Low temperature sintering of ZnTiO3/TiO2 based dielectric with controlled temperature coefficient

https://doi.org/10.1016/j.jeurceramsoc.2006.09.015Get rights and content

Abstract

Structure, microstructure and dielectric properties of ZnTiO3 and rutile TiO2 mixtures (ZnTiO3 + xTiO2 with x = 0, 0.02, 0.05, 0.1, 0.15 and 0.2) sintered using ZnO–B2O3 glass phase (5 wt.% added) as sintering aid have been investigated. For all compounds, the sintering temperature achieves 900 °C. The X-ray diffraction patterns indicate for x = 0.1 that the material is composed by three phases identified as ZnTiO3 hexagonal, TiO2 rutile and ZnO. The presence of ZnO is explained by the introduction of Ti into Zn site to form the (Zn1−xTix)TiO3+x solid solution in resulting the departure of ZnO from the ZnTiO3 structure. The ZnTiO3 + 0.15TiO2 composition sintered at 900 °C with glass addition exhibits attractive dielectrics properties (ɛr = 23, tan(δ) < 10−3 and a temperature coefficient of the dielectric constant near zero (τɛ = 0 ppm/°C)) at 1 MHz. It is also shown that the introduction of TiO2 allows to tune the temperature coefficient of the permittivity. All these properties lead this system compatible to manufacture silver based electrodes multilayer dielectrics devices.

Introduction

The rapid development of the wireless communication implies to design new ceramics sinterable at low temperature, e.g. at around 900 °C and exhibiting good dielectric properties. This low sintering temperature is of primary importance to produce silver co-sintering devices such as silver based multilayer ceramic capacitors or hybrid circuits.1, 2 The required specifications in term of dielectrics properties are a high dielectric constant (ɛr > 20), a high quality factor (Q > 10,000) which corresponds to a low dielectric loss (tan(δ) = 1/Q) and a temperature coefficient of the permittivity close to zero ppm/°C. Two temperature coefficients are commonly used; the first one is the temperature coefficient of the resonant frequency (τf) and the second one is the temperature coefficient of the permittivity (τɛ). The two both coefficients are linked by the well-known relation τf =(−1/2)τɛ + α in which α is the thermal expansion coefficient.3 The control of one coefficient means that the second one is also tuned. All these requirements must be fulfilled at high frequency range (from MHz to GHz) that will allow to produce more performer and more miniaturized electronic devices needed in the telecommunication system. Golovchanski et al.4 reported that ZnTiO3 ceramic is a promising microwave dielectric material because it only requires a sintering temperature of about 1100 °C in the absence of sintering additives and exhibits attractive dielectrics properties (ɛr = 19, QXf = 30,000 GHz, τɛ  +120 ppm/°C). In a recent paper,5 the glass phase addition on ZnTiO3 phase has been carefully examined in terms of sinterability and dielectric properties. It was precisely shown that the formulation ZnTiO3 + 5 wt.% of (ZnO–B2O3) could be sintered at 900 °C. The resulting sintered samples exhibit attractive dielectric properties at high frequency range, e.g. a relative permittivity around 22 and low dielectric losses (tan(δ) < 10−3). However, the τɛ value has been measured to be higher than +100 ppm/°C on this sample. The difficulty in controlling the temperature coefficient stands in the presence of secondary phases, as Zn2TiO4 and TiO2. It is indeed well established that Zn2TiO4 and TiO2 phases are stable at high temperature leading very difficult the obtaining of ZnTiO3 as single phase.6 One strategy developed for controlling the temperature coefficient is to add a compound with a very high value of the temperature coefficient with an opposite sign. Haga et al.7 have studied the mixture (1  x) ZnTiO3 (τɛ  + 100)8 + xTiO2 (τɛ  −500 ppm/°C)9 to tailor the temperature coefficient according to the well-known mixing rule. This rule links the resulting temperature coefficient (τɛ) of a composite versus the temperature coefficients of the compounds belonging to the material and characterised by their volume fractions (respectively τɛ1, ν1 and τɛ2, ν2): τɛ = ν1τɛ1 + ν2τɛ2. Haga et al. have shown that the temperature coefficient tuning of ZnTiO3 + xTiO2 mixture is not trivial because of ZnTiO3 decomposition at high temperature (>945 °C) into Zn2TiO4 + TiO2. This leads very difficult the control of ZnTiO3/TiO2 ratio at the end of the sintering stage. In this context, the purpose of our work is to investigate the dependence of τɛ versus x in the system ZnTiO3 + xTiO2 + 5 wt.% (ZnO–B2O3) glass phases. The low temperature sintering (900 °C), obtained via the glass phase addition, should permit to avoid the ZnTiO3 decomposition, resulting in a better control of the ZnTiO3/TiO2 ratio. Hence, a control of the temperature coefficient could be expected. Practically, various compounds consisting in a ZnTiO3 + xTiO2 + 5 wt.% (ZnO–B2O3) mixture with x = 0, 0.02, 0.05, 0.1, 0.15, and 0.2 have been prepared. Specimens were sintered at 900 °C and characterised in terms of structure, microstructure, density and dielectric properties.

Section snippets

Experimental procedure

The ZnTiO3 compound was prepared by solid state reaction using reagent grades powders of ZnO and TiO2 (purity >99%). The precursors were appropriately weighted according equimolar ratio. The mixing was performed in ammoniac solution at pH 11 using zircon balls in a Teflon jar for 3 h. These conditions were found to be optimal to obtain a very well-dispersed slurry.10, 11 The slurry was subsequently dried and the powder was manually reground and heat treated at 800 °C for 2 h in air. The powder was

Results and discussion

After the calcination step at 800 °C, the powder is mainly composed by ZnTiO3 and Zn2Ti3O8 phases. Takai et al.12 have indeed reported that pure ZnTiO3 phase is very difficult to synthesise. After the grinding process, ZnTiO3 based powder presents a very fine microstructure and a very narrow distribution of grains size centred at around 400 nm (Fig. 1). The shrinkage versus temperature curves of the ZnTiO3 based powder, with and without glass phase addition, are given in Fig. 2. The temperature

Conclusion

The structure, microstructure and dielectric properties of the ZnTiO3 (hexagonal) + TiO2 (rutile) system have been investigated. Low temperature sintering has been allowed by the addition of ZnO–B2O3 glass phase. The ZnTiO3 based material was sintered at 900 °C adding 5 wt.% of glass phase. Otherwise, the introduction of TiO2 does not modify the TMA curves. Usual densities of 95–96% of the theoretical are routinely obtained in these sintering conditions. The composition of the ceramics analysed by

References (14)

  • S. Liufu et al.

    Powder Technol.

    (2004)
  • S. Takai et al.

    Solid State Ionics

    (2000)
  • A. Molson et al.

    Electro Ceramics

    (1990)
  • K. Wakino et al.

    Dielectric resonator materials and their applications for mobile communication systems

    Br. Ceram. Trans. J.

    (1990)
  • D.H. Idles et al.

    J. Mater. Sci.

    (1992)
  • A. Golovchanski et al.

    J. Korean Phys. Soc.

    (1998)
  • Chaouchi, A., d’Astorg, S., Marinel, S. and Aliouat, M., Mat. Phys. Chem., submitted for...
There are more references available in the full text version of this article.

Cited by (0)

View full text