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Polyethylene/metal oxide nanocomposites for electrical insulation in future HVDC-cables: probing properties from nano to macro
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials. (Polymeric Materials)ORCID iD: 0000-0003-4774-4341
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanocomposites of polyethylene and metal oxide nanoparticles have shown to be a feasible approachto the next generation of insulation in high voltage direct current cables. In order to reach an operationvoltage of 1 MV new insulation materials with reduced conductivity and increased breakdown strengthas compared to modern low-density polyethylene (LDPE) is needed.In this work polyethylene MgO nanocomposites for electrical insulation has been produced andcharacterized both from an electrical and material perspective. The MgO nanoparticles weresynthesized into polycrystalline nanoparticles with a large specific surface area (167 m2 g–1). Meltprocessing by extrusion resulted in evenly dispersed MgO nanoparticles in LDPE for the silane surfacemodified MgO as compared to the unmodified MgO. All systems showed a reduction in conductivityby up to two orders of magnitude at low loading levels (1–3 wt.%), but where the surface modifiedsystems were able to retain reduced conductivity even at loading levels of 9 wt.%. A maximuminteraction radius to influence the conductivity of the MgO nanoparticles was theoretically determinedto ca. 800 nm. The interaction radius was in turn experimentally observed around Al2O3 nanoparticlesembedded in LDPE using Intermodulation electrostatic force microscopy. By applying a voltage on theAFM-tip charge injection and extraction around the Al2O3 nanoparticles was observed, visualizing theexistence of additional localized energy states on, and around, the nanoparticles. Ptychography wasused to reveal nanometre features in 3D of electrical trees formed under DC-conditions. Thevisualization showed that the electrical tree grows by pre-step voids in front of the propagatingchannels, facilitating further growth, much in analogy to mechanical crack propagation (Griffithconcept). An electromechanical effect was attributed as possible mechanism for the formation of the voids.

Abstract [sv]

Nanokompositer av polyeten och metalloxidpartiklar anses vara möjliga material att använda i morgondagens isolationshölje till högspänningskablar för likström. För att nå en transmissionsspänning på 1 MV behövs isolationsmaterial som i jämförelse med dagens polyeten har lägre elektrisk ledningsförmåga, högre styrka mot elektriskt genomslag och som kan kontrollera ansamling av rymdladdningar. De senaste årens forskning har visat att kompositer av polyeten med nanopartiklar av metalloxider har potential att nå dessa egenskaper.

I det här arbetet har kompositer av polyeten och nanopartiklar av MgO för elektrisk isolation producerats och karaktäriserats. Nanopartiklar av MgO har framställts från en vattenbaserad utfällning med efterföljande calcinering, vilket resulterade i polykristallina partiklar med en mycket stor specifik ytarea (167m2 g-1). MgO-nanopartiklarna ytmodifierades i n-heptan genom att kovalent binda oktyl(trietoxi)silan och oktadekyl(trimetoxi)silan till partiklarna för att skapa en hydrofob och skyddande yta. Extrudering av de ytmodifierade MgO nanopartiklarna tillsammans med polyeten resulterade i en utmärkt dispergering med jämnt fördelad partiklar i hela kompositen, vilket ska jämföras med de omodifierade partiklarna som till stor utsträckning bildade agglomerat i polymeren. Alla kompositer med låg fyllnadsgrad (1–3 vikt% MgO) visade upp till 100 gånger lägre elektrisk konduktivitet jämfört med värdet för ofylld polyeten. Vid högre koncentrationer av omodifierade MgO förbättrades inte de isolerande egenskaperna på grund av för stor andel agglomerat, medan kompositerna med de ytmodifierade fyllmedlen som var väl dispergerade behöll en kraftig reducerad elektrisk konduktivitet upp till 9 vikt% fyllnadshalt. Den minsta interaktionsradien för MgO-nanopartiklarna för att minska den elektriska konduktiviten i kompositerna fastställdes med bildanalys och simuleringar till ca 800 nm. Den teoretiskt beräknade interaktionsradien kompletterades med observation av en experimentell interaktionsradie genom att mäta laddningsfördelningen över en Al2O3-nanopartikle i en polyetenfilm med intermodulation (frekvens-mixning) elektrostatisk kraftmikroskop (ImEFM), vilket är en ny AFM-metod för att mäta ytpotentialer. Genom att lägga på en spänning på AFM-kantilevern kunde det visualiseras hur laddningar, både injicerades och extraherades, från nanopartiklarna men inte från polyeten. Det tolkades som att extra energinivåer skapades på och runt nanopartiklarna som fungerar för att fånga in laddningar, ekvivalent med den gängse tolkningen att nanopartiklar introducera extra elektronfällor i den polymera matrisen i nanokompositer. Nanotomografi användes för att avbilda elektriska träd i tre dimensioner. Avbildningen av det elektriska trädet visade att tillväxten av trädet hade skett genom bildning av håligheter framför den framväxande trädstrukturen. Håligheterna leder till försvagning av materialet framför det propagerande trädet och förenklar på det sättet fortsatt tillväxt. Bildningen av håligheter framför trädstrukturen uppvisar en analogi till propagering av sprickor vid mekanisk belastning, i enlighet med Griffiths koncept. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:34
Keyword [en]
Nanocomposites, polyethylene, particle dispersion, high voltage direct current cables, HVDC, HVDC insulation, MgO, intermodulation electrostatic force microscopy, ptychography, electrical tree
Keyword [sv]
Nanokompositer, polyeten, HVDC, HVDC-kabel, HVDC-isolering, MgO, elektrisk isolation, partikeldispergering, elektriska träd
National Category
Polymer Technologies Nano Technology Textile, Rubber and Polymeric Materials Composite Science and Engineering
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-193591ISBN: 978-91-7729-135-0 (print)OAI: oai:DiVA.org:kth-193591DiVA: diva2:1032932
Public defence
2016-10-28, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20161006

Available from: 2016-10-06 Created: 2016-10-05 Last updated: 2017-04-12Bibliographically approved
List of papers
1. Formation and the structure of freeze-dried MgO nanoparticle foams and their electrical behaviour in polyethylene
Open this publication in new window or tab >>Formation and the structure of freeze-dried MgO nanoparticle foams and their electrical behaviour in polyethylene
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2015 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 14, 7523-7534 p.Article in journal (Refereed) Published
Abstract [en]

Electrically insulating low-density polyethylene (LDPE) nanocomposites based on dispersed MgO nanoparticle foams are reported. The foams were obtained via freeze-drying aqueous suspensions of precipitated ca. 40 nm wide and 10 nm thick Mg(OH)(2) nanoparticles and dewatering (calcining) at 400 degrees C, resulting in a 25 times more voluminous powder compared to conventionally dried nanoparticles. This powder handling prior to extrusion melt-processing greatly facilitated the nanocomposite preparation since no particle grinding was necessary. Large quantities of particles were prepared (>5 g), and the nanoparticle foams showed improved dispersion in the LDPE matrix with 70% smaller aggregate sizes compared to the conventionally dried and ground nanopowders. The nature of the nanoparticle foams was evaluated in terms of their dispersion on Si-wafers using ultrasonication as a dispersing aid, which showed to be detrimental for the nanoparticle separation into solitary particles and induced severe aggregation of the calcined nanoparticles. The grind-free MgO nanoparticles/LDPE-composite was evaluated by electrical measurement. The prepared composite showed an initial ca. 1.5 orders of magnitude lower charging current at 10(2) s, and a 4.2 times lower charging current after 16 hours compared to unfilled LDPE. The results open a way for improved insulation to be implemented in the future high-voltage cable system and present a new promising nanoparticle powder handling technique that can be used on a large scale.

National Category
Chemical Engineering Physical Sciences
Identifiers
urn:nbn:se:kth:diva-165239 (URN)10.1039/c4ta06362g (DOI)000351845400042 ()2-s2.0-84925651646 (Scopus ID)
Note

QC 20150505

Available from: 2015-05-05 Created: 2015-04-24 Last updated: 2017-12-04Bibliographically approved
2. The impact of MgO nanoparticle interface in ultra-insulating polyethylene nanocomposites for high voltage DC cables
Open this publication in new window or tab >>The impact of MgO nanoparticle interface in ultra-insulating polyethylene nanocomposites for high voltage DC cables
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2016 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 4, no 22, 8590-8601 p.Article in journal (Refereed) Published
Abstract [en]

Low density polyethylene (LDPE) nanocomposites with a reduced conductivity of two orders of magnitude are reported as a novel insulation material for high voltage distribution of renewable energy. The key to the high insulation capacity was to provide 70 nm hexagonal MgO nanoparticles with relatively tong, preferably 18 units long, hydrocarbon functional silsesquioxane coatings. This rendered the surface of the particles completely hydrophobic and also served as a protective layer against adsorption of polar low molecular weight atmospheric substances (H2O and CO2). The elimination of trace amounts of water, in combination with the provided carbon functionality, dramatically improved the dispersion of MgO nanoparticles. The lowest volume conductivity was ca. 7 x 10(-16) s m(-1) for 3 wt% surface coated nanoparticles. Extensive electron microscopy characterization was further used to relate the measured volume conductivity, acquired under conditions that resemble 800 kV high voltage direct current (HVDC) cables, to the distribution of the nanoparticles in the polymer matrix. The results show that an appropriate surface-modification approach yielded uniformly dispersed MgO nanoparticles up to contents as high as 9 wt%, with maintained 10-100 times reduced volume conductivity. Simulations of the MgO nanoparticles distribution revealed that the required interaction radius of the MgO-phase was 775 nm, setting a lower limit of particle amount to effectively work as electrical insulation promoters. The reduced volume conductivity values and scalable processing chemistry reported allow for the production of the next generation insulation material for HVDC cables.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2016
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-189933 (URN)10.1039/c6ta02041k (DOI)000378583200011 ()2-s2.0-84963937777 (Scopus ID)
Note

QC 20160728

Available from: 2016-07-28 Created: 2016-07-25 Last updated: 2017-11-28Bibliographically approved
3. Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture Resistant MgO Nanoparticles
Open this publication in new window or tab >>Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture Resistant MgO Nanoparticles
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2016 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 23, 14824-14835 p.Article in journal (Refereed) Published
Abstract [en]

The use of MgO nanoparticles in polyethylene for cable insulation has attracted considerable interest, although in humid media the surface regions of the nanoparticles undergo a conversion to a hydroxide phase. A facile method to obtain MgO nanoparticles with a large surface area and remarkable inertness to humidity is presented. The method involves (a) low temperature (400 degrees C) thermal decomposition of Mg(OH)(2), (b) a silicone oxide coating to conceal the nanoparticles and prevent interparticle sintering upon exposure to high temperatures, and (c) heat treatment at 1000 degrees C. The formation of the hydroxide phase on these silicone oxide-coated MgO nanoparticles after extended exposure to humid air was assessed by thermogravimetry, infrared spectroscopy, and X-ray diffraction. The nanoparticles showed essentially no sign of any hydroxide phase compared to particles prepared by the conventional single-step thermal decomposition of Mg(OH)(2). The moisture-resistant MgO nanoparticles showed improved dispersion and interfacial adhesion in the LDPE matrix with smaller nanosized particle clusters compared with conventionally prepared MgO. The addition of 1 wt % moisture-resistant MgO nanoparticles was sufficient to decrease the conductivity of polyethylene 30 times. The reduction in conductivity is discussed in terms of defect concentration on the surface of the moisture-resistant MgO nanoparticles at the polymer/nanoparticle interface.

Keyword
MgO nanoparticles, thermal decomposition, surface coating, humidity-resistance, HVDC cable
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-189808 (URN)10.1021/acsami.6b04188 (DOI)000378195000064 ()27203860 (PubMedID)2-s2.0-84975246468 (Scopus ID)
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20160719

Available from: 2016-07-19 Created: 2016-07-15 Last updated: 2017-11-28Bibliographically approved
4. Local Charge Injection and Extraction on Surface-Modified Al2O3Nanoparticles in LDPE
Open this publication in new window or tab >>Local Charge Injection and Extraction on Surface-Modified Al2O3Nanoparticles in LDPE
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2016 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 9, 5934-5937 p., 10.1021/acs.nanolett.6b02920Article in journal (Refereed) Published
Abstract [en]

We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
Keyword
HVDC, intermodulation, KPFM, nanodielectrics, polyethylene nanocomposites, surface potential
National Category
Nano Technology
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-193640 (URN)10.1021/acs.nanolett.6b02920 (DOI)000383412100091 ()27532486 (PubMedID)2-s2.0-84987718577 (Scopus ID)
Funder
Swedish Foundation for Strategic Research , EM11-0022Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20161006

Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2017-11-30Bibliographically approved
5. Three-dimensional nanometre features of direct current electrical trees in low-density polyethylene
Open this publication in new window or tab >>Three-dimensional nanometre features of direct current electrical trees in low-density polyethylene
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(English)Manuscript (preprint) (Other academic)
Keyword
electrical tree, ptychography, DC-tree, HVDC, polyethylene
National Category
Textile, Rubber and Polymeric Materials Nano Technology Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-193642 (URN)
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20161006

Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-06Bibliographically approved

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