P. Griesbach

Optimization of a conversion electron Mössbauer spectroscopy gas flow He/CH4 proportional counter

A new detector for CEMS has been built and optimized with respect to the statistical quality of spectra obtained. The optimization has been performed by measuring Mößbauer and pulse height spectra at in- and off-resonance. Single channel analyzer settings were calculated by a new optimization routine. A comparison of different detector designs has been performed using the statistical utility rate of spectra obtained from a stainless steel foil. A procedure for determining optimal operating parameters for ICEMS gas flow proportional counters is proposed.

Electrochemical and Conversion Electron Mössbauer Studies of Low Carbon Steel Polarized in Aqueous Sulfate Solution Containing Sulfite in Low Concentration

The passivation of low carbon steel was studied in aqueous solution of 0.5M and of at . The electrochemical methods (potentiodynamic, chronopotentiometric, and voltammetric) showed that the presence of 0.001M increased the thickness of the oxide film by about one order of magnitude. The spectrophotometric analysis of the solutions demonstrated that the rate of the dissolution of iron was much higher through the thick passive film, in the presence of ions, than through the thin passive film formed in the solution of 0.5M without ions. This is good evidence for the protective properties of the passive film being very weak if sulfite ions are present in the solution, even in a very low concentration. The conversion electron Mossbauer spectroscopy (CEMS) provides information about the compositions and thicknesses of the passive films formed in a solution containing both sulfate and sulfite ions. The major components found are and , and also could be identified on the surface of the low carbon steel as a minor component.

Problems of electron detection in Depth-Selective Conversion Electron Mössbauer Spectroscopy

Applications of Depth-Selective Conversion Electron Mössbauer Spectroscopy (DCEMS) are limited by the long measuring times needed for collecting sufficient data statistics. To shorten the recording time, the background should be reduced and the detection efficiency for conversion electrons should be improved. For57Co/57Fe DCEMS, systematic studies were performed to investigate the origin, shape, and structure of the background components in DCEMS data distributions for various samples using channeltrons and low-noise scintillation counters as electron detectors.

Quantitative determination of fayalite layers on iron by CEMS

In the processing of silicon iron (Fe-3%) Si), so-called ‘fayalite layers’ are formed. By CEMS, they were found to consist of an outer Fe3+-oxide layer and an inner Fe2SiO4 (fayalite) layer. Sometimes an additional wüstite contribution was found. Thef-factor of fayalite was determined experimentally (ffayalite/fα-Fe=0.47±0.04) and, by use of it, the thicknesses of the layers on some silicon iron samples could be calculated from CEMS data.

Low noise scintillation detectors with a P-47 thin layer screen for electrons of several keV

Abstract The applicability of a low-noise scintillation detector (ScD) for the registration of electrons of several keV energy has been studied employing photomultipliers (PM) of different types and sizes. With the application of a sedimented P-47 scintillation screen, the values of the low-energy sensitivity limit and those of the light conversion coefficient were determined as about 2.7–4.7 keV and 2.8–6.6 photoelectrons/keV, respectively, for the set of PMs (Philips-Valvo XP 2020, Philips-Valvo XP 2052, Philips-Valvo XP 2972, EMI 9124a) studied. It is concluded that such scintillation detectors might be used advantageously as electron counters in the range of E > 5 keV. Applications below this kinetic energy value are also feasible when applying a floating acceleration of several kV to the ScD — a voltage much lower than the values required for Everhart-Thornley detectors.

Layer formation on silicon steel by processing in H2/H2O at elevated temperatures

Silicon steel (Fe-3wt%Si), as used for transformers and generators, has been annealed in wet hydrogen at elevated temperatures. The composition, sequence, and thicknesses of the layers found by conversion electron Mössbauer spectroscopy (CEMS) and Auger electron spectroscopy (AES) depth profiling for a 10 minutes anneal in different atmospheres are reported. In the range from 500°C to 720°C we observed carbide formation, indicating that the decarburization is hindered. Above 800°C, the layers consist of fayalite or of fayalite and iron oxides, depending on the oxygen potentialaO. At 843°C, the onset of iron oxide formation was found ataO=0.33.

The state of iron in andalusite

Iron in andalusite (Al2SiO5) with concentrations of 1.5 … 4.3 wt-% has been reported in the literature to occupy mainly Al1 sites as Fe3+, and only small portions occupy Al2 sites as Fe3+ and Fe2+. In no case a magnetic hyperfine splitting has been found at temperatures above 77 K.We have studied natural samples of andalusite containing 0.3 and 0.5 wt-% only. Mössbauer spectra recorded at 80 and 300 K show magnetically split spectra and, for the 0.5% sample, an additional doublet can be seen.The spectra are interpreted as being due to a slow paramagnetic relaxation resulting in Fe3+ and Fe2+ hyperfine multiplets. An increase in the Fe concentration reduces the paramagnetic relaxation time. It seems that the andalusite structure can contain a limited number of Fe2+ only, and a similar limitation does not exist for Fe3+ ions.

61Ni Mössbauer study of the surface hyperfine magnetic field in nickel

Abstract61Ni Mössbauer measurements have been performed at 4.2 K on spherical Ni particles with an average diameter of 100 and 30 Å, covered with a protective layer of SiO. Their spectra contain a surface component with a significantly reduced hyperfine magnetic field as compared with the field in the bulk. This result confirms recent theoretical predictions.

57Fe,61Ni, and121Sb Mössbauer study of oxidic spinel ferrites CoxNi5/3−xFeSb1/3O4

Antimony substituted nickel ferrites CoxNi5/3−xFeSb1/3O4 withx=0,1/3,1,4/3, and 5/3 were prepared by the common ceramic technique, and the single-phase structure was confirmed by x-ray powder diffraction.57Fe Mössbauer spectra were recorded at 4.2 K with and without an applied magnetic fieldHlong=60 kOe in order to determine the cation distribution and the spin structure. Utilizing the cation distributions and the Yafet-Kittel spin structures for B-site Fe3+ ions, derived from Mössbauer experiments the calculated magnetic moments are in excellent agreement with the experimental values at 4.2 K. The existence of Ni2+ ions in crystallographic A-sites was confirmed by61Ni Mössbauer spectroscopy. The magnitude of the supertransferred hyperfine field at121Sb nuclei is determined mainly by a covalent spin charge transfer from A-sites, occupied by Fe3+, Ni2+, and Co2+ ions, into the 5s orbital of Sb5+ ions. The different contributions of various magnetic A-site ions to HSTHFSb may be understood by combining the energy differences between the ionic configurations, connected with the electron transfer process and the related exchange integrals.

AES and CEMS analysis of the formation of layers on Si steel under thermal treatment in a flux of H2/water vapour

SummaryThe near surface diffusion and reaction processes in iron-silicon steel (3.1 wt.%Si) during 10 min decarburization in water vapour/hydrogen have been studied. The decarburization temperature has been varied between 506 and 714°C for the fixed partial pressure ratio pH2O/pH2=0.017. An outer layer of SiO2 forms on the surface with its thickness increasing with temperature. From 600°C upwards, the decarburization process is hindered and a cementite layer is formed below the SiO2 layer. The formation of fayalite at the surface has been studied at a fixed decarburization temperature with pH2O/pH2 ranging from 0.017 to 0.49. The scale thickness reduces abruptly just before the ratio pH2O/pH2 necessary for the wüstite formation is reached. Obviously, a new layer is formed which reduces the oxygen diffusion that much that a transition from internal to external oxidation occurs.