Morphotropic phase boundary in (1-x)Bi0.5Na0.5TiO3-xK(0.5)Na(0.5)NbO(3) lead-free piezoceramics

The electromechanical behavior of (1-x)Bi0.5Na0.5TiO3-xK(0.5)Na(0.5)NbO(3) (BNT-KNN) lead free piezoelectric ceramics is investigated for 0 <= x <= 0.12 to gain insight into the antiferroelectric-ferroelectric (AFE-FE) phase transition on the basis of the giant strain recently observed in BNT-based systems. At x approximate to 0.07, a morphotropic phase boundary (MPB) between a rhombohedral FE phase and a tetragonal AFE phase is found. While the piezoelectric coefficient is largest at this MPB, the total strain further increases with increasing KNN content, indicating the field-induced AFE-FE transition as the main reason for the large strain.open756

Knowledge and Attitudes Towards Classroom Active Breaks Among Brazzaville’s Middle-Schoolers and Teachers of the Antonio Agostinho NETO General Education College

Background: Although classroom activity breaks (CAB) are essential for improving the learner's level of moderate to vigorous physical activity, they are not implemented and the impact of their effects is very poorly understood. Purpose: To determine students and teachers' knowledge on CAB and analyze the factors related to AA Neto middle school teachers' CAB knowledge. Method: A total of 90 students and 52 teachers Participated in this study. Student’s knowledge on CPAB and teacher’s knowledge and attitude on CAB and factors related to teacher’s knowledge on CAB were recorded with a questionnaire for about 20 minutes respectively. Results: The prevalence of student’s unawareness of CAB was high (87.8%) with a predominance of girls (92%). The prevalence of teacher’s unawareness of CAB was also high (78.8%) with a predominance of men (9 out of 2). Teachers had an unfavorable attitude towards the introduction of CAB. Lack of awareness of CAB was found to be significantly associated with age (OR = 0.95, CI = 0.92–0.99) and middle school diplomas (OR = 2.24, CI = 1, 03–4.84) and high school diploma (OR = 5.06, IC = 1.72–14.86). Conclusion: Overall, these results demonstrate a significant lack of CAB knowledge among students and teachers at AA Neto middle school. Similarly, teachers have a negative attitude towards CAB. Finally, the age, the diploma and the behavior change at the individual level were predictors of teachers' unawareness

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. I. Structure and room temperature properties

Lead-free piezoelectric ceramics, (1-x-y)Bi0.5Na0.5TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (0.05 <= x <= 0.07 and 0.01 <= y <= 0.03), have been synthesized by a conventional solid state sintering method. The room temperature ferroelectric and piezoelectric properties of these ceramics were studied. Based on the measured properties, the ceramics were categorized into two groups: group I compositions having dominant ferroelectric order and group II compositions displaying mixed ferroelectric and antiferroelectric properties at room temperature. A composition from group II near the boundary between these two groups exhibited a strain as large as similar to 0.45% at an electric field of 8 kV/mm. Polarization in this composition was not stable in that the piezoelectric coefficient d(33) at zero electric field was only about 30 pm/V. The converse piezoelectric response becomes weaker when the composition deviated from the boundary between the groups toward either the ferroelectric or antiferroelectric compositions. These results were rationalized based on a field induced antiferroelectric-ferroelectric phase transition.open12510

Lead-free piezoceramics with giant strain in the system Bi0.5Na0.5TiO3-BaTiO3-K0.5Na0.5NbO3. II. Temperature dependent properties

The temperature dependence of the dielectric and ferroelectric properties of lead-free piezoceramics of the composition (1-x-y)Bi0.5Na0.5TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (0.05 <= x <= 0.07, 0.01 <= y <= 0.03) was investigated. Measurements of the polarization and strain hystereses indicate a transition to predominantly antiferroelectric order when heating from room temperature to 150 degrees C, while for 150 < T < 200 degrees C both remnant polarization and coercive field increase. Frequency-dependent susceptibility measurements show that the transition is relaxorlike. For some samples, the transition temperature T-d is high enough to allow mostly ferroelectric ordering at room temperature. These samples show a drastic increase of the usable strain under an external electric field just after the transition into the antiferroelectric state at high temperatures. For the other samples, T-d is so low that they display significant antiferroelectric ordering already at room temperature. In these samples, the usable strain is relatively stable over a wide temperature range. In contrast to T-d, the temperature T-m of the transition into the paraelectric high-temperature phase depends far less on the sample composition. These results confirm that the high strain in this lead-free system is due to a field-induced antiferroelectric-ferroelectric phase transition and that this effect can be utilized in a wide temperature range.open8

Danielle Galliano, Les groupes industriels de l'agro-alimentaire français. Une approche statistique de la dynamique des organisations. Paris, Economica, 1995

Alcouffe Alain, Kounga Makele J. Danielle Galliano, Les groupes industriels de l'agro-alimentaire français. Une approche statistique de la dynamique des organisations. Paris, Economica, 1995. In: Cahiers d'Economie et sociologie rurales, N°42-43, 1er et 2e trimestres 1997. économie du développement. Education ; pauvreté ; commerce international. pp. 207-211

Shielding Stresses for Soft PZT

Crack opening displacements (COD) were obtained on fully developed cracks in compact tension specimens manufactured from a commercial, soft lead zirconate titanate (PZT). The data were evaluated using the weight function method to provide crack closures stresses as a function of distance to the crack tip. As ferroelastic toughening in PZT shows time-dependent effects, data were taken at different stages of unloading. The peak closure stress was determined as 20 MPa

Crack opening displacements of Vickers indentation cracks

Vickers indentation cracks are an appropriate tool to determine the crack-tip toughness KI0 and, possibly, the bridging relation of ceramics with an R-curve behaviour from the total crack opening displacements. Two contributions to the total crack opening displacement field are addressed. First, the residual stresses occurring in the uncracked body are considered and then, the contact stresses generated by preventing crack face penetration are computed. The COD solution resulting from the superposition of residual and contact displacements is given and an analytical expression is provided. Near-tip displacements are represented by the first terms of series expansions. As an example of application, an evaluation of the actual stress intensity factor is presented for a window glass 1 h after Vickers indentation