Gelcasting of Aluminum Nitride Ceramics

Jianfeng Xue

Graduate School of Chinese Academy of Science, Beijing 100039, China

Manjiang Dong, Jun Li, Guohong Zhou, and Shiwei Wangw

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Aluminum nitride (AlN) ceramics has been prepared by non-aqueous gelcasting in air and sintering in nitrogen atmosphere.The influence of pH and dispersant on the f potential of AlN–ethanol slurry was studied. And the rheological behavior of theslurry was evaluated to optimize the dispersant content and solidloading. Concentrated AlN slurry (53.8 vol%) with low viscositywas prepared using polyethyleneimine as the dispersant. Sorbitolpolyglycidyl ether and tetraethylenepentamine were used as gel-ling and hardening agents, respectively. A green body with arelative density of 63.3% was obtained. After sintering at18001C for 4 h, AlN ceramics with a relative density of 99.6%and a thermal conductivity of 200 W . (m .K)

�1were produced.

I. Introduction

ALUMINUM NITRIDE (AlN) is one of the typical ceramics thathave special properties such as high thermal conductivity

(eight to 10 times that of Al2O3), low dielectric coefficient (about8.1 at 1MHz), high electrical resistance, and a thermal expansioncoefficient matched with silicon.1 Because of these advantageousproperties, it is used in various engineering applications and hasattracted intense interest of researchers. However, as ceramicsare introduced in new areas and applications, the requirements ofthe materials such as performance, reliability, shape, and dimen-sional tolerances are continuously increasing. This means thatseveral forming techniques traditionally used do not meet theserequirements, especially in producing complex shapes.2 Gelcast-ing is an attractive forming process for fabricating complex-shaped ceramic parts, which was first developed by Janney andcolleagues.3–5 Gelcast green bodies have high homogeneity andmechanical strength. It has been applied to various ceramic ma-terials in the past decade, such as alumina,6 silicon nitride,7 zir-conia,8 and silicon carbide.9 But the preparation of AlN bygelcasting has not been reported. One possible reason is that AlNis sensitive to aqueous gelcasting. Another reason is that thepresent gelcasting process is somehow complicated because theoxygen in air should be isolated during gelcasting.

Recently, Mao and colleagues developed a new gelcastingsystem with epoxy resin as a gelling agent.10,11 Polymerization ofsuch a system is a nucleophilic addition reaction instead of a freeradical reaction. Not only does the new system avoid problemswith inhibition of gelation due to oxygen in the air but also itdoes not cause problems due to inhibition of gelation by themold materials.

The aim of this work was to investigate the nonaqueous gel-casting of AlN ceramics with epoxy resin as a gelling agent. Therheological behavior of ceramic slurry, mechanical and thermalproperties, and microstructure of gelcast AlN ceramics werepresented.

II. Experimental Procedure

(1) Materials and Procedures

A commercial AlN powder (Grade F, Tokuyama Soda Co. Ltd.,Tokuyama, Japan) was used as the raw material, with an averageparticle size of 1.2 mm. Slurries with different solid loadings (48.4–55.4 vol%) were prepared by ball milling the AlN powder with anethanol premix solution containing 15 wt% sorbitol polyglycidylether (SPGE) (EX614-B, Nagase Chemtex, Osaka, Japan). Poly-ethyleneimine (PEI) (50% in water, Tokyo Kasei Kogyo Co.Ltd., Tokyo, Japan) was added as a dispersant. One milliliter oftetraethylenepentamine based on 3 g SPGE was added to theslurries as a hardener. The slurries were degassed in a vacuumchamber to reduce gas bubbles before casting to a rubber mold.After consolidation and demolding, the wet green bodies weregradually dried in an air oven at 401C for several hours. Binderburnout was operated in a muffle furnace in air at 6001C for 2 h,at a heating rate of 1.51C/min. Sintering was carried out at18001C for 4 h in a graphite tube furnace in nitrogen atmosphere.

(2) Measurements

AlN is easy to react with water. Therefore, the AlN powder wassuspended not in water but in ethanol. Zeta potential of theslurries was measured by a Zetaplus analyzer (Zetaplus, Brook-haven Instruments Corp, NY). Rheological properties of theslurries were determined by a rotational rheometer (SR5, Rheo-metric Scientific, Piscataway, NY) at a shear rate ranging from0.1 to 1000 s�1. Pore-size distribution of the green body wasevaluated by mercury porosimetry (PoreSizer 9320, Micromeri-tics Instrument Corp, Norcross, GA). Microstructures of thegreen bodies and sintered ceramics were observed by scanningelectron microscopy (SEM) (JSM-6390, JEOL, Tokyo, Japan).Thermal conductivity of sintered ceramics (3 mm thick) wasmeasured through the laser flash technique by a thermal con-stant analyzer (LFA427, Netzsch, Selb, Germany).

Here, the definition of pH in nonaqueous media is addressed.The theoretical background and method involving ‘‘operationalpH value’’ (O.pH) was described by Bates et al.12 They ex-plained that in organic medium the O.pH differs from the actualpH (paH) by the residual liquid-junction potential (DEj):

pHpaH ¼DEj

0:05916ðat 25�CÞ (1)

For the ethanol suspension, DEj/0.059165�2.91.12 For con-venience, the so-called O.pH was directly determined using a pHmeter (Leici PHSj-4a, Shanghai, China).

G. Franks—contributing editor

This work was supported by the Knowledge Innovation Program of the Chinese Acad-emy of Sciences (No.SCX200706).

wAuthor to whom correspondence should be addressed. e-mail: swwang51@mail.sic.ac.cn

Manuscript No. 26451. Received July 13, 2009; approved October 15, 2009.

Journal

J. Am. Ceram. Soc., 93 [4] 928–930 (2010)

DOI: 10.1111/j.1551-2916.2009.03489.x

r 2009 The American Ceramic Society

928

III. Results and Discussion

(1) Zeta Potential

Figure 1 shows the z potential of AlN slurries with and withoutdispersant at different O.pH values. It can be seen that the ad-dition of 1 wt% PEI shifted z value of the slurries positively. Forexample, the system without PEI has a z of �30 mv at aboutO.pH5 8.3 and adding 1 wt% PEI resulted in a z of 40 mv atabout O.pH5 8.5. Meanwhile, the isoelectric point (IEP) shiftedfrom 5.9 to 11.7. The change of z and IEP shift were attributedto the absorption of the positively charged PEI on the surface ofthe AlN particles. On the other hand, the z value showed littlechange over a wide O.pH range, from O.pH5 7 to 10 and theoriginal O.pH of AlN slurry was about 8.1, suggesting that ad-justment of O.pH value was not necessary in the present exper-iment.

(2) Slurries Viscosity

Generally, a key factor for the successful production of ceramicsby gelcasting technique is producing a flowable and stable slurrywith as high solid loading as possible (at least 50 vol%).WithoutPEI dispersant, the AlN slurry shows a high viscosity even at alower solid loading (48.4 vol%), which is not suitable for cast-ing. In order to achieve a AlN slurry with high solid loading andlow viscosity, PEI was selected as the dispersant for the AlNslurry. The effect of PEI addition on the viscosity of the slurry isshown in Fig. 2. The addition of 0.18 wt% PEI (by weight of

AlN) can greatly decrease the viscosity, indicating that PEI is avery effective dispersant for AlN. With the increase of PEI con-tent, the viscosity decreased. When PEI content was 0.23 and0.25 wt%, the viscosities of the two slurries were almost thesame.

The rheological curve of slurries with different solid loadingsis shown in Fig. 3. All these slurries were prepared by adding thesame amount of PEI 0.23 wt% as dispersant. It can be seen fromthe curves that all slurries show shear thinning behavior, and theviscosity increases with the increase of solid loading. For theslurry with a solid loading up to 53.8 vol%, the viscosity at 100s�1 is about 0.6 Pa � s, which is suitable for both casting and for ahigh density of green body.

(3) AlN Ceramics

Figure 4 shows the pore-size distribution of the green body.According to Krell and Klimke,13 the pore-size distribution canreveal the existing state of pores . It can be seen that there wereno large pores in the green body and the slope at the center ofthe pore-size distribution was very steep, indicating that thepore-size distribution was narrow. The green body without ca-lcining exhibits a tail of small pores (0.1 mm) and the tail of smallpores disappeared after calcining at 6001C for 2 h. This is animportant indicator of a homogeneous coordination of the par-ticles. From Fig. 5(a), we can see that the AlN particles were

Fig. 1. Zeta potential of the aluminum nitride (AlN) suspension withand without polyethyleneimine (PEI) addition in ethanol.

Fig. 2. Influence of dispersant contents on the viscosity of aluminumnitride (AlN) slurry with a solid loading of 48.4 vol%.

Fig. 3. Viscosity versus shear rate for slurries with different solid load-ings (polyethyleneimine [PEI]: 0.23 wt%; sorbitol polyglycidyl ether[SPGE]: 15 wt%).

Fig. 4. Pore-size distribution of aluminum nitride (AlN) green bodyfrom a slurry of 53.8 vol% solid loading.

April 2010 Rapid Communications of the American Ceramic Society 929

packed homogeneously and no large defects were found in thegreen body under this magnification level. This result is in agree-ment with that of the pore-size distribution (Fig. 4). The greendensities of the formed bodies rose as the solid loading of theslurries increased. When the solid loading increased from 48.4 to53.8 vol%, the relative density of green bodies were from 62.4%to 63.3% (theoretical density of AlN: 3.31 g/cm3). When thegreen body with a relative density of 63.3% was sintered at18001C for 4 h in nitrogen atmosphere, the sintered bodyreached a theoretical density of 99.6%. The flexural strengthand the thermal conductivity of the sintered body was 350740MPa and 200 W � (m �K)�1, respectively. Fracture surface of thesintered ceramics (Fig. 5(b)) showed a small amount of poreswith a diameter of approximately 0.1 mm at the triple points anda few within the grains. However, the residual pores may beeliminated if we modify the slurry and the sintering process. Theremoval of small pores will be an advantage for the enhance-ment of the thermal conductivity of AlN ceramics.

We attempted to prepare AlN parts with a complicated shapeusing the above slurry. Figure 6 shows some complex-shapedAlN green bodies consolidated by gelcasting and the sinteredceramics. It shows that gelcasting in air is a convenient andpractical way to fabricate AlN ceramics with complex shape.

IV. Conclusion

Nonaqueous gelcasting was developed to prepare AlN ceramicsusing a kind of epoxy resin (SPGE) as a gelling agent and tetra-ethylenepentamine as hardener. The AlN slurry with a solidloading of 53.8 vol% was obtained for gelcasting using 0.23wt% PEI as the dispersant. Pore-size distribution and SEM ob-servation revealed the homogeneity of particle packing in thegelcast green body. After sintering at 18001C for 4 h in nitrogenatmosphere, the green body with a relative density of 63.3% re-

sulted in nearly full densed AlN ceramics. The flexural strengthand thermal conductivity of the ceramics were 350MPa and 200W � (m �K)�1, respectively.

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Fig. 5. Scanning electron microscopy photographs of the fracture sur-face of the (a) green body and (b) sintered ceramics.

Fig. 6. (a) Aluminum nitride (AlN) green bodies made by gelcastingprocess and (b) sintered ceramics.

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