Positron annihilation properties

Network properties and microscopic structures of various epoxy resins cross-linked by phenolic novolacs were investigated by Suzuki et al.97 Positron annihilation spectroscopy (PAS) was utilized to characterize intermolecular spacing of networks and the results were compared to bulk polymer properties. The lifetimes (t3) and intensities (/3) of the active species (positronium ions) correspond to volume and number of holes which constitute the free volume in the network. Networks cured with flexible epoxies had more holes throughout the temperature range, and the space increased with temperature increases. Glass transition temperatures and thermal expansion coefficients (a) were calculated from plots of t3 versus temperature. The Tgs and thermal expansion coefficients obtained from PAS were lower titan those obtained from thermomechanical analysis. These differences were attributed to micro-Brownian motions determined by PAS versus macroscopic polymer properties determined by thermomechanical analysis. 

Sufficient atomic particle research has been accomplished to warrant discussion of possible methods of applying energy available from particle mass annihilation to rocket propulsion. Complete conversion of matter to energy would allow exhaust velocities near that of light to be obtained from a propulsion device. Antimatter, by definition is matter made up of antiparticles, such as antineutrons, negatrons (antiprotons), and positrons (anheledrons). An annihilation property is known to exist between particles with one particle termed the anhparticle of the other. 

The areas of inorganic and organic positron chemistry deal mainly with material characterization and industrial applications using PAS. Both chemical and electronic industries have found PAS to be a powerful method. In addition to the traditional solution chemistry of the positron and Ps [11], PAS has been developed to determine the free volume Bom-Oppenheimer approximation, such as molecular solids [14] and polymers [15]. The unique localization property of Ps in free volumes and holes has opened new hope in polymer scientific research that determination of atomic-level free volumes at the nanosecond scale of motion is possible. During the last ten years, most positron annihilation research has involved a certain amount of polymer chemistry, polymers and coatings, which will be discussed in Chapters 12 and 13. For inorganic systems, oxides are mostly studied using the positron and Ps. Silicon oxides and zeolites are the most important systems in positron and Ps chemistry. The developments in this area have on the cavity structure and chemical states of inner surfaces. Chapters 8 and 14 will discuss this subject. 

Numerous applications of positron annihilation spectroscopy (PAS) in investigations of the physico-chemical properties of matter require a precise understanding of the process of Ps formation. Usually it proceeds on a picosecond time scale and is strongly influenced by early (pico- and femtosecond) processes initiated by ionizations in the track of a fast positron. These early intratrack processes initiate all subsequent chemical transformations and, consequently, take a key position in radiation chemistry. 

The annihilation characteristics of a positron in a medium is dependent on the overlap of the positron wavefunction with the electron wavefunction [9]. From a measurement of the two photon momentum distribution, information on the electron momentum distribution can be obtained and this forms the basis of extensive studies on electron momentum distribution and Fermi surface of solids [9]. In the presence of defects, in particular, vacancy type defects, positrons are trapped at defects and the resultant annihilation characteristics can be used to characterize the defects [9, 10], Given these inherent strengths of the technique, in the years following the discovery HTSC, a large number of positron annihilation experiments have been carried out [11, 12]. These studies can be broadly classified into three categories (1) Studies on the temperature dependence of annihilation characteristics across Tc, (2) Studies on structure and defect properties and (3) Investigation of the Fermi surface. In this chapter we present an account of these investigations, with focus mainly on the Y 1 2 3 system (for an exhaustive review, see Ref. 11). 

Positronium formation and annihilation behavior in Si and Si02 thin films are reviewed. Positronium is highly sensitive to pore (or void) sizes, surface properties of pores, defects near pore surfaces, etc., in various Si and Si02 samples. Therefore, not only positron annihilation spectroscopy but also positronium annihilation spectroscopy is useful for characterization of Si and Si02 materials. 

Djermouni, B., Ache, H.J. (1980) Effect of casting solvent on the properties of styrene-butadiene-styrene block copolymers studied by positron annihilation techniques . Macromolecules, 13,168. 

Venkateswaran, K., Cheng, K.L., Jean, Y.C. (1984) Application of positron annihilation to study the surface properties of porous resins . J. Phys. Chem. 88,2465. 

Jean, Y.C., Yuan, J.-P., Liu, J., Deng, Q., Yang, H. (1995) Correlations between gas permeation and free volume hole properties probed by positron annihilation spectroscopy . J. Poly. Sci..Part B Poly. Phys.33,1. 

At the same time investigations using light scattering, electron microscopy, positron annihilation, dielectricity and transport properties 30,- ) indicated the surfactant molecules not to be Involved in associations to colloidal size aggregates at these low water contents. The low light scattering intensity rather points to the surfactant molecules not to be inter-associated (Fig. 7). 

Positron annihilation lifetime spectroscopy (PALS) is a commonly used technique for the investigation of the electronic properties of condensed matter. The first application of positrons in condensed matter was in the study of electronic structure of metals and in the characterization of defects in solids [1]. 

Wang, Z.F. Wang, B. Yang, Y.R. Hu, C.P. Investigation of gas permeation and free volume hole properties of polyurethane membranes by positrons. In Positron Annihilation, ICPA-13, Proceedings, 2004 Vol. 445, 352. 

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

Furthermore, in addition to the bulk thermal properties of polymers and resists, determination of Tg of film interfaces and of ultrathin films has become an important issue in thin film imaging (bilayer, 157 nm, and EUV). Various techniques have been employed, which include ellipsometry [481,482], positron annihilation spectroscopy (PALS) [483], QCM [484], scanning viscoelasticity microscope (SVM) [485],x-ray reflectivity [486,487], and thermal probe [488]. 

In this chapter we discuss briefly most of these applications. Furthermore, the S-S equations of state are used to describe surface tension in Chapters 8, for analysis of positron annihilation lifetime spectroscopy in Chapters 10 to 12, for analysis of glassy and molten polymeric nanocomposites in Chapters 4 and 14, and to describe flow properties in Chapter 16. 

Dlubek, G., Stejny, J., and Alam, M. A., Effect of cross-linking on the free-volume properties of diethylene glycol bis(allyl carbonate) polymer networks a positron annihilation lifetime study. Macromolecules, 31,4574-4580 (1998b). 

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