Investigation On The Preparation And Thermal Management Capacity Of High Thermal Conductivity Composites Based On The Rational Design Of The Microstructure/Interface Of 2D Fillers

With the advent of 5G era and rapid development of third generation semiconductors,the power density of electronic components increases dramatically.If the heat generated during the device operations cannot be timely extracted,the rising temperatures originated from the horrible heat accumulation will lead to a significant degradation in the system lifetimes and reliabilities.Noting that the highly efficient heat dissipation is the key to address this issue.Accompanying with the fast innovation for the preparation technologies of nano-materials,some 2D materials with ultrahigh intrinsic thermal conductivity,such as graphene and boron nitride nanosheets（BNNS）,have attracted a great deal of attentions,and are widely used to enhance the thermal management capacity of polymer-based composites.However,due to the anisotropic thermal conductivity of graphene and BNNS,the heat transfer capacity of traditional polymer-based composites is still seriously limited by some technical problems,such as the random distribution and poor alignment of fillers in the polymer matrix,as well as the horrible phonon scattering at the filler/filler and filler/matrix interfaces.Therefore,the key scientific and technical issues of this filed are to ensure the consistency of the preferred direction for heat conduction between the composites and fillers by adjusting the distribution and orientation of fillers in matrix,as well as enhance the phonon transfer based on the control and modification of contact interfaces.Given that,in this dissertation,we aim at enhancing the heat transfer capacity of the polymer matrix by adjusting the micro-structure and micro-interfaces of filler skeletons to prepare high-performance polymer-based composites for thermal management.Simultaneously,we also study the correlation of microscopic alignment of fillers and the component of interfaces versus the thermal conductivity and thermal management capacity of the composites.The main results as shown below:Chapter 1:Soft and self-adhesive thermal interface materials based on vertically aligned,covalently bonded graphene nanowalls.In view of the great challenge for the traditional methods to prepare vertically aligned,covalently bonded graphene architectures,a 120μm-thick film composed by graphene nanowalls（GNWs）was synthesized by the mesoplasma chemical vapor deposition（CVD）technology.The vertically aligned GNWs are composed by high-quality graphene with covalently bonded structure,according to the results of reverse non-equilibrium molecular dynamics simulation,the interfacial thermal resistance of covalently bonded graphene is two orders of magnitude lower than that based on van der Waals interactions,which is very suitable for the heat transfer along the GNW skeletons.After compositing with polydimethylsiloxane（PDMS）,the obtained product presents a high through-plane thermal conductivity of 20.4 W m^-1 K^-1with a graphene loading of 5.6 wt%,showing an ultrahigh thermal conductivity enhancement per 1 wt%graphene of 2006,which is much more superior than that of previous studies.The thermal interface material（TIM）performance evaluation indicates that the cooling efficiency of our sample is≈1.5 times higher than that of the state-of-the-art thermal pad(Bergquist,5000S35,≈5 W m^-1 K^-1),which is attributed to not only higher through-plane thermal conductivity,but also lower thermal contact resistance（37.4%）due to the low compressive modulus of our sample originated from very low filler loading.Our sample also shows superior cooling efficiency in the real operating conditions,which can reduce the running temperature and suppress the degradation of luminous properties of commercial LED chips,providing an insight to future research to fabricate high performance TIM for efficient cooling of electronic/optoelectronic devices.Chapter 2:The preparation of high-aspect-ratio BNNS and the insulating composite films with high in-plane thermal conductivity based on these nanosheets.Currently,it remains a great challenge to fabricate high-aspect-ratio BNNS by mechanical exfoliation,which is seriously restricted the capacity of nanosheets to enhance the thermal conductivity of polymer matrix.To address this issue,a microfluidization process was employed to exfoliate h-BN based on the physical mechanism that a high-pressure fluid jet was forced to pass through a Z-type microchannels,where the flow with a high shear rete of≈8.77×10⁷ s^-1may directly act on the h-BN to effectively exfoliate it into BNNS with negligible damage to the lateral size.The method has the advantages of high-yield（70–76%）,high efficiency,and enables the preparation of BNNS with ultrahigh aspect ratio of 1500（BNNS-1500）.Changing the aspect ratio of BNNS can accurately control the contact area of adjoining BNNS,as well as the interface densities in the composites,so as to reduce the interfacial resistance between the contact interfaces.Comparing with the BNNS with average aspect ratio of 1000（BNNS-1000）,the average overlapping area of adjoining BNNS-1500 are≈3.75 times higher than that of former sample,and the contact resistance is only half of that of BNNS-1000.The composite film composed by BNNS-1500 and poly（vinyl alcohol）（PVA）presents a high in-plane thermal conductivity of 67.6 W m^-1 K^-1 with a filler loading of 83 wt%,which is 33%higher than that of BNNS-1000/PVA(50.9 W m^-1 K^-1),showing an ultrahigh thermal conductivity enhancement per 1 wt%BNNS-1500 of 427,which is much higher than that of relative studies.Moreover,the BNNS-1500/PVA used as heat spreader exhibit better heat dissipation performance than commercial flexible copper clad laminate in the real operating conditions for cooling of LED chips.Our sample combines the advantages of light weight,flexibility,insulation,and high thermal conductivity,which is expected to replace the commercial products.Chapter 3:Covalent functionalization of high-aspect-ratio BNNS and the insulating composite films with high in-plane thermal conductivity based on these nanosheets.It is well known that the covalent functionalization of BNNS can enhance the interfacial interaction between filler and polymer matrix,resulting in a significant decrease of interfacial thermal resistance.Especially,the high-aspect-ratio BNNS with edge functionalization has unparalleled advantages in improving thermal conductivity of composites,but it is still a great challenge to fabricate this kind of nanosheets by traditional methods.Herein,a new strategy was employed to hydroxylate the edges of ultrahigh-aspect-ratio BNNS obtained by microfluidization via a simple heat treatment process in atmosphere,with negligible damages to the lateral seize.Comparing with the pristine BNNS,the functionalized BNNS（BNNS-OH）can enhance the phonon transfer across the filler/matrix interface,resulting in a higher thermal conductivity of corresponding composites.At a filler loading of 83.7 wt%,the composites composed by BNNS-OH and cellulose nanofibers（CNF）exhibit a high in-plane thermal conductivity of 101.2 W m^-1 K^-1,which is 37%higher than that of BNNS/CNF with the same filler loading.Moreover,the tensile strength of the former（28.4 MPa）is also 58%higher than that of the latter.The printing circuit will tightly bond on the BNNS-OH/CNF composite film when the sample is used as a heat spreader,in which only a tiny increase of circuit’s resistance can be found after suffering from 10000 cycles of bending.During the operating process of the circuit,the film shows an excellent capacity in cooling,where the heat generated by the circuit can be transmitted to the environment in very short time.Our results provide a new route for the preparation and commercial application of the insulating composites with high thermal conductivity.