Low-temperature Solidifiable Liquid Metal With Ultrahigh Thermal Conductivity Enabled By Spontaneous Phase Transition For Electronics’ Safety And Long-life Cooling

With the development of microelectronics,the power of chips has grown,making thermal interface materials（TIMs）with high heat-conducting properties and great conformability essential for thermal management in the electronic industry.Commercially available TIMs are typically divided into two categories:polymer-based and metal-based.Achieving a thermal conductivity higher than 9 W/（m·K）is challenging for polymer-based TIMs due to the inferior heat-transfer properties of polymer matrices.In contrast,metal-based TIMs have superior heat-transfer performance but are not suitable for applications where high-temperature soldering processes are impractical,such as in laptop computer central processing units and some flexible electronic devices.Gallium-based liquid metals,as a new generation of functional materials,have shown many advantages as TIMs for high-power electronic devices.However,due to their excellent rheological properties,liquid metal droplets can easily be extruded from the narrow gap between two mating surfaces,potentially causing short circuits or even failure of electronic systems.This is widely considered to be the biggest obstacle limiting the large-scale industrial application of liquid metals as TIMs.To address this problem,we propose a low-temperature solidifiable liquid metal-a biphasic（liquid-solid）composite material consisting of a eutectic Gallium-Indium-Tin alloy and copper particles.We have found that the state of this composite can spontaneously transform from liquid to solid even at room temperature.We subsequently investigated the liquid-solid transition behavior and phase composition evolution of liquid-metal composites during solidification.We found that the solidifying reaction is triggered by the formation of Cu Ga₂ intermetallic compounds and the precipitation of In₃Sn solid solutions.Additionally,only residual copper particles,Cu Ga₂intermetallic compounds（IMCs）,and In₃Sn solid solutions were detected in the connecting layer of the sandwich structure sample of“copper substrate-EGa In Sn/Cu-copper substrate,”indicating that the liquid metal has been exhausted.We further investigated the thermal properties of this liquid-metal composite.Our results confirm that the composite,with only a 36%volume ratio of copper fillers,can achieve an ultrahigh thermal conductivity of 86.7 W/（m·K）,far greater than that of commercial polymer-based TIMs and other analogous liquid-metal composites.When the thickness of the connecting layer is 100μm,the total thermal resistance can be as low as 3.1K·mm²/W,and the thermal conductivity of the material remains stable during the liquid-solid diffusion reaction.Through simulation calculations,we determined that the interface thermal conductivity value?(8 is approximately 6.5×10⁸ W/（m²·K）.Finally,using a theoretical model of thermal conductivity,we found that the change in thermal conductivity of composites is most sensitive to changes in the thermal conductivity of liquid metal.We also discussed the mechanical properties of the composites.Our experimental results show that with increasing temperature or time,the high melting point Cu Ga₂ and In₃Sn phases in EGa In Sn/Cu thermal interface materials continue to form,and the shear strength of Gallium-Copper liquid phase diffusion bonding increases continuously.The shear strength of the materials after complete curing can reach 13.7 MPa,equivalent to that of common low-temperature solders such as 100In and Sn48In.Additionally,the fracture position of the lap sample is always near the interface between the EGa In Sn/Cu thermal conductive interface material and the copper substrate.This is because residual copper particles prevent crack propagation in the composites,and the strength of the material itself is improved by a second phase strengthening mechanism.Finally,we measured the hardness and Young’s modulus of the Cu Ga₂ phase using a nano-indentation technique and found them to be 2.49 and 86.4 GPa,respectively.The Vickers hardness of EGa In Sn/Cu thermal conductive interface materials is between 35.1 and 39.3 HV,indicating that the Cu Ga₂ phase is a soft material and that Gallium-Copper liquid phase diffusion bonding may have high anti-fatigue reliability.More importantly,in thermal cycling tests,the mechanical and thermal properties of this material after solidification remained steady,demonstrating that the low-temperature solidifiable liquid metal could meet the needs of electronic safety and long-life cooling applications.In summary,the heat-conducting property of the obtained composite is far greater than that of commercial polymer-based TIMs.Additionally,compared to conventional metal-based TIMs,liquid-metal composites do not require high-temperature processing.It can be predicted that this material may have broad applications in high-power electronic systems such as high-performance computers,5G communications,and power electronic devices.