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SmCo for polymer bonded magnets: Corrosion, silanization, rheological, mechanical and magnetic properties
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Ceramics.
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 [en]
Organofunctional alkoxysilanes, High temperature oxidation, Rare-earth magnetic alloys, SmCo, Isothermal, Non-isothermal, Microstructure, Magnetic properties
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-106809ISBN: 978-91-7501-582-8 (print)OAI: oai:DiVA.org:kth-106809DiVA: diva2:574145
Public defence
2012-12-14, B2, Brinellvägen 23, KTH, Stockholm, 14:30 (English)
Opponent
Supervisors
Note

QC 20121205

Available from: 2012-12-05 Created: 2012-12-04 Last updated: 2016-01-13Bibliographically approved
List of papers
1. Anomalous high temperature oxidation of Sm 2(Fe,Co,Cu,Zr) 17 particles
Open this publication in new window or tab >>Anomalous high temperature oxidation of Sm 2(Fe,Co,Cu,Zr) 17 particles
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.

Keyword
Intermetallic phases, Oxidation, Permanent magnets, Sm 2Co 17
National Category
Corrosion Engineering
Identifiers
urn:nbn:se:kth:diva-104931 (URN)10.1016/j.corsci.2012.08.049 (DOI)000311192800049 ()2-s2.0-84867234354 (Scopus ID)
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-14 Last updated: 2017-12-07Bibliographically approved
2. Improved oxidation resistance of SmCo magnetic alloy powders by silanization
Open this publication in new window or tab >>Improved oxidation resistance of SmCo magnetic alloy powders by silanization
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).

Keyword
Sm2Co17, Silanes, Intermetallic phases, Oxidation resistance, Microstructure
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-106974 (URN)10.1016/j.porgcoat.2012.08.014 (DOI)000312510900011 ()2-s2.0-84869099060 (Scopus ID)
Note

QC 20121205

Available from: 2012-12-05 Created: 2012-12-05 Last updated: 2017-12-07Bibliographically approved
3. The Effect of Silanisation on Microstructural Stability and Magnetic Properties of the Intermetallic Sm2(Co, Fe, Cu, Zr)17
Open this publication in new window or tab >>The Effect of Silanisation on Microstructural Stability and Magnetic Properties of the Intermetallic Sm2(Co, Fe, Cu, Zr)17
2011 (English)In: Materials Research Society Symposium Proceedings, ISSN 0272-9172, E-ISSN 1946-4274, Vol. 1295, 443-448 p.Article in journal (Refereed) Published
Abstract [en]

The effects of silanising using the coupling agent γ-glycidoxpropyltrimethoxysilane on microstructural stability and magnetic properties of Sm-Co powder particles have been investigated. The silanisation provides structural stability by improving the oxidation resistance at 400oC for 10 hours. The untreated particles undergo microchemical changes by redistribution of alloying elements which mainly accumulate in parallel black and grey streaks in the interior of the particles. The silanised particles after heat treatment show coercivity of 836 Oe and the untreated particles show a much lower coercivity of 376 Oe. The difference in magnetic properties of uncoated particles is caused by diffusion of oxygen and microstructural instability.

Keyword
magnetic properties, microstructure, coating
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-106976 (URN)2-s2.0-80053192060 (Scopus ID)
Conference
2010 MRS Fall Meeting; Boston, MA; United States
Note

QC 20121205

Available from: 2012-12-05 Created: 2012-12-05 Last updated: 2017-04-28Bibliographically approved
4. High temperature magnetic properties of Sm-Co and Sm-Co/polyamide-12 materials: effects of temperature, particle size, and silanization
Open this publication in new window or tab >>High temperature magnetic properties of Sm-Co and Sm-Co/polyamide-12 materials: effects of temperature, particle size, and silanization
2013 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 48, no 23, 8163-8170 p.Article in journal (Refereed) Published
Abstract [en]

There is an increasing demand for polymer-bonded magnets (PBM) in high temperature applications. While most research deals with high temperature properties of NdFeB-PBM, only a few studies consider Sm-Co PBM. Therefore, this study, on the thermal and magnetic properties of Sm-Co alloy powders and blends of these with polyamide-12 (PA12), was undertaken. Since the Sm-Co powders were the product of ball milling, they contained a variety of shapes and sizes. Studies on size fractions of these showed that the thermal stability and magnetic properties were improved as the particle size increased. It was suggested that higher residual strains and smaller crystallite sizes in the small particles were responsible for a decrease in the thermal stability and magnetic properties. In addition, energy dispersive X-ray spectroscopy revealed that the oxygen content increased with decreasing particle size (larger specific surface area) and higher oxygen content was possibly also responsible for a decrease in the magnetic properties. It was shown that, in general, the surface modification by silanization, using (3-aminopropyl)trimethoxsilane, increased the saturation magnetization and remanence of both the particles and the Sm-Co/PA12 composite. The silanization also improved the thermal stability of the particles.

Keyword
FE-B Magnets, Permanent-Magnets, NDFEB Magnets, Corrosion, Oxidation, Coatings
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-106980 (URN)10.1007/s10853-013-7629-6 (DOI)000324111000014 ()2-s2.0-84884284353 (Scopus ID)
Note

QC 20131011. Updated from manuscript to article in journal.

Available from: 2012-12-05 Created: 2012-12-05 Last updated: 2017-12-07Bibliographically approved
5. Rheological and dynamic mechanical properties of polymer-bonded magnets based on Sm2Co17 and polyamide-12
Open this publication in new window or tab >>Rheological and dynamic mechanical properties of polymer-bonded magnets based on Sm2Co17 and polyamide-12
2014 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 49, no 21, 7529-7538 p.Article in journal (Refereed) Published
Abstract [en]

The rheological and dynamic mechanical properties of polymer-based composites of Sm2Co17 and polyamide-12 with different particle loadings, sizes, and surface treatments are reported. Sm2Co17 particles were surface-treated with three different silanes: 3-glycidoxy(propyl)trimethoxysilane, 3-amino(propyl)trimethoxysilane (APTMS), and methyltrimethoxysilane (MTMS). It was shown, for the composites with untreated particles, that the viscosity and storage modulus increased with increasing filler content (0-60 vol%) and decreasing filler particle size. In addition, the glass transition temperature increased significantly and the damping decreased with increasing filler content. Of the silanes, the MTMS, which yielded only a thin surface layer, had in general the least effect on the rheological properties of the composite. The composite containing the APTMS-coated filler showed the highest storage modulus. The results give new insights on how to prepare polymer-bonded magnets with optimal process conditions (rheology) and dynamic mechanical properties, by varying the amount of particles, their size, and surface treatment.

Keyword
Elastic moduli, Fillers, Permanent magnets, Silanes, Surface treatment, Dynamic mechanical property, Filler particles, Methyltrimethoxysilane, Polymer based composite, Polymer-bonded magnets, Rheological property, Thin surface layer, Trimethoxysilane
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-151322 (URN)10.1007/s10853-014-8460-4 (DOI)000340679000021 ()2-s2.0-84906315426 (Scopus ID)
Note

QC 20140918

Available from: 2014-09-18 Created: 2014-09-18 Last updated: 2017-12-05Bibliographically approved

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