Gerhard Krexner | University of Vienna (original) (raw)
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Papers by Gerhard Krexner
Applied Physics A: Materials Science & Processing, 2002
Short-range order (SRO) in model γ-phases of several Ni-based superalloys is investigated by elas... more Short-range order (SRO) in model γ-phases of several Ni-based superalloys is investigated by elastic diffuse neutron scattering as a function of temperature. The composition of the single-crystalline samples has been modified by small amounts of additional elements like Mo, Re, Ru and W in order to study their influence on SRO, which is clearly dominated by peaks of the {1 1/2 0} type. The intensity of SRO scattering remains nearly unchanged up to 600 • C, above this temperature SRO decreases gradually and finally vanishes around 1000 • C. Correlation lengths do not change significantly with temperature.
Acta Physica Hungarica A) Heavy Ion Physics, 2006
In recent years, direct rapid soft tooling based on inserts manufactured additively by light reac... more In recent years, direct rapid soft tooling based on inserts manufactured additively by light reactive photopolymer curing has become a solid alternative to classic tooling based on subtractive machining of metals for prototyping and pilot production of polymer parts by injection moulding. Reported numbers of injection-moulded parts per insert are in the low to medium three-digit range. This, however, is based on using additive manufacturing for the production of both mould halves. It was found that combining a soft tooling insert on one mould half and a metal insert on the other half can increase insert lifetime to more than 10 000 parts in acrylonitrile butadiene styrene (ABS) with a negligible influence of insert wear on the dimensional stability of the injection-moulded parts. Photopolymers are known for their very low heat conductivity compared to metals and their sensitivity to elevated temperatures leading to degradation and negative effects on their mechanical properties. Con...
Physical Review B, 1987
ABSTRACT The recently reported direct observation of charge-density waves in potassium by neutron... more ABSTRACT The recently reported direct observation of charge-density waves in potassium by neutron diffraction was checked by high-resolution neutron-diffraction experiments. The observed diffraction pattern is not due to charge-density-wave satellites, but due to double-scattering processes. After eliminating these processes we searched for the charge-density-wave satellites in the theoretically predicted region, but found none. The intensity of any undetected satellites must be at least 2 orders of magnitude lower than predicted.
Physical Review B, 1994
A series of diffuse-neutron-scattering measurements on C60 powder material is presented. The resu... more A series of diffuse-neutron-scattering measurements on C60 powder material is presented. The results give evidence for the presence of significant static disorder below the 260-K ordering transition where besides the well-known simple cubic Bragg peaks elastic diffuse scattering is observed. Additional measurements on a small C60 crystal likewise show elastic diffuse scattering below the ordering temperature.
There is the widespread opinion within the research community of hydrogen storage materials, that... more There is the widespread opinion within the research community of hydrogen storage materials, that their nanocrystalline structure is a precondition for enhancement of the kinetics of hydrogen absorbtion/desorption, by means of enhanced diffusion of H2 along the grain boundaries. Examples for this behavior are presented, including ball milled, filed and/or SPD processed Mg, Mg- and Fe-alloys. SPD (Severe Plastic Deformation) represents a new method processing method to achieve bulk nanocrystalline materials. Considering more than one storage cycles in pure Mg, the kinetics and even the storage capacity is drastically decreased. One may find the reason in the strongly increased grain size because of the comparably high absorption/desorption temperature of 350°C. However, in the SPD processed Mg-alloys like ZK60, the kinetics and the storage capacity do not decrease with repeated absorption/desorption cycles, although the average grain size significantly increases. Thus it is concluded...
Materials Science Forum, 1992
Physical Review B, 1999
The compressibility of solid C 60 has been investigated by single-crystal neutron diffraction at ... more The compressibility of solid C 60 has been investigated by single-crystal neutron diffraction at pressures up to 5 kbar and temperatures between 70 and 300 K. Ar and He were used as pressure-transmitting mediums at temperatures above and below 200 K, respectively. In this way, a large temperature region could be explored while avoiding intercalation of gaseous species. At room temperature, a very sharp fcc-to-sc transition was observed proving fully hydrostatic loading conditions. Measurements slightly above the glass transition showed that the reduction of the bulk modulus due to the pressure-induced reorientation of the C 60 molecules ͑as monitored by the diffraction intensity͒ may be as large as 30%. Measurements below the glass transition showed that the bulk modulus of the pentagon phase is about 10% larger than that of the hexagon phase. ͓S0163-1829͑99͒01415-0͔
Materials Science Forum, 2001
Physical Review B, 1989
ABSTRACT
Physical Review B, 1987
ABSTRACT Phonon frequencies of longitudinal- and transverse-acoustic modes have been determined i... more ABSTRACT Phonon frequencies of longitudinal- and transverse-acoustic modes have been determined in the mixed-valence compound CeSn3 at 295 and 10 K. For comparison, temperature-induced phonon-frequency shifts were also measured in the integral-valence isostructural compound LaSn3. No evidence has been found that the mixed valence significantly alters the lattice dynamics of CeSn3 as a function of temperature. The difference between the thermal expansion coefficients of CeSn3 and LaSn3 below room temperature is in agreement with differences in the respective mode Grüneisen parameters of the two substances.
Hydrogen ordering in the solid-solution phase of LuD/sub x/ is investigated by diffuse-neutron-sc... more Hydrogen ordering in the solid-solution phase of LuD/sub x/ is investigated by diffuse-neutron-scattering techniques. The results show a linear ordering tendency of hydrogen atoms forming chains along the hexagonal c direction. Within the chains, second-neighbor tetrahedral sites are occupied. The scattering pattern in both the c/sup */-a/sup */ and c/sup */-b/sup */ reciprocal-lattice planes gives evidence for interchain correlations.
Neutron holography constitutes a novel technique to obtain structural information on an atomic sc... more Neutron holography constitutes a novel technique to obtain structural information on an atomic scale. It is based on the recording of the interference of neutron waves coherently scattered by atomic nuclei located on a crystal lattice with a suitable reference wave. Neutron holograms can be recorded using two complementary schemes, one of which is discussed in detail. In this approach, a point-like source of spherical neutron waves is required inside a single crystal which can be realized by making use of the large incoherent neutron scattering cross section of hydrogen nuclei, i.e.protons. When a sample containing substantial amounts of hydrogen is placed in a monochromatic beam of slow neutrons the protons will emit spherical neutron waves as a result of incoherent scattering. The interference between the undisturbed spherical wave field and that part of the wave which is scattered by neighboring atoms is recorded over a wide range of different orientations of the sample with resp...
Physical Review B, 1998
We present refined diffuse neutron scattering measurements performed in the long-range-ordered ph... more We present refined diffuse neutron scattering measurements performed in the long-range-ordered phase of potassium thiocyanate (TϽT c). The analysis of the data yields that the diffuse scattering is due to the presence of order-parameter fluctuations that appear as a precursor to the order-disorder phase transition. The diffuse intensity increases when approaching T c from below, whereas the width remains constant in the whole longrange-ordered phase. Molecular-dynamics simulations indicate that the size of the precursor clusters is stabilized by the order-parameter-strain interactions. ͓S0163-1829͑98͒06537-0͔ PHYSICAL REVIEW B PRB 58 O. BLASCHKO et al.
Physical Review B, 1986
ABSTRACT Lattice deformation in TaTx caused by transmutation-produced helium is investigated by n... more ABSTRACT Lattice deformation in TaTx caused by transmutation-produced helium is investigated by neutron-scattering techniques. The evolution of the position and width of Debye-Scherrer lines is followed in two polycrystalline samples over a period of more than two years. The observed relative lattice parameter expansion with helium concentration d(Δa/a)/dcHe=0.10 yields d(Δa/a)/dcHe=0.15 if corrected for tritium decay. Broadening of Debye-Scherrer lines indicates the presence of an evolving dislocation network. In addition, some long-wavelength phonons are investigated in a TaT0.05 single crystal. Comparison with a single crystal of pure Ta taken as reference shows that the phonon intensities in the tritium-charged sample are considerably reduced, however, without any noticeable change in energy and linewidth. Technologically significant, macroscopic fracture of both polycrystalline samples occurs already at helium concentrations below 1 at. %.
Scripta Materialia, 2010
ABSTRACT Samples of partially hydrogenated palladium were heavily deformed at low homologous temp... more ABSTRACT Samples of partially hydrogenated palladium were heavily deformed at low homologous temperatures by means of high-pressure torsion and subsequently investigated by differential scanning calorimetry along with scanning and transmission electron micros-copy. The results provide for the first time clear evidence for the formation of vacancy–hydrogen complexes from deformation-induced vacancies and for enhanced thermal stability of the nanostructure achieved under such conditions.
Physica B: Condensed Matter, 2006
The influence of instrumental effects has to be taken into account in assessing various limitatio... more The influence of instrumental effects has to be taken into account in assessing various limitations to the quality of holographic images obtained with thermal neutrons. In comparison with the traditional treatment of resolution effects in the field of neutron scattering this requires a somewhat different approach. The reconstruction of the position of a nucleus from a hologram leads to a spot-like distribution of finite width in real space and the knowledge of the functional shape of this distribution is of crucial importance. In this context, the choice of the angular range covered by the recording process, the wavelength as well as the wavelength spread of the neutrons have to be considered. In planning a particular experiment, a proper choice for each of these quantities has to be worked out. Instrumental resolution also defines a limiting distance in real space up to which neighbouring object nuclei can be distinguished. The mathematical background of the above considerations is discussed.
Neutron News, 2012
Owing to their wave properties, for most elementary particles including neutrons, almost all char... more Owing to their wave properties, for most elementary particles including neutrons, almost all characteristic phenomena of wave optics have been successfully demonstrated – except for the concept of holographic imaging. The main difficulty hindering a realization of this concept arises from the fact that holography requires either a point-like source or a point-like detector of neutrons having wavelengths comparable to
Le Journal de Physique IV, 1993
Sitering in compacted Molybdenum powders is investigated using small angle neutron scattering. Th... more Sitering in compacted Molybdenum powders is investigated using small angle neutron scattering. The scattering data permits to follow the evolution of the total pore surface F together with the total pore volume V in the course of the entire densilication process. Characteristic changes in the relation between F and V suggest that, in particular, final stages of sintering (when the porosity can already be considered isolated from each other) are governed by a universal relation between F and V [I].
Physical Review Letters, 1992
Applied Physics A: Materials Science & Processing, 2002
Short-range order (SRO) in model γ-phases of several Ni-based superalloys is investigated by elas... more Short-range order (SRO) in model γ-phases of several Ni-based superalloys is investigated by elastic diffuse neutron scattering as a function of temperature. The composition of the single-crystalline samples has been modified by small amounts of additional elements like Mo, Re, Ru and W in order to study their influence on SRO, which is clearly dominated by peaks of the {1 1/2 0} type. The intensity of SRO scattering remains nearly unchanged up to 600 • C, above this temperature SRO decreases gradually and finally vanishes around 1000 • C. Correlation lengths do not change significantly with temperature.
Acta Physica Hungarica A) Heavy Ion Physics, 2006
In recent years, direct rapid soft tooling based on inserts manufactured additively by light reac... more In recent years, direct rapid soft tooling based on inserts manufactured additively by light reactive photopolymer curing has become a solid alternative to classic tooling based on subtractive machining of metals for prototyping and pilot production of polymer parts by injection moulding. Reported numbers of injection-moulded parts per insert are in the low to medium three-digit range. This, however, is based on using additive manufacturing for the production of both mould halves. It was found that combining a soft tooling insert on one mould half and a metal insert on the other half can increase insert lifetime to more than 10 000 parts in acrylonitrile butadiene styrene (ABS) with a negligible influence of insert wear on the dimensional stability of the injection-moulded parts. Photopolymers are known for their very low heat conductivity compared to metals and their sensitivity to elevated temperatures leading to degradation and negative effects on their mechanical properties. Con...
Physical Review B, 1987
ABSTRACT The recently reported direct observation of charge-density waves in potassium by neutron... more ABSTRACT The recently reported direct observation of charge-density waves in potassium by neutron diffraction was checked by high-resolution neutron-diffraction experiments. The observed diffraction pattern is not due to charge-density-wave satellites, but due to double-scattering processes. After eliminating these processes we searched for the charge-density-wave satellites in the theoretically predicted region, but found none. The intensity of any undetected satellites must be at least 2 orders of magnitude lower than predicted.
Physical Review B, 1994
A series of diffuse-neutron-scattering measurements on C60 powder material is presented. The resu... more A series of diffuse-neutron-scattering measurements on C60 powder material is presented. The results give evidence for the presence of significant static disorder below the 260-K ordering transition where besides the well-known simple cubic Bragg peaks elastic diffuse scattering is observed. Additional measurements on a small C60 crystal likewise show elastic diffuse scattering below the ordering temperature.
There is the widespread opinion within the research community of hydrogen storage materials, that... more There is the widespread opinion within the research community of hydrogen storage materials, that their nanocrystalline structure is a precondition for enhancement of the kinetics of hydrogen absorbtion/desorption, by means of enhanced diffusion of H2 along the grain boundaries. Examples for this behavior are presented, including ball milled, filed and/or SPD processed Mg, Mg- and Fe-alloys. SPD (Severe Plastic Deformation) represents a new method processing method to achieve bulk nanocrystalline materials. Considering more than one storage cycles in pure Mg, the kinetics and even the storage capacity is drastically decreased. One may find the reason in the strongly increased grain size because of the comparably high absorption/desorption temperature of 350°C. However, in the SPD processed Mg-alloys like ZK60, the kinetics and the storage capacity do not decrease with repeated absorption/desorption cycles, although the average grain size significantly increases. Thus it is concluded...
Materials Science Forum, 1992
Physical Review B, 1999
The compressibility of solid C 60 has been investigated by single-crystal neutron diffraction at ... more The compressibility of solid C 60 has been investigated by single-crystal neutron diffraction at pressures up to 5 kbar and temperatures between 70 and 300 K. Ar and He were used as pressure-transmitting mediums at temperatures above and below 200 K, respectively. In this way, a large temperature region could be explored while avoiding intercalation of gaseous species. At room temperature, a very sharp fcc-to-sc transition was observed proving fully hydrostatic loading conditions. Measurements slightly above the glass transition showed that the reduction of the bulk modulus due to the pressure-induced reorientation of the C 60 molecules ͑as monitored by the diffraction intensity͒ may be as large as 30%. Measurements below the glass transition showed that the bulk modulus of the pentagon phase is about 10% larger than that of the hexagon phase. ͓S0163-1829͑99͒01415-0͔
Materials Science Forum, 2001
Physical Review B, 1989
ABSTRACT
Physical Review B, 1987
ABSTRACT Phonon frequencies of longitudinal- and transverse-acoustic modes have been determined i... more ABSTRACT Phonon frequencies of longitudinal- and transverse-acoustic modes have been determined in the mixed-valence compound CeSn3 at 295 and 10 K. For comparison, temperature-induced phonon-frequency shifts were also measured in the integral-valence isostructural compound LaSn3. No evidence has been found that the mixed valence significantly alters the lattice dynamics of CeSn3 as a function of temperature. The difference between the thermal expansion coefficients of CeSn3 and LaSn3 below room temperature is in agreement with differences in the respective mode Grüneisen parameters of the two substances.
Hydrogen ordering in the solid-solution phase of LuD/sub x/ is investigated by diffuse-neutron-sc... more Hydrogen ordering in the solid-solution phase of LuD/sub x/ is investigated by diffuse-neutron-scattering techniques. The results show a linear ordering tendency of hydrogen atoms forming chains along the hexagonal c direction. Within the chains, second-neighbor tetrahedral sites are occupied. The scattering pattern in both the c/sup */-a/sup */ and c/sup */-b/sup */ reciprocal-lattice planes gives evidence for interchain correlations.
Neutron holography constitutes a novel technique to obtain structural information on an atomic sc... more Neutron holography constitutes a novel technique to obtain structural information on an atomic scale. It is based on the recording of the interference of neutron waves coherently scattered by atomic nuclei located on a crystal lattice with a suitable reference wave. Neutron holograms can be recorded using two complementary schemes, one of which is discussed in detail. In this approach, a point-like source of spherical neutron waves is required inside a single crystal which can be realized by making use of the large incoherent neutron scattering cross section of hydrogen nuclei, i.e.protons. When a sample containing substantial amounts of hydrogen is placed in a monochromatic beam of slow neutrons the protons will emit spherical neutron waves as a result of incoherent scattering. The interference between the undisturbed spherical wave field and that part of the wave which is scattered by neighboring atoms is recorded over a wide range of different orientations of the sample with resp...
Physical Review B, 1998
We present refined diffuse neutron scattering measurements performed in the long-range-ordered ph... more We present refined diffuse neutron scattering measurements performed in the long-range-ordered phase of potassium thiocyanate (TϽT c). The analysis of the data yields that the diffuse scattering is due to the presence of order-parameter fluctuations that appear as a precursor to the order-disorder phase transition. The diffuse intensity increases when approaching T c from below, whereas the width remains constant in the whole longrange-ordered phase. Molecular-dynamics simulations indicate that the size of the precursor clusters is stabilized by the order-parameter-strain interactions. ͓S0163-1829͑98͒06537-0͔ PHYSICAL REVIEW B PRB 58 O. BLASCHKO et al.
Physical Review B, 1986
ABSTRACT Lattice deformation in TaTx caused by transmutation-produced helium is investigated by n... more ABSTRACT Lattice deformation in TaTx caused by transmutation-produced helium is investigated by neutron-scattering techniques. The evolution of the position and width of Debye-Scherrer lines is followed in two polycrystalline samples over a period of more than two years. The observed relative lattice parameter expansion with helium concentration d(Δa/a)/dcHe=0.10 yields d(Δa/a)/dcHe=0.15 if corrected for tritium decay. Broadening of Debye-Scherrer lines indicates the presence of an evolving dislocation network. In addition, some long-wavelength phonons are investigated in a TaT0.05 single crystal. Comparison with a single crystal of pure Ta taken as reference shows that the phonon intensities in the tritium-charged sample are considerably reduced, however, without any noticeable change in energy and linewidth. Technologically significant, macroscopic fracture of both polycrystalline samples occurs already at helium concentrations below 1 at. %.
Scripta Materialia, 2010
ABSTRACT Samples of partially hydrogenated palladium were heavily deformed at low homologous temp... more ABSTRACT Samples of partially hydrogenated palladium were heavily deformed at low homologous temperatures by means of high-pressure torsion and subsequently investigated by differential scanning calorimetry along with scanning and transmission electron micros-copy. The results provide for the first time clear evidence for the formation of vacancy–hydrogen complexes from deformation-induced vacancies and for enhanced thermal stability of the nanostructure achieved under such conditions.
Physica B: Condensed Matter, 2006
The influence of instrumental effects has to be taken into account in assessing various limitatio... more The influence of instrumental effects has to be taken into account in assessing various limitations to the quality of holographic images obtained with thermal neutrons. In comparison with the traditional treatment of resolution effects in the field of neutron scattering this requires a somewhat different approach. The reconstruction of the position of a nucleus from a hologram leads to a spot-like distribution of finite width in real space and the knowledge of the functional shape of this distribution is of crucial importance. In this context, the choice of the angular range covered by the recording process, the wavelength as well as the wavelength spread of the neutrons have to be considered. In planning a particular experiment, a proper choice for each of these quantities has to be worked out. Instrumental resolution also defines a limiting distance in real space up to which neighbouring object nuclei can be distinguished. The mathematical background of the above considerations is discussed.
Neutron News, 2012
Owing to their wave properties, for most elementary particles including neutrons, almost all char... more Owing to their wave properties, for most elementary particles including neutrons, almost all characteristic phenomena of wave optics have been successfully demonstrated – except for the concept of holographic imaging. The main difficulty hindering a realization of this concept arises from the fact that holography requires either a point-like source or a point-like detector of neutrons having wavelengths comparable to
Le Journal de Physique IV, 1993
Sitering in compacted Molybdenum powders is investigated using small angle neutron scattering. Th... more Sitering in compacted Molybdenum powders is investigated using small angle neutron scattering. The scattering data permits to follow the evolution of the total pore surface F together with the total pore volume V in the course of the entire densilication process. Characteristic changes in the relation between F and V suggest that, in particular, final stages of sintering (when the porosity can already be considered isolated from each other) are governed by a universal relation between F and V [I].
Physical Review Letters, 1992