Short Communication
A method to observe the structure of the interface between mesocarbon microbeads and pitch

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Highlights

  • An interface between MCMBs and pitch matrix was observed by a new method.

  • The interface absorbs molecules from pitch.

  • The absorbed molecules change their orientations in the interface.

Abstract

It is known that mesocarbon microbeads (MCMBs) have ordered structure and grow after absorbing molecules from pitch. However, the structure of pitch used for preparing MCMBs is disordered. Thus question of whether there is a region where the molecules absorbed from pitch change their orientations is a very interesting problem. In this paper, an interface between MCMBs and their pitch matrix has been found and the structure of the interface was observed by scanning electron microcopy. There are some ordered regions in the interface but the number of ordered regions in the interface is much fewer than the number inside the MCMBs. This indicates that the interface between the MCMBs and the pitch is the region where the orientations of the molecules absorbed from pitch change.

Introduction

Mesocarbon microbeads (MCMBs) are widely used in lithium batteries due to their ordered graphitic structure [1], [2], [3]. It is well known that MCMBs appear when pitch is heated and the MCMBs can grow after absorbing molecules from pitch [4]. MCMBs have an ordered structure but the structure of pitch used for preparing the MCMBs is disordered. Therefore, we think that there is an interfacial region where the molecules absorbed form pitch change their orientations (i.e. the structure becomes more ordered). In order to find and observe this region, a proper method is needed. Recently, a new method was developed to study the structure of MCMBs [5], [6]. In this method, the MCMBs were carbonized, which enabled the structure of the MCMBs to be clearly seen using scanning electron microcopy (SEM). However, this method involved washing the MCMBs away from their pitch matrix with an organic solvent before carbonization. This may destroy the MCMBs-pitch interface (if any exists). In order to study the interface, the MCMBs and their pitch matrix need to be carbonized together. However, as the carbonization temperature increases, the MCMBs tend to coalesce with each other and eventually form bulk mesophase. Therefore, in this paper, a method that uses a thermosetting phenol resin to anchor the MCMBs and their pitch matrix is reported. The resin restricts the movement of the MCMBs during carbonization. In addition, Ca(OH)2 was used to prevent the MCMBs from coalescing with each other.

Section snippets

Experimental

Some properties of the coal tar pitch used in the study are shown in Table 1. Initially, 0.8 g of Ca(OH)2 was added to 200 g of coal tar pitch at 90 °C with stirring. The mixture was heated to 395 °C in N2 for 6 h with continuous stirring. During this time, MCMBs appear in the mixture. After cooling the mixture to room temperature, some of the mixture was placed in the middle of a thermosetting phenol resin and then the resin was cured at 90 °C for 5 h. Next, the sample was carbonized in N2 at 950 °C

Results and discussion

The SEM images of the MCMBs in the sample in Fig. 2a and b shows that there are two kinds of MCMBs in the sample. One is a typical Brooks–Taylor type [4] with two poles (Fig. 2a) and the other is an X type with four poles (Fig. 2b), which forms when the Brooks–Taylor type MCMBs coalesce [7]. Both types can be divided into three regions: the pitch matrix (blue triangles), the interface between the MCMBs and the pitch (regions between the two green lines) and the regions inside the MCMBs (red

Conclusions

A novel method was used to study structure of MCMBs. In this method, MCMBs were prevented from coalescing with each other when they were carbonized together with their pitch matrix by using resin and Ca(OH)2. An interface between MCMBs and their pitch matrix was found, which has never been reported by previous studies. Results showed that molecules from pitch matrix are absorbed by MCMBs from the interface and ordered structures appear from the interface to inside of MCMBs. However, the number

Acknowledgments

This work was supported by the National Basic Research Program of China (973 Program No. 2012CB720302).

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