Unconventional acoustic wave propagation transitions induced by resonant scatterers in the high-density limit.

Experiments on ultrasound propagation through a gel doped with resonant encapsulated microbubbles provided evidence for a discontinuous transition between wave propagation regimes at a critical excitation frequency. Such behavior is unlike that observed for soft materials doped with non-resonant air or through liquid foams, and disagrees with a simple mixture model for the effective sound speed. Here, we study the discontinuous transition by measuring the transition as a function of encapsulated microbubble volume fraction. The results show the transition always occurs in the strong-scattering limit (l/λ < 1, l and λ are the mean free path and wavelength, respectively), that at the critical frequency the effective phase velocity changes discontinuously to a constant value with increasing microbubble volume fraction, and the measured critical frequency shows a power law dependence on microbubble volume fraction. The results cannot be explained by multiple scattering theory, viscous effects, mode decoupling, or a critical density of states. It is hypothesized the transition depends upon the microbubble on-resonance effective properties, and we discuss the results within the context of percolation theory. The results shed light on the discontinuous transition's physics, and suggest soft materials can be engineered in this manner to achieve a broad range of physical properties with potential application in ultrasonic actuators and switches.

Performance of a combined electrotrophic and electrogenic biofilm operated under long-term, continuous cycling.

OBJECTIVES: Evaluate electrochemically active biofilms as high energy density rechargeable microbial batteries toward providing persistent power in applications where traditional battery technology is limiting (, remote monitoring applications). RESULTS: Here we demonstrated that an electrochemically active biofilm was able to store and release electrical charge for alternating charge/discharge cycles of up to 24 h periodicity (50% duty cycle) with no significant decrease in average current density (0.16 ± 0.04 A/m2) for over 600 days. However, operation at 24 h periodicity for > 50 days resulted in a sharp decrease in the current to nearly zero. This current crash was recoverable by decreasing the periodicity. Overall, the coulombic efficiency remained near unity within experimental error (102 ± 3%) for all of the tested cycling periods. Electrochemical characterization here suggests that electron transfer occurs through multiple routes, likely a mixture of direct and mediated mechanisms. CONCLUSIONS: These results indicate that bidirectional electrogenic/electrotrophic biofilms are capable of efficient charge storage/release over a wide range of cycling frequency and may eventually enable development of sustainable, high energy density rechargeable batteries.

Critical Role of a Nanometer-Scale Microballoon Shell on Bulk Acoustic Properties of Doped Soft Matter.

A material's acoustic properties depend critically upon porosity. Doping a soft material with gas-filled microballoons permits a controlled variation of the porosity through a scalable fabrication process while generating well-tailored spherical cavities that are impermeable to liquids. However, evidence is lacking of how the nanometer-scale polymeric shell contributes to the overall effective material properties in the regime where the wavelength is comparable to the sample thickness. Here, we measure ultrasound transmission through a microballoon-doped soft material as a function of microballoon and impurity concentration, sample thickness, and frequency. The measured longitudinal wave speeds are an order of magnitude larger than those in similar systems where no shell is present, while the transverse wave speed is found to linearly increase with microballoon concentration, also in contrast to systems with no shell. Furthermore, we find the results are independent of the soft material's elastic moduli as well as a lesser contribution of the microballoon shell on material attenuation. The results are validated with a multiple scattering model and suggest the shell contributes significantly to the material's bulk acoustic properties despite its thickness being 4 orders of magnitude smaller than the acoustic wavelength. Our results demonstrate how a nanometer-scale interface between a gas cavity and a soft polymer can be used in the submicrometer design of acoustic materials, and are important for observations of such phenomena as strong interference effects in soft matter.

Cylindrical heat conduction and structural acoustic models for enclosed fiber array thermophones.

Calculation of the heat loss for thermophone heating elements is a function of their geometry and the thermodynamics of their surroundings. Steady-state behavior is difficult to establish or evaluate as heat is only flowing in one direction in the device. However, for a heating element made from an array of carbon fibers in a planar enclosure, several assumptions can be made, leading to simple solutions of the heat equation. These solutions can be used to more carefully determine the efficiency of thermophones of this geometry. Acoustic response is predicted with the application of a Helmholtz resonator and thin plate structural acoustics models. A laboratory thermophone utilizing a sparse horizontal array of fine (6.7 µm diameter) carbon fibers is designed and tested. Experimental results are compared with the model. The model is also used to examine the optimal array density for maximal efficiency.

The Effect of Copper Addition on the Activity and Stability of Iron-Based CO2 Hydrogenation Catalysts.

Iron-based CO2 catalysts have shown promise as a viable route to the production of olefins from CO2 and H2 gas. However, these catalysts can suffer from low conversion and high methane selectivity, as well as being particularly vulnerable to water produced during the reaction. In an effort to improve both the activity and durability of iron-based catalysts on an alumina support, copper (10-30%) has been added to the catalyst matrix. In this paper, the effects of copper addition on the catalyst activity and morphology are examined. The addition of 10% copper significantly increases the CO2 conversion, and decreases methane and carbon monoxide selectivity, without significantly altering the crystallinity and structure of the catalyst itself. The FeCu/K catalysts form an inverse spinel crystal phase that is independent of copper content and a metallic phase that increases in abundance with copper loading (>10% Cu). At higher loadings, copper separates from the iron oxide phase and produces metallic copper as shown by SEM-EDS. An addition of copper appears to increase the rate of the Fischer-Tropsch reaction step, as shown by modeling of the chemical kinetics and the inter- and intra-particle transport of mass and energy.

Electronic Transport in Bilayer MoS2 Encapsulated in HfO2.

The exact nature of the interface between a two-dimensional crystal and its environment can have a significant impact on the electronic transport within the crystal, and can place fundamental limitations on transistor performance and long-term functionality. Two-dimensional transition-metal dichalcogenides are a new class of transistor channel material with electronic properties that can be tailored through dielectric engineering of the material/environmental interface. Here, we report electrical transport measurements carried out in the insulating regime of bilayer molybdenum disulfide, which has been encapsulated within a high-κ hafnium oxide dielectric. Temperature- and carrier-density-dependent measurements show that for T < 130 K the transport is governed by resonant tunneling, and at T = 4.2 K the tunneling peak lineshape is well-fitted by a Lorentzian with an amplitude less than e2/h. Estimates of tunneling time give τ ∼ 1.2 ps corresponding to a frequency f ∼ 0.84 THz. The tunneling processes are observable up to T ∼ 190 K (more than a factor of 6 higher than that previously reported for MoS2 on SiO2) despite the onset of variable range hopping at T ∼ 130 K, demonstrating the coexistence of the two transport processes within the same temperature range. At constant temperature, varying the Fermi energy allows experimental access to each transport process. The results are interpreted in terms of an increase in charge carrier screening length and a decrease in electron-phonon coupling induced by the hafnium oxide. Our results represent the first demonstration of the intermediate tunneling-hopping transport regime in a two-dimensional material. The results suggest that interface engineering may be a macroscopic tool for controlling quantum transport within such materials as well as for increasing the operating temperatures for resonant-tunneling devices derived from such materials, with applications in high-frequency electronics and logic devices.

Energy Dissipation Pathways in Few-Layer MoS2 Nanoelectromechanical Systems.

Free standing, atomically thin transition metal dichalcogenides are a new class of ultralightweight nanoelectromechanical systems with potentially game-changing electro- and opto-mechanical properties, however, the energy dissipation pathways that fundamentally limit the performance of these systems is still poorly understood. Here, we identify the dominant energy dissipation pathways in few-layer MoS2 nanoelectromechanical systems. The low temperature quality factors and resonant frequencies are shown to significantly decrease upon heating to 293 K, and we find the temperature dependence of the energy dissipation can be explained when accounting for both intrinsic and extrinsic damping sources. A transition in the dominant dissipation pathways occurs at T ~ 110 K with relatively larger contributions from phonon-phonon and electrostatic interactions for T > 110 K and larger contributions from clamping losses for T < 110 K. We further demonstrate a room temperature thermomechanical-noise-limited force sensitivity of ~8 fN/Hz1/2 that, despite multiple dissipation pathways, remains effectively constant over the course of more than four years. Our results provide insight into the mechanisms limiting the performance of nanoelectromechanical systems derived from few-layer materials, which is vital to the development of next-generation force and mass sensors.

Potassium-Promoted Molybdenum Carbide as a Highly Active and Selective Catalyst for CO2 Conversion to CO.

The high concentration of CO2 bound in seawater represents a significant opportunity to extract and use this CO2 as a C1 feedstock for synthetic fuels. Using an existing process, CO2 and H2 can be concurrently extracted from seawater and then catalytically reacted to produce synthetic fuel. Hydrogenating CO2 directly into liquid hydrocarbons is exceptionally difficult, but by first identifying a catalyst for selective CO production through the reverse water-gas shift (RWGS) reaction, CO can then be hydrogenated to fuel through Fischer-Tropsch (FT) synthesis. Results of this study demonstrate that potassium-promoted molybdenum carbide supported on γ-Al2 O3 (K-Mo2 C/γ-Al2 O3 ) is a low-cost, stable, and highly selective catalyst for RWGS over a wide range of conversions. These findings are supported by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations.

Evidence for Spin Glass Ordering Near the Weak to Strong Localization Transition in Hydrogenated Graphene.

We provide evidence that magnetic moments formed when hydrogen atoms are covalently bound to graphene exhibit spin glass ordering. We observe logarithmic time-dependent relaxations in the remnant magnetoresistance following magnetic field sweeps, as well as strong variances in the remnant magnetoresistance following field-cooled and zero-field-cooled scenarios, which are hallmarks of canonical spin glasses and provide experimental evidence for the hydrogenated graphene spin glass state. Following magnetic field sweeps, and over a relaxation period of several minutes, we measure changes in the resistivity that are more than 3 orders of magnitude larger than what has previously been reported for a two-dimensional spin glass. Magnetotransport measurements at the Dirac point, and as a function of hydrogen concentration, demonstrate that the spin glass state is observable as the zero-field resistivity reaches a value close to the quantum unit h/2e(2), corresponding to the point at which the system undergoes a transition from weak to strong localization. Our work sheds light on the critical magnetic-dopant density required to observe spin glass formation in two-dimensional systems. These findings have implications to the basic understanding of spin glasses as well the fields of two-dimensional magnetic materials and spintronics.

Differences in Physical and Biochemical Properties of Thermus scotoductus SA-01 Cultured with Dielectric or Convection Heating.

A thermophile, Thermus scotoductus SA-01, was cultured within a constant-temperature (65°C) microwave (MW) digester to determine if MW-specific effects influenced the growth and physiology of the organism. As a control, T. scotoductus cells were also cultured using convection heating at the same temperature as the MW studies. Cell growth was analyzed by optical density (OD) measurements, and cell morphologies were characterized using electron microscopy imaging (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]), dynamic light scattering (DLS), and atomic force microscopy (AFM). Biophysical properties (i.e., turgor pressure) were also calculated with AFM, and biochemical compositions (i.e., proteins, nucleic acids, fatty acids) were analyzed by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Gas chromatography-mass spectrometry (GC-MS) was used to analyze the fatty acid methyl esters extracted from cell membranes. Here we report successful cultivation of a thermophile with only dielectric heating. Under the MW conditions for growth, cell walls remained intact and there were no indications of membrane damage or cell leakage. Results from these studies also demonstrated that T. scotoductus cells grown with MW heating exhibited accelerated growth rates in addition to altered cell morphologies and biochemical compositions compared with oven-grown cells.

Residency preceptor development and evaluation: a new approach.

PURPOSE: The design and implementation of a preceptor development program involving close collaboration by the pharmacy residency program director (RPD) and members of a residency advisory committee (RAC) are described. SUMMARY: In 2011 the Nebraska Medical Center (NMC) implemented a program to foster the development of preceptors' teaching and clinical skills, as required by accreditation standards of the American Society of Health-System Pharmacists (ASHP). The RPD and RAC worked closely to define expectations of resident preceptors and a pathway for their transition from "conditional" to "primary" status. Among other program requirements, prospective (and new incoming) preceptors must (1) submit to the RAC a letter of interest in becoming a resident preceptor accompanied by an academic and professional record, (2) complete a new preceptor orientation session in which teaching concepts and principles are presented and discussed, and (3) obtain 10 preceptor education (PE) credits every two years. PE credits are earned through defined preceptor development opportunities including a teaching core and a clinical core focused on enhancing skills such as the application of therapeutic knowledge and conduct of clinical research. All preceptors are required to submit updated academic and professional records to the RAC every two years. The RAC audits 25% of preceptors to document attainment of PE credits and also conducts annual reviews of ASHP Resi-Trak summary evaluations of all preceptors and learning experiences. CONCLUSION: The NMC program may serve as a model for other institutions in planning and evaluating professional development programs to ensure that preceptors maintain the skills and knowledge necessary to provide high-quality resident training.

Hydrogenation and fluorination of graphene models: analysis via the average local ionization energy.

We have investigated the use of the average local ionization energy, I[combining overline](S)(r), as a means for rapidly predicting the relative reactivities of different sites on two model graphene surfaces toward the successive addition of one, two, and three hydrogen or fluorine atoms. The I[combining overline](S)(r) results were compared with directly computed interaction energies, at the B3LYP/6-311G(d,p) level. I[combining overline](S)(r) correctly predicts that the edges of graphene sheets are more reactive than the interior portions. It shows that added hydrogens activate the adjoining (ortho) sites and deactivate those that are separated by one site (meta). Overall, I[combining overline](S)(r) is effective for rapidly (single calculations) estimating the relative site reactivities of these large systems, although it reflects only the system prior to an interaction and cannot take into account postinteraction factors, e.g., structural distortion.

Surface doping and band gap tunability in hydrogenated graphene.

We report the first observation of the n-type nature of hydrogenated graphene on SiO(2) and demonstrate the conversion of the majority carrier type from electrons to holes using surface doping. Density functional calculations indicate that the carrier type reversal is directly related to the magnitude of the hydrogenated graphene's work function relative to the substrate, which decreases when adsorbates such as water are present. Additionally, we show by temperature-dependent electronic transport measurements that hydrogenating graphene induces a band gap and that in the moderate temperature regime [220-375 K], the band gap has a maximum value at the charge neutrality point, is tunable with an electric field effect, and is higher for higher hydrogen coverage. The ability to control the majority charge carrier in hydrogenated graphene, in addition to opening a band gap, suggests potential for chemically modified graphene p-n junctions.

Ultrathin single crystal diamond nanomechanical dome resonators.

We present the first nanomechanical resonators microfabricated in single-crystal diamond. Shell-type resonators only 70 nm thick, the thinnest single crystal diamond structures produced to date, demonstrate a high-quality factor (Q ≈ 1000 at room temperature, Q ≈ 20 000 at 10 K) at radio frequencies (50-600 MHz). Quality factor dependence on temperature and frequency suggests an extrinsic origin to the dominant dissipation mechanism and methods to further enhance resonator performance.

Substance use attitudes and behaviors at three pharmacy colleges.

The objective of this study was to profile and compare alcohol and other drug (AOD) use attitudes and behaviors in three pharmacy colleges. Student surveys of AOD use attitudes and behaviors were conducted at one southwestern and two midwestern pharmacy colleges. Response was 86.5% (566/654). Reported past-year use included alcohol 82.8%, tobacco 25.4%, and marijuana 6.9%. Past-year AOD-associated events included blackouts 18.2%, class or work under influence 7.8%, patient care under influence 1.4%, lowered grades or job evaluations 5.8%, legal charges 2.7%, and heavy drinking 29.0%. Family histories of alcohol or drug problems were reported by 35.5% and 13.1%, respectively. A number of significant differences in AOD use attitudes and behaviors between the three colleges were identified. Pharmacy students reported a number of risky drug-use attitudes and behaviors in this survey. Student AOD prevention, assistance, and education should be proactively addressed by pharmacy colleges.