Temperature-dependent Contact Research Articles

SYNTHESIS of P(N-ISOPROPYL ACRYLAMIDE - HYDROXYPROPYL METHACRYLATE) THERMO RESPONSIVE COPOLYMER FILMs BY INITIATED CHEMICAL VAPOR DEPOSITION METHOD

This study illustrates the deposition of thermo responsive p(N-isopropyl acrylamide-hydroxypropyl methacrylate) p(NIPAAm-HPMA) copolymer thin films by initiated chemical vapor deposition (iCVD) method using tert-butyl peroxide (TBPO) as the initiator. Copolymers were deposited at three different HPMA flow rates and the effects of NIPAAm/HPMA flow rate ratio on the deposition rate, structure and responsive properties of the as-deposited films were investigated. The highest deposition rate of 50 nm/min was observed for the copolymer deposited using lowest NIPAAm/HPMA monomer ratio studied. The deposition rate showed a significant increase with decreasing NIPAAm/HPMA flow ratio. Results of FTIR and XPS spectroscopy analyses revealed a significant preservation of structural retention in iCVD p(NIPAAm-HPMA) thermo-responsive films. Lower critical solution temperatures (LCST) of p(NIPAAm-HPMA) films were determined by carrying out a temperature-dependent contact angle analysis. Accordingly, it was shown that LCST was varied between 19 and 23 oC, which was observed to be dependent on the NIPAAm/HPMA monomer ratio. That LCST range is considerably below the literature- reported values for pNIPAAM, which makes the as-deposited copolymer suitable for applications that require thermos-responsive properties at lower temperatures.

Contact rheological DEM model for visco-elastic powders during laser sintering

Laser sintering is a widely used process for producing complex shapes from particulate materials. However, understanding the complex interaction between the laser and particles is a challenge. This investigation provides new insights into the sintering process by simulating the laser source and the neck growth of particle pairs. First, a multi-physics discrete element method (DEM) framework is developed to incorporate temperature-dependent contact rheological and thermal properties, incorporating heat transfer and neck formation between the particles. Next, energy transport by ray tracing is added to allow for computing the amount of laser energy absorbed during sintering. The DEM model is calibrated and validated using experimental data on neck growth and temperature evolution of particle pairs made of polystyrene and Polyamide 12. The findings show that the proposed DEM model is capable of accurately simulate the neck growth during the laser sintering paving the way for better controlling and optimizing the process.Graphical

Influence of temperature on wettability of micro hierarchical rough surfaces: A three-dimensional thermodynamic model

Inspired by the micrometer-scale hierarchical microstructures on the surface of cicada wings, this paper presents a three-dimensional thermodynamic analytical model to comprehensively analyze the superhydrophobicity of rough surfaces with micro-hierarchical structures. Emphasis is placed on the variation of normalized free energy and free energy barrier as a function of apparent contact angle for different wettability states. The effect of temperature-dependent intrinsic contact angle on the intertransition between different wettability states is explored. Additionally, the model allows obtaining the equilibrium contact angle and contact angle hysteresis through corresponding calculations. Importantly, the present model elucidates the influence of the height of the micropillar of the paraboloid on the intertransition between different wettability states at a specific temperature. The geometrical parameters of a droplet in a certain environment, when it is in the final stable wettability state, can be designed based on this model. By conducting a thorough investigation into the effect of temperature on the contact angle of rough surfaces with micro-hierarchical structures, this study offers a fresh perspective for understanding the wettability behavior of droplets at the micrometer scale. Moreover, it provides a theoretical basis for designing rough surfaces with micro-hierarchical structures for application in various environments.

Temperature sensitive contact modes allosterically gate TRPV3.

TRPV Ion channels are sophisticated molecular sensors designed to respond to distinct temperature thresholds. The recent surge in cryo-EM structures has provided numerous insights into the structural rearrangements accompanying their opening and closing; however, the molecular mechanisms by which TRPV channels establish precise and robust temperature sensing remain elusive. In this work we employ molecular simulations, multi-ensemble contact analysis, graph theory, and machine learning techniques to reveal the temperature-sensitive residue-residue interactions driving allostery in TRPV3. We find that groups of residues exhibiting similar temperature-dependent contact frequency profiles cluster at specific regions of the channel. The dominant mode clusters on the ankyrin repeat domain and displays a linear melting trend while others display non-linear trends. These modes describe the residue-level temperature response patterns that underlie the channel's functional dynamics. With network analysis, we find that the community structure of the channel changes with temperature. And that a network of high centrality contacts connects distant regions of the protomer to the gate, serving as a means for the temperature-sensitive contact modes to allosterically regulate channel gating. Using a random forest model, we show that the contact states of specific temperature-sensitive modes are indeed predictive of the channel gate's state. Supporting the physical validity of these modes and networks are several residues identified with our analyses that are reported in literature to be functionally critical. Our results offer high resolution insight into thermo-TRP channel function and demonstrate the utility of temperature-sensitive contact analysis.

Ultraviolet and visible light active photothermal copper bismuth oxide nanostructures for solar steam generation

The CuBi2O4 (CBO) microspheres comprising 1D nanostructures crystallized in a tetragonal crystal system with P4/ncc space group. The Cu and Bi ions in the CBO photothermal harvesters remained in stoichiometric 2+ and 3+ oxidation states, respectively. The CBO showed excellent absorption in the entire ultra-violet (UV) and visible spectrum, covering the prominent solar spectrum. The CBO microsphere formed by assembling 1D CBO nanorods possessed a specific surface area of 1.45 m2/g, and the temperature-dependent contact angle of water over CBO surface decreases upon increasing temperature, making it an excellent choice in solar steam generation (SSG) process. Thus, the presence of CBO-loaded cellulose paper at the air-water interface has demonstrated three times accelerated evaporation rate of 1.7 kg/m2h. Notably, the CBO-loaded cellulose paper delivered high stability with no change in evaporation rate even after consecutive cycles. The CBO exposed to the natural sunlight in the customized desalination setup provided >50 % CR/ER ratio for the salinity of 3.5, 10, and 20 wt % salty waters. Our findings establish that CBO is a highly desirable photo-material for SSG and has the potential to act as a solar light harvester for photovoltaic systems.

Experiment and Analysis on Temperature-Dependent Electric Contact Resistivity of an NI HTS Coil

This paper reports experimental and numerical analysis results of a no-insulation (NI) high-temperature superconductor (HTS) coil in terms of temperature-dependent electric contact resistivity. A test coil was designed, constructed, and operated. The coil is divided into four sections according to the radial locations of inserted voltage taps to measure local voltages. A preliminary experiment was performed to evaluate the electromagnetic properties of the coil in liquid nitrogen. Then, the temperature-dependent contact resistance of the coil was evaluated from 10K to 80K at every 10K increment in a cooling conduction facility. To investigate the impact of the contact resistivity on the charge/discharge dynamics of the coil, we selected two arbitrary current ramp rates values, namely fast (0.5) and slow (0.01) charge/discharge. In this study, the experimental results were analyzed with equivalent circuit and finite element models. The analysis results suggest the following conclusions. First, the contact resistivity of the test coil calculated from the experiment at the coil terminals level increases by a factor 2 with respect to the temperature, ranging between 1.16 to 2.59. Second, although a constant winding tension was applied during the coil fabrication, the contact resistivity was found to increase within different sections (inner to outer radius). Third, the impact of the ramp rate on temperature dependency is negligible.

The microscopic contact morphology of ice crystal on substrate and its effect on heterogeneous nucleation/icing

As one of the long-standing crucial theoretical assumptions of the heterogeneous nucleation and substrate icing theory, the tiny shape of an ice tip on the substrate commands further detailed experimental observation. In this paper, the morphology and growth processes of ice on different near-thermal insulation surfaces at a low supercooling degree were observed by microscope. Experimental results show that the contact angles of the dendritic ice tip on the substrates are all obtuse under a lower supercooling degree, and decrease with temperature as a Sigmoid function. Moreover, the analysis of ice growth on different substrates reveals that the surface property can affect substrate icing by altering the ice tip; that is, as the ice contact angle increases, the radius will increase and the influence of the thermal conduction of substrate on the ice tip will decrease, ultimately leading to a slower growth rate. Significantly, heterogeneous nucleation experiments illustrate that the obtained ice contact angle is close to the ice nucleus contact angle. Consequently, compared with previous theories, the temperature-dependent ice contact angle could be used to accurately predict the nucleation rate and ice growth.

Chitosan Grafted with Thermoresponsive Poly(di(ethylene glycol) Methyl Ether Methacrylate) for Cell Culture Applications

Chitosan is a polysaccharide extracted from animal sources such as crab and shrimp shells. In this work, chitosan films were modified by grafting them with a thermoresponsive polymer, poly(di(ethylene glycol) methyl ether methacrylate) (PMEO2MA). The films were modified to introduce functional groups useful as reversible addition-fragmentation chain transfer (RAFT) agents. PMEO2MA chains were then grown from the films via RAFT polymerization, making the chitosan films thermoresponsive. The degree of substitution of the chitosan-based RAFT agent and the amount of monomer added in the grafting reaction were varied to control the length of the grafted PMEO2MA chain segments. The chains were cleaved from the film substrates for characterization using 1H NMR and a gel permeation chromatography analysis. Temperature-dependent contact angle measurements were used to demonstrate that the hydrophilic-hydrophobic nature of the film surface varied with temperature. Due to the enhanced hydrophobic character of PMEO2MA above its lower critical solution temperature (LCST), the ability of PMEO2MA-grafted chitosan films to serve as a substrate for cell growth at 37 °C (incubation temperature) was tested. Interactions with cells (fibroblasts, macrophages, and corneal epithelial cells) were assessed. The modified chitosan films supported cell viability and proliferation. As the temperature is lowered to 4 °C (refrigeration temperature, below the LCST), the grafted chitosan films become less hydrophobic, and cell adhesion should decrease, facilitating their removal from the surface. Our results indicated that the cells were detached from the films following a short incubation period at 4 °C, were viable, and retained their ability to proliferate.

A temperature dependent power-law drain current model for coplanar OFETs

We present a drain current model for coplanar OFETs considering temperature dependent power-law mobility and contact resistance. This model enables analyzing the electrical characteristics of an OFET based on the spatially random Gaussian disorder model.

Numerical and experimental determination of contact heat transfer during orthogonal cutting

During the cutting process, a large part of the mechanical work is converted into heat, which is dissipated to the environment via the chip, the tool and the workpiece in dry machining. The heat flux partition in the cutting zone depends on the contact heat transfer between the workpiece and the tool. In most cutting simulations to date, the contact heat transfer is assumed to be thermal resistance-free contact, which leads to an overestimation of the heat flux into the tool. In this work, a pressure and temperature dependent contact heat transfer model was derived and implemented in the numerical cutting simulation. The model parameters for dry machining of AISI 1045 with carbide tools were calibrated experimentally. The heat distribution determined with the model provided assured accuracy for orthogonal machining with low uncut chip thickness and thus established a basis for the investigation of the process parameters on the tool temperature.

Exploiting the Nanostructural Anisotropy of β-Ga2O3 to Demonstrate Giant Improvement in Titanium/Gold Ohmic Contacts

Here we demonstrate a dramatic improvement in Ti/Au ohmic contact performance by utilizing the anisotropic nature of β-Ga2O3. Under a similar doping concentration, Ti/Au metallization on (100) Ga2O3 shows a specific contact resistivity 5.11 × 10-5 Ω·cm2, while that on (010) Ga2O3 is as high as 3.29 × 10-3 Ω·cm2. Temperature-dependent contact performance and analyses suggest that field emission or thermionic field emission is the dominant charge transport mechanism across the Ti/Au-(100) Ga2O3 junction, depending on whether reactive ion etching was used prior to metallization. Cross-sectional high-resolution microscopy and elemental mapping analysis show that the in situ-formed Ti-TiOx layer on (100) Ga2O3 is relatively thin (2-2.5 nm) and homogeneous, whereas that on (010) substrates is much thicker (3-5 nm) and shows nanoscale facet-like features at the interface. The anisotropic nature of monoclinic Ga2O3, including anisotropic surface energy and mass diffusivity, is likely to be the main cause of the differences observed under microscopy and in electrical properties. The findings here provide direct evidence and insights into the dependence of device performance on the atomic-scale structural anisotropy of β-Ga2O3. Moreover, the investigative strategy here─combining comprehensive electrical and materials characterization of interfaces on different semiconductor orientations─can be applied to assess a variety of other anisotropic oxide junctions.

Shear-assisted contact aging of single-asperity nanojunctions

Contact aging, i.e., the strengthening of a tribological contact with time, is a phenomenon that is commonly attributed to an increase of the effective contact area between multiasperity surfaces based on contact mechanical processes. Only recently, atomic force microscopy experiments and simulations on single silica nanojunctions have also demonstrated logarithmic aging with contact time for single-asperity contacts and the effects were attributed to a gradual formation of chemical bonds in the contact, thereby suggesting an atomistic mechanism of ``contact aging.'' Generally, this atomic bond-formation process is considered as a shear-assisted thermally activated process, where both temperature and loading conditions are expected to influence the aging behavior. A direct link between aging and temperature has recently been substantiated by temperature-dependent aging experiments and complementary molecular dynamics simulations, but the potential influence of shear stress on single-asperity aging still remains largely elusive. To analyze the role of shear stress, we now apply a simple and semianalytical model for thermally activated bond formation, which explicitly considers the reduction of energy barriers by a shear-stress-related Eyring term. Based on this model, characteristic fingerprints of mechanical shear can be identified in our temperature-dependent contact aging experiments, including, for instance, the counterintuitive decrease of aging effects with increasing temperature. Our results thereby hint at a path to introduce this form of qualitative contact aging to the phenomenological rate and state friction, where the local aging effects might be closely linked to the shear-stress distribution within multiasperity contacts.

Mechanisms of charge carrier transport in polycrystalline silicon passivating contacts

We use temperature-dependent contact resistivity ( ρ c ) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature ( T ann ), (ii) the oxide thickness ( t ox ), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3–1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiO x layer for increasing T ann . This transition occurs for T ann in the range of 850°C-950 °C for a 1.5 nm thick thermal oxide, and 700°C-750 °C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiO x with t ox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for T ann = 1050 °C, whereas no electric current through the SiO x layer could be detected for lower T ann . Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of ρ c were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing T ann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality. • Electron transport mechanisms through the oxide are investigated by temperature-dependent contact resistivity measurements. • A transition from tunneling to pinhole dominant transport was observed at T ann = 900–950°C for a 1.5 nm thick thermal oxide. • For a 1.3 nm thick wet-chemical oxide, the transition was observed at T ann = 700–750°C. • Good passivation quality is preserved even in the pinhole regime, although it degrades at extremely high T ann .

Unraveling the synergistic effects of solid surface material and temperature on the contact angle of water under an elevated pressure: An experimental study

HypothesisIn terms of the Young’s equation, the temperature dependence of liquid-solid contact angle is affected by the surface material, so the wetting behavior could be tuned by both changing the temperature and surface material. However, the synergistic effects of surface material and temperature on the water contact angle remain unclear, especially at elevated temperatures. ExperimentsIn this study, a systematic characterization of water contact angle against various smooth metallic and nonmetallic surfaces was conducted for temperatures up to 300 ℃ in a high-pressure chamber at 15 MPa. The measured results were finally compared with the predictions made by the sharp-kink approximation model. FindingsNot surprisingly, it was observed the temperature-dependent water contact angle is sensitive to the type of solid surface. The temperature coefficients and critical temperature points on the contact-angle-temperature curves can be manipulated by altering the surface material. However, the influence of surface material is weakened by raising temperature, thus leading to the nearly consistent temperature-dependent water contact angle over 120℃. Additionally, the necessity of investigating the internal flows within the water drops was highlighted to unravel the positive temperature correlation of the water contact angle at high temperatures, in view of the presence of non-spherical-cap-shaped drops.

Hybrid quasi-steady thermal lattice boltzmann model for investigating the effects of thermal, surfactants and contact angle on the flow characteristics of oil in water emulsions between two parallel plates

In this work, a two-dimensional hybrid quasi-steady thermal lattice Boltzmann model with temperature-dependent interfacial tension, surfactant and contact angle modules, is used to study the combined multi-physics effects on oil in water (O/W) systems flowing between two-parallel oil-wet plates. The proposed model is used to execute a parametric study for investigating the effects of the droplet radius, source terms, temperature, and surfactants parameters such as concentrations, elasticity, and diffusion coefficients on the flow characteristics of oil in water (O/W) emulsions inside microchannels. Flow conditions such as droplet Reynolds number0.41≤Re≤48, Webber number0.041≤We≤28 dimensionless surfactant surface concentration0.1≤Γ∗≤0.5, and surface Péclet number0.011≤Pes≤31are used in the simulations. The presence of surfactant noticeably enhances the droplets' transportation inside the channel, due to increased droplets' contact angle and deformation. Contaminated droplets with surfactant retain a greater bulk mass center velocity than clean ones to the tune of 2–5 times depending on the simulation conditions. The rise in temperature has a major impact on improving the suspended phase bulk velocity and power number ratio (system's efficiency indicator) for cases with and without surfactant. However, the presence of surfactant at elevated temperatures ameliorates the amount of useful work absorbed by the droplets from the flow significantly. Using specific conditions from the parametric study such as high temperature and surfactants characterized by low diffusion coefficient, results in an effective droplets breakup. The daughter droplets depart from the channel walls by shear lift, which enhances the suspended fluid transportation and increases the power number ratio up to 5.7 times.

Theoretical characterization of the temperature dependence of the contact mechanical properties of the particulate-reinforced ultra-high temperature ceramic matrix composites in Hertzian contact

The novel theoretical models for the temperature dependence of the contact mechanical properties of the particulate composites in Hertzian contact are proposed in this paper, based on an assumption of the temperature-independent constant energy storage capacity for a specific brittle particulate composite, associating with the material yielding, and the theory of Hertzian contact. The yield stress, critical loads for the onset of the brittle and quasi-plastic damage modes, indentation strength, and the more commonly used fracture strength of the composites are expressed in terms of the basic material properties—the temperature-dependent Young’s modulus and specific heat capacity, and the critical flaw size. The models enable the priori and quantitative predictions of the contact damage behavior of the composites during the whole process of the Hertzian contact. The models are validated by comparing against the experimental measurements of the temperature-dependent contact mechanical properties of the particulate-reinforced ultra-high temperature ceramic matrix composites in the temperature range of 25 ∼ 800 °C, which were found probably to be the only reported experimental studies of the particulate composites. The temperature dependence of the contact damage behavior of the composites is revealed to be mainly governed by the temperature-dependent Young’s modulus and specific heat capacity, and the change of the flaw size above a certain temperature point. The models provide additional adequate methods for the determination of the nature and the underlying physical control mechanisms of the damage behavior of the composites at high temperatures.

Temperature-dependent noise tendency prediction of the disc braking system

In this paper, with the temperature-dependent contact coupling stiffness and friction coefficient, a modified closed-loop coupling disc braking model was established. Considering the effective temperature range during the actual braking process, the proposed modified coupling model could predict the high-frequency braking noise tendency with satisfied accuracy. Regarding above, the finite element models of main braking parts were firstly established, all the parts were integrated and connected with the friction coefficient and imaginary springs, and the complex eigenvalue analysis was applied on the closed-loop coupling model to calculate the braking noise tendency, etc. In consequence, the relationships between temperature and two key factors (coupling stiffness and friction coefficient) were investigated. The temperature-dependent contact coupling stiffness and friction coefficient were substituted into the proposed model to predict the noise tendencies, the noise tendencies in different frequency bands varying with temperature were obtained. Finally, the effective temperature range during the actual braking process was extracted from the thermodynamic simulation. Considering the effective temperature range, the modified closed-loop coupling model could accurately identify 87.5% braking noise tendencies, and reach a good consistency with test results.

Drift\u2013diffusion simulation of S-shaped current\u2013voltage relations for organic semiconductor devices

We present an electrothermal drift–diffusion model for organic semiconductor devices with Gauss–Fermi statistics and positive temperature feedback for the charge carrier mobilities. We apply temperature-dependent Ohmic contact boundary conditions for the electrostatic potential and discretize the system by a finite volume based generalized Scharfetter–Gummel scheme. Using path-following techniques, we demonstrate that the model exhibits S-shaped current–voltage curves with regions of negative differential resistance, which were only recently observed experimentally.

The Effects of Aluminum-Nitride Nano-Fillers on the Mechanical, Electrical, and Thermal Properties of High Temperature Vulcanized Silicon Rubber for High-Voltage Outdoor Insulator Applications.

AlN nanoparticles were added into commercial high-temperature-vulcanized silicon rubber composites, which were designed for high-voltage outdoor insulator applications. The composites were systematically studied with respect to their mechanical, electrical, and thermal properties. The thermal conductivity was found to increase greatly (>100%) even at low fractions of the AlN fillers. The electrical breakdown strength of the composites was not considerably affected by the AlN filler, while the dielectric constants and dielectric loss were found to be increased with AlN filler ratios. At higher doping levels above 5 wt% (~2.5 vol%), electrical tracking performance was improved. The AlN filler increased the tensile strength as well as the hardness of the composites, and decreased their flexibility. The hydrophobic properties of the composites were also studied through the measurements of temperature-dependent contact angle. It was shown that at a doping level of 1 wt%, a maximum contact angle was observed around 108°. Theoretical models were used to explain and understand the measurement results. Our results show that the AlN nanofillers are helpful in improving the overall performances of silicon rubber composite insulators.

Influence of the surface temperature on the spreading and receding dynamics of an impacting biodiesel drop

The paper presents an experimental study of the impact of a Jatropha biodiesel drop on a heated flat stainless steel surface at various temperatures and Weber numbers (We). The effect of the surface temperature (TS) on the spreading and the receding of the drop is explored. Entire impact process is recorded using a high-speed camera and the captured images are processed to obtain the evolution of the drop profile and impact morphology at various surface temperatures. The analysis of the morphology provides physical changes in the drop during impact. The maximum spreading factor, βmax (maximum spreading diameter normalized by pre-impact diameter), increases with both We and TS. The receding is found to be dependent on the surface temperature. The average velocity of the receding increases with TS, because of the decrease in surface free energy at higher temperatures. The decrease is explained from the measurement of the temperature-dependent static contact angles. The non-dimensional time taken to reach βmax increases with TS and this increment is more pronounced at higher We. βmax is found to be highly sensitive to We and shows a sharp rise at higher surface temperatures.