Article pubs.acs.org/jced
Thermophysical Properties of Potassium Fluoride Tetrahydrate from (243 to 348) K Patrick J. Shamberger*,† and Timothy Reid†,‡ †
Thermal Sciences & Materials Branch, Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433, United States ‡ University of Dayton Research Institute, University of Dayton, Dayton, Ohio 45469, United States ABSTRACT: Potassium fluoride tetrahydrate is of interest as a thermal energy storage material, due to its large specific and volumetric enthalpy of fusion and its low melting temperature. Here, we report the thermophysical properties of solid and liquid potassium fluoride tetrahydrate at temperatures from (243 to 348) K and compare this compound to water and octadecane, two other potential thermal energy storage materials with similar melting temperatures. Furthermore, we present a modified potassium fluoride−water phase diagram and accurately determine the enthalpies of fusion and melting temperatures for potassium fluoride tetrahydrate, ΔHfus = (246 ± 2) J·g−1 and Tfus = 291.6 K, and the potassium fluoride tetrahydrate−potassium fluoride dehydrate eutectic, ΔHfus = (203 ± 2) J·g−1 and Tfus = 282.2 K.
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INTRODUCTION Thermal management of aerospace systems and components is a critical factor in meeting both current and future technological goals for the aerospace industry.1 For thermal management purposes, thermal energy storage (TES) materials are of great utility, as they absorb transient pulses of heat, averaging heat loads over greater time scales, thereby decreasing the mass and volume of remaining thermal management elements. In practice, materials which undergo a solid−liquid phase transition (commonly referred to as “phase change materials”) are observed to reversibly absorb and release large quantities of heat over very small temperature ranges.2 The paraffins have been widely adopted as engineering phase change materials, due to the wide range of melting temperatures observed in different paraffins (93 to 353 K), their predictable melting and crystallization behaviors, and the workability and nontoxicity of the basic materials. However, solidification behavior of paraffins is known to depend strongly on their purity, and the paraffins have only moderate specific and volumetric thermal storage capabilities. In comparison, a number of salt hydrates have attracted interest which have volumetric storage densities nearly double those of paraffins (due principally to the higher density of the salt hydrates) as well as higher thermal conductivities.2−4 However, very few of the thermophysical parameters of these salt hydrate systems are known within a reasonable degree of certainty. This limits comparison with other known TES materials, as well as highfidelity computational simulations of TES components based on salt hydrates. This paper describes the thermophysical properties of one candidate low melting temperature (292 K) TES material, potassium fluoride tetrahydrate (KF·4H2O). KF·4H2O is of great interest due to its near room-temperature melting point, © 2013 American Chemical Society
which is ideal for certain thermal management applications, and because it offers double the volumetric energy densities (≈350 MJ·m−3) of comparable melting temperature paraffins.2−4 Here, we report the heat capacity, thermal conductivity and diffusivity, density, and viscosity of KF·4H2O between (243 and 348) K, as determined by a number of analytical techniques. The thermophysical properties of this compound are compared against the properties of water and a paraffin with a similar melting temperature (octadecane, C18H38). Furthermore, we investigate the melting temperature and enthalpy of fusion of KF·4H2O and the KF·4H2O/KF·2H2O eutectic. This work complements a previous study on the lithium nitrate− water system which melts at temperatures ≈10 K greater than KF·4H2O.5
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BRIEF REVIEW OF EXPERIMENTAL DATA FOR POTASSIUM FLUORIDE TETRAHYDRATE
Equilibrium solubility in the potassium fluoride−water system has been determined by Jatlov and Poljakava by isothermal solubility measurements.6 The system consists of congruently melting compound, KF·4H2O, at a mass fraction of KF w = 0.446 with a melting temperature Tfus ≈ 291 K. The KF·4H2O/ H2O eutectic is reported at w = 0.215 with a eutectic temperature Teut = 251.4 K, while the KF·4H2O/KF·2H2O eutectic is reported at w = 0.477 with a eutectic temperature Teut = 290.9 K. Furthermore, a peritectic is reported between KF·2H2O/KF at w = 0.581 with a peritectic temperature Tper = 313.4 K. Magin reviewed the peritectic temperature for use as a Received: August 1, 2012 Accepted: December 7, 2012 Published: January 2, 2013 294
dx.doi.org/10.1021/je300854w | J. Chem. Eng. Data 2013, 58, 294−300
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calibration standard and determined it to be (314.57 ± 0.04) K.7 These data are reviewed in Figure 1.
Additional thermophysical properties of KF·4H2O remain minimally characterized in the literature. The heat capacities of liquid and solid KF·4H2O have been determined at temperatures just below and above Tfus by multiple authors with significant deviation (fractional deviations from these two sources are >0.2).9,10 To our knowledge, the thermal transport properties of KF·4H2O have not been previously reported. The equilibrium vapor pressure of KF·4H2O has been reported and is lower than that of water ( 0.99, Alfa Aesar) dried at 500 K for >4 h (Table 2). This
The melting temperature and enthalpy of fusion of KF·4H2O has been measured by a number of authors using calorimetric techniques; these values are summarized in Table 1.6,8−12
Table 2. Specification of the Products Used for Measurements
Table 1. Tfus and ΔHfus of KF·4H2O and wKF and Teut of the KF·4H2O/KF·2H2O Eutectic at 0.1 MPa as Reported in the Literature Tfus/K 292 ± 1 291.9 ± 0.1 291.7 ± 0.1 291.7 ± 0.1 291.9 ± 0.1 a
ΔHfus/J·g−1 a
232 231 ± 19 231.0a 330a 201 ± 27
wKF
Teut/K
ref
0.477a
290.8a
6 8 9 10 11b 12
a
composition
CAS no.
supplier
wKFa
wvol.b
potassium fluoride, ACS
7789-23-3
Alfa Aesar
0.997
20 K, and the average ΔT of KF·4H2O/KF·2H2O is > 30 K, with considerable variation from sample to sample (Table 9). Previous studies have demonstrated a decrease in ΔT of KF·4H2O with the addition of volcanic pumice.10 Research into stable nucleation agents to promote heterogeneous nucleation is ongoing in the authors’ research group.26
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(2)
CONCLUSIONS The thermophysical properties of KF·4H2O are measured at temperatures from (243 to 348) K. These are compared against the properties of water and octadecane, two other potential thermal energy storage materials and commonly utilized
Here, η is absolute viscosity (in Pa·s), T is the temperature (in K), and A = 4.6502·10−2, B = −33.434, and C = 6215.0 are parameters fit to the data. Deviation of experimental data from the polynomial fit the linear fit (|ηexp − ηfit|/ηfit) is < 0.02. The 299
dx.doi.org/10.1021/je300854w | J. Chem. Eng. Data 2013, 58, 294−300
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Table 9. Undercooling, ΔT, of KF·4H2O and the KF·4H2O/ KF·2H2O Eutectic at 0.1 MPa
(10) Gawron, K.; Schröder, J. Properties of Some Salt Hydrates for Latent Heat Storage. Energy Res. 1977, 1, 351−363. (11) Guion, J.; Sauzade, J. D.; Laügt, M. Critical Examination and Experimental Determination of Melting Enthalpies and Entropies of Salt Hydrates. Thermochim. Acta 1983, 67, 167−179. (12) Reznitskii, L. A.; Filippova, S. E. Potassium fluoride tetrahydratereversible heat storage material at room temperature. Vestn. Mosk. U. Khim. 1997, 38, 175−176. (13) Beurskens, G.; Jeffrey, G. A. Crystal Structure of Potassium Fluoride Tetrahydrate. J. Chem. Phys. 1964, 41, 917−923. (14) Preisinger, A.; Zottl, M.; Mereiter, K.; Mikenda, W.; Steinböck, S.; Dufek, P.; Schwarz, K.; Blaha, P. Hydrogen Bonding in Potassium Fluoride Dihydrate: A Crystallographic, Spectroscopic, and Theoretical Study. Inorg. Chem. 1994, 33, 4774−4780. (15) Günther, E.; Mehling, H.; Werner, M. Melting and nucleation temperatures of three salt hydrate phase change materials under static pressures up to 800 MPa. J. Phys. D: Appl. Phys. 2007, 40, 4636−4641. (16) Archer, D. G.; Rudtsch, S. Enthalpy of Fusion of Indium: A Certified Reference Material for Differential Scanning Calorimetry. J. Chem. Eng. Data 2003, 48, 1157−1163. (17) Chase, M. W., Jr. NIST-JANAF Themochemical Tables, 4th ed.; J. Phys. Chem. Ref. Data Monograph 9; American Institute of Physics: Washington, DC, 1998. (18) ASTM Standard D2717, 1995 (2009). Standard Test Method for Thermal Conductivity of Liquids; ASTM International: West Conshohocken, PA, 2009; DOI: 10.1520/D2717-95R09. http://www. astm.org (accessed Oct 24, 2011). (19) Lemmon, E. W.; McLinden, M. O.; Friend, D. G. Thermophysical Properties of Fluid Systems. In NIST Chemistry WebBook; Linstrom, P. J.,Mallard, W. G., Eds.; NIST Standard Reference Database Number 69; NIST: Gaithersburg, MD. http:// webbook.nist.gov (accessed Oct 24, 2011). (20) ASTM Standard D445, 2011a. Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamics Viscosity); ASTM International: West Conshohocken, PA, 2011; DOI: 10.1520/D0445-11A. http://www.astm.org (accessed Oct 24, 2011). (21) Lide, D. R. CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press: Boca Raton, FL, 2010. (22) Tsederberg, N. V. Thermal Conductivity of Gases and Liquids; MIT Press: Cambridge, MA, 1965. (23) Iyoki, S.; Iwasaki, S.; Kuriyama, Y.; Uemura, T. Densities, Viscosities, and Surface Tensions for the Two Ternary Systems H2O + LiBr + LiI and H2O + LiCl + LiNO3. J. Chem. Eng. Data 1993, 38, 302−305. (24) Porter, D. A.; Easterling, K. E.; Sherif, M. Y. Phase Transformations in Metals and Alloys, 3rd ed.; CRC Press: Boca Raton, FL, 2009. (25) Turnbull, D. The Undercooling of Liquids. Sci. Am. 1965, 212, 38−46. (26) Shamberger, P. J.; Forero, D. E. Towards High Energy Density, High Conductivity Thermal Energy Storage Composites. In Proceedings of the ASME 2012 3rd Micro/Nanoscale Heat & Mass Transfer International Conference, Atlanta, GA, Mar 3−6, 2012; ASME: New York, NY, 2012; MNHMT2012-75039.
Δ T/Ka KF·4H2O KF·4H2O/KF·2H2O eutectic
avg
2σ
Nb
28.1 34.0
19.7 16.6
9 6
a
u(T) = 0.2 K. bReported values are averages of N independent measurements.
reference points. Both KF·4H2O (246 J·g−1, 353 MJ·m−3sol, 358 MJ·m−3liq) and the KF·4H2O/KF·2H2O eutectic (203 J·g−1, 310 MJ·m−3sol, ≈296 MJ·m−3liq) have moderately large specific energy storage densities and very high volumetric energy storage densities which are comparable with or even exceed those of water. Furthermore, these two compositions have a higher specific energy density, thermal conductivity, and thermal diffusivity than that of comparable paraffins. Thus, KF-based salt hydrates are very competitive as thermal energy storage materials in the temperature range (283 to 293) K and excel in applications where weight and volume are at a premium. Further studies of other salt hydrate systems are needed to evaluate their potential as TES materials and to establish compositional trends in material properties.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +1 (937) 255-6809. Fax: +1 (937) 255-2176. E-mail:
[email protected]. Funding
The authors thank the Air Force Research Laboratory, Materials and Manufacturing Directorate for providing necessary financial support to carry out the present work. Notes
The authors declare no competing financial interest.
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REFERENCES
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