Katsuhiko Kanari

Linearity and non-linearity in DSC: A critique on modulated DSC☆

Abstract Recently a new interesting technique, modulated differential scanning calorimetry (MDSC), was published. This technique is based on Fourier transformation, and its sound applicability depends on linearity in thermal processes, because the Fourier transformation is based on linearity or the superposition principle. If a process under observation is a non-linear process, the physical meaning of the Fourier transformation should be closely examined. The DSC output signals of some processes, such as melting and reaction, have been revealed to be non-linear. Therefore, differentiation of linearity from non-linearity in DSC thermal responses is very important for modulated DSC, and they are discussed in this paper. In this discussion, methods to analyze oscillating output signals are also proposed.

Comparative study of thermal behaviors of various lithium-ion cells

Heat generation behaviors during charge and discharge are examined by calorimetric measurement for several lithium-ion cells that are commercially available. Heat flow from the cell shows interesting variation reflecting the characteristics of the electrode reactions. Heat generation of the cell with graphitic carbon anode draws extremely complicated curve caused by variation of stage structure of lithium intercalated graphite. The cell with hard carbon anode dissipates excess heat resulting in a voltage hysteresis between charge and discharge. In the LiCoO2 cathode, thermal behaviors due to the crystal phase transition are clearly observed.

Entropy change in lithium ion cells on charge and discharge

Open circuit voltage (OCV) was measured as a function of temperature and state of charge (SOC) for six kinds of lithium ion cells. The following cells were used: four kinds of commercial cell using a LiCoO2 cathode and a graphite or hard carbon anode; a trial manufacture cell using a Li–Ni–Co complex oxide cathode and a graphite-coke hybrid carbon anode; and a trial manufacture cell using a LiMn2O4 cathode and a graphite anode. The entropy change in the cell reaction was determined by calculating the derivative of the OCV with temperature. Results were compared and discussed to determine the influence of the phase transition in the electrode materials due to cell reaction. It was clarified that the entropy change in cells using a LiCoO2 cathode is negative except for the part of the SOC region where LixCoO2 phase transition occurred. An endothermic reaction then occurs during discharge and an exothermic reaction during charge. In cells using LiCoO2 cathodes, there was a fluctuation in the entropy change originating from the LixCoO2 phase transition in the SOC range between 70% and 90%. This fluctuation was influenced by temperature and by additives or excess lithium in the cathode material. The entropy change in both cells using a Li–Ni–Co complex oxide cathode or a LiMn2O4 cathode was comparatively small.

Heat capacity measurements by dynamic differential scanning calorimetry

Abstract Dynamic differential scanning calorimetry (d-DSC) has attracted much attention, and it was postulated that high-precision heat capacity measurements can be made by this technique. However, its applicability, accuracy and precision have not yet been made clear. Because it is based on Fourier transformation, it can be applied to linear thermal responses. The dynamic steady state and the base line, from which the heat capacity is estimated, demonstrate a linear response in DSC, so that the dynamic steady state has been theoretically considered by analytical methods, and analytical solutions have been derived for the dynamic steady state and hence for the heat capacity measurement. For further examination, numerical solutions have also been obtained by taking account of other factors, which may potentially influence the measurements. In conclusion, a few points should be considered in the high-accuracy heat capacity measurements, because some factors, which have not been considered before, affect the dynamic steady state or the base line. Furthermore comparison has been made between d-DSC and conventional DSC in this research.

Investigation of latent heat thermal energy storage materials: V. thermoanalytical evaluation of binary eutectic mixtures and compounds of NAOH with NaNO3 OR NaNO2

Abstract Differential scanning calorimetry was applied to the evaluation of binary eutectic mixtures and compounds of NaOH with NaNO 3 or NaNO 2 as latent heat thermal energy storage materials. There are two compounds and three eutectic mixtures consisting of NaOH and NaNO 3 and one compound and two eutectic mixtures consisting of NaOH and NaNO 2 . The heats of fusion of all compounds and mixtures were more than 200 J g −1 . The thermal behaviour of these materials was observed, and some compounds and mixtures show a great tendency to supercool. The specific heat capacity was also measured. Because the eutectic mixture 18.5 mol.% NaNO 3 -81.5 mol.% NaOH is the most promising material for use in a nuclear power plant, this mixture made from industrial products was examined further. After 1000 cycles of melting and crystallization in bench-scale equipment the material shows little change.

Investigation of latent heat-thermal energy storage materials. I. Thermoanalytical evaluation of modified polyethylene

Abstract Differential scanning calorimetry is used to evaluate polyethylene for latent heat-thermal energy storage. Polyethylene is a suitable material from the points of view of latent heat, behavior of melting and crystallization, and material cost. The thermal stability is satisfactory when it is used in a closed system with heat transfer media, such as silicone oil, alkyldiphenyl, alkyldiphenylethane, Caloria HT and ethylene glycol, which shield polyethylene from air. Surface-crosslinking by ion bombardment prevents polyethylene from mutual adhesion and it retains its form after melting. Surface-crosslinked polyethylene with silicone oil or ethylene glycol is the most promising, while the heat transfer media, such as alkyldiphenyl, alkyldiphenylethane and Caloria HT, which dissolve polyethylene, decrease the melting point, but have little effect on the latent heat and sharp DSC peaks of melting and crystallization. Thus, the composite of polyethylene with these media can also be used at an adjusted and desirable operating temperature.

A calorimetric study on a cylindrical type lithium secondary battery by using a twin-type heat conduction calorimeter

Thermochemical properties of a lithium secondary battery are calorimetrically studied. A twin-type heat conduction calorimeter was used and its accuracy was determined by the specific heat capacity measurement of a standard material, synthetic sapphire. The inaccuracy was smaller than 1.1% in the range of 303–343 K. The thermal transitions corresponding to crystal phase transitions of the positive electrode material, Li1−xCoO2, between hexagonal and monoclinic symmetry were observed. An attempt was made to measure the apparent specific heat capacity of the battery and to study its dependence on the state of charge. The influences of the diffusion of lithium ions in the electrode materials and the self-discharge to the apparent specific heat capacity are discussed.

Investigation of latent heat-thermal energy storage materials. IV. Thermoanalytical evaluation of binary eutectic mixtures of NaOH with LiOH or KOH

Abstract Differential scanning calorimetry is used to measure the heat capacity and heat of fusion of binary eutectic mixtures of NaOH with LiOH or KOH, which are promising materials for latent heat-thermal energy storage materials. These binary mixtures are concluded to be useful for thermal energy storage, when residual water, which has a large influence on their thermal characteristics, is exclusively eliminated. The thermophysical data obtained are also useful in R&D of the technology as well as design of the equipment.

Charge and Discharge Characteristics of a Direct Contact Latent Thermal Energy Storage Unit Using Form-Stable High-Density Polyethylene

For solar thermal energy utilization in relatively low temperatures, a lab-scale direct contact, latent thermal energy storage unit using a form-stable high-density polyethylene (HDPE) was developed. The phase change material (PCM), the form-stable HDPE, does not fluidize nor adhere even after melting, and this particular property permits a direct contact heat transfer between the PCM and a heat transfer fluid (HTF). The storage column in the present study consists of a bundle of vertically arranged thin HDPE rods, where HTF flows in the axial direction and contacts with the HDPE surface directly. A series of experiments were performed for both charge and discharge modes under conditions of different flow rates, initial temperatures in the column, and HTF inlet temperatures. A numerical simulation was also made to study further detailed performance of the storage unit. The charge and discharge characteristics of the storage unit are discussed.

Investigation of latent heat-thermal energy storage materials. II. Thermoanalytical evaluation of urea

Abstract Urea is a very suitable material for latent heat-thermal energy storage in the temperature range 100–150°C with respect to the heat of fusion and material cost. Differential scanning calorimetry and thermogravimetry were applied to observe melting and crystallization behavior and to evaluate the thermal stability. Heating and cooling curve methods were also used. It has been revealed that urea has two serious drawbacks; one is a large supercooling tendency and the other is its low thermal stability. By forming a eutectic mixture with potassium chloride or ammonium chloride, the supercooling tendency is remarkably decreased and addition of sebacic acid is also effective for the purpose, but the low thermal stability is not improved by the addition of these compounds. Urea is thus unsuitable for latent heat-thermal energy storage because of its low thermal stability, though the latent heat and the material cost of urea are superior to those of polyethylene which is selected to be a promising material for latent heat-thermal energy storage in the same temperature range.