Browsing by Author "Lanceros-Mendez, S."

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Dielectric behavior in an oriented β-PVDF film and chain reorientation upon transverse mechanical deformation
(2002) Lanceros-Mendez, S.; Moreira, S. C.; Mano, J. F.; Schmidt, V. Hugo; Bohannan, Gary W.
Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the g -phase have been studied by means of dielectric measurements and Fourier Transform Infrared Spectroscopy (FTIR). The main goal of the study was to compare the dielectric relaxations of f - and g -PVDF and to improve the understanding of the structural changes that occur in g -PVDF during a mechanical deformation process and their impact in the electromechanical properties of the polymer. A reorientation of the chains and a decrease in the degree of crystallinity with increasing deformation was observed.
Dielectric, NMR and X-ray diffraction study of Cs1-x(NH4)xH2PO4
(1998) Meschia, S. C.; Lanceros-Mendez, S.; Zidansek, Aleksander; Schmidt, V. Hugo; Larson, R.
Mixed crystals Cs1−x(NH4)xH2PO4 of the ferroelectric CsH2PO4 (CDP) and the antiferroelectric (NH4)H2PO4 were grown with x=0.2 (CADP0.2) in solution. The structural properties of the crystal were analyzed by means of x-ray diﬀraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). NMR experiments were also performed.
Dielectric, NMR and x-ray diffraction study of pseudo-one-dimensional Cs1-x(NH4)xH2PO4
(1999) Meschia, S. C.; Lanceros-Mendez, S.; Zidansek, Aleksander; Schmidt, V. Hugo; Larsen, R.
Mixed crystals Cs1−x (NH4) x H2PO4 of the ferroelectric CsH2PO4 (CDP) and the antiferroelectric NH4H2PO4 were grown with x = 0.2 (CADP0.2) in solution. The structural properties of the crystal were analyzed by means of X-ray diffraction. Dielectric measurements at several temperatures and frequencies have been performed along the three crystallographic axes in this sample and also in the fully deuterated CADP0.2 sample (DCADP0.2). Dielectric and NMR experiments in a powdered sample were also performed. The shift of the transition temperature as a function of x and deuteration, and the changes in the properties of the different phases together with, the thermally-activated conductivity found in the paraelectric phase will be discussed and related with the several relaxation mechanisms measured in the NMR experiments.
FTIR and DSC studies of mechanically deformed β-PVDF films
(2001) Lanceros-Mendez, S.; Mano, J. F.; Costa, A. M.; Schmidt, V. Hugo
Films of semicrystalline poly(vinylidene fluoride) (PVDF) in the βphase were studied by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The main goal of this study was to improve the understanding of the structural changes that occur in β-PVDF during a mechanical deformation process. FTIR spectroscopy was used to examine the structural variations as a function of strain. DSC data allowed measurement of the melting temperatures and enthalpies of the material before and after deformation, providing information about the changes in the crystalline fraction. After the molecular vibrations were assigned to the corresponding vibrational modes, we investigated the energy and intensity variations of these vibrations at different deformations. A reorientation of the chains from perpendicular to parallel to the stress direction was observed to occur in the plastic region. During the deformation, a decrease in the degree of crystallinity of the material was observed, but the thickness of the lamellae did not change significantly.
Influence of NH4-Rb substitution on the phase transitions with different kinds of proton disorder in mixed [(NH4)1-xRbx]3H(SO4)2 crystals
(1998-09) Baranov, A. I.; Dolbinina, V. V.; Yakushkin, E. D.; Vinnichenko, Yu. V.; Schmidt, V. Hugo; Lanceros-Mendez, S.
The role of ammonium ions in order–disorder ferroelastic, ferroelectric and structural phase transitions was studied in mixed [(NH4)1–x Rb x ]3H(SO4)2 0 < x < 1 crystals by dielectric spectroscopy and ac calorimetry. A new superprotonic phase was detected in compounds with x > 0·6. It was found that small Rb concentrations, x < 0·05, strongly smear out the temperature anomalies of dielectric constant and specific heat for phase transitions II–III, III–IV, IV–V and V–VIII. At x > 0·09 these phase transitions are fully suppressed. Finally, for Rb concentrations 0·05 < x < 0·9 a proton glass phase appears below 35 K.
Paraelectric-antiferroelectric phase coexistence in the deuteron glass Rb0.5(ND4)0.5D2AsO4
(2004) Lanceros-Mendez, S.; Schmidt, V. Hugo; Shapiro, S. M.
Neutron diffraction was used to study the paraelectric (PE) to antiferroelectric (AFE) phase transition in a deuteron glass crystal Rb 0.5 (ND 4 ) 0.5 D 2 AsO 4 (DRADA-50). Coexistence of AFE and PE phases was proven in a temperature range 7-12 K wide.
Phase coexistence in the deuteron glass Rb0.9(ND4)0.1D2AsO4 proven by neutron diffraction
(1999-01) Lanceros-Mendez, S.; Schmidt, V. Hugo; Shapiro, S.A.
Neutron diffraction technique has been used to study the transitions from the paraelectric (PE) phase to the ferroelectric (FE) one in the deuteron glass crystal Rb0.9(ND4)0.1D2AsO4 (DRADA-10). The crystal shows PE hk0 diffraction spots splitting into spot patterns from domains, 4 spots from 4 FE domain types. A more complex phase behavior that the one corresponding to a typical PE-FE phase transition has been found. Coexistence of FE and PE features together with incommensurate correlations in the low temperature phase has been discovered. The intensity of the spots associated with the FE domains indicates the fraction of the crystal in the low-temperature phase. For DRADA-10 there is a fairly wide temperature range (7-10 K) over which these spots coexisted with spots remaining from the paraelectric phase
Phase diagram and dielectric properties of mixed Cs1-x(NH4)xH2PO4 crystals
(2002) Baranov, A. I.; Dolbinina, V. V.; Lanceros-Mendez, S.; Schmidt, V. Hugo
Influence of NH4 M Cs substitution has been studied in the mixed Cs 1 m x (NH 4 ) x H 2 PO 4 crystals system by means of dielectric measurements. The x-T phase diagram was determined: for ammonium concentrations up to 0.1 the system has a monoclinic paraelectric phase with a transition into a ferroelectric phase whose temperature increases with increasing ammonium concentration. For ammonium concentration 0.4 to 1.0 the system shows a tetragonal paraelectric phase and a transition into an antiferroelectric phase. No solid solutions with concentrations between 0.1 and 0.4 could be grown. The system was further studied with the quasi-one-dimensional Ising model and the three-dimensional anisotropic Ising model.