R. A. Brand

Metal-doped sodium aluminium hydrides as potential new hydrogen storage materials

Abstract Thermodynamics and kinetics of the reversible dissociation of metal-doped NaAlH 4 as a hydrogen (or heat) storage system have been investigated in some detail. The experimentally determined enthalpies for the first (3.7 wt% of H) and the second dissociation step of Ti-doped NaAlH 4 (3.0 wt% H) of 37 and 47 kJ/mol are in accordance with low and medium temperature reversible metal hydride systems, respectively. Through variation of NaAlH 4 particle sizes, catalysts (dopants) and doping procedures, kinetics as well as the cyclization stability within cycle tests have been substantially improved with respect to the previous status [B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253–254 (1997) 1]. In particular, using combinations of Ti and Fe compounds as dopants, a cooperative (synergistic) catalytic effect of the metals Ti and Fe in enhancing rates of both de- and rehydrogenation of Ti/Fe-doped NaAlH 4 within cycle tests, reaching a constant storage capacity of ∼4 wt% H 2 , has been demonstrated. By means of 57 Fe Mossbauer spectroscopy of the Ti/Fe-doped NaAlH 4 before and throughout a cycle test, it has been ascertained that (1) during the doping procedure, nanosize metallic Fe particles are formed from the doping agent Fe(OEt) 2 and (2) already after the first dehydrogenation, the nanosize Fe particles with NaAlH 4 present are probably transformed into an Fe–Al-alloy which throughout the cycle test remains practically unchanged.

The crucial role of particle morphology in the magnetic properties of haematite

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and disk shaped) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4, 7.4, 0.2, and 0.12 μm, respectively) were determined by transmission electron microscopy combined with electron diffraction. The haematite samples were studied by transmission Mossbauer spectroscopy in the temperature range 4.2–300 K. In all cases, a weak ferromagnetic (WF) phase was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic (AF) phase below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology.

A cubic approximant of the icosahedral phase in the (Al - Si) - Cu - Fe system

We present a stable simple cubic structure in the quaternary system (Al - Si) - Cu - Fe with the composition which corresponds to the p/q = 1/1 cubic approximant of the icosahedral Al - Cu - Fe phase. This cubic structure, with lattice parameter a = 12.330 A and density , contains roughly 135 atoms per unit cell. Preliminary studies of NMR and ME spectra as well as electric conductivity properties strongly suggest that this alloy shares most of the basic structural and physical properties of the parent icosahedral phase I-(Al - Cu - Fe).

Ferrimagnetic to spin glass transition in the mixed spinel Mg1+tFe2-2tTitO4: a Mossbauer and DC susceptibility study

Mossbauer and DC susceptibility studies are reported on the mixed spinel Mg1+tFe2-2tTitO4 at various temperatures and in an external magnetic field. Results are obtained indicating that on increasing dilution t, the collinear ferrimagnetic phase breaks down before reaching the ferrimagnetic percolation limit as a result of the presence of competing exchange interactions. In this dilution region there is a second transition, at Tf, to a spin-glass-like (SGL) state, well below the ferrimagnetic ordering temperature TN. The lower limit of re-entrant ferrimagnetism is determined by observing the spontaneous spin canting which occurs in the presence of the external magnetic field on going into the SGL state. At higher dilution a transition to a pure spin-glass state occurs. The results are discussed in terms of the model proposed by Villain (1979) based on the competition between dilution and frustration in the spinel lattice. The re-entrant ferrimagnetism found in this insulating system with near-neighbour interactions is similar to the re-entrant ferromagnetism found in some metallic alloys, and the authors propose a canting order parameter for the low-temperature state as in the metallic case.

Magnetic hyperfine fields near the Fe / Cr (100, 110) interface studied by CEMS

Abstract Magnetic hyperfine (hf) fields near the Fe/Cr-interface in epitaxial thin film structures with (110)- and (100)-orientation were analyzed by means of in situ conversion electron Mossbauer spectroscopy (CEMS). The hf field at room temperature in the first Fe-monolayer (ML) at the interface is strongly reduced to 22.0 T (110) and 20.9 T (100) compared with the Fe-bulk value of 33.4 T. In contrast, in the second and third Fe-monolayer with respect to the interface the hf field was enhanced by 0.3 T to 33.7 for both film orientations.

Solution-processed oxide semiconductor SnO in p-channel thin-film transistors

A p-type inorganic oxide semiconductor, tin monoxide (SnO), is developed by a solution process. SnO thin-film transistors (TFTs) in the p-channel enhancement mode are fabricated by spin-coating a precursor solution followed by postannealing, showing a highest field-effect mobility of 0.13 cm2 V−1 s−1, threshold voltage of −1.9 V, and on/off drain current ratio of 85.

The sign of the EFG in semiconducting and grain-oriented high-Tc YBa2(Cu1−χ57Feχ)3Oy by Mössbauer spectroscopy

Abstract The structure of both the orthorhombic (y→7, high Tc) and tetragonal (y→6, semiconducting) YBa2Cu3Oy phases includ es two Cu sites, denoted Cu1 and Cu2. From NQR/NMR measurements [1] on the high-Tc sample the ratio of 63Cu quadrupole frequencies ν(Cu2) /ν(Cu1) ⋍ 1.43 was determined. The results were consistent with the structural hypothesis that the asymmetry parameter η and the orientation of Vzz obey the following: Cu2 (η⋍0, Vzz|c axis); Cu1 (η⋍1). These conclusions have been checked, and the sign of the EFG determined, using Mossbauer spectroscopy from 57Fe-doped semiconducting and grain-oriented high-Tc samples. We have shown that for y=6.9: (1) the 57Fe splitting ΔE Q (2)/ ΔE Q (1)⋍1.39 , consistent with the above NQR/ NMR results; (2) that for Fe on Cu2 sites, the large line intensity asymmetry shows that the EFG is negative, and implies that η is small (η⋍0 and Vzz|c); (3) that for Fe on site Cu1, the symmetrical doublet implies η⋍1 (and so no EFG sign). For y=6.0: (1) there is a large decrease in ΔEQ(2), and (2) Vzz(2) is now positive. Thus the sign of the EFG for 57Fe on Cu2 sites different for the superconducting and the semiconducting states.

Intercalation‐Driven Reversible Control of Magnetism in Bulk Ferromagnets

An extension in magnetoelectric effects is proposed to include reversible chemistry-controlled magnetization variations. This ion-intercalation-driven magnetic control can be fully reversible and pertinent to bulk material volumes. The concept is demonstrated for ferromagnetic iron oxide where the intercalated lithium ions cause valence change and partial redistribution of Fe(3+) cations yielding a large and fully reversible change in magnetization at room temperature.

Evidence for enhanced ferromagnetism in an iron-based nanoglass

The possibility to synthesize bulk amorphous materials with an internal nanostructure—nanoglasses—leads to yet another class of materials potentially with modified properties. Here, evidence is presented that the nanoglass model system Fe90Sc10 exhibits enhanced magnetic properties: it is shown that this nanoglass (prepared by cold compaction of glassy nanospheres) is a ferromagnet at ambient temperature although the isolated nanospheres are paramagnetic. Structural studies reveal that it consists of glassy nanospheres connected by regions with reduced atomic density. The ferromagnetism is explained by the presence of such regions of low atomic density.

On the Mössbauer site assignment in YBa2(Cu1−xFex)3Oy

The assignment of the four57Fe Mössbauer sites which are observed in Fe-substituted YBa2Cu3Oy is presented. We present a systematic study of highly-controlled samples with low Fe-concentration over the whole oxygen concentration range. Two sites are found to dominate in the limit of high oxidation (y→7), indentified as Fe4+ on the two crystallographic Cu sites: Cu(1) and Cu(2). These are replaced at lower oxidation (y→6) by two others, which we identify with Fe3+ on these same two sites. Consistency is found with the sign of the electric field gradient (EFG), the asymmetry parameter η and the isomer shiftIS for this model. The magnetic structure in the limit y→6 shows changes in the antiferromagnetic (AF) structure dependent on iron concentration. Mössbauer and spin-echo NMR results show a change in the Cu(2) AF layer sequence at low temperatures, without any magnetic moments on the Cu(1) chain sites.