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Spark Plasma Sintering Enhancing Grain Sliding, Deformation and Grain Size Control: Studies of the Systems Ti, Ti/TiB2, Na0.5 K0.5 NbO3, and Hydroxyapatite
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The unique features of the Spark plasma sintering (SPS) were used to investigate the sintering and deformation behaviour of titanium and titanium–titanium diboride composites, and to control the sintering and grain growth of ferroelectric Na0.5K0.5NbO3 (NKN) and of hydroxyapatite (HAp). In the SPS the samples experience a temperature different from that recorded by the thermocouple (pyrometer) used and this temperature difference has been estimated for Ti and NKN.

 

Sintering and deformation of titanium was investigated. Increasing heating rate and/or pressure shifted the sintering to lower temperatures, and the sintering and deformation rates changed when the α→β phase transition temperature was passed. Fully dense Ti/TiB2 composites were prepared. The Ti/TiB2 composites could be deformed at high temperatures, but the hardness decreased due to the formation of TiB. 

 

The kinetic windows within which it is possible to obtain fully dense NKN and HAp ceramics and simultaneously avoid grain growth are defined. Materials have a threshold temperature above which rapid and abnormal grain growth takes place. The abnormal grain growth of NKN is due to a small shift in the stoichiometry, which in turn impairs the ferroelectric properties. Fully transparent HAp nanoceramics was prepared, and between 900 and 1050 oC elongated grains are formed, while above 1050 oC abnormal grain growth takes place.NKN samples containing grains of the sizes 0.35–0.6 µm yielded optimum ferroelectric properties, i.e. a high remanent polarization (Pr = 30 µC/cm2) and high piezoelectric constant (d33= 160 pC/N). The ferroelectric domain structure was studied, and all grains exhibited a multi-domain type of structure.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University , 2010. , 81 p.
Keyword [en]
Spark plasma sintering, plastic deformation, grain growth, titanium, TiB2, hydroxyapatite, Na0.5K0.5NbO3, ferroelectric, transparent
National Category
Materials Chemistry
Research subject
Materials Chemistry
Identifiers
URN: urn:nbn:se:su:diva-38681ISBN: 978-91-7447-072-7 (print)OAI: oai:DiVA.org:su-38681DiVA: diva2:312512
Public defence
2010-05-25, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of doctoral defense the following articles were unpublished and had a status as follows: Article 4: Manuscript; Article 5 : Manuscript Available from: 2010-05-04 Created: 2010-04-26 Last updated: 2010-04-27Bibliographically approved
List of papers
1. Fast densification and deformation of titanium powder
Open this publication in new window or tab >>Fast densification and deformation of titanium powder
2005 (English)In: Powder Metallurgy, ISSN 0032-5899, Vol. 48, no 3, 231-236 p.Article in journal (Refereed) Published
Abstract [en]

 A coarse titanium powder containing 0.2 wt-% oxygen with an average particle size of 45 micrometers was rapidly densified using the spark plasma sintering (SPS) technique. The fully consolidated specimens were also deformed in compression in the SPS unit and in a conventional hot pressing (HP) furnace. The densification and deformation behaviours were investigated by microstructural observation and examining the shrinkage and shrinkage rate recorded in real time. The efforts were focused on clarifying to what extent the possible spark discharging and intensive Joule heating at the particle contact points contribute to the densification, and what effect if any the pulsed electrical heating has on the densification. The results showed that what contributed to the densification were neither spark discharging nor anisotropic heating, but the particle deformation. Deformation occurs throughout the entire particle both in SPS and HP conditions, with a higher strain rate under SPS condition especially in the a phase region.

Place, publisher, year, edition, pages
Maney Publishing, 2005
Keyword
Spark Plasma Sintering, Electrical Field, Densification, Deformation, Titanium
National Category
Environmental Sciences
Identifiers
urn:nbn:se:su:diva-26119 (URN)10.1179/174329005X71939 (DOI)
Available from: 2009-03-10 Created: 2009-03-10 Last updated: 2010-04-27Bibliographically approved
2. Spark plasma sintering and deformation of Ti-TiB2 composites.
Open this publication in new window or tab >>Spark plasma sintering and deformation of Ti-TiB2 composites.
2008 (English)In: Materials Science & Engineering, A: Structural Materials: Properties, Microstructure and Processing, ISSN 0921-5093, Vol. 1-2, no A475, 101-104 p.Article in journal (Refereed) Published
Abstract [en]

Spark plasma sintering (SPS) was used to investigate the densification and deformation behaviour of Ti–TiB2 composites. Fully densified samples were prepared with Ti addition larger than 5%. The prepared composites can be deformed under compression at 1700 °C to achieve a strain of 50% without cracking. At lower temperatures, cracks were initiated due to low ductility of TiB2 and low content of Ti. During the sintering and deformation, TiB is formed via a reaction between Ti and TiB2. To elucidate the formation mechanism of TiB in the SPS process, reactive sintering of TiB using element precursors was also performed. Fully dense samples were prepared but it was not possible to prepare pure uniphase TiB. The reactive sintering resulted in the formation of TiB and TiB2 mixtures at low temperatures and a mixture of TiB2 and Ti3B4 at high temperature

Place, publisher, year, edition, pages
Elsevier, 2008
Keyword
Spark plasma sintering; Titanium; Titanium diboride; Grain growth; Reactive sintering
National Category
Environmental Sciences
Identifiers
urn:nbn:se:su:diva-17243 (URN)10.1016/j.msea.2007.01.161 (DOI)000253693100020 ()
Available from: 2009-01-10 Created: 2009-01-10 Last updated: 2010-04-26Bibliographically approved
3. Low temperature consolidated lead-free ferroelectric niobateceramics with improved electrical properties
Open this publication in new window or tab >>Low temperature consolidated lead-free ferroelectric niobateceramics with improved electrical properties
Show others...
2010 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 25, no 2, 240-247 p.Article in journal (Refereed) Published
Abstract [en]

There is a concerted effort to develop lead-free piezoelectric ceramics. ((Na0.5K0.5)NbO3 based ceramics have good electrical properties, and are a potential replacement material for lead zirconate titanate piezoelectric ceramics. In this work a commercial powder based on (Na0.5K0.5)NbO3 with an initial particle size of 260 nm was consolidated by plasma sintering (SPS). To avoid volatilization, high mechanical pressures were used to minimize the densification temperature. It was found that under a uniaxial pressure of 100 MPa, fully densified compacts can be prepared at 850. Ceramics densified at such a low temperature demonstrate an unusually high remanent polarization (30 mC/cm2) and high d33 (146 pC/N). The improved ferroelectric properties are ascribed to the homogeneous, dense, and submicron grained microstructure achieved.

Place, publisher, year, edition, pages
Elsevier, 2010
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-38660 (URN)10.1557/JMR.2010.0034 (DOI)000274114400008 ()
Available from: 2010-04-23 Created: 2010-04-23 Last updated: 2017-12-12Bibliographically approved
4. Effect of grain size on ferroelectric domain and electrical properties of submicron sized sodium potassium niobate ceramics
Open this publication in new window or tab >>Effect of grain size on ferroelectric domain and electrical properties of submicron sized sodium potassium niobate ceramics
Show others...
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

Piezoelectric ceramics of the composition Na0.5K0.5NbO3 (NKN) with grain sizes in the range of 0.2 - 1 mm were fabricated by Spark Plasma Sintering using normal pressure dies and a high pressure cell designed for pressures up to 500 MPa  with the purpose of investigating the effect of grain size on domain structures and electrical properties. Optimized processing conditions enabled ceramics of high densities (>99.5%TD) to be made at T≥850°C. It was found that domain size decreases with decreasing grain size and that non-180° ferroelectric domains walls were still visible in 200 nm sized grains. The room temperature dielectric constant firstly increased with decreasing grain size and then decreased in the low grain size regime. The materials with finer grain size displayed a broad ferro-paraelectric phase transition and a depression of the dielectric maximum at the Curie point. They also displayed an increase in the coercive field and approximately unchanged remnant polarization. The material sintered at 850°C represents a very good candidate for lead-free piezoelectric applications, because of its high piezoelectric constant (d33 = 160±2 pC/N).

Keyword
lead-free, domain size, grain size, ferroelectric, piezoelectric
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-38654 (URN)
Available from: 2010-04-23 Created: 2010-04-23 Last updated: 2010-04-27Bibliographically approved
5. Transparent hydroxyapatite nanoceramics by high pressure spark plasma sintering at the minimized sintering temperature
Open this publication in new window or tab >>Transparent hydroxyapatite nanoceramics by high pressure spark plasma sintering at the minimized sintering temperature
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

For direct observation of the bio-interfacial reactions with improved spatial and temporal resolution by confocal microscopy transparent hydroxyapatite nanoceramics are demanded. The aim of the present study was to, through detailed kinetics study and micostructural characterization, define a processing window within which transparent HAp nanoceramics can be produced by spark plasma sintering of dry powders.  A lab-made hydrothermally processed bulky powder composed of loosely aggregated nanorods and a commercial granulated-powder composed of irregular shaped nanorods were tested.  The use of a high pressure cell allows the application of pressure up to 500 MPa. It was found that applying of high pressure is beneficial for widen up the processing window for attaining dense HAp ceramics with nano grained microstructure. The high transparency of HAp nanoceramics obtained in this study is ascribed to the high density and homogeneous nano-structure achieved besides the unique intrinsic optical properties of the HAp crystal, i.e. its low refractive index and very small birefringence. Achieving full densification at the minimized sintering temperature allows for the first time the preparation of transparent HAp nanoceramics with stoichiometric composition, i.e. avoiding the loss of water that commonly encountered  during the conventional ways of sintering.

Keyword
Hydroxyapatite, nanoceramics, optical transparency, sintering
Research subject
Materials Chemistry
Identifiers
urn:nbn:se:su:diva-38653 (URN)
Available from: 2010-04-23 Created: 2010-04-23 Last updated: 2010-04-27Bibliographically approved

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