Magnetic and magnetocaloric properties of Gd3-xTbxGa5O12 (x=0, 1, 2, 3) garnets

The structure and magnetocaloric properties of Tb substituted Gd3Ga5O12 garnets have been investigated. The structural refinement of X-ray diffraction patterns show that Gd3-xTbxGa5O12 (x=0, 1, 2,3) are isostructural, with the symmetry of a cubic space group la (3) over bard. The temperature dependence of magnetization studies indicate that all the samples follow Curie-Weiss paramagnetic behavior above 25 K and the effective magnetic moment increases with Tb concentration. The low temperature magnetization studies suggest that there exists a field dependent antiferromagnetic ordering of rare earth ions in Tb substituted Gd3Ga5O12, which in turn leads to a lower magnetic moment in Gd3-xTbxGa5O12 (x=1, 2, 3), compared to that of Gd3Ga5O12. Magnitude of field corresponding to the antiferromagnetic ordering is found to be dependent on temperature as well as on Tb concentration. Magnetocaloric effect analysis confirms that Tb substitution enhances the magnetic entropy change at higher temperatures and Gd3-xTbxGa5O12 (x=1, 2, 3) are better candidates than Gd3Ga5O12 for low fields and low temperature magnetic refrigeration applications. (C) 2012 Elsevier B.V. All rights reserved

Room temperature magnetocaloric properties of Ni substituted La0.67Sr0.33MnO3

The magnetocaloric effect (MCE) in Ni substituted La0.67Sr0.33MnO3 (LSMO) has been studied to tune the magnetic transition temperature (T-C) to room temperature and to increase the operating temperature range for MCE. The maximum magnetic entropy change (Delta S-M) and relative cooling power (RCP) for La0.67Sr0.33Mn0.9Ni0.1O3 are found to be 3 J/kg K, 132 J/kg for a 50 kOe field change at 290 K, making it a promising candidate for magnetic refrigeration near room temperature. The effect of Ni on the structural, magnetic and critical exponent properties of polycrystalline La0.67Sr0.33Mn1-xNixO3 (x = 0.05, 0.1, 0.33) powder samples prepared by the standard solid state reaction method was also carried out. The structural analysis using Rietveld refinement shows that all the compositions crystallize in the rhombohedral symmetry with R (3) over barc space group. A systematic decrease in the transition temperature is observed up on Ni substitution and a near room temperature T-C (290 K) is achieved with x = 0.1 composition. The temperature dependant magnetization data of x = 0.05 and 0.1 shows a well-defined transition near T-C. The addition of Ni weakens the Mn3+-O-Mn4+ double exchange interaction, which leads to a decrease in T-C and magnetic moment in all the samples. The critical exponent analysis of x = 0.1 composition was done from the field dependence of magnetic entropy change and the obtained values are [beta = 0.54, gamma = 1.55 and delta = 3.85] close to the theoretical mean-field values. (C) 2013 Elsevier Masson SAS. All rights reserved

Near room temperature magnetocaloric properties of Fe substituted La0.67Sr0.33MnO3

The effect of Fe substitution in the polycrystalline La0.67Sr0.33MnO3 has been investigated. Substitution of Fe decreases the Curie temperature of La0.67Sr0.33MnO3 from 370 K to near room temperature in x 0.3 exhibit a frustrated glassy behaviour, evident from the ZFC and FC magnetization data. The magnetic entropy change calculated from the M-H-T data of x = 0.05 and x = 0.1 show broad peak and the respective values of -Delta S-M and RCP are found to be about 2.8 J/kg K, 166 J/kg (at T-C = 275 K) and 3.3 J/kg K, 153 J/kg (at T-C = 266 K) under a field of 50 kOe. Due to the wider distribution of the magnetic entropy, these two compounds can be used as refrigerants for room temperature magnetic refrigeration applications. The critical behaviour of the two compositions was also estimated from the field dependent magnetic entropy change. (C) 2012 Elsevier Ltd. All rights reserved

Co-existence of magnetocaloric effect and magnetoresistance in Co substituted La0.67Sr0.33MnO3 at room temperature

Magnetization (M), Magnetocaloric effect (MCE), and Magnetoresistance (MR) in Co substituted La0.67Sr0.33Mn1-xCoxO3 (x = 0.03, 0.05, and 0.1) samples have been investigated. All the studied samples were crystallized into a single phase rhombohedral structure with R (3) over barc space group. The Curie temperature (T-C) is found to decrease with the increase of Co content and for the composition x = 0.1, a cluster glass-like nature is observed. Temperature variation of magnetic entropy change (Delta S-M) shows a positive peak at TC for the composition x = 0.1 with Delta S-M = 3.1 J/kg K and relative cooling power, RCP = 155 J/kg at a magnetic field of 50 kOe. Again, the critical analysis of the paramagnetic to ferromagnetic transition and the scaling behaviour of field dependence of MCE for x = 0.1 composition is also reported. Furthermore, the temperature dependence of electrical resistivity under the magnetic fields up to 50 kOe is reported along with the values of 20% enhancement in MR at room temperature for the Co substituted samples. (C) 2013 AIP Publishing LLC

Glycidyl azide polymer crosslinked through triazoles by click chemistry: curing, mechanical and thermal properties

Glycidyl azide polymer (GAP) was cured through click chemistry by reaction of the azide group with bispropargyl succinate (BPS) through a 1,3-dipolar cycloaddition reaction to form 1,2,3-triazole network. The properties of GAP-based triazole networks are compared with the urethane cured GAP-systems. The glass transition temperature (T-g), tensile strength, and modulus of the system increased with crosslink density, controlled by the azide to propargyl ratio. The triazole incorporation has a higher T-g in comparison to the GAP-urethane system (T-g-20 degrees C) and the networks exhibit biphasic transitions at 61 and 88 degrees C. The triazole curing was studied using Differential Scanning Calorimetry (DSC) and the related kinetic parameters were helpful for predicting the cure profile at a given temperature. Density functional theory (DFT)-based theoretical calculations implied marginal preference for 1,5-addition over 1,4-addition for the cycloaddition between azide and propargyl group. Thermogravimetic analysis (TG) showed better thermal stability for the GAP-triazole and the mechanism of decomposition was elucidated using pyrolysis GC-MS studies. The higher heat of exothermic decomposition of triazole adduct (418kJmol(-1)) against that of azide (317kJmol(-1)) and better mechanical properties of the GAP-triazole renders it a better propellant binder than the GAP-urethane system

Inter molecular azide-diisocyanate coupling: new insights for energetic solid propellants

Hydroxyl terminated azide binders can undergo a spurious reaction with diisocyanates to form tetrazoline-5-one via an inter molecular 1,3-dipolar cycloaddition reaction apart from urethane/allophanate groups which has been overlooked. This has serious implications on solid propellants. The computed activation barrier using density functional theory (DFT) for urethane formation reaction is 28.4 kJ mol(-1) and that for tetrazoline-5-one formation reaction is 108.0 kJ mol(-1). DFT studies reveal that the rate limiting step of the reaction is 1,3-dipolar cycloaddition between azide and isocyanate. A dual cure was observed in the temperature ranges 42-77 degrees C and 78-146 degrees C by differential scanning calorimetry (DSC) and rheological studies, confirming multiple reactions. Tetrazoline-5-one formation was confirmed by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magnetic resonance spectroscopy (NMR)