Helena Bruncková

Effect of PMN modification on structure and electrical response of xPMN–(1−x)PZT ceramic systems

The structural and electrical properties of xPb(Mg1/3Nb2/3)O3–(1−x)Pb(Zr,Ti)O3 ternary ceramic system with the composition near to the morphotropic phase boundary (MPB) and of xPb(Mg1/3Nb2/3)O3–(1−x)Pb(Zr0.47Ti0.53)O3 ceramics were investigated as a function of the Pb(Mg1/3Nb2/3)O3 (PMN) content by scanning electron microscopy (SEM), X-ray diffraction (XRD), dielectric and piezoelectric spectroscopy and polarization-electric field measurement technique. Studies were performed on the samples prepared by a columbite precursor method for x=0.125, 0.25 and 0.5. Room temperature SEM investigations revealed common trends in the grain structure with increasing PMN content. XRD analysis demonstrated that with increasing PMN content in xPb(Mg1/3Nb2/3)O3–(1−x)Pb(Zr0.47Ti0.53)O3, the structural change occurred from the tetragonal to the pseudocubic phase at room temperature. Changes in the dielectric and ferroelectric behavior were then related to these structural trends and further correlated with the piezoelectric properties. The results of ferroelectric hysteresis measurements, in conjunction with dielectric spectroscopy, demonstrated an intermediate, relaxor-like behavior between normal and relaxor ferroelectrics in the solid solution system, depending on the PMN content.

Dielectric properties and phase transition behavior of xPMN-(1 - x)PZT ceramic systems

The dielectric properties and phase transition behavior of the pseudo-ternary xPb(Mg1/3Nb2/3)O3-(1 − x)Pb(Zr,Ti)O3 solid solution system were investigated as a function of the Pb(Mg1/3Nb2/3)O3 (PMN) content and Ti/Zr ratio for selected compositions. The investigations have demonstrated a general trend in broadening of the phase transition and increasing diffusivity with increasing PMN content. For the morphotropic phase boundary (MPB) compositions, the dielectric permittivity maximum, its temperature (Tm) and the Curie-Weiss constant were found to decrease with increasing Mg1/3Nb2/3 concentration. When a Ti/Zr ratio was constant and equal to 53/47, temperature-dependent investigations demonstrated that the dielectric parameters involved in a modified Curie-Weiss law increase monotonically with increasing PMN content and Tm moves toward room temperature with average rate of ≈ −4.1°C/mol% as well. A phase transition in 0.5PMN-0.5Pb(Zr0.47Ti0.53)O3 and 0.25PMN-0.75Pb(Zr0.60Ti0.40)O3 ceramic systems exhibited a diffused behavior with a characteristic frequency dependence of Tm. From pyroelectric measurement, an unusual spontaneous polarization behavior at about 215 K is reported for some MPB compositions.

Effect of substrate on phase formation and surface morphology of sol-gel lead-free KNbO3, NaNbO3, and K0.5Na0.5NbO3 thin films

Environmentally acceptable lead-free ferroelectric KNbO3 (KN) or NaNbO3 (NN) and K0.5Na0.5NbO3 (KNN) thin films were prepared using a modified sol-gel method by mixing potassium acetate or sodium acetate or both with the Nb-tartrate complex, deposited on the Pt/Al2O3 and Pt/SiO2/Si substrates by a spin-coating method and sintered at 650°C. X-ray diffraction (XRD) analysis indicated that the NN and KNN films on the Pt/SiO2/Si substrate possessed a single perovskite phase, while NN and KNN films on the Pt/Al2O3 substrate contained a small amount of secondary pyrochlore phase, as did KN films on both substrates. Scanning electron microscopic (SEM) and atomic force microscopic (AFM) analyses confirmed that roughness Rq of the thin KNN/Pt/SiO2/Si film (≈ 7.4 nm) was significantly lower than that of the KNN/Pt/Al2O3 film (≈ 15 nm). The heterogeneous microstructure composed of small spherical and larger needle-like or cuboidal particles were observed in the KN and NN films on both substrates. The homogeneous microstructure of the KNN thin film on the Pt/SiO2/Si substrate was smoother and contained finer spherical particles (≈ 50 nm) than on Pt/Al2O3 substrates (≈ 100 nm). The effect of different substrates on the surface morphology of thin films was confirmed.

PZT Ceramics Prepared from Mechanically Activated Calcinate

0.025Nb-0.0125Sr-PZT ceramics was prepared by mechanical milling of Pb3O 4 , TiO 2 ,ZrO 2 , SrCO 3 and Nb 2 O 5 (all reactants were reagent grade). The powder mixture was calcined at temperature of 900°C for 1 hour and calcinate was mechanically activated by wet or dry milling in planetary ball mill (Fritsch, Pulversiette 7) using the ZrO 2 milling elements (vesell and balls) at various milling times. The activated calcinate was pressed into pellets form and sintered at 1150 and 1200°C for 1 hour in air. In final ceramic samples, both the microstructure and dielectric properties (at 1, 10 and 100 kHz) were studied. Ceramics prepared from 20 and 30 minutes wet milled calcinates (sintering at 1200°C) had microstructures with coarse grains (average grain size approximatelly 6 μ m) in comparison with the microstructure of the sample prepared from non-milled calcinate (NMC) (avg. grain size of 3 μ m). Densities of all ceramic samples were similar to each other and they were very close to value of 96% of theoretical density. The avg. grain size of NMC sintered at 1100°C was 1–2 μ m and one was about 4 μ m in ceramic sample prepared from 20 min. milled precursor. Specific surface of mechanically activated PZT precursors increased about 60% after 30 min. milling in comparison with non-milled sample. The specific surfaces of dry milled precursors were twice of ones of wet milled calcinates. The maximum of relative dielectric permitivity (ϵ r ) of the ceramic samples in dependence of ϵ r on temperature rise with milling time of wet milled samples and one was approximately about 25% higher after 20 min. milling than in NMC. It has not been found similar dependence in dry milled samples and in this case, the values of maximum ϵ r in the dependence of ϵ r on temperature did not change with milling time.

Effect of sol-gel preparation method on particle morphology in pure and nanocomposite PZT thin films

Double-scale composite lead zirconate titanate Pb(Zr0.52Ti0.48)O3 (PZT) thin films of 360 nm thickness were prepared by a modified composite sol-gel method. PZT films were deposited from both the pure sol and the composite suspension on Pt/Al2O3 substrates by the spin-coating method and were sintered at 650°C. The composite suspension formed after ultrasonic mixing of the PZT nanopowder and PZT sol at the powder/sol mass concentration 0.5 g mL−1. PZT nanopowder (≈ 40–70 nm) was prepared using the conventional sol-gel method and calcination at 500°C. Pure PZT sol was prepared by a modified sol-gel method using a propan-1-ol/propane-1,2-diol mixture as a stabilizing solution. X-ray diffraction (XRD) analysis indicated that the thin films possess a single perovskite phase after their sintering at 650°C. The results of scanning electron microscope (SEM), energy-dispersive X-ray (EDX), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses confirmed that the roughness of double-scale composite PZT films (≈ 17 nm) was significantly lower than that of PZT films prepared from pure sol (≈ 40 nm). The composite film consisted of nanosized PZT powder uniformly dispersed in the PZT matrix. In the surface micrograph of the film derived from sol, large round perovskite particles (≈ 100 nm) composed of small spherical individual nanoparticles (≈ 60 nm) were observed. The composite PZT film had a higher crystallinity degree and smoother surface morphology with necklace clusters of nanopowder particles in the sol-gel matrix compared to the pure PZT film. Microstructure of the composite PZT film can be characterized by a bimodal particle size distribution containing spherical perovskite particles from added PZT nanopowder and round perovskite particles from the sol-matrix, (≈ 30–50 nm and ≈ 100–120 nm), respectively. Effect of the PZT film preparation method on the morphology of pure and composite PZT thin films deposited on Pt/Al2O3 substrates was evaluated.

Acetate Sol-Gel Synthesis of PMN Ceramics from Nb-Ethyleneglycol-Tartarate Complex

PbMgNb(OAc) x (OR) y precursor was prepared from Pb and Mg acetates by dissolution in acetic acid (AcOH) and by mixing with Nb-ethyleneglycol-tartarate (Pechini) complex. The sol was formed after hydrolysis by various addition of water at 80°C and it was transformed by polycondensation to PMN two-phase gel. By the calcination of gel at the temperature of 500°C for 2 hours, the two-phase system was created with perovskite and undesirable pyrochlore phases. The content of the Pb(Mg 1/3 Nb 2/3 )O 3 perovskite phase in PMN calcinates was increased from 0 to 60 vol.% by decreasing of water addition as it results from DTA, TG analysis, IR spectroscopy and XRD analysis. The Pb 1.83 (Mg 0.29 Nb 1.71 )O 6.39 pure pyrochlore phase was found in the calcinate prepared from gel with higher addition of water and hydrolyzing factor (h) equals 43. The content of perovskite phase in calcinate was approximately 60 vol.% at h = 21 and without water addition to sol. The partial decomposition of the pyrochlore phase with transformation into the perovskite phase (the maximal volume fraction of perovskite phase ∼ 70 vol.%) was observed above the sintering temperature of 1100°C. Densities of PMN ceramics sintered at 1200°C decreased with the content of py phase in PMN calcinates. The maximal value of ϵ r = 4200 (at 1 kHz) was found in the ceramic sample with the highest content of perovskite phase and density.

Microstructure of Composites Based on Phosphated Iron Powder

The commercial carbonyl iron powder coated with iron phosphate (20 wt.%) was dried (60°C for 2 h in air), calcinated at 400°C for 3 h in air, compacted at 600 MPa into cylindrical samples and subsequently sintered at 820, 900 and 1110°C for 30 min in N2-10%H2 atmosphere. By means of EDX and XRD analyses the phase composition of the coating and sintered microstructure was studied. Microstructure resulting from sintering at 820 and 900°C was formed by initial iron particles surrounded with the crystalline FePO4 and α-Fe2O3 phases. Due to liquid phase sintering at 1110°C a mixed microstructure containing spheroidized α-Fe phase surrounded by solidified liquid phase consisting of iron oxides and phosphorous compounds has been formed. In order to prepare a network composite microstructure the compacts based on spherical iron particles size of 100-160 µm coated with 2 wt.% of iron phosphate were dried, calcined at 400°C, compacted and liquid phase sintered at 980°C.

Effect of Sintering Time on the Phase Composition in PFN Ceramics Prepared by Sol-Gel Process

Lead iron niobate Pb(Fe0.5Nb0.5)O3 (PFN) ceramics were prepared using sol-gel synthesis by mixing acetates Pb and Fe with Nb-ethylene glycol-tartarate (Pechini) complex at 80°C, calcination of gels at 600°C and sintering at 1150°C for various times. The metastable pyrochlore phase Pb3Nb4O13 in stoichiometric precursor was partially decomposed to perovskite phase Pb(Fe0.5Nb0.5)O3 in ceramics sintered at temperature of 1150°C for 2, 4 and 6 hours. Excess of Pb in molar ratio (Pb:Fe:Nb = 1.2:0.5:0.5) caused the increase of the content of the perovskite phase (~50 vol.%) in nonstoichiometric PFN ceramics sintered at 1150°C for 6 hours while the decrease in perovskite phase content was found in stoichiometric PFN ceramics (~16 vol.%). In microstructures of PFN ceramics sintered at 1150°C for different times, the bimodal grain size distribution was observed with small spherical grains of perovskite phase and larger octahedral grains of pyrochlore phase. EDX analysis confirm that complex types of pyrochlore phases that differ in iron content were present in ceramics.

PZT Ceramics Prepared from Two Phase Gels

[Pb(Zr,Ti)(OAc)x(OR)y] acetate-alkoxide precursor was prepared by chelating from organic Pb, Zr and Ti alkoxides by their dissolution in various amounts of acetic acid (AcOH). The optimum mole ratio of (Pb+Zr+Ti) to AcOH for preparation gel is 1:7 and a pure perovskite phase is formed at 500 °C. At a low AcOH concentration, two-phase (pyrochlore and perovskite) regions are formed during gellation. During sintering, the decomposition of a pyrochlore phase and the formation of a perovskite phase takes place Dependences of dielectric permitivities and losses on temperature in final PZT ceramics were very similar and they were not influenced by the type of calcinate.