Journal ArticleDOI

# Measurements of Heat Capacity and Enthalpy of Phase Change Materials by Adiabatic Scanning Calorimetry

20 Apr 2011-International Journal of Thermophysics (Springer US)-Vol. 32, Iss: 5, pp 913-924

Abstract: Phase change materials (PCMs) are substances exhibiting phase transitions with large latent heats that can be used as thermal storage materials with a large energy storage capacity in a relatively narrow temperature range. In many practical applications the solid–liquid phase change is used. For applications accurate knowledge of different thermal parameters has to be available. In particular, the temperature dependence of the enthalpy around the phase transition has to be known with good accuracy. Usually, the phase transitions of PCMs are investigated with differential scanning calorimetry (DSC) at fast dynamic scanning rates resulting in the effective heat capacity from which the (total) heat of transition can be determined. Here we present adiabatic scanning calorimetry (ASC) as an alternative approach to arrive simultaneously at the equilibrium enthalpy curve and at the heat capacity. The applicability of ASC is illustrated with measurements on paraffin-based PCMs and on a salt hydrate PCM.
Topics: , Calorimetry (57%), Phase transition (56%), Heat capacity (55%), Latent heat (55%)

Content maybe subject to copyright    Report

Int J Thermophys (2011) 32:913–924
DOI 10.1007/s10765-011-0984-0
Measurements of Heat Capacity and Enthalpy of Phase
Change Materials by Adiabatic Scanning Calorimetry
Patricia Losada-Pérez · Chandra Shekhar Pati Tripathi ·
Jan Leys · George Cordoyiannis · Christ Glorieux ·
Jan Thoen
Received: 26 August 2010 / Accepted: 23 March 2011 / Published online: 20 April 2011
Abstract Phase change materials (PCMs) are substances exhibiting phase transi-
tions with large latent heats that can be used as thermal storage materials with a large
energy storage capacity in a relatively narrow temperature range. In many practical
applications the solid–liquid phase change is used. For applications accurate knowl-
edge of different thermal parameters has to be available. In particular, the temperature
dependence of the enthalpy around the phase transition has to be known with good
accuracy. Usually, the phase transitions of PCMs are investigated with differential
scanning calorimetry (DSC) at fast dynamic scanning rates resulting in the effective
heat capacity from which the (total) heat of transition can be determined. Here we
present adiabatic scanning calorimetry (ASC) as an alternative approach to arrive
simultaneously at the equilibrium enthalpy curve and at the heat capacity. The appli-
cability of ASC is illustrated with measurements on parafﬁn-based PCMs and on a
salt hydrate PCM.
Keywords Adiabatic scanning calorimetry · Enthalpy · Heat capacity ·
Latent heat · Phase change materials
P. Losada-Pérez · C. S. P. Tripathi · J. Leys (
B
) · G. Cordoyiannis · C. Glorieux · J. Thoen
Laboratorium voor Akoestiek en Thermische Fysica, Departement Natuurkunde en Sterrenkunde,
Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: jan.leys@fys.kuleuven.be
J. Thoen
e-mail: jan.thoen@fys.kuleuven.be
G. Cordoyiannis
Condensed Matter Physics Departement, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
123

914 Int J Thermophys (2011) 32:913–924
1 Introduction
Phase change materials (PCMs) are substances exhibiting phase transitions with large
latent heats and can be used as thermal storage materials with a large energy stor-
age capacity in a relatively narrow temperature range [13]. In principle latent heat
storage can be achieved through a solid–solid, solid–liquid, solid–gas, or liquid–gas
phase change. In practical applications where temperature control is important, the
solid–liquid phase change is mostly used. In nowadays applications mainly two types
of PCMs are in use. A ﬁrst category includes organic materials, mainly parafﬁns and to
some extent fatty acids. The second large category includes the inorganic salt hydrates.
Recently, also eutectic mixture combinations of organic and non-organic compounds
are being considered. The choice of material depends not only on the intrinsic prop-
erties of the PCM but also on the practical application envisioned. A large number of
PCMs are available in the temperature range from well below 0
C to 200
C.
For proper design of application devices an accurate knowledge of different ther-
mal parameters has to be available. In particular, the temperature dependence of the
enthalpy around the phase transition has to be known with good accuracy. Usually,
the phase transitions of PCMs are investigated with differential scanning calorimetry
(DSC). In DSC a reference sample is made to increase (or decrease) its temperature at
a constant rate and the PCM sample is forced to follow this rate by changing the power
delivered to it. This allows one to extract the (effective) heat capacity as a function of
temperature. The transition heat is then determined by integrating the heat capacity
curve. Moreover, DSC uses (for sufﬁcient resolution) fast scanning rates (1 K · min
1
to 10 K · min
1
), quite often resulting in (apparent) overheating and undercooling
effects. Efforts to (partly) overcome these problems for latent heat measurements of
PCMs have resulted in running a DSC in an isothermal step mode and/or by applying
a T -history method [4].
In this paper we present adiabatic scanning calorimetry (ASC) as an interesting
complementary tool to measure simultaneously the temperature dependence of the
enthalpy as well as of the heat capacity near the phase transitions in PCMs. ASC has
been extensively used to discriminate between ﬁrst-order (exhibiting a discontinuous
step in the enthalpy) and second-order (with a continuous temperature dependence
of the enthalpy) phase transitions and to detect heat-capacity anomalies near critical
points in several types of soft matter systems, such as, e.g., liquid crystals and critical
mixtures [58]. With ASC the problems with superheating or supercooling can in
many cases be avoided and true equilibrium data can be obtained by using very slow
rates as slow as 2 to 3 orders of magnitude slower than in DSC. After a description of
the ASC technique, we present results for the temperature dependence of the enthalpy
and of the (effective) heat capacity of two parafﬁn-based PCMs and of one pure salt
hydrate. In addition, also results for a parafﬁn-based powder PCM are given.
2 Experimental Method
The so-called ASC technique was introduced around 1980 [5] and extensively used
for the study of many different types of phase transitions, in particular in liquid
123

Int J Thermophys (2011) 32:913–924 915
mixtures and liquid crystals. An extensive description of the technique and major
results can be found in recent overviews [7,8]. Here we will only give a description
of the two principal modes of operation of an ASC and some key features of the data
analysis.
2.1 Principal Modes of Operation of an ASC
Since the beginning of the twentieth century, several different calorimetric techniques
with varying degrees of accuracy and precision have been developed. Traditionally,
heat-capacity measurements are carried out by means of the adiabatic heat pulse
method, where a known amount of heat, Q, is (usually electrically) applied to the
sample and the corresponding temperature rise, T , is measured. The heat capacity
(at constant pressure) of a sample at a given temperature is then obtained from
C
p
=
Q
T
(1)
In this way one looks at the derivative of the enthalpy H(T ) curve and no information
can be obtained on enthalpy discontinuities or latent heats (and thus on the order of a
given phase transition). Rewriting Eq. 1 in the following way:
C
p
=
dQ
dT
=
dQ/dt
dT/dt
=
P
˙
T
(2)
(with t time and P power), shows the possibility of operating in dynamic modes.
By keeping P or
˙
T constant, while increasing or decreasing the temperature of the
sample (P and
˙
T positive or negative), four practical modes of operation are obtained.
These modes require different settings for the (adiabatic) thermal environment (thermal
shields) of the sample. The most interesting modes (and for enthalpy measurements
where latent heats are present, the only feasable ones) are the ones with constant heat-
ing or cooling power P. Since in the PCM phase transitions substantial latent heats are
present, we will only give a general description of the constant power modes. Detailed
information on all modes can be found elsewhere [8].
In the heating mode with constant (electrically applied) power P
e
to the sample
(cell), in order to maintain the adiabatic conditions, one has to arrange for negligibly
small leaking power P
l
to the environment, measure P
e
lution of the sample temperature T(t) with time t. Because the heating rate is inversely
proportional to C
p
, the increase of C
p
at a second-order phase transition will result
in a decrease of the rate and facilitate thermodynamic equilibrium and servo-control
of adiabatic conditions. At ﬁrst-order transitions, in principle, the rate is zero at the
transition for a time interval given by
t = t
f
t
i
=
L
P
e
(3)
123

916 Int J Thermophys (2011) 32:913–924
where L is the latent heat of the transition, and t
i
and t
f
are the times during the scan
at which the transition is reached and left. The direct experimental result T (t) gives
the enthalpy as a function of temperature by
H = H(T
0
) + P
e
(t t
0
) (4)
with T
0
the starting temperature of the scanning run at the time t
0
. Implementing a
cooling run with constant (negative) power is less obvious and has to be realized by
imposing a constant leaking power between the sample (cell) and its isothermal envi-
ronment. This can be done by imposing a constant temperature difference between
the cell and the isothermal environment. These conditions have to be veriﬁed and usu-
ally involve calibration (certainly for scans over large temperature ranges) to arrive
at absolute values for the heat capacity or enthalpy. This type of cooling run (with
negative power and negative rate) is very similar to the constant power heating mode
and also easily allows one to deal with ﬁrst-order transitions. Although an ASC is
normally optimized for scanning, it can easily be operated as a normal heat pulse step
calorimeter as well. This can be very practical for calibration purposes and veriﬁcation
of absolute heat-capacity values.
2.2 Implementation of the ASC Concept
Figure 1 gives a schematic diagram of a four-stage ASC that can operate between room
temperature and about 470 K. The centrally located cylindrical sample cell for liquids
is surrounded by three concentric (copper) thermal shields. Each of the stages (1 to 4)
has its own thermometer (Th
i
) and its own electrical (e.g., constantan) heating wires.
On stages 1 to 3 the heating wires are evenly distributed and wound in grooves and
thermally anchored with a good thermal conductive and electrically insulating epoxy.
Stage 4 of this calorimeter is composed of a hot air oven and the outer thermal and
vacuum shield of the actual calorimeter with three internal stages. The temperature
of the oven is measured and controlled by means of the thermistor Th
4
and computer
regulated power delivery to the heater of the oven. The stages are in very poor thermal
contact, and the space between them is vacuum pumped. The sample cell is suspended
by thin nylon threads inside stage 2. To minimize further thermal transfer between
stages, all electric connecting wires are, on passing from one stage to another, several
thermal diffusion lengths long (for temperature variations at relevant time scales),
and neatly coiled not to touch the wall of either stage. These wires are also thermally
anchored at each stage. Different sizes of sample cells can be suspended in the calo-
rimeter. For the PCM measurements we used cells with a volume of 5 cm
3
to 11 cm
3
.
For liquid samples it is also possible to stir the samples inside the cell. Stirring in the
horizontally mounted cylindrical cells is achieved by means of a metal ball that can
roll back and forth inside the cell by changing periodically the inclination of the plate
supporting the calorimeter.
123

Int J Thermophys (2011) 32:913–924 917
Fig. 1 Schematic diagram of a
four-stage ASC with typical
modern measurement and PC
controlled instrumentation. (1)
sample in sample holder with
stirring ball, thermistor Th
1
,and
heater (not shown); (2) shield
with thermistor Th
2
, platinum
reference thermometer Pt
2
,and
heater (not shown); (3) shield
with platinum thermometer Pt
3
and heater (not shown); and (4)
external vacuum tight shield in
hot air oven with thermistor Th
4
and heater. K-2010 (7.5 digit
multiplexer) and HP-34401 (6.5
digits) are multimeters. There
are two HP-6181B power
2.3 Analysis of the Direct Experimental Data
The basic data measured very frequently as a function of time (typically every 3 s to
5 s) during a heating run in an ASC are the temperature of the sample and the holder
as well as the (constant) power. These results are graphically displayed in the two
central boxes of Fig. 2 for a weakly ﬁrst-order transition. After a long temperature
stabilization time of stage 2 (shield around the sample cell) with zero power to the cell
(stage 1), the cell attains the same temperature (within a few tenths of a mK). Then
the power to the sample cell is switched on, at t
0
, to a chosen value depending on the
desired overall scanning rate. Depending on the temperature range to be covered, a
typical run can take several days or weeks (for very slow scans). The temperature is
measured with µK resolution, and the extremely large number of T (t) data allows
averaging (if desired) and determination of local derivatives resulting in nearly as
many
˙
T values as T (t) data points by using several (consecutive) data points in a
moving time derivative (adding one data point at one end and leaving out one at the
other end). A simple division of P by
˙
T at a given T (t) results immediately in a C
p
(T )
value at that temperature. These C
p
(T ) values are total heat capacities for the sample
and the sample holder together. Proper calibration of the (only weakly T dependent)
heat capacity of the empty cell and knowing the total amount of sample allows one to
calculate the speciﬁc heat capacity of the sample. In the case of a ﬁrst-order transition,
the transition is reached at time t
i
. From that moment until its end at t
f
the temperature
remains constant over the time interval given by Eq. 3. According to Eq. 4, the direct
combination of t(T ) and P(t) data immediately results in the enthalpy as a function
123

##### Citations
More filters

Journal ArticleDOI
Abstract: A general problem of most solar thermal systems is the need for thermal storage in order to balance supply and demand of heat over a certain period of time. A possibility to employ latent heat of fusion in phase change materials (PCMs) for thermal energy storage in air-based solar thermal systems was investigated using laboratory experiments and numerical simulations. A heat storage unit containing 100 aluminium panels filled with a paraffin-based PCM was used in the investigations. The experiments were carried out in a lab environment with an electric air heater as a heat source. A numerical model of the unit was developed and implemented as a type in the TRNSYS 17 simulation tool. The results of the simulations with the developed model show a good agreement with experimental results. Subsequently, the model was used for a parametric study analysing the influence of certain parameters. The performed investigations showed a potential of the use of latent heat thermal storage in air-based thermal systems with a narrow temperature operation range.

94 citations

Journal ArticleDOI
29 Sep 2016-PLOS ONE
TL;DR: Results indicate that interactions are mainly driven by the hydrophobic components of the IL, which significantly distort the layer and promote vesicle rupture, and evidence the gradual decrease of the main phase transition temperature upon increasing IL concentration, reflecting increased disorder by weakening of lipid chain interactions.
Abstract: Despite the environmentally friendly reputation of ionic liquids (ILs), their safety has been recently questioned given their potential as cytotoxic agents. The fundamental mechanisms underlying the interactions between ILs and cells are less studied and by far not completely understood. Biomimetic films are here important biophysical model systems to elucidate fundamental aspects and mechanisms relevant for a large range of biological interaction ranging from signaling to drug reception or toxicity. Here we use dissipative quartz crystal microbalance QCM-D to examine the effect of aqueous imidazolium-based ionic liquid mixtures on solid-supported biomimetic membranes. Specifically, we assess in real time the effect of the cation chain length and the anion nature on a supported vesicle layer of the model phospholipid DMPC. Results indicate that interactions are mainly driven by the hydrophobic components of the IL, which significantly distort the layer and promote vesicle rupture. Our analyses evidence the gradual decrease of the main phase transition temperature upon increasing IL concentration, reflecting increased disorder by weakening of lipid chain interactions. The degree of rupture is significant for ILs with long hydrophobic cation chains and large hydrophobic anions whose behavior is reminiscent of that of antimicrobial peptides.

30 citations

Journal ArticleDOI
Yudong Zhang1, Kai Du1, Jiapeng He2, Longxiang Yang1  +1 moreInstitutions (2)
Abstract: An experimental method was used for validating the heat transfer within phase change materials (PCMs). The two models, enthalpy and effective heat capacity models, were implemented and three impact factors were analyzed. The results showed that it should not calculate using the same temperature range for the PCMs with different melting and freezing ranges. Narrower temperature range can make the enthalpy method more accurate. Also, the liquid fraction should be taken into account if the phase change had not completed. In addition, the proper treatment of latent heat made the effective heat capacity method more accurate than the enthalpy method.

30 citations

Journal ArticleDOI
Abstract: The (solid + liquid) phase equilibrium for eight {x diphenyl ether + (1 − x) biphenyl} binary mixtures, including the eutectic mixture were studied by using a differential scanning calorimetry (DSC) technique. A good agreement was found between previous literature and experimental values here presented for the melting point and enthalpy of fusion of pure compounds. The well-known equations for Wilson and the non-random two-liquid (NRTL) were used to correlate experimental solid liquid phase equilibrium data. Moreover, the predictive mixture model UNIFAC has been employed to describe the phase diagram. With the aim to check this equipment to measure heat capacities in the quasi-isothermal Temperature-Modulated Differential Scanning Calorimetry method (TMDSC), four fluids of well-known heat capacity such as toluene, n-decane, cyclohexane and water were also studied in the liquid phase at temperatures ranging from (273.15 to 373.15) K. A good agreement with literature values was found for those fluids of pure diphenyl ether and biphenyl. Additionally, the specific isobaric heat capacities of diphenyl ether and biphenyl binary mixtures in the liquid phase up to T = 373.15 K were measured.

28 citations

Journal ArticleDOI
Igor Medved1, Anton Trník1, Libor VozárInstitutions (1)
Abstract: We consider three phase-change materials (PCMs) in which a change between two phases may be used to store/release thermal energy. Their enthalpy and heat capacity were measured in a quasistatic regime by adiabatic scanning calorimetry and show, within a certain temperature range, a single distinct jump and peak, respectively. We present a microscopic development from which the jumps and peaks can be accurately fitted and that could be analogously applied even to other PCMs. In addition, we determine the baseline and excess part of the heat capacity and thus the latent heat associated with the phase change. The development is based on the observation that PCMs often have polycrystalline structure, being composed of many single-crystalline grains. The enthalpy and heat capacity measured in experiments are therefore interpreted as superpositions of many contributions that come from the individual grains.

20 citations

##### References
More filters

Book
10 Oct 2008-
Abstract: Basic thermodynamics of thermal energy storage.- Solid-liquid phase change materials.- Determination of physical and technical properties.- Heat transfer basics.- Design of latent heat storages.- Integration of active storages into systems.- Applications in transport and storage containers.- Applications for the human body.- Applications for heating and cooling in buildings.

732 citations

### "Measurements of Heat Capacity and E..." refers background in this paper

• ...Phase change materials (PCMs) are substances exhibiting phase transitions with large latent heats and can be used as thermal storage materials with a large energy storage capacity in a relatively narrow temperature range [1–3]....

[...]

• ...Apparently, as pointed out by one of the reviewers, this PCM melts incongruently due to a peritectic transition [1]....

[...]

Journal ArticleDOI
Abstract: Phase change materials (PCMs) are thermal storage materials with a high storage density for small temperature range applications. In the design of latent heat storage systems, the enthalpy change of the PCM has to be known as a function of temperature with high precision. During dynamic measurements, the sample is not in thermal equilibrium, and therefore the measured value is not the equilibrium value. The influence of non-equilibrium on the measurement results can be quantified by doing measurements during heating and cooling with any measurement instrument. Measurements carried out by differential scanning calorimetry (DSC) and by the T-history method are presented and discussed. To characterize encapsulated PCM objects, measurements on the whole objects should be carried out. A measurement setup for this purpose is also presented. The obtained precision meets typical application requirements, and good agreement between results obtained with the different methods is demonstrated.

226 citations

Journal ArticleDOI
Jan Thoen1, H. Marynissen1, W. Van Dael1Institutions (1)
01 Nov 1982-Physical Review A
Abstract: An adiabatic scanning calorimeter has been used to study the thermal behavior of the liquid-crystal octylcyanobiphenyl (8CB) in the temperature range between 10 and 50\ifmmode^\circ\else\textdegree\fi{}C. The solid---to---smectic-$A$ ($\mathrm{KA}$), the smectic-$A$---to---nematic ($\mathrm{AN}$), as well as the nematic-to-isotropic (NI) phase transitions, which fall in this temperature range, have been investigated in great detail. From our measuring procedure the enthalpy behavior (including latent heats) as well as the heat capacity have been obtained. For the KA transition the latent heat was 25.7\ifmmode\pm\else\textpm\fi{}1.0 kJ/mol and for the NI transition it was 612\ifmmode\pm\else\textpm\fi{}5 J/mol. Within the resolution of our experiment we find that the $\mathrm{AN}$ transition is a continuous one. For the latent heat, if any, we arrive at an upper limit of 0.4 J/mol (or 1.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ J/g). The observed anomaly in the heat capacity for the $\mathrm{AN}$ transition is not consistent with a nearly logarithmic singularity as predicted by the $\mathrm{XY}$ model, instead we obtain a critical exponent $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=0.31\ifmmode\pm\else\textpm\fi{}0.03$. This result is consistent with the anisotropic scaling relation ${\ensuremath{ u}}_{\ensuremath{\parallel}}+2{\ensuremath{ u}}_{\ensuremath{\perp}}=2\ensuremath{-}\ensuremath{\alpha}$. The pretransitional effects near the NI transition are in qualitative agreement with the hypothesis of quasitricritical behavior.

168 citations

Journal ArticleDOI
Zongrong Liu1, D.D.L. Chung1Institutions (1)
24 Jan 2001-Thermochimica Acta
Abstract: The phase change behavior of organic and inorganic phase change materials, namely paraffin wax, microcrystalline wax, Na2SO4·10H2O and CaCl2·6H2O, with melting temperatures close to room temperature, was evaluated by differential scanning calorimetry. The melting and solidification temperatures, supercooling, heat of fusion and thermal cycling stability of these materials, with and without additives, were determined. Paraffin wax, with or without α-Al2O3 or BN particles, are potentially good thermal interface materials, because of the negative supercooling (down to −7°C), large heat of fusion (up to 142 J/g) and excellent thermal cycling stability. Microcrystalline wax is not suitable, due to its unclear endothermic and exothermic peaks and wide melting temperature range. The addition of 20–60 wt.% α-Al2O3 to paraffin wax decreases the melting temperature by 7°C. Beyond 60 wt.% α-Al2O3, the melting temperature of paraffin wax increases toward the value without α-Al2O3. The heat of fusion of paraffin wax attains a minimum at 20 wt.% α-Al2O3. The addition of BN has little effect on the phase change behavior of paraffin wax. The inorganic materials Na2SO4·10H2O and CaCl2·6H2O, with and without nucleating additives, are not suitable for use as thermal interface materials, due to the incongruent melting and decomposition behavior, large supercooling (8°C or more) and thermal cycling instability.

83 citations

Journal ArticleDOI
A. Genovese1, Gandara Amarasinghe1, M. Glewis1, David E. Mainwaring1  +1 moreInstitutions (1)
15 Apr 2006-Thermochimica Acta
Abstract: Phase change materials (PCM) provide thermoregulation originating from the latent heat exchanged during melting or crystallisation. Linear hydrocarbons have weak interactions, but high symmetry, providing an effective quantity of latent heat over the most acceptable temperature range for applications. The ability to both melt and crystallise over a narrow range is made complex by nucleation, polymorphism and the kinetic nature of these changes. Differential scanning calorimetry (DSC), optical microscopy and temperature modulated DSC (TMDSC) was used to study the melting of n-eicosane. This PCM has a low degree of supercooling and conversion to the most stable crystalline state (triclinic) that occurs rapidly from a metastable phase (rotator) state on cooling. TMDSC revealed a small, yet similar degree of thermodynamic reversibility in the melting of each of the crystalline phases.

60 citations

##### Network Information
###### Related Papers (5)
01 Nov 2012, Applied Energy

Eduard Oró, A. de Gracia +3 more

01 Apr 2011, Renewable & Sustainable Energy Reviews

Luisa F. Cabeza, Albert Castell +4 more

01 Dec 2007, Renewable & Sustainable Energy Reviews

Murat Kenisarin, Khamid Mahkamov

26 Aug 2009, International Journal of Thermophysics

Eva Günther, Stefan Hiebler +2 more

01 Apr 2013, Energy and Buildings

N. Soares, José J. Costa +2 more

##### Performance
###### Metrics
No. of citations received by the Paper in previous years
YearCitations
20211
20202
20196
20182
20173
20163