Papers by Keyword: Curie Temperature

Abstract: The structure and magnetic properties of the Nd₃Fe_27.7-xNi_xTi_1.3 compounds prepared by arc-melting method have been investigated by means of X-ray diffraction and magnetization measurement. It was found that all the compounds crystallized in Nd₃(Fe, Ti)₂₉-type structure with monoclinic symmetry and A2/m space group in concentration range 0.2 ≤ x ≤ 1.2. The unit cell volume decreased slightly with increasing Ni content. The X-ray diffraction patterns of the magnetically aligned samples showed that all compounds investigated had planar anisotropy. The Spin reorientation phenomena occurred in all the compounds. As the increase of Ni content, the Curie temperature T_C monotonously increased, but the spin reorientation temperature T_sr did not obviously change. The saturation magnetization Ms of the compounds decreased monotonously with increasing Ni content at 5 K, 105 K, 205 K, and 305 K. The anisotropy fields B_a at 5 K, 305 K and anisotropy constant K₂ at 305K appeared as a minimum, but the anisotropy constant K₁ at 305 K appeared as a maximum on x = 0.6.

Abstract: Solid state sintered pellets of barium hexaferrite were performed. The Curie temperature of BaFe₁₂O₁₉ pellets were measured with differential scanning colorimetry (DSC). The coefficient of thermal expansion (CTE) of pellets was defined above and below the Curie temperature. The Curie temperature calculated basedon the dilatometry is are in good agreement with the DSC measurements.

Abstract: In this work, the effects of substitution of Nd by Gd on the magnetic properties of Nd-Fe-B magnets were investigated. The results demonstrate that the size of grain decreases with the addition of Gd element. The Curie temperatures (Tc) are found to improve with the increase in Gd content, which means that the thermal stability of the magnets is enhanced. The coercivity and saturation magnetization decrease with the increase in Gd content due to the low magnetocrystalline anisotropy and saturation magnetization of Gd₂Fe₁₄B.

Abstract: In this work, the microstructure, crystal structure and magnetic properties of (Nd_0.7Pr_0.15RE_0.15)_2.28Fe_13.58B_1.14 (RE=La, Ce, Y) alloys prepared by arc-melting were investigated experimentally. The experimental results show that all alloys annealed at 1173 K for 360 hrs contain the isotropic Nd₂Fe₁₄B structure phase. The coercivities (H_c) of (Nd_0.7Pr_0.15RE_0.15)_2.28Fe_13.58B_1.14 (RE=La, Ce, Y) melt-spun ribbons are 12.3 kOe, 13.2 kOe, 8.5 kOe, and the Curie temperatures (T_c) of annealed (Nd_0.7Pr_0.15RE_0.15)_2.28Fe_13.58B_1.14 (RE=La, Ce, Y) alloys are 569 K, 552 K and 576 K, respectively. Meanwhile, the remanences (Br) of (Nd_0.7Pr_0.15RE_0.15)_2.28Fe_13.58B_1.14 (RE=La, Ce, Y) melt-spun ribbons are about 67 emu/g, 74 emu/g and 72 emu/g, respectively. It was indicated that the coercivity, remanence and Curie temperatures of (Nd_0.7Pr_0.15RE_0.15)_2.28Fe_13.58B_1.14 alloys are dependent on light rare earth elements.

Abstract: Ferrites nanopowder of spinel Mg_xMn_1-xFe₂O₄ (with x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) synthesized by the coprecipitation method and pellets of nanopowder sintered at temperature 1250 °C. The X-ray diffraction results confirmed the single phase formation of the samples. The lattice constant and interionic distances decreased with increase in magnesium content that can be originated by strengthening of A–B interaction. The substitution effect of nonmagnetic Mg²⁺ ions was studied on Curie temperature of sintered pellets. Curie temperature measurements exhibit increasing trend with increase in magnesium content. Enhancement in Curie temperature can be explained on the basis of strengthening of A–B interaction and Neel’s two sub-lattice models.

Abstract: In order to investigate the influence of (Bi_0.5M_0.5)TiO₃ (M=Li, Na, K, Rb) addition on the Curie temperature (T_c) and positive temperature coefficient (PTC) effect of BaTiO₃-based ceramics, BaTiO₃-(Bi_0.5M_0.5)TiO₃ (M=Li, Na, K, Rb) solid solutions were prepared by a conventional solid sintering reaction using high-purity reagents. It was found that the T_c of the samples would vary with (Bi_0.5M_0.5)TiO₃ of different alkali ions, in which BT-BKT ceramics had the highest value (about 148 °C). Moreover, the incorporation of alkali ions would influence the PTC effect of the sample, which can be defined by the resistivity jump with the ratio of maximum to minimum resistivity (ρ_max/ρ_min). Under the present conditions, the ρ_max/ρ_min of BT-BRT and BT-BLT ceramics were almost equal and higher than those of BT-BNT and BT-BKT ceramics.

Abstract: The rare-earth (RE) permanent magnets based on Nd₂Fe₁₄B with excellent magnetic properties have been widely used in industrial applications. In this work, the crystal structure, microstructure and magnetic properties of Nd_2.28Fe_13.58B_1.14, Ce_2.28Fe_13.58B_1.14 and Pr_2.28Fe_13.58B_1.14 alloys prepared by arc-melting were investigated. The results show that all alloys are single phase with tetragonal Nd₂Fe₁₄B-type (space group P42/mnm). The Curie temperatures (T_c) of RE_2.28Fe_13.58B_1.14 (RE=Nd, Ce, Pr) alloys are 583 K, 423 K and 557 K, respectively. On the other hand, the coercivities of Nd_2.28Fe_13.58B_1.14 and Pr_2.28Fe_13.58B_1.14 alloys are about 1.05 T and 1.23 T, respectively, while that of Ce_2.28Fe_13.58B_1.14 alloy is only about 0.25 T due to the poor squareness of hysteresis loop. Meanwhile, the saturation magnetizations of Nd_2.28Fe_13.58B_1.14 and Pr_2.28Fe_13.58B_1.14 alloys are about 135 emu/g and 113 emu/g, respectively, while that of Ce_2.28Fe_13.58B_1.14 alloy is about 97 emu/g. It was indicated that the Curie temperatures and magnetic properties of RE_2.28Fe_13.58B_1.14 alloys with the same crystal structure are dependent on light rare earth elements.

Abstract: Three series of NiMgCuZn ferrites were prepared by conventional double sintering ceramic process. The formation of single phase in these ferrites was confirmed by X-ray diffraction. A brief review of the important investigations carried out on the internal friction behaviour of NiMgCuZnFe₂O₄, in the temperature range 40oC to 360oC, has been reported. In the present investigation, the composite piezoelectric resonator method has been used. The effect of compositional changes of ferrites on Curie temperature and internal friction are reported. Results and discussions on the temperature variation of internal friction of the three series of NiMgCuZnFe₂O₄ samples are discussed. In all the series studied, only single stress induced relaxation peaks are observed. These studies were carried out to develop a ferrite composition for their use as core materials for microinductor applications. The results are explained in the light of structural phase transitions.

Abstract: The transport and magnetic properties of cation-substituted manganese sulphides CexMn_1-ХS in the 4K - 450K temperature range in magnetic fields up to 90 kOe are studied. The hysteresis of curve magnetization for X=0.01 and nonlinear field behavior of the magnetization at X = 0.05, the sharp Curie temperature drop were found. The sharp maximum in the temperature dependence of resistivity was observed. The shift of the maximum temperature to low temperatures at cerium ion concentration increasing and in magnetic field was established. Model of orbital polaron for explanation of experimental datа was used.

Abstract: Electronic control and operation in almost all advanced devices or machines involve use of various sensors and actuators, many of which are based on piezoelectric (PE) effect. Ferroelectric (FE) materials forming a sub-group of piezoelectric materials have additional applications. Subject to success in materials and related developments, PE and FE devices perform competitively with alternative devices but at lower cost in most cases. There is increasing commercial and technical interest for PE actuators (ranging from electronic muscles, fuel injectors and inkjet printers to various vibrators), PE sensors (pressure and other sensors and motion detection to energy recovery), and ultrasonic imaging devices. PE to non-PE transition temperature (Curie temperature for FE PE materials) and piezoelectric coefficients together decide the choice of the right material for any particular application. Since most of these applications, including medical ultrasonic imaging, are done at or near room temperature, low Curie temperature (but otherwise attractive) piezoelectric materials, based on barium titanate (BT), lead zirconate titanate (PZT) and relaxor ferroelectric ceramics, have served us well. However, a few important applications, in automobile and rocket exhausts, in some engines and gadgets, and inside high pressure molten metal in nuclear Fast Breeder Reactors (FBRs) involve high temperatures (HTs), higher than or nearing the Curie temperature of even PZT. These applications including FBRs, generating nuclear fuel and power, demand development of high temperature piezoelectric materials. FBRs can close the nuclear fuel cycle by partially using the nuclear waste (containing U-238) and thus minimize waste disposal problem. That makes nuclear energy a better green energy. Working on Th-232 from monazite sand, FBRs can breed Th-233, a nuclear fuel, with simultaneous generation of electricity. Ranging and imaging of nuclear fuel rods and control rods through the liquid metal coolant in FBRs, especially during insertion and withdrawal, help correct positioning of the rods to avoid any misalignment and possible nuclear accident. This “viewing” through the optically opaque liquid metal or alloy coolant, is possible by ultrasonic imaging of the rods using HT PE ultrasonic-generators and-detectors, an active area of research. Lithium niobate with T(Curie) > 1000°C and orthorhombic PbNb₂O₆ with T(Curie) > 570°C are two of many HT PE materials under development or in trial runs. In the present work, world-wide R & D on HT piezoelectric materials has been reviewed after an outline of the basics.