Same particle size from different milling time

The parallel evolution of the MT characteristics and the magnetic properties on varying the particle size and in particular the similar ΔH and magnetization values obtained for each size range (irrespectively of the milling time), point to a close relationship between milling-induced deformation and particle size. The particle size seems to be then the most accurate parameter to assess the impact of milling, as long as the average behavior of the unsieved as-milled powders appears as just a mere consequence of different size distributions achieved after each milling time. This point has been analyzed from the comparison between the magneto-structural properties of the "M" particles (63µm < φ < 100µm) coming from the soft hand-crushing and from the more severe 15 minutes ball milling. Apart from being the narrowest studied size range (hence with the lowest intrinsic particle size dispersion), this is the most suitable range for the production of printable 3D composites since (a) particles are compatible with the standard printer nozzles, (b) particles above 100µm are presumably too large to generate homogeneous and ductile enough printable wires, and (c) particles below 63µm exhibit weak magnetic response, which may compromise the functionality of the material. Figure 4.15 shows the DSC thermograms (on cooling-heating after destabilization) and the M(H) curves at 350K obtained in

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the M0M and the M15M samples, alongside with the measurement corresponding to the bulk sample. It can be seen that, in effect, the enthalpy change at the MT and the austenite magnetization are almost same in both the samples in spite of very different milling intensity, thus confirming the proposed closed correlation between particle size and milling-induced plastic deformation.

Fig. 4.15. a) DSC thermograms, and b) magnetic-field dependence of magnetization for the M0M and M15M

powders.

Since the analyzed size interval ranges from 63µm to 100µm, the slight differences observed in the curves for the M0M and M15M powders could be attributable to a small difference in the distribution of the sizes inside the respective samples. This fact has been checked from Scanning Electron Microscopy (SEM) observations. The obtained micrographs and the particle size histograms calculated from it, are shown in figure 4.16. The obtained histograms indeed reveal a

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slight variation in the size distribution between both samples. In particular, the M0M sample consists of larger particles in average than the M15M samples, which contains more particles below 80µm. Such a small difference in the size distribution, in turn, result in small differences on the amount of non-transforming martensite and the internal strains present in each sample, as revealed by XRD measurements performed in both samples.

Fig. 4.16. SEM micrographs and particle-size histograms on the M0M and M15M powders.

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Fig. 4.17. X-ray diffractogram at 400K (black dots), calculated (red line), and the difference between experimental and calculated diffractogram (blue line) for the 0´, 15´; the green marks indicate the Bragg

reflections.

Figure 4.17 shows the XR diffractograms obtained at 400K, which confirm the predominance of the austenite cubic L21 structure together with the presence of a small amount of martensitic phase. As shown in figure 4.17b, the intensity of the reflection peaks linked to the retained martensite is slightly higher in the M15M sample than in the M0M sample. Likewise, the austenite peak is slightly broader in the M15M sample (see figure 4.17c), which is compatible with a higher value of internal strains. So, all the analyzed measurement is in line with an intimate relationship between particle size and degree of milling-induced deformation, which is fulfilled regardless of the milling duration. This means that particles of the same size must exhibit the same magneto-

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structural features irrespectively of the milling time. Even though a more detailed work (for which more and narrower particle sizes should be analyzed) is obviously required to establish a quantitative correlation between the particle size and the magneto-structural properties of the powders obtained after milling. The present finding is particularly intriguing from an applied point of view, since it implies that specific MT and magnetization properties may be attained by selecting particles with particular sizes, regardless of the milling process (hand crushing or ball milling), milling duration or even milling equipment.

- Microstructure recovery

As widely reported in the literature, the MT characteristics and the magnetic properties of the as- milled MSMA particles may be significantly enhanced after high temperature annealing treatments leading to microstructural recovery [83, 296, 343, 345, 346]. In order to explore this point, the particles between 63µm and 100µm were subjected to a 150 minutes annealing treatments at 873K. Such a relatively low temperature was chosen for the first attempt to avoid the risk of oxidation.

Fig. 4.18. Magnetic-field dependence of magnetization of the particles lying on the 63µm < φ < 100µm range, after annealing.

Figure 4.18 shows the comparison between the high-field magnetization at 350K in the as-milled samples and after annealing. It can be seen that the magnetization at 60KOe, M60kOe, experiences a rise around 50% while the magnetization in martensite remains almost unaffected, which results in a ΔM in the annealed sample twice that in the as-milled state. Therefore, a considerable improvement in the MCE should be expected after annealing.

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Finally, the MCE has also been assessed in the annealed powders. Figure 4.19 displays the isothermal entropy change as a function of both temperature and applied magnetic field. A maximum ΔSiso ≈ 8J/kgK for the annealed powders in 63 < μm < 100 size range has been obtained under a 60kOe magnetic field. This entropy change is considerably higher than that obtained in as-milled powders with the same particle size (∆𝑆_𝑖𝑠𝑜^{𝑎𝑠−𝑚𝑖𝑙𝑙}≈ 5.5J/kgK, see figure 4.12). A direct second order MCE (ΔSiso< 0) linked to the Curie temperature can also be observed. Due to the large ΔSiso and the relatively wide MT temperature range, the calculated RCP = 300J/kg is similar to that found in the bulk alloy. Therefore, it can be concluded that selecting the particle size after milling (independently of the milling time) and performing subsequent annealing treatments seem to be an efficient procedure to get controlled powders, suitable for the production of composites for magnetic refrigeration.

Fig. 4.19. Isothermal magnetically-induced entropy change as a function of temperature and applied magnetic field for the particles between 63µm and 100µm after annealing.