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The synthesis, surface modification and use of metal-oxide nanoparticles in polyethylene for ultra-low transmission-loss HVDC cable insulation materials
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.ORCID iD: 0000-0001-5867-0531
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polyethylene composites which contain low concentrations of metal-oxide nanoparticles e.g. ZnO and MgO are emerging materials for the use in insulations of extruded high-voltage direct-current (HVDC) cables. The challenge in the development of the composites with ultra-low electrical conductivity is to synthesize uniform and high-purity metal-oxide nanoparticles, which are functionalized with hydrophobic groups in order to make them compatible with polyethylene. The thesis reports different approaches to prepare this new generation of insulation materials.

Different reaction parameters/conditions – zinc salt precursor, precursor concentrations and reaction temperature – were varied in order to tailor the size and morphology of the ZnO nanoparticles. It was shown that different particle sizes and particle morphologies could be obtained by using different zinc salt precursors (acetate, nitrate, chloride or sulphate). It was shown that 60 °C was a suitable reaction temperature in order to yield particles with different morphologies ranging from nano-prisms to flower-shaped superstructures. For removal of reaction residuals from the particles surfaces, a novel cleaning method based on ultrasonication was developed, which was more efficient than traditional water-replacement cleaning. After cleaning, the presence of one atomic layer of zinc-hydroxy-salt complex (ZHS) on the nanoparticle surfaces was suggested by thermogravimetry and infrared spectroscopy. A method involving three steps – silane coating, heat treatment and silica layer etching – was used to remove the last trace of the ZHS species from the nanoparticle surface while preserving its clean and active hydroxylated surface. The surface chemistry of these nanoparticles was further tailored from hydroxyl groups to hydrophobic alkyl groups with different lengths by reactions involving methyltrimethoxysilane (C1), octyltriethoxysilane (C8) and octadecyltrimethoxysilane (C18).

MgO nanoparticles were prepared by aqueous precipitation of Mg(OH)2 followed by a partial transformation to MgO nanoparticles via heat treatment at 400 °C. The surface regions of the MgO nanoparticles convert into a hydroxide phase in humid media. A novel method to obtain large surface area MgO nanoparticles with a remarkable inertness to humidity was also presented. The method involved three steps:  (a) thermal decomposition of Mg(OH)2 at 400 °C; (b) silicone oxide coating of the nanoparticles to prevent inter-particle sintering and (c) a high temperature heat treatment at 1000 °C. These MgO nanoparticles showed essentially no sign of formed hydroxide phase even after extended exposure to humid air.

The functionalized metal-oxide nanoparticles showed only a minor adsorption of phenolic antioxidant, which is important in order to obtain nanocomposites with an adequate long-term stability. Tensile testing and scanning electron microscopy revealed that the surface-modified metal-oxide nanoparticles showed improved dispersion and interfacial adhesion in the polyethylene matrix with reference to that of unmodified metal-oxide nanoparticles. The highly “efficient” interfacial surface area induced by these modified nanoparticles created the traps for charge carriers at the polymer/particle interface thus reducing the DC conductivity by more than 1 order of magnitude than that of the pristine polyethylene.

Abstract [sv]

Polyetenkompositer med mycket låga halter av ZnO och MgO metalloxid nanopartiklar är en växande kategori material för användning som isolering av extruderade kablar avsedda för likriktad högspänning. En utmaning i utvecklingen av dessa material kan relateras till den praktiska kompositframställningen, vilken innefattar framställning av högrena metalloxid nanopartiklar som ytmodifieras med hydrofoba molekylstrukturer för att möjliggöra blandning med den hydrofoba polyetenplasten. Denna avhandling behandlar olika metoder för att framställa denna generation av isoleringsmaterial.

Vid syntesen av de rena nanopartiklarna krävdes optimering av ett antal olika reaktionsparametrar för att uppnå tillfredställande slutresultat i form av partikelstorlekar och partikelmorfologier. Dessa inkluderade val av zinksalt, zinksaltkoncentration vid utfällning, samt reaktionstemperatur vid framställningen. Experimenten avslöjade att olika partikelstorlekar och partikelmorfologier kunde framställas som endast korrelerat mot källan av zinkjonerna, och berodde av vilka motjoner som zinkatomerna haft i zinksaltet (acetat, nitrat, klorid eller sulfat). Optimering av reaktionstemperaturen visade att ca 60 °C utgjorde en lämplig start för utvärdering av synteserna, som resulterade i olika partikelmorfologier i form av pyramidformade nanopartiklar till blomformationer. Utöver de specifika reaktionsparametrarna utvecklades även en ny ultrasonikeringsmetod för att rena ytorna hos partiklarna från motjoner relaterade till de valda specifika salterna. Metodiken som visade sig avsevärt mer effektiv än sedvanlig rening att utfällda nanopartiklar via repetitivt vattenutbyte, och skapade förutsättningar etablering av kolloidal stabilitet och fragmentering av aggregat i vattensuspensionerna. Efter ultrasonikeringsreningen beräknades de kvarvarande zinkhydroxidsalterna (ZHS) utgöra endast ett atomlager ZHS utifrån termogravimetriska data kompletterade med infraröd spektroskopi. En metod att eliminera de kvarvarande ZHS-komplexen från ytan av partiklarna tillämpades/utvecklades, inkluderade ytbeläggning av partiklarna med silan, följt av värmebehandling samt etsning av den resulterande kiseloxidytan, för att uppnå en ren hydroxylyta på partiklarna. Ytkemin hos dessa partiklar modifierades från att bestå av hydroxylgrupper till att utgöras av hydrofoba alkylgrupper med olika längder relaterade metyltrimetoxysilan (C1), oktyltrietoxysilan (C8), eller oktadekyltrimetoxysilan (C18).

Även MgO nanopartiklar framställdes via vattenutfällning av Mg(OH)2 partiklar, vilka omvandlades till MgO nanopartiklar via en lågtemperatur värmebehandling vid 400°C. Ytan av dessa partiklar omvandlades dock till hydroxid i fuktig miljö. En ny metod att bibehålla den stora ytarean av MgO nanopartiklarna med anmärkningsvärd motståndskraft mot att omvandlas till hydroxid utvecklades således. Metoden består av (a) en låg temperatur omvandling av Mg(OH)2, (b) en kiseloxidytbehandling av nanopartiklarna för att undvika partikelsintring vid högre temperaturer och (c) en hög temperaturbehandling vid 1000 °C. De framställda partiklarna uppvisade ingen anmärkningsvärd känslighet mot luftfuktighet och bibehöll MgO sammansättningen efter exponering mot fukt.

De modifierade metalloxid nanopartiklarna visade mycket liten adsorption av fenoliska antioxidanter, vilket medförde en långtidsstabilitet hos polyeten nanokompositerna. De ytmodifierade metalloxidpartiklarna visade även förbättrade möjligheter för dispergering och yt-kompatibilitet med/i polyetenmatrisen i jämförelse med omodifierade metalloxidpartiklar, utifrån mätningar baserade på dragprovning och svepelektronmikroskopi. Slutligen, de utvecklade ytorna på de modifierade nanopartiklarna skapade ett polymer/nanopartikel gränssnitt som kunder fungera som laddningsansamlingsområden i nanokompositerna, vilket resulterade i en storleksordning minskad ledningsförmåga hos kompositerna jämfört med den rena polyetenen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 57 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:30
Keyword [en]
polyethylene, metal-oxide nanoparticles, heat treatment, surface coating, humidity resistance, interfacial adhesion, nanocomposite, HVDC insulation
Keyword [sv]
polyeten, metalloxid nanopartiklar, värmebehandling, ytmodifiering av partiklarna, fukt inverkan, gränsskiktsvidhäftning, nanokomposit, HVDC isolering
National Category
Polymer Technologies
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-190808ISBN: 978-91-7729-059-9 (print)OAI: oai:DiVA.org:kth-190808DiVA: diva2:952950
Public defence
2016-09-23, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20160829

Available from: 2016-08-29 Created: 2016-08-16 Last updated: 2016-10-14Bibliographically approved
List of papers
1. Water-based synthesis and cleaning methods for high purity ZnO nanoparticles - comparing acetate, chloride, sulphate and nitrate zinc salt precursors
Open this publication in new window or tab >>Water-based synthesis and cleaning methods for high purity ZnO nanoparticles - comparing acetate, chloride, sulphate and nitrate zinc salt precursors
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2014 (English)In: RSC Advances, ISSN 2046-2069, Vol. 4, no 67, 35568-35577 p.Article in journal (Refereed) Published
Abstract [en]

A low temperature (60 degrees C) aqueous synthesis method of high purity ZnO nanoparticles intended as fillers for ultra-low electrical conductivity insulations is described. Particles were prepared under identical conditions from different zinc salts based on nitrate, chloride, sulphate or acetate to compare their abilities to form high yields of sub-50 nm particles with narrow size distribution. The acetate salt gave uniform 25 nm ZnO particles with a conical prism shape. The chloride and sulphate derived particles showed mixed morphologies of nanoprisms and submicron petals, whereas the nitrate salt yielded prisms assembled into well-defined flower shapes with spiky edges. The micron-sized flower shapes were confirmed by Xray diffraction to consist of the smaller prism units. Photoluminescence spectroscopy showed emission in the blue-violet region with little variation depending on precursor salt, suggesting that the spectra were dependent on the primary nanoprism formation and rather independent of the final particle morphology. Microscopy revealed that the salt residuals after the reaction showed different affinity to the particle surfaces depending on the type of salt used, with the acetate creating ca. 20 nm thick hydrated shells; and in falling order of affinity: chloride, sulphate and nitrate. An acetate ion shielding effect during the synthesis was therefore assumed, preventing nanoparticle fusion during growth. Varying the concentrations of the counter-ions confirmed the shielding and only the acetate anions showed an ability to stabilize solitary nanoprisms formation in reaction yields from 2 to 10 g L-1. Ultrasonic particle surface cleaning was significantly more efficient than water replacement, resulting in a stable aqueous dispersion with a high zeta potential of 38.9 mV at pH 8.

Keyword
Chlorine compounds, Ions, Metal nanoparticles, Morphology, Nanostructures, Nitrates, Photoluminescence spectroscopy, Prisms, Salts, Shielding, Sulfur compounds, Volatile fatty acids, X ray diffraction, Aqueous synthesis, Electrical conductivity, Identical conditions, Narrow size distributions, Particle morphologies, Shielding effect, Stable aqueous dispersions, ZnO nanoparticles
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-153415 (URN)10.1039/c4ra06651k (DOI)000341288100039 ()2-s2.0-84906544319 (Scopus ID)
Note

QC 20141008

Available from: 2014-10-08 Created: 2014-10-03 Last updated: 2016-08-25Bibliographically approved
2. Heat treatment of ZnO nanoparticles: new methods to achieve high-purity nanoparticles for high-voltage applications
Open this publication in new window or tab >>Heat treatment of ZnO nanoparticles: new methods to achieve high-purity nanoparticles for high-voltage applications
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2015 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 33, 17190-17200 p.Article in journal (Refereed) Published
Abstract [en]

Novel methods based on orienting and coating of ZnO nanoparticles were studied in order to obtain uniform, nano-sized and ultra-pure ZnO grains/particles after heat treatment. A 1 nm zinc-hydroxy-salt complex layer on the nanoparticle surfaces was revealed by thermogravimetry and infrared spectroscopy. This 'phase' gradually decomposed into ZnO during the heat treatment while sintering occurred above 600 degrees C, as revealed by scanning-and transmission-electron microscopy. The c-axis alignment of the nanoparticles provided smaller pores than those associated with non-oriented nanoparticles, presenting the means to obtain high-density ceramics. The orientation resulted in a smaller grain size after heat treatment than that of the nonaligned nanoparticles. Another method that involved three steps - silane coating, heat treatment and silica layer etching - was used to remove the ionic species from the nanoparticle surface while preserving its hydroxylated surface. These ultra-pure nanoparticles are expected to be key components in the development of HVDC insulation polyethylene nanocomposites.

National Category
Energy Engineering Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-173293 (URN)10.1039/c5ta03120f (DOI)000359459900033 ()2-s2.0-84939196366 (Scopus ID)
Note

QC 20150909

Available from: 2015-09-09 Created: 2015-09-09 Last updated: 2016-12-06Bibliographically approved
3. Aqueous synthesis of (21̅0) oxygen terminated defect free hierarchical ZnO particles and their heat treatment for enhanced reactivity
Open this publication in new window or tab >>Aqueous synthesis of (21̅0) oxygen terminated defect free hierarchical ZnO particles and their heat treatment for enhanced reactivity
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2016 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 32, no 42, 11002-11013 p.Article in journal (Refereed) Published
Abstract [en]

A controlled aqueous growth of 1 µm flower-shaped ZnO particles with a hierarchical subset of exposed nano-sheets represented by {21̅0} crystal faces, followed by annealing at temperatures up to 1000 °C, is presented. The flower-shaped particles showed superior photocatalytic performance compared to the crystal faces of 20 nm ZnO nanoparticles. The photocatalytic reaction rate of the flower-shaped particles before annealing was 2.4 times higher per m2 compared with that of the nanoparticles with double specific surface area. Crystal surface defects and nano-sized pores within the flower-shaped particles were revealed by porosity measurement and electron microscopy. A heat treatment at 400 °C was found to be optimal for removal of nanoporosity/surface defects and impurities while retaining the hierarchical superstructure. The heat treatment resulted in a photo-degradation efficiency that increased by an additional 43 %, although the specific surface area decreased from 16.7 to 13.0 m2g-1. The enhanced photocatalytic effect remained intact under both acidic and alkaline environments owing to the {21̅0} crystal surfaces, which were less prone to dissolution than the nanoparticles. The photocatalytic performance relied on primarily three factors: the removal of surface impurities, the oxygen termination of the {21̅0} crystal faces, and the promotion of charge carrier lifetime by removal of lattice defects acting as recombination centres. The synthesis presented is an entirely hydrocarbon- and surfactant free ('green') preparation scheme, and the formation of the flower-shaped particles was favored solely by optimization of the reaction temperature after the correct nitrate salts precursor concentrations had been established.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016
Keyword
Carrier lifetime, Crystal defects, Crystal impurities, Heat treatment, Nanoparticles, Photodegradation, Porosity, Removal, Specific surface area, Zinc oxide, Alkaline environment, Defects and impurities, Photocatalytic effect, Photocatalytic performance, Photocatalytic reactions, Photodegradation efficiency, Porosity measurement, Recombination centers
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-193667 (URN)10.1021/acs.langmuir.6b03263 (DOI)000386422300024 ()27689906 (PubMedID)2-s2.0-84994008685 (Scopus ID)
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20161007

Available from: 2016-10-07 Created: 2016-10-07 Last updated: 2016-11-21Bibliographically approved
4. 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: 2016-10-06Bibliographically approved
5. Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene
Open this publication in new window or tab >>Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene
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2016 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 125, 21-32 p.Article in journal (Refereed) Published
Abstract [en]

Polyethylene composites based on metal oxide nanoparticles are emerging materials for use in the insulation of extruded HVDC cables. The short-term electrical performance of these materials is adequate, but their stability for extended service needs to be assessed. This study is focussed on the capacity of the nanoparticles to adsorb polar species (water, dicumyl peroxide and byproducts from peroxide-vulcanisation, acetophenone and cumyl alcohol) that have an impact on the electrical conductivity of nanocomposites, the oxidative stability by adsorption of phenolic antioxidants on the nanoparticles and the potential transfer of catalytic impurities from the nanoparticles to the polymer. The adsorption of water, dicumyl peroxide, acetophenone, cumyl alcohol and Irganox 1076 (phenolic antioxidant) on pristine and coated (hydrophobic silanes and poly(lauryl methacrylate)) Al2O3, MgO and ZnO particles ranging from 25 nm to 2 gm was assessed. Composites based on low-density polyethylene and the particles mentioned (<= 12 wt.%) were prepared, the degree of adsorption of Irganox 1076 onto the particles was assessed by OIT measurements, and the release of volatile species at elevated temperature was assessed by TG. The concentration of moisture adsorbed on the particles at 25 degrees C increased linearly with both increasing hydroxyl group concentration on the particle surfaces and increasing relative humidity. Dicumyl peroxide showed no adsorption on any of the nanoparticles. Acetophenone and cumyl alcohol showed a linear increase in adsorption with increasing concentration of hydroxyl groups, but the quantities were much smaller than those of water. Irganox 1076 adsorbed only onto the uncoated nanoparticles. Uncoated ZnO nanoparticles that contained ionic species promoted radical formation and a lowering of the OIT. This study showed that carefully coated pure metal oxide nano particles are not likely to adsorb phenolic antioxidants or dicumyl peroxide, but that they have the capacity to adsorb moisture and polar byproducts from peroxide vulcanisation, and that they will not introduce destabilizing ionic species into the polymer matrix. Low contents of dry, equiaxed ZnO and MgO particles strongly retarded the release of volatile species at temperatures above 300 degrees C.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Polyethylene, Metal oxide nanoparticles, Water, Acetophenone, Cumyl alcohol, Phenolic antioxidant, Adsorption
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-184031 (URN)10.1016/j.polymdegradstab.2015.12.014 (DOI)000370894900003 ()2-s2.0-84954186925 (Scopus ID)
Note

QC 20160324

Available from: 2016-03-24 Created: 2016-03-22 Last updated: 2016-09-15Bibliographically approved
6. Highly Efficient Interfaces in Nanocomposites Based on Polyethylene and ZnO Nano/Hierarchical Particles: A Novel Approach toward Ultralow Electrical Conductivity Insulations.
Open this publication in new window or tab >>Highly Efficient Interfaces in Nanocomposites Based on Polyethylene and ZnO Nano/Hierarchical Particles: A Novel Approach toward Ultralow Electrical Conductivity Insulations.
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2016 (English)In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 28, no 39, 8651-8657 p.Article in journal (Refereed) Published
Abstract [en]

Polyethylene nanocomposites based on functionalized ZnO nano/hierarchical particles with highly effective interfacial surface area are presented, for the next generation of ultralow transmission-loss high-voltage DC insulating materials.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2016
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-190824 (URN)10.1002/adma.201603291 (DOI)000387147200009 ()27502081 (PubMedID)2-s2.0-84980605549 (Scopus ID)
Note

QC 20160823

Available from: 2016-08-16 Created: 2016-08-16 Last updated: 2017-01-10Bibliographically approved

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