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Thermal Conductivity of Hygroscopic Foams Based on Cellulose Nanomaterials
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0003-3036-8730
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biobased super-insulating materials could mitigate climate change by minimizing the use of petroleum-based materials, creating artificial carbon sinks and minimizing the energy needed to maintain pleasant interior conditions. Cellulose nanomaterials (CNM) produced from abundantly available cellulose sources constitute versatile, highly anisotropic raw materials with tunable surface chemistry and high strength. This thesis includes the evaluation of the thermal conductivity of isotropic and anisotropic CNM-based foams and aerogels and analysis of the dominant heat transfer mechanisms. 

We have developed a customized measurement cell for hygroscopic materials in which the humidity and temperature are carefully controlled while the thermal conductivity is measured. Anisotropic cellulose nanofibrils (CNF) foams with varying diameters showed a super-insulating behavior perpendicular (radial) to the nanofibril direction, that depended non-linearly on the relative humidity (RH) and foam density. Molecular simulations revealed that the very low thermal conductivity is related to phonon scattering due to the increase of the inter-fibrillar gap with increasing RH that resulted in a 6-fold decrease of the thermal boundary conductance. The moisture-induced swelling exceeds the thermal conductivity increase due to water uptake at low and intermediate RH and resulted in a minimum thermal conductivity of 14 mW m-1 K-1 at 35% RH and 295 K for the foams based on the thinnest CNF.

The density-dependency of the thermal conductivity of cellulose nanocrystal (CNC) foams with densities of 25 to 129 kg m-3 was investigated and a volume-weighted modelling of the solid and gas thermal conductivity contributions suggested that phonon scattering was essential to explain the low radial thermal conductivity, whereas the replacement of air with water and the Knudsen effect related to the nanoporosity in the foam walls had a small effect. Intermediate-density CNC foams (34 kg m-3) exhibited a radial thermal conductivity of 24 mW m-1 K-1 at 295 K and 20% RH, which is below the value for air.

The moisture uptake of foams based on CNMs with different degree of crystallinity and surface modifications decreased significantly with increasing crystallinity and temperature. Molecular simulations showed that the narrow pore size distribution of the amorphous cellulose film, and the relatively low water adsorption in the hydration cell around the oxygen of the carboxyl group play an important role for the moisture uptake of amorphous and crystalline CNM-based materials.

Isotropic CNF- and polyoxamer based foams as well as CNF-AL-MIL-53 (an aluminum‑based metal-organic framework) foams were both moderately insulating (>40 mW m-1 K-1) and comparable with commercial expanded polystyrene. The thermal conductivity of CNF and polyoxamer foams displayed a very strong RH dependency that was modelled with a modified Künzel’s model. The presence of hydrophobic AL-MIL-53 decreased the moisture uptake of CNF-AL-MIL-53 aerogels by 42% compared to CNF-polyoxamer foams.

Solid and gas conduction are the main heat transfer mechanisms in hygroscopic nanofibrillar foams and aerogels that depend on the interfacial phonon scattering, Knudsen effect and water uptake. It is essential that the thermal conductivity measurements of hygroscopic CNM-based foams and aerogels are determined at controlled RH and that parameters such as the temperature, density, nanoporosity, fibril dimensions and alignment are characterized and controlled for systematic development and upscaling of biobased foams for applications in building insulation and packaging.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University , 2021.
Keywords [en]
thermal conductivity, cellulose nanomaterials, foams, hygroscopic, super-insulating, phonon scattering, moisture uptake, heat transport
National Category
Materials Engineering Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-190212ISBN: 978-91-7911-406-0 (print)ISBN: 978-91-7911-407-7 (electronic)OAI: oai:DiVA.org:su-190212DiVA, id: diva2:1527441
Public defence
2021-03-26, digitally via Zoom, public link will be made available at https://www.mmk.su.se/, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2021-03-03 Created: 2021-02-10 Last updated: 2022-02-25Bibliographically approved
List of papers
1. Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams
Open this publication in new window or tab >>Humidity-Dependent Thermal Boundary Conductance Controls Heat Transport of Super-Insulating Nanofibrillar Foams
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2021 (English)In: Matter, ISSN 2590-2393, E-ISSN 2590-2385, Vol. 4, no 1, p. 276-289Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
super-insulation, nanocellulose, thermal conductivity, foam, phonon scattering, moisture uptake, anisotropic heat transport, thermal boundary conductance, hygroscopic
National Category
Chemical Sciences Materials Engineering
Identifiers
urn:nbn:se:su:diva-189663 (URN)10.1016/j.matt.2020.11.007 (DOI)000608248900007 ()
Available from: 2021-01-29 Created: 2021-01-29 Last updated: 2022-02-25Bibliographically approved
2. Effect of density, phonon scattering and nanoporosity on the thermal conductivity of anisotropic cellulose nanocrystal foams
Open this publication in new window or tab >>Effect of density, phonon scattering and nanoporosity on the thermal conductivity of anisotropic cellulose nanocrystal foams
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:su:diva-190211 (URN)
Available from: 2021-02-10 Created: 2021-02-10 Last updated: 2022-02-25
3. Moisture uptake in nanocellulose: The effect of relative humidity, temperature and degree of crystallinity
Open this publication in new window or tab >>Moisture uptake in nanocellulose: The effect of relative humidity, temperature and degree of crystallinity
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(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:su:diva-190210 (URN)
Available from: 2021-02-10 Created: 2021-02-10 Last updated: 2022-02-25
4. Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer
Open this publication in new window or tab >>Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer
2018 (English)In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 25, no 2, p. 1117-1126Article in journal (Refereed) Published
Abstract [en]

Nanocellulose-based lightweight foams are promising alternatives to fossil-based insulation materials for energy-efficient buildings. The properties of cellulose-based materials are strongly influenced by moisture and there is a need to assess and better understand how the thermal conductivity of nanocellulose-based foams depends on the relative humidity and temperature. Here, we report a customized setup for measuring the thermal conductivity of hydrophilic materials under controlled temperature and relative humidity conditions. The thermal conductivity of isotropic foams based on cellulose nanofibrils and a nonionic polyoxamer, and an expanded polystyrene foam was measured over a wide range of temperatures and relative humidity. We show that a previously developed model is unable to capture the strong relative humidity dependence of the thermal conductivity of the hygroscopic, low-density nanocellulose- and nonionic polyoxamer-based foam. Analysis of the moisture uptake and moisture transport was used to develop an empirical model that takes into consideration the moisture content and the wet density of the investigated foam. The new empirical model could predict the thermal conductivity of a foam with a similar composition but almost 3 times higher density. Accurate measurements of the thermal conductivity at controlled temperature and relative humidity and availability of simple models to better predict the thermal conductivity of hygroscopic, low-density foams are necessary for the development of nanocellulose-based insulation materials.

Keywords
Thermal conductivity, Nanocellulose, Isotropic foams, Moisture transport, Hygroscopic, Empirical modelling
National Category
Materials Engineering Chemical Sciences
Identifiers
urn:nbn:se:su:diva-153629 (URN)10.1007/s10570-017-1633-y (DOI)000425318000018 ()
Available from: 2018-03-13 Created: 2018-03-13 Last updated: 2022-05-10Bibliographically approved
5. Elastic Aerogels of Cellulose Nanofibers@Metal-Organic Frameworks for Thermal Insulation and Fire Retardancy
Open this publication in new window or tab >>Elastic Aerogels of Cellulose Nanofibers@Metal-Organic Frameworks for Thermal Insulation and Fire Retardancy
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2020 (English)In: Nano-Micro Letters, ISSN 2150-5551, Vol. 12, no 1, article id 9Article in journal (Refereed) Published
Abstract [en]

Metal-organic frameworks (MOFs) with high microporosity and relatively high thermal stability are potential thermal insulation and flame-retardant materials. However, the difficulties in processing and shaping MOFs have largely hampered their applications in these areas. This study outlines the fabrication of hybrid CNF@MOF aerogels by a stepwise assembly approach involving the coating and cross-linking of cellulose nanofibers (CNFs) with continuous nanolayers of MOFs. The cross-linking gives the aerogels high mechanical strength but superelasticity (80% maximum recoverable strain, high specific compression modulus of similar to 200 MPa cm(3) g(-1), and specific stress of similar to 100 MPa cm(3) g(-1)). The resultant lightweight aerogels have a cellular network structure and hierarchical porosity, which render the aerogels with relatively low thermal conductivity of similar to 40 mW m(-1) K-1. The hydrophobic, thermally stable MOF nanolayers wrapped around the CNFs result in good moisture resistance and fire retardancy. This study demonstrates that MOFs can be used as efficient thermal insulation and flame-retardant materials. It presents a pathway for the design of thermally insulating, superelastic fire-retardant nanocomposites based on MOFs and nanocellulose.

Keywords
Metal-organic frameworks, Nanocellulose, Superelastic aerogel, Thermal insulation, Fire retardancy
National Category
Materials Engineering Chemical Sciences
Identifiers
urn:nbn:se:su:diva-179689 (URN)10.1007/s40820-019-0343-4 (DOI)000510847500009 ()
Available from: 2020-03-05 Created: 2020-03-05 Last updated: 2022-02-26Bibliographically approved
6. Thermally Insulating Nanocellulose-Based Materials
Open this publication in new window or tab >>Thermally Insulating Nanocellulose-Based Materials
2021 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 33, no 28, article id 2001839Article, review/survey (Refereed) Published
Abstract [en]

Thermally insulating materials based on renewable nanomaterials such as nanocellulose could reduce the energy consumption and the environmental impact of the building sector. Recent reports of superinsulating cellulose nanomaterial (CNM)-based aerogels and foams with significantly better heat transport properties than the commercially dominating materials, such as expanded polystyrene, polyurethane foams, and glass wool, have resulted in a rapidly increasing research activity. Herein, the fundamental basis of thermal conductivity of porous materials is described, and the anisotropic heat transfer properties of CNMs and films with aligned CNMs and the processing and structure of novel CNM-based aerogels and foams with low thermal conductivities are presented and discussed. The extraordinarily low thermal conductivity of anisotropic porous architectures and multicomponent approaches are highlighted and related to the contributions of the Knudsen effect and phonon scattering.

Keywords
aerogels, heat transfer, nanocellulose, phonon scattering, thermal insulation
National Category
Chemical Sciences Materials Engineering
Identifiers
urn:nbn:se:su:diva-185410 (URN)10.1002/adma.202001839 (DOI)000555852400001 ()32761673 (PubMedID)
Available from: 2020-10-16 Created: 2020-10-16 Last updated: 2022-02-25Bibliographically approved

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Apostolopoulou-Kalkavoura, Varvara

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