Graphite contains a layered structure. Under certain external factors, some molecules, atoms, ions or clusters can be inserted between the graphite layers to form interlayer compounds. Under the action of instantaneous high temperature, the graphite intercalation compound is called swelled graphite because the contained intercalation material is rapidly decomposed and vaporized and violently expanded by several tens to hundreds of times. Expanded graphite is a kind of loose porous graphite material. It can be made into porous materials with different pores through rolling and other processes. It has wide applications in the fields of environment, chemical industry, energy, fire protection and construction.
The expanded graphite prepared by the traditional high-temperature expansion method has a very large porosity and a large pore size, and most of the pores limit the application of the porous graphite and reduce the application effect. In this paper, the novel and microwave methods are used to puff the graphite. Due to the different mechanism of action of the microwave method and the high temperature expansion method on graphite, different expansion effects can be produced. By controlling parameters such as power and time of microwave heating, the degree of strength of the graphite treatment can be easily adjusted. In addition, the microwave method is convenient to start and stop, and has the advantages of high efficiency and energy saving.
1. Experimental method 1 Graphite intercalation treatment The graphite is intercalated by chemical method. The intercalant selected is 98% sulfuric acid and the oxidant is potassium permanganate. Graphite is natural flake graphite 890# and 592#, which is provided by Baoding Lianxing Carbide Co., Ltd. of Hebei Province, with fineness of 80 mesh and 50 mesh respectively, and carbon content of 90% and 92%, respectively.
Adjust the temperature of the water bath between 40 and 60 ° C, then place a glass beaker into the water bath. First, 30 g of concentrated sulfuric acid was poured into a glass beaker, and under a continuous stirring, a mixture of pre-mixed potassium permanganate and graphite was poured. The mixture contained 10 g of graphite and the powder was off 2 g of potassium permanganate. Stir using a magnetic bar. The temperature of the water bath was controlled to be constant, and the intercalation reaction was continued for 1 h. The graphite scales were then filtered using a 200 mesh stainless steel wire mesh and washed with water. The washing process is as follows: First, the filtered graphite scales are placed in a 1000 ml and beaker, 500 ml of water is added, and the mixture is stirred with a glass rod for about 2-3 minutes, and the pH is measured, and then filtered through a 200 mesh stainless steel screen. The above water washing process was repeated, and the pH of the washing liquid was measured and recorded. As the number of washes increases, the pH gradually approaches neutrality. In general, the pH of the second washing liquid is about 00.4, and the pH of the fifth washing liquid is 6-7. The cleaned graphite intercalate was dried in an oven at 70 ° C for a drying time of about 12 h.
1. 2 Microwave puffing The dried graphite intercalate was taken to be expanded to prepare expanded graphite. The ordinary household microwave oven is selected as the microwave generating device, and the model of the microwave oven is Galanz WP800, and its rated input power is 1200w, and the microwave output power is 800w. The microwave generation power is continuously adjusted by the knob method, from 136w to 800w.
Graphite has conductive properties, and its inside generates a strong eddy current under the action of microwaves, which has a very fast and intense heating effect. Therefore, the use of microwave heating of graphite is very efficient. Under the microwave power of about 500w, the microwave can be fully expanded by the action of microwave for about 10s. The advantages of down-wave method for expanding graphite: simple operation, high efficiency, safety, uniform expansion, low equipment cost, etc.
1. 3-hole structure characterization The pore structure of porous graphite was measured by mercury intrusion. The instrument used was an Autoscan-60 mercury intrusion porosimeter manufactured by uantachrome. The instrument has a test pressure range of 0.003~414Mpa, a test aperture range of 2~8000nm, a volume resolution of 0.001cm2, and a pressure accuracy better than full scale and 0.25%.
2 experimental results and discussion 2. 1 Microwave method and high temperature folk furnace method The intercalated material was prepared by chemical method, and the graphite pore structure measurement results of porous graphite were prepared by microwave expansion, as shown in Fig. 1. As a comparison, the pore structure measurement results of the conventional high-temperature electric furnace method for the preparation of porous graphite were also given. 1a is the hole number step with the hole diameter and distribution curve, and FIG. 1b is the hole volume with the hole diameter and distribution curve. It can be seen that the pore structure of the porous graphite prepared by the two expansion methods differs greatly.
According to the angle analysis of the number of holes, the pore size distribution of the porous graphite prepared by the microwave method is below 100 nm, and the pore size distribution of the porous graphite prepared by the high-temperature electric furnace method is between 30 and 800 nm. If pore volume analysis is used, the pore size distribution of porous graphite prepared by microwave method is below 1000 nm, most of which is distributed below 100 nm; and the pore size distribution of my pore graphite prepared by high temperature electric furnace method is between 30 and 8000 nm. It can be considered that the pores in the porous graphite prepared by the microwave method are substantially nano-scale, and the pores in the porous graphite prepared by the conventional high-temperature electric furnace are between the nanometer scale and the micrometer.
2. 2 Effect of microwave action time Figure 2 shows the test results of the effect of microwave action time on the pore structure of my pore graphite. In the figure, the graphite intercalation under the action of microwaves, curve 1 represents the case where the sparks that have just been emitted immediately turn off the microwave. Curve 2 represents the case of full expansion. In actual operation, under the microwave power condition of 500w, the former case of the microwave action time is generally about 2s, and the latter case is about 10s.
It can be seen from Fig. 2a that as the microwave action time is prolonged, the maximum aperture can be slightly increased, but the change is small, but the larger variation of the pore size distribution range is increased from 4 to 20 nm to 4 to 100 nm. As can be seen from Fig. 2b, the effect of microwave action time is very significant: on the one hand, the volume of the pores increases significantly with the extension of the microwave action time, and on the other hand, the pore size distribution range is greatly expanded, from 4 to 20 nm to 4~ 1000nm.
2. The effect of 3 scale graphite fineness As shown in Fig. 3, the most suitable pore size decreases with the decrease of the particle size of the scale, both from the angle of the number of holes and from the angle of the volume of the pore. Therefore, porous graphite prepared using a graphite material having a small particle size has more pores. As can be seen from Figure 3b, the particle size has little effect on the total pore volume.
2. 4 Effect of pH Figure 4 shows the structure and effect of the pH of the water wash on the porous graphite pores during the water washing of the graphite intercalate. Among them, curve 1 represents water washing twice, the final water washing solution has a pH of 0.4; curve 2 represents water washing 5 times, and the final water washing liquid has a pH of 6.2.
As can be seen from Fig. 4, the number of times of washing and the pH of the washing liquid have little effect on the pore structure of the porous graphite, and the most pore diameter is hardly affected, and the total pore volume does not change much. The pore size distribution curve of porous graphite with a small number of washings and a low pH of the washing liquid is slightly flatter than the washing time and the pH of the washing liquid is slightly flat.
3 Conclusion Deep-method and microwave micro-prepared nano-scale porous graphite, compared with high-temperature electric furnace scale, has good nanoporosity; and its pore structure is washed by water (pH of washing liquid), microwave action time and scales The effect of graphite particle size.
Microwave-expanded graphite is highly efficient, and it takes only a few seconds for the graphite to fully expand. Therefore, in the industrial production line, a microwave cavity may be included, and an inlet and an outlet are opened on both sides thereof, and the expandable graphite is continuously fed into the cavity by the crawler to control the traveling speed of the crawler so that the graphite is in the cavity. Fully expanded, the expanded graphite is transmitted from the outlet, which can improve the production efficiency of expanded graphite.
(Institute of Environmental Materials, School of Materials Science and Engineering, Tongji University, Zhang Dongtian, Sheng Li, Xiao Deyan)
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