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      Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose

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          Abstract

          Researchers transformed natural wood into a super thermal insulation structural material with aligned cellulose nanofibers.

          Abstract

          There has been a growing interest in thermal management materials due to the prevailing energy challenges and unfulfilled needs for thermal insulation applications. We demonstrate the exceptional thermal management capabilities of a large-scale, hierarchal alignment of cellulose nanofibrils directly fabricated from wood, hereafter referred to as nanowood. Nanowood exhibits anisotropic thermal properties with an extremely low thermal conductivity of 0.03 W/m·K in the transverse direction (perpendicular to the nanofibrils) and approximately two times higher thermal conductivity of 0.06 W/m·K in the axial direction due to the hierarchically aligned nanofibrils within the highly porous backbone. The anisotropy of the thermal conductivity enables efficient thermal dissipation along the axial direction, thereby preventing local overheating on the illuminated side while yielding improved thermal insulation along the backside that cannot be obtained with isotropic thermal insulators. The nanowood also shows a low emissivity of <5% over the solar spectrum with the ability to effectively reflect solar thermal energy. Moreover, the nanowood is lightweight yet strong, owing to the effective bonding between the aligned cellulose nanofibrils with a high compressive strength of 13 MPa in the axial direction and 20 MPa in the transverse direction at 75% strain, which exceeds other thermal insulation materials, such as silica and polymer aerogels, Styrofoam, and wool. The excellent thermal management, abundance, biodegradability, high mechanical strength, low mass density, and manufacturing scalability of the nanowood make this material highly attractive for practical thermal insulation applications.

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          Most cited references59

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          Characteristics of hemicellulose, cellulose and lignin pyrolysis

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            Wood-Derived Materials for Green Electronics, Biological Devices, and Energy Applications.

            With the arising of global climate change and resource shortage, in recent years, increased attention has been paid to environmentally friendly materials. Trees are sustainable and renewable materials, which give us shelter and oxygen and remove carbon dioxide from the atmosphere. Trees are a primary resource that human society depends upon every day, for example, homes, heating, furniture, and aircraft. Wood from trees gives us paper, cardboard, and medical supplies, thus impacting our homes, school, work, and play. All of the above-mentioned applications have been well developed over the past thousands of years. However, trees and wood have much more to offer us as advanced materials, impacting emerging high-tech fields, such as bioengineering, flexible electronics, and clean energy. Wood naturally has a hierarchical structure, composed of well-oriented microfibers and tracheids for water, ion, and oxygen transportation during metabolism. At higher magnification, the walls of fiber cells have an interesting morphology-a distinctly mesoporous structure. Moreover, the walls of fiber cells are composed of thousands of fibers (or macrofibrils) oriented in a similar angle. Nanofibrils and nanocrystals can be further liberated from macrofibrils by mechanical, chemical, and enzymatic methods. The obtained nanocellulose has unique optical, mechanical, and barrier properties and is an excellent candidate for chemical modification and reconfiguration. Wood is naturally a composite material, comprised of cellulose, hemicellulose, and lignin. Wood is sustainable, earth abundant, strong, biodegradable, biocompatible, and chemically accessible for modification; more importantly, multiscale natural fibers from wood have unique optical properties applicable to different kinds of optoelectronics and photonic devices. Today, the materials derived from wood are ready to be explored for applications in new technology areas, such as electronics, biomedical devices, and energy. The goal of this study is to review the fundamental structures and chemistries of wood and wood-derived materials, which are essential for a wide range of existing and new enabling technologies. The scope of the review covers multiscale materials and assemblies of cellulose, hemicellulose, and lignin as well as other biomaterials derived from wood, in regard to their major emerging applications. Structure-properties-application relationships will be investigated in detail. Understanding the fundamental properties of these structures is crucial for designing and manufacturing products for emerging applications. Today, a more holistic understanding of the interplay between the structure, chemistry, and performance of wood and wood-derived materials is advancing historical applications of these materials. This new level of understanding also enables a myriad of new and exciting applications, which motivate this review. There are excellent reviews already on the classical topic of woody materials, and some recent reviews also cover new understanding of these materials as well as potential applications. This review will focus on the uniqueness of woody materials for three critical applications: green electronics, biological devices, and energy storage and bioenergy.
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              Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide.

              High-performance thermally insulating materials from renewable resources are needed to improve the energy efficiency of buildings. Traditional fossil-fuel-derived insulation materials such as expanded polystyrene and polyurethane have thermal conductivities that are too high for retrofitting or for building new, surface-efficient passive houses. Tailored materials such as aerogels and vacuum insulating panels are fragile and susceptible to perforation. Here, we show that freeze-casting suspensions of cellulose nanofibres, graphene oxide and sepiolite nanorods produces super-insulating, fire-retardant and strong anisotropic foams that perform better than traditional polymer-based insulating materials. The foams are ultralight, show excellent combustion resistance and exhibit a thermal conductivity of 15 mW m(-1) K(-1), which is about half that of expanded polystyrene. At 30 °C and 85% relative humidity, the foams retained more than half of their initial strength. Our results show that nanoscale engineering is a promising strategy for producing foams with excellent properties using cellulose and other renewable nanosized fibrous materials.
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                Author and article information

                Journal
                Sci Adv
                Sci Adv
                SciAdv
                advances
                Science Advances
                American Association for the Advancement of Science
                2375-2548
                March 2018
                09 March 2018
                : 4
                : 3
                : eaar3724
                Affiliations
                [1 ]Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA.
                [2 ]Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA.
                [3 ]Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA.
                [4 ]Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
                [5 ]Wallenberg Wood Science Centre, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
                Author notes
                [*]

                These authors contributed equally to this work.

                []Corresponding author. Email: binghu@ 123456umd.edu (L.H.); ronggui.yang@ 123456colorado.edu (R.Y.)
                Author information
                http://orcid.org/0000-0002-1087-0662
                http://orcid.org/0000-0002-0212-1397
                http://orcid.org/0000-0001-7263-2080
                http://orcid.org/0000-0001-9553-554X
                http://orcid.org/0000-0002-9191-2982
                http://orcid.org/0000-0003-2621-590X
                http://orcid.org/0000-0001-8622-0386
                http://orcid.org/0000-0002-3602-6945
                http://orcid.org/0000-0002-9456-9315
                Article
                aar3724
                10.1126/sciadv.aar3724
                5844708
                29536048
                ebffefef-7978-4ef2-bf42-645467ca50d0
                Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

                This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

                History
                : 01 November 2017
                : 02 February 2018
                Categories
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                Materials Science
                Applied Sciences and Engineering
                Materials Science
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                Earl Rosopa

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