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Al2O3 powders by DSC testing
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COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2954–2959 www.elsevier.com/locate/compscitech
Thermal decomposition of carbon nanotube/Al2O3 powders by DSC testing Chih-Wei Chang, Jo-Ming Tseng, Jao-Jia Horng, Chi-Min Shu * Department of Safety, Health, and Environmental Engineering, National Yunlin University of Science and Technology, 123, University Road, Sec. 3, Douliu, Yunlin 64002, Taiwan, ROC Received 14 November 2007; accepted 14 November 2007 Available online 28 November 2007
Abstract Concerns over unsafe or unknown properties of carbon nanotubes (CNTs) have been raised by many researchers. The thermal stability characteristics, which may represent potential hazards during the production or utilization stage, could be determined by calorimetry testing for various thermokinetic parameters. In this study, differential scanning calorimetry (DSC) was employed to analyze thermal decomposition and to calculate thermodynamic characters and dynamic parameters for activated carbon powder (C), commercial CNT (CBT), and an innovative composite of CNT on Al2O3 powders (CCNT) during the time-effective experiments. The higher heating temperature revealed a ‘‘thermal delay effect” that increased the exothermic onset temperature and decreased the heat of decomposition for C, CBT and CCNT. The temperature shifts were irregular at exothermic onset temperatures and at about 25 °C for peak exothermal temperatures of C, CBT, and CCNT as heating rate increased from 2 to 4 °C min1. From a comparison of results on C, CBT, and CCNT, CCNT had medium exothermic onset temperature, lowest heat of decomposition, and is a safer material (less thermal hazard) than C and CBT. In addition, the heating exothermic reaction was the first order for CCNT and the activation energy increased as the heating rate increased. Our results also implied that different types of carbon contained in C, CBT, and CCNT could yield different energies during thermal decomposition. Ó 2008 Published by Elsevier Ltd. Keywords: Carbon nanotubes (CNTs); Thermokinetic parameters; Differential scanning calorimetry (DSC); Thermal decomposition, material, dynamic, equipment, runaway reaction phenomenon
1. Introduction Since Iijima discovered carbon nanotubes (CNTs) by arc-discharge [1], their applications in various fields have been studied widely and intensely. CNTs have been the subject of much research interest in recent years owing to their attractive physical, chemical, and material characteristics [2]. However, the literature on research on thermal analysis for CNTs has been slim. Kashiwagi et al. [3] analyzed the thermal and flammability properties of polypropylene/multi-walled carbon nanotube (PP/MWNT), nanocomposites and found an increase in the radiation *
Corresponding author. Tel.: +886 534 2601; fax: +886 531 2069. E-mail address: [email protected] (C.-M. Shu).
0266-3538/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2007.11.011
in-depth absorption coefficient by the addition of MWNT. Furthermore, the effects of residual iron particles (catalyst) and of defects in the MWNTs on the heat release rate of the nanocomposite were not significant. Yu et al. [4] used thermogravimetry–differential scanning calorimetry–mass spectrometry (TG–DSC–MS) coupling techniques to study the thermal decomposition process of CNT/SiO2 precursor powders prepared by rapid sol–gel method. The results showed that the oxidation of CNT began from 530 and combusted at about 678 °C at a heating rate of 10 °C min1 in air. Moreover, the faster the heating rate, the higher the combustion temperature of CNT. Pritchard [5] concluded that an increasing range of materials as nanopowders capable of generating explosive dust clouds were being produced. In addition, there is a growing concern over the
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
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98.69 m2 g1 (Table 1). For comparison, active carbon powders (denoted C) from Merck, commercial multi-wall CNT of MWCNT 2040 from ConYuan Biochemical Technology Co. Ltd., Taiwan (denoted CBT), and a-Al2O3 particles with Fe–Ni nanoparticles were also tested. Those materials were observed by TEM and FE-SEM as shown in Fig. 1. The contents of CCNT that adsorbed Pb+2 were also analyzed by XRD and we found Al2O3, graphite, Ni, and Fe/ Ni alloy (Fig. 2).
impact of the increased use of nanopowders and other nanomaterials will have negative effects on health, safety and the environment. MWNTs were grown on micron-sized Al2O3 particles methane and hydrogen at 700 °C under the catalysis of Fe–Ni nanoparticles. This composite MWNT/Al2O3 (CCNT) exhibits excellent adsorption capacities of Pb+2, Cu+2, and Cd+2 from aqueous solution as well as good operation properties [6]. CCNT’s thermal and flammability properties should also be tested in order to expand its applications. The principal purpose of DSC is to analyze data of thermokinetic parameters via experimental thermal curves [7,8]. The aim of this study was to explore the thermal and flammability properties of CCNT and to compare with the commercial CNT and powdered activated carbon in order to learn more about the possible applications of the material. Finally, the background material of Al2O3, along with the effects of the catalysis of Fe–Ni nanoparticles, was also tested.
2.2. Tests by differential scanning calorimetry (DSC) Scanning experiments were performed on a Mettler TA8000 system coupled with a DSC821e measuring cell that can withstand pressure up to about 100 bar. STARe software was used for acquiring curve traces [10]. An aluminum standard pan was used to avoid evaporation of the CNTs during scanning. We found that the data with heating rates of 2 and 4 °C min1 are the best in terms of accuracy and reproducibility. For accuracy, the scanning rates chosen for the temperature range from 30 to 640 °C were 2 and 4 °C min1 under atmospheric air. We discovered that thermal curves of CCNT for 4 mg were most complete and accurate. Therefore, the amounts of materials that were tested were 1.28 mg for C and CBT, 4.00 mg for CCNT (with carbon content 1.28 mg same as C and CBT) and 2.72 mg for Al2O3, as indicated in Table 2.
2. Experimental 2.1. CCNT and related materials The micro-sized Al2O3 particles were initially cleaned in acid aqueous solution with ultrasonic vibration. After cleaning, the Al2O3 particles were sensitized in sensitization solution (SnCl2 + HCl) and then were activated in activation solution (PdCl2 + HCl). After sensitization and activation, a film of Fe–Ni nanoparticles form electroless deposits on the Al2O3 particles in an aqueous solution containing Fe+2, Ni+2 and various ions as the composition and operation parameters listed elsewhere [6]. The Al2O3 particles, with which Fe–Ni nanoparticles were deposited on the surface, were heated to 700 °C and held at this temperature for a half hour in an N2 atmosphere with a flow rate of 120 mL min1 [9]. Then, the N2 atmosphere was then replaced by CH4 gas at the same flow rate. With the as catalyst and under CH4 atmosphere, CNTs were grown with Fe–Ni nanoparticles on the tips at the surface of Al2O3 particles. The contents and characteristics of CCNT, CNTs, Fe–Ni nanoparticles and Al2O3 were measured and observed by electron microscopy for morphology, BET method for special surface area, and Zeta potential test for surface charge and could be found elsewhere [6]. The weight percents of CNTs, Fe–Ni nanoparticles and Al2O3 particles were 32%, 12% and 56%, respectively. The specific surface areas of CCNT and Al2O3 particles were 31.58 and 9.30 m2 g1. From the weight percent of CNTs, we deduced that the specific surface area of CNTs alone would be
2.3. Kinetic analysis by the borchardt & daniels method This technique assumes the reaction follows nth order kinetics and the Arrhenius equation. The instantaneous reaction rate can be acquired by using single dynamic scanning with DSC and expressed as follows da n ¼ kðT Þð1 aÞ dt
ð1Þ
where da/dt, k(T), a, n are the rate of reaction (sec1), the rate constant at temperature T(K), the conversion of reactant (0–1) and the reaction order, individually. From the Arrhenius equation kðT Þ ¼ k 0 eEa =RT
ð2Þ 1
1n
where k0, Ea and R are the frequency factor (sec M ), activation energy (kJ mol1) and universal gas constant (8.314 J mol1 K1), respectively. On substituting the Arrhenius equation into Eq. (1), we can obtain a multiple linear equation which can be written in logarithmic form
Table 1 The physical and chemical properties of materials Material
Sharp/size
Mass (%)
SBET (m2 g1)
Structure
Al2O3 Fe–Ni CNTs
Irregular, 3–5 lm Particular, 40 nm Tublar, lm long, diameter < 100 nm
56 12 32
9.30 10.87 98.69
a-Type Nano-bimetal Multi-walled, tangles
2956
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
Fig. 1. The morphology by TEM and FE-SEM for (a) powder activated carbon, (b) commercial CNT, (c) Al2O3 with nano-Fe–NI, and (d) the composite CNT (CCNT) with nano-Fe–Ni.
Fig. 2. The XRD diagram of CCNT with adsorbed Pb+2 (no detection of Pb here).
ln
da Ea þ n lnð1 aÞ ¼ ln k 0 RT dt
ð3Þ
Multiple linear regression analysis provides the desired kinetic data by using measured values (da/dt, a and T) directly from thermal curve data. Those required
parameters (da/dt and a) can be readily obtained by the basic assumption that a fraction of reacted (da) corresponds to the heat flow change (dH) from DSC [7,8]. a¼
DH T DH 0
ð4Þ
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
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Table 2 Scanning tests and results of C, CBT, and CCNT by DSC 821e Material
M (mg)
Heating rate (C min1)
T0 (°C)
Tmax (°C)
DH (J g1)
CCNT CCNT CBT CBT C C
4.00 4.00 1.28 1.28 1.28 1.28
2 4 2 4 2 4
367.20 445.70 437.30 470.10 300.10 394.40
556.50 581.00 615.00 >640.00 548.90 575.90
6817.35 3813.24 24.08e + 03 Over-range 34.33e + 03 13.48e + 03
ð5Þ
where HT, DHT, H0 and DH0 are the heat flow at temperature T (kJ mol1), partial area of DSC curve (kJ mol1), initial heat flow (kJ mol1) and total peak area of DSC curve (kJ mol1), respectively. 3. Results and discussion CNTs have been reported to be hazardous to the environment, as well as to humans and animals [5]. SWNT and MWNT are all composed by carbon. They could be readily ignited if exposed to a thermal source. Because of their large surface areas, they could initiate a spontaneous oxidation reaction as indicated by the observation of SWNT evaporating at the tip due to local heating at high current–voltage application under vacuum. However, recent research has indicated that MWNT even could retard flame by being uniformly laid on polypropylene surface [4]. Therefore, this study used the non–isothermal method of DSC to determine the thermal decomposition hazards of different types of carbon-activated carbon powder, CBT, and CCNT. The tests on Al2O3 powder were used to compare the baseline for the heating property of CCNT. Experimental data on C, CBT, and CCNT by DSC with different scanning heating rates of 2 and 4 °C min1 are summarized in Table 2. The temperature ‘‘shift” was irregular at exothermic onset temperatures and at about 25 °C for peak temperatures of C, CBT, and CCNT as heating rate increased from 2 to 4 °C min1. The exothermic onset temperature of CCNT for decomposition was at 367.2 °C, and higher exothermic onset temperature at 445.7 °C with higher heating rates in the thermal curve appear to show a ‘‘thermal delayed effect” phenomenon. Similar results were observed for CBT with higher exothermic onset temperatures and for C with lower exothermic onset temperature. The values for the heat of decomposition demonstrated the order of C > CBT > CCNT, with CCNT only 20 mass% of C at low heating rate (38 mass% at 4 °C min1). The heating of C has the lowest exothermic onset temperature of exothermal reaction and highest heat of decomposition than CBT and CCNT. The novel material CCNT yielded the lowest heat of decomposition and medium exothermic onset temperature.
Although CBT had the highest exothermic onset temperature, its heat of decomposition is moderate (70 mass% of C). Those results were compatible with MWNT and CNT/SiO2 precursor powders by Yu et al. [3]. They obtained the peak temperature of MWNT at 5 °C min1 to be 659 °C (738 °C at 10 °C min1) and experienced a similar thermal delayed effect. Their CNT/SiO2 yielded a peak temperature at 677.5 °C. By doping 0.5–4 mass% of MWNT to PP surface, the heat release rate could greatly reduced for a cone calorimeter at 50 kW m2 according to Kashiwagi et al. [4]. They observed that 7.1 mass% catalyst iron particles in MWNT revealed no effect on heat release except for smoldering at the end of combustion. They further concluded that the thermal conductivity of MWNT on PP surface re-emitted the incident radiation and reduced the transmitted flux. Our CCNT probably demonstrated a similar deflection effect as the lower peak temperatures of CCNT depict in Figs. 3 and 4 of the DSC thermal scans. Besides, there were much lower heat flows of CCNT compared to those of C and CBT in Figs. 3 and 4. The baseline of heating Al2O3 indicates that almost no effect occurred at heating, as illustrated in Fig. 4. The higher heating temperature showed a ‘‘thermal delay effect” that alleviated the exothermic onset temperature for various materials and lessened the heat of decomposition. 14 12 10
HeatFlow (W/g)
dH T da ¼ dt dt DH 0
CCNT CBT C
8 6 4 2 0 -2 0
100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. Comparison of heat flow vs. temperature by DSC tests on CCNT, CBT and C at heating rate: 2 °C min1.
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C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959 Table 3 Kinetic parameters for the decomposition of CCNT by DSC 821e
14 12
Material Condition CCNT CBT C Al2O3
HeatFlow (W/g)
10 8
CCNT CCNT
Ea ln A n M Heating (kJ mol1) (min1 M1n) (mg) rate (°C min1)
CNT/Al2O3 4.00 CNT/Al2O3 4.00
2 4
154.24 162.66
13.33 16.60
1 1
6 4 2 0 -2 0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 4. Comparison of heat flow vs. temperature by DSC tests on CCNT, CBT, C and Al2O3 at heating rate of 4 °C min1.
The comparison of the DSC thermal curves for CCNT measured at the heating rates of 2 and 4 °C min1 are diagrammed in Fig. 5. Here, two corresponding DSC curves appear to initiate the oxidation of CNT beginning at 406.5 °C and to combust at exothermic peaks about 556 and 581 °C, respectively. In addition, the DSC exothermic peak moved toward higher temperature and heat flow enhanced with increasing heating rate. By comparing the results for DSC thermal scans on C, CBT, CCNT, and CCNT had medium exothermic onset temperature and lowest heat of decomposition, indicating it is a safer material (with less thermal hazard) than C and CBT. In addition, the heating exothermal reaction is first order for CCNT and it had its activation energy augmented as the heating rate increased (Table 3). Moreover, our results implied that different types of carbon contained in C, CBT and CCNT could yield different energies during thermal decomposition. Thus, our MWNT in CCNT 4
HeatFlow (W/g)
3
o
-1
CCNT-2 C min o -1 CCNT-4 C min
2
1
0
-1 0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 5. Comparison of DSC thermal curves of CCNT at heating rates of 2 and 4 °C min1.
would have much lower heat of decomposition than MWNT in commercial CNT. CCNT should be safe and have lower heat of decomposition. Although recognized as a thermally stable material, CCNT’s heat of decomposition of reaction far exceeds our expectations. Therefore, it is prone to accidents under higher temperature, with the possibility of causing disasters. 4. Conclusions According to the DSC results, we drew the following conclusions: (i) The heating of activated carbon powder (C) had the lowest exothermic onset temperature (T0) and heat of decomposition than CBT and CCNT. The new material CCNT yielded the lowest heat of decomposition and medium exothermic onset temperature. The baseline of heating Al2O3 indicated that almost no effect occurred at heating. (ii) The higher heating temperature disclosed a ‘‘thermal delay effect” that augmented the exothermic onset temperature and lessened the heat of decomposition for C, CBT, and CCNT. The temperature ‘‘shifts” were irregular at exothermic onset temperatures and at about 25 °C for peak exothermal temperatures of C, CBT and CCNT as heating rate increased from 2 to 4 °C min1. (iii) By comparing results on C, CBT, and CCNT, CCNT had medium exothermic onset temperature and the lowest heat of decomposition, indicating it is a safer material (with less thermal hazard) than C and CBT. Furthermore, the exothermic reaction was the first order for CCNT and had the activation energy increase as the heating rate increased. Our results also implied that different types of carbon contained in C, CBT and CCNT yielded different energies during the excursion of heating composition. Thus, our MWNT in CCNT would have much lower heat of decomposition than MWNT in commercial CNT, demonstrating an advantage in loss prevention. (iv) Although CCNT’s onset temperature is 367.2 °C, it should not be exposed to a thermal source in order to prevent a catastrophe. CCNT’s thermal hazard during preparation, manufacturing, transportation, storage and even disposal is often ignored.
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
Acknowledgment We are indebted to Dr. Shu-Huei Hsieh, Department of Materials Science and Engineering, National Formosa University (NFU), for her considerable technical assistance. References [1] Iijima S. Nature 1991;354:56–8. [2] Hsieh SH, Horng JJ. J Univ Sci Technol, 1. Beijing: PRC; 2007 [p. 1–8].
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[3] Kashiwagi T, Grulke E, Hilding J, Groth K, Harris R, Butler K, et al. Polymer 2004;45:4227–39. [4] Yu H, Yu C, Xi T, Luo L, Ning J, Xiang C. J Therm Anal Calorim 2005;82:97–101. [5] Pritchard DK. Literature review – explosion hazards associated with nanopowders. Crown Copyright; 2004. [6] Hsieh SH, Horng JJ, Tsai CK. J Mater Res 2006:1269–73. [7] Wang YW, Shu CM, Duh YS, Kao CS. Ind Eng Chem Res 2001;40:1125–32. [8] Shu CM, Yang YJ. Thermochim Acta 2002 [392–3, 257–69]. [9] Hsieh SH, Horng JJ. Appl Surf Sci 2006;253(3):1660–5. [10] Mettler Toledo. STARe thermal analysis, Sweden; 2005.
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 2954–2959 www.elsevier.com/locate/compscitech
Thermal decomposition of carbon nanotube/Al2O3 powders by DSC testing Chih-Wei Chang, Jo-Ming Tseng, Jao-Jia Horng, Chi-Min Shu * Department of Safety, Health, and Environmental Engineering, National Yunlin University of Science and Technology, 123, University Road, Sec. 3, Douliu, Yunlin 64002, Taiwan, ROC Received 14 November 2007; accepted 14 November 2007 Available online 28 November 2007
Abstract Concerns over unsafe or unknown properties of carbon nanotubes (CNTs) have been raised by many researchers. The thermal stability characteristics, which may represent potential hazards during the production or utilization stage, could be determined by calorimetry testing for various thermokinetic parameters. In this study, differential scanning calorimetry (DSC) was employed to analyze thermal decomposition and to calculate thermodynamic characters and dynamic parameters for activated carbon powder (C), commercial CNT (CBT), and an innovative composite of CNT on Al2O3 powders (CCNT) during the time-effective experiments. The higher heating temperature revealed a ‘‘thermal delay effect” that increased the exothermic onset temperature and decreased the heat of decomposition for C, CBT and CCNT. The temperature shifts were irregular at exothermic onset temperatures and at about 25 °C for peak exothermal temperatures of C, CBT, and CCNT as heating rate increased from 2 to 4 °C min1. From a comparison of results on C, CBT, and CCNT, CCNT had medium exothermic onset temperature, lowest heat of decomposition, and is a safer material (less thermal hazard) than C and CBT. In addition, the heating exothermic reaction was the first order for CCNT and the activation energy increased as the heating rate increased. Our results also implied that different types of carbon contained in C, CBT, and CCNT could yield different energies during thermal decomposition. Ó 2008 Published by Elsevier Ltd. Keywords: Carbon nanotubes (CNTs); Thermokinetic parameters; Differential scanning calorimetry (DSC); Thermal decomposition, material, dynamic, equipment, runaway reaction phenomenon
1. Introduction Since Iijima discovered carbon nanotubes (CNTs) by arc-discharge [1], their applications in various fields have been studied widely and intensely. CNTs have been the subject of much research interest in recent years owing to their attractive physical, chemical, and material characteristics [2]. However, the literature on research on thermal analysis for CNTs has been slim. Kashiwagi et al. [3] analyzed the thermal and flammability properties of polypropylene/multi-walled carbon nanotube (PP/MWNT), nanocomposites and found an increase in the radiation *
Corresponding author. Tel.: +886 534 2601; fax: +886 531 2069. E-mail address: [email protected] (C.-M. Shu).
0266-3538/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.compscitech.2007.11.011
in-depth absorption coefficient by the addition of MWNT. Furthermore, the effects of residual iron particles (catalyst) and of defects in the MWNTs on the heat release rate of the nanocomposite were not significant. Yu et al. [4] used thermogravimetry–differential scanning calorimetry–mass spectrometry (TG–DSC–MS) coupling techniques to study the thermal decomposition process of CNT/SiO2 precursor powders prepared by rapid sol–gel method. The results showed that the oxidation of CNT began from 530 and combusted at about 678 °C at a heating rate of 10 °C min1 in air. Moreover, the faster the heating rate, the higher the combustion temperature of CNT. Pritchard [5] concluded that an increasing range of materials as nanopowders capable of generating explosive dust clouds were being produced. In addition, there is a growing concern over the
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
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98.69 m2 g1 (Table 1). For comparison, active carbon powders (denoted C) from Merck, commercial multi-wall CNT of MWCNT 2040 from ConYuan Biochemical Technology Co. Ltd., Taiwan (denoted CBT), and a-Al2O3 particles with Fe–Ni nanoparticles were also tested. Those materials were observed by TEM and FE-SEM as shown in Fig. 1. The contents of CCNT that adsorbed Pb+2 were also analyzed by XRD and we found Al2O3, graphite, Ni, and Fe/ Ni alloy (Fig. 2).
impact of the increased use of nanopowders and other nanomaterials will have negative effects on health, safety and the environment. MWNTs were grown on micron-sized Al2O3 particles methane and hydrogen at 700 °C under the catalysis of Fe–Ni nanoparticles. This composite MWNT/Al2O3 (CCNT) exhibits excellent adsorption capacities of Pb+2, Cu+2, and Cd+2 from aqueous solution as well as good operation properties [6]. CCNT’s thermal and flammability properties should also be tested in order to expand its applications. The principal purpose of DSC is to analyze data of thermokinetic parameters via experimental thermal curves [7,8]. The aim of this study was to explore the thermal and flammability properties of CCNT and to compare with the commercial CNT and powdered activated carbon in order to learn more about the possible applications of the material. Finally, the background material of Al2O3, along with the effects of the catalysis of Fe–Ni nanoparticles, was also tested.
2.2. Tests by differential scanning calorimetry (DSC) Scanning experiments were performed on a Mettler TA8000 system coupled with a DSC821e measuring cell that can withstand pressure up to about 100 bar. STARe software was used for acquiring curve traces [10]. An aluminum standard pan was used to avoid evaporation of the CNTs during scanning. We found that the data with heating rates of 2 and 4 °C min1 are the best in terms of accuracy and reproducibility. For accuracy, the scanning rates chosen for the temperature range from 30 to 640 °C were 2 and 4 °C min1 under atmospheric air. We discovered that thermal curves of CCNT for 4 mg were most complete and accurate. Therefore, the amounts of materials that were tested were 1.28 mg for C and CBT, 4.00 mg for CCNT (with carbon content 1.28 mg same as C and CBT) and 2.72 mg for Al2O3, as indicated in Table 2.
2. Experimental 2.1. CCNT and related materials The micro-sized Al2O3 particles were initially cleaned in acid aqueous solution with ultrasonic vibration. After cleaning, the Al2O3 particles were sensitized in sensitization solution (SnCl2 + HCl) and then were activated in activation solution (PdCl2 + HCl). After sensitization and activation, a film of Fe–Ni nanoparticles form electroless deposits on the Al2O3 particles in an aqueous solution containing Fe+2, Ni+2 and various ions as the composition and operation parameters listed elsewhere [6]. The Al2O3 particles, with which Fe–Ni nanoparticles were deposited on the surface, were heated to 700 °C and held at this temperature for a half hour in an N2 atmosphere with a flow rate of 120 mL min1 [9]. Then, the N2 atmosphere was then replaced by CH4 gas at the same flow rate. With the as catalyst and under CH4 atmosphere, CNTs were grown with Fe–Ni nanoparticles on the tips at the surface of Al2O3 particles. The contents and characteristics of CCNT, CNTs, Fe–Ni nanoparticles and Al2O3 were measured and observed by electron microscopy for morphology, BET method for special surface area, and Zeta potential test for surface charge and could be found elsewhere [6]. The weight percents of CNTs, Fe–Ni nanoparticles and Al2O3 particles were 32%, 12% and 56%, respectively. The specific surface areas of CCNT and Al2O3 particles were 31.58 and 9.30 m2 g1. From the weight percent of CNTs, we deduced that the specific surface area of CNTs alone would be
2.3. Kinetic analysis by the borchardt & daniels method This technique assumes the reaction follows nth order kinetics and the Arrhenius equation. The instantaneous reaction rate can be acquired by using single dynamic scanning with DSC and expressed as follows da n ¼ kðT Þð1 aÞ dt
ð1Þ
where da/dt, k(T), a, n are the rate of reaction (sec1), the rate constant at temperature T(K), the conversion of reactant (0–1) and the reaction order, individually. From the Arrhenius equation kðT Þ ¼ k 0 eEa =RT
ð2Þ 1
1n
where k0, Ea and R are the frequency factor (sec M ), activation energy (kJ mol1) and universal gas constant (8.314 J mol1 K1), respectively. On substituting the Arrhenius equation into Eq. (1), we can obtain a multiple linear equation which can be written in logarithmic form
Table 1 The physical and chemical properties of materials Material
Sharp/size
Mass (%)
SBET (m2 g1)
Structure
Al2O3 Fe–Ni CNTs
Irregular, 3–5 lm Particular, 40 nm Tublar, lm long, diameter < 100 nm
56 12 32
9.30 10.87 98.69
a-Type Nano-bimetal Multi-walled, tangles
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C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
Fig. 1. The morphology by TEM and FE-SEM for (a) powder activated carbon, (b) commercial CNT, (c) Al2O3 with nano-Fe–NI, and (d) the composite CNT (CCNT) with nano-Fe–Ni.
Fig. 2. The XRD diagram of CCNT with adsorbed Pb+2 (no detection of Pb here).
ln
da Ea þ n lnð1 aÞ ¼ ln k 0 RT dt
ð3Þ
Multiple linear regression analysis provides the desired kinetic data by using measured values (da/dt, a and T) directly from thermal curve data. Those required
parameters (da/dt and a) can be readily obtained by the basic assumption that a fraction of reacted (da) corresponds to the heat flow change (dH) from DSC [7,8]. a¼
DH T DH 0
ð4Þ
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
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Table 2 Scanning tests and results of C, CBT, and CCNT by DSC 821e Material
M (mg)
Heating rate (C min1)
T0 (°C)
Tmax (°C)
DH (J g1)
CCNT CCNT CBT CBT C C
4.00 4.00 1.28 1.28 1.28 1.28
2 4 2 4 2 4
367.20 445.70 437.30 470.10 300.10 394.40
556.50 581.00 615.00 >640.00 548.90 575.90
6817.35 3813.24 24.08e + 03 Over-range 34.33e + 03 13.48e + 03
ð5Þ
where HT, DHT, H0 and DH0 are the heat flow at temperature T (kJ mol1), partial area of DSC curve (kJ mol1), initial heat flow (kJ mol1) and total peak area of DSC curve (kJ mol1), respectively. 3. Results and discussion CNTs have been reported to be hazardous to the environment, as well as to humans and animals [5]. SWNT and MWNT are all composed by carbon. They could be readily ignited if exposed to a thermal source. Because of their large surface areas, they could initiate a spontaneous oxidation reaction as indicated by the observation of SWNT evaporating at the tip due to local heating at high current–voltage application under vacuum. However, recent research has indicated that MWNT even could retard flame by being uniformly laid on polypropylene surface [4]. Therefore, this study used the non–isothermal method of DSC to determine the thermal decomposition hazards of different types of carbon-activated carbon powder, CBT, and CCNT. The tests on Al2O3 powder were used to compare the baseline for the heating property of CCNT. Experimental data on C, CBT, and CCNT by DSC with different scanning heating rates of 2 and 4 °C min1 are summarized in Table 2. The temperature ‘‘shift” was irregular at exothermic onset temperatures and at about 25 °C for peak temperatures of C, CBT, and CCNT as heating rate increased from 2 to 4 °C min1. The exothermic onset temperature of CCNT for decomposition was at 367.2 °C, and higher exothermic onset temperature at 445.7 °C with higher heating rates in the thermal curve appear to show a ‘‘thermal delayed effect” phenomenon. Similar results were observed for CBT with higher exothermic onset temperatures and for C with lower exothermic onset temperature. The values for the heat of decomposition demonstrated the order of C > CBT > CCNT, with CCNT only 20 mass% of C at low heating rate (38 mass% at 4 °C min1). The heating of C has the lowest exothermic onset temperature of exothermal reaction and highest heat of decomposition than CBT and CCNT. The novel material CCNT yielded the lowest heat of decomposition and medium exothermic onset temperature.
Although CBT had the highest exothermic onset temperature, its heat of decomposition is moderate (70 mass% of C). Those results were compatible with MWNT and CNT/SiO2 precursor powders by Yu et al. [3]. They obtained the peak temperature of MWNT at 5 °C min1 to be 659 °C (738 °C at 10 °C min1) and experienced a similar thermal delayed effect. Their CNT/SiO2 yielded a peak temperature at 677.5 °C. By doping 0.5–4 mass% of MWNT to PP surface, the heat release rate could greatly reduced for a cone calorimeter at 50 kW m2 according to Kashiwagi et al. [4]. They observed that 7.1 mass% catalyst iron particles in MWNT revealed no effect on heat release except for smoldering at the end of combustion. They further concluded that the thermal conductivity of MWNT on PP surface re-emitted the incident radiation and reduced the transmitted flux. Our CCNT probably demonstrated a similar deflection effect as the lower peak temperatures of CCNT depict in Figs. 3 and 4 of the DSC thermal scans. Besides, there were much lower heat flows of CCNT compared to those of C and CBT in Figs. 3 and 4. The baseline of heating Al2O3 indicates that almost no effect occurred at heating, as illustrated in Fig. 4. The higher heating temperature showed a ‘‘thermal delay effect” that alleviated the exothermic onset temperature for various materials and lessened the heat of decomposition. 14 12 10
HeatFlow (W/g)
dH T da ¼ dt dt DH 0
CCNT CBT C
8 6 4 2 0 -2 0
100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. Comparison of heat flow vs. temperature by DSC tests on CCNT, CBT and C at heating rate: 2 °C min1.
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C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959 Table 3 Kinetic parameters for the decomposition of CCNT by DSC 821e
14 12
Material Condition CCNT CBT C Al2O3
HeatFlow (W/g)
10 8
CCNT CCNT
Ea ln A n M Heating (kJ mol1) (min1 M1n) (mg) rate (°C min1)
CNT/Al2O3 4.00 CNT/Al2O3 4.00
2 4
154.24 162.66
13.33 16.60
1 1
6 4 2 0 -2 0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 4. Comparison of heat flow vs. temperature by DSC tests on CCNT, CBT, C and Al2O3 at heating rate of 4 °C min1.
The comparison of the DSC thermal curves for CCNT measured at the heating rates of 2 and 4 °C min1 are diagrammed in Fig. 5. Here, two corresponding DSC curves appear to initiate the oxidation of CNT beginning at 406.5 °C and to combust at exothermic peaks about 556 and 581 °C, respectively. In addition, the DSC exothermic peak moved toward higher temperature and heat flow enhanced with increasing heating rate. By comparing the results for DSC thermal scans on C, CBT, CCNT, and CCNT had medium exothermic onset temperature and lowest heat of decomposition, indicating it is a safer material (with less thermal hazard) than C and CBT. In addition, the heating exothermal reaction is first order for CCNT and it had its activation energy augmented as the heating rate increased (Table 3). Moreover, our results implied that different types of carbon contained in C, CBT and CCNT could yield different energies during thermal decomposition. Thus, our MWNT in CCNT 4
HeatFlow (W/g)
3
o
-1
CCNT-2 C min o -1 CCNT-4 C min
2
1
0
-1 0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 5. Comparison of DSC thermal curves of CCNT at heating rates of 2 and 4 °C min1.
would have much lower heat of decomposition than MWNT in commercial CNT. CCNT should be safe and have lower heat of decomposition. Although recognized as a thermally stable material, CCNT’s heat of decomposition of reaction far exceeds our expectations. Therefore, it is prone to accidents under higher temperature, with the possibility of causing disasters. 4. Conclusions According to the DSC results, we drew the following conclusions: (i) The heating of activated carbon powder (C) had the lowest exothermic onset temperature (T0) and heat of decomposition than CBT and CCNT. The new material CCNT yielded the lowest heat of decomposition and medium exothermic onset temperature. The baseline of heating Al2O3 indicated that almost no effect occurred at heating. (ii) The higher heating temperature disclosed a ‘‘thermal delay effect” that augmented the exothermic onset temperature and lessened the heat of decomposition for C, CBT, and CCNT. The temperature ‘‘shifts” were irregular at exothermic onset temperatures and at about 25 °C for peak exothermal temperatures of C, CBT and CCNT as heating rate increased from 2 to 4 °C min1. (iii) By comparing results on C, CBT, and CCNT, CCNT had medium exothermic onset temperature and the lowest heat of decomposition, indicating it is a safer material (with less thermal hazard) than C and CBT. Furthermore, the exothermic reaction was the first order for CCNT and had the activation energy increase as the heating rate increased. Our results also implied that different types of carbon contained in C, CBT and CCNT yielded different energies during the excursion of heating composition. Thus, our MWNT in CCNT would have much lower heat of decomposition than MWNT in commercial CNT, demonstrating an advantage in loss prevention. (iv) Although CCNT’s onset temperature is 367.2 °C, it should not be exposed to a thermal source in order to prevent a catastrophe. CCNT’s thermal hazard during preparation, manufacturing, transportation, storage and even disposal is often ignored.
C.-W. Chang et al. / Composites Science and Technology 68 (2008) 2954–2959
Acknowledgment We are indebted to Dr. Shu-Huei Hsieh, Department of Materials Science and Engineering, National Formosa University (NFU), for her considerable technical assistance. References [1] Iijima S. Nature 1991;354:56–8. [2] Hsieh SH, Horng JJ. J Univ Sci Technol, 1. Beijing: PRC; 2007 [p. 1–8].
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[3] Kashiwagi T, Grulke E, Hilding J, Groth K, Harris R, Butler K, et al. Polymer 2004;45:4227–39. [4] Yu H, Yu C, Xi T, Luo L, Ning J, Xiang C. J Therm Anal Calorim 2005;82:97–101. [5] Pritchard DK. Literature review – explosion hazards associated with nanopowders. Crown Copyright; 2004. [6] Hsieh SH, Horng JJ, Tsai CK. J Mater Res 2006:1269–73. [7] Wang YW, Shu CM, Duh YS, Kao CS. Ind Eng Chem Res 2001;40:1125–32. [8] Shu CM, Yang YJ. Thermochim Acta 2002 [392–3, 257–69]. [9] Hsieh SH, Horng JJ. Appl Surf Sci 2006;253(3):1660–5. [10] Mettler Toledo. STARe thermal analysis, Sweden; 2005.