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Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
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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.

Place, publisher, year, edition, pages
2011. Vol. 412, no 2, 227-232 p.
Keyword [en]
DOPED AMMONIUM PARATUNGSTATE, HELIUM-COOLED DIVERTOR, GRAINED TUNGSTEN, CO POWDER, IRRADIATION, FABRICATION, ALLOYS, TEMPERATURE, PRECURSOR, DEFECTS
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-34656DOI: 10.1016/j.jnucmat.2011.03.007ISI: 000291086900003Scopus ID: 2-s2.0-79955481393OAI: oai:DiVA.org:kth-34656DiVA: diva2:426945
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
In thesis
1. Development of Nanostructured Tungsten Based Composites for Energy Applications
Open this publication in new window or tab >>Development of Nanostructured Tungsten Based Composites for Energy Applications
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
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:nbn:se:kth:diva-101319 (URN)
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
2. Tungsten-Based Nanocomposites by Chemical Methods
Open this publication in new window or tab >>Tungsten-Based Nanocomposites by Chemical Methods
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Tungsten based-materials find use in many different fields of engineering, particularly in applications where good temperature and/or erosion resistance is important. Nanostructuring of tungsten composites is expected to dramatically improve the materials’ properties and enhancing the performance in present applications but also enabling totally new possibilities. Nanostructured WC-Co materials have been the focus of researchers and engineers for over two decades. New fabrication methods have been developed. But, the fabrication of true nanograined WC-Co composites is still a challenge. Nanostructured tungsten-based materials for applications as plasma facing materials in fusion reactors have attracted a growing interest. This Thesis summarizes work on the development of chemical methods for the fabrication of two different types of nanostructured tungsten-based materials; WC-Co composites mainly for cutting tools applications and W-ODS materials with yttria particles, intended as plasma facing materials in fusion reactors. The approach has been to prepare powders in two steps: a) synthesis of uniform powder precursors containing ions of tungsten and cobalt or yttrium by precipitation from aqueous solutions and b) processing of the precursors into WC- or W-based nano-composite powders.

Highly homogenous W- and Co- containing precursors for WC-Co composites were prepared via two different routes. Keggin-based precursors ((NH4)8[H2Co2W11O40]) were made from sodium tungstate or ammonium metatungstate and cobalt acetate. The powder composition corresponded to 5.2 % Co in the final WC-Co material. In a second approach, paratungstate-based precursors (Cox(NH4)10-2x[H2W12O42]) were prepared from ammonium paratungstate (APT) and cobalt hydroxide with different compositions corresponding to 3.7 to 9.7 % Co in WC-Co. Both precursors were processed and sintered into uniform microstructures with fine scale (<1μm). The processing of paratungstate-based precursors was also further investigated. WC-Co powders with grains size of less than 50 nm were obtained by decreasing processing temperatures and by applying gas phase carburization.

W-ODS materials were fabricated starting from ammonium paratungstate and yttrium salts. Paratungstate-based precursors were prepared with different homogeneities and particle sizes. The degree of the chemical uniformity varied with the particle size from ca 1 to 30 μm. Tungsten trioxide hydrate-based precursors made from APT and yttrium salts under acidic conditions had higher uniformity and smaller particle size. The tungsten oxide crystallite size was decreased to a few nanometers. Yttrium was included either by doping or in a nanocomposite structure as yttrium oxalate. The nanocomposite precursor was found to be more reactive during hydrogen reduction, facilitating its conversion to pure W-Y2O3 nanopowder. The doped precursor were further processed to nanopowders and sintered to highly uniform W-1.2%Y2O3 composites.

In  summary, APT was converted to highly homogenous or uniform powder precursors ofdifferent compositions. The transformations were carried out in aqueous suspensions as a water-mediated  process. These precursors were processed  further in to nano-sized  powders  and sintered to highly uniform tungsten composites with fine microstructures.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. 46 p.
Series
TRITA-ICT/MAP AVH, ISSN 1653-7610 ; 2014:20
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-156604 (URN)
Public defence
2014-12-11, Sal/hall 205, Electrum, KTH-ICT, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20141201

Available from: 2014-12-01 Created: 2014-12-01 Last updated: 2014-12-04Bibliographically approved

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