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稀土催化材料的制备、结构及催化性能 总被引:2,自引:0,他引:2
稀土催化材料的研究和发展为La和Ce等高丰度轻稀土元素的高质、高效利用提供了有效的途径.稀土元素具有未充满电子的4f轨道和镧系收缩等特征,作为催化剂的活性组分或载体使用时表现出独特的催化性能.本文从稀土氧化物、稀土复合氧化物、稀土-贵金属催化剂、稀土改性多孔催化材料等稀土催化材料出发,重点介绍和讨论了稀土的添加对催化剂的结构、活性和稳定性等的影响,阐述了稀土与过渡金属及氧化物、稀土与贵金属之间的相互作用,及对催化剂催化性能的影响.并对稀土催化材料的研究和发展提出了思考和展望. 相似文献
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随着航空航天、核技术等领域的发展和核能的广泛利用,对核辐射屏蔽材料的性能也提出更高的要求。核反应中产生的中子、伽马射线的穿透能力较强,危害较大,所以对于中子、伽马辐射屏蔽材料的研究成为辐射防护研究的重点。稀土元素具有较高的中子吸收截面和高原子序数,逐渐被科研人员重视并应用于中子、伽马辐射屏蔽材料的研发中。本文简述了稀土材料在辐射屏蔽材料领域的应用,介绍了稀土元素与中子和伽马射线的作用原理;根据基体材料的类别将稀土基中子和伽马辐射屏蔽材料分为稀土金属基、稀土聚合物基、稀土玻璃基三类,分别介绍了三类稀土基复合屏蔽材料的研究进展,并分析了稀土材料用于中子、伽马辐射屏蔽存在的问题与展望。 相似文献
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古代铜矿冶炼过程中稀土元素的变化研究 总被引:1,自引:1,他引:1
主要研究稀土元素在铜矿冶炼过程中的变化规律及其在冶金考古中的潜在应用。实验采用电感藕合等离子体发射光谱仪(ICP-AES)测试了湖北铜绿山、宁夏照壁山、山西中条山和内蒙古林西大井古代矿冶遗址的铜矿石,炼渣和铜锭的稀土元素含量,通过对其稀土总量和稀土配分模式的对比研究,探讨了稀土元素在古代铜矿冶炼过程中的化学行为。结果发现稀土元素在铜矿冶炼过程主要富集在炼渣中,金属铜中的稀土元素含量很低。因此,要想根据青铜器中的稀土元素特征示踪铜料来源,是很难实现的。通过对比各矿冶遗址的铜矿石、炼渣和铜锭的稀土配分曲线,发现炼渣的稀土配分曲线为判断古代所使用的冶炼技术及铜矿石的输出路线提供了可能。 相似文献
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本文探讨了共存稀土离子及温度对镨、钕、钐、铕、镝、钬、铒、铥等稀土离子的高氯酸溶液的二阶导数吸收光谱的影响。在所选定的分析峰位,共存稀土离子或不干扰或仅存在轻微干扰,而对于待测稀土离子导数峰的这种轻微干扰及温度的影响,都可用校正系数法消除。文中提出了一种在混合稀土(十五个稀土元素共存)中直接、快速测定上述八个稀土元素的导数光度法,并在合成样品及实际样品分析中获得了满意的结果。 相似文献
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氮氧自由基-金属配合物的合成和功能性研究,已成为分子磁性材料的一个热点课题。本文主要介绍了近年来氮氧自由基-金属配合物型分子基磁体、单分子磁体、单链磁体、分子自旋转换配合物等的最新研究成果,并就这一领域的发展前景做一展望。 相似文献
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研究了Ti和C添加对Nd9.4Fe79.6-xTixB11-yCy(x=0,1,2,4,6;y=0.5,1.5,3)合金晶化方式、显微结构和磁性能的影响规律。结果表明,适量Ti和C添加改变了合金的晶化方式,使-αFe相和Nd2Fe14B相同时从Nd9.4Fe75.6Ti4B10.5C0.5非晶基体中析出,避免了先析出相晶粒的长大,利于获得细小均匀的显微结构。适量Ti和C添加的Nd-Fe-B-Ti-C非晶合金在退火过程中易析出细小弥散的TiC和TiB2相,可作为形核质点促进形核,且可抑制晶粒长大,最终形成细小均匀的显微结构。综合性能较佳的Nd9.4Fe75.6Ti4B10.5C0.5合金退磁曲线具有优异的方形度,最佳退火条件下合金薄带的剩磁Br为0.91 T,矫顽力iHc为976 kA.m-1,磁能积(BH)max达135 kJ.m-3。文章最后对Ti和C添加合金微结构的形成机制进行了探讨。 相似文献
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Joel S. Miller Arthur J. Epstein 《Angewandte Chemie (International ed. in English)》1994,33(4):385-415
Magnets composed of molecular species or polymers and prepared by relatively low-temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular-species-based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low-frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin-containing moieties, and dipole—dipole, through-space interactions) which enable the design of new molecular-based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro- or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular-based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …?D?+ A?? D?+ A??…?, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]?+ [TCNE]?? (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean-field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one-dimensional ferrimagnetic materials based on metalloporphins is discussed. 相似文献
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Sakellariou D Hugon C Guiga A Aubert G Cazaux S Hardy P 《Magnetic resonance in chemistry : MRC》2010,48(12):903-908
We introduce a cylindrical permanent magnet design that generates a homogeneous and strong magnetic field having an arbitrary inclination with respect to the axis of the cylinder. The analytical theory of 3 D magnetostatics has been applied to this problem, and a hybrid magnet structure has been designed. This structure contains two magnets producing a longitudinal and transverse component for the magnetic field, whose amplitudes and homogeneities can be fully controlled by design. A simple prototype has been constructed using inexpensive small cube magnets, and its magnetic field has been mapped using Hall and NMR probe sensors. This magnet can, in principle, be used for magic angle field spinning NMR and MRI experiments allowing for metabolic chemical shift profiling in small living animals. 相似文献
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单分子磁体及其磁学表征* 总被引:2,自引:0,他引:2
单分子磁体是介于分子基磁体和纳米磁性材料的学科交叉点.对其不同寻常磁特性的研究不仅有助于纳米磁性离子物理学和化学的发展,而且有望最终用于高密度信息储存设备.本文就单分子磁体的研究背景和意义、单分子磁体的种类、结构及磁学表征作一概述. 相似文献
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Miyasaka H Nezu T Sugimoto K Sugiura K Yamashita M Clérac R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1592-1602
Mn(III)-Ni(II)-Mn(III) linear-type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn2(5-Rsaltmen)2(H2O)2](ClO4)2 (5-Rsaltmen2-=N,N'-(1,1,2,2-tetramethylethylene) bis(5-R-salicylideneiminate); R=Cl, Br) with [Ni(pao)2(phen)] (pao-=pyridine-2-aldoximate; phen=1,10-phenanthroline) in methanol/water: [Mn2(5-Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [Mn(III)-ON-Ni(II)-NO-Mn(III)] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn2(saltmen)2Ni(pao)2(L1)2](A)(2) (L(1)=pyridine, 4-picoline, 4-tert-butylpyridine, N-methylimidazole; A=ClO4-, BF4-, PF6-, ReO4-). Recently, these 1D compounds have attracted a great deal of attention for their magnetic properties, since they exhibit slow relaxation of the magnetization (also called single-chain magnet (SCM) behavior). This unique magnetic behavior was explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these 1D compounds, the [Mn(III)-ON-Ni(II)-NO-Mn(III)] unit was considered as an S(T)=3 anisotropic spin. Direct-current magnetic measurements on 1 and 2 confirm their S(T)=3 ground state and strong uniaxial anisotropy (D/k(B) approximately -2.4 K), in excellent agreement with the magnetic characteristic deduced in the study on the SCM series. The ac magnetic susceptibility of these trimers is strongly frequency-dependent and characteristic of single-molecule magnet (SMM) behavior. The relaxation time tau shows a thermally activated (Arrhenius) behavior with tau0 approximately 1x10(-7) s and Delta(eff)/k(B) approximately 18 K. The effective energy barrier for reversal of the magnetization Delta(eff) is consistent with the theoretical value (21 K) estimated from |D| S2T. The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 series and opens new perspectives to design single-chain magnets. 相似文献
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Thomas Walker Steffen Wirth Dietrich Schläfer Norbert Mattern Karl -Hartmut Müller Axel Handstein 《Mikrochimica acta》1997,125(1-4):355-360
NdFeB samples of different texture degree were prepared by uniaxial pressing in a magnetic field of different strength. Afterwards, the texture formation was investigated by means of X-ray diffraction. Usually, a fibre texture with the basal plane (001) perpendicular to the applied magnetic field is expected from the used compaction process. However, by introducing a two-parameter Gauss-function describing the texture, deviations from the rotational symmetry of the texture are found. The texture parameters are compared with those obtained by a fit of demagnetization curves. Knowing the intrinsic magnetic properties of the investigated material, demagnetization curves in the first quadrant of the J-H-plane can be calculated on the basis of the texture parameters.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday 相似文献
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We study the magnetic properties of the erbium based compounds, Na[Er(WO)] and [(Pc)Er{Pc{N(CH)}}], in the framework of an effective spin exchange model involving delocalized electrons occupying molecular orbitals. The calculations successfully reproduce the experimental data available in the literature for the magnetic spectrum, magnetization and molar susceptibility in dc and ac fields. Owing to their similar molecular geometry, the compounds’ magnetic behaviors are interpreted in terms of the same set of active orbitals and thus the same effective spin coupling scheme. For all three complexes, the model predicts a prompt change in the ground state from a Kramer’s doublet at zero fields to a fully polarized quartet one brought about by the action of an external magnetic field without Zeeman splitting. This alteration is attributed to the enhancement of the effect of orbital interactions over the spin exchange as the magnitude of the external magnetic field increases. 相似文献