Gary A. Carlson

Intercalation of molecular species into the interstitial sites of fullerene

Molecular species were found to diffuse readily into the octahedral interstitial sites of the fcc lattice of C{sub 60}. The {sup 13}C NMR spectrum of C{sub 60} under magic angle spinning (MAS) conditions consisted of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The downfield shift obeys Curies law and is attributed to the Fermi-contact interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C{sub 60} molecules. Exposure of C{sub 60} to 1 kbar oxygen for 1.75 h at room temperature resulted in a spectrum of seven evenly spaced resonances corresponding to the filling of 0 to 6 of the adjacent octahedral interstitial sites with oxygen molecules. The distribution of site occupancies about a C{sub 60} molecule provided evidence that the intercalation process is controlled by diffusion kinetics. Exposure to 0.14 kbar hydrogen gas at room temperature for 16 h filled a substantial fraction of the interstitial sites of C{sub 60} and C{sub 70} with hydrogen molecules.

A three-dimensional model for lignocellulose from gymnospermous wood

Abstract The three-dimensional characteristics of lignocellulose complexes are examined by the use of molecular mechanics and dynamics. The model created is composed of cellulose, hemicellulose, and lignin as would be present in gymnospermous wood tracheids. Molecular mechanics and dynamics show that the hemicellulose is hydrogen bonded and oriented parallel to the cellulose microfibrils. Energy minimization of lignin units linked by the major lignin type bond reveals a helical structure. On the basis of these findings we propose a model for three-dimensional organization of lignocellulose.

Dynamic tensile strength of glycerol

By use of a tensile stress wave 0.1 to 0.4 μsec in duration, generated by pulsed electron−beam energy deposition, the dynamic tensile strength (TS) of glycerol was determined in the 220−350 K temperature range. The TS curve is clearly divided into high− and low−temperature segments at about 262 K. Above this temperature, the TS decreases uniformly from 250 MPa (2.5 kbar) to 34 MPa at 350 K and failure appears to occur by void nucleation and growth. The experimental results are explained by a theory of homogeneous bubble nucleation which takes into account the non−steady−state nature of the experiment. Below the transition temperature, the TS is constant at about 250 MPa and failure is believed to occur by crack nucleation and growth.

A computer-aided molecular model for high volatile bituminous coal☆

Abstract This study defines a three dimensional chemical representation of a coal macromolecule based on data from physiochemical analyses. Both atomic and molecular information is introduced in the computerized model ( signature program). The atomic information includes elemental analysis and 13C NMR data. The molecular information involves analyses of the pyrolysis products of the macromolecule. The components analyzed are molecular fragments of the coal and their structures are determined by gas chromatography/mass spectrometry. The main task of the program is to assemble these fragments to obtain a molecule which is consistent with the atomic information. The computerized method, based on a combinatorial program is able to compute one, several or all non-isomorphic models which can be built. An example of such an approach is presented for vitrinite from high volatile bituminous coal. The signature program was used to construct one of the many possible structural isomers in three dimensions. An energy minimization program was subsequently applied to identify the structure as it might exist in it lowest energy conformation. The resulting model forms a reasonable representation in terms of energy, density, and number of cross-links.

Dynamic tensile strength of mercury

The dynamic tensile strength of mercury has been determined using very‐short‐duration (∼100 ns) stress pulses generated by an electron‐beam machine. Tensile strengths of 1900 MPa (19 kbar) are observed for mercury at maximum tensile stress rates of ∼75 MPa/ns. These results are somewhat lower than the value of 3000 MPa predicted by homogeneous nucleation theory.

Dynamics of electron flow in extended planar-anode diode operating at 19 MV and 700 kA

The electron flow in a planar‐anode diode having an extended anode‐cathode gap operating on the HERMES III accelerator is characterized and compared with predictions of a computational model. The model combines a particle‐in‐cell code with Monte Carlo radiation transport. The comparisons confirm the model and show that the diode provides both a matched load and a versatile large‐area source of γ rays for the study of nuclear radiation effects. Electrical and spatial parameters of the beam at the diode and the downstream radiation fields from a graphite target are presented as a function of the anode‐cathode gap.

Technique for studying dynamic tensile failure in liquids: application to glycerol

A new experimental technique has been developed for the study of the dynamic tensile strength of liquids. The liquid is confined between a solid plate and a stretched aluminized Mylar film. Pulsed electron beam deposition in the plate generates a stress wave which traverses the liquid and gives a high tensile stress during reflection at the Mylar‐covered free surface. Interferometric analysis of the motion of the Mylar film provides a sensitive measure of the stress‐wave‐liquid interaction. The technique has been used to determine a dynamic tensile strength for glycerol at room temperature of 0.6±0.1 kbar from 5 to 23 kbar/μsec. No variation of tensile strength with stress rate was found.

Characterization of flash γ-ray detectors that operate in the Trad/s range

Abstract Compton-diode detectors, scintillator-photodiode detectors, and Cherenkov-photodiode detectors, designed to measure the intense pulsed bremsstrahlung field of HERMES III, are described and characterized in the field of HERMES III. Measurements and modeling show that (1) the Compton-diode detector measures dose rate and is capable of linear operation up to 2.5×1012 rad/s, (2) the scintillator-photodiode detector measures dose rate only when the rate is less then 2×1011 rad/s, and (3) the Cherenkov photodiode detector can be used o place limits on the radiation pulse over the range ∼ 2.5×1010 to ∼ 2.5×1012 rad/s. A by-product of the measurements and modeling is the determination of the temporal behavior of the forward field of HERMES III near the beam axis.

Computer-aided structure elucidation for arylene-bridged polysilsesquioxanes

Abstract Phenylene- and terphenylene-bridged polysilsesquioxane networks are modeled using computer-aided structure elucidation and molecular-dynamics (MD) simulations. The models are matched with analytical results such as elemental analysis, solid-state 29 Si NMR, and gas-sorption porosimetry. Models which are cross-linked in every direction in three-dimensional space do not contain pore volume and are rejected. Models which are cross-linked in only two dimensions fit well with all analytical data. In conclusion, the bridged polysilsesquioxane networks seem to be formed by an aggregation of two-dimensional structures covalently or hydrogen bonded together.

Operation of a bidirectional voltage‐dividing bremsstrahlung diode

Operation of a new type of high‐power electron diode, the bidirectional voltage‐dividing (BVD) diode, has been demonstrated on a 1‐MV, 2.5‐MA, 20‐ns accelerator. This diode consists of a forward‐ and an inverse‐triaxial diode in series. The diode impedance and electron flow pattern agreed reasonably well with particle‐in‐cell code calculations. The radiation spectrum was measured using two different diagnostics, a differential absorption spectrometer and the time‐projection Compton spectrometer. Operating at a voltage of 850–1000 kV, the BVD diode produced a bremsstrahlung spectrum similar to that produced by a standard diode operating at 350–500 kV.