수열합성법을 이용한 BaO-PbO-Nd2O3-TiO2계 분말제조

조윤석, 서경원

아주대학교 화학·생물공학부

Preparation of BaO-PbO-Nd2O3-TiO2 Powder

by Hydrothermal Synthesis

Yun Seok Cho, Kyung Won Seo

School of Chemical Engineering & Biotechnology, Ajou Univ.

Introduction

Microwave dielectric materials in using high frequency must have a high dielectric constant (□r), a good quality constant (Q factor) and a low temperature coefficient of resonance frequency (□f) [1]. In order to satisfy these properties, many researchers have investigated mainly BaO-TiO2, BaO-Re2O3-TiO2 (Re: Nd, Sm, La), (Mg, Ca)TiO3, (Zr, Sn)TiO4, (Ca, Sr, Ba)ZrO3 and the complex perovskite compounds system of A(B1/3B2/3)O3 (A=Ba, Sr, B=Mg, Zn, Mn, Ni, Ca, Co, B=Nb, Ta) structures [1, 2]. Among these dielectric materials, BaNd2Ti5O14 has a high dielectric constant and high Q value, but its temperature coefficient is large. Wakino et al. investigated BaO-PbO-Nd2O3-TiO2 system that was modified from BaNd2Ti5O14 by substituting Pb for Ba in order to improve its dielectric constant and Q value. In particular, temperature dependency was close to zero in this substitution composition [3].

Hydrothermal synthesis, as applied to ceramic powder production, is an aqueous chemical process for direct preparation of crystalline, anhydrous ceramic powders [4]. As compared with a conventional method, hydrothermal synthetic method could be prepared homogeneous and mono-dispersive particles by simple processing than conventional method. This fact expected to improve electrical properties without many processing steps such as a milling and calcination step.

In this study, the preparation of microwave dielectric materials BaO-PbO-Nd2O3-TiO2 system (referred to as BPNT in this paper) gave high dielectric constants and good temperature stability that was studied under hydrothermal condition. Also, we compared characteristics of the products with conventional method.

Experiment

First, we prepared microwave dielectric BPNT powder by the conventional method. Commercial-grade BaTiO3, TiO2, Nd2O3 and PbO powders were used as the starting materials. The formulated powders was composed according to (Ba0.5Pb0.5)O-Nd2O3-5TiO2. The formulated powder was ball-milled for 24hrs in distilled water with zirconia media, dried. In order to form compounds, it was calcined at 1050℃ for 1.5hrs with 5℃/min rate in air. After adding an organic binder (PVA 5wt% solution) 5wt%, the calcined powders were re-milled for 2hrs. Then, the mixtures were dried, pulverized and sieved. The product powders were cold-pressed into disk-shaped pellets (about 15mm in diameter and 7mm thick) under a pressure of 2.0 ton/cm2. In order to remove organic component, samples were heated at 550℃ for 1hr prior to sintering at 1280℃ for 2.5hrs with 5℃/min rate in air.

In the next process, we prepared by hydrothermal method in order to synthesize BPNT powder. Barium hydroxide [Ba(OH)2·8H2O], lead nitrate [PbNO3], neodymium (III) chloride hexahydrate [NdCl3·6H2O], titanium butoxide [Ti(OC4H9)4] were used as law materials. The chemicals used in this work were of extra pure reagent grade. The feedstock solution was prepared to accommodate the molar ratio of Ba and Pb as 0.1M, Nd as 0.4M and Ti as 1.0M in order to obtain the same powder composition as prepared by the conventional method. The prepared feedstock solution was put into an autoclave with the addition of a mineralizer, KOH. Reaction temperature was ranged from 140℃ to 200℃, and reaction time was ranged from 6 to 48hrs. Mineralizer was added from 1.0M to 4.0M. After reaction was finished, the product was filtered and then washed with deionized water, acetic acid and ethanol, and again with deionized water successively to remove impurities. Then, the synthetic product was dried in a freezing dryer in order to be not agglomerated. In preparing disk-shaped pellet, the same experimental method mentioned above was introduced. Samples were heated at 550℃ for 1hr in order to remove organic impurities and sintered in air at 1280℃ for 2.5hrs without calcination step.

The crystallinity of the product powder was interpreted by an X-ray diffractometer (XRD: MXP3, Mac Science Co.) using a monochromatic CuK radiation, 40kV, 30mA. The morphology of the product particles was observed by using a TEM (CMS, Philips, US) and the surface morphology of the disk-shape pellet samples was observed by using a SEM (JEOL, JSM-840A). Dielectric constant (□r) and quality constant (Q factor) of the prepared pellets were measured at 3∼5GHz by Hakki and Coleman's method [5]. The temperature coefficient of resonant frequency (□f) measured in the temperature range from 20℃ to 80℃ by a resonant cavity method.

Result and Discussion

Fig. 1 shows XRD patterns of BPNT powder prepared in 24 hours reaction at 3.0 M mineralizer with different reaction temperatures from 140℃ to 200℃. The crystallite peaks of BPNT powder were not fully developed at 140℃, but when the reaction temperature was over 180℃, the peaks showed the better crystallinity. Therefore, we fixed the reaction temperature at 180℃ throughout the experiments.

Fig. 2 shows TEM micrographs of the synthetic BPNT powders prepared at 180℃ for different reaction times with 3.0M KOH. We can confirm from these micrographs in fig. 1 that the grains were crystallized as the reaction times increased. That is to say, as shown in fig. 2(A), the particles reacted for 12 hours are almost amorphous. However, when reaction time increased, we could see needle-like particles that shown in fig. 2(B). This result was identified by selected area diffraction (SAD) patterns as shown in this figure. The SAD patterns of prepared powders for a 24 hour reaction were more distinct picture than those for a 12 hour reaction.

Fig. 3 shows XRD patterns of (Ba1-xPbx)Nd2Ti5O14 with increasing Pb addition that prepared under optimum hydrothermal conditions and were sintered at 1280℃ for 2 hours. The crystalline phase shown in the figure is the (Ba,Pb) Nd2Ti5O14 with the value of x is in the range of 0.0 to 0.5. The figure shows that (Ba,Pb)ONd2O35TiO2 was formed by substituting Pb for Ba. However, the secondary phase was detected weakly when the value of x was above x=0.6. When the mole fraction of Pb was 1.0 (x=1.0), the crystalline phase was transformed to new crystalline PNT, as shown in this figure.

References

[1] Wu, Y. J. and Chen, X. M., □"Modified Ba6-3xNd8+2xTi18O54 Microwave Dielectric Ceramics", J. Eur. Ceram. Soc., 19, 1123 (1999).

[2] Ploude, J. K., LInn, D. F., O'Brayn, H. M. and Thomson. J., "Ba2Ti9O20 as a microwave dielectirc resonator", J. Am. Ceram. Soc., 58, 418 (1975)

[3] Wakino, K., Minai, K. and Tamura, H., J. □"Microwave Characteristics of (Zr, Sn)TiO4 and BaO-PbO-Nd2O3-TiO2 Dielectric Resonators",□Am. Ceram. Soc., 67(4), 278 (1984).

[4] Dawson. W. J., □"Hydorthermal Synthesis of Advaned Ceramic Powders", Ceramic Bulletin, 67(10) 1673 (1988).

[5] Hakki, B.W. and Coleman,P. D., □"A Dielectric Resonator Method of Measuring Inductive Capacities in the Millinmeter Range", IRE Trans. Microwave Theory Tech., 8, 402 (1960).

Fig. 1. XRD patterns of synthetic BPNT powder prepared at different reaction temperatures for 24 hrs reaction with 3.0M KOH.

Fig. 2. TEM micrographs of synthetic BPNT powder prepared for   different reaction times at 180℃ with 3.0 M KOH.

Fig. 3. XRD patterns of BPNT product with various compositions that prepared under optimum hydrothermal conditions and were sintered at 1280 C for 2 hrs.