Previous studies have demonstrated that it is possible to use the ultrasound radiation force in air for modal excitation of objects ranging in size from microcantilevers that are a few hundred microns in length to hard drive suspensions and other cantilevers a few centimeters long. The current study demonstrates that the ultrasound radiation force excitation technique can also be used for modal excitation of significantly larger objects, in this case an acoustic guitar. It was demonstrated that the noncontact combination of ultrasound radiation force excitation and a scanning vibrometer allowed measurements of both the frequency response and operating deflection shapes of a Cordoba 45R classical guitar in the range from 70 to 800 Hz. The resonance frequencies and deflection shapes are similar to those measured using a conventional mechanical shaker. By using a pair of ultrasound transducers and adjusting their relative phase difference, it was possible to selectively enhance or suppress different resonances. This is a substantial extension over previous studies because the guitar is several orders of magnitude larger than devices used previously.
The ultrasound radiation force has been used for noncontact excitation of devices ranging from microcantilevers to acoustic guitars. For ultrasound radiation force excitation, one challenge is formation of standing waves between the ultrasound transducer and the device under test. Standing waves result in constructive/destructive interference causing significant variations in the intensity of the ultrasound field. The standing-wave induced intensity variations in the radiation force can result from minor changes in the transducer position, carrier frequency, or changes in the speed of sound due to changes in ambient temperature. The current study demonstrates that by randomly varying the ultrasound carrier frequency in packets, it is possible to eliminate the negative consequences resulting from the formation of standing waves. A converging ultrasound transducer with a central frequency of 550 kHz was focused onto a brass cantilever. The 267 Hz resonance was excited with the ultrasound radiation force with a carrier frequency that randomly varied between 525 kHz to 575 kHz in packets of 10 cycles. Because each packet had a different carrier frequency, the amplitude of standing wave artifacts was reduced by a factor of 20 compared to a constant frequency excitation of 550 kHz.
Molybdenum dioxide (MoO2) has appealing properties as an alternative to graphite for Li-ion battery anodes. It has a low electrical resistivity (8.8⋅10⁻⁵ Ω⋅cm for bulk MoO2 at room temperature[1] vs. about 1⋅10⁻¹ Ω⋅cm for graphite powder electrodes[2]) and a high theoretical capacity (840 mAh/g[3] vs 372 mAh/g for graphite[4]). The MoO2 structure accommodates 4 lithium atoms for every Mo atom, whereas graphite can only accommodate 1 lithium atom per 6 carbon atoms. MoO2 has a 1.1 V chemical potential versus lithium ions,[5] suitable for an anode material. Graphite lithiates at about 0.1 V, yielding a higher cell voltage than MoO2, but also making it susceptible to hazardous lithium metal deposition.[6] Because MoO2 undergoes a large volume change while intercalating Li-ions (12% according to DFT calculations[5]), its bulk form has limited capacity and cycling stability. This can be significantly enhanced by changing its morphology, but its performance is strongly dependent on synthesis method.[7] Li-ion diffusion kinetics in MoO2 are relatively slow, so the material benefits from nano-scale particles that reduce the diffusion length.[1] Many studies have examined synthesis routes for nano-sized MoO2 oxides[8][9] or hybrid materials.[10][11][12] This work focuses on synthesizing particles of micron to nano-scale using a low-temperature, wet chemical synthesis, and characterizing the effect of particle size on the electrochemical performance of MoO2 materials, providing insight into the lithium intercalation mechanism. As a comparison to MoO2, MoO3 was also investigated.[13] It shares morphology dependence and slow kinetics with MoO2, but is an insulator.[14] Its higher chemical potential makes it more suitable as a cathode material.[5] The electrode materials were characterized using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), and X-ray absorption spectroscopy fine structure (XAFS). XAFS is an element-specific technique, probing the local electronic and atomic environment. As XAFS measurements do not require long-range crystalline order, they yield information for both crystalline and amorphous phases, making XAFS a valuable technique for battery material characterization. XAFS spectra were taken in situ during charge and discharge cycles of bulk and nanoscale MoO2 half-cells vs. Li metal. This helps elucidate the charge and discharge mechanisms and provides insight into the cycling behavior of the cell. [1] Shi, Y. et al. Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity. Nano Letters 9, 4215-4220 (2009). doi:10.1021/nl902423a. [2] Imoto, K., et al. High-performance carbon counter electrode for dye-sensitized solar cells. Solar Energy Materials and Solar Cells 79, no. 4, 459-469 (2003). doi:10.1016/S0927-0248(03)00021-7 [3] Wang, Z., et al. One-pot synthesis of uniform carbon-coated MoO2 nanospheres for high-rate reversible lithium storage. Chemical Communications 46, 6906 (2010). doi:10.1039/c0cc01174f. [4] Shu, Z. X., et al. Electrochemical intercalation of lithium into graphite. Journal of The Electrochemical Society 140, no. 4 (1993): 922-927. doi: 10.1149/1.2056228 [5] Dillon, A., et al. High Capacity MoO3 Nanoparticle Li-Ion Battery Anode. Vehicle Technologies Program AMR, Feb. 27, 2008. http://energy.gov/sites/prod/files/2014/03/f11/merit08_dillon.pdf [6] Jansen, A. N., et al. Development of a high-power lithium-ion battery. Journal of power sources 81 (1999): 902-905. doi:10.1016/S0378-7753(99)00268-2 [7] Wang, Z., et al. One-pot synthesis of uniform carbon-coated MoO2 nanospheres for high-rate reversible lithium storage. Chemical Communications 46, 6906 (2010). doi:10.1039/c0cc01174f. [8] Ellefson, Caleb A., et al. Synthesis and applications of molybdenum (IV) oxide. Journal of Materials Science 47, no. 5 (2012): 2057-2071. doi:10.1007/s10853-011-5918-5 [9] Manthiram, A., A. Dananjay, and Y. T. Zhu. New route to reduced transition-metal oxides. Chemistry of materials 6, no. 10 (1994): 1601-1602. doi: 10.1021/cm00046a006 [10] Hirsch, Ofer, et al. Aliovalent Ni in MoO2 Lattice— Probing the Structure and Valence of Ni and Its Implication on the Electrochemical Performance. Chemistry of Materials 26, no. 15 (2014): 4505-4513. doi:10.1021/cm501698a [11] Huang, Z. X., et al. 3D graphene supported MoO 2 for high performance binder-free lithium ion battery. Nanoscale 6, no. 16 (2014): 9839-9845. doi:10.1039/C4NR01744G [12] Bhaskar, A., et al. MoO2/multiwalled carbon nanotubes (MWCNT) hybrid for use as a Li-ion battery anode. ACS applied materials & interfaces 5, no. 7 (2013): 2555-2566. doi:10.1021/am3031536 [13] Luigi, C., and Pistoia, G. MoO3: A New Electrode Material for Nonaqueous Secondary Battery Applications. Journal of The Electrochemical Society 118, no. 12 (1971): 1905-8. doi: 10.1149/1.2407864 [14] Sunu, S. S., et al. Electrical conductivity and gas sensing properties of MoO3. Sensors and Actuators B: Chemical 101, no. 1 (2004): 161-174. doi:10.1016/j.snb.2004.02.048
Development of transformational electrochemical energy storage technologies is an imperative for enabling sustainable technologies such as vehicle electrification and renewable energy generation. Major advances in these technologies are made possible by developing the ability to predict how structural changes correlate with the functionality of new materials. Characterization by x-ray absorption spectroscopy (XAS) is an important tool in developing a better understanding of the structure-function relationship in battery materials, including structural changes upon cycling and aging mechanisms. XAS is commonly used to study electrochemical reactions in situ and is particularly valuable when working with nanomaterials or highly disordered or doped crystalline systems. A successful in situ XAS experiment on battery materials requires a careful choice of experimental conditions including design of the in situ cell, choice of beamline, energy scan mode, and detectors. An overview of these considerations for various types of battery materials will be presented along with results which illustrate the pitfalls inherent in these experiments. Results will be presented on a variety of Li-ion battery cathode and anode materials, including metals and metal oxides where there is a high capacity but rapid aging due to severe swelling of the anode material upon lithiation. Our results have identified the structural signature of metal-Li bonds and their persistence upon discharge, which imply a loss of electrical contact as the primary failure mode of these materials. In situ results on aqueous battery materials such as Fe2O3 and NiOOH will also be presented and discussed with particular emphasis on the comparison between nanoparticles in solid state electrodes and in nanofluid suspension electrodes known as nanoelectrofuel (NEF). NEF are stable dispersions of battery active nanoparticles in electrolyte that effectively charge/discharge as they are pumped through custom-designed flow cell(s) and represent a high-energy-density rechargeable, renewable, and recyclable electrochemical fuel.
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