Publications Search Results

Now showing 1 - 10 of 67
  • Publication
    Influence of the Sputtering Technique and Thermal Annealing on YSZ Thin Films for Oxygen Sensing Applications
    ( 2021)
    Alpuche, E.P.
    ;
    Gröger, P.
    ;
    Wang, X.
    ;
    Kroyer, T.
    ;
    Fasoulas, S.
    Yttria-stabilized zirconia (YSZ) thin films were deposited using direct current (reactive and metallic) and radio frequency magnetron sputtering. The effect of the deposition technique and annealing treatment on the microstructure and crystallinity of the thin films was assessed. Using the films produced in this work, oxygen gas sensors were built and their performance under vacuum conditions was evaluated. All the films exhibited a cubic crystalline structure after a post-deposition thermal treatment, regardless of the sputtering technique. When the annealing treatment surpassed 1000 °C, impurities were detected on the thin film surface. The oxygen gas sensors employing the reactive and oxide-sputtered YSZ thin films displayed a proportional increase in the sensor current as the oxygen partial pressure was increased in the evaluated pressure range (5 × 10−6 to 2 × 10−3 mbar). The sensors which employed the metallic-deposited YSZ films suffered from electronic conductivity at low partial pressures.
  • Publication
    On the scaling laws for low-temperature plasmas at macro and micro scales
    ( 2021)
    Fu, Y.
    ;
    Wang, X.
    ;
    Zheng, B.
    ;
    Zhang, P.
    ;
    Fan, Q.H.
    ;
    Verboncoeur, J.P.
    The theoretical background and historical development of the similarity theory during the past decades are reviewed. We demonstrate similar discharges in local and nonlocal kinetic regimes, taking the low-pressure capacitive radio frequency (rf) discharges and microdischarges as examples. By using fully kinetic particle-in-cell simulations, we verify the similarity law (SL) and show a violation of frequency scaling (f-scaling) in the low-pressure capacitive rf plasmas. The similarity relations for electron density and electron power absorption are confirmed in similar rf discharges. With only the driving frequency varied, the f-scaling for electron density is also validated, showing almost the same trend as the similarity scaling, across most of the frequency regime. However, violations of the f-scaling are observed at lower frequencies, which are found to be relevant to the electron heating mode transition from stochastic to Ohmic heating. The scaling characteristics have also been comprehensively studied for microdischarges with dimensions from hundreds to several microns, with transition from secondary electron dominated regime to field emission regime. Finally, practical applications of the similarity and scaling laws are summarized.
  • Publication
    Generalizing Similarity Laws for Radio-Frequency Discharge Plasmas across Nonlinear Transition Regimes
    ( 2021)
    Fu, Y.
    ;
    Wang, H.
    ;
    Zheng, B.
    ;
    Zhang, P.
    ;
    Fan, Q.H.
    ;
    Wang, X.
    ;
    Verboncoeur, J.P.
    We generalize similarity theory based on the scaling and solution invariance of the Boltzmann equation, coupled with the Poisson equation, and demonstrate similarity laws for radio-frequency (rf) discharge plasmas across three nonlinear transitional regimes, namely, the alpha-gamma mode transition, the stochastic-Ohmic-heating mode transition, and the bounce-resonance-heating mode transition. Fundamental plasma parameters, e.g., the electron power absorption, under similar discharge conditions are examined via fully kinetic particle-in-cell simulations, and electron-kinetic invariance is exemplified in similar rf discharge plasmas. The results unambiguously confirm the applicability of similarity laws for rf plasmas in extended operating regimes, and strengthen the foundations and framework of similarity physics with universality.
  • Publication
    Trial of upadacitinib and adalimumab for psoriatic arthritis
    ( 2021)
    McInnes, I.B.
    ;
    Anderson, J.K.
    ;
    Magrey, M.
    ;
    Merola, J.F.
    ;
    Liu, Y.
    ;
    Kishimoto, M.
    ;
    Jeka, S.
    ;
    PachecoTena, C.
    ;
    Wang, X.
    ;
    Chen, L.
    ;
    Zueger, P.
    ;
    Liu, J.
    ;
    Pangan, A.L.
    ;
    Behrens, F.
    Background The Janus kinase inhibitor upadacitinib is a potential treatment for psoriatic arthritis. The efficacy and safety of upadacitinib as compared with adalimumab, a tumor necrosis factor a inhibitor, in patients who have an inadequate response to nonbiologic disease-modifying antirheumatic drugs are unclear. Methods In a 24-week, phase 3 trial, we randomly assigned patients in a 1:1:1:1 ratio to receive oral upadacitinib at a dose of 15 mg or 30 mg once daily, placebo, or subcutaneous adalimumab (40 mg every other week). The primary end point was an American College of Rheumatology 20 (ACR20) response (>20% decrease in the number of tender and swollen joints and >20% improvement in at least three of five other domains) at week 12 with upadacitinib as compared with placebo. Secondary end points included comparisons of upadacitinib with adalimumab. Results A total of 1704 patients received an active drug or placebo. The percentage of patients who had an ACR20 response at week 12 was 70.6% with 15-mg upadacitinib, 78.5% with 30-mg upadacitinib, 36.2% with placebo (P<0.001 for both upadacitinib doses vs. placebo), and 65.0% with adalimumab. The difference between groups for 15-mg upadacitinib as compared with adalimumab was 5.6 percentage points (95% confidence interval [CI], −0.6 to 11.8) and for 30-mg upadacitinib as compared with adalimumab was 13.5 percentage points (95% CI, 7.5 to 19.4). Both upadacitinib doses were noninferior to adalimumab for the ACR20 response at week 12; the 30-mg dose but not the 15-mg dose was superior to adalimumab. The incidence of adverse events through week 24 was 66.9% with 15-mg upadacitinib, 72.3% with 30-mg upadacitinib, 59.6% with placebo, and 64.8% with adalimumab. There were serious infections in 1.2%, 2.6%, 0.9%, and 0.7% of the patients, respectively. Hepatic disorders occurred in 9.1% of patients in the 15-mg upadacitinib group and 12.3% in the 30-mg upadacitinib group, but grade 3 increases in aminotransferase levels occurred in 2% of patients or fewer in all groups. Conclusions The percentage of patients with psoriatic arthritis who had an ACR20 response at week 12 was significantly higher with 15-mg or 30-mg upadacitinib than with placebo. The 30-mg dose but not the 15-mg dose was superior to adalimumab. Adverse events were more frequent with upadacitinib than with placebo. (Funded by AbbVie; SELECT-PsA 1 ClinicalTrials.gov number, NCT03104400. opens in new tab.)
  • Publication
    Direct current microplasma formation around microstructure arrays
    ( 2021)
    Fu, Y.
    ;
    Wang, H.
    ;
    Zheng, B.
    ;
    Zhang, P.
    ;
    Fan, Q.H.
    ;
    Wang, X.
    ;
    Verboncoeur, J.P.
    We demonstrate the formation and transition behaviors of a microplasma around microstructure arrays at different gas pressures via two-dimensional particle-in-cell/Monte Carlo collision simulations. It is found that the microdischarge occurs outside the cathode microcavities at the lowest pressure and starts penetrating the microcavities with a curved sheath edge as the pressure increases. At higher pressure, coupled periodic microhollow cathode discharges (MHCDs) are formed inside the microcavities. Further increasing the gas pressure results in the disappearance of the MHCDs, and the dominant discharge shifts outside of the microcavity, locating above the protrusion tips. The effect of the space charge shielding on the discharge and the conditions for MHCD formation are discussed. The macroscopic discharge parameter scalings with the gas pressure and the electron kinetics are also examined. The results are helpful for deeply understanding the microplasma formation with nonplanar electrodes, which inform the scaling, design, and optimization of microplasma array devices across a wide range of pressure regimes in practical applications.
  • Publication
    Similarity properties in capacitive radio frequency plasmas with nonlinear collision processes
    ( 2021)
    Yang, D.
    ;
    Fu, Y.
    ;
    Zheng, B.
    ;
    Wang, H.
    ;
    Fan, Q.H.
    ;
    Zou, X.
    ;
    Wang, X.
    ;
    Verboncoeur, J.P.
    Similarity laws (SL) are essential for correlating the characteristics of plasmas at different dimensional scales, which have been validated for radio frequency (rf) discharges at low pressure but under limited conditions. In this work, we evaluate the effects of nonlinear collisions (e.g. stepwise ionization) on the similarity properties in capacitive rf discharges in argon across a wide range of pressure regimes via fully kinetic particle-in-cell simulations. The SL scalings of fundamental discharge parameters, e.g. the electron power absorption and electron energy probability function, are examined with and without nonlinear collisions, respectively. Without the nonlinear collisions, the similarity scalings are found to be rigorously valid. When the nonlinear collisions are considered, the similarity properties in rf discharges still exist approximately, which indicates that the violations caused by the nonlinear collisions are still minor in the studied cases. The reasons for the effectiveness of SL scalings with nonlinear collision processes are also discussed. The results from this study confirmed the validity of similarity transformations with more complex reaction kinetics, which may promote the applicability of the SL scalings for the design and fabrications of plasma devices.
  • Publication
    Similarity of capacitive radio-frequency discharges in nonlocal regimes
    ( 2020)
    Fu, Y.
    ;
    Zheng, B.
    ;
    Zhang, P.
    ;
    Fan, Q.H.
    ;
    Verboncoeur, J.P.
    ;
    Wang, X.
    Similarity transformations are essential for correlating discharges at different scales, which are mostly utilized with local field or local energy approximations. In this work, we report the fully kinetic results from particle-in-cell/Monte Carlo collision simulations that unambiguously demonstrate the similarity of radio frequency (rf) discharges in nonlocal regimes where the electron energy relaxation length is much larger than the gap dimension. It is found that at a constant rf voltage amplitude, discharges will be similar if the gas pressure, inverse of gap distance, and rf driving frequency are all changed by the same scaling factor. The scaling relations of fundamental parameters are illustrated for rf discharges in the alpha-mode with secondary electron emission ignored, and the temporal electron kinetics are shown to have invariance in similar discharges. The results explicitly validate the scaling laws in nonlocal kinetic regimes, indicating promising application potentials of the similarity transformations across a wide range of kinetic regimes.
  • Publication
    Fractional order PID control with rate-limited anti-windup for the pitch system of wind turbines
    ( 2020)
    Wang, X.
    ;
    Gambier, A.
    ;
    Vinagre, B.M.
    This contribution analyses the performance of a fractional order PID (FOPID) controller with an anti-windup strategy for magnitude and rate applied to the pitch control of a wind turbine. Due to the fact that the operating point of wind turbines changes with the wind speed and that the plant is nonlinear, the controller includes a gain scheduling procedure to adjust the controller parameters. In order to avoid undesired oscillations on the tower due to the pitching activity, an active tower damping control is coupled with the pitch control. A 20 MW reference wind turbine is used as virtual plant, which is implemented in the simulation software FAST. Simulation results show that the control system based on the FOPID approach contributes to an improvement in the control performance of the wind turbine.
  • Publication
    Shaping Metallic Nanolattices: Design by Microcontact Printing from Wrinkled Stamps
    ( 2020)
    Wang, X.
    ;
    Sperling, M.
    ;
    Reifarth, M.
    ;
    Böker, A.
    A method for the fabrication of well‐defined metallic nanostructures is presented here in a simple and straightforward fashion. As an alternative to lithographic techniques, this routine employs microcontact printing utilizing wrinkled stamps, which are prepared from polydimethylsiloxane (PDMS), and includes the formation of hydrophobic stripe patterns on a substrate via the transfer of oligomeric PDMS. Subsequent backfilling of the interspaces between these stripes with a hydroxyl‐functional poly(2‐vinyl pyridine) then provides the basic pattern for the deposition of citrate‐stabilized gold nanoparticles promoted by electrostatic interaction. The resulting metallic nanostripes can be further customized by peeling off particles in a second microcontact printing step, which employs poly(ethylene imine) surface‐decorated wrinkled stamps, to form nanolattices. Due to the independent adjustability of the period dimensions of the wrinkled stamps and stamp orientation with respect to the substrate, particle arrays on the (sub)micro‐scale with various kinds of geometries are accessible in a straightforward fashion. This work provides an alternative, cost‐effective, and scalable surface‐patterning technique to fabricate nanolattice structures applicable to multiple types of functional nanoparticles. Being a top‐down method, this process could be readily implemented into, e.g., the fabrication of optical and sensing devices on a large scale.
  • Publication
    Density functional theory calculation of the properties of carbon vacancy defects in silicon carbide
    ( 2020)
    Wang, X.
    ;
    Zhao, J.
    ;
    Xu, Z.
    ;
    Djurabekova, F.
    ;
    Rommel, M.
    ;
    Song, Y.
    ;
    Fang, F.
    As a promising material for quantum technology, silicon carbide (SiC) has attracted great interest in materials science. Carbon vacancy is a dominant defect in 4H-SiC. Thus, understanding the properties of this defect is critical to its application, and the atomic and electronic structures of the defects needs to be identified. In this study, density functional theory was used to characterize the carbon vacancy defects in hexagonal (h) and cubic (k) lattice sites. The zero-phonon line energies, hyperfine tensors, and formation energies of carbon vacancies with different charge states (2−, −, 0,+ and 2+) in different supercells (72, 128, 400 and 576 atoms) were calculated using standard Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof methods. Results show that the zero-phonon line energies of carbon vacancy defects are much lower than those of divacancy defects, indicating that the former is more likely to reach the excited state than the latter. The hyperfine tensors of VC+(h) and VC+(k) were calculated. Comparison of the calculated hyperfine tensor with the experimental results indicates the existence of carbon vacancies in SiC lattice. The calculation of formation energy shows that the most stable carbon vacancy defects in the material are VC2+(k), VC+(k), VC(k), VC−(k) and VC2−(k) as the electronic chemical potential increases.