Abstract: New thermal management solutions are required to provide cost-effective means of dissipating heat from next generation microelectronic devices. In this paper, fabrication of heat sink nanocomposite made of copper reinforced by multiwalled carbon nanotubes up to 10 Vol. % using metal injection molding technique is presented. A mixture of Cu-MWNTs was compounded using internal mixer machine for homogenous dispersion of the solid powder in the binder. To ensure a flow able feedstock during injection molding process, different binder systems and their Cu-MWNTs feedstocks were examined using capillary rheometer machine. In order 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, and then the samples were isothermally sintered at different sintering temperatures (900-1050°C) in argon atmosphere. The results showed that the sintered samples experienced an isotropic shrinkage around 17% with relatively homogeneous dispersion of MWNTs in the copper matrix which is expected to enhance the physical and mechanical properties of the final product.
INTRODUCTION
Improvement of thermal management in electronic devices is the most relevant problem to be urgently solved for current and future electronic industry, specifically for space applications where micro devices sizes are being decreased and thus power densities increased (Carubelli and Khatir, 2003). In order to overcome this problem, a unique nanocomposite material having high thermal conductivity, light weight and low coefficient of thermal expansion is needed (Cho et al., 2010).
Carbon nanotube (CNT) is one of the most promising nano-material in the field of advanced composites due to its unique mechanical, electrical and thermal properties among others. These outstanding properties are expected to contribute strongly in improving the thermal conductivity of the traditional heat sink materials, such as copper (Gao et al., 2004).
Multiwalled carbon nanotubes (MWNTs) are not only cheaper and easier to be produced, but also generate surprising thermal properties compared to other nano-materials. The average thermal conductivity of MWNTs is around 3000 W m-1 K-1, which is approximately 7 times of that of the traditional copper (Cho et al., 2010).
Therefore, adding small amount of MWNTs is expected to enhance the thermal conductivity of copper remarkably.
On other hand, fabrication of carbon nanotubes reinforced metal matrix nanocomposites is facing series of challenges. Where, MWNTs might expose to high temperature and stress or contact with the molten metal matrix. This might lead to chemical reaction, which could lead to loss of MWNTs through carbide formation. The applied stress during processing might also damage the MWNTs or may align them in the matrix in undesired way (Agarwal et al., 2011).
Metal Injection Molding (MIM) technique is the most suitable method for fabrication small and high detailed metallic parts with high volume production. Big amount of products with superb mechanical properties and high precision in shape can be obtained by using this recently advanced technique, under cost much lower than using traditional methods (German and Bose, 1997; German, 2003). Due to their widespread use in various applications such as producing automobile parts, office machinery and medical and dental instruments, MIM technique has been restricted to iron, steel and their alloy powders.
Unfortunately, only few works have been done on improving the thermal conductivity Cu-CNTs. nanocomposites using traditional techniques (Cho et al., 2010; Ang et al., 2000). Most of the researchers have focused on powder metallurgy and some other techniques and they required ball milling process to disperse CNTs in Cu powder which usually lead CNTs to be agglomerated due to the Van der Waals forces (Chung et al., 2010).
Therefore, to ensure a uniform dispersion of CNTs in Cu matrix with high details product having minor defects, metal injection molding of MWNTs reinforced Cu matrix nanocomposites is presented.
MATERIALS AND PROCESSES
In this study, we have used pure copper powder (99.95% purity) having a spherical shape, produced by gas atomization and supplied by Sandvik Osprey LTD, UK. The copper particle size distribution was = 22 μm. Particle size analysis was performed using CILAS 1190 DRY and results are shown in the Table 1.
Binder systems preparation: In order to select a suitable binder system to achieve optimum volume loading of copper and MWNTs with flow able feedstock, three binder systems were formulated and compounded using twin screw internal mixer machine. The main ingredients of the binder system are: paraffin wax (PW), which is the core binder part, was supplied by Merck, high density polyethylene (HDPE) was supplied by Titan Pet chem.
| Table 1: | Copper particle size analyses |
| MWNTs used in this research were supplied by Shenzhen Nano-Technologies Port Co., Ltd., China | |
Sdn Bhd, Johor, Malaysia as a minor binder component and Stearic Acid (SA) which has been supplied by ACROS organics, as a surface lubricant agent (Ahmad et al., 2011). The physical properties of the binder components provided by the supplier and binder formulations prepared for viscosity measurements are shown in Table 2. Binder formulations were termed as BS1, BS2 and BS3 and were compounded using internal mixer machine. The barrel temperature was sustained at 140-150 and 160°C. The formulation was chopped using a chopper machine to get granule size of 3~4 mm for viscosity inspection of the binder systems. Since mold filling depends on viscous flow into the die cavity, all binder formulations were tested using rheometer capillary machine to measure their rheological characteristics (flow rate, viscosity, shear rate and shear stress) (Moballegh et al., 2005; Raza et al., 2011).
Viscosity measurement of Cu-MWNTs feedstock: The viscosity-shear rate relationship measured for volume loading was maintained to 59 vol.%. To study the effect of increasing contents of CNTs on viscosity, volume contents of CNTs were varied from 2.5 to 10% and the measured values were compared. This test has been described with details in reference (Ali Samer et al., 2011).
Thermal gravity analysis (TGA) of Cu-MWNTs feedstock: The thermal degradation properties were determined by Perkin Elmer, thermal gravimetrical analyzer, under nitrogen atmosphere and heating rate of 3°C min-1.
| Table 2: | Physical properties and volume fraction of binder components used in this study |
| Fig. 1: | TGA of Cu-MWNTs feedstock at 500 |
| Fig. 2: | Processes of MIM technique, 1: Solid powder, 2: Binder system, 3: Internal mixer, 4: Injection molding machine, 5: Moulded samples, 6: Solvent debinding, 7: Thermal debinding and sintering, 8: Final product |
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 (Goudah et al., 2010).
Figure 1 shows the TGA curve 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 170°C.
Feedstock preparation (Cu-MWNTs-Binder): The formulations of copper powder with binder system, BS3 were prepared using co-rotating twin screw mixer machine. Binder BS3 was selected due to its lower viscosity and with an objective to achieve higher solid loadings. The viscosity of copper mixtures was measured using same arrangement of capillary rheometer and data was collected for four mixtures to study the effects of increasing copper contents on the viscosity. Based on the data, one copper mix was identified for inclusion of MWNTs and the selection was based on the processability of the mixtures (Ali Samer et al., 2011). The viscosity of Cu-MWNTs mixtures and effects of increasing MWNTs in copper were also measured at 160°C. Figure 2 shows a schematic of the individual steps involved in MIM forming.
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.
Solvent debinding process: Solvent debinding was carried out using heptane as a chemical solvent agent at 60°C for 5 h. It is considered that after removing some percent of the binder, interconnected capillary pores inside the solvent debinded samples will be created. This capillary pores will assist in leaving of gaseous products in subsequent thermal debinding in a short period of time. Since approximately 70% of binder system was removed in solvent debinding step, subsequent thermal debinding can be finalized with higher speed in comparison with usual thermal debinding process.
Thermal debinding and sintering process: In this stage the thermal debinding and sintering process were carried out at the same time. Different heating rates, dwell time and inner gases were performed to optimize the sintering process parameters. The following procedures were used for both thermal debinding and sintering process: From room temperature to 450°C, the heating rate was 1°C min-1. To thermally remove the binder, the samples were held at that temperature for 1 h. From 450°C to the sintering temperature, the heating rate was performed to be 3°C min-1 and then the samples were held at 900°C for 1 h and cooled down slowly to the room temperature.
RESULTS AND DISCUSSION
Raw materials characterization Fig. 3a show the SEM micrograph of copper powder. It can be seen clearly that the copper particles are in spherical in shape, with wide particle size distribution which makes it proper to attain higher volume loading during densification process of the sintered parts.
| Fig. 3(a-b): | FESEM of copper particles, FESEM of MWNTs |
| Fig. 4: | Rheological behavior of Cu-CNTs feedstock |
Figure 3b demonstrates the FESEM micrograph of MWNTs which showed a dimension of: 10-20 nm in diameter and length of 5-15 μm. The purity of MWNTs was 95-98% with ash content of = 0.2 wt. %.
Feedstock characterization
Rheological behavior of Cu-MWNTs feedstock: As demonstrated in Fig.
4, the viscosity of CuMWNT-1 mixture at a shear rate 230 sec-1
was measured to be 900 Pa.s, which is 28% higher than that of without MWNTs.
This high increasing in viscosity was mostly due to the huge surface area of
MWNTs and interstitial type of packing within the powder particles which has
a tendency to increase the viscosity of the mixture. An increase on MWNTs contents
to 5 vol.%, mixture CuMWNT-2, the viscosity was increased to 1090 Pa.s which
is approximately 16% higher than CuMWNT-1, Composite mixture, CuMWNT-3 with
7.5 vol.% MWNTs showed only 8% higher than the earlier mixture and this may
be due the alignment of MWNTs in capillary die. The viscosity measured for composite
mixture, CuMWNT-4 containing 10 vol. % MWNTs, was 1360 Pa.s and showed 14% high
viscosity. Stimulatingly, mixtures with viscosity higher than 1000 Pa.s have
been injection molded successfully except one where the injection pressure was
increased and molded samples produced were free from defects (Ali
Samer et al., 2011).
Microstructural analysis of Cu-MWNTs feedstock: Figure 5a and b show the FESEM micrograph of Cu-MWNTs feedstock with two different magnifications (μm 20-1 μm).
| Fig. 5(a-b): | FESEM micrograph of Cu-MWNTs-Binder feedstock, MWNTs dispersion in Cu-MWNTs-Binder feedstock |
| Fig. 6: | Cu-MWNTs part in different MIM stages |
It can be clearly seen that the polymer binder was properly binding the copper particle and filling the voids between copper powder and MWNTs with relatively homogenous dispersion of MWNTs in the feedstock.
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 Fig. 6.
Microstructural analysis of sintered Cu-MWNTs composites: The scanning electron micrograph of Fig. 7a 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 in grain boundaries.
Multiwalled carbon nanotubes cannot be seen in the SEM images with 1000x magnification since the particles size about 5-20 μm length and 1-1.2 nm in diameter. However, the porosity is visible on the microstructure surface.
In Fig. 7b, the micrograph was taken by FESEM to fractured surface of low temperature sintered Cu-MWNTs sample. It can be seen from the figure below that CNTs are uniformly dispersed in the copper matrix and there is only few agglomerations of CNTs was noted.
| Fig. 7(a-c): | (a) SEM of sintered sample surface, (b) FESEM of Cu-MWNTs Fractured surface and (c) FESEM of the fractured surface of sintered Cu-MWNTs with 10.00KX magnification |
On the other hand, Fig. 7c shows the same micrograph with higher magnification. It also shows a uniform dispersion of MWNTs in the copper matrix. It also shows that MWNTs are making a better bonding between copper particles that expected to be an affected factor on improving the physical and mechanical properties of the final product.
CONCLUSION
In summary, for the first time, fabrication of carbon nanotubes reinforced Cu matrix composites by means of metal injection molding technique (MIM) has been successfully achieved. 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 of 60 MPa 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 free defects samples. The results displayed that the sintered samples experience isotropic shrinkage around 17%, while the scanning electronic microscopy images showed that MWNTs has dispersed uniformly into the copper matrix composites which expected to be an affected factor in improving the physical and mechanical properties of the final product.
ACKNOWLEDGMENT
Authors would like to acknowledge Universiti Teknologi PETRONAS (UTP) for supporting this work.