Structural and magnetic properties of hydrides R3Fe29-xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and intrinsic magnetic properties of the hydrides R3Fe29 - xVxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed in this work. The lattice constants a, b, and c and the unit cell volume of R3Fe29 - xVxHy decrease with increasing rare-earth atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in regular anisotropic expansions along the a-, b-, and c-axes in this series of hydrides. Abnormal crystallographic and magnetic properties of Ce3Fe27.5V1.5H6.5, like Ce3Fe27.5V1.5, suggest that the Ce ion is non-triply ionized. Hydrogenation leads to the increase in both Curie temperature for all the compounds and in the saturation magnetization at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Sm, Gd, and Dy, except for Tb. Hydrogenation also leads to a decrease in the anisotropy field at 4.2 K and RT for R3Fe29 - xVx with R = Y, Ce, Nd, Gd, Tb, and Dy, except for Sm. The Ce3Fe27.5V1.5 and Gd3Fe28.4V0.6 show the larger storage of hydrogen with y = 6.5 and 6.9 in these hydrides. (C) 1998 Elsevier Science B.V. All rights reserved.

Crystallographic and magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic study of the structural and intrinsic magnetic properties of the hydrides R3Fe29-xCrxHy (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Hydrogenation lends to a relative volume expansion of the unit cell and a decrease in x-ray density for each compound. Anisotropic expansions mainly along the n- and b-axes rather than along the c-axis for all of the compounds upon hydrogenation are observed. The lattice constants and the unit-cell volume of R3Fe29-xCrx and R3Fe29-xCrxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Hydrogenation results in an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. After hydrogenation a decrease of 0.34 mu(B)/Fe in the average Fe atomic magnetic moment and a slight increase in the anisotropy field for Y3Fe27.2Cr1.8 are achieved at 4.2 K. First-order magnetization processes (FOMP) occur in magnetic fields of around 1.5 T and 4.0 T at 4.2 K for Nd3Fe24.5Cr4.5H5.0 and TD3Fe27.0Cr2.0H2.8, and around 1.4 T at room temperature for Gd3Fe28.0Cr1.0H4.2. The abnormal crystallographic and magnetic properties of Ce3Fe25.0Cr4.0 and Ce3Fe25.0Cr4.0H5.4 suggest that the Ce ion non-triply ionized.

Crystallographic and magnetic properties of R3Fe29-xVxN4 (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy)

A systematic investigation of crystallographic and magnetic properties of nitride R3Fe29-xVxN4 (R = Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. Nitrogenation leads to a relative volume expansion of about 6%. The lattice constants and unit cell volume decrease with increasing rare-earth atomic number from Nd to Dy, reflecting the lanthanide contraction. On average, the Curie temperature increases due to the nitrogenation to about 200 K compared with its parent compound. Generally speaking, nitrogenation also results in a remarkable improvement of the saturation magnetization and anisotropy fields at 4.2 K and room temperature for R3Fe29-xVxN4 compared with their parent compounds. The transition temperature indicates the spin reorientations of R3Fe29-xVxN4 for R = Nd and Sm are at around 375 and 370 K which are higher than that of R3Fe29-xVx, for R = Nd and Sm 145 and 140 K, respectively. The magnetohistory effects of R3Fe29-xVxN4 (R = Ce, Nd, and Sm) are observed in low fields of 0.04 T. After nitrogenation the easy magnetization direction of Sm3Fe26.7V2.3 is changed from an easy-cone structure to the b-axis. As a preliminary result, a maximum remanence B-r of 0.94 T, an intrinsic coercivity mu(0)H(C) of 0.75 T, and a maximum energy product (B H)(max) of 108.5 kJ m(-3) for the nitride magnet Sm3Fe26.7V2.3N4 are achieved by ball-milling at 293 K.

Synthesis Gas Generation by Chemical-Looping Reforming Using Ce-Based Oxygen Carriers Modified with Fe, Cu, and Mn Oxides

Chemical-looping reforming (CLR) is a technology that can be used for partial oxidation and steam reforming of hydrocarbon fuels. It involves the use of a metal oxide as an oxygen carrier, which transfers oxygen from combustion air to the fuel. Composite oxygen carriers of cerium oxide added with Fe, Cu, and Mn oxides were prepared by co-precipitation and investigated in a thermogravimetric analyzer and a fixed-bed reactor using methane as fuel and air as oxidizing gas. It was revealed that the addition of transition-metal oxides into cerium oxide can improve the reactivity of the Ce-based oxygen carrier. The three kinds of mixed oxides showed high CO and H-2 selectivity at above 800 degrees C. As for the Ce-Fe-O oxygen carrier, methane was converted to synthesis gas at a H-2/CO molar ratio close to 2:1 at a temperature of 800-900 degrees C; however, the methane thermolysis reaction was found on Ce-Cu-O and Ce-Mn-O oxygen carriers at 850-900 degrees C. Among the three kinds of oxygen carriers, Ce-Fe-O presented the best performance for methane CLR. On Ce-Fe-O oxygen carriers, the CO and H-2 selectivity decreased as the Fe content increased in the carrier particles. An optimal range of the Ce/Fe molar ratio is Ce/Fe > 1 for Ce-Fe-O oxygen carriers. Scanning electron microscopy (SEM) analysis revealed that the microstructure of the Ce-Fe-O oxides was not dramatically changed before and after 20 cyclic reactions. A small amount of Fe3C was found in the reacted Ce-Fe-O oxides by X-ray diffraction (XRD) analysis.

中空纤维膜基萃取稀土的研究：中空纤维膜基萃取Ce~(4+), Th~(4+)和RE~(3+)及其传质动力学

本文用中空纤维膜基萃取法研究了铈（IV）、钍和 RE（III）的膜基萃取；铈（IV）与 RE（III）、钍和 RE（III）的膜基萃取分离；铈（IV）与 RE（III）的界面化学反应动力学；膜基萃取中的流体动力学；萃取操作中乳化发生的机理；测定了中空纤维膜的孔率和孔径。在铈（IV）、钍和 RE（III）的膜基萃取过程中，研究了水相流量、油相流量、原料液中硫酸浓度、萃取剂 N1923浓度和水相溶质浓度对基于水相总传质系数的影响。钍的总传质系数受水相流量影响较大，不受油相流量影响，从而提出水相扩散层控制的膜基萃取传质机理；铈（IV）的传质系数受水相流量影响，油相流量影响较小，主要为水相扩散层控制的传质机理；RE（III）的传质系数受水相和油相的影响都比较小，为膜内传质过程的传质机理。酸浓度对钍和铈（IV）传质系数影响较小，是由于酸浓度影响分配系数，而对于水相扩散控制的传质过程，水相分传质系数与分配系数无关；酸浓度对 RE（III）的影响，是由于膜内传质阻力与分配系数在关；N1923 对 RE（III）萃取传质系数的影响进一步证明膜内过程控制的传质机理，同时对数曲线的斜率大于1 也说明界面反应为一复杂过程；水相溶质浓度不影响基于水相的总传质系数，进一步证实了传质速度与初始浓度无关，也从一个侧面反应了膜基萃取实验有较好的重复性。实验研究的结果，对铈（IV）的膜基萃取选择油相组成为 10%N1923+4%异辛醇+正庚烷（或煤油），水相酸浓度为 1～2mol/L，水相和油相流量可以在较大的范围内选择；钍的膜基萃取条件相似，只是油组成中萃取剂浓度为 1%N1923。在流体动力学部分，通过作用力分析，利用能量守恒原理，推导出了膜基萃取操作中的雷诺数，建立了层流流动模型；腔内外流体流速的径向分布模型和平均流速的计算式；腔内外压力沿管长度分布的数学模型，并得到了两相流体的压力差计算式。流体的动力学分析对于研究乳化发生的机理、传质的数学模型和扩散层厚度等皆有理论价值。通过流体动力学理论，结合膜破裂压和界面张力的实验方法研究了乳化发生的机理。水相进入有机相的乳化，对于非同级萃取，外压作用是乳化的主要原因，提出了类似于重力液滴形成的乳化模型；对于有机相进入水相的乳化，亲油膜的表面易形成油膜及界面张力形成的附加压力是乳化的基本原因，而随着有机相内溶质浓度增加界面张力降低使乳化在萃取操作的后期较易发生。膜破裂压和界面张力的研究方法使乳化的研究变得可以进行实验测定，这对于膜材的选择是很有价值的。界面反应动力学采用了上升单滴法，这种方法一般具有实验重复性较差的缺点。通过实验发现，影响重复性的因素主要是单滴形成速率的稳定性、聚结界面处油水界面位置的恒定和扩散传质的消除等。我们采用盘管式油加液管的设计，比较简单地解决了单滴形成速率稳定的问题，油水界面恒定是一个技术性问题，在实验中得到了较好的解决，通过传质时间与控制聚结界面的面积解决了消除扩散传质的难题。本论文的创新之处有如下几个方面：1．根据氟碳铈矿中钍、铈（IV）与稀土（III）分离的总目标，首次实现了伯胺 N1923对上述离子的中空纤维膜基萃取，提出了利用动力学差异的新型分离模式；2．对流量改变对传质系数的影响，所有文献报道的流量范围都没有达到流量增加使传质系数减小，而这一较高的流量揭示了油水界面随压力增加向膜内移动的事实，这对防止乳化时的压力控制是非常重要的。流量对扩散层厚度影响是对传质系数影响的主要原因，而混流的影响是次要的因素；3．铈（IV）和 RE（III）、钍和 RE（III）的分离表明，分离系数远远大于由总传质系数预计的结果，从而提出了动力学竞争萃取分离的机理；4．膜破裂压的测定方法是一个创新的设计。这种方法对于膜材选择和萃取时操作压力的控制是非常重要的。在现有文献中，都是直接在膜萃取操作时测定水相中的游离油或油相中游离水的体积的方法进行研究，这即不能研究乳化的机理，也不能测定准确的乳化时间。因此膜破裂压测定为乳化过程提供了一个新的研究方法；5．乳化机理的研究得到了较新的结论，尤其对于油相进入水相的乳化机理，较好地解释了在水相有超压存在下油相漏液的原因；6．界面反应动力学实验装置中盘管式油相加样解决了油滴形成稳定性的问题，提高了实验测定的可靠性的重复性。

新型绿色荧光体ABLa(PO_4)_2:Ce, Tb研究(A = K, Na; B = Mg, Zn)

本工作完成了磷酸盐化合物ABLa(PO_4)_2的合成，这些磷盐均可在900 ℃左右合成；对其进行了结构测试与表征，发现这些磷酸盐属于单斜晶系独居石结构，与LaPO_4同构，具有很相近的晶胞参数；系统地研究了RE~(3+)离子(RE = Ce,Tb,Dy)在ABLa(PO_4)_2基质中的发光与能量传递规律，研究了Ce~(3+)、Tb~(3+)离子发光中心与基质晶格之间的相互作用，计算了这些稀土离子之间能量传递的临界距离Rc(dd)，结果表明ABLa(PO_4)_2基质中Ce~(3+)离子与基质晶格之间的相互作用属于中等程度耦合，Tb~(3+)离子与基质晶格之间的相互作用属于无辐射多声子过程，Ce~(3+)→Ce~(3+)、Ce~(3+)→Tb~(3+)能量迁移临界距离均与LaPO_4中相近，Ce~(3+) → Ce~(3+)相对于Ce~(3+) → Tb~(3+)属于快过程，Ce~(3+) → Ce~(3+)能量传递对ABLa(PO_4)_2:Ce,Tb荧光体的Tb~(3+)绿色发光起了重要的作用，ABLa(PO_4)_2基质是Ce~(3+)，Ce~(3+)-Tb~(3+)，Ce~(3+)-Dy~(3+)的优良发光基质；最后探讨了绿色荧光体ABLa(PO_4)_2:Ce，Tb的调制途径，主要研究了Ce~(3+)、Tb~(3+)离子的浓度效应，掺杂B_2O_3、Dy~(3+)、SiO_2对荧光体发光的影响及NH_4Cl的作用，结果表明Ce~(3+)、Tb~(3+)离子的适宜浓度分别为0.2～0.5和0.08～0.2，掺杂适量的B_2O_3、Dy~(3+)能很好地提高荧光体的发光，掺杂SiO_2、NH_4Cl不利荧光体发光。