W. Meisel

XPS study of leached glass surfaces

Abstract By use of a special deconvolution algorithm, it was possible to decompose the XPS O 1s signal into several components reflecting different oxygen bonds. The influence of exposure in vacuo, environmental atmosphere, distilled water, and in a polish solution as corrosive media on the surface of different glasses has been studied. The O 1s signals of fused silica, Na 2 O· n SiO 2 glasses, a BaO·SiO 2 and a BaOB 2 O 3 SiO 2 glass (Schott SK 16) were analyzed. On the glass surfaces, leached layers were formed and analyzed qualitatively and semiquantitatively by XPS and optical and infrared methods. As a reference, virgin surfaces of samples broken in ultrahigh vacuum have been used. The absolute binding energies as well as the shifts relative to the bridging oxygen in ≡SiOSi≡ have been determined for the first for silanol groups ≡SiOH in the surface layer (532.9±0.2 eV and 0.56±0.10 eV, respectively) and for water and/or hydronium bound oxygen (533.6±0.2eV and 1.25±0.10 eV, respectively). Silanol groups are formed in atmosphere as well as by the residual gas in vacuo. Details of the growth of the leached layer were studied, e.g. the dissolution of Ba ++ ions and the growth of the thickness of the leached layer.

The influence of phosphoric acid on steel and on its corrosion products: a Mössbauer spectroscopic approach

Mossbauer spectroscopy was used to study corroded iron after treatment with phosphoric acid or commercial rust transformers. It was found that treatment produced normal iron phosphates from metallic iron, hematite and FeO, and acid ferric phosphate FeH 3 (POI4I)I2I. 2.5H 2 O from alphaand gamma Fe 2 O 3 and alpha, beta, and gammaFeOOH. When these last are present, phosph oric acid treatment does not produce a protective, inhibiting layer. See also AATA 19-702. -- AATA

Discrete bond model (DBM) of sodium silicate glasses derived from XPS, Raman and NMR measurements

In sodium silicate glasses, the fraction of differently bound Si species Q[i] (i = 0−4), depending on the number i of bridging oxygens bound to the quarternary silicon, is a function of stoichiometry and the Na/Si ratio. Sodium silicate glasses were investigated by high resolution X-ray photoelectron spectroscopy. To explain the differences in chemical shifts and linewidths of the O 1s signal of the bridging and the non-bridging oxygen as a function of alkali concentration, and extended glass model was developed. This new model takes into account the influence of the alkali concentration on the Q[i] distribution and on the appearance and concentrations of differently bound bridging oxygens as well as non-bridging oxygens. In principle, four different bonds for the non-bridging oxygens (NaOQ[i], i = 0−3) and seven different bonds for the bridging oxygens (Q[i]OQ[j], i, j = 1−4, |i − j| < 2), linking the different Q[i] species could exist. According to this ‘discrete bond model’, for each glass composition the number of these possible bridging and non-bridging oxygen species is limited to a maximum of two for each of them. An approximation of the measured X-ray photoelectron O 1s signal by a superposition of O 1s signals as calculated on the basis of this model allows an excellent reproduction of the experimental results if the glasses are non-phase-separated. In the case of phase separation, the measured spectrum shows only Q[i]OQ[i] bonds (with i = 1−4) identical to the bonding behaviour in the corresponding well-known crystalline phases. The chemical shifts of the non-bridging oxygens relative to the brodging oxygens (as well as the absolute binding energies), as measured by X-ray photoelectron spectroscopy, can be explained by changes in the relative concentrations and intensities of the different oxygen bonds as a function of glass composition. The energy differences between two energetically neighbouring bridging oxygens (i.e., Q[i]OQ[i] and Q[i]OQ[j] with |i − j| = 1) are equal (within the experimental error) for all bridging oxygen bonds in sodium silicate glasses and can be correlated with the cation field strengths of the modifying cations.

Corrosion processes and their inhibition as studied by Mössbauer conversion and other electron spectroscopies

To study corrosion processes of iron and steel and measures of their inhibition, a detailed knowledge of the phase composition and of phase transformations in very thin layers close to the attacked surface of the material is necessary. The information depths of integral (ICEMS) and depth selective (DCEMS) conversion electron Mössbauer spectroscopy are well suited for such investigations, but some effort is necessary if technical samples, i.e. nonenriched in57Fe, are to be studied. In many cases of practical importance, full information on the corroded surfaces cannot be got from Mössbauer spectra only, and a combination with Auger and photoelectron spectroscopies, in-including scanning and sputter options, is found to be most informative. This is demonstrated by three examples.The high corrosion resistance of stainless steel X1 CrNiSi 18 15 against boiling HNO3 is found to be due to a SiO2 layer formed on the surface and growing with duration of exposure. Nearly no iron oxides are formed, although the composition of the alloy close to the metal surface is slightly influenced.Transformer sheet steel (Fe−3%Si), commercially available after a thermal pretreatment, is covered by an insulating layer containing iron oxides and a large amount of Fe2SiO4. By the method combination it was found that a very thick layer below the metallic surface is also modified by the pretreatment. There, one finds pure α-iron containing clusters of SiO2 which have been formed by internal oxidation.The passivation of iron and steel (DIN 1623) in sulphate solution and in a phosphate buffer was studied in detail. Phase composition and thickness of the passive layer, as thin as a few nm only, were analyzed in dependence on the applied anodic potential and the duration of the passivating procedure. From the experiments, a model of the passive layer was derived, which is a modification of a p-i-n junction proposed elsewhere. The real passivation is ascribed to a layer, only a few monolayers thick, which has a highly disordered structure and provides the transition from cubic to orthorhombic structure.Some thermodynamical considerations show that immediately on an iron substrate, an oxide layer should always contain a high concentration of Fe2+ forming an equilibrium oxide. The absence of Fe2+ in Mössbauer spectra is interpreted by the assumption that this layer may be extremely thin, i.e. a few monolayers only.

CEMS/XPS study of iron stearate Langmuir-Blodgett layers

SummaryLangmuir-Blodgett mono- and multilayers of ferric stearate have been formed on oxidized silicon wafers. Thermodesorption of these layers was investigated by conversion electron Mössbauer and photoelectron spectroscopy and some complementary methods. Heating of samples in air up to 523 K leads to a desorption of the fatty acid chains, while the ferric ions are left on the substrate surface. These ions do not cluster laterally like it was found for Cd ions. They form a rather homogeneous, closed oxidic layer. This well defined layer may be used afterwards for further studies of surface reactions as well as interface and intra-layer interactions. The surface iron ions were found to exhibit a reasonably high recoilless fraction. Therefore, Mössbauer spectroscopy allows to follow chemical, structural, and magnetic changes of the iron ions even if the surface is covered by less than one monolayer.

Study of very thin oxide layers by conversion and Auger electrons

Oxidic layers as thin as 20–30 Å on α-Fe and stainless steel are studied by57Fe-DCEMS with K-conversion electrons and ICEMS. No indication of a vanishingf-factor could be found. Mössbauer spectra, recorded by use of LMM-Auger electrons (AEMS) and by electrons emitted with energies below 15 eV (LEEMS), contain information on the surface layer as well as on the bulk material, showing that part of these electrons are due to secondary effects and the high escape depths of K-conversion electrons.

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.

Surface spectroscopic study of the corrosion of ultrathin 57Fe-evaporated and Langmuir–Blodgett films in humid SO2 environments

Integral conversion electron Mossbauer spectroscopy (ICEMS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) have been used to carry out a comparative study of the corrosion resistance against humid SO2-aggressive environments of ultrathin 57Fe films. These films, having a thickness ≤ 2.5 nm, have been prepared by evaporation of 57Fe under vacuum and by 57Fe coating by a Langmuir–Blodgett (LB) technique on SiO2/Si wafers. The results have shown that the corrosion resistance of the evaporated films is similar to that of massive Fe. However, the LB films show a remarkably higher corrosion resistance. Thus, although the Fe films prepared by evaporation have been completely corroded after 6 h of exposure to the deleterious environment, the LB films remain unaltered even after 18 h of exposure to the same aggressive conditions. The higher corrosion resistance of the LB films appears to be related to the existence of a thin surface layer containing Si, Fe2+ and Fe3+ formed on top of the Fe metal film during the thermal treatments subsequent to the LB deposition. Copyright

Magnetic pigments for recording media

CrO2 doped with 1--2 wt% Fe3+ on Cr4+ positions is one of the most important materials for magnetic recording in audio, data and video tapes. 57Fe Mössbauer spectroscopy was applied to determine the level of iron doping in newly developed high coercivity CrO2 particles. It was found that, compared with the conventional preparation process, the new particles contain increased amounts of Fe3+ ions in the CrO2 crystal lattice giving rise to higher magnetocrystalline anisotropy. This is reflected in coercivities of up to over 900 Oe. All samples contain as a secondary iron-containing phase α-(Cr1-xFex)2O3, the amount of which is higher in samples of the conventional preparation process than in those of the new BASF process. Details about the mechanism of the development of doped CrO2 particles in both types of processes were obtained by means of Mössbauer, XRD and wet chemical investigations making possible an optimization of the new BASF process.

Formation and characterization of oxidic and metallic Fe/Ni multilayers prepared from Langmuir-Blodgett films

Oxidic and metallic iron, nickel, and mixed iron/nickel mono- and multilayers were prepared by some treatments of iron- and nickel-containing Langmuir-Blodgett films. The layers were characterized by several surface-sensitive methods. Interactions between the iron and the nickel could be observed. The results are different to those obtained from disordered bulk alloys and from simple diffusion calculations.