Microwave Assisted Synthesis of Dy2ti2o7 Ultrafine Powders by Sol

Ceramics International 42 (2016) 11177–11183 Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Microwave assisted synthesis of Dy2Ti2O7 ultrafine powders by sol–gel method Jincheng Yu a,b, Xiaojun Liu c,LiMaa,b, Haijiao Zheng a, Yubai Zhang a,b, Dezhi Gao a,b, Hongyu Gong a,b, Yujun Zhang a,b,n a Key Laboratory for Liquid-Solid Structural Evolut ion & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China b Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250061, PR China c Shandong Industrial Ceramics Research & Design institute Co., Ltd., Zibo 255000, PR China article info abstract Article history: Dy2Ti2O7 ultrafine powders ranging from 100 to 300 nm were successfully synthesized by sol–gel Received 6 February 2016 method. Particularly, the dried gel precursor was treated at different temperatures (700–1000 °C) via Received in revised form microwave-heating, which contributed to decreasing the grain size and reaction time. The phase com- 2 April 2016 position and structural evolution of the final products were examined by X-ray diffraction (XRD), Accepted 6 April 2016 scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the Available online 7 April 2016 resultant powders were selected to fabricate ceramics and rubber based absobers. Their sinterability, Keywords: mechanical properties and neutron absorption ability were also studied. Results showed that the highest Dysprosium titanate flexural strength of 99.0 MPa were obtained for Dy2Ti2O7 samples when sintered at 1600 °C for 2 h in air Ultrafine powders atmosphere. Meanwhile, the neutron absorption rate of Dy Ti O ceramics and rubber based absorbers Sol–gel 2 2 7 could reach 97.39% and 80.00% respectively when the thickness of samples was set as 5.0 mm. Microwave-heating & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 1. Introduction complex processes, is used to prepare equally distributed powders. And the microwave synthesis, which heats samples Dysprosium titanate with a typical pyrochlore structure [1– on molecular level by the interaction of microwaves with 4] has been widely used as solid electrolytes, catalysts, di- materials themselves [20], offers uniform heating and saves electric materials, magneto-optical materials and radiation calcination time. – absorbers [5 8], because of the excellent properties of anion In this study, Dy2Ti2O7 ultrafine powders were successfully and mixed conduction, superconductivity, giant magneto-re- prepared by sol–gel and microwave-heating methods. The – sistance, ferrimagnetism and neutron absorption [1,9 11]. X-ray pure Dy2Ti2O7 powders were selected to prepare cera- Therefore, it has received a great deal of attention from en- mics and rubber based absorbers subsequently. Meanwhile, ormous researchers. several related tests were performed to investigate the com- Various methods have been used to synthesize rare-earth position and microstructure of the treated samples. pyrochlore such as solid state reaction [12,13], single crystal growth method [14–16], coprecipitation [17], sol–gel method [18,19] and so on. However, the conventional method still has 2. Materials and methods some limitations. For example, solid state reaction results in huge losses of energy and large grain size [12,13], while single 2.1. Synthesis of Dy2Ti2O7 powders crystal growth method relies on the equipment of sophisti- cated instruments [14]. The sol–gel method, which realizes the Tetrabutyl titanate (Ti(OBu)4, 98.0 wt% purity), dysprou- Á synthesis of ultrafine powders at low temperature without sium nitrate (Dy(NO3)3 5H2O, 99.0 wt% purity), glacial acetic acid (CH3COOH, 99.0 wt% purity), hydrochloric acid (HCl, 38.0 wt% purity) and ethyl alcohol (C H OH, 99.5 wt% purity) n 2 5 Corresponding author at: Key Laboratory for Liquid-Solid Structural Evolut ion were used as the raw materials for the sol–gel procedure. & Processing of Materials of Ministry of Education, Shandong University, Jinan 250061, PR China. Firstly, a certain amount of tetrabutyl titanate was dissolved E-mail address: [email protected] (Y. Zhang). in the ethyl alcohol, namely solution A, where the volume ratio http://dx.doi.org/10.1016/j.ceramint.2016.04.026 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 11178 J. Yu et al. / Ceramics International 42 (2016) 11177–11183 of Ti(OBu)4/C2H5OH was fixed at 2/1. At the same time, ethyl alcohol and distilled water were mixed uniformly at the volume ratio of 2/1. Then, stoichiometric dysprousium nitrate with the mole ratio of nDy/nTi ¼1, and 6.0 vol% glacial acetic acid were added into the mixture to form solution B. Subse- quently, solution B was carefully poured into solution A and stirred slowly to realize the hydrolysis, where a spot of hydrochloric acid was added until the pH value was 3. After aging at the room temperature for 10 h, a transparent and uniform gel was obtained. The gel was transferred to a pallet and dried at 60 °C in a vacuum drying oven and then calcined at 700– 1000 °C for 30 min in a microwave furnace. Finally, the products was ground in an alumina crucible to obtain Dy2Ti2O7 powders. Fig. 1. TG-DTA curves for the precursor of Dy2Ti2O7 (a. TG curve, b. DTA curve). 2.2. Fabrication of Dy2Ti2O7 ceramics Dy2Ti2O7 powders with better crystallinity and finer particle size were selected for the subsequent experiment. At first, 3. Results and discussion the resultant powders were crushed and prilled through a 60 3.1. Thermogravimetric analysis of the precursor mesh sieve. Then they were uniaxially dry pressed into circular sheet samples with a diameter of 50.0 mm and a thickness of The precursor of Dy2Ti2O7 was analyzed for burnout be- 5.0 mm under 30.0 MPa. Further, the samples were iso- haviors by TG-DTA. Fig. 1 exhibited the TG-DTA curves of the statically pressed under 160.0 MPa for 3 min. Eventually, green dried gel. Three weight loss stages were observed in the TG bodies were placed into a tube furnace and heated at 1300– curve obviously. The first stage ranged from 50 to about 210 °C, 1600 °C for 2 h in air atmosphere. accompanied by an endothermic peak in the DTA curve due to the removal of water from phosphor precursors with around 2.3. Fabrication of rubber based neutron absorbers 8% weight loss. The second stage from 210 to 400 °C with about 20% mass loss might be attributed to the oxidative de- At first, low density polyethylene (LDPE) was mixed with composition of organic compounds, and obvious exothermic butyl rubber (IIR), where the mass ratio of LDPE/IIR was fixed peaks at 247.36 °C and 348.98 °C were found at the corre- at 2/3. After that, different amounts of dysprosium titanate sponding differential thermal curve. The last stage occurring (1.0–10.0 wt%), 0.5 wt% carbon black (C), 0.1 wt% deca- between 400 to 850 °C with about 5% weight loss was prob- bromodiphenyl ether (DBDPO) and 0.3 wt% antimonous oxide ably correlated with phase transition of Dy2Ti2O7, which could (Sb2O3) were added into the matrix, and then refined by the be confirmed by the exothermic peak at 808.31 °C. When the mixing mill. Eventually, the mixture was fashioned into neu- temperature was above 850 °C, no further mass loss occurred. tron absorbers with a diameter of 50.0 mm and a thickness of The TG-DTA results indicated that the crystallization of 20.0 mm. Dy2Ti2O7 taken place above 850 °C was a slow reaction 2.4. Characterization techniques Thermal behaviors of the dried gel were performed by thermogravimetric and differential thermal analysis (Shi- madzu TG/DTA-40) under synthetic air in the range of 20– 100 °C with a heating rate of 10 °C minÀ1. Phase composition was characterized by an X-ray diffractometer (EVO-18, CARL ZEISS SMT Ltd) with Cu Kɑ radiation (λ¼1.5406 Å) over a 2θ range of 10 to 70°. Infrared spectra analysis was carried out by Fourier transform infrared spectroscopy (FTIR, Bruker Ver- tex70) in a wave number from 400 to 4000 cmÀ1. Micro- structure of the powders and ceramics was examined by scanning electron microscope (SEM, RIGAKU ULTIMA Ⅳ model) and transmission electron microscope (TEM, JEM- 1200EX). The flexural strength of the sintered samples with dimensions of 3.0 mm Â 4.0 mm Â 30.0 mm were measured by three point bend testing (CMT 5105, Shenzhen SANS Mea- surement Technology Co., Ltd., China) with a span of 20.0 mm. And the neutron absorption rate of synthesized materials was Fig. 2. X-ray diffraction patterns of samples heated at different temperatures by also simulated by the Monte Carlo software (MCNP) [21–23]. microwave-heating (a. 700 °C, b. 800 °C, c. 900 °C, d. 1000 °C). J. Yu et al. / Ceramics International 42 (2016) 11177–11183 11179 crystallized grains. When the temperature increased to 800 °C, well-defined peaks of Dy2Ti2O7 (JCPDS card No.17-0453) pro- vided the evidence of crystallization, shown in Fig. 2(b). However, the weak intensity of Dy2Ti2O7 peaks partly indicated the incomplete grain-growth. As Fig. 2(c) showed, X-ray pure Dy2Ti2O7 could be successfully obtained at 900 °C, and the broadened peaks and increased intensity suggested a finer particle size and better crystallinity. As the temperature rose up to 1000 °C, the intensity of Dy2Ti2O7 peaks increased further, whereas the half peak width became narrow, which implied the further growth of grains. As a consequence, it could be confirmed that the optimal calcination temperature for the synthesis of Dy2Ti2O7 powders was 900 °C.

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