Calorimetric study of phase transitions in the thiourea carbon tetrachloride inclusion compound

Heat capacities of the inclusion compound (thiourea)_3.00CCl₄ have been measured in the temperature range 15–300 K. A first-order phase transition was found at 41.3 K and a second-order transition at 67.17 K. The enthalpy and entropy of the transition are 149 J mol^–1 and 3.7 J K^–1 mol^–1 for the former, and 241 J mol^–1 and 3.9 J K^–1 mol^–1 for the latter. A divergent expression C = A{(T _c –T)/T _c} ^– was fitted to the excess heat capacity of the upper phase transition. The best-fit parameters wereA = 7.4 J K^–1 mol^–1,T _c = 67.166 K and = 0.31. Possible types of molecular disorder in the high temperature phase are discussed in relation to the transition entropy and the molecular and site symmetries of the guest molecule. The heat capacity of the lowest temperature phase was unusually large and may indicate the existence of very low frequency vibrational modes or labile configurational excitation of the guest molecule. Standard thermodynamic functions were calculated from the heat capacity data and are tabulated in the appendix.Contribution No. 11 from the Microcalorimetry Research Center.     

Heat of transformation of chains in amorphous sulphur

The heatQ of transformation of the chains in amorphous sulphur was measured calorimetrically. The mean value ¯Q for samples remelted atT _f=443 K increases from 31.5 to 45.9 J g^–1 in the measurement temperature range from 288 to 303 K.For samples remelted in theT _f range from 443 to 573 K, the ¯Q values are from 30.6 to 24.0 J g^–1.The results are discussed on the basis of the theory of nucleation and growth of nuclei.     

On the expansion of hole in main chain polymers determined by positron annihilation

The temperature dependence of positron annihilation characteristics, ₃ andI ₃, has been studied on sample of poly(butadiene), poly(isobutylene) and poly(chloroprene). The temperature range was between 15 and 470 K. The rate of expansion of holes or free-volume in all samples was deduced belowT _g as well aboveT _g as appr. 3·10^–3 K^–1 and 2·10^–2 K^–1, respectively. These values are very close to the rate of the mean squared displacement of scatterer<r ²>observed in neutron scattering experiments. A possibility to use an inverse value of free-volume,V _f ^–1 for study of viscoelastic state of polymers is demonstrated.     

Molecular dynamics study of phase transition and nucleation in supercooled clusters of potassium iodide

In a series of molecular dynamics (MD) runs on (KI)₁₀₈ clusters, the Born–Mayer–Huggins potential function is employed to study structural, energetic, and kinetic aspects of phase change and the homogeneous nucleation of KI clusters. Melting and freezing are reproducible when clusters are heated and cooled. The melted clusters are not spherical in shape no matter the starting cluster is cubic or spherical. Quenching a melted (KI)₁₀₈ cluster from 960 K in a bath with temperature range 200–400 K for a time period of 80 ps both nucleation and crystallization are observed. Nucleation rates exceeding 10³⁶ critical nuclei m⁻³ s⁻¹ are determined at 200, 250, 300, 350, and 400 K. Results are interpreted in terms of the classical theory of nucleation of Turnbull and Fisher and of Buckle. Interfacial free energies of the liquid–solid phase derived from the nucleation rates are 7–10 mJ m⁻². This quantity is 0.19 of the heat of transition per unit area from solid to liquid, or about two-thirds of the corresponding ratio which Turnbull proposed for freezing transition. The temperature dependence of σ_sl(T) of (KI)₁₀₈ clusters can be expressed as σ_sl(T)∝T^0.34.     

Thermal and dielectric studies of 2,2’-dihydroxybenzophenone

Thermal and dielectric properties of 2,2’-dihydroxybenzophenone were studied in relation with the potential progress of crystal nucleation and growth below the ordinary glass transition temperature, T_gα. Differential scanning calorimetry was carried out in a range 100-350 K. The α glass transition was found to occur at T_gα=239 K. Crystallization and fusion were observed to take place when the sample was cooled down to 103 K, but not observed when cooled to 203 K. Crystal nucleation was interpreted as having happened during annealing for a short time at 103 K which is much below the T_gα. Heat capacities were measured in a range 7-350 K by an intermittent heating method with an adiabatic calorimeter. The temperature, enthalpy and entropy of fusion were determined to be 334.46 K, 20.07 kJ mol^-1 and 60.01 J K^-1mol^-1, respectively. Crystal growth was found to proceed even at 220 K below the T_gα, but no glass transition was detected below 220 K. Dielectric losses were measured in a temperature range of 100-250 K and a frequency range of 30Hz-10 kHz. β-Relaxation process was found dielectrically with the activation energy of 22.6 kJ mol^-1, and the corresponding glass transition was expected to occur at 76.9 K. It is discussed, based on the “structurally ordered clusters aggregation” model for supercooled liquids and glasses, that the β process is potentially attributed to the crystal nucleation progressing at 103 K.     

Electrical Conductance and Volumetric Studies in Aqueous Solutions of DL-Pyroglutamic Acid

Conductivity measurements of DL-pyroglutamic acid and sodium pyroglutamate in dilute aqueous solutions were performed in the 288.15–323.15 K temperature range. The limiting molar conductances of pyroglutamate anion, λ^o(pGlu⁻, T) and the dissociation constants of pyroglutamic acid, K(T) were derived from the Onsager, and the Quint and Viallard conductivity equations. Densities of aqueous solutions with molalities lower than 0.5 mol-kg⁻¹ were determined at 5 K intervals from T = 288.15 K to 333.15 K. Densities served to evaluate the apparent molar volumes, V_2,φ(m, T), the cubic expansion coefficients, α (m,T) and the changes of the isobaric heat capacities with respect to pressure, (∂ C_P/∂ P)_T,m. They were correlated qualitatively with the changes in the structure of water when pyroglumatic acid is dissolved in it.     

Electron spin-lattice relaxation of Cu(II) in Zn(II)-bis(diethyl-diselenocarbamate) single crystals

Using the pulse saturation method the spin-lattice relaxation rate T₁⁻¹ for Cu(II) in Zn(II)-bis(diethyl-diselenocarbamate) was measured in the temperature range 1.5 < T < 33 K. From the T-dependence of T₁⁻¹ the Debye temperature of the host crystal was obtained. The angular dependence of T₁⁻¹ as well as the influence of the hyperfine interaction on T₁⁻¹ are discussed.     

Kinetic Studies of Bulk Ge22Se78−xBix (x=0, 4 and 8) Semiconducting Glasses

Results of phase transformations, enthalpy released and specific heat of Ge₂₂Se_78–xBi_x(x=0, 4 and 8) chalcogenide glasses, using differential scanning calorimetry (DSC), under non-isothermal condition have been reported and discussed. The glass transition temperature, T _g, is found to increase with an average coordination number and heating rates. Following Gibbs—Dimarzio equation, the calculated values of T _g (i.e. 462.7, 469.7 and 484.4 K) and the experimental values (i.e. 463.1, 467.3 and 484.5 K) increase with Bi concentration. Both values of T _g, at a heating rate of 5 K min^–1, are found to be in good agreement. The glass transition activation energy increases i.e. 102±2, 109±3 and 115±8 kJ mol^–1 with Bi concentration. The demand for thermal stability has been ensured through the temperature difference T _c–T _g and the enthalpy released during the crystallization process. Below T _g, specific heat has been observed to be temperature independent but highly compositional dependent. The growth kinetic has been investigated using the Kissinger, Ozawa, Matusita and modified JMA equations. Results indicate that the crystallization ability is enhanced, the activation energy of crystallization increases with increasing the Bi content and the crystal growth of these glasses occur in 3 dimensions.This revised version was published online in November 2005 with corrections to the Cover Date.     

Oxygen reduction in acid media at the amorphous Mo–Os–Se carbonyl cluster coated glassy carbon electrodes

An amorphous Mo–Os–Se carbonyl cluster compound has been synthesized in 1,2-dichlorobenzene (b.p.≈180°C) to be tested as an electrocatalyst for molecular oxygen reduction in 0.5 M H₂SO₄. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) performed for the powder supported on pyrolytic carbon show a distribution of nanometer-scale amorphous particles with agglomerations in cluster forms. The catalytic activity was studied by the rotating disc electrode technique. Kinetic studies show a first-order reaction with a Tafel slope of −0.118 V dec⁻¹ and dα/dT=1.55×10⁻³ K⁻¹. In the temperature range 298–343 K, an activation energy of 32 kJ mol⁻¹ was determined.     

Vapor pressure, kinetics and enthalpy of sublimation of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II)

The kinetics of sublimation of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II), [Cu(tmhd)₂] was studied by non-isothermal and isothermal thermogravimetric (TG) methods. The non-isothermal sublimation activation energy values determined following the procedures of Friedman, Kissinger, and Flynn–Wall methods yielded 93 ± 5, 67 ± 2, and 73 ± 4 kJ mol⁻¹, respectively and the isothermal sublimation activation energy was found to be 97 ± 3 kJ mol⁻¹ over the temperature range of 375–435 K. The dynamic TG run proved the complex to be completely volatile and the equilibrium vapor pressure (p_e)_T of the complex over the temperature range of 375–435 K determined by a TG-based transpiration technique, yielded a value of 96 ± 2 kJ mol⁻¹ for its standard enthalpy of sublimation (Δ_subH°).     

Electrical Conductance and Volumetric Studies in Aqueous Solutions of Nicotinic Acid

Conductivity measurements of nicotinic acid and sodium nicotinate in dilute aqueous solutions were performed in the (288.15 to 323.15) K temperature range. The limiting equivalent conductances of the nicotinate anion, λ⁰(Nic⁻, T), and the dissociation constants of nicotinic acid, K(T), were derived by the use of the Onsager and the Quint and Viallard conductivity equations. Densities of aqueous solutions with molalities lower than 0.2 mol-kg⁻¹ were determined at 5 K temperature intervals, from T = (288.15 to 333.15) K. The measured densities were used to evaluate the apparent molar volumes, V_{2, φ}(m, T), the cubic expansion coefficients, α(m, T), and the changes of isobaric heat capacities with respect to pressure, (∂C_P/∂p)_{T, m}. They were qualitatively correlated with the changes in the structure of water when nicotinic acid is dissolved in it.     

Pyrolysis of eicosane in supercritical water

Pyrolysis of eicosane and redox reactions of the pyrolysis products in supercritical water (SCW) were studied in a batch reactor at 30 MPa, in the temperature range from 450 to 750 °C and with reaction times ranging from 75 to 600 s. The rate constants for eicosane pyrolysis (k" = 10^16.5±0.5exp[–(32000±2000)/T] s^–1) and for the formation of H₂ (k = 10^25±0.8exp[–(64000±4000)/T] s^–1) were determined. The time and temperature dependences of the heat of reaction were elucidated. Water accelerates pyrolysis and participates in the subsequent transformations of the pyrolysis products. The yield of H₂ sharply increases for T > 700 °C.     

Heat Capacities and Thermodynamic Properties of Fenpropathrin (C22H23O3N)

The heat capacities of fenpropathrin in the temperature range from 80 to 400 K were measured with a precise automatic adiabatic calorimeter. The fenpropathrin sample was prepared with the purity of 0.9916 mole fraction. A solid—liquid fusion phase transition was observed in the experimental temperature range. The melting point, T_m, enthalpy and entropy of fusion, _fusH_m, _fusS_m, were determined to be 322.48±0.01 K, 18.57±0.29 kJ mol^–1 and 57.59±1.01 J mol^–1 K^–1, respectively. The thermodynamic functions of fenpropathrin, H_(T)—H_(298.15), S_(T)—S_(298.15) and G_(T)—G_(298.15), were reported with a temperature interval of 5 K. The TG analysis under the heating rate of 10 K min^–1 confirmed that the thermal decomposition of the sample starts at ca. 450 K and terminates at ca. 575 K. The maximum decomposition rate was obtained at 558 K. The purity of the sample was determined by a fractional melting method.This revised version was published online in November 2005 with corrections to the Cover Date.     

An optical, EPR and electrical conductivity study of blue barium titanate, BaTiO3−δ

The electronic UV–VIS–NIR absorption spectra of single crystalline BaTiO_3−δ (BTO) are studied in the temperature range of 102–1173 K in pure oxygen and at conditions of moderate and strong reduction of the material. The strongly reduced crystals are of deep blue colour. The optical spectra of blue BTO are characterised by a strong absorption in the NIR region at around 7000 cm⁻¹, which is attributed to polaronic defects associated with the formation of Ti³⁺ in the material. This assumption is supported by fits of the spectra using polaronic line shape functions appropriate for disordered systems and also by the electrical conductivity of blue BTO which, in agreement with results from the optical spectra, exhibits an activation energy of 0.20 eV. The EPR spectra of moderately reduced BTO powders show an anisotropic g-factor, which is compatible with the optical spectrum. The temperature dependence of the band gap energy of BTO was found to be given as dE_g/dT = −7.21 × 10⁻⁴ eV/K.     

Solubility of 1-Pentene Ice in Liquid Nitrogen and Argon at the Standard Boiling Points of the Solvents

The solubilities of 1-pentene ice in liquid nitrogen at a temperature of 77.4 K and in liquid argon at 87.3 K have been measured by the filtration method. The 1-pentene content in solution was determined using gas chromatography. The experimental value of the mole fraction solubility of 1-pentene ice in liquid nitrogen at 77.4 K is: (1.28±0.25)×10^–7 and (4.11±0.44)×10^–7 in liquid argon at 87.3 K. The Preston–Prausnitz method was used for calculation of the solubilities of 1-pentene ice in liquid nitrogen in the temperature range 64.0–90.0 K and in liquid argon in the temperature range 84.0–90.0 K. The parameters l ₁₂ were also calculated. At 90.0 K liquid argon is the better solvent for 1-pentene ice than is liquid nitrogen.     

Extended NMR Study of Spin-Crossover Compounds [Fe(1-alkyl-1H-tetrazole)6](BF4)2 and Their ZnII Analogs

A systematic NMR study was performed on several alkyl–tetrazole complexes of iron(II) and zinc(II) in the 10–300 K temperature range. The experiments were designed to separate the electronic and reorientational phase transitions caused by the spin crossover of the iron compounds from those independent of unpaired electrons. The ¹⁹F spectral data on the propyl-tetrazole compounds show that the electronic spin-transition has a strong effect on the spectra, and their behavior can be explained as a combined response to molecular reorientations and the spin transition. For these complexes, second-moment calculations revealed the strength of the interaction between resonant and nonresonant nuclei. Both of the applied NMR methods show irregularities at the temperature region between 70 and 120 K, suggesting the presence of a phase transition. The data also suggest two kinds of reorientational behavior for the BF₄ ^– counter ions. In the iron–ethyl–tetrazole compound, unlike in the propyl–tetrazole complex, a significant amount of unpaired electrons remains in their original high-temperature HS state. Above their effect, the behavior of the nuclear spins of the iron compound is basically governed by the same structural factors as in its zinc analog. The two-exponential behavior of the ¹H-T ₁ in case of the zinc–methyl–tetrazole compound can be explained on the basis of cross relaxation with the ¹⁹F nuclei due to the low ¹H/¹⁹F ratio. The presence of the two types of methyl reorientation is assumed to be the sign of the two different lattice sites known to be present in the Fe^II compound. The single-exponential T ₁ above T _c in the case of [Fe(mtz)₆](BF₄)₂ is consistently the sign of the strength of the paramagnetic relaxation observed in the ethyl and propyl compounds.     

A method for determining the mean translational energy of particles desorbed from solid surfaces

A method for determinig the mean molecular translational energy in gas flows of low intensity (10¹²–10¹⁴ molec. s^–1) has been proposed. The method was verified using various gases (H₂, N₂, O₂, and CO₂ flowing into a vacuum out of a heated capillary. The translational energies were determined for CO and N₂ molecules desorbing from the surface of polycrystalline Ir. The translational temperature (T _tr) measured for CO equals 650±90 K and almost coincides with the surface temperature (T _s = 600 K). In the case of nitrogen molecules,T _tr = 4600±500 K atT _s = 500 K. The method proposed is applicable to the determination of the spatial distribution of molecular beam particles.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 30–34, January, 1995.The authors express their gratitude to A. V. Sklyarov for fruitful discussions during the elaboration of the theoretical basis and technical realization of the method.The reseach was carried out with the partial financial support of the International Science Foundation (Grant MBN 000).     

Heat capacity and other thermodynamic properties of CoTe2 from 5 to 1 030 K and of CoTe2.315 from 300 to 1 040 K

The heat capacity of orthorhombic (marcasite-type structure) cobalt ditelluride has been measured from 5 to 1 030 K by adiabatic-shield calorimetry with alternate energy inputs and equilibrations. Above 900 K a marked increase in heat capacity occurs which probably signals a change in the composition of the CoTe₂-phase towards higher tellurium content. Values at 298.15 and 1 000 K in J K^–1 mol^–1 of the heat capacity (C _p,m), entropy [S _m ^° (T)–S _m ^° (0)], andGibbs energy function – [G _m ^° (T)–H _m ^° (0)]T ^–1 are 75.23, 114.5, 49.93, and 132.4, 216.2, 139.17, respectively. Consistent with the metallic behavior of CoTe₂, deviation of the heat capacity from theDebye T ³-law was found at low temperatures. Comparison with the heat capacity of FeTe₂ shows aSchottky-like deviation with a maximum of 7.3 J K^–1 mol^–1 at 80 K and evidences the influence of the additional 3 d-electron in cobalt compared to iron. Heat capacity measurements were made on CoTe_2.33 to ascertain the existence range of the CoTe_2+x -phase and the entropy of the associated structural disorder.The portion of this research done at Ann Arbor was supported in part by the Structural Chemistry and Chemical Thermodynamics Program of the Division of Chemistry of the National Science Foundation under Grant No. CHE-7710049.     

Recombination of HCO and DCO ions with electrons

Recombination of HCO⁺ and DCO⁺ ions with electrons was studied in afterglow plasma. The flowing afterglow with Langmuir probe (FALP) apparatus was used to measure the recombination rate coefficients and their temperature dependencies in the range 150–270 K. To obtain a recombination rate coefficient for a particular ion, the dependencies on partial pressures of gases used in the ion formation were measured. The variations of α_HCO⁺(T) and α_DCO⁺(T) seem to obey the power law: α_HCO⁺(T) = (2.0 ± 0.6) × 10⁻⁷ (T/300)^−1.3 cm³ s⁻¹ and α_DCO⁺(T) = (1.7 ± 0.5) × 10⁻⁷ (T/300)^−1.1 cm³ s⁻¹ over the studied temperature range.     

Constant-Volume Heat Capacity of H2O + Na2CO3 Solutions Near the Critical Point of Pure Water

The results of new isochoric heat capacity measurements of H₂O + Na₂CO₃ solutions as a function of temperature along several isochores in the near-critical region of pure water are reported. The measurements cover temperatures from 331 to 661 K at Na₂CO₃ mole fraction 0.008869. The experiments were performed at seven densities between 245 and 875 kg-m^–3. The measurements were made in a high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter. Uncertainties of the isochoric heat capacity measurements are estimated to be 1–2% in the liquid phase, 2–3% in the vapor phase, and 4–5% near the critical point. Measurements were made in the two-phase (vapor–liquid, liquid–solid, vapor–solid) and three-phase (vapor–liquid–solid) regions. Two peaks in isochoric heat capacity have been found near the critical point of pure water. One of the peaks at T _S1 occurs on the three-phase (L–V–S) curve and another peak at T _S2 corresponds to a two-phase (L–S or V–S) curve. The experimental values of phase transition temperatures T _S() on each isochore was determined. Uncertainty in the phase transition temperature measured was no greater than ±0.03 to 0.05 K.