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Anomalous high temperature oxidation of Sm 2(Fe,Co,Cu,Zr) 17 particles
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2012 (English)In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 65, 453-460 p.Article in journal (Refereed) Published
Abstract [en]

Sm 2Co 17 alloys are attractive as high temperature permanent magnets; however their magnetic properties are prone to degrade by oxidation. The oxidation mechanism is not clearly understood, and inconsistent results are reported in the literature.This work correlates the oxidation of Sm 2(Co,Fe,Cu,Zr) 17 with microstructural and microchemical changes as a consequence of chemical reaction of alloy particles with their environment at high temperature. Thermogravimetric analysis suggests a change in oxidation mechanism occurs at 550°C. Energy dispersive X-ray spectroscopy suggests this is related to the loss of Sm by sublimation.

Place, publisher, year, edition, pages
2012. Vol. 65, 453-460 p.
Keyword [en]
Intermetallic phases, Oxidation, Permanent magnets, Sm 2Co 17
National Category
Corrosion Engineering
URN: urn:nbn:se:kth:diva-104931DOI: 10.1016/j.corsci.2012.08.049ISI: 000311192800049ScopusID: 2-s2.0-84867234354OAI: diva2:568076

QC 20121115

Available from: 2012-11-15 Created: 2012-11-14 Last updated: 2013-01-09Bibliographically approved
In thesis
1. SmCo for polymer bonded magnets: Corrosion, silanization, rheological, mechanical and magnetic properties
Open this publication in new window or tab >>SmCo for polymer bonded magnets: Corrosion, silanization, rheological, mechanical and magnetic properties
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents the study of organofunctional alkoxysilane coatings to prevent high temperature oxidation of Sm-Co powders. Sm-Co are important permanent magnetic alloys, owing to their high Curie temperature and large values of magnetocrystalline anisotropy. They possess stable magnetic properties in the temperature range -40 to 120 °C which makes them very attractive candidates for automobile’s electric motors. However, the environmental conditions for such applications are a sum of high temperatures, humidity, fuels and salts which provide perfect breeding ground for corrosion.

In this study we report the high temperature oxidation resistance of Sm2Co17 powders coated with four common commercially available organofunctional silanes; (3-aminopropyl)trimethoxysilane (APTMS), (3-aminopropyl)triethoxysilane (APTES), methyltrimethoxysilane (MTMS) and (3-glycidyloxypropyl)trimethoxysilane (GPTMS).

The as received powder was a multimodal mixture of many sizes and shapes which represented a typical ball milling product. The thermal analyses of the powders suggested that the powders without surface coatings had profound affinity towards oxidation. The thermal properties of sieved uncoated powders revealed that the small powders were more susceptible to oxidation than the large powders due to their large specific surface area.

The isothermal properties of coated powders revealed that the powders coated with silanes had at least 10 times higher resistance to oxidation as compared to uncoated powders heated at 400 °C for 10 h. The non-isothermal tests conducted from room temperature to 500 °C also revealed that the uncoated powders gained 6 times more mass as compared to the powders coated with an ideal (MTMS) silane.

The microstructural analysis of the uncoated powders heated from 400 °C to 550 °C revealed diffusion of oxygen, instable intermetallic phases which resulted in a redistribution of alloying elements, precipitation of alloying elements and formation of a featureless shell (approximately 20 µm in thickness) that surrounded the unreacted core. The coated powders on the other hand showed homogenous distribution of alloying elements, stable intermetallic phases and limited the shell thickness (1 µm).

The thermo-magnetic properties of Sm-Co powders showed that the thermal instability also affected the magnetic properties adversely. It was found that the magnetic properties were deteriorated with a decrease in powder size. The energy dispersive spectroscopic (EDS) analyses showed that the small powders contained higher oxygen content than the large powders. Moreover XRD analysis also revealed that the small powders contain higher residual strains and smaller crystallite size which can play their role in deteriorating magnetic properties.

It was found that surface modification by silanization improve the thermo-magnetic properties by effectively shielding the powder surfaces from surface oxidation.

The rheological properties Sm-Co/PA12 composites revealed that the viscosity of the composites was increased with decreasing powder size due to the presence of rough surfaces and sharp corners in small powders. The rheological properties of the melts containing coated powders revealed that the silane layer acted as a lubricant and decreased the melt viscosity. It was found that coating the powders with silanes not only improve the rheological properties but also improve the other physical properties such as glass transition temperature the loss modulus by modifying the interfacial layer between the polymer matrix (PA12) and the powder. It results in a decrease in viscosity, a broadening of the glass transition temperature and a change in the damping properties of the composites.

The dynamic mechanical properties of Sm-Co/PA12 composites showed that the storage modulus was increased with decreasing powder size. The results were expected as the rough surfaces act as local welding points between the powder and the polymer matrix. It was found that the surface modification improve the storage modulus. It is assumed that the silanes modify the interfacial properties which not only resulted in increasing the storage modulus but also broadened the glass transition temperature, Tg and damping, tanδ peaks.

From the thermogravimetric, microstructural, rheological and magnetic analyses it can be concluded that the silanes are the effective coatings in preventing high temperature oxidation, stabilizing microstructure, enhancing mechanical properties, and improving rheological and magnetic properties.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 65 p.
Organofunctional alkoxysilanes, High temperature oxidation, Rare-earth magnetic alloys, SmCo, Isothermal, Non-isothermal, Microstructure, Magnetic properties
National Category
Materials Engineering
urn:nbn:se:kth:diva-106809 (URN)978-91-7501-582-8 (ISBN)
Public defence
2012-12-14, B2, Brinellvägen 23, KTH, Stockholm, 14:30 (English)

QC 20121205

Available from: 2012-12-05 Created: 2012-12-04 Last updated: 2016-01-13Bibliographically approved

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Qadeer, Muhammad I.Azhdar, BruskaHedenqvist, Mikael S.
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