Single-phase Materials Research Articles

Load-induced local phase transformation and modulus of shape memory alloys under spherical indentation by finite element method

Shape memory alloys are a unique class of materials that are capable of large reversible deformations under external stimuli such as stress or temperature. The present study examines the phase transformations and mechanical responses of NiTi and NiTiHf shape memory alloys under the loading of a spherical indenter by using a finite element model. It is found that the indentation unloading curves exhibit distinct changes in slopes due to the reversible phase transformations in the SMAs. The normalized contact stiffness (F/S2) of the SMAs varies with the indentation load (depth) as opposed to being constant for conventional single-phase materials. The load-induced phase transformation that occurred under the spherical indenter was simulated numerically. It is observed that the phase transformation phenomenon in the SMA induced by an indentation load is distinctly different from that induced by a uniaxial load. A pointed indenter produces a localized deformation, resulting in a stress (load) gradient in the specimen. As a result, the transformation of phases in SMAs induced by an indenter can only be partially completed. The overall modulus of the SMAs varies continuously with the indentation load (depth) as the average volumetric fraction of the martensite phase varies. For NiTi (Ea > Em), the modulus decreases with the depth, while for NiTiHf (Ea < Em), the modulus increases with the depth. The predicted young modules during indentation modeling agree well with experimental results. Finally, the phase transformation of the SMAs under the indenter is not affected by the post-yield behavior of the materials.

Electrochemical Decalcification-Exfoliation of Two-Dimensional Siligene, SixGey: Material Characterization and Perspectives for Lithium-Ion Storage.

A two-dimensional (2D) silicene-germanene alloy, siligene (SixGey), a single-phase material, has attracted increased attention due to its two-elemental low-buckled composition and unique physics and chemistry. This 2D material has the potential to address the challenges caused by low electrical conductivity and the environmental instability of corresponding monolayers. Yet, the siligene structure was studied in theory, demonstrating the material's great electrochemical potential for energy storage applications. The synthesis of free-standing siligene remains challenging and therefore hinders the research and its application. Herein we demonstrate nonaqueous electrochemical exfoliation of a few-layer siligene from a Ca1.0Si1.0Ge1.0 Zintl phase precursor. The procedure was conducted in an oxygen-free environment applying a -3.8 V potential. The obtained siligene exhibits a high quality, high uniformity, and excellent crystallinity; the individual flake is within the micrometer lateral size. The 2D SixGey was further explored as an anode material for lithium-ion storage. Two types of anode have been fabricated and integrated into lithium-ion battery cells, namely, (1) siligene-graphene oxide sponges and (2) siligene-multiwalled carbon nanotubes. The as-fabricated batteries both with/without siligene exhibit similar behavior; however there is an increase in the electrochemical characteristics of SiGe-integrated batteries by 10%. The corresponding batteries exhibit a 1145.0 mAh·g-1 specific capacity at 0.1 A·g-1. The SiGe-integrated batteries demonstrate a very low polarization, confirmed by their good stability after 50 working cycles and a decrease in the solid electrolyte interphase level that occurs after the first discharge/charge cycle. We anticipate the growing potential of emerging two-component 2D materials and their great promise for energy storage and beyond.

Magnetic Properties of Melt-Spun CoMnSi(B) Alloys

MnCoSi and MnCoSiB0.5 alloys were obtained by melt-spinning technique in order to attain polycrystalline microstructures with average grain sizes well below 10 microns. Phase distribution studied by X-ray analysis indicated single-phase materials with orthorhombic structure and TiNiSi-type crystal symmetry. Chemical homogeneity was verified by means of scanning electron microscopy, whereas magnetic behavior corresponded to soft magnetic materials with coercivity values lower than 20 Oe and saturation magnetization of 80 emu/g. Magnetocaloric response of these MnCoSiB alloys was characterized by a maximum magnetic entropy change of 1.0 J/kg K (for a magnetic field variation of 2.0 Tesla) and attractive refrigerant capacity over 100 J/kg, which represents a significant enhancement respect to equivalent MnCoGe alloys.

Thermochemical Activity of Single- and Dual-Phase Oxide Compounds Based on Ceria, Ferrites, and Perovskites for Two-Step Synthetic Fuel Production.

This study focuses on the generation of solar thermochemical fuel (hydrogen, syngas) from CO2 and H2O molecules via two-step thermochemical cycles involving intermediate oxygen-carrier redox materials. Different classes of redox-active compounds based on ferrite, fluorite, and perovskite oxide structures are investigated, including their synthesis and characterization associated with experimental performance assessment in two-step redox cycles. Their redox activity is investigated by focusing on their ability to perform the splitting of CO2 during thermochemical cycles while quantifying fuel yields, production rates, and performance stability. The shaping of materials as reticulated foam structures is then evaluated to highlight the effect of morphology on reactivity. A series of single-phase materials including spinel ferrite, fluorite, and perovskite formulations are first investigated and compared to state-of-the-art materials. NiFe2O4 foam exhibits a CO2-splitting activity similar to its powder analog after reduction at 1400 °C, surpassing the performance of ceria but with much slower oxidation kinetics. On the other hand, although identified as high-performing materials in other studies, Ce0.9Fe0.1O2, Ca0.5Ce0.5MnO3, Ce0.2Sr1.8MnO4, and Sm0.6Ca0.4Mn0.8Al0.2O3 are not found to be attractive candidates in this work (compared with La0.5Sr0.5Mn0.9Mg0.1O3). In the second part, characterizations and performance evaluation of dual-phase materials (ceria/ferrite and ceria/perovskite composites) are performed and compared to single-phase materials to assess a potential synergistic effect on fuel production. The ceria/ferrite composite does not provide any enhanced redox activity. In contrast, ceria/perovskite dual-phase compounds in the form of powders and foams are found to enhance the CO2-splitting performance compared to ceria.

Electrodeposited NiFe2O4/Cu2O heterostructure thin films with enhanced photocurrent generation

In comparison with single-phase materials, heterostructures have been known for superior water splitting applications. In this study, Cu2O and NiFe2O4 are chosen to fabricate thin film heterostructures. Cu2O is electrodeposited at 60 °C for 5 min on ITO-coated glass substrates using three-electrode system. After deposition, the phase formation is confirmed using powder x-ray diffraction studies. The NiFe2O4 (NFO) thin films are deposited using RF sputtering method at room temperature for 2 h on Cu2O/ITO substrates to obtain NFO/Cu2O/ITO Type-II heterostructure. The scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) cross-sectional images show that the thickness of NFO layer is 120 nm and Cu2O layer is 1.5 µm. The photocurrent density of Cu2O on ITO is 0.08 ± 0.002 mA/cm2, and it increased to 0.12 ± 0.002 mA/cm2 after adding NFO layer on Cu2O film due to Type-II heterojunction formation.

Fabrication of Architectured Biomaterials by Multilayer Co‐Extrusion and Additive Manufacturing

AbstractTissue engineering benefits from advances in 3D printing and multi‐material assembly to attain certain functional benefits over existing man‐made materials. Multilayered tissue engineering constructs might unlock a unique combination of properties, but their fabrication remains challenging. Herein, a facile process is reported to manufacture biomaterials with an engineered multilayer architecture, via a combination of co‐extrusion and 3D printing. Polymer filaments containing 5, 17, or 129 alternating layers of poly(lactic acid)/thermoplastic polyurethane (PLA/TPU) are produced, and explored for their use in fused deposition modeling (FDM) to fabricate scaffolds for cardiomyocyte culture. The co‐extruded filaments exhibit a layered architecture in their cross‐section with a continuous interface, and the integrity and alignment of the layers are preserved after 3D printing. The 17 alternating layers PLA/TPU composites exhibit excellent mechanical properties. It is envisaged that the multilayered architecture of the fabricated scaffolds can be beneficial for aligning cardiomyocytes in culture. It is found that the 17 alternating layers PLA/TPU significantly improve cardiomyocyte morphology and functionality compared to single phase materials. It is believed that this biomaterials fabrication scheme, combining a top‐down and bottom‐up approach, offers tremendous flexibility in producing a broad class of novel‐architectured materials with tunable structural design for tissue engineering applications and beyond.

Co-Cr-Fe-Mn-Ni Oxide as a Highly Efficient Thermoelectric High-Entropy Alloy.

Among the existing materials for heat conversion, high-entropy alloys are of great interest due to the tunability of their functional properties. Here, we aim to produce single-phase high-entropy oxides composed of Co-Cr-Fe-Mn-Ni-O through spark plasma sintering (SPS), testing their thermoelectric (TE) properties. This material was successfully obtained before via a different technique, which requires a very long processing time. Hence, the main target of this work is to apply spark plasma sintering, a much faster and scalable process. The samples were sintered in the temperature range of 1200-1300 °C. Two main phases were formed: rock salt-structured Fm3̅m and spinel-structured Fd3̅m. Comparable transport properties were achieved via the new approach: the highest value of the Seebeck coefficient reached -112.6 μV/K at room temperature, compared to -150 μV/K reported before; electrical properties at high temperatures are close to the properties of the single-phase material (σ = 0.2148 S/cm, σ ≈ 0.2009 S/cm reported before). These results indicate that SPS can be successfully applied to produce highly efficient TE high-entropy alloys in a fast and scalable way. Further optimization is needed for the production of single-phase materials, which are expected to exhibit an even better TE functionality.

Quantifying Co–Zn contents for compositional tailoring of strontium W-type hexaferrites for useful applications

Structural, optical, dielectric, and magnetic characteristics of Sr–Ni–W hexaferrites were probed to justify their potential use in various applications. Sr1-xCoxZnyNi2-yFe16O27 (WHFs) ferrites were synthesized by the self-combustible sol-gel method. XRD analysis confirmed a single-phase material with different Co–Zn contents. Fourier transform infrared spectroscopic (400-4000 cm−1) analysis was performed. Variations in Raman intensities indicated the induction of local strain due to a mismatch of ionic radii. The energy band gap (1.879 eV) was calculated from photoluminescence spectra, indicating the semiconducting nature of materials. SEM images showed the presence of grains with hexagonal platelet morphology oriented irregularly, which is highly beneficial for microwave absorption. The high coercivity values indicated that the materials are magnetically hard. Magnetic analysis revealed the potential use of materials in high-storage applications. Dielectric properties were determined using a vector network analyzer within a frequency span (0–6 GHz), indicating materials' usefulness in constructing resonant circuits and MLCIs.

Temperature-dependent defect evolution and electrochemical performance enhancement of Na3V2(PO4)2F3

In this paper, we reported on the lattice distortion, surface morphologies, vacancy defects and electrochemical performance that had been observed in Na3V2(PO4)2F3 prepared at different annealing temperatures. X-ray diffraction indicated that all the samples were single phase materials with tetragonal structure and exhibited lattice distortion with the increase of annealing temperatures. A possible mechanism causing the strain-induced lattice distortion had been discussed. Moreover, scanning electron microscopy and positron annihilation techniques were used to study the grain size and vacancy defects as a function of annealing temperatures. The superior electrochemical performance of Na3V2(PO4)2F3 electrode was obtained at the annealing temperature of 350 °C with 167.73 F·g−1 specific capacitance and 85% capacitance retention. The better electrochemical performance was due to the synergistic effects of grain size and vacancy defect regulated by the annealing temperatures. These results could provide experimental basis for enhancing electrochemical performance of Na3V2(PO4)2F3 in sodium-ion battery area applications.

White-light emission from yttrium iron garnet (YIG)

Single-phase phosphors that emit broadband white-light are needed for white-light-emitting diodes (wLEDs) to reach their full potential. However, it is challenging to achieve broad white-light emission from single-phase materials. Consequently, polycrystalline inorganic bulk compounds that emit white-light sans doping are rare. We report on broadband white-light emission from a well-known garnet compound, i.e., yttrium iron garnet (YIG), without activator-ion doping. Upon near-UV excitation at 370 nm, polycrystalline bulk YIG emits broadband white-light with (1931) Commission Internationale de L’Eclairage (CIE) coordinates as (0.28, 0.35) and correlated color temperature (CCT) as 8029 K. Variable excitation wavelengths ranging from 280 to 600 nm enable color-tunable emission as cyan-white-blue-green-yellow-orange-red, including near-white-light emission for a broad range of excitation from 325 to 390 nm. Moreover, a short lifetime (sub-nanosecond) is obtained, which is desirable for LED and other applications. We demonstrated the propriety of YIG as a single-phase converting phosphor for illumination by fabricating prototype wLEDs using commercial InGaN UV-LED chips (λ = 380 nm) for excitation. The CIE coordinates and CCT of prototype wLEDs were obtained as (0.34, 0.37) and 5284 K, respectively. We believe that the reported findings signify the great potential of YIG as a single-phase white-light-emitting phosphor for broadband emission, which offers a new perspective and a viable approach for the development of wLEDs.

Design and preparation of high-entropy fluorite oxides based on [formula omitted]-S diagram

Since high-entropy oxide has a huge compositional space, it is urgent to develop efficient methods to quickly find the compositions that can form single-phase materials. In this paper, taking (Ce, Zr, Hf, Yb, Nb)O2 system as an example, we carried out the design and preparation of high-entropy fluorite oxides (HEFOs) by constructing a R‾-S diagram. Three single-phase HEFOs have been successfully design and synthesized. The impedance measurement showed that the three materials are electronic conductors, which exhibited relatively low thermal conductivities ranging between 1.6 W/m·K ∼1.9 W/m·K. The current study suggests that R‾-S diagram is a useful tool for quickly selecting compositions that can form single-phase high-entropy oxides.

Scaling studies of temperature dependence of electric conductivity and electric modulus spectra of solid solution Sr2-xLaxSnO4 (x ≤ 0.10) Ruddlesden Popper Oxide

The mixed ionic and electronic conductor oxide with formula Sr2-xLaxSnO4 (S2LS) within x ≤ 0.10 were synthesized via the conventional ceramic route and investigated. The structural analysis revealed the formation of single-phase materials with a tetragonal crystal structure. The presence of Sn in the form of Sn4+/Sn2+ ions and oxygen vacancies in the synthesized samples were investigated by X-ray photoelectron spectroscopy. The alternating current (AC) conductivity and modulus spectra of S2LS systems were obtained as functions of frequency and temperature and further scaled. The AC conductivity data were analyzed using Jonscher's power law (JPL) fitting to determine the hopping frequencies, DC conductivities, and power exponents. The obtained activation energy value suggests the dependence of conductivity primarily on the mobility of charge carriers. The conductivity spectra were analyzed using various formalisms, viz., Ghosh scaling and Rolling scaling, to understand the conduction mechanism. The scaling behaviors of AC conductivity and modulus spectra reflect the temperature-independent conduction mechanism of the system.

Zero thermal expansion in non-stoichiometric and single-phase (Hf,Nb)Fe2.5 alloy

Zero thermal expansion (ZTE) alloys are dimensionally stable under the influence of heat, which is critical in high-precision components but remains rare among single phase materials. This work reports a single phase and nearly isotropic ZTE alloy (Hf0.8Nb0.2)Fe2.5 over a wide temperature range of 5–460 K (αl≈1.07 × 10−6 K−1) via non-stoichiometric regulation. The introduction of excess Fe breaks the dual-phase nature of parent alloy, extends the magnetic ordering temperature, and induces a spin reorientation. The evolution of magnetic structure versus temperature gives rise to the ZTE behavior. The design concept of changing the internal magnetic interaction through non-stoichiometric regulation provides support for the discovery of novel ZTE materials.

Dependence of the hardness of WC - Co alloys on the character of the distribution of WC grains in size

The paper presents the results of studying the dependence of the hardness of WC - Co alloy with a Co content of 10 wt.% on the character of the size distribution of WC grains. An equivalent diameter of the circle with an area matching the cross-section area of the grain was chosen as a grain size of the carbide phase. The WC grain sizes were averaged over their number, area and volume. It is shown that for the alloys with a narrow WC grain size distribution the hardness of the alloys as function of the average grain size follows the Hall - Petch relation. As for a wide distribution, a deviation from this function is observed. It is shown that the use of the area-averaged WC grain size enables description of the hardness as a function of the grain size via a unified Hall - Petch equation, regardless of the nature of the grain size distribution. In this case, the average scatter of hardness values around the trend line does not exceed 12 HV. The use of the volume-averaged grain size in description of the hardness dependence results in practice in a large scatter from the regression line, which is attributed to the error in determination of the content of coarse and very coarse grains, when one needs to measure significantly larger number of grains, as compared to averaging over the area. The results obtained may be used in analysis of single-phase and two-phase materials with grain size distributions with a varying width.

Gradient Strain-Induced Room-Temperature Ferroelectricity in Magnetic Double-Perovskite Superlattices.

Single-phase multiferroics suffer from a fundamental contradiction between polarity and magnetism in d0 electronic configuration, motivating studies of unconventional ferroelectricity in magnetic oxides. However, low critical temperature and polarization still need to be overcome. Here, it is reported that the switchable polarization behavior at room temperature in [(La2 NiMnO6 )/(La2 CoMnO6 )]n double-perovskite magnetic superlattice films is achieved by engineering a microstructure with gradient strains, and the ferromagnetic Curie temperature did not show a rapid decrease. The synergy of gradient strains and superlattice components plays a decisive role in inducing ferroelectricity via the tilting or rotation of various oxygen octahedra. Such distortion responses to gradient strains are accompanied by slight magnetic fluctuations, maximizing the preservation of the initial magnetic exchange interactions, which alleviates the contradiction of multiferroic coexistence to a certain extent. This work confirms the room-temperature ferroelectricity in double-perovskite superlattices and provides a preferred strategy for confronting the difficulty of multiferroic coexistence in single-phase materials.

Thin-film (Al)BCN materials synthesized by sequential precursor pulses to mimic atomic layer deposition

This work brings novel insights into the existing knowledge on the deposition of films containing boron (B), carbon (C), nitrogen (N), and aluminum (Al). The (Al)BCN films are obtained at low substrate temperatures (TS) of 250–400 °C from triethylborane, ammonia (NH3), and trimethylaluminum. For BCN films, a nearly similar elemental composition of B0.42C0.41N0.15O0.02, with 1–2 at. % variations, is observed for substantial ranges of TS and NH3-exposure time. This can indicate a similar growth mechanism and/or formation of a single-phase material. While excluding precursor underdosing, a remarkable dependence of growth rate per cycle (GPC) on total gas pressure (Ptot) is observed. The GPC approaches near saturation regime for Ptot between 1 and 10 mbar, for TS = 330–375 °C, which might support the occurrence of a surface-adduct assisted pathway. The level of GPC saturation is influenced by TS. For a wide range of process conditions, N-share in the films slightly varies between 12 and 16 at. %. C-share only changes between 40 and 42 at. %. The attempt to increase N-share by dissociating NH3 into NH2 radicals by hot wire assistance remarkably shows the opposite effect, i.e., a decrease in the N-share from 15 to 6 at. %. This is accompanied by a corresponding increase in the B- and, in particular, C-shares, suggesting that the removal of carbon can occur via the incorporation of nitrogen. For AlBCN films, changing Ptot has a strong effect on their elemental composition. At Ptot = 10 mbar, Al-deficient films grow, whereas a Ptot of 0.2 mbar leads to mainly AlN-containing films with some inclusions of BN.

Ultrashort pulsed laser ablation of zirconia toughened alumina: Material removal mechanism and surface characteristics

Zirconia toughened alumina (ZTA) is commonly used in hip joint implants and cutting tools. The difference in material properties of the zirconia and alumina phases makes the composite inhomogeneous at the grain size level. Due to the strong nonlinear laser absorption mechanisms of wide band gap materials under intense laser field, the smaller band gap zirconia phase can absorb a significantly larger amount of laser energy than the alumina phase. The nonuniform energy deposition and lattice heating takes place at a comparable timescale as the laser pulse duration, which is too short for thermal diffusion to play a role. As a result, the temperature in the zirconia phase can be significantly higher than in the alumina phase, leading to a unique selective phase melting phenomenon, i.e. localized melting of the higher melting point zirconia phase (2715 °C) surrounded by a lower melting point alumina phase (2072 °C) which remains solid. This phenomenon makes the material removal mechanism of the nanocomposite significantly different from that of single-phase materials under ultrashort pulsed laser processing. On the basis of experimental analysis and theoretical modelling, material ablation by the disintegration of alumina grains due to the localized melting of the zirconia phase was identified.

TEMPERATURE DEPENDENCE OF ACOUSTIC BIREFRINGENCE OF SHEAR ELASTIC WAVES IN POLYCRYSTALLINE ALUMINUM

The article presents the results of a study of the temperature dependences of the shear elastic waves velocity of polycrystalline aluminum alloy AD0 in the temperature range from 20 °C to 45 °C. For the studied material the temperature dependence of the acoustic birefringence parameter characterizing the a shear ultrasonic wave splitting into two orthogonally polarized waves propagating at different velocities is determined. The calculation of the acoustic birefringence parameter was carried out according to the data of the propagation time precision measurement of shear elastic waves (the measurement error of the propagation time is 1-2 ns), polarized along and across the direction of rolling. Theoretical studies have shown that for materials with weak anisotropy, the temperature dependence of the acoustic birefringence parameter is mainly related to the influence of temperature on the elastic anisotropy of the crystals that make up the polycrystalline material and the characteristic of the crystallographic texture – the coefficient of the orientation distribution function W420. Corrections to the value of the birefringence parameter with temperature change are given. It was experimentally obtained that the acoustic birefringence parameter increases by 9% with increasing temperature, which must be taken into account by using it as a diagnostic parameter to evaluation changes in the characteristics of static and fatigue strength. An algorithm is proposed for estimating the prediction of the temperature dependence of the acoustic birefringence parameter of a single-phase material when the crystallographic texture changes as a result of plastic deformation, for example. A good coincidence of the predicted temperature dependence of the acoustic birefringence parameter with the experimental one was obtained – the deviation error of the calculated curve from the experimental one was 0.2–2% in the temperature range of 20 °С to 45 °С.

Fatigue free relaxor ferroelectric and electrocatalytic hydrogen evolution of yttrium substituted BaBi2Nb2O9 layered structured nanoceramics for energy storage application

Lead-free bismuth layered structured ferroelectric ceramics are highly important in superior energy storage applications. The slim hysteresis curve with high energy storage density and efficiency is expected for wide application. In this regard, the suitable electrocatalyst for the generation of hydrogen gas through water-splitting show high importance as an alternative energy source. The current article aims to investigate the above goals by incorporating 5% yttrium at the place of bismuth for BaBi 2 Nb 2 O 9 relaxor ferroelectrics. The solid-state synthesized method successfully prepared the single-phase material and is confirmed through spectroscopic and morphological analysis. The dielectric characteristics follow diffused phase transition with broad temperature dependence that indicated the space charge polarization in the material owing to the presence of Y 3+ at different domains. The higher degree of diffuseness and low dielectric loss implies the significant role of charge carriers during frequency and temperature variation in the dielectric study. Impedance spectroscopy analysis indicates the non-Debye type relaxation and NTCR behavior of the material that is correlated to the multiple relaxation processes. The slim P-E hysteresis curve and fatigue-free properties of the materials up to 10 6 cycles reflect its usefulness in the potential device as NVRAM applications. The materials are also used for hydrogen evolution reaction (HER) under an electrochemical condition. A lower overpotential value of 180 mV is obtained at 10 mA cm −2 current density. The lower Tafel slope and charge transfer resistance value support its superior HER activity. The above results demonstrated the potential application of the studied material (BaBi 1 · 9 Y 0 · 1 Nb 2 O 9 ) in the energy storage application and as an electrocatalyst for alternative energy source.

Simultaneous reactant accessibility and charge transfer engineering in Co-doped RuO2-supported OCNT for robust rechargeable zinc-air batteries

The sluggish oxygen reduction/evolution reaction (ORR/OER) kinetics greatly limits the application of zinc-air batteries (ZAB). Regarding this, searching for high-efficient electrocatalysts should be emphatically considered, while reactant accessibility and charge transfer on catalysts are the decisive factors for ORR/OER activities. Here we propose the incorporation of excess Co into (101) plane of RuO2 to form a single-phase material with abundant Ru/Co dual-atom sites on the active (110) surface to simultaneously optimize the charge transfer and reactant accessibility in the electrode reactions, while oxidized carbon nanotube applied as supports for electron transfer. Ru and Co atoms serve as the main ORR and OER catalytic sites respectively, while Ru/Co dual atoms tend to co-adsorbs the intermediate *OOH to produce the synergistic effects. As a result, the optimal electrocatalysts achieve an ultralow overpotential gap of 0.67 V in 0.1 M KOH and superior stability over 800 cycles at 10 mA cm-2 for ZAB.