Preparation and characterization of Si-doped barium titanate nanopowders and ceramics

Si-doped barium titanate nanopowders and ceramics were prepared through the sol-gel process. The powders and ceramics were characterized by methods of XRD, SEM and TEM. The dielectric properties of the ceramics were also determined. The results indicated that the powders were nanopowders and they were all cubic BaTiO₃ phase with the concentration of Si ?5.0 mol%. When the concentration of Si increased to 10.0 mol%, another phase of Ba₂TiSi₂O₈ appeared. After sintering, the cubic BaTiO₃ phase was transformed into tetrahedron BaTiO₃ phase. Si doping with low concentration resulted in improving grain growth and reduced dielectric loss. As the sintering temperature increased, the dielectric properties of the ceramics decreased. BaTiO₃ ceramics doped with Si all had a small peak at room temperature in ε−T cures. Specially, the ceramic doped with 0.3 mol% Si had evident double peaks, the maximum room temperature permittivity (4081) and the minimum dielectric loss (0.004).     

High-Temperature Capacitor Materials Based on Modified BaTiO<Subscript>3</Subscript>

High-temperature capacitor materials sintered at 1120°C were prepared in a BaTiO₃ (BT)-Na_0.5Bi_0.5TiO₃ (NBT)-Nb₂O₅-ZnO-CaZrO₃ system. The Curie temperature of BaTiO₃ was increased by NBT doping, and a secondary phase occurred when adding ≥5 mol% NBT. The effects of Nb₂O₅, ZnO, and CaZrO₃ on the dielectric properties and the microstructure of BT ceramics doped with 1 mol% NBT were analyzed. The overall dielectric constant decreased when the Nb₂O₅ content increased, and increased when the ZnO content increased. The dielectric constant peak at the Curie temperature was effectively depressed, and a broad secondary dielectric constant peak appeared at 60°C when the ZnO concentration was ≥4.5 mol%. Significant grain growth was observed by scanning electron microscope (SEM) analysis as the amount of ZnO increased. The high-temperature capacitor specification (−55°C to +175°C, ΔC/C _25°C less than ±15%) is met when 7 mol% to 8 mol% CaZrO₃ is added.     

Nanoscale Insight Into Lead‐Free BNT‐BT‐xKNN

Piezoresponse force microscopy (PFM) is used to afford insight into the nanoscale electromechanical behavior of lead‐free piezoceramics. Materials based on Bi_1/2Na_1/2TiO₃ exhibit high strains mediated by a field‐induced phase transition. Using the band excitation technique the initial domain morphology, the poling behavior, the switching behavior, and the time‐dependent phase stability in the pseudo‐ternary system (1–x)(0.94Bi_1/2Na_1/2TiO₃‐0.06BaTiO₃)‐xK_0.5Na_0.5NbO₃ (0 <= x <= 18 mol%) are revealed. In the base material (x = 0 mol%), macroscopic domains and ferroelectric switching can be induced from the initial relaxor state with sufficiently high electric field, yielding large macroscopic remanent strain and polarization. The addition of KNN increases the threshold field required to induce long range order and decreases the stability thereof. For x = 3 mol% the field‐induced domains relax completely, which is also reflected in zero macroscopic remanence. Eventually, no long range order can be induced for x >= 3 mol%. This PFM study provides a novel perspective on the interplay between macroscopic and nanoscopic material properties in bulk lead‐free piezoceramics.     

Fabrication and Dielectric Characterization of Advanced BaTiO3/Polyimide Nanocomposite Films with High Thermal Stability

Barium titanate/polyimide (BaTiO₃/PI) nanocomposite films with high dielectric permittivity (20), high breakdown strength (67 MV m^?1), and high thermal stability are prepared by an in‐situ polymerization process. A very thin polymer layer (about 5 nm) is coated on the surface of nanosized BaTiO₃ particles to form a core–shell‐like structure, which can guarantee homogeneous dispersion of the BaTiO₃ particles in the PI matrix. It is confirmed that the core–shell‐like structure originates from both the electrostatic attraction between the precursor poly(amic acid) (PAA) and the BaTiO₃ particles and the hydrogen bond interaction between PI and the BaTiO₃ particles. Such a structure also has some influence on the dielectric properties and breakdown strength of films. After casting and degassing of the sticky film, the dielectric permittivity of the nanocomposite film is close to or even higher than that of submicrocomposite films, which is attributed to the advanced interfacial structure between the BaTiO₃ and PI phases.     

Dielectric Properties and Microstructure of MgO-TiO<Subscript>2</Subscript>-ZnO-CaO Ceramics

The effects of CaTiO₃ addition on the microstructure, phase formation, and dielectric properties of MgO-TiO₂-ZnO ceramics were investigated. The sintering temperature of CaTiO₃-doped (Mg_0.63Zn_0.37)TiO₃ ceramics can be lowered to 1290°C when the additive is used. The dielectric properties are found to be strongly correlated with the amount of CaTiO₃ addition. At 1290°C, (Mg_0.63Zn_0.37)TiO₃ ceramic with 1.0 mol% CaTiO₃ exhibited a dielectric constant ε _r of 23.3, dielectric loss tan δ of 1 × 10⁻⁵, and temperature coefficient of capacitance (TCC) of 10 ppm/°C.     

Domain Wall Displacement is the Origin of Superior Permittivity and Piezoelectricity in BaTiO3 at Intermediate Grain Sizes

The dielectric and piezoelectric properties of ferroelectric polycrystalline materials have long been known to be strong functions of grain size and extrinsic effects such as domain wall motion. In BaTiO₃, for example, it has been observed for several decades that the piezoelectric and dielectric properties are maximized at intermediate grain sizes (≈1 μm) and different theoretical models have been introduced to describe the physical origin of this effect. Here, using in situ, high‐energy X‐ray diffraction during application of electric fields, it is shown that 90° domain wall motion during both strong (above coercive) and weak (below coercive) electric fields is greatest at these intermediate grain sizes, correlating with the enhanced permittivity and piezoelectric properties observed in BaTiO₃. This result validates the long‐standing theory in attributing the size effects in polycrystalline BaTiO₃ to domain wall displacement. It is now empirically established that a doubling or more in the piezoelectric and dielectric properties of polycrystalline ferroelectric materials can be achieved through domain wall displacement effects; such mechanisms are suggested for use in the design of new ferroelectric materials with enhanced properties.     

New dielectric material system of Nd(Mg1/2Ti1/2)O3–CaTiO3 at microwave frequency

The microwave dielectric properties of (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ (0.1  x  1.0) ceramics prepared by the conventional solid state method have been investigated. The system forms a solid solution throughout the entire compositional range. The dielectric constant decreases from 152 to 27 as x varies from 0.1 to 1.0. In the (1 − x)CaTiO₃–xNd(Mg_1/2Ti_1/2)O₃ system, the microwave dielectric properties can be effectively controlled by varying the x value. At 1400 °C, 0.1CaTiO₃–0.9Nd(Mg_1/2Ti_1/2)O₃ has a dielectric constant (ε_r) of 42, a Q × f value of 35 000 GHz and a temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 43 000 GHz for x = 0.9 is achieved at the sintering temperature 1500 °C.     

Preparation and characterization of BaTiO3 powders and ceramics by sol-gel process using hexanoic and hexanedioic acid as surfactant

The BaTiO₃ xerogels, powders and ceramics were prepared through the sol-gel process using hexanedioic acid as surfactant. The xerogels, powders and ceramics were characterized by methods of thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscope, and transmission electron microscope. The dielectric properties of the ceramics were also measured. The results indicated that the powders calcined at 900 °C for 2 h were pure BaTiO₃ phase partly consisted of the tetrahedron BaTiO₃. The powders were nanometer scale particles. After sintering, the ceramics mainly consisted of the tetrahedron BaTiO₃ phase. Compared with the powders prepared using hexanoic acid as surfactant, the particle size of the powders prepared using hexanedioic acid obviously increased, and as well as the grain size, the relative density and the whole permittivity of the ceramics increased. Furthermore, the forming process of the powders with better dispersibility and the influence of the carboxyl number on the particle size of the powders can be explained using the “organic acid micro-capsules” model. Relatively, the ceramic prepared using hexanedioic acid as the surfactant had higher relative density (96.6%), room temperature permittivity (3089) and lower dielectric loss (0.015).     

A study on the temperature dependence of epoxy/BaTiO<Subscript>3</Subscript> composite embedded capacitor films

In this study, the temperature dependence of capacitance, one of the most important properties of embedded capacitor films (ECFs), was investigated. The temperature dependence of the capacitance of ECFs was determined by the temperature dependence of the dielectric constant and thickness, and among these, the main factor was the dielectric constant of ECFs. The dielectric constant of ECFs was determined by that of epoxy and BaTiO₃ powders. Below 130°C, the dielectric constant of ECFs increased as temperature increased, and was mainly affected by an epoxy matrix. However, above 130°C (the Curie temperature of BaTiO₃), the increased rate of the dielectric constant of ECFs started decreasing. This was due to the fact that BaTiO₃ powder undergoes a phase transition from a tetragonal to a cubic structure, and its dielectric constant decreases at 130°C. The dielectric constant of BaTiO₃ powder was obtained from measured dielectric constants of ECF and application of the Lichtenecker logarithmic rule.     

Good Thermal Stability,High Permittivity,Low Dielectric Loss and Chemical Compatibility with Silver Electrodes of Low-Fired BaTiO<Subscript>3</Subscript>–Bi(Cu<Subscript>0.75</Subscript>W<Subscript>0.25</Subscript>)O<Subscript>3</Subscript> Ceramics

(1 ? x)BaTiO₃–xBi(Cu_0.75W_0.25)O₃ [(1 ? x)BT–xBCW, 0 ≤ x ≤ 0.04] perovskite solid solutions ceramics of an X8R-type multilayer ceramic capacitor with a low sintering temperature (900°C) were synthesized by a conventional solid state reaction technique. Raman spectra and x-ray diffraction analysis demonstrated that a systematically structural evolution from a tetragonal phase to a pseudo-cubic phase appeared near 0.03 < x < 0.04. X-ray photoelectron analysis confirmed the existence of Cu⁺/Cu²⁺ mixed-valent structure in 0.96BT–0.04BCW ceramics. 0.96BT–0.04BCW ceramics sintered at 900°C showed excellent temperature stability of permittivity (Δε/ε _25°C ≤ ±15%) and retained good dielectric properties (relative permittivity ～1450 and dielectric loss ≤2%) over a wide temperature range from 25°C to 150°C at 1 MHz. Especially, 0.96BT–0.04BCW dielectrics have good compatibility with silver powders. Dielectric properties and electrode compatibility suggest that the developed materials can be used in low temperature co-fired multilayer capacitor applications.     

Effects of Ca and Mn Additions on the Microstructure and Dielectric Properties of (Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript> Ceramics

Dielectric ceramics based on the solid solution (1 − x)Bi_0.5Na_0.5TiO₃ (BNT)-xCaTiO₃ (CT) were synthesized by the conventional solid-state route. BNT with various contents of CT formed a complete solid solution and exhibited a rhombohedral structure. CT in this solid solution with BNT was observed to decrease the dielectric constant at higher temperatures and raise the dielectric constant at lower temperatures. On the other hand, decreased ferroelectricity was observed with increasing CT concentration, resulting in a downward shift of the depolarization temperature and a decrease of the dissipation factor. With the addition of Mn²⁺ to 0.86BNT-0.14CT, the temperature characteristics of capacitance were improved (−55°C to 250°C, ΔC/C _25°C ≤ ±15%). By doping with 1.5 wt.% Mn²⁺, the dielectric constant at room temperature reached over 900, with a dielectric loss of less than 1%.     

Interface-Charge-Coupled Polarization Response of Pt-BaTiO<Subscript>3</Subscript>-ZnO-Pt Heterojunctions: A Physical Model Approach

Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferroelectric) BaTiO₃ are very interesting because of the previously observed ionic lattice polarization coupling at their interfaces. We report electric Sawyer-Tower polarization hysteresis measurements and analysis of a ZnO-BaTiO₃ heterostructure with Pt front and back contacts deposited by pulsed laser deposition onto a (001) silicon substrate. The ZnO layer is n-type (N _c = 5.5 × 10¹⁶ cm⁻³), and the BaTiO₃ (BTO) layer is highly resistive. We observe a strong asymmetric ferroelectric hysteresis, which we attribute to a rectifying depletion layer formation between the ZnO and BaTiO₃ layers. The coupling between the wurtzite-structure and perovskite-structure interface polarization influences the depletion layer formation. We develop a physical model for the electric Sawyer-Tower measurements. Our model includes the effects of the depletion layer formation inside the ZnO layer, the interface charge coupling between the ZnO and BaTiO₃ layers, and the field-dependent ferroelectric polarization inside the BTO. We obtain a very good agreement between our model-generated data and our experiment. We identify voltages in forward and reverse direction at which the depletion layer opens or closes. These voltages are asymmetric, and reveal the effect of the spontaneous piezoelectric (nonswitchable) interface charge of ZnO, which we determine from our analysis here as P _sz = −4.1 μC/cm².     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.     

Polymer nanocomposite dielectric based on P(VDF-TrFE)/PMMA/BaTiO3 for TIPs-pentacene OFETs

TIPs-pentacene OFETs were fabricated on a plastic substrate using polymer nanocomposite dielectric. The blend polymer P(VDF-TrFE)/PMMA (30 wt%) was used as polymer matrix and BaTiO₃ nanoparticles modified by 3-glycidoxypropyltrimethoxysilane (GPTMS) were dispersed as ceramic fillers. The effects of different loadings of BaTiO₃ on the surface morphology and electrical properties of dielectric films were investigated. The formulation of screen-printable dielectric ink of P(VDF-TrFE)/PMMA/BaTiO₃/Silica (SII) was achieved by adding fumed silica as the viscosity modifier. TIPs-pentacene OFETs using SII as the gate dielectric features a mobility of 0.01 cm²/V s, and having a threshold voltage of −6 V. This screen-printable dielectric ink is promising for low operating-voltage fully-printed OFETs.     

Raman characterization and electrical properties of poly(3-hexylthiophene) doped electrochemically in an ionic liquid-gated transistor geometry

Using Raman spectroscopy, we observed carriers, polarons and bipolarons formed in an ionic-liquid-gated P3HT electrochemical transistor with an ionic liquid [BMIM][TFSI] as a gate dielectric. The relationships between the source−drain current (I_D), the gate voltage (V_G) at a constant source−drain voltage (V_D), and injected charges at each V_G were investigated. An increase in I_D is attributed to the formation of positive polarons, whereas a decrease in I_D corresponded to positive bipolarons. Thus, positive polarons are efficient carriers in P3HT electrochemical transistors. Charge densities, doping levels, electrical conductivities, and mobilities of polarons in P3HT were calculated from the electrochemical measurements. Only positive polarons exist below the dopant level x = 27 mol%, whereas at higher doping levels, polarons and bipolarons coexist. The mobility of polarons was dependent on the doping level. The highest mobility was 0.31 cm² V⁻¹ s⁻¹ at x = 15 mol%.     

Electrical and structural properties of hafnium silicate thin films

Thin (4 nm) hafnium silicate (HfO₂)_x(SiO₂)_1−x/SiO₂ gate stacks (0 < x < 1) grown by metal organic chemical vapour deposition (MOCVD) are investigated in this study. The focus is on extracting the optical constants, and hence bandgaps as well as dielectric constants. The VUV (vacuum ultraviolet) spectroscopic ellipsometry (VUV-SE) technique in the spectral range 140–1700 nm, together with current–voltage and capacitance–voltage techniques were used for studying the optical and electrical properties of the layers, respectively. The bandgap was found to increase from 5.24 eV for HfO₂ to 6 eV for Hf-silicate with 30% Hf. The permittivity was reduced from 21 for HfO₂ layers to 8 for Hf-silicate with x = 0.3. The results suggest that the optimal Hf content is above 0.6, for which the permittivity higher than 10 can be achieved.     

Dielectric properties of Fe-doped BaxSr1−xTiO3 thin films on polycrystalline substrates at temperatures between −35 and +85 °C

Large-area Ba_xSr_1−xTiO₃ (BSTO-x) thin films, partially Fe-doped, have been grown by pulsed laser deposition (PLD) on technically relevant polycrystalline alumina based ceramics. The capacity (dielectric constant _r) and Q-factor of planar Pt/BTO:Fe/Pt capacitors were investigated within a temperature range from −35 to +85 °C. The applied DC-bias voltages were up to 10 V and the measurement frequency was 1 kHz.Although operating in the ferroelectric state below the Curie temperature, pure BaTiO₃ (BTO) thin films showed the smallest variation of _r within the temperature range from −35 to +85 °C compared to BSTO-0.6 and BSTO-0.8. The temperature dependence of _r below the Curie temperature (ferroelectric state) seems to be smaller than above the Curie temperature (paraelectric state) for the BSTO-x system. A homogeneous tunability of the capacity of about 60% was achieved for applied electrical DC voltages resulting in electrical field strengths between 0 and 5 V/μm within the whole temperature range. The Q-factor of 2 μm thick BTO films increases with increasing DC bias voltage. Furthermore, by Fe-doping of BTO films Q-factors could be increased by a factor of three up to about 70 compared to the not doped films. In addition, the temperature dependence of capacity is considerably influenced by Fe-doping.At a microwave frequency of 30 GHz high _r values of about 1500 were measured for large-area BSTO-0.45 films at room temperature deposited directly on microwave ceramic substrates. Low values of tanδ of about 0.003 were measured for the PLD-BSTO-0.45 films which corresponds to a Q-factor of more than 300. The results show the potential of ferroelectric BTO thin films for applications as tunable electronic devices in a wide temperature range.     

Electrical characterization of high-k gate dielectrics fabricated using plasma oxidation and post-deposition annealing of a Hf/SiO2/Si structure

High permittivity (high-k) gate dielectrics were fabricated using the plasma oxidation of Hf metal/SiO₂/Si followed by the post-deposition annealing (PDA), which induced a solid-phase reaction between HfO_x and SiO₂. The oxidation time and PDA temperature affected the equivalent oxide thickness (EOT) and the leakage current density of the high-k dielectric films. The interfacial structure of the high-k dielectric film/Si was transformed from HfO_x/SiO₂/Si to HfSi_xO_y/Si after the PDA, which led to a reduction in EOT to 1.15 nm due to a decrease in the thickness of SiO₂. These high-k dielectric film structures were investigated by X-ray photoelectron spectroscopy. The leakage current density of high-k dielectric film was approximately four orders of magnitude lower than that of SiO₂.     

Effects of Sm<Superscript>3+</Superscript> Doping on Dielectric Properties of Anatase TiO<Subscript>2</Subscript> Nanoparticles Synthesized by a Low-Temperature Hydrothermal Method

Samarium-doped thermally stable TiO₂ nanoparticles in the anatase phase have been synthesized by a low-temperature hydrothermal method. The formation of the anatase phase has been investigated by x-ray diffraction. Thermogravimetry and differential thermal analysis have been used for thermal studies. The morphology and composition of synthesized powders have been studied using scanning electron microscopy, transmission electron microscopy, and energy-dispersive spectroscopy. Surface areas were studied by the Brunauer–Emmett–Teller method. Dielectric properties were studied for dopant levels of 0.2 mol% and 0.5 mol% at 300 K in the frequency range of 42 Hz to 5 MHz. At low frequency, charge carriers at the grain boundary produce interfacial polarization giving rise to a high dielectric constant (ε′), which is significantly reduced by doping with samarium ions (Sm³⁺). Strong frequency dependence of the dielectric loss was also observed for each concentration. Conductivity studies showed that the reduction in conductivity is due to the decrease in particle size with the increase in Sm³⁺ dopant level.     

Effect of substitution of Ti by Zr in BaTiO3 thin films grown by MOCVD

Thin films of the solid solution BaTiO₃–BaZrO₃ were studied. Such lead-free, environmental friendly materials are known, from dielectric measurements, to exhibit relaxor behaviour in bulk materials with increasing the Zr content. Ba[Ti_(1−x)Zr_x]O₃ thin films with various x values were prepared by liquid injection MOCVD by varying the corresponding x′ value in solution (from 0.00 to 0.80). The films were studied by X-ray diffraction, Raman spectroscopy and microprobe analysis. A single perovskite phase with a linear evolution of the out-of-plane lattice parameter was identified by X-ray diffraction up to x′=0.25. For higher Zr contents a secondary ZrO₂ phase was detected. Raman spectroscopy was used to follow the subtle evolution of the crystal structure as a function of the chemical composition. The dielectric properties of single-phase layers were investigated in the range 20–600 K and 0.02–100 kHz. For some composition, the measured dielectric constant displayed a frequency-dependent behaviour.