E. Jartych

Synthesis and characterization of goethite and goethite-hematite composite: experimental study and literature survey

Aging of synthetic goethite at 140 degrees C overnight leads to a composite material in which hematite is detectable by Mössbauer spectroscopy, but X-ray diffraction does not reveal any hematite peaks. The pristine point of zero charge (PZC) of synthetic goethite was found at pH 9.4 as the common intersection point of potentiometric titration curves at different ionic strengths and the isoelectric point (IEP). For the goethite-hematite composite, the common intersection point (pH 9.4), and the IEP (pH 8.8) do not match. The electrokinetic potential of goethite at ionic strengths up to 1 mol dm(-3) was determined. Unlike metal oxides, for which the electrokinetic potential is reversed to positive over the entire pH range at sufficiently high ionic strength, the IEP of goethite is rather insensitive to the ionic strength. A literature survey of published PZC/IEP values of iron oxides and hydroxides indicated that the average PZC/IEP does not depend on the degree of hydration (oxide or hydroxide). Our material showed a higher PZC and IEP than most published results. The present results confirm the allegation that electroacoustic measurements produce a higher IEP than the average IEP obtained by means of classical electrokinetic methods.

X-ray diffraction, magnetization and Mössbauer studies of nanocrystalline Fe–Ni alloys prepared by low- and high-energy ball milling

Abstract Fe–Ni alloys were prepared both by low- and high-energy ball milling processes. Structure and magnetic properties were studied by using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy and magnetization measurements. Mechanical treatment influenced the magnetic properties of Fe–Ni alloys as compared with the equilibrium alloys. Reduction of grain size resulted in the increase of magnetization. Invar anomaly for 35 at% Ni was not detected.

Local atomic order in nanocrystalline Fe-based alloys obtained by mechanical alloying

Abstract Using the 57 Fe Mossbauer spectroscopy, a local atomic order in nanocrystalline alloys of iron with Al, Ni, W and Mo has been determined. Alloys were prepared by mechanical alloying method. Analysis of Mossbauer spectra was performed on the basis of the local environment model in terms of Warren–Cowley parameters. It was shown that impurity atoms are not randomly distributed in the volume of the first and the second co-ordination spheres of 57 Fe nuclei and they form clusters.

Hyperfine interactions in nanocrystalline Fe-Al alloys

Nanocrystalline powder samples of Fe-30 at.% Al, Fe-40 at.% Al and Fe-50 at.% Al alloys were prepared by the mechanical alloying method. X-ray diffraction studies indicated that the solid solution with bcc structure was formed with increasing milling time for all investigated compositions. The magnetic ordering temperature of the nanocrystalline mechanically synthesized alloys was larger than that of the corresponding alloys on a micrometric scale. The magnetization curves as well as the Mossbauer spectra revealed that the Fe-Al alloys formed during the low energy ball milling process contained different magnetic phases.

Structural and magnetic study of crystalline Fe80Ni20 alloys with nanometer-sized grains

Abstract Samples of the Fe80Ni20 alloys were synthesized in the low energy conventional horizontal and high energy planetary ball mills. Both in the low and high energy processes, the solid bcc Fe(Ni) solutions with a similar structure are formed. The lattice constants are slightly larger than for the pure iron. Thermal stability of alloys was measured upto 1100 K. The magnetization and ferromagnetic Curie temperatures confirm a ferromagnetic ordering. The magnetic moment equal to 2.07 μB for the longest milling periods proves that the electronic structure of the crystalline alloy with nanometer-sized grains does not differ from that of polycrystalline samples. The distributions of the hyperfine magnetic fields revealed that an iron atom has one to two Ni atoms in the nearest neighborhood.

A Mossbauer study of electrodeposited Fe1-xCox alloys

The specimens of Fe1-xCox alloys obtained by electrodeposition onto graphite were tested by Mossbauer spectroscopy and X-ray diffraction. Two structural transformations from the body-centred cubic to the face-centred cubic and further to the hexagonal, as the Co concentration increases, were observed. How the hyperfine field depends on the occupancies of the 3d and 4s states was deduced. Changes in the preferred direction of domain magnetization caused by the variable Co concentration have also been observed.

Structure and hyperfine interactions in mechanosynthesized iron–molybdenum alloys

Abstract X-ray diffraction and Mossbauer spectroscopy were used to study the structure and hyperfine interactions in Fe 80 Mo 20 and Fe 50 Mo 50 prepared by mechanical alloying. Two solid solutions, i.e. Fe(Mo) and Mo(Fe) with b.c.c. lattice were formed during milling of Fe 80 Mo 20 . Mossbauer spectra revealed different magnetic arrangements in these solid solutions. In the case of Fe 50 Mo 50 no amorphisation was observed, as literature data suggest. During mechanosynthesis of Fe 50 Mo 50 a paramagnetic Mo(Fe) solid solution was probably formed.

Hyperfine Interactions in Amorphous Fe–Nb Alloys Prepared by Mechanical Alloying

Single-phase amorphous Fe52Nb48 and Fe36Nb64 alloys were synthesized during mechanical alloying (MA) processes. X-ray diffraction and Mössbauer spectroscopy were used to study the structure and hyperfine interactions of the milling products. Some differences in the formation mechanism of a single-phase amorphous alloy were observed between Fe52Nb48 and Fe36Nb64. However, in both cases the final product of MA processes were amorphous alloys in paramagnetic state.

Structure and magnetic properties of mechanosynthesized iron–tungsten alloys

Abstract X-ray diffraction, Mossbauer spectroscopy and magnetization measurements were used to study the mechanical alloying processes in the Fe–W system. Two solid solutions of W in Fe lattice [Fe(W)] and Fe in W lattice [W(Fe)] were produced during the early stage of the mechanical alloying. The results are inconsistent with those obtained by the calculations of phase diagram (CALPHAD) method. It was assumed that Vegard law and simple dilution model were approximately realized in the Fe-rich and W-rich regions. On the basis of this assumption and Mossbauer measurements, Fe(W) solid solution was recognized as a ferromagnetic phase while W(Fe) solid solution as a paramagnetic one. Nearest neighborhood of 57 Fe atoms was determined using the hyperfine interactions parameters.

Mössbauer and X-ray diffraction studies of mechanically alloyed Fe-Al

Powder samples of Fe25Al75 were prepared by the mechanical alloying method. Mössbauer effect, X-ray diffraction and DSC measurements indicate that Fe and Al crystalline powder transform into Fe-Al amorphous powder with increasing milling time. The X-ray diffraction patterns of the milled Fe25Al75 do not clearly show a sign of the existence of the intermetallic phases or Fe-Al solid solution. However, Mössbauer measurements reveal two sites with hyperfine magnetic fields 30.2 and 26.0 T. These sites form locally during the milling process and then they disappear.