Influence of short time milling in R5(Si,Ge)4, R = Gd and Tb, magnetocaloric materials

Abstract

The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 µm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 minutes of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process.

Magnetocaloric materials: From micro- to nanoscale

“…59 One of the most popular approaches for down-sizing magnetocaloric materials has been ball-milling as Blasquez and coworkers have reviewed, 60 and this technique has been intensively used in particular for the R 5 (Si,Ge) 4 materials. [61][62][63] It was observed that for materials with first-order magnetostructural transitions, the ball-milling and consequent sizereduction (downwards to the 0.5-5 lm range) lead to a strong attenuation of the respective MCE, which is explained by the increasing amorphization with increasing milling time. However, our group unveiled the mechanism that is ruling the different behaviors: it is an induce internal strain arising from the ball-milling process during the formation of microparticles.…”

Freestanding and flexible composites of magnetocaloric Gd5(Si,Ge)4 microparticles embedded in thermoplastic poly(methyl methacrylate) matrix

“…Similarly, the amount of detected eutectic phase was found to increase for Gd 5 Si 4 compound when the particle size reduces from 700 to 80 nm through ball milling 29 . Previously, it was shown an enlargement of secondary R 5 M 4 phases, such behavior was observed for milled Gd 5 Si 1.3 Ge 2.7 and Tb 5 Si 2 Ge 2 followed by a reduction on the unit cell volume of the main phase 11 . In our case, by sieving the GSG powders, a slight increase on the unit cell volume of the main O(I) phase from 865 A 3 for the bulk to 869Å 3 for the 3.4 µm powder was noted, while M and 5:3 unit cells remain unchanged, as summarized on Table I.…”

“…Such observation is confirmed through the appearance of a bump highlighted at the dM/dT curves 25 . It is worth to point out that the reduction of particle size seems to be affecting only the secondary M-phase since there is no shift on T C for the main O(I) phase, as usually observed in magnetic materials at the micro/nanoscale 11,35,36 . Such effect can be rising from the higher sensitivity of the M structure over O(I) to applied magnetic field, hydrostatic pressure and particle size 31,37 .…”

“…At a first glance, one highlight is the broadening of the diffracted peaks on the used powder in comparison the bulk counterpart. This is a direct consequence on the reduction of particle size after sieving 11,28,29 . From Rietveld refinements it was found the formation of expected O(I) structure, M-phase and eutectic R 5 M 3 -phase (that will be simply denoted as 5:3) 25 , with the results summarized for all samples on Table I.…”

“…However, most of the published research is performed with bulk samples that are brittle and fragile 8,9 , which represents a significant drawback in shape design for arXiv:1908.03188v1 [cond-mat.mtrl-sci] 8 Aug 2019 technological applications. Besides that, when implemented into devices, the material is submitted to several thermal cycles that can lead to its degradation 10,11 . An approach to improve the material mechanical and chemical stability can be achieved by incorporating the inorganic magnetocaloric materials (MCM) into organic polymers with suitable properties.…”