Fe8Nx Thin Films and Nanoparticles: from Intrinsic Properties Towards Magnetic Applications

Iron nitride Fe8Nx could potentially provide an environmentally friendly and resource-efficient functional magnetic material in the areas of permanent magnets, magnetic recording as well as biomedical applications. Despite the amount of research within the last decades, questions remain on whether or not the intrinsic magnetic properties are sufficient and if they can, by sustainable means, be engineered into the useful extrinsic properties. Another key issue is the phase stability in different environments which needs a thorough investigation. In this thesis, the Fe8Nx material synthesis, an analysis of structure and the corresponding magnetic properties, particularly in thin films and nanoparticles, are presented. The focus lies first on the fabrication of buffer-free, phase-pure α'-Fe8Nx and α''-Fe16N2 samples in order to converge towards an unambiguous interpretation of the observed physical phenomena. The main aim of this work is to study the magnetic properties, the thermal stability and consequently feasibility for the proposed applications, by performing advanced synthesis and in-depth characterization of high-quality α'-Fe8Nx and α''-Fe16N2 samples. α'-Fe8Nx thin films are deposited in the full range of 0 ≤ x ≤ 1. The nitrogen incorporation leads to a gradually induced tetragonal unit cell expansion of the compounds which is accompanied by an increase in the magnetic moment, reaching 2.50 ± 0.09μB per Fe atom at 10 K. The origin of the increased magnetic moment is solely the lattice expansion. The uniaxial anisotropy constant increases with c/a ratio (or resp. nitrogen content) reaching a value of 0.54MJm3 for c/a ≈1.1. The interstitial N atoms play a decisive role in stabilizing the enhanced perpendicular magnetocrystalline anisotropy. These findings can be generalized to other nitrogen containing interstitial Fe alloys. The second major activity is the development of a novel route with a high-pressure hydrogen reduction step for the synthesis of α''-Fe16N2 nanoparticles. With this route, phase-pure α''-Fe16N2 nanoparticles are successfully synthesized and characterized. The Ms(0) for α''-Fe16N2 nanoparticles is found to be 215Am2kg-1 and coercivity μ0Hc = 0.22T. Fe-O shells form around the particles when exposed to atmosphere which leads to a reduced magnetization. Overall the Fe8Nx alloys are shown to possess semi-hard magnetic properties as well as relatively poor phase stability, which has direct consequences on applications, such as bulk permanent magnets, nanocomposites and magnetic nanoparticle hyperthermia

The role of Debye temperature in achieving large adiabatic temperature changes at cryogenic temperatures: a case study on Pr2InPr_2In

The excellent magnetic entropy change ($\Delta S_T$) in the temperature range of 20 $\sim$ 77 K due to the first-order phase transition makes $Pr_2In$ an intriguing candidate for magnetocaloric hydrogen liquefaction. As an equally important magnetocaloric parameter, the adiabatic temperature change ($\Delta T_{ad}$) of $Pr_2In$ associated with the first-order phase transition has not yet been reported. In this work, the $\Delta T_{ad}$ of $Pr_2In$ is obtained from heat capacity measurements: 2 K in fields of 2 T and 4.3 K in fields of 5 T. While demonstrating a $\Delta T_{ad}$ that is not as impressive as its remarkable $\Delta S_T$, $Pr_2In$ exhibits an unusual low Debye temperature ($T_D$) of around 110 K. Based on these two observations, an approach that combines the mean-field and Debye models is developed to study the correlation between $\Delta T_{ad}$ and $T_D$. The role of $T_D$ in achieving large $\Delta T_{ad}$ is revealed: materials with higher $T_D$ tend to exhibit larger $\Delta T_{ad}$, particularly in the cryogenic temperature range. This discovery explains the absence of an outstanding $\Delta T_{ad}$ in $Pr_2In$ and can serve as a tool for designing or searching materials with both a large $\Delta S_T$ and a $\Delta T_{ad}$

Evaluation of Fe-nitrides, -borides and -carbides for enhanced magnetic fluid hyperthermia with experimental study of α″-Fe₁₆N₂ and ϵ-Fe₃N nanoparticles

In this work, we investigate alternative materials systems that, based on their intrinsic magnetic properties, have the potential to deliver enhanced heating power in magnetic fluid hyperthermia. The focus lies on systems with high magnetization phases, namely iron-nitrogen (Fe-N), iron-boron (Fe-B) and iron-carbon (Fe-C) compounds, and their performance in comparison to the conventionally used iron oxides, γ-Fe₂O₃, Fe₃O₄ and non-stoichiometric mixtures thereof. The heating power as a function of the applied alternating magnetic field frequency is calculated and the peak particle size with the maximum specific loss power (SLP) for each material is identified. It is found that lower anisotropy results in larger optimum particle size and more tolerance for polydispersity. The effect of nanoparticle saturation magnetization and anisotropy is simulated, and the results show that in order to maximize SLP, a material with high magnetization but low anisotropy provides the best combination. These findings are juxtaposed with experimental results of a comparative study of iron nitrides, namely α″-Fe₁₆N₂ and ϵ-Fe₃N nanoparticles, and model nanoparticles of iron oxides. The former ones are studied as heating agents for magnetic fluid hyperthermia for the first time

Microstructure of a spark-plasma-sintered Fe2VAl-type Heusler alloy for thermoelectric application

The influence of microstructure on thermoelectricity is increasingly recognized. Approaches for microstructural engineering can hence be exploited to enhance thermoelectric performance, particularly through manipulating crystalline defects, their structure, and composition. Here, we focus on a full-Heusler Fe2VAl-based compound that is one of the most promising thermoelectric materials containing only Earth-abundant, non-toxic elements. A Fe2VTa0.05Al0.95 cast alloy was atomized under a nitrogen-rich atmosphere to induce nitride precipitation. Nanometer- to micrometer-scale microstructural investigations by advanced scanning electron microscopy and atom probe tomography (APT) are performed on the powder first and then on the material consolidated by spark-plasma sintering for an increasing time. APT reveals an unexpected pick-up of additional impurities from atomization, namely W and Mo. The microstructure is then correlated with local and global measurements of the thermoelectric properties. At grain boundaries, segregation and precipitation locally reduce the electrical resistivity, as evidenced by in-situ four-point probe measurements. The final microstructure contains a hierarchy of structural defects, including individual point defects, dislocations, grain boundaries, and precipitates, that allow for a strong decrease in thermal conductivity. In combination, these effects provide an appreciable increase in thermoelectric performance

Magneto-active composites with locally tailored stiffness produced by laser powder bed fusion

Additive manufacturing technologies enable the production of complex and bioinspired shapes using magneto-responsive materials, which find diverse applications in soft robotics. Particularly, the development of composites with controlled gradients in mechanical properties offers new prospects for advancements in magneto-active materials. However, achieving such composites with gradients typically involves complex multi-material printing procedures. In this study, a single-step laser powder bed fusion (LPBF) process is proposed that enables precise local adjustments of the mechanical stiffness within magneto-active composites. By utilizing distinct laser parameters in specific regions of a composite containing thermoplastic polyurethane and atomized magnetic powder derived from hard magnetic Nd-Fe-B, the stiffness of the composite can be modified within the range of 2 to 22 MPa. Various magneto-responsive actuators with locally tailored stiffness are fabricated and their magnetic performance is investigated. The enhanced response exhibited by actuators with locally adjusted mechanical properties in comparison to their homogeneous counterparts with identical geometries is shown. As a demonstration of a biomedical application, a magnetically responsive stent with localized adjustment is presented with the ability to meet specific requirements in terms of geometry and local stiffness based on an individual's anatomy and disease condition. The proposed method presents an approach for creating functionally graded materials using LPBF, not only for magneto-active materials but also for several other structural and functional materials

Development of theoretical model and selection of appropriate materials for switchable magnetic flux

Darbs veltīts magnētiskās plūsmas elektrisko motoru pētījumiem un to potenciālās pielietošanas iespējām tautsaimniecībā. Darbā apkopoti rezultāti pētījumiem par pārslēdzamas magnētiskas plūsmas elektrisku mašīnu: izveidots un atrisināts tuvināts analītisks un skaitlisks modelis, izveidots eksperimentāls makets pārslēdzamas magnētiskās plūsmas elektriskajai mašīnai, kuru izmantojot veikti induktivitātes, spriegumu profilu, griezes momenta un citi mērījumi. Sniegts ieskats optimālu magnētisku materiālu izvēlei pārslēdzamas magnētiskās plūsmas elektriskajām mašīnām, kā arī analizēta vadības elektronika. Iegūtie eksperimentu rezultāti salīdzināti ar analītiskā un skaitliskā modeļu rezultātiem, izdarīti secinājumi. Formulēti turpmākie soļi pētījumiem konkrētajā virzienā.The research deals with magnetic flux electric motor research and its potential applications in the national economy. Master's thesis summarizes the results of studies of the switchable magnetic flux electrical machine. Analytical and numerical theoretical models are set up and solved, an experimental model of switchable magnetic flux electrical machine is constructed and tested. Studies on selecting the optimal magnetic materials for switchable magnetic flux electrical machine are made. Obtained experimental results are compared with outputs from analytical and numerical models. Conclusions are made and further research steps are outlined

Fe8Nx Thin Films and Nanoparticles: from Intrinsic Properties Towards Magnetic Applications

Iron nitride Fe8Nx could potentially provide an environmentally friendly and resource-efficient functional magnetic material in the areas of permanent magnets, magnetic recording as well as biomedical applications. Despite the amount of research within the last decades, questions remain on whether or not the intrinsic magnetic properties are sufficient and if they can, by sustainable means, be engineered into the useful extrinsic properties. Another key issue is the phase stability in different environments which needs a thorough investigation. In this thesis, the Fe8Nx material synthesis, an analysis of structure and the corresponding magnetic properties, particularly in thin films and nanoparticles, are presented. The focus lies first on the fabrication of buffer-free, phase-pure α'-Fe8Nx and α''-Fe16N2 samples in order to converge towards an unambiguous interpretation of the observed physical phenomena. The main aim of this work is to study the magnetic properties, the thermal stability and consequently feasibility for the proposed applications, by performing advanced synthesis and in-depth characterization of high-quality α'-Fe8Nx and α''-Fe16N2 samples. α'-Fe8Nx thin films are deposited in the full range of 0 ≤ x ≤ 1. The nitrogen incorporation leads to a gradually induced tetragonal unit cell expansion of the compounds which is accompanied by an increase in the magnetic moment, reaching 2.50 ± 0.09μB per Fe atom at 10 K. The origin of the increased magnetic moment is solely the lattice expansion. The uniaxial anisotropy constant increases with c/a ratio (or resp. nitrogen content) reaching a value of 0.54MJm3 for c/a ≈1.1. The interstitial N atoms play a decisive role in stabilizing the enhanced perpendicular magnetocrystalline anisotropy. These findings can be generalized to other nitrogen containing interstitial Fe alloys. The second major activity is the development of a novel route with a high-pressure hydrogen reduction step for the synthesis of α''-Fe16N2 nanoparticles. With this route, phase-pure α''-Fe16N2 nanoparticles are successfully synthesized and characterized. The Ms(0) for α''-Fe16N2 nanoparticles is found to be 215Am2kg-1 and coercivity μ0Hc = 0.22T. Fe-O shells form around the particles when exposed to atmosphere which leads to a reduced magnetization. Overall the Fe8Nx alloys are shown to possess semi-hard magnetic properties as well as relatively poor phase stability, which has direct consequences on applications, such as bulk permanent magnets, nanocomposites and magnetic nanoparticle hyperthermia

Research of possibility to use water electrolysis gases as fuel in internal combustion engine

Bakalaura darbā ir apskatīts ūdens elektrolīzes process tā tradicionālajā formā, izmantojot līdzstrāvas elektroenerģijas avotu un ūdeni ar tajā disociētiem joniem kā elektrolītu. Analizēti elektrolīzes efektivitāti ietekmējoši faktori, veikta elektrolīzeru optimizēšana un salāgošana ar standarta vieglās automašīnas parametriem. Darba mērķis ir pārbaudīt ūdens elektrolīzes procesā radušos gāzu kā, iespējams, fosilajam kurināmajam alternatīvas degvielas potenciālu, vieglo automašīnu iekšdedzes dzinējos. Darbā tiek apskatītas šī procesa teorētiskās iespējas, kā arī uzbūvēts, uzstādīts un praksē pārbaudīts elektrolīzera prototips uz reālas vieglās automašīnas ar benzīna iekšdedzes dzinēja. Pirmie eksperimenti parādīja degvielas ekonomijas iespējas, bet ir nepieciešami turpmāki mērījumi izplūdes gāzu sastāva izmaiņu noteikšanai.This work deals with a classical water electrolysis using a DC current power and water based electrolyte solution. There is made an analysis of factors that are influencing an efficiency of electrolysis. Optimization of a self built electrolyser is performed accordingly to technical demands of standard gasoline car. An aim of this work is to test gases of water electrolysis as possible alternative for the fossil fuel (gasoline) in the internal combustion engine cars. Theoretical aspects are discussed and a practical electrolyser is built in a car powered with a gasoline fuel. The first test showed a good fuel economy, but further experiments are necessary to test an influence of addition of water electrolysis gases to fuel – how the composition of exhausts will change

Stability of Fe3N nanoparticles as possible candidates for biomedical applications

Superparamagnetic iron oxide nanoparticles (SPIONs) have been the material of choice for the biomedical industry as well as scientific community due to their extreme stability, well-known metabolism of iron in the human body, acceptable magnetic properties, easiness, low cost and scalability of production[1]. However, recently alternative iron-based[2] materials with enhanced magnetic properties have been considered for improved performance in biomedical applications. In this work, we study iron nitride Fe3N nanoparticles, as alternative candidates in biomagnetic applications due to their larger saturation magnetization values(Ms (Fe3N) = 128 Am2/kg vs Ms (Fe3O4) = 61 Am2/kg). Calorimetry results demonstrate significantly enhanced magnetic fluid hyperthermia heating performance compared to iron oxides as shown in Fig. 1. Crucial question for further development of Fe3N particles is stability in biological environment. The as-synthesized spherical Fe3N nanoparticles (13.5 nm) demonstrate very good stability of magnetic properties in water (Fig 2.). Transmission electron microscopy studies show that the reason for this is formation of a thin oxide layer that protects particles from further oxidation