Advanced Arc Plasma Synthesis of Biomorphic Silicon Carbide Using Charcoal and Silicon Dioxide in Air

This work presents the results revealing the possibility of obtaining a cubic phase of silicon carbide with features of a biomorphic structure. Renewable plant raw materials were used as a source of carbon, in particular, pyrolyzed sawdust, which is a waste of a timber enterprise. Silicon dioxide powder was used as a source of silicon. The synthesis was realized using DC arc discharge plasma initiated in an open air. In this case, the oxidation of the synthesis products was prevented due to the effect of the reaction volume self-shielding from atmospheric oxygen. It was possible due to the generation of protective gaseous medium predominantly consisting of carbon dioxide and monoxide. The dependences of the product phase composition on the supplied energy and composition of initial components were established. The synthesis product was characterized by a significant excess of carbon, which was a caused by the erosion of the electrodes. After removal of chemically unbound carbon from synthesis product by annealing in an atmospheric furnace at 850 °C, obtained powder was sintered by the spark plasma sintering method. In the result, a bulk ceramic sample was obtained in which the only one crystalline phase of silicon carbide with a lattice parameter of 4.359 Å was identified.


Introduction
Wood waste is available and ecological renewable raw material for charcoal production. In turn, charcoal is a raw material for obtaining refractory materials based on metal and non-metal carbides [1]. Additionally, the microstructure of charcoal could affect the morphology of the synthesis product, including the so-called "biomorphic" structure (structure with signs of the morphology of a living organism) of the material. In particular, such porous biomorphic materials could be used to solve various problems, for example, to create high-temperature filters. A large number of types of wood and other biomass, as well as its waste, allows the application of feedstock for the synthesis of the carbides of metals and non-metals with different shape and size of pores and channels, as well as with different physical and mechanical characteristics. In this case, it is possible to involve organic waste in the technological cycle of carbide production. Thus, the use of charcoal as a raw material for the synthesis of metal and non-metal carbides is an important direction of the scientific development in field of synthesis methods and the application of refractory carbides.
Silicon carbide (SiC) is one of the most studied among such materials due to the unique set of properties, like the resistance to oxidation, high hardness and relatively low density [2]. SiC is used in the aerospace industry, medicine and other fields [3,4]. The synthesis of silicon carbide using wood has already been studied for beech, oak, sapele, maple, pine and linden [5][6][7][8][9][10][11]. A variety of morphological features of natural precursors provided wide possibilities for implementation of the synthesized ceramic materials due to different morphological characteristics and, accordingly, properties [12][13][14]. The synthesis of silicon carbide using wood as a raw material is often realized in two stages: in the first stage, the wood is pyrolized to produce charcoal. At the second stage its infiltration is carried out with a silicon melt or vapor and with the formation of SiC [1,[15][16][17]. A well-known method for the synthesis of carbide with a biomorphic structure is a sol-gel [18,19]. Despite of progress in this area, methods for silicon carbide synthesis using charcoal are still undeveloped. Among the disadvantages of the available methods, the following features could be distinguished: the long duration of the synthesis process, the need for prolonged infiltration of the initial biochar, the necessity to create an inert atmosphere for the synthesis process, the uneven distribution of the initial components and the complexity of the morphology controlling of the obtained particles. Accordingly, the development of new methods for the synthesis of silicon carbide using charcoal is an important task.
The silicon carbide powders production in plasma generated by electric arc is a well-known solution. Arc discharge plasma allows to reach high (on the order of several thousand degrees) temperatures in the reaction zone which is suitable for the synthesis of silicon carbide [20][21][22]. However, plasma methods are still relatively complex, has high energy and resource consumption. Usually the synthesis takes place in the inert gas atmosphere of a closed tight reactor chamber. In this case, significant part of energy is spent by vacuum system working in continuous regime. The vacuum pump, auxiliary vacuum equipment and reactor chamber itself contribute significantly into reactor cost. A group of vacuum-free electric arc methods has appeared recently. In our previous works the synthesis of silicon and tungsten carbides in the plasma of a DC arc discharge in air [23] was reported. Realization of this synthesis method was possible due to self-shielding of the reaction zone by formed carbon monoxide and dioxide [24]. Thus, air didn't oxidize the obtained reaction products until they cool down and the oxidation reaction rate dropped to a negligibly low value. Such synthesis technique is simpler compared to the ones requiring arc reactor with a vacuum pump and other necessary equipment. Thus, the cost of this synthesis method realization could be lower while and the productivity of an open-type DC electric arc reactors is higher [25][26][27].
In the present work the electric arc synthesis of the cubic phase of silicon carbide in air has been realized using silicon dioxide mixed with charcoal obtained from the wood waste (sawdust). The paper presents results of a study of the obtained product characteristics dependences on the parameters of the synthesis process: the duration of arc discharge, the carbon to silicon dioxide ratio in the feedstock. A silicon carbide powder with biomorphic structure was obtained. The ceramic samples of silicon carbide with a density of ~ 2.0 g/ cm 3 were obtained by the of spark plasma sintering method using SiC powder.

Electric Arc Discharge Synthesis
The biochar was obtained by steam gasification of waste pine sawdust from a timber enterprise. The gasification was carried out in the superheated steam atmosphere at 400 °C and 1 kg/h flow rate for 30 min. The mass of the biochar residue was 28% to the initial sawdust mass. The gasification product was ground in a drum mill in the following grinding mode: the ratio of the mass of grinding balls to the material was 1:1 and the grinding time was 8 h. After grinding the screening was performed to obtain a powder fraction with a particle size < 200 μm.
Electric arc synthesis was carried out in an open air [23,24]. A direct current energy source with 220 A maximum operating current was used. An anode (graphite rod) and a cathode (graphite crucible) were connected to the power source. A mixture in form of the gel consisting of produced biochar powder with an amorphous structure and an amorphous SiO 2 with particle size < 20 nm was placed in the cathode cavity. A gel was preliminarily prepared using the particles of silicon dioxide: 15 ml of water and biochar powder were added to 1 g of silicon dioxide. The obtained gel was stored at room temperature in a vacuum chamber for 2 h at < 0.01 MPa to remove air. The resulting powder was dried in a corundum crucible in an oven at 200 °C for one hour in air. Then the powder was loaded into the cathode cavity for the arc treatment. A maximum current up to 220 A (power density up to ~ 19 W/mm 2 , arc voltage ~ 30 V) was used to add more energy into the system for set period of time. The three series of parallel repetitive experiments were carried out: 1. The duration of the synthesis process was varied from 5 to 25 s (with a 5 s step). See supplementary video V1 (color image with temperature distribution) and V2 (video image). 2. The mass ratio of SiO 2 :C was varied and was 3:1, 2:1, 1:1 and 1:2. 3. The synthesis was carried out at abovementioned conditions in order to accumulate a sufficient amount of material for its subsequent enrichment and sintering.

Analytical Methods
The elemental compositions of biochar and its ash residue were determined using a Flash 2000 CHNS elemental analyzer (Thermo Fisher Scientific, USA) and the iCAP 6300 Duo inductively coupled plasma optical emission spectrometer (Thermo Scientific, USA), respectively. The specific surface area was determined using an ASAP 2400 specific surface area analyzer (Micromeritics, USA). The synthesized materials were analyzed by X-ray diffractometry (Shimadzu XRD 7000 s, CuKα radiation). The qualitative X-ray phase analysis was performed using the PDF4+database on the basis of the integrated intensity of the diffraction maximums.
Samples were studied by TESCAN VEGA 3 SBU scanning electron microscope (SEM) with OXFORD X-Max 50 energy-dispersive adapter (EDS). The SEM Hitachi TM 3000 was used for auxiliary SEM images as well.
The thermal decomposition of the obtained samples was carried out using a differential scanning calorimetry and thermogravimetry (DSC/TG) Netzsch STA 449 F3 Jupiter (Netzsch, Germany) at atmospheric pressure, 10 °C/min heating rate in corundum crucibles with perforated lids in the temperature range from room temperature to 1200 °C.
The synthesized powdery material was annealed in an atmospheric furnace (EKSP 10, Russia) to purify the synthesized silicon carbide phase. As-synthesized powders were placed in a mullite crucible and heated up to 850 °C at 25 °C/min heating rate. The powders were kept at this temperature for 2 h and cooled to 150 °C before retrieving.
Sintering of the obtained material was carried out in the SPS-10-4 system of spark plasma sintering (Advanced Technologies) at 10 MPa axial pressure and 1800 °C temperature for 10 min in an argon atmosphere. The samples were ground and polished using Forcipol 1 V grinding and polishing machine with diamond discs.

Characteristics of Obtained Biochar
The technical characteristics and elemental composition for the C, H, N, S, O elements of the obtained biochar are presented in Table 1. The biochar sample was characterized by a relatively high content of volatile matter (about 32 wt%) and a relatively low ash content (about 1 wt%). The high moisture content could be caused by the activation of pore structure of carbon samples during the steam treatment. Also, this sample was characterized by high oxygen content and the absence of sulfur, which is typical for products of the wood waste thermal processing.
The elemental composition of the ash residue (according to the iCAP 6300 Duo analyzer data (Thermo Fisher Scientific, USA)) was characterized by the highest content of silicon (39.0 wt%), calcium (28.9 wt%) and magnesium (14.6 wt%).

Arc Discharge Synthesis
Typical X-ray patterns of the products synthesized by electric arc method by (C charcoal +SiO 2 ) treatment are shown in Fig. 1. According to X-ray diffractometry data, the initial biochar was characterized by an amorphous structure. In Fig. 1a the diffraction patterns of the samples obtained in a series of experiments with a different duration of the synthesis time varied in the range from 5 to 25 s (which corresponded to the amount of supplied energy from 30 to 150 kJ) are presented. The diffraction pattern (line 1 in Fig. 1a) was typical for samples obtained at a synthesis process duration from 5 to 10 s. In the diffraction pattern the amorphous halo was identified in the 2Ɵ range from 16 deg. to 25 deg which indicated the presence of an amorphous fraction in the synthesis product. The amorphous fraction could be referred to both biochar and amorphous silicon dioxide. The crystalline component was represented by two phases: graphite and a cubic phase of SiC. The lattice parameters of the graphite phase were the following: a = 2.465 Å, c = 6.871 Å. The lattice parameter of the cubic phase of silicon carbide was estimated to be a = 4.354 Å. The diffraction patterns (line 2 in Fig. 1a) and (line 3 in Fig. 1a) were characteristic for the synthesis products obtained at the duration of the arc discharge from 15 to 25 s. Their characteristic feature was the absence of an amorphous halo. This could be an evidence of the initial amorphous biochar graphitization as well as the consumption of silicon dioxide for the silicon carbide synthesis. It should be noted that with an increase in the duration of the arc discharge, the energy consumption of the synthesis was increased as well as the anode consumption, which also contributed to the increasing in the amount of graphite in the synthesis product [28]. Thus, it was experimentally established that for the complete processing of the feedstock at 220 A current the 15-20 s required arc discharge duration is required.
The experiments with a change in the SiO 2 :C mass ratio were carried out at a synthesis duration of 20 s. Figure 1b shows the results of this experimental series. All X-ray diffraction patterns were characterized by the presence of two crystalline phases: graphite and a cubic phase of silicon carbide. In this case, with an increase in the mass ratio of SiO 2 :C, the intensity of the diffraction maxima corresponding to the silicon carbide phase was also increased reaching its maximum at SiO 2 :C ratio equal to 3:1. A further increase in the proportion of silicon dioxide was seen as inappropriate, since the volume occupied by biochar in the composition of the initial components, which served as a matrix for the formation of the possible desired microstructure, was too small in comparison with the volume of the initial silicon dioxide and gel based on it.
The experimental data obtained corresponded to the known concept of the silicon carbide synthesis in electric arc. Silicon dioxide (SiO 2 ) was placed in the zone of initiation and combustion of the arc discharge. It was decomposed, thus, due to the influence of high temperatures it was losing oxygen atoms with the silicon carbide formation according to the reaction (1). The released oxygen and air oxygen reacted with carbon with the release of carbon monoxide CO according to the reaction (2). Carbon monoxide CO was further oxidized to form carbon dioxide CO 2 (reaction (3)). The formation of SiO was also possible according to the reaction (4), but the yield of volatile silicon dioxide was low under atmospheric pressure.
CO and CO 2 formed a gaseous layer that protected the synthesized product SiC from oxidation by atmospheric oxygen. A similar explanation for the synthesis of silicon carbide was given in [29] which described a method for obtaining silicon carbide nanorods in constant current arc discharge plasma in a helium atmosphere. The dynamics of the CO and CO 2 release with the formation of a protective gas barrier during the synthesis was described in our previous work [24]. Thus, it could be concluded that when synthesizing silicon carbide in an electric arc discharge plasma, regardless of the composition of a gaseous medium, the synthesis takes place in a protective atmosphere of CO and CO 2 .
However, a low yield of the silicon carbide phase (10-15 wt%) according to the method of quantitative XRD was obtained. It is known that silicon carbide is highly resistant to oxidation in air. Thus, the investigation on the possibility of graphite removing from the synthesis products by calcination in an atmospheric furnace with the transition of carbon to gaseous CO 2 was performed. A differential thermal analysis was carried out to choose the heating mode of the obtained materials.
The results of the oxidation process of a typical produced powder via the DTA-TG method are shown in Fig. 2. The mass loss of the studied sample was proceeded in one stage, which was associated with the oxidation of graphite phase particles of the synthesis product. Since the sample was subjected to high-temperature treatment during synthesis, there was no need to remove the adsorbed moisture (which is typical for biochar obtained by the steam gasification). The temperature of the onset of intense oxidation was about 700 °C. The total mass loss of the sample was 84 wt%, which was observed in the temperature range 620-940 °C. Even though the ash content in the initial sample was about 1 wt% (Table 1), the residue obtained after the oxidation was close to 16 wt%. It was associated with the cubic silicon carbide phase presence.
According to DTG data (Fig. 2), the oxidation process of the studied sample was characterized by a monomodal peak. The maximum reaction rate in this point was 0.62 wt%/min at the T max = 895 °C.
It should be noted that the temperature of the exothermic maximum on the DSC profile (Fig. 2) coincided with T max . The observed exothermic effect was associated with the release of heat during the oxidation of the graphite phase.
According to the data of scanning electron microscopy combined with an energy dispersive analysis, the synthesis product mainly consisted of carbon (up to 51 wt%), oxygen (up to 22 wt%) and silicon (up to 18 wt%). The presence of the trace amounts of other chemical elements such as calcium was observed as well. The identification of impurities in the synthesis product, in particular, calcium, was explained by their presence in the composition of the obtained biochar. The prevailing of carbon in the sample according to the energy dispersive analysis data is natural due to the dominance of the graphite phase in the synthesis product. The relatively high oxygen content could be associated with its presence in the initial biochar, incomplete processing of the initial silicon dioxide, and the presence of an amorphous oxide shell on the surface of silicon carbide particles. The formation of an amorphous oxide layer on the surface of micro-and nanoparticles of silicon carbide is a well-known phenomenon, and it is characteristic for materials both obtained by electric arc discharge plasma in atmospheric air [23] or inert gas [30][31][32]. Scanning electron microscopy was carried out using several different detectors: the BSE (back-reflected electrons) images obtained are given in Fig. 3a, c, d and f and the SE (reflected electrons) images are given in Fig. 3b and e. According to the data of scanning electron microscopy, the obtained silicon carbide was represented by both crystal shapes of regular outlines of flattened form (typical for micron-sized silicon carbide crystals [33]) and complex shapes obtained by the authors for the first time in the framework of a vacuum-free electric arc method. These structures differed from the traditional ones because of their porous structure, which had a certain similarity in textural features with the initial biochar material. In Fig. 3 the number 1 indicates the porous and capillary structures of charcoal; number 2 denotes agglomerates of silicon dioxide particles deposited on the surface of charcoal; number 3 denotes silicon carbide structures. Structures of silicon carbide are indicated on Fig. 3d and e. SiC phase, thus, was isolated from the synthesis product by the annealing in an atmospheric furnace and it had the morphological similarities with silicon carbide identified as biomorphic SiC [1]. The porous structure of the material obtained had a certain regularity and order which allowed us to conclude that the experiment with the synthesis of the biomorphic silicon carbide was successful.

SiC Purification and SPS Treatment
To isolate the phase of silicon carbide from the synthesis products, considering the data of differential thermal analysis, a thermal treatment regime was chosen for annealing of the synthesis products. The samples were placed in an atmospheric oven, heated up to 850 °C and exposed at this temperature for 2 h. The materials obtained were analyzed by XRD. XRD patterns of synthesis products purified from the graphite phase are shown on Fig. 4a. The diffraction pattern on Fig. 4a corresponded to a sintered ceramic sample. The material consisted of silicon carbide only within the sensitivity limits of the XRD method. The lattice parameter of the main phase of silicon carbide in the obtained sample was 4.359 Å, which coincides with the lattice parameter of the reference silicon carbide phase. This trace of the diffraction maximum at 2ʘ = 34.06 might correspond to the presence of an insignificant amount of the hexagonal phase of α-SiC in the product [20]. According to EDX measurements, the sintered ceramic sample contained on average 34.1 wt% of carbon, 62.4 wt% of silicon, 3.0 wt% of oxygen and other chemical elements with a total content up to 0.5 wt%. As it was mentioned above, elemental oxygen could be contained in a thin X-ray amorphous oxide layer of ceramic sample. At the same time, energy dispersive analysis (taken for individual crystals from the image in Fig. 4b with dimensions about 10 μm) showed that the mass ratio of silicon to carbon was varied from 1.7 to 1.9, while the oxygen content tended to decrease up to zero. According to the data of scanning electron microscopy, the structure of the ceramic sintered sample was inhomogeneous. Pores with sizes of up to several micrometers were observed. The presence of these pores was the cause of the relatively low density-~ 2.0 g/ cm 3 -of the sintered bulk sample.

Conclusion
Combining all abovementioned data the following conclusions could be formulated: 1. Cubic silicon carbide was synthesized for the first time using charcoal (obtained from timber waste) and silicon dioxide in atmospheric electric arc plasma by a nonvacuum method; 2. The dependences of the phase composition of the synthesis product on the initial parameters of the synthesis process, in particular, duration of the synthesis and the silicon dioxide to charcoal mass ratio, were obtained; 3. By annealing the obtained material in an atmospheric furnace at 850 °C, it was possible to isolate the cubic silicon carbide phase from the extra graphite; 4. Obtained powder was sintered by SPS method. The ceramic sample was characterized by the low density ~ 2.0 g/cm 3 ; 5. The morphological analysis of the obtained materials made it possible to identify multiple pores and individual crystals of silicon carbide with size varied in a range from 1 to 10 µm; similar to biomorphic silicon carbide structures could be found into obtained powder material.

Supplementary Information
The online version contains supplementary material available at https:// doi. org/ 10. 1007/ s12649-021-01517-8. Author Contributions AYP-direct current plasma arc processing of samples, writing-original draft. KBL-wood waste biochar samples acquisition and characterization. APK-XRD data acquisition and interpretation, plasma arc processing data analysis. TYY-SEM and EDS results acquisition and interpretation. SAY-DTA-TG data acquisition and interpretation. VEG-idea and design of article, general supervision. KVS-writing. AAG-review and editing.
Funding Synthesis of the studied samples was carried out with the financial support of Tomsk Polytechnic University development program. The physicochemical characteristics of the studied samples was studied with the financial support of the Ministry of Science and Higher Education of the Russian Federation within the framework of the project No. 075-00268-20-02 (ID: 07180-2020-0040).
Data Availability Data available within the article or its supplementary materials.

Conflict of interest
The authors declare that they have no known conflict or competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.