Thermal Properties Of Graphene Research Articles

Thermal Properties of Graphene and Graphene-Based Nanocomposites: A Review

Thermal Properties of Graphene and Graphene-Based Nanocomposites: A Review

Investigating the initial pressure effect on Brownian displacement, thermophoresis, and thermal properties of graphene/ water nanofluid by molecular dynamics simulation

The concept of nanofluid includes suspensions containing nanoparticles, metallic and non-metallic materials. Nanofluids have many potentials in different environments and conditions that make them exist in industries and food industries. Considering their high thermal conductivity, the nanoparticles increased the fluid’s thermal conductivity, one of the basic heat transfer parameters, when distributed in the base fluid. The present research investigated the thermal properties, Brownian motion, and thermophoresis of water/ graphene nanofluid affected by different ratios of initial pressure (1, 2, 3 and 5 bar) by molecular dynamics simulation. This study reported the changes in heat flux, thermal conductivity, average Brownian displacement, and thermophoresis. The results depict that by increasing the initial pressure from 1 to 5 bar, average Brownian displacement and thermophoresis values decrease from 06.3 and 23.88 to 2.91 and 23.53 Å, respectively. Also, by raising the initial pressure (1 to 5 bar), the heat flux and thermal conductivity after 10 ns decrease from 39.54 and 0.36 to 35.12 W/m2 and 0.28 W/m.K, and the maximum temperature reduces from 1415 K to 1033 K. These results can be useful in different industries, especially for improving the thermal properties of different nanofluids.

Emerging Trends in Thermal Properties of Graphene

We go over the thermal characteristics of graphene, few-layer graphene, and graphene nanoribbons, and we go over some real-world uses for graphene in energy storage and thermal control. The first section of the paper discusses the most recent developments in the graphene thermal area with an emphasis on the experimental and theoretical data on heat conduction in graphene and graphene nanoribbons that have recently been published. In the summary, the effects of the sample's size, shape, quality, strain distribution, isotope composition, and point-defect concentration are discussed. The thermal characteristics of materials used in energy storage that have been increased by graphene are described in the second section of the review. It has been established that the usage of liquid-phase-exfoliated graphene as a filler in phase change materials has promise for the thermal control of high-power density battery parks. The experimental and modelling findings have been disclosed.

Probing elastic properties of graphene and heat conduction in graphene bubbles above 1000∘C

The elastic and thermal properties of graphene were investigated by illuminating graphen bubbles with a laser spot. Tempertures above $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ were obtained in large ($>10\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{m}$) graphene bubbles. The formation of standing optical waves lead to laser heating depending on the height of the garphene bubble, which results in Raman band oscillations when scanning the laser spot across the bubble. The profile of the bubble under laser illumination can be deduced from the Raman G band oscillations. A distinct swelling at the center of the bubble is observed which is attributed to the strong softening of graphene above $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. From the size and height of the swelling it is deduced that the elastic modulus is reduced by at least $40%$ at $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. On solving the heat equation for the heat dissipation through the graphene only, analytical expressions are obtained for the isotherms on the bubble for both symmetrical and asymmetrical positions of the laser spot. It was found that a large fraction of the absorbed heat dissipates through the gas in the bubble using the finite volume method. Analytical expressions for the temperature distribution in the bubble are deduced from the numerical results. Heat conduction through the gas in the bubble influences the temperature distribution and needs to be taken into account in the heat dissipation in graphene bubbles.

A review on graphene oxide: 2D colloidal molecule, fluid physics, and macroscopic materials

Graphene oxide (GO), a mostly known oxidized derivative of graphene, which possesses two-dimensional (2D) topological nature and good dispersity in multiple common solvents as a single layer, has shown unique molecular science and fluid physics. Assembling 2D GO macromolecules into a variety of carbonaceous architectures is recognized as an important nanotechnology to address the challenge of translating the unprecedented mechanical, electrical, and thermal properties of graphene into a macroscopic level. To realize real-world applications of graphene-based materials, sophisticated architecture manipulation spanning from the nanoscale, mesoscale to macroscale is essential to make sure every atom is at the right place. It takes comprehensive understanding of the compositional chemistry, fluid physics, and solid-state physics of 2D GO and graphene. Much effort in studying the graphene solid-state materials has helped people build perspectives on their structure-property relations. Nevertheless, the molecular science and fluid physics of GO that governs the single molecular behavior and collective effects of sheets still lack exploration. Single GO sheet exhibits both colloid behaviors and molecule conformations, which can be viewed as a 2D colloidal macromolecule with special dynamic aggregate and transition behaviors in solvents. Focusing on this topic, we have summarized recent progress in the science, technology, and engineering of 2D GO colloidal macromolecules with particular focus on intriguing features of molecular conformation, lyotropic liquid crystal, slow relaxation behavior, reversible fusion and fission, etc. Novel solvation-triggered hydroplastic processing for graphene-based macroscopic materials will be introduced, followed by the structural principles for high-performance graphene macroscopic materials. Finally, we will wrap up the topic with some perspectives on future research directions and give our opinions on the roadmap toward graphene industrialization.

Manipulation of mechanical and thermal properties of graphene oxide/nanoclay/unsaturated polyester hybrid nanocomposites by the surface chemistry and nanofiller composition

Dispersion of platelet-like nanominerals in low-viscous resins is a serious challenge; which arises from the strong interactions among the large surface area of the layers. Herein, we improved the dispersion of graphene oxide (GO) within unsaturated polyester (UP) resin and synergistically enhanced the mechanical and thermal properties of the resulting nanocomposites. In a multi-stage manner, GO was incorporated into a novel hybrid structure before adding to the polymer matrix alongside with benefited from the advantage of surface functionalization. Functional hybrid nanofiller skeletons comprising GO and nanoclay (NC) were designed and prepared at different composition ratios (25:75, 50:50 and 75:25) and different surface chemistries via modification with two thermal-resistant silane coupling agents, namely (3-Aminopropyl) triethoxysilane (APTES) and 3-(trimethoxysilyl) propyl methacrylate (MPS). The optimal loading of different hybrid structures into UP has been examined by applying three loading levels (0.25, 0.50 and 1.00 wt%). The results indicated the synergistic effects of GO and NC on the mechanical properties of UP, where the composition ratio of the hybrid constituents played a key role. Moreover, it was demonstrated that the range of composition ratio for the realization of synergistic reinforcement is strongly affected by the surface chemistry of the GO and NC. For UP matrix reinforced with APTES-modified hybrid nanofillers of GD:CD at 50:50 w/w composition, a higher tensile modulus compared to blank UP (≈+21.5%) was obtained due to boosted dispersion of nanoplatelets in UP matrix. The results of DMTA and TGA analyses suggested that there are two different UP/GO and UP/NC interfacial interactions in the composites containing unmodified hybrid nanofillers. However, after modification of hybrid nanofillers, the homogeneous dispersion of nanoplatelets boosted the interfacial interactions between nanofillers and polymer, such that higher properties were obtained.

Correlation between Orbital Hybridizations, Phonon Spectra, and Thermal Properties of Graphene, Germanene, and Plumbene

Based on first‐principles calculations within the harmonic approximation, the phonon properties and specific heat capacities of 2D honeycomb lattices are investigated. Graphene, germanene, and plumbene manifest interesting phonon properties. The similarities and differences in their nature, for example, the total phonon bandwidth, low‐frequency phonon dispersion, and phonon bandgap mostly arise from the significant orbital interactions. These features are reflected in the phonon branch‐dependent specific heat capacity. The information obtained in this work about the relative contributions of distinct phonons to different temperature ranges is crucial for energy storage and heat management applications.

Characterization of graphene reinforced 3C-SiC composite as a metal-free friction material using molecular dynamics simulation

In this work, the tribological, mechanical and thermal properties of graphene (Gr)-based 3C-SiC nanocomposite as a metal-free friction material were first studied by using molecular dynamics (MD) simulations. In tribological properties, the frictional force, normal force, effective coefficient of friction and wear rate were investigated. The effects of scratching depth, temperature, scratching velocity and the number of Gr layers in the composite to the friction process were considered. Young’s modulus, ultimate tensile stress (UTS) and failure strain in the mechanical properties, and thermal conductivity in the thermal property were also included to understand the tribological performances of the composite. The interfacial interaction energy was evaluated to characterize the effect of interface on material properties. The validations were performed on some representative results. In the simulation results with the selected configurations which are listed in the subsequent content, the nanocomposite showed the lowest wear rate with an appreciable coefficient of friction at the scratching depth of 10 Å. 10 m/s was the optimal scratching speed to maintain the best wear performance. A temperature of 300 K yielded the highest coefficient of friction and the lowest wear rate. The model with three Gr layers showed a relatively high coefficient of friction and a relatively low wear rate. The model with five Gr layers stacked in 3C-SiC exhibited the best mechanical performance. The nanowire model with two Gr layers showed a higher thermal conductivity. The research findings will provide guidance in design and manufacturing of Gr/3C-SiC friction materials. The study will also help understand the effect of Gr as a potential reinforcement for metal-free friction materials.

Effects of interfaces and ordered microstructures on thermal properties of graphene flakes/polyethylene composites

AbstractThe lightweight type IV hydrogen storage tanks put forward higher requirements for the thermal properties of the polymer liners. To improve the thermal properties, graphene flakes (GFs) were used as fillers to reinforce high‐density polyethylene (HDPE). The effects of the interfaces between GFs and polymer matrix and the ordered microstructures on the thermal properties of composites were discussed. The results showed that the ordered arrangement of the composite microstructures can significantly improve its thermal conductivity (kc). At 0.5 wt% GFs loading, the ordered composite kc reached 6.052 W/(m K), corresponding to 11.8 times that of unoriented pure HDPE. The effect of interfaces on kc was qualitatively analyzed by scratching GFs on the cross‐section of the composites. Although GFs form continuous networks with high kc in the polymer matrix, the interface defects between GFs and HDPE matrix lead to increased interfacial thermal resistance at high GFs concentrations. Therefore, the thermal enhancement coefficient of the composites decreases with the increase in GFs concentration. Moreover, mixing GFs can increase the initial thermal degradation temperature of the composites by 15.9 °C and the Vicat softening point by 1.6 °C. This work will provide an essential reference for the application of polymers in type IV tanks.

Thermodynamics of Graphene beyond the Linear Approximation

Abstract In this paper, we study the thermodynamic correction due to the deformed Dirac equation in a homogeneous magnetic field. We study the deformation of both the massive and massless Dirac equation. The deformation will occur from generalized uncertainty principle, which occurs due to a minimal length. As the massless Dirac equation in (2+1) gives the behaviour of electrons in systems like sheet of graphene, this deformed Dirac equation will be used to analyze the thermal properties of graphene in a magnetic eld. Due to the existence of minimal length the degeneracy of systems gets modied, as there exist states which vanish when minimal length considerations are ignored. Thus, we will also analyze the implication of this additional degeneracy due to these states to the thermodynamics of the systems. In the case of graphene the minimal length or zero point length is the inter-atomic distance, which occurs by going beyond the linear regime of graphene.

Grafting Ink for Direct Writing: Solvation Activated Covalent Functionalization of Graphene.

Covalent functionalization of graphene (CFG) has shown attractive advantages in tuning the electronic, mechanical, optical, and thermal properties of graphene. However, facile, large‐scale, controllable, and highly efficient CFG remains challenging and often involves highly reactive and volatile compounds, requiring complex control of the reaction conditions. Here, a diazonium‐based grafting ink consisting of only two components, i.e., an aryl diazonium salt and the solvent dimethyl sulfoxide (DMSO) is presented. The efficient functionalization is attributed to the combination of the solvation of the diazonium cations by DMSO and n‐doping of graphene by DMSO, thereby promoting electron transfer (ET) from graphene to the diazonium cations, resulting in the generation of aryl radicals which subsequently react with the graphene. The grafting density of CFG is controlled by the reaction time and very high levels of functionalization, up to the failing of the Tuinstra–Koenig (T–K) relation, while the functionalization layer remains at monolayer height. The grafting ink, effective for days at room temperature, can be used at ambient conditions and renders the patterning CFG by direct writing as easy as writing on paper. In combination with thermal sample treatment, reversible functionalization is possible by subsequent writing/erasing cycles.

Carbon-Based Micro/Nano Devices for Transistors, Sensors, and Memories

The ballistic transport of electrons and unique structural characteristics of graphene and carbon nanotubes enable them to play an important role in nano electronical appliances. Nanodevices based on carbon nano materials can further reduce device size without affecting performance. Here, this paper analyzes Fin Field-effect transistor (FinFET) and Tunnel Field-effect transistor (TFET) based on graphene nanoribbon (GNR) and carbon nanotube which could be used for reducing power consumption. Then it summarizes the applications of graphene in micro/nano sensors based on the electrical, mechanical, optical, and thermal properties of graphene. Graphene’s single-atom thickness and charge storage mechanism provide itself with great potential in the field of resistive memory. Graphene is also widely used in flexible electronic devices.

Computational prediction of electrical and thermal properties of graphene and BaTiO3 reinforced epoxy nanocomposites

Graphene based materials e.g., graphene oxide (GO), reduced graphene oxide (RGO) and graphene nano platelets (GNP) as well as Barium titanate (BaTiO3) are emerging reinforcing agents which upon mixing with epoxy provides composite materials with superior mechanical, electrical and thermal properties as well as shielding against electromagnetic (EM) radiations. Inclusion of the aforementioned reinforcing agents has shown to improve the performance, however, the extent of improvement has remained uncertain. In this study, a computational modelling approach was adopted using COMSOL Multiphysics software in conjunction with Bayesian statistical analysis to investigate the effects of including various filler materials e.g. GO, RGO, GNP and BaTiO3 in influencing the direct current (DC) conductivity (σ), dielectric constant (ε) and thermal properties on the resulting epoxy polymer matrix composites. The simulation of epoxy composites were performed for different volume percentage of the filler materials by varying the geometry of the filler material. It was observed that the content of GO, RGO, GNPs and the thickness of graphene nanoplatelets can alter the DC conductivity, dielectric constant, and thermal properties of the epoxy matrix. The lower thickness of GNPs was found to offer the larger value of DC conductivity, thermal conductivity and thermal diffusivity than rest of the graphene nanocomposites, while, the RGO showed better dielectric constant value than neat epoxy, and GO, GNP nanocomposites. Similarly, BaTiO3 nanoparticles content and diameter were observed to alter the dielectric constant, DC conductivity and thermal properties of modified epoxy in several order magnitude than neat epoxy. In this way, the higher diameter particles of BaTiO3 showed better DC conductivity properties, dielectric constant value, thermal conductivity and thermal diffusivity. Moreover, this research provides guidance for further computational examination on the selection of GNP and BaTiO3 materials for the enhancement of the electrical and thermal properties of the epoxy matrix.

Thermogravimetric Analysis (TGA) of Graphene Materials: Effect of Particle Size of Graphene, Graphene Oxide and Graphite on Thermal Parameters

Thermogravimetric analysis (TGA) has been recognized as a simple and reliable analytical tool for characterization of industrially manufactured graphene powders. Thermal properties of graphene are dependent on many parameters such as particle size, number of layers, defects and presence of oxygen groups to improve the reliability of this method for quality control of graphene materials, therefore it is important to explore the influence of these parameters. This paper presents a comprehensive TGA study to determine the influence of different particle size of the three key materials including graphene, graphene oxide and graphite on their thermal parameters such as carbon decomposition range and its temperature of maximum mass change rate (Tmax). Results showed that Tmax values derived from the TGA-DTG carbon combustion peaks of these materials increasing from GO (558–616 °C), to graphene (659–713 °C) and followed by graphite (841–949 °C) The Tmax values derived from their respective DTG carbon combustion peaks increased as their particle size increased (28.6–120.2 µm for GO, 7.6–73.4 for graphene and 24.2–148.8 µm for graphite). The linear relationship between the Tmax values and the particle size of graphene and their key impurities (graphite and GO) confirmed in this study endows the use of TGA technique with more confidence to evaluate bulk graphene-related materials (GRMs) at low-cost, rapid, reliable and simple diagnostic tool for improved quality control of industrially manufactured GRMs including detection of “fake” graphene.

Microstructure, mechanical, and thermal properties of graphene and carbon nanotube-reinforced Al2O3 nanocomposites

Microstructure, mechanical, and thermal properties of graphene and carbon nanotube-reinforced Al2O3 nanocomposites

Analysis of Heat Transfer Characteristics of a GnP Aqueous Nanofluid through a Double-Tube Heat Exchanger.

The thermal properties of graphene have proved to be exceptional and are partly maintained in its multi-layered form, graphene nanoplatelets (GnP). Since these carbon-based nanostructures are hydrophobic, functionalization is needed in order to assess their long-term stability in aqueous suspensions. In this study, the convective heat transfer performance of a polycarboxylate chemically modified GnP dispersion in water at 0.50 wt% is experimentally analyzed. After designing the nanofluid, dynamic viscosity, thermal conductivity, isobaric heat capacity and density are measured using rotational rheometry, the transient hot-wire technique, differential scanning calorimetry and vibrating U-tube methods, respectively, in a wide temperature range. The whole analysis of thermophysical and rheological properties is validated by two laboratories. Afterward, an experimental facility is used to evaluate the heat transfer performance in a turbulent regime. Convective heat transfer coefficients are obtained using the thermal resistances method, reaching enhancements for the nanofluid of up to 13%. The reported improvements are achieved without clear enhancements in the nanofluid thermal conductivity. Finally, dimensionless analyses are carried out by employing the Nusselt and Péclet numbers and Darcy friction factor.

Study of Thermal Expansion Coefficient of Graphene via Raman Micro-Spectroscopy: Revisited.

The thermal expansion coefficient (TEC) of a 2D material is a fundamental parameter for both material property and applications. A joint study is hereby reported, using Raman microspectroscopy and molecular dynamics (MD) simulations, of the substrate effects on thermal properties of graphene. It is found that besides the lateral strain induced by the substrate, out-of-plane coupling strongly affects the temperature-dependent vibrational modes and TEC of graphene. MD simulation shows significant reduction of the density of states for longer wavelength out-of-plane vibrations when the graphene is supported on an alkane substrate. The negative TEC of freestanding graphene becomes smaller when out-of-plane rippling is suppressed. In order to measure TEC of 2D materials with the out-of-plane coupling being taken into consideration, a Raman microspectroscopic scheme to separate the contributions of lateral strain and out-of-plane coupling to TEC is developed. The TEC of graphene on octadecyltrichlorosilane substrate is found to be (-0.6 ± 0.5)× 10-6 K-1 at room temperature, which is fundamentally smaller than that of freestanding graphene. These results shed light on the fundamental understanding of the interaction between 2D material and substrate, and offer a general recipe for studying separately in-plane and out-of-plane couplings on supported materials.

Preparation of silk fibroin and graphene composite membrane and its response driving performance test

With the development of materials, intelligent materials have the functions of sensing, controlling and driving objects, self-adaptive repair and so on, so they have been widely used. Silk fibroin fiber is a kind of natural protein, which is formed by silk degumming, insoluble in water, and can be decomposed gradually in alkaline solution without irritation and side effects, so it has a good prospect of biological application. The silk fibroin film was mixed with graphene composites, and the intelligent materials for functional response were prepared by means of the excellent optical and thermal properties of graphene. The biaxially oriented polypropylene film (BOPP) / silk fibroin bilayer composite film was prepared by the simple composite method, which showed good thermal response bending deformation properties. The graphene solution was dripped on one side of the fibroin film by a dripping method. After low-temperature drying, the final BOPP/SF/RGO/PEDOT:PSS composite film was prepared by adding a layer of poly (3meme 4-ethylenedioxythiophene / polystyrene sulfonate) (PEDOT:PSS). The flexible film driver was prepared, which can respond to multiple stimuli under light, heat and humidity.

Mechanical and thermal properties of graphene and carbon nanotube reinforced epoxy/boron nitride adhesives

Outstanding mechanical properties and high thermal conductivity epoxy composites are being developed and are becoming urgent in the electronic and aerospace industries. Boron nitride nanosheet (BN), a nanomaterial with high thermal conductivity, and its epoxy composite adhesive have high thermal conductivity. However, due to its characteristics, the mechanical properties of BN/epoxy will decrease when more BN is added. Therefore, this study graphene platelet and carbon nanotube are selected and packed epoxy/BN composite adhesives to develop adhesives with excellent mechanical properties and thermal conductivity at the same time. In this study, an enhancement effect between a graphene platelet (GnP) and a carbon nanotube (CNT) within an epoxy/BN composite is reported. In the three-phase composite adhesive, the 2 D graphene platelets and 1D carbon nanotubes form a global bearing force network, and the boron nitride nanosheets provide high heat conduction inside the epoxy. They both complement each other in developing thermal conductive and high mechanical energy epoxy composite adhesives. At filler ranging from 0 to 1.0 wt%, epoxy/BN-CNT can improve the fracture toughness (K 1c ), critical stress-energy release rate (G 1c ) and Young's modulus of pure epoxy, are 89, 484, and 67% compared to 52, 286, and 42% in epoxy/BN-GnP composites, respectively. When adding 5.0 wt% of BN-CNT, the epoxy composites' thermal conductivity is up to 0.8 W/m%K, 400% higher than pure epoxy. This article also tested the electrical conductivity of epoxy/BN-CNT and epoxy/BN-GnP composites and found that the electrical conductivity was increased by 12 and 10 orders of magnitude, respectively, and it could be used as an antistatic material. Investigating the enhanced mechanism of epoxy/BN/CNT and epoxy/BN/GnP composites by scanning electron microscopy analysis. Lap shear strength tests on epoxy composite adhesives confirm the reinforcement effect of GnP and CNT of epoxy/BN composites.

Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams

•Anisotropic cellulose nanofibril foams are super-insulating also at high humidity•Moisture-induced swelling offsets thermal conductivity increase due to water uptake•Thermal boundary conductance decreases 6-fold by increasing inter-fibrillar gap•Thinner fibrils enhance phonon scattering and reduce the thermal conductivity Efficient thermal insulation using biobased materials could reduce the energy consumption and minimize the carbon footprint of buildings. Biobased materials are sensitive to moisture, and there is a need to better understand how moisture controls the thermal conductivity. Here, we show that anisotropic cellulose nanofibril foams can be super-insulating also at high humidity. The relative humidity dependence of the thermal conductivity of super-insulating nanocellulose foams is controlled by moisture-induced phonon scattering and the replacement of air with water. The moisture-induced swelling and increase of the inter-fibrillar separation distance results in a reduction of the thermal boundary conductance that exceeds the thermal conductivity increase due to water uptake up to high relative humidity. Humidity-dependent phonon engineering could be used to tailor the heat transfer properties of biobased nanofibrillar materials for packaging and thermal management in buildings. Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications. Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications. 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Mater. 2017; 27: 1-9Google Scholar and CNF-nanozeolites composite aerogels24Bendahou D. Bendahou A. Seantier B. Grohens Y. Kaddami H. Nano-fibrillated cellulose-zeolites based new hybrid composites aerogels with super thermal insulating properties.Ind. Crops Prod. 2015; 65: 374-382Crossref Scopus (79) Google Scholar can display thermal conductivities significantly lower than those of traditional EPS insulation and even below that of air (=25.7 mW m−1 K−1) at ambient conditions. Cellulose and CNM-based materials are hygroscopic,11Li T. Song J. Zhao X. Yang Z. Pastel G. Xu S. Jia C. Dai J. Chen C. Gong A. et al.Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose.Sci. Adv. 2018; 4: 1-10Crossref Scopus (180) Google Scholar,25Guo X. Wu Y. Xie X. Water vapor sorption properties of cellulose nanocrystals and nanofibers using dynamic vapor sorption apparatus.Sci. Rep. 2017; 7: 1-12Crossref PubMed Scopus (40) Google Scholar and the thermal conductivity of isotropic foams at room temperature (RT) increases with increasing relative humidity (RH) as adsorbed water replaces air.1Jelle B.P. Traditional, state-of-the-art and future thermal building insulation materials and solutions - properties, requirements and possibilities.Energy Build. 2011; 43: 2549-2563Crossref Scopus (695) Google Scholar,26Apostolopoulou-Kalkavoura V. Gordeyeva K. Lavoine N. Bergström L. Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer.Cellulose. 2018; 25: 1117-1126Crossref Scopus (23) Google Scholar The elasticity and strength of CNF films are also strongly reduced at high RH.27Benítez A.J. Torres-Rendon J. Poutanen M. Walther A. Humidity and multiscale structure govern mechanical properties and deformation modes in films of native cellulose nanofibrils.Biomacromolecules. 2013; 14: 4497-4506Crossref PubMed Scopus (162) Google Scholar Molecular dynamics simulations suggest that inter-fibrillar hydrogen bonds may weaken or break with increasing RH due to competition with the adsorbed water molecules.27Benítez A.J. Torres-Rendon J. Poutanen M. Walther A. Humidity and multiscale structure govern mechanical properties and deformation modes in films of native cellulose nanofibrils.Biomacromolecules. 2013; 14: 4497-4506Crossref PubMed Scopus (162) Google Scholar, 28Kulasinski K. Guyer R. Derome D. Carmeliet J. Water adsorption in wood microfibril-hemicellulose system: role of the crystalline-amorphous interface.Biomacromolecules. 2015; 16: 2972-2978Crossref PubMed Scopus (73) Google Scholar, 29Kulasinski K. Effects of water adsorption in hydrophilic polymers.Polym. Sci. Res. Adv. Pract. Appl. Educ. Asp. 2016; : 217-223Google Scholar Recently, it was demonstrated that low-density CNF-based foams with aligned nanofibrils and columnar macropores display strongly anisotropic30Wicklein B. Kocjan A. Salazar-Alvarez G. Carosio F. Camino G. Antonietti M. Bergström L. et al.Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide.Nat. Nanotechnol. 2015; 10: 277-283Crossref PubMed Scopus (784) Google Scholar,31Kriechbaum K. Munier P. Apostolopoulou-Kalkavoura V. Lavoine N. Analysis of the porous architecture and properties of anisotropic nanocellulose foams: a novel approach to assess the quality of cellulose nanofibrils (CNFs).ACS Sustain. Chem. Eng. 2018; 6: 11959-11967Crossref Scopus (25) Google Scholar and RH-dependent32Munier P. Apostolopoulou-Kalkavoura V. Persson M. Bergström L. Strong silica-nanocellulose anisotropic composite foams combine low thermal conductivity and low moisture uptake.Cellulose. 2019; : 1-12Google Scholar heat transport properties, with a minimum radial thermal conductivity of the CNF foams, perpendicular to the fibril and macropore direction, of 18 mW m−1 K−1. Such low thermal conductivities have only been attained in nanoporous materials, such as silica aerogels,33Lu X. Arduini-Schuster M.C. Kuhn J. Nilsson O. Fricke J. Pekala R.W. Thermal conductivity of monolithic organic aerogels.Science. 1992; 255: 971-972Crossref PubMed Scopus (358) Google Scholar,34Budtova T. Bio-based aerogels : a new generation of thermal superinsulating materials.in: Cellulose Science and Technology: Chemistry, Analysis, and Applications, First Edition 2019. 2019: 371-392Google Scholar where pore sizes below the mean free path length of air can result in thermal conductivities substantially below the value for air. Phonon scattering at the interfaces between nanosized materials can also reduce the thermal conductivity, and it was recently shown that tuning the separation distance in, for example, few-layer graphene17Luo T. Lloyd J.R. Enhancement of thermal energy transport across graphene/graphite and polymer interfaces: a molecular dynamics study.Adv. Funct. Mater. 2012; 22: 2495-2502Crossref Scopus (245) Google Scholar or multi-layer graphene,35Sadeghi M.M. Jo I. Shi L. Phonon-interface scattering in multilayer graphene on an amorphous support.Proc. Natl. Acad. Sci. U S A. 2013; 110: 16321-16326Crossref PubMed Scopus (120) Google Scholar and the water density at weakly bonded interfaces of self-assembled monolayers36Hung S.W. Kikugawa G. Shiomi J. Mechanism of temperature dependent thermal transport across the interface between self-assembled monolayer and water.J. Phys. Chem. C. 2016; 120: 26678-26685Crossref Scopus (29) Google Scholar,37Losego M.D. Grady M.E. Sottos N.R. Cahill D.G. Braun P.V. Effects of chemical bonding on heat transport across interfaces.Nat. Mater. 2012; 11: 502-506Crossref PubMed Scopus (469) Google Scholar can have a strong influence on the thermal boundary conductance of nanomaterials. Hence, it would be important to determine the contribution and importance of phonon scattering to the moisture-dependent thermal conductivity of anisotropic foams. However, the understanding of how RH and moisture uptake controls the thermal conductivity of hygroscopic anisotropic nanofibrillar-based foams is poor and the thermal boundary conductance of CNM-based materials has not been investigated previously. Here, we have combined thermal conductivity measurements and molecular simulations to quantify the effect of RH on the anisotropic heat transfer and thermal boundary conductance of ice-templated CNF foams with highly aligned nanofibrils of different diameters in the foam walls. The RH dependence of the thermal conductivity of hygroscopic nanocellulose foams was shown to be controlled by moisture-induced phonon scattering and the replacement of air with water. The moisture-induced swelling and increase of the inter-fibrillar separation distance can result in a 6-fold reduction of the thermal boundary conductance that exceeds the thermal conductivity increase due to water uptake up to high RH. Foams made from thinner fibrils display lower thermal conductivities due to enhanced phonon scattering. Understanding how heat transport of biobased nanofibrillar foams can be tuned by moisture uptake and release could enable novel ways to engineer hygroscopic super-insulating nanomaterials in packaging and building applications. Low-density foams were produced by ice templating aqueous suspensions of CNFs. We used three different CNFs that differed primarily in the degree of carboxylation at the C6 positions in the anhydroglucose units and the diameter (d) and length (L) of the fibrils (Figure 1A). X-ray diffraction (XRD) and atomic force microcopy (AFM) image analysis (Figure S1), together with conductometric titration and sedimentation measurements38Varanasi S. He R. Batchelor W. Estimation of cellulose nanofibre aspect ratio from measurements of fibre suspension gel point.Cellulose. 2013; 20: 1885-1896Crossref Scopus (102) Google Scholar of non-oxidized CNFs with an average diameter of 19 nm (CNF19), medium-charge 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized CNFs with an average diameter of 4.4 nm (CNF4.4), and high-charge TEMPO-oxidized CNFs with an average diameter of 2.3 nm (CNF2.3) showed that more intensive oxidation led to smaller fibril diameter, higher aspect ratio (L/d; Equation S1 and S2), and lower crystallinity index (Equation S3) (Table 1).Table 1Physical Properties of CNF19, CNF4.4, and CNF2.3 at 295 KCDaCD was measured by conductometric titration. (mmol COO–g−1)Crystallinity indexbCrystallinity was measured by XRD.(%)Diametercd was measured by AFM image analysis. d (nm)Aspect ratiodAspect ratio was measured by sedimentation.L/dCNF190.02 ± 0.0045419 ± 7.9110 ± 26CNF4.40.30 ± 0.020524.4 ± 1.8140 ± 4CNF2.31.60 ± 0.010452.3 ± 0.7200 ± 16a CD was measured by conductometric titration.b Crystallinity was measured by XRD.c d was measured by AFM image analysis.d Aspect ratio was measured by sedimentation. Open table in a new tab Directional growth of the ice crystals during freeze-casting resulted in strongly anisotropic foams with the CNF particles aligned in the growth direction of the ice crystals (Figure 1B). The nanofibril alignment was confirmed by 2D XRD patterns and azimuthal integration (Figure 1C, Equations S4 and S5), and is consistent with previous studies on CNM-based freeze-cast foams.39Munier P. Gordeyeva K. Bergström L. Fall A.B. Directional freezing of nanocellulose dispersions aligns the rod-like particles and produces low-density and robust particle networks.Biomacromolecules. 2016; 17: 1875-1881Crossref PubMed Scopus (104) Google Scholar,40Deville S. The lure of ice-templating Recent trends and opportunities for porous materials.Scr. Mater. 2018; 147: 119-124Crossref Scopus (58) Google Scholar The SEM images of the cross section along the radial direction (Figure 1D) of the CNF foams showed a porous honeycomb-like architecture with a narrower pore size distribution in the CNF2.3 foams (Figure 1E) than in the CNF19 (Figure S2A) or CNF4.4 (Figure S2B) foams. The macroporous structure of freeze-cast materials is determined predominantly by the confined growth of the ice crystals, and the nanoporous structure within the pore walls is strongly influenced by the ability of the particles to be transported and assembled as the freezing front moves through the dispersion.40Deville S. The lure of ice-templating Recent trends and opportunities for porous materials.Scr. Mater. 2018; 147: 119-124Crossref Scopus (58) Google Scholar The nanoporosity, Πnp, of the foams corresponds to pores with sizes between 2 and 100 nm. CNFs with low charge density (CD) display electrostatic repulsions that may be of insufficient magnitude to prohibit the CNF particles from aggregating during ice templating and therefore result in foams with disordered structures.41Anderson A.M. Worster M.G. Periodic ice banding in freezing colloidal dispersions.Langmuir. 2012; 28: 16512-16523Crossref PubMed Scopus (42) Google Scholar CNFs with high CD (i.e., 1.6 mmol COO–g−1) display strong interparticle repulsion41Anderson A.M. Worster M.G. Periodic ice banding in freezing colloidal dispersions.Langmuir. 2012; 28: 16512-16523Crossref PubMed Scopus (42) Google Scholar and produce ordered ice-templated structures (Figure 1D). High-resolution scanning electron microscopy (HRSEM) showed that the foam walls of the highly charged CNF2.3 foam were compact and thin, with a thickness of about 200 nm (Figure 1E). The difference in the Brunauer-Emmett-Teller (BET) surface area between the CNF2.3 foam (15 m2/g) and the CNF19 foam (8.7 m2/g; Figure S3) is much smaller than the >60-fold difference predicted based upon the fibril diameters, which suggests that the fibrils in the walls are tightly packed. Nitrogen sorption experiments (Figure S3) performed at dry conditions show that the foam walls contain a low fraction (3.5%–5%) (Equation S6 and S7, Table S1) of pores up to 12 nm (Figure S3C) in diameter. The radial (λr) and axial (λa) thermal conductivities of the CNF19, CNF4.4, and CNF2.3 ice-templated foams (Figure 2A) with a dry density between 5.9 and 6.4 kg m−3 were measured using the anisotropic mode of the Hot Disk at controlled temperature and RH (Figures S4A and S4B). The hot disk records the time-dependent temperature increase in response to a transient power pulse and determines the radial thermal diffusivity (αr, Figure S4C). The radial thermal conductivity, λr is calculated by Equation 142Lagüela S. Bison P. Peron F. Romagnoni P. Thermal conductivity measurements on wood materials with transient plane source technique.Thermochim. Acta. 2015; 600: 45-51Crossref Scopus (46) Google Scholar:λr=αrρCP(Equation 1) where CP is the specific heat capacity of the foam, and ρ is the density of the foam. The “dry” CP (RH = 0) is determined from differential scanning calorimetry (DSC) measurements (Figure S5), and the RH-dependent CP at RH > 0 is estimated by the rule of mixtures (Equation S12) taking into account the water uptake (Figure 3A) of the foam and the CP of water. The CP at dry (RH = 0) conditions decreased with increasing CD (and decreasing diameter) of the CNFs, from 1,180 to 753 J kg−1 K−1 for CNF19 and CNF2.3, respectively (Figure S5). The axial thermal conductivity is measured in the direction of the ice crystal growth and thus the direction of the columnar macropores, while the radial thermal conductivity is measured perpendicularly to the macropores and the aligned CNFs.Figure 3Experimental and Hybrid GCMC/MD Simulations of Moisture Uptake and Foam-Wall-Sorption-Induced SwellingShow full caption(A) Experimental moisture content (H2Ow) by mass of ice-templated foams prepared from CNF2.3, CNF4.4, and CNF19 compared with the moisture content obtained from hybrid GCMC/MD simulations for the CNF2.3 (CNF2.3-S) as a function of RH% at 295 K.(B) The estimated swelling (continuous line-○) and average inter-fibril gap (dashed line-Δ) of a CNF2.3 foam as a function of RH%, and the inter-fibril gap calculated by hybrid GCMC/MD (▲) as a function of RH%.(C–E) (C) Initial arrangement of four individually equilibrated fibrils before drying. Snapshots of cellulose bundle of (D) aligned CNF2.3 fibril after the drying when they come close to each other, and (E) the same fibrils subjected to RH that have swelled as water molecules have entered their interstitial sites. Cellulose chains are colored in orange, COO− groups in green, counterions in pink, and water in blue.View Large Image Figure ViewerDownload (PPT) (A) Experimental moisture content (H2Ow) by mass of ice-templated foams prepared from CNF2.3, CNF4.4, and CNF19 compared with the moisture content obtained from hybrid GCMC/MD simulations for the CNF2.3 (CNF2.3-S) as a function of RH% at 295 K. (B) The estimated swelling (continuous line-○) and average inter-fibril gap (dashed line-Δ) of a CNF2.3 foam as a function of RH%, and the inter-fibril gap calculated by hybrid GCMC/MD (▲) as a function of RH%. (C–E) (C) Initial arrangement of four individually equilibrated fibrils before drying. Snapshots of cellulose bundle of (D) aligned CNF2.3 fibril after the drying when they come close to each other, and (E) the same fibrils subjected to RH that have swelled as water molecules have entered their interstitial sites. Cellulose chains are colored in orange, COO− groups in green, counterions in pink, and water in blue. The thermal conductivities of the ice-templated CNF foams were anisotropic and depended strongly on the RH. The λr (Figure 2B) of the CNF foams were 3–10 times lower than λa (Figures S4D and S4E) depending on the RH and the diameter of the CNFs. The anisotropy of the thermal conductivity of the ice-templated foams is related to the alignment of the nanofibrils in the freezing direction (Figures 1C and S6) and the intrinsic anisotropy of the thermal conductivity of cellulose. The λr of the ice-templated foams decreased with an increase of RH up to 35%–50% RH and increased as the RH increased from 65% to 80% RH (Figure 2B). Freeze-cast foams prepared from CNFs with the smallest diameter (CNF2.3) displayed a lower λr compared with foams prepared from CNFs with larger diameter and lower CD (Figure 2B), and the lowest thermal conductivity (14 mW m−1 K−1) was attained at 35% RH for the CNF2.3 foam. This value of the radial thermal conductivity is significantly below the value for air (λ = 25.7 mW m−1 K−1), which is surprising considering that the majority of the pores of ice-templated foams are much larger than the mean free path of air (about 70 nm in open space).19Sakai K. Kobayashi Y. Saito T. Isogai A. Partitioned airs at microscale and nanoscale: thermal diffusivity in ultrahigh porosity solids of nanocellulose.Sci. Rep. 2016; 6: 20434Crossref PubMed Scopus (72) Google Scholar,43Zeng S.Q. Hunt A. Greif R. Transport properties of gas in silica aerogel.J. Non. Cryst. Sol. 1995; 186: 264-270Crossref Scopus (85) Google Scholar,44Ruckdeschel P. Philipp A. Retsch M. Understanding thermal insulation in porous, particulate materials.Adv. Funct. Mater. 2017; 27: 1-11Google Scholar The convection contribution should be negligible because the macropores are sufficiently small to minimize gas transport over large distances and the radiation contribution is also small at RT (295 K).44Ruckdeschel P. Philipp A. Retsch M. Understanding thermal insulation in porous, particulate materials.Adv. Funct. Mater. 2017; 27: 1-11Google Scholar,45Collishaw P.G. Evans J.R.G. An assessment of expressions for the apparent thermal-conductivity of cellular materials.J. Mater. Sci. 1994; 29: 2261-2273Crossref Scopus (99) Google Scholar The presence of nanopores in the foam walls is expected to reduce the gas conduction contribution, but it is clear that additional effects, such as a substantial decrease of the thermal boundary conductance by phonon scattering, need to be invoked to explain the very low thermal conductivities. The λr of the anisotropic freeze-cast CNF foams remained below the thermal conductivity of air between 10% and 70% RH (Figure 2B) and was always several times lower than λa (Figures S4D and S4E). In contrast, λa increased with increasing RH over the entire measured range (7%–80% RH) for all of the CNF foams (Figures S4D and S4E), similar to isotropic CNF- and polyoxamer-based foams.26Apostolopoulou-Kalkavoura V. Gordeyeva K. Lavoine N. Bergström L. Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer.Cellulose. 2018; 25: 1117-1126Crossref Scopus (23) Google Scholar Figure 2C shows that the effect of RH on λr was reversible between 7% and 50% RH, which suggests that the moisture uptake is reversible29Kulasinski K. Effects of water adsorption in hydrophilic polymers.Polym. Sci. Res. Adv. Pract. Appl. Educ. Asp. 2016; : 217-223Google Scholar and that the structure of the foam walls is not irreversibly affected by the moisture uptake within the investiga