Interplay between Transport, Magnetism and Structural Properties of Transition Metal Oxides under High Pressure

The effect of external pressure on the electronic, magnetic and structural properties of two novel types of transition metal oxides (RNiO_3 and (La,Sr)CoO_3) that allows one to investigate the influence of charge ordering, spin-state transition, magnetic ordering and structure on the electrical transport and in particular the mechanism of the metal insulator (MI) transition. The applied experimental methods were electrical resistance, x-ray diffraction, 151^Eu nuclear resonance scattering, magnetization, neutron diffraction and K_beta x-ray emission spectroscopy. The major part of this thesis was devoted to the high pressure investigation of the RNiO_3 series (R = Sm, Eu, Y and Lu), in which the temperature-induced MI transition (at a temperature T_MI) is connected with an orthorhombic-monoclinic structural phase transition and simultaneous charge ordering of the Ni^3+ ions. At temperatures lower than T_MI, all these compounds undergo antiferromagnetic ordering. In all investigated compounds we find a pressure-induced insulator metal (IM) transition for 5.4 0.2) the conductivity and ferromagnetic coupling are suggested to be related to the double exchange of LS Co^4+ and intermediate-spin (IS, S = 1) Co^3+ states. We have investigated the effect of pressure on the electronic, magnetic and structural properties on a single crystal sample of conducting, ferromagnetic La_{0.82}Sr_{0.18}CoO_3. Contrary to the results reported on related systems, we find a transition from the conducting to an insulating state and a reduction of the magnetic ordering temperature T_C with increasing pressure while the lattice structure remains unchanged. The investigation of the effect of pressure on the Co magnetic moment both by magnetization measurements and Co K_beta x-ray emission spectroscopy prove that the pressure-induced metal insulator transition is driven by a gradual change of the spin-state of Co^3+ ions from magnetic IS to nonmagnetic LS state

MEASUREMENT OF DENDRITE GROWTH ON Al-Ni ALLOYS IN REDUCED GRAVITY

It is well known that the growth kinetics in metallic melts controls microstructure evolution. If the melt is cooled below its equilibrium melting temperature prior to solidification, the state of a metastable undercooled melt is created. An undercooled melt possesses an enhanced free energy that enables the liquid to choose solidification pathways into various metastable solids of properties being different to their stable counterparts. A very efficient method to undercool a metallic liquid is the application of containerless processing a liquid drop such that heterogeneous crystal nucleation on container walls is completely avoided. Electro-magnetic levitation is a power-full technique to produce a freely suspended drop without any contact to a solid or liquid medium with the extra benefit that it is accessible for direct observation of solidification far from equilib-rium by proper diagnostic means. Under terrestrial conditions, strong electromagnetic fields are needed to compensate the gravitational force. That, in turn, causes forced convection inside the liquid drop and influences mass and heat transport, and consequently, crystal growth in undercooled melts. If the reduced gravity environment is utilized the forces to compensate residual accelerations are several orders of magnitude smaller than the levitation force on Earth. In the present paper we report on results obtained in the Earth laboratory, during parabolic flight missions and during TEXUS 44 flight in 2008 using the TEMPUS facility for containerless processing of metals in space. The results are discussed within dendrite growth theory and give evidence for strong effects of gravitational driven effects in the solidification dynamics

Anomalous dendrite growth in undercooled melts of Al-Ni alloys

We have measured dendrite growth velocities as a function of undercooling on liquid Al-Ni alloys using electromagnetic levitation on Earth and in reduced gravity. In total, six differently concentrated alloys are investigated, one of them for comparative investigations during the sounding rocket mission TEXUS 44. While on the Ni-rich side of Al-Ni alloys growth velocity is increasing with increasing undercooling, Al-rich Al-Ni alloys show an unusual decrease of the growth velocity with increasing undercooling in the terrestrial investigations. The comparison of the results of complementary terrestrial and microgravity experiments suggests that the anomalous growth behavior of Al-rich Al–Ni alloys may be caused by fluid-flow related processes during non-equilibrium solidification of undercooled melts. Support by ESA within contract number 15236/02/NL/SH (NEQUISOL) and by the European Commission EC under contract FP6-500635-2 (IMPRESS) is gratefully acknowledged

Anomalous dendrite growth in undercooled melts of Al–Ni alloys in relation to results obtained in reduced gravity

Dendrite growth velocities are measured as a function of undercooling on Al–Ni alloys using electromagnetic levitation, both on Earth and in reduced gravity. Six alloys of various compositions are investigated, one of them for comparative investigations in reduced gravity. Whereas on the Ni-rich side, growth velocity increases with increasing undercooling, Al-rich alloys show an unusual decrease in growth velocity with increasing undercooling. Experiments in reduced gravity suggest that this anomalous behavior is caused by forced convection

Microstructural analysis of rapidly solidified particles of Al-Ni alloys

Particles of Al-Ni alloys with different compositions (Al – 50 wt% Ni and Al – 36 wt% Ni) were produced using a drop tube-impulse system, known as Impulse Atomization. The microstructure of these rapidly solidified particles was compared with those solidified in a DSC at low cooling rates (5 and 20 ̊C/minutes). Also, the effects of cooling rate on the microstructure and the phase formation of the rapidly solidified droplets were investigated using scanning electron microscope and neutron diffraction. Rietveld analysis was performed to estimate the phase fractions of Al3Ni2, Al3Ni and eutectic Al. The results were compared to those achieved from electromagnetic levitation under terrestrial and microgravity conditions (TEXUS 44). Effect of cooling rate and microgravity condition on the crystal structure of Al3Ni2 was also studied. It was shown that increasing cooling rate as a result of decreasing particle size or using helium as a cooling gas, rather than nitrogen, would result in a refined microstructure. From Rietveld analysis on neutron diffraction data, it was shown that the increasing cooling rate increases the weight fraction Al3Ni in Al – 36 wt% Ni, while it has an opposite effect in Al – 50 wt% Ni. Also, from Rietveld analysis studies, a striking difference between the samples solidified in the drop tube-impulse system and those produced in microgravity was observed. The former always contain eutectic aluminum, while the latter showed no sign of this element. Crystal structure studies on Al3Ni2 showed that increasing cooling rate changes the c/a ratio in Al – 50 wt% Ni and Al – 36 wt% Ni. It was found that in the sample with higher nickel content, increasing cooling rate increased the c/a ratio, while in the sample with lower nickel content, it showed opposite effect. This work is part of NEQUISOL project supported by ESA within contract 15236/02/NL/SH and CSA within contract number 9F007-08-0154 and SSEP Grant 2008. The authors thank Stefan Schneider for assistance in conducting the TEXUS experiments

Microstructure evolution in undercooled Al–8 wt%Fe melts: Comparison between terrestrial and parabolic flight conditions

Al–8 wt%Fe, a hypereutectic alloy, was studied under electromagnetic levitation (EML) solidification conditions in both terrestrial and reduced gravity conditions. The latter was carried out on the A300 aircraft using the TEMPUS facility. The solidified samples were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and neutron diffraction techniques. The results are interpreted in the light of the temperature–time measurements taken in situ during the solidification process in the EML. It is shown that both samples experienced some undercooling for the solidification of the primary Al–Fe intermetallic phase, which is likely AlmFe. The solidification path continues with the nucleation and growth of Al13Fe4 followed by primary a-Al. These last two phases do not seem to show any measureable undercooling and recalescence events. Finally, the metastable AlxFe (where x = 5) nucleates starting with the formation of eutectic. This metastable intermetallic continues the eutectic growth as Al13Fe4. The morphology differences of the intermetallics growing under terrestrial and reduced gravity conditions are clear with acicular morphology for the former and a star like morphology for the latter. The primary a-Al has a clear strong textured structure in the reduced gravity sample, while a weak one is observed in the terrestrially processed sample. The difference in texture is attributed to the weaker fluid flow occurring in the droplet under reduced gravity conditions while the difference in the morphology of the primary intermetallic is attributed to the higher cooling rate experienced by the reduced gravity sample compared to that for the terrestrially processed sample

Dendrite growth in undercooled melts of Al–Ni alloys solidified on Earth and under reduced gravity

Fluid flow decisively influences the heat and mass transport during solidification of melts and consequently the properties of the as-solidified materials. In this work we present investigations on the influence of convection on the non-equilibrium solidification of different undercooled Al-Ni alloy melts. The dendrite growth velocities in Al-Ni melts containerlessly processed by electromagnetic levitation have been measured on Earth and under the conditions of reduced gravity. While under terrestrial conditions strong electromagnetic fields are required for levitation against gravity, the fields necessary to compensate disturbing accelerations under reduced gravity conditions are smaller by orders of magnitude. Consequently, convective fluid flow induced by electromagnetic stirring effects is significantly decreased in experiments performed under reduced gravity. Comparative experiments on congruently melting Al50Ni50 alloys revealed that the dendrite growth velocities measured at small undercoolings in the electromagnetic positioning facility TEMPUS during parabolic flight campaigns are significantly lower as compared to terrestrial results [1]. At elevated undercoolings, when the growth velocity is exceeding the fluid flow velocity, the influence of convection becomes negligible. Finally, at very high undercoolings and growth velocities a transition from growth of the ordered superlattice structure of the intermetallic AlNi phase to solidification of a disordered superlattice structure due to the nonequilibrium effect of disorder trapping is observed for Al50Ni50 [2]. This transition is associated with a strong rise of the growth velocity. The growth velocity - undercooling relation measured on Earth and during parabolic flight, respectively, is well described by modelling of the dedrite growth using a sharp interface approach with and without considering convective fluid flow. While on the Ni-rich side of the Al-Ni phase diagram and around the equiatomic composition the growth velocity during non-equilibrium solidification is increasing with increasing undercooling, Al-rich Al-Ni alloys show an unusual decrease of the growth velocity with increasing undercooling in terrestrial investigations. The comparison with complementary microgravity experiments performed during the sounding rocket mission TEXUS 44 suggests that the anomalous growth behavior of Al-rich Al–Ni alloys may be caused by fluid-flow related processes [3]. Support by ESA within contract numbers 15236/02/NL/SH (NEQUISOL), by DLR Space Agency under contract number 50 WM 036, by Deutsche Forschungsgemeinschaft (DFG) under contract number HE1601/18 and by the European Commission EC under contract FP6-500635-2 (IMPRESS) is gratefully acknowledged. References: [1] S. Reutzel, H. Hartmann, P.K. Galenko, S. Schneider, and D.M. Herlach, Appl. Phys. Lett. 91, 041913 (2007). [2] H. Hartmann, D. Holland-Moritz, P.K. Galenko, and D.M. Herlach, Europhys. Lett. 87, 40007 (2009). [3] R. Lengsdorf, D. Holland-Moritz, and D.M. Herlach, Scripta Materialia 62, 365 (2010)

Containerless Solidification and Characterization of Industrial Alloys (NEQUISOL)

Containerless solidification using electromagnetic levitator, gas atomization and an instrumented drop tube, known as impulse atomization is investigated. Effect of primary phase undercooling on dendrite growth velocity of Al-Ni alloys under terrestrial and reduced-gravity condition is discussed. It is shown that with increasing undercooling in the Ni-rich alloys the growth velocity increases, whereas in the Al-rich alloys the growth velocity decreases. However, the Al-rich alloy in microgravity shows similar behavior to that of Ni-rich alloys. Furthermore, the effect of cooling rate on the phase fractions, metastable phase formation and Al3Ni2 lattice parameter of impulse-atomized Al-Ni alloys is discussed. In addition, the effects of primary phase and eutectic undercooling on the microstructure of Al-Fe alloys are investigated. The TEM characterization on the eutectic microstructure of impulse-atomized Al-Fe powders with two compositions shows that the metastable AlmFe forms in these alloys. Also, the growth undercooling that the dendritic front experiences in the solidification of the droplet results in variation of dendrite growth direction from to . For Al-4 at%Fe, it is found that in the sample solidified in reduced-gravity and in the impulse-atomized droplets the primary intermetallic forms with a flower-like morphology, whereas in the terrestrial EML sample it has a needle like morphology. A microsegregation model for the solidification of Al-Ni alloys is presented that accounts for the occurrence of several phase transformations, including one or several peritectic reactions and one eutectic reaction