Computer Simulation of Anionic Structures of Molten CaO-SiO2-P2O5 System JIANG DIAO, GUOZHENG FAN, XUAN LIU, and BING XIE Molecular dynamics simulations were carried out to investigate the anionic structures of the molten CaO-SiO2-P2O5 system. The results show that the average first nearest-neighbor distances for Si-O and P-O pairs are 1.61 and 1.53 A˚, respectively. As expected, above 98 pct P and 95 pct Si show fourfold coordination and form tetrahedral structures. Due to the high basicity, nonbridging oxygen occupies a predominant position in Si and P tetrahedron. Based on the oxygen number of different types, the structures of both Si and P tetrahedron were classified as Q0, Q1, Q2, Q3, and Q4, where the superscript referred to the number of bridging oxygen atoms. With the substitution of P2O5 for SiO2, Q0 decreased and other type of Qi units increased. For Si tetrahedron, Q2 and Q3 show most notable change, for P tetrahedron, Q1and Q2 show the most notable change. The change of Qi units for Si tetrahedron is larger than that for P tetrahedron. The concentration of free oxygen decreases remarkably with the increase of P2O5 content. The Si-O-P linkage is energetically more favorable than Si-O-Si and P-O-P linkages. P ion has a tendency to promote the polymerization of phosphosilicate melts. DOI: 10.1007/s11663-014-0092-1 Ó The Minerals, Metals & Materials Society and ASM International 2014
I.
INTRODUCTION
THE structural information of molten slag, molten salt, and amorphous metal is of great interest for understanding their physical–chemical and transport behaviors in steelmaking and other scientific and industrial fields. The structures of these substances have been investigated through various sorts of techniques, e.g., nuclear magnetic resonance (NMR), X-ray diffraction (XRD), X-ray absorption (XAS), neutron diffraction, Raman and infrared spectroscopy, etc.[1,2] Meanwhile, taking into account the difficulties of experiments at high temperatures, molecular dynamics (MD) simulation has been widely employed as a very useful method for the calculation of structural and transport properties for molten slags and liquid phases at high temperatures.[3–5] It is well known that phosphorus is one of the most detrimental impurities in steels, and very considerable quantities of phosphorus are eliminated into the steelmaking slags. Dicalcium silicate (2CaOÆSiO2) and tricalcium phosphate (3CaOÆP2O5) form a solid solution during dephosphorization treatment in the steelmaking process.[6,7] This implies that the CaO-SiO2-P2O5 system is one of the most important systems in the molten converter slag. So far, most of the previous studies were mainly concentrated on the structure of silicate systems and aluminate systems.[8–19] However, only a few studies have reported on the phosphate systems. Belashchenko and Ostrovski[20] investigated the structure of the CaO-P2O5 JIANG DIAO, Lecturer, GUOZHENG FAN, Bachelor, XUAN LIU, Master, and BING XIE, Professor, are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China. Contact e-mail:
[email protected] Manuscript submitted March 27, 2014. Article published online June 20, 2014. 1942—VOLUME 45B, OCTOBER 2014
system using MD simulation. It was found that the structure of phosphates is inhomogeneous, with broad distributions of ion charges, bond lengths, and coordination numbers. The simulated structures are rather loose and contain large voids. Tang et al.[21] carried out a firstprinciples MD simulation of ternary phosphate-based glasses CaO-Na2O-P2O5. The coordination statistics of the phosphate tetrahedral network shows a shift of the Qn distribution from 100 pct Q3 species in vitreous P2O5 to a prevalence of a mixture of metaphosphate Q3 and orthophosphate Q2 species. The local arrangement of each PO4 is essentially constant with change in glass composition. However, the distribution of the P-P distances and P-O-P angles changes significantly with Ca content. P5+ is generally supposed to be a network former in most of phosphate melts and occurs in tetrahedral coordination under most conditions. Because of the importance of CaO-SiO2-P2O5 system, it is necessary to clarify the effect of P5+ on the polymerization of silicate melts. With this background, this article presents a new molecular dynamics study of the structure of molten CaO-SiO2-P2O5 system. The analysis of the anionic structures is the main focus of interest.
II.
MOLECULAR DYNAMICS SIMULATION
A. Interatomic Potential In the molecular dynamics simulations of a molten metallurgical slag system, the choice of a suitable potential function and its corresponding parameters is critical for the success of a calculation. In the current study, the interatomic potential used in simulation was Born–Mayer–Huggins form of Eq. [1].[22] METALLURGICAL AND MATERIALS TRANSACTIONS B
Uij ðrÞ ¼
qi qj Cij þ Aij expðBij rij Þ 6 ; rij rij
½1
where Uij(r) is the interatomic pair potential; qi, qj are the charge of the ions; and rij denotes the distance between atoms i and j. The first term of the right-hand side corresponds to the coulombic interaction, which can be calculated using the standard charge of each ion. The second term denotes short-range repulsion interaction. Aij and Bij are adjustable parameters related to the size and softness of the ions. The third term represents the van der Waals interactions and Cij is the van der Waals interaction parameter. The interatomic potential parameters of CaO and SiO2 in this study were taken from potential model by Hirao and Kawamura.[23] The potential model and parameters have successfully reproduced the structure and dynamic properties for various silica melts, glasses and molten slags. The interatomic potential parameter for P-O ion pair for P2O5 was reported by Belashchenko and Ostrovski.[24] The parameter have been proven successful in the simulation of CaO-SiO2-P2O5 system as it reproduced best nearest neighbor distance and coordination for P and O ions. The potential parameters used in present study are shown in Table I. B. Calculation Method The MD simulations were carried out using the canonical ensemble (NVT). Three-dimensional periodic boundary conditions were employed for each simulation and the basic cells containing approximately 5000 atoms. The long-range coulomb interactions were calculated by the Ewald summation. The equation of motion was integrated by the Verlet algorithm.[25] All the noncoulombic parts of the pair potentials were subjected to a short-range cutoff of 10 A˚. At the start of calculation, random configurations were used. The calculations were carried out at a constant pressure of 101 kPa. The initial temperature and running time were set to 5000 K (4727 °C) and 10,000 steps to agitate the atoms and eliminate the effect of the initial distribution on the final results. Then, the temperature was decreased of 30,000 time steps from 5000 K to 1673 K (4727 °C to 1400 °C). Finally, the calculation was carried out for 10,000 time steps at 1673 K (1400 °C) and the structure
Table I. i-j Ca-Ca Ca-Si Ca-P Ca-O Si-Si Si-P Si-O P-P P-O O-O
data of CaO-SiO2-P2O5 system were accumulated. The equation of motion was integrated at time interval of 1 fs. The calculated compositions of CaO-SiO2-P2O5 system are presented in Table II and Figure 1. The binary basicity (CaO/SiO2) of the basic slag sample (7CaOÆ3SiO2) is 2.33, which is within the range of the converter slag and 2CaOÆSiO2-3CaOÆP2O5 solid-solution composition. The compositions of other slag samples in the current study have been obtained by replacing SiO2 by P2O5.
III.
RESULTS AND DISCUSSION
A. PCF and CCN for Si-O and P-O Pairs The calculated pair correlation functions (PCFs) and cumulative coordination numbers (CCNs) for Si-O and P-O pairs in 7CaOÆSiO2Æ2P2O5 system at 1673 K (1400 °C) are shown in Figure 2. The average first nearest-neighbor distances of the Si-O and P-O pairs are found to be 1.61 and 1.53 A˚, respectively. The bond length of the P-O pair is shorter than that of the Si-O pair because the coulombic interaction of P-O pair is stronger than that of the Si-O pair. Both the PCF curves are almost symmetric and show negligible tails, suggesting that the O atoms around Si and P are tightly bounded and O ions cannot move out easily from the influence area of Si or P ion’s coulombic force. Both the cumulative coordination numbers of O for Si and P are Table II.
Composition (mol pct) of Slag Samples
7CaOÆ3SiO2 7CaOÆ2.5SiO2Æ0.5P2O5 7CaOÆ2SiO2ÆP2O5 7CaOÆ1.5SiO2Æ1.5P2O5 7CaOÆSiO2Æ2P2O5
CaO
SiO2
P2O5
70 70 70 70 70
30 25 20 15 10
0 5 10 15 20
Potential Parameters Used in Present Study Aij (eV)
Bij (1/A˚)
Cij (eV A˚6)
329,171.51 26,684.39 164,585.76 718,088.63 2163.18 1081.59 62,817.23 0 1847.66 1497,594.32
6.25 6.25 12.50 6.06 6.25 12.50 6.06 0 3.45 5.88
4.34 0 0 8.67 0 0 0 0 0 17.35
METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 1—Slag compositions in CaO-SiO2-P2O5 system. VOLUME 45B, OCTOBER 2014—1943
Table III.
70C30S 70C25S5P 70C20S10P 70C15S15P 70C10S20P
O/Si and O/P Ratios of Slag Samples O/Si
O/P
4 6 8 12 19
— 15 8 6 5
Fig. 2—PCF and CCN for Si-O and P-O pairs in 7CaOÆSiO2Æ2P2O5 system.
Fig. 4—Coordination statistics of Si and P atoms.
Fig. 3—The first peaks of PCF for Si-O and P-O pairs. 70C30S: 7CaOÆ3SiO2, 70C25S5P: 7CaOÆ2.5SiO2Æ0.5P2O5, 70C20S10P: 7CaOÆ 2SiO2ÆP2O5, 70C15S15P: 7CaOÆ1.5SiO2Æ1.5P2O5, 70C10S20P: 7CaOÆ SiO2Æ2P2O5.
found to be 4. Namely, the formation of Si and P tetrahedral structures is clearly identified from Figure 2. The values agree well with the previous measured data. 1944—VOLUME 45B, OCTOBER 2014
Figure 3 shows the first peaks of PCF for Si-O and P-O pairs in CaO-SiO2-P2O5 system. There is an interesting decreasing tendency of both the first peaks of PCF for Si-O and P-O pairs. This result is related to the O/Si and O/P ratios in the simulated CaO-SiO2-P2O5 system. The values of the O/Si and O/P ratios are shown in Table III. It can be seen from Figure 3 and Table III that with the decrease of O/P ratios, the first peak of PCF for P-O pair was gradually decreased. However, although the O/ Si ratios increase from 4 to 19, the first peak of PCF for Si-O pair was also showed a slight decreasing tendency. This is caused by the interaction between P and O. The affinity between P and O is stronger than the affinity between Si and O. The coordination statistics of Si and P atoms are shown in Figure 4. Above 98 pct P and 95 pct Si shows fourfold coordination. A small amount of fivefoldcoordinated Si and P are also found. Zhang et al.[26] reported the coordination statistics of Si in the CaOSiO2-TiO2 system (CaO/SiO2 < 1). The results indicated that the fourfold-coordination Si preponderates over others with a fraction of more than 80 pct. The CaO/ SiO2 in the current study is much higher than that in Zhang’s work. And the stability of Si4c is also much higher than that in Zhang’s work. Meanwhile, the higher coordinated Si of Si5c and Si6c were also found. Tilocca[27] investigated the coordination statistics of Si and P in the 11CaOÆ10Na2OÆ19SiO2ÆP2O5 system. It was found that both Si and P show ideal fourfold coordination at 300 K (27 °C), whereas about 11 pct of Si atoms are miscoordinated in the melt at 3000 K (2727 °C). Unlike Si, P atoms essentially maintain an ideal tetrahedral even at a high temperature; 99.8 pct of METALLURGICAL AND MATERIALS TRANSACTIONS B
Table IV.
Linkage Relations between Si and P Tetrahedron (a) 70C30S
Si 0 1 2 3 4
0 49.06 38.49 11.32 1.13 0
1 0 0 0 0 0
2 0 0 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
(b) 70C25S5P Si 0 1 2 3 4
0 42.92 34.91 12.26 1.89 0
1 4.72 1.89 0.47 0 0
2 0 0.94 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
P 0 1 2 3 4
0 72.50 20.00 1.25 0 1.25
1 5.00 0 0 0 0
2 0 0 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
0 78.70 17.16 1.78 0 0
1 1.78 0.59 0 0 0
2 0 0 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
0 61.16 18.30 1.79 0 0.45
1 15.18 3.13 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
3 0.35 0 0 0 0
4 0 0 0 0 0
(c) 70C20S10P Si 0 1 2 3 4
0 39.74 27.56 12.18 0.64 0
1 13.46 5.13 0.64 0 0
2 0 0.64 0 0 0
3 0 0 0 0 0
4 0 0 0 0 0
P 0 1 2 3 4
(d) 70C15S15P Si 0 1 2 3 4
0 31.82 26.36 7.27 0.91 0
1 17.27 6.36 1.82 0 0
2 3.64 2.73 0 0 0
3 1.82 0 0 0 0
4 0 0 0 0 0
P 0 1 2 3 4
2 0 0 0 0 0
(e) 70C10S20P Si 0 1 2 3 4
0 17.39 10.14 4.35 0 0
1 26.09 11.59 7.25 0 0
2 15.94 5.80 0 0 0
3 0 1.50 0 0 0
4 0 0 0 0 0
P atoms exist as P4c. The results shown in Figure 4 are agree well with the previous study.[21,26,27] B. Linkage Relations between Si and P Tetrahedron The cutoff radii for Si and P tetrahedron are 2.2 and 2.3 A˚, respectively. When oxygen atoms were found to exist within the distances around Si or Al, they were decided to belong to Si and P tetrahedron. For each oxygen atom belonging to a tetrahedron, it was examined whether another Si or P existed within the cutoff radius. If Si or P existed, then the oxygen was determined to bridging oxygen. Based on the number of bridging oxygens, each Si and P tetrahedron was classified into five types of structural units. The structures are referred as Q0, Q1, Q2, Q3, and Q4, where the superscript referred to the number of bridging oxygen atoms in the unit. The calculated linkage relations between Si and P tetrahedron are shown in Table IV.The left-side block METALLURGICAL AND MATERIALS TRANSACTIONS B
P 0 1 2 3 4
0 61.13 15.19 1.77 0 0
1 15.90 4.59 0.71 0 0
2 0.35 0 0 0 0
shows the linkages for Si tetrahedron and the right-side block shows the linkages for P tetrahedron. The vertical column denotes the type of Qi in Si tetrahedron. The value of i indicates the number of bridging oxygen in the Si tetrahedron. Similarly, the horizontal row denotes the type of Qi in P tetrahedron, and the value of i indicates the number of bridging oxygens in P tetrahedron. The values in the cells indicate the percentage of Si or P tetrahedron that have the specific linkage patterns to other Si and P tetrahedron. For example, the value of 4.59 in column 2 and rank 2 (referred as Q4 (2, 2)) in Table IV(e) means the amount of P tetrahedron connected to 1 Si tetrahedron and 1 P tetrahedron is 4.59 pct of the whole P tetrahedron. In Table IV, each summation of cells parallel to the diagonal corresponds to the percentage of structural unit Qi in Si or P tetrahedron. For example, the percentage of Q3 in Table IV(c) can be expressed by Q3 = Q3 (1, 4) + Q3 (2, 3) + Q3 (3, 2) + Q3 (4, 1) = 1.92 pct. VOLUME 45B, OCTOBER 2014—1945
Fig. 5—Distribution of Qi for Si tetrahedron.
Fig. 6—Distribution of Qi for P tetrahedron.
Fig. 8—Distribution of different types of BO.
Qi units for Si and P tetrahedron, Q0 are in the majority, which means nonbridging oxygen occupy a predominant position in the tetrahedron. This occurs as a result of the high basicity of the CaO-SiO2-P2O5 system. With the substitution of P2O5 for SiO2, the remarkable characteristic of the change of Qi units for Si and P tetrahedron is that Q0 decreased and other type of Qi units increased. This result indicates that the degree of polymerization in the molten CaO-SiO2-P2O5 system was enhanced by the increase of P2O5 content. In other words, P can be more effective network former than Si. Among the Qi units except Q0, for Si tetrahedron, Q2and Q3 show the most notable change, for P tetrahedron, Q1and Q2 show most notable change. As a large number of Ca atoms exist in the system, the changes of Q4 in both Si and P tetrahedron are very small. In addition, the change of Qi units for Si tetrahedron is larger than that for P tetrahedron. C. Effect of P on the Polymerization of Phosphosilicate Melts
Fig. 7—Distribution of different types of O atoms.
Based on the aforementioned analysis, the calculated distributions of Qi units for Si and P tetrahedron are plotted in Figures 5 and 6. It can be seen that among the 1946—VOLUME 45B, OCTOBER 2014
The distributions of different oxygen atoms are shown in Figure 7. Free oxygen (FO) denotes the oxygen atom that links two network modifiers (Ca-O-Ca). The concentration of FO decreases remarkably with the increase of P2O5 content or decrease of the fraction of Ca atoms NCa/(NSi+NP) in a molten CaO-SiO2-P2O5 system. NBO contains two types of units, i.e., Ca-O-Si and Ca-O-P. Under the condition of sufficient Ca atoms, the number of Ca-O-P seems to increase with the increase of P2O5 content. This led to an increasing tendency of NBO in Figure 7. Likewise, the BO has three types, i.e., Si-O-Si, P-O-P, and Si-O-P. The distribution of different types of BO is shown in Figure 8. A previous study[27] also exploded a relevant fraction of Q1 phosphate groups forming a Si-O-P link with an adjacent silicate. And the Si-O-P links can occasionally be broken and formed in the melt. The concentration of Si-O-P increases obviously with P2O5 content increase in the present study. To further reveal the relationship among the three types of BO, the METALLURGICAL AND MATERIALS TRANSACTIONS B
changing tendency of concentration can be interpreted by the following reaction: Si O Si þ P O P ! 2 ðSi O PÞ:
½2
The calculated equilibrium constant KBO of Eq. [4] for the slag samples in the current study is 0.55, 1.62, 2.02, and 2.35, respectively. It could be considered that with the increase of P2O5 content, the Si-O-P linkage is energetically more favorable than Si-O-Si and P-O-P linkages. In other works, P ion has a tendency to promote the polymerization of phosphosilicate melts. For the silicates, the average BO could be simplified estimated from the compositions, Y ¼ 8 2ðNO =NSi Þ;
½4
where NSi+P is numbers of silicon and phosphorus atoms. With the increase of P2O5 content, the NO/NSi+P decreased from 4.3 to 3.8 in the current study. Namely, Y or the degree of polymerization is increased with the decrease of O in the melt. Therefore, the promotion of the polymerization of phosphosilicate melts could be obtained by increase the P2O5 content. However, the change of Y is very small as the change of NO/NSi+P is slight.
IV.
CONCLUSIONS
In the current study, focusing the anionic structures of phosphosilicate melts, a molecular dynamics simulation was carried out on a series of molten ternary CaO-SiO2P2O5 systems. The following conclusions can be drawn from the calculation, 1. The average first nearest-neighbor distances of the Si-O and P-O pairs are found to be 1.61 and 1.53 A˚, respectively. With the increase of P2O5 content, both the first peaks of PCF for P-O pair and Si-O pair gradually decreased. 2. Both the cumulative coordination numbers of O for Si and P are found to be 4. Above 98 pct P and 95 pct Si show fourfold coordination. Si and P mainly form a tetrahedron structure in the phosphosilicate melts. 3. As a result of the high basicity of the CaO-SiO2-P2O5 system, nonbridging oxygen occupies a predominant position in Si and P tetrahedron. With the substitution of P2O5 for SiO2, Q0 decreased and other type of Qi units increased. For Si tetrahedron, Q2and Q3 show the most notable change, for P tetrahedron, Q1and Q2 show the most notable change. The change of Qi units for Si tetrahedron is larger than that for P tetrahedron.
METALLURGICAL AND MATERIALS TRANSACTIONS B
ACKNOWLEDGMENT Financial support from the Fundamental Research Funds for the Central Universities (Project CDJZR 14130001) is greatly appreciated.
½3
where Y is the number if BO in silicate system. NO and NSi are the numbers of oxygen atoms and silicon atoms, respectively. When P worked as network former in phosphosilicate melts, the relation could be changed into Y ¼ 8 2ðNO =NSiþP Þ;
4. The concentration of FO decreases remarkably with the increase of P2O5 content. With the increase of P2O5 content, the Si-O-P linkage is energetically more favorable than Si-O-Si and P-O-P linkages. The calculated equilibrium constant KBO is 0.55, 1.62, 2.02, and 2.35, respectively. P ion has a tendency to promote the polymerization of phosphosilicate melts.
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