李琪, 黃羿, 錢濱, 許貝貝, 陳莉英, 肖文戈, 邱建榮

橙黃光玻璃陶瓷的光固化成型與無壓燒結

李琪1, 黃羿1, 錢濱2, 許貝貝1, 陳莉英1, 肖文戈1, 邱建榮1

(1. 浙江大學 光電科學與工程學院, 杭州 310027; 2. 寧波匠心快速成型技術有限公司, 寧波 315000)

傳統(tǒng)“熒光粉+有機硅脂”熒光轉換體的熱導率低, 且物理化學穩(wěn)定性差, 不能應用于高功率白光LED領域。全無機熒光塊體材料可以規(guī)避有機封裝, 具有更高的熱導率, 但這類材料面臨著成本高且極難實現(xiàn)立體結構的問題。本工作基于非晶態(tài)納米二氧化硅, 得到一種包含(Gd,Y)AG:Ce熒光粉、可在紫外光下固化的漿料, 并通過光固化成型、空氣排脂、無壓燒結, 制備了一種(Gd,Y)AG:Ce熒光粉–石英玻璃復合材料。該熒光玻璃陶瓷在藍光激發(fā)下發(fā)射峰值位于575 nm的寬帶橙黃光, 且內(nèi)量子效率大于90%。研究結果表明, 在致密化燒結過程中, (Gd,Y)AG:Ce熒光粉與石英玻璃之間的界面反應非常微弱, 因此熒光粉能夠完好地嵌入到石英玻璃中。該全無機熒光轉換體可以用于封裝相關色溫小于4500 K、顯色指數(shù)大于75和流明效率為74 lm·W–1的高功率暖白光LED。所構建的激光照明器件的飽和激光功率密度可達2.84 W·mm–2, 此時光通量為180 lm。此外, 所提出的制備方法與3D打印兼容, 可以批量化制造出具有復雜立體結構的熒光轉換體。該技術有望推動高功率白光LED朝著個性化和模塊化發(fā)展。

熒光粉; 玻璃陶瓷; 暖白光; 3D打印

半導體發(fā)光二極管(Light emitting diodes, LED)因體積小、耗能低、壽命長等優(yōu)點, 已廣泛應用于通用照明和背光顯示領域, 其應用場景也在進一步拓展[1-6]。目前獲得白光LED的主流方案是藍光LED激發(fā)黃色熒光粉(YAG:Ce), 且大部分采用傳統(tǒng)的“環(huán)氧樹脂/有機硅脂+熒光粉”方式進行封裝。這些有機透明材料的熱導率極低且高溫下易老化[7], 限制了其在高功率照明領域的應用。解決上述問題的有效方法是改用熒光陶瓷、單晶、玻璃陶瓷等塊體材料作為全無機熒光轉換體[8-15]。作為一類新型熒光玻璃陶瓷, Phosphor-in-Glass(PiG)復合體可以兼具熒光粉的高量子效率和玻璃基體的優(yōu)良物理化學穩(wěn)定性和較高熱導率[14-18]。相比于熒光單晶或陶瓷, PiG的制造工藝更簡單、成本更低。因此, PiG在大功率白光LED或激光照明領域具有巨大的應用潛力。

基于YAG:Ce黃色熒光粉的白光LED, 由于光譜中缺乏紅光成分, 只能得到相關色溫(Correlated color temperature, CCT)大于5500 K的冷白光[9,19-20]。CCT小于5500 K的正白光或暖白光更加符合人們對照明光源的需求, 特別適合于家居照明。盡管添加紅色熒光粉可以獲得高顯色性的暖白光, 但是目前氮化物/氟化物紅色熒光粉的物理化學穩(wěn)定性差, 不能應用于高功率照明領域[21-26]。另一種策略是通過晶格工程降低Ce3+的5d能級高度使其發(fā)射光譜紅移[27-33], 例如Gd3+取代Y3+可以獲得橙黃色熒光粉(Y,Gd)AG:Ce[30-32]。已有的研究工作均是關于(Y,Gd)AG:Ce透明陶瓷或者單晶[12,30-32], 已經(jīng)報道的平板型全無機熒光轉換體會導致白光LED的出光角小、效率低以及“黃環(huán)”等發(fā)光不均勻現(xiàn)象[8,15,34-35]。將遠程熒光結構從傳統(tǒng)的平板改為半球、鐘型和鈸形等曲面立體結構不僅可以顯著改善LED器件的角度顏色均勻性, 而且能提高器件的出光效率, 沿中心角具有較厚熒光粉層的幾何形狀可以更好地匹配藍光LED芯片的朗伯發(fā)射[36-39]。但是, 傳統(tǒng)粉末燒結和單晶生長均極難實現(xiàn)三維立體結構, 增材制造(3D打印)作為一種快速成形技術, 具有高度的可定制性, 并已在無機材料的個性化和模塊化生產(chǎn)方面展示出巨大潛力[14,40-41]。

本工作在前期研究的基礎上[14], 選取非晶態(tài)納米二氧化硅(SiO2)和(Gd,Y)AG:Ce橙黃色熒光粉為主要原料, 設計了一種可光固化復合漿料, 然后通過光固化成型、排脂和還原氣氛燒結等步驟獲得了一系列高效率的(Gd,Y)AG:Ce-PiG熒光體, 并對其晶體結構、形貌、發(fā)光特性和LED/LD器件性能等進行詳細表征和分析, 還探索了不同摻雜熒光粉濃度對LED器件的流明效率、顯色指數(shù)(Color rendering index, CRI)和CCT等光學參數(shù)的影響。最后, 通過DLP 3D打印技術演示了本制造方法在實現(xiàn)三維立體遠程熒光體上的可行性。

1 實驗方法

1.1 實驗原料

原料主要包含: 甲基丙烯酸羥乙酯(HEMA, 96%,阿拉丁)、聚乙二醇二丙烯酸酯(PEGDA400, 阿拉丁)、二甘醇二苯甲酸酯(DEDB, 99.5%, 佛山今佳新材料)、2,2-二甲氧基-2-苯基苯乙酮(DMPA, 99%, 阿拉丁)、光引發(fā)劑Irgacure819(德國Basf)和蘇丹紅G(95%, 阿拉丁)組成、氣相二氧化硅(Aerosil OX50, 德國Evonik)、(Gd,Y)AG:Ce熒光粉(中心波長為575 nm)。

1.2 (Gd,Y)AG-PiG的制備

可光固化漿料的制備:將體積分數(shù)67%的HEMA、3%的PEGDA400和30%的DEDB混合均勻后加入平均粒徑為40 nm的氣相二氧化硅并充分攪拌, 其中溶液和氣相二氧化硅的體積約比為6 : 4。然后, 在上述漿料中添加質(zhì)量分數(shù)0.26%的紫外光引發(fā)劑DMPA(若用于3D打印則替換為Irgacure819, 并額外加入質(zhì)量分數(shù)0.004%的蘇丹紅G)。最后加入一定質(zhì)量分數(shù)的橙色熒光粉(Gd,Y)AG:Ce, 并充分攪拌和除泡。

光固化成型:將所得漿料倒入特定形狀的模具中, 放置在1000 W的365 nm紫外燈下照射30 s進行固化成型, 或者倒入DLP 3D打印機(MoonRay-S, 浙江迅實科技)料槽中進行前驅(qū)體的3D打印成型。

脫脂: 將成型后的前驅(qū)體放入高溫箱式爐中, 緩慢加熱到600 ℃, 并保溫10 h。

燒結:將上述多孔坯體放入高溫管式爐中, 并通入弱還原氣體((N2) :(H2)=95 : 5), 在1250 ℃下燒結3 h, 即得到完全致密化的(Gd,Y)AG-PiG樣品。

1.3 晶體結構與發(fā)光特性表征

采用紫外–可見分光光度計(U-4100, 日立)測量樣品的透射光譜。樣品的晶體結構由粉末X射線衍射(XRD)譜儀(D/MAX 2550/PC, Rigaku)確認, 由配有能量色散光譜儀(INCA EnergyCoater, 牛津儀器)的掃描電子顯微鏡(SEM, Utral-55,卡爾·蔡司)分析樣品的微觀形貌和元素分布。用光學顯微鏡(BX53M,奧林巴斯)拍攝光學顯微圖, 使用激光掃描共聚焦顯微鏡(SP5, 徠卡)分析石英玻璃中熒光粉顆粒的分布情況。用FLS920P光譜儀(愛丁堡)獲得發(fā)射和激發(fā)光譜, 其中在測試變溫發(fā)射光譜時, 使用高溫熒光測試裝置(TAP02, 東方科捷)控制溫度。使用紫外近紅外絕對量子產(chǎn)率測量儀(Quantaurus-QY Plus C13534-12, 濱松)測量樣品的內(nèi)量子效率(Internal quantum efficiency, IQE)和吸收率(Absorption efficiency, AE)。

1.4 白光LED/LD器件的封裝與測試

將制得的PiG圓片拋光至指定厚度, 并切割成10 mm×10 mm的正方形, 直接嵌入到高功率(10 W)藍光LED芯片, 并用導熱硅膠密封, 得到高功率白光LED器件。采用配有積分球(SPEKTRON R98,50 cm)的LED綜合測試系統(tǒng)(LHS-1000, 杭州遠方光電)測量LED/LD器件的光學性能, 如CCT、CRI、流明效率和色坐標等?；谏鲜鰷y試系統(tǒng), 本課題組自行搭建了激光照明測試系統(tǒng)對藍光激光激發(fā)下的光色度參數(shù)進行測試, 其中激發(fā)光源為功率可調(diào)的450 nm半導體激光器(Laser diodes, LD)(寧波遠明光電, LSR450CP-15W)。

2 結果與討論

2.1 晶體結構和形貌分析

如圖1(a)所示, 摻雜了(Gd,Y)AG:Ce熒光粉的樣品均能在藍光激發(fā)下發(fā)射明亮的橙黃光。圖1(b)為(Gd,Y)AG:Ce-PiG樣品在300～800 nm的透過率曲線。隨著(Gd,Y)AG:Ce摻雜濃度的增加, 樣品的全透過率逐漸降低。這是因為熒光粉顆粒(≈1.85)和石英玻璃(≈1.46)之間存在較大的折射率差異, 當熒光粉濃度增大時, 樣品對入射光的散射作用增強, 透過率下降。此外, 添加熒光粉之后, 樣品在450 nm處出現(xiàn)一個很強的吸收帶, 這來源于(Gd,Y)AG:Ce 中Ce3+離子對藍光的吸收。從樣品的XRD圖譜(圖1(c))可知, (Gd,Y)AG:Ce顆粒在致密化燒結后仍保留石榴石立方相, 從而形成一種熒光粉–玻璃復合體(PiG)。

(a) (Gd,Y)AG:Ce-PiG (0.5 mm in thickness) with different doping concentrations under daylight and blue light (using a 480 nm filter to filter out blue light when taking pictures); (b) Transmittance spectra of (Gd,Y)AG:Ce-PiG samples; (c) XRD patterns of silica glass, (Gd,Y)AG:Ce phosphors and (Gd,Y)AG:Ce-PiG

Colorful figures are availuable on the website

如圖2(a～c)所示, 熒光粉均勻分布在石英玻璃的表面和內(nèi)部, 沒有出現(xiàn)明顯的團聚現(xiàn)象。光固化成型所需時間極短(30 s), 有效避免了傳統(tǒng)熱固化成型中存在的熒光粉沉降現(xiàn)象。從SEM照片(圖2(d))可以看出, (Gd,Y)AG:Ce顆粒與SiO2玻璃的界面非常清晰, 表明二者在高溫燒結時沒有發(fā)生明顯的反應。對樣品進行元素能譜分析(EDS面掃描), 如圖2(e, f)所示, Al元素(代表(Gd,Y)AG:Ce顆粒)存在的區(qū)域沒有Si元素(代表SiO2玻璃基質(zhì)), 反之亦然。這些結果表明在高溫燒結時(Gd,Y)AG:Ce顆粒幾乎沒有受到石英玻璃的侵蝕, 即完好地嵌入到石英玻璃中, 使得PiG樣品可以同時擁有熒光粉的發(fā)光性能和石英玻璃的物理化學穩(wěn)定性。

2.2 (Gd,Y)AG:Ce-PiG的發(fā)光性能

圖3(a)為(Gd,Y)AG:Ce-PiG的激發(fā)和發(fā)射光譜。在460 nm藍光激發(fā)下, PiG發(fā)射峰值波長為575 nm的寬帶橙黃光, 兩個激發(fā)帶位于在340和460 nm附近, 這些均源于(Gd,Y)AG:Ce中Ce3+離子的4f-5d躍遷。相對于YAG:Ce黃色熒光粉(≈540 nm), (Gd,Y)AG:Ce的發(fā)射峰明顯紅移, 主要是因為Gd3+取代Y3+導致5d能級的晶場劈裂增加, 5d與4f之間的能量差變小[30]。如圖3(b)所示, (Gd,Y)AG:Ce-PiG樣品對藍光(450 nm)的吸收率隨著熒光粉摻雜濃度先增加后減小, 而內(nèi)量子效率均在90%左右, 其中, 質(zhì)量分數(shù)5%和7%摻雜的PiG的IQE高達91.2%。相比于(Gd,Y)AG:Ce熒光粉(IQE=93.3%), (Gd,Y)AG:Ce- PiG樣品的內(nèi)量子效率僅下降了2%。進一步測試了(Gd,Y)AG:Ce-PiG的變溫熒光光譜(圖3(c))。隨著溫度升高, 發(fā)光強度單調(diào)下降, 這是因為在高溫下激發(fā)態(tài)電子更容易被熱激活到導帶或者通過位形坐標的交叉點無輻射弛豫[12]。PiG和熒光粉的積分發(fā)光強度隨著溫度的變化曲線幾乎重合(圖3(d))。這些結果說明在還原氣氛的保護下, 即使在1250 ℃(3 h)下(Gd,Y)AG:Ce-PiG仍保留了相應熒光粉的發(fā)光性能, 也證明了(Gd,Y)AG:Ce與石英玻璃之間非常有限的界面反應。

2.3 高功率LED/LD性能測試

為展示(Gd,Y)AG:Ce-PiG在高功率領域的應用潛力, 使用(Gd,Y)AG:Ce-PiG薄片與高功率(10 W) 460 nm LED芯片組裝成白光LED原型器件。圖4(a)為基于不同摻雜濃度PiG的LED器件及其電致發(fā)光光譜(100 mA)。表1列出了基于不同摻雜濃度PiG的LED器件的流明效率、CCT和CRI等參數(shù)。

圖2 3%(質(zhì)量分數(shù))摻雜(Gd,Y)AG:Ce-PiG的光學照片和能譜分析

(a) Fluorescence microscope image; (b, c) 2D and 3D confocal laser scanning microscope images; (d) SEM image; (e, f) EDS spectra of Si and Al

圖3 (Gd,Y)AG:Ce-PiG的發(fā)光性能

(a) Excitation and emission spectra of 5% (mass fraction) (Gd,Y)AG:Ce-PiG; (b) Values of internal quantum efficiency (IQE), absorption efficiency (AE) and external quantum efficiency (EQE) of (Gd,Y)AG:Ce-PiG with different doping concentrations; (c) Temperature-dependent emission spectrum of 5% (mass fraction) (Gd,Y)AG:Ce-PiG PiG; (d) Temperature dependences of integrated emission intensity of (Gd,Y)AG:Ce-PiG and (Gd,Y)AG:Ce phosphor

圖4 白光LED器件的電致發(fā)光光譜及其相應的CIE色坐標

(a) Electroluminescence spectra; (b) Corresponding CIE color coordinates. White LEDs fabricated by using (Gd,Y)AG:Ce-PiG (0.8 mm in thickness) with different doping concentrations under the current of 100 mA; The inset shows the picture of LED device

表1 白光LED器件的光學性能

相比于傳統(tǒng)“藍光LED+黃色YAG:Ce熒光粉”白光LED, 基于(Gd,Y)AG:Ce的白光LED, 其光譜中紅光成分更多, 因此, 可以通過熒光粉濃度調(diào)節(jié)LED和熒光粉的光譜比例實現(xiàn)從冷白光到暖白光的調(diào)控(表1和圖4(b))。摻雜濃度為質(zhì)量分數(shù)5%的(Gd,Y)AG:Ce-PiG制成的白光LED器件的流明效率可達74.2 lm/W, CCT為4444 K, CRI為78.4, 而YAG:Ce僅能實現(xiàn)CCT > 5500 K冷白光輸出[9,19-20]。使用如圖5(a)所示的反射式激光照明測試系統(tǒng)進一步探究了樣品在高功率密度激發(fā)下的熒光轉換能力。如圖5(b)所示, 摻雜濃度為質(zhì)量分數(shù)5%的(Gd,Y)AG:Ce-PiG樣品(厚度0.8 mm)在藍色激光輻照下的發(fā)射強度隨激光功率的增加而增大, 在2.84 W·mm–2下達到最大值, 此時LD照明器件的流明通量達到180 lm。(Gd,Y)AG:Ce-PiG的輸出飽和閾值隨著熒光粉摻雜濃度的增加而變小(圖5(c)), 這是因為PiG樣品對激發(fā)光的吸收增強, 進而產(chǎn)生更多的熱量, 最終因樣品溫度升高而導致(Gd,Y)AG:Ce熒光猝滅[8, 24]。相應的CCT、CRI和輻射流明效率(Luminous efficiency of radiation, LER)如圖5(d)所示。CCT和CRI在PiG未達到發(fā)光飽和時均變化較小, 而LER則呈單調(diào)下降趨勢, 主要是因為在高功率激光激發(fā)下, PiG的工作溫度上升, Ce3+的非輻射躍遷幾率增加。這些結果表明(Gd,Y)AG:Ce-PiG適合用作中高功率暖白光LED/LD的熒光轉換 材料。

2.4 3D打印熒光轉換體

目前基于“熒光粉+有機硅脂”以及熒光無機塊體材料(如陶瓷、單晶以及PiG)的高功率LED均采用平面型結構進行封裝[8,15]。研究表明, 曲面立體結構的熒光轉換體不僅能提高LED器件的出光效率, 還能改善其顏色均勻性[36-39]。采用一臺桌面DLP 3D打印機演示了三種立體結構的3D打印制造, 分別為半球形、半球–圓柱型和半橢球形, 如圖6(a～c)。3D打印的遠程熒光體前驅(qū)體在燒結后能夠保持原始形狀, 表面亦無明顯裂紋, 且內(nèi)量子效率均在90%左右。將得到的PiG直接覆蓋在1 W的460 nm LED芯片上, 即可組裝成相應的白光LED(圖6(d))?？紤]到目前3D打印PiG前驅(qū)體的成品率較低, 相關工藝還有待進一步優(yōu)化。通過合理的光學設計和優(yōu)化的3D打印和燒結工藝, 該方法將大幅改善高功率白光LED的光學性能。

3 結論

本研究基于非晶態(tài)納米復合漿料的光固化成型和無壓致密化燒結, 制備了一種與3D打印兼容的熒光玻璃陶瓷(PiG)復合材料。得益于石英玻璃和(Gd,Y)AG:Ce熒光粉之間微弱的界面反應, 成功制備了一種橙黃色全無機熒光轉換體, 其不僅內(nèi)量子效率高(>90%)和熱穩(wěn)定性較好, 還能夠?qū)崿F(xiàn)曲面立體結構。利用(Gd,Y)AG:Ce-PiG可以封裝得到CCT<4500 K、CRI>75暖白光LED, 且能夠耐受2.84 W·mm–2的藍光激光密度輻照, 在中高功率暖白光固態(tài)光源領域展示出較大潛力。全無機熒光轉換體的3D打印將推動高功率白光LED進入個性化和模塊化階段。

圖5 高功率下(Gd,Y)AG:Ce-PiG的光學性能

(a) Schematic of reflective LD device; (b) Luminous flux of (Gd,Y)AG:Ce-PiG (0.8 mm in thickness) with different doping concentrations as a function of the laser power density; (c) Emission spectra of 5% (mass fraction) (Gd,Y)AG:Ce-PiG under different laser powers densities; (d) Values of CCT, CRI and luminous efficacy of radiation (LER) of 5% (mass fraction) (Gd,Y)AG:Ce-PiG under different laser power densities

Colorful figures are availuable on the website

圖6 3D打印熒光轉換體

(a) Photos of 5% (mass fraction) doped 3D printed precursor; (b) Photos of sintered (Gd,Y)AG:Ce-PiG; (c) Sintered (Gd,Y)AG:Ce-PiG under 450 nm blue light irradiation; (d) Device demonstration of white LED when combined with blue LED chip

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Photo Curing and Pressureless Sintering of Orange-emitting Glass-ceramics

LI Qi1, HUANG Yi1, QIAN Bin2, XU Beibei1, CHEN Liying1, XIAO Wenge1, QIU Jianrong1

(1. School of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China; 2. Ningbo Ingenuity Rapid Prototyping Technology Co., Ltd., Ningbo 315000, China)

Because of low thermal conductivity and weak physical and chemical stabilities, traditional “phosphor in silicone” color converters are precluded from high-power white LED applications. All-inorganic bulk luminescence materials not only can circumvent organic encapsulation, but also have higher thermal conductivity.However, those bulk materials are high in cost and very difficult to be shaped into three-dimensional structures. Here, based on amorphous silica nanoparticles, a slurry, containing (Gd,Y)AG:Ce phosphor powders and can be polymerized under UV light, were developed. Bulk (Gd,Y)AG:Ce-silica glass composites were prepared successfully through photo curing, debinding in air and pressureless sintering. Under excitation of blue light, these luminescence glass-ceramics exhibit broadband orange emission peaking at 575 nm with internal quantum efficiency higher than 90%. Our results show that the interfacial reaction between (Gd,Y)AG:Ce and silica glass is very weak, and thus the former can be well embedded into bulk silica glass. Such all-inorganic color converters were further used to fabricate high-power warm white LEDs with correlated color temperature smaller than 4500 K, color rendering index higher than 75, and luminous efficiency of 74 lm·W–1. Luminescence saturation threshold of the as-fabricated laser lighting device is as high as 2.84 W·mm–2, where its luminous flux can achieve 180 lm. Moreover, preparation of (Gd,Y)AG: Ce-silica glass composites is compatible to 3D printing technology, thus allowing the mass manufacturing of color converters with complex 3D structures, which may promote personalization and modularization of high-power white LEDs.

phosphors; glass-ceramics; warm white light; 3D printing

TQ174

A

1000-324X(2022)03-0289-08

10.15541/jim20210518

2021-08-23;

2021-09-24;

2021-11-01

浙江省重點研發(fā)計劃(2021C01024); 中國博士后科學基金(2021M692840); 浙江大學現(xiàn)代光學儀器國家重點實驗室開放基金

Provincial Key R&D Program of Zhejiang (2021C01024); China Postdoctoral Science Foundation (2021M692840); Open Fund of the State Key Laboratory of Modern Optical Instrumentation, Zhejiang University

李琪(1998–), 女, 碩士研究生. E-mail: 22030030@zju.edu.cn

LI Qi (1998–), female, Master candidate. E-mail: 22030030@zju.edu.cn

肖文戈, 助理研究員. E-mail: wengsee@zju.edu.cn; 邱建榮, 教授. E-mail: qjr@zju.edu.cn

XIAO Wenge, lecturer. E-mail: wengsee@zju.edu.cn; QIU Jianrong, professor. E-mail: qjr@zju.edu.cn