Papers by Author: Bożena Zgardzińska

Abstract: Glasses with the compositions (100-x)(0.16Na₂O/0.10MnO/0.74SiO₂)/xFe₂O₃ (x = 0-15 mol%) were prepared and characterized using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) spectroscopy. The PALS method applied seems to be sensitive to verify the Verwey phase transition, discovered for bulk magnetite (Fe₃O₄), from a high-temperature ‘bad metal’ conducting phase to a low-temperature insulating phase occurring at about 120 K in the glasses examined. It means that at relatively low concentration of Fe₂O₃ up to 15 mol%, the magnetite crystals could be synthesized in the amorphous matrix of Na₂O/MnO/SiO₂/Fe₂O₃ glass that is important for numerous practical applications. The CDB measurements showed that the majority of positrons in the glass samples studied are annihilated in the vicinity of oxygen anions and iron-oxide containing glasses have lower concentration of oxygen-vacancy defects compared to the non-iron containing base glass.

Abstract: The positron annihilation lifetime spectroscopy (PALS) was applied to investigate the properties of capsules composed of n-alkanes (filling material) and polymer (shell) in the broad range of pressures up to 450 MPa. These microcapsules aggregate into the grains having about 200 μm in diameter. Their properties were investigated as a function of pressure (p) at several selected temperatures: when the filling material is in liquid, rotator and solid phase. Pressure experiments were performed without gas access to the sample and in an argon atmosphere. Two o-Ps components were found, the longer-lived correspond to the filler material, and the shorter-lived one – to the shell. These components change with p; even a small pressure (6 MPa) reduces considerably the o-Ps lifetimes (τ). At 303 K the o-Ps lifetime in the core changes non-monotonically, and at 60 MPa τ is higher than at 20 MPa. The increase of pressure induces the phase changes in the filling material, and also produces the deformation of microcapsule aggregates and crash of small capsules at the grain boundary region. Internal structure of the microcapsules was observed by SEM.

Abstract: The terminal part of the e₊ track (the positron blob) is formed during ionization slowing down and subsequent ion-electron recombinations produced by a positron. It releases up to 1 keV of energy, which is converted into heat within few picoseconds. If a bulk temperature of a medium is below, but close enough to its melting point, some region of a substance may melt, yielding a peculiar temperature dependence of the lifetime (LT) spectra. We have estimated properties of the molten region with a help of macroscopic heat con- duction equation and suggested a model describing temperature dependence of the ortho- positronium lifetime in frozen methanol, ethanol, butanol and water close to their melting points.

Abstract: Ortho-positronium lifetime in alkanes and alcohols was measured in a broad range of temperatures. The results were compared with predictions of the bubble model. The bubble radius can be determined from the Tao-Eldrup equation as well as calculated using the model. The depth of potential well and surface tension are the main factors ruling the Ps bubble size. The best fit of the bubble model to the experimental data is obtained assuming the rectangular potential well to be 1 eV deep (equivalent to an infinite one broadened by 0.166 nm) and the microscopic surface tension increased about 3 times comparing to the macroscopic one.

Abstract: A review of data about positron annihilation lifetime in normal alkanes is presented. It was found that positronium in rigid phase locates in the interlamellar gap, while in the rotator one in free volumes inside lamellae. Interesting effects seem to be: identity of o-Ps lifetime in rotator and liquid phase in long chain alkanes; appearance of “premature” rotator phase in nonadecane with low tricosane admixture, long-term relaxation after the changes of temperature or pressure in some binary mixtures, formation of porous structure at solidification of argon saturated alkane under high pressure.

Abstract: The PALS spectra of alkanes and their binary mixtures were measured as a function of temperature and pressure. It was found that in rigid phase positronium locates in the interlamellar gap, while in the rotator phase in the vicinity of nonplanar (kinked) conformers. An interesting feature is equality of o-Ps lifetimes in rotator and liquid phases in spite of different mechanisms of Ps formation. In binary mixture of nonadecane and tricosane an instability of phase transition from rigid to rotator in the range of tricosane admixture (3-10)% was observed. The pressure and temperature dependences of o-Ps lifetime and intensity are similar, 1 K is roughly equivalent to 4 MPa. The effect of “overpressing” the rotator phase was demonstrated in the mixture containing 10% of tricosane. Unstable rotator phase was observed up to pressure of 110 MPa at 300 K, while at this pressure the stable rotator phase needs the temperature over 310 K.

Abstract: Undamaged and damaged tungsten were investigated by means of positron spectroscopy and AFM surface images. Three different lifetimes (corresponding to dislocation loops, mono-vacancies, vacancy clusters and large size voids) were observed with different intensities. AFM shows the transition from hillocks to pinholes after W self-implantation.

Abstract: Positron lifetime spectra were measured for several alkanes under high pressure of argon and nitrogen. In the case of argon intercalation, the dependence of melting temperature on pressure is nonmonotonous, while in nitrogen this effect does not appear.