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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: urn:nbn:se:kth:diva-156604OAI: oai:DiVA.org:kth-156604DiVA: diva2:767377
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
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. 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
Show others...
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
5. 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
6. Processing and sintering of yttrium-doped tungsten oxide nanopowders to tungsten-based composites
Open this publication in new window or tab >>Processing and sintering of yttrium-doped tungsten oxide nanopowders to tungsten-based composites
Show others...
2014 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 16, 5703-5713 p.Article in journal (Refereed) Published
Abstract [en]

Innovative chemical methods are capable of fabricating nanoscale tungsten oxide compounds doped 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 we provide 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 A degrees C for 3 min holding time to achieve higher density composites with an optimum finer grain size (3 A mu m) and a hardness value up to 420 H (V). Major grain growth takes place at temperatures above 1300 A degrees C during sintering. From the density values obtained, it is recommend to apply higher pressure before 900 A degrees C to obtain maximum density. Oxides inclusions present in the matrix were identified as Y2O3 center dot 3WO(3) and Y2O3 center dot WO3 during high resolution microscopic investigations.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-147716 (URN)10.1007/s10853-014-8289-x (DOI)000336983600021 ()2-s2.0-84903534208 (Scopus ID)
Note

QC 20140707

Available from: 2014-07-07 Created: 2014-07-03 Last updated: 2017-12-05Bibliographically approved
7. Solution chemical synthesis of W-Y2O3 nanocomposites
Open this publication in new window or tab >>Solution chemical synthesis of W-Y2O3 nanocomposites
(English)Manuscript (preprint) (Other academic)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-156603 (URN)
Note

QS 2014

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

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