Activation Energy Of Viscous Flow Research Articles

Effect of temperature and composition on density and dynamic viscosity of (lanthanide metal salts + urea) deep eutectic solvents

• Lanthanide metal-based type IV deep eutectic solvents (DESs) are prepared. • Density of type IV metal DESs are higher compared to the other urea-based DESs. • Dynamic viscosities of these DESs are lower compared to other urea-based DESs. • Viscosity of the DESs exhibits Vogel − Fulcher − Tammann (VFT) temperature dependence. • Density and viscosity both decrease with increase in mole ratio of urea. Deep eutectic solvents (DESs) are rapidly becoming solvent media of prominence due not only to their unique properties but also to their promising environmentally-benign nature. For effective utilization of these neoteric media, assessment of their physical properties is essential. Density and dynamic viscosity of a class of DESs constituted of a hydrated lanthanide salt and urea at varying composition in the temperature range 293.15 to 363.15 K are presented. Specifically, different possible eutectic mixtures of lanthanum nitrate hexahydrate: urea (named La : Urea), cerium nitrate hexahydrate: urea (named Ce : Urea) and gadolinium nitrate hexahydrate: urea (named Gd : Urea) are investigated. DES (Gd : Urea) at 1: 2 mole ratio exhibits the highest density (1.98783 g.cm −3 at 293.15 K) of all the DESs investigated; density is found to decrease with increasing urea. Density of these DESs decreases linearly with increasing temperature. Dynamic viscosity of these DESs also decrease with increasing urea content. The temperature dependence of the dynamic viscosity follows Vogel − Fulcher − Tammann (VFT) expression. While the activation energy of viscous flow ( E a,η ) for these (lanthanide salt : Urea) DESs is closer to those of (choline chloride + H-bond donor) DESs, their viscosity-temperature dependence is similar to that of common imidazolium ionic liquids.

Effect of B<sub>2</sub>O<sub>3</sub> on ESR Slag Viscosity for Remelting 9CrMoCoB Steel

The viscosity of the slag constituted by 55%CaF2–20%CaO–3%MgO–22%Al2O3–x%B2O3 (mass fraction: x ≤ 3) was measured by rotating cylinder method during the electroslag remelting (ESR) of 9CrMoCoB steel to analyze the effects of the B2O3 content on slag viscosity, break point temperature, and activation energy for viscous flow. Meanwhile, the slag’s structural characteristics were studied by Raman spectrometry. Results indicated that when T ≥ 1663 K, the viscosity changed gently with temperature, showing the value of about 0.055 Pa·s, which was rarely affected by w(B2O3). However, with the increase of w(B2O3) in the slag, the Ca3B2O6 with a low melting point was formed, reducing the slag’s break point temperature from 1628 K to 1563 K. According to the NBO/T value and Raman spectra, B2O3, as a network generating oxide, improved the slag’s degree of polymerization, thus strengthening and complicating its network structure at high temperatures. In addition, as the w(B2O3) increased from 0 to 3%, the activation energy for viscous flow rose from 64 kJ·mol−1 to 94 kJ·mol−1 in the slag.

Life at low Reynolds number Re-visited: The efficiency of microbial propulsion

It has for over 40 years been taken as a truth universally acknowledged that microbial swimming efficiency by flagellar propulsion is fixed by fluid mechanical limitations at 1–2%. And that the missing 98% dissipated as heat is inefficient or wasted. Estimates of such low swimming efficiency make no sense. Microbes have had billions of years to evolve highly efficient swimming; images of microbes in motion show movement with alacrity and maximum speeds of up to 10 body lengths per second, equivalent to the running and swimming speeds of far larger animals. This paradox can be resolved by taking into account the hydrogen-bonded nature of water and how efficient viscous flow over the surface of the animal is established. The minimal requirement for viscous flow is that the activation energy barrier be overcome. The activation energy for viscous flow in water and sea water is the amount of energy required to break 2 hydrogen bonds—breaking apart the dominant water pentamer into the single H2O species, thus greatly reducing the size of the molecular hole required for flow. Microbial swimming efficiency is made highly efficient by devoting some 95% of the energy expended (some must be lost to entropy) into the breaking of hydrogen bonds.

A predictive viscosity model for aqueous electrolytes and mixed organic–inorganic aerosol phases

Abstract. Aerosol viscosity is determined by mixture composition and temperature, with a key influence from relative humidity (RH) in modulating aerosol water content. Aerosol particles frequently contain mixtures of water, organic compounds, and inorganic ions, so we have extended the thermodynamics-based group-contribution model AIOMFAC-VISC to predict viscosity for aqueous electrolyte solutions and aqueous organic–inorganic mixtures. For aqueous electrolyte solutions, our new, semi-empirical approach uses a physical expression based on Eyring's absolute rate theory, and we define activation energy for viscous flow as a function of temperature, ion activities, and ionic strength. The AIOMFAC-VISC electrolyte model's ion-specific expressions were simultaneously fitted, which arguably makes this approach more predictive than that of other models. This also enables viscosity calculations for aqueous solutions containing an arbitrary number of cation and anion species, including mixtures that have never been studied experimentally. These predictions achieve an excellent level of accuracy while also providing physically meaningful extrapolations to extremely high electrolyte concentrations, which is essential in the context of microscopic aqueous atmospheric aerosols. For organic–inorganic mixtures, multiple mixing approaches were tested to couple the AIOMFAC-VISC electrolyte model with its existing aqueous organic model. We discuss the best-performing mixing models implemented in AIOMFAC-VISC for reproducing viscosity measurements of aerosol surrogate systems. We present advantages and drawbacks of different model design choices and associated computational costs of these methods, of importance for use of AIOMFAC-VISC in dynamic simulations. Finally, we demonstrate the capabilities of AIOMFAC-VISC predictions for phase-separated organic–inorganic particles equilibrated to observed temperature and relative humidity conditions from atmospheric balloon soundings. The predictions for the studied cases suggest liquid-like viscosities for an aqueous electrolyte-rich particle phase throughout the troposphere, yet a highly viscous or glassy organic-rich phase in the middle and upper troposphere.

Effect of MgO/Al2O3 and CaO/SiO2 on the Metallurgical Properties of CaO–SiO2–Al2O3–MgO–TiO2 Slag

Melting performance and viscosities of CaO–SiO2–17.5wt.%Al2O3–MgO–2wt.%TiO2 slags were studied. Results show that viscosity and the activation energy for viscous flow (Eη) decrease with an increase of MgO/Al2O3 from 0.20 to 0.54. Liquidus temperature (TL), deformation temperature (TD), flow temperature (TF), and breakpoint temperature (TBr) increase, whereas soft melting interval (TS) increases and then decreases. With the increase of CaO/SiO2 from 1.05 to 1.35, viscosity and Eη decrease. TL, TD, TF, and TBr increase, whereas TS decreases and then remains unchanged. CaO/SiO2 has little effect on the thermal stability of the slags. When CaO/SiO2 is 1.24, MgO/Al2O3 should be 0.31–0.43; when MgO/Al2O3 is 0.37, CaO/SiO2 should be 1.15–1.25. Viscosity in the range of slag discharge temperature is 0.3–0.5 Pa•s, and the fluidity and stability of the slags are superior.

Simulation of Tubular Film Extrusion of Polymer Melts

Abstract A model of the tubular film process is presented which includes power law non-linear rheological behaviour with temperature and crystallinity dependent properties. The heat transfer modelling uses an experimental correlation for the heat transfer coefficient. It is shown that except at low activation energies of viscous flow, the power law exponents have little influence on bubble shape. For activation energies of viscous flow of 11 kcal/mol and more, the temperature dependence of rheological properties dominates bubble shapes at fixed drawdown ratios, blowup ratios and frostline heights. Low activation energies of viscous flow produced wine glass shaped bubbles, while high activation energies cause rapid increases in bubble radius after the die exit.

Investigation of cooling processes of molten slags to develop multilevel control method for cleaner production in mineral wool

Direct use of molten slags in production of mineral wool is considered a positive approach for large-scale absorption of this waste, both economically and environmentally. However, this application has been limited by the poor production stability caused by the complex and variable compositions and properties of molten slags. To improve it, this study developed a novel multilevel control method based on molecular dynamics simulations and experimental investigation of structure and thermophysical properties of molten slags during cooling processes over a broad basicity range. The structural analysis showed as the basicity increased, the Si-related network was depolymerized and the Al-related structure was polymerized, and as a comprehensive result the degree of polymerization of melt decreased. The reflection of this structural change on the process properties was the flowing and crystallization abilities of molten slags increased. Especially, the viscous flow activation energy decreased from 188.43 to 185.15 kJ/mol and the critical cooling rate increased from 5 to 30 °C/s. The mineral wool prepared from molten slags with basicity of 0.8 showed the optimal morphology and mechanical properties overall, with the fibrosis rate of 91%, the mean fiber diameter of 5.39 μm, the tensile strength of 2518 MPa. We further analyzed the relationship between structure, rheological and crystallographic properties, and product quality. Based on these, method of determining initial composition, temperature schedule, and cooling rates for cleaner and high-efficient production was developed from the structural simulation, viscosity-temperature curve and time-temperature-transformation curve.

Evaluation of rheological and aging behavior of modified asphalt based on activation energy of viscous flow

Styrene–butadiene–styrene (SBS) is one of the most widely used polymer modifiers for the improvement of the durability of asphalt pavement. SBS functionalization can improve the low-and high-temperature performance and aging resistance of asphalt binders. This paper systematically explores the feasibility of the activation energy of viscous flow (Ea) in evaluating the rheological and aging behavior of functionalized SBS-modified asphalt binders, including SBS-, styrene–ethylene/butylene–styrene (SEBS)-, SEBS-grafted maleic anhydride (MAH)-, and SBS/organic montmorillonite (OMMT) nanocomposites- modified asphalts (SBS-A, SEBS-A, MAH-A, and OSBS-A for short). The Ea values of different functionalized materials were calculated by Arrhenius equation based on Dynamic Shear Rheometer (DSR) and Rotational Viscosimeter (RV) tests, which means that the viscous flow behavior were analyzed at different temperature ranges. Ea based on RV test, the rutting factor and rutting factor aging index came to the same conclusion, that is, SEBS-A and OSBS-A with higher Ea values (lower temperature sensitivity) show excellent rheological properties and better aging resistance, followed by SBS-A, MAH-A, and base asphalt (BA). Besides, Ea based on DSR test was higher than Ea based on RV test, which is related to physical phase (visco-elastic phase and viscous phase). From the perspective of the viscous flow behavior of the binders, Ea based on RV test provides scientific and effective methods for probing the effect of SBS functionalization and designing high capability materials for pavement.

Viscometric and thermodynamic study of aspirin in aqueous ethanol media at four different temperatures

The present study was conducted to investigate the effect of concentration and temperature on viscosity of aspirin solution. In order to achieve better bioavailability of drug, it is necessary to make it soluble. Aqueous ethanol solutions have been taken as solvents. Aspirin solutions with different concentrations were prepared in aqueous ethanol. Viscosity values of the solutions were determined experimentally by using Ostwald viscometer at four different temperatures. The viscosity (η) data were used to calculate relative viscosity (ηr), Jone-Dole’s coefficient (BJ) and Falken Hagen coefficient (AF). Activation energy of viscous flow for the solvent (Δμ10∗), as well as for the solute (Δμ20∗) and other thermodynamic parameters like entropies (ΔS20∗) and enthalpies (ΔH20∗) of viscous flow were also calculated. All these physico-chemical parameters are important for understanding the molecular interactions present in the drug solutions. It has been found that strong solute–solvent interactions prevail over solute–solute interactions in the drug solutions. Negative values of (dBJ/dT) reveal that the drug aspirin acts as a structure promoter for the solutions under study.

Thermophysical properties of hexapropyl and hexabutyl phosphoramides in n-dodecane

Hexapropyl phosphoramide (HPPA) and Hexabutyl phosphoramide (HBPA), which are structural analogs of Tributyl phosphate, are the novel extractants developed for the processing of spent nuclear fuel. For the commercial use of the above extractants in reprocessing plants, data on the thermophysical properties such as density, viscosity, interfacial tension (IFT) and radiation stability are needed to develop engineering-level flow sheets for liquid-liquid extraction process. In this work, we estimated the density, viscosity and IFT of binary mixtures of HPPA and HBPA in n-dodecane pre-equilibrated with 3.5 mol.kg−1 HNO3 and also without pre-equilibration. In addition, the effect of concentration and temperature on the thermophysical properties of these solvent systems was also investigated. Both the density and viscosity of HPPA and HBPA decrease with an increase in temperature and dilution with n-dodecane. The thermal expansion coefficients of neat extractants were lower than their diluted counterpart. From the variation of viscosities with temperature, the activation energy for viscous flow was calculated and increased with extractant concentration. The IFT of HBPA and HPPA systems was higher and the zirconium retention capacities of the irradiated phosphoramides were lower than the TBP system, indicating the feasibility of HPPA and HBPA for actinide separation processes.

Effect of TiO2 substituting SiO2 on the rheological and crystallization behavior of mold slags for casting Ti-containing steel

In the current study, the influence of TiO 2 substituting SiO 2 on the rheological and crystallization behavior of mold slags for casting Ti-containing steel was investigated through the measurement of the viscosity-temperature relationship, crystallization process and initial crystallization temperature. The structure of molten slags from the Raman spectrum and other parameters (NBO/T and Optical basicity) was compared to explain the difference of slag properties. With the substituting of SiO 2 by TiO 2 in slags from 5% to 20%, the viscosity of these slags at 1573 K decreased from 0.36 Pa s to 0.10 Pa s, and the viscous flow activation energy below the break temperature had the tendency to decrease. With the increase of the TiO 2 /SiO 2 ratio from 0.25 to 4.00, the initial crystallization temperature of slags increased and the crystallization of slags were enhanced, which was related to the difference of the crystallization mode and the precipitation of dispersed crystals as nuclei. With the substituting of SiO 2 by TiO 2 , the parameter NBO/T and optical basicity of molten slags increased, the complex network structure by the Raman spectra depolymerized gradually and the degree of polymerization decreased, which was related to the destruction of silicate/aluminate structure by the addition of TiO 2 in slags. Hence, the substitution of SiO 2 by TiO 2 in slags simplified the network structure of mold slags, then lowered the viscosity and promoted the crystallization of slags.

Effect of Fe/SiO2 Ratio and Fe2O3 on the Viscosity and Slag Structure of Copper-Smelting Slags

Secondary copper smelting is an effective means of treating waste resources. During the smelting process, the viscous behavior of the smelting slags is essential for smooth operation. Therefore, the effects of Fe/SiO2 ratio and Fe2O3 contents on the viscous behavior of the FeO−Fe2O3−SiO2−8 wt%CaO−3 wt%MgO−3 wt%Al2O3 slag system were investigated. The slag viscosity and activation energy for viscous flow decrease with increasing Fe/SiO2 from 0.8 to 1.2, and increase as the Fe2O3 content increases from 4 wt% to 16 wt% at Fe/SiO2 ratio of 1.2. However, under the conditions of Fe/SiO2 of 0.8 and 1.0, the viscosity and activation energy for viscous flow show a minimum value at Fe2O3 content of 12 wt%. Fe2O3 exhibits amphoteric properties. In addition, the increase in Fe2O3 content raises the breaking temperature of the slag, while the Fe/SiO2 ratio has the opposite effect. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy show that increases in Fe/SiO2 ratio lead to simplification of the silicate network structure, while increases in Fe2O3 content improves the formability of the network. This study provides theoretical support for the related research and application of secondary copper smelting.

Effect of water on amine-based deep eutectic solvents (choline chloride + monoethanolamine): Structure and physicochemical properties

Water, can not only regulate the thermodynamic properties of deep eutectic solvent (DES), but also affect its supramolecular structure. Therefore, it is necessary and meaningful to study the effect of water on DES. In this work, amine functionalized DES (choline chloride + monoethanolamine) was successfully prepared, and the densities and viscosities of DES + H2O were measured from 293.15 K to 323.15 K. The effects of DES composition, water content and temperature on the excess molar volume, partial molar volume, apparent molar volume, limiting molar volume, coefficient of thermal expansion, viscosity deviation and activation energy for viscous flow were investigated. The hydration and structural changes of DES were analyzed. Additionally, the Redlich-Kister equation was used to correlate the excess properties of DES + H2O. Finally, a three-stage theory of effect of water on DES, namely the pure DES stage, the destruction stage and the thorough destruction stage, was proposed. Conclusively, it can be seen that an appropriate amount of water is very helpful to improve the properties of DES, otherwise the structural advantages of DES will be lost. It is hoped that this study will lay a foundation for the application of DES.

Evaluating the Effect of MgO/Al2O3 Ratio on Thermal Behaviors and Structures of Blast Furnace Slag with Low Carbon Consumption

In order to clarify the effect of the MgO/Al2O3 ratio on the fluidity of a low-alumina blast furnace slag system, the influence law of slag fluidity with different MgO/Al2O3 ratios was studied based on the composition of blast furnace slag through a viscosity experiment and themodynamic software. By studying the effect of the MgO/Al2O3 ratio on the activation energy of viscous flow of slag combined with FT-IR, the effect of the MgO/Al2O3 ratio on the thermal-stability of low-aluminum slag was interpreted from the microstructure level. Results indicated that the viscosity and the melting temperature of slag both showed a gradual downward trend due to the increase of the MgO/Al2O3 ratio. Besides, the temperature stability of the low aluminum slag became more stable due to the depolymerization of the complex structure of slag. Considering the actual operating conditions of blast furnace, the MgO/Al2O3 ratio of slag was suggested to be controlled to 0.60 and the basicity to be no higher than 1.20 under the conditions of this investigation. Industrial test results showed that the coke rate could be saved as 3.49 kg/t when the MgO/Al2O3 ratio decreased from 0.70 to 0.58.

DIRECTED REGULATION OF THE STRUCTURAL AND RHEOLOGICAL PROPERTIES OF HEAT-INSULATING ACRYLIC AQUEOUS DISPERSIONS USING THE COMBINED USE OF HYDROPHILIC AND HYDROPHOBIC SILICATE FILLERS

The formation technology and performance characteristics of coatings based on aqueous dispersions are largely determined by the properties of the initial film-forming materials, which should ensure uniform thin-layer distribution on the substrate surface and the formation of coatings with the required technological complex of properties. Among them, due to their functional properties and relatively low cost, the most widespread are water-dispersion polymer coatings based on acrylic film-formers. In this paper, mathematical models of the structural and rheological dependences of heat-insulating acrylic aqueous dispersions are considered depending on the combined content of hydrophilic-hydrophobic fillers. To describe these dependencies, it is advisable to use equations of the second degree. According to the mathematical theory of experiment, the second-order orthogonal central compositional design makes it possible to predict the behavior of the response function. Carrying out an experiment in accordance with this plan makes it possible to establish the analytical dependence of the response function on the corresponding factors in the form of a polynomial equation of the second degree. The main response functions were: conditionally static yield stress, viscosity at the minimum rate of onset of fracture (initial effective viscosity), viscosity of the “destroyed” structure according to the Newtonian nature of the flow, activation energy of viscous flow at minimum, average and maximum shear rates. On the basis of the established dependences, the optimal ratios of hydrophobized aerosil and aluminosilicate microspheres were selected, the combined use of which makes it possible to reduce shear stresses to create a homogeneous aqueous acrylic dispersion, to predict the activation energy at various technological stages of preparation and application of heat-insulating coatings. The established results made it possible to create a hydrophilic-hydrophobic aqueous acrylic dispersion, which, without the use of surfactants, makes it possible to simplify the production technology of heat-insulating water-dispersion coatings, namely, to exclude the stage of pretreatment of fillers, to reduce the rotation speed of the frame mixer, and also to increase the kinetic stability of the finished dispersion.

Rheological Behavior of High Density Polyethylene (HDPE) Filled with Corn Stalk Biochar

AbstractCorn stalk (CS) was used as raw material to prepare biochar. Corn stalk biochar /HDPE composite (CSB/HDPE) was prepared by extrusion method with 3 wt% MA added. The effect of corn stalk biochar (CSB) on the rheological properties of HDPE under different conditions was studied. The interface performance of each component with HDPE was studied by using the Harmonic‐Mean interface equation. The purpose is to provide guidance for the processing and molding of materials by studying the rheological properties and interface properties of the materials. The results showed that, compared with CS, the interfacial tension between CSB and HDPE was lower; Research on dynamic rheological properties showed that an appropriate amount of CSB can improve the processing performance of HDPE; With the increase of CSB content, the stability of the composite system decreased, but the elasticity and viscosity increased correspondingly. The steady‐state rheological properties showed that shear induced compatibility occurs in the composite system at high shear rates; Compared with HDPE, 50 %CSB/HDPE was more sensitive to temperature and had higher viscous flow activation energy. In addition, the tensile strength and flexural strength of the composite material reached the maximum when the CSB content was 50 %, which were 28.69 MPa and 30.14 MPa, respectively. Overall, the study of rheological properties of materials provides guidance for the application of CSB in polymers and broadens the way of waste resources utilization.

Viscosity and structure evolution of bearing‐BaO slag melt with the low CaO/SiO2 mass ratio of 0.7

AbstractThe effect of BaO on the viscosity of experimental slag with the CaO/SiO2 ratio of 0.7 was studied based on the rotating cylinder method, and the structure evolution analysis was performed using Fourier transform infrared (FT‐IR), X‐ray photoelectron spectroscopy (XPS) and 27Al MAS‐NMR spectra. The results indicated that the viscosity of molten slag decreased gradually with BaO content rising from 0 wt.% and 5 wt.% due to the dominant effect of free O2‐ rather than Ba2+. The viscous flow activation energy (Eη) of molten slag was calculated, presenting a similar change trend with that of viscosity. The structure analysis demonstrated that, with the increasing BaO content, the [SiO4]‐tetrahedral structures and Si‐O‐Al bonds were destroyed due to an increase in the relative fraction of free oxygen (O2‐). For the Al‐related structural units, the 27Al magic angles spinning nuclear magnetic resonance (27Al MAS NMR) spectra analysis illustrated that the concentration of [AlO4] units reduced, whereas that of [AlO5] and [AlO6] units increased because of the increase of free oxygen and nonbridged oxygen (O0). The results of structure analysis agreed well with the viscosity variations of experimental slag.

Development of natural deep eutectic solvents (NADESs) as anti-freezing agents for the frozen food industry: Water-tailoring effects, anti-freezing mechanisms and applications

Nature-inspired natural deep eutectic solvents (NADESs) as anti-freezing agents including Pro:Glc (5:3), Pro:Glc (1:1), Pro:Sor (1:1), and Urea:Glc:CaCl2 (3:6:1) were prepared. Viscosity (η), conductivity (σ), activation energy of viscous flow (Eη) and conduction (Eᴧ), transverse relaxation time (T2), thermal behaviours, and anti-freezing capacities of the NADESs were investigated. A critical T2 of 24.60 ms for η changes was obtained, and the relationship between η and T2 was determined as lnη = -1.398lnT2 + 10.688. Differentialscanningcalorimetry and low-field nuclear magnetic resonance analyses indicated NADESs could hinder the molecular motion as temperature decreased through enhancing the hydrogen-bonding strength, endowing them with excellent anti-freezing capacity. NADESs showed varied Eη (41.58 ∼ 45.72 kJ mol−1) and Eᴧ (48.31 ∼ 63.08 kJ mol−1), of which Pro:Sor (1:1) possessed the greatest ones, showing its greatest temperature sensitivity and best anti-frosting capacity. Applications in frozen chicken breast further announced the potentials of NADESs as anti-freezing agents for the industry.

Viscosity of silica and doped silica melts: evidence for a crossover temperature

AbstractSilica is known as the archetypal strong liquid, exhibiting an Arrhenius viscosity curve with a high glass transition temperature and constant activation energy. However, given the ideally isostatic nature of the silica network, the presence of even a small concentration of defects can lead to a significant decrease in both the glass transition temperature and activation energy for viscous flow. To understand the impact of trace level dopants on the viscosity of silica, we measure the viscosity‐temperature curves for seven silica glass samples having different impurities, including four natural and three synthetic samples. Depending on the type of dopant, the glass transition temperature can vary by nearly 300 K. A common crossover is found for all viscosity curves around ~2200–2500 K, which we attribute to a change of the transport mechanism in the melt from being dominated by intrinsic defects at high temperature to dopant‐induced defects at low temperatures.

Relationship between Kinematic Viscosity and Cluster Size in Multicomponent Metal Melts

We analyzed the temperature dependences of the kinematic viscosity and density of Fe73.5Cu1M3Si13.5B9 melts, where M = Nb, Mo, V, and Cr, in the temperature range from 1450 to 1950 K using the transition state theory. It is shown that the activation energy of viscous flow is proportional to the particle size on a natural logarithmic scale. The lowest viscosity and the highest free volume has the Nb melt. In melts with Mo, V, and Cr, the structural units of viscous flow upon heating and cooling are clusters about 0.6 nm in size. In a melt with Nb, at the initial stage of heating, the vibrations of individual atoms prevail, the movement of which creates viscosity. After heating the Nb melt above the critical temperature of 1770 K, the viscous flow is associated with clusters about 1 nm in size. At the cooling stage, the cluster structure of the Nb melt is retained up to a temperature of 1450 K.