Synthesis and Characterization of New MAX Phase Alloys
نویسنده
چکیده
This Thesis explores synthesis and characterization of new MAX phase alloys (M = early transition metal, A = A-group element, and X = C or N), based on incorporation of M and X elements previously not considered. My primary focus is on M = Mn for attaining magnetic properties, and on X = O for potential tuning of the transport properties. A recent theoretical study predicted (Cr1-xMnx)2AlC MAX phase to be a stable magnetic nanolaminate. I aimed at realizing this material and through a combinatorial approach based on magnetron sputtering from elemental targets, the first experimental evidence of Mn incorporation (x = 0.16) in a MAX phase is presented. The corresponding MAX phase was also synthesized using cathodic arc film deposition (x = 0.20) and bulk synthesis methods (x = 0.06). The primary characterization techniques were X-ray diffraction and high-resolution (scanning) transmission electron microscopy in combination with energy dispersive X-ray spectroscopy and/or electron energy loss spectroscopy, to obtain a precise local quantification of the MAX phase composition and to perform lattice resolved imaging. For epitaxial film growth of (Cr1-xMnx)2AlC, evidence is presented for the formation of (Cr1-yMny)5Al8, exhibiting a bcc structure with an interplanar spacing matching exactly half a unit cell of the hexagonal MAX phase. Consequently, routinely performed X-ray diffraction symmetric θ-2θ measurements result in peak positions that are identical for the two phases. As (Cr1-yMny)5Al8 is shown to display a magnetic response, its presence needs to be taken into consideration when evaluating the magnetic properties of the MAX phase. Methods to distinguish between (Cr,Mn)5Al8 and (Cr,Mn)2AlC are also suggested. As different A-element in the MAX phase is theoretically predicted to influence phase stability, attainable level of Mn incorporation, as well as magnetic properties, thin films of (Cr0.75Mn0.25)2GeC and bulk (Cr0.7Mn0.3)2GaC have also been synthesized. Vibrating sample magnetometry measurements display a magnetic response for all these materials, identifying (Cr,Mn)2AlC, (Cr,Mn)2GeC, and (Cr,Mn)2GaC as the first magnetic MAX phases. The results presented in this Thesis show that A = Al displays the highest magnetic transition temperature (well above room temperature) and A = Ga allows the highest Mn content. The attainable O incorporation in Ti2Al(C1-xOx) MAX phase was explored by arc deposition of Ti2AlC1-y thin films under high vacuum conditions, and solidstate reactions following deposition of understoichiometric TiCz on Al2O3. Ti2Al(C1-xOx) thin films with up to 13 at.% O (x = 0.52) were synthesized, and O was shown to occupy the C lattice site. The obtained O concentration is enough to allow future experimental investigations of the previously suggested (from theory) substantial change in anisotropic electronic properties with increasing O content. The experimental results obtained in this Thesis expand the MAX phase definition and the materials characteristics into new research areas, towards further fundamental understanding and functionalization. Populärvetenskaplig sammanfattning Materialvetenskap inkluderar forskning på material, dess syntes, struktur och sammansättning samt resulterande egenskaper, med fokus på att utveckla nya material skräddarsydda för specifika ändamål. En viktig del inom materialvetenskapen är forskning på tunna filmer, d.v.s. lager av material med tjocklek från ett atomlager till några mikrometer. Egenskaperna hos en yta, till exempel friktion, slitmotstånd, ledningsförmåga, eller utseende, kan förbättras genom att applicera en lämplig tunnfilm. Den här avhandlingen handlar om en grupp material som kallas MAX-faser. M står för en övergångsmetall (t.ex. Ti, Cr, Nb, Sc), A för ett element från grupp A i det periodiska systemet (t.ex. Al, Si, Ge, Ga), och X står för C eller N. Atomer av tre sådana olika element staplas i en struktur bestående av rena atomlager, t.ex. M-X-M-A-M-X-M-A. Ti2AlC, Cr2AlC, Ti3SiC2 och Ti4AlN3 är några exempel av mer än 60 hittills upptäckta MAX-faser. Det extra spännande med MAX-faser är att de kombinerar metalliska och keramiska egenskaper. De leder alltså ström och värme, men tål samtidigt höga temperaturer och de står emot oxidation. Dock, oavsett hur bra något är, kan det alltid göras ännu bättre. Nyligen har det publicerats teoretiska beräkningar som visar att tillsatser av nya atomslag i MAX-faser kan leda till helt nya egenskaper. Till exempel skulle syre (O) i Ti2AlC kunna styra ledningsförmågan, och mangan (Mn) i Cr2AlC skulle kunna resultera i magnetiska egenskaper. Det sistnämnda är särskilt intressant, då MAX-fasernas lagrade struktur gör att de skulle kunna passa för magnetisk datalagring och dataöverföring på ett bättre sätt än den teknologi som används i stor skala idag. I min forskning har jag utgått från ovan nämnda beräkningar och undersökt hur mycket O och Mn det är möjligt att få in i tunna filmer av Ti2AlC respektive Cr2AlC, samt vilken effekt de nya elementen har på materialets struktur och egenskaper. Jag har lyckats byta ut hälften av kolatomerna mot syre i Ti2AlC, vilket enligt teoretiska beräkningar är det högsta nåbara och dessutom tillräckligt för att möjliggöra framtida studier av hur ledningsförmågan förändras. Med olika förbättrade framställningsprocesser har jag också lyckats inkorporera Mn i tunna filmer av Cr2AlC och i de relaterade MAX-faserna Cr2GeC och Cr2GaC, samt även undersökt möjligheten att använda Mn i bulksyntes. Det visar sig att minst en fjärdedel av Cr kan bytas mot Mn i Cr2AlC eller Cr2GeC och att minst en tredjedel ryms i Cr2GaC. Mätningar rörande magnetiska egenskaper hos dessa material bevisar att alla tre är magnetiska, d.v.s. helt nya slags material.
منابع مشابه
Fabrication of Ti3SiC2-SiC max phase composites via in-situ and ex-situ synthesis
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