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Despite all progresses made so far, in-stent restenosis (ISR) is still a vital problem after angioplasty and stenting with permanent stents. Therefore, the effects of microblasting on laser powder bed fusion (LPBF) manufactured biodegradable Fe-based stents with regard to surface topography and its effect on smooth muscle cell (SMC) adherence, which could be interpreted as an early hallmark for ISR, are characterized. The LPBF-processed Fe-30Mn-1C-0.025S stents are microblasted with spherical glass beads and angular corundum particles. On the microscale, the partially molten particles on the stents are significantly reduced after the surface treatments, especially after microblasting with glass beads. Angular corundum particles lead to a rougher surface on the nanoscale as demonstrated by scanning electron microscopy and atomic force microscopy. With the aim to reduce migration and proliferation of SMC, which contribute to ISR after stenting, the interactions of microblasted stent surfaces with SMC are assessed by fluorescence microscopy. Both microblasted surfaces reduce SMC adhesion and change SMC morphology compared to the as-built state as well as to commercially available 316L stents. In conclusion, microblasting treatment shows a high potential for the postprocessing of additively manufactured, biodegradable stents due to the reduction of the surface roughness and possible beneficial effect regarding ISR.

As the magnetocaloric characterization of polycrystalline Tb1−xHoxNi2 intermetallic compounds (with x = 0.25, 0.5, and 0.75), their relative cooling power (RCP) and refrigerant capacity (RC) are estimated based on comprehensive investigations of the magnetic and thermodynamic properties of the compositions, the structure of which was studied in detail by x-ray diffraction analysis and scanning electron microscopy. The taken x-ray diffraction patterns demonstrate the similarity between the Ho-depleted compound (x = 0.25) and the parent TbNi2 and HoNi2 compounds, i.e., the regular C15 cubic structure forms in them, whereas, in the case of x = 0.5 and 0.75, the C15-based superstructure is found. Both magnetic and heat capacity measurements showed that all of the studied compounds exhibit the transition from the ferromagnetic to paramagnetic state, which is confirmed to be the second order magnetic phase transition with the Curie temperature that decreases from 31.1 to 18.6 K in passing from the Tb-enriched Tb0.75Ho0.25Ni2 to the Ho-enriched Tb0.25Ho0.75Ni2 composition. The high values of RCP and RC are shown can be reached for the compositions and experimentally are demonstrated based on the performed measurements of magnetic and thermodynamic parameters. Under an external magnetic field change of 0–5 T, the maximum magnetic entropy change -ΔSmag for the Tb1−xHoxNi2 compounds increases from 13.9 to 19.5 J/kgK as x increases from 0.25 to 0.75. These experimental results are compared with and confirmed by available theoretical data. The maximum adiabatic temperature change ΔTad for Tb0.75Ho0.25Ni2 and Tb0.5Ho0.5Ni2 at the 2 T magnetic field change reaches 3.0 K, and substantially increases, namely, to 4.5 K in the case of the Tb0.25Ho0.75Ni2 composition near its Curie temperature TC. The effect of increasing Ho content in the Laves-phase compounds on their magnetic and magnetocaloric properties is discussed.

Composites of polymer and micron-sized particles of carbonyl-iron were investigated in terms of their magnetization behavior. Thermoplastic elastomers with varying Young's modulus ( E Polymer =0.14–14.6 MPa) were used as matrix material. Field dependent magnetization curves reveal that the hysteretic behavior of the composites strongly depends on both the particle fraction (7, 10, 14, 21, 31 vol%) and on the mechanical properties of the polymer. It is shown that hysteresis only appears above a certain fraction of magnetic particles which can be accounted to the magnetic exchange between the particles. However, hysteresis is suppressed in the composite with largest Young's modulus of the polymer matrix, even at largest particle fraction.

Present work unveils novel magnetic resonance imaging (MRI) compatible glassy Ti-Zr-Nb-Hf-Si alloys designed based on a high entropy alloys approach, by exploring the central region of multi-component alloy phase space. Phase analysis has revealed the amorphous structure of developed alloys, with a higher thermal stability than the conventional metallic glasses. The alloys exhibit excellent corrosion properties in simulated body fluid. Most importantly, the weak paramagnetic nature (ultralow magnetic susceptibility) and superior radiopacity (high X-ray attenuation coefficients) offer compatibility with medical diagnostic imaging systems thereby opening unexplored realms for biomedical applications.

Magnetocaloric La(Fe,Si)13-based alloys are promising materials for magnetic cooling systems but their limited corrosion resistance in water-based heat transfer fluids is critical. The corrosion behavior of as-cast and annealed La-Fe-Si alloy samples was analyzed in comparison to that of La and Fe for evaluation of the impact of alloy chemistry and microstructure. Electrochemical studies were conducted in defined electrolytes starting with aerated distilled water (pH = 6) for assessing the influence of pH value changes and anion contaminations. Specifically, forced flow electrolyte conditions were applied which are closer to operation conditions of real magnetocaloric regenerator beds than stagnant ones. Corroded sample surfaces were analyzed with SEM to assess damage mechanisms. The reactive nature of the alloy constituents determines the high corrosion activity and limited passivation ability of La-Fe-Si alloys. Their exposure to distilled water is particularly detrimental under stagnant conditions as local fluid acidification enhances corrosion processes. These are based on galvanic coupling between the phases with different corrosion activities: La-rich phases > La(Fe,Si)13-based matrix > alpha-Fe(Si). Laminar fluid flow is beneficial for alloy surface passivation. But anion contaminants like sulfate or hydrogen phosphate ions counteract the weak passivity in flowing distilled water. While acidic conditions lead to instable corrosive states, a pH value control of the heat transfer fluid at alkaline conditions is effective for stable passivity of the alloy surface. Also, the applicability of a phosphate conversion coating treatment in 0.15 M NaH2PO4 (pH = 4) was evaluated and prospects of this approach are discussed.

The magnetostructural transition (MST) can be tuned close to room temperature for an isostructurally alloyed (MnNiGe)1−x(Fe2Ge)x (x = 0.1) compound by partially substituting a small amount of Si for Ge (7 at. %). In this study, the effect of hydrostatic pressure (p) on MST is investigated. In comparison to purely magnetically induced phase transition, pressure initiates structural transition more abruptly, which results in an increase in the isothermal entropy change by a factor of 2 from −Δs = 25.6 (p = 0) to 45.6 J/kg K (p = 190 MPa) for a magnetic field change of 2 T. Since the direct assessment of the adiabatic temperature change, ΔTad, is difficult due to the large volume change and subsequent structural breakdown at MST, an indirect method has been employed to estimate ΔTad.

Magnetocaloric composite wires have been studied by pulsed-field measurements up to μ0ΔH = 10 T with a typical rise time of 13 ms in order to evaluate the evolution of the adiabatic temperature change of the core, ΔTad, and to determine the effective temperature change at the surrounding steel jacket, ΔTeff, during the field pulse. An inverse thermal hysteresis is observed for ΔTad due to the delayed thermal transfer. By numerical simulations of application-relevant sinusoidal magnetic field profiles, it can be stated that for field-frequencies of up to two field cycles per second heat can be efficiently transferred from the core to the outside of the jacket. In addition, intense numerical simulations of the temperature change of the core and jacket were performed by varying different parameters, such as frequency, heat capacity, thermal conductivity and interface resistance in order to shed light on their impact on ΔTeff at the outside of the jacket in comparison to ΔTad provided by the core.

Magnetocaloric ${\mathrm{LaFe}}_{13\ensuremath{-}x}{\mathrm{Si}}_{x}$-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent $^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering measurements (in a field ${\ensuremath{\mu}}_{0}H$ of $\ensuremath{\sim}0.7$ T) of the vibrational (phonon) density of states, VDOS, in ${\mathrm{LaFe}}_{11.6}{\mathrm{Si}}_{1.4}$ across the metamagnetic isostructural first-order phase transition at ${T}_{C}\ensuremath{\sim}192$ K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at ${T}_{C}$. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across ${T}_{C}$, occurring within a small temperature interval of $\mathrm{\ensuremath{\Delta}}T\ensuremath{\le}12$ K around ${T}_{C}$. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to ${T}_{C}$, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states $D({E}_{F}$) at the Fermi energy ${E}_{F}$. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}$ of $+6.9\ifmmode\pm\else\textpm\fi{}2.6$ J/(kg K) and in the vibrational specific heat $\mathrm{\ensuremath{\Delta}}{C}_{\mathrm{vib}}$ of $+2.7\ifmmode\pm\else\textpm\fi{}1.6$ J/(kg K) upon heating. The increase in magnitude of the vibrational entropy $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}|=6.9$ J/(kg K) of the Fe sublattice at ${T}_{C}$ upon heating is substantial, if compared with the magnitude of the isothermal entropy change $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{iso}}|$ of 14.2 J/(kg K) in a field change $\mathrm{\ensuremath{\Delta}}B$ from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. We demonstrate that $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{vib}}$ obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change $\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{iso}}$ at the phase transition of ${\mathrm{LaFe}}_{13\ensuremath{-}x}{\mathrm{Si}}_{x}$.

Magnetocaloric composites made from La-Fe-Co-Si particles and an epoxy binder matrix exhibit mechanical stability and good magnetocaloric properties, but also a large characteristic time for thermal transport. Here, the origin of this large time constant is examined by comparing two measurement techniques, direct and contactless, to finite-element simulations based on a tomographic dataset of the sample. The combination of the low thermal conductivity of the epoxy matrix and a thermal resistance at the interface between epoxy and La-Fe-Co-Si is shown to be in good agreement in simulations and experiments. The findings help to disentangle the role of the thermal conductivity and the interfacial thermal resistance for the heat flow in magnetocaloric composites. It is shown that the low thermal conductivity of the epoxy alone cannot explain the large time constant and possibilities for using the interfacial thermal resistance to tailor anisotropic thermal conductivity for directional heat transfer in magnetocaloric composites are presented.

Magnetocaloric La (Fe, Si)13 intermetallic powder is dry coated with ductile metallic copper. The dry coating is attempted by two different processes using a high energy impact coater “Nara Hybridization System (NHS)” and Fritsch P7 Planetary Ball Mill. LaFe11.6Si1.4 was used as core particle and metallic copper as the coating. SEM and EDX images show the homogeneous coating obtained by the NHS while partial coating was observed by the ball milling process.

We report on novel magnetocaloric composites based on La(Fe,Si)13 particles in an amorphous metallic matrix. Hot compaction at the glass transition temperature, Tg, of the matrix prevents crack formation as commonly occurs in conventional hot-compacted La(Fe,Si)13 material. This damage can affect the magnetocaloric performance. The approach in the present paper shows that the La(Fe,Si)13 particles stay intact due to the buffer-effect of the amorphous and ductile matrix at Tg. Therefore, the magnetocaloric properties of the composites are almost independent of the compaction pressures.

Fully hydrogenated Mn containing LaFeSi alloys provide long time stability of attractive magnetocaloric properties; issues of previously reported induced phase co-existence can be avoided. In this paper, two series LaFe11.8 xSi1.2Mnx and LaFe11.6 xSi1.4Mnx (x = 0, 0.1, 0.2, 0.3, 0.4), before and after hydrogenation are presented. In the non-hydrogenated parent samples, microstructural evolution and phase formation with increasing Mn content are studied. It is found that materials combination within the sample series can provide constant MCE over a large working temperature range. As a peculiarity, the hydrogenated compounds show a different dependence of the transition temperature as a function of Si content than the parent compounds. This behaviour is explained by comparing the amount of hydrogen incorporated in samples with different Si content. Moreover, an estimation for the maximum adiabatic temperature change and optimal magnetic field is given exemplarily for a non-hydrogenated and a hydrogenated compound. (C) 2014 Elsevier B.V. All rights reserved.

Composites of two magnetocaloric components have been prepared by melt spinning. A special nozzle modification allows to separate the melts of two different materials which solidify simultaneously as rapidly quenched composite. Two materials combinations were chosen: (a) Fe-based amorphous materials with different Cr-contents and (b) Fe-based alloys with different Si-content resulting in crystalline La(Fe,Si)13 phase with large magnetocaloric effect after subsequent annealing. By melt spinning crystalline segments of several cm length and long amorphous ribbons were obtained. The magnetocaloric properties of the composites were assessed by magnetometry and direct measurements of the adiabatic temperature change. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe$_{1-x}$Si$_x$)$_{13}$ by means of first-principles calculations combined with the fixed-spin moment approach. We find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. In the same spirit, the dependence of average local Fe moment on the Si content can account for the occurence of first- and second-order transitions in the system.

Advanced magnetic refrigerants such as La(Fe,Si)13 materials require large entropy and adiabatic temperature changes based on the control of phase change physics and hysteresis. In order to advance their incorporation in prototypes and industrial applications, processing of single phase materials with graded working temperatures needs to be up-scaled and important engineering properties such as the thermal transport properties, corrosion protection and mechanical stability need to be optimized. These issues, including a last step of near net-shaped manufacturing of complex geometries, are discussed in this Viewpoint paper.

By comparing the microstructural and texture evolution with tensile stress–strain response of an Fe–24Mn–7Ni–8Cr (mass%) alloy, a slip-dominated deformation process and, at a later stage of deformation, twinning-induced plasticity are observed. The occurrence of deformation twinning is texture sensitive and occurs only in the 〈1 1 1〉 fibre texture component. Based on these experimental observations, a model is presented, which reflects an orientational and configurational peculiarity of face-centred cubic stacking faults bordered by two Shockley partials. With this model, the onset point of stacking fault growth, i.e. movement of the leading partial and stopping of the trailing partial, is evaluated. This point reflects the formation of twins in the sense that a twin is regarded as an arrangement of stacking faults on every consecutive slip plane. Furthermore, based on the tensile test results, a model-compatible description of the mechanical behaviour is shown and a reasonable stacking fault energy of about 8 mJ m −2 is calculated for the onset of partial dislocation breakaway, i.e. the onset of deformation twinning.

In an austenitic Fe 61.5 Mn 23 Ni 7 Cr 8.5 antiferromagnetic (afm) alloy deformation results in a splitting between magnetisation-vs.-temperature curves measured during field cooling (FC) below the Neel temperature and those measured at the same field after zero-field cooling (ZFC). Furthermore, a thermoremanent magnetisation (TRM) appears that corresponds to the splitting between zero-field and field cooled thermomagnetic measurements for a given cooling field and scales with the degree of deformation. This TRM is attributed to the deformation induced defects which act as a source of uncompensated magnetic moments and interact with the bulk afm moments. This interpretation is also supported by the fact that the net magnetic moments vanish above the Neel temperature. The TRM does not saturate in fields of up to 7 T, applied during cooling, and cannot be switched by fields up to 7 T. Within the investigated field range of −7 T to 7 T the magnetisation-vs.-field curves show a stable shift along the magnetisation axis. Thus, the uncompensated moments appear to be strongly exchange coupled to the afm matrix surrounding them. Within the series of experiments the maximum TRM reached after FC in 7 T corresponds to about 4 × 10 −4 μ B /atom (Bohr magneton) of the maximum deformed afm sample. The Neel temperature decreases due to deformation.

By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe(13-x)Si(x). These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.

Manganese containing La–Fe–Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La–Fe–Mn–Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μ Fe is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe11.3Mn0.3Si1.4 and, for comparison, LaFe11.6Si1.4 by 57Fe Mossbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn13 structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (FeII) of low local symmetry, and the highly symmetrical (8b) sites (FeI). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(B hf) of hyperfine magnetic fields B hf. The average hyperfine field 〈B hf〉, originating predominantly from FeII sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe11.6Si1.4, and the approximate relation 〈B hf〉 = Aμ Fe is confirmed for FeII sites, with A = 14.2 T/μ B. 〈B hf〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe11.3Mn0.3Si1.4, providing evidence for a reduction by 9.7% of the average Fe atomic moment μFe from ~2.16 μ B to a value of ~1.95 μ B by Mn substitution of Fe. Our Mossbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of M s = 163.1(1) Am2 kg−1 of LaFe11.6Si1.4 by 10.5% due to Mn substitution.

By means of time- and temperature-dependent magnetization measurements, we demonstrate that the timescale of hydrogen diffusion in partially-hydrogenated LaFe$_{13-x}$Si$_x$H$_y$ is of the order of hours, when the material is held at temperatures close to its as-prepared Curie temperature, $T_{C0}$. The diffusion constant is estimated to be $D \approx 10^{-15}$ - $10^{-16}$ m$^2$s$^{-1}$ at room temperature. We examine the evolution of a magnetically phase-separated state upon annealing for 3 days at a range of temperatures around $T_{C0}$, and show that the thermodynamic driving force behind hydrogen diffusion and phase segregation may be attributed to the lower free energy of hydrogen interstitials in the ferromagnetic state relative to the paramagnetic state., Comment: 4 pages, 3 color figures

Thermal management is one of the crucial issues in the development of magnetocaloric refrigeration technology for application. In order to ensure optimal exploitation of the materials “primary” properties, such as entropy change and temperature lift, thermal properties (and other “secondary” properties) play an important role. In magnetocaloric composites, which show an increased cycling stability in comparison to their bulk counterparts, thermal properties are strongly determined by the geometric arrangement of the corresponding components. In the first part of this paper, the inner structure of a polymer-bonded La(Fe, Co, Si)13-composite was studied by X-ray computed tomography. Based on this 3D data, a numerical study along all three spatial directions revealed anisotropic thermal conductivity of the composite: Due to the preparation process, the long-axis of the magnetocaloric particles is aligned along the xy plane which is why the in-plane thermal conductivity is larger than the thermal conductivity along the z-axis. Further, the study is expanded to a second aspect devoted to the influence of particle distribution and alignment within the polymer matrix. Based on an equivalent ellipsoids model to describe the inner structure of the composite, numerical simulation of the thermal conductivity in different particle arrangements and orientation distributions were performed. This paper evaluates the possibilities of microstructural design for inducing and adjusting anisotropic thermal conductivity in magnetocaloric composites.

We report the co-existence of longitudinal ferrimagnetic behavior with Neel temperature TN ∼ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For T > TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Neel's expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ∼ 15.44, NAB ∼ 32.01, and NAA ∼ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν” = 6.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of critical-exponent, respectively). This system exhibits unusual hysteresis loops with large bipolar exchange-bias effect (HEB ∼ 13.6 kOe at 7 K) after zero-field cooling process from an un-magnetized state, and a dramatic collapse of remanence (MR) and coercive field (HC) across TCOMP. The possible origins of such anomalous characteristics were discussed.

In magnetocaloric La(Fe,Si)13 alloys, it is known that hydrogen addition shifts the magnetic transition temperature TC near room temperature. By partial hydrogen desorption, TC can be adjusted precisely towards the working temperature of a magnetic cooling device. In this work, we studied the stability of partially desorbed LaFe11.6Si1.4Hy and show that the large volume difference of ferro- and para-magnetic phases drives the system from a single phase to a stable two-phase configuration. The hydrogen concentration separates on a macroscopic scale. The phase-coexistence is visualized by Kerr Microscopy. Differential scanning calorimetry measurements show that hydrogen can be reversibly recombined and separated again. We explain the separation by the coexistence of a high-volume ferromagnetic and low-volume paramagnetic phase that can be induced either by temperature or other external stimuli. This phenomenon can be applied to material systems that show a coupling of physical and structural transitions.

La(Fe,Si)13 alloys display a giant magnetocaloric effect when a magnetic field is applied near the Curie temperatureT C. However, to use these alloys for domestic refrigeration based on magnetic cooling, it is vital to increase T C near to the room-temperature range while simultaneously maintaining a large magnetocaloric effect. With this aim, we studied the effect of interstitialcarbon on the microstructure and magnetocaloric effect in LaFe11.6Si1.4C x (x = 0–0.4). The investigation was carried out in cast samples annealed for seven days at 1323 K. The study of microstructure shows that annealing led to about 90 wt. % of 1:13 magnetocaloric phase. Magnetization data revealed that the addition of carbon leads to an increase in T C and a decrease of the thermal hysteresis width. For x > 0.2, the magnetic transition changes from first-order to second-order, with a corresponding reduction in magnetocaloric effect. A small amount of C (x up to 0.2) improves the magnetocaloric properties of the parent alloy La(Fe,Si)13, and, furthermore, the carbon addition leads to an increase in the thermal stability of hydrided LaFe11.6Si1.4C x . The onset of hydrogen desorption increases from 460 K for the x = 0 (carbon-free alloy) to 500 K and 540 K, respectively, for x = 0.1 and x = 0.2.