Journal of the European Ceramic Society 27 (2007) 2835–2839 Low sintering BaNd 2 Ti 4 O 12 microwave ceramics prepared by CuO thin layer coated powder M.-C. Wu a , M.-K. Hsieh b , C.-W. Yen a , Y.-C. Huang a , W.-T. Huang c , W.-F. Su a,d,∗ a Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan b Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan c Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan d Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan Abstract Recently, BaO–Nd 2 O 3 –TiO 2 systems are widely studied for microwave applications because of their high dielectric constant and high quality factor. However, pure BaNd 2 Ti 4 O 12 ceramics without additives have to be sintered above 1300 ◦ C to achieve densification. Copper oxide has been known as a good sintering aid for electronic ceramics and less reactive toward silver. We have introduced the CuO into BaNd 2 Ti 4 O 12 by modifying the surface of BaNd 2 Ti 4 O 12 by CuO thin layer on the calcined powder instead of mixing CuO directly with BaNd 2 Ti 4 O 12 powder. The process reduces the amount of sintering aid and minimized the negative impact of sintering aid on dielectric properties such as quality factor. The CuO precursor solution of Cu(CH 3 COO) 2 , Cu(NO 3 ) 2 and CuSO 4 , were used to prepare CuO thin layer. They were investigated individually to determine their effects on the densification, crystalline structure, microstructure and microwave dielectric properties of BaNd 2 Ti 4 O 12 . The CuSO 4 coated BaNd 2 Ti 4 O 12 sintered at 1150 ◦ C has exhibited better dielectric properties than those of CuO doped BaNd 2 Ti 4 O 12 (k, 62.5 versus 61.2; Q × f, 11,500 GHz versus 10,500 GHz). The thin layer dopant coating process has been found to be a very effective way to lower ceramic sintering temperature without scarifying its dielectric properties. © 2006 Elsevier Ltd. All rights reserved. Keywords: Powders-solid state reaction; Dielectric properties; Microstructure 1. Introduction Low temperature co-firable ceramics (LTCC) possessing good microwave dielectric properties have been widely investi- gated due to the necessity for miniaturization of devices in order to reduce the size of wireless communication system. Because of the high sintering temperature, Ag–Pd electrode is the only choice for multilayer ceramic components (MLCCs). Thus, it is desirable to replace the poor conductivity and high cost Ag–Pd electrode with the better properties and lower cost of silver or copper electrodes. BaO–Nd 2 O 3 –TiO 2 series materials possess marvelous microwave dielectric properties such as high dielectric constant and high quality factor, and were extensively investigated for the applications in microwave devices. 1–8 Recently, the chemical ∗ Corresponding author at: Department of Materials Science and Engineer- ing, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, 106-17 Taipei, Taiwan, ROC. Tel.: +866 2 33664078; fax: +866 2 33664078. E-mail address: suwf@ntu.edu.tw (W.-F. Su). composition of BaNd 2 Ti 4 O 12 has emerged as a good microwave material because it exhibits high dielectric constant (k ∼ 84) and high quality factor (Q × f ∼ 7800 GHz), but its high sintering temperature (>1350 ◦ C) is difficult for LTCC process. 8 Oxide dopants (B 2 O 3 , Bi 2 O 3 ,V 2 O 5 , etc.) and glasses (low temperature melting glasses) have been used as sintering aids. However, some studies have reported that oxide dopants and the glasses exhib- ited pronounced effects on the microstructure and microwave dielectric properties of the materials. Large and interconnected pores were observed due to the agglomeration of dopants during cofiring process. 9–13 Copper oxide has been known as a good sintering aid and less reactive toward silver. 14–16 We propose to introduce the CuO into BaNd 2 Ti 4 O 12 by modifying the surface of BaNd 2 Ti 4 O 12 by CuO thin layer on the calcined powder instead of mixing CuO directly with BaNd 2 Ti 4 O 12 powder. The process will reduce the amount of sintering aid and eliminate the negative impact of sintering aid on dielectric properties such as quality factor. The CuO layer was prepared from three kinds of Cu precur- sors: Cu(CH 3 COO) 2 , Cu(NO 3 ) 2 and CuSO 4 , respectively. We 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.11.057