As shown in Figure 1, the non-graphitic carbon is composed of a small hexagonal network, showing a disordered structure slightly along the c-axis structure. The crystal grains are cross-linked with each other and coexist with the amorphous phase.
According to the ability of graphitization, non-graphitizable carbon is divided into graphitizable carbon and nongraphitizable carbon. Figure 2 shows the structure of easily graphitized carbon and that of difficult-graphitizable carbon. The difference in these structures is attributed to the rearrangement of crystal grains during the carbonization of the carbon precursor.
In the non-graphitizable carbon, the carbonization process suppresses the accumulation of graphite layers and causes cross-linking between crystal grains. Due to the small crystal grains and disordered structure, it is very difficult for crystals to reorganize to achieve graphitization even at temperatures higher than 2500°C. On the other hand, the graphite layers in the graphitizable carbon are arranged in a parallel manner, so graphitization is easy. At the same time, due to different carbon materials produced by different raw materials or carbonization processes, their carbon grain size (La and Lc) increases with the increase of the heat treatment temperature.
Figure 3 shows the relationship between La and Lc of easily graphitized carbon and difficult graphitized carbon as a function of heat treatment temperature. From the curve, we can see that the La and Lc values of graphitizable carbon are higher than those of difficult graphitizable carbon, and this difference becomes more obvious with the increase of temperature. In a certain temperature range, the graphitization of easily graphitizable carbon will be very rapid, while for difficult graphitizable carbon, the increase in temperature will not help its graphitization.
When the carbonization temperature is 3000°C, the La value of easily graphitizable carbon can reach 100 nm, while the maximum La value of non-graphitizable carbon can only reach 10 nm. Similarly, the Lc value of graphitizable carbon can reach 100 nm, while the Lc value of non-graphitizable carbon can only reach 4 nm.
Figure 4 shows the Raman spectra of crystalline graphite and different carbon materials with different grain sizes. In crystalline graphite, the asymmetric C=C bond appears at about 1582 cm﹣1 (G mode), and the harmonic peak of 1355 cm﹣1 (D mode) appears at 2708 cm﹣1. For materials with smaller grain size (La) or amorphous carbon, at 1355 cm ﹣ 1 (D mode), ~ 1622 cm -1 (D’ mode) and ~ 2950 cm ﹣ 1 (D and D’ A wave crest is observed at the mode). The wave crests at ~ 1355 cm ﹣ 1 and ~ 1622 cm ﹣ 1 are caused by the diamond structure. The larger the crystal grain, the narrower the width of the Raman spectrum peak. The peak intensity at 1355 cm-1 increases as the size of the graphite crystal decreases or the degree of disorder increases. As shown in Figure 5, the ratio of the peak intensity of 1355 cm﹣1 to 1575 cm-﹣1 (1582 cm﹣1 in G mode) is inversely proportional to the graphite crystal size, that is, the smaller the peak intensity ratio, the larger the graphite grain size. Big. Based on this inverse relationship, we can infer the size of the graphite grains from the Raman wave peak.
Through transmission electron microscopy, the strong microstructures of easy-to-stone fruiting carbon and hard-to-stone fruiting carbon can be observed. Figures 6 and 7 show the TEM images of carbon precursor, anthracene and sucrose after heat treatment at 1000 ℃ and 2300 ℃, respectively.
The non-graphitizable carbon obtained by heat treatment at 1000 °C exhibits an anisotropic structure of cross-linking between graphite layers. The arrangement of graphitizable carbon heat-treated at the same temperature is similar to graphite and has anisotropy. structure. For the carbon material obtained by heat treatment at 2300°C, the non-graphitizable carbon will lead to the disordered arrangement of fine curved graphite layers, while the graphitizable carbon shows a well-developed graphite layer structure.