J. A. Roberts

Electromagnetic wave properties of polymer blends of single wall carbon nanotubes using a resonant microwave cavity as a probe

A resonant microwave cavity operating in the TM010 mode was used to determine the microwave susceptibility of single walled carbon nanotubes (SWNT) that are blended in polymer matricies. The frequencies of the probe signal were 9.8, 11.4, and 35.93 GHz. Samples of 3%–19% blends of SWNT in polycarbonate were tested to determine the best blends for shielding of devices from microwaves at these frequencies. It appears that blends of 9%–11% are very effective in shielding the electric vector of electromagnetic waves. Both the electric vector and the magnetic vectors were probed by the process to determine the nature of coupling between the SWNTs and the applied fields. Some details are given about the apparatus design that enables computer collection and processing of the data to be achieved. An electronic differentiation technique was used to allow the second derivative of the cavity absorption profile to be displayed for precise measurement. Data are presented to show the relative microwave absorption for d...

Electromagnetic wave absorption properties of carbon nanotubes-epoxy composites at microwave frequencies

This paper focuses on the absorption properties of electromagnetic waves, both electric and magnetic, from multiwalled carbon nanotubes (MWCNTs)-epoxy composites. A microwave resonant cavity was utilized as a probe. Six groups of MWCNT-epoxy composite samples with different MWCNT outside diameters (ODs) were fabricated and tested. The weight percentages of MWCNTs in each sample group were in the range of 1%–10%. The line widths at half-power-maxima, the shifts in resonant frequency under load, and the amplitudes of the absorption profiles for the six groups of composite samples were compared. In general, the MWCNTs with smaller ODs had higher microwave absorption efficiency at 9.968 GHz than the large diameter ones. However, the sample group M5 (OD<8 nm) showed unusual results, a lower microwave absorption than other sample groups, even though the OD of the MWCNTs in the sample group M5 was smaller than those of other groups. The scanning electron microscope analysis and microwave absorption measurements ...

Effective dielectric constants of photonic crystal of aligned anisotropic cylinders and the optical response of a periodic array of carbon nanotubes

We calculate the static dielectric tensor of a periodic system of aligned anisotropic dielectric cylinders. Exact analytical formulas for the effective dielectric constants for the E- and H- eigenmodes are obtained for arbitrary 2D Bravais lattice and arbitrary cross-section of anisotropic cylinders. It is shown that depending on the symmetry of the unit cell photonic crystal of anisotropic cylinders behaves in the low-frequency limit like uniaxial or biaxial natural crystal. The developed theory of homogenization of anisotropic cylinders is applied for calculations of the dielectric properties of photonic crystals of carbon nanotubes.

The microwave spectra of propyne in the 17–70 GHz region for the ground V10=1 and V9=1 vibrational states

The rotational spectra of the ground V10=1 and V9=1 vibrational states of propyne have been determined for the frequency range of 17–70 GHz. The ground and V10=1 measurements have been shown to be in excellent agreement with constants derived from higher J values by other authors. This was the first time that a sufficient number of lines in the V9=1 state had been measured to make a reasonable assignment. It was found that the data could be fit to Amat’s theory for Vt=1 states in C3V molecules only if an additional term of 0.653 (J+1) was added to the frequencies predicted for the K=±1, l=∓1 lines. Our assignments are in disagreement with those of Jaseja based only on the J=2→3 transitions measured by Gordy. Our data completes the low J rotational spectra for the ground V10=1 and V9=1 states of CH3CCH.

Linewidth parameters in the rotational spectrum of nitrogen dioxide

Contributions to the second order collision efficiency function S ⁽²⁾ (b), used in semiclassical perturbation approaches to pressure broadening of microwave and infrared spectra, due to several leading terms, dipole and quadrupole components, in the expansion of the intermolecular interaction energy are derived by method of irreducible spherical tensor operators for molecules of arbitrary symmetry. Results are given explicitly in terms of dipole and quadrupole line strengths. General expressions for dipole moment line strength in the asymmetric rotor basis as well as quadrupole moment line strength for the special case of molecules with two independent quadrupole moment components are derived. Computer programs for calculating linewidth parameters in the rotational spectrum of ¹⁴NO₂ based on Anderson and Murphy and Boggs theories are presented.

Linewidth parameters for Δ J=1, 0≤ J ≤ 43, rotational transitions of the sulfur dioxide molecule

A systematic study was made of the rotational quantum number (J,K) dependence of the linewidth parameters for sulfur dioxide (SO2) at 300 °K. Linewidth parameters were used to calculate effective hard‐sphere collision diameters for SO2 over the pressure range 0.001 ≤ P ≤ 0.06 torr. Linewidth parameters were calculated using the Anderson collision theory with appropriate modifications for dominant dipole‐dipole interaction between these molecules. These calculations compare favorably with the experimental values obtained. The linewidth parameter trend was explored for transitions Δ J=1, 0≤ J ≤ 43, with all transitions lying within the frequency range of the instrument 4–80 GHz.

Experimental microwave collision diameters in the rotational spectrum of H2CO

Abstract Collision diameters for some select transitions in the rotational spectrum of H 2 CO have been determined using pressure broadening of the spectral lines. Transitions of the type Δ J = 0, K −1 = 1, and Δ K +1 = 1 with 1 ≤ J ≤ 5 were investigated for both self-broadening and foreign gas broadening (He and H 2 ) of the spectral lines. Pressure ranges from 0.001 Torr to 0.1 Torr were explored in obtaining the line width parameters Δ ν p for each transition. Collision diameters were found to be very nearly constant (14 A) over the J states studied for H 2 COH 2 CO interaction, 2.5–5.8 A for H 2 COH 2 interaction and 2.7–3.5 A for H 2 COHe interaction.

Microwave properties of liquids and solids using a resonant microwave cavity as a probe

The frequency shifts and Q changes of a resonant microwave cavity were utilized as a basis for determining microwave susceptibilities and loss tangents for solids and liquids. The method employed consisted of varying the depth of penetration of a continuous cylindrical sample of the material into a cavity operating in the TM010 mode with the resonant frequency and quality factor determined at each incremental setting of the perturbing sample. The perturbation of the cavity was achieved by advancing the sample into the cavity along the symmetry axis by employing a micrometer drive appropriately calibrated for depth of penetration of the sample. A differentiation method was used to obtain the half‐power points of the cavity resonance profile at each depth of penetration. The perturbation techniques for resonant cavities were used to reduce the experimental data obtained to physical parameters for the samples. The probing frequency employed was near 10 GHz.

Theoretical and experimental pressure shifts in some select rotational transitions of the methyl cyanide molecule

The Anderson–Tsao–Curnutte–Frost and Murphy–Boggs theories of linewidth and line shift are briefly discussed, and the significance of certain ’’resonance’’ functions is noted. Power series approximations for these functions, which facilitate numerical evaluation of linewidths and line shifts using these two theories, are given and discussed. For completness, experimental line shift parameters for several rotational transitions in the CH3C14N and CH3C15N molecules are presented with a comparison of the theoretical results derived using the Murphy–Boggs and the modified Anderson theories.

DETERMINATION OF HYDROGEN BONDING IN WATER CLUSTERS THROUGH CONDUCTIVITY (DC) MEASUREMENTS OF AQUEOUS SOLUTIONS OF NACL

Conductivity (dc) measurements over the temperature range 273≤T≤360 K of aqueous solutions of NaCl at various concentrations were made and used to establish transport properties of ions in solution. The dynamical properties of the electrolytes were used to establish the nature of the hydrogen bonding in these aqueous systems. Rate equations for ion formation and recombination were used to establish the temperature ranges in which hydrogen bonding dominated in forming polymeric species. Experimental data obtained on the aqueous systems demonstrate that over the range of temperatures 273≤T≤323 K the water system maintains a structure which appears to be independent of solute concentration. This is evidenced by the nearly constant slope in the Arrhenius plot of the data over that range. For the higher range of temperatures, 323≤T≤360 K, the slope of the Arrhenius plot varied more than one order of magnitude for the concentration levels studied. An average value of hydrogen bond energy of 2.8 kcal mol−1 was o...