Metastable gamma-iron nickel nanostructures for magnetic refrigeration near room temperature
Başlık çevirisi mevcut değil.
- Tez No: 401669
- Danışmanlar: PROF. MICHAEL MCHENRY, PROF. DAVID LAUGHLIN
- Tez Türü: Doktora
- Konular: Metalurji Mühendisliği, Metallurgical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2013
- Dil: İngilizce
- Üniversite: Carnegie Mellon University
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 118
Özet
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Özet (Çeviri)
The observation of a giant magnetocaloric effect in Gd5Ge1:9Si2Fe0:1 has stimulated the magnetocaloric research in the last two decades. However, the high price of Gd and its proclivity to corrosion of these compounds have prevented their commercial use. To reduce raw materials cost, transition metal-based alloys are investigated to replace rare earth-based materials. Environmental considerations, substitution for scarce and strategic elements, and cost considerations all speak to potential contributions of these new materials to sustainability. Efforts in improving the refrigeration capacity (RC) of refrigerants mainly rely on broadening the magnetic entropy change. One promising technique is to couple two phases of magnetic materials with desirable properties. Second is the investigation of nanoparticle synthesis routes, with ball milling being the most widely used one. The motivation for the nanoparticles synthesis is rooted in their inherent tendency to have distributed exchange coupling, which will broaden the magnetic entropy curve. As proven with the cost analysis, the focus is believed to shift from improving the RC of refrigerants toward finding the most economically advantageous magnetic refrigerant with the highest performance. Mechanically alloyed Fe70Ni30 and Fe72Ni28 alloys were characterized in terms of their structural and magnetic properties. Previous studies showed that single phase FCC -FeNi alloys with 26-30 at. % Ni have Curie temperatures, TC, near room temperature. Having TC near room temperatures along with large magnetization makes -FeNi alloys attractive for room temperature magnetocaloric cooling technologies. To obtain a single -phase, particles were solution annealed in the -phase field and water quenched. The preferential oxidation of Fe during ball milling was used as a means to tune the TC of the alloy. Refrigeration capacities, RCFWHM, of the Fe70Ni30 and the Fe72Ni28 alloys were calculated to be 470 J/kg and 250 J/kg at 5 T, with peak temperatures 363 K and 333 K, respectively. The RCFWHM for the Fe70Ni30 is higher than the previously reported Nanoperm (Fe70Ni30)89Zr7B4 type alloy and on the same order of magnitude with other Fe-based alloys. The maximum magnetic entropy change values observed for the Fe70Ni30 and the Fe72Ni28 are 0.65 and 0.5 Jkg 1K 1, respectively, at a field of 5 T. These are smaller than those of rare earth magnetic refrigerants showing first order transformation behavior. The larger RCFWHM value results mainly from the width of the magnetic entropy curve in these types of materials. In a follow up study, nanocrystalline powders of (Fe70Ni30)100 xMox (x=1 to x=4) were produced by high energy (SPEX) mechanical alloying. Increasing the Mo content was found to stabilize the FCC phase in mechanically alloyed nanopowders. The TC of the alloys was lowered with Mo additions, without decreasing the Refrigeration Capacity (RC), due to the additional temperature broadening of the magnetic entropy change. Based on the previous study on the role of disorder, the additional temperature broadening was attributed to the increased positional disorder introduced by the Mo additions into the -FeNi system. Alloy with (Fe70Ni30)96Mo4 composition was observed to have RCFWHM of 432 J/kg at 5T which is comparable to other prominent magnetic refrigerants operating near room temperatures. The economic viability of these rare-earth-free alloys, along with respectable magnetocaloric properties and potential for scalable production, make them good candidates for magnetic refrigeration applications.
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