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Nanostructured Tungsten Materials by Chemical Methods
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Tungsten based-materials are used in many different technical fields, particularly in applications requiring good temperature and/or erosion resistance. Nanostructuring of tungsten alloys and composites has the potential to dramatically improve the materials’ properties, enhancing the performance in present applications or enabling totally new possibilities. Nanostructured WC-Co composites have been the focus of researchers and industries for over two decades. New methods for powder fabrication and powder consolidation have been developed. However, the fabrication of true nanograined WC-Co materials is still a challenge. Nanostructured tungsten composites for applications as plasma facing materials in fusion reactors have in recent years 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 materials mainly aimed at cutting tools applications and W-ODS composites with rare earth oxide particles, intended as plasma facing materials in future fusion reactors. The approach has been to prepare powders in two steps: a) synthesis of uniform powder precursors containing ions of tungsten and the doping elements by co-precipitation from aqueous solutions, and b) further processing of the precursors into W or WC based nano-composite powders.

Highly homogenous W and Co containing powder precursors for WC-Co composites were prepared via two different routes. Keggin-based precursors ((NH4)8[H2Co2W11O40]) with agglomerates of sizes up to 50 μm, were made from sodium tungstate or ammonium metatungstate and cobalt acetate. The powder composition corresponded to 5.2 % Co in the final WC-Co composites. 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. These particles had a plate-like morphology with sides of 5-20 μm and a thickness of less than 1 μm. Both precursors were processed and sintered into highly uniform microstructures with fine scale (<1μm). The processing of paratungstate-based precursors was also further investigated. Nanostructured WC-Co powders with grains size of less than 50 nm by decreasing processing temperatures and by applying gas phase carburization.

W-ODS materials were fabricated starting from ammonium paratungstate and rare earth elements (Y or La). Paratungstate-based precursors were prepared with different homogeneity 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 nitrate under acidic conditions had dramatically higher homogeneity and smaller particle size. The crystallite size was decreased to a few nanometers. These precursors were further processed to composite nanopowder and sintered to a nanostructured W-1.2%Y2O3 composite with 88% relative density.

In summary, APT can be converted to highly homogenous powder precursors of different compositions. The transformations are carried out in aqueous suspensions as a solvent mediated process, in which the starting material dissolves and the precursor precipitates. Powders with fine scale morphologies are obtained, e.g. plate-like particles with thickness less than 1 μm or spherical particles with size of a few nanometers. These precursors were processed further in to nano-sized composite powders and sintered to highly uniform tungsten composites with fine microstructures.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 47 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2011:13
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-42702ISBN: 978-91-7501-130-1 (print)OAI: oai:DiVA.org:kth-42702DiVA: diva2:447629
Presentation
2011-11-02, sal 539, Forum, Isafjordsgatan 39, Kista, 10:00 (English)
Opponent
Supervisors
Note
QC 20111013Available from: 2011-10-13 Created: 2011-10-11 Last updated: 2011-10-25Bibliographically approved
List of papers
1. Nanostructured hard material composites by molecular engineering .1. Synthesis from soluble tungstate salts
Open this publication in new window or tab >>Nanostructured hard material composites by molecular engineering .1. Synthesis from soluble tungstate salts
1997 (English)In: Nanostructured materials, ISSN 0965-9773, E-ISSN 1872-9150, Vol. 9, no 1-8, 105-108 p.Article in journal (Refereed) Published
Abstract [en]

A key issue, in the development of very fine grades, is to produce materials with high uniformity. The aqueous chemistry of tungsten offers several possibilities for the synthesis of homogenous molecular precursors containing W and Co mixed on the atomic scale. Powders containing (NH4)(8)[H2Co2W11O40] have been prepared starting from sodium tungstate or ammonium metatungstate. Nanophase powders of W-Co and WC-Co powders were obtained after reduction at 750 degrees C under H-2 and carburisation at 1000 degrees C under H-2/CH4. Hard materials with submicron microstructures were obtained after sintering at 1410 degrees C.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-43125 (URN)10.1016/S0965-9773(97)00029-9 (DOI)A1997XB96900018 ()
Note
QC 20111013Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-12-08Bibliographically approved
2. Processing of nanostructured WC-Co powder from precursor obtained by co-precipitation
Open this publication in new window or tab >>Processing of nanostructured WC-Co powder from precursor obtained by co-precipitation
1999 (English)In: Nanostructured materials, ISSN 0965-9773, E-ISSN 1872-9150, Vol. 12, no 1-4, 163-166 p.Article in journal (Refereed) Published
Abstract [en]

The paper reports the processing of a homogenous cobalt tungstate salt synthesised by co-precipitation starting from ammonium paratungstate and cobalt hydroxide. Different processing parameters such as temperatures and times of reduction and carburization and the composition of the carburization gas were studied. Powders of W and Co were obtained after the reduction of the precursor at 600 'C, 650 'C and 700 'C. The specific surface area of W-Co powders increased with decreasing reduction temperature. The carburization was carried out at 700 'C for 3 hours in different CO/CO2 - mixtures. The extent of the reaction was found to depend on the CO/CO2 - ratio. A nanophase WC-Co powder agglomerated at the micrometer scale was obtained after carburization in a 90%CO/10%CO2 gas-mixture.

National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-43226 (URN)10.1016/S0965-9773(99)00089-6 (DOI)000081912100036 ()
Note
QC 20111013Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2017-12-08Bibliographically approved
3. Oxide Dispersed Tungsten Powders from Rare Earth Doped Ammonium Paratungstate
Open this publication in new window or tab >>Oxide Dispersed Tungsten Powders from Rare Earth Doped Ammonium Paratungstate
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-43228 (URN)
Note
QS 2011Available from: 2011-10-13 Created: 2011-10-13 Last updated: 2014-12-01Bibliographically approved
4. 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
Show others...
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

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