Effect Of Size On The Phase Transition Of Several Nanomaterials

With the size reduction down to nanometer scale, owing to the small size effect,quantum confinement effect and surface and interface effect, many physical andchemical properties of a solid, such as thermodynamic, electric, magnetic, optic, andcatalytic performance expose novel properties that are different from bulk materials.Although the classical approaches such as thermodynamics and molecular dynamicstheory can study the size effect on the properties of various nanomaterials, bothencounter some difficulties, for instance, without building a whole physicalmechanism for nanomaterials. Due to the large ratio of undercoordinated surfaceatoms and ite interaction, nanomaterials are thus different than corresponding bulkmaterials and single atom. According to the Bond Order-Length-Strength correlation(BOLS), if one bond breaks, the remainder between the undercoordinated atomsbecome shorter and stronger, the BOLS is one important point of explaining thephysical mechanism of novel performance of nanomaterialsUtilizing the BOLS correlation and Local Bond Average approach (LBA), thisthesis focuses on the size effect on the phase transition temperature and pressure ofseveral nanomaterias, major progress are followed:1. For the phase transition temperature is proportional the atom cohesive energy,we report the modeling of the finite size effect on the two phase transition (theorder-disorder phase transition of FePt, FePb, CoPt nanoparticles and the solid-solidphase transition of Cu2S and ZnS nanosolids) in the nanometer scale from theperspective of the BOLS correlation. Consistency between the theory predictions andthe measured size dependence of the phase transition temperature of two typenanostructures clarify that: i) the phase transition temperature is intrinsicallyproportional to the mean cohesive energy of atoms for nanocrystals; ii)undercoordination atoms at sites surrounding atomic defects and at the surface inducethe loss in cohesive energy; iii) in the core-shell structure, only the outermost threeatomic layers effect on the decline of TCyet atoms in core interior remain as they arein corresponding bulk materials; and iv) the size induced trend of TCdepressiondepends on the skin-to-body volume ratio.2. Take advantage of LBA, we build the function expression between themeasurable physical quantities and identities of its representative bonds, and theenergetic responses of these bonds (bond nature, order, length and strength) to external stimuli; and then,combing the BOLS correlation and LBA approach, wederive a general rule for the size, pressure and temperature dependence of phasetransition; at last, we obtain theory predictions and the measured size effect on thephase transition pressure of CdSe and ZnS nanosolids, the consistency affirms that theincrease of the pressure enlarge the atomic bond energy and atomic cohesive energy isalso getting higher, thereby the structure stability of nanomaterials enhance and thephase transition pressure increase.The reconciliation of the theoretical predictions and measurements verified thecorrectness of the BOLS correlation in explaining the size dependence ofnanomaterials, and provided a powerful tool for us to further exploring the novelperformance of nanomaterials..