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Characteristic and dispersion of a treated AlN powder in aqueous solvent
Materials Science and Engineering A368 (2004) 126–130
Characteristic and dispersion of a treated AlN powder in aqueous solvent Xiao-Jun Luo, Xin-Rui Xu, Bao-Lin Zhang∗ , Wen-Lan Li, Han-Rui Zhuang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 21 July 2003; received in revised form 7 October 2003
Abstract Characteristic and dispersion of the aluminum nitride (AlN) treated by phosphoric acid were studied in aqueous solvent. The treated AlN powder displayed a good stabilization in aqueous ball-milling media. After 72 h ball-milling, the oxygen content of AlN powder with DP270, NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA and DP270. These two dispersants were proved to be effective dispersants by the apparent viscosity and sedimentation tests and could play a dual role in preparation of aqueous AlN suspensions. A dispersant is needed in aqueous suspensions of AlN powder treated by phosphoric acid. © 2003 Elsevier B.V. All rights reserved. Keywords: AlN; Ball-milling; Aqueous characteristic; Dispersion
1. Introduction Aluminum nitride (AlN) has greatly been noted in the area of high density package due to its attractive properties [1–3]. However, aluminum nitride is very easy to react with water, which restricts the development of AlN aqueous processing. The mechanism and kinetics of AlN hydrolysis have been illustrated in many researches [4–8]. In order to limit AlN hydrolysis, both organic long-chain molecules and inorganic acid have been used as a surface-modifier of AlN [9–11]. Thus, the aqueous processing of AlN has been possible, but little information is available for the characteristic of AlN powder in aqueous ball-milling media, which is important for AlN aqueous processing, such as aqueous tape casting and aqueous slip casting. Dispersion of slurries is very important in its preparation processing because it determines the structure and various characteristics of the green sheet, and furthermore, influences the sintering behavior and the final properties. In order to attain a well dispersed stable suspension, an
∗ Corresponding author. Tel.: +86-21-52414322; fax: +86-21-52413903. E-mail address: [email protected] (B.-L. Zhang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.274
efficient dispersant is usually introduced to coat the ceramics particles and keeps them in a stable suspension in the slurries. The mechanisms of powder particle dispersion in suspensions have been widely discussed in many reports and reviews [12–14]. The deflocculation and stabilization behavior of powder particle dispersions in a liquid medium, either by a long chain polymer (steric separation) or by polyelectrolytes (electrostatic separation) onto the particle surface, have been established recently [15,16]. However, different surface-treated methods of AlN can result in a different coater onto AlN powder. It makes AlN powder dispersion in aqueous media complicated. Hotza [17] reported that there was no need for a dispersant in aqueous suspensions of AlN powder treated by stearic acid. It is interesting to evaluate that a dispersant is necessary or unnecessary for AlN powder treated by other method. In this work, AlN powders were anti-hydrolysis treated by phosphoric acid and its characteristic were investigated in aqueous ball-milling media. The characteristic include pH value and oxygen content. Zeta potentials were also determined. Two dispersants NH4 PAA and DP270 were employed in aqueous AlN suspensions. Viscosity and sedimentation methods were used to evaluate whether a dispersant is needed or not for aqueous suspensions of AlN powder treated by phosphoric acid.
X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130
2. Experimental procedure
127
11.5 11.0
2.1. Materials
10.5
non-treated AlN Powder Treated AlN Powder
10.0
2.2. Characterization of the treated AlN in aqueous ball-milling media Three AlN slurries (20 g AlN powder + 20 g deionized water + 40 g AlN ball) were prepared by the method of ball-milling in polyethylene jar. One was not with dispersant. NH4 PAA and DP270 were, respectively, added to the other two slurries as controlled test. The pH value was tested every 12 h. After different ball-milling time, the slurries were centrifuged, and the sedimentation powders were then dried at microwave oven for phase composition analysis and oxygen content analysis (TCH600, LECO Co.).
9.5
pH
AlN powder used in this study was synthesized by self-propagation high temperature synthesis method and anti-hydrolysis treated simultaneously by phosphoric acid. The average diameter of the powder is 0.5 m. The content of oxygen and nitrogen are 1.44 and 32.31 wt.%, respectively. Two types of commercially available dispersants, NH4 PAA (Tianjin Institute of Chemical Industry, Tianjin, China) and DP270 (a kind of ester of polyacrylate, provided by Rhodia Co., France), were used to prepare AlN aqueous suspensions. NH4 PAA and DP270 contain 30 and 43 wt.% of the active components, respectively. The contents of dispersants in this experiment were expressed in wt.% with respect to the AlN powder. The deionized water (pH 5.60–6.0) was used as solvent.
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 0.1
1
10
Time (hour)
Fig. 1. pH values of different AlN suspensions as a function of time at 80 ◦ C.
3. Results and discussion 3.1. Characteristic of the treated AlN in aqueous ball-milling media The reaction between AlN and water precludes costeffective aqueous processing techniques. Bowen et al. [18] studied the mechanisms and kinetics of AlN hydrolysis more thoroughly and the following reaction scheme was proposed: AlN + 2H2 O → AlOOH (amorphous) + NH3 NH3 + H2 O ↔ NH4 + + OH−
2.3. Zeta potential test
AlOOH (amorphous) + H2 O → Al(OH)3
Zeta potential of suspensions was measured by Zetaplus analyzer (Brookhaven Instruments Corp.). Suspensions were prepared by dispersing AlN powders (0.01 vol.%) in 0.001 mol/l NaCl, which was used to adjust ionic strength. pH value was adjusted by adding HCl (0.01, 0.1 and 1 M) or NaOH (0.01, 0.1 and 1 M). The suspensions were mixed ultrasonically before the tests to ensure that only the mobility of single particle was measured. 2.4. Apparent viscosity measurements and sedimentation test
The reaction kinetics was described by an unreacted core model. In order to limit the hydrolysis reaction of AlN, the AlN powder used in this study was anti-hydrolysis treated by phosphoric acid. Fig. 1 plots the pH value of different AlN suspensions as a function of time at 80 ◦ C. It indicated that AlN powder treated in phosphoric acid showed good protection ability and almost no hydrolysis took place. The prevention from water was attributed to the formation of impermeable, water-insoluble phosphate on the AlN surface. The mechanism of phosphate layer formation could be possibly presented as follows:
Seventy-five grams of AlN powder, 25 g deionized water and various amounts of dispersants were mixed with 150 g AlN ball for 6 h. Then, the rotating cylinder viscometer (model NDJ-7, Shanghai Balance Instrument Plant) was used to measure the apparent viscosity of the suspensions. Sedimentation tests were made on suspensions of 20 wt.% AlN in the absence and presence of dispersants. The suspensions were ultrasonicated for 10 min and settled in 10 ml grade tube. The readings of sedimentation volume were recorded after 3 weeks.
which was similar to that of silicic acid onto AlN powder [19]. In Fig. 2, pH variation of three aqueous AlN suspensions as a function of time is presented. It can be seen that pH values of three aqueous suspensions gradually ascend with the increase of ball-milling time. The phenomena are attributed
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X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130 12
AlN
milled without dispersant milled with 0.6wt% NH4PAA
11
milled with 0.4wt% DP270
10
(a)
pH
9 8 7
(b) 6 5 4 0
10
20
30
40
50
60
70
80
(c)
Time (hours)
to hydrolysis of the treated AlN. Milling ball could destroy the coated of AlN powder, and thus new surface was exposed to water. Accordingly, AlN hydrolysis became easier. The suspension without dispersant exhibited a notable rise in pH value, increasing from the initial 5.96–9.92. In contrast, the suspension retaining 0.6 wt.% NH4 PAA showed a smaller increase in pH. Its final pH value was 8.88, which was one pH unit lower than that of the suspension without dispersant. The smaller increase of pH value in the presence of the dispersant was due to the limited hydrolysis of the AlN powder. The destroyed surface quickly absorbed the long-chain polymer or the dissociated ionic of the dispersant to form steric or electrostatic coats, and effectually prevent continual hydrolysis of the AlN powder. In absence of dispersant, the destroyed surface was easy to react with water and a greater pH increase was observed. Table 1 shows the oxygen content of AlN powders with and without dispersants after different ball-milling time. After 72 h ball-milling in aqueous media, the oxygen content of AlN powder with 0.4 wt.% DP270, 0.6 wt.% NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The oxygen content of the treated AlN powder with a dispersant is lower than that of the treated AlN without a dispersant. It indicated that the existence of DP270 and NH4 PAA could limit continual hydrolysis of AlN powder. It agrees well with the results of pH tests.
Table 1 Oxygen content of the treated AlN powder after ball-milling in aqueous media
20
30
40
50
60
70
2θ (Degree) Fig. 3. XRD patterns for AlN with DP270, NH4 PAA and without dispersant after 72 h ball-milling: (a) without dispersant; (b) with 0.6 wt.% NH4 PAA; (c) with 0.4 wt.% DP270.
It was noted that no ammonia was smelled though the pH values of suspensions had a notable increase. X-ray patterns of AlN powders after 72 h ball-milling were shown in Fig. 3. No other crystalline phase was observed except AlN. AlN peak intensity had no significant reduction no matter what dispersants exist or not. These results indicated the treated AlN displayed a good stabilization in aqueous ball-milling media. 3.2. Zeta potential Fig. 4 shows the zeta potential of AlN suspensions with and without dispersant at different pH value. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA, and with DP270. Obviously, dispersants made the IEP of AlN shift to the
30 20
(without dispersant) (with NH4PAA)
10
(with DP270)
0
zeta potential
Fig. 2. pH changes of three aqueous AlN suspensions with ball-milling time.
-10 -20 -30 -40
Ball-milling time
-50
0h
24 h
48 h
72 h -60
Oxygen content (wt.%) Without dispersant With 0.4 wt.% DP270 With 0.6 wt.% NH4 PAA
1.44 1.44 1.44
1.57 1.52 1.54
1.69 1.59 1.64
2.08 1.84 1.92
2
4
6
8
10
12
pH
Fig. 4. The relationship between the pH value and the zeta potential.
X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130
3.3. Apparent viscosity and sedimentation
2.0
1.8
Sediment Volume (mL)
smaller pH value. Meantime, the absolute values of zeta potential notably elevated. It could be explained by the chemical nature of dispersants. Both NH4 PAA and DP270 can dissociate in water when pH >3 (according to the supplier). The dissociated R–COO− could easily adsorb onto the surface of AlN and make the AlN load the negative charge. NH4 PAA and DP270 made the surface of AlN powder produce a high surface-charged density, which made the double electric layer become strong. Due to the formation of impermeable, water-insoluble phosphate, not Al2 O3 on AlN powder surface, the zeta potential of the treated AIN without dispersant is different from that of Al2 O3 [20].
129
(NH4PAA) (DP270) 1.6
1.4
1.2
1.0
0.8 0.0
0.2
0.4
0.6
0.8
1.0
Dispersant Concentration (wt%)
Fig. 5 plots viscosities of suspensions (75 wt.%) with different dispersants at 350 s−1 as a function of dispersant content. pH values was about 6.38–6.58. It can be seen that high viscosity suspensions were obtained at low and high dispersant content. At 0.4 wt.% dispersant content for DP270 and 0.6 wt.% for NH4 PAA, the viscosity of suspensions reached minimum 16 and 10 mPa s, respectively. The reduction in viscosity was well explained by electro-steric mechanism in many works [12,15,21]. The adsorption of NH4 PAA and DP270 onto AlN particles made surface of particles produce large magnitude double layer repulsion, which efficiently screened the van der Walls force between particles. Moreover, the high molecular polymer produced some steric stability. Consequently, the viscosity of suspensions gradually decreased. When the surface charge of particles reached the saturation, the minimum viscosity was achieved. With the further increasing dispersant, higher amount of free polyelectrolyte remaining in the solution caused an increase in the ionic strength of the solution and functioned as screening electrolyte to reduce the range and strength of the double layer repulsion. Thus, the double layer compression happened and the attraction between particles was significant. The particles tended to form floc with 50
with NH4PAA
45
with DP270
40
Viscosity (mPa.s)
35 30
Fig. 6. Sediment volume of AlN suspensions vs. concentration of NH4 PAA and DP270.
some volume of deionized water immobilized. Therefore, the suspensions showed higher viscosities. Fig. 6 describes the sediment volume of AlN suspensions versus the concentration of NH4 PAA and DP270 after resting for 3 weeks. The effects of either flocculated or well-dispersed suspensions on the sedimentation and consolidation behavior of suspensions were discussed in details in many literatures [22,23]. As Fig. 6 shows, the sediment volume reached a minimum at 0.6 wt.% for NH4 PAA and 0.4 wt.% for DP270. It indicated that the aqueous AlN suspensions with 0.6 wt.% NH4 PAA and 0.4 wt.% DP270 stayed in a well-dispersing state. This result agreed well with that of viscosity test though there were different solid loading involved in the sedimentation and apparent viscosity experiments. Apparent viscosity and sedimentation tests verified that both DP270 and NH4 PAA are efficient dispersants for AlN powder treated by phosphoric acid. It could not coincide with Hotza’s result [17], possibly because the anti-hydrolysis treatment processing of AlN powders were different. The AlN powder used in Hotza’s study was treated by stearic acid, which has long hydrophobic groups and could produce some steric stability. However, for AlN powder treated by phosphoric acid, the dispersant DP270 and NH4 PAA could play a dual role. It not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder.
25 20
4. Conclusions 15 10 5 0.0
0.2
0.4
0.6
0.8
1.0
Dispersant Amount (wt%)
Fig. 5. Apparent viscosity of AlN suspensions as a function of dispersant concentration.
The characteristic of the treated AlN were studied in aqueous ball-milling media. Only AlN crystalline phase was found though the pH value of suspensions changed greatly. The increase in the final pH of AlN suspension with dispersants was smaller than that of suspension without dispersant. After 72 h ball-milling, the oxygen content of
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AlN powder with 0.4 wt.% DP270, 0.6 wt.% NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA and DP270. The dispersants not only made the isoelectric point shift to smaller pH value, but also made the absolute values of zeta potential become higher. Two commercial dispersants NH4 PAA and DP270 were demonstrated to be effective dispersants by the apparent viscosity and sedimentation studies and could play a dual role in preparation of aqueous AlN suspensions. A dispersant was needed for an aqueous suspension of AlN powder treated by phosphoric acid.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Acknowledgements The authors would like to thank the National Natural Science Foundation of China for providing financial support ((No. 69836030).
References
[15] [16] [17] [18] [19] [20] [21]
[1] L.M. Sheppard, Am. Ceram. Soc. Bull. 69 (1990) 1801. [2] R.R. Tummala, Am. Ceram. Soc. Bull. 67 (1988) 752. [3] K. Niwa, E. Horikoshi, Y. Imanaka, in: J.-H. Jean, T.K. Gupta, K.M. Nair, K. Niwa (Eds.), Multilayer Electronic Ceramic Devices, Ceram
[22] [23]
Transaction, vol. 97, The America Ceramic Society, 735 Ceramic Place, Westerville, Ohio, 1998, pp. 171–182. P. Bowen, J.G. Highfeild, A. Mocellin, T.A. Ring, J. Am, Ceram. Soc. 73 (1990) 724. C.-D. Young, J.-G. Duh, J. Mater. Sci. 30 (1995) 185. Y. Zhang, J.G.P. Binner, Int. J. Inorg. Mater. 1 (1999) 219. L.M. Svedberg, K.C. Arndt, M.J. Cima, J. Am. Ceram. Soc. 83 (2000) 41. S. Fukumoto, T. Hookabe, H. Tsubakino, J. Mater. Sci. 35 (2000) 2743. U. Masatoshi, H. Yoshiki, Y. Takamasa, United States Patent: 4,923,689 (1990). D. Hotza, O. Sahling, P. Greil, J. Mater. Sic. 20 (1995) 127. R. Metselaar, R. Reenis, M. Chen, H. Gorter, H.T. Hintzen, J. Eur. Ceram. Soc. 15 (1995) 1079. R.G. Horn, J. Am. Ceram. Soc. 73 (1990) 1117. R. Moreno, Am. Ceram. Soc. Bull. 71 (1992) 1521. A.U. Khan, P.F. Luckham, S. Manimaaran, J. Mater. Chem. 7 (1997) 1849. F.F. Lange, J. Am. Ceram. Soc. 72 (1989) 3. G.V. Franks, F.F. Lange, J. Am. Ceram. Soc. 79 (1996) 3161. D. Hotza, Dachamir, Fortschr.-Ber. VDI, Reihe 5 (1996) 4401 (in German). P. Bowen, J.G. Highfield, A. Mocellin, T.A. Ring, J. Am. Ceram. Soc. 73 (1990) 724. K. Krnel, T. Kosmaˇc, J. Am. Ceram. Soc. 83 (2000) 1375. Z. Yuping, J. Dongliang, P. Greil, J. Eur. Ceram. Soc. 20 (2000) 1691. L. Bergstrom, Surface and Colloid Chemistry in Advance Ceramic Processing, Surface Science Serials, vol. 51, Marcel Dekker, New York, 1994. F.F. Lange, K.T. Miller, Am. Ceram. Soc. Bull. 66 (1987) 1498. G.V. Franks, F.F. Lange, J. Am. Ceram. Soc. 79 (1996) 3161.
Characteristic and dispersion of a treated AlN powder in aqueous solvent Xiao-Jun Luo, Xin-Rui Xu, Bao-Lin Zhang∗ , Wen-Lan Li, Han-Rui Zhuang Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Received 21 July 2003; received in revised form 7 October 2003
Abstract Characteristic and dispersion of the aluminum nitride (AlN) treated by phosphoric acid were studied in aqueous solvent. The treated AlN powder displayed a good stabilization in aqueous ball-milling media. After 72 h ball-milling, the oxygen content of AlN powder with DP270, NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA and DP270. These two dispersants were proved to be effective dispersants by the apparent viscosity and sedimentation tests and could play a dual role in preparation of aqueous AlN suspensions. A dispersant is needed in aqueous suspensions of AlN powder treated by phosphoric acid. © 2003 Elsevier B.V. All rights reserved. Keywords: AlN; Ball-milling; Aqueous characteristic; Dispersion
1. Introduction Aluminum nitride (AlN) has greatly been noted in the area of high density package due to its attractive properties [1–3]. However, aluminum nitride is very easy to react with water, which restricts the development of AlN aqueous processing. The mechanism and kinetics of AlN hydrolysis have been illustrated in many researches [4–8]. In order to limit AlN hydrolysis, both organic long-chain molecules and inorganic acid have been used as a surface-modifier of AlN [9–11]. Thus, the aqueous processing of AlN has been possible, but little information is available for the characteristic of AlN powder in aqueous ball-milling media, which is important for AlN aqueous processing, such as aqueous tape casting and aqueous slip casting. Dispersion of slurries is very important in its preparation processing because it determines the structure and various characteristics of the green sheet, and furthermore, influences the sintering behavior and the final properties. In order to attain a well dispersed stable suspension, an
∗ Corresponding author. Tel.: +86-21-52414322; fax: +86-21-52413903. E-mail address: [email protected] (B.-L. Zhang).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.274
efficient dispersant is usually introduced to coat the ceramics particles and keeps them in a stable suspension in the slurries. The mechanisms of powder particle dispersion in suspensions have been widely discussed in many reports and reviews [12–14]. The deflocculation and stabilization behavior of powder particle dispersions in a liquid medium, either by a long chain polymer (steric separation) or by polyelectrolytes (electrostatic separation) onto the particle surface, have been established recently [15,16]. However, different surface-treated methods of AlN can result in a different coater onto AlN powder. It makes AlN powder dispersion in aqueous media complicated. Hotza [17] reported that there was no need for a dispersant in aqueous suspensions of AlN powder treated by stearic acid. It is interesting to evaluate that a dispersant is necessary or unnecessary for AlN powder treated by other method. In this work, AlN powders were anti-hydrolysis treated by phosphoric acid and its characteristic were investigated in aqueous ball-milling media. The characteristic include pH value and oxygen content. Zeta potentials were also determined. Two dispersants NH4 PAA and DP270 were employed in aqueous AlN suspensions. Viscosity and sedimentation methods were used to evaluate whether a dispersant is needed or not for aqueous suspensions of AlN powder treated by phosphoric acid.
X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130
2. Experimental procedure
127
11.5 11.0
2.1. Materials
10.5
non-treated AlN Powder Treated AlN Powder
10.0
2.2. Characterization of the treated AlN in aqueous ball-milling media Three AlN slurries (20 g AlN powder + 20 g deionized water + 40 g AlN ball) were prepared by the method of ball-milling in polyethylene jar. One was not with dispersant. NH4 PAA and DP270 were, respectively, added to the other two slurries as controlled test. The pH value was tested every 12 h. After different ball-milling time, the slurries were centrifuged, and the sedimentation powders were then dried at microwave oven for phase composition analysis and oxygen content analysis (TCH600, LECO Co.).
9.5
pH
AlN powder used in this study was synthesized by self-propagation high temperature synthesis method and anti-hydrolysis treated simultaneously by phosphoric acid. The average diameter of the powder is 0.5 m. The content of oxygen and nitrogen are 1.44 and 32.31 wt.%, respectively. Two types of commercially available dispersants, NH4 PAA (Tianjin Institute of Chemical Industry, Tianjin, China) and DP270 (a kind of ester of polyacrylate, provided by Rhodia Co., France), were used to prepare AlN aqueous suspensions. NH4 PAA and DP270 contain 30 and 43 wt.% of the active components, respectively. The contents of dispersants in this experiment were expressed in wt.% with respect to the AlN powder. The deionized water (pH 5.60–6.0) was used as solvent.
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 0.1
1
10
Time (hour)
Fig. 1. pH values of different AlN suspensions as a function of time at 80 ◦ C.
3. Results and discussion 3.1. Characteristic of the treated AlN in aqueous ball-milling media The reaction between AlN and water precludes costeffective aqueous processing techniques. Bowen et al. [18] studied the mechanisms and kinetics of AlN hydrolysis more thoroughly and the following reaction scheme was proposed: AlN + 2H2 O → AlOOH (amorphous) + NH3 NH3 + H2 O ↔ NH4 + + OH−
2.3. Zeta potential test
AlOOH (amorphous) + H2 O → Al(OH)3
Zeta potential of suspensions was measured by Zetaplus analyzer (Brookhaven Instruments Corp.). Suspensions were prepared by dispersing AlN powders (0.01 vol.%) in 0.001 mol/l NaCl, which was used to adjust ionic strength. pH value was adjusted by adding HCl (0.01, 0.1 and 1 M) or NaOH (0.01, 0.1 and 1 M). The suspensions were mixed ultrasonically before the tests to ensure that only the mobility of single particle was measured. 2.4. Apparent viscosity measurements and sedimentation test
The reaction kinetics was described by an unreacted core model. In order to limit the hydrolysis reaction of AlN, the AlN powder used in this study was anti-hydrolysis treated by phosphoric acid. Fig. 1 plots the pH value of different AlN suspensions as a function of time at 80 ◦ C. It indicated that AlN powder treated in phosphoric acid showed good protection ability and almost no hydrolysis took place. The prevention from water was attributed to the formation of impermeable, water-insoluble phosphate on the AlN surface. The mechanism of phosphate layer formation could be possibly presented as follows:
Seventy-five grams of AlN powder, 25 g deionized water and various amounts of dispersants were mixed with 150 g AlN ball for 6 h. Then, the rotating cylinder viscometer (model NDJ-7, Shanghai Balance Instrument Plant) was used to measure the apparent viscosity of the suspensions. Sedimentation tests were made on suspensions of 20 wt.% AlN in the absence and presence of dispersants. The suspensions were ultrasonicated for 10 min and settled in 10 ml grade tube. The readings of sedimentation volume were recorded after 3 weeks.
which was similar to that of silicic acid onto AlN powder [19]. In Fig. 2, pH variation of three aqueous AlN suspensions as a function of time is presented. It can be seen that pH values of three aqueous suspensions gradually ascend with the increase of ball-milling time. The phenomena are attributed
128
X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130 12
AlN
milled without dispersant milled with 0.6wt% NH4PAA
11
milled with 0.4wt% DP270
10
(a)
pH
9 8 7
(b) 6 5 4 0
10
20
30
40
50
60
70
80
(c)
Time (hours)
to hydrolysis of the treated AlN. Milling ball could destroy the coated of AlN powder, and thus new surface was exposed to water. Accordingly, AlN hydrolysis became easier. The suspension without dispersant exhibited a notable rise in pH value, increasing from the initial 5.96–9.92. In contrast, the suspension retaining 0.6 wt.% NH4 PAA showed a smaller increase in pH. Its final pH value was 8.88, which was one pH unit lower than that of the suspension without dispersant. The smaller increase of pH value in the presence of the dispersant was due to the limited hydrolysis of the AlN powder. The destroyed surface quickly absorbed the long-chain polymer or the dissociated ionic of the dispersant to form steric or electrostatic coats, and effectually prevent continual hydrolysis of the AlN powder. In absence of dispersant, the destroyed surface was easy to react with water and a greater pH increase was observed. Table 1 shows the oxygen content of AlN powders with and without dispersants after different ball-milling time. After 72 h ball-milling in aqueous media, the oxygen content of AlN powder with 0.4 wt.% DP270, 0.6 wt.% NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The oxygen content of the treated AlN powder with a dispersant is lower than that of the treated AlN without a dispersant. It indicated that the existence of DP270 and NH4 PAA could limit continual hydrolysis of AlN powder. It agrees well with the results of pH tests.
Table 1 Oxygen content of the treated AlN powder after ball-milling in aqueous media
20
30
40
50
60
70
2θ (Degree) Fig. 3. XRD patterns for AlN with DP270, NH4 PAA and without dispersant after 72 h ball-milling: (a) without dispersant; (b) with 0.6 wt.% NH4 PAA; (c) with 0.4 wt.% DP270.
It was noted that no ammonia was smelled though the pH values of suspensions had a notable increase. X-ray patterns of AlN powders after 72 h ball-milling were shown in Fig. 3. No other crystalline phase was observed except AlN. AlN peak intensity had no significant reduction no matter what dispersants exist or not. These results indicated the treated AlN displayed a good stabilization in aqueous ball-milling media. 3.2. Zeta potential Fig. 4 shows the zeta potential of AlN suspensions with and without dispersant at different pH value. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA, and with DP270. Obviously, dispersants made the IEP of AlN shift to the
30 20
(without dispersant) (with NH4PAA)
10
(with DP270)
0
zeta potential
Fig. 2. pH changes of three aqueous AlN suspensions with ball-milling time.
-10 -20 -30 -40
Ball-milling time
-50
0h
24 h
48 h
72 h -60
Oxygen content (wt.%) Without dispersant With 0.4 wt.% DP270 With 0.6 wt.% NH4 PAA
1.44 1.44 1.44
1.57 1.52 1.54
1.69 1.59 1.64
2.08 1.84 1.92
2
4
6
8
10
12
pH
Fig. 4. The relationship between the pH value and the zeta potential.
X.-J. Luo et al. / Materials Science and Engineering A368 (2004) 126–130
3.3. Apparent viscosity and sedimentation
2.0
1.8
Sediment Volume (mL)
smaller pH value. Meantime, the absolute values of zeta potential notably elevated. It could be explained by the chemical nature of dispersants. Both NH4 PAA and DP270 can dissociate in water when pH >3 (according to the supplier). The dissociated R–COO− could easily adsorb onto the surface of AlN and make the AlN load the negative charge. NH4 PAA and DP270 made the surface of AlN powder produce a high surface-charged density, which made the double electric layer become strong. Due to the formation of impermeable, water-insoluble phosphate, not Al2 O3 on AlN powder surface, the zeta potential of the treated AIN without dispersant is different from that of Al2 O3 [20].
129
(NH4PAA) (DP270) 1.6
1.4
1.2
1.0
0.8 0.0
0.2
0.4
0.6
0.8
1.0
Dispersant Concentration (wt%)
Fig. 5 plots viscosities of suspensions (75 wt.%) with different dispersants at 350 s−1 as a function of dispersant content. pH values was about 6.38–6.58. It can be seen that high viscosity suspensions were obtained at low and high dispersant content. At 0.4 wt.% dispersant content for DP270 and 0.6 wt.% for NH4 PAA, the viscosity of suspensions reached minimum 16 and 10 mPa s, respectively. The reduction in viscosity was well explained by electro-steric mechanism in many works [12,15,21]. The adsorption of NH4 PAA and DP270 onto AlN particles made surface of particles produce large magnitude double layer repulsion, which efficiently screened the van der Walls force between particles. Moreover, the high molecular polymer produced some steric stability. Consequently, the viscosity of suspensions gradually decreased. When the surface charge of particles reached the saturation, the minimum viscosity was achieved. With the further increasing dispersant, higher amount of free polyelectrolyte remaining in the solution caused an increase in the ionic strength of the solution and functioned as screening electrolyte to reduce the range and strength of the double layer repulsion. Thus, the double layer compression happened and the attraction between particles was significant. The particles tended to form floc with 50
with NH4PAA
45
with DP270
40
Viscosity (mPa.s)
35 30
Fig. 6. Sediment volume of AlN suspensions vs. concentration of NH4 PAA and DP270.
some volume of deionized water immobilized. Therefore, the suspensions showed higher viscosities. Fig. 6 describes the sediment volume of AlN suspensions versus the concentration of NH4 PAA and DP270 after resting for 3 weeks. The effects of either flocculated or well-dispersed suspensions on the sedimentation and consolidation behavior of suspensions were discussed in details in many literatures [22,23]. As Fig. 6 shows, the sediment volume reached a minimum at 0.6 wt.% for NH4 PAA and 0.4 wt.% for DP270. It indicated that the aqueous AlN suspensions with 0.6 wt.% NH4 PAA and 0.4 wt.% DP270 stayed in a well-dispersing state. This result agreed well with that of viscosity test though there were different solid loading involved in the sedimentation and apparent viscosity experiments. Apparent viscosity and sedimentation tests verified that both DP270 and NH4 PAA are efficient dispersants for AlN powder treated by phosphoric acid. It could not coincide with Hotza’s result [17], possibly because the anti-hydrolysis treatment processing of AlN powders were different. The AlN powder used in Hotza’s study was treated by stearic acid, which has long hydrophobic groups and could produce some steric stability. However, for AlN powder treated by phosphoric acid, the dispersant DP270 and NH4 PAA could play a dual role. It not only efficiently dispersed AlN powder in water to form a stable suspension, but also formed a coat onto AlN surface to limit hydrolysis of the AlN powder.
25 20
4. Conclusions 15 10 5 0.0
0.2
0.4
0.6
0.8
1.0
Dispersant Amount (wt%)
Fig. 5. Apparent viscosity of AlN suspensions as a function of dispersant concentration.
The characteristic of the treated AlN were studied in aqueous ball-milling media. Only AlN crystalline phase was found though the pH value of suspensions changed greatly. The increase in the final pH of AlN suspension with dispersants was smaller than that of suspension without dispersant. After 72 h ball-milling, the oxygen content of
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AlN powder with 0.4 wt.% DP270, 0.6 wt.% NH4 PAA and without a dispersant were 1.84, 1.92 and 2.08 wt.%, respectively. The isoelectric points were 3.9, 3.5 and 3.35, respectively, for the treated AlN without dispersant, with NH4 PAA and DP270. The dispersants not only made the isoelectric point shift to smaller pH value, but also made the absolute values of zeta potential become higher. Two commercial dispersants NH4 PAA and DP270 were demonstrated to be effective dispersants by the apparent viscosity and sedimentation studies and could play a dual role in preparation of aqueous AlN suspensions. A dispersant was needed for an aqueous suspension of AlN powder treated by phosphoric acid.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Acknowledgements The authors would like to thank the National Natural Science Foundation of China for providing financial support ((No. 69836030).
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