石墨和纳米碳中缺陷和电子密度随温度的变化

测量了石墨和纳米碳在不同温度下的正电子寿命谱,研究了石墨和纳米碳中缺陷和电子密度随温度的变化.结果表明,纳米碳中缺陷的开空间和缺陷浓度分别大于和高于石墨晶体;纳米碳的平均自由电子密度低于石墨晶体.当温度从25K升至295K时,石墨和纳米碳中的平均自由电子密度随温度的升高而下降:石墨晶体中的自由电子密度随温度的升高变化较小;纳米碳的自由电子密度随温度的升高变化较大.随着温度的升高,石墨和纳米碳中的热空位数量增多,而且这些空位可迁移至微孔洞的内表面使微孔洞的开空间增大.     

Inelastic cross sections for pentane isomers by positron impact

ABSTRACT

This work aims at the calculation of various inelastic cross sections for three pentane isomers, namely normal pentane, isopentane and neopentane. The direct ionisation, positronium formation, total ionisation and total inelastic cross section are reported for these targets using modified spherical complex optical potential (mSCOP) and complex scattering potential-ionisation contribution (CSP-ic) method. The cross sections are computed for a wide energy range from their respective thresholds to 5?keV. We have also attempted to probe the isomeric effect in the inelastic scattering of positrons from the pentane isomers. The cross sections for the three isomers were found to overlap for the entire comparative energy range except at the peak region. Hence, in general no appreciable isomeric effect was beheld for the pentane isomers.     

Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 °C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature increasing up to 1400 °C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200 °C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 °C. However, after 1400 °C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.     

Mesoporous titania films investigated by positron annihilation based on a slow positron beam

Tunable mesoporous titania (TiO₂) thin films were synthesized via a sol-gel method using an amphiphilic triblock copolymer F38 as the structural template. The dependence of crystalization, pore morphology and interconnectivity of TiO₂ films on the weight ratio of F38 was studied by wide-angle X-ray diffraction, field emission scanning electron microscopy and Doppler broadening of positron annihilation radiation spectroscopy based on a slow positron beam. By loading more F38, the crystallization of TiO₂ films is enhanced, accompanied by a decrement in oxygen vacancies/grain boundaries. Smaller and isolated mesopores are formed in the films prepared with F38 less than 15?wt%. The pore percolation occurs when the weight ratio of F38 is up to 20?wt% and larger and interconnected worm-like pores are formed.     

Evidence of formation of tetravacancies in uniformly oxygen irradiated n-type silicon

High purity n-type silicon single crystal with resistivity in the order of 4000 Ω cm has been irradiated with high-energy oxygen ions at room temperature up to a fluence of 5E15 ions/cm². The energy of the beam was varied from 3 to 140 MeV using a rotating degrader to achieve a depthwise near-uniform implantation profile. Radiation induced defects and their dynamics have been studied using positron annihilation spectroscopy along with isochronal annealing up to 700 °C in steps of 50 °C for 30 min. After annealing the sample at 200 °C for 30 min, formation of silicon tetravacancies has been noticed. The formation of the tetravacancies was found to be due to agglomeration of divacancies present in the irradiated sample. An experimentally obtained positron lifetime value of 338±10 ps has been reported for silicon tetravacancies, which has a very close agreement with the value obtained from recent theoretical calculations. The tetravacancies were found to dissociate into trivacancy clusters upon further annealing. The trivacancies thus obtained were observed to agglomerate beyond 400 °C to form larger defect clusters. Finally, all the defects were found to anneal out after annealing the sample at 650 °C.     

Defect engineering of ZnO

The defect responsible for the transparent to red color change of nominally undoped ZnO bulk single crystals is investigated. Upon annealing in the presence of metallic Zn as reported by Halliburton et al. and also Ti and Zr a native defect forms with an energy level about 0.7 eV below the conduction band. This change is reversible upon annealing in oxygen. Optical transmission data along with positron depth profiles and annealing studies are combined to identify the defect as oxygen vacancies. Vacancy clustering occurs at about 500 °C if isolated zinc and oxygen vacancies. In the absence of zinc vacancies, clusters form at about 800 °C.     

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55 nm at 15 MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350 °C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410 °C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ₁). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τ_av), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.     

Simultaneous acquisition of magnetic resonance spectroscopy (MRS) data and positron emission tomography (PET) images with a prototype MR-compatible, small animal PET imager

Multi-modality imaging (such as PET-CT) is rapidly becoming a valuable tool in the diagnosis of disease and in the development of new drugs. Functional images produced with PET, fused with anatomical images created by MRI, allow the correlation of form with function. Perhaps more exciting than the combination of anatomical MRI with PET, is the melding of PET with MR spectroscopy (MRS). Thus, two aspects of physiology could be combined in novel ways to produce new insights into the physiology of normal and pathological processes. Our team is developing a system to acquire MRI images and MRS spectra, and PET images contemporaneously. The prototype MR-compatible PET system consists of two opposed detector heads (appropriate in size for small animal imaging), operating in coincidence mode with an active field-of-view of approximately 14 cm in diameter. Each detector consists of an array of LSO detector elements coupled through a 2-m long fiber optic light guide to a single position-sensitive photomultiplier tube. The use of light guides allows these magnetic field-sensitive elements of the PET imager to be positioned outside the strong magnetic field of our 3T MRI scanner. The PET scanner imager was integrated with a 12-cm diameter, 12-leg custom, birdcage coil. Simultaneous MRS spectra and PET images were successfully acquired from a multi-modality phantom consisting of a sphere filled with 17 brain relevant substances and a positron-emitting radionuclide. There were no significant changes in MRI or PET scanner performance when both were present in the MRI magnet bore. This successful initial test demonstrates the potential for using such a multi-modality to obtain complementary MRS and PET data.     

Planned positron experiments at FRM-II

The new research reactor FRM-II near Munich has a strong positron source, which delivers an intense, nearly monoenergetic positron beam. Our positron systems, the pulsed low energy positron source (PLEPS) and the scanning positron microscope (SPM) will be operated at this beam. Some aspects of matching these systems to the new positron source will be discussed.Considerable improvements are expected, e.g. more than 10⁵ s⁻¹ recorded events at PLEPS and sub-micrometre resolution at SPM. They will enable investigations in so far inaccessible problems like the evaluation of annihilation characteristics and trapping constants of individual defects or studies of fast dynamical processes. In applied materials science complex defect structures will be studied which demand a resolution into many differing lifetimes, e.g. fractured specimens, wear, corrosion, etc. Also large series of measurements at small systematic modifications are planned. There is also the opportunity to analyse in addition the chemical microstructure of the specimens by means of a hydrogen microprobe and other ion beam techniques available close to FRM-II at the Technical University of Munich.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.