Search results for "Phase Transition"
showing 10 items of 1281 documents
Thermoreversible gelation of kappa-carrageenan: relation between conformational transition and aggregation.
2003
Abstract We have studied, by optical rotation dispersion, light scattering and rheology, the κ-Carrageenan system to elucidate the processes involved in gel formation (on decreasing the temperature) and gel melting (on increasing the temperature). Our results show that, on decreasing the temperature, a conformational transition from coils to double helices first occurs, followed by aggregation of the double helices into domains and gel formation at appropriate polymer concentration. Structural details of this sequence are better revealed by re-heating the system. Melting appears as a two-step process characterized by first a conformational change of helices involved in junction zones betwee…
Ethyl Hexanoate Transfer Modeling in Carrageenan Matrices for Determination of Diffusion and Partition Properties.
2007
; Aroma compound properties in food matrices, such as volatility and diffusivity, have to be determined to understand the effect of composition and structure on aroma release and perception. This work illustrates the use of mass transfer modeling to identify diffusion and partition properties of ethyl hexanoate in water and in carrageenan matrices with various degrees of structure. The comparison of results obtained with a diffusive model to those obtained with a convective model highlights the importance of considering the appropriate transfer mechanism. Modeling of the preliminary experimental steps ensures correct estimation of the conditions for the main aroma release step. The obtaine…
Analysis of variability in divergence and turn-over induced by three idealized convective systems with a 3D cloud resolving model
2020
Abstract. The sensitivity of upper tropospheric and lower stratospheric convective outflows and related divergence fields is analysed using an ensemble of cloud resolving model (CM1) simulations in LES-mode including various physically manipulated simulations for three different convective systems initialized with an idealized trigger. The main goal of this study is to assess to what extend the divergence field depends on cloud microphysical processes, the mode of convection and on the processes of convective momentum transport and moist static energy redistribution. We find that latent heat release (representing the microphysical uncertainty) plays an essential role by explaining much of m…
Lateral indirect exchange coupling in a two-dimensional nanostripe array
2002
We observe a lateral indirect exchange coupling in a multistripe system of parallel Fe nanostripes grown by self-organized growth on a vicinal single-crystal W(110) surface. The width and the distance between the stripes of monolayer height were modified via the total amount of deposited Fe. The easy axis of the magnetization lies perpendicular to the stripe edges and in the surface plane, thus resulting in a ferromagnetic dipolar coupling. In addition to the dipolar coupling we observe an antiferromagnetic indirect exchange coupling for the nanostripe system covered by Au. The coupling causes a superferromagnetic phase transition in the nanostripe array.
DSC, Dilatometric, Dielectric, and1H NMR Studies of Phase Transitions and Molecular Motions in [N(C2H5)4]3M2Cl9 (M = Sb, Bi) Crystals
1995
Results in the dependence of stoichiometry of obtained tetraethylammonium (TEA) chloroantiomonate and chlorobismuthate salts on the molar ratio of reactants used in the synthesis are presented. Seven tetraethylammonium salts are obtained: (TEA) 6 M 8 Cl 30 , TEAMCI 4 , (TEA) 3 M 2 Cl 9 (M = Sb, Bi) and (TEA) 2 SbCl 5 . Preliminary X-ray diffraction studies on (TEA) 3 M 2 Cl 9 (M = Sb, Bi) show that they are isomorphous, crystallizing at room temperature in monoclinic symmetry. The dilatometric, dielectric, and DSC studies show that (TEA) 3 Bi 2 Cl 9 undergoes two phase transitions at T c2 = 144 K and at T c1 = 322 K while (TEA) 3 Sb 2 Cl 9 undergoes three transitions at T c3 = 185, T c2 = 2…
On structural phase transitions inn-butylammonium chloroantimonate(III) and chlorobismuthate(III) crystals: x-ray, differential scanning calorimetry,…
1997
Numerous structural phase transitions are detected in new crystals of the n-butylammonium compounds: , and by means of differential scanning calorimetry, dilatometric and dielectric dispersion (1 kHz - 1 MHz) studies. For the transitions the basic thermodynamic data are determined. Interesting dielectric properties are found in a metastable form of the crystals. Debye-like dispersion of the electric permittivity between 30 and 800 MHz is observed around the 310 K phase transition for this crystal. The activation energy of the reorientation of the n-butylammonium cations is found to be 16 kJ . The structure of ( form) has been refined to R = 0.0439 and shows isolated units and two non-equiva…
Ferroelectricity in {N(CH3)4}2ZnI4
1988
Dielectric properties of the {N(CH 3 ) 4 } 2 ZnI 4 crystal were studied between room temperature and 4.2 K. Two phase transitions were seen from dielectric constant anomalies at T 1 =254 K and T c =210 K on cooling. Ferroelectric hysteresis loops were observed along the c -direction below T c with a low frequency applied field of 0.0025 Hz. The spontaneous polarization obtained by pyroelectric charge measurement tended to be 2.5×10 -3 C/m 2 at 0 K.
Structure, phase transitions and molecular dynamics in ferroelastic crystal pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6]
2005
Abstract The crystal structure of the pyrrolidinium hexachloroantimonate(V), [C4H8NH2][SbCl6], abbreviated PCA, has been determined by means of X-ray diffraction at 300 and 340 K. The space groups are monoclinic P 2 1 / n (phase III) and orthorhombic Pmnb (phase II), respectively. The crystal undergoes two structural phase transitions: first-order type at 356/329 K (heating/cooling) from phase (I) to (II) and second-order type at 323 K from phase (II) to (III). Dielectric studies suggest the plastic crystals behaviour above 356 K (phase I). Proton spin-lattice relaxation time ( T 1 ) and second moment ( M 2 ) of polycrystalline [C4H8NH2][SbCl6] have been determined at 77–370 K, at 90 and 25…
The structure and phase transition of mixed crystals
1996
The phase transition and molecular motions of methylammonium cations are studied in the temperature range from 110 to 450 K in the (x = 0.22, 0.345) (MACAB) crystal by x-ray DSC, dilatometric, dielectric and NMR techniques. It is shown that MACAB (x = 0.22) undergoes a structural phase transition at 222 K. The second moment of the NMR line () and the temperature dependence of spin lattice relaxation time () results are interpreted in terms of ( ion dynamics. In the high-temperature phase (I) the cations undergo isotropic reorientations. In the low-temperature phase (II) only one of three cations is still disordered, whereas the two others perform a type of reorientation about their C - N ax…
The structure, phase transition and molecular dynamics of [C(NH2)3]3[Sb2Br9]
2005
The crystal structures of [C(NH2)3]3[Sb2Br9] (Gu3Sb2Br9) at 300 K and of [C(NH2)3]3[Sb2Cl9] (Gu3Sb2Cl9) at 90 and 300 K are determined. The compounds crystallize in the monoclinic space group: C 2/c. The structure is composed of Sb2X93− (X = Cl, Br) ions, which form two-dimensional layers through the crystal, and guanidinium cations. In Gu3Sb2Br9 the structural phase transformation of the first-order type is detected at 435/450 K (on cooling/heating) by the DSC and dilatometric techniques. The dielectric relaxation process in the frequency range between 75 kHz and 5 MHz over the low temperature phase indicates reorientations of weakly distorted guanidinium cations. The proton 1H NMR second-…