Grain Boundary Resistance Research Articles

Grain-boundary migration induced abnormally increased flow stress under large strain in a high-temperature compressed Ti-22Al-25Nb alloy

• Impeded grain boundary (GB) slip results in visibly increased flow stress. • The GB slip mechanism was impeded by zigzag GBs. • The zigzag GB result from the GB migration. • Energy difference between <001> and <111> fiber textures leads GB migration. • DDRX is a concomitant phenomenon with increased flow stress. The flow stress of Ti-22Al-25Nb alloy abnormally increased during uniaxial compression at large strains in the B2 phase field. This phenomenon is dependent on the influence of oriented grain boundary (GB) migration on GB sliding. In the early stages of deformation, straight and smooth GBs gradually evolved facilitating GB sliding, and the <001>/<111> double fiber texture has formed. There is a stored energy difference between <001> and <111> grains due to their different Taylor factors. Under low strain rates, the energy difference between the two fibers will drive GB to migrate, making the GBs zigzag and impeding GB sliding. Thus, greater stress is required to achieve further deformation, namely, the flow stress increased. The dislocation density in the shear band near the GB rapidly increased. Although the current alloy has a high stacking fault energy (SFE), the high dislocation density is sufficient to cause discontinuous dynamic recrystallization (DDRX).

A correlation between microstructural and impedance properties of MnFe2-xCoxO4 nanoparticles

A comprehensive study of physical properties and impedance analysis of MnFe2-xCoxO4 (x = 0.00, 0.40, 0.80, 1.20, 1.60, 2.00) nanoparticles prepared by citrate-nitrate method has been carried out. Structural analysis by Rietveld refinement indicated the formation of pure cubic structure with the space group Fd3‾m. The chemical composition and approximate grain size of the samples were determined by energy dispersive X-ray spectroscopy and field-emission scanning electron microscopy, respectively. The presence of Co has a strong influence on space charge modulation and affect the grain-boundary resistance. The results showed that sample x = 1.20 has less mobility, less polarizability, and therefore lower dielectric constant which is preferred for high frequency or power applications to minimize electric power loss. Impedance analysis demonstrate that small polaron hopping are responsible for the short-range translation hopping in conduction process of the samples (except for x = 1.60).

Surface and interfacial conduction using gadolinium-doped ceria electrolyte for advanced low temperature 400–500 °C fuel cell

Lowering the operating temperature with enhanced performance and better efficiency is considered a universal R&D bottleneck for the progress of low-temperature (<520 °C) solid oxide fuel cells (SOFCs) or ceramic fuel cells (CFCs) to avail the opportunities of commercialization. Bulk doping is the primary ionic conduction mechanism in traditional electrolyte materials for CFC, mainly influenced by the operating temperature and bulk density, thus resulting in high operational temperature. Herein, we proposed a new mechanism for tunning Ce0.9Gd0.1O2-δ (GDC) electrolyte based on a nanocrystalline structure from surface or grain boundary conduction to interfacial conduction by introducing the carbonates to form the heterostructure, exhibiting an extremely high ionic conductivity of (0.2 S·cm−1) at 520 °C. At first, we noticed that the grain boundary conduction of GDC is causing to enhance the charge carrier leading to establishing surface conduction. We further modify the GDC with carbonates to form the core–shell heterostructure to boost ionic conduction. The constructed cell with core–shell heterostructure has exhibited a remarkable performance of 968 mW/cm2.In contrast, 578 mW/cm2 was achieved from the surface conduction of GDC, and at the same time, we observed immensely low grain-boundary resistance from the impedance analysis. The mechanism of core–shell structure formation has been discussed in detail, and the ionic conduction at the surface and interfaces can be considered the dominant conduction mechanism for GDC electrolytes. This work has explored a new approach by tuning the ionic property from surface to interface to develop advanced CFCs, significantly improving the state-of-art GDC electrolyte via enhancing surface/interface conductions. This may be developed as a more functional approach for advanced energy materials, e.g., battery and electrolysis devices.

Effects of grain boundary characteristics changing with cold rolling deformation on intergranular corrosion resistance of 443 ultra-pure ferritic stainless steel

The relationship between evolution in grain boundary characteristics and intergranular corrosion properties of cold rolled 443 ultra-pure ferritic stainless steel was investigated. The results showed that low-angle (LA) grain boundaries, special grain boundaries (SGB), and high-angle (HA) grain boundaries had a high intergranular corrosion sensitivity after cold deformation in reducing acid medium. In oxidative acid medium, the LA and SGB boundaries had a high intergranular corrosion resistance in comparison to HA boundaries, and their inhibitory effect on the corrosion continuance of the dynamic recrystallization high-angle (RHA) grain boundaries can improve the corrosion resistance of the RHA grain boundaries.

Colossal dielectric response and complex impedance analysis of LaFeO3 ceramics

The present investigations mainly focused on the colossal dielectric response and complex impedance analysis of LaFeO3 ceramics. The studied sample was prepared by a citrate gel method. Structural and microstructural properties are analyzed from the XRD pattern and SEM micrograph. The anomalies in the dielectric constant versus temperature plots are analyzed on the basis of polarization induced by the Maxwell-Wagner mechanisms and ferromagnetic interaction between the Fe[Formula: see text] ions driven by the oxygen vacancy mediated Fe[Formula: see text]–V[Formula: see text] –Fe[Formula: see text] exchange interaction A giant dielectric permittivity in the order of [Formula: see text]105 was observed in the sample even at the room temperature for 100 Hz. The colossal dielectric constant in LaFeO3 is mainly driven by the internal barrier layer capacitor (IBLC) formation. The formation of IBLC was explained on the basis of highly insulating grain boundary and less resistive/semiconducting grain, which was confirmed from both the resistance and capacitance of grain and grain boundary from the impedance analysis. The non-Debye-type relaxation process associated with the grain and grain boundary effect was investigated from the broad and asymmetric relaxation peak. The relaxation time for both the grain and grain boundary effect was also calculated. In addition to this, we have also analyzed the normalized bode plot of imaginary part of impedance and electrical modulus which suggests the relaxation process dominated by the short-range movement of charge carriers.

Development and characterization of (1-x)BiYO3-xBiMnO3 ceramics for Ferro-photovoltaic applications

The rare combination of ferroelectricity, low band gap and high carrier mobility in a material can facilitate exceptional photovoltaic efficiency. In this paper, we attempted to reduce the band gap and improve the conductivity of ferroelectric solid solution BiY(1-x)MnxO3 by varying composition. The BiY(1-x)MnxO3 (x = 0.0, 0.10, 0.25, 0.50, 0.75) samples were prepared by high energy ball milling method and structural, dielectric, optical, electrical and ferroelectric properties were analyzed systematically. Herein, we show that band gap of the material is drastically reduced from 3.0 eV (BiYO3) to 1.76 eV (x = 0.50) and the samples exhibit ferroelectric behavior with significant polarization. Compound resistance of grain and grain boundaries were also found to reduce with increase in Mn concentration in the BiY(1-x)MnxO3 solid solution, demonstrating improvement in semiconducting behaviour of the material. Thus, solid solution formation of BiMnO3 with BiYO3, tunes band gap in useful region, improves electrical conduction and enhances ferroelectric polarization showing good potential for high performance solar cell and thermoelectric applications.

High Formability Bromide Solid Electrolyte with Improved Ionic Conductivity for Bulk-Type All-Solid-State Lithium–Metal Batteries

Solid electrolytes with excellent formability and high room-temperature Li-ion conductivities used in bulk-type all-solid-state lithium-metal batteries (ASSLBs) are expected to suppress the formation of Li dendrites that affects the application of metal Li anodes with high energy density. However, voids and cracks within sintered electrolytes lead to short-circuiting of ASSLBs due to the formation of Li dendrites. This work aimed to develop solid electrolytes with high formability and high room-temperature Li-ion conductivities that can suppress Li dendrites’ formation in ASSLBs. Three indicators of bulk moduli, lithium-ion migration energies, and energies above the hull were investigated: formability, ionic conductivity, and thermodynamic stability. The 109 inorganic materials containing elements of Li and Br listed in the open web-based database of the Materials Project were surveyed due to their weak Coulombic interaction with Li ions and high polarizability. LiGaBr4 with monoclinic symmetry (LGB, S.G.: P121/a1) was focused on due to its lower values for all three analyzed indicators. A ball milling mechanochemical synthesis method was adopted to synthesize LGB. It indicated that the obtained LGB possessed a monoclinic structure with a lithium ionic conduction of 2.0 × 10–5 S cm–1 (cold isostatic pressed powder) at 25 °C. The ionic conductivity was nearly three times higher than that of the conventional sintered LGB and comparable to that of a typical oxide-type electrolyte of LISICON-type Li3.55(Ge0.45Si0.10V0.45)O4 (LGSV) at 25 °C, which is appealing for applying the sintered samples for Li-metal anodes. The SEM and the distribution of relaxation times (DRT) analysis demonstrated that LGB contributed minimal grain-boundary resistance due to tight connections between the particles, while the LGSV oxides contributed an extremely large grain-boundary resistance. An ASSLB using LGB powder as an electrolyte, bare LiCoO2 as a cathode, and lithium metal as the anode exhibited the ability to function smoothly, while short-circuiting occurred at the second cycle with LGSV, demonstrating that LGB is a pure ionic conductor and may be suitable for use in ASSLB that can prevent Li dendrites with electrochemical stability.

Low-temperature fabrication of NASICON-type LATP with superior ionic conductivity

NASICON-type Li1.3Al0.3Ti1.7(PO4)3 (LATP) has attracted much attention as a fast lithium-ion conductor for all-solid-state lithium-based batteries. However, the large grain-boundary resistance and high sintering temperature above 950 °C for pure LATP electrolytes hinder their practical application in lithium-based batteries. Here, a low-temperature preparation approach for the superionic conductor LATP electrolyte with sintering aids is proposed. By introducing sintering aids Li3PO4, LiBO2·0.3H2O, and binary 0.32Li3PO4-0.68LiBO2·0.3H2O, LATP-based pellets can be fabricated at optimized sintering temperatures below 900 °C. The resultant LATP-based pellets maintain a high relative density and homogeneous microstructure, which arises from the sintering aid to facilitate the grain rearrangement and fill the interstices between grains. The LATP pellets using sintering aid Li3PO4 achieve a high ionic conductivity of 5.2 × 10−4 S cm−1 at a low temperature of 800 °C, 7.3 times that (7.1 × 10−5 S cm−1) of pure LATP pellets sintered at 950 °C, which demonstrates the effectiveness of low-temperature preparation approach to reducing the preparation cost and optimizing the ionic conductivity of LATP-based electrolytes.

Band gap tunning to enhance photovoltaic response in NaNbO3-based bulk ferroelectrics

The observation of anomalous photovoltaic effect and switchable photocurrent in ferroelectrics receive extensive attention for its great advantages in next-generation high-performance photovoltaic devices, but the key factor, the wide band gap results in low photocurrent output and photoelectric conversion efficiency. Focusing on narrowing band gap to improve photovoltaic performance in NaNbO3, in this work, we fabricated the NaNbO3 and 0.8NaNbO3-0.2La(Mn0.5Ni0.5)O3 (abbreviated as 0.2NLMNO) ferroelectric ceramics through a solid state method. The optical absorption spectra reveal the band gap of ceramics reduce from 3.42 eV to 2.11 eV and the light absorption in visible region is significantly enhanced. A higher symmetry of crystal structure and great reduced resistance of grain and grain boundary induced by introducing La(Mn0.5Ni0.5)O3. What's more, the photocurrent density of 0.2NLMNO improves approximately 10 times larger than that of pure ferroelectric NaNbO3, it is expected to explore a new ferroelectric material with photovoltaic effect.

Microstructure and mechanical properties of Y2O3 strengthened Inconel 625 alloy fabricated by selective laser melting

Selective laser melting (SLM) has unique advantages in manufacturing oxide dispersion strengthened alloys due to its high cooling rate and tiny molten pool. In this study, the microstructure and mechanical properties of Inconel 625 alloy and Y2O3 strengthened Inconel 625 alloy prepared by SLM were investigated comparatively. The highest density for Y2O3/Inconel 625 specimen with a value of 99.6% was achieved when the laser volume energy density is around 145.8 J/mm3. Compared with Inconel 625 specimen, there presents a larger molten pool, finer grain structure, and weaker texture along the deposition direction in Y2O3/Inconel 625 specimen due to the active flux effect of Y2O3. During the heat treatment, the grain growth and annealing twin formation are inhibited in Y2O3/Inconel 625 specimen due to the pinning effect of Y2O3 particles. The yield strength of Inconel 625 specimen increases by 11.4%, 22.0%, and 12.5% at 20, 800, and 1100 °C, respectively, due to the dispersion strengthening and grain boundary strengthening induced by the addition of 0.48 wt % Y2O3 particles. In addition, the elongation of Y2O3/Inconel 625 specimen at high temperature is improved because of the contribution of Y2O3 to the strengthening and oxidation resistance of grain boundaries.

Origin of excellent giant dielectric performance of rutile–TiO2 ceramics codoped with isovalent/pentavalent dopants

The excellent dielectric performance of an Sn4+–isovalent/Ta5+–pentavalent doped TiO2 (SnTTO) ceramic was investigated. Substitution of Sn4+ into TiO2 caused slight changes in the dielectric permittivity (ε′) and loss tangent (tanδ). Although doping TiO2 with Ta5+ can cause a substantial increase in ε′, tanδ was very large. Notably, the SnTTO ceramic exhibited a high ε′ value of 44,112 and a low tanδ value of 0.011 at 1 kHz and 25 °C. Furthermore, the temperature coefficient of ε′ (Δε′(%)) was less than 15% in the temperature range of −60–170 °C. The microstructure of the codoped SnTTO consisted of semiconducting grains and grain boundaries (GBs) with very large GB resistance (Rgb) value, thereby leading to effective retention of a low tanδ value and a small Δε′(%) value. Evidently, the primary roles of the Ta5+ and Sn4+ doping ions were to increase the interfacial polarization via increased free charges and increased Rgb by reducing the oxygen vacancies at the GBs.

Structural, FTIR, optical and dielectric properties of Zn1-xAlxO ceramics for advanced applications

We report here the structural, FTIR, optical and dielectric properties of Zn1-xAlxO with x = 0.00 ≤ x ≤ 0.20)). The samples are synthesized by the solid state reaction method and the phase purity, structural morphology, and absorption spectra are examined by XRD, SEM, and FTIR techniques. The optical and dielectric measurements are obtained by a double beam spectrophotometer and an impedance analyzer. The wurtzite structure is confirmed for all samples, and the lattice parameters, crystallite diameter, and porosity are decreased by Al, whereas the Debye temperature and elastic modulus are increased. The residual stress is compressive for ZnO, but it is changed to tensile for the doped samples. Two different values of energy gap (Egh and Egl) are apparent for each sample. The Egh and Egl are, respectively, 3.60, 3.56, 3.43, 3.53, 3.68 eV and 3.02, 2.95, 2.85, 2.92, 3.07 eV for all samples such that ΔE = (Egh – Egl) ~ 0.60. The residual dielectric lattice constant is decreased by increasing x to 0.10, followed by a sharp increase at x = 0.20, while the opposite behavior is obtained for (N/m*). The dielectric constant and ac conductivity are slightly increased as x increases to 0.025, followed by a sharp increase with more increase of x to 0.20. The conduction is electronic for pure and x = 0.025 samples, but it changes to hole with increasing x to 0.20. The binding energy was decreased as x increased to 0.20, but there is no exact trend for the hopping distance and density of localized states against x. The F-factor for solar cell design was increased as x increased to 0.10, but it is almost constant at x = 0.20. The Cole–Cole plot is a straight line for x = 0.00, a semicircle arc for x = 0.025, and a complete semicircle for x ≥ 0.05. The impedance resistance of grain and grain boundaries decreases as x increases to 0.20. These outcomes indicate that the addition of Al to ZnO shifts the mechanical, optical, and dielectric medium to higher values, which is strongly recommended for the design of optoelectronic and solar cell instruments.

Significantly enhanced varistor properties of CaCu3Ti4O12 based ceramics by designing superior grain boundary: Deepening and broadening interface states

Significantly enhanced varistor properties via tailoring interface states were obtained in Ca 1–2 x /3 Y x Cu 3 Ti 4 O 12 -SrCu 3 Ti 4 O 12 composite ceramics. The breakdown field was improved to 35.8 kV cm −1 and the nonlinear coefficient in 0.1–1 mA cm −2 was enhanced to 14.6 for Ca 0.67 Y 0.5 Cu 3 Ti 4 O 12 -SrCu 3 Ti 4 O 12 . Noticeably, the withstand voltage of single grain boundary reached up to 24 V while the reported ones were constant to about 3 V. Greatly improved properties were attributed to the formation of superior grain boundary rather than the reduced grain size. Surprisingly, with distinct discrepancy of nonlinear performance in the composites, the resistance and activation energy of grain boundary exhibited little differences. Based on the double Schottky barrier at grain boundary and the field-assisted thermal emission model, it was found that the excellent electrical nonlinearity arose from the formation of deeper and broader interface states at grain boundary. In this case, interface states were not easily entirely filled and the barrier could maintain its height under applied voltage. This work provides a novel routine for enhancing the varistor properties of CaCu 3 Ti 4 O 12 based ceramics by manipulating interface states at grain boundary.

Electrical responses and dielectric properties of (Zn2+ + F−) co–doped CaCu3Ti4O12 ceramics

The effects of Zn2+ and F− ions on the ceramic microstructure, electrical properties, and colossal dielectric response of CaCu3Ti4O12 were investigated. The dopants influenced the microstructure of the CaCu3Ti4O12. The replacement of Zn2+ and F− ions reduced the low–frequency loss tangent while retaining the high dielectric constant of CaCu3Ti4O12. In co–doped ceramics, the high dielectric permittivity of around 5.76 – 7.59 × 104 and low loss tangent of 0.082 – 0.085 can be attained. The decrease in the loss tangent of the co–doped ceramic samples is commensurate with the significant increase in the grain boundary (GB) resistance and breakdown electric field. The Cu+ and Ti3+ ions in CaCu3Ti4O12 were identified using X–ray photoelectron spectroscopy and were found to be decreased by co–doping with Zn2+ and F−. The presence of these ions may play a crucial role in the semiconducting grains. The substitution of Zn2+ and F− ions can increase the electrostatic potential barriers at the GBs, resulting in improved GB resistance.

Pressure-induced transition from pure electronic to mixed ionic-electronic conduction in strontium hydride

The heavier alkaline-earth hydrides AeH2 (Ae = Ca, Sr, and Ba) are considered as promising materials for hydrogen energy storage. Pressure-induced structural changes in AeH2 materials could improve hydrogen transport properties and result in a better understanding of the structure-property relationship. In this work, pressure evolution of carrier transport properties of SrH2 was investigated using impedance spectroscopy measurements at room temperature and first-principles calculations. The pressure-induced structure phase transition from a Pnma phase to a P63/mmc phase was accompanied by a transition from pure electronic conduction to mixed ionic-electronic conduction, which was related to the ionic migration barrier energy. In the P63/mmc phase, the H− ionic and electronic resistances of bulk and grain boundaries were distinguished, respectively. The total resistance of SrH2 decreased by about four orders of magnitude after the phase transition. This work provides critical insight into the structure-conduction relationship and the role of grain boundaries in the transport process of alkaline-earth hydrides under high pressure.

Simultaneous Extraction of the Grain Size, Single-Crystalline Grain Sheet Resistance, and Grain Boundary Resistivity of Polycrystalline Monolayer Graphene.

The electrical properties of polycrystalline graphene grown by chemical vapor deposition (CVD) are determined by grain-related parameters—average grain size, single-crystalline grain sheet resistance, and grain boundary (GB) resistivity. However, extracting these parameters still remains challenging because of the difficulty in observing graphene GBs and decoupling the grain sheet resistance and GB resistivity. In this work, we developed an electrical characterization method that can extract the average grain size, single-crystalline grain sheet resistance, and GB resistivity simultaneously. We observed that the material property, graphene sheet resistance, could depend on the device dimension and developed an analytical resistance model based on the cumulative distribution function of the gamma distribution, explaining the effect of the GB density and distribution in the graphene channel. We applied this model to CVD-grown monolayer graphene by characterizing transmission-line model patterns and simultaneously extracted the average grain size (~5.95 μm), single-crystalline grain sheet resistance (~321 Ω/sq), and GB resistivity (~18.16 kΩ-μm) of the CVD-graphene layer. The extracted values agreed well with those obtained from scanning electron microscopy images of ultraviolet/ozone-treated GBs and the electrical characterization of graphene devices with sub-micrometer channel lengths.

Dielectric and Electric Properties of Ba0.996La0.004Ti0.999O3 Ceramics Doped with Europium and Hafnium Ions.

Lanthanum-modified BaTiO3 electroceramic materials have superior dielectric and piezoelectric properties. Ba0.996La0.004Ti0.999O3 (BLT4) seems to be a serious candidate for ultracondensator applications. This manuscript describes the results of hafnium and europium modification of BLT 4 ceramics. The pure and doped ceramic materials were synthesized by the conventional mixed oxide method. The microstructure of obtained samples was examined by scanning electron microscope. The investigations reveal strong correlations between the presence of admixture and the grain size, which was especially visible in the case of the hafnium dopant. The frequency and temperature dielectric characteristics measurements revealed a decrease in electric permittivity. Moreover, the impedance spectroscopy investigations showed severe changes in grains and grain-boundary resistivity, which was connected with changes in electric conductivity.

Processing and characterisation of BaZr0.8Y0.2O3−δproton conductor densified at 1200 °C

BaZr0.8Y0.2O3−δdensifies at 1200 °C on employing a combination of Ba excess, ZnO sintering additive and nanopowders prepared by a sol–gel route. The low densification temperature decreases grain-boundary resistance despite the lower grain size.

Non-equilibrium molecular dynamics study on atomistic origin of grain boundary resistivity in NASICON-type Li-ion conductor

Grain boundary (GB) resistance to ion conduction in solid-state electrolytes is one of the main issues on next-generation, high-performance rechargeable batteries. Thus, it is required to understand the origin of the GB resistance from the atomistic point of view. In this paper, a method to investigate the local ion flux using the non-equilibrium molecular dynamics (NEMD) is proposed, and it is demonstrated that the atomistic origin of the GB resistance in NASICON-type LiZr2(PO4)3 is clarified by the local ion-flux analysis of poly-crystalline system containing over half-million atoms in combination with Li-ion site potential energy analysis. The local ion-flux analysis enables us to visualize where Li ions migrate fast or slow in poly-crystalline structures, and it is observed, for the first time, that Li ions move toward lower reaches within grains and pass through specific regions of GBs. The Li-ion site potential energy analysis provides atomic-level details of the differences between high-flux and low-flux regions. It is confirmed from the analyses that the GB resistance comes from deep Li-ion traps and high-energy Li-ion migration paths made of rings of PO4 and ZrO6 polyhedra that do not exist in the crystalline structure.

Enhanced rolling contact fatigue behavior of selective electron beam melted Ti6Al4V using the ultrasonic surface rolling process

This paper investigates the influence of building directions and ultrasonic surface rolling processing (USRP) on rolling contact fatigue (RCF) performances of selective electron beam melted Ti6Al4V. Results show the RCF performance in the xoz direction is 54% higher than that of the xoy direction due to the poor crack propagation resistance of the prior-β columnar grain boundary α layer. The USRP treated samples show ∼ 30% longer RCF life than that of untreated samples. The increased RCF performance is attributed to large work hardening, surface nano-crystallization, surface residual compressive stress, and the changed lubrication condition from boundary lubrication to mixed lubrication, which suppresses crack initiation and propagation.