Photon and Ion-Beam Induced Processes in Materials Studied by Hyperfine Interactions

Laser nitriding and laser cementation are investigated by Mössbauer spectroscopy and complementary methods. It is demonstrated how the backscattering versions of Conversion Electron and Conversion X-ray Mössbauer spectroscopy can contribute to the investigation of surface processes, like the laser-induced formation of nitrides and carbides. Additionally, the formation of semiconducting iron disilicide can be achieved by ion-beam mixing and pulsed laser irradiation of Fe/Si bilayers. The results of both processes are compared.

     

Laser-produced iron nitrides seen by Mössbauer spectroscopy

Mössbauer spectroscopy is a very powerful tool to investigate technological processes performed mainly at the surface of materials. Nitriding of metals and steel is well established in surface engineering, and gas nitriding is used most frequently. Laser nitriding, i.e. the nitrogen take-up from the ambient gas upon irradiation of a steel surface with short laser pulses, is presented in its application to iron, stainless steel and plain carbon steels. It will be demonstrated how Mössbauer spectroscopy in combination with complementary methods (Rutherford backscattering spectroscopy, Resonant nuclear reaction analysis, Nanoindentation) can help to reveal basic mechanisms in these processes.     

Experience with a toroidal proportional detector for backscattered Mössbauer ψ-rays and X-rays

Because backscattering geometry in Mössbauer spectroscopy is particularly useful for non-destructive testing interesting technical applications arise. The reported toroidal proportional detector for backscattered Mössbauer ψ-rays and X-rays enables non-destructive testing of technical surfaces since no sample preparation is required. Improvements in assembling, shielding, wire-fixing and gas-filling-in comparison to the first version of the detector-are reported. The performance of the improved detector is demonstrated by measurements of steel surfaces which are modified by plasma nitriding and laser irradiation.     

Iron nitrides and laser nitriding of steel

Nitriding of iron and steels is well known to improve hardness, wear and corrosion resistance of surfaces. Recently, it has been found that a simple irradiation of iron and steel with pulses of an excimer laser in air or nitrogen leads to a significant nitrogen take-up into the surface and to improvements in the surface properties. The mechanisms of the laser-nitriding process are not yet understood but the nitrification will be proved by several methods. Among these, Mössbauer spectroscopy is very powerful, especially when Conversion Electron and Conversion X-ray Mössbauer Spectroscopy (CEMS and CXMS) are applied simultaneously. Nevertheless, only a combination of methods gives a satisfying overview of the laser induced nitriding processes: Rutherford Backscattering Spectrometry (RBS) and Resonant Nuclear Reaction Analysis (RNRA) were also used to characterize the treated surfaces. First measurements show an increase of the hardness and the wear resistance with the number of pulses. From the results of the CEMS analyses this should be correlated with the increase in the fraction of the ε-phase. This may be important for industrial applications.     

A Mössbauer and X-ray investigation of ξ-Fe2N

Mössbauer spectra of ξ-Fe₂N at 4·2°K, 78°K and room temperature are presented. The Mössbauer parameters are compatible with the nitrogen donor model previously found to be applicable to the lower iron nitrides. X-ray analyses of this phase confirm the structure previously reported. Mössbauer and X-ray spectra for partially nitrided iron foils containing γ′, ε and ξ iron nitrides are also discussed.     

Clays and clay minerals: The firing process

⁵⁷Fe Mössbauer spectroscopy reveals changes in iron valence and iron site geometry when clays and clay minerals are heated, and allows a distinction to be made between paramagnetic and magnetically ordered phases. Mössbauer spectra can thus reveal the extent of iron retention in silicate structures upon heating, the identity of iron oxides initially present or formed during the heating process and their transformations, and the character of the atmosphere under which heating was carried out. This makes Mössbauer spectroscopy the most effective tool for the characterization of changes induced by heating phyllosilicates and iron oxides.     

Mössbauer spectroscopy of iron meteorite Dronino and products of its corrosion

The results of the first study of iron meteorite Dronino and products of its corrosion by Mössbauer spectroscopy are presented. Mössbauer parameters for iron phase, sulfide inclusions, products of terrestrial corrosion and iron oxides in concretion are determined and analyzed.     

Characterization of magnetic nano-fluids via Mössbauer spectroscopy

The laser pyrolysis became a useful tool, providing various ways, in production of nano materials. The iron Mössbauer spectroscopy is one very accurate method in evidencing the physical properties and related processes in the nano scale compounds. The effect of pressure, laser spot area and induced combustion, of gas mixture and laser power on the phase composition and inside particle distribution, grain size as well as the related phenomena were investigated by temperature dependent Mössbauer spectroscopy. A selection of most relevant properties is presented and discussed in details.     

Clays and clay minerals: What can Mössbauer spectroscopy do to help understand them?

Mössbauer spectroscopy is a powerful technique for the characterization of materials formed in the weathering environment. Mössbauer studies of clay-sized phyllosilicates, however, are burdened with several problems: the samples are rarely monomineralic, they may be poor in iron, and only few iron-rich species order magnetically above 4.2 K. Site occupancies are difficult to determine, and cis and trans octahedral-OH site assignments are normally not possible. Unequivocal information that can be gained from such work thus is often restricted to the determination of the oxidation state of iron and average structural site distortions. Mössbauer data on iron oxides are generally more straightforward to interpret because these can be studied in the magnetically ordered state. A further asset of Mössbauer spectroscopy when studying iron oxides lies in its high sensitivity for magnetically ordered phases. Adverse effects ensuing from small particle size, interparticle interactions, non-stoichiometry and foreign-element substitution that often affect the Mössbauer parameters of iron oxides occurring in clays and soils can be at least partly offset by taking spectra at low temperatures.     

Artificial neural networks in Mössbauer material science

Mössbauer spectroscopy is a useful technique for characterizing the valencies, electronic and magnetic states, coordination symmetries and site occupancies of the cation. The Mössbauer parameters of isomer shift and quadrupole splitting are useful to distinguish paramagnetic ferrous and ferric iron in several substances, while the internal magnetic field provides information on the crystallinity. In recent years artificial neural networks have shown to be a powerful technique to solve problems of pattern recognition of a mineral from its Mössbauer spectrum, Mössbauer parameters data bank, crystalline structure and magnetic phases of soil from Mössbauer parameters. A computer software named Mössbauer Effect Assistant has been developed. It uses learning vector quantization neural network linked to a Mössbauer data bank that contains Mössbauer parameters of isomer shift, quadrupole spliting, internal magnetic field and the references of the substances. The program identifies the substance under study and/or its crystalline structure when fed with experimental Mössbauer parameters. It can also list the references from the literature by feeding the name of the substance or the author of the publication. Typical application of Mössbauer Effect Assistant in iron-bearing materials Mössbauer spectroscopy is present in user friendly Microsoft Windows environment.     

Low temperature nuclear orientation studies of the magnetic structures of RNiAl4 in applied magnetic fields

The accumulation of iron and arsenic from aqueous solution by lettuce leaves biomass was investigated using Mössbauer and EXAFS spectroscopic techniques. Mössbauer spectroscopy results show that iron is oxidized during sorption while EXAFS results indicate that iron is coordinated by approximately 6 oxygen and 2 carbon atoms while arsenic is coordinated by approximately 4 oxygen atoms with iron as a second neighbor.     

Mössbauer spectroscopy on the surface of Mars. Why?

A Mössbauer spectrometer is included in the preliminary payload of a rover to be placed on the surface of Mars in the Soviet mission to the planct in 1996/1,2/. In counection with the American planctary program it has also been suggested to construct a Mössbauer spectrometer to be landed on Mars /3, 4/. The objective is to study the iron compounds of the Martian soil and rocks by backscattering Mössbauer spectroscopy. The paper describes the significance of the element iron in the study of the evolution of the planetary system and what we might expect to learn from Mössbauer spectroscopy of the surface materials of Mars. The study of Mars is expected to expand substantially in the coming decades, probably culminating with a manned flight to the planet. The international Mössbauer community may contribute significantly to the preparation of these events.     

In situ characterisation of supported iron-iridium catalysts by iron-57 and iridium-193 Mössbauer spectroscopy

Some silica-supported iron-iridium catalysts with different iron to iridium ratios and formed by the incipient wetness technique have been examined in situ by⁵⁷Fe and¹⁹³Ir Mössbauer spectroscopy following pretreatment in hydrogen. The results show that the reduction of the iron component is enhanced by the presence of iridium metal. The pretreated catalysts were evaluated for the hydrogenation of carbon monoxide at 270°C and 50 atmospheres pressure. The presence of iridium, which was shown by Mössbauer spectroscopy to result in the formation of reduced iron, iridium, and iron iridium alloy in the pretreated catalyst, was found to increase the catalytic activity and also influence selectivity. However, the systematic variation of the iridium content which was shown by Mössbauer spectroscopy to determine the exact phase composition of the pretreated catalysts, appeared to have little effect on catalytic performance. The⁵⁷Fe Mössbauer spectra recorded from all the used catalysts showed the formation of large and small particle ε′-Fe_2.2C under the high pressure of the catalytic reaction. The results suggest that the formation of iron carbides on reduced iron, perhaps at the surface, and the adsorption of hydrogen on reduced iridium are important features of this catalytic system.     

Magnetic and quadrupolar studies of the iron storage overload in livers

Absorption⁵⁷Fe Mössbauer spectra, performed directly on tissues of liver with iron overload due to an excessive intestinal iron absorption or induced by hypertransfusional therapeutics, have pointed out a new high spin ferric storage iron besides the ferritin and hemosiderin. Mössbauer studies, carried out on ferritin and hemosiderin fractions isolated from normal and overloaded livers, show that this compound, only present in the secondary iron overload (transfusional pathway), seems characteristic of the physiological process which induces the iron overload.     

The South African industry use of Mössbauer spectroscopy to solve operational problems

South Africa is a country that is very rich in mineral resources but the use of Mössbauer spectroscopy to solve operational industrial problems is however very limited. In the Bushveld Igneous Complex the main minerals extracted from the ore are the platinum group metals and chromium, but secondary recovery of base metals such as nickel, copper and cobalt forms an integral part of the process. Losses of nickel in the slag can amount to about 4 % and subsequent a slag cleaning furnace is used to reduce the loss to less than 0.5 % nickel oxide. The Fe²⁺/Fe³⁺ ratio and mineralogy was used to determine the partial oxygen pressure in the furnaces and also the efficiency of the nickel recovery. From the Mössbauer results, augmented with XRD, SEM, EMP-WDX and MLA analyses, optimum conditions were determined to ensure minimum metal losses. The use of Mössbauer spectroscopy in the coal industry, to investigate mineral changes that occur during its use, is also of importance. The main minerals present in coal were determined with the aid of various techniques, such as Mössbauer, XRD, SEM and HR-TEM, with the major iron minerals found to be pyrite, illite, ankerite and jarosite. A large quantity of coal is used to produce syngas via gasification plants for the production of synthetic fuels. The change of the mineral matter during gasification was studied and the changes occurring during the gasification process were followed. The syngas produced, is further treated by means of the Fischer–Tropsch process where an iron catalyst is incorporated in the process. The usefulness and fouling of the catalyst is being studied with the aid of Mössbauer spectroscopy. The calibration of equipment to determine work hardening in mining equipment was also investigated and found to be a useful tool in industry. From the above few examples it is evident that, although used on a limited base, Mössbauer spectroscopy, augmented by various other spectroscopic tools, still ensures optimal recovery and production of the vast resource base of South Africa.     

In-situ identification of iron--zinc intermetallics in galvannealed steel coatings and iron oxides on exposed steel

Identification of all the compounds present in various coatings on steels is particularly difficult. Non-destructive, in-situ analysis is necessary if the fraction of each compound as well as its probable layering within the coating, is to be determined. Mössbauer spectroscopy is one valuable probe capable of uniquely identifying all iron compounds which form as coatings on steel and other iron alloy surfaces. To investigate a complete coating several criteria need to be considered. Removing the coating inevitably leaves a small and perhaps important component intact on the substrate. Therefore investigating the coating as it remains intact on the steel is important if complete identification of the iron compounds is to be made. This also preserves crystalline texture or preferred growth orientation within the coating to which the Mössbauer effect is sensitive. Mössbauer spectroscopy is a non-destructive technique which allows the integrity of the coating to be maintained during analysis. The combined transmission and scattering Mössbauer geometries generally result in accurate analysis of the coating composition. For the scattering geometry added information on compound layering is obtained if separate Mössbauer spectra are recorded using the re-emitted gamma rays as well as the conversion electrons and subsequently emitted X-rays. In-situ scattering Mössbauer spectroscopy has been used to characterize the iron--zinc alloys which form in the coatings of commercially produced corrosion resistant galvannealed sheet steel, a product of great interest to automotive producers. The results show that different amounts of four iron--zinc phases are present depending on the production conditions of the coating. The different phases are also distinctly layered. Mössbauer analyses of corrosion coatings formed on the surface of steels which have been exposed to different environments has also been undertaken. Materials include structural steels exposed for up to 25 years in marine, rural and industrial environments, and the interior surfaces of boiler pipes subjected to adverse chemical and temperature environments.     

Determination of the Hyperfine Parameters of Iron Silicides by Angle Dependent Conversion Electron Mössbauer Spectroscopy on Single Crystals

Hyperfine Interactions - The Mössbauer spectra of single crystals of the iron silicides ε-FeSi, β-FeSi2 and α-FeSi2 are collected by conversion electron Mössbauer...     

Mössbauer effect phase determination in iron oxide–polyaniline nanocomposites

Mössbauer effect spectroscopy and thermal analysis techniques were applied to characterize polyaniline composites successfully synthesized by embedding Fe oxide nanoparticles (about 10–13 nm) in a polymeric matrix in the presence of dodecyl benzene sulfonic acid and HCl (dopant). Thermal techniques provided quantitative information on iron oxide content and on polyaniline stability and transformations. Mössbauer results indicated that for the whole studied composition range, 3.4 to 100 iron oxide wt.%, composites hold maghemite particles. A preliminary study of the conductivity of the nanocomposites was performed. The largest conductivity was observed for a 8 wt.% maghemite composite where all particles are magnetically unblocked at room temperature within the Mössbauer time window.     

Air pollution studies by Mössbauer spectroscopy

Magnetic properties of industrial and urban aerosols which were collected in various places in Poland are studied by Mössbauer spectroscopy. Data concerning the concentration of iron in atmospheric air and the size of iron containing particles as determined by Mössbauer spectroscopy is discussed.     

Seasonal variations of iron concentration in atmospheric aerosols

Mössbauer spectroscopy has been used for determination of the chemical composition of iron containing particles of atmospheric aerosol. Seasonal varlations of the iron concentration in the atmosphere were investigated. The Mössbauer technique allowed to suggest the possible sources of iron in the air.