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Improved oxidation resistance of SmCo magnetic alloy powders by silanization
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
2013 (English)In: Progress in organic coatings, ISSN 0300-9440, E-ISSN 1873-331X, Vol. 76, no 1, 94-100 p.Article, review/survey (Refereed) Published
Abstract [en]

The thermal stability of Sm2Co17 powders coated with four different silanes was studied between 25 degrees C and 500 degrees C and isothermally at 400 degrees C. Thermogravimetry data indicated that the silane-based coatings provided improved oxidation resistance. The microstructural analysis of uncoated powders oxidized for 10 h at 400 degrees C revealed the formation of a featureless ca. 10 mu m thick shell, surrounding the unreacted core. The development of this shell was attributed to the inward diffusion of oxygen, decomposition of intermetallic phases and redistribution of alloying elements. The EDS elemental maps revealed that the shell was rich in O, Fe and Co, and depleted in Sm, Zr and Cu. In the presence of the silane-based coatings the thickness of the shell was reduced by more than 80% (to less than 2 mu m) and the redistribution of alloying elements was insignificant. Based on the thermogravimetric analysis at or above ca. 400 degrees C and the nnicrostructural analysis it was possible to assess the relative effectivity of the different silanes in preventing the oxidation of the SmCo powder. Methyltrimethoxysilane (MTMS), which also formed the thinnest coating, was the best silane. (3-Glycidyloxypropyl)trimethoxysilane, forming a thicker coating, was less effective than MTMS, but superior to the two amine-functionalized silanes ((3-aminopropyl)triethoxysilane and (3-aminopropyl)trimethoxysilane).

Place, publisher, year, edition, pages
2013. Vol. 76, no 1, 94-100 p.
Keyword [en]
Sm2Co17, Silanes, Intermetallic phases, Oxidation resistance, Microstructure
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-106974DOI: 10.1016/j.porgcoat.2012.08.014ISI: 000312510900011Scopus ID: 2-s2.0-84869099060OAI: oai:DiVA.org:kth-106974DiVA: diva2:574248
Note

QC 20121205

Available from: 2012-12-05 Created: 2012-12-05 Last updated: 2017-12-07Bibliographically 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.
Keyword
Organofunctional alkoxysilanes, High temperature oxidation, Rare-earth magnetic alloys, SmCo, Isothermal, Non-isothermal, Microstructure, Magnetic properties
National Category
Materials Engineering
Identifiers
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)
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Note

QC 20121205

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

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