Magnetic Properties and Magnetocaloric (MC) Effects in the Antiferromagnetic Gd2NiGa3 Compound

Journal of Superconductivity and Novel Magnetism - The magnetocaloric (MC) properties in various magnetic materials were well determined within last three decades due to its own potential...     

Synthesis, Magnetic Properties, Magnetic Entropy and Arrot Plot of Antiferromagnetic Frustrated Er2Ti2O7 Compound

The pyrochlore $\mathrm{Er}_{2}^{3 +} \mathrm{Ti}_{2}^{4 +}\mathrm{O}_{7}^{2 -}$ was synthesized via a ceramic method using two different oxides (Er₂O₃ and TiO₂). The compound was found to crystallize in the cubic system with the $\mathrm{Fd}\overline{3}\mathrm{m}$ space group (No. 227). A magnetic study, carried out at 2 and 300 K under an applied magnetic field ?? ₀ H=0.05?T, has revealed a complex magnetic structure at low temperature. The effective paramagnetic moment $\mu_{\mathrm{eff}}^{\exp}$ , deduced from (????? ₀)^?1=f(T) curve, was found by assuming a zero moment on the transition metal atom Ti⁴⁺. The paramagnetic Curie?CWeiss temperature ?? _CW=?21.54?K, the nearest neighbor interaction J _nn =?2.30?K, the classical nearest neighbor J ^cl=?8.65?K and the dipolar D _nn =3.76?K interactions?? values have revealed an antiferromagnetic behavior for Er₂Ti₂O₇ compound at low temperature. We have also studied the effects of the magnetic field splitting of rare-earth atom Er³⁺ in the compound Er₂Ti₂O₇ curved Arrott plots.     

TiC-Al2O3/Fe3Al复合粉末的燃烧合成

以天然钛铁矿为主要原料,采用燃烧合成技术制备了TiC-Al2O3/Fe3Al金属间化合物/陶瓷基复合材料.研究了预热时间和热处理对燃烧合成过程及产物的影响.研究结果表明:随着预热时间的延长,燃烧温度和燃烧波速率都增加,产物晶胞参数增大,合成更为完全,无序固溶相进行有序化转变的程度增大.当预热5min时,Fe3Al有序金属间化合物的量明显高于无序固溶相,但继续延长预热时间很难将无序相消除;在750℃下进行热处理,可以制备出以Fe3Al金属间化合物为主要成分的复合粉体.     

SiO2/Al2O3复合包覆TiH2颗粒发泡剂的制备

采用非均匀成核法制备包覆有SiO2和Al2O3的TiH2发泡剂,观察了包覆层形貌,进行了释氢实验.结果表明: SiO2/Al2O3的包覆层均匀、致密,形成内松外紧的结构,具有较大的热阻,降低了TiH2表面的热流密度,有效延迟释放氢气时间达160s.     

Impurity‐Induced First‐Order Phase Transitions in Highly Crystalline V2O3 Nanocrystals

A first‐order phase transition in a bulk material is generally considered to arise at extended defects such as grain boundaries or dislocations, where the energetic barrier between the two phases is reduced. Downsizing a crystal to the nanoscale can exclude the number of defects, leading to enhanced kinetic stabilization of the metastable phase. Here, the disappearance of the first‐order metal–insulator transition in defect‐free V₂O₃ nanocrystals and the revival of the transition by introducing a certain Cr or Ti impurity content are investigated. The hysteresis width of the transition corresponding to the barrier height decreases with the impurity content. It is proposed that homogeneous impurity doping is a universal method that can control the occurrence of a first‐order phase transition in nanoscale materials.     

Some phase relationships within the system Na2O/B2O3/Nb2O5

Phase behaviour over regions of the ternary system Na₂O/B₂O₃/Nb₂O₅ has been explored.Liquidus temperatures and the stability regions of primary phases have been determined over selected composition ranges by high temperature microscopy. Crystallisation processes in melts and corresponding glasses have been followed using both conventional methods of thermal analysis and newly developed micro techniques combined with hot stage microscopy.An electron microscope has been employed to follow changes in the microstructure of quenched glasses after controlled heat treatments.It has been shown that the system contains a liquid immiscibility gap, and some attention is given in the discussions to the influence that can be assigned to cations in determining the extent of such gaps and general structural relationships in borate/oxide systems.     

Optical properties of the Al2O3/SiO2 and Al2O3/HfO2/SiO2 antireflective coatings

Investigations of bilayer and trilayer Al₂O₃/SiO₂ and Al₂O₃/HfO₂/SiO₂ antireflective coatings are presented in this paper. The oxide films were deposited on a heated quartz glass by e-gun evaporation in a vacuum of 5 × 10^?3 [Pa] in the presence of oxygen. Depositions were performed at three different temperatures of the substrates: 100 °C, 200 °C and 300 °C. The coatings were deposited onto optical quartz glass (Corning HPFS). The thickness and deposition rate were controlled with Inficon XTC/2 thickness measuring system. Deposition rate was equal to 0.6 nm/s for Al₂O₃, 0.6 nm ? 0.8 nm/s for HfO₂ and 0.6 nm/s for SiO₂. Simulations leading to optimization of the thin film thickness and the experimental results of optical measurements, which were carried out during and after the deposition process, have been presented. The optical thickness values, obtained from the measurements performed during the deposition process were as follows: 78 nm/78 nm for Al₂O₃/SiO₂ and 78 nm/156 nm/78 nm for Al₂O₃/HfO₂/SiO₂. The results were then checked by ellipsometric technique. Reflectance of the films depended on the substrate temperature during the deposition process. Starting from 240 nm to the beginning of visible region, the average reflectance of the trilayer system was below 1 % and for the bilayer, minima of the reflectance were equal to 1.6 %, 1.15 % and 0.8 % for deposition temperatures of 100 °C, 200 °C and 300 °C, respectively.     

Phonon Dispersion Measurements of YBa2Cu3O6.15 and YBa2Cu3O6.95 by Time-of-Flight Neutron Spectroscopy

We measured the phonon dispersions of YBa₂Cu₃O_6.15 and YBa₂Cu₃O_6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.     

Microstructure and Strength of Al2O3/Al2O3 Joints Bonded with ZnO-Al2O3-B2O3-SiO2 Glass-Ceramic

ZnO-Al2O3-B2O3-SiO2 （ZABS） glass powder was used as interlayer to join alumina ceramics. The effect of joining temperature on the microstructure and strength of joints was investigated. The results showed that the ZABS glass can react with alumina substrate to form a layer of ZnAl2O4 at Al2O3/glass interface. Bending test exhibited that low joining temperature （1150℃） led to low joint strength due to the formation of pores in the interlayer, originated by high viscosity of the glass. High joining temperature （1250 ℃） also resulted in low joint strength, because of large CTE （coefficient of thermal expansion） mismatch between amorphous interlayer and alumina substrate. Therefore, only when the joining temperature was appropriate （1200℃）, defect-free interface and high joint strength can be obtained. The optimum joint strength reached 285 MPa, which was the same as the base material strength.     

Clues Obtained from the Oxygen Isotope Effect on NMR/NQR Parameters Observed in YBa2Cu4O8

Nuclear magnetic and nuclear quadrupole resonance (NMR, NQR) techniques have a precision allowing one to determine rather small isotope effects. Well-defined oxygen stoichiometry and negligible oxygen diffusion makes YBa₂Cu₄O₈ an ideal compound for studies of small isotope effects that require experimental results not hampered by reproducibility problems. We report on high-precision measurements of the temperature dependence of plane-^63,65Cu NMR/NQR parameters such as Knight shift, spin–lattice relaxation rate R = 1/T ₁, NQR line frequency ν_Q and NQR linewidth δν_Q, as well as ⁸⁹Y Knight shift, performed in normal and superconducting ¹⁶O and ¹⁸O exchanged YBa₂Cu₄O₈.     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

CdS/Nd2O3 and CdSe/Nd2O3 thin film transistors

Thin film transistors were grown by the multiple-pumpdown method of vacuum deposition. The behaviour of CdS and CdSe thin film transistors with Nd₂O₃ as the gate insulator was studied, and the various characteristics and parameters of the fabricated devices were determined. The performance of CdSe-Nd₂O₃ thin film transistors was found to be superior to that of CdS/Nd₂O₃ thin film transistors. The devices showed little change over a period of 4 months.     

Thermodynamics of the Spin-1/2 Two-Leg Ladder Compound (C_5H_{12}N)_2CuBr_4

The thermodynamic behavior of the spin \(S=1/2\) antiferromagnetic two-leg ladder compound (C \(_5\) H \(_{12}\) N) \(_{2}\) CuBr \(_{4}\) in a uniform magnetic field is studied using numerical and analytical approaches. The entropy \(S(H,T)\) and specific heat \(C(H,T)\) are calculated. The specific heat shows various behaviors in different regions of the magnetic field. The field dependence of the specific heat is almost symmetric about the average of quantum critical fields in complete agreement with experimental results. In addition, it is found that during an adiabatic demagnetization process, temperature drops in the vicinity of the field induced zero-temperature quantum phase transitions.     

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of T_N (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature.