Mihaela Albu - Academia.edu (original) (raw)

Papers by Mihaela Albu

Research paper thumbnail of 合金926オーステナイト系ステンレス鋼の空乏帯と粒界腐食【Powered by NICT】

Materials and Corrosion-werkstoffe Und Korrosion, 2017

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Research paper thumbnail of In-Situ Studies on the Formation and Tunable Properties of Nanoporous Metals Produced by Electrochemical Dealloying

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Research paper thumbnail of Sensitization of stable and as rolled annealed Alloy 625

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Research paper thumbnail of Characterization of Ni-nanostructures in porous silicon by SEM and TEM - more problems than results

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Research paper thumbnail of Ageing of Al-Mn-Cu-Be Alloys for Stimulating Precipitation of Icosahedral Quasicrystals

Metals

In this work, the ageing of some Al-Mn-Cu-Be alloys was investigated in the temperature range in ... more In this work, the ageing of some Al-Mn-Cu-Be alloys was investigated in the temperature range in which predominantly icosahedral quasicrystalline (IQC) precipitates can form. The alloys were cast into a copper mould, directly aged (T5 heat treatment) between 300 and 440 °C for different times. Afterwards, they were examined using scanning and transmission electron microscopy, X-ray diffraction and hardness testing. The main aim of the work was to determine the conditions at which a high number density of spherical icosahedral quasicrystalline precipitates can form. The highest number density of IQC precipitates was obtained at 300 °C after prolonged ageing. The spheroidal precipitates had a diameter less than 20 nm. The size of IQC precipitates increased with the increasing temperature, and in addition, decagonal quasicrystalline precipitates appeared. The time to maximum hardness decreased strongly with increasing ageing temperature. The IQC precipitates can form in a fairly broad ...

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Research paper thumbnail of Copper‐alumina nanocomposites derived from CuAlO 2 : Phase transformation and microstructural coarsening

Journal of the American Ceramic Society

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Research paper thumbnail of Organic coating on biochar explains its nutrient retention and stimulation of soil fertility

Nature communications, Oct 20, 2017

Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to ... more Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to address climate change and soil degradation by carbon sequestration, reducing soil-borne greenhouse-gas emissions and increasing soil nutrient retention. Biochar was shown to promote plant growth, especially when combined with nutrient-rich organic matter, e.g., co-composted biochar. Plant growth promotion was explained by slow release of nutrients, although a mechanistic understanding of nutrient storage in biochar is missing. Here we identify a complex, nutrient-rich organic coating on co-composted biochar that covers the outer and inner (pore) surfaces of biochar particles using high-resolution spectro(micro)scopy and mass spectrometry. Fast field cycling nuclear magnetic resonance, electrochemical analysis and gas adsorption demonstrated that this coating adds hydrophilicity, redox-active moieties, and additional mesoporosity, which strengthens biochar-water interactions and thus enh...

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Research paper thumbnail of Single grain analysis on a nanoscale in ZrO2:Al2O3nano-composites by means of high-resolution scanning transmission electron Microscopy

Materials Research Express

Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by micr... more Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by microwave plasma process synthesis and studied by high resolution scanning transmission electron microscopy. The ZrO2:Al2O3 nanocomposite samples showed a structure of randomly arranged, crystalline grains of both ZrO2 and Al2O3 with a size in the range of 3–5 nm. For each constituent, both cubic and monoclinic grains were observed. Similarly prepared, single phase Al2O3 nanoparticles were found to be completely amorphous. In the case of ZrO2, the crystallite size and the fraction of the monoclinic ZrO2 phase is noticeably reduced in the ZrO2:Al2O3 nanocomposite in comparison to single phase samples. The observation of ultra-small crystalline Al2O3 in the composite samples, is tentatively ascribed to a modification of the interface energy by the contact of Al2O3 and ZrO2 as well as a possible uptake of ZrO2 into the Al2O3 grains from the gas phase during synthesis.

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Research paper thumbnail of Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying

Langmuir, 2016

The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying ... more The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying was applied for tuning electrical resistance by surface charging. In the as-dealloyed state, a characteristic sign-inversion of the charging-induced resistance variation occurs, which can be associated with the electronic structure of PtO. After electrochemical reduction, the relative resistance variations of np-Pt of up to 58% could be generated by electrochemically induced adsorption and desorption, which was 1 order of magnitude larger compared with that of cluster-assembled nanocrystalline Pt. Although the maximum resistance variation was also higher than that of dealloyed nanoporous gold (np-Au), the resistance variation related to the imposed charge was reduced owing to the higher bulk resistance of Pt compared with that of Au. The sign-inversion behavior of the resistance could be recovered by re-oxidation.

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Research paper thumbnail of Gefügeauswirkungen und atomistische Vorgänge bei der Veredelung von Al-Si-Legierungen mit Sr, Ca und Yb

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Research paper thumbnail of Multimodal size distributions of ?' precipitates during continuous cooling of UDIMET 720 Li

Acta Mater, 2009

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Research paper thumbnail of Multimodal size distributions of y' precipitates during continuous cooling of UDIMET 720 Li

Acta Materialia, 2009

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Research paper thumbnail of EELS quantification of complex nitrides in a 12% Cr steel

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

In 9–12% Cr steels, nitrides (MX, M2X and modified Z-phase ((Cr,V,Nb,Fe)N)) are of special intere... more In 9–12% Cr steels, nitrides (MX, M2X and modified Z-phase ((Cr,V,Nb,Fe)N)) are of special interest because of their different contribution to the creep strength of the material. The changes in the chemical composition and their crystallography were investigated using transmission electron microscopy (TEM).

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Research paper thumbnail of Comprehensive analysis of precipitates in rich chromium steels by means of electron energy loss spectrometry spectrum imaging

Yejin Fenxi/Metallurgical Analysis

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Research paper thumbnail of Investigation of Ni and Co Deposition into Porous Silicon and the Influence of the Electrochemical Parameters on the Physical Properties

ECS Transactions, 2012

not Available.

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Research paper thumbnail of Interaction of Recrystallization and Precipitation during Hot Forming of Alloy 80A

Materials Science Forum, 2007

In the high temperature deformation window of the nickel base Alloy 80A the lower temperature reg... more In the high temperature deformation window of the nickel base Alloy 80A the lower temperature region during open die forging was examined with regard to the materials formability. For that purpose, hot compression samples were investigated by means of EBSD and TEM in order to look at recrystallization, precipitations and ductile damage as well as their reciprocal effects. Further a microstructure model was used, which calculates the materials strengthening, softening and the particle kinetics. A micro mechanical damage model of the effective stresses was coupled with the grain structure development in order to describe a retarded damage rate due to the ongoing recrystallization.

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Research paper thumbnail of Metallic Nanotubes in Porous Silicon

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Research paper thumbnail of Three dimensional quasi-regular arrangement of ferromagnetic nanostructures within porous silicon

ABSTRACT The presented hybrid material consists of a porous silicon matrix and precipitated metal... more ABSTRACT The presented hybrid material consists of a porous silicon matrix and precipitated metal nanostructures within the pores. The porous silicon is achieved by wet etching of a silicon wafer, offering straight pores grown perpendicular to the surface. Ferromagnetic metals are electrochemically deposited within the pores leading to a composite material merging the electronic properties of silicon and the magnetic behaviour of the metal precipitates. Considering the metal deposition, samples with various spatial distributions of the metal structures within the pores can be fabricated and also the geometry of the precipitates can be modified. Thus the magnetic behaviour of the samples can be tuned during the fabrication process. The physical properties of the system strongly depend on the structural features wherefore its precise investigation is of great importance.

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Research paper thumbnail of Modification of Eutectic Si in Al-Si Based Alloys

Materials Science Forum, 2014

The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To d... more The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To date, impurity-induced twinning mechanism and twin plane re-entrant edge mechanism are the well-accepted theories. However, neither IIT nor TPRE can be used to interpret all modification observations. Therefore, a re-consideration of modification mechanisms is still required. In this contribution, recent advances on the understanding the modification of eutectic Si are reviewed. Two different cases are highlighted. In the case of Sr, Na and Eu addition, eutectic Si was modified from a faceted to a fibrous morphology, which involves the formation of multiple Si twinning. In the case of Yb and Ca addition, eutectic Si was refined to a smaller size, but still maintained a plate-like morphology. The possible modification mechanism was thus discussed in terms of (i) adsorption of atoms at twin re-entrant edge, and (ii) segregation across {111}Si growth planes. Furthermore, solute entrainment o...

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Research paper thumbnail of Nanoscale voxel spectroscopy by simultaneous EELS and EDS tomography

Nanoscale, 2014

3D elemental maps and EELS and EDS spectrum volumes of an Al–Si-based alloy are reconstructed usi... more 3D elemental maps and EELS and EDS spectrum volumes of an Al–Si-based alloy are reconstructed using analytical electron tomography.

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Research paper thumbnail of 合金926オーステナイト系ステンレス鋼の空乏帯と粒界腐食【Powered by NICT】

Materials and Corrosion-werkstoffe Und Korrosion, 2017

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Research paper thumbnail of In-Situ Studies on the Formation and Tunable Properties of Nanoporous Metals Produced by Electrochemical Dealloying

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Sensitization of stable and as rolled annealed Alloy 625

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Characterization of Ni-nanostructures in porous silicon by SEM and TEM - more problems than results

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Ageing of Al-Mn-Cu-Be Alloys for Stimulating Precipitation of Icosahedral Quasicrystals

Metals

In this work, the ageing of some Al-Mn-Cu-Be alloys was investigated in the temperature range in ... more In this work, the ageing of some Al-Mn-Cu-Be alloys was investigated in the temperature range in which predominantly icosahedral quasicrystalline (IQC) precipitates can form. The alloys were cast into a copper mould, directly aged (T5 heat treatment) between 300 and 440 °C for different times. Afterwards, they were examined using scanning and transmission electron microscopy, X-ray diffraction and hardness testing. The main aim of the work was to determine the conditions at which a high number density of spherical icosahedral quasicrystalline precipitates can form. The highest number density of IQC precipitates was obtained at 300 °C after prolonged ageing. The spheroidal precipitates had a diameter less than 20 nm. The size of IQC precipitates increased with the increasing temperature, and in addition, decagonal quasicrystalline precipitates appeared. The time to maximum hardness decreased strongly with increasing ageing temperature. The IQC precipitates can form in a fairly broad ...

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Research paper thumbnail of Copper‐alumina nanocomposites derived from CuAlO 2 : Phase transformation and microstructural coarsening

Journal of the American Ceramic Society

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Research paper thumbnail of Organic coating on biochar explains its nutrient retention and stimulation of soil fertility

Nature communications, Oct 20, 2017

Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to ... more Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to address climate change and soil degradation by carbon sequestration, reducing soil-borne greenhouse-gas emissions and increasing soil nutrient retention. Biochar was shown to promote plant growth, especially when combined with nutrient-rich organic matter, e.g., co-composted biochar. Plant growth promotion was explained by slow release of nutrients, although a mechanistic understanding of nutrient storage in biochar is missing. Here we identify a complex, nutrient-rich organic coating on co-composted biochar that covers the outer and inner (pore) surfaces of biochar particles using high-resolution spectro(micro)scopy and mass spectrometry. Fast field cycling nuclear magnetic resonance, electrochemical analysis and gas adsorption demonstrated that this coating adds hydrophilicity, redox-active moieties, and additional mesoporosity, which strengthens biochar-water interactions and thus enh...

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Research paper thumbnail of Single grain analysis on a nanoscale in ZrO2:Al2O3nano-composites by means of high-resolution scanning transmission electron Microscopy

Materials Research Express

Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by micr... more Nano-particulate powders of Al2O3:ZrO2 composites and the pure constituents were prepared by microwave plasma process synthesis and studied by high resolution scanning transmission electron microscopy. The ZrO2:Al2O3 nanocomposite samples showed a structure of randomly arranged, crystalline grains of both ZrO2 and Al2O3 with a size in the range of 3–5 nm. For each constituent, both cubic and monoclinic grains were observed. Similarly prepared, single phase Al2O3 nanoparticles were found to be completely amorphous. In the case of ZrO2, the crystallite size and the fraction of the monoclinic ZrO2 phase is noticeably reduced in the ZrO2:Al2O3 nanocomposite in comparison to single phase samples. The observation of ultra-small crystalline Al2O3 in the composite samples, is tentatively ascribed to a modification of the interface energy by the contact of Al2O3 and ZrO2 as well as a possible uptake of ZrO2 into the Al2O3 grains from the gas phase during synthesis.

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Research paper thumbnail of Electrochemically Tunable Resistance of Nanoporous Platinum Produced by Dealloying

Langmuir, 2016

The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying ... more The extremely high surface-to-volume ratio of nanoporous platinum (np-Pt) produced by dealloying was applied for tuning electrical resistance by surface charging. In the as-dealloyed state, a characteristic sign-inversion of the charging-induced resistance variation occurs, which can be associated with the electronic structure of PtO. After electrochemical reduction, the relative resistance variations of np-Pt of up to 58% could be generated by electrochemically induced adsorption and desorption, which was 1 order of magnitude larger compared with that of cluster-assembled nanocrystalline Pt. Although the maximum resistance variation was also higher than that of dealloyed nanoporous gold (np-Au), the resistance variation related to the imposed charge was reduced owing to the higher bulk resistance of Pt compared with that of Au. The sign-inversion behavior of the resistance could be recovered by re-oxidation.

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Research paper thumbnail of Gefügeauswirkungen und atomistische Vorgänge bei der Veredelung von Al-Si-Legierungen mit Sr, Ca und Yb

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Multimodal size distributions of ?' precipitates during continuous cooling of UDIMET 720 Li

Acta Mater, 2009

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Research paper thumbnail of Multimodal size distributions of y' precipitates during continuous cooling of UDIMET 720 Li

Acta Materialia, 2009

Bookmarks Related papers MentionsView impact

Research paper thumbnail of EELS quantification of complex nitrides in a 12% Cr steel

EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008

In 9–12% Cr steels, nitrides (MX, M2X and modified Z-phase ((Cr,V,Nb,Fe)N)) are of special intere... more In 9–12% Cr steels, nitrides (MX, M2X and modified Z-phase ((Cr,V,Nb,Fe)N)) are of special interest because of their different contribution to the creep strength of the material. The changes in the chemical composition and their crystallography were investigated using transmission electron microscopy (TEM).

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Comprehensive analysis of precipitates in rich chromium steels by means of electron energy loss spectrometry spectrum imaging

Yejin Fenxi/Metallurgical Analysis

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Research paper thumbnail of Investigation of Ni and Co Deposition into Porous Silicon and the Influence of the Electrochemical Parameters on the Physical Properties

ECS Transactions, 2012

not Available.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Interaction of Recrystallization and Precipitation during Hot Forming of Alloy 80A

Materials Science Forum, 2007

In the high temperature deformation window of the nickel base Alloy 80A the lower temperature reg... more In the high temperature deformation window of the nickel base Alloy 80A the lower temperature region during open die forging was examined with regard to the materials formability. For that purpose, hot compression samples were investigated by means of EBSD and TEM in order to look at recrystallization, precipitations and ductile damage as well as their reciprocal effects. Further a microstructure model was used, which calculates the materials strengthening, softening and the particle kinetics. A micro mechanical damage model of the effective stresses was coupled with the grain structure development in order to describe a retarded damage rate due to the ongoing recrystallization.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Metallic Nanotubes in Porous Silicon

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Three dimensional quasi-regular arrangement of ferromagnetic nanostructures within porous silicon

ABSTRACT The presented hybrid material consists of a porous silicon matrix and precipitated metal... more ABSTRACT The presented hybrid material consists of a porous silicon matrix and precipitated metal nanostructures within the pores. The porous silicon is achieved by wet etching of a silicon wafer, offering straight pores grown perpendicular to the surface. Ferromagnetic metals are electrochemically deposited within the pores leading to a composite material merging the electronic properties of silicon and the magnetic behaviour of the metal precipitates. Considering the metal deposition, samples with various spatial distributions of the metal structures within the pores can be fabricated and also the geometry of the precipitates can be modified. Thus the magnetic behaviour of the samples can be tuned during the fabrication process. The physical properties of the system strongly depend on the structural features wherefore its precise investigation is of great importance.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Modification of Eutectic Si in Al-Si Based Alloys

Materials Science Forum, 2014

The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To d... more The paper provides a new insight into the modification of eutectic Si in Al-Si based alloys. To date, impurity-induced twinning mechanism and twin plane re-entrant edge mechanism are the well-accepted theories. However, neither IIT nor TPRE can be used to interpret all modification observations. Therefore, a re-consideration of modification mechanisms is still required. In this contribution, recent advances on the understanding the modification of eutectic Si are reviewed. Two different cases are highlighted. In the case of Sr, Na and Eu addition, eutectic Si was modified from a faceted to a fibrous morphology, which involves the formation of multiple Si twinning. In the case of Yb and Ca addition, eutectic Si was refined to a smaller size, but still maintained a plate-like morphology. The possible modification mechanism was thus discussed in terms of (i) adsorption of atoms at twin re-entrant edge, and (ii) segregation across {111}Si growth planes. Furthermore, solute entrainment o...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Nanoscale voxel spectroscopy by simultaneous EELS and EDS tomography

Nanoscale, 2014

3D elemental maps and EELS and EDS spectrum volumes of an Al–Si-based alloy are reconstructed usi... more 3D elemental maps and EELS and EDS spectrum volumes of an Al–Si-based alloy are reconstructed using analytical electron tomography.

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