Richard C. Bell

Formation, structure and bond dissociation thresholds of gas-phase vanadium oxide cluster ions

The formation and structure of gas-phase vanadium oxide cluster anions are examined using a guided ion beam mass spectrometer coupled with a laser vaporization source. The dominant peaks in the anion total mass distribution correspond to clusters having stoichiometries of the form (VO2)n(VO3)m(O2)q−. Collision-induced dissociation studies of the vanadium oxide species V2O4–6−, V3O6–9−, V4O8–10−, V5O11–13−, V6O13–15−, and V7O16–18− indicate that VO2, VO3, and V2O5 units are the main building blocks of these clusters. There are many similarities between the anion mass distribution and that of the cation distribution studied previously. The principal difference is a shift to higher oxygen content by one additional oxygen atom for the stoichiometric anions (VxOy−) as compared to the cations with the same number of vanadium atoms, which is attributed to the extra pair of electrons of the anionic species. The oxygen-rich clusters, VxOy(O2)−, are shown to more tightly adsorb molecular oxygen than those of the co...

Magnetorheology of submicron diameter iron microwires dispersed in silicone oil

We investigate the magnetorheological (MR) properties of suspensions containing iron microwires with 260 nm diameter and two distinct length distributions of 5.4 ± 5.2 µm and 7.6 ± 5.1 µm suspended in silicone oil (0.45 Pa s). The rheological properties of these fluids were determined using a parallel plate rheometer equipped with a variable strength electromagnet. The shear stress was measured as a function of shear rate for increasing applied magnetic fields. These results were modeled using the Bingham-plastic constitutive model to determine the apparent yield stress and viscosity as a function of increasing volume fraction and length of microwires. At a saturated magnetic flux density, the yield stress using the 5.4 µm microwires was found to be 0.65, 2.23, and 4.76 kPa for the 2, 4, and 6 vol% suspensions, respectively. For the 7.6 µm wires, the yield stress increases to 8.2 kPa for the 6 vol% suspension. Compared with conventional MR fluids employing spherical particles, the degree of settling is markedly decreased in the microwire-based fluids. At 6 vol%, conventional fluids display appreciable settling whereas the microwire-based fluids display no discernable settling. Moreover, the rod-shaped microwires are shown to increase the yield stress of the fluids and enhance the MR performance.

Dimorphic magnetorheological fluids: exploiting partial substitution of microspheres by nanowires

Magnetorheological (MR) fluids typically are suspensions of spherical micron-sized ferromagnetic particles suspended in a fluid medium. They are usually thought of as Bingham-plastic fluids characterized by an apparent yield stress and viscosity. Partial substitution of the micron-sized iron particles with rod-shaped nanowires constitutes a dimorphic MR fluid. In this study, we investigate the influence that nanowires have on the magnetorheological and sedimentation properties of MR fluids. A variety of conventional and dimorphic MR fluid samples were considered for this study with iron loading ranging from 50 to 80 wt%. The nanowires used in this study have mean diameters of 230 nm and a length distribution of 7.6 ± 5.1 µm, while the spherical particles have a mean diameter of 8 ± 2 µm. Flow curves were measured using a parallel disk rheometer and a sedimentation measuring instrument was constructed for quantifying sedimentation velocity. The Bingham yield strength and sedimentation velocity of the dimorphic MR fluids are then compared to those of conventional MR fluids incorporating spherical particles.

Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice

There is limited time for the dust in the nebula around a newborn star to form planetesimals: in a few million years or less the stars stellar winds will disperse most of the unagglomerated dust. It has been difficult to explain the efficiency by which dust grains must have agglomerated to form planetesimals in circumstellar disks. A major obstacle is the fragility of aggregates, leading to collisional fragmentation, which makes it difficult for them to grow to, and beyond, meter-sized bodies. The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains in the cooler parts of the nebulae (Jovian distances), may be able to account for the rapid agglomeration. Measurements are presented that show that this ice readily acquires persistent macroscopic electric dipoles, strongly enhancing grain-grain adhesion. In addition, measurements were made showing that vapor-deposited amorphous water ice is also highly mechanically inelastic (≈10% rebound). Together these may explain this efficient net sticking and net growth. Similar properties of higher temperature grains may aid agglomeration in the inner regions of the nebulae.

Stiffness and Damping in Fe, Co, and Ni Nanowire-Based Magnetorheological Elastomeric Composites

The stiffness and damping properties of the aligned magnetorheological (MR) elastomer composites filled with 10 wt% Fe, Co, and Ni nanowires were investigated under normalized strain amplitude of 1, 2, and 3%, cyclic deformation frequency of 1 Hz, and magnetic flux density of 0, 0.1, and 0.2 T. The highest values of the dynamic stiffness are observed for the Ni- and the lowest for the Fe-based composites within the whole range of strain amplitude and magnetic flux density. The MR effect on the dynamic stiffness is the most significant for 1% strain amplitude and it almost completely disappears for 3% amplitude for all composites. The equivalent damping coefficient values have maxima for 1% strain amplitude for all composites. These values abruptly drop with an increase of strain amplitude to 2% and only slightly change as strain amplitude is further increased to 3%. The MR effect on the equivalent damping coefficient is high for all composites and strain amplitudes.

Elastic percolation transition in nanowire-based magnetorheological fluids

We observe an elastic percolation transition in the yield stress (τy) of cobalt-nanowire magnetorheological fluids, with a critical volume fraction of ferromagnetic particles (pc) that increases with the applied magnetic field (H). Unlike studies of static percolation phenomena, our observations reveal percolation in a dynamic, fluid-semisolid system. The elastic critical exponent (f) appears to be independent of H, having a value in the range of 1.0–1.2, near that seen in various two-dimensional networks. The superelastic exponent (c) decreases with increasing H and is smaller than that seen in typical networks.

Pyroelectricity of Water Ice

Water ice usually is thought to have zero pyroelectricity by symmetry. However, biasing it with ions breaks the symmetry because of the induced partial dipole alignment. This unmasks a large pyroelectricity. Ions were soft-landed upon 1 mum films of water ice at temperatures greater than 160 K. When cooled below 140-150 K, the dipole alignment locks in. Work function measurements of these films then show high and reversible pyroelectric activity from 30 to 150 K. For an initial approximately 10 V induced by the deposited ions at 160 K, the observed bias below 150 K varies approximately as 10 Vx(T/150 K)2. This implies that water has pyroelectric coefficients as large as that of many commercial pyroelectrics, such as lead zirconate titanate (PZT). The pyroelectricity of water ice, not previously reported, is in reasonable agreement with that predicted using harmonic analysis of a model system of SPC ice. The pyroelectricity is observed in crystalline and compact amorphous ice, deuterated or not. This implies that for water ice between 0 and 150 K (such as astrophysical ices), temperature changes can induce strong electric fields (approximately 10 MV/m) that can influence their chemistry, ion trajectories, or binding.

Field dependent response of magnetorheological elastomers utilizing spherical Fe particles versus Fe nanowires

This study compares the dynamic response of nanowire-based magnetorheological elastomers (MREs), to those containing conventional spherical particles. MRE samples were fabricated by curing the iron particle laden elastomeric material in a magnetic field. Material characteristics of the MRE samples were evaluated using a material test machine that was modified to measure static and frequency dependent characteristics of these samples under different magnetic fields. The MRE samples consisted of a silicone rubber matrix containing various weight fractions of iron particles of differing morphology. Nanowires were used to enhance the interaction forces and contact area between particles. The static and dynamic properties of the MREs were evaluated under a compressive load for the various compositions and weight fractions. The stress vs. strain characteristics were measured for each sample. The equivalent damping coefficient of the MRE samples was measured and characterized under magnetic fields of differing intensities. The dynamic characteristic (dynamic stiffness) was measured under sinusoidal excitation in the frequency domain.

Impact of Nanowires on the Properties of Magnetorheological Fluids and Elastomer Composites

The authors acknowledge funding support from the National Science Foundation (NSF-CBET-0755696), The Pennsylvania State University, and Altoona College. Additional support provided by a DARPA SBIR Phase 2 Contract No. W31P4Q-06-C-0400 (N. M. Wereley). This publication was supported by the Pennsylvania State University Materials Research Institute Nano Fabrication Network, the National Science Foundation Cooperative Agreement No. 0335765, and the National Nanotechnology Infrastructure Network through with Cornell University.

Impact of Nanowire Versus Spherical Microparticles in Magnetorheological Elastomer Composites

This study presents static and dynamic characterization of nanowire-based magnetorheological elastomer (MRE) composites. MRE composites were synthesized using a silicone rubber filled with magnetizable particles. Fe and Co particles of varying weight fraction (10, 30, and 50 wt%) were dispersed in the elastomeric matrix. To assess particle morphology, nanowire-based MRE composites were compared with spherical microparticle- based MRE composites. Under static and sinusoidal compressive loads, the field-dependent properties of the MRE composites such as static and dynamic stiffness, elastic modulus, yield stress, and equivalent damping were measured using a modified material testing machine. To investigate particle alignment effects in nanowire-based MRE composites, samples were cured in the presence of a magnetic field (aligned nanowires) and in the absence of a field (unaligned nanowires).