Preparation and dielectric properties for temperature-stable Pb(Ni1 /3Nb2/3)O3-based composite ceramics

Abstract

Many lead-based relaxor ferroelectrics have been developed for multilayer ceramics (MLCs) capacitors because of their excellent dielectric properties and low sintering temperature compared with BaTiO3based dielectric materials [1]. Despite these advantages in lead-based relaxor materials, temperature dependence of capacitance is relatively large, which is unfavorable to temperature-stable MLCs. Several attempts have been made to improve the temperature dependence [2±4]. Although mixed sintering of two components with different Curie temperatures is one of the methods [2±5], the two mixed lead-based constituents can very easily react to form a solid solution in the sintering process. Inhibiting the solution of two different constituents is an important problem to address. Pb(Zn1=3Nb2=3)O3-based biphasic dielectric ceramics that satisfy X7R (nC=C , 15%, ±55 to 125 8C) speci®cations of the Electronic Industries Association (EIA) standards have been prepared in our laboratory by the SiO2 ±B2O3 gel glass-coating technique [6]. A SiO2 ±B2O3 gel glass layer is useful for inhibiting the reaction between two PZN-based constituents in PZN-based composite ceramics. In preparing Pb(Ni1=3Nb2=3)O3-based composite ceramics in a Pb(Ni1=3Nb2=3)O3 ±PbTiO3 system, we also used the SiO2-B2O3 gel glass-coating technique, although it failed. To gain temperature-stable PNNbased composite ceramics, the PbTiO3 particlecoating method was used. PNN-based composite ceramics with X7R characteristics were obtained successfully. The preparation of composite ceramics and their dielectric properties are reported in this letter. Two solid solutions with different transition temperatures, y60 8C and 100 8C, were chosen in the Pb(Ni1=3Nb2=3)O3-PbTiO3 system as the lowtemperature constituent (LTC) and high-temperature constituent (HTC), respectively. Reagent grade PbO, NiO, Nb2O5, TiO2 were used. The Columbite method was employed to form NiNb2O6 by calcination of NiO and Nb2O5 at 1000 8C for 6 h. LTC and HTC were synthesized from PbO; NiNb2O6 and TiO2 powders by the conventional solid-state reaction method. The procedure for formation of PbTiO3 coating on PNN powder particles was as follows. First, the precursor for PT was prepared. Lead acetate was added to ice acetic acid, which was used as solvent. The mixture was heated, and the lead acetate was dissolved. Titanium-butoxide was mixed with acetylacetone at the same time, adding an amount of ethanol that was used as diluent to make the reaction complete. Two kinds of solutions were mixed by stirring, and the resulting solution was brown and transparent. Second, a weighed quantity of pulverized powder was mixed with PT precursor to obtain a slurry, and the mixture was blended for 4 h. After being dried, the powder was calcined at 550 8C for 1 h. The coated HTC and LTC powders were mixed following xLTCy(1y x)HTC with different (x ˆ 0:3, 0.4, 0.5) values, while at the same time, a small amount of sintering aid was added. The resultant powders were pressed using PVA binder into pellets 15 mm in diameter and 2 to 3 mm in thickness. The pellets were then ®red at 1000±1050 8C for 1±2 h in a sealed alumina crucible. To compensate for the PbO loss from pellets, a PbO-rich atmosphere was maintained by placing an equimolar mixture of PbO and ZrO2 inside the crucible. For dielectric measurements, silver paste was painted onto both sides of the pellets and ®red at 550 8C for 15 min as electrodes. Dielectric measurements were conducted on an automated system, where a temperature box and an HP4274A LCR meter were controlled by a computer. Dielectric properties were measured at 1, 10 and 100 kHz, in the temperature range of y80 to 160 8C with a heating rate of 3 8C miny1. The dielectric constants versus temperature behavior for LTC and HTC are shown in Fig. 1. The Curie temperature of LTC is y60 8C and that of HTC is 100 8C. Both exhibit relaxor ferroelectric characteristics and have a dielectric constant maximum of 6400 and 8000 at 1 kHz, respectively.