The Experts below are selected from a list of 255 Experts worldwide ranked by ideXlab platform
Eckhard Müller - One of the best experts on this subject based on the ideXlab platform.
-
influence of mg loss on the phase stability in mg2x x si sn and its correlation with Coherency Strain
Acta Materialia, 2021Co-Authors: Mohammad Yasseri, Kunal Mitra, Aryan Sankhla, Johannes De Boor, Eckhard MüllerAbstract:Abstract Understanding of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for their applicability in thermoelectric modules. A miscibility gap was reported for the quasi-binary Mg2Si–Mg2Sn series and the exact compositions of its limits are disputed. In this work we study the phase evolution and stability of Mg2SixSn1-x with x = 0.5. Samples were annealed at 600°C, 525°C, and 450°C both with and without excess elemental Mg in quartz ampules in order to manipulate the Mg vapor pressure. This led to two qualitatively different evolution routes of phase constitution, namely, (I) progressive phase separation and material degradation related to intense Mg loss accompanied by formation of side phases such as elemental Si and (II) much slower phase separation without formation of elemental precipitates when the sample was kept under Mg vapor atmosphere. Accordingly, XRD and EDAX gave evidence that the phase evolution and demixing behavior in magnesium silicide stannide depend sensitively on the amount and rate of Mg loss. We also observe stabilization of solid solutions against demixing by Coherency Strain and can show that the phase separation which will occur in thermodynamic equilibrium related to the miscibility gap, can be inhibited if Mg loss is suppressed. Then Mg2Si0.5Sn0.5 shows improved stability at typical application temperatures (450 – 600°C) which are far below the previously reported upper limit of the coherent miscibility gap (720°C). The improvement of the phase stability of thermoelectric Mg2(Si,Sn) by controlling the Mg vapor pressure is of essential importance for long-term utilization of the material in thermogenerators at elevated temperatures.
-
Influence of Mg loss on the phase stability in Mg2X (X = Si, Sn) and its correlation with Coherency Strain
Acta Materialia, 1Co-Authors: Mohammad Yasseri, Kunal Mitra, Aryan Sankhla, Johannes De Boor, Eckhard MüllerAbstract:Abstract Understanding of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for their applicability in thermoelectric modules. A miscibility gap was reported for the quasi-binary Mg2Si–Mg2Sn series and the exact compositions of its limits are disputed. In this work we study the phase evolution and stability of Mg2SixSn1-x with x = 0.5. Samples were annealed at 600°C, 525°C, and 450°C both with and without excess elemental Mg in quartz ampules in order to manipulate the Mg vapor pressure. This led to two qualitatively different evolution routes of phase constitution, namely, (I) progressive phase separation and material degradation related to intense Mg loss accompanied by formation of side phases such as elemental Si and (II) much slower phase separation without formation of elemental precipitates when the sample was kept under Mg vapor atmosphere. Accordingly, XRD and EDAX gave evidence that the phase evolution and demixing behavior in magnesium silicide stannide depend sensitively on the amount and rate of Mg loss. We also observe stabilization of solid solutions against demixing by Coherency Strain and can show that the phase separation which will occur in thermodynamic equilibrium related to the miscibility gap, can be inhibited if Mg loss is suppressed. Then Mg2Si0.5Sn0.5 shows improved stability at typical application temperatures (450 – 600°C) which are far below the previously reported upper limit of the coherent miscibility gap (720°C). The improvement of the phase stability of thermoelectric Mg2(Si,Sn) by controlling the Mg vapor pressure is of essential importance for long-term utilization of the material in thermogenerators at elevated temperatures.
D J Dunstan - One of the best experts on this subject based on the ideXlab platform.
-
Strength of Coherently Strained Nanolayers Under High Temperature Nanoindentation
MRS Proceedings, 2006Co-Authors: Xiaodong Hou, D J Dunstan, A J BushbyAbstract:AbstractSemiconductor Strained layer superlattices are an ideal model material to study the effects of Coherency Strain in plasticity, due to the fine control of nanolayer thickness and internal Strain afforded by MBE deposition. Previously, nanoindentation of bulk InGaAs at 300K gave a yield pressure of 6GPa (Jayawera et al Proc. Roy Soc, A459, 2049, 2003) while bending at 500 centigrade gave a yield value of 30MPa (Pp’ ng et al Phil. Mag. 85, 4429, 2005). In contrast, coherently Strained InGaAs superlattices gave nanoindentation values of 3GPa at room temperature and bending at 500oC gave a yield value also around 3GPa. It appears that the Coherency Strain can impart an athermal strengthening to the superlattice. It is clearly necessary to do mechanical testing over the range 300-800K that will be able to link the room temperature nanoindentation with the results from the high temperature bending experiment and to determine the relationship between strength, Coherency Strain and temperature. Preliminary experiments on these samples at elevated temperatures using a hot stage and the UMIS nanoindentation system is difficult but feasible with the help of AFM to verify the contact area.
-
Strength of coherently Strained layered superlattices
Philosophical Magazine, 2005Co-Authors: A J Bushby, D J DunstanAbstract:International audienceElectronic-grade single-crystal semiconductor structures provide a means to study the mechanical effects of Coherency Strain in isolation. In this work, thin coherently Strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500oC. The force-deflection curves of the specimens were measured and the beams were found to be significantly strengthened by the presence of the superlattices. Analysis provides yields estimates of the stress supported by the thin superlattices in the plastic regime. The superlattices are found to display mechanical strength up to a hundred times greater than the strength of the bulk substrate material. This effect can be attributed only to the Coherency Strain in the superlattices
-
Effect of Coherency Strain on the deformation of InxGa1−xAs superlattices under nanoindentation and bending
Philosophical Magazine, 2005Co-Authors: S. J. Lloyd, A J Bushby, William J. Clegg, D J DunstanAbstract:It has been shown elsewhere that the room temperature yield pressure of In x Ga1− x As superlattices measured by nanoindentation, decreases from a high value as the volume averaged Strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing Strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing Strain modulation. The Coherency Strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The Strain energy released by the loss of Coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and d...
-
Deformation of small volumes of material using nanostructured Strained layered superlattices
Materials Science and Technology, 2004Co-Authors: K M Y Png, A J Bushby, D J DunstanAbstract:Abstract A key aspect of nanostructured materials is that large Coherency Strains can readily exist between nano-sized phases. This can result in strengthening or in improved ductility. However, in conventional materials, it can be very difficult to separate the effects of Coherency Strain from other phenomena. Electronic grade single crystal semiconductor structures provide a means to study the effects of Coherency Strain in isolation. In this work, thin coherently Strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500°C. The stress–Strain curves of the specimens were measured, and from them, analysis yields the actual stress supported by the thin superlattice. The superlattices display extraordinary strength compared to the corresponding bulk material. This effect can be attributed only to the Coherency Strain in the superlattices.
-
Coherency Strain and a New Yield Criterion
MRS Proceedings, 2000Co-Authors: N B Jayaweera, A J Bushby, P Kidd, D J Dunstan, J.r. Downes, Anthony KellyAbstract:ABSTRACTWe have studied the onset of plasticity in coherently-Strained semiconductor superlattices, using nano-indentation with spherical indenter tips to observe the full stress-Strain curve. The yield pressure is reduced by as much as a factor of two by the presence of the Coherency Strain. By varying the thicknesses and Strains of the superlattice layers, we provide a proof that yield commences over a finite volume. It is properties averaged or summed over this volume which determine the yield pressure. We show that the relevant yield criterion for our experimental data is the rate of change of elastic Strain energy with plastic relaxation, integrated over a volume of the order of a micron across. This result is expected to be valid for other systems with highly inhomogenous Strain fields, and hence to be applicable to modelling of point contact, and to the design and understanding of structural materials which have coherently-Strained microstructure.
Mohammad Yasseri - One of the best experts on this subject based on the ideXlab platform.
-
influence of mg loss on the phase stability in mg2x x si sn and its correlation with Coherency Strain
Acta Materialia, 2021Co-Authors: Mohammad Yasseri, Kunal Mitra, Aryan Sankhla, Johannes De Boor, Eckhard MüllerAbstract:Abstract Understanding of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for their applicability in thermoelectric modules. A miscibility gap was reported for the quasi-binary Mg2Si–Mg2Sn series and the exact compositions of its limits are disputed. In this work we study the phase evolution and stability of Mg2SixSn1-x with x = 0.5. Samples were annealed at 600°C, 525°C, and 450°C both with and without excess elemental Mg in quartz ampules in order to manipulate the Mg vapor pressure. This led to two qualitatively different evolution routes of phase constitution, namely, (I) progressive phase separation and material degradation related to intense Mg loss accompanied by formation of side phases such as elemental Si and (II) much slower phase separation without formation of elemental precipitates when the sample was kept under Mg vapor atmosphere. Accordingly, XRD and EDAX gave evidence that the phase evolution and demixing behavior in magnesium silicide stannide depend sensitively on the amount and rate of Mg loss. We also observe stabilization of solid solutions against demixing by Coherency Strain and can show that the phase separation which will occur in thermodynamic equilibrium related to the miscibility gap, can be inhibited if Mg loss is suppressed. Then Mg2Si0.5Sn0.5 shows improved stability at typical application temperatures (450 – 600°C) which are far below the previously reported upper limit of the coherent miscibility gap (720°C). The improvement of the phase stability of thermoelectric Mg2(Si,Sn) by controlling the Mg vapor pressure is of essential importance for long-term utilization of the material in thermogenerators at elevated temperatures.
-
Influence of Mg loss on the phase stability in Mg2X (X = Si, Sn) and its correlation with Coherency Strain
Acta Materialia, 1Co-Authors: Mohammad Yasseri, Kunal Mitra, Aryan Sankhla, Johannes De Boor, Eckhard MüllerAbstract:Abstract Understanding of the thermochemical stability of Mg2(Si,Sn) thermoelectric materials is crucial for their applicability in thermoelectric modules. A miscibility gap was reported for the quasi-binary Mg2Si–Mg2Sn series and the exact compositions of its limits are disputed. In this work we study the phase evolution and stability of Mg2SixSn1-x with x = 0.5. Samples were annealed at 600°C, 525°C, and 450°C both with and without excess elemental Mg in quartz ampules in order to manipulate the Mg vapor pressure. This led to two qualitatively different evolution routes of phase constitution, namely, (I) progressive phase separation and material degradation related to intense Mg loss accompanied by formation of side phases such as elemental Si and (II) much slower phase separation without formation of elemental precipitates when the sample was kept under Mg vapor atmosphere. Accordingly, XRD and EDAX gave evidence that the phase evolution and demixing behavior in magnesium silicide stannide depend sensitively on the amount and rate of Mg loss. We also observe stabilization of solid solutions against demixing by Coherency Strain and can show that the phase separation which will occur in thermodynamic equilibrium related to the miscibility gap, can be inhibited if Mg loss is suppressed. Then Mg2Si0.5Sn0.5 shows improved stability at typical application temperatures (450 – 600°C) which are far below the previously reported upper limit of the coherent miscibility gap (720°C). The improvement of the phase stability of thermoelectric Mg2(Si,Sn) by controlling the Mg vapor pressure is of essential importance for long-term utilization of the material in thermogenerators at elevated temperatures.
Suk-joong L. Kang - One of the best experts on this subject based on the ideXlab platform.
-
Coherency Strain enhanced dielectric temperature property of rare earth doped batio3
Applied Physics Letters, 2013Co-Authors: Sangchae Jeon, Suk-joong L. KangAbstract:Core/shell-grained BaTiO3 samples were prepared with addition of rare earth elements. The core/shell interface was semi-coherent, and many misfit dislocations formed in Dy-doped samples. In contrast, a coherent interface and few dislocations were observed in Ho- and Er-doped samples. Dy-doped samples exhibited poor temperature stability, showing a peak with no frequency dispersion. Ho- and Er-doped samples exhibited a broad curve with frequency dispersion. This improved temperature stability is attributed to the Coherency Strain, which leads to the formation of polar nano-regions in the shell. Coherency at the core/shell interface is critical to improve the temperature stability of core/shell-structured BaTiO3.
-
Diffusion Induced Grain-Boundary Migration and Enhanced Grain Growth in BaTiO3
Interface Science, 2000Co-Authors: Ho Yong Lee, Jae-suk Kim, Suk-joong L. KangAbstract:The effect of diffusion induced grain-boundary migration (DIGM) on grain growth has been studied in a model system of BaTiO3-PbTiO3. When sintered BaTiO3 samples of two different grain sizes were heat-treated in contact with PbTiO3, DIGM occurred in the coarse-grained samples (∼200 μm in average size) while fast grain growth was observed in the fine-grained samples (∼4 μm). Energy dispersive spectroscopy (EDS) analysis confirmed that the fast growth of BaTiO3 grains was accompanied by the alloying of Pb and thus related to DIGM. A calculation of Coherency Strain energy for the BaTiO3-PbTiO3 system showed that the Coherency Strain energy of a coherent (Ba0.8Pb0.2)TiO3 layer on BaTiO3 was between 2 and 3 MJ/m3 depending on the surface orientation. The calculated Coherency Strain energy values are much higher than the capillary energy due to the grain boundary curvature of 4 μm grains in the fine-grained sample. The observed enhancement of grain growth appears therefore to be a result of DIGM. Such grain growth enhancement by DIGM is thought to occur in materials processing under chemical inhomogeneity or inequilibrium, for example, in the sintering of powder mixtures and in the annealing of chemically inhomogeneous polycrystals.
-
Coherency Strain energy and the direction of chemically induced grain boundary migration in al2o3 cr2o3 and al2o3 fe2o3
Journal of the American Ceramic Society, 1994Co-Authors: Ho Yong Lee, Suk-joong L. Kang, Duk Yong YoonAbstract:Using various pairs of Al2O3 crystals of known surface orientation and two solute species of Cr2O3 and Fe2O3, the direction of chemically induced grain boundary migration has been investigated on the basis of a calculation of Coherency Strain energy. For all the grain boundaries studied, the chemically induced migration always occurred in the direction of the grain with the surface orientation corresponding to higher Coherency Strain energy. The migration directions of some grain boundaries were different depending on the solute species because of different Strain energy for the same orientation. The zigzag migration was also observed but only in grain boundaries between grains whose Coherency Strain energies were comparable to each other. These observations have confirmed the prediction of the migration initiation by the Coherency Strain theory.
-
Coherency Strain Energy and the Direction of Chemically Induced Grain Boundary Migration in Al2O3–Cr2O3 and Al2O3–Fe2O3
Journal of the American Ceramic Society, 1994Co-Authors: Ho Yong Lee, Suk-joong L. Kang, Duk Yong YoonAbstract:Using various pairs of Al2O3 crystals of known surface orientation and two solute species of Cr2O3 and Fe2O3, the direction of chemically induced grain boundary migration has been investigated on the basis of a calculation of Coherency Strain energy. For all the grain boundaries studied, the chemically induced migration always occurred in the direction of the grain with the surface orientation corresponding to higher Coherency Strain energy. The migration directions of some grain boundaries were different depending on the solute species because of different Strain energy for the same orientation. The zigzag migration was also observed but only in grain boundaries between grains whose Coherency Strain energies were comparable to each other. These observations have confirmed the prediction of the migration initiation by the Coherency Strain theory.
Long-qing Chen - One of the best experts on this subject based on the ideXlab platform.
-
Computer simulation of spinodal decomposition in conStrained films
Acta Materialia, 2003Co-Authors: D. J. Seol, Jie Shen, Long-qing ChenAbstract:The morphological evolution during spinodal decomposition of a binary alloy thin film elastically conStrained by a substrate is studied. Elastic solutions, derived for elastically anisotropic thin films subject to the mixed stress-free and conStraint boundary conditions, are employed in a three-dimensional phase-field model. The Cahn–Hilliard diffusion equation for a thin film boundary condition is solved using a semi-implicit Fourier-spectral method. The effect of composition, Coherency Strain, film thickness and substrate conStraint on the microstructure evolution was studied. Numerical simulations show that in the absence of Coherency Strain and substrate conStraint, the morphology of decomposed phases depends on the film thickness and the composition. For a certain range of compositions, phase separation with Coherency Strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Similar to bulk systems, the negative elastic anisotropy in the cubic alloy results in the alignment of phases along 100 elastically soft directions. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
-
A three-dimensional phase-field model for computer simulation of lamellar structure formation in γTiAl intermetallic alloys
Acta Materialia, 2001Co-Authors: Y.h. Wen, Long-qing Chen, P.m. Hazzledine, Y. WangAbstract:A three dimensional phase-field model of α′2→α2+γ transformation is developed to simulate the formation of coherent multi-domain lamellar structures in γTiAl intermetallic alloys. The model takes into account the effect of Coherency Strain associated with the lattice rearrangement accompanying the phase transformation, and the anisotropy in interfacial energy. Simulation studies based on the model successfully predicted the essential features associated with the multi-domain lamellar structures observed experimentally. It is shown that the Coherency Strain accommodation is the dominating factor responsible for the formation of the lamellar structure. The neighboring lamellae of γ phase are found to have either a twin or a pseudo-twin relationship, with the former being dominant. It is found that Strain-induced correlated nucleation plays an important role in the formation of the twined lamellae. The lamellar thickness is determined by the interplay among the elastic Strain energy, interfacial energy and bulk chemical free energy. Domains within individual lamellae are isotropic and domain boundaries are smoothly curved. No special self-accommodating morphological patterns are observed on the (0001)α2 plane, which is very different from the pattern formation predicted for the coherent hexagonal → O-phase transformations.