Ian L. Spain

Graphite fibers and filaments

Contents: Introductory Material on Graphite Fibers and Filaments.- Synthesis of Graphite Fibers and Filaments.- Structure.- Lattice Properties.- Thermal Properties.- Mechanical Properties.- Electronic Structure.- Electronic and Magnetic Properties.- High Temperature Properties.- Intercalation of Graphite Fibers and Filaments.- Ion Implantation of Graphite Fibers and Filaments.- Applications of Graphite Fibers and Filaments.- References.- Subject Index.

Phases of silicon at high pressure

Abstract X-ray diffraction studies are reported on silicon at pressures up to 250 kbar (25 GPa). A transition to the β-Sn structure (II) initiates at 112 ± 2 kbar and two phases (I + II) coexist to 125 ± 2 kbar. At 132 ± 2 kbar a new phase (V) initiates, and the transition is complete at 164 ± 5 kbar. This phase persists to 250 kbar. Its structure is tentatively assigned as primitive hexagonal with c / a = 0.941 ± 0.002 at 250 kbar. On release of pressure, the sequence is V → (V + II) (145 - 110 kbar) → II → (II + III) (108 - 85 kbar) → III, the last phase persisting to room pressure.

Band offset of GaAs/In0.48Ga0.52P measured under hydrostatic pressure

Low‐temperature photoluminescence spectra of an In0.48Ga0.52P alloy and a p‐type GaAs/In0.48Ga0.52P multiple quantum well, both grown by molecular beam epitaxy, have been obtained under hydrostatic pressures from 0 to 6 GPa. The zero‐pressure extrapolation of the InGaP(X) to GaAs(Γ) transitions yields a 0.40±0.02 valence‐band offset, and hence only a small, 0.06 ± 0.02 eV, conduction‐band offset. These offset values are in agreement with measured values of the confinement energy versus well width.

New metastable phases of silicon

Abstract It has been known since 1963 that a metastable phase (III,BC-8) results from the decompression of silicon from its high pressure metallic phases. However recent theoretical studies suggest the possibility of several metastable phases with similar total energies. Upon rapid release of pressure from the metallic state, two new, metastable, phases of Si have been discovered, with tetragonal structures. X-ray diffraction data on these new phases are presented.

Properties and Characterization of Codeposited Boron-nitride and Carbon Materials

The physical properties of carbon‐boron nitride (C‐BN) prepared from mixtures of BCl3, NH3, and selected hydrocarbons by codeposition methods have been investigated using x‐ray diffraction, electron diffraction, transmission electron spectroscopy, x‐ray photoelectron spectroscopy, Raman scattering, optical reflectivity, thermal conductivity, thermopower, and electrical resistivity. Taken collectively, the results of these experiments indicate that the compression‐annealed C‐BN materials studied here consist of separated domains of pyrolytic boronated graphite and pyrolytic boron nitride, while as‐deposited samples may possibly be a single‐phase mixture of C, B, and N. As‐deposited materials containing more than 20% carbon were found to be more highly oriented than unannealed pyrolytic graphite, and the crystallinity of these materials was greatly enhanced by uniaxial compression annealing. Results of the thermal conductivity, thermopower, and electrical resistivity measurements are consistent with a netwo...

Hall effect measurement in the diamond anvil high‐pressure cell

Hall effect measurements have been carried out in the diamond anvil cell to pressures of 6 GPa. A plasma sprayed coating has been used to insulate the electrical leads from the gasket, similar to the configuration employed by Tozer and King [Rev. Sci. Instrum. 56, 260 (1985)], but a van der Pauw geometry was used to make the Hall and mobility measurements. Application of the technique to GaAs at 300 K is described, for which the Γ‐X conduction band crossover is observed at 4.0 GPa, in good agreement with recent photoluminescence measurements.

Effects of Pressure on the Spectra and Lifetimes of Nd(x)Y(1-x)P55O14 and Ruby,

Variations in the fluorescence line positions, linewidth, and lifetimes with hydrostatic pressure up to about 80 kbar are reported for samples of NdP5O14 and Nd0.1Y0.9P5O14. The results for the lightly doped sample are consistent with changes in the radiative decay rate while the very different results obtained on the heavily doped sample are interpreted in terms of increased radiationless quenching due to energy migration to trapping sites. Results are also reported on the variations in the fluorescence lifetimes of the R lines, and the positions of the N lines and vibronic peaks of ruby as a function of hydrostatic pressure.

ELECTRONIC PROPERTIES OF PAN-BASED CARBON FIBERS-I EXPERIMENT AND COMPARISON WITH PROPERTIES OF BULK CARBONS

Abstract Measurements of the resistivity of PAN-based fibers are reported for temperatures between 1.5 and 300 K. Particular care was taken to avoid sample heating problems during measurements. Fibers were heat-treated between ~ 1300°C to above 3000°C, corresponding to a wide range of elastic moduli (~ 35–112 Msi : 240–770 GPa). A particularly surprising result was the observation of a resistivity maximum for low modulus (35 Msi ~240 GPa) fiber below 50 K. Results are compared to those reported on bulk heat treated carbons and pyrocarbons. A qualitative discussion of the results is also given in terms of recent models of the electronic energy levels of carbon ribbons and turbostratic carbons.

Unusual electrical resistivity behavior of carbon fibers

Abstract PAN-based fibers, heat-treated to ≈1300°C having elastic moduli ≈35 Msi (240 GPa), exhibit an unusual maximum in the resistivity at low temperature (≈30 K), and the temperature coefficient of resistance remains positive to 1.5 K. Measurement of resistivity are reported, and the results discussed in terms of models for the electronic states of this disordered material. It is speculated that the behavior is related to anomalies observed in the specific heat of bulk carbons heat treated to similar temperatures.

Graphitic nature of chemical vapor-deposited carbon filaments grown on silicon surfaces from acetylene

Abstract Debye-Scherer and diffractometer X-ray studies have been carried out on carbon filaments grown on carbon and silicon surfaces from the catalytic chemical vapor deposition of acetylene on Ni and Fe catalysts. From these data it is concluded that filaments grown on carbon substrates with diameters less than ~0.3 μm are essentially amorphous, or highly disordered, whereas those grown on Si substrates have a partially graphitic, turbostratic structure. Diffraction lines obtained from the cooled catalyst particle at the tip of the filaments grown on carbon surfaces could not be indexed on the basis of known Ni-C compounds.