K. V. S. Rama Rao

Effect of Al, Cu, Ga, and Nb additions on the magnetic properties and microstructural features of sintered NdFeB

This study describes the relative effect on the permanent magnet characteristics viz. remanence (Br), intrinsic coercivity (Hci), Curie temperature (TC), and rectangularity of the intrinsic demagnetization curve, when Al, Cu, Ga, and Nb are added individually to NdFeB. Each elemental addition causes significant improvement in Hci but the level of improvement differs from one additive element to the other. The addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve. The microstructural features of the sintered samples of NdFeB with elemental addition show the formation of a new phase, in addition to the phases (φ,η, and Nd-rich) generally found in the ternary sample. The factors influencing the permanent magnet characteristics of sintered samples are the distribution of the Nd-rich phase in the intergranular region, the size and distribution of the minor phases at the grain junctions, the formation and distribution of new phases...

EFFECTS OF CO SUBSTITUTION ON MAGNETIC PROPERTIES OF PR3(FE1-XCOX)27.5TI1.5 (X=0-0.3)

X-ray diffraction carried out on random and oriented samples of Pr3(Fe1−xCox)27.5Ti1.5 (x=0, 0.1, 0.2, 0.3) showed that the easy magnetization direction is near the b axis for x=0, 0.2, and 0.3 and is almost along the b axis for x=0.1. Magnetic hysteresis data taken on oriented samples showed that the anisotropy field (HA) varies from 12 kOe for x=0 to 25 kOe for x=0.3 at 300 K. At 10 K, HA increases from 55 kOe for x=0.1 to 70 kOe for x=0.3. An indication of spin reorientation transition has been observed at ∼250 K in x=0.3.

Structural and magnetic properties of (Sm1-xPrx)3Fe27.5Ti1.5 [x=0.2, 0.5, 0.8, 1.0] and their nitrides

Abstract (Sm 1− x Pr x ) 3 Fe 27.5 Ti 1.5 [ x =0.2, 0.5, 0.8, 1.0] compounds belonging to the novel 3:29 class have been prepared in order to investigate the effects of the combination of rare earths having opposite signs of second-order Stevens coefficient on their structural and magnetic properties. The amounts of different phases have been estimated from the Rietveld analysis of powder X-ray diffractograms. The changes in the easy magnetization direction in these compounds are explained as the resultant of two competing anisotropies from the rare earths occupying the two crystallographically inequivalent sites, 2 a and 4 i . Nitrogenation has increased the saturation magnetization and Curie temperature which could be due to magnetovolume effect and the changes in the densities of states in the 3d band. Changes in the easy magnetization direction upon nitrogenation are due to the modification of rare earth anisotropy. 57 Fe Mossbauer spectra have been analyzed in a Wigner–Seitz cell approach.

Hydrogen absorption characteristics in the Tb1-xZrxFe3 (x=0.1, 0.2, 0.3) system

The hydrogen absorption pressure-composition-isotherms of the PuNi3 type rhombohedral Tb1−xZrxFe3 (x=0.1, 0.2, 0.3) alloys are investigated by Sievert’s apparatus, in the temperature and pressure ranges 75≤T(°C)≤150 and 0.1≤P(bar)≤25, respectively, with a view to study the dependence of hydrogen solubility and the plateau pressure on Zr content. The P-C isotherms show the presence of a single plateau region in the temperature and pressure ranges studied and it is found that the plateau pressure at any given temperature increases with increase of Zr content along with a reduction in the hydrogen capacity. The absorption of hydrogen is accompanied by a maximum of 17.4% expansion in the unit cell volume of the host material without any change in PuNi3 type structure as evidenced by the X-ray diffraction studies on the alloy– hydrides. The presence of single and two phase regions as seen in the P-C isotherms are confirmed from the powder X-ray diffractogram of the alloy–hydrides with appropriate hydrogen concentration. The powder X-ray diffractograms on the alloy–hydrides with maximum hydrogen concentration further show the absence of hydrogen induced amorphization (H1A) upon hydrogenation.

Structural and magnetic properties of ErPrFe17−xMx (M=Ga, Si; x=0–3.5)

Abstract The alloys ErPrFe 17− x Ga x and ErPrFe 17− x Si x ( x =0–3.5) have been prepared and their structure and magnetization have been investigated. Powder X-ray diffraction patterns show that these compounds crystallize in hexagonal Th 2 Ni 17 (2:17) structure. The lattice parameters are found to increase as the Ga concentration is increased and decrease with increase of Si concentration. The saturation magnetization ( M s ) measured at 20 K is found to increase initially and then decrease as the Ga and Si concentration are increased beyond 0.1. The Curie temperatures of ErPrFe 15 Ga 3.5 and ErPrFe 15 Si 3.5 are 528 and 537 K which are 241 and 250 K, respectively, higher than that of the parent compound ErPrFe 17 and this has been explained on the basis of band model. The X-ray diffraction patterns taken for the magnetically aligned samples suggest that the easy magnetization direction (EMD) is in the basal plane in the Si substituted samples whereas in the case of the Ga substituted samples the EMD tend to be away from the basal plane. The anisotropy fields ( H A ) calculated from the magnetization measurements on aligned samples show that in the case of the Ga substituted compounds there is a significant increase in H A compared to the ones that are seen in the Si substituted compounds.

Neutron diffraction studies on (Ho1−xErx)2Fe15Ga2Cy compounds

Powder neutron diffraction studies were carried out on (Ho1−xErx)2Fe15Ga2Cy (x=0, 0.5, 1; y=0, 2) with a view to study the effect of substitution of Ga for Fe and the insertion of carbon at the interstitial sites on the structural and magnetic properties. All Ga substituted compounds (y=0) possess hexagonal Th2Ni17 type structure (space group: P63/mmc). However, the carbides adopt rhombohedral Th2Zn17 type structure (Space group: R3m). The unit cell volume increases with Ga substitution as well as with interstitial modification by carbon. An increase in the Curie temperature of about 260 K is observed for the compound HoErFe15Ga2C2 vis-a-vis HoErFe17. The increase is attributed to the preferential occupancy of Ga atoms in 18f(12j) and 18h(12k) sites and the presence of interstitial carbon. The magnetization increases with carbon addition. X-ray diffraction studies on magnetically aligned samples and neutron diffraction studies show that the easy magnetization direction lies in the basal plane at room tem...

Kinetics of hydrogen absorption and thermodynamics of dissolved hydrogen in Tb1-xZrxFe3 system

Abstract The kinetics of hydrogen absorption in the PuNi 3 type rhombohedral Tb 1− x Zr x Fe 3 ( x =0.1, 0.2, 0.3) system are studied in the temperature range 75≤ T (°C)≤150 at an initial pressure of 20 bar. By fitting the experimental kinetic data to a second order rate equation, the different phases [ α , ( α + β ), β ] and phase boundaries [ α →( α + β ), ( α + β )→ β ] of the alloy-hydrogen system are identified. The thermodynamical parameters viz. the relative molar enthalpy (Δ H H ) and the relative molar entropy (Δ S H ) of dissolved hydrogen are evaluated using the vant Hoff equation and are found to be in the ranges −(8 to 28) kJ (mol H) −1 and −(32 to 66) J K −1 (mol H) −1 respectively. From the dependence of Δ H H with hydrogen concentration of the alloy-hydrides, the different phases [ α , ( α + β ), β ] and phase boundaries [ α →( α + β ), ( α + β )→ β ] of the alloy-hydrogen system are identified. The different phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms of the respective alloy.

On the structural and magnetic properties of R2Fe17-x(A, T)x (R = rare earth; a = Al, Si, Ga; T = transition metal) compounds

R 2 Fe 17 (R = rare earth) intermetallic compounds constitute one of the most important classes of materials identified as high-energy permanent magnet materials. They crystallize either in the rhombohedral Th 2 Zn 17 structure (for light R) or in the hexagonal Th 2 Ni 17 structure (for heavy R). In this article, we discuss the variations in the lattice parameters (unit cell volume), site occupancies and Curie temperature when non-transition and transition metals are substituted for Fe in R 2 Fe 17 compounds.

Structural, magnetic and exchange interaction studies on R2Fe17−xGax (R=Tm, Er and Sm) compounds

Abstract A detailed investigation of the structure and magnetic properties of R 2 Fe 17− x Ga x (R=Tm, Er and Sm and x =0–7) was carried out by means of powder X-ray diffraction, magnetization and AC magnetic susceptibility measurements. With increasing Ga content, a structural transformation from hexagonal Th 2 Ni 17 -type structure to rhombohedral Th 2 Zn 17 -type structure is observed in compounds Er 2 Fe 17− x Ga x ( x =0–7) and Tm 2 Fe 17− x Ga x ( x =0–7). The compounds Sm 2 Fe 17− x Ga x ( x =0–7) crystallize in rhombohedral Th 2 Zn 17 -type structure. The Curie temperature increases in the initial Ga concentration range, reaches a maximum and then decreases at higher Ga concentration. The exchange interaction parameter J FeFe reaches a maximum and then decreases whereas the value of J RFe is almost independent of Ga concentration. X-ray diffraction measurements on magnetically aligned powder samples show a uniaxial anisotropy for compounds with at x=2, 3 and 4 in the Sm 2 Fe 17− x Ga x series whereas, it was observed for x =7 in the Tm 2 Fe 17− x Ga x compounds.

Hydrogen absorption characteristics in Zr0.2Ho0.8Fe0.5Co1.5

Hydrogen absorption studies have been carried out on the C15 cubic Laves phase alloy of Zr0.2Ho0.8Fe0.5Co1.5. The p–c data were collected in the pressure and temperature ranges of 0.05 ≤ P (bar) ≤ 45 and 27 ≤ T (°C) ≤ 100, respectively. A maximum hydrogen uptake of 3.6 hydrogen atoms per formula unit at 33 bar and 27°C is observed. The p–c isotherms show one plateau (α + β) region. This is confirmed from the dependence of the thermodynamics of dissolved hydrogen as a function of hydrogen concentration. ΔHH and ΔSH are found to be in the ranges −(5–15) kJ (mol H)−1 and −(23–48) J K−1 (mol H)−1, respectively. Volume expansion of about 20% is found at the maximum hydrogen concentration at RT, without a change in its crystal structure.