2,371 research outputs found
Computational study on microstructure evolution and magnetic property of laser additively manufactured magnetic materials
Additive manufacturing (AM) offers an unprecedented opportunity for the quick
production of complex shaped parts directly from a powder precursor. But its
application to functional materials in general and magnetic materials in
particular is still at the very beginning. Here we present the first attempt to
computationally study the microstructure evolution and magnetic properties of
magnetic materials (e.g. Fe-Ni alloys) processed by selective laser melting
(SLM). SLM process induced thermal history and thus the residual stress
distribution in Fe-Ni alloys are calculated by finite element analysis (FEA).
The evolution and distribution of the -Fe-Ni and FeNi phase
fractions were predicted by using the temperature information from FEA and the
output from CALculation of PHAse Diagrams (CALPHAD). Based on the relation
between residual stress and magnetoelastic energy, magnetic properties of SLM
processed Fe-Ni alloys (magnetic coercivity, remanent magnetization, and
magnetic domain structure) are examined by micromagnetic simulations. The
calculated coercivity is found to be in line with the experimentally measured
values of SLM-processed Fe-Ni alloys. This computation study demonstrates a
feasible approach for the simulation of additively manufactured magnetic
materials by integrating FEA, CALPHAD, and micromagnetics.Comment: 20 pages, 15 figure
A phase-field model of relaxor ferroelectrics based on random field theory
A mechanically coupled phase-field model is proposed for the first time to
simulate the peculiar behavior of relaxor ferroelectrics. Based on the random
field theory for relaxors, local random fields are introduced to characterize
the effect of chemical disorder. This generic model is developed from a
thermodynamic framework and the microforce theory and is implemented by a
nonlinear finite element method. Simulation results show that the model can
reproduce relaxor features, such as domain miniaturization, small remnant
polarization and large piezoelectric response. In particular, the influence of
random field strength on these features are revealed. Simulation results on
domain structure and hysteresis behavior are discussed and compared with
related experimental results.Comment: 8 figure
Multiscale examination of strain effects in Nd-Fe-B permanent magnets
We have performed a combined first-principles and micromagnetic study on the
strain effects in Nd-Fe-B magnets. First-principles calculations on Nd2Fe14B
reveal that the magnetocrystalline anisotropy (K) is insensitive to the
deformation along c axis and the ab in-plane shrinkage is responsible for the K
reduction. The predicted K is more sensitive to the lattice deformation than
what the previous phenomenological model suggests. The biaxial and triaxial
stress states have a greater impact on K. Negative K occurs in a much wider
strain range in the ab biaxial stress state. Micromagnetic simulations of
Nd-Fe-B magnets using first-principles results show that a 3-4% local strain in
a 2-nm-wide region near the interface around the grain boundaries and triple
junctions leads to a negative local K and thus decreases the coercivity by
~60%. The local ab biaxial stress state is more likely to induce a large loss
of coercivity. In addition to the local stress states and strain levels
themselves, the shape of the interfaces and the intergranular phases also makes
a difference in determining the coercivity. Smoothing the edge and reducing the
sharp angle of the triple regions in Nd-Fe-B magnets would be favorable for a
coercivity enhancement.Comment: 9 figure
Insight into perovskite antiferroelectric phases: Landau theory and phase field study
Understanding the appearance of commensurate and incommensurate modulations
in perovskite antiferroelectrics (AFEs) is of great importance for material
design and engineering. The dielectric and elastic properties of the AFE domain
boundaries are lack of investigation. In this work, a novel Landau theory is
proposed to understand the transformation of AFE commensurate and
incommensurate phases, by considering the coupling between the oxygen
octahedral tilt mode and the polar mode. The derived relationship between the
modulation periodicity and temperature is in good agreement with the
experimental results. Using the phase field study, we show that the
polarization is suppressed at the AFE domain boundaries, contributing to a
remnant polarization and local elastic stress field in AFE incommensurate
phases
Determination of optimal reversed field with maximal electrocaloric cooling by a direct entropy analysis
Application of a negative field on a positively poled ferroelectric sample
can enhance the electrocaloric cooling and appears as a promising method to
optimize the electrocaloric cycle. Experimental measurements show that the
maximal cooling does not appear at the zero-polarization point, but around the
shoulder of the P-E loop. This phenomenon cannot be explained by the theory
based on the constant total entropy assumption under adiabatic condition. In
fact, adiabatic condition does not imply constant total entropy when
irreversibility is involved. A direct entropy analysis approach based on work
loss is proposed in this work, which takes the entropy contribution of the
irreversible process into account. The optimal reversed field determined by
this approach agrees with the experimental observations. This study signifies
the importance of considering the irreversible process in the electrocaloric
cycles
Positive and negative electrocaloric effect in BaTiO in the presence of defect dipoles
The influence of defect dipoles on the electrocaloric effect (ECE) in
acceptor doped BaTiO is studied by means of lattice-based Monte-Carlo
simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen
vacancy-acceptor associates are described by fixed defect dipoles with
orientation parallel or anti-parallel to the external field. By a combination
of canonical and microcanoncial simulations the ECE is directly evaluated. Our
results show that in the case of anti-parallel defect dipoles the ECE can be
positive or negative depending on the density of defect dipoles. Moreover, a
transition from a negative to positive ECE can be observed from a certain
density of anti-parallel dipoles on when the external field increases. These
transitions are due to the delicate interplay of internal and external fields,
and are explained by the domain structure evolution and related field-induced
entropy changes. The results are compared to those obtained by MD simulations
employing an {\it{ab initio}} based effective Hamiltonian, and a good
qualitative agreement is found. In addition, a novel electrocaloric cycle,
which makes use of the negative ECE and defect dipoles, is proposed to enhance
the cooling effect
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