Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.     

Evidence of room temperature ferromagnetism in Co-doped transparent CuAlO2 semiconductor

We synthesized CuAl_1−xCo_xO₂ (x = 0.00-0.07) thin films by solid-state reaction from Cu₂O and Al₂O₃ on a sapphire (0 0 1) substrate by a simple and cost-effective spin-on technique. X-ray diffraction and Raman spectroscopy confirm the formation of single impurity-free delafossite CuAlO₂ structure for all the compositions. We observed diamagnetism for pristine CuAlO₂ and ferromagnetism for Co-doped CuAlO₂ at room temperature. Specially, the coercivity (H_c) and saturation magnetization (M_s) are significantly enhanced with Co composition from 1 at.% to 5 at.% but show the reverse tendency with higher Co content.     

Charge compensation, electrical and dielectric behavior of lanthanum doped CaCu3Ti4O12

Mechanism of charge compensation on lanthanum, (La³⁺) substitution on Ca site in calcium copper titanate (CaCu₃Ti₄O₁₂), and its effect on resulting electrical and dielectric properties has been studied in the present investigation. For this purpose samples were prepared according to two stoichiometries viz. La_xCa_(1−3x/2)Cu₃Ti₄O₁₂ (x ≤ 0.09) and La_xCa_(1−x)Cu₃Ti₄O₁₂ (x = 0.03) by solid state ceramic route. The former represents ionic compensation while the later is in accordance with electronic compensation. Nature of charge carriers is identified by measuring Seebeck coefficient which is found to be negative in the entire range of measurement. In order to understand the mechanism of conduction, ac conductivity is measured as a function of temperature and frequency. Space charge polarization is the dominant polarization mechanism phenomenon at low frequency and high temperature while orientation polarization dominates at low temperature and high frequency. Impedance analysis confirms the formation of internal barrier layers which is responsible for high dielectric constant in these samples.     

A new temperature stable microwave dielectric ceramics: ZnTiNb2O8 sintered at low temperatures

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈-xTiO₂ composite ceramics with different weight percentages of BaCu(B₂O₅) additive prepared by solid-state reaction method have been investigated using the X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results showed that the microwave dielectric properties were strongly dependent on densification, grain sizes and crystalline phases. The sintering temperature of ZnTiNb₂O₈ ceramics was reduced from 1250 °C to 950 °C by doping BaCu(B₂O₅) additive and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative value of −52 ppm/°C to 0 ppm/°C by incorporating TiO₂. Addition of 2 wt% BaCu(B₂O₅) in ZnTiNb₂O₈-xTiO₂ (x = 0.8) ceramics sintered at 950 °C showed excellent dielectric properties of ?_r = 38.89, Q × f = 14,500 GHz (f = 4.715 GHz) and τ_f = 0 ppm/°C, which represented very promising candidates as LTCC dielectrics for LTCC applications.     

Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Coexistence of room temperature ferroelectricity and ferrimagnetism in multiferroic BiFeO3-Bi0.5Na0.5TiO3 solid solution

The structure, ferroelectric and magnetic properties of (1 − x)BiFeO₃-xBi_0.5Na_0.5TiO₃ (x = 0.37) solid solution fabricated by a sol-gel method have been investigated. X-ray diffraction and Raman spectroscopy measurements show a single-phase perovskite structure with no impurities identified. Compared with pure BiFeO₃, the coexistence of ferroelectricity and ferrimagnetism have been observed at room temperature for the solution with remnant polarization P_r = 1.41 μC/cm² and remnant magnetization M_r = 0.054 emu/g. Importantly, a magnetic transition from ferrimagnetic (FM) ordering to paramagnetic (PM) state is observed, with Curie temperature T_C ∼ 330 K, being explained in terms of the suppression of cycloid spin configuration by the structural distortion.     

Direct synthesis and characterization of mesoporous Fe3O4 through pyrolysis of ferric nitrate-ethylene glycol gel

Mesoporous magnetite (Fe₃O₄) was successfully synthesized on a large scale by direct pyrolysis of ferric nitrate-EG (EG = ethylene glycol) gel in a one-end closed horizontal tube furnace in the air without using any template, additions, and carrier gas. The as-synthesized mesoporous Fe₃O₄ were characterized by powder X-ray diffraction (XRD), infrared spectra (IR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and thermal gravimetric analysis (TGA). Results from TEM showed that the as-obtained Fe₃O₄ has mesoporous structure formed by the loose agglomeration of nanoparticles with diameter of about 6 nm, which was also confirmed by small-angle XRD and nitrogen adsorption analysis. Furthermore, vibrating sample magnetometer (VSM) measurements indicated that the saturated magnetization of the as-obtained mesoporous Fe₃O₄ was ferromagnetic with the saturation magnetization (Ms) and coercivity (Hc) of 46 emu/g and 136 Oe, respectively. In addition, a possible growth mechanism of mesoporous Fe₃O₄ was also discussed.     

Multiferroic and magnetoelectric properties of single-phase Bi0.85La0.1Ho0.05FeO3 ceramics

Single-phase Bi_0.85La_0.1Ho_0.05FeO₃ multiferroic ceramics were prepared by a rapid liquid sintering method. The ceramics exhibited an obvious ferroelectric loop with a remnant polarization of 11.2 μC/cm² and also showed weak ferromagnetism with the remnant magnetization of 0.179 emu/g at room temperature. A considerable enhancement of the polarization on magnetic poling and a dielectric anomaly in the vicinity of the antiferromagnetic transition temperature due to the intrinsic magnetoelectric coupling effect were observed in Bi_0.85La_0.1Ho_0.05FeO₃ ceramics. The dielectric constant for the Bi_0.8La_0.1Ho_0.05FeO₃ samples at room temperature decreases with increasing applied magnetic fields, and the coupling coefficient (?′(H) − ?′(0))/?′(0) reaches −1.04% at H = 10 kOe.     

Electrical properties of binder-free thick film BaYxBi1−xO3 NTC thermistors

The microstructure and electrical properties of BaY_xBi_1−xO₃ thick film negative temperature coefficient thermistors, fabricated by screen printing, were investigated. The sintered thick films were the single-phase solid solutions of the BaY_xBi_1−xO₃ compounds with a monoclinic structure. The added Y₂O₃ led to a significant decrease in the grain size of the thermistors. The resistivity and coefficient of temperature sensitivity for the BaY_xBi_1−xO₃ (0 ≤ x ≤ 0.15) thick film NTC thermistors decreased first with increasing x in the range of x < 0.04 and then increased with further increase in x.     

Effect of α-Al2O3 on in situ synthesis low density O′-sialon multiphase ceramics

In this paper, the effect of α-Al₂O₃ on in situ synthesis low density O′-sialon multiphase ceramics was investigated. Thermodynamics analysis was used to illustrate the feasibility of synthesizing O′-sialon at a low temperature of 1420 °C. The crystalline phase and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The actual substitution parameter x value of O′-sialon was estimated via lattice correction. The results showed that, O′-sialon multiphase ceramics with different x values could be synthesized successfully through varying α-Al₂O₃ content. Bulk densities of samples ranging from 1.64 to 2.11 g cm⁻³ were adjusted with the percentage of α-Al₂O₃ increasing from 5.21 wt.% to 15.62 wt.%. Formation of nearly single-phase O′-sialon was obtained in the sample containing 10.42 wt.% α-Al₂O₃. The actual substitution parameter x increased with the increase of α-Al₂O₃, whereas it was lower than the original designation, and the O′-sialon with a low x value was achieved.     

Effects of Al and Al4C3 contents on combustion synthesis of Cr2AlC from Cr2O3-Al-Al4C3 powder compacts

Preparation of the ternary carbide Cr₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the Cr₂O₃-Al-Al₄C₃ powder compact. Effects of the contents of Al and Al₄C₃ on the product composition and combustion behavior were studied by formulating the reactant mixture with a stoichiometric proportion of Cr₂O₃:Al:Al₄C₃ = 3:5x:y, where x and y varied from 1.0 to 1.5. When compared to those of the powder compact with Cr₂O₃:Al:Al₄C₃ = 3:5:1 (i.e., x = y = 1.0), the combustion temperature and reaction front velocity increased with content of Al, but decreased with that of Al₄C₃. Besides Cr₂AlC and Al₂O₃, the final products always contained a secondary phase Cr₇C₃ that was substantially reduced by adopting additional Al and Al₄C₃ in the reactant compacts. For the sample with Cr₂O₃:Al:Al₄C₃ = 3:7.5:1 (x = 1.5), solid state combustion reached a peak temperature of 1245 °C and yielded Cr₂AlC with a trivial amount of Cr₇C₃. Although Cr₇C₃ was lessened by introducing extra Al₄C₃, the increase of Al₄C₃ from y = 1.1 to 1.5 produced almost no further reduction of Cr₇C₃ in the final product. This is partly attributed to the low combustion temperature in the range of 1065-1095 °C for the samples with additional Al₄C₃, and in part, due to the role of Al₄C₃ which might react with Cr to form Cr₇C₃, Cr₂Al, and Cr₂AlC.     

Enhanced thermoelectric properties of n-type Bi2Te3-based nanocomposite fabricated by spark plasma sintering

Highly dense n-type Bi₂Te₃-based thermoelectric materials dispersed with x vol.% γ-Al₂O₃ nanoparticles (x = 0, 0.4, 1.0, 1.5) were fabricated by spark plasma sintering method. The effects of γ-Al₂O₃ addition on microstructure and the thermoelectric properties were studied. It was found that γ-Al₂O₃ nanoparticles locate both at grain boundaries and inside Bi₂Se_0.3Te_2.7 grains. The nanoparticles induce both potential barrier scattering effect and additional phonon scattering effect, which simultaneously enhance the Seebeck coefficient and reduce the lattice thermal conductivity of the nanocomposites in the measured temperature range of 300-500 K, respectively. The maximum dimensionless figure of merit (ZT_max) reaches up to 0.99 for the sample with x = 1.0 at 400 K, which is 35% improvement over the Bi₂Te₃-based matrix. More importantly, the average ZT value of the sample increases from 0.65 to 0.91 in the temperature range 300-500 K, making the nanocomposites much more applicable in cooling and power generation.     

Low temperature sintering and microwave dielectric properties of (Mg0.7Zn0.3)0.95Co0.05TiO3 ceramics with BaCu(B2O5) additions

The effects of BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were investigated. The (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were not able to be sintered below 1000 °C. However, when BaCu(B₂O₅) were added, they were sintered below 1000 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics. Good microwave dielectric properties of Q × f = 35,000 GHz, ?_r = 18.5.0 and τ_f = −51 ppm/°C were obtained for the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics containing 7 wt.% mol% BaCu(B₂O₅) sintered at 950 °C for 4 h.     

DC magnetization investigations in Ti1−xMnxO2 nanocrystalline powder

In the present paper, DC magnetization investigation on the insulating nanocrystalline powder samples of Ti_1−xMn_xO₂ (x = 0, 0.05, 0.10, and 0.15) prepared by simple chemical route is reported. Structural measurements revealed phase pure anatase structure of TiO₂ when x ≤ 0.05 and a mixture of anatase and rutile TiO₂ along with the signature of Mn₃O₄ phase for x > 0.05. Magnetic measurements exhibited the presence of ferromagnetic ordering at room temperature in samples having either small fraction of Mn or no Mn at all. This ferromagnetic signature is accompanied with paramagnetic contribution which is found to dominate with increase in Mn concentration. The Ti_1−xMn_xO₂ sample having highest Mn concentration of x = 0.15 showed nearly paramagnetic behavior. However, at low temperatures, additional ferrimagnetic ordering arising due to Mn₃O₄ (T_C = 42 K) is evidenced in the doped samples. Consistent with the XRD investigations, the isofield DC-magnetization measurements under field cooled and zero field cooled (FC-ZFC) histories corroborated the presence of Mn₃O₄ phase. Also, distinct thermomagnetic irreversibility has been observed above 42 K. These results are suggestive of presence of weak ferromagnetic ordering possibly due to defects related with oxygen vacancies.     

Mg-substituted ZnNb2O6-TiO2 composite ceramics for RF/microwaves ceramic capacitors

Microstructure and microwave dielectric properties of Mg-substituted ZnNb₂O₆-TiO₂ microwave ceramics were investigated. Mg acted as a grain refining reagent and columbite phase stabilization reagent. With an increasing Mg content, the amount of ixiolite (Zn, Mg) TiNb₂O₈ decreased, and the amount of (Zn_0.9Mg_0.1)_0.17Nb_0.33Ti_0.5O₂ and columbite increased. ZnO-Nb₂O₅-1.75TiO₂-5 mol.%MgO exhibited excellent dielectric properties (at 950 °C): ?_r = 35.6, Q × f = 16,000 GHz (at 5.6 GHz) and τ_f = −10 ppm/°C. The material was applied successfully to make RF/microwaves ceramic capacitor, whose self-resonance frequency was 19 GHz at low capacitance of 0.13 pF.     

Microwave dielectric properties and its compatibility with silver electrode of Li2MgTi3O8 ceramics

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Li₂MgTi₃O₈ ceramic have been investigated. The pure Li₂MgTi₃O₈ ceramic shows a relative high sintering temperature (∼1000 °C) and good microwave dielectric properties as Q × f of 40,000 GHz, ?_r of 27.2, τ_f of 2.6 ppm/°C. It was found that the addition of a small amount of BCB can effectively lower the sintering temperature of Li₂MgTi₃O₈ ceramics from 1025 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 0.5 wt% BCB added Li₂MgTi₃O₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 36,200 GHz (f = 7.31 GHz), ?_r = 26 and τ_f = −2 ppm/°C. Compatibility with Ag electrode indicates this material can be applied to low temperature-cofired ceramics (LTCC) devices.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

Preparation and electrical properties of perovskite ceramics in the system BaBi1−xSbxO3 (0 ≤ x ≤ 0.5)

The effect of the composition on the electrical properties of BaBi_1−xSb_xO₃ (0 ≤ x ≤ 0.5) negative temperature coefficient (NTC) thermistors was studied. Major phases present in the sintered bodies of BaBi_1−xSb_xO₃ (0 < x < 0.5) ceramics were BaBi_0.5Sb_0.5O₃ compounds with a rhombohedral structure and BaBiO₃ compounds with a monoclinal structure. Most pores were located in the grains of BaBiO₃ and BaBi_0.5Sb_0.5O₃ ceramics. It was apparent that the ρ₂₅ and B_25/85 constant of the thermistors increased with increasing Sb content.     

Structural, morphological and optical properties of Bi2−xSbxSe3 thin films grown by electrodeposition

Ternary single-phase Bi_2−xSb_xSe₃ alloy thin films were synthesized onto Au(1 1 1) substrates from an aqueous solution containing Bi(NO₃)₃, SbCl₃, and SeO₂ at room temperature for the first time via the electrodeposition technique. The electrodeposition of the thin films was studied using cyclic voltammetry, compositional, structural, optical measurements and surface morphology. It was found that the thin films with different stoichiometry can be obtained by controlling the electrolyte composition. The as-deposited films were crystallized in the preferential orientation along the (0 1 5) plane. The SEM investigations show that the film growth proceeds via nucleation, growth of film layer and formation of spherical particles on the film layer. The particle size and shape of Bi_2−xSb_xSe₃ films could be changed by tuning the electrolyte composition. The optical absorption spectra suggest that the band gap of this alloy varied from 0.24 to 0.38 eV with increasing Sb content from x = 0 to x = 0.2.