Uniaxial Pressure Research Articles

Final‐stage densification kinetics of direct current–sintered ZrB2

AbstractFinal‐stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high‐purity ZrO2. Analysis was conducted on ZrB2 ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB2 ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB2. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10−6 cm2/s at 1900°C and 5.1 × 10−5 cm2/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final‐stage sintering of ZrB2 ceramics is dislocation motion.

Phase Stability, Microstructure, and Mechanical Properties of Spark Plasma Sintered Nanocrystalline Boron-Doped AlCoFeMnNi High-Entropy Alloy

The microstructure and mechanical properties of mechanically alloyed and spark plasma sintered AlCoFeMnNi-xB (x = 0, 0.5, 1, and 5 at. %) high-entropy alloys (HEAs) have been investigated. Boron-doped HEAs were synthesized using mechanical alloying up to 50 h of milling. Synthesized powders were then consolidated at 850, 900, and 950 °C for 10 min under a uniaxial pressure of 40 MPa using spark plasma sintering (SPS). A scanning electron microscope, which was equipped with energy dispersive spectroscopy (EDS), together with an optical microscope (OM) were used to analyze the microstructural evolution. X-ray diffraction analysis was used to differentiate the phases formed in the solution. The mechanical properties of the sintered specimens were analyzed using the shear-punch test (SPT). The fracture surface of the SPT samples was studied using SEM. Thermodynamic calculations revealed that by employing this process, it is possible to produce solid solution HEAs with a duplex FCC + BCC structure. It was shown that boron-doped AlCoFeMnNi high-entropy alloys contain some unique attributes. SPS at 900 °C for a sample with boron up to 0.5 at. % leads to the formation of an alloy with the highest shear strength. A further increase in the boron content in the boron-doped HEAs exhibited a decrease in the maximum shear strength. Finally, the correlations between the microstructural and mechanical characteristics of the sintered boron-containing high-entropy alloys are discussed.

Microstructure, domain structure, ferroelectric and piezoelectric properties of textured bismuth-containing ceramics

In this report, the processes of texture formation in grain-oriented ferroelectric ceramics based on layer-structured ferroelectric Bi4Ti3O2 (LSBT) prepared by hot forging method are considered. The microstructural and X-ray methods revealed the axial textured formation in ferroelectric ceramic that are used to estimate the orientation factor of ceramics. For the first time, the domain structure changes when poling the anisotropic ferroelectric ceramics are investigated. The anisotropy of electromechanical, piezoelectric and ferroelectric properties of ferroelectric ceramics due to the crystal texture existence in it is studied. The aim of this study is to study the processes of crystalline texture formation in polycrystalline BLSF and to establish the dependence of the electrophysical properties of ceramics on the degree of texturing. Ceramics were textured using the hot stamping (HS) method developed at the Research Institute of Physics. The mechanism of the method is that the workpiece is subjected to uniaxial pressure and free radial deformation occurs due to the plastic flow of the material until the workpiece fills the free volume of the mold, which is created by placing the workpiece in the mold with a gap. The study of the microstructure of ceramics showed that an increase in the firing temperature in the range 950–1050[Formula: see text]C causes a sharp decrease in porosity and increases the density to 7.95 g/[Formula: see text], which is 98% of theoretical. An X-ray analysis was performed and microstructural studies were carried out, which revealed the formation of an axial texture in ceramics. The features of the switching processes of textured ceramics are revealed. The characteristics of the polarization switching of ceramics in the directions parallel and perpendicular ([Formula: see text]) of the pressure axis during hot processing were obtained from the dielectric hysteresis [Formula: see text]([Formula: see text]) loops, i.e., axis axial texture. The [Formula: see text]-cut ceramics are characterized by a more complete polarization switching, which is associated with the additional orientation of the (001) crystallographic planes in the textured material, as well as the presence of a threshold switching field. In the temperature range from -196 to + 600[Formula: see text]C, the anisotropy of the electro physical properties of ceramics due to the presence of a crystalline texture in it was studied. The dielectric constant, electrical conductivity, piezoelectric and elastic coefficients were measured for sections of ceramics of different orientations relative to the axis of the texture. The anisotropy of the dielectric constant and electrical conductivity manifests itself weakly at room temperature and increases sharply when approaching the Curie temperature. In the temperature range +20–400[Formula: see text]C, the high thermal stability of the piezoelectric module [Formula: see text], measured by the quasistatic method, was established.

Cold Sintering Process for a BaTiO3/Poly(vinylidene difluoride) Ceramic-Polymer Composite: Evaluation of the Structural and Microwave Dielectric Properties.

The sintering process is essential to transforming particular material into dense ceramics. Although several sintering techniques have emerged over the past few years, the process is still performed at high temperatures. The alternative cold sintering process (CSP) is a potential strategy for producing an advance high dielectric permittivity and enables densification at low temperature. In this process, the CSP technique has been applied to successfully prepare the BaTiO3/poly(vinylidene difluoride) (PVDF) nanocomposite. The inorganic material of the BaTiO3/PVDF nanocomposite was confirmed by various physical characterizations, and the densification studies were performed using a semiautomated press, indicating a dissolution-precipitation mechanism. When a uniaxial pressure of 350 MPa was applied, with transient liquid could be sintered at 190 °C, and a relative density of 94.8%. The nanocomposite exhibits an excellent dielectric properties like εr = 71.1 and tan δ = 0.04 with frequency range at 1 GHz for various dwelling times and maximize the electrical resistivity. The BaTiO3/PVDF composite indicating a breakthrough opportunity toward an increase of the high dielectric constant will be significantly impacted by cold sintering. It enables innovative materials design and integrated devices to advance modern electronic industry applications.

Enhanced thermoelectric properties of bismuth telluride (Bi2Te3) and multiwall carbon nanotube (MWCNT) composites

Solid solutions based on bismuth telluride are the traditional thermoelectric materials that are commercially utilized in the industry close to room temperature applications, where energy scavenging is a big challenge due to low quality and energy density. However, their broader applications are still limited due to low efficiency. The focus of the present work is to enhance the energy conversion efficiency of Bi2Te3 by mixing one-dimensional (1D) multiwall carbon nanotubes (MWCNTs) via facile powder processing by synergistically utilizing the nanostructuring and quantum confinement effects. Thus, different vol% of MWCNTs ranging from 0.5, 1.0, 1.5, and 2 were uniformly dispersed in a fine powder of Bi2Te3 that was ball milled from course Bi2Te3 in an inert environment. The coarse Bi2Te3 and fine composite powders were consolidated in a spark plasma-sintering furnace at ∼400 °C under uniaxial pressure of ∼40 MPa. Thermoelectric properties of Bi2Te3-based MWCNTs composites were evaluated in the temperature range from ∼300 to ∼525 K. The electrical conductivity of 1.5 vol% MWCNT composite has significantly enhanced from pristine Bi2Te3. This increase indicates that electrical transport dominantly involves a percolating network of MWCNTs. The concomitant increase in the Seebeck coefficient of 1.5 vol% Bi2Te3 composite suggests the enhanced energy-dependent scattering of charge carriers. Consequently, a substantially improved power factor is observed for the composite. The thermal conductivity of all the composites decreases, and considerable reduction is achieved for 0.5 vol% due to enhanced phonon scattering through nanostructuring and uniform distribution of the nanotubes. Hence, the significant decrease in thermal conductivity with improved power factor leads to substantial improvement in the thermoelectric figure of merit of 1.5 vol% Bi2Te3 composite from pristine Bi2Te3.

Tensoelectric properties irradiated by the fast electrons n-Ge single crystals

Tensoelectric properties of the uniaxially deformed along the crystallographic direction [100] n-Ge single crystals, irradiated by the flows of electrons , , and , with the energy of , were investigated. Mechanisms of changes in resistivity and mobility of current carriers under the uniaxial deformation for the studied n-Ge single crystals based on the measurements of tenso-Hall effect and infrared Fourier spectroscopy were established.The presence of tensoresistance for n-Ge single crystals, irradiated by the flow of the electrons , is associated with changes under the uniaxial pressure of electron concentration in the conduction band and holes in the valence band due to a decrease of the ionization energy of the deep level of the radiation defects which belongs to the complexes , at the uniaxial deformation. A significant increase of the effective mobility of carriers current in uniaxially deformed single crystals n-Ge is primarily related to the deforming redistribution of electrons between the branch of ‘light’ holes and the level . For single crystals n-Ge irradiated by the flows of electrons , which lead to the n-p conversion of the conductivity type of germanium, significant tensoresistance is observed only at the pressures . In this case, the resistivity of n-Ge decreases both due to an increase in the concentration of holes and their mobility under the deformation. Mobility of holes does not depend on deformation for uniaxial pressures , and a slight decrease in the resistivity of the researched single crystals n-Ge is caused by an increase in their concentration. Radiation-induced tensosensitivity at the room temperature for single crystals n-Ge can be used in technologies for creating uniaxial pressure sensors based on germanium. Obtained radiation effect of a significant increase in the mobility of current carriers for the uniaxially deformed n-Ge single crystals can find its practical application in the creation of ‘germanium’ channels of n-MOSFET and p-MOSFET transistors.

Seebeck coefficient of two-dimensional Dirac electrons in an organic conductor under pressure

The Seebeck coefficient, which is proportional to a ratio of the thermoelectric conductivity to electrical conductivity has been examined for Dirac electrons in the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF denotes a molecule given by bis(ethylenedithio)tetrathiafulvalene] under a uniaxial pressure using a two-dimensional tight-binding model with both impurity and electron--phonon (e--p) scatterings. We calculate an anomalous temperature ($T$) dependence of the Seebeck coefficient $S_{\nu}$ with $\nu = x$ (perpendicular to the molecular stacking axis) and $y$, which shows $S_\nu > 0$ with a maximum at high temperatures and $S_{\nu} < 0$ with a minimum at low temperatures.The microscopic mechanism of such a sign change of $S_\nu$ is clarified in terms of the spectral conductivity. The result is compared with experiments on $\alpha$-(BEDT-TTF)$_2$I$_3$.

The Influence of Preparation Temperature on the Different Facets of Bulk MgB2 Superconductors.

Two MgB2 samples were prepared using the spark plasma sintering (SPS) technique at different temperatures-950 °C (S1) and 975 °C (S2)-for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB2 samples. We analyzed the superconducting properties of the PeF and PaF of two MgB2 samples prepared at different temperatures from the curves of the critical temperature (TC), the curves of critical current density (JC), the microstructures of MgB2 samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, Tc,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher JC compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h0 and Kn of the PeF were lower than those of the PaF, except for Kn of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (JC) was 503 kA/cm2 self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the JC of MgB2. However, in high field, S2-PeF had the highest JC value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher JC.

Fabric-like Electrospun PVAc-Graphene Nanofiber Webs as Wearable and Degradable Piezocapacitive Sensors.

Flexible piezocapacitive sensors utilizing nanomaterial-polymer composite-based nanofibrous membranes offer an attractive alternative to more traditional piezoelectric and piezoresistive wearable sensors owing to their ultralow powered nature, fast response, low hysteresis, and insensitivity to temperature change. In this work, we propose a facile method of fabricating electrospun graphene-dispersed PVAc nanofibrous membrane-based piezocapacitive sensors for applications in IoT-enabled wearables and human physiological function monitoring. A series of electrical and material characterization experiments were conducted on both the pristine and graphene-dispersed PVAc nanofibers to understand the effect of graphene addition on nanofiber morphology, dielectric response, and pressure sensing performance. Dynamic uniaxial pressure sensing performance evaluation tests were conducted on the pristine and graphene-loaded PVAc nanofibrous membrane-based sensors for understanding the effect of two-dimensional (2D) nanofiller addition on pressure sensing performance. A marked increase in the dielectric constant and pressure sensing performance was observed for graphene-loaded spin coated membrane and nanofiber webs respectively, and subsequently the micro dipole formation model was invoked to explain the nanofiller-induced dielectric constant enhancement. The robustness and reliability of the sensor have been underscored by conducting accelerated lifetime assessment experiments entailing at least 3000 cycles of periodic tactile force loading. A series of tests involving human physiological parameter monitoring were conducted to underscore the applicability of the proposed sensor for IoT-enabled personalized health care, soft robotics, and next-generation prosthetic devices. Finally, the easy degradability of the sensing elements is demonstrated to emphasize their suitability for transient electronics applications.

Boosting the Ceramics with In Situ MOF-Derived Nanocarbons

Metal–organic framework (MOF)-derived nanocarbons have emerged as promising materials for energy and environmental applications owing to their high surface area, structural and chemical tunability, and hierarchical porosity. Although various carbon-based materials such as graphene and carbon nanotubes have been extensively used as secondary sintering additives to develop advanced ceramics with improved mechanical, thermal, and electrical properties, the potential of MOF-derived nanocarbon-based materials has not been explored. Here, we report the first use of MOF-derived nanocarbons as a reinforcement phase in ceramic composites. To this end, Al2O3 and zeolitic imidazolate framework (ZIF-8) are used as the ceramic matrix and nanocarbon source, respectively. The ceramic composites are produced by densifying Al2O3 and ZIF-8 powder mixtures using spark plasma sintering (SPS) at 1550 °C and uniaxial pressure of 50 MPa. The fracture toughness of the composite increases up to 67% in comparison to an alumina monolith as ZIF-derived nanocarbons form interlayers to assist the dissipation of energy during the crack propagation and inhibit grain growth. The room-temperature electrical conductivity of the sintered samples drastically increases with the in situ formed nanocarbon-based fillers, reaching as high as 1410 S/m for 10 wt % ZIF-8 content. These results constitute an excellent initial step toward boosting the mechanical and electrical properties of ceramic matrix composites with in situ MOF-derived nanocarbons.

Effect of pressure on the scattering of electrons by phonons in niobium triselenide at low temperatures

The paper analyzed the data on the temperature dependence in the region 4.2-300 K of the electrical resistance along the direction of the highest conductivity of the NbSe3 single crystal at different values of the uniaxial pressure perpendicular to the indicated direction. This pressure suppresses the charge density wave states without shifting their temperatures. In regions of metallic conductivity, the resistance can be approximated within the framework of the Bloch- Gruneisen-Wilson model. The Debye temperature decreases due to the size effect, which is stimulated by both cooling and pressure.

Analysis of long-term deformations of the ultimately stressed samples of rocks

The results of ongoing laboratory observations of long-term plastic deformations of an extremely stressed sample of rocks, which have been carried out from September 2009 to September 2022, are presented. A sample in the form of a cube with sides of 7 cm, made in the form of a cement-sand conglomerate, is subjected to a constant uniaxial pressure of 7 MPa. With the use of rubber suspension dampers, the press installation was isolated from the influence of microseisms and other mechanical vibrations from external sources. The laboratory ensured regular temperature stabilization of the environment within daily temperatures of the order of 1°C, and hermetic conditions were observed with minimal influence of humidity. For the measurement, a special method was used to register impulses of deformation jumps, which are converted into electromagnetic signals by virtue of a highly sensitive installation. The trend components of the creep rate, its annual periodicities and four stages, reflecting the features of changes in the state of the sample, are identified. It is shown that the magnitude of the large-scale, positive trend over the entire observation period is at least an order of magnitude greater than the standard one. At a qualitative level, a correlation was found between the amplitudes of diurnal variations in the creep rate and the distinguished stages of the ultimate deformed state. Estimates are made for the seasonal temperature influence of the surrounding laboratory environment on the annual periodicity of the creep in comparison with seasonal changes in atmospheric pressure.

Strain-induced massless Dirac fermion state of the molecular conductor α-(BEDT-TTF)2I3

Uniaxial pressure can dramatically change the properties of low-dimensional systems and induce electronic phase transitions. The first-discovered molecular massless Dirac electron system α-(BEDT-TTF)2I3 has intensively been studied because of its unique features, such as the Fermi energy being very close to the Dirac point, strong electron correlation, and the quantum phase transition from the charge-ordered insulating state. However, the Dirac state is realized only under high pressure of 15 kbar, which limits the measurement of physical properties (e.g., experimental determination of band dispersion and density of states). Here, we demonstrate that the Dirac state of α-(BEDT-TTF)2I3 can be realized by applying uniaxial bending strain without confining it in a pressure cell. Uniaxial strain below 1% completely suppresses the metal–insulator transition observed at ambient pressure. Under strain, a characteristic temperature dependence of magnetoresistance associated with the formation of the n = 0 Landau level is observed, indicating the realization of the massless Dirac state.

Effects of dispersoid preforming via multistep sintering of oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy

• A newly designed ODS manufacturing process, multi-step sintering process, consisting of a dispersoid preforming heat treatment followed by sintering to achieve the dispersoid refinement in HEA matrix was suggested. • An atypical form of Y 2 O 3 , FCC-Y 2 O 3 , formed adjacent to the dislocation inside the matrix grain during the dispersoid preforming heat treatment of multi-step sintering process with a cube-on-cube orientation relationship with the matrix. • The FCC-Y 2 O 3 formed in the multi-step sintered bulk acts as an effective obstacle to grain boundary and dislocation movement resulting in a microstructure refinement and the increment of mechanical strength. • In the bulk prepared by the conventional ODS manufacturing process, coarsened dispersoids consisting of a complex structured oxide with BCC-Y 2 O 3 as the core were predominantly formed along the grain boundary. • Cr-C-O region with a crystal structure of Cr 23 C 6 formed in the HEA matrix serves as an oxygen-supplying reservoir for dispersoids, promoting coarsening of the dispersoids. Dispersoid formation and microstructural evolution in an oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy (HEA) using a newly designed multistep sintering process are investigated. The proposed multistep sintering consists of a dispersoid preforming heat treatment of as-milled 0.1wt% Y 2 O 3 -CoCrFeMnNi high-entropy alloy powders at 800°C, followed by sintering at 800–1000°C under uniaxial pressure. In the conventional single-step sintered bulk, the coarsened BCC Y 2 O 3 dispersoids mainly form with an incoherent interface with the HEA matrix. In contrast, finer FCC Y 2 O 3 dispersoids, an atypical form of Y 2 O 3 , are formed in the matrix region after multistep sintering. Nucleation of FCC Y 2 O 3 dispersoids is initiated on the favorable facet, the {111} plane of the austenitic matrix, with the formation of a semi-coherent interface with the matrix during the dispersoid preforming heat treatment and it maintains its refined size even after sintering. It is found that dispersoid preforming prior to sintering appears promising to control the finer dispersoid formation and refined grain structure.

Densification of Two Forms of Nanostructured TATB under Uniaxial Die Pressures: A USAXS-SAXS Study.

Sequential ultra-small-angle and small-angle and X-ray scattering (USAXS and SAXS) measurements of hierarchical microstructure of a common energetic material, the high explosive 2,4,6-Triamino-1,3,5-trinitrobenzene (TATB), were performed to follow the microstructure evolution upon applied pressure. The pellets were prepared by two different routes-die pressed from a nanoparticle form and a nano-network form of TATB powder. The derived structural parameters, such as void size, porosity, and the interface area, reflected the response of TATB under compaction. Three populations of voids were observed in the probed q range from 0.007 to 7 nm-1. The inter-granular voids with size larger than 50 nm were sensitive to low pressures and had a smooth interface with the TATB matrix. The inter-granular voids with size of ~10 nm exhibited a less volume-filling ratio at high pressures (>15 kN) as indicated by a decrease of the volume fractal exponent. The response of these structural parameters to external pressures implied that the main densification mechanisms under die compaction were the flow, fracture, and plastic deformation of the TATB granules. Compared to the nanoparticle TATB, the applied pressure strongly influenced the nano-network TATB due to its more uniform structure. The findings and research methods of this work provide insights into the structural evolution of TATB during densification.

Cyclic bond deterioration of deformed steel bar in well-confined concrete

The cyclic bond deterioration mechanism of deformed steel bars embedded in concrete is critical for establishing the hysteretic bond stress-slip relationship. For a deformed steel bar, well-confined in concrete, the bond failure model under monotonic loading is governed by pullout, the bond deterioration trend under cyclic loading is remains unclear, especially in the case of applied lateral pressure. In this paper, the effect of uniaxial lateral pressure on the bond performance under different loading histories was studied. The test results indicate that the variation in bond parameters under varied lateral pressure is negligible. Therefore, the monotonic and reversed bond stress-slip constitutive relationship of a deformed bar failed by pullout, without lateral pressure confinement, can be used for simulating the bond response under different uniaxial lateral pressures. Cubic concrete specimens with small openings were used to quantitatively analyze the failure process under reversed loading through digital imaging correlation (DIC). The bond stress was observed to be nearly uniformly distributed along the interface at the initial loading stage. After increasing the pull-out or push-in loading to 75 %–85 % of the peak bond strength, we observed minute shear cracks between the interfaces. Subsequently, the length and width of the cracks were then gradually developed with the controlling slip and number of cycles, leading to reduced mechanical interaction and friction between the shear cracks.

Upper critical field of Sr2RuO4 under in-plane uniaxial pressure

In-plane uniaxial pressure has been shown to strongly tune the superconducting state of Sr2RuO4 by approaching a Lifshitz transition and associated Van Hove singularity (VHS) in the density of states. At the VHS, $T_c$ and the in- and out-of-plane upper critical fields are all strongly enhanced, and the latter has changed its curvature as a function of temperature from convex to concave. However, due to strain inhomogeneity it has not been possible so far to determine how the upper critical fields change with strain. Here, we show the strain dependence of both upper critical fields, which was achieved due to an improved sample preparation. We find that the in-plane upper critical field is mostly linear in $T_c$. On the other hand, the out-of-plane upper critical field varies with a higher power in $T_c$, and peaks strongly at the VHS. The strong increase in magnitude and the change in form of $H_\mathrm{c2||c}$ occur very close to the Van Hove strain, and points to a strong enhancement of both the density of states and the gap magnitude at the Lifshitz transition.

Spark plasma sintering of translucent polycrystalline α/β‐silicon nitride ceramics with Y2O3 additive

Translucent polycrystalline α/β‐silicon nitride ceramics had been fabricated for the first time by spark plasma sintering from α-Si3N4 starting powder with 2 wt% Y2O3. It aimed to investigate the influence of process parameters on the phase evolution, microstructural development, and mechanical properties of sintered ceramics. The increase in sintering temperature from 1750 °C to 1800 °C under 25 MPa considerably raised relative density, promoted α-β phase transformation, increased hardness and fracture toughness values, and exhibited translucency in the visible region. Furthermore, the highest relative density (98%), hardness (∼HV: 14.9 GPa), fracture toughness (∼9.8 MPa m1/2) values, and a higher degree of translucency were achieved with the increment of uniaxial pressure from 25 to 50 MPa at 1750 °C. These results indicated that translucent polycrystalline α/β‐silicon nitride ceramics, produced by liquid phase assisted spark plasma sintering at 1750 °C under 50 MPa, can be used as transparent armors and visible windows under harsh conditions.

Rhombic skyrmion lattice coupled with orthorhombic structural distortion in EuAl4

The centrosymmetric tetragonal itinerant magnet EuAl$_{4}$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl$_{4}$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim$0.10% and $\sim$0.03% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in EuAl$_{4}$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

The effect of intermediate layer densification on the critical current of a Bi-2223 superconducting joint

The effect of intermediate layer densification on the critical current (I c) of Bi-2223 superconducting joints was quantitatively studied. First, we evaluated the phase purity, density, and intergrain critical current density (J c) of Bi-2223 thick film samples simulating the intermediate layer. The samples were uniaxially pressed to increase the film density. After two heat treatments of the pressed film, an increase in J c was achieved. Second, we fabricated superconducting joints by synthesizing an intermediate layer between two Bi-2223 tapes. Applying a uniform uniaxial pressure on the joint resulted in the formation of a homogeneous structure. This process enables the reproducible fabrication of superconducting joints with high n values. The I c of the superconducting joint was increased by intermediate pressing (IP) and two heat treatments. However, pressing at high pressures can mechanically damage filaments in the Bi-2223 tapes, leading to a decrease in I c. Sample characterization showed that the optimum IP pressure range to produce high I c was 1.5–2 × 108 Pa. We confirmed that pressing densified the intermediate layer of the superconducting joints. Our experimental results and analyses reveal that densification of the intermediate layer increases the I c of Bi-2223 superconducting joints.