Crystallization of poly(ethylene terephthalate) under tensile strain: crystalline development versus mechanical behaviour

The crystallization of poly(ethylene terephthalate) (PET) under tensile strain was investigated using wide angle X-ray diffraction. Real-time investigation of the crystallization state, including the crystalline ratio and the crystallite orientation, of the material could be undertaken due to the high brilliance of the synchrotron X-ray source used in our study. Initially amorphous PET specimens were stretched at different strain rates and draw ratios at the same temperature (above and close to T_g). Our experimental set-up was designed to undertake simultaneous recording of the X-ray diffraction patterns and the mechanical parameters. Up to the draw ratio of 500% and draw rate of 0.75 s⁻¹, the crystalline development dynamics corresponded to three different regimes. (i) For small enough extension rates, there was no measurable crystallinity during the drawing process. The crystallization developed after cessation of deformation. (ii) For intermediate extension rates, the whole crystallization process took place during the deformation. (iii) For the highest extension rate involved in our experiments, the crystallization started during the drawing process and continued after cessation of deformation. The mechanical behaviour of the polymer was simultaneously recorded and correlated with the induced crystalline microstructure. In particular, we were able to discriminate the influence of crystallite orientation and crystallization growth on the mechanical behaviour of the material.     

Experimental and DFT Study of Structural and Optical Properties of Kesterite-Type Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> Compound for Solar Cell Applications

In this work, Cu₂ZnSnS₄ (CZTS) thin films were prepared by thermal evaporation from Cu₂SnS₃ and ZnS initially mixed by a mechanical alloying process. Structural and optical properties of CZTS films have been studied. X-ray diffraction results showed that the semiconductor has the Kesterite structure, and the optical absorption coefficient and band gap energy of the thin films were about 10⁴ cm^?1 and 1.46 eV, respectively. The structural and optical properties of Kesterite CZTS, studied by using the full potential linearized augmented plane wave method within the density functional theory, showed good agreement with our experimental results. The surface morphological studies revealed the formation of a smooth, compact and uniform CZTS surface.     

First-Principles Calculations of the Dynamical and Thermodynamic Properties of Yttrium Compounds YX (X = N,P, As and Sb)

The structural, dynamical and thermodynamical properties of the yttrium pnictides YN, YP, As and YSb compounds in the NaCl (B1) stable phase and the CsCl (B2) unstable phase structures are studied by performing ab initio calculations with the help of the generalized gradient approximation (GGA). The computed lattice parameters; lattice constants, static bulk modulus and corresponding first-order pressure derivative of the bulk modulus are obtained in the two structures NaCl and CsCl and compared with other theoretical and experimental values. The transition pressure from B1 type to B2 type phases is presented and it is about 171, 72, 75 and 34 GPa for YN, YP, YAs and YSb compounds respectively. To obtain the dispersion relation curves and density of states we used the linear-response approach to the density functional theory in both structures. The values of phonon frequencies are used to obtain the thermodynamic quantities such as entropy and heat capacity as function of temperature at different pressures.