173 research outputs found
Efficient and affordable thermomagnetic materials for harvesting low grade waste heat
Industrial processes release substantial quantities of waste heat, which can
be harvested to generate electricity. At present, the conversion of low grade
waste heat to electricity relies solely on thermoelectric materials, but such
materials are expensive and have low thermodynamic efficiencies. Although
thermomagnetic materials may offer a promising alternative, their performance
remains to be evaluated, thereby hindering their real-world application. Here
we evaluate the efficiency and cost effectiveness of thermomagnetic materials
that can be used in motors, oscillators and generators for converting waste
heat to electricity. Our analysis reveals that up to temperature differences of
several 10 K the best thermomagnetic materials outperform thermoelectric
materials. Importantly, we find that the price per watt for some thermomagnetic
materials are much lower compared to present-day thermoelectrics and can become
competitive with conventional power plants. The materials library that we
present here enables the selection of the best available thermomagnetic
materials for harvesting waste heat and gives guidelines for their future
development.Comment: 24 pages, 5 figures (incl. 2 Ashby plots
Magnetic nanostructures by adaptive twinning in strained epitaxial films
We exploit the intrinsic structural instability of the Fe70Pd30 magnetic
shape memory alloy to obtain functional epitaxial films exhibiting a
self-organized nanostructure. We demonstrate that coherent epitaxial straining
by 54% is possible. The combination of thin film experiments and large-scale
first-principles calculations enables us to establish a lattice relaxation
mechanism, which is not expected for stable materials. We identify a low twin
boundary energy compared to a high elastic energy as key prerequisite for the
adaptive nanotwinning. Our approach is versatile as it allows to control both,
nanostructure and intrinsic properties for ferromagnetic, ferroelastic and
ferroelectric materials.Comment: Final version. Supplementary information available on request or at
the publisher's websit
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Mechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe-Pd magnetic shape memory alloy films on MgO
Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning
Magnetic domain structure of epitaxial Ni-Mn-Ga films
For the magnetic shape memory effect, knowledge about the interaction between
martensitic and magnetic domain structure is essential. In the case of Ni-Mn-Ga
bulk material and foils, a staircase-like magnetic domain structure with
90{\deg}- and 180{\deg}-domain walls is known for modulated martensite. In the
present paper we show that the magnetic domain pattern of thin epitaxial films
is fundamentally different. Here we analyze epitaxial Ni-Mn-Ga films by atomic
and magnetic force microscopy to investigate the correlation between the
twinned martensitic variants and the magnetic stripe domains. The observed
band-like domains with partially perpendicular outof-plane magnetization run
perpendicular to the microstructure domains defined by twinning variants. These
features can be explained by the finite film thickness, resulting in an
equilibrium twinning period much smaller than the domain period. This does not
allow the formation of a staircase domain patter. Instead the energies of the
magnetic and martensitic microstructures are minimized independently by
aligning both patterns perpendicularly to each other. By analyzing a thickness
series we can show that the observed magnetic domain pattern can be
quantitatively described by an adapted band domain model of Kittel.Comment: 12 pages, 4 figure
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What is the speed limit of martensitic transformations?
Structural martensitic transformations enable various applications, which range from high stroke actuation and sensing to energy efficient magnetocaloric refrigeration and thermomagnetic energy harvesting. All these emerging applications benefit from a fast transformation, but up to now their speed limit has not been explored. Here, we demonstrate that a thermoelastic martensite to austenite transformation can be completed within 10 ns. We heat epitaxial Ni-Mn-Ga films with a nanosecond laser pulse and use synchrotron diffraction to probe the influence of initial temperature and overheating on transformation rate and ratio. We demonstrate that an increase in thermal energy drives this transformation faster. Though the observed speed limit of 2.5 × 1027 (Js)1 per unit cell leaves plenty of room for further acceleration of applications, our analysis reveals that the practical limit will be the energy required for switching. Thus, martensitic transformations obey similar speed limits as in microelectronics, as expressed by the Margolus–Levitin theorem
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