@article{xu_zheng_hu_2021, title={Estimation of Joint Kinematics and Fingertip Forces using Motoneuron Firing Activities: A Preliminary Report}, ISSN={["1948-3546"]}, DOI={10.1109/NER49283.2021.9441433}, abstractNote={A loss of individuated finger movement affects critical aspects of daily activities. There is a need to develop neural-machine interface techniques that can continuously decode single finger movements. In this preliminary study, we evaluated a novel decoding method that used finger-specific motoneuron firing frequency to estimate joint kinematics and fingertip forces. High-density electromyogram (EMG) signals were obtained during which index or middle fingers produced either dynamic flexion movements or isometric flexion forces. A source separation method was used to extract motor unit (MU) firing activities from a single trial. A separate validation trial was used to only retain the MUs associated with a particular finger. The finger-specific MU firing activities were then used to estimate individual finger joint angles and isometric forces in a third trial using a regression method. Our results showed that the MU firing based approach led to smaller prediction errors for both joint angles and forces compared with the conventional EMG amplitude based method. The outcomes can help develop intuitive neural-machine interface techniques that allow continuous single-finger level control of robotic hands. In addition, the previously obtained MU separation information was applied directly to new data, and it is therefore possible to enable online extraction of MU firing activities for real-time neural-machine interactions.}, journal={2021 10TH INTERNATIONAL IEEE/EMBS CONFERENCE ON NEURAL ENGINEERING (NER)}, author={Xu, Feng and Zheng, Yang and Hu, Xiaogang}, year={2021}, pages={1035–1038} }
 @article{cheng_miao_qin_li_xu_haftbaradaran_dickey_gao_zhu_2015, title={Large anelasticity and associated energy dissipation in single-crystalline nanowires}, volume={10}, ISSN={1748-3387 1748-3395}, url={http://dx.doi.org/10.1038/NNANO.2015.135}, DOI={10.1038/nnano.2015.135}, abstractNote={Anelastic materials exhibit gradual full recovery of deformation once a load is removed, leading to dissipation of internal mechanical energy. As a consequence, anelastic materials are being investigated for mechanical damping applications. At the macroscopic scale, however, anelasticity is usually very small or negligible, especially in single-crystalline materials. Here, we show that single-crystalline ZnO and p-doped Si nanowires can exhibit anelastic behaviour that is up to four orders of magnitude larger than the largest anelasticity observed in bulk materials, with a timescale on the order of minutes. In situ scanning electron microscope tests of individual nanowires showed that, on removal of the bending load and instantaneous recovery of the elastic strain, a substantial portion of the total strain gradually recovers with time. We attribute this large anelasticity to stress-gradient-induced migration of point defects, as supported by electron energy loss spectroscopy measurements and also by the fact that no anelastic behaviour could be observed under tension. We model this behaviour through a theoretical framework by point defect diffusion under a high strain gradient and short diffusion distance, expanding the classic Gorsky theory. Finally, we show that ZnO single-crystalline nanowires exhibit a high damping merit index, suggesting that crystalline nanowires with point defects are promising materials for energy damping applications.}, number={8}, journal={Nature Nanotechnology}, publisher={Springer Science and Business Media LLC}, author={Cheng, Guangming and Miao, Chunyang and Qin, Qingquan and Li, Jing and Xu, Feng and Haftbaradaran, Hamed and Dickey, Elizabeth C. and Gao, Huajian and Zhu, Yong}, year={2015}, month={Jul}, pages={687–691} }
 @article{xu_yuan_hu_qiu_2014, title={Miniature horizontal axis wind turbine system for multipurpose application}, volume={75}, ISSN={["1873-6785"]}, DOI={10.1016/j.energy.2014.07.046}, abstractNote={A MWT (miniature wind turbine) has received great attention recently for powering WISP (Wireless Intelligent Sensor Platform). In this study, two MHAWTs (miniature horizontal axis wind turbines) with and without gear transmission were designed and fabricated. A physics-based model was proposed and the optimal load resistances of the MHAWTs were predicted. The open circuit voltages, output powers and net efficiencies were measured under various ambient winds and load resistances. The experimental results showed the optimal load resistances matched well with the predicted results; the MHAWT without gear obtained higher output power at the wind speed of 2 m/s to 6 m/s, while the geared MHAWT exhibited better performance at the wind speed higher than 6 m/s. In addition, a DCM (discontinuous conduction mode) buck-boost converter was adopted as an interface circuit to maximize the charging power from MHAWTs to rechargeable batteries, exhibiting maximum efficiencies above 85%. The charging power reached about 8 mW and 36 mW at the wind speeds of 4 m/s and 6 m/s respectively, which indicated that the MHAWTs were capable for sufficient energy harvesting for powering low-power electronics continuously.}, journal={ENERGY}, author={Xu, F. J. and Yuan, F. G. and Hu, J. Z. and Qiu, Y. P.}, year={2014}, month={Oct}, pages={216–224} }
 @article{xu_zhu_2012, title={Highly Conductive and Stretchable Silver Nanowire Conductors}, volume={24}, ISSN={["0935-9648"]}, DOI={10.1002/adma.201201886}, abstractNote={IO N Materials that are both conductive and stretchable could enable a spectrum of applications such as stretchable displays, [ 1 ] stretchable radiofrequency antennas, [ 2 ] artifi cial muscles [ 3 ] and conformal skin sensors. [ 4–7 ] A variety of such materials have been recently developed, such as wavy thin metals, [ 8 , 9 ] metal-coated net-shaped plastic fi lm, [ 10 ] graphene fi lms [ 11 ] and carbon nanotube (CNT)-based composites. [ 12–19 ] But several limitations typically exist in these materials including low conductivity, [ 13 , 15 , 16 , 19 ]}, number={37}, journal={ADVANCED MATERIALS}, author={Xu, Feng and Zhu, Yong}, year={2012}, month={Sep}, pages={5117–5122} }
 @article{qin_xu_cao_ro_zhu_2012, title={Measuring True Young's Modulus of a Cantilevered Nanowire: Effect of Clamping on Resonance Frequency}, volume={8}, ISSN={1613-6810}, url={http://dx.doi.org/10.1002/smll.201200314}, DOI={10.1002/smll.201200314}, abstractNote={Abstract}, number={16}, journal={Small}, publisher={Wiley}, author={Qin, Qingquan and Xu, Feng and Cao, Yongqing and Ro, Paul I. and Zhu, Yong}, year={2012}, month={May}, pages={2571–2576} }
 @article{zhu_qin_xu_fan_ding_zhang_wiley_wang_2012, title={Size effects on elasticity, yielding, and fracture of silver nanowires: In situ experiments}, volume={85}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.85.045443}, abstractNote={Thispaperreportsthequantitativemeasurementofafullspectrumofmechanicalpropertiesoffivefoldtwinned silver (Ag) nanowires (NWs), including Young’s modulus, yield strength, and ultimate tensile strength. In-situ tensile testing of Ag NWs with diameters between 34 and 130 nm was carried out inside a scanning electron microscope (SEM). Young’s modulus, yield strength, and ultimate tensile strength all increased as the NW diameter decreased. The maximum yield strength in our tests was found to be 2.64 GPa, which is about 50 times the bulk value and close to the theoretical value of Ag in the � 110� orientation. The size effect in the yield strength is mainly due to the stiffening size effect in the Young’s modulus. Yield strain scales reasonably well with the NW surface area, which reveals that yielding of Ag NWs is due to dislocation nucleation from surface sources. Pronounced strain hardening was observed for most NWs in our study. The strain hardening, which has not previously been reported for NWs, is mainly attributed to the presence of internal twin boundaries.}, number={4}, journal={PHYSICAL REVIEW B}, author={Zhu, Yong and Qin, Qingquan and Xu, Feng and Fan, Fengru and Ding, Yong and Zhang, Tim and Wiley, Benjamin J. and Wang, Zhong Lin}, year={2012}, month={Jan} }
 @article{xu_wang_zhu_zhu_2012, title={Wavy Ribbons of Carbon Nanotubes for Stretchable Conductors}, volume={22}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201102032}, abstractNote={Abstract}, number={6}, journal={ADVANCED FUNCTIONAL MATERIALS}, author={Xu, Feng and Wang, Xin and Zhu, Yuntian and Zhu, Yong}, year={2012}, month={Mar}, pages={1279–1283} }
 @article{xu_lu_zhu_2011, title={Controlled 3D Buckling of Silicon Nanowires for Stretchable Electronics}, volume={5}, ISSN={["1936-0851"]}, DOI={10.1021/nn103189z}, abstractNote={Silicon (Si) nanowire (NW) coils were fabricated on elastomeric substrates by a controlled buckling process. Si NWs were first transferred onto prestrained and ultraviolet/ozone (UVO)-treated poly(dimethylsiloxane) (PDMS) substrates and buckled upon release of the prestrain. Two buckling modes (the in-plane wavy mode and the three-dimensional coiled mode) were found; a transition between them was achieved by controlling the UVO treatment of PDMS. Structural characterization revealed that the NW coils were oval-shaped. The oval-shaped NW coils exhibited very large stretchability up to the failure strain of PDMS (∼104% in our study). Such a large stretchability relies on the effectiveness of the coil shape in mitigating the maximum local strain, with a mechanics that is similar to the motion of a coil spring. Single NW devices based on coiled NWs were demonstrated with a nearly constant electrical response in a large strain range. In addition to the wavy shape, the coil shape represents an effective architecture in accommodating large tension, compression, bending, and twist, which may find important applications for stretchable electronics and other stretchable technologies.}, number={1}, journal={ACS NANO}, author={Xu, Feng and Lu, Wei and Zhu, Yong}, year={2011}, month={Jan}, pages={672–678} }
 @article{xu_durham_wiley_zhu_2011, title={Strain-Release Assembly of Nanowires on Stretchable Substrates}, volume={5}, ISSN={["1936-086X"]}, DOI={10.1021/nn103183d}, abstractNote={A simple yet effective method for assembly of highly aligned nanowires (NWs) on stretchable substrates is reported. In this method, NWs were first transferred to a strained stretchable substrate. After the strain was released, the NWs aligned in the transverse direction and the area coverage of the NWs on the substrate increased. This method can be applied to any NWs deposited on a stretchable film and can be repeated multiple times to increase the alignment and density of the NWs. For silver (Ag) and silicon (Si) NWs on poly(dimethylsiloxane) (PDMS) substrates, the probability of NW alignment increased from 29% to 90% for Ag NWs, and from 25% to 88% for Si NWs after two assembly steps; the density increased by 60% and 75% for the Ag and Si NWs, respectively. The large-strain elasticity of the substrate and the static friction between the NWs and the substrate play key roles in this assembly method. We find that a model that takes into account the volume incompressibility of PDMS reliably predicts the degree of NW alignment and NW density. The utility of this assembly method was demonstrated by fabricating a strain sensor array composed of aligned Si NWs on a PDMS substrate, with a device yield of 95%.}, number={2}, journal={ACS NANO}, author={Xu, Feng and Durham, John W., III and Wiley, Benjamin J. and Zhu, Yong}, year={2011}, month={Feb}, pages={1556–1563} }
 @article{xu_qin_mishra_gu_zhu_2010, title={Mechanical Properties of ZnO Nanowires Under Different Loading Modes}, volume={3}, ISSN={["1998-0124"]}, DOI={10.1007/s12274-010-1030-4}, abstractNote={A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.}, number={4}, journal={NANO RESEARCH}, author={Xu, Feng and Qin, Qingqun and Mishra, Ashish and Gu, Yi and Zhu, Yong}, year={2010}, month={Apr}, pages={271–280} }
 @article{zhu_xu_qin_fung_lu_2009, title={Mechanical Properties of Vapor-Liquid-Solid Synthesized Silicon Nanowires}, volume={9}, ISSN={["1530-6984"]}, DOI={10.1021/nl902132w}, abstractNote={The Young's modulus and fracture strength of silicon nanowires with diameters between 15 and 60 nm and lengths between 1.5 and 4.3 mum were measured. The nanowires, grown by the vapor-liquid-solid process, were subjected to tensile tests in situ inside a scanning electron microscope. The Young's modulus decreased while the fracture strength increased up to 12.2 GPa, as the nanowire diameter decreased. The fracture strength also increased with the decrease of the side surface area; the increase rate for the chemically synthesized silicon nanowires was found to be much higher than that for the microfabricated silicon thin films. Repeated loading and unloading during tensile tests demonstrated that the nanowires are linear elastic until fracture without appreciable plasticity.}, number={11}, journal={NANO LETTERS}, author={Zhu, Yong and Xu, Feng and Qin, Qingquan and Fung, Wayne Y. and Lu, Wei}, year={2009}, month={Nov}, pages={3934–3939} }