Pinning Effect Research Articles

Ultralow dielectric loss in Tb + Ta‐modified TiO2 giant dielectric ceramics via designing defect chemistry

AbstractThe environment‐friendly materials exhibiting colossal permittivity play an increasingly important role in the electronics industry. In this work, (Tb0.5Ta0.5)xTi1−xO2 (x = 0, 0.005, 0.01, 0.02, and 0.04) ceramics with new defect clusters were fabricated to enhance the dielectric response. Significantly, all ceramic samples exhibit large dielectric constants (εr > 104), whereas the ceramic with x = 0.005 exhibits an ultralow loss (tan δ) of about 0.008 at room temperature and 1 kHz. The origins of its excellent dielectric properties were revealed by XRD, X‐ray photoelectron spectroscopy, and dielectric response analysis, which were mainly caused by defective dipoles associated with oxygen vacancies (). These defective dipoles limiting the long‐range hopping of electrons lead to the electron pinning effect. Additionally, the frequency spectrum under DC bias and conductivity behavior suggests that grain boundary and electrode effects also contribute to the dielectric constant. This study not only explores the dielectric response properties of a new giant dielectric (Tb0.5Ta0.5)xTi1−xO2 ceramics but also offers a candidate material suitable for ceramics capacitors.

Investigation of the hot deformation behavior and microstructure evolution of TiB2 +TiAl3/2024Al composite

In this study, the hybrid aluminum matrix composite reinforced with TiB2 and TiAl3 particles was fabricated by in situ synthesis. Hot compression tests were performed at a deformation temperature range of 300 − 450 ℃ and strain rates of 0.001 − 1 s−1 on a Gleeble-3500 system to investigate the hot deformation behavior of the fabricated composite. The experimental results show that the flow stress was sensitively dependent on the strain rate and temperature, and the peak stress level increased with the decrease of deformation temperature or the increase of strain rates. A sine-hyperbolic constitutive equation with the hot deformation activation energy of 244.844 kJ/mol was established to describe the flow stress of the composite. Further, the effects of TiB2 and TiAl3 particles on the flow stress levels of the composite under different hot deformation conditions were described by the developed strengthening model. It found that the predicting values of yield strength calculated by the established model agreed well with the experimental data, and the nano-sized TiB2 had a greater contribution to the stress level of the composite compared to the micro-sized TiAl3. Based on the microstructure characterization, dynamic recovery (DRV) was found to be the main flow softening mechanism during hot deformation and the evolution of dynamic recrystallization (DRX) was insufficient. Micro-sized TiAl3 particles and TiB2 particle clusters can promote DRX nucleation and growth of newly formed grains, meanwhile, separated nano-sized TiB2 particles had a pinning effect on the dislocation migration leading to substructure formation, which was in favor of the development of DRV.

Unveiling the self-annealing phenomenon and texture evolution in room-temperature-rolled Cu-Fe-P alloy sheets

The present study addresses the microstructural and texture evolution of Cu-Fe-P alloy sheets rolled at room temperature (RT). Cu-Fe-P specimens were subjected to 20, 40, 60, and 80% reduction ratios (RR) through room-temperature rolling (RTR). Nano-sized Fe2P precipitates were observed in the initial and deformed specimens. Continuous-rolling deformation causes strain hardening, whereas exposing severely deformed specimens to RT causes the self-annealing phenomenon. Deformed specimens to 60 and 80% RRs exhibited strain localizations (SLs) that formed only in Copper-type grains. In this study, a visco-plastic self-consistent (VPSC) polycrystal model was used to calculate the relative slip activity for major texture orientations. Copper-type grains showed the occurrence of predominant single-slip system which was the primary reason for the formation of SLs. Static recovery (SRV) and static recrystallization (SRX) were the main softening phenomena found in the RTR specimens. Both discontinuous SRX (DSRX) and continuous SRX (CSRX) were observed as the SRX phenomenon in RTR specimens. Major SRX grains were elongated in shape due to the combined effects of a higher orientation gradient (CSRX), to the larger SE differences between the matrix grains (restricted growth), and to a pinning effect by Fe2P particles. The as-received specimen showed weak plane-strain texture components at maxima around the S component ((123)<634>). As the RR increased, the volume fractions of Copper ((112)<111>), Brass ((110)<112>), and S components were likewise increased (except at 60% RR). The highest fraction of plane-strain components was observed for the RTR-80 specimen. The large number of SLs and the higher orientation gradient decreased the intensity of the plane-strain texture in the RTR-60 specimen, whereas a higher fraction of SRX in the RTR-80 specimen enhanced the plane-strain texture components. Time-dependent decay in the hardness values in the RTR specimens were attributed to SRV and SRX.

Spintronic Integrate-Fire-Reset Neuron with Stochasticity for Neuromorphic Computing.

Spintronics has been recently extended to neuromorphic computing because of its energy efficiency and scalability. However, a biorealistic spintronic neuron with probabilistic "spiking" and a spontaneous reset functionality has not been demonstrated yet. Here, we propose a biorealistic spintronic neuron device based on the heavy metal (HM)/ferromagnet (FM)/antiferromagnet (AFM) spin-orbit torque (SOT) heterostructure. The spintronic neuron can autoreset itself after firing due to the exchange bias of the AFM. The firing process is inherently stochastic because of the competition between the SOT and AFM pinning effects. We also implement a restricted Boltzmann machine (RBM) and stochastic integration multilayer perceptron (SI-MLP) using our proposed neuron. Despite the bit-width limitation, the proposed spintronic model can achieve an accuracy of 97.38% in pattern recognition, which is even higher than the baseline accuracy (96.47%). Our results offer a spintronic device solution to emulate biologically realistic spiking neurons.

Effect of Resin Pins on the Failure Behavior of Foam Core Sandwich Beams under Three-Point Loading.

Studies show limitations in increasing the mass fraction of resin pins in aviation components. The main objective of the present study is to investigate the effect of resin pins with different diameters and numbers on the bending performance and damage mode of sandwich structures with a constant mass fraction of resin pins. The obtained results show that the Φ2 specimen (i.e., the diameter of the resin pin in the sample is 2 mm) increases the load-bearing capacity by 41.8%. The deformation process of sandwich beams with and without resin pins is similar. However, the failure modes are different due to the diameters, amounts, and spacing of resin pins. Moreover, the uncertainty analysis of the experimental results is carried out by CT three-dimensional reconstruction of foam and finite element method (FEM). The obtained results show that the resin absorbed by the cells on the foam surface is the primary source of simulation errors. To resolve this problem, a new method is proposed to estimate the range of experimental results and improve the simulation accuracy by compensating for the diameter of the resin pin.

Size effect of local single-crystal-layered ultrathin nickel-based superalloy GH4145 strip

Austenitic nickel-based superalloys with very few grain layers or even a single-crystal layer in the thickness direction of the ultrathin strips can be achieved by controlling the cold rolling and subsequent annealing process, which leads to mechanical properties and deformation mechanisms that differ from those of macroscopic polycrystalline layered materials as a result of the size effect. In this study, nickel-based superalloy GH4145 with the thickness of 0.2 mm was used as the raw material. By controlling the cold rolling and annealing process, local single-crystal and multi-layer-crystal ultrathin strips with the thicknesses of 70 μm and 100 μm were successfully prepared. The effects of annealing on the microstructural evolution of the ultrathin strips with different grain layers on the deformation mechanism and room-temperature mechanical properties were studied. At annealing temperatures above 1000 °C, the γ′-Ni3 (Al, Ti) phase dissolved, and the main precipitates in the microstructure changed from γ′ and carbide at low-temperature annealing to only carbide. The weakened pinning effect on the grain-boundary effect and abnormal coarse grains owing to secondary recrystallization generated a local single-grain layer in the thickness direction of the strips. Furthermore, because the grain boundary decreases and the proportion of free surface increases, the abnormal decrease in elongation with increasing annealing temperature is called the size effect, which critically influences the mechanical properties of the strips. The ultrathin strips, which have a partially deformed microstructure and a large number of fine recrystallized grains, show better comprehensive mechanical performance.

An analytical model of grain growth considering the conjoint effects of precipitate pinning and solute drag in steel

ABSTRACT An analytical model is developed to explain the kinetics of grain growth in steel that takes into account all feasible prime phenomena; namely, surface energy reduction, precipitate pinning effect and solute drag effect. The shapes of matrix grains and the precipitates are assumed to be tetrakeidecahedron and spherical, respectively. A single solute and a single type of precipitate are further assumed to be responsible for the solute drag and the precipitate pinning, respectively. An explanation of drag energy is provided in terms of Gibbs free energy change for solute segregation that eventually merges to the well-known Langmuir–McLean relationship. The developed model is validated in view of the existing database in available literature. A new methodology of grain growth analysis is accordingly proposed on the basis of the conceptualised ‘effective migration coefficient’ of the system.

Analysis of the mechanism of chromatic aberration defects on A510L surface of high quality hot rolled pickled strip

In view of the chromatic aberration defects on the surface of hot-rolled strip A510L after pickling, the microscopic morphology features of different positions of the pickled strip surface were compared and analyzed by using laser confocal microscope (CLSM), and the generation mechanism of chromatic aberration defect was studied by using high-resolution scanning electron microscope and energy dispersive spectrometer (EDS). The results show that the main cause of surface chromatic aberration defects on the surface of pickling plate A510L is closely related to the interface state between hot-rolled raw material substrate and iron oxide scale, and there is an enrichment area of Fe2SiO4/FeO binary eutectic near the hot-rolled raw material substrate and iron oxide scale, and the proportion of Fe2SiO4 is higher as it approaches the strip steel matrix, and Fe2SiO4/FeO has a strong pinning effect, which leads to the adhesion of iron oxide scale. The existence of Fe2SiO4/FeO changes the structure of carbon steel iron oxide scale, which makes the bonding force between iron oxide scale and strip steel matrix increase the difficulty of pickling process, and finally presents the surface chromatic aberration defects after pickling.

Effects of large strain pre-deformation on creep age behavior and microstructural evolution of an Al-Cu-Li alloy

• The creep strain has been significantly increased by large strain pre-deformation. • The dislocation motion overcame the pinning effect of strengthening precipitates. • Part of the grain boundaries became serrated when increasing pre-deformation. The creep age behaviors and tensile properties of an Al-Cu-Li alloy pre-deformed by cold-rolling with different reductions were tested, combined with the characterization of their microstructural evolution during creep age tests. It is shown that increasing the pre-deformation can bring large number of dislocations into the lattice. These dislocations can detach from the pinning effect of strengthening precipitates under stress field. Thus, the creep strain is promoted by dislocation motion mechanism and the strength increases from 500.2 MPa to 567.8 MPa due to dislocation hardening effect. Further increasing total reduction to 75% makes creep strain increase from 0.14% to 0.24%, which is suggested to be a result of decreasing hindrance effect on dislocation movement due to severe serrations to sub-grain scale in grain boundaries.

Development of AA6061/316 stainless steel surface composites via friction stir processing: Effect of tool rotational speed

Metallic and alloy particles are recognized as alternatives to ceramic reinforcements to improve the overall characteristics of Al metal matrix composites. The present study aimed to investigate the friction stir processing/manufacturing of the 316 stainless steel particles reinforced AA6061 Al matrix composites by varying the tool rotational speeds. The microstructure, tensile, hardness, wear, corrosion behavior, and shear punching tests of the composites were studied. The results showed that the friction stir processing route produced no particle-matrix reaction in the composite and dislocation entanglements were formed close to the inherent 316 stainless steel particles within the Al matrix. The improved mechanical action of the tool aided particle fragmentation as the average 316 stainless steel particle sizes decreased from 41.88 to 26.89 μm. As a result of the dynamic recrystallization and particle-assisted pinning effects, the mean grain sizes of the composite decreased from 7.48 to 4.31 μm when the tool rotational speed was raised. An increment in the tool rotational speed (800–1200 rpm) triggered a rise in the maximum shear force (1198–1562 N), tensile strength (200.09–279.98 MPa), and maximum hardness value (121–139 HV) of the composite while a favorable decrease in the composite's wear rate (0.63–0.20 mg/m) and the friction coefficient (0.38 ± 0.01–0.21 ± 0.01) ensued. The composite's improvement is attributable to the homogeneous spread of dislocation.

Unexpected effect of pre-aging prior to extrusion on microstructure and mechanical properties of extruded Mg–8Sn–6Zn–0.1Mn alloy

Pre-aging prior to extrusion (AE), aiming to achieve high strength without sacrificing ductility, has shown considerable potential in Mg-Sn alloys containing low content of Zn element. For Mg–8Sn–6Zn–0.1Mn alloy containing high contents of Sn and Zn elements, however, an unexpected decrease in mechanical properties after AE treatment is observed and comprehensively investigated in this paper. We demonstrate that after pre-aging treatment, the size and volume fraction of bulk Mg2Sn particles are not decreased as expected, but some fine particles are segregated around the bulk Mg2Sn phases due to high contents of Sn and Zn in this alloy. The bulk phases are prone to facilitate the occurrence of particle-stimulated nucleation and continuous dynamic recrystallization mechanisms during extrusion. Since pre-aging treatments produce a high number of fine particles, these particles exert a pinning or drag effect on grain boundaries, inhibiting the nucleation of dynamic recrystallization, which reduce the volume fraction of dynamic recrystallized grains (DRXed). Unexpectedly, solution-extruded (SE) alloy has finer and higher volume fraction of DRXed grains than AE alloy. Additionally, the fine particles produced by dynamic precipitation in SE alloy are densely distributed and smaller than those in AE alloy. The coarse particles in AE alloy serve as the crack initiation sites under external force, and reduce the ductility of the alloy. Consequently, SE alloy with excellent strength-ductility synergy can be attributed to a combination of refinement of grain structure and fine densely distributed particles.

Novel approach for augmented carrier transfer and reduced Fermi level pinning effect in InAs surface quantum dots

The present work introduces an in-situ technique to tune the inter-dot coupling between vertically aligned InAs surface and buried quantum dots (SQD and BQD). Here, we utilize the self-assembly growth kinetics to get the most stable SQDs with lowest possible surface states. We vary the monolayer (ML) coverage of top SQD layer (from 2.2 ML to 1.6 ML), keeping monolayer coverage of BQD constant (2.7 ML). The reduced Fermi level pinning effect is observed through the monolayer coverage minimization, and the upshifting of energy level in the conduction band is obtained. The degeneracy between BQD and SQD energy levels is maximized in case of the 1.6 ML SQD and an enhanced coupling between the two QD families is observed through the photoluminescence (PL) result analysis. The type-II carrier distribution with effect of the Fermi level pinning is reduced in the 1.6 ML SQD structure, which claims the highest level of Coulomb coupling between the two QD families. Also, AFM images show lower dispersion in size and shape of the surface dot with 1.6 ML coverage. The proposed optimum QD heterostructure would promote a QD based sensor with high efficiency in effect with higher level of inter- dot carrier tunneling.

Statics and dynamics of skyrmions interacting with disorder and nanostructures

Magnetic skyrmions are topologically stable nanoscale particle-like objects that were discovered in 2009. Since that time, intense research interest has led to the identification of numerous compounds that support skyrmions over a range of conditions spanning cryogenic to room temperatures. Skyrmions can be set into motion under various types of driving, and the combination of their size, stability, and dynamics makes them ideal candidates for numerous applications. Skyrmions represent a new class of system in which the energy scales of the skyrmion-skyrmion interactions, sample disorder, temperature, and drive can compete. A growing body of work indicates that the static and dynamic states of skyrmions can be influenced strongly by pinning or disorder in the sample; thus, an understanding of such effects is essential for the eventual use of skyrmions in applications. In this article we review the current state of knowledge regarding individual skyrmions and skyrmion assemblies interacting with quenched disorder or pinning. We outline the microscopic mechanisms for skyrmion pinning, including the repulsive and attractive interactions that can arise from impurities, grain boundaries, or nanostructures. This is followed by descriptions of depinning phenomena, sliding states over disorder, the effect of pinning on the skyrmion Hall angle, the competition between thermal and pinning effects, the control of skyrmion motion using ordered potential landscapes such as one- or two-dimensional periodic asymmetric substrates, the creation of skyrmion diodes, and skyrmion ratchet effects. We highlight the distinctions arising from internal modes and the strong gyroscopic or Magnus forces that cause the dynamical states of skyrmions to differ from those of other systems with pinning. We also discuss future directions and open questions related to the pinning and dynamics in skyrmion systems.

Interface induced transition from Schottky-to-Ohmic contacts in single-walled carbon nanotube-based van der Waals Schottky heterostructures

The unique advantages of one-dimensional (1D) van der Waals (vdW) Schottky heterostructures in electronic devices have gradually been highlighted with the improvement of experimental technology in recent years. However, it is inevitable that metal and semiconductor contact will be in the 1D nanodevices, leading to many challenges in the formation of low-resistance Ohmic contacts due to the Fermi level pinning (FLP) effect. Herein, we employ density functional theory to explore a series of 1D armchair carbon nanotube (CNT)-based vdW Schottky heterostructures. By comparing three kinds of Schottky barrier heights (SBH) of S2-NT/CNT, S-NT/CNT, and Se-NT/CNT under different interface distances, we find that only S-NT/CNT can form an Ohmic contact when the interface distance is reduced to 3.01 Å. By analyzing the interface charge transfer, band alignment, and flexoelectric effect, we find that flexoelectric polarization can promote more charge transfer at the S-NT/CNT interface when interface distance becomes smaller, and energy band at the interface bends upwards and overlaps the Fermi level, resulting in the interface to change from a depletion region to an inversion layer and leading to the formation of p-type Ohmic contacts. The results achieved in this study provide a new avenue for the design of 1D vdW electronic devices.

Effects of Nb addition and heat treatment on the microstructure, mechanical property and internal friction behavior of FeCrAlMo cladding alloys

FeCrAl based alloy was a good candidate as an accident tolerant fuel (ATF) cladding material, this research investigated the influence of Nb addition and heat treatment on the evolution of dispersed Fe2Nb precipitates, the mechanical property and Internal Friction (IF) behavior of Nb containing Fe-13Cr-4.5Al-2Mo alloys. Fe-13Cr-4.5Al-2Mo-1.5Nb (wt%) annealed at 1000 °C exhibited the highest ultimate tensile strength (UTS) and an acceptable ductility at each tested temperature (room temperature, 300 °C, 500 °C or 800 °C). For example, the UTS of FeCrAlMo-1.5 wt% Nb at 800 °C was 95 MPa, 73% higher than that of Nb-free FeCrAlMo alloy at the same test temperature. Comparing with IF results of the cold-rolled Nb-free FeCrAlMo alloy in the first heating cycle, a high IF peak (∼P2) located at about 680 °C disappeared in the Nb containing FeCrAlMo alloy. Based on the optical micrographs analysis, the reason may be the pinning effect by the high density nanoscale Fe2Nb particles on dislocations, P2 peak would be expected to find at higher temperature; Likewise, the disappearance of P3 in the Nb containing FeCrAlMo alloys proved the pinning effect of Fe2Nb particles on grain boundaries. Based on the IF theory and microstructure characterization, the disappearance of P2 (recrystallization peak) and P3 (grain boundary peak) in the Nb containing FeCrAlMo alloys well proved that this pinning effect by high density Fe2Nb nanoscale particles was the main reason for improving the tensile strength of the Nb containing FeCrAlMo alloys.

Investigation of the Heat Treatment Process and Formation Mechanism of Grain Boundary Serration for GH4795 Superalloy

Heat treatments, including solution treatment and isothermal heat treatment, were conducted to investigate the grain boundary serration of GH4975 superalloy. The two different heat treatment processes could both promote the formation of serrated grain boundaries within the present temperature and soaking time ranges, provided that the cooling rates were controlled to be quite slow. The samples subjected to furnace cooling exhibited a more obvious serrated grain boundary morphology by comparison with those subjected to air cooling. The interaction between precipitated phases and grain boundaries was focused to explore the formation mechanisms of serrated grain boundaries within GH4975 superalloy. Heat treatment temperature and soaking time strongly affected the morphology and size of precipitated phases, and consequently influenced the formation of serrated grain boundaries. The directional growth of grain boundary precipitates and its pinning effects on the migration of grain boundaries also affected the grain boundary morphology.

Spirally Ag-coated short carbon fiber as a reinforcing filler for rigid polyurethane

High-performance materials often require high toughness and strength. Unfortunately, in most composites, toughness and strength are almost mutually exclusive. The simple combination of brittle carbon fibers (CFs) and rigid polyurethane (RPU) obviously cannot meet the standards of high-performance materials. In this paper, the spiral CFs (SS-CFs) were obtained by the electrodeposition. From the point of view of fracture mechanics, fracture resistance increases with crack propagation. This spiral structure provides more crack propagation paths, resulting in more energy consumption, which can lead to materials properties that are much higher than simple mixture. The results showed that the interfacial shear strength, tensile toughness, tensile strength, and impact toughness of SS-CFs/RPU composites were increased by 73.6%, 77.2%, 103.3% and 85.0%, respectively, compared with the pristine CFs/RPU composites. The main strengthening mechanisms of the materials were the pinning effect and the effective increase of the crack propagation pathway by the convex SS-CFs. At the same time, due to the deposition of the silver coating on the CF surface, the modified CFs composites also had excellent electrical conductivity and electromagnetic shielding performance. This means that our materials have a wide range of applications.

Secondary Recrystallization Texture and Magnetostriction in Fe-Ga Alloy Ultra-Thin Sheet

The 0.15 mm thick Fe-Ga alloy ultra-thin thin sheets with high magnetostriction were successfully produced by the conventional processing route including hot rolling and two-stage cold rolling with intermediate annealing and primary and secondary recrystallization annealing. The evolution of microstructure, texture, and inhibitor was briefly investigated. The primary recrystallization texture is characterized by strong <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula> fiber with the peak at {111} <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\langle 112\rangle $ </tex-math></inline-formula> component and weak Goss component through sheet thickness. The dispersedly distributed nano-precipitates provide a sufficient pinning effect on the primarily recrystallized grains and induce abnormal grain growth of Goss grains. Well-developed secondary recrystallized Goss grains with centimeter size and maximum magnetostriction (3/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\lambda \text{s}$ </tex-math></inline-formula> ) of 229 ppm were achieved in Fe-Ga alloy ultra-thin sheet.

Pinocembrin suppresses oxidized low-density lipoprotein-triggered NLRP3 inflammasome/GSDMD-mediated endothelial cell pyroptosis through an Nrf2-dependent signaling pathway

Pinocembrin (Pin) has been confirmed to exert anti-inflammatory and antiatherosclerotic effects. Here we have explored whether and how Pin would protect vascular endothelial cells against pyroptosis elicited by the exposure to oxidized low density lipoprotein (oxLDL). Our results showed that Pin preconditioning dose-dependently suppressed oxLDL-stimulated HUVEC injury and pyroptosis, which were manifested by improved cell viability, lower lactate dehydrogenase (LDH) levels and DNA damage as well as decreased expression of pyroptosis-related markers, such as NOD-like receptor pyrin domain-containing 3 (NLRP3), apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC), pro-Caspase-1, cleaved Caspase-1, N-terminus of Gasdermin D-N (GSDMD-N), pro-interleukins-1β (pro-IL-1β), IL-1β and inflammatory cytokines (IL-18 and IL-1β). All of the effects were similar to those of MCC950 (an NLRP3 inhibitor). As expected, Pin distinctly activated the Nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidative signaling pathway assessed through increased expressions of Nrf2, heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase 1 (NQO1). Furthermore, after transfection with small interfering RNA of Nrf2, the inhibitory effects of Pin on oxLDL-triggered NLRP3 inflammasome/GSDMD-mediated pyroptosis and oxidative stress in HUVECs were weakened. Additionally, Pin up-regulated Nrf2/HO-1 axis and down-regulated NLRP3 inflammasome/GSDMD-mediated pyroptosis signals in Apoe−/− mice fed with high fat diet. These results contribute to the understanding of the anti-pyroptosis mechanisms of Pin and provide a reference for future research on the anti-atherosclerotic effect of Pin.

Fabrication of ultra-fine grained Hf-based materials with superior hardness and temperature-independent electrical conductivity by a combination of high-energy ball milling and spark plasma sintering

Ultra-fine grained (UFG) hexagonal-close-packed (HCP) hafnium (Hf) and Hf-5 wt%Y2O3 (HYO) were prepared by a combination of high-energy ball milling and spark plasma sintering (SPS), and both possess high relative density (95–99%), high hardness, and low electrical conductivity. The HYO sample shows superior hardness of 12.11 GPa, which is about 6–7 times of that of coarse-grained Hf, and electrical conductivity of 2 × 105 S/m, which keeps constant and is almost independent of temperature. It is very possible for HYO with these good properties to become an excellent plasma torch cathode. The analysis of the microstructure under transmission electron microscope (TEM) and scanning electron microscope (SEM) shows that the superior hardness of the samples originates from the grain boundary (GB) strengthening and the pinning effects of the Y2O3 particles, while their almost temperature-independent electrical conductivity originates from the combining effects of the dispersion of large electrical resistivity particles Y2O3 and the dense high-angle grain boundaries (GBs).