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5.2.2 MSMA/polymer composites
5.2.2.3 PCL-based composites
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FILLING LOAD
The mechanical consistency of different PCL/PLA composites elaborated with different particles concentration was analyzed on the basis of both visual and touch direct analysis (hard to quantify). After tens of trials (not shown here), it was confirmed particles concentrations above 10wt% while keeping constant the PCL90wt%-PLA10wt% polymers ratio resulted in a critical increase of brittleness that made the material not adequate for 3D-printing.
Fig. 5.12. TGA curves for P10-90(black line), P10-9095 MP5(red line) and P10-9090 MP10(blue line) samples.
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Fig. 5.13. SEM images of P0-10050-MP50 composites at different magnification.
5.2.2.3.1 High filling-load printable composite (80wt% particles)
As mentioned, composites with 80wt% particles and acceptable printability were obtained from the dispersion of particles in pure PCL. Nevertheless, in the case of the previously studied MSMA/PCL composites, an additional problem arose from the fact the MT in the used particles took place at approximately the same temperature as the melting of the polymer (compare figures 4.1 and 5.2). This implies that the composite will lose the structural integrity at working temperatures, thus hindering any possible practical application.
Fig. 5.14. DSC thermogram for ordered powdered alloy (black line) and P0-10020-MP80 (red line).
260 280 300 320 340
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
Temperature (K)
crystallization
melting MPSC P0-10020-MP80
He at F low
(W/ g
)exo up
martensitic transformation
a )
b )
c )
f ) d
)
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In order to overcome this setback, new composites were produced from powders sieved from the MAHC samples studied in Chapter 3, whose MT take place well below the PCL melting point. Both the MT in the particles and the melting of the polymer in a P0-10020MP80 composite can be clearly seen in the DSC thermograms shown in figure 5.14. In effect, the reverse martensitic transformation temperature in the particles lies around 40K below the melting point of PCL, in such a way that there is no overlapping at all.
Microstructure and thermal stability
The distribution of sizes and geometry of the particles used for P0-10020MP80 composite can be seen in the SEM images displayed in figure 5.15. The images show the morphological homogeneity and the arrangement of the particles in the PCL matrix. The mixture of small and large particles in the composite show that the smaller particles are located in the gaps between the bigger particles. Many studies involving metallic particles, soft magnetic materials, permanent magnet composites or concrete in the construction area report that such distribution of the particles along the composite leads to an increase in the packing fraction [197, 366–368].
In particular, Palmero et al. report slight increase in the magnetization values in the MnAlC- based permanent magnet composites by increasing the fine to coarse particle ratio [197].
Therefore, the observed particle-size distribution along the composite could be behind the high achieved filling factor.
Fig. 5.15. SEM images of a) MPSC of Ni45Co5Mn36.7In13.3 powder with particle size <100μm, b)-d) P0- 10020MP80wire at different magnifications, and e) dispersion of Ni element in composite wires.
The effect of such a high concentration of particles on the thermal stability of the P0-10020MP80 has been analyzed by comparing the behavior of the composite with pure PCL polymer. Figure 5.16 shows the TGA curves for raw PCL and P0-10020MP80. The thermal degradation in the P0- 10020MP80 composite takes place at lower temperature than in pure PCL, as it occurred for lower micro-particle concentration in previous cases (figure 5.12). On the other hand, the weight loss
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in the P0-10020MP80 occurs at lower temperatures than in the P0-10050MP50 one, thus evidencing a considerable influence of the amount of particles on the thermal decomposition in the composites. In particular, the higher the concentration of micro-particles, the lower the evaporation temperature of the polymer matrix.
Fig. 5.16. TGA curves for P0-100 (black line), P0-10050MP50 (red dotted line) and P0-10020MP80 (blue line) samples.
Magnetic response
In order to assess the functionality of the elaborated composite, the magnetic properties have been also evaluated. Figure 5.17a, shows the M(H) magnetization curves at 343K for the milled micro-particles and the P0-10050MP50 and P0-10020MP80 composites. The saturation magnetization for the micro-particles is around 63 emu/g, whereas it decreases to 52 emu/g for P0-10020MP80 (around 20%) and almost 50% for P0-10050-MP50 (~33emu/g). Obviously, the magnetization reduction is in perfect agreement with the fraction of non-magnetic polymer in the composite.
Similar behaviors have been reported by Wang et al.. with composites of PCL-Fe3O4@graphene oxide [359], where the nano-composites exhibit a super-paramagnetic behavior. However, by increasing the concentration of MP, the magnetic properties can be improved.
Likewise, the influence of the micro-particle concentration on the functional properties (active magnetic refrigeration) of the composite has been evaluated by the estimation of the MCE.
Figure 5.17b shows the isothermal entropy change (∆Siso) calculated from a set of zero-cooled
500 600 700 800
0 20 40 60 80 100
Temperature (K)
P0-100
P0-10050-MP50 P0-10020-MPSC80
Weig ht (% )
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thermo-magnetization curves measured on heating under different magnetic fields. It has been calculated for fields ranging from 100Oe to 60kOe using equation [314]:
∆𝑆𝑖𝑠𝑜= 𝑆(𝑇, 𝐻) − 𝑆(𝑇, 0) = ∫ (𝜕𝑀𝜕𝑇)
𝐻𝑑𝐻
𝐻
0 5.3 The temperature dependence of the entropy change (60kOe) for free MP, P0-10050MP50 and P0- 10020MP80 composites are shown in figure 5.17b.
Fig. 5.17. a) M(H) curves at 70°C, b) Magnetically-induced entropy change at 60kOe as a function of temperature for free MP, P0-10020MP80 andP0-10050-MP50 samples.
The ∆𝑆𝑖𝑠𝑜corresponding to the micro-particles reaches a maximum value of around 9.5J/kgK at martensitic temperature. The MCE in polymer-composites however is relatively much lower. For P0-10050MP50 and P0-10020MP80 composites, the MCE value is almost half (4.5J/kgK). On the other side, as previously stated (see chapter 4), the relative cooling power (RCP) is a primary
0 20000 40000 60000
10 20 30 40 50 60 70
343K
Magnet iz ati on ( em u/g)
H (Oe) a)
P0-10050-MP50 P0-10020-MP80
MP
200 225 250 275 300 325 350 375 400
-6 -4 -2 0 2 4 6 8 10
Temperature (K)
S
iso(J/ kgK )
MP P0-10020-MP80 P0-10050-MP50
b) 60k Oe
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performance indicator to rank the MCM for active refrigeration. The RCP for MP, P0-10050MP50 and P0-10020MP80 composites are given in table 5.3. The reduction in the RCP is mainly linked to the reduction in the MP content. However,P0-10020MP80 shows relatively higher RCP values than free MP.
Table. 5.4. Refrigeration Cooling Power for different samples
The entropy change (under 60kOe) as a function of temperature for MP and P0-10020MP80 are shown in figure 5.18. The RCP for MP is around 195J/kg and reduces by 13% to 171J/kg for P0- 10020MP80 composite. The decrease in RCP must be expected around 20% (polymer concentration). However, the wider peak for P0-10020-MP80 compensates the decrease in ∆Siso and results in RCP relatively comparable to the MP. In this regard, the polymeric composites with high concentration of MP are seen to be promising candidates for magnetic refrigeration applications.
Fig. 5.18. Magnetically-induced entropy change at 60kOe as a function of temperature for MP and P0-
10020MP80.
Samples RCP (J/kg) P0-1000-MP100 154 P0-10020- MP80 171 P0-10050-MP50 78
200 220 240 260 280 300 320 340
-4 -2 0 2 4 6 8
Temperature (K)
S
iso(J/ kgK )
MP
P0-10020-MPSC80
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3D printing
Some simple objects have been successfully printed using the above wires with a commercial Artillery Sidewinder-X 3D printer. The nozzle used for printing is 0.6mm in diameter to ensure there is no clogging of the particle the tip of the nozzle (the maximum size of the particles is 0.006mm). The bed temperature has been kept constant at 303K to make ensure the printed layers have sufficient time to cool down. A relatively low printing speed (12.5mm/s) has been used to ensure the cooling of previous layers. The desired shape or form is designed by the CURA software, where the parameters like printing speed, filling density of the printed shape, thickness of each layer, etc. are adjusted and transferred to the printer. The printing temperature has been set at 473K. These parameters have been kept constant in all cases. Figure 5.19 show the obtained wires from P0-10050MP50 composites and some simple printed objects.
Fig. 5.19. Polymer-composite wire and 3D-printed objects with P0-10050MP50.
The heat exchanger represents the active part of the magnetic refrigeration and therefore, their design is crucial to get a good effectiveness and performance. The development of the technology to produce heat exchanger in different geometries with large area/volume ratio and so a high capacity to remove or absorb heat is the final objective of this Ph.D. thesis. Numerous computational and experimental investigations have been conducted to optimize the MCM heat transfer regenerator geometry, including taking into account stacked thin parallel plates, micro- channels with various geometries, fin structures, woven screens, packed bed etc. [365, 369, 370]. A porous structure like honeycomb-like shape of a heat exchanger used in magnetic refrigeration provides several advantages over conventional designs [371]. Such structure allows for a greater surface area to be exposed to the working magnetic field, leading to better heat transfer efficiency. This, in turn, leads to faster cooling and heating times, making the system more energy efficient. The honeycomb shape also facilitates the removal of excess heat