Dielectric properties of barium titanate–molybdenum composite

The barium titanate–molybdenum composites were prepared through solid state reaction method in argon atmosphere. The microstructure, resistivity, and dielectric properties of the composites were investigated. XRD results indicated that chemical reactions between barium titanate (BaTiO₃:BT) and molybdenum (Mo) have taken place during sintering, resulting in the formation of BaMoO₄ (BM) and BaTi₂O₅ (BT₂). The resistivity decreased with the increasing amount of Mo in the composites. The composites (when x = 5 and 20 wt.%) showed lower dielectric constant than pure BaTiO₃, especially, the dielectric constant (when x = 20 wt.%) reached a minimum value (<10⁴), while composites (when x = 10 and 15 wt.%) showed rather high dielectric constant at temperatures range from 25 °C to 160 °C. The dielectric constant of the composite gradually decreased with increase in frequency at the room temperature. The dielectric constant of composite (when x = 5 wt.%) comes up to 10⁴, and the T_c (Curie temperature) of the composite was relatively higher than that of BT (120 °C).     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

THz region dielectric properties of barium titanate fine particles using infrared reflection method

The THz region dielectric properties of barium titanate (BaTiO₃) fine particles were measured using the infrared (IR) reflection method. Prior to this measurement, to minimize the scattering light caused by surface roughness, the preparation of the dense 3D colloidal sphere arrays (colloidal crystals) with a flat surface was tried. First, the BaTiO₃ fine particles were well dispersed into diethylene glycol as an organic solvent, and this BaTiO₃ slurry was dried very slowly at 80 °C. Finally, the dense BaTiO₃ colloidal crystals were successfully prepared. The IR reflection method was performed using these dense BaTiO₃ colloidal crystals, and their IR reflection spectra were measured from 100 to 1200 cm⁻¹. As a result, by the use of the dense BaTiO₃ colloidal crystals, the high intensity reflection spectra of the BaTiO₃ fine particles were successfully obtained. The spectra were analyzed using a four-parameter semi-quantum (FPSQ) model, and finally, the THz region dielectric properties were estimated for the BaTiO₃ fine particles.     

Rare-earth nitrate additives for the sintering of silicon carbide

Fourteen rare earth elements in their nitrate form were evaluated as sintering additives for β-SiC. All rare earth nitrates transformed to oxides by a reaction with the surface-adsorbed thin SiO₂ during heat treatment, which enhanced the density of the SiC monolith without decomposing SiC. In particular, Sc, Yb, Tm, Er and Ho were quite effective sintering additives; a > 99% relative density was observed by the addition of 5 wt.% rare earth oxide, whereas the other rare earth additives (Lu, Dy, Tb, Gd, Eu, Sm, Nd, Ce and La) revealed 77–92% density. Moreover, a fine 156 nm-sized SiC grain could be acquired by Sc addition, whereas the other additives showed a SiC grain size of approximately 1 μm. The mean hardness and K_Ic of the dense SiC containing rare earth elements were 24–27 GPa and 3.3–5.0 MPa m^1/2, respectively.     

Microstructural evolution and electric properties of mechanically activated BaTiO3 ceramics

Ceramic materials with a perovskite related structures such as non-doped and doped barium titanate ceramics are attracting much interest for their application as capacitor dielectrics, resistors, thermal sensors, etc. Since mechanical activation can be used in order to modify properties of these materials, in this study microstructure evolution and electric properties of mechanically activated BaTiO₃ have been analyzed. The sintering process of high purity non-doped mechanically activated BaTiO₃ was monitored using a sensitive dilatometer with a heating rate of 10 °C/min. Investigation of the microstructure evolution of mechanically activated BaTiO₃ was performed using scanning electron microscope (SEM) and digital pattern recognition (DPR) methods. A dielectric study of the paraelectric–ferroelectric phase transition in the barium titanate ceramics was performed by recording the temperature dependence of dielectric permittivity.     

Nanoscaled BaTiO3 powders with a large surface area synthesized by precipitation from aqueous solutions: Preparation,characterization and sintering

Nanosized BaTiO₃ powders with a specific surface area of 60–75 m²/g have been prepared by precipitation of a titanium ester with Ba(OH)₂ solution at temperatures less than 100 °C. The effects of the Ba(OH)₂ concentration, isopropanol mixing with water as a solvent, the Ba:Ti ratio and surface modifiers on the surface area, the particle size, the crystalline phase, the agglomeration and aggregation degree of the synthesized powders as well as dielectric properties of sintered pellets have been investigated. The properties of the obtained powders have been characterized with XRD, BET, TG-DTA, ICP-AES, HRTEM and dilatometer. A high concentration of Ba(OH)₂ can increase the agglomeration and aggregation degree of the particles while the addition of isopropanol in water is beneficial for lowering it. To obtain stoichiometrical barium titanate, the ratio of Ba:Ti should be 1.1. The leaching of barium ions during processing can be limited by washing the powder with ammonia solution at pH10.2. A BaTiO₃ ceramic (95.8% of the theoretic density) has been fabricated by sintering the powders at 1250 °C for 2 h.     

Sol–gel synthesis and characterization of Ce doped-BaTiO3

Barium titanate (BaTiO₃) have been doped “in situ” with 5.5 mol% cerium by a sol–gel method using barium acetate, titanium (IV) isopropoxide, and cerium (III) acetylacetonate as starting materials. The dried gel showed a microstructure consisting of nano-sized grains (∼140 nm) with great tendency to agglomeration. Several thermal analysis techniques were used to study the decomposition process of the gel. The presence of hydroxyls up to 720 °C suggests a strong bonding TiOH that is responsible for the existence of aggregates even at high temperatures. The as-prepared gel powder was found to be amorphous, and then decomposes through oxides and barium carbonate around 500 °C and crystallizes on the perovskite structure of tetragonal BaTiO₃ at 1100 °C for 3h in air. A small influence of the frequency on the dielectric properties of the Ba_0.945Ce_0.055TiO₃ ceramics was observed in 100 Hz to 1 MHz domain. At the Curie temperature point (22 °C) the dielectric constant was 10130 at 100 Hz while the dielectric loss (tan δ) was 0.018.     

Low sintering of X7R ceramics based on barium titanate with SiO2–B2O3–Li2O sintering additives in reducing atmosphere

Development of a low-temperature sintered dielectric material derived from barium titanate for X7R characterized dielectric ceramics application is discussed in this paper. By addition of SiO₂–B₂O₃–Li₂O sintering additives to commercial BaTiO₃ powder, more than 95% of the theoretical density was obtained at a sintering temperature of 950 °C in H₂/N₂ atmosphere. The influence of the composition and procedures on the microstructures, lattice parameters and properties of ceramics materials were systemically studied. After explaining the reason for lower isolated resistivity (IR) in the previous experiment, several methods are tried out to improve the IR properties, which have reached the application requirement level of 10¹² Ω cm. These ceramics sintered between 900 °C and 950 °C in H₂/N₂ atmosphere are promising candidates for fabrication of Cu electrode MLCCs.     

Control of dispersion frequency of BaTiO3-based ceramics applicable to thin absorber for millimeter electromagnetic wave

Elements such as B, Li and Na were doped to barium titanate, BaTiO₃ in order to control dielectric dispersion. Addition of 3 mol% Li₂O lowered the dispersion at frequency of 0.53 MHz, while addition of 3 mol% B₂O₃ or Na₂O did not affect dispersion frequency. BaTiO₃ doped with 0.3 mol% Li₂O showed dielectric dispersion at around 2.5 GHz. An electromagnetic (EM) wave absorber using the doped BaTiO₃ plate was tried to produce for millimeter frequency range. A matching layer of 0.5 mm thick ceramic plate with relative permittivity 21 was attached to it to suppress reflection of incident EM wave due to the discontinuity at the boundary between the BaTiO₃ and air. The obtained EM wave absorber had reflectivity of −45 dB at 31 GHz and −25 dB at 95 GHz, respectively.     

Facile synthesis of nanorods of tetragonal barium titanate using ethylene glycol

Tetragonal barium titanate (BaTiO₃) nanorods were synthesized from hydroxide precursor by a hydrothermal/solvothermal method with 10 vol% ethylene glycol as solvent. The hydroxide precursor slurry was prepared by the addition of 10 M NaOH to a mixed solution of BaCl₂ and TiCl₄. When the above aqueous slurry was heated with water only at 200 °C, cubic BaTiO₃ nanocrystals formed, whereas tetragonal BaTiO₃ nanorods were obtained when heated with 10 vol% ethylene glycol. The crystallization of cubic BaTiO₃ via dissolution–reprecipitation of precursor could be suppressed by the addition of ethylene glycol, resulting in the formation of tetragonal BaTiO₃ under hydrothermal treatment at 200 °C.     

The dielectric properties of BaTiO3 based ceramics co-doped with Bi/Mn

BaTiO₃ based ceramics (with some additives such as ZrO₂, SnO₂, etc.) were prepared by solid state reaction. Mn²⁺ or Mn³⁺ as an acceptor substituting for Ti⁴⁺ in B site and Bi³⁺ as a donor substituting for Ba²⁺ in A site were co-doped in BaTiO₃ based ceramics. The dielectric properties of BaTiO₃ based ceramics co-doped with Bi/Mn were investigated. The results show that the dielectric properties of BaTiO₃ based ceramics co-doped with Bi/Mn are affected by the mole ratio of donor and acceptor (Bi/Mn). When the mole ratio of donor and acceptor is high, dielectric dispersion behavior was observed and the dielectric constant decrease and remnant polarization, coercive field and piezoelectric constant will varied. When Bi varied from 1.0% to 2.0 mol% (Mn = 0.8 mol%), remnant polarization from 10.35 to 2.25 μC/cm², coercive field from 4 to 2.75 kV/cm, and piezoelectric constant d₃₃ from 137 to 36 pC/N respectively.     

Development of novel magnetic-dielectric ceramics for enhancement of reflection loss in X band

The purpose of this research was to develop BaFe_9.5Al_1.5CrO₁₉-xCaCu₃Ti₄O₁₂ nanocomposites (x=10%, 20%, 30%, 40%, 50%) and investigate their structural and magnetic features. The substituted barium hexaferrite (BaFe_9.5Al_1.5CrO₁₉) nanoparticles and calcium copper titanate (CaCu₃Ti₄O₁₂) particles were synthesized by the auto-combustion sol-gel method. The structural, chemical composition and morphology of CaCu₃Ti₄O₁₂ (CCTO) and the nanocomposites were investigated by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy, respectively. The magnetic and microwave properties of nanocomposites were also investigated by vibrating sample magnetometer and vector network analyzer, respectively. The results confirmed that 1100 °C is the optimum synthesis temperature for CCTO, the mean particles size of the CCTO particles changing from 220 nm (at 850 °C) to 2.18 µm (1250 °C). The SEM micrograph revealed that in all of the BaM-xCCTO nanocomposites (x=10%, 20%, 30%, 40%, 50%), the CCTO dielectric particles were attached to the substituted barium hexaferrite nanoparticles, indicating the effectiveness of the adopted synthesis method. Due to the presence of a dielectric phase in the nanocomposites the saturation magnetization decreases from 22 emu/g to 12 emu/g. The coercive field was a slightly larger than substituted barium hexaferrite and increased from 5.558 kOe for substituted barium hexaferrite to 5.813 kOe for BaM-50CCTO due to hindered motion of the domain walls by the dielectric phase and also to the collective behavior of agglomerated barium ferrite nanoparticles. The BaM-30CCTO nanocomposite shows the highest value of reflection loss compared to other nanocomposites. The reflection dip frequency of BaM-30CCTO nanocomposite was −48.85 dB at 10.93 GHz.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Hydrothermal synthesis and formation mechanism of tetragonal barium titanate in a highly concentrated alkaline solution

Tetragonal cube-shaped barium titanate (BaTiO₃) was produced by the hydrothermal treatment of a peroxo-hydroxide precursor, a single-source amorphous barium titanate precursor, in a highly concentrated sodium hydroxide solution. Phase pure barium titanate with cube-shaped morphology and particle-sizes in the 0.2–0.5 µm range were formed at temperatures above 80 °C. Also, the cube-shaped morphology of the BaTiO₃ product was preceded by spherical- and plate-like morphologies with, respectively, a Ti-excess and Ba-excess. Coinciding with these morphological observations, changes in the reaction product were also observed. The formation of crystalline BaTiO₃ proceeded alongside secondary BaTi₂O₅ and Ba₂TiO₄ phases. These secondary phases disappeared as the reaction time was increased leaving only BaTiO₃ as the sole reaction product. Kinetic analysis of the formation of hydrothermal BaTiO₃ crystallization by the Johnson-Mehl-Avrami method showed that BaTiO₃ crystallization is a homogeneous dissolution-precipitation reaction. The mechanism is governed by nucleation and growth in the beginning of the reaction and dissolution-precipitation dominating throughout the hydrothermal reaction process.     

Doping behaviors of NiO and Nb2O5 in BaTiO3 and dielectric properties of BaTiO3-based X7R ceramics

Doping behaviors of NiO and Nb₂O₅ in BaTiO₃ in two doping ways and dielectric properties of BaTiO₃-based X7R ceramics were investigated. When doped in composite form, the additions rendered higher solubility than that doped separately due to the identical valence between the complex (Ni_1/3²⁺Nb_2/3⁵⁺)⁴⁺ and Ti⁴⁺. NiO–Nb₂O₅ composite oxide was more effective in broadening dielectric constant peaks which was responsible for the temperature-stability of BaTiO₃ ceramics. A reduction in grain size was observed in the specimens with 0.5–0.8 mol% NiO–Nb₂O₅ composite oxide, whereas the abnormal growth of individual grains took place in the 1.0 mol% NiO–Nb₂O₅ composite oxide-doped specimen. When the specimen of BaTiO₃ doped with 0.8 mol% NiO–Nb₂O₅ composite oxide was sintered at 1300 °C for 1.5 h in air, good dielectric properties were obtained and the requirement of (EIA) X7R specification with a dielectric constant of 4706 and dielectric loss lower than 1.5% were satisfied.     

Lanthanide coordination polymers constructed from 5-(4-pyridyl)-isophthalic acid: Synthesis,structure and photoluminescent properties

Nine lanthanide coordination polymers [Ln₂(pyip)₃(H₂O)₄·DMF·3H₂O]_n (Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) based on the pyip^2 − ligand {H₂pyip = 5-(4-pyridyl)-isophthalic acid} have been synthesized under solvothermal conditions. X-ray crystallographic studies reveal that 1–9 are isostructural and crystallize in the triclinic system, space group P2₁/c, and exhibit a 3D framework. Topological analysis reveals that the 3D framework can be simplified to a uninodal 6-connected pcu alpha-Po primitive cubic type structure. Meanwhile, the luminescent properties of these nine coordination polymers in the solid state are also investigated. Especially the Eu and Tb compounds show bright red and green luminescence with luminescence lifetimes of 0.39 and 0.80 ms, respectively.     

Syntheses,structural characterization and luminescent properties of M2(ATPA)3(DMF)2(H2O)2 (M = Nd,Sm, Eu,Gd, Tb,Dy; ATPA = 2-aminoterephthalate,DMF = N,N-dimethylformamide)

Six isomorphous metal–organic frameworks: M₂(ATPA)₃(DMF)₂(H₂O)₂ (M = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6); ATPA = 2-aminoterephthalate, DMF = N,N-dimethylformamide), were synthesized by the self-assembly of lanthanide ions, 2-aminoterephthalate, DMF and H₂O. Their crystal structures determined by X-ray crystallography reveal interpenetrating frameworks. Compounds 2, 3, 5 and 6 exhibit mainly ligand luminescence at room temperature while 3 exhibits enhanced Eu³⁺ emissions at 77 K. Magnetic studies indicate 4 is paramagnetic with slight Gd³⁺–Gd³⁺ couplings.     

Lanthanide coordination polymers for multi-color luminescence and sensing of Fe3 +

Luminescent lanthanide coordination polymers [H₂NMe₂]₃[Ln(DPA)₃] (Ln = Eu, Tb, Sm, Dy; [H₂NMe₂]⁺ = dimethyl amino cation; H₂DPA = 2,6-dipicolinic acid) are synthesized, whose multi-color can be tuned and even white color luminescence can be integrated. Besides, the fluorescent sensing property of the [H₂NMe₂]₃[Tb(DPA)₃] system is checked, which shows selective fluorescent quenching effect for Fe^3 +.     

Effect of mechanical milling on structural,dielectric and magnetic properties of BaTiO3–Ni0.5Co0.5Fe2O4 multiferroic nanocomposites

The coexistence of ferroelectricity and ferromagnetism has triggered great interest in multiferroic materials. Multiferroic with strong room temperature magnetoelectric (ME) coupling can provide a platform for future technologies. In this paper, we have investigated the effect of mechanical milling on the properties of multiferroic nanocomposites synthesized by mixing barium titanate (BaTiO₃) (BT) and nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) (NCF). This process has resulted into reliable disposal of a given quantity of NCF nanoparticles in BT grid and composite samples of different particle sizes (<500 nm) have been obtained by varying the duration of ball-milling for 12, 24, and 48 h. The presence of NCF within BT powder has been confirmed by X-ray Diffraction (XRD) and magnetization measurements (MH). Structural analysis was performed by using Reitveld refinement method that shows that the tetragonality of BaTiO₃ structure get reduced in submicron range. Variations in ferroelectric and dielectric properties with reduction in particle size/milling duration have been studied by P-E loop tracer and Impedance analyzer. The dielectric constant value of 400 has been observed for BT-NCF0 that increases to 9.7 K for composite sample ball mill at 48 h whereas remnant polarization increases to 4.2 μC/cm². These composites with high dielectric constant that changes with temperature and particles size find application in energy storage devices, sensor and memory devices.     

Effect of BiMO3 (M=Al,In, Y,Sm, Nd,and La) doping on the dielectric properties of BaTiO3 ceramics

In this study, in order to enhance the energy storage density, 10% BiMO₃ doping is performed in BaTiO₃ ceramics (M=Al, In, Y, Sm, Nd, La) by a traditional solid-state method. The effects of different M³⁺ radii on the structural characteristics, dielectric properties, and energy storage are investigated systematically. The locations of the M-ions gradually shift from B-site substitutions to A-site substitutions with the increase in the ionic radius, which affect the structural characteristics and the dielectric properties. When 80<R_M3+<95.5 pm, the ceramic has a cubic phase which shows the highest energy density; while out of this range, the dielectric properties of the ceramics are degraded. Specially, the change rate of permittivity of the Sm substituted composition reaches 70% at 100 kV/cm, which might be good for high frequency tunable device application. Typically, combined with the suppression of nonlinearity, polarization maximum (P_m) and remnant polarization (P_r), 0.9BaTiO₃–0.1BiInO₃ exhibits the maximum energy density of 0.753 J/cm³ and the highest energy efficiency of 89.4%, which exhibits slim P-E hysteresis loops for energy storage applications.