D. C. Lorents

Mechanical and thermal properties of carbon nanotubes

Abstract This chapter discusses some aspects of the mechanical and thermal properties of carbon nano-tubes. The tensile and bending stiffness constants of ideal multi-walled and single-walled carbon nano-tubes are derived in terms of the known elastic properties of graphite. Tensile strengths are estimated by scaling the 20 GPa tensile strength of Bacons graphite whiskers. The natural resonance (fundamental vibrational frequency) of a cantilevered single-wall nanotube of length 1 micron is shown to be about 12 MHz. It is suggested that the thermal expansion of carbon nanotubes will be essentially isotropic, which can be contrasted with the strongly anisotropic expansion in “conventional” (large diameter) carbon fibers and in graphite. In contrast, the thermal conductivity may be highly anisotropic and (along the long axis) perhaps higher than any other material. A short discussion of topological constraints to surface chemistry in idealized multi-walled nanotubes is presented, and the importance of a strong interface between nano-tube and matrix for formation of high strength nanotube-reinforced composites is highlighted.

Single crystal metals encapsulated in carbon nanoparticles

Single-domain microcrystals of LaC2 encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as α-LaC2, the phase of LaC2 that is stable at room temperature. Bulk α-LaC2 is metallic and hydrolytic. Observation of crystals of pure encapsulated α-LaC2 that were exposed to air for several days before analysis indicates that the LaC2 is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC2 single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.

Characteristics of electron‐beam‐excited Xe*2 at low pressures as a vacuum ultraviolet source

The performance of Xe*2 as a 172‐nm fluorescence or laser source when pumped by a low‐current, long‐pulse electron beam was determined. The fluorescence efficiency of Xe*2 is near the theoretical limit of ∼50% at modest pressures over a range of pump rates up to 106 W/cm. The laser efficiency is limited to values <1% by a very strong medium absorption that is probably due to Xe*2 photoionization. Laser performance is further degraded by early pulse termination that appears related to mirror degradation. An improved kinetics and extraction code was developed to model the performance of the Xe*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Rates for these processes were derived as described herein. With this model, good absolute agreement was obtained between experiments and calculated parameters at pressures as low as 0.5 atm.

Time resolved spectroscopy of xenon excimers excited by synchrotron radiation

Formation and decay rates of the lowest 0u+ and 1u states of Xe2*, excited by monochromatized synchrotron radiation from the Stanford storage ring (SPEAR), have been measured in pure xenon and in xenon–argon mixtures over the pressure range 102 to 104 Torr. The results are interpreted to yield radiative lifetimes (4.6±0.3 and 99±2 nsec, respectively, for vibrationally relaxed 0u+ and 1u molecules), vibrational relaxation rates [7×10−11 and 6×10−12 cm3/sec for Xe2* (1u) in collisions with xenon and argon, respectively], the Xe2* (0u+) three‐body formation rate from Xe(3P1) (5.3×10−32 cm3/sec), and rates for 0u+–1u mixing by collisions with xenon and argon.

MECHANICAL AND THERMAL PROPERTIES OF CARBON NANOTUBES

Abstract This chapter discusses some aspects of the mechanical and thermal properties of carbon nanotubes. The tensile and bending stiffness constants of ideal multi-walled and single-walled carbon nanotubes are derived in terms of the known elastic properties of graphite. Tensile strengths are estimated by scaling the 20 GPa tensile strength of Bacons graphite whiskers. The natural resonance (fundamental vibrational frequency) of a cantilevered single-wall nanotube of length 1 micron is shown to be about 12 MHz. It is suggested that the thermal expansion of carbon nanotubes will be essentially isotropic, which can be contrasted with the strongly anisotropic expansion in “conventional” (large diameter) carbon fibers and in graphite. In contrast, the thermal conductivity may be highly anisotropic and (along the long axis) perhaps higher than any other material. A short discussion of topological constraints to surface chemistry in idealized multi-walled nanotubes is presented, and the importance of a strong interface between nanotube and matrix for formation of high strength nanotube-reinforced composites is highlighted.

Visible absorption by electron-beam pumped rare gases

The temporal and spectral behavior of absorption in the visible by e -beam excited argon, krypton, and xenon have been investigated with and without additives. The probe light used was from an argon-ion pumped, tunable dye laser, or from the visible lines of the argon-ion laser itself. Strong absorption was observed at all wavelengths investigated (450-620 nm). Structured absorptions out of atomic and molecular excited states have been identified by their temporal, spectral, and pressure behavior. The ubiquity of these absorptions, which is not fully understood, should have serious implications for the development of new visible lasers that operate during the electron-beam pumping pulse.

Optical emissions of triatomic rare gas halides

We report in this paper the observation of optical emissions from triatomic rare gas halide molecules formed in excited high density rare gas–halogen mixtures. The broad‐band continuum emissions are identified as transitions between ionically bonded excited states (Rg+2X−) and repulsive covalent lower states that dissociate to ground state atoms. These emissions become predominant at high rare gas densities.

A new blue‐green excimer laser in XeF

A new blue‐green excimer laser has been demonstrated on the C‐A transition in XeF, with an output energy of greater than 1 mJ. Xe*2 excimer fluorescence at 172 nm was used to photodissociate XeF2, producing XeF[B (1/2)] and XeF[C (3/2)]. The B (1/2) state was collisionally relaxed to the C (3/2) state with an Ar buffer. Lasing then occurs sequentially on the B (1/2) ‐X (1/2) and C (3/2) ‐A (3/2) transitions. Measurements of the C‐A laser spectrum showed a peak wavelength of 483 nm with a bandwidth of 12 nm. This new laser is potentially highly efficient and scalable and should be tunable over a bandwidth greater than 40 nm.

Quenching of Ne, F, and F2 in Ne/Xe/NF3 and Ne/Xe/F2 mixtures

As part of a kinetic study of XeF laser media, we have followed the time evolution of the Ne*, F*, and F*2 densities in e‐beam excited Ne/Xe/NF3(F2) mixtures. The Ne* and F* densities were monitored by laser absorption while the F2* was observed in emission at 158 nm. Rate coefficients for the quenching of these excited states by Ne, Xe, NF3, and F2 have been determined. The radiative lifetime of the F2* was measured to be 41±4×10−9 sec.

Magnetic separation of GdC2 encapsulated in carbon nanoparticles

Abstract Single crystal particles of α-GdC2 have been encapsulated into carbon polyhedral shells by the carbon arc-discharge technique. These particles range in size from 20 to 50 nm and are paramagnetic in nature. The phase and stoichiometry of these crystals have been determined using a combination of techniques, such as X-ray diffraction and high-resolution electron microscopy. As reported for α-LaC2 earlier, the crystal structure of bulk α-GdC2 is also tetragonal, and it is metallic and undergoes hydrolysis. However, the encapsulation of the nanoscale crystals in carbon shells prevents their hydrolysis indefinitely. These nanometer-sized particles, which can be preferentially extracted using powerful magnetic fields, may have useful applications in several fields of science.