Search Results (16 total)

We investigate the magnetostriction of field-structured magnetoelastomers, which are an important class of materials that have great potential as both sensors and actuators. Field-structured magnetoelastomers are synthesized by suspending magnetic particles in a polymeric resin and subjecting these to magnetic structuring fields during polymerization. These structuring fields can consist of as many as three orthogonal ac components, allowing a wide variety of particles structures—chains, sheets, or networks—to be formed. A principal issue is how particle structure and loading affects the magnetostriction of these materials. To investigate magnetostriction in these field-structured composites we have constructed a constant stress, optical cantilever apparatus capable of 1  ppm strain resolution. Magnetoelastomers having a wide range of particle loadings and structures are investigated, and it is shown that the observed deformation depends strongly on composite structure. The best magnetoelastomers exhibit a contractive strain of 10  000  ppm, the worst materials exhibit a negative, tensile response, which we show is due to the dominance of demagnetizing field effects over magnetostriction. Finally, some discussion is given to the surprising finding that magnetostriction is proportional to the sample prestrain. Simulations of a chain of particles in an elastomer show that particle clumping transitions can occur, but this does not account for the dependence of magnetostriction on prestrain.

We have conducted a series of experiments designed to measure the flashover strength of various azimuthally symmetric 45° vacuum-insulator configurations. The principal objective of the experiments was to identify a configuration with a flashover strength greater than that of the standard design, which consists of a 45° polymethyl-methacrylate (PMMA) insulator between flat electrodes. The thickness d and circumference C of the insulators tested were held constant at 4.318 and 95.74 cm, respectively. The peak voltage applied to the insulators ranged from 0.8 to 2.2 MV. The rise time of the voltage pulse was 40–60 ns; the effective pulse width [as defined in Phys. Rev. ST Accel. Beams 7, 070401 (2004)] was on the order of 10 ns. Experiments conducted with flat aluminum electrodes demonstrate that the flashover strength of a crosslinked polystyrene (Rexolite) insulator is (18±7)% higher than that of PMMA. Experiments conducted with a Rexolite insulator and an anode plug, i.e., an extension of the anode into the insulator, demonstrate that a plug can increase the flashover strength by an additional (44±11)%. The results are consistent with the Anderson model of anode-initiated flashover, and confirm previous measurements. It appears that a Rexolite insulator with an anode plug can, in principle, increase the peak electromagnetic power that can be transmitted across a vacuum interface by a factor of [(1.18)(1.44)]²=2.9 over that which can be achieved with the standard design.

We have developed a statistical model for the flashover of a 45° vacuum-insulator interface (such as would be found in an accelerator) subject to a pulsed electric field. The model assumes that the initiation of a flashover plasma is a stochastic process, that the characteristic statistical component of the flashover delay time is much greater than the plasma formative time, and that the average rate at which flashovers occur is a power-law function of the instantaneous value of the electric field. Under these conditions, we find that the flashover probability is given by 1-exp(-E_p^βt_effC/k^β), where E_p is the peak value in time of the spatially averaged electric field E(t), t_eff≡∫[E(t)/E_p]^βdt is the effective pulse width, C is the insulator circumference, k∝exp(λ/d), and β and λ are constants. We define E(t) as V(t)/d, where V(t) is the voltage across the insulator and d is the insulator thickness. Since the model assumes that flashovers occur at random azimuthal locations along the insulator, it does not apply to systems that have a significant defect, i.e., a location contaminated with debris or compromised by an imperfection at which flashovers repeatedly take place, and which prevents a random spatial distribution. The model is consistent with flashover measurements to within 7% for pulse widths between 0.5 ns and 10   μs, and to within a factor of 2 between 0.5 ns and 90 s (a span of over 11 orders of magnitude). For these measurements, E_p ranges from 64 to 651  kV/cm, d from 0.50 to 4.32 cm, and C from 4.96 to 95.74 cm. The model is significantly more accurate, and is valid over a wider range of parameters, than the J. C. Martin flashover relation that has been in use since 1971 [J. C. Martin on Pulsed Power, edited by T. H. Martin, A. H. Guenther, and M. Kristiansen (Plenum, New York, 1996)]. We have generalized the statistical model to estimate the total-flashover probability of an insulator stack (i.e., an assembly of insulator-electrode systems connected in series). The expression obtained is consistent with the measured flashover performance of a stack of five 5.72-cm-thick, 1003-cm-circumference insulators operated at 100 and 158  kV/cm. The expression predicts that the total-flashover probability is a strong function of the ratio E_p/k, and that under certain conditions, the performance improves as the capacitance between the stack grading rings is increased. In addition, the expression suggests that given a fixed stack height, there exists an optimum number of insulator rings that maximizes the voltage at which the stack can be operated. The results presented can be applied to any system (or any set of systems connected in series) subject to random failures, when the characteristic statistical delay time of a failure is much greater than its formative time.

Composites of conductive particles in an insulating phase are conductive if the particle volume fraction exceeds the percolation threshold. Composites prepared slightly above the percolation threshold have a conductivity that is sensitive to small volume changes, and thus have potential as temperature, pressure, or chemical sensors. In practice it is difficult to prepare composites close to the percolation threshold, and the critical current-carrying path gives a rather low sample conductivity. We find that magnetic-field-structured composites, consisting of gold-coated magnetic particle chains in a polymeric resin, can be reproducibly brought to the percolation threshold, regardless of particle concentration. The low-dimensionality conducting chains form a dense population of critical current paths with extreme sensitivity to composite volume changes. These field-structured composites thus exhibit giant thermoresistance, piezoresistance, and chemiresistance, and should be useful as sensor materials.

Magnetic field-structured composites (FSCs) are made by structuring magnetic particle suspensions in uniaxial or biaxial (e.g., rotating) magnetic fields, while polymerizing the suspending resin. A uniaxial field produces chainlike particle structures, and a biaxial field produces sheetlike particle structures. In either case, these anisotropic structures affect the measured magnetic hysteresis loops, with the magnetic remanence and susceptibility increased significantly along the axis of the structuring field, and decreased slightly orthogonal to the structuring field, relative to the unstructured particle composite. The coercivity is essentially unaffected by structuring. We present data for FSCs of magnetically soft particles, and demonstrate that the altered magnetism can be accounted for by considering the large local fields that occur in FSCs. FSCs of magnetically hard particles show unexpectedly large anisotropies in the remanence, and this is due to the local field effects in combination with the large crystalline anisotropy of this material.

We report dielectric breakdown experiments on electric-field-structured composites of high-dielectric-constant BaTiO₃ particles in an epoxy resin. These experiments show a significant increase in the dielectric standoff strength perpendicular to the field structuring direction, relative to control samples consisting of randomly dispersed particles. To understand the relation of this observation to microstructure, we apply a simple resistor-short breakdown model to three-dimensional composite structures generated from a dynamical simulation. In this breakdown model the composite material is assumed to conduct primarily through particle contacts, so the simulated structures are mapped onto a resistor network where the center of mass of each particle is a node that is connected to neighboring nodes by resistors of fixed resistance that irreversibly short to perfect conductors when the current reaches a threshold value. This model gives relative breakdown voltages that are in good agreement with experimental results. Finally, we consider a primitive model of the mechanical strength of a field-structured composite material, which is a current-driven, conductor-insulator fuse model. This model leads to a macroscopic fusing behavior and can be related to mechanical failure of the composite.

The time dependence of Schottky-barrier depletion layer charge density is monitored after injection of holes in P- and As-doped Si containing donor/hydrogen pairs. The resultant hole capture event leads to the reactivation of hydrogenated donors. In contrast to the observations of Johnson et al., we see an essentially abrupt change in this capacitance, and little, if any, transient behavior that can be attributed to the sweepout of H⁺ or the conversion of H^- to neutral or positively charged hydrogen. Since we observe that much of the hole-induced charge density disappears after a few days wait under dark conditions, it appears that this reaction is at least partially reversible. We conclude that this experiment provides no direct confirmation of the existence of positively or negatively charged hydrogen.

A Comment on the Letter by N. M. Johnson, C. Herring, and Chris G. Van de Walle, Phys. Rev. Lett. 73, 130 (1994). The authors of the Letter offer a Reply.

When silicon implanted with >10¹⁶ He cm^-2 is annealed at 700 °C or above, the He forms bubbles and then diffuses out leaving voids (nanocavities). We have prepared n-type and p-type Si samples with nanocavity layers and characterized their structure and their effect on the local band structure and the transport of charge. The ambipolar Si dangling orbitals at the nanocavity walls trap majority carriers in both types of silicon and form quasi-one-dimensional potential barriers which impede transport of charge across the cavity layer. Using dc conductance and high-frequency capacitance techniques we have characterized the height and width of these electrostatic barriers. Capacitance measurements have also been employed to study the evolution of trapped dangling-bond charge as the void containing layers are depleted of carriers in n-type and p-type Schottky-barrier structures. With transient capacitance techniques we have characterized the emission of holes from the unoccupied dangling-bond localized states and also the emission of electrons from the doubly occupied states. The lower dangling-bond level is 0.17 eV above the valence-band maximum, while the upper level lies 0.38 eV below the conduction-band minimum; these energies are qualitatively consistent with broader spectral features observed in ultrahigh-vacuum photoemission experiments on clean reconstructed Si surfaces.

Electron paramagnetic resonance (EPR) has been employed to observe unpaired spins associated with dangling orbitals on the cavity walls. The strength of the EPR signal corresponds to ∼0.1 unpaired spin per dangling orbital, and this reduced amplitude is interpreted in terms of charge redistributions among inequivalent sites which are well known to occur on reconstructed Si surfaces. A simple, one-electron model yields semiquantitative agreement with much of the experimental data on electrical properties and helps explain some of the unusual emission-rate prefactors seen in the capacitance-transient experiments. We nevertheless conclude that a more realistic treatment, including electron-electron repulsion within the cavities as well as charge redistributions on the neutral surface, is probably needed for quantitative prediction.

The energies of systems composed of a point negative charge (e.g., antiproton, p¯, or negative muon, μ^-) and an atom or small molecule (He,Ne,Ar,C₆H₆) have been calculated using large-basis-set, correlated ab initio electronic-structure methods. By invoking the Born-Oppenheimer approximation, these energies are used to generate potential surfaces for the interaction of a negative particle with an atom or small molecule. The results indicate that the particle-induced polarizations are significant even at 10-bohr separations in neon. Unusual behavior of the dipole moments of both argon and neon was calculated to occur as a function of separation. The molecular-orbital approach used here may also be useful in analyzing recent ionization behavior in low-energy p¯–inert-gas scattering experiments. Furthermore, the calculated large degree of polarization further constrains the design of materials for long-term normal matter storage of antiprotons for advanced energy sources.

Mechanical stress in a dielectric solid from application of a uniform electric field is usually assumed to be described by ‘‘Maxwell stress,’’ proportional to the first power of the relative dielectric constant, κ. Significant corrections are found from energy minimization when the dependence of permittivity on strain is included. Electrostriction coefficients are evaluated by the use of a model dielectric consisting of a simple-cubic lattice of linearly polarizable point dipoles. Compressive stress in the applied-field direction is larger than expected by more than a factor of κ. The force density exerted on internal space charge needs to be corrected by the same factor. Stress components also have been calculated, with identical results, through direct summation of microscopic forces. This method permits identification of the origins of electrically induced stress. The dominant contribution is a compressive stress in the field direction, proportional to κ², from attraction between free charge at the electrodes. This component can attain tens of MPa at fields approaching the intrinsic dielectric strength. A lateral tensile stress independent of κ also is present, which may assist electrical breakdown in some crystalline dielectrics. These stress components are augmented by short-range, dipolar forces throughout the bulk of the dielectric. Deformations accompanying poling of poly(vinylidene fluoride) are considered and found to be influenced by electrically induced stress.

Measurements of the angular distributions of the single charge exchange reactions to isobaric analog states for ¹³C(π⁺, π⁰)¹³N and ¹⁵N(π⁺, π⁰)¹⁵O at 165 MeV are described. The two angular distributions are very similar. The shapes are reproduced by semiphenomeno-logical isobar-doorway calculations and by second-order coupled channels calculations. The calculated magnitudes are low by about a factor of 2. The measurements are consistent with older angle-integrated cross sections for ¹³C.

NUCLEAR REACTIONS ¹³C, ¹⁵N(π⁺, π⁰), T_π=164 MeV; measured σ(θ); IAS transitions; enriched targets; test of reaction models.

It is shown that the crystallinity of poled films of poly(vinylidene fluoride) can be changed by the application of an electric field. This is the first time that electric-field-induced changes of crystallinity in a polymer have been reported, and this observation confirms the hypothesis that reversible changes in crystallinity with temperature contribute significantly to the pyroelectric effect in poly(vinylidene fluoride).

The forward-angle differential cross sections of pion single charge exchange on ⁷Li and ¹³C were measured at 70, 100, 150, 165, and 180 MeV. The cross sections rise steeply up to 150 MeV and remain almost constant between 150 and 180 MeV. Comparisons with theoretical calculations and with the free charge-exchange cross sections are presented. There is poor agreement with the data. Only phenomenological calculations can fit the resonance region. The isobaric analog excitation functions rise more steeply than the continuum single-charge-exchange cross sections.

We have measured the absolute value of the penetration depth in superconducting indium films. Applying a dc axial magnetic field to the exterior of a hollow superconducting thin-film cylinder, we determined the ratio ΔH_i/ΔH₀ of the variation of the magnetic field penetrating into the interior region to the variation of the applied magnetic field. Applied magnetic fields of small amplitude were used so that ΔH_i was linear in ΔH₀; a superconducting magnetometer was used to measure ΔH_i. Film thicknesses were between 95 and 160 Å, and the temperature range was from 1.3 K to the critical temperature near 4.0 K. From the actual penetration depths deduced from these measurements, which were performed on four samples, values of the London penetration depth at absolute zero λ_L(0) were calculated to be in the range 397 ± 22 Å. This indicates that λ_L(0) in thin films is larger than typical published measurements for bulk indium, in accordance with other evidence. Except for one sample and for temperature within about 0.3 K of the critical temperature, the data were fitted moderately well by the theoretical temperature dependence. Flux creep depending logarithmically on elapsed time was also observed for values of ΔH₀ sufficiently large to induce currents in the film exceeding the critical-state current. The temperature dependence of the logarithmic flux-creep rate was approximately that of the product of the critical current, the absolute temperature, and the square of the penetration depth over a wide range of temperatures.

An investigation of the sensitized fluorescence of a mercury thallium mixture without and with the addition of argon and helium gases is discussed. Data taken without the addition of foreign gases are used to extend the theory of Frish and Karaulinya to a mercury thallium mixture. Relative collision cross sections for the excitation of the thallium energy states, 9 ²S_{1 / 2}, 7 ²D_{5 / 2,3 / 2}, 8 ²S_{1 / 2}, 6 ²D_{5 / 2}, and 6 ²D_{3 / 2} were calculated for mercury thallium collisions. Data taken with the addition of argon and helium gases are given to indicate the variation of the intensity of the thallium lines as a function of argon and helium gas pressures at one constant thallium and three mercury temperatures. The explanation of the results depends on the role of mercury 6 ³P₁ excited atoms, metastable atoms, and mercury molecules in collision with thallium atoms for energy exchange, and also requires the use of Winans' partial selection rule and other generally accepted ideas concerning energy transfer, emission, and absorption.