Wen Deng

Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

Abstract The microdefects and free electron densities in B 2, R and B 19′ phases of Ni 50.78 Ti 49.22 alloy were studied by positron lifetime measurements. Comparing the lifetime parameters of the Ni 50.78 Ti 49.22 alloy measured at 295 K and 225 K, it is found that the free electron density of the R phase is lower than that of the B 2 phase; the open volume of the defects of the R phase is larger, while the concentration of these defects is lower than that of the B 2 phase. The Ni 50.78 Ti 49.22 alloy exhibits B 19′ phase at 115 K. In comparison with the R phase, the free electron density of the B 19′ phase increases, the open volume of the defects of the B 19′ phase reduces, and the concentration of these defects increases. The microdefects and the free electron density play an important role during the multi-step transformations ( B 2→ R → B 19′ phase transformations) in Ni 50.78 Ti 49.22 alloy with the decrease of temperature.

Effects of Nb and Si on densities of valence electrons in bulk and defects of Fe3Al alloys

Positron lifetime measurements have been performed in binary Fe3Al and Fe3Al doping with Nb or Si alloys. The densities of valence electrons of the bulk and microdefects in all tested samples have been calculated by using the positron lifetime parameters. Density of valence electron is low in the bulk of Fe3Al alloy. It indicates that, the 3d electrons in a Fe atom have strong-localized properties and tend to form covalent bonds with Al atoms, and the bonding nature in Fe3Al is a mixture of metallic and covalent bonds. The density of valence electron is very low in the defects of Fe3Al grain boundary, which makes the bonding cohesion in grain boundary quite weak. The addition of Si to Fe3Al gives rise to the decrease of the densities of valence electrons in the bulk and the grain boundary thus the metallic bonding cohesion. This makes the alloy more brittle. The addition of Nb to Fe3Al results in the decrease of the ordering energy of the alloy and increases the density of valence electron and the bonding cohesion of the grain boundary. However, since the radius of Nb atom is larger than that of Fe atom, when Nb atoms substitute for Fe atoms, they will distort the lattice and enlarge the volume of the lattice, which decreases the density of valence electron and the cohesion of metallic bond in the bulk of the alloy.

Preparation, characterization and optical properties of Dy-doped yttrium aluminum garnet

The Y2.94Dy0.06Al5O12 (Dy:YAG) ceramic rods were sintered at different temperatures from 1000∘C to 1450∘C by solid-state reaction method and the Dy:YAG single crystals were grown by the optical floating zone method. The XRD found the Y4Al2O9 (YAM) phase after sintering at 1000∘C and disappeared after sintering at 1450∘C. The YAG phase and the YAlO3 (YAP) phase were found in the ceramic sintered at >1200∘C. As the sintering temperature increases, the amount of the YAG phase increases, while YAP phase decreases. There is only a single YAG phase in the Dy:YAG single crystal. The SEM images showed that the grain size in the Dy:YAG ceramic increases with the sintering temperature. There are several emission bands in the photoluminescence spectrum of the Dy:YAG single crystal located at 483 nm in the blue region, 580 nm in yellow region and 670 nm in red region. Dy:YAG is a good candidate crystal for white light emission.

The 3d Electrons and Microdefects in Binary Ti-Al Alloys Studied by Positron Annihilation

The behavior of 3d electrons and microdefects in binary Ti-Al alloys with Al contents from 47at.% to 53at.% have been studied by coincidence Doppler broadening and positron lifetime techniques. It has been found that the 3d electron signals in the spectra of binary Ti-Al alloys increase with Ti content. In Al-rich Ti-Al alloys, on increasing Al content, the open volume of defect on grain boundary increases while the density of free electron decreases. On the contrary, in Ti-rich Ti-Al alloys, on increasing Ti content, the open volume of defect on grain boundary decreases, and the electron density of the grain boundary increases.

Positron Annihilation on the Surfaces of Single Crystals of Si, SiO2, Graphite and Cu

Slow positron beam technique has been applied to measure the Doppler broadening spectra for single crystals of Cu, SiO2, graphite, virgin Si, and Si without oxide film. The results show that the Cu ratio curve shows a high peak due to Cu having 10 electrons in the 3d shells. The ratio curve of SiO2 is higher than that of graphite. For the single crystal of Cu, SiO2, graphite, and Si without oxide film, the S (W) parameters decrease (increase) as positron implantation energy increasing. Defects on the surface lead to higher S (lower W) value. For the virgin Si and the thermally grown SiO2-Si samples, the S (W) parameters increase (decrease) as positron implantation energy increasing. It can be due to Si atom at surface, with two dangling bonds, tend to form silicon oxide with O. The W parameter for the single crystal of Cu is relatively high as compared with that of the single crystals of SiO2 and graphite.

Influence of Fe on electronic density of defects in NiAl alloys

Conclusions1)The density of free electrons in the bulk of NiAl alloy is lower than that of Ni or Al metal, indicating that the 3d electrons in a Ni atom have strong-localized properties and tend to form covalent bonds with A1 atoms.2)The open volume of a defect on grain boundary of NiAl alloy is larger than that of a monovacancy in A1 or Ni metal. The density of free electrons of a defect on the grain boundary of NiAl alloy is much lower than that in the bulk of NiAl alloy, and the bonding cohesion is weak in the grain boundary of NiAl alloy.3)The addition of Fe element to NiAl results in the increase of the density of free electrons in the bulk and the grain boundary when Fe atoms substitute for Al atoms. Fe is a benefit element to improving the structure of the grain boundary and the brittleness of NiAl alloy.

Influence of TiO2 content on microstructures and optoelectronic properties of titanium-doped ZnO nanofilms

Thin films of TiO2-doped ZnO (TZO) with TiO2 contents from 0.5 to 3.0 wt.% were deposited on glass substrates by RF magnetron sputtering. The microstructures and optoelectronic properties of the TZO films were characterized by XRD, Hall effect analyzer, UV–VIS spectrophotometry and physical property measurement (PPMS-9). Results indicate that the microstructure and optoelectronic properties of TZO films are strongly affected by the TiO2 content. The best optoelectronic properties were obtained with the film having 2.0 wt.% TiO2. This film had superior crystal properties, high average optical transmittance (89.0%), and the lowest resistivity (9.58 × 10−4 Ω ⋅ cm). Furthermore, the resistivity of this film changed with temperature between 10 and 350 K, they experienced an initial decrease followed by an increase as the temperature increased.

Micro-defects and optical properties of YAG:Ce crystals prepared by optical floating zone method

The Y3Al5O12 (YAG):Ce ceramic rods and single crystals were prepared by solid state reaction and optical floating zone method, respectively. The XRD results showed that the phases in the YAG:Ce ceramics are mixture of YAG phase and YAlO3 (YAP) phase. There is only a single YAG phase in the single crystals of YAG:Ce. The mean positron lifetime of the YAG:Ce crystal decreases with increasing Ce content, and reaches minimum at 1 at.% Ce, then increases with Ce content. The mean positron lifetime of the YAG:Ce crystal is higher than that of YAG:Ce ceramic with the same chemical composition. The UV–visible absorption spectra of the YAG:Ce crystals indicated that the transmittances of the YAG:Ce crystals are higher than 90% in the wavelength range from 550 nm to 800 nm. The photoluminescence spectra of the YAG:Ce crystals showed wide luminescence emission peaks located in the range of 525–529 nm. A relatively high luminescence intensity is found in the YAG:Ce crystal with 1 at.% Ce. With further increase of Ce content, the luminescence decreases due to the fluorescence quenching effect.

Studies of Microstructure and Electro-Magnetic Transport Property of La 0.67 Ca 0.33 MnO 3 Ceramic

The positron annihilation techniques and X-ray diffraction have been used to study the microstructure of the La0.67Ca0.33MnO3 ceramics prepared by the solid-state reaction method at different sintered temperatures (T=1573K, 1623K, 1673K, 1723K, 1773K, 1823K). And the electro-magnetic transport behavior of the samples was measured by VSM and Resistivity modular on PPMS. According to these results, all samples show a perovskite structure, the ferromagnetic-paramagnetic and metal-insulator transitions occur at the transition temperature Tc and TMI, respectively, which is almost the same. For La0.67Ca0.33MnO3 sintered at 1673K, the mean positron lifetime is the largest, the maximum value of the magnetization is achieved on the magnetization-temperature curve at H=0.2mT, while the transition temperature occurs at about 244K.

Studies of Microdefects, Thermal Expansion and Magnetic Properties of Fe-Ni-Co Invar Alloys

The microdefects, the thermal expansion coefficients and the magnetization -temperature curves of the Fe64Ni36-xCox (x=1~10) were characterized by means of positron lifetime, X-ray diffraction, Michelsons interferometer and VSM modular on PPMS, respectively. The Fe64Ni30Co6 alloy is a mixture of BCC and FCC structures. With the Co content increasing in Fe64Ni36-xCox alloys, the BCC phase increases, while the FCC phase decreases. In comparison with other Fe64Ni36-xCox alloys, the Fe64Ni31Co5 alloy has a rather high magnetization at temperature lower than Tc, a relatively large change of the magnetization with the temperature near Tc, and a rather low thermal expansion coefficient.