Interfacial Structure And Thermal Conductivity Of Cu/diamond Composites

The rapid development of electronics and information industry has led to a sharp increase in power density of electronic devices,and thus highly thermally conductive materials for electronic packaging applications are urgently required to ensure efficient heat dissipation from these devices.Diamond possesses excellent thermo-physical properties,but it can hardly be used directly.Copper has a high thermal conductivity and it also has good processing performance.Therefore,diamond particles reinforced copper matrix(Cu/diamond)composites have become a focus of the new generation of thermal management materials.However,the poor wettability between Cu and diamond makes it difficult to obtain sound interfacial bonding in Cu/diamond composites,and it is thus impossible to take full advantage of the outstanding thermal properties of diamond.Researchers in this field have adopted Cu matrix alloying and diamond surface metallization to modify the Cu/diamond interface,and the thermal conductivity of Cu/diamond composites is improved.However,there is a lack of in-depth study on the interfacial structure of the composites,and the understanding on the interfacial formation mechanism is not consistent.In this thesis,Cu/diamond composites are fabricated by gas pressure infiltration(GPI),and the interface of the composite is modified by both metal matrix alloying and diamond surface metallization with carbide-forming elements.The interfacial structure of the Cu/diamond composite modified by different interfacial modification methods is thoroughly characterized by focused ion beam(FIB)and scanning transmission electron microscope(STEM).Based on the characterizations and analyses,the interfacial formation mechanism of Cu/diamond composites is elucidated,and the relationship between interfacial structure evolution and composite thermal conductivity is established.The alloying element Zr in the range of 0.25-1.0 wt.%was added into the Cu matrix by vacuum induction melting,and the Cu-Zr/diamond composites were prepared by GPI using synthetic single-crystalline diamond particles.The nucleation and growth mechanisms of interfacial reaction products were systematically studied.The interfacial structure evolution and composite thermal conductivity were elucidated in relation to Zr content in the Cu matrix.The results show that the formation of ZrC at the interface is an inhomogeneous nucleation process,and the nucleation and growth of ZrC nuclei are controlled by the diamond surface features.On diamond(111)surface,the ZrC particles nucleate on the less dense edges of the perpendicular steps and then grow mainly through surface diffusion.On diamond(100)surface,the ZrC particles nucleate on the high-density surface pits that show an inclination angle of?55° with diamond(100)surface,and the ZrC nuclei grow by bulk diffusion.As a result,low-density ZrC particles with large sizes are formed on diamond(111)surface,while high-density ZrC particles with small sizes are formed on diamond(100)surface.The crystallographic orientation relationship between diamond and ZrC is identified as(111)diamond//(111)ZrC and[110]diamond//[110]ZrC The Zr content in the Cu matrix has a significant influence on the interfacial carbide morphology.With increasing Zr content from 0.25 wt.%to 1.0 wt.%,the interfacial structure evolves from small-sized and discrete ZrC particles into continuous carbide layer.As a result,the composite thermal conductivity firstly increases and then decreases,giving the maximum value at an intermediate Zr content of 0.5 wt.%.At this composition,the coefficient of thermal expansion near room temperature is 5.16×10-6 K-1 that matches semiconductors.Introducing an interfacial layer through diamond surface metallization is another commonly used method of interfacial modification.Magnetron sputtering was employed to deposit Zr coating layers 47-430 nm in thickness on the single-crystalline diamond particles.The phase structure and morphology evolution of the Zr-coating during preparation of the Cu/diamond(Zr)composites were systematically studied.The results reveal that the Zr-coating layer can protect the diamond particles from graphitization during infiltration,and the Zr-coating gradually transforms to interfacial ZrC layer with the diffusion of C atoms to the Zr-coating.The crystallographic orientation relationship between diamond and ZrC is(111)diamond//(111)ZrC and[110]diamond//[110]ZrC.The interfacial ZrC layer can greatly strengthen the interfacial bonding bentween the diamond and the Cu matrix,which improves the composite thermal conductivity.When the thickness of interfacial ZrC layer is 50 nm(Zr-coating thickness is 47 nm),the thermal conductivity of the Cu/diamond(Zr)composite is 735 W m-1 K-1.With increasing the thickness of interfacial ZrC layer,the composite thermal conductivity decreases.This is attributed to the low thermal conductivity of ZrC and the interfacial de-bonding at the interfacial ZrC layer with large thickness.The maximum thermal conductivity in the Cu-Zr/diamond composites is 930 W m-1 K-1 obtained at 0.5 wt.%Zr,which is much higher than the maximum value of 735 W m-1 K-1 in the Cu/diamond(Zr)composites.This is closely related to the strong pinning effect of wedge-shaped carbides growing into the Cu matrix.For comparison ,Ti coating layers 65-850 nm in thickess were deposited on the single-cpystalline diamond particles by magnetron sputtering. The phase stucure and morphology evolution of the Ti-coating during fabrication of the Cu/diamond (Ti) composites evolution were systematically studied.Similar to the formation Similar to the formation mechanism of interfacial ZrC layer in the Cu/diamond(Zr) composites,the diffusion of C atoms makes the Ti-coating convert to interfacial TiC layer,whichr enhances the interfacial bonding and improves the thermal conductivity of Cu/diamond(Ti) composites. The crystallographic orientation relationship between diamond and TiC is (lxns//(il1)nc and [110]a mno///IOnc. The maximum thermal conductivity of the Cu/diamond(T) compositeis 811 W m-1K-1when the thickness of the uniform Ti-coating is 220 nm. With the further Increase of Ti-coating thickness, the composite thermal condaictivity decreases. By optimizing the plating process tp reduce coating defects, it is expected that thethermal conductivity of Cu/diamond(Ti) composites could be increased at a thinner Ti-coating The maximum thermal conductivity in the Cu/diamond(Zr) composites is 735 W m-l K-1 when the thickness of Zrycoating is 47 nm, which is lower than the maximum thermal conductivity of 811 w m-1 K-1 in the Cu/diamond(Ti)composites with the Ti-coating thickness of 220 nm, and it is Possible to further improve the thermal conductivity by reducing the thickness ofTi-coating. Compared with Zr-coating, Ti-coating on diamond surface is more effpctive to bridging the a difference in acoustic impedance between Cu and dismond, and thus Ti-coating is more beneficial to imiprove the composite thermalconductivity. The coefficient of therphal expansiork of the Cudiamond(Ti) composite modified by 220 nm Ti-coating is 5.55X 10-6 K near room temperature, vHich is compatible with semiconductors.To summarize, the interfacial carbide layers wereintroduced into Cu/diamond composites through Cu matrix alloying and diamond surface metallization, which both enhance the interfacial bonding and improve the compositc thermal conductivity. The interfacial formation mechanisms in the Cu/diamond composites modified by the TWo different routes were charified. The interfacial structure characteristics beneficial to composite thermal conductivity were revealed. The /results provide a theoretical basis for the destgn and preparation of Cu/diamond composites with high thermal conductivity. The modified Cu/diagiond composites have excellent thermo-physical properties and are promising for heat dissipation applications of high-power clectronic devicas such as lasers/LED and CPU chips.