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https://doi.org/10.29013/AJT-22-3.4-44-51
Nurov Kurbonali Bozorovich, Candidate of Chemical Sciences, Associate Professor of the Department of Experimental Physics, TSPU named S. Ayni
Dzhuraev Tukhtasun Dzhuraevich, Doctor of Chemical Sciences, Professor, of the Department of Metallurgy of the TTU named acad. M. S. Osimi Jafarov Amirsho Sayobidovich, PG student, of the Department of Technology and Mechanical Engineering TSPU named after. S. Ayni
INVESTIGATION OF THE REGION OF EXAMINATION OF MELTS IN SYSTEMS In - BVI (BVI - S, Se, Te) BY THE ACOUSTIC METHOD
Abstract. The temperature dependences of the propagation velocity of ultrasound in exfoliating melts of the In-S, In-Se and In-Te systems have been studied. Supercritical phenomena have been found, consisting in an anomalous decrease in the speed of ultrasound with decreasing temperature as one approaches the delamination dome in a short temperature range. It has been established that these anomalies increase gradually as the melt concentration approaches the critical one. The coordinates of the critical point have been established.
Keywords: ultrasonic velocity, delamination, melt, system, delamination dome, critical composition and temperature.
Introduction
The study of stratification of liquids is of interest for at least two important areas: the first is the construction of the stratification dome on the phase diagram and the second is the study of the features of the second-order phase transition liquid-liquid.
The optical method is widely used to study the phenomenon of stratification and to study the structure in transparent liquids. The difference in refractive indices in separating liquids, and the resulting specificity of light propagation, makes it possible to visually observe the boundary between them. Opaque liquids (metal and semiconductor) lack such an effective research method as optical. However, the generality of the laws of wave processes makes it possible to use the propagation of not only electromagnetic waves, but also other types ofwaves, in particular elastic ones. Elastic waves have even greater
possibilities than light in studying the atomic structure, as well as in studying the separation of liquids, since all real liquid media 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 strongly depends on the inertial properties of the particles that make up the medium and, consequently, on the concentration of the components. Considering that the speed of propagation of ultrasound is currently measured with an accuracy of 10-4, it is possible to effectively use this characteristic of the propagation of elastic waves for the precision study of liquids.
Although diffraction methods are methods of direct study of the structure, nevertheless, they do not provide direct information about the structure of melts of metals and semiconductors. Side maxi-
ma or sagging on the scattered X-ray intensity curve only indicate that a superposition of two structures is possible. Processing of experimental X-ray data requires well-known assumptions [1-2]. At the same time, it should be borne in mind that X-ray measurements in high-temperature and chemically aggressive melts are rather complicated, and sometimes not realizable.
Currently, acoustic research is a powerful tool for obtaining information about the structure of melts of metals and semiconductors. In condensed media, the elastic impulse propagates from atom to atom through interatomic bonds, and therefore the change in the latter significantly affects the rate of its propagation. Therefore, the speed of propagation of ultrasound is a fine characteristic, sensitive to changes in the nature of the chemical bond. To study the stratification of liquids and the stratification process itself, a method is needed that makes it possible to measure the ultrasound velocity in each of the existing layers, moreover, at different distances from the boundary between the layers. This method is pulsephase, which allows you to work on a transmitted wave and change the acoustic base [3-4].
Technique for measuring the velocity of ultrasound in exfoliating melts
The essence of the acoustic method lies in the fact that the propagation velocity of ultrasound $s is measured depending on the height h of the liquid column and the so-called $s - h characteristic is analyzed at a fixed temperature. The set of $s - h characteristics at different temperatures and for melts of different initial concentrations provides complete experimental information on the melt separation, i. e., all the necessary data for constructing a separation dome from experimental points.
On (pic. 1) A diagram of the installation for measuring the velocity of propagation of ultrasound is presented. The pulse generator 1 modulates the RF voltage of the sinusoidal signal generator 2 into a sequence of rectangular packets with high-frequency filling, which excite the piezoelectric elements 3 of the comparison cell and the measuring cell in parallel (dashed lines). Ultrasonic pulses through the lower fixed sound ducts 4 probe the reference liquid (distilled water) 5 in the container 6 and the investigated melt 7 in the container 8.
Picture1. Functional diagram of the installation for measuring the velocity of propagation of ultrasound in melts
On (pic. 2) representations the scheme ofthe mea- of ultrasound in a separating melt and the nature of the suring cell for determining the velocity of propagation change in velocity with height are presented.
Picture 2. Scheme of the measuring cell for determining the velocity of propagation of ultrasound in the melt and the nature of its change in the height of the melt in the presence of stratification into two liquid phases.
By moving the upper movable sound duct with the help ofa micrometer screw at a distance A h — nk, we get the opportunity to fix the value of the velocity of propagation of ultrasound in the area An.
By successively probing the melt by moving the upper sound duct, one can establish a change in the velocity of propagation of ultrasound along the height of the investigated melt and detect its jump when passing through the phase boundary between regions 4 and 5. In (pic. 2) on the left schematically shows how a sinusoidal plane wave propagates in the melt from the lower to the upper sound duct, the length of which in the lower layer is greater than in the upper one. The spatial distribution of the wave phases has a stationary character, i.e. at any moment of time, a multiple of the period of oscillation nr in a plane located at an arbitrary distance from the bottom of the container, the same phase of the wave is realized.
When moving the upper acoustic duct down to a distance n X (in this case n = 2 ) on the screen of an oscilloscope with a differential amplification unit, the second input is fed with a coherent sinu-
soidal signal from the same generator that generates the total displacement A h = nX probing the melt and setting frequency f, speed of ultrasound, we find the voltage, n extinctions of the total signal are observed. Registering according to the ratio A h
$s = f-, which is identical to the obvious for-
n
mula Ss= f • Since the wavelength, defined as A h
X =-, is a component of the melt thickness Ah,
the velocity Ss also refers to this melt volume. By probing the melt in the region of delamination at different temperatures, one can detect the disappearance or appearance of a boundary between the layers, i.e. to fix the temperature of the beginning of delamination, and also, using the values of the velocity of propagation of ultrasound in the first and second layers at each given temperature, it is possible to plot the vs dependence above the delamination region for alloys of a certain composition and extrapolate it to the intersection with the vs dependence curve along the delamination dome.
Experimental results
The authors of[l-6] carried out the study using the methods of differential thermal, microstructural and X-ray analysis and the method of measuring mi-crohardness. The alloys were synthesized in evacuated quartz ampoules using In and elemental Se with a purity above 99.9% (by weight).
For the In- Se system, indium (ln-000) and selenium-was were used as starting materials for the preparation of alloys. h. Samples were fused in evacuated to - 10-4 Pa and sealed quartz ampoules. At the melting temperatures of indium and selenium, the samples were kept for 2 hours, the main fusion was carried out at 950K for 3 hours with intensive mechanical stirring, and finally, they were cooled in air while shaking the ampoules until the samples solidified. The measurements were carried out in a high-purity argon atmosphere in the frequency range 1-3 MHz.
On (pic. 3) presents the results of an experimental study of the ultrasound propagation velocity Ss as a function of the height h of the liquid column of a
sample with the initial composition InQ 83 Se0 at various temperatures. It can be seen that at 930 and 917 K (lines 1 and 2, respectively), the $-h characteristics are straight lines parallel to the h axis, the speed of propagation of ultrasound does not depend on height, which indicates the homogeneity of the melt.
Picture 3. The change in the velocity of propagation of ultrasound along the height of the liquid column in the melt of the initial composition In0 83 Se017 1-5 correspond to temperatures of 930, 917, 910, 903, 893 K
But at 910 K on the $s - h - characteristic (line 3) there is a velocity jump of ultrasound. The step unambiguously establishes the fact of stratification of the melt into two liquid phases, which differ in the velocity of ultrasound propagation. Further, at 903 and 893 K, the step size A$s increases successively (Pic. 3, lines 4 and 5), which indicates an increase in the concentration gap in the coexisting layers with decreasing temperature.
As seen in (pic. 3), steps $s - h - characteristics 3-5 are fixed with great accuracy at the same height. The fact that the boundary between the layers is kept in one position with a change in temperature only indicates a redistribution of the atoms of the components without changing the volume of the phases. Therefore, it can be suggested that this composition is critical. Since the delamination temperature is fixed quite accurately, the temperature $s - h
-characteristic 3 (910 K) can be considered close to critical.
The data presented in (pic. 3), make it possible to plot the dependence of the velocity of propagation of ultrasound along the line of monovariant liquid-liquid equilibrium. This requires values corresponding to the upper and lower branches of the $s - h - characteristics in (pic. 3), submit according to temperature. As a result, we obtain a general curve corresponding to the dependence of the ultrasound propagation velocity along the delamination dome (see pic. 4).
Picture 4. Temperature - concentration dependence of the speed propagation of ultrasound in melts of the In-Se system. Polytherms 1-7 correspond to samples containing 4, 10, 15, 17, 20, 25, and 30 at.% Selena
This result, in turn, makes it possible to construct a line of monovariant liquid-liquid equilibrium on the state diagram of the In-Se system. To do this, it is necessary to establish the dependence of the ultrasound propagation velocity on temperature for melts of various compositions at various temperatures above the delamination region and extrapolate this dependence to the intersection with the curve characterizing the dependence of $s along the delamination dome. The intersection points and determine the coordinates of the figurative points of the line of monovariant equilibrium liquid - liquid.
Based on this, we studied the temperature dependence of the propagation velocity of ultrasound Ss 7 compositions (see pic. 4). It can be seen from the figure that the temperature dependences of the ultrasound propagation velocity above the delami-nation temperature (pic. 4, polytherms 1-7, respectively) are linear, which allows them to be reliably extrapolated to the intersection with the curve of the dependence of the ultrasound propagation velocity along the delamination dome.
From (pic. 4) shows that at T > Tcr all polytherms have a negative slope to the temperature axis. Such a change in the speed of propagation of ultrasound at T > T is quite understandable, since above T (critical
cr -1 ' cr v
temperature) there is a homogeneous solution. As can be seen from the figure, no anomalies were found on the polytherms. They decrease linearly with temperature starting from the delamination temperatures.
In accordance with the conclusions in [5-6], this fact indicates that there is no pronounced development of large-scale concentration fluctuations in the melts of this system. Built according to the data presented in (pic. 4), the curve of monovariant liquid-liquid equilibrium in the In-Se system is shown in (pic. 5) as a fragment of the phase equilibrium diagram.
Picture 5. Fragment of the state diagram of the In-Se system in the area of melt separation
It can be seen that the curve of monovariant liquid-liquid equilibrium in the indium-selenium system is characterized by a symmetrical binodal. In
this system, the coordinates of the critical point are set: temperature - 910 K, composition -17 at.% Se.
Alloys of the In - S system were studied in [1112] in the concentration range from 0 to 70% at. S by microstructural, X-ray, and differential thermal analyses. Alloys were prepared by heating mixtures of components in evacuated quartz ampoules. The study used In with a purity of 99.999% (by mass) and crystalline S with a purity of99.999% (by mass).
State diagrams of the In-S system were constructed in a similar way. On picture 6 shows the state diagrams of the In-S system in the region of melt separation. According to the obtained experimental data, the delamination dome in this system is symmetrical and rests on a monotectic horizontal with extreme points at compositions of 7 and 37 at.% sulfur. In the In-S system, the following coordinates of the critical point are set: temperature -1018 K, composition - 21at.% S.
Picture 6. A fragment of the diagram of phase equilibria of the In-S system illustrating the position of the curve of monovariant liquid-liquid equilibrium in combination with a monotectic horizontal
The In - Te state diagram is described in [1316]. It has been established that the interaction between In and Te is characterized by limited mis-
cibility of the components in the liquid state with monotectics at a temperature of423 °C and a content of 28% (at.) Te and the formation of a number of compounds in the solid state, regarding which there are discrepancies in the literature.
In the In-Te system, this method was used to study the temperature dependence (Ss-h) - characteristics in melts containing 6, 11, 15, 18, 18.35, 22, 26, 29 at.% Te.
In the In-Te system, anomalies were found on the Ss polytherms, the essence of which is that the dependence Ss ~f(T) passes through a gentle maximum before becoming linear. These anomalies extend over a fairly wide temperature range and are most pronounced in the alloy of critical composition (18.35 at% Te).
Picture 7. A fragment of the diagram of phase equilibria of the In-Te system illustrating the position of the curve of the
monovariant liquid-liquid equilibrium in combination with the monotectic horizontal
According to [7-8], melts of the In-Te system are semi-metallic; interatomic bonds in them are realized not only by the metallic type, but also by the covalent one. The presence of covalent bonds in melts is due to the presence of tellurium atoms, which are prone to the formation of these bonds due to the well-known specificity of the structure of the
outer electron shells. We suggest that the observed anomalies in the velocity of ultrasound propagation in the melts of this system are due precisely to the possibility of the coexistence of two types of interatomic bonds, which, along with concentration fluctuations, should contribute to the formation of significant density fluctuations. The nature of the changes in the latter during heating of the melts, in fact, should be the physical reason for the occurrence of anomalies in the temperature dependence of the ultrasound propagation velocity.
The results of the studies carried out provide complete experimental information, on the basis of which it is possible to construct a monovariant equilibrium curve that limits the separation region on the phase equilibrium diagram of the In-Te system.
On (pic. 7) shows the results ofsuch a construction.
For the In-Te system, the following critical point coordinates are established: temperature - (803 ± 2) K, composition - 18.35 at.% Te. the rest In.
Conclusions
Thus, using the ultrasonic method, the area of delamination in the systems In - BVI (where BVI - S, Se, Te) was investigated and refined. Curves of monovariant equilibrium were constructed, limiting the indicated area. Experimental result We have shown that the delamination dome for all the studied systems is practically symmetrical.
It is shown that the study of the propagation velocity of ultrasound is an effective and reliable method for constructing curves of monovariant liquid-liquid equilibrium in high-temperature melts of binary metal and semiconductor systems.
In conclusion, we note that the acoustic method has sufficient reliability and high information content in the case of: studies of the stratification of melts of metals and semiconductors, supercritical phenomena occurring in them; construction and refinement of state diagrams; finally, the study of the structure of metallic and semiconductor melts.
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