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.     

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.     

SHI induced modification in CR-39 polycarbonate by positron annihilation lifetime studies

CR-39 is a polycarbonate widely used as SSNTD for recording nuclear charged particles and in other applications. Latent ion tracks produced in the polymers due to the damage produced by the passage of Swift Heavy Ions contain amorphous material with highest degree of disorder, changing the free volume properties which have strong correlation with the macroscopic properties of the material. Positron annihilation lifetime spectroscopy (PALS) provides direct information about the dimension, content and hole size distribution of free volume in polymers. The effect of irradiation of ⁴⁰Ar (14.9 MeV/n) ions on CR-39 polycarbonate by Positron Annihilation Lifetime Spectroscopy (PALS) is reported here. PALS provides a non-destructive and non-interfering probe, having high detection efficiency for free volume hole properties. From o-Ps lifetime mean free volume hole radius and average free volume of the micro-voids have been calculated. PAL measurement shows an increase in free volume on irradiation.     

Effects of oxygen in gamma irradiated aromatic polyesters in film

The effect of gamma irradiation in air is investigated on four thermoplastic polyesters (PET, PBT, PEN, PCT-co-ET) in films containing aromatic rings, in order to evaluate the influence of aromatic density and the role of oxygen on radiation resistance. Physical-chemical-nuclear analyses were used to this purpose. EPR measurements were carried out to detect radical stability against oxygen permeation. Viscometric investigations reported a very similar trend for all the investigated polyesters: a chain break effect that decreases at the highest doses. FT-IR analyses focused on the formation of oxidized species. Positron annihilation spectra pointed out a decrease of the intensity of ortho-positronium formation, while its lifetime remains unchanged with radiation dose.     

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.     

冲击波处理的结晶MgO的化学分析正电子谱研究

冲击波处理的结晶ＭｇＯ的化学分析正电子谱研究＊张菊郑小明（杭州大学催化研究所，杭州３１００２８）常志向（抚顺石油化工研究院，抚顺１１３００１）关键词冲击波，化学分析正电子谱，结晶氧化镁，丁烯，脱氢冲击波是在介质中以超音速传播的压力脉冲．气体中的冲击波...     

钆铁磁－顺磁相变的正电子寿命研究

通过测量金属钆在居里点附近的正电子湮没寿命谱随温度变化的关系，发现钆的正电子湮没寿命在居里点近邻Ｔ＜ＴＣ一侧随温度有显著变化，而在此温区以外几乎不随温度变化     

侧链型丙烯酸酯共聚物相变的正电子湮没谱研究

利用正电子湮没技术对侧链型热致高分子液晶丙烯酸酯共聚物进行了变温相变研究．除实验标识出样品的相变温度点外，根据试样中自由体积随温度的变化关系，对高分子液晶材料内部立链、侧链以及介晶基元的相变行为特点进行了探讨，并就与小分子液晶变化特点的一些不同做了解释．     

用正电子湮没寿命谱法研究锡镀层缺陷

光亮镀锡工艺由于成本低、无毒、镀层稳定性好而广泛应用于电子元器件引线的电镀.但因锡镀层与基体之间会发生原子的相互扩散,致使产品在储存、焊接或使用期间因温度效应而使某些重要的物理和化学性能,如钎焊性、导电性和防腐性变差。金属界面原子的热扩散行为与金属晶体的结构缺陷密切相关,因此应用正电子湮没寿命谱法研究锡镀层在热处理过程中缺陷的运动规律,在理论和工艺方面都具有重要的意义。