高AR涂層的硬度。通過溶膠-凝膠法,成功合成了具有核-殼結(jié)構(gòu)的HSNs@TiO2納米粒子,并將其與少量的酸性硅溶膠(ACSS)復(fù)合,制備了HSNs@TiO2/ACSS減反射液。由此制備的AR涂層不僅有良好的透射性,而且相對(duì)于HSNs/ACSS AR涂層,硬度有明顯提高。同時(shí),作為粘合劑存在的ACSS,可以使最終制備的AR涂層的致密性提高,有望進(jìn)一步提高該AR涂層的環(huán)境適應(yīng)性。
選用正硅酸乙酯(TEOS,質(zhì)量分?jǐn)?shù)為99%,分析純)作為SiO2前驅(qū)體,購(gòu)自成都市科龍化工試劑廠;異丙醇鈦(TTIP,99%,分析純)作為TiO2前驅(qū)體,聚丙烯酸(PAA,W≈5000,50%)作為HSNs制備前的核材料,購(gòu)自上海麥克林生化科技有限公司;催化劑為氨水(25%水溶液)和濃鹽酸(HCl,36%~38%的水溶液,分析純),分別購(gòu)自廣東省化學(xué)試劑工程技術(shù)研究開發(fā)中心和成都市科隆化學(xué)品有限公司。
HSNs/ACSS減反射液的制備:HSNs膠體溶液和ACSS的制備參考文獻(xiàn)[27]、[29]曾報(bào)道的方法,將兩者按一定比例混合均勻即可制備得到HSNs/ACSS減反射液。
HSNs@TiO2/ACSS減反射液的制備如下:1)在2 h內(nèi)分5次將0.05 g異丙醇鈦(TTIP,Ti(OC3H7)4)加入50 mL上述HSNs膠體溶液中;2)以500 r/min的攪拌速率繼續(xù)攪拌使其反應(yīng)6 h,即可制得HSNs@TiO2膠體溶液;3)將HSNs@TiO2膠體溶液和ACSS按3∶1的質(zhì)量比混合,混合后持續(xù)攪拌4 h使其反應(yīng)完全;4)將該混合膠體溶液在室溫下老化1 d后備用,即得到HSNs@TiO2/ACSS減反射液。
所用膠粘劑為自制的ACSS,玻璃基板為25 mm× 25 mm×1 mm的載玻片。
AR涂層的制備步驟如下:1)將載玻片置于去離子水和乙醇中分別超聲清洗10 min;2)采用KW-4A型旋涂?jī)x以4000 r/min的旋涂速率在載玻片上旋涂HSNs/ACSS或HSNs@TiO2/ACSS減反射液;3)將涂覆AR涂層的載玻片在室溫下晾干,然后置于450 ℃的馬弗爐中焙燒1.5 h;4)焙燒后的載玻片自然冷卻至室溫,得到涂覆HSNs/ACSS或HSNs@TiO2/ ACSSAR涂層的玻璃基板。
采用FEI Talos F200X型場(chǎng)發(fā)射高分辨透射電鏡(HRTEM)觀察HSNs@TiO2納米粒子。將HSNs@ TiO2納米粒子用無水乙醇稀釋到0.5%左右,超聲分散10 min,然后滴加到銅網(wǎng)上,紅外燈干燥后進(jìn)行HRTEM觀測(cè),加速電壓為200 kV。采用Verios G4型特高分辨率場(chǎng)發(fā)射掃描電鏡(FESEM)觀察HSNs@TiO2納米粒子和HSNs@TiO2/ACSS減反射膜的表面形貌,測(cè)量前作噴金處理,噴金時(shí)間60 s。采用Dimension Fastscan and Dimension Icon型原子力顯微鏡(AFM)觀察HSNs@TiO2/ACSS減反射膜的表面形貌和粗糙度。原子力測(cè)試模式為非接觸模式,頻率為5 Hz,針尖上的力為0.1 nN。采用UV-3100型紫外-可見(UV-Vis)分光光度計(jì)測(cè)量HSNs/ACSS減反射膜的透過率。儀器采用空氣為背景校零,樣品表面與測(cè)量光線保持垂直,波長(zhǎng)范圍為300~800 nm,掃描步長(zhǎng)為2 nm。采用TI980型納米壓痕儀(Hysitron公司)測(cè)量AR涂層的硬度和損耗模量r。測(cè)試的熱漂移率低于0.05 nm/s,每個(gè)樣品的測(cè)試范圍為50 μm×50 μm的1×5陣列。
圖1為HSNs及HSNs@TiO2納米粒子的微觀形貌。由圖1a、b可知,HSNs表面光滑,具有明顯的中空結(jié)構(gòu),形狀呈規(guī)則球形且粒徑分布均一,平均粒徑約50 nm。由圖1c、d可知,HSNs@TiO2納米粒子表面凹凸不平,存在明顯的顆粒狀小粒子,內(nèi)部中空結(jié)構(gòu)明顯,粒徑大多在50~80 nm范圍內(nèi)。與HSNs相比,HSNs@TiO2納米粒子表面的顆粒明顯,且粒徑明顯增大,表明納米TiO2成功沉積在HSNs表面。從HSNs@TiO2納米粒子的EDS圖(圖1e)中可獲得單個(gè)粒子的元素組成,C、O、Si、Ti四種元素的定量結(jié)果如表1所示。其中Ti元素在HSNs@TiO2納米粒子中的質(zhì)量分?jǐn)?shù)為14.59%,進(jìn)一步證明了納米TiO2已成功沉積在HSNs表面。Ti元素出現(xiàn)的兩個(gè)峰則是由核外電子的不同躍遷造成。納米TiO2與HSNs、納米TiO2與納米TiO2之間的結(jié)合力主要為范德華力。
圖1 HSNs及HSNs@TiO2納米粒子的微觀形貌圖
表1 單個(gè)HSNs@TiO2納米粒子的EDS測(cè)量結(jié)果
圖2為HSNs/ACSS AR涂層及HSNs@TiO2/ACSS AR涂層的微觀形貌圖。由圖2a、b可以看出,HSNs/ACSS AR涂層是由連續(xù)的ACSS相和非連續(xù)的HSNs顆粒相組成。其成形原理為,ACSS以膠粘劑的形式將HSNs粒子粘結(jié)起來,并在干燥的過程中進(jìn)一步完成縮聚成為凝膠,而HSNs粒子則分散于ACSS形成的凝膠中。HSNs/ACSS AR涂層的表面粗糙度為3.62 nm。由圖2c、d可見,HSNs@TiO2/ACSS AR涂層是由連續(xù)的ACSS相和HSNs@TiO2粒子相組成。其成形原理與HSNs/ACSS AR涂層一致,但其表面的顆粒及團(tuán)簇明顯,RMS值高達(dá)9.61 nm。這是因?yàn)楸患{米TiO2包覆的HSNs在膠體溶液中的穩(wěn)定性差,易團(tuán)聚,且游離在減反射液中的未包覆在HSNS上的納米TiO2也因范德華力作用聚集成簇。團(tuán)聚的HSNs@TiO2粒子和成簇的納米TiO2分散于ACSS中,仍保持其聚集狀態(tài),導(dǎo)致由該減反射液制備的AR涂層出現(xiàn)粒徑較大的團(tuán)聚顆粒。與HSNs/ACSS AR涂層相比,HSNs@TiO2/ACSS AR涂層的粗糙度明顯增加。
圖2 HSNs/ACSS AR涂層及HSNs@TiO2/ACSS AR涂層的微觀形貌圖
圖3為HSNs@TiO2/ACSS AR涂層、HSNs/ACSS AR涂層和載玻片的透射圖??梢钥闯觯扛睞R涂層的載玻片的透射率上升階段主要集中在425~ 675 nm,HSNs@TiO2/ACSS AR涂層的透射率雖較載玻片有所提高,但低于HSNs/ACSS AR涂層。在550 nm波長(zhǎng)處,HSNs@TiO2/ACSS AR涂層的透射率為87.0%,較載玻片的透射率85.7%提高了1.3%,而HSNs/ACSS AR涂層的透射率提高到了88.5%,較載玻片的透射率提高了2.8%。這是因?yàn)閱螌覣R涂層的透射率曲線呈“Λ”形,這意味著涂層存在峰值透射率,僅在峰值處的透射率高,在其他處的透射率會(huì)降低,即/4光學(xué)AR涂層的增透帶寬較小。本次試驗(yàn)的峰值透射率出現(xiàn)在約525 nm波長(zhǎng)處,結(jié)合單層AR涂層透射率的曲線特征,發(fā)現(xiàn)其增透寬度主要集中在550 nm波長(zhǎng)前后,即425~675 nm之間。
圖3 HSNs@TiO2/ACSS AR涂層、HSNs/ACSS AR涂層和載玻片的透射圖
影響AR涂層透射率的直接因素是AR涂層的折射率,根據(jù)Lorentz-Lorentz公式,薄膜的折射率與其孔隙率相關(guān),薄膜孔隙率越大,其折射率越低。在本研究中,具有較大孔隙率的HSNs@TiO2/ACSS AR涂層折射率較低,因而提高了載玻片的透射率,但該AR涂層較高的表面粗糙度導(dǎo)致其表面漫反射增加,且TiO2自身具有較高的折射率[30](銳鈦礦的折射率≈2.52),因此在一定程度上,透射率較HSNs/ACSS AR涂層的透射率有所降低。
圖4為HSNs@TiO2/ACSS AR涂層和HSNs/ACSS AR涂層的典型載荷-位移曲線??梢钥闯觯谙嗤妮d荷作用于AR涂層表面時(shí),探針在HSNs@TiO2/ ACSS AR涂層內(nèi)的位移均小于在HSNs/ACSS AR涂層內(nèi)的位移,表明HSNs@TiO2/ACSS AR涂層的硬度高于HSNs/ACSS AR涂層。在施加200 μN(yùn)的載荷時(shí),HSNs@TiO2/ACSS AR涂層的位移、平均硬度和損耗模量分別為32 nm、3.3 GPa和54.1 GPa,而HSNs/ACSS AR涂層的位移、平均硬度和損耗模量分別為55 nm、2.3 GPa和56.3 GPa。200 μN(yùn)的壓入載荷是通過基礎(chǔ)標(biāo)準(zhǔn)法在一個(gè)壓痕點(diǎn)上進(jìn)行一系列加載-卸載過程測(cè)試后選取而來的。AR涂層的彈性模量s可通過公式(1)計(jì)算:
圖4 HSNs@TiO2/ACSS AR涂層和HSNs/ACSS AR涂層的典型載荷-位移曲線
式中:r為試樣材料的損失彈性模量;i和i分別為壓頭的彈性模量和泊松比;s和s分別為試樣材料的彈性模量和泊松比。
由公式(1)計(jì)算可知,HSNs@TiO2/ACSS AR涂層和HSNs/ACSS AR涂層的彈性模量分別為57.6 GPa和55.2 GPa。與HSNs/ACSS AR涂層相比,HSNs@TiO2/ ACSS AR涂層的硬度增加,這是AR涂層內(nèi)高硬度的納米TiO2所致。
在本研究當(dāng)中,納米TiO2包覆在HSNs表面,且大部分隨HSNs均勻分布于ACSS中,形成減反射液。由該減反射液制備的HSNs@TiO2/ACSS AR涂層與HSNs/ACSS AR涂層相比,表現(xiàn)出更好的力穩(wěn)定性和牢固性,因此其硬度更高。HSNs@TiO2/ACSS AR涂層和HSNs/ACSS AR涂層在200 μN(yùn)載荷下的位移、硬度和彈性模量如表2所示。
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表2 HSNs@TiO2/ACSS AR涂層和HSNs/ACSS AR涂層的位移、硬度、損耗模量和彈性模量
Tab.2 Displacement, hardness, reduced elastic modulus and elastic modulus of HSNs@TiO2/ACSS AR coating and HSNs/ ACSS AR coating
1)通過溶膠-凝膠法,可成功將納米TiO2沉積在HSNs上,從而制備得到核-殼結(jié)構(gòu)的HSNs@TiO2納米粒子。此種方法引入的納米TiO2在AR涂層中分布均勻,有利于提高AR涂層的硬度。
2)納米TiO2的引入顯著提高了AR涂層的硬度,當(dāng)HSNs@TiO2膠體溶液與ACSS的質(zhì)量比為3∶1時(shí),HSNs@TiO2/ACSS AR涂層的硬度可達(dá)3.3 GPa,同時(shí)該涂層在550 nm波長(zhǎng)處的透射率較玻璃基板提高了1.3%。
3)納米TiO2的引入會(huì)導(dǎo)致納米HSNs@TiO2和納米TiO2團(tuán)聚粒子的出現(xiàn),增加AR涂層表面粗糙度,使HSNs@TiO2/ACSS AR涂層透射率較HSNs/ ACSS AR涂層有所降低。
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Effect of TiO2on the Hardness of Hollow Silica Antireflection Coating
1,2,1,2,2
(1.Xi’an ChaoMa Technology Co., Ltd, Xi’an 710025, China; 2.Northwestern Polytechnical University, Xi’an 710129, China)
The purpose is to improve the hardness of hollow silica antireflection (AR) coatings. In this paper, a colloidal solution of hollow silica nanospheres (HSNs) is prepared by sol-gel method, and HNSs@TiO2colloidal solution is prepared by depositing nano-TiO2on the surface of HSNs through hydrolysis and condensation of titanium isopropoxide (TTIP). The HSNs@TiO2/ACSS AR solution is prepared by mixed the HSNs@TiO2colloidal solution and acidic silica sol (ACSS). The morphology of HSNs and HSNs@TiO2nanoparticles are analyzed by ultra-high resolution field emission scanning electron microscope, high-resolution transmission electron microscope and atomic force microscope. The transmittance, hardness and elastic modulus of HSNs/ACSS AR coating and HSNs@TiO2/ACSS AR coating are analyzed by UV-visible spectrophotometer and nanoindenter respectively. After the nano-TiO2is deposited on the surface of HSNs, the particle size of the HSNs@TiO2nanoparticles in antireflection liquid increased by 1~30 nm compared with the particle size of the HSNs; Particles and clusters on the surface of AR coating that prepared by HSNs@TiO2/ACSS AR liquid are obvious, and the surface roughness (RMS) of the AR coating could reach 9.61 nm, which is much higher than 3.62 nm of HSNs/ACSS AR coating; HSNs@TiO2/ACSS AR coating with larger HSNs@TiO2nanoparticles increased the transmittance of glass at 550 nm by 1.3%, which is lower than 2.8% of HSNs/ACSS AR coating; Before the nano-TiO2deposited, the hardness and elastic modulus of the HSNS/ACSS AR coating are 2.3 GPa and 56.3 GPa, respectively, the hardness of the AR coating is significantly improved after the nano-TiO2deposited, the hardness and elastic modulus of the HSNs@TiO2/ACSS AR coating are 3.3 GPa and 55.2 GPa, respectively. The nano-TiO2deposited on HSNs by sol-gel method could effectively improve the hardness of AR coatings, so the environmental applicability of AR coatings is expected to be further improved.
AR coatings; sol-gel; hollow SiO2; nano-TiO2; hardness; transmittance
2020-03-23;
2020-05-26
HUANG Fen-chao(1994—), Female, Master, Assistant engineer, Research focus: nano functional coating.
焦劍(1970—),女,博士,教授,主要研究方向?yàn)榧{米復(fù)合材料和介孔材料。郵箱:jjiao@nwpu.edu.cn
Corresponding author:JIAO Jian (1970—), Female, Doctor, Professor, Research focus: nanocomposite and mesoporous materials. E-mail: jjiao@ nwpu.edu.cn
黃粉超, 焦劍, 程皓, 等. TiO2對(duì)中空硅減反射涂層硬度的影響[J]. 表面技術(shù), 2021, 50(4): 191-197.
TB332
A
1001-3660(2021)04-0191-07
10.16490/j.cnki.issn.1001-3660.2021.04.018
2020-03-23;
2020-05-26
黃粉超(1994—),女,碩士,助理工程師,主要研究方向?yàn)榧{米功能涂層。
HUANG Fen-chao, JIAO Jian, CHENG Hao, et al. Effect of TiO2on the hardness of hollow silica antireflection coating[J]. Surface technology, 2021, 50(4): 191-197.