Search NASA⌕ Search

SEARCH · Search NASA

Results for “Capacitance”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

MnO{sub 2}-wrapped hollow graphitized carbon nanosphere electrode for supercapacitor

Highlights: • MnO{sub 2}/HGC nanospheres are prepared by a cooperative template wrapping method. • MnO{sub 2}/HGC nanospheres possess large specific surface area. • MnO{sub 2}/HGC nanospheres are benefit for transmission of ions and electrons. • MnO{sub 2}/HGC electrodes exhibit a high specific capacitance. - Abstract: MnO{sub 2}-wrapped hollow graphitized carbon nanospheres (MnO{sub 2}/HGC) electrodes are prepared by a cooperative template wrapping method. hollow Graphitized carbon nanospheres (HGC) are firstly obtained by carbonizing phenolic resin followed by etching the SiO{sub 2} template, then the MnO{sub 2} ultrathin nanoplates are coated on the surfaces of the HGC nanospheres through a redox reaction between KMnO{sub 4} and HGC nanospheres. The as-prepared MnO{sub 2}/HGC hollow nanospheres possess porous structure and large specific surface area (∼230 m{sup 2} g{sup −1}). The specific capacitances of MnO{sub 2}/HGC nanosphere electrodes with different mass ratios of MnO{sub 2} to HGC are about 340–380 F g{sup −1} at a scan rate of 5 mV s{sup −1} in Na{sub 2}SO{sub 4} solution, and shows relative good cycling performance of the initial capacitance after 1000 cycles. The good specific capacitance is ascribed to the novel hollow nanosphere structure, which possesses high surface-to-volume ratio, and makes it easy for the mass diffusion of electrolyte and transmission of ions and electrons and also maintains the mechanical integrality.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ceria nanoparticles uniformly decorated on graphene nanosheets with coral-like morphology for high-performance supercapacitors

Graphical abstract: The study reported the synthesis of coral-like CeO{sub 2}/GNs for supercapacitors. The unique architecture with overall connected framework and good conducting network of the graphene greatly boosts the Faradaic redox reaction. Therefore, the CeO{sub 2}/GNs delivers an excellent electrochemical performance. - Highlights: • The study reported the synthesis of CeO{sub 2}/GNs. • The graphene was woven into CeO{sub 2}, acting as a good conducting network. • The CeO{sub 2}/GNs shows a coral-like structure. • The architecture creates an overall connected framework. • The CeO{sub 2}/GNs delivers good capacitive performances. - Abstract: CeO{sub 2}/graphene was synthesized by a simple microwave method along with subsequent calcination. CeO{sub 2} nanoparticles with an average size of 68.8 nm are uniformly decorated on graphene nanosheets (CeO{sub 2}/GNs). The CeO{sub 2}/GNs displays a like-coral morphology. The architecture including overall connected framework, abundant intercrossed and interconnected nanochannels and perfect conducting network of the graphene, endows the CeO{sub 2}/GNs material with a superior electron and mass transport. As a result, the CeO{sub 2}/GNs gives a high specific capacitance of 503.4 F/g at 2 A/g and good cycle performance with 91.8% capacitance retention after 3000 cycles. Further, an asymmetric supercapacitors was assembled by using CeO{sub 2}/GNs as the positive electrode and activated carbon as the negative electrode, the asymmetric device demonstrate a favorable energy density of 30.2 Wh/kg at the power density of 750.0 W/kg and superior cycle life with 86.4% the capacitance retenion at 5 A/g after 3000 cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Synthesis of MnO{sub 2}/C composite nanotubes as advanced cathode of supercapacitors

Graphical abstract: Self-supported porous MnO{sub 2}/C nanotube arrays are prepared by combined hydrothermal plus carbonization methods and demonstrated with high capacitance and good cycling life as cathode of supercapacitors. - Highlights: • MnO{sub 2}/C nanotube arrays are synthesized by a hydrothermal method. • MnO{sub 2}/C nanotube arrays show good high-rate performance. • MnO{sub 2}/C nanotube arrays show an excellent cycling performance. - Abstract: Controllable synthesis of high-capacity cathode is the research hotspot in the field of supercapacitors. In this work, with the help of sacrificial ZnO template, MnO{sub 2}/C nanotube arrays are formed by hydrothermal synthesis plus glucose carbonization. MnO{sub 2}/C composite nanotubes of 120–130 nm are formed and grow vertically to the substrate forming the final nanotube arrays. Fast ion/electron transfer is realized in this unique porous nanotube architecture. As cathode of supercapacitors, the MnO{sub 2}/C nanotube arrays exhibit high specific capacitance (790 F g{sup −1} at 5 A g{sup −1}), excellent rate performance (570 F g{sup −1} at 30 A g{sup −1}) and long-term cycling stability (90.7% capacitance retention after 20,000 cycles). The proposed electrode construction strategy is favorable for fabrication of other capacitive cathodes for energy storage.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Single-step hydrothermal synthesis of wrinkled graphene wrapped TiO2 nanotubes for photocatalytic hydrogen production and supercapacitor applications

Highlights: • Wrinkle rGO wrapped TiO{sub 2} nanotubes were prepared by alkanine hydrothermal method. • Improved visible absorption and reduced recombination was witnessed in rGO-TiO{sub 2}. • H{sub 2} production of 24,880 μmole/g/h was achieved by rGO-TiO{sub 2}, 12.9 folds to P25 TiO{sub 2}. • Double layer and pseudo capacitance of rGO-TiO{sub 2} showed specific capacitance 160 F/g. - Abstract: Herein, we discuss the synthesis of reduced graphene oxide and TiO{sub 2} (rGO-TiO{sub 2}) nanocomposites with varying ratios of rGO to TiO{sub 2} by hydrothermal method. Photocatalytic ability of the nanocomposites was assessed for H{sub 2} production under natural sunlight. At 5 wt% GO loading, the rGO-TiO{sub 2} exhibited 24,880 μmol/g/h H{sub 2}, 12.9 times to commercial P25-TiO{sub 2} (1920 μmol/g/h). The symmetric supercapacitor device fabricated using rGO-TiO{sub 2} demonstrated 160 F/g specific capacitance with 99% retention. The efficient charge carrier separation and transportation between TiO{sub 2} nanotubes and rGO resulted high photocatalytic activity. The synergistic double layer pseudo capacitor behavior of rGO-TiO{sub 2} is the reason for improved specific capacitance.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Facile preparation of porous nickel oxide membrane for flexible supercapacitors electrode via phase-separation method of polymer

Highlights: • Polymer/metal oxide hybrid membrane is fabricated for supercapacitors electrode. • Effects of membrane components on structure and performance are investigated. • The prepared electrode membrane shows high flexible property. • A capacitance of 794 F/g is obtained at a current density of 1.0 A/g. • Symmetric supercapacitor device exhibits an energy density of 39.6 Wh/kg. - Abstract: In this article, a porous flexible membrane electrode of nickel oxide is developed by a facile method based on liquid-liquid phase-separation. The effects of the membrane components on structures and electrochemical behaviors of membrane electrode are investigated. The results reveal that nano NiO and conducting graphite are imbedded into the polymer membrane of polyethersulfone to form a flexible hybrid membrane (M-NiO-G), which shows the highest specific capacitance of 794.0 F/g at a current density of 1.0 A/g. The asymmetrical supercapacitor device based on the as-synthesized of M-NiO-G (positive electrode) and activated carbon membrane (negative electrode) exhibits a high specific capacitance of 154.0 F/g at a current density of 0.2 A/g. The coulombic efficiency remains about 95.0%, and the capacitance retention is 84.8% after 5000 charging-discharging cycles at 1.0 A/g. The energy density of the asymmetrical supercapacitor device reaches 39.6 Wh/kg at a power density of 516.9 W/kg, still remains 20.0 Wh/kg at a high power density of 2567.4 W/kg in the potential window from 0 to 1.36 V in 6 M KOH aqueous solution.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fabrication of CeO2/PANI composites for high energy density supercapacitors

Highlights: • Fabrication of CeO{sub 2}/PANI composite was successfully prepared using easy solution mixing method. • Combinational approaches of rare earth oxides with conducting polymer composites posses’ excellent pseudocapacitive nature. • Superior specific capacitance, rate capacitance and cyclic stability. • Outstanding performance of asymmetric and symmetric devices. - Abstract: Combinational approaches of rare earth oxides with conducting polymer composites have gained vast attention in supercapacitor field due to the outstanding pseudocapacitive property. Here we detail the redox reaction based on CeO{sub 2}/polyaniline nanostructure to grasp the superior electrochemical nature of the composite. The CeO{sub 2}/polymer nanostructure showed the improved specific capacitance of 1452 F g{sup −1} at 2 A g{sup −1} when compared to pristine CeO{sub 2} (927 F g{sup −1} at 2 A g{sup −1}). The composite also maintains excellent rate capacitance (875 F g{sup −1} at 10 A g{sup −1}) and good cyclic stability upto 1500 cycles at high current density of 20 A g{sup −1}. Symmetric and asymmetric supercapacitor devices were fabricated using CeO{sub 2}/ PANI as an electrode. Moreover, the asymmetric supercapacitor possesses excellent specific capacity (256 F g{sup −1} at 2 mV s{sup −1}) with high energy density (75 Whkg{sup −1}) and high power density (566 Wkg{sup −1}). These results suggest a potential usefulness of CeO{sub 2}/ PANI composite exploit as an efficient electrode for supercapacitor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Morphological behaviour, electronic bond formation and electrochemical performance study of V2O5-polyaniline composite and its application in asymmetric supercapacitor

Highlights: • Interlinked V{sub 2}O{sub 5}-PANI composites are synthesized by chemical polymerization. • V{sub 2}O{sub 5}-PANI exhibits the highest capacitance of 529.56 F g{sup −1} with wide window of 1.6 V. • Correlation between electrochemical performances with IR peak shifts is studied. • Inorganic-organic synergistic effect plays crucial role for wide working window. • Asymmetric supercapacitor exhibits high energy and power in wide potential windows. - Abstract: To meet the demand of high-energy density supercapacitors, development of suitable electrode material with a wide potential window and high specific capacitance is most important. In this report, we have synthesized V{sub 2}O{sub 5}-PANI nanocomposite through in-situ intercalation and polymerization reaction of aniline with V{sub 2}O{sub 5}, which shows a wide potential window as well as high specific capacitance. XRD, SEM and FTIR have been used to characterize the electrode materials. Their electrochemical activity and pseudocapacitive performances were investigated. The correlation between morphology and electrochemical performances were investigated which is a vital way for the selection of electrode materials. Due to the organic-inorganic compatibility, V{sub 2}O{sub 5}-PANI exhibits good electrochemical charge storage in a wide potential window of 1.6 V (ranging between −0.4 V to 1.2 V) with excellent stability of 84.2% capacitance retention after 5000 GCD cycles. Moreover, asymmetric supercapacitor was assembled V{sub 2}O{sub 5}-PANI composite and NiMn{sub 2}O{sub 4} electrodes which yield remarkable energy and power density.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

V2O5 nanowires-graphene composite as an outstanding electrode material for high electrochemical performance and long-cycle-life supercapacitor

Highlights: • V{sub 2}O{sub 5} nanowires anchored on the surface of rGO sheets using facile hydrothermal method for energy storage application. • High gravimetric capacitance of 1002 F g{sup −1} has been achieved at the current density of 1 A g{sup −1} in an aqueous electrolyte. • Exceptionally high energy density of 116 Wh kg{sup −1} has been obtained using supercapacitor coin cell. • Composite (V{sub 2}O{sub 5}-NW-G) exhibits excellent cyclic stability with 83.94 % capacitance retention after 5000 GCD cycles. - Abstract: A composite of V{sub 2}O{sub 5} nanowires with reduced graphene oxide (V{sub 2}O{sub 5}-NW-G) has been prepared using facile hydrothermal method and tested for charge storage supercapacitor application. Morphological and structural aspects of as prepared materials has been studied using material characterization techniques like transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The nanosized wires like structure of V{sub 2}O{sub 5} anchored on the graphene (rGO) sheets possesses improved electrical conductivity and high surface area. Electrochemical properties of the synthesized materials have been studied using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Composite (V{sub 2}O{sub 5}-NW-G) exhibits exceptional properties like high gravimetric capacitance of 1002 F g{sup −1} along with good cyclic stability of 83.94% capacitance retention after 5000 repetitive GCD cycles. Excellent energy density of 116 Wh kg{sup −1} has been obtained at the power density of 1520 W kg{sup −1}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Bimodal wireless sensing with dual-channel wide bandgap heterostructure varactors

A capacitive wireless sensing scheme is developed that utilizes an AlN/GaN-based dual-channel varactor. The dual-channel heterostructure affords two capacitance plateaus within the capacitance-voltage (CV) characteristic, owing to the two parallel two-dimensional electron gases (2DEGs) located at respective AlN/GaN interfaces. The capacitance plateaus are leveraged for the definition of two resonant states of the sensor when implemented in an inductively-coupled resonant LRC network for wireless readout. The physics-based CV model is compared with published experimental results, which serve as a basis for the sensor embodiment. The bimodal resonant sensor is befitting for a broad application space ranging from gas, electrostatic, and piezoelectric sensors to biological and chemical detection.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Study of the operation optical power in modified uni-traveling carrier photodetector for low amplitude to phase conversion

The phase noise known as the amplitude to phase conversion (AM–PM) in modified uni-traveling carrier photodetector (MUTC-PD) is severely power-dependent. To achieve low AM–PM operation, applying proper injection optical power will be of crucial importance. For this purpose, the characterization of the phase change between the 60-GHz sinusoidal input and RF output of a MUTC-PD is analyzed under different incident optical power. Moreover, the equation of the phase change is deduced by applying an equivalent circuit model. The results demonstrate that the power-dependent nonlinear transit time plays a key role in the phase change. Besides, the absolute phase change is observed smallest when the differential capacitance equals to zero in a typical optical power. To the best of our knowledge, this is the first demonstration of the relationship between the differential capacitance and the phase change. When the differential capacitance equals to zero, the total capacitance of the MUTC exhibits a valley which could be a measurable feature of the optimum optical power for low phase noise operation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Distance-dependent dielectric constant at the calcite/electrolyte interface: Implication for surface complexation modeling

Hypothesis: The electrical double layer formed at the mineral/electrolyte interface is often modeled using mean-field approaches based on a continuum description of the solvent whose dielectric constant is assumed to decrease monotonically with decreasing distance to the surface. In contrast, molecular simulations show that the solvent polarizability oscillates near the surface similar to the water density profile - as shown previously, for example, by Bonthuis et al. (D.J. Bonthuis, S. Gekle, R.R. Netz, Dielectric Profile of Interfacial Water and its Effect on Double-Layer Capacitance, Phys Rev Lett 107(16) (2011) 166102). Here, we showed that molecular and mesoscale pictures agree by spatially averaging the dielectric constant obtained from molecular dynamics simulations over the distances relevant to the mean-field representation. In addition, the values of capacitances used to describe the electrical double layer in Surface Complexation Models (SCMs) of the mineral/electrolyte interface can be estimated using molecularly informed spatially averaged dielectric constants and positions of hydration layers. Experiments: First, we used molecular dynamics simulations to model the calcite 101¯4/electrolyte interface. Next, by using atomistic trajectories, we calculated the distance-dependent static dielectric constant and water density in the direction normal to the. Finally, we applied spatial compartmentalization consistent with the model of parallel-plate capacitors connected in series to estimate SCM capacitances. Findings: Computationally expensive simulations are required to determine the dielectric constant profile of interfacial water near the mineral surface. On the other hand, water density profiles are readily assessable from much shorter simulation trajectories. Our simulations confirmed that dielectric and water density oscillations at the interface are correlated. Here, we parametrized linear regression models to estimate the dielectric constant directly from the local water density. This is a significant computational shortcut compared to slowly converging calculations relying on total dipole moment fluctuations. The amplitude of the interfacial dielectric constant oscillation can exceed the dielectric constant of the bulk water, suggesting an ice-like frozen state, but only if there are no electrolyte ions. The interfacial accumulation of electrolyte ions causes a decrease in the dielectric constant due to the reduction of water density and re-orientation of water dipoles in ion hydration shells. Finally, we show how to use the computed dielectric properties to estimate SCM's capacitances.

58 GEOSCIENCES↗
DOE OSTI2018

Efficient 3D Printed Pseudocapacitive Electrodes with Ultrahigh MnO 2 Loading

Retaining sound electrochemical performance of electrodes at high mass loading holds critical importance to energy storage. Pseudocapacitive materials such as manganese oxide (MnO 2 ) deposited on current collectors have achieved outstanding gravimetric capacitances, sometimes even close to their theoretical values. However, this is only achievable with very small mass loading of active material typically less than 1 mg cm -2 . Increasing mass loading often leads to drastic decay of capacitive performance due to sluggish ion diffusion in bulk material. In this work, we demonstrate a 3D printed graphene aerogel electrode with MnO 2 loading of 182.2 mg cm -2 , which achieves a record-high areal capacitance of 44.13 F cm -2 . Most importantly, this 3D printed graphene aerogel/MnO 2 electrode can simultaneously achieve excellent capacitance normalized to area, gravimetry, and volume, which is the trade-off for most electrodes. This work successfully validates the feasibility of printing practical pseudocapacitive electrodes, which might revolutionize pseudocapacitor fabrication.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Broad Applicability of Electrochemical Impedance Spectroscopy to the Measurement of Oxygen Nonstoichiometry in Mixed Ion and Electron Conductors

Oxygen non-stoichiometry is a fundamental feature of mixed ion and electron conductors (MIECs). In this work a general electrochemical method for determining non-stoichiometry in thin film MIECs, via measurement of the chemical capacitance, is demonstrated using ceria and ceria-zirconia (Ce 0.8 Zr 0.2 O 2-δ ) as representative materials. A.C. impedance data are collected from both materials at high temperature (750-900 °C) under reducing conditions with oxygen partial pressure (pO 2 ) in the range 10-13 to 10-20 atm. Additional measurements of ceria-zirconia films are made under relatively oxidizing conditions with pO 2 in the range 0.2 to 10 -4 atm and temperatures of 800-900 °C. Under reducing conditions, the impedance spectra are described by a simple circuit in which a resistor is in series with a resistor and capacitor in parallel, and thickness dependent measurements are used to resolve the capacitance into interfacial and chemical terms. Under more oxidizing conditions, the impedance spectra (of Ce 0.8 Zr 0.2 O 2-δ ) reveal an additional diffusional feature, which enables determination of the ionic resistance of the film in addition to the capacitance, and hence the transport properties. A generalized mathematical formalism is presented for recovering the non-stoichiometry from the chemical capacitance, without recourse to defect chemical models. The ceria non-stoichiometry values are in good agreement with literature values determined by thermogravimetric measurements but display considerably less scatter and are collected on considerably shorter timescales. Here, the thermodynamic analysis of Ce 0.8 Zr 0.2 O 2-δ corroborates earlier findings that introduction of Zr into ceria enhances its reducibility.

08 HYDROGEN↗
DOE OSTI2014

Metal-oxide-semiconductor capacitors and Schottky diodes studied with scanning microwave microscopy at 18 GHz

We measured the DC and RF impedance characteristics of micrometric metal-oxide-semiconductor (MOS) capacitors and Schottky diodes using scanning microwave microscopy (SMM). The SMM consisting of an atomic force microscopy (AFM) interfaced with a vector network analyser (VNA) was used to measure the reflection S11 coefficient of the metallic MOS and Schottky contact pads at 18 GHz as a function of the tip bias voltage. By controlling the SMM biasing conditions, the AFM tip was used to bias the Schottky contacts between reverse and forward mode. In reverse bias direction, the Schottky contacts showed mostly a change in the imaginary part of the admittance while in forward bias direction the change was mostly in the real part of the admittance. Reference MOS capacitors which are next to the Schottky diodes on the same sample were used to calibrate the SMM S11 data and convert it into capacitance values. Calibrated capacitance between 1–10 fF and 1/C{sup 2} spectroscopy curves were acquired on the different Schottky diodes as a function of the DC bias voltage following a linear behavior. Additionally, measurements were done directly with the AFM-tip in contact with the silicon substrate forming a nanoscale Schottky contact. Similar capacitance-voltage curves were obtained but with smaller values (30–300 aF) due to the corresponding smaller AFM-tip diameter. Calibrated capacitance images of both the MOS and Schottky contacts were acquired with nanoscale resolution at different tip-bias voltages.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Charge storage in β-FeSi{sub 2} nanoparticles

We report on the observation of a surprisingly high specific capacitance of β-FeSi{sub 2} nanoparticle layers. Lateral, interdigitated capacitor structures were fabricated on thermally grown silicon dioxide and covered with β-FeSi{sub 2} particles by drop or spin casting. The β-FeSi{sub 2}-nanoparticles, with crystallite sizes in the range of 10–30 nm, were fabricated by gas phase synthesis in a hot wall reactor. Compared to the bare electrodes, the nanoparticle-coated samples exhibit a 3–4 orders of magnitude increased capacitance. Time-resolved current voltage measurements show that for short times (seconds to minutes), the material is capable of storing up to 1 As/g at voltages of around 1 V. The devices are robust and exhibit long-term stability under ambient conditions. The specific capacitance is highest for a saturated relative humidity, while for a relative humidity below 40% the capacitance is almost indistinguishable from a nanoparticle-free reference sample. The devices work without the need of a fluid phase, the charge storing material is abundant and cost effective, and the sample design is easy to fabricate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Electro-physical characterization of individual and arrays of ZnO nanowires

Capacitance measurements were made on an array of parallel ZnO nanowires embedded in a polymer matrix and provided with two electrodes perpendicular to the nanowires. The capacitance monotonically increased, and saturated at large negative (depleting) and large positive (accumulating) voltages. A qualitative explanation for this behavior is presented, taking into account specific features of quasi-one-dimensional screening. The increasing or decreasing character of the capacitance-voltage characteristics were determined by the conductivity type of the nanowires, which in our case was n-type. A dispersion of the experimental capacitance was observed over the entire frequency range of 1 kHz to 5 MHz. This phenomenon is explained by the slow discharge of the nanowires through the thin dielectric layer that separates them from the top electrode. Separate measurements on individual identical nanowires in a field effect transistor configuration yielded an electron concentration and mobility of approximately 10{sup 17 }cm{sup −3} and 150 cm{sup 2}/Vs, respectively, at room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Linking stem rehydration kinetics to hydraulic traits using a novel method and mechanistic model

Background: Despite the recognized importance of hydraulic capacitance as a mechanism used by plants to maintain hydraulic functioning during high transpiration, characterizing the dynamics of capacitance remains a challenge. Methods: For this work, we used a novel ‘two-balance method’ to investigate relationships between stem rehydration kinetics and other hydraulic traits in multiple tree species, and we developed a model to explore stem rehydration kinetics further. Key Results: We found that: (1) rehydration time constants and the amount of water uptake occurring during rehydration differed significantly across species; (2) time constants did not change with declining water potential (Ψ), while water uptake increased at lower Ψ in some species; (3) longer time constants were associated with lower wood density, higher capacitance and less negative stem pressures causing 50 % loss of hydraulic conductivity (P 50 ); (4) greater water uptake occurred in stems with lower wood density and less negative P 50 values; and (5) the model could estimate the total hydraulic resistance of the rehydration path, which cannot be measured directly. Conclusions: Overall, the two-balance method can be used to examine rehydration dynamics quickly and thoroughly in detached woody stems. This method has the potential to improve our understanding of how capacitance functions across tree species, which is an often-overlooked component of whole-plant hydraulics.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Simulation and Experimental Results of Interdigital Capacitor (IDC) Sensors to Monitor Insulation Degradation of Cables

The efficacy of using interdigital capacitor (IDC) sensors to monitor cable insulation aging was studied through electromagnetic simulations and experimental measurements. Simulation results on both planar and conformal dielectric materials indicated that IDCs on thin substrates and with conducting backplanes provide superior performance in terms of higher capacitance. An airgap study showed that even a very small airgap between the sensor and the cable’s insulation results in significant reduction in capacitance; therefore, airgaps should be minimized. Finally, cables that underwent accelerated aging exposure in an oven were tested using fabricated IDC sensors. Measurement results showed a 12.5% increase in capacitance when the IDC was pressed against the insulation versus a 91.3% increase in capacitance when the IDC was pressed against the jackets of the cables for cable specimens ranging from unaged to aged 35 days.

Cable NDE, Interdigital Capacitance↗