Polymorphic Phase Transition Region Research Articles

Силикаты магния при высоких динамических нагрузках

Research on the dynamic compression of geological materials is important for understanding composition and physical condition of the deep interior of the Earth and other planets. It also provides some data on the interaction processes related to the formation and evolution of planets. Magnesium silicates dominate in Earth's mantle and, thus, are expected to become the major phases in rocky exoplanets. In particular, enstatite Mg2[Si2O6] and forsterite Mg2SiO4 are essential constituents of Earth's mantles. Strong emphasis is put on the phase transition possibility for magnesium silicates under study. A remarkable fact is the dissociation of Mg2SiO4 into the following oxides: MgO and SiO2 (stishovite). The experiments have been carried out at a pressure value of 33 GPa, which corresponds to that in Earth's mantle at a depth of 1000 km. In this paper, the results of modeling the shock-wave loading of enstatite and forsterite as the mixtures of quartz SiO2 and periclase MgO are presented. The proposed model assumes that the components of the mixture under shock-wave loading are in thermodynamic equilibrium. The components of the material under study are considered in a phase transition region as a mixture of low- and high-pressure phases. The model is also valid for a polymorphic phase transition region. The calculations of magnesium silicates are performed with account for the polymorphic phase transition of quartz and periclase. The results are validated using the data obtained in dynamic experiments.

Relation of the phase transition and electrical, photoluminescence properties in (1 − x) Na0.5K0.5NbO3–xLiSbO3:0.006Dy3+ lead free ceramics

The (1 − x) Na0.5K0.5NbO3–xLiSbO3:0.006Dy3+ were synthesized by a solid state reaction method. The phase structure, microstructure and electric properties were systematically investigated. The orthorhombic–tetragonal (O–T) phase was identified in the composition of x = 0.04 with the help of Raman spectra and photoluminescence response. The ceramic with x = 0.04 mol% LiSbO3 demonstrates optimal properties: Pr = 32.2 μC/cm2, Ec = 12.26 kV/cm, TC = 325 °C, tan δ ≤ 1.5% (250–400 °C). We believe that this work can provide an effective method to investigate the phase transition near polymorphic phase transition (PPT) region. The advantages of simple operation, quick response and pollution-free make it potential to be used in investigating the phase transition near the PPT region of lead-free ceramics.

Phase structures, PPT region and electrical properties of new lead-free KNLNTS-BCTZ ceramics fabricated via the solid-state combustion technique

Abstract (1−x)[(K0.44Na0.52Li0.04)(Nb0.84Ta0.10Sb0.06)O3]-x[(Ba0.85Ca0.15)(Ti0.90Zr0.10)O3]; (abbreviate to (1−x)KNLNTS-xBCTZ) lead-free piezoelectric ceramics were prepared via the solid state combustion technique and the effects of adding BCTZ compound (0≤x≤0.030 wt%) on the phase formation, morphology and electrical behavior of the compound ceramics were investigated. The X-ray diffraction (XRD) results at room temperature of all samples exhibited a single perovskite phase and the BCTZ efficiently diffused into the KNLNTS lattice during the sintering process to produce a solid solution. The crystal structure of these ceramics changed from O+T phases to higher T phase (0≤x≤0.020) and then began to change to a cubic phase when the x content increased from 0.025 to 0.030. A polymorphic phase transition (PPT) of (1−x)KNLNTS-xBCTZ ceramics was observed at an x content of 0.015 and the ratio of the orthorhombic and tetragonal phase was close to equal. Adding BCTZ compound at a ratio ≤ 0.015 promoted better morphology and higher densification. When the BCTZ compound was > 0.015, the microstructure and density of the samples deteriorated. The TO-T and TC of (1−x)KNLNTS-xBCTZ samples reduced continuously with increasing x content. Moreover, the (1−x)KNLNTS-xBCTZ showed a typical relaxor ferroelectric behavior with high BCTZ content. The optimum electrical properties, such as; the highest dielectric behavior (er at Tr = 2194 and em at TC = 6812), excellent ferroelectric properties (Pr = 27.9 µC/cm2 and EC= 9.9 kV/cm) and the highest d33 (295 pC/N) were obtained by the sample with x content of 0.015 wt%, this sample composition was confirmed to be in the PPT region. The addition BCTZ is a new alternative for improving densification, microstructure, ferroelectric and piezoelectric properties of KNLNTS ceramics.

Effect of phase structure changes on the lead-free Er3+-doped (K0.52Na0.48)1−xLixNbO3 piezoelectric ceramics

Er3+ doped (K0.52Na0.48)1−xLixNbO3 ceramics had been prepared through a conventional solid state reaction routine and their phase structure, ferroelectric, piezoelectric, and photoluminescence properties were systematically investigated. X-ray diffraction results clearly illustrated a phase transition from the orthorhombic to coexistence and further to tetragonal phase with the increase of Li content. Via measuring the ferroelectric and piezoelectric properties of the ceramics, it found that the polymorphic phase transition (PPT) effect contributed the obvious enhancement in electrical properties of the Er3+ doped (K0.52Na0.48)0.94Li0.06NbO3 ceramic which lay in the composition region of coexistence phase structure. The up-conversion photoluminescence intensity of green emission and red emission first increased as x increase from 0 to 0.06 and then decreased for x = 0.08. The most obvious enhancement in the up-conversion photoluminescence intensity was also achieved in the composition lying in the vicinity of PPT region. And the above circumstance caused the consideration that here the essence of the coexistence phase may be a phase structure with lower crystal symmetry instead of just physical combination of orthorhombic and tetragonal phase.

Inverted electro-mechanical behaviour induced by the irreversible domain configuration transformation in (K,Na)NbO3-based ceramics.

Miniaturization of domains to the nanometer scale has been previously reported in many piezoelectrics with two-phase coexistence. Despite the observation of nanoscale domain configuration near the polymorphic phase transition (PPT) regionin virgin (K0.5Na0.5)NbO3 (KNN) based ceramics, it remains unclear how this domain state responds to external loads and influences the macroscopic electro-mechanical properties. To this end, the electric-field-induced and stress-induced strain curves of KNN-based ceramics over a wide compositional range across PPT were characterized. It was found that the coercive field of the virgin samples was highest in PPT region, which was related to the inhibited domain wall motion due to the presence of nanodomains. However, the coercive field was found to be the lowest in the PPT region after electrical poling. This was related to the irreversible transformation of the nanodomains into micron-sized domains during the poling process. With the similar micron-sized domain configuration for all poled ceramics, the domains in the PPT region move more easily due to the additional polarization vectors. The results demonstrate that the poling process can give rise to the irreversible domain configuration transformation and then account for the inverted macroscopic piezoelectricity in the PPT region of KNN-based ceramics.

Effect of Yb Codoping on the Phase Transition, and Electrical and Photoluminescence Properties in KNLN:Er/xYb Ceramics

Ceramics 0.94(K0.5Na0.5)NbO3−0.06LiNbO3:Er/xYb with x = 0, 0.002, 0.004, 0.006, 0.008 were fabricated in this study, and phase structure, dielectric, piezoelectric, ferroelectric, and upconversion photoluminescence properties of the ceramics were systematically studied. Results show that all ceramics are in the polymorphic phase transition region near room temperature. However, a relaxor‐like phase transition was observed as Yb increasing to over x = 0.004. The optimized dielectric, piezoelectric, and ferroelectric properties, and maximized photoluminescence intensity ratio are obtained at x = 0.004. And the electrical and photoluminescence properties of the ceramics were discussed from the point view of the normal to relaxor‐like ferroelectric phase transition. The present study demonstrates that both the electrical and upconversion photoluminescence properties of the ceramics have an intimate correlation with the normal to relaxor‐like ferroelectric transition induced by Yb doping.

Strong domain configuration dependence of the nonlinear dielectric response in (K,Na)NbO3-based ceramics

The nonlinear dielectric response in (Na0.52K0.4425Li0.0375)(Nb0.92−xTaxSb0.08)O3 ceramics with different amounts of Ta was measured using subcoercive electric fields and quantified by the Rayleigh model. The irreversible extrinsic contribution, mainly caused by the irreversible domain wall translation, was strongly dependent on the domain configuration. The irreversible extrinsic contributions remained approximately the same within the single-phase regions, either orthorhombic or tetragonal, due to the similar domain morphology. However, in the polymorphic phase transition region, the domain wall density was increased by minimized domain size, as observed by transmission electron microscopy. This resulted in constrained domain wall motion due to self-clamping and reduced the irreversible extrinsic contribution.

Structure and electrical properties of IrO2-doped 0.5Ba0.7Ca0.3TiO3–0.5BaTi0.8Zr0.2O3 ceramics via low-temperature sintering

Lead-free ceramics 0.5Ba0.7Ca0.3TiO3–0.5BaTi0.8Zr0.2O3–xIrO2 (x = 0–1.2 %) were prepared at a low temperature of 1260 °C via sintering the as-synthesized nanoparticles, which were synthesized by a modified Pechini polymeric precursor method. All samples featured high levels of densification under the synthesis conditions; particularly, the ceramic prepared at IrO2 content of 0.4 % achieved the highest relative density (~97.4 %). Phase transition from tetragonal to orthorhombic and a polymorphic phase transition (PPT) region were observed, which were influenced by the IrO2 content. The dielectric properties, temperature coefficient of capacitance, ferroelectric properties, and piezoelectric properties as a function of IrO2 content were thoroughly studied. Optimal electrical properties (remnant polarization, piezoelectric constant, and planar electromechanical coupling factors, i.e., Pr = 6.28 μC/cm2, d33 = 199 pC/N, and kp = 0.258) were obtained around the PPT region. The findings of the ceramic electrical properties by IrO2 doping are believed to be insightful in the development of lead-free dielectric and ferroelectric materials.

Strong Red Emissions in Pr3+‐Doped (K0.5Na0.5)NbO3–CaTiO3 Diphase Ceramics

The photoluminescence and temperature sensing properties based on down‐shifting emission of Pr3+‐doped (K0.5Na0.5)NbO3–yCaTiO3 (KNN: yCT) diphasic materials were systematically investigated. Under 447‐nm excitation, Pr3+‐doped KNN: yCT samples exhibited significantly enhanced red emission at 603 nm assigned to 1D2→3H4 transitions of Pr3+ ions. The red emission intensities reached the optimum value with y = 0.05 near the polymorphic phase transition region. The origin of the enhanced red emission is mainly ascribed to the doping‐induced lattice symmetry change. The energy level transitions from the typical f–f transitions to the valence‐to‐conduction transitions were observed as CaTiO3 concentration increases above a critical concentration of y = 0.05. Furthermore, the sample with y = 0.05 also possessed excellent temperature response properties in a wide temperature range 300–473 K and the maximum sensing sensitivity was 0.016 K−1. The Pr3+‐doped (K0.5Na0.5)NbO3–yCaTiO3 red emission materials with admirable intrinsic piezoelectric properties may have important technological promise in novel multifunctional devices.

Y2O3-modified Ba(Ti0.96Sn0.04)O3 ceramics with improved piezoelectricity and raised Curie temperature

Ba(Ti0.96Sn0.04)O3–xY2O3 (BTS–Y) lead-free piezoceramics are fabricated by the conventional solid state reaction technique. The influences of Y3+ substitution on the phase structure, dielectric properties and piezoelectricity of BTS–Y piezoceramics are investigated. XRD patterns show the ceramics possess a pure perovskite structure. The orthorhombic–tetragonal (O–T) phase transition dielectric peak shifts toward lower temperature with increasing Y3+ content. The compositions x=0.15−0.25% that are at O–T polymorphic phase transition region possess excellent piezoelectricity and the maximum piezoelectric constant d33 of 454pC/N is obtained at x=0.2%. The ceramics’ Curie temperatures (TC) are monotonically increased from 71°C for x=0−86°C for x=0.4% due to Y3+ substitution and the usage temperature range of BTS–Y piezoceramics is broadened. Compared with Ca2+ which also decreases the O–T phase transition temperature (TO–T) but hardly affects the TC of BaTiO3-based ceramics, Y3+ substitution seems to be more beneficial for the practical application of the piezoceramics.

Displacement of Ta-O bonds near polymorphic phase transition in Li-, Ta-, and Sb-modified (K, Na)NbO3 ceramics

Excellent piezoelectric properties can be obtained near the polymorphic phase transition (PPT) region in (K, Na)NbO3 based piezoceramics. The enhanced piezoelectric properties resulted from the 18-fold spontaneous polarization directions caused by the coexistence of orthorhombic and tetragonal phases. In this study, the various polarization directions derived from frequently changing Ta-O bonds in Li-, Ta-, and Sb-modified (K, Na)NbO3 ceramic were directly observed by extended X-ray absorption fine structure. More than three peaks were observed and represented the nearest neighbor Ta-O bonds because of the coexistence of tetragonal and orthorhombic phases as well as Ta displacements along the [001]c and [110]c directions. Hence, the domains rotated easily and responded actively to an external electric or force field. Large Pr and optimized piezoelectric properties were obtained near the PPT region.

Investigation on transition behavior and electrical properties of (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 around polymorphic phase transition region

(K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 (KNLNTS) lead free ceramics with different Li concentration were fabricated by conventional solid-state reaction method. By increasing Li ions in KNLNTS, the grains grow up and the crystal structure changes from orthorhombic to tetragonal. When 0.03 ≤ x ≤ 0.05, the ceramics structure lays in PPT region. Polarization versus electric field (P-E) hysteresis loops at room temperature show good ferroelectric properties and the remnant polarization decreases by increasing Li content while coercive electric keeps almost unchanged. In PPT region, taking x = 0.04 as an example, the sample shows excellent dielectric properties: the dielectric constant is 1159 and loss tangent is 0.04, while the piezoelectric constant d33 is 245 pC/N and kp is 0.44 at room temperature, it is promising for (K0.5Na0.5)1-xLixNb0.84Ta0.1Sb0.06O3 with 4 at. % Li to substitute PZT.

Effects of CaAl2O4 on the electrical properties and temperature stability of (Na0.53K0.404Li0.066)Nb0.92Sb0.08O3 ceramics

Abstract Lead-free (1 − x)(Na0.53K0.404Li0.066)Nb0.92Sb0.08O3 + xCaAl2O4 (NKLNS–xCA) ceramics were prepared by the conventional mixed oxide method. The effects of the addition of CaAl2O4 on the phase transitions and electrical properties of the KNN-based ceramics were studied and the temperature stability of their piezoelectric and electromechanical properties was also discussed. It is found that 1.5 mol% CaAl2O4 doped NKLNS ceramic exhibits stable piezoelectric and electromechanical properties in −40 to 60 °C due to the extended polymorphic phase transition region induced by the B-site composition fluctuation. The piezoelectric constant d33, dielectric loss tanδ, and electromechanical coupling factor kp of the NKLNS–0.0150CA ceramic are found to be 254 pC/N, 3.5%, and 45%, respectively, demonstrating that the NKLNS–0.0150CA ceramic is a very promising lead-free piezoelectric material for practical applications.

Improvement in temperature stability and modified polymorphic phase transition of La-doped (Na0.52K0.44Li0.04) Nb0.8Ta0.2O3 lead-free piezoelectric ceramics

Lead-free ceramics (Na0.52K0.44Li0.04)1-3xLaxNb0.8Ta0.2O3 (KNLNT-Lax, x=0.00–1.25mol%) were prepared by traditional method. The piezoelectric proprieties and the polymorphic phase transition behaviors around the room temperature were investigated. The results show that high density (ρ~95%) and outstanding piezoelectric performance (d33≥200pC/N, kp≥40%) were obtained with 0.5–0.75mol% La doping. By the research of temperature dependent εr, Δfr/fr20°C, and kp, we acquired the extended and mild polymorphic phase transition region, and obvious improvement of temperature stability in −50–80°C with the maximum derivation d(Δfr/fr20°C)max less than 2%, as the La doping content exceeds 0.5mol%. The possible mechanism of enhanced properties and temperature stability was analyzed. Both excellent piezoelectric and temperature stability characteristics make the La-doped KNLNT ceramics feasible for practical applications.

Phase transition and electrical properties of LiNbO3-modified K0.49Na0.51NbO3 lead-free piezoceramics

Lead-free (1-x)K0.49Na0.51NbO3-xLiNbO3 (KNN-LN, x = 0 ~ 0.08) piezoelectric ceramics were prepared by the conventional solid-state sintering method. The effects of LiNbO3 doping amount x on the phase transition behavior and the electrical properties of KNN-LN ceramics were investigated. By increasing LiNbO3 doping amount x, the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature (T o–t) of KNN-LN ceramics shifted downwards, however, the Curie temperature (T c) slightly moved upwards. The room temperature phase structure thus changed from orthorhombic to tetragonal across the compositions with 0.05 ≤ x ≤ 0.06, named as PPT region. The composition with x = 0.06 in the tetragonal side of PPT region exhibited optimized electrical properties of d 33 = 246pC/N, k p = 41.6%, e r = 679, tgδ = 0.028, and Q m = 52. In addition to its very high T c = 467 °C, this ceramic can be an excellent candidate for replacing the lead-based piezoceramics in high temperature applications.

An anomaly in the kinetic properties of tin in the polymorphic phase transition region

Experimental data are presented on the resistivity and thermo-emf of tin over the temperature range −183 to 700°C, on thermal conductivity from −183 to +300°C, and on the speed of ultrasound from 20 to 400°C. Anomalies appear in the temperature curves corresponding to polymorphic transitions in the tin.