Materials Letters 173 (2016) 211–213
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ﬁber composites prepared by a simple electroless plating technique Huiyu Chen, Guilin Liu, Chunju Xu n, Xin Hou, Yaqing Liu School of Materials Science and Engineering, North University of China, Taiyuan 030051, China
art ic l e i nf o
a b s t r a c t
Article history: Received 20 February 2016 Accepted 11 March 2016 Available online 12 March 2016
The synthesis of copper-coated carbon ﬁbers (CFs) by a simple electroless deposition method in aqueous solution was conducted in this work, and hydrazine hydrate was employed as reducing reagent. The composites were characterized by X-ray diffraction and scanning electron microscopy techniques. The experiment results showed that the reaction temperature and dosage of sodium hydroxide played important roles in the deposition of copper layers. The CFs covered by a compact and continuous copper coating possessed an excellent conductivity with volume resistivity of 6.63 10 3 Ω cm. Such [email protected]
ﬁbers can be used as shielding materials, ﬁber-matrix composites, and so on. & 2016 Elsevier B.V. All rights reserved.
Keywords: Carbon materials Composite materials Electrical properties Thin ﬁlms
1. Introduction Carbon ﬁbers (CFs) are well known for their unique properties including high speciﬁc strength, speciﬁc modulus, high thermal and electric conductivity, so they were widely used as reinforcements to fabricate composites with excellent performance [1–3]. Cu/CFs composites possess superior mechanical, thermal and electrical properties, and are promising functional materials used as electrical packing, electrical devices, heat exchangers, etc. [4,5]. However, neither does liquid copper wet carbon nor does chemical reaction occur at the interface between copper and CFs. It was very difﬁcult to fabricate [email protected]
composites with excellent interface bonding, and thus their further application was greatly limited . It is anticipated that the above problem of wettability between CFs and copper during fabrication could be overcome by coating the CFs with copper before the consolidation process. Several copper coating techniques including sputtering, electrodeposition, and electroless plating have been developed so far. Among them, the electroless plating method was regarded as a promising one because it was low cost, simple handle, and environmental friendly. Typically, Kim et al. fabricated [email protected]
composite powders by electroless plating, and found that the copper particles were aggregated during the thermal exposure in vacuum and hydrogen in order to reduce surface energy . Xu et al. studied the wear behavior of [email protected]
CFs with low load, and they found that the addition of CFs improved the wear resistance obviously . Silvain et al. prepared [email protected]
composites by conventional n
Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.matlet.2016.03.055 0167-577X/& 2016 Elsevier B.V. All rights reserved.
powder metallurgy (PM) without ﬁber coating, electroless coating of ﬁbers with PM, and salt decomposition method with PM, and found that the salt decomposition route is capable of achieving the desired high thermal conductivity values . Herein, we prepared Cu-coated CFs composites via an electroless plating approach. The quality and conductivity of copper coatings were found to be closely related to the bath temperature and dosage of sodium hydroxide. The results demonstrated that the sample coated with a condensed copper layer exhibited a conductivity of 6.63 10 3 Ω cm.
2. Experimental procedure 2.1. Materials and method All reagents were in analytical grade and used without further puriﬁcation. The raw CFs need pretreatment including burning, etching, sensitizing, and activating procedures. In a typical synthesis, raw CFs were ﬁrstly burned in a Mufﬂe furnace at 420 °C for 30 min. Secondly, 2 g of the CFs were etched in100 mL of aqueous solution containing 15 mL of nitric acid (HNO3, 65– 68 wt%), stirred at 90 °C for 30 min. Subsequently, the sensitizing treatment was carried out in 100 mL of solution containing 2 g of tin(II) chloride (SnCl2) and 2 mL of hydrochloric acid (HCl, 37 wt%) for 15 min. Then the sensitized CFs were transferred to 100 mL of solution composed of 0.01 g palladium(II) chloride (PdCl2) and 0.1 mL of HCl (37 wt%) for activation, stirred for 15 min, rinsed with distilled water and collected. Finally, the pretreated CFs were dispersed in a copper ELD bath at 80 °C. The copper ELD bath was
H. Chen et al. / Materials Letters 173 (2016) 211–213
a mixture of solution containing 2.0 g of CuSO4 5H2O, 5.0 g of sodium citrate (C6H5Na3O7 2H2O), and 0.3 g of NaOH in 90 mL of deionized water. When the bath temperature was increased to 80 °C, a dilute reducing solution, composed of 4 mL of N2H4 H2O (80 wt%) and 6 of mL distilled water, was added dropwise. The mixture was mechanically stirred for 40 min. Then the product was collected, washed, and dried for characterization. Controlled experiments were conducted by changing reaction temperature and the dosage of NaOH, respectively, while kept other synthetic parameters constant. 2.2. Characterizations The surface morphology and crystalline structure of samples were investigated by a Hitachi SU-1500 scanning electron microscope (SEM) and X-ray diffractometer (XRD) using Cu Kα radiation (λ ¼1.5406 Å). The electrical resistivity of the Cu-coated CFs was determined by a SB120 four-point-probe instrument. Fig. 2. SEM images of the samples obtained at (a) 50, (b) 60, (c) 70, and (d) 90 °C.
3. Results and discussion Fig. 1a showed the XRD pattern of the obtained composites. The four diffraction peaks at 43.3°, 50.4°, 74.1°, and 89.9° correspond to the diffraction of (111), (200), (220), and (311) planes of FCC Cu metal, respectively, and were in good agreement with the standard data (JCPDS no. 4-836). The XRD result clearly indicated that the Cu layers were successfully deposited on the surface of CFs by electroless plating process. SEM image of the ﬁbers covered with copper was shown in Fig. 1b, which evidently revealed to a relatively homogeneous distribution of ﬁne copper ﬁlm on CFs surface. No scattered copper particles could be observed to be separated from the Cu-coated CFs, implying that the sample possessed a strong adherence between the copper layers and CFs. Temperature was one of the critical factors that affect the reduction rate during the electroless plating process. In general, with the increase of temperature, the copper coating tended to be more uniform and complete. Low temperature means that less energy was supplied for the reaction, and the reaction would not occur if the temperature was too low. As shown in Fig. 2a, a few copper particles were observed on the surface of CFs when the experiment was conducted at 50 °C. When the temperature was increased to 60 and 70 °C, copper nuclei was easy to form and subsequently formed particles, meanwhile, the reaction was so intense that a large amount of nitrogen gas bubbles as byproduct were released, which contributed to the movement of copper particles, and ﬁnally greatly improve the chance to form a complete coating (Fig. 2b and c). However, if the plating was carried out at 90 °C, not only produced large particles obviously on the CFs
surface, the color of the plating solution became black within 10 min, which indicated that copper coatings were seriously oxidized (Fig. 2d). As important as temperature, the concentration of hydrazine hydrate also inﬂuenced the rate of reduction. If the hydrazine hydrate with a concentration of 80% was added directly into the plating solution, it would lead to an immediate reduction of the Cu2 þ around them, which resulted in large sized copper particles and their inhomogeneous distribution . Hence, dilute reductant was necessary for preparation of the perfect coating. Yet, reducing efﬁciency might be lower with decreasing the hydrazine hydrate concentration. We could compensate through increasing the plating temperature to assure a fast enough reaction rate. A series of experiments were conducted and it was found that at a temperature of 80 °C, the reduction rate was suitable to obtain an excellent distribution of copper particle based coatings. The dosage of sodium hydroxide also had a remarkable effect on the quality of copper coatings. As we could see from Fig. 3a, the copper coating was relatively uniform and compact in the absence of NaOH. With increasing the NaOH content, the copper coating became coarse and incomplete (Fig. 3b and c). Under such condition, the nucleation speed of Cu0 and the subsequent growth process of Cu colloids were so fast that the generated copper particles were not uniform in size and distribution. When the dosage of NaOH was further increased to 0.9 g, as shown in Fig. 3d, only a small portion of ﬁbers possessed the broken copper coating, and fast deposition resulting in poor adhesion between the copper ﬁlm and ﬁber. Thus, most of the copper layer was peeled off from the surface. The reduction reaction of metallic ions was sensitive to the pH value of the solution. Moreover, it may also inﬂuence the
Fig. 1. (a) XRD pattern and (b) SEM images of the product obtained in the typical synthesis.
H. Chen et al. / Materials Letters 173 (2016) 211–213
Fig. 3. SEM images of the products prepared with different dosage of sodium hydroxide: (a) 0, (b) 0.5, (c) 0.7, and (d) 0.9 g.
Fig. 4 showed the plot of volume resistivity of Cu-coated CFs synthesized with different dosage of NaOH during the electroless copper plating at 80 °C. The sample obtained with 0.3 g of NaOH possessed the minimum volume resistivity of 6.63 10 3 Ω cm, due to the perfect copper coatings formed on the surface of CFs. Both higher and lower NaOH content could weaken the conductivity of Cu-coated CFs. The increase of electrical resistivity at high OH content was probably related to the insufﬁcient conductive path. Cu grains aggregated into bigger particles at the expense of smaller nanoparticles if more OH was added, and the conducting Cu clusters were separated by dielectric gaps which weakened electrical transport through the coating . The unevenness in the layers was interpreted by the presence of uncoated CFs areas and some isolated spots. Therefore, the resistivity of Cu-coated CFs began to rise gradually. However, with the further increase of OH , the volume resistivity of Cu-coated CFs appeared a downward trend due to that the large number of ﬁne copper particles scattered around the CFs build a conductive path. The above phenomenon was in good agreement with the SEM observations.
4. Conclusions In conclusion, Cu-coated CFs have been successfully synthesized via an electroless deposition method, which could effectively prevent copper particles from aggregation and oxidization. Both the bath temperature and dosage of NaOH played very important roles in determining copper conversion. Higher temperature would increase the reducing power of hydrazine hydrate, and the increase of solution pH improves the rate of conversion. However, too high temperature tended to oxidation and much alkaline solution would reduce the stability of bath. The CFs coated with a condensed copper ﬁlm exhibited an excellent conductivity. The current approach can be extended to the fabrication of copper coatings on other substrates, and hence it is expected to have various applications in catalysis, sensors, and other ﬁelds. Fig. 4. Plot of volume resistivity of Cu-coated CFs as a function of sodium hydroxide content.
References product's morphology via the formation of certain species . The standard potentials of hydrazine were supplied in formula 1– 2. Hydrazine itself was a strong reducing agent and the reducing power was increased under alkaline condition. The total plating reaction can be expressed as formula 3. Therefore, the reduction rate and the extent of decoration were dependent on the OH concentration of the Cu bath. N2H4-N2 þ 4H þ þ 4e E0 ¼ 0 23 V
N2H4 þ 4OH -N2 E0 ¼ 1 16 V
N2H4 þ 4OH þ2Cu2 þ ¼N2↑ þ4H2Oþ2Cu
        
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