Microwave dielectric properties of barium substituted screen printed CaBi2Nb2O9 ceramic thick films

In the present study, Aurivillius-structured Ba²⁺ substituted CaBi₂Nb₂O₉ (CBNO) ceramic powder was synthesized by co-precipitation method. The CBNO thick films were delineated by screen printing method on alumina substrates using co-precipitated ceramic powder. The overlay method was adopted to measure the microwave dielectric properties of prepared thick films. Single phase layered perovskite structure of the prepared thick films was confirmed by X-ray Diffraction. The effects of Ba²⁺ substitution on the surface morphology, bonding, and microwave dielectric properties of thick films were systematically presented. The maximum value of microwave dielectric constant for the CBNO thick films at 11.8 GHz is 15.6 for Ba²⁺=0.8 substitution. The shift in the stretching vibration modes of the Nb-O bond of NbO₆ octahedron in the Raman spectra with a substitution of Ba²⁺ in CBNO was observed. The substitution of Ba²⁺ on A-site of CBNO improves the microwave dielectric properties of prepared thick films. This work may provide a new approach to enhance the microwave dielectric performance of Aurivillius-structured ceramic thick films.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Sintering behavior,microstructure and thermoelectric properties of calcium cobaltite thick films for transversal thermoelectric multilayer generators

The sintering behavior and the thermoelectric performance of Ca₃Co₄O₉ multilayer laminates were studied, and a multilayer thermoelectric generator was fabricated. Compacts and multilayer samples with anisotropic microstructure and residual porosity were obtained after conventional sintering at 920 °C, whereas dense and isotropic multilayer samples were prepared by firing at 1200 °C and reoxidation at 900 °C. A hot-pressed sample has a dense and anisotropic microstructure. Samples sintered at 920 °C exhibit low electrical conductivity due to the low density, whereas the Seebeck coefficient is not sensitive to preparation conditions. However, thermal conductivity of multilayers is very low, and, hence acceptable ZT values are obtained. A transversal multilayer thermoelectric generator (TMLTEG) was fabricated by stacking layers of Ca₃Co₄O₉ green tapes, AgPd conductor printing, and co-firing at 920 °C. The TMLTEG has a power output of 3 mW at ΔT = 200 K in the temperature interval of 25 °C to 300 °C.     

Microstructural,structural, dielectric and piezoelectric properties of potassium sodium niobate thick films

In the framework of a systematic study, we present the influence of processing parameters – in particular the presence of a packing powder during sintering and the sintering temperature – on the microstructural and structural properties of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. These KNN thick films were prepared with a 1 mass% addition of potassium sodium germanate (KNG), which serves as a liquid-phase sintering aid. The sintered films exhibited preferential crystallographic orientations along [100]_pc and [10−1]_pc, the origin of which lies in the compressive stresses developed during cooling as a result of the thermal expansion mismatch between the film and the substrate. In addition, the dielectric permittivity, dielectric losses and the piezoelectric d₃₃ coefficient of the obtained films were compared with those of KNN bulk ceramics.     

Synthesis, characterization and electrochemical properties of copper phosphide (Cu3P) thick films prepared by solid-state reaction at low temperature: a probable anode for lithium ion batteries

Copper phosphide (Cu₃P) was produced as thick films over copper foils. The synthesis was performed by solid-state reaction at low temperature (400 °C). Similar attempts were carried out for other transition metals of the first series without success. Scanning electron microscopy (SEM) revealed the formation mechanism of the Cu₃P thick films. First, phosphorus diffuses into the copper foil followed by the subsequent formation of the binary compound. During this process, the Cu₃P particles seem to dig the copper foil, producing holes, where the Cu₃P crystallites nucleate and growth. Then, the thick films are formed by the conjugation of several agglomerates and their morphology is not homogeneous. Oxidation of Cu₃P occurs to a small extend on the top surface of the films. The electrochemical behaviour of the thick film was compared with a standard Cu₃P composite electrode, in which the active material is mixed with carbon and a binder. Although the two different electrodes presented some differences in their electrochemical behaviour, both electrodes showed promising qualities to be used as anode materials in lithium ion batteries or hybrid devices.     

Ternary doping of Na+, Bi3+ and La3+ to improve piezoelectric constant and electrical resistivity simultaneously in calcium bismuth niobate ceramics

High Curie-temperature layer-structured calcium bismuth niobate (CaBi₂Nb₂O₉) piezoelectric ceramics are promising for important application in high-temperature vibration sensors. However, such application is currently limited due to not only poor high-temperature piezoelectric constant (d₃₃), which is attributable to spontaneous polarization along a-b plane and high coercive fields, but also inferior high-temperature electrical resistivity, which results from volatilization of Bi₂O₃ during the sintering process that increases defect concentration of oxygen vacancies. Herein, we report a Na⁺, Bi³⁺ and La³⁺ ternary-doping-strategy to obtain Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics, which exhibited higher piezoelectric constant and larger electrical resistivity as accompanied by a better thermal stability at high-temperatures. The piezoelectric constant was enhanced from 8.8 pC/N in pristine CaBi₂Nb₂O₉ to 13.4 pC/N in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics, which is ascribed to the presence of pseudo-tetragonal structural distortion after La³⁺ doping. In addition, the electrical resistivity at 600 °C was increased by more than one-order of magnitude from 3.7 × 10⁴ Ω cm in pristine CaBi₂Nb₂O₉ to 1.4 × 10⁶ Ω cm in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics. Such significant improvement in electrical resistivity results from the reduction in oxygen vacancies due to ternary doping of Na⁺, Bi³⁺ and La³⁺ and stronger binding interaction between La³⁺ dopants and O^2? in (Bi₂O₂)²⁺ layers in Ca_0.8(Na_0.5La_0.3Bi_0.2)_0.2Bi₂Nb₂O₉ ceramics. This work demonstrates an important way of employing chemical doping to improve piezoelectric constant and electrical resistivity simultaneously at high-temperatures to tune structural distortion in bismuth-layered structural CaBi₂Nb₂O₉ ceramics.     

Effect of Ag+ doping and Ag addition on the thermoelectric properties of KSr2Nb5O15

The electron and phonon thermal transport behavior of Ag ⁺ doped KSr₂Nb₅O₁₅ were discussed by using the first-principles calculations. The band gap was reduced after Ag⁺ doping, and the electrons near the Fermi level had stronger transition capability, which effectively increased the carrier concentration and electrical conductivity and reduced the thermal conductivity, thereby improving the ZT of the doped KSr₂Nb₅O₁₅ from 0.6298 to 0.7214 (1200 K) under ideal conditions. In addition, the solid-state reaction method was used to prepare Ag nanoparticle added KSr₂Nb₅O₁₅ samples, and their thermoelectric performance was tested. The experimental results and the calculated results showed a good consistent trend in which Ag improved the thermoelectric properties of KSr₂Nb₅O₁₅. When the amount of addition of nanosized Ag was 20 wt%, the power factor and ZT of the material were the highest at 1073 K, which were 0.228 mW/(K²·m) and 0.1090, respectively. This research shows how to improve the thermoelectric performance of KSr₂Nb₅O₁₅ ceramics and broaden their temperature range for application.     

Improved conductivity in tape casted Li-NASICON supported thick films: Effect of temperature treatments and lamination

In this work, the influence of different thermal sintering treatments on Li_1.3Al_0.3Ti_1.7(PO₄)₃ NASICON thick films has been investigated. The isostatic lamination step performed before the thermal sintering of thick films has demonstrated to improve film density and grain size, increasing "bulk" and grain boundary Li-conductivities. The confocal Raman spectroscopy characterization allowed the observation of the connectivity of the particles present in the ceramic samples and so a deeper understanding of ionic conductivity. The dependence of total and "bulk" Li conductivity on the samples microstructure is discussed. The films sintered by slow heating sintering with a previous lamination step, displayed an overall Li- conductivity >10^?4 Ω^-1 cm^-1, that is superior to that reported in commercial OHARA Li- NASICON glass ceramics. The tape casting deposition method is scalable for preparation of large area thick supported electrolyte films with high conductivity for novel Li ion all solid state batteries (ASSB) architectures.     

Effect of substrate on the microstructure and thermoelectric performances of Sr-doped Ca3Co4O9 thick films

Ca₃Co₄O₉ thermoelectric materials in form of thick films are very promising in practical applications due to their low costs and relatively high performance. In this work, two different suspensions have been used to produce different coatings on Al₂O₃ polycrystalline substrates with theoretical green thickness of 360 and 2000?µm. Moreover, the effect of substrate has also been investigated using Al₂O₃ monocrystalline substrates and a 360?µm green thickness. Sintering procedure at 900?°C for 24?h has drastically decreased coating thickness. XRD performed on the coatings surface has shown the formation of small amounts of Ca₃Co₂O₆ secondary phase on the polycrystalline substrates, while it was more abundant, and accompanied by Ca₂Co₂O₅ on the monocrystalline substrates. In spite of the higher secondary phases content, monocrystalline substrates produced a slight grain orientation which led to the highest thermoelectric properties between the samples (0.38?mW/K²m at 800?°C), and very close to the best reported values in the literature.     

Fabrication and characterization of porous mullite ceramics with ultra-low shrinkage and high porosity via sol-gel and solid state reaction methods

Porous mullite ceramics with ultra-low shrinkage and high porosity were prepared by solid state reaction between MoO₃ and mullite precursor powders which were synthesized from tetraethylorthosilicate and aluminium nitrate nonahydrate via sol-gel methods. The synthetic process of mullite precursor powder and effects of MoO₃ amount on the phase composition, microstructure, physical properties such as firing shrinkage, open porosity, bending strength, water absorption and bulk density of porous mullite ceramics were investigated. The results indicated that the addition of MoO₃ not only lowered the mullite forming temperature from 985.4 to 853.3 °C, but also restrained densification behavior of samples due to the formation of mullite and Al₂O₃–MoO₃ solid solution, besides, MoO₃ also improves the formability, open porosity and bending strength of samples. The optimal amounts of MoO₃ is 8 wt%, and the resultant samples exhibit outstanding properties, including a low shrinkage rate of 1.86 ± 0.07%, an open porosity of 61.91 ± 0.16% and a bending strength of 9.35 ± 1.11 MPa.     

Phase formation and crystallization kinetics in cordierite ceramics prepared from kaolinite and magnesia

In this work, Algerian kaolinite, a naturally occurring clay mineral, was used as low-cost precursor for the synthesis of cordierite ceramics. The kaolinite was mixed with synthetic magnesia, and the mixture was ball milled and reaction sintered in the temperature range 900–1350 °C for 2 h. Thermogravimetry (TG), differential thermal analysis (DTA), dilatometry, high temperature x-ray powder diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) complementary techniques were used to analyze sintering behavior, characterize phase transformations, and investigate crystallization kinetics. Milling the kaolinite and magnesia mixture for 10 h yielded a homogenous powder, decreased the average particle size, and improved the roundness of particles. Different crystalline phases were present in the samples sintered in the temperature range 900–1150 °C, the cordierite phase started to crystallize at 1200 °C, and the formation of highly dense cordierite (99%) was complete at 1250 °C. The activation energy values for cordierite formation calculated using Kissinger, Boswell, and Ozawa methods were found to be equal to 577, 589, and 573 kJ/mol, respectively. The kinetic parameters n and m had values close to 2. Bulk nucleation with a constant number of nuclei was the dominant mechanism in cordierite crystallization, followed by two-dimensional growth controlled by interface reaction.     

Effects of boron addition and intensive grinding on synthesis of anorthite ceramics

Anorthite ceramics were synthesized starting from mixtures prepared by using mechanochemical methods with boron oxide addition. The raw materials used in this study were Sivas Kaolin, calcined alumina/Al(OH)₃ and calcite. Statistical experimental design techniques (SED) were used in order to determine and analyze the more important process variables for synthesizing anorthite ceramics. Phase characterizations of synthesized powders were performed by XRD using Cu Kα radiation. Microstructural characterization was performed by SEM. The results of screening experimental design clarified that the temperature was the most important process variable. Second most important process variable was grinding speed of starting mixture which was followed by additive amount and additive type. The effect of both additive use and grinding on anorthite synthesis helped decrease the synthesis temperature down to 900 °C.     

Structural and thermal studies of silver nanoparticles and electrical transport study of their thin films

This work reports the preparation and characterization of silver nanoparticles synthesized through wet chemical solution method and of silver films deposited by dip-coating method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy dispersive spectroscopy (EDX) have been used to characterize the prepared silver nanoparticles and thin film. The morphology and crystal structure of silver nanoparticles have been determined by FESEM, HRTEM, and FETEM. The average grain size of silver nanoparticles is found to be 17.5 nm. The peaks in XRD pattern are in good agreement with that of face-centered-cubic form of metallic silver. TGA/DTA results confirmed the weight loss and the exothermic reaction due to desorption of chemisorbed water. The temperature dependence of resistivity of silver thin film, determined in the temperature range of 100-300 K, exhibit semiconducting behavior of the sample. The sample shows the activated variable range hopping in the localized states near the Fermi level.     

Effect of Y concentration and film thickness on microstructure and electrical properties of HfO2 based thin films

In this work, we introduced a simple solution processing method to prepare yttrium (Y) doped hafnium oxide (HfO₂) based dielectric films. The films had high densities, low surface roughness, maximum permittivity of about 32, leakage current < 1.0 × 10^?7 A/cm² at 2 MV/cm, and breakdown field >5.0 MV/cm. In addition to dielectric performance, we investigated the influence of YO_1.5 fraction on the electronic structure between Y doped HfO₂ thin films and silicon (Si) substrates. The valence band electronic structure, energy gap and conduction band structure changed linearly with YO_1.5 fraction. Given this cost-effective deposition technique and excellent dielectric performance, solution-processed Y doped HfO₂ based thin films have the potential for insulator applications.     

Effects of Ta5+ doping on microstructure evolution,dielectric properties and electrical response in CaCu3Ti4O12 ceramics

The effects of Ta⁵⁺ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The mean grain size decreased with increasing Ta⁵⁺ concentration, which was ascribed to the ability of Ta⁵⁺ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu₃Ti₄O₁₂ ceramics were reduced by doping with Ta⁵⁺. This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti⁴⁺ with Ta⁵⁺ are discussed. The experimental data and variation caused by the substitution of Ta⁵⁺ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.     

Reactive synthesis for porous Ti3AlC2 ceramics through TiH2, Al and graphite powders

Porous Ti₃AlC₂ ceramics were fabricated by reactive synthesis. The process of fabrication involved five steps: (i) the pyrolysis of stearic acid at 450 °C; (ii) the decomposition of TiH₂ at 700 °C, which leads to the conversion of TiH₂ to Ti; (iii) the solid–liquid chemical reaction of solid Ti and molten Al at 800–1000 °C, which converts the mixture to Ti–Al compounds; (iv) the newly synthesized Ti–Al compounds that react with surplus Ti and graphite to form ternary carbides and TiC at 1100–1200 °C; and (v) reactive synthesized ternary carbides and TiC that yield porous Ti₃AlC₂ at 1300 °C.     

Preparation, structural and electrical characteristics of praseodymium modified lead titanate

Praseodymium modified lead titanate ceramics (Pb_1−xPr_xTi_1−2yMo_yFe_yO₃) with x = 0.02, 0.04, 0.06, 0.08 and 0.10 and y = 0.02 have been prepared by high temperature solid state reaction technique. Cold pressed pellets were sintered at 1100 °C for 2 h. Lattice parameters and crystal tetragonality was determined by X-ray diffraction analysis. All the synthesized samples show single phase with tetragonal structure. Tetragonality decreases and relative density increases with the increase in praseodymium substitution. Electrical characteristics which include dielectric properties and ac conductivity were studied as a function of temperature and frequency. Variation of dielectric constant with temperature shows a ferroelectric phase transition and transition temperature (T_C) decreases with increase in the praseodymium content. Nature of transition was studied and found to be diffused. Conductivity (σ_ac) was measured as a function of frequency in the range 10³–10⁶ Hz at different temperature suggesting hopping mechanism.     

Elastic modulus and surface tension of a polyurethane rubber in nanometer thick films

Estane is a polyurethane with thermodynamically incompatible hard and soft segments and behaves like a thermoplastic elastomer. In the present work the mechanical properties of Estane at the nano- and macro-scales have been characterized. The macroscopic viscoelastic properties were found to follow time–temperature superposition. Also we used a nano bubble inflation method to obtain the compliance of the Estane ultra thin films over thicknesses ranging from 220 nm to 22 nm. The results have been analyzed using both a direct stress-strain analysis and an energy balance method to separate the rubbery plateau regime modulus and the surface tension of the ultra thin films. It is found that the rubbery regime modulus stiffens significantly as thickness decreases, while the surface tension is independent of film thickness. The rubbery stiffening dependence on film thickness is found to be consistent with a new analysis that suggests that it is related to the shape of the macroscopic segmental relaxation.     

Effect of Ba2+ substitution on the structural and electrical properties of SrBi4Ti4O15 ceramic

We studied the effect of Barium doping on the structural and electrical properties of SrBi₄Ti₄O₁₅ (SBBT) ferroelectric ceramic. The samples were synthesized by the conventional solid-state reaction method. X-ray diffraction (XRD) and Raman scattering techniques have been employed to characterize the structural property. Scanning electron microscope images showed plate like grain morphology with random orientation of platelets. Lower Curie temperature and enhanced dielectric constant at the transition temperature were clearly observed with increasing the concentration of Ba²⁺ ion. Detailed studies on the effect of barium on electrical behavior of the SBBT systems have been carried out by the non destructive complex impedance spectroscopy (CIS technique). The Nyquist plots suggest that the grain and grain boundary were responsible in the conduction mechanism of the materials. Ferroelectric measurements revealed that Ba²⁺ doping leads to reduction in the remnant polarization. The piezoelectric coefficients (d₃₃) of the ceramics were enhanced with Ba²⁺ doping.     

The influence of fluorine doping on the structural and electrical properties of the LiFePO4 powder

Low intrinsic electronic conductivity is the main disadvantage of LiFePO₄ when used as a cathode material in lithium ion batteries. The paper offers experimental proofs of the theoretical prediction that fluorine doping of LiFePO₄ can enhance its electrical conductivity. The LiFePO₄ and fluorine-doped LiFePO₄ olivine type, carbon-free powders are synthesized and examined. The crystal structure refinements in the Pnma space group reveal that doping with fluorine ions preserves the olivine structure, while reducing both the lattice parameters and the antisite defect, and increasing the crystallite size. A small amount of incorporated fluorine enhances the electrical conductivity from 4.6×10⁻⁷ S cm⁻¹ to 2.3×10⁻⁶ S cm⁻¹ and has a positive impact on the electrochemical performance. Several spectroscopy techniques (Mössbauer, FTIR, and Raman) reveal differences between the two powders and additionally support the findings of both the Rietveld refinement and the conductivity measurements.