High-temperature Structural Behaviour Research Articles

Revealing high temperature structural behavior behind the enhanced piezoelectric properties of lead-free 0.7BiFeO3-0.3BaTiO3 ceramics by in-situ process

Local domain evolution plays a vital role in modulating the high-temperature piezoelectricity of ferroelectric ceramics. Herein, 0.7BiFeO3-0.3BaTiO3-xBi(Zn0.5Ti0.5)O3 (referred to as BF-BT-xBZT, 0.00≤x≤0.05) high temperature lead free piezoelectric ceramics were prepared by a solid-phase method. All ceramics are at the morphotropic phase boundary (MPB), with an overall decreasing trend of the R/Pc phase fraction upon heating. STEM reveals the in-situ heterogeneity of polar nanodomains. Due to the modulation with proper R/PC ratio and a multilayer nanodomain ratio, the in-situ d33 and Tdr values increase from 495 pC/N and 314 °C to 502 pC/N and 360 °C with increasing content of (Zn1/2Ti1/2)3+ from x=0 to x=0.04 while maintaining a high Curie temperature (Tc>470 °C). This study provides a new paradigm for studying the in-situ depolarization behavior of lead-free perovskite materials and to optimizing the high-temperature piezoelectric properties by tuning the domain scale.

Structural behavior of a stuffed derivative of α-quartz, Mg0.5AlSiO4, at high temperature: an in situ synchrotron XRD study

High-temperature structural behavior of a stuffed derivative of α-quartz, Mg0.5AlSiO4, has been investigated using in situ synchrotron-based angle-dispersive powder X-ray diffraction (XRD) from 299 to 1273 K. Rietveld analysis of the XRD data indicates that the framework of Mg0.5AlSiO4 remains isostructural with α-quartz throughout the temperature range tested. As in α-quartz, unit-cell parameters a and c and cell volume V of Mg0.5AlSiO4 increase with increasing temperature, primarily due to progressive tilting of [(Al,Si)O4] tetrahedra along the a axes. However, the rates of increase in the cell parameters and the rate of decrease in the tetrahedral tilt angle (δ) are much smaller for Mg0.5AlSiO4 than for α-quartz. This behavior can be attributed to the occupancy of Mg2+ over the octahedral channel sites in the α-quartz-type framework, effectively hindering the [(Al,Si)O4] tetrahedral tilting. As a result, the α- to β-quartz phase transformation, which exists in silica at 846 K, does not occur in Mg0.5AlSiO4 up to 1273 K, and probably beyond, to its melting point.

Thermally induced reversible phase transformations of boroleucite, KBSi2O6

The high-temperature structural behaviour of the monoclinic and cubic phases of ANA-type boroleucite, KBSi2O6, was studied by X-ray powder diffraction. Boroleucite crystallizes with different symmetry depending on the method of preparation: the P 21/ a monoclinic modification was obtained by dry high-pressure synthesis at 600 °C and 0.5 GPa, whereas the cubic I ![Formula][1] 3 d phase was prepared by solid-state reaction at 700 °C, 1 bar, for 72 h. Both modifications undergo reversible transformations at higher temperatures, viz . P 21/ a ↔ (at 800 °C) Ia ![Formula][2] d and I ![Formula][3] 3 d ↔ (at 300 °C) P 21/ a ↔ (at 550 °C) Ia ![Formula][4] d , respectively. The values of the thermal-expansion coefficients are reported. The high-temperature Ia ![Formula][5] d modification of KBSi2O6 exhibits the lowest thermal expansion (6–10 × 10−6 °C−1); it decomposes above 1000 °C with formation of SiO2. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif [5]: /embed/mml-math-5.gif

The role of π-bonding on the high temperature structure of the double perovskites Ba2CaUO6 and BaSrCaUO6.

The high temperature structural behaviour of the uranium perovskites Ba2CaUO6 and BaSrCaUO6 has been investigated using a combination of synchrotron X-ray and neutron powder diffraction. Ba2CaUO6 undergoes a complex sequence of structures associated with the progressive loss of cooperative octahedral tilting: P21/n → I2/m → I2/m → I4/m → Fm3[combining macron]m. The observation of the intermediate tetragonal structure, I4/m, in this, contrasts with the previously reported rhombohedral R3[combining macron] intermediate formed by the Ba2SrUO6 oxide. The importance of π-bonding in determining the structural sequence is discussed.

In-situ high-temperature diffraction studies of ion-exchanged umbite

Microporous materials such as zirconium silicates have the potential to be of importance in the nuclear industry for the selective uptake of cationic radionuclides and environmental pollutants. The structural behaviour of these materials at elevated temperatures is of interest for two reasons, the first is the densification of the exchanged materials prior to long term storage and the second is the formation of new porous phases which may have increased ion exchanged affinity for certain cations. The work presented here focuses on the umbite system. Umbite is a naturally occurring microporous zirconium titanium silicate found in northern Russia and synthetic analogues, K2ZrSi3O9·H2O, can be prepared using hydrothermal methods. It has an orthorhombic cell with a = 10.2977(2)Å, b = 13.3207(3)Å and c = 7.1956(1)Å. The ion-exchange of umbite with cations such as rubidium, caesium and strontium and the structures of the resulting exchanged materials have been studied. Exchanges with certain cations were found to cause a change in crystal system to a monoclinic cell. Recently Rocha and co-workers found that synthetic umbite will undergo a topotactic transformation when heated 9100C to form a new microporous zirconium silicate (AV-15) with the formula K2ZrSi3O9·2H2O, but to date no in-situ work has been carried out on this phase transition. In this work the high temperature structural behaviour of five umbite samples with different exchanged cations (K+, Na+, Mg2+Ca2+and Cu2+) was studied up to a temperature of 10000C. All samples behaved very differently, indicating that the nature and location of the charge balancing cation plays an important part in determining which high temperature phases are formed. Certain general trends were observed, with group 1 cations the samples remain crystalline to high temperatures. With group 2 cations dense phases are formed at high temperatures and with transition metal cations there is a loss of crystallinity at low temperature.

High-Pressure and High-Temperature in situ X-Ray Diffraction Study of FeP2 up to 70 GPa

The high-pressure and high-temperature structural behavior of FeP2 is investigated by means of synchrotron x-ray powder diffraction combined with a laser heating technique up to 70 GPa and at least 1800 K. No phase transition of FeP2 occurs up to 68 GPa at room temperature. While a new phase of FeP2 assigned to the CuAl2-type structure (I4/mcm, Z = 4) is observed at 70 GPa after laser-heating. This new phase presents a quenchable property on decompression to ambient conditions. Our results update previous experimental data and are consistent with theoretical studies.

The role of microstrain on the thermostructural behaviour of industrial kaolin deformed by ball milling at low mechanical load

The high-temperature structural behaviour of kaolin deformed by compaction and shear in a ball mill was investigated by extending the temperature range to 1500 °C. The deformation was induced for the first time with steps of low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25 °C and at a vacuum of 0.13 Pa). The investigated kaolin was made of about 65% of a well-ordered kaolinite, 10% of dickite and 25% of quartz. The mechanical energy transfer to the material was measured via the microstrain < ε 2> 1/2 because we wanted to investigate the details of the evolution of the kaolin modification as a function of the microstrain. At the very early stage of milling (up to 1 h), no strain was accumulated in the kaolinite structure which, however, presented lamination, layer flattening and texturing. Further milling induced a progressive reduction of the stacking layer coherence and an increase of the microstrain, in both cases as a non linear function of the deformation time. The thermo-structural behaviour of kaolin was investigated by TG-DTA in a helium atmosphere at 10 °C/min of heating rate. In the medium temperature range (400–1000 °C), the mechanical milling affected the dehydroxylation reaction of kaolinite by a significant anticipation of about 300 °C of the temperature range, which usually occurs between 400 and 800 °C. Other reactions related to the formation of a Si-spinel and other intermediate phases were observed. The mechanical deformation severely influenced of the high-temperature reactions related to mullite and cristobalite formation. All thermal reactions linearly correlate to the microstrain < ε 2> 1/2 accumulated in the kaolinite structure. The reported data are of particular usefulness in industrial applications involving grinding or milling of kaolin.

On the high-temperature structural behaviour of talc (Mg3Si4O10(OH)2) to 1600°C: Effect of mechanical deformation and strain

The high-temperature structural behaviour of talc, Mg3Si4O10(OH)2, deformed by compaction and shear was investigated for the first time by extending the temperature range to 1600°C. The deformation was induced with low mechanical load through a specifically built planetary ball milling working in a controlled thermodynamic environment (25°C and at a vacuum of 0.13 Pa). The mechanical energy transfer to the material was measured via the microstrain ⟨ϵ 2⟩1/2. In our experimental set-up, amorphisation was not deliberately reached since we wanted to investigate the details of the evolution of the talc structure as a function of the microstrain. At the very early stage of milling (up to 1 h), no strain was accumulated in the talc structure which, however, presented lamination, layer flattening and texturing. Further milling induced a progressive reduction of the stacking layer coherence and an increase of the microstrain, in both cases as a non linear function of the deformation time. The thermo-structural behaviour of talc was investigated by TG-DTA in a helium atmosphere at 10°C/min of heating rate. In the medium temperature range (400–1100°C), the mechanical milling affected the dehydroxylation reaction by a significant anticipation of about 200–300°C of the temperature range, which usually occurs between 750 and 1050°C and gave rise to a clear exothermic reaction at about 840°C, related to an increase of the recrystallisation kinetics of MgSiO3 (orthoenstatite). The mechanical deformation strongly influenced also the kinetics of the high-temperature endothermic reactions at about 1555°C, involving cristobalite and MgSiO3 polymorphs melting and transformation. All thermal reactions linearly correlate to the microstrain ⟨ϵ 2⟩1/2 accumulated in the talc structure.

Hydration/Dehydration and Cation Migration Processes at High Temperature in Zeolite Chabazite

High temperature structural behavior of a natural chabazite of composition (Ca1.1Na0.4K0.7)[Si8.6Al3.4O24]·14.4H2O has been characterized by means of in situ HT single-crystal X-ray diffraction (SC-XRD) and thermogravimetric analysis. Lattice dimensions have been measured in the 25−700 °C range and crystal structure refined from XRD data collected at T = 25, 100, 125, 175, 250, 300, 425, and 600 °C. Variations of unit-cell parameters as a function of temperature reveal two discontinuities at 100 and 200 °C, to which no symmetry changes are associated, and an overall volume reduction of 2.8%. Between 200 and 250 °C, a steep contraction of cell volume is associated with a significant broadening of diffraction profiles, which turn sharp and narrow at higher temperatures. As the dehydration process proceeds with increasing T, cationic sites partly coordinated by extraframework water molecules become unstable and cations migrate toward new positions. Cations occupying the C2, C3, and C4 sites at room temperatu...

Pure Iron Compressed and Heated to Extreme Conditions

The results of a first-principles study supported by the temperature-quenched laser-heated diamond anvil-cell experiments on the high-pressure high-temperature structural behavior of pure iron are reported. We show that in contrast to the widely accepted picture, the face-centered cubic (fcc) phase becomes as stable as the hexagonal-close-packed (hcp) phase at pressures around 300-360 GPa and temperatures around 5000-6000 K. Our temperature-quenched experiments indicate that the fcc phase of iron can exist in the pressure-temperature region above 160 GPa and 3700 K, respectively. This, in particular, means that the actual structure of the Earth's core may be a complex phase with a large number of stacking faults.

High temperature structural behaviour of Li2VOSiO4

Structural investigations at high temperature were carried out on compound Li2VOSiO4, which crystallises with a natisite-type structure. Unit-cell parameters were measured and diffracted intensities collected at regular intervals in the temperature range 25–500 °C using single crystal X-ray diffraction techniques. Thermal expansion coefficients showed positive expansion of lattice constants, greater along the c direction, and of cell volume. Reversibility of thermal expansion in the investigated temperature range was checked by measuring unit-cell parameters after the crystal was cooled down from 500 °C to room temperature. Structure refinements revealed that the [VOSi4]n 2− layers are almost rigid and that the most relevant modifications occur at the intercalated ion layer.

Phase transitions in Cd 3P 2 at high pressures and high temperatures

The high-pressure, high-temperature structural behaviour of Cd 3P 2 has been studied using electrical resistance measurements, differential thermal analysis, thermo baric analysis and X-ray diffraction. At room temperature, a phase transformation is observed at 4.0 GPa in compression. The experimental zero-pressure bulk modulus of the low-pressure phase is 64.7(7) GPa, which agrees quite well with the calculated value of 66.3 GPa using the tight-binding linear muffin-tin orbital method within the local density approximation. Tentatively, the high-pressure phase has an orthorhombic crystal structure with space group Pmmn (#59). The relative volume change at the phase transition is Δ V/ V = −5.5%. Amorphization of the sample occurs above 25 GPa. A P– T phase diagram of Cd 3P 2 has been constructed. A metastable phase is observed at ambient conditions after heating the sample to above 600 K at about 3 GPa. A hexagonal unit cell can describe the crystal structure of the metastable phase.

Layered Intergrowth Phases Bi4MO8X (X=Cl, M=Ta and X=Br, M=Ta or Nb): Structural and Electrophysical Characterization

The high-temperature structural behavior of the layered intergrowth phase Bi4TaO8Cl, belonging to the Sillén-Aurivillius family, has been studied by powder neutron diffraction. This material is ferroelectric, space group P21cn, at TC<640 K. An order–disorder transition to centrosymmetric space group Pmcn is found around 640 K, which involves disordering of TaO6 octahedral tilts. A second phase transition, of a first-order nature, to space group P4/mmm occurs at a temperature of ∼1038 K. The crystal structures of the bromide analogs Bi4MO8Br (M=Nb, Ta) have also been determined at room temperature; both are isomorphous with Bi4TaO8Cl and exhibit maxima in dielectric constant at temperatures of approximately 588 and 450 K, respectively.

High-temperature structural behavior of SrRuO 3

The unusual metal SrRuO 3 is perhaps the only known 4d transition-metal-based ferromagnet ( T c = 162 K) with a sizable moment. To complement low- T polarized neutron diffraction measurements of the magnetization density, high- T neutron diffraction measurements are reported here. Two structural phase transitions are observed. Between 10 and 800 K, SrRuO 3 is orthorhombic and at 800 K it appears to be tetragonal until 975 K, where it becomes cubic. The temperature variation of the lattice parameters is reported along with a structural description of the tetragonal phase.

The high-temperature structural behaviour of copper(I) iodide

The structural behaviour of copper(I) iodide, CuI, has been investigated between room temperature and its melting point (TM=878 K) using neutron powder diffraction. Detailed measurements have been made in the vicinity of the two known structural phase transitions gamma to beta and beta to alpha , which are observed at 643+or-2 K and 673+or-8 K. Within the zinc-blende-structured gamma -phase (space group F43 m) increasing disorder of the Cu+ ion sublattice is observed as temperature approaches the gamma to beta transition, in addition to a non-linear thermal expansion. The hexagonal beta -phase (space group P3m1) is observed as a single phase in the temperature range 645-668 K, but on first heating it is found to coexist with a rhombohedral phase. This transient phase was observed in isolation for only a short time but this was sufficient to show that its structure was that of CuI-IV (space group R3m), which had only been observed previously at elevated pressures. The high-temperature phase alpha -CuI has Fm3m symmetry with Cu+ ions distributed randomly over all the tetrahedral sites within the cubic close-packed I$sublattice.

Fluoromica coated Nicalon fiber reinforced glass-ceramic composites

High temperature structural behavior was demonstrated in Nicalon fiber reinforced glass-ceramic matrix composites by means of a functional weak fluoromica interface. Sheet silicate interfaces were evaluated from both the mica matrix and sol-gel derived fluoromica fiber coatings in a calcium aluminosilicate (CAS) Matirx. The Nicalon fiber reinforced mica matrix composites were found to fail early in shear owing to the cleavage dominated mica matrix, whereas the potassium-phlogopite mica coated composite provided much higher strength with tensile failure mode. The oxidative stability inherent in the fluoromica interface led to the enhanced oxidation enbrittlement resistance of the composites at high temperatures in an aerobic environment, in contrast to the severe embrittlement seen in uncoated Nicalon fiber reinforced composites with the in situ carbon-rich interface. Microstructural and mechanical studies of potassium-phlogopite coated N/CAS II composites are presented, along with the synthesis and characterization of potassium-phlogopite precursor sol.