Key Engineering Materials Vols. 512-515

Abstract: Doping effect of Sb on Pb-free barium strontium titanate was investigated. The Sb doped (Ba,Sr)TiO3 ceramics were prepared by solid reactions in 1473-1623 K by employing 0~1.2 wt% Sb2O5 as main additives. Density changes with sintering temperature were determined. The morphology and microstructure of the Sb doping samples were analyzed by SEM. The dielectric properties such as permittivity and dielectric loss were measured. It was found that addition of Sb benefited to low temperature high density sintering process. When the addition amount of Sb2O5 was about 0.6 wt%, the dielectric loss of (Ba,Sr)TiO3 was decreased from 1.9% to about 0.4% while the dielectric constant was kept over 2000. Possible mechanism of the improvement by Sb addition was discussed too.

Abstract: Both BaTi₄O₉ and Ba₂Ti₉O₂₀ have the best microwave properties among many compounds in BaO-TiO₂ system. The microwave performance of the ceramics in BaO-TiO₂ system, which were prepared by conventional solid state method, was investigated in this paper. The study indicates that the (Ti,Zr)/Ba ratio has a great effect on the dielectric properties of the ceramic due to different BaTi₄O₉/Ba₂Ti₉O₂₀ ratio. The pure BaTi₄O₉ ceramic exhibits the most outstanding microwave properties, with ε_r=35.55 and Q×f=43300GHz. On the contrary, the dielectric property of the pure Ba₂Ti₉O₂₀ ceramic was not good enough, which should be doped with some additive additions. WO₃ was incorporated into the system and the sintering temperature was adjusted to improve the microwave dielectric property. The Q×f was increased to 38000GHz by adding 1.35mol% WO₃ due to the formation of a little BaWO₄ phase. When sintered at a higher temperature (1380°C), the BaO-TiO₂ system ceramic has ε_r =37.04 and Q×f =44874GHz.

Abstract: BaTiO₃-(Bi_0.5Na_0.5)TiO₃ (BTBNT) solid solution ceramics with the Curie temperature higher than 150°C were prepared, which were promising for X9R MLCCs application. (Bi0.5Na0.5)TiO3 (BNT) was first synthesized by the conventional solid state reaction and then it was mixed with BaTiO₃ (BT) with increasing BNT content from 0 to 12 mol%. BaTiO₃-(Bi_0.5Na_0.5TiO₃ solid solutions were obtained after calcining at 1100°C. The structural and dielectric properties of BTBNT and Nb-doped BTBNT ceramics were investigated.

Abstract: In this paper, titanium ammonium fluoride ((NH₄)TiF₆), strontium nitrate (Sr(NO₃)₂) and boric acid (H₃BO₃) were used as raw materials, the precursor solution was prepared with molar ratio of AHFT/SN/BA=1/1/3. SrTiO₃ dielectric thin films were deposited with the self-assembled monolayers (SAMs) by the liquid-phase deposition on FTO substrate. X-ray diffraction (XRD), scanning election microscopy (SEM) and Agilent E4980A precision LCR Meter were used to characterize the SrTiO₃ films. The precursor solution concentration and the pH values of precursor solution had the effects on the dielectric properties of the as-prepared thin films. When the precursor concentration was 0.0125mol/L, the crystallization of as-prepared SrTiO₃ thin films was high and the grain sizes on the film surface were even and dense. When the frequency was 15~100KHz, the optimal dielectric constant was up to 1060, the minimal dielectric loss was 4.053. As pH=3.30, the frequency of the as-prepared SrTiO₃ thin films was 15~100KHz. The optimal dielectric constant was up to 1060, too. The minimal dielectric loss was 1.914. The optimal dielectric constants were 346.3 and 424.1 when the pH was 3.1 and below 3.5 respectively. The minimal dielectric losses were respectively 18.10 and 54.82.

Abstract: This paper describes physical properties of (Ca0.8Sr0.2)TiO3 were deposited by sol-gel method with a fix per-heating temperature of 400oC for 60 min at various annealing temperatures from 600oC to 700oC for 60 min. Particular attention will be paid to the effects of an annealing treatment in air ambient on the physical properties. The annealed films were characterized using X-ray diffraction (XRD). The surface morphologies of annealed film were examined by atomic force microscopy and scanning electron microscopy.

Abstract: Using (NH₄)₂TiF₆, Bi(NO₃)₃•5H₂O and H₃BO₃ as raw materials, Bi₄Ti₃O₁₂ thin films were synthesized with liquid phase self-assembled monolayers on the glass substrate. The different precursor solution concentrations and acid contents had the effects on the physical phase and morphology of Bi₄Ti₃O₁₂ thin films. The Bi₄Ti₃O₁₂ thin films were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The results indicate that the precursor solution with 10.0 mmol/L concentration was prepared by 3:4:9 molar ratio of (NH₄)₂TiF₆: Bi(NO₃)₃•5H₂O: H₃BO₃ and 6.0 ml acid content, depositing at 50°C for 20h, and heat treating at 590°C for 2h. The as-prepared thin films are well-crystal. The surface is even and dense. The formation mechanism of the Bi₄Ti₃O₁₂ thin films is as follows: [TiF_6-n(OH)_n]^2- complex ions are formed in the (NH₄)₂TiF₆ aqueous solution. Then H₃BO₃ continually consumed F^- ions of the solution, which made the amounts of [TiF_6-n(OH)_n]^2- ions increase gradually. Under the induction of the electrostatic force, Bi³⁺ ions and [TiF_6-n(OH)_n]^2- ions would grow naturally on the substrate surface in the form of Bi₂[TiF_6-n(OH)_n]₃ or Bi₂[TiF_6-nO_m/2(OH)_n-m]₃. The samples lost the bound water after the heat treatment and finally the Bi₄Ti₃O₁₂ thin films with pure phase were obtained.

Abstract: The (Ba, Sr)TiO₃ (hereafter BST) ceramics are promising candidate for applying in tunable devices. MgO coated BST-Mg₂TiO₄ (BSTM-MT) composite ceramics were prepared to obtain the low dielectric constant, low dielectric loss, good dielectric constant temperature stability, and high tunability of BST ceramics. The Ba_0.55Sr_0.40Ca_0.05TiO₃ nanoparticles were coated with MgO using the precipitation method and then mixed with Mg₂TiO₄ powders to fabricate BSTM-MT composite ceramics. The morphologies, phases, elements, and dielectric properties of the sintered ceramics were investigated. The core-shell structure of BST powder wrapped with MgO was clearly observed from the TEM image. After sintered at 1100 °C for 2 h, the composite ceramics expressed dense microstructures from SEM images and two main phases BST and Mg2TiO4 were detected in the XRD patterns. The dielectric constant and loss tangent were both reduced after the coating. The reduced dielectric constant and loss tangent of BSTM-MT were 190, 0.0011 (2MHz), respectively. The ceramics exhibited the diffuse phase transition near the Curie temperature and the Curie temperature shifted from 10 °C to 5 °C after the coating. Since the continuous Ti-O bonds were disconnected with the MgO coating, the tunability was reduced to 15.14 % under a DC bias field of 1.1 kV/mm. The optimistic dielectric properties made it useful for the application of tunable capacitors and phase shifters.

Abstract: The influence of ZnO and Nb₂O₅ additions on the sinterability, microstructure and microwave dielectric properties of (Mg_0.95Ca_0.05)TiO₃ (abbreviated as 95MCT hereafter) ceramic is investigated. XRD patterns indicate that MgTi₂O₅ secondary phase can be effectively suppressed by ZnO and Nb₂O₅ additions, which is beneficial for improving the microwave dielectric properties. Appropriate amount of Nb₂O₅ addition can effectively improve the Qf value of 95MCT ceramic, which is suggested to be ascribed to the reduced oxygen vacancies. When the ZnO: Nb₂O₅ mole ratio is 1.5 and the co-doping content is 0.25wt%, the optimal microwave dielectric properties can be obtained Qf=72730GHz(6.8GHz), ε_r=20.29 and τ_f=-6.84ppm/°C and the sintering temperature of 95MCT is lowered from 1400°C to 1320°C.

Abstract: The microstructure and dielectric properties of the complex perovskite Nd(Co_l/2Ti_1/2)0₃ ceramics with 60P₂O₅-15ZnO-5La₂O₃-5Al₂O₃-5N₂O-5MgO-5Yb₂O₃ glass additions from 1-2 wt% were prepared with the conventional solid-state route have been investigated. The resultant microwave dielectric properties were analyzed based upon the densification, the X-ray diffraction patterns and the microstructures of the ceramics. Doping with 60P₂O₅-15ZnO-5La₂O₃-5Al₂O₃- 5N₂O-5MgO-5Yb₂O₃ glass (up to 2 wt%) can effectively promote the densification of Nd(Co_1/2Ti_1/2)O₃ ceramics at low sintering temperature. A distinct sintering temperature reduction (100-200°C) can be achieved by adding 60P₂O₅-15ZnO-5La₂O₃-5Al₂O₃-5N₂O-5MgO-5Yb₂O₃ glass to the Nd(Co_1/2Ti_1/2)O₃ ceramics. The correlation between the microstructures and the microwave dielectric properties was also examined. The Nd(Co_1/2Ti_1/2)O₃ ceramics with 60P2O₅-15ZnO-5La2O₃- 5Al2O₃-5N₂O-5MgO-5Yb2O₃ glass additions can find applications in microwave devices requiring low sintering temperature.

Abstract: Impedance spectroscopy is known as an important technique used for describing the electrical processes occurring in a system on applying an ac signal as input perturbation. In the present paper results of a study of BiNbO₄ ceramics fabricated by mixed oxide method and sintered by free sintering are reported. Results on the ac response of the electroceramic samples by impedance spectroscopy at temperature T= 100 – 400 °C are given. The usual representation (i.e. Z” vs. Z’ where Z’ and Z” are the real and imaginary parts of the complex impedance, respectively) as well as the alternative representations of the impedance measurement (electrical modulus representation) was used to interpret the impedance spectra of BiNbO4 ceramics in order to obtain separate contributions of the bulk, grain boundary and electrode processes. The Kramers-Kronig data validation test was employed in the impedance data analysis. Experimental data of impedance spectroscopy were fitted to the corresponding equivalent circuit using the complex non-linear least squares method. Agreement between experimental and simulated data was established.