Corresponding author: Izatullo N. Ganiev ( ganiev48@mail.ru ) © 2020 Izatullo N. Ganiev, Aslam P. Abulakov, Jamshed H. Jayloev, Umarali Sh. Yakubov, Amirsho G. Safarov, Vladimir Dz. Abulkhaev.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Ganiev IN, Abulakov AP, Jayloev JH, Yakubov USh, Safarov AG, Abulkhaev VDz (2020) Effect of bismuth additions on the thermophysical and thermodynamical properties of EAlMgSi (Aldrey) aluminum semiconductor alloy. Modern Electronic Materials 6(3): 107112. https://doi.org/10.3897/j.moem.6.3.63734

The economic feasibility of using aluminum as a conductive material is explained by the favorable ratio of its cost to the cost of copper. In addition, one should take into account that the cost of aluminum has remained virtually unchanged for many years.
When using conductive aluminum alloys for the manufacture of thin wire, winding wire, etc., certain difficulties may arise in connection with their insufficient strength and a small number of kinks before fracture. Aluminum alloys have been developed in recent years which even in a soft state have strength characteristics that allow them to be used as a conductive material.
The electrochemical industry is one of the promising application fields of aluminum. EAlMgSi (Aldrey) conductor aluminum alloys represent this group of alloys. This work presents data on the temperature dependence of heat capacity, heat conductivity and thermodynamic functions of the EAlMgSi (Aldrey) aluminum alloy doped with bismuth. The studies have been carried out in "cooling" mode. It has been shown that the heat capacity and thermodynamic functions of the EAlMgSi (Aldrey) aluminum alloy doped with bismuth increase with temperature and the Gibbs energy decreases. Bismuth additions of up to 1 wt.% reduce the heat capacity, heat conductivity, enthalpy and entropy of the initial alloy and increase the Gibbs energy.
EAlMgSi (Aldrey) aluminum alloy, bismuth, heat capacity, heat conductivity, "cooling" mode, enthalpy, entropy, Gibbs energy.
Aluminum and its alloys are widely used in electrical engineering as conductor and structural materials. As a conductive material, aluminum is characterized by high electrical and thermal conductivity (after copper, the maximum level among all technically used metals) [
Aluminum also has a low density, high atmospheric corrosion resistance and resistance to chemicals. However, aluminum alloys in specific states and under severe operation conditions may undergo dangerous corrosion types. Of special interest is aluminum corrosion in closetoneutral solutions (6 < pH < 8). This corrosion occurs in natural environments: seawater, lacustrine and river water, potable water and atmospheric precipitation. Under these conditions and normal temperatures, the mobility of H^{+} ions or H_{2}O molecules during hydrogen emission is negligibly low [
The economic feasibility of using aluminum as a conductive material is explained by the favorable ratio of its cost to the cost of copper. In addition, one should take into account that the cost of aluminum has remained virtually unchanged for many years [
When using conductive aluminum alloys for the manufacture of thin wire, winding wire, etc., certain difficulties may arise in connection with their insufficient strength and a small number of kinks before fracture.
Aluminum alloys have been developed in recent years which even in a soft state have strength characteristics that allow them to be used as a conductive material [
One of conductor aluminum alloys is EAlMgSi (Aldrey) pertaining to thermally hardened alloys. It has high strength and ductility. This alloy acquires high electrical conductivity upon appropriate heat treatment. Products made from it are used almost exclusively for overhead power lines [
Since overhead power lines made from aluminum and its alloys are used in open air, increasing the corrosion resistance of these alloys is an urgent task.
The aim of this work is to study the effect of bismuth additions on the thermophysical and thermodynamical properties of EAlMgSi (Aldrey) aluminum conductor alloy, chemical composition (wt.%): 0.5 Mg, 0.5 Si, balance Al.
The alloys were synthesized in a SShOL type resistance laboratory shaft furnace at 750–800 °C. A6 grade aluminum which was additionally doped with the calculated amount of silicon and magnesium was used as a charge in the preparation of the EAlMgSi alloy. When doping aluminum with silicon, the metallic (0.1 wt.%) silicon present in primary aluminum was taken into account. Magnesium wrapped in aluminum foil was introduced into the molten aluminum using a bell. Bismuth was introduced into the melt in a form wrapped in aluminum foil. The alloys were chemically analyzed for silicon and magnesium contents at the Central Industrial Laboratory of the State Unitary Enterprise Tajikistan Aluminum Company. The alloy compositions were controlled by weighing the charge and the alloys. Synthesis was repeated if the alloy weight deviated from the target one by more than 1–2% rel.u. Then the alloys were cleaned from slag and cast into graphite molds in order to obtain samples for thermophysical study. The cylindrical samples had a diameter of 16 mm and a length of 30 mm.
The heat capacity of the alloys was measured using the setup shown in Fig.
The temperature was measured with a multichannel digital thermometer allowing direct recording of the data on the computer in tabular form. The temperature measurement accuracy was 0.1 °C. The relative temperature measurement error in the 40 to 400 °C range was ± 1%. The heat capacity measurement error for this method is within 4–6% depending on temperature.
The measurement data were processed in MS Excel and plotted in Sigma Plot. The correlation coefficient was R_{corr.} > 0.989 which confirms the correct choice of the approximating function.
To determine the cooling rate we plotted specimen cooling curves. The cooling curves are specimen temperature vs time functions for air cooling [
The experimental temperature vs time functions of the specimens (Fig.
T = ae^{b}^{τ} + pe^{k}^{τ}, (1)
where a, b, p, k are constants and t is cooling time.
(a) temperature and (b) cooling rate for samples of EAlMgSi alloy (Aldrey) doped with bismuth and reference (A5N grade Al) as functions of time
Differentiating Eq. (1) by t we obtained the following equation for specimen cooling rate determination:
$\frac{\mathrm{d}T}{\mathrm{d}\tau}=ab{e}^{b\tau}+pk{e}^{k\tau}$, (2)
Based on Eq. (2) we calculated the cooling rates for the specimens of EAlMgSi alloy (Aldrey) doped with bismuth (Fig.
Coefficients a, b, p, k, ab, pk in Eq. (2) for EAlMgSi alloy (Aldrey) doped with bismuth.
Bismuth content in EAlMgSi alloy, wt.%  a, K  b × 10^{3}, s^{1}  p, K  k × 10^{5}, s^{1}  ab × 10^{1}, K × s^{1}  pk × 10^{3}, K × s^{1} 

EAlMgSi alloy  165.61  4.46  314.72  2.27  7.38  7.14 
+0.05 Bi  164.02  4.46  314.82  2.31  7.32  7.26 
+0.1 Bi  160.75  4.74  315.21  2.11  7.61  6.64 
+0.5 Bi  158.44  4.72  314.99  2.06  7.48  6.48 
+1.0 Bi  159.23  4.73  315.17  2.10  7.54  6.62 
Reference (A5N grade Al)  494.26  5.01  319.92  2.57  0.25  8.23 
Then based on the calculated cooling rates for the alloys and the A5N grade Al reference specimens we calculated the specific heat capacity of EAlMgSi alloy (Aldrey) doped with bismuth:
${C}_{{P}_{2}}^{0}={C}_{{P}_{1}}^{0}\frac{{m}_{1}}{{m}_{2}}\frac{{\left(\frac{\mathrm{d}T}{\mathrm{d}\tau}\right)}_{1}}{{\left(\frac{\mathrm{d}T}{\mathrm{d}\tau}\right)}_{2}}$ (3)
where m_{1} = ρ_{1}V_{1} is the weight of the reference; m_{2} = ρ_{2}V_{2} is the weight of the specimen and
${\left(\frac{\mathrm{d}T}{\mathrm{d}\tau}\right)}_{1},{\left(\frac{\mathrm{d}T}{\mathrm{d}\tau}\right)}_{2}$
are reference and alloy specimen cooling rates for a specific temperature.
Using polynomial regression we obtained an equation for the temperature dependence of the heat capacity of EAlMgSi alloy (Aldrey) doped with bismuth:
$\mid {C}_{{P}_{0}}^{0}=a+bT+c{T}^{2}+d{T}^{3}$ (4)
The coefficients а, b, c, d in Eq. (4) are summarized in Table
Coefficients a, b, c, d in Eq. (4) for samples of EAlMgSi alloy (Aldrey) doped with bismuth and the reference (A5N grade Al)
Bismuth content in EAlMgSi alloy, wt.%  а, J/(kg × К)  b, J/(kg × К^{2})  с, J/(kg × К^{3})  d × 10^{4}, J/(kg × К^{4})  Correlation coefficient R, % 

EAlMgSi alloy  10394.96  84.30  0.21  1.71  0.9925 
+0.05 Bi  8928.68  72.90  0.18  1.48  0.9899 
+0.1 Bi  11529.79  89.00  0.21  1.71  0.9950 
+0.5 Bi  11560.07  89.54  0.216  1.75  0.9980 
+1.0 Bi  10548.49  81.60  0.20  1.57  0.9989 
Reference (A5N grade Al)  645.88  0.36  0  0  1.0 
The data on the heat capacity of the alloys calculated using Eq. (3) with 25 K intervals are summarized in Table
$a=\frac{{C}_{P}^{0}m\frac{\mathrm{d}T}{\mathrm{d}\tau}}{\left(T{T}_{0}\right)S}$ (5)
where Т, Т_{0} are the specimen and environment temperatures, respectively, and S, m are the specimen surface area and weight, respectively. The temperature dependence of the heat conductivity coefficient of EAlMgSi alloy (Aldrey) doped with bismuth is shown in Fig.
Temperature dependence of specific heat capacity (kJ/(kg×K)) of EAlMgSi alloy (Aldrey) doped with bismuth and reference (A5N grade Al)
Bismuth content in EAlMgSi alloy, wt.%  Heat capacity, kJ/(kg·К)  

300 K  325 K  350 K  375 K  400 K  450 K  500 K  
EAlMgSi alloy  751.00  855.36  907.62  923.83  920.00  916.37  1025.00 
+0.05 Bi  737.65  832.24  882.20  901.42  903.76  913.31  1021.87 
+0.1 Bi  590.20  735.49  822.57  867.46  886.20  909.32  1020.20 
+0.5 Bi  560.05  701.29  785.45  828.91  848.10  879.25  1010.15 
+1.0 Bi  557.44  690.13  769.55  810.40  827.42  848.78  951.40 
Reference (A5N grade Al)  854.62  877.90  901.55  925.45  949.48  997.46  1044.57 
Temperature dependence of (a) heat capacity and (b) heat conductivity coefficient of EAlMgSi alloy (Aldrey) doped with bismuth and reference (A5N grade Al)
To calculate the enthalpy, entropy and Gibbs energy as functions of temperature using Eqs. (6)–(8), we integrated the specific heat capacity calculated using Eq. (4):
$\begin{array}{l}\left[{H}^{0}\left(T\right){H}^{0}\left({T}_{0}\right)\right]=a\left(T{T}_{0}\right)+\frac{b}{2}\left({T}^{2}{T}_{0}^{2}\right)+\\ +\frac{c}{3}\left(T{T}_{0}^{3}\right)+\frac{d}{4}\left({T}^{4}{T}_{0}^{4}\right)\end{array}$ (6)
$\begin{array}{l}\left[{S}^{0}\left(T\right){S}^{0}\left({T}_{0}\right)\right]=a\mathrm{ln}\frac{T}{{T}_{0}}+b\left(T{T}_{0}\right)+\\ +\frac{c}{2}\left({T}^{2}{T}_{0}^{2}\right)+\frac{d}{3}\left({T}^{3}{T}_{0}^{3}\right)\end{array}$ (7)
$\begin{array}{l}\left[{G}^{0}\left(T\right){G}^{0}\left({T}_{0}\right)\right]=\\ =\left[{H}^{0}\left(T\right){H}^{0}\left({T}_{0}\right)\right]T\left[{S}^{0}\left(T\right){S}^{0}\left({T}_{0}\right)\right]\end{array}$ 8)
where T_{0} = 298.15.
The calculation data for the enthalpy, entropy and Gibbs energy as functions of temperature obtained using Eqs. (6)–(8) with 25 K intervals are summarized in Table
Temperature dependence of the thermodynamical functions of EAlMgSi alloy (Aldrey) doped with bismuth and reference (A5N grade Al)
Bismuth content in EAlMgSi alloy, wt.%  Thermodynamical functions  

300 К  325 К  350 К  375 К  400 К  450 К  500 К  
[H^{0}(T) – H^{0}(T_{0}^{*})], kJ/kg for alloys  
EAlMgSi alloy  1.3799  21.5847  43.7138  66.6654  89.7383  135.4471  183.2466 
+0.05 Bi  1.3562  21.0873  42.5965  64.9414  87.5269  132.7727  180.5087 
+0.1 Bi  1.0794  17.7886  37.3691  58.5658  80.5244  125.3141  172.9194 
+0.5 Bi  1.0240  16.9277  35.6147  55.8629  76.8600  119.9037  166.4520 
+1.0 Bi  1.0199  16.7410  35.0828  54.8974  75.4046  117.2165  161.6372 
Reference (A5N grade Al)  1.5795  23.2351  45.4777  68.3149  91.7514  140.4266  191.4833 
[S^{0}(T) – S^{0}(T_{0}^{*})], kJ/kg ·K for alloys  
EAlMgSi alloy  0.0046  0.0692  0.1348  0.1982  0.2577  0.3654  0.4660 
+0.05 Bi  0.0045  0.0676  0.1313  0.1930  0.2513  0.3579  0.4584 
+0.1 Bi  0.0036  0.0570  0.1150  0.1735  0.2302  0.3357  0.4359 
+0.5 Bi  0.0034  0.0543  0.1096  0.1655  0.2197  0.3210  0.4190 
+1.0 Bi  0.0034  0.0537  0.1080  0.1627  0.2156  0.3141  0.4076 
Reference (A5N grade Al)  0.0053  0.0746  0.1405  0.2035  0.2640  0.3786  0.4862 
[G^{0}(T) – G^{0}(T_{0}^{*})], kJ/kg for alloys  
EAlMgSi alloy  0.0043  0.9209  3.4739  7.6429  13.3499  28.9837  49.7672 
+0.05 Bi  0.0042  0.9014  3.3917  7.4534  13.0161  28.2969  48.7033 
+0.1 Bi  0.0033  0.7460  2.8923  6.5011  11.5525  25.7408  45.0265 
+0.5 Bi  0.0031  0.7084  2.7516  6.1912  11.0094  24.5605  43.0455 
+1.0 Bi  0.0031  0.7030  2.7201  6.1057  10.8391  24.1202  42.1591 
Reference (A5N grade Al)  0.0049  1.0111  3.7068  8.0133  13.8629  29.9625  51.6098 
^{*} T_{0} = 298.15 К. 
Aldrey alloy consists of aluminum with the following impurities: 0.3–0.5% Mg, 0.4–0.7% Si and 0.2–0.3% Fe. Compulsory impurities which determine the properties of Aldrey are magnesium and silicon the content ratio of which should meet that for the Mg_{2}Si compound which forms in the alloy and acts as a strengthening agent ensuring the high mechanical strength of the alloy. However one should take into account that in practice the melt always contains iron which is a still unavoidable but often detrimental impurity in any technical grade aluminum, forming the silicon containing compound Al_{6}Fe_{2}Si_{3}. Therefore to completely ensure the formation of the Mg_{2}Si compound one should compose the melt with a certain excess of silicon (0,4–0,5%) above the theoretical level [
The strengthening action of the Mg_{2}Si compound stems from the fact that its solubility is solid aluminum decreases with a decrease in temperature. For example the maximum solubility of Mg_{2}Si in aluminum is 1.85% at 595 °С and only 0.2% at 200 °С. Therefore rapid cooling (quenching) of an Aldrey type alloy heated to above 500 °С at which all Mg_{2}Si is in the solid solution produces a supersaturated Mg_{2}Si solid solution in aluminum [
Longterm tempering causes precipitation of excess Mg_{2}Si from the solid solution in the form of a finegrained structural component which increases the mechanical strength of the alloy (precipitation hardening). This tempering of the alloy is referred to as natural ageing. Ageing can be accelerated by slightly heating the alloy (to 150–200 °С), i.e., artificial ageing. In the course of ageing Mg_{2}Si impurity precipitates from the solid solution causing an increase in the electrical conductivity of the alloy [
Bismuth doping of EAlMgSi alloy modifies the primary precipitates of binary and ternary phases in the alloy, providing a generally positive contribution to its performance.
Thus we determined the heat capacity of bismuth doped EAlMgSi alloy (Aldrey) in "cooling" mode based on the known heat capacity of a reference A5N grade aluminum specimen. Using the experimentally obtained polynomial dependences we showed that with an increase in temperature the heat capacity, enthalpy and entropy of the alloys increase while the Gibbs energy decreases. Bismuth additions in the experimentally studied concentration range 0.05–1.0 wt.% reduce the heat capacity, heat conductivity coefficient, enthalpy and entropy of the initial EAlMgSi alloy (Aldrey) and increase the Gibbs energy. The increase in the heat capacity, heat conductivity coefficient, enthalpy and entropy with an increase in the bismuth concentration in the alloy is caused by the modification of the αAl solid solution structure, i.e., a higher heterogeneity of the structure of the multicomponent alloys [
Experimental data on the temperature dependence of the heat capacity, heat conductivity coefficient and thermodynamical functions of EAlMgSi aluminum alloy (Aldrey) doped with bismuth were presented for "cooling" mode measurements.
We show that with an increase in temperature the heat capacity and the thermodynamical functions of EAlMgSi alloy (Aldrey) with bismuth increase and the Gibbs energy decreases. Bismuth additions of up to 1 wt.% reduce the heat capacity, heat conductivity, enthalpy and entropy of the initial alloy but increase the Gibbs energy.