Thermochimica Acta 531 (2012) Contents lists available at SciVerse ScienceDirect. Thermochimica Acta

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1 hermochimica Acta 531 ( Contents lists available at SciVerse ScienceDirect hermochimica Acta journa l h o me page: hermodynamic properties of potassium nitrate magnesium nitrate compound [2KNO 3 Mg(NO 3 2 ] Ramana G. Reddy, ao Wang, Divakar Mantha Department of Metallurgical and Materials Engineering, he University of Alabama, uscaloosa, AL , USA a r t i c l e i n f o Article history: Received 10 April 2011 Received in revised form 8 December 2011 Accepted 14 December 2011 Available online 24 December 2011 Keywords: DSC Mg(NO 3 2 KNO 3 binary system 2KNO 3 Mg(NO 3 2 compound Melting point Heat capacity hermodynamic properties a b s t r a c t he Mg(NO 3 2 KNO 3 binary system phase diagram has a congruent melting compound, 2KNO 3 Mg(NO 3 2. he thermodynamic properties for this compound are not available in the literature. In this study, the nitrate compound was synthesized and the melting point and heat capacity were determined using differential scanning calorimetry (DSC. wo endothermic peaks were observed at K and K corresponding to solid state transition and melting of the compound with the enthalpies of transitions as 2.71 kj/mol and kj/mol, respectively. he heat capacity data as function of temperature are fit to polynomial function and thermodynamic properties like enthalpy, entropy and Gibbs energies of the compound as function of temperature are subsequently deduced Elsevier B.V. All rights reserved. 1. Introduction Molten salts have been used as thermal energy storage media for solar energy applications. Nitrates are being used in the solar energy applications for their low melting point, low unit cost, high heat capacity, high thermal stability, negligible vapor pressure and high energy storage density [1]. Solar salt (NaNO 3 /KNO 3 : 60/40 is the most popular thermal energy storage medium which is currently being used with the freezing point of K [2]; another ternary system HIEC which contains NaNO 3, KNO 3 and NaNO 2 has freezing point of K [3]. Newer nitrate salt mixtures are being studied and projected as potential candidates for thermal energy storage (ES and heat transfer (H applications. Based on these favorable features, molten salt can work directly as the energy storage medium below K [1]. Development and synthesis of newer molten salt mixtures with freezing point lower than those currently used for thermal energy storage applications is necessary for sustained utilization of solar energy. he approach to develop lower freezing point molten salt mixtures is by the prediction of new eutectic mixtures and also by the development of new nitrate compounds. In this context, the congruently melting compound, 2KNO 3 Mg(NO 3 2 can be a promising additive. Corresponding author. el.: ; fax: address: rreddy@eng.ua.edu (R.G. Reddy. he phase diagram of Mg(NO 3 2 KNO 3 binary system has two eutectics and one congruently melting solid phase, 2KNO 3 Mg(NO 3 2. he compound 2KNO 3 Mg(NO 3 2 will be labeled as MgKN in this article. he two eutectic points appear on either side of the congruently melting solid. he melting point of the congruently melting compound, MgKN that can be read from the phase diagram is 498 K [4]. However, no experimental data are available in the literature on the accuracy of this melting point. hermodynamic properties such as heat capacity, enthalpy, and entropy and Gibbs energy are also not available in the literature. In this paper, we determine the melting point and heat capacity of MgKN using the differential scanning calorimetry (DSC technique to re-verify the melting point given in the phase diagram and also to deduce the thermodynamic parameters as function of temperature. 2. Experimental 2.1. Materials he MgKN compound is synthesized from magnesium nitrate hexahydrate (98%, Alfa Aesar and potassium nitrate (ACS, 99.0% min, Alfa Aesar. Potassium nitrate is used without further purification whereas the magnesium nitrate hexahydrate is dehydrated before synthesizing the 2KNO 3 Mg(NO 3 2 compound. he synthesized 2KNO 3 Mg(NO 3 2 compound has 98% of purity /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.tca

2 R.G. Reddy et al. / hermochimica Acta 531 ( Dehydration of Mg(NO 3 2 6H 2 O Weighted amount of magnesium nitrate hexahydrate was taken in a stainless steel crucible and placed on a hot plate in argon atmosphere. emperature of the salt is measured with a thermocouple immersed in the salt. he salt was held at K for 2 h after which the salt solidifies to a white mass. he temperature is then raised slowly to K to remove any traces of moisture from the salt and to ensure complete dehydration. he complete removal of water is ascertained by weight loss Apparatus and calibration Perkin-Elmer Diamond differential scanning calorimeter (DSC was use to measure the melting point and heat capacity of the MgKN compound. Endothermic heat flow and temperature can be recorded in the instrument with an accuracy of mw and 0.01 K, respectively. Before the actual measurements, pure indium, zinc metal were used to calibrate the DSC temperature as well as the heat flow curve based on the GEFA calibration procedure [5,6]. he measurements were made under purified nitrogen atmosphere with a flow rate of 20 cc min 1 and at a heating rate of 5 K min Salt preparation he 2KNO 3 Mg(NO 3 2 nitrate salt compound is composed of mol% KNO 3 and mol% Mg(NO 3 2. Potassium nitrate and dehydrated magnesium nitrate are the two salt components that are used to synthesize the MgKN compound. Weighed amounts of the two salts according to the above mentioned stoichiometry are measured to an accuracy of 0.1 mg with the electrical balance and mixed thoroughly in a stainless steel crucible. he mixture is heated up to a certain temperature, which is about 50 K more than the melting temperature of the salt mixture. At this temperature the salt mixture was held for about 30 min. he salt mixture is allowed to air cool to ambient temperature. his procedure is repeated 3 4 times to get a homogeneous MgKN compound Experimental procedure Standard aluminum pan with lid used for DSC measurements are weighed before the experiment. For the determination of melting point and heat capacity of the synthesized MgKN compound mg of the sample was used. he salt compound is placed carefully in the aluminum pan and closed with the lid. he lid is crimped by a sample press and the pan is weighed. he weight of the sample is determined by removing the weight of the pan and lid. he crimped ample pan was immediately put inside the sample chamber of DSC after preparation and held at K for 10 h to remove the trace amount of moisture that might have possibly caught in the process of loading sample and also to ensure a homogeneous mixture. In the experimental procedure for melting point determination, a temperature range from K to K was set with a heating rate of 5 K min 1 followed by a cooling cycle at the same rate. his cycle is repeated for at least 6 times to ensure good mixture of the sample and reproducibility of the results. For heat capacity measurement, the same procedure as that followed for melting point determination is employed with addition of iso-scan-iso steps to the heating cooling cycles program. he iso-scan-iso steps with a step width of 25 K are introduced into the program cycle after five temperature-scan cycles. Starting from K, the temperature was held for 5 min before and after each scanning step. he small temperature scan range is chosen to decrease the thermal resistance between the device and the sample. he upper limit for the heat capacity (C p measurement was set to K in our experiments. o get the value of molar Fig. 1. Endothermic peaks of 2KNO 3 Mg(NO 3 2 compound determined by Diamond DSC showing solid state phase transition and melting. heat capacity of the sample, heat flow curve for the baseline of the empty sample pan also needs to be obtained immediately following the identical iso-scan-iso steps which were used for the actual sample run. he difference of heat flow between the actual crimpled sample and the empty sample pan is the absolute heat absorbed by the test sample. 3. Results and discussion 3.1. Melting point determination Differential scanning calorimetry (DSC was used to determine the melting point and any solid state phase transitions of the 2KNO 3 Mg(NO 3 2 compound. A low scanning rate was chosen to record the heat flow curve as function of temperature in order to improve the sensitivity of detection. It helps to pick up any small endothermic peaks and also reduces the temperature difference between the internal furnace and sample. Fig. 1 shows the DSC plot of one run (sixth cycle for the MgKN compound. DSC plots for the compound were collected for at least three runs (each run with fresh MgKN preparation to ensure the reproducibility. wo endothermic peaks were identified in the figure. he first endothermic peak refers to solid state phase transition of the MgKN compound. he onset of phase transition begins at ± 0.20 K and the peak transition temperature is ± 0.18 K. Fig. 2 shows the DSC plot of endothermic peaks of the heating cycle in KNO 3. wo endothermic peaks appear in the figure, the first refers to solid state phase transition and the second peak refers to the melting of KNO 3. Solid state phase transition from -KNO 3 to -KNO 3 occurs at a peak temperature of ± 0.12 K and melting peak occurs at ± 0.10 K. Similar to the solid state phase transition that occurs in KNO 3, a solid state phase transition too occurs in the MgKN compound. he temperature of solid state phase transition in the MgKN compound occurs at K which is about 2.6 K lower than that in the KNO 3 compound. he enthalpy of solid state phase transition determined from the area under the endothermic peak is 2.71 ± 0.03 kj/mol. No information on the solid state phase transition of MgKN compound is available in the literature. In this study, we label the two solid states of MgKN compound as solid state and solid state ˇ for convenience. he second endothermic peak refers to the melting of the MgKN compound with an onset temperature of K and peak temperature of K. Normally, the onset temperature of transition

3 8 R.G. Reddy et al. / hermochimica Acta 531 ( able 1 emperatures and enthalpies of phase transitions in the 2KNO 3 Mg(NO 3 2 compound. Compound ransition emperature (K Enthalpy (kj/mol 2KNO 3 Mg(NO 3 2 Solid solid Solid liquid Fig. 2. Endothermic peaks of KNO 3 determined by Diamond DSC showing solid state phase transition and melting. is taken as the experimental transition point for any metallic sample. However, in case of molten salts mixtures, since the thermal conductivity is low, the complete transition is ensured only at the peak transition temperature. he thermal gradient which exists due to the low thermal conductivity of the salt results in internal heat flow which enhances the mixing in the salt. hus, the transition temperature is defined as the peak temperature of phase transition. he peak values of temperatures for all transitions of the MgKN compound are taken as the transition temperatures. he only possible melting point data of the MgKN compound that can be found prior to our study was from the previously reported binary Mg(NO 3 2 KNO 3 phase diagram [4] which showed 30 K higher melting point than that obtained in this study. o ensure the validity of the congruent temperature of the MgKN compound, the Mg(NO 3 2 KNO 3 phase diagram between two eutectic points were reproduced using melting temperatures of the binary system with different constituent salt compositions. he partial phase diagram is illustrated in Fig. 3, the melting points of two eutectic composition were determined as ± 0.1 K and ± 0.3 K. wo points were chosen between eutectic composition and Fig. 3. Phase diagram of KNO 3 Mg(NO 3 2 binary system. congruent point on each side of the phase diagram and the melting temperatures are all found to be lower than the congruent temperature while higher than those of the eutectic points. As a result of that, the congruent temperature determined using DSC technique was verified. Moreover, the melting temperatures of two composition points beyond the portion of the partial phase diagram limited by two eutectic points were also tested. Each point shows higher melting temperatures than that of the eutectic point on the same side, which indicates the validity of the measured eutectic temperatures. he enthalpy of fusion of the MgKN compound determined from the area under the melting endothermic peak is kj/mol. able 1 lists the peak transition temperatures and the corresponding enthalpies for the MgKN compound Heat capacity measurement In the DSC equipment, the heat flow through the sample is tested as function of temperature. In the heating process, the temperature profile can be expressed in (K as: = o h (1 where o is the initial temperature in K, h is the set heating rate (K min 1 and is the temperature in K at a certain time t (min. Based on the thermodynamic definition, heat capacity is the amount of heat required to change a body s temperature by a given amount. Combined with the equation above, the expression for C p (J/K mol can be shown as follows: [7] C p = 1 m ( dh d = 1 m ( dh/dt d/dt = 1 m ( P where the P is the actual DSC heat flow signal which has the unit as mw, m is the mass for the sample of interest. his way the heat capacity with the DSC heat flow data are calculated. he molar heat capacity of the MgKN compound was measured by the DSC equipment from room temperature to K. he heat flow is recorded as a function of temperature in iso-scan-iso steps at intervals of 25 K. he iso stage refers to isothermal holding at a particular temperature, scan stage refers to the heat flow recording at a heating rate of 5 K min 1 up to a an increment of 25 K, followed by another isothermal holding stage. his is a standard procedure followed in the measurement of heat capacity of materials using the DSC equipment. his procedure of heat capacity measurement has two advantages; (i any heat fluctuations during the recording are avoided by the isothermal steps and (ii any phase transition can be highlighted by the choice of temperature range. he absolute heat flow to the sample is determined by subtracting the heat flow collected by running a baseline curve with an empty pan. Fig. 4 shows the heat capacity of the MgKN compound measured as function of temperature in the range K. he figure shows two phase transitions corresponding to the solid state phase transition and melting of the MgKN compound. able 2 lists the values of heat capacity as function of temperature for the entire temperature range of this study. he heat capacity data can be divided into three sections; (i solid state ( K (ii solid state ( K (iii liquid state ( K. Accordingly, the heat capacity data are fit to three separate polynomial equations corresponding to the three phases of the compound. ˇ (2

4 R.G. Reddy et al. / hermochimica Acta 531 ( able 2 Heat capacity of 2KNO 3.Mg(NO 3 2 at different temperatures ( K. (K C p (J/K mol (K C p (J/K mol (K C p (J/K mol (K C p (J/K mol Heat capacity of solid state : ( K he heat capacity data for MgKN compound in the solid state 1 in the temperature range of K is given in able 3. he data are fit to a second order polynomial equation. Eq. (3 gives the polynomial equation along with the least square fit parameter (R 2 in the temperature range for the solid state of the compound. C p (soild state (J/K mol = ; ( K, R 2 = ( Heat capacity of solid state ˇ: ( K he heat capacity data for MgKN compound in the solid state in the temperature range of K is given in able 4. he data are fit to a second order polynomial equation. Eq. (4 gives the able 3 hermodynamic properties of 2KNO 3 Mg(NO 3 2 compound in solid state ( K. (K C p (J/mol K S (J/mol K H (kj/mol G (kj/mol able 4 hermodynamic properties of 2KNO 3 Mg(NO 3 2 compound in solid state ( K. (K C p (J/mol K S (J/mol K H (kj/mol G (kj/mol Fig. 4. Heat capacity of 2KNO 3 Mg(NO 3 2 compound as function of temperature determined by Diamond DSC

5 10 R.G. Reddy et al. / hermochimica Acta 531 ( able 5 hermodynamic properties of 2KNO 3 Mg(NO 3 2 compound in liquid state ( K. (K C p (J/mol K S (J/mol K H (kj/mol G (kj/mol below: S S = H H = m ( CP d H t ( CP d H m m C p d H t m C p d H m ( Cp m d (6 m C p d (7 G G = (H H (S S (8 he standard thermodynamic properties, entropy, enthalpy and Gibbs energies as function of temperature for the three phases of the MgKN compound are expressed in the following sections Solid state ( K Standard entropy as a function of temperature of the MgKN ( S = S Cp = d = ln J/K mol (9 polynomial equation along with the least square fit parameter (R 2 in the temperature range for the solid state of the compound. C p (solid state (J/K mol = ; ( K, R 2 = (4 Standard enthalpy as a function of temperature of the MgKN H H = C p d = J/mol (10 Standard Gibbs energy as a function of temperature of the MgKN Heat capacity of liquid state: ( K he heat capacity data for MgKN compound in the liquid state in the temperature range of K is given in able 5. he data are fit to a linear equation. Eq. (5 gives the polynomial equation along with the least square fit parameter (R 2 in the temperature range for the liquid state of the compound. C p (liquid(j/k mol = ; ( K, R 2 = (5 Heat capacity data of the MgKN compound in the two solid states follows a second order polynomial curve whereas the heat capacity is linear in the liquid state hermodynamic properties he standard thermodynamic properties such as entropy, enthalpy, and Gibbs energy for MgKN compound are determined from the experimental data of heat capacity as function of temperature for each phase of the compound. In thermodynamics, all these three properties are expressed in terms of heat capacity as function of temperature [8]. In the present study, the expressions for the standard thermodynamic properties are given G G = (H H (S S = ( ln J/mol ( Solid state ˇ ( K Standard entropy as a function of temperature of the MgKN ( S S Cp = d H ( t Cp d = 4.02 t ln J/K mol (12 Standard enthalpy as a function of temperature of the MgKN H H = C p d H t C p d = J/mol (13

6 R.G. Reddy et al. / hermochimica Acta 531 ( Standard Gibbs energy as a function of temperature of the MgKN G G = (H H (S S = ( ln J/mol ( Liquid state ( K Standard entropy as a function of temperature of the MgKN compound in the liquid state is given by: S S = ( Cp m ( Cp d H t m ( Cp d H m m = ln J/K mol (15 Standard enthalpy as a function of temperature of the MgKN compound in the liquid state is given by: H H = H m C p d H t m C p d C p d = J/mol m (16 Standard Gibbs energy as a function of temperature of the MgKN compound in the liquid state is given by: G G = (H H (S S = ( ln J/mol (17 Among the equations above, Eqs. (9 (11 refer to the thermodynamic properties for solid ; Eqs. (12 (14 refer to the thermodynamic properties for solid ; Eqs. (15 (17 refer to thermodynamic properties of the liquid. he entropy and Gibbs energy curves have logarithmic dependence with temperature while that of the enthalpy has a linear relationship with temperature. hese curves are all smooth in the three temperature ranges. 4. Conclusions he melting point and heat capacity of the 2KNO 3 Mg(NO 3 2 compound were experimentally determined by DSC. here exists a solid state phase transition in the compound. Solid state phase transition occurs at K and melting of the compound takes place at K. Heat capacity data showed two endothermic peaks corresponding to the phase transitions of the MgKN compound. Heat capacity data in the entire temperature range of study were fit to three polynomials corresponding to the three phases of the compound. hermodynamic properties such as entropy, enthalpy, and Gibbs energy of the MgKN compound were deduced from the heat capacity expressions. Acknowledgements he authors are pleased to acknowledge the financial support from DOE, Grant No. DE-FG36-08GO18153, for this research project. We also thank the University of Alabama for providing the experimental facilities. References [1] R.W. Bradshaw, N.P. Siegel, Molten Nitrate Salt Development for hermal Energy Storage in Parabolic hrough Solar Power System, Sandia National Laboratory, ES , [2] O. Greis, K.M. Bahamdan, B.M. Uwais, he phase diagram of the system NaNO 3 KNO 3 studied by differential scanning calorimetry, hermochim. Acta 86 ( [3] Badger Energy Corporation, Design, Handling, Operation and Maintenance Procedures for Hitec Molten Salt, Sandia National Laboratories Contractor Report, SAND , [4] Von Ernst Jänecke, Das quaternäre system der nitrate von Na K Ca Mg und seine teilsysteme, Zeitschrift fur elektrochemie und angewandte physikalische chemie Bd. 48 ( [5] E. Gmelin, S.M. Sarge, emperature, heat and heat flow rate calibration of differential scanning calorimeters, hermochim. Acta 347 ( [6] S.M. Sarge, E. Gmelin, G.W.H. HoÈhne, H.K. Cammenga, W. Hemminger, W. Eysel, he caloric calibration of scanning calorimeters, hermochim. Acta 247 ( [7] M.M. Zhang, R.G. Reddy, hermodynamic properties of C4mim[f2 N] ionic liquids, rans. Inst. Min. Metall. C 119 ( [8] O. Kubaschewski, C.B. Alcock, P.J. Spencer, Materials hermochemistry, sixth edition, Pergamon Press, New York, 1993.