Cold Substrate Research Articles

Evolutionary selection growth of silver films for low-loss nanophotonic devices

When talking about nanophotonics, quantum computing and sensing high-quality factor plasmonic devices are playing crucial role. In most cases for making such devices one has to use non-lattice matched or transparent amorphous substrates. Plasmonic devices quality factor is mainly determined by ohmic losses, scattering losses at grain boundaries, and in-plane plasmonic scattering losses of a metal – substrate system. We demonstrate here the e-beam evaporation method to deposit ultralow-loss silver thin films on transparent lattice-mismatched substrates. This process corresponds to evolutionary selection growth mode. The key feature of our approach is a precise control during the whole process consisting of film deposition on a cold substrate, self-crystallization and subsequent annealing for stress relaxation. In particular, the stress relaxation leads to further grains growth. We are able to deposit 100 nm thick polycrystalline silver films with micrometer-scale grains, low roughness and ultralow optical losses. Finally, we show ultrahigh-quality factor plasmonic silver nanostructures on transparent lattice-mismatched substrate. This can be of the great interest for high performance and single-molecule optical sensors applications.

Particle-based approach for modeling phase change and drop/wall impact at thermal spray conditions

The ability to model the impact of small, high-velocity molten droplets on a cold substrate is a key to designing the thermal spray process. Such drop-wall interactions involve complex phenomena such as droplet splash, phase change, solidification, substrate melting, and re-solidification. In this study, a particle-based approach, Smoothed Particle Hydrodynamics (SPH), is used to simulate the impact of molten droplets on a cold substrate at the thermal spray conditions, i.e., small droplets with high temperature and velocities. A solidification model is developed to simulate the phase change of the droplet and substrate. The present SPH method is validated against the experimental results using tin droplets on stainless-steel and aluminum substrate, and copper droplet on copper substrate. The histories of the spread factor, the substrate temperature, and the splat height at the impingement point are validated. A parametric study on the impact of the high melting point molybdenum droplets on different substrate materials (including tin, stainless steel, zinc, yttrium stabilized zirconia, and aluminum) is performed. The temporal evolution of the solidification interface and the height/thickness of the solidified splat are reported. Temperature distributions across the splat, substrate, and the corresponding melting/re-solidification are investigated. It was found that the cooling rates for the same impingement velocity were nearly the same for the same substrate regardless of the initial substrate temperatures but were higher for higher impingement velocities. A modified Biot number and thermal diffusivity were used to describe the heat transfer characteristics of the splat substrate combination. Results show that a higher modified Biot number for the splat-substrate combination indicates a faster cooling of the splat and a higher maximum substrate temperature. Furthermore, a substrate with high thermal diffusivity also causes heat at the impingement location to be dissipated quickly, resulting in a fast-cooling rate and accelerated solidification.

Hydrogen Bonding with Hydridic Hydrogen–Experimental Low-Temperature IR and Computational Study: Is a Revised Definition of Hydrogen Bonding Appropriate?

Spectroscopic characteristics of Me3Si-H···Y complexes (Y = ICF3, BrCN, and HCN) containing a hydridic hydrogen were determined experimentally by low-temperature IR experiments based on the direct spectral measurement of supersonically expanded intermediates on a cold substrate or by the technique of argon-matrix isolation as well as computationally at harmonic and one-dimensional anharmonic levels. The computations were based on DFT-D, MP2, MP2-F12, and CCSD(T)-F12 levels using various extended AO basis sets. The formation of all complexes related to the redshift of the Si-H stretching frequency upon complex formation was accompanied by an increase in its intensity. Similar results were obtained for another 10 electron acceptors of different types, positive σ-, π-, and p-holes and cations. The formation of HBe-H···Y complexes, studied only computationally and again containing a hydridic hydrogen, was characterized by the blueshift of the Be-H stretching frequency upon complexation accompanied by an increase in its intensity. The spectral shifts and stabilization energies obtained for all presently studied hydridic H-bonded complexes were comparable to those in protonic H-bonded complexes, which has prompted us to propose a modification of the existing IUPAC definition of H-bonding that covers, besides the classical protonic form, the non-classical hydridic and dihydrogen forms.

Freezing characters study of the sessile seawater drop on a cold substrate

Sea spray icing poses risks to vessels and offshore structures in cold ocean regions. Compared to many research works dealing with the freezing of fresh water, the freezing process of sessile seawater drop was less discussed. The coupled level set and volume of fluid method combined with the enthalpy–porosity method is used to solve the Stefan problem. A two-dimensional (2D) axis-symmetric model is adopted to describe the freezing and temperature variation process. Numerical results were verified by our experiment results. The initial geometric profile of sessile drops was characterized by the Young–Laplace equation. Various salinities within the oceanographic range (10–40 g/kg) were adopted, and results showed that the freezing time increases dramatically with increasing salinity. The influence of the contact angle and substrate temperature in the freezing process was also concentrated. All these results contributed to a better understanding of the icing mechanism on marine surfaces.

Crystallization and photophysical dynamics of Tris(8-hydroxyquinoline) aluminum crystals fabricated by micro-spacing sublimation method

The crystallization and photophysical properties of organic semiconductors have a great influence on the device performance. In this work, micro-spacing sublimation method was used to regulate the crystallization and optical properties of tris(8-hydroxyquinoline) aluminum (Alq3) molecules. It is found that when Alq3 gas molecules contact with the cold substrate, Alq3 molecules aggregate to form small nuclei, and then grow into fiber structure. Micro-spacing distance supplies enough time for Alq3 molecules to assembly into ordered structure. The fibers will further grow to large crystals with the increase of deposition time. The density of trap states is significantly reduced as the crystal size increases owing to the strong π-π molecule interactions, leading to the higher luminescence efficiency. This work will be beneficial to the preparation of organic semiconductor crystals and study of their crystallization process, thus applied to the optimization of device performance.

Early freezing dynamics of an aqueous foam.

Phase-change is an essential and unavoidable process to obtain solid foam. We study experimentally the solidification dynamics of a model aqueous foam in contact with a cold substrate. The substrate temperature, the foam bubble radius and the liquid fraction are changed. We show that the freezing dynamics always starts by following a self-similar square root of time diffusive dynamics. These early dynamics are then predicted as a function of the control parameters using a 1D diffusion model and by treating our foam as a homogeneous fluid with equivalent thermophysical properties. In particular, we build a new expression for the foam conductivity. Finally, experimental and theoretical results are compared and interpreted. This study paves the way towards the understanding of the complex foam freezing dynamics at longer times, when the freezing is then coupled to water migration in the foam.

Karakterizacija prevlake cirkonijum oksida stabilizovanog cerijum i itrijum oksidom deponovane na veznoj prevlaci nikl hrom aluminijum kobalt itrijum oksid

Zirconium dioxide ZrO2 is used as a basic material for thermal barriers and as a biomaterial for manufacturing parts for hip implants in orthopedic surgery and in dentistry for making crowns. Use of ZrO2 ceramics as an insulating material and biomaterial is due to its good chemical and dimensional stability, mechanical strength, toughness and elastic modulus. The aim of this study was to analyze the characteristics of zirconium oxide coating of stabilized cerium and yttrium oxide deposited by atmospheric plasma spraying (APS) process on the bond coating of nickel chromium cobalt yttrium oxide. Coatings were deposited on cold substrates and on substrates preheated to 180°C. Composite powder nickel chromium/aluminum/cobalt/yttrium oxide was used for the production of bond layers and for the ceramic layers zirconium oxide stabilized with cerium and yttrium oxide powder. Testing the quality of coatings was done by measuring microhardness of layers using the HV method and for bond tensile strength of the coating system tensile testing was applied. Metallographic evaluation of the share of pores in the bond and ceramic layers was performed with image analysis - processing of images from a light microscope. Powder particle morphology and EDS analysis were performed on the SEM. The obtained results confirm that the preheating temperature of the substrate has a significant impact on structural mechanical properties of the tested coatings system.

Structure studies of graded amorphous carbon obtained by liquid carbon quenching

A new method for obtaining graded amorphous carbon using quenching of a graphite melt on a diamond substrate is proposed. Using molecular dynamics modeling of liquid carbon quenching on a cold diamond substrate, it is shown that the amorphous carbon obtained in the experiment is a material with a strongly gradient structure and properties along the depth of the sample. This is due to the quenching rate decrease with the distance from the substrate in the range of 1014-1012 K/s. In this case, the density of amorphous carbon varies from 1.50 g/cm3 to 1.93 g/cm3. The spatial change in the structural characteristics of the obtained amorphous carbon was studied: the distribution of carbon atoms according to the degree of chemical bond hybridization (sp1-, sp2-, sp3-), the radial distribution function, the angular distribution function, and a statistical analysis of carbon rings were carried out. It is shown that at a pressure in liquid of 1 GPa, the carbon structure within the quenched zone changes from a highly porous structure with a large number of sp1 chains of carbon atoms near the substrate to an amorphous graphene structure at the periphery. Keywords: amorphous carbon, liquid carbon, quenching, molecular dynamics, radial distribution function.

Main properties of silicides of the Mo - Si system

Using magnetron-ion sputtering, a layer of metallic molybdenum 1–2 μm thick was deposited on the surface of a silicon single crystal grown by the Czochralski method in an atmosphere of an inert argon gas at ambient temperature. According to the results of the experiment, pure Mo layers 2 μm thick deposited by magnetron reactive sputtering from a highly pure metal molybdenum target onto a cold silicon wafer substrate with a thickness of 1.5 mm. They have simultaneously high conductivity and transparency only in the case when a strictly defined deposition rate of molybdenum metal corresponds to a given partial pressure of argon in the volume. The sputtered targets are disks 40 mm in diameter with a thickness of 3-4 mm. The technological cycle of product processing includes the stage of target cleaning. Sputtering of a metal Mo target in pure argon Ar without the addition of oxygen promotes the formation of opaque metal films with very good conductivity. X-ray diffraction analysis of silicon monocrystals with a Mo metal-coated surface revealed compounds Mo3Si and MoSi0.65 in the molybdenum-silicon system. Silicon silicide MoSi2 was found to undergo an allotropic transformation in the temperature range 1850 ÷ 1900 °C, and the low- temperature variety α-MoSi2 has a tetragonal structure. The high- temperature form of β-MoSi2 has a hexagonal structure. The results of the study with an atomic scanning microscope showed that the chains of silicon atoms, connecting with Mo atoms, form zigzags passing through the prisms of the Mo structure along parallel X and Y axes.

Исследование структуры градиентного аморфного углерода, полученного закалкой жидкого углерода

A new method for obtaining graded amorphous carbon using quenching of a graphite melt on a diamond substrate is proposed. Using molecular dynamics modeling of liquid carbon quenching on a cold diamond substrate, it is shown that the amorphous carbon obtained in the experiment is a material with a strongly gradient structure and properties along the depth of the sample. This is due to the quenching rate decrease with the distance from the substrate in the range of 10^14-10^12 K/s. In this case, the density of amorphous carbon varies from 1.50 g/cm^3 to 1.93 g/cm^3. The spatial change in the structural characteristics of the obtained amorphous carbon was studied: the distribution of carbon atoms according to the degree of chemical bond hybridization (sp^1, sp^2, sp^3), the radial distribution function, the angular distribution function, and a statistical analysis of carbon rings were carried out. It is shown that at a pressure in liquid of 1 GPa, the carbon structure within the quenched zone changes from a highly porous structure with a large number of sp^1 chains of carbon atoms near the substrate to an amorphous graphene structure at the periphery.

Reactions in CCl4 films deposited onto a cold gold substrate induced by charge transfer vs (0–5) eV free electrons

Reactions induced by electrons may arise from the charge transfer when molecules are deposited onto a metal substrate or from bombardment. Here, we comparatively study, by using the same experimental set-up, the reactions induced to carbon tetrachloride via the two processes. We found that free electrons bombardment with energy below 5 eV trigger the production of Cl2 as for the charge transfer reaction. From the present measurements, no formation of chlorocarbon chains from the irradiation of carbon tetrachloride films by electrons were observed at low energies.

Asymmetric freezing behaviors of inclined impact of water droplets on cold hydrophobic surfaces

Solidification of impacting water droplets on surfaces impairs the performance and safety of various applications. Prior studies mostly focused on the freezing behaviors of normal impact of water droplets on cold surfaces. In this work, we explore the freezing morphologies and self-peeling behaviors of water droplets when impacting cold hydrophobic substrates with an inclination angle (45°). To improve the coupling between the solidification and drop-scale hydrodynamics, we thus conducted the experiments in a regime where the heat transfer rate is significantly enhanced for fast solidification. In contrast to hydrophilic surfaces, the final freezing morphologies on hydrophobic surfaces are easier to exhibit a rivulet, whose length has a non-monotonic relation with Weber number. The rivulet morphologies can be transited into an asymmetric fingering pattern when Weber number is high enough. More intriguingly, we observe the asymmetric self-peeling behaviors of water droplets upon impacting cold inclined surfaces, where both ice titled direction and degree are highly dependent on Weber number. Moreover, the maximum ice deflection at the edge is found to agree well with a scaling law.

Spreading and shrinking behaviors of oil films with different viscosities on a cold immiscible liquid substrate

The spreading behavior of an oil film on an immiscible liquid substrate plays an important role in practical phenomena such as oil spills and thermal spraying. Especially in extremely cold regions such as the Arctic regions, the spreading behavior and balanced state of the spilled oil may be significantly affected due to the varying oil physical properties caused by the low temperature. The effect of low-temperature conditions on the spreading behavior remains poorly understood. The spreading experiments of silicone oils with viscosities of 50, 100, 350, and 1000 cs were carried out on cold substrates by controlling the liquid substrate temperature in the range of − 20 °C to 20 °C through a circulating bath. The effects of substrate temperature on the spreading morphology, equilibrium thickness, spreading rate and other parameters of the oil film were investigated. When the temperature was greater than 10 °C, silicone oils spread continuously, whereas when the temperature was less than 0 °C, some of them first exhibited a short period of shrinking behavior before spreading. The surface tensions at three interfaces, namely oil–air, oil–substrate, and substrate–air, were analyzed comprehensively. A novel equilibrium equation of the gravity-surface tension based on Neumann’s triangle rule was derived to calculate the stable thickness of the oil film, in which the contact angle at the three-phase junction was measured experimentally, and the quantitative relationship between the surface tension and temperature was considered. The calculated results were in excellent agreement with the experimental results. A modified prediction model was fitted to accurately describe the spreading velocity of the silicone oil by considering the effect of temperature on the viscosity and density. Finally, the variations in the temperature inside the oil film were discussed to explain the shrinking phenomenon. The study provides theoretical support for the creation of oil spill clean-up methods for low-temperature waters.

Contact Line Catch Up by Growing Ice Crystals

The effect of freezing on contact line motion is a scientific challenge in the understanding of the solidification of capillary flows. In this Letter, we experimentally investigate the spreading and freezing of a water droplet on a cold substrate. We demonstrate that solidification stops the spreading because the ice crystals catch up with the advancing contact line. Indeed, we observe the formation and growth of ice crystals along the substrate during the drop spreading, and show that their velocity equals the contact line velocity when the drop stops. Modeling the growth of the crystals, we predict the shape of the crystal front and show that the substrate thermal properties play a major role on the frozen drop radius.

Solidification of liquid metal droplet during impact in the presence of vertical magnetic field

We report a liquid metal droplet impacting onto a cold substrate under the influence of vertical magnetic field numerically. During the impacting dynamics, the spreading and the solidification of the droplet are seriously influenced by the magnetohydrodynamic effects. The numerical methodology is implemented by coupling the volume of fluid method and the implicit enthalpy approach, the former is used to track the liquid/solid–gas interface, while the latter is employed to simulate the solidification process. At first, the numerical method is validated against a series of benchmark problems. Then, by varying the impacting velocities, the thermal contact resistance and the magnetic strengths, the variations of the maximum spreading diameter against different dimensionless parameters are reported. An interpolation scheme between the impacting effect, the thermal effect, and the magnetohydrodynamic effect is proposed to predict the maximum spreading factor, and very good agreement is observed compared to our numerical results. After that, we identify different impacting behaviors in different parameter regimes. For non-isothermal cases, we find that the solidification makes the droplet transit from full rebound to adhesion on the cold substrate, and the participation of the magnetic field promotes the pinch off phenomena during the retraction of the liquid drop. Mechanisms for the transitions between different impacting regimes are discussed, and the comparisons with the available experimental results and analytical solutions are also delivered. At last, we identify that the thickness growth of the solidified splat can be predicted by solving the simple one-dimensional Stefan problem, implying that the thermal dynamics is dominating over the hydrodynamic or the magnetohydrodynamic effects during the melting process of the spreading droplet. Our work therefore provides a general framework to model and study more complex configurations, such as the droplet impacting problems in the metallurgical industry and Tokamak devices, in which environment the droplet dynamics significantly depend on the non-isothermal magnetohydrodynamic effects.

Self-peeling of frozen water droplets upon impacting a cold surface

Freezing of water droplets impacting a cold substrate is a commonly encountered circumstance impairing the performance and safety of various applications. Active methods of ice removal such as heating or mechanical means are energy intensive and inconvenient. Here, we report a passive ice removal method via harvesting the thermal-mechanical stress of ice, leading to the self-peeling of frozen water droplets upon impacting a cold substrate. We find that the frozen ice completely self-peels and is then easily removable from a cold hydrophobic surface whiles the ice exhibits cracking and remains firmly sticky to a hydrophilic surface. The peeling behaviors of frozen water droplets are then scrutinized by varying the subcooling degree, impact parameters and wettability. Moreover, we develop a theoretical model to characterize the peeling and bending behaviors of the ice and also provides a simple criterion to predict the occurrence of complete self-peeling, facilitating the design of anti-icing surfaces.

Modeling of liquid detachment and fragmentation during the impact of plasma spray particles on a cold substrate

The flattening of a droplet impacting a solid surface plays a fundamental role in several industrial applications. In plasma spray, the splats resulting from the impact, spreading, and solidification of molten particles are the building blocks of the spray coatings. Fragmented splats are formed on substrates held at room temperature and atmospheric pressure. Although the formation of a fragmented splat is attributed to the presence of adsorbates on the substrate surface, its dynamics have not been adequately addressed. In this study, a numerical model is developed to investigate the formation of fragmented splats during droplet flattening and solidification in plasma spraying conditions. Compressible Navier-Stokes equations are solved, and the volume of fluid (VOF) method is used to capture the liquid and gas interface. In addition, the source term method is used to capture the solidification process during droplet flattening. In addition, a new boundary condition is defined to consider the effect of gas desorption on the substrate surface after droplet impact. The numerical results show that gas desorption from the surface produces a barrier layer between the droplet and the substrate. This high-pressure region detaches the edge of the spreading droplet from the surface and forms a liquid sheet. The liquid sheet rises above the substrate and spreads up to 2 times more than droplets impacting surfaces without gas desorption. The fragmentation of the liquid film follows the overspreading of the droplet. Moreover, the barrier gas layer reduces the heat transfer rate between the molten particle and the substrate. As a result, only a portion of the initial droplet remains at the location of the impact, which forms a small solidified splat.

Unlocking the sulphur chemistry in intermediate-mass protostars of Cygnus X

Context. The chemistry of sulphur-bearing species in the interstellar medium remains poorly understood, but might play a key role in the chemical evolution of star-forming regions. Aims. Coupling laboratory experiments to observations of sulphur-bearing species in different parts of star-forming regions, we aim to understand the chemical behavior of the sulphur species in cold and warm regions of protostars, and we ultimately hope to connect them. Methods. We performed laboratory experiments in which we tested the reactivity of hydrogen sulfide (H2S) on a cold substrate with hydrogen and/or carbon monoxide (CO) under different physical conditions that allowed us to determine the products from sulphur reactions using a quadrupole mass spectrometer. The laboratory experiments were complemented by observations. We observed two luminous binary sources in the Cygnus-X star-forming complex, Cygnus X-N30 and N12, covering a frequency range of 329–361 GHz at a spatial resolution of 1′′5 with the SubMillimeter Array (SMA). This study was complemented by a 3 mm line survey of Cygnus X-N12 covering specific frequency windows in the frequency ranges 72.0–79.8 GHz at a spatial resolution of 34′′0–30′′0 and 84.2–115.5 GHz at a spatial resolution of 29′′0–21′′0, with the IRAM-30 m single-dish telescope. Column densities and excitation temperatures were derived under the local thermodynamic equilibrium approximation. Results. We find that OCS is a direct product from H2S reacting with CO and H under cold temperatures (T < 100 K) from laboratory experiments. OCS is therefore found to be an important solid-state S-reservoir. We identify several S-species in the cold envelope of Cyg X-N12, principally organo-sulphurs (H2CS, CS, OCS, CCS, C3S, CH3SH, and HSCN). For the hot cores of Cyg X-N12 and N30, only OCS, CS and H2CS were detected. We found a difference in the S-diversity between the hot core and the cold envelope of N12, which is likely due to the sensitivity of the observations toward the hot core of N12. Moreover, based on the hot core analysis of N30, the difference in S-diversity is likely driven by chemical processes rather than the low sensitivity of the observations. Furthermore, we found that the column density ratio of NCS/NSO is also an indicator of the warm (NCS/NSO > 1), cold (NCS/NSO < 1) chemistries within the same source. The line survey and molecular abundances inferred for the sulphur species are similar for protostars N30 and N12 and depends on the protostellar component targeted (i.e., envelope or hot core) rather than on the source itself. However, the spatial distribution of emission toward Cyg X-N30 shows differences compared to N12: toward N12, all molecular emission peaks on the two continuum sources, whereas emission is spatially distributed and shows variations within molecular families (N, O, and C families) toward N30. Moreover, this spatial distribution of all the identified S-species is offset from the N30 continuum peaks. The sulphur-bearing molecules are therefore good tracers to connect the hot and cold chemistry and to provide insight into the type of object that is observed.

Vacuum-UV Photodesorption from Compact Amorphous Solid Water: Photon Energy Dependence, Isotopic and Temperature Effects

Photodesorption from water-rich interstellar ice mantles is known to play an important role on the gas-to-ice ratio in stars and planet formation regions. Quantitative determination of the photodesorption yields in the laboratory is crucial to astrochemical models. This study presents for the first time, the photon-energy dependence of the photodesorption yields from water ice samples in the vacuum-UV (VUV) range. Experiments have been performed with the Surface Processes and ICES (SPICES) setup coupled to the DESIRS beamline at the SOLEIL synchrotron facility (St. Aubin, France). Thick (20–100 ML) compact amorphous solid water ices (H2O and D2O) grown onto a cold Au substrate have been irradiated at sample temperatures ranging from 15 to 110 K. Photodesorption yields of water and photoproducts have been obtained by mass-spectrometry from 7 to 13.5 eV. In interstellar conditions, average H2O photodesorption yields are (5 ± 2) × 10–4 molecule/photon at low temperature (15 K) whereas, lower yields, by a factor of ∼6–9 on average, were found for D2O. This strong isotopic effect can be explained by a differential chemical recombination between OH (OD) and H (D) photofragments at the surface of the samples. In addition, an enhancement of the yields above (70 ± 10) K suggests a thermally induced ice restructuration at this threshold temperature.

Анизотропное насыщение спектра ЭПР радикала DCO, стабилизированного в твердом CO при 4.2 K, и его ориентационное движение

Formyl radicals, DCO, were produced in solid CO matrix and studied by EPR using gas deposition technique on a cold substrate at 4.2 K. The radical electron spin-lattice relaxation was measured to be anisotropic. The anisotropy was found to originate from the anisotropic orientational motion of the radical with the most intensive librations performed around a z axis corresponding to the smallest moment of inertia of the molecule. It was shown that, in solid CO, the libration motion of small molecules is modeled by considering a particle rotating in the orientational potential box. A suggestion was substantiated about the complicated nature of the orientational motion of HCO in the solid CO matrix including the intensive librations and slow tunneling rotation of the molecule around the z axis.