Changxing Cui

Fullereneynes: a new family of porous fullerenes

Abstract We propose a new family of C n cage molecules which have the same highest possible symmetry of I h and the same number of sp 2 carbon atoms as either C 60 or related larger fullerenes, but in addition contain sixty or more sp-bonded carbon atoms. The cages of these fullereneynes are porous, which should facilitate the reversible formation of endohedral compounds, as well as complexes in which dopants reside in the cluster shell. We determine the optimized geometries and formation energies of two prototypical examples, namely the C 120 and C 180 molecules, using a tight-binding Hamiltonian which has been applied successfully to large carbon clusters. The C 120 and C 180 fullereneynes are expected to be stable despite a binding energy which is about 1 eV per atom lower than for graphite. Moreover, the per atom binding energy dramatically increases with increasing size of fullereneynes containing equal number of sp 2 and sp carbon atoms and approaches a value which is only 0.2 eV/atom lower than for the C 60 fullerene.

Quantum‐mechanical oligomer approach for the calculation of vibrational spectra of polymers

The force‐constant matrix of an oligomer, composed of five or more repeat units with appropriate terminal groups, can be used to construct the force‐constant matrix of the corresponding planar or helical polymer. The calculations on some typical polymers (polymeric sulfur, polyacetylene, and polyethylene) show that the oligomer approach is accurate enough to duplicate the vibrational frequencies of frozen‐phonon calculations. The oligomer approach has much more predictive power if used in conjunction with some form of empirical scaling (scaled quantum‐mechanical oligomer force field for polymers). ir and Raman selection rules for helical polymers are also discussed. Vibrational frequencies and intensities of polymeric sulfur and polyethylene have been calculated at the ab initio STO‐3G, 4‐31G, 6‐31G, and 6‐31G* basis‐set levels. The agreement of frequencies with experiment is excellent. The quality of calculation is limited by the basis set and theoretical model used rather than the oligomer approach.

A carbon phase that graphitizes at room temperature

Abstract The observed properties and diffraction data for a diamond-like carbon phase that graphitizes at room temperature are shown to be consistent with predictions for “rectangulated diamond” carbon phases. These phases contain eclipsed sp 3 carbon layers having the intra-layer connectivity of graphite - just like the case for hexagonal diamond. However, in contrast with known diamond polytypes, these phases contain four-carbon rings of the type found in cyclobutane and in the bridges between cages in C 60 polymers.

Improved piezoelectric ceramic/polymer composites for hydrophone applications

Composites of piezoelectric ceramic powders and organic polymers are described that provide highly desirable properties for sonar sensors: (1) high figures of merit, g h d h = 50(±5)x10 -13 m 2 /N and g h d h /tanδ = 3.3xl0 -10 m 2 /N, (2) pressure-independent sensitivity up to high pressures (14 MPa, which is 2000psi), (3) thermal stability of the poled state for months at 100°C, (4) high dielectric constants (60-70), (5) convenient melt processibility, and (6) robust mechanical properties.

Fabrication and characterization of three-dimensional periodic ferroelectric polymer-silica opal composites and inverse opals

Using a high temperature solution infiltration process, ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer is infiltrated into three-dimensional (3D) periodic opal lattices with the silica opal diameters of 180, 225, and 300 nm to form periodic composite structures. By etching out the silica opal, inverse copolymer opals can be fabricated, which retains the 3D periodic structure of the original silica opal lattice. In addition to the optical observation, x-ray diffraction and dielectric study were carried out to characterize the change in the ferroelectric behavior of the composites and inverse opals. Although the copolymer in the composites and inverse opals remains ferroelectric, the ferroelectric transition in the composites and inverse opal becomes diffused and moves to a lower temperature, which is due to the random stress introduced by the irregular voids and interfaces and may be made use of to facilitate the transformation of the copolymer into a relaxor. These results suggest the...

Improved piezoelectric s for hydrophone applications based on calcium-modified lead titanate/poly(vinylidene fluoride) composites

Abstract Piezoelectric composites comprising dispersions of free-flowing calcium-modified lead titanate powders in a poly(vinyiidene fluoride) matrix have been developed and characterized as materials for hydrophone applications. We find that these composites provide highly desirable properties: (1) high figures of merit, g h d h =50(±5) × 10 −13 m 2 N −1 , and a relatively low dielectric loss tangent, less than 0.018; (2) convenient melt processibility; (3) no significant sensitivity changes in going to high pressures (14 MPa or 2 kpsi); (4) thermal stability of the poled state for months at 100 °C; and (5) easy fabrication. This performance results from material choices and from special ceramic powder processing and composite preparation methods that optimize the achievable degree of ceramic poling, the remanent polarization, and the transfer of mechanical energy to the dispersed ceramic. Materials design concepts, composite processing, and device construction and evaluation are described.

AB initio study of the dynamical properties of polythiophene

Abstract The dynamical properties of polythiophene (PT) have been studied by our newly developed oligomer approach on the basis of ab initio calculations on PT oligomers. The vibrational frequencies and intensities of PT for spectroscopically active modes have been obtained, and are in reasonable agreement with the available experimental results. The predicted vibrational spectrum can be fully tested by the measurements of the polarized IR and Raman spectra, which have not been carried out yet.

Hinged and chiral polydiacetylene carbon crystals

Abstract Structures and properties are calculated for hinged and chiral polydiacetylene carbon phases, which consist entirely of carbon chains found in organic polydiacetylenes. Diffraction and spectroscopic results are consistent with those for the controversial reported carbon phases of unknown structure, which might have accidentally formed by the reaction of acetylenic chains. Predicted properties include (1) a semiconductor-to-metal transition upon doping, (2) a soft deformation mode that results in negative linear and area compressibilities, negative Poisson ratios and expected shape-memory and ferroelastic behavior for the hinged phases, and (3) optical activity, second harmonic generation and torsional and transverse compressional piezoelectricity for the chiral phase.

Nanostructured thermoelectrics based on periodic composites from opals and opal replicas. I. Bi-infiltrated opals

We have developed a variety of templating processes for the fabrication of three-dimensionally periodic, nanostructured thermoelectrics from opal and inverse opal matrices. These opals and inverse opals are periodic at optical wavelengths and have extremely high interfacial area. It was hoped that scattering processes at the interface between opal and infiltrated thermoelectric material would increase the thermoelectric figure of merit (ZT) by having a greater effect on phonon-mediated (lattice) thermal conductivity than on electronic conductivity. We provide the first demonstration that this approach can increase ZT by evaluating a simple prototype system: bismuth infiltrated into porous SiO/sub 2/ opal. We find a larger fractional decrease in thermal conductivity than for electrical conductivity (relative to bulk polycrystalline Bi). Since the thermopower is little changed, the overall effect we observe is as much as a two-fold increase of ZT compared with that for polycrystalline bulk bismuth. However, the observed ZT is much smaller than for single crystal bismuth.

Improved piezoelectric 0-3 ceramic particle/polymer composites

Methods and compositions are described for the preparation of piezoelectric ceramic-polymer composites having improved properties for applications in piezoelectric sensors. These composites consist of piezoelectric ceramic particles embedded in a polymer matrix. The improvements result from investigations of the effects of particle size, particle size dispersity, volumetric loading levels, ceramic and polymer dielectric constants, particle percolation, and processing conditions on performance.