J. Font

The effects of temperature on the behaviour of an apatitic calcium phosphate cement

An apatitic calcium phosphate cement is obtained by mixing α-tricalcium phosphate (α-TCP) and precipitated hydroxyapatite into a cement powder, and by then mixing this powder with an aqueous solution of Na2HPO4 as an accelerator. Setting times were reduced by about 30% by increasing the temperature from 22 to 37°C. Compressive strength reached higher intermediate and final values at 37 °C. Degrees of transformation of the α-TCP in the resulting calcium-deficient hydroxyapatite (CDHA) were much higher at 37 °C after 24 h of storage in Ringers solution according to X-ray diffraction. Differential scanning calorimetry indicated that the rate of reaction increased by a factor of about 5 when the temperature was increased from 25 to 37 °C. Scanning electron microscopy showed that the microstructure was more homogeneous and that a more tight entanglement of the precipitated CDHA crystals occurred after storage at 37 °C than at room temperature.

Thermomechanical cycling in Cu /Al /Ni-based melt-spun shape- memory ribbons

Temperature-induced transformations under constant tensile loads (thermomechanical cycling) have been performed on a set of Cu � /Al � /Ni-based shape-memory ribbons prepared by melt spinning. The rapidly solidified ribbons being already characterized in what concerns martensite and parent phase ageing and their effects on the thermal (stress free) martensitic transformation, the purpose of this study was to complete the characterization of the ribbons, particularly in their shape-memory effect. The results presented for a Cu */13.0 Al */4.0 Ni */0.3 Ti */0.05 B (wt.%) ribbon show larger recoverable strains in the strain � /temperature cycles (thus larger shape-memory effect) for ribbons aged at 200 8C than for the as-spun samples. The dependence of the transformation temperatures T on the applied load, quantified through the slope dT /ds , increases for aged alloys. Both results can be related to the recovery of internal stresses produced by the rapid solidification and some recovery of short-range disorder, both mechanisms favouring higher values of the effective transformation strain. This behaviour is in agreement with the evolution of transformation temperatures in stress free transformation and related microstructural changes produced by ageing the ribbons in parent phase. # 2003 Elsevier Science B.V. All rights reserved.

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.

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.

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.

Preliminary study of a heat storage unit using a solid-solid transition

A preliminary study for the design of a device for domestic water has been performed. A phase change material in solid state has been used to store solar energy. We have determined some characteristics physical parameters necessary to construct this device. The results obtained using a numerical simulation, considering an unidirectional model, have been verified with those obtained experimentally. The concordance between both experimentall and simulation results show that this model is available to study the heat transfer phenomenon in the PCM in order to optimize the design of the device.

Miscibility and molecular interactions in plastic phases: Binary system pentaglycerin/ tris(hydroxymethyl)aminomethane

Abstract The phase diagram of the plastic crystals pentaglycerin (PG) and tris(hydroxymethyl)-aminomethane has been established by means of thermal analysis and X-ray powder diffraction from room temperature to the liquid state. The binary system is characterized by two eutectoid invariants corresponding to solid to plastic phase transitions and by one eutectic invariant corresponding to the melting process. The phase diagram exhibits a demixing region in the plastic state as a consequence of the difference between the disordered high-temperature forms of the pure compounds. The determination of the solubility boundaries in the plastic state shows a narrow two phase domain (about 0.13 in molar fraction). The evolution of the volume occupied by a molecule in the molecular alloys in the plastic phases can be interpreted taking into account the different intermolecular interactions in these phases and by geometrical effects.

Determination of an intermediate cubic phase in the PE/NPG binary system by X-ray powder diffraction

Abstract An intermediate cubic phase was detected in the binary system PE/NPG by accurate powder X-ray diffraction at various constant temperatures. The solubility boundaries between this new phase and the biphasic adjacent domains were determined and related to the isobaric thermal expansion. The results obtained by X-ray diffraction are supported by calorimetric measurements and from these we determine a temperature invariant when the intermediate cubic phase appears in the PE/NPG diagram.

Miscibility in plastic phases: binary system NPG (neopentylglycol)/AMP (2-amino,2-methyl-1,3-propanediol)

Abstract The phase diagram of the plastic crystals neopentylglycol (NPG) and 2-amino, 2-methyl-1,3-propanediol (AMP) has been determined. The phase diagram shows an extensive diversity of two-phase regions, due to the polymorphism of the pure substances. The binary system is characterized by two eutectoid invariants corresponding to the solid to plastic transitions and by one eutectic invariant corresponding to the melting process. The solubility boundaries, determined by means of X-ray powder diffraction, strongly depend on which phases are involved. The demixing region in the plastic state shows a very narrow two-phase domain (approximately 0.05 in mole fraction). The study of the mole-fraction dependence of the cell dimensions of the molecular alloys in the plastic phase is explained by means of the existence of different intermolecular interactions and geometrical effects.

Characterization of a hot-rolled CuAlNiTi shape memory alloy

Abstract The changes in the martensitic transformation of a CuAlNiTi alloy hot-rolled at different temperatures have been studied in detail, covering different aspects such as ageing in the parent phase at temperatures ranging between 250 and 350 o C, stabilisation of the martensite and betatization of the previously hot-rolled specimens. Besides the evolution of transformation temperatures upon different thermal treatments, special attention has been paid to the changes in mechanical properties of the alloy, such as elastic modulus and internal friction. These results are analysed in relation to the microstructural changes as observed by transmission electron microscopy.