Transformation Kinetics of Ba2Ti9O20 with ZrO2 Additive

Pure Ba₂Ti₉O₂₀ (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO₂ powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO_2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO₂. The addition of ZrO₂ possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled.     

Influence of Rare-Earth Dopants on Barium Titanate Ceramics Microstructure and Corresponding Electrical Properties

In this article, ytterbium and erbium oxides are used as doping materials for barium titanate (BaTiO₃) materials. The amphoteric behavior of these rare-earth ions leads to the increase of dielectric permittivity and decrease of dielectric losses. BaTiO₃ ceramics doped with 0.01–0.5 wt% of Yb₂O₃ and Er₂O₃ were prepared by conventional solid-state procedure and sintered at 1320°C for 4 h. In BaTiO₃ doped with a low content of rare-earth ions (0.01 wt%) the grain size ranged between 10 and 25 μm. With the higher dopant concentration of 0.5 wt%, the abnormal grain growth is inhibited and the grain size ranged between 2 and 10 μm. The measurements of capacitance and dielectric losses as a function of frequency and temperature have been carried out in order to correlate the microstructure and dielectric properties of doped BaTiO₃ ceramics. The temperature dependence of the dielectric constant as a function of dopant amount has been investigated.     

Low-Fire Processing of Microwave BaTi4O9 Dielectric with BaO–ZnO–B2O3 Glass

The effects of BaO—ZnO–B₂O₃ (BZB) glass addition on the densification and dielectric properties of BaTi₄O₉ (BT4) have been investigated. With increasing BaO content in the BZB glass, the softening and melting points of the resulting BZB glass decrease, but the wetting between BZB and BT4 improves cosiderably. Although the densification temperature is reduced from 1300°C for pure BT4 to 925°C for BT4+BZB dielectric ceramics, the enhancement in densification becomes less significant with increasing BaO content in the BZB glass. The above result is attributed to a chemical reaction taking place at the interface of BZB/BT4 during firing, which becomes less extensive with increasing BaO content in the BZB glass. For the BZB glass with a BaO content in the range of 0–20 mol%, the resulting 90 vol% BT4+10 vol% BZB microwave dielectric has a dielectric constant of 28–33, and a product ( Q × f _r) of quality factor ( Q ) and a resonant frequency ( f _r) of 15 000–20 000 GHz at 6.6 GHz.     

The Effect of Deposition Temperature and Postanneal on the Bi3.63Pr0.3Ti3O12 thin Films Prepared by RF-Magnetron Sputtering Method

Praseodymium doped Bi₄Ti₃O₁₂ (BTO) thin films with composition Bi_3.63Pr_0.3Ti₃O₁₂ (BPT) were successfully prepared on Pt/Ti/SiO₂/Si substrates by RF-magnetron sputtering method at substrate temperatures ranging between 500° and 750°C. The structural phase and orientation of the deposited films were investigated in order to understand the effect of the deposition temperature on the properties of the BPT films. As the substrate temperature was increased to 700°C, the films started showing a tendency of assuming a c -axis preferred orientation. At lower temperatures, however, polycrystalline films were formed. The Pt/BPT/Pt capacitor showed an interesting dependence of the remnant polarization (2 P _r) as well as dc leakage current values on the growth temperature. The film deposited at 650°C showed the largest 2 P _r of 29.6 μC/cm². With the increase of deposited temperature, the leakage current densities of films decreased at the same applied field and the film deposited at 750°C exhibited the best leakage current characteristics. In addition, the ferroelectric fatigue and Raman measurements were carried out on the as-prepared, postannealed in air and postannealed in oxygen BPT films. It was revealed that the BPT film postannealed in air exhibited the weakest fatigue-resistance characteristics and highest frequency shifted Raman vibration modes, indicating the highest oxygen vacancy concentration in this film.     

Preparation and Electrical Properties of an Anodized Al2O3–BaTiO3 Composite Film

A highly stable, water-based barium titanate BaTiO₃, BT, sol was synthesized using a sol–gel route through a chelate lactate technique. Dried BT precursor powders were measured by thermal gravimetry–differential thermal analysis and X-ray diffraction. It was found that BT powders first converted into barium carbonate BaCO₃, Ti complex, and intermediate phase Ba₂Ti₂O₅CO₃, and then transformed into perovskite phase BaTiO₃. The crystallization temperature was about 550°C. The low-voltage etched aluminum foils were covered with BT sol by dip coating, and then annealed at 600°C for 30 min in air. After that, the samples were anodized in a 15 wt% aqueous solution of ammonium adipate. The voltage–time variations during anodizing were monitored, and the electrical properties of the anodic oxide film were examined. It was shown that the specific capacitance, the product of specific capacitance and withstanding voltage, and leakage current of samples with a BT coating were about 48.93%, 38.50%, and 167% larger than that without a BT coating, respectively.     

Phase Stability of Ln2BaCuO5 (Ln = Dy, Er, Eu, Gd, Ho, Sm, Yb)

Phase stability of a series of Ln₂BaCuO₅ (Ln=Dy, Er, Eu, Gd, Ho, Sm, Yb) has been studied as a function of oxygen partial pressure and temperature by X-ray diffraction, thermal gravimetric, and differential thermal analyses. Above the critical temperature T_d, phase decomposition with a decrease in sample weight took place. In the case of Ln=Dy, Er, Ho, and Yb, the Ln₂BaCuO₅ was decomposed into Ln₂O₃BaCuO₃ and BaCuO₂. The critical temperature T_d (K) was changed depending heavily on the oxygen partial pressure P (Pa) and the effective ionic radius R (0.1-nm unit) of the Ln atom which was sevenfold coordinated by oxygen atoms. The observed T_d values for all of the systems, together with those of Y₂BaCuO₅ reported previously, closely followed the relation T_d=−17493R²+ 34582R − 15489 + 239.47 In P with a reliability index R_p=0.5%. [Key words: superconductors, barium, phases, X-ray diffraction, differential thermal analysis.]     

Low-Temperature Sintering and Piezoelectric Properties of CuO-Added 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

The sintering temperature of 0.95(Na_0.5K_0.5)NbO₃–0.05BaTiO₃ (NKN–BT) ceramics needs to be decreased below 1000°C to prevent Na₂O evaporation, which can cause difficulties in poling and may eventually degrade their piezoelectric properties. NKN–BT ceramics containing CuO were well sintered at 950°C with grain growth. Poling was easy for all specimens. Densification and grain growth were explained by the formation of a liquid phase. The addition of CuO improved the piezoelectric properties by increasing the grain size and density. High piezoelectric properties of d ₃₃=230 pC/N, k _p=37%, and ɛ₃^T/ɛ₀=1150 were obtained from the specimen containing 1.0 mol% of CuO synthesized by the conventional solid-state method.     

Electromechanical Properties of Some Pb (Mg1/3Nb2/3–PbTiO3–(Ba,Sr)TiO3 Ceramics: I

Compositional variation within the Pb(Mg_1/3Nb_2/3) O₃–PbTiO₃–(Ba, Sr)TiO₃ (hereafter PMN–PT–BT,ST) ternary (6.4% PT% 14.1%, 1.25% BT,ST% 2.5%) results in major changes in induced strain and hysteresis. For the 1.25% BT family, the increase in strain correlates with an increase in T _max, while the dielectric loss is uncorrelated with hysteresis and strain. In addition, weak field aging (which is not reset by application of field) shows little effect on strain and hysteresis for drive fields of > 0.2 MV/m. The vary narrow polarization-fields loops (virgin curvesnearly indistinguishable from subsequent cycles) show that weak-field permittivity is a good approximation to the high-field permittivity. is a good approximation to the high-field Permittivity. Although these data clarify the frequency ( T _max is linearly dependent on the logarithm of the frequency) effect on weak-field dielectric behavior, they do not directly address the question of meaningful extrapolation of high-field strain with frequency. In particular, the question remains as to whether the high-field permittivity and strain are frequency dependent. In future papers we will address this question by a combination of measurement techniques as functions of frequency.     

Mechanical Property and Oxidation Behavior of Self-Reinforced Si3N4 Doped with Re2O3 (Re=Yb, Lu)

In this work, self-reinforced silicon nitrides with β-Si₃N₄ seeds doped with Re₂O₃ (Re=Yb, Lu) were investigated. Firstly, the two kinds of seeds were obtained by heating α-Si₃N₄ powder with Yb₂O₃ or Lu₂O₃, respectively. Then the self-reinforced silicon nitride ceramics were prepared by HP-sintering of α-Si₃N₄ powder, Re₂O₃ as additive, and the as-prepared seeds. Oxidation test was carried out at 1400°C in air for 100 h with thermogravimetry analysis (TGA) measurement. Mechanical properties, scanning electronic microscopy microstructures, and X-ray diffraction patterns were measured before and after oxidation. The results indicated that the introduction of the seeds doped with Re₂O₃ (Re=Yb, Lu) could obviously increase the toughness and keep the room temperature and high-temperature strength of the ceramics at high values. After oxidation, the crystalline phase in grain boundary changed and the mechanical properties decreased. TGA showed a parabolic weight gain and the oxidation mechanism was discussed.     

Microwave Dielectric Loss of Titanium Oxide

The dielectric loss (tan δ) of titanium dioxide (TiO₂) disks has been measured at a frequency of 3 GHz. High-purity TiO₂ sintered to almost-full density exhibits a very high tan δ, which is interpreted to be due to oxygen deficiency. To counter this, doping with stable divalent and trivalent cations, such as Mg and Al, leads to a low tan δ, probably by preventing Ti⁴⁺ reduction. The tan δ of polycrystalline TiO₂ doped with divalent and trivalent ions with ionic radii in the range of 0.5–0.95 Å at 3 GHz can be very low: 6 × 10⁻⁵ ( Q ∼ 17 000) at a temperature of 300 K. The tan δ of undoped pure TiO₂ disks increases when the disks are cooled from 300 K to ∼100 K. At temperatures <100 K, the tan δ decreases rapidly, which is interpreted as carrier freeze-out. The tan δ for all the high- Q doped TiO₂ polycrystalline samples smoothly decrease to ∼5 × 10⁻⁶ ( Q ∼ 200 000) at 15 K, comparable to that of single crystals.     

Preparation and Analysis of a Silicon Carbide Composite Membrane

Silicon carbide (SiC) porous substrates, containing alumina (Al₂O₃) dopant levels of 3, 5, and 8 wt%, are prepared by slip casting and sintering in the temperature range of 1450°–1800°C. The linear shrinkage, bulk density, and pore size of the sintered substrate increase as the sintering temperature and the amount of dopant increase. A large amount of β-phase SiC is transformed to α-phase SiC if the dopant concentration is 5 or 8 wt%. The flexural strength of the substrate doped with 8 wt% Al₂O₃ is higher than that of the substrate doped with 3 wt% Al₂O₃; however, the Weibull modulus of the former is lower. SiC composite membranes of improved selectivity and strength are fabricated by coating the porous substrate with layers of lower Al₂O₃ contents at lower sintering temperatures.     

Effect of Cation Substitution and Non-Stoichiometry on the Microwave Dielectric Properties of Sr(B'0.5Ta0.5)O3 [B'=Lanthanides] Perovskites

The Sr(B'_0.5Ta_0.5)O₃ ceramics where B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, and Yb have been prepared by the conventional solid-state ceramic route and their microwave dielectric properties have been investigated. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscope techniques. The relative permittiviy (ɛ_r) varies linearly with B'-site ionic radii, except for La, and the temperature coefficient of resonant frequency (τ_f) varies linearly with the tolerance factor. The Sr(B'_0.5Ta_0.5)O₃ ceramics have ɛ_r in the range 25.9–32, Q _u× f =4500–54 300 GHz, and τ_f=−79 to −42 ppm/°C. A slight deviation from stoichiometry affects the dielectric properties of these double perovskites. Partial substitution of Ba for Sr could tune the dielectric properties. Addition of rutile (TiO₂) lowered the sintering temperature and improved the dielectric properties of Sr(B'_0.5Ta_0.5)O₃ ceramics.     

The Effect of Dopants on the Dielectric Properties of Ba(B'1/2Ta1/2)O3 (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) Microwave Ceramics

Low-loss dielectric ceramics based on Ba(B'_1/2Ta_1/2)O₃ (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) complex perovskites have been prepared by the solid-state ceramic route. The dielectric properties (ɛ_r, Q _u, and τ_f) of the ceramics have been measured in the frequency range 4–6 GHz by the resonance method. The resonators have a relatively high dielectric constant and high quality factor. Most of the compounds have a low coefficient of temperature variation of the resonant frequencies. The microwave dielectric properties have been improved by the addition of dopants and by solid solution formation. The solid solution Ba[(Y_{1− x} Pr _x )_1/2Ta_1/2]O₃ has x =0.15, with ɛ_r=33.2, Q _u× f =51,500 GHz, and τ_f≈0. The microwave dielectric properties of Ba(B'_1/2Ta_1/2)O₃ ceramics are found to depend on the tolerance factor ( t ), ionic radius, and ionization energy.     

Adhesive-Bonded Ca(Mg1/3Nb2/3)O3/Ba(Zn1/3Nb2/3)O3 Layered Dielectric Resonators with Tunable Temperature Coefficient of Resonant Frequency

Ca(Mg_1/3Nb_2/3)O₃ and Ba(Zn_1/3Nb_2/3)O₃ ceramic cylinders with the same diameter were bonded by adhesive with low dielectric loss to yield the layered dielectric resonators, and the microwave dielectric characteristics were evaluated with TE_01δ mode. With increasing the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction, the resonant frequency ( f ₀) decreased, while the effective dielectric constant (ɛ _{r ,eff}) and temperature coefficient of resonant frequency (τ _f ) increased. Good microwave dielectric characteristics were attained for the samples with the Ba(Zn_1/3Nb_2/3)O₃ thickness fraction of 0.5: ɛ _{r ,eff}=34.33, Q × f =57 930 GHz and τ _f =2.6 ppm/°C. Finite-element method was used to predict the microwave dielectric characteristics of the layered resonators and good agreements were attained between the experimental results and predicted ones. Also, both experiment and finite-element analysis indicated that the effects of the adhesive on f ₀, ɛ _{r ,eff}, and τ _f were slight, while that on Q × f value was significant.     

Dielectric Properties of Ceramics in Ba (Co1/3 Nb2/3)O3–Ba(Zn1/3Nb2/3)O3 Solid Solution

The dielectric properties of the Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ system were determined. Ba (Co_1/3 Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ has a complex perovskite structure, a high dielectric constant, a low dielectric loss, and a low temperature coefficient of the resonant frequency. A solid-solution ceramic with 0.7Ba (Co_1/3 Nb_2/3)O₃·0.3 Ba(Zn_1/3Nb_2/3)O₃ has a dielectric constant of K=33.5, Q=11000 at 6.5 GHz, and a temperature coefficient of the resonant frequency of τ_f=0 ppm/°C. The temperature coefficient of resonant frequency can be varied by changing the composition. The Q values of the ceramics can be increased by annealing in a nitrogen atmosphere. These ceramics can be used for resonant elements and stabilized oscillators.     

Influence of TiO2 Solid Solution on the Thermal Property and Ionic Conductivity Of Partially Stabilized Zirconia

TiO₂(0–20 mol%)-3 mol% yttria-stabilized zirconia (3YSZ) ceramics were prepared by a solid-state reaction. With increasing TiO₂ content in 3YSZ, the structure of the main phase changed from a monoclinic, tetragonal, and cubic mixture to a tetragonal single phase. Increasing TiO₂ content in 3YSZ caused an increase in the average grain size of these ceramics. The thermal conductivity decreased from 4.1 to 2.1 at room temperature with an increase in the TiO₂ content. The specific heat of non-TiO₂-doped 3YSZ was slightly larger than all the doped TiO₂–3YSZ at room temperature. When the TiO₂ content was >8 mol% in 3YSZ, no abrupt expansion, shrinkage, or cracks were observed on heating and cooling these samples; thus, the thermal stability of 3YSZ was improved by TiO₂ solid solution. The ionic conductivity of the samples decreased with increasing TiO₂ solid solution.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Phase Evolution in Heat-Treated Si3N4 with Additions of Yb2O3

The heat treatment of silicon nitride (Si₃N₄) ceramics with additions of 8, 12, and 16 wt% Yb₂O₃ was carried out at different temperatures and the evolution of grain boundary (GB) phase was investigated systematically by X-ray diffraction (XRD) as well as scanning electron and transmission electron microscopic analyses. XRD results reveal that the extent and the ease of GB crystallization increase with increasing the Yb₂O₃ content, and that high heat-treatment temperatures in general favor crystallization of the quaternary compounds such as the Yb₄Si₂O₇N₂ phase. These results provide an insight into the GB phase evolution in the Yb-system Si₃N₄ ceramics subjected to a postsintering heat treatment.     

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.     

Semiconducting–Insulating Transition for Highly Donor-Dopod Barium Titanate Ceramics

BaTiO₃ ceramics doped with La (0.01–0.84 at.%) were prepared only with the addition of La and stoichiometric TiO₂. As a result, even when BaTiO₃ was doped with 0.53 at.% La, it could be converted to a semiconductor by sintering at 1540°C for 2 h in air and cooled slowly in the furnace. Differential thermal analysis data clearly demonstrated that the Curie point in the materials shifted toward lower temperatures with increased content of La substituted at the Ba site up to a critical concentration that varied with the sintering temperature. The obtained results suggest that the semiconducting–insulating transition for highly donor-doped BaTiO₃ was closely related to the incorporation of donor into the grains and to the resultant grain size, which were significantly affected by the sinterability of the BaTiO₃ starting powders and sintering conditions used.