Grain Boundary Conductivity Research Articles

Discovery of a Rare-Earth-Free Oxide-Ion Conductor Ca3Ga4O9 by Screening through Bond Valence-Based Energy Calculations, Synthesis, and Characterization of Structural and Transport Properties.

In this work, we have discovered Ca3Ga4O9 as a rare-earth-free oxide-ion conductor by a combined technique of bond valence (BV)-based energy calculations, synthesis, and characterization of structural and transport properties. Here, the energy barriers for oxide-ion migration (Eb) of 217 Ga-containing oxides were calculated by the BV method to screen the candidate materials of oxide-ion conductors. We chose the orthorhombic calcium gallate Ca3Ga4O9 as a candidate of oxide-ion conductors, because Ca3Ga4O9 had a relatively low Eb. Ca3Ga4O9 was synthesized by a solid-state-reaction method. Rietveld analyses of time-of-flight neutron and synchrotron X-ray powder diffraction data of Ca3Ga4O9 indicated an orthorhombic Cmm2 layered crystal structure consisting of Ca18 and (Ga4O9)6 units where the (Ga4O9)6 units form the two-dimensional (2D) corner-sharing GaO4 tetrahedral network. The electromotive force measurements with an oxygen concentration cell showed that the transport numbers of the oxide ion were 0.69 at 1073 K and 0.84 at 973 K in Ca3Ga4O9, which indicates that the major carrier of Ca3Ga4O9 is the oxide ion. The oxide-ion conductivity was estimated to be 1.03(8) × 10-5 S cm-1 at 1073 K. The total electrical conductivity and impedance spectroscopy measurements of this Ca3Ga4O9 sample indicated that the bulk conductivity was much higher than the grain-boundary conductivity and that the total conductivity was equivalent to the bulk conductivity. The bond valence-based energy landscape calculated using the refined crystal parameters of Ca3Ga4O9 indicated 2D oxide-ion diffusion in the layered tetrahedral network [(Ga4O9)6 unit]. It was found that the structural and transport properties of Ca3Ga4O9 are similar to those of LaSrGa3O7 melilite.

Phonon-Grain-Boundary-Interaction-Mediated Thermal Transport in Two-Dimensional Polycrystalline MoS2.

Although dislocations and grain boundaries (GBs) are ubiquitous in large-scale MoS2 samples, their interaction with phonons, which plays an important role in determining the lattice thermal conductivity of polycrystalline MoS2, remains elusive. Here, we perform a systematic study of the heat transport in two-dimensional polycrystalline MoS2 by both molecular dynamics simulation and atomic Green's function method. Our results indicate that the thermal boundary conductance of GBs of MoS2 is in the range from 6.4 × 108 to 35.3 × 108 W m-2 K-1, which is closely correlated with the overlap between the vibrational density of states of GBs and those of the pristine lattice, as well as the GB energy. It is found that the GBs strongly scatter the phonons with frequency larger than 2 THz, accompanied by a pronounced phonon localization effect and significantly reduced phonon group velocities. Furthermore, by comparing the results from realistic polycrystalline MoS2 to those from different theoretical models, we observe that the Casimir model is broken down and detailed phonon dynamics at a specific GB should be taken into account to accurately describe the phonon transport in polycrystalline materials. Our findings will provide useful guidelines for designing efficient thermoelectric and thermal management materials based on phonon-GB interaction.

Conductivity behavior of Fe, Ti singly and doubly doped CaO-ZrO2

10 mol% CaO stabilized ZrO2 (CSZ) singly and doubly doped with various amounts of FeO1.5 and TiO2 was prepared by a co-precipitation method, and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), electrochemical impedance spectroscopy (EIS) and Hebb-Wagner method. The results show that relative densities of samples are above 97% and close to each other except CSZT0.29. FeO1.5 single doping improves the grain-boundary conductivity, but decreases the bulk conductivity, while TiO2 single doping decreases both the bulk and grain-boundary conductivities. FeO1.5 and TiO2 double doping increases the grain-boundary and bulk conductivities. The increase in bulk and total conductivities of doped CSZ is mainly due to the high relative density, which provides more migration pathway for O2−, while the increase in co-doping is attributed to the scavenge effect of TiO2 on the grain boundary. The electronic conductivities of all samples measured in air are low, being 10−7−10−6 S cm−1 in the temperature range of 670–850 °C.

Synthesis and Comparative Study of the Microstructure and Properties of LiNbO3 and LiNbO3:Zn Ceramics Manufactured by Sol–Gel Processes

The manufacture of LiNbO3 and LiNbO3:Zn fine-grained powders by two variants of the sol–gel process was studied. The microstructure, mechanical and electric properties of LiNbO3 and LiNbO3:Zn ceramics prepared from powders of various origins were studied. The ceramics prepared from powders that had been fractionated by elutriation had uniform micro- and submicrostructure, improved strength characteristics, and ionic grain-boundary conductivity. Doping LiNbO3 with Zn2+ reduced the electronic conductivity of LiNbO3:Zn ceramics, and this allowed the ceramics to be efficiently polarized and their piezoelectric properties to be improved.

Evaluation of ion-transport in composite polymer-in-ceramic electrolytes. Case study of active and inert ceramics

Inorganic ceramics- and organic polymer-based solid electrolytes (SE) could revolutionize battery technology because of their nontoxicity, stability during operation and enhanced safety. Of particular interest are electrolytes, which contain high concentrations of ion charge carriers with a minimum polymer concentration, and enable the major ion-conduction path through the inorganic material. In this article we compare ion-conduction mechanisms in two solid electrolytes composed of either an inert or active ceramic matrix with imbedded LiI:P(EO)n electrolytes. On the basis of AC-impedance and NMR data, we suggest that the high-ionic-conductivity (0.5 mS/cm) and low-activation-energy (2.3 kJ/mol) ion paths are brought about by the grain boundaries between the excess of LiI and inert LiAlO2 ceramic nanoparticles. Both confined-in-ceramic polymer electrolyte (PE) and ceramic LiAlO2 grains impede the total ion mobility. The fast ion transport in polymer-in-ceramic electrolytes composed of high-conductivity active Li10SnP2S12, goes through lithium-iodide-rich glass ceramics, and is restricted by slow ion transport via the imbedded polymer electrolyte. Unexpectedly, it was found that at 1:3 salt-to-polymer ratio, the contribution of grain-boundary conductivity in an inert-ceramic–based composite electrolyte is stronger than that of bulk conductivity via active ceramic matrix. One of the possible reasons of the reduced relative contribution of the active ceramics to the total conductivity of polymer-in-ceramic electrolyte is that the ceramic powder was not densified.

Modifying Grain Boundary Ionic/Electronic Transport in Nano-Sr- and Mg- Doped LaGaO3-δ by Sintering Variations

Perovskite La0.9Sr0.1Ga0.9Mg0.1O3-δ (LSGM) is one of the fastest known oxide ion conductors, with reported enhanced p-type electronic transference numbers at grain boundaries, attributed to space charge effects. As this material is applied as a solid oxide fuel/electrolysis cell electrolyte, it is of interest to learn how its mixed conductivity may be tailored. Field assisted sintering technique/spark plasma sintering (FAST/SPS) and conventional sintering without field or pressure were employed to prepare pellets with various grain sizes, in order to systematically assess the influence of processing route on the mixed conductivity. AC-impedance spectroscopy and the brick layer model were applied to determine local conductivities as a function of temperature, oxygen partial pressure, and dc bias. With increasing sintering temperature and grain size, the following trends were observed: larger electrical grain boundary (GB) widths, higher GB potentials, lower specific GB conductivity, greater dc-bias dependence of GB conductivity, higher pO2-dependence of GB conductivity indicating higher electronic transference numbers, and lower pre-exponential factor for specific GB conductivity. These results suggest an increasing GB space charge effect with increasing sintering temperature/grain size, which coincided with increasing compositional uniformity by TEM and EDS. The results confirm that sintering route is an important variable for tailoring mixed conduction.

Manipulating the grain boundary properties of BaCeO3-based ceramic materials through sintering additives introduction

BaCeO3-based materials represent a well-known family of proton-conducting electrolytes, which can be used in different solid oxide electrochemical devices. An effective operation of the latter across an intermediate-temperature range requires improved transport of PCEs, including their grain (G) and grain boundary (GB) components. In the present work, some 3d-elements in a small amount were used as sintering additives to verify the possibility of improving the GB conductivity of BaCe0.9Gd0.1O3–δ. It is shown that copper oxide (CuO) can be considered as one of the most effective sintering agents, since its use enables decreasing the GB density of the BCG ceramic material at the reduced sintering temperatures. The obtained results form a new tactic for designing new protonic electrolytes, whose conductivity might be prevail over ones containing Ni-based modifiers.

Low temperature sintering of crystallized Li1.5Al0.5Ge1.5(PO4)3 using hot-press technique

The hot-press technique is applied to lower the sintering temperature and shorten the sintering process of Li1.5Al0.5Ge1.5(PO4)3 (LAGP). The hot-press conditions are studied to obtain well sintered LAGP pellet with high Li ion conductivity. The highest Li ion conductivity is obtained at 550 oC for 30 min at 7 MPa and the values are 3.8, 3.4 and 1.8×10-4 S cm-1 in bulk, grain-boundary and total conductivities, respectively. These values are comparable to reported values, however, sintering temperature is more than 250 oC lower and holding time is more than several hours shorter than conventional sintering processes.

Thermal conductivity of graphene grain boundaries along arbitrary in-plane directions: A comprehensive molecular dynamics study

The thermal conductivity of polycrystalline graphene is expected to be lower than that of pristine graphene, due to the existence of defects, such as grain boundaries (GBs). To study the thermal transport behavior in polycrystalline graphene, it is crucial to understand the thermal conductivity of graphene GBs as a function of the tilt GB misorientation angle and in-plane thermal loading angle. However, existing studies of thermal conductivity of graphene GBs only consider the case where the thermal flux is perpendicular or parallel to the graphene GB. To address this issue, here we perform systematic non-equilibrium molecular dynamics simulations and investigate the thermal conductivity of graphene GBs for all possible tilt GB misorientation angles (23 cases) under arbitrary in-plane thermal loading directions. The findings from the present study can offer quantitative guidance for using polycrystalline graphene in thermal devices and flexible electronics applications.

Effect of grain boundary and Ar–H2 atmosphere on electrical conductivity of bulk a-La2Mo2O9 studied by impedance and x-ray photoelectron spectroscopy

The effect of grain boundary on electrical conductivity of α-La2Mo2O9 (LAMOX) over the temperature range of 300 °C to 400 °C has been investigated in this paper. Grain boundary effect characterisation for the mentioned temperature range has been done by electrochemical impedance spectroscopy (EIS). The calculated values of grain boundary width and specific grain boundary conductivity for α- La2Mo2O9 are 14.6 nm and 0.0146 μS/cm respectively. Total ionic conductivity calculated by EIS for α-La2Mo2O9 at 300 °C is 1 μS/cm. Resulting electrical conductivity of grain boundary comes out to be two orders less than the total electrical conductivity suggesting high activation energy requirement for grain boundary conduction. The electrical conductivity of α-La2Mo2O9 treated in Ar-H2 atmosphere (2.5 mS cm−1 at 200 °C) is approximately three orders higher than the electrical conductivity of LAMOX treated in air atmosphere. This may be attributed to reduction of Mo6+ to lower valance states in LAMOX as confirmed by x-ray photoelectron spectroscopy (XPS). Therefore, the low ionic conductivity nature of monoclinic phase of La2Mo2O9 has been explained on the basis of grain boundary conduction mechanism in the present study.

Nanocomposite Ceramics on the Basis of Magnesium, Cerium, and Samarium Oxides

A model of conductivity of nanocomposite ceramics consisting of solid-electrolyte and dielectric phases is proposed based on the assumption that the conductivity of grain boundaries between the solid-electrolyte and dielectric phases is higher than the conductivity of the volume of particles in the solid-electrolyte phase and its grain boundaries. Taking into account the size of particles, the thickness of grain boundaries, and the bulk and grain-boundary conductivities, the grain size of composite ceramics for which the conductivity may exceed the conductivity of single-phase solid-electrolyte ceramics is assessed. For testing this model, the composite samples are synthesized based on dielectric magnesium oxide and solid-electrolyte cerium oxide doped with samarium oxide. It is shown that introduction of 50 mol % magnesium oxide into composite ceramics has virtually no effect on its conductivity as compared with single-phase solid-electrolyte ceramics. This result can be explained by assuming the appearance of accelerated transport routes for oxygen ions in grain boundaries between dielectric and solid-electrolyte phases. Further dispersion, optimization of the ratio, and increase in distribution homogeneity of components can confirm the validity of the proposed conductivity model and open up the possibility of preparation of oxide solid-electrolyte materials with higher conductivity.

Sodium-ion conduction in Na2Zn2TeO6 solid electrolytes

The ionic conduction in a novel solid sodium-ion conductor of Na2Zn2TeO6 (NZTO) is investigated from the point of view of defect chemistry. NZTO shows an ionic conductivity of 0.57 mS cm−1 at room temperature, and the grain bulk conductivity and the grain-boundary conductivity are individually measured using the AC impedance spectroscopy at temperatures down to −30 °C. The grain-boundary conductivities are about two orders of magnitude lower than those of the grain bulk; such a phenomenon can be ascribed to the Schottky barrier at the grain boundaries of the NZTO electrolyte. The concentration and mobility of the charge carriers in the grain bulk are calculated from the grain bulk conductivity. The concentration and mobility of the charge carriers and the Schottky barrier height can be tuned by doping; the ionic conductivity of NZTO is enhanced to 0.83 mS cm−1 by the doping of 2.5 mol% Ga at the Zn sites, because the Ga-doping increases the concentration and mobility of the charge carriers, and lowers the Schottky barrier height.

7Li NMR diffusion studies in micrometre-space for perovskite-type Li0.33La0.55TiO3 (LLTO) influenced by grain boundaries

Perovskite-type oxide solid electrolyte Li0.33La0.55TiO3 (LLTO) samples are known to have high ionic conductivities and grain-boundary resistances from AC impedance spectroscopy. In addition, the existence of domain structures has been shown by transmission electron microscopy and other techniques. In this study Li+ migration in tetragonal and cubic samples (t- and c-LLTO) was studied in micrometre space by pulsed-gradient spin-echo (PGSE) NMR from the room temperature up to 100 °C. The grain-boundary effects appeared clearly in the 7Li echo attenuation plots, which were not linear and included at least two components. Previously, we reported that the Li+ diffusion in inorganic solid electrolytes (garnets, NASICON, sulfides) distributes widely in time and space, illustrated by the dependences of the measuring parameters such as observation time (Δ) and pulsed-field gradient (PFG) strength (g). In addition to the complexities of parameter dependent Li+ diffusion, grain-boundary disturbance effects were observed for Li+ diffusion phenomena in LLTO samples. We indicated that a unique DLi value could be estimated from the linear echo attenuation plot at convergent measuring conditions with long Δ and large g for garnets and NASICON. The LLTO samples showed curved echo attenuation plots at the convergent measuring conditions, which were analysed by two components to give two Li+ diffusion constants (DLi-fast and DLi-slow). These values are consistent with the tracer diffusion constants measured at higher temperatures (>150 °C). For c-LLTO, DLi-fast and DLi-slow showed good correspondences with the bulk ionic conductivity (σbulk) and grain-boundary ionic conductivity (σgb), respectively. The Li carrier numbers estimated from DLi-fast and σbulk for c- and t-LLTO showed similar values, whereas those estimated from DLi-slow and σgb for c-LLTO were smaller by approximately one-order of magnitude.

Effect of grain size on the electrical performance of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ solid electrolytes with addition of NiO

In order to clarify the effect of grain size on the electrical performance of BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) solid electrolytes with addition of NiO, microcrystalline (~1.5 µm) and ultrafine-grained (~280 nm) BZCYYb electrolytes (with 1 wt% NiO) were fabricated by the conventional and two-step sintering method, respectively. The results show that compared with microcrystalline electrolytes, the ultrafine-grained electrolytes have similar grain-interior conductivities, but much lower grain-boundary conductivities, illustrating that the grain boundary is not conducive for ionic transport. As a result, the electrical conductivity of microcrystalline electrolytes (1.9 × 10−2 S cm−1 at 600 °C in wet air) is higher than that of ultrafine-grained electrolytes (1.1 × 10−2 S cm−1 at 600 °C in wet air). In addition, the OCV (open-circuit voltage) values of electrolyte-supported single cells show that the undesired electronic conduction exists in the electrolytes due to the BaY2NiO5 impurity formed by the reaction of NiO and BZCYYb. The ultrafine-grained electrolytes show lower OCV values than that of microcrystalline ones, due to the prolonged electronic transport paths. Therefore, large-grained or grain boundary-free microstructure are necessary for improving the electrical performance of BZCYYb electrolytes.

Temperature dependent conductivity of Bi4Ti3O12 ceramics induced by Sr dopants

Bi4Ti3O12 is an important lead-free ferroelectric material. Doping modification of Bi4Ti3O12 has attracted great attention to improving its performances. In this work, the effect of Sr dopants on the microstructure, dielectric, and conductivity of Bi4Ti3O12 ceramic was investigated by XRD, SEM, and AC impedance spectroscopy. Substitution of 1 at% Sr for Bi decreased the grain size, suppressed the dielectric dispersion of Bi4Ti3O12 ceramic at room temperature, and resulted in different effects on the conductivity of grains and grain boundaries. The conductivity of grains in Bi4Ti3O12 ceramic was increased by the small amount of Sr dopants in the whole experimental temperature range. While the grain boundaries of 1 at% Sr-doped Bi4Ti3O12 exhibited lower conductivity than pure Bi4Ti3O12 below ~380 °C and higher conductivity above ~380 °C. The experimental phenomena were interpreted in term of compensating defects for Sr dopants.

Structural and electrical properties of carbon-ion-implanted ultrananocrystalline diamond films**Project supported by the National Natural Science Foundation of China (Grant Nos. 50972129 and 50602039), the International Science Technology Cooperation Program of China (Grant No. 2014DFR51160), the National Key Research and Development Program of China (Grant No. 2016YFE0133200), European Unio’s Horizon 2020 Research and Innovation Staff Exchange (RISE) Scheme (Grant No. 734578),

We investigate the structural and electrical properties of carbon-ion-implanted ultrananocrystalline diamond (UNCD) films. Impedance spectroscopy measurements show that the impedance of diamond grains is relatively stable, while that of grain boundaries (GBs) (Rb) significantly increases after the C+ implantation, and decreases with the increase in the annealing temperature (Ta) from 650 °C to 1000 °C. This implies that the C+ implantation has a more significant impact on the conductivity of GBs. Conductive atomic force microscopy demonstrates that the number of conductive sites increases in GB regions at Ta above 900 °C, owing to the formation of a nanographitic phase confirmed by high-resolution transmission electronic microscopy. Visible-light Raman spectra show that resistive trans-polyacetylene oligomers desorb from GBs at Ta above 900 °C, which leads to lower Rb of samples annealed at 900 and 1000 °C. With the increase in Ta to 1000 °C, diamond grains become smaller with longer GBs modified by a more ordered nanographitic phase, supplying more conductive sites and leading to a lower Rb.

The influence of water on the electrical conductivity of aluminum-substituted lithium titanium phosphates

Li1+xAlxTi2−x(PO4)3 (0.1 ≤ x ≤ 0.6) powders were prepared by a novel sol-gel method with high phase purity, densification activity and conductivity. Diffraction analyses showed that the solubility limit of Al3+ was reached at x = 0.5. The highest ionic conductivity was obtained for Li1.5Al0.5Ti1.5(PO4)3, which reached 1.0 × 10−3 S cm−1 at 25 °C when tested in ambient air. However, measurements in dry argon resulted in a conductivity of only 5 × 10−4 S cm−1 at 25 °C. Hence, the influence of moisture or water on microstructure and grain-boundary conductivity was investigated by impedance spectroscopy, scanning electron microscopy, Raman spectroscopy, X-ray and neutron diffraction. An ion exchange of Li+ by protons could not be unambiguously achieved by exposure of LATP powder in water. Neutron diffraction of humidified samples did not clearly indicate the presence of water or protons in the crystal structure of LATP, whereas μ-Raman measurements confirmed the presence of water/protons on the sample surface and in the bulk material. A higher signal of the vibrational modes of H2O was measured on grain boundaries than in the grain interior on the sample surface as well as on the fractured surface of sintered specimens. Therefore the higher apparent conductivity of LATP samples may predominantly result from adsorbed water from ambient air at grain boundaries. Hence, the conductivity tested in dry argon represents the correct conductivity of LATP samples. The highest density after sintering is not necessarily leading to high conductivity but rather the microstructure plays the dominant role.

Lithium superionic conductors Li10MP2O12 (M = Ge, Si)

The exploration for superionic conductors with new structures and compositions is challengeable for developing safer and high-energy-density batteries. Here, we report lithium superionic conductors with the compositions of Li[Formula: see text]GeP2O[Formula: see text] and Li[Formula: see text]SiP2O[Formula: see text]. The materials are prepared by melt-quenching and characterized by X-ray diffraction (XRD), AC impedance, DC polarization and cyclic voltammetry. Both germanium and silicon members demonstrate same orders of magnitude of bulk and grain-boundary conductivities of 10[Formula: see text] S[Formula: see text]cm[Formula: see text] at room temperature. The crystal structure is investigated by the Rietveld refinement of powder XRD and the migration mechanism of lithium ions is proposed.

Temperature-dependence of electrical properties for the ceramic composites based on potassium polytitanates of different chemical composition

The samples of ceramic materials based on potassium polytitanate (PPT) characterized with various TiO2/K2O molar ratio, are produced by calcination at 900 °C and investigated. AC conductivity (σ ac ) of the obtained ceramics is measured at different temperatures between 200 and 800 °C in frequency range of 0.1 Hz–1 MHz. The method of combined impedance and modulus spectroscopy is used to analyze the obtained results. The activation energies of DC conductivity, bulk and grain-boundary conductivity as well as relaxation frequency for studied composites are estimated. Using the correlated barrier hopping (CBH) model, the energies of potential barrier between neighboring defect sites for all kinds of investigated materials are presented. The bulk and grain boundary parameters of the produced ceramic materials based on potassium polytitanates are calculated. The mechanism of different vacancies formation in the investigated ceramic system is discussed. The influence of precursor chemical composition on electrical properties for the ceramic composites based on potassium polytitanates is studied.

Effect of sintering aids on the densification and electrical properties of SiO2 –containing Ce0.8Sm0.2O1.9 ceramic

In this work, five different metal-oxide additives (metal = Ba, Co, Fe, Li, and Mn) were examined as sintering aids and SiO2 impurity scavengers for Ce0.8Sm0.2O1.9 (SDC). 2 mol% additives were loaded into the SDC with ∼150 ppm (moderately impure) and ∼2000 ppm (highly impure) SiO2. Ba-, Co-, Fe- and Mn-oxides showed comparative sintering-aid effect on both moderately- and highly-impure SDC specimens, but the sintering-assisting effect of Li-oxide was completely neutralized in highly impure SDC. Regarding electrical property, the deleterious effect of 2000 ppm SiO2 impurity on the grain-boundary conduction of SDC can be effectively alleviated by adding Ba-, Co-, Fe-, or Mn-oxides. Microstructure analysis revealed that Ba-oxide reacted directly with SiO2 and consequently enhanced grain-boundary conduction. By contrast, with the addition of Co-, Fe-, and Mn-oxides, the improved grain-boundary conductions of impure SDC were related to the scavenging reactions between Si, Ca (another original impurity) and Sm components.