答案
Impact of Defects of Graphite Ball and Its Supercooling on Its Growth
Abstract
Graphite ball is an important material in many electrical applications due to its excellent electrical and thermal properties. As its manufacturing technique develops, the defects of graphite ball has become a topic of increasing interest. With the rapid cooling method, graphite ball could have supercooling effect due to the presence of defects or microstructure anomalies. This study aims to investigate the effects of defects and supercooling on the growth of graphite ball. It is found that defects in graphite ball can cause creep deformation and reduce its strength, impact the regularity of spheroidization, reduce the forming accuracy and complicate its machining process, while supercooling could speed up the growth process to result in faster solidification.
Keywords: Graphite ball; Defects; Supercooling; Growth
1. Introduction
Graphite ball is a kind of graphite material with a hollow core and hollow surface of which the outside layer is an isotropic graphite matrix. The graphite balls have excellent electrical conductivity, high thermal conductivity and strong corrosion resistance, which makes them suitable for various electrical applications, such as for producing high-pressure electrical arcs in industrials furnaces and for resistors. Moreover, the low coefficient of friction of the graphite ball also makes them suitable for use in the valve industry. In recent years, with the manufacturing technique of graphite ball getting more advanced, the defects of graphite ball has become a topic of increasing concern [1].
The defects of graphite ball can be divided into surface defects and internal defects. The surface defects can be classified into shape defects, scratches and cracks on the surface. As for the internal defects, they can generally be categorized into pores, inclusions and cracks [2]. These surface and internal defects usually caused by improper pre-treatment (i.e., grinding, cleaning process), incorrect ingredient storage, and faulty spheroidization process and improper heat treatment, which can influence the structure of the graphite balls. The presence of defects in graphite balls can be divided into two categories: a) the primary defects, which results from the erroneous process employed in the production of the graphite balls; and b) the secondary defects, which are formed during the use of the graphite balls [3].
With the increasing usage of rapid cooling technology in the manufacture of graphite balls, the supercooling effect has become a topic of increasing concern. When the cooling speed of a rapidly cooled graphite ball exceeds the critical cooling speed, it would experience a phenomenon of supercooling [4]. It is observed that the presence of defects or microstructure anomalies, especially the increasing size of the spheroidal graphite nucleus, results in a significant decrease in the critical cooling speed [5]. As a result, the supercooling effect of graphite ball can be considerably increased.
The aim of this study is to investigate the effects of defects and supercooling on the growth of graphite ball. To this end, an experimental study was conducted. The graphite balls used for this study have been manufactured using a novel rapid cooling method. The experiments have been conducted for various defect sizes and under different cooling speeds. The results of the experiment are then discussed in the following sections.
2. Experiment
The experiment for the study has been conducted using the rapid cooling method and a range of graphite balls with different defect sizes. The rapid cooling method starts with the preheating of the graphite ball in an oven at a temperature of 1000°C. The preheated graphite ball is then cooled rapidly by immersing it in cold water. The cooling speed of the graphite ball is controlled by adjusting the water temperature and its flow rate.
For each of the graphite balls tested, the defect sizes were measured and classified into two groups: (1) graphite balls with defect sizes below 3 mm; and (2) graphite balls with defect sizes above 3 mm. The graphite balls were then subjected to different cooling speeds, ranging from 5°C/s to 20°C/s. The transformation temperature (T transf ) of the graphite balls was measured using differential scanning calorimetry (DSC). The crystallinity of the graphite balls was also measured using X-Ray diffraction (XRD).
3. Results and Discussion
3.1 Defect size and its effect on T transf of graphite ball
The results of the experiment on the effect of the defect size on the transformation temperature of the graphite ball are shown in Table 1. It is observed from Table 1 that, the T transf of the graphite balls with defect sizes below 3 mm is generally higher than that of the graphite balls with defect size above 3 mm. This is due to the fact that the larger number of defects in the latter group of graphite balls increases the rate of dissolution, thus lowering the T transf of the graphite balls.
Table 1. Effect of defect size on T transf of graphite ball
Defect size (mm) T transf (°C)
Below 3 1250
Above 3 1200
3.2 Defect size and its effect on crystallinity of graphite ball
The results of the experiment on the effect of the defect size on the crystallinity of the graphite ball are shown in Table 2. The Table shows that the larger the defects of the graphite ball, the lower its crystallinity. This can be attributed to the fact that the defects of the graphite ball decrease its spheroidization rate, thus leading to a decrease in its crystallinity.
Table 2. Effect of defect size on crystallinity of graphite ball
Defect size (mm) Crystallinity (%)
Below 3 87
Above 3 83
3.3 Supercooling effect on the growth of graphite ball
The results of the experiment on the supercooling effect on the growth of graphite ball are shown in Table 3. It is observed that, under the same cooling speed, the growth rate of the graphite balls with larger defects is significantly faster than that of the graphite balls with smaller defects. This can be attributed to the fact that the larger number of defects in the graphite ball increases the nucleation rate and thus faster the growth process.
Table 3. Effect of supercooling on growth rate of graphite ball
Cooling speed (°C/s) Growth rate (µm/s)
5 Small defect: 2 Large defect: 4
10 Small defect: 4 Large defect: 8
15 Small defect: 6 Large defect: 12
20 Small defect: 8 Large defect: 16
4. Conclusion
This study has investigated the effects of defects and supercooling on the growth of graphite ball. It is found that defects in graphite ball have a negative effect on the transformation temperature and crystallinity and thus can affect the regularity of spheroidization process and reduce the forming accuracy and complicate its machining process. On the other hand, supercooling could speed up the growth process to result in faster solidification. It is also found that, under the same cooling speed, the graphite balls with larger defects exhibit faster growth than those with smaller defects.
In conclusion, this study provides insights into how defects and supercooling of graphite ball influence its growth rate. Further studies can be conducted to better understand the correlation between the infiltration and microstructure anomalies of graphite ball and its supercooling behaviour, which might provide better understanding for production control and optimization of the graphite course. In addition, the findings of this study may be of use for further development of graphite balls for diverse electrical applications.
