Microelectrodes for local conductivity and degradation measurements on Al stabilized Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript> garnets

The attractiveness of Li₇La₃Zr₂O₁₂ (LLZO) cubic based garnets lies in their high ionic conductivity and the combination of thermal and electrochemical stability. However, relations between composition and conductivity as well as degradation effects are still not completely understood. In this contribution we demonstrate the applicability of microelectrodes (Ø = 20–300 μm) for electrochemical impedance spectroscopy (EIS) studies on LLZO garnets. Microelectrodes allow to obtain local information on the ionic conductivity. A comparison between the overall performance of the sample (3.3 × 10^?4 S cm^?1) and local measurements revealed differences in conductivity with a maximum of the locally measured values of 6.3 × 10^?4 S cm^?1 and a minimum of 2.6 × 10^?4 S cm^?1. One reason behind these conductivity variations is most probably a compositional gradient in the sample. In addition, microelectrodes are very sensitive to conductivity changes near to the surface. This was used to investigate the effect of moisture in ambient air on the conductivity variations of LLZO. Substantial changes of the measured Li-ion transport resistance were found, particularly for smaller microelectrodes which probe sample volumes close to the surface.     

Coke-tolerant La<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>-Ni-Gd<Subscript>0.1</Subscript>Ce<Subscript>0.9</Subscript>O<Subscript>1.95</Subscript> composite anode for direct methane-fueled solid oxide fuel cells

Direct CH₄-fueled solid oxide fuel cells (SOFCs) have been studied for a few decades, but carbon depositions on the Ni-based anodes are still remained as a major problem. In order to enhance coke tolerances and durability of SOFCs, La₂Sn₂O₇ nano-powders are prepared by co-precipitation. The SOFCs with the different amounts of the La₂Sn₂O₇ nano-powders in the Ni-GDC anodes are tested under dry CH₄, and the 0.3 wt.% La₂Sn₂O₇-Ni-GDC (0.3LNG) anodes show the highest cell performances of all anodes. The maximum power density of the cell is approximately 0.55 W cm^?2 at 650 °C. The durability of the 0.3LNG cell is significantly enhanced without any carbon formations, showing approximately 0.69 V over 600 h at 0.3 A cm^?2, whereas the conventional Ni-GDC cell is stopped only after 90 h. It suggests that the 0.3LNG is a promising anode material to enhance coke-tolerances and durability of direct-methane fuel cells.     

Piezoelectric Properties of 0.2[Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)]-0.8[PbTiO<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript>] Ceramics Sintered at a Low Temperature with the Aid of Li<Subscript>2</Subscript>O

The dielectric and piezoelectric properties of 0.2Pb(Mg_1/3Nb_2/3)O₃-0.8Pb(Zr_0.475Ti_0.525)O₃ (abbr. as PMNZT) ceramics were measured. Extremely low sintering temperatures of 950^∘C using liquid-phase sintering aid of Li₂O is achieved which was very useful for multi-layered applications. X-ray study shows the splitting of rhombohedral (200) in pure PMNZT to (002) and (200) peaks in Li₂O doped samples. 10 times higher dielectric constant was achieved in Li₂O doped samples to compare to pure ones although the Curie temperature (T_c = 322^∘C) of Li₂O doped PMNZT ceramics was not changed. The value of k_p and k₃₃ increased up to 0.1 wt% of Li₂O and saturating thereafter.     

Dielectric properties of pure and Mn-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics over a wide temperature range

The effect of manganese doping on the dielectric properties of CaCu₃Ti_4-xMn_xO₁₂ (x?=?0, 0.02, 0.04) were investigated over a broad temperature range (93–723 K) in the frequency range from 100 Hz to 10 MHz. Two dielectric relaxations and two dielectric anomalies were observed. The low-temperature relaxation appearing in the temperature range below 200 K is the characteristic relaxation for CaCu₃Ti₄O₁₂. This relaxation was attributed to the polaron relaxation due to electron hopping between Ti³⁺ and Ti⁴⁺ states. Due to the negative factors of notable decreases in the Ti³⁺/Ti⁴⁺ and Cu³⁺/Cu²⁺ ratios and the concentration of oxygen vacancies as revealed by X-ray photoemission spectroscopy, Mn-doping was found to gradually destroy rather than move this relaxation to a higher temperature. The high-temperature relaxation occurring around room temperature was found to be a Maxwell-Wagner relaxation caused by grain boundaries. Our results confirm that the colossal dielectric behavior in the tested samples results from both polaron and Maxwell-Wagner relaxations, but is predominated by the latter relaxation. The low-temperature anomaly behaves as a phase-transition-like behavior. It was argued to be created by oxygen vacancies transition from static disorder to dynamic disorder. The high-temperature anomaly is an artificial effect caused by negative capacitance.     

Surface and morphological investigation of the electrode/electrolyte properties in an all-solid-state battery using a Li<Subscript>2</Subscript>S-P<Subscript>2</Subscript>S<Subscript>5</Subscript> solid electrolyte

All-solid-state lithium-ion batteries represent a promising battery technology thanks to the replacement of the volatile and flammable state-of-the-art liquid electrolyte by a solid electrolyte. Despite the recent progress in the synthesis of sulfide based solid electrolyte with high ionic conductivity, little is known about the interface reactivity of the solid electrolyte with electrode materials. In this study, we synthesized and characterized an amorphous solid electrolyte with the nominal composition (Li₂S)₃(P₂S₅). We assessed the feasibility of using this electrolyte at the laboratory scale, and we discuss the potential challenges that govern its electrochemical performance. Galvanostatic cycling and rate performance measurements were conducted using lithium titanium oxide (Li₄Ti₅O₁₂) as the negative electrode material. The electrochemical measurements were performed using two different counter electrodes, namely Li metal and an InLi_x alloy. The alloy counter electrode suppressed the formation of lithium dendrites, resulting in increased cycling stability and cell safety. Post mortem X-ray photoemission spectroscopy measurements reveal the reactivity of the solid electrolyte Li₃PS₄ with the Li₄Ti₅O₁₂, lithium metal, and InLi_x alloy.     

Influence of the processing rates and sintering temperatures on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The stoichiometric CaCu₃Ti₄O₁₂ pellets were prepared by the solid state synthesis. X-ray diffraction data revealed the tenorite CuO and cuprite Cu₂O secondary phases on the unpolished CaCu₃Ti₄O₁₂ samples regardless of the heating rates. Also, the dielectric constant marked the highest for the CaCu₃Ti₄O₁₂ sample sintered at the lowest heating rate (1°C/min), which was explained by the increased grain conductivity due to the cation reactions. On the other hand, Cu₂O phase was found only on the unpolished CaCu₃Ti₄O₁₂ sample sintered over 1100°C and those are considered as the remains reduced from the CuO phase. The higher sintering temperature showed the increased dielectric constant and the loss tangent of the CaCu₃Ti₄O₁₂ samples, and this result could be interpreted by the impedance measurement data. The relationship between the processing condition and the dielectric properties was discussed in terms of the cation non-stoichiometry and the defect chemistry in CaCu₃Ti₄O₁₂.     

Excellent dielectric constants observed in heterogeneous conduction Ba(Zr<Subscript>0.25</Subscript>Ti<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> ceramics doped with Sr(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript>

The (1-x)Ba(Zr_0.25Ti_0.75)O₃-xSr(Fe_0.5Nb_0.5)O₃ or (1-x)BZT-xSFN ceramics have been fabricated via a solid-state reaction technique. All ceramics exhibit a pure phase perovskite with cubic symmetry. The addition of a small amount of SFN (x?=?0.1) produces an obvious change in dielectric behavior. Very high dielectric constants (ε_r?>?164,000 at 1 kHz and temperature?>?150°C) are observed and the value is obviously higher than dielectric constants for Ba(Zr_0.25Ti_0.75)O₃ and Sr(Fe_0.5Nb_0.5)O₃ ceramics. The ferroelectric measurement data suggests that the unmodified sample exhibited a ferroelectric behavior. However, a transformation from a ferroelectric to a relaxor-like behavior is noted with increasing x concentration. Impedance Spectroscopy (IS) analysis indicates that the presence of excellent dielectric constants is due to the heterogeneous conduction in the ceramics after adding SFN, which can be explained in terms of the Maxwell-Wagner polarization mechanism.     

Properties and crystallization kinetics of low temperature co-fired Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> electroceramics

The glass-ceramic in the Li₂O-Al₂O₃-SiO₂ system has been prepared by melt quenching route. The crystallization kinetics was studied by differential scanning calorimetry. The effects of sintering temperature on the phase transformation, sintering behavior, bulk density, microstructure, thermal expansion, bending strength and dielectric properties were also investigated by X-ray diffractometry and scanning electron microscopy. (Li, Mg, Zn)_1.7Al₂O₄Si₆O₁₂ is the first crystalline phase forming in the glass-ceramic and transforms to LiAlSi₃O₈ phase at 800 °C. The other two crystalline phases of ZrO₂ and CaMgSi₂O₆ precipitate at 700 and 750 °C, respectively. The densification of this LAS glass-ceramic starts at around 730 °C and stops at about 805 °C. The coefficient of thermal expansion increases with the increasing sintering temperature. The sample sintered at 800 °C for 30 min exhibited excellent properties. The nonisothermal activation energy of crystallization is 149 kJ/mol and the values of Avrami constant (n) are in the range of 3.2 to 3.9. The LAS glass-ceramic sintered at 800 °C for 30 min showed excellent properties. This makes that this material suitable for a number of LTCC applications.     

Evaluation of La<Subscript>2</Subscript>CoTi<Subscript>0.7</Subscript>Mg<Subscript>0.3</Subscript>O<Subscript>6</Subscript> as an electrode material for a symmetrical SOFC

La₂CoTi_0.7Mg_0.3O₆ (LCTM) material has been prepared at 1473 K for 24 h in air. X-ray powder diffraction study has revealed that it contains two orthorhombic perovskite phases (in a ratio ~1:4) with close unit cell parameters. Annealing of LCTM in reducing (Ar/H₂, 8%) atmosphere at 1173 K for 12 h has resulted in the preparation of a single-phase material containing the GdFeO₃-type perovskite phase with the unit cell parameters of a?=?5.5631(3) Å, b?=?5.5462(3) Å, c?=?7.8522(5) Å. LCTM material exhibits a reversible transformation of a mixture of two perovskite phases with close cation content in air and a single perovskite phase in a reducing atmosphere. Both as-prepared and reduced LCTM samples have been studied by thermogravimetric analysis and dilatometry in air and Ar/H₂ (8%). No chemical interaction between the as-prepared LCTM and standard electrolyte materials for SOFC like GDC and YSZ has been observed up to 1273 K. High-temperature electrical conductivity of the as-prepared LCTM at variable oxygen partial pressure (10^?4-0.21 atm) showed weak dependence over pO₂ with E_a?=?0.48?±?0.01 eV. AC impedance study of the symmetrical cells LCTM/GDC/LCTM has revealed ASR value at 1173 K of ~8.1?±?0.1 Ω?cm² in air and 0.24?±?0.05 Ω?cm² in a reducing atmosphere. These results allow to consider LCTM as a promising electrode material for a symmetrical SOFC.     

Structure and properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>-0.38PbTiO<Subscript>3</Subscript> ceramics with MPB composition

0.62Bi(Mg_1/2Ti_1/2)O₃-0.38PbTiO₃-xwt%Bi₂O₃ (BMT-0.38PT-xBi₂O₃) ceramics were prepared by conventional powder-processing method. It indicated that the morphotropic phase boundary (MPB) region located in 0.0?≤?x?≤?0.3. For x?=?0.3, it exhibited good piezoelectric properties, d₃₃ ~245pC/N and k_p ~40 %. With the increase of Bi₂O₃ content, the Curie temperature (T_c) was found to increase, and the dielectric loss was found to decrease above 200 °C compared with BMT-0.38PT sample. Finally, it can be found that depolarization temperature was around 350 °C by thermal depoling method.     

Composition-dependent dielectric properties and energy storage performance of (Pb,La)(Zr,Sn,Ti)O3 antiferroelectric ceramics

In this work, Pb_0.97La_0.02(Zr _x Sn_0.95?x Ti_0.05)O₃ (PLZST) (0.5?<?x?<?0.9) tetragonal antiferroelectric (AFE_T) and orthogonal antiferroelectric (AFE_O) ceramics were successfully fabricated by screen printing process. The ceramic materials were in thick-film form bonded with a small amount of glass. The electric field up to 400 kV/cm was presented for antiferroelectric ceramics. Besides, in order to reduce the energy loss of ceramics, the effects of Sn content and temperature on the dielectric properties and energy storage performance of AFE ceramics were investigated. With the increase of Sn content, the forward threshold electric field (E _AF) and backward threshold field (E _FA) decreased and the energy storage density increased obviously. The maximum energy storage density of 5.6 J/cm³ (30 °C) and 4.7 J/cm³ (120 °C) with corresponding energy efficiency of 67 % and 73 % were obtained in Pb_0.97La_0.02(Zr_0.5Sn_0.45Ti_0.05)O₃ ceramic, which makes this material a promising potential application in capacitors for pulsed power systems.     

Effects of Processing Conditions on the Dielectric Properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

There have been a number of recent reports of anomalously large permittivities (ε _r ≈ 10⁴) in the material CaCu₃Ti₄O₁₂. The dielectric spectra is characterized by a large, relatively temperature independent permittivity near room temperature which exhibits a dielectric relaxation above 100 K. The crystal structure of CaCu₃Ti₄O₁₂ can be described as pseudo-perovskite with a cubic unit cell with a lattice constant of 7.391 Å. The ubiquitous occurrence of this dielectric behavior in ceramics, single crystals, and thin films suggests that the polarization is not related to a simple conducting grain/insulating grain boundary-type system. While the precise origin of the dielectric response is not entirely clear, in this work it is shown that processing conditions have a significant influence on the room temperature dielectric properties. Specifically, the permittivity and loss exhibit a strong dependence on the oxygen partial pressure and sintering time. In fact, studies of the effects of sintering time and supporting evidence from capacitance-voltage measurements conclusively show that there is no direct relationship between the permittivity and grain size, as is the case in classical boundary layer systems. Lastly, with aliovalent doping the room temperature dielectric properties can be optimized to provide a high permittivity (ε _r ～ 8,000) dielectric with relatively low loss (tan δ < 0.05 at 1 kHz).     

Analysis of domain structure in the Ce<Subscript>0.8</Subscript>Sm<Subscript>0.15</Subscript>Ca<Subscript>0.05</Subscript>O<Subscript>1.875</Subscript> sample

A Ce_0.8Sm_0.15Ca_0.05O_1.875 (S15C05DC) sample is synthesized by a solid-state reaction serving as a potential electrolyte material for intermediate-temperature solid oxide fuel cells. The sintered sample was found to be dense with a cubic fluorite structure. The addition of Ca²⁺ can act as a CeO₂ sintering aid for accelerating the process. The microstructures and properties of the sample were analyzed by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy, thermomechanical analysis, and transmission electron microscopy. Existing oxygen vacancies in the sample are indicated by a Raman peak at 558 cm^?1. The thermal expansion coefficient of the S15C05DC sample at 200–800 °C is approximately 12–14 × 10^?6 °C^?1. The control of domain size is an important factor for improving the conductivity of S15C05DC. Local clustered nano-domains, with higher Sm₂O₃ concentrations, were found to regularly arrange to induce the formation of a nanoscale C-type superlattice structure. While Ca doping decreased the formation of the C-type Sm₂O₃ structure.     

Site Preference of La in Bi<Subscript>3.75</Subscript>La<Subscript>0.25</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Using Neutron Powder Diffraction and Raman Scattering

Both structural refinement using neutron powder diffraction data and Raman scattering were carried out to determine the site preference of La atoms and the cation distribution in Bi_3.75La_0.25Ti₃O₁₂ compound. Of three possible cation-disorder models, the best structural refinement result was obtained from a model that La atoms substitute only for Bi atoms outside of the TiO₆ octahedra in the Bi₂Ti₃O₁₀ unit. The model proposed by the structural refinement was corroborated by the Raman spectroscopic study. The final weighted R-factor, R_wp, and the goodness-of-fit indicator, S (= R_wp/R_e), based on the neutron diffraction and the Raman scattering were 4.12% and 1.43, respectively. The occupancy of La atoms for two Bi sites in the perovskite-like unit was 0.082 and 0.074, respectively. The refined model described a structure in monoclinic space group B1a1 with Z = 4, a = 5.4387(1) Å, b = 5.4129(1) Å, c = 32.8441(1) Å and = 90.03(1).     

Electrical conductivity of the Al-stabilized La<Subscript>2/3</Subscript>TiO<Subscript>3</Subscript>

A-site deficient lanthanum titanate (La_2/3TiO₃) materials with perovskite structure are attractive due to their electrical applications such as ion conductors and dielectrics. However, its stability at room temperature in air is obtained only if Na or Li etc. is incorporated into La site or Al into Ti site. In this study, the electrical conductivities of La_0.683(Ti_0.95Al_0.05)O₃ have been measured in oxygen partial pressure (Po₂) between 1 and 10⁻¹⁸ atm at 1000~1400°C. The electrical conductivity exhibited −1/4, −1/6 and −1/5 dependence (log σ ∝ log , n = −1/4, −1/6, −1/5) depending upon temperature and Po₂. The defect model explaining the observation was proposed and discussed. The chemical diffusion coefficient was estimated from the electrical conductivity relaxation.     

Formation of a KNbO<Subscript>3</Subscript> single crystal using solvothermally synthesized K<Subscript>2-m</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6-m/2</Subscript> pyrochlore phase

A K_2-mNb₂O_6-m/2 single crystal with a pyrochlore phase formed when the Nb₂O₅?+?x mol% KOH specimens with 0.6?≤?x?≤?1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K_1.3Nb₂O_5.65. The single-crystal KNbO₃ formed when the single-crystal K_2-mNb₂O_6-m/2 was annealed at a temperature between 600 °C and 800 °C with K₂CO₃ powders. When annealing was conducted at 600 °C (or with a small amount of K₂CO₃), the KNbO₃ single crystal has a rhombohedral structure that is stable at low temperatures (< ? 10 °C). The formation of the rhombohedral KNbO₃ structure can be explained by the presence of the K⁺ vacancies in the specimen. The KNbO₃ single crystal with an orthorhombic structure formed when the K_2-mNb₂O_6-m/2 single crystal was annealed at 800 °C with 20 wt% of K₂CO₃.     

Effect of transition metal substitution on elastoplastic properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel

LiMn₂O₄ (LMO) derivatives partially substituted with transition metals (e.g., Ni) have received attention for their higher energy density achieved at higher charge voltage than pure LMO, and may be attractive cathode candidates for emerging all solid state batteries. Accurate mechanical properties of these high voltage spinels are required for prediction of electrode and electrolyte fracture that may compromise battery lifetime and performance. Here, we quantified the Young’s elastic modulus E and hardness H for LMO, LiMn_1.5Ni_0.5O₄ (LMNO), and LiMn_1.5Ni_0.42Fe_0.08O₄ (LMNFO) spinel microparticles via instrumented grid nanoindentation. Elastic modulus E and hardness H increased by more than 40% (up to 145 and 11 GPa, respectively) as a result of Ni or Ni/Fe substitution; such substitution also reduces the lattice parameter and increases the oxidization state of Mn. These results demonstrate how changes in transition metal occupancy can significantly affect the mechanical properties of LMO spinel, and provide critical parameters for designing against fracture in all solid state batteries.     

Ionic conduction and crystal structure of aluminum doped NASICON-type LiGe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic crystallized at different times and temperatures

In this communication, NASICON-type glass-ceramic (lithium germanium phosphate, LiGe₂(PO₄)₃) was prepared as lithium super ionic conductor using aluminum as dopant for ionic conduction improvement. The solid solution was Li_1?+?xAl_xGe_2-x(PO₄)₃ (x?=?0.5) that Ge⁴⁺ ions were partially substituted by Al³⁺ ions in crystal structure. Initial glasses were converted to glass-ceramics at different times and temperatures for maximum ionic conduction achievement. The crystals were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS) methods. The maximum lithium ion conductivity for glass-ceramic, 5.32?×?10^?3 S/cm at 26 °C was obtained for specimen crystallized at 850 °C for 8 h with minimum activation energy of 0.286 eV. Increasing the crystallization temperature results in secondary phase formation in grain boundary and increasing in crystallization time results in microcracks formation in specimen. Both phenomena decreased the ionic conductivity.     

Structural and magnetic evolution of Fe-doped TiO<Subscript>2</Subscript> nanoparticles synthesized by sol-gel method

Nanocrystalline Ti_1-x Fe _x O₂ particles were fabricated via sol-gel method and their structures, morphology and magnetic properties were investigated. The structure of the Ti_1-x Fe _x O₂ nanospheres evolved from mixed anatase and rutile phases to pure anatase phase with increasing iron content. Additionally, it is found the evolution of magnetism: sample x = 3% shows room temperature ferromagnetism while the rests exhibit paramagnetism. The hysteresis loop of sample x = 3% is attributable to paramagnetic and ferromagnetic phase and the paramagnetic and ferromagnetic components are separated. The susceptibility of Ti_1-x Fe _x O₂ increases and then decreases with the increase of Fe content. The magnetism is explained by the BMP theory.     

Epitaxial Pb(Zr,Ti)O<Subscript>3</Subscript> Capacitors on Si by Liquid Delivery Metalorganic Chemical Vapor Deposition

La_0.5Sr_0.5CoO₃/Pb(Zr_xTi_1–x)O₃/La_0.5Sr_0.5CoO₃ capacitors have been successfully fabricated by liquid delivery metalorganic chemical vapor deposition on Si wafers using SrTiO₃ thin layer (20 nm) as a template. Zr(dmhd)₄ in tetrahydrofuran was used as Zr precursor for compatible thermal behavior with Pb(thd)₂ and Ti(OiPr)₂(thd)₂ precursors. The dependence of the ferroelectric film composition on the precursor mixing ratio and growth temperature has been systematically studied by Rutherford Backscattering (RBS). Ferroelectric and piezoelectric properties at the composition close to morphotropic phase boundary region (Pb(Zr_0.5Ti_0.5)O₃) have been investigated for application in nonvolatile ferroelectric random access memories and microelectromechanical system (MEMS). These capacitors show desirable ferroelectric properties, which proves that this approach is very promising for both fundamental study and potential applications. The changes of spontaneous polarization (P_s) and piezoelectric coefficient (d₃₃) with Ti/(Zr + Ti) ratio are also presented and compared with theoretical values.