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Grain growth by Ordered Coalescence of crystallites in Ceramics: Grain Growth Mechanisms, Microstructure Evolution and Sintering
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). (Zhijian Shen)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Grain growth and densification process play the two most crucial roles on the microstructure evolution and the achieved performances during sintering of ceramics. In this thesis, the grain growth of SrTiO3, BaTiO3-SrTiO3 solid solutions and Si3N4 ceramics during spark plasma sintering (SPS) were investigated by electron microscopy.

SrTiO3 ceramics starting from nanopowders were fabricated by SPS. A novel grain growth mechanism was discovered and named as ordered coalescence (OC) of nanocrystals. This mechanism involved nanocrystals as building blocks and is distinguished from atomic layer epitaxial growth (AEG) in classical sintering theory. The results also revealed that the dominant grain growth mechanism can be changed by varying heating rates. Low rate (10°C/min) gives AEG, whereas high rates (≥ 50°C/min) yields three-dimensional coalescence of nanocrystals, i.e. OC.

BaTiO3-SrTiO3 sintered bodies were made by SPS of BaTiO3 and SrTiO3 nanopowders mixtures. A novel Sr1-xBaxTiO3 “solid solution” with mosaic-like single crystal structure was manufactured by OC of the precursor crystallites. This reveals a new path for preparation of solid solution grains or composites.  

Si3N4 ceramics were prepared from α- or β-Si3N4 nanopowders at the same SPS conditions. The anisotropic OC of precipitated β-Si3N4 crystallites gives elongated β-Si3N4 grains at 1650°C using α-Si3N4 nanopowder. In contrast, AEG leads to the equi-axed β-Si3N4 grains using β-Si3N4 nanopowder. The metastable α- to β-Si3N4 phase transformation and OC accelerates anisotropic grain growth.

Grain motions contribute to the densification process during pressureless sintered 3Y-ZrO2 (>87%TD) or SPS of SrTiO3 (>92%TD) ceramics. This extends the sintering range for active grain re-arrangement over that predicted by classical theory.

In this thesis a new grain growth mechanism (OC) is proved by using SPS and nanopowders. By OC the microstructural evolution can be manipulated.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University , 2013. , p. 80
National Category
Materials Chemistry Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-88628ISBN: 978-91-7447-677-4 (print)OAI: oai:DiVA.org:su-88628DiVA, id: diva2:612646
Public defence
2013-06-13, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted. Paper 3: Submitted. Paper 4: Manuscript. Paper 7: Accepted.

Available from: 2013-05-16 Created: 2013-03-22 Last updated: 2013-06-14Bibliographically approved
List of papers
1. Grain growth by multiple ordered coalescence of nanocrystals during spark plasma sintering of SrTiO3 nanopowders
Open this publication in new window or tab >>Grain growth by multiple ordered coalescence of nanocrystals during spark plasma sintering of SrTiO3 nanopowders
2012 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 18, p. 6405-6412Article in journal (Refereed) Published
Abstract [en]

Sintering is the most widely applied densification process for manufacturing polycrystalline materials in powder metallurgy and ceramic industries. Grain growth behavior during sintering has a crucial influence on the final microstructure and thus the achieved performance. So far, it has been accepted that grain growth, based on classic crystal growth theory, takes place via atomic diffusion driven by excess interfacial energy. This paper presents a novel grain growth mechanism resulting from multiple ordered coalescence of nanocrystals via the activation of rapid grain motions. In rapid solid-state sintering of a strontium titanate (SrTiO3) nanopowder, individual SrTiO3 nanocrystals can act as the building blocks and self-assemble to form larger grains. A quasi-liquid interfacial film achieved by surface melting of the nanocrystals plays an essential role in this new process by facilitating the grain motion and ordered coalescence of nanocrystals. The imperfect ordered coalescence of nanocrystals introduce deep structural heterogeneities characterized by the unique quasi-interfaces inside grown grains of single crystal signature. The quasi-interfaces consist of the vacancy arrays and/or aggregated line defects.

Keyword
Coalescence, Interface, Grain growth mechanism, Rapid sintering, Nanocrystals
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-83785 (URN)10.1016/j.actamat.2012.08.027 (DOI)000310663300017 ()
Note

AuthorCount:2;

Available from: 2012-12-18 Created: 2012-12-14 Last updated: 2017-12-06Bibliographically approved
2. Ordered coalescence of nano crystallites contributing to the rapid anisotropic grain growth in silicon nitride ceramics
Open this publication in new window or tab >>Ordered coalescence of nano crystallites contributing to the rapid anisotropic grain growth in silicon nitride ceramics
2013 (English)In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 69, no 3, p. 270-273Article in journal (Refereed) Published
Abstract [en]

Microstructural characterization is performed on two dense Si3N4 ceramic samples consolidated by spark plasma sintering (SPS): one fabricated using alpha-Si3N4 and the other using beta-Si3N4 as the starting powder. A novel mechanism is revealed where ordered coalescence of nano beta-crystallites accelerate the rapid beta-Si3N4 anisotropic grain growth. The rapid alpha- to beta-Si3N4 phase transformation via a high supersaturation of dissolved Si3N4 in the melt favors this mechanism. The high heating rate by SPS is essential for achieving such supersaturation.

Keyword
Abnormal grain growth, Phase transformations, Interface defects, Spark plasma sintering, HREM
National Category
Nano Technology Materials Chemistry Metallurgy and Metallic Materials
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-89296 (URN)10.1016/j.scriptamat.2013.04.017 (DOI)000320841900017 ()
Funder
Swedish Research Council
Note

AuthorCount: 2;

Available from: 2013-04-19 Created: 2013-04-19 Last updated: 2017-12-06Bibliographically approved
3. Grain growth kinetics determined by the heating rate during spark plasma sintering: 2Dnucleation versus ordered coalescence of nanocrystals
Open this publication in new window or tab >>Grain growth kinetics determined by the heating rate during spark plasma sintering: 2Dnucleation versus ordered coalescence of nanocrystals
2013 (English)Article in journal (Refereed) Submitted
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-89297 (URN)
Available from: 2013-04-19 Created: 2013-04-19 Last updated: 2017-03-08Bibliographically approved
4. Mosaic-like structure in Barium Strotium Titanate solid solution
Open this publication in new window or tab >>Mosaic-like structure in Barium Strotium Titanate solid solution
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-89298 (URN)
Available from: 2013-04-19 Created: 2013-04-19 Last updated: 2013-05-02Bibliographically approved
5. Silicon nitride nanoceramics densified by dynamic grain sliding
Open this publication in new window or tab >>Silicon nitride nanoceramics densified by dynamic grain sliding
Show others...
2010 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 25, p. 2354-2361Article in journal (Refereed) Published
Abstract [en]

The densification behaviors of two silicon nitride nanopowder mixtures based respectively on a-Si3N4 and ß-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of a- to ß-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.

 

National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-53049 (URN)10.1557/jmr.2010.0313 (DOI)
Available from: 2011-01-19 Created: 2011-01-19 Last updated: 2017-12-11Bibliographically approved
6. Preparation of Transparent Nanoceramics by Suppressing Pore Coalescence
Open this publication in new window or tab >>Preparation of Transparent Nanoceramics by Suppressing Pore Coalescence
Show others...
2011 (English)In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 94, no 12, p. 4269-4273Article in journal (Refereed) Published
Abstract [en]

Microstructural developments in nanoceramics were investigated in 3Y-TZP compacts with relative density (RD) exceeding 93%. Special attentions were paid to the evolutions of pore structures. It was found that the densification process of nanoceramic compacts with apparently close porosity was greatly jeopardized by pore coalescence. This observation was interpreted by the coalescence of locally interconnected pores originated from inhomogeneous packing of particles. The pore coalescence can be suppressed by application of an external pressure. The processing principle was demonstrated by spark plasma sintering (SPS) with extended holding at a minimized sintering temperature. The highly dense 3Y-TZP nanoceramics containing no large pores became optically transparent.

National Category
Environmental Sciences Materials Chemistry Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-74053 (URN)10.1111/j.1551-2916.2011.04829.x (DOI)000297848100032 ()
Note
authorCount :5Available from: 2012-02-29 Created: 2012-02-28 Last updated: 2017-12-07Bibliographically approved
7. Dynamic Pore Coalescence in Nanoceramic Consolidated by Two-Step Sintering Procedure
Open this publication in new window or tab >>Dynamic Pore Coalescence in Nanoceramic Consolidated by Two-Step Sintering Procedure
2013 (English)In: Journal of the European Ceramic Society, ISSN 0955-2219, E-ISSN 1873-619X, Vol. 33, no 11, p. 2087-2092Article in journal (Refereed) Published
Abstract [en]

Two-step sintering (TSS) concept was adopted in the consolidation of 3 mol% yttria doped zirconia nanopowder. Partially densified bodies with 87% theoretical density (TD) were firstly prepared using high-pressure spark plasma sintering (SPS) technique and followed by second-step pressureless sintering. The samples achieved only 96% TD final density after 30 h soaking. It was found that the densification process was impeded by dynamic pore coalescence with a pore growth factor of 10. The phenomenon was explained by the coalescence of interconnected small pores generated by differential sintering of nanoceramic green compacts. Such pore coalescence was accompanied with particle movement, which resulted in sintering state deviating from the frozen state. Present results indicated that the active range for particle rearrangement was greatly extended during nanoceramic sintering and the efficiency of TSS approach was greatly dependent on the homogeneity of green bodies

Keyword
Nanoceramics, two-step sintering (TSS), Densification, Grain growth
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-89299 (URN)10.1016/j.jeurceramsoc.2013.03.015 (DOI)000319539500004 ()
Funder
Swedish Research Council, 621-2008-5730
Note

AuthorCount: 3;

Available from: 2013-04-19 Created: 2013-04-19 Last updated: 2017-12-06Bibliographically approved

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