煤系针状焦原料在成焦过程中的XRD结构分析

低QI(原生喹啉不溶物)含量的软沥青（SCTP）是制备煤系针状焦的优选原料，研究其在成焦过程中的结构变化有助于高品质针状焦的研制。基于样品的X射线衍射（XRD）数据，利用Smarsly团队开发的CarbX软件对其全谱拟合，定量出SCTP在不同炭化温度（400，500，600，800，1 000，1 200和1 400 ℃）下的微晶结构参数，进而在纳米尺度下研究SCTP的热致结构变化情况。结果表明，随炭化温度升高，微晶堆垛的石墨烯层大小L_a从初始沥青的10.3 Å逐步增大到1 400 ℃的47.9 Å，但在500 ℃前L_a增加缓慢，只有当温度超过800 ℃后，L_a才显著增大，这表明需要800 ℃以上的高温才能使交联石墨烯层内的原子重组，进而导致微晶长大。然而，石墨烯碳网的C—C键长l_cc受温度的影响很小，在1.41~1.42 Å范围内变化。由于SCTP在液相炭化成半焦过程中存在中间相转化，导致微晶堆垛高度L_c在500 ℃前逐步增大，在500 ℃时达到最大（L_c=31.1 Å），随后由于半焦进一步热解缩聚，使L_c逐步减小，在1 000 ℃时达到最低点（L_c=15.4 Å），超过1 000 ℃后又开始增大。与L_c的变化趋势相同，堆垛的石墨烯层数N从原始沥青的2.66层增加到500 ℃的约9.05层，随后减小到1 000 ℃的4.55层，超过1 000 ℃后又开始增大。由于500 ℃前样品仍处于沥青态，所以此阶段微晶的石墨烯层间距a₃都较大，约为3.50 Å。当在500 ℃变为半焦后，a₃迅速减小至3.44 Å。随后温度升高，a₃在1 000 ℃达到最小（a₃=3.39 Å），1 000 ℃后又开始增大，这表明焦炭经历了收缩再膨胀过程。通过CarbX软件拟合样品的XRD数据，除了可得到样品炭微晶的主要尺寸（L_a，L_c，N，a₃）信息外，还可获得这些参数的分散性（k_a，k_c，σ₃，ε₃）以及堆垛的取向性（q）、均匀性（η）和无序碳含量（c_un）等信息，有利于深入了解样品的微观结构，有助于优质针状焦的生产。

XRD Structural Analysis of Raw Material Used as Coal-Based Needle Coke in the Coking Process

Soft coal tar pitch (SCTP) with low QI content is the preferred raw material for preparing coal-based needle coke. The study on its structure changes in the cooking process is helpful to prepare high-quality needle coke. In this paper, the CarbX software developed by the Smarsly team was used to fit the full spectrum X-ray diffraction (XRD) data of the samples toquantify the microcrystalline structure parameters of SCTP at different carbonization temperatures (400, 500, 600, 800, 1 000, 1 200 and 1 400 ℃), and then investigate the thermally induced structural changes of SCTP at the nanoscale. The results show that the average graphene layer size of microcrystalline stack L_a gradually increases from 10.3 Å for the pristine pitch to 47.9 Å at 1 400 ℃ with the rising of the carbonization temperature, but L_a increases slowly before 500 ℃. A significant increase of L_a is found only when the temperature exceeds 800 ℃, indicating that high temperatures above 800 ℃ are needed to recombine the atoms in the cross-linked graphene layers and lead to the growth of the microcrystals. However, the C—C bond length (l_cc) of the graphene carbon network is slightly affected by temperature and varies in the range of 1.41~1.42 Å. Because of mesophase transformation during the liquid-phase carbonization of SCTP into semi-coke, the average stack size L_c gradually increases before 500 ℃ and reaches the maximum at 500 ℃ (L_c=31.1 Å). Subsequently, due to further pyrolysis and polycondensation of semi-coke, L_c gradually decreases and reaches the lowest point (L_c=15.4 Å) at 1 000 ℃, and increases again after 1 000 ℃. Similar to L_c, the average number of graphene layers per stack N increases from 2.66 layers for the raw pitch to 9.05 layers at 500 ℃, then decreases to 4.55 layers at 1 000 ℃, and then begins to increase after 1 000 ℃. The samples are still in the pitch state before 500 ℃ the average graphene interlayer spacing a₃ is large, about 3.50 Å at this stage. When the pitch becomes semi-coke at ca. 500 ℃, a₃ rapidly decreases to 3.44 Å, continues to decrease, reaches the minimum at 1 000 ℃ (a₃=3.39 Å), and begins to increase again after 1 000 ℃, indicating that the coke has undergone a shrinkage and re-expansion process. By using CarbX software to fit the XRD data of the sample, the main size (L_a, L_c, N, a₃) of carbon microcrystals of the sample can be obtained, as well as the dispersion (k_a，k_c，σ₃，ε₃) of these parameters and the orientation (q), homogeneity (η) of per stack and disordered carbon content (c_un). It is helpful to deeply understand the sample’s microstructure and to produce high-quality needle coke.