Shao Jie Wang

Evidence of defect associates in yttrium-stabilized zirconia ☆

Abstract After Nb 2 O 5 was added to Y 2 O 3 -stabilized ZrO 2 , the bulk conductivity decreased distinctly. Positron annihilation lifetime and Doppler broadening spectra were measured to study the effects of Nb 2 O 5 on electrical properties of Y 2 O 3 -stabilized ZrO 2 . The result shows that the additions of Nb 2 O 5 can decrease the V 2+ O concentration and increase the ( Y − Zr V 2+ O Y − Zr ) 0 concentration, which is why bulk conductivity decreases. The additions of Nb 2 O 5 were expected to suppress the formation of defect associates, however, only adverse experimental results were found in the present work.

Study of structural characteristics of HDPE/CaCO3 nanocomposites by positrons

Abstract The positron lifetime spectra were measured as a function of weight fraction Φw of CaCO3 nanoparticles in the high-density polyethylene/CaCO3 (HDPE/CaCO3) (polymer) nanocomposites. It is observed that, first, positronium (Ps) is formed and annihilates in the free-volume hole of HDPE and the free-volume concentration decreases with increasing fraction of the CaCO3 nanoparticles in the HDPE/CaCO3; second, the positron lifetime τ2 and its intensity I2 are strongly affected by the structure of the interface layer between CaCO3 nanoparticles and HDPE matrix.

Investigation of ionic conductivity of polymeric electrolytes based on poly (ether urethane) networks using positron probe

Abstract Positron-lifetime measurements have been made for poly (ether urethane) undoped and doped with [LiClO 4 ]/[Unit]=0.05 in the temperature range of 120–340 K. The measured lifetime spectra were resolved into three components. The lifetime and the intensity of orthopositronium were used to evaluate the amount of the free volume in poly (ether urethane). It was found that the variation of ionic conductivity with temperature and salt concentration can be rationalised in terms of free volume consideration.

Growing process of CdS nanoclusters in zeolite Y studied by positron annihilation

Zeolite Y has been used as the host to generate CdS nanoclusters. The location of CdS nanoclusters inside zeolite hosts was confirmed by the blue-shifted reflection absorption spectra with respect to that of bulk CdS materials. But which kind of cage inside zeolite Y, sodalite cage or supercage, was preferred for the CdS clusters remained unclear. In this paper, we conducted positron annihilation spectroscopy (PAS) measurements for the first time on a series of CdS/Y zeolite samples and concluded that CdS clusters were not located in supercages but in smaller sodalite cages. The stability of CdS clusters inside the sodalite units was due to the coordination of Cd atoms with the framework oxygen atoms of the double six-ring windows. Moreover, PAS revealed some important information of surface states existing on the interfacial layers between CdS clusters and zeolite Y.

Effects of deformation on the microstructure of PTFE polymer studied by positron annihilation

The free-volume properties of uniaxially stretched polytetrafluoroethylene (PTFE) have been studied by positron lifetime spectroscopy as a function of residual deformation epsilon in the range of 0-180%. According to the variation of ortho-positronium (o-Ps) intensity, the stretch process can be divided into three stages, i.e. elastic region ( epsilon = 0-20%), plastic-flow region ( epsilon = 20-80%) and strain-strengthening region ( epsilon = 80-180%). In each stage the free-volume concentration is constant, but it has a step increase at the transition points ( epsilon = 20% and 80%). The average radius of free-volume holes increases with deformation in the elastic region, has a maximum in the plastic-flow region, and finally tends to a lower level in the strain-strengthening region. These results show that positron annihilation lifetime is a sensitive means to probe the microstructural change of polymers during deformation.

Compositional and temperature dependence of free volume in segmented copolymer PET/PEO studied by positron annihilation lifetime measurement

Abstract Positron lifetime measurements were performed for segmented copolymer poly(ethylene oxide)–poly(ethylene terephthalate) (PEO/PET) as a function of the hard segment content (PET) and the temperature, which is used to study the free volume properties, structural transition and miscibility behaviour of segmented copolymer PEO/PET. From the variation of the ortho -positronium ( o -Ps) lifetimes with changes in the hard segment content, we find that the free volume decreases with increase of the hard segment, this fact means that the hard segment microdomains act as physical cross-links that suppress movement of the chains and that the interactions between the hard segment (PET) and soft segment (PEO) also plays an important role. On the other hand, the temperature dependence of positron lifetimes reveals the existence of two glass transition temperatures. This experimental results show that the hard segment PET is immiscibility with soft segment PEO in segmented copolymer PEO/PET (PEO molecular weight 4000, PET content 30 wt.%). The maximum entropy lifetime method (MELT) is used to detect the distributions of the free volume holes. Large differences in width of the free volume hole distributions as a function of temperature are observed.

The Microstructure of EPOXY-Layered Silicate Nanocomposite Studied by Positron Annihilation

A series of epoxy-rectorite nanocomposites with rectorite content w ranged from 0 to 2.0wt% were prepared and the sample with w=0.5% exhibits high mechanical performance and thermal stability. XRD, TEM and positron annihilation lifetime (PAL) were used to study the microstructure of materials under study. The exfoliation of rectorite platelets in epoxy matrix was observed by XRD patterns and TEM micrograph. The PAL results show that the interfacial layers between silicate layers and polymer were formed and play an important role for improvement of material performance. Introduction Recently the polymer-layered silicate nanocomposites have been attracted considerable attention because of their unique properties such as increased modules, improved gas permeability and thermal stability [1,2]. The microstructure of polymer-layered silicate nanocomposite can be broadly divided into two types: intercalated nanocomposites, in which the silicate is well dispersed in a polymer matrix, and exfoliated nanocomposites where the silicate platelets become fully separated. The rectorite is a mica-like layered silicate with an interlayer distance of 2.2-2.4nm, which is twice as that of regular montmorillonite and helpful for exfoliation of rectorite platelets. So the rectorite fillers have shown significant improvement in material performance at very low contents. To date, very little is known about the free volume properties of polymer-layered silicate nanocomposites. This work investigated the interaction between layered silicate and epoxy, molecular packing and free volume properties in epoxy-rectorite nanocomposites by X-ray diffraction(XRD), transmission electron microscope(TEM) and positron annihilation lifetime(PAL). Experiment The organically modified rectorite (org-rct) was made by cation exchange reaction between rectorite with a particle size of 30-60 μm and Octadecylammonium Salt [3]. A series of epoxy-rectorite nanocomposites with rectorite contents ranged from 0 to 2.0wt% were prepared by dispersing the org-rct in epoxy resin. The impact strength and softening temperature were measured, and the XRD, TEM and PAL were used to study the microstructure. For the sample of w=0.5%, both of impact strength and softening temperature were significantly increased. The positron lifetime spectra were measured for epoxy, rectorite, org-rct, and nanocomposites containing different rectorite content by using a conventional fast-fast coincidence system with a 22 Na source and a time resolution of 260ps. Each spectrum with million counts was collected twice. To whom correspondence should be sent. Email address: wsjh@public.wh.hb.cn Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 355-357 doi:10.4028/www.scientific.net/MSF.445-446.355

Evidence for charge transfer in Bi-based superconductors studied by positron annihilation☆

Abstract We have measured Doppler-broadening annihilation radiation (DBAR) spectra and positron lifetimes in normal and superconducting states for three kinds of Bi-based superconductors: Bi2212, Pb-doped Bi2223, Pb-and F-doped Bi2223. The difference spectra after deconvolution between two states show a sharpening effect with increasing temperature; the F-doped sample has the greatest amplitude in difference spectra but nearly the same positron lifetimes as the Pb-doped sample. The results are interpreted in terms of charge transfer between the Cu-O and Bi-O planes. The role of oxygen defects in charge transfer is discussed.

Structural instability in the Bi(Pb)-Sr-Ca-Cu-O superconductor between 80 and 300 K studied by positron annihilation

The authors have measured the positron lifetime as a function of temperature from 80 to 300 K for the Bi(Pb)-Sr-Ca-Cu-O superconductor (the Bi(Pb)2:2:2:3 phase). They have observed that the positron lifetime results display three narrow valleys near 120, 140 and 160 K and two wide valleys near 240 and 270 K. The anomaly in the positron lifetime indicates the structural instability of the Bi2:2:2:3 superconductor in the normal state, which may be caused by structural changes, such as the softening of the lattice, or the motion of vacancies in Bi-O layers. The authors have also carried out positron lifetime measurements as a function of temperature from 80 to 180 K in the Bi2:2:1:2 superconductor; on comparison with the results for the Bi(Pb)2:2:2:3 system, they found that adding Pb impurities can stabilize the Bi2:2:2:3 phase.

ON THE AGING CHARACTERIZATION OF POLY(ETHYLENE TEREPHTHALATE) BY POSITRON SPECTROSCOPY

Abstract Positron spectroscopy was used to study the free volume properties of glassy poly(ethylene terephthalate) as a function of temperature and aging time. The experimental results indicate that the glass transition temperature increases with increasing aging time. This is confirmed by differential scanning calorimetry measurements. From the aging time dependence of the intensity of the ortho-positronium (o-Ps) component I3 and the fractional free volume, we observe that the concentration of free volume holes and the fractional free volume show a continuous decrease at a fixed aging temperature, which suggests that structural relaxation and segmental rearrangement occur during physical aging.