Browsing by Author "Wang, S."

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Cohesive cracking in negative geometries
(2019-11-01) Wang, S.; Gharamti, I. E.; Dempsey, J. P.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
An isolated cohesive crack loaded by a central concentrated load and a cohesive edge crack loaded by concentrated loading at the crack mouth are examined. In the cohesive zones of both cases, rectangular softening and linear softening are investigated. A distinct peak load does not occur for these negative geometries. The traction-free crack grows at the instant that the process zone is fully developed. In a negative geometry, in order to maintain traction-free crack propagation, the load ratio must always increase, ensuring that stable crack growth always occurs.
High-pressure neutron study of the morphotropic lead-zirconate-titanate
(2012-07-01) Frantti, J.; Fujioka, Y.; Zhang, Jinsuo; Wang, S.; Vogel, Sven C.; Nieminen, Risto; Asiri, A. M.; Zhao, Y.; Obaid, A. Y.; Mkhalid, I. A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate ceramic with composition Pb(Zr 0.54Ti 0.46)O 3 at the Zr-rich side of the morphotropic phase boundary at which two phases co-exists. The pressure-induced changes in the phase fractions were studied by high-pressure neutron powder diffraction technique up to 3 GPa and 773 K. The two co-existing phases were rhombohedral R 3 c and monoclinic C m at room temperature and R 3 c and P 4 m m above 1 GPa and 400 K. The experiments show that pressure favors the R 3 c phase over the C m and P 4 m m phases, whereas at elevated temperatures entropy favours the P 4 m m phase. At 1 GPa pressure, the transition to the cubic P m 3 m phase occurred at around 600 K. Pressure lowers the C m → P 4 m m transition temperature. The C m phase was found to continuously transform to the P 4 m m phase with increasing pressure, which is inline with the usual notion that the hydrostatic pressure favours higher symmetry structures. At the same time, thephase fraction of the R 3 c phase was increasing, implying discontinuous C m → R 3 c phase transition. This is in clear contrast to the polarization rotation model according to which the C m would link the tetragonal and rhombohedral phases by being a phase in which the polarization would, more or less continuously, rotate from the tetragonal polarization direction to the rhombohedral direction. Pressure induces large changes in phase fractions contributing to the extrinsic piezoelectricity. The changes are not entirely reversible, as was revealed by noting that after high-pressure experiments the amount of rhombohedral phase was larger than initially, suggesting that on the Zr-rich side of the phase boundary the monoclinic phase is metastable. An important contribution to the intrinsic piezoelectricity was revealed: a large displacement of the B cations (Zr and Ti) with respect to the oxygen anions is induced by pressure.
High-pressure neutron study of the morphotropic lead-zirconate-titanate: Phase transitions in a two-phase system
(2012) Frantti, J.; Fujioka, Y.; Zhang, J.; Wang, S.; Vogel, S. C.; Nieminen, Risto M.; Asiri, A. M.; Zhao, Y.; Obaid, A. Y.; Mkhalid, I. A.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate ceramic with composition Pb(Zr0.54Ti0.46)O3 at the Zr-rich side of the morphotropic phase boundary at which two phases co-exists. The pressure-induced changes in the phase fractions were studied by high-pressureneutronpowder diffraction technique up to 3 GPa and 773 K. The two co-existing phases were rhombohedral R3c and monoclinic Cm at room temperature and R3c and P4mm above 1 GPa and 400 K. The experiments show that pressure favors the R3c phase over the Cm and P4mm phases, whereas at elevated temperatures entropy favours the P4mm phase. At 1 GPa pressure, the transition to the cubic Pm3¯m phase occurred at around 600 K. Pressure lowers the Cm→P4mm transition temperature. The Cm phase was found to continuously transform to the P4mm phase with increasing pressure, which is inline with the usual notion that the hydrostatic pressure favours higher symmetry structures. At the same time, the phase fraction of the R3c phase was increasing, implying discontinuous Cm→R3cphase transition. This is in clear contrast to the polarization rotation model according to which the Cm would link the tetragonal and rhombohedral phases by being a phase in which the polarization would, more or less continuously, rotate from the tetragonal polarization direction to the rhombohedral direction. Pressure induces large changes in phase fractions contributing to the extrinsic piezoelectricity. The changes are not entirely reversible, as was revealed by noting that after high-pressure experiments the amount of rhombohedral phase was larger than initially, suggesting that on the Zr-rich side of the phase boundary the monoclinic phase is metastable. An important contribution to the intrinsic piezoelectricity was revealed: a large displacement of the B cations (Zr and Ti) with respect to the oxygen anions is induced by pressure.
Point defect balance in epitaxial GaSb
(2014) Segercrantz, N.; Slotte, J.; Makkonen, I.; Kujala, J.; Tuomisto, Filip; Song, Y.; Wang, S.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Positron annihilation spectroscopy in both conventional and coincidence Doppler broadening mode is used for studying the effect of growth conditions on the point defect balance in GaSb:Bi epitaxial layers grown by molecular beam epitaxy. Positron annihilation characteristics in GaSb are also calculated using density functional theory and compared to experimental results. We conclude that while the main positron trapping defect in bulk samples is the Ga antisite, the Ga vacancy is the most prominent trap in the samples grown by molecular beam epitaxy. The results suggest that the p–type conductivity is caused by different defects in GaSb grown with different methods.