nanostructured materials volume 10 issue 4 1998 [doi 10.1016_s0965-9773(98)00095-6] w.m. zeng; l....

8/13/2019 Nanostructured Materials Volume 10 Issue 4 1998 [Doi 10.1016_s0965-9773(98)00095-6] W.M. Zeng; L. Gao; J.K.

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Pergamon

NanoStnrctured Materials. Vol. 10, No. 4. pp. 543-550.1998Elsevier Science Ltd

0 1998 Acta Metdlurnica Inc.Printed in he USA. All rights &rwd

0965-9773198 19.00 + .OO

PI1 SO9659773 98)00095-6

A NEW SOL-GEL ROUTE USIN G INORGANI C SALT FORSYNTHESIZING AI,O, NANOPOWDERS

W.M. Zeng, L. Gao, and J .K. Guo

State Key Lab on High Performance Ceramics and Superfine Microstructure Studies,Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,

Peoples Republic of China

A ccept ed M ar ch 261998)

Abstr aat - Boehmit e pAl OO H) powder was first prepared by sol-f reeze dry i ng met hodusing i nexpensiv e A1Clj .6H 20 as raw mat eri al , and t hen yA l Z03 nanopow der w i t h averagedi amet er of 6 nm and a-A l ,O, nanopow der w i t h mean diameter of 30 nm w ere obtai ned by thecalcinat ion oj he above pA100H pow der at 500 and l l O O C, respect i vel y. Potenti al- pHdiagramsofAl-HZ0 system at 25 C nd 90 c w ere obtai ned, separat el y. Thermody nami c anal ysi s

of t he mai n react i ons i n he process of t herm al decomposi t i on of A I w as al so made usingTemk i n-Schw ~rr zmansmet hodandbymeansofregressionanal ysi s.I nvest i gati onsoft hepot ent i al -pH diagrams of Al -H O syst em and the t hernw dynami c analysis of thermal decomposi t i on ofA 1 O H )3prov i de a guide to he ormat i on of boehmi t e sol and heat reat ment of boehmite powderrespecti vely . @1998 Acta M et al l urgica Inc.

INTRODUCTION

A&O, nanopowderis an important superfine ceramic powder with many excellent properties,and it can be ,widely used in the fields of electronics, metallurgy, fine ceramics, and compositematerials et al. As to preparing nanoscale ceramic powder, recently, a lot of approaches andprocesses have been developed, such as coprecipitation, heterogeneous azeotropic distillation,hydrothermal., thermal spraying, laser induced chemical vapour deposition (LICVD), and sol-gelprocess (1). Varma et al . prepared a spherical powder with the average diameter of 2pm by sol-spray drying technique using Al(NO,), as raw materials (2). Up to now, most of Al,O,nanopowders were prepared by sol-gel process using expensive aluminum alkoxides as rawmaterials (3). In this work, boehmite (y-AlOOH) powder was first prepared by sol-freeze dryingmethod using cheapAlC1,.6H,O as raw materials, and then y-A&O, nanopowder with the averagediameter of 6 nm and o-A1203 nanopowder with the mean diameter of 30 nm were obtained by

thecalcinationoftheabove y-AlOOHpowder at 500C and 1 OOC,respectively. Thermodynamicsin the process of preparation was also investigated.

543


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544 WM ZENG L GAO ANDJK Guo

POTENTIAL-pH DIAGRAMS OF Al-H,0 SYSTEM

Hydrolysis conditions of aluminum inorganic salt must be strictly controlled in order toobtain a high quality beohmite sol, while potential-pH diagrams of Al-H,0 system can provide acertain guide for determination of the hydrolysis conditions.

Few studies have been reported on potential-pH diagrams of Al-H,0 system at ambienttemperature, especially at high temperatures for lack of thermodynamic data of some species.Therefore, the known thermodynamic data for Al-H,O system have been summarized, evaluatedand calculated in the present work, and using these data, potential-pH diagrams of Al-H,0 systemat 25C and 90C have been presented, respectively (see Figures 1 and 2) (4). The number of eachline in Figures 1 and 2 responds to one of the various equilibrium reactions in Table 1.

It can be seen from Figures 1 and 2 that the hydrolysis of A13(aq) produces either

Al(OH),(gibbsite) or AlOOH(boehmite).

THERMODYNAMIC ANALYSIS OF THERMAL DECOMPOSITIONOF Al(OH),

The thermal decomposition process of Al(Ol-l), is very complicated due to manydecomposition andtransformationreactions in its thermal decomposition process.Thermodynamicanalysis of the main reactions in the process of thermal decomposition of Al(OH), can provide acertain guide for the heat treatment of boehmite powder prepared by hydrolysis using aluminum

PH

AlOOH boehmite

P

Figure 1. Potential-pH diagram of Al-H20 Figure 2. Potential-pH diagram of Al-H20

system (responding to reactions 1,2,3,4,8, H,O system (responding to reactions 1,5,6,7,9), solid line - 25C; dashed line - 90C. 8,9), solid line - 25C; dashed line - 90C.


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A NEW SOL-GEL ROUTE USING NOROANIC ALT FOR SYNTHESIZING 1203 POWDERS 545

TABLE 1Main Reactions Between the Species of Al-H,0 System

No. Reaction

1. A13+ + 3e2. Al(OH), + 3H+3. AKOH), + 3H+ + 3e4. Al(OH), + H,O5. AlOOH + 3H+6. AlOOH + 3H+ + 3e

7. AlOOH + 2H,O8. 2H+ + 2e9. l/20, + 2H+ + 2e

Note: Tbe state of the various species is obvious.

= Al= A13 3H,O= Al + 3H,O= Al(OH); + H+= A13+ + 2H,O= Al + 2H,O

= Al(OH)4- + H+= H,= H,O

inorganic salt. In this work, using the values of standard molarenthalpy of formation and standardmolar entropy of Al(OH),(gibbsite), y-AlGGH(boehmite),Y-A1203, cr-A O, and H*O(g), andcombining the relationships between molar heat capacity and temperature of Al(OH),,y-AlGGH,

y-Al,O,, cx-A1203, and H,O(g), the polynomials between standard molar Gibbs energy AG,O(T)and temperature (T) for the main reactions in the process of thermal decomposition of Al(OI-I),were first calculated using Temkin-Schwarzmans method. Then, the polynomials were convertedinto the binomials by means of regression analysis (see Table 2) (5).

TABLE 2Standard Molar Gibbs Energies for the Main Reactions in the Process of Thermal

Decomposition of Al(OH),

AG,(T)Reaction ___________

kJ .moP

(1) AI L(OH) ) = y-AlOOH(s) + H*O(g)

(2) 2yAlOOH (s) = y-Al,O, +HzO(g)

(3) y-Al,O,(s)= a-A1203(s)

(4) 2/3Al(W3(s) = 1/3r-A1@ + 0(g)(5) 2/3A1(OH )3(s) = 1/3a-Al,O, + H,O(g)

48.52 - 0.1535T (298-480K)

93.68 - O.l684T(298-600K)

-19.31 + O.O049T(298-NOOK )

63.57 - O.l585T(298-480K)

57.14 - O.l568T(298-480K)


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546 WM ZENG, L GAO ANO K Guo

-10

-20

-30 I I 1 1 I I *300 400 500 600 700 800 900 1000

T(K)Figure 3. Standard Gibbs energy diagram of main reactions in the process of

thermal decomposition of AI(O

In terms of Table 2, standard Gibbs energy diagram of main reactions in the process of thermaldecomposition of Al(OH), are shown in Figure 3.

From Figure 3 it can be seen boehmite (y-AlGGH) begins to decompose at 283.15C andy-A&O, can transforms into cr.-A1203 t lower temperature.

SYNTH ESIS AND CHARACTERI ZATI ON OF Al,O, NANOPOWDERS

Boehmite was prepared by reaction of aqueous AlCl, solution with NH, solution. Theprecipitated boehmite was aged for 24 h, filtered, and washed with distilled water. Then, it wasmade into a translucent sol by peptizing with acetic acid. The acid addition alone does not causethe peptization; it must be heated and the heat treatment significantly enhances the rate ofpeptization. During the peptization the shape and size of the sol particle change. Boehmite sol wasrapidly frozen by ethanol, and placed in the vacuum container of freeze dryer. On freezing anddrying for 72h, a white boehmite powder was obtained. After boehmite powder was calcined for2h at 500C and 1 100C, y-Al,O, and a-A120, powder was obtained, separately. Characterizationof the powders were made with DTA, XRD, BET, TEM, and centrifugal particle size analyzer.

Figure 4 is DTA curve of AlGGH powder. From DTA curve it can be seen that the only

endothermic peak (2852C) from room temperature to 500C corresponds to the endothermicheats that AlGGH powder decomposed into y-Al,O,. It can be also known from DTA curve that


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A NEW SOL-GEL ROUTE USING NORGANIC ALT FOR SYNTHESIZING I,O, POWOERS 547

0.25.-

0.15-

G9CY 0.09

si

-0.05-

0. 155

0 100 200 300 400 500

-I (C)

Figure 4. DTA curve of y-AlOOH powder.

10 20 30 40 50 60

29 (degree)

Figure 5. X-ray diffraction patterns of Al,O, powdersafter calcination at 500C and 1100C.


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5 8 WM ZENG, L GAO AND JK Guo

Figure 6. TEM photograph of y-Al,O, powder.

Figure 7. TEM photograph of a-Al,O, powder.

boehmite powder began to transform into y-A&O, at 268.7C, which is in agreement with theabove result of thermodynamic analysis (283.15C). The thermal decomposition temperature ofAlOOH powder prepared by this method decreased by about 250C as compared with that ofAlOOH powder synthesized by hydrothermal method (6).

Figure 5 is the phase evolution of AlOOH powder after calcination at 500C and 1100C.From Figure 5 it can be seen that there are no obvious peaks on XRD pattern after calcination at


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A NEW SOL-GEL ROUTE USING NORGANIC ALT FOR SYNTHESIZING 120, POWDERS 549

1.3 1.1 0.8 0.5 0.2 0.01

Equivalent spherical particle (pm)

Figure 8. Size distribution curves of Al,03 powders by Sedimentation technique.

TABLE 3Characterization Results of A1,O1 Powders

Specific surface dBET O EM dSO1 dBETPowder area

(m*.g-) nm nm nm

I-W, 291.95 6 531 4 89.5o-Al*03 51.21 29 755 25 26.0

5OOC,and sharp peaks of o-Al203 on XRD pattern after calcination at 1100C. y-Al203 is stablebelow 1050C, and transforms into o-Al203 at 105OC-1500C (7). In alumina industry, thecakination temperature of y-Al203 is 1400C. The transformation temperature of y-Al203prepared by tlhis method deceased by 300C as compared with that of alumina industry. This showsthat the smaller y-Al203 particle is, the easier y-Al203 particle transforms into o-Al2O3; inagreement with the above results of thermodynamic analysis that y-Al203 can transform into cr-Al*03 at lower temperature.

Figures 6 and 7 are TEM photographs of Al203 powder after calcination at 5OOCand lOOOC, espectively. It can be seen that the powder after calcination at 500C is still in thestate of amorphous soft agglomerates with the mean diameter of 4 nm, and the powder aftercalcination at 1100C forms a-Al203 particles of partial agglomerates with a average diameterof 25 nm.

Figure 8 is the size distribution curves of Al,O, powders. From Figure 8, dsoof y-Al,O, and

dso of a-Al,O, can be obtained as 537 nm and 755 nm, respectively. The specific surface areas ofy-Al,O, andl o.-A1203 were measured as 29 1.95 and 5 l.21m2/g by BET method, separately.


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550 WM ZENG L GAO AND JK Guo

The characterization results of Al,O, powders are summarized in Table 3. From Table 3 itcan be seen that the average diameter of singleA1203 particle measured by BETmethodis basicallyconsistent with its mean diameter obtained by TEM method, showing the reliability of theseresults. d,, (average diameter of agglomerate ) / dBET ( average diameter of single particle)represents the agglomerate size of the powder (8). From Table 3 it can be shown that theagglomerate size of cx-Al,O, powder is 3.4 times lower than that of y-A&O, powder. This may bebecause the heat provided by increasing temperature can partly destroy the soft agglomeratesobtained by freeze drying method.

CONCLUSIONS

Boehmite (y-AlOOH) powder was first prepared by sol-freeze drying method using cheap

AlCl,dH,O as raw materials, and then y-Al,O, nanopowder with the average diameter of 6 nmand a-A&O, nanopowder with the mean diameter of 30 nm were obtained by the calcination ofthe above y AlOOH powder at 500C and 1 OOC, espectively. Investigations of the potential-pHdiagrams of Al-H20 system and the thermodynamic analysis of thermal decomposition ofAl(OH), provide a certain guide for the formation of boehmite sol and the heat treatment ofboehmite powder, separately. Acidity has larger influence on the formation of boehmite sol, andtherefore the addition of acetic acid must be strictly controlled in order to obtain a high qualityboehmite sol.

1.

2.

3.4.5.6.7.

8.

REFERENCES

Guo, J.K., and Feng, C.D., Fabrication and Characterization OfAdvancedMaterials The MaterialResearch Society of Korea, 1995, 1,227.Varma, H.K., Mani, T.V., Damodaran, A.D., Warrier, K.G., and Balachandra, U., Journal of theAmerican Ceramic Society 1994 77 1597.Messing, G.L., and Kumagai, M., American Ceramic Society Bulletin 1994 73 88.

Zeng, W.M., and Chen, N.Y., Transactions of Nonferrous Metals Society of China 1997 7 34.Zeng, W.M., and Chen, N.Y., Journal of Materials Science and Technology 1997 13 446.Zeng, W.M., Doctorate Thesis, Central South University of Technology, 1994, p. 37 (in Chinese).Shi, Y.T., Electronic Ceramics Technology Basis Shanghai Peoples Press, 1975, p. 120 (inChinese).

Shi, J.L., Modern Inorganic Nonmetallic Materials Technology Jilin Science &Technology Press,1993, p. 99. (in Chinese).

nanostructured materials volume 10 issue 4 1998 [doi 10.1016_s0965-9773(98)00095-6] w.m. zeng; l....

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