Carbon Nanotube Reinforced Copper Matrix Composites With High Thermal Conductivity

Copper is widely used as heat dissipatings in various elcctronic fields for it’s excellent thermal conductivity.With the development of electronic components in the direction of high power density,the copper-based thermal management materials is required to further improve its thermal conductivity.Carbon nanotubes(CNTs)are ideal reinforcements for copper-based composites because of its excellent mechanical properties,electrical and thermal conductivity,and low thermal expansion coefficient.Carbon nanotubes reinforced copper matrix composites(CNTs/Cu)are expected to improve both the thermal conductivity and mechanical properties of copper.In this dissertation,carbon nanotubes were dispersed by Noise isolating chamber and Highenergy ball mill.The effect of dispersion process on the dispersibility of carbon nanotubes was studied.Three kinds of CNTs/Cu composite powders were prepared by mechanical mixing or high energy ball milling,respectively.The influence of the preperation methods for CNTs/Cu coposites powders on the morphology of composite powders was studied.The CNTs/Cu composite powders were consolidated by Spark Plasma Sintering(SPS)technology.The influence of the sintering parameters on the microstructure,mechanical properties and thermal properties of the CNTs/Cu composites was studied.By optimizing the mold structure and sintering process parameters,large size CNTs/Cu composites were prepared.The hot deformation process of the as-sintered CNTs/Cu composites was performed by SPS,and the whole process of hot deformation was simulated by the Deform software.The effects of thermal deformation on the microstructure,mechanical properties and thermal properties of the composites were also investigated.The morphology,phase and Raman spectra of carbon nanotubes and CNTs/Cu composite powders were examined by Scanning Electron Microscopy(SEM),X-ray diffraction(XRD)and microscopic confocal laser Raman spectroscopy.The results showed the dispersed carbon nanotubes by Noise isolating were uniformly dispersed in water after being dried,and without precipitation after a long stewing time when 1% TritonX-100 acetone solution was used as a dispersant.The carbon nanotubes milled for 240 minutes still maintained their basic structure and did not agglomerate,but the length of the carbon nanotubes decreased.The CNTs/Cu composite powders prepared by the traditional mixed powder process were evenly coated on the spherical copper powder.There were no carbon nanotubes agglomeration in the composites powders.The CNTs/Cu composite powder prepared by the high-energy ball milling process(ball milling 720 min)showed morphologies of lamellar,and the CNTs were broken and evenly distributed within the copper powder.No other impurity phases occured in the composite powders prepared by these two methods.When adopting gradient heating method and holding time is 10 min,the optimal sintering temperature and sintering pressure to consolidate the CNTs/Cu composite powders which are prepared by the mechanical mixed powder process were 800°C and 50 MPa,respectively.Carbon nanotubes are evenly distributed in the Cu matrix after SPS.The relative density of the CNTs/Cu composites reaches up to 98.8%,and the thermal conductivity is up to 330W/(m·K).However,the tensile strength and hardness of the CNTs/Cu composites are lower than that of pure copper.For the composite powder prepared by high-energy ball milling process,the distribution of carbon nanotubes is uniform and non-agglomerated after SPS.The density of composites is about 98%.Compared with pure copper,the thermal conductivity and tensile strength of the composites decrease,but the maximum hardness increases by 89.2%.By optimizing the structure of the sintering mold,a large-sized(?60×60mm)CNTs/Cu composites with high density(98%)and uniform microstructure was successfully prepared by SPS.Hot deformation has a significant effect on the microstructure and properties of CNTs/Cu composites.After hot deformation of CNTs/Cu composites,carbon nanotubes agglomerate with the change of the shape of the copper powder,resulting in the increased porosity and lower density.After deformation the hardness and thermal conductivity of CNTs/Cu composites prepared by conventional mixing process + SPS are lower than those before deformation,but the tensile strength increased by nearly 100%.Howerer,in contract to the CNTs/Cu composites prepared by high energy ball milling + SPS,the hot-deformed CNTs/Cu composites showed decread hardness(still higher than that of pure copper).Hot deformation improved the arrangement of the carbon nanotubes,so the thermal properties slightly increased,and the tensile strength increased by 51.7%.