The Experts below are selected from a list of 60 Experts worldwide ranked by ideXlab platform
Anthony R. West - One of the best experts on this subject based on the ideXlab platform.
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oxygen stoichiometry Chemical expansion or Contraction and electrical properties of rutile tio2 δ ceramics
Journal of the American Ceramic Society, 2019Co-Authors: Yun Dang, Anthony R. WestAbstract:Rutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; Chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a Chemical Contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.
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Oxygen stoichiometry, Chemical expansion or Contraction, and electrical properties of rutile, TiO2±δ ceramics
Journal of the American Ceramic Society, 2018Co-Authors: Yun Dang, Anthony R. WestAbstract:Rutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; Chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a Chemical Contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.
Yun Dang - One of the best experts on this subject based on the ideXlab platform.
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oxygen stoichiometry Chemical expansion or Contraction and electrical properties of rutile tio2 δ ceramics
Journal of the American Ceramic Society, 2019Co-Authors: Yun Dang, Anthony R. WestAbstract:Rutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; Chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a Chemical Contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.
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Electrical properties of pure and doped rutile ceramics
2018Co-Authors: Yun DangAbstract:Rutile, TiO2, has drawn significant attention due to its unique properties. In this project, the structural and electrical properties of undoped and Cr/Al/Ga/Zn-doped rutile have been investigated. Undoped rutile is increasingly oxygen-deficient on heating in air above ~700 °C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The lattice parameters of quenched samples show an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increase with a small oxygen loss, δ; Chemical expansion of the lattice occurs and is attributed to increase in average Ti-O bond lengths. At higher δ, lattice parameters start to decrease, giving a Chemical Contraction effect attributed to partial collapse of columns of edge-sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear, CS, plane formation. Oxygen-deficient rutile samples quenched from above 700 °C are n-type, whereas samples annealed and measured at 450-500 °C are p-type and believed to be slightly oxygen-rich. Holes are located on oxide ions at or near the sample surface and arise from redox electron transfer between under-bonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600 °C show cross-over between n- and p-type behavior. Cr-doped rutile, Ti1-xCrxO2-x/2-δ, samples show a volume expansion at low Cr content, which is associated with the larger size of Cr3+ and the increase in average M-O bond length. At relatively higher Cr content, samples show an unusual volume Contraction effect, which is also believed to be attributed to the formation of shear planes. The electrical properties of Cr-doped rutile are very dependent on composition x. Impedance results show that at low dopant concentration (0.001 ≤ x ≤ 0.01), samples are two-phase mixtures. The phase identified by R2C2 is more resistive than R1C1 and has larger activation energy. R2C2 represents a phase with very small Cr3+ dopant concentration, i.e. x = 0.001 ± 0.0005, and R1C1 represents a phase with relatively higher dopant content, i.e. x = 0.008 ± 0.002. In addition, since Cr content of both phases are different, R2C2 may have random point defects, such as oxygen vacancies. With relatively higher dopant content, oxygen vacancies collapse to form shear planes, i.e. defects present in the phase R1C1 may be random shear planes. At low x, Cr-doped rutile samples show mixed conduction, with p-type electronic conduction and oxide ion conduction. The location of holes is thought to be related with O2- ions as well. At higher x, a p-n transition is observed; samples are semiconducting and have small activation energy (~0.2 eV). Such semiconductivity is attributed to intrinsic hopping of charge carriers associated with Cr-Cr or Cr-Ti. Since orbital overlap may occur, charge carriers could hop between Cr3+ and Ti4+ and thus, Cr4+ and Ti3+ are generated and samples become semiconducting. Moreover, if oxygen vacancies collapse to form shear planes, the distance between Cr-Cr or Cr-Ti in face-sharing octahedra in the shear planes are smaller, and hopping of charge carriers becomes easier and the conductivity increases. Moreover, polaron hopping is an alternative description of the amount of semiconductivity. Electrical properties of Al-doped TiO2 are dependent on cooling condition. Samples show a p-n transition with increasing quench temperature. The p-type conductivity is driven by surface absorption of oxygen and associated with O- at the sample surface. Ga-doped TiO2 shows an n-p transition with increasing dopant concentration, whereas such electronic conduction is quite weak. Maybe with Ga3+ doping in TiO2, ionic compensation is the main compensation mechanism. With Zn2+ doping, it is clear that the homogeneity of rutile ceramics improves in some degree. However, Zn-doped samples still show evidence of a constriction resistance or a dipole orientation-related impedance. Zn-doped rutile samples show n-type conduction.
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Oxygen stoichiometry, Chemical expansion or Contraction, and electrical properties of rutile, TiO2±δ ceramics
Journal of the American Ceramic Society, 2018Co-Authors: Yun Dang, Anthony R. WestAbstract:Rutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; Chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a Chemical Contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.
Markus Böl - One of the best experts on this subject based on the ideXlab platform.
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A Coupled Chemomechanical Model for Smooth Muscle Contraction
Computer Models in Biomechanics, 2013Co-Authors: Markus Böl, André SchmitzAbstract:This manuscript presents a chemomechanically coupled three-dimensional model, describing the contractile behavior of smooth muscles. It bases on a strain-energy function, additively decomposed into passive parts and an active calcium-driven part related to the Chemical Contraction of smooth muscle cells. For the description of the calcium phase the four state cross-bridge model of Hai and Murphy (Am. J. Physiol. 254:C99–106, 1988) has been used. Before the features and applicability of the proposed approach are illustrated in terms of three-dimensional boundary-value problems, the model is validated by experiments on porcine smooth muscle tissue strips.
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a three dimensional chemo mechanical continuum model for smooth muscle Contraction
Journal of The Mechanical Behavior of Biomedical Materials, 2012Co-Authors: Markus Böl, André Schmitz, Gotz Nowak, Tobias SiebertAbstract:Based on two fields, namely the placement and the calcium concentration, a chemo-mechanically coupled three-dimensional model, describing the contractile behaviour of smooth muscles, is presented by means of a strain energy function. The strain energy function (Schmitz and Bol, 2011) is additively decomposed into a passive part, relating to elastin and collagen, and an active calcium-driven part related to the Chemical Contraction of the smooth muscle cells. For the description of the calcium phase the four state cross-bridge model of Hai and Murphy (Hai and Murphy, 1988) has been implemented into the finite element method. Beside three-dimensional illustrative boundary-value problems demonstrating the features of the presented modelling concept, simulations on an idealised artery document the applicability of the model to more realistic geometries.
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On a phenomenological model for active smooth muscle Contraction
Journal of biomechanics, 2011Co-Authors: André Schmitz, Markus BölAbstract:This paper presents a three-dimensional phenomenological model for the description of smooth muscle activation. A strain energy function is proposed as sum of the strain energy stored in the passive tissue, consisting of elastin and collagen, and an active calcium-driven energy related to the Chemical Contraction of the smooth muscle cells. Further, the proposed model includes the dispersions of the orientations of smooth muscle cells and collagen. These dispersions, measured in experiments, can be directly inserted into the model. The approach is implemented into the framework of the finite element method. Consequently, beside a validation with experiments the modelling concept is used for a three-dimensional numerical study.
André Schmitz - One of the best experts on this subject based on the ideXlab platform.
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A Coupled Chemomechanical Model for Smooth Muscle Contraction
Computer Models in Biomechanics, 2013Co-Authors: Markus Böl, André SchmitzAbstract:This manuscript presents a chemomechanically coupled three-dimensional model, describing the contractile behavior of smooth muscles. It bases on a strain-energy function, additively decomposed into passive parts and an active calcium-driven part related to the Chemical Contraction of smooth muscle cells. For the description of the calcium phase the four state cross-bridge model of Hai and Murphy (Am. J. Physiol. 254:C99–106, 1988) has been used. Before the features and applicability of the proposed approach are illustrated in terms of three-dimensional boundary-value problems, the model is validated by experiments on porcine smooth muscle tissue strips.
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a three dimensional chemo mechanical continuum model for smooth muscle Contraction
Journal of The Mechanical Behavior of Biomedical Materials, 2012Co-Authors: Markus Böl, André Schmitz, Gotz Nowak, Tobias SiebertAbstract:Based on two fields, namely the placement and the calcium concentration, a chemo-mechanically coupled three-dimensional model, describing the contractile behaviour of smooth muscles, is presented by means of a strain energy function. The strain energy function (Schmitz and Bol, 2011) is additively decomposed into a passive part, relating to elastin and collagen, and an active calcium-driven part related to the Chemical Contraction of the smooth muscle cells. For the description of the calcium phase the four state cross-bridge model of Hai and Murphy (Hai and Murphy, 1988) has been implemented into the finite element method. Beside three-dimensional illustrative boundary-value problems demonstrating the features of the presented modelling concept, simulations on an idealised artery document the applicability of the model to more realistic geometries.
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On a phenomenological model for active smooth muscle Contraction
Journal of biomechanics, 2011Co-Authors: André Schmitz, Markus BölAbstract:This paper presents a three-dimensional phenomenological model for the description of smooth muscle activation. A strain energy function is proposed as sum of the strain energy stored in the passive tissue, consisting of elastin and collagen, and an active calcium-driven energy related to the Chemical Contraction of the smooth muscle cells. Further, the proposed model includes the dispersions of the orientations of smooth muscle cells and collagen. These dispersions, measured in experiments, can be directly inserted into the model. The approach is implemented into the framework of the finite element method. Consequently, beside a validation with experiments the modelling concept is used for a three-dimensional numerical study.
Tobias Siebert - One of the best experts on this subject based on the ideXlab platform.
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a three dimensional chemo mechanical continuum model for smooth muscle Contraction
Journal of The Mechanical Behavior of Biomedical Materials, 2012Co-Authors: Markus Böl, André Schmitz, Gotz Nowak, Tobias SiebertAbstract:Based on two fields, namely the placement and the calcium concentration, a chemo-mechanically coupled three-dimensional model, describing the contractile behaviour of smooth muscles, is presented by means of a strain energy function. The strain energy function (Schmitz and Bol, 2011) is additively decomposed into a passive part, relating to elastin and collagen, and an active calcium-driven part related to the Chemical Contraction of the smooth muscle cells. For the description of the calcium phase the four state cross-bridge model of Hai and Murphy (Hai and Murphy, 1988) has been implemented into the finite element method. Beside three-dimensional illustrative boundary-value problems demonstrating the features of the presented modelling concept, simulations on an idealised artery document the applicability of the model to more realistic geometries.