Large Lattice Distortion Research Articles

In situ neutron diffraction study of a new type of stress-induced confined martensitic transformation in Fe22Co20Ni19Cr20Mn12Al7 high-entropy alloy

We successfully prepared an Fe22Co20Ni19Cr20Mn12Al7 alloy consisting of a face-center cubic (fcc) phase and body-center cubic (bcc) phase that exhibits an outstanding combination of true strength of 1430 MPa and ductility of 19.9% at room temperature (RT). The micromechanical behavior at RT and 77 K for the studied alloy during tensile deformation was investigated using in situ time-of-flight (TOF) neutron diffraction in combination with synchrotron-based high-energy X-ray diffraction (HE-XRD). Here, the striking finding of a large elastic strain of 7.0% and 5.6% is reported for the {200} bcc crystal plane, which was achieved at RT and 77 K, respectively. Such a large lattice distortion observed in the bcc phase was attributed to a new type of stress-induced confined martensitic transformation. We attributed the physical origin of this specific martensitic transformation to the intrinsic microstructural feature of a nano-scale continuous distribution of an ordered-to-disordered crystal structure within the bcc phase, i.e., the disordered A2-structure was distributed continuously in the ordered B2-structure matrix. The stress-induced martensitic transformation from the metastable nano-sized disordered A2-phase was confined by the stable B2-ordered matrix. The new findings in this study provide additional understanding of the deformation mechanisms of high-entropy alloys and insights into alloy design for further enhancement of the mechanical properties of high-performance structural materials used at cryogenic temperatures.

Oxidation behaviours of high-entropy transition metal carbides in 1200 °C water vapor

High-entropy (Zr,Ti,Hf,Nb,Ta)C ceramic with single-phase rock salt structure was prepared by spark plasma sintering and their oxidation behavior was investigated at 1200 °C water vapor atmosphere. Results indicate that high-entropy carbides exhibit high oxidation resistance with a small mass gain per unit area of 6.6 × 10−2 kg/m2 after oxidation for 5 h. The parabolic rate constant kp of (Zr,Ti,Hf,Nb,Ta)C ceramic is almost two order of magnitude lower than that of ZrC ceramic. The high oxidation resistance is generally ascribed to the large lattice distortion and the formation of dense oxide scale which significantly suppressed the metal elements and H2O diffusion.

Magnetic, dielectric and optical properties of five-layered Aurivillius phase Bi6Fe2Ti3O18-based ceramics

The structural, magnetic, dielectric and optical properties of Aurivillius Bi6Fe2Ti3O18-based ceramics were investigated in detail. The replacement of Co for Fe/Ti ions obviously varies the grain morphology. Compared with Bi6Fe2Ti3O18 (BFTO) with antiferromagnetic ordering, a spin glass state can be observed in Bi5.25La0.75Fe2Ti3O18 ceramic, while other samples exhibit the ferromagnetic behavior. The specimen Bi6Fe2Ti2(NbCo)0.5O18 (BFTNCO) represents the largest remanent magnetization Mr of 0.93emu/g among all the samples, which can be attributed to the combination of a short lattice parameter c and a large lattice distortion as well as more magnetic ions in a unit cell. The room-temperature dc conductivity of BFTNCO is one order magnitude lower than that of BFTO. In addition, the band gaps of Co-doped samples are about 0.2eV smaller than these of Co-free samples. This work provides a promising path forward to tailor the multiferroic and optical properties in five-layered Aurivillius compounds.

A coupled microstructural-structural mechanism governing thermal depolarization delay in Na0.5Bi0.5TiO3-based piezoelectrics

Driven by environmental concerns and governmental directives, a sustained research effort in the last decade and half has led to the development of lead-free alternatives which can potentially replace the commercial lead-based piezoceramics in niche applications. Na0.5Bi0.5TiO3 (NBT)-based lead-free piezoceramics have found acceptance as promising lead-free transducers in high power ultrasonic devices. An issue of concern however is the low depolarization temperature which limits the device's tolerance for temperature rise during operation. While several strategies have been reported to improve thermal depolarization in NBT-based piezoceramics, there is a lack of consensus regarding the most fundamental factor/mechanism which enhances the depolarization temperature. In this paper we unravel a coupled microstructural-structural mechanism which controls the thermal depolarization in NBT-based piezoceramics. First, we demonstrate the phenomenon of a considerable increase in the depolarization temperature, without significantly losing the piezoelectric property in unmodified NBT by increasing the grain size. We then establish a grain size controlled structural mechanism and demonstrate that the rise in depolarization temperature is primarily associated with the bigger grains allowing relatively large lattice distortion to develop in the poling stabilized long range ferroelectric phase. We reconfirmed the validity of this mechanism in the model morphotropic phase boundary (MPB) composition 0.94Na0.5Bi0.5TiO3-0.06BaTiO3. For the sake of generalization, we demonstrate that the same mechanism is operative in another lead-based relaxor-ferroelectric system 0.62PbTiO3-0.38Bi(Ni0.5Hf0.5)O3. Our study provides the fundamental structural basis for understanding thermal depolarization delay in relaxor ferroelectric based piezoceramics.

Effects of ion irradiation on chromium coatings of various thicknesses on a zirconium alloy

Recently, the development of accident tolerant fuel is a critical issue in light water reactors. Cr coatings display a promising application in protecting Zr alloys from oxidation and maintaining the fuel performance. In this work, an attempt has been made to systematically investigate the dose-dependent and thickness-related irradiation responses of Cr coatings on the Zr alloy. Three sets of Cr coatings on a Zr alloy with thickness of 5, 10 and 12 μm have been irradiated at 400 °C with 6 MeV Au ions to average doses of 10, 25 and 50 dpa. Pre- and post-irradiation microstructures were characterized with transmission electron microscopy and radiation hardening was evaluated by nano-indentation. An obvious thickness dependent columnar grain size is observed before radiation with smaller grains in thinner coatings. Although the sizes of dislocation loops increase with irradiation dose in all samples, the evolution of irradiation defects is delayed in thinner coatings that can be attributed to the smaller columnar grains and larger lattice distortion.

Modification of Electrochemical Behavior of Li-Rich Layered Oxide Cathode through Defect Consideration

Cathode materials formulated as xLi2MnO3-(1-x)LiMO2(M = Mn, Ni, Co, etc.) with layered structure received much attention due to their surprisingly high reversible capacity for Li-ion battery applications.[1,2] For better understanding of such high-capacity layered cathode, a systematic study on powder morphology, crystal structures and reaction mechanisms of Li1.2Mn0.54Ni0.13Co0.13O2 during electrochemical charge/discharge cycles were conducted. First, the capacity of Li-rich layered cathode powder was strongly affected by its particle size. To obtain various particle sizes of cathode powder, different powder synthesis routes have been adopted. By usng sol-gel process, particle size of Li1.2Mn0.54Ni0.13Co0.13O2 powder obtained is around 200 nm. On the other hand, Li1.2Mn0.54Ni0.13Co0.13O2 powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place. Although Li-rich layer-structured cathode material show a reversible capacity as high as 250 mAh/g at low rate, its electrochemical properties such as capacity loss at first cycle, rate capability and capacity fading still need to be examined and characterized. In this study, Li1.2Mn0.54Ni0.13Co0.13O2 were investigated. The TEM structural analysis shows that the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.

Mechanism of electrostrain enhancement in the single rhombohedral phase region of Ba(Ti1 –xZrx)O3 ceramics

A large electrostrain usually occurs at ferroelectric-ferroelectric phase boundaries. However, in the Ba(Ti1 –xZrx)O3 (BT-xZr) system prepared by the conventional solid-state reaction method, we discovered a local electrostrain maximum in the single rhombohedral phase region at the quasi-quadruple point with a composition of BT-0.12Zr that had a relatively weak temperature sensitivity below 50 °C. Mechanisms of the local electrostrain maximum in the single-phase region of the BT-xZr system were studied. The large coercive field and large domain size of BT-0.12Zr indicate that domain switching was not facilitated. Nevertheless, the large lattice distortion of BT-0.12Zr suggests a large intrinsic lattice strain, which predominantly contributed to the observed macroscopic electrostrain enhancement at the quasi-quadruple point composition in the single-phase region. Compared to nearby sample compositions, the largest lattice distortion of BT-0.12Zr was found to be correlated with the highest electrostrictive coefficient at the tricritical quasi-quadruple point, as deduced from phenomenological Landau analysis. Our work may shed light on the development of weakly temperature-dependent large electrostrain materials by identifying the tricritical quasi-quadruple point in ferroelectric phase diagrams.

Investigation of Capacity Fading on Li-Rich Layered Composite Cathode Materials for Li Ion Batteries

the capacity of Li-rich layered cathode powder was strongly affected by its particle size. By using sol-gel process, particle size of Li1.2Mn0.54Ni0.13Co0.13O2 powder obtained is around 200 nm. On the other hand, Li1.2Mn0.54Ni0.13Co0.13O2 powder synthesized by solid-state reaction method gives particle size as coarse as 1.0μm. The initial discharge capacity of sol-gel processed Li-rich cathode was 250 mAh/g. After 40 cycles, the discharge capacity from the same cathode still shows a capacity as high as 205 mAh/g. However, for cathode powder from solid-state reaction, its initial capacity was 190 mAh/g that is 24% lower than that of sol-gel processed cathode. After 40 cycles, the capacity degraded to 137 mAh/g. The enhanced capacity of sol-gel processed Li-rich cathode is attributed to its enormous surface area and short Li diffusion distance provided for electrochemical reaction to take place. Furthermore, the TEM diffraction analysis of cycled Li[Li0.2Mn0.54Ni0.13Co0.13]O2 shows the evidence of spinel phase after cycling. It is believed that the transition from layered structure to spinel structure may also induce a large lattice distortion resulting in lattice breakdown and capacity fading. In additions, the phase transition may be caused by the redox reaction of transition metal ions through charging/discharging tests.

Low-temperature sintered (Na1/2Bi1/2)TiO3-based incipient piezoceramics for co-fired multilayer actuator application

Low-temperature sintered (Na1/2Bi1/2)0.935Ba0.065Ti0.975(Fe1/2Nb1/2)0.025O3 (NBT-BT-0.025FN) lead-free incipient piezoceramics were investigated using high-purity Li2CO3 as sintering aids. With the ≤0.5 wt% Li2CO3 addition, the introduced Li+ cations precede to enter the A-sites of the perovskite lattice to compensate for the A-site deficiencies. Once the addition exceeds 0.5 wt%, the excess Li+ cations will occupy B-sites and give rise to the generation of oxygen vacancies, which accelerate the mass transport and thus lower the sintering temperature effectively from 1100 °C down to 925 °C. It was also found that a small amount of Li+ addition has little effect on the phase structure and electromechanical properties of the system, but overweight seriously disturbs these characteristics because of the large lattice distortion. The sintered NBT-BT-0.025FN incipient piezoceramics with 1.25 wt% Li2CO3 addition at 925 °C provides a large strain of 0.33% and a corresponding large signal piezoelectric coefficient d*33 of 550 pm/V at 60 kV/cm, indicating this system is a very promising candidate for lead-free co-fired multilayer actuator application.

Large reversible magnetostriction and improved mechanical properties in epoxy-reinforced MnCoSi1-xGex cast ingots

Textured MnCoSi-based bulks, prepared by high-magnetic-field-solidification technique, are considered to be promising magnetostrictive materials for the large anisotropic lattice distortions during the metamagnetic transition. Here, we demonstrate that these magnetostrictive bulks can be obtained easily by vacuum-impregnating and solidifying a small amount of epoxy in the cracks of the naturally textured cast ingot. Large reversible magnetostriction effects and improved mechanical properties were observed in the epoxy-reinforced MnCo(Si,Ge) cast ingots under relatively low field at room temperature. Compared with the high-magnetic-field-solidification, epoxy-reinforcing has the features of low equipment requirement, low energy/time consumption, and high feasibility.

Investigation of Gilbert damping of a tetragonally distorted ultrathin Fe0.5Co0.5 epitaxial film with high magnetic anisotropy

We have investigated the Gilbert damping, α, of a tetragonally distorted, perpendicular magnetic anisotropic (PMA) ultrathin Fe0.5Co0.5 film grown on a Rh-buffered MgO(100) substrate fabricated by magnetron sputtering at room temperature by means of the time-resolved magneto-optical Kerr effect. We obtained the highest PMA value of 0.573 MJ/m3 ever reported for the Fe0.5Co0.5/Rh film. The PMA strongly depends on the lattice distortion which originates from the epitaxial growth in the large lattice misfit system of Fe0.5Co0.5 and Rh. We have estimated an unusually high value of α = 0.041 ± 0.002 for a 1 nm thick Fe0.5Co0.5 film. Based on the microstructural observation and the first-principles calculation, we conclude that the large α in the ultrathin Fe0.5Co0.5 film comes from the minority-spin electron transition around the Fermi level mediated by the spin-orbit interaction, which is caused by the large lattice distortion.

Morphology-Controlled Discharge Profile and Reversible Cu Extrusion and Dissolution in Biomimetic CuS.

Metal sulfide materials such as CuS, SnS2, Co9S8, and MoS2 are high-capacity anode materials for Li-ion batteries with high capacity. However, these materials go through a conversion reaction with Li+, which is accompanied by inevitably huge volume expansions, thereby causing performance degradation. Here, we report ananoscale engineering route to efficiently control the overall volume expansion for enhanced performance. We engineered CuS with nanoplate assembly on a nanostring, leading to a nanostructure mimicking the crassula baby necklace (CBN) in the natural plant. Using in situ transmission electron microscopy, we probed the lithiation kinetics and dynamic structural transformations. Due to the linkage of the central nanostring, the CuS CBN exhibited a fast Li+ diffusion along the axial direction and high mechanical stability during lithiation. The volume expansion was minimal for our CuS CBN due to the pre-engineered gap and pores between these plates. The CuS followed a two-step lithiation process, with Cu2S and Li2S formation as the first step and Cu extrusion in the later stage. Interestingly, during the Cu2S-to-Cu conversion, we observed an incubation period before the metallic Cu extrusion, which is featured by the formation of an amorphous structure due to the large lattice strain and distortion associated with the displacement of Cu by Li ions. In the final stage, the lithiated amorphous phase recrystallized to a composite of Cu nanocrystals in a polycrystalline Li2S matrix. Associated with the nanoscale size, the Cu nanocrystals can reversibly dissolve into the matrix upon delithiation. The present work demonstrates tailoring of desired functionality in electrodes using bionic engineering methods.

Effect of Lanthanide on Bi3.25Ln0.75Ti3O12 (Ln=La, Ce, Nd, Sm) Nanofibers for Humidity Sensing Properties

Bi3.25Ln0.75Ti3O12 (Ln= La, Ce, Nd, Sm) (BLnT) nanofibers are prepared by the electrospinning method and characterized by X-ray diffraction, field-emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The humidity sensors based on BLnT nanofibers are fabricated and their humidity sensing properties are measured at room temperature within the relative humidity (RH) range of 11-95%. The optimum working frequencies of all BLnT humidity sensors are 10 Hz, and the impedance variation of Bi3.25Sm0.75Ti3O12 (BST) humidity sensor is about six orders of magnitude within the whole humidity range from 11% to 95% RH, which is larger than it of other BLnT humidity sensors. The response time of BST humidity sensor is about 3 s and the maximum humidity hysteresis is 3% RH. The results indicate that the BST humidity sensor is of the best humidity sensitivity compared to other BLnT humidity sensors due to the low Ln-O binding energy and large lattice distortion. We expect that our work may offer the guidelines for the investigation of the relationship between binding energy and humidity sensing properties.

Enhanced multiferroicity and narrow band gap in B-site Co-doped Aurivillius Bi5FeTi3O15

The effects of Co ions substitution on the structural, multiferroic, dielectric, and optical properties of Aurivillius Bi5FeTi3O15 are investigated. High resolution transmission electron microscope measurements imply that Bi5FeTi2Nb0.5Co0.5O15 (BFTNCO) exhibits a typical four-layered Aurivillius structure. The co-doping of Nb/Co ions at Ti sites is found to induce the lattice distortion. In comparison with Bi5FeTi3O15 and Bi5Fe0.5Co0.5Ti3O15, the sample BFTNCO shows a more superior multiferroicity with the remanent magnetization Mr and polarization Pr of 76.4 memu/g and 10.2 μC/cm2, respectively. The improved multiferroicity can be ascribed to reduced oxygen vacancy, large lattice distortion and large grain size. In addition, a relatively narrow optical band gap Eg can be obtained as 2.30 eV for BFTNCO. The decreased Eg in BFTNCO can be attributed to large lattice distortion and the overlap of Co 3d state with Ti t2g state. These results reveal that Ti-site doped Aurivillius phase may be the potential candidates for exploring excellent room-temperature multiferroics with narrow band gap.

A novel method to enhance CSL fraction, tensile properties and work hardening in complex concentrated alloys ― Lattice distortion effect

A framework has been developed to reduce the stacking fault energy of complex concentrated alloys (CCAs) or high entropy alloys based on increasing lattice distortion by choosing principal elements with large differences in moduli and atomic size. The framework and selection criteria have resulted in the inclusion of five CCA compositions for validation of the lattice distortion and three Fe-containing CCAs. Orientation imaging microscopy (OIM) was used to examine the fraction of special boundaries in all of the CCAs. Alloys with large lattice distortion contained high density of annealing twins. Both tensile properties and work hardening rate characteristics were evaluated. CCAs that were designed based on lattice distortion demonstrated improved ultimate tensile strength. Work hardening rate curves revealed the underlying variation in the tensile properties of various CCAs. For selected alloys, post-deformation OIM analysis of the lateral surface was carried out for establishing defect density variation.

Study of the properties of SBT ceramics doped with different concentrations of Ca

This study investigated how Ca doping affects the electrical properties of Sr2Bi4Ti5O18 (SBT) ceramics. Among the samples tested, a sample with a substitution level of 0.15 showed the highest a-axis orientation and the largest lattice distortion. Through XPS analysis, it was found that the formation of oxygen vacancies is suppressed by Ca substitution in SBT. A large remnant polarization (2Pr) of 17.7 μC/cm2 and small coercive field (2Ec) of 91 kV/cm was observed for the Ca0.15Sr1.85Bi4Ti5O18 (CSBT-0.15) ceramic, which indicated that this sample possessed the best ferroelectric properties. As the substitution level increased, the variation of the leakage current density first decreased and then increased, and the sample with the substitution level of 0.15 showed the smallest leakage current density of J= 1.1 × 10−6A/cm2 at ~80 kV/cm. For the investigated CaxSr2-xBi4Ti5O18 (CSBT) ceramic samples, space-charge-limited conduction (SCLC) was determined to be the main conduction mechanism.

Room Temperature Exchange Bias in Structure-Modulated Single-Phase Multiferroic Materials

Single-phase materials with room temperature (RT) exchange bias (EB) are very important for future applications in spintronic devices, magnetic storage, and sensors. However, such materials are rare because EB normally occurs with ferromagnetic (FM)/antiferromagnetic (AFM) boundaries. In this work, RT EB was realized by breaking the homogeneous structure of long-period Aurivillius oxides through a simple Fe/Ti content modulation. Fantastic intergrowth structures, with different neighboring layer numbers larger than 1, were first observed in this material system with optimized compositions. The intergrowth structure introduces a much larger lattice distortion and probability of Fe–O–Fe interaction, which can largely enhance the EB effect and the EB temperature. Most importantly, RT EB was finally realized in Bi10Fe6+yTi3–yO30+δ (x = 0.2, 0.4) samples with the largest HE of ∼38 Oe measured at 300 K. Significantly, the concurring room temperature multiferroic behavior in such materials may add a new regulati...

Chemical intermixing at oxide heterointerfaces with polar discontinuity

The stability of the (4 × 4) reconstruction is studied on SrTiO3(111) and LaTiO3(111) surfaces by scanning tunneling microscopy. While the reconstruction may effectively compensate the surface polarity, the associated large lattice distortion on SrTiO3 with relatively strong polarity has to be relaxed by extra adsorbates. Alternatively, substituting Sr by La can reduce the polarity and stabilize the clean reconstructed surface. This provides the driving force for La segregating towards the surface during the heteroepitaxy of SrTiO3 on LaTiO3-covered SrTiO3(111), and thus, the interface sharpness will be deteriorated. To improve the quality of heterointerfaces where the surface polarity of substrates tends to be increased by the epitaxial films, the growth kinetics must be optimized to play the major role.

Room temperature ferroelectricity in donor-acceptor co-doped TiO2 ceramics using doping-engineering

Conventional ferroelectric oxides are of generally non-centrosymmetric crystals resulting from paraelectric-ferroelectric transitions. Here, we present a new donor-acceptor ion co-doping route to room-temperature ferroelectricity in rutile TiO2, where ionic pair of co-doping ions plays very important roles in generating local broken symmetry and polarization. The (Nb + Al) co-doped TiO2 sintered with a two-step method exhibits ferroelectricity and piezoelectric response, and it is believed that Nb5+-Al3+ pairs along TiO2 [001] direction play important role in the ferroelectricity. A large lattice distortion can be induced in the vicinity of the ionic pair, which results in local broken-symmetry. The relative displacements of cations and anions around the ionic pair, induced by the lattice distortion, cause the local polarization. The results and mechanisms presented in this work provide a new approach to the design of new ferroelectrics via doping engineering.

Enhanced thermal stability of Cu alloy films by strong interaction between Ni and Zr (or Fe)

Low resistivity, phase stability and nonreactivity with surrounding dielectrics are the key to the application of Cu to ultra-large-scale integrated circuits. Here, a stable solid solution cluster model was introduced to design the composition of barrierless Cu–Ni–Zr (or Fe) seed layers. The third elements Fe and Zr were dissolved into Cu via a second element Ni, which is soluble in both Cu and Zr (or Fe). The films were prepared by magnetron sputtering on the single-crystal p-Si (1 0 0) wafers. Since the diffusion characteristics of the alloying elements are different, the effects of the strong interaction between Ni and Zr (or Fe) on the film’s stability and resistivity were studied. The results showed that a proper addition of Zr–Ni (Zr/Ni ⩽ 0.6/12) into Cu could form a large negative lattice distortion, which inhibits Cu–Si interdiffusion and enhances the stability of Cu film. When Fe–Ni was co-added into Cu, the lattice distortion of Cu reached a lower value, 0.0029 Å ⩽ |Δa| ⩽ 0.0046 Å, and the films showed poor stability. Therefore, when the model is applied to the composition design of the films, the strong interaction between the elements and the addition ratio should be taken into consideration.