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- Composition of samples and their corresponding average liquidus temperature ( T L ), the solidus temperature ( T
- and Δ T S - relative to temperature T L and T
- O – CH 3 COONa·3H 2 O»
- Crystal lattice parameters of decahydrate sodium carbonate and trihydrate sodium acetate [10–11]
- Molecular weights of components of crystalline hydrates, g/mol
2 СO 3 ·10H 2 O – CH 3 COONa·3H 2 O The phase diagram in the system of crystalline hydrates Na 2 CO
·10H 2 O – CH 3 COONa·3H
2 O is constructed with methods of thermal analysis, this diagram is of the eutectic type. Eutectic composition is 66 wt.% Na 2 CO 3 ·10H 2 O + 34 wt.% CH 3 COONa·3H
2 O. Eutectic temperature is of –11±0.7 °C. The enthalpy of melt- ing and crystallization of mixtures relatively to liquidus and solidus lines were measured, which were used for specify the type of phase diagrams. As the eutectic composition is approached, the reduction of supercooling was established. The mixture of eutectic composition was proposed for using as a heat storage material.
melting, crystallization, phase diagram, liquidus, solidus, eutectic supercooling, enthalpy of phase transition, Tamman triangle, heat storage material.
Crystalline hydrates of sodium acetate trihydrate (AH-3) and sodium carbonate ten hydrates (KH-10) are widely used as a heat storage material (HSM) based on the phase transitions [1–7]. In this case we can predict the compositions, the most satisfying the requirements for HSM. Information about this diagram is not available in the literature. So basic aim of this paper is construction of phase diagrammes of crystalline hydrates КН-10 – АН-3, also investigation of super cooling processes and enthalpy of phase transition of mixtures as basic parameters.
Mixtures containing 0 (I), 10 (II), 20 (III), 30 (IV), 34 (V), 40 (VI), 50 (VII), 60 (VIII), 70 (IX), 80 (X), 90 (XI), 95 (XII), 97 (XIII), 100 (XIV) wt. % AH-3 were investigated. These mixtures were prepared from components of KH-10 and AH-3 (model is «pure»). Additional investigations for compositions different from the eutectic at 0.5 % were carried out in the vicinity of the eutectic composition. Samples were pre- pared according to procedure [6–7]. All samples having the same weight of 0.1 g, were placed in a test tube with ground-glass lid. Heating and cooling of the samples were performed by a resistance furnace in a tem- perature range from –25 °C to +80 °C. For this the furnace was placed in a freezer operating at a temperature of –30 °C. Heating and cooling rates were chosen approximately the same and ranged from 0.1–0.2 K/sec. Liquidus and solidus temperatures of mixtures were mainly determined by the cyclic thermal analysis (CTA) in coordinates «temperature–time», also endo- and exothermic effects were determined by differential thermal analysis (DTA). All mixtures in the system «KH-10 – AH-3» were investigated under conditions in which the individual crystalline hydrates were enough super cooling. These conditions were achieved after heating mixtures of 10÷12 degrees above the corresponding liquidus temperatures. Recording of DTA and CTA curves, also heating-cooling processes was made using meter-regulator TRM-202 from «OVEN» firm and PC. Samples of each composition were prepared not less than 3 times, the number of thermal cycles of each composition consists of at least 10. The statistical processing was carried out, and the average values of liquidus (T L ), solidus (T S ) and min- imum temperatures (T L min ) in cooling at the beginning of crystallization, crystallization enthalpy relatively liquidus (ΔH L ) and solidus (ΔH S ) temperatures were found on the basis of these experiments. Results and discussion In a first step the samples were heated and cooled in order to determine the liquidus (T L ) and the solidus (T S ) temperatures. Results of mean values of T L and T S are shown in Table 1. Construction of phase diagram … Серия «Химия». № 3(75)/2014 27 T a b l e 1 Composition of samples and their corresponding average liquidus temperature (T L ), the solidus temperature (T S ), the minimum temperature (T min ), cooling ΔT L - and ΔT S - relative to temperature T L and T S in system of crystalline hydrates «Na 2 СO 3 ·10H 2 O – CH 3 COONa·3H 2 O» Sample № Amount of АН-3 in КН-10, wt.% < T L > < T L min > <Δ T L - > < T S > < T S min > <Δ T S - > Δ
L
H S
ºC kJ/kg
kJ/kg I 0.0
32.5 19
13.5
286 0 II 10.0 20.6 9 11.6 –11.2 –14.0 2.8 247 38 III 20.0 11.2 1 10.2 –11.0 –13.8 2.8 225 59 IV 30.0 –2.5 –9
6.5 –10.8 –14.2 3.4 152 131 V(E) 34.0 –11.5 –14 2.5 –11.0 –14.0 3.0 124 124 VI 40.0 7.1 –3
10.1 –10.0 –14.0 4.0 130 151 VII 50.0 19.5 8 11.5 –10.5 –14.1 3.6 156 124 VIII 60.0 31.0 18
13.0 –11.0 –14.2 3.2 184 95 IX 70.0 38.0 22 16.0 –11.5 –14.0 2.5 200 78 X 80.0 47.0
28 19.0 –11.3 –14.0 2.7 218 59 XI 90.0 54.0 31
23.0 –11.7 –13.8 2.1 245 31 XII 95.0 57.5 22.3 35.2 –10.5 –13.5 3.0 247 28 XIII 97.0 57.8 18
39.8 –11.0 –13.5 2.5 263 12 XIV 100 58.0 –20 78.0
274
0
The liquidus and solidus lines for the system Na 2 CO 3 ·10H 2 O — CH 3 COONa·3H
2 O were constructed on evidence derived from Table 1. Obtained phase diagram is was eutectic type diagram (Fig. 1).
A = 58 ºC; B = –75;
C = 104;
D = –223 for x >
E
Figure 1. Phase diagram of crystalline hydrates «Na 2 СO 3 ·10H 2 O – CH 3 COONa·3H
2 O»
Eutectic composition consists of ~ 66 wt.% KH-10 + 34 wt.% AH-3. Line corresponding to the temper- atures T S , is practically a straight line, corresponding to an average value –11.0 ± 0.7 ºC. Obtained liquidus lines were fairly well described by the equation:
L = A + Bx + Cx 2 + Dx 3 , (1)
where A = 32.5 ºC; B = –160; C = 532; D = –1294.7 for x < x E .
The enthalpy of melting ΔH LS and crystallization ΔH SL were measured by DTA. As has already been in- timated that the measured values of melting enthalpy of pure crystalline hydrates KH-10 (286 kJ/kg) and AH-3 (274 kJ/kg) were close to the literature data: it is 286.6 kJ/kg for КН-10 [8, 9], and 280.0 kJ/kg for AH-3 [6]. Bifurcate of exothermic effects of crystallization of mixtures relative to the lines of the liquidus T L and
solidus T S was established. DTA-curves of compositions with 10 wt.% (II) and 70 wt.% (IX) were given as Sh.K.Amerkhanova, V.D.Alexandrov, А. 28
examples (Fig. 2). Each curve has tw the crystallization occurs in the regio Investigations have shown that enthalpy of crystallization, i.e. ΔH L
LS ≈ ΔH SL .
а Figure 2. DTA-curves of m Such bifurcating of thermal ef take place in two stages: the first st the hypoeutectic region, and AH-3 c Then this process is terminate And the residues of these molecule hydrates in the form of eutectic whe Average values ΔH L and ΔH S s tion according to Tamman triangle m Figure 3. Dependence of th The correlation of eutectic com seen from figure 3. Stability of the t sample of eutectic composition and The results showed some decr value with further thermal cycling u and ~5 % for AN-3, and ~ 3 % for t be due to partial dehydration of aqu that the eutectic composition compa Yu.Sobolev Вестник Караг wo peaks of exo-effects, and the main proportion on near the liquidus temperature. t the sum of the enthalpies ΔH L and ΔH S approxi
+ ΔH S ≈ ΔH SL , which in turn is equal to the me b melting and crystallization for compositions (II) — а a ffects ΔH LS on ΔH L and ΔH S may mean that the tage is relative to liquidus line when crystals of crystals in the hypereutectic region. ed due to depletion of solutions in molecules Na es in their own water of crystallization form fine en approaching the eutectic temperature. show in table. Data of ΔH S were used for refinem method [8] (Fig. 3).
he enthalpy ΔH S of components’ concentration in the K mposition with data shown in the diagram (Fig. 1) thermal effects values ΔH LS and ΔH SL to prolong pure crystalline hydrates KH-10 and AH-3 was c rease in value of ΔH SL after 10 cycles initially, a up to 100 thermal cycles. The decrease value of Δ the eutectic composition. The decrease of enthalp ueous solutions with repeated thermal cycling. It ares favorably to some crystalline hydrates. гандинского университета n of the thermal effect in imately equal to the total elting enthalpy ΔH LS , i.e.
and (IX) — b crystallization mixtures KH-10 begin to form in a 2 CO 3 and CH 3 COONa.
e mixed crystals of both ment of eutectic composi- KH-10 – AH-3 in the range of 2–3 % is ged thermal cycling on a hecked.
and then almost constant ΔH SL is ~17 % for KN-10 py of crystallization may is observed, in this case, Серия «Химия». № 3(75)/2014 At the third stage the precrysta mixtures) and the melting temperatu peratures in hypothermia field T L
CTA method. The average values < liquidus line ΔT L - = T L – T L min and
<ΔT L
> from concentration of x in th
Figure 4. Dependence The figure shows, that as the m larly reduce: in hypoeutectic field fr tic field from ~78 °C (for pure trihy mixtures the hypothermia ΔT S - wit <ΔT S
> ranged 3.5 ± 0.5 ºC and did vides a comparative graphs of dep compositions: (I), (V) and (XV).
Figure 5. Dependence of t As seen in Figure 4, the eutec overheating, which makes it more at The crystal structure of the te Both crystalline hydrates have mon Constr
allization hypothermia ΔT L of samples relatively l ure of pure crystalline hydrates was studied. For n , in which the spontaneous crystallization was <T L
> are shown in Table. It also gives the hy solidus line ΔT S -
S – T S
. Dependence of th he mixture is shown in Figure 4. of average hypothermia of mixture from concentration mixture composition to approach the eutectic (poin rom ~13 °C (for pure decahydrate of sodium carb ydrate of sodium acetate) until ~3÷4 ºC in eutect th respect to solidus temperature T S were recor dn’t depend on prior overheating of the liquid p endence of hydrothermia from pre-overheating
f hidrotermia from overheating for three compositions the eutectic composition (V), AN-3 (XV) ctic composition has the lowest hydrothermia w ttractive for use as HAM, acting at low temperatu
st substances (Fig. 6) is considered for the inter noclinic type of crystalline lattices [10–11]. Acco ruction of phase diagram … 29 liquidus temperature (for this, the minimum tem- began, was recorded by ypothermia relatively to
n of AN-3 nt E), hypothermia regu- bonate), and hypereutec- tic. In addition to all the rded. Average values of phase ΔT
. Figure 5 pro- of liquid phase to three s KH-10 (I), which doesn’t depend on ures.
rpretation of the results. ording to classical ideas, Sh.K.Amerkhanova, V.D.Alexandrov, А.Yu.Sobolev 30
Вестник Карагандинского университета mixtures of similar substances should lead to the formation of a continuous series of solid solutions. Howev- er, in this case two similar crystalline hydrates form mixtures of eutectic type.
а b Figure 6. Crystal lattices Na 2 СO
·10H 2 O ( a ) and CH
3 COONa·3H
2 O (
b ) Consider the lattice parameters of the crystalline hydrates KH-10 and AN-3 are given in Table 2. From data shown in Table 2, the difference between the lattice parameters of the KH-10 and AN-3 is not so great. For example, the volume of cells unit difference is only 4.6 %. Same syngony of crystals with similar ratios of parameters of lattices at best could lead to the formation of limited solubility of the compo- nents.
T a b l e 2 Crystal lattice parameters of decahydrate sodium carbonate and trihydrate sodium acetate [10–11]
Lattice type Coordination number a , Å
b , Å
c , Å
β, ° V, Å
3
КН-10 Monoclinic 4 12,83
9,03 13,44
123 1305,31
АН-3 Monoclinic 4 12,34 10,45 10,41
111,65 1247,8
Parameters differences 4,0 %
13,6 % 29,1 %
10,2 % 4,6 %
The possible reason of formation eutectic mixtures in the system of crystalline hydrates Na 2 СO
·10H 2 O and CH 3 COONa·3H 2 O can be a significant difference between the structure of molecules, the content of crystallization water in corresponding hydrates, molecular configuration Na 2 СO 3 ·10H
2 O and
CH 3 COONa·3H 2 O and quantities of hydrogen bonds. You can see this if you look at the projections of the lattices of these materials (Fig. 7).
а b Figure 7. Projections of the crystal lattices on the x –
plane of decahydrate sodium carbonate (
)
and trihydrate sodium acetate ( b ). The dotted lines show the hydrogen bonds [10–11]
Construction of phase diagram … Серия «Химия». № 3(75)/2014 31 By incongruent melting of KH-10 and AN-3, there are saturated salt solution Na 2 СO 3 and CH 3 COONa in their own crystallization water, i.e. their dehydration is by scheme CH 3 COONa·3H 2 O → CH
3 COONa +
+ 3H 2 O and Na 2 СO 3 ·10H 2 O → Na 2 СO 3 + 10H 2 O. Upon cooling these solutions occurs a reverse process — the water molecules attaching to the respective ions. For solutions in hypoeutectic region dominated Na 2 СO 3
molecules, so KN-10 crystals begin to form at the liquidus temperature, and in hypereutectic region AN-3 crystals form, because of the predominance of CH 3 COONa molecules. At the same molecules of H 2 O in re-
spective crystals form the complex of hydrogen bonds (see dashed lines in Fig. 7). For example, acetate ions form clusters (in the x–y plane, four ion into cluster), and methyl groups are oriented inside the cluster, but carboxyl groups are to outward. With regard to crystallization water in turn to molecular weights of hydrates M and components: anhy- drous salts of М 1 and water М 2 . Their comparison shows (Table 3) that the water amount in the CN-10 is more than water amount of the AN-3 is approximately in 2.6 times. The relative water content in the AN-3 is ~39 %, and RH 10 is ~63 %. The good correlation is observed when comparing these dates with the percent- age of AN-3 and KH-10 in the eutectic (~34 wt.% AN-3 + 66 wt.% KH-10). T a b l e 3 Molecular weights of components of crystalline hydrates, g/mol Crystalline hydrates M of hydrate M 1 of dry salt M 2 of water М 1 /М М 2 /М М 1 /М 2
Na 2 СO 3 ·10H
2 O 286
106 180
0,37 0,63
1,70 CH 3 COONa·3H 2 O 136 82 54 0,61 0,39
0,66 Eutectic of 66 wt.% KH-10 + 34 wt.% AN-3 236 98,84 137,16 0,42 0,58 1,40
Having a large amount of total crystallization water in mixtures of KH-10 and AN-3 in the range to ~80 %, which facilitates mobility of molecules of Na 2 СO 3 , CH
3 COONa and H 2 O, their association and for- mation of crystal hydrates, which helps to reduce hypothermia. For example, in the eutectic water content is 58 % in a mixture with crystalline hydrates, and subcooling reaches a minimum value (~3÷4 K). At high concentrations (> 80 %) of AN-3 in the system, the mobility of molecules (especially CH 3 COONa) in solu- tion with a low water content creates steric interferences with respect to crystallization and sharply increases supercooling of these solutions, including clean trihydrate sodium acetate. Conclusion The phase diagram of two crystalline hydrates: carbonate and sodium acetate is constructed by methods of thermal analysis. The thermal effects of melting and crystallization, and pre-crystallization supercooling of mixtures in the system were investigated. Based on the analysis of the obtained results it is proposed to use a mixture of eutectic composition (66 wt.% of Sodium carbonate + 34 wt % of sodium acetate) as a heat accumulating material.
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Вестник Карагандинского университета 11 Efremov V.A., Endeladze N.O., Agre V.M., Trunov V.K. Refinement of the crystal structure of sodium acetate Trihydrate // Journal of Structural Chemistry — Translated from Zhurnal Struktornoi Khimii. — May–June, 1986. — Vol. 27, No. 3. — P. 177– 180.
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