FEATURES OF SYNTHESIS OF COMPOSITE MATERIAL BASED ON SILICON DIOXIDE AND CARBON NANOTUBES

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Introduction. The unique properties of carbon nano- tubes such as their strength and chemical resistance [1; 2] determine the interest in their use as an additive to various materials for the production of composites. At present time, there are a lot of works [3-8] showing the efficiency of the use of carbon nanotubes as additives to composites. Their application in polymers is especially successful [9-12], but the effectiveness of their use in ceramics leaves many questions. For example, direct mixing of ceramic particles with carbon nanotubes leads to a dete- rioration in the strength of a composite with an increase in the proportion of nanotubes [13] and only using them in materials with a large proportion of the amorphous phase, including glass and crystalline glass [14], increases the strength of such a composite. Materials used and experiment. Carbon nanotubes were obtained from ethanol [15] on a [Ni (NH3)6]Cl2 pre- cursor at a temperature of 600 °C. As a model ceramics, we used quartz ceramics pro- duced from a slurry of aqueous ammonia and tetraethox- ysilane obtained by annealing in colloid suspension. The structure of such ceramics is grains of amorphous SiO2 (fig. 1) with the size of 200-500 nm. Ceramic material was obtained by hydrolysis of tetraethoxysilane followed by sol-gel precipitation and annealing at 900 °C. Since the efficiency of direct mixing of nanotubes with ceramic particles does not always give an effective result, and it is very difficult to achieve uniformity of dis- tribution of nanotubes in this case, the authors used only two methods, which guaranteed uniform mixing. On the one hand, carbon nanotubes can get into ceramics during its formation from a suspension, and on the other hand, carbon nanotubes can already be placed in ceramics by growing them in pores and cavities of the material. To implant nanotubes into ceramics at the growth stage, the following method was used. The nanotubes were dispersed by ultrasound in 2-propanol to obtain a stable colloid. Then dispersion containing 0.1 mas. % of nanotubes was added to tetraethoxysilane in a volume ratio of 1:10, respectively. After that, ammonia was added to the mixture, and the suspension was recieved by agitation. The suspension, after drying, was annealed at 900 °C. The growth of nanotubes in ceramics was provided by introducing a catalyst into the pores of ceramics annealed at 900 °C and was carried out at a temperature of 600 °C from ethanol. The catalyst was added into ceramics by impregnating the finished material with a precursor. Results. In fig. 2 photographs of received materials are presented. On the left, there is a composite obtained by synthesizing ceramics in a dispersion of carbon nano- tubes, on the right, there is a composite with carbon nano- tubes grown in the pores of ceramics. As you can see, with the same amount of substance, completely different volumes of material are obtained. It should be noted that the density of ceramics obtained by synthesis from a dispersion with carbon nanotubes is less than without nanotubes (or with nanotubes grown in the pores of the finished ceramics). This decrease in density is determined by the addition of carbon nanotubes during the synthesis of ceramics and is explained by the fact that carbon nanotubes, having a high specific surface, lower the free energies of the formation of ceramics on their surface. Thus, they act as a catalyst for the growth of ce- ramics. Measuring the hardness of ceramic composites clearly indicate the hardening of ceramics obtained on carbon nanotubes, compared with the original, at the same time, the growth of carbon nanotubes reduced the strength of carbon nanotubes (see table). Vickers ceramic hardness Type of ceramic Vickers hardness number Without CNT 300 MPa Ceramics made with carbon nanotube dispersion 1 GPa Ceramics with CNT grown in pores 200 MPa Fig. 1. Quartz ceramics. Scanning electron microscopy Рис. 1. Кварцевая керамика. Растровая электронная микроскопия Fig. 2. Quartz ceramics, the samples of equal mass. On the left there is a composite obtained by synthesizing ceramics in a dispersion of carbon nanotubes, on the right there is a composite with carbon nanotubes grown in ceramic pores Рис. 2. Кварцевая керамика, образцы равной массы. Слева - композит, полученный синтезом керамики в дисперсии углеродных нанотрубок, справа - композит с углеродными нанотрубками, выращенными в порах керамики A significant increase in the hardness of ceramics grown on the basis of carbon nanotubes (fig. 3, a) is ex- plained by the more dense structure of the original ce- ramic grains, which was created by a matrix of carbon nanotubes. That is in the colloid, the formation of silica particles proceeded not only in volume, but also on the surface of carbon nanotubes, which led to a significant compaction of the entire material after annealing. а b Fig. 3. Composite based on quartz ceramics: a - ceramics obtained with carbon nanotubes; b - ceramics with grown carbon nanotubes in it Рис. 3. Композит на основе кварцевой керамики: а - керамика, выращенная на основе углеродных нанотрубок; б - керамика с выращенными углеродными нанотрубками The decrease in the hardness of ceramics with carbon nanotubes grown (fig. 3, b) compared to the initial one can be explained by the deformations in ceramics that occur with the growth of carbon nanotubes, which have a wedging effect on grains. Conclusion. In this work, carbon nanotubes were placed in ceramics either during its formation from a sus- pension, or by growing them in pores and cavities of ce- ramics. In the first case, it is important to note that the synthe- sis of nanotubes does not affect the formation of ceramics. Nanotubes always influence the process of forming ce- ramics. This influence leads to a change in the structure of the ceramic particles, and as a result of the mechanism of interaction among them, which changes the density and strength of ceramics. . In the second case, in order to grow nanotubes in the pores and cavities of a ceramic material, you should first saturate the ceramic material with a catalyst. Thus, there is a requirement for a catalyst precursor - the lack of its interaction with ceramics. The second requirement is the inertness of ceramics, both to the catalyst and to the entire synthesis process. In addition, it is necessary that the pore structure does not change during the synthesis, i.e. they did not close during the synthesis of nanotubes, but pro- vided the transport of starting materials and reaction products.

About the authors

M. M. Simunin

Federal Research Center KSC SB RAS; Siberian Federal University

Email: michanel@mail.ru
50, Akademgorodok, Krasnoyarsk, 660036, Russian Federation; 79, Svobodny Av., Krasnoyarsk, 660041, Russian Federation

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