CC BY
17iSHS 2019
Moscow, Russia
BEHAVIOURS OF ELECTROTHERMAL EXPLOSION OF TITANIUM-CARBON BLACK MIXTURE UNDER QUASI-ISOSTATIC
COMPRESSION
V. A. Shcherbakov*" and A. V. Shcherbakov"
aMerzhanov Institute of Structural Macrokinetics and Materials Science Russian Academy of
Sciences, Chernogolovka, 142432 Russia *e-mail: vladimir@ism.ac.ru
DOI: 10.24411/9999-0014A-2019-10156
Electrothermal explosion (ETE) of heterogeneous condensed systems under quasi-isostatic compression is an effective method for producing ceramic composites [1, 2]. The method includes Joule heating of the reaction heterogeneous mixture of metal and non-metal powders to the ignition temperature and pressing the hot synthesis product. The combined electrochemical heating source allows control of the thermal synthesis and consolidation regime to produce composites with minimal residual porosity. The search for optimal conditions of exothermic synthesis to obtain composites with minimal residual porosity, submicron microstructure and high physical and mechanical characteristics stimulated the study of regularities and mechanism of fast exothermic interaction.
In the present work the results of experimental diagnostics of thermal modes of ETE of titanium and carbon black mixture under conditions quasi-isostatic compression summarizes. Studying of the ETE behavior of a heterogeneous mixture under pressure is based on the measurement of thermal and electrical parameters in conditions of subcritical and supercritical heating. This temperature was measured using a tungsten-rhenium thermocouple in the center of the sample. This technique under pressure is described in [3]. ETE thermograms obtained at different pressures of quasi-isostatic compression are shown in Fig. 1. Sharp bends in ETE thermograms separate the stages of preheating (7b < T < Tign), thermal explosion (Tign < T < Tmax) and post-process (Tmax < T < Ts) occuring at cooling of the final product (Fig. 1). Increasing the pressure from 8 to 96 MPa leads to Fig. 1 ETE thermograms of titanium and decrease in ignition time from 3.1 to 1.2 s, carbon black mixture obtained at U = 9 V and and in maximum temperature of the P = 8 (1X 48 (2X 96 (3) MPa. thermal explosion Tmax from 3bbb to 2500 K. The ignition temperature Tign reaches the maximum value of 700 K at pressures of 24 MPa, and the abnormally low value of 400 K at 8 MPa.
The dependencies of the electric current force I on the time obtained at the ETE of titanium and carbon black mixture at different quasi-isostatic compression pressures are shown in Fig. 2. It is seen that at the stage of thermal explosion, the electrical parameters, like the thermal parameters changed sharply (curves 1-3). The value of the electric current Tign and /max are achieved at Tign and Tign, respectively. Dotted lines drawn through the characteristic points /ign and /max separate the stages of the pre-heating (/0 < / < /ign), thermal explosion
XV International Symposium on Self-Propagating High-Temperature Synthesis
(iign < I < Imax) and cooling (Imax < I < Is). In extrapolation, the dotted lines intersect at the point at which the change in the electric current AI« 0.
The decrease in electrical resistance is due to the increase in the number of contacts between the particles, as the particles of titanium and soot during cold pressing are poorly deformed due to the high stiffness. At high pressure, electrical contacts are formed in the sample, including almost all the particles of the mixture, and at low pressure - only a small fraction of the particles.
Ratio of the times of exothermic interaction and the change in the electric current at the stage of a thermal explosion is criterion to determine a thermal regime of ETE. In the case of a homogeneous heating mode, the ratio of the time of exothermic interaction and the change in the electric current is close to one, and in the case of inhomogeneous heating, the time of exothermic interaction is much less than the time of the change in the electric current force.
Figure 3 shows the ETE thermograms of the titanium and carbon black mixture at quasi-isostatic compression pressure of 48 MPa and various electrical voltages. It is seen that the parameters of the ETE depend on the magnitude of the applied electric voltage U. In subcritical conditions (U = 3 V), the sample temperature increases smoothly to a temperature of 1600 K (Fig. 3, curve 1). Stable ignition of the mixture was carried out at U > 4 V.
Fig. 2. Dependencies of electric current force on time, obtained at ETE of titanium and carbon black mixture at U = 9 V and P = 8 (1), 48 (2), 96 (3) MPa.
T, K
3000
2000.
1000
J _».
: TmpOTi)
5 4 3 2 1 / ^
t, c
0,1
10
100
Fig. 3. ETE thermograms of titanium and carbon black mixture obtained at P = 48 MPa and U(V): 1 3; 2 4; 3 6; 4 9; and 5 11.
In supercritical conditions (U > 4 V) at the stage of thermal explosion the ETE temperature increased abruptly. The characteristic points in the thermograms are ignition temperature (Tign), ignition time (iign), as well as the maximum temperature of the ETE (Tm) and the reaction time (im). At Tm the reagents mixture is complete transformed into the final product. The ignition time and reaction time correspond to the duration of the temperature rise in the range To < T < Tign, and Tign < T < Tm, respectively. As the electric voltage increases, iign and Tign decrease, and Tm increases.
Changing the parameters of ETE is associated with the interaction of reagents at the stage of preheating and the formation of the final product. The heat released at this stage is removed from the sample to the environment. Therefore, the decrease in Tm is due to a decrease in the chemical energy reserve in the sample, and the increase in the ignition temperature of the iign reaction mixture is due to an increase in the concentration of thermal ballast. The melting point of titanium is in the area of linear temperature increase. It is important to note that when T < Tmp and T > Tmp, dT/di « 60000 K/c. Therefore, the melting of titanium does not influence on the power of chemical heat release.
The dependences of electric current force on time are shown in Fig. 4. In subcritical conditions (U = 3 V) for 50 s of heating curve 1 gradually increases. In supercritical conditions (U > 4 V) at the stage of thermal explosion electrical parameters like thermal ones change abruptly (curves 2-5). Parameters Iign and Im correspond to the values of the electric current force when the ignition temperature (Tign) and the maximum temperature of the ETE (Tm) are achieved, respectively.
_SI IS 2019_Moscow, Russia
/, kA
1,6 -
1,2 -
0,8 -
0,4 -
0 -0
Fig. 4. Dependencies of electric current force on time, obtained at ETE of titanium and carbon black mixture at P = 48 MPa and U (V): 1 3; 2 4; 3 6; 4 9; and 5 11.
Thus, the results obtained showed that the measurement of thermal and electrical parameters allows to determine the separate stage of ETV, but also the modes of homogeneous or inhomogeneous heating of the test sample.
1. V.A. Shcherbakov, V.T. Tilopa, A.V. Shcherbakov, Preparation of composite materials based on fused titanium carbide by the electro-thermal explosion under pressure, Compos. Nanostruct, 2016, vol. 8, no. 1, pp. 70-80.
2. V.T. Telepa, V.A. Shcherbakov, A.V. Shcherbakov, Production of composite TiC-30 weight % Fe by electrothermal explosion under pressure, Mater. Lett., 2016, vol. 6, no. 4, pp. 286-289.
3. V.A. Scherbakov, A.V. Shcherbakov, S.A. Bostandzhiyan, Electrothermal explosion of titanium and carbon black mixture under quasi-isostatic compression. Part I - Thermal and electrical parameters, Combust. Shock Wave, 2019, no. 1, pp. 83-91.
