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Frequency and voltage response of a wind-driven fluttering triboelectric nanogenerator
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.ORCID iD: 0000-0002-4376-2676
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.ORCID iD: 0000-0003-2873-7875
Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.ORCID iD: 0000-0003-2965-0288
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2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, no 1, article id 5543Article in journal (Refereed) Published
Abstract [en]

Triboelectric nanogenerators (TENG:s) are used as efficient energy transducers in energy harvesting converting mechanical energy into electrical energy. Wind is an abundant source of mechanical energy but how should a good triboelectric wind harvester be designed? We have built and studied a TENG driven by air flow in a table-top sized wind tunnel. Our TENG constitutes of a plastic film of size10 cm × 2 cm which is fluttering between two copper electrodes generating enough power to light up a battery of LED:s. We measured the voltage and frequency of fluttering at different wind speeds from zero up to 8 m/s for three electrode distances 6 mm, 10 mm and 14 mm. We found that the frequency increases linearly with the wind speed with a cutoff at some low speed. Power was generated already at 1.6 m/s. We seem to be able to explain the observed frequency dependence on wind speed by assuming excitation of the film into different harmonics in response to von Kármán vortices. We also find that the voltage increase linearly with frequency. We anticipate that TENG:s of this design could be useful both as generators and speed sensors because they work at low air speeds.

Place, publisher, year, edition, pages
2019. Vol. 9, no 1, article id 5543
National Category
Natural Sciences Engineering and Technology
Identifiers
URN: urn:nbn:se:miun:diva-35936DOI: 10.1038/s41598-019-42128-7ISI: 000463178500004PubMedID: 30944397Scopus ID: 2-s2.0-85063884794OAI: oai:DiVA.org:miun-35936DiVA, id: diva2:1302008
Funder
J. Gust. Richert stiftelseSwedish Energy AgencyKnowledge FoundationEuropean Regional Development Fund (ERDF)
Note

Forskningsfinansiär: Länsstyrelsen Västernorrland

Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-07-08Bibliographically approved
In thesis
1. Nanomechanics – Quantum Size Effects, Contacts, and Triboelectricity
Open this publication in new window or tab >>Nanomechanics – Quantum Size Effects, Contacts, and Triboelectricity
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanomechanics is different from the mechanics that we experience in everyday life. At the nano-scale, typically defined as 1 to 100 nanometers, some phenomena are of crucial importance, while the same phenomena can be completely neglected on a larger scale. For example, the feet of a gekko are covered by nanocontacts that yield such high adhesion forces that the animal can run up on walls and even on the ceiling. At small enough distances, matter and energy become discrete, and the description of the phenomena occurring at this scale requires quantum mechanics. However, at room temperature the transitions between quantized energy levels may be concealed by the thermal vibrations of the system. As two surfaces approach each other and come into contact, electrostatic forces and van der Waals forces may cause redistribution of matter at the nano level. One effect that may occur upon contact between two surfaces is the triboelectric effect, in which charge is transferred from one surface to the other.This effect can be used to generate electricity in triboelectric nanogenerators (TENGs), where two surfaces are repeatedly brought in and out of contact, and where the charge transfer is turned into electrical energy.

This thesis concerns nanomechanics addressing whether quantum mechanics play a role in elastic deformation, as well as various mechanical aspects of nanocontacts including electric charging. The objectives are to contribute to the understanding when quantum effects are of importance at the nanolevel, increase the fundamental understanding of the mechanisms responsible for triboelectric phenomena and apply the triboelectric effect to a wind harvesting device.

For more insight into whether quantum effects are of importance in nanomechanics, we use a one dimensional jellium model and the standard beam theory allowing the spring constant of an oscillating nanowire cantilever to be calculated. As the nanowire bends, more electron states fit in its cross section, giving rise to an amplitude dependent resonance frequency of the nanowire oscillations.

Furthermore, a model for electric field induced surface diffusion of adatoms was developed. The model takes electrostatic forces and van der Waals forces into account as a voltage is applied between a scanning tunneling microscope tip and a sample. The calculated force on the adatoms at the surface of the sample, which is stemming from the inhomogeneous electric field and the dipole moment of the adatoms, is relatively small, but due to thermal vibrations adatoms diffuse and form mounds at the sample.

When bringing two different materials into contact, the difference in triboelectric potentials between the materials results in electric charging. To increase the understanding of triboelectricity, a two-level Schottky model, assuming ion transfer, was developed to describe the temperature dependence of the triboelectric effect for a TENG. The two levels correspond to the binding energy for ions on the two surfaces that are brought into contact, where the difference in binding energy enters the Boltzmanndistribution. The model describes the decreasing triboelectric effect in TENG:s with increasing temperature as described in the literature, and results in a separation energy, which is of the right order of magnitude for physically adsorbed atoms.

It was recently demonstrated that TENGs can convert wind energy into electrical energy. Here, a TENG based on a plastic film fluttering between two copper electrodes was constructed. It was found that the frequency of the the fluttering film increases linearly with the wind speed. TENG:s designed in this way generate electricity already at low wind speed, and we therefore expect such TENG:s to be useful both as generators and speed sensors in the future.

While quantum mechanics is of importance in a limited number of nanomechanical systems, nanocontacts have a broader meaning, and are crucial for the understanding of triboelectric phenomena. We anticipate that the findings in this thesis will contribute to a better understanding of nanomechanics, in particular the mechanism of triboelectricity.

Abstract [sv]

Nanomekanik är annorlunda än den mekanik vi upplever i vardagen. I nanoskalan, som definieras som storleksområdet 1 -100 nanometer, är vissa fenomen viktiga medan dessa helt kan försummas i den större skalan i vardagslivet. Till exempel har evolutionen på nanoskalan designat en gekkoödlas fötter så att de har en så kraftigvidhäftning till omgivningen att gekkon kan gå omkring i taket. Tittar man på den lilla skalan finner man att materien är diskret och att beskrivning av fenomen i denna skala kräver kvantmekanik. Övergångar mellan kvantnivåer döljs dock ofta vid rumstemperatur av termiska vibrationer hos systemet. När två ytor närmar sig varandra och kommer i kontakt kan elektrostatiska krafter och van der Waalskrafter orsaka omfördelning av materia på nanonivå. En effekt som kan uppträda vid kontakt mellan två ytor är den triboelektriska effekten vid vilken det sker överföring av laddning från den ena ytan till den andra. Denna effekt kan användas för att generera elektrisk energi i triboelektriska nanogeneratorer (TENG:s) där två ytor omväxlande förs ihop och dras isär.

Denna avhandling behandlar nanomekanik med fokus på om kvantmekaniken spelar en viktig roll vid elastisk deformation och även olika mekaniska aspekter hos nanokontakter inklusive elektrisk uppladdning. Målsättningen är att bidra till kunskap om när kvanteffekter är viktiga i nanoskalan och att öka den grundläggande förståelsen för triboelektriska fenomen samt att tillämpa den triboelektriska effekten för en vinddriven energiutvinnande apparat.

För att få bättre förståelse om kvanteffekter är betydelsefulla i nanomekanik har vi gjort en endimensionell fri elektronmodell och använder vanlig balkteori för att beräkna fjäderkonstanten för en nanotråd utgörande en svängande bladfjäder. När nanotråden böjs kommer fler elektrontillstånd att passa i trådens tvärsnittsyta vilket ger upphov till en amplitudberoende resonansfrekvens hos tråden.

Dessutom har vi utvecklat en modell för elektriskt fältinducerad ytdiffusion av adatomer. Modellen tar hänsyn till elektrostatiska krafter samt till van der Waalskrafter när en spänning påläggs mellan en spets i ett sveptunnelmikroskop och en provyta. Den beräknade kraften på adatomerna på provets yta - vilken kommer av det inhomogena elektriska fältet och adatomernas dipolmoment - är relativt liten, men på grund av de termiska vibrationerna kan adatomerna ändå diffundera och bilda en kulle under tippen.

När man sammanför två olika material kommer skillnaden i deras triboelektriska potentialer att orsaka uppladdning. För att öka förståelsen av triboelektriciteten har vi utvecklat en två-nivå Schottkymodell som antar joner som laddningsbärare för att beskriva den triboelektriska uppladdningens temperaturberoende för en TENG. De två nivåerna motsvarar de två kontaktande ytorna. Skillnaden i bindningsenergi mellan joner på ytorna kommer därför in i Boltzmannfaktorn. Modellen beskriver den minskande triboelektriska effekten vid ökande temperatur som rapporterats ilitteraturen, och stöds av att den separationsenergi man finner vid anpassning av modellen till data är av samma storleksordning som skillnaden i bindningsenergi för fysikaliskt ytadsorberade atomer (adatomer).

Nyligen visades det att TENG:s kan omvandla vindenergi till elektrisk energi.Vi har konstruerat en TENG baserad på ett flappande plastband löst inspänt mellan två kopparelektroder. Vi fann att frekvensen hos det flappande plastbandet ökar linjärt med vindhastigheten. TENG designade på detta sätt alstrar elektrisk energi redan vid låga vindhastigheter vilket gör att apparaten kan komma att användas både som generator och som vindhastighetsmätare i framtiden.

Medan kvantmekanik bara är betydelsefull i ett begränsat antal nanomekaniska system har nanokontakter en mer allmän tillämpning och är viktig för förståelsen av triboelektriska fenomen. Vi förutser att resultaten i denna avhandling kan komma att bidra till en bättre förståelse för nanomekanik i allmänhet och för mekanismen för triboelektricitet i synnerhet.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2019. p. 50
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 299
Keywords
triboelectricity, adatom, ion, charge, mound, quantum size effect, triboelctric nanogenerator, TENG, wind driven, contact, nanomechanics, fri electron model, nanowire cantilever
National Category
Natural Sciences
Identifiers
urn:nbn:se:miun:diva-36153 (URN)978-91-88947-02-4 (ISBN)
Public defence
2019-06-05, O102, Holmgatan 10, Sundsvall, 10:15 (English)
Opponent
Supervisors
Available from: 2019-05-20 Created: 2019-05-18 Last updated: 2019-05-20Bibliographically approved

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Olsen, MartinZhang, RenyunÖrtegren, JonasAndersson, HenrikOlin, Håkan

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