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Preparation and characterisation of refractory whiskers and selected alumina composites
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A whisker is a common name of single crystalline inorganic fibre of small dimensions, typically 0.5-1 μm in diameter and 20-50 μm in length. Whiskers are mainly used as reinforcement of ceramics. This work describes the synthesis and characterisation of new whisker types. Ti0.33Ta0.33Nb0.33CxN1-x, TiB2, B4C, and LaxCe1-xB6 have been prepared by carbothermal vapour–liquid–solid (CTR-VLS) growth mechanisms in the temperature range 900-1800°C, in argon or nitrogen. Generally, carbon and different suitable oxides were used as whisker precursors. The oxides reacted via a carbothermal reduction process. A halogenide salt was added to form gaseous metal halogenides or oxohalogenides and small amount of a transition metal was added to catalyse the whisker growth. In this mechanism, the whisker constituents are dissolved into the catalyst, in liquid phase, which becomes supersaturated. Then a whisker could nucleate and grow out under continuous feed of constituents.

The syntheses of TiC, TiB2, and B4C were followed at ordinary synthesis conditions by means of mass spectrometry (MS), thermogravimetry (TG), differential thermal analysis (DTA) and quenching. The main reaction starting temperatures and reaction time for the different mixtures was revealed, and it was found that the temperature inside the crucible during the reactions was up to 100°C below the furnace set-point, due to endothermic nature of the reactions. Quench experiments showed that whiskers were formed already when reaching the temperature plateau, but the yield increased fast with the holding time and reached a maximum after about 20-30 minutes. Growth models for whisker formation have been proposed.

Alumina based composites reinforced by (2-5 vol.%) TiCnano and TiNnano and 25 vol.% of carbide, and boride phases (whiskers and particulates of TiC, TiN, TaC, NbC, (Ti,Ta)C, (Ti,Ta,Nb)C, SiC, TiB2 and B4C) have been prepared by a developed aqueous colloidal processing route followed by hot pressing for 90 min at 1700°C, 28 MPa or SPS sintering for 5 minutes at 1200-1600°C and 75 MPa. Vickers indentation measurements showed that the lowest possible sintering temperature is to prefer from mechanical properties point of view. In the TiNnano composites the fracture mode was typically intergranular, while it was transgranular in the SiCnano composites. The whisker and particulate composites have been compared in terms of e.g. microstructure and mechanical properties. Generally, additions of whiskers yielded higher fracture toughness compared to particulates. Composites of commercially available SiC whiskers showed best mechanical properties with a low spread but all the other whisker phases, especially TiB2, exhibited a great potential as reinforcement materials.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi , 2004. , 107 p.
Keyword [en]
VLS-growth, Characterisation, Whiskers, Sintering, Ceramic composites
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-113ISBN: 91-7265-866-5 (print)OAI: oai:DiVA.org:su-113DiVA: diva2:189462
Public defence
2004-05-11, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 14:00
Opponent
Supervisors
Available from: 2004-04-20 Created: 2004-04-20Bibliographically approved
List of papers
1. Synthesis and characterisation of Ti0.33Ta0.33Nb0.33C and Ta0.33Ti0.33 Nb0.33CxN1-x whiskers
Open this publication in new window or tab >>Synthesis and characterisation of Ti0.33Ta0.33Nb0.33C and Ta0.33Ti0.33 Nb0.33CxN1-x whiskers
1999 (English)In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 82, no 8, 1969-1976 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-22783 (URN)10.1111/j.1151-2916.1999.tb02028.x (DOI)
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2017-12-13Bibliographically approved
2. Vapour-Liquid-Solid growth of TiB2 whiskers
Open this publication in new window or tab >>Vapour-Liquid-Solid growth of TiB2 whiskers
2002 (English)In: Journal of Materials Science, ISSN 0022-2461, Vol. 37, no 14, 2917-2925 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-22784 (URN)10.1023/A:1016000510721 (DOI)
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2010-08-16Bibliographically approved
3. Synthesis and characterisation of boron carbide whiskers and thin elongated platelets
Open this publication in new window or tab >>Synthesis and characterisation of boron carbide whiskers and thin elongated platelets
2002 (English)In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 236, no 1-3, 466-476 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-22785 (URN)10.1016/S0022-0248(01)02272-2 (DOI)
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2017-12-13Bibliographically approved
4. Colloidal processing of Al2O3-based composites reinforced with TiN and TiC particulates, whiskers and nanoparticles
Open this publication in new window or tab >>Colloidal processing of Al2O3-based composites reinforced with TiN and TiC particulates, whiskers and nanoparticles
Show others...
2001 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 21, no 8, 1027-1035 p.Article in journal (Refereed) Published
Abstract [en]

A colloidal processing route has been developed for the preparation of dense and homogeneous Al2O3–TiN/TiC composites. The dispersion and rheological properties of mixtures of TiN or TiC particulates and Al2O3 particles were investigated using electrokinetics and steady-shear rheology. We found that well-dispersed aqueous suspensions with low viscosity could be prepared by adding a poly(acrylic acid) dispersant and controlling pH in the alkaline range. This processing scheme was also suitable for preparation of whisker and nanoparticle composite suspensions. The alumina-based composite suspensions with a secondary-phase concentration of 25 vol.% were freeze-granulated and hot-pressed, and the resulting bodies were fully densified with well-dispersed secondary phases. Homogeneous Al2O3–TiN nanoparticle composites could only be prepared with additions of up to 5 vol.% nanoparticles; higher additions resulted in agglomeration and subsequent grain growth of the nanoparticles. 

National Category
Chemical Sciences Physical Chemistry Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-22786 (URN)10.1016/S0955-2219(00)00302-2 (DOI)
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2017-12-13Bibliographically approved
5. Preparation and Characterization of Alumina based TiNn and SiCn Composites
Open this publication in new window or tab >>Preparation and Characterization of Alumina based TiNn and SiCn Composites
2003 (English)In: Nanomaterials for Structural Applications / [ed] C. Berndt, T.E. Fischer, I. OvidÕko, G. Skandan, T. Tsakalakos, New York; North-Holland: Materials Research Society , 2003, 173-178 p.Conference paper, Published paper (Other academic)
Place, publisher, year, edition, pages
New York; North-Holland: Materials Research Society, 2003
Series
Materials Research Society symposia proceedings, ISSN 0272-9172 ; 740
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-22787 (URN)1-55899-677-X (ISBN)
Conference
Boston, Massachusetts on December 2-6, 2002
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2010-11-19Bibliographically approved
6. Synthesis of LaxCe1-xB6 whiskers
Open this publication in new window or tab >>Synthesis of LaxCe1-xB6 whiskers
2004 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 40, no 11, 2991-2994 p.Article in journal (Refereed) Published
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-22788 (URN)10.1007/s10853-005-2387-8 (DOI)
Note
Part of urn:nbn:se:su:diva-113Available from: 2004-04-20 Created: 2004-04-20 Last updated: 2017-12-13Bibliographically approved

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