The Research Of Magnetocaloric Effect In The Low And Medium Temperature

Magnetic refrigeration based on the magnetocaloric effect (MCE) has been paid muchattention due to its friendly environment and higher energy efficiency. To improve theapplication of this cooling technology, it is important to explore efficient magneticrefrigerants. In this thesis, magnetic properties and magnetocaloric effects have beenstudied for Fe3C type Er3-xGdxCo, MgZn2type Er1-xGdxCoAl, ZrNiAl type (TmNi1-xCuxAland HoNi1-xCuxIn), ThCr2Si2type RCu2X2(R=Er, Ho and X=Si, Ge) and CrB typeTmGa compounds. The main results are as follows:1. The results show that with increasing the Gd content, the transition temperaturechanges to the higher temperature and the values of-ΔSMtrend to decrease in theEr3-xGdxCo (x=0-3) compounds. But the values of RC are not monotonous (increase andthen decrease), the most value of RC is629J/kg in the Er1.5Gd1.5Co compounds under fieldchange of5T. There are tow successive transition, it makes broaden temperature span,when the Gd substituted Er in the Er3Co. Although the value of-ΔSMwas decreased, thevalue of RC was increased.2. X-ray diffractions of TmNi1-xCuxAl (x=0-1) and HoNi1-xCuxIn compounds wererefined and identified the crystal structure and the lattice parameters by using GSASprogram. The results indicated that Cu gives priority to take place of Ni1, which belongs tothe layer of all R atoms and one-third of T atoms and it is important for the phase transitionin the HoNi1-xCuxIn compounds. For the TmNi1-xCuxAl (x=0-1) compounds, the Cu doesnot give priority to take place of Ni1or Ni2. The discontinuity of lattice parameter (a and c)observed for x within the interval (0.6,0.65). With the substitution of Ni by Cu, theTm-magnetic moment rotates its direction from basal plane to the c-axis, which makes thechange of transition temperature and magnetic properties.3. TmCuAl has a giant low field reversible magnetocaloric effect in the TmNi1-xCuxAl(x=0-1) compounds. The maximum value of-ΔSMis24.3J/Kg K and without thermal andfield hysteresis loss, for field changes of0-5T. Especially, the giant values of-ΔSM(12.2J/Kg K and17.2J/kg K) are obtained for the field change of0-1T and0-2T, respectively.Additionally, under the field change of0-1T and0-2T, the values of ΔTadare4.6K and9.4K, respectively. The large reversible-ΔSMis20.2J/kg K and the RC reaches356.7J/kg forfield changes of5T for HoNi0.7Cu0.3In. Especially, the value of-ΔSM(12.5J/kg K) and thelarge RC (132J/kg) are observed for field changes of2T for HoNi0.9Cu0.1In. Additionally, although the value of-ΔSMdecreases to9.4J/kg K, the value of RC is improved to149J/Kfor the field changes of2T due to a wide temperature span for the mix of HoNi0.9Cu0.1Inand HoNi0.7Cu0.3In compounds with the mass ratio of1:1.4. The RCu2Si2and RCu2Ge2(R=Ho, Er) possess an AFM-PM transition and theirrespective Neel temperatures are below10K. It is found that the RCu2Si2compoundsundergo spin-glass behavior above Neel temperature. Especially, ErCu2Si2exhibits largeMCE with no thermal hysteresis and magnetic hysteresis loss. The values of-ΔSMand RCreach22.8J/Kg K and166.5J/kg for magnetic field changes from0to5T. In particular,for field changes of1and2T, the giant reversible-ΔSMare8.3J/kg K and15.8J/kg K. Afield-induced metamagnetic transition from AFM to FM states is observed in thesecompounds.5. TmGa exhibits two successive magnetic transitions: PM-AFM and AFM-FMtransition. There is a field-induced AFM-FM transitionin the AFM range. Under fieldchanges of1and2T, giant reversible values of-ΔSM(12.9and20.6J/kg K) and largevalues of RC (69J/kg and149J/kg) are observed, respectively. Additionally, the maximalvalues of adiabatic temperature change are3.2and5K for field changes of1and2T,respectively. The TmGa compound with excellent magnetocaloric effect is expected to haveeffective applications in low temperature magnetic refrigeration.