Optimization of dispersing agents for preparing YAG transparent ceramics

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013, P. 507

Optimization of dispersing agents for preparing YAG transparent ceramics BA Xuewei (ᏈᄺᎡ)1,2,3, LI Jiang (ᴢ∳)1,*, PAN Yubai (┬㺩ᶣ)1, LIU Jing (߬ျ)1,4, JIANG Benxue (ྰᴀᄺ)1, LIU Wenbin (߬᭛᭠)1, KOU Huamin (ᆛढᬣ)1, GUO Jingkun (䛁᱃സ)1 (1. Key Laboratory of Transparent Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 2 Graduate School of the Chinese Academy of Sciences, Beijing 100039, China; 3. School of Materials Science and Engineering, Qiqihar University, Qiqihar 161006, China; 4. Graduate school of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China) Received 10 January 2013; revised 25 February 2013

Abstract: The suspensions of the powder mixture of yttria and alumina were prepared by the aqueous tape casting method. Rodia DP270, Dolapix CE64, citric acid and Dammonium 3008 were used as dispersing agents. The morphologies of the powders and the fracture surface of the green body were observed by scanning electron microscopy (SEM). The zeta potential test was employed to characterize the surface charge states of different suspensions. The dispersion of the suspensions was analyzed by the rheological tests and the sedimentation experiments. The results indicated that the yttria and alumina powder mixtures were well dispersed by the dispersing agents. The rheological curves showed shear thinning behavior. The most efficient dispersing agent was Dammonium 3008. The optimum usage of Dammonium 3008 was 1.0 wt.%. The green body was dense and no interface between the adjacent layers was found. The in-line transmittance of the as-sintered YAG ceramic was higher than 80% between 400 and 1100 nm. Keywords: Y3Al5O12; tape casting; dispersion; ceramic; rare earths

Laser ceramics are important laser gain media using in many potential fields, such as industry, science research, military, information delivery, and so on. Compared with single crystals, polycrystalline ceramics behave the similar chemical and physical properties, shorter preparing time, larger scale, higher doping concentration and lower cost[1–5]. A particular characteristic is that ceramics can be formed to composite structures to reduce the thermal effects[6]. The laser gain media with gradient structure are believed as a solution to adjust the thermal distribution and improve the quality of the laser beam. The molding process is an important technology for fabricating transparent ceramics. Ikesue et al.[7] and Messing et al.[8] prepared Nd:YAG ceramics by using the dry pressing method. Wu et al.[9] fabricated Yb,Cr:YAG ceramics and the molding method was also dry pressing. Konoshina Chemical Co., Ltd. obtained high quality RE:YAG ceramics by using a slip casting method[10–12]. Kupp et al.[13,14] prepared composite Er:YAG ceramics by the tape casting method and the optical quality was comparable to that of single crystals. Tang et al.[15–17] prepared Yb:YAG and Nd:YAG composite ceramics and they achieved laser output. The forming technology they used was also the tape casting method. The dry pressing method is an easier process and suitable for preparing green bodies with regular shapes. Slip casting method can be used to fabricate green bodies with higher density. The compos-

ite green bodies prepared by the dry pressing and slip casting methods have simple geometric configurations. The single layer is at least several millimeters. Another problem is the irregular interfaces between the adjacent layers which decreases the optical quality significantly. The tape casting method is used in electrical ceramics industry for many years. The thickness of ceramic layer can be controlled at tens of micrometers. By designing the doping concentration of the active rare earths ions, gradient structural ceramics can be prepared. In all of the above molding methods, slurries of YAG powders or the powder mixture of Al2O3 and Y2O3 will be prepared. The slurries were then used for spraying granulation, slip casting, tape casting, and so on. In order to prepare slurries with high quality, the first step and the most important process is to select a proper dispersing agent and make it clear the best amount of the dispersing agent. The dispersing agents are used to disperse primary particles and to hold them in a homogeneous suspension by steric hindrance and ionic repulsion. The dispersant can reduce the viscosity and increase solid loading of the suspension. In order to prepare transparent ceramics, higher solid loading is desired. In this work, we used the powder mixture of yttria and alumina as the solid composition. The efficiency of different dispersing agents was analyzed for preparing suspension with high solid loading. Zeta potential, sedimen-

Foundation item: Project supported by National Natural Science Foundation of China (51002172, 90122035) and the Development Program of China (2010AA03015887003) * Corresponding author: LI Jiang (E-mail: [email protected]; Tel.: +86-21-52412816) DOI: 10.1016/S1002-0721(12)60310-X

508

tary experiment and rheological properties were employed to study the dispersion of the dispersing agents. The green body and the ceramics prepared by the aqueous tape casting method were also discussed.

1 Experimental Commercial yttria and alumina powders (99.99%, Alfa Aesra, USA) were used as the raw materials. Y2O3 powder and Al2O3 powder were weighed by the formula of Y3Al5O12. The powders were ball milled at 120 r/min for 10 h. The slurry was dried and sieved through a sieve of 200 meshes. HNO3 and NaOH solutions were used to adjust the pH value of the suspensions. The solid loading for selecting the dispersing agents was controlled at 35 vol.%. A certain amount of dispersing agent was firstly dissolved into deionized water. Rhodia DP270 (an ammonium polyacrylate, Rhodia Co., Ltd., Boulogne-Billancourt, France), Dolapix CE64 (an ammonium salt of polymethacrylate, Zschimmer & Schwarz Chemische Fabriken, Germany), citric acid (Sinopharm Chemical Reagent Co., Ltd., China) and Dammonium 3008 (an ammonium polyacrylate, Shanghai Juesheng Co., Ltd., China) were used as the dispersing agents. The solution was poured into the Al2O3 jar. The powder mixture was mixed with the solution and ball milled for 8 h. The pH value of the obtained suspension was adjusted at 11 to compare the rheological property. The velocity of tape casting was controlled at 100 mm per minute and the height of the blade was 500 Pm. The tape was dried at room temperature. The green body was prepared by laminating the tapes at 80 ºC for 30 min. And the body was heat treated at 600 ºC for 10 h to eliminate the organic materials. The cold isostatic pressed green body was sintered at 1750 ºC for 6 to 10 h under vacuum condition of <10–3 Pa. The microscopic morphology of the powder mixture was observed by a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL, Japan). The zeta potential of the powder mixture in diluted suspension was measured by an electro-acoustic method (Zeta Plus, Brookhaven, USA). The rheological properties of the suspensions were studied by a rotational rheometer (Anton Paar Physica MCR-301, Germany). The sedimentation experiments were carried out to evaluate the degree of powder dispersion in 5 vol.% suspensions. The suspensions were kept standing in 10 mL cylinders for one month. The sedimentary height of the powders was adopted to analyze the dispersion of the suspensions. UV-Vis-NIR spectrophotometer (Cary-5000, Varian, USA) was used to test the in-line transmittance of the ceramics.

2 Results and discussion

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013

2.1 Micro-morphologies of the powders Commercial Y2O3 and Al2O3 powders were selected for preparing the slurries in order to fabricate YAG ceramics by a solid state reactive sintering method. The reactions were described as below. 2Y2O3+Al2O3Y4Al2O9 (YAM) 900–1100 ºC (1) Y4Al2O9+Al2O34YAlO3 (YAP) 1100–1250 ºC (2) 3YAlO3+Al2O3Y3Al5O12 (YAM) 1400–1600 ºC (3) Fig. 1 shows the microscopic morphologies of the raw materials of Al2O3, Y2O3 and the powder mixture. Fig. 1(a) and (b) are the micrographs of alumina and yttria powders, respectively. The primary particle size of alumina is about 100 nm. The particles form aggregates with the particle size from 200 to 500 nm. The yttria powders are large particles and the mean particle size is 5 m. The configurations of yttria powders are irregular and the edges are sharp. Fig. 1(c) is the secondary electron image of powder mixture. The finer powders are Al2O3 particles and the larger powders are Y2O3 powders. The yttria powders were ball milled down from 5 m to less than 2 Pm. Fig. 1(d) is the back-scattered electron image of the powder mixture. The darker powders are Al2O3 powders and the lighter powders are Y2O3 powders. From the figure, it can be seen that the two powders distribute homogeneously and no hard aggregate is found. 2.2 Properties of the suspensions Fig. 2 shows the relationship of zeta potential and pH value of different dispersing agents. The curves of (1), (2), (3), (4) and (5) represent the slurries using no dispersant, citric acid, Dolapix CE64, Rhodia DP270 and Dammonium 3008 as the dispersants, respectively. From the curves it can be seen that the isoelectric point (IP) of the mixture powders locates at pH=3. It is well known that the IP of Al2O3 is at pH=7–8. This means that Y2O3 plays the main role in the powder mixture. With the increase of pH value, zeta potential decreases and meets a minimum potential at pH=11. If the value is further increased, the zeta potential increases. The dispersing agents provide greater zeta potential for the powder mixture of Y2O3 and Al2O3. By comparing the four types of dispersing agent, the most efficient one is Dammonium 3008, the zeta potential at pH=11 decreases from –37 to –49 mV. The low zeta potential provides stable suspension for ceramic powders. Rhodia DP270 has the similar dispersing effect of Dammonium3008, but the decrease of zeta potential is a little smaller than the latter. Citric acid and Dolapix CE64 can also act as the dispersing agent, but the decrease of zeta potential is relatively lower. The relationship of viscosity and shear rate of different dispersing agents is shown in Fig 3. The curves of (1), (2), (3), (4) and (5) represent the slurries using no dispersant, citric acid, Dolapix CE64, Rhodia DP270 and

BA Xuewei et al., Optimization of dispersing agents for preparing YAG transparent ceramics

509

Fig. 1 SEM images of the raw materials and the powder mixture of yttria and alumina (a) Al2O3; (b) Y2O3; (c) Secondary electron image of the powder mixture; (d) Back-scattered electron image of the powder mixture

Fig. 2 Zeta potential curves for suspensions with various dispersing agents (1) No dispersant; (2) Citric acid; (3) Dolapix CE64; (4) Rhodia DP270; (5) Dammonium 3008

Fig. 3 Rheological curves as a function of various dispersing agents (1) No dispersant; (2) Citric acid; (3) Dolapix CE64; (4) Rhodia DP270; (5) Dammonium 3008

Dammonium 3008 as the dispersants, respectively. The solid loadings of these suspensions were 35 vol.%. The suspensions show shear thinning behavior. At the shear rate between 0.1 and 1000 1/s, the viscosities of the suspension without the dispersing agent have the highest value. The suspensions using the ammonium of polyacrylic show lower viscosities, such as Rhodia DP270 and Dammonium 3008. Among all of the four types of dispersing agents, Dammonium 3008 is the best for the powder mixture of Y2O3 and Al2O3. Sedimentary experiments also proved the dispersion

change of the five suspensions. As shown in Fig. 4, the smallest sedimentary fractions of the dispersing agents located at pH=10 to 11. While the highest ones appear at pH=7 to 8. The results indicate that alkaline condition is suitable for achieving high quality suspension with good dispersion. This can be attributed to the existence of a great amount of negative charges adsorbed around the oxides’ surface. Among the four types of dispersing agents, Dammonium 3008 was selected as the best one. The influences of different Dammonium 3008 levels on the dispersion of the suspension prepared from the

510

Fig. 4 Sedimentary behavior of suspensions using different dispersing agents (1) No dispersant; (2) Citric acid; (3) Dolapix CE64; (4) Rhodia DP270; (5) Dammonium 3008

JOURNAL OF RARE EARTHS, Vol. 31, No. 5, May 2013

Fig. 6 Relationship between the settled height and Dammonium 3008 level

amount of the dispersant to 1.0 wt.%. But the height increased again if the amount of the dispersant was over 1.0 wt.%. According to the figure, it can be seen that the tendency of these suspensions is consistent with the rheological test in Fig. 5. The results indicate that the optimum Dammonium 3008 level is 1.0 wt.%. Lower or higher dispersing agent levels created higher viscosity and poorer dispersibility. The pores in the suspensions were hard to be eliminated. These factors are harmful to transparent ceramics. High solid loadings were achieved by using Dammonium 3008 as the dispersing agent and the highest solid loading reached at 53 vol.%. 2.3 Preparation of YAG transparent ceramics

Fig. 5 Viscosity of 35 vol.% slurry as a function of the amount of dispersant

mixture powders of Y2O3 and Al2O3 were studied. Fig. 5 shows the relationship between viscosity and shear rate of the suspensions by changing the Dammonium 3008 levels from 0.2 to 1.6 wt.%. The dispersant level was calculated based on the dry powder mixture. All the samples show shear thinning behavior. With the increase of Dammonium 3008 levels, the viscosities decrease evidently. When the Dammonium 3008 level arrives at 1.0 wt.%, the viscosity is the minimum. Further increasing Dammonium 3008 level, the viscosity of the suspensions will increase. This is the result that excess PAA molecules will twine together and form continuous structures in the suspension. Some of the water molecules are wrapped in these structures and the fluidity of the suspension gets worse. The influences of Dammonium 3008 levels on the dispersion of the suspensions were also analyzed by sedimentary experiments. The results are shown in Fig. 6. The pH values were all adjusted at 11 and the solid loadings were 35 vol.%. The settled height of the suspension with 0.2 wt.% dispersant is above 18%. The height decreased to the minimum point of 16% by increasing the

Fig. 7 is the fracture surface micrograph of the green body prepared from a suspension with a solid loading of 45 vol.%. 20 pieces of dried tapes were laminated at 90 ºC for 30 min. The body underwent a de-waxing process at 600 ºC for 10 h to eliminate the organic components. The green body was prepared by cold isostatic pressing at 200 MPa for 2 min. The green body has a dense microstructure and no gaps between the adjacent layers are found in the image. Fig. 8 is the in-line transmittance curve of pure YAG ceramic prepared by an aqueous tape casting and vacuum

Fig. 7 Fracture surface image of the green body

BA Xuewei et al., Optimization of dispersing agents for preparing YAG transparent ceramics

Fig. 8 In-line transmittance curve of YAG ceramic

sintering method. It can be seen that the sample has high optical property and the transmittance is higher than 80% from visible to near infrared band. The values of transmittance at 400 and 1064 nm are 80.2% and 82.8%, respectively.

3 Conclusions The powder mixture of Y2O3 and Al2O3 was obtained by ball milling firstly for preparing YAG transparent ceramics. Rhodia DP270, Dolapix CE64, citric acid and Dammonium 3008 were compared to determine as the suitable dispersing agent. Among the four kinds of agents, Dammonium 3008 showed the lowest viscosity and sedimentary fraction. The best usage level of Dammonium 3008 was 1.0 wt.%. YAG transparent ceramics with high quality were prepared and the in-line transmittance was higher than 80% from 400 to 1100 nm.

References: [1] Ikesue A, Aung Y L, Taira T, Kamimura T, Yoshida K, Messing G L. Progress in ceramic lasers. Annu. Rev. Mater. Res., 2006, 36: 397. [2] Maitre A, Salle C, Boulesteix R, Baumard J, Rabinovitch Y. Effect of silica on the reactive sintering of polycrystalline Nd:YAG ceramics. J. Am. Ceram. Soc., 2008, 91: 406. [3] Kochawattana S, Stevenson A, Lee S, Ramirez M, Gopalan V, Dumm J, Castillo V K, Quarles G J, Messing G L. Sintering and grain growth in SiO2 doped Nd:YAG. J. Eur. Ceram. Soc., 2008, 28: 1527. [4] Chen J, Huang X G, Wang L X, Zhang Q T. Preparation

511

and properties of Nd:YAG ultra-fine powders. J. Rare Earths, 2011, 29: 44. [5] Zhang X L, Liu D, Liu H, Wang J Y, Qin H M, Sang Y H. Microstructural characteristics of Nd:YAG powders leading to transparent ceramics. J. Rare Earths, 2011, 29: 585. [6] Ikesue A, Aung Y L. Ceramic laser materials. Nat. Photonics, 2008, 2: 721. [7] Ikesue A, Kinoshita T, Kamata K, Yoshida K. Fabrication and optical properties of high-performance polycrystalline Nd:YAG ceramics for solid-state laser. J. Am. Ceram. Soc., 1995, 78: 1033. [8] Lee S, Kochawattana S, Messing G L, Dumm J Q, Quarles G, Castillo V. Solid-state reactive sintering of transparent polycrystalline Nd:YAG ceramics. J. Am. Ceram. Soc., 2006, 89: 1945. [9] Wu Y S, Li J, Qiu F G, Pan Y B, Liu Q, Guo J K. Fabrication of transparent Yb,Cr:YAG ceramics by a solid-state reaction method. Ceram. Int., 2006, 32: 785. [10] Lu J, Yagi H, Takaichi K, Uematsu T, Bisson J-F, Feng Y, Shirakawa A, Ueda K I, Yanagitani T, Kaminskii, A A. 110 W ceramic Nd3+:Y3Al5O12 laser. Appl. Phys. B, 2004, 79: 25. [11] Yagi H, Yanagitani T, Takaichi K, Ueda K I, Kaminskii A A. Characterization and laser performances of highly transparent Nd:Y3Al5O12 laser ceramics. Opt. Mater., 2007, 29: 1258. [12] Samuel P, Yanagitani T, Yagi H, Nakao H, Ueda K I, Babu S M. Efficient energy transfer between Ce3+ and Nd3+ in cerium codoped Nd:YAG laser quality transparent ceramics. J. Alloys Compd., 2010, 507: 475. [13] Kupp E R, Messing G L, Anderson J M, Gopalan V, Dumm J Q, Kraisinger C, Ter-Gabrielyan N, Merkle L D, Dubinskii M, Castillo V K, Quarles G J. Co-casting and optical characteristics of transparent segmented composite Er:YAG laser ceramics. J. Mater. Res., 2010, 25: 476. [14] Ter-Gabrielyan N, Merkle L D, Kupp E R, Messing G L, Dubinskii M. Efficient resonantly pumped tape cast composite ceramic Er:YAG laser at 1645 nm. Opt. Lett., 2010, 35: 922. [15] Tang F, Cao Y G, Huang J G, Liu H, Guo W, Wang W C. Fabrication and laser behavior of composite Yb:YAG ceramic. J. Am. Ceram. Soc., 2012, 95: 56. [16] Tang F, Cao Y G, Huang J Q, Guo W, Liu H G, Wang W C, Huang Q F, Li J T. Diode-pumped multilayer Yb:YAG composite ceramic laser. Laser Phys. Lett., 2012, 9: 564. [17] Tang F, Cao Y G, Huang J Q, Guo W, Liu H G, Huang Q H, Wang W C. Multilayer YAG/Re:YAG/YAG laser ceramic prepared by tape casting and vacuum sintering method. J. Eur. Ceram. Soc., 2012, 32: 3995.