CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International ] (]]]]) ]]]–]]] Influence of TiO 2 additive on the microwave dielectric properties of α-CaSiO 3 –Al 2 O 3 ceramics Wei Hu, Hanxing Liu n , Hua Hao, Zhonghua Yao, Minghe Cao, Zhijian Wang, Zhe Song State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, PR China Received 26 October 2014; accepted 4 March 2015 Abstract The effects of TiO 2 additives on microstructure and microwave dielectric properties of α-CaSiO 3 –2 wt%Al 2 O 3 ceramics have been investigated in detail. Specifically, the TiO 2 addition results in the presence of two distinct phases, e.g. CaTiO 3 and CaTiSiO 5 , which can inhibit the growth of α-CaSiO 3 grains by surrounding their boundaries and also improve the bulk density of α-CaSiO 3 –2 wt%Al 2 O 3 ceramics. However, the TiO 2 addition undermines the microwave dielectric properties of the title ceramics because the derived phases of CaTiO 3 and CaTiSiO 5 have poor quality factor. The τ f value is strongly correlated to the compositions and can be controlled through the compositional ratio. The α-CaSiO 3 –2 wt% Al 2 O 3 ceramics containing 2.5 wt% of TiO 2 sintered at 1250 1C showed excellent microwave dielectric properties (ε r ¼ 7.88, Q  f ¼ 24,412 GHz and τ f ¼ 0.52 ppm/1C). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Microstructure; Dielectric properties; α-CaSiO 3 ceramic; TiO 2 addition 1. Introduction Recently, microwave dielectric materials have attracted much attention for their applications in communication systems such as substrates, resonators, filters, global positioning systems and intelligent transport systems, which has led to an increase of investigations on microwave dielectric materials. [1–4] These dielectric materials for microwave communication require a low dielectric constant (ε r o10) which increases the signal propaga- tion velocity as well as reduces the cross-coupling effect, a high quality factor (Q  f) which increases selectivity of the fre- quency, and a near-zero temperature coefficient of resonant frequency (τ f r 710 ppm/1C) which ensures the stability of the frequency against the temperature. [5–10] CaSiO 3 ceramic has been proven to be an excellent dielectric material with a low dielectric constant and a low dielectric loss, indicating that CaSiO 3 ceramic might be regarded as sui- table candidate for microwave dielectric resonators. As it is known, CaSiO 3 has mainly two common phases; one is the low-temperature phase wollastonite (β-CaSiO 3 ), and the other is the high-temperature phase pseudo-wollastonite (α-CaSiO 3 ). [11,12] β-CaSiO 3 ceramics have been investigated by the traditional solid-state process and possesses microwave dielec- tric properties of ε r =6.59 and Q  f=13,109 GHz. Meanwhile for α-CaSiO 3 ceramics obtained by a sol–gel method, it exhi- bits good microwave dielectric properties of ε r =6.69 and Q  f=25,398 GHz. [13] However, the sintering temperature range of pure CaSiO 3 ceramic is very narrow. Chakradhar et al. pointed out that it was difficult to obtain dense CaSiO 3 ceramic since its grains grew exceptionally and the bulk CaSiO 3 ceramic became more porous with the increase of the calcination temperature. [14] In order to improve the sintering characteristic and microwave dielectric properties, considerable researches about β-CaSiO 3 ceramics have been investigated, Wang et al. found that Li 2 CO 3 addition can lower the sintering temperature of β-CaSiO 3 –Al 2 O 3 ceramics and also improve the bulk density of ceramics. When the addition was 2 wt%, the β-CaSiO 3 –1 wt % Al 2 O 3 ceramics exhibited excellent dielectric properties of ε r ¼ 7.73, Q  f ¼ 21,904 GHz and τ f ¼ 34 ppm/1C. Then, 10 wt% of CaTiO 3 was added to adjust the τ f value to  1.22 ppm/1C. [15] So far, methods to improve the microwave www.elsevier.com/locate/ceramint http://dx.doi.org/10.1016/j.ceramint.2015.03.138 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author. Tel.: þ86 27 87653330; fax: þ 86 27 87885813. E-mail address: lhxhp@whut.edu.cn (H. Liu). Please cite this article as: W. Hu, et al., Influence of TiO 2 additive on the microwave dielectric properties of α-CaSiO 3 –Al 2 O 3 ceramics, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.138