Al3Ti1B1RE细化剂对罐用铝材的细化效果及稀土的作用

采用XRD，OM，SEM，EDAX等探讨了一种新型Al3Ti1B1RE中间合金细化剂对罐用铝材的细化效果及RE的作用。结果表明，该细化剂对罐用铝材的细化效果优于进口和国产Al3Ti1B，且具有长效性，达6h后仍未见明显衰退，明显提高了该材料的强度和塑性；其细化效果及稳定性好的主要原因是：RE可降低铝液的表面能，增加铝液对细化核心(如TiAl3，TiB2)的润湿性，既充分发挥了异质形核作用，又防止了TiB2聚集沉淀倾向；RE也极易在结晶前沿富集造成成分过冷，阻碍了α-Al晶粒生长，并促进其在细化核心上形核；此外RE还兼有一定的净化、细化和变质作用，尤其是净化作用提高了该细化剂的冶金质量。     

混合稀土对共晶Al-2%Fe合金组织形态的影响

研究了富La混合稀土对共晶Al-2?合金组织形态的影响. 当混合稀土加入量较少时, 合金中的α-Al相为明显的胞状枝晶;随着混合稀土加入量的增加, α-Al枝晶优先形核生长得到抑制, 共晶Al3Fe相得到细化;当稀土加入量增至0.6% (质量分数)共晶Al3Fe相尺寸逐渐增大. 并对混合稀土对共晶Al-2?合金组织形态的影响机制进行了探讨.     

Structural refinement of Nd[Fe(CN)6]·4H2O and study of NdFeO3 obtained by its oxidative thermal decomposition at very low temperatures

The crystal structure of Nd[Fe(CN)₆]·4H₂O has been refined by Rietveld analysis using high resolution synchrotron powder X-ray diffraction data. It belonged to the orthorhombic crystal system, Cmcm space group, with cell parameters: , and . The change in space group from P6₃/m which is observed in the pentahydrates (LnFe(CN)₆·5H₂O) to Cmcm in the tetrahydrates has been analyzed to be a consequence of the change in 9-fold coordination of Nd³⁺ in the pentahydrates to 8-fold coordination in the tetrahydrates, which changes the Nd³⁺ environment from tricapped trigonal prism to a distorted tricapped trigonal prism or square antiprism. Its decomposition process in air to produce NdFeO₃ has been followed by thermogravimetric and differential thermal analysis, IR spectroscopy and laboratory powder XRD. We found that it is possible to synthesize crystalline NdFeO₃ at temperatures as low as 380 °C and refine the structure of single phase crystalline NdFeO₃ synthesized by this method at 600 °C.     

Synthesis and structure determination of Co(HNCN)2 and Ni(HNCN)2

Well-crystallized cobalt and nickel hydrogencyanamide, Co(HNCN)₂ and Ni(HNCN)₂, were synthesized from the corresponding ammonia complexes [M(NH₃)₆]²⁺ under aqueous cyanamide conditions. The X-ray and neutron powder data evidence the orthorhombic system and space group Pnnm. The cell parameters for Co(HNCN)₂ are a=6.572(1), b=8.805(2), c=3.267(1) Å, and Z=2; for the isotypic Ni(HNCN)₂, the cell parameters arrive at a=6.457(1), b=8.768(2), c=3.230(1) Å. The octahedral coordinations of the metal ions are marginally squeezed, with interatomic distances of 4×Co-N=2.134(5) Å, 2×Co-N=2.122(9) Å, and 4×Ni-N=2.133(6) Å, 2×Ni-N=2.035(11) Å. The HNCN⁻ units appear as slightly bent (177.5(2)° for Co(HNCN)₂ and 175.7(2)° for Ni(HNCN)₂) and exhibit cyanamide shape character due to triple- and single-bond C-N distances (1.20(2) vs. 1.33(2) Å for Co(HNCN)₂ and 1.15(2) vs. 1.38(2) Å for Ni(HNCN)₂). The infrared vibration data compare well with those of the three existing alkali-metal hydrogencyanamides.     

Synthesis, Electrical Properties, and Powder Neutron Crystal Structure Refinement of Pb1−xBixPt2O4 Compounds (0≤x≤0.3)

Substitution of Pb for Bi in the recently characterized mixed-valence lead-platinum oxide PbPt₂O₄ was attempted and a Pb_1−xBi_xPt₂O₄ solid solution was obtained for 0≤x≤0.3. Powder X-ray diffraction study showed that all substituted compounds crystallize with similar triclinic unit cell and PbPt₂O₄ lattice parameters. The structural model of Pb_0.7Bi_0.3Pt₂O₄ was refined from powder X-ray diffraction data using the Rietveld method and the results indicate the same crystal structure than PbPt₂O₄ with one mixed Pb/Bi atomic site. Neutron diffraction realized on the two limit compositions of the solid solution (x=0 and 0.3) allowed to confirm the PbPt₂O₄ and Pb_0.7Bi_0.3Pt₂O₄ stoichiometries. Mean oxidation degree of Pt atoms in the [PtO₄] infinite chains decreases from +3 for PbPt₂O₄ to +2.7 for Pb_0.7Bi_0.3Pt₂O₄. Conductivity measurements show a metallic behavior for all the compositions except the limit composition x=0.3 for which a semiconducting behavior appears.     

Chemical shift driven geometry optimization

A new method for refinement of 3D molecular structures by geometry optimization is presented. Prerequisites are a force field and a very fast procedure for the calculation of chemical shifts in every step of optimization. To the energy, provided by the force field (COSMOS force field), a pseudoenergy, depending on the difference between experimental and calculated chemical shifts, is added. In addition to the energy gradients, pseudoforces are computed. This requires the derivatives of the chemical shifts with respect to the coordinates. The pseudoforces are analytically derived from the integral expressions of the bond polarization theory. Single chemical shift values attributed to corresponding atoms are considered for structural correction. As a first example, this method is applied for proton position refinement of the D-mannitol X-ray structure. A crystal structure refinement with 13C chemical shift pseudoforces is carried out.     

The mixed valent tellurate SrTe3O8: Electronic lone pair effect of Te

A novel mixed valent tellurium oxide, SrTe₃O₈, has been synthesized and its crystal structure was determined ab initio from powder X-ray diffraction data. This oxide, which crystallizes in a tetragonal unit-cell, P4₂/m space group, with very close a and c cell parameters (6.8257(1) and 6.7603(1) Å, respectively), exhibits a very original structure built up of corner-sharing TeO₆ (Te⁶⁺) octahedra and Te₂O₈ (Te⁴⁺) twin-pyramidal units. The latter ones form [Te₃O₈]_∞ chains running along the [001] and the [110] directions. Besides the four sided tunnels where the Sr²⁺ cations are located, there are very large four sided tunnels running along the c-axis which are obstructed by the electronic lone pairs of the Te⁴⁺ cations.     

Synthesis, crystal structure, and magnetic properties of the manganate La2Ca2MnO6(O2) related to the hexagonal perovskite-type structure

The compound La₂Ca₂MnO₆(O₂) has been synthesized from La₂Ca₂MnO₇ heated at 1123 K under high pressure (4 GPa) with KClO₃ as oxygen source. The crystal structure has been refined from X-ray powder data in the space group. The unit-cell parameters are a=5.6335(2) Å and c=17.4879(8) Å. Perpendicular to the c-axis, the structure is built up by the periodic stacking of two close packed [LaO₃] layers separated by a layer of composition [Ca₂O₂] containing (O₂)²⁻ peroxide ions. This oxide belongs to the family of compounds formulated as [A′₂O_2−δ][A_nB_n−1O_3n] for n=2 and δ=0. It is the first member of the series where the thickness of the perovskite slab corresponds to one [BO₆] (B=Mn) octahedron. The structural relationships with La₂Ca₂MnO₇ are discussed and the magnetic properties show that in both phases manganese is tetravalent.     

光学透明频率选择表面的设计研究

以氟化镁为基底材料, 采用基底、金属网栅与频率选择表面一体化设计方法设计了一种雷达波与光学波段双带通的结构. 利用模式匹配法对设计结构的传输特性进行了仿真研究, 并将设计结果与制备样件的测试结果进行了对比分析. 结果发现: 采用一体化设计的方法设计光学透明频率选择表面, 不仅能够快速得到电场基函数而且还能够准确预估其谐振尺寸, 从而在提高计算效率的同时避免了模式互作用零点的出现. 采用一体化设计方法获得了具有稳定滤波特性的光学透明频率选择表面, 为雷达/红外双模制导头罩的电磁屏蔽技术和隐身技术提供了一种有效技术方案. 关键词： 复合制导 频率选择表面 金属网栅 一体化设计