Low temperature synthesis and characterization of BiNbO 4 powders R. Radha a , H. Muthurajan b , N. Koteswara Rao c , Sivaram Pradhan c , U.N. Gupta c , R.K. Jha c , S.A. Mirji a , V. Ravi a, ⁎ a Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008 India b Armament Research & Development Establishment, Pune-411 021, India c Catalysis Division, National Chemical Laboratory, Pune 411008, India ARTICLE DATA ABSTRACT Article history: Received 9 May 2007 Received in revised form 3 August 2007 Accepted 28 August 2007 BiNbO 4 powders, a potential microwave dielectric ceramic were prepared by both co- precipitation and solid state techniques. In the former method, an aqueous ammonium hydroxide was used to precipitate Bi 3+ and Ta 5+ cations as hydroxides simultaneously under basic conditions. It is observed that the co-precipitation produces BiNbO 4 phase on heating at 600 °C, whereas complete phase formation occurs only at 800 °C in the case of solid state method. The phase contents and lattice parameters were studied by powder X-ray diffraction. The sequence of the reaction and phase evolution of BiNbO 4 were also studied by the X-ray diffraction. The samples were characterized by the differential–thermal/ thermo-gravimetric analyses and scanning electron microscopy techniques. © 2007 Elsevier Inc. All rights reserved. Keywords: Ceramics Oxides Chemical synthesis Electron microscopy X-ray diffraction BiNbO 4 1. Introduction The applications of microwave dielectric ceramics in the mobile communication system, such as resonators, band-pass filters and duplexers, have been tremendously increasing in the last decade, due to their high reliability, high integration potential, good dielectric properties. The requirements of microwave dielectric ceramic resonators used at microwave frequencies are a high dielectric constant, a high Q value (reciprocal of dielectric loss) and a low temperature coefficient of resonant frequency [1–5]. Most of the known commercial microwave dielectrics such as Sn x Zr 1-x TiO 4 and Ba(Mg,Ta)O 3 can be sintered only at very high temperatures greater than 1400 °C. The so called passive integration for miniaturization is performed using multiplayer ceramic technology where green ceramic tapes of different materials corresponding to different passive functions are laminated and co-fired at low temperatures. Therefore it is necessary to develop microwave dielectric materials that can be sintered at temperatures lower than the melting point of internal metal conductor such as Cu or Ag. Recently, BiNbO 4 was reported to be promising microwave dielectric material with relatively lower melting point [1–5]. The reported values for BiNbO 4 are the tempera- ture coefficient of resonant frequency ∼ 50 ppm/°C, the dielectric constant ∼ 45, and Q = 14000 [1–5]. Generally, the traditional solid state method was used for the preparation of oxides and its drawbacks are well docu- mented in the literature. The wet–chemical techniques such as combustion, alkoxide and citrate sol–gel and co-precipita- tion were found to produce these oxides with several advantages namely, nano-sized particles (hence high surface area), high reactivity and very good homogeneity in compo- sition. Occasionally metastable phases also formed by these methods. Recently the citrate gel method was reported to MATERIALS CHARACTERIZATION 59 (2008) 1083 – 1087 ⁎ Corresponding author. Tel.: +91 020 25902539; fax: +91 020 25902636. E-mail address: r.venkat@ncl.res.in (V. Ravi). 1044-5803/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.08.026