寬光譜透過Mg0.9Al2.08O3.97N0.03透明陶瓷的制備與性能研究

李文俊, 王皓, 涂兵田, 諶強國, 鄭凱平, 王為民, 傅正義

寬光譜透過Mg0.9Al2.08O3.97N0.03透明陶瓷的制備與性能研究

李文俊, 王皓, 涂兵田, 諶強國, 鄭凱平, 王為民, 傅正義

(武漢理工大學 材料復合新技術國家重點實驗室, 武漢 430070)

MgAl2O4透明陶瓷具有優異的光學性能, 但其較差的機械性能和成型過程中的水解問題限制了實際應用, 通過組成設計MgAlON四元尖晶石可以有效調節其綜合性能。本研究采用凝膠注模成型、無壓燒結和熱等靜壓處理制備了一種具有寬光譜透過范圍的新型Mg0.9Al2.08O3.97N0.03透明陶瓷, 系統比較其與MgAl2O4透明陶瓷的光學性能和機械性能, 分析了低應力下裂紋的緩慢擴展并預測使用壽命。研究表明：固相體積分數為50%的陶瓷漿料粘度最低, 為124 mPa·s, 滿足凝膠注模成型的需求; 2 mm厚的Mg0.9Al2.08O3.97N0.03透明陶瓷樣品在3.7 μm處的直線透過率達86.2%, 光學透過范圍與MgAl2O4相比擬, 折射率和阿貝數略高于MgAl2O4; 同時, 該陶瓷具有和MgAl2O4相近的Weibull模數, 盡管裂紋緩慢擴展系數比MgAl2O4小, 但特征強度(210.6 MPa)和惰性強度(227.5 MPa)均高于MgAl2O4。包含少量N的MgAlON尖晶石較好地克服了陶瓷粉體的水解問題, 并在保持優越光學性能的前提下顯著提高了透明陶瓷的機械性能。本研究為尖晶石型透明陶瓷的制備與性能的改善提供了新的途徑。

透明陶瓷; 凝膠注模成型; 機械性能; 光學性能

MgAl2O4透明陶瓷是一種在0.3～5.5 μm波段具有重要應用的光學材料, 其光學透過范圍在0.11～7 μm, 理論透過率可達到87%, 適用于紅外窗口、條形碼掃描儀及夜視系統等領域[1-4]。然而該材料的抗彎強度約為150 MPa, 斷裂韌性通常低于2 MPa·m1/2[5], 迫切需要在保持優異光學性能的同時提升材料的機械性能。目前, 減少燒結助劑用量和細化晶粒尺寸是改善MgAl2O4透明陶瓷機械性能的主要手段[6-8], 但容易對材料的光學性能產生不利影響。另外, MgAl2O4粉體易產生水解行為, 難以通過水基凝膠注模成型制備高強度的大尺寸陶瓷[9-11]。如何獲得綜合性能優異且易于制備的尖晶石型透明陶瓷仍是亟待解決的問題。

MgAlON是一種四元尖晶石固溶體, 其中無序混合的O2–和N3–呈面心緊密堆積, Mg2+和Al3+分布于四面體和八面體空隙。MgAlON晶體結構復雜、組成變化范圍寬, 可在較大范圍內調控材料性能[12-13]。Liu等[14]采用無壓燒結制備了Mg0.27Al2.58O3.73N0.27透明陶瓷, 發現該陶瓷的光學透過范圍介于AlON和面藍寶石之間。Zhang等[15]系統評價了該透明陶瓷的機械性能, 其顯著優于MgAl2O4。Zong等[16]還發現Mg0.27Al2.58O3.73N0.27粉體不會發生水解, 適合用水基凝膠注模成型制備大尺寸、復雜形狀的部件。Zong等[13]通過材料組成設計與調控, 制備出一種光學透過域與面藍寶石相當的新型Mg0.55Al2.36O3.81N0.19透明陶瓷。上述研究表明, 隨著MgAlON透明陶瓷中N含量降低和Mg含量上升, 陶瓷的光譜透過范圍變寬, 且機械性能始終優于MgAl2O4, 同時還易于水基膠態成型。

本研究通過調控組成設計與晶體結構, 采用高溫固相反應合成Mg0.9Al2.08O3.97N0.03陶瓷粉體, 通過凝膠注模成型、無壓燒結結合熱等靜壓處理制備透明陶瓷。研究陶瓷漿料的流變學性能, 評價Mg0.9Al2.08O3.97N0.03透明陶瓷的光學性能和機械性能, 進而分析該陶瓷的裂紋擴展情況和疲勞特性。

1 實驗方法

以高純度的-Al2O3、AlN和實驗室自制的MgAl2O4為原料, 經過高溫固相反應制得單相Mg0.9Al2.08O3.97N0.03粉體。然后通過水基凝膠注模成型制備陶瓷坯體, 具體過程如下: 以甲基丙烯酰胺(MAM, 98%, Ourchem Chem, 中國)作為單體,,?-亞甲基雙丙烯酸酰胺(MBAM, 97%, Alfa Aesar Chem, 美國)作為交聯劑, 檸檬酸三銨(TAC, 97%, Alfa Aesar Chem, 美國)作為分散劑, 攪拌2 h制得固相體積分數為50%的漿料, 隨后加入體積分數1%的引發劑過硫酸銨(APS, 98%, Alfa Aesar Chem, 美國)并真空攪拌除泡30 min, 加入體積分數0.25%的催化劑,,?,?-四甲基乙烯二胺(TEMED, 99%, Alfa Aesar Chem, 美國), 再把漿料注入到模具中使之固化成型獲得陶瓷坯體。脫模后將坯體置于室溫下干燥48 h, 冷等靜壓(200 MPa, 5 min)后在馬弗爐中加熱至630 ℃以排除有機物。將坯體在1.01×105Pa N2氣氛下1675 ℃保溫2 h, 所得陶瓷預燒體在180 MPa Ar氣氛中1800 ℃熱等靜壓燒結5 h,然后對樣品進行切割、兩面研磨和拋光, 備用。

使用X射線衍射儀(Model χ’Pert PRO of Panal-ytical, Almelo, 荷蘭)對粉體和陶瓷的物相進行鑒定。通過旋轉黏度計(Brookfield Viscometer, 美國)測試漿料的流變學性能, 剪切速率范圍為0.01～ 56 s–1。用氧氮分析儀(TC600, Leco, 美國)和電感耦合等離子體原子發射光譜(ICP-AES Optima4300DV, PerkinElmer, 美國)確定粉體的實際組成; 使用掃描電子顯微鏡(Hitachi S-3400N,日本)觀察坯體、陶瓷斷口及刻蝕表面的形貌特征, 其中表面拋光的陶瓷樣品在200 ℃的濃磷酸中刻蝕15 min。分別采用紫外–近紅外分光光度計(Lambda750 S, PerkinElmer, 美國)和智能傅里葉紅外光譜儀(Model Nexus, Thermo Nicolet Corporation, Madison, WI)測試透明陶瓷樣品在200～2500 nm和2500～7000 nm范圍的光學直線透過率; 通過橢圓偏振光譜儀(M-2000V, J.A. Woollam, 美國)測試樣品在不同波長下的折射率。用維氏硬度計(Model 430 SVD, Wolpert, 中國)測試樣品的維氏硬度、測試載荷9.8 N、保壓時間15 s; 利用壓痕法測試樣品的斷裂韌性; 使用彈性模量測試儀(GrindoSonic MK7, GrindoSonic, 比利時)測量樣品的楊氏模量, 測試方法為脈沖激勵法; 采用萬能試驗機(MTS810 100KN, MTS, 美國)測量陶瓷的抗彎強度, 測試方法為四點彎曲法[17](GB/T 6569-2006, 樣品尺寸為3 mm×4 mm×35 mm)。利用熱膨脹儀(DIL402SE, Netzsch, 德國)測定樣品的熱膨脹系數。

2 結果與討論

2.1 Mg0.9Al2.08O3.97N0.03透明陶瓷的制備

通過氧氮分析儀結合ICP-AES測定陶瓷粉體的實際組成為Mg0.9Al2.08O3.97N0.03, 與設計組成(Mg0.94Al2.05O3.98N0.02)差別不大, 差別主要來源于Mg、N的少量揮發[18]。圖1(a)是所得粉體和陶瓷預燒體的XRD圖譜, 從圖中可以看到, 粉體和陶瓷的衍射峰位置均與尖晶石的PDF#99-0098標準卡片一一對應, 說明它們是由單一尖晶石相組成。

圖1(b)顯示了固相體積分數為50%時, 分散劑含量對漿料黏度的影響。在剪切速率50 s–1下, TAC質量分數為0.4%時, 漿料的黏度最低, 為124 mPa·s, 說明該漿料可滿足水基凝膠注模成型制備陶瓷坯體的要求。

從圖1(c)可以看到, 坯體中陶瓷顆粒堆積較緊密、分布較均勻, 粒徑約為0.4 μm。圖1(d)是熱等靜壓燒結后樣品的表面SEM照片, 平均晶粒尺寸約為100 μm、沒有出現晶粒的異常長大, 但是殘留少量的小氣孔。

2.2 光學性能

圖2(a)為Mg0.9Al2.08O3.97N0.03、MgAl2O4透明陶瓷、面藍寶石[1]和Mg0.27Al2.58O3.73N0.27透明陶瓷[14]的光學透過譜。從插圖中可以看到, 制備的樣品表觀透光性良好。2 mm厚的Mg0.9Al2.08O3.97N0.03陶瓷樣品在3.7 μm處的透過率為86.2%, 在可見光波段樣品的透過率略低于MgAl2O4透明陶瓷, 這是因為陶瓷中殘留少量的小氣孔(圖1(d))。最終樣品的光譜透過范圍為0.2～6.7 μm, 與MgAl2O4透明陶瓷接近, 較面藍寶石和Mg0.27Al2.58O3.73N0.27陶瓷更寬。在尖晶石晶體中, 隨著Mg含量增大和N含量降低, 晶格振動頻率降低, 聲子吸收紅移, 光譜透過范圍變寬[13]。

圖1 Mg0.9Al2.08O3.97N0.03陶瓷粉體、漿料、坯體以及透明陶瓷的表征

(a) XRD patterns of powder and ceramic; (b) Relationship between viscosity of slurry and contents of TAC; (c) Morphology of green body; (d) SEM image of etched surface of transparent ceramic

(a) In-line transmittance of Mg0.9Al2.08O3.97N0.03, MgAl2O4,-plane sapphire[1], Mg0.27Al2.58O3.73N0.27transparent ceramics[14]; (b) Refractive index of Mg0.9Al2.08O3.97N0.03, MgAl2O4transparent ceramics[19]

2.3 斷裂強度的Weibull統計分析

圖3(a)是Mg0.9Al2.08O3.97N0.03透明陶瓷的兩參數Weibull統計分布圖, 可以看到對樣品的實測強度線性擬合較好。從擬合結果可得, Mg0.9Al2.08O3.97N0.03透明陶瓷的Weibull模數為5.97±0.25, 特征強度為210.6 MPa。文獻[25]報道了MgAl2O4透明陶瓷(平均晶粒尺寸約為5 μm, 下同)的Weibull模數為5±2, 特征強度為169 MPa; 而Mg0.27Al2.58O3.73N0.27透明陶瓷[15]的Weibull模數為4.5, 特征強度為255.54 MPa。本研究的材料特征強度介于MgAl2O4和Mg0.27Al2.58O3.73N0.27透明陶瓷之間, 但Weibull模數高于它們。這說明在相同載荷下, Mg0.9Al2.08O3.97N0.03陶瓷的斷裂概率比MgAl2O4低。造成這三種材料特征強度和Weibull模數差異的原因可能來自于它們組成和顯微結構的不同[26]。

圖3(b)是Mg0.9Al2.08O3.97N0.03陶瓷的斷口形貌, 可以看出, 樣品中存在明顯的穿晶和沿晶混合斷裂過程以及臺階式斷層, 具備陶瓷的脆性斷裂特征, 晶界處的小氣孔可能是該材料的斷裂源。

圖3 Mg0.9Al2.08O3.97N0.03透明陶瓷的抗彎強度Weibull統計圖(a)和斷面SEM照片(b)

2.4 裂紋緩慢擴展

基于陶瓷材料在長時間、低應力下發生裂紋擴展的行為, 研究不同載荷速率下Mg0.9Al2.08O3.97N0.03透明陶瓷的四點抗彎強度。材料在低應力下微裂紋擴展對抗彎強度的影響可采用下式進行表征[28]：

在實際應用中, 陶瓷的使用壽命是一個非常重要的性能指標。將抗彎強度的Weibull統計結果和裂紋緩慢擴展相結合[27], 可得到表征陶瓷疲勞特性的關系式。

為了預測材料在不同使用時間下的實際強度和失效概率, 基于式(5)獲得Mg0.9Al2.08O3.97N0.03和MgAl2O4透明陶瓷[27]的強度–概率–時間(Strength- probability-time, SPT)關系圖。從圖4(b)中可以看出, 材料的斷裂概率為63.2%時, 隨著使用時間從1 s延長到10 a, Mg0.9Al2.08O3.97N0.03透明陶瓷對應的承受應力從298 MPa降低到112 MPa。這說明隨著材料使用時間延長, 裂紋緩慢擴展導致材料可承受的應力降低。當Mg0.9Al2.08O3.97N0.03和MgAl2O4透明陶瓷都使用10 a后, Mg0.9Al2.08O3.97N0.03在應力112 MPa下斷裂概率為63.2%, 而MgAl2O4在該應力下的斷裂概率超過99%。這是因為Mg0.9Al2.08O3.97N0.03透明陶瓷的惰性強度遠大于MgAl2O4透明陶瓷, 不易在外應力沖擊下破壞。因此在實際應用中, Mg0.9Al2.08O3.97N0.03透明陶瓷的使用壽命更長, 適用于更苛刻的工作環境。

表1給出了Mg0.9Al2.08O3.97N0.03和MgAl2O4透明陶瓷的其它相關性能, 從表中可以看到, Mg0.9Al2.08O3.97N0.03透明陶瓷的硬度、斷裂韌性和楊氏模量略高于MgAl2O4; 熱膨脹系數略低于MgAl2O4透明陶瓷。這是因為兩者組成和結構不同, 隨著尖晶石晶體中Mg含量增大和N含量減少, 其四面體鍵的硬度和鍵力常數急劇下降, 而八面體鍵的硬度和鍵力常數幾乎不變, 因此硬度、斷裂韌性和楊氏模量降低[34]。

圖4 Mg0.9Al2.08O3.97N0.03和MgAl2O4透明陶瓷[27]在不同載荷速率下的抗彎強度(a)與強度–概率–時間關系圖(b)

Colorful figures are available on website

表1 Mg0.9Al2.08O3.97N0.03和MgAl2O4透明陶瓷的性能

3 結論

采用水基凝膠注模成型、無壓預燒結合熱等靜壓燒結制備了新型Mg0.9Al2.08O3.97N0.03透明陶瓷。所得漿料的固相量高、粘度低, 適合水基凝膠注模成型制備陶瓷坯體。2 mm厚的陶瓷樣品在3.7 μm處的直線透過率達86.2%, 且具有與MgAl2O4透明陶瓷可比擬的光學透過域(0.2～6.7 μm)和略大的阿貝數。通過抗彎強度的Weibull統計以及裂紋緩慢擴展研究, 發現Mg0.9Al2.08O3.97N0.03透明陶瓷的Weibull模數、特征強度和惰性強度均大于MgAl2O4透明陶瓷, 但是裂紋緩慢擴展系數略小。另外, 較高的惰性強度使Mg0.9Al2.08O3.97N0.03透明陶瓷的使用壽命更長。

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Preparation and Property of Mg0.9Al2.08O3.97N0.03Transparent Ceramic with Broad Optical Transmission Range

LI Wenjun, WANG Hao, TU Bingtian, CHEN Qiangguo, ZHENG Kaiping, WANG Weiming, FU Zhengyi

(State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China)

MgAl2O4transparent ceramics possess excellent optical property, but their practical applications are somewhat restricted by the hydrolysis problem during the shaping process and limited mechanical property. Meanwhile, it has been demonstrated that the property of quaternary MgAlON spinel can be effectively adjustedvarying their composition. Accordingly, a novel Mg0.9Al2.08O3.97N0.03transparent ceramic with a broad transmit-tance range was prepared by combining aqueous gel-casting, pressureless sintering, and hot isostatic pressing treatment. Optical and mechanical property of this transparent ceramic were systematically investigated and compared with those of MgAl2O4transparent ceramic. Furthermore, the slow crack growth under low stress were analyzed, and the service life of transparent ceramic was predicted. It is shown that the viscosity of ceramic slurry with 50% (in volume) solid load was 124 mPa·s, which could meet the requirement of aqueous gel-casting. The in-line transmittance of 86.2% at 3.7 μm was obtained in the Mg0.9Al2.08O3.97N0.03transparent ceramic sample with thickness of 2 mm, and the optical transmittance range was comparable to that of MgAl2O4, with slightly higher refrac-tive index and Abbé number. Further, this ceramic showed a Weibull modulus similar to MgAl2O4, and although its crack slow growth coefficient is lower than MgAl2O4,but both the characteristic strength (210.6 MPa) and inert strength (227.5 MPa) were much higher. Therefore, in the quaternary MgAlON spinel with low nitrogen content, the hydrolysis problem of ceramic powders could be well overcome, while the mechanical property of transparent ceramic was remarkably improved without degradation of optical property. This research provides a new pathway toward obtaining the novel spinel transparent ceramics with improved preparation and property.

transparent ceramics; aqueous gel-casting; mechanical property; optical property

1000-324X(2022)09-0969-07

10.15541/jim20210771

TQ174

A

2021-12-17;

2022-02-22;

2022-06-16

國家重點研發計劃(2017YFB0310500); 國家自然科學基金(51472195); 三亞科教創新園開放基金(2020KF0018)

National Key Research and Development Program of China (2017YFB0310500); National Natural Science Foun-dation of China (51472195); Sanya Science and Education Innovation Park Open Fundation (2020KF0018)

李文俊(1996–), 男, 碩士研究生. E-mail: 15826911464@163.com

LI Wenjun (1996–), male, Master candidate. E-mail: 15826911464@163.com

王皓, 教授. E-mail: shswangh@whut.edu.cn

WANG Hao, professor. E-mail: shswangh@whut.edu.cn