Thermal conductivity and phonon transport in suspended few-layer hexagonal boron nitride

Miracle in "White":Hexagonal Boron Nitride

The exploration of 2D materials has captured significant attention due to their unique performances, notably focusing on graphene and hexagonal boron nitride (h-BN). Characterized by closely resembling atomic structures arranged in a honeycomb lattice, both graphene and h-BN share comparable traits, including exceptional thermal conductivity, impressive carrier mobility, and robust pi-pi interactions with organic molecules. Notably, h-BN has been extensively examined for its exceptional electrical insulating properties, inert passivation capabilities, and provision of an ideal ultraflat surface devoid of dangling bonds. These distinct attributes, contrasting with those of h-BN, such as its conductive versus insulating behavior, active versus inert nature, and absence of dangling surface bonds versus absorbent tendencies, render it a compelling material with broad application potential. Moreover, the unity of such contradictions endows h-BN with intriguing possibilities for unique applications in specific contexts. This review aims to underscore these key attributes and elucidate the intriguing contradictions inherent in current investigations of h-BN, fostering significant insights into the understanding of material properties.

Multifunctional Applications Enabled by Fluorination of Hexagonal Boron Nitride

2D materials exhibit exceptional properties as compared to their macroscopic counterparts, with promising applications in nearly every area of science and technology. To unlock further functionality, the chemical functionalization of 2D structures is a powerful technique that enables tunability and new properties within these materials. Here, the successful effort to chemically functionalize hexagonal boron nitride (hBN), a chemically inert 2D ceramic with weak interlayer forces, using a gas-phase fluorination process is exploited. The fluorine functionalization guides interlayer expansion and increased polar surface charges on the hBN sheets resulting in a number of vastly improved applications. Specifically, the F-hBN exhibits enhanced dispersibility and thermal conductivity at higher temperatures by more than 75% offering exceptional performance as a thermofluid additive. Dispersion of low volumes of F-hBN in lubricating oils also offers marked improvements in lubrication and wear resistance for steel tribological contacts decreasing friction by 31% and wear by 71%. Additionally, incorporating numerous negatively charged fluorine atoms on hBN induces a permanent dipole moment, demonstrating its applicability in microelectronic device applications. The findings suggest that anchoring chemical functionalities to hBN moieties improves a variety of properties for h-BN, making it suitable for numerous other applications such as fillers or reinforcement agents and developing high-performance composite structures.

Fabrication of Novel 3-D Nanocomposites of HAp-TiC-h-BN-ZrO2: Enhanced Mechanical Performances and In Vivo Toxicity Study for Biomedical Applications

Due to excellent biocompatibility, bioactivities, and osteoconductivity, hydroxyapatite (HAp) is considered as one of the most suitable biomaterials for numerous biomedical applications. Herein, HAp was fabricated using a bottom-up approach, i.e., a wet chemical method, and its composites with TiC, h-BN, and ZrO2 were fabricated by a solid-state reaction method with enhanced mechanical and biological performances. Structural, surface morphology, and mechanical behavior of the fabricated composites were characterized using various characterization techniques. Furthermore, transmission electron microscopy study revealed a randomly oriented rod-like morphology, with the length and width of these nanorods ranging from 78 to 122 and from 9 to 13 nm. Moreover, the mechanical characterizations of the composite HZBT4 (80HAp-10TiC-5h-BN-5ZrO2) reveal a very high compressive strength (246 MPa), which is comparable to that of the steel (250 MPa), fracture toughness (14.78 MPa m1/2), and Young's modulus (1.02 GPa). In order to check the biocompatibility of the composites, numerous biological tests were also performed on different body organs of healthy adult Sprague-Dawley rats. This study suggests that the composite HZBT4 could not reveal any significant influence on the hematological, serum biochemical, and histopathological parameters. Hence, the fabricated composite can be used for several biological applications, such as bone implants, bone grafting, and bone regeneration.

First principles predictions of structural, electronic and topological properties of two-dimensional Janus Ti2N2XI (X = Br, Cl) structures

Motivated by the report of the giant Rashba effect in ternary layered compounds BiTeX, we consider two Janus structured compounds Ti2N2XI (X = Br, Cl) of the same ternary family exhibiting a 1 : 1 : 1 stoichiometric ratio. Broken inversion symmetry in the Janus structure, together with its unique electronic structure exhibiting anti-crossing states formed between Ti-d states and strong spin-orbit coupled I-p states, generates large Rashba cofficients of 2-3 eV Å for these compounds, classifying them as strong Rashba compounds. The anti-crossing features of the first-principles calculated electronic structure also result in non-trivial topology, combining two quantum phenomena - Rashba effect and non-trivial topology - in the same materials. This makes Janus TiNI compounds candidate materials for two-dimensional composite quantum materials. The situation becomes further promising by the fact that the properties are found to exhibit extreme sensitivity and tunability upon application of uniaxial strain.

Enhanced Thermal Conductivity of Single-Walled Carbon Nanotube with Axial Tensile Strain Enabled by Boron Nitride Nanotube Anchoring

Thermal conductivity measurements are conducted by optothermal Raman technique before and after the introduction of an axial tensile strain in a suspended single-walled carbon nanotube (SWCNT) through end-anchoring by boron nitride nanotubes (BNNTs). Surprisingly, the axial tensile strain (<0.4 %) in SWCNT results in a considerable enhancement of its thermal conductivity, and the larger the strain, the higher the enhancement. Furthermore, the thermal conductivity reduction with temperature is much alleviated for the strained nanotube compared to previously reported unstrained cases. The thermal conductivity of SWCNT increases with its length is also observed.

Ultra-fast switching memristors based on two-dimensional materials

The ability to scale two-dimensional (2D) material thickness down to a single monolayer presents a promising opportunity to realize high-speed energy-efficient memristors. Here, we report an ultra-fast memristor fabricated using atomically thin sheets of 2D hexagonal Boron Nitride, exhibiting the shortest observed switching speed (120 ps) among 2D memristors and low switching energy (2pJ). Furthermore, we study the switching dynamics of these memristors using ultra-short (120ps-3ns) voltage pulses, a frequency range that is highly relevant in the context of modern complementary metal oxide semiconductor (CMOS) circuits. We employ statistical analysis of transient characteristics to gain insights into the memristor switching mechanism. Cycling endurance data confirms the ultra-fast switching capability of these memristors, making them attractive for next generation computing, storage, and Radio-Frequency (RF) circuit applications.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Tunability of electronic and thermoelectric properties of hexagonal boron nitride with carbon impurities under magnetic field: Tight binding investigation

Utilizing the Kubo-Greenwood formula, Tight Binding calculations were employed to examine the electronic and thermoelectric properties of hexagonal boron nitride (h-BN) with carbon impurity instead of boron, nitrogen and pairs boron-nitrogen. The electronic properties of the pristine monolayer BN are markedly impacted by the introduction of carbon dopants and its band gap reduction is directly correlated with the concentration of carbon impurities. The electronic properties of doped h-BN are influenced by the presence of a magnetic field, leading to subband separation and band gap narrowing, independent of the impurity types. The thermal conductivity and magnetic susceptibility of the CBN-doped monolayer BN structure are higher than those of the BC and NC doped h-BN structures and for all structures, their properties have a strong dependence on the magnetic field. The Lorenz Number for all structures has peak at the TM temperature which shifts to a lower temperature as the impurity concentration decreases.

Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe2

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe2 have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe2 with a pentagonal fold structure. The thermal conductivity of PdSe2 flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04 W mk-1 for 60 nm PdSe2 to 34.51 W mk-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe2 becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.

Thermal Conductivity Enhancement of Polymeric Composites Using Hexagonal Boron Nitride: Design Strategies and Challenges

With the integration and miniaturization of chips, there is an increasing demand for improved heat dissipation. However, the low thermal conductivity (TC) of polymers, which are commonly used in chip packaging, has seriously limited the development of chips. To address this limitation, researchers have recently shown considerable interest in incorporating high-TC fillers into polymers to fabricate thermally conductive composites. Hexagonal boron nitride (h-BN) has emerged as a promising filler candidate due to its high-TC and excellent electrical insulation. This review comprehensively outlines the design strategies for using h-BN as a high-TC filler and covers intrinsic TC and morphology effects, functionalization methods, and the construction of three-dimensional (3D) thermal conduction networks. Additionally, it introduces some experimental TC measurement techniques of composites and theoretical computational simulations for composite design. Finally, the review summarizes some effective strategies and possible challenges for the design of h-BN fillers. This review provides researchers in the field of thermally conductive polymeric composites with a comprehensive understanding of thermal conduction and constructive guidance on h-BN design.

Transfer-Free Analog and Digital Flexible Memristors Based on Boron Nitride Films

The traditional von Neumann architecture of computers, constrained by the inherent separation of processing and memory units, faces challenges, for instance, memory wall issue. Neuromorphic computing and in-memory computing offer promising paradigms to overcome the limitations of additional data movement and to enhance computational efficiency. In this work, transfer-free flexible memristors based on hexagonal boron nitride films were proposed for analog neuromorphic and digital memcomputing. Analog memristors were prepared; they exhibited synaptic behaviors, including paired-pulse facilitation and long-term potentiation/depression. The resistive switching mechanism of the analog memristors were investigated through transmission electron microscopy. Digital memristors were prepared by altering the electrode materials, and they exhibited reliable device performance, including a large on/off ratio (up to 106), reproducible switching endurance (>100 cycles), non-volatile characteristic (>60 min), and effective operating under bending conditions (>100 times).

Interface thermal conductivities induced by van der Waals interactions

The interface heat transfer of two layers induced by van der Waals (vdW) contacts is theoretically investigated, based on first-principles calculations at low temperatures. The results suggest that out-of-plane acoustic phonons with low frequencies dominate the interface thermal transport due to the vdW interaction. The interface thermal conductivity is proportional to the cubic of temperature at very low temperatures, but becomes linearly proportional to temperature as temperature increases. We show that manipulating the strain alters vdW coupling, leading to increased interfacial thermal conductivity at the interface. Our findings provide valuable insights into the interface heat transport in vdW heterostructures and support further design and optimization of electronic and optoelectronic nanodevices based on vdW contacts.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Theoretical insights into the structural, electronic and thermoelectric properties of the inorganic biphenylene monolayer

Being motivated by a recently synthesized biphenylene carbon monolayer (BPN), using first principles methods, we have studied its inorganic analogue (B-N analogue) named I-BPN. A comparative study of structural, electronic and mechanical properties between BPN and I-BPN was carried out. Like BPN, the stability of I-BPN was verified in terms of formation energy, phonon dispersion calculations, and mechanical parameters (Young's modulus and Poisson's ratio). The chemical inertness of I-BPN was also investigated by adsorbing an oxygen molecule in an oxygen-rich environment. It has been found that the B-B bond favours the oxygen molecule to be adsorbed through chemisorption. The lattice transport properties reveal that the phonon thermal conductivity of I-BPN is ten times lower than that of BPN. The electronic band structure reveals that I-BPN is a semiconductor with an indirect bandgap of 1.88 eV, while BPN shows metallic behaviour. In addition, we investigated various thermoelectric properties of I-BPN for possible thermoelectric applications. The thermoelectric parameters, such as the Seebeck coefficient, show the highest peak value of 0.00289 V K-1 at 300 K. Electronic transport properties reveal that I-BPN is highly anisotropic along the x and y-axes. Furthermore, the thermoelectric power factor as a function of chemical potential shows a peak value of 0.057 W m-1 K-2 along the x-axis in the p-type doping region. The electronic figure of merit shows a peak value of approximately unity. However, considering lattice thermal conductivity, the peak value of the total figure of merit (ZT) reduces to 0.68(0.46) for p-type and 0.56(0.48) for n-type doping regions along the x(y) direction at 900 K. It is worth noting that our calculated ZT value of I-BPN is higher than that of many other reported B-N composite materials.

A New Laser-Combined H-Type Device Method for Comprehensive Thermoelectrical Properties Characterization of Two-Dimensional Materials

Two-dimensional nanomaterials have obvious advantages in thermoelectric device development. It is rare to use the same experimental system to accurately measure multiple thermoelectrical parameters of the same sample. Therefore, scholars have developed suspended microdevices, T-type and H-type methods to fulfill the abovementioned requirements. These methods usually require a direct-current voltage signal to detect in Seebeck coefficient measurement. However, the thermoelectric potential generated by the finite temperature difference is very weak and can be easily overwritten by the direct-current voltage, thereby affecting the measurement accuracy. In addition, these methods generally require specific electrodes to measure the thermoelectric potential. We propose a measurement method that combines laser heating with an H-type device. By introducing a temperature difference in two-dimensional materials through laser heating, the thermoelectric potential can be accurately measured. This method does not require specific electrodes to simplify the device structure. The thermoelectrical parameters of supported graphene are successfully measured with this method; the results are in good agreement with the literature. The proposed method is unaffected by material size and characteristics. It has potential application value in the characterization of thermoelectric physical properties.

Freeze-casting production of thermal insulating and fire-retardant lightweight aerogels based on nanocellulose and boron nitride

Cellulose aerogels exhibit biocompatibility and biodegradability, rendering them promising candidate for application in building energy conservation and insulation materials. However, the intrinsic inflammability of pristine cellulose aerogel causes unneglectable safety concerns, hindering their application in energy-efficient buildings. Herein, a thermal insulating, fire-retardant, strong, and lightweight aerogel was produced via freeze-casting suspensions of cellulose nanofibril (CNF) and l-glutamine functionalized boron nitride nanosheets (BNNS-g). The aerogel with a BNNS-g:CNF concentration ratio of 15:5 exhibited outstanding mechanical strength owing to the strong interaction between BNNS-g and CNF as well as satisfactory thermal insulating performance (0.052 W/m·K). Particularly, this aerogel showed excellent fire-retardant and self-extinguishing capabilities in the vertical burning test, which remained unscathed after over 60 s of burning in a butane flame. Further, the limit oxygen index (LOI) of this aerogel was 36.0 %, which was better than the LOIs of traditional petrochemical-based insulating materials. This study provides a promising strategy for producing aerogels with excellent properties using cellulose and other inorganic nano-fillers.

Optimizing thermoelectric performance of carbon-doped h-BN monolayers through tuning carrier concentrations and magnetic field

The thermoelectric properties of carbon-doped monolayer hexagonal boron nitride (h-BN) are studied using a tight-binding model employing Green function approach and the Kubo formalism. Accurate tight-binding parameters are obtained to achieve excellent fitting with Density Functional Theory results for doped h-BN structures with impurity type and concentration. The influence of carbon doping on the electronic properties, electrical conductivity, and heat capacity of h-BN is studied, especially under an applied magnetic field. Electronic properties are significantly altered by doping type, concentration, and magnetic field due to subband splitting, merging of adjacent subbands, and band gap reduction. These modifications influence the number, location, and magnitude of DOS peaks, generating extra peaks inside the band gap region. Heat capacity displays pronounced dependence on both magnetic field and impurity concentration, exhibiting higher intensity at lower dopant levels. Electrical conductivity is increased by double carbon doping compared to single doping, but is reduced at high magnetic fields because of high carrier scattering. The electronic figure of merit ZT increases with lower impurity concentration and is higher for CB versus CN doping at a given field strength. The power factor can be improved by increasing magnetic field and decreasing doping concentration. In summary, controlling doping and magnetic field demonstrates the ability to effectively engineer the thermoelectric properties of monolayer h-BN.

Hexagonal boron nitride exfoliation and dispersion

Research on hexagonal boron nitride (hBN) 2-dimensional nanostructures has gained traction due to their unique chemical, thermal, and electronic properties. However, to make use of these exceptional properties and fabricate macroscopic materials, hBN often needs to be exfoliated and dispersed in a solvent. In this review, we provide an overview of the many different methods that have been used for dispersing hBN. The approaches that will be covered in this review include solvents, covalent functionalization, acids and bases, surfactants and polymers, biomolecules, intercalating agents, and thermal expansion. The properties of the exfoliated sheets obtained and the dispersions are discussed, and an overview of the work in the field throughout the years is provided.

Constructing Hierarchical Polymer Nanocomposites with Strongly Enhanced Thermal Conductivity

The rapid advancement of communication technology has substantially increased the demand for advanced electronic packaging materials with high thermal conductivity and outstanding electrical insulation properties. In this study, we design polyvinyl alcohol/polydopamine-modified boron nitride nanosheet (PVA/BNNS@PDA) nanocomposites with hierarchical structures by combining electrospinning, vacuum filtration deposition, and hot pressing. The modified BNNS@PDA improves the interaction between the filler and the polymer matrix while reducing the interfacial thermal resistance, resulting in superior thermal conductivity, excellent insulation, and perfect flexibility. The PVA/BNNS@PDA nanocomposites possess an ultrahigh in-plane thermal conductivity of 16.6 W/(m·K) at 35.54 wt % BNNS@PDA content. Even after 2000 folds, the nanocomposites do not undergo any crack, showing their ultrahigh thermal conductivity behavior. Furthermore, the nanocomposites exhibit a volume resistivity above 1014 Ω·cm, which is well above the standard for insulating materials. Based on these results, this work provides a novel method to produce nanocomposites with high thermal conductivity, offering a new perspective to design advanced thermal management materials.

Bias sputtering of granular L10-FePt films with hexagonal boron nitride grain boundaries

In this paper, we present an experimental study of L10-FePt granular films with crystalline boron nitride (BN) grain boundary materials for heat assisted magnetic recording (HAMR). It is found that application of a RF substrate bias (VDC = -15 V) yields the formation of hexagonal boron nitride (h-BN) nanosheets in grain boundaries, facilitating the columnar growth of FePt grains during sputtering at high temperatures. The h-BN monolayers conform to the side surfaces of columnar FePt grains, completely encircling individual FePt grains. The resulting core-shell FePt-(h-BN) nanostructures appear to be highly promising for HAMR application. The high thermal stability of h-BN grain boundaries allows the deposition temperature to be as high as 650℃ such that high order parameters of FePt L10 phase have been obtained. For the fabricated FePt-(h-BN) thin film, excellent granular microstructure with FePt grains of 6.5 nm in diameter and 11.5 nm in height has been achieved along with good magnetic hysteresis properties.

Centrifuge-Free Separation of Solution-Exfoliated 2D Nanosheets via Cross-Flow Filtration

Solution-processed graphene is a promising material for numerous high-volume applications including structural composites, batteries, sensors, and printed electronics. However, the polydisperse nature of graphene dispersions following liquid-phase exfoliation poses major manufacturing challenges, as incompletely exfoliated graphite flakes must be removed to achieve optimal properties and downstream performance. Incumbent separation schemes rely on centrifugation, which is highly energy-intensive and limits scalable manufacturing. Here, cross-flow filtration (CFF) is introduced as a centrifuge-free processing method that improves the throughput of graphene separation by two orders of magnitude. By tuning membrane pore sizes between microfiltration and ultrafiltration length scales, CFF can also be used for efficient recovery of solvents and stabilizing polymers. In this manner, life cycle assessment and techno-economic analysis reveal that CFF reduces greenhouse gas emissions, fossil energy usage, water consumption, and specific production costs of graphene manufacturing by 57%, 56%, 63%, and 72%, respectively. To confirm that CFF produces electronic-grade graphene, CFF-processed graphene nanosheets are formulated into printable inks, leading to state-of-the-art thin-film conductivities exceeding 10⁴ S m^-1 . This CFF methodology can likely be generalized to other van der Waals layered solids, thus enabling sustainable manufacturing of the diverse set of applications currently being pursued for 2D materials.

Boron Nitride/Polyurethane Composites with Good Thermal Conductivity and Flexibility

Thermal insulating composites are indispensable in electronic applications; however, their poor thermal conductivity and flexibility have become bottlenecks for improving device operations. Hexagonal boron nitride (BN) has excellent thermal conductivity and insulating properties and is an ideal filler for preparing thermally insulating polymer composites. In this study, we report a method to fabricate BN/polyurethane (PU) composites using an improved nonsolvent-induced phase separation method with binary solvents to improve the thermal performance and flexibility of PU. The stress and strain of BN60/PU are 7.52 ± 0.87 MPa and 707.34 ± 38.34%, respectively. As prepared, BN60/PU composites with unordered BN exhibited high thermal conductivity and a volume resistivity of 0.653 W/(m·K) and 23.9 × 10¹² Ω·cm, which are 218.71 and 39.77% higher than that of pure PU, respectively. Moreover, these composite films demonstrated a thermal diffusion ability and maintained good integrity after 1000 bending cycles, demonstrating good mechanical and thermal reliability for practical use. Our findings provide a practical route for the production of flexible materials for efficient thermal management.

Thermal conductivity of ethylene glycol and propylene glycol nanofluids with boron nitride nano-barbs

This study investigates the potential of composite allotrope boron nitride nanobarbs (BNNBs) as nanoparticles for enhancing the thermal conductivity of nanofluids based on mixtures of ethylene glycol and propylene glycol with water. BNNBs are allotrope composites composed of boron nitride nanotube cores with walls decorated with attached hexagonal boron nitride crystals, creating a jagged morphology that facilitates the formation of a connected network and contributes to the enhancement of thermal conductivity in nanofluids. BNNBs exhibit high thermal conductivity due to efficient phonon transfer and they are electrical insulators owing to their wide bandgap. The effect of BNNB concentration in carrier fluids on nanofluid thermal conductivity was investigated by introducing BNNBs into ethylene glycol-water and propylene glycol-water mixtures at 0-10 wt%. The results showed that BNNBs enhanced thermal conductivity of carrier fluids up to 45%, and the enhancement was proportional to the concentration of BNNBs in the carrier fluid. The study also investigated the dispersion stability of BNNBs in different solvents using Hansen Solubility Parameters, revealing that propylene glycol mixtures demonstrated better long-term stability compared to ethylene glycol mixtures. The findings suggest that BNNBs have great potential for use as thermally conductive nanoparticles in nanofluids for various heat transfer applications. Future research should focus on enhancing the dispersion stability of BNNB nanofluids and exploring the influence of BNNB morphology on the thermal conductivity and other thermophysical properties of nanofluids.

Significant Improvement of Thermal Conductivity of Polyamide 6/Boron Nitride Composites by Adding a Small Amount of Stearic Acid

This study investigates the effect of adding stearic acid (SA) on the thermal conductivity of polyamide 6 (PA6)/boron nitride (BN) composites. The composites were prepared by melt blending, and the mass ratio of PA6 to BN was fixed at 50:50. The results show that when the SA content is less than 5 phr, some SA is distributed at the interface between BN sheets and PA6, which improves the interface adhesion of the two phases. This improves the force transfer from the matrix to BN sheets, promoting the exfoliation and dispersion of BN sheets. However, when the SA content was greater than 5 phr, SA tends to aggregate and form separate domains rather than being dispersed at the interface between PA6 and BN. Additionally, the well-dispersed BN sheets act as a heterogeneous nucleation agent, significantly improving the crystallinity of the PA6 matrix. The combination of good interface adhesion, excellent orientation, and high crystallinity of the matrix leads to efficient phonon propagation, resulting in a significant improvement in the thermal conductivity of the composite. The highest thermal conductivity of the composite is achieved when the SA content is 5 phr, which is 3.59 W m^-1 K^-1. The utilization of a composite material consisting of 5phr SA as the thermal interface material displays the highest thermal conductivity, and the composite also demonstrates satisfactory mechanical properties. This study proposes a promising strategy for the preparation of composites with high thermal conductivity.

Thickness-Dependent Cross-Plane Thermal Conductivity Measurements of Exfoliated Hexagonal Boron Nitride

Submicrometer-thick layers of hexagonal boron nitride (hBN) exhibit high in-plane thermal conductivity and useful optical properties, and serve as dielectric encapsulation layers with low electrostatic inhomogeneity for graphene devices. Despite the promising applications of hBN as a heat spreader, the thickness dependence of its cross-plane thermal conductivity is not known, and the cross-plane phonon mean free paths (MFPs) have not been measured. We measure the cross-plane thermal conductivity of hBN flakes exfoliated from bulk crystals. We find that submicrometer thick flakes exhibit thermal conductivities up to 8.1 ± 0.5 W m^-1 K^-1 at 295 K, which exceeds previously reported bulk values by more than 60%. Surprisingly, the average phonon mean free path is found to be several hundred nanometers at room temperature, a factor of 5 larger than previous predictions. When planar twist interfaces are introduced into the crystal by mechanically stacking multiple thin flakes, the cross-plane thermal conductivity of the stack is found to be a factor of 7 below that of individual flakes with similar total thickness, thus providing strong evidence that phonon scattering at twist boundaries limits the maximum phonon MFPs. These results have important implications for hBN integration in nanoelectronics and improve our understanding of thermal transport in two-dimensional materials.

Ultra-High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂ , WSe₂ , WS₂ ) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX₂ /hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂ and hBN reaches 74 ± 25 MW m^-2 K^-1 , which is at least ten times higher than the interfacial thermal conductance of MX₂ in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂ /hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.

Low-Temperature Direct Synthesis of Multilayered h-BN without Catalysts by Inductively Coupled Plasma-Enhanced Chemical Vapor Deposition

Low-temperature direct synthesis of thick multilayered hexagonal-boron nitride (h-BN) on semiconducting and insulating substrates is required to produce high-performance electronic devices based on two-dimensional (2D) materials. In this study, multilayered h-BN with a thickness exceeding 5 nm was directly synthesized on quartz and Si at low temperatures, between 400 and 500 °C, by inductively coupled plasma-enhanced chemical vapor deposition using borazine as the precursor material. The quality and thickness of the h-BN crystals were investigated with respect to synthesis parameters, namely, temperature, radio frequency power, N₂ flow rate, and H₂ flow rate. Introducing N₂ and H₂ carrier gases critically affected the deposition rate, and increasing the carrier gas flow rate enhanced the h-BN crystal quality. The typical optical band gap of synthesized h-BN was approximately 5.8 eV, consistent with that of previous studies. The full width at half-maximum of the h-BN Raman peak was 32-33 cm^-1, comparable to that of commercially available multilayered h-BN on Cu foil. These results are expected to facilitate the development of 2D materials for electronics applications.

Environmentally sustainable implementations of two-dimensional nanomaterials

Rapid advancement in nanotechnology has led to the development of a myriad of useful nanomaterials that have novel characteristics resulting from their small size and engineered properties. In particular, two-dimensional (2D) materials have become a major focus in material science and chemistry research worldwide with substantial efforts centered on their synthesis, property characterization, and technological, and environmental applications. Environmental applications of these nanomaterials include but are not limited to adsorbents for wastewater and drinking water treatment, membranes for desalination, and coating materials for filtration. However, it is also important to address the environmental interactions and implications of these nanomaterials in order to develop strategies that minimize their environmental and public health risks. Towards this end, this review covers the most recent literature on the environmental implementations of emerging 2D nanomaterials, thereby providing insights into the future of this fast-evolving field including strategies for ensuring sustainable development of 2D nanomaterials.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Thermal Transport in 2D Materials

In recent decades, two-dimensional materials (2D) such as graphene, black and blue phosphorenes, transition metal dichalcogenides (e.g., WS₂ and MoS₂), and h-BN have received illustrious consideration due to their promising properties. Increasingly, nanomaterial thermal properties have become a topic of research. Since nanodevices have to constantly be further miniaturized, thermal dissipation at the nanoscale has become one of the key issues in the nanotechnology field. Different techniques have been developed to measure the thermal conductivity of nanomaterials. A brief review of 2D material developments, thermal conductivity concepts, simulation methods, and recent research in heat conduction measurements is presented. Finally, recent research progress is summarized in this article.

BN-PVDF/rGO-PVDF Laminate Nanocomposites for Energy Storage Applications

The increasing demand for high energy storage devices calls for concurrently enhanced dielectric constants and reduced dielectric losses of polymer dielectrics. In this work, we rationally design dielectric composites comprising aligned 2D nanofillers of reduced graphene oxide (rGO) and boron nitride nanosheets (BNNS) in a polyvinylidene fluoride (PVDF) matrix through a novel press-and-fold technique. Both nanofillers play different yet complementary roles: while rGO is designed to enhance the dielectric constant through charge accumulation at the interfaces with polymer, BNNS suppress the dielectric loss by preventing the mobility of free electrons. The microlaminate containing eight layers each of rGO/PVDF and BNNS/PVDF films exhibits remarkable dielectric performance with a dielectric constant of 147 and an ultralow dielectric loss of 0.075, due to the synergistic effect arising from the alternatingly electrically conductive and insulating films. Consequently, a maximum energy density of 3.5 J/cm³-about 18 times the bilayer composite counterpart-is realized. The high thermal conductivities of both nanofillers and their alignment endow the microlaminate with an excellent in-plane thermal conductivity of 6.53 Wm^-1K^-1, potentially useful for multifunctional applications. This work offers a simple but effective approach to fabricating a composite for high dielectric energy storage using two different 2D nanofillers.

Electrical Gating of the Charge-Density-Wave Phases in Two-Dimensional h-BN/1T-TaS2 Devices

We report on the electrical gating of the charge-density-wave phases and current in h-BN-capped three-terminal 1T-TaS₂ heterostructure devices. It is demonstrated that the application of a gate bias can shift the source-drain current-voltage hysteresis associated with the transition between the nearly commensurate and incommensurate charge-density-wave phases. The evolution of the hysteresis and the presence of abrupt spikes in the current while sweeping the gate voltage suggest that the effect is electrical rather than self-heating. We attribute the gating to an electric-field effect on the commensurate charge-density-wave domains in the atomic planes near the gate dielectric. The transition between the nearly commensurate and incommensurate charge-density-wave phases can be induced by both the source-drain current and the electrostatic gate. Since the charge-density-wave phases are persistent in 1T-TaS₂ at room temperature, one can envision memory applications of such devices when scaled down to the dimensions of individual commensurate domains and few-atomic plane thicknesses.

Microstructured BN Composites with Internally Designed High Thermal Conductivity Paths for 3D Electronic Packaging

Miniaturized and high-power-density 3D electronic devices pose new challenges on thermal management. Indeed, prompt heat dissipation in electrically insulating packaging is currently limited by the thermal conductivity achieved by thermal interface materials (TIMs) and by their capability to direct the heat toward heat sinks. Here, high thermal conductivity boron nitride (BN)-based composites that are able to conduct heat intentionally toward specific areas by locally orienting magnetically functionalized BN microplatelets are created using magnetically assisted slip casting. The obtained thermal conductivity along the direction of alignment is unusually high, up to 12.1 W m^-1 K^-1 , thanks to the high concentration of 62.6 vol% of BN in the composite, the low concentration in polymeric binder, and the high degree of alignment. The BN composites have a low density of 1.3 g cm^-3 , a high stiffness of 442.3 MPa, and are electrically insulating. Uniquely, the approach is demonstrated with proof-of-concept composites having locally graded orientations of BN microplatelets to direct the heat away from two vertically stacked heat sources. Rationally designing the microstructure of TIMs to direct heat strategically provides a promising solution for efficient thermal management in 3D integrated electronics.

Direct growth of h-BN multilayers with controlled thickness on non-crystalline dielectric substrates without metal catalysts

We report an rGO-assisted CVD approach that enables the direct growth of high-quality single crystalline h-BN films with adjustable thickness and layered order on amorphous quartz and SiO₂/Si substrates at relatively low temperatures. This work demonstrates a viable prototype for growing continuous ultrathin h-BN films on desired substrates without the requirement of lattice orientation, thus offering a great opportunity for their appealing applications.

Magnetically assisted drop-on-demand 3D printing of microstructured multimaterial composites

Microstructured composites with hierarchically arranged fillers fabricated by three-dimensional (3D) printing show enhanced properties along the fillers’ alignment direction. However, it is still challenging to achieve good control of the filler arrangement and high filler concentration simultaneously, which limits the printed material’s properties. In this study, we develop a magnetically assisted drop-on-demand 3D printing technique (MDOD) to print aligned microplatelet reinforced composites. By performing drop-on-demand printing using aqueous slurry inks while applying an external magnetic field, MDOD can print composites with microplatelet fillers aligned at set angles with high filler concentrations up to 50 vol%. Moreover, MDOD allows multimaterial printing with voxelated control. We showcase the capabilities of MDOD by printing multimaterial piezoresistive sensors with tunable performances based on the local microstructure and composition. MDOD thus creates a large design space to enhance the mechanical and functional properties of 3D printed electronic or sensing devices using a wide range of materials.

3D printed composites with hierarchically arranged fillers have been challenging to fabricate. Here, the authors make use of magnetically assisted droplet-based printing to 3D print voxelated structures with high filler content, localized control of filler material, and orientation.

Near-Isotropic Polariton Heat Transport along a Polar Anisotropic Nanofilm

The heat transport of surface phonon-polaritons propagating along a polar uniaxial anisotropic nanofilm is studied for different orientations of its optical axis, film thicknesses, and temperatures. For an hBN nanofilm, it is shown that i) the propagation of polaritons can be described in terms of even and odd modes that generalize the transverse magnetic and transverse electrical ones that typically appear in isotropic films. ii) The frequency spectrum of polaritons can efficiently be tuned with the angle between the film optical axis and their propagation direction. iii) The polariton thermal conductivity takes higher values for a thinner or hotter nanofilm. iv) The even and odd modes have a remarkable contribution to the total polariton thermal conductivity, which takes a value higher than 5.6 Wm⁻¹K⁻¹ for a 25-nm-thick nanofilm at 500 K. The obtained results thus uncover some key features of the propagation and heat transport of polaritons in uniaxial nanofilms.

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Polariton thermal conductivity of a uniaxial anisotropic nanofilm

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Polariton heat transport driven by the optical axis of a polar anisotropic nanofilm

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Higher thermal conductivities for thinner or hotter nanofilms

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Near-isotropic thermal response of a strongly anisotropic nanofilm

Failure modes and mechanisms of layered h-BN under local energy injection

Layered h-BN may serve as an important dielectric and thermal management material in the next-generation nanoelectronics, in which its interactions with electron beam play an important role in device performance and reliability. Previous studies report variations in the failure strength and mode. In this study, using molecular dynamics simulations, we study the effect of local heat injection due to the electron beam and h-BN interaction on the failure start time and failure mode. It is found that at the same heat injection rate, the failure start time decreases with the increase in the layer number. With the introduction of point defects in the heating zone, the failure always starts from the defect site, and the start time can be significantly shortened. For monolayer h-BN, failure always starts within the layer, and once failure starts, its propagation is through melting or vaporization of the h-BN atoms, and no swelling occurs. For multiple layers, once failure starts within the h-BN film, swelling occurs first. With continued heating, the large pressure induced by melting and vaporization can cause the burst of the layers above, leading to the formation of a pit. In the presence of multiple defects within the heating zone, these defects can interact, causing a further reduction in the failure start time. We also reveal the relation of beam power with layer-by-layer failure mode and swelling/pit formation mode. The present work not only reproduces many interesting experimental observations, but also reveal several interesting mechanisms responsible for the failure processes and modes. It is expected that the findings revealed here may provide useful references for the design and engineering of h-BN for device applications.

Thermally Conductive and Electrically Insulated Silicone Rubber Composites Incorporated with Boron Nitride−Multilayer Graphene Hybrid Nanofiller

Thermally conductive and electrically insulating composites are important for the thermal management of new generation integrated and miniaturized electronic devices. A practical and eco−friendly electrostatic self−assembly method was developed to prepare boron nitride−multilayer graphene (BN−MG) hybrid nanosheets. Then, BN−MG was filled into silicone rubber (SR) to fabricate BN−MG/SR composites. Compared with MG/SR composites with the same filler loadings, BN−MG/SR composites exhibit dramatically enhanced electrical insulation properties while still maintaining excellent thermal conductivity. The BN−MG/SR with 10 wt.% filler loading shows a thermal conductivity of 0.69 W·m⁻¹·K⁻¹, which is 475% higher than that of SR (0.12 W·m⁻¹·K⁻¹) and only 9.2% lower than that of MG/SR (0.76 W·m⁻¹·K⁻¹). More importantly, owing to the electron blocking effect of BN, the electron transport among MG sheets is greatly decreased, thus contributing to the high−volume resistivity of 4 × 10¹¹ Ω cm for BN−MG/SR (10 wt.%), which is fourorders higher than that of MG/SR (2 × 10⁷ Ω·cm). The development of BN−MG/SR composites with synergetic properties of high thermal conductivity and satisfactory electrical insulation is supposed to be a promising candidate for practical application in the electronic packaging field.

Significant enhancement of lattice thermal conductivity of monolayer AlN under bi-axial strain: a first principles study

Using rigorous ab initio calculations within the framework of phonon Boltzmann transport theory, we have carried out a detailed investigation to probe the effects of uniform bi-axial strain and finite size on the lattice thermal conductivity (κ) of monolayer AlN. We show that implementation of bi-axial tensile strain can shoot up the value of κ of monolayer AlN by a large amount unlike in the case of analogous 2D materials. The value of κ for monolayer AlN is calculated to be 306.5 W m^-1 K^-1 at room temperature (300 K). The value of κ can be raised by one order of magnitude to up to 1500.9 W m^-1 K^-1 at 300 K by applying a bi-axial strain of about 5%. A similar trend persists when the finite size effect is incorporated in the calculation. As the sample size is varied from 10 nm to 10 000 nm along with increasing tensile strain, a huge variation of κ (from 20.7 W m^-1 K^-1 to 558.9 W m^-1 K^-1) is observed. Our study reveals that the major part of the lattice thermal conductivity of monolayer AlN comes from the contribution of the flexural acoustic (ZA) phonon modes. The anomalous trend of drastic increment in the value of κ with tensile strain is found to be a direct effect of interaction between nitrogen lone-pair electrons and bonding electrons in the ionic lattice which results in the reduction of phonon anharmonicity with increasing tensile strain. Our study provides a detailed analysis of the strain modulated and size-tuned thermal transport properties of monolayer AlN revealing that it is an impactful 2D material to be used in thermal management devices.

Efficient Preconstruction of Three-Dimensional Graphene Networks for Thermally Conductive Polymer Composites

Electronic devices generate heat during operation and require efficient thermal management to extend the lifetime and prevent performance degradation. Featured by its exceptional thermal conductivity, graphene is an ideal functional filler for fabricating thermally conductive polymer composites to provide efficient thermal management. Extensive studies have been focusing on constructing graphene networks in polymer composites to achieve high thermal conductivities. Compared with conventional composite fabrications by directly mixing graphene with polymers, preconstruction of three-dimensional graphene networks followed by backfilling polymers represents a promising way to produce composites with higher performances, enabling high manufacturing flexibility and controllability. In this review, we first summarize the factors that affect thermal conductivity of graphene composites and strategies for fabricating highly thermally conductive graphene/polymer composites. Subsequently, we give the reasoning behind using preconstructed three-dimensional graphene networks for fabricating thermally conductive polymer composites and highlight their potential applications. Finally, our insight into the existing bottlenecks and opportunities is provided for developing preconstructed porous architectures of graphene and their thermally conductive composites.

Low-Temperature Synthesis of Boron Nitride as a Large-Scale Passivation and Protection Layer for Two-Dimensional Materials and High-Performance Devices

Two-dimensional materials (2DMs) with extraordinary electronic and optical properties have attracted great interest in optoelectronic applications. Due to their atomically thin feature, 2DM-based devices are generally sensitive to oxygen and moisture in ambient air, and thus, practical application of durable 2DM-based devices remains challenging. Here, we report a novel strategy to directly synthesize amorphous BN film on various 2DMs and field-effect transistor (FET) devices at low temperatures by conventional chemical vapor deposition. The wafer-scale BN film with controllable thickness serves as a passivation and heat dissipation layer, further improving the long-term stability, the resistance to laser irradiation, and the antioxidation performance of the underneath 2DMs. In particular, the BN capping layer could be deposited directly on a WSe₂ FET at low temperature to achieve a clean and conformal interface. The high performance of the BN-capped WSe₂ device is realized with suppressed current fluctuations and 10-fold enhanced carrier mobility. The transfer-free amorphous BN synthesis technique is simple and applicable to various 2DMs grown on arbitrary substrates, which shows great potential for applications in future two-dimensional electronics.

Recent advances in thermally conductive polymer composites

Polymer matrix composites (PMCs) with high thermal conductivity (TC) play an important role in improving the heat dissipation capacity of a new generation of electronic devices, particularly for 5G and aviation applications. Over the last few decades, considerable efforts have been made in the fabrication of highly thermally conductive PMCs. Advances in the thermal conduction mechanism of polymer composites are induced to, and then commonly used thermally conductive fillers are presented. In the following, the factors affecting the TC of polymer composites are discussed in detail, including fillers, interfaces, polymer matrices and processing technologies. Special attention is paid to the thermally conductive fillers. Then, some application areas of thermally conductive polymer composites are introduced. Finally, the deficiencies and future development trends in this research field are put forward. It is expected that this review will provide some beneficial inspiration in improving the TC of PMCs.

Growth of 2D Materials at the Wafer Scale

Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered 2D materials in high-end electronics and optoelectronics. Most vdW 2D materials are initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. This review briefly introduces several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer scale are reviewed in depth. In particular, three strategies are summarized that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy is reviewed. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

Characteristic and Synthesis of High-Temperature Resistant Liquid Crystal Epoxy Resin Containing Boron Nitride Composite

Five liquid crystal epoxy resins and composites containing flat boron nitride (f-BN) and spherical boron nitride (s-BN) were successfully synthesized. The chemical structures, crystal diffraction, and thermal conductivity of the liquid crystal (LC) epoxy composites were measured using Nuclear Magnetic Resonance (NMR), Differential Scanning Calorimetry (DSC), X-ray, and Discovery Xenon Flash. In this study, the molecular arrangement of five LC epoxy resins and the thermal conductivity of their composites were carefully discussed. Several different amounts of flat boron nitride and spherical boron nitride were added to the five LC epoxy resins. The influence of nano-scale ceramic materials, f-BN, and s-BN, on the thermal conductivity of the LC epoxy resins, was studied. It is worth noting that the thermal conductivity of the spherical boron nitride composite demonstrated a better result than that of the flat boron nitride composite. In simpler terms, the thermal conductivity of the composites is closely related to the molecular arrangement of the LC resin and the amount of BN added. The results demonstrate that the SBPDAE/s-BN (60%) composite shows the highest thermal conductivity of 9.36 W/mK in the vertical direction. These data prove that the LC alignment of the matrix will greatly enhance the thermal conductivity of the composites.