Chapter 4: Properties of am orphous F e-(Co,M n)-Z r-B alloy ribbons
4.4. Magnetocaloric effect of amorphous Fe-(Co,Mn)-Zr-B ribbons
The temperature change of a magnetic material, associated with an external magnetic field change in an adiabatic process is defined as magnetocaloric effect (MCE). The magnetic phase transition from FM to PM state in magnetic materials results in a change in the magnetic entropy value associated with magnetization and demagnetization of the material. This change is used in the MCE. The current prototype for magnetic refrigeration is based on rare-earth based materials [PHAN2007, PROV2004, ZIMM1998]. For high temperature magnetic refrigeration, materials other than these rare-earth materials are under intensive research. Particularly, soft magnetic amorphous alloys have received a lot of attention as refrigerant materials due to their low cost, low energy loss, higher electrical resistivity and tunable TC [FRAN1996, FRAN2006, FIG. 4.18: Magnetic domain structures of amorphous Fe89-x-yCoyZr11Bx alloy ribbons (a) x = 5, y = 5; (b) x = 5, y = 10, (c) x = 10, y = 5, and (d) x = 10, y = 10.
Chapter 4: Properties of am orphous F e-(Co,M n)-Z r-B alloy ribbons
magnetic properties, the effect of substituting elements on the MCE in a-Fe89-x-y(Co,Mn)yZr11Bx
(x = 0, 5, 10; y = 5, 10) alloys has been investigated.
Results discussed in the previous sections suggest that the TC of a-Fe-(Co,Mn)-Zr-B alloys can
be well-controlled with the addition of B, Co and Mn. In addition, these samples exhibit very low HC in the range of 0.015 Oe to 0.063 Oe, indicating good soft magnetic properties in the
0 15 30 45 60
0 15 30 45 60
0 15 30 45
0 6 12 18 0
15 30 45 60 75 0 15 30 45 60 75
0 6 12 18
0 15 30 45
Fe84Zr11B5 295K
383K
Fe79Zr11B10 319K
419K Fe79Co5Zr11B5 352K
445K
M a g n e ti z a ti o n ( e m u /g )
H (kOe)
Fe69Co10Zr11B10 429K
529K 439K
539K Fe74Co10Zr11B5
373K
473K Fe74Co5Zr11B10
FIG. 4.19: Isothermal magnetization curves of B and Co substituted Fe89-x-yCoyZr11Bx amorphous alloy ribbons around TC.
Chapter 4: Properties of am orphous F e-(Co,M n)-Z r-B alloy ribbons
amorphous ribbons. The good soft magnetic properties in these samples would prove to be advantageous in magnetic refrigeration applications [PROV2004]. Fig.4.19 shows the isothermal magnetization curves of B and Co substituted a-Fe89-x-yCoyZr11Bx alloys around TC. With increasing temperature, all the samples show a gradual change in magnetization from nonlinear to linear variations with field. MCE of a magnetic material can be evaluated from the field dependent magnetization curves using a numerical approximation to the equation,
∆6 7 89*
9+:3;+
3 ,
(4.6) where the partial derivative is replaced by finite differences and the integration is performed numerically. The change in the entropy was calculated from the area under the M – H curves at different temperatures using eqn.(4.6). Fig.4.20 shows the variations of ∆SM with temperature calculated up to 18 kOe field for B, Co and Mn substituted a-Fe-Zr alloys. With increasing B content in Fe-Zr alloys [Fig.4.20(a)], the ∆SM value increases from 1.34 J/kg/K for Fe89Zr11 sample to 1.73 J/kg/K for Fe79Zr11B10 sample. In addition, the width of the ∆SM curves decreases
350 400 450 500 550 200 250 300 350 400
0.0 0.5 1.0 1.5 2.0
180 210 240 270 300
(b)
a-Fe89-x-yCo
yZr
11B
x
x=0 x=5 x=10
(a)
∆S M (J/kg/K)
a-Fe89-xZr11Bx
x=5;y=5; (c)
x=5;y=10;
x=10;y=5;
x=10;y=10
Temperature (K)
a-Fe89-x-yMnyZr11Bx
FIG. 4.20: Variations of ∆SM with temperature calculated up to 18 kOe field for B, Co and Mn substituted amorphous Fe89-x-y(Co,Mn)yZr11Bx alloys.
Chapter 4: Properties of am orphous F e-(Co,M n)-Z r-B alloy ribbons
with increasing B content. The increase in the ∆SM value is mainly due to the enhanced FM properties of a-Fe-Zr-B alloys with B addition [BARA1994]. It is to be noted that the ∆SM values obtained in the currently investigated systems are higher compared to those (1.4 J/kg/K) reported for a similar system with low Zr (~6 at.%) content [FRAN2008] and FINEMET (1.1 J/kg/K) alloys [FRAN1996]. Taking note of the enhancement in FM nature of a-Fe-Zr alloy with the addition of Zr [BARA1994], one can attribute this observed increase in ∆SM to the relative influences of both Zr and B on the stabilization of local magnetic structure of a-Fe-Zr alloy.
Figs.4.20(b) and 4.20(c) show the variations of ∆SM with temperature for Co and Mn substituted a-Fe-Zr-B alloys. With increasing Co content, the ∆SM values increase to a maximum of about 1.93 J/kg/K for Fe74Co10B5Zr11 sample. This is due the enhanced magnetic exchange interaction between Fe and Co with the addition of Co [SHEN1991]. On the other hand, the substitution of Mn in Fe84Zr11B5 alloy results in a large decrease in ∆SM value down to 0.8 J/kg/K [Fig.4.20(c)]
for alloys with 10 at.% Mn. This could be due to the loss of FM properties, resulting from the formation of AFM interactions between Mn-Mn and Fe-Mn. However, the rate of decrease of
∆SM in Fe89-x-yMnyZr11Bx decreases with increasing B content [Fig.4.20(c)] due to the increase in the relative competition between B and Mn in stabilizing the FM properties of a-Fe-Mn-Zr-B alloys. Min et al [MINS2005] also reported that the ∆SM at 50 kOe decreases from 2.78 J/kg/K to 2.33 J/kg/K with increasing Mn content from 8 to 10 at.% in a-Fe90-xMnxZr10 alloys. A close observation of Fig.4.20(c) suggests that the width of ∆SM versus T curve broadens with the addition of Mn and the maximum value of ∆SM lies just above TC of the respective amorphous alloys in contrast to the Co added a-Fe-Zr-B alloys. This broadening can be attributed to the increase in magnetoelastic coupling with Mn substitution [AMRA2004]. The effects of Co and Mn substitutions on the ∆SM of a-Fe-Zr-B alloys suggest that the temperature of the peak in the
∆SM versus temperature graph can be tuned without changing its magnitude by manipulating the B/Co(Mn) ratio in a-Fe-(Co,Mn)-Zr-B alloys with high B content.