Barium titanate thick films prepared by screen printing technique

The barium titanate (BaTiO3) thick films were prepared by screen printing technique using powders obtained by soft chemical route, modified Pechini process. Three different barium titanate powders were prepared: i) pure, ii) doped with lanthanum and iii) doped with antimony. Pastes for screen printing were prepared using previously obtained powders. The thick films were deposited onto Al2O3 substrates and fired at 850°C together with electrode material (silver/palladium) in the moving belt furnace in the air atmosphere. Measurements of thickness and roughness of barium titanate thick films were performed. The electrical properties of thick films such as dielectric constant, dielectric losses, Curie temperature, hysteresis loop were reported. The influence of different factors on electrical properties values was analyzed

Microstructural analysis of Bulk Molding Compounds and correlation with the flexural strength

In this study, the influence of the glass fiber (GF) content on the microstructure and flexural strength of bulk molding compounds (BMCs) is investigated. Three sets of BMCs with different weight fractions of GF (5/10/12.5 wt%) were commercially prepared and compression molded into test specimens. The microstructure of the composites was analysed by scanning electron microscopy and further quantitatively characterized by Voronoi analysis in order to define the degree of the fiber distribution homogeneity. The experimental results were compared to the modelled microstructures. The results revealed that the fiber distribution in the composite with 5 wt% of GF is considered as the most homogeneous. Through the obtained microstructural descriptors, the fiber weight content and their distribution were correlated to the flexural strength of BMCs. The flexural strength was the highest for the composite with 10 wt% of GF

Microstructure, mechanical and electrical properties of Glass Fiber Reinforced Composites (GFRC)

In this study, the influence of E-glass fiber and mineral filler content on the microstructure, physical, mechanical and dielectric properties of Glass Fiber Reinforced Composites (GFRC) was investigated. Five sets of GFRC, based on polymer resin with varying E-grade glass fibers and CaCO3 mineral filler weight fractions (15/64, 20/59, 25/54, 30/49, 35/44), were commercially prepared. Test specimens were prepared by compression molding. Scanning Electron Microscope images revealed that at higher concentrations, the fibers clustered together, resulting in heterogeneous microstructures. Characterization of the composites showed that glass fiber content and distribution significantly affects the mechanical properties. The flexural strength of the composites decreased with increasing glass fiber content. The dielectric constant ε` decreased with increasing fiber content

Preparation and dielectric properties of CuAlO2 ceramics

Within this work, the focus was on preparation of the delafossite CuAlO2 single phase powder and ceramic with a high density by the solid state synthesis, and on dielectric properties of the as-synthesized ceramic. The reaction between the nanoboehmite γ-AlOOH, with a high specific surface area, and the Cu2O, with the particles below 1 μm, was enhanced by comminution in a high energy mill, which resulted in reduction of the particle size and consequently shorter diffusion paths between constituent powders. The phase pure CuAlO2 powder was synthesized upon heating the reagent powder mixture two times for 10 h at 1100oC in the inert argon atmosphere as confirmed by the X-ray analysis. The ceramic with 86% of theoretical density was obtained after sintering the CuAlO2 powder compact at 1100oC for 2 h in air. According to the X-ray analysis the ceramic sample was single-phase. The bulk of the sample revealed a dense microstructure with a uniform distribution of porosity within the delafossite matrix. However, traces of Cu-rich impurities have been identified at the surface of the pellets by the EDXS analysis. The semiconducting nature of the ceramic sample was confirmed by the temperature dependent dielectric parameters measurements (ε’ and tgδ) in the 10 kHZ-1MHz frequency range between 297 and 473 K

Chemical synthesis of nanocrystalline CuAlO2 via nitrate-citrate combustion route

The nanocrystalline delafossite CuAlO2 powder was synthesised by a sol-gel nitrate-citrate self-combustion route. Citric acid was introduced both as the chelating and reducing agent or fuel. The citric acid/metal ion ratio was adjusted to provide fuel-lean, stoichiometric or fuel-rich conditions of the redox reaction. Equimolar amounts of copper and aluminium nitrates and the citric acid were dissolved in deionized water. The sol was dried at 80 oC to obtain the gel. By increasing the temperature above 250 oC, the gel immediately ignited, forming the precursor powder. According to the X-ray diffraction analysis the phase pure delafossite was obtained only when the precursor powder was prepared from the stoichiometric redox reaction, and after the calcination for 4 h in Ar atmosphere at 920 oC. The field emission scanning electron micrographs revealed the cauliflower aspect of the calcined powder, where small primary particles formed the agglomerates. The formation of the phase pure CuAlO2 powder was also confirmed by Fourier transformed infrared spectroscopy

Strontium-doping effects in solution derived lead-free ferroelectric K(0.5)Na(0.5)NbO3 thin films

Potassium sodium niobate, K0.5Na0.5NbO3 (KNN) is an environment-friendly lead-free alternative to highly efficient lead-based piezoelectrics. The poor functional properties of the KNN thin films prepared by chemical solution deposition are frequently related to the volatilisation of alkali species during processing, which hinders control over the stoichiometry, contributes to formation of secondary phases and deterioration of the microstructure. The problem can be overcome by adding alkalis in excess and/or by partial substitution of the A- and B- site atoms, such as in the case of the solid state synthesized KNN ceramics. Therefore, in this contribution, the influence of the alkaline-earth A- site dopant, Sr2+ on the microstructure, structure, and functional properties were examined for the solution-derived KNN thin films with alkaline excess. Liquid precursors of (K0.5Na0.5)1-ySryNbO3 (KNN-ySr) thin-films, where the Sr- dopant content was set at y = 0, 0.005, 0.01, were prepared from potassium and sodium acetates and niobium ethoxide in 2-methoxyethanol solvent with 5 mol% of potassium acetate excess. Strontium was introduced as acetate or nitrate. The approximately 250 nm thick KNN-ySr thin films on Pt/TiOx/SiO2/Si substrates were obtained by rapid thermal annealing at 650 oC for 5 min. According to X-ray diffraction analysis, all synthesized KNN thin films crystallize in pure perovskite phase with random orientation. The surface and cross-section microstructure analysis, performed by the field emission scanning electron microscopy, reveals that the KNN-ySr films consist of equiaxed grains, the average size of which gradually decreases from about 90 nm to a few tens of nm by increasing the Sr-dopant content. In the contribution we discuss the influence of the chemical modification on the functional response, i.e., dielectric properties versus frequency and temperature, polarisation – electric field dependence, leakage current and piezoelectric response of the as-prepared films

Multifunctional Cantilevers as Working Elements in Solid-State Cooling Devices

Despite the challenges of practical implementation, electrocaloric (EC) cooling remains a promising technology because of its good scalability and high efficiency. Here, we investigate the feasibility of an EC cooling device that couples the EC and electromechanical (EM) responses of a highly functionally, efficient, lead magnesium niobate ceramic material. We fabricated multifunctional cantilevers from this material and characterized their electrical, EM and EC properties. Two active cantilevers were stacked in a cascade structure, forming a proof-of-concept device, which was then analyzed in detail. The cooling effect was lower than the EC effect of the material itself, mainly due to the poor solid-to-solid heat transfer. However, we show that the use of ethylene glycol in the thermal contact area can significantly reduce the contact resistance, thereby improving the heat transfer. Although this solution is most likely impractical from the design point of view, the results clearly show that in this and similar cooling devices, a non-destructive, surface-modification method, with the same effectiveness as that of ethylene glycol, will have to be developed to reduce the thermal contact resistance. We hope this study will motivate the further development of multifunctional cooling devices

Influence of Synthesis-Related Microstructural Features on the Electrocaloric Effect for 0.9Pb(Mg1/3Nb2/3)O3−0.1PbTiO3 Ceramics

Despite having a very similar electrocaloric (EC) coefficient, i.e., the EC temperature change divided by the applied electric field, the 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN-10PT) ceramic prepared by mechanochemical synthesis exhibits a much higher EC temperature change than the columbite-derived version, i.e., 2.37 °C at 107 °C and 115 kV/cm. The difference is due to the almost two-times-higher breakdown field of the former material, 115 kV/cm, as opposed to 57 kV/cm in the latter. While both ceramic materials have similarly high relative densities and grain sizes (>96%, ≈5 µm) and an almost correct perovskite stoichiometry, the mechanochemical synthesis contributes to a lower level of compositional deviation. The peak permittivity and saturated polarization are slightly higher and the domain structure is finer in the mechanochemically derived ceramic. The secondary phases that result from each synthesis are identified and related to different interactions of the individual materials with the electric field: an intergranular lead-silicate-based phase in the columbite-derived PMN-10PT and MgO inclusions in the mechanochemically derived cerami

Initial stage sintering mechanism of NaNbO3 and implications regarding the densification of alkaline niobates

The behavior of submicron- and nano-sized NaNbO3 powder compacts during conventional sintering was studied using optical dilatometry and microstructure analysis. Microstructure-development trajectories revealed the dominance of grain growth during the initial sintering stage, while densification occurred only during later stages. Surface diffusion with low activation energy in the range of 50–60 kJ/mol was found to be the dominant material-transport mechanism during the initial sintering stage. The early activation of surface diffusion reduced the sintering driving force, decreased the rate of the densifying mechanisms and was thus identified as the main cause for poor densification of NaNbO3. Same explanation could be valid also for other alkaline niobate based lead-free piezoelectric ceramics. Finally, alternative sintering methods are discussed and the efficiency of the pressure-assisted sintering was demonstrated in successful production of highly-dense fine-grained NaNbO3 ceramics, with relative density and grain size of 98% and 700 nm, respectively