Contact Properties Research Articles

Investigation of coated hydrophobic granular materials by means of computed tomography and environmental scanning electron microscopy

Hydrophobic materials in geotechnical engineering and soil science can have natural or artificial origin. They can be applied, e. g., to waterproof structures in the industry. In this contribution, hydrophobic granular material was manufactured through a cold plasma polymer coating procedure. The monomer used was C4F8 (octafluorocyclobutane) and the material to be coated was Hamburg sand, a coarse grained sand. In this context, computed microtomography and environmental scanning electron microscopy were used to investigate the materials in their unsaturated state. The tools are applied to visualize unsaturated phenomena on the microscale. The hydrophobic and untreated materials were imaged by both techniques at different saturation degrees in order to understand the influence of the coating on the sample’s hydraulic behaviour. The chosen environmental scanning electron microscope is able to provide relative humidity in the sample chamber, and so water drops were condensed on the grain surface, allowing to also observe the initial contact of water and the hydrophobic coating. It was observed how the capillary menisci, their geometry and contact properties evolve at different degrees of saturation. The measurements obtained and respective analyses state qualitatively the influence of the hydrophobic coatings on the pore water dynamics at different saturation degrees, which dictates the material’s hydraulic behaviour. Contact angles were also analysed were it was physically possible.

Electrical contact properties of 2D metal-semiconductor heterojunctions composed of different phases of NbS<sub>2</sub> and GeS<sub>2</sub>

Metal-semiconductor heterojunction (MSJ) is the basis for developing novel devices. Here, we consider different two-dimensional van der Waals MSJs consisting of different-phase metals H- and T-NbS<sub>2</sub> and semiconductor GeS<sub>2</sub>, and conduct an in-depth study of their structural stabilities, electronic and electrical contact properties, with an emphasis on exploring the dependence of the electrical contact properties of the MSJs on the different phases of metals. Calculation results of their binding energy, phonon spectra, AIMD simulations, and mechanical properties show that both heterojunctions are highly stable, which implies that it is possible to prepare them experimentally and feasible to use them for designing electronic devices. The intrinsic H-NbS<sub>2</sub>/GeS<sub>2</sub> and T-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunctions form p-type Schottky contacts and quasi-n-type Ohmic contacts, respectively. It is also found that their Schottky barrier heights (SBHs) and electrical contact types can be effectively modulated by an applied electric field and biaxial strain. For example, for the H-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, Ohmic contact can be achieved regardless of applying a positive/negative electric field or planar biaxial compression, while for the T-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, Ohmic contact can be achieved only at a very low negative electric field. The planar biaxial stretching can achieve quasi-Ohmic contact. In other words, when the semiconductor GeS<sub>2</sub> monolayer is used as the channel material of the field effect transistor and contacts different metal NbS<sub>2</sub> monolayers to form the MSJ, the interfacial Schottky barriers are distinctly different, and each of them has its own advantages in different situations (intrinsic or physically regulated). Therefore, this study is of great significance for understanding the physical mechanism of the electrical contact behaviors for H(T)-NbS<sub>2</sub>/GeS<sub>2</sub> heterojunction, especially for providing the theoretical reference for selecting suitable metal electrodes for the development of high-performance electronic devices.

Tunable ohmic van der Waals-type contacts in monolayer C3N field-effect transistors.

Monolayer (ML) C3N, a novel two-dimensional flat crystalline material with a suitable bandgap and excellent carrier mobility, is a prospective channel material candidate for next-generation field-effect transistors (FETs). The contact properties of ML C3N-metal interfaces based on FETs have been comprehensively investigated with metal electrodes (graphene, Ti2C(OH/F)2, Zr2C(OH/F)2, Au, Ni, Pd, and Pt) by employing ab initio electronic structure calculations and quantum transport simulations. The contact properties of ML C3N are isotropic along the armchair and zigzag directions except for the case of Au. ML C3N establishes vertical van der Waals-type ohmic contacts with all the calculated metals except for Zr2CF2. The ML C3N-graphene, -Zr2CF2, -Ti2CF2, -Pt, -Pd, and -Ni interfaces form p-type lateral ohmic contacts, while the ML C3N-Ti2C(OH)2 and -Zr2C(OH)2 interfaces form n-type lateral ohmic contacts. The ohmic contact polarity can be regulated by changing the functional groups of the 2D MXene electrodes. These results provide theoretical insights into the characteristics of ML C3N-metal interfaces, which are important for choosing suitable electrodes and the design of ML C3N devices.

Contact Analysis of CNT-FGM Nanocomposite Using Indentation Contact Model

In the present paper, carbon nanotube (CNT) reinforced functionally graded (FG) material matrix nanocomposite (CNT-FGM) is indented with a rigid conical indenter and the contact behaviour of the CNT-FGM nanocomposite is analyzed in the context of variation in gradation parameter and wall thickness of the CNTs. The study is conducted on the basis of finite element model, which is developed utilizing Ansys Parametric Design Language (APDL) codes in commercial finite element modelling platform, ANSYS. The material model is developed considering the CNTs are uniformly distributed in the graded matrix, whose elastic properties vary along the direction of indentation. The gradation model, corresponding to the matrix material, adopted for the present study is a power law function. In this study, elastically graded material (EGM)matrix CNT reinforced CNT-FGM nanocomposite has been examined, varying the elastic gradation parameter (+2, 0 and -2) of the matrix material. Additionally, effect of the variations in CNT wall thickness are explored, while maintaining other parameter constant in the nanocomposite. A frictionless contact between the CNT-FGM substrate and rigid indenter is simulated for both loading and unloading process. Prior to extracting pertinent results, the current model undergoes validation by comparing its outcomes with those published in the literature. It is observed that the contact force-displacement plots align satisfactorily for both sets of data. Various contact parameters, such as contact force, contact area, contact pressure distribution, stress distribution, and deformation behavior during the indentation contact, are then extracted from the analysis. The observation indicates that positive gradation parameter result in superior contact properties compared to negative gradation parameters as well as without gradation.

Enhancing the Efficiency of Indoor Perovskite Solar Cells through Surface Defect Passivation with Coplanar Heteroacene Cored A–D–A‐type Molecules

AbstractThe passivation of perovskite interfacial defects by the electron transport layer (ETL) has emerged as an effective strategy for enhancing the performance of perovskite solar cells (PSCs). Dithieno[2,3‐d:2′,3′‐d′]thieno[3,2‐b:3′,2′‐b′]dipyrrole (DTPT)‐based acceptor‐donor‐acceptor (A–D–A) molecules composed of coplanar heteroacene as electron‐donating core end‐capped with various electron‐accepting moieties are designed and examined as ETL modifiers for PSCs. Employing PCBM:DTPTCY as the ETL results in passivation perovskite defects, facilitation energy alignment at the ETL/perovskite interface, and enhancement of carrier transport efficiency. The optimized blended ETL‐based Cs0.18FA0.82Pb(I0.8Br0.2)3 p‐i‐n PSC exhibit performances of 37.2% and 39.9% under TL84 and 3000K LED (1000 lux), respectively. The DTPTCY‐based device demonstrates remarkable stability, retaining 87% of its initial power conversion efficiency (PCE) after 30 days of storage in a 40% relative humidity (RH) ambient air environment without any encapsulation, surpassing the control device, which retains only 67% of its original PCE. These findings underscore the potential of A–D–A‐type molecule‐based interface modification to enhance passivation and contact properties, ultimately leading to high‐efficiency and stable PSCs.

Flexible Perovskite Solar Cells on Ultra‐Thin Stainless‐Steel with a Power‐to‐Weight Ratio over 3000 W kg−1

Ultra‐thin stainless‐steel substrates with excellent water‐oxygen barrier properties and high thermal and electrical conductivities are suitable for the fabrication of lightweight and flexible perovskite solar cells (FPSCs). However, the deposition of dense perovskite films on stainless steel by the solution method is crucial because short circuits caused by perovskite holes are fatal to parallel structures. Herein, a single crystal (SC) is incorporated into the precursor solution to reduce the formation of holes in perovskite films on smooth stainless‐steel substrates. Additionally, a magnetic method is developed based on the properties of stainless steel to fix and fabricate FPSCs nondestructively on ultra‐thin stainless‐steel films with a thickness as low as 5 μm. Furthermore, 4,6‐dimethyl‐2‐mercaptopyrimidine (DMI) was introduced to passivate the surface of the perovskite film, optimizing the contact properties of the perovskite heterojunction and adjusting the energy level of the perovskite/C60 interface. Finally, ultra‐thin FPSCs achieved a champion power conversion efficiency (PCE) of 20.24% on an active area of 1.012 cm2 and a power‐to‐weight ratio over 3000 W kg−1. Moreover, under continuous illumination, the stainless‐steel substrates exhibited better photothermal stability than the polymer substrates. This method provides a basis for the fabrication of lightweight, low‐cost, and large‐area FPSCs.

Design of ohmic contacts between Janus MoSSe and two-dimensional metals

Two-dimensional semiconductors are considered as channel materials for field-effect transistors to overcome short-channel effects and reduce the device size. As the contacts to the metallic electrodes are decisive for the device performance, we study the electronic properties of contacts between Janus MoSSe and various two-dimensional metals. We demonstrate that weak interactions at these van der Waals contacts suppress Fermi level pinning and show that ohmic contacts can be formed for both terminations of Janus MoSSe, generating favorable transport characteristics.

Hydrogenation of silicon-nanocrystals-embedded silicon oxide passivating contacts

We investigate the effect of hydrogen passivation of dangling bonds in silicon oxide passivating contacts with embedded silicon nanocrystals (NAnocrystalline Transport path in Ultra-thin dielectrics for REinforced passivation contact, NATURE contact). We first investigated the differences in electrical properties of the samples after hydrogen gas annealing and hydrogen plasma treatment (HPT). The results show that the NATURE contact was efficiently passivated by hydrogen after HPT owing to the introduction of hydrogen radicals into the structure. Furthermore, we examined the dependence of process parameters such as HPT temperature, duration, and H2 pressure, on the electrical properties and hydrogen depth profiles. As a result, HPT at 500 °C, 15 min, and 0.5 Torr resulted in a large amount of hydrogen inside the NATURE contact and the highest implied open-circuit voltage of 724 mV. Contact resistivity and surface roughness hardly increased when HPT was performed under the optimized condition, which only improved the passivation performance without deteriorating the electron transport properties of the NATURE contact.

Numerical and experimental study of the conical connection of the blade of a rotary machine

The paper presents an experimental and numerical study of the tapered socket of the aluminum blade root of the mine main ventilation fan, which is based on the tests of a simplified full-scale model with a discarded airfoil and its subsequent finite element analysis. The calculation model takes into account elastoplastic properties of materials and non-linear contacts with friction. The proposed joint consists of an aluminum tapered blade root, two steel retainers with similar tapered surfaces, and two steel bolts that join the retainers around the root. Pre-tightening the bolts allows fixing the blade in an unloaded state in the socket and prevents its unwanted turns. Such a tightening is taken into account in the finite element analysis by means of determining, in compliance with special rules, the axial force of the pretension of the bolts. With the help of a hydraulic press acting on the lower surface of the airfoil root, the effect of the centrifugal load on the conical joint from the side of the blade airfoil is simulated. Nonlinear static analysis of the elastoplastic behavior of the structure allows determining the destructive loads that cause the bolts to break with the subsequent disconnection of the fasteners and the blade to fly out of the seat. The graphs of the equivalent von Mises stresses indicate that the maximum stresses are reached in the working part of the bolts, which fully corresponds to the nature of the destruction of the structure upon reaching the maximum equivalent load on it. The experimental study confirms the correctness of the determination of contact stresses at the tapered socket location. Correspondence of the results of the static analysis with the results of the full-scale experiment makes it possible to draw a conclusion about the correctness of the conducted finite element modelling. This allows using the developed formulation of the problem to determine the strength of rotor structures with conical connections of blades without performing preliminary experimental studies. In addition, the developed technique can be extended to a larger range of conical and cylindrical joints due to the simplicity of the approach and the versatility of the formulation of the nonlinear finite-element problem which models structures with preloaded or tensioned elements.

Electronic, mechanical and contact properties of Mo4/3B2T2 (T=F, O, OH) monolayer: A first principles study

Single-layer 2D molybdenum boride sheets, Mo4/3B2T2 (T = F, O and OH), have been realized in recent experiment. Here, the electronic structure and mechanical flexibility of its most stable configurations are investigated by using first principles calculations. Mo4/3B2F2 and Mo4/3B2(OH)2 perform as 2D metal, while Mo4/3B2O2 is a semiconductor with indirect band gap ∼0.36 eV. These three monolayers can individually sustain the maximum stress and the strain limits at least ∼15 Gpa and 15 %, such mechanic flexibility is comparable to borophene and MoS2. When forming heterojunction with MoS2, p-type and n-type Schottky contact with barrier height ∼0.16 eV and 0.23 eV occur in Mo4/3B2F2 and Mo4/3B2(OH)2, respectively. Fortunately, the contact barrier can be eliminated by external field. In addition, a type-II heterojunction with a large conduction band offset is found in Mo4/3B2O2/MoS2, which may be a potential candidate for optoelectronics.

Effect of CNT radius on flattening contact behaviour of CNT-Al nanocomposite: A numerical approch

In the present paper, a flattening contact analysis of CNT-Al nanocomposite is presented for both loading and unloading phases. APDL code is used in ANSYS framework to create the FE model to analyse the contact behaviour between a frictionless cylinder (CNT-Al) and a rigid flat. The developed model has been validated against established results of certain problems with lesser complexity. The contact force, contact area, von Mises stresses and nodal displacements are extracted from the simulated solution. These parameters are noted at the end of the loading step and also once unloading is completed from a certain interference. It has been found that as CNT radius increases, the contact properties get decreased. It's also observed that beyond a certain CNT radius, the contact properties of CNT-Al nanocomposite materials start to resemble those of the matrix material and eventually fall below the matrix material.

Extremely Low Contact Resistivity of Bi2Te3-Based Modules Enabled by NiP-Based Alloy Barrier.

Electrode diffusion barrier plays an important role in thermoelectric cooling devices. Compared with p-type Bi0.5Sb1.5Te3, the compatibility between commercial Ni barrier and n-type Bi2Te2.7Se0.3 is a key bottleneck to enhance the performance of Bi2Te3-based cooling devices. This paper proposed a NiP alloy barrier to improve the compatibility with n-type Bi2Te2.7Se0.3, and systemically investigated the contact and interfacial dynamics properties. Due to the low diffusion rate of NiP alloy, the initial interfacial contact resistivity of Bi2Te2.7Se0.3/NiP is as low as 0.90 μΩ cm2, and it further can be depressed below 1.98 μΩ cm2 even after aging at 423 K for 35 days, indicating the superior thermal stability of the NiP barrier layer compared to the commercial Ni barrier layer. Based on the NiP barrier, a 15-pair bismuth telluride device is prepared and a high cooling temperature difference of 71.5 K at a hot-side temperature of 304 K is achieved, which proves the practical applications potential of NiP barrier for Bi2Te3-based modules.

Effective modeling of the chromatin structure by coarse-grained methods

The interphase chromatin structure is extremely complex, precise and dynamic. Experimental methods can only show the frequency of interaction of the various parts of the chromatin. Therefore, it is extremely important to develop theoretical methods to predict the chromatin structure. In this publication, we implemented an extended version of the SBS model described by Barbieri et al. and created the ChroMC program that is easy to use and freely available (https://github.com/regulomics/chroMC) to other users. We also describe the necessary factors for the effective modeling of the chromatin structure in Drosophila melanogaster. We compared results of chromatin structure predictions using two methods: Monte Carlo and Molecular Dynamic. Our simulations suggest that incorporating black, non-reactive chromatin is necessary for successful prediction of chromatin structure, while the loop extrusion model with a long range attraction potential or Lennard-Jones (with local attraction force) as well as using Hi-C data as input are not essential for the basic structure reconstruction. We also proposed a new way to calculate the similarity of the properties of contact maps including the calculation of local similarity. Communicated by Ramaswamy H. Sarma

Achieving highly efficient kesterite solar cells using simultaneous surface Ge substitution and rear interface engineering strategies

The progress in the device performance of kesterite-based thin-film solar cells (TFSCs) have been stagnant due to unstable rear contact properties and large open circuit voltage (Voc)-deficit characteristics. Several individual strategies have been developed to solve these issues. In this work, we investigate the simultaneous impact of Ge substitution and rear interface engineering with a MoOx intermediate layer on Mo back contact. The simultaneous application of both strategies delivers a synergetic effect, improving microstructure, Voc-deficit, band-tailing states, and device parameters and reducing Sn-related deep-level defects, resulting in enhanced power conversion efficiency (from 8.1 to 11.55%). This effective strategy offers new insight and a way to solve unstable rear contact properties and large Voc-deficit, further improving the device performances of kesterite-based TFSCs.

An approach for the creep-curve assessment using a new rail tribometer

The friction properties of the wheel-rail contact are characterized by a coefficient of traction (CoT), which can be affected by many contaminants. Numerous devices for assessing CoT are available; however, only a small number of them are capable of recording creep curves. This work introduced a rail-mounted tribometer that uses controlled changes in braking torque on a measuring wheel to induce creep in the contact. A methodology for assessing creep curve parameters was proposed, utilizing both analytical and numerical models of the contact. The experiments were performed both in the field and in the laboratory to investigate the effect of railhead conditions under various contact pressures and to track the evolution of the CoT during a measurement.

Thermoelastoplastic hydrodynamic lubrication contact analysis of three dimensional rough tooth surface considering micro-scale effect and thermal effect

The contact properties of gear rough surfaces during meshing are influenced by various factors. The analysis of rough surface thermoelastoplastic hydrodynamic lubrication (TPEHL) considering micro-scale effect and thermal effect has not been addressed in the literature. This study establishes a new TEPHL model with multi-factor coupling of thermal field, fluid lubrication, stress-strain, and geometric profile. Based on strain gradient theory (SG), a new thermoelastoplastic material constitutive considering micro-scale effect is developed. Subsequently, the three-dimensional rough surface TPEHL-SG analysis of gear is conducted. The paper investigates the impact of thermal and micro-scale effects on various parameters during the meshing of gear in the full oil film lubrication condition, including surface pressure, oil film thickness, surface temperature, surface deformation, and friction coefficient.

Isotropic Contact Properties in Monolayer GeAs Field-Effect Transistors

Owing to the tunable bandgap and high thermodynamic stability, anisotropic monolayer (ML) GeAs have arisen as an attractive candidate for electronic and optoelectronic applications. The contact properties of ML GeAs with 2D metal (graphene, Ti2CF2, V2CF2, and Ti3C2O2) and Cu electrodes are explored along two principal axes in field-effect transistors (FET) by employing ab initio electronic structure calculations and quantum transport simulations. Weak van der Waals interactions are found between ML GeAs and the 2D metal electrodes with the band structure of ML GeAs kept the same, while there is a strong interaction between ML GeAs and the Cu metal electrode, resulting in the obvious hybridization of the band structure. Isotropic contact properties are seen along the two principal directions. P-type lateral Schottky contacts are established in ML GeAs FETs with Ti3C2O2, graphene, and Ti2CF2 metals, with a hole Schottky barrier height (SBH) of 0.12 (0.20), 0.15 (0.11), and 0.29 (0.21) eV along the armchair (zigzag) direction, respectively, and an n-type lateral Schottky contact is established with the Cu electrode with an electron SBH of 0.64 (0.57) eV. Surprisingly, ML GeAs forms ideal p-type Ohmic contacts with the V2CF2 electrode. The results provide a theoretical foundation for comprehending the interactions between ML GeAs and metals, as well as for designing high-performance ML GeAs FETs.

Pore surface engineering of FeNC for outstanding power density of alkaline hydrazine fuel cells

In alkaline hydrazine fuel cells (AHFCs), the rapid kinetics of hydrazine oxidation contribute to their superior power density compared to other liquid fuel cells. However, the optimization of oxygen reduction reaction (ORR) electrodes in AHFCs has received limited attention, despite the potential importance of ORR as a rate-determining step. The wettability and pore structure of ORR catalysts are critical factors in facilitating reactant supply (O2 and H2O) and improving catalyst utilization. In this study, we synthesized a hierarchical pore structure in a FeNC catalyst by excess iron incorporation and steam activation, and adjusted its water/oxygen gas contact property by oxalic acid treatment. The modified catalyst exhibited improved wettability and optimized triple-phase boundaries. Remarkably, utilizing a 25 cm2 electrode size and non-noble electrocatalysts, the resulting AHFC achieved a significant power density improvement from 630 to 1240 mW cm−2, setting a new record for AHFCs. This research highlights the importance of pore structure and surface engineering, including the role of excess iron, in enhancing the performance of ORR electrodes in AHFCs.

Tunable electronic properties and Schottky barrier in Janus Ti3C2FO and TMD heterostructures by interface atomic species and disorder

In two-dimensional (2D) Janus MXenes materials, asymmetric disordered surface termination can greatly enhance material functionality. In this work, considering surface termination composition and disorder in 2D Janus MXenes, we designed five models of Janus Ti3C2FO, and then systematically investigated the electronic and contact properties of the corresponding Janus Ti3C2FO and TMD (TMD=WS2, MoS2 and WSe2) heterostructures using the density functional theory method. When the top and bottom surfaces of Janus Ti3C2FO are purely occupied by F and O atoms, respectively, Janus Ti3C2FO shows a pseudogap, which disappears in the presence of the mixed F and O atoms. It was found that the surface termination atomic species and disorder of Janus Ti3C2FO can effectively control the contact nature and Schottky barrier height (SBH) of these Ti3C2FO/TMD heterostructures and lead to a different response of their contact properties to the interlayer distance and external electric field. The presence of pure F atoms at the interface induces in the SBHs a strong dependence on the decrease in interlayer distance, while the pure O atoms causes both ΦB,n and ΦB,p to be almost pinned. Our study provides important new insight into the tuning of contact properties of MXenes/TMD heterostructures.

High temperature operation of Beta-Ga2O3 transistors

We studied the application of Beta-Ga2O3 for high temperature operation up to 500C. Field effect transistors were fabricated using epitaxial films grown on insulating Beta-Ga2O3 substrates. Variable temperature DC measurements were performed in vacuum and air ambient. Measurements revealed a reduction in on/off ratio for the devices due to increase in thermionic emission over the gate/dielectric barrier of MOSFET and over the gate/semiconductor barrier of MESFET. Devices also exhibited detrapping of electrons from interface traps with the increase in temperature. After the devices were tested intermittently at different high temperatures in vacuum or in air ambient, suggesting no change in semiconductor and contact properties.