- Email: [email protected]
Dynamic simulation and experimental study of a novel Al extraction method from AlN under vacuum
Vacuum 119 (2015) 102e105
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Dynamic simulation and experimental study of a novel Al extraction method from AlN under vacuum Yong Lu a, c, Yuezhen Zhou a, c, Chunhan Wu a, c, Xiumin Chen a, b, c, *, Jiaju Wang a, c, Qingchun Yu a, c, Dachun Liu a, c, Bin Yang a, c a b c
National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, PR China State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming, Yunnan 650093, PR China Key Laboratory of Vacuum Metallurgy of Non-ferrous Metals of Yunnan Province, Kunming, Yunnan 650093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2015 Accepted 30 April 2015 Available online 14 May 2015
AlN chlorination route, in the present work, was proposed to extract Aluminum from aluminum nitride under vacuum. Density functional theory (DFT) were implemented to study the interaction of AlCl3 molecule and AlN (1010) surface. The results of DFT indicate that chemisorbed AlCl3 adsorption configuration was observed on the clean AlN (1010) surface after structure optimization, and adsorbed AlCl molecules were generated after 1 ps dynamic simulation time. The phase and composition of condensate were examined by means of XRD and EDS. It was found that 97.76 wt% of Al metal was obtained in the experiment B (in the presence of AlCl3), however, no condensate was collected in the experiment A (without AlCl3) at 1760 K under pressure of average 60 Pa. The results show that AlN chlorination route is an alternative Al production method from aluminum nitride. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ab initio molecular dynamics AlN chlorination reaction Al extraction Vacuum
Considerable theoretical and experimental efforts have been made in order to provide an insight into the carbothermic reduction of alumina to metallic Al [1e3], owing to the apparent disadroult process [4e6]. The carbothermic vantages of the Hall-He reduction process, however, remains to be a formidable technical challenge, because of the high temperature required and the byproducts (aluminum carbide and oxycarbide) formed [7]. For avoiding these undesired mixtures, researchers have directed their attention to carbothermic reduction and nitridation of alumina for the synthesis of aluminum nitride [8e10], following by Al extraction processes which are focused in two major directions: (1) thermal dissociation and (2) electrolysis. The theoretical dissociation temperature is still up to 1978 K at 100 Pa [11]. Unfortunately, no correspondingly experimental study of thermal dissociation have been presented up to date. The theoretical decomposition voltage of AlN is 0.75 V at 973 K which is lower than that of Al2O3(s) at the same temperature, but the major challenges is to find the appropriate electrolytes that can dissolve the very stable AlN [12].
* Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, PR China. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.vacuum.2015.04.040 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Considering that aluminum can be extracted by disproportionation from aluminum monochloride at low temperature [13,14] and the problems are associated with the thermal dissociation and electrolysis, we propose a novel Al extraction method of AlN chlorination route according to the reactions:
2AlNðsÞ þ AlCl3ðgÞ ¼ 3AlClðgÞ þ N2ðgÞ
(1)
3AlClðgÞ ¼ 2Alðs;lÞ þ AlCl3ðgÞ
(2)
The reformed AlCl3 which can be recirculated is not condensed in the vacuum chlorination furnace, but it is solid and N2 is still gas at temperature below 373 K, so further separation process of N2, Al and AlCl3 is not necessary. The objective of this work was to preliminarily investigate the feasibility and mechanism of AlN chlorination process for providing a new, efficient and energy-saving way of Al extraction. Thermodynamic calculations were performed before dynamic simulation and experimental study. The dissociation temperature of AlN calculated is 1977 K at 100 Pa, which is in very good agreement with 1978 K in Ref. 11. AlN theoretical chlorination temperature is 1553 K at 100 Pa, which is obviously reduced over 400 K compared with thermal dissociation at the same pressure.
Y. Lu et al. / Vacuum 119 (2015) 102e105
In this study, the DFT calculations were performed by using the program package Cambridge Sequential Total Energy Package (CASTEP) in the Materials Studio software [15]. Exchange and correlation effects were treated with generalized gradient approximation (GGA) implemented in the Perdew-Burke-Ernzerhof functional (PBE) as exchange-correlation functional [16]. The AlN (1010) surface was modeled by 5 atomic layers periodic slab with a p(22) supercell. In order to avoid the interaction of periodic configurations, a vacuum layer of 20 Å was added to the surface. Brillouin-zone sampling of 4 4 1 k-points and cutoff energy of 400 eV in plane wave basis sets were verified to be sufficient to obtain good numerical accuracy with reasonable computational costs. Optimizations were performed before and after the AlCl3 molecule was added to the supercell as the initial structure. The convergence criteria for geometry optimization and energy calculation were (1) an energy tolerance of 1.0 105 eV/ atom, (2) maximum force tolerance of 0.03 eV/Å and (3) maximum displacement tolerance of 0.001 Å. The ab initio molecular dynamics (AIMD) simulations were carried out on the optimized AlN surface in contact with AlCl3 molecule, using the NPT ensemble at 1760 K, 60 Pa, a time step of 1 fs and the simulation time 1 ps. The [18] thermostat were used to Andersen [17] barostat and Nose control pressure and temperature, respectively. The bond lengths and angles calculated were 2.08 Å and 120 for free AlCl3 molecule after structure optimization, respectively. The optimized structure of AlCl3 adsorbed on the AlN (1010) surface at the low monolayer coverage (0.25 ML) was shown in Fig. 1. Compared with the free AlCl3, however, the adsorbed AlCl3 changed considerably during the interaction process. The bond lengths were 2.38 Å for Al(1)-Cl(2) and Al(1)-Cl(3), 2.23 Å for Al(1)-Cl(1) after interaction. Meanwhile, the bond angles of Cl(1)-Al(1)-Cl(2) and Cl(1)-Al(1)-Cl(3) both decreased by 4 , and Cl(2)-Al(1)-Cl(3) decreased to 79 . It was indicated that the AlCl3 spontaneously adjusted its molecular structure when bonding with the Al and N atoms of AlN surface. The bond lengths after interaction were 2.35 Å for Al(2)-Cl(2) and Al(3)-Cl(3), 2.01 Å for Al(1)-N(1) and
Fig. 1. Schematic of AlCl3 molecule interaction with AlN (1010) surface.
103
Al(1)-N(2). According to Eq. (3), chemisorption energy of AlCl3 on AlN surface was 630.95 kJ/mol, This negative adsorption energy confirmed the spontaneous process of AlCl3 interaction with Al and N atoms of AlN (1010) surface.
Eadsorption ¼ EðsurfaceþadsorbateÞ Esurface þ Eadsorbate
(3)
Where Eadsorption is the adsorption energy of the adsorbate, E(surfais the total energy of the surface-adsorbate system, Esurface is the total energy of the clean surface, and Eadsorbate is the total energy of the isolated adsorbate. The chemical interaction, at the atomic level, is achieved by the interaction of various electrons in the orbital of the interactional atoms. Hence, it is necessary to analyze the partial density of states (PDOS) of the bonding atoms, which can aid in understanding the surface chemical mechanism of AlCl3 and AlN surface. Fig. 2 showed the PDOS of the bonding atoms between AlCl3 molecule and AlN (1010) surface. As shown in Fig. 2a, the Al 3s and Al 3p orbitals of Al(2), Al(3) atoms and Cl 3p orbital of Cl(2), Cl(3) atoms are apparently overlapped between 8.8 eV and 1.9 eV (valence band) with the overlapped peak of PDOS occurring at 6.6 eV, indicating a strong hybridization between them and the major reason for the Al(2)Cl(2), Al(3)-Cl(3) formed. The Cl 3s orbital between 17.7 eV ceþadsorbate)
Fig. 2. PDOS of (a) Cl(2), Cl(3) atoms in AlCl3 and its interactional Al atoms of AlN and (b) Al(1) atom and N(1), N(2) atoms.
104
Y. Lu et al. / Vacuum 119 (2015) 102e105
Fig. 3. Structure of AlCl3 molecule interaction with AlN (1010) surface after 1 ps dynamics simulation.
and 14.2 eV was located at a deep energy level of valence band and far from the Fermi level (EF). Therefore, this orbital had a negligible effect on the formed AleCl bonds. The Al 3p orbital of Al(1) atom and N 2p orbital of AlN surface were just located at a similar energy level (Fig. 2b) with the overlapped peak of PDOS occurring at 5.1 eV nearby the Fermi level. It was indicated that the hybridization between Al(1) 3p orbital and N 2p orbital of N(1), N(2) atoms was very strong. The interaction of the Al(1) 3s orbital and N 2p orbital was weaker than that of Al(1) 3p orbital and N 2p orbital because of the narrow energy level (8.8 eV to 5.1 eV) of Al(1) 3s orbital. The strong hybridization of orbitals indicated that AlCl3 on the AlN (1010) surface was stable chemisorption, as confirmed by the larger adsorption energy (630.95 kJ/mol). A
comparison of the overlapped area of PDOS showed that the overlapped area of the Al 3s, Al 3p orbitals and Cl 3p orbital (Fig. 2a) was more significant than that of the Al 3s, Al 3p orbitals and N 2p orbital (Fig. 2b), which suggested that the interaction of Al(2) and Cl(2), Al(3) and Cl(3) atoms was stronger than that of Al(1) and N(1), N(2) atoms in this system. The structure of AlCl3 molecule interaction with AlN (1010) surface after 1 ps dynamics simulation was shown in Fig. 3. This structure changed obviously by compared with Fig. 1 (before dynamics simulation). It was found that the adsorbed AlCl molecules were generated when the adsorbed AlCl3 molecule on substrate AlN (1010) surface was dissociated, with the fracture of Al(1)-Cl(2) and Al(1)-Cl(3). The bond length after dynamics simulation was 2.23 Å for Al(1)-Cl(1), 2.09 Å for Al(2)-Cl(2) and 2.28 Å for Al(2)Cl(2). As observed from Fig. 3, the adsorbed AlCl molecules did not desorb from AlN surface because of the short dynamics simulation time. The schematic details of the vacuum chlorination furnace were shown in Fig. 4 describing the Al extraction process of AlN using graphite condensing tower to collect metallic aluminum. Analytical grade of wurtzite AlN grains and anhydrous aluminum chloride were used as the raw materials in our experiment. They were put in different graphite crucibles (No.5 and No.11 in Fig. 4) of the vacuum chlorination furnace, which was then evacuated to a vacuum of average 60 Pa. The system temperature was raised from room temperature to 1760 K for 30 min at a heating rate of about 20K min1 in the absence of AlCl3 (Experiment A). The furnace temperature was also maintained at 1760 K for 30 min in the presence of AlCl3 which sublimated at 373e403 K and transported to No.5 crucible, where the chlorination process occurred (Experiment B). The product collected in graphite condensing tower was identified by XRD (D/max-3B, Rigaku, Japan). The chemical composition of the condensate was determined with EDS, which was made by Philips of Holland. No condensate was found in the experiment A, suggesting that thermal dissociation of AlN did not occur at 1760 K for 30 min under pressure of average 60 Pa. The product in condensation tower, on the contrary, was collected (Experiment B) in the presence of AlCl3. Fig. 5 showed the XRD and EDS images of the condensate obtained in AlN chlorination process. As seen in Fig. 5a, the crystallinity of product was clearly evident by the sharp and well-defined
Fig. 4. Schematic diagram of the vacuum chlorination furnace.
Y. Lu et al. / Vacuum 119 (2015) 102e105
105
of aluminum surface during sample collection process. As oxygen was not from chlorination process, it could be considered that high purity aluminum was attained by AlN chlorination route in the vacuum chlorination furnace. Combined with the results of dynamic simulation and experimental investigation, it could be certainly deduced that the new adsorbed AlCl molecules which would desorb from AlN surface were generated between Al atoms (the substrate surface) and Cl atoms dissociated from the AlCl3 molecule. The AlCl(g) was then disproportionate into Al at low temperature in the graphite condensing tower. According to the analyses above, we can conclude that AlN chlorination route is an alternative Al production method from aluminum nitride.
Acknowledgments This work was supported by the youth fund of the NSFC (No. 51104078), the Joint Funds of the NSFC-Yunnan province (No. U1202271), the region fund of the NSFC (No. 51264023), the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT1250), and supported of national supercomputing center in shenzhen.
References
Fig. 5. The XRD pattern (a) and EDS image (b) of product collected in condensation tower.
diffraction peaks at the respective diffraction angles, which can be readily indexed to the cubic structure and are consistent with the standard JCPDS aluminum (Card No. 85e1327). Therefore, the final condensate was aluminum metal. There was no denying that AlCl(g) was formed by reaction (1) and Al metal was generated by disproportionation reaction (2) at low temperature under vacuum. From Fig. 5b, the aluminum peak was detected by EDS and the average purity of Al metal obtained was 97.76 wt%, in which 2.24 wt % of oxygen should be ignored because it originate from oxidation
[1] Yu WZ, Yang B, Chen XM, Jiang WL, Yu QC, Xu BQ. Vacuum 2012;86:2005e9. [2] Halmann M, Steinfeld A, Epstein M, Vishnevetsky I. Mineral Process Extr Metall. Rev 2014;35:126e35. [3] Vishnevetsky I, Epstein M. Sol Energy 2015;111:236e51. [4] Halmann M, Frei A, Steinfeld A. Mineral Process Extr Metall. Rev 2011;32: 247e66. [5] Murry JP. Sol Energy 1999;66:133e42. [6] Namboothiri S, Taylor MP, Chen JJJ, Hyland1 MM, Cooksey M. Asia-Pacific J Chem Eng 2007;2:442e7. [7] Choate W, Green J. Light Met 2006;2:445e50. [8] Wang Q, Cui W, Ge YY, Chen KX, Xie ZP. J Am Ceram Soc 2015;98:392e7. [9] Joo HU, Jung WS. J Mater Process Technol 2008;204:498e501. [10] Ravenel P, Disson JP, Bachelard R. [P] US 5538705 Jul. 23, 1996. [11] Dewan MA, Rhamdhani MA, Brooks GA, Monaghan BJ, Prentice L. Mineral Process Extr Metallurgy 2013;122:113e21. [12] Rhamdhani MA, Dewan MA, Brooks GA, Monaghan BJ, Prentice L. Mineral Process Extr Metallurgy 2013;122:87e104. [13] Zhu FL, Yang B, Li Y, Yuan HB, Yu QC, Xu BQ, et al. Vacuum 2011;85:968e71. [14] Feng YB, Yang B, Dai YN. Trans Nonferrous Metals Soc China 2012;22:215e21. [15] Segall MD, Lindan PJD, Probert MJ, Pickard CJ, Hasnip PJ, Clark SJ, et al. J Phys Condens Matter 2002;14:2717e44. [16] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865e8. [17] Andersen HC. J Chem Phys 1980;72:2384e93. S. J Chem Phys 1984;81:511e9. [18] Nose
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
Dynamic simulation and experimental study of a novel Al extraction method from AlN under vacuum Yong Lu a, c, Yuezhen Zhou a, c, Chunhan Wu a, c, Xiumin Chen a, b, c, *, Jiaju Wang a, c, Qingchun Yu a, c, Dachun Liu a, c, Bin Yang a, c a b c
National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, PR China State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming, Yunnan 650093, PR China Key Laboratory of Vacuum Metallurgy of Non-ferrous Metals of Yunnan Province, Kunming, Yunnan 650093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2015 Accepted 30 April 2015 Available online 14 May 2015
AlN chlorination route, in the present work, was proposed to extract Aluminum from aluminum nitride under vacuum. Density functional theory (DFT) were implemented to study the interaction of AlCl3 molecule and AlN (1010) surface. The results of DFT indicate that chemisorbed AlCl3 adsorption configuration was observed on the clean AlN (1010) surface after structure optimization, and adsorbed AlCl molecules were generated after 1 ps dynamic simulation time. The phase and composition of condensate were examined by means of XRD and EDS. It was found that 97.76 wt% of Al metal was obtained in the experiment B (in the presence of AlCl3), however, no condensate was collected in the experiment A (without AlCl3) at 1760 K under pressure of average 60 Pa. The results show that AlN chlorination route is an alternative Al production method from aluminum nitride. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ab initio molecular dynamics AlN chlorination reaction Al extraction Vacuum
Considerable theoretical and experimental efforts have been made in order to provide an insight into the carbothermic reduction of alumina to metallic Al [1e3], owing to the apparent disadroult process [4e6]. The carbothermic vantages of the Hall-He reduction process, however, remains to be a formidable technical challenge, because of the high temperature required and the byproducts (aluminum carbide and oxycarbide) formed [7]. For avoiding these undesired mixtures, researchers have directed their attention to carbothermic reduction and nitridation of alumina for the synthesis of aluminum nitride [8e10], following by Al extraction processes which are focused in two major directions: (1) thermal dissociation and (2) electrolysis. The theoretical dissociation temperature is still up to 1978 K at 100 Pa [11]. Unfortunately, no correspondingly experimental study of thermal dissociation have been presented up to date. The theoretical decomposition voltage of AlN is 0.75 V at 973 K which is lower than that of Al2O3(s) at the same temperature, but the major challenges is to find the appropriate electrolytes that can dissolve the very stable AlN [12].
* Corresponding author. National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming, Yunnan 650093, PR China. E-mail address: [email protected] (X. Chen). http://dx.doi.org/10.1016/j.vacuum.2015.04.040 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
Considering that aluminum can be extracted by disproportionation from aluminum monochloride at low temperature [13,14] and the problems are associated with the thermal dissociation and electrolysis, we propose a novel Al extraction method of AlN chlorination route according to the reactions:
2AlNðsÞ þ AlCl3ðgÞ ¼ 3AlClðgÞ þ N2ðgÞ
(1)
3AlClðgÞ ¼ 2Alðs;lÞ þ AlCl3ðgÞ
(2)
The reformed AlCl3 which can be recirculated is not condensed in the vacuum chlorination furnace, but it is solid and N2 is still gas at temperature below 373 K, so further separation process of N2, Al and AlCl3 is not necessary. The objective of this work was to preliminarily investigate the feasibility and mechanism of AlN chlorination process for providing a new, efficient and energy-saving way of Al extraction. Thermodynamic calculations were performed before dynamic simulation and experimental study. The dissociation temperature of AlN calculated is 1977 K at 100 Pa, which is in very good agreement with 1978 K in Ref. 11. AlN theoretical chlorination temperature is 1553 K at 100 Pa, which is obviously reduced over 400 K compared with thermal dissociation at the same pressure.
Y. Lu et al. / Vacuum 119 (2015) 102e105
In this study, the DFT calculations were performed by using the program package Cambridge Sequential Total Energy Package (CASTEP) in the Materials Studio software [15]. Exchange and correlation effects were treated with generalized gradient approximation (GGA) implemented in the Perdew-Burke-Ernzerhof functional (PBE) as exchange-correlation functional [16]. The AlN (1010) surface was modeled by 5 atomic layers periodic slab with a p(22) supercell. In order to avoid the interaction of periodic configurations, a vacuum layer of 20 Å was added to the surface. Brillouin-zone sampling of 4 4 1 k-points and cutoff energy of 400 eV in plane wave basis sets were verified to be sufficient to obtain good numerical accuracy with reasonable computational costs. Optimizations were performed before and after the AlCl3 molecule was added to the supercell as the initial structure. The convergence criteria for geometry optimization and energy calculation were (1) an energy tolerance of 1.0 105 eV/ atom, (2) maximum force tolerance of 0.03 eV/Å and (3) maximum displacement tolerance of 0.001 Å. The ab initio molecular dynamics (AIMD) simulations were carried out on the optimized AlN surface in contact with AlCl3 molecule, using the NPT ensemble at 1760 K, 60 Pa, a time step of 1 fs and the simulation time 1 ps. The [18] thermostat were used to Andersen [17] barostat and Nose control pressure and temperature, respectively. The bond lengths and angles calculated were 2.08 Å and 120 for free AlCl3 molecule after structure optimization, respectively. The optimized structure of AlCl3 adsorbed on the AlN (1010) surface at the low monolayer coverage (0.25 ML) was shown in Fig. 1. Compared with the free AlCl3, however, the adsorbed AlCl3 changed considerably during the interaction process. The bond lengths were 2.38 Å for Al(1)-Cl(2) and Al(1)-Cl(3), 2.23 Å for Al(1)-Cl(1) after interaction. Meanwhile, the bond angles of Cl(1)-Al(1)-Cl(2) and Cl(1)-Al(1)-Cl(3) both decreased by 4 , and Cl(2)-Al(1)-Cl(3) decreased to 79 . It was indicated that the AlCl3 spontaneously adjusted its molecular structure when bonding with the Al and N atoms of AlN surface. The bond lengths after interaction were 2.35 Å for Al(2)-Cl(2) and Al(3)-Cl(3), 2.01 Å for Al(1)-N(1) and
Fig. 1. Schematic of AlCl3 molecule interaction with AlN (1010) surface.
103
Al(1)-N(2). According to Eq. (3), chemisorption energy of AlCl3 on AlN surface was 630.95 kJ/mol, This negative adsorption energy confirmed the spontaneous process of AlCl3 interaction with Al and N atoms of AlN (1010) surface.
Eadsorption ¼ EðsurfaceþadsorbateÞ Esurface þ Eadsorbate
(3)
Where Eadsorption is the adsorption energy of the adsorbate, E(surfais the total energy of the surface-adsorbate system, Esurface is the total energy of the clean surface, and Eadsorbate is the total energy of the isolated adsorbate. The chemical interaction, at the atomic level, is achieved by the interaction of various electrons in the orbital of the interactional atoms. Hence, it is necessary to analyze the partial density of states (PDOS) of the bonding atoms, which can aid in understanding the surface chemical mechanism of AlCl3 and AlN surface. Fig. 2 showed the PDOS of the bonding atoms between AlCl3 molecule and AlN (1010) surface. As shown in Fig. 2a, the Al 3s and Al 3p orbitals of Al(2), Al(3) atoms and Cl 3p orbital of Cl(2), Cl(3) atoms are apparently overlapped between 8.8 eV and 1.9 eV (valence band) with the overlapped peak of PDOS occurring at 6.6 eV, indicating a strong hybridization between them and the major reason for the Al(2)Cl(2), Al(3)-Cl(3) formed. The Cl 3s orbital between 17.7 eV ceþadsorbate)
Fig. 2. PDOS of (a) Cl(2), Cl(3) atoms in AlCl3 and its interactional Al atoms of AlN and (b) Al(1) atom and N(1), N(2) atoms.
104
Y. Lu et al. / Vacuum 119 (2015) 102e105
Fig. 3. Structure of AlCl3 molecule interaction with AlN (1010) surface after 1 ps dynamics simulation.
and 14.2 eV was located at a deep energy level of valence band and far from the Fermi level (EF). Therefore, this orbital had a negligible effect on the formed AleCl bonds. The Al 3p orbital of Al(1) atom and N 2p orbital of AlN surface were just located at a similar energy level (Fig. 2b) with the overlapped peak of PDOS occurring at 5.1 eV nearby the Fermi level. It was indicated that the hybridization between Al(1) 3p orbital and N 2p orbital of N(1), N(2) atoms was very strong. The interaction of the Al(1) 3s orbital and N 2p orbital was weaker than that of Al(1) 3p orbital and N 2p orbital because of the narrow energy level (8.8 eV to 5.1 eV) of Al(1) 3s orbital. The strong hybridization of orbitals indicated that AlCl3 on the AlN (1010) surface was stable chemisorption, as confirmed by the larger adsorption energy (630.95 kJ/mol). A
comparison of the overlapped area of PDOS showed that the overlapped area of the Al 3s, Al 3p orbitals and Cl 3p orbital (Fig. 2a) was more significant than that of the Al 3s, Al 3p orbitals and N 2p orbital (Fig. 2b), which suggested that the interaction of Al(2) and Cl(2), Al(3) and Cl(3) atoms was stronger than that of Al(1) and N(1), N(2) atoms in this system. The structure of AlCl3 molecule interaction with AlN (1010) surface after 1 ps dynamics simulation was shown in Fig. 3. This structure changed obviously by compared with Fig. 1 (before dynamics simulation). It was found that the adsorbed AlCl molecules were generated when the adsorbed AlCl3 molecule on substrate AlN (1010) surface was dissociated, with the fracture of Al(1)-Cl(2) and Al(1)-Cl(3). The bond length after dynamics simulation was 2.23 Å for Al(1)-Cl(1), 2.09 Å for Al(2)-Cl(2) and 2.28 Å for Al(2)Cl(2). As observed from Fig. 3, the adsorbed AlCl molecules did not desorb from AlN surface because of the short dynamics simulation time. The schematic details of the vacuum chlorination furnace were shown in Fig. 4 describing the Al extraction process of AlN using graphite condensing tower to collect metallic aluminum. Analytical grade of wurtzite AlN grains and anhydrous aluminum chloride were used as the raw materials in our experiment. They were put in different graphite crucibles (No.5 and No.11 in Fig. 4) of the vacuum chlorination furnace, which was then evacuated to a vacuum of average 60 Pa. The system temperature was raised from room temperature to 1760 K for 30 min at a heating rate of about 20K min1 in the absence of AlCl3 (Experiment A). The furnace temperature was also maintained at 1760 K for 30 min in the presence of AlCl3 which sublimated at 373e403 K and transported to No.5 crucible, where the chlorination process occurred (Experiment B). The product collected in graphite condensing tower was identified by XRD (D/max-3B, Rigaku, Japan). The chemical composition of the condensate was determined with EDS, which was made by Philips of Holland. No condensate was found in the experiment A, suggesting that thermal dissociation of AlN did not occur at 1760 K for 30 min under pressure of average 60 Pa. The product in condensation tower, on the contrary, was collected (Experiment B) in the presence of AlCl3. Fig. 5 showed the XRD and EDS images of the condensate obtained in AlN chlorination process. As seen in Fig. 5a, the crystallinity of product was clearly evident by the sharp and well-defined
Fig. 4. Schematic diagram of the vacuum chlorination furnace.
Y. Lu et al. / Vacuum 119 (2015) 102e105
105
of aluminum surface during sample collection process. As oxygen was not from chlorination process, it could be considered that high purity aluminum was attained by AlN chlorination route in the vacuum chlorination furnace. Combined with the results of dynamic simulation and experimental investigation, it could be certainly deduced that the new adsorbed AlCl molecules which would desorb from AlN surface were generated between Al atoms (the substrate surface) and Cl atoms dissociated from the AlCl3 molecule. The AlCl(g) was then disproportionate into Al at low temperature in the graphite condensing tower. According to the analyses above, we can conclude that AlN chlorination route is an alternative Al production method from aluminum nitride.
Acknowledgments This work was supported by the youth fund of the NSFC (No. 51104078), the Joint Funds of the NSFC-Yunnan province (No. U1202271), the region fund of the NSFC (No. 51264023), the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT1250), and supported of national supercomputing center in shenzhen.
References
Fig. 5. The XRD pattern (a) and EDS image (b) of product collected in condensation tower.
diffraction peaks at the respective diffraction angles, which can be readily indexed to the cubic structure and are consistent with the standard JCPDS aluminum (Card No. 85e1327). Therefore, the final condensate was aluminum metal. There was no denying that AlCl(g) was formed by reaction (1) and Al metal was generated by disproportionation reaction (2) at low temperature under vacuum. From Fig. 5b, the aluminum peak was detected by EDS and the average purity of Al metal obtained was 97.76 wt%, in which 2.24 wt % of oxygen should be ignored because it originate from oxidation
[1] Yu WZ, Yang B, Chen XM, Jiang WL, Yu QC, Xu BQ. Vacuum 2012;86:2005e9. [2] Halmann M, Steinfeld A, Epstein M, Vishnevetsky I. Mineral Process Extr Metall. Rev 2014;35:126e35. [3] Vishnevetsky I, Epstein M. Sol Energy 2015;111:236e51. [4] Halmann M, Frei A, Steinfeld A. Mineral Process Extr Metall. Rev 2011;32: 247e66. [5] Murry JP. Sol Energy 1999;66:133e42. [6] Namboothiri S, Taylor MP, Chen JJJ, Hyland1 MM, Cooksey M. Asia-Pacific J Chem Eng 2007;2:442e7. [7] Choate W, Green J. Light Met 2006;2:445e50. [8] Wang Q, Cui W, Ge YY, Chen KX, Xie ZP. J Am Ceram Soc 2015;98:392e7. [9] Joo HU, Jung WS. J Mater Process Technol 2008;204:498e501. [10] Ravenel P, Disson JP, Bachelard R. [P] US 5538705 Jul. 23, 1996. [11] Dewan MA, Rhamdhani MA, Brooks GA, Monaghan BJ, Prentice L. Mineral Process Extr Metallurgy 2013;122:113e21. [12] Rhamdhani MA, Dewan MA, Brooks GA, Monaghan BJ, Prentice L. Mineral Process Extr Metallurgy 2013;122:87e104. [13] Zhu FL, Yang B, Li Y, Yuan HB, Yu QC, Xu BQ, et al. Vacuum 2011;85:968e71. [14] Feng YB, Yang B, Dai YN. Trans Nonferrous Metals Soc China 2012;22:215e21. [15] Segall MD, Lindan PJD, Probert MJ, Pickard CJ, Hasnip PJ, Clark SJ, et al. J Phys Condens Matter 2002;14:2717e44. [16] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865e8. [17] Andersen HC. J Chem Phys 1980;72:2384e93. S. J Chem Phys 1984;81:511e9. [18] Nose