Papers by Keyword: Free Volume

Abstract: Ultra-high-temperature ceramics (UHTC) such as ZrB₂ and HfB₂ with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB₂ and HfB₂ with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a ²²Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB₂ has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB₂ and SiC. The SEM observations showed that HfB₂ has larger particles compared to ZrB₂ and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB₂-SiC and HfB₂-SiC composites. HfB₂-SiC composites seem to be more brittle compared to ZrB₂-SiC composites. This may be due to larger size and smoother surface of HfB₂ particles (600 nm) compared to ZrB₂ particles (240 nm).

Abstract: Structural relaxation through isothermal annealing below the glass transition temperature was conducted on a Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass. Differential scanning calorimetry was used to quantify enthalpy differences between the as-cast and relaxed samples, which were then related to average free volume differences. The influence of structural relaxation on plasticity was examined. While the free volume decreasement can be clearly observed between the as-cast and relaxed samples, structural relaxation is not accompanied by severe embrittlement.

Abstract: Nano inorganic particles, as a kind of rigid inorganic particles, can combine the characteristics of organic polymers and inorganic to obtain the composite materials with excellent comprehensive properties. In this thesis, nanosilica after surface modification was used as the modified agent, adding to polypropylene materials to get a series of polypropylene films with different content of nanosilica. Free volume characterizations of different polypropylene films were measured using the positron annihilation lifetime spectroscopy technique, and the residual solvents values were measured. Researches show that the free volume of polypropylene films changes result from the changes of nanosilica content. With the free volume of polypropylene films decreases, the residual solvents values become lower.

Abstract: There is a growing interest for investigation of free-volume type defects in SPD-processed ultrafine-grained materials. In the present work, excess free volume in pure Fe prepared by shaped rolling is studied by high-resolution differential dilatometer as well as Archimedes method. The results show the formation of excess volume with increasing deformation strain. The volume fraction of free volume detected by dilatometer is below the evaluation of Archimedes method maybe because of the existence of void in the deformed metal. Microstructural study by field emission type scanning electron microscope confirms the presence of voids.

Abstract: Depth profile of subsurface chemical composition and free volume in segmented polyurethane-urea/clay nanocomposites was studied by angle resolved X-ray photoelectron spectroscopy (ARXPS) and Doppler broadening energy spectroscopy (DBES) using slow positron beam. The ARXPS studies revealed increasing N/C atomic ratio (hard segment to soft segment ratio) at any given depth for the clay loaded samples compared to the neat polymer. DBES study revealed significant microstructure modification with clay loading. Self segregation of hard and soft segments in neat polymer and their interspersing with clay loading was observed from DBES measurements.

Abstract: In this work positron annihilation lifetime spectroscopy (PALS) is employed to study the ortho-Positronium lifetime parameters τ₃>, σ₃ and I₃ in two structurally simple amorphous polymers 1,2-poly(butadiene) and cis-1,4-poly(isoprene). The ortho-Positronium mean lifetime parameter τ₃> is compared with dynamics data from broadband dielectric spectroscopy (BDS) and electron spin resonance (ESR) experiments. Coincidences of characteristic temperatures from PALS with BDS and ESR experiments show close relationships between positron lifetime and dynamics of polymers.

Abstract: The Poly N-Isopropyl acrylamide (PNIPA) gels were prepared in different synthesis media by crosslinking with acrylate monomers with varying chain length. The microstructural characterization in terms of the free volume fractions and distributions were studied using positron annihilation lifetime spectroscopy (PALS). The equilibrium swelling of the PNIPA in distilled water was measured. An attempt has been made to understand the correlations between microscopic free volumes and the equilibrium swelling of the gels. The free volume fraction of the gels prepared in Dimethyl formamide (DMF) was directly correlated with swelling while it varied inversely with chain length of the crosslinker. The microstructure of the gels was found to be very sensitive to the solvent/medium used for polymerization. Our studies show that the swelling property is influenced by the free volumes, chain relaxation as well as the chemical nature of the cross linkers.

Abstract: Two selected solvents were incorporated into the olanzapine structure. Molecules located inside the voids in the structure caused the changing of the o-Ps lifetime and intensity. After removing the n-butanol●H₂O molecules, the materials returned to its initial structure. Creating the complex with the methanol●H₂O leads to the change of the crystallographic structure of the olanzapine.

Abstract: The subnanoscopic structures of polyrotaxanes, prepared from α-cyclodextrins, poly(ethylene glycol), and bulky adamantane end groups, were examined by means of the positron annihilation lifetime technique, in consideration of the free-volume hole, quantified from the long-lived ortho-positronium (o-Ps) lifetimes. The influence of the chemical structure on the temperature dependence of the o-Ps lifetimes are discussed.

Abstract: Polypropylene (PP) derivatives, prepared with different α-olefin comonomers of hexane and hexenol, were investigated using the positron annihilation lifetime technique. The free-volume hole sizes for the present polymers were quantified from the long-lived ortho-positronium (o-Ps) lifetimes. In contrast to the case of nonpolar, introduction of the polar hexenol into PP reduced the o-Ps lifetimes and the hole sizes derived from them. This indicates that the interchain interaction due to the hydrogen bonds in the amorphous region is much stronger than the enhancement of the segmental motion due to the butyl long branch. A comparison of the o-Ps lifetimes with the corresponding tensile strength suggested significant contribution of the subnanostructure in the amorphous region to the mechanical property.