Influence of Ammonium Hydrogen Carbonate to Metal Ions Molar Ratio on Co-precipitated Nanopowders for TGG Transparent Ceramics

LI Xiao-Ying, LIU Qiang, HU Ze-Wang, JIANG Nan, SHI Yun, LI Jiang

Influence of Ammonium Hydrogen Carbonate to Metal Ions Molar Ratio on Co-precipitated Nanopowders for TGG Transparent Ceramics

LI Xiao-Ying1,2, LIU Qiang2, HU Ze-Wang1,3, JIANG Nan1,3, SHI Yun1, LI Jiang1,3

(1. Key Laboratory of Transparent Opto–Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2. School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China; 3. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China)

Terbium gallium garnet (TGG) ceramics were successfully fabricated by air sintering at 1500 ℃ for 3 h combined with HIP post-treating at 1550 ℃ for 3 h under 150 MPa argon gas, where the TGG powders were synthesized by the co-precipitation method employing ammonium hydrogen carbonate (AHC) as precipitant. The influences of ammonium hydrogen carbonate to metal ions molar ratio (value) on phase composition and morphology of the resultant powders as well as optical transmittance and Verdet constant of the TGG ceramics were investigated systematically. The precursors with=3.6, 4.0 and 4.4 calcined at 1100 ℃ form pure TGG phase, whereas the precursor with=3.2 treated at the same temperature yields the mixed phases of TGG and Ga2O3. The TGG powder with=4.0 shows the best dispersity and homogeneity, giving rise to ceramic with the best optical quality. On the contrary, the powder with=4.4 exhibits a strong agglomeration, which is closely related to the morphology of its precursor. High quality TGG transparent ceramics with the transmittance of 80.1% at 1064 nm can be fabricated by the nanopowder with=4.0, and the Verdet constant of the TGG ceramics at 633 nm is rather close to that of the commercial TGG single crystals (-134 rad·T-1·m-1).

Faraday material; TGG ceramics; co-precipitation method; AHC/M3+molar ratio

Faraday rotator is one of the key optics for the isolation, the polarization control and the birefringence compen-sation of a laser amplifier for the high energy and high average power laser driven application[1-3]. The most significant properties of the Faraday material are the high Verdet constant and good optical quality, as well as a high thermal conductivity and superior size scalability for handling thermal effects[4-7]. To date, Tb-doped phosphate and silicate glasses are common Faraday material used in large aperture laser systems because of the superior size scalability[8-9]. However, amorphous glasses cannot meet the demand of high average power lasers due to its low thermal conduc-tivity[10-11]. Terbium gallium garnet is considered as a promising magneto-optical material used in the visible and near- infrared isolators because of its twice Verdet constant of Terbium-doped glass. Furthermore, the thermal conductivity of crystalline TGG is an order of magnitude greater than a typical glass[12-14]. The combination of the above factors makes TGG better suited for high average power applications. Unfortunately, the growth of high quality and large size TGG crystal is not an easy task for the evaporation of Ga2O3from the melt during the crystal growth process, which leaves it far from application in high-average- power lasers[15-17].

With the advancement of transparent ceramics fabrication technology, ceramics with high optical quality are usually preferred over single crystal for high power applications by virtue of its outstanding optical quality and excellent size scalability[18-19]. In general, transparent ceramics can be fabricated by two main methods. In conventional solid-state reaction between the commercial oxide, high calcination temperature and prolonged calcination time are necessary to gain the pure phase[20-21]. Furthermore, the extensive milling leads to possible contamination, thus degrading the optical quality[22-23]. For this reason, several wet-chemical methods, such as Sol–Gel process[24], microemulsion[25], co-precipitation[26-27]and hydrothermal synthesis[28], exhibit considerable advantages such as intimate mixing of the starting materials, excellent chemical homogeneity and low synthesis temperature[29-30]. Among the wet-chemical methods, co-precipitation method using ammonium hydrogen carbonate as precipitant is a promising route to synthesize TGG powders possessing the proper characteristics toward transparent ceramics. Characteristics of the co-precipitated powders depend on the nature of salts solution and precipitant as well as synthesis conditions. According to Dai,[31]and Dulina,[32], precipitant to metal ions molar ratio (defined asvalue) can influence the terminal pH during the precipitation process, and the terminal pH must be controlled strictly to optimize the chemical composition and morphology of nanopowders. However, the synthesis of TGG powders using co-precipitation method is rarely studied in detail, especially the influences of ammonium hydrogen carbonate to metal ions molar ratio on the properties of precursors and TGG powders.

In this work, TGG nanopowders were prepared by a co-precipitation route using AHC as the precipitant. With the aim of preparing pure TGG nanopowders beneficial to the fabrication of transparent ceramics, the role of thevalues on the phase composition and dispersity of the as-synthesized powders and final microstructures, optical quality and magneto-optical property of the TGG ceramics were systematically studied.

1 Experimental

Nanosized TGG powders, produced by a co- precipitation method, were used as starting materials for ceramic sample. Highly pure Tb and Ga nitrate solutions are prepared by dissolving appropriate amounts of Tb4O7(99.99%, Yuelong New Materials Co., Ltd., Shanghai, China) and Ga2O3(99.995%, Jining Zhongkai New Materials Co., Ltd., Shandong, China) powders in hot HNO3/H2O. Then, the metal nitrates were mixed together to form a homogeneous solution according to the stoichiometric ratio of Tb3Ga5O12and the Ga3+concentration was set to 0.3 mol/L. Precipitant solution with a concentration of 1.5 mol/L was obtained by dissolving ammonium hydrogen carbonate (Analytical grade, Aladdin) in deionized water. Ammonium sulfate (99.0%, Sinopharm Chemical Reagent Co., Ltd.) was added into the precipitant solution as the dispersant. The precursor precipitate was performed by the reverse- strike method at room temperature. The molar ratiowas chosen as 3.2, 3.6, 4.0 and 4.4, respectively. The white precipitate formed, and the reaction mixture was washed four times with deionized water and rinsed twice with absolute ethanol. Then, the precursor was dried at 70 ℃ for 48 h. After that, the dried precursor was sieved through a 75 μm screen and then calcined at 1100 ℃ for 4 h. Finally, the powders were dry- pressed to a 20-mm-diameter pellet at 20 MPa, then further cold isostatically pressed (CIP) under 250 MPa. The pellets were sintered at 1500 ℃ for 3 h in muffle furnace followed by hot isostatic pressing (HIP) post-treated at 1550 ℃for 3 h under 150 MPa in Ar atmosphere. The specimens were mirror-polished on both surfaces into 1.2-mm-thickness for further test.

Phase identification of the as-synthesized powders was performedX-ray diffraction (XRD) analysis using a Diffractometer (XRD, Mode-l D/max2200PC, Rigaku, Japan). Specific surface area analysis was carried out by Norcross ASAP 2010 micromeritics at 77 K, using N2as the absorbate gas. The compositions of the precursors were examined by the Fourier transform infrared spectrometer (FT-IR, Bruker VERTEX 70 spectrophotometer, Ettlingen, Germany). The mor- phologies of powders and thermally-etched surfaces of the ceramic sample were submitted to FESEM characterization (S-8220, Hitachi, Japan). Grain size of the sintered sample was determined by image analysis, carried out on several micrographs acquired by FESEM and using the linear intercept method. The in-line transmittance of the specimens was measured over the wavelength region from 300 nm to 1800 nm using a spectrometer (Model Cray-5000 UV-VIS-NIR Spectrophotometer, Varian, CA, USA). The Verdet constant of the ceramics at 633 nm was measured using an instrument consisting of a He–Ne laser, two polarizers, and an electromagnet at room temperature.

2 Results and discussion

The FT-IR spectra of precursors synthesized with differentvalues are shown in Fig. 1. It can be seen that the positions of the main absorption peaks are almost the same in the FT-IR spectra with=3.2-4.4, which indicates that the molar ratio R has little impact on the chemical compositions of the precursors. The FT-IR spectra of precursors exhibit broad absorption bands at 3400 cm-1corresponding to the stretching vibrations of O-H bond[33]. The weaker absorption peak at 1630 cm?1can be attributed to H-O-H bending mode of molecular water. The peak at about 1520 cm-1results from the bond-stretching of NH4+. The stretching vibrations of CO32-appear as absorption peaks at 1416 cm-1, whereas the nonplanar bending CO32-vibrations are observed at 841 cm-1, indicating the presence of carbonate group in the precursors. The weak peak at 2350 cm-1is attributed to the asymmetrical stretch of CO2absorbed in air[34].The abnormal absorption peaks at 2350 cm-1of the precursors with=3.2 and 3.6 are caused by baseline scan and background subtraction, which is independent of the precursors with differentvalues.

Fig. 1 FT-IR spectra of the precursors synthesized with different R values

Fig. 2 shows the FESEM micrographs of the precursors synthesized with different R values. It can be seen that the precursors with=3.2 and 3.6 are composed of sub-micrometer sized near-spherical shaped particles. Additionally, the precursors with=3.2 and 3.6 are loosely agglomerated and characterized by a high homogeneity. For the precursor with=4.0, the needlelike shaped particles occur and the slight agglomeration can be observed. However, with the increase ofvalue to 4.4, large-sized aggregate with nubby morphology occurs, which is probably due to the relatively higher pH resulting in the enhancement of agglomeration between particles[35].

Fig. 2 FESEM micrographs of the precursors synthesized with different R values

(a)=3.2; (b)=3.6; (c)=4.0; (d)=4.4

Fig. 3 shows the XRD results of the calcined powders with differentvalues. The results indicate that the peaks of the powders with differentvalues match well with the standard diffraction of TGG (JCPDS 88-0575), except for the powder with=3.2. The powder with=3.2 shows a feeble trace of peak consisted with Ga2O3. It might be due to the partial Ga3+ions begin to precipitate from the nitrate solutions at about pH=4.2, while the production of Tb precipitates requires a higher pH value. Asvalue decreases to 3.2, the lower terminal pH results in segregation of Tb precipitate and Ga precipitate. The average crystallite size (XRD) of the synthesized powders can be calculated from the XRD spectra using the Scherrer's formula. The average crystallite size value of TGG powders with=3.6, 4.0 and 4.4 are 125.9, 106.3 and 115.2 nm, respectively.

Fig. 4 shows the dispersion state of TGG powders calcined at 1100 ℃ for 4 h with variousvalues. For the powder with=3.2, the obvious secondary phase can be observed and the EDS measurement demonstrates that the composition of the secondary phase is a gallium riched phase, which is in good accordance with the XRD results. As can be seen, the synthesized TGG nanopowder with=3.6 consists of loosely agglomerated dumbbell shape particles and the average particle size is about 157.4 nm. Whenvalue is 4.0, the powder exhibits good homogeneity and dispersity, and the morphology and average particle size of the powder are similar to the powder with=3.6. The specific surface areas of TGG powder with=4.0 is 5.33 and the average particle size is about 157.7 nm. Further increase ofvalue to 4.4 results in severe agglomeration, accompanying the increase of average particle size (~175.1 nm), which is detrimental to the densification of green body. The morphology of TGG powder with=4.4 is closely related to the agglomeration of its precursor. In general, these results highlight the key role of thevalue on the purity of the TGG phase, in addition,value is also used mainly for the purpose of optimizing morphology of final TGG particles.

Fig. 3 XRD patterns of powders calcined at 1100 ℃ for 4 h with different R values

Fig. 4 FESEM micrographs of TGG powders calcined at 1100 ℃ for 4 h with different R values

(a)=3.2; (b)=3.6; (c)=4.0; (d)=4.4

Fig. 5 shows the photograph and the in-line trans-mittance of the double-polished TGG ceramics (1.2 mm thick) fabricated by the nanopowders with differentvalues. It can be seen that the specimen with=3.2 is almost opaque and the in-line transmittance is less than 5% through the entire range from the visible to 1.8 μm. It is owing to a mass of second phase particles in the ceramics, which is evidenced by the FESEM micrograph shown in Fig. 6(a). Whenvalue is 3.6, the TGG ceramic sample exhibits the better transparency than the sample with=3.2, but the drastic decrease in the visible wavelength range can be observed. For the ceramic sample with=4.0, the in-line transmittance exceeds 75% in the region of 500-1600 nm, reaching about 80.1% at 1064 nm, which is equal to the theoretical value. The excellent optical quality can be attributed to the high chemical purity as well as the better dispersity of TGG powder, which result in minimum optical loss arising from the absorption or scattering in the ceramic. The result shows that there is an absorption peak centered at about 487 nm corresponding to7F6-5D4transition of Tb3+. The sample with=4.4 is opaque as the result of a large number of residual pores acted as scattering centers after the HIP post-treatment.

Fig. 6 displays the SEM micrographs of the thermally etched surfaces of the TGG ceramics with different R values pre-sintered in a muffle furnace at 1500 ℃ for 3 h. It can be noticed that the specimen with=3.2 contains not only a small amount of intergranular pores and intragranular pores, but also a small amount of second phases. The EDS measurement reveals that the secondary phase is gallium oxide. The appearance of gallium oxide results from the composition segregation of the corresponding TGG powders. For the pre-sintered ceramic samples with=3.6, 4.0 and 4.4, all of the pre-sintered samples are opaque because there are quite a few pores in the samples. The average grain size of TGG ceramics with=3.6, 4.0 and 4.4 are 1.44, 1.63 and 1.59 μm, respectively. However, it can be obviously seen that the average grain size of TGG ceramics with=3.2 is larger than those of other samples. We deduce that the superfluous Ga2O3can enhance the migration rate of the grain boundary, which leads to the faster grain growth and the formation of intragranular pore.

Fig. 5 (a) Photograph of TGG transparent ceramics (1.2 mm thick) sintered from powders with different R values and in-line transmission curves of (b) the samples pre-sintered at 1500 ℃ for 3 h in air with HIP post-treatment at 1550 ℃ for 3 h with different R values

Fig. 6 FESEM micrographs of the thermally etched surfaces of TGG ceramics pre-sintered at 1500 ℃ for 3 h with different R values

(a)=3.2; (b)=3.6; (c)=4.0; (d)=4.4

The FESEM micrographs of the thermally etched surfaces of the HIP-treated TGG ceramics with differentvalues are shown in Fig. 7. After the HIP-treatment, a slight grain growth occurred in all the samples, and the average grain sizes of TGG ceramics with=3.2, 3.6, 4.0 and 4.4 are 9.4, 1.9, 1.8 and 2.0 μm, respectively. A mass of secondary Ga2O3phase grains are also observed in the specimen with=3.2, which act as scattering centers and lead to the low transmittance. The pores are remarkably reduced and the grain boundaries are clean without any secondary phases in the sample with=3.6. However, a small number of residual pores with sub-micrometer sized are observed in the sample with=3.6. The size of pores is comparable to incident wavelength, giving rise to the occurrence of Mie scattering[22]. Therefore, the transmittances of the TGG ceramics are still far from the theoretical value. For the specimen with=4.0, it shows a nearly pore- free microstructure without abnormal grain growth. On the opposite, no secondary phases but large quantity of residual pores are observed in the ceramic samples with=4.4, which is mainly caused by the agglomeration of the original powders.

Faraday effect leads to the rotation of polarized light and the Faraday rotation angle can be expressed by the formula:

=(1)

For magneto-optical materials, the Faraday rotationis linear ratio to the Verdet constant when the length of sampleand magnetic induction intensityare fixed, so the Verdet constant is the main parameter for eva-luating the magneto-optical property. In this work, the Verdet constant of TGG ceramics with=3.2 and 4.4 was not measured, since the ceramics are non-transparent. The Verdet constants of the TGG ceramics with=3.6 and 4.0 are-137.4 and-136.5 rad·T-1·m-1, respectively, obviously indicating no significant difference between the ceramics and single crystals (-134 rad·T-1·m-1).

Fig. 7 SEM micrographs of the mirror-polished and thermal etched surfaces of TGG ceramics pre-sintered at 1500 ℃ for 3 h in air followed by HIP at 1550 ℃for 3 h with different R values

(a)=3.2; (b)=3.6; (c)=4.0; (d)=4.4

3 Conclusion

TGG precursors with differentvalues were co-precipitated using ammonium hydrogen carbonate as the precipitant. By controlling thevalue to a reasonable degree, the segregation of Tb precipitate and Ga precipitate was eliminated and single phase TGG powders were obtained. Thevalue has a significant impact on the morphology of the powders. The powder with=4.0 shows the best dispersity, giving rise to denser ceramic with finer microstructures. Using these powders as raw materials, TGG transparent ceramics were successfully fabricated by air pre-sintering at 1500 ℃ for 3 h and then HIP post-treatment at 1550 ℃for 3 h. For the TGG ceramic from the powder prepared with=4.0, the in-line transmittance is 80.1% at 1064 nm. The prepared TGG magneto-optical ceramics show excellent magneto-optical properties, which is close to the TGG crystals. In future work, further efforts will concentrate on the production of high quality TGG ceramics, able to provide large aperture as well as good transparency.

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碳酸氫銨與金屬陽離子摩爾比對共沉淀法合成鋱鎵石榴石納米粉體及陶瓷性能的影響

李曉英1,2, 劉強2, 胡澤望1,3, 姜楠1,3, 石云1, 李江1,3

(1. 中國科學院 上海硅酸鹽研究所，透明光功能無機材料重點實驗室，上海 200050；2. 江蘇大學 材料科學與工程學院，鎮(zhèn)江 212013; 3．中國科學院大學 材料科學與光電工程中心，北京 100049)

本研究以碳酸氫銨(AHC)為沉淀劑, 采用共沉淀法制備了TGG粉體。以上述粉體為原料, 將素坯于1500 ℃空氣預燒3 h, 然后于1550 ℃, 150 MPa氬氣氣氛下HIP后處理3 h獲得TGG陶瓷。系統(tǒng)研究了碳酸氫銨與金屬離子摩爾比(值)對合成粉體的相組成、形貌以及TGG陶瓷的透光率和Verdet常數的影響。=3.6, 4.0和4.4的前驅體在1100 ℃煅燒形成純相TGG粉體, 而=3.2的前驅體經相同溫度煅燒后形成了TGG和Ga2O3的混合相粉體。=4.0的TGG粉體分散性和均勻性最好, 故制備的陶瓷光學質量最佳。=4.4的粉體具有較嚴重的團聚, 這與其前驅體形貌密切相關。以=4.0的粉體為原料, 制備的TGG透明陶瓷在1064 nm處的直線透過率為80.1%。制備的TGG陶瓷在633 nm處的Verdet常數和商業(yè)TGG單晶(-134 rad·T-1·m-1)幾乎相等。

法拉第材料; 鋱鎵石榴石陶瓷; 共沉淀法; 碳酸氫銨與金屬陽離子摩爾比

TQ174

A

2018-12-05;

2018-12-19

National Natural Science Foundation of China (61575212)

LI Xiao-Ying (1993-), female, candidate of Master degree. E-mail: lxy113712@163.com

LI Jiang, professor. E-mail: lijiang@mail.sic.ac.cn

1000-324X(2019)07-0791-06

10.15541/jim20180574