Microstructure and Dielectric Properties of a LaAlO3–TiO2 Diffusion Couple

Near-field scanning microwave microscopy was applied to investigate the dielectric properties and microstructure in a polycrystalline LaAlO₃–TiO₂ diffusion couple, which included three regions containing different phases and microstructures. Relatively low (La₂Ti₄Al₁₈O₃₈), high (α-La_2/3TiO₃), and intermediate (La₄Ti₉O₂₄) dielectric constant phases were distinguished at the inter-diffusion interface in optical, backscattered electron scanning electron microscopy, and scanning microwave microscopy (SMM) images. The relative ranking of dielectric constants based on SMM examination was as follows: TiO₂>α-La_2/3TiO₃>La₄Ti₉O₂₄>LaAlO₃>La₂Ti₄Al₁₈O₃₈. La_2/3TiO₃ and LaAlO₃ will form solid solutions in the LaAlO₃-rich region. The reaction paths leading to phase development are discussed.     

Temperature Coefficient of Microwave Resonance Frequency of a Low-Temperature Cofired Ceramic (LTCC) System

A family of low-temperature cofired ceramics (LTCC) based on mixtures of a commercial dielectric, MgTiO₃–CaTiO₃(designated MMT-20) and ZnO, SiO₂, and B₂O₃, has been investigated for microwave applications. The main objective was to optimize the three key properties—relative permittivity (ɛ_r), dissipation factor (DF), and the temperature dependence of the microwave resonance frequency (τ_f)—through adjustment of the composition. A further objective was to estimate the limits on compositional variability while maintaining acceptable properties. The developed microstructures, after firing at 900°C, were studied using X-ray diffractometry and scanning electron microscopy/energy dispersive spectrometry techniques and compared with the dielectric parameters. The optimum composition (wt%) was found to lie in the ranges 45.8–44.9, ZnO; 17.25–17.55, B₂O₃; 6.95–7.05, SiO₂; and 30–30.5, MMT-20, yielding values of ɛ_r= 8.5–9.5, DF < 0.93 × 10⁻³ ppm/K, and τ_f< ±10 ppm/K.     

A Chemical Route to BiNbO4 Ceramics

The liquid phase sintering of fine BiNbO₄ powders allows to obtain dense ceramics with excellent microwave dielectric properties (ɛ=44–46; Q × f =16,500–21,600 GHz) at T ≥700°C. The thermal decomposition of freeze-dried precursors results in the crystallization of a metastable β'-BiNbO₄ polymorph that transforms into a stable orthorhombic α-modification at T ≥700°C. The dependence of sinterability on the powder synthesis temperature shows the maximum at 600°C, corresponding to the formation of crystalline BiNbO₄ powders with a grain size 80–100 nm. Sintering temperature reduction to 700°C prevents the deterioration of silver contacts during co-firing with BiNbO₄ ceramics. In situ scanning electron microscopy observation of the morphological evolution during sintering shows that the intense shrinkage soon after the appearance of a CuO–V₂O₅ eutectics-based liquid phase is accompanied by complete transformation of the ensemble of primary BiNbO₄ particles.     

Finite Size Effect on the Sinterability and Dielectric Properties of ZnNb2O6–ZBS Glass Composites

ZnNb₂O₆ (ZN) is a columbite-structured niobate compound showing excellent dielectric properties and comparatively low sintering temperatures (∼1200°C). Hence it is a good candidate for possible low-temperature cofired ceramics (LTCC) applications. In the present investigation, ZnNb₂O₆ was synthesized in the form of micrometer-sized powder using a conventional solid-state ceramic synthesis route as well as in the form of nanosized powder by a polymer complex method. The finite size effect of ZN particles on sinterability and microwave dielectric properties of sintered pellets was evaluated. The phase formation was confirmed from the X-ray diffraction (XRD) analysis. The particle size distribution of the nanoparticles was found to be of the order of 18–20 nm by using high-resolution transmission electron microscopy analysis and 30 nm by analyzing the XRD patterns using Debye Scherrer's formula, after correcting for the instrument broadening effects. A ZN–60ZnO–30B₂O₃–10SiO₂ (ZBS) composite was made by adding predetermined amounts of glasses. The microstructures of the sintered pellets of ZN and ZN–ZBS composites were examined using scanning electron microscopy and analyzed using image analysis. The nano-ZN–ZBS composites were sintered to 93% of the reported density at 925°C/2 h, with microwave dielectric properties of ɛ_r=22.5, Q × f ∼12 800 GHz, and τ_f=−69.6 ppm/°C, emerging as a potential material for possible LTCC applications.     

Dielectric Properties and Microstructure of Nb-Co Codoped BaTiO3–(Bi0.5Na0.5)TiO3 Ceramics

X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, and an impedance analyzer were used to examine the Nb–Co codoping effects on the densification, crystalline phase, microstructure development, and dielectric–temperature characteristics of BaTiO₃–(Bi_0.5Na_0.5)TiO₃ ceramics. The results indicate that the Curie temperature shifted to a higher temperature (above 140°C) by adding BNT. The dielectric constant–temperature (ɛ– T ) curve broadened at the Curie temperature due to the small grain size (0.3–0.4 μm). A core-shell structure was developed, which is helpful to flatten the ɛ– T curve of BaTiO₃ ceramics at high temperatures.     

Sintering and Electrical Properties of Lead-Free Na0.5K0.5NbO3 Piezoelectric Ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) piezoelectric ceramics were fairly well densified at a relatively low temperature under atmospheric conditions. A relative density of 96%–99% can be achieved by either using high-energy attrition milling or adding 1 mol% oxide additives. It is suggested that ultra-fine starting powders by active milling or oxygen vacancies and even liquid phases from B-site oxide additives mainly lead to improved sintering. Not only were dielectric properties influenced by oxide additives, such as the Curie temperature ( T _c) and dielectric loss ( D ), but also the ferroelectricity was modified. A relatively large remanent polarization was produced, ranging from 16 μC/cm² for pure NKN to 23 μC/cm² for ZnO-added NKN samples. The following dielectric and piezoelectric properties were obtained: relative permittivity ɛ ^T ₃₃/ɛ ₀ =570–650, planar mode electromechanical coupling factor, k _p=32%–44%, and piezoelectric strain constant, d ₃₃=92–117 pC/N.     

Microstructure and Microwave Dielectric Properties of (1−x)Ca(Mg1/3Ta2/3)O3/xCaTiO3 Ceramics

Dense (1− x )Ca(Mg_1/3Ta_2/3)O₃/ x CaTiO₃ ceramics (0.1≤ x ≤0.9) were prepared by a solid-state reaction process. The crystal structures and microstructures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Single-phase solid solutions were obtained in the entire composition range. Long-range 1:2 ordering of B-site cations and oxygen octahedra tilting lead to the monoclinic symmetry with space group P 2₁/ c for x =0.1. For x above 0.1, the long-range ordering was destroyed and the crystal structure became the orthorhombic with space group Pbnm . The microwave dielectric properties showed a strong dependence on the composition and microstructure. The dielectric constant and temperature coefficient of resonant frequency increased nonlinearly as the CaTiO₃ content increased while the Qf values decreased approximately linearly. Good combination of microwave dielectric properties was obtained at x =0.45, where ɛ_r=45.1, Qf =34 800 GHz, and τ_f=17.4 ppm/°C.     

Processing and Mechanical Properties of Ti3SiC2: I, Reaction Path and Microstructure Evolution

In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti₃SiC₂, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiC _x and Ti₅Si₃C _x . Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti₃SiC₂ grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiC _x , which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti₃SiC₂ is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.     

Preparation of Lead Zirconate Titanate Thin Films Derived by Hybrid Processing: Sol-Gel Method and Laser Ablation

Thin films of Pb(Zr_0.52Ti_0.48)O₃ (PZT) were prepared by hybrid processing (sol-gel and excimer laser ablation) on Pt/Ti/SiO₂/Si substrates. Crystalline phases and microstructures of the PZT films were investigated by X-ray diffraction analysis and scanning electron microscopy, respectively. Electrical properties of the films were evaluated by measuring their P - E hysteresis loops and dielectric constants. The temperature of postdeposition annealing in hybrid processing was lower than that in the case of direct film deposition by laser ablation on a Pt/Ti/SiO₂/Si substrate. The preferred orientation of the films derived by hybrid processing could be controlled using the seeding layer deposited by the sol-gel process. The films fabricated by hybrid processing consisted of the perovskite phase with a (111) preferred orientation and had good ferroelectric properties.     

Effect of Alumina Additions on Microstructural Aspects of the β to α Transformation in Tantalum (V) Oxide

Tantalum (V) oxide (Ta₂O₅) has potential applications as part of an environmental barrier coating system for Si₃N₄-based turbine components. However, at elevated temperatures, Ta₂O₅ undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta₂O₅ microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X-ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta₂O₅, transformation of the low-temperature β-phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta₂O₅ was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO₄ second and phase particles were seen to be highly effective at inhibiting grain growth.     

Effect of Silver Addition on the Dielectric Properties of Barium Titanate-Based X7R Ceramics

Different amounts of silver (0.5–10 wt%) have been mixed with EIA X7R-type ceramic powders based on barium titanate. The XRD analysis indicated that no phases other than BaTiO₃ and silver were present in the doped ceramics; it further suggested that no reaction took place between BaTiO₃ and silver during calcination and sintering. SEM observation showed that the silver particles presented island distribution in the BaTiO₃ ceramic matrix. The densification and dielectric properties of the silver-doped ceramics in disk form were investigated. A large amount of silver addition (>1 wt%) was found to improve the sintered density and dielectric properties. The temperature coefficient of capacitors of the ceramics doped with 10 wt% silver still met the X7R characteristics, and the dielectric constant of the ceramics at room temperature was >6000, which is the highest dielectric constant in the BaTiO₃-based X7R system.     

Base-Metal-Electroded BaTiO3 Capacitor Materials with Duplex Microstructures

The effect of dopants and processing conditions on the dielectric properties of base-metal-electroded materials was investigated. BaTiO₃ materials simultaneously doped with MgO and Y₂O₃ additives can achieve small capacitance variation (Δ C / C ), which meets the X7R specification, when the proportion of additives is abundant enough and the materials are not over-fired. Presumably, small Δ C / C values of thus obtained materials are the result of the formation of core–shell structure, which requires stringent control of material processing conditions. In contrast, X7R-type materials can be obtained in a much wider processing window, when prepared by mixing two BaTiO₃ materials of suitable dielectric constant–temperature ( K – T ) characteristics. Duplexed materials prepared from these two end-point BaTiO₃ materials with ratios ranging from 3:1 to 1:2 exhibit K – T behavior within the X7R specification, provided that one of the components possesses flat K – T behavior. Moreover, the dielectric properties of these materials were simulated using a simplified microstructural model. Simulation results indicate that the effective dielectric constant of core–shell materials, ( K _e)_CS, varies significantly not only with the dielectric properties of cores and shells, but also with the shell-to-core thickness ratio, whereas the effective dielectric constant of duplexed materials, ( K _e)_D, can be maintained at a very small Δ C / C value for a wide range of end-point constituent ratios, which agrees very well with the measured K – T properties for the materials.     

High-Q Microwave Dielectrics in the (Mg1−xCox)2TiO4 Ceramics

The microwave dielectric properties and the microstructures of (Mg_{1− x} Co _x )₂TiO₄ ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg_{1− x} Co _x )₂TiO₄ ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τ_f∼−52.5 ppm/°C) was achieved for (Mg_0.95Co_0.05)₂TiO₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg_0.95Co_0.05)TiO₃ was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.     

In Situ Whisker-Reinforced AlPO4-Modified β-Eucryptite Glass-Ceramic: I, Morphology and Crystallization Kinetics

A whisker-reinforced glass-ceramic composite in the Li₂O-Al₂O₃-SiO₂-P₂O₅ system has been fabricated by a single-stage process that simultaneously forms the glass-ceramic material and whiskers of TiO₂ in situ . The method utilizes typical glass-ceramic processing techniques and requires precise addition of a TiO₂ nucleation agent during the glass-melting operation, followed by controlled nucleation and crystallization. The maximum nucleation temperature, 740°C, was determined by differential thermal analysis (DTA), and the result was confirmed by scanning electron microscopy (SEM) of the microstructures formed by nucleation/isothermal crystallization heat treatments. The apparent activation energy for crystallization of the material was determined to be 285 ± 3 kj/mol. The average aspect ratio of the TiO₂ whiskers depends on temperature and time during crystallization. The X-ray diffraction patterns of the in situ composite show that eucryptite(ss) and rutile exist as two different phases; no additional phases were observed. Elemental X-ray mapping by electron microprobe indicates that these highly crystallized composites consist of modified β-eucryptite glass-ceramic matrix and acicular grains of TiO₂.     

Phase Transformations of Plasma-Sprayed Zirconia–Ceria Thermal Barrier Coatings

Phase constituents and transformations of plasma-sprayed thermal barrier coatings (TBCs) with CeO₂-stabilized ZrO₂ (CSZ; 16–26 wt% CeO₂) have been investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The as-coated CSZ coatings with 16 and 18 wt% CeO₂ consisted only of the nonequilibrium tetragonal ( t ') phase. A mixture of the t ' and the nonequilibrium cubic ( c ') phases was observed for the as-coated CSZ coatings containing 20–26 wt% CeO₂. During 65 min cyclic oxidation at 1135°C (45 min hold time) in air, the t ' or the mixture of the t ' and the c ' phases decomposed to the equilibrium tetragonal ( t ) and the equilibrium cubic ( c ) phases. Some of the t phase transformed to the monoclinic ( m ) phase on cooling. More m phase was observed to develop in the CSZ coating containing 16 wt% CeO₂ than in the other coatings. More m phase was observed on the top surface than on the bottom surface of the CSZ coating. Spalling of the plasma-sprayed CSZ coating during thermal cycling occurred after 230 cycles for the CSZ coating containing 16 wt% CeO₂, whereas the lifetime of the CSZ coatings with 18–26 wt% CeO₂ ranged between 320 and 340 cycles.     

Electrophoretic Deposition and Characterization of Micrometer-Scale BaTiO3 Based X7R Dielectric Thick Films

Uniform and relatively dense BaTiO₃ thick films of 1–5 μm were prepared by an electrophoretic deposition process using submicrometer BaTiO₃ powders (mean particle size: ∼0.2 μm). Two different BaTiO₃ powders and solvent media were used to investigate the film quality and thickness control. The surface charge mechanism of BaTiO₃ particles was explained according to the observed results. The microstructures were examined by means of SEM. The experimental results show that the thickness could be controlled independently of suspension concentration by keeping a constant applied voltage and a constant current drop in a given suspension. BaTiO₃ thick films have good insulation resistance and dielectric properties such as a dielectric constant and a dissipation factor that are compatible with the data from conventional tape-cast BaTiO₃ thin layers.     

Effects of Excess Bi2O3 on the Ferroelectric Behavior of Nd-Doped Bi4Ti3O12 Thin Films

Effects of excess Bi₂O₃ content on formation of (Bi_3.15Nd_0.85)Ti₃O₁₂ (BNT) films deposited by RF sputtering were investigated. The microstructures and electrical properties of BNT thin films are strongly dependent on the excess Bi₂O₃ content and post-sputtering annealing temperature, as examined by XRD, SEM, and P – E hysteresis loops. A small amount of excess bismuth improves the crystallinity and therefore polarization of BNT films, while too much excess bismuth leads to a reduction in polarization and an increase in coercive field. P – E loops of well-established squareness were observed for the BNT films derived from a moderate amount of Bi₂O₃ excess (5 mol%), where a remanent polarization 2P _r of 25.2 μC/cm² and 2E _c of 161.5 kV/cm were shown. A similar change in dielectric constant with increasing excess Bi₂O₃ content was also observed, with the highest dielectric constant of 304.1 being measured for the BNT film derived from 5 mol% excess Bi₂O₃.     

Conductive, Polypyrrole Coating on Mullite/Alumina Fibers for Electrophoretic Deposition of Oxide Matrices

Mullite/alumina fibers (Nextel™ 720) have been rendered conductive for electrophoretic deposition (EPD) by coating with polypyrrole (Ppy) from an aqueous solution of pyrrole (Py). The polymer coating was characterized by thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and conductivity measurements. The Ppy coating thickness is ∼0.1 μm and the fiber resistance 1–2 (kΩ/cm)/fiber tow. A maximum conductivity of ∼48 S/cm was achieved from a 0.005 M pyrrole solution. A Nextel 720/Al₂O₃ composite was synthesized using Ppy-coated fibers as the cathode in an EPD cell. The green and sintered microstructures of the resultant composites are reported.     

Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 Ceramics

(Ca_{1+ x} Sm_{1− x} )(Al_{1− x} Ti _x )O₄ (0≤ x ≤0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca_{1+ x} Sm_{1− x} ) (Al_{1− x} Ti _x )O₄ and minor amount of (Ca,Sm)(Al,Ti)O₃ secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO₄ ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ɛ_r=19–23, Q × f =49 100–118 700 GHz, and τ_f=−15–15 ppm/°C.     

Fabrication and Properties of Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films near the morphotropic phase boundary were successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates via an aqueous sol–gel method. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, ferroelectric, and piezoelectric properties of the sol–gel-derived BSPT thin films were investigated. A remanent polarization of 74 μC/cm² and a coercive field of 177 kV/cm were obtained. The local effective piezoelectric coefficient d ^*₃₃ was 23 pC/N at 2 V, measured by a scanning probe microscopy system. The dielectric peak appeared at 435°C, which was 80°C higher than that of Pb(Ti, Zr)O₃ thin films.