THERMOPHYSICAL PROPERTIES AND THERMODYNAMIC FUNCTIONS OF ZINC ALLOY Zn5Al ALLOYED WITH MOLIBDAENUM

ABSTRACT

The paper shows a method for comparing the cooling curves of the test sample with the reference. Cooling of the measured sample heated to a certain temperature will change the speed when the ambient temperature is exceeded. It has been shown that with an increase in temperature, the specific heat capacity, enthalpy, and entropy of the zinc-aluminum alloy Zn5Al with molybdenum increase, while the value of Gibbs energy decreases. The presence of molybdenum slightly decreases the specific heat capacity of the original zinc-aluminum alloy Zn5Al.

АННОТАЦИЯ

В работе показан метод сравнения кривых охлаждения исследуемого образца с эталоном. Охлаждение измеряемого образца, нагретого до определенной температуры будет менять скорость, при превышении температуры окружающей среды. Показано, что с ростом температуры удельная теплоёмкость, энтальпия и энтропия цинк-алюминиевого сплава Zn5Al с молибденом увеличиваются, а значение энергии Гиббса уменьшается. От содержания молибдена теплоемкость исходного цинк-алюминиевого сплава Zn5Al незначительно уменьшается.

Keywords: molibdaenum, zinc-aluminum alloy Zn5Al, copper, tellurium, specific heat, heat transfer coefficient, enthalpy, entropy, Gibbs energy.

Ключевые слова: молибден, цинк-алюминиевого сплава Zn5Al, удельная теплоёмкость, коэффициент теплоотдачи, энтальпия, энтропия, энергия Гиббса.

For many materials, it is very difficult to systematize all the available data obtained on samples that differ in composition, condition and production technology. In our opinion, in such cases it is necessary to provide only information about the range of possible changes in thermal and thermophysical properties. According to the main purpose, there are defining properties for each group of materials. Thus, for semiconductor materials, the most characteristic are the coefficient of thermal conductivity and its components, for building materials – the coefficient of thermal expansion, for structural metals – almost all thermophysical properties (their role may vary depending on the specific purpose of the material). In our case, in order to develop alloy protective coatings, it is necessary to study the heat capacity and the change in the thermodynamic functions of the alloys being developed, depending on the optimal composition and the range of certain temperatures. Data on the method of measuring properties is also very important. In addition, for practical calculations, information about the reliability of the recommended data is always necessary. Further, in cases where physico-chemical transformations occur in materials, leading to abrupt changes in properties, thermal characteristics are additionally given at characteristic temperatures [1-3].

The paper shows a method for comparing the cooling curves of the test sample with the reference. Cooling of the measured sample heated to a certain temperature will change the speed when the ambient temperature is exceeded. The cooling rate depends on the heat capacity of the sample material. To determine the heat capacity. of another, i.e. unknown substance [4-6], comparisons of cooling curves are used – thermograms (temperature versus time) of two samples, one of which serves as a standard with a known heat capacity.

The value of the heat flux entering through the heat meter is estimated, which is calculated from the thermal conductivity of the heat meter and the temperature difference on the heat meter, which are determined by independent calibration experiments using a copper sample. The temperature range is up to 400 °C. The error in the method does not exceed 6%.

The aim of the work is to study the effect of molybdenum additives on the properties of zinc-aluminum alloy Zn5Al with improved characteristics by establishing temperature dependences of heat capacity and changes in thermodynamic functions in the "cooling" mode.

The study of the heat capacity of metals was carried out at the installation, the scheme of which is shown in Figure 1.

Figure 1. Installation for determining the heat capacity of solids in the "cooling" mode

1 – autotransformer; 2 – thermostat; 3 – electric furnace; 4 – sample to be measured; 5 – standard; 6 – stack of an electric furnace; 7 – digital thermometer of the measured sample; 8 – general–purpose digital thermometer; 9 – digital thermometer of the standard; 10  –  registration device.

The curves obtained during the experiment for the dependence of temperature on the cooling time of samples made of Zn5Al alloy with molybdenum and the reference (copper grade M00) are shown in Fig. 2 and are described by an equation of the form

                                                      (1)

here a, b, p, k are the coefficients; τ is the time.

The cooling rates of the samples for the studied alloys were calculated according to the following equation:

                                                 (2)

The values of the coefficients of equation (2) are presented in Table 1. Fig 2 shows the temperature dependence dT/dt for the studied alloys. To determine the specific heat capacity С⁰_р zinc-aluminum alloy Zn5Al with molybdenum used the equation

                                                      (3)

Table 1

Results of the coefficients a, b, p, k, ab, pk (2.9) for zinc-aluminum alloy Zn5Al with molybdenum

Figure 2. Graph of temperature dependence of samples (T) on the cooling time (τ) of the zinc-aluminum alloy Zn5Al alloyed with molybdenum.

where m₁=ρ₁V₁ – is the mass of the standard, m₂=ρ₂V₂ – is the mass of the studied sample, - the cooling rate of the reference sample under study at a given temperature.

Graphical representation of the temperature dependence of the specific heat capacity of Zn5Al alloys with molybdenum is shown in Fig 3. The values of the specific heat capacity are presented in Table 2.

The temperature dependence of the specific heat capacity for Zn5Al alloy with molybdenum is described by the general equation

                                                  (4)

Figure 2. Temperature dependence of the cooling rate of the zinc-aluminum alloy Zn5Al alloyed with molybdenum.

Table 2

Temperature dependence of the specific heat capacity of  zinc-aluminum alloy Zn5Al with molybdenum and copper standard

The values of the coefficients of equation (4) obtained by processing using the Sigma Plot curves program in Fig 4 and are presented in Table 3.

To calculate the temperature dependence of changes in the enthalpy H⁰ (T), entropy S⁰ (T) and Gibbs energy G⁰ (T) of Zn5Al alloy with molybdenum, integrals from the heat capacity polynomial according to equation (4) were used:

                 (5)

                       (6)

                                 (7)

где Т₀ = 298,15 К.

Figure 3.  Specific heat capacity dependence on temperature for zinc-aluminum alloy Zn5Al, alloyed with molybdenum

Table 3

Results of the values of coefficients a, b, c, d for zinc-aluminum alloy Zn5Al, alloyed with molybdenum and copper standard

The results of calculating the changes in enthalpy, entropy, and Gibbs energy for the Zn5Al alloy with molybdenum over 50 K are presented in table 4.

Table 4

Dependence of specific heat capacity, enthalpy, entropy, and Gibbs energy on temperature for zinc-aluminum alloy Zn5Al with molybdenum and standard (Cu M00)

Conclusions. In the "cooling" mode, the temperature dependence of the heat capacity of the Zn5Al alloy with molybdenum is established based on the known heat capacity of the copper grade M00 standard. It is shown that with increasing temperature and molybdenum concentration, the heat capacity of the Zn5Al alloy slightly increases. Using the temperature dependence of the heat capacity, thermodynamic functions of the alloys are calculated. It has been shown that with increasing temperature and molybdenum content, the enthalpy and entropy of alloys increase, while the values of Gibbs energy decrease.

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