J. Ll. Tamarit

Applicability for heat storage of binary systems of neopentylglycol, pentaglycerine and pentaerythritol: A comparative analysis

Abstract We have studied the solid-solid transitions of mixtures of the polyols neopentylglycol (NPG), pentaglycerine (PG) and pentaerythritol (PE). Values of temperatures and heat of transition for the PE/PG binary system are given. We have observed an increase of these values with the concentration of PE. Our calorimetric results show that PE/PG, PG/NPG and PE/NPG systems are suitable for heat storage. Depending on the transition temperatures, these systems are available for different applications (solar heat, industrial process heat, airborne and waterborne heat for space heating). We have also analysed the thermal hysteresis of the transitions and its influence on the utilization of these mixtures for energy storage.

X-ray and molecular dynamics study of liquid structure in pure methylchloromethane compounds ((CH3)4−nCCln)

We have studied liquid structure for a whole family of methylchloromethane compounds ((CH3)4−nCCln), exploiting the interplay of x-ray diffraction measurements and molecular dynamics (MD) computations. To this end we report for the first time x-ray spectra for 1,1,1-trichloroethane (n=3), and 2,2-dichloropropane (n=2), together with a new determination for carbon tetrachloride (n=4). A consistent set of molecular models for MD simulation has also been developed for the full family, providing excellent accord with thermodynamic properties (vaporization enthalpy and density over the full liquid phase), and with diffraction data alike. The theoretical results have allowed the interpretation of the salient features in the experimental spectra and of the trends peculiar to this family of compounds, basically characterized by the suppression of one of the two main peaks in the spectrum as the number of chlorines is diminished. A numerical method that constructs radial correlation functions for ideal dimer geome...

Decagonal C60 crystals grown from n-hexane solutions: solid-state and aging studies

Abstract Decagonal C60 crystals grown from n-hexane solutions correspond to an orthorhombic 1:1 solvate (a=10.249 A, b=31.308 A, c=10.164 A). It forms with negative excess volume ( −55.5 A 3 per formula unit) and transforms on heating into fcc C60 (desolvation enthalpy of +50.6 kJ per solvate mole, close to the sublimation enthalpy for pure n-hexane) while n-hexane desorption from fcc C60 is accompanied by an enthalpy of +48.6 kJ per solvent mole. Thus solvate formation is preferred to solvent adsorption. Orthorhombic C 60 ·1 n -hexane undergoes no degradation when stored in air for 9 years at room temperature in the dark.

Dielectric studies on orientationally disordered phases of neopentylglycol ((CH3)2C(CH2OH)2) and tris(hydroxymethyl aminomethane) ((NH2)C(CH2OH)3)

The dielectric spectra up to frequencies of 1 GHz of the substances 2,2-dimethyl-1,3-propanediol (or neopentylglycol, NPG) and 2-amino-2-(hydroxymethyl)-1,3-propanediol (or tris(hydroxymethyl)aminomethane, TRIS) have been determined in the orientationally disordered FCC and BCC phases respectively, in the ranges 305 - 353 K for NPG and 343 - 375 K for TRIS (in this case, in the supercooled BCC orientationally disordered phase). The dielectric relaxation of both substances has been studied by considering modified Debye theories as well as the Jonscher equation. Significant deviations from the single Debye relaxation were found in both cases. The temperature dependence of the relaxation time, assuming Arrh?nius or Eyring activation models, allows one to evaluate of the activation enthalpies and entropies. Large values were obtained for both parameters, showing that there are strong intermolecular interactions via hydrogen bonds, as had been proved previously for solid phases of alicyclic and branched (tetrahedral molecules) alcohols.

Floor radiant system with heat storage by a solid-solid phase transition material

Abstract Heat storage by means of solid-solid phase transition has been used in a floor radiant system. Tests have been performed in order to establish a comparison between a system storing energy as latent heat with another one using sensible heat. The first system is much more efficient in temperature regulation, allowing an adequate utilization of the off-peak electricity for the charge period. Results obtained in this study show the promising perspectives of the solid-solid phase transition for thermal storage in building materials.

Influence of grain growth on the martensitic transformation in βCuZnAl memory alloys

Abstract The effect of grain size on martensitic transformation in CuZnAl shape memory alloys subjected to two thermal treatments in order to retain β phase has been studied by means of calorimetry and acoustic emission. The dependence of transformation temperatures and thermal hysteresis on grain size is interpreted from a thermodynamic model in terms of elastic enthalpy and frictional work contributions; these are non-chemical terms present in thermoelastic martensitic transformation.

Calorimetric study of the mixtures PE/NPG and PG/NPG

Abstract The polyalcohols neopentylglycol (NPG), pentaglycerine (PG) and pentaerythritol (PE) present reversible solid-solid transitions with properties (enthalpy change, temperature) that make them suitable for energy storage at relatively low temperatures (40–180°C). In this article we present a calorimetric study of the binary mixtures PE/NPG and PG/NPG. We have analyzed the dependence of the enthalpy changes and the transition temperatures on the concentration of the compounds, and the effect of cycling on the transition characteristics. Our results indicate that, when the concentration of the minoritary compound in the mixture is less than about 40% there is a total inhibition of the transition corresponding to this compound. We observe that the transition from crystalline solid to plastic crystal corresponding to NPG can be advanced in temperature by addition of PE or PG. This fact increases the possibilities of the utilization of these mixtures for solar energy storage.

Liquid crystal binary mixtures 8CB+8OCB: critical behaviour at the smectic A-nematic transition

The experimental determination by means of DSC together with a thermodynamic analysis of the two-component phase diagram of octylcyanobiphenyl (8CB)+ octyloxycyanobiphenyl (8OCB) has been performed. The thermodynamic consistency of the two-component system has been proved and similar excess Gibbs energies for mixtures in the isotropic, nematic and smectic phases have been obtained. In order to study the order of the nematic to smectic A phase transition, specific heat measurements obtained using modulated DSC have been made on three mixtures of 8CB+ 8OCB as well as on the pure components. An evolution of the α -critical exponent has been found when the composition of the mixture changes between pure 8CB and pure 8OCB showing crossover behaviour between 3D-XY universal class and the tricritical point. As the 8OCB content in the mixture is further increased, the α -value decreases with the decrease in the McMillan ratio (T NA/T NI) giving rise to a value of about 0.96 at which the 3D-XY mode would be hypothetically reached. The tricritical point would be hypothetically reached at a McMillan ratio of about 0.99 and the extrapolated tricritical amplitude ratio A −/A + is estimated to be 1.6, a value very different from that obtained for the 3He-4He tricritical transition, but comparable with other experimental results reported in the literature.

Dynamic heterogeneity in the glass-like monoclinic phases of CBrnCl4−n, n = 0,1,2

Glassy dynamics of rigid molecules is still a matter of controversy: the physics behind the relaxation process at time scales faster than that ruled by the viscosity, the so called Johari-Goldstein process, is not known. In this work we unravel the mechanism of such a process by using a simple molecular model in which the centers of mass of the molecules are forming an ordered lattice, and molecular reorientation is performed by jumps between equilibrium orientations. We have studied the dynamics of simple quasi-tetrahedral molecules CBr(n)Cl(4-n), n = 0, 1, 2, in their monoclinic phases by means of dielectric spectroscopy and nuclear quadrupole resonance: the first technique allows to measure in a broad time scale but it is insensitive to molecular particularities, while the second has a restricted time window but senses the movement of each chlorine atom separately. The dynamic picture emerging from these techniques is that the secondary relaxation process is related to the different molecular surroundings around each nonequivalent atom of the molecule. Dynamical heterogeneities thus seem to be the cause of the secondary relaxation in this simple model of glass.

Miscibility study in stable and metastable orientational disordered phases in a two-component system (CH3)CCl3+CCl4

The orientationally disordered stable and metastable mixed crystals of the two-component system methylchloroform ((CH3)CCl3)+carbon tetrachloride (CCl4) have been characterised from a crystallographic and thermodynamic point of view. The monotropic behaviour of the metastable phase in the pure components is maintained for the whole range of composition. The lattice symmetry of the stable orientationally disordered phase of methylchloroform has been found to be isostructural with that of the carbon tetrachloride compound. Continuous series of both stable and metastable mixed crystals give rise to a double isomorphism relationship, one for the stable state and another for the metastable stable of the pure components.