微纳级银包铜颗粒的快速制备及其导电性能*

才滨, 张哲娟, 孙卓

华东师范大学物理与材料科学学院纳光电集成与先进装备教育部工程研究中心上海 200062

CAI Bin, ZHANG Zhejuan^*^,, SUN Zhuo

School of Physics and Materials Science, Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, East China Normal University, Shanghai 200062, China

**To whom correspondence should be addressed, Tel: (021)62232054, E-mail: zjzhang@phy.ecnu.edu.cn
本文联系人: 张哲娟

基金资助: * 国家青年自然基金资助项目11204082, 上海市自然基金项目16ZR1410700, 上海市闵行区企校合作项目2015MH218和华东师范大学研究生科研创新实践YJSKC2015-32资助项目;

*Supported by National Natural Science Foundation of China No.11204082, Shanghai Natural Fund Project No.16ZR1410700, School-enterprise Cooperation in Minhang District of Shanghai No. 2015MH218 and Postgraduate Scientific Research and Innovative Practice Funding Program of ECNU No. YJSKC2015-32.;

中图分类号: 分类号 TB383 文章编号 1005-3093(2016)12-0903-06 文献标识码: 分类号 TB383 文章编号 1005-3093(2016)12-0903-06

摘要:

提出一种制备银包铜(Cu-Ag)核壳颗粒的一步化学置换法。使用成本较低的柠檬酸三纳(SC)作为还原剂和螯合剂, 用明胶(Gelatin)作为分散剂, 硫酸银(Ag₂SO₄)为Ag源, 用一步化学置换法制备银包铜(Cu-Ag)核壳颗粒, 研究了Ag₂SO₄和SC用量对Cu-Ag颗粒包覆效果和抗氧化性能的影响。结果表明, SC的剂量直接影响表面包裹的Ag颗粒形貌和均匀度。Ag₂SO₄剂量越大则Cu表面的Ag包裹量越大, 导电性越好。当SC的剂量为1.5 g, Ag₂SO₄的剂量为8.0 g时Cu-Ag颗粒包覆效果好且电阻较低(仅为1.1 Ω), 因此可尽量降低Ag的消耗量提高颗粒的导电性。

关键词: 金属材料 ; 银包铜金属颗粒 ; 核壳结构 ; 螯合剂 ; 导电性

Abstract:

Micro- and nano-sized Cu and Ag core-shell particles were fabricated by a simple and quick-making method with copper micro-particles, gelatin and silver sulfate (Ag₂SO₄) as raw materials, and citric acid trisodium (SC) as reducing- and chelating-agent. The influence of SC and Ag₂SO₄ on the morphologies and oxidation resistance of Cu-Ag particles were investigated. The results showed that the dosage of SC directly affected the uniformity and morphology of the Ag coated Cu particles. The more the Ag₂SO₄ was used, the lower the conductive resistance was for the prepared particles. With dosages of 1.5 g and 8.0 g for SC and Ag₂SO₄respectively, the Cu- particles could be covered by Ag nanoparticles completely to form core-shell structured Cu-Ag particles, thereafter, the electrical resistance of sheets made of which can reach as low as 1.1Ω.

Key words: metallic materials ; Ag coated Cu particles ; core-shell particles ; chelating agent ; conductivity

有壳覆盖的粒子在许多领域受到关注, 因为覆盖层提高和改变了原粒子的表面性质和功能, 比如催化活性, 硬度, 渗透性, 粘性和导电性^[1]。银粒子具有低电阻和高导热的特性, 还有很好的抗氧化性能, 但是较高的价格限制了它的应用; Cu粒子具有与Ag粒子相似的导电性能, 但是钠米级Cu粉的比表面积较大, 化学活性很高, 在空气中暴露极易被氧化成氧化亚铜, 失去原有的物理化学特性^[2]。在Cu粉表面镀Ag形成铜银双金属粉, 既提高了Cu粉的抗氧化性能又保留了Cu粉优良的物理化学特性。因此, 铜银双金属粉是Cu和Ag的最佳替代品^{[3, 4]}。

制备纳米级银包铜的方法有化学置换法, 溅射法, 电镀法等, 但是有的方法效率较低且需要较贵重的实验装置^[5]。Valérie Mancie等^[6]用超声辅助电化学法制备了尺寸小于100 nm的银包铜颗粒。D.S. Jung等^[1]用超声喷雾热解法将液相银铜溶液加热, 制备出固态银铜双金属粉。本文提出一步化学置换法, 以纳微米级Cu粉置换Ag⁺制备铜银双金属粉。使用SC为络合剂, 与Ag⁺形成络合离子以控制Ag的生长速率, 利用明胶作为分散剂, 一次性制得纳微米银包铜粉末。

1 实验方法

实验用材料: 微纳纯Cu粉, 分析纯硫酸银(Ag₂SO₄), 柠檬酸三钠(SC), 明胶(Gelatin)、盐酸(HCl)和无水乙醇。

将5.0 g Cu粉用稀HCl进行酸洗; 将5.0 g明胶加入1000 ml去离子水后搅拌均匀, 配成溶液A。将适量的SC和Ag₂SO₄加入2500 ml去离子水中, 然后放入棕色瓶中, 搅拌均匀配成溶液B。在实验中, 先固定Ag₂SO₄用量为11.7 g, 分别加入0 g, 0.5 g, 1.5 g, 6.0 g SC; 再固定SC用量为1.5 g, 分别加入3.0 g, 4.0 g, 5.0 g和8.0 g的Ag₂SO₄。在27℃在1000rps磁力搅拌条件下将溶液B缓慢倒入溶液A中, 直到溶液变成深绿色, 反应停止。将反应后的样品用去离子水和无水乙醇反复清洗3次, 得到银包铜颗粒产物(Cu-Ag颗粒), 将其放在无水乙醇中保存。

为了研究Ag₂SO₄用量和还原剂SC用量对Ag包裹作用的影响, 用激光扫描粒径分布仪(LA-950V2)测试Cu核粒径, 用冷场发射扫描电子显微镜(FESEM, JEOL-5600LV) 测试不同产物的表面形貌, 用X射线衍射仪(XRD, D/tax-Rigaku Ultima IV X-ray diffraction)分析Cu-Ag颗粒的晶体结构。为了分析比较不同包覆条件下Cu-Ag颗粒的导电性, 选择相同固含量的Cu-Ag颗粒乙醇溶液, 用滴涂法将其均匀涂在载玻片上, 在150℃烧结30 min, 用四探针测试仪(RTS-8)测量涂层的方块电阻。

2 结果和讨论

2.1 SC的用量对Cu颗粒表面Ag包覆效果的影响

将原始Cu粉浸入EG溶液中, 用超声使其分布均匀。用激光扫描粒径分布仪测试原始Cu的平均粒径为2.63 μm和粒径分布(图1a), 原始Cu粉的SEM图像如图1b所示。由图1b可见, 原始Cu颗粒的粒径不均匀, 颗粒表面形貌也不均匀, 其中大部分颗粒表面不光滑缺陷较多且不十分圆整, 有少量Cu颗粒表面光滑且圆整。图2给出了不同SC用量下合成的Cu-Ag颗粒的SEM照片。可以看出, 不添加SC时Cu颗粒表面包裹了椭球形或圆形的颗粒状Ag, Ag颗粒尺寸约为500 nm。但是, Cu颗粒表面仅为部分包裹, 分布不均匀。随着螯合剂的加入及用量的增加, Ag颗粒的生长速度减慢, 且包裹均匀度也有所改善。当SC增加到0.5 g时包裹呈现为两种现象(图2b中的箭头), 小颗粒Cu的表面包裹了一层均匀光滑的Ag层, 大颗粒Cu表面以堆积方式包裹了粒径约为200 nm的Ag颗粒。当SC继续增加到1.5 g以上时产物中的Cu颗粒被大量的纳米Ag颗粒包裹, Ag颗粒尺寸约为100nm(图2c, d)。但是, 当SC增加到6.0 g时Cu颗粒表面Ag析出过多。除大部分粒径约为50 nm的颗粒外, 还出现了部分弯曲的Ag纤维。Ag纤维的粗细为150~200 nm, 长度约2.0~2.5 μm。由此可见, SC的用量直接影响产物中Ag原子的析出浓度从而改变了Ag颗粒的尺寸和密度, 当Ag原子浓度过高时Ag向一维尺度生长。

图1 原始Cu粉的粒径分布和SEM照片

Fig.1 Size distribution (a) and SEM image (b) of the raw Cu particles

图2 不同SC剂量下制备的Cu-Ag颗粒SEM照片

Fig.2 SEM images of Cu-Ag particles prepared with different dosages of SC (a) 0 g; (b) 0.5 g; (c) 1.5 g; (d) 6.0 g

为了证明产物是Ag包裹的Cu颗粒, 分别对SC为1.5 g条件下制备的Cu-Ag颗粒进行了XRD和EDS能谱扫描测试。由样品的XRD谱图(图3)可见, Ag的峰窄且峰值很高, Cu的峰值相对很低。这说明Ag和Cu的结晶度很高, 也说明Cu颗粒表面的Ag层过厚, 产物中Ag含量大。另外, 在图谱中还出现了AgO的特征峰, 这主要是样品在空气中干燥所致。

图3 SC的含量为1.5 g时制备的Cu-Ag粉末的XRD衍射图谱

Fig.3 XRD patterns of the Cu-Ag powder prepared with 1.5 g SC

样品的线扫描SEM和EDS能谱, 如图4所示。为了便于分析, 将衬底的背景元素去除, 只保留了Cu和Ag元素的EDS能谱。由图4b可见, 颗粒边缘Ag的能谱强度高, 而颗粒中心位置的Ag能谱强度较低; 这表明, 用这种方法合成的Cu-Ag样品为核壳结构。样品中Ag的含量明显高于Cu的含量, Ag的含量高有利于提高Cu-Ag粉末的导电性能^[7]。

图4 SC含量为1.5 g时制备的样品线扫描SEM和线扫描能谱分布

Fig.4 Cross-sectional SEM images (a) and Cross-sectional EDS analysis (b) of the Cu-Ag sample prepared with 1.5 g SC

2.2 AgSO对Cu-Ag包覆效果的影响

固定SC剂量为1.5 g, 改变Ag₂SO₄的质量分别为3.0 g, 4.0 g, 5.0 g, 8.0 g, 11.7 g, Cu-Ag颗粒的SEM形貌如图5所示。从图5可以看出, Ag颗粒包裹在Cu颗粒表面形成包覆结构。当Ag₂SO₄含量较低时Ag层包裹较薄, 有部分Cu核裸露在外(图5a, b中的方框)。随着Ag₂SO₄含量的提高Ag颗粒的包裹越来越致密, 含量越来越高。当Ag₂SO₄剂量增加到11.7 g时Ag颗粒大量堆积包裹在Cu颗粒表面(图5e)。图6给出了不同Ag₂SO₄剂量下Cu-Ag样品中Ag含量的EDS能量分布, 可见随着Ag₂SO₄含量的增加颗粒中Ag的含量增加。

图5 不同Ag₂SO₄剂量下制备的Cu-Ag产物的形貌

Fig.5 SEM images of the Cu-Ag particles prepared with different dosages of Ag₂SO₄ (a) 3.0 g, (b) 4.0 g, (c) 5.0 g, (d) 8.0 g and (e) 11.7 g

图6 不同Ag₂SO₄剂量下制备的Cu-Ag产物中Ag的EDS能量比重

Fig.6 EDS proportion of Ag in Cu-Ag production with different dosages of Ag₂SO₄

2.3 Cu-Ag颗粒的电性能

将在不同条件下制备的Cu-Ag颗粒用超声均匀分散在乙醇溶液中, 采用滴涂法均匀涂在玻璃衬底上, 将样品在150℃烧结30 min后采用四探针测试仪(RTS-8)测量其方块电阻。

表1列出了不同SC和Ag₂SO₄剂量下制备的Cu-Ag样品的电阻。由表1可见, 不添加螯合剂SC时表面方块电阻值为6.0 Ω。这说明, Cu的包裹并不完整, 有Cu裸露在空气中形成了氧化铜, 阻碍了导电性能。随着SC剂量的增加样品的方块电阻逐渐减小, 在SC剂量为1.5 g时达到最小值(0.4 Ω)。SC剂量继续增加至6.0 g, 样品的导电性能反而下降至4.5Ω。这些结果表明, SC质量为1.5 g时Cu-Ag颗粒的包裹均匀性和导电性最佳。当固定SC用量时, Ag₂SO₄含量越高Cu-Ag颗粒的电阻值越小。当Ag₂SO₄剂量小于4.0 g时, Cu-Ag样品的电阻较大导电性能很差。这说明, 在Ag源用量较少的情况下有大量的Cu核包裹不完全裸露在外, 或包裹的Ag层很薄, 样品在高温烧结后有Cu被氧化成氧化铜, 降低了导电性能。当Ag₂SO₄含量大于5.0 g即Cu-Ag中Ag的比重大于78.67%时, Ag的包裹相对完整, Ag层较厚, 此时电阻阻值降低到30.6 Ω。随着Ag₂SO₄剂量继续增加达到8.0 g阻值降低至1.1 Ω, Cu-Ag颗粒表现出了较好的导电性能。图7给出了Ag₂SO₄剂量为8.0 g条件下制备的Cu-Ag样品150℃烧结30 min后的SEM图。可以看出, Cu-Ag颗粒在150℃处理后依然包裹完好, Cu核颗粒尺寸较大, 不会在低温下出现熔融状态。在该温度处理后Cu颗粒表面Ag层没有发生脱落, 也没有出现Ag层的熔融互联。这些结果表明, 纳米Ag包裹量直接影响了Cu-Ag颗粒的导电性能。

Sample	Dosage of SC/g	Dosage of Ag₂SO₄/g	Sheet resistance/(Ω/□)
(a)	0	11.7	6.0
(b)	0.5	11.7	0.5
(c)	1.5	3.0	-
(d)	1.5	4.0	200.0
(e)	1.5	5.0	30.6
(f)	1.5	8.0	1.1
(g)	1.5	11.7	0.4
(h)	6.0	11.7	4.5

表1 不同SC和Ag₂SO₄剂量Cu-Ag样品的方块电阻

Table 1 Sheet resistance of Cu-Ag samples prepared with different dosages of SC and Ag₂SO₄

图7 Ag₂SO₄剂量为8.0 g时制备的Cu-Ag样品在150℃烧结30 min后的SEM照片

Fig.7 SEM images of Cu-Ag particles prepared with 8.0 g Ag₂SO₄ after being heated under 150℃ for 30 min

在实验过程中Ag₂SO₄, SC, Cu粉之间发生如下化学反应

$\begin{matrix} C_{6} H_{5} {O_{7}}^{3 -} + A g^{+} \to C_{5} H_{4} {O_{5}}^{2 -} + H^{+} + C O_{2} + 2 Ag \end{matrix}$ (1)

$\begin{matrix} {(Ag)}_{2}^{+} + citrat e^{-} \leftrightarrow [{(Ag)}_{2}^{+} ... {(citrate)}^{-}] \end{matrix}$ (2)

$\begin{matrix} Cu + 2 A g^{+} \leftrightarrow 2 Ag + C u^{2 +} \end{matrix}$ (3)

$\begin{matrix} 3 C u^{2 +} + 2 C_{6} H_{5} {O_{7}}^{3 -} \leftrightarrow C u_{3} (C_{6} H_{5} O_{7})_{2} \end{matrix}$ (4)

Ag₂SO₄先与柠檬酸根发生螯合作用生成乳白色柠檬酸银螯合物, Cu颗粒再置换柠檬酸银。反应方程式(1)–(4)表明, 要将Ag离子在Cu表面较好的还原, Ag源的用量至少在还原剂的3倍以上。随着螯合剂SC用量的增加Cu-Ag颗粒的抗氧化性能增加, SC剂量为1.5 g时抗氧化性能最佳。SC与Ag⁺形成了稳定的螯合物, 降低了溶液中游离的Ag⁺浓度, 从而有效地降低了Ag离子被还原的速率, 并降低了Ag晶核的形成和生长速度, 有利于Ag晶核还没长大时就附着在Cu颗粒表面, 改善包覆情况。SC剂量越大则Ag的生成越慢, 但当SC剂量太多时Ag的生成会很慢。先沉积在Cu粉表面的Ag与Cu形成大量的微电池, 在其作用下Ag的缓慢生成增加其在点缀镀层部位沉积的几率, 更多的Ag沉积在点缀部位, 使Ag在Cu粉表面沉积得愈加不均匀, 降低了Cu-Ag粉的抗氧化性^[3]。

3 结论

用柠檬酸三钠还原法可制备核壳结构的纳微米Cu-Ag颗粒。改变SC和Ag₂SO₄的剂量可优化Cu-Ag颗粒表面包覆Ag层的均匀性, 使纳微米Cu-Ag颗粒具有较好的导电性能。当SC的剂量为1.5 g、Ag₂SO₄剂量为8.0 g时Cu-Ag颗粒的电阻达到1.1 Ω, 包覆效果比较完整。

The authors have declared that no competing interests exist.

参考文献

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4	H. T. Hai, H.Takamura, J. Koike.Oxidation behavior of Cu-Ag core-shell particles for solar cell applications. J. Alloys Compd., 564, 71(2013) ABSTRACT Cu–Ag core–shell particles with 5 μm-diameter Cu core and 260 nm-thick Ag shell were prepared by an electroless plating method in an aqueous system. The obtained core–shell particles were investigated for their thermal oxidation behavior during baking in air ambient. Thermogravimetric analysis indicated that the oxidation was started at about 200 °C. This oxidation temperature was a little higher than that (150 °C) of Cu particles without Ag shell. Microstructure observation of the core–shell particles baked in Ar ambient revealed the agglomeration of Ag shell on the Cu core surface after heating the sample above 200 °C, which resulted in direct exposure of the Cu core to ambient. Thus in air ambient, the exposed Cu core was destined to be oxidized. The temperature dependent growth of the agglomerated Ag shell was characterized with an activation energy of 37.56 kJ/mol, which could be assigned to a surface-diffusion-controlled mechanism. This characteristic of the Ag shell would limit the applications of the Cu–Ag core–shell particles to conductive paste for solar cells, wherein baking temperature beyond 200 °C in air ambient is generally required to promote good sintering of the paste particles as well as to obtain good electrical contact between metal and silicon. DOI:10.1016/j.jallcom.2013.02.048 URL [本文引用:1]
5	Y. H.Peng, C. H. Yang, K. T. Chen, S. R. Popuri, C. H. Lee, B. S. Tang, Study on synthesis of ultrafine Cu-Ag core-shell powders with high electricalconductivity, Appl. Surf. Sci., 263, 38(2012) Cu-Ag composite powders with high electrical conductivity were synthesized by electroless plating of silver sulfate, copper powders with eco-friendly sodium citrate as reducing agent, dispersant and chelating agent in an aqueous system. The influences of sodium citrate/Ag ratio on Ag coatings of Cu powders were investigated. Ag was formed a dense coating on the surface of Cu powders at a molar ratio of sodium citrate/Ag = 0.07/1. SEM showed an uniformity of Ag coatings on Cu powders. SEM-EDX also revealed that Cu cores were covered by Ag shells on the whole. The surface composition analysis by XPS indicated that without Cu or Ag atoms in the surface were oxidized. The resistivity measurements of Cu-Ag paste shows that they have closer resistivity as the pure silver paste's after 250 °C for 30 min heat-treatment (2.55 × 10Ω cm) and 350 °C for 30 min heat-treatment (1.425 × 10Ω cm). DOI:10.1016/j.apsusc.2012.08.066 URL [本文引用:1]
6	V. Mancie, C. Rousse-Bertrand, J. Dille, J. Michel, P. Fricoteaux, Sono and electrochemical synthesis and characterization of copper core-silvershell nanoparticles, Ultrason. Sonochem., 17(4), 690(2010) Abstract Cu-Ag core-shell nanopowders have been prepared by ultrasound-assisted electrochemistry followed by a displacement reaction. The composition of the particles has been determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The XRD patterns versus time displacement show that higher are the silver peaks intensities, weaker are the copper ones. That exhibits the progressive recovering of copper by silver. EDX results and quartz crystal microbalance results indicate that various reaction mechanisms are implied in this process. Transmission electron microscopy (TEM) points out variable nanometric diameter grain and some small agglomerates. Elemental mapping obtained by electron energy-loss spectroscopy (EELS) underlines the core-shell structure. DOI:10.1016/j.ultsonch.2009.12.009 PMID:20074993 URL [本文引用:1]
7	Hu Lei, Zhu Xiaoyun, Plating Layer Structure and Property of Silver-CoatedCopper Power with High Silver Content, Rare Metal Materials and Engineering, 11, 2017(2012) [本文引用:1] (胡磊, 朱晓云, 高银含量银包铜粉镀层结构及性能研究, 稀有金属材料与工程, 11, 2017(2012)) Magsci URL

Air-stable silver-coated copper particles ofsub-micrometer size, J

2011

... 有壳覆盖的粒子在许多领域受到关注, 因为覆盖层提高和改变了原粒子的表面性质和功能, 比如催化活性, 硬度, 渗透性, 粘性和导电性^[1].银粒子具有低电阻和高导热的特性, 还有很好的抗氧化性能, 但是较高的价格限制了它的应用; Cu粒子具有与Ag粒子相似的导电性能, 但是钠米级Cu粉的比表面积较大, 化学活性很高, 在空气中暴露极易被氧化成氧化亚铜, 失去原有的物理化学特性^[2].在Cu粉表面镀Ag形成铜银双金属粉, 既提高了Cu粉的抗氧化性能又保留了Cu粉优良的物理化学特性.因此, 铜银双金属粉是Cu和Ag的最佳替代品^{[3, 4]}. ...

... 制备纳米级银包铜的方法有化学置换法, 溅射法, 电镀法等, 但是有的方法效率较低且需要较贵重的实验装置^[5].Valérie Mancie等^[6]用超声辅助电化学法制备了尺寸小于100 nm的银包铜颗粒.D.S. Jung等^[1]用超声喷雾热解法将液相银铜溶液加热, 制备出固态银铜双金属粉.本文提出一步化学置换法, 以纳微米级Cu粉置换Ag⁺制备铜银双金属粉.使用SC为络合剂, 与Ag⁺形成络合离子以控制Ag的生长速率, 利用明胶作为分散剂, 一次性制得纳微米银包铜粉末. ...

Ethylene glycol-based Ag plating for the wet chemical fabrication of onemicrometer Cu/Ag core/shell particles, J

2015

Preparation and oxidation-resistant property research of micron Cu-Ag pimetallicpowder,

2006

... Ag₂SO₄先与柠檬酸根发生螯合作用生成乳白色柠檬酸银螯合物, Cu颗粒再置换柠檬酸银.反应方程式(1)–(4)表明, 要将Ag离子在Cu表面较好的还原, Ag源的用量至少在还原剂的3倍以上.随着螯合剂SC用量的增加Cu-Ag颗粒的抗氧化性能增加, SC剂量为1.5 g时抗氧化性能最佳.SC与Ag⁺形成了稳定的螯合物, 降低了溶液中游离的Ag⁺浓度, 从而有效地降低了Ag离子被还原的速率, 并降低了Ag晶核的形成和生长速度, 有利于Ag晶核还没长大时就附着在Cu颗粒表面, 改善包覆情况.SC剂量越大则Ag的生成越慢, 但当SC剂量太多时Ag的生成会很慢.先沉积在Cu粉表面的Ag与Cu形成大量的微电池, 在其作用下Ag的缓慢生成增加其在点缀镀层部位沉积的几率, 更多的Ag沉积在点缀部位, 使Ag在Cu粉表面沉积得愈加不均匀, 降低了Cu-Ag粉的抗氧化性^[3]. ...

微米级铜银双金属粉的制备及其抗氧化性能研究,

2006

Oxidation behavior of Cu-Ag core-shell particles for solar cell applications. J

2013

Study on synthesis of ultrafine Cu-Ag core-shell powders with high electricalconductivity

2012

Sono and electrochemical synthesis and characterization of copper core-silvershell nanoparticles

2010

Plating Layer Structure and Property of Silver-CoatedCopper Power with High Silver Content,

2012

... 样品的线扫描SEM和EDS能谱, 如图4所示.为了便于分析, 将衬底的背景元素去除, 只保留了Cu和Ag元素的EDS能谱.由图4b可见, 颗粒边缘Ag的能谱强度高, 而颗粒中心位置的Ag能谱强度较低; 这表明, 用这种方法合成的Cu-Ag样品为核壳结构.样品中Ag的含量明显高于Cu的含量, Ag的含量高有利于提高Cu-Ag粉末的导电性能^[7]. ...

高银含量银包铜粉镀层结构及性能研究,

2012