Andrew R. McGhie

Orientational disorder in solvent-free solid c70.

The high-temperature structure of solvent-free C70 has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

Structural phase transitions and orientational ordering in C70

Abstract The thermal behavior of solid C70 has been studied by synchroton X-ray powder diffraction and differential scanning calorimetry. The equilibrium solid state structures formed by C70 were solved by full profile refinement techniques in which orientational and packing disorder were explicitly accounted for. Above 345 K, C70 forms a plastic crystal, with an equilibrium face-centered cubic structure. At lower temperatures, orientational freezing occurs in two stages. Between 295 and 345 K, disorder persists only about the long axis of the molecule, and the lattice undergoes a rhombohedral distortion. Below 295 K, the rhombohedral lattice undergoes a further distortion, resulting in a previously unobserved monoclinic structure, in which the molecules are presumed to be essentially static. At all temperatures, however, the structure of C70 retains an ABC packing sequence.

The orientational phase transition in solid buckminsterfullerene epoxide (C60O)

Abstract Crystalline C 60 O, the first fullerene epoxide, has been studied using calorimetry and high resolution powder X-ray diffraction. At room temperature, C 60 O is orientationally disordered with a face-centered cubic lattice, a = 14.185 A. Upon cooling an orientational ordering transition at 278 K leads to a low temperature simple cubic phase. At 19 K, this phase is qualitatively similar to the orientationally ordered Pa 3 phase of C 60 , with a =14.062 A, but with additional randomness due to a distribution of orientations of the oxygen bonds.

Identification of a growth defect in solid C 60 by electron diffraction

The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid C 60 is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the 〈111〉 axis normal to the surface but not along other 〈111〉 axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 A radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C 60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.

X-ray Powder Diffraction Structure of Triclinic C60Br24(Br2)2

This paper reports the results of X-ray powder diffraction experiments on C60Br24(Br2)2. This material is a C60 derivative and shows rhombohedral symmetry at room temperature. At low temperature, a rhombohedral-to-triclinic phase transition can be observed that involves a large distortion of the cell while the volume of the unit cell remains almost constant. This is a sufficiently complicated system that high-resolution synchrotron data and a combination of different tactics were required to reach a structure solution.

Thermogravimetric apparatus temperature calibration using melting point standards

Abstract Temperature calibration of thermogravimetric apparatus is demonstrated using a dropping weight and fusible links made from melting point temperature standards of the International Practical Temperature Scale of 1968. Precision of the individual measurements is ±1.1°C. When used with a DuPont 951 Thermogravimetric Analyzer, temperature calibration to within ±2°C is obtained with a single linear calibration curve over the full 1200°C temperature range of the instrument.

A study of the thermal elimination reaction in a poly(p-phenylene vinylene) precursor

Poly(p-phenylene vinylene) (PPV) and its derivatives have recently received considerable attention in the field of electroactive organic materials chiefly because of their potential to replace the standard inorganic semi-conducting materials in light-emitting devices. Currently efforts are being made to reduce the carbonyl and hydroxyl defects in these materials which act as quenching sites for the injected charge. Thus, the understanding of the origin of these defects in PPV is extremely important in order to improve its electroluminescence efficiency. These defects are found to appear during the thermal conversion of PPV from its precursor. In this paper we have studied the thermal elimination reaction sequence in a PPV precursor using thermogravimetry and mass spectrometry and the observed results have been associated with standard reaction mechanisms to explain the formation of the aforementioned defects.

Solid State Polymerization of a Diacetylene: 2,4-Hexadiyne-1,6-Diol bis (p-Toluenesulfonate) (PTS)

Abstract Thermal polymerization of 2,4-hexadiyne-1,6-diol bis (p-toluene-sulfonate) in the solid state has been studied by differential scanning calorimetry. Slow polymerization during an induction period is followed by a fast polymerization regime which appears to obey first order kinetics. The induction period is almost sample independent whereas the fast polymerization is very sample dependent. The heat of polymerization is constant for single crystal samples, ΔH p= −31.2 kcal/mole, but increases for polycrystalline samples and for single crystals which have undergone some slow polymerization (<5%).

On the Stability of Mg Nanograins to Coarsening after Repeated Melting

Herein we report on the extraordinary thermal stability of approximately 35 nm Mg-nanograins that constitute the matrix of a Ti(2)AlC-Mg composite that has previously been shown to have excellent mechanical properties. The microstructure is so stable that heating the composite three times to 700 degrees C, which is 50 degrees C over the melting point of Mg, not only resulted in the repeated melting of the Mg, but surprisingly and within the resolution of our differential scanning calorimeter, did not lead to any coarsening. The reduction in the Mg melting point due to the nanograins was approximately 50 degrees C. X-ray diffraction and neutron spectroscopy results suggest that thin, amorphous, and/or poorly crystallized rutile, anatase, and/or magnesia layers separate the Mg nanograins and prevent them from coarsening. Clearly that layer is thin enough, and thus mechanically robust enough, to survive the melting and solidification stresses encountered during cycling. Annealing in hydrogen at 250 degrees C for 20 h, also did not seem to alter the grain size significantly.

Isothermal Oxidation of Ti2SC in Air

The oxidation behavior of fully dense Ti 2 SC was studied thermogravimetrically in air in the 500-800°C temperature range. The oxidation product was a single-layer of rutile in all cases. At 800°C, the oxide layer was not protective and the oxidation kinetics were rapid. At 600 and 700°C, and up to ∼50 h, the kinetics were parabolic before they became linear. It was only at 500°C that the weight gain reached a plateau after a 50 h initial parabolic regime. Mass spectrometry of the gases evolved during oxidation confirmed that both CO 2 and SO 2 are oxidation products. The overall oxidation reaction is thus Ti 2 SC + 4O 2 ― 2TiO 2 + SO 2 + CO 2 . On the basis of this and previous work, we conclude that oxidation occurs by the outward diffusion of titanium, sulfur, and carbon, the latter two either as atoms or in the form of CO 2 and SO 2 and, most probably, the inward diffusion of oxygen. Mesopores and microcracks were found in all rutile layers formed except those formed at 500°C. The presence of these defects is believed to have led to significantly higher oxidation rates as compared to other rutile-forming ternary carbides, such as Ti 3 SiC 2 .