Shuai Ren

Large piezoelectricity in Pb-free 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3−0.04BaZrO3 ceramic: A perspective from microstructure

We employ transmission electron microscopy to explore the reason for large piezoelectricity (d33≈400pC/N) in a Pb-free 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3 −0.04BaZrO3 ceramic from microstructure. The result shows that the high piezoelectricity corresponds to a miniaturized nanodomain configuration in a domain hierarchy. The nanodomains disappear on heating accompanied by a reduction in d33 value. Further convergent beam electron diffraction study reveals a coexistence of tetragonal and orthorhombic phase, which indicates that large piezoelectricity of KNLNS0.07-BZ may stem from easy polarization rotation due to low polarization anisotropy on the tetragonal-orthorhombic phase boundary.

Phase transition sequence in Pb-free 0.96(K0.5Na0.5)0.95Li0.05Nb0.93 Sb0.07O3−0.04BaZrO3 ceramic with large piezoelectric response

The piezoceramic 0.96(K0.5Na0.5)0.95Li0.05Nb0.93Sb0.07O3−0.04BaZrO3 (KNLNS0.07-BZ), which shows large piezoelectric response (d33 ≈ 425 pC/N), has been considered as one of the promising Pb-free substitutions for Pb(Zr,Ti)O3. In this paper, we investigate the phase transition sequence for KNLNS0.07-BZ by employing the dielectric measurement, mechanical spectroscopy, as well as Raman spectroscopy. Two ferroelectric-ferroelectric transitions have been detected by inspecting anomalies in the spectra, indicating the existence of three ferroelectric phases. Moreover, in-situ X-ray diffraction study has been further performed on KNLNS0.07-BZ to identify the crystal structure for each phase. The result reveals that the phase sequence for KNLNS0.07-BZ evolves from tetragonal (T) to rhombohedral (R) via an intermediate orthorhombic (O) phase. And the piezoelectric-optimal region for KNLNS0.07-BZ locates on a T-O boundary rather than the previously reported T-R boundary. Strong piezoelectricity may stem from the ea...

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.

Ferroelastic and strain glass transition in (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 solid solution

The electrical and structure behaviors are somehow deviated in (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (BNT-xBT), which is unusual for regular ferroelectrics whose structure transition is always accompanied with the ferroelectric one. To understand this problem, a dynamic ferroelastic measurement was used as a new way (besides ferroelectric and structure views) to determine its transition. It is found that for x < 5%, they showed a typical ferroelastic phase transition, whereas it was a strain glass transition for 5% < x < 8% (previously regarded as morphotropic phase boundary region). The new ferroelastic characterization provides a unique aspect to understand complex ferroelectric materials, such as BNT-xBT.

Suppression of martensitic transformation in Fe50Mn23Ga27 by local symmetry breaking

Defects-induced local symmetry breaking has led to unusual properties in nonferromagnetic ferroelastic materials upon suppressing their martensitic transformation. Thus, it is of interest to discover additional properties by local symmetry breaking in one important class of the ferroelastic materials, i.e., the ferromagnetic shape memory alloys. In this letter, it is found that local symmetry breaking including both tetragonal nano-inclusions and anti-phase boundaries (APBs), suppresses martensitic transformation of a body-centered-cubic Fe50Mn23Ga27 alloy, however, does not affect the magnetic ordering. Large electrical resistivity is retained to the low temperature ferromagnetic state, behaving like a half-metal ferromagnet. Lower ordering degree at APBs and local stress fields generated by the lattice expansion of tetragonal nanoparticles hinder the formation of long-range-ordered martensites. The half-metal-like conducting behavior upon suppressing martensitic transformation extends the regime of ferr...

Evidence for crossover martensite in Ti50Ni45Fe5: An intermediate state between normal martensite and strain glass

We report that the crossover from normal martensite to strain glass in the TiNi-Fe alloy system does not proceed directly as indicated by previous studies but is actually mediated by an intermediate state named crossover martensite, appearing in the crossover composition of Ti50Ni45Fe5. The martensitic transition from the parent phase to the crossover martensite is a thermodynamic phase transition, being similar to a normal martensitic transition. However, its produced crossover martensite shows an obvious kinetic feature such as a time-dependent behavior in internal friction (aging effect), resembling that of the strain glass. The microstructure of the crossover martensite exhibits a special morphology of coexisting macro-sized martensite plates and nano-sized martensitic domains. The displaying of both thermodynamic and kinetic features in the crossover martensite demonstrates that it is an intermediate state between normal martensite and strain glass; this finding provides a complete physical picture for the crossover phenomenon from normal martensite to strain glass.

Evolution from successive phase transitions to "morphotropic phase boundary" in BaTiO3-based ferroelectrics

Obtaining superior physical properties for ferroic materials by manipulating the phase transitions is a key concern in solid state physics. Here, we investigated the dielectric permittivity, piezoelectric coefficient d33, storage modulus, and crystal symmetry of (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba1−yCay)TiO3 (BZT-xBCyT) systems to demonstrate the gradual evolution process from successive phase transitions in BaTiO3 to the morphotropic phase boundary (MPB) regime in BZT-xBC0.3T. Furthermore, we analysed with a Landau-type theoretical model to show that the high field-sensitive response (dielectric permittivity) originates from a small polarization anisotropy and low energy barrier at the quadruple point. Together, the intermediate orthorhombic phase regime and the tetragonal-orthorhombic and orthorhombic-rhombohedral phase boundaries constitute the MPB. Our work not only reconciles the arguments regarding whether the structural state around the MPB corresponds to a single-phase regime or a multiple-phase-coexistence regime but also suggests an effective method to design high-performance functional ferroic materials by tailoring the successive phase transitions.Obtaining superior physical properties for ferroic materials by manipulating the phase transitions is a key concern in solid state physics. Here, we investigated the dielectric permittivity, piezoelectric coefficient d33, storage modulus, and crystal symmetry of (1-x)Ba(Ti0.8Zr0.2)O3-x(Ba1−yCay)TiO3 (BZT-xBCyT) systems to demonstrate the gradual evolution process from successive phase transitions in BaTiO3 to the morphotropic phase boundary (MPB) regime in BZT-xBC0.3T. Furthermore, we analysed with a Landau-type theoretical model to show that the high field-sensitive response (dielectric permittivity) originates from a small polarization anisotropy and low energy barrier at the quadruple point. Together, the intermediate orthorhombic phase regime and the tetragonal-orthorhombic and orthorhombic-rhombohedral phase boundaries constitute the MPB. Our work not only reconciles the arguments regarding whether the structural state around the MPB corresponds to a single-phase regime or a multiple-phase-coexistenc...

Temperature invariable magnetization in Co-Al-Fe alloys by a martensitic transformation

Recently, it was reported that in a dual-phase ferromagnetic Fe-Ga alloy, highly thermally stable magnetization is achieved by a diffusional transformation between the two phases with different magnetizations, which compensates for the inevitable magnetization reduction. Contrasting with the compensation mechanism through a diffusional transformation, here we report that a diffusionless martensitic transition can also be used as a compensation mechanism for achieving nearly temperature invariable magnetization. We found that ferromagnetic Co-Al-Fe alloys with a non-thermoelastic martensitic transformation exhibit almost unchanged magnetization up to 830 K (about 81% TC). In-situ heating transmission electron microscopy reveals that the gradual transformation from a hexagonal close-packed (hcp) martensite phase with lower magnetization into a face-centered-cubic (fcc) parent phase with higher magnetization occurs over a wide temperature range, giving rise to such temperature invariable magnetization. The electrical resistivity measurement reveals that the fcc → hcp transformation can be repeated during heating-cooling cycles, which can be used to design temperature invariable magnets with a long cycling life.Recently, it was reported that in a dual-phase ferromagnetic Fe-Ga alloy, highly thermally stable magnetization is achieved by a diffusional transformation between the two phases with different magnetizations, which compensates for the inevitable magnetization reduction. Contrasting with the compensation mechanism through a diffusional transformation, here we report that a diffusionless martensitic transition can also be used as a compensation mechanism for achieving nearly temperature invariable magnetization. We found that ferromagnetic Co-Al-Fe alloys with a non-thermoelastic martensitic transformation exhibit almost unchanged magnetization up to 830 K (about 81% TC). In-situ heating transmission electron microscopy reveals that the gradual transformation from a hexagonal close-packed (hcp) martensite phase with lower magnetization into a face-centered-cubic (fcc) parent phase with higher magnetization occurs over a wide temperature range, giving rise to such temperature invariable magnetization. The e...