Inhomogeneous Contacts Research Articles

Electrochemical-Mechanical Coupling Experiments on Solid-State Battery Materials and Interfaces

The interaction of mechanical and electrochemical effects in the areas of thermodynamics, kinetics, and transport are known, but accurately measuring those couplings can be complicated by the difficulty of getting a uniform stress at a solid-solid interface where surface roughness, voids and inclusion result in inhomogeneous contacts.1 This is a particular challenge for solid-state batteries where both the active material and electrolyte are in the solid phase. In addition, cycling of metal anodes (e.g., Na and Li) results in processes such as dendrite and void formation in which electrochemical and mechanical can be amplified.2 This talk will focus on two improvements to typical experiments in which electrochemical-mechanical coupling strongly influences results: (1) The use of a solid-state reference electrode for solid Na / solid electrolyte interfacial studies, and (2) the use of a nanoindentation platform to assess electrochemical-mechanical coupling behaviors in thin-film solid-state materials.For topic (1), we will discuss our use of a solid Na metal reference electrode to inform the cycling of Na metal against a Na-β″-Al2O3 solid electrolyte using cycling protocols of commercial relevance (e.g., 5 mAh/cm2, <5 MPa external pressure).2 Our work identifies the current density at which stripping shifts from stable to unstable, and shows that large (100s of mV) of interfacial overpotential can persist for hours in some stripping test protocols, which, according to models indicates that >80% of the initial electrochemical active interfacial area has been lost. We will also present recent results on how the applied pressure affects the interfacial behavior of Na metal / Na-β″-Al2O3 interfaces, as well as the role of applied pressure and the cycling protocol for an interface that starts as only current collector / Na-β″-Al2O3.For topic (2), we will present on our electrochemical-mechanical coupling studies of thin-film (i.e., <1 um thick) V2O5 samples on an Ohara Li+-conducting glass ceramic disc. This platform allows us to simultaneously control electrochemical behavior (e.g., Li insertion and removal, current) and the applied force, to study thermodynamic and kinetic electrochemical-mechanical couplings. This platform enables the application of a uniform stress on the thin-film V2O5 sample.1. Joule 7, 652–674, April 19, 2023.2. ACS Appl. Mater. Interfaces 2023, 15, 42, 49213–49222

Research on sound field characteristics of full-ceramic angular contact ball bearing

This paper focuses on the distribution characteristics of the radiation noise for a full-ceramic angular contact ball bearing (FCACBB). A model for calculating the radiation noise of a FCACBB was established based on the bearing dynamic characteristics and sound radiation theory, and the effectiveness and accuracy of the method were verified by corresponding experiments. The variation of the radiation noise for a FCACBB with the rotational speed was analyzed. The directivity of the sound field in the circumferential direction was found. The variation trends of the radiation noise in the friction load zone and the impact load zone were discussed. The attenuation of the radiation noise in the radial direction and in the axial direction was investigated. The directivity angle and the degree of directivity of the sound field were explored at different radial and axial distances. The results show that the inhomogeneous contacts between the bearing components in the circumferential direction result in different degrees of friction and impact vibration at different locations, which makes the radiation noise of the FCACBB have a larger component in the radial direction than in the axial direction. The conclusions can provide a reference for predicting the radiation noise of FCACBBs and further optimizing the sound field.

Спектроскопия электрон-фононного взаимодействия сверхпроводящих микроконтактов: экспериментальные аспекты

The recovering procedure of the electron-phonon interaction (EPI) functions from the additional nonlinearities of the current-voltage curve ($I-V$ curve) of point contacts associated with an excess current is considered. The approach proposed takes into account both inelastic scattering, which causes suppression of the excess current in the reabsorption of nonequilibrium phonons by electrons undergoing Andreev reflection (Andreev electrons), and elastic processes associated with the electron-phonon renormalization of the energy spectrum in a superconductor. The results obtained are systematically expounded for both the ballistic contacts, wherein the second derivatives of the $I-V$ curve in the normal state are proportional to the EPI functions, and inhomogeneous contacts (with dirty constrictions and clean banks), whose second derivatives in the normal state are either free of phonon singularities or weakly pronounced.

Application of electrical impedance tomography to an anisotropic carbon fiber-reinforced polymer composite laminate for damage localization

Electrical impedance tomography (EIT) is an emerging method for assessing the structural condition of composite structures. In this work, EIT's damage localization capability is investigated through its application upon a commercial laminated anisotropic carbon fiber reinforced polymer plate. Aluminum rivets were used as electrodes. EIT reconstructs the spatial conductivity within a defined space by using its boundary voltage response. The algorithm starts with a forward simulation of the voltage distributions within the boundary, hence the orthotropic conductivity of the CFRP plate was first characterized experimentally. It was found that the inhomogeneous fiber contacts through the thickness are influencing the reconstruction results. A detailed 3D finite element simulation was built to model the conductivity distribution. Different current injection patterns were attempted with their reconstruction results being evaluated via four criteria. It was found that the diagonal current injection and the difference evaluation lead to the best results.

Operando current mapping on PEM water electrolysis cells. Influence of mechanical stress

Proton Exchange Membrane Water Electrolysis (PEM WE) is considered as a key technology for large scale storage of renewable electricity, using hydrogen as an energy vector. To achieve economically viable MW-scale storage units, there is a strong trend to increase both current density and electrode surface area. Such upscaling raises concerns regarding the homogeneity of compression forces and current lines distribution throughout the electrolysis stack. Mechanical and thermal stresses resulting from any heterogeneity can result in faster local ageing and accelerated degradation. Thus, there is a major interest to measure and understand the nature of such coupled heterogeneities. We report here on results obtained with a non-disturbing tool for the current and the temperature mapping of PEM WE electrodes. A complete description of the setup is discussed in order to avoid some mapping artefacts that could come from the measurement device. Measurements made on 250 cm2 surface area cells show that the clamping pressure homogeneity over the entire geometrical surface area of the cell is one of the first mandatory requirement for an appropriate operation of electrolysis cells. In particular, it was found that even homogeneous pressure distribution could lead to excessive current maldistribution in some cases, due to overriding inhomogeneous electric contacts. This situation has been corrected by using non-flat end plates and/or an accurate thickness of the gaskets. Current mapping has been performed. Results are discussed.

Characterization technique for inhomogeneous 4H-SiC Schottky contacts: A practical model for high temperature behavior

In this paper, a characterization technique for 4H-SiC Schottky diodes with varying levels of metal-semiconductor contact inhomogeneity is proposed. A macro-model, suitable for high-temperature evaluation of SiC Schottky contacts, with discrete barrier height non-uniformity, is introduced in order to determine the temperature interval and bias domain where electrical behavior of the devices can be described by the thermionic emission theory (has a quasi-ideal performance). A minimal set of parameters, the effective barrier height and peff, the non-uniformity factor, is associated. Model-extracted parameters are discussed in comparison with literature-reported results based on existing inhomogeneity approaches, in terms of complexity and physical relevance. Special consideration was given to models based on a Gaussian distribution of barrier heights on the contact surface. The proposed methodology is validated by electrical characterization of nickel silicide Schottky contacts on silicon carbide (4H–SiC), where a discrete barrier distribution can be considered. The same method is applied to inhomogeneous Pt/4H–SiC contacts. The forward characteristics measured at different temperatures are accurately reproduced using this inhomogeneous barrier model. A quasi-ideal behavior is identified for intervals spanning 200 °C for all measured Schottky samples, with Ni and Pt contact metals. A predictable exponential current-voltage variation over at least 2 orders of magnitude is also proven, with a stable barrier height and effective area for temperatures up to 400 °C. This application-oriented characterization technique is confirmed by using model parameters to fit a SiC-Schottky high temperature sensor's response.

Magnetic properties of terminal iodinated nitroxide radical liquid crystals

We designed and synthesized terminal iodinated nitroxide radical liquid crystalline compounds (NR-LCs) with octyloxy side chain (±)-8-I and dodecyloxy side chain (±)-12-I to understand the substitution effects of an iodine atom on the phase transition behavior and on the magnetic properties in crystalline (Cr), liquid crystalline (LC) and isotropic (Iso) phases. These NR-LCs show monotropic nematic phases because the iodine atom is too short and apolar as a terminal group to stabilize LC phases. Furthermore, the SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy indicate that the magnetic properties in the Cr phase depend on the length of the alkoxy side chain; antiferromagnetic interactions occur in (±)-8-I, whereas ferromagnetic interactions occur in (±)-12-I, and the paramagnetic susceptibility of (±)-8-I slightly decreases (J¯<0) and that of (±)-12-I abruptly increases (J¯>0) at each of the Cr-to-Iso phase transitions. These results suggest that the most frequent type of the magnetic interactions in LC and Iso phases is similar to those in the Cr phases among the various types of intermolecular magnetic interactions induced by the inhomogeneous intermolecular contacts.

An Investigation of SiC Schottky Contact Barrier Inhomogeneity for Temperature Sensing Applications

This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.

Magneto-LC Effects in Hydrogen-Bonded All-Organic Radical Liquid Crystal

To identify the origin of the magneto-LC effects, which refer to the enhancement of intermolecular magnetic interactions in the LC phases of all-organic radical compounds, we have designed and synthesized H-bonded all-organic radical compounds showing nematic and cholesteric phases for the first time. They show stronger magneto-LC effects than in the same LC phases of analogous covalent-bonded all-organic radical LC compounds. Variable-temperature electron paramagnetic resonance spectroscopy of the hydrogen-bonded compounds reveals that the inhomogeneous intermolecular contacts give rise to the magneto-LC effects.

Inhomogeneous ohmic contacts: Barrier height and contact area determination

The current-voltage characteristics at a uniform ohmic contact are, by definition, dominated by its series resistance and ordinarily offer little information, beyond an upper-bound, on the Schottky barrier height (SBH) of the metal-semiconductor junction. We demonstrate through temperature-dependent measurements of Au “ohmic” contacts on n-type Si(100)1 × 1-S, however, that quantitative information on not only the magnitude of the SBH, to within ± 0.03 eV, but also the effective conduction area can still be deduced. Details of these analyses, which require the use of a thin Si epi-layer, and the “partisan interlayer” mechanism, which lead to the formation of ohmic contacts, are discussed.

Pd/Au/SiC Nanostructured Diodes for Nanoelectronics: Room Temperature Electrical Properties

Pd/Au/SiC nanostructured Schottky diodes were fabricated embedding Au nanoparticles (NPs) at the metal-semiconductor interface of macroscopic Pd/SiC contacts. The Au NPs mean size was varied controlling the temperature and time of opportune annealing processes. The electrical characteristics of the nanostructured diodes were studied as a function of the NPs mean size. In particular, using the standard theory of thermoionic emission, we obtained the effective Schottky barrier height (SBH) and the effective ideality factor observing their dependence on the annealing time and temperature being the signature of their dependence on the mean NP size. Furthermore, plotting the effective SBH as a function of the effective ideality factor we observe a linear correlation, indicating that the Au NPs act as lateral inhomogeneities in the Schottky diodes according to the Tung's model. Therefore, we can control the size, fraction of covered area, and surface density of such intentionally introduced inhomogeneities. The application of the Tung's model for the electronic transport in inhomogeneous Schottky contacts allow us to obtain, in particular, the homogeneous SBH. These nanostructured diodes are proposed as possible components of integrated complex nanoelectronic devices.

The theoretical and experimental study on double-Gaussian distribution in inhomogeneous barrier-height Schottky contacts

We have considered multi-Gaussian distribution of barrier-heights for non-interactive barrier inhomogeneities in the inhomogeneous Schottky diodes, and we have shown the presence of the intersecting behavior in the forward-bias current–voltage ( I– V) curves for the double-Gaussian distribution model at low temperatures. We have tried to eliminate this effect by generating I– V curves at lower temperatures with the bias-dependent barrier-height expression which leads to the ideality factors greater than unity. For this calculation, we have obtained the expressions for the barrier-height change and ideality factor, and for bias-dependency of the BH for the multi-Gaussian model by following the literature. We have shown that the experimental forward-bias I– V curves coincide with the theoretical ones using the bias-dependent inhomogeneous BH expression at low and high temperatures in the double-Gaussian distribution of BHs.

Interphase interactions and the mechanism of current flow in Au-TiB x -AuGe-n-GaP ohmic contacts

Au-TiBx-AuGe-n-GaP ohmic contacts have been investigated before and after rapid thermal annealing at T = 723, 773, and 873 K for 60 s in a hydrogen atmosphere. It is shown that the contact resistivity decreases with an increase in temperature in the range 77–232 K due to the thermionic nature of current flow in inhomogeneous ohmic contacts, while in the range 232–386 K the contact resistivity increases, which can be related to the conduction through metal shunts.

A theoretical analysis together with experimental data of inhomogeneous Schottky barrier diodes

We have shown that the current at lower temperatures may exceed the current at higher temperatures using the approximation of the apparent BH in the inhomogeneous SDs, as determined by some authors in the literature. Then, in the simulated forward I–V curves of non-interactive inhomogeneous SDs using the approximation of the apparent BH, it has been showed that the current at lower temperatures does not exceed the current at higher temperatures when considering the bias voltage dependence of apparent BH. In the calculations, the parameters from the experimental forward bias I–V characteristics, in the temperature range of 60–300 K, of the Ni/ n-GaAs Schottky contacts with the inhomogeneous BH have been used by means of the thermionic emission theory of inhomogeneous Schottky contacts, considering a Gaussian distribution of barrier heights with a linear bias dependence.

Electrical characterization of thiols self-assembled layers on GaP (1 1 1) structures

The experimental studies on III–V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol—CH 3(CH 2) 11SH) and aromatic thiol (4, 4′ tiobisbenzenthiol-S (C 6H 4SH) 2 have been deposited on the surface of GaP (1 1 1) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current–voltage ( I–V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in “reverse bias” way, the I–V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and “in forward bias” way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes’ ideality factors determined from I–V characteristics are unusually high ( n⪢2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation.

Effect of superconductivity on the electronic transport and capacitance of La1.85Sr0.15CuO4∕Nb doped SrTiO3 heterojunction

The authors report the effect of superconductivity on the electronic transport and capacitance of La1.85Sr0.15CuO4∕Nb doped SrTiO3 heterojunction. This heterojunction shows good rectifying property. With the occurrence of superconductivity, the capacitance, junction resistance, and diffusion voltage of the heterojunction show jumps, which correlate with the resistance change of La1.85Sr0.15CuO4. Analysis shows that the current-voltage behavior of the heterojunction is consistent with that of the inhomogeneous Schottky contacts. The results were discussed by considering the effect of superconductivity on the inhomogeneous heterojunction. This work is helpful for the understanding of superconductor based heterojunctions and inhomogeneous Schottky contacts.

Current-voltage and noise characteristics of an inhomogeneous schottky-barrier contact

A Schottky-barrier (SB) metal/semiconductor contact (MSC) is considered. A model contact characterized by the normal distribution of the SB height and the corresponding distribution of the series resistance over the contact area is investigated numerically. A general approach to the analysis of inhomogeneous and homogeneous contacts is demonstrated. The approach is based on the concept of the nonlinear dependence of the barrier height (effective height in the case considered) on the bias voltage. An analytic approximation corresponding to the known empirical expression is obtained for the current-voltage characteristic of the contact. The model considered is applied to interpret experimental current and temperature dependences of the 1/f-noise level in SB contacts.

Determination of the characteristic parameters of Sn/n-GaAs/Al–Ge Schottky diodes by a barrier height inhomogeneity model

A study on parameters of the Sn/n-GaAs Schottky barrier diode (SBD) fabricated on an n-type GaAs substrate has been made. The Sn/n-GaAs SBD has shown a nearly ideal behaviour with ideality factor and barrier height (BH) values of 1.081 and 0.642 eV, respectively, from the experimental forward-bias current–voltage (I–V) characteristics. A BH value of 0.724 eV has been obtained from the experimental reverse-bias capacitance–voltage (C–V) characteristics. An accurate theoretical modelling of the effect of the presence of inhomogeneities on the electron transport across the metal–semiconductor interface has been applied. This model attempts to explain abnormal experimental results obtained on ‘real’ Schottky diodes. Our results clearly demonstrate that the electron transport at the metal–semiconductor interface is significantly affected by low-barrier regions (patches). When the experimental data are described by the thermionic emission theory of inhomogeneous Schottky contacts, it has been concluded that both the experimental forward and reverse I–V characteristics and the difference between the values of the experimental I–V and C–V SBHs should be considered. An experimental BH difference of Δ = 0.082 V has been obtained for the Sn/n-GaAs SBD that is less than the critical value; therefore, it has been seen that the potential in front of the patch is not pinched off.

Theoretical evidence for random variation of series resistance of elementary diodes in inhomogeneous Schottky contacts

The current–voltage ( I– V) characteristics of Schottky diodes measured at low temperature shows an abnormal behavior, which is attributed to the presence of spatial variation of barrier heights of Gaussian type at metal–semiconductor interface. The simulation studies assuming Gaussian distribution of barrier heights also show similar temperature dependence of diode parameters as observed in real Schottky contacts fabricated on various metal–semiconductor systems. The simulation studies performed so far on inhomogeneous Schottky contacts assumes constant series resistance ( R S) to all elementary diodes because of the complexity in programming. We have simulated I– V characteristics of inhomogeneous Schottky diodes with Gaussian distribution of Barrier heights (BHs) assuming different R S for different elementary diodes in the distribution. It is found that the simulation performed using randomly varying R S having Gaussian distribution yields I– V characteristics similar to those obtained using constant R S with its value about which variation in R S is considered for all elementary diodes. The simulation results provide theoretical evidence about the occurrence of random fluctuation of R S of Gaussian type in elementary diodes in inhomogeneous Schottky contact.

A comparative study of numerical and analytical methods of simulating inhomogeneous Schottky diode characteristics

The current–voltage characteristics of inhomogeneous Schottky diodes can be generated using either analytically solved equations for total current through all elementary diodes, or by numerical integration over the entire barrier height range. The simulation performed using these two methods yields current–voltage characteristics with different features. In this paper we have analysed the nature of elementary diodes representing a Gaussian distribution of barrier heights in inhomogeneous Schottky contact, and found that it is the constant series resistance considered for all elementary diodes in the distribution which makes the two methods yield current–voltage characteristics with different features. It apparently indicates that the two methods of analysing inhomogeneous Schottky contacts are inconsistent. It is shown that each elementary barrier in the distribution has a different series resistance associated with it, which has a direct correlation with its probability of occurrence in the distribution. Performing the numerical integration with varying series resistance yields current–voltage characteristics with features similar to those obtained by the analytical method. The analysis made in this paper confirms that the two methods of modelling inhomogeneous Schottky contacts yield consistent current–voltage characteristics.