Vol. 4 No.2/ Feb. 2010
硫酸钠水合盐在微结构中的相变
Phase Change of Confined Sodium
Sulfate Hydrates in Micro Regions
吴晓琳　孙蓉　于淑会　王依海　杜如虚
摘 要 无机水合盐作为固液相变材料因具有高的储能密度特性而受到关注。然而，其应用受到了限制。因为它存在过冷
和相分离的缺陷。为了解决这些问题，我们提议，通过添加硅胶的方法实现防过冷及相分离。硅胶通过原位聚合制备而
成，并应用在水合盐饱和溶液中。通过实验发现，硅胶中的无机水合盐的潜热为238.1 J•g-1，相变温度为30℃。通过某
种表面活性剂的添加，材料在亚热带的春夏季可以经受冷热循环五个多月。通过扫描电镜和傅立叶变换红外光谱仪检测发
现，硅胶的微结构主要是无定型的。并发现，存在一些小的晶体分布在硅胶中。我们相信，硅胶的加入提供了多孔结构。
这种结构有利于硫酸钠水合盐的晶体生长，因此，可以实现对无机水合盐的防过冷及相分离的效果。
关键词 相变材料；水合盐；硅胶；过冷；相分离；晶体
ABSTRACT Hydrate salt is an attractive solid-liquid Phase Change Material (PCM) because of its high energy storage
density. However, its applications have been limited owing to the supercooling and phase segregation. In order to solve
these problems, we propose to add silica gel, which provides in in-situ-polymerization for saturated solution of sodium
sulfate. According to our experiments, the latent heat of saturated solution of sodium sulfate in silica gel is about 238.1 J•g-1
and the phase change temperature is 30oC. With some surfactant, it can endure more than five months of heating-cooling
cycles stably in subtropical spring and summer. Using Scanning Electron Microscope (SEM) and Fourier Transformation
Infrared (FTIR) Spectroscopy, it is found that the microstructure of silica gel mainly is amorphous with some small
crystals distributed in it. We believe that adding the silica gel produces a porous structure, which helps the crystal growth
of sodium sulfate hydrate and hence, suppress supercooling, and phase segregation.
KEYWORDS Phase Change Material (PCM); hydrated salt; silica gel; supercooling; phase segregation; crystal
1 Introduction
I
needed to diminish supercooling and phase separation.
It is possible to use direct contact heat transfer between
an immiscible heat transfer fluid and the salt hydrate
solution without additives [8, 9]. The agitation caused
by the heat transfer fluid minimized the subcooling
and prevented phase segregation. However, several
controlling systems were needed. Until now, there is no
effective method to solve the problems of supercooling
and phase segregation. Moreover, the mechanics of phase
transition of hydrate salt is not well understood.
In this paper, we investigate the phase transition of
salt hydrate with silica gel. The rest of the paper is
organized as follows: Section 2 describes the materials
and methods. Section 3 presents the experiment results.
Finally, Section 4 contains conclusions.
n recent years, as the energy cost is skyrocketing,
phase change material (PCM) is attracting much
renewed attention. PCMs use latent heat to store energy
and hence shall have high energy storage density and
small temperature variation in the process of storing
and releasing heat energy. Various organic, inorganic,
polymeric and eutectic PCMs have been studied [1–4].
Among them, salt hydrate is one of most attractive
PCMs owing to its moderate cost, easy production,
high volumetric energy storage density, high thermal
conductivity, and environment safety. However, its
applications have been limited by the problems of
supercooling and phase segregation.
In order to solve the problems of supercooling and phase
segregation, many studies have been done. Biswas [5],
for example, suggested the use of the extra water to
prevent the formation of the heavy anhydrous salt. Telkes [6]
introduced borax as a nucleating agent to minimize
subcooling. The use of some thickening agents [7], such
as polymer and gel, has been suggested to overcome the
phase segregation. In general, at least two additives are
2　Experimental Method
I
n our experiments, the raw materials used include
Na2SO4, Na2SiO3 and H2SO4. Na2SiO3 is dissolved into
saturated Na2SO4 solution at 40 ℃ (All the raw solution
40
硫酸钠水合盐在微结构中的相变
used in our experiments are saturated Na2SO4 solution.).
Through chemical reaction of Na 2 SiO 3 with acid,
silica sol is obtained. After ripening it for a moment,
homogeneous silica gel is obtained and saturated Na2SO4
solution could be dispersed in the silica gel uniformly.
Two methods were used to test the long-time cycling
stability of silica gel. One was natural heating-cooling
process which worked by the temperature change of a
day in the subtropical spring and summer in the lab (the
lab is located in Shenzhen, Guangdong, China, a typical
subtopic region where the temperature ranges from 18 ℃
to 35 ℃). The other was forcible heating-cooling process
which worked by two prepared constant temperature
water baths (one was at 10 ℃, the other was at 40 ℃).
0
o
T/ C
10
1
0
00
1000
100
000
00
000
00
t/s
Fig. 1 The t-T curves of saturated sodium sulfate solution
with and without
Sample silica gel 1
2
3
Testedphase
cycle times
5
10
> 10
Table 1 summarizes the
change temperature
and
Phase change
latent heat of the 7 samples.
From the table,
it
is
seen
that
17.5
27.5
30.5
temperature(ć)
except Sample 1, the other
samples
have pretty much the
-1
Latent heat(JΦg )
90.2
108.2
210
0
0
o
T/ C
0
1
1
10
10
0
4
5
> 10
> 10
29.0
29.9
186.2
238.1
o
T/ C
1- hydrated salts without additives
- hydrated salts with silica gel
T/ C
0
same characteristics.
To test the long time stability of saturated sodium sulfate
1.
after 10 times cycles
solution with silica
gel, the same
using
. sample
after 0 times
cycles forcible
.
after
0
times
cycles
heating-coolingmethod was tested in many cycles.
salts without additives
Fig. 21-- hydrated
is the
t-T
of a sample. Curves 1, 2 and 3
hydrated
salts with
silicacurves
gel
after 10, 20 and 30
1
are the t-T curves
heating-cooling
cycles respectively.
Each 10 heating-cooling
cycles is
completed in a day.
Then, the next 10 heating-cooling
cycles are done after 10 days. It is seen that the phase
change temperature is stabilized
30 ℃,
and the
at process
t/s
is very stable.
o
1
1
00 0010001000100100 000
00
000
000
00
000
0
t/s t/s
00
00
Table 1. Summary of the experiment results
Sample
Sample
11
22
33
Tested
cycle
times
Tested
cycle
times
Phase change
Phase
change
temperature(ć)
temperature(ć) -1
Latent heat(JΦg )
Latent heat(JΦg-1)
55
10
10
>>1010
o
27.5
27.5
30.5
29.0
29.9
29.5
108.2
210
186.2
238.1
139.7
90.2
108.2
30.5
210
7
6
90.2
7
29.0
186.2
29.9
238.1
29.5
32
32
182.4
139.7
182.4
o
T/ C
6
1
1
o
T/ C
o
T/ C
5 5
17.5
after 0 times cycles
after
10 times cycles
after 0 times cycles
after 0 times cycles
after 0 times cycles
1.
.
.
.
4 4
> 10
> 10 > 10> 10 > 10> 10> 10 > 10
17.5
after 10 times cycles
1.
.
T/ C
o
0
1
t/s
Fig.
2
t/s
t/s
Fig. 3
t/s
Fig. 2: The t-T cooling curves of saturated sodium sulfate
with silica gel in different cycles
Fig. 3: The t-T curve of saturated sodium sulfate hydrate
with silica gel and surfactant
3　Results and Discussions
3.1 The Time-temperature Characteristics
of the Sodium Sulfate Hydrate
Fig. 1 shows two time-temperature (t-T) curves. The
first one corresponds to the saturated solution of sodium
sulfate without silica gel. The phase change temperature
is about 28℃. Though, the supercooling to 13℃ is
obvious. The second curve corresponds to the saturated
sodium sulfate solution with silica gel. The phase
change phenomenon is clearly seen at 30℃ and there is
no supercooling. It is also found that after 30 heating-
1- hydrated salts without additives
- hydrated salts with silica gel
T/ C
We made a simple device to record the temperature
in time. The samples were placed on test tubes with a
thermal couple in the center. The tubes were initially
heated to 40 ℃ to reach the liquid phase. Then the
tubes were immersed into a 10 ℃ water cooler. The
temperature histories were recorded using a multipoint
recorder (IDAQ-8018+) and the data were stored in a
computer via an RS-232 port.
In order to analyze the phase transition in microscopic
level, Differential Scanning Calorimeter (DSC) was
used with nitrogen atmosphere. Samples were placed in
a sealed aluminum pan with a mass of about 15.00 mg.
The pan was placed in DSC and analyzed from 10 to 60
℃ with a linear heating rate of 7 ℃ /min. The nitrogen
flow rate was 50 ml/min. The accuracy of the DSC
temperature measurements is ± 0.02 ℃. The latent heat
was calculated as the total area under the peaks of solidliquid transitions.
To study the microstructure of the samples, we used
Scanning Electron Microscopy (SEM) and Fourier
Transformation Infrared (FTIR) Spectroscopy. For SEM
analysis, the surface of sample was first coated by gold.
SEM images were acquired on a HITACHI S-4700
microscope with a constant ambient temperature at 22
℃. For FTIF spectroscopy, viscous samples were mixed
with KBr at 10 ℃. Then the functional groups of samples
were obtained.
cooling cycles, the silica gel is still homogeneous and
stable without phase separation.
41
To further improve the long-time recycling stability of
silica gel, span 80 as a surfactant was added. Fig. 3 is the
t-T curve of a sample. It can be seen that the surfactant
does not affect the phase change characteristics.
Fig. 4(a) is the picture of saturated sodium sulfate
hydrate without additives. The sample has clear phase
segregation and bulky crystals can be observed at the
bottom of the test tube. Fig. 4(b) is the saturated sodium
sulfate hydrate in silica gel with surfactant. The natural
heating-cooling method is used to test the long-time
stability of the sample. After five months, it is seen that
1
.1°C
-
10
-1
-1
-
10
0
Temperature (°C)
Exo Up
Transmittance/%
Universal V4.A TA Instruments
Wavenumber/cm
-1
Based on the testing results above, it is clear that
adding silica gel can suppress supercooling and phase
segregation of hydrated salt. The main reasons are firstly,
the complex structure of silica gel can seed the saturated
solution of sodium sulfate during the solidification
providing effective crystallization. Secondly, the
existence of the stable micro-regions between the
crystals can hinder convection and hence, decrease
dynamic resistance of the saturated solution of sodium
sulfate. Thirdly, as Rosa [12] reported, the Gibbs energy
necessary for heterogeneous nucleation was diminished
by the presence of foreign substances. Therefore, the
phase change process can occur homogeneously with
less impetus.
Also, silica gel is a good thickening agent. The saturated
hydrate salt solution is dispersed homogeneously in
the micro regions, which effectively reduces the effect
of gravity. Thus, phase segregation can be reduced. It
may be argued that those micro regions are connected.
Though, the densely packed structure of silica gel
minimizes the connection channels between the adjacent
micro regions. In other words, the hydrate salt in each
micro region can be considered as a single system. Thus,
0
Fig. 6 shows the SEM images of a sample situated
sodium sulfate hydrate with silica gel, magnified by
10,000, 30,000 and 200,000 times respectively. Fig.
6(a) shows the micro structure is complex. The crystals
are randomly packed with many pores about several
micrometers in between. Fig. 6(b) shows a couple of
typical needle-shaped crystals and their lengths are about
10 µm. Fig. 6(c) zooms in a pore, which is full of tiny
holes about 20 nm.
3.5 Discussions
3.3 SEM Results
Fig. 7 FTIR spectrum of the salt hydrate with silica gel
Fig. 5 DSC curve of a sample hydrate salts in silica gel
The above analysis indicates that there should be two
kinds of components in silica gel with hydrate salts:
amorphous silicon dioxide and crystal silicon dioxide.
This would be the reason that with the silica gel, the
bond strength is improved.
.1°C
Wavenumber/cm
-1
Temperature (°C)
.1°C
40.14°C
Exo Up
-1
1
0
40.14°C
––––––– hydrated salts
––––
hydrated salts in silica gel
10
(c)
9.°C
.1J/g
1.41min
.4°C
-0.1W/g
9.0J/g
1.41min
.4°C
-0.1W/g
9.0J/g
(b)
0
Wavenumber/cm
Fig. 5 shows the DSC curves of samples with and
without additives. The curve with solid line depicts the
peak of heat absorption of saturated hydrate salt without
additives. The latent heat can be measured by the area
of the peak. It is only 96.7 J•g-1 and the phase change
temperature is 25.4 ℃. The curve with dash line is the
peak of heat absorption of saturated hydrate salt with
additives (silica gel). It is seen that the latent heat is
increased to 238.1 J•g-1 and the phase change temperature
is 29.9 ℃. This indicates that the addition of silica gel
improves the heat energy storage capability.
1
3.2 The DSC Results
9.°C
.1J/g
α-cristobalite [11]. The band at 1618.22 cm-1 is associated
to the bending of the absorbed H2O molecules. The band
at 1448.54 cm -1 is attributed to the stretching (S=O)
groups. The band for stretching (OH) groups is found
at 3477.66 cm -1. From these observations, no other
bonds are formed. So, it can be concluded that sodium
sulfate hydrate should be lying in the matrix without any
interaction with silica gel or only with the interaction of
hydrogen bond connecting.
(b)
Fig. 4: Pictures of sodium sulfate hydrate
(a) saturated solution without additives;
(b) saturated solution with silica gel
-
––––––– hydrated salts
––––
hydrated salts in silica gel
Fig. 6 SEM images of a sample hydrate salt with silica
gel: (a) ×10,000; (b) ×30,000; (c) ×200,000
(a)
Heat Flow (W/g)
(a)
Transmittance/%
0
Universal V4.A TA Instruments
0
Temperature (°C)
the sample is still without phase segregation. This also
t/s
t/s
t/s
t/s
shows the stability of the samples.
Vol. 4 No.2/ Feb. 2010
0
Exo Up
Heat Flow (W/g)
-1
40.14°C
1
1.41min
.4°C
-0.1W/g
9.0J/g
0
1
Heat Flow (W/g)
Transmittance/%
1
o
T/ C
o
1.
after 10
after
times
10 cycles
times cycles
.
after 0
after
times
0 cycles
times cycles
.
after 0
after
times
0 cycles
times cycles
1.
.
.
T/ C
o
o
T/ C
T/ C
3.4 FTIR Spectroscopy Results
Fig. 7 shows the FTIR spectroscopy of a sample hydrate
salt with silica gel in solid phase. The absorption bands
at 470.63 and 1099.43 cm-1 are associated to the bending,
symmetric and asymmetric stretching of amorphous SiO2
[10]
. The band at 617.22 and 796.60 cm-1 are associated to
42
0
Universal V4.A TA Instrument
硫酸钠水合盐在微结构中的相变
silica gel can hinder crystal growth. The small crystal
of hydrate salt with large surface area can contact to the
water around and hence, improve the stability. Therefore,
the porous structure of silica gel is effective. In fact,
comparing to the results obtained by other researchers,
the gel-salt hydrate system is more stable and the results
are comparable.
Na2CO3•10H2O solution, Energy 2004, 29: 2573-2583.
[9]
[10] Bruni S, Cariati F, IR and NMR study of nanoparticle-support
interactions in a Fe2O3–SiO2 nanocomposite prepared by a sol–
gel method, Nanostructured Materials 1999, 11: 573–586.
4　Conclusions
[11] Chen HS, Ji SF, Vibration spectroscopy on transformation of
amorphous silica to –cristobalite, ACTA Physico-Chimica Sinica
1999, 15: 454-457.
T
his paper presents a new process for making
sodium sulfate hydrate with silica gel. Based on the
discussions above, following conclusions can be made:
(a)Adding the silica gel helps to suppress supercooling
and phase separation.
(b)The latent heat of the sodium sulfate hydrate with
silica gel is about 238 J•g-1, its phase transition
temperature is around 30 ℃ and its life is more than
5 months.
(c)The silica gel can seed the saturated sodium sulfate
solution during the solidification process. Its stable
pores structure generates many micro regions which
can hinder convection, reduce thermal resistance and
improve the uniformity of the hydrate salt. Therefore,
it is effective in preventing supercooling and phase
segergation.
The future work includes the study of the mechanics of
the phase change and the development of an effective
process to produce stable PCM.
[12] Rosa ME, Lutz F, Phase changes of salts in porous materials:
Crystallization, hydration and deliquescence, Construction and
Building Materials 2008, 22: 1758-1773.
作者简介
吴晓琳 2007级博士研究生，硕士毕业于大连交通大
学材料(化学方向)专业。研究方向：微纳米
水合盐相变材料。
孙　蓉 作者简介见本期封2页。
于淑会 博士，2005年毕业于新加坡国立大学。研究
方向为功能薄膜材料、纳米复合材料、功能
陶瓷及电子器件，目前致力于面向高密度封
装的关键材料及嵌入式器件的研发和应用。
王依海 理学博士，物理化学专业，现任职于材料实
验室。研究方向：储能微结构单元及应用；
二维界面组装和表面处理技术；微纳米金属
材料的制备和应用；功能高分子复合材料的
制备和性能研究。
5　Acknowledgement
T
he presented research is partially supported by
a research grant from Shenzhen/ Hong Kong
Innovation Circle Fund.
杜如虚 作者简介见本期封2页。
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43