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Basic and Applied Research: Section I

Experimental Determination of NH4NO3-KNO3 Binary Phase Diagram Wen-Ming Chien, Dhanesh Chandra, Abdel K. Helmy, Jennifer Franklin, and Claudia J. Rawn

(Submitted July 18, 2004; in revised form January 21, 2005) The solid-state phase transitions in ammonium nitrate (AN)-potassium nitrate (KN) system, and the equilibrium AN-KN phase diagram have been determined by using differential scanning calorimetry and high-temperature in situ x-ray diffractometry. Sample preparation was performed in a special “dry room” with very low humidity. A single phase region (AN III) with no phase transitions to 373 K was observed in the composition range 5 to 20% KN; this is critical for use in air bag gas generators. The high-temperature KN phase (KN I) has a wide range of stability from 20 to 100 wt.% KN. There are one eutectic, two eutectoid, three peritectoid, and one congruent transformations in this phase diagram. Two new nonstoichiometric phases were found at lower temperatures in the mid-composition range between the AN and KN terminal solid solutions. Details of the phase equilibria are presented.

1. Introduction Ammonium nitrate (NH4NO3 [AN]) is used as an oxidizer in propellants, explosives, and gas generators. It is well known that pure AN exhibits several solid-state phase transitions. In applications in the automotive industry, such as air bags, in which AN is used as an oxidizer, it is important to avoid these phase transitions because they cause volume changes with possible disintegration of the pellet or disk forms containing the oxidizer [1983Cad, 1983Chol and 1999Cha]. [1983Cad], [1975Hol], and [1992Dei] reported a method to stabilize the ANs by the addition of potassium nitrate (KNO 3 [KN]) to AN. [1932Jan] and [1949Jan] showed an AN-KN phase diagram for the entire AK-KN composition range; this is perhaps the first comprehensive study of this system. [1947Ber], [1955Mor], and [1963Coa] developed alternative AN-KN phase diagrams. [1981Cad] and [1983Cad] developed a phase diagram that was close to that of [1932Jan] in the range 0 to 30 wt.% KN. Comparison of these proposed phase diagrams show discrepancies among the phase transitions at lower temperatures, particularly so for the AN IV-AN III phases. In one of the phase diagrams, there is a simple line denoting a metastable phase change from AN IV to AN II. [1981Cad] and [1932Jan] have shown a two-phase AN IV + AN II region whereas the other four phase diagrams do not show this region. [1983Cad] and [1947Ber] showed two cubic phases, CI and CII, occurring at ∼16 to 30 wt.% KN above 373K, but [1932Jan] reported only one cubic phase (C). In addition, there are uncertainties in the invariant equilibria temperatures and compositions. A more accurate determination of the phase diagram is important to

W.-M. Chien and D. Chandra, Metallurgical and Materials Engineering, MS 388, University of Nevada-Reno, Reno, NV 89557; A.K. Helmy, TRW Incorporated, Lockwood, NV 89434; J. Franklin, Sandia National Laboratory, Albuquerque, NM 87123; and