Papers by Keyword: Microwave Dielectric

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Authors: Ki Hyun Yoon, Ji Won Choi
Abstract: The microwave dielectric properties of (300-X) nm MgTiO3/(X) nm CaTiO3 thin films have been investigated with correlation between the interface and stress induced by dielectric layers with heattreatment. As the thickness (X) of CaTiO3 film increased, the dielectric constant increased and the temperature coefficient of the dielectric constant changed from the positive to the negative values by the dielectric mixing rule. The dielectric loss of (300-X) nm MgTiO3/(X) nm CaTiO3 thin films increased with an increase of the thickness (X) of CaTiO3 film because of higher thermal stress induced by the higher thermal expansion coefficient of CaTiO3 than that of MgTiO3.
Authors: N. Qin, X.M. Chen
Abstract: Ba6-3xSm8+2xTi18O54 microwave dielectric ceramics were modified by Nd/Bi co-substitution for Sm on A-site. According to XRD and SEM analysis, a single-phase solid solution with new tungsten bronze-type structure was formed in low-Bi-substituted region. Bi was effective on increasing dielectric constant, while led to a decreased Qf value and an increased negative tf. The co-substituting approach exhibited the advantage in improving e and compensating Qf and tf value.
Authors: F. Azough, Colin Leach, Robert Freer
Abstract: Ba(Me1/3Nb2/3)O3 (Me=Zn, Co, Ni and Mg) ceramics were prepared using the conventional mixed oxide route; additives included Al2O3, Ga2O3, SiO2, WO3, B2O3 and V2O5. Powders were mixed, milled for 18h, calcined at 1100°C, remilled pressed into pellets at 100 MPa, sintered in air at temperatures in the range 1350-1550°C and then cooled at 360C h–1 to 5°C h–1. Products were characterised in terms of phase analysis (X-ray diffraction), microstructure (SEM and TEM) and electrical properties (relative permittivity, εr, dielectric Q value and temperature coefficient of resonant frequency,τf). The Q values of the Ba(Me1/3Nb2/3)O3 ceramics depend on the degree of cation ordering and the additives. Slow cooling leads to 1:2 ordering of the B sites and enhanced dielectric Q values. For samples cooled at 5°C h–1 after sintering the Qxf values are in the range 28000 to 98000 GHz, and are in the sequence Ba(Ni1/3Nb2/3)O3, Ba(Co1/3Nb2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3. Additions of BaO-4WO3 or V2O5 yield higher Qxf values than Al2O3. Highly ordered Ba(Zn1/3Nb2/3)O3 has a relative permittivity of 39.4, but most other Ba(Me1/3Nb2/3)O3 ceramics exhibit εr of 31-32. The temperature coefficient of resonant frequency, τf, varies from –18 ppm/°C (Ba(Ni1/3Nb2/3)O3) to +32 ppm/°C (Ba(Mg1/3Nb2/3)O3); the sintering additives (Al2O3 and BaO-4WO3) change τf by typically 10-16 ppm/°C.
Authors: Hitoshi Ohsato, Eiichi Koga, Isao Kagomiya, Kenichi Kakimoto
Abstract: Origins of high Q are considered on intrinsic as high symmetry, ordering structure and high density crystal structure. It was concluded that the high symmetry brings high Q instead of ordering comparing some cases as follows: As if ordering ratio of Ba(Zn1/3Ta2/3)O3 (BZT) is high of about 80%, Q values are distributed from low to high Q. Disordered BZT ceramics with high density obtained for short sintering time by spark plasma sintering (SPS) showed high Q. Ba(Zn1/3Nb2/3)O3 (BZN) with order-disorder transition showed high Q at disorder form sintered over the transition temperature. And, the disordered BZN with high Q annealed at lower temperature changed to order structure without improvement of Q.
Authors: Zhong Qing Tian, You Li Yang, Lin Lin
Abstract: LaAlO3 has gained attention in the last few years because of its favorable microwave dielectric properties, excellent lattice matching and a good matching for thermal expansion. Nanoparticles of LaAlO3 were produced by using gelatin as an organic precursor. The optimum temperature for the phase formation of the precursor sample is found out by TG/DTA analysis. The specific surface area of the powder was measured by the BET technique with nitrogen. X-ray diffraction (XRD) was used to determine the phases present in the calcined powders and to estimate particle size. The morphology of LaAlO3 powder was studied by transmission electron microscopy (TEM). Microwave dielectric properties were measured using a HP8722ET network analyzer. The results demonstrate that pure perovskite LaAlO3 powder formed at 800 °C for 2 h. And the crystallite size was in the range of 17-48 nm. The specimen sintered at 1450 °C for 8 h shows 97.4% of the theoretical density and gave the excellent microwave dielectric properties: εr=23.3 and Qf=32300 GHz.
Authors: N. Chaiyo, R. Muanghlua, A. Ruangphanit, Wanwilai C. Vittayakorn, Naratip Vittayakorn
Abstract: A corundum-type structure of cobalt niobate (Co4Nb2O9) has been synthesized by a solid-state reaction. The formation of the Co4Nb2O9 phase in the calcined powders was investigated as a function of calcination conditions by differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. Morphology and particle size have been determined by scanning electron microscopy (SEM). It was found that the minor phases of unreacted Co3O4 tend to form together with the columbite CoNb2O6 phase at a low calcination temperature and short dwell time. It seems that the single-phase of Co4Nb2O9 in a corundum phase can be obtained successfully at the calcination conditions of 900°C for 60 min, with heating/cooling rates of 20°C /min.
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