J. R. Gladden

Reconciliation of ab initio theory and experimental elastic properties of Al2O3

Ab initio calculations of the basic properties of solids have advanced significantly, and it is now possible to simply access a crystal structure database, and from the given constituent atoms and their positions, calculate reasonably accurate values for elastic constants and thermomechanical properties that may be derived from them. However, progress has been impeded by a unique discrepancy involving the sign of one of the elastic constants of an important material: Al2O3. In this letter, this longstanding discrepancy is resolved with experimental measurements.

PERMITTIVITY MEASUREMENT WITH A NON-STANDARD WAVEGUIDE BY USING TRL CALIBRATION AND FRACTIONAL LINEAR DATA FITTING

Modifications in the measurement of the complex permittivity are described, based on the transmission and reflection coefficients of a dielectric slab. The measurement uses TRL two- port calibration to bring the reference planes accurately to the sample surface. The complex permittivity as a function of frequency is computed by minimizing the difference between the measured and the ideal scattering parameters. An alternative procedure for determining the complex permittivity uses the fractional linear data fitting to a Q- circle of the virtual short-circuit and/or virtual open circuit data. In that case, the sample must be a multiple of one-quarter wavelength long within the measured range of frequencies. Comparison with results obtained by other traditional procedures is provided.

Dynamic Buckling and Fragmentation in Brittle Rods

We present experiments on the dynamic buckling and fragmentation of slender rods axially impacted by a projectile. By combining the results of Saint-Venant and elastic beam theory, we derive a preferred wavelength lambda for the buckling instability, and experimentally verify the resulting scaling law for a range of materials including teflon, dry pasta, glass, and steel. For brittle materials, buckling leads to the fragmentation of the rod. Measured fragment length distributions show two peaks near lambda/2 and lambda/4. The nonmonotonic nature of the distributions reflect the influence of the deterministic buckling process on the more random fragmentation processes.

Motion of a Viscoelastic Micellar Fluid around a Cylinder: Flow and Fracture

We present an experimental study of the motion of a viscoelastic micellar fluid around a moving cylinder, which ranges from fluidlike flow to solidlike tearing and fracture, depending on the cylinder radius and velocity. The observation of crack propagation driven by the cylinder indicates an extremely low tear strength, approximately equal to the steady state surface tension of the fluid. At the highest speeds a driven crack is observed in front of the cylinder, propagating with a fluctuating speed equal on average to the cylinder speed, here as low as 5% of the elastic wave speed.

Shape and size of highly concentrated micelles in CTAB/NaSal solutions by Small Angle Neutron Scattering (SANS).

Highly concentrated micelles in CTAB/NaSal solutions with a fixed salt/surfactant ratio of 0.6 have been studied using Small Angle Neutron Scattering (SANS) as a function of temperature and concentration. A worm-like chain model analysis of the SANS data using a combination of a cylindrical form factors for the polydisperse micellar length, circular cross-sectional radius with Gaussian polydispersity, and the structure factor based on a random phase approximation (RPA) suggests that these micelle solutions have a worm-like micellar structure that is independent of the concentration and temperature. The size of the micelle decreases monotonically with increasing temperature and increases with concentration. These observations indicate that large micelles are formed at low temperature and begin to break up to form smaller micelles with increasing temperature.

Characterization of Multiwalled Carbon Nanotube (MWCNT) Composites in a Waveguide of Square Cross Section

Multiwalled carbon nanotube (MWCNT) composites are characterized within 8-10 GHz using a waveguide of square cross section. The complex permittivity is extracted and empirical expressions are obtained as a function of frequency by minimizing the difference between the measured and theoretical values of the scattering parameters. Composites with CNT concentrations ranging from 0%-20% are investigated. The square cross section of the waveguide allows for different orientations of samples to test the anisotropy of MWCNT composites.

High Temperature Resonant Ultrasound Spectroscopy: A Review

The measurement of elastic constants plays an important role in condensed matter physics and materials characterization. This paper presents the resonant ultrasound spectroscopy (RUS) method for the determination of elastic constants in a single crystal or amorphous solid. In RUS, the measured resonance spectrum of a properly prepared sample and other information such as geometry, density, and initial estimated elastic constants are used to determine the elastic constants of the material. We briefly present the theoretical background and applications to specific materials; however, the focus of this review is on the technical applications of RUS, especially those for high-temperature measurements.

Role of phonon scattering by elastic strain field in thermoelectric Sr1−xYxTiO3−δ

Perovskite-type SrTiO3−δ ceramics are multifunctional materials with significant potential as n-type thermoelectric (TE) materials. The electronic and thermal transport properties of spark plasma sintered polycrystalline Sr1−xYxTiO3−δ (x = 0.05, 0.075, 0.1) ceramics are systematically investigated from (15–800) K. The Sr0.9Y0.1TiO3−δ simultaneously exhibits a large Seebeck coefficient, α > −80 μV/K and moderately high electrical resistivity, ρ ∼ 0.8 mΩ-cm at a carrier concentration of ∼1021 cm−3 at 300 K resulting in a high TE power factor defined herein as (α2σT) ∼ 0.84 W/m-K at 760 K. Despite the similar atomic masses of Sr (87.6 g/mol) and Y (88.9 g/mol), the lattice thermal conductivity (κL) of Sr1−xYxTiO3−δ is significantly reduced with increased Y-doping, owing to the smaller ionic radii of Y3+ (∼1.23 A, coordination number 12) compared to Sr2+ (∼1.44 A, coordination number 12) ions. In order to understand the thermal conductivity reduction mechanism, the κL in the Sr1−xYxTiO3−δ series are phenomeno...

Acoustic modes of finite length homogeneous and layered cylindrical shells: Single and multiwall carbon nanotubes

We present a numerical study of the normal modes of vibration of both homogeneous and heterogeneous finite length cylindrical shells of arbitrary wall thickness with applications toward single and multiwall carbon nanotubes in the continuum limit. The method is checked by comparison of computed and measured resonance spectra for a machined aluminum cylindrical shell. The dependence of the natural frequencies of various radial modes with the length and radius of single wall tubes is investigated and compared to atomistic models and Raman spectroscopy data. The radial dependence for the radial breathing mode and four harmonics of the squash mode are found to be well fitted by power laws and agree with analytical solutions in the thin wall limit. A general model for an elastically heterogeneous layered cylindrical shell is applied to multiwall tubes with graphene sheets and gaps between the sheets represented by two different materials. The frequency dependence on length and diameter is investigated for tube...

Measurement of Mechanical Properties of Hydrated Cement Paste Using Resonant Ultrasound Spectroscopy

The resonant ultrasound spectroscopy (RUS) experimental technique had been used successfully to measure the modulus of elasticity and Poisson’s ratio of hydrated cement paste. RUS is a modern nondestructive acoustic technique, which can be applied to determine the elastic properties of solids with high precision and great efficiency. By measuring the natural resonance frequencies for a single small parallelepiped cement paste sample with water cement ratio of 0.4, the elastic constants and, subsequently, the modulus (E) and Poisson’s ratio (v) were obtained. The results from RUS measurement, E=21.55 GPa and v=0.225, agree with the numerical modeling values.