Siyuan Zhang | Max Planck Institute for Iron Research (original) (raw)
Papers by Siyuan Zhang
Journal of Physics: Condensed Matter, 2015
Scripta Materialia, 2020
A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composit... more A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composite, in which each soft 100 nm Cu or Zr layer is surrounded on both sides by several hard 10 nm Cu/Zr bilayers. This design aims to impose a full geometrical constraint on all of the soft layers. Micropillar compression tests demonstrate that the composite deforms in a compatible fashion among the layers, in which no extrusion of the soft layers occurs. An elevated strength of 730 MPa is achieved in the composite compared with the strength prediction based on the linear rule of mixtures.
Advanced Functional Materials
Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for... more Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>10 12 cm-2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na 0.025 Eu 0.03 Pb 0.945 Te is demonstrated at a moderate dislocation density of 1 × 10 10 cm-2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon-scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge-type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal-insulating applications.
JOURNAL OF PHYSICS-CONDENSED MATTER, 2014
High energy-resolution fluorescence-detected X-ray absorption spectroscopy and density functional... more High energy-resolution fluorescence-detected X-ray absorption spectroscopy and density functional theory calculations were used to investigate the local bonding and electronic structure of Sc in epitaxial wurtzite-structure Sc x Ga 1-x N films with x ≤ 0.059. Sc atoms are found to substitute for Ga atoms, accompanied by a local distortion involving an increase in the internal lattice parameter u around the Sc atoms. The local bonding and electronic structure at Sc are not affected strongly by the strain state or the defect microstructure of the films. These data are consistent with theoretical predictions regarding the electro nic structure of dilute Sc x Ga 1-x N alloys.
Microscopy (Oxford, England), Jan 9, 2017
Multivariate analysis is a powerful tool to process spectrum imaging datasets of electron energy ... more Multivariate analysis is a powerful tool to process spectrum imaging datasets of electron energy loss spectroscopy. Most spatial variance of the datasets can be explained by a limited numbers of components. We explore such dimension reduction to facilitate quantitative analyses of spectrum imaging data, supervising the spectral components instead of spectra at individual pixels. In this study, we use non-negative matrix factorization to decompose datasets from Fe2O3 thin films with different Sn doping profiles on SnO2 and Si substrates. Case studies are presented to analyse spectral features including background models, signal integrals, peak positions and widths. Matlab codes are written to guide microscopists to perform these data analyses.
Materials Science Forum
An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size ... more An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size by severe plastic deformation (SPD). Microstructure evolution during SPD and mechanical behaviour after SPD of the alloy were characterized by electron microscopy and tensile, compression as well as nanoindentation tests. The influence of η' precipitates on microstructure and mechanical properties of ultrafine grained Al-Zn-Mg alloy is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.
Nano Letters, 2015
Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high... more Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.
Physical Review Letters, 2013
Aberration-corrected scanning transmission electron microscopy was used to investigate the core s... more Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in undoped GaN films with both high and low dislocation densities, and in a comparable high dislocation density Mg-doped GaN film. All a-type dislocations in all samples have a 5/7-atom core structure. In contrast, most (a+c)-type dislocations in undoped GaN dissociate due to local strain variations from nearby dislocations. In contrast, Mg doping prevents (a+c)-type dislocation dissociation. Our data indicate that Mg affects dislocation cores in GaN significantly.
Physical Review Letters, 2013
The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by ab... more The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5:4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeN x. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5:4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5:4 lattice-matching growth of GaN.
physica status solidi (a), 2012
... Status Solidi B 238, 127 (2003). [14] V. Ranjan, S. Bin-Omran, D. Sichuga, RS Nichols, L. Bel... more ... Status Solidi B 238, 127 (2003). [14] V. Ranjan, S. Bin-Omran, D. Sichuga, RS Nichols, L. Bellaiche, and A. Alsaad, Phys. Rev. B 72, 085315 (2005). ... Lett. 78, 2891 (2001). [18] P. de Rouffignac, APYousef, KH Kim, and RG Gordon, Electrochem. Solid-State Lett. 9, F45 (2006). ...
Journal of Applied Physics, 2013
Acta Materialia, 2010
The paper presents a new extrusion method, alternate forward extrusion, in which the punch was re... more The paper presents a new extrusion method, alternate forward extrusion, in which the punch was replaced with double-split structures so as to achieve the grain refinement for material near the interface of double-split structures. The results showed that the unique loading mode made metal flow sequence and behavior significantly changed during alternate forward extrusion. The additional shear deformation produced by the double-split punch structures resulted in a refining effect on the microstructure of the blank, which was then further refined during flow through the die orifice owing to shear deformation. Compared with the conventional extrusion, the recrystallization process in the alternate forward extrusion process produced grains that were smaller and more homogeneous in size. The recrystallization process was more abundant, and the dislocation density was significantly increased. It can be concluded that the alternate forward extrusion process could achieve fine-grained strengthening, which provided technical support and scientific guidance for the engineering application of magnesium alloy extrusion forming technology.
Journal of Applied Physics, 2013
Sc-based III-nitride alloys were studied using Density Functional Theory with special quasi-rando... more Sc-based III-nitride alloys were studied using Density Functional Theory with special quasi-random structures and were found to retain wide band gaps which stay direct up to x = 0.125 (Sc x Al 1-x N) and x = 0.375 (Sc x Ga 1-x N). Epitaxial strain stabilization prevents spinodal decomposition up to x = 0.3 (Scx Al 1-x N on GaN) and x = 0.24 (Sc x Ga 1-x N on GaN), with critical thicknesses for strain relaxation ranging from 3 nm to near-infinity. The increase in Sc content introduces compressive in-plane stress with respect to AlN and GaN, and leads to composition-and stress-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in Sc x Ga 1-x N at x ≈ 0.625.
ACS Applied Materials & Interfaces
Capturing and converting waste heat into electrical power through thermoelectric generators based... more Capturing and converting waste heat into electrical power through thermoelectric generators based on the Seebeck effect is a promising alternative energy source. Among thermoelectric compounds, PbTe can be alloyed and form precipitates by aging at elevated temperatures, thus reducing thermal conductivity by phonon scattering. Here, PbTe is alloyed with Ag to form Ag-rich precipitates having a number density controlled by heat treatments. We employ complementary scanning transmission electron microscopy and atom probe tomography to analyze the precipitate number density and the PbTe-matrix composition. We measure the temperature-dependent transport coefficients and correlate them with the microstructure. The thermal and electrical conductivities, as well as the Seebeck coefficients, are found to be highly sensitive to the microstructure and its temporal evolution, e.g., the number density of Ag-rich precipitates increases by ca. 3 orders of magnitude and reaches (1.68 ± 0.92) × 10 m upon aging at 380 °C for 6 h, which is pronounced by reduction in thermal conductivity to a value as low as 0.85 W m K at 300 °C. Our findings will help to guide predictive tools for further design of materials for energy harvesting.
Scientific Reports
Nanolayered metallic composites are much stronger than pure nanocrystalline metals due to their h... more Nanolayered metallic composites are much stronger than pure nanocrystalline metals due to their high density of hetero-interfaces. However, they are usually mechanically instable due to the deformation incompatibility among the soft and hard constituent layers promoting shear instability. Here we designed a hybrid material with a heterogeneous multi-nanolayer architecture. It consists of alternating 10 nm and 100 nm-thick Cu/Zr bilayers which deform compatibly in both stress and strain by utilizing the layers' intrinsic strength, strain hardening and thickness, an effect referred to as synergetic deformation. Micropillar tests show that the 6.4 GPa-hard 10 nm Cu/Zr bilayers and the 3.3 GPa 100 nm Cu layers deform in a compatible fashion up to 50% strain. Shear instabilities are entirely suppressed. Synergetic strengthening of 768 MPa (83% increase) compared to the rule of mixture is observed, reaching a total strength of 1.69 GPa. We present a model that serves as a design guideline for such synergetically deforming nano-hybrid materials.
ACS Applied Materials & Interfaces, 2017
Thermoelectric materials are of utmost significance for conversion of heat flux into electrical p... more Thermoelectric materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency thermoelectric materials suitable for the mid-temperature range. Herein, we explore a Ag16.7Sb30Te53.3 alloy (at. %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured thermoelectric transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations of the matrix composition, highlighting the importance of material&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s thermal stability under service conditions.
Journal of Materials Chemistry A, 2014
ScAlN and ScGaN alloys are wide band-gap semiconductors which can greatly expand the options for ... more ScAlN and ScGaN alloys are wide band-gap semiconductors which can greatly expand the options for band gap and polarisation engineering required for efficient III-nitride optoelectronic devices, high-electron mobility transistors and energy-harvesting devices.
Journal of Physics: Condensed Matter, 2015
Scripta Materialia, 2020
A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composit... more A topologically heterogeneous microstructure design is introduced in a Cu/Zr nanolayered composite, in which each soft 100 nm Cu or Zr layer is surrounded on both sides by several hard 10 nm Cu/Zr bilayers. This design aims to impose a full geometrical constraint on all of the soft layers. Micropillar compression tests demonstrate that the composite deforms in a compatible fashion among the layers, in which no extrusion of the soft layers occurs. An elevated strength of 730 MPa is achieved in the composite compared with the strength prediction based on the linear rule of mixtures.
Advanced Functional Materials
Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for... more Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>10 12 cm-2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na 0.025 Eu 0.03 Pb 0.945 Te is demonstrated at a moderate dislocation density of 1 × 10 10 cm-2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon-scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge-type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal-insulating applications.
JOURNAL OF PHYSICS-CONDENSED MATTER, 2014
High energy-resolution fluorescence-detected X-ray absorption spectroscopy and density functional... more High energy-resolution fluorescence-detected X-ray absorption spectroscopy and density functional theory calculations were used to investigate the local bonding and electronic structure of Sc in epitaxial wurtzite-structure Sc x Ga 1-x N films with x ≤ 0.059. Sc atoms are found to substitute for Ga atoms, accompanied by a local distortion involving an increase in the internal lattice parameter u around the Sc atoms. The local bonding and electronic structure at Sc are not affected strongly by the strain state or the defect microstructure of the films. These data are consistent with theoretical predictions regarding the electro nic structure of dilute Sc x Ga 1-x N alloys.
Microscopy (Oxford, England), Jan 9, 2017
Multivariate analysis is a powerful tool to process spectrum imaging datasets of electron energy ... more Multivariate analysis is a powerful tool to process spectrum imaging datasets of electron energy loss spectroscopy. Most spatial variance of the datasets can be explained by a limited numbers of components. We explore such dimension reduction to facilitate quantitative analyses of spectrum imaging data, supervising the spectral components instead of spectra at individual pixels. In this study, we use non-negative matrix factorization to decompose datasets from Fe2O3 thin films with different Sn doping profiles on SnO2 and Si substrates. Case studies are presented to analyse spectral features including background models, signal integrals, peak positions and widths. Matlab codes are written to guide microscopists to perform these data analyses.
Materials Science Forum
An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size ... more An aged Al-5Zn-1.6Mg alloy with fine η' precipitates was grain refined to ~100 nm grain size by severe plastic deformation (SPD). Microstructure evolution during SPD and mechanical behaviour after SPD of the alloy were characterized by electron microscopy and tensile, compression as well as nanoindentation tests. The influence of η' precipitates on microstructure and mechanical properties of ultrafine grained Al-Zn-Mg alloy is discussed with respect to their effect on dislocation configurations and deformation mechanisms during processing of the alloy.
Nano Letters, 2015
Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high... more Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.
Physical Review Letters, 2013
Aberration-corrected scanning transmission electron microscopy was used to investigate the core s... more Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in undoped GaN films with both high and low dislocation densities, and in a comparable high dislocation density Mg-doped GaN film. All a-type dislocations in all samples have a 5/7-atom core structure. In contrast, most (a+c)-type dislocations in undoped GaN dissociate due to local strain variations from nearby dislocations. In contrast, Mg doping prevents (a+c)-type dislocation dissociation. Our data indicate that Mg affects dislocation cores in GaN significantly.
Physical Review Letters, 2013
The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by ab... more The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5:4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeN x. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5:4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5:4 lattice-matching growth of GaN.
physica status solidi (a), 2012
... Status Solidi B 238, 127 (2003). [14] V. Ranjan, S. Bin-Omran, D. Sichuga, RS Nichols, L. Bel... more ... Status Solidi B 238, 127 (2003). [14] V. Ranjan, S. Bin-Omran, D. Sichuga, RS Nichols, L. Bellaiche, and A. Alsaad, Phys. Rev. B 72, 085315 (2005). ... Lett. 78, 2891 (2001). [18] P. de Rouffignac, APYousef, KH Kim, and RG Gordon, Electrochem. Solid-State Lett. 9, F45 (2006). ...
Journal of Applied Physics, 2013
Acta Materialia, 2010
The paper presents a new extrusion method, alternate forward extrusion, in which the punch was re... more The paper presents a new extrusion method, alternate forward extrusion, in which the punch was replaced with double-split structures so as to achieve the grain refinement for material near the interface of double-split structures. The results showed that the unique loading mode made metal flow sequence and behavior significantly changed during alternate forward extrusion. The additional shear deformation produced by the double-split punch structures resulted in a refining effect on the microstructure of the blank, which was then further refined during flow through the die orifice owing to shear deformation. Compared with the conventional extrusion, the recrystallization process in the alternate forward extrusion process produced grains that were smaller and more homogeneous in size. The recrystallization process was more abundant, and the dislocation density was significantly increased. It can be concluded that the alternate forward extrusion process could achieve fine-grained strengthening, which provided technical support and scientific guidance for the engineering application of magnesium alloy extrusion forming technology.
Journal of Applied Physics, 2013
Sc-based III-nitride alloys were studied using Density Functional Theory with special quasi-rando... more Sc-based III-nitride alloys were studied using Density Functional Theory with special quasi-random structures and were found to retain wide band gaps which stay direct up to x = 0.125 (Sc x Al 1-x N) and x = 0.375 (Sc x Ga 1-x N). Epitaxial strain stabilization prevents spinodal decomposition up to x = 0.3 (Scx Al 1-x N on GaN) and x = 0.24 (Sc x Ga 1-x N on GaN), with critical thicknesses for strain relaxation ranging from 3 nm to near-infinity. The increase in Sc content introduces compressive in-plane stress with respect to AlN and GaN, and leads to composition-and stress-tunable band gaps and polarization, and ultimately introduces ferroelectric functionality in Sc x Ga 1-x N at x ≈ 0.625.
ACS Applied Materials & Interfaces
Capturing and converting waste heat into electrical power through thermoelectric generators based... more Capturing and converting waste heat into electrical power through thermoelectric generators based on the Seebeck effect is a promising alternative energy source. Among thermoelectric compounds, PbTe can be alloyed and form precipitates by aging at elevated temperatures, thus reducing thermal conductivity by phonon scattering. Here, PbTe is alloyed with Ag to form Ag-rich precipitates having a number density controlled by heat treatments. We employ complementary scanning transmission electron microscopy and atom probe tomography to analyze the precipitate number density and the PbTe-matrix composition. We measure the temperature-dependent transport coefficients and correlate them with the microstructure. The thermal and electrical conductivities, as well as the Seebeck coefficients, are found to be highly sensitive to the microstructure and its temporal evolution, e.g., the number density of Ag-rich precipitates increases by ca. 3 orders of magnitude and reaches (1.68 ± 0.92) × 10 m upon aging at 380 °C for 6 h, which is pronounced by reduction in thermal conductivity to a value as low as 0.85 W m K at 300 °C. Our findings will help to guide predictive tools for further design of materials for energy harvesting.
Scientific Reports
Nanolayered metallic composites are much stronger than pure nanocrystalline metals due to their h... more Nanolayered metallic composites are much stronger than pure nanocrystalline metals due to their high density of hetero-interfaces. However, they are usually mechanically instable due to the deformation incompatibility among the soft and hard constituent layers promoting shear instability. Here we designed a hybrid material with a heterogeneous multi-nanolayer architecture. It consists of alternating 10 nm and 100 nm-thick Cu/Zr bilayers which deform compatibly in both stress and strain by utilizing the layers' intrinsic strength, strain hardening and thickness, an effect referred to as synergetic deformation. Micropillar tests show that the 6.4 GPa-hard 10 nm Cu/Zr bilayers and the 3.3 GPa 100 nm Cu layers deform in a compatible fashion up to 50% strain. Shear instabilities are entirely suppressed. Synergetic strengthening of 768 MPa (83% increase) compared to the rule of mixture is observed, reaching a total strength of 1.69 GPa. We present a model that serves as a design guideline for such synergetically deforming nano-hybrid materials.
ACS Applied Materials & Interfaces, 2017
Thermoelectric materials are of utmost significance for conversion of heat flux into electrical p... more Thermoelectric materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency thermoelectric materials suitable for the mid-temperature range. Herein, we explore a Ag16.7Sb30Te53.3 alloy (at. %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured thermoelectric transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations of the matrix composition, highlighting the importance of material&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s thermal stability under service conditions.
Journal of Materials Chemistry A, 2014
ScAlN and ScGaN alloys are wide band-gap semiconductors which can greatly expand the options for ... more ScAlN and ScGaN alloys are wide band-gap semiconductors which can greatly expand the options for band gap and polarisation engineering required for efficient III-nitride optoelectronic devices, high-electron mobility transistors and energy-harvesting devices.