Microwave Assisted Synthesis of Dy2ti2o7 Ultrafine Powders by Sol

Ceramics International 42 (2016) 11177–11183

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Ceramics International

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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- 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 min1. 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 cm1. 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 4.0 mm 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.

3.3. Fourier transform infrared spectra analysis of treated samples

The FTIR spectra were utilized to monitor the structural changes during the synthesis process, shown in Fig. 3. The FTIR Fig. 3. FTIR spectra of the precursor and samples heated at different temperatures by microwave-heating (a. precuror, b. 800 °C, c. 900 °C, d. 1000 °C). spectrum of the precursor, as Fig. 3(a) presented, clearly exhibited characteristic stretching vibrations of O–Hat 3550 cm1,O–C¼O groups at 1480 (symmetrical stretching process. vibration) and 1637 cm 1 (antisymmetric stretching vibra- 1 tion), –NO2 at 1307 cm and C–O in ethyl alcohol at 1 3.2. X-ray diffraction analysis of resultant powders 1037 cm , respectively. Compared with Fig. 3(b, c and d), it was obvious that the intensity of above characteristic absorp- Fig. 2 showed the X-ray diffraction patterns of resultant tion bands decreased a lot and even disappeared when the powders heated at different temperatures for 30 min. As calcination temperature increased from 800 to 1000 °C. Be- clearly seen from Fig. 2(a), the precursor remained almost sides, a new band ranging at 500–1000 cm 1 was revealed unreacted, perhaps because the calcination temperature was probably owing to the formation of the stretching vibration not high enough to promote the diffusion to form fully between high metal and oxygen, which indicated that

Fig. 4. SEM images of samples heated at different temperatures by microwave-heating. (a. 700 °C, b. 800 °C, c. 900 °C, d. 1000 °C, all were magniﬁed 20.0 k times, c was magniﬁed 50.0 k times in the insert). 11180 J. Yu et al. / Ceramics International 42 (2016) 11177–11183

Fig. 5. TEM images of samples heated at different temperatures by microwave-heating (a. 800 °C, b. 900 °C, inserts were corresponding SAED patterns).

coordination interaction of Dy–O and Ti–O–Ti existed. And the evolution of samples heated at different temperatures for intensity of the new absorption band increased with the 30 min. As Fig. 4(a) showed, the crystal grain of Dy2Ti2O7 was temperature rising up, suggesting that a good crystal of not formed because of the unclear grain boundaries. When the

Dy2Ti2O7 powders could be obtained at a higher temperature. temperature increased to 800 °C, spherical and granular grains ranging from 100 to 300 nm were demonstrated in a uniform 3.4. Microstructural analysis of resultant powders distribution, with edges and shapes relatively evident, shown in Fig. 4(b). However, great changes had taken place in grain Fig. 4 presented the morphology and microstructural morphology when held at 900 °C for 30 min. The grain

Fig. 6. SEM images of fracture surfaces of Dy2Ti2O7 ceramics sintered at different temperatures (a. 1300 °C, b. 1400 °C, c. 1500 °C, d. 1600 °C). J. Yu et al. / Ceramics International 42 (2016) 11177–11183 11181 boundary migration also occurred and slight agglomerates were intuitively observed, shown in Fig. 4(c). Combined with the SEM image under different magnification, hexagonal and irregular grains were aligned in a stacked structure, probably due to the nucleation growth mechanism of two-dimensional smooth interface. According to the law of Bravais [24], crystal planes evolved and even disappeared because of differences in growth rate of each crystal face, which led to various morphology when exposed to microwave radiation. Detailed ana- lyses for corresponding TEM images were shown in Fig. 5. Granular and hexagonal grains could be identified clearly in Figs. 5(a) and (b) respectively. Corresponding SAED patterns were attributed to different planes in Dy2Ti2O7, which mani- fested the formation of a polycrystalline structure. Obviously, the quadrilateral Dy2Ti2O7 powders synthesized via traditional heat treatments [25] differed largely from those prepared by microwave-heating. From Fig. 4(d), the trace of abnormal growth of grains and the melting of grain boundaries indicated the occurrence of oversinering. Based on these results, it could Fig. 7. The relationship between the flexural strength of Dy2Ti2O7 ceramics and be concluded that the optimal temperature to synthesize sintering temperature. Dy2Ti2O7 powders was above 800 °C.

3.5. Microstructural analysis of sintered ceramics

SEM images of the crystal morphology and features of fracture surfaces were displayed in Fig. 6.FromFig. 6(a), the uniform and coarse section of Dy2Ti2O7 ceramics with small pores roughly estimated to be about 1.0 μm presented a typical brittle fracture, which indicated the incomplete sintering. When the sintering temperature increased to 1400 and 1500 °C, grains grew up gradually accompanied with the de- crease of pores, shown in Figs. 6(b) and (c). However, a few closed pores still existed and a river-like fracture surface was revealed, which satisfied the demands of extending the crack propagation path for saving energy. As Fig. 6(d) exhibited, the Fig. 8. The model of the neutron absorption simulation. porosity of the ceramics decreased while the size of crystal grains increased further when heated at 1600 °C for 2 h. Atomic Energy [26]. A passive critical mode was used in the 3.6. Mechanical properties of sintered ceramics simulation, using Kcode as the counting card. And the sample was exposed to the radiation for 30 min with the radiant flux 11 2 The test results on the flexural strength of sintered ceramics of 5 10 n/cm s and the thermal neutron fluence rate of 9 2 were summarized clearly in Fig. 7. The flexural strength of the 1 10 n/cm s. samples heated from 1350 to 1600 °C ranged among 49.0 to The neutron absorption rate can be calculated by Eq. (1). fl 99.0 MPa. As the temperature rose up, the exural strength ΦΦ01− increased stably, whereas there were no significant changes A = × 100% Φ0 ()1 when the temperature exceed 1550 °C. Furthermore, the highest flexural strength of 99.0 MPa was adequate for the where A is the neutron absorption rate, Φ0 is the detector necessity of absorbers in the practical application. count (or flux) when absorbers are used and Φ1 is the detector count (or flux) when absorbers are not used. 3.7. Simulation of the neutron absorption The relationship between the neutron absorption rate and

thickness of Dy2Ti2O7 ceramics was simulated. As Fig. 9 The simulation model was shown in Fig. 8. Isotropic point showed, the neutron absorption rate sharply increased from d1, which demonstrated a partial abnormal continuous energy 93.24% to 96.77% and then slowly rose up to 97.65% when the distribution, was chosen as the neutron source. F5 point de- thickness varied from 2.0 to 8.0 mm. In theory, the neutron tector was selected to detect the neutron ﬂux. The sample was absorption rate of Dy2Ti2O7 ceramic could reach 97.39% when a cylinder with a radius of 25.0 mm (d¼50.0 mm). The dis- the thickness was 5.0 mm. tance between neutron detector and the sample was set as The thickness of the sample was set as 20.0 mm

1675.0 mm (a¼1675.0 mm). And the sample was placed in (t¼20.0 mm) and the Dy2Ti2O7 content was considered as the 50.0 mm away from the radiation source (b¼50.0 mm). As- variable quantity in the simulation model. The inﬂuences of suming the point O was the center of the ball, the entire space different Dy2Ti2O7 content on the neutron absorption rate of could be considered as a spherical model with a radius of rubber based absorbers were shown in Fig. 10. It was obvious 10000.0 mm. The radiation conditions took example by the that the neutron absorption rate monotonically increased from data of In-Hospital Neutron Irradiator in China Institute of 60.00% to 85.41% with increasing Dy2Ti2O7 content up to 11182 J. Yu et al. / Ceramics International 42 (2016) 11177–11183

Dy2Ti2O7 samples when sintered at 1600 °C for 2 h in air atmosphere. Meanwhile, the neutron absorption rate of

Dy2Ti2O7 ceramics was above 93.0% when the thickness was over 2.0 mm. And the absorption rate of rubber based absor-

bers was more than 80.00% when the Dy2Ti2O7 content was 5.0 wt%.

Acknowledgments

We are highly grateful to the National Natural Science Foundation of China (No. 51472146) for providing ﬁnancial support for this work. In addition, we acknowledge teachers in inorganic nonmetal institute of Shandong University for their help with this study.

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