Critical Film Thickness Research Articles

Mixed-lubrication mechanism considering thermal effect on high-pressure to reciprocating water seal

This paper establishes the mixed-lubrication and wear model to reciprocating of water seal under the high-pressure of 100 MPa. Reciprocating movement thermal sources (RMTS) from seal frictional force coupled with the mixed-lubrication and wear contact are investigated. Taking fluid-solid-thermal and wear couple four modules of multi-scale simulates the RMTS effect at the boundary of the water inlet initial to 5 ℃ to 60 ℃. Results show initial temperature with the specific speed significantly changes the seal ratio of the film and solid pressure of bearing capacity, and that of the film pressure still dominates more than 80%. The micro convex-body forms the secondary local elastic-hydrodynamic film pressure distribution appears with the wear process with forming the wedge effect. When the convex-body of wear height exceeds about 25% of the critical film thickness, the seal leakage rate increases with transiting to the water slider bearing to elastic-hydrodynamic lubrication. This systematic multi-scale simulation and measurement of seal surface performance with the micro and macro to RMTS effect improves previous biased point that seal is totally under the wear stage only focusing on the macro aspect of simulation.

Critical Thicknesses of Free-Standing Thin Films of Molten Polymers: A Multiscale Simulation Study.

The free-standing thin films of melted poly(ethylene oxide) have been extensively simulated by the chemically specific coarse-grained (CG) molecular dynamics (MD) method. It is revealed that if the polymer thin film becomes thinner than some critical value, it would initially turn into a fiber, accompanied by an increase in the free surface area and a decrease in surface tension. A simple but efficient scheme is proposed to determine the critical interfacial thickness and the film thickness from the non-intrinsic density and pressure profiles, and the ratio of the two thicknesses is defined as the interfacial fraction. The critical film thickness is found to increase with the number of chains or equivalently the transverse area. With increasing temperature, the critical interfacial thickness increases a bit whereas the critical film thickness slightly decreases, highlighting the important role of the interfacial fraction. For both of the "critical" and "thick" films, the outermost surface layers are confirmed to undergo the greatest movements. The "critical" film exhibits the intrinsic interfacial thickness and bulk density almost identical to those of the "thick" film, dictating the thickness independence of the surface tension. Therefore, the phase stability of the film is essentially determined from the intrinsic thickness of the bulk layer, and the identified temperature dependence of the critical film thickness can be mainly explained by the surface tension.

Manifold death: A Volume of Fluid implementation of controlled topological changes in thin sheets by the signature method

A well-known drawback of the Volume-Of-Fluid (VOF) method is that the breakup of thin liquid films or filaments is mainly caused by numerical aspects rather than by physical ones. The rupture of thin films occurs when their thickness reaches the order of the grid size and by refining the grid the breakup events are delayed. When thin filaments rupture, many droplets are generated due to the mass conserving properties of VOF. Thus, the numerical character of the breakup does not allow obtaining the desired convergence of the droplet size distribution upon grid refinement. In this work, we present a novel algorithm to detect and perforate thin structures. First, thin films or ligaments are identified by taking quadratic moments of an indicator obtained from the volume fraction. A multiscale approach allows us to choose the critical film thickness independently of the mesh resolution. Then, the breakup is induced by making holes in the films before their thickness reaches the grid size. We show that the method improves the convergence upon grid refinement of the droplets size distribution and of enstrophy.

Micro-particle entrainment from density mismatched liquid carrier system

Micro-scale inorganic particles (d > 1 µm) have reduced surface area and higher density, making them negatively buoyant in most dip-coating mixtures. Their controlled delivery in hard-to-reach places through entrainment is possible but challenging due to the density mismatch between them and the liquid matrix called liquid carrier system (LCS). In this work, the particle transfer mechanism from the complex density mismatching mixture was investigated. The LCS solution was prepared and optimized using a polymer binder and an evaporating solvent. The inorganic particles were dispersed in the LCS by stirring at the just suspending speed to maintain the pseudo suspension characteristics for the heterogeneous mixture. The effect of solid loading and the binder volume fraction on solid transfer has been reported at room temperature. Two coating regimes are observed (i) heterogeneous coating where particle clusters are formed at a low capillary number and (ii) effective viscous regime, where full coverage can be observed on the substrate. ‘Zero’ particle entrainment was not observed even at a low capillary number of the mixture, which can be attributed to the presence of the binder and hydrodynamic flow of the particles due to the stirring of the mixture. The critical film thickness for particle entrainment is {h}^{*}=0.16a for 6.5% binder and {h}^{*}=0.26a for 10.5% binder, which are smaller than previously reported in literature. Furthermore, the transferred particle matrices closely follow the analytical expression (modified LLD) of density matching suspension which demonstrate that the density mismatch effect can be neutralized with the stirring energy. The findings of this research will help to understand this high-volume solid transfer technique and develop novel manufacturing processes.

Investigation of Adhesion Properties of Tire-Asphalt Pavement Interface Considering Hydrodynamic Lubrication Action of Water Film on Road Surface.

To obtain the tire–pavement peak adhesion coefficient under different road states, a field measurement and FE simulation were combined to analyze the tire–pavement adhesion characteristics in this study. According to the identified texture information, the power spectral distribution of the road surface was obtained using the MATLAB Program, and a novel tire hydroplaning FE model coupled with a textured pavement model was established in ABAQUS. Experimental results show that here exists an “anti-skid noncontribution area” for the insulation and lubrication of the water film. Driving at the limit speed of 120 km/h, the critical water film thickness for the three typical asphalt pavements during hydroplaning was as follows: AC pavement, 0.56 mm; SMA pavement, 0.76 mm; OGFC pavement, 1.5 mm. The road state could be divided into four parts dry state, wet sate, lubricated state, and ponding state. Under the dry road state, when the slip rate was around 15%, the adhesion coefficient reached the peak value, i.e., around 11.5% for the wet road state. The peak adhesion coefficient for the different asphalt pavements was in the order OGFC > SMA > AC. This study can provide a theoretical reference for explaining the tire–pavement interactions and improving vehicle brake system performance.

An empirical model for the Backscattering coefficient of 1-30 keV electrons from thin film targets

In this paper, the electron backscattering coefficient for normally incident beams with energy up to 30 keV impinging on thin film targets is stochastically modeled using a Monte Carlo simulation.Accordingly, a generalized model describing the realisticbackscattering behaviortaking into account both the atomic number and the thickness for energy up to 30keV is proposed. The obtained results are compared to the experimental and theoretical data, where an excellent agreement is achieved. Moreover, the usefulness of the proposed model as a probe for investigating the electrons backscattered behavior of several materials is thoroughly discussed. It is revealed that the developedmodel allowsidentifying the critical thickness of thin film exhibiting the same electron backscattering behavior as that of a semi-infinite solid, which contributes to an accurate assessment of surface properties of various thin-films.The use of our empirical model enables reducing the simulation time as compared to that of complicated Monte Carlo time consuming simulation.Therefore, the presented model can be implemented to accurately determinatethe electron backscattering coefficient of various thin-film materials with dissimilar thicknesses, making it appropriate for surface analysis applications.

High-mobility two-dimensional electron gas in γ−Al2O3/SrTiO3 heterostructures

The origin of the two-dimensional electron gas (2DEG) in the interface between $\gamma$-Al$_2$O$_3$ (GAO) and SrTiO$_3$ (STO) (GAO/STO) as well as the reason for the high mobility of the 2DEG is still in debate. In this paper, the electronic structures of [001]-oriented GAO/STO heterostructures with and without oxygen vacancies are investigated by first-principle calculations based on the density functional theory. The calculation results show that the necessary condition for the formation of 2DEG is that the GAO/STO heterostructure has the interface composed of Al and TiO$_2$ layers. For the heterostructure without oxygen vacancy on the GAO side, the 2DEG originates from the polar discontinuity near the interface, and there is a critical thickness for the GAO film, below which the 2DEG would not present and the heterostructure exhibits insulator characteristics. For the case that only the GAO film contains oxygen vacancies, the polar discontinuity near the interface disappears, but the 2DEG still exists. In this situation, the critical thickness of the GAO film for 2DEG formation does not exist either. When the GAO film and STO substrate both contain oxygen vacancies, it is found that the 2DEG retains as long as the oxygen vacancies on the STO side are not very close to the interface. The low-temperature mobilities of the 2DEGs in these GAO/STO heterostructures are considered to be governed by the ionized impurity scattering, and $\sim$3 to $\sim$11 times as large as that in LaAlO$_3$/SrTiO$_3$ heterojunction. The high mobility of the 2DEG is mainly due to the small electron effective mass in GAO/STO heterostructure.

Magnetic domain structure of epitaxial Gd films grown on W(110)

We present a detailed real-space spin-polarized scanning tunneling microscopy (SP-STM) study of the magnetic domain structure of Gd(0001) films epitaxially grown on W(110). To find optimal preparation conditions, the influence of the substrate temperature during deposition and of the postgrowth annealing temperature was investigated. Our results show that the lowest density of surface defects, such as step edges as well as screw and edge dislocations, is obtained for room-temperature deposition and subsequent annealing at 900 K. SP-STM data reveal small-size magnetic domains at lower annealing temperatures, evidently caused by pinning at grain boundaries and other crystalline defects. The coverage-dependent magnetic domain structure of optimally prepared Gd films was systematically investigated. For low coverage up to about 80 atomic layers (AL), we observe $\ensuremath{\mu}\mathrm{m}$-size domains separated by domain walls which are oriented approximately along the $[1\overline{1}0]$ direction of the underlying W substrate. Above a critical film thickness ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}\ensuremath{\approx}(100\ifmmode\pm\else\textpm\fi{}20)$ AL, we identify stripe domains, indicative of a spin reorientation transition from in plane to out of plane. In agreement with existing models, the periodicity of the stripe domains increases the further the coverage exceeds ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}$. While the orientation of the stripe domains is homogeneous over large distances just above ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}$, we find a characteristic zigzag pattern at $\mathrm{\ensuremath{\Theta}}\ensuremath{\gtrsim}200$ AL and irregular stripe domains beyond 500 AL. Intermediate minima and maxima of the magnetic signal indicate the nucleation of branching domains. The results are discussed in terms of various contributions to the total magnetic energy, such as the magnetocrystalline, magnetostatic, and magnetoelastic energy density.

Suitability of metallic materials for constructing metal-coated dielectric terahertz waveguides

We aimed to identify metallic materials that could be used to construct metal-coated dielectric terahertz (THz) waveguides. We examined seven different metals: gold (Au), copper (Cu), silver (Ag), aluminum (Al), nickel (Ni), chromium (Cr), and titanium (Ti). The propagation losses of our in-house metal-coated dielectric parallel-plate waveguide (PPWG) were experimentally determined. We developed a physical model to estimate the two key parameters determining the performance of metal-coated waveguides: the critical film thickness required for bulk material-like behavior and the propagation loss in a film with a thickness greater than critical film thickness. Film quality, as revealed by the thickness-dependent electrical conductivity of the metal film, was measured prior to experiments and used for model calculations because propagation loss is influenced by film conductivity, which differs from bulk conductivity and depends on film thickness. After experimentally validating the applicability of the model to different metals, suitable metals were identified based on the two key parameters calculated by the model, assuming the same high film quality. Cu was identified as the optimal metal. The effect of film quality on the two key parameters is discussed in this paper. The impact of the surface oxide (CuOx) layer on THz wave propagation was experimentally evaluated using CuOx/Cu-coated PPWG; no detectable transmittance decrease was observed regardless of the CuOx thickness (1.5–176 nm), when the underlying Cu film was of sufficient thickness. Our model also indicated that a CuOx layer <1 μm-thick had a negligible impact on THz wave propagation. Thus, native oxidation is not an issue when using Cu.

Size effects in the plastic deformation of NiAl thin films

Abstract This paper presents the first study of the plastic behavior of NiAl thin films with thicknesses in the micron and sub-micron range. The influence of geometrical and microstructural constraints was studied for different Al contents (45 to 50 at.% Al) and film thicknesses (0.2 to 3.1 μm) in the temperature range from 20 to 700 °C. The films were deposited on Si substrates and subjected to temperature changes resulting in thermal strains of up to 0.84%. The stress evolution in the films was measured by the substrate curvature method. The residual stress at room temperature of nearstoichiometric NiAl films significantly increased with decreasing film thickness indicating an increase of the film strength. At temperatures above 400 °C strong stress relaxation was found, which was more pronounced in thinner films suggesting the contribution of diffusional creep processes. The existence of a critical film thickness is suggested at which a transition from diffusion-controlled to dislocation-controlled plasticity occurs. The results have potential implications for the reliability of NiAl films as protective coatings in thermal engines.

Misfit stress relaxation in wide bandgap semiconductor heterostructures with trigonal and hexagonal crystal structure

In this work, we consider film/substrate semiconductor heterostructures with a hexagonal (wurtzite) and trigonal (corundum) crystal structure. We show that the differences between the stress level in the α-Ga2O3/α-Al2O3 heterostructure with the corundum crystal structure and the stress level in the GaN/AlN heterostructure with the wurtzite crystal structure do not exceed 50%. We study the effect of Al composition x and growth direction of the heterostructure on the critical film thickness for misfit dislocation formation in α-(AlxGa1−x)2O3/α-Al2O3 heterostructures. We provide a comparison between theoretical calculations of the critical film thickness and experimental data on the film thickness, at which the misfit dislocations were observed in α-(AlxGa1−x)2O3/α-Al2O3 heterostructures.

Analytical solutions of critical oil film thickness of negative spreading coefficient in a capillary corner

In this paper, by combining applications of minimum surface free energy with capillary force balance equations, and considering that the total free energy change is zero at thermodynamic equilibrium, an analytical equation of the oil film critical thickness to spread over a water film in a three-dimensional capillary corner is derived. The derivation is based on the analysis proposed by Dong et al. [Journal of Colloid and Interface Science 172 (1995) 21–36]. This analytical solution allows the critical oil film thickness in three-phase systems to be calculated conveniently without lengthy calculation and be readily applied in simulation of three-phase systems in porous media using predictive models. The equation was examined by comparing the analytical solutions with the numerical results by Dong et al. Using the analytical solution, the reason why the oil phase spreads between the gas and water phases in three-phase systems in porous media was analyzed. The spreading of oil phase is controlled by the free energy change, and the spreading of oil phase only occurs in the system when the free energy change during the oil spreading process is negative. Also, there is a critical oil film thickness controlling this process that is a function of interfacial tension, contact angles, and corner angles. This analytical solution of critical oil film thickness can also be utilized to find the correlation of three-phase capillary pressure when the system is in thermodynamic equilibrium for water-wet porous media. This correlation should be useful in pore-scale simulation of three-phase flow of spreading oil.

Morphology of island structures formed by self-organization processes during melting of lead films

The paper presents the results of the study of the formation of arrays of metal particles arising due to the phenomenon of self-organization during melting of lead films. The lead films from 15 to 300 nm thickness were studied, condensed onto amorphous carbon substrates at room temperature. The excess energy of the initial film, which is the driving force of thermal dispergation, has been determined. It was found that the arrays of particles formed during the melting of lead films with a thickness of less than 150–200 nm have a unimodal size distribution. The existence of a critical mass thickness of the initial film has been found, at which the form of the size dependences of the coverage of the substrate with the film and the density of particles in the sample changes. The presence of the critical thickness is explained by a change in the morphological structure of the initial film at a given thickness.

Stress Relaxation Due to Dislocation Formation in Orthorhombic Ga2O3 Films Grown on Al2O3 Substrates

We analyze the preference of various types of misfit dislocation (MD) formation in film/substrate κ-Ga2O3/α-Al2O3 and κ (AlxGa1–x)2O3/κ-Al2O3 heterostructures. We consider two possibilities for variation in films growth orientation (defined by inclination angle ϑ) for these heterostructures with inclination axes about either [100] or [010] crystallographic directions. We study dependences of the critical film thickness for MD formation on the inclination angle ϑ for heterostructures under consideration. We find the presence of two special orientations (ϑ ~ 26° for [100] heterostructure, ϑ ~ 28° for [010] heterostructure, and ϑ = 90° for both inclination types) of κ-Ga2O3/α-Al2O3 heterostructures, for which the formation of MDs is energetically unfavorable. We show that formation of pure edge MDs is easier for [010] κ-(AlxGa1–x)2O3/κ-Al2O3 heterostructures than for [100] heterostructures, and it is vice versa for mixed MDs in these heterostructures.

Highly Tunable Magnetic and Magnetotransport Properties of Exchange Coupled Ferromagnet/Antiferromagnet-Based Heterostructures.

Antiferromagnets (AFMs) with zero net magnetization are proposed as active elements in future spintronic devices. Depending on the critical film thickness and measurement temperature, bimetallic Mn-based alloys and transition-metal oxide-based AFMs can host various coexisting ordered, disordered, and frustrated AFM phases. Such coexisting phases in the exchange coupled ferromagnetic (FM)/AFM-based heterostructures can result in unusual magnetic and magnetotransport phenomena. Here, we integrate chemically disordered AFM γ-IrMn3 thin films with coexisting AFM phases into complex exchange coupled MgO(001)/γ-Ni3Fe/γ-IrMn3/γ-Ni3Fe/CoO heterostructures and study the structural, magnetic, and magnetotransport properties in various magnetic field cooling states. In particular, we unveil the impact of rotating the relative orientation of the thermally disordered and reversible AFM moments with respect to the irreversible AFM moments on the magnetic and magnetotransport properties of the exchange coupled heterostructures. We further reveal that the persistence of thermally disordered and reversible AFM moments is crucial for achieving highly tunable magnetic properties and multilevel magnetoresistance states. We anticipate that the presented approach and the heterostructure architecture can be utilized in future spintronic devices to manipulate the thermally disordered and reversible AFM moments at the nanoscale.

Wettability and thermal performance of Ga62.5In21.5Sn16 liquid metal alloy on W-coated Cu substrates with varying film thickness

The Ga62.5In21.5Sn16 liquid metal alloy (LMA) has been considered as a promising thermal interface material for electronics cooling. In this work, the Cu substrates were deposited with tungsten (W) films or copper (Cu) films of various thicknesses to form different morphology and composition. The surface morphology and wettability of the filmed Cu substrates were investigated by the scanning electron microscope and drop shape analyzer, respectively. Thermal performances of the Ga62.5In21.5Sn16 sandwiched between filmed Cu substrates were studied using the laser flash analyzer. It was observed that the rough Cu substrate becomes smooth after being deposited with metal film. A decrease in surface roughness leads to the decrease in the contact angle of Ga62.5In21.5Sn16 droplet on filmed Cu substrate and the thermal resistance of Ga62.5In21.5Sn16 sandwiched between filmed Cu substrates. The wettability and thermal performance are strongly dependent on the surface composition. Thermal performance of the Ga62.5In21.5Sn16 sandwiched between the W-coated Cu substrates initially varies dramatically with W film thickness, and then changes slowly beyond a critical film thickness of ∼60 nm. This trend originates from the changing of surface composition arising from the thickening of W films.

Effect of film growth thickness on the refractive index and crystallization of HfO2 film

A series of Hafnium dioxide (HfO2) thin films with nominal thickness of 5–350 nm were reactively deposited on silicon（100）substrates at 200 °C using electron-beam evaporation. Based on the measurement and characterization techniques of spectroscopic ellipsometry, X ray diffraction, scanning electron microscopy and atomic force microscopy, the effect of growth thickness on the refractive index and crystallization of HfO2 film grown under the same conditions was studied. The results indicate that the change of refractive index of HfO2 film is closely related to the change of microstructure i.e., grain size and crystallization which change obviously with the increasing growth thickness. The average refractive index at 633 nm decreases about from 1.97 to 1.84 with the increase of the average size about from 0.5 nm to 7~8 nm and the crystallinity from zero to 74% when the thickness of HfO2 film increases from 5 nm to 350 nm. Meanwhile, a critical growth thickness of HfO2 film, somewhere on order of 130 nm, is confirmed at which the film transforms from an amorphous to a polycrystalline monoclinic structure. The sharp change of microstructure near the critical thickness value of HfO2 film leads to the abrupt change of optical properties of HfO2 film, such as the turning behaviors of the refractive index curve. The correlation between phase transformation, size and orientation of crystallite, packing density and hence the refractive index is established. The possible interpretations are proposed to well understand the underlying mechanism of the evolution of optical properties in HfO2 film-forming process.

Energy of a Drop Required to Break a Liquid Film

An external disturbance can destabilize and break a liquid film on a nonwettable surface. Previous studies focused on evaluating critical film thickness for a spontaneous breakup, but the required energy has been unknown. We experimentally found that the energy of a drop to break a liquid film is an order of magnitude more than that predicted by a free energy balance. Here, we show how to evaluate the energy needed to rupture a liquid film by considering the formation of a crater with a critical size.

Development of a model for evaluating propagation loss of metal-coated dielectric terahertz waveguides

A simple physical model for evaluating propagation loss of a metal-coated dielectric terahertz (THz) waveguide with different metal film thicknesses was developed for those fabricated by three-dimensional printing and film coating techniques. Our model enables a comprehensive understanding of the propagation loss mechanism and two key values: the critical film thickness to behave like the bulk material and loss in a sufficiently thick film. To develop the model, in addition to reflection at the metal–dielectric interface, the thickness-dependent electrical conductivity of the metal film was considered. The model was validated by an in-house multi-channel Au-coated THz parallel-plate waveguide in the lowest transverse-electric mode. The estimated critical thickness of our Au film was 171–207 nm at 0.72–1.4 THz. Our model clarified the contribution of three loss components to the overall loss: penetration loss, ohmic loss by bulk conductivity, and ohmic loss by a decrease in conductivity due to thin-film effects. Evaluation of loss over a broader frequency range (0.03–3.0 THz), which corresponds to fifth- to sixth-generation mobile network, revealed that the critical thickness decreased by up to 1.0 THz but increased above this range due to the transition of the dominant loss component from penetration loss to ohmic loss by a decrease in conductivity. As all three loss components and the critical thickness depend on film quality, a deposition process to yield high-quality films is necessary for high-performance waveguides. Our model is applicable to various waveguides, including rectangular waveguides, at any frequency and with any metal film.

Experimental investigation on effect of water film thickness in unsaturated sandstone cores on CO2 transport during geologic storage

Characteristics of the water distribution and film thickness within pores control CO2 transport and determine the effectiveness of geologic storage. Although studies of adsorbed water films on solid surfaces are abundant, reports about the dependence of CO2 transport behavior on film thickness under geologic CO2 storage conditions are still lacking. In this study, the water film thickness was estimated through water adsorption equilibrium experiments on four tight sandstone samples at varying relative humidities. The calculated results reveal that the adsorbed water exerts a greater influence on smaller pores, as micro- and mesopores are first filled with condensed water, while macropores only contain absorbed water films, and that the measured critical brine film thicknesses are strongly controlled by the clay mineral content. The influence of water film thickness on CO2 transport was evaluated based on the DLVO theory for cylindrical pores. Considering the mobility difference of water films in different pores, the effective disjoining pressure measured by the most probable pore radius can be regarded as the preliminary basis for the selection of reservoir or caprock and evaluation of the sealing performance of rock. Increasing the water film thickness results in a smaller effective pore radius and flowable pore space, and the movement of water in pore channel causes an electroviscous effect. Both conditions have a negative effect on CO2 movement and eventually decrease the CO2 effective permeability.