Critical Current Density and Meissner Effect of Smart Meta-Superconductor MgB2 and Bi(Pb)SrCaCuO

The smart meta-superconductor MgB₂ and Bi(Pb)SrCaCuO increase the superconducting transition temperature (T_C), but the changes in the transport critical current density (J_C) and Meissner effect are still unknown. Here, we investigated the J_C and Meissner effect of smart meta-superconductor MgB₂ and Bi(Pb)SrCaCuO. The use of the standard four-probe method shows that Y₂O₃:Eu³⁺ and Y₂O₃:Eu³⁺+Ag inhomogeneous phase significantly increase the J_C, and J_C decreases to a minimum value at a higher temperature. The Meissner effect was measured by direct current magnetization. The doping of Y₂O₃:Eu³⁺ and Y₂O₃:Eu³⁺+Ag luminescent inhomogeneous phase causes a Meissner effect of MgB₂ and Bi(Pb)SrCaCuO at a higher temperature, while the non-luminescent dopant reduces the temperature at which samples have Meissner effect. The introduction of luminescent inhomogeneous phase in conventional MgB₂ and copper oxide high-temperature Bi(Pb)SrCaCuO superconductor increases the T_C and J_C, and Meissner effect is exerted at higher temperature. Therefore, smart meta-superconductivity is suitable for conventional and copper oxide high-temperature superconductors.     

An Overview of Some Nonpiezoelectric Properties of BaTiO3 Ceramics Doped by Eu Ions

Ferroelectric ceramics BaTiO₃:x%Eu (x = 0, 0.1, 1, 2, 3) were synthesized by a conventional method. Structural investigation confirmed that all ceramics possessed tetragonal (P4mm) symmetries at room temperature for the undoped ceramics as well as for the doped ceramics. Furthermore, a slight downshifting of the Curie temperature (T_C) with an increasing Eu³⁺ doping amount has been noted. The Raman spectra unveiled the existence of new modes for higher-doped BaTiO₃:x%Eu (BTEx) which are related to local disorders and defects. The ferroelectric properties were found to depend on both doping and the microstructure. The electrocaloric effect was also studied for those ceramics. It was observed that ΔT decreases with doping; however, the temperature range of its occurrence widens considerably.     

Synthesis of Dense MgB2 Superconductor via In Situ and Ex Situ Spark Plasma Sintering Method

In this study, high-density magnesium diboride (MgB₂) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (J_c-B) in MgB₂ bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and J_c-B using two methods, ex situ (sintering MgB₂ synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB₄ and MgO were found to be effective in improving the J_c-B characteristics. In the ex situ method, the degradation of MgB₂ due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the J_c-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and J_c-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (f_P) analysis revealed the possibility of refined MgO inclusions and MgB₄ phase as new pinning centers, which greatly contributed to the J_c-B properties. The contributions of the sintering conditions on f_P for both synthesis methods were analyzed.     

Facile Synthesis of Island-like ZrO2-VO2 Composite Films with Enhanced Thermochromic Performance for Smart Windows

VO₂-based film, as a very promising thermochromic material for smart windows, has attracted extensive attention but has not been widely applied because it is difficult to simultaneously improve in terms of both solar-modulation efficiency (ΔT_sol) and visible transmittance (T_lum) when made using the magnetron-sputtering method, and it has poor durability when made using the wet chemical method. Herein, island-like ZrO₂-VO₂ composite films with improved thermochromic performance (ΔT_sol: 12.6%, T_lum: 45.0%) were created using a simple approach combining a dual magnetron-sputtering and acid-solution procedure. The film’s ΔT_sol and T_lum values were increased initially and subsequently declined as the sputtering power of the ZrO₂ target was raised from 30 W to 120 W. ΔT_sol achieved its maximum of 12.6% at 60 W, and T_lum reached its maximum of 51.1% at 90 W. This is likely the result of the interaction of two opposing effects: Some VO₂ nanocrystals in the composite film were isolated by a few ZrO₂ grains, and some pores could utilize their surface-plasmon-resonance effect at high temperature to absorb some near-infrared light for an enhanced ΔT_sol and T_lum. More ZrO₂ grains means fewer VO₂ grains in the composite film and increased film thickness, which also results in a decrease in ΔT_sol and T_lum. As a result, this work may offer a facile strategy to prepare VO₂-based films with high thermochromic performance and promote their application in smart windows.     

Combustion Synthesis and Photoluminescence Properties of Red-Emitting CaAlSiN3:Eu2+ Phosphor for White-LEDs

A combustion synthesis method has been developed for synthesis of Eu²⁺ doped CaAlSiN₃ phosphor and its photoluminescence properties were investigated. Ca, Al, Si, and Eu₂O₃ powders were used as the Ca, Al, Si and Eu sources. The addition of NaN₃, NH₄Cl and Si₃N₄ powders was found to increase significantly the product yield. These powders were mixed and pressed into a compact, which was then wrapped up with an igniting agent (i.e., Mg+Fe₃O₄). The compact was ignited by electrical heating under a N₂ pressure of ≤1.0 MPa. Effects of these experimental parameters on the product yield were investigated and a reaction mechanism was proposed. The synthesized CaAlSiN₃:Eu²⁺ phosphor absorbs light in the region of 200–600 nm and shows a broad band emission in the region of 500–800 nm due to the 4f⁶5d¹ → 4f⁷ transition of Eu²⁺. The sample doped with Eu²⁺ at the optimized molar ratio of 0.04 is efficiently excited by the blue light (460 nm) and generates emission peaking at ~650 nm with peak emission intensity ~106% of a commercially available phosphor, YAG:Ce³⁺(P46-Y3).The internal quantum efficiency of the synthesized phosphor was measured to be 71%, compared to 69% of the YAG:Ce³⁺ (P46-Y3).     

Modulation of Structure and Optical Property of Nitrogen-Incorporated VO2 (M1) Thin Films by Polyvinyl Pyrrolidone

VO₂, as a promising material for smart windows, has attracted much attention, and researchers have been continuously striving to optimize the performance of VO₂-based materials. Herein, nitrogen-incorporated VO₂ (M1) thin films, using a polyvinylpyrrolidone (PVP)-assisted sol–gel method followed by heat treatment in NH₃ atmosphere, were synthesized, which exhibited a good solar modulation efficiency (ΔT_sol) of 4.99% and modulation efficiency of 37.6% at 2000 nm (ΔT_{2000 nm}), while their visible integrated transmittance (T_lum) ranged from 52.19% to 56.79% after the phase transition. The crystallization, microstructure, and thickness of the film could be regulated by varying PVP concentrations. XPS results showed that, in addition to the NH₃ atmosphere-N doped into VO₂ lattice, the pyrrolidone-N introduced N-containing groups with N–N, N–O, or N–H bonds into the vicinity of the surface or void of the film in the form of molecular adsorption or atom (N, O, and H) filling. According to the Tauc plot, the estimated bandgap of N-incorporated VO₂ thin films related to metal-to-insulator transition (E_g1) was 0.16–0.26 eV, while that associated with the visible transparency (E_g2) was 1.31–1.45 eV. The calculated E_g1 and E_g2 from the first-principles theory were 0.1–0.5 eV and 1.4–1.6 eV, respectively. The Tauc plot estimation and theoretical calculations suggested that the combined effect of N-doping and N-adsorption with the extra atom (H, N, and O) decreased the critical temperature (τ_c) due to the reduction in E_g1.     

Fabrication and Optical Characterization of VO2-Based Thin Films Deposited on Practical Float Glass by Magnetron Sputtering and Professional Annealing

In this paper, VO₂ thin films with good optical properties are fabricated on practical float glass by magnetron sputtering and a professional annealing method. The near-infrared switching efficiency (NIRSE) of the prepared film reaches 39% (@2000 nm), and its near-infrared energy modulation ability (ΔT_ir) reaches 10.9% (780–2500 nm). Further, the highest integral visible transmittance T_lum is 63%. The proposed method exhibits good reproducibility and does not cause any heat damage to the magnetron sputtering machine. The crystalline structure of the VO₂ film is characterized by X-ray diffraction (XRD). The lattice planes (011) and (−211) grow preferentially (JCPDS 65-2358), and a large number of NaV₂O₅ crystals are detected simultaneously. The microstructures are characterized by scanning electron microscopy (SEM), and a large number of long sheet crystals are identified. The phase transition temperature is significantly reduced by an appropriate W doping concentration (T_c = 29 °C), whereas excessive W doping causes distortion of the thermal hysteresis loop and a reduction in the NIRSE. Oxygen vacancies are created by low pressure annealing, due to which the phase transition temperature of VO₂ film decreases by 8 °C. The addition of an intermediate SiO₂ layer can prevent the diffusion of Na⁺ ions and affect the preparation process of the VO₂ thin film.     

Observation of a multiferroic critical end point

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a “metaelectric” critical end point, has been rarely studied. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (H) as a tuning parameter. Herein, we report the discovery of a magnetic-field-induced metaelectric transition in multiferroic BiMn₂O₅, in which the electric polarization (P) switches polarity along with a concomitant Mn spin–flop transition at a critical magnetic field H_c. The simultaneous metaelectric and spin–flop transitions become sharper upon cooling but remain a continuous cross-over even down to 0.5 K. Near the P = 0 line realized at μ₀H_c ≈ 18 T below 20 K, the dielectric constant (ɛ) increases significantly over wide field and temperature (T) ranges. Furthermore, a characteristic power-law behavior is found in the P(H) and ɛ(H) curves at T = 0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn₂O₅ near zero temperature.     

Magnetic and Magneto-Caloric Properties of the Amorphous Fe92−xZr8Bx Ribbons

Magnetic and magnetocaloric properties of the amorphous Fe_92−xZr₈B_x ribbons were studied in this work. Fully amorphous Fe₈₉Zr₈B₃, Fe₈₈Zr₈B₄, and Fe₈₇Zr₈B₅ ribbons were fabricated. The Curie temperature (T_c), saturation magnetization (M_s), and the maximum entropy change with the variation of a magnetic field (−ΔS_m^peak) of the glassy ribbons were significantly improved by the boron addition. The mechanism for the enhanced T_c and −ΔS_m^peak by boron addition was studied.     

Interacting cytoplasmic loops of subunits a and c of Escherichia coli F1F0 ATP synthase gate H+ transport to the cytoplasm

H⁺-transporting F₁F₀ ATP synthase catalyzes the synthesis of ATP via coupled rotary motors within F₀ and F₁. H⁺ transport at the subunit a–c interface in transmembranous F₀ drives rotation of a cylindrical c₁₀ oligomer within the membrane, which is coupled to rotation of subunit γ within the α₃β₃ sector of F₁ to mechanically drive ATP synthesis. F₁F₀ functions in a reversible manner, with ATP hydrolysis driving H⁺ transport. ATP-driven H⁺ transport in a select group of cysteine mutants in subunits a and c is inhibited after chelation of Ag⁺ and/or Cd⁺² with the substituted sulfhydryl groups. The H⁺ transport pathway mapped via these Ag⁺(Cd⁺²)-sensitive Cys extends from the transmembrane helices (TMHs) of subunits a and c into cytoplasmic loops connecting the TMHs, suggesting these loop regions could be involved in gating H⁺ release to the cytoplasm. Here, using select loop-region Cys from the single cytoplasmic loop of subunit c and multiple cytoplasmic loops of subunit a, we show that Cd⁺² directly inhibits passive H⁺ transport mediated by F₀ reconstituted in liposomes. Further, in extensions of previous studies, we show that the regions mediating passive H⁺ transport can be cross-linked to each other. We conclude that the loop-regions in subunits a and c that are implicated in H⁺ transport likely interact in a single structural domain, which then functions in gating H⁺ release to the cytoplasm.The F₁F₀-ATP synthase of oxidative phosphorylation uses the energy of a transmembrane electrochemical gradient of H⁺ or Na⁺ to mechanically drive the synthesis of ATP via two coupled rotary motors in the F₁ and F₀ sectors of the enzyme (1). H⁺ transport through the transmembrane F₀ sector is coupled to ATP synthesis or hydrolysis in the F₁ sector at the surface of the membrane. Homologous ATP synthases are found in mitochondria, chloroplasts, and many bacteria. In Escherichia coli and other eubacteria, F₁ consists of five subunits in an α₃β₃γδε stoichiometry. F₀ is composed of three subunits in a likely ratio of a₁b₂c₁₀ in E. coli and Bacillus PS3 (2, 3) or a₁b₂c₁₁ in the Na⁺ translocating Ilyobacter tartaricus ATP synthase (1, 4) and may contain as many as 15 c subunits in other bacterial species (5). Subunit c spans the membrane as a hairpin of two α-helices, with the first transmembrane helix (TMH) on the inside and the second TMH on the outside of the c ring (1, 4). The binding of Na⁺ or H⁺ occurs at an essential, membrane-embedded Glu or Asp on cTMH2. High-resolution X-ray structures of both Na⁺- and H⁺-binding c-rings have revealed the details and variations in the cation binding sites (4–8). In the H⁺-translocating E. coli enzyme, Asp-61 at the center of cTMH2 is thought to undergo protonation and deprotonation, as each subunit of the c ring moves past the stationary subunit a. In the functioning enzyme, the rotation of the c ring is thought to be driven by H⁺ transport at the subunit a/c interface. Subunit γ physically binds to the cytoplasmic surface of the c-ring, which results in the coupling of c-ring rotation with rotation of subunit γ within the α₃β₃ hexamer of F₁ to mechanically drive ATP synthesis (1).E. coli subunit a folds in the membrane with five TMHs and is thought to provide aqueous access channels to the H⁺-binding cAsp-61 residue (9, 10). Interaction of the conserved Arg-210 residue in aTMH4 with cTMH2 is thought to be critical during the deprotonation–protonation cycle of cAsp-61 (1, 11, 12). At this time, very limited biophysical or crystallographic information is available on the 3D arrangement of the TMHs in subunit a. TMHs 2–5 of subunit a pack in a four-helix bundle, which was initially defined by cross-linking (13), but now, such a bundle, packing at the periphery of the c-ring, has been viewed directly by high-resolution cryoelectron microscopy in the I. tartaricus enzyme (14). Previously published cross-linking experiments support the identification of aTMH4 and aTMH5 packing at the periphery of the c-ring and the identification of aTHM2 and aTMH3 as the other components of the four-helix bundle seen in these images (13, 15, 16). More recently, published cross-linking experiments identify the N-terminal α-helices of two b subunits, one of which packs at one surface of aTMH2 with close enough proximity to the c-ring to permit cross-linking (17). The other subunit b N-terminal helix packs on the opposite peripheral surface of aTMH2 in a position where it can also be cross-linked to aTMH3 (17). The last helix density shown in Hakulinen and colleagues (14) packs at the periphery of the c-ring next to aTMH5 and is very likely to be aTMH1.The aqueous accessibility of Cys residues introduced into the five TMHs of subunit a has been probed on the basis of their reactivity with and inhibitory effects of Ag⁺ and other thiolate-reactive agents (18–20). Two regions of aqueous access were found with distinctly different properties. One region in TMH4, extending from Asn-214 and Arg-210 at the center of the membrane to the cytoplasmic surface, contains Cys substitutions that are sensitive to inhibition by both N-ethylmaleimide (NEM) and Ag⁺ (18–20; Fig. 1). These NEM- and Ag⁺-sensitive residues in TMH4 pack at or near the peripheral face and cytoplasmic side of the modeled four-helix bundle (11, 13). A second set of Ag⁺-sensitive substitutions in subunit a mapped to the opposite face and periplasmic side of aTMH4 (18, 19), and Ag⁺-sensitive substitutions were also found in TMHs 2, 3, and 5, where they extend from the center of the membrane to the periplasmic surface (19, 20). The Ag⁺-sensitive substitutions on the periplasmic side of TMHs 2–5 cluster at the interior of the four-helix bundle predicted by cross-linking and could interact to form a continuous aqueous pathway extending from the periplasmic surface to the central region of the lipid bilayer (11, 13, 19, 20). We have proposed that the movement of H⁺ from the periplasmic half-channel and binding to the single ionized Asp61 in the c-ring is mediated by a swiveling of TMHs at the a–c subunit interface (16, 21–24). This gating is thought to be coupled with ionization of a protonated cAsp61 in the adjacent subunit of the c-ring and with release of the H⁺ into the cytoplasmic half-channel at the subunit a–c interface. The route of aqueous access to the cytoplasmic side of the c subunit packing at the a–c interface has also been mapped by the chemical probing of Cys substitutions and, more recently, by molecular dynamics simulations (22, 25, 26).Open in a separate windowFig. 1.The predicted topology of subunit a in the E. coli inner membrane. The location of the most Ag⁺-sensitive Cys substitutions are highlighted in red (>85% inhibition) or orange (66–85% inhibition). The five proposed TMHs are shown in boxes, each with a span of 21 amino acids, which is the minimum length required to span the hydrophobic core of a lipid bilayer. The α-helical segments shown in loops 1–2 and 3–4 are consistent with the predictions of TALOS, based on backbone chemical shifts seen by NMR (29). Others have also predicted extensive α-helical regions in these loops (12, 30), but the possible positions remain largely speculative. aArg210 is highlighted in green. Figure is modified from those shown previously (21, 23, 24, 27).We have also reported Ag⁺-sensitive Cys substitutions in two cytoplasmic loops of subunit a (27) and, more recently, in the cytoplasmic loop of subunit c (28). The mechanism by which Ag⁺ inhibited F₁F₀-mediated H⁺ transport was uncertain. Several of these substitutions were also sensitive to inhibition by Cd⁺², and these substitutions provided a means of testing whether Cd⁺² directly inhibits passive H⁺ transport through F₀ (28). In the case of two subunit c loop substitutions, Cd⁺² was shown to directly inhibit passive F₀-mediated transport activity. In this study, we have extended the survey to Cd⁺²-sensitive Cys substitutions in cytoplasmic loops of subunit a. We report four loop substitutions in which Cd⁺² inhibits passive F₀-mediated H⁺ transport. Further, in two cases, we show cross-linking between pairs of Cys substitutions, which lie in subunits a and c, respectively, and which individually mediate passive H⁺ transport activity. These results suggest that the a and c loops, which gate H⁺ release to the cytoplasm, fold into a single domain at the surface of F₀.     

Re-Examination of the Microstructural Evolution in Undercooled Co-18.5at.%B Eutectic Alloy

The undercooling (∆T) dependencies of the solidification pathways, microstructural evolution, and recalescence behaviors of undercooled Co-18.5at.%B eutectic alloys were systematically explored. Up to four possible solidification pathways were identified: (1) A lamellar eutectic structure consisting of the FCC–Co and Co₃B phase forms, with extremely low ΔT; (2) The FCC–Co phase primarily forms, followed by the eutectic growth of the FCC–Co and Co₂B phases when ΔT < 100 K; (3) As the ΔT increases further, the FCC–Co phase primarily forms, followed by the metastable Co₂₃B₆ phase with the trace of an FCC–Co and Co₂₃B₆ eutectic; (4) When the ΔT increases to 277 K, the FCC–Co phase primarily forms, followed by an FCC–Co and Co₃B eutectic, which is similar in composition to the microstructure formed with low ΔT. The mechanisms of the microstructural evolution and the phase selection are interpreted on the basis of the composition segregation, the skewed coupled zone, the strain-induced transformation, and the solute trapping. Moreover, the prenucleation of the primary FCC–Co phase was also detected from an analysis of the different recalescence behaviors. The present work not only enriches our knowledge about the phase selection behavior in the undercooled Co–B system, but also provides us with guidance for controlling the microstructures and properties practically.     

Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe1?xTex thin films

We demonstrate the successful fabrication on CaF₂ substrates of FeSe_1−xTe_x films with 0 ≤ x ≤ 1, including the region of 0.1 ≤ x ≤ 0.4, which is well known to be the “phase-separation region,” via pulsed laser deposition that is a thermodynamically nonequilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature, T_c, in the region of 0.1 ≤ x ≤ 0.4: The maximum value reaches 23 K, which is ∼1.5 times as large as the values reported for bulk samples of FeSe_1−xTe_x. We present a complete phase diagram of FeSe_1−xTe_x films. Surprisingly, a sudden suppression of T_c is observed at 0.1 < x < 0.2, whereas T_c increases with decreasing x for 0.2 ≤ x < 1. Namely, there is a clear difference between superconductivity realized in x = 0 ? 0.1 and in x ≥ 0.2. To obtain a film of FeSe_1−xTe_x with high T_c, the controls of the Te content x and the in-plane lattice strain are found to be key factors.Since the discovery of superconductivity in LaFeAs(O,F) (1), many studies concerning iron-based superconductors have been conducted. FeSe is the iron-based superconductor with the simplest crystal structure (2). The T_c of FeSe is ∼8 K, which is not very high in comparison with other iron-based superconductors. However, the value of T_c strongly depends on the applied pressure, and the temperature at which the resistivity becomes zero, Tczero, reaches as high as  ～ 30 K at 6 GPa (3). This suggests that FeSe samples with higher T_c are available by the fabrication of thin films because we can introduce lattice strain. Indeed, we have previously reported that FeSe films fabricated on CaF₂ substrates exhibit T_c values ∼1.5 times higher than those of bulk samples because of in-plane compressive strain (4). On the other hand, superconductivity with T_c of 65 K has recently been reported in a monolayer FeSe film on SrTiO₃ (5, 6). It is unclear whether this superconductivity results from the characteristics of the interface. However, this finding indicates that FeSe demonstrates potential as a very-high-T_c superconductor.The partial substitution of Te for Se in FeSe also raises T_c to a maximum of 14 K at x = 0.5 ? 0.6 (7). In FeSe_1−xTe_x, it is well known that we cannot obtain single-phase samples with 0.1 < x < 0.4 because of phase separation (7). Here, we focus on this region of phase separation. Generally, the process of film deposition involves crystal growth in a thermodynamically nonequilibrium state. Thus, film deposition provides an avenue for the synthesis of a material with a metastable phase. In this paper, we report the fabrication of epitaxial thin films of FeSe_1−xTe_x with 0 ≤ x ≤ 1 on CaF₂ substrates, using the pulsed laser deposition (PLD) method. We demonstrate that single-phase epitaxial films of FeSe_1−xTe_x with 0.1 ≤ x ≤ 0.4 are successfully obtained and that the maximum value of T_c is as large as 23 K, which is higher than the previously reported values for bulk and film samples of FeSe_1−xTe_x (7–12), except for those of the monolayer FeSe films (5, 6). Our results clearly show that the optimal Te content for the highest T_c for FeSe_1−xTe_x films on CaF₂ is different from the widely believed value for this system.Fig. 1A presents the X-ray diffraction patterns of FeSe_1−xTe_x films for x = 0 ? 0.5 on CaF₂. Here and hereafter, the Te content x of our films represents the nominal Te composition of the polycrystalline target. With the exception of an unidentified peak in the FeSe_0.5Te_0.5 film, only the 00l reflections of a tetragonal PbO-type structure are observed, which indicates that these films are well oriented along the c axis. Fig. 1 B and C presents enlarged segments of these plots near the 001 and 003 reflections, respectively. The 2θ values of the peak positions decrease with increasing x in a continuous manner, which is consistent with the fact that the c-axis length increases with increasing x. It should be noted that the values of the full widths at half maximum (FWHM), δ(2θ), of the FeSe_1−xTe_x films with x = 0.1 ? 0.4, which is known as the region of phase separation in the bulk samples (7), are δ(2θ) = 0.2°?0.3° for the 001 reflection and δ(2θ) = 0.4°?0.6° for the 003 reflection, which are nearly the same as the values for the FeSe and FeSe_0.5Te_0.5 films. This result is in sharp contrast to the previously reported result that the FWHM was broad only in films of FeSe_1−xTe_x with x = 0.1 and 0.3 (13), where phase separation has been believed to occur. The results presented in Fig. 1 A–C indicate the formation of a single phase in our FeSe_1−xTe_x films with x = 0.1 ? 0.4.Open in a separate windowFig. 1.(A) Out-of-plane X-ray diffraction patterns of FeSe_1−xTe_x thin films for x = 0 ? 0.5 with film thicknesses of 120–147 nm. The # symbols represent peaks associated with the substrate. The * represents an unidentified peak. Enlarged segments of the plots presented in A near the 001 and 003 peaks are shown in B and C, respectively. (D) The c-axis lengths of FeSe_1−xTe_x films, where the x value indicated on the horizontal axis is the nominal Te content. (E) Relations between the a-axis and c-axis lengths in FeSe_1−xTe_x films. The colors and shapes of the symbols correspond to the Te content x, as shown. The dashed lines are guides for the eye. The data for x = 0 and 0.5 presented in A–E are cited from refs. 4, 9, and 14.In Fig. 1D, the c-axis lengths of 29 films of FeSe_1−xTe_x are plotted as a function of x. The values of the c-axis lengths vary almost linearly with the nominal Te contents of the targets in the whole range of x, including both end-member materials. The evident formation of a single phase and the systematic change in the c-axis length strongly indicate that the nominal Te content of the polycrystalline target is nearly identical to that of the final FeSe_1−xTe_x film. Note that the compositional analysis of grown films using scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis is impossible for FeSe_1−xTe_x films on CaF₂ substrates because the energies of the K edge of Ca and the L edge of Te are very close to each other. The above-mentioned features indicate that phase separation is suppressed in our FeSe_1−xTe_x films with x = 0.1 ? 0.4 on CaF₂ substrates. To our knowledge, this result is the first manifestation of the suppression of phase separation in FeSe_1−xTe_x with x = 0.1 ? 0.4.Fig. 1E presents the relations between the a-axis and c-axis lengths in films of FeSe_1−xTe_x. At first glance, there seem to be no relations between the a-axis length and x, in sharp contrast to the behavior of the c-axis length. The a-axis and c-axis lengths of films with the same x show a weak negative correlation. This behavior cannot be explained by a difference in Te content of a film, which should result in a positive correlation. By contrast, if variations in c are caused by a difference in in-plane lattice strain, this behavior can be explained in terms of the Poisson effect. Indeed, the a-axis lengths of films of FeSe and FeSe_0.5Te_0.5 are smaller than those of bulk samples with the same composition. Thus, we consider that the a-axis length predominantly depends on the in-plane lattice strain rather than the Te content x. One might think that this behavior looks strange, because the lattice constant of CaF₂(aCaF2/2) is longer than the a of FeSe_1−xTe_x, which usually leads to a tensile strain. In a previous paper, the penetration of F⁻ ions from the CaF₂ substrates into the films was proposed as a possible mechanism for nontrivial compressive strain in FeSe_1−xTe_x films on CaF₂ substrates (15). Because of the smaller ionic radius of F⁻ than that of Se²⁻, this peculiar compressive strain can be explained by the partial substitution of F⁻ for Se²⁻ near the interface between a film and a substrate.Fig. 2 A–D presents the temperature dependences of the electrical resistivities, ρ, of 16 films of FeSe_1−xTe_x for x = 0.1 ? 0.4. The value of T_c depends on the film thickness, even in films with the same x. The highest Tconset, which is defined as the temperature where the electrical resistivity deviates from the normal-state behavior, and the Tczero of the FeSe_1−xTe_x films are 13.2 K and 11.5 K, respectively, for x = 0.1; 22.8 K and 20.5 K, respectively, for x = 0.2; 20.9 K and 19.9 K, respectively, for x = 0.3; and 20.9 K and 20.0 K, respectively, for x = 0.4. Compared with the results for bulk samples, a drastic enhancement of T_c is observed in these FeSe_1−xTe_x films. Surprisingly, the values of Tczero in the films with x = 0.2 and 0.4 exceed 20 K. These values are larger than those reported for FeSe_0.5Te_0.5 films (8, 10, 11, 14). In particular, the T_c of the FeSe_0.8Te_0.2 film with a thickness of 73 nm is ∼1.5 times as high as those of bulk crystals of FeSe_1−xTe_x with the optimal composition, x ≈ 0.5 (7). Based on the measurement of the ρ of the FeSe_0.8Te_0.2 film under a magnetic field applied along the c axis, we estimate an upper critical field at 0 K of μ₀H_c2 = 55.4 T, using the Werthamer–Helfand–Hohenberg (WHH) theory (16), which yields a Ginzburg–Landau coherence length at 0 K of ξ_ab(0) ～ 24.4 Å (SI Appendix). This value of μ₀H_c2 is approximately half the value for an FeSe_0.5Te_0.5 film on CaF₂ with a T_c of ∼16 K (9).Open in a separate windowFig. 2.Temperature dependences of the electrical resistivities, ρ, of FeSe_1−xTe_x thin films for (A) x = 0.1, (B) x = 0.2, (C) x = 0.3, and (D) x = 0.4 with different film thicknesses. Insets present enlarged views of the plots near the superconducting transition.Using the data shown above, we present the phase diagram of FeSe_1−xTe_x films on CaF₂ substrates in Fig. 3. For comparison, the data for bulk samples of FeSe_1−xTe_x (7, 17) are also plotted in Fig. 3. In bulk crystals, the optimal Te content to achieve the highest T_c is considered to be x ≈ 0.5, and phase separation occurs in the region of 0.1 ≤ x ≤ 0.4 (7). However, our data clearly demonstrate that this phase separation is absent and that the optimal composition for an FeSe_1−xTe_x film on a CaF₂ substrate is not x ≈ 0.5 but x ≈ 0.2. It should be noted that the dependence of T_c on x suddenly changes at the boundary defined by 0.1 < x < 0.2. Unlike the “dome-shaped” phase diagram that is familiar in iron-based superconductors, the values of T_c in films with 0.2 ≤ x ≤ 1 increase with decreasing x, whereas the strong suppression of T_c is observed at 0.1 < x < 0.2. The behavior in films with x ≥ 0.2 can be explained by the empirical law that shows the relation between T_c and structural parameters. In iron-based superconductors, it is well accepted that the bond angle of (Pn, Ch)-Fe-(Pn, Ch) (Pn, ?Pnictogen; Ch, ?Chalcogen), α (18, 19), and/or the anion height from the iron plane, h (20), are the critical structural parameters that determine the value of T_c. In bulk samples of FeSe_1−xTe_x, α and h approach their optimal values, i.e., α = 109.47° (18, 19) and h = 1.38 Å (20), with decreasing x (down to x = 0), which should be the same in FeSe_1−xTe_x films. Therefore, the increase of T_c in films with 0.2 ≤ x ≤ 1 with decreasing x can be explained by the optimization of α and/or h based on the empirical law. However, the sudden suppression of T_c in films with 0 ≤ x < 0.2 is not consistent with this scenario, and its origin should be sought among other factors. We consider there are two candidates for this origin from the structural analysis of bulk samples of FeSe_1−xTe_x. One is the effect of the orthorhombic distortion. In a bulk sample of FeSe, a structural phase transition from tetragonal to orthorhombic occurs at 90 K (21). However, in bulk samples of FeSe_1−xTe_x with x ～ 0.4 ? 0.6 where T_cs take optimum values, there are papers with different conclusions on the presence/absence of a similar type of structural transition to that of FeSe (22–24). It should be noted that a structural transition temperature is lower and that the orthorhombicity is much smaller than those of FeSe even in the report where the structural transition is present (24). These results on crystal structures suggest that the orthorhombic distortion results in a suppression of T_c. This scenario is applicable to the behavior of our films, if a large orthorhombic distortion is observed only in films with x = 0 ? 0.1. The other candidate is the change in the distance between the layers of Fe-Ch tetrahedra, δ. As shown in SI Appendix, in polycrystalline samples of FeSe_1−xTe_x, the δ value of FeSe is much smaller than those of FeSe_1−xTe_x with x ≥ 0.5 where δ is nearly independent of x (22). We speculate that the decrease of δ in FeSe is related with the suppression of T_c. Indeed, in polycrystalline samples, FeSe exhibits smaller values of δ and T_c than does FeSe_0.5Te_0.5 (7, 22), and the intercalation of alkali metals and alkaline earths into FeSe results in the c-axis length as large as ∼20 Å and T_c as high as 45 K (25, 26). At this moment, the origin of the suppression of T_c at 0.1 < x < 0.2 is unclear. Regardless of its origin, we believe that it is reasonable to distinguish between superconductivity in x = 0 ? 0.1 and in x ≥ 0.2. In other words, our phase diagram in Fig. 3 provides a previously unidentified view for superconductivity in FeSe_1−xTe_x, that is, a discontinuity in superconductivity of FeSe_1−xTe_x. We are able to come to this picture only after the data for x = 0.1 ? 0.4 become available in this study. If we remove a cause for the suppression of T_c in x=0,0.1 in some way, a further increase in T_c can be expected because of the optimization of structural parameters.Open in a separate windowFig. 3.Dependence of T_c on x. The red and blue circles represent the Tconset and Tczero values of the FeSe_1−xTe_x thin films, respectively. The black triangles represent the Tconset values obtained in measurements of the magnetic susceptibility of bulk samples (7, 17). The dashed curve is a guide for the eye.In conclusion, we prepared high-quality epitaxial thin films of FeSe_1−xTe_x on CaF₂ substrates, using the pulsed laser deposition method. We successfully obtained FeSe_1−xTe_x films with 0.1 ≤ x ≤ 0.4, which has long been considered to be the “phase-separation region,” using a thermodynamically nonequilibrium growth of film deposition. From the results of electrical resistivity measurements, a complete phase diagram is presented in this system, in which the maximum value of T_c is as high as 23 K at x = 0.2. Surprisingly, a sudden suppression of T_c is observed at 0.1 < x < 0.2, whereas T_c increases with decreasing x for 0.2 ≤ x < 1. This behavior is different from that of the dome-shaped phase diagram that is familiar in iron-based superconductors.     

Influence of Ti3C2Tx MXene and Surface-Modified Ti3C2Tx MXene Addition on Microstructure and Mechanical Properties of Silicon Carbide Composites Sintered via Spark Plasma Sintering Method

This article presents new findings related to the problem of the introduction of MXene phases into the silicon carbide matrix. The addition of MXene phases, as shown by the latest research, can significantly improve the mechanical properties of silicon carbide, including fracture toughness. Low fracture toughness is one of the main disadvantages that significantly limit its use. As a part of the experiment, two series of composites were produced with the addition of 2D-Ti₃C₂T_x MXene and 2D-Ti₃C₂T_x surface-modified MXene with the use of the sol-gel method with a mixture of Y₂O₃/Al₂O₃ oxides. The composites were obtained with the powder metallurgy technique and sintered with the Spark Plasma Sintering method at 1900 °C. The effect adding MXene phases had on the mechanical properties and microstructure of the produced sinters was investigated. Moreover, the influence of the performed surface modification on changes in the properties of the produced composites was determined. The analysis of the obtained results showed that during sintering, the MXene phases oxidize with the formation of carbon flakes playing the role of reinforcement. The influence of the Y₂O₃/Al₂O₃ layer on the structure of carbon flakes and the higher quality of the interface was also demonstrated. This was reflected in the higher mechanical properties of composites with the addition of modified Ti₃C₂T_x. Composites with 1 wt.% addition of Ti₃C₂T_x M are characterized with a fracture toughness of 5 MPa × m^0.5, which is over 50% higher than in the case of the reference sample and over 15% higher than for the composite with 2.5 wt.% addition of Ti₃C₂T_x, which showed the highest fracture toughness in this series.     

Acid Solution Processed VO2-Based Composite Films with Enhanced Thermochromic Properties for Smart Windows

As a typical thermochromic material, VO₂ coatings can be applied to smart windows by modulating the transmission of near infrared (NIR) light via phase transition. However, the inherent undesirable luminous transmittance (T_lum) and solar modulation efficiency (ΔT_sol) of pure VO₂ impede its practical application. In order to solve this problem, the porous VO₂ based composite film was prepared by magnetron sputtering and subsequent acid solution process with Zn₂V₂O₇ particles used as a sacrificial template to create pores, which showed excellent T_lum (72.1%) and enhanced ΔT_sol (10.7%) compared with pure VO₂ film. It was demonstrated that the porous structure of the film caused by acid solution process could improve the T_lum obviously and the isolated VO₂ nanoparticles presented strong localized surface plasmon resonance (LSPR) effects to enhance the ΔT_sol. Therefore, this method will provide a facile way to prepare VO₂ based films with excellent thermochromic performance and thus promote the application of the VO₂ based films in smart windows.     

Red-Emitting Hybrid Based on Eu3+-dbm Complex Anchored on Silica Nanoparticles Surface by Carboxylic Acid for Biomarker Application

Luminescent organic-inorganic hybrids containing lanthanides (Ln³⁺) have been prominent for applications such as luminescent bio-probes in biological assays. In this sense, a luminescent hybrid based on dense silica (SiO₂) nanospheres decorated with Eu³⁺ β–diketonate complexes using dibenzoylmethane (Hdbm) as a luminescent antenna was developed by using a hierarchical organization in four steps: (i) anchoring of 3-aminopropyltriethoxysilane (APTES) organosilane on the SiO₂ surface, (ii) formation of a carboxylic acid ligand, (iii) coordination of Eu³⁺ to the carboxylate groups and (iv) coordination of dbm⁻ to Eu³⁺. The hybrid structure was elucidated through the correlation of thermogravimetry, silicon nuclear magnetic resonance and photoluminescence. Results indicate that the carboxylic acid-Eu³⁺-dbm hybrid was formed on the surface of the particles with no detectable changes on their size or shape after all the four steps (average size of 32 ± 7 nm). A surface charge of −27.8 mV was achieved for the hybrid, assuring a stable suspension in aqueous media. The Eu³⁺ complex provides intense red luminescence, characteristic of Eu^{3+ 5}D₀→⁷F_J electronic transitions, with an intrinsic emission quantum yield of 38%, even in an aqueous suspension. Therefore, the correlation of luminescence, structure, particle morphology and fluorescence microscopy images make the hybrid promising for application in bioimaging.     

Nanorod Self-Assembly in High Jc YBa2Cu3O7?x Films with Ru-Based Double Perovskites

Many second phase additions to YBa₂Cu₃O_7−x (YBCO) films, in particular those that self-assemble into aligned nanorod and nanoparticle structures, enhance performance in self and applied fields. Of particular interest for additions are Ba-containing perovskites that are compatible with YBCO. In this report, we discuss the addition of Ba₂YRuO₆ to bulk and thick-film YBCO. Sub-micron, randomly oriented particles of this phase were found to form around grain boundaries and within YBCO grains in bulk sintered pellets. Within the limits of EDS, no Ru substitution into the YBCO was observed. Thick YBCO films were grown by pulsed laser deposition from a target consisting of YBa₂Cu₃O_y with 5 and 2.5 mole percent additions of Ba₂YRuO₆ and Y₂O₃, respectively. Films with enhanced in-field performance contained aligned, self-assembled Ba₂YRuO₆ nanorods and strained Y₂O₃ nanoparticle layers. A 0.9 µm thick film was found to have a self-field critical current density (J_c) of 5.1 MA/cm² with minimum J_c(Θ, H=1T) of 0.75 MA/cm². Conversely, J_c characteristics were similar to YBCO films without additions when these secondary phases formed as large, disordered phases within the film. A 2.3 µm thick film with such a distribution of secondary phases was found to have reduced self-field J_c values of 3.4 MA/cm² at 75.5 K and J_c(min, Θ, 1T) of 0.4 MA/cm².     

The Dielectric Constant of Ba6−3x(Sm1−yNdy)8+2xTi18O54 (x = 2/3) Ceramics for Microwave Communication by Linear Regression Analysis

The electronics related to the fifth generation mobile communication technology (5G) are projected to possess significant market potential. High dielectric constant microwave ceramics used as filters and resonators in 5G have thus attracted great attention. The Ba_6−3x(Sm_1−yNd_y)_8+2xTi₁₈O₅₄ (x = 2/3) ceramic system has aroused people’s interest due to its underlying excellent microwave dielectric properties. In this paper, the relationships between the dielectric constant, Nd-doped content, sintering temperature and the density of Ba_6−3x(Sm_1−yNd_y)_8+2xTi₁₈O₅₄ (x = 2/3) ceramics were studied. The linear regression equation was established by statistical product and service solution (SPSS) data analysis software, and the factors affecting the dielectric constant have been analyzed by using the enter and stepwise methods, respectively. It is found that the model established by the stepwise method is practically significant with Y = −71.168 + 6.946x₁ + 25.799x₃, where Y, x₁ and x₃ represent the dielectric constant, Nd content and the density, respectively. According to this model, the influence of density on the dielectric constant is greater than that of Nd doping concentration. We bring the linear regression analysis method into the research field of microwave dielectric ceramics, hoping to provide an instructive for the optimization of ceramic technology.     

Influence of Amorphous Boron Grain Size,High Isostatic Pressure,Annealing Temperature,and Filling Density of Unreacted Material on Structure,Critical Parameters,n-Value,and Engineering Critical Current Density in MgB2 Wires

Our results show that a lower density of unreacted Mg + B material during an Mg solid-state synthesis reaction leads to a significant reduction in the quantity of the superconducting phase and lowers the homogeneity of the superconducting material. It also significantly reduces the irreversible magnetic field (B_irr), critical temperature (T_c), upper magnetic field (B_c2), engineered critical current density (J_ec), and n-value, despite high isostatic pressure (HIP) treatment and the use of nanoboron in the sample. Our measurements show that samples with large boron grains with an 8% higher density of unreacted Mg + B material allow better critical parameters to be achieved. Studies have shown that the density of unreacted material has little effect on B_irr, T_c, B_c2, J_ec, and the n-value for an Mg liquid-state synthesis reaction. The results show that the critical parameters during an Mg liquid-state synthesis reaction depend mainly on grain size. Nanoboron grains allow for the highest B_irr, T_c, B_c2, J_ec, and n-values. Scanning electron microscopy (SEM) images taken from the longitudinal sections of the wires show that the samples annealed under low isostatic pressure have a highly heterogeneous structure. High isostatic pressure heat treatment greatly improves the homogeneity of MgB₂.     

Coordination of {Mo142} Ring to La3+ Provides Elliptical {Mo134La10} Ring with a Variety of Coordination Modes

A 28-electron reduced C_2h-Mo-blue 34Ǻ outer ring diameter circular ring, [Mo₁₄₂O₄₂₉H₁₀(H₂O)₄₉(CH₃CO₂)₅(C₂H₅CO₂)]^30- (≡{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}) comprising eight carboxylate-coordinated (with disorder) {Mo₂} linkers and six defect pockets in two inner rings (four and three for each, respectively), reacts with La³⁺ in aqueous solutions at pH 3.5 to yield a 28-electron reduced elliptical C_i-Mo-blue ring of formula [Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]^10- (≡{Mo₁₃₄La₁₀}), isolated as the Na₁₀[Mo₁₃₄O₄₁₆H₂₀(H₂O)₄₆{La(H₂O)₅}₄{La(H₂O)₇}₄{LaCl₂(H₂O)₅}₂]·144 H₂O Na⁺ salt. The elliptical structure of {Mo₁₃₄La₁₀} showing 36 and 31 Å long and short axes for the outer ring diameters is attributed to four (A-D) modes of LaO₉/LaO₇Cl₂ tricapped-trigonal-prismatic coordination (TTP) geometries. Two different LaO₂(H₂O)₇ and one LaO₂(H₂O)₂Cl₂ TTP geometries (as A-C modes) for each of two inner rings result from the coordination of all three defect pockets of the inner ring for {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}, and two LaO₄(H₂O)₅ TTP geometries (as D mode) result from the displacement of two (acetate/propionate-coordinated) binuclear {Mo₂} linkers with La³⁺ in each inner ring. The isothermal titration calorimetry (ITC) of the ring modification from circle to ellipsoid, showing the endothermic reaction of [La³⁺]/[{Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)}] = 6/1 with ΔH = 22 kJ⋅mol^-1, ΔS = 172 J⋅K^-1⋅mol^-1, ΔG = −28 kJ⋅mol^-1, and K = 9.9 × 10⁴ M^-1 at 293 K, leads to the conclusion that the coordination of the defect pockets to La³⁺ precedes the replacement of the {Mo₂} linkers with La³⁺. ¹³⁹La- NMR spectrometry of the coordination of {Mo₁₄₂(CH₃CO₂)₅(C₂H₅CO₂)} ring to La³⁺ is also discussed.     

Fatigue Damage Evaluation of Short Carbon Fiber Reinforced Plastics Based on Thermoelastic Temperature Change and Second Harmonic Components of Thermal Signal

Short fiber reinforced plastics (SFRPs) have excellent moldability and productivity compared to continuous fiber composites. In this study, thermoelastic stress analysis (TSA) was applied to detect delamination defects in short carbon fiber reinforced plastics (SCFRPs). The thermoelastic temperature change ΔT_E, phase of thermal signal θ_E, and second harmonic temperature component ΔT_D were measured. In the fatigue test of SCFRP, it was confirmed that changes in ΔT_E, θ_E, and ΔT_D appeared in the damaged regions. A staircase-like stress level test for a SCFRP specimen was conducted to investigate the generation mechanism of the ΔT_D. The distortion of the temperature change appeared at the maximum tension stress of the sinusoidal load—and when the stress level decreased, the temperature change returned to the original sinusoidal waveform. ΔT_D changed according to the change in the maximum stress during the staircase-like stress level test, and a large value of ΔT_D was observed in the final ruptured region. A distortion of the temperature change and ΔT_D was considered to be caused by the change in stress sharing condition between the fiber and resin due to delamination damage. Therefore, ΔT_D can be applied to the detection of delamination defects and the evaluation of damage propagation.