Priprava in karakterizacija novega tipa Nd-Fe-B trajnih magnetov na osnovi recikliranih mikrokristaliničnih prahov : magistrsko delo

Sotelšek, Tim (author), Vrabec, Mirijam (mentor), Šturm, Sašo (comentor), Tomše, Tomaž (comentor)

V magistrskem delu je predstavljen nov pristop k pripravi trajnih magnetov tipa Nd-Fe-B na osnovi recikliranih mikrokristaliničnih prahov z dodatkom zlitine Nd70Cu30 (at. %). Magneti so bili pripravljeni z uporabo alternativnega postopka sintranja s t. i. tehniko sintranja v pulzirajočem električnem polju (spark-plasma sintering – SPS). Glavne prednosti uporabe tovrstne tehnike pred konvencionalnimi metodami so visoka hitrost segrevanja ter krajši časi in nižja temperatura sintranja. To je pomembno z ekonomskega vidika in vidika zagotavljanja dobrih magnetnih lastnosti v sintranem materialu. Kot osnovni material za izdelavo magnetov smo uporabili reciklirani prah, izdelan iz izmeta pri proizvodnji magnetov, ki je bil glede na referenčne vrednosti močno oksidiran. Sintrani magnet, pripravljen iz recikliranega prahu brez dodatka Nd-Cu, je imel izjemno slabe magnetne lastnosti (Br = 0,40 T; Hci = 26 kA/m; (BH)MAX = 3 kJ/m3) in neustrezno mikrostrukturo. Izhodnemu prahu smo dodali 5 ut. % evtektične zlitine Nd-Cu in opravili parametrično študijo, da smo določili optimalne pogoje sintranja. Slednji so: končna temperatura sintranja 1000 °C z 1 min držanja pri tej temperaturi; hitrost segrevanja 25 °C/min; izotermni korak tekom sintranja 20 min na 550 °C. Vzorec, izdelan pri optimalnih pogojih sintranja z dodatkom Nd-Cu, ima v primerjavi z začetnim vzorcem za 2-krat povišano remanenco in kar za 40-krat povišano koercitivnost (remanenca 0,88 T; maksimalni energijski produkt 141 kJ/m3; koercitivnost 1053 kA/m). Slednjo smo z naknadno enostopenjsko toplotno obdelavo po sintranju povišali še za 5 % oziroma z dvostopenjsko za 7 % glede na začetno vrednost. Z izdelavo magneta iz magnetno anizotropnega in izostatsko stisnjenega zelenca, smo remanenco povišali na 0,98 T. Minimalna količina dodatka Nd-Cu za doseganje dobrih lastnosti je 3,5 ut. %. Konvencionalno sintranje izostatsko stisnjenih zelencev ni bilo uspešno. Magnetne lastnosti so bile slabe (Br = 0,20 T; Hci = 20 kA/m; (BH)MAX = 1 kJ/m3) – podobne SPS sintranim vzorcem brez dodatka Nd-Cu, poleg tega vzorci niso bili dovolj gosti. To kaže, da sintranje oksidiranih recikliranih prahov na konvencionalen način ni možno, z uporabo SPS tehnike pa se da izdelati magnete z uporabnimi magnetnimi lastnostmi.

• Language: Slovenian
• Keywords: magneti, Nd-Fe-B, reciklirani prahovi, Nd-Cu, sintranje v pulzirajočem električnem polju - SPS
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [T. Sotelšek]
• Year: 2021
• Number of pages: XVI, 71 str.
• PID: 20.500.12556/RUL-129698
• UDC: 55
• COBISS.SI-ID: 84856835
• Publication date in RUL: 07.09.2021

Secondary language

• Language: English
• Title: Preparation and characterisation of new type of Nd-Fe-B permanent magnets based on recycled microcrystalline powders : master's thesis

Abstract

The master's thesis presents a new approach to the preparation of Nd-Fe-B type magnets based on recycled microcrystalline powders with an addition of alloy with composition Nd70Cu30 (at. %). The magnets were prepared by using an alternative sintering technique called spark-plasma sintering (SPS). The main advantages of this technique are the possibility of high of sintering speeds and lower sintering temperatures. Both are very important aspects from an economic point of view, furthermore magnetic properties of initial powder are retained. To prepare magnets, we used recycled powder, which was made from production scrap and was strongly oxidized compared to a commercial reference magnet. The sintered magnet from recycled powder without the addition of Nd-Cu had extremely poor magnetic properties (Br = 0.40 T; Hci = 26 kA/m; (BH)MAX = 3 kJ/m3) and an inappropriate microstructure. We added 5 wt. % of the eutectic alloy Nd-Cu to the recycled powder and performed a parametric study to determine the optimal sintering conditions. These are as follows: final sintering temperature 1000 °C with 1 min holding time; heating rate 25 °C/min; isothermal step during sintering for 20 min at 550 °C. The sample produced under optimal sintering conditions and with the addition of Nd-Cu has a 2 × increased remanence and a 40 × increased coercivity (remanence 0.88 T; coercivity 1053 kA/m; maximum energy product 141 kJ/m3) compared to one without Nd-Cu. The coercivity was increased by 5 % with single-stage and by 7 % with two-stage annealing. By preparing a magnet from magnetically anisotropic and isostatically compressed green compact, we increased the remanence to 0.98 T. The minimum addition amount of Nd-Cu to achieve good properties was found to be 3.5 wt. %. Conventional sintering of isostatically compressed green compacts was not successful. The magnetic properties were poor (Br = 0.20 T; Hci = 20 kA/m; (BH)MAX = 1 kJ/m3) as in case of SPS sintered magnet without addition of Nd-Cu. Moreover, the samples were not dense. This implies that sintering of oxidised recycled powders is not possible with conventional high-temperature approach. On the other hand, it is possible to produce magnets with useful magnetic properties using the SPS technique.

• Keywords: magnets, Nd-Fe-B, recycled powders, Nd-Cu, spark-plasma sintering (SPS)