岳孟斌,陳穎芝,白 宇,黃正宏*,許德平,康飛宇
(1.中國(guó)礦業(yè)大學(xué)(北京)化學(xué)與環(huán)境工程學(xué)院,北京 100083;2.清華大學(xué)材料學(xué)院,北京 100084)
20世紀(jì)60年代碳纖維已經(jīng)成為重要的工業(yè)材料。近年來(lái)隨著納米材料的興起,碳納米纖維得到快速發(fā)展。碳納米纖維因其比表面積大、導(dǎo)電和導(dǎo)熱性好,被廣泛地應(yīng)用于吸附材料、儲(chǔ)能材料、電極材料等。制備納米纖維的方法有模板合成法[1]、相分離[2]、自組裝[3]和靜電紡絲法[4]等。 靜電紡絲裝置簡(jiǎn)單、成本低廉、工藝可控,可大批量制備出形狀規(guī)則、直徑分布均勻的納米纖維,已成為有效制備納米纖維材料的主要途徑之一。將聚合物溶液或熔體通過(guò)靜電紡絲過(guò)程制備成的無(wú)紡布纖維,進(jìn)行高溫碳化處理即得到碳納米纖維。Fom Illals[6]于1934年發(fā)表了靜電紡絲的第1個(gè)專(zhuān)利以來(lái),已有百余種高分子納米纖維通過(guò)靜電紡絲的方法制得[5],近年來(lái)靜電紡絲受到越來(lái)越多的關(guān)注。
靜電紡絲裝置(圖1)一般由高壓直流電源發(fā)生器、注射泵、收集裝置組成,靜電紡絲時(shí)高分子溶液或者熔體裝在注射泵中,高壓直流電源正極連接注射針頭,負(fù)極連接收集裝置,收集裝置一般直接接地,施加電壓后在注射針頭和收集裝置間形成一個(gè)靜電場(chǎng),聚合物前驅(qū)體溶液以一定流速?gòu)募徑z針頭流出,當(dāng)針頭處液滴所受電荷的排斥力可以克服表面張力時(shí),液滴在靜電力作用下被拉伸形成Taylor錐[7],當(dāng)電壓達(dá)到臨界電壓時(shí),在Taylor錐尖端產(chǎn)生噴射液流,噴射液流表面電荷之間作用力和電場(chǎng)力的合力形成切向應(yīng)力,抵抗掉液流的黏性應(yīng)力后,使液流加速經(jīng)過(guò)一個(gè)穩(wěn)定拉伸階段[8-9],在穩(wěn)定運(yùn)動(dòng)一段時(shí)間后,電場(chǎng)力對(duì)射流的作用開(kāi)始減弱,電荷間的相互作用力成為液流運(yùn)動(dòng)的主要推動(dòng)力,液流中每一點(diǎn)電荷都受到相鄰電荷的作用力,形成沿法向向外的力和沿軸向的斥力,法向向外地力使液流偏離軸向,沿軸向的斥力使液流拉伸變細(xì),引起液流不穩(wěn)定運(yùn)動(dòng)[10-11],在電場(chǎng)中形成螺旋運(yùn)動(dòng),在運(yùn)動(dòng)過(guò)程中高分子溶液的溶劑揮發(fā),高分子固化,形成纖維,收集在收集裝置上。
圖1 靜電紡絲的實(shí)驗(yàn)裝置[18]Fig.1 A setup for electrosp ining[18]
靜電紡絲纖維收集方式對(duì)纖維收集狀態(tài)有很大影響,采用平板收集時(shí),靜電紡絲制備的纖維是雜亂地堆積在一起,形成纖維膜。很多研究致力于改進(jìn)收集裝置,以便獲得平行的纖維束、紗線或其他形狀,如:采用滾筒收集法[12],使用一個(gè)圓筒以一定速度轉(zhuǎn)動(dòng)可以收集到一定排列形式的連續(xù)纖維;采用尖端收集法[13],制備排列度很高的纖維集合體;采用平行磁場(chǎng)法[14],使在收集裝置區(qū)域形成一個(gè)磁場(chǎng),可以制備出有序的納米纖維。改變注射裝置,采用兩種溶液和兩個(gè)注射泵可以進(jìn)行同軸紡絲[15],制備核殼結(jié)構(gòu)的纖維,應(yīng)用在生物醫(yī)學(xué)、電子元件等方面。采用多組針頭電紡,可以提高聚合物納米纖維的產(chǎn)率[16],采用1個(gè)注射針頭和2種不相溶聚合物溶液可以電紡出雙組分纖維[17]。
靜電紡絲的前驅(qū)體有聚合物溶液和高溫熔融聚合物。聚合物溶液是將聚合物溶于一種合適的溶劑,形成具有一定濃度、黏度、電導(dǎo)率的溶液,然后進(jìn)行靜電紡絲制備納米纖維,表1列舉了一些用于靜電紡絲的聚合物種類(lèi);還有一部分高分子聚合物如聚乙烯[19]、尼龍 _12[20]等,能在高溫熔融狀態(tài)下進(jìn)行靜電紡絲,但需要在真空條件下進(jìn)行。已有數(shù)十種高分子聚合物通過(guò)靜電紡絲過(guò)程成功制備成納米纖維。
表1 用于靜電紡絲的聚合物Table 1 Polymers used for electrospinning
靜電紡絲過(guò)程中聚合物溶液的性質(zhì)、電紡參數(shù)以及環(huán)境因素都會(huì)對(duì)紡絲過(guò)程和產(chǎn)物形貌產(chǎn)生影響,調(diào)整好參數(shù),才能得到均勻、連續(xù)的碳纖維。聚合物溶液性質(zhì)包括表面張力、電導(dǎo)率和黏度,聚合物的相對(duì)分子質(zhì)量、溶液濃度和溶劑種類(lèi)以及添加劑對(duì)影響溶液的電導(dǎo)率、黏度等有很大影響,濃度越高,黏度越大,一般濃度要在一定的范圍內(nèi)紡絲才能順利,過(guò)低時(shí)纖維有串珠,或者只是液滴,過(guò)高時(shí)也不能形成纖維。Koshi等[21]研究了不同重均相對(duì)分子質(zhì)量的聚乙烯醇對(duì)纖維結(jié)構(gòu)的影響,發(fā)現(xiàn)纖維直徑隨著聚合物相對(duì)分子質(zhì)量的增大而變大,Wang[22]對(duì)熱固性酚醛樹(shù)脂/乙醇體系靜電紡絲研究表明,當(dāng)質(zhì)量分?jǐn)?shù)低于35%時(shí),黏度調(diào)節(jié)至15~30 mPa·s時(shí),可以紡出均勻連續(xù)的纖維。電紡參數(shù)包括溶液流速、施加電壓、接受距離,每個(gè)聚合物溶液體系都有合適的接受距離、電壓及流速范圍。環(huán)境因素包括溫度和濕度,靜電紡絲過(guò)程很容易受到溫度、濕度的影響[23],溫度和濕度影響溶劑揮發(fā)速度,濕度會(huì)對(duì)纖維形貌和孔結(jié)構(gòu)產(chǎn)生影響[24]。
靜電紡絲制備的納米纖維要經(jīng)過(guò)高溫碳化過(guò)程轉(zhuǎn)化成碳納米纖維,碳化過(guò)程通常在惰性氣氛保護(hù)下進(jìn)行,使纖維發(fā)生氧化、交聯(lián)等反應(yīng),纖維內(nèi)部結(jié)構(gòu)向碳碳結(jié)構(gòu)轉(zhuǎn)換。聚丙烯腈、瀝青和酚醛纖維在碳化之前還要進(jìn)行預(yù)氧化或穩(wěn)定化過(guò)程,使纖維中分子環(huán)化和脫氫,轉(zhuǎn)化為耐熱結(jié)構(gòu)[43]。聚丙烯腈通常在空氣氣氛中280℃進(jìn)行預(yù)氧化處理,然后在惰性氣氛中600~1 100℃碳化處理。經(jīng)過(guò)穩(wěn)定化和碳化處理,纖維重量減小,直徑變小,有所收縮。
纖維碳化過(guò)程中由于纖維中前驅(qū)體的裂解和易揮發(fā)組分的揮發(fā),在纖維中形成一些微孔(孔徑<2 nm)甚至中孔(2~50 nm),使碳化后的碳納米纖維比表面積變大,尤其是以酚醛樹(shù)脂為原料,所得到的碳納米纖維具有豐富孔結(jié)構(gòu)。王磊[44]使用靜電紡絲制備的熱固性酚醛纖維氈,在600、800、1 000、1 200、1 400℃碳化處理后發(fā)現(xiàn)碳收率從53.8%下降到50.5%,比表面積由600℃時(shí)的530 m2/g上升到了1 000℃時(shí)的838 m2/g,隨后又迅速下降至1 400℃時(shí)的8 m2/g,碳化過(guò)程中隨著前驅(qū)體逐漸裂解形成越來(lái)越多的微孔[45],而溫度過(guò)高時(shí)碳原子重排消除微孔形成大孔(孔徑>50 nm)。為了增加比表面積和調(diào)整孔徑分布,常使用水蒸氣、CO2和KOH等試劑對(duì)碳納米纖維進(jìn)行活化處理,以獲得不同比表面積和孔徑分布的活性碳納米纖維。
在聚合物前驅(qū)體溶液中加入不同的添加劑,可以獲得不同孔徑分布和比表面積的碳纖維。Zhang等[46]在聚丙烯腈前驅(qū)體溶液中加入聚乙烯吡咯烷酮,靜電紡絲制備出納米纖維,100℃水熱條件下除去聚乙烯吡咯烷酮,再1 000℃碳化處理,聚乙烯吡咯烷酮的加入提高了聚丙烯腈碳纖維的比表面積和孔徑分布。在紡絲前驅(qū)體溶液中加入模板、表面活性劑或MgO前驅(qū)體進(jìn)行紡絲,可以調(diào)節(jié)纖維的中孔分布[47]。
在用作電極材料、催化劑負(fù)載材料等時(shí)需要靜電紡絲制備的碳納米纖維具備高導(dǎo)電性。碳化溫度較低時(shí)碳納米纖維大多是無(wú)序結(jié)構(gòu),還包含很多非碳原子,高溫處理可以使碳納米纖維中非碳原子大大減少,同時(shí)提高碳納米纖維中的石墨化結(jié)構(gòu),從而提高碳納米纖維的導(dǎo)電性;但會(huì)使碳納米纖維中的微孔、中孔減少。聚丙烯腈基碳納米纖維由于需要預(yù)氧化處理以保持碳化后的纖維形貌,而不容易獲得高度石墨化的碳納米纖維。在2 800℃處理后電導(dǎo)率20 S/cm,但石墨化程度仍較低[48]。而聚酰亞胺碳納米纖維不需要預(yù)氧化處理,高溫處理后石墨化程度較高[49]。
在碳納米纖維上負(fù)載多壁碳納米管,可以提高碳納米纖維的導(dǎo)電性。Guo[50]在聚丙烯腈靜電紡絲前驅(qū)體溶液中加入多壁碳納米管,制備出摻雜碳納米管的碳納米纖維,摻雜質(zhì)量分?jǐn)?shù)8%時(shí)使碳納米纖維的導(dǎo)電性從0.86 S/cm提高到5.32 S/cm。
靜電紡絲制備的碳納米纖維具有高導(dǎo)電性、高比表面積等特點(diǎn),使其在電極材料、吸附材料、催化劑載體等方面得到廣泛應(yīng)用。
靜電紡絲制備的碳纖維布具有較大比表面積,優(yōu)異的導(dǎo)電性,柔韌性好,常用于電容式脫鹽裝置電極吸附材料。電容式脫鹽是利用類(lèi)似于電容器的裝置去除水中離子的水處理技術(shù),在用作電容式脫鹽電極時(shí),靜電紡絲碳纖維布不需要額外處理,可以直接使用。Wang[51]通過(guò)靜電紡絲制備出聚丙烯腈納米纖維在800℃碳化、活化處理后,用作電容式脫鹽電極材料,對(duì)初始濃度60 mg/L的氯化鈉溶液的電容式吸附量達(dá)到3.1 mg/g,遠(yuǎn)高于未活化的碳纖維。王磊[44]使用靜電紡絲制備的酚醛碳纖維氈作為電容式脫鹽的電極材料,用以電吸附脫除水中初始濃度100 mg/L的氯化鈉離子,電容式脫鹽量達(dá)到5.4 mg/g。使用活化后的酚醛碳纖維,電容式脫鹽量達(dá)到7.6 mg/g。Bai[52]在聚丙烯腈前驅(qū)體溶液中加入氧化石墨烯,將靜電紡絲制備的纖維經(jīng)過(guò)預(yù)氧化、碳化、活化處理后,制備出含氧化石墨烯的碳納米纖維,氧化石墨烯的加入提高了纖維的中孔比率和導(dǎo)電性,用作電容式脫鹽電極材料時(shí)比未添加氧化石墨烯的碳納米纖維具有更高的吸附量。
多孔碳材料常用作電容器電極材料,商用的電容器一般使用活性碳材料。最新研究發(fā)現(xiàn)模板碳、碳納米管和碳納米纖維可用于提高電容器性能。靜電紡絲可以制備網(wǎng)狀碳納米纖維,通過(guò)控制納米纖維孔結(jié)構(gòu)、負(fù)載離子到納米碳纖維從而增加形成贗電容,提高電容器的性能。Iijima等[53]通過(guò)靜電紡絲制備的聚丙烯腈基碳納米纖維,在700、750和800℃水蒸氣活化后,用作質(zhì)量分?jǐn)?shù)30%KOH水溶液的電容器電極材料,700℃活化樣品比表面積達(dá)到1 230 m2/g,含有一些微孔,在10 mA/g的低放電電流密度下容量達(dá)到173 F/g,800℃活化樣品比表面積為850 m2/g,含有一些中孔,在1 000 mA/g的高電流密度時(shí)電容達(dá)到120 F/g。Ju等[54]在聚丙烯腈溶液中摻入質(zhì)量分?jǐn)?shù)15%的醋酸纖維素靜電紡絲制備的碳納米纖維微孔比表面積919 m2/g,中孔比表面積241 m2/g,在6 mol/L的 KOH水溶液中電流密度1 mA/cm時(shí)電容達(dá)到245 F/g;在PAN/DMF前驅(qū)體溶液中摻入多壁碳納米管靜電紡絲制備的碳納米纖維提高了電容器的性能,摻入質(zhì)量分?jǐn)?shù)3%的多壁碳納米管使碳納米纖維比表面積提高到1 170 m2/g,電導(dǎo)率達(dá)到0.98 S/cm,電容器電容值在6 mol/L的 KOH水溶液中達(dá)到 180 F/g[55]。為了增加贗電容,在PAN/DMF溶液中添加乙酰丙酮釕進(jìn)行靜電紡絲制備出負(fù)載釕離子的碳納米纖維,負(fù)載質(zhì)量分?jǐn)?shù)7.31%釕的碳納米纖維在6 mol/L的KOH中電容達(dá)到391 F/g,而未負(fù)載釕的碳納米纖維電容僅為 140 F/g[56]。 Wang[57]在聚丙烯腈前驅(qū)體溶液中加入碳納米管,制備出復(fù)合了碳納米管的的碳納米纖維,將制得的纖維浸入0.1 mol/L的KMnO4溶液中超聲30 min,制備出負(fù)載了 MnO2的纖維,用作電容器電極時(shí)電容達(dá)到374 F/g。
靜電紡絲制備的碳納米纖維由于不需要添加導(dǎo)電添加劑和黏結(jié)劑,在用作鋰離子二次電池陽(yáng)極材料方面有一定優(yōu)勢(shì),但是由于大部分纖維石墨化程度低,不能像天然石墨一樣提供低的不可逆容量。Kim等[58]將制備的 PAN基碳納米纖維1 000℃熱處理后,電流密度30 mA/g時(shí),可逆放電容量450 mA·h/g,略微高于天然石墨,而不可逆容量高達(dá)500 mA·h/g。Ji等[59]在PAN/DMF溶液中加入聚乳酸制備的碳納米纖維,微孔孔容0.086 cm3/g,在電流密度50 mA/g,50次循環(huán)后可逆放電容量達(dá)到435 mA·h/g。Ji等[60]在PAN/DMF溶液中加入SiO2顆粒制備碳納米纖維,然后濾除SiO2造孔,比表面積只有92 m2/g,但首次循環(huán)不可逆容量超過(guò)1 000 mA·h/g。在 PAN/DMF溶液中加入質(zhì)量分?jǐn)?shù)15%硅納米顆粒,靜電紡絲制備納米纖維在700℃碳化處理,首次循環(huán)可逆放電容量高達(dá)855 mA·h/g,不可逆容量312 mA·h/g,比不加硅納米顆粒的碳納米纖維高的多。Ji等[61]在PAN/DMF前驅(qū)體溶液中加入乙酸錳,靜電紡絲制備納米纖維700℃碳化處理得到含有MnO和Mn3O4的納米纖維,循環(huán)性能穩(wěn)定。加入質(zhì)量分?jǐn)?shù)50%乙酸錳的碳納米纖維穩(wěn)定容量在600 mA·h/g,比不加乙酸錳的碳納米纖維高的多。Steven等[62]將硅包覆在垂直碳納米纖維表面,并將其用于鋰離子二次電池陽(yáng)極材料,結(jié)果顯示該材料的電容量(以 1 g硅計(jì))達(dá)到了 3 000~3 650 mA·h/g,而且經(jīng)過(guò)100次充放電循環(huán)后電容量仍然保持89%。Kim等[63]以靜電紡絲技術(shù)制備出可用于鋰離子二次電池陽(yáng)極材料的碳纖維,比表面積500~1 200 m2/g,比電容達(dá)到 35 ~202 F/g。 Chen[64]利用紡絲技術(shù)制備出高導(dǎo)電性的碳/鈷納米復(fù)合纖維,可逆容量大于 800 mA·h/g。Nan[65]在聚丙烯酸 /二甲基乙酰胺前驅(qū)體溶液中加入正硅酸乙酯和鹽酸,通過(guò)靜電紡絲制備出含有SiO2的碳納米纖維,使用氫氟酸去除掉SiO2納米粒子,得到的纖維比表面積達(dá)到950 m2/g,用作鋰離子二次電池陽(yáng)極材料時(shí)首次循環(huán)容量達(dá)到730 mA·h/g。
靜電紡絲制備的碳纖維薄膜具有大比表面積,孔徑發(fā)達(dá),易于回收,可以重復(fù)使用,常用作催化劑載體。Wang[66]將靜電紡絲制備的聚丙烯腈纖維進(jìn)行活化和石墨化處理后用于室溫下對(duì)NO的催化氧化,發(fā)現(xiàn)活性碳納米纖維及1 900和2 400℃石墨化處理的碳納米纖維對(duì)NO的催化氧化率分別是11%,38%和45%,石墨化的碳納米纖維為NO催化氧化為NO2提供了更多的活性點(diǎn)位。陳瑩瑩[67]以PVP/Ti(OC4H9)4前驅(qū)體溶液電紡制備出納米線,再在不同溫度下煅燒制備出具有光催化性能的TiO2納米線,對(duì)羅丹明B降解效果明顯。Sundaramurthy[68]使用 Fe(acac)3/PVP乙醇溶液體系電紡出纖維,在500℃退火5 h,制備出a-Fe2O3纖維,對(duì)有機(jī)染料剛果紅的光催化降解率達(dá)到90%。
靜電紡絲制備的碳納米纖維形成的纖維膜氣流阻力小,孔隙度高而孔徑小,比表面積高,表面黏結(jié)性好,是高效的過(guò)濾材料。Bai[69]將靜電紡絲制備的活性碳納米纖維使用硝酸和硫酸進(jìn)行氧化處理,處理后的活性碳納米纖維表面官能團(tuán)更加豐富,表面極性變大,對(duì)丁酮、乙醇等極性有機(jī)組分的吸附能力更強(qiáng)。將碳納米纖維與其他機(jī)體復(fù)合,制成防護(hù)服,用作分子過(guò)濾、生化阻隔、吸收和降解有害氣體。Gibson[70]在電紡法的纖維薄膜中引入把活性碳、碳納米管和酶粒子引入電紡絲薄膜,制備化學(xué)生物保護(hù)膜,有效地捕捉空氣中的粒子,具有防生化武器的基本功能。
靜電紡絲作為一種簡(jiǎn)便高效的制備方法,制備的碳納米纖維所具有的獨(dú)特優(yōu)勢(shì),使其在功能材料領(lǐng)域里大有作為,已經(jīng)引起人們廣泛的關(guān)注。目前已有多種原材料可以用于靜電紡絲,但最終產(chǎn)品大多都處于實(shí)驗(yàn)階段,對(duì)其進(jìn)行深入的基礎(chǔ)和實(shí)用性研究具有重要意義,碳納米纖維將會(huì)得到更廣泛的應(yīng)用。
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