Surface magnetic disorder in nanostructured Ni0.5Zn0.5Fe2O4 particles

Nanostructured ferroxide particles with initial formula Ni_0.5Zn_0.5Fe₂O₄ are investigated. The aim was to explore the monodomain and the superparamagnetic states of the ferrospinel and the impact of the surface magnetic disorder on the magnetization processes. Mössbauer spectroscopy (MöS) demonstrated that the ion distribution follows the general formula (Zn_0.5Fe_0.5)_A[Ni_0.5Fe_1.5]_BO₄, where A is the tetrahedral and B, the octahedral sublattice. MöS in an external magnetic field (5 T) at 4.2 K shows non-collinearity of the sublattices’ magnetic moments and deviations in the hyperfine magnetic field that could be related to a canting effect. Magnetic measurements were applied to characterize the temperature behavior of the magnetic properties and the a.c. complex magnetic susceptibility.     

Hyperfine interactions and site population in the spinel compounds Ca x Co1−x Fe2O4

Mössbauer spectra of the spinel series Ca _x Co_1–x Fe₂O₄, withx=0.0, 0.05 and 0.10, have been collected between 80 and 300 K and in an applied field of 6 T at 4.2 K. From the shape-independent hyperfine field distribution fits of the external field spectra, and from the binomial distribution fits of the temperature-dependent spectra, the hyperfine field and the center shift could be unambiguously assigned to octahedrally and tetrahedrally coordinated ferric ions. The importance of the application of an external field in conjunction with a reasonable physical model to obtain a correct assignment of hyperfine parameters to a specific site is demonstrated.     

The electric field gradient at the iron sites inβFeOOH

The temperature behaviour of the hyperfine parameters of iron in paramagnetic chlorine containingΒFeOOH is studied by⁵⁷Fe Mössbauer spectroscopy. Applying external magnetic fields, the sign and asymmetry parameterη of the electric field gradient (EFG) at the compound's two iron sites is determined. It is shown that an external field enhances the distributive character of the hyperfine parameters. Finally, heat treatment experiments allow new data on the nature of both iron coordination types to be presented.     

Extractive Fermentation of Xylanase from Aspergillus tamarii URM 4634 in a Bioreactor

Of the many reported applications for xylanase, its use as a food supplement has played an important role for monogastric animals, because it can improve the utilisation of nutrients. The aim of this work was to produce xylanase by extractive fermentation in an aqueous two-phase system using Aspergillus tamarii URM 4634, increasing the scale of production in a bioreactor, partially characterising the xylanase and evaluating its influence on monogastric digestion in vitro. Through extractive fermentation in a bioreactor, xylanase was obtained with an activity of 331.4 U mL^?1 and 72 % yield. The xylanase was stable under variable pH and temperature conditions, and it was optimally active at pH 3.6 and 90 °C. Xylanase activity potentiated the simulation of complete monogastric digestion by 6 %, and only Mg²⁺ inhibited its activity. This process provides a system for efficient xylanase production by A. tamarii URM 4634 that has great potential for industrial use.     

壳聚糖膜固定生物活性短肽RGDS的研究

为改善壳聚糖对细胞的特异性吸附,采用水溶性碳二亚胺将生物活性短肽精氨酸-甘氨酸-天冬氨酸-丝氨酸(RGDS)固定到壳聚糖膜的表面,采用X射线光电子能谱检测固定多肽前后的壳聚糖膜表面,发现反应后壳聚糖膜表面氮元素含量增大,Nls和Cls曲线拟合谱中酰胺键增多,表明RGDS短肽已固定到壳聚糖膜的表面;人角膜缘上皮细胞体外培养实验表明,固定RGDS后壳聚糖膜的细胞黏附率有了明显提高,固定RGDS后的壳聚糖膜在角膜组织工程支架等方面有更好的应用潜力。     

Surface oxidation control of nanosized magnetite and Mössbauer measurements

Selected highly homogeneous powders of Fe₃O₄ with different particle size on the nanometer scale (10?±?2 and 3?±?2 nm) obtained by soft-chemical methods were studied by Mössbauer spectroscopy. The study shows clearly the powerful possibilities of Mössbauer spectroscopy to analyze the surface oxidation of nanostructured powders of magnetite. On the other hand, it is shown that for very small superparamagnetic particles the spectrum of magnetite might be quite similar to that of maghemite, making it difficult to distinguish between both phases.     

About the Morin transition in hematite in relation with particle size and aluminium substitution

The results of a M?ssbauer study of the Morin transition behaviour in three series of hematite and Al-hematite samples are reviewed and discussed. The first two series comprise small-particle hematites and Al-hematites prepared from decomposition of lepidocrocite, whereas in the third series Al-hematites up to the μm range are obtained from co-precipitated oxinates. It is demonstrated that the Morin transition temperature follows quite well the overall properties of the samples such as particle size and Al substitution, while the transition region is rather determined by all kind of distributive effects. A model involving intermediate states is suggested for the Morin transition behaviour in non-ideal hematite. Presented at the International Colloquium “M?ssbauer Spectroscopy in Materials Science”, Velké Losiny, Czech Republic, 3–8 September 2000. Finacial support from the FWO-Flanders (project G.000797) and the Belgian Federal Interuniversity Attraction Pole (PAI/UIAP) on reduced dimensionalities is acknowledged.     

The role of k0-NAA in the assessment of the annual radiation dose for use in TL/OSL dating of sediments

An account is given of the role and the strength of k ₀-INAA to assess the annual radiation dose (contribution of U, Th, K and Rb) for use in TL/OSL dating of sediments. Mention is made of: (1) its simplicity of sample preparation, (2) its accuracy and even its reference character for other elemental determination methods, (3) the possibility of obtaining the Rb content with no extra effort and (4) its capability to easily and reliably quantify U and Th in a small amount of etched quartz grains, followed by evaluating the importance of their presence in the quartz inclusion technique.     

共振辐射的理论研究——Holstein方程的解

解决介质中共振辐射的关键问题是求解Holstein方程。本文直接利用算符运算将方程积分,给出了激发态粒子集居数的含无限项的级数表达式。基于对这个级数高阶系数的极限行为的研究,成功地克服了无穷项求和的困难,整理出一个在任意给定精度下都只含有限项的解式,由它可以计算出任何时刻的结果。 关键词：     

57Fe Mössbauer effect study of well-crystallized goethite (α-FeOOH)

Well-crystallized, natural goethite has been investigated as a function of temperature. The saturation magnetic hyperfine field was found to be 507 kOe and the Néel temperatureT _N=(400±2) K. The antiferromagnetic structure is stable in external fields of up to 6O kOe and the spin direction is at right angles with respect to the EFG's principal axis. AtT/T _N<0.5, the reduced hyperfine field varies asT ² and in the range 0.80<T/T _N<0.97 asT ^1/3. From the temperature dependence of the isomer shift, the Debije temperature was found to be 440 K from which a relative Mössbauer fraction of 0.96 against hematite was evaluated.