3.1. 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 gasification 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.
Table 1 Technical characteristics of the obtained biochar (Flash 2000 CHNS)
Parameter
|
Value
|
Moisture content, wt.%
|
5.1
|
Ash content, wt.%
|
1.1
|
Volatiles content, wt.%
|
31.8
|
Net calorific value, MJ/kg
|
28.1
|
Specific surface area, m2/g
|
0.008
|
Element composition, wt. %
|
С
|
76.0
|
H
|
3.3
|
N
|
0.2
|
S
|
0
|
O
|
19.4
|
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.
3.2. Arc discharge synthesis
Typical X-ray patterns of the products synthesized by electric arc method by
(Ccharcoal + SiO2) treatment are shown in Figure 1. In Figure 1a the diffraction patterns were corresponded to the samples obtained in a series of experiments with a different duration of the synthesis varied in the range from 5 s to 25 s (which corresponded to the amount of supplied energy from 30 kJ to 150 kJ). The diffraction pattern (1, Figure 1, a) was typical for samples obtained with a synthesis process duration from 5 s to 10 s. In the diffraction pattern an amorphous halo was identified in the 2Ɵ range from 16 deg. to 25 deg. which indicates 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 as a = 4.354 Å. The diffraction patterns (2, 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 SiO2: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 SiO2:C, the intensity of the diffraction maxima corresponding to the silicon carbide phase was also increased. The largest ratio used was SiO2: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 microstructure, was too small in comparison with the volume of the initial silicon oxide and gel based on it.
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.
The experimental data obtained corresponded to the known concept of the silicon carbide synthesis in electric arc. Silicon dioxide SiO2 was placed in the zone of initiation and combustion of the arc discharge. It was decomposed, thus, due to the influence 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 CO2 (reaction (3)). The formation of SiO is also possible under reaction (4), but the yield of volatile silicon oxide is low under atmospheric pressure.
SiO2 + C ® SiC + O2 (1)
2 C + O2 ® 2 CO (2)
2 CO + O2 ® 2 CO2 (3)
SiO2 ® SiO + 0.5 O2 (4)
CO and CO2 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 CO2 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 CO2.
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 CO2. 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 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 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 Tmax = 895 °C.
It should be noted that the position of the exothermic maximum on the DSC profile (Figure 2) was coincided with the value of Tmax 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 identification 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 [30-32].
Scanning electron microscopy was carried out using several different detectors: the BSE (back-reflected electrons) images obtained are given in Figure 3 (a, c, d, f) and the SE (reflected 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 flattened habit (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 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 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.
3.3. SiC purification 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 purified 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 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 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/cm3 of the sintered bulk sample.