Preparation of high-porous SiC ceramics from

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Preparation of high-porous SiC ceramics from polymeric composites based on diatomite powder E. P. Simonenko, N. P. Simonenko, M. A. Zharkov, N. L. Shembel, I. D. Simonov-Emel’yanov, V. G. Sevastyanov & N. T. Kuznetsov Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 Volume 50 Number 2 J Mater Sci (2015) 50:733-744 DOI 10.1007/s10853-014-8633-1

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Author's personal copy J Mater Sci (2015) 50:733–744 DOI 10.1007/s10853-014-8633-1

Preparation of high-porous SiC ceramics from polymeric composites based on diatomite powder E. P. Simonenko • N. P. Simonenko • M. A. Zharkov • N. L. Shembel • I. D. Simonov-Emel’yanov V. G. Sevastyanov • N. T. Kuznetsov



Received: 15 April 2014 / Accepted: 25 September 2014 / Published online: 8 October 2014 Ó Springer Science+Business Media New York 2014

Abstract High-porous SiC ceramics (density 0.50–0.58 g/cm3, porosity 82–84 %, compressive stress at break 3.7–6.3 MPa) was prepared by means of polymeric technology and natural raw material (diatomite powder, ‘‘Biosilica’’ grade) at the temperature of carbothermal synthesis (1400 °C). It was shown that the main phase was silicon carbide with a small (\5 %) impurity of FeSi; SiC crystallite size was found to be 23–30 nm. Using scanning electron microscopy, X-ray computerized microtomography, and dynamic light scattering in aqueous suspensions of powders obtained at ultrasonic exposure, it was shown that SiC nanoparticles in the samples were aggregated to a great extent. The degree of aggregation strongly depends on SiO2-C ratio in the starting samples.

Introduction Silicon carbide is one of the most widely used materials, and finds application in various fields of science and technology. Owing to high decomposition temperature, good mechanical characteristics, chemical inertness and the highest oxidation resistance among refractory carbides,

E. P. Simonenko  N. P. Simonenko  V. G. Sevastyanov (&)  N. T. Kuznetsov Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky pr., Moscow 119991, Russia e-mail: [email protected] E. P. Simonenko e-mail: [email protected] E. P. Simonenko  M. A. Zharkov  N. L. Shembel  I. D. Simonov-Emel’yanov Lomonosov Moscow State University of Fine Chemical Technology, 86 pr. Vernadskogo, Moscow 119576, Russia

SiC is the main and the most important component of many composite and ceramic materials for high-temperature applications [1–9]. Silicon carbide is used for production of wear-resistant parts due to its high hardness [10, 11]. Application of silicon carbide as a material for active elements of electronic and sensory devices is widely known from literature [12–15]. Porous ceramic materials based on silicon carbide find application as filters for aggressive gases and molten salts, and also as sorbents [16– 22]. Earlier published papers [23–25] deals with methods of superfine silicon carbide preparation through the procedure combining sol–gel synthesis of the chemically active starting mixture SiO2-C and relatively low-temperature (B1500 °C) carbothermal synthesis at reduced pressure. These studies were performed within the program devoted to the production of ultra-refractory carbides [26–29] as components of high-temperature materials. There are several papers [30–32] concerning application of sol–gel processes for the preparation of porous (density: 0.1–0.3 g/ cm3), including mesoporous, samples of SiC ceramics. However, expensive silicon alkoxides or alkylalkoxides were used in such cases as sources of silicon. Preparation of porous ceramic silicon carbide articles, described in this paper, was performed via polymeric technology [33–37]. This technology includes the following stages: production of polymeric composites based on phenol–formaldehyde resin and affordable silicon dioxide source-diatomite powder of ‘‘Biosilica’’ grade; pressing of samples (articles); thermal treatment of samples (articles) for resin carbonization with the subsequent carbothermal synthesis of silicon carbide. Diatomite powders were previously used [38–47] as SiO2 sources for carbothermal production of SiC and Si3N4 powders (at temperatures above 1450–1500 °C), as well as for reduction by

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Table 1 Some properties of the polymeric composite samples based on biosilica Composition

1

n(C):n(SiO2) ratioa

3.18

Compressive stress at break(MPa)

145.5

a

2

3 3.23

157.2

4 3.44

189.1

5 3.49

193.3

4.13 194.2

According to the DTA data of the starting SiO2-C samples under air

hydrogen or active metals; however, the challenge to develop the procedure, allowing preparation powders with the subsequent formation of porous ceramic articles within one cycle, was not formulated. The goal of this manuscript is to prepare porous silicon carbide ceramics based on diatomite powder using polymeric technology.

Experimental section Reagents. Diatomite powder of ‘‘Biosilica’’ grade (Diamix group, Inzensky deposit, Ul’yanovsk, Russian Federation) was used as a silicon-bearing component (SiO2: 89.2, Al2O3: 5.75, Fe2O3: 2.26, K2O: 1.23, MgO: 0.76, TiO2: 0.36, CaO: 0.33, Na2O: 0.06 wt.%). Phenolic powdery binder SFP-012 K (OJSC ‘‘Karbolit’’) was used as a polymeric source of carbon. Zinc stearate (degree of purity [98 %) was used as a lubricant, required for product forming. Preparation of porous SiC articles was carried out in accordance with the following procedure. At first diatomite powder, phenolic binder SFP-012 K and zinc stearate were mixed in a vibrating mill for 1 h. Five series of samples with different diatomite powder/phenol resin ratios were obtained. In the first series mole ratio of SiO2, contained in diatomite powder, and carbon formed as a result of resin pyrolysis, was close to stoichiometric one (some excess was provided, assuming formation of carbides of other metals presented in diatomite powder). In the other series, phenol resin was taken in an excess amount so that carbon, formed as a result of resin carbonization, not only could serve as a reducing agent during the subsequent carbothermal synthesis of silicon carbide, but also could form the matrix, preventing aggregation and sintering of particles during high-temperature holding. Weight ratio of zinc stearate was in the range of 3.3–3.5 %. The mixture, obtained in a vibrating mill, was rollforged on rolls heated up to 120–130 °C until film delamination, with the subsequent milling for 10 min. The resulting powder was pressed into cylindrical samples (1.0 cm in diameter, 1.5 cm in height) at a pressure of 60 MPa and at a temperature of 170 °C.

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Fig. 1 X-ray diffraction patterns of the starting mixture and the products of carbothermal reduction

Carbonization of polymeric samples was performed in a chamber furnace in carbon filling at 800 ± 30 °C for 1 h; heating rate was equal to 400°/h. As a result, the starting mixtures SiO2:C with different carbon contents were formed (see Table 1). Carbothermal reduction under dynamic vacuum (residual pressure in the reactor was maintained within 1 9 10-1–1 9 10-2 mm Hg) was performed at a temperature of 1400 °C (heating rate *20°/min) with the subsequent holding for 4 h. Cooling was carried out in a furnace; as soon as the temperature reached 200–250 °C, the reactor was filled with argon. X-ray diffraction analysis (XRD) was performed with a DRON-2 instrument (Huber camera, Imaging Plate detector, germanium monochromator, CuKa1-radiation, angle increment 0.005°). Infrared spectra of the samples, suspended in mineral oil and placed between KBr plates, were recorded with a Fourier transform infrared spectrometer InfraLUM FT-08. The thermal behavior of the starting samples and the products was studied using an integrated TG/DSC/DTA analyzer SDT Q-600 (heating rate 20°/min in air flow rate of 100 ml/min). Scanning electron microscopy (SEM) data were obtained with a triple-beam workstation NVision 40 (Carl Zeiss); the elemental composition of microdomains was determined with an EDX system (Oxford Instruments). Transmission electron microscopy (TEM) data were obtained with a JEM-1001 (JEOL) instrument. X-ray computerized microtomography was carried out using a desktop X-ray microtomograph SkyScan 1172 with computing cluster.

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735 Table 2 Properties of the porous SiC samples Composition

1

2

3

4

5

Total linear shrinkage (%) In diameter

10 ± 0.3

In height

10 ± 0.5

Apparent density (g/cm3)a

0.58

Total porosity(%)a

82.0

82.3

82.8

83.2

84.3

Compressive stress at break (MPa)a

5.0

6.2

6.3

6.2

3.7

Specific surface area (m2/g)a Crystallite size (nm)b

18.2 23

19.6 30

19.2 25

19.2 25

20.6 24

0.57

0.55

0.54

0.50

a

After annealing of excess carbon in air at a temperature of 700 °C for 4 h

b

Calculated as a result of analysis of X-ray diffraction patterns by Scherer method

Fig. 2 Thermal analysis data of the SiC samples: composition no. 1 (1), composition no. 2 (2), composition no. 3 (3), composition no. 4 (4), and composition no. 5 (5); a DSC curves, b TG curves

analyzer ASAP 2020 (Micrometrix) at the Scientific Center for New Catalytic Technologies (SCNCT) of the Moscow State University of Fine Chemical Technologies. Size distribution of pores with diameter from 10 nm to 10 lm was examined using mercury porosimetry method with an AutoPore IV 9500 instrument (Micromeritics). Values of hydrodynamic diameter and f-potential of SiC suspensions were estimated using analyzer of particle size, f-potential, and molecular weight Brookhaven 90 Plus (angle of measurement 90°, temperature 25 °C, 10 runs of 30 s each, dispersion medium—MilliQ water, dilution of the initial sample—by 125 times). Analysis was performed using unimodal and multimodal size distribution (MSD) software. f-potential was evaluated by means of ZETA PALS application under the above-mentioned conditions. Suspensions of silicon carbide were prepared by means of ultrasonic generator I10-0.63 (operating frequency 22 ± 10 % kHz) with magnetostrictor (titanium).

Results and Discussion

Fig. 3 Appearance of the SiC samples

Compressive stress at break was determined with a ZIM-UM-5A instrument (lifting speed of plates 5 mm/min) for polymeric composite samples, and with a Plast-BendTester instrument (up to 50 kg, lifting speed 2 mm/min) for porous silicon carbide samples. Specific surface area of the samples was obtained by means of Sorbtometer-M analyzer (BET method, N2, 77 K). Detailed investigation of size distribution for pores with 5–250 nm in diameter was made by means of gas adsorption

It is known that diatomite powders are finely dispersed and porous [37–39]. According to SEM data, treatment of diatomite powder from Inzensky deposit (Russian Federation, Ul’yanovsk city), resulting in the formation of biosilica, does not lead to considerable aggregation; microand nanoporous powder consist of remains of diatom frustules. The values of compressive stress at break (see Table 1) were determined for prepared polymeric composites, filled with biosilica. As may be seen from Table 1, r values fall in the range from 145 to 194 MPa; increase in n(C)/n(SiO2) ratio leads to increase in strength. X-ray diffraction analysis of the starting SiO2-C mixtures obtained as a result of carbonization of polymeric

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Fig. 4 Nitrogen adsorption/desorption isotherms for SiC samples: composition no. 1 (a), and composition no. 5 (b). Insets contain the corresponding pore size distribution curves

Table 3 Average hydrodynamic diameter and f-potential of SiC particles in aqueous suspensions Composition

Dispergating time (min)

1 mode 1

2

Fig. 5 Pore size distribution of the SiC samples measured by mercury porosimetry method (composition no. 4)

composites based on biosilica revealed that the samples were mainly X-ray amorphous with a small impurity of quartz. Infrared spectra of the samples have the characteristic broadened absorption band related to stretching vibrations of Si–O groups, with the maximum being at 1100 cm-1. SiO2:C ratio was determined by means of integrated TG/DSC/DTA analyzer STD Q-600 under air in the temperature range of 25–1200 °C (see Table 1). In case of the samples with composition no. 1, SiO2:C mole ratio is close to stoichiometric one (the excess is about 5 mol. %), and for the samples with composition no. 5, the carbon excess is about 38 %. SiO2-C samples, obtained as a result of carbonization, were used for carbothermal synthesis of SiC ceramics – high-temperature treatment under dynamic vacuum at a temperature of 1400 °C. Infrared spectra of the products revealed disappearance of the absorption bands with maxima at 1100 and 470 cm-1, characteristic for silicon

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3

4

5

f-potential (mV)

Average hydrodynamic diameter (nm) 2 mode

1

200

760

-39 ± 3

3

110

465



8

100

460

-29 ± 8 -38 ± 5

1

90

450

3

130

460



8

180

740

-27 ± 2

1

120

460

-30 ± 2

3

110

470

-26 ± 3

8

105

455

-24 ± 1

1 3

70 70

160/450 230

-28 ± 4 -23 ± 2

8

105

440

-28 ± 2

1

65

450



3

100

440

-25 ± 1

8

30



-30 ± 2

dioxide, and appearance of intense characteristic absorption band m(Si–C) at 750–950 cm-1, indicating the formation of silicon carbide. X-ray diffraction patterns of the products are presented in Fig. 1. It was found that the main phase was cubic silicon carbide, and the crystal lattice did not depend on the composition of the starting mixture. In addition, X-ray diffraction patterns contained small reflections of impurity phase (iron silicide FeSi, \5 %), formed during reduction of iron oxide present in diatomite powder. Crystallite sizes

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Fig. 6 The average hydrodynamic diameter distribution for silicon carbide particles in aqueous suspensions: composition no. 1 (a), composition no. 5 (b)

of silicon carbide calculated by Scherer method were found to be 26 ± 4 nm. To select technique of removal of excess carbon from the obtained SiC-C materials, thermal analysis of such materials in air was performed (air flow rate 100 ml/min, heating rate 20°/min, temperature range 25–1400 °C). Figure 2 illustrates DTA and TG curves for all samples. As Fig. 2 suggests, oxidation of carbon begins for all samples at 530–630 °C and fully completes at a temperature of 800 °C. In case of the sample with the maximum content of excess carbon, oxidation, which is accompanied by weight loss, begins at lower temperatures (composition no. 5, weight loss onset temperature is about 530 °C) compared with the sample with the minimum content of excess carbon (composition no. 1, weight loss onset temperature is about 630 °C). Maximum of the exo-effect lies in the range 705–730 °C for all samples prepared. At temperatures above 800–850 °C, oxidation of silicon carbide, formed during carbothermal reduction, is observed for all compositions. It is related with increase in test charge weight. As evident from thermograms, oxidation takes place in two stages (two partially overlapped maximums are present); geometry and position of these maxima depend on composition of the starting mixture SiO2-C. For example, in case of composition no. 1 with the minimum content of

excess carbon, the maximum is at *1340 °C and has lowresolved and low-intensity shoulder at a temperature of *1240 °C. In case of composition no. 5 with the maximum content of excess carbon, bifurcated exo-effect is observed, and both maxima (at *1190 °C and at 1290 °C) have almost equal intensities. It may indicate that silicon carbide, obtained with an excess of carbon, is more reactive due to smaller values of the average size of particles and their aggregates. Apart from change in the form of the exo-effects due to oxidation of silicon carbide, increase in content of carbon in the starting samples results in shift of the peak position toward lower temperatures; this points to the formation of smaller SiC particles in an excess of carbon. Since substantial oxidation of silicon carbide was observed for all samples at temperatures above 800 °C, excess carbon was removed from the obtained ceramic samples by annealing in air at a temperature of 700 ± 20 °C for 4 h (Fig. 3). Density and estimated porosity of the samples are given in Table 2. As evident from Table 2, density of SiC ceramics appropriately decreases from 0.58 (composition no. 1) to 0.50 g/cm3 (composition no. 5) as carbon content in the starting samples SiO2-C increases. Estimated porosity of the samples increases from 82 to 84 %, while specific

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Fig. 7 Microstructure of the obtained silicon carbide samples: composition no. 1 (a), no. 2 (b), no. 3 (c), no. 4 (d), and no. 5 (e)

surface area values increase from 18.0 to 20.6 m2/g. Nitrogen adsorption/desorption isotherms, obtained using ASAP 2020 instrument, are similar for all compositions; the isotherms for the end points are shown in Fig. 4. As may be seen from Fig. 4, the isotherms are characterized by hysteresis H1, indicating the presence of mesopores. Pore size distribution in the range 10–250 nm, calculated by means of BJH method, betokens the wine-bottle form of pores. It was stated that size of small pores for the obtained

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samples slightly increased as excess carbon content in the starting compositions increased. For example, pore size distribution curves for compositions no. 1 and no. 5, obtained on the basis of adsorption and desorption branches (see Fig. 4), suggest that dominant size of pore site, evaluated from the adsorption branch, increases from 51 to 60 nm, while neck diameter, calculated from the desorption data, increases from 36 to 40 nm. According to the mercury porosimetry data, pore diameter falls in the range

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Fig. 8 Morphology of some fragments of silicon carbide ceramics, retaining microstructure of diatom frustules: composition no. 1 (a), composition no. 2 (b), and composition no. 3 (c, d)

Fig. 9 TEM images of the SiC samples (composition no. 3)

from *300 nm to 2 lm, the average diameter being equal to 770 nm (Fig. 5). Based on powders, obtained as a result of milling of the SiC ceramic samples, aqueous suspensions were prepared,

used for determination of hydrodynamic diameter and fpotential of particles. Suspensions were subjected to dispergating, performed during 1, 3, and 8 min by ultrasonic generator with magnetostrictor (operating frequency

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Fig. 10 X-ray two-dimensional cuts for composition no. 1 (a) and composition no. 5 (b)

22 ± 10 % kHz). The obtained experimental data are presented in Table 3. Bimodal particle size distribution is observed for the majority of suspensions; small aggregates with the hydrodynamic diameter of *100 nm, as well as large aggregates with the average hydrodynamic diameter of *450–550 nm (low-intensity maximum) are detected after ultrasonic treatment for 8 min (examples are presented in Fig. 6). In case of silicon carbide suspension based on composition no. 5 (the maximum content of carbon), it should be noted that for the longest dispergating time disaggregation of powder occurs with the formation of particles of 30–45 nm in diameter; fraction of large aggregates is very small. As may be seen from Fig. 6, ultrasonic exposure for SiC suspension based on composition no. 5 results in the formation of much smaller particles compared with composition no. 1. This result lends support to the view that silicon carbide particles of smaller size and smaller association are formed under carbon excess conditions; it is also in good agreement with the results of thermal analysis. Suspensions based on compositions no. 2 and no. 4 tend to substantial aggregation with the course of time as a result of ultrasonic exposure. f-potentials of silicon carbide particles lie in the range from -23 to -39 mV; in general, these values decrease with the course of time, except for composition no. 5. According to the scanning electron microscopy data obtained for cleavages of SiC ceramic samples, such cleavages consist of strongly aggregated nanoparticles (size of aggregates falls in the range 0.5–2 lm, the average

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particle size falls in the range 20–100 nm, see Fig. 7). Degree of aggregation of particles decreases as carbon content in the starting samples SiO2-C increases from composition no. 1 to composition no. 5. Small amount of impurity phase is detected in the phase contrast mode; such phase, possibly, relates to impurities in natural raw material or impurities formed during carbothermal high-temperature reduction (for example, FeSi). As the microphotographs illustrate, increase in carbon content in the starting samples SiO2-C results in the formation of SiC particles of more prolate shape during carbothermal synthesis. As Fig. 8 suggests, in spite of a large number of chemical processing stages, some fragments of SiC ceramics retain morphology which is typical for biomorphic diatomite powder, and represent the shape of diatom frustules. According to the transmission electron microscopy data of the powders obtained as a result of milling of the samples, the average diameter of separate particles forming the ceramic samples amounts 40 nm for all compositions, while particle size falls in the range of 15–150 nm (see Fig. 9a). In addition, application of this method makes it possible to conclude that the samples, synthesized from the starting compositions with an excess carbon, also contain elongated particles of max. 250 nm in length and *25 nm in diameter (see Fig. 9b). The values of compressive stress at break (r) for all SiC samples were found to fall in the range of 3.7–6.3 MPa (Table 2). The determined values are large enough for such porous samples.

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Fig. 11 3D-models of the pore structure for composition no. 1 (a) and composition no. 5 (c) of SiC ceramics, as well as mutual volumetric distribution of all phases for composition no. 1 (b) and composition no. 5 (d)

According to the results of X-ray computerized microtomography (with resolution of 2.9 lm), cracks and large inhomogeneity domains are absent in the bulk of all samples. As a case in point, Fig. 10 gives X-ray two-dimensional cuts for the samples with the minimum (composition no. 1) and the maximum (composition no. 5) porosity. Scanning of the porous SiC samples allowed selecting characteristic area, used for calculation of the statistical parameters, and constructing 3D-models of the pore structure and volumetric distribution of all phases presented in SiC ceramics (see Fig. 11). Large pores are marked with gray color; silicon carbide distribution is marked with blue color. Models testify that the number and size of large pores, as well as their fraction in the bulk of the samples increase from composition no. 1 to composition no. 5; this is in good agreement with the calculated porosity values.

The total porosity values (for pores with [2.9 lm in size) were calculated to be 6.7 % for composition no. 1 and 11.5 % for composition no. 5. Small increase in the average size of large pores was also observed. According to the results of the analysis, impurity level (more roentgendense, possibly, iron silicide) was found to be 0.01–0.02 % for all samples. Figure 12 illustrates the pore structure using the model describing pores by balls (red color), and connecting channels by cylinders (blue color). As may be seen from Fig. 12, the more porous sample (composition no. 5) is characterized by the more extensive channel network. Calculations revealed that increase in the carbon content in the samples resulted in increase in pore diameter from 9.5 to 10.8 lm, as well as increase in length (from 36 to 45 lm) and diameter (from 5.5 to 6.3 lm) of connecting channels.

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Fig. 12 Model of the pore structure for composition no. 1 (a) and composition no. 5 (b), where pores and channels are depicted as balls (red color) and cylinders (blue color), respectively (Color figure online)

In general, X-ray computerized microtomography data support the conclusion that introduction of excess carbon at the initial stage, which is annealed under mild conditions after carbothermal synthesis, makes it possible to prepare more porous material without loss of integrity of the samples and without formation of large inhomogeneity domains and inclusions.

Conclusions Samples of high-porous (82–84 %) SiC ceramics with density of 0.50–0.58 g/cm3 were prepared by means of polymeric technology and natural raw material (diatomite powder, ‘‘Biosilica’’ grade) at moderate temperatures (1400 ± 20 °C) and reduced pressure. The elemental and phase compositions of the samples were determined. It was shown that iron silicide (FeSi, \5 %) was formed as an impurity phase. The influence of SiO2C preset ratio (carbon is formed as a result of pyrolysis of phenol resin) on properties of SiC ceramics was studied. Notwithstanding the fact that crystallite size values are close to each other for all samples (23–30 nm; these values are predominately dictated by the process temperature), size of aggregates of SiC particles, degree of aggregation, and dispergating ability in aqueous suspensions under ultrasonic exposure depend on the content of excess carbon introduced at the stage of formation of polymeric material filled with biosilica. Thermal

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behavior of powders under air also varies: in case of the sample obtained with the minimum content of excess carbon, maxima of exo-effects related to the oxidation of formed SiC particles are shifted toward higher temperatures by 50–100 °C compared with the sample, obtained with the maximum content of excess carbon. According to the SEM and TEM data, microstructure of all samples contains aggregates (0.5–2.0 lm in size) of nanoparticles (the average diameter 20–100 nm). Degree of aggregation of SiC particles decreases as the content of excess carbon in the starting samples SiO2-C increases; particles of elongated form are present in the samples based on the starting compositions with an excess carbon. Studies of pore structure revealed that the samples contained mesopores (low-temperature nitrogen adsorption, the average diameter (BJH method) was found to be 36–60 nm), and pore diameter fell in the range from *300 nm to 2 lm (mercury porosimetry, the average diameter was found to be 770 nm). X-ray computerized microtomography (with resolution of 2.9 lm) allowed revealing the peculiarities of 3D microstructure of ceramic samples. It was shown that cracks, large inhomogeneity domains, and inclusions were absent in the bulk of all samples. In addition, it was found that volume and diameter of pores increased as the content of excess carbon (which was introduced at the initial stage and subsequently removed) increased. The values of compressive stress at break for SiC samples were found to fall in the range 3.7–6.3 MPa.

Author's personal copy J Mater Sci (2015) 50:733–744 Acknowledgements This study was supported by the Russian Foundation for Basic Research (Grants No. 12-03-33005-mol_a_ved, 13-03-12206-ofi_m, 14-03-31002-mol_a) and a grant of the President of the Russian Federation MK-1435.2013.3. The authors are grateful to Scientific Center for New Catalytic Technologies (SCNCT) of the Moscow State University of Fine Chemical Technologies for assistance in determination of surface areas of the specimens.

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