978-1-4577-1884-7/11/$26.00 ©2011 IEEE

Development of Nanocomposites Heat Sink (MWCNTs/Cu) Using Powder Injection Moulding for Electronic Applications

Ali S. Muhsan* *Mechanical Engineering Department,

Universiti Teknologi PETRONAS, Bandar Seri-Skandar, Perak- Darul Rudzuan, Malaysia

E-mail: albatreeq2007@yahoo.com

Faiz Ahmad Mechanical Engineering Department,

Universiti Teknologi PETRONAS, Bandar Seri-Skandar, Perak- Darul Rudzuan, Malaysia

E-mail: faizahmad@petronas.com.my

sink composite material made of copper matrix reinforced by multiwalled carbon nanotubes (MWCNTs) up to 2.5 Vol. % using powder injection molding technique (PIM) has been done successfully. A mixture of Cu/MWCNTs was compounded using a Z-blade mixer for homogenous dispersion of solids in the binder. The flow properties were measured using a capillary rheometer in the shear rate range which expected to occur during metal injection molding. To avoid binder degradation, TGA test was carried out. The TGA results showed that the processing temperature such as mixing and injection molding should be lower than 170°C. The injection molding was carried out at low pressure. A combination of solvent and thermal debinding was used for binder removal from the samples and then the sintering process has taken place in argon gas at 900°C. The results show that the sintered samples experience isotropic shrinkage around 21% with 14% weight loss and the density of the composites was decreased after sintering process.

I. INTRODUCTION Currently, the electronic industries have a serious concern to achieve optimum performance of smart electronic chips which attained high temperature during operation resulting in reduced performance [1]. Improvement of heat management in electronic devices is the most relevant problem to be urgently solved for current and future electronic industry, specifically for space applications where microdevices sizes are being decreased and thus powder densities increases [2]. Therefore, selection of appropriate reinforcement material is a critical problem in enhancing the physical properties of the composites such as thermal conductivity. Carbon nanotubes (CNTs) are one of the most promising nanomaterials in the area of advanced composites due to the remarkable mechanical, electrical and thermal properties among others [3-4]. These outstanding properties are expected to contribute in improving the properties of composites. Multiwalled carbon nanotubes (MWCNTs) are not only cheaper and easier to produce, but also generate surprising mechanical and thermal properties compared with SWCNTs.

The average estimated strain modulus is over 1.8 Tpa. The average thermal conductivity is approximately around 3000 W/m.k. However, addition of MWCNTs is predicted to remarkably enhance the thermal conductivity in thermal management metals such as copper (Cu) because of their extremely high axial thermal conductivity [5-6].

II. LITERATURE SURVEY Metal injection molding (MIM) is expected to be very efficient for manufacturing small and complex metallic components in large batch. Large amount of components with excellent mechanical properties and high geometrical accuracy may be obtained by this newly developed technology, under a cost much lower than traditional techniques [7]. The research on MIM concerns three main stages: injection molding of a binder/powder mixture, thermal or catalytic debinding to get a shaped porous metallic part, and sintering to get the condensed part in pure metallic material [8-9]. The geometrical accuracy and mechanical properties of the final parts after sintering depend strongly on the choice of process parameters in the different stages. The production of complex shape heat sink using MIM requires a flow able feedstock in the molding machine. In principle, injection molding is a relatively simple process. A feedstock is plasticating unit, which is comprised of a heated until the injection temperature, and then a controlled volume of the molten feedstock is injected from the plasticator under pressure into a closed mold, with solidification beginning on the mold’s cavity wall [10]. Although, MIM technique can be used for shaping various metal powders, most of researchers in this field have been limited to iron, steel and their alloy powders due to their widespread use in various applications such as producing automobile parts, office machinery and medical and dental instruments. Unfortunately, only few papers have aimed at improving the thermal conductivity of copper matrix composites reinforced by CNTs [11]. Most of the researchers have focused on improving the mechanical properties by incorporating CNTs into the matrix composite. Where some of them have reported that powder metallurgy technique, comprising of compaction and sintering, helps increasing the hardness up to 20% with 15 vol. % CNT additions [12-17]. Carbon nanotube reinforcement coated with Ni improved bonding with the Cu matrix and resulting in about 80–100% increase in the hardness for 9–12 vol. %CNT addition [18]. Spark plasma sintering of Cu-10 vol. %CNT composite has improved the hardness by 79% with a

Abstract - A fabr icat ion of high performance heat

Keywords ; Carbon nanotubes reinforced copper composites, Powder injection molding, Compounding, Feedstock, Binder, Viscosity, Heat Sink.

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

further improvement up to 207% resulted from rolling of the SPS composite [19]. Daoushb, B.K.L., Chan B. et al. [20] have prepared a multiwalled carbon nanotubes/copper (MWCNTs/Cu) nanocomposite powder with different CNTs volume fraction using electroless copper deposition. Copper was deposited in a form of layer on CNTs surface and then the nanocomposite powder was sintered by spark plasma sintered. They found that the electrical conductivity, hardness and young’s modulus have been increased by increasing the volume fraction of CNTs in the copper matrix. Ping et al. [21] have employed molecular level mixing to produce Cu-CNT composite in order to prevent agglomeration of Cu nano particles by coating them on CNTs as shown in Fig 1. Thus they claim to provide more surface area of Cu nanoparticles to be used for catalytic performance in the thermal decomposition of ammonium perchlorate. Molecular level mixing has shown a capability to improve distribution and interfacial bond strength of CNT with metal matrix. Still, there are only few studies of this technique and the technique needs to be explored for other systems.

Figure 1: Molecular Level Mixing [21]

Some researchers have tried to prepare MM-CNT composites by putting alternate layers of CNT and metal like a sandwich structure and then consolidating by applying severe pressure. Y.-H. Li and W. Housten Li et al. have arranged 20 layers of 10 mm Cu foil with alternate CNT layers of 450 nm thickness and cold rolled the assembly with intermittent annealing steps to form a Cu-CNT composite [6]. They have reported good bonding between CNT and Cu, and improvement of Young’s modulus by 13¡5% by addition of 3?1 vol.-%CNTs. But no image of the cross-section of the composite has been provided to show the bonding between Cu layers. The authors also have not reported about possible damage to the CNTs, due to severe plastic deformation. However, the main objective of the present study was to fabricate a high performance heat sink composite material made of copper matrix reinforced by multiwalled carbon nanotubes up to 2.5 Vol. % using metal injection molding technique.

III. MATERIALS AND PROCESSES The copper powder (99.95% purity) of spherical in shape was used in this study. The copper powder produced by gas atomization and supplied by Sandvik Osprey LTD, UK. Particle size distribution of copper powder was >22 μm.

Particle size analysis was performed using CILAS 1190 DRY and results are shown in the Table-1.

Table 1: Results from Particle size analyzer

Copper powder D10 D50 D90

Size (µm) 3.31 8.96 17.19 SEM micrograph of the copper powder, Figure 1-a shows a spherical shape of particles, and a wide particle size distribution is considered suitable to achieve higher volume loading during PIM and densification in the sintered parts. The multi-walled carbon nano tubes (MWCNTs) used for this research was supplied by Shenzhen Nano-Technologies Port Co., Ltd., China. The dimensions of MWCNTs were: diameter 10-20nm, length 5-15 μm. The purity is 95-98% and ash content of ≤0.2 wt %. An image of MWCNTs was taken from Field Emission Scanning Electron Microscope (FESEM) and is shown in Figure 1-b.

Figure 2-a: SEM image of copper powder shows spherical shape.

Figure 2-b: FESEM image of MWCNTs

A. Preparation of Binder Systems

In order to study the flow behavior and to select a suitable binder system with lower viscosity to achieve optimum volume loading of copper and MWCNTs, three binder systems have been formulated and extrusion compounded.

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

The binder system consists of three ingredients. Paraffin wax is a major binder component supplied by Merck. High density polyethylene (HDPE) was supplied by Titan Pet chem. (M) Sdn Bhd, Johor, Malaysia as a minor binder component and stearic acid was supplied by ACROS organics as a surface active agent or flow promoter. The physical properties of the binder components provided by the supplier are listed in Table 2-a and binder formulations prepared for viscosity measurements are shown in Table 2-b. Binder formulations were named as B1, B2 and B3 and were extrusion compounded using co rotating twin screw extruder. The barrel temperature profile was maintained at 140-150-160°C. A die of 2.5-mm diameter was used to produce extruded binder. The extruded binder was cooled in a water tank and chopped using an automated chopper to granule size of 3~4 mm for measurement of viscosity of binder.

Table 2-a: Physical properties of binder components used in this study

Binder ingredient

Density

(g/cm3)

Melting Temperature

(°C)

Parafine wax 0.90 60-65

HDPE 0.95 130

Stearic Acid 0.96 67-69

Table 2-b: Binder formulations developed for viscosity measurement in this study

Binder ingredient

B1 B2 B3

Parafine wax 55 65 70

HDPE 40 30 25

Stearic Acid 5 5 5

Table 1: Properties of the research materials [22]

Properties

Materials Copper MWCNTs

Thermal Conductivity

401 W/m.K 1200~ 3000 W/m.K

CTE 17.0x 10-6 /ºC (20-100°C)

Theoretical negligible

Density 8.96 g/cm3 1.33~1.4 g/cm3

B. Feedstock preparation of Cu/MWCNTs

Metal injection molding technique is composed of four basic steps consisting of: feedstock preparation, injection molding, binder debinding and sintering process. Figure 3-shows a schematic of the individual steps involved in MIM forming.

The formulations of copper powder with binder system, B3 were prepared using Z-blade mixer. Binder B3 was selected due to its lower viscosity and with an objective to achieve higher solid loadings. The viscosity of copper mixes was measured using same arrangement of capillary rheometer and data was acquired for four mixes to study the effects of increasing copper contents on the viscosity. Based on the data, one copper mix was identified for substitution of MWCNTs and the selection was based on the process ability of the mixes. The viscosity of MWCNTs/copper mixes and effects of increasing MWCNTs in copper were also measured at 160 °C.

Figure 3-a: Flow chart for MWCNTs reinforced Cu-PIM process [24]

After mixing process, the feedstock was injected at 160°C and 4 bars with the mold temperature 30-40 °C and samples with dumbbell and strip were obtained. The injection molding was performed in a short time to match the injected amount with the mold size. No sign of defects within the samples observed after the injection molding process. Figure 3-b shows the FESEM micrograph of Cu/MWCNTs feedstock

Figure 3-b: FESEM micrograph of composite feedstock

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

1- Thermal Gravity Analysis (TGA) The thermal degradation properties were determined by Perkin Elmer, thermal gravimetrical analyzer, under nitrogen atmosphere and heating rate The TGA curves can be used for establishing the upper limit of the melt temperature during injection molding and guidelines for a suitable debinding process [24].

Figure 4 shows the TGA cure of the feedstock, and indicates that the degradation starts at 170°C and ends at 500°C. Thus, to avoid the degradation during mixing or injection molding processes the processing temperature must be lower than that temperature (170°C).

Figure 4: TGA of Cu/2.5Vol.%MWCNTs feedstock at 500°C for 30 minutes with a heating rate of (3°C/ min)

2- Rheology As shown in Figure 5, a capillary Rheometer (Shimodzu flow tester CFT-500D), , using a capillary die of 1-mm diameter and length of 10-mm was used to measure the viscosity of the feedstock [23].

Figure 5: Rheometer Machine

Feedstock were filled into rheometer barrel heated heated to 150°C and different loads were applied using rheometer ram and allowed for 5-10 minutes to attain equilibrium temperature throughout the materials. The materials were extruded through the capillary die and time taken was recorded as shown in Fig.6.

Figure 6: Rheological behavior of Cu/2.5 Vol. % CNTs

feedstock

Volumetric flow of melt was calculated for shear rate and corresponding shear stress was calculated to determine the viscosity of the binder [25].

3- Debinding Process To minimize the possibility of defects with safe and fast binder removal, the multi-component binder chosen includes the lower stability components of paraffin wax and stearic aced, which are removed in early stage of debinding, and generate pore channels [8].

Solvent debinding was carried out by means of Heptane as a solvent at 60°C only for few hours. It is considered that after removing some percent of the binder, there exists some interconnected capillary porosity inside of the samples which makes leaving of gaseous products in subsequent thermal debinding easy in short time. Since nearly most of binder was removed in solvent debinding step, subsequent thermal debinding can be performed with higher speed in comparison with usual thermal debinding process [22].

4- Sintering In this stage the thermal debinding and sintering process were carried out at the same time. To optimize the sintering heating rates and environments, different heating rates and environments were performed [22]. The following procedures were used for both thermal debinding and sintering processes:

1- From room temperature to 450°C, the heating rate was 1°C/min.

2- To thermally debinding the binder, the samples were held on that temperature for 1 hour.

3- From 450°C to the sintering temperature, the heating rate performed to be 3°C/min.

4- Then the samples were held at (900°C) for 1 hour and cooled down slowly to the room temperature.

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

IV. RESULT AND DISCUSSION

A. Characterizations 1- Dimensional Measurements

After the sintered samples were measured and compared in three dimensions to study the shrinkage quality, it found that the sintered samples experience isotropic shrinkage around 21% with 14% weight loss which indicates that the homogeneity of the feedstock was relatively good as shown in Figure 7-a, 7-b and 7-c.

Figure 7: Injected samples before and after sintering process, (a) green sample, (b) tthermal debinded sample and (c) sintered sample

2- Scanning Electron Microscopy

The scanning electron micrograph (SEM) image (Figure 8) shows the surface of the sintered sample. It can be easily noted that there is some defects occur on the surface which contributes to voids contents. Multiwalled carbon nanotubes cannot be seen in the SEM images with 1000x magnification since the particles size about (5-20) μm length and 1 to 1.2 nm in diameter. However, the porosity is visible on the microstructure surface.

Figure 8: SEM of the sintered sample surface

In Figure 9-a, the Micrograph was taken from FESEM of the fractured surface of sintered Cu/MWCNTs sample. It can be seen the figure below that CNTs are uniformly dispersed in the copper matrix and there is only few agglomerations of CNTs was noted.

Figure 9-a: SEM of the fractured surface of sintered Cu/MWCNTs with 300x and 2.00KX Mag.

On the other hand, Figure 9-b shows the same micrograph but with higher magnification. This Figure also shows clearly the diffusion bonding between copper-copper particles as well as it shows that MWCNTs are bonded with the copper particles.

Figure 9-b: SEM of the fractured surface of sintered Cu/MWCNTs with 2.00 and 10.00KX Mag.

V. CONCLUSION In summary, for the first time, fabrication of carbon nanotubes reinforced Cu matrix composites by means of powder injection molding technique (PIM) has been done successfully. The binder system with formulation of 70 % PW, 25 % HDPE and 5 % SA weight percent was used base on the rheological properties. From TGA testes we concluded that the degradation temperature of the binder starts at 170°C, which means that the processing temperature such as mixing and injection molding temperature must be lower in order that binder degradation doesn’t occur. The injection molding was carried out at law pressure and the binder of the molded samples was debinded using solvent and thermal debinding, followed by sintering underperformed heating rates and environment. Also the debinded samples were sintered with slower heating rate in order to get samples with free defects. The electronic microscope images showed uniform distribution of the CNTs into the copper matrix composites. The results also suggested that the sintered samples experience isotropic shrinkage around 21% with 14% weight loss and the density

978-1-4577-1884-7/11/$26.00 ©2011 IEEE

of the composites was decreased after sintering process as well as the composites exhibited volume expansion due to grain growth which can be seen in the microstructure images result from Scanning Electron Microscope.

AKNOWLEDGMENT

I am thankful to University Technology PETRONAS (UTP) for accept me as a PhD candidate. I would also like to thank my supervisor Asst. Prof. Dr. Faiz Ahmad for his guidance and encouragement.

REFERENCES [1] Ryan J. McGlen a, Roshan Jachuck a, Song Lin,

Integrated thermal management techniques for high power electronic devices. Applied Thermal Engineering, 2003.

[2] Zauner, R., Micro powder injection moulding. Microelectronic Engineering, 2006.

[3] CHA S I, K.K.T., ARSHAD S N, MO C B, HONG S H., Extraordinary strengthening effect of carbon nanotubes in metal-matrix nanocomposites processed by molecular-level mixing. 2005.

[4] D. Qian, G.J.W., W.K. Liu, M.F. Yu, R.S. Ruof, Mechanics of carbon nanotubes. 2002.

[5] Soon-Wook Kim1, M.-J.S., *, Young-Do Kim1, and In-Hyung Moon, Micro Metal Powder Injection Molding of W-Cu Nanocomposite Powder. METALS AND MATERIALS, 2005.

[6] Y.-H. Li, W.H., Y. Zhao and Y. Q. Zhu, Multiwalled Carbon nanotubes layers reinforced Cu foil matrix composite. 2007.

[7] Æ, P.C.Y.Æ.Q.F.L.Æ.J.Y.H.F.Æ.T.L. and P.W. Ho, Micro injection molding of micro gear using nano-sized zirconia powder. TECHNICAL PAPER, 2008.

[8] Ahmad, F., Orientation of short fibers in powder injection molded aluminum matrix composites. Journal of Materials Processing Technology, 2005.

[9] Eric Nyberga, Megan Millerb, Kevin Simmonsa, K. Scott Weila, Microstructure and mechanical properties of titanium components fabricated by a new powder injection molding technique. Materials Science and Engineering, 2004.

[10] L. Liua, N.H. Loha, B.Y. Tayb, S.B. Tora, Y. Murakoshic, R. Maeda, Mixing and characterisation of 316L stainless steel feedstock for micro powder injection molding. Materials Characterization, 2004.

[11] POP E, M.D., WANG Q. , Thermal conductance of an individual single-wall carbon nanotube above room temperature. 2006.

[12] Dai, H., Carbon nanotubes: opportunities and challenges. Surface Science, 2001.

[13] KIM K T, C.S.I., HONG S H., Hardness and wear resistance of carbon nanotube reinforced Cu matrix nanocomposites. 2007.

[14] Peigney A, L.C., Dumortier O, Rousset A, Carbon nanotubes-Fe-alumina nanocomposites. Part II: microstructure and mechanical properties of the hot-pressed composites. Journal of the European Ceramic Society, 2005.

[15] Peigney A, L.C., Dumortier O, Rousset A, Carbon nanotubes-Fe-alumina nanocomposites. Part I: influence of the Fe content on the synthesis of

powders, Journal of the European Ceramic Society, 2004.

[16] Sheikh M. Uddin a, e., *, Tanvir Mahmudb, Christoph Wolf b, Carsten Glanz b, Ivica Kolaric b, and H.H.c. Christoph Volkmer c, Ulrich Wienecke c, Siegmar Roth a,d, Hans-Jörg Fecht, Effect of size and shape of metal particles to improve hardness and electricalproperties of carbon nanotube reinforced copper and copper alloy composites. Composites Science and Technology, 2010.

[17] W.X. Chena, J.P.T., *, L.Y. Wangb, H.Y. Gana, Z.D. Xua, X.B. Zhang, T ribological application of carbon nanotubes in a metal-based composite coating and composites. Carbon, 2002.

[18] AN J W, Y.D.H., LIM D S. , Tribological properties of hot-pressed alumina-CNT composite. 2003.

[19] Hansang Kwon, M.E., Kenta Takagi, Takamichi Miyazaki, Akira Kawasaki, Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix composites. CA R B O N, 2008.

[20] Daoushb, B.K.L., Chan B. Moa, Dong H. Nama, Soon H. Honga,, Electrical and mechanical properties of carbon nanotube reinforced copper nanocomposites fabricated by electroless deposition processWalid M. Materials Science and Engineering 2009.

[21] C. Ping, F.L., Z. Jian and J. Wei: Propellants, Explos., Pyrotech.,, producing Cu-CNT composite using molecular level mixing method. 2006.

[22] Ahmad, F., MICROSTRUCTURAL STUDIES OF SINTERED CARBON NANOTUBES REINFORCED COPPER MATRIX COMPOSITE. Journal of Engineering Science and Technology, 2010.

[23] F. Ahmad, O.M., N. Muti, Afian Omer and R.M German, Rheology study of carbon nanotubes reinforced copper feedstock for powder injection molding. international conference on powder metallurgy & particulate materials, june 2010 F.L, USA, 2010.

[24] Ahmad, F., preparation and characterization of copper feedstock for metal injection molding. Journal of applied sciences, 2010.

[25] F. Ahmad, G.G., O Mamat, N. Muti, and R.M German, developement of carbon nanotubes reinforced copper matrix compostes for heat sinking application. 2010.