Determination of foreign phases in Fe–As based superconducting systems

The recently discovered superconducting materials, containing Fe and As, have raised huge interest. However most materials prepared to date, suffer from a varying degree of content of foreign phases, Fe₂As, FeAs₂ and FeAs, which can lead to wrong conclusions concerning the properties of these materials. Mössbauer Spectroscopy determines quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of eight samples of three Fe–As based families of materials, RAsFeO_1???x F _x , Sr_1???x K _x Fe₂As₂ and LiFeAs, prepared in three different laboratories.     

Practical application of thermal ionization mass spectrometry for the determination of plutonium for the LANL Bioassay Program (LA-UR-00-1697)

Utilization of thermal ionization mass spectrometry as a routine analytical service provided to the Los Alamos National Laboratory Bioassay Program has evolved significantly since its implementation just over three years ago. Converting this unique research tool designed to support nuclear weapons testing to a quasi-production mode for the routine analysis of ~300 urine samples/year for ultra-low levels of plutonium has required resolution of numerous practical issues. These issues include clean-room sample preparation, adequate tracer recovery, customer specified turn-around times, throughput, water and urine blank values, statistical data reduction, and quality control and performance evaluation sample requirements.     

毛菊苣种子精油化学成分的气相色谱-质谱法分析

研究毛菊苣种子精油的化学成分并为进一步的生理活性的研究打下基础,以水蒸气蒸馏法提取毛菊苣种子的精油,并进行GC-MS仪器分析,用面积归一化方法测定其含量。分离得到24个化合物,对所有分离得到的化合物进行鉴定,鉴定率达100%。通过毛菊苣种子精油化学成分的研究,为开发和利用这一维吾药用植物提供了科学依据。     

Mechanism by which Tungsten Oxide Promotes the Activity of Supported V2O5/TiO2 Catalysts for NOX Abatement: Structural Effects Revealed by 51V MAS NMR Spectroscopy

The selective catalytic reduction (SCR) of NO_x with NH₃ to N₂ with supported V₂O₅(‐WO₃)/TiO₂ catalysts is an industrial technology used to mitigate toxic emissions. Long‐standing uncertainties in the molecular structures of surface vanadia are clarified, whereby progressive addition of vanadia to TiO₂ forms oligomeric vanadia structures and reveals a proportional relationship of SCR reaction rate to [surface VO_x concentration]², implying a 2‐site mechanism. Unreactive surface tungsta (WO₃) also promote the formation of oligomeric vanadia (V₂O₅) sites, showing that promoter incorporation enhances the SCR reaction by a structural effect generating adjacent surface sites and not from electronic effects as previously proposed. The findings outline a method to assess structural effects of promoter incorporation on catalysts and reveal both the dual‐site requirement for the SCR reaction and the important structural promotional effect that tungsten oxide offers for the SCR reaction by V₂O₅/TiO₂ catalysts.     

2019年2期 电子期刊

用化学共沉淀法一次煅烧工艺制备Ce,Pr:GAGG粉体,利用XRD、SEM、荧光光谱仪等对样品表面形貌及发光性能进行表征,研究煅烧温度、沉淀剂引入草酸根对粉体发光性能的影响。结果表明,前驱体经950℃煅烧3 h后全部转变为GAGG相,Pr³⁺、Ce³⁺共掺未改变基质的物相结构;沉淀剂引入草酸铵后,粉体发射光谱积分强度由333 573 a.u上升至420 894 a.u,沉淀剂引入草酸根能提高粉体的发光性能。荧光寿命测试表明,在Ce:GAGG中掺入Pr³⁺可使Ce³⁺的荧光寿命降低,衰减时间为35.43 ns。     

非线性Af一Bg型自由基交替共聚反应的固化理论

利用高分子反应统计理论,给出了非线性Af-Bg型自由基交替共聚反应的溶胶-凝胶分配方程和反应体系的凝胶化条件,这些结果是进一步研究与凝胶网络性能相关的网络结构参数的基础．     

非线性Af—Bg型自由基交替共聚反应的凝胶网络结构参数

利用高分子反应统计理论,计算了非线性Ａｆ－Ｂｇ型自由基交替共聚反应凝胶网络中的弹性有效链和悬吊链的链数及相应的链节数,进而计算出弹性有效链的数量和质量分数、有效交联点数和弹性量等网络结构参数。     

火焰原子吸收光谱法测定蜂蜜中的钙和锌

报道了用直接消解法处理蜂蜜,在选定光谱条件下,用火争原子吸收分光光度计测定其中的钙和锌,与湿法,干法消解处理的试样相比较,该法操作简单,结果令人满意,加标回收率钙为99－101．9％。锌为98．8％－105％,相对标准偏差钙为4．4％－5．9％,锌为2．1％－3．8％检出限钙为0．138ug.mL^-1,锌为0．084ug.mL^-1。     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.     

Aryl-halide versus aryl-aryl reductive elimination in Pt(IV)-phosphine complexes

Upon the addition of Br2 to complexes (P-P)Pt(Ar)2, two different products were observed, depending on the bite angle of the bidentate phosphine ligand: a Pt(II) aryl bromide complex, the product of C-Br reductive elimination, and Pt(IV) oxidative addition complex. At high temperatures, the latter exclusively gave the product of the C-C reductive elimination.