Thermophysical Characteristics Of Materials Research Articles

О среднеквадратичной амплитуде нулевых колебаний атомов в кристалле

It is shown that the values of the energy and amplitude of zero-point vibrations of atoms in a crystal, due to the uncertainty principle, depend on the dynamic characteristics of atoms in the crystal. It was found that the root-mean-square amplitude of thermal and zero-point vibrations of atoms, like other properties, has a periodic dependence on the ordinal number of elements in the Mendeleev's Periodic Table. It is shown that the value of the root-mean-square amplitude of thermal vibrations of atoms in a lattice of elements with a high value of the Debye temperature at room temperature does not differ much from the value of the amplitude of zero-point vibrations of atoms (at T = 0 K). This is explained by the small number of excited vibrations with the maximum frequency in these crystals at room temperature, since the room temperature is much lower than their Debye temperature, at which the entire spectrum of thermal vibrations of atoms in the crystal is excited. The results can be used in materials science and technology to assess the strength and thermo physical characteristics of materials at cryogenic temperatures, without resorting to measuring them directly at absolute zero.

DEPENDENCE OF THE MEDIUM SQUARE AMPLITUDE OF ZERO VIBRATION OF ATOMS IN THE ELEMENTS OF THE PERIODIC SYSTEM OF MENDELEEV ON THE ORDER NUMBER

According to the theory of thermal vibrations of atoms in crystals and the theory of scattering of X-rays and thermal neutrons by real crystals, the root-mean-square amplitude of the zero vibration of an atom in a lattice at T = 0 K is determined by the mass of the atom and the strength of the interatomic relationship. It is inversely proportional to the product of the mass and temperature of the Debye crystal. It was found that the amplitudes of thermal (at T = 300 K) and zero (at T = 0 K) atomic vibrations in elements, like other physical properties, at moderate temperatures have the same periodic dependence on the number of elements. This is due to the weak dependence of the configuration of the external electronic shells of atoms on the temperature. The results can be used in materials science and technology to assess the strength and thermo physical characteristics of materials at cryogenic temperatures through the Debye temperature, without measuring them directly at an absolute zero temperature.

Porous Structures and their Effect on Thermophysical Properties of Thermal Protection Elements

The improvement of the thermal insulating material thermophysical characteristics of the thermal protection elements by studying the porous structure is a promising direction of research. The article describes the effects of the porosity and coupling of the porous structure on the thermophysical characteristics of thermal insulating materials. The article uses standard systematized techniques and instruments of scientific research applied in thermophysics. The research methodology of highly-porous material thermophysical properties is based on performance of empirical laboratory investigations of the samples obtained. It was found that for the pore structure effect on the material characteristics it is rational to use the following complex indices: porosity, number of pores, pore position in space, the pore form, pore formation energy. The article shows the effect of the porous structure on the thermophysical characteristics of the material. The complex parameters of the porous structure, which will allow to develop a new method of control of the porous structure, are proposed. As a result of the experiment planning method, the regression equation of an effective coefficient of thermal conductivity for porous thermal protection structures was developed. It was established that for a more even distribution of the mixture in a volume it is necessary to minimize the size of the dispersed components, thereby increasing the area of their contacts. The experimental method revealed that the moisture evaporation caused the formation of pores inside the clay. The shape of the pores was determined using electron microscope MMP-2P, both on the sample section and surface. The clearest clay has the greatest porosity (no iron oxide and calcium oxide). The pores have a spherical shape in it. The presence of impurities reduces the material porosity due to the increased clay viscosity.

Investigating the features of color laser marking process of galvanic chrome plating in order to create a controlled color image formation at given marking

Abstract The article is devoted to the problems of laser marking of metal surfaces. The factors affecting the color formation during laser processing are considered. To assess the degree of influence of the material thermophysical characteristics on the final result, samples with different properties were used: steel and brass. In order to exclude the influence of other factors on the result, besides the material characteristics, the samples were chrome-plated to obtain the same thickness of the chromium layer. Samples were processed with the same laser parameters and the same marking modes. The results of the experiment showed the formation of various colors on the treated surface, which proves the influence of the thermophysical characteristics of the material being processed on the formation of the color gamma during laser processing.

Methodological Errors of Defining the Thermophysical Characteristics of Materials Using the Laser Pulse Method at High Temperatures

Methodological errors of defining the thermophysical characteristics of construction materials have been evaluated. Methodological errors are caused by heat exchange of a sample with the environment in conditions corresponding to execution of the laser pulse method at high temperatures. Analysis was done of the effect on methodological errors of heat exchange (as a result of natural convection) with the environment. It has been shown that the laser pulse method can be used to determine thermal diffusivity and thermal capacity of highly heat-conductive wire materials (metals and alloys) at temperatures to 1173 K.

Technique of studying thermal diffusivity of metallics in different directions in a field of centrifugal accelerations and forces

Study of the thermal diffusivity of metallics in a field of centrifugal accelerations and forces is essential for aerospace engineering. Characteristics of thermal diffusivity of materials are used in calculations of thermal state of blades and disks of turbine rotors. An original technique and a device on semiconductors have been developed for determination of thermophysical characteristics of materials on an acceleration bench using a vacuum chamber, under centrifugal forces and accelerations. Presented are results on nonstationary heating of heat conductors in the radial and circumferential directions in a field of centrifugal forces and accelerations. Analysis of experimental results shows that the thermal diffusivity of heat conductors grows with rotational speed as compared with a static state without rotation. The thermal diffusivity phenomenon of concern has two components: from centrifugal acceleration and from centrifugal tensile load. From experimental data on the effect of tensile forces it follows that the second component is small. Thus, said thermal diffusivity growth is strongly associated with increase in the velocity of electron drift in ametal under centrifugal acceleration forces.

Thermal Stability Influence of the Enclosure Structure on the Building’s Energy Efficiency

Thermal stability of the enclosure structures is one of the most important objective in the building design. Thermal processes in the wall depend not only on the internal and external air temperature, but also on many other factors. Therefore, complexity of this process make heat conservation in the room to be an actual problem. This paper presents dependence between thermal stability of the enclosure structure and its design. It was shown that thermophysical characteristics of materials directly affect the thermal processes in the wall. For the research, three frequently used types of enclosure structure in Russia were taken. For each wall was found the average temperature and cooling time. As a result, it was found that the higher values of thermal conductivity, specific heat and material density are, the higher average temperature of the wall is.

Quasi-Static Problem of Thermoelasticity for Thermosensitive Infinite Circular Cylinder of Complex Heat Exchange

Nonlinear nonstationary heat conduction problem for infinite circular cylinder under a complex heat transfer taking into account the temperature dependence of thermophysical characteristics of materials is solved numerically by the method of lines. Directing it to the Cauchy’s problem for systems of ordinary differential equations studied feature which takes place on the cylinder axis. Taken into account the dependence on the temperature coefficient of heat transfer that the different interpretation of its physical content makes it possible to consider both convective and convective-ray or heat ray. Using the perturbation method, the corresponding thermoelasticity problem taking into account the temperature dependence of mechanical properties of the material is construed. The influence of the temperature dependence of the material on the distribution of temperature field and thermoelastic state of infinite circular cylinder made of titanium alloy Ti-6Al-4V by radiant heat transfer through the outer surface has been analyzed.

Investigation of the thermophysical properties of oxide ceramic materials at liquid-helium temperatures

The main regularities in the transport of thermal phonons in oxide ceramic materials are investigated at liquid-helium temperatures. The dependences of the thermophysical characteristics of ceramic materials on their structural parameters (such as the grain size R, the grain boundary thickness d, and the structure of grain boundaries) are analyzed. It is demonstrated that, in dense coarse-grained ceramic materials with qR≫1 (where q is the phonon wave vector), the grain boundaries and the grain size are the main factors responsible for the thermophysical characteristics of the material at liquid-helium temperatures. A comparative analysis of the thermophysical characteristics of optically transparent ceramic materials based on the Y3Al5O12 (YAG) and Y2O3 cubic oxides synthesized under different technological conditions is performed using the proposed criterion.

Modeling of Heat Outflows to a Probe in Thermophysical Testing

The nondestructive method developed by the author with the use of a linear pulsed heat source makes it possible to determine the thermophysical characteristics of solid materials on the basis of the portions of thermograms that correspond to the regularization of the thermal regime in the region of the heater and temperature detectors. Consideration has been given to the influence of the outflows of heat to a probe on the error in determining the thermophysical characteristics of materials in the case where two semiinfinite bodies are in ideal contact. A mathematical expression upon the fulfillment of which one may disregard the heat loss to the material of the probe substrate has been obtained. In the work, consideration has also been given to the problem of heat loss to the probe material in the case where it occurs only in the region of the heater. It has been shown that the temperatures of the first and second bodies in the contact area will become closer with time.

Influence of heat flowmeters on thermal measurements

The measurement of heat flows in the engineering field is relevant to thermophysical characteristics of materials in the study of the behaviour of a wall or the achievement of an energy balance in a building. Nevertheless, the utilisation of heat flowmeters in a thermal circuit has to be accompanied by some precautions, notably for materials which are slightly heat conducting. The perturbation induced by the presence of the heat flowmeter is in fact often difficult to evaluate. An analysis method is proposed that allows the definition of optimum conditions of use of two sensor types in the case of conductive exchanges. This analysis is based on a simplified model considering the heat flowmeter as a resistive and capacitive thermal layer placed in series in the studied system. The modelling of these perturbations demonstrates three influential factors: the type of heat flowmeter, the ratio of the temperature of the surface of the studied system to the outgoing heat flow, and the time scale characterising the studied phenomena. An example of the use of heat flowmeters to measure the thermal effusivity of building materials and the measurement of the heat flow inside walls of building is presented.

Mathematical model of thermal destruction of polymer composition materials

Considered is the process of heat propagation in armored plastic both under moderate tem­peratures and binder thermal destruction. Worked out is the method of determining kinematic pa­rameters of the thermal destruction process and thermophysical characteristics of materials when chemical reaction is going on in then. As the result a mathematical model was built which allows to carry out tests of polymer composition materials with the help of computer and to exercise optimal collection of components which would meet the desired conditions

Nondestructive techniques for determining the thermophysical characteristics of materials by the instantaneous-heat-source method

Using the method of a linear instantaneous heal source, we consider nondestructive techniques for determining the thermophysical characteristics of materials. A procedure for estimating errors in the techniques suggested is given.

Contactless nondestructive inspection method and measuring system for the thermophysical characteristics of materials, with adaptive compensation for heat loss

A new method with loss compensation is proposed for contactless measurements of thermal conductivity and heat capacity. A microprocessor unit for implementing the method is described in detail.

Determination of the dynamic thermophysical properties of media

The problems of determining the dynamic thermophysical characteristics of materials and the solution of such problems by harmonic thermal spectroscopy are discussed.

Identification of the constants of heat transfer in arbitrary thermal modes using local heat sources

Exact explicit relations connecting thermophysical characteristics of materials with results of measuring nonstationary values of primary parameters in heating of specimens by local heat sources with power varying arbitrarily in time are presented.

Nondestructive flash measurement of the thermophysical characteristics of materials

Nondestructive flash measurement of the thermophysical characteristics of materials

Investigation of the thermophysical properties of polymers by the temperature wave technique

The technique of temperature waves to measure the complex thermophysical characteristics of materials has been proposed. The relations for the moduli and arguments of these characteristics for the case of plane temperature waves have been calculated. The temperature dependences of the moduli and arguments of complex thermophysical characteristics of polyvinylacetate (PVAc) at different frequencies and also the temperature dependences of its vibrational and configurational heat capacities have been determined. It has been shown that the vibrational heat capacity of PVAc in the region of softening grows sharply and its configurational heat capacity goes through a peak. The segmental mobility has been found to affect the heat transfer in the polymer in an ambivalent way causing both a decrease and increase of thermal conductivity.

Determination of a set of material thermophysical characteristics from data of nonstationary measurements during heating by local heat sources

Exact explicit dependences connecting material thermophysical characteristics to the results of measuring nonstationary values of the primary parameters during specimen heating by local thermal sources with power varying arbitrarily in time are presented.

Mechanical and thermophysical characteristics of materials in fracture

On the basis of aspects of thermodynamics and methods of the theory of dimensions in relative form for solids, relations have been obtained linking the change of the functions of state, i.e., enthalpy and relative volume, with the heat of melting and the work of fracture in single loading. The presented relations describe satisfactorily the conditions of failure in melting various chemical elements, their oxides, nitrides, salts, as well as in single loading of various structural materials in a wide range of temperature changes.