| Sažetak | Istraživan je utjecaj dopiranja odabranim amorfnim magnetskim nanočesticama NiCoB u masenom udjelu 2.67 % (2.67 mas.%), te kristalnim magnetskim nanočesticama Ni u ugljikovoj košuljici, Ni/C, u masenim udjelima 2.5 % i 10 % (2.5 mas.% i 10 mas.%), na kristalnu strukturu i mikrostrukturu magnezijeva diborida, MgB2. Radi sustavne analize utjecaja dopiranja, uz dopirane uzorke MgB2+NiCoB, 2.67 mas.%, MgB2+Ni/C, 2.5 mas.% i 10 mas.%, provedeno je i istraživanje nedopiranoga uzorka MgB2, kao i samih magnetskih nanočestica. U svrhu strukturne/mikrostrukturne analize istraživanih uzoraka korištene su eksperimentalne metode rendgenske difrakcije u polikristalnom uzorku, pretražne elektronske mikroskopije, energijski razlučive spektroskopije, elementnog mapiranja te transmisijske elektronske mikroskopije i računske metode uključujući Rietveldovo utočnjavanje. Utvrđeno je da većinski sastav dopiranih uzoraka, kao i u slučaju nedopiranog uzorka MgB2, čine MgB2 i MgO (rezultat oksidacije uzoraka) te manji udjel Mg (ostatak od priprave uzoraka). U uzorcima MgB2+NiCoB, 2.67 mas.%, i MgB2+Ni/C, 10 mas.%, dodatno je opažan i manji udjel CoNi (0.24 mas.%) i Ni (0.20 mas.%). U uzorku MgB2+Ni/C, 2.5 mas.%, nije opažen Ni, što se objašnjava vrlo malim udjelom dopiranja. Sve se opažene morfologije u obliku prizmolikih zrna MgO, kuglastih nanočestica Mg, CoNi i Ni, nalaze na granicama heksagonskih zrna MgB2, što je utvrđeno temeljem elektronske mikroskopije. To je dodatno potvrđeno izostankom značajnije promjene bridova jedinične ćelije a i c u svim dopiranim uzorcima, u odnosu na nedopirani uzorak MgB2, te manjom veličinom zrna MgB2 i kristalita koji čine ta zrna (uslijed ograničenja rasta radi okolnih precipitata tipa MgO, Mg…). Nanočestice novoformirane kristalne faze CoNi u uzorku MgB2+NiCoB, 2.67 mas.%, veličine ∼6 nm (nastale uslijed mikrostrukturnih promjena, zbog termičkoga popuštanja pri temperaturi 650 °C tijekom sinteze dopiranog uzorka) te nanočestice Ni u uzorku MgB2+Ni/C, 10 mas.%, veličine ~ 2 nm (magnetska jezgra), karakterizirane jednodomenskom magnetskom strukturom i temperaturom blokiranja TB ispod sobne temperature, smještene na granicama zrna MgB2, mogu poslužiti kao centri magnetskoga zapinjanja u supravodiču MgB2, poboljšavajući time njegova elektromagnetska svojstva. |
| Sažetak (engleski) | The doping effect on the crystal structure and microstructure of magnesium diboride, MgB2, with selected amorphous magnetic nanoparticles NiCoB in weight fraction 2.67 % (2.67 wt.%) and crystal nanoparticles Ni with carbon coating, Ni/C, in weight fractions 2.5 % and 10 % (2.5 wt.% i 10 wt.%), was investigated. As a part of systematic study of the effect of doping, the undoped MgB2 sample and as-prepared samples with the magnetic nanoparticles were additionally investigated, along with the doped samples MgB2+NiCoB, 2.67 wt.%, and MgB2+Ni/C, 2.5 wt.% and 10 wt.%. The structural/microstructural analysis of the investigated samples was performed by various experimental methods: X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, elemental mapping and transmission electron microscopy as well as computational methods, including Rietveld refinement. It was found that the doped samples are mostly composed of MgB2 and MgO (due to oxidation), along with small fraction of Mg (the remnant from the samples preparation), as in the case of the undoped MgB2 sample. The small fractions of CoNi (0.24 wt.%) and Ni (0.20 wt.%) were also detected in the doped samples MgB2+NiCoB, 2.67 wt.% and MgB2+Ni/C, 10 wt.%, respectively. Due to the small amount of doping, Ni was not detected in MgB2+Ni/C sample, 2.5 wt.%. According to the electron microscopy observations, all of the morphologies in the investigated samples, present in form of the prism-like MgO grains, spherical nanoparticles Mg, CoNi and Ni, are located at grain boundaries of hexagonal MgB2 grains. This is confirmed by the lack of any significant change in the unit cell parameters a and c of the doped samples in respect to the undoped MgB2 sample, as well as with the decreased MgB2 grain sizes, and constituent MgB2 crystallites sizes, in doped samples (confirmed by the inhibited growth due to the adjacent precipitates presence, like MgO, Mg…). The new crystalline CoNi phase formed in the MgB2+NiCoB sample, 2.67 wt.% (due to the annealing at 650 °C, during the synthesis of the sample), consisting of spherical nanoparticles with size ∼ 6 nm, along with Ni nanoparticles in the MgB2+Ni/C sample, 10 wt.% (with size of magnetic core ∼ 2 nm), characterized by the single domain magnetic structure and blocking temperature TB below room temperature, located at MgB2 grain boundaries, could serve as effective magnetic pinning centers in MgB2, thus improving its electromagnetic properties. |
