- Email: [email protected]
Preparation and characterization of low-temperature expandable graphite
Materials Research Bulletin 43 (2008) 2677–2686 www.elsevier.com/locate/matresbu
Preparation and characterization of low-temperature expandable graphite Ying Zongronga,*, Lin Xuemeia, Qi Yua, Luo Jieb a
b
School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, PR China Work 699 of the Second Academy of China Aerospace Science & Industry Corporation, Beijing 100039, PR China Received 13 May 2007; received in revised form 11 October 2007; accepted 24 October 2007 Available online 1 November 2007
Abstract The low-temperature expandable graphite was successfully prepared with perchloric acid, phosphoric acid and KMnO4 by chemical process. The optimum weight ratio of perchloric acid to phosphoric acid in mixed acid was 1:0.2, and the weight ratio of the mixed acid, KMnO4 and natural flake graphite was preferably 1.5:0.1:1. The expanded volume can reach 260 mL/g at a relatively low temperature of 300 8C. Meanwhile, the prepared samples were characterized by means of Fourier transform infrared, thermogravimetry-differential scanning calorimetry and X-ray diffraction. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Intercalation reactions; C. Differential scanning calorimetry; C. X-ray diffraction; D. Thermal expansion
1. Introduction Expandable graphite as a sort of graphite intercalation compound (GIC) can be applied to various fields such as airproof material, oil absorbing material, fire retardant, high-power battery, electrode, military material, etc. [1–9]. The properties of expandable graphite prepared with H2SO4 and strong oxidants (like KMnO4, K2MnF6, HNO3 or K2Cr2O7) have been widely studied [10–20]. Recently, the research on expandable graphite with low-sulfur content [21] or nosulfur [22–24] has attracted much attention due to erosion and environmental pollution caused by sulfur-containing compounds. On the other hand, the high treatment temperature (1000 8C) required to obtain an expanded volume above 300 mL/g is a disadvantage in the fields of multi-band smoke composition, expandable fire retardant and extinguishing agent, etc. High treatment temperature will also lead to enormous energy consumption and require adequate equipments and techniques. Therefore, low-temperature expandable graphite presents great interest as compared to products available on the market. Few papers on this subject have been reported in the literature. In the present work, sulfur-free low-temperature expandable graphite is successfully prepared with perchloric acid, phosphoric acid and KMnO4. 2. Experimental Natural flake graphite (99 wt%) was purchased from Qingdao Yingshida Graphite Co. Ltd. Perchloric acid (AR, 72 wt%), phosphoric acid (AR, 85 wt%) and KMnO4 (CP) were used as received. * Corresponding author. Tel.: +86 25 84315949; fax: +86 25 84315949. E-mail address: [email protected] (Z. Ying). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.027
2678
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
The mixed acid was prepared with phosphoric acid and perchloric acid. KMnO4 was dissolved into the mixed acid, and then natural flake graphite was added to the solution. The graphite was oxidized for a specified time under continuous stirring. Afterwards, the resulting mixture was washed with water, the precipitate was filtered off and the prepared samples were dried at 60 8C. For characterization, samples were prepared with those conditions: for sample A, B, C and D, the weight ratio of perchloric acid to phosphoric acid in mixed acid was 1:0, 1:0.2, 1:0.75 and 1:1.5, respectively, the weight ratio of mixed acid, KMnO4 and natural flake graphite was 1.5:0.1:1 and the reaction was carried out at 30 8C for 60 min. Expansion volume was determined as follows: pouring 0.5 g of prepared expandable graphite into a graduated quartz glass beaker heated in a furnace to 300 8C, removing the beaker from the furnace after 60 s, and then reading the expansion volume. The Fourier transform infrared (FT-IR) spectroscopy was carried out using a Bomem-made MB154S-type spectrometer with KBr pellet, recorded in the 4000–500 cm1 region. Thermogravimetry-differential scanning calorimetry (TG–DSC) was performed using a Mettler-Toledo-made 823e-type apparatus in the range from 50 to 500 8C at a heating rate of 20 8C/min under flowing N2 gas. Power X-ray diffraction (XRD) pattern was obtained on a D8-Advance Bruker X-Ray operating at 40 kV, 30 mA, using Ni-filtered Cu Ka radiation. 3. Results and discussion 3.1. The process and mechanism of reactions In the presence of a proper oxidant system, natural flake graphite is oxidized and carried on positive charge or form C–O bond. Due to the distortion of conjugate system and the exclusive function of positive charge, the gap between graphite layers is extended. Under these conditions, radicles (even polymer [25]) could intercalate into graphite. The reactions consist of three steps as shown in the following: (a) Oxidizing reaction. KMnO4 is used as the main oxidizing agent for higher electric potential with high concentration of H+ (or H3O+). Perchloric acid also can react with graphite if KMnO4 is not sufficient or oxidizing capacity of KMnO4 decreases, which should be avoided. The reaction can be expressed by the equation: 5Cn þ MnO4 þ 8Hþ ¼ 5Cn þ þ Mn2þ þ 4H2 O (b) Intercalating reaction. The paths of exchange reactions in GICs with different Brönsted acids depend on the nature of acids [26–28]: a strong acid rapidly replaces a weaker one in the order of HClO4 > H2SO4 > HNO3 H3PO4. Among them, we selected perchloric acid as a very efficient intercalating agent, and phosphoric acid with little intercalating capacity as an additive. The main intercalating reaction is: Cn þ þ ClO4 þ xHClO4 ¼ Cn þ ClO4 xHClO4 (c) Expanding reaction. The low-temperature expansion capacity is largely due to the intercalation of perchloric acid. At a heat treatment temperature of 255–300 8C, graphite perchlorate will be decomposed to release O2 and HClO4 will be decomposed into HCl, Cl2, Cl2O, ClO2 and O2. 3.2. Influence of composition and processing condition on expanded volume 3.2.1. Influence of mixed acid As shown in Fig. 1, the weight ratio of phosphoric acid to perchloric acid greatly influences on the expanded volume, which could be explained by the function of phosphoric acid. At first, phosphoric acid as a critical additive adjusts the oxidizing capacity of system. It is well known that the nature of KMnO4 largely depends on the acidity of solution. HClO4 is the strongest inorganic acid whose acidity is much higher than that of H2SO4. In the absence of phosphoric acid, KMnO4 in concentrated perchloric acid would perform an extremely strong oxidizing capacity, and the graphite would be over oxidized at the beginning of reaction. Besides, along with the reaction, large amounts of H+ are consumed, which results in fast increase of the pH value and decrease of the oxidizing capacity of KMnO4. The assumption of H+ is proved by the fact that brown MnO2 would appear after about 15 min reaction. Phosphoric acid not only dilutes perchloric acid but also acts as buffer for its weak acidity. Therefore, the concentration of H+ would decrease slowly, and the reaction would proceed placidly, which prevents over oxidization at first and incomplete
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2679
Fig. 1. Effect of the composition of mixed acid on expanded volume at 300 8C, where the weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1 and the preparation reaction is carried out at 30 8C for 60 min.
reaction at last. In addition, perchloric acid, which is not expected to take part in the oxidizing reaction, exhibits high oxidizing capacity in concentrated solution. Phosphoric acid is thus required to dilute HClO4, which is essential to efficiently oxidize the natural flake graphite with KMnO4 to obtain a high expanded volume. Secondly, excessive phosphoric acid diminishes the intercalating capacity of perchloric acid. The degree of intercalating reaction is found to depend on the concentration of intercalating agent, in agreement with literature [26]. When perchloric acid is diluted by phosphoric acid, fewer ClO4 and HClO4 intercalate into graphite layers, and even ClO4 or HClO4 in graphite would be partially replaced by phosphoric acid. In summary, phosphoric acid in proper amount is required to obtain a high expanded volume while the excessive amount should be avoided. Fig. 2 shows the optimum amount of mixed acid. A saturation point is observed beyond which the expanded volume begins to decrease. It is observed that even the weight ratio of mixed acid to graphite is as low as 0.8:1, an expanded volume of 115 mL/g is obtained, which could not be achieved in other oxidizing–intercalating systems. According to Fig. 2, the optimum ratio of 1.5:1 is lower than other systems, which is an advantage for industrialization since low amount of acid required means less acidic wastes and lower cost.
Fig. 2. Effect of the amount of mixed acid on expanded volume at 300 8C, where the weight ratio of KMnO4 to natural flake graphite is 0.1:1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C for 60 min.
2680
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Fig. 3. Effect of the amount of KMnO4 on expanded volume at 300 8C, where the weight ratio of mixed acid to natural flake graphite is 1.5:0.1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C for 60 min.
3.2.2. Influence of KMnO4 While natural flake graphite is added into the solution, the system turns into slop, and it is restricted for KMnO4 to diffuse freely within the slop. As a consequence, KMnO4 could hardly maintain saturated everywhere in mixed acid during the reaction. The amount of KMnO4 should be largely beyond solubility. However, excessive KMnO4 would lead to over oxidization. Fig. 3 reveals the optimum amount of KMnO4. 3.2.3. Influence of reaction temperature and time The reaction temperature and time required are interrelated. The higher the temperature is, the faster the reaction proceeds. From Table 1, at 5–90 8C, the natural flake graphite can be oxidized effectively, and the expanded volume is more than 210 mL/g. The wide range of temperature is very favorable for industrialization. For instance, in most areas, production can be carried out at room temperature. Besides, the reaction time is shorter as compared to other systems, which would help to improve the production. An optimum point always exists concerning the reaction time for an excessive oxidization would erode the layers of graphite and destroy the structure of GICs. However, natural flake graphite is not easily over oxidized in this system. As shown in Fig. 4, though 20 min is enough at 30 8C, a further oxidizing, even as long as 150 min, would not decrease the expanded volume. For industrial process, the expansion capacity would not be varied with the difference of reaction time resulting from factors such as whisking. The stability of quality would be high using this oxidizing– intercalating system. 3.3. Characterization 3.3.1. Analysis of IR spectra Fig. 5 shows Fourier transform infrared spectra of the prepared low-temperature expandable graphite. The peak at 3420 and 1635 cm1 are derived of adsorbed water, though the later may also contain components from the skeletal Table 1 Expanded volume at 300 8C of expandable graphite prepared under different oxidizing conditions Oxidizing temperature (8C) Oxidizing time (min) Expanded volume at 300 8C (mL/g)
5 80 210
20 60 240
30 40 245
50 25 240
70 10 225
90 4 215
Note: Weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1. Weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2681
Fig. 4. Effect of the reaction time on expanded volume at 300 8C, where the weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C.
vibrations of un-oxidized graphitic domains [29]. The peak at 1384 cm1 is ascribed to hydroxyl groups from the oxidized graphite [29,30]. The absorption peaks of ClO4 are at 629 and 1089 cm1, and other peaks in the region 1150–1050 cm1 is considered as oxygen-containing groups’ absorption. Although absorption of H2PO4 is in the 1150–1040 cm1 range, it does not necessarily means that peaks at 1115 and 1146 cm1 belong to phosphoric acid, because sample A without using phosphoric acid shows intense absorption at 1115 and 1146 cm1, while sample D prepared with the largest amount of phosphoric acid only shows very weak absorption in this region. Intensity of peaks at 626 and 1089 cm1 indicate the amount of ClO4 and HClO4 in graphite. The order A < B > C > D of their intensity is consistent with the order A < B > C > D of the expanded volume at 300 8C. So it is revealed that the decomposition of expandable graphite under heat treatment of low temperature such as 300 8C is mainly due to the intercalated ClO4 and HClO4, and that fewer ClO4 and HClO4 enter into graphite layers when perchloric acid is diluted by phosphoric acid. 3.3.2. DSC analysis The differential scanning calorimetry thermograms are shown in Fig. 6. A strong peak is detected at about 220 8C with onset temperature of 210 8C and endset temperature of 255 8C. The data of further analysis are summarized in
Fig. 5. IR spectra of the prepared low-temperature expandable graphite.
2682
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Fig. 6. DSC thermograms of the prepared low-temperature expandable graphite.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2683
Table 2. The onset temperature is almost independent of the weight ratio of phosphoric acid to perchloric acid, especially for sample A (without H3PO4). This finding clearly shows that the decomposition is mainly derived from graphite perchlorate. On the other hand, the value of enthalpy change (DH) reflects the amount of ClO4 and HClO4 in graphite. Consistent with the analysis of IR spectra, DSC analysis supports the conclusion: more radicles intercalate
Fig. 7. Powder XRD patterns of the prepared low-temperature expandable graphite.
2684
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Table 2 DSC analysis of the prepared low-temperature expandable graphite Sample
Peak (8C)
DH (J/g)
Expanded volume (mL/g)
A B C D
223.0 233.6 219.6 222.0
311.7 639.0 256.4 236.6
155 215 145 95
Table 3 Parameter analysis of the prepared low-temperature expandable graphite Sample
di (nm)
Ic (nm)
n
Expanded volume (mL/g)
A B C
0.761 0.763 0.760 0.801 0.797
2.101 1.678 2.100 1.471 2.472
5 4 5 3 6
155 215 145
D
95
into graphite while the oxidization is more sufficient (see sample A and B), fewer radicles intercalate into graphite while the intercalating agent is diluted by excessive additive (see sample B, C and D). 3.3.3. XRD analysis The powder X-ray diffraction pattern for natural flake graphite (see Fig. 7E) reveals that the 0 0 2 peak is at 26.558 (d = 0.335 nm) and 0 0 4 peak is at 54.58 (d = 0.168 nm). For expandable graphite samples, the most distinct peaks are detected at 25.58 and 29.58, namely ‘‘a’’ and ‘‘b’’, respectively (see Fig. 7 A–D). The strong peak of a (da 0.35 nm) is due to slightly oxidized structure, similar to natural flake graphite, and other peaks are ascribed to GICs structures in expandable graphite samples derived from oxidizing–intercalating process. The fact that peak a exists in all samples indicates that the crystal phase of natural flake graphite is partially conserved in the samples [31]. For most GICs, Ic = di + (n 1)d0, where Ic is the identity period of the crystal structure along axis-c, di the thickness of the layer filled by the intercalating agents, d0 = 0.335 nm the interplanar spacing in graphite and n is the stage index. As shown in Table 3, the samples tested are high stage GICs [32–35]. Sample C shows a new GIC phase which might result from the intercalation of phosphoric acid. The mechanism of the gap between layers widening from d0 to di is illustrated by the structural model (see Fig. 8). In period I, in a proper oxidizing system, surface or edge of graphite crystal is oxidized and positive charge is formed without destroying the layer structure. Due to conjugate system, positive charge would be dispersed onto carbon atoms in one layer, and the surface of graphite could be further oxidized. It is likely that, with the oxidizing reaction proceeding, the positive charge continuously transfers to interior and would be cumulated to a certain amount. The result (period II) is that the distance between some layers slightly widens for exclusion among positive charge. The agents can thus more easily intercalate into graphite and finally the gap between layers widens to di (period III). According to the mechanism described above (Fig. 8), peak a is relative to the phase of graphite while peak b is relative to the phase of GIC, and the ratio of peak intensity Ia/Ib could reflect degree of oxidizing reaction (Ia = Kaxa,
Fig. 8. Structural model for the oxidizing–intercalating process.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2685
Fig. 9. Structural model for amount of radicles in the gap between graphite layers.
Table 4 Oxidizing reaction degree analysis Sample
2u (8)
d (nm)
Intensity (cps)
Ia/Ib
Concentration of perchloric acid (%)
Expanded volume (mL/g)
A
25.7 29.7 25.3 30.4 25.9 29.2 25.2 29.2
0.346 0.300 0.352 0.294 0.344 0.305 0.353 0.305
11226.7 723.3 3046.7 443.3 11580.0 973.3 3413.3 266.7
15.5
72.0
155
6.9
60.0
215
11.9
41.1
145
12.8
28.8
95
B C D
Ib = Kbxb, Ka and Kb are constants, xa and xb are proportions of GIC phase and graphite phase in the samples, respectively, xa + xb = 1). The Ia/Ib ratio could not indicate the degree of intercalating reaction because the intensity of peak b is only relative to ratio of GIC phase, not to the amount of radicles in the gaps between graphite layers. Fig. 9 shows that increasing the amount of intercalated radicles does not necessarily increase the GIC phase ratio. Along with the oxidizing reaction, peak a becomes weaker and peak b stronger, the ratio of Ia/Ib turning lower. As shown in Table 4, sample B is largely oxidized, thus offering best conditions for intercalation, and the expanded volume is the highest. However, there seems to be an exception: sample A is oxidized less than sample D, but the expanded volume of sample A is higher. It can be assumed that the expanded volume depends on the amount of radicles in graphite and the amount of ClO4 and HClO4 intercalated into graphite depends on the concentration of perchloric acid. 4. Conclusions The low-temperature expandable graphite can be prepared with oxidizing–intercalating system of perchloric acid, phosphoric acid and KMnO4. The optimum weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the weight ratio of the mixed acid, KMnO4 and natural flake graphite is preferably 1.5:0.1:1. The expanded volume can reach 260 mL/g at a relatively low temperature of 300 8C. In the reactions, KMnO4 acts as the main oxidizing agent and perchloric acid is the effective intercalating agent. Phosphoric acid as a critical additive adjusts the oxidizing capacity of system. The characterization of the prepared low-temperature expandable graphite leads to an important conclusion: the degree of intercalation depends on the degree of oxidization and concentration of intercalating agent. In this paper, the decomposition of expandable graphite at 300 8C is mostly derived from graphite perchlorate. XRD analysis shows that the prepared samples are high stage GICs with stage three to six structures. References [1] [2] [3] [4]
M.I. Nagaki, J. Mater. Res. 4 (1989) 1560–1568. N.E. Sorokina, A.V. Redchitz, S.G. Ionov, V.V. Avdeev, J. Phys. Chem. Solids 67 (5–6) (2006) 1202–1204. A. Karaipekli, A. Sarˇ, K. Kaygusuz, Renew. Energy 32 (13) (2007) 2201–2210. A. Sarˇ, A. Karaipekli, Appl. Therm. Eng. 27 (8–9) (2007) 1271–1277.
2686 [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686 B. Sˇljukic´, C.E. Banks, A. Crossley, R.G. Compton, Anal. Chim. Acta 587 (2) (2007) 240–246. Z.Y. Wang, E.H. Han, W. Ke, Corros. Sci. 49 (5) (2007) 2237–2253. L.N. Song, M. Xiao, Y.Z. Meng, Compos. Sci. Technol. 66 (13) (2006) 2156–2162. Z.Z. Li, B.J. Qu, Polym. Degrad. Stabil. 81 (3) (2003) 401–408. Y.F. Shih, Y.T. Wang, R.J. Jeng, K.M. Wei, Polym. Degrad. Stabil. 86 (2) (2004) 339–348. S.Y. Lee, D.H. Cho, J. Mater. Sci. Mater. Med. 40 (2005) 231–234. W.C. Katinonkul, M.M. Lerner, Carbon 45 (3) (2007) 499–504. X. Zhang, M.M. Lerner, Chem. Mater. 11 (4) (1999) 1100–1109. M. Inagaki, N. Iwashita, E. Kouro, Carbon 28 (1) (1990) 49–55. V.V. Avdeev, L.A. Monyakina, I.V. Nikol’skaya, N.E. Sorokina, K.N. Semenenko, A.I. Finaenov, Carbon 30 (6) (1992) 825–827. B. Tryba, J. Przepiórski, A.W. Morawski, Carbon 41 (10) (2003) 2012–2015. A. Moissette, H. Fuzellier, A. Burneau, J. Dubessy, M. Lelaurain, Carbon 33 (2) (1995) 123–128. J.M. Skowronski, H. Shioyama, Carbon 33 (10) (1995) 1473–1478. N.V. Maksimova, V.S. Leshin, V.V. Avdeev, N.E. Sorokina, J. Phys. Chem. Solids 67 (5–6) (2006) 1198–1201. B. Tryba, A.W. Morawski, K. Kaucki, J. Phys. Chem. Solids 65 (2–3) (2004) 165–169. F.Y. Kang, Y. Leng, T.Y. Zhang, J. Phys. Chem. Solids 57 (6–8) (1996) 889–892. X.L. Chen, K.M. Song, J.H. Li, J.P. Liu, Carbon 34 (12) (1996) 1599–1600. P. Scharff, Z.Y. Xu, E. Stumpp, Carbon 29 (1) (1991) 31–37. F. Kang, Y. Leng, T.Y. Zhang, Carbon 35 (8) (1997) 1089–1096. K.M. Song, H.J. Dun, Mater. Res. Bull. 35 (3) (2000) 425–430. R. Bissessur, K.Y. Liu, W. White, S.F. Scully, Langmuir 22 (2006) 1729–1817. V.V. Avdeev, N.E. Sorokina, N.V. Maksimova, I.Y. Martynov, A.V. Sezemin, Inorg. Mater. 37 (4) (2001) 366–371, Translated from Neorg. Mater. 37 (4) (2001) 448–453. W. Rudorff, U. Hofmann, Z. Anorg. Allg. Chem. 238 (1) (1938) 1–50. N.E. Sorokina, I.V. Nikol’skaya, S.G. Ionov, V.V. Avdeev, Russ. Chem. B: Int. Ed. 54 (8) (2005) 1749–1767. S. Sasha, D.P. Richard, T.N. SonBinh, S. Rodney, Carbon 44 (15) (2006) 3342–3347. M. Yoshiaki, M. Tadaaki, F. Tomokazu, S. Yosohiro, Carbon 45 (5) (2007) 1005–1012. S.S. Tzeng, P.L. Wang, J. Mater. Sci. 36 (2001) 497–501. V.A. Kulbachinski, N.E. Sorokina, S.V. Kuvshinnikov, S.G. Ionov, S. Haas, Phys. Solid State 45 (12) (2003) 2264–2270, Translated from Fizi Tverd Tela 45 (12) (2003) 2161–2167. N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikol’skaya, Russ. J. Gen. Chem. 75 (2) (2005) 162–168, Translated from Zh. Obshch. Khim. 75 (2) (2005) 184–191. Z.H. Liu, Z.M. Wang, X.J. Yang, K. Ooi, Langmuir 18 (2002) 4926–4932. Y.H. Chu, Z.M. Wang, M. Yamagishi, H. Kanoh, T. Hirotsu, Y.X. Zhang, Langmuir 21 (2005) 2545–2551.
Preparation and characterization of low-temperature expandable graphite Ying Zongronga,*, Lin Xuemeia, Qi Yua, Luo Jieb a
b
School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, PR China Work 699 of the Second Academy of China Aerospace Science & Industry Corporation, Beijing 100039, PR China Received 13 May 2007; received in revised form 11 October 2007; accepted 24 October 2007 Available online 1 November 2007
Abstract The low-temperature expandable graphite was successfully prepared with perchloric acid, phosphoric acid and KMnO4 by chemical process. The optimum weight ratio of perchloric acid to phosphoric acid in mixed acid was 1:0.2, and the weight ratio of the mixed acid, KMnO4 and natural flake graphite was preferably 1.5:0.1:1. The expanded volume can reach 260 mL/g at a relatively low temperature of 300 8C. Meanwhile, the prepared samples were characterized by means of Fourier transform infrared, thermogravimetry-differential scanning calorimetry and X-ray diffraction. # 2007 Elsevier Ltd. All rights reserved. Keywords: B. Intercalation reactions; C. Differential scanning calorimetry; C. X-ray diffraction; D. Thermal expansion
1. Introduction Expandable graphite as a sort of graphite intercalation compound (GIC) can be applied to various fields such as airproof material, oil absorbing material, fire retardant, high-power battery, electrode, military material, etc. [1–9]. The properties of expandable graphite prepared with H2SO4 and strong oxidants (like KMnO4, K2MnF6, HNO3 or K2Cr2O7) have been widely studied [10–20]. Recently, the research on expandable graphite with low-sulfur content [21] or nosulfur [22–24] has attracted much attention due to erosion and environmental pollution caused by sulfur-containing compounds. On the other hand, the high treatment temperature (1000 8C) required to obtain an expanded volume above 300 mL/g is a disadvantage in the fields of multi-band smoke composition, expandable fire retardant and extinguishing agent, etc. High treatment temperature will also lead to enormous energy consumption and require adequate equipments and techniques. Therefore, low-temperature expandable graphite presents great interest as compared to products available on the market. Few papers on this subject have been reported in the literature. In the present work, sulfur-free low-temperature expandable graphite is successfully prepared with perchloric acid, phosphoric acid and KMnO4. 2. Experimental Natural flake graphite (99 wt%) was purchased from Qingdao Yingshida Graphite Co. Ltd. Perchloric acid (AR, 72 wt%), phosphoric acid (AR, 85 wt%) and KMnO4 (CP) were used as received. * Corresponding author. Tel.: +86 25 84315949; fax: +86 25 84315949. E-mail address: [email protected] (Z. Ying). 0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2007.10.027
2678
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
The mixed acid was prepared with phosphoric acid and perchloric acid. KMnO4 was dissolved into the mixed acid, and then natural flake graphite was added to the solution. The graphite was oxidized for a specified time under continuous stirring. Afterwards, the resulting mixture was washed with water, the precipitate was filtered off and the prepared samples were dried at 60 8C. For characterization, samples were prepared with those conditions: for sample A, B, C and D, the weight ratio of perchloric acid to phosphoric acid in mixed acid was 1:0, 1:0.2, 1:0.75 and 1:1.5, respectively, the weight ratio of mixed acid, KMnO4 and natural flake graphite was 1.5:0.1:1 and the reaction was carried out at 30 8C for 60 min. Expansion volume was determined as follows: pouring 0.5 g of prepared expandable graphite into a graduated quartz glass beaker heated in a furnace to 300 8C, removing the beaker from the furnace after 60 s, and then reading the expansion volume. The Fourier transform infrared (FT-IR) spectroscopy was carried out using a Bomem-made MB154S-type spectrometer with KBr pellet, recorded in the 4000–500 cm1 region. Thermogravimetry-differential scanning calorimetry (TG–DSC) was performed using a Mettler-Toledo-made 823e-type apparatus in the range from 50 to 500 8C at a heating rate of 20 8C/min under flowing N2 gas. Power X-ray diffraction (XRD) pattern was obtained on a D8-Advance Bruker X-Ray operating at 40 kV, 30 mA, using Ni-filtered Cu Ka radiation. 3. Results and discussion 3.1. The process and mechanism of reactions In the presence of a proper oxidant system, natural flake graphite is oxidized and carried on positive charge or form C–O bond. Due to the distortion of conjugate system and the exclusive function of positive charge, the gap between graphite layers is extended. Under these conditions, radicles (even polymer [25]) could intercalate into graphite. The reactions consist of three steps as shown in the following: (a) Oxidizing reaction. KMnO4 is used as the main oxidizing agent for higher electric potential with high concentration of H+ (or H3O+). Perchloric acid also can react with graphite if KMnO4 is not sufficient or oxidizing capacity of KMnO4 decreases, which should be avoided. The reaction can be expressed by the equation: 5Cn þ MnO4 þ 8Hþ ¼ 5Cn þ þ Mn2þ þ 4H2 O (b) Intercalating reaction. The paths of exchange reactions in GICs with different Brönsted acids depend on the nature of acids [26–28]: a strong acid rapidly replaces a weaker one in the order of HClO4 > H2SO4 > HNO3 H3PO4. Among them, we selected perchloric acid as a very efficient intercalating agent, and phosphoric acid with little intercalating capacity as an additive. The main intercalating reaction is: Cn þ þ ClO4 þ xHClO4 ¼ Cn þ ClO4 xHClO4 (c) Expanding reaction. The low-temperature expansion capacity is largely due to the intercalation of perchloric acid. At a heat treatment temperature of 255–300 8C, graphite perchlorate will be decomposed to release O2 and HClO4 will be decomposed into HCl, Cl2, Cl2O, ClO2 and O2. 3.2. Influence of composition and processing condition on expanded volume 3.2.1. Influence of mixed acid As shown in Fig. 1, the weight ratio of phosphoric acid to perchloric acid greatly influences on the expanded volume, which could be explained by the function of phosphoric acid. At first, phosphoric acid as a critical additive adjusts the oxidizing capacity of system. It is well known that the nature of KMnO4 largely depends on the acidity of solution. HClO4 is the strongest inorganic acid whose acidity is much higher than that of H2SO4. In the absence of phosphoric acid, KMnO4 in concentrated perchloric acid would perform an extremely strong oxidizing capacity, and the graphite would be over oxidized at the beginning of reaction. Besides, along with the reaction, large amounts of H+ are consumed, which results in fast increase of the pH value and decrease of the oxidizing capacity of KMnO4. The assumption of H+ is proved by the fact that brown MnO2 would appear after about 15 min reaction. Phosphoric acid not only dilutes perchloric acid but also acts as buffer for its weak acidity. Therefore, the concentration of H+ would decrease slowly, and the reaction would proceed placidly, which prevents over oxidization at first and incomplete
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2679
Fig. 1. Effect of the composition of mixed acid on expanded volume at 300 8C, where the weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1 and the preparation reaction is carried out at 30 8C for 60 min.
reaction at last. In addition, perchloric acid, which is not expected to take part in the oxidizing reaction, exhibits high oxidizing capacity in concentrated solution. Phosphoric acid is thus required to dilute HClO4, which is essential to efficiently oxidize the natural flake graphite with KMnO4 to obtain a high expanded volume. Secondly, excessive phosphoric acid diminishes the intercalating capacity of perchloric acid. The degree of intercalating reaction is found to depend on the concentration of intercalating agent, in agreement with literature [26]. When perchloric acid is diluted by phosphoric acid, fewer ClO4 and HClO4 intercalate into graphite layers, and even ClO4 or HClO4 in graphite would be partially replaced by phosphoric acid. In summary, phosphoric acid in proper amount is required to obtain a high expanded volume while the excessive amount should be avoided. Fig. 2 shows the optimum amount of mixed acid. A saturation point is observed beyond which the expanded volume begins to decrease. It is observed that even the weight ratio of mixed acid to graphite is as low as 0.8:1, an expanded volume of 115 mL/g is obtained, which could not be achieved in other oxidizing–intercalating systems. According to Fig. 2, the optimum ratio of 1.5:1 is lower than other systems, which is an advantage for industrialization since low amount of acid required means less acidic wastes and lower cost.
Fig. 2. Effect of the amount of mixed acid on expanded volume at 300 8C, where the weight ratio of KMnO4 to natural flake graphite is 0.1:1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C for 60 min.
2680
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Fig. 3. Effect of the amount of KMnO4 on expanded volume at 300 8C, where the weight ratio of mixed acid to natural flake graphite is 1.5:0.1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C for 60 min.
3.2.2. Influence of KMnO4 While natural flake graphite is added into the solution, the system turns into slop, and it is restricted for KMnO4 to diffuse freely within the slop. As a consequence, KMnO4 could hardly maintain saturated everywhere in mixed acid during the reaction. The amount of KMnO4 should be largely beyond solubility. However, excessive KMnO4 would lead to over oxidization. Fig. 3 reveals the optimum amount of KMnO4. 3.2.3. Influence of reaction temperature and time The reaction temperature and time required are interrelated. The higher the temperature is, the faster the reaction proceeds. From Table 1, at 5–90 8C, the natural flake graphite can be oxidized effectively, and the expanded volume is more than 210 mL/g. The wide range of temperature is very favorable for industrialization. For instance, in most areas, production can be carried out at room temperature. Besides, the reaction time is shorter as compared to other systems, which would help to improve the production. An optimum point always exists concerning the reaction time for an excessive oxidization would erode the layers of graphite and destroy the structure of GICs. However, natural flake graphite is not easily over oxidized in this system. As shown in Fig. 4, though 20 min is enough at 30 8C, a further oxidizing, even as long as 150 min, would not decrease the expanded volume. For industrial process, the expansion capacity would not be varied with the difference of reaction time resulting from factors such as whisking. The stability of quality would be high using this oxidizing– intercalating system. 3.3. Characterization 3.3.1. Analysis of IR spectra Fig. 5 shows Fourier transform infrared spectra of the prepared low-temperature expandable graphite. The peak at 3420 and 1635 cm1 are derived of adsorbed water, though the later may also contain components from the skeletal Table 1 Expanded volume at 300 8C of expandable graphite prepared under different oxidizing conditions Oxidizing temperature (8C) Oxidizing time (min) Expanded volume at 300 8C (mL/g)
5 80 210
20 60 240
30 40 245
50 25 240
70 10 225
90 4 215
Note: Weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1. Weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2681
Fig. 4. Effect of the reaction time on expanded volume at 300 8C, where the weight ratio of mixed acid, KMnO4 and natural flake graphite is 1.5:0.1:1, the weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the preparation reaction is carried out at 30 8C.
vibrations of un-oxidized graphitic domains [29]. The peak at 1384 cm1 is ascribed to hydroxyl groups from the oxidized graphite [29,30]. The absorption peaks of ClO4 are at 629 and 1089 cm1, and other peaks in the region 1150–1050 cm1 is considered as oxygen-containing groups’ absorption. Although absorption of H2PO4 is in the 1150–1040 cm1 range, it does not necessarily means that peaks at 1115 and 1146 cm1 belong to phosphoric acid, because sample A without using phosphoric acid shows intense absorption at 1115 and 1146 cm1, while sample D prepared with the largest amount of phosphoric acid only shows very weak absorption in this region. Intensity of peaks at 626 and 1089 cm1 indicate the amount of ClO4 and HClO4 in graphite. The order A < B > C > D of their intensity is consistent with the order A < B > C > D of the expanded volume at 300 8C. So it is revealed that the decomposition of expandable graphite under heat treatment of low temperature such as 300 8C is mainly due to the intercalated ClO4 and HClO4, and that fewer ClO4 and HClO4 enter into graphite layers when perchloric acid is diluted by phosphoric acid. 3.3.2. DSC analysis The differential scanning calorimetry thermograms are shown in Fig. 6. A strong peak is detected at about 220 8C with onset temperature of 210 8C and endset temperature of 255 8C. The data of further analysis are summarized in
Fig. 5. IR spectra of the prepared low-temperature expandable graphite.
2682
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Fig. 6. DSC thermograms of the prepared low-temperature expandable graphite.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2683
Table 2. The onset temperature is almost independent of the weight ratio of phosphoric acid to perchloric acid, especially for sample A (without H3PO4). This finding clearly shows that the decomposition is mainly derived from graphite perchlorate. On the other hand, the value of enthalpy change (DH) reflects the amount of ClO4 and HClO4 in graphite. Consistent with the analysis of IR spectra, DSC analysis supports the conclusion: more radicles intercalate
Fig. 7. Powder XRD patterns of the prepared low-temperature expandable graphite.
2684
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
Table 2 DSC analysis of the prepared low-temperature expandable graphite Sample
Peak (8C)
DH (J/g)
Expanded volume (mL/g)
A B C D
223.0 233.6 219.6 222.0
311.7 639.0 256.4 236.6
155 215 145 95
Table 3 Parameter analysis of the prepared low-temperature expandable graphite Sample
di (nm)
Ic (nm)
n
Expanded volume (mL/g)
A B C
0.761 0.763 0.760 0.801 0.797
2.101 1.678 2.100 1.471 2.472
5 4 5 3 6
155 215 145
D
95
into graphite while the oxidization is more sufficient (see sample A and B), fewer radicles intercalate into graphite while the intercalating agent is diluted by excessive additive (see sample B, C and D). 3.3.3. XRD analysis The powder X-ray diffraction pattern for natural flake graphite (see Fig. 7E) reveals that the 0 0 2 peak is at 26.558 (d = 0.335 nm) and 0 0 4 peak is at 54.58 (d = 0.168 nm). For expandable graphite samples, the most distinct peaks are detected at 25.58 and 29.58, namely ‘‘a’’ and ‘‘b’’, respectively (see Fig. 7 A–D). The strong peak of a (da 0.35 nm) is due to slightly oxidized structure, similar to natural flake graphite, and other peaks are ascribed to GICs structures in expandable graphite samples derived from oxidizing–intercalating process. The fact that peak a exists in all samples indicates that the crystal phase of natural flake graphite is partially conserved in the samples [31]. For most GICs, Ic = di + (n 1)d0, where Ic is the identity period of the crystal structure along axis-c, di the thickness of the layer filled by the intercalating agents, d0 = 0.335 nm the interplanar spacing in graphite and n is the stage index. As shown in Table 3, the samples tested are high stage GICs [32–35]. Sample C shows a new GIC phase which might result from the intercalation of phosphoric acid. The mechanism of the gap between layers widening from d0 to di is illustrated by the structural model (see Fig. 8). In period I, in a proper oxidizing system, surface or edge of graphite crystal is oxidized and positive charge is formed without destroying the layer structure. Due to conjugate system, positive charge would be dispersed onto carbon atoms in one layer, and the surface of graphite could be further oxidized. It is likely that, with the oxidizing reaction proceeding, the positive charge continuously transfers to interior and would be cumulated to a certain amount. The result (period II) is that the distance between some layers slightly widens for exclusion among positive charge. The agents can thus more easily intercalate into graphite and finally the gap between layers widens to di (period III). According to the mechanism described above (Fig. 8), peak a is relative to the phase of graphite while peak b is relative to the phase of GIC, and the ratio of peak intensity Ia/Ib could reflect degree of oxidizing reaction (Ia = Kaxa,
Fig. 8. Structural model for the oxidizing–intercalating process.
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686
2685
Fig. 9. Structural model for amount of radicles in the gap between graphite layers.
Table 4 Oxidizing reaction degree analysis Sample
2u (8)
d (nm)
Intensity (cps)
Ia/Ib
Concentration of perchloric acid (%)
Expanded volume (mL/g)
A
25.7 29.7 25.3 30.4 25.9 29.2 25.2 29.2
0.346 0.300 0.352 0.294 0.344 0.305 0.353 0.305
11226.7 723.3 3046.7 443.3 11580.0 973.3 3413.3 266.7
15.5
72.0
155
6.9
60.0
215
11.9
41.1
145
12.8
28.8
95
B C D
Ib = Kbxb, Ka and Kb are constants, xa and xb are proportions of GIC phase and graphite phase in the samples, respectively, xa + xb = 1). The Ia/Ib ratio could not indicate the degree of intercalating reaction because the intensity of peak b is only relative to ratio of GIC phase, not to the amount of radicles in the gaps between graphite layers. Fig. 9 shows that increasing the amount of intercalated radicles does not necessarily increase the GIC phase ratio. Along with the oxidizing reaction, peak a becomes weaker and peak b stronger, the ratio of Ia/Ib turning lower. As shown in Table 4, sample B is largely oxidized, thus offering best conditions for intercalation, and the expanded volume is the highest. However, there seems to be an exception: sample A is oxidized less than sample D, but the expanded volume of sample A is higher. It can be assumed that the expanded volume depends on the amount of radicles in graphite and the amount of ClO4 and HClO4 intercalated into graphite depends on the concentration of perchloric acid. 4. Conclusions The low-temperature expandable graphite can be prepared with oxidizing–intercalating system of perchloric acid, phosphoric acid and KMnO4. The optimum weight ratio of perchloric acid to phosphoric acid in mixed acid is 1:0.2, and the weight ratio of the mixed acid, KMnO4 and natural flake graphite is preferably 1.5:0.1:1. The expanded volume can reach 260 mL/g at a relatively low temperature of 300 8C. In the reactions, KMnO4 acts as the main oxidizing agent and perchloric acid is the effective intercalating agent. Phosphoric acid as a critical additive adjusts the oxidizing capacity of system. The characterization of the prepared low-temperature expandable graphite leads to an important conclusion: the degree of intercalation depends on the degree of oxidization and concentration of intercalating agent. In this paper, the decomposition of expandable graphite at 300 8C is mostly derived from graphite perchlorate. XRD analysis shows that the prepared samples are high stage GICs with stage three to six structures. References [1] [2] [3] [4]
M.I. Nagaki, J. Mater. Res. 4 (1989) 1560–1568. N.E. Sorokina, A.V. Redchitz, S.G. Ionov, V.V. Avdeev, J. Phys. Chem. Solids 67 (5–6) (2006) 1202–1204. A. Karaipekli, A. Sarˇ, K. Kaygusuz, Renew. Energy 32 (13) (2007) 2201–2210. A. Sarˇ, A. Karaipekli, Appl. Therm. Eng. 27 (8–9) (2007) 1271–1277.
2686 [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
Z. Ying et al. / Materials Research Bulletin 43 (2008) 2677–2686 B. Sˇljukic´, C.E. Banks, A. Crossley, R.G. Compton, Anal. Chim. Acta 587 (2) (2007) 240–246. Z.Y. Wang, E.H. Han, W. Ke, Corros. Sci. 49 (5) (2007) 2237–2253. L.N. Song, M. Xiao, Y.Z. Meng, Compos. Sci. Technol. 66 (13) (2006) 2156–2162. Z.Z. Li, B.J. Qu, Polym. Degrad. Stabil. 81 (3) (2003) 401–408. Y.F. Shih, Y.T. Wang, R.J. Jeng, K.M. Wei, Polym. Degrad. Stabil. 86 (2) (2004) 339–348. S.Y. Lee, D.H. Cho, J. Mater. Sci. Mater. Med. 40 (2005) 231–234. W.C. Katinonkul, M.M. Lerner, Carbon 45 (3) (2007) 499–504. X. Zhang, M.M. Lerner, Chem. Mater. 11 (4) (1999) 1100–1109. M. Inagaki, N. Iwashita, E. Kouro, Carbon 28 (1) (1990) 49–55. V.V. Avdeev, L.A. Monyakina, I.V. Nikol’skaya, N.E. Sorokina, K.N. Semenenko, A.I. Finaenov, Carbon 30 (6) (1992) 825–827. B. Tryba, J. Przepiórski, A.W. Morawski, Carbon 41 (10) (2003) 2012–2015. A. Moissette, H. Fuzellier, A. Burneau, J. Dubessy, M. Lelaurain, Carbon 33 (2) (1995) 123–128. J.M. Skowronski, H. Shioyama, Carbon 33 (10) (1995) 1473–1478. N.V. Maksimova, V.S. Leshin, V.V. Avdeev, N.E. Sorokina, J. Phys. Chem. Solids 67 (5–6) (2006) 1198–1201. B. Tryba, A.W. Morawski, K. Kaucki, J. Phys. Chem. Solids 65 (2–3) (2004) 165–169. F.Y. Kang, Y. Leng, T.Y. Zhang, J. Phys. Chem. Solids 57 (6–8) (1996) 889–892. X.L. Chen, K.M. Song, J.H. Li, J.P. Liu, Carbon 34 (12) (1996) 1599–1600. P. Scharff, Z.Y. Xu, E. Stumpp, Carbon 29 (1) (1991) 31–37. F. Kang, Y. Leng, T.Y. Zhang, Carbon 35 (8) (1997) 1089–1096. K.M. Song, H.J. Dun, Mater. Res. Bull. 35 (3) (2000) 425–430. R. Bissessur, K.Y. Liu, W. White, S.F. Scully, Langmuir 22 (2006) 1729–1817. V.V. Avdeev, N.E. Sorokina, N.V. Maksimova, I.Y. Martynov, A.V. Sezemin, Inorg. Mater. 37 (4) (2001) 366–371, Translated from Neorg. Mater. 37 (4) (2001) 448–453. W. Rudorff, U. Hofmann, Z. Anorg. Allg. Chem. 238 (1) (1938) 1–50. N.E. Sorokina, I.V. Nikol’skaya, S.G. Ionov, V.V. Avdeev, Russ. Chem. B: Int. Ed. 54 (8) (2005) 1749–1767. S. Sasha, D.P. Richard, T.N. SonBinh, S. Rodney, Carbon 44 (15) (2006) 3342–3347. M. Yoshiaki, M. Tadaaki, F. Tomokazu, S. Yosohiro, Carbon 45 (5) (2007) 1005–1012. S.S. Tzeng, P.L. Wang, J. Mater. Sci. 36 (2001) 497–501. V.A. Kulbachinski, N.E. Sorokina, S.V. Kuvshinnikov, S.G. Ionov, S. Haas, Phys. Solid State 45 (12) (2003) 2264–2270, Translated from Fizi Tverd Tela 45 (12) (2003) 2161–2167. N.E. Sorokina, M.A. Khaskov, V.V. Avdeev, I.V. Nikol’skaya, Russ. J. Gen. Chem. 75 (2) (2005) 162–168, Translated from Zh. Obshch. Khim. 75 (2) (2005) 184–191. Z.H. Liu, Z.M. Wang, X.J. Yang, K. Ooi, Langmuir 18 (2002) 4926–4932. Y.H. Chu, Z.M. Wang, M. Yamagishi, H. Kanoh, T. Hirotsu, Y.X. Zhang, Langmuir 21 (2005) 2545–2551.