Rhombohedral Phase Transformation Research Articles

B-site ion driven morphotropic phase transformation in NN-BT-ABO3 lead-free ceramics through chemical pressure

Forming morphotropic phase boundary (MPB) is crucial to improve the piezoelectric properties of lead-free NaNbO3-BaTiO3-xABO3 (NN-BT-xABO3) ceramics. In this work, four NN-BT-xABO3 systems (ABO3 = K0.5Bi0.5TiO3 (KBT), Bi0.5Na0.5TiO3 (BNT), K0.5Bi0.5ZrO3 (KBZ), and Bi0.5Na0.5ZrO3 (BNZ)) were constructed to analyze the relationship between ABO3 characteristic and phase transformation behavior. The results showed that the NN-BT-xKBT/xBNT systems underwent ferroelectric tetragonal (T)-relaxor ferroelectric T phase transformation with d33 ∼ 150 pC/N and kp ∼ 0.2 in NN-BT-0.11KBT and NN-BT-0.09BNT compositions, while the NN-BT-xKBZ/xBNZ systems underwent ferroelectric T- ferroelectric rhombohedral (R) morphotropic phase transformation with improved d33 ∼ 256 pC/N and kp ∼ 0.31 in NN-BT-0.04BNZ composition. The B-site ion radius was found to be a key factor in inducing phase transformation. The incorporation of small Ti4+ ions tends to break the long ordering of ferroelectrics, inducing the normal-relaxor ferroelectric phase transformation, however, the appearance of the R phase in NN-BT-xAZrO3 compositions was attributed to the chemical pressure caused by buckled Zr-O-Zr bonds as larger Zr4+ ions enter the NN-BT matrix lattices. This study will deepen the understanding of the ferroelectric phase transformation mechanism in the lead-free NaNbO3 material system and provide a guidance for designing morphotropic NN-based lead-free piezoelectric ceramics.

Sc, Ge co-doping NASICON boosts solid-state sodium ion batteries' performance

Solid-state sodium ion batteries (SSSIBs) have been proposed to resolve the safety hazards of traditional liquid batteries. The sodium superionic conductor (NASICON), one of the most promising solid electrolyte candidates, has received much attention. However, the practical use of NASICON has been impeded by low ion mobility at room temperature (RT) and poor interfacial connectivity. Here, the improvement of both bulk and grain boundary conductivity has been achieved simultaneously via Sc and Ge co-doping. Bulk conductivity is increased by the Sc-doping, which increases the stability of the rhombohedral phase at RT, and the Ge-doping, which reduces the monoclinic to rhombohedral phase transformation temperature. The highest total conductivity of 4.64 × 10−3 S cm−1 was obtained for the Na3.125Zr1.75Sc0.125Ge0.125Si2PO12 structure. The improvement in conductivity due to the stabilisation of the rhombohedral phase was verified by DFT calculations. Finally, two different solid-state batteries using Na3V2(PO4)3 and Sn4P3@CNT as electrodes display impressive cycling capacities of 98 and 629 mAh g − 1 after 250 cycles, respectively. This co-doping principle also provides a framework to explore multiple-doping for other materials with similar structures.

Maximum self‐generated electric field induced by pyroelectricity of PZT 95/5

AbstractElectric field induced by pyroelectric effect is often observed in the ferroelectric devices when they subject to temperature change. But the maximum value of ferroelectric field is still unknown. If this value exceeds the coercive field or even reaches breakdown field, the ferroelectric devices may be damaged. In this study, pyroelectricexperimental of PbZr0.95Ti0.05O3 (PZT 95/5) ferroelectric ceramics was conducted to understand maximum self‐generated pyroelectric field. An equivalent circuit and path of pyroelectricity was built to analyze the pyroelectric field of PZT 95/5. Results indicated that maximum pyroelectric field is about 1.1 kV/mm. This value is controlled by the internal resistivity of PZT 95/5. A mean internal resistivity of PZT 95/5 is about 1 × 109 Ω·m during the low‐temperature to high‐temperature rhombohedral phase transformation.

Flexural properties, bond ability, and crystallographic phase of highly translucent multi-layered zirconia.

This study investigated the mechanical properties, bond ability, and crystallographic forms of different sites in a highly translucent, multi-layered zirconia disk. Flexural properties, bond ability to resin cement, and phase composition were investigated at three sites of a highly translucent, multi-layered zirconia disk: incisal, middle, and cervical. Flexural strength (FS) and flexural modulus (FM) were measured with static three-point flexural test. Shear bond strength (SB) to resin cement was measured after 24 h storage (37°C). Phase composition under mechanical stress was analyzed using X-ray diffraction. Without air abrasion, FS at the incisal site yielded the lowest value and was significantly lower than the middle and cervical sites. Air abrasion lowered the FS of each site. FM at the incisal site without air abrasion showed the significantly lowest value, and air abrasion increased its FM value. At the middle and cervical sites, their FM values were higher than the incisal site but were not significantly affected by air abrasion. SB value did not show significant differences among the sites. After sintering, cubic zirconia was detected at each site. Rhombohedral phase transformation occurred after mirror polishing. In highly translucent, multi-layered zirconia which was mainly composed of cubic zirconia, rhombohedral phase transformation occurred under mechanical stress and resulted in weakened mechanical properties.

Impact of fabrication temperature on the stability of yttria doped bismuth oxide ceramics

Bi2O3 doped with Y2O3 in the cubic-δ structure is a promising electrolyte material to bring the operating temperatures of solid oxide fuel cells (SOFCs) down to ≤650 °C. However, even in the doped state, the stability of its fast-ionic conductor phase (cubic-δ) at this targeted operating temperature is questionable. In this work, the stability of the electrical conductivity of 28 mol% Y2O3 doped Bi2O3 (YDB) ceramics sintered at different temperatures was investigated. YDB ceramics sintered at 800 °C exhibited a fast, exponential-type conductivity decay with a concomitant cubic to rhombohedral phase transformation upon a 100-hour exposure to 650 °C, in stagnant air. On the other hand, all the YDB ceramics sintered at higher temperatures (900–1100 °C) experienced a slower, linear conductivity decay under the same conditions, but with no observable phase transformation. This difference was explained in the premise that, based on the most recent Y2O3-Bi2O3 phase diagram, YDB sintered at 800 °C had a higher amount of metastable cubic-δ phase than those sintered at higher temperatures. Practically, it was suggested that for SOFC applications, the conventional approach of lowering the sintering temperature to reduce the manufacturing costs would result in poor stability in YDB electrolytes.

Polaronic Effect on Electrical Conductivity and Thermoelectric Power in Ga(Cu)V4S8

Polycrystalline V4-cluster compounds of GaV4S8 and its derivatives Ga0.90 Cu0.10V4S8 and Ga0.90Cu0.20V4S8 have been prepared at 800°C by solid-state reaction method. Although the cubic–rhombohedral phase transformation at 45 K was found to be absent in the derivatives of GaV4S8, low-temperature hopping conduction occurred in all the materials. In the present context, we explain the conduction mechanism for all the materials using polaron theory. The polaron size was found to be large above 260 K but small below 260 K in GaV4S8, as confirmed by the Seebeck coefficient. From the activation energies and polaron size, the anomaly at 260 K is interpreted as associated with crossover from thermally activated to nearest-neighbor hopping upon cooling.

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO2F are carried out up to 19GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4GPa with rhombohedral (R-3c) structure above 4GPa, but the structure between 0.7GPa and 4GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8GPa and gets completed at 4.4GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO2F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO2F. Disorder-induced first order Raman modes at 176, 212, 381 and 485cm−1 soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91ppmK−1) is in fair agreement with the reported values (−1 to +1ppmK−1). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.

Sulfur-catalyzed phase transition in MoS2 under high pressure and temperature

We report phase transition and stability of MoS2 with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS2 at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS2 and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers.

Elastic stability of high coupling ternary single crystals

A sub-hertz harmonic force was used to investigate the elastic response of ternary (1-y-x)Pb(In1/2Nb1/2)O3-yPb(Mg1/3Nb2/3)O3-xPbTiO3 (PIN-PMN-PT) single crystal with compositions (x = 0.30 and x = 0.33) near the Morphotropic Phase Boundary (MPB) under isothermal and zero-field conditions. The observed instability under mechanical compression is attributable to a ferroelectric rhombohedral FR-ferroelectric orthorhombic FO phase transformation. The associated strain is a function of proximity to MPB and is consistent with the prediction of the classical Devonshire theory. Isothermal resonance-antiresonance frequency sweeps versus temperature revealed a non-vertical MPB with (∂TRT/∂x)<0, where TRT is the rhombohedral-tetragonal transition temperature. These results provide insights into the role of PT content on the stability of PIN-PMN-PT crystals used for electromechanical devices and sound projectors.

Electric-field-driven monoclinic-to-rhombohedral transformation in Na1/2Bi1/2TiO3

The lead free ferroelectric Na${}_{1/2}$Bi${}_{1/2}$TiO${}_{3}$ (NBT) is shown to exhibit electric-field-induced monoclinic ($Cc$) to rhombohedral ($R$3$c$) phase transformation at room temperature. This phenomenon has been analyzed both from the viewpoint of the intrinsic polarization rotation and adaptive phase models. In analogy with the morphotropic phase boundary systems, NBT seems to possess intrinsic competing ferroelectric instabilities near room temperature.

Piezoresponse force microscopy and vibrating sample magnetometer study of single phased Mn induced multiferroic BiFeO3 thin film

The effect of Bi excess (0 to 15%) and subsequently Mn doping (0 to 20%) on the structural, ferroelectric, and magnetic properties of BiFeO3 (BFO) thin films deposited by chemical solution deposition technique on low cost substrates are reported in the present work. No significant changes in crystallographic structure of BFO were observed with 5% excess Bi. With further increasing the Bi concentrations (> 10%), however, rhombohedral phase transformation occurs with appearance of the other Bi excess phases. Piezoresponse force microscopy studies show that the BFO thin film having 5% excess Bi and 10% Mn yields good ferroelectric behavior. The ferromagnetic behavior of 10% Mn doped BFO thin film is confirmed with vibrating sample magnetometer studies. The results indicate that the 10% Mn doped BFO thin films deposited by chemical solution deposition technique on low cost substrate exhibit very good multiferroic properties, suitable for magnetic memories applications.

Thermomechanical behaviors of Ni–Ti shape memory alloy ribbons and their numerical modeling

The thermomechanical behaviors of Ni–Ti shape memory alloy (SMA) ribbons, associated with stress and temperature-induced transformations, are experimentally and numerically investigated. A 2-D incremental formulation of an SMA model is proposed and implemented in an ABAQUS finite element program through a user-defined material (UMAT) subroutine for the purpose of predicting the thermomechanical characteristics of SMA films. The evolution of stress–strain curves and phase transformation temperatures is examined with various annealing temperatures. The rhombohedral phase (R-phase) transformation appears in the specimen of an Ni–Ti SMA ribbon. The uncommon behaviors of permanent elongation and partial pseudoelastic behavior are observed. The 2-D SMA model with a new transformation function shows a good prediction for the hysteresis of the strain–stress curve and the recovery stress. For application of the developed numerical model of 2-D SMAs, the feasibility of the gripper actuator with the SMA ribbon is numerically demonstrated.

SINGLE GRAINS HOSTING TWO SPACE GROUPS — A TRANSMISSION ELECTRON MICROSCOPY STUDY OF A LEAD-FREE FERROELECTRIC

In this letter, conclusive experimental evidence is presented for the coexistence of two phases within individual grains in the system ( Bi 0.5 Na 0.5 TiO 3)0.91-( BaTiO 3)0.06-( K 0.5 Na 0.5 NbO 3)0.03. Electron diffraction data unequivocally reveal the presence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively, both on the nanoscale level. Superlattice reflections, observed in the corresponding selected area electron diffraction, were used to identify the rhombohedral and tetragonal phase present at different regions within one grain. It is thought that the rhombohedral phase acts as a nucleation site for the phase transformation monitored under applied electrical field. The coexistence of these two phases in conjunction with the triggered tetragonal to rhombohedral phase transformation rationalizes the high bipolar and unipolar strain recorded for this lead-free ferroelectric material.

Phase Stability and Sintering Behavior of 10mol%Sc2O3–1mol%CeO2–ZrO2 Ceramics

The phase composition and sintering behavior of two commercially available 10mol%Sc2O3–1mol%CeO2–ZrO2 ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft (∼100μm) spherical agglomerates containing 60–100nm crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to 30μm long and 3–4μm thick, which consisted of smaller 100–200nm crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic (c) and rhombohedral (r) phases: 79% cubic +21% rhombohedral for DKKK powders and 88% cubic +12% rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93–95% of theoretical density when sintered at 1300°C for 2h. Comparatively, the Praxair powders required high sintering temperatures at 1500–1600°C. However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at 1300°C or above exist in a cubic phase at room temperature. However, if sintered at 1100°C and 1200°C, the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at 1300°C or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a 300–500°C temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to 900°C. However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in ScCeZrO2 electrolyte upon heating and cooling down from the operation temperatures.

Investigation of BiFeO 3 modified PbTiO 3–Bi(MgZr)O 3-based complex perovskite ceramics

Bismuth containing crystalline solutions of (1 − x)Bi(MgZr) 0.5O 3– xPbTiO 3 (BMZ–PT) and [(BiFeO 3) y − (BiMg 0.5Zr 0.5) 1− y ] x − [PbTiO 3] 1− x (BMZ–BF–PT) have been developed using conventional ceramic technology. X-ray diffraction analysis reveals that both the systems possess a perovskite structure, in which tetragonal to rhombohedral phase transformation appears for x = 0.55 in BMZ–PT and y = 0.20 in BMZ–BF–PT systems. SEM photographs reveal a uniform grain size distribution in the solid solution matrix with the presence of ferroelectric domains in few of the compositions. Ferroelectric hysteresis (polarization–electric field, P–E) loops reveal that increase in BiFeO 3 in BMZ–PT systems results in a decrease in residual polarization of the system with change and distortion in the shape of the (P–E) loops.

Elastic constants and volume changes associated with two high-pressure rhombohedral phase transformations in vanadium

We present results from ab initio calculations of the mechanical properties of the rhombohedral phase $(\ensuremath{\beta})$ of vanadium metal reported in recent experiments, and other predicted high-pressure phases ($\ensuremath{\gamma}$ and bcc), focusing on properties relevant to dynamic experiments. We find that the volume change associated with these transitions is small: no more than 0.15% (for $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$). Calculations of the single crystal and polycrystal elastic moduli (stress-strain coefficients) reveal a remarkably small discontinuity in the shear modulus and other elastic properties across the phase transitions even at zero temperature where the transitions are first order.

Deformation Behavior of Shape-Control Plate Using NiTi Shape Memory Alloy Wire

This study deals with deformation behavior of a shape-control plate which consists of an aluminum alloy plate and a pre-strained NiTi shape memory alloy (SMA) wire. The shape-control plate exhibits reciprocating bending deformation by heating and cooling. Deformation behavior of the plate is examined by electric heating and natural cooling of the SMA wire. Experimental results exhibit that the bending deformation of the plate is considerably stable over more than two thousand heating-cooling cycles and can be well controlled by electric current. Furthermore, the deformation behavior of the plate is analyzed by a simple beam theory for the aluminum alloy plate and Brinson’s one-dimensional constitutive model for the SMA wire taking account of not only martensitic transformation but also rhombohedral-phase transformation. Numerical results describe well the deformation behavior of the shape-control plate observed in the experiments.

Phase transformation and low temperature sintering of manganese oxide and scandia co-doped zirconia

The powders of 89 mol% ZrO 2–11 mol% Sc 2O 3 (11ScSZ) doped with various Mn 2O 3 contents were prepared by a co-precipitation method combined with a SCFD (supercritical fluid drying) route. Inhibition of the cubic–rhombohedral phase transformation in both oxidation and reduction atmospheres is achieved for 11ScSZ by the addition of 2.0 mol% Mn 2O 3. The Mn 2O 3 addition can lower the sintering temperature of 11ScSZ ceramics, and the 11ScSZ–2Mn 2O 3 compact can be sintered to nearly full density at 850 °C. 11ScSZ–2Mn 2O 3 ceramic with the cubic structure sintered at 900 °C possesses the conductivity of ∼ 0.10 S cm − 1 at 800 °C, which is very promising for intermediate temperature solid oxide fuel cell (SOFC) applications.

Electric field effect on the rhombohedral–rhombohedral phase transformation in tin modified lead zirconate titanate ceramics

The ferroelectric–to–ferroelectic phase transformation between the high temperature (FERH) and the low temperature (FERL) rhomobohedral phases in lead based perovskite under the dc bias conditions was investigated. Dielectric measurements show that an external electric field stabilizes the FERL phase and moves the phase transformation to a higher temperature. The observation has been further verified by an in situ microfocused x-ray study where an external field can effectively induce the oxygen octahedral tilting in the crystalline lattice and extends the thermal stability region of the FERL phase to a higher temperature. An analysis based on the combination of the Clausius–Clapeyron relationship with lattice dynamic principles suggests that the transformation from FERH to FERL is driven by a short-range interaction in the crystalline lattice. The origin of this short-range interaction is proposed, based on the structural evolution during the phase transformation. Experimental evidence suggests that such interaction driving the structural instability can be exploited by an external electric field near the phase transformation temperature and leads to an unusual, transient field-enhanced deformation near the FERH/FERL phase transformation.

Dynamic modelling of a shape memory alloy adaptive tuned vibration absorber

Shape memory alloy (SMA) adaptive tuned vibration absorbers show great potential for adaptive–passive vibration control. Heating the SMA spring element in an ATVA changes the SMA elastic modulus, which results in changing the device natural frequency. With proper design, SMA ATVA may be tuned to attenuate harmonic vibration of a primary system for a range of frequencies. Continuous tuning of the SMA ATVA is realized through modulating the temperature of the SMA spring elements. A perceived limitation of SMA in vibration control applications has been a potentially large temperature hysteresis accompanying the martensitic transformation in SMA. However, with appropriate heat treatment, a rhombohedral phase (R-phase) transformation may be achieved in the material without significant hysteresis. This paper discusses the development and modelling of a continuously tuned SMA ATVA utilizing an R-phase transformation to realize online variation of the ATVA natural frequency. The models are developed for use in feedback control of an SMA ATVA. An appropriate control goal for such controllers is defined and verified.