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

The paper presents the results of experimental studies on the advanced synthesis method for a cubic phase of silicon carbide using charcoal and silicon dioxide as precursors. The charcoal used for the synthesis was obtained by steam pyrolysis of wood wastes (sawdust). It was found that the arc synthesis could be executed in air due to the protective CO and CO 2 environment formation by charcoal oxidation. With an increase of the amount of supplied energy by direct current arc plasma synthesis, the products of two crystalline phases were formed: graphite and cubic phase of silicon carbide. The phase of silicon carbide was extracted from the synthesis product by its annealing in air at 850 °C. The resulting cubic SiC phase was characterized by an elementary cell parameter of 4.359 Å. According to the data of scanning electron microscopy, the morphology of crystals obtained is common for the biomorphic silicon carbide which was identied in the synthesis product. The ceramics synthesized by spark plasma sintering from the obtained material was characterized by a density of ~2.0 g/cm 3 .


Introduction
Wastes from the wood are a renewable raw material for processing and further application for charcoal production. In turn, charcoal is a raw material for refractory materials obtaining based on metal and nonmetal carbides [1]. The use of charcoal for the production of metal and nonmetal carbides provides the synthesis with renewable, environmentally friendly raw materials. 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 can be used to solve various problems, for example, to create hightemperature lters. 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 scienti c development in eld of synthesis methods and the application of refractory carbides. A silicon carbide is the most studied among them as a material with an important set of properties, such as the resistance to oxidation, high hardness and relatively low density [2]. SiC is used in the aerospace industry, medicine, and other elds [3][4]. The synthesis of silicon carbide using various types of wood has already been studied for a beech, an oak, a sapele, a maple, a pine, and a linden [5][6][7][8][9][10][11]. A variety of morphological features of natural precursors provides almost unlimited implementation of the synthesis of ceramic materials with different morphological characteristics and, accordingly, properties [12][13][14]. The synthesis of silicon carbide using wood as an initial material is often carried out in two stages: in the rst stage, the wood is pyrolized to produce charcoal. At the second stage its in ltration 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 in ltration 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.
A well-known approach to produce silicon carbide powders is a usage of an electric arc. Arc discharge plasma provides 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]. Usually the synthesis takes place in an inert gas atmosphere in a closed reactor. However, 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 executed. This synthesis method application was possible because of selfshielding of the reaction zone by formed carbon monoxide and dioxide [24]. Thus, air cannot oxidize the obtained reaction products until they became cold and the oxidation reaction rate achieved a negligibly low value. This synthesis technique is simpli ed compared to the one assumed arc reactor construction with a vacuum pump, closed chamber and the entire vacuum-gas system of the reactor. Thus, the cost of this synthesis method is lower in general 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 implemented using charcoal obtained from the wood waste (sawdust) and silicon dioxide. The paper presents results of a study of the characteristics of synthesis product versus initial parameters of the process: the duration of arc discharge, the ratio of carbon to silicon dioxide in the initial composition. A powder of silicon carbide with biomorphic structure was obtained, which was then sintered by the method of spark plasma sintering. As a result, ceramic samples of silicon carbide with a density of ~2.0 g/cm 3 was obtained.

Biochar obtaining from wood wastes
The biochar was obtained because of steam gasi cation of waste from a wood processing enterprise, pine sawdust. The process was carried out at superheated steam temperature of 400 °C with a steam ow rate of 1 kg / h. The process was carried out for 0.5 hours. The mass of the biochar residue was 28 wt. % in relation to the initial mass of sawdust. The gasi cation 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 hours. After grinding the screening was performed to obtain a powder fraction with a particle size < 200 μm. The elemental composition of biochar and its ash residue were determined using a Flash 2000 CHNS elemental analyzer (Thermo Fisher Scienti c, USA) and the iCAP 6300 Duo inductively coupled plasma optical emission spectrometer (Thermo Scienti c, USA), respectively. The speci c surface area was determined using an ASAP 2400 speci c surface area analyzer (Micromeritics, USA).

Electric arc discharge synthesis
Electric arc synthesis was carried out in an open air [23][24]. A direct current energy source with a maximum operating current of 220 A was used. An anode (graphite rod) and a cathode (graphite crucible) were connected to the power source. A mixture of reagents was placed in the cathode cavity: biochar produced from sawdust with particles of < 200 μm with an amorphous structure and an amorphous SiO 2 with particle size < 20 nm. A gel was prepared from the particles of silicon dioxide preliminarily: 15 ml of water and biochar powder were added to 1 g of silicon dioxide. The resulting gel was stored at room temperature in a vacuum chamber for 2 hours at a pressure < 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 which added more energy into the system for a time given. Three series of parallel experiments were carried out: 1. The duration of the synthesis process was varied from 5 s to 25 s (with a 5 s step). See supplementary video V1 (color image with temperature distribution) and V2 (video image).
3. The synthesis was carried out for equal conditions in order to accumulate a su cient amount of material for its subsequent enrichment and sintering.

Analytical methods
The synthesized materials were analyzed by X-ray diffractometry (Shimadzu XRD 7000s, CuKα radiation). The X-ray patterns were analyzed using standard X-ray diffractometer software (PC Suit XRD-6100/7000 Ver. 7.00: main). In particular, a component of the program for the analysis of the crystallinity degree (XRD: Crystallinity) was used. To carry out a qualitative X-ray phase analysis, the PDF4+ database was used. The quantitative analysis was carried out on the basis of the integrated intensity of the diffraction maximums.
Samples were studied under a TESCAN VEGA 3 SBU scanning electron microscope (SEM) and an OXFORD X-Max 50 energy-dispersive adapter (EDS) with a 10-20 kV accelerating voltage, specimen current of 12 nA, and a spot diameter of approximately 2 µm. The scanning electron microscope Hitachi TM 3000 was used for SEM as well.
The study of thermal decomposition (oxidation) of the obtained samples was carried out using a differential scanning calorimetry and thermogravimetry (DSC/TG/DTG) Netzsch STA 449 F3 Jupiter (Netzsch, Germany). The analysis was carried out at a heating rate of 10 °C/min in corundum crucibles with perforated lids in the temperature range from room temperature to 1200 °C. Powdery samples with a mass of ~ 20 mg were analyzed in air. The gas ow rate was 150 ml/min (air) and 10 ml/min (nitrogen).
In the latter case, nitrogen was used as a shielding gas to ensure reliable operation of the analyzer and the correct recording of the data obtained. All DTA-TG experiments were carried out under atmospheric pressure. A comparative assessment of the parameters of the oxidation process was carried out on the basis of temperature, time and rate of the oxidation reaction calculated graphically using TG, DTG and DSC curves.
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 boat and heated up to 850 °C at 25 °C/min heating rate. The powders were kept at this temperature for 2 hours. The obtained material after cooling was collected for further research and sintering.
Sintering of the obtained material was carried out in the SPS-10-4 system of spark plasma sintering (Advanced Technologies) at an axial pressure of 10 MPa and a temperature of 1800 °C for 10 minutes in an argon atmosphere. The ceramic samples obtained were processed for further research, in particular, on the surface microstructure. The samples were ground and then polished using Forcipol 1 V grinding and polishing machine via diamond discs with a 54 μm, 18 μm, 6 μm, 3 μm roughness and polishing cloths with diamond suspensions with abrasives with an average particle size of 6 μm, 3 μm, 1 μm, 0.25 μm.

Characteristics of biochar
The technical characteristics and elemental composition for the C, H, N, S, O elements of the produced biochar are presented in Table 1. The sample of biochar obtained because of steam gasi cation was characterized by a relatively high value of the volatile substances content (about 32 wt.%) and a relatively low ash content (about 1 wt.%). The high value of the adsorbed moisture content could be caused by the activation of pore structure of carbon samples during the steam treatment of initial pine sawdust. 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) was characterized by the highest content of silicon (39.0 wt.%), calcium (28.9 wt.%) and magnesium (14.6 wt.%).
According to X-ray diffractometry data, the obtained biochar was characterized by an amorphous structure.

Arc discharge synthesis
Typical X-ray patterns of the products synthesized by electric arc method by (C charcoal + SiO 2 ) treatment are shown in Figure 1. In Figure 1a Figure 1, a) and (3, Figure 1, a) were characteristic for the synthesis products obtained with 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 during the silicon carbide formation. It should be noted that with an increase in the duration of the arc discharge, the energy consumption for the synthesis process was increased as well as the anode in the process of its consumption, which was also increased the amount of graphite contaminating the synthesis product [28]. Thus, it was experimentally established that for the complete processing of the feedstock the required arc discharge duration at current of 220 A must be 15 -20 seconds. Further experiments with a change in the SiO 2 :C mass ratio were carried out with a synthesis process duration of 20 s. Figure 1,b 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. The largest ratio used was SiO 2 :C = 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 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 in uence of high temperatures and was losing oxygen atoms with the silicon carbide formation under reaction (1). The evolved oxygen and air oxygen were reacted with carbon with the release of carbon monoxide CO under reaction (2). Carbon monoxide CO was further oxidized to form carbon dioxide CO 2 (reaction (3)). The formation of SiO is also possible under reaction (4), but the yield of volatile silicon oxide is 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 process was described in our previous work [24]. Thus, we can conclude 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, just a low yield of the silicon carbide phase (10-15 wt.%) according to the method of quantitative XRD was determined. It is known that silicon carbide is highly resistant to oxidation in air. Thus, we investigated the possibility of graphite removing from the synthesis products by calcination in an atmospheric furnace with the transition of carbon to gaseous CO 2 . 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 Figure 2. The mass loss of the studied sample was proceeded in one stage, which was associated with the oxidation of graphite phase particles in the composition of the synthesis product. Since the sample was subjected to high-temperature treatment at the stage of synthesis, there was no stage of the adsorbed moisture removal (which is typical for biochar obtained by the method of steam gasi cation). 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 in an amount of 16 wt.% was associated with the cubic silicon carbide phase presence. The mass loss was recorded in the temperature range 700-900 °C.
According to DTG data (Figure 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 position of the exothermic maximum on the DSC pro le (Figure 2) was coincided with the value of T max corresponding to the maximum rate of the oxidation reaction (calculated from the DTG data). 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.%), silicon (up to 18 wt.%). The presence of trace amounts of other of chemical elements such as calcium was observed as well. The identi cation of impurities in the synthesis product, in particular, calcium, was explained by their presence in the composition of the obtained biochar. The dominance of carbon in the sample according to the data of energy dispersive analysis 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 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-re ected electrons) images obtained are given in Figure 3 (a, c, d, f) and the SE (re ected electrons) the images are given in Figure 3 (b, e). According to the data of scanning electron microscopy, the obtained silicon carbide was represented by both crystal shapes of regular outlines of attened habit (typical for micronsized silicon carbide crystals [33]) and complex shapes obtained by the authors for the rst time in the framework of a vacuum-free electric arc method. These structures were different from the traditional because of their porous structure, which had a certain similarity in textural features with the initial biochar material. On Figure 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 is indicated on Figure 3 d, 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 identi ed 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 puri cation and SPS treatment
To isolate the phase of silicon carbide from the synthesis products, considering the data of differential thermal analysis, a heat treatment regime was chosen for annealing of the synthesis products. The samples were placed in an atmospheric oven, heated up to 850 °C and held at this temperature for 2 hours. The materials obtained in this way were analyzed by XRD. XRD patterns of synthesis products puri ed from the graphite phase are shown on Figure 4a. The X-ray diffraction pattern on Figure 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 phase of silicon carbide. This trace of the diffraction maximum at 2 = 34.06 might correspond to the presence of an insigni cant 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 at.% 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 contain a thin X-ray amorphous oxide layer of ceramic sample. At the same time, energy dispersive analysis (taken from individual crystals from the image in Fig. 4b with dimensions of about 10 μm) showed that the mass ratio of silicon and carbon was within 1.7-1.9, and the oxygen content tended to a value close to zero. Judging by 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 causes of the relatively low density ~2.0 g/cm 3 of the sintered bulk sample.

Conclusion
In this work powders and a ceramic sample of SiC were obtained using the electric arc method for plasma processing a mixture of silicon oxide and charcoal obtained from sawdust. The dependences of the phase composition of the synthesis product from the initial parameters of the process, in particular, duration of the synthesis and the silicon dioxide to charcoal mass ratio, were obtained. By annealing the obtained material in an atmospheric furnace at 850 °C, it was possible to purify the phase of cubic silicon carbide from the graphite. The resulting powder was sintered by SPS method. Ceramic sample was characterized by the rather low density ~2.0 g/cm 3 . The morphology of the obtained materials was made it possible to identify multiple pores and individual crystals of silicon carbide with size varied in a range from 1 to 10 µm. The similar to biomorphic silicon carbide structures could be found into obtained powder material.
Thus, the article revealed the possibility of obtaining a SiC powder with a biomorphic structure, as well as low-density ceramics based on silicon carbide using charcoal. The vacuum-free electric arc method was used which was realized in an ambient air environment. Cubic silicon carbide was synthesized for the rst time using charcoal (obtained from woodworking waste) and silicon oxide in atmospheric electric arc plasma by a non-vacuum method.

Declarations
Funding: This research was supported by TPU development program.
Con icts of interest/Competing interests: The authors declare that they have no known con ict or competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper. Figure 1 XRD patterns of the obtained products (λ = 1.54060 Å): a) synthesized products obtained at different electric arc discharge durations: 1) 5-10 s, 2) 15 s, 3) 20-25 s; b) typical diffraction patterns of the synthesis products obtained at the 20 s arc discharge duration at various mass ratios of the silicon dioxide to carbon in the initial mixture: 4) SiO2:C = 1:2, 5) SiO2:C = 1:1, 6) SiO2:C = 3:1.

Figure 2
TG, DTG and DSC data for the oxidation process of the as-synthesized product (SiC + C). Air ow rate 150 ml/min, heating rate 10 °C/min, sample weight 20 mg.