High-Loading Boron Nitride-Based Bio-Inspired Paper with Plastic-like Ductility and Metal-like Thermal Conductivity

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

Durable, Low-Cost, and Efficient Heat Spreader Design from Scrap Aramid Fibers and Hexagonal Boron Nitride

Aramid, chemically known as para phenylene terephthalamide or PPD-T, has been widely used in the reinforcement of telecommunication cables, rubber materials (transmission belts, pneumatic belts), ballistic clothing, and frictional materials primarily due to their high tensile resistance, high elastic modulus, and excellent thermal stability (−80–200 °C). These unique properties of aramid originate from its chemical structure, which consists of relatively rigid polymer chains linked by benzene rings and amide bonds (-CO-NH-). Here, in this work inspired by these properties, a heat spreader called Thermal Interface Material (TIM) is developed by synthesizing a resin from scrap aramid fibers. When hexagonal boron nitride (h-BN) as filler is introduced into the as-synthesized aramid resin to form a thin film of thermal sheet (50 μm), an in-plane thermal conductivity as high as 32.973 W/mK is achieved due to the firmly stacked and symmetric arrangement of the h-BN in the resin matrix. Moreover, the influence of h-BN platelet size is studied by fabricating thermal sheets with three different sizes of h-BN (6–7.5 μm, 15–21 μm, and 30–35 μm) in the aramid resin. The results of the study show that as platelet size increases, thermal conductivity increases significantly. Since the resin reported herein is developed out of scrap aramid fibers, the cost involved in the manufacture of the thermal sheet will be greatly lower. As the thermal sheet is designed with h-BN rather than graphene or carbonaceous materials, this high heat spreading sheet can be employed for 5G antenna modules where properties like a low dielectric constant and high electrical insulation are mandated.

Thermally Conductive but Electrically Insulating Polybenzazole Nanofiber/Boron Nitride Nanosheets Nanocomposite Paper for Heat Dissipation of 5G Base Stations and Transformers

The rapid development of 5G equipment and high-power density electronic devices calls for high thermal conductivity materials for heat dissipation. Dielectric polymer composites are highly promising as the electrical insulation, mechanical property, thermal stability, and even fire retardance are also of great importance for electrical and electronic applications. However, the current thermal conductivity enhancement of dielectric polymer composites is usually at the cost of lowering the mechanical and electrical insulating properties. In this work, we report the facile preparation of highly thermally conductive and electrically insulating poly(p-phenylene benzobisoxazole) nanofiber (PBONF) composites by incorporating a low weight fraction of functionalized boron nitride nanosheets (BNNSs). With strong electrostatic interaction, the BNNSs are encapsulated by PBONFs, and the constructed robust interconnected network makes the nanocomposites exhibit a nacre-like structure. Accordingly, the nanocomposite paper has a high in-plane thermal conductivity of 21.34 W m^-1 K^-1 at a low loading of 10 wt % BNNSs and exhibits an ultrahigh strength of 206 MPa. Additionally, the nanocomposite paper exhibits superior electrical insulation properties up to higher than 350 °C and excellent fire retardance. The strong heat dissipation capability of the nanocomposite paper was demonstrated in 5G base stations and control transformers, showing wide potential applications in high power density electrical equipment and electronic devices.

AWI-Assembled TPU-BNNS Composite Films with High In-Plane Thermal Conductivity for Thermal Management of Flexible Electronics

Thermal management of flexible/stretchable electronics has been a crucial issue. Mass supernumerary thermal heat is created in the repetitive course of deformation because of the large nanocontact resistance between electric conductive fillers, as well as the interfacial resistance between fillers and the polymer matrix. Here, we report a stretchable thermoplastic polyurethane (TPU)-boron nitride nanosheet (BNNS) composite film with a high in-plane thermal conductivity based on an air/water interfacial (AWI) assembly method. In addition to rigid devices, it was capable for thermal management of flexible electronics. During more than 2000 cycles of the bending-releasing process, the average saturated surface temperature of the flexible conductor covered with composite film with 30 wt % BNNSs was approximately 40.8 ± 1 °C (10.5 °C lower than that with pure TPU). Moreover, the thermal dissipating property of the composite under stretching was measured. All the results prove that this TPU-BNNS composite film is a candidate for thermal management of next-generation flexible/stretchable electronics with high power density.

Emerging Flexible Thermally Conductive Films: Mechanism, Fabrication, Application

Effective thermal management is quite urgent for electronics owing to their ever-growing integration degree, operation frequency and power density, and the main strategy of thermal management is to remove excess energy from electronics to outside by thermal conductive materials. Compared to the conventional thermal management materials, flexible thermally conductive films with high in-plane thermal conductivity, as emerging candidates, have aroused greater interest in the last decade, which show great potential in thermal management applications of next-generation devices. However, a comprehensive review of flexible thermally conductive films is rarely reported. Thus, we review recent advances of both intrinsic polymer films and polymer-based composite films with ultrahigh in-plane thermal conductivity, with deep understandings of heat transfer mechanism, processing methods to enhance thermal conductivity, optimization strategies to reduce interface thermal resistance and their potential applications. Lastly, challenges and opportunities for the future development of flexible thermally conductive films are also discussed.

Advances in Studies of Boron Nitride Nanosheets and Nanocomposites for Thermal Transport and Related Applications

Hexagonal boron nitride (h-BN) and exfoliated nanosheets (BNNs) not only resemble their carbon counterparts graphite and graphene nanosheets in structural configurations and many excellent materials characteristics, especially the ultra-high thermal conductivity, but also offer other unique properties such as being electrically insulating and extreme chemical stability and oxidation resistance even at elevated temperatures. In fact, BNNs as a special class of 2-D nanomaterials have been widely pursued for technological applications that are beyond the reach of their carbon counterparts. Highlighted in this article are significant recent advances in the development of more effective and efficient exfoliation techniques for high-quality BNNs, the understanding of their characteristic properties, and the use of BNNs in polymeric nanocomposites for thermally conductive yet electrically insulating materials and systems. Major challenges and opportunities for further advances in the relevant research field are also discussed.

Hydrogel foams from liquid foam templates: Properties and optimisation

Hydrogel foams are an important sub-class of macroporous hydrogels. They are commonly obtained by integrating closely-packed gas bubbles of 10-1000 μm into a continuous hydrogel network, leading to gas volume fractions of more than 70% in the wet state and close to 100% in the dried state. The resulting wet or dried three-dimensional architectures provide hydrogel foams with a wide range of useful properties, including very low densities, excellent absorption properties, a large surface-to-volume ratio or tuneable mechanical properties. At the same time, the hydrogel may provide biodegradability, bioabsorption, antifungal or antibacterial activity, or controlled drug delivery. The combination of these properties are increasingly exploited for a wide range of applications, including the biomedical, cosmetic or food sector. The successful formulation of a hydrogel foam from an initially liquid foam template raises many challenging scientific and technical questions at the interface of hydrogel and foam research. Goal of this review is to provide an overview of the key notions which need to be mastered and of the state of the art of this rapidly evolving field at the interface between chemistry and physics.

Artificial Nacre Epoxy Nanomaterials Based on Janus Graphene Oxide for Thermal Management Applications

Owing to the development of microelectronics, demands for excellent thermal dissipation materials have substantially increased. Learning from natural nacre, thermally conductive epoxy nanocomposites were prepared based on asymmetrically polydopamine-functionalized Janus graphene oxide (JPGO) scaffolds. The required highly oriented JPGO scaffolds were prepared via the bidirectional freeze-casting method. With the addition of epoxy resin, the resulting nanocomposite reveals anisotropic thermal properties. With the total content of the JPGO scaffold being 0.93 wt %, almost 35 times enhancement of in-plane thermal conductivity (perpendicular to the lamellar structure) (∼5.6 W m^-1 K^-1) has been obtained. The single-side-functionalized JPGO scaffolds play an important role in forming thermal conductive networks for the epoxy nanocomposites. Importantly, the nanocomposites present electrically insulating properties (>10¹⁴ Ω cm). Such high-performance nanocomposites have promising applications for thermal management in electronic devices.