陳 強(qiáng),劉 翔,謝清水,王來森,彭?xiàng)澚?/p>

(廈門大學(xué)材料學(xué)院,福建廈門361005)

近年來,隨著無線通訊技術(shù)的發(fā)展以及雷達(dá)等微波系統(tǒng)的廣泛應(yīng)用,電磁輻射和污染日益嚴(yán)重[1]．國內(nèi)外科研工作者研究了大量不同種類的吸波材料,如碳材料[2-4]、鐵氧體[5-6]以及導(dǎo)電高分子[7-8]等．如今,納米技術(shù)的進(jìn)步和發(fā)展為吸波材料的研究提供了新的思路．納米介電材料可通過介電極化弛豫效應(yīng)來衰減電磁波的能量,在高頻波段對(duì)電磁波的吸收效果尤為顯著[9-10]．納米材料具有巨大的比表面積,當(dāng)處于交變電磁場(chǎng)中時(shí),材料表面將產(chǎn)生多種極化損耗衰減電磁波能量[11-12]．鈦酸鋇(BaTiO3)納米顆粒作為一種典型的納米介電材料,具有良好的化學(xué)穩(wěn)定性以及高介電常數(shù)和介電響應(yīng)[13-14],是一種很有發(fā)展前景的電磁波吸收材料．Zhu等[15]利用液相法制備了BaTiO3納米管,當(dāng)厚度為2.0 mm時(shí)其最小反射損耗達(dá)到了-21.8 dB;Tian等[16]將BaTiO3納米顆粒在氫氣氣氛中700 ℃下煅燒4 h后,其最小反射損耗達(dá)到了-36.9 dB．但由于在高頻時(shí)BaTiO3吸波材料的反射系數(shù)較大,導(dǎo)致其微波吸收性能不夠理想[17]．

還原氧化石墨烯(RGO)具有低密度、高比表面積、高電子遷移速率和優(yōu)異的導(dǎo)熱性等優(yōu)點(diǎn)[18-19],且其特殊的二維片狀結(jié)構(gòu)也有利于電磁波的衰減[20-21],可作為輕重量級(jí)的吸波材料[22-23]．Wen等[24]研究發(fā)現(xiàn)RGO的微波衰減性能優(yōu)于納米石墨片(GNs)．但RGO作為吸波材料使用時(shí),其過高的介電常數(shù)會(huì)導(dǎo)致反射系數(shù)較大而吸收系數(shù)較小,不能滿足電磁波吸收的阻抗匹配特性[25-26]．因此,國內(nèi)外研究者通過將RGO與不同材料復(fù)合來調(diào)控介電常數(shù),從而改善其阻抗匹配并提高電磁波吸收性能．Singh等[27]制備了RGO/丁腈橡膠復(fù)合物,其最小反射損耗達(dá)到-57 dB．Zong等[28]制備了Fe3O4/RGO納米復(fù)合物,其最小反射損耗達(dá)到-44.6 dB．Wen等[29]制備了RGO/SiO2納米復(fù)合物,在溫度為473 K時(shí)其最小反射損耗達(dá)到-38 dB．因此,將納米介電材料BaTiO3和RGO進(jìn)行復(fù)合制備的BaTiO3/RGO納米復(fù)合物,不僅可以通過改變其復(fù)介電常數(shù)來改善材料的阻抗匹配特性,而且隨著RGO的引入,BaTiO3/RGO納米復(fù)合物將形成更多的表界面,由于界面極化效應(yīng)以及協(xié)同效應(yīng)的作用,其微波吸收性能將得到有效加強(qiáng),比單一組分的吸波材料具有明顯的優(yōu)勢(shì)．

本研究采用溶劑熱法制備BaTiO3納米顆粒,將不同質(zhì)量BaTiO3納米顆粒與氧化石墨烯(GO)進(jìn)行復(fù)合,并在氬氣保護(hù)下經(jīng)過煅燒得到一系列BaTiO3/RGO納米復(fù)合物,并對(duì)其物相結(jié)構(gòu)和表面形貌進(jìn)行表征,測(cè)試其微波吸收特性．

1 實(shí)驗(yàn)部分

1.1 試 劑

乙醇、四氯化鈦(TiCl4)、八水合氫氧化鋇(Ba(OH)2·8H2O)、氫氧化鈉(NaOH)、十六烷基三甲基溴化銨(CTAB)以及乙二胺均為分析純,購自國藥集團(tuán)化學(xué)試劑有限公司．采用Hummers法[25]合成GO．去離子水為實(shí)驗(yàn)室自制．

1.2 BaTiO3/RGO納米復(fù)合物的制備

取220 μL TiCl4分散于20 mL乙醇中,配制成TiCl4的乙醇溶液;取1.241 6 g Ba(OH)2·8H2O溶于20 mL去離子水中,超聲30 min使其完全溶解;然后將TiCl4的乙醇溶液逐滴加入Ba(OH)2溶液中,并滴加0.2 mol/L的NaOH溶液,使混合溶液pH>13;攪拌120 min后將溶液轉(zhuǎn)入100 mL的聚四氟乙烯內(nèi)襯的反應(yīng)釜中,并將反應(yīng)釜置于真空干燥箱中240 ℃下保溫24 h進(jìn)行溶劑熱反應(yīng)．反應(yīng)結(jié)束后,離心收集產(chǎn)物并用去離子水和無水乙醇交替洗滌3次,冷凍干燥24 h后得到BaTiO3納米顆粒．

將40.0 mg GO在超聲條件下分散于20 mL 去離子水中,形成均勻的棕褐色溶液;然后加入160 mg BaTiO3納米顆粒和6.0 mg CTAB,超聲2 h使其形成均勻的懸浮液;往懸浮液中滴加5 mL乙二胺,攪拌12 h后轉(zhuǎn)入50 mL的聚四氟乙烯內(nèi)襯的反應(yīng)釜中,并將反應(yīng)釜置于真空干燥箱中200 ℃下保溫180 min進(jìn)行溶劑熱反應(yīng)．反應(yīng)結(jié)束后,離心收集產(chǎn)物并用去離子水和無水乙醇交替洗滌3次,冷凍干燥24 h后得到樣品．隨后將樣品在氬氣氣氛下600 ℃退火處理120 min得到最終產(chǎn)物,記為S1．其他實(shí)驗(yàn)條件不變,只改變加入的BaTiO3質(zhì)量為93和60 mg,制得的產(chǎn)物分別記為S2和S3;不加入GO時(shí)得到的產(chǎn)物記為S0．

1.3 樣品測(cè)試

采用德國Bruker公司D8 Advance X射線衍射(XRD)儀分析樣品的物相結(jié)構(gòu),Cu Kα的波長為0.154 18 nm,工作電壓為40 kV,工作電流為40 mA,測(cè)試的角度范圍為5°～90°,測(cè)量模式為平板掃描模式．使用日本日立儀器公司SU70場(chǎng)發(fā)射掃描電子顯微鏡(SEM)和日本電子株式會(huì)所JEM-1400透射電子顯微鏡(TEM)表征樣品表面形貌特征．

采用美國TA儀器公司SDT Q600熱重/差熱聯(lián)用熱分析儀,在空氣氣氛下對(duì)樣品進(jìn)行熱重分析,加熱速率為10 ℃/min．

采用同軸線法測(cè)量樣品的微波吸收特性,測(cè)量時(shí)先將樣品與石蠟混合(樣品的質(zhì)量分?jǐn)?shù)為40%),壓制成外徑為7.00 mm,內(nèi)徑為3.04 mm的圓環(huán);然后使用Agilent Technologies公司N5222A型矢量網(wǎng)絡(luò)分析儀(vector network analyzer,VNA)測(cè)試樣品的電磁參數(shù),其測(cè)試頻率范圍為2～18 GHz,并根據(jù)電磁參數(shù)計(jì)算出樣品的反射損耗．

2 結(jié)果及討論

2.1 BaTiO3/RGO納米復(fù)合物的結(jié)構(gòu)與形貌

圖1(a)和(b)分別為BaTiO3納米顆粒的TEM和SEM圖,從圖中可以看出,制備的BaTiO3呈規(guī)則的球形,其尺寸約為70 nm,粒徑較為均一,且分散性良好,并未發(fā)現(xiàn)團(tuán)聚現(xiàn)象．圖1(c)和(d)分別為BaTiO3/RGO納米復(fù)合物的TEM和SEM圖,從圖中可以看出,BaTiO3納米顆粒均勻負(fù)載在RGO納米片上且其尺寸保持不變．

圖1 BaTiO3納米顆粒(a、b)和BaTiO3/RGO納米復(fù)合物(c、d)的TEM(a、c)和SEM(b、d)圖 Fig.1TEM (a,c) and SEM (b,d) images of BaTiO3 nanoparticles (a,b) and BaTiO3/RGO nanocomposites (c,d)

圖2 GO、RGO、BaTiO3納米顆粒和BaTiO3/RGO納米復(fù)合物(S2)的XRD譜圖(a)和樣品S1、S2和S3的熱重曲線(b) Fig.2XRD patterns of GO,RGO,BaTiO3 nanoparticles and BaTiO3/RGO nanocomposites (S2) (a) and the thermogravimetric curves of the samples of S1,S2 and S3 (b)

圖2(a)分別為GO、RGO、BaTiO3納米顆粒和BaTiO3/RGO納米復(fù)合物(S2)的XRD譜圖．GO在2θ=9.8°處有一個(gè)強(qiáng)烈衍射峰,表明GO具有有序的層狀結(jié)構(gòu)[26-27]．RGO在2θ=26.0°存在一個(gè)較弱的衍射峰,這與文獻(xiàn)[30]中RGO特征峰的位置相符．BaTiO3納米顆粒的衍射峰都較為尖銳,且在2θ=21.99°,31.56°,38.87°,45.20°,50.89°,56.09°,65.77°,70.30°,74.92°,79.06°以及83.30°的衍射峰對(duì)應(yīng)于立方相BaTiO3(JCPDS:31-0174)的(100),(110),(111),(200),(210),(211),(221),(300),(310),(311)和(222)晶面,這表明溶劑熱法制備的BaTiO3納米顆粒的結(jié)晶性良好且為立方相結(jié)構(gòu)．在BaTiO3/RGO納米復(fù)合物的XRD譜圖中只觀察到BaTiO3的衍射峰,這是由于BaTiO3納米顆粒的衍射峰很強(qiáng)而RGO衍射峰強(qiáng)度較低[31];而BaTiO3衍射峰的峰位沒有改變，說明復(fù)合后BaTiO3納米顆粒的晶體結(jié)構(gòu)并沒有發(fā)生變化．由圖2(b)可知,溫度高于550 ℃后樣品的熱重曲線趨于水平,說明S1、S2和S3中BaTiO3的質(zhì)量分?jǐn)?shù)分別為92.3%,80.9%和77.8%,對(duì)應(yīng)的RGO的質(zhì)量分?jǐn)?shù)分別為7.7%,19.1%和22.2%．

2.2 BaTiO3/RGO納米復(fù)合物的吸波特性

圖3(a)和(b)顯示了不同樣品的復(fù)介電常數(shù)實(shí)部ε′和虛部ε″在頻率2～18 GHz內(nèi)的變化規(guī)律．從圖中可以看出S0、S1、S2和S3的ε′和ε″的值依次增大,表明ε′和ε″隨著樣品中RGO質(zhì)量分?jǐn)?shù)的升高而增大,這是由于與BaTiO3納米顆粒相比,RGO的介電常數(shù)較大[22]．由圖3(a)可知,隨著頻率升高,樣品的ε′值整體上呈減小趨勢(shì)．其中,S0的ε′值變化幅度較小,在3.70附近波動(dòng);S1的ε′值從7.20減至6.41;S2的ε′值從18.67減至12.51;而S3的ε′值則從23.98減至12.91．由此可以看出,S0和S1的弛豫現(xiàn)象并不明顯,而隨著RGO質(zhì)量分?jǐn)?shù)的增加,S2和S3產(chǎn)生了明顯的弛豫現(xiàn)象．由圖3(b)可知,隨著頻率升高,S1、S2和S3的ε″值整體上呈先減小后增大的趨勢(shì)．上述結(jié)果表明,可通過調(diào)整樣品中BaTiO3和RGO的質(zhì)量分?jǐn)?shù),來調(diào)控BaTiO3/RGO納米復(fù)合物的復(fù)介電常數(shù),進(jìn)而增加材料的極化特性,改善其阻抗匹配．

圖3 樣品S0、S1、S2和S3的ε′(a)、ε″(b)、tan δε(c)和Cole-Cole半圓(d) Fig.3The ε′ (a),ε″ (b),tan δε (c) and the Cole-Cole semicircles (d) of the samples of S0,S1,S2 and S3

吸波材料的介電損耗角正切值tanδε=ε″/ε′,可用來表征其介電損耗的大小[32]．圖3(c)為不同樣品的損耗角正切值在頻率2～18 GHz內(nèi)的變化規(guī)律．當(dāng)頻率升高時(shí),S1、S2和S3的損耗角正切值呈現(xiàn)先減小后增大的趨勢(shì)．當(dāng)頻率為2 GHz時(shí),S1、S2和S3的損耗角正切值依次為0.23,0.59和0.74．這表明隨著GO質(zhì)量分?jǐn)?shù)的增大,制得的BaTiO3/RGO納米復(fù)合物的損耗角正切值逐漸增大．這是由于GO與BaTiO3復(fù)合時(shí),BaTiO3/RGO納米復(fù)合物在電磁波中將發(fā)生界面極化[33],RGO與其負(fù)載的BaTiO3納米顆粒間產(chǎn)生電荷轉(zhuǎn)移后引入載流子的運(yùn)動(dòng),引起介電損耗．當(dāng)GO質(zhì)量分?jǐn)?shù)增大時(shí),制備的BaTiO3/RGO納米復(fù)合物形成更豐富的表界面,從而界面極化產(chǎn)生更強(qiáng)的介電損耗．圖3(d)展示了樣品S0、S1、S2和S3的復(fù)介電常數(shù)虛部ε″隨實(shí)部ε′的變化關(guān)系．一般通過Cole-Cole半圓來判斷材料是否產(chǎn)生德拜弛豫[34]．從圖中可以看出,樣品S2和S3存在明顯的弛豫現(xiàn)象．

為了進(jìn)一步衡量樣品的吸波性能,先計(jì)算出微波吸收層的歸一化輸入阻抗Zin,其計(jì)算式為

再由下式計(jì)算出樣品的反射損耗RL[35-36]:

其中,c為微波在真空中的傳播速度,μr和εr分別為吸波材料的復(fù)磁導(dǎo)率和復(fù)介電常數(shù),t為微波吸收層的厚度,i為復(fù)介電常數(shù)表達(dá)式εr=ε′-iε″中的虛數(shù)單位.考慮到BaTiO3/RGO納米復(fù)合物的磁性極為微弱,樣品的復(fù)磁導(dǎo)率μr可近似為1．當(dāng)反射損耗低于-10 dB時(shí),可認(rèn)為約10%的微波被反射,剩余的90%被吸收．

圖4 樣品的反射損耗曲線 Fig.4Microwave reflection loss curves of the samples

圖4(a)～(d)分別為S0、S1、S2和S3在2～18 GHz頻率范圍內(nèi),不同厚度樣品的理論反射損耗．樣品S0的最小反射損耗僅為-9.47 dB,S1、S2和S3的微波吸收性能相較于S0有明顯的提高．其中,當(dāng)厚度為5.5 mm時(shí),S1的最小反射損耗為-23.52 dB,且在14.96～17.68 GHz頻段內(nèi)反射損耗小于-10 dB;當(dāng)厚度為2.0 mm時(shí),S2的最小反射損耗為-26.06 dB,且在9.32～11.54 GHz頻段內(nèi)反射損耗小于-10 dB;當(dāng)厚度為1.5 mm時(shí),S3的最小反射損耗為-18.16 dB,且在11.35～15.29 GHz頻段內(nèi)反射損耗小于-10 dB．S1與S2的最小反射損耗和反射損耗小于-10 dB對(duì)應(yīng)的頻段寬度較為接近,但S1的厚度大于S2,故S1的微波吸收性能不如S2．雖然S3的厚度小于S2,且S3反射損耗小于-10 dB對(duì)應(yīng)的頻段寬度大于S2,但S3的最小反射損耗大于S2,故S3的微波吸收性能也不如S2．綜合考慮樣品的厚度、反射損耗的大小和反射損耗小于-10 dB對(duì)應(yīng)的頻段寬度3個(gè)參數(shù),可以發(fā)現(xiàn),樣品S2展現(xiàn)了最好的微波吸收性能．此外,由圖4(c)可知,當(dāng)吸波層厚度增大,S2的反射損耗峰往低頻方向移動(dòng),這是由于1/4波長相消理論產(chǎn)生的頻移現(xiàn)象．樣品S2能表現(xiàn)出優(yōu)異的吸波性能,主要由于:1) 高介電常數(shù)的RGO可以調(diào)節(jié)介電常數(shù)的大小,從而改善阻抗匹配,使更多的電磁波進(jìn)入吸波體內(nèi),拓寬吸收頻帶范圍;2) 化學(xué)法合成的RGO表面帶有官能團(tuán)并存在缺陷,在電磁場(chǎng)下易產(chǎn)生極化[37-38];3) 二維RGO與BaTiO3納米顆粒構(gòu)建的三維導(dǎo)電網(wǎng)絡(luò)有利于電磁波能量的轉(zhuǎn)化與衰減;4) BaTiO3納米顆粒與RGO產(chǎn)生的界面能有效地增加界面極化效應(yīng),增加電磁波能量的吸收[39]．表1顯示了不同材料的微波吸收性能,綜合對(duì)比可知,在厚度較小的情況下,本研究制備的BaTiO3/RGO納米復(fù)合物(S2)具有較好的吸波性能．

表1 不同材料的微波吸收性能

Tab.1 Microwave absorption properties of different materials

材料厚度/mm最小反射損耗/dB反射損耗小于-10dB對(duì)應(yīng)的頻段寬度/GHz參考文獻(xiàn)BaTiO3納米管2-21.81.7[15]BaTiO3納米顆粒3-36.92.7[16]RGO/丁腈橡膠復(fù)合物3-573.5[27]Fe3O4/RGO納米復(fù)合物3.9-44.64.3[28]BaTiO3/RGO納米復(fù)合物(S2)2.0-26.062.22本研究

3 結(jié) 論

本研究中采用去離子水和乙醇作為溶劑進(jìn)行溶劑熱反應(yīng),成功制備出尺寸約為70 nm的BaTiO3納米顆粒．利用CTAB和乙二胺作為表面活性劑,將不同質(zhì)量的BaTiO3納米顆粒與GO復(fù)合并在氬氣氣氛中煅燒120 min后得到BaTiO3/RGO納米復(fù)合物．研究發(fā)現(xiàn)，通過改變加入的BaTiO3和GO的質(zhì)量比,可以調(diào)控BaTiO3/RGO納米復(fù)合物復(fù)介電常數(shù)的大小和介電損耗角正切值．當(dāng)制備的BaTiO3/RGO納米復(fù)合物中BaTiO3的質(zhì)量分?jǐn)?shù)為80.9%時(shí)展現(xiàn)出良好的微波吸收性能,當(dāng)其厚度為2.0 mm時(shí),在頻率為10.48 GHz處的反射損耗達(dá)到為-26.06 dB,且在9.32～11.54 GHz頻段內(nèi)的反射損耗小于-10 dB．實(shí)驗(yàn)結(jié)果表明,BaTiO3/RGO納米復(fù)合物是一種有應(yīng)用前景的微波吸收材料．

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