Enhanced grain boundary ionic conductivity of LiTa2PO8 solid electrolyte by 75Li2O-12.5B2O3-12.5SiO2 sintering additive

LiTa₂PO₈(LTPO) has low electrolyte density and many pores at grain boundaries, and it is easy to precipitate dielectric phase LiTa₃O₈ at grain boundaries. The performance can be improved by adding 75Li₂O-12.5B₂O₃-12.5SiO₂ (LBS) sintering additive with low melting point during sintering. The effects of LBS addition on the microstructure and grain boundary ionic conductivity of LTPO electrolytes were studied. The results showed that the addition of LBS sintering additives reduced the sintering temperature, improved the density and stability of LTPO electrolyte samples, effectively inhibited the precipitation of LiTa₃O₈ phase, reduced the grain boundary impedance of samples, and improved the total ionic conductivity of electrolytes. When LBS was added at 0.4 wt%, the relative density of LTPO reached 93.54%, the grain boundary impedance decreased from 1243 Ω to 248.2 Ω, the total ionic conductivity increased from 1.55 × 10⁻⁴ S cm⁻¹ to 6.51 × 10⁻⁴ S cm⁻¹, and the ionic activation energy was 0.137 eV.     

Effect of excess TiO2 addition on the PTC properties of Ba0.99Y0.01TiO3-0.001MnO based ceramics

BaTiO₃ is a typical ceramic with a positive temperature coefficient of resistivity (PTC). The grain conductivity of BaTiO₃ is associated with doping, and the grain boundary plays an important role in the PTC effect. In this study, Ba_0.99Y_0.01TiO₃-0.001MnO-xTiO₂ (x=0.01–0.07) ceramics were synthesized by the conventional solid-state reaction method in air atmosphere. The influence of excess TiO₂ on the crystal structure, microstructure, binding energy and the PTC characteristics were investigated by XRD, SEM, EDS, XPS and impedance spectroscopy, respectively. The results showed that excellent room-temperature resistivity and resistance jump ratio (R_max/R_min) up to 84 Ω cm and 1.6×10⁵ could be obtained possibly associated with the two different roles played by Ti ions at the grain boundaries.     

Microstructural evolution and dielectric properties of SiO2-doped CaCu3Ti4O12 ceramics

The abnormal grain growth (AGG) behavior of undoped and SiO₂-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. With the addition of 2 wt.% SiO₂, the AGG-triggering temperature decreased from 1100 to 1060 °C, and the temperature for obtaining a uniform and coarse microstructure decreased from 1140 to 1100 °C. The lowering of the AGG temperature by SiO₂ addition was attributed to the formation of a CuO-SiO₂-rich intergranular phase at lower temperature. The apparent dielectric permittivity of coarse SiO₂-doped CCTO ceramics was ∼10 times higher than that of fine SiO₂-doped CCTO ceramics at the frequency of 10³–10⁵ Hz. The doping of SiO₂ to CCTO ceramics provides an efficient route of improving the dielectric properties via grain coarsening. The correlation between the microstructure and apparent permittivity suggests the presence of a barrier layer near the grain boundary.     

Effect of mixing glass frits on electrical property and microstructure of sintered Cu conductive thick film

The resistivity of the sintered Cu thick film decreases with the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits up to 50 wt%. As the weight percentage of the SiO₂–ZnO–B₂O₃ additive in the mixing glass frits is over 50 wt%, the resistivity of the sintered Cu thick films is quite similar. The lowest resistivity (6.62 × 10⁻⁶ Ω-cm) of the sintered Cu thick films occurs at 75 wt% of the SiO₂–ZnO–B₂O₃ additive. Also, we observe the extensive glass phase framing around the large Cu grains in the Cu thick films sintered with low SiO₂–ZnO–B₂O₃ additives (less than 50 wt%) narrows the cross-section area of the electrical path. On the contrary, the round-shaped glass phase solidified among the small Cu grains allows a larger cross-section of the electrical path (a possible lower resistivity) for the Cu thick films sintered with higher SiO₂–ZnO–B₂O₃ additives (larger than 50 wt%). The above results imply that the resistivity of the sintered Cu thick film correlates well with the microstructure (Cu grain size and the glass/Cu composite structure) of the sintered Cu thick films. Twin grain boundaries can clearly be observed in the sintered Cu thick films, especially for the Cu thick film sintered with the higher SiO₂–ZnO–B₂O₃ additives. Owing to small Cu grains size and high density of Cu grain boundary, the probability of the grain boundaries with a high grain-boundary energy in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive would be much larger, comparing to that in the Cu thick film sintered with low SiO₂–ZnO–B₂O₃ additive. Thus, more annealing twin boundaries formed in the Cu thick film sintered with high SiO₂–ZnO–B₂O₃ additive. Hence, the formation of the twin boundary in the sintered Cu thick film helps reducing the resistivity of the sintered Cu films.     

Microstructural design and high temperature tensile deformation behaviour of 8 mol% yttria stabilized cubic zirconia (8YCSZ) with SiO2 additions

In the present study, the microstructural evolution and high temperature deformation behaviours of 8 mol% Y₂O₃ stabilized cubic zirconia (8YCSZ) containing up to 10 wt% SiO₂ is investigated. The experimental results show that the SiO₂ doped specimens sintered at 1400 °C contain only the cubic crystalline phase and SiO₂ has the very limited solubility of 0.3 wt% in cubic zirconia. This suggests that only small part of the SiO₂ dissolves in the cubic zirconia and the rest of SiO₂ segregates at grain boundaries and multiple junctions as amorphous (glassy) phase. This glassy phase prevents the grain growth by minimizing grain boundary energy and mobility, which results from solute segregation at the grain boundary and its drag. The deformation of the undoped 8YCSZ is characterized by large strain hardening with limited elongation. This is mainly due to severe grain growth during high temperature deformation. The addition of the SiO₂ results in a decrease in strain hardening and enhanced tensile elongation. These effects have been further improved with the increase of the SiO₂ addition reaching the elongation to failure of 152% for 10 wt% SiO₂ doped specimen in tension at a temperature of 1400 °C and strain rate of 1.3 × 10⁻⁴ s⁻¹. The decreased strain hardening and increased ductility in the SiO₂ doped specimens are due to the segregation of amorphous glassy phase to the grain boundaries, thus hindering grain growth and facilitating grain boundary sliding, which is the primary mechanism of deformation in fine grained materials at high temperatures.     

Effects of Y2O3 additives and powder purity on the densification and grain boundary composition of Al2O3/SiC nanocomposites

Sub-micron sized SiC additions can be used to increase the wear resistance and change the fracture mode of Al₂O₃. However, these additions also restrict sintering.Al₂O₃ and Al₂O₃–5%SiC ‘nanocomposites’ were prepared from alumina powders of high purity and of commercial-purity, with or without the addition of Y₂O₃. The effects of these compositional variables on sintering rate, final density and grain boundary composition were investigated. A direct comparison with Al₂O₃–SiO₂ composites was also made, as it has been proposed that SiC partially oxidises during processing of Al₂O₃–SiC nanocomposites.The addition of 5 vol.% SiC to Al₂O₃ hindered densification, as did addition of 0.15 wt.% Y₂O₃ or 0.1 wt.% SiO₂. In contrast, the addition of 0.15 wt.% Y₂O₃ to Al₂O₃–5% SiC nanocomposites improved densification.The composition of Al₂O₃–Al₂O₃ grain boundaries in these materials was studied using STEM and EDX microanalysis. The addition of SiC and SiO₂ caused segregation of Si, and Y₂O₃ addition caused segregation of Y. The segregation of each element was equivalent to <10% of a monolayer at the grain boundary. However, if SiC and Y₂O₃ were simultaneously added the segregation increased to 40% of a monolayer. The enhanced segregation was attributed to increased oxidation of SiC in the presence of Y₂O₃ allowing formation of a SiO₂–Al₂O₃–Y₂O₃ eutectic phase or a segregated layer which may explain the improvement in sintering rate when Y₂O₃ was added to nanocomposites.     

Effects of SiC content on the densification,microstructure, and mechanical properties of HfB2–SiC composites

To investigate the effects of SiC on microstructure, hardness, and fracture toughness, 0, 10, 20, and 30 vol% SiC were added to HfB₂ and sintered by SPS. Upon adding SiC to 30 vol%, relative density increased about 4%; but HfB₂ grain growth had a minimum at 20 vol% SiC. This may be due to grain boundary silicate glass, responsible for surface oxide wash out, enriched in SiO₂ with higher fraction of SiC. By SiO₂ enrichment, the glass viscosity increased and higher HfO₂ remained unsolved which subsequently lead to higher grain growth. Hardness has increased from about 13 to 15 GPa by SiC introduction with no sensible variation with SiC increase. Residual stress measurements by Rietveld method indicated high levels of tensile residual stresses in the HfB₂ Matrix. Despite the peak residual stress value at 20 vol% SiC, fracture toughness of this sample was the highest (6.43 MPa m^0.5) which implied that fracture toughness is mainly a grain size function. Tracking crack trajectory showed a mainly trans-granular fracture, but grain boundaries imposed a partial deflection on the crack pathway. SiC had a higher percentage in fracture surface images than the cross-section which implied a weak crack deflection.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x = 0.10, Y_2/3?0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.     

Powder chemistry effects on the sintering of MgO‐doped specialty Al2O3

In this work, we investigate the effects of powder chemistry on the sintering of MgO‐doped specialty alumina. The stages at which MgO influences densification of Al₂O₃ were identified by comparing dilatometry measurements and the sintering kinetics of MgO‐free and MgO‐doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO₂ in alumina grains near the boundaries using EDS. First‐principles DFT calculations demonstrate that the co‐dissolution of MgO and SiO₂ in alumina is thermodynamically favored over the dissolution of MgO or SiO₂ individually in alumina. This study experimentally demonstrates for the first time that removal of SiO₂ from the grain boundaries is a key process by which MgO enhances the sintering of alumina.     

Liquid phase sintering of Al2O3/SiC nanocomposites

Al₂O₃/SiC nanocomposites are usually prepared by hot pressing or using high sintering temperatures, viz. 1700°C. This is due to the strong inhibiting effect of the nano-sized SiC particles on the densification of the material. Liquid phase sintering (LPS) can be used to improve densification. This work explored two eutectic additive systems, namely MnO₂.SiO₂ (MS) and CaO.ZnO.SiO₂ (CZS). The additive content in Al₂O₃/5 wt% SiC nanocomposite material varied from 2 to 10 wt%. Densities of up to 99% of the theoretical value were achieved at temperatures as low as 1300°C. Characterisation of the materials by XRD, indicated the formation of secondary crystalline phases in addition to Al₂O₃ and SiC. SEM and TEM analysis showed the presence of a residual glassy phase in the grain boundaries, and an increase in the average grain size when compared to nanocomposites processed without LPS additives.     

The transition from non-ohmic to ohmic characteristics in La2O3 doped ZnO–MgO–TiO2 linear resistance ceramics

La₂O₃ doped ZnO–MgO–TiO₂ based linear resistance ceramics were prepared by the solid phase sintering method. The doping content of La₂O₃ is from 0.0 wt% to 2.5 wt%. The solubility of La₂O₃ in ZnO is less than 0.06 mol% (0.5 wt%), La_0.66TiO_2.993 phases will be formed at grain boundary and change the distribution of spinel phase when La₂O₃ is excessive. For I–V test, undoped sample exhibits typical non-ohmic characteristics, but La-doped samples show excellent ohmic behaviors under low DC and high pulse current (PC). The complex impedance spectrum and the frequency dependent conductivity furtherly demonstrate that La-doped samples possess linear characteristics because there is no grain boundary effect which can affect the electron transmission at grain boundaries. Besides, the decrease of the grain boundary barrier from 0.2135eV for undoped sample to 0.0031eV for 0.5 wt% La doped samples can account for the elimination or reduction of grain boundary effect. In this work, the transition from non-ohmic to ohmic properties by doping La₂O₃ in ZnO–MgO–TiO₂ multiphase ceramics is realized.     

Effect of SiO2 additive on dielectric response and energy storage performance of Ba0.4Sr0.6TiO3 ceramics

SiO₂-added barium strontium titanate ceramics Ba_0.4Sr_0.6TiO₃-xwt%SiO₂ (x=0, 0.5, 1, 3, BSTSx) were prepared via a traditional solid state reaction method. The effect of SiO₂ additive on the microstructure, dielectric response and energy storage properties was investigated. The results confirmed that with the increase of SiO₂ additive, diffuse phase transition arises and the dielectric constant decreases. An equivalent circuit model and Arrhenius law were used to calculate the activation energy of grain and grain boundary, which indicated that the dielectric relaxation at high temperature was caused by oxygen vacancy. While appropriate SiO₂ additive led to improve the breakdown strength, further increase of SiO₂ deteriorated the energy storage because of the low densification. Finally, optimized energy storage performance was obtained for BSTS0.5 ceramics: dielectric constant of 1002, dielectric loss of 0.45%, energy density of 0.86 J/cm³ and energy storage efficiency of 79% at 134 kV/cm.     

The effect of SiO2 on electrical properties of low‐temperature‐sintered ZnO–Bi2O3–TiO2–Co2O3–MnO2‐based ceramics

ZnO–Bi₂O₃–TiO₂–Co₂O₃–MnO₂‐based (ZBTCM) varistors were fabricated via the conventional solid‐state method, and the effect of SiO₂ content on the phase transformation, microstructure, and electrical properties of the ZBTCM had been investigated. Results showed that this varistor can be sintered at a low temperature of 880°C with a high sintering density above 0.95 of the ZnO theoretical density. In these components, SiO₂ acts as a controller in ZnO grain growth, decreasing the grain size of ZnO from 3.67 to 1.92 μm, which in turn results in an increase in breakdown voltage E_1mA from 358.11 to 1080 V/mm. On the other hand, SiO₂ has a significant influence on the defect structure and component distribution at grain‐boundary regions. When SiO₂ content increases from 0 to 4 wt%, the value of the interface state density (N_s) increases sharply. At the same time, the electrical properties are improved gradually, and reach an optimized value with the nonlinear coefficient (α) up to 54.18, the barrier height (?_b) up to 2.90 eV, and the leakage current (I_L) down to 0.193 μA/cm².     

Grain-boundary engineering inducing thermal stability,low dielectric loss and high energy storage in Ta+Ho co-doped TiO2 ceramics

How to achieve a giant dielectric constant and high energy storage density at the same time has been the problem to be solved for donor-acceptor co-doped TiO₂ ceramics. In this work, (Ho_0.5Ta_0.5)_0.01Ti_0·99O₂ - x SiO₂, where x = 0, 1, 3, 5 and 7 wt% (HTTO - x wt% SiO₂), nanocomposites were prepared via a conventional mixed oxide technique. Significantly, the HTTO - 5 wt% SiO₂ composite ceramic exhibits a low dielectric loss (tanδ ～ 0.012) and an ultrahigh permittivity (ε_r ～ 1.29 × 10⁴) at 1 kHz. Also, excellent energy storage property with a high breakdown field strength (E_b ～1.86 kV/cm) and energy storage density (η ～ 1.97 mJ/cm³) was obtained in HTTO - 5 wt% SiO₂ ceramic. Besides, the enhancement of E_b is attributed to the finer grains and the presence of SiO₂ blocking layers in the grain boundaries, which hinder the long-range motion of electrons. It can be concluded that the CP and high energy storage properties arise from the combined contribution of enhanced grain boundary effects and electron-pinning type of defect dipole (EPDD) effects. This study not only proposes an effective method improving E_b, but also offers a new routine for how to simultaneously achieve CP and high η in TiO₂ dielectric materials.     

Influence of WO3‐Doping on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO₃‐doped ZnO–Bi₂O₃‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W⁶⁺ in the β‐Bi₂O₃ acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO₃ equal to that of Bi₂O₃, the electrical properties dramatically increased, which means the W⁶⁺ donor effect is removed at the grain boundaries because a new Bi₂WO₆ phase was formed in the grain‐boundary regions. The Bi₂WO₆ phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO₃ that is greater than the content of Bi₂O₃), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO₄ phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.     

Improved Energy Storage Properties of Fine‐Crystalline BaTiO3 Ceramics by Coating Powders with Al2O3 and SiO2

The multilayer structure of capacitor demands for fine grain size of dielectric ceramics in devices, because the thinner layer which needs ceramics with fine grain size is helpful in enlarging the capacitance. In this paper, the aqueous chemical coating method was utilized to modify the BaTiO₃ particles. The fine‐crystalline BaTiO₃ ceramics with an average grain size below 200 nm without abnormal grain growth by co‐coating Al₂O₃ and SiO₂ has been prepared. The phase composition, microstructures of coated particles and ceramics, and dielectric properties were investigated. For samples containing 3 wt% of Al₂O₃ and 1 wt% of SiO₂, the energy storage density is 0.725 J/cm³ and the efficiency of the ceramic samples can keep above 80%. The breakdown strength was improved to about 190 kV/cm.     

Structural and electrical characterisation of silica-containing yttria-stabilised zirconia

Zirconia stabilised by yttria has a high oxide ion conductivity at high temperature and, therefore, is currently used as electrolyte in Solid Oxide Fuel Cells. Silica is normally avoided in this material because formation of amorphous silica phases along the grain boundaries causes an increased grain boundary impedance. The present study examines the effect of SiO₂ and SiO₂ with Mn-oxide on the structure and resistivity of yttria stabilised zirconia electrolyte materials. During fabrication of Solid Oxide Fuel Cells, Mn readily diffuses from the manganite-based cathode into the electrolyte. It is shown that a grain boundary phase which causes an insulating layer in the grain boundaries is formed when both SiO₂ and Mn-oxide coexist in the samples, whereas such effects are much less pronounced when only SiO₂ is present.     

Characterisation of the R–T response of BaTiO3 thermistors on three different length scales

Scatter in the resistance–temperature (R–T) characteristic of BaTiO₃-based PTC thermistors was investigated on three scales of measurement, corresponding to bulk pellet, a region approximately six grains wide, and single grain boundaries. On the bulk pellet scale the R–T responses were highly consistent with only small variations in peak resistance. Comparison of R–T measurements across pairs of electrodes separated by approximately six grain diameters revealed systematic changes with position across the pellet, as well as local scatter, corresponding to local resistivity differences of up to a factor of five on the 50 μm scale in the PTC region. These were attributed to differences both in local composition and the local level of porosity, which affects oxidation during cooling. The R–T responses of individual high angle grain boundaries showed significant variations in PTC curve shape and degree of field dependence between boundaries.     

Preparation,oxidation property and mechanism of Si3N4/O′-SiAlON composite ceramics

The applications of Si₃N₄ ceramics were significantly restricted because of the disastrous failure resulted from the oxidation weight gain. The generation of O′-SiAlON could effectively address this issue. The effect of N₂ gas pressure on the phase evolution of the Si₃N₄/O′-SiAlON was studied. It was found that high N₂ gas pressure (3 MPa) was more favorable for the formation of the O′-SiAlON than low N₂ gas pressure (0.6 MPa). Furthermore, the effects of SiO₂ content on the phase evolution, microstructure, oxidation properties and mechanism of the Si₃N₄/O′-SiAlON ceramics were investigated. The results revealed that the relative content of the O′-SiAlON phase evidently enhanced from 0 wt% to 18.15 wt%, and the bulk density decreased from 3.01 g/cm^?3 to 2.62 g/cm^?3 with an increase in SiO₂ from 0 wt% to 12.5 wt%. Additionally, the weight gain, oxide layer thickness and roughness similarly reduced from 2.02 mg/cm² to 0.85 mg/cm², 133.87 μm to 2.31 μm and 21.91 μm to 6.34 μm, respectively. The addition of SiO₂ could also reduce bubbles and cracks formation and hinder the diffusion of Al and Y elements from the interior to the surface. Finally, the oxidation resistance mechanism was mainly credited to the pinning effect of O′-SiAlON phases at the grain boundaries.