x-ray diffractometry studies and lattice parameter calculation on kno3-nh4no3 solid ... · ·...

X-RAY DIFFRACTOMETRY STUDIES AND LATTICE PARAMETER CALCULATION ON KNO3-NH4NO3 SOLID SOLUTIONS

Wen-Ming Chien1, Dhanesh Chandra1, Jennifer Franklin1, Claudia J. Rawn2 and Abdel K. Helmy3

1Metallurgical and Materials Engineering, University of Nevada, Reno, Reno, Nevada 89557 2Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6064

3Special Devices Inc., 14370 White Sage Road, Moorpark, CA 93021

ABSTRACT

The solid-state phase transitions of the KNO3-NH4NO3 solid solutions have been determined by high temperature X-ray diffractometry, and lattice parameter calculation has also been performed. Ammonium nitrate (AN) is of great use for gas generators of automobile air bag systems. The X-ray diffraction results showed the single (AN) phase III from 5% to 20% KNO3 in NH4NO3 and up to 373 K, which is the important temperature range for the air bag gas generator applications. The X-ray diffraction patterns of the low temperature KNO3 phase (KN II) are from 92%-100% KNO3 composition range and up to 393 K temperature. The hightemperature KNO3 phase (KN I) showed very broad composition range from 20% up to 100% KNO3 at various temperature ranges. The lattice parameters of the NH4NO3-rich (AN III) and KNO3-rich (KN II and KN I) solid solutions have been calculated at different temperature range. The volumes of AN III phase decrease from 0.3201(4) to 0.3166(1) nm3 at the room temperature and from 0.3250(6) to 0.3215(3) nm3 at 373 K as the compositions increase from 5% to 20%KNO3. The lattice constants of the hexagonal KN I phase show that there is no significant change in a-direction when the temperature increases. Details of X-ray results, lattice expansions and equations during heating are presented.

INTRODUCTION

Ammonium nitrate (NH4NO3, AN) is used as an oxidizer in the propellants, explosives and gas generators. Addition of Potassium nitrate (KNO3, KN) will avoid the solid state phase transitions of AN near room temperature, as the solid state phase transitions will lead to change in volume and possible disintegration of the pellets or disks made of the propellant [1-6]. Pure AN has five different solid phases (AN V, AN IV, AN III, AN II and AN I phases), and undergoes four different solid state phase transitions before melting. Pure KN has two different solid phases (KN II and KN I phases), and undergoes one solid-state phase transition. The detail experimental crystal structure information of AN and KN are given in references 7-12. The crystal structure of AN III phase of pure NH4NO3 was determined as orthorhombic structure and stable between 305.3 K to 357 K [13]. Goodwin et al. [13] determined the lattice parameters of this orthorhombic structure by X-ray diffraction method. Lucas et al. [9] reported the crystal structure of AN III phase as orthorhombic, space group Pnma, and a=0.77184(1) nm, b=0.58447(1) nm and c=0.71624(1) nm (Z=4) by the neutron diffraction study. Solid solution of the 5%KNO3 in NH4NO3 has also been determined as AN III phase at room temperature, and the detail structure studies are performed by X-ray diffraction [1] as well as neutron diffraction [14-15] methods. Potassium nitrate (KN) undergoes one solid-state phase transition during heating

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and contains the KN II and KN I solid-state phases. The structure of KNO3 low temperature KN II (α) phase has been studied by Edwards [16] and redetermined by Nimmo et al. [12] with as an orthorhombic structure with the space group of Pmcn by using single-crystal neutron diffraction data. The unit cell of this orthorhombic structure is with a=0.54142 nm, b=0.91659 nm, and c=0.64309 nm (Z=4) at room temperature and confirmed by Holden and Dickinson [1] with single-crystal X-ray data. When heated the KNO3 from room temperature, the KN II (α)→KN I (β) phase transition occurs at ~401 K at atmospheric pressure. The high temperature KN I phase structure has been studied by Tahvonen [17], Shinnaka [18], Stromme [19], and determined by Nimmo and Lucas [11] as hexagonal (R-3m) structure with a=0.5425(1) nm and c=0.9836(4) nm (Z=3) at 424 K. The purpose of this study is to determine the lattice constants of the AN III, KN II, and KN I phases of AN-KN solid solutions at various temperature ranges. EXPERIMENTAL

Binary powder samples of different compositions were made using pure NH4NO3 and KNO3 powders. As the samples of ammonium nitrate (AN) are very sensitive to moisture, these samples were made in special walk-in “Dry room” with extremely low humidity at TRW (air bag propellant facility in Lockwood, Nevada). X-ray diffraction studies of the binary mixtures were performed at University of Nevada, Reno (UNR) and Oak Ridge National Laboratories (ORNL). Room temperature powder diffraction work was performed using a Philips Bragg-Brentano powder diffractometer in nitrogen environment at UNR. A plastic enclosure was installed around the room temperature diffractometer. In addition, a desiccant, Drierite (Hammond Drierite Company, Xenia, OH) anhydrous CaSO4 (white), and 97%CaSO4-3%CoCl2 (blue), was placed in the enclosure with constant nitrogen flow around the diffractometer, and a gas flow was maintained within the sample holder. The humidity was monitored by change in color of 97%CaSO4-3%CoCl2 (blue) to purple (if humidity is present). A cylinder of nitrogen is placed close to the chamber and gas released into the enclosure for approximately an hour before the experiment. The sample preparation (mixed with the internal standard Si or LaB6) was done in the enclosure. After placing the sample in the diffractometer, an additional dry nitrogen gas flow was maintained directly to the sample holder housing so that there was minimal contact of water with the sample. The X-ray data was recorded by the MDI “DataScan” program and analyzed by MDI “Jade” program. The high temperature X-ray diffraction studies were performed by the high-temperature X-ray diffractometer at High Temperature Material Laboratory (HTML), Oak Ridge National Laboratory (ORNL). The samples were mixed with the internal standard (Si or LaB6) and prepared in a glove bag with flowing nitrogen gas. The samples were placed on the heat strip (Rh,Pt) and heated to the desired temperatures by the heat strip. After placing the sample in the diffractometer, we purged and vacuumed the chamber three times with UHP He before the experiment run and kept the He gas flowing through the chamber during the experiment. The experiment run for each sample was from room temperature to the temperature near melting point of the solid solutions. The data were recorded by the Scintag software. The heating rate of the heat strip is 10°C/min. There is a 5 minutes delay before the experiment starts at each temperature run.


RESULTS AND DISCUSSIONS The X-ray diffraction patterns show the orthorhombic AN III phase for 5% to 20%KNO3 in NH4NO3 solid solutions at room temperature. Figure 1 shows the AN III X-ray diffraction patterns of 5%, 14%, and 20% KNO3 solid solutions with indexing of AN III phase, internal standards (Si or LaB6) and heat strip (Rh,Pt). The room temperature XRD patterns do not show significant change as the compositions increase. The single AN III phase is stable up to 373 K at this composition range. This is confirmed by the results reported by Holden et al. [1] and Cady [2] that the AN III phase will extend stability when temperature increases from very low to higher temperature by adding the KNO3 to form the AN-KN solid solution.

Figure 1. The X-ray diffraction patterns for (a) 5%, (b) 14%, and (c) 20% KNO3 in NH4NO3

solid solutions show the orthorhombic AN III phase. Si and LaB6 are the internal standards. (Rh.Pt) peaks are the referred heat strip.

The lattice constants and volumes of the room temperature AN III phase for 5%-20% KNO3 compositions are calculated and listed in Table 1. The results are also compared to the data reported by Holden et al. [1] and Choi et al. [15], which are also listed in Table 1. The lattice constants of the orthorhombic AN III phase are a=0.7686(8) nm, b=0.5828(4) nm, and c=0.7146(4) nm, and volume is V=0.3201(4) nm3 for 5% KNO3 solid solution at room temperature in this study; and these results are reasonable matched the Holden et al. [1] and Choi et al. [15] results. It should be noted that the values of lattice constants and volumes decrease as the composition (%KNO3) increases from 5% to 20%KNO3 composition. The volumes of AN III phase decrease from 0.3201(4) to 0.3166(1) nm3 at the room temperature and from 0.3250(6) to 0.3215(3) nm3 at 373 K as the compositions increase from 5% to 20%KNO3. The decrease in the values of the volume in the orthorhombic structure maybe because that more NH4

+ ions (larger) are replaced by the K+ ions (smaller) as compositions (KNO3) increase. The lattice constant results at the various temperatures (up to 373 K) for the AN III phase show that the values of volume per unit cell increase as the temperature increases up to 373 K, but decrease as the composition increases at the 5%-20% KNO3 composition range. The equations of the volume/unit versus temperature for the AN III phase are obtained by using the least-square method, and are listed in Table 2.


Table 1. Room temperature AN III phase lattice constants and volumes for 5%, 10%, 14%, 16%, and 20% KNO3 compositions as well as the data reported by Choi and Prask [15] and Holden and Dickinson [1].

Lattice Parameter Volume Composition

a (nm) b (nm) c (nm) V (nm3) 5% KNO3 0.7686(8) 0.5828(4) 0.7146(4) 0.3201(4) 10% KNO3 0.7660(4) 0.5824(5) 0.7131(7) 0.3182(4) 14% KNO3 0.7662(9) 0.5822(5) 0.7121(6) 0.3177(5) 16% KNO3 0.7679(3) 0.5829(3) 0.7121(3) 0.3187(2) 20% KNO3 0.7668(1) 0.5805(1) 0.7114(2) 0.3166(1) 5% KNO3 [15] 0.76772(4) 0.58208(4) 0.71396(5) 0.3191 5% KNO3 [1] 0.7694(4) 0.5827(3) 0.7158(3) 0.3209

Table 2. Thermal expansion equations of volume/unit vs. temperature for the AN III, KN II and

KN I phases at various temperature ranges. Phase Composition Equations for Volume/unit cell vs. Temperature Temperature

range 5%KNO3 V(nm3) = (1.446±0.208)×10-5×T(K) + (75.903±0.690)×10-3 293K - 373K

10%KNO3 V(nm3) = (1.324±0.411)×10-5×T(K) + (75.907±1.441)×10-3 293K - 393K 14%KNO3 V(nm3) = (1.634±0.409)×10-5×T(K) + (74.665±1.401)×10-3 293K - 383K 16%KNO3 V(nm3) = (1.165±0.197)×10-5×T(K) + (76.296±0.679)×10-3 293K - 373K

AN III

20%KNO3 V(nm3) = (1.465±0.220)×10-5×T(K) + (74.753±0.736)×10-3 293K - 373K 95%KNO3 V(nm3) = (1.178±0.336)×10-5×T(K) + (76.966±1.125)×10-3 293K - 393K 98%KNO3 V(nm3) = (1.994±0.243)×10-5×T(K) + (74.058±0.836)×10-3 293K - 393K KN II 100%KNO3 V(nm3) = (1.585±0.220)×10-5×T(K) + (75.288±0.749)×10-3 293K - 393K 95%KNO3 V(nm3) = (2.270±0.161)×10-5×T(K) + (73.356±0.755)×10-3 408K - 553K 98%KNO3 V(nm3) = (2.071±0.146)×10-5×T(K) + (74.428±0.687)×10-3 408K - 553K KN I 100%KNO3 V(nm3) = (2.422±0.349)×10-5×T(K) + (72.842±1.690)×10-3 423K - 553K

The X-ray diffraction patterns of the KN-rich region show the low temperature KN II phase with orthorhombic structure, and the high temperature KN I phase with hexagonal structure. Figure 2(a) shows the KN II and KN I phase X-ray diffraction patterns with indexing for the 98% KNO3 solid solution at various temperatures. As temperature increases, the orthorhombic structure KN II phase transits to the hexagonal structure KN I phase. Figure 2(b) shows the lattice parameter changes during the KN II→KN I phase transition for the 98% KNO3 composition. The lattice expansion on c-axis direction of the KN I phase is much greater than the a-axis direction of KN I phase, and a-axis, b-axis, and c-axis directions of the KN II phase. The lattice constant data of the KN II phase show the similar results to the AN III phase. The values of the volume increase as the temperature increases up to 393 K, but decrease as the composition increases at the 95%-100% KNO3 composition range. The values of volumes decrease from 0.3208(3) nm3 to 0.3195(3) nm3 as the composition increases from 95% to 100%KNO3. Holden et al. [1] reported the lattice constants and volume of the 95.2%KNO3 solid solution as a=0.5444 nm, b=0.9211 nm, c= 0.6458 nm and V=0.3238 nm3 which are close to the results of 95%KNO3 KN II phase in this study as a=0.5426(2) nm, b=0.9167(5) nm, c= 0.6449(5) nm, and V=0.3208 nm3. The lattice


constants and volume of the hexagonal KN I phase at 523 K are a=0.5409(4) nm, c=1.0082(9), and V=0.2554(4) nm3.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

273 323 373 423 473 523 573

Temperature (K)

Lat

tice

Para

met

er (n

m)

KN II Phase KN I Phase

a - KN II

c - KN II

b - KN II c - KN I

a - KN I

Figure 2. (a) XRD patterns for 98% KNO3 solid solution show the KN II phase at 293 K and 393

K. (b) The lattice parameters of the KN II and KN I phases in the 98%KNO3 composition during heating.

The lattice constant calculation results show that the lattice expansion is decreasing in the a-axis direction of the KN I phase during heating, but it can be considered that there is no change in the a-axis direction because of the small amount of decrease in comparison to the large amount of lattice expansion in c-axis direction. This means when the temperature increases, the lattice expansion occurs only in c-axis direction, and the basal plane (a-axis) of the hexagonal structure remains the same. Figure 3 shows the values of the volume/unit increase as the compositions (%KNO3) increase strongly depending on the c-axis lattice expansion at 433 K, 483 K, and 523 K for the composition range of 50%-100% KNO3 solid solution. The thermal expansion equations of volume/unit vs temperature for the KN II and KN I phases are also listed in Table 2.

0.97

0.98

0.99

1.00

1.01

1.02

40 50 60 70 80 90 100

%KNO3

c - L

attic

e Pa

ram

eter

(nm

)

433 K

523 K

483 K

0.082

0.083

0.084

0.085

0.086

40 50 60 70 80 90 100

%KNO3

Vol

ume/

unit

(nm

3 )

433 K

523 K

483 K

Figure 3. The (a) c-axis lattice constant and (b) volume/unit cell vs composition plots for the KN

I phase at 433 K, 483 K, and 523 K.

(a) (b)

(a) (b)


CONCLUSIONS The phase transitions and lattice constants of the AN-KN solid solutions have been determined. The low temperature AN III phase is stable up to 373 K at the composition range of 5%-20% KNO3 by adding KNO3 to NH4NO3. The lattice constant calculation results show that the values of the volume for the AN III and KN II phases decrease as the compositions increase, but the values increase as the temperature increases. The KN II phase is stable up to 393 K. The lattice expansion of KN I phase occurs only in c-axis direction and the basal plane (a-axis) of the hexagonal structure remains the same when heating.

REFERENCES [1] Holden, J.R.; Dickinson, C.W., J. Phys. Chem., 1975, 79(3), 249-256. [2] Cady, H.H., Phase Stabilization of Ammonium Nitrate, CPIA Publication No. 377, Johns

Hopkins University, Applied Physics Laboratory, 1983, 9-14. [3] Choi, C.S.; Prask, H.J., Phase Stabilization of Ammonium Nitrate, CPIA Publication No.

377, Johns Hopkins University, Applied Physics Laboratory, 1983, 87-96. [4] Deimling, A.; Engel, W.; Eisenreich, N., J. Therm. Anal., 1992, 38, 843-853. [5] Chandra, D.; Helmy, A.K., X-ray Diffraction and Differential Calorimetry Investigation of

Ammonium Nitrate Solid Solutions, Interim Report to TRW Vehicle Safety Systems, 1999. [6] Chien, W., Solid State Phase Transition and Vapor Pressure Studies in NH4NO3-KNO3

Binary System, Ph.D. Dissertation, University of Nevada, Reno, 2003. [7] Brown, R.N.; McLaren, A.C., Proc. Royal Soc., 1962, 266, 329-343. [8] Lucas, B.W.; Ahtee, M.; Hewat, A.W., Acta Crystallogr., Sect. B: Struc. Cryst., 1979,

B35(5), 1038-1041. [9] Lucas, B.W.; Ahtee, M.; Hewat, A.W., Acta Crystallogr., Sect. B: Struc. Cryst., 1980,

B36(9), 2005-2008. [10] Ahtee, M.; Smolander, K.J.; Lucas, B.W.; Hewat, A.W., Acta Crystallogr., Sect. C: Cryst.

Struct. Commun., 1983, C39(6), 651-655. [11] Nimmo, J.K.; Lucas, B.W., Acta Crystallogr. Sect. B: Struc. Cryst., 1976, B32(7), 1968-

1971. [12] Nimmo, J.K.; Lucas, B.W., J. Phys. C: Solid State Phys., 1973, 6(2), 201-211. [13] Goodwin, T.H.; Whetstone, J., J. Chem. Soc. Abs., 1947, 1455-1461. [14] Choi, C.S.; Prask, H.J.; Prince, E., J. Appl. Crystallogr., 1980, 13(5), 403-409. [15] Choi, C.S.; Prask, H.J., Acta Crystallogr., Sect. B: Struc. Sci., 1982, B38(9), 2324-2328. [16] Edwards, D.A., Z. Kristallogr., 1931, 80, 154-163. [17] Tahvonen, P.E., Ann. Acad. Sci. Fennicae, 1947, Ser. A.(I. Math.-Phys. No. 44), 20. [18] Shinnaka, Y., J. Phys. Soc. Jpn, 1962, 17, 820-828. [19] Stromme, K.O., Acta Chem. Scand., 1969, 23(5), 1625-1636.


x-ray diffractometry studies and lattice parameter calculation on kno3-nh4no3 solid ... · ·...

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