Ultrahigh Curie Temperature Research Articles

Significantly enhanced piezoelectric response in Aurivillius Bi3TiNbO9 ceramics simply by Ce doping

Bi3-xCexTiNbO9 (BTN-100xCe, x = 0, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11) piezoelectric ceramics with ultra-high Curie temperature were fabricated by a solid-state sintering method, and the effect of Ce doping on the structure, dielectric and piezoelectric properties of the ceramics was investigated. All ceramic samples possess a single Aurivillius phase and Ce ions enter both A- and B-sites. Dense microscopic morphology with a remarkable decrease of grain size can be observed due to Ce doping. The resistivity markedly increases after Ce modification, and a significantly enhanced piezoelectric constant d33 of 17.1 pC/N can be achieved in the optimized BTN-7Ce ceramics, which is nearly 6 times that of pure Bi3TiNbO9 (d33 ≈ 3 pC/N). In addition to ultra-high Curie temperature (TC = 886.5 °C) and relatively high resistivity (ρ = 1.2 × 106 Ω cm @ 500 °C), the BTN-7Ce ceramics should be very promising for piezoelectric sensor under the application in high temperature conditions.

Realization of ultra-high temperature stability and excellent electrical properties in LTO-based ceramics via A/B-site co-doing

La2Ti2O7 (LTO) ceramics, ideal for ultra-high temperature applications with a Curie temperature near 1460 °C, were previously limited by a low piezoelectric coefficient (d33) of 1.5 pC/N. In this work, the incorporation of Bi3+ and V5+ dopants have been employed to augment the piezoelectric properties of LTO. An enhanced d33 ∼ 4.0 pC/N is achieved in La1.91Bi0.09Ti1.97V0.03O7 (LBTV9) ceramics, while maintaining an ultra-high Curie temperature of 1437 °C. This improvement is attributed to local heterogeneity in the lattice structure induced by the doping. LBTV9 ceramics also exhibit excellent high-temperature stability and insulation properties, maintaining a d33 at 4.0 pC/N after annealing at 1000 °C for 48 h and stabilizing at 4.2 pC/N across temperatures from 200 °C to 1600 °C, with a DC resistivity of 1.7 × 106 Ω·cm at 1000 °C. These enhancements make LBTV9 ceramics suitable for extreme condition applications, thus unlocking new possibilities for their use in ultra-high temperature piezoelectric devices.

Superfine Nanodomain Engineering Unleashing Ferroelectricity in Incipient Ferroelectrics.

Incipient ferroelectrics have emerged as an attractive class of functional materials owing to their potential to be engineered for exotic ferroelectric behavior, holding great promise for expanding the ferroelectric family. However, thus far, their artificially engineered ferroelectricity has fallen far short of rivaling classic ferroelectrics. In this study, we address this challenge by developing a superfine nanodomain engineering strategy. By applying this approach to representative incipient ferroelectric of SrTiO3-based films, we achieve unprecedentedly strong ferroelectricity, not only surpassing previous records for incipient ferroelectrics but also being comparable to classic ferroelectrics. The remanent polarization of the thin film reaches up to 17.0 μC cm-2 with an ultrahigh Curie temperature of 973 K. Atomic-scale investigations elucidate the origin of this robust ferroelectricity in the emergent high-density superfine nanodomains spanning merely 3-10 unit cells. Combining experimental results with theoretical assessments, we unveil the underlying mechanism, where the intentionally introduced diluted foreign Fe element creates a deeper Landau energy well and promotes a short-range ordering of polarization. Our developed strategy significantly streamlines the design of unconventional ferroelectrics, providing a versatile pathway for exploring new and superior ferroelectric materials.

Ultra-high piezoelectric properties and ultra-high Curie temperature of Li/Ce-doped La2Ti2O7 ceramics.

The demand for ultra-high-temperature piezoelectric sensors in industrial applications has witnessed a rapid upsurge. In this study, the piezoelectric properties of La2Ti2O7 (LTO) piezoelectric ceramics with a perovskite-like layered structure were enhanced by doping with Li/Ce ions. It was found that a remarkable 300% enhancement in the piezoelectric constant (d33) value was achieved in Li/Ce-doped LTO ceramics compared to their pristine counterparts, reaching 6.4 pC N-1 at room temperature with an ultra-high Curie temperature of 1408 °C. After annealing at 500 °C, the d33 value of the samples can be further improved to 7.4 pC N-1. Moreover, temperature-dependent resistivity measurements indicate that even at 1000 °C, the ceramics exhibit a high resistivity of 8.9 × 105 Ω cm. By combining X-ray diffraction, Raman spectra, transmission electron microscopy and piezoresponse force microscopy data, the enhanced piezoelectricity of the ceramics is attributed to local heterogeneity induced by Li/Ce doping. Our results unequivocally demonstrate the suitability of modified LTO ceramics for ultra-high-temperature piezoelectric applications.

Simultaneous achievement of high performance and low sintering temperature in ultra-high Curie temperature La2Ti2O7 ceramics by V2O5 additive

This study uses V2O5 as a sintering aid for La2Ti2O7 ceramics, lowering the sintering temperature, increasing density and resistivity, enhancing piezoelectric performance and maintaining thermal stability.

Ultra-high Curie temperature transparent piezoelectric Bi doped Ca2Nb2O7 single crystals

Ca2Nb2O7 is a promising material for high-temperature piezoelectric applications due to its ultra-high Curie temperature, which makes it a suitable candidate for use in harsh environments. However, their piezoelectric performance needs improvement. Here, through chemical engineering methods, we have successfully grown Bi3+ doped Ca2Nb2O7 single crystals using the optical floating zone technique. The Ca1.94Bi0.06Nb2O7 single crystals achieved better high-temperature stability, transparency and a higher piezoelectric constant of 12.8 pC/N at 298 K and 20.0 pC/N at 660 K. In addition to the dopant effects, mechanisms for enhancing the piezoelectric properties of perovskite-like layer structure (PLS) materials have been identified, highlighting the significant contribution of the NbO6 octahedron and interlayer ions. These findings offers valuable insights into improving the performance of PLS materials, which is crucial for promoting their practical applications as high-temperature lead-free piezoelectric materials.

First-Principles Study on Mechanical, Electronic, and Magnetic Properties of Room Temperature Ferromagnetic Half-Metal MnNCl Monolayer.

Two-dimensional ferromagnetic (FM) half-metals are highly desirable for the development of multifunctional spintronic nano-devices due to their 100% spin polarization and possible interesting single-spin electronic states. Herein, using first-principles calculations based on density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional, we demonstrate that the MnNCl monolayer is a promising FM half-metal for spintronics. Specifically, we systematically investigated its mechanical, magnetic, and electronic properties. The results reveal that the MnNCl monolayer has superb mechanic, dynamic, and thermal (ab initio molecular dynamics (AIMD) simulation at 900 K) stability. More importantly, its intrinsic FM ground state has a large magnetic moment (6.16 μB), a large magnet anisotropy energy (184.5 μeV), an ultra-high Curie temperature (952 K), and a wide direct band gap (3.10 eV) in the spin-down channel. Furthermore, by applying biaxial strain, the MnNCl monolayer can still maintain its half-metallic properties and shows an enhancement of magnetic properties. These findings establish a promising new two-dimensional (2D) magnetic half-metal material, which should expand the library of 2D magnetic materials.

Enhanced piezoelectric performance in Na0.5Bi4.5Ti4O15‐based bismuth‐layered ceramics by Nb/Mn doping modification

AbstractIn this work, Na0.5Bi4.5Ti3.94–xMn0.06NbxO15+y bismuth‐layered ferroelectric ceramics were prepared by a solid‐state reaction method. The effect of Nb5+ content on crystal morphology, electrical properties, and piezoelectric performance were systematically investigated. The results show that the introduction of Nb5+ into Na0.5Bi4.5Ti3.94–xMn0.06NbxO15+y ceramics to replace Ti4+ increases the ratio of b/a lattice parameter, leading to the TiO6 octahedral distortion and the structural transformation tendency from the orthorhombic to tetragonal phase, which facilitates dipole movements of Na0.5Bi4.5Ti3.94–xMn0.06NbxO15+y ceramics. Therefore, the ferroelectric properties of Na0.5Bi4.5Ti3.94–xMn0.06NbxO15+y ceramics are improved, and an enhanced piezoelectric coefficient of 30 pC/N combining great temperature stability with d33 value higher than 25 pC/N in the temperature range of 25°C–450°C has been realized in Na0.5Bi4.5Ti3.94–xMn0.06NbxO15+y ceramics with x = 0.08 mol. Our work provides a good model for designing lead‐free ultrahigh Curie temperature piezoelectric devices that can be practically applied in extremely harsh environments.

Lead-Free BF-BT Ceramics With Ultrahigh Curie Temperature for Piezoelectric Accelerometer.

Piezoelectric ceramics have been widely used in high precision sensors such as vibration detection, but piezoelectric accelerometers in high-temperature applications are very rare. We prepared ( 1- x ) BiFeO3- x BaTiO3-0.0035MnO2-0.001Li2CO3(BF- x BT) ceramics by a solid state approach, and investigated the effect of BT content( x ) on the phase structure, microstructure, dielectric properties, ferroelectric properties, piezoelectric properties, temperature stability, and especially the sensitivity of the piezoelectric accelerometer. The crystal structure of the sample is pure perovskite structure with the MPB (R and P phases) locating in a composition range of 0.28 ≤ x ≤ 0.32 for BF-xBT ceramics, and the single R phase exist at . When x = 0.30, the ceramic presents both high Curie temperature and d₃₃ . Notably, the sensitivity of BF-0.30BT piezoelectric accelerometer reaches the highest value of about 40 pC/g and shows an excellent stability until 400 °C, indicating that this material is a promising candidate for high-temperature piezoelectric accelerometer applications.

Microstructure and Electrical Properties of Lanthanides-doped CaBi2Nb2O9 Ceramics

Calcium bismuth niobate (CaBi2Nb2O9, CBN) is a promising candidate for high-temperature applications in aerospace, shipping, and nuclear industries, owing to its ultra-high Curie temperature (TC∼x223C 940 °C). Lanthanide ions modified CBN based ceramics were prepared by a conventional solid-state sintering route. The crystal structure, microstructure, and high-temperature electrical properties of CBN ceramics doped by different lanthanide dopants (La, Ce, Nd, Sm, Dy, Er, Yb) were studied and the underlying mechanism was also discussed. Among these doping ions, Ce3+/Ce4+ doping CBN ceramics exhibits anomalous electrical properties. Impedance analysis was introduced to clarify the difference in electrical properties of lanthanide-doped CBN ceramics at high temperatures. As a result, the anomalous electrical properties are attributed to the extrinsic oxygen vacancies induced by the Ce doping. These results could assist to tailor the electrical properties of Aurivillius phase ceramics with lanthanide modifications.

Enhanced piezoelectric properties and high electrical resistivity in (Li0.5Pr0.5) co-substituted bismuth calcium tantalate (CaBi2Ta2O9) ceramics

Bismuth calcium tantalate (CaBi2Ta2O9) ferroelectric ceramic has not been studied adequately though it shows potential applications in high-temperature piezoelectric devices due to its ultrahigh Curie temperature (TC~929 °C). In this paper, (Li0.5Pr0.5) co-substitution at A site was proposed to increase piezoelectricity activity (d33) and electrical resistivity (ρ) of CaBi2Ta2O9-based ceramics and thereby a study of effect of the co-substitution on their structure and electrical properties was conducted. The composition Ca1-x(Li0.5Pr0.5)xBi2Ta2O9+δ for x= 0.04 shows the highest d33 of 15.1 pC/N which is about twice than that of CaBi2Ta2O9. At 700 °C, ρ of this composition reaches up to 1.5 MΩ cm and hence the dielectric loss (tanδ) is lower than 3%. These results combined with high TC (~925 °C) and stable d33 (up to 800 °C), suggest that it is a very promising material for high-temperature piezoelectric applications at 700 °C or higher.

Ultrahigh-temperature ferromagnetism in MoS2 Moiré superlattice/graphene hybrid heterostructures

Realizing high-temperature ferromagnetism in two-dimensional (2D) semiconductor nanosheets is significant for their applications in next-generation magnetic and electronic nanodevices. Herein, this goal could be achieved on a MoS2 Moire superlattice grown on the reduced graphene oxide (RGO) substrate by a hydrothermal approach. The as-synthesized bilayer MoS2 superlattice structure with rotating angle (ϕ = 13° ± 1°) of two hexagonal MoS2 lattices, possesses outstanding ferromagnetic property and an ultra-high Curie temperature of 990 K. The X-ray absorption near-edge structure and ultraviolet photoelectron spectroscopies combined with density functional theory calculation indicate that the covalent interactions between MoS2 Moire superlattice and RGO substrate lead to the formation of interfacial Mo-S-C bonds and complete spin polarization of Mo 4d electrons near the Fermi level. This design could be generalized and may open up a possibility for tailoring the magnetism of other 2D materials.

Effects of (Li0.5Sm0.5)/W co-substitution and sintering temperature on the structure and electrical properties of ultrahigh Curie temperature piezoceramics, Ca0.92(Li0.5Sm0.5)0.08Bi2Nb2-xWxO9

With co-substitution of (Li0.5Sm0.5) at A site and W at B site, the electrical properties of modified Ca0.92(Li0.5Sm0.5)0.08Bi2Nb2-xWxO9 [(CLS)BN-xW, x = 0, 0.015 and 0.03] piezoceramics with ultrahigh Curie temperature (TC) of > 930 °C were enhanced dramatically. The increased resistivity induced by the co-substitution ensure them to be polarized under an enough high field. Combined with the increase of spontaneous ferroelectric polarization (PS), the significant enhancements in the piezoelectric, dielectric and ferroelectric properties can be obtained in the composition x = 0.015. Furthermore, the piezoelectric activity (d33) and bulk resistivity (ρb) of (CLS)BN-0.015 W can be further enhanced at an appropriate sintering temperature. This optimum composition sintered at 1170 °C shows ultrahigh TC of ∼948 °C, d33 of ∼17.3 pC/N and ρb of ∼6.9 MΩ cm at 600 °C, which are comparable to those of the reported high-temperature Aurivillius piezoceramics with TC > 850 °C.

Ultra-low Young’s modulus and high super-exchange interactions in monolayer CrN: A promising candidate for flexible spintronic applications* *Project supported by the National Natural Science Foundation of China (Grant No. 61888102), the National Key Research and Development Program of China (Grant No. 2016YFA0202300), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB30000000).

Monolayer CrN has been predicted to be half-metallic ferromagnet with high Curie temperature. Due to bulk CrN’s biocompatibility, the monolayer is a promising candidate for bio-related devices. Here, using first-principles calculations based on density functional theory, we find that the formation energy of the bulk CrN stacking from layers with square lattice is only 68 meV/atom above the convex hull, suggesting a great potential to fabricate the monolayer CrN in a square lattice by using molecular beam epitaxy method. The monolayer CrN is then proved to be a soft material with an ultra-low Young’s modulus and can sustain very large strains. Moreover, the analysis of the projected density of states demonstrates that the ferromagnetic half-metallicity originates from the splitting of Cr-d orbitals in the CrN square crystal field, the bonding interaction between Cr–N, and that between Cr–Cr atoms. It is worth noting that the super-exchange interaction is much larger than the direct-exchange interaction and contributes to the ultra-high Curie temperature, which is obtained from Monte Carlo simulations based on Heisenberg model. Our findings suggest that the monolayer CrN can be an indispensable candidate for nanoscale flexible spintronic applications with good biocompatibility and is considerable appealing to be realized in experiment.

Significantly enhanced piezoelectric performance in Bi4Ti3O12-based high-temperature piezoceramics via oxygen vacancy defects tailoring

Bismuth titanate (Bi4Ti3O12, BIT) piezoelectric materials have attracted increasing attention due to their high-temperature applications. However, it is quite challenging to simultaneously achieve outstanding piezoelectric properties and high Curie temperature in BIT-based systems. In this study, oxygen vacancy defects tailoring strategy was utilized to solve this problem, excellent piezoelectric coefficient (32.1 pC/N), and ultrahigh Curie temperature (659 °C) are gotten in Bi4Ti3-x(Mn1/3Nb2/3)xO12 (BTMN) ceramics, which are among the top values in the BIT-based ceramics. More importantly, the (Mn1/3Nb2/3)(4+δ)+ complex-ion modified Bi4Ti3O12-based ceramics are characterized with excellent piezoelectric stability up to 500 °C (d33 > 30.0 pC/N at 500 °C)) and significantly reduced conductivity (only ∼ 10−7 Ω−1 cm−1 at 500 °C). Moreover, enhanced ferroelectricity and good dielectric stability were also obtained. The better comprehensive properties can be ascribed to two aspects. First, the concentration of oxygen vacancy defects is obviously reduced, and their distribution is effectively controlled in BITMN ceramics. Second, the introduction of (Mn1/3Nb2/3)(4+δ)+ complex-ion gives rise to the antiphase boundaries and massive ferroelectric domain walls. This works not only reveal the high potential of BITMN ceramics for high-temperature piezoelectric applications but also deepen the understanding of the structure-properties relationship in BIT-based materials.

Critical Control of Highly Stable Nonstoichiometric Mn-Zn Ferrites with Outstanding Magnetic and Electromagnetic Performance for Gigahertz High-Frequency Applications.

Pristine nonstoichiometric Mn-Zn ferrites were synthesized using a facile "heating-up" method. A route for achieving very stable single-crystal spinel Mn-Zn ferrites with enhanced magnetic performance and Curie temperature was explored using annealing procedures, where the protective gas flow velocities, heating rates, and annealing temperatures were critical in determining the phase structures and performance. The annealed Mn-Zn ferrites showed ultrahigh saturation magnetizations of ∼120 emu/g and an ultrahigh Curie temperature of ∼750 K. Mössbauer spectra indicated that the valence state of Mn was maintained at Mn2+, and both Mn and Zn were located at A sites in the inverse spinel structure for the highly stable Mn-Zn ferrites. An excellent microwave-absorbing capability in a broad frequency range of 0.1-18 GHz was realized owing to the large magnetic and dielectric losses. The optimal match thickness of Mn-Zn ferrites was found to be 1.5 mm, corresponding to a maximum reflection loss of 22 dB at 16 GHz. These results indicate that the synthesized Mn-Zn ferrites exhibit significant advantages in microwave absorption in the high-frequency range. The demonstrated multicomponent ferrites with high magnetic performance and Curie temperature may find broad applications in various complicated environments, such as those having elevated temperatures.

Switchable polar spirals in tricolor oxide superlattices

There are increasing evidences that ferroelectric states at the nanoscale can exhibit fascinating topological structures including polar vortices and skyrmions, akin to those observed in the ferromagnetic systems. Here we report the discovery of a new type of polar topological structure, an ordered array of nanoscale spirals, in the PbTiO3/BiFeO3/SrTiO3 tricolor ferroelectric superlattice system obtained via phase-field simulations. This polar spiral structure is composed of fine ordered semi-vortex arrays with vortex cores forming a wavy distribution. It is demonstrated that the tricolor system has an ultrahigh Curie temperature of ∼1000 K and a temperature of ∼800 K for the phase transformation from spiral structure to in-plane orthorhombic domain structure, demonstrating a great thermal stability. The periodicity phase-diagram is constructed, showing multiple phases from inplane domain, polar spiral, and polar vortex to flux-closure with increasing ferroelectric layer thickness. Moreover, the spiral structure has a net in-plane polarization that could be switched by an experimentally-feasible irrotational in-plane field. The switching process involves a metastable vortex state and is fully reversible. This discovery could open up a new routine to design novel multiferroic topological structures with enhanced stability and tunability towards potential future applications in next-generation electronics.

Effect of excess Bi on the structure and electrical properties of CaBi2Nb2O9 ultrahigh temperature piezoceramics

CaBi2Nb2O9 + x wt% Bi2O3 (x = 0, 1, 2, 3) ceramics with bismuth layer structure were prepared by solid state reaction route. The effect of excess bismuth on the crystal structure, microstructure and electrical properties of the ceramics was investigated. At all compositions, orthorhombic symmetric bismuth layer structure was formed without secondary phases. Excess bismuth caused more uniform grains. The ceramic with excess Bi2O3 of 1 wt% exhibited optimal electrical properties as follows: piezoelectric constant d33 = 6.4 pC/N, planar electromechanical coupling coefficient kp = 11.0%, dielectric constant $$\varepsilon _{{33}}^{T}/{\varepsilon _0}$$ = 93.5, dielectric loss tanδ = 0.24%, and resistivity ρ = 2.9 × 106 Ω cm (@ 500 °C). This ceramic also possess ultrahigh Curie temperature Tc = 940 °C, indicating its potential application for high temperature sensors and actuators.

Upconversion photoluminescence properties of Er3+ doped CaBi2Nb2O9 phosphors for temperature sensing

The CaBi2Nb2O9 materials have ultrahigh Curie temperature (T c ∼ 1216 K). Therefore, such compound as the host would make their luminescent materials have wide temperature range for application. In this work, the Er3+ doped CaBi2Nb2O9 phosphors were synthesized to investigate upconverion emission properties and their optical sensing behavior. The analyses based on photoluminescence and decay time upon the 980 nm excitation demonstrated that the phosphors had an intense green emission by two-photo process. Using the fluorescence intensity ratio technique, temperature sensing performances based on the green emission were evaluated in the wide-range 90–570 K, which fitted well with the theoretical equation. These results indicate that Er doped CaBi2Nb2O9 could be promising temperature sensor for wide range and high temperature application.

Significantly enhanced piezoelectricity in low-temperature sintered Aurivillius-type ceramics with ultrahigh Curie temperature of 800 °C

We report an Aurivillius-type piezoelectric ceramic (Ca1−2x (LiCe)xBi4Ti3.99Zn0.01O15) that has an ultrahigh Curie temperature (Tc) around 800 °C and a significantly enhanced piezoelectric coefficient (d33), comparable to that of textured ceramics fabricated using the complicated templating method. Surprisingly, the highest d33 of 26 pC/N was achieved at an unexpectedly low sintering temperature (Ts) of only 920 °C (~200 °C lower than usual) despite the non-ideal density. Study of different synthesized samples indicates that a relatively low Ts is crucial for suppressing Bi evaporation and abnormal grain growth, which are indispensable for high resistivity and effective poling due to decreased carrier density and restricted anisotropic conduction. Because the layered structure is sensitive to lattice defects, controlled Bi loss is considered to be crucial for maintaining structural order and spontaneous polarization. This low-Ts system is very promising for practical applications due to its high piezoelectricity, low cost and high reproducibility. Contrary to our usual understanding, the results reveal that a delicate balance of density, Bi loss and grain morphology achieved by adjusting the sintering temperature is crucial for the enhancing performance in Aurivillius-type high-Tc ceramics.