UDC:546.3.12
INFLUENCE OF TYPES OF CHEMICAL BANDS ON THE STRUCTURE AND PHYSICAL-CHEMICAL PROPERTIES OF METALS AND SEMICONDUCTORS AFTER MELTING.
1Nurov Q.B.., 2Juraev T.J., 3Rustami T.
1 Tajik State Pedagogical University named after S. Ayni
2 Tajik Technical University named after M.S. Osimi
3 Bokhtar State University named after N.Khusrav
Introduction. The type of bonds formed between the elementary particles in the crystal is determined by the electronic structure of the interacting atoms. The elementary particles in the crystal are close to a certain distance, which gives the crystal thermodynamic stability. The distance of particles approaching each other is determined by the interaction of the forces acting on the crystal. Gravitational forces arise as a result of the interaction of electrons with the positively charged nuclei of their own atoms, as well as with the positively charged nuclei of neighboring atoms. Repulsive forces are caused by the interaction of the positively charged nuclei of neighboring atoms when they are brought close together.
Repulsive forces appear at small distances and increase more than gravitational forces. The balance of forces occurs when elementary particles approach each other at a certain distance. This approach corresponds to the minimum continuous energy that makes the crystal thermodynamically stable. It determines the melting point, evaporation temperature, elastic modulus, linear thermal expansion coefficient, etc. All the events mentioned above are in fact physical causes of thermodynamic stability of crystals if the substance exists in the solid state. In this article, we will discuss the effect of the type of bond on the structure and properties of metals and semiconductors after melting. This is how the appearance of bonds changes (or does not change) during the transition from a solid to a liquid (first-floor phase transition).
Currently, the following methods are used to study the structure and properties of substances in solid and liquid states: measurement of electrical conductivity, viscosity, density, differential thermal analysis, metallographic analysis, etc. The listed methods are not reliable enough for objective reasons, they are labor-intensive and often their results are contradictory. Optical methods are widely used to study the structure and properties of transparent liquids. Opaque liquids (metals and semiconductors) do not have such an effective method of optical research. However, the generality of the laws of wave processes allows using the propagation of not only electromagnetic waves, but also other types of waves, including elastic ones. Elastic waves are more effective than light waves in studying atomic and microheterogeneous (microheterogeneity) structure, since all real liquids are always "transparent" in the acoustic sense, and not always in the optical sense. Elastic waves are also distinguished by the fact that the speed of their propagation depends on the physical properties of the particles that make up the medium, and therefore strongly depends on the concentration of the components (components). Considering that the speed of ultrasound propagation is currently determined with an accuracy of 10-4, this characteristic of elastic wave propagation can be effectively used for precise study of the structure of metal and semiconductor melts (including the microheterogeneous region).
Diffraction methods are direct methods of studying the structure, but they cannot provide direct information about the structure of metal and semiconductor melts. Side maxima or peaks on the X-ray scattering intensity curve indicate only the presence of a superposition of two structures in the melt. Processing experimental X-ray data requires certain assumptions. It should be borne in mind that X-ray measurements in high-temperature and chemically active melts are very complex and sometimes impossible.
In [1-4] it is proposed to solve this problem by measuring the speed of ultrasound propagation. At present, acoustic methods (ultrasound) are a powerful tool for obtaining specific information about the types of chemical bonds and structural changes during the transition from a solid to a liquid state. In a condensed state, an elastic pulse propagates from atom to atom via interatomic bonds, so a change in the latter significantly affects the speed of its propagation. As a result, it is clear that the speed of ultrasound propagation is a fine characteristic sensitive to changes in the nature of a chemical bond. Information about the dependence of temperature on the speed of ultrasound propagation, on its structure and changes provides complete and useful information. The use of ultrasonic waves allows us to significantly expand the range of phenomena studied in the study of the equilibrium and kinetic properties of substances, as well as to obtain new information about the features of their molecular structure.
Statistical physics [5-6] shows the dependence of compression and the speed of sound on the molecular structure of substances. Thus, adiabatic compression is equivalent to the following expression:
s V( )s ,
where s is the modulus of adiabatic compression.
Similarly, the isothermal compressibility is related to the Helmholtz energy by a similar method:
T V ( )
In practice, acoustic measurements are used to determine a number of thermodynamic quantities, and primarily to determine adiabatic compression, since there are no other direct methods for measuring in an experiment. The quantity of is related to the speed of propagation of ultrasound ϑ by a simple relation (the Laplace coefficient).
, )
where ρ is the density.
Modern methods of ultrasonic method allow to determine the speed of sound with high accuracy, therefore the values of , calculated by the formula (*), are widely used to determine other thermodynamic quantities associated with it:
+ Т 2( Ср) ,
Where α is the volume expansion coefficient.
From the known values of adiabatic and isothermal compression, it is possible to calculate the thermal compression ratio:
=
Because the speed of sound and adiabatic compression determine the structure of melts and the interactions between ions and electrons. In this regard, studying their temperature dependence provides very valuable information about various types of structural rearrangements and changes in the concentration of conductors.
Methodology of ultrasonic speed measurement
and experimental results
The equipment and methodology for measuring the speed of ultrasound propagation is described in detail in [7]. The functional block diagram of the equipment for studying the structure and properties of melts of metals and semiconductors is shown in Figure 1.
Figure 1. Block diagram of the installation for studying the ultrasonic properties of metal and semiconductor melts.
A high-frequency electric signal from a sinusoidal signal generator (G4-102A) 1 is fed to device 2, in which rectangular pulses with a high-frequency charge of a certain duration are formed from sinusoidal pulses. The pulses are amplified using amplifier 3. The electric waves reach piezoelectric element 6, are converted into ultrasonic waves by elastic waves and pass through the lower sound-conducting tube 7 to the melt 8 located in container 9. Then the ultrasonic wave signals are received through the upper sound-conducting tube 10 and are converted into electric signals by the receiving piezoelectric element 11. After this, the electric signal is fed to one of the inputs of the oscilloscope (C1-70) 4 with a differential amplification unit. A constant signal of the same frequency as that of oscilloscope 1 is fed to the second input of the oscilloscope. These signals are collected in the differential amplification unit, which makes it possible to observe their interference when the phase of the pulse signal changes. This observation is carried out by moving the upper sound tube 10 relative to the lower tube 7 by several ultrasound wavelengths during the melting process. The frequency is determined by an electronic frequency meter 5 (Ч3-34А).
Measurements are made both during heating and during cooling (to avoid a temperature gradient). In this case, the melt is kept at any temperature for at least 30 minutes before measurement. The measurement consists of determining the wavelength, which at a given frequency allows us to determine the speed of propagation of ultrasound. Then the process of determining the speed of propagation of ultrasound is carried out in a similar way, that is, when the distance nλ is reached on the oscilloscope screen, we observe the silence of the general signal.
After recording the total displacement Δh=nλ and setting the frequency f, the speed of propagation of ultrasound is determined by the expression ϑs =f ∆h/n, which is similar to the following formula:
ϑs = f λ ,
where it is λ=∆h/n .
Figure 2 shows the results of measuring the speed of ultrasound propagation depending on the temperature in the indium melt with a purity of 99.999%. Samples of five different melts were studied. Measurements were made at a frequency of 2-4 MHz. Boron anhydride - В2О3, which has the degree of purity "OCЧ" was used as an acoustic binder.
Figure 2. – Results of measuring the speed of ultrasound propagation for In in the liquid state.
Indium is a metal whose melting temperature is 429.7 K. The boiling point for In- is 2345K. As can be seen from Figure 2, the speed of ultrasound propagation decreases with increasing temperature. The measurement results are the same for the 5 investigated samples. The temperature coefficient of ultrasound speed is equal to 0.26 m/s K. For comparison, Figure 2 shows the results of another author [8]. It can be seen that the obtained results are absolutely consistent with the ultrasonic velocity polytherm approach.
It can be concluded from here that the way of bonding of chemical bonds in liquid indium also remains unchanged when the temperature is increased. That is, during the phase transition of the first sex, the crystal-liquid passes without changing the chemical bond. This mode of transition is characteristic of almost all metals.
Figure 3 shows the results of the study of the dependence of temperature on the speed of ultrasound propagation in the TA-1 tellurium melt from 5 different melts.
Tellurium is a silvery white solid with a metallic luster. In the molten state, tellurium is very inactive, therefore, graphite and quartz are used as a container material for studying physico-chemical properties and when melting. Therefore, we used quartz as a vessel or to study the ultrasonic properties of tellurium.
Figure 3. – Results of measuring the speed of ultrasound propagation in liquid tellurium.
The results of measuring the speed of ultrasound propagation for tellurium are presented graphically in Figure 3. It can be seen that at a temperature of 1098 K, the velocity of ultrasound propagation has a maximum, which indicates the presence of several structures during its melting. With a gradual decrease in the maximum values of the dependence of temperature on the speed of ultrasound, the intensity of changes in the structure decreases. At the same time, these structural changes end at certain temperatures.
Therefore, it appears that the melt of tellurium in the liquid state is microheterogeneous and has clusters (formation with a finite lifetime) of different structure and density. In these clusters, after melting, elements similar to the long order structure with the structure of solid tellurium are stored [9].
According to the Hall model, the tellurium structure can be considered as a combination of two structures with different coordination numbers. This is consistent with the conclusions of the chemical-crystalline theory of heredity, according to which, in our opinion, these structures consist of two forms of clusters. They have non-uniform low and high density, and their structure consists of tetrahedral and icosahedral configurations. An experiment to study the speed of ultrasound showed that with increasing temperature, the proportion of denser clusters increases, and this leads to an increase in the speed of ultrasound propagation.
The results of measuring the speed of ultrasound propagation for silicon are shown in Figure 4. In one of the latest studies of the electrophysical properties of silicon in the liquid state [10], it was confirmed that silicon melts like a semiconductor metal. However, these studies do not provide an exact answer to the question of the existence of structural changes with further heating in the liquid state, since the electrophysical properties are determined by both the concentration of charge carriers and their mobility, changes in which are often imperceptible. This problem can be solved by directly studying the structure using diffraction methods, but at such high temperatures these methods are not accurate enough.
The measurements were carried out in a high-purity argon atmosphere. The piezoelectric elements were 10 mm diameter PZT (lead zirconate titanate) ceramic disks with a fundamental harmonic resonance frequency of 2 MHz. Silicon with a purity of 99.9999% Si was used. The materials of the sound-insulating tubes and container were made of quartz. The temperature was determined with a VR 5/20 tungsten-rhenium thermocouple. Acoustic coupling was achieved using 0.01 g of boron anhydride B2O3 according to the technique developed in [11]. When receiving radio-frequency voltage pulses with an amplitude of 20 V on the transmitting element of the piezoelectric, a stable signal with an amplitude of 0.3 V was formed in the receiving piezoelectric element. In the temperature range of ~1950 K, the amplitude of the output signal sharply decreased to 0.003 V. It was still possible to perform measurements with such a signal, but we consider measurements up to 1940K to be reliable, since the amplitude of the output signal was 0.1 V. At each temperature, the sample is kept for 15-20 minutes before evaluation.
The spread of experimental points at a given temperature was 5-7 m/s.
Figure 4 shows the results of our study together with literature data.
Figure 4. – Results of measuring the speed of ultrasound propagation in liquid silicon.
- Information on [12]
- Results of our work
- Information on [14]
Furthermore, the dashed line shows the temperature dependence of the adiabatic compression calculated using Laplace's correlation. Density values are taken from monograph [13]. It can be seen that our results are closer to the data of [12]. However, the speed of sound does not increase linearly as in [12], but with a gradual decrease of the temperature coefficient from 0.65 to 0.15 m/s K. The clear polytherm trend of the ultrasonic propagation velocity reaches its maximum in the temperature range of ~2000K. A gradual decrease in the slope of the temperature dependence of the ultrasound propagation speed indicates that the intensity of structural changes decreases with temperature.
Probably, at the maximum temperature ϑ of the polytherm, which is estimated to be in the region of ~2000K, structural changes are completed. This is especially evident in the dependence of temperature on adiabatic compression. Its slope, as can be seen in Fig. 4, changes faster and, starting from ~1930K, practically does not depend on temperature. Obviously, up to the 2000K region it can be expected to increase with further heating.
Let us consider the results we have obtained for silicon based on the two-structure cluster model of the melt. On the basis of this model, the liquid cement can be represented as a solution of clusters in an atomic matrix, and in the clusters the interatomic bonds are mainly of the covalent type, and in the matrix - the metallic type. According to this model, melt compression
= 0 ( ),
where 0 compression of the atomic matrix, N - the number of atoms in the medium-sized cluster, m - the number of atoms in the matrix, n - the number of clusters, a - dimensionless positive parameter. In addition, a → 1 for N → 1 and a → 0 for N → N pr.. Npr. is a limited number of atoms in a cluster, which will be dispersed upon reaching those clusters. Therefore, it is easy to see from its relation that > 0 for example for any 1 < N < Nпр. As a result, as long as the clusters are dissolved in liquid silicon (or rather temperatures), its compressibility decreases with increasing temperature. This is qualitatively consistent with the nature of the experimental compression curve. Also, when N=1 is reached, the fractional factor in the expression (**) becomes one and becomes = 0 . From this moment, the temperature dependence of the compression should be similar to the compression polytherms of ordinary metals, should increase.
Thus, as a result of studying the dependence of temperature on the speed of ultrasound propagation in liquid silicon, it was determined that in silicon [15]:
1.The restructuring process does not end at the melting temperature.
2.Structural changes continue with further heating in the temperature range ∆Т= ~300 K.
REVIEWER: Qurbonova M.Z.,
Candidate of Chemical Sciences
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INFLUENCE OF TYPES OF CHEMICAL BONDS ON THE STRUCTURE AND PHYSICAL-CHEMICAL PROPERTIES OF METALS AND SEMICONDUCTORS AFTER MELTING
Conditions for thermodynamic stability of the structure of some metals and semiconductors have been identified. Based on experimental results, the conditions of thermodynamic stability are explained from the scientific point of view of modern ideas about the structure of liquids. It can be concluded that the type of chemical bond in liquid indium does not change with increasing temperature. That is, during a first-order phase transition, the crystal-liquid transition occurs without changing the type of chemical bond.
This type of transition is characteristic of all metals. Experimental results for tellurium indicate the existence of several structures during its melting. With a gradual decrease in the maximum values of the temperature dependence of the ultrasound speed, the intensity of their structure changes decreases. At the same time, these structural changes end at certain temperatures. For silicon in the range – 250K above the melting point of silicon, the temperature dependence of the speed of propagation of ultrasound with a frequency of 2 MHz was studied in detail. Analysis of the results obtained using the model theory of compressibility of two-structure cluster liquids shows that structural changes in liquid silicon continue in the studied temperature range.
Key words: thermodynamic stability, solid and liquid state, ultrasonic speed, structure, adiabatic compression, temperature, indium, tellurium, silicon.
Information about the authors: Nurov Kurbonali Bozorovich – Tajik state pedagogical university named after S. Aini, Candidate of chemical sciences, Associate Professor of department of experimental physics. Address: 734003, Dushanbe, Tajikistan, Rudaki Avenue, 121. Phone: (+992) 938-23-65-65. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..
Juraev Tukntasun Juraevich - Tajik technical university named after academician M. Osimi, doctor of chemistry, professor departments of metallurgy. Address: 734042, Dushanbe, Tajikistan, academicians Rajabov avenue 10. Phone: (+992) 919-94-89-24. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..
Rustami Tagaimurod – Bokhtar state university named after N. Khusrav, Post-graduate student of the department of general Physics. Address: 735140, Bokhtar, Republic of Tajikistan, Aini Street, 67. Phone: (+992) 886-66-68-66. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..
Article received 26.08.2024
Approved after review 15.09.2024
Accepted for publication 07.10.2024