Dense Vertically Aligned Copper Nanowire Composites as High Performance Thermal Interface Materials

Liquid-infused nanostructured composite as a high-performance thermal interface material for effective cooling

Effective heat dissipation remains a grand challenge for energy-dense devices and systems. As heterogeneous integration becomes increasingly inevitable in electronics, thermal resistance at interfaces has emerged as a critical bottleneck for thermal management. However, existing thermal interface solutions are constrained by either high thermal resistance or poor reliability. We report a strategy to create printable, high-performance liquid-infused nanostructured composites, comprising a mechanically soft and thermally conductive double-sided Cu nanowire array scaffold infused with a customized thermal-bridge liquid that suppresses contact thermal resistance. The liquid infusion concept is versatile for a broad range of thermal interface applications. Remarkably, the liquid metal infused nanostructured composite exhibits an ultra-low thermal resistance <1 mm² K W-1 at interface, outperforming state-of-the-art thermal interface materials on chip-cooling. The high reliability of the nanostructured composites enables undegraded performance through extreme temperature cycling. We envision liquid-infused nanostructured composites as a universal thermal interface solution for cooling applications in data centers, GPU/CPU systems, solid-state lasers, and LEDs.

2D Materials-Based Thermal Interface Materials: Structure, Properties, and Applications

The challenges associated with heat dissipation in high-power electronic devices used in communication, new energy, and aerospace equipment have spurred an urgent need for high-performance thermal interface materials (TIMs) to establish efficient heat transfer pathways from the heater (chip) to heat sinks. Recently, emerging 2D materials, such as graphene and boron nitride, renowned for their ultrahigh basal-plane thermal conductivity and the capacity to facilitate cross-scale, multi-morphic structural design, have found widespread use as thermal fillers in the production of high-performance TIMs. To deepen the understanding of 2D material-based TIMs, this review focuses primarily on graphene and boron nitride-based TIMs, exploring their structures, properties, and applications. Building on this foundation, the developmental history of these TIMs is emphasized and a detailed analysis of critical challenges and potential solutions is provided. Additionally, the preparation and application of some other novel 2D materials-based TIMs are briefly introduced, aiming to offer constructive guidance for the future development of high-performance TIMs.

Fillers and methods to improve the effective (out-plane) thermal conductivity of polymeric thermal interface materials - A review

The internet of things and growing demand for smaller and more advanced devices has created the problem of high heat production in electronic equipment, which greatly reduces the work performance and life of the electronic instruments. Thermal interface material (TIM) is placed in between heat generating micro-chip and the heat dissipater to conduct all the produced heat to the heat sink. The development of suitable TIM with excellent thermal conductivity (TC) in both in-plane and through-plane directions is a very important need at present. For efficient thermal management, polymer composites are potential candidates. But in general, their thermal conductivity is low compared to that of metals. The filler integration into the polymer matrix is one of the two approaches used to increase the thermal conductivity of polymer composites and is also easy to scale up for industrial production. Another way to achieve this is to change the structure of polymer chains, which fall out of the scope of this work. In this review, considering the first approach, the authors have summarized recent developments in many types of fillers with different scenarios by providing multiple cases with successful strategies to improve through-plane thermal conductivity (TPTC) (k⊥). For a better understanding of TC, a comprehensive background is presented. Several methods to improve the effective (out-plane) thermal conductivity of polymer composites and different theoretical models for the calculation of TC are also discussed. In the end, it is given a detailed conclusion that provides drawbacks of some fillers, multiple significant routes recommended by other researchers to build thermally conductive polymer composites, future aspects along with direction so that the researchers can get a guideline to design an effective polymer-based thermal interface material.

High Thermal Conductivity of Sandwich-Structured Flexible Thermal Interface Materials

Thermal interfaces are vital for effective thermal management in modern electronics, especially in the emerging fields of flexible electronics and soft robotics that impose requirements for interface materials to be soft and flexible in addition to having high thermal performance. Here, a novel sandwich-structured thermal interface material (TIM) is developed that simultaneously possesses record-low thermal resistance and high flexibility. Frequency-domain thermoreflectance (FDTR) is employed to investigate the overall thermal performance of the sandwich structure. As the core of this sandwich, a vertically aligned copper nanowire (CuNW) array preserves its high intrinsic thermal conductivity, which is further enhanced by 60% via a thick 3D graphene (3DG) coating. The thin copper layers on the top and bottom play the critical roles in protecting the nanowires during device assembly. Through the bottom-up fabrication process, excellent contacts between the graphene-coated CuNWs and the top/bottom layer are realized, leading to minimal interfacial resistance. In total, the thermal resistance of the sandwich is determined as low as ~0.23 mm²  K W^-1 . This work investigates a new generation of flexible thermal interface materials with an ultralow thermal resistance, which therefore renders the great promise for advanced thermal management in a wide variety of electronics.

3D Graphene-Nanowire "Sandwich" Thermal Interface with Ultralow Resistance and Stiffness

Despite the recent advancements of passive and active cooling solutions for electronics, interfaces between materials have generally become crucial barriers for thermal transport because of intrinsic material dissimilarity and surface roughness at interfaces. We demonstrate a 3D graphene-nanowire "sandwich" thermal interface that enables an ultralow thermal resistance of ∼0.24 mm²·K/W that is about 1 order of magnitude smaller than those of solders and several orders of magnitude lower than those of thermal greases, gels, and epoxies, as well as a low elastic and shear moduli of ∼1 MPa like polymers and foams. The flexible 3D "sandwich" exhibits excellent long-term reliability with >1000 cycles over a broad temperature range from -55 °C to 125 °C. This nanostructured thermal interface material can greatly benefit a variety of electronic systems and devices by allowing them to operate at lower temperatures or at the same temperature but with higher performance and higher power density.

Ultralow Interfacial Thermal Resistance of Graphene Thermal Interface Materials with Surface Metal Liquefaction

Developing advanced thermal interface materials (TIMs) to bridge heat-generating chip and heat sink for constructing an efficient heat transfer interface is the key technology to solve the thermal management issue of high-power semiconductor devices. Based on the ultra-high basal-plane thermal conductivity, graphene is an ideal candidate for preparing high-performance TIMs, preferably to form a vertically aligned structure so that the basal-plane of graphene is consistent with the heat transfer direction of TIM. However, the actual interfacial heat transfer efficiency of currently reported vertically aligned graphene TIMs is far from satisfactory. In addition to the fact that the thermal conductivity of the vertically aligned TIMs can be further improved, another critical factor is the limited actual contact area leading to relatively high contact thermal resistance (20-30 K mm2 W-1) of the "solid-solid" mating interface formed by the vertical graphene and the rough chip/heat sink. To solve this common problem faced by vertically aligned graphene, in this work, we combined mechanical orientation and surface modification strategy to construct a three-tiered TIM composed of mainly vertically aligned graphene in the middle and micrometer-thick liquid metal as a cap layer on upper and lower surfaces. Based on rational graphene orientation regulation in the middle tier, the resultant graphene-based TIM exhibited an ultra-high thermal conductivity of 176 W m-1 K-1. Additionally, we demonstrated that the liquid metal cap layer in contact with the chip/heat sink forms a "liquid-solid" mating interface, significantly increasing the effective heat transfer area and giving a low contact thermal conductivity of 4-6 K mm2 W-1 under packaging conditions. This finding provides valuable guidance for the design of high-performance TIMs based on two-dimensional materials and improves the possibility of their practical application in electronic thermal management.

Computer vision-assisted investigation of boiling heat transfer on segmented nanowires with vertical wettability

The boiling efficacy is intrinsically tethered to trade-offs between the desire for bubble nucleation and necessity of vapor removal. The solution to these competing demands requires the separation of bubble activity and liquid delivery, often achieved through surface engineering. In this study, we independently engineer bubble nucleation and departure mechanisms through the design of heterogeneous and segmented nanowires with dual wettability with the aim of pushing the limit of structure-enhanced boiling heat transfer performances. The demonstration of separating liquid and vapor pathways outperforms state-of-the-art hierarchical nanowires, in particular, at low heat flux regimes while maintaining equal performances at high heat fluxes. A deep-learning based computer vision framework realized the autonomous curation and extraction of hidden big data along with digitalized bubbles. The combined efforts of materials design, deep learning techniques, and data-driven approach shed light on the mechanistic relationship between vapor/liquid pathways, bubble statistics, and phase change performance.

Enhanced Heat Dissipation Performance of Automotive LED Lamps Using Graphene Coatings

Graphene heat-dissipating coating (GNHC) of 0.6 wt % GN concentration is utilized to promote the cooling performance of automotive light-emitting diode (LED) lamps. Three cases are studied as follows: Case 0 is the original automotive LED lamp as the baseline. Case 1 is to apply GNHC to reduce the thermal resistance of the junction surfaces between the components of automotive LED lamps. The aluminum fin radiator of Case 1 is further coated with GNHC on the surface that becomes Case 2. The spectrum, illuminance, power consumption, and surface temperature are measured at different ambient temperatures (T_a) to fully evaluate the feasibility of applying GNHC to improve cooling performance and the impacts on the related characteristics of automotive LED lamps. The results show that the maximum illuminance efficacy of Case 1 and Case 2 with high beam, irradiation angle of 0 degrees, and T_a of 80 °C is 11.03% and 8.70% higher than that of Case 0, respectively. The minimum temperature difference of heat dissipation path of Case 1 and Case 2 with high beam, irradiation angle of 90 degrees, and T_a of 80 °C is 6.41% and 5.33% lower than that of Case 0, respectively, indicating GNHC as a promising coating material for improving the cooling performance of automotive LED lamps.

Controlled Vertically Aligned Structures in Polymer Composites: Natural Inspiration, Structural Processing, and Functional Application

Vertically aligned structures, which are a series of characteristic conformations with thickness-direction alignment, interconnection, or assembly of filler in polymeric composite materials that can provide remarkable structural performance and advanced anisotropic functions, have attracted considerable attention in recent years. The past two decades have witnessed extensive development with regard to universal fabrication methods, subtle control of morphological features, improvement of functional properties, and superior applications of vertically aligned structures in various fields. However, a systematic review remains to be attempted. The various configurations of vertical structures inspired from biological samples in nature, such as vertically aligned structures with honeycomb, reed, annual ring, radial, and lamellar configurations are summarized here. Additionally, relevant processing methods, which include the transformation of oriented direction, external-field inducement, template method, and 3D printing method, are discussed in detail. The diverse applications in mechanical, thermal, electric, dielectric, electromagnetic, water treatment, and energy fields are also highlighted by providing representative examples. Finally, future opportunities and prospects are listed to identify current issues and potential research directions. It is expected that perspectives on the vertically aligned structures presented here will contribute to the research on advanced multifunctional composites.

Beyond homogeneous dispersion: oriented conductive fillers for high κ nanocomposites

Rational design of structures for regulating the thermal conductivities (κ) of materials is critical to many components and products employed in electrical, electronic, energy, construction, aerospace, and medical applications. As such, considerable efforts have been devoted to developing polymer composites with tailored conducting filler architectures and thermal conduits for highly improved κ. This paper is dedicated to overviewing recent advances in this area to offer perspectives for the next level of future development. The limitations of conventional particulate-filled composites and the issue of percolation are discussed. In view of different directions of heat dissipation in polymer composites for different end applications, various approaches for designing the micro- and macroscopic structures of thermally conductive networks in the polymer matrix are highlighted. Methodological approaches devised to significantly ameliorate thermal conduction are categorized with respect to the pathways of heat dissipation. Future prospects for the development of thermally conductive polymer composites with modulated thermal conduction pathways are highlighted.

Multifunctional composite films with vertically aligned ZnO nanowires by leaching-enabled capillary rise infiltration

Nanocomposite films (NCFs) with vertically aligned nanowires (NWs) provide several useful properties owing to their unique morphology. One of the key challenges in producing such an NCF is retaining the vertical alignment of NWs during NCF fabrication. Although current methods such as layer-by-layer assembly and solution-based processes with field-induced alignment of NWs have been successfully demonstrated, these approaches require multiple steps thus are time-consuming, and only suitable for lab-scale production, consequently limiting their widespread applicability. Herein, we describe a new method for fabricating an NCF with vertically aligned ZnO NWs by inducing leaching-enabled capillary rise infiltration (LeCaRI) of uncross-linked and mobile oligomer chains from a poly(dimethylsiloxane) (PDMS) slab into the space between the vertically aligned ZnO NWs. PDMS-infiltrated ZnO NW NCFs have a suite of useful properties including superhydrophobicity, self-cleaning, solvent resistance, and anti-icing properties as well as high transparency and anti-reflection properties. The NCF can easily recover its superhydrophobicity after it has been compromised through repeated plasma treatments or even exposure to intense UV irradiation. Moreover, our approach represents a straightforward, efficient, and potentially scalable strategy to produce multifunctional NCFs with vertically aligned NW arrays which could be easily extended to other types of materials and NW arrangements toward a wide range of properties and applications.

Metal-Level Thermally Conductive yet Soft Graphene Thermal Interface Materials

Along with the technology evolution for dense integration of high-power, high-frequency devices in electronics, the accompanying interfacial heat transfer problem leads to urgent demands for advanced thermal interface materials (TIMs) with both high through-plane thermal conductivity and good compressibility. Most metals have satisfactory thermal conductivity but relatively high compressive modulus, and soft silicones are typically thermal insulators (0.3 W m^-1 K^-1). Currently, it is a great challenge to develop a soft material with the thermal conductivity up to metal level for TIM application. This study solves this problem by constructing a graphene-based microstructure composed of mainly vertical graphene and a thin cap of horizontal graphene layers on both the top and bottom sides through a mechanical machining process to manipulate the stacked architecture of conventional graphene paper. The resultant graphene monolith has an ultrahigh through-plane thermal conductivity of 143 W m^-1 K^-1, exceeding that of many metals, and a low compressive modulus of 0.87 MPa, comparable to that of silicones. In the actual TIM performance measurement, the system cooling efficiency with our graphene monolith as TIM is 3 times as high as that of the state-of-the-art commercial TIM, demonstrating the superior ability to solve the interfacial heat transfer issues in electronic systems.

Patterning Cu nanostructures tailored for CO2 reduction to electrooxidizable fuels and oxygen reduction in alkaline media

Due to the limited availability of noble metal catalysts, such as platinum, palladium, or gold, their substitution by more abundant elements is highly advisable. Considerably challenging is the controlled and reproducible synthesis of stable non-noble metallic nanostructures with accessible active sites. Here, we report a method of preparation of bare (ligand-free) Cu nanostructures from polycrystalline metal in a controlled manner. This procedure relies on heterogeneous localized electrorefining of polycrystalline Cu on indium tin oxide (ITO) and glassy carbon as model supports using scanning electrochemical microscopy (SECM). The morphology of nanostructures and thus their catalytic properties are tunable by adjusting the electrorefining parameters, i.e., the electrodeposition voltage, the translation rate of the metal source and the composition of the supporting electrolyte. The activity of the obtained materials towards the carbon dioxide reduction reaction (CO2RR), oxygen reduction reaction (ORR) in alkaline media and hydrogen evolution reaction (HER), is studied by feedback mode SECM. Spiky Cu nanostructures obtained at a high concentration of chloride ions exhibit enhanced electrocatalytic activity. Nanostructures deposited under high cathodic overpotentials possess a high surface-to-volume ratio with a large number of catalytic sites active towards the reversible CO2RR and ORR. The CO2RR yields easily electrooxidizable compounds - formic acid and carbon monoxide. The HER seems to occur efficiently at the crystallographic facets of Cu nanostructures electrodeposited under mild polarization.

Reduction of the Lorenz Number in Copper at Room Temperature due to Strong Inelastic Electron Scattering Brought about by High-Density Dislocations

In this work, heat and charge transport were measured in a series of deformed bulk Cu samples where dislocation density was tuned but dislocation character generally remained unchanged. We observed a notable violation of the Wiedemann-Franz law at room temperature for such a conventional metal. We show that high-density dislocations introduce strong inelastic electron scattering, which relax heat and charge currents to different extents. A reduction of Lorenz number by 15% was observed. We reveal that the contribution from elastic scattering to the incremental thermal resistivity scarcely varies with dislocation density, but the contribution due to inelastic scattering monotonically increases and becomes overwhelmingly dominant for dislocation density above 1.0 × 10¹⁵ m^-2.

Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m-1 K-1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake-PU and Ag nanosphere-PU TIMs provide significantly smaller κ (7.9 and 15.0 W m-1 K-1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF-PU TIM dramatically increases (κlat = 22.2 W m-1 K-1) compared with Ag nanoflake-PU and Ag nanosphere-PU (κlat = 0.2 and 1.2 W m-1 K-1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF-PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.

Capillary Wicking in Hierarchically Textured Copper Nanowire Arrays

Capillary wicking through homogeneous porous media remains challenging to simultaneously optimize due to the unique transport phenomena that occur at different length scales. This challenge may be overcome by introducing hierarchical porous media, which combine tailored morphologies across multiple length scales to design for the individual transport mechanisms. Here, we fabricate hierarchical nanowire arrays consisting of vertically aligned copper nanowires (∼100 to 1000 nm length scale) decorated with dense copper oxide nanostructures (∼10 to 100 nm length scale) to create unique property sets that include a large specific surface area, high rates of fluid delivery, and the structural flexibility of vertical arrays. These hierarchical nanowire arrays possess enhanced capillary wicking ( K/ R_eff = 0.004-0.023 μm) by utilizing hemispreading and are advantageous as evaporation surfaces. With the advent and acceleration of flexible electronics technologies, we measure the capillary properties of our freestanding hierarchical nanowire arrays installed on curved surfaces and observe comparable fluid delivery to flat arrays, showing the difference of 10-20%. The degree of effective inter-nanowire pore and porosity is shown to govern the capillary performance parameters, thereby this study provides the design strategy for capillary wicking materials with unique and tailored combinations of thermofluidic properties.

Ultracompliant Heterogeneous Copper-Tin Nanowire Arrays Making a Supersolder

Due to the substantial increase in power density, thermal interface resistance that can constitute more than 50% of the total thermal resistance has generally become a bottleneck for thermal management in electronics. However, conventional thermal interface materials (TIMs) such as solder, epoxy, gel, and grease cannot fulfill the requirements of electronics for high-power and long-term operation. Here, we demonstrate a high-performance TIM consisting of a heterogeneous copper-tin nanowire array, which we term "supersolder" to emulate the role of conventional solders in bonding various surfaces. The supersolder is ultracompliant with a shear modulus 2-3 orders of magnitude lower than traditional solders and can reduce the thermal resistance by two times as compared with the state-of-the-art TIMs. This supersolder also exhibits excellent long-term reliability with >1200 thermal cycles over a wide temperature range. By resolving this critical thermal bottleneck, the supersolder enables electronic systems, ranging from microelectronics and portable electronics to massive data centers, to operate at lower temperatures with higher power density and reliability.

Integrated nanomaterials for extreme thermal management: a perspective for aerospace applications

Nanomaterials will play a disruptive role in next-generation thermal management for high power electronics in aerospace platforms. These high power and high frequency devices have been experiencing a paradigm shift toward designs that favor extreme integration and compaction. The reduction in form factor amplifies the intensity of the thermal loads and imposes extreme requirements on the thermal management architecture for reliable operation. In this perspective, we introduce the opportunities and challenges enabled by rationally integrating nanomaterials along the entire thermal resistance chain, beginning at the high heat flux source up to the system-level heat rejection. Using gallium nitride radio frequency devices as a case study, we employ a combination of viewpoints comprised of original research, academic literature, and industry adoption of emerging nanotechnologies being used to construct advanced thermal management architectures. We consider the benefits and challenges for nanomaterials along the entire thermal pathway from synthetic diamond and on-chip microfluidics at the heat source to vertically-aligned copper nanowires and nanoporous media along the heat rejection pathway. We then propose a vision for a materials-by-design approach to the rational engineering of complex nanostructures to achieve tunable property combinations on demand. These strategies offer a snapshot of the opportunities enabled by the rational design of nanomaterials to mitigate thermal constraints and approach the limits of performance in complex aerospace electronics.