Direct Current Measurements Research Articles

Insight into the Structural Optimization and Electrical Conductivity of NiII/MnII Bipyridine–Dicyanamide Complexes

AbstractTwo monomeric octahedral complexes {[Ni(N(CN)2)2(bpy)2]2·H2O (1) and [Mn(N(CN)2)2(bpy)2] (2) where bpy = 2,2′‐bipyridine} were synthesized and characterized by single crystal X‐ray diffraction, elemental analysis, UV–visible, and IR spectra. The geometry optimization through DFT measurements predicts that both the complexes have similar monomeric structures with hexa‐coordinated metal centers having a couple of bpy and mononegative dca anions and thus satisfying the octahedral geometry. Moreover, it is found that the HOMO–LUMO energy gaps are ΔE = 4.981 and 5.563 eV for complexes 1 and 2, respectively, which are responsible for the stabilization of the complex formation. The calculated absorption bands are located at 304 and 235 nm, which are in excellent agreement with the experimental result. Moreover, the DFT study of the electronic spectra of both complexes shows that the calculated absorption bands are in well agreement with the experimental results. In the electrical studies, direct current (dc) measurements on the copper/complex (1 or 2)/copper structure confirm a definite development in the device current with applied bias. Complex 1 has shown better electrical conductance compared to complex 2 based on the reduction of device resistance.

Oxygen-Plasma-Treated Al/TaOX/Al Resistive Memory for Enhanced Synaptic Characteristics.

In this study, we investigate the impact of O2 plasma treatment on the performance of Al/TaOX/Al-based resistive random-access memory (RRAM) devices, focusing on applications in neuromorphic systems. Comparative analysis using scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the differences in chemical composition between O2-plasma-treated and untreated RRAM cells. Direct-current measurements showed that O2-plasma-treated RRAM cells exhibited significant improvements over untreated RRAM cells, including higher on/off ratios, improved uniformity and distribution, longer retention times, and enhanced durability. The conduction mechanism is investigated by current-voltage (I-V) curve fitting. In addition, paired-pulse facilitation (PPF) is observed using partial short-term memory. Furthermore, 3- and 4-bit weight tuning with auto-pulse-tuning algorithms was achieved to improve the controllability of the synapse weight for the neuromorphic system, maintaining retention times exceeding 103 s in the multiple states. Neuromorphic simulation with an MNIST dataset is conducted to evaluate the synaptic device.

DC current sensor based upon a fused fiber optic Fabry-Perot interferometer (FPI) and copper wire

In order to meet the demand for direct current (DC) current measurements in industrial production, a new type of fiber optic DC current sensor, combining thin copper wire and fiber optic Fabry-Perot interferometer (FPI), is reported. FPI is a sandwich structure formed by splicing a single-mode fiber (SMF) and a quartz capillary, with a cavity length of approximately 200 μm. The copper wire, with a length of 3.2 cm and a diameter of 1 mm, is attached to the FPI to form the sensor. Due to the excellent thermal conductivity of copper, the results showed that the sensor increased the sensitivity from 4.1 pm/°C to 11 pm/°C. After applying direct current to the sensor, a large quantity of heat generated by the copper wire was absorbed by the FPI, resulting in a significant shift in the resonant wavelength of FPI, which can indirectly measure the current. Three current experiments were performed, and parabolic curves have been used in fitting of the Dip wavelength and the square of the current. The results provide high correlation coefficients with values exceeding 0.99. Therefore, this curve may be used to measure the square of the current, further obtaining the current. In summary, the sensor is compact and durable, easy to manufacture, and provides high correlation coefficients, providing an alternative for measuring DC current.

Transport diagrams of germanium double quantum dots/Si barriers using photocurrent measurement

We reported transport diagrams of self-assembled germanium (Ge) double quantum-dots (DQDs) using direct current measurement under illumination at wavelength (λ) of 850 nm and at the base temperature of 4.5 K. Ge DQDs with a coupling-barrier of Si, tunneling-barriers of Si3N4, and self-aligned p+-Si reservoirs were fabricated in a self-organized CMOS approach. Charge transport through the Ge-DQDs is facilitated by photon-assisted tunneling. Characteristic gate-controlled hexagonal-shaped cells over a wide range of hole occupancy are acquired thanks to hard-wall confinement. Large dimensions (ΔVG > 200 mV) of hexagonal-shaped cells are favored for the operation of charge states, indicating that our Ge DQDs system is less susceptible to shot noises arising from external voltage sources. Estimated intra-QD and inter-QD charging energies are EC,R/EC,L = 48.9 meV/42.7 meV and ECm = 7.8 meV, respectively.

A life prediction model for P110S steel in deep-well environments with H2S/CO2 coexistence based on multi-factor chemometric drive

A mechanistic-chemometrics model for life prediction of P110S steel in deep-well environments with H₂S/CO₂ coexistence was proposed. The model was developed by considering the interaction mechanism between uniform and pitting corrosion, then modified using a multi-factor chemometric drive incorporating temperature/pressure, flow velocity, stress. Finally, the pit-to-crack transition was modeled using finite element design and direct current potential drop measurements, completing the life prediction process. The model predicts a lifespan of 2.28–5.25 years at different well depths, and this result was validated with on-site data, indicating the model’s accuracy. The knowledge paradigm provided herein will assist in corrosion prediction.

Impact of Synthetic Variables on the Structural Diversity of TbIII-Carboxylate Frameworks: Gas Adsorption, Magnetism, and Organocatalysis Investigations.

In this work, three unique TbIII-carboxylate frameworks with the formula {[Tb2(OH)2(H2O)2(abtc)]·2H2O}n (1), {[Tb2(abtc)1.5(H2O)3(DMA)]·H2O}n (2) and {[Tb3(abtc)2.5(H2O)4]·H3O}n (3), each displaying structural variations, have been successfully synthesized by the solvothermal reactions of Tb(NO3)3·6H2O with the azo-containing ligand 3,3',5,5'-azobenzene tetracarboxylic acid (H4abtc) under varying conditions. Detailed single-crystal X-ray diffraction (SC-XRD) analysis manifested a remarkable diversity in these structures, demonstrating various coordination patterns of TbIII-metal nodes with the carboxylate groups of the organic linker, which contributed to the generation of intricate three-dimensional (3D) coordination networks with remarkable chemical resistance. Furthermore, frameworks 2 and 3, characterized by porous networks containing two and three independent TbIII-metal nodes, respectively, were both demonstrated to be efficient heterogeneous catalysts toward the cyanosilylation of imines under mild conditions with excellent reusability. In addition, direct current (Dc) magnetic susceptibility measurements conducted on frameworks 1, 2, and 3 indicated that there were obvious antiferromagnetic interactions among the TbIII-metal nodes, which suggests the involvement of intricate intra- and intertrimer exchange channels, adding another fascinating dimension to their physical properties.

Syntheses, structures and magnetisms of dimethyl phosphate-bridged dinuclear lanthanide complexes with pentadentate macrocyclic ligand

A series of air-stable dinuclear complexes, [HNEt3]2[Ln2(L)2((CH3O)2PO2)4](BPh4)4 (L = 2,14-dimethyl-3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca1(19), 2,13,15,17-pentene, Ln = Dy, 1-Dy; Tb, 2-Tb; Gd, 3-Gd), has been synthesized and characterized using single-crystal X-ray crystallography and magnetic measurements. The crystal structures disclose that two dimethyl phosphate anions act as bridging ligands, linking two Ln(III) ions. Each Ln(III) ion is eight-coordinated by five nitrogen atoms from a pentadentate Schiff based ligand L and three oxygen atoms from three dimethyl phosphates, forming a triangular dodecahedral coordination geometry. Direct-current magnetic susceptibility measurements indicate an extremely weak antiferromagnetic exchange between Gd(III) ions in 3-Gd. Complex 1-Dy exhibits slow magnetic relaxation with an effective energy barrier (Ueff) of 13 K under an applied field of 1000 Oe, whereas 2-Tb does not exhibit single-molecule magnet behavior. This work presents a novel strategy for constructing polynuclear single-molecular magnets by using a pentadetate macrocycle as magnetic building blocks integrated with replaceable axial ligands.

A novel numerical approach for analyzing experimental data on critical current degradation in Nb3Sn wires caused by transverse deformations preceding heat treatment

In the framework of studies on high-field magnets for future accelerators, a specific project called ASTRACT focuses on the effect of transverse strain on the critical current of Nb3Sn wires. The first phase of the project addresses the effects of strain imposed on Nb3Sn wires before heat treatment and the development of a procedure to directly compare critical current measurements with values extracted from magnetization cycles. A Nb3Sn RRP-Ti wire was deformed to different values of transverse strain (10%, 15%, 20%, and 25%), and long samples were collected for transport measurements at 4.2 K, in a background field ranging from 10 to 14 T. Short samples were used for magnetization measurements (VSM technique). Additionally, SEM images of the cross-section were collected at different longitudinal positions along the wire for each strain value. This paper proposes a method based on SEM image analysis and magnetization measurements analysis to study the effect of bundle deformation on transport properties. The procedure requires morphological information provided by SEM images after appropriate numerical processing. Verification through statistical comparison between the Ic transport and VSM data is also conducted. Direct critical current measurements showed no degradation due to deformation up to 25%. The method introduced, independent of transport data, can reach the same conclusions by considering the real shape of the bundles.

Electrical stimulation for brighter persistent luminescence

An immature understanding of the mechanisms of persistent luminescence (PersL) has hindered the development of new persistent luminescent materials (PersLMs) with increased brightness. In this regard, in-situ direct current (DC) electric field measurements were conducted on a layered structure composed of the SrAl2O4:Eu2+,Dy3+ phosphor, and an electrode. In this study, the photoluminescence (PL) and afterglow properties were investigated with respect to voltage by analyzing the current signal and thermoluminescence (TL) spectroscopy. The intensity of PersL increased due to a novel phenomenon known as “external electric field stimulated enhancement of initial brightness of afterglow”. This dynamic process was illustrated via the use of a rate equation approach, where the electrons trapped by the ultra-shallow trap at 0.022 eV could be transferred through the conduction band during long afterglow. The afterglow intensity could reach 0.538 cd m−2 at a 6 V electric voltage. The design of an electric field stimulation technique enables the enhancement of the intensity of PersLMs and provides a new perspective for exploring the fundamental mechanics of certain established PersLMs.

Multi-technique characterization and thermal degradation study of epoxy modified resins designed for multifunctional applications

Tetraglycidyl methylene dianiline (TGMDA) was mixed with 1,4-Butanediol diglycidyl ether (BDE) (in a 4:1 mass ratio) and with a stoichiometric amount of the curing agent diaminodiphenyl sulfone which was solubilized at 120 °C for 20 min in the liquid mixture TGMDA + BDE. The so obtained unfilled epoxy resin matrix, denoted as ER, was blended with glycidyl polyhedral oligomeric silsesquioxane and carbon nanotubes in suitable proportions to obtain binary and ternary mixtures. Characterization of the formulated materials was performed using different experimental techniques, such as Dynamic mechanical analysis, Thermogravimetry (TG), Field emission scanning electron microscopy. Furthermore, the investigation of the flame behavior was carried out by the limiting oxygen index and mass loss calorimeter measurements. Direct current measurements and investigation by Tunneling atomic force microscopy of the conductive nanodomain map allowed the evaluation of the electrical properties of the developed nanofilled systems. The TG data related to thermal decomposition of ER and its binary and ternary mixtures were processed according to isoconversional kinetic analysis by assuming a non-Arrhenian behavior of the temperature function, and lifetime prediction was estimated at suitable relatively low temperatures and possible relation between the thermal stability and the presence of each component was discussed. This method of kinetic analysis paves the way for the possibility of evaluating in a more realistic way, on the basis of thermal stability, the potential application of structural resins with primary load functions in contact with hot areas of aeronautical aircraft engines.

Quantifying the State of the Art of Electric Powertrains in Battery Electric Vehicles: Comprehensive Analysis of the Tesla Model 3 on the Vehicle Level

Data on state-of-the-art battery electric vehicles are crucial to academia; however, these data are not published due to non-disclosure policies in the industry. As a result, simulation models and their analyses are based on assumptions or insider information. To fill this information gap, we present a comprehensive analysis of the electric powertrain of a Tesla Model 3 Standard Range Plus (SR+) from 2020 with lithium iron phosphate (LFP) cells, focusing on the overall range. On the vehicle level, we observe the resulting range in multiple test scenarios, tracing the energy path from source to sink by conducting different test series on the vehicle dynamometer and through alternating current (AC) and direct current (DC) charging measurements. In addition to absolute electric range tests in different operating scenarios and electric and thermal operation strategies on the vehicle level, we analyze the energy density and the power unit’s efficiency on the component level. These tests are performed through procedures on the chassis dynamometer as well as efficiency analysis and electric characterization tests in charge/discharge scenarios. This study includes over 1 GB of attached measurement data on the battery pack and vehicle level from the lab to the real-world environment available as open-source data.

Characterization of a novel TORT cable wound of stabilized striated REBCO tapes for reduced magnetization AC losses

REBCO (rare-earth barium copper oxide) high-temperature superconducting tapes, have a high potential for winding of large magnet coils. Tapes on round tube (TORT) cables represent a promising option for achieving a conductor suitable for the winding of magnet coils. However, certain applications, such as accelerator magnets, require the use of superconducting cables with low magnetization alternative current (AC) losses. There are several methods to reduce AC losses in TORT cables. Our first approach was to get rid of eddy currents by replacing the copper former with dielectric materials based on polymers and composites, i.e. polyethylene terephthalate glycol-modified reinforced with carbon fibers. Additional reduction of hysteretic loss was achieved by striating of copper coated REBCO tapes. We employed chemo-mechanical striating, for these objectives. However, the superconductor is exposed during the striating process, which may lead to later moisture-related degradation. Hence multilayers based on Ti/AlN were deposited using magnetron sputtering in order to protect the superconductor immediately after the striating process from water and atmospheric moisture corrosion. Subsequently, striated tapes as well as the non-striated tapes were then wound onto formers with diameters of 10 mm, 7 mm and 5.5 mm, and then on the short TORT cables bending tests were performed. After each technological step, direct current measurements were performed on the samples and finally the AC losses were measured.

Oxygen vacancy migration and impact on high voltage DC polarization in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3

AbstractElectrical polarization and defect transport are examined in 0.8BaTiO3–0.2BiZn0.5Ti0.5O3, an attractive capacitor material for high power electronics. Oxygen vacancies are suggested to be the majority charge carrier at or below 250°C with a grain conduction hopping activation energy of 0.97 eV and 0.92 eV for thermally stimulated depolarization current (TSDC) and impedance spectroscopy measurements, respectively. At higher temperature, thermally generated electronic conduction with an activation energy of 1.6 eV is dominant. Significant oxygen vacancy concentration is indicated (up to ∼1%) due to cation vacancy formation (i.e., acceptor defects) from observed Bi (and likely Zn) volatility. Oxygen vacancy diffusivity is estimated to be 10−12.8 cm2/s at 250°C. Low diffusivity and high activation energies are indicative of significant defect interactions. Dipolar oxygen vacancy defects are also indicated, with an activation energy of 0.59 eV from TSDC measurements. The large oxygen vacancy content leads to a short lifetime during high voltage (30 kV/cm), high temperature (250°C) direct current (DC) electrical measurements.

Forming-Free, Low-Voltage, and High-Speed Resistive Switching in Ag/Oxygen-Deficient Vanadium Oxide(VOx)/Pt Device through Two-Step Resistance Change by Ag Filament Formation.

Forming-free, low-voltage, and high-speed resistive switching is demonstrated in an Ag/oxygen-deficient vanadium oxide (VOx)/Pt device via the facilitated formation and rupture of Ag filaments. Direct current (DC) voltage sweep measurements exhibit forming-free switching from a high-resistance state (HRS) to a low-resistance state (LRS), called SET, at an average VSET of +0.23 V. The reverse RESET transition occurs at an average VRESET of -0.07 V with a low RESET current of <1 mA. Reversible switching operations are stable with an HRS/LRS resistance ratio >103 during repeated measurements for thousands of cycles. In pulse measurements, switching occurs within 100 ns at an amplitude of +1.5 V. Notably, a two-step resistance change is observed in the SET operation, where the resistance first partially decreases due to Ag+ ion accumulation in VOx and then further decreases to the LRS after hundreds of nanoseconds upon complete filament formation. The VOx layer deposited to be mostly amorphous with oxygen deficiency from V2O5 has abundant vacancies and expedites Ag+ ion migration, thus realizing forming-free, high-speed, and low-voltage switching. These characteristics of the facilitated Ag filament formation using the substoichiometric VOx layer are highly beneficial for use as stand-alone nonvolatile memory and in-memory computing elements.

Synthesis and Properties of La14TME6CuS24O4, Exhibiting Multivalent Spin Chains (TME = Cr, Fe).

Two homologues in a series of quinary oxysulfides, La14TME6CuS24O4 (TME = Cr or Fe), have been synthesized by solid-state synthesis in sealed ampules, and their structures are homologue to assume a novel crystal structure. X-ray diffraction analyses of single crystal and powder samples give a monoclinic lattice, described in the C2/m (No. 12) space group, with lattice parameters a = 15.3853(5) Å, b = 13.9729(5) Å, c = 10.5074(4) Å, and β = 116.227(3)° for the Cr analogue and a = 15.4303(2) Å, b = 14.0033(2) Å, c = 10.4909(2) Å, and β = 116.261(2)° for Fe. The crystal structure contains one-dimensional (1D) chains consisting of interconnected transition metal element (TME) trimers, which are further arranged into two-dimensional (2D) layers. These spin-chain planes are interspaced with 1D chains of lanthanum-oxygen coordinations and an apparent disordered occupation of copper sites. Alternating current (AC) and direct current (DC) magnetic susceptibility measurements show that the Cr and Fe analogues exhibit what is best described as spin-domain formation. Density functional theory (DFT) calculations suggest the formal oxidation state of the species is best represented in the form La14TME52+TME3+Cu+S24O4.

Spatial variability of water mass transports in the Bransfield Strait based on direct current measurements

The Bransfield Strait connects the Bellingshausen and Weddell seas and is strongly affected by intense inflows of their water masses. In this work we analyzed Shipborne Acoustic Current Doppler Profiler (SADCP) measurements carried out in the Bransfield Strait from 2015 to 2022. The new dataset includes over 100 transects, crossing the main currents in the strait (namely, the Bransfield Current and the Transitional Weddell Water (TWW) flow). Based on these data, we studied spatial variability of water mass transports in the Bransfield Strait during austral summer season. As shown below in this research, the Bransfield Current was observed down to the 600 m depth. By considering deeper layers, we managed obtaining more precise assessments of mean water mass transports of the Bransfield Current. They were estimated at 1.4–2.1 Sv. We found that the transports of the Bransfield Current and TWW flow in the austral summer season (November–March) do not show significant variability. The water mass transport of the Bransfield Current and the TWW flow was stable within the central basin of the Bransfield Strait and differed in its western basin. We also found that in austral summer the heat transport into the Bransfield Strait varied between 0.2 and 0.4 Sv·°C and salinity transport was −0.07 to −0.06 Sv·psu, their sign reflecting a dominant northeastward flow of relatively warm and fresh water. In addition, we compared Lowered Acoustic Current Doppler Profiler (LADCP) and SADCP measurements and proved that the technical limitations of the SADCP do not significantly affect the estimates of the transports in this region.

Investigation of sulphate hydride anti-perovskite as solid electrolyte

Over the last decade, anti-perovskites have drawn significant attention as potential solid-electrolytes for solid-state batteries. Due to the increase in consumption of lithium, there has been a push towards next generation batteries, including sodium-ion batteries. The first representative of the material class of sulphate hydride anti-perovskites, Na3SO4H, was synthesized by solid-state methods as a possible electrolyte for sodium solid-state batteries. Structural characterization confirms the results reported in literature with P4/nmm space group Thermal measurements (DSC and TGA) reveal the stability of the material up to 633 K with H2 release beginning shortly after. Here were report the first electrochemical measurements of this new sulphate hydride anti-perovskite with room temperature conductivity of 4.0 × 10−7 S/cm. Similarly, the electronic conductivity was also measured by Direct Current (DC) experiments to understand a non-linear Arrhenius plot of the conductivity. From the EIS and DC measurements, it is suggested that the electronic and ionic conductivities of this material fall in the same range at room temperature. Upon heating, the material becomes a mainly ionic conductor, explaining the change in the activation energy values in the Arrhenius plot (0.83 eV at low T and 0.24 eV at high T). Solid-state NMR hints at defects in the structure that correspond to Na1 and Hb-c-d dynamics.

The dependence of electrical conductivity of MgxZn1–xO ceramics on phase composition

The structural and electrical characteristics of (Mg,Zn)O ceramics produced using the solid state reaction at 1100 °C for 3 hours were studied applying X-ray diffraction and IR reflection spectroscopy as well as means of direct current measurements versus MgO content in initial charge (varied from 0 to 100 mol.%). It has been shown that electrical conductivity extracted from the IR reflection spectra corresponds to that of hexagonal phase in a solid solution, while plasmon in cubic phase was not observed. The electron concentration in the hexagonal grains of solid solution prepared with MgO content below 30 mol. % in the charge was found to be close to that of ZnO grains. It shows the tendency to decrease with further growth of the MgO content, which was explained by extraction of zinc interstitials, responsible for ZnO conductivity, from ZnO under formation of the MgZnO cubic phase. The direct current measurements have shown the lower conductivity as compared to the value estimated from IR reflection spectra. This fact along with the superlinearity of current-voltage characteristics has been explained by the presence of intergranular barriers, which does not allow obtaining information on the concentration of free electrons in the grain by this method. The possible nature of intergranular barriers as well as the role of grain boundaries in the DC conductivity of samples has been discussed.

Electrical and Electro-Thermal Characteristics of (Carbon Black-Graphite)/LLDPE Composites with PTC Effect.

Electrical properties and electro-thermal behavior were studied in composites with carbon black (CB) or hybrid filler (CB and graphite) and a matrix of linear low-density polyethylene (LLDPE). LLDPE, a (co)polymer with low crystallinity but with high structural regularity, was less studied for Positive Temperature Coefficient (PTC) applications, but it would be of interest due to its higher flexibility as compared to HDPE. Structural characterization by scanning electron microscopy (SEM) confirmed a segregated structure resulted from preparation by solid state powder mixing followed by hot molding. Direct current (DC) conductivity measurements resulted in a percolation threshold of around 8% (w) for CB/LLDPE composites. Increased filler concentrations resulted in increased alternating current (AC) conductivity, electrical permittivity and loss factor. Resistivity-temperature curves indicate the dependence of the temperature at which the maximum of resistivity is reached (Tmax(R)) on the filler concentration, as well as a differentiation in the Tmax(R) from the crystalline transition temperatures determined by DSC. These results suggest that crystallinity is not the only determining factor of the PTC mechanism in this case. This behavior is different from similar high-crystallinity composites, and suggests a specific interaction between the conductive filler and the polymeric matrix. A strong dependence of the PTC effect on filler concentration and an optimal concentration range between 14 and 19% were also found. Graphite has a beneficial effect not only on conductivity, but also on PTC behavior. Temperature vs. time experiments, revealed good temperature self-regulation properties and current and voltage limitation, and irrespective of the applied voltage and composite type, the equilibrium superficial temperature did not exceed 80 °C, while the equilibrium current traversing the sample dropped from 22 mA at 35 V to 5 mA at 150 V, proving the limitation capacities of these materials. The concentration effects revealed in this work could open new perspectives for the compositional control of both the self-limiting and interrupting properties for various low-temperature applications.

Emission characteristics of pulse CNT cold cathode X-ray source combined with channel electron multiplier

Carbon nanotube (CNT)-based field emission X-ray source with the advantage of fast start-up response offers the chance to achieve high-frequency X-ray emission. In this study, a high-frequency random pulse X-ray source of CNT cold cathode combined with a channel electron multiplier (CEM) was built, and its direct current (DC) and pulse emission characteristics were tested. The DC measurement results were used for parameter selection for performing pulse experiments. During the DC test, with the conditions of 2.2 kV CEM bias voltage and 25 kV anode voltage, the anode currents are 141, 250, and 300 μA at grid voltages of 290, 387.6, and 432.2 V, respectively; the corresponding grid field values are 1.45, 1.94, and 2.16 V/μm. During the pulse test, the amplitude–frequency response of the X-ray source reaches 3.58 MHz at 3 dB. The developed pulse X-ray source was introduced into the X-ray communication (XCOM), and the experimental communication rate reached 6 Mbps with the bit-error-rate of 1.1 × 10−3. The developed high-frequency pulse CNT-CEM X-ray source has potential applications in XCOM, high-speed X-ray imaging, and other fields.