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7SH-SYNTHESIS OF CERAMIC MATERIALS BASED ON PRE-ACTIVATED AND MODIFIED SYSTEMS
N. N. Mofa*", B. S. Sadykov", G. Kaiypbek", T. B. Osserov", and D. Shaltykova"
aAl-Farabi Kazakh National University, The Institute of Combustion Problems, Almaty, Kazakhstan *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10100
One of the progressive ways to create materials both compact and with varying degrees of dispersion up to formation of nanostructured systems today is self-propagating high-temperature synthesis (SHS), or in other words - technological combustion [1]. The results of numerous experimental studies and theoretical calculations showed that the burning rate and temperature profile of the combustion wave during SHS are a function of many variables: the heating value of the mixture, the thermal conductivity of the combustion products, the volumetric heat release rate, the heat capacity of the products, the concentration of the initial substances and the reaction products, the diffusion coefficient of the reacting substance, activation energy of the reaction and its rate constant. With the help of various influences, it is possible to control the combustion process. The use of ultrasonic action [2] and mechanochemical treatment [3], which allow to change not only the dispersion, but also the energy state of the material, turned out to be effective.
Preliminary MCT in mechanical reactors (dynamic action mills) allows to achieve a high degree of dispersion of particles (up to 100 nm) and to change the structure, energy intensity and, consequently, reactivity of the material. When pre-activated systems are used in combustion processes, the task is to preserve the stored energy of the material for quite a long time, since it is well known that activated systems undergo aging, that is, a gradual relaxation of the energy stored in them. A real way to solve this problem is to modify the surface of particle with compounds that stabilize the energy state of the material, i.e. creating a capsule shell.
Another effective way to influence the structure and condition of various materials (liquid, amorphous, crystalline) is ultrasonic treatment (UST) of a different range of frequency, power and time, depending on the material and processing medium [4]. The main mechanisms for ultrasonic dispersion of powder materials in aqueous medium are cavitation and flows that occur in the working fluid during the collapse of cavitation cavities [5]. Erosion of the surface and destruction of particles under the influence of ultrasound leads to an increase in the chemical activity of their surface and accelerate the processes of interaction of particles of the material being processed with the processing medium. Thus, during UST, a mechanochemical effect is rendered on the material being processed. In both cases, there is a change in the structure, first of all, of the surface layers, the state and, consequently, the chemical activity of the material, which is then realized in various processes of synthesizing new materials for a specific purpose.
In this work, it is shown how, when using organic compounds containing bound water as well as carbon and ammonia groups as modifying additives during MCT, one can influence the thermokinetic characteristics of the technological combustion process (self-propagating high-temperature synthesis (SHS)), ensuring the completeness of the reaction between the main components of the reaction mixture. Mechanochemical and ultrasonic treatment of quartz, calcite (calcium carbonate) and wollastonite was carried out both individually and as a mixture of mineral powders.
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Moscow, Russia
Thus, MCT of a triple system including quartz, wollastonite and calcium carbonate, pre-activation and modification of components (a mixture of succinic acid with ammonia and polyvinyl alcohol) lead to a significant change in the thermokinetic characteristics of the combustion process (Fig. 1).
0 20 40 60 80 100 120
Time, s
Fig. 1. Thermograms of the combustion system [(30% CaCO3 + 70% SiO2) + 5% wollastonite] (1) in the initial state, (2) after MCT and (3) modification with compound C4H6O4, (4) with a mixture of 5% C4H6O4 + 5% NH4OH and (5) (C2H3OHX MCT time was 20 min.
The modifiers used and the presence of calcium carbide in the charge after MCT enhance the role of gas-phase reactions in the combustion process of the activated system. Such conditions lead to the possibility of synthesis gas formation in the process of sample heating, which is actively involved in the combustion process, changing its thermokinetic characteristics. The result of these processes is a change in the strength of the SHS samples, which is determined by the phase composition of the synthesis products (Table 1).
Table 1. Phase composition of the combustion products of (SiO2 + CaCO3 + Al) system samples
depending on the presence of wollastonite, modifier and conditions of MCT._
_Phase content, %_
Phases _Samples
1 2 3 4 5 6 7
Al2O3 42.7 20.8 32.8 30.7 32.8 37.3 30.4
Si 20.3 10.3 12.1 13.2 17.7 11.8 10.5
Al 7.8 11.1 5.7 11.6 8.2 3.5 9.6
SiO2 9.2 3.4 1.9 1.5 10.6 1.1 4.5
Ca(Al2Si2Os) 26.1 43.9 28.2 30.9 18.4
Ca2Al(AlSiO7) 10.5 8.8 3.5 6.5 11.0 5.8 14.4
CaO 5.8 1.0
CaAUOv 13.7 16.2
Ca3Si3O9 5.7
FeAl3Si2 5.7 6.7 3.5 7.2 5.3
FeSi2 1.5 2.3
1 (30% CaCO3 + 70% SiO2) + Al;
2 (30% CaCOs + 70% S^mct 10 mm + Al;
3 [(30% CaCO3 + 70% SiO2) + 5% B*]mct 10 mm + Al;
4 [(30% CaCO3 + 70% S^mct 10 mm + (5% W + 5% SA**)mct 20 mm] + Al;
5 [(30% CaCO3 + 70% SîO2)mct 10 mm + (5% W + 5% SA + 5% Ammonia)MCT 20 min] + Al;
6 [(30% CaCO3 + 70% SÎO2)mct 10 mm + (5% W + 5% PVA****) mct 20 mm] + Al;
7 [(30% CaCO3 + 70% SîO2)mct 10 mm + (10% W + 5% PVA****)mct 20 min] + Al; *Wollastonite (W), **Succinic acid (SA), ***Polyvinyl alcohol (PVA)_
Activation and modification of the mixture components contribute to formation of anorthite and helenite in the combustion process. After modifying wollastonite with succinic acid and activating a mixture of quartz and marble, silicon is more completely realized in combustion reactions, wollastonite decomposes during the synthesis process, and decay products are involved in formation of anorthite, helenite, and CaAUO7.
A comprehensive study of the technological combustion of systems containing wollastonite after ultrasonic treatment (UST) was also carried out. After ultrasonic treatment in aqueous solution in the presence of various modifying additives, highly dispersed particles and their aggregates are encapsulated in sufficiently dense polymer films [6]. Depending on the conditions of UST (time, frequency), the nature and level of structural changes are somewhat different, this being indicated in thermokinetic characteristics of the combustion process. Introduction of wollastonite modified during UST into the charge mixture (SiO2 + Al) in an amount of 10% contributes to a decrease in the induction period of the mixture ignition, an increase in the rate and temperature of combustion as well as a change in temperature at the post-process stage (Fig. 2).
Time, s
(a) (b)
Fig. 2. Thermograms of the combustion system [(SiO2 + 37.5%Al]) with 10% FW100 wollastonite (1) in the initial state and after ultrasonic treatment in water for (2) 20, (3) 40, and (4) 60 min at (a) a frequency of 40 kHz and (b) simultaneous exposure to 40 and 10 kHz.
The level of these characteristics varies with both the processing time and the frequency of ultrasound exposure. With introduction of various modifying additives into the medium of ultrasonic treatment of wollastonite, the general tendency to intensify the combustion process is preserved, only the parameters of the SH-synthesis process change somewhat.
One of the indicators of the quality of synthesized samples is the tensile strength. If for a synthesized sample of [SiO2 + 37.5% Al] system with quartz activated for 10 min in a planetary centrifugal mill the strength is 33 MPa, its values for SHS samples obtained with wollastonite modified at different conditions of UST vary significantly depending on the conditions of ultrasound modification. The measurement results are presented in Table 2. The highest strength was shown by samples containing wollastonite after ultrasonic treatment in water and in an aqueous solution of glycerin (73.15 MPa). In all cases, introduction of wollastonite into the charge after UST contributes to a more complete conversion of the initial components during the combustion of the samples [7]. The greatest amount of corundum and silicon is formed, when using wollastonite treated in water, an aqueous solution of polyvinyl alcohol and salicylic acid.
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Table 2. Strength of SHS-samples [SiÜ2 + 37.5% Al] with 10% wollastonite modified under different conditions of UST depending on the frequency and time in different liquid media.
Conditions of US-treatment Strength of SHS-samples, MPa
Frequency f kHz Time, min The medium of US-treatment of wollastonite
H2O H2O + 5% CsH5(OH)3 H2O + 5% H2SiÜ3nH2Ü H2O + 5% (C2H3ÜH)n H2O + 5% C9H8Ü4
20 41.8 50.16 25.08 20.9 62.70
40 40 71.06 39.71 21.62 22.99 62.70
60 62.70 41.80 18.81 35.53 52.25
20 33.44 58.52 10.45 14.63 73.15
100 40 31.35 12.54 16.75 33.44 62.70
60 37.62 14.63 45.98 31.35 52.25
20 41.80 18.81 52.25 39.71 73.15
100 + 40 40 73.15 73.15 31.35 41.8 62.70
60 29.26 20.90 27.17 20.9 62.70
The results of the studies clearly show that preliminary mechanochemical treatment both in dynamic mills and under ultrasonic action in liquid media is an effective way to influence the structure and the state of the material being processed, increases its reactivity, which is manifested in the changes in the thermokinetic characteristics of the combustion process (SHS mode), conversion of the reagents used, the phase composition of synthesis products and the properties of the material obtained.
1. A.E. Levashov, A.S. Rogachev, V.I. Yukhvid, I.P. Borovinskaya, Physico-chemical and technological bases of self-propagating high-temperature synthesis, M.: BINOM, 1999, 176 p.
2. N.Z. Lyakhov, T.L. Talako, T.F. Grigorieva, The influence of mechanical activation on the phase and structure formation processes during self-propagating high-temperature synthesis, Novosibirsk: Parallel, 2008, 168 p.
3. O.V. Abramov, I.G. Kharbenko, S. Shvegla, Ultrasonic treatment of materials, M.: Mashinostroenie, 1984, 243 p.
4. A.E. Baranchikov, B.K. Ivanov, Yu.D. Tretyakov, Sonochemical synthesis of inorganic materials, Adv. Chem., 2007, vol. 76, no. 2, pp. 147-168.
5. Z.A. Mansurov, N.N. Mofa, B.S. Sadykov, Zh.Zh. Sabaev, Ultrasonic treatment of wollastonite and obtaining SHS-composite systems, III Int. Sci. Conf. Modern Problems of Condensed Matter Physics, Nanotechnologies and Nanomaterials, 2014, Almaty, pp.18-20.
6. Z.A. Mansurov, N.N. Mofa, T.A. Shabanova, Hybride, nano-structurized materials of special purpose on the basis of silicon dioxide, Adv. Eng. Ceram. Compos., 2011, vol. 484, pp. 230-240.
