The paper presents an experimental study of heat transfer during boiling in a thin horizontal layer of liquid on a capillary-porous coating. A sample of a stainless steel coating was manufactured using additive 3D printing technology with selective laser sintering. An inversion of the boiling curve was observed on the coating, which led to a fivefold decrease in temperature difference compared to an uncoated surface. After 350 h of the test cycle, the heat flux rate at which the boiling curve inversion occurred decreased by a factor of 7.

В работе представлено экспериментальное исследование теплообмена при кипении в тонком горизонтальном слое жидкости на капиллярно-пористом покрытии. Образец покрытия из нержавеющей стали изготовлен с помощью аддитивной технологии 3D-печати методом селективного лазерного спекания. На покрытии наблюдалась инверсия кривой кипения, которая приводила к пятикратному снижению температурного напора по сравнению с поверхностью без покрытия. После 350 ч испытательного цикла значение плотности теплового потока, при котором возникала инверсия кривой кипения, уменьшалось в семь раз.

The parameters of the synthesis of catalysts by the solution combustion method using oxalic acid as a reducing agent were investigated. The activity of the catalysts was determined in the production of hydrogen and carbon nanofibers by the catalytic decomposition of methane. The efficiency of oxalic acid was demonstrated in the preparation of a nickel catalyst (90% Ni/10% Al 2 O 3 ), which does not require the preliminary reduction with hydrogen. Regression analysis identified that the yields of carbon and hydrogen are most strongly affected by temperature among other synthesis parameters.

In this work, experimental data on the heat transfer in a horizontal layer of liquid are obtained for wide ranges of the layer height and reduced pressure. Explosive boiling-up of liquid at low reduced pressures, when significant fluctuations of the pressure, heating surface temperature, and heat flux were observed, is considered. For this condition, the problem of determination of the temperature head is analyzed, as well as uncertainties of its measurement. The experimental data obtained at different layer heights and pressures were compared with the known calculation formulas derived for determination of the heat transfer coefficient during nucleate boiling for analysis of the range of applicability of these formulas. It is shown that at low reduced pressures, the experimental data are generalized well by Yagov’s formula, and the experimental data obtained at moderate reduced pressures at nucleate boiling are generalized well by Gogonin’s formula. Both formulas were obtained for pool boiling. With the help of Pioro’s formula in which we corrected the coefficients and exponents at the Prandtl number and heat flux, we managed to summarize the experimental data over the entire range of the reduced pressure and the liquid layer height.

Carbon nanofibers obtained by catalytic decomposition of methane in a vibrated fluidized bed reactor were chemically treated to increase the specific capacitance of supercapacitors. The material was studied by transmission electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform IR spectroscopy, and cyclic voltammetry. The treatment was conducted in various media (H 2 SO 4 , HNO 3 , H 2 Cr 2 O 7 , and HCl) at 80°C for 6 h. The chemical treatment was accompanied by a significant weight loss (14–67 wt %) and oxidation of the material. Treatment in diluted nitric acid mainly led to the transfer of surface layers of disordered carbon into solution with their removal during washing, while treatment in concentrated acid was more conducive to oxidation of the material surface. The highest specific capacitance (50.6 F g –1 at 2 mV s –1 in 3.5 M H 2 SO 4 electrolyte) was attained when carbon nanofibers were treated with H 2 Cr 2 O 7 .

The AC electrical properties of epoxy composites based on carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) were determined. The conductivity of the composites increases with an increase in the carbon content. It is established that an increase in the specific conductivity is observed when very low concentrations of MWCNTs are added to the epoxy resin/CB composite. The addition of small loadings of MWCNTs makes it possible to increase the low-frequency permittivity values by several orders of magnitude.

Based on analysis of phase diagram data and thermodynamic modeling, we have evaluated the optimal temperature ranges of the processes underlying the preparation of B4C–TiB2 and B4C–ZrB2 composite powders via boron carbide reactions in the presence of excess boron carbide: 2MO2 + (n + 1)B4C + 3C = 2MB2 + 4CO + nB4C (M = Ti, Zr). The values of n have been taken so as to obtain composite powders with the following compositions (mol %): 90B4C–10MB2 (n = 19), 80B4C–20MB2 (n = 9), 75B4C–25MB2 (n = 7), and 70B4C–30MB2 (n = 5.67). We have found temperatures that ensure the preparation of composite powders with tailored composition at various CO pressures. At a CO pressure of 0.0773 MPa, these temperatures for both reactions are 1816 K (~1540°C), independent of the composition of the synthesized powders. The eutectic temperature in the B4C–TiB2 system is ~2200°C and that in the B4C–ZrB2 system is ~2280°C. Thus, at a nearly atmospheric pressure in the reactor, the optimal synthesis temperature of B4C–TiB2 composite powder lies in the range 1540–2200°C and that of B4C–ZrB2 composite powder lies in the range 1540–2280°C. Such powders are potentially attractive for the fabrication of ceramics with improved performance parameters.

The paper discusses the problem of determining the temperature head for calculating the heat transfer coefficient during explosive boiling of a liquid on a smooth horizontal surface under conditions of low pressure. The task of the work is obtaining the method for determining the characteristic pressure for calculating the temperature of saturated vapors. As a result, it is found that the difference between the heat transfer coefficients calculated at the pressure specified at the beginning of the experiment and the pressure averaged over time differ on average by 5%.

The work is devoted to the synthesis and study of porous carbon materials: thermally expanded graphite (TEG), carbon nanofibres, and activated carbon. The possibility of using them in electrochemical current sources, in particular, in supercapacitors, was investigated. The physicochemical properties of the obtained samples were studied by low-temperature nitrogen adsorption, cyclic voltammetry, and electron microscopy. The modification of carbon materials was carried out in three ways, and the effect of the modification on the values of specific capacitance and surface area was studied. It is revealed that the samples are characterized by the well developed porous structure: for initial materials, the maximum surface area was 759 m(2)/g, while for the composites obtained as a result of the modification, it was 187 m(2)/g. It was shown that the highest specific capacitance was typical for the materials possessing micro- and mesoporous structure, with an average pore size of 3-5 nm. Modification of the surface of carbon materials with nickel led to a decrease in the specific surface area, an increase in the average pore diameter, and also, in some cases, to an increase in the specific capacitance. Among the materials with deposited nickel particles, the maximum specific capacitance (116 F/g) was exhibited by a composite of 20 mass % NiO/80 mass % TEG. Treatment with nitric acid and mechanical activation led to an increase in the fraction of micropores in the samples and to an increase in the specific capacitance. The data obtained prove that the studied carbon materials are highly promising for electrochemical applications.

This paper presents experimental data on pressure fluctuations during evaporation/boiling of a thin liquid film under conditions of reduced pressure. The experimental data were obtained as a result of studying heat transfer on a smooth horizontal surface in a wide range of changes in the height of the liquid layer. Using the fast Fourier transform, the power spectra of pressure pulsations versus frequency were obtained. It was found that the power spectra of pressure pulsations differ depending on the mode of evaporation/boiling in the system.

The electrical conductivity of carbon nanofiber–thermally expanded graphite compacted systems at various component mass ratios was investigated. Because the initial carbon nanofibers were not pressed, thermally expanded graphite was added as a binder. The compaction was carried out at a pressure of 11 MPa for 30 min. The AC electrical conductivity was measured in a variable-field frequency range of 25 Hz to 1 MHz during heating from 30 to 100°C. It was found that a decrease in the binder content leads to a narrowing of the electrical conductivity range of the system.

The paper presents the results of the study of evaporation and boiling in a thin horizontal layer of liquid on microstructured surfaces in a wide range of changes in pressure. It is found that the thermal conductivity of materials of microstructured surfaces significantly affects the mechanism of steam removal from the pores and circulation of liquid along the heat transfer surface. It is determined that the pressure change leads to three regimes of heat transfer: evaporation, transition regime, and bubble boiling. The lowest values of the heat transfer coefficients and CHF were obtained in the transition regime; the highest ones were obtained in the bubble regime on both surfaces. Due to the higher thermal conductivity, the higher heat transfer coefficients and CHF were obtained on the bronze coating than on stainless steel over the entire pressure range.

This review paper is devoted to an extended analysis of ammonia gas sensors based on carbon nanomaterials. It provides a detailed comparison of various types of active materials used for the detection of ammonia, e.g., carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and related materials. Different parameters that can affect the performance of chemiresistive gas sensors are discussed. The paper also gives a comparison of the sensing characteristics (response, response time, recovery time, operating temperature) of gas sensors based on carbon nanomaterials. The results of our tests on ammonia gas sensors using various techniques are analyzed. The problems related to the recovery of sensors using various approaches are also considered. Finally, the impact of relative humidity on the sensing behavior of carbon nanomaterials of various different natures was estimated.

The results of studying the effect of temperature and pressure on conversion of methane and the catalyst lifetime during the catalytic decomposition of methane with the formation of hydrogen and nanofibrous carbon on a Ni–Cu catalyst are presented. The pressure varied in the range of 1–10 atm at the temperatures of 600 and 675°C. It was found that when increasing pressure, the total yield of hydrogen increases from the start of synthesis to the deactivation of the catalyst. This effect is manifested the stronger, the higher the temperature of process. It is shown that increasing pressure allows expanding the temperature range of the process without reducing the total yield of useful products.

The paper reports the results of the study of evaporation and boiling regimes of a thin horizontal liquid layer on a modified surface. The formation of various structures was observed at the different heat fluxes, pressures, and heights of the liquid layer. The paper presents maps of the hydrodynamic regimes of evaporation and boiling of thin liquid layers on a modified surface. The regions of existence of various structures observed in the layer are indicated depending on the pressure and heat flux. The results are compared with the calculation dependencies.

The set of epoxy composites based on Alfa Aesar (TM) carbon black (CB) (0-1.5 wt%) has been prepared. Dependence of shaft torque on the rotation frequency in batch mixer with overhead stirrer for epoxy/CB system was obtained. Chemical resistance tests of epoxy resin/carbon black composites were carried out by sample immersion in 30% H2SO4 and 95% H2SO4. The increase of apparent viscosity of epoxy/CB mixture was observed for relatively low temperatures (30-40 degrees C). Immersion of epoxy/CB composite in concentrated H2SO4 induced strong oxidation of sample and mass loss. Dilution of sulfuric acid led to the mass gain of the composite sample due to the increased uptake. The improvement of chemical resistance of epoxy/CB composites was observed in 95% H2SO4. (C) 2020 Elsevier Ltd. All rights reserved.