phase change materials for energy storage nucleation to prevent supercooling

Solar Energy Materials and Solar Cells 2’7 (1992) 135-160 North-Holland

War Enewgy Materials and Solar Ceils

Phase change materials for energy storage nucleation to prevent supercooling

George A. Lane Central Research & Development Laboratories, The Dow Chemical Company, Midland, MI, USA

Received 12 September 1991

Phase change materials (PCMs) are useful for storing heat as the latent heat of fusion. Such storage has potential in heating and cooling buildings, waste heat recovery, off-peak power utilization, heat pump systems, and many other applications.

Among the PCMs that have proven useful in heat storage applications are calcium chloride hexahydrate, CaCl,.6H,O, magnesium chloride hexahydrate, MgCl,.6H,O, and magnesium nitrate hexahydrate, Mg(N0,),.6H,O.

Many salt hydrate PCMs, including those listed above, have the disadvantage that during extraction of stored heat the material supercools before freezing. This reduces the utility of the material, and if too severe can completely prevent heat recovery.

Many factors determine whether a particular additive will promote nucleation, for example, crystal structure, solubility, and hydrate stability. Candidate isomorphous and isotypic nucleating additives, with crystal structures that fit well with the PCM structure, were selected from tables of crystallographic data.

Bpitaxial nucleators, with less obvious lattice structure features that promote nucleation, were selected mostly by intuition. Effective nucleators were discovered by both methods.

Based on laboratory test results, promising materials were developed into formulations based on CaCI,.6H,O, MgCI,.6H,O, Mg(N0,),.6H,O, Mg(N0,),~6H,O-MgCIz6H,O eutectic, and Mg(N03)z.6H,0-NH,NOs eutectic salt hydrate PCMs. Subsequently, attempts were made to correlate crystal structure and hydrate stability with nucleating efficacy, and to speculate about active nucleating structures.

1. Introduction

Understanding crystallization and melting processes is fundamental to PCM research and technology [1,2]. Heat is stored in the PCM by melting (or phase transition), and recovered by freezing. The crystallization process involves several steps: an induction period, a crystal growth phase, and a period of recrystallization or crystal regrowth. During the induction phase, nuclei are formed,and grow to a sufficient size to be stable. Initially, nucleation centers are formed. Next, material diffuses to a nucleus and is adsorbed on its surface. Then, the adsorbed material

Correspondence to: G.A. Lane, Central Research & Development Laboratories, The Dow Chemical Company, Midland, MI, USA.

0927-0248/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights resewed

136 G.A. Lane / Nucleation to precent PCM supercooling

migrates along the surface and is incorporated into the crystal at preferred locations.

These small crystals continue to grow by this process, eventually becoming large enough and numerous enough to sustain a rapid rate of crystal growth. The rate of crystallization slows as the freezing process nears completion. Even after the material is totally frozen, redistribution processes continue to modify the particle shape and the size distribution.

Freezing, withdrawal of the stored heat, is the more critical process in PCM technology. The attendant problems of supercooling, nucleation, and conduction of heat through the frozen crystalline mass must be addressed. Melting is not dependent on nucleation or supercooling and convective heat transfer enhances the process.

Much of our knowledge of nucleation and crystal growth comes from theoretical and experimental studies of crystallization from solution. This is applicable to salt hydrate PCMs, which in the liquid state are concentrated aqueous solutions.

1.1. Nucleation

Crystal growth originates at extremely tiny nucleation centers. These centers can be spots located on the container walls, dust or impurity particles in the liquid, small "seed" crystals of the PCM either added to initiate crystallization or maintained in a frozen state in contact with the liquid, nucleating additive crystals incorporated in the PCM, or nuclei formed from the PCM itself. The latter case is termed homogeneous nucleation, the others are examples of heterogeneous nucleation.

In order for any nucleation and crystal growth to occur, the melted PCM must be supersaturated or supercooled. In crystallization from solutions, these two terms are interchangeable. If the PCM is above its melting point (above the temperature at which the solution is saturated), added crystals tend to dissolve. If the PCM is below the melting point, added crystals (above the critical size) tend to initiate crystallization. At the melting point, added crystals (with no heat gained or lost) cause no net growth or dissolution.

1.1.1. Supersaturation and supercooling The concepts of supersaturated solutions and supercooled solutions represent

two ways of considering the same phenomenon, solutions which, at a given temperature, contain more solute than permitted by stable thermodynamic equilibrium. Given proper nucleation, these systems will move toward equilibrium conditions. It is cgnvenient to use the concept of supersaturation when considering isothermal processes, employing supercooling for polythermal cases.

Various investigators have searched unsuccessfully for an abrupt or discontinu- ous change in the thermal or physical properties of solutions upon passing from saturation to supersaturation. It is most likely that the supersaturated or supercooled condition is a metastable extension of the dissolved solution or liquid state.

G.4. Lane / Nucleation to prevent PCM supercooling 137

If properly nucleated, the system returns to the thermodynamically stable condition.

1.1.2. Formation of nuclei Homogeneous nucleation involves the formation of nucleating particles in the

liquid phase from the PCM material itself. Heterogeneous nucleation occurs on foreign particles or sites which are present accidentally or intentionally in the system. Obviously, homogeneous nucleation is difficult to study experimentally because of the difficulty of completely freeing a solution of heterogeneous nucleation centers, such as impurities, dust, surface imperfections, and the like.

There are quite a few potential PCMs, particularly organic compounds, that display little or no supercooling. Whether this is an innate property of the pure material, or a result of impurities, is an open question. Most PCMs, however, supercool excessively from the standpoint of practical heat storage systems and depend on added heterogeneous nucleators to initiate freezing.

Three classes of nucleating materials generally are recognized: isomorphous, isotypic, and epitaxial. Isomorphous and isotypic nucleators have crystal structures and lattice parameters nearly identical with the substrate salt. Isomorphous nucleators, in addition, have a chemical structure similar enough to the substrate that composite crystals can form over at least some range of compositions. Epitaxial nucleators have a different crystal morphology than the substrate, but the nucleating crystal's surface provides in its lattice structure preferred positions for deposi- tion of the substrate crystal.

In developing nucleators for a given PCM, two approaches have been successful: the "scientific" and the "Edisonian" methods. The "scientist" chooses candidate isomorphous and isotypic materials from tables of crystallographic data, and tests their nucleation efficacy. Rarely are there more than a few, if any, compounds of suitable crystal structure. And a good match of morphology does not guarantee nucleation activity.

The disciple of Edison tests many materials as nucleators, guided by intuition and by the stock of leftover chemicals on the laboratory shelf. Often he is more successful than the "scientist". In retrospect, many of the successful nucleators can be rationalized to operate by epitaxy. In some cases, no logical explanation of their effectiveness is apparent.

Several prior investigators have approached the problem of nucleation from a crystal structure standpoint. Brown and Preckshot [3] studied the effect of crystal- lographically similar additives in nucleating supersaturated KCI solutions. Telkes' classic study on PCM nucleation [4] showed that borax, Na2B40 7 • 10H20 , is an effective isomorphous nucleator for Glauber's salt, Na2SO 4 • 10H20. The present study also used known crystallographic data as a starting point.

1.2. Crystallography

Seven crystal lattice systems are used to describe and classify crystals. While the fundamental characteristic that distinguishes these systems is symmetry, it is

138 G.A. Lane / Nucleation to prevent PCM supercooling

convenient to think of them as three-dimensional coordinate systems. The lengths of the unit cell edges are designated a, b, and c, and the angle between a and b is designated a, be tween b and c, /3, and between a and c, 3'.

(1) Cubic or isometric. The three crystal axes are all equal and at right angles to one another , i.e., a = b = c (or a 1 = a 2 = a3), and a =/3 = y = 90 °.

(2) Tetragonal. Two crystal axes are equal while the third is not, a = b ~ c (or a~ = a 2 4: c); all crystal axes are at right angles, i.e., a =/3 = 3' = 90 °.

(3) Orthorhombic. The three crystal axes are unequal, a 4: b 4: c; all axes are at right angles, a =/3 = 3' = 90 °.

(4) Trigonal or rhombohedral. The three crystal axes are equal, a = b = c; none is at right angles to another , but all the angles are equal, a =/3 = y 4:90 °. This is now t rea ted in the U S A as a subsystem of the hexagonal crystal system.

(5) Monoclinic. The three axes are unequal, a 4: b 4: c; two are at right angles and the third is not. By convention, the b axis is that which forms right angles with the o ther two, so that a =/3 = 90°4: 3'; the a and c axes are selected so that y > 90 °.

(6) Triclinic or anorthic. The three axes are unequal , a 4: b 4: c; each angle is different, a 4:/3 4: y, and may have any value, though usually not a right angle.

(7) Hexagonal. Two crystal axes are equal while the third is not, a = b 4: c (or a I = a 2 ~ C); two axes are at right angles, a =/3 = 90 °, and the third angle, y is 120 ° .

For this study, the most convenient source of crystal s tructure data was found to be the " D o n n a y " series, published jointly by the US Depa r tmen t of Commerce , NIST; and the JCPDS Internat ional Centre for Diffract ion Data, Volumes II and IV [5,6]. The data are a r ranged within each crystal system by axial ratio, and lists of possible nucleators were developed by scanning the tables within a 15% variance of the axial ratio of the PCM. The lists were next fur ther restricted by eliminating exotic, unstable, reactive, toxic, etc., compounds .

2. Calcium chloride hexahydrate

High purity C a C 1 2 " 6 H 2 0 supercools excessively during freezing, render ing it useless as a phase change material (PCM) for heat storage. A variety of additives to suppress supercool ing by nucleat ing CaCI2- 6 H 2 0 was studied. Addit ives with favorable crystal structures, as well as compounds chosen intuitively, were investigated.

2.1. Crystal structure

Calcium chloride hexahydrate crystallizes in the hexagonal system, with the lattice parameters shown in table 1. A li terature search was done to identify promising hexagonal crystalline nucleators with similar lattice parameters . Fortu- nately, several candidates were uncovered. Table 1 summarizes the promising nucleators that were identified. O the r materials of isotypic structure were rejected

G.A. Lane / Nucleation to prevent PCM supercooling

Table 1 Crystal structures of CaCI2.6H 2 ° and potential nucleating additives

139

Compound a (,~) c (,~) c / a

Hexagonal system CaCI 2' 6H 2 ° 7.860 3.87 0.4924 CaBr 2 • 6H 2 ° 8.138 4.015 0.4934 CaI 2' 6H 20 8.4 4.25 0.5060

SrCI 2" 6H 2 O 7.940 4.108 0.5173 SrBr 2 • 6H 20 8.2051 4.146 0.5053 SrI 2" 6H z O 8.51 4.29 0.5041

Ba lz .6H20 8.9 4.6 0.5169

because of cost, reactivity, toxicity, unavailability, or similar reasons. Were not so many practical candidates unearthed, some of these others would have been studied.

2.2. Nucleation tests

The candidate nucleating materials were studied by comparing the freezing behaviour of the reagent grade phase change material (PCM) with and without the test additive. A sample of about 80g was prepared in a two-ounce glass vial, and 0.01-1.0% by weight of the test nucleator, if used, was added. The sample was heated to at least 10°C above the melting point, thoroughly mixed, and allowed to cool to room temperature. A glass-sheathed thermocouple, inserted through the cap of the vial, was used to obtain a time-temperature record of the cooling material. Thus, supercooling could be detected and measured readily.

For CaCI2.6H20 (m.p. 29°C), the nucleators studied were either of the isomorphous variety shown in table 1, or other materials suggested by intuition and experience.

2.2.1. Isomorphous nucleators Each of the additives listed in table 1 was tested for nucleating efficacy. Up to

ten freeze-thaw cycles were run for each nucleator. Later, more runs were made for the most effective additives. Table 2 shows the results for candidate isomorphous nucleators.

For the nucleator to be effective it should be present in the PCM as the desired hydrate. However, nucleator addition as the hexahydrate salt does not assure its persistence, since each additive will equilibrate to form the hydrate which is stable under the exisIing conditions.

In this study, neat, purified CaCI2.6H20 usually supercooled about 10-25°C. Barium iodide was the most effective nucleating additive, completely suppressing supercooling. Its lattice parameters deviate more from the ideal, 13% for axis a and 18.9% for the c axis, than the other candidates, however. It is the only barium


Table 2 Freeze test results for candidate isomorphous nucleators with CaCI2.6H20

Nucleator added Wt% Number of Supercooling (°C)

cycles Average Maximum

None - 10 22.5 23.0 CaBr 2- 6H20 0.5 4 17.4 24.0 Cal 2" 6H 2 ° 0.5 3 24.2 29.5

SrCl2.6H zO 0.1 11 9.4 23.5 SrCl 2" 6H 20 0.5 15 6.2 18.0 SrCl z" 6H 2 ° 1.0 13 0.0 0.0

SrBr 2 "6H20 0.5 10 14.0 29.5 SrBr z '6H20 1.0 12 2.1 25.5

SrI2"6H20 0.5 6 12.4 24.5 Srl 2" 6H z O 1.0 6 11.9 25.5

BaI 2' 6H 20 0.5 20 0.0 0.0

halide to form a stable hexahydrate, the peritectic point being 35°C at about 69% BaI 2 (compared with 78.35% BaI 2 in the hexahydrate). By analogy with dilute solutions, one might expect the metathetical reaction:

B a I 2 " 6 H 2 0 + CaCI2 • 6 H 2 0 ~ , BaCI 2 • 2 H 2 0 + C a I 2 • 6 H 2 0 + 3 H 2 0 , (1 )

however, in molten CaC12.6H20 the waters of hydration are probably too firmly bound.

As discussed later, BaCI 2 is a good nucleator for CaCI 2 • 6H20 , and CaI 2 • 6H20 is ineffective. These tests show BaI 2 • 6H20 superior to BaC12, however. As will be shown latter, barium salts in general seem to be effective CaCI 2 • 6H20 nucleators.

Next in effectiveness is SrCI2.6H20. At the 0.1% level supercooling was eliminated for several cycles, but then returned. At 0.5% no supercooling was observed for ten cycles, but then the additive lost its effectiveness. Doubling the SrCI2.6H20 level to 1.0% then completely suppressed supercooling.

Assarsson and Balder [7] report that SrCI 2 is soluble in CaC12.6H20 to the extent of about 0.8% (1.3%, expressed as the hexahydrate). On freezing, the solid solution SrCI2.6H 20 /CaCI z" 6H 20 crystallizes. Apparently, near-saturation with the strontium salt is needed to effect nucleation. The lattice parameters are rather close to CaCI 2 • 6H20, within 1% for the a axis and 6% for the c axis. One would expect nearly identical parameters for CaCI 2 • 6H20 and the solid solution which is being nucleated. The peritectic point of SrCI2.6H20 is 61.3°C at 46.65 wt% SrCI 2 (compared with 59.46 wt% in the hexahydrate). However, Assarsson [7] reports that in CaC12.6H20 solution SrCI2.6H20 is converted to the dihydrate above 29.4°C.

The effectiveness of strontium chloride as a nucleator for CaCI2.6H20 has been reported by Danilin and coworkers at the Krasnodar Polytechnic Institute in

G.A. Lane / Nucleation to prevent PCM supercooling 141

the USSR [8], who also disclosed the effectiveness of BaCI 2. Miyoshi and Tanaka at the Sekisui laboratories in Japan [9,10] also patented the use of SrCI 2.

Strontium bromide hexahydrate also shows nucleating ability. Addition of 0.5% reduced supercooling for a few cycles before the additive became ineffective. Doubling the nucleator level to 1.0% suppressed supercooling in all but one of 11 subsequent cycles. Lattice parameters for SrBr 2 • 6 H 2 0 are within 4% for axis a and 7% for axis b. The SrBr 2 • 6 H 2 0 peritectic point at 88°C, 68.3 wt% SrBr z, lies very close to hexahydrate stoichiometry, 60.60 wt%, giving a near-congruent melting composition. The solubility in CaCI2" 6 H 2 0 is not known, but could be close to that of the chloride, due to metathesis:

SrBr 2 • 6 H 2 0 + CaCI 2 • 6 H 2 0 ~, SrC12 • 6 H 2 0 + CaBr 2 • 6H20 . (2)

Again, however, in molten CaCI 2 • 6 H 2 0 the waters of hydration may be too firmly bound.

The above-mentioned Sekisui patents [9,10] also disclose strontium bromide as a CaCI 2 • 6 H 2 0 nucleator. Strontium iodide hexahydrate shows some nucleating properties, which do not persist. Addition of 0.5% SrI z • 6 H 2 0 minimized supercooling for a few cycles, but became ineffective. Doubling the level of additive to 1.0% gave a few more good cycles, followed by nucleation failure. The lattice parameters of SrI 2 • 6 H 2 0 are within 8% for dimension a and 11% for b of the ideal values. SrI2" 6 H 2 0 is a congruent melting composition with 75.95 wt% SrI 2, melting at 84°C. As in the case of the bromide, there may be metathesis in CaCI 2 • 6 H 2 0 solution. It is not known whether the solubility is limited by SrCI 2 or CaI 2 solubility:

SrI 2 • 6 H 2 0 + CaCI 2 • 6 H 2 0 ~ SrC12 • 6 H 2 0 + CaI 2 • 6 H 2 0 . (3)

Strontium iodide was claimed as a CaCI 2 • 6 H 2 0 nucleator in one of the Japanese patents mentioned above [9].

A low level of nucleation activity was shown for CaBr 2 • 6H20 , but this additive proved unsatisfactory. It has lattice parameters quite close to CaC12 • 6H20 , within 4% for both dimensions a and b. The two hydrates are miscible in all proportions, forming solid solutions. CaBr 2 • 6 H 2 0 itself is a congruent melting compound with 64.90 wt% CaBr 2, melting at 34°C.

Calcium iodide hexahydrate apparently displays no nucleating activity. Its lattice parameters are within 7% for dimension a and 10% for b of the desired values. It is a congruent melting material with 73.11 wt% CaI 2, melting at 42°C.

2.2.2. Non-isostructural nucleators A variety of salts, mainly alkaline earth compounds, was tested for nucleating

CaCI2-6H20 . Table 3 summarizes these results. In these experiments, neat C a C I 2 . 6 H 2 0 supercooled up to 20°C.

In general, the hydroxides, oxides, and carbonates of strontium and barium show activity as nucleators. Strontium compounds are good and barium compounds excellent. Actually, all the barium salts tested seem to have a high level of activity. Nucleation of CaCI 2 • 6 H 2 0 by barium hydroxide and strontium hydroxide


Table 3

Freeze test results for candidate non-isostructural nucleators with CaCIz .6H20

Nucleator added Wt% Number of Supercooling (°C) cycles Average Maximum

none - 10 11.9 20 Sr(OH) 2 0.1 100 2.4 14

Ba(OH) 2 0.01 24 1.9 13 Ba(OH) 2 0.05 100 0.7 15 Ba(OH) 2 0.1 100 0.2 4 Ba(OH) 2 0.5 6 0.0 0

BaO 0.1 25 0.3 3 BaCO 3 0.5 4 0.0 0 BaSO 4 0.1 572 0.2 8

BaC12 0.01 24 1.4 14 BaCI2 0.1 100 1.0 8 BaCI 2 0.5 7 0.0 0

was reported by Ishihara and Nonogaki of the Hitachi laboratories [11], and by Kai and coworkers at Mitsubishi [12,13]. Schroeder and others at N.V. Phillips reported the effectiveness of BaCO 3, SrCO3, BaF2, BaF 2 • HF, and SrF 2 [14].

Examination of the crystal structures of the nucleators of table 3 provides a limited understanding of their effectiveness. SrO and BaO have cubic structures with dimension a ranging from 4.2 to 5.5 ,~. They have no readily apparent CaC12 • 6 H 2 0 nucleating structure.

Of the hydroxides Ba(OH) 2 and Sr(OH)2, neither has an obvious nucleating feature. Strontiumohydroxide has a tetragonal structure, with c dimension of 11.58

and a = 9.00 A. B a ( O H ) 2 . 8 H 2 0 has a monoclinic structure, with a = 6.4, o

b = 7.0 A, 3' = 99. 00o. It is likely that in liquid CaC12.6H20 these additives undergo reactions, and the products are the actual nucleating species.

Reactions. In dilute solution, taking M to represent an alkaline earth metal, one would expect hydration of an oxide additive:

MO + H 2 0 --~ M(OH)2 , (4)

reaction of a metal hydroxide additive to form Ca(OH)2:

CaC12(aq ) + M(OH)2 -~ Ca(OH)2 + MCl2(aq ), (5)

or a combination of the two to yield the same products:

CaCl2(aq ) + MO + H 2 0 --* Ca(OH)2 + MC12(aq ) . (6)

For the cases where BaC12 and SrC12 are formed, they themselves have been shown to be excellent nucleators.

However, caution is advised in predicting reaction products since the well-known reactions that occur in dilute aqueous solution to not necessarily proceed in highly concentrated salt hydrates.


In the crystalline state a salt hydrate consists of an array of ions and water molecules, interconnected by ionic and hydrogen bonds, with long-range crystalline order. A mode m view of melting is that long-range order is destroyed, but that the interconnecting bonds are largely preserved. As the temperature increases above the melting point, these bonds are increasingly disrupted, and the liquid becomes more like a solution. As a consequence, the water of hydration may be unavailable or only partially available for reactions that occur readily in dilute solution.

Barium carbonate is about equal to the oxide and hydroxide for suppressing supercooling. Orthorhombic BaCO 3 has a = 6.4, b = 8.8, c = 5.3 ,~. Reaction of the carbonate with calcium chloride hexahydrate could lead to:

CaC12 • 6 H 2 0 + BaCO 3 ~ CaCO 3 + BaC12(aq) + 6 H 2 0 , (7)

CaCI 2 • 6 H 2 0 + 2BaCO 3 ~ Ca(OH)2 + Ba(HCO3) 2 + BaCIz(aq ) + 4 H 2 0 ,

(8) in both cases forming the known nucleator barium chloride. In dilute aqueous solution eq. (7) would predominate, because calcium carbonate is much less soluble than calcium hydroxide. In the molten PCM, however, it is not clear which reaction is favored.

The effectiveness of the other barium salts, the sulfate and chloride, is remark- o

able. BaSO 4 has an orthorhombic structure with a = 7.2, b = 8.9, c = 5.5 A, not an expected nucleating structure.

Barium chloride dihydrate crystallizes in the monoclinic system with a = 7.1, o

b = 10.9, c = 6.7 A, /3 = 90°34 '. One would not expect nucleation from this structure. There is a hexagonal form of anhydrous barium chloride, reported to be stable above 38°C, with a = 8.113, c = 4.675 ,~. While the a axis is within 3.2% of that of CaCI z • 6H20, the c axis is 20.8% longer, outside the 15% range usually accepted. Isotypic nucleation might be possible, and certainly epitaxial nucleation is a genuine possibility. The stability of the hexagonal form in molten PCM is not known, but not precluded. The stability of anhydrous BaC12 in the PCM at first may seem unlikely. However, calcium chloride is strongly dehydrating, as evi- denced by its dehydration of S r C I 2 . 6 H 2 0 to the dihydrate, discussed above.

2.3. Discussion

The isomorphous nucleators B a I 2 - 6 H 2 0 , SrCI2- 6H20, and S r B r 2 . 6 H 2 0 are effective additives to suppress the supercooling of CaCl 2 • 6 H 2 0 PCM. SrI 2 • 6 H 2 0 and C a B r 2 . 6 H 2 0 show weak nucleating properties.

Several other additives were tested for nucleation ability. Ba(OH) 2, BaO, BaCO3, BaSO4, BaC1E.2H20 , and Sr(OH) 2 are very effective. There is yet no completely satisfying explanation of the efficacy of these additives.

3. Magnesium chloride hexahydrate

High purity MgCI 2 • 6I-I20 supercools on freezing, reducing the usefulness as a phase change material (PCM) for heat storage. A variety of additives to suppress

144 G.A. Lane / Nucleation to preuent PCM supercooling

Table 4 Crystal structures of MgCI2"6H20 and potential nucleating additives

Compound a (~,) b (A) c (A) fl a / b c / b

Monoclinic system MgCI2.6H20 9.90 7.15 6.10 94°00 ' 1.3846 0.8531 MgBr 2 .6HzO 10.25 7.40 6.30 93o30 ' 1.3851 0.8514 Na3A1F 6 7.80 5.61 5.46 90011 ' 1.3904 0.9733 (NH 4)2 SO 4. NHaNO 3 10.31 7.95 5.91 98o59 ' 1.2969 0.7434

Ca(NH4)2(SO4)2' HzO 10.17 7.15 6.34 102045 ' 1.4224 0.8867 CaK2(SO4)2.H20 9.78 7.16 6.25 104o0 ' 1.3660 0.8735 CaC204.H20 9.89 7.26 6.25 107o00 ' 1.3623 0.8609

Orthorhombic system AgNO 3 7.32 10.12 7.0 - 0.7239 0.6915 (NH4)2SO 4 7.78 10.62 5.98 - 0.7326 0.5631 K2SO4(II) 7.46 10.08 5.78 - 0.7397 0.5730 K2CrO 4 7.61 10.40 5.92 - 0.7317 0.5692

supercooling by nucleating M g C I 2 . 6 H 2 0 was studied. Additives with favorable crystal structures, as well as compounds chosen intuitively, were investigated. Attempts were made to correlate crystal structure and hydrate stability with nucleating efficacy, and to speculate about active nucleating structures.


Magnesium chloride hexahydrate crystallizes in the monoclinic system, with the lattice parameters shown in table 4. Since the angle/3, 94°00 ', is close to a right angle, either monoclinic or orthorhombic nucleators might be successfully em- ployed. A literature search was performed to identify promising monoclinic and orthorhombic crystalline nucleators with similar lattice parameters. Fortunately, several candidates were uncovered. Table 4 summarizes the promising nucleators that were identified.

It should be noted that by convention for orthorhombic crystals the longest dimension is designated as the b axis. For monoclinic crystals the angle y between the a and c axis is not a right angle. In this instance, we need to match the orthorhombic a axis with the monoclinic b axis.

Because of cost, reactivity, toxicity, unavailability, or similar reasons, several other materials of isotypic structure were rejected. Some of these might have been studied if the more practical candidates proved ineffective.


The candidate nucleating materials were studied by comparing the freezing bebaviour of the reagent grade phase change material (PCM) with and without the test additive. In the case of reagent grade M g C I 2 . 6 H 2 0 supercooling of about


Table 5 Freeze test results for candidate isomorphous and isotypic nucleators a) with MgCI 2 • 6H20

145

Nucleator added Supercooling

Average (*C) b~ Maximum (*C) % of cycles c)

none 19.8 32 100

MgBr 2 • 6H 2 ° 21.3 33 90 Na3AIF 6 1.6 8 40 (NH4)2SO4.NH4NO3 11.1 11 100

C a ( N H 4 )2(504)2 . H 2 ° 10.0 17 100 CaK 2(504)2 ' H 2 ° 9.6 20 70 CaC 204 • H 20 0.0 0 0

AgNO 3 20.3 28 100 (NH4)2SO 4 10.2 18 100 K2SO 4 15.9 22 100 K 2CRO4 11.8 16 100

a) 0.5% additive by weight. b) 10 cycles. c) % of cycles with supercooling.

20°C is typical. Tests were conducted in the manner described above for CaCI 2 • 6H20.

For MgCl2" 6H20 the nucleators studied were either of the isomorphous and isotypic varieties shown in table 4, or other materials which intuition and experience led us to try.

3.2.1. Isomorphous and isotypic nucleators Each of the additives listed in table 4 was tested for nucleating efficacy.

Although the nucleator should best be present as the specified anhydrous or hydrated form, each additive will equilibrate to the degree of hydration which is stable under the existing conditions. Therefore, for convenience available hydrates were used rather than those listed in table 4.

Ten freeze-thaw cycles were run for each nucleator. Table 5 shows the results for candidate nucleators (listed as in table 4, rather than as added). Only the first, MgBr 2 • 6H20, is of the isomorphous variety, the rest are isotypic additives. The most effective of these nucleators is calcium oxalate, the monohydrate of which has lattice parameters a within 0.1%, b within 1.5%, and c within 2.5% of those of MgC12 • 6H20. The angle/3, however (107°), is not a good match with the desired 94 ° .

Second most effective is cryolite, Na3AIF 6. Lattice parameter deviations from the ideal are 21% for a, 22% for b, and 10.5% for c. The angle /3 is in good agreement. In view of the shrunken lattice constants, however, the nucleation activity is somewhat surprising.


Next in nucleation efficacy come several materials which are only weakly effective. C a K 2 ( S O 4 ) ' H 2 0 , Ca(NHa)2(SO4)2.H20 , (NH4)2SO4, (NH4)2SO 4 • NH4NO3, and K2CrO 4 diminish the supercooling of MgCl 2 • 6 H 2 0 by about half.

The double salt monohydrate of CaSO 4 and K2SO 4 has a very close fit of lattice parameters a (1.2%), b (0.1%), and c (2.5%), but/3 (104 °) is not a good match. We did not prepare the double salt separately before addition to MgCI 2 -6H20 , reasoning that it might form in situ if conditions favoured its stability. The separate sulfates were added in the proper proportion. It is possible that pre-preparing the double salt would give better results. Nucleation activity is not attributable to K z S O 4 or CaSO 4 by themselves, as these have been shown ineffective.

The double salt monohydrate of CaSO 4 and (NH4)zSO 4 also matches the ideal lattice parameters closely for a (2.7%), b (0.0%), and c (3.9%), but /3 (102.75 °) is only a fair fit. Again, the separate sulfates were added, not the double salt. Activity could arise from (NH4)2SO 4 itself, which shows moderate activity.

Ammonium sulfate has lattice parameters within 7.3% of the desired values for a, 8.8% for b, and 2.0% for c, a reasonable fit.

The double salt of (NH4)2SO 4 and NH4NO 3 has lattice parameters deviating from target values of 4.1% for a, 11.2% for b, and 3.1% for c. The angle/3 (99 °) is reasonable. The separate salts were added in the hope of forming the double salt in situ. Once again, activity may arise from the ammonium sulfate content.

Potassium chromate's lattice values are within 5.1% of ideal for a, 8.8% for b, and 2.0% for c. This is a reasonable match.

Potassium sulfate, with lattice parameters for the II form within 1.8% for a, 4.3% for b, and 5.2% for c, presents a favorable fit. It has only very weak nucleation properties, if any. Perhaps the orthorhombic form II is not stable in MgCI 2 • 6 H 2 0 at 117°C.

Silver nitrate appears completely ineffective in nucleating M g C l z ' 6 H 2 0 . Its lattice values deviate from the desired value by 2.2% for a, 2.4% for b, and 14.8% for c. However, as one could anticipate, upon mixing AgNO 3 with MgCI 2 • 6 H 2 0 , AgCI is precipitated and metallic silver is slowly formed by photoreduction.

M g B r 2 . 6 H 2 0 is totally ineffective as a nucleator. It is the only isomorphous material studied, and the lattice parameter fit is excellent, within 3.5% for a and b axes, and 3.3% for c, and /3 within half a degree. MgBr 2 • 6HzO melts incongru- ently at 164°C and would normally be a solid at the temperatures of this study. Its inability to nucleate is probably due to complete solubility in MgCI 2 • 6 H 2 0 at the 0.5% level tested.

3.2.2. Non-isostructural nucleators

A variety of non-isostructural materials was tested for nucleating MgCI z • 6H20. In each case 0.5% additive was used. Table 6 summarizes these results.

In general, the hydroxides, oxides, a n d / o r carbonates of strontium, barium, calcium, and magnesium show activity as nucleators. The best results were obtained with Sr(OH)2, SrCO3, Ca(OH)z, CaO, Mg(OH)2, Ba(OH): , and BaO. Examination of the crystal structures of these nucleators provides a limited understanding of their effectiveness. CaO and BaO have cubic structures with


Table 6 Freeze test results for candidate non-isostructural nucleators a) with MgCI2"6H20

147

Nucleator added Supercooling (°C)

Average b) Maximum % of cycles e~

Sr(OH) 2 0 0 0 SrCO 3 0 0 0

Ca(OH) 2 0.6 2 50 CaO 0 0 0

Ba(OH) 2 0 0 0 BaO 0.2 2 10

Mg(OH) 2 0.2 2 10 CaSO 4 16.8 25 100

none 19.8 32 100 none 20.1 24 100

a) 0.5% by weight additive. b) 10 cycles. c) % of cycles with supercooling.

o

dimensions a = 4.8 and 5.5 A, respectively, neither a good fit for nucleation. Strontium hydroxide is also a poor fit, with a tetragonal structure, a = 9.00 and c = 11.58 ,~. B a ( O H ) E . 8 H 2 0 has a monoclinic structure, with a = 6.4, b - -7 . 0 , c = 3.9 .~, and /3 = 90 °, not a promising fit; Ca(OH) 2 has hexagonal crystals, and the lattice parameters , a -- 3.6 and c = 4.9 A, do not appear favorable. Ma.gnesium hydroxide has an orthorhombic form with a = 3.8, b = 10.2, and c = 2.8 A. The b value is within 2.6% of the a paramete r of MgCl 2 • 6H20 . If doubled, the a value is within 6.1% of the b paramete r of MgCI2" 6H20 . I f doubled, the c value is within 6.8% of the target. Thus, epitaxial nucleation of MgC12 • 6H 20 by Mg(OH) 2 is conceivable.

Strontium carbonate has the orthorhombic structure with a -- 6.1, b = 8.4, and c = 5.1 A. This is, respectively, 14.9%, 15.1%, and 16.1% less than the desired parameters . These divergences are somewhat larger than usually associated with good nucleation. Another possibility is epitaxy involving matching of a 6.1 × 8.4,4, template of SrCO 3 with the 6.1 × 9.9 ,~ spacing of MgCI 2 • 6H20 .

3.3. Discussion

Several effective nucleators for MgCI 2 • 6 H 2 0 were discovered, and others were found with limited nucleating ability. The only candidate nucleator with isomorphous crystal structure, M g B r E . 6 H 2 0 , was ineffective in our tests. Effective isotypic nucleators discovered w e r e C a C 2 0 4 • H 2 0 , N a 3 A I F 6 , and N a E S O 4.

Limited activity was found for KECrO 4, (NH4)2SO 4, Ca(NH4)2(SO4)2- H 2 0 , CaK2(SO4)E .H20 , and (NH4)ESO4-NH4NO 3. The activity of the double salts


might be attributable to one of the components, e.g., (NH4)ESO 4 or CaSO 4. Surprisingly, KESO 4 has only weak, if any, nucleating power.

Other additives were uncovered with very good nucleating properties: Sr(OH) z, SrCO3, Ca(OH)E, Ca*, Ba(OH)2, BaO, and Mg(OH) z. Some of these, BaCO3, SrCO3, BaSO4, and Mg(OH) a have crystal structures compatible with isotypic or epitaxial nucleation.

4. Magnesium nitrate hexahydrate

High purity Mg(NO3)2" 6H20 (m.p. 89°C), supercools on freezing, reducing its usefulness as a PCM. A variety of additives to suppress supercooling by nucleating was studied. Additives with favourable crystal structures, as well as compounds chosen instinctively, were investigated. Attempts were made to correlate crystal structure and hydrate stability with nucleating efficacy, and to speculate about active nucleating structures.


Magnesium nitrate hexahydrate (nitromagnesite) crystallizes in the monoclinic system, with the lattice parameters shown in table 7. Also shown in table 7 are promising monoclinic crystalline nucleators, identified through a literature search, with lattice parameters similar to Mg(NO3)2.6HEO. Fortunately, quite a few candidates were uncovered. Since the angle/3 is near 90 °, orthorhombic crystalline salts with similar lattice constants also were searched. No practical possibility was found. Also, since dimensions a and c differ by only about 6%, tetragonal crystalline nucleators also were sought. Again, no practical candidate was discovered.

Table 7 Crystal structures of Mg(NO3) 2 • 6H 20 and potential nucleating additives

Compound a (.A) b (.~) c (.A) /3 a / b c / b

Monoclinic system Mg(NO3)2.6HEO 6.600 12.707 6.194 92°35 ' 0.5194 0.4874 CuSO4"3H2 O 7.34 13.03 5.59 97*06' 0.5633 0.4290 KF. 4H 2 O 6.80 13.29 6.64 90*40' 0.5117 0.4996 CuNaE(SO4) 2 -2HE* 5.805 12.675 5.52 108"36' 0.4580 0.4355 CaSO4.2HEO 6.28 15.15 5.67 114°12 ' 0.4145 0.3743 CaHPO4.2H20 6.239 15.180 5.812 116"25' 0.4110 0.3829

NaBO2.2H20 6.07 10.51 5.86 111"6' 0.5775 0,5575 MgSO4-4H20 7.905 13,604 5.922 90051 ' 0.5811 0,4353 COSO4.4H20 7.90 13.56 5.94 90°30 ' 0.5826 0,4381 ZnSO4.4H20 7.96 13.60 5.95 90"18' 0.5853 0.4375 NiSO4.4HEO 7.902 13.476 5.931 91004 ' 0.5864 0.4388 MgCIz-2HzO 8.59 14.40 7.38 114°05 ' 0.5965 0.5125

G~. Lane / Nucleation to preoent PCM supercooling

Table 8 Freeze test results for candidate isotypic nucleators with Mg(NO3)2.6H20

149

Nucleator W t % Cycles Supercooling (°C) added Average a) Maximum % of cycles

MgSO 4" 4H 2 ° 0.5 10 - 0.0 0 CuSO 4" 3H 2 ° 0.5 10 0.5 0.5 10 NaBO 2 • 2H 2 ° 0.5 10 0.5 0.5 10 NiSO 4 ' 4 H 2 0 0.5 10 0.9 1.0 40 CaSO 4.2H 20 0.5 10 1.0 1.5 30 COSO4" 4H 2 O 0.5 10 1.1 2.0 50

CuNA2(SO4)2.2H 20 0.5 10 1.9 4.0 70 ZnSO 4 • 4H 2 ° 0.5 10 2.8 4.5 90 MgC! 2 • 2H 20 0.5 10 10.9 15.0 100 KF" 4H 20 0.5 10 10.9 16.0 100 CaHPO 4 • 2H 2 O 0.5 10 13.7 16.0 100 none - 10 11.1 19.0 100

a) Average supercooling (°C) of those cycles showing supercooling.

Several other materials of isotypic structure were rejected because of cost, reactivity, toxicity, unavailability, or a similar reason. Some of these might have been studied were there not so many more practical candidates.


The candidate nucleating materials were studied by comparing the freezing behaviour of the reagent grade phase change material (PCM) with and without the test additive, using the method described previously. For Mg(NOa)2.6H20 the nucleators studied were either of the isotypic variety shown in table 7, or non-isostructural materials.

4.2.1. Isotypic nucleators Table 8 shows the results for candidate isotypic nucleators. Once again, though

the nucleator should be present as the specified hydrate form, no effort was made to introduce the additive in the correct hydrate state. The most convenient form of the chemical was added, in the hope that the hydrate stable in the presence of Mg(NO3)2.6H20 would be the form favoured for nucleation. In the case of CuNa2(SO4)2, the separate sulfate salts were added in the proper ratio.

A minimum of ten freeze-thaw cycles was run for each nucleator between room temperature and 100°C. Later, considerably more runs were made for the most effective additives. In these tests, neat Mg(NO3) 2 • 6H20 supercooled about 19°C.

Magnesium sulfate was the most effective additive, completely suppressing supercooling. This is somewhat surprising, since the desired tetrahydrate nucleator is observed as a metastable phase only above 75°C and in a narrow range of

150 G.A. Lane / Nucleation to prel,ent PCM supercooling

M g S O 4 concentrations from 38.5% to about 41%. It is possible that the tetrahydrate stability and composition range is increased in the presence of a high concentration of Mg(NO3) ~. Alternatively, hydration of a stable monohydrate solid phase:

MgSO 4 • H 2 0 + 3 H 2 0 --* MgSO 4 • 4 H 2 0 , (9)

may take place at about 90°C, facilitated by the presence of the nitrate. The tetrahydrate lattice dimensions a, b, and c are within 20%, 7%, and 4% of those of Mg(NO3) 2 • 6H20 .

Another possibility is that the heptahydrate or monohydrate acts as an epitaxial nucleator. The monohydrate, expected to be stable at 90°C at MgSO4 concentrations above 56%, forms monoclinic crystals with a = 7.5, b = 7.7, and c = 6.9 .A. The a and c dimensions are within 14% and 11% of the desired values, and the b dimension, if doubled, is within 21%. The heptahydrate is usually not stable above 48°C. Its orthorhombic structure, a = 11.9, b = 12.0, c = 6.9 A, is within 6% and 11% for b and c dimensions, and within 10% is the a dimension is halved. Similar speculations can be raised for many of the other nucleators studied.

Copper sulfate proved nearly as effective as MgSO 4. In this case, the desired phase, trihydrate, should be stable above 95.5°C and 43% CuSO4. Below this tempera ture and concentration, the pentahydrate should be stable. At high Mg(NO3) 2 concentrations, however, the expected stability range could be altered. The trihydrate lattice dimensions a, b, and c are within 11%, 3%, and 10% of ideal. Values for the pentahydrate are also reasonably close, a = 6.1, b = 10.7, and

o

c = 6.0 A, within 7%, 16%, and 4% of the desired values. However, the pentahydrate forms triclinic crystals with a = 97038 ' , /3 = 107°18% y = 77o26 '. It is hard to guess whether this diverges too much from the M g ( N O 3 ) 2 " 6 H 2 0 monoclinic structure with /3 = 92°35'.

In addition, one of several structures suggested for copper sulfate monohydrate o

is orthorhombic with a = 7.9, b = 12.6, and c = 6.8 A, within 20%, 1%, and 10% of target. One would expect it to be formed at a considerably higher temperature, however. Best guess is that the trihydrate is the active nucleating structure.

Sodium metabora te is equally as effective as copper sulfate in nucleating Mg(NO3) 2 • 6H20. The desired dihydrate has lattice dimensions within 8%, 17%, and 5% of the ideal. The most recent work indicates a triclinic crystal structure, but very close to monoclinic, with a = 91°30% and y = 89°40 '. The dihydrate is stable above 54°C and 37.85% NaBO2 concentration. The tetrahydrate, forming hexagonal crystals, is stable below 54°C. The anhydride is cubic in habit. Most probably, the dihydrate is the active nucleator.

The next rank of effective nucleators consists of nickel, calcium, and cobalt sulfates. The desired N i S O 4 . 4 H 2 0 has lattice dimensions within 20%, 6%, and 5% of the target values. It has been obtained as a metastable phase in a narrow concentration range about 97°C. The monoclinic monohydrate, which should be

o

stable above 85°C, has lattice parameters of a = 7.4, b = 7.6, and c = 6.8 A, /3 = 116053 ' . These are within 12% and 10% for a and c, and 20% for b, if doubled. Either hydrate could be the active nucleating material.


The favored C a S O 4 . 2 H 2 0 has a monoclinic crystal structure with dimensions within 5%, 19%, and 8% of those of Mg(NO3) 2 • 6H20 . Above 42°C is is metastable with respect to the anhydrous salt, and above 97°C less stable than the hemihy- drate. For anhydrous CaSO4, the orthorhombic form could nucleate epitaxially. Lattice dimensions of a -- 6.95, b = 6.96, and c = 6.21 A are within 5% and 0.3% of ideal for a and c, and 10% for b, if doubled. One monoclinic form reported for

o

CaSO 4 • H 2 0 has a -- 12.60, b = 11.88, and c = 6.85 A, within 7% and 11% for b and c, and 5% for a, if halved. The most likely nucleator seems to be CaSO 4 • 2H20 .

The preferred COSO4 • 4 H 2 0 has lattice parameters within 20%, 7%, and 4% of target. Above 80°C it exists as a metastable form in a limited composition range, the monohydrate being favoured. CoSO 4 • H 2 0 has a = 7.5, b = 7.6, and c = 7.0 A, /3 = 116°16 ', within 14% and 13% for a and c, and 19% for b, if doubled. The alpha form of the anhydride has lattice parameters of a = 6.5, b = 7.9, and c = 5.2 A, within 1% and 16% for a and c, and 24% for b, if doubled. Either tetrahydrate or monohydrate seems the probable nucleator.

CuNa2(SO4) 2 was next in nucleation effectiveness. It appears most likely, comparing its activity with that of CuSO 4, that the former is effective solely because of its content of CuSO 4, not through formation of CuNa2(SO4) 2 • 2H20 . In other words, the Na2SO 4 content is of no benefit.

Following in efficacy is zinc sulfate. ZnSO 4 • 4H 2 ° has crystal lattice dimensions within 21%, 7%, and 4% the ideal values. Above 70°C the monohydrate is the

o

stable form. The latter has a = 7.5, b = 7.6, and c = 6.9 A, within 14% and 12% for a and c, and within 19% for b, if doubled. The orthorhombic heptahydrate is also conceivably the nucleator. With a = 11.8, b = 12.1, and c = 6.9 A, its lattice parameters are within 5% and 10% for b and c, and within 10% for a, if halved. It is not certain which of these is the active nucleator.

The other candidate nucleators tested, MgCl2, KF, and CaHPO4, showed no nucleation tendencies. In the case of KF, the desired monoclinic tetrahydrate, which has lattice parameters within 3%, 5%, and 7% of target, melts congruently at 17°C, and thus would not furnish seed crystals at the PCM freezing temperature . The orthorhombic dihydrate, which melts semi-congruently at about 40°C, or the cubic anhydride, the stable form above 40°C, would not be expected to nucleate

Mg(NO3)2 " 6H20. The desired monoclinic nucleator MgCI .12H20 has lattice values within 30%,

13%, and 19% of the desired parameters , perhaps outside the workable range. In addition, it is stable only below -17°C. The stable monoclinic hexahydrate does not have promising lattice parameters . In addition, since M g C I 2 " 6 H 2 0 forms a eutectic with Mg(NO3) 2 • 6 H 2 0 (discussed below) which melts at 58°C, it would be liquid at the PCM melting point and not furnish nuclei.

C a H P O 4 • 2H20 , with monoclinic parameters within 5%, 19%, and 6% of ideal, probably does not exist in the 90°C range of interest, as dehydration proceeds at a little over 40°C. The resulting triclinic anhydride could serve to nucleate, with a = 6.9, b = 7.9, c = 6.65 A, a = 96021 ', /3 = 91°16 ', and 3' = 76 °06". Parameters are within 5% and 7% for a and c, and 10% for b, if doubled. Possibly, y may be too acute.


Table 9 Freeze test results for candidate non-isostructural nucleators with Mg(NO3) 2. 6H20

Nucleator added Wt% Number of Supercooling (°C)

cycles Average Maximum

St(OH) 2 0.1 5 0 0 SrCO 3 0.1 5 0 0

Ca(OH) 2 0.1 5 0 0 CaO 0.1 5 0 0 CaCO 3 0.1 5 0 0

Mg(OH) 2 0.1 68 0.17 1.0 MgO 0.1 66 0.12 1.5 MgCO 3 0.1 5 0 0 Mg3(PO4)2 0.1 5 2.6 7

Ba(OH) z 0.1 13 0 0 BaO 0.1 12 0 0 BaCO 3 0.1 18 0.11 2 BaHPO 4 0.1 5 5.4 11 BaSO 4 0.1 4 5.0 9 BaCI 2 0.1 3 7.7 11 BaF 2 0.1 4 4.5 6 Ba(NO3) z 0.1 5 7.2 9

Zn(NO3) 2 0.1 4 10.0 17 Fe203 0.1 4 7.8 11 none - 6 11.0 15

4.2.2. Non-isostructural nucleators A variety of salts, mainly alkaline earth compounds, was tested for nucleating

Mg(NO3)2.6H20. Table 9 summarizes these results. In these experiments, neat Mg(NO3)2" 6H20 supercooled about ll°C.

In general, the hydroxides, oxides, and carbonates of strontium, barium, calcium, and magnesium are highly effective nucleators. Certain other barium and magnesium salts seem to have a lower level of activity. Zinc nitrate and ferric oxide have little, if any, effect.

Examination of the crystal structures of these active nucleators provides a limited understanding of their effectiveness. MgO~ CaO, and BaO have cubic structures with dimension a ranging from 4.2 to 5.5 A. In liquid Mg(NO3) 2 • 6H20 there may be hydrolysis:

M g ( N O 3 ) 2 • 6 H 2 0 + MO ~ M g ( O H ) 2 + M ( N O 3 ) 2 + 5 H 2 0 . ( 1 0 )

The demonstrated nucleator, magnesium hydroxide, formed in eq. (10) is orthorhombic with a = 3.8, b = 10.2, and c = 2.8 A. The b dimension fits within 20% of ideal. I f doubled, the a and c values are within 15% and 8%. While the fit is not good, epitaxy is possible. This might account for the activity of all the oxide and hydroxide nucleators.

Of the possible metal nitrate products of eq. (10), barium, strontium, and calcium nitrate all crystallize to a cubic structure, a = 8.1, 7.78, and 7.60 ,~,


respectively, none of them a good fit for nucleation• Barium nitrate additive did show limited nucleation effectiveness per se.

No hydrated structures were found for the first two, but four hydrates were found for calcium nitrate. C a ( N O 3 ) 2 - 4 H 2 0 (nitrocalcite) is monoclinic, with a = 14.48, b = 9.16, c = 6.28 A, and/3 = 98.4 °, not a good match with the PCM. No structure is listed for Ca(NO3) 2 • 3H20. .Also of the monoclinic system is Ca(NO3) 2 • 2H20 , with a = 7.8, b = 6.9, c = 12.2 A, and /3 = 90•0 °. Both structures are close to orthorhombic, and axis b of the additive is within 11.1% of the c axis of the PCM, axis c within 4% of the b axis, and the a axes within 18%. The differences may be too great for isotypic nucleation, but epitaxial growth seems poss!ble. Another hydrate, Ca(NO3)2 .1 .24H20, is hexagonal, a = 13•23, c = 32.37 A, a poor fit.

Of the hydroxide additives, strontium hydroxide has a tetragonal structure, with c dimension of 11.58 ~,, close to the desired figure, but a = 9.00 ,~ is a poor fit.

Ba(OH) 2 • 8 H 2 0 has the desired monoclinic structure, with a = 6.4, b = 7.0 ,~, satisfactorily close, and c = 3.9 ,~, fits if tripled. Epitaxial nucleation is conceivable. However, above 80°C the octahydrate dehydrates to B a ( O H ) 2 - 3 H 2 0 , an orthorhombic material with a = 7.64, b = 11.40, and c = 5.97 A. The a axis is within 16%, the b axis within 10%, and the c axis within 4% of the PCM values. This is a good fit. Epitaxial nucleation is suggested, and isotypic growth is possible• Another hydrate, Ba(OH)2. H 2 0 , has orthorhombic (a = 6.37, b = 6.96, and c = 3.89 ,~) and monoclinic forms (a = 7.05, b = 4.18, and c = 6.33)• In either case, epitaxial growth is conceivable.

Ca(OH) 2 has hexagonal crystals (a = 3.593, and c = 4•909) with no readily apparent nucleating feature•

The hydroxide additives may react with the PCM to yield the same products as eq. (10):

Mg(NO3) 2 "6H20 + M(OH)2 --~ Mg(OH)2 + M(NO3) 2 + 6H20. (11)

The carbonates are equal to or bet ter than the oxides and hydroxides for suppressing supercooling. Orthorhombic BaCO 3 has a -- 6.4, b = 8.8, and c = 5.3 A, within 3%, 31%, and 15% of the desired value. The depar ture is too great to propose isotypic nucleation, but epitaxy is plausible.

The orthorhombic form of SrCO 3 has a = 6.1, b = 8.4, and c = 5.1 A, within 8%, 34%, and 17% of target, again too divergent, but with possible epitaxy.

Calcium carbonate exists either as the monohydrate or anhydrous salt. The hydrate displays several hexagonal crystalline forms which do not seem capable of nucleating activity. The anhydride has a multiplicity of hexagonal and orthorhombic forms, of which the most promising has a = 5.7, b = 7.9, and c = 4.9 ,~, within 13% and 20% for a and c, and within 25% for b, if doubled.

Magnesium carbonate may form in the liquid Mg(NO3) 2 • 6H20:

Mg(NO3)2 • 6 H 2 0 + MCO3 -~ MgCO 3 + M(NO3) 2 + 6 H 2 0 . (12)

Magnesium carbonate can exist as the anhydride, dihydrate, trihydrate, pentahydrate, and in several basic forms. The most likely nucleator in the group is


o r t h o r h o m b i c M g C O 3 • 3 H 2 0 , with a = 7.7, b = 12.0, and c = 5.4 .~, within 16%, 6%, and 13% of the des i r ed values. Epi taxy is conceivable with the tr icl inic d ihydra te , a = 6.2, b = 9.2, c = 6.1 A, cr = 95°32 ', /3 = 94°00 ', y = 108°42'; or with

o

the monocl in ic p e n t a h y d r a t e , a = 12.5, b = 7.6, c = 7.3 A, 13 = 100047 '. In the p r e s e n c e of large amoun t s of m a g n e s i u m salt, hydrolysis is possible:

M g ( N O 3 ) 2 • 6 H e O + 2 M C O 3

--* M g ( O H ) 2 + M ( N O 3 ) 2 + M ( H C O 3 ) z + 4 H 2 0 , (13)

forming the known nuc l ea to r m a g n e s i u m hydroxide , a meta l n i t ra te , and a meta l b i ca rbona te . A search [15] for b a r i u m of s t ron t ium b i ca rbona t e s found no structures.

M a g n e s i u m and b a r i u m p h o s p h a t e s have m o d e r a t e nuc lea t ion activity. Mg3(PO4) 2 • 8 H 2 0 has a monocl in ic s t ruc ture with a = 9.966, b = 27.71, c = 4.65 A, and 13 = 104.0 °, not a l ikely nuc lea t ing s t ructure . In Mg(NO3)2 solut ion, loss or gain of wa te r to form a lower or h igher hyd ra t e is possible. Unfo r tuna te ly , the s t ruc tures of the decahyd ra t e , p e n t a h y d r a t e , and t e t r a h y d r a t e a re not avai lable . Mg3(PO4) 2 • 2 2 H 2 0 is tr icl inic, a = 6.94, b = 6.93, c = 16.13 A, a = 82.2 °, 13 = 89.7 °, and 3' = 119.5 °, not a good match to the PCM. In Mg(NO3) 2 solut ion, pa r t i a l hydrolysis is possible:

Mg3(PO4)2 • 8 H 2 0 ~ 2 M g H P O 4 + M g ( O H ) 2 + 6 H z O

M g ( H z P O 4 ) 2 + 2 M g ( O H ) 2 + 4 H 2 0 , (14)

fo rming a known nuc lea tor , Mg(OH)2 , and monobas i c or dibasic magnes ium phospha t e . M g H P O 4 . 7 H 2 0 is monocl in ic (a = 11.61, b = 25.4, c = 6.63 ,~, and 13 = 94.87°), and not a good match , though epi taxia l nuc lea t ion is ,plausible. M g H P O 4 • 3 H 2 0 is o r t h o r h o m b i c (a = 10.21, b = 10.68, and c = 10.013 A), a poor fit. M g ( H z P O 4 ) 2 . 2 H 2 0 is monocl in ic (a = 7.29, b = 9.94, c = 5.34 A, and 13 = 95.5 ° ) with no a p p a r e n t nuc lea t ing fea ture .

B a H P O 4 crystal l izes in the o r t h o r h o m b i c form, a = 14.1, b = 17.1, and c = 4.6 o

A, not ap t to nuc lea t e M g ( N O 3 ) 2 . 6 H z O . Again , M g ( O H ) z fo rma t ion is possible:

2 B a H P O 4 + 2 M g ( N O 3 ) 2 • 6 H 2 0

2 M g H P O 4 + 2 B a ( N O 3 ) 2 + 6 H 2 0

M g ( H 2 P O 4 ) 2 + M g ( O H ) 2 + 2 B a ( N O 3 ) 2 + 4 H 2 0 . (15)

It is also conce ivable tha t monobas i c b a r i u m p h o s p h a t e s will form:

2 B a H P O 4 + 2 M g ( N O 3 ) 2 • 6 H 2 0 .-, B a ( H 2 P O 4 ) 2 + M g ( O H ) 2 + M g ( N O 3 ) 2

+ B a ( N O 3 ) 2 + 4 H 2 0 . (16)

B a ( H z P O 4 ) 2 has an o r t h o r h o m b i c (a = 10.3, b = 7.8, and c = 8.6 ]k) and a tr icl inic (a = 8.0, b = 7.0, c = 7.2 .~, a = 109.4 °, 13 = 104.5 °, and 3' = 96.0 °) form. N e i t h e r r ep r e sen t s a ma tch for the PCM.

Several o t h e r b a r i u m salts, the sulfate, chlor ide , f luor ide , and n i t ra te , showed l i t t le to m o d e r a t e nuc lea t ion activity. B a S O 4 has an o r t h o r h o m b i c s t ruc ture with


a = 7.2, b = 8.9, and c = 5.5 .~, within 8%, 30%, and 12% of ideal, probably too wide of the mark for isotypic nucleation, but perhaps allowing epitaxy.

Barium chloride dihydrate crystallizes in the monoclinic system with a = 7.1, b = 10.9, c = 6.7 .~, and /3 = 90°34 ', within 8%, 15%, and 9% of the desired axial values. BaC1E • 2HEO should be stable to over 100°C. In view of this, it is surprising that it is not a much better nucleator• Only a sight activity was observed.

Barium fluoride forms cubic crystals of a = 6.2 A. Epitaxial nucleation is conceivable.

Barium nitrate crystallizes to a cubic structure, a = 8.1 ,~. Its slight nucleation activity, if real, cannot be explained readily.

For several of these nucleation additives, metathesis is a definite possibility, forming, e.g., a metal nitrate and a magnesium salt. If the additive is insufficiently soluble in the melted PCM, little reaction will occur, while an insoluble reaction product will promote metathesis. This can have a desirable or negative result for nucleation.

For example, in the case of BaSO 4 this would form the extremely effective isotypic nucleator magnesium sulfate tetrahydrate:

Mg(NO3) 2 • 6 H 2 0 + BaSO 4 ~ MgSO 4 • 4 H 2 0 + Ba(NO3) E + 2H20 . (17)

Conversely, for barium chloride dihydrate, its promising nucleating structure is reacted away:

Mg(NO3) 2 • 6HEO + BaCI E • 2HEO (--) MgCI 2 • 6HEO + Ba(NO3) 2 + 2HEO.

(18)

Other such metathesis reactions are shown in eqs. (11), (12), and (15).

4.3. Discuss ion

The number of effective nucleators discovered for Mg(NO3) 2- 6 H 2 0 was sub- stantial, and others were found with limited nucleating ability. No candidate nucleator with isomorphous crystal structure was identified.

Several effective isotypic nucleators were discovered: M g S O 4 . 4 H 2 0 , CuSO 4 - 3H20, NaBOE. 2H20 , N i S O 4 . 4 H 2 0 , CaSO4.2HEO, COSO4.4H20 , and ZnSO 4 • 4H E O.

Non-isostructural additives were uncovered with very good nucleating properties: Sr(OH)E, SrCO3, Ca(OH)E, CaO, CaCO3, Mg(OH)2, MgO, MgCO3, Mga(PO4)E, Ba(OH)E, BaO, and BaCO 3.

5. M a g n e s i u m nitrate h e x a h y d r a t e - a m m o n i u m nitrate eutectic

A eutectic PCM freezes to a polycrystalline mixture of the separate frozen components of the eutectic. Nucleation is theoretically possible by initiating freezing of one of these components. Thus, for nucleating, e.g., the eutectic Mg(NO3) E. 6HEO-MgCIE.6HEO we tested nucleators already found effective


Table 10

Freeze test results for effective isotypic Mg(NO3) 2 .6H20 nucleators with Mg(NO3)z-6H20-NH4NO 3 eutectic


CaSO 4 • 2H 20 0.5 5 4.8 6.5 ZnSO4"4H20 0.5 5 5.6 7.0 CuSO 4"3H2 ° 0.5 5 5.9 6.5 CoSO 4- 4H20 0.5 5 6.4 8.0 NiSO 4 • 4H 2 ° 0.5 5 6.6 7.5 CuNa z(SO4)2 • 2H 2 ° 0.5 5 7.2 8.0 MgSO 4-4H 20 0.5 5 7.5 8.0 NaBO 2 • 2H 2 ° 0.5 5 9.1 10.5 none - 3 6.3 7.0

both for Mg(NO3) 2 • 6 H 2 0 and for MgCI 2 • 6H20. Success of this approach is not guaranteed, however, a fact which our results demonstrate.

The magnesium nitrate hexahydrate-ammonium nitrate eutectic, consisting of 61.5% Mg(NO3) 2 • 6 H 2 0 and 38.5% NH4NO3, melts at 52°C. The additives tested were those found effective in suppressing the supercooling of Mg(NO3)2 • 6H20. A literature search revealed no practical candidates for nucleating NH4NO 3.

Table 10 shows the results on isotypic nucleators which successfully reduce the supercooling of Mg(NO3) 2 • 6H20. The neat eutectic showed about 6-8°C supercooling in various tests. The additives tested show a low nucleation effectiveness, or none at all. Calcium sulfate may be the best. One can speculate on the causes of this poor nucleation efficiency. The desired nucleator M g S O 4 . 4 H 2 0 is not expected to be stable below 75°C. The melting point of the substrate PCM, 52°C is near the transition point of MgSO 4 • 7 H 2 0 to MgSO 4 • 6H20, 48°C. The hexahydrate structure is not promising for nucleation, but, as stated previously, the heptahydrate is conceivably epitaxial.

For CuSO4, the pentahydrate seems the most likely form at the temperature studied. As mentioned above, CuSO 4 . 5 H 2 0 could have a desirable structure, except for an acute angle, 77°26 ', between the a and b axes.

The transition point between NaBO 2 • 4 H 2 0 and the desired NaBO 2 • 2 H 2 0 is about 54°C, close to the Mg(NO3) 2 • 6HEO-NH4NO 3 eutectic melting point. In the presence of large amounts of nitrate salts this transition could change. The lack of nucleation activity, however, is not readily explained.

The previously discussion of NiSO 4 remains valid in the vicinity of 52°C. Reasons for the poor nucleation are not apparent. For COSO4, the desired tetrahydrate is not expected at 52°C. The heptahydrate to hexahydrate transition is expected at about 43°C, and the hexahydrate to monohydrate transition at about 64°C. Neither CoSO 4 • 6 H 2 0 nor CoSO 4 • 7 H 2 0 has a crystal structure that should nucleate Mg(NO3)2 • 6H20.

CaSO 4 may have the best activity of the additives discussed here, but is still only moderately effective. The previous discussion of hydrate structures is valid for the


Table 11 Freeze test results for other effective Mg(N0&.6H,O nucleators with MgPJ03)2~6H~O-NH4NO~ eutectic

Nucleator added

Sr(OI-0, Sr(OH), SrCO,

Ca(OH), Ca(OH), CaO

Mg(OH), Mg(OH), MgO

Ba(OH), Ba(OH), BaO

none

Wt%

0.1 0.5 0.5

0.1 0.5 0.1

0.1 0.5 0.1

0.1 0.5 0.5

Number of cycles

3 5 5

3 113 40

6 132 131

2 3 4

3

Supercooling PC)

Average

4.3 5.2 2.8

4.0 1.7 2.8

6.8 1.3 1.8

6.0 4.0 2.8

8.3

52°C range. In fact, nucleation by CaSO, * 2H,O seems more likely at the lower temperature.

Zinc sulfate shows a possible weak nucleation effect. In the 52°C region the hexahydrate is the most likely form. Transitions expected are ZnSO, * 7H,O to 6H,O at 39°C 6H,O to 4H,O at 63”C, and 6H,O to lH,O at 70°C. As discussed above, the heptahydrate and monohydrate could conceivably act as nuclei.

Generally speaking, the altered effectiveness of these isotypic nucleators with the Mg(NO,), - 6H,O-NH,NO, eutectic, compared with neat Mg(NO,), * 6H,O, can be attributed to different hydrated forms of the additive being present at the lower freezing point of the eutectic.

Table 11 presents the results with non-isostructural nucleators which are effective on Mg(NO,), - 6H,O. They are not nearly as efficient at reducing supercooling of the eutectic. The best are MgO and CaO. Sr(OH),, Ca(OH),, and Mg(OH), are moderately effective. SrCO,, BaO, and Ba(OH), are marginally effective.

With these additives, most of the previous speculations about epitaxial nucleation still apply. The fact that only one component of the eutectic is being nucleated could reduce the effectiveness. But in many cases there is no ready explanation for the discrepancy.

6. Magnesium nitrate hexahydrate-magnesium chloride hexahydrate eutectic

This eutectic consists of 53.0 mol% Mg(NO,), - 6H,O and 47.0 mol% MgCl, * 6H,O, and it melts at 59°C. Three groups of nucleators were investigated: effective


Table 12

Freeze test results for effective isotypic Mg(NO3)2.6H20 nucleators with Mg(NO3)2.6H20-MgC12. 6H20 eutectic


NaBO 2 • 2H 2 ° 0.5 5 2.0 2.0 CoSO 4 - 4H 2 ° 0.5 5 4.4 5.0 CuSO 4" 3H Z ° 0.5 5 4.4 5.0 MgSO 4" 4H 2 ° 0.5 5 4.4 5.0 NiSO 4 • 4H 20 0.5 5 4.8 5.0 CuNa 2(SO4)2' 2H 2 ° 0.5 5 4.9 5.0 ZnSO 4 • 4H z ° 0.5 5 5.4 6.0 CaSO 4. 2H 20 0.5 5 6.3 7.0 none - 5 11.8 16.0

isotypic Mg(NO3)2.6H20 nucleators, other effective Mg(NO3)2- 6H20 additives, and candidate crystallization initiators for MgCI2.6H20.

Table 12 shows the results with effective isotypic nucleators for Mg(NO3) 2 • 6H20. All have a moderate nucleation activity, but much weaker than with the pure PCM. Sodium metaborate is substantially better than the rest.

Table 13 Freeze test results for other effective Mg(NOa)2.6H20 nucleators with Mg(NO3)2-6H20-MgCI 2. 6H20 eutectic

Nucleator added Wt% Cycles Supercooling (°C)

Average a) Maximum % of cycles

Sr(OH) 2 0.1 10 2.2 3.0 100 Sr(OH) 2 0.5 10 0.9 1.5 50 SrCO 3 0.5 10 5.2 6.5 100

Ca(OH) 2 0.1 10 1.4 2.0 80 Ca(OH) 2 0.5 10 2.0 2.0 10 CaO 0.1 10 0.9 1.0 90 CaO 0.5 10 1.6 2.0 70 CaCO 3 0.5 10 9.5 14.5 100

Mg(OH) 2 0.5 10 1.5 - 100 MgO 0.5 10 1.4 - 100 MgCO 3 0.5 10 11.2 15.0 100

Ba(OH) 2 0.1 10 2.2 3.0 100 Ba(OH) 2 0.5 10 1.2 2.0 60 BaO 0.1 10 1.1 2.0 100 BaO 0.5 10 1.2 2.0 100 BaCO 3 0.5 1 0 5.0 5.5 100

none - 10 14.0 18.0 100

a) Average supercooling (°C) of those cycles showing supercooling.


Table 14 Freeze test results for candidate isomorphous and isotypic MgCI2.6H20 nucleators with Mg(NO3) 2. 6H 20-MgCl2 • 6H 2 ° eutectic

Nucleator added Wt% Number of Average cycles supercooling

(°c)

CaK 2(SO4)2 • H 2 ° 0.5 10 5.0 (NH4)2SO4.NH4NO 3 0.5 10 5.2 Ca(NH 4)2(504)2- H 2 ° 0.5 10 6.2 MgBr 2 - 6H 20 0.5 10 7.2 CaC 204 • H 2 ° 0.5 10 9.8 none - 1 11.8

Table 13 displays the results with other workable Mg(NO3) 2 • 6H20 nucleators• All are effective at reducing supercooling, except MgCO 3 and CaCO 3. SrCO 3 and BaCO 3 are only moderately active• None of the additives suppress supercooling of the eutectic as well as they work with the Mg(NO3) 2 • 6H20 PCM.

The previous speculative discussions of nucleator crystal structure, hydrate stability, and nucleation efficiency are applicable to the Mg(NO3)2 • 6H20-MgC12 • 6H20 eutectic.

Table 14 presents the results for candidate MgC12- 6H20 nucleators. MgBr 2- 6H20 is isomorphous with MgCIE.6H20. The others are isotypic or have an isotypic hydrate• All these materials, except perhaps calcium oxalate, have a weak to moderate activity in decreasing supercooling.

7. Summary

Using the additives discovered in this investigation, practical, non-segregating PCMs with minimum supercooling behaviour can be formulated.

References

[1] G.A. Lane, Ed., Solar Heat Storage: Latent Heat Materials, Vol. 1. (CRC Press, Boca Raton, FL, 1983).

[2] G.A. Lane, Ed., Solar Heat Storage: Latent Heat Materials, Vol. 2 (CRC Press, Boca Raton, FL, 1986).

[3] G.W. Preckshot and G.G. Brown, Ind. Eng. Chem. 44 (1952) 1314. [4l M. Telkes, Ind. Eng. Chem. 44 (1952) 1308. [5] J.D.H. Donnay and H.M. Ondik, Crystal Data Determinative Tables, Vol. 2, 3rd Ed. (National

Bureau of Standards, Washington, DC, 1973). [6] H.M. Ondik and A,D. Mighell, Crystal Data Determinative Tables, Vol. 4, 3rd Ed. (National

Bureau of Standards, Washington, DC, 1978). [7] Assarsson and Balder, J. Phys. Chem. 57 (1953) 717. [8] V. Danilin, A. Dolesov, R. Petrenko and B. Shaposhnikov, USSR Patent 568, 669, (1977). [9] H. Miyoshi and K. Tanaka, Japanese Patent Kokai 53-70, 989, (1978).


[10] H. Miyoshi and K. Tanaka, Japanese Patent Kokai 53-191, 183, (1978). [11] H. Ishihara and S. Nonogaki, German Offenlegungsschrift 2, 550, 106, (1976). [12] J. Kai, H. Kimura and K. Mutoh, Japanese Patent Kokai 51-70, 193, (1976). [13] J. Kai, H. Kimura and K. Mutoh, Japanese Patent Kokai 51-76, 183, (1976). [14] J. Schroeder and K. Gawron, German Offenlegungsschrift 2, 731,572, (1979). [15] JCPDS CD ROM Searching Program, JCPDS International Centre for Diffraction Data, JCPDS-

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phase change materials for energy storage nucleation to prevent supercooling

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