Low Voltage Region Research Articles

A P2/P3 composite-layered cathode material with low-voltage decay for sodium-ion batteries

Average discharge voltage of the P-type-layered oxides cathode materials is one of the key factors that determine the sodium-ion batteries (SIBs) performance, especially for the high energy density performance. Inspired by the strategy of lithium and magnesium doping in the transition metal layer to promote the anionic oxygen redox and the synergistic effect of composite materials for SIBs, we develop a layered P2/P3-Na0.75Li0.2Mg0.05Al0.05Mn0.7O2 composite cathode material enabled by the anionic oxygen redox. The synthesized composite delivers a reversible capacity of 180 mAh g−1 between 2.0 and 4.5 V at 0.2 C and keep ≈ 3.1 V stable average voltage above 2.5 V the discharge cutoff voltage. With a combination of the electrochemical performance, hard and soft X-ray absorption spectroscopy, we demonstrate that the irreversible anionic oxygen activity in the high-voltage region results in the capacity fading and the irreversible cationic manganese activity in the low-voltage region is responsible for the voltage decay. These findings provide a simple and effective method for designing low-voltage decay and high-energy density Na-deficient layered oxide composite.

Sleep apnoea has a dose-dependent effect on atrial remodelling in paroxysmal but not persistent atrial fibrillation: a high-density mapping study.

Obstructive sleep apnoea (OSA) associates with atrial fibrillation (AF), but the relationship of OSA severity and AF phenotype with the atrial substrate remains poorly defined. We sought to define the atrial substrate across the spectrum of OSA severity utilizing high-density mapping. Sixty-six consecutive patients (male 71%, age 61 ± 9) having AF ablation (paroxysmal AF 36, persistent AF 30) were recruited. All patents underwent formal overnight polysomnography and high-density left atrial (LA) mapping (mean 2351 ± 1244 points) in paced rhythm. Apnoea-hypopnoea index (AHI) (mean 21 ± 18) associated with lower voltage (-0.34, P = 0.005), increased complex points (r = 0.43, P < 0.001), more low-voltage areas (r = 0.42, P < 0.001), and greater voltage heterogeneity (r = 0.39, P = 0.001), and persisted after multivariable adjustment. Atrial conduction heterogeneity (r = 0.24, P = 0.025) but not conduction velocity (r = -0.09, P = 0.50) associated with AHI. Patchy regions of low voltage that co-localized with slowed conduction defined the atrial substrate in paroxysmal AF, while a diffuse atrial substrate predominated in persistent AF. The association of AHI with remodelling was most apparent among paroxysmal AF [LA voltage: paroxysmal AF -0.015 (-0.025, -0.005), P = 0.004 vs. persistent AF -0.006 (-0.017, 0.005), P = 0.30]. Furthermore, in paroxysmal AF an AHI ≥ 30 defined a threshold at which atrial remodelling became most evident (nil-mild vs. moderate vs. severe: 1.92 ± 0.42 mV vs. 1.84 ± 0.28 mV vs. 1.34 ± 0.41 mV, P = 0.006). In contrast, significant remodelling was observed across all OSA categories in persistent AF (1.67 ± 0.55 mV vs. 1.50 ± 0.66 mV vs. 1.55 ± 0.67 mV, P = 0.82). High-density mapping observed that OSA associates with marked atrial remodelling, predominantly among paroxysmal AF cohorts with severe OSA. This may facilitate the identification of AF patients that stand to derive the greatest benefit from OSA management.

Structure and reaction mechanism of binary Ni-Al oxides as materials for lithium-ion battery anodes.

A nanometer sized solid solution of NiO and Al2O3 was synthesized by calcination of Ni-Al layered double hydroxides (LDHs). The crystal structure of the obtained compound was determined by XRD and XAFS analyses: Ni2+ and Al3+ ions are located at the metal ion site of the rock salt structure and a certain amount of cation vacancies are also introduced for charge compensation. The electrochemical properties of the Ni-Al binary metal oxide as an anode material for lithium ion batteries were examined by the constant current charge-discharge test. Ni-Al oxide showed higher charging capacity in comparison with pristine NiO. In particular, the capacity in the lower voltage region (below 1.5 V), the limited capacity in this region is the weak point of the conversion anode, was improved to 540 mA h g-1 that is about twice that of pristine NiO. This improvement in the capacity in the lower voltage region is concluded to be due to the redox activity of Al3+ ions during the charge-discharge on the basis of the results of electrochemical measurements and ex situ XAFS measurements at the Ni and Al edge. The reaction mechanism of this compound is investigated using ex situ XRD and XAFS methods. For the charge (reduction) in the higher voltage region (OCV-1.0 V), lithium ion intercalation into the cation vacancy sites and/or lithium ion adsorption on the surface of particles are proceeding. For the charge in the lower voltage region (1.0-0.03 V), conversion reaction occurs by the reduction of Ni2+ and Al3+ ions to metal particles with surface electrolyte interface (SEI) layer formation. For the discharge in the lower voltage region (0.03-1.5 V), only Al metal particles are oxidized to Al3+ ions and some intermediate complexes are formed. For the discharge in the higher voltage region (1.5-3.0 V), the lattice of the Ni-Al binary oxide solid solution is reconstructed with the oxidation of Ni to Ni2+.

Swept Langmuir probe investigation of a time varying DC discharge

The paper reports on the development and application of a swept Langmuir probe to characterize plasma between two disc-like electrodes. A battery was added to a probe circuit to offset against the cathode fall voltage, and to make the sweep voltage effective at the probe tip. This arrangement allowed the collection of the electron and ion parts of the probe current and the subsequent construction of time-resolved current–voltage IP(V) characteristics with a time resolution of about one millisecond. The probe collected electron current in the lower voltage region of the discharge waveform where it surmounted the cathode fall voltage, whereas the ion current was collected continuously due to an accelerating field for the ions. The results highlighted how the cathode fall voltage limits the collection of the electron and ion parts of the probe current and how to handle the problem with a series battery in the probe circuit. In addition to the swept single-probe, a triple-probe was used simultaneously to compare and validate the results.

Improvement of Multilevel DC/DC Converter for E-Mobility Charging Station

Considerable efforts are being made to reduce CO2 emissions and thereby solve the problems of environmental pollution and global warming. Technologies for environmentally friendly transportation are being developed using batteries. In particular, with the increase in urbanization and one-person households, e-mobility products are drawing increasing attention as short-distance transportation devices. Among these vehicles, personal mobility devices (PMDs) are receiving attention as new transportation devices that are simple to operate. This paper proposes a new multilevel charging system that is advantageous in responding to the charging voltage specifications of various mobile devices with a single charging system while ensuring a low charging current ripple. The proposed diode-parallel multilevel converter consists of an independent Buck converter in series. The switch of the buck converter is configured at the negative terminal of the input power source so that the gate amplifier voltage is used as the power supply voltage; it can therefore be simply configured without a separate gate amplifier power supply. In addition, it is improved so as to have a wider charging voltage range in a low output voltage region and a better efficiency than the existing diode series multilevel converter. To verify the feasibility of the proposed system, simulations were performed using the software PowerSIM(PSIM), and, in order to verify the validity, a prototype charging system was fabricated to compare and analyze losses according to operating conditions.

Understanding the Role of Electrochemically Inactive Substituent in Layered Oxide Cathode Materials for Advanced Sodium Ion Batteries

The importance of electrochemical energy storage systems for large-scale application has been growing increasingly over the last decades. Due to high cost, limited source and several technical barriers of the lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) are considered as one of promising candidates for large-scale energy storage systems based on their cost-effective, earth abundant and recyclable characteristics.[1] As promising cathode materials for SIBs, P2-type sodium iron manganese oxide with layered structures (Na x Mn y Fe1-y O2, 0.5 < x < 0.8) demonstrates similar high energy density analogous to the LiBs.[2] However, the capacity retention of the cell with layered structure cathode is not optimized especially when it is charged and discharged with wide voltage window to obtain higher capacity. The major reason for this is a phase transition induced by gliding of transition metal slabs, which causes irreversible structural changes gradually during cycling.[3]To address this phase transition issue, we introduce electrochemically inactive component (e.g., Mg2+) in the material. The introduction of the Mg2+ by substituting with existing transition metal(s) is able to provide improved capacity retention due to their enhanced structural stability. We systematically analyzed the capacity fading trends at high and low voltage regions, where each different transition metal dominantly contributes to the redox reactions, which provides mechanistic understanding of the structural degradation of P2-type NaxMnyFe1-yO2 and the effect of Mg2+ on transition metal redox behaviors. In addtion, by tracing the structural changes in the cathode, we demonstrated that the introduction of Mg2+ effectively abbreviates the unwanted phase transitions. The results can provide the insights of the phase transition mechanism and redox behaviors of layered type cathode materials and the strategies to stabilize the structure by employing electrochemically inactive substituent(s), thus enabling rational design of new cathode materials for the development of next generation SIBs.[1] M. D. Slater, D. Kim, E. Lee, C. S. Johnson, Adv. Funct. Mater. 23 (2013) 947-958.[2] C. Delmas, C. Fouassier, P. Hagenuller, Physica B+C 99 (1980) 81-85.[3] E. Talaie, V. Duffort, H. L. Smith, B. Fultz, L. F. Nazar, Energy Environ. Sci., 5 (2015) 2512. Acknowledgement J. Yang and S.-D. Han acknowledge funding support by the Laboratory Directed Research and Development (LDRD) program at National Renewable Energy Laboratory (NREL).The work done at Brookhaven National Laboratory was supported by Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE, through the Advanced Battery Materials Research (BMR) Program under contract DE-SC0012704.

(Invited) Heterojunction Channel Engineering for in-Ga-Zn-O Thin-Film Transistors

Heterojunction channel engineering approach is presented as a means of enhancing the performance and reliability of In–Ga–Zn–O thin-film transistors (IGZO TFTs). A bottom-gate IGZO TFT with the heterojunction channel was formed by depositing an In-rich IGZO on a typical composition of IGZO111. A potential well of 0.4 eV was expected to form for electrons at the heterojunction interface. Field effect mobilities (μFE) of the TFTs were 12 cm2/Vs and 23 cm2/Vs for homogeneous IGZO111 and high-In IGZO channels, respectively. In addition, threshold voltage stability (ΔVth) of the TFTs showed big a difference between homogeneous IGZO111 and high-In IGZO channels under positive bias and temperature stress (PBTS). The IGZO111 TFT exhibited good Vth stability (ΔVth of +0.4 V) after the PBTS of +20 V for 104 sec at 60 °C. In contrast, although the homogeneous high-In IGZO TFT exhibited a higher μFE than the IGZO111 TFT, the PBTS stability noticeably degraded (ΔVth of +7.0 V) from the IGZO111 TFT.For the heterojunction channel TFTs, when the high-In IGZO layer was deposited on the IGZO layer, μFE increased from the IGZO111 TFT especially in low gate voltage (VGS) region. In addition, the PBTS stability of the heterojunction channel TFT was clearly improved from the high-In IGZO TFT by inserting IGZO111 layer at the interface between HI-IGZO and the GI. A maximum μFE of 24.7 cm2/Vs and improved PBTS stability (ΔVth of +0.3 V) were achieved by the heterojunction channel TFT with a high-In IGZO/IGZO111 thicknesses of 10/5 nm.Device simulation results revealed that quantum confinement effect in the heterojunction channel played an important role in determining carrier transport properties in the TFT, result in improving μFE and PBTS stability. A channel engineering approach will provide a method of overcoming the trade-off between μFE and the PBTS stability of oxide TFTs.

Deep-subthreshold Schottky barrier IGZO TFT for ultra low-power applications

This study reports the deep subthreshold characteristics (≤1 V) of low thermal budget Indium Gallium Zinc Oxide (IGZO) thin film transistors (TFTs) with Schottky barrier source/drain contacts. The Schottky barrier was analyzed and a consistent ideality factor was observed across the devices. A deep subthreshold region was extracted from the nominal characteristics and barrier influence was observed in the low voltage region. This operation led to high output impedance (~1012Ω) and excellent trans-conductance leading to a high voltage gain (>100) due to hard saturation of the output characteristics. Positive bias stress and stability tests were conducted within this region that showed minimal drift in the transfer characteristics. Such characteristics make these devices an excellent choice for low-power deep subthreshold and weak signal applications.

Regression Models for the Field Electron Emission Signal

The field electron emission signal is described using the regression approach with three models. Each regression function has its own theoretical justification. The three-parameter models make it possible to take into account the deviations of the I‒V characteristics from a straight in the Fowler–Nordheim coordinates in both the high- and low-voltage region. For clear comparison, investigated data samples are obtained in the numerical experiment with specified signal parameters. A special noise model is considered, in which the use of conventional linearization over the parameters seems fair. Attention is paid to checking the regression residuals for normality of their distribution. The parameters of the proposed models are cross estimated and their statistical significance with a confidence level of 95% is verified. It is shown that, for each set of experimental data, a separate model may be necessary. The adjusted coefficient of determination is used as a measure of the dependence of the response on the factor.

Quantitative evaluation of different high-density 3D mapping modes for atrial and ventricular substrate assessment of cardiac arrhythmias with the HD grid catheter

BackgroundAtrial and ventricular arrhythmias significantly contribute to morbidity and mortality of patients with cardiac disease. Ablation of these arrhythmias has shown to improve clinical outcomes, yet targeted ablation strategies rely on proper mapping capabilities. In the present study, we compare different modes of high-resolution mapping in clinically relevant arrhythmias using HD grid. Methods and resultsUsing the Advisor™ HD Grid Mapping Catheter in either the standard, the wave (bipolar along spline and bipolar orthogonal) or the wave diagonal setting, low-voltage areas were determined. Low-voltage was defined as local electrograms with an amplitude <0.5 mV (bipolar; atria/ventricle) or <4 mV (unipolar; ventricle). Ultra high-density mapping in 47 patients with ventricular tachycardia, ventricular premature beats, atrial fibrillation and atrial tachycardia provided reliable information for the understanding of the arrhythmia mechanism resulting in safe ablation procedures. Regions of low voltage were significantly decreased by 14 ± 2% and 31 ± 3% with wave and wave diagonal settings as compared to standard settings, respectively. ConclusionSubstrate mapping and risk stratification relies on proper low voltage discrimination. Even though the Advisor™ HD Grid Mapping Catheter was safely used in all cases, the extent of low voltage areas was mapping-mode dependent.

Structural Stabilization of P2-type Sodium Iron Manganese Oxides by Electrochemically Inactive Mg Substitution: Insights of Redox Behavior and Voltage Decay.

Layered P2-type Na0.8 Mn0.5 Fe0.5 O2 cathode material is a promising candidate for next-generation sodium-ion batteries due to the economical and environmentally benign characteristics of Mn and Fe. The poor cycling stability of the material, however, is still a problem that must be solved. To address the problem, electrochemically inactive Mg2+ was introduced into the structure by substituting some of the Fe ions. It was shown that Mg substitution led to a smoother voltage profile with improved cycling performance and rate capability. These observations were attributed to the suppressed structural changes during electrochemical processes. Detailed redox mechanisms, associated local structural changes, and phase transitions were investigated by X-ray absorption spectroscopy and X-ray diffraction. From the detailed analysis of electrochemical behaviors, it was also identified how the redox reactions and structural disordering occurred in the high- and low-voltage regions and how Mg substitution stabilized the structure.

An analytical study on low voltage regime of natural organic semiconductor based device: Physics of trap energy and ideality factor

Present investigation demonstrates the physics at low voltage region of natural organic semiconductor based single layer device. Theoretical explanation of diffusion driven current equations has been implemented as an efficient way to study the charge transport mechanism in that regime. GPVDM (abbreviated form of General purpose photovoltaic model), a renowned software simulation technique of organic devices has been introduced to obtain current voltage relationship and fitted with diffusion assisted equation. The plot shows high consistency with theoretical explanation which indicates reliability of the plot. Simulation based observation on trap assisted conduction at low voltage and its relation with corresponding ideality factor at that region has been explained on the basis of theoretical point of view. It has been found that ideality factor decays exponentially with increasing trap energy at low voltage. Experimental approach has finally been employed on Turmeric and Indigo dye based single layer natural semiconducting diodes to validate the outcome of the proposal. Results of the experiment lead to great similarity with the outcomes of both theoretical and simulation approach.

Toward high-performance hard carbon as an anode for sodium-ion batteries: Demineralization of biomass as a critical step

Biomass is a promising precursor for producing high-performance hard carbon as an anode for sodium-ion batteries (SIBs) because of its high low-voltage plateau capacity. However, the effect of residual ash in biomass on the electrochemical performance of hard carbons has rarely been investigated. This work describes an effective ash-removal approach as a critical step for preparing high-performance anodes for SIBs. A strong correlation between the ash removal techniques with structural and electrochemical properties of hard carbon was revealed. By examining various ash-removal techniques prior to carbonization and after carbonization using aqueous acid, neutral, and alkaline solutions, it was demonstrated that the removal of ash from raw cocoa pod husk (CPH) using aqueous acid and subsequent carbonization at 1300°C can produce hard carbon with high Na+ ion uptake in the low-voltage plateau region. During the acid pretreatment, ash and some hemicellulose fractions were removed, and carbonization of the acid-treated CPH resulted in hard carbon with a high degree of graphitization and reduced surface area. When tested as an anode in SIBs, the hard carbon produced from the acid-treated CPH exhibited an exceptionally high capacity of 317mAhg−1 and high plateau capacity of 244mAhg−1 at 0.05Ag−1, with a high initial Coulombic efficiency of 87%. At a high current density of 250mAg−1, a high capacity of 134mAhg−1 was maintained after 800 cycles. Post-treatment of hard carbon did not enhance the electrochemical performance. The physicochemical and electrochemical properties of hard carbons produced with the various pre- and post-treatment techniques were presented.

Resistive Switching Characteristics of Nonvolatile Memory with HSQ Film Using Microwave Irradiation.

In this study, we fabricated a resistive random access memory (ReRAM) of metal-insulator-metal structures using a hydrogen silsesquioxane (HSQ) film that was deposited by a low-cost solution process as a resistance switching (RS) layer. For post-deposition annealing (PDA) to improve the switching performance of HSQ-based ReRAMs, we applied high energy-efficient microwave irradiation (MWI). For comparison, ReRAMs with an as-deposited HSQ layer or a conventional thermally annealed (CTA) HSQ layer were also prepared. The RS characteristics, molecular structure modification of the HSQ layer, and reliability of the MWI-treated ReRAM were evaluated and compared with the as-deposited or CTA-treated devices. Typical bipolar RS (BRS) behavior was observed in all the fabricated HSQ-based ReRAM devices. In the low-voltage region of the high-resistance state (HRS) as well as the low-resistance state, current flows through the HSQ layer by an ohmic conduction mechanism. However, as the applied voltage increases in HRS, the current slope increases nonlinearly and follows the Poole-Frenkel conduction mechanism. The RS characteristics of the HSQ layer depend on the molecular structure, and when the PDA changes from a cage-like structure to a cross-linked network, memory characteristics are improved. In particular, the MWI-treated HSQ ReRAM has the largest memory window at the smallest operating power and demonstrated a stable endurance during the DC cycling test over 500 times and reliable retention at room (25 °C) and high (85 °C) temperatures for 10⁴ seconds.

Electroanatomic guidance versus conventional fluoroscopy during transseptal puncture for atrial fibrillation ablation.

Technological advancement in the setting of atrial fibrillation (AF) ablation has decreased radiation exposure and complications associated with the procedure. Yet, transseptal puncture (TSP) remains a challenging step that necessitates accurate guidance. We describe our experience performing TSP under electroanatomic (EA) guidance. The analysis included 145 consecutive EA-guided ablation procedures performed between June 2018 and April 2019 and 145 consecutive standard ablations performed before June 2018. EA guidance utilized the CARTO 3 three-dimensional mapping system to reconstruct anatomic and electrical characteristics of the right atrium and fossa ovalis. Patients with a history of previous cardiac surgery were excluded. For EA-guided procedures, the mean patient age was 60 ± 10 years, 75.2% were male, and 69.0% had paroxysmal AF. Similarly, the mean age for conventional procedures was 60 ± 11 years, 71.0% were male, and 71.7% had paroxysmal AF. The fossa ovalis was detected as a region of low voltage, <0.75 mV. EA guidance yielded shorter fluoroscopy times (EA vs. conventional, 3.6 ± 2.5 vs. 13.5 ± 10.5 min; p < .001) and a lower dose area product than conventional guidance (13 ± 11 Gy* cm2 vs. 28 ± 27 Gy* cm2 ; p < .001).The total procedure duration was similar between groups (146 ± 48vs. 148 ± 54 min). There were no significant complications related to TSP. During AF ablation, TSP with EA guidance facilitated safe access to the left atrium while reducing radiation risk to both patients and operators.

Analysis and Design Considerations of a Contactless Magnetic Plug for Charging Electric Vehicles Directly From the Medium-Voltage DC Grid With Arc Flash Mitigation

Electric vehicle charging has shifted to higher voltages to achieve higher power for more rapid charging capabilities. This article provides a contactless magnetic plug solution that enables medium-voltage grid connections for electric vehicle charging to achieve 3.5-kVDC-to-400-VDC, 150-kW rapid charging capabilities. This novel magnetic plug improves upon existing electric vehicle charging solutions by guaranteeing safe operation and connection through galvanic and physical separation from the medium-voltage side. It achieves this with a gap and barrier in the transformer core. We introduce a unique asymmetry in the core to localize parasitic capacitance, fully separating the medium- and low-voltage regions. This approach eliminates arcing risk and allows rapid charging capabilities to be delivered to the general public. This gapped core constitutes the plug action of our proposed charging system. We present solutions for the unique challenges of this solution through a detailed analysis of the magnetic design. We confirm this analysis in finite-element analysis and experimentation. The solution is verified through a scaled laboratory prototype of 20 kW, 1 kVDC to 50 VDC that is representative of the proposed 150-kW design. We demonstrate safe, arc-free, disconnection in included active content, a new solution for high-power electric vehicle rapid charging.

Investigation on capacity loss mechanisms of lithium-ion pouch cells under mechanical indentation conditions

Capacity loss was observed in Li-ion cells after mechanical deformation approaching the onset of internal short circuit (ISCr). In this paper, a series of indentation tests were carried out on commercial Li-ion cells of three capacities (500, 1500 and 2000 mAh). Both in-situ and ex-situ methods were used to investigate the mechanisms of indentation-induced capacity loss. After indentation test, the cell capacity reduced by 0.5%–6% of its original value. The incremental capacity (IC) analysis results showed that IC curves generally shifted to lower voltage region, indicating the increase in cell internal resistance. In addition, the fitting results of electrochemical impedance spectroscopy (EIS) indicated that mechanical indentation can result in a reduction in ohmic resistance and the increase in polarization resistance. Scanning electron microscopy (SEM) and X-ray computed tomography (XCT) results showed crushing of graphite, mud cracking of copper current collectors and enlarged pores in separator, which is proposed to be the main reasons for the increase in polarization resistance and permanent capacity loss. The rapid capacity loss due to mechanical abuse was compared with the long-term capacity fading.

Interfacial Characterization and Transport Conduction Mechanisms in Al|HfO2|p-Ge Structures: Energy Band Diagram

Ge-based metal-oxide semiconductor structures exhibiting thin ALD-grown high-k dielectric HfO2 films were fabricated and characterized chemically, structurally, and electrically. X-ray photoelectron (XP) spectroscopy confirms the good stoichiometry of the ALD-grown HfO2 films. Furthermore, through the analysis of the XP spectra, the conduction and valence band offsets of HfO2|p-Ge were calculated to be equal to 1.8 ± 0.2 eV and 2.8 ± 0.2 eV, respectively. C(V) and G(V) analysis reveals structures with a well-defined MOS behavior with Dit values in the range of 1011 eV–1 cm–2 and a dielectric constant of HfO2 films of 20. The dominant carrier transport conduction mechanisms were studied through J(V) analysis, performed at both substrate and gate electron injection. Specifically, in the low voltage region (V 3.0 V) and high temperatures (>450 K). Applying Schottky’s emission model the energy barrier heights of HfO2|p-Ge and Al|HfO2 interfaces were evaluated equal to 1.7 ± 0.2 eV and 1.3 ± 0.2 eV, respectively. Combining the XPS and J(V) analysis results, the energy band diagram of Al|HfO2|p-Ge structures is constructed. The calculated values of conduction and valence band offsets via XPS and J(V) measurements are in very good agreement.

Low-Frequency Noise Parameter Extraction Method for Single-Layer Graphene FETs

In this paper, a detailed parameter extraction methodology is proposed for low-frequency noise (LFN) in single layer (SL) graphene transistors (GFETs) based on a recently established compact LFN model. Drain current and LFN of two short channel back-gated GFETs (L=300, 100 nm) were measured at lower and higher drain voltages, for a wide range of gate voltages covering the region away from charge neutrality point (CNP) up to CNP at p-type operation region. Current-voltage (IV) and LFN data were also available from a long channel SL top solution-gated (SG) GFET (L=5 um), for both p- and n-type regions near and away CNP. At each of these regimes, the appropriate IV and LFN parameters can be accurately extracted. Regarding LFN, mobility fluctuation effect is dominant at CNP and from there the Hooge parameter aH can be extracted while the carrier number fluctuation contribution which is responsible for the well-known M-shape bias dependence of output noise divided by squared drain current, also observed in our data, makes possible the extraction of the NT parameter related to the number of traps. In the less possible case of a Lambda-shape trend, NT and aH can be extracted simultaneously from the region near CNP. Away from CNP, contact resistance can have a significant contribution to LFN and from there the relevant parameter SDR^2 is defined. The LFN parameters described above can be estimated from the low drain voltage region of operation where the effect of Velocity Saturation (VS) mechanism is negligible. VS effect results in the reduction of LFN at higher drain voltages and from there the IV parameter hOmega which represents the phonon energy and is related to VS effect can be derived both from drain current and LFN data.

Experimental investigation on PEM fuel cell using serpentine with tapered flow channels

The performance comparison of various flow fields for practical application can be better understood when loading the cell at a lower voltage region for an operational duration of more than an hour. In this paper, the performance of serpentine and serpentine with tapered channels are compared by loading the Polymer Electrolyte Membrane (PEM) fuel cell at a constant voltage of 0.5 V for an operational duration of 5 h. Purging with nitrogen at the inlets is done to recover the drop in current over the period of operation and flush out water. The presence of tapered flow fields in a serpentine flow channel shows an improvement of 15% in the current obtained due to better reactant gas transport in the gas diffusion layer. The performance of both flow fields were studied with polarization and power density curve obtained after 5 h of testing. Further, to confirm the performance improvement at lower voltage region, impedance study is done and obtained Nyquist plot confirms the better transport phenomenon of reactant gases to catalyst site and better removal of water in Gas Diffusion Layer (GDL).