Study of crystallization behavior and kinetics of ZrO 2 -3Y 2 O 3 -xSrO (1 r x r 3) precursor powders using an isothermal process Hsueh-Liang Chu a , Weng-Sing Hwang a,b , Je-Kang Du c,d , Ker-Kong Chen c,d,n , Moo-Chin Wang e,nn a Department of Materials Science and Engineering, National Cheng Kung University,1 Ta-Hsueh Road, Tainan 70101, Taiwan b Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University,1 Ta-Hsueh Road, Tainan 70101, Taiwan c Department of Dentistry, Kaohsiung Medical University,100 Shih-Chuan 1st Road, Kaohsiung 80708, Taiwan d Department of Dentistry, Kaohsiung Medical University, Chung Ho Memorial Hospital,100 Tzyou 1st Road, Kaohsiung 80708, Taiwan e Department of Fragrance and Cosmetic Science, Kaohsiung Medical University,100 Shih-Chuan 1st Road, Kaohsiung 80708, Taiwan article info Article history: Received 31 May 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online 24 June 2016 Keywords: ZrO 2 -SrO mixed oxide Crystallization kinetics Crystallinity Isothermal method abstract Zirconium nitrate (Zr(NO 3 ) 4  xH 2 O), yttrium nitrate (Y(NO 3 ) 3  6H 2 O) and strontium nitrate (Sr(NO 3 ) 2 ) were used as the starting materials for synthesized ZrO 2 -3Y 2 O 3 -xSrO (1 rx r3) precursor powders. The isothermal crystallization behavior and kinetics were investigated using an isothermal process. The characterization used X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano-beam electron diffraction (NBED) and high-resolution TEM (HRTEM). The XRD results show that all patterns only contain the single phase of tetragonal ZrO 2 (t-ZrO 2 ) for ZrO 2 - 3Y 2 O 3 -xSrO precursor powders after calcination between 773 and 923 K for various durations. When the ZrO 2 -3Y 2 O 3 -1SrO precursor powders were calcined at 923 K, the crystallinity of t-ZrO 2 increased from 29.98% to 81.64% with rose calcined time from 15 min to 120min. When calcination time was held at 60 min, the crystallinity of t-ZrO 2 increased from 52.49% to 73.41% of ZrO 2 -3Y 2 O 3 -1SrO precursor pow- ders with increasing calcination temperature from 773 K to 923 K. Moreover, when the precursor powders were maintained at the same calcination temperature and duration, the crystallinity of t-ZrO 2 decreased with increasing addition of SrO. The activation energy and kinetics formula for t-ZrO 2 crys- tallization of ZrO 2 -3Y 2 O 3 -xSrO precursor powders by an isothermal method were obtained. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction Pure ZrO 2 ceramics used as advanced structural materials are limited by the spontaneous stress-induced martensitic transfor- mation from high temperature to room temperature. With the addition of 3–5 mol% Y 2 O 3 to ZrO 2 , fine-grain tetragonal ZrO 2 (t- ZrO 2 ) polycrystals (Y-TZP) and partially stabilized ZrO 2 (Y-PSZ) have excellent strength and fracture toughness due to the stress- induced martensitic transformation from the tetragonal- to monoclinic-phase [1,2]. Additionally, the stress-induced phase transformation with the associated volume increase, leading to compressive stress formation on the surface. This phenomenon leads to an increase in flexural strength, but also affects the phase integrity of the material and increases its susceptibility to aging [3]. To overcome these problems, the addition of small quantities of different metal oxides such as BaO, CaO, MgO, Gd 2 O 3 , La 2 O 3 , and SrO to ZrO 2 has been reported by many studies [4–8]. Cheng et al. [9] also investigated the effect of Mg 2 þ , Ca 2 þ , Sr 2 þ ions and pH value on the formation of zirconia, they found Sr 2 þ and Ca 2 þ as mineralizer at pH  10. Moreover, Drennan and Hannink have reported SrO addition has improvement in the mechanical prop- erties of Mg-PSZ [10], and SrO is also found to be effective in densification [11] and mitigates the harmful effects on the grain boundary [12]. Due to its high strength and toughness compared to other ceramics, tetragonal yttria-stabilized zirconia (YSZ) is regarded as an important engineering ceramic. The microstructure, in parti- cular the crystallite size and the crystallinity, affects the properties significantly [13–16]. An understanding of the crystallization characteristics is thus essential for microstructural engineering. However, the technological importance and the crystallization kinetics of ZrO 2 have only been reported by few studies [17–21]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ceramint Ceramics International http://dx.doi.org/10.1016/j.ceramint.2016.06.156 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. n Corresponding author at: Department of Dentistry, Kaohsiung Medical Uni- versity, 100 Shih-Chuan 1st Road, Kaohsiung 80708, Taiwan. nn Corresponding author. E-mail addresses: enamel@kmu.edu.tw (K.-K. Chen), mcwang@kmu.edu.tw (M.-C. Wang). Ceramics International 42 (2016) 15041–15048