M. Carbucicchio

Spin-order and magnetic properties of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite

Mössbauer and magnetic measurements have been carried out both on single crystals and polycrystalline samples of BaZn2Fe16O27 (Zn2-W) hexagonal ferrite. The saturation magnetization at 0 K and at room temperature turns out to be very high, that is, 123 and 79 Gauss·cm3/g, respectively. The results have been interpreted by assuming a local reversal or a weakening of the Fe3+ magnetic moments due to the perturbing action of Zn2+ ions. The magnetic anisotropy is confirmed to be uniaxial with an anisotropy field at room temperature of 12.500 Oe.

Mechanism of thermochemical growth of iron borides on iron

The mechanism of thermochemical growth of iron borides on iron was studied by analysing the products which form on samples placed in contact with B4C-base powder mixtures with different boronising potential. The analyses were carried out by means of metallographic and X-ray diffraction techniques. Three subsequent stages of growth were observed which explain the occurrence of typical properties of the thermochemically grown boride coatings, such as preferred crystallographic orientations and textures, as well as mechanical compactness and hardness, which play a determining role in the tribological behaviour of borided components.

Surface and bulk redox processes in iron-molybdate-based catalysts

Abstract The bulk reduction process of iron-molybdate-based catalysts as a function of temperature has been investigated by Mossbauer measurements on pure Fe2(MoO4)3 and Fe2(MoO4)3 · MoO3 which had been held in a nitrogen-methanol stream for different times and temperatures. The reduction takes place at temperatures T ⩾ 230 °C; the whole catalyst participates in the reaction and its reduced form has been proved to be β-FeMoO4. The reversibility of the process has also been studied by Mossbauer measurements carried out on the catalysts which were prereduced and then held in an oxygen atmosphere for different times and temperatures. The reduced catalyst can be completely reoxidized and this process occurs at T ⩾ 270 °C with a rate which increases with temperature. These data are discussed together with catalytic results obtained for the oxidation of methanol in the presence of the same catalysts in a stirred-tank flow reactor.

Synthesis and properties of α″‐Fe16N2 in magnetic particles

The α‘‐Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3. Acicular γ‐Fe2O3 was first reduced to α‐Fe under H2, then it was converted to γ‐austenite at 650–700 °C by nitriding using a NH3‐H2 mixed gas. A martensitic transformation to α’‐martensite occurred when the sample was quenched in liquid nitrogen. Finally, the α‘‐Fe16N2 phase was formed from the α’‐martensite by tempering in the temperature range 120–200 °C. In order to increase the extent of the martensitic transformation, acicular Fe2O3 particles with 10–15 at % MnO2 were prepared by oxidizing Fe++ and Mn++ in alkaline aqueous solution. When a quenched sample, which contains martensite, α‐Fe, and austenite, was tempered at 200 °C for different times, the magnetic moment first increased (transformation of α’‐martensite to α‘‐Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering. Using even longer tempering times, the magnetic moment decreased (decomposition of α‘‐Fe16N2 to α‐F...

Influence of the Cation Distribution on the Magnetization of Y-type Hexagonal Ferrites

Mössbauer absorption and magnetization measurements have been made in Mg2−Y and Co2−Y ferrites. The magnetizations of the various iron sublattices did not show any marked difference in Mg2−Y, whilst in the case of Co2−Y three different behaviours of the sublattice magnetizations have been detected.Low-temperature magnetization measurements in Mg2Y gave evidence of a noncollinear magnetic order. The different magnetic properties of these compounds have been explained on the ground of a preferential occupation by Mg2+ and Co2+ ions of the inner octahedral sites of the T crystallographic block. The presence in these sites of a non-magnetic ion such as Mg2+ is responsible for the peculiar magnetic order observed in Mg2−Y.

Magnetism of α ″‐Fe16N2 (invited)

The metastable α‘‐Fe16N2 phase may have a magnetic moment up to 50% higher than that of pure bulk α‐Fe. This article addresses the following issues. (i) Can epitaxial films of α‘‐Fe16N2 be prepared phase pure? Yes, but there are some doubts. (ii) Can powders of α‘‐Fe16N2 be prepared phase pure? Not yet. (iii) Is the Mossbauer spectrum due to α‘‐Fe16N2, to martensite, or to something else? Most assign it to α‘‐Fe16N2. (iv) What is the specific saturation magnetic moment of α‘‐Fe16N2? Some claim it is close to that of α‐Fe, most claim that it is much larger. (v) Is the high moment due to α‘‐Fe16N2, or to some other phase?

Surface Mössbauer analysis of 38NCD4 steel ion implanted with nitrogen

Surface Mossbauer measurements have been carried out for 38NCD4 steel samples, ion implanted with nitrogen at different doses and currents. It has been found that Fe‐N compounds form in a wide range of compositions as aggregates in the steel matrix: from the unaffected base metal to the external surface, steel layers follow one upon the other in which first Fe2+yN with 0.3 ⩽ y < 2.7, then Fe2+xN with 0 ⩽x < 0.3, and finally austenite containing ∼10 at. % nitrogen, predominantly form. Dose decrease decreases the thickness of the above layers, while current variations produce differences in the relative amounts of the various Fe‐N compounds.

On the morphology of thermochemically produced Fe2B/Fe interfaces

Etude de la formation des interfaces Fe 2 B/Fe dans le cas de systemes differant fortement en reactivite thermochimique. On peut considerer differents cas suivant la reactivite fer-bore et la morphologie revetement-substrat, qui dependent de la composition du milieu de boruration

Mssbauer and metallographic analysis of borided surface layers on Armco iron

Armco iron samples boronized at 850 and 1000° C in crystalline boron powder have been studied. Scattering and transmission Mössbauer measurements, supported by optical and electron scanning metallography and by X-ray diffraction analysis, enabled the surface phases to be identified, the multi-layer structure of the coatings to be defined and the average thickness of each layer to be measured. In addition to an inner Fe2B layer and to an intermediate FeB layer, the presence of an outer layer of a third phase richer in boron that FeB has been ascertained in the boride coatings. The morphology of the reaction products and their mechanical consistency have also been examined and discussed.

The role of excess antimony as a promoter of activity and selectivity in selective oxidation in the FeSb system

Abstract The modifications of two iron-antimony oxides with SbFe ratios of 1 and 2 during calcination up 1000 °C and after catalytic tests (1-butene oxidation to butadiene) were studied by X-ray diffraction, infrared, and Mossbauer analyses. The results show that a nonstoichiometric (NS) rutile structure with excess antimony held within the structure itself is formed on both catalysts calcined up to 900 °C. The strong increase both in selectivity and activity in allylic oxidation showed by the catalyst with an Sb:Fe ratio of 2 is attributed to the formation of a NS-rutile that is stable during catalytic tests. It is suggested that the excess antimony held within the structure creates SbO double bonds which are responsible for the increase of selectivity and activity. The fundamental role of the method of preparation by coprecipitation in a basic medium is also suggested.