Zujian Wang

The formation of PSB-like shear bands in cyclically deformed ultrafine grained copper processed by ECAP

Cyclic deformation was performed on ultrafine grained copper processed by ECAP. Shear bands (SBs) and adjacent microstructures were investigated using electron channeling contrast in scanning electron microscope. The possible formation mechanism of SB was discussed based on the characteristic distribution of defects introduced by ECAP

Dependence of intergranular fatigue cracking on the interactions of persistent slip bands with grain boundaries

Intergranular fatigue cracking mechanisms in various copper crystals with different grain boundaries (GBs) were systematically investigated and summarized. In the present investigation, the GBs are classified into three types, i.e. (1) random large-angle GBs parallel, perpendicular or tilting to the stress axis in various copper bicrystals; (II) low-angle GBs parallel or perpendicular to the stress axis in copper columnar crystals; (III) large-angle Sigma19b GB in a special [(4) over bar 1520]/[18 (2) over bar7]copper bicrystal. The slip planes of the adjacent crystals are coplanar across the later two types of GBs, but the slip directions of the two component grains are different beside the Sigma19b GB. With the help of electron channeling contrast (ECC) technique in scanning electron microscopy (SEM), fatigue cracks and the interactions of dislocations with the GBs in all the fatigued crystals were observed and revealed. The results show that all the large-angle GBs (type I) in copper bicrystals always become the preferential sites to initiate fatigue cracks, independent of the interaction angle between the GB plane and the stress axis. This intergranular fatigue cracking mechanism can be attributed to the piling-up of dislocations at the large-angle GBs. For the columnar crystals containing low-angle GBs (type II), it is observed that persistent slip bands (PSBs), which transfer through low-angle GBs continuously, are the preferential sites for the nucleation of fatigue cracks. However, fatigue cracks were never observed at the low-angle GBs, no matter whether they were perpendicular or parallel to the stress axis. The non-cracking behavior of the low-angle GBs can be explained by the continuity of the dislocations, which led to the disappearance of piling-up of dislocations. For the Sigma19b GB (type III), it is found that the favorable fatigue cracking mechanism is still intergranular type in comparison with PSB cracking even though the two component grains have a coplanar slip system. The corresponding GB cracking mechanism should be attributed to the difference in the slip directions between two component grains, which only allows for partial passing through of dislocations across the Sigma19b GB. Based on the results above, it is suggested that intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. Among all the GBs, only the low-angle GBs are intrinsically strong to resist the nucleation of fatigue cracks under cyclic loading

Growth and piezo-/ferroelectric properties of PIN-PMN-PT single crystals

Ternary solid solution crystals of 0.19Pb(In1/2Nb1/2)O3− 0.46Pb(Mg1/3Nb2/3)O3− 0.35PbTiO3 [PIMNT(19/46/35)] with dimensions of 35u2009×u200938u2009×u200915 mm3 were grown by the top-seeded solution growth (TSSG) method. The dielectric, piezo- and ferroelectric properties of the grown crystals were characterized. The ternary piezocrystals exhibit a Curie temperature TCu2009=u2009190u2009°C and a tetragonal-rhombohedral phase transition temperature TR-Tu2009=u2009130u2009°C, which are increased significantly compared with TC ≈ 155u2009°C and TR-T ≈ 80u2009°C of PMN-PT crystals. The dielectric constant (ɛ′) and dielectric loss tangent (tan δ) are 4300 and 0.40 at room temperature. The piezoelectric coefficient d33 is found to be 2380 pC/N. The longitudinal electromechanical coupling factor k33 reaches 90% at room temperature. A peak-to-peak bipolar strain value of 0.13% is obtained at E ≈u2009±u200914u2009kV/cm. The coercive field Ec and remanent polarization Pr are 5.50u2009kV/cm and 27.10u2009μC/cm2, respectively, which are also improved from the vales of PMN-PT crystals.

New observation on the luminescence of CdS clusters in zeolite-Y

The excitation spectrum of CdS dusters in zeolite-Y is consistent with their absorption spectrum, both showing two absorption bands that are assigned to the Is-is and Is-lp transitions, respectively. A new emission at 400 nn is considered to be the recombination of the bounded excitons. The emission firstly increases then decreases with increasing cluster size or loading. The emission by excitation into the Is-is band is stronger and sharper than that by excitation into the Is-lp band. This phenomenon is attributed to the size inhomogeneity and the strong electron-phonon interaction of the dusters. Copyright (C) 1996 Elsevier Science Ltd

A new (1 − x)Pb(Lu1/2Nb1/2)O3–xPbTiO3 binary ferroelectric crystal system with high Curie temperature

A series of new (1 − x)Pb(Lu1/2Nb1/2)O3–xPbTiO3 (PLN–xPT) binary ferroelectric crystals with high Curie temperature have been successfully obtained firstly by top-seed solution growth technique. A solid-state phase diagram in the low temperature range has been established based on the form of single crystals, which reveals a morphotropic phase boundary (MPB) in the range of 49–51% PT. The polarized light microscope analysis also confirms the coexistence of rhombohedral and tetragonal phase within the MPB. Furthermore, those compositions within MPB exhibit good comprehensive piezoelectric properties. For example, the crystal with composition PLN–0.49PT, showing high Curie temperature Tc = 360 °C and high rhombohedral–tetragonal phase transition temperature TRT = 110 °C, and the piezoelectric coefficient d33, electromechanical coupling coefficient k33, coercive field Ec and remanent polarization Pr are found to be 1630 pC N−1, 81%, 13.8 kV cm−1 and 26.6 μC cm−2, respectively. The large coercive field and high Tc make it a promising candidate for high power transducers in a wider temperature range.

Compositional dependence of properties of Pb(Yb1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ferroelectric crystals

A series of Pb(Yb1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ferroelectric crystals with different compositions have been grown by the top-seeded solution growth technique in order to investigate systemically this ternary system based on the single crystal form. The compositional segregation and the compositional dependence of electric properties have been studied for the selected compositions in the vicinity of the morphotropic phase boundary (MPB) region. All the samples exhibit perovskite structure with rhombohedral symmetry at room temperature. The lattice parameter (a), coercive field Ec, Curie temperature TC, the rhombohedral–tetragonal phase transition temperature Trh–te increase gradually with increasing PYN content. All the samples exhibit quite high piezoelectric performance. In particular, the crystal with composition 0.20PYN–0.40PMN–0.40PT possesses excellent electric properties such as TC = 180 °C, Trh–te = 130 °C, k33 = 90.9%, d33 = 2240–2580 pC N−1, Pr = 28.4 μC cm−2 and Ec = 5.4 kV cm−1, indicating that the PYN–PMN–PT ternary crystals will be a promising candidate for electromechanical applications.

Growth of Pb(Fe1/2Nb1/2)O3–Pb(Yb1/2Nb1/2)O3–PbTiO3 piezo-/ferroelectric crystals for high power and high temperature applications

In order to develop new piezoelectric crystals with high TC and high performance for high-power and high-temperature applications, the Pb(Fe1/2Nb1/2)O3–Pb(Yb1/2Nb1/2)O3–PbTiO3 [PFN–PYN–PT(10/38/52)] ternary piezo-/ferroelectric single crystals have been grown by the flux method for the first time. The crystal structure has been characterized by means of X-ray diffraction technique, showing a perovskite phase with rhombohedral symmetry at room temperature. The as-grown PFN–PYN–PT crystal possesses a high Curie temperature (TC) of 325 °C and a high rhombohedral-tetragonal phase transition temperature (Trt) of 155 °C. Complete electric characterization has revealed the high performance of the ternary crystals, with a dielectric constant e′ = 1620, a piezoelectric coefficient d33 = 1100 pC N−1, a longitudinal electromechanical coupling factor k33 = 70%, a remnant polarization Pr = 25.8 μC cm−2, a coercive field Ec = 10.3 kV cm−1, and a mechanical quality factor Qm = 110 for (001)-oriented crystals at room temperature. These results show that the PFN–PYN–PT crystals are a promising candidate for uses in electromechanical transducers for high-power and high-temperature applications.

Synthesis, structure and electric properties of Pb(Yb1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ceramics

The ternary ceramics of Pb(Yb1/2Nb1/2)O3?Pb(Mg1/3Nb2/3)O3?PbTiO3 (PYMNT) solid solution system is prepared by a two-step synthetic process. Its structure and electrical properties are studied. All the obtained ceramics have pure perovskite structure. It is noted that the morphotropic phase boundary (MPB) of the PYMNT ternary system is not a linear region which is between the MPB compositions of PMNT and PYNT binary systems. The Curie temperature TC of the ternary system varies in the range 165?370??C and the piezoelectric coefficient d33 varies in the range 335?505?pC?N?1. With increasing PYNT content, the dielectric constants ?? decrease and the coercive field Ec increases. Therefore, the electric properties and TC can be adjusted by selection of composition. The results show that the PYMNT ceramic exhibits higher Curie temperature, larger coercive field and good piezoelectric properties, making it a promising material for high-power electromechanical transducers that can operate in a large temperature range.

IN-SITU INVESTIGATION OF STRESS-INDUCED MARTENSITIC-TRANSFORMATION IN THE TI-NI SHAPE-MEMORY ALLOY DURING DEFORMATION

An investigation of the stress-induced martensitic transformation produced during deformation in the Ti-Ni shape memory alloy has been made. The results show that the stress-induced martensitic transformation occurs near the crack-tip and this kind of transformation is reversible. Electron diffraction analysis reveals that there is a transitional R-phase that forms during the transformation from the parent phase (B-2) to the martensitic phase (B-19), accompanying a rhombohedral lattice distortion of the alloy.

Effects of volume expansion and electron correlation in some R(Fe,M)12 compounds (M=Nb, Ti, V) and their nitrides

Abstract In the present paper, a study of the effect of the volume change on the Curie temperature ( T C ) of R(Fe,M) 12 (M=Nb, Ti and V) due to nitrogenation has been performed. The dln T C /dln V values were derived by comparing the Curie temperatures and volumes before and after nitrogen. Our results indicate that the relationship between dln T C /dln V and the Curie temperature T C agrees with an itinerant-model formalism. The effects of volume expansion and the increasing electron correlation play important roles in the enhancement of the Curie temperature upon nitrogenation of intermetallic compounds.