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The practical importance of alloy surfaces in catalysis, corrosion andother aspects of materials performance is widely recognized. What is needed now is sufficient knowledge of the relationship between externally controllable factors — alloy composition, temperature, environment — and surface properties — composition, structure, chemical activity — to control materials performance in these applications. Our purpose here is to review progress in determining and predicting the relationship between one surface property, composition, and certain externally controllable variables: overall composition, temperature, environment and physical form.We find that theoretical treatments of metal alloy surface composition now include essentially all significant physical effects and can predict values for most parameters of interest. Though improvements are still possible, the accuracy of predictions is more often limited by uncertainties or absence of the basic data for the calculations (e.g., thermochemical values) than by the models themselves.Alloy surface composition can now be measured well. The first monolayercomposition of large alloy slabs can be determined quantitatively over a wide temperature range in ultra-high vacuum. Difficulties with specimens of practical interest still challenge experimentalists. Among these are supported catalysts, surfaces under chemisorbed layers and composition of layers below the first. Significant progress is being made and we expect the next few years will see success. 相似文献
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H. Zhang Y. Long Ya. Mudryk K.A. Gschneidner Jr. V.K. Pecharsky 《Journal of magnetism and magnetic materials》2010,322(13):1710-8541
Phase formation, structure, and the magnetocaloric effect (MCE) in as-cast LaFe11.5Si1.5Bx (x=0.5, 1.0) compounds have been studied. The Curie temperatures, TC, are ∼211 and 230 K for x=0.5 and 1.0, respectively, which are higher than that of annealed LaFe11.5Si1.5 (TC=183 K), while the maximum magnetic entropy changes at the respective TC under a magnetic field change of 0-5 T are 7.8 and 5.8 J/(kg K). Wavelength dispersive spectrometry (WDS) analysis shows that only a small fraction of boron atoms is dissolved in the NaZn13-type structure phase, and that the compositions of the as-cast LaFe11.5Si1.5Bx (x=0.5, 1.0) alloys are much different from the intended nominal compositions. These as-cast alloys exhibit second-order magnetic phase transitions and low MCEs. However, based on the relative cooling power, the as-cast LaFe11.5Si1.5Bx alloys are promising candidates for magnetic refrigerants over a wide temperature range. 相似文献
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Interferences (overlaps) occurring when lines of other elements affect either peak or background measurements can cause errors
in quantitative WD analysis, but may be minimised by suitable choices of analysis conditions such as spectrometer crystal,
background offsets, and pulse-height analyser settings. Computer spectrum-simulation is much more effective than reference
to wavelength tables for investigating interferences. The ‘Virtual WDS’ simulation program developed by the present authors,
hitherto applied only to ‘ordinary’ elements (Z ≥ 11), has been extended to light elements for which evaporated multilayers are used in place of true crystals. ‘Virtual
WDS’ utilises experimentally recorded light-element K spectra and L and M spectra of heavier elements in the same wavelength
range. It is impractical to record all high-order peaks, so computed line profiles are used, with widths and intensities interpolated
from a limited set of observations. The relative positions of first and higher order peaks are affected significantly by the
refractive index of the multilayer, requiring modification of the Bragg equation. Suppression of high orders by pulse-height
analysis is less effective than for ‘normal’ wavelengths, owing to the breadth of the pulse-height distribution for low X-ray
energies. Simulation using a Gaussian expression aids optimisation of threshold and window-width settings. 相似文献
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Batobolot Zhalsaraevich Zhalsaraev 《X射线光谱测定》2020,49(4):480-492
Background suppression is discussed in wavelength-dispersive polarization X-ray spectrometer (WDPXRS), in which the goniometer scans in plane perpendicular to primary and secondary beams. Background suppression coefficients in WDPXRS and energy-dispersive polarization X-ray spectrometer are determined by different expressions (in “The scattering suppression of X-rays with energy of 20–200 keV in spectrometers with Barkla polarizers,” doi: 10.1002/xrs.3046 ). It is proposed to install silicon drift detectors in WDPXRS and implement energy-dispersive and wavelength-dispersive modes in one channel. 相似文献
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Stephen J. B. Reed 《Mikrochimica acta》2000,132(2-4):145-151
“Trace” elements may be defined as elements whose concentrations are of a similar order to the detection limit. In WD analysis
the detection limit is a function of the ‘figure of merit’ P2/B, where P is the pure-element peak intensity and B the background intensity. With normal analysis conditions detection limits
of ∼100 ppm are typical, but substantial improvements can be achieved by using higher values of accelerating voltage and beam
current. Long counting times are also advantageous, but should preferably be divided into relatively short alternating peak
and background measurements to minimise the effect of instrumental drift. Using separate routines for trace and major element
analysis is desirable owing to their different requirements. As the statistically defined detection limit is reduced, errors
due to background nonlinearity and interferences (overlaps) from other elemental peaks become more probable. Spectrum simulation
is useful for optimising background offsets and choice of crystal to minimise interferences, and estimating interference corrections
when these are necessary. ‘Blank’ standards containing none of the trace elements of interest are also useful for quantifying
background nonlinearity. 相似文献
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Valdek Mikli 《Mikrochimica acta》2006,155(1-2):205-208
The study covers a problem frequently encountered in the quantification of the results of wavelength-dispersive spectrometry
(WDS) for the composition analysis of thin films. The characteristics of a Parallel Beam Spectrometer and traditional WDS
systems were examined and olivine mineral – (Mg, Fe)2SiO4 (O – 44.03 wt%, Mg – 31.1 wt%, Si – 19.56 wt%, Fe – 5.06 wt%, Ni – 0.16 wt%, Mn – 0.09 wt%) was used as a reference material.
Low accelerating voltage at 7 kV and beam current 400 nA were applied. Both methods yielded 30–35% of Mn. This is attributed
to the overlapping of the MnLα first-order and the MgKα second-order lines. Studies of the influences of the second- and the
third-order lines show that the second-order lines from Kα and Lα X-ray counts affected significantly the obtained WDS spectra
when the influence of Mα counts was insignificant. Furthermore, the third-order lines did not have a marked effect on the
analysis results. 相似文献
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环带钾长石、榍石和锆石的显微结构与微区组成特征分析 总被引:1,自引:0,他引:1
南秦岭沙河湾石英二长岩中的钾长石、榍石和锆石具有明显的成分环带。利用电子探针(EPMA)波谱仪(WDS)、扫描电子显微镜(SEM)及其附带的能谱仪(EDS)和激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)对造岩矿物环带钾长石、榍石和锆石的显微结构与微区组成进行表征。结果表明:钾长石中K+被Na+,Ca2+,Ba2+,Fe2+和Ce3+不同程度替代。榍石中Ca2+被V3+,Ce3+和Ba2+等替代,Ti4+被Fe2+和Al3+等替代。锆石含Fe,Th,U,Nb,Ta,Y,Hf,Yb和Pb等稀土和微量元素。钾长石中元素浓度由高到低为Si,Al,K,Ca,Na,Mg和Ba,其中K和Na互补,较亮处Ba含量高,越靠近边缘,Si升高、K升高与Na降低趋势越明显。榍石中元素浓度由高到低为Ca,Si,Ti,Ba,V,Ce,Al和Fe,较亮处Fe含量较高。锆石中元素浓度由高到低为Zr,Si,Nd,Ce,Hf,U,Pb和Th,Hf与Zr呈明显的互补关系,核部Zr含量较边部高,核部Hf、U和Th含量较边部低。 相似文献
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用实验测量数据对波谱分析与能谱分析这两种技术在元素定性与定量分析方面的性能差异进行了比较,说明各自的长处与不足,为材料科学工作者在分析测试中如何选择提供参考。 相似文献
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Djordje Mandrino Martin Lamut Matjaž Godec Matjaž Torkar Monika Jenko 《Surface and interface analysis : SIA》2007,39(5):438-444
Protective surface layers on AISI 321 stainless steel were prepared by thermal treatments at two different temperatures in air and two controlled atmospheres. Different oxide and/or nitride layers were formed. Surface morphology of the layers was investigated by scanning electron microscopy (SEM). Auger electron spectroscopy (AES) depth profiling of the samples was performed. Since depth profiling suggested layer thicknesses of the order of hundreds of nanometres, an attempt was made to obtain some fast, averaged information about the layer compositions using wavelength dispersive spectroscopy (WDS) at two different beam energies to obtain probing depths best suited to the layer thickness. X‐ray photoelectron spectroscopy (XPS) profiling of one layer was also performed to obtain information about the chemical states of the elements inside the layer. The analysed samples showed considerable differences with respect to their surface morphology, oxide/nitride layer thicknesses, compositions and layer–metal interface thickness. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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