Microstructure of X7R Type Base-Metal-Electroded BaTiO3 Capacitor Materials Co-Doped with MgO/Y2O3 Additives

Detailed microstructure of MgO/Y₂O₃ co-doped BaTiO₃ materials were examined using transmission electron microscopy (TEM). For the 1250_C-sintered BaTiO₃ samples possessing flat K-T characteristics, which meet the X7R specification, the granular structure is complicated. Most of the grains are very small ( 150 nm) and are highly strained. The small grains contain large proportion of Y₂O₃ species and are paraelectric, whereas the large grains contain Y₂O₃ species unevenly distributed and are of core-shell structure. In contrast, for the 1300C-sintered BaTiO₃ samples, which have K-T properties slightly off the X7R specification, the grains grew larger to around 300 nm. The core-shell structured grains are seldom observed. Apparently, it is the existence of such a non-equilibrium core-shell microstructure, which renders the dielectric properties of the BaTiO₃ materials extremely sensitive to the processing parameters.     

Effect of Cr additions on the electrical properties of Ni–BaTiO3 ultra-thin multilayer capacitors

Multilayer ceramic capacitors based on BaTiO₃ dielectric compositions and Ni inner electrodes have complex interfacial reactions that impact the continuity of the inner electrode microstructure. Previously we demonstrated that through the addition of Cr to Ni, a significant improvement in the continuity of ultra-thin Ni electrodes in Ni–BaTiO₃ multilayer capacitors could be achieved. Here, the effect of the Cr addition to the nickel electrode pastes is studied with regard to the electrical properties. Low-field electrical measurements demonstrate no major differences between Cr doped Ni and undoped Ni. However, high-field measurements show a significant decrease to the total capacitor resistance. Under a critical electrical bias the conductivity significantly increases due to a Fowler–Nordheim tunneling conduction though the interfacial Schottky barrier at the dielectric–electrode interface; the onset voltage of this conduction is much lower than with the undoped nickel. Based on these results, we evaluate criteria for the selection of an appropriate refractory metal in order to improve the Ni electrode continuity.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Nanocomposite ceramics based on La-doped BaTi2O5 and BaTiO3 with high temperature-independent permittivity and low dielectric loss

Nanocomposite ceramics containing a mixture of two ferroelectric phases, La-doped BaTi₂O₅ and BaTiO₃, with carefully-controlled phase amounts and ceramic microstructure have temperature-independent permittivity and low dielectric loss over very wide temperature ranges: ɛ = 95 ± 10 from 25 to 600 °C; tan δ = 0.02(2) from 25 to 400 °C, measured at 100 kHz. Further optimisation of properties should be possible.     

Microwave Sintering of Base-Metal-Electroded BaTiO3 Capacitor Materials Co-Doped with MgO/Y2O3 Additives

In this paper, we systematically investigated the effect of microwave sintering parameters on the characteristics of BaTiO₃ capacitor materials co-doped with Y₂O₃/MgO species. It is observed that the granular structure of the materials is relatively insensitive to the sintering temperature and soaking time such that the BaTiO₃ capacitor materials possessing X7R dielectric constant-temperature (K-T) characteristics can be obtained in a wide range of sintering conditions. TEM examinations reveal that the detailed microstructure of these materials is extremely complicated. The unique K-T properties of these materials are ascribed to the duplex structure of the samples, viz. fine grains of paraelectric phase and large grains of ferroelectric phase.     

Doping effects of Dy and Mg on BaTiO3 ceramics prepared by hydrothermal method

Fine Dy and Mg-doped barium titanate powders were prepared by hydrothermal method at 240^∘C with BaCl₂⋅ 2H₂O, TiCl₄ and NaOH as the main reactants, Dy₂O₃ and MgCl₂⋅ 6H₂O as additives respectively. The substitution style, microstructure and electrical properties of the two kinds of samples were comparatively studied by X-ray diffractometer (XRD), atomic emission spectrum (AES) and scanning electron microscopy (SEM) characterizations. It is confirmed that Dy or Mg enters into the BaTiO₃ lattice. Ba site is replaced if a little Dy₂O₃ is doped but some more Dy will take up the positions of Ti. Mg always substitutes for Ti site. Dy and Mg are both useful to obtain the microstructure with small grains and high density, and the former results in a better microstructure. Dy has no influence on shifting and depressing Curie peak. When Dy₂O₃ content is 0.6 wt%, the dielectric constant rises up to 4250. However, Mg has evident effect on shifting Curie peak. The value of Δ T _c is −40^∘C in the doping range of 0 to 0.15 wt%. When Mg/Ti atomic ratio is 0.06 in the precursors, the dielectric constant rises up to 4100.     

Preparation,microstructure, and ferroelectric properties of laser-deposited thin BaTiO3 and lead zirconate-titanate films

Abstract

Ferroelectric thin films of BaTiO₃ and lead zirconate titanate, PbZr_0.53Ti_0.47O₃ (PZT), have been prepared by pulsed excimer laser deposition. The microstructure and crystallography of these films have been studied by scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDX), transmission electron microscopy (TEM), x-ray diffraction (XRD), and differential scanning calorimetry (DSC). Electrical properties, including remanent polarization, dielectric loss, and dielectric constant, have been measured. Also, switched remanent polarization has been measured under conditions of continuous cycling.     

Sintering Behavior and Interfacial Analysis of Ni/Cu Electrode with BaTiO<Subscript>3</Subscript> Particulates

The sintering behavior of Ni electrode alloyed with Cu and the interfacial structure between Ni/Cu to BaTiO₃ (BT) have been investigated. The quantitative properties, which include thermal shrinkage, thermal expansion, wetting behaviors of Ni/Cu alloys on BT sheet, and composition distribution were measured by several thermal analysis techniques (TGA/DTA/TMA) and microstructural techniques (SEM/TEM/ HRTEM) with energy-dispersive spectroscopy (EDS). The shrinkage of the Ni/Cu/ BaTiO₃ composite tested in 5%H₂/N₂ atmosphere showed strong influence by the addition of Cu, and retarded slightly due to the addition of the BT particulates. The Cu alloyed with Ni improves the continuity of the electrode and does not trigger mutual reaction between Ni and BT.     

Dispersion and rheology of BaTiO3 nanoparticles in ethanol–isopropanol solvents

Barium titanate (BaTiO₃) nanoparticles were dispersed in ethanol–isopropanol mixtures and their rheological behaviors were examined in terms of surfactant concentration (0–5 wt.% of the solids) and volumetric solids loading () over a shear-rate () range 1 to 1,000 s⁻¹. An oxyethylene-based polymeric surfactant was used to facilitate the nanoparticle dispersion. A pronounced viscosity reduction, >95% when compared to the suspensions without the dispersant, resulted with a surfactant concentration of 4 wt.% at a constant shear rate of 100 s⁻¹. This finding was in parallel with a simultaneous reduction in the mean “floc” size of the suspensions. Shear-thinning flow character resulted over most of the shear-rate range examined, especially for the concentrated suspensions with ϕ ≥ 0.25. The concentrated suspensions were indeed flocculated. This increased instability was partly due to the compression of electrical double layer as the particulate solids became more crowded in the carrier solvents, and also to the increased “effective” solids concentration because of the preferential adsorption of the surfactant molecules on the nanoparticle surface.     

Microstructural Properties of Ba0.6Sr0.4TiO3/RuO2 Multi-Layers Grown on MgO and YSZ by Pulsed-Laser Deposition

The microstructure of Ba_0.6Sr_0.4TiO₃ (BST)/RuO₂ multi-layers grown on (100) MgO and (100) YSZ substrates, respectively, by pulsed-laser deposition (PLD) has been studied by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The RuO₂ films deposited at 700°C adopt epitaxial relationships with both substrates. The epitaxial films on (100) MgO consist of two variants with an orientation relationship given by (110) RuO₂//(100) MgO and [001] RuO₂//[011] MgO. The epitaxial films on (100) YSZ contain four variants with an orientation relationship given by (200) RuO₂//(100) YSZ and [011] RuO₂//[001] YSZ. The BST films deposited on the RuO₂ electrode are epitaxial on the (200) RuO₂ films deposited on YSZ, and non-epitaxial on the (110) RuO₂ films deposited on MgO. The epitaxial relationship between the BST and (200) RuO₂ films can be described as (111) BST//(200) RuO₂ and [1&1macr;0] BST//[011] RuO₂. The BST films contain at least four variants. The growth and microstructural properties of the multi-layer structures can be understood based on geometrical consideration of the crystal structures.     

Effects of Oxide Additives Coating on Microstructure and Dielectric Properties of BaTiO3

Effects of mixing methods (mechanical mixing, chemical coating) on microstructure and dielectric properties of Ho, Mg and Mn doped BaTiO₃ have been studied. BaTiO₃ particles coated with Ho, Mn and Mg were prepared by a homogeneous precipitation method using urea, and then silica was coated by the sol-gel technique. The adsorption of additives on the BaTiO₃ surface was confirmed. Temperature characteristics of capacitance were satisfied by mechanical mixing and chemical coating techniques, both of which yield different sintering and microstructure behaviors. Pyrochlore phase (Ho₂Ti₂O₇) was observed on the mechanically mixed sample, whereas none of pyrochlore phase was observed at the coated sample after thermal etching. Those different behaviors caused by the degree of homogeneous distribution of the additives in BaTiO₃ matrix were confirmed by EDS analysis.     

The Defect Chemistry of Donor-Doped BaTiO3: A Rebuttal

It has recently been asserted that the donor charge in La⁺³ -doped BaTiO₃ is always compensated by Ti vacancies, and that electrons are never the primary compensating defect. It was also stated that the conductivity observed in reduced, donor-doped BaTiO₃ results from the loss of a very small amount of oxygen not directly related to the donor content. However, the observed reproducible and reversable weight loss on reduction, or gain on oxidation, is exactly that expected for a change between ionic and electronic compensation. It corresponds to the loss or gain of the excess oxygen contained in the donor oxide, e.g. LaO_1.5 vs. the BaO it replaces. The amount of this weight change is proportional to the donor concentration. This is in agreement with the observation that the equilibrium conductivity in the P(O₂)-independent region of electronic compensation is proportional to the donor concentration. Thus the conductivity observed in reduced samples is directly coupled to the donor concentration, and the carrier concentration is equal to the net donor content. In fact, the equilibrium conductivity of donor-doped BaTiO₃ conforms to the behavior expected from classical defect chemistry, and exhibits regions of both ionic and electronic compensation of the donor charge, as expected. Phase studies by TEM have shown that donor-doped BaTiO₃ sintered in air self-adjusts its composition, by splitting out a second phase if necessary, so that the appropriate amount of compensating Ti vacancies are formed. However, when sintered in a reducing atmosphere, the composition self-adjusts to accommodate charge compensation by electrons.     

Dielectric properties of the BaTiO3-AlN-additive system

The mixed system of BaTiO₃ and AlN has been investigated in terms of dielectric properties and microstructure. Two different types of additives, bismuth oxide and bismuth borosilicate glass, were used to lower sintering temperature. First, the addition of a fixed content (3 wt.%) of Bi₂O₃ provided densification at 1200^∘C where monotonous decreases of dielectric constant were found with increasing the content of AlN. On the other hand, the bismuth borosilicate glass was effectively used to decrease firing temperature to 850^∘C, which is suitable for thick film capacitor applications. A practical demonstration of thick film capacitors using a Ag electrode on a 96% alumina substrate indicated that the optimum composition of 76BaTiO₃-20AlN-4glass may be adequate for generating k of 79.4 and tan δ of 0.014 at 1 MHz as a result of the low temperature firing of 850^∘C in air atmosphere.     

Effects of Mn-doping on TSDC and degradation of BaTiO3

The effects of Mn-doping on TSDC (Thermally Stimulated Depolarization Current) and electrical degradation of BaTiO₃ have been investigated. TSDCs of un-doped BaTiO₃ and Ba(Ti_1−x Mn_x)O_3−δ exhibited the three sharp TSDC peaks around phase transition temperatures. TSDC of Ba(Ti_0.995Mg_0.005)O_2.995 increased gradually from 50^∘C and this anomalous depolarization current kept going up well above the Curie temperature (∼130^∘C). TSDCs of un-doped BaTiO₃ and Ba(Ti_0.995Mn_0.005)O_3−δ decreased in the temperature range above the Curie point, whereas a slight increase in TSDC was confirmed at the specimen of Ba(Ti_0.99Mn_0.01)O_3−δ. TSDCs of Ba(Ti_0.995−y Mg_0.005Mn_y)O_3−δ (y = 0.005, 0.01) were lower than that of Ba(Ti_0.995Mg_0.005)O_2.995.     

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi_2.5Na_3.5Nb₅O₁₈ particles were used as templates to fabricate grain-oriented Na_0.5Bi_0.5TiO₃—BaTiO₃ (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.     

Low-temperature sintering of BaTiO3 with Mn-Si-O glass

In order to reduce sintering temperature and prevent adverse dielectric effects, pure BaTiO₃ powder with the addition of Mn-Si-O glass was sintered in the temperature range of 1175–1300°C. Microstructural observation showed that BaTiO₃ grains of the sintered samples only grew from the initial 400 nm to an average of 430 nm between 1175–1275°C for 1 h, or sintered at 1250°C as long as 27 h. Abnormal BaTiO₃ grains are not found in the sintered samples. The microstructure and phase analysis showed that the dielectric properties, tetragonality, and grain growth of BaTiO₃ are closely controlled by the formation of the liquid phase, newly formed Ba₂TiSi₂O₈ grains, and Mn solid solution in BaTiO₃ grains.     

Dielectric behavior of Bi2/3Cu3Ti4O12 ceramic and thick films

The paper reports on synthesis, sintering and microstructure of Bi_2/3Cu₃Ti₄O₁₂, a lead-free, high-permittivity material with internal barrier layer capacitor behavior. Complex impedance and capacitance of the ceramic and thick films were studied as a function of frequency (10 Hz–2 MHz) and temperature (−170 to 400°C). Dc electrical conductivity of the samples was measured in the temperature range 20–400°C. Broad and high maxima of dielectric permittivity versus temperature plots were observed reaching 60,000 for ceramic and 5,000 for thick films. The maxima decrease and shift to higher temperatures with increasing frequency. Two arcs ascribed to grains and grain boundaries were found in the plots of imaginary part versus real part of impedance. Analysis of the impedance spectra indicates that Bi_2/3Cu₃Ti₄O₁₂ ceramic could be regarded as electrically heterogeneous system composed of semiconducting grains and less conducting grain boundaries. The developed thick film capacitors with dielectric layers based on Bi_2/3Cu₃Ti₄O₁₂ exhibit dense microstructure, good cooperation with Ag electrodes, high permittivity up to 5,000 and relatively low temperature coefficient of capacitance in the temperature range 100–300°C. Broad maxima in the dielectric permittivity versus temperature curves may be attributed to Maxwell–Wagner relaxation.     

Relaxor ferroelectric behavior of “A” site deficient Bismuth doped Barium Titanate ceramic

A site deficient Bi doped BaTiO₃ ceramic with general formula Ba_1-xBi_2x/3TiO₃ (x?=?0.00, 0.01, 0.025) is prepared by solid state reaction route. The phase formation and structural property of all compositions are studied by X-Ray Diffraction pattern. The pattern reports single phase tetragonal crystal system with space group of P4mm. The XRD study also reveals that bismuth (Bi) replaces ??A?? site (Ba) of the BaTiO₃ pervoskite. The surface morphology of the sintered pellets is studied by scanning electron microscopy which shows a decrease in grain size with an increase in Bi concentration. The temperature and frequency dependent dielectric behaviors of the compositions are studied to show the effect of Bi ion on the ??A?? (Ba) site of BaTiO₃ perovskite. The dielectric constant decreases and transition temperature increases with increase in Bi concentration. Substitution of Bi ion induces diffuse ferroelectric behavior and the degree of diffuseness increases with increase in doping concentration. The ferroelectric behavior is also confirmed by the P-E loop study.     

Structural and electrical properties of Sr(Ti, Fe)O3-δ materials for SOFC cathodes

Doped strontium titanates are very versatile materials. Iron doped SrTiO₃ can be used, for example, as a material for resistive gas sensors and fuel cell electrodes. In this paper, two compositions based on Fe doped SrTiO₃ were studied as possible candidates for cathode application in SOFCs. Namely, SrTi_0.65Fe_0.35O₃ and SrTi_0.50Fe_0.50O₃ were examined. A chemical reactivity between electrode and YSZ electrolyte material was investigated, since Sr containing cathode materials in contact with YSZ electrolyte are prone to form insulating phases. Electrical conductivity of bulk samples showed relatively low total conductivities of 0.4 S cm⁻¹ and ~2 S cm⁻¹ for STF35 and STF50 respectively. Suitability for cathode application was studied by Electrochemical Impedance Spectroscopy in a symmetrical electrode configuration. Area specific resistance (ASR) was determined in the temperature range from 600°C to 800°C. At 790°C samples show polarization ASR of approximately 0.1 Ω cm². It can be expected that further reduction of electrode ASR can be obtained by introduction of ceria barrier layer and tailoring of the electrode microstructure.     

Preparation and electrochemical properties of La<Subscript>1.0</Subscript>Sr<Subscript>1.0</Subscript>FeO<Subscript>4+δ</Subscript> as cathode material for intermediate temperature solid oxide fuel cells

Cathodic material La_1.0Sr_1.0FeO_4+δ for an intermediate temperature solid oxide fuel cell (IT-SOFC) was prepared via the glycine-nitrate process and characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM). XRD results showed that no reaction occurred between the La_1.0Sr_1.0FeO_4+δ electrode and Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1000 °C. SEM results showed that the electrode formed good contact with the SDC electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of La_1.0Sr_1.0FeO_4+δ were measured using electrochemical impedance spectroscopy (EIS) and steady state polarization measurement. At 700 °C, the polarization resistance was about 3.90 Ωcm², and the lowest polarization overpotential was 57 mV at a current density of 55 mA cm⁻².