徐 毅,劉志宏

1.深圳市中金嶺南有色金屬股份公司丹霞冶煉廠,廣東仁化512300;2.中南大學(xué)冶金科學(xué)與工程學(xué)院,湖南長沙410083

凝膠-溶膠法制備單分散菱柱形草酸鎳的研究

徐 毅1,劉志宏2

1.深圳市中金嶺南有色金屬股份公司丹霞冶煉廠,廣東仁化512300;2.中南大學(xué)冶金科學(xué)與工程學(xué)院,湖南長沙410083

以Ni(OH)2凝膠為前驅(qū)體,采用凝膠-溶膠法制備了單分散菱柱形二水草酸鎳.采用SEM,XRD等方法及Imagetool圖像分析軟件,對所制備的NiC2O4·2H2O粉末形貌及形成機理等進行了研究,結(jié)果表明,控制適當(dāng)條件,可使NiC2O4·2H2O顆粒在低過飽和度下均勻生長,其形成機理為溶解-再結(jié)晶;隨溫度升高,粉末粒度增大,單分散性降低;溫度較低時,粉末顆粒接近球形;在313K制備的粉末為α型NiC2O4·2H2O,粉末的粒度分布范圍為0.45～0.90μm.

制備;二水草酸鎳;凝膠-溶膠法;菱柱狀形貌

微米或納米尺度的鎳及其化合物粉末,在電子、磁性材料、催化劑等領(lǐng)域應(yīng)用廣泛,其制備技術(shù)一直是研究的熱點[1-4].為滿足不同應(yīng)用的要求,制備中必須對粉末形貌、粒度及晶體結(jié)構(gòu)等進行控制,單分散粉末的制備是研究者們長期追求的目標(biāo)之一[5].LaMer[6]提出了濕化學(xué)法制備單分散粉末的模型,其要點為:通過對體系過飽和度的精確控制,將形核與晶核長大過程分開,粉末顆粒在一次形核的基礎(chǔ)上,按生長而非團聚方式長大.LaM er模型一般只適用于極稀溶液體系,難以應(yīng)用于工業(yè)生產(chǎn).在濃度較高的體系中,顆粒間靜電斥力較弱,其相互間團聚難以避免.為了解決這一問題,Tadao Sagamoto[7]提出了制備單分散粉末的凝膠-溶膠法(Gel-Sol),其思路為:首先通過水解或其他沉淀方法,將高濃度金屬鹽溶液制備成凝膠,然后再將凝膠進一步轉(zhuǎn)化為溶解度更低的溶膠,實現(xiàn)粉末顆粒形核與生長在低的過飽和度下進行,滿足LaM er模型制備單分散粉末條件的要求.此外,在溶膠顆粒形成中,由于凝膠的阻隔作用,也可避免其顆粒間的團聚發(fā)生.截至目前,已采用凝膠-溶膠法制備出單分散

Al3(SO4)2(OH)5·2H2O[8], CdS[9-11], Fe2O3[7,12-17],TiO2[18-22]等粉末.

金屬鎳及其氧化物粉末可通過前驅(qū)體草酸鎳熱分解制備,其形貌與粒度對前驅(qū)體具有“繼承性”.因此,單分散草酸鎳粉末的制備,是制備高質(zhì)量金屬鎳及其氧化物粉末的關(guān)鍵步驟.本文研究了一種以Ni(OH)2凝膠為前驅(qū)體,加入 H2C2O4使其轉(zhuǎn)化為NiC2O4·2H2O溶膠,即凝膠-溶膠法制備單分散二水草酸鎳粉末的方法.同時,研究了粉末顆粒的形成機理,以及溫度對粉末形貌與粒度的影響.

1 實驗部分

1.1 原料

NiCl2·6H2O,NaOH,Na2C2O4·2H2O,H2C2O4·2H2O均為試劑級(由日本 Nacalai Tesque公司生產(chǎn),未進一步提純);去離子水.

1.2 粉末制備

粉末樣品制備分為3步,各步驟及其固定條件為:

第1步:將100m L 0.2mol/L NiCl2溶液加入反應(yīng)器中加熱至313K,溶液p H313K值為5.75.然后,在攪拌條件下,快速加入30 m L 1 mol/L NaOH溶液,按反應(yīng)(1)生成淺綠色Ni(OH)2凝膠,其溶度積KSP,298K,Ni(OH)2=5.48 ×10-16,平衡后體系 p H313K值增至6.64.

第2步:由于NaOH的加入量不足以使全部Ni2+水解沉淀,根據(jù)加入的 NaOH量,可計算出Ni2+剩余濃度為0.0385 mol/L;以1 m L/min的速度滴加20 mL 0.25 mol/L Na2C2O4溶液,剩余的游離Ni2+依反應(yīng)(2)生成 NiC2O4·2H2O晶核,其溶度積 KSP,298K,NiC2O4=4×10-10,至此,體系 p H313K增加至7.36.

第3步:在控制體系p H313K值為6.0的條件下,將0.2mol/L H2C2O4溶液通過帶p H控制器的計量泵(EH/W,日本 Iwaki公司),滴入反應(yīng)器中,使Ni(OH)2凝膠通過下列反應(yīng)(3)～(6)逐步轉(zhuǎn)化為NiC2O4·2H2O溶膠.

H2C2O4溶液的滴加速度,可根據(jù)p H值偏離設(shè)定值的大小,通過反應(yīng)(3)～(6)的耦合自動調(diào)節(jié).當(dāng)H2C2O4溶液的滴加速度快,使體系的p H值達到或低于設(shè)定值時,自動停止滴加,此時,反應(yīng)(3)會加速正向進行,體系p H值增高,又恢復(fù)滴加,滴加速度與p H值正向偏離設(shè)定值的大小成正比.因此,反應(yīng)可在基本恒定的p H值下進行,保證了NiC2O4·2H2O顆粒在穩(wěn)定的過飽和度下生長.

1.3 樣品表征

采用SEM(Hitachi S-800)觀察樣品形貌;采用Imagetool圖像分析軟件,通過測量 SEM照片中100個粉末顆粒來確定其長度、寬度及軸向比(長度/寬度);采用 XRD(Shimadzu XRD-600,銅靶Kα)對樣品進行物相鑒定.

2 實驗結(jié)果及分析

2.1 粉末顆粒的形成機理

圖1(a)為第1步制得的Ni(OH)2凝膠的形貌圖.圖1(a)與圖3 XRD分析表明,凝膠的物相為β型Ni(OH)2(JCPDS 14-117),其形貌為直徑30～100 nm的球形顆粒,顆粒間呈網(wǎng)狀結(jié)構(gòu).圖1(b),(c)分別為第3步加入 H2C2O4溶液20,40 min后,制得的NiC2O4·2H2O顆粒的SEM照片.圖1(b),(c)及圖 3 XRD分析表明,隨 H2C2O4加入,Ni(OH)2凝膠逐漸消失,至40 min后,已完全轉(zhuǎn)化為α型NiC2O4·2H2O(JCPDS25-0281).圖1(b)還表明,二水草酸鎳顆粒在生長過程中被Ni(OH)2凝膠所包裹,避免了顆粒團聚.對圖1(c)所示樣品陳化2 h后,其SEM和XRD分析表明,粉末顆粒的形貌與晶體結(jié)構(gòu)沒有變化.以上研究結(jié)果表明,NiC2O4·2H2O粉末形成的機理為溶解-再結(jié)晶(Dissolution-Recrystallization),如式 (3)～ (6)所示.

圖1 固定條件下樣品的SEM照片(a)加入NaOH溶液后;(b)加入 H2 C2 O4溶液20min后;(c)加入 H2 C2O4溶液40min后Fig.1 SEM photographs of p recipitates under control conditions(a)after adding NaOH solution;(b),(c)after adding H2 C2 O4 solution fo r 20 and 40 min,respectively

2.2 粉末的表征

所制備的粉末形貌如圖2所示.圖2(b)為采用圖像處理軟件 Imagetool,對圖2(a)中的顆粒進行測量,以此為依據(jù)繪制的粉末顆粒形貌示意圖.圖2表明,顆粒形貌為菱柱形,菱形底面的兩對夾角分別為106°和74°,已知菱邊長b和柱高l,可推導(dǎo)出顆粒體積V的計算公式,如式(7)所示.

對Imagetool測定結(jié)果進行統(tǒng)計處理得到,固定條件下制備的粉末顆粒底面菱邊長平均值為0.32μm,柱高平均值為0.65μm,菱邊長與柱高的比值(b/l)為0.49,考慮測量與統(tǒng)計誤差,取值0.5.

圖3為所制備的NiC2O4·2H2O顆粒的XRD圖.圖3表明,制備的樣品為α型 NiC2O4·2H2O(JCPDS 25-281),屬單斜(Monoclinic)晶系,a=1.1775×10-9m,b=5.333×10-10m,c=9.833×10-10m,β=127.2°.

測量粉末SEM照片中100個顆粒的粒徑,所得結(jié)果繪制成粉末粒度頻率分布圖,如圖4所示.由圖4可知,粉末的平均粒度為0.65μm,粒度分布范圍為0.45～0.90μm.根據(jù)統(tǒng)計結(jié)果計算,其單分散性指標(biāo)σ(σ=D84.13/D50)值為 1.22,這表明按1.2節(jié)所述條件制備的粉末的單分散性較好.

圖2 粉末形貌(a)SEM照片;(b)顆粒形貌示意圖Fig.2 Morphology of pow ders(a)SEM photographs;(b)Schematic diagram of typical particle morphology

圖3 粉末的XRD圖譜Fig.3 XRD pattern of pow der p repared under control conditions

圖4 粉末的粒度分布Fig.4 Size distribution of pow ders

2.3 溫度的影響

在1.2節(jié)粉末的制備中,將反應(yīng)溫度分別改為293K和333K進行試驗,所制得的粉末形貌如圖5所示.粉末粒度與形貌參數(shù)的統(tǒng)計結(jié)果列于表1.

圖5與表1表明,溫度對顆粒的形貌與粒度均有較大影響.隨溫度升高,粉末粒度增大、單分散性降低.圖5顯示,不同溫度下,顆粒形貌均為菱柱形,但在293K低溫下,菱邊長與柱高的比值(b/l)較大,形貌接近類球形,而在313K和333K下,其值均為0.5左右.

表1 溫度對粉末形貌及粒度的影響Table 1 Effect of temperatures onmorphology and size of pow ders p repared

圖5 不同溫度下樣品的SEM照片F(xiàn)ig.5 SEM photographs of pow ders p repared at different temperatures

3 結(jié) 論

(1)二水草酸鎳粉末顆粒的形成機理為溶解-再結(jié)晶.控制適當(dāng)條件,可使顆粒在低過飽和度下均勻生長.顆粒生長過程中,Ni(OH)2凝膠包覆在NiC2O4·2H2O顆粒表面,一方面作為 Ni2+的“緩釋源”,另一方面,起到了阻隔顆粒碰撞團聚的作用.

(2)在 313K制備的粉末為α型 NiC2O4·2H2O,呈菱柱形,菱形底面的兩對夾角分別為74°和106°.粉末的粒度分布范圍為 0.45～0.90μm,D50=0.65μm,單分散性指標(biāo)為1.22.

(3)溫度對顆粒的形貌與粒度均有較大影響.隨溫度升高,粉末粒度增大,單分散性降低;溫度較低時,粉末顆粒接近球形.

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Preparation of mono-dispersed rhombohedra-type n ickel oxalate dehydrate by a gel-sol process

XU Yi1,L IU Zhihong2
1.Dan X ia Smelter,Shenzhen Zhong jin L ingnan N onfemet Co.L TD,Renhua 512300,China;2.College of Metallurgical Science and Engineering,Central South University,Changsha 410083,China

Monodisperse nickel oxalate dihydrate pow ders of rhom bohedron mo rphology were p repared from condensed Ni(OH)2p recurso r by the novel gel-sol p rocess.The mo rphologies and fo rmation mechanism of the particles p repared were investigated by SEM,XRD and Imagetool softw are,and the results show ed that by controlling app rop riate conditions,nickel oxalate dehydrate particles grew equably at a low er super saturation through dissolution-recrystallization p rocess.With the increase of temperature,the size of the pow ders increased,accompanied w ith the decrease of theirmono-dispersity.A t a lower temperature,the mo rphology of the pow ders was similar to spheroid.The pow ders made at 313 K were in the form ofαtype NiC2O4·2H2O w ith a size range of 0.45-0.90μm.

p reparation;nickel oxalate dehydrate;gel-sol p rocess;rhombohedron morphology

TF123.121

A

1673-9981(2011)02-0159-05

2011-05-16

徐毅(1958—),男,重慶人,高級工程師,碩士.