Polyamide 6 (PA6) was studied using positron annihilation lifetime spectroscopy (PALS). From the ortho-positronium lifetime 3 the size of local free volumes (holes) was estimated. In dry PA6 the mean hole volume v varied between 70 Å 3 and 128 Å 3 when the temperature increased from 25 to 220°C. From the comparison of the coefficient of thermal expansion of macroscopic volume with that of hole volume a number density of holes N h of 0.8 nm Ϫ3 was estimated. The mean hole volume of PA6 exposed to atmospheres of different relative humidity (RH) was compared with the v(T) behavior of dry PA6. We propose a phenomenological model in which T g and v g , the glass transition temperature and the mean hole volume at T g , are allowed to vary with RH. Assuming T g (RH) as given in the literature, the v g (RH) were estimated from our experiments. In the range 0 Ͻ RH Ͻ 90%, the v g of the humid PA6 was always smaller than in the dry polymer, while near RH ϭ 100% both values became approximately identical. Two ranges of sorption behavior were observed. For small and medium RH, v g decreases by up to 19 Å 3 for RH ϭ 45%, which corresponds to more than half of the volume of a water molecule. This behavior of v g was interpreted as antiplasticization (loss of free volume) of the glassy polyamide. For larger RH, v g increases again toward the value of dry PA6, indicating a plasticization (gain of free volume) behavior that compensates and finally nullifies the former antiplasticization. The behavior of the local free volume in moist PA6 corresponds well to the known variation of the specific volume. Our results deliver experimental evidence for the decrease in the unrelaxed free volume of the (glassy) water-PA6 mixture with respect to that of the dry glassy polymer. The results are also discussed in terms of a simple hole-filling model and of the hypothesis of firmly and loosely bound water molecules in polyamides.

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Two-dimensional angular correlation of annihilation radiation ͑2D-ACAR͒ and coincidence Doppler broadening ͑CDB͒ of annihilation radiation measurements have been performed on electron-irradiated n-type 3C -SiC in which isolated silicon vacancies are responsible for positron trapping. After irradiation, the intensity of the CDB spectrum increased and decreased in low-and high-momentum regions, respectively. These features were explained by a theoretical calculation considering silicon vacancies. The central region of the 2D-ACAR spectra became isotropic after iradiation, while the overall anisotropies extending within the Jones zone were conserved suggesting that isolated silicon vacancies have tetrahedral symmetry, as expected from a previous electron spin resonance study.

The annealing behavior of defects in n-type 6H SiC epilayers irradiated with 2 MeV electrons have been studied using positron annihilation and deep level transient spectroscopy. Vacancy-type defects are annealed at 500–700 °C and 1200–1400 °C. From the analysis of Doppler broadening spectra (core electron momentum distribution), the latter annealing process is attributed to the disappearance of complexes related to silicon vacancies and not to nearest neighbor divacancies. Among the observed deep levels, the E1/E2 levels show similar annealing behavior to that of positron annihilation centers above 1000 °C. It is thus proposed that the E1/E2 levels originate from complexes containing silicon vacancies.

Annealing behavior of vacancies and the Z1/2 levels in n-type 4H–SiC epilayers after 2 MeV electron irradiation has been studied using positron annihilation and deep-level transient spectroscopy. Isochronal annealing studies indicate that silicon vacancy-related defects are primarily responsible for positron trapping. The Z1/2 levels are the predominant deep centers after irradiation and subsequent annealing at 1200 °C. Both the positron-trapping rate at vacancies and the Z1/2 concentration decrease in a similar manner while annealing from 1200 to 1500 °C. It is thus proposed that the Z1/2 levels originate from silicon vacancy-related defects.