113 research outputs found
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 Ī±'-Fe8Nx and Ī±''-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 Ī±'-Fe8Nx and Ī±''-Fe16N2 samples.
Ī±'-Fe8Nx thin films are deposited in the full range of 0 ā¤ x ā¤ 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 Ā± 0.09Ī¼B 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 ā1.1. 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 Ī±''-Fe16N2 nanoparticles. With this route, phase-pure Ī±''-Fe16N2 nanoparticles
are successfully synthesized and characterized. The Ms(0) for Ī±''-Fe16N2 nanoparticles is found to be
215Am2kg-1 and coercivity Ī¼0Hc = 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
The excellent magnetic entropy change () in the temperature range
of 20 77 K due to the first-order phase transition makes an
intriguing candidate for magnetocaloric hydrogen liquefaction. As an equally
important magnetocaloric parameter, the adiabatic temperature change () of associated with the first-order phase transition has not
yet been reported. In this work, the of 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 that is not as impressive as its
remarkable , exhibits an unusual low Debye temperature
() 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 and . The role of in achieving large is revealed: materials with higher tend to exhibit larger , particularly in the cryogenic temperature range. This discovery
explains the absence of an outstanding in and can
serve as a tool for designing or searching materials with both a large and a
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
Evaluation of Fe-nitrides, -borides and -carbides for enhanced magnetic fluid hyperthermia with experimental study of Ī±ā³-FeāāNā and Ļµ-FeāN 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, Ī³-FeāOā, FeāOā 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 Ī±ā³-FeāāNā and Ļµ-FeāN nanoparticles, and model nanoparticles of iron oxides. The former ones are studied as heating agents for magnetic fluid hyperthermia for the first time
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 veltÄ«ts magnÄtiskÄs plÅ«smas elektrisko motoru pÄtÄ«jumiem un to potenciÄlÄs pielietoÅ”anas iespÄjÄm tautsaimniecÄ«bÄ. DarbÄ apkopoti rezultÄti pÄtÄ«jumiem par pÄrslÄdzamas magnÄtiskas plÅ«smas elektrisku maŔīnu: izveidots un atrisinÄts tuvinÄts analÄ«tisks un skaitlisks modelis, izveidots eksperimentÄls makets pÄrslÄdzamas magnÄtiskÄs plÅ«smas elektriskajai maŔīnai, kuru izmantojot veikti induktivitÄtes, spriegumu profilu, griezes momenta un citi mÄrÄ«jumi. Sniegts ieskats optimÄlu magnÄtisku materiÄlu izvÄlei pÄrslÄdzamas magnÄtiskÄs plÅ«smas elektriskajÄm maŔīnÄm, kÄ arÄ« analizÄta vadÄ«bas elektronika. IegÅ«tie eksperimentu rezultÄti salÄ«dzinÄti ar analÄ«tiskÄ un skaitliskÄ modeļu rezultÄtiem, izdarÄ«ti secinÄjumi. FormulÄti turpmÄkie soļi pÄtÄ«jumiem konkrÄtajÄ virzienÄ.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
Sinhrono masinu specialie rezimi
Summary in English, RussianAvailable from Latvian Academic Library / LAL - Latvian Academic LibrarySIGLELVLatvi
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