DSC Calibration During Cooling

A number of compounds are investigated for DSC calibration during cooling. Adamantane and Zn show fast reversible transitions and can be applied both for temperature and for heat calibration. A third compound, namely 4,4′-azoxyanisole, has a liquid crystal to isotropic liquid transition at 409 K. This compound can be used for temperature calibration. Heat calibration with this compound is more problematic because of the small heat effect and the construction of the baseline. Two other compounds, namely Hg and Pb, show a slight undercooling. Nevertheless they can be used for heat calibration, and possibly also for temperature calibration during cooling. This revised version was published online in August 2006 with corrections to the Cover Date.     

DSC Investigation of some Testing and Calibration Compounds, Particularly During Cooling

Two compounds are described with interesting properties for use in DSC. The first compound is adamantane (C₁₀H¹⁶), with a reversible solid-solid transition at 208.62 K [1], suitable for DSC calibration at this low temperature [2]. The second compound is 4,4'-azoxyanisole (C₁₄H₁₄N₂O₃), with a liquid crystal range between 390 and 407 K [3]. This compound shows two transitions on heating, with a large heat effect at 390 K and a small heat effect at 407 K. For this reason, this substance is well suitable for testing the sensitivity and the resolution of DSC instruments [4]. For both compounds not only the heating, but also the cooling behaviour is investigated.This revised version was published online in November 2005 with corrections to the Cover Date.     

Identification of Superoxide O2- during Thermal Decomposition of Molten KNO3-NaNO2-NaNO3 Salt by Electron Paramagnetic Resonance and UV-Vis Absorption Spectroscopy

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O₂^2- and O₂^- were considered as intermediates in molten KNO₃-NaNO₃ while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UVVis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO₃/40 wt% NaNO₂/7 wt% NaNO₃). It is found that the superoxide is more easily generated from molten NaNO₂ compared to NaNO₃, and it has an absorption band at 420-440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO₂ or KO₂, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO₂ and NaNO₃ are also obtained and compared with that of HTS.     

Molar enthalpy at five compositions in the NaNO3NaOH system in the liquid and solid states

The molar enthalpies of the two compounds and three eutectics in the NaNO₃NaOH system were determined over the 450–630 K temperature range by means of drop calorimetry. The enthalpy changes over the 500–550 K range, which in all cases includes melting, are usefully large from the viewpoint of energy storage: 5.7 (79), 5.2 (83), 5.1 (87), 5.1 (93) and 3.5 (73) kcal mole^?1 (cal g^?1) in 0.710 NaNO₃·0.290 NaOH, 0.500 NaNO₃·0.500 NaOH, 0.411 NaNO₃·0.589 NaOH, 0.327 NaNO₃·0.673 NaOH and 0.178 NaNO₃·0.822 NaOH, respectively. At χ_NaOH = 0.822, a high, negative heat of mixing (0.7–0.8 kcal mole^?1) was noted. This is contrary to what has been observed in other common cation-mixed anion molten salt solutions.     

Photoinduced Reversible Solid‐to‐Liquid Transitions for Photoswitchable Materials

Heating and cooling can induce reversible solid‐to‐liquid transitions of matter. In contrast, athermal photochemical processes can induce reversible solid‐to‐liquid transitions of some newly developed azobenzene compounds. Azobenzene is photoswitchable. UV light induces trans‐to‐cis isomerization; visible light or heat induces cis‐to‐trans isomerization. Trans and cis isomers usually have different melting points (T_m) or glass transition temperatures (T_g). If T_m or T_g of an azobenzene compound in trans and cis forms are above and below room temperature, respectively, light may induce reversible solid‐to‐liquid transitions. In this Review, we introduce azobenzene compounds that exhibit photoinduced reversible solid‐to‐liquid transitions, discuss the mechanisms and design principles, and show their potential applications in healable coatings, adhesives, transfer printing, lithography, actuators, fuels, and gas separation. Finally, we discuss remaining challenges in this field.     

Thermodynamic performance of the NaNO3–NaCl–NaNO2 ternary system

Calculations of phase diagram for additive ternary molten salt system are carried out by the conformal ionic solution theory. The liquidus temperatures of the system NaNO₃–NaCl–NaNO₂ are determined according to different types of solid–liquid equilibrium and different values of the binary interaction coefficients. For the system NaNO₃–NaCl–NaNO₂, the calculated and experimental temperatures differ are very small below 673 K, but the oxidation and decomposition of the mixed salts are found when the temperature is higher than 673 K. Meanwhile, the eutectic point is obtained from calculated phase diagram and the eutectic temperature is 507 K. Thermal stability of this eutectic mixture is investigated by thermo-gravimetric analysis device. Experimental results show this kind of molten salt has a lower melting point (501.28 K), similar to solar salt (493 K). It is thermally stable at temperatures up to 819 K, and may be used up to 827 K for short periods.     

Isolation of free phenolic compounds from arboreal leaves by use of the Florisil/C<Subscript>18</Subscript> system

In studies of the phenolic compounds present in leaves and needles, GC and GC–MS have so far been applied only sporadically. This is probably because of the greater difficulties encountered in preparing the samples for this method than those used for liquid chromatography. When preparing a sample for gas chromatography the analyst is faced with two difficult stages—separation of the compound from the matrix without losses (stage 1) so that the final sample can be derivatized to make it suitable for analysis on a non-polar capillary column of the gas chromatograph (stage 2). This paper presents a procedure for extraction of phenolic compounds from the matrix by means of a Florisil/C₁₈ sorbent system and their analysis by GC. After passage through the adsorbents the recovery ranges from 32% for ferulic acid to 88% for gentisic acid. It was found that this extraction method and the GC analysis are very precise (particularly for samples of a mass <1 g) and can be used for quantification. The high-precision quantification of 15 phenolic acids, shikimic acid, and six other compounds present in pine needles has been achieved. The conditions used for GC analysis and construction of calibration curves for quantitative determination are given.     

Viscosity of liquid Cu–Sn alloys

We investigated of the kinematic viscosity of liquid Cu–Sn alloys upon heating and subsequent cooling by the method of the oscillating cylinder. For the liquids alloys Cu75Sn25, Cu50Sn50, Cu48Sn52, Cu32Sn68, and Cu17Sn83, the temperature dependencies of the viscosity upon heating deviate from the Arrhenius relation. The temperature dependencies of viscosity show the Arrhenius-like behaviour upon cooling for all investigated alloys. A discrepancy between the temperature dependencies of viscosity obtained upon heating and cooling arised. We built the concentration dependences of the kinematic viscosity of liquid Cu–Sn alloys upon cooling. The increase of the values of viscosity and activation energy of viscous flow in the concentration range corresponding to the existence of intermetallic compounds Cu₃Sn in the solid state was observed. These results were qualitatively interpreted using the concept of microheterogeneities of liquid alloys.     

Heats of melting of sodium nitrate and indium by differential scanning calorimetry: a suggestion for a new calibration substance

Measurments with a Perkin-Elmer DSC-2 differential scanning calorimeter that has been calibrated with indium, tin, and lead have yielded values for the heat of melting of sodium nitrate. Analysis of these results along with those from earlier investigations gives ΔH_m ? 3615 cal mol^?1 as a recommended “best” value for the heat of melting of NaNO₃. The consistency of our results over a period of three years in which many samples were investigated, in combination with results of earlier investigations, leads us to suggest that NaNO₃ is a useful substance for calibration of differential scanning calorimeters. Results of our measurements of the heat of melting of indium are presented and discussed in relation to earlier investigations.     

Rotating Magnetocaloric Effect in an Anisotropic Molecular Dimer

In contrast to the mainstream research on molecular refrigerants that seeks magnetically isotropic molecules, we show that the magnetic anisotropy of dysprosium acetate tetrahydrate, [{Dy(OAc)₃(H₂O)₂}₂]?4 H₂O ( 1 ), can be efficiently used for cooling below liquid‐helium temperature. This is attained by rotating aligned single‐crystal samples in a constant applied magnetic field. The envisioned advantages are fast cooling cycles and potentially compact refrigerators.     

Evaluation of the calibration errors on cooling of a differential scanning calorimeter using different sets of standard metals

The temperature calibration on cooling of thermal analysis instruments is important for the accurate study of the non-isothermal crystallization kinetics of semi-crystalline polymers. In previous works, a methodology was proposed for performing the calibration on cooling of differential scanning calorimeters (DSCs) with standard metals, and the calibration errors were checked using transitions of high-purity liquid crystals. In this work, alternative, physically meaningful, procedures for carrying out the calibration on cooling are analyzed and validated. The calibration errors are evaluated also with liquid crystalline transitions, when the calibration is performed with standard metals, in a wide temperature range, and when due account is taken for the need of isothermal corrections to the temperature measurements. It is shown that any pair of standard metals may be used to calibrate on cooling, that the calibration errors increase for wider working temperature ranges and that, providing that certain conditions are fulfilled, both calibration procedures yield similar results.     

Phase transitions in potassium nitrate

It is shown that the heat of transition of the phase change II → I at 129° on heating KNO₃ is dependent on the thermal history of the sample, since it involves two steps, viz., II→ III and III→ I at 2° interval. During cooling, the latter step is fast and truly reversible, though with a temperature hysteresis. The former step is sluggish and is dependent both on temperature and time. Our results indicate that KNO₃ can be used for calibration purpose only if the material has not been heated beyond 128° in the immediately preceding three hours.     

Mass-spectrometric examination of vaporization of sodium nitrite and sodium and potassium nitrates

Vaporization of sodium nitrite and sodium and potassium nitrates was examined by high-temperature mass spectrometry. The inverse temperature dependences of the vapor pressure logarithm for these compounds were presented. Enthalpies of vaporization and standard enthalpies of formation of gaseous NaNO₂, NaNO₃, and KNO₃ were determined.     

Modeling phase equilibria of alkanols with the simplified PC-SAFT equation of state and generalized pure compound parameters

The simplified PC-SAFT equation of state has been applied to liquid–liquid, vapor–liquid and solid–liquid equilibria for mixtures containing 1- or 2-alkanols with alkanes, aromatic hydrocarbons, CO₂ and water. For the alkanols we use generalized pure compound parameters. This means that two of the physical pure compound parameters, m (segment number) and σ (segment diameter), are obtained from linear extrapolations, since m and mσ³, increase linearly with respect to the molar mass, and moreover, the two association parameters (association energy and association volume) were assumed to be constant for all alkanols. Only the dispersion energy is fitted to experimental data. Thus it is possible to estimate parameters for several 1- and 2-alkanols. The final aim is to develop a group contribution approach for PC-SAFT which is suitable for complex compounds, considering that the motivation of this project is to obtain a thermodynamic model which can be used in the development of sophisticated products such as pharmaceuticals, polymers, detergents or food ingredients. One of the severe limitations in applying SAFT-type equations of state to these compounds is that the procedure for obtaining the pure compound parameters is usually based on fitting to saturated vapor pressure and liquid density data over an extended temperature range. However, such data are rarely available for complex compounds. To verify the new pure compound parameters, comparisons to ordinary optimized alkanol parameters, where all five pure compound parameters were fitted to experimental liquid density and vapor pressure data, were made. The results show that the new generalized alkanol parameters from this work perform at least as well as other alkanol parameter sets.     

Low temperature lattice parameters and expansion coefficients of AI2Au and LiF Gruneisen constants of LiF

There is no difference in the thermal expansion behavior of an intermetallic compound (Al₂Au) and of an ionic (LiF), except in the magnitude of the expansion coefficients, which for both compounds upon cooling below 40°K approach zero. The lattice parameters of the two compounds mentioned decrease uniformly and without anomalies with lowering the temperature, approaching a constant value below 40°K. The a–T relationship between 40 and 180° is given in form of equations. A pump working on the Joule-Thomson principle was used for cooling. The GRÜNEISEN parameter, γ, of LiF between 40 and 180°K is a constant, contrary to theoretical prediction. The values of a, α and γ agree well with previous measurements, where liquid gases were used for cooling and lattice parameter determinations and a dilatometer for expansivity measurements.     

Hydrolysis of (CH3)3Sn+ in Various Salt Media

The hydrolysis of trimethyltin(IV) has been studied by potentiometry (H⁺ -glass electrode) and calorimetry in various salt media (NaNO₃, NaCl, KCl, Na₂SO₄, and NaNO₃—NaCl mixtures). The effect of ionic strength on the hydrolysis constants is accounted for by a simple Debye–Hückel type equation and by Pitzer equations. The results allow us to obtain H for hydrolysis and the temperature dependence of the Pitzer parameters. The resulting coefficients can be used to examine the speciation of (CH₃)₃Sn⁺ in multicomponent electrolyte solutions, such as natural waters, over a wide range of temperature and ionic strength.     

A facile synthesis of NaNbO<Subscript>3</Subscript> powders by decomposition of ammonium niobium oxalate with sodium salt at low temperature

A facile synthesis of NaNbO₃ powders was performed by solid-state reaction at low temperature. Stoichiometric ammonium niobium oxalate and Na₂CO₃ were mixed in water and then calcined at different temperatures for various times after drying. A combination of X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared (FTIR) spectroscopy and thermogravimetric (TG) analysis was used to characterize the product and precursor compound. The XRD patterns show that single-phase NaNbO₃ powders with high crystallinity can be synthesized at 425 °C for 15 min. The particle size from XRD data is found to be about 40 nm for NaNbO₃ powders calcined at 500 °C for 3 h, which is in good agreement with SEM data. The SEM photograph shows that NaNbO₃ powders are cuboid-like and well crystallized when calcination at 800 °C for 3 h. The product compositions prepared using other sodium reactants, such as HCOONa and NaNO₃, are also discussed.     

Enthalpies of Phase Transitions and Heat Capacity of TbCl3 and Compounds Formed in TbCl3–MCl Systems (M=K, Rb, Cs)

The molar enthalpies of the solid–solid and solid–liquid phase transitions were determined by differential scanning calorimetry for pure TbCl₃ and KTb₂Cl₇, RbTb₂Cl₇, CsTb₂Cl₇, K₃TbCl₆, Rb₃TbCl₆ and Cs₃TbCl₆ compounds. Both types of compounds, i.e. M₃TbCl₆ and MTb₂Cl₇ (M=K, Rb, Cs) melt congruently and show additionally a solid–solid phase transition with a corresponding enthalpy Δ_trs H ⁰ of 6.1, 7.6 and 7.0 kJ mol^–1 for potassium, rubidium and caesium M₃TbCl₆ compounds andΔ_trs H ⁰ of 17.1 (rubidium) and of 12.1 and 10.9 kJ mol^–1 (caesium) for MTb₂Cl₇ compounds, respectively. The enthalpies of fusion were measured for all the above compounds with the exception of Rb₃TbCl₆ and Cs₃TbCl₆. The heat capacities of the solid and liquid compounds have been determined by differential scanning calorimetry (DSC) in the temperature range 300–1100 K. The experimental heat capacity strongly increases in the vicinity of a phase transition, but varies smoothly in the temperature ranges excluding these transformations. C _p data were fitted by an equation, which provided a satisfactory representation up to the temperatures of C _p discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the liquid phase, which had been previously measured. The results obtained agreed satisfactorily with these experimental data. This revised version was published online in August 2006 with corrections to the Cover Date.     

The rigid amorphous fraction in semicrystalline macromolecules

The first experimental evidence of the existence of the rigid amorphous fraction (RAF) was reported by Menczel and Wunderlich for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate) (PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC _p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated from the ΔC _p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional) amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling. Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions. After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration.     

Thermal properties characterization of two promising phase change material candidates

Solar absorption cooling is a wonderful method to provide cold energy by exploiting solar energy. Phase change materials (PCMs) that store latent thermal energy are indispensible in solar absorption cooling system. It is worthwhile to find new PCMs due to the demanding on the temperature of the stored thermal energy which in turn would power the absorption chiller. In this paper, two compounds: 1-bromo-2-methoxynaphthalene (compound 1) and 2,2′-diphenyl-4,4′-bi(1,3-dioxane)-5,5′-diol (compound 2), were selected as potential PCMs. Their thermal energy storage properties and thermal stability were characterized by differential scanning calorimetry and thermogravimetric analysis. The results showed that both compounds could be applied as good PCMs in solar absorption cooling systems. Compound 1 melted at 356.82 K with the ΔH of 98.81 J g^?1, while compound 2 melted in a broad temperature range with the melting point of 466.26 K and the ΔH of 101.4 J g^?1. Both compounds exhibited good thermal stability. Furthermore, the molar specific heat capacities of these two compounds were measured by temperature-modulated differential scanning calorimetry from 198.15 K to the temperature that they started to decompose, and the thermodynamic functions of [H _T–H _298.15] and [S _T–S _298.15] were calculated based on the specific heat capacities data.