Influence of sintering temperature on phase formation and optical properties of lead-Free ferroelectric bi0.5na0.5tio3 materials - Le Thi Hai Thanh

Journal of Science and Technology 54 (1A) (2016) 104-111 INFLUENCE OF SINTERING TEMPERATURE ON PHASE FORMATION AND OPTICAL PROPERTIES OF LEAD-FREE FERROELECTRIC BI0.5NA0.5TIO3 MATERIALS Le Thi Hai Thanh 1, 3 , Dang Ba Tung 1 , Nguyen Hoang Viet 2 , Luong Huu Bac 1 , Phung Quoc Bao 3 , Nguyen Hoang Tuan 1 , and Dang Duc Dung 1, * 1 School of Engineering Physics, Ha Noi University of Science and Technology, 1 Dai Co Viet road, Ha Noi, Viet Nam 2 School of Materials Science and Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet road, Ha Noi, Viet Nam 3 Department of Quantum Optics, Faculty of Physics, VNU University of Science, 334 Nguyen Trai road, Ha Noi, Viet Nam * Email: dung.dangduc@hust.edu.vn Received: 24 October 2015; Accepted for publication: 30 November 2015 ABSTRACT Pb(Zr,Ti)O3 based ferroelectric materials have been widely used in electronic devices even though they were banned due to the toxicity of lead on health and environment. Among the lead- free ferromagnetic materials, Bi0.5Na0.5TiO3 has received much attention because their ferroelectric and piezoelectric characteristics are comparable to Pb(Zr,Ti)O3. In this report, Bi0.5Na0.5TiO3 was fabricated by sol-gel method. The influence of fabricating conditions such as gelization, sintering temperature on crystallinity was studied. The result showed that Bi0.5Na0.5TiO3 was in single phase when compenstation amount of Na in gelization is 40 mol%; sintering temperature higher than 800 o C and sintering time is 2 h. The bandgap of Bi0.5Na0.5TiO3 which estimated from absorption spectroscopy is in the range of 3.01 - 3.18 eV. Keywords: Bi0.5Na0.5TiO3, sol-gel, optical properties, lead-free, ferroelectric. 1. INTRODUCTION Pb(Zr,Ti)O3 based ferroelectric materials (PZT) have been widely used in daily life, science and technology [1]. Although PZT has good ferroelectric and piezoelectric characteristics, PZT- based materials have a serious problem due to the high amount of Pb (60 %) which can affect ecosystem and human health. According to the World Health Organization (W.H.O) report, lead has a harmful effect on children, especially on the mental development. This report also shows that about 600000 new cases of child patients having fluence on the mental development, 143000 deaths are related to lead poisoning every year [2]. In Vietnam, according to the news of the Ministry of Natural Resources and Environment, the environmental pollution related to Influence of sintering temperature on phase formantion and 105 heavy-metals containing Pb in the traditional villages such as a lead recycling village of Dong Mai, Chi Dao, Hung Yen (Vietnam) is quite alarming. The blood tests of 335 children in this village show that 207 children have been lead poisoning [3]. Therefore, removing poisonous elements in electronics devices is really necessary. To encourage the development of the new ferroelectric material which is friendly environmental and do not affect human health, many laws of ban and resctriction of using electronics devices containing poisonous element such as Pb were enacted by EU, Japan, Korea and China etc [4]. However, PZT-based ferroelectric materials have excellent ferroelectric and piezoelectric properties which make it not be ready to replace by other lead-free materials. Jo’s survey about the market of ferroelectric and piezoelectric ceramics is presented in Fig. 1 in which the market share for PZT-based ceramics is very high, around 94.5%.The survey also shows that the lead-free piezoelectric ceramics only makes up a very small market share for applications [1]. From the upward tendency of lead-free piezoelectric ceramics, Bi0.5Na0.5TiO3 based ceramics has been focused attention due to their properties which are comparable with traditional materials based on PZT-based [4]. Lead-free ferroelectric Bi0.5Na0.5TiO3 ceramic was fabricated for the first time by two Soviet scientists (Smolenskii and Agranovskaya) in 1959 [5]. The ferroelectric property of Bi0.5Na0.5TiO3 was announced by Smolenskii’s group [6]. Bi0.5Na0.5TiO3 has perovskite structure with a remanent polarization of 34 C/cm 2 at room temperature and the Curie temperature of 320 o C [6]. The theoritical calculation for Bi0.5Na0.5TiO3 suggests this material has a wide direct bandgap of 1.97 eV [7]. The experimental results from Parija et al. show that the bandgap is around 2.94 eV [8]. Another suggestion from Wang et al. is that the bandgap varies in a range of 2.82 eV and 2.92 eV, depending on the concentration of NaOH in hydrothermalization [9]. Kushawaha et al. found that Bi0.5Na0.5TiO3 has the good photocatalytic and antibacterial abilities [10]. Additionally, Ai et al. confirmed that Bi0.5Na0.5TiO3 has a better detoxification of NOx compared to a commercial material of TiO2 P25 (Degussa) [11]. The ability to H2 evolution from water with an evolution rate of 325.4 molh - 2 gcat -1 by using a 500W Xe lamp was studied by Wang et al. [9]. Lin et al. found that the photocatalytic property of Bi0.5Na0.5TiO3 can be improved by treatment of metyl orange [12]. . Figure1. The survey of the market share for electronics devices using lead-free piezoelectric ceramics (a) and using ferroelectric and piezoelectric materials in common [1] (License Number #3762260532932). Recently, besides the study of ferroelectric, piezoelectric and insulating properties, Bi0.5Na0.5TiO3 has been futher studied in the other directions as photocatalysis, water splitting and antibacterial ability. This is due to the existence of spontaneous ferroelectric domains which L. T. H. Thanh, Đ. B. Tùng, N. H. Việt, P. Q. Bảo, N. H. Tuấn, Đ. Đ. Dũng 106 can reduce the posibility of electron-hole pair recombination, making the photocatalytic efficiency very high [13]. Kang et al. found that the phase formation, ferroelectronic and insulating properties of Bi0.5Na0.5TiO3 film synthesized by sol-gel method strongly depended on the primary ratio of Bi/Na because these elements are easy to evaporate during gelization and annealing processes [14]. However, these reports about the evaporating amount of Na during gelization,the influence of sintering temperture on the phase information and optical properties of this material have not been performed in a systematic manner. In this report, lead-free ferroelectric material Bi0.5Na0.5TiO3 was synthesized by sol-gel method. The result shows that the single-phase Bi0.5Na0.5TiO3 material is obtained when the compensating amount of Na is above 40% and the annealing temperature is higher than 800 C. The bandgap of Bi0.5Na0.5TiO3 varies with the change of the Na amount and the annealing temperature. 2. EXPERIMENT Bi0.5Na0.5TiO3 was synthesized by sol-gel method in which the precursors are bismuth nitrate (Bi(NO3)2.5H2O), sodium nitrate (NaNO3), tetraisopropoxytitanium (IV) C12H28O4Ti and the solvents are acetic acid (CH3COOH) and acetylacetone (CH3COCH2COCH3). The Bi(NO3)2.5H2O and NaNO3 materials were first dissolved in deionised water and acetic acid. The solution was stirred with a magnetic stirrer at room temperature in 1 hour to completely dissolved the salts of Bi(NO3)2.5H2O and NaNO3. When the solution became transparent, the solvent of acetylacetone was added to prevent Bi 3+ ions from reverse precipitation and form C12H28O4Ti-dissolved environment. The C12H28O4Ti amount was suitably calculated before adding into the solution which was then stirred in 1 day until it became transparent. After that, the solution was dried at a temperature of 110 o C to form gel. Dried gel was annealed in air at different temperatures from 500 o C to 1000 o C for 2 hour then furnace cooled. The Na residual amount was varied from 0 to 50 mol% to investigate the influence on the phase information. Structural phase of sample was studied by X-ray diffraction using the wavelength of CuKα (1.5406 Å), the 2θ angles from 20 to 70o and the step of 0.02o. The morphology was monitored by scanning electron microscope (SEM) and the optical properties were studied by UV-visible absorption spectroscopy. 3. RESULT AND DISCUSSION Figure 2 shows XRD pattern of Bi0.5Na0.5TiO3 sample annealed at 800 C in air for 2 h with different compensated amount of Na. Bi0.5Na0.5TiO3 has a single phase with rhombohedral structure when the Na amount is above 40 %. In case of amount of Na less than 40 %, the gel- forming vaporization results in the lack of a necessary amount of Na, hence the new phase of Bi2Ti2O7 is formed to. This observation is in a good agreement with the report of Kang et al. [14]. In addition, when the Na amount in range of 10 - 30 mol%, other imputity phases are observed and difficult to identify the origin of these phases. The influence of sintering temperature on the phase formation was illustrated on XRD patterns (Fig. 3) of Bi0.5Na0.5TiO3 with the Na amount of 40 mol% at different sintering temperatures from 500 to 1000 C for 2 h in air. Fig. 3 shows that, after sintering at 500 o C, some diffraction peaks related to a parasite phase of Bi2Ti2O7 are observed in addition to the peak of Bi0.5Na0.5TiO3 phase. The intensity of Bi0.5Na0.5TiO3-phase peak increases with an increase of sintering temperature; meanwhile, the intensity of Bi2Ti2O7-phase peak decreases. At Influence of sintering temperature on phase formantion and 107 temperature above 800 o C, most of peaks related to Bi2Ti2O7 phase are removed and only some peaks from Bi0.5Na0.5TiO3 are left. At 1000 o C, no observation of parasite phase suggests that Bi0.5Na0.5TiO3 is stable. As a result, the phase-formation temperature of Bi0.5Na0.5TiO3 should be in range of 800 - 1000 o C. \ Figure2. XRD patterns of Bi0.5Na0.5TiO3sample annealed at 800 C for 2 hours in air with different Na compensated amounts. Figure 3. XRD patterns of Bi0.5Na0.5TiO3 sample with the Na amountof 40 mol at different sintering temperatures from 500 o C to 800 o C for 2 hours in air. The effect of sintering temperature on the morphology of Bi0.5Na0.5TiO3 having 40 mol% Na amount was studied at different temperatures: 500 C, 600 C, 900 C and 1000 C, seen Fig 4(a)-(d). The result shows that the particle grain size is small and uniform. When increasing sintering temperature to 600 o C, particles have a tendency of forming cubic shape with larger size as compared to the 500 o C sintered sample. At 900 o C, particles were sintered and cubic structure was destroyed. This trend was observed more clearly at the sintering temperature above 1000 o C. In conclusion, the sintering temrature plays a very important role in controlling the structure of Bi0.5Na0.5TiO3 which influences on the Bi0.5Na0.5TiO3 development in applications related to H2 splitting and also for photocatalysis [7]. L. T. H. Thanh, Đ. B. Tùng, N. H. Việt, P. Q. Bảo, N. H. Tuấn, Đ. Đ. Dũng 108 Figure 4. The morphology of Bi0.5Na0.5TiO3samples with the same Na amount of 40 mol% annealed at (a) 500 C, (b) 600 C, (c) 900 C and (d) 1000 C for 2 h in air. Figure 5. (a) absorption spectra of Bi0.5Na0.5TiO3 sample and (b) the dependence of ( h ) 2 on the photon energy (h ) corresponding to different Na amounts from 0 to 50 mol.% Figure 5 shows absorption spectra of Bi0.5Na0.5TiO3 sample corresponding to different Na amounts. From Fig. 5, the sample has an absorption edge of 420 nm when Na amount varies from 10 to 40 mol%. In case of no Na compensation, beside absorption edge of 420 nm, there is another absorption edge of 520 nm. In Bi0.5Na0.5TiO3 sample with 50 mol% Na compensated amount, a new small absorption edge of 450 nm was detected. This appearance is expected to be related to the absorption edge of Bi2Ti2O7 or contaminants in case of no or too high Na- compensated amount. Fig. 6(b) shows the relation between ( hv) 2 and the photon energy (h ) in Bi0.5Na0.5TiO3 samples corresponding to different Na-compensated amount varying in range of 0 - 50 mol%. The bandgap (Eg) is determined by interpolating the linear part of the plot of ( hv) 2 versus hv. The bandgap value is estimated around 3.05 - 3.18 eV. Influence of sintering temperature on phase formantion and 109 Figure 6.(a) absorption spectra of Bi0.5Na0.5TiO3 sample and (b) the dependence of ( h ) 2 on the photon energy (h )corresponding to Na-compensated amount of 40 mol% at different temperature from 500 o C to 1000 o C for 2 hours in air. Figure 6(a) presents absorption spectra of Bi0.5Na0.5TiO3 sample with Na-amount of 40 mol% corresponding to different sintering temperature from 500 C to 1000 o C for 2 h in air. One absorption edge of 350 nm was detected in sample sintered at 500 C. From XRD pattern and absorption spectroscopy of Bi0.5Na0.5TiO3 with different Na compensated amount, this edge may be the attribution of Bi2Ti2O7, or defects. When increasing sintering temperature, the absorption edge shifts to the short wavelength, and the secondary absorption edge disappears. This result is consistent with the microstructural analysis of the sintering temperature. Fig. 7(b) depicts the photon-energy dependence of ( hv) 2 . The interpolated bandgap of samples sintered at 500 C and 1000 C is 3.01 eV and 3.14 eV, respectively. These values are in a good agreement with previous reports in which the bandgap of Bi0.5Na0.5TiO3 varies in range of 2.82- 2.94 eV [8-12]. 4. CONCLUSION Bi0.5Na0.5TiO3 was fabricated by sol-gel method. The influence of technological condition such as the Na-compensated amount in gelization and the sintering temperature on the phase formation and the optical properties was studied. The results suggest that the optimized conditions are: Na-compensated amount in gelization of 40 mol% and the sintering temperature range of 800 – 1000 oC for 2 h in air. The bandgap value varies from 3.01 to 3.18 eV. Acknowledgement. This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2012.62. REFERENCES 1. Jo W., Dittmer R., Acosta M., Zang J., Groh C., Sapper E., Wang K., and Rodel J. - Giant electric-field-induced strains in lead-free ceramics for actuator applications-status and prespective, J. Electroceram. 29 (2012) 71-93. 2. Report of World Health Organization, Lead poisoning and health, August 2015. 3. Vũ Vân - Giúp dân giải độc chì ở làng nghề Đông Mai, Môi trường và phát triển, Tài nguyên và môi trường online, Ấn phẩm của Báo tài nguyên và Môi trường, tháng 5 năm 2015, Cơ quan ngôn luận của Bộ Tài nguyên và Môi trường. L. T. H. Thanh, Đ. B. Tùng, N. H. Việt, P. Q. Bảo, N. H. Tuấn, Đ. Đ. Dũng 110 4. Quan N. D., Bac L. H., Thiet D. V., Hung V. N., and Dung D. D. - Current development in lead-free Bi0.5(Na,K)0.5TiO3-based piezoelectric materials, Adv. Mater. Sci. Eng. 2014 (2014) Article ID 365391, and reference therein. 5. 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TÓM TẮT ẢNH HƯỞNG CỦA NHIỆT ĐỘ NUNG THIÊU KẾT ĐẾN VIỆC TẠO PHA VÀ TÍNH CHẤT QUANG CỦA VẬT LIỆU SẮT ĐIỆN KHÔNG CHÌ Bi0.5Na0.5TiO3 Lê Thị Hải Thanh1, Đặng Bá Tùng1, Nguyễn Hoàng Việt2, Phùng Quốc Bảo3, Nguyễn Hoàng Tuấn1, Đặng Đức Dũng1, * 1 Bộ môn Vật lý Đại cương, Viện Vật lý Kỹ thuật, Trường Đại học Bách khoa Hà Nội, số 1 đường Đại Cồ Việt, Hà Nội 2 Viện Khoa học và Kỹ thuật Vật liệ , Trường Đại học Bách khoa Hà Nội, số 1 đường Đại Cồ Việt, Hà Nội 3 Bộ môn Q ang lượng tử, Khoa Vật lý, Trường Đại học Khoa học tự nhiên, Đại học Quốc gia Hà Nội, số 334 đường Nguyễn Trãi, Hà nội * Email: dung.dangduc@hust.edu.vn Influence of sintering temperature on phase formantion and 111 Vật liệu sắt điện nền Pb(Zr,Ti)O3 đã và đang được ứng dụng rộng rãi trong các linh kiện điện tử mặc dù vật liệu này đã bị cấm sử dụng do tính độc hại của nguyên tố chì (chiếm khoảng ~ 60 % khối lượng) đến sức khỏe và môi trường. Trong số các họ vật liệu sắt điện không chì được nghiên cứu phát triển thì vật liệu sắt điện không chì Bi0.5Na0.5TiO3 được quan tâm do chúng có các đặc trưng sắt điện và áp điện so sánh được với vật liệu nền Pb(Zr,Ti)O3. Trong báo cáo này, vật liệu Bi0.5Na0.5TiO3 được chế tạo bằng phương pháp sol-gel. Các điều kiện ảnh hưởng từ điều kiện công nghệ chế tạo như quá trình gel hóa, quá trình trung thiêu kết đến pha kết tinh của vật liệu được khảo sát. Kết quả nghiên cứu cho thấy vật liệu sắt điện không chì Bi0.5Na0.5TiO3 đơn pha cấu trúc khi hàm lượng Na bù trong quá trình gel hóa tối ưu khoảng 40 mol. và nhiệt độ nung tạo pha là trên 800 C trong khoảng thời gian 2 giờ ngoài không khí. Kết quả phân tích phổ hấp thụ cho thấy độ rộng vùng cấm của vật liệu Bi0.5Na0.5TiO3 có giá trị trong khoảng từ 3,01 - 3,18 eV. Từ khóa: Bi0.5Na0.5TiO3, sol-gel, tính chất quang, sắt điện không chì.