Toughening Ceramics down to Cryogenic Temperatures by Reentrant Strain-Glass Transition.

Ceramics, often exhibiting important functional properties like piezoelectricity, superconductivity, and magnetism, are usually mechanically brittle at room temperature and even more brittle at low temperature due to their ionic or covalent bonding nature. The brittleness in their working temperature range (mostly from room down to cryogenic temperatures) has been a limiting factor for the usefulness of these ceramics. In this Letter, we report a surprising "low-temperature toughening" phenomenon in a La-doped CaTiO_{3} perovskite ceramic, where a 2.5× increase of fracture toughness K_{IC} from 1.9 to 4.8 MPa m^{1/2} occurs when cooling from above room temperature (323 K) down to a cryogenic temperature of 123 K, the lowest temperature our experiment can reach. In situ microscopic observations in combination with macroscopic characterizations show that this desired but counterintuitive phenomenon stems from a reentrant strain-glass transition, during which nanosized orthorhombic ferroelastic domains gradually emerge from the existing tetragonal ferroelastic matrix. The temperature stability of this unique microstructure and its stress-induced transition into the macroscopic orthorhombic phase provide a low-temperature toughening mechanism over a wide temperature range and explain the observed phenomenon. Our finding may open a way to design tough ceramics with a wide temperature range and shed light on the nature of reentrant transitions in other ferroic systems.

Direct Observation of Evolution from Amorphous Phase to Strain Glass.

The amorphous phase and strain glass are both disordered states of solids. The amorphous phase is an atomic packing disordered phase, while strain glass is a glassy state with transformation strain disorder in a crystalline matrix, which both bring extraordinary properties to alloys. Previous studies have mostly focused on the properties and structure of single glass; however, the link between them has seldom been considered. In this work, the specimen of the almost amorphous state was obtained from the heavy-defects-doping Fe67.8Pd32.2 strain glass ingot by arc melting and 90% cold rolling, which were characterized by amorphous packages in X-ray diffraction and amorphous rings in transmission electron microscope diffraction. The evolution from the amorphous phase (metallic glass) back to strain glass was directly observed by an in situ high-resolution transmission electron microscope, which revealed that strain nanodomains began to form on the amorphous matrix below the crystallization temperature of the amorphous phase. Here, direct observation of the evolution process provides a theoretical basis for achieving precise control of crystallinity to obtain the desired microstructure, while the study of the unusual crystallization process offers a possible way to tailor the mechanical and functional properties through tuning the amorphous and strain glass coexistence. This work presents the specific pathway and realization possibilities for the design of glass composite materials with enhanced properties.

Enhanced Piezoelectric Properties in a Single-Phase Region of Sm-Modified Lead-Free (Ba,Ca)(Zr,Ti)O3 Ceramics.

In ferroelectric materials, phase boundaries such as the morphotropic phase boundary (MPB) and polymorphic phase boundary (PPB) have been widely utilized to enhance the piezoelectric properties. However, for a single-ferroelectric-phase system, there are few effective paradigms to achieve the enhancement of piezoelectric properties. Herein, we report an unexpected finding that largely enhanced piezoelectric properties occur in a single-tetragonal-ferroelectric-phase region in the Sm-modified (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT-xSm) system. An electrostrain maximum (0.13%) appears in the single-phase region of the BZCT-0.5Sm composition with the maximum polarization (Pm = 18.37 µC/cm2) and piezoelectric coefficient (d33 = 396 pC/N) and the minimum coercive field (EC = 3.30 kV/cm) at room temperature. Such an enhanced piezoelectric effect is due to the synergistic effect of large lattice distortion and domain miniaturization on the basis of the transmission electron microscope (TEM) observation and X-ray diffraction (XRD) Rietveld refinement. Our work may provide new insights into the design of high-performance ferroelectrics in the single-phase region.

Trirelaxor Ferroelectric Material with Giant Dielectric Permittivity over a Wide Temperature Range.

Advanced ferroelectrics with a combination of large dielectric response and good temperature stability are crucial for many technologically important electronic devices and electrical storage/power equipment. However, the two key factors usually do not go hand in hand, and achieving high permittivity is normally at the expense of sacrificing temperature stability. This trade-off relation is eased but not fundamentally remedied using relaxor-type materials which are known to have a diffuse permittivity peak at their relaxor transition temperatures. Here, we report an anomalous trirelaxor phenomenon in a barium titanate system and show that it can lead to a giant dielectric permittivity (εr ≈ 18 000) over a wide temperature range (Tspan ≈ 34K), which successfully overcomes a long-standing permittivity-stability trade-off. Moreover, the enhancement in the dielectric properties also yields a desired temperature-insensitive electrocaloric performance for the trirelaxor ferroelectrics. Microstructure characterization and phase-field simulations reveal a mixture of tetragonal, orthorhombic, and rhombohedral polar nanoregions over a broad temperature window in trirelaxor ferroelectrics, which is responsible for this combination of giant dielectric permittivity and good temperature stability. This finding provides an effective approach in designing advanced ferroelectrics with high performance and thermal stability.

Electric field control of magnetism through modulating phase separation in (011)-Nd0.5Sr0.5MnO3/PMN-PT heterostructures.

Large and non-volatile electric field control of magnetization is promising to develop memory devices with reduced energy consumption. Herein, we report the electric field control of magnetization with a non-volatile memory effect in an intermediate band Nd0.5Sr0.5MnO3 film grown on a (011)-cut 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystal. Applying an electric field across the ferroelectric PMN-PT increases the magnetization of the Nd0.5Sr0.5MnO3 film along both in-plane [100] and [011[combining macron]] directions. Moreover, the magnetization does not recover to its original state after withdrawal of the electric field at temperatures below 70 K, demonstrating a non-volatile memory effect. Detailed investigation showed that (011)-PMN-PT exhibits an anisotropic in-plane strain due to an electric field-induced rhombohedral to orthorhombic phase transition. This electric field-induced anisotropic strain can dynamically transfer to Nd0.5Sr0.5MnO3 film and modulate the magnetization of the Nd0.5Sr0.5MnO3 film through adjusting its phase balance between ferromagnetic (FM) and charge-orbital ordered antiferromagnetic (COO AFM) phases. The non-volatile memory effect can be ascribed to the competition of thermal energy and energy barriers between the FM and COO AFM phases at low temperatures. This work broadens the knowledge of electric field control of magnetism in the intermediate band-manganite ferromagnetic/ferroelectric multiferroic heterostructures, and may also pave a way for the control of antiferromagnetism and to design antiferromagnet-based memories.

General Synthesis of Amorphous PdM (M = Cu, Fe, Co, Ni) Alloy Nanowires for Boosting HCOOH Dehydrogenation.

Noble metal-based nanomaterials with amorphous structures are promising candidates for developing efficient electrocatalysts. However, their synthesis remains a significant challenge, especially under mild conditions. In this paper, we report a general strategy for preparing amorphous PdM nanowires (a-PdM NWs, M = Fe, Co, Ni, and Cu) at low temperatures by exploiting glassy non-noble metal (M) nuclei generated by special ligand adsorption as the amorphization dictator. When evaluated as electrocatalysts toward formic acid oxidation, a-PdCu NWs can deliver the mass and specific activities as high as 2.93 A/mgPd and 5.33 mA/cm2, respectively; these are the highest values for PdCu-based catalysts reported thus far, far surpassing the crystalline-dominant counterparts and commercial Pd/C. Theoretical calculations suggest that the outstanding catalytic performance of a-PdCu NWs arises from the amorphization-induced high surface reactivity, which can efficiently activate the chemically stable C-H bond and thereby significantly facilitate the dissociation of HCOOH.

Highly thermal-stable ferromagnetism by a natural composite.

All ferromagnetic materials show deterioration of magnetism-related properties such as magnetization and magnetostriction with increasing temperature, as the result of gradual loss of magnetic order with approaching Curie temperature TC. However, technologically, it is highly desired to find a magnetic material that can resist such magnetism deterioration and maintain stable magnetism up to its TC, but this seems against the conventional wisdom about ferromagnetism. Here we show that a Fe-Ga alloy exhibits highly thermal-stable magnetization up to the vicinity of its TC, 880 K. Also, the magnetostriction shows nearly no deterioration over a very wide temperature range. Such unusual behaviour stems from dual-magnetic-phase nature of this alloy, in which a gradual structural-magnetic transformation occurs between two magnetic phases so that the magnetism deterioration is compensated by the growth of the ferromagnetic phase with larger magnetization. Our finding may help to develop highly thermal-stable ferromagnetic and magnetostrictive materials.