Papers by Keyword: Dielectric Ceramics

Abstract: The 0.85(Mg_0.7Zn_0.3)TiO₃-0.15(Ca_0.61La_0.26)TiO₃(referred to as 85M₇Z₃CLT) ceramics were fabricated by microwave sintering(MWS) and conventional sintering(CS) techniques. The effects of MWS and CS on the sintering property, crystal phase structure and dielectric properties of 85M₇Z₃CLT were discussed. Compared with CS, MWS had a short sintering period and a high volume density. The results indicated that the phase composition of 85M₇Z₃CLT ceramics were not changed after MWS, with (Mg_0.7Zn_0.3)TiO₃ and (Ca_0.61La_0.26)TiO₃ structure as the main crystal phase were also obtained by MWS. However, the grain growth was significantly inhibited due to the rapid sintering speed of MWS, which resulted in a fine, uniform microstructure. Furthermore, microwave-sintered ceramics had excellent dielectric properties due to the miscellaneous phase (Mg_0.7Zn_0.3)Ti₂O₅ being suppressed. As a result, excellent dielectric properties of 27.11 for ε_r, 103,500GHz for Q·f and +2ppm/°C for τ_f were obtained by microwave sintering at 1275°C for 20 min. Therefore, the ceramic quality factor was increased by 22% compared with the conventional sintering.Key words: microwave technique; (Mg,Zn)TiO₃; dielectric ceramics

Abstract: A detailed study on the structural and dielectric characterization of the Nd doped SrTiO₃ ceramics at radio frequencies were conducted in this paper. Sr_1-xNd_xTiO₃ (x from 0 to 0.13) ceramics were synthesized using the conventional solid state ceramic route and the phase purity was confirmed through XRD analysis. XRD patterns of the ceramics showed almost similar peaks with no additional phases. A reduction in the lattice parameter can be observed with increase in the Nd content, which can be attributed to the possible lattice shrinkage due to the substitution by a smaller sized ion. The SEM images also showed a reduction in grain size which can be due the fact that Nd doping induces Sr and Ti vacancies in the system which inhibits the grain growth. The dielectric characterization at 1 MHz was done and optimal values of ε_r=5496 and tanδ=0.0925 were observed at x=0.1. Thus the synthesized ceramics can be used for energy storage applications.

Abstract: Microwave filters require dielectric ceramic resonators attached to ceramic supports via joining. Among the BaTiO₃, CaTiO₃, and Ba₂Ti₉O₂₀ based-dielectric ceramics prepared in our labs., the CaTiO₃ ceramics modified with NdAlO₃ and Al₂O₃ gave satisfactory dielectric properties: dielectric constant DK = 40.69, quality factor Qxf = 18842 GHz (fo = 6.551 GHz), and frequency temperature coefficient t_f = 2.542 ppm °C^-1. For replacing the hard-to-sinter Al₂O₃ based-ceramic supports, a glass-ceramic of the SiO₂-B₂O₃-P₂O₅-ZnO system was developed, leading to a low sintering temperature of 900 °C, and a low dielectric constant (<4). Finally, a Bi₂O₃-ZnO-SiO₂-Al₂O₃-CaO glass was used as a sealing agent to replace the traditional gluing consisting of an adhesive. The present work hinters that with proper sintering aids, one can produce multilayered and integrated resonators with and without embedded passive elements.

Abstract: The Mg2SiO4-MgTiO3-CaTiO3 composite dielectric ceramics with different Mg2SiO4 addition amounts were prepared by solid state reaction method. The effects of Mg2SiO4 addition amounts on the microstructure and dielectric properties as well as sintering temperature of xMg2SiO4-(0.95-x)MgTiO3-0.05CaTiO3 (abbreviated as xMSTC, 0.25≦x≦0.75) composite ceramics were investigated. The results indicated that the sintering temperature of MgTiO3-CaTiO3 based ceramics with Mg2SiO4 addition could be lowered effectively to 1320~1340°C, and the dielectric constant decreased and dielectric loss increased gradually with the increase of Mg2SiO4 content. The 0.45MSTC ceramics containing 45 wt% Mg2SiO4 and sintered at 1340°C showed desirable dielectric properties with dielectric constant εr=13.3，dielectric loss tanδ=4.5×10-4 and temperature coefficient of relative permittivity τε =10 ppm/°C.

Abstract: AST-doped ZnO-MgTiO3-SrTiO₃ ceramics were prepared using a solid-state sintering process. The effects of AST (Al2O3-SiO2-TiO2) on the dielectric properties of MgTiO₃-based ceramics were investigated. The results indicate that AST-doped MgTiO3-based ceramics could be obtained after 1205~1280°C sintering for 3 h. The XRD results indicate that the obtained MgTiO₃-based ceramics contain high percentage of MgTi2O5 phase and the percentage increased with the increase of AST content. It was found that the occurred liquid-phase sintering by adding AST glass could effectively lower the sintering temperature and decrease the dielectric loss of MgTiO₃-based ceramics. A minimum tanδ of 1.5×10-4 associated with εr=19.0 was achieved for 6.0 wt% AST-doped samples sintered at 1255°C.

Abstract: (Bi_2-xLi_x) (Mg_2/3Nb_1.1833Sn_0.15)O₇ piezoelectric ceramics were prepared by conventional solid state reaction method. The microstructure and dielectric properties of dielectric ceramics were studied which was doped by different amount of Li₂CO₃. The experiment results showed that the density of the ceramics would decrease, but have little effect on the microstructure of the ceramics when the amount of the Li₂CO₃ doped increases. With the increasing of Li₂CO₃ doped, the dielectric constant(ε_r) and dielectric loss(tanδ) of dielectric ceramics first decreases and then increases. When tested in 1MHz frequency, the piezoelectric ceramics display the optimum properties tanδ is 0.00009, ε_r is 163 while Li₂CO₃ doped amount is 0.05 mol%.

Abstract: The nano-sized Ba(Zr0.2Ti0.8)O3 powder was prepared by citrate sol-gel method. The crystal structure and the particle size of the powder were characterized by XRD. The morphology of calcined powder and the sintered ceramics was observed by TEM and SEM, respectively. The particle size of the powders calcined at 600oC is about 10 nm. The dense ceramics can be obtained at sintering temperature of 1220 oC, which is much lower than that of the powder prepared by the conventional solid method. The effects of Al2O3 additive on the sintering behavior, microstructure and dielectric properties of Ba(Zr0.2Ti0.8)O3 ceramics were also investigated.

Abstract: Two measuring techniques, the parallel plate method and the cylinder cavity perturbation method, for complex permittivity of microwave dielectric ceramics were introduced. Attention was focused on the theoretical consideration of measurement precision and suitable applications of the techniques.

Abstract: The copper-niobates, M2+Cu¬2Nb2O8 (M2+ = Zn, Co, Ni, Mg or Ca) have good microwave dielectric properties when sintered between 985 – 1010oC, and 1110oC for CaCu¬2Nb2O8. Therefore they would be potential dielectric Low Temperature Co-fired Ceramic (LTCC) materials if they could be made to sinter below 960oC (melting point of silver). To this end, additions of 3wt% V2O5 were made to ZnCu2Nb2O8, CoCu2Nb2O8, NiCu2Nb2O8, MgCu2Nb2O8 and CaCu2Nb2O8, and their sintering and dielectric behaviour was investigated for samples fired between 800 and 950oC. Doping lowered sintering temperatures to below the 960oC limit in all cases. Doping had the general effect of reducing r, density, Qf and f, although doped CaCu2Nb2O8 had a Qf value of 9300 GHz, nearly four times that of the best undoped sample. When doped and undoped samples all fired to 935oC were compared, all doped samples had greater r and density, and all except ZnCu2Nb2O8 had a smaller f.

Abstract: The effect of the electric field on the dielectric response of the Sr1-xCaxTiO3 ceramic system was analysed and complemented by structural and microstructural studies. A monotonic decrease of the lattice parameter and an increase of the average grain size from 11 to 23 μm with increasing x were observed by X-ray diffraction and scanning electron microscopy analysis respectively. Samples with x = 0.003 and 0.01 reveal a monotonous increase of low-temperature dielectric constant, corresponding to an enhancement of the tunability and a reduction of the driving electric field at 10-30 K for x = 0.01. For Sr1-xCaxTiO3 ceramic samples with x = 0.04 and 0.10 the ferroelectric phase transition of around 20 K was observed together with the enhanced tunability of the dielectric constant at 30-85 K for x = 0.10.