Materials Science Forum Vol. 765 (2013) pp 353-357 (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/msf.765.353 Manufacture of CNTs-Al Powder Precursors for Casting of CNTs-Al Matrix Composites Sehyun Ko 1,a, Boyoung Kim 1,b, Yongin Kim 1,c, Taehyeong Kim 1,d, Kitae Kim 1,e, Brian McKay 2,f and Jesik Shin 1,g 1 Korea Institute of Industrial Technology, Incheon, South Korea 2 BCAST, Brunel University, Uxbridge, Middlesex, UK a shko@kitech.re.kr, b kby0827@kitech.re.kr, c kimyi@kitech.re.kr, d kkurie012@kitech.re.kr, e kitae@kitech.re.kr, f brian.mckay@brunel.ac.uk, g jsshin@kitech.re.kr Keywords: CNTs-Al powder precursor, Thermal conductivity, Powder processing, Electroless plating Abstract. CNTs-Al matrix composites are considered to be promising heat dissipating materials because thermal conductivity can potentially be improved whilst their density is reduced. Although casting has many advantages in the fabrication of large, complex components, this process cannot be easily employed when manufacturing CNTs-Al composites. In order to produce CNTs-Al matrix composites by casting a CNTs-Al powder precursor was manufactured using mechanical milling and electroless plating processes. Aluminium powder with CNTs of 10 wt.% and 20 wt.% were mixed and ball milled using a horizontal mill. After milling for 3 hrs., the milled powder exhibits a flattened morphology with a band-type distribution of CNT clusters observed within the aluminium particles. Prolonged milling of up to 24 hrs. introduces an equiaxed particle shape for the milled powders with a uniform distribution of CNTs within the aluminium particles. However, as milling time increases, the CNTs become fractured by ball-to-ball collisions. There was no reaction evident between the aluminium and the CNTs at milling times up to 24 hrs. In order to improve the wettability between the CNTs-Al powder precursor and Al melt during the casting process, electroless Ni plating was performed. The processing time for the Ni-plating affects the uniformity of the coating layer. For a uniform coating condition, the average thickness of the coating was ~1.87 µm, with no evidence of gaps between the milled powders and coatings observed. Introduction Carbon nanotubes (CNTs) are good candidates for use as reinforcement material in metal, polymer and ceramic matrices due to its exceptional thermal, electrical and mechanical properties [1]. Depending on their structure, morphology and orientation, CNTs show approximately 10 times the thermal conductivity of copper and close to 100 times the tensile strength of high strength steel [2]. Among many candidate matrix materials for heat dissipating applications, aluminium is preferential, due to its good thermal coefficient, relative low density and low material cost. Therefore, CNTs-Al composites are a promising material for thermal management industries. A number of methods using liquid-state or solid-state techniques have been developed in order to attempt to disperse the CNTs within the metal matrix [1]. Casting is a suitable process to manufacture relatively low cost, complex and net shape components. The squeeze casting process, which involves pouring molten metal containing CNTs into a pre-heated die and subsequent solidification of the melt under pressure, is often used to fabricate composites [1]. However, it is known that casting processes have difficulty in achieving a homogeneous distribution of CNTs within the aluminium matrix without interfacial reactions [1,3]. The other problem in the casting process is a poor wettability between aluminium and CNTs, which can deteriorate the thermal and the mechanical properties of the composites. In order to overcome the poor wettability, CNTs are surface-coated with SiC [4], Al [5], Cu [6], Ni [7] before the melting process. This surface coating process can be a drawback in their practical application due to a long processing time and a high cost caused by the large surface area of CNTs. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 134.83.1.243, BCAST, Brunel University,, Uxbridge UB8 3PH, United Kingdom-03/06/13,16:04:55)
354 Light Metals Technology 2013 Since solid-state techniques, such as powder metallurgy methods, are performed at relatively low processing temperatures, unexpected reactions between the matrix and reinforcement can be avoided. Mechanical milling, especially, is well known to be an effective method to make a uniform distribution of reinforcement within the metal matrix [8]. However, mechanical milling and sintering processes are not suitable for the production of large and complicated components. In this study, CNTs-Al powder precursors were produced by mechanical milling, to create a uniform distribution of CNTs within the Al powder matrix, followed by electroless Ni plating on the surface of the milled powder precursors that will improve the wettability of the particles in the Al melt during casting. Experimental Procedure The compositions of CNTs-Al powder precursors were formulated to contain 10 and 20 wt.% of CNTs. As starting materials, MWCNTs (Applied Carbon Nano Technology Co., Ltd), 1~10 µm in length and 99.9 wt.% atomized aluminium powder (Kojundo Chemical Laboratory Co., Ltd) of 30 µm diameter, were used in this study. Since the CNTs provided were agglomerated in clusters several micrometers in size, a given composition of CNTs and Al powder was mixed in 200 ml of acetone using an ultrasonic cleaner. The mixed powders with the acetone were dried in a vacuum oven at 2.7 Pa and 50 C until the acetone was completely evaporated. Mechanical milling was performed at room temperature using a horizontal mill with a cylindrical stainless steel container and stainless steel balls, 6.35 mm and 9.53 mm in diameter. To avoid contamination, the inner surface of the container and balls were coated with aluminium. Powders together with balls were sealed into a container in a glove box under a high purity Ar gas atmosphere in order to minimize the oxygen contamination. A process control agent of 0.4 ml of methanol was added to prevent excessive cold welding of the powders. The volume fraction of the ball charge in the vial was 0.6. In total, 25 g of powder was charged and the weight ratio of ball-to-powder was 200:1. Milling was conducted from 3 hrs. to 24 hrs. at a speed of 87 rpm. Milled powder precursors were Ni-coated using electroless plating. The conditions for electroless Ni plating are summarized in Table 1. The solution for each procedure was provided by MSC Co., Ltd. Characterization of the milled and electroless Ni plated powders was conducted using scanning electron microscope-energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD). Table 1. Conditions of electroless Ni plating procedure. Procedure Solution Processing time Temperature Degreasing Zincating Ni-plating HNO 3 750 ml 5 min 50 C 10 s 25 C 6 min 12 min 32 C Drying Dried in Vacuum oven for 3 hrs. 50 C Results and Discussion The morphology of the Al-10%CNT milled powders, shown in Fig. 1, varied according to milling time. After 3 hrs. milling, the powders became flattened due to the ball-to-ball collisions and the size of powders was about 100 to 300 µm. With further milling to 24 hrs., the size of the powders decreased to ~10-30 µm and the morphology of the powders changed to equiaxed. The mechanical milling process is characterized by the repeated welding and fracture of powder particles. If the
Materials Science Forum Vol. 765 355 powder component is ductile material, the milling process can be conveniently divided into five stages, that is, flattening by microforging (1st stage), extensive cold welding (2nd stage), formation of equiaxed particles with decrease in particle size (3rd stage), random welding orientation of particles (4th stage) and a homogenization stage in terms of saturation of hardness and size of powders (final stage) [8]. From the appearance of the size and morphology, the powder milled for 3 hrs. corresponds to the first or the second stage. The third stage was achieved after milling for 24 hrs.. Since the purpose of this study is the uniform distribution of CNTs within the aluminium powders, the second or third stage of milling is recommended. Prolonged milling time can induce the fracture of CNTs and mechanical alloying between aluminium and CNTs. Fig. 1. SEM morphology of Al-10wt.%CNTs milled for (a) 3hrs. and (b) 24hrs. In order to investigate the distribution of CNTs within the aluminium powders, the milled powders were mounted and polished, and the subsequent microstructure examined. Fig. 2 shows the microstructure of the Al-10%CNTs powders milled for 3 hrs. and 24 hrs.. Although several bands of CNTs clusters appeared in the powder milled for 3 hrs., the powder milled for 24 hrs. showed a uniform distribution of CNTs within the aluminium matrix. Fig. 2. Cross-sectional SEM images of Al-10wt.%CNTs milled for (a) 3h and (b) 24h. Fig. 3 shows the XRD patterns from the mixed and milled Al-10wt.%CNTs powders. Although an Al peak shift was not observed, the CNT peak disappeared after milling for 24 hrs.. In addition, peak broadening increased with an increase in milling time. The disappearance of the CNT peak can occur as a result of the CNTs alloying with aluminium or their fracture into very small particles. If CNTs reacted with aluminium atoms, the peak of a new phase, such as Al 4 C 3, will appear. Furthermore, the alloying of aluminium and CNTs may result in an peak shift of the aluminium. It can therefore be deduced, that the CNT peak disappeared due to the fracture of CNTs into smaller particles, which are beyond the detection limits of the diffractometer. The peak broadening which is evident may be due to severe plastic deformation of the aluminium powder during milling.
356 Light Metals Technology 2013 Fig. 3. XRD results of Al-10wt.%CNTs of (a) mixed, (b) milled for 3 hrs. and (c) milled for 24 hrs.. The fracture of CNTs during milling has been reported in earlier research [1,3,9]. In these studies, milling parameters, such as milling time and intensity, affected the fracture as well as the dispersion of the CNTs. The contents of CNTs in this work are higher than in other studies. Table 2 shows the volume fraction of CNTs calculated by real density and apparent density in this study. The apparent densities of aluminium and CNTs were measured according to ASTM B 212. During the milling, each powder will occupy the space of the container according to the volume fraction calculated by the apparent density. A higher volume fraction of CNTs means that the CNTs have a higher probability of interacting in a ball-to-ball collision. During the milling, once the CNTs are embedded within the aluminium particles, the possibility of fracture will decrease. Therefore, the fracture of CNTs may dominate in the initial stages of milling. As mentioned above, however, a prolonged milling time induced the severe fracture of CNTs in this study. Table 2. CNT volume fractions calculated by real density and apparent density. Compositions Vol.% of CNTs in real density Vol.% of CNTs in apparent density Al-10wt.%CNTs 19 vol.% 49 vol.% Al-20wt.%CNTs 34 vol.% 68 vol.% The milled powders were Ni-coated using electroless plating. Degreasing, zincating, Ni-plating and drying processes were conducted according to the conditions given in Table 1. As shown in Fig. 4, the processing time for Ni-plating affected the uniformity of the coating layer. While an island-type coating layer was obtained after 6 min of Ni-plating time, a process time of 12 min achieved a uniform Ni-coated layer on the surface of milled powder. There were no gaps between the milled powder and Ni-coating layer indicating that the coating layer was well-deposited on the surface of the milled powders. The average thickness of the coating layer after a process time of 12 min was ~1.87 µm.
Materials Science Forum Vol. 765 357 Fig. 4. Cross-sectional SEM images of Ni-coated powders: Ni-plating processed for (a) 6 min and (b) 12 min. Summary CNTs-Al matrix composites can be made by adding a CNTs-Al powder precursor into an Al melt during the casting process. In this study, CNTs-Al powder precursors were manufactured with an aim to make large, complex components from CNTs-Al matrix composites. Mechanical milling process was an effective process in order to disperse CNTs within the aluminium particles. It was found that a uniform distribution of CNTs strongly depends on the milling time. As milling time increases, CNTs were fractured by ball-to-ball collisions. In order to improve wettability, the surface of the milled powder was Ni-coated using an electroless plating method. A uniform Ni layer coating was achieved with short processing times. Acknowledgement This research was supported by a grant from Industrial Strategic Technology Development Program funded by the Ministry of Knowledge Economy, Republic of Korea. References [1] H.J. Choi, J.H. Shin, D.H. Bae, Composites: Part A 43 (2012) 1061-1072. [2] C.F. Deng, D.Z. Wang, X.X. Zhang, A.B. Li, Mat. Sci. and Eng. A 444 (2007) 138-145. [3] L. Wang, H. Choi, J.M. Myoung, W. Lee, Carbon 47 (2009) 3427-3433. [4] K.P. So, J.C. Jeoun, J.G. Park, H.K. Park, Y.H. Choi, D.H. Noh, D.H. Keum, H.Y. Jeong, C. Biswas, C.H. Hong, Y.H. Lee, Composites Sci. Tech. 74 (2013) 6-13. [5] K.P. So, I.H. Lee, D.L. Duong, T.H. Kim, S.C. Lim, K.H. An, Y.H. Lee, Acta Mater. 59 (2011) 3313-3320. [6] P. Chen, X. Wu, J. Lin, K.L. Tan, J. Phys. Chem. 103 (1999) 4559-4561. [7] G.S. Cho, J.K. Lim, K.H. Choe, W. Lee, Mater. Sci. Forum 658 (2010) 360-363. [8] B.S. Murty, S. Ranganathan, Int. Mater. Rev. 43 (1998) 101-141. [9] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493-497.