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Development of Nanostructured Tungsten Based Composites for Energy Applications
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tungsten (W) based materials can be used in fusion reactors due to several advantages. Different fabrication routes can be applied to develop tungsten materials with intended microstructure and properties for specific application including nanostructured grades. Therein, innovative chemical routes are unique in their approach owing numerous benefits. This thesis summarizes the development of W-based composites dispersed-strengthened by rare earth (RE) oxides and their evaluation for potential application as plasma facing armour material to be used in fusion reactor. Final material development was carried out in two steps; a) fabrication of nanostructured metallic tungsten powder dispersed with RE-oxides and b) powder sintering into bulk oxide-dispersed strengthened (ODS) composite by spark plasma process. With the help of advanced characterization tools applied at intermediate and final stages of the material development, powder fabrication and sintering conditions were optimized. The aim was to achieve a final material with a homogenous fine microstructure and improved properties, which can withstand under extreme conditions of high temperature plasma.

Two groups of starting materials, synthesized via novel chemical methods, having different compositions were investigated. In the first group, APT-based powders doped with La or Y elements in similar ways, had identical particles’ morphology (up to 70 μm). The powders were processed into nanostructured composite powders under different reducing conditions and were characterized to investigate the effects on powder morphology and composition. The properties of sintered tungsten materials were improved with dispersion of La2O3 and Y2O3 in the respective order. The oxide dispersion was less homogeneous due to the fact that La or Y was not doped into APT particles. The second group, Ydoped tungstic acid-based powders synthesized through entirely different chemistry, contained nanocrystalline particles and highly uniform morphology. Hydrogen reduction of doped-tungstic acid compounds is complex, affecting the morphology and composition of the final powder. Hence, processing conditions are presented here which enable the separation of Y2O3 phase from Y-doped tungstic acid.

Nevertheless, the oxide dispersion reduces the sinterability of tungsten powders, the fabricated nanostructured W-Y2O3 powders were sinterable into ultrafine ODS composites at temperatures as low as 1100 °C with highly homogeneous nano-oxide dispersion at W grain boundaries as well as inside the grain. The SPS parameters were investigated to achieve higher density with optimum finer microstructure and higher hardness. The elastic and fracture properties of the developed ODS-W have been investigated by micro-mechanical testing to estimate the materials’ mechanical response with respect to varying density and grain size. In contrast from some literature results, coarse grained ODS-W material demonstrated better properties. The developed ODS material with 1.2 Y2O3 dispersion were finally subjected to high heat flux tests in the electron beam facility “JUDITH-1”. The samples were loaded under ELM-like thermal-shocks at varying base temperatures up to an absorbed power density of 1.13 GW/m2, for armour material evaluation. Post mortem characterizations and comparison with other reference W grades, suggest lowering the oxide contents below 0.3 wt. % Y2O3.

As an overview of the study conducted, it can be concluded that innovative chemical routes can be potential replacement to produce tungsten based materials of various composition and microstructure, for fusion reactor applications. The methods being cheap and reproducible, are also easy to handle for large production at industrial scale.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 48 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:13
Keyword [en]
Tungsten, Nano-tungsten, ODS tungsten, W-La2O3, W-Y2O3, SPS, Plasma-facing materials, Armour material, fusion material
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-101319OAI: oai:DiVA.org:kth-101319DiVA: diva2:547136
Public defence
2012-09-04, Sal/Hall C2, KTH-Electrum, Isafjordsgatan 26, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20120827

Available from: 2012-08-27 Created: 2012-08-27 Last updated: 2012-11-19Bibliographically approved
List of papers
1. Chemically produced nanostructured ODS-lanthanum oxide-tungsten composites sintered by spark plasma
Open this publication in new window or tab >>Chemically produced nanostructured ODS-lanthanum oxide-tungsten composites sintered by spark plasma
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2011 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 408, no 2, 129-135 p.Article in journal (Refereed) Published
Abstract [en]

High purity W and W-0.9La(2)O(3) (wt.%) nanopowders were produced by a wet chemical route. The precursor was prepared by the reaction of ammonium paratungstate (APT) with lanthanum salt in aqueous solutions. High resolution electron microscopy investigations revealed that the tungstate particles were coated with oxide precipitates. The precursor powder was reduced to tungsten metal with dispersed lanthanum oxide. Powders were consolidated by spark plasma sintering (SPS) at 1300 and 1400 degrees C to suppress grain growth during sintering. The final grain size relates to the SPS conditions, i.e. temperature and heating rate, regardless of the starting powder particle size. Scanning electron microscopy revealed that oxide phases were mainly accumulated at grain boundaries while the tungsten matrix constituted of nanosized sub-grains. The transmission electron microscopy revealed that the tungsten grains consist of micron-scale grains and finer sub-grains. EDX analysis confirmed the presence of W in dispersed oxide phases with varying chemical composition, which evidenced the presence of complex oxide phases (WO-La) in the sintered metals.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-30527 (URN)10.1016/j.jnucmat.2010.10.060 (DOI)000286788700001 ()2-s2.0-78650726734 (Scopus ID)
Funder
EU, FP7, Seventh Framework ProgrammeSwedish Research Council
Note
QC 20110304Available from: 2011-03-04 Created: 2011-02-28 Last updated: 2017-12-11Bibliographically approved
2. Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization
Open this publication in new window or tab >>Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization
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2011 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 412, no 2, 227-232 p.Article in journal (Refereed) Published
Abstract [en]

Nano-crystalline W-1%Y2O3 (wt.%) powder was produced by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. The precursor powder was found to consist of particles of bimodal morphology i.e. large APT-like particles up to 20 pm and rectangular yttrium containing ultrafine plates. After thermal processing tungsten crystals were evolved from W-O-Y plate like particles. spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200 degrees C for different holding times in order to optimize the sintering conditions to yield high density but with reduced grain growth. Dispersion of yttrium oxide enhanced the sinterability of W powder with respect to lanthanum oxide. W-1%Y2O3 composites with sub-micron grain size showed improved density and mechanical properties as compared to W-La2O3 composites. Sample sintered in two steps showed improved density, due to longer holding time at lower temperature (900 degrees C) and less grain growth due to shorter holding time at higher temperature i.e. 1 min at 1100 degrees C.

Keyword
DOPED AMMONIUM PARATUNGSTATE, HELIUM-COOLED DIVERTOR, GRAINED TUNGSTEN, CO POWDER, IRRADIATION, FABRICATION, ALLOYS, TEMPERATURE, PRECURSOR, DEFECTS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-34656 (URN)10.1016/j.jnucmat.2011.03.007 (DOI)000291086900003 ()2-s2.0-79955481393 (Scopus ID)
Funder
EU, FP7, Seventh Framework ProgrammeSwedish Research Council
Note
QC 20110627Available from: 2011-06-27 Created: 2011-06-13 Last updated: 2017-12-11Bibliographically approved
3. Fabrication of Nanostructured W-Y2O3 Materials by Chemical Methods
Open this publication in new window or tab >>Fabrication of Nanostructured W-Y2O3 Materials by Chemical Methods
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2012 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 25, 12622-12628 p.Article in journal (Refereed) Published
Abstract [en]

A novel method for the fabrication of highly uniform oxide dispersion-strengthened (ODS) materials made by chemical processing is presented. The powders are fabricated by a two-step route starting with a chemical synthesis at room temperature, producing nanocrystalline yttrium doped tungsten trioxide hydrate precursor powders. Thermogravimetric analysis with evolved gas analysis revealed the presence of ammonium nitrate in the precursors. The second step is the reduction of the precursor in a hydrogen atmosphere at 600 and 800 degrees C. The reduced powders, containing W-1.2%Y2O3, showed two types of tungsten particles, cube-shaped with a size less than 250 nm and finer particles (<50 nm) of both spherical and cubic shape. The powder was consolidated by spark plasma sintering at 1100 degrees C, producing a bulk material with a relative density of 88%. Characterization of the sintered materials by high resolution scanning electron microscopy revealed a uniform microstructure with tungsten grains of less than 300 nm and nanosized oxide particles uniformly dispersed at the tungsten grain boundaries, as well as inside the tungsten grains. Experimental determination of the elastic properties was conducted by nanoindentation tests and fracture toughness was studied by radial indentation cracking.

Keyword
doped ammonium paratungstate, tungsten blue oxide, mechanical-properties, technical reduction, youngs modulus, co powder, composites, decomposition, indentation, precursor
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-43229 (URN)10.1039/c2jm30652b (DOI)000304884000030 ()2-s2.0-84862231536 (Scopus ID)
Funder
EU, FP7, Seventh Framework ProgrammeSwedish Research Council
Note
Updated from manuscript to article in journal. QC 20120702Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-12-08Bibliographically approved
4. Processing and Sintering of Yttrium-Doped Tungsten Oxide Nano-powders
Open this publication in new window or tab >>Processing and Sintering of Yttrium-Doped Tungsten Oxide Nano-powders
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(English)In: Journal of nanoparticle research, ISSN 1388-0764, E-ISSN 1572-896XArticle in journal (Other academic) Submitted
Abstract [en]

Innovative chemical methods are capable of fabricating nanoscale tungsten oxide compoundsd oped with various rare-earth elements with high purity and homogeneity, which can be processed under hydrogen into nanostructured oxide-dispersed tungsten composite powders having several potential applications. However, hydrogen reduction of doped-tungsten oxide compounds is rather complex, affecting the morphology and composition of the final powder. In this study we have investigated the reduction of tungstic acid in the presence of Y and weprovide the experimental evidence that Y2O3 can be separated from Y-doped tungstic acid via hydrogen reduction to produce Y2O3-W powders. The processed powders were further consolidated by spark plasma sintering at different temperatures and holding times at 75 MPa pressure and characterized. The optimized SPS conditions suggest sintering at 1400 °C for 3 min holding time to achieve higher density composites with an optimum finer grain size (3 μm) and a hardness value up to 420 HV. Major grain growth takes place at temperatures above 1300 °C during sintering. From the density values obtained, it is recommend to apply higher pressure before 900 °C to obtain maximum density. Oxides inclusions present in the matrix were identified as Y2O3•3WO3 and Y2O3•WO3 during high resolution microscopici nvestigations.

Keyword
nanostructured W, ODS-tungsten, Nano W-Y2O3, tungsten oxide reduction, Tungstic acid
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-101166 (URN)
Note

QS 2012

Available from: 2012-08-24 Created: 2012-08-24 Last updated: 2017-12-07Bibliographically approved
5. Micro-mechanical and high heat load testing of W-Y2O3 ODS armourmaterials fabricated by novel chemical method and SPS
Open this publication in new window or tab >>Micro-mechanical and high heat load testing of W-Y2O3 ODS armourmaterials fabricated by novel chemical method and SPS
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(English)In: Journal of nanoparticle research, ISSN 1388-0764, E-ISSN 1572-896XArticle in journal (Other academic) Submitted
Abstract [en]

Oxide-dispersed strengthened (ODS) - tungsten based composites can be fabricated using several methods. In this study W-Y2O3 composite powders were synthesized by an innovative chemical process yielding ultrafine to micron range grains that were subsequently compacted using spark plasma sintering (SPS). Micro-mechanical tests were conducted to investigate the elastic and fracture properties of sintered compacts with grain size from ultrafine to several microns. For the evaluation of the developed material for plasma facing armour application in the fusion reactor, high heat load tests have been performed in an electron beam test facility. Surface effects, i.e. roughening, particle erosion and crack formation in dependence of base temperature and power density, were determined for an applied number of 100 ELM like loads with a pulse duration of 1 ms. The crack paths at the surface and particularly versus the bulk material were investigated to determine the resistance of the material to the formation of cracks parallel to the surface which finally would limit the thermal transfer and lead to local overheating and probably melting. Furthermore, the thermal stability of material, i.e. the resistance to recrystallization was determined by thermal annealing up to 1800 °C and during the electron beam tests by applying heat loads that lead to a surface temperature increase of> 2000°C.

Keyword
ODS-tungsten, W-Y2O3, Thermal shock test, nano indentation, armour material, PFC, high heat load
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-101164 (URN)
Note

QS 2012

Available from: 2012-08-24 Created: 2012-08-24 Last updated: 2017-12-07Bibliographically approved

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