纳米晶复相Nd-Fe-B合金粉末的制备及组织结构研究

研究了在氢气氛下机械球磨铸态Nd8Fe86B6合金,利用机械力驱动Nd-Fe-B合金发生歧化反应,并随后在一定温度及真空度下进行脱氢-再结合处理,制备纳米晶Nd2Fe14B/α-Fe复相材料.利用X射线衍射检测(XRD)、差示扫描量热法(DSC)、透射电镜(TEM)及磁力显微镜(MFM)等测试方法,对球磨和脱氢过程中合金粉末相变及微观组织进行分析观察.实验结果表明,Nd8Fe86B6合金中基体Nd2Fe14B相发生了吸氢并歧化反应,球磨20h后,Nd2Fe14B相峰基本消失,获得了晶粒大小为10nm左右的Nd2H5、FeB和α-Fe歧化组织.在700℃下进行30min真空脱氢处理,获得了平均晶粒尺寸约为37nm左右的Nd2Fe14B/α-Fe复相组织.通过对材料磁畴结构的观察,发现获得粉末中两相晶粒之间具有明显交换耦合作用.     

Nd2Fe14B/α-Fe纳米复相永磁合金微结构的形成

研究了使用不同快淬速度制备的Nd3 6Pr5 4Fe83Co3B5合金中Nd2Fe14B/α-Fe复合纳米晶结构的形成.采用X射线(XRD)、透射显微(TEM)分析技术和振动样品磁强计(VSM)观测和测量了材料的微结构和磁性.结果表明,使用最佳淬速(20m/s)形成的Nd2Fe14B/α-Fe复合纳米晶结构晶粒细小,晶粒尺寸均匀Nd2Fe14B相和α-Fe相的平均晶粒尺寸分别为14nm、16nm.合金中α-Fe相的体积分数为48.6%.纳米晶合金的磁性能为Jr=1.108T,Hc=446.5kA/m,(BH)max=193.6kJ/m3,剩磁比Jr/Js=0.736.     

Nd2Fe14B/α-Fe纳米晶复合永磁材料的中子衍射结构分析及磁性研究

用真空快淬法制备Nd12Fe84B6非晶态薄带,经700℃真空退火处理,获得Nd2Fe14B/α-Fe双相纳米晶复合永磁材料.用中子衍射的方法对两种不同退火时间的样品进行室温相结构分析,并根据尺寸效应引起的衍射线展宽得到了晶粒的平均尺寸.研究结果表明:对退火时间为20min和90min的样品, Nd2Fe14B硬磁性相的平均晶粒大小分别为52.1nm和55.3nm,α-Fe软磁性相的平均晶粒大小分别为61.5nm和66.2nm;软、硬磁性相的晶粒大小几乎没有变化.随着退火时间的增加,Nd2Fe14B硬磁性相的体积分数增加,而α-Fe软磁性相的体积分数则减少.磁性测量及剩磁分析表明:90min的样品磁性能优于20min的样品,90min样品软、硬磁性相晶粒之间的交换耦合作用＞20min的样品.     

α-Fe/Nd2Fe14B复合纳米晶的织构形成研究

采用X射线衍射分析(XRD)技术,研究了NdprFeCoB熔体快淬所制备的α-Fee/Nd2Fe14B纳米晶的织构形成过程.结果表明,Nd2Fe14B纳米晶具有较强的(001)织构,而α-Fe纳米晶具有(110)织构,这些织构垂直于薄带表面,且从自由面到激冷面逐渐减弱.从自由面到激冷面,Nd2Fe14B相在(001)晶面法线方向的尺寸和α-Fe相在(220)晶面法线方向的尺寸逐渐减小.在激冷面各晶面法线方向的尺寸相近,表明纳米晶的形状近似于等轴晶.α-Fe/Nd2Fei4B纳米晶织构的形成和生长受合金薄带在凝固过程中热流梯度的控制.     

α-Fe/Nd2Fe14B复合纳米晶合金的X射线定量相分析

采用联立方程组确立了α-Fe/Nd2Fe14B复合纳米晶合金中相含量的计算方法.该方法运用最小二乘回归法和抛弃平均法消除病态方程组.将其应用于α-Fe/Nd2Fe14B复合纳米晶合金的相含量计算,所得结果与直接对比法一致.     

Nd3Fe14B／α—Fe纳米晶复合永磁材料的中子衍射结构分析及磁性研究

用真空快淬法制备Nd12Fe84B6非晶态薄带,经700℃真空退火处理,获得Nd2Fe14B/α-Fe双相纳米晶复合永磁材料.用中子衍射的方法对两种不同退火时间的样品进行室温相结构分析,并根据尺寸效应引起的衍射线展宽得到了晶粒的平均尺寸.研究结果表明:对退火时间为20min和90min的样品, Nd2Fe14B硬磁性相的平均晶粒大小分别为52.1nm和55.3nm,α-Fe软磁性相的平均晶粒大小分别为61.5nm和66.2nm;软、硬磁性相的晶粒大小几乎没有变化.随着退火时间的增加,Nd2Fe14B硬磁性相的体积分数增加,而α-Fe软磁性相的体积分数则减少.磁性测量及剩磁分析表明:90min的样品磁性能优于20min的样品,90min样品软、硬磁性相晶粒之间的交换耦合作用＞20min的样品.     

纳米晶双相Nd3.6Pr5.4Fe83-xGaxCo3B5合金磁性能的研究

用熔体快淬法制备Nd3.6Pr5.4Fe83-xGaxCo3B5(%(原子分数),x=0～1.0)纳米晶双相永磁材料,研究其磁性能的变化.结果表明适量的添加Ga(x=0.25)可以细化晶粒,使快淬薄带中的Nd2Fe14B硬磁相和α-Fe软磁相发生较充分的交换耦合作用,获得较优异的磁性能,Ga含量过多或过少的快淬薄带的磁滞回线出现缩颈现象,低温退火使得该现象得以改善.     

Nd2Fe14B/(α-Fe,Fe3B)复合纳米晶的生长动力学研究

采用X射线衍射分析(XRD)研究了Fe-Co-Nd-Dy-B非晶合金晶化过程中α-Fe、Fe3B纳米晶的生长动力学.根据纳米晶生长达到稳定状态所需的时间常数tE与退火温度Ta的关系,计算了α-Fe和Fe3B纳米晶的生长激活能为Egα-Fe=95±2 kJ/mol和E Fe3B=133±13 kJ/mol.该值远小于α-Fe和Fe3B两相的表观晶化激活能Ecα-Fe=555 kJ/mol和EcFe3B=481 kJ/mol.这表明Fe-Co-Nd-Dy-B非晶合金晶化过程中α-Fe和Fe3B纳米晶的形成主要由成核所控制.     

Nd8Fe86B6合金非晶带的快速晶化与磁性能

利用高频感应加热，对熔体快淬Nd8Fe86B6非晶薄带进行了快速晶化。结果表明，快速加热可使非晶带迅速晶化，从而得到高性能的纳米双相Nd2Fe14B／α-Fe永磁体。加热速度对薄带的磁性能影响较大。不同的加热速度下，合适的加热时间可以得到较高磁学性能。最佳工艺所得Nd8Fe86B6薄带磁性能可达(BH)max=105．55kJ／m^3，Br=0．93T，Hci=258．26kA／m。     

强磁场对Fe65Co10Nd8B17非晶合金薄带晶化行为的影响

通过X射线衍射和透射电镜手段研究了强磁场对Fe65Co10Nd8B17非晶合金晶化行为的影响。结果显示：在Fe65Co10Nd8B17非晶合金的晶化过程中,磁场抑制Nd2Fe23B3相的析出,促进Nd2Fe14B和α-Fe相的析出。通过对显微结构的分析,发现在晶化过程中,磁场起到了细化晶粒的作用,同时使晶粒分布更加均匀。通过考虑强磁场对晶化过程中体系自由能的影响,探讨了强磁场抑制晶粒尺寸长大的机理。     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.     

Varistor properties of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-In2O3 ceramics

The varistor properties of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-In₂O₃ ceramics were investigated for different concentrations of In₂O₃. The increase of In₂O₃ concentration slightly increased the sintered density (5.60-5.63 g/cm³) and slightly decreased the average grain size (3.4-2.9 μm). The breakdown field increased from 6023 to 14822 V/cm with increasing concentration of In₂O₃. The nonlinear coefficient increased from 17.6 to 44.6 for up to 0.005 mol%, whereas the further doping caused it to decrease to 36.8. In₂O₃ acted as an acceptor due to the donor concentration, which decreases in the range of 1.02 × 10¹⁷ to 0.24 × 10¹⁷/cm³ with increasing concentration of In₂O₃.     

Nanocrystallization of SrBi2Nb2O9 from glasses in the system Li2B4O7---SrO---Bi2O3---Nb2O5

Transparent glasses in the system (100−x)Li₂B₄O₇–x(SrO---Bi₂O₃---Nb₂O₅) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi₂Nb₂O₉ (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (_r) and the loss factor (D) for the lithium borate, Li₂B₄O₇ (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.     

PbSbBiS4-Sb2S3 and PbSbBiS4-Bi2S3 joins in the PbS-Sb2S3-Bi2S3 system

Phase equilibria along the PbSbBiS₄-Sb₂S₃ and PbSbBiS₄-Bi₂S₃ joins of the PbS-Sb₂S₃-Bi₂S₃ system have been studied for the first time using differential thermal analysis, X-ray diffraction, microstructural analysis, microhardness tests, and density measurements, and the phase diagrams of the joins have been mapped out. The joins are shown to be pseudobinary with limited series of terminal solid solutions. The solid solutions are p-type semiconductors.     

Syntheses of hollandite type Rb2Cr8O16, K2Cr2V6O16 and K2V8O16

Hollandite-type compounds, Rb₂Cr₈O₁₆, K₂Cr₂V₆O₁₆ and K₂V₈O₁₆, were synthesized under high P-T conditions up to 1200°C and 7GPa. The structural refinement using a single crystal of Rb₂Cr₈O₁₆ confirms that the structure is similar to that of K₂Cr₈O₁₆. Magnetic measurements indicate that Rb₂Cr₈O₁₆ is ferromagnetic below 295K, K₂Cr₂V₆O₁₆ paramagnetic down to 77K and K₂V₈O₁₆ has susceptibility anomaly at 175K. These compounds are all semiconductive and show discontinuities in temperature-resistivity curves at points corresponding to magnetic anomalies.     

Microstructures of YBa2Cu3O7/La0.7Ca0.3MnO3 and La0.7Ca0.3MnO3/YBa2Cu3O7 bi-layers grown on (001)LaAlO3

Epitaxial YBa₂Cu₃O₇/La_0.7Ca_0.3MnO₃ (YBCO/LCMO) bi-layers and La_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (LCMO/YBCO) bi-layers were grown on (001)LaAlO₃ by pulsed laser deposition, and their microstructures were compared by transmission electron microscopy investigation. In the YBCO(100 nm)/LCMO(150 nm) bi-layers, the LCMO layer consists of columnar grains of ~ 17 nm in diameter and contains mixed orientation domains of [100]_c, [010]_c and [001]_c. The YBCO layer is totally c-axis oriented and the YBCO lattices are tilted − 2.5° to + 2.5° as they grew on the rough surfaces of LCMO columnar grains. For the LCMO(140 nm)/YBCO(140 nm) bi-layers, the LCMO/YBCO interface is sharp and flat. The initial 12-nm thickness of the YBCO layer is composed of c-axis oriented domains, and the upper part of YBCO layer is [100] oriented. The LCMO layer was predominantly [001]_c oriented while [100]_c-oriented domains were occasionally observed.     

Characterization of YBa2Cu3O7/PrBa2Cu3O7 superlattices

We investigated the structural and superconducting properties ofc-axis oriented (YBa₂Cu₃O₇) _nY /(PrBa₂Cu₃O₇) _npr superlattices with thicknesses of the individual layers down to one unit cell (10nY1; 18>nPr 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa₂Cu₃O₇ layers of two unit cell thickness we find a highT _c value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.     

Vapour-liquid equilibria in the ternary mixtures N2---CH4---C2H6 and N2---C2H6---C3H8

A systematic study was performed with mixtures consisting of N₂, CH₄, C₂H₆ and C₃H₈, to investigate experimentally phase equilibria and caloric properties and to test the accuracy of thermodynamic correlations. The first part of this Paper reports results of T---p---x---y measurements on ternary systems in the range 20 < p < 120 bar and 140 < T < 220 K. The results are compared with data calculated by generalized equations of state.     

Structural study of Sr0.3Ba0.7Nb2O6 and La0.030Sr0.255Ba0.700Nb2O6 ceramic systems

Sr_0.3Ba_0.7Nb₂O₆ (SBN) and La_0.030Sr_0.255Ba_0.700Nb₂O₆ (LSBN) ceramic compounds have been prepared using the traditional ceramic method at two different calcination temperatures (900 and 1000 °C) and later sintered both at 1400 °C. A study of the effects of the calcination temperatures and La substitution on the morphological, compositional, and structural properties of SBN and LSBN is presented using scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. From Rietveld refinement processes, the XRD patterns were interpreted to evaluate such effects in the structural parameters and the site occupation factors of the heavy metals and oxygen atoms. The effect of the incorporation of La resulted in a 0.25% cell contraction and turned out to be higher than the 0.08% dilation effect produced by the increase of calcination temperature. The La ion with similar effective ionic radius and higher electronegativity is incorporated into the structure occupying the A₁ site just like the Sr ions in the SBN compound. Differences in the site occupation factors between the SBN and LSBN samples lead to substantial changes in the physical properties such as temperature of relative dielectric constant maximum, relative dielectric constant, and dielectric loss, correlated with the distortion and the relative orientation of the oxygen octahedra.     

The effect of Bi1.5Zn0.5Nb0.5Ti1.5O7 cover layer on the dielectric and tunable properties of Ba0.6Sr0.4TiO3/Bi1.5Zn0.5Nb0.5Ti1.5O7 bilayered thin films

Bi_1.5Zn_0.5Nb_0.5Ti_1.5O₇ (BZNT) thin films with different thicknesses as cover layers were deposited on the Ba_0.6Sr_0.4TiO₃ (BST) thin films on the Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering method. The microstructure, surface morphology, dielectric and tunable properties of BST/BZNT heterogeneous bilayered films were investigated as a function of the thickness of BZNT films and the effect of BZNT films on the asymmetric electrical properties of BST/BZNT bilayered films was discussed. It was found that BZNT cover layer significantly improved the leakage current and the dielectric loss, and the dielectric constant and tunability of BST/BZNT bilayered thin films simultaneously decreased with the increasing thickness of BZNT films. The BST/BZNT bilayered thin film with a 50 nm BZNT cover layer gave the largest figure of merit (FOM) of 33.48 with the upper tunability of 55.38%. The asymmetric electrical behavior of BST/BZNT bilayered films is probably related to an internal electric field caused by built-in voltages at Pt/BST and BZNT/Au interfaces.