Accepted Manuscript Title: Synthesis and Characteristics of Hygroscopic Phase Change Material: Composite Microencapsulated Phase Change Material (MPCM) and Diatomite Author: Zhi Chen Menghao Qin Jun Yang PII: DOI: Reference:
S0378-7788(15)30008-6 http://dx.doi.org/doi:10.1016/j.enbuild.2015.05.033 ENB 5881
To appear in:
Received date: Revised date: Accepted date:
25-1-2015 19-5-2015 22-5-2015
Please cite this article as: Z. Chen, M. Qin, J. Yang, Synthesis and Characteristics of Hygroscopic Phase Change Material: Composite Microencapsulated Phase Change Material (MPCM) and Diatomite, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.05.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and Characteristics of Hygroscopic Phase Change Material:
Composite Microencapsulated Phase Change Material (MPCM) and
Zhi Chen, Menghao Qin*, Jun Yang
Division of Building Physics, School of Architecture and Urban Planning,
Nanjing University, Nanjing 210093, China
Tel: +86 25 83593020
*Corresponding author: Prof. Menghao Qin
E-mail address: [email protected]
Abstract: This paper prepared a new kind of hygroscopic phase change material using
MPCM and diatomite. The composite can absorb/release not only thermal heat, but also
moisture. The shell material of MPCM was prepared with methyl triethoxysilane
(MTES) by sol–gel method, and a kind of alkane mixture was used as the core material.
The diatomite was used as hygroscopic material. The morphology of the microcapsules
and the diatomite were measured by the scanning electron microscopy (SEM). The
thermal properties of MPCM and the composite MPCM/diatomite materials (CMPCM)
were analyzed with differential scanning calorimetry (DSC). The thermal gravimetric
analysis (TGA) was used to study the thermal stability of MPCM and CMPCM. The
moisture transfer coefficient and moisture buffer value (MBV) of the diatomite and
CMPCM were measured. The DSC results showed that the microcapsules were
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encapsulated in the SiO2 shell. The TGA results showed that the microcapsules and
CMPCM have a good thermal stability. The measurements of the moisture transfer
coefficient and moisture buffer value (MBV) of CMPCM, diatomite, gypsum board and
wood showed that CMPCM has a better hygroscopic performance. The hygroscopic
phase change material can moderate both the indoor temperature and moisture.
Keywords: Microencapsulated phase change material; Silicon dioxide shell; Diatomite;
Thermal energy storage material; Hygroscopic material; Building energy conservation
With the development of economy, energy demand is increasing quickly.
Conventional fossil energy sources are limited, and the use of them lead to climate
changes and environment pollution. Buildings account for about 40 percent of the
world’s total energy consumption, and more than 30% of the primary energy consumed
in buildings is for the heating and air-conditioning system . In order to ensure
adequate supplies of energy and to curtail the growth of CO2 emissions, it is essential
that building energy consumption is significantly reduced [2,3]. One way this can be
achieved is through the introduction of passive building design enabled by innovative
sustainable building materials, for example the hygroscopic phase change material
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The indoor temperature, relative humidity and air quality are important
environmental parameters of human comfort. To control the indoor environment at a
comfort level, the internal sensible and latent heat loads must be removed by either
active technologies (for example: air-conditioning, dehumidification system etc.) or
passive strategies (for example: hygroscopic thermal energy storage materials, natural
ventilation etc.) The research of coupled heat and moisture transfer in buildings and
using passive strategies to create comfort indoor environment are timely and important
research topics [4-6].
The ambient temperature during a day is fluctuant. In summer, the temperature in
daytime may be high during the daytime, so it’s not comfortable. But at night the
temperature drops below the comfort temperature range. Phase change material (PCM)
can absorb thermal energy when the temperature is high, and release thermal energy
when temperature is low. The temperature of the phase change material is a constant
during the phase change process. So using the phase change material to realize the
regulation of temperature is an ideal way of energy saving [7-10]. But some
disadvantages of the PCM limit the use of it, for example the low conductivity,
super-cooling degree and the difference between the phase change temperature during
heating and cooling.
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Microencapsulated Phase change material is a kind of shape stabilized phase
change material . The shell material of the microcapsules can prevent the leakage of
phase change material in the phase change process, but also can improve the thermal
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conductivity of phase change materials, and reduce the super-cooling degree of phase
change materials. Many preparation methods of microencapsulated phase change
microcapsules have been developed [12, 13], but the shell material is usually organic
material. Organic materials are usually flammable, and some shell material can release
toxic substances. Studies on Preparation of shell material using inorganic material are
not too many. Fang  and Chen [15, 16] prepared some microcapsules with SiO2 as
shell material. He  developed a new silica encapsulation technique using sodium
silicate precursor. Cao [18, 19] and Chai  prepared some microcapsules with TiO2
as shell material.
Relative humidity is related to human comfort and health closely .
Experiments show that people feel most comfortable and are not easy to get disease
when the relative humidity of the air is 50% - 60%. People may suffer from rheumatic
and rheumatoid arthritis if working in high humidity areas for long time. When
humidity is too low, dry air makes people skin chapped, and dry cough and hoarseness
may occur [22-24]. In order to maintain a relatively stable relative humidity, air
conditioning system is used to dehumidify or humidify the indoor environment.
Humidity is also fluctuant during a day like temperature, so hygroscopic material can be
used to regulate the indoor relative humidity, which can keep the indoor environment
healthy, but also reduce energy demand . However, the sorption and desorption
process of the hygroscopic material are different, and the performance of the
hygroscopic material is dependent on climate conditions, which all limit the real
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application of the hygroscopic material. There is a need to develop new materials or
products with better hygrothermal performance for passive building applications. A hygroscopic phase change material was prepared in this paper. SiO2 prepared
with MTES by sol–gel method was used as shell material of the microcapsules . A
kind of alkane mixture was used as core material. Diatomite has a good moisture
sorption property, and it is a kind of healthy organic material. So the diatomite was used
as hygroscopic material . The composite microcapsules and diatomite material were
prepared to be a composite thermal regulating and humidity controlling materials. The
materials can effectively reduce the daily fluctuations of indoor air temperature and
relative humidity. At the same time the microcapsule particles can improve the moisture
sorption performance of the diatomite.
d 2. Experimental
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Methyl triethoxysilane (Reagent grade, Tokyo chemical industry CO., LTD) was
used as the precursor. Anhydrous ethanol (Reagent grade, Sinopharm Chemical Reagent
Company) and distilled water were used as solvent. Hydrochloric acid (Reagent grade,
Nanjing Chemical Reagent CO., LTD) and ammonia solution (Reagent grade, Nanjing
Chemical Reagent CO., LTD) were used to control the PH degree. PCM (Industrial
grade, Ruhr Technology Company) was used as core material. Sodium dodecyl sulfate
(SDS) (Reagent grade, Shanghai Chemical Reagent CO., LTD) was used as oil–water
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emulsifier . The melting temperature and the latent heat of the alcane mixture are listed
in Table 1. More detailed characteristics could be found in . Diatomite (Industrial
grade, Shanghai Liangjiang Titanium White Product Company) was used as
hydroscopic material, the density is 0.47 g/cm3 (loose weight) (lit.), the specific surface
area is 38m2/g, the pore volume is 0.6cm3/g and the porosity is 80%.
2.2 Preparation of the PCM Oil/Water emulsion
20g PCM and 2.5g SDS were added into 100ml distilled water in a beaker. The solution
was heated at 35 ℃ to melt the phase change material. Then the temperature of the
solution was maintained at 35 ℃ and the solution was stirred at a rate of 600 rpm for
0.5 h with a magnetic stirrer. And then the temperature was maintained at 25 , and the
solution was stirred for 0.5h.
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2.3 Preparation of the microencapsulated PCM with SiO2 shell
20g MTES, 20g ethanol and 30ml distilled water were mixed in a beaker to form the
solution. The PH degree of the MTES solution was controlled at 2–3 by using
hydrochloric acid. Then the solution was stirred by a magnetic stirrer at 50 ℃ a rate of
500 rpm for 20 min. Then the sol solution was gained as microencapsulation precursor
with the hydrolysis reaction of the MTES. The temperature of the PCM micro-emulsion
was maintained at 35℃ and stirred at a rate of 400 rpm. And the PH degree of the PCM
micro-emulsion was maintained at 9-10 by adding ammonia solution. Then, the sol
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solution was added into the PCM micro-emulsion in drops. The emulsion was kept
reacting and being stirred for 2 h. The condensation reactions between the methyl
silicate and methyl silicate took place to form the SiO2 shell. And then the SiO2
polymerized to build SiO2 shell on the surface of the PCM droplet in the polymerization
process of the sol mixture. Finally, the filter paper was used to collect the microcapsules.
And the microcapsules were washed with distilled water and dried in a low temperature
vacuum oven at 0 ℃ for 24 h. The microencapsulated PCM composites were gained,
and then named as MPCM.
2.4 Preparation of the endothermal-hygroscopic material The diatomite was dried in a vacuum oven for 10 h at 100 ℃. Then 5g MPCM, 20g
diatomite and 80g water were mixed in a beaker. The composites were stirred at a rate
of 200 rpm at the room temperature for 5 min, and were formed as a brick. Then the
composites were dried in a vacuum oven at 25 ℃ for 48 h. The MPCM/diatomite
composites were acquired, and then were named as CMPCM. 20g diatomite and 40g
water were mixed in a beaker, stirred at a rate of 200 rpm at the room temperature for 5
min, and then also formed as a brick. The brick formed by single diatomite, gypsum and
wood will be as the control samples for the CMPCM.
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3 Characterization of the MPCM/diatomite composites
3.1 Morphology of the MPCM and diatomite
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The microstructure and morphology of the MPCM and diatomite were investigated by a scanning electronic microscope (SEM, S-3400NⅡ, Hitachi Inc., Japan). Fig. 1 shows the SEM profiles of the MPCM and the diatomite. As shown in Fig.
1a, the microcapsules have a compact surface to encapsulate the PCM in the SiO2 shell,
and the SiO2 shell can keep the PCM from leaking when the PCM is melted. The
microcapsules have a spherical shape without edges or dents, and the size of the
microcapsules is about 60-80µm. It can be seen from Fig.1b that the diatomite has a
porous structure. There are many nano-pores in the particle of the diatomite, which
make it absorb water vapor. Fig.2 shows the photos of the brick formed by CMPCM,
the brick formed by single diatomite, gypsum and wood. It can be seen that the bricks
are blocky structural integrity. The mass ratio of the MPCM and diatomite ensures that
the brick of CMPCM is not too loose to be broken. When increasing the mass ratio of
the MPCM to 30%, the brick will be likely to be broken.
161 162 163 164
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Fig. 1 Fig. 2
3.2 Thermal properties of the MPCM/diatomite composites A differential scanning calorimeter (Pyris 1 DSC, Perkin-Elmer) was used to
measure the thermal properties of the MPCM/diatomite composites. The heating and
cooling temperature rate was 5 ℃/min with a constant stream of argon at a flow rate of
20 ml/min. The accuracy of temperature measurements was ±2 ℃ and the enthalpy
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accuracy was ±5%. The DSC results of the PCM, MPCM and CMPCM are shown in Fig.3, Fig.4 and
Table 1. The thermal absorbing process is shown in Fig.3, and the thermal releasing
process is shown in Fig.4. Table 1 shows the melting temperature, solidifying
temperatures, latent heat and the super-cooling degree of the PCM, MPCM and
CMPCM. As can be seen in Table 1, the melting and solidifying temperatures of the
PCM are measured to be 28.1℃and 26.2℃, the melting and solidifying latent heat are
145.7kJ/kg and 144.3kJ/kg, and the super-cooling degree is 1.9℃. The melting and
solidifying temperatures of the MPCM are measured to be 27.2 ℃ and 26.7 ℃, the
melting and solidifying latent heat are 94.4 kJ/kg and 89.6 kJ/kg, and the super-cooling
degree is 0.5 ℃. The melting and solidifying temperatures of the CMPCM are measured
to be 27.0 ℃ and 26.8 ℃, the melting and solidifying latent heat are 19.0 kJ/kg and
18.4 kJ/kg, and the super-cooling degree is 0.3 ℃. It can be seen that the super-cooling
degrees of MPCM and CMPCM is lower than that of the PCM, which makes the
composite more suitable for building applications.
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The latent heat of the MPCM and CMPCM are lower than that of the PCM because
the PCM and diatomite have no phase change process. Higher content of the PCM in
the MPCM or CMPCM mean higher thermal heat storage capacity. The latent heat of
the MPCM or the CMPCM is proportional to the content of the PCM. The mass ratio of
the PCM can be calculated by the following Eq. (1):
∆H × 100 % ∆H PCM
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The η is the mass ratio of the PCM in MPCM or the CMPCM, the ∆H means the
mean latent heat of the MPCM or the CMPCM, and the ∆HPCM represents the mean
latent heat of the PCM. The mass ratios of the PCM in the MPCM and the CMPCM are
shown in Table 1. The mass ratios of the PCM in the MPCM and the CMPCM are 63.4%
3.3 Thermal stability of the MPCM/diatomite composites The thermo-gravimetric analyzer (Pyris 1 TGA, Perkin-Elmer) was used to
investigate the thermal stability of the MPCM and CMPCM. The temperature in
measurement is from 25 ℃ to 700 ℃. The heating rate is 20 ℃/min with a constant
nitrogen stream, and the flow rate of nitrogen is 20 ml/min.
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Fig.5 presents the TGA curves of the PCM, MPCM and CMPCM. Table 2 shows
the residual weight at 700 ℃ and the starting temperature of the maximum weight loss.
It can be seen from Fig. 5 that there is a two-step thermal degradation process. When
the temperature is between 130 ℃ and 250 ℃, the first step occurs, which is
corresponding to the thermal degradation of the PCM. When the temperature is between
250 ℃ and 700 ℃, the second step occurs, which is corresponding to the thermal
degradation of the SiO2 molecular. The starting temperature of maximum weight loss
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for MPCM or CMPCM is higher than that of PCM, and this result means that the SiO2
shell can protect the core material and act as a fire retardant to improve the kindling
3.4 Hygroscopic properties of the CMPCM and diatomite
In order to evaluate the hygroscopic performance (especially the moisture buffer
ability) of the composite, both the moisture transfer coefficient and the moisture buffer
value were measured. The former represents the rate of moisture transfer; the latter
mainly represents the moisture buffer capacity in a dynamic RH variation. The sorption
and desorption isotherms of the CMPCM were also measured.
Moisture transfer in a porous building material can be analyzed using the so-called
thermal–moisture analogy. The moisture diffusivity is analogous to the thermal
conductivity in heat transfer process. The material used for humidity controlling should
have a higher moisture transfer coefficient, which means that the material can quickly
respond to the indoor relative humidity change. The mass ratio of the saturated moisture
content to the diatomite is relatively high, can reach to 10% . But the moisture
transfer coefficient of a material which has large saturated moisture may be small,
which means that the material may need to take a long time to absorb the water vapor.
So the material cannot respond to the indoor relatively humidity quickly, and cannot act
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as a moisture controlling material. Moisture transfer coefficient can be defined by
following Eq. (2): qm ∂RH ∂x
The λ is the moisture transfer coefficient, q m represents the flux of water vapor
in a unit time and a unit area, RH means the relative humidity and x is the thickness of
The classic cup method was adopted to measure the moisture transfer coefficient of
samples. Areas of 3*3cm on the two sides of the brick are exposed to make the water
vapor transfer from the bottle to outside, and the other samples were also operated like
this. The saturated NaCl solution is kept in the bottle to keep the relative humidity of the
air in container as 75%. The relative humidity outside is maintained at 52%. The
measurement accuracy of the relative humidity was ±2%.
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The moisture transfer coefficients of CMPCM and the diatomite are shown in Fig.6
and Table 3. It can be seen that the moisture transfer coefficient of CMPCM is 5×10-8
kg/(m ∙ s ∙ %RH). It is higher than that of others, which is mainly due to the fact that
adding MPCM increases the porosity of CMPCM.
Moisture buffer value is a characteristic of a material based on a period of moisture
uptake/release. The practical moisture buffer value indicates the amount of water that is
transport in or out a material per open surface during a certain period of time, when it is
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subjected to variations in relative humidity of surrounding air . The moisture buffer
value can be defined as Eq. (3): (3)
The MBV is the moisture buffer value, the G is the moisture uptake of the material,
The A is the area of the material surface to exchange moisture with the air, and ∆RH is
the difference value between the high relative humidity and the low relative humidity.
Fig.7 shows that there are two bottles containing some saturated salt solution. One
bottle contains saturated KCl solution, which keeps the relative humidity inside at 88%.
Another one contains saturated NaBr solution, which keeps the relative humidity inside
at 62%. The sample hangs in the bottle with KCl solution, and then the bottle is sealed
for 8 hours. Then the sample hangs in the bottle with NaBr solution, and the bottle is
sealed for 16 hours. The process is alterative operated for at least 6 days. And the mass
of the sample is measured with analytical balance.
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Fig.8 shows the mass change curves of the CMPCM, diatomite, wood and gypsum.
The moisture buffer values of the samples are presented in Fig.9 and Table 4. It can be
seen that the MBV of CMPCM is 1.57 g/m2∙%RH, which is much higher than that of
others. Rode  classified the MBV using five different categories. The good class
ranges from 1 g/m2∙%RH to 2 g/m2∙%RH. It can be seen that the MBV of CMPCM is
within good class, and the MBV of other samples are all within limited class, which
ranges from 0.2 g/m2∙%RH to 0.5 g/m2∙%RH. The microcapsules in CMPCM will
increase the porosity of the composite, and consequently increase the moisture transfer
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coefficient of the composite. The CMPCM can absorb more water vapor in a given time (e.g. 8 h) than pure diatomite, which leads to a larger MBV as shown in Fig. 9. In addition, Fig. 10 shows that the sorption and desorption isotherms of the
CMPCM are nearly linear in normal hygroscopic range (between 20% and 85% RH).
The MBV could be regarded as a constant within this range for building application.
M Table 4
3.7 The effects of the CMPCM in buildings applications
The CMPCM can be used in interior wallboard of buildings. The PCM in the
composites can effectively reduce the daily fluctuations of indoor air temperatures and
maintains it at the desired comfort level for a longer period of time. The hygroscopic
material (diatomite) in the composites can regulate the relative humidity of the indoor
environment by uptake/release moisture.
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A following example about the practical application is given to show how the
CMPCM can be used to reduce the air conditioning load. An unventilated and insulated
room has dimensions 4m×5m×3m= 60 m3. At the initial time, the indoor temperature is
25 ℃, and the relative humidity is 50%. In the room is released 100g moisture per hour,
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and heat power is 1000W. To maintain the indoor temperature is below 27 ℃ and the
relative humidity is below 60 %, how much will the air conditioning load be in 8 hours? If there is no CMPCM, the sensible heat load can be calculated from Eq. (4):
∙ 8 ∙ 27 ∙ 25 ∙
E" h ∙ $q& ∙ 8 SW ∙ 60% SW" ∙ 50% ∙ V+
The air conditioning load is from Eq. (6):
E E, - E" /COP
The latent heat load can be calculated from Eq. (5):
Where E, is the sensible heat load, E" is the latent heat load and E is the total load
of the air conditioning system. The q and q& are heat power and moisture releasing rate
respectively. The c and c" are specific heat capacity of air at 27 ℃ and 25 ℃
respectively. The ρ and ρ" are air density at 27 ℃ and 25 ℃ respectively. The SW
and SW" are saturation water vapor concentration at 27 ℃ and 25 ℃ respectively. The
h is the latent heat of water vapor. The V is the volume of the room, and the COP is the
coefficient of performance of the air conditioning system . The values of the
parameters are listed in Table 5. It can be calculated that the E= 2.54 kWh.
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If the inner wall of the room is clad with A=54 m2 of the CMPCM and the
thickness of the coating is d=2 cm . The E456 represents the thermal heat absorbed
by PCM, which can be calculated from Eq. (7):
E456 α ∙ A ∙ d ∙ ρ56456 ∙ h56456
Where, α is the ratio of the enthalpy available between the high and low
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temperatures by the total enthalpy, ρ56456 is the density of the CMPCM and h56456
is the latent heat of the CMPCM, which are listed in Table 5. And the relative humidity RH in 8 hours can be calculated from Eq. (8):
SW ∙ RH ∙ V - MBV ∙ RH 50% ∙ 100 ∙ A SW" ∙ 50% ∙ V - q& ∙ 8
When α =20%, It can be determined that the E456 =0.74 kWh and RH=57.1%.
It can be known that the relative humidity is always below 60% when the inner
wall is clapped with CMPCM. So air condensation dehumidification is not needed. The
air conditioning load is calculated from Eq. (9):
E ; E, E456 /COP
It can be determined that the E; =2.09 kWh. The energy saving rate is 17.7%. Table 5
In reality, the room experiences an air and heat change with outside environment, so the energy load should be investigated comprehensively.
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This paper presents the synthesis, thermal properties and the hygroscopic
properties of the MPCM/diatomite composites. The SiO2 prepared with MTES was used
as the shell material, and a kind of alkane mixture was used as core material. The
measurement results of thermal properties show that the melting temperature of
CMPCM is 27.0℃ and the solidifying temperature is 26.8℃. The super-cooling degree
of CMPCM is 0.3℃. The SiO2 shell can improve the thermal stability of the PCM. The
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measurement results of the hygroscopic properties show that the MPCM can improve
the hygroscopic performance of CMPCM. The MBV of CMPCM is within good class.
The overall hygrothermal performance of the composite is better than simple
combination of two separate layers of PCM and diatomite. An example about practical
application shows that the CMPCM can moderate both the indoor temperature and
relative humidity. The CMPCM has the potential to be an energy saving material.
For future work, the performance and effect of the CMPCM in real building
applications under different climates will be studied by detailed experimental
This work was supported by the National Natural Science Foundation of China
(Grant No. 51108229), and Research Fund for the Doctoral Program of Higher
Education of China (Grant No. 20130091110053).
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Table(s) with Caption(s)
Tables with Captions
Table 1 DSC data of the PCM, MPCM and CMPCM Melting （5 ℃/min）
Solidifying （5 ℃/min）
Sample ratio of the name
Ac ce p
Charred residue amount (%) (700 ℃)
Table 2 TGA data of PCM, MPCM and CMPCM
Table 3 Moisture transfer coefficient of CMPCM, diatomite, gypsum and wood Samples
moisture transfer coefficient -8
(10 kg/m ∙ s)
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Table 4 MBV of CMPCM, diatomite, gypsum and wood CMPCM
ρ (kg/m3) ρ (kg/m3)
ρ (kg/m3) η (-)
Ac ce p
Table 5 The values of parameters for the CMPCM application
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List of Figure Captions
List of Figure Captions
Fig.1 SEM photographs of the (a) CMPCM, (b) Diatomite
Fig.5 TGA curve of PCM, MPCM and CMPCM
Fig.4 Solidifying DSC curve of PCM, MPCM and CMPCM
Fig.3 Melting DSC curve of PCM, MPCM and CMPCM
Fig.2 Photos of the (a) Wood, (b) CMPCM, (c) gypsum, (d) diatomite
Fig.6 Moisture transfer coefficient of CMPCM, diatomite, wood and gypsum Fig.7 Photos of bottles containing saturated salt solution of (a) KCl, (b) NaBr
Fig.8 Mass change curves of the CMPCM, diatomite, wood and gypsum
Fig.9 MBV of the CMPCM, diatomite, wood and gypsum
Fig.10 Sorption and desorption isotherms of the CMPCM
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ip t cr us an M ed ce pt Ac
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ip t cr us an M Ac
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ip t cr us an M Ac
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ip t cr us an M Ac
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ip t cr us an M Ac
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ip t cr us an M Ac
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ip t cr us an M ed ce pt Ac
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ip t cr us an
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Moisture content by mass (g/g)
Relative humidity (%)
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1. A new kind of hygroscopic phase change material was prepared. 2. The SiO2 shell can improve the thermal properties of the composite.
4. The composite has a good hygrothermal performance.
3. The SiO2 shell can prevent the melted phase change material from leaking.
5. The composite can moderate both the indoor temperature and relative humidity.
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