Preparation And Deposition Process Of Cu/Al Powder In Conductive Polymer For EMI Shielding

Xing Zhang, Zhidong Xia, Yong Gao, Shaofan Zhao Beijing University of Technology

100 Pingle Yuan, Chaoyang District, Beijing 100124 Email: zhangxingl22@ sohu.com

Phone: 13466310648

Abstract Copper-aluminum powders were prepared by displacement

method in aqueous system at 40°C. Composite powder's surface topography was investigated by varying the addition of sodium fluoride (NaF). It was noted that dense and uniform copper coating was obtained when the mass ratio of NaF and aluminum powder was approximate 0.1. The mechanism of copper coating formation was discussed in detail. The deposition process was summarized to four stages, including initial reaction stage, grid formation stage, two-dimensional coating spread stage and final epitaxial growth stage.

1 Introduction It is more and more important to pay attention to the

influence of electromagnetic interference (EMI). Electromagnetic shielding polymer consists of highly conductive filler powders dispersed in a flexible, insulating rubber substrate. Electro-magnetic property of electromagnetic shielding polymer is closely dependent upon conductive fillers. Silver may be the most effective but more expensive filler, while aluminum shows good conductivity and low density but is susceptible to oxidation. The substitution of Ag/Al composite powders for silver and aluminum powder is currently considered to be a desirable option in electronic industry to solve the problems of oxidation of aluminum and high cost of silver[1]. Since aluminum is a strongly active metal that reacts with both acid and alkali, depositing silver on aluminum powder directly is difficult because the pH level in the plating environment is too high and aluminum powder would autolyze. Therefore some metals, such as copper[2], are chosen as the transition materials.

Generally, the composite powder of aluminum-based is obtained by electroplating[3] and electroless plating processes[4], while electroplating requires precision equipment and has low efficient deposition for aluminum powder[5]. Electroless copper coating can be successfully achieved. Zhang[6] dispersed active aluminum powders in copper-bath, and sodium potassium tartrate was added as reducing agent to form copper coats on aluminum powders. Chen[7] used fluorinion as complexing agent, and copper layer was formed by displacement reaction.

In this paper, copper coated aluminum (Cu/Al) composite powder was prepared and the formation process of copper layer on aluminum powder had been studied. Meanwhile, the copper deposition process was analyzed by observation of powders at different stages during the plating process. Deposition and growth of copper was speculated to include four stages,

including initial reaction stage, grid formation stage, two-dimensional coating spread stage and heteroepitaxy stage. Based on this speculation, experimental parameters and conditions were controlled to obtain sound coverage Cu/Al composite powders.

2 Experiments 2.1 Copper coating process

5g aluminum powders (30-50|im, chemical purity > 99%) were dispersed in 10ml of acetone and ultrasonically cleaned for 5mins. The cleaned powders were filtered and washed with deionized water. After the pretreatment, the powders were added to approximately 100ml of copper sulfate and ethylene diamine tetraacetic acid disodium (EDTA-2Na). The slurry was stirred at the speed of 400rpm for 2mins, and 100ml of a solution containing gelatin was added to the slurry as dispersing agent. Sodium hydroxide and buffer solution were used to adjust the pH value to be 4 in the process. The mixture then were heated up to 40 °C. Finally, an aqueous solution containing sodium fluoride which functioned as complexant was added to the mixture till the copper coating formed. The composite powders were separated and washed with deionized water and anhydrous alcohol for several times. The powders were finally dried in a vacuum oven at 80 °C for 3h. All the chemicals used throughout the study were regent grade in purity.

2.2 Characterization The characterization of the composite powders was

performed by scanning electron microscopy (SEM, HITACHI S-3400N), and the cross-section of the powders were also investigated. The compositions of initial and terminal powders were determined by energy dispersive analysis (EDS).

3 Results and Discussion 3.1 Mechanism of copper coating

In a copper sulfate solution, the displacement reaction can be described as follows:

E/^-1.66V E2°= 0.34V

2A1 + 3Cu2+= 2A13+ + 3Cu A E=2.00V

Al -3 e = Af+

Cu2+ + 2e =Cu (1) (2) (3)

During the coating process, copper is deposited by reduction-oxidation reaction as described in Eq.(3), where the reduction-oxidation potentials indicates that copper can be deposited on aluminum by displacement reaction.

But because aluminum is a strongly active metal, there is a layer of oxide on its surface which impedes the displacement

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reaction. Alumina reacts with both acid and alkali as expressed in the following equations:

A1203 + 6H+ = 2A13+ + 3H20 (4) A1203 + 20H = 2A102"+ H20 (5)

Al3+ + 6F = [AlF6f (6) In fact, aluminum also reacts with acid and alkali. We can't

find the destination of the reactions in pretreatment. It is suggested that the reduction process is mainly controlled by the dissolution of surface oxides; furthermore, if the pH of plating system is either too high or too low, the cleansed aluminum powder area would react with OH" or H+ and form H2, which leads to the formation of copper species. F" works as the complexing of Al3+, as described in Eq.(6). The existence of F" allows the oxides dissolve in a neutral environment in the plating system, and the displacement reaction would occur rapidly wherein the aluminum surface become cleansed and activated. In addition the uniform copper nuclei distribution on aluminum powder surfaces is necessary to form a uniform copper coating layer. 3.2 Effect of the addition of NaF

The quality of composite powders depended much on the addition of NaF. In this test, the mass ratio of NaF and Al (MNaF/Ai) varied from 0 to 0.2, where the effect of addition of

NaF was investigated by keeping aluminum powder constant while NaF was varied.

Fig.l showed the surface morphology of copper coating with the variation of MNaF/Ai, It showed that the copper coating structure changed from porous to dense.

Without any addition of NaF, no copper but alumina was found on the powder surface according to EDS data, as shown in Fig.la. From Fig.l(b-c), it could be seen that the uniformity of copper coating was improved with the increase of addition of NaF. These changes were also proved by the increasing of copper weight percent in the composite powder as the ratio MNaF/Ai varied from zero to 0.1. However, aluminum fluoride formed when the MNaF/Ai was greater than 0.1, which was due to the excess fluorinion could promote the complex reaction as shown in Eq.(6). The formation of aluminum fluoride made the powder surface unsmooth. Because of the light scattering on the uneven surface, the powder appeared darker as the MNaF/Ai increased from 0.1 to 0.2(Fig.l(c-e)). Fig.If showed the cross-section images of composite powders when the mass ratio MNaF/Ai was 0.1. It showed that the thickness of the copper coating was approximate 1-2 urn, and the layer was continuous and free of copper.

Fig.l. The surface topography and sectional drawing of copper coated aluminum by different addition of NaF (a) MNaF/Ai=0, (b)MNaF/Ai=0.05, (c)MNaF/Ai=0.1, (d)MNaF/Ai=0.15, (e)MNaF/Ai=0.2, (f)Cu /Al sectional drawing at MNaF/A1 = 0.1

3.3 Developing process of copper layer The powders prepared at the optimum conditions with the

mass ratio MNaF/Ai as 0.1, the concentration of NaF as 0.5g, and the pH level as 4. The copper deposition process was investigated and the images of the composite powders with different reaction time were shown in Fig.2.

The micro-sized aluminum particles with a sphere shape were showed in Fig.2a. The powder's surface was feature with

smoothness with some satellite particles around it. EDS data showed that all aluminum surfaces stored in aerobic

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environment were covered by thin layer of the oxide(alumina). Table 1 showed the chemical compositions of the aluminum powder.

Table 1. EDS results of aluminum surface Elements

A1K O K

Weight fraction (%) 94.25 5.75

Atom fraction (%) 93.29 6.71

Aluminum powders were added into the plating solution and stirred at speed of 400rpm. The process started with the reaction of surface oxide with hydrion and then followed by a displacement reaction between aluminum and copper ions (Fig.2b). It is believed that the place such as the edge, the lug boss and the surface discontinuity show high energy potential, where low activation energy is required, therefore the dissolved reaction occurred more favorably at these places. And the removal of the oxides provided more activated points on aluminum particles to attract copper ions to deposit in situ. As the plating continuous, more and more

tiny copper particles were deposited at the convex of the particles and the copper coverage areas enlarged(Fig.2c). Once the copper particles deposited on aluminum surfaces, the subsequent copper deposited priority on the borders of substrate and the initial copper nodules. This was the reason that why copper coating structure showed the same characteristics as that of the substrate, which was obviously indicated in Fig.2d. The discontinuous of surface morphology on aluminum particles reflected by the discontinuous of the coating, which made the layer show a grid structure. As reaction time went on, the copper plating with grid features kept on absorbing new copper particles from the solution to fill in the vacancy of the structure, and the copper nodules grows up in a two-dimensional way to form the initial layer with less vacancy as shown in Fig.2e.

Fig.2. Characteristics of composite powder with plating time (a) Osecs, (b) 5secs, (c) 15secs, (d) 30secs, (e) 5mins, (f) 30min

Following the two-dimensional growing mode, copper coating grew up in an epitaxial mode. The most area of the surface was covered by copper, and the subsequent copper made the coating dense and uniform. Copper could be reduced by the successive reaction as shown in Eq.(3), and the bare aluminum area reduced gradually as the increasing coverage of copper coating. A dense and uniform layer(Fig.2f) was finally synthesized when the reaction process finished. 3.4 Modeling the coating process

According to Kamasaki's researches on electroplating Au layer growing on Fe[8], the initial growth of the layer contained three models, including Volmer-Weber(V-W model), Frank-van

der Merwe(F-M model) and Stranski-Krastanov(S-K model). In electroless plating process, the deposited metal on substrate also has different growth models. Fig.3 shows schematically the process of copper deposition. It mainly consists of four stages: (l)initial reaction stage, (2)grid formation stage, (3) two-dimensional coating spread stage, and (4)fmal heteroepitaxy stage. The deposition of copper is caused by a series of chemistry reactions happen on the aluminum surfaces, and the final morphology of copper is closely related to the surface energy. At the initial stage, the high energy points such as the edge and the lug boss, firstly remove the oxidation film and react with copper ions rapidly. In this stage, the quantity of F"

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controls the rate of removel of the oxidation and liquid solution is the preferred form to ensure the surface has the same opportunities to contact with F". Meanwhile, the distribution of copper nuclei is the key factor for the following steps. The more uniform distribution of the nuclei, the better quality of the layer. The first stage spends about 5 seconds(Fig.3a). The second stage is a grid formation stage, in which the copper particles generated in the following reaction are attracted by the copper nuclei which have been formed in the first stage. They grow up in some directions which trend to decrease the surface energy and form grid shape. Coating grows up in the two-dimensional spread model in the third stage. In this stage, the surface of aluminum gradually shows the same energy in the solution, and the copper produced by displacement reaction(Eq.(3)) covers the whole aluminum surface(Fig.3c). It is known that the migration of metal ions from solution to the substrate surface is controlled by the concentration gradient of ions and the convection caused by stirring, and the surfaces between covered and uncovered ones

Fig. 3. The schematic of copper coating deposition (a) Initial reaction stage, (b) Grid formation stage,

(c) Two-dimensional spread stage, (d) Heteroepitaxy stage have height difference. The copper concentrations in different places are not the same, so the stirring speed should be noticed in this stage. The final heteroepitaxy growth stage is that the layer grows up in the thickness direction. The copper combines each other with metallic bond and finally forms the smooth copper coating.

4 Conclusions Copper-coated aluminum powders with uniform and dense

layer were synthesized successfully by using displacement method. With the optimal mass ratio MNaF/Ai as 0.1, the copper coating was continual without free copper fines. Copper particle firstly deposited at the cleaned points on the aluminum surface, and then acted as the active sites for the further deposition of

copper. Copper coating grew up in the direction of reducing of the surface energy, which finally formed grid structure in the second stages. Continuously, copper particles attracted each other in a two-dimensional spread model, which resulted in the increasing coverage area on aluminum. Finally, the whole coating grew up in the thickness direction in the heteroepitaxy growth stage. As a result of the four growth stages, uniform copper coating formed on aluminum with 1-2 urn layer thickness.

5 Acknowledgments The research is supported by the special funds of Chaoyang

District and MOTOROLA (No.Q5009012200902), the platform of science and technology innovation (No.0090005466009), and talent teaching plan (No.009000543111506).

References 1. Xinrui Xu. et al, "Electroless silver coating on fine copper

powder and its effects on oxidation resistance" Materials Letters No.57 (2003), pp.3987- 3991

2. Li Chuan-you. et al, "Fabricating Ag/Al composite powder by electroless silver plating", Gongneng Cailiao/Journal of Functional Materials Vol. 41, No.9(2010),pp. 1525-1528.

3. Takeshima. et al, "Electroplating Of Fine Particles With Metal" US Patent 4954235

4. Djokic. et al, Process For The Production Of Silver Coated Particles US Patent 5945158

5. H.T.Hai. et al, "Developing Process For Coating Copper Particles With Silver By Electroless Plating Method" Surface & Coatings Technology 201 (2006), pp.3788-3792.

6. Zhang Zhen-hua. et al, "Study on technics of silver coated aluminum powder" Electroplating & Finishing, Vol. 26, No. 1(2007), pp 23-25

7. Cheng Zhipeng et al. Preparation of Core-shell Cu/Al Powders by Displacement Method. Acta Chimica Sinica, 2007,65(1):81

8. Tohru Watanabe. Nano-Plating : Microstructure Control Theory Of Plated Film And Data Base Of Plated Film Microstructure,Elservier Pte Ltd.(Singapore 2006), pp74-75.

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