Ferroelectric to relaxor crossover in Li solid solutions derived from Ba2Nd□2FeNb4O15

Three solid solutions of Tetragonal Tungsten Bronze structure containing Li were prepared by solid state route starting from the parent compound Ba₂NdFeNb₄O₁₅. These solid solutions involve Li insertion compensated by cationic removal. All cationic sites were involved to investigate their influence on the dielectric transitions observed in Ba₂NdFeNb₄O₁₅, i.e. from relaxor (R) to ferroelectric (FE) and finally to paraelectric (PE). All solid solutions display the same pattern, which is an evolution from a relaxor to ferroelectric crossover, leading to a single transition (relaxor to paraelectric). This evolution is accompanied by a sudden increase of the ordering temperature (T_m). A focus on structure refinements shows the slight differences induced by the insertion of Li and points towards the role of anionic framework as responsible for the dielectric properties of these materials. This study completes the work performed on other cationic substitutions performed in the title material, and represents a new example of Li substituted TTB materials.     

Ferroelectric–relaxor crossover characteristics in Ba(ZrxTi1−x)O3 ceramics investigated by AFM-piezoresponse study

Ba(Zr_xTi_1?x)O₃ ceramics with x = 0.10 and 0.18 prepared via solid state, with high-density, homogeneous microstructures and similar grain size were investigated. The dielectric data showed that the relaxor properties are induced by increasing the Zr vs. Ti fraction x, both compositions showing a combined relaxor–ferroelectric character. The AFM-piezoresponse experiments revealed a few interesting characteristics, as follows: (1) none of the samples could be totally poled or switched, like the normal ferroelectrics; (2) the samples locally present different types of responses (ferroelectric with a strong piezoresponse, field-induced ferroelectric, polar but non-switchable and non-polar regions); (3) both the regions with a natural strong piezoresponse and the ones obtained after poling are larger in size and more stable in time for the sample with x = 0.10 than for x = 0.18; (4) the sample having x = 0.18 has a smaller piezoresponse than the one with x = 0.10. The observed local features are confirming the ferroelectric–relaxor crossover with increasing x, as observed by the analysis of the dielectric data.     

Multiscale study of ferroelectric–relaxor crossover in BaSnxTi1−xO3 ceramics

A systematic study of BaSn_xTi_1−xO₃ solid solutions (x = 0–0.20) by a combined field-induced dielectric and ferroelectric analysis with Raman and PFM investigations was realized, in order to obtain new insights concerning the composition-induced modification of the structural phase transitions and ferroelectric–relaxor crossover induced by the increase of Sn addition. The ceramics prepared via solid state reaction and sintering at 1400 °C/4 h showed average tetragonal symmetry for x ≤ 0.15 and cubic for x = 0.20. However, the dielectric and Raman analysis demonstrated that x = 0.05 and x = 0.15 are characterized by a coexistence of phases, which enhances their macroscopic properties (polarization for x = 0.05 and permittivity for x = 0.15). The domain structure shows a gradual modification when increasing Sn addition. No detectable domain structure has been found for x ≥ 0.15. All the compositions show local d₃₃(V) hysteresis loops at room temperature.     

Near room temperature giant negative and positive electrocaloric effects coexisting in lead–free BaZr0.2Ti0.8O3 relaxor ferroelectric films

BaZr_0.2Ti_0.8O₃ (BZT) relaxor ferroelectric (FE) films with tetragonal structure were deposited on Pt/TiO₂/SiO₂/Si(100) substrates via a sol-gel technique. A giant electrocaloric effect (ECE) was observed in the compact films with ΔT = 43.6 K and ΔS = 61.9 J/K kg at near room temperature of 366.5 K, which was attributed to the high breakdown electric field of E = 1010 kV/cm. Meanwhile, large negative ECE (ΔT = ? 5.2 K and ΔS = ? 8.8 J/K kg at room temperature of 305 K, ΔT = ? 5.1 K and ΔS = ? 7.2 J/K kg at near room temperature of 375 K) were achieved. The abnormal negative ECE is originated from structural transformation in relaxor ferroelectrics. The coexisting of giant negative and positive electrocaloric effects makes it more effective to realize the refrigeration during the application or removal of an electric field. The maximum electrocaloric coefficient (ζ_max = 0.043 K cm/kV) and refrigeration efficiency (COP = 35.18) of the films were obtained at near room temperature of 366.5 K. The giant electroelectric effect makes BZT films promising as lead-free materials for application in environment-friendly refrigeration equipment at near room temperature.     

Stabilization of the relaxor phase by adding CuO in lead-free (Bi1/2Na1/2)TiO3‒SrTiO3‒BiFeO3 ceramics

We investigated microstructures, crystal structures, and electrical properties of high temperature sintered (1‒x)BiNaTiO₃‒xSrTiO₃‒2BiFeO₃ (BNST100x‒2BF, x = 0.20 and 0.22) ceramics without added CuO and compared these results with low temperature sintered BNST100x‒2BF ceramics with 1 mol% of added CuO. The sintering temperature and average grain size of BNST100x‒2BF ceramics decreased after addition of CuO. We found that changes in the stabilized relaxor phase with decreasing sintering temperature resulted in changes in the temperature dependent dielectric properties and provided the enhanced electromechanical strain properties in BNST100x‒2BF ceramics. Therefore, we suggest that the nonergodicity to ergodicity transition is able to induce by changes in sintering temperature with added CuO and this result is important to tailor the electrical properties of BNT‒based lead‒free ceramics in proceeding with practical application.     

Achieving ultrabroad temperature stability range with high dielectric constant and superior energy storage density in KNN–based ceramic capacitors

Multilayer ceramic capacitors (MLCCs) had become an important component of many electronic devices on account of its miniaturization, high capacitance and reliability. To satisfy the requirements of MLCCs, the temperature–insensitivity and dielectric properties of the dielectric ceramics were urgent to be enhanced. In our work, (1–x)K_0.5Na_0.5NbO₃–xBi(Li_0.5Nb_0.5)O₃ (abbreviated to KNN–xBLN) were successfully synthesized by traditional solid state reaction method. On the one hand, the doping BLN induced the diffused phase transition and broadened the dielectric anomaly peaks, which improved the temperature insensitivity of KNN-based ceramics. On the other hand, the nanosized grains and dense microscopy boosted the breakdown electric field. Ultimately, the KNN–0.175BLN samples presented the excellent dielectric properties with high dielectric constant (1735) and low dielectric loss (1.9%) at room temperature with a wide temperature stability range (–62 – 300 °C), which exhibited the wider temperature stability range than X9R specification. Meanwhile, the x = 0.175 samples also achieved a high recoverable energy storage density of 3.71 J/cm³ under the breakdown electric field of 360 kV/cm. The designed KNN–based dielectric materials were expected to be applicable to the energy storage capacitor with standed high operating temperature.     

Large strain response in (Mn,Sb)–modified (Bi0.5Na0.5)0.935Ba0.065TiO3 lead–free piezoelectric ceramics

Lead–free piezoelectric ceramics (Bi_0.5Na_0.5)_0.935Ba_0.065Ti_1–x(Mn_0.5Sb_0.5)_xO₃ (BNBT6.5–xMS, x=0.005, 0.010, 0.015, 0.020) were prepared by conventional solid state reaction sintering technique. All ceramics present a pure perovskite phase structure, indicating that (Mn, Sb) has completely diffused into the BNBT6.5 lattice in the studied components. The addition of (Mn, Sb) disrupted the ferroelectric long–range order and promoted the electric field induced strain response. At x=0.015, a large electric field–induced unipolar strain of 0.48% (at an applied electric field of 80 kV/cm) with normalized strain d₃₃^*(S_max/E_max) of 602 pm/V are achieved. Temperature dependent measurements of both polarization and strain from room temperature to 120 °C were also studied, and the results suggest that the origin of the large strain is due to a reversible field–induced non–polar relaxor phase to polar ferroelectric phase transformation.     

Lipid autoxidation in fishery products postharvest: Is cold to freezing oxygen‐free packaged storage the better remedy?

The progress of lipid oxidation in fishery products is complex, as it involves a whole lot of reactions that deter product quality. On the other hand, the use of modified atmosphere packaging (MAP) has, over the years, been effective in improving food product safety and nutritional quality. In addition, as an oxygen‐free and protective food packaging technique, MAP has been shown to effectively minimize overall lipid damage. In this article we discuss how cold to freezing/sub zero‐temperatures could be promising for reducing the development of lipid oxidation in fishery products.     

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N₂ adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N₂O‐containing stream and in one containing also SO₂ and O₂, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO₂ and O₂ are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Predicting the dynamics and performance of a polymer–clay based composite in a fixed bed system for the removal of lead (II) ion

A polymer–clay based composite adsorbent was prepared from locally obtained kaolinite clay and polyvinyl alcohol. The composite adsorbent was used to remove lead (II) ions from aqueous solution in a fixed bed mode. The increase in bed height and initial metal ion concentration increased the adsorption capacity of lead (II) and the volume of aqueous solution treated at 50% breakthrough. However, the adsorption capacity was reduced by almost 16.5% with the simultaneous presence of Ca²⁺/Pb²⁺ and Na⁺/Pb²⁺ in the aqueous solution. Regeneration of the adsorbent with 0.1 M of HCl also reduced its adsorption capacity to 75.1%. Adsorption of lead (II) ions onto the polymer–clay composite adsorbent in the presence of Na⁺ and Ca²⁺ electrolyte increased the rate of mass transfer, probably due to competition between cationic species in solution for adsorption sites. Regeneration further increased the rate of mass transfer as a result of reduced adsorption sites after the regeneration process. The length of the mass transfer zone was found to increase with increasing bed height but did not change with increasing the initial metal ion concentration. The models of Yoon–Nelson, Thomas, and Clark were found to give good fit to adsorption data. On the other hand, Bohart–Adams model was found to be a poor predictor for the column operation. The polymer–clay composite adsorbent has a good potential for the removal of lead (II) ions from highly polluted aqueous solutions.     

Cells integration onto scaffolds prepared from polyester based polymers – importance of polymer thermal properties in addition to hydrophilicity

Abstract

Three-dimensional (3D) scaffolds represent valuable tools for biological studies and tissue engineering applications. They offer better biological environment that better mimic 3?D dynamic in vivo conditions compared with conventional bidimentional (2D) cell culture systems. The integration of cells within the scaffolds is, however, dependent on their properties. In the present study, porous scaffolds were prepared from poly lactide (PLA), poly lactide polyethylene glycol copolymer (PLA-PEG) and poly lactide-co-glycolide – polyethylene glycol copolymer (PLGA-PEG) using porogen leaching method with NaCl particles as porogen. The three scaffolds were prepared with identical conditions (concentration, dimensions, porogen weight fraction and particle size) in order to evaluate the impact of polymer composition and properties on scaffold characteristics (internal architecture and pores distribution), as well as fibroblasts integration and proliferation within the scaffold 3?D network. The impact of hydrophilicity, water uptake and protein adsorption are discussed. The data shows that amorphous polymers, which have glass transition temperature close to cell culture temperature, may be advantageous for scaffold construction and cell integration. This fact needs to be considered when interpreting data of cell interaction with scaffolds obtained using amorphous polymers developed for 3?D cell culture and tissue engineering applications.     

Team formation on the basis of Belbin’s roles to enhance students’ performance in project based learning

This paper presents a method that instructors have designed and implemented to form balanced teams based on Belbin's roles, with the aim of boosting positive interdependence and individual accountability within the teams and improving their performance in a project-based learning environment. Students’ performance has been measured through the scores obtained during the project, individual exam and Individual Accountability Factor (IAF) and compared with cohorts of previous years, in which team composition was self-selected by students. Belbin teams (18/19–19/20) have performed significantly better than self-selected teams (16/17–17/18). Additionally, students’ feedback experience and opinion has been collected. Students belonging to Belbin teams acknowledge that they attend classes more regularly, they need less time for study outside the classes and they show a higher interest for the subject at the end of the course. They also agree that working on Belbin teams has helped them to mainly improve interpersonal relationships and social skills, followed by positive interdependence and individual accountability. This team forming method gives students the opportunity to identify their own strengths and weaknesses and understand the roles (behaviours) of their teammates as well as their strengths and weaknesses. Besides, it encourages learners to focus explicitly on group work skills.     

Influence of Mg and Ce addition to ruthenium based catalysts used in the selective hydrogenation of α,β-unsaturated aldehydes

Ce and Mg were used as promoters in two series of Ru based catalysts supported on alumina (Al₂O₃) and activated carbon (AC). The catalysts were characterized by H₂ chemisorption and temperature-programmed reduction (TPR), and studied in the crotonaldehyde (gas phase) and the citral (liquid phase) hydrogenations. Addition of MgO and CeO₂ decreased the catalytic activity in crotonaldehyde and citral hydrogenations. With regard to the selectivity towards unsaturated alcohols, similar trends were observed for the two reactions. MgO did not influence the selectivity, but CeO₂ increased the selectivity to unsaturated alcohols, especially on carbon supported catalyst. Bulk CeO₂ and Ce/AC catalyst showed low activity but very high selectivity (93 and 100%, respectively) to the unsaturated alcohols. Based on these results and the calorimetric experiments of CO adsorption it was suggested that defect sites on the surface of the promoter are the active and highly selective sites for unsaturated aldehydes due to their influence on the CO bond activation.     

Phase–composition and temperature dependence of electrocaloric effect in lead–free Bi0.5Na0.5TiO3–BaTiO3–(Sr0.7Bi0.2□0.1)TiO3 ceramics

The (0.94–x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT, 0 ≤ x ≤ 0.24) solid solution ceramics were synthesized via a conventional solid–state reaction method and the correlation of phase structure, piezoelectric, ferroelectric properties and electrocaloric effect (ECE) was investigated in detail. The ECE in lead–free BNT–BT–xSBT ceramics was measured directly using a home–made adiabatic calorimeter with maximum adiabatic temperature change ΔT = 0.4 K with x = 0.08 under the electric field E = 6 kV/mm at room temperature. The position of maximum ECE was found in the vicinity of nonergodic and ergodic phase boundary, where the maximum change in entropy occurs as a result of the field–induced phase transformation between the ergodic and long–range ferroelectric phase. Besides, the mechanism for the shift of ECE peak is discussed in detail. Finally, the temperature dependence of ECE for BNT–BT–xSBT (x = 0, 0.04 and 0.08) was also investigated. This work may present a guideline for designing BNT–based ferroelectric relaxor ceramics for EC cooling technologies.     

Manganese dioxide electrode—X. A theoretical treatment based on the concept of two solid solutions in the range γ-MnO2 to δ-MnOOH

Models are presented which account for the electrode potential of electrodeposited gamma manganese dioxide and its reduction products, MnOOH_r in the range 0 < r < 1 and which in particular explain the distinct change in potential vs r slope at the mid-reduction point.Two solid solutions are proposed, one with end-members MnO₂ and MnOOH_0.5 operating in the range r < 0.5 and the other with end-members MnO₂ and MnOOH operating in the range r > 0.5. The Gibbs Free Energy of the solid solutions and thence electrode potential is derived on the basis of the sites available for placement of inserted protons and electrons, the number of possible arrangements of protons and electrons on the sites and the independence of proton and electron arrangements.Standard Free Energies are derived for γ-MnO₂, MnOOH_0.5 and δ-MnOOH at 25°C.     

Use of the combined Lifshitz–van der Waals and Lewis acid–base approaches in determining the apolar and polar contributions to surface and interfacial tensions and free energies

Recently, a number of authors have been rearranging the various combinations and permutations of the different apolar and polar liquids with which contact angles can be measured on polar surfaces and in so doing have arrived at bizarre results. The rational order and procedures to be followed in the determination of the apolar and polar surface tension properties of polar materials, according to the van Oss-Chaudhury-Good components and parameters of the approach, are reiterated.     

Exploring the processing conditions to optimize the interface in 2–2 composites based on Pb(Zr,Ti)O3 and NiFe2O4

The processing of laminated composites based on lead zirconate titanate and nickel ferrite was investigated. Ceramic powders were prepared by solid-state reaction and sintered by conventional and hot pressing method, under different time and temperature conditions. As expected, it is found that differences in the grain size and thermal expansion coefficients between the constituent phases can cause a shear breaking of the material and excessive crack formation. Such problems were considerably reduced with the manufacture of several layers of particulate composites to create a stepwise crossover at the interface between both laminated phases. The results in this work show the way to obtain hot-pressed magnetoelectric composites with optimized interface, and they demonstrate that micro-cracks may lower the magnetoelectric effect by several orders of magnitude.