- Email: [email protected]
Preparation and characterization of Mg nanoparticles
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4–5 1 8
Preparation and characterization of Mg nanoparticles Mei-Rong Song a,b,c , Miao Chen a,⁎, Zhi-Jun Zhang b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China Laboratory for Special Functionary Materials, Henan University, Kaifeng, 475001, P. R. China c Graduate School of the Chinese Academy of Sciences, Beijing, 100049, P. R. China b
AR TIC LE D ATA
ABSTR ACT
Article history:
In this paper, Mg nanoparticles were prepared in tetrahydrofuran via lithium reduction of the
Received 2 December 2006
corresponding Mg salt. X-ray diffraction and scanning electron microscope investigations
Received in revised form
confirm the formation of hexagonal phase Mg particles with an average size of 300 nm. X-ray
10 March 2007
photoelectron spectrometer analysis indicates that the as-prepared Mg nanoparticles are
Accepted 12 March 2007
covered with a protecting layer consisting of residue solvents, naphthalene and Mg(OH)2, which slows down further oxidation under ambient conditions. Thermal analysis shows that
Keywords:
the rapid oxidation and nitridation processes of the particles take place at around 500 °C and
Magnesium nanoparticles
553 °C, respectively. Furthermore, the addition of a small amount of magnesium
Preparation
nanoparticles remarkably catalyzes the decomposition process of ammonium perchlorate
Characterization
by lowering the decomposition temperature and enhancing its heat output. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
In recent years, magnesium nanoparticles (Mg NPs) have received intense attention due to their novel properties different from bulk materials and various applications in fields of H2 storage, battery, propellant, composite fillers, etc. [1–5]. Usually, Mg NPs are prepared by two main techniques, namely, high energy ball milling and the gas-condensation method [5]. Here, we describe a novel method to prepare Mg NPs without using complicated pieces of equipment. Mg NPs were prepared in tetrahydrofuran (THF) via lithium reduction of the corresponding Mg salt. The reactions can be illustrated as follows:
A small amount of naphthalene was added as an electron carrier [6]. We provide detailed characterization to investigate the structure and composition of the resulting Mg NPs. The thermal decomposition of ammonium perchlorate (AP), an important energetic material in solid rocket propel-
lant, is remarkably sensitive to additives. More recently, new additives based on metal nanoparticles are reported to show good catalytic effect on thermal decomposition of AP [7]. Nanosized Mg powders are considered as good additives to enhance the heat release and improve the ignition properties of propellant [4]. In the present work, the effect of the as-prepared Mg NPs as additives on the thermal decomposition of AP was investigated using differential scanning calorimetry (DSC).
2.
Experimental
2.1.
Chemicals and Preparation
Lithium (99.9%) and analytical naphthalene were used as received. THF is distilled from its Na/benzophenone solution. Anhydrous magnesium chlorides (MgCl2) were purchased from Aldrich. AP was kindly presented by Nanjing Science and Technology University in China. A typical procedure preparation is as follows: A 100-mL conical flask was equipped with a Teflon-coated magnetic
⁎ Corresponding author. Tel.: +86 931 4968189. E-mail address: [email protected] (M. Chen). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.03.008
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4 –5 1 8
515
stirring bar, rubber septum and a condenser connected to an argon inlet. The flask was charged with 0.18 g of lithium, 1.46 g of anhydrous magnesium chlorides, 0.37 g of naphthalene, and 50 mL of THF. The mixture was stirred vigorously for 24 h at room temperature. After complete reaction, the mixture was filtered through a medium frit and the resulting dark gray powders were washed with ethanol and dried in a desiccator. To prepare the samples for the thermal decomposition experiments, the as-prepared Mg nanopowders and AP were dispersed in acetone by ultrasonication for several minutes, filtered through medium frit, and then dried in a vacuum desiccator. The resulting mixtures were ground for 10 min, and hence the samples comprised of AP and Mg NPs powders were obtained.
2.2.
Characterization
The morphology of Mg particles was observed on a JSM-5600 scanning electron microscope (SEM). Quantitative analysis of elemental composition was investigated by energy-dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction (XRD) measurement was performed on an X'pert Philips diffractometer using Cu Kα radiation, operating at 40 kV and 40 mA. Surface analysis was conducted on an AXIS ULTRO X-ray photoelectron spectrometer (XPS, Kratos). The Al Kα radiation was used as the excitation source and a Flood gun was used to reduce the charge effect. The binding energy was calibrated with reference to the C 1s level of carbon (284.8 eV). Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were conducted on the EXSTAR 6000 instrument in a flow of air and N2, respectively. Differential scanning calorimetry (DSC) studies were performed with Mettler Toledo DSC822e at a heating rate of 20 °C/min and under a nitrogen flow of 20 mL/min.
3.
Results and Discussion
3.1.
XRD Pattern
Fig. 1 shows the XRD pattern of the resulting Mg NPs. The XRD spectrum contains multiple peaks which are clearly distinguishable. All of the diffraction peaks are readily indexed to various crystal planes of the hexagonal phase Mg (JCPDS 040770). No peaks from other phase can be detected, which indicates that the product prepared via this route is of highly purity. In addition, the crystalline size of Mg NPs calculated from XRD is 26.4 nm according to Scherrer's equation.
3.2.
SEM Observation and EDS Analysis
Fig. 2 displays a representative SEM image and EDS spectrum of the as-prepared Mg NPs. The shape of the particles is irregular and their average size is about 300 nm. Compared to the result of XRD, it can be concluded that every Mg NP seen from SEM is congeries of smaller microcrystals. The EDS spectrum shows that the product is mainly composed of Mg, O and a small amount of Cl, and their respective atomic content are 60.38%, 36.67% and 2.95%. The O peak might come from magnesium oxides species and residue solvent on the surface
Fig. 1 – XRD pattern of Mg nanoparticles. All of the peaks are indexed to the hexagonal phase of Mg (JCPDS 04-0770).
of the particles, while Cl might derive from the unreacted MgCl2.
3.3.
XPS Analysis
The results of XRD, SEM and EDS clearly show the formation of Mg NPs. To investigate the surface composition, the sample was further characterized by X-ray photoelectron spectroscopy (XPS). The theoretical fitting of the spectra by the Gaussian function and the background calculation according to a linear algorithm are obtained using XPSPEAK95 Version 3.1. The C 1s spectrum is fit by four components as shown in Fig. 3a. Besides the peak of adventitious carbon at 284.8 eV, the peak at 286.3 eV is assigned to ether bonding (C⁎–O–C) [8], indicating the presence of residue solvent, THF, on the surface of the particles. In addition, a small amount of naphthalene remaining on the surface is verified by the peak at 289.6 eV, which is close to the datum reported previously [9]. The peak at 287.9 eV is ascribed to C⁎fO which might derive from absorption of CO2 in air. Fig. 3b gives the XPS spectrum of Mg 2p. There is only one peak with a binding energy of 49.5 eV, which is in good agreement with datum previously observed for Mg(OH)2 [10]. This result indicates that the main specie on the surface of Mg NPs is Mg(OH)2, which might be caused by the atmosphere moisture in air. On the basis of the XPS results, the as-prepared Mg NPs are covered with a protecting layer consisting of residue solvent, naphthalene and Mg(OH)2, which could slow down their further oxidation under ambient conditions. In addition, from the results of XPS and EDS, it is estimated that the content of zero-valent metal is more than 67.2% [(60.38 − 36.67/ 2 − 2.95/2) / 60.38] if assuming the particles are completely composed of Mg, Mg(OH)2 and MgCl2.
3.4.
Thermal Analysis
Fig. 4a shows the TG and DTA curves of the product obtained in air atmosphere. The weight changes of the sample can be divided into three regions, initial weight loss followed by
516
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4–5 1 8
Fig. 2 – SEM image and EDS spectrum of Mg nanoparticles. The average size is about 300 nm, and the atomic contents of Mg, O and Cl are 60.38%, 36.67% and 2.95%, respectively.
weight gain in two distinct steps. A small amount of weight loss (about 3.2 wt.%) before 100 °C results from the volatilization of residual solvent and naphthalene on the surface of the particles. Following the desorption process, the sample begins to gain weight and nearly level off until 480 °C. It corresponds to the oxidation of the skin of the particles. Accordingly, an exothermic slope exists between 100 and 480 °C in the curve of DTA. Subsequently, a sharp weight gain takes place between
480 and 610 °C, and it exceeds the initial weight by 32.2 wt.%. Meanwhile, two sharp exothermic peaks in DTA curve are found at 499.5 °C and 553 °C, respectively. This remarkable weight increase is ascribed to violent reactions between magnesium core and air. The two exothermic peaks are presumably related to the rapid oxidization and nitridation processes of the particles. To confirm the above supposition, TG and DTA measurements of the particles under a N2 atmosphere are carried out and the results are shown in Fig. 4b. The weight increase behavior of the particles in N2 is similar to that in air. But for DTA curve, there is only one sharp exothermic peak at 556 °C
Fig. 3 – XPS of Mg nanoparticles: (a) XPS spectrum of C 1s; (b) XPS spectrum of Mg 2p.
Fig. 4 – TG and DTA curves of Mg nanoparticles in air (a) and N2 (b), respectively.
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4 –5 1 8
due to the rapid nitridation of Mg NPs in N2, which is very close to the temperature of the second exothermic peak in air. Therefore, the two exothermic peaks in the DTA curve measured in air could be ascribed to the rapid oxidization and nitridation processes, respectively. In addition, compared to the theoretical total weight gain (38.9 wt.%) of combusting Mg in N2 to form Mg2N3, the high experimental value (35.8 wt. %) indicates the high content of metallic Mg in the sample. However, the exact calculation is difficult since the formation of MgO cannot be avoided due to the presence of trace amount of O2 and H2O in the atmosphere. It should be noted that the weight loss caused by Mg(OH)2 and MgCl2 is not observed in the curves either in air or N2. The reason might be that the TG curves are an overall result of the weight loss and gain, while the weight gain is the dominant process.
3.5.
Effect of Mg NPs on Thermal Decomposition of AP
Fig. 5 shows the DSC results of AP in the absence and presence of as-prepared Mg NPs. The thermal decomposition of pure AP is characterized by three events. First, the endothermic peak at around 250 °C is ascribed to the transition from the orthorhombic to the cubic phase. Secondly, the exothermic peak at the low temperature 325 °C corresponds to the partial decomposition of AP and formation of an intermediate product. Finally, the main exothermic peak along with a shoulder peak at higher temperatures 395 °C and 419.7 °C, respectively, corresponds to the complete decomposition of the intermediate product into volatile product. Addition of Mg NPs does not change the phase transition but remarkably sensitize the decomposition process. In the presence of 0.5 wt.% and 2 wt.% Mg NPs, the low-temperature decomposition peak of AP is hardly observed, whereas the high-temperature decomposition peaks turn into two strongly exothermic peaks below 380 °C. When the content of Mg NPs is increased to 5 wt.%, the low-temperature decomposition at 280 °C becomes significant as well as the high-temperature decomposition at 380 °C. Additionally, there is one small exothermic peak at 535 °C which might be caused by the reaction between Mg and N2. For the heat release, all of
Fig. 5 – DSC curves of AP and AP in the presence of Mg nanoparticles.
517
samples of AP in the presence of Mg NPs show higher release than the pure AP, and the sample of AP containing 0.5 wt.% Mg NPs has the highest value. As a result, Mg NPs catalyze the thermal decomposition of AP by lowering the decomposition temperature and enhancing its heat output.
4.
Conclusions
In summary, the method described in this paper offers a convenient method for the synthesis of Mg NPs without using complicated apparatus. The as-prepared Mg nanoparticles with an average size of 300 nm are covered with a protecting layer consisting residue solvents, naphthalene and Mg(OH)2, which slows down further oxidation under ambient conditions. The addition of a small amount of magnesium nanoparticles remarkably catalyzes the decomposition process of ammonium perchlorate by lowering the decomposition temperature and enhancing its heat output. The rapid oxidation and nitridation processes take place at around 500 °C and 553 °C, respectively. This method is a general method to synthesize a series of metal nanoparticles, which is of importance to study the catalysis, sensitivity and other properties.
Acknowledgments The authors are grateful to Prof. H. Wang for help in experimental section and Prof. Y. Yang for presenting AP. This work was supported by the National Nature Science Foundation of China (Grant No. 20473106) and the Innovation Group Project of Chinese Ministry of Science and Technology (Grant No. 50421502).
REFERENCES [1] Kooi BJ, Palasantzas G, Hosson JTMD. Gas-phase synthesis of magnesium nanoparticles: a high-resolution transmission electron microscopy study. Appl Phys Lett 2006;89:161914-1–3. [2] Zaluska A, Zaluski L, Strom-olsen JOS. Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage. Appl Phys A Mater Sci Process 2001;72:157–65. [3] Li WY, Li CS, Zhou CY, Ma H, Chen J. Metallic magnesium nano/ mesoscale structures: their shape-controlled preparation and Mg/air battery applications. Angew Chem Int Ed Engl 2006;45:6009–12. [4] Pang WQ, Zhang JQ, Zhu F. Research and application of a new-type of nanometer materials in solid propellants. Fiber Compos 2005;1:12–5. [5] Peng HR, Zhu LC, Zhang ZK. Preparation, structure and property research of nano-Mg/PP composites. Compos Interfaces 2004;112:231–43. [6] Kruizinga WH, Strijveen B, Kellogg RM. Preparation of highly reactive metal powders, new procedure for the preparation of highly reactive zinc and magnesium powders. J Org Chem 1981;46:4323–4. [7] Liu L, Li F, Tan L, Li M, Yang Y. Effects of nanometer Ni, Cu, Al and NiCu powders on the thermal decomposition of ammonium perchlorate. Prop Explos Pyrotech 2004;29:34–8.
518
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4–5 1 8
[8] Dufrêne YF, van der Wal A, Norde W, Rouxhet PG. X-ray photoelectron spectroscopy analysis of whole cells and isolated cell walls of Gram-positive bacteria: comparison with biochemical analysis. J Bacteriol 1997;179:1023–8. [9] Minkov F, Friedlein GR, Osikowicz W, Suess C. Core excitations of naphthalene: vibrational structure versus chemical shifts. J Chem Phys 2001;121:5733–9.
[10] Haycock DE, Kasrai M, Nicholls CJ, Urch DS. The electronic structure of magnesium hydroxide (brucite) using X-ray emission, X-ray photoelectron, and auger spectroscopy. J Chem Soc Dalton Trans 1978:1791–6.
Preparation and characterization of Mg nanoparticles Mei-Rong Song a,b,c , Miao Chen a,⁎, Zhi-Jun Zhang b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China Laboratory for Special Functionary Materials, Henan University, Kaifeng, 475001, P. R. China c Graduate School of the Chinese Academy of Sciences, Beijing, 100049, P. R. China b
AR TIC LE D ATA
ABSTR ACT
Article history:
In this paper, Mg nanoparticles were prepared in tetrahydrofuran via lithium reduction of the
Received 2 December 2006
corresponding Mg salt. X-ray diffraction and scanning electron microscope investigations
Received in revised form
confirm the formation of hexagonal phase Mg particles with an average size of 300 nm. X-ray
10 March 2007
photoelectron spectrometer analysis indicates that the as-prepared Mg nanoparticles are
Accepted 12 March 2007
covered with a protecting layer consisting of residue solvents, naphthalene and Mg(OH)2, which slows down further oxidation under ambient conditions. Thermal analysis shows that
Keywords:
the rapid oxidation and nitridation processes of the particles take place at around 500 °C and
Magnesium nanoparticles
553 °C, respectively. Furthermore, the addition of a small amount of magnesium
Preparation
nanoparticles remarkably catalyzes the decomposition process of ammonium perchlorate
Characterization
by lowering the decomposition temperature and enhancing its heat output. © 2007 Elsevier Inc. All rights reserved.
1.
Introduction
In recent years, magnesium nanoparticles (Mg NPs) have received intense attention due to their novel properties different from bulk materials and various applications in fields of H2 storage, battery, propellant, composite fillers, etc. [1–5]. Usually, Mg NPs are prepared by two main techniques, namely, high energy ball milling and the gas-condensation method [5]. Here, we describe a novel method to prepare Mg NPs without using complicated pieces of equipment. Mg NPs were prepared in tetrahydrofuran (THF) via lithium reduction of the corresponding Mg salt. The reactions can be illustrated as follows:
A small amount of naphthalene was added as an electron carrier [6]. We provide detailed characterization to investigate the structure and composition of the resulting Mg NPs. The thermal decomposition of ammonium perchlorate (AP), an important energetic material in solid rocket propel-
lant, is remarkably sensitive to additives. More recently, new additives based on metal nanoparticles are reported to show good catalytic effect on thermal decomposition of AP [7]. Nanosized Mg powders are considered as good additives to enhance the heat release and improve the ignition properties of propellant [4]. In the present work, the effect of the as-prepared Mg NPs as additives on the thermal decomposition of AP was investigated using differential scanning calorimetry (DSC).
2.
Experimental
2.1.
Chemicals and Preparation
Lithium (99.9%) and analytical naphthalene were used as received. THF is distilled from its Na/benzophenone solution. Anhydrous magnesium chlorides (MgCl2) were purchased from Aldrich. AP was kindly presented by Nanjing Science and Technology University in China. A typical procedure preparation is as follows: A 100-mL conical flask was equipped with a Teflon-coated magnetic
⁎ Corresponding author. Tel.: +86 931 4968189. E-mail address: [email protected] (M. Chen). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.03.008
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4 –5 1 8
515
stirring bar, rubber septum and a condenser connected to an argon inlet. The flask was charged with 0.18 g of lithium, 1.46 g of anhydrous magnesium chlorides, 0.37 g of naphthalene, and 50 mL of THF. The mixture was stirred vigorously for 24 h at room temperature. After complete reaction, the mixture was filtered through a medium frit and the resulting dark gray powders were washed with ethanol and dried in a desiccator. To prepare the samples for the thermal decomposition experiments, the as-prepared Mg nanopowders and AP were dispersed in acetone by ultrasonication for several minutes, filtered through medium frit, and then dried in a vacuum desiccator. The resulting mixtures were ground for 10 min, and hence the samples comprised of AP and Mg NPs powders were obtained.
2.2.
Characterization
The morphology of Mg particles was observed on a JSM-5600 scanning electron microscope (SEM). Quantitative analysis of elemental composition was investigated by energy-dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction (XRD) measurement was performed on an X'pert Philips diffractometer using Cu Kα radiation, operating at 40 kV and 40 mA. Surface analysis was conducted on an AXIS ULTRO X-ray photoelectron spectrometer (XPS, Kratos). The Al Kα radiation was used as the excitation source and a Flood gun was used to reduce the charge effect. The binding energy was calibrated with reference to the C 1s level of carbon (284.8 eV). Thermogravimetric (TG) analysis and differential thermal analysis (DTA) were conducted on the EXSTAR 6000 instrument in a flow of air and N2, respectively. Differential scanning calorimetry (DSC) studies were performed with Mettler Toledo DSC822e at a heating rate of 20 °C/min and under a nitrogen flow of 20 mL/min.
3.
Results and Discussion
3.1.
XRD Pattern
Fig. 1 shows the XRD pattern of the resulting Mg NPs. The XRD spectrum contains multiple peaks which are clearly distinguishable. All of the diffraction peaks are readily indexed to various crystal planes of the hexagonal phase Mg (JCPDS 040770). No peaks from other phase can be detected, which indicates that the product prepared via this route is of highly purity. In addition, the crystalline size of Mg NPs calculated from XRD is 26.4 nm according to Scherrer's equation.
3.2.
SEM Observation and EDS Analysis
Fig. 2 displays a representative SEM image and EDS spectrum of the as-prepared Mg NPs. The shape of the particles is irregular and their average size is about 300 nm. Compared to the result of XRD, it can be concluded that every Mg NP seen from SEM is congeries of smaller microcrystals. The EDS spectrum shows that the product is mainly composed of Mg, O and a small amount of Cl, and their respective atomic content are 60.38%, 36.67% and 2.95%. The O peak might come from magnesium oxides species and residue solvent on the surface
Fig. 1 – XRD pattern of Mg nanoparticles. All of the peaks are indexed to the hexagonal phase of Mg (JCPDS 04-0770).
of the particles, while Cl might derive from the unreacted MgCl2.
3.3.
XPS Analysis
The results of XRD, SEM and EDS clearly show the formation of Mg NPs. To investigate the surface composition, the sample was further characterized by X-ray photoelectron spectroscopy (XPS). The theoretical fitting of the spectra by the Gaussian function and the background calculation according to a linear algorithm are obtained using XPSPEAK95 Version 3.1. The C 1s spectrum is fit by four components as shown in Fig. 3a. Besides the peak of adventitious carbon at 284.8 eV, the peak at 286.3 eV is assigned to ether bonding (C⁎–O–C) [8], indicating the presence of residue solvent, THF, on the surface of the particles. In addition, a small amount of naphthalene remaining on the surface is verified by the peak at 289.6 eV, which is close to the datum reported previously [9]. The peak at 287.9 eV is ascribed to C⁎fO which might derive from absorption of CO2 in air. Fig. 3b gives the XPS spectrum of Mg 2p. There is only one peak with a binding energy of 49.5 eV, which is in good agreement with datum previously observed for Mg(OH)2 [10]. This result indicates that the main specie on the surface of Mg NPs is Mg(OH)2, which might be caused by the atmosphere moisture in air. On the basis of the XPS results, the as-prepared Mg NPs are covered with a protecting layer consisting of residue solvent, naphthalene and Mg(OH)2, which could slow down their further oxidation under ambient conditions. In addition, from the results of XPS and EDS, it is estimated that the content of zero-valent metal is more than 67.2% [(60.38 − 36.67/ 2 − 2.95/2) / 60.38] if assuming the particles are completely composed of Mg, Mg(OH)2 and MgCl2.
3.4.
Thermal Analysis
Fig. 4a shows the TG and DTA curves of the product obtained in air atmosphere. The weight changes of the sample can be divided into three regions, initial weight loss followed by
516
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4–5 1 8
Fig. 2 – SEM image and EDS spectrum of Mg nanoparticles. The average size is about 300 nm, and the atomic contents of Mg, O and Cl are 60.38%, 36.67% and 2.95%, respectively.
weight gain in two distinct steps. A small amount of weight loss (about 3.2 wt.%) before 100 °C results from the volatilization of residual solvent and naphthalene on the surface of the particles. Following the desorption process, the sample begins to gain weight and nearly level off until 480 °C. It corresponds to the oxidation of the skin of the particles. Accordingly, an exothermic slope exists between 100 and 480 °C in the curve of DTA. Subsequently, a sharp weight gain takes place between
480 and 610 °C, and it exceeds the initial weight by 32.2 wt.%. Meanwhile, two sharp exothermic peaks in DTA curve are found at 499.5 °C and 553 °C, respectively. This remarkable weight increase is ascribed to violent reactions between magnesium core and air. The two exothermic peaks are presumably related to the rapid oxidization and nitridation processes of the particles. To confirm the above supposition, TG and DTA measurements of the particles under a N2 atmosphere are carried out and the results are shown in Fig. 4b. The weight increase behavior of the particles in N2 is similar to that in air. But for DTA curve, there is only one sharp exothermic peak at 556 °C
Fig. 3 – XPS of Mg nanoparticles: (a) XPS spectrum of C 1s; (b) XPS spectrum of Mg 2p.
Fig. 4 – TG and DTA curves of Mg nanoparticles in air (a) and N2 (b), respectively.
M A TE RI A L S CH A RACT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4 –5 1 8
due to the rapid nitridation of Mg NPs in N2, which is very close to the temperature of the second exothermic peak in air. Therefore, the two exothermic peaks in the DTA curve measured in air could be ascribed to the rapid oxidization and nitridation processes, respectively. In addition, compared to the theoretical total weight gain (38.9 wt.%) of combusting Mg in N2 to form Mg2N3, the high experimental value (35.8 wt. %) indicates the high content of metallic Mg in the sample. However, the exact calculation is difficult since the formation of MgO cannot be avoided due to the presence of trace amount of O2 and H2O in the atmosphere. It should be noted that the weight loss caused by Mg(OH)2 and MgCl2 is not observed in the curves either in air or N2. The reason might be that the TG curves are an overall result of the weight loss and gain, while the weight gain is the dominant process.
3.5.
Effect of Mg NPs on Thermal Decomposition of AP
Fig. 5 shows the DSC results of AP in the absence and presence of as-prepared Mg NPs. The thermal decomposition of pure AP is characterized by three events. First, the endothermic peak at around 250 °C is ascribed to the transition from the orthorhombic to the cubic phase. Secondly, the exothermic peak at the low temperature 325 °C corresponds to the partial decomposition of AP and formation of an intermediate product. Finally, the main exothermic peak along with a shoulder peak at higher temperatures 395 °C and 419.7 °C, respectively, corresponds to the complete decomposition of the intermediate product into volatile product. Addition of Mg NPs does not change the phase transition but remarkably sensitize the decomposition process. In the presence of 0.5 wt.% and 2 wt.% Mg NPs, the low-temperature decomposition peak of AP is hardly observed, whereas the high-temperature decomposition peaks turn into two strongly exothermic peaks below 380 °C. When the content of Mg NPs is increased to 5 wt.%, the low-temperature decomposition at 280 °C becomes significant as well as the high-temperature decomposition at 380 °C. Additionally, there is one small exothermic peak at 535 °C which might be caused by the reaction between Mg and N2. For the heat release, all of
Fig. 5 – DSC curves of AP and AP in the presence of Mg nanoparticles.
517
samples of AP in the presence of Mg NPs show higher release than the pure AP, and the sample of AP containing 0.5 wt.% Mg NPs has the highest value. As a result, Mg NPs catalyze the thermal decomposition of AP by lowering the decomposition temperature and enhancing its heat output.
4.
Conclusions
In summary, the method described in this paper offers a convenient method for the synthesis of Mg NPs without using complicated apparatus. The as-prepared Mg nanoparticles with an average size of 300 nm are covered with a protecting layer consisting residue solvents, naphthalene and Mg(OH)2, which slows down further oxidation under ambient conditions. The addition of a small amount of magnesium nanoparticles remarkably catalyzes the decomposition process of ammonium perchlorate by lowering the decomposition temperature and enhancing its heat output. The rapid oxidation and nitridation processes take place at around 500 °C and 553 °C, respectively. This method is a general method to synthesize a series of metal nanoparticles, which is of importance to study the catalysis, sensitivity and other properties.
Acknowledgments The authors are grateful to Prof. H. Wang for help in experimental section and Prof. Y. Yang for presenting AP. This work was supported by the National Nature Science Foundation of China (Grant No. 20473106) and the Innovation Group Project of Chinese Ministry of Science and Technology (Grant No. 50421502).
REFERENCES [1] Kooi BJ, Palasantzas G, Hosson JTMD. Gas-phase synthesis of magnesium nanoparticles: a high-resolution transmission electron microscopy study. Appl Phys Lett 2006;89:161914-1–3. [2] Zaluska A, Zaluski L, Strom-olsen JOS. Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage. Appl Phys A Mater Sci Process 2001;72:157–65. [3] Li WY, Li CS, Zhou CY, Ma H, Chen J. Metallic magnesium nano/ mesoscale structures: their shape-controlled preparation and Mg/air battery applications. Angew Chem Int Ed Engl 2006;45:6009–12. [4] Pang WQ, Zhang JQ, Zhu F. Research and application of a new-type of nanometer materials in solid propellants. Fiber Compos 2005;1:12–5. [5] Peng HR, Zhu LC, Zhang ZK. Preparation, structure and property research of nano-Mg/PP composites. Compos Interfaces 2004;112:231–43. [6] Kruizinga WH, Strijveen B, Kellogg RM. Preparation of highly reactive metal powders, new procedure for the preparation of highly reactive zinc and magnesium powders. J Org Chem 1981;46:4323–4. [7] Liu L, Li F, Tan L, Li M, Yang Y. Effects of nanometer Ni, Cu, Al and NiCu powders on the thermal decomposition of ammonium perchlorate. Prop Explos Pyrotech 2004;29:34–8.
518
MA TE RI A L S CH A R A CT ER IZ A TI O N 59 ( 20 0 8 ) 5 1 4–5 1 8
[8] Dufrêne YF, van der Wal A, Norde W, Rouxhet PG. X-ray photoelectron spectroscopy analysis of whole cells and isolated cell walls of Gram-positive bacteria: comparison with biochemical analysis. J Bacteriol 1997;179:1023–8. [9] Minkov F, Friedlein GR, Osikowicz W, Suess C. Core excitations of naphthalene: vibrational structure versus chemical shifts. J Chem Phys 2001;121:5733–9.
[10] Haycock DE, Kasrai M, Nicholls CJ, Urch DS. The electronic structure of magnesium hydroxide (brucite) using X-ray emission, X-ray photoelectron, and auger spectroscopy. J Chem Soc Dalton Trans 1978:1791–6.