Propriétés électromécaniques des céramiques PMN-PT et PZT à très basses températures

International audienceSpace borne observation requires extremely high resolution systems with large dimensions. The design of such systems implies associating sensing and actuation systems to insure a highly stable positioning of optical components. However, the currently used PZT materials for piezoelectric actuation show strongly diminished strain at cryogenic temperatures. This work explores the performances of PMN-xPT piezoelectric materials at cryogenic temperatures. Temperature dependence of PMN-xPT ceramics is compared with hard PZT (PZT-4) and soft PZT (PZT5H) ceramics from RT to 10 K. PMN-PT tetragonal compositions exhibit stable piezoelectric constant in the 250-100 K range. The highest values of piezoelectric constant are observed for PMN-38PT in the 200-50 K range. All ceramics exhibit decreasing performances versus cooling temperature. This behaviour is attributed to a "freezing out" of extrinsic contributions to piezoelectricity [12] but is also due to a contribution of quantum effects [13, 14]. The mechanical quality factor (Q m) increases very quickly from 50 K to 10 K for all samples. This fact could be due to low relaxation processes at very low temperatures

Electromechanical properties of single domain PZN-12%PT measured by three different methods

International audienceIn this paper, we compare the electromechanical properties of tetragonal single domain PZN-12%PT single crystals obtained by different methods resonance-antiresonance method, Brillouin scattering, and resonant ultrasound spectroscopy. The agreement between the different measurements is found satisfactory for many elastic constants within experimental uncertainties. Differences are notable for the elastic constants associated to the propagation of shear waves (c 66 E and c 44 E ). This can be accounted for by imperfections of the sample in acoustic spectroscopy and specific difficulties of shear resonators used for the resonance method. Strong discrepancies are noted for the hardened elastic constant c 33 D associated to longitudinal waves propagating along the polarization direction; we suggest this can arise from a frequency dependence of the dielectric constant. © 2009 Elsevier Masson SAS. All rights reserved

Average vs. local structure and composition-property phase diagram of K0.5Na0.5NbO3−Bi½Na½TiO3\mathrm{K_{0.5}Na_{0.5}NbO_{3}-Bi_{½}Na_{½}TiO_{3}} system

tPhase diagram of the solid solution system K$_{0.5}$Na$_{0.5}$NbO$_{3}$-Bi$_{½}$Na$_{½}$TiO$_{3}$[(1-x)KNN-xBNT] has beenestablished from dielectric permittivity measurement and structure analyses. The unit cell volumescontinuously decrease depending on the composition, while the local structure maintains distortionsaway from the cubic average structure in the range 0.10 ≤ x ≤ 0.90. No clear correspondence for thetemperatures of phase transition exists between structural studies and physical properties. The dielec-tric behavior is depicted successively from normal ferroelectric, diffuse phase transition, re-entrant-likerelaxor, relaxor + dipolar glass-like relaxor, BNT-like relaxor with the increase of BNT. A comprehensivecomposition-property phase diagram for this system has been given to understand the various ferro-electric phenomena. The result could be mainly elucidated by the nanoclusters and the disorder drivennucleation of polar nanoregions contributed by a valence mismatch at one of the cation sites

Does BMI Really Alter the Hormonal Profile in Infertile Women? Retrospective Study in the Region of Sidi Bel Abbes (West Algeria)

Background: In last few years .; it’s  increasingly being recognized  that  Reproductive function is controlled by the hypothalamic–pituitary– gonadal axis, which is regulated by numerous endogenous and environmental factors  such us adipose accumulation in obesity  contributing to reproductive failure  such as menstrual disorders and infertility, gestational failure and obstetric complications, and infertility , Distinct changes in circulating sex hormones appear to underline these abnormalities. The objective: The aim of this study was to elucidate the possible correlation between body mass index as fatness indicator and hormonal profile in infertile women from the west of Algeria. To identify the impact of overweight  and obesity on female hormonal profile ; we  conducted a prospective study  measuring pituitary hormones  (FSH  and LH and prolactin ) , steroid sex hormones ( progesterone , testosterone and estradiol ) , anti miulleian hormone , and thyroidal hormones (FT3 , FT4 and TSH) in 360 women  consulting for subfertility in private genecologycal and obstetrical centers in SID BELABESS (West of Algeria ) Result: Our study showed that the majority of patients were aged between 20 and 29 years, representing a percentage of 47.8%, with the average age in sample (31.65 ± 6,93ans). The majority of subjects was obese   46.4%, or overweight (39.4%) with an average BMI of (29.76 ± 4,85Kg / m2). No statistically significant association was found between the BMI as obesity indicator with hormonal levels of pituitary hormones  (FSH  and LH and prolactin ) , steroid sex hormones ( progesterone , testosterone and estradiol ) , anti miulleian hormone , and thyroidal hormones (FT3 , FT4 and TSH). A negative and statistically significant correlation between the age of the patients and the AMH level (R = -0.60, P <0.01) was noted. Conclusion: The results of our study showed that the majority of women included in our study had a high BMI but no statistical significant difference was found between underweight , normal , overweight and obese women , that’s why more studies should be conducted to elucidate the in which level does obesity impair the reproductive outcomes. Keywords: Female infertility, hormonal profile, BMI, SBA, Algeria

Average vs. local structure and composition-property phase diagram of K 0.5 Na 0.5 NbO 3 -Bi ½ Na ½ TiO 3 system

Phase diagram of the solid solution system K0.5Na0.5NbO3- Bi½Na½TiO3 [(1-x)KNN-xBNT] has been established from dielectric permittivity measurement and structure analyses. The unit cell volumes continuously decrease depending on the composition, while the local structure maintains distortions away from the cubic average structure in the range 0.10 ≤ x ≤ 0.90. No clear correspondence for the temperatures of phase transition exists between structural studies and physical properties. The dielectric behavior is depicted successively from normal ferroelectric, diffuse phase transition, re-entrant-like relaxor, relaxor + dipolar glass-like relaxor, BNT-like relaxor with the increase of BNT. A comprehensive composition-property phase diagram for this system has been given to understand the various ferroelectric phenomena. The result could be mainly elucidated by the nanoclusters and the disorder driven nucleation of polar nanoregions contributed by a valence mismatch at one of the cation sites