Dielectric properties of La-substituted Pb 0.5 Ca 0.5 [(Mg 1/3 Nb 2/3 ) 0.5 Ti 0.5 ]O 3 ceramics
- PDF / 272,245 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 113 Downloads / 159 Views
Modification of Pb0.5Ca0.5[(Mg1/3Nb2/3)0.5Ti0.5]O3 dielectric ceramics was performed by up to 20 mol% La substitution for Pb and Ca. The temperature coefficient of dielectric constant was significantly reduced by the present approach, while an increased Qf factor was achieved. Good microwave dielectric properties were obtained in a composition Pb0.425Ca0.425La0.1[(Mg1/3Nb2/3)0.5Ti0.5]O3: ⑀ ⳱ 125; Qf ⳱ 3150 GHz; calculated temperature coefficient of resonant frequency f ⳱ +253 ppm/°C.
MgO+ Nb2O5 → MgNb2O6
I. INTRODUCTION
The lead-based complex perovskite Pb(Mg1/3Nb2/3)O3 (designated as PMN) is a relaxor ferroelectric with a high dielectric constant, a diffuse phase transition, and relatively small temperature coefficient.1–3 Generally, PMN ceramics are used as capacitors, transducers, and memory devices.4–6 With rapid development of wireless communication, the modified lead-based perovskite ceramics have received increasing scientific interest as candidates for high-⑀ microwave dielectric ceramics, where Ca substitution for Pb was investigated.7–9 Chen et al.10,11 have proposed alternative approaches for such modification, where the high-Tc low-loss ferroelectric phase CaTiO3 and paraelectric phase SrTiO3 were incorporated into the Pb(Mg1/3Nb2/3)O3 matrix to reduce the dielectric loss and temperature coefficient of dielectric constant. Dielectrics with low loss and reduced temperature coefficient of the dielectric constant were obtained in the previous work,10,11 and good microwave dielectric properties were achieved in the composition 0.5Pb(Mg 1 / 3 Nb 2 / 3 )O 3 ⭈ 0.5CaTiO 3 : ⑀ ⳱ 197.8; Qf ⳱ 1550 GHz; f ⳱ +483 ppm/°C.10 For the application of such materials as microwave dielectric resonators, the primary issue is to suppress the temperature coefficient into an acceptable level and to improve the Qf factor further. In the present work, 0.5Pb(Mg1/3Nb2/3)O3 ⭈ 0.5CaTiO3 dielectric ceramics were modified by La substitution for A-site Pb and Ca ions, and effects of such modification upon the dielectric properties were determined together with microstructure characterization. II. EXPERIMENTAL PROCEDURE
(Pb0.5Ca0.5)1−3xLa2x[(Mg1/3Nb2/3)0.5Ti0.5]O3 ceramics (x ⳱ 0.01, 0.05, 0.1) were prepared by the solid-state reaction process according to the following reactions: J. Mater. Res., Vol. 16, No. 7, Jul 2001
http://journals.cambridge.org
Downloaded: 26 Mar 2015
,
⁄ 共1 − 3x兲PbO + 3⁄2共1 − 3x兲CaCO3 + 3xLa2O3 + 3⁄2TiO2 + 1⁄2MgNb2O6 → 3共Pb0.5Ca0.5兲1−3x La2x关共Mg1 Ⲑ 3Nb2 Ⲑ 3兲0.5 Ti0.5兴O3 + 1⁄2共3 − 9x兲CO2
(1)
32
. (2)
The raw powders all had high purity (more than 99%). First, a columbite precursor method was used to prepare MgNb2O6.10,12,13 MgO and NbO5 were batched in the proper ratio and calcined at 1100 °C for 4 h. This precursor was then mixed in a stoichiometric ratio with PbO, CaCO3, TiO2, and La2O3 powders by ball milling with ZrO2 media in ethanol for 24 h and calcined at 850 °C in air for 4 h. The synthesized powders were remilled and pressed into the compacts of 12 mm in diameter at a pressure of 98 MPa after
Data Loading...
