MECHANISM CONTROLLING THERMAL CONDUCTIVITY AND COEFFICIENT OF COPPER METAL MATRIX COMPOSITES

Transcription

1 Materials Science and Technology (MS&T) 2009 October 25-29, 2009 Pittsburgh, Pennsylvania Copyright 2009 MS&T FUNDAMENTALS AND CHARACTERIZATION: Modeling and Mechanical and Physical Behavior Session Chair(s): Carl Boehlert; Characterization of Metal Matrix Composite Materials MECHANISM CONTROLLING THERMAL CONDUCTIVITY AND COEFFICIENT OF COPPER METAL MATRIX COMPOSITES David Esezobor, Samuel Fatoba Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria Keywords: copper silicon composite, thermo-physical property, electronic Abstract The potential demands for reliable materials in electronic industries are ever increasing. The main pronounced failure that occurs during microelectronic circuits application involves thermal fatigue. This occurs due to the different thermal expansion coefficient of semi conductor chip and packaging material. Thus, the search for appropriate coefficient of thermal expansion (CTE) of packaging materials in combination with a high thermal conductivity is inevitable in the design and selections of it sink material. An attempt as been made in this research work to produce copper matrix composites using silicon carbide (SiC) as reinforcement. This is aimed at getting a material with high thermal conductivity using non convectional liquid metallurgy. Copper silicon carbide composites were produced in 80%Cu 20%SiC, 70%Cu 30%SiC, 60% Cu 40% SiC, 50% Cu 50%SiC, 40%Cu 60%SiC ratios with an average grain size of 212μm, 425μm and 710μm respectively via liquid metallurgy route. The result revealed that increasing volume fraction and increasing particle size of the particulate had significant effect on the thermo-physical properties. This phenomenon is explained using the evolving microstructures. Introduction Composite materials are emerging chiefly in response to unprecedented demands from technology due to rapidly advancing activities in aircrafts, aerospace, automotive industries and electronic devices. These materials have low specific gravity that makes their properties particularly superior in strength and modulus to many as metals. As a result of intensive studies into the fundamental nature of the composite materials [1-8] and better understanding of their structure property relationship [5-10], it has become possible to develop new composite materials with improved physical and mechanical properties. Metal matrix composites (MMCs) are types of material which have great potential because of the unique property combinations that can be achieved. These include: low thermal expansion (CTE), better electrical conductivity and superior thermal conductivity. An important consideration in MMC manufacture is the nature of the interface between the matrix and the 1795

2 reinforcement. This often depends on the processing route and since this occurs at high temperature, it is more chemical than mechanical. This paper investigates the mechanism controlling high thermal conductivity of Cu SiC particulate composite produced by liquid metallurgy route. The effect of particle size, volume fraction and other processing parameters is studied alongside with the development of microstructures suitable for high thermal conductivity in Cu alloy based MMC. Furthermore, the relationship between microstructures, particle sizes, volume fractions and the production process are evaluated for high thermal conductivity and low CTE to be achieved. Materials and Preparation Materials Methodology Commercial pure copper wire C10200 (98.8% Cu) used for experimentation was obtained from Cable Metal Nig. Ltd. The chemical composition as well as the density, resistivity, thermal conductivity and electrical resistivity of the copper wire C10200 are presented in Tables 1 and 2. Table 1: Chemical Composition of Copper Wire C10200 Element % Composition Element % Composition Element % Composition Cu Si Cr Pb 0.49 Mn Sb Zn Fe 0.45 Mg Table 2: Density, Thermal Conductivity and Electrical Resistivity and Thermal Expansion of Copper Wire C10200 Variables Unit Values Density kg/m Thermal Conductivity W/mK 380 Electrical Conductivity S/m 5.18 x 10 7 Resistivity Ωm 1.72 x 10-8 Thermal expansion K The alloys are produced in 80% Cu 20% SiC, 70% Cu 30% SiC, 50%Cu 50%SiC, 40%Cu 60%SiC from pure copper wire (C10200) with variation in the SiC particle grain size in each volume fraction. The grain sizes used were 212, 425 and 710 microns. The materials were synthesized by liquid metallurgy route in an oil fired pit furnace. The copper wires C10200 were properly cleaned prior to melting so as to eliminate any surface impurities and superheated to C. The addition of SiC particulates to the molten copper was done first at the tail end of melting. The mixture was continuously stirred before casting. The casting of the melt was done in a metal mould with outer diameter of 1.45 cm x 14.5 cm. The resulting Cu SiC metal matrix composite was later heat treated in muffle furnace to C for 8 hours. Characterization Thermal conductivity, electrical conductivity, resistivity, rolling and metallographic examinations were carried out on these materials. 1796

3 The resistivity of a material depends on many factors including the way in which it is produced, its heat treatment and so on. The resistivity of annealed copper is often quoted as 1.72 x 10-8 Ωm where as pure copper has a slightly different value. Tests were carried out on the various samples of 1.5cm diameter and gauge length of 15cm. The test was conducted on DV power micro-ohmmeter RM0600 at 10 amperes and 20 amperes. An average of three readings was taken. The value of resistivity of a wire can be evaluated from the expression below: Resistivity, RA ρ = (1) l ρ - Resistivity in ohm-meter (Ω - m - ) R is the resistance of material in ohms A is the cross section area in square meter l is the length in meter. The value of electrical conductivity in per ohms per meter was derived from the relationship K = ρ 1 (2) Tests were conducted using thermal transport sample station (TT0) P670. An average of three readings was taken. The cold rolling tests were carried out on Buhler machine (DW 80φ x 150). An average of three passes was taken and a total of 15 samples were rolled. Metallographical Examination The specimens for optical metallography were prepared using standard techniques. A mixture of 10g of ferric chloride in 80ml of water and 30ml of hydrochloric acid was used as the etchant. The microstructures of the specimens were examined using a digital metallurgical microscope with 100X magnification. Results and Discussion 11. The results of the thermo physical properties are presented in Tables 3 8 and figures 1 - Table 3: Thermal Conductivity of Copper-SiC Matrix in W/mk PARTICLE SIZE 212μm 425μm 710μm 80/ / / / / /

4 Table 4: Thermal Conductivity of Heat Treated Copper-SiC Matrix Composites in W/mk PARTICLE SIZE 212μm 425μm 710μm 80/ / / / / / Table 5: Electrical Conductivity of Copper-SiC Matrix Composites in 10 7 Siemens/meter PARTICLE SIZE 212μm 425μm 710μm 80/ / / / / / Table 6: Electrical Conductivity of Heat Treated Copper-SiC Matrix Composite in 10 7 Siemens/meter PARTICLE SIZE 212μm 425μm 710μm 80/ / / / / / Table 7 Resistivity of Copper-SiC Matrix Composite in μωm PARTICLE SIZES 212μm 425μm 710μm 80/ / / / / / Table 8: Resistivity of Heat Treated Copper-SiC Matrix Composite in μωm PARTICLE SIZES 212μm 425μm 710μm 80/ / / / / /

5 Figure 1: Thermal conductivity of copper matrix composites Figure 2: Thermal conductivity of heat treated copper matrix composites 1799 Figure 3: Resistivity of non-heat treated copper matrix composites

6 Figure 4: Resistivity of heat treated of copper matrix composites Figure 5: Resistivity of non- heat treated of copper matrix composites at 27 degrees Celsius 1800

7 Figure 6: Electrical Conductivity of non- heat treated of copper matrix composites Figure 7: Electrical Conductivity of heat treated of copper matrix composites CONTROL CONTROL HEAT TREATED Figure 8: Microstructures of Control Samples 1801

8 (212μm) (425μm) (710μm) Figure 9: Microstructures of Non-Heat Treated Cu-SiC Particulate Composite with Varied Volume Fraction 1802

9 (212μm) (425μm) (710μm) Figure 10: Microstructures of Heat Treated Cu-SiC Particulate Composite with Varied Volume Fraction 1803

10 (212μm) (425μm) (710μm) Figure 11: Microstructures of Rolled Cu-SiC Particulate Composite with Varied Volume Fraction 1804

11 In the micrographs, fine crystal of the base metal with sparce dispersion of the SiC particles show marked clustering of its grains. Crystals of the base metal in 80/20 CuSiC when the SiC particle grain size is 212µm are very fine. The particles of SiC are seen as dark spot and forming thin unconnected grain boundaries. There are few spots of the SiC clustering. As the grain size of SiC particles further increases (80/20-425µm), it forms distinct grain boundaries around fine crystal of the base metal (see fig. 10). Furthermore, there is an increase in the grain boundaries formed by the particles of SiC (80%/20% - 710µm) in the base metal matrix (see fig.11). There are marked areas of SiC particles clustering. Similarly, grains boundaries are formed with clustering of SiC particles for 425µm SiC grain size in the CuSiC matrix (70%/30%), round the crystal of the base metal. SiC Particles of sizes 710µm in (70%/30%) CuSiC are seen well distributed in the base metal matrix forming thin grains boundaries. Uniform dispersion of second phase particles is observed in composites consisting of large size particles (212µm) of the reinforcement phase. The grain boundary of the primary matrix phase is not clearly discernible, this feature is due to the presence of large volume fraction of the particulate phase (40%/60% - 212µm) (see fig 9) Decrease size of the particulate phase leads to clustering of particles at the grain junction and their continuous network along the grain boundaries (see fig 12). At (50%/50% - 710µm) indicated formation of coarse precipitate phase. The white phase which shows a tendency to nucleate at SiCp surface. However more often they are observed to basically decorate the grain boundaries. The grain size of the primary copper phase varies with fraction and size of the SiCp particulate. Conclusion A low cost of copper-silicon carbide particulate composite was developed using liquid metallurgy. This study has shown that the Electrical Conductivity of the Composite decreases with increase in size of particles as well as volume fraction of SiC particles in the copper matrix. However, when resistivity is the requirement fine particulate of SiC (710µm) will be preferred at high volume fraction of particles. Also with decrease in grains size high thermal conductivity can be achieved by increasing the volume fraction. The strengthening of the C10200 alloy with the particulate SiC shows to an appreciable extent the influence of the volume fraction and particle size on the resistivity, electrical conductivity and thermal conductivity. Thus high volume Cu-SiC composites have the best thermo physical property, which has a high potent for electronic devices applications. This will be found useful in sectors where high thermal conductivity, low resistivity and high electrical conductivity as well as low thermal expansion (CTE) are required such as aerospace industry, defence technology and electronics industry. References [1] P.R. Kleth, J.M. Qemisset and R. Naslain, Composite science Technology, Vol , p.155. [2] Qicing Zhang, Composite Material Technology, vol. 37, 1991, p.191. [3] D.Z. Yang, S.L. Dong and J.F. Mao, High Conductivity Copper for Electrical Engineering, Comp. Sci. Tech.; Vol. 35, 1989, pp

12 [4] J. Zhang, R.J. Perez, M.N. Gungor and E.J. Lavernia, Damping Characteristics of Graphite Particulate Reinforced Copper Composites, Development in Ceramics and Metal - Matrix Composites, Kamleshwar Upadhya, ed; Warrendale, PA: TMS Publication, 1992, pp [5] D. Huda, M.A. El Baradie and M.J.S Hashni, Meta; Matrix Composites: Materials Aspects. Part II Journal of Material Processing Technology, Vol. 37(1993), pp [6] J. Hashim, L. Looney and M.S.J. Hashmi, Particle distribution in cast metal matrix composites Part I. Journal of Materials Processing Technology, Vol 123, No 2, 2002, pp [7] H.J. Rack, Metal Matrix Composites, Advance Material Processes, Vol. 137 No 1, 1990, pp [8] M. Taya and R.J. Arsenault, Metal Matrix Composites Thermo Mechanical Behaviour (1989) New York, Pergmon Press, pp [9] S. Kochira, Fabrication of Sic p -Al Composite Materials Material and Manufacturing Processes, Vol. 5, 1990, p.51. [10] Margaret Hunt, Aerospace Composites, Journal of Material Engineering. Cleveland, Vol. 108 No , pp