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We used a finite element method for the computer simulation of the temperature field in the apparatus of hot vacuum pressing (HVP) of alumina powder blanks. We considered the temperature dependence of the properties of the materials of the apparatus elements and the contact thermal and electrical resistances. The results of the calculations agreed to the laboratory temperature measurements, which confirms the reliability of the model. The results of computer experiments show that by selecting the materials of the HVP apparatus elements, we can create a practically uniform temperature field in the sample at the final stage of the sintering process and decrease significantly energy consumption in the entire sintering process. Depending on the arrangement of the equipment of the working cell of the HVP apparatus, the consideration of contact resistances can significantly affect the results of predicting the homogeneity of the temperature field of the sample during HVP. [ABSTRACT FROM AUTHOR]

A computer model is developed on the basis of the finite-element method for the temperature field formed during the sintering of large-size boron carbide products in the working cell of a high-pressure apparatus at a pressure of 0.5–1.0 GPa and temperatures of 1700–1750°C. The dependence of the thermophysical properties of a sample on the porosity and temperature in the process of sintering is taken into account. It is shown that the selection of geometric parameters for the constructional elements of the working cell of a high-pressure apparatus makes it possible to find such a cell structure, whose application may provide a nearly uniform temperature field in a large-size sample under sintering and uniformity of the density and physicomechanical properties of the sintered sample. [ABSTRACT FROM AUTHOR]

The reaction of sulfoxides with peroxy acids in various organic media was studied. The reaction mechanism involves the rapid formation of a sulfoxide-–peroxy acid intermediate which decomposes in the second stage to form carboxylic acid and the corresponding sulfone. The second stage is the rate-limiting step. The reaction medium significantly affects the rate of oxidation. The calculated activation parameters of the oxidation process indicate a compensation effect in the investigated reaction. Correlations between the main physicochemical parameters of solvents and the effective rate constants (k) of dimethyl sulfoxide oxidation with peroxy acids were found. Depending on the reaction conditions, the main factors affecting the k values are specific and nonspecific solvation of the reactants and structural factors. [ABSTRACT FROM AUTHOR]

We study the influence of temperature of a working medium and loading frequency on the contact fatigue of 20KhN3A and ShKh15 steels in oil and in a 3% NaCl aqueous solution. It is shown that, as the loading frequency increases, the contact fatigue of steels increases in a 3% NaCl aqueous solution but does not change in oil. At the same time, as temperature increases, the contact fatigue decreases in a 3% NaCl aqueous solution but remains almost constant in oil. It is shown that the outlined behaviors of the analyzed steels is explained by the action of the components of working media and their concentrations in the surface layers in the course of the tests. [ABSTRACT FROM AUTHOR]

The computer simulation of the temperature field in the working cell of the production unit during high-speed sintering under pressure of boron carbide powder samples is performed by the finite elements method. Consideration is given to the dependence of the thermophysical properties of a powder sample on the porosity and temperature. In order to minimize the temperature drop in the powder sample, the effect of the working cell design on the temperature drop in the sample is studied by computational modeling. Based on the computational experiments, the working cell design of the unit for high-speed sintering under pressure has been optimized and the heating conditions have been selected such that the sample sintering temperature of 2000°C is reached within 80 to 150 s and the temperature drop in the sample during the holding period for 100–150 s is within 50–30 deg. The laboratory experiments have revealed that the optimized working cell design provides an almost uniform distribution of microhardness in the sintered product. [ABSTRACT FROM AUTHOR]

The kinetics of thermal decomposition of didecanoyl diperoxyadipate in different organic solvents have been studied. The primary homolytic dissociation of the peroxide bond (O-O) is accompanied by secondary induced chain decomposition processes. The reaction medium affects both the rate of primary homolytic decomposition and secondary decomposition processes. Correlation equations have been proposed for the rate constants of the reactions under study and physicochemical parameters of the solvents. [ABSTRACT FROM AUTHOR]

Thermal decomposition of peroxydecanoic acid has been investigated in various organic solvents. The reaction medium significantly affects the rate of peroxydecanoic acid thermal decomposition. It has been shown that primary homolytic decomposition of peroxy group is followed by induced radical chain decomposition reactions. The solvent nature influences both primary homolytic decomposition and secondary induced reactions. The correlation equations relating kinetic parameters of peroxydecanoic acid thermolysis and basic solvent properties have been derived. [ABSTRACT FROM AUTHOR]

The paper presents a numerical model for infiltration of porous preforms of ceramic materials in the process of sintering. The mathematical model of infiltration is based on the Richards equation and Van Genuchten formulas describing permeability properties of the porous structure and the infiltrant. A finite-element method and stable implicit finite-difference time scheme are applied for discretization of the Richards equation. Based on the outcome of solving test problems, a comparison of the numerical results with the results obtained by other researchers and with the experimental data, the numerical model has been proved adequate. The paper provides some findings of computer modeling of infiltration of liquid silicon into porous silicon carbide preforms of various geometries and dimensions. [ABSTRACT FROM AUTHOR]

Reaction of epoxidation of α-pinene and Δ-carene with peroxydecanoic acid in various organic solvents was studied. Effective activation energies of oxidation of α-pinene and Δ-carene with peroxydecanoic acid in various media are evaluated. It is shown that reaction medium significantly affects the rate of the process. Correlation dependences connecting the rate of epoxidation with main parameters of solvents are found. [ABSTRACT FROM AUTHOR]

The adsorption of oligo(sebacoyl peroxide) on Aerosil and MgO and benzoyl peroxide on Fe 2O 3, Cr 2O 3, and V 2O 5 has been studied. It has been established that peroxide adsorption on the considered adsorbents is described by the Langmuir equation. Benzoyl-peroxide adsorption increases in the series Fe 2O 3 < Cr 2O 3 < V 2O 5. The process of thermal decomposition of peroxides in the presence of the listed adsorbents has been studied. The overall reaction of peroxide decomposition comprises two components, i.e., the decomposition processes occurring in a solution and on an adsorbent surface. Kinetic and activation parameters of the thermal-decomposition reactions in the solutions and on the surfaces have been determined. [ABSTRACT FROM AUTHOR]

A computer model has been elaborated for the temperature field in a high-temperature furnace for sintering ceramic powder samples under induction and radiation heating conditions. For numerical solution of the set of equations describing the induction and radiation heating processes a finite-element method is used in combination with a finite-difference procedure in time, which is based on Newton-Raphson technique, and a radiosity method is used in modeling the radiation heat transfer. The temperature field in the furnace in the process of sintering has been numerically studied. The numerical results are compared to the data of temperature measurements at the infiltrator surface during the sintering of a composite blend with a silicon-carbide matrix. Based on the agreement between the calculated and experimental results, the computer model is inferred to be adequate. [ABSTRACT FROM AUTHOR]

Bomb calorimetry was used to determine the standard enthalpies of combustion and formation of eight crystalline aliphatic diacyldiperoxides with high molecular weights and low decomposition temperatures. A comparison of the calculated peroxide bond energy with the enthalpies of sublimation of these substances shows that the latter cannot in principle be determined experimentally. [ABSTRACT FROM AUTHOR]

The paper presents the results of numerical and experimental investigations of hardness distribution in specimens of a cutting tool steel body upon induction hardening in media that essentially differ in cooling properties. The heating conditions for the tool body were chosen the same as in the case of induction brazing of a hardmetal insert to a tool body with subsequent hardening. The hardness calculations for various zones of the specimen axial section agree well with the experimental data, thus demonstrating the adequacy of the numerical procedure. [ABSTRACT FROM AUTHOR]

The thermal decomposition reactions of aliphatic peroxy acids containing from 8 to 16 carbon atoms in a molecule were studied. It was found that the carbon radical length had no effect on the thermal stability of peroxide groups. The apparent rate constants of thermolysis of peroxydecanoic acid in various solvents and the activation energies of the test reaction were found. The thermal degradation of peroxy acids involved secondary reactions of induced chain degradation in addition to the primary homolysis of the peroxide group. The rate constants of induced chain degradation were found. [ABSTRACT FROM AUTHOR]

The influence of different solvents on the oxidation reaction rate of pyridine (Py), quinoline (QN), acridine (AN), α-oxyquinoline (OQN) and α-picolinic acid (APA) by peroxydecanoic acid (PDA) was studied. It was found that the oxidation rate grows in the series Py < QN < AN, and the rate of the oxidation reaction of compounds containing a substituent in the α position from a reactive center is significantly lower than for unsubstituted analogues. The effective energies of activation of the oxidation reaction were found. It was shown that in the first stage, the reaction mechanism includes the rapid formation of an intermediate complex nitrogen-containing compound, peroxyacid, which forms products upon decomposing in the second stage. A kinetic equation that describes the studied process is offered. The constants of equilibrium of the intermediate complex formation ( K) and its decomposition constant ( k) in acetone and benzene were calculated. It was shown that the nature of the solvent influences the numerical values of both K and k. It was established that introduction of acetic acid (which is able to form compounds with Py) into the reaction medium slows the rate of the oxidation process drastically. Correlation equations linking the polarity, polarizability, electrophilicity, and basicity of solvents with the constant of the PDA oxidation reaction rate for Py were found. It was concluded that the basicity and polarity of the solvent have a decisive influence on the oxidation reaction rate, while the polarizability and electrophilicity of the reaction medium do not influence the oxidation reaction rate. [ABSTRACT FROM AUTHOR]

The catalytic decomposition of peroxy acids is studied in various solvents in the presence of manganese, cobalt, chromium, nickel, and copper acetates and cerium benzoate. The catalytic activity of the salts in peroxy acid decomposition decreases in the order Mn2+ > Co2+ > Ce3+ > Cr3+ > Ni2+ > Cu2+. The apparent activation energies of the catalytic decomposition of peroxydecanoic acid range between 45.9 and 88.0 kJ/mol. The reaction medium has an effect both on the apparent rate constant and on the activation energy of the reaction. The reaction mechanism includes the fast formation of a catalyst-peroxy acid intermediate complex, which decomposes to release a catalyst molecule and forms the reaction products. The oxidation state of the metal ion of the catalyst can change. The introduction of compounds capable of forming complexes with metal ions of the catalyst substantially slows down the catalytic decomposition. [ABSTRACT FROM AUTHOR]

A scheme of intensive cooling of a portion of the steel holder of a carbide-tipped tool bit during its quenching after induction brazing is put forward. The scheme implies that the cooling of various portions of the holder working surface is carried out in such a way that they should have different martensite content, resulting in a differential of mechanical properties. The scheme is illustrated by some examples of computer modeling of the mechanical properties differential for a 35KhGSA steel holder of a carbide-tipped tool bit in the cases of quenching in aqueous solutions of Na-KMTs polymer and in quenching oil I-20. It is demonstrated that by choosing an appropriate quenchant and cooling scheme one can impart high hardness to the major-wear portion of the holder and high impact toughness to the zone of the tool bit fixing in a milling cutter. [ABSTRACT FROM AUTHOR]

The paper provides some results of prediction of hardness and hardened layer thickness of a 35KhGSA steel holder of carbide tool upon quenching in aqueous solution of salts and bases, in water, and in oils at various quenchant temperatures. Thermokinetic cooling paths of the holder surface points are shown to differ quantitatively and qualitatively between the first two quenchants and oils at room temperature. Numerical investigation of quenching in water at 100°C has revealed an abrupt increase, by a factor of 2–3, in the holder surface cooling rate as soon as the cooling curve enters the austenite-to-bainite phase transformation region. For the case of quenching in an aqueous solution of salts and bases a functional relation has been found between the thickness of the holder surface layer quenched to maximum hardness and the quenchant temperature. [ABSTRACT FROM AUTHOR]

The paper presents some results of numerical prediction of the possibility of hardening a working surface of a steel holder of a carbide tool through quenching in aqueous solutions of salts and bases upon the insert-to-holder induction brazing. The investigation is performed using a previously developed computer model of a temperature field in a carbide tool during its induction brazing, air cooling as it is carried from the induction heater to a quenchant, and cooling in the quenchant. Predictive estimates of the hardened surface layer thickness and the hardness of the holder working surface are made for various quenching conditions. [ABSTRACT FROM AUTHOR]

The paper summarizes the results of numerical experiments to study a temperature field formation in the holder surface of a cutting tool during its induction brazing and further quenching. The author of the paper uses a previously developed computer model of a temperature field in a carbide cutting tool during its induction brazing. It is demonstrated that the entire brazing time can be divided into two stages in terms of the heating rate: the first stage—fast heating—lasting from the beginning until the magnetic transformation temperature is reached, while the second one covers the remaining brazing time. At the first stage the tool holder surface is heated 5–6 times faster than at the second stage. Based on the calculated results, a surface of parameters has been plotted in the three-dimensional space of time—frequency—current in the induction coil; it can be used for choosing appropriate induction brazing conditions. [ABSTRACT FROM AUTHOR]

The process variables (heating rate and duration) of pressure-assisted electrical sintering of a diamond-containing layer to a tool body are proposed to be determined based on the assessment of thickness of the diffusion layers that arise in the course of diffusion in the tool body-powder composition contact zones. A previously developed numerical model for electrical heating of an assembly unit (a macromodel) and a model for the diffusion process (a micromodel), which allows for the temperature dependence of the diffusion coefficient, are employed for that purpose. An example of choosing optimum heating conditions for the pressure-assisted sintering is presented. The results obtained can be used for the optimization of process variables for sintering powder-composition products. [ABSTRACT FROM AUTHOR]

A computer model is proposed for the diffusion process that occurs in the contact zone between a composite powder and a metal base during intensive pressure-assisted electrosintering of the powder onto the base. The model consists of two parts: (i) a model of the temperature field in the powder sample volumes under study, and (ii) a model of the diffusion process in the contact zone between the powder particles and the metal base during the sintering. The data on the time variation of the contact temperature, as calculated in the first part, are then used in the second one for solving the diffusion problem. The model adequacy has been verified by comparing the calculated data and the results of laboratory investigations of the diffusion between a titanium powder sample and a copper plate during the pressure-assisted sintering. The model proposed can be used to study structural alterations in powder composites being bonded to a metal base by intensive pressure-assisted electrosintering as well as to optimize the sintering process variables for powder composites. [ABSTRACT FROM AUTHOR]

We present a complete description of Lie symmetries for the nonlinear diffusive Lotka-Volterra system. The results are used for the construction of exact solutions of the Lotka-Volterra system, which, in turn, are used for solving the corresponding nonlinear boundary-value problems with zero Neumann conditions. The analytic results are compared with the results of computation based on the finite-element method. We conclude that the obtained exact solutions play an important role in solving Neumann boundary-value problems for the Lotka-Volterra system. [ABSTRACT FROM AUTHOR]

A variant of the statistical theory of brittle fracture proposed earlier by V. T. Golovchan is used to calculate the probability of fracture of annular graphite specimens with allowance for the volumetric distribution of thermal stresses in the specimens. The calculated results are compared with data from laboratory studies of the heat resistance of graphite within a broad range of temperatures. It is established that the given variant of the theory is suited for evaluating the reliability of products made of structural graphite when subjected to thermomechanical loading. [ABSTRACT FROM AUTHOR]

It is established that the use of a previously proposed variant of the finite-element method for solving nonlinear nonstationary heat-conduction problems is efficient (in the sense of computer time expenditures) when the half-width of the band β of the resulting matrix in the system of linear algebraic equations does not exceed a certain value of βcr. As a result of a numerical experiment for an axially symmetric case we obtain βcr ≈ 47, which demonstrates the effectiveness of this variant of the finite-element method for solving a wide range of practical problems of heat-conduction theory. [ABSTRACT FROM AUTHOR]