Precipitation of finely divided A1,0, powders by a molten salt method Yuansheng Du and Douglas Inman Department of Materials, Imperial College, London, UK SW7 2BP Finely divided A120, powders have been prepared by reactions of Al,( SO,), or AlC1, with molten nitrates, nitrites or nitrates containing Lux-Flood bases. The reactions in different melts were compared and the possible stoichiometries of the reactions have been proposed. The powders precipitated were characterised by XRD, AAS and TEM. A120, as an important ceramic material has attracted a great deal of research interest. The traditional routes for the prep- aration of advanced A120, powders normally employ aqueous solutions such as in precipitation and sol-gel methods. As has been shown by a number of researchers,lP7 alternative low- temperature methods using molten salts have led to ultrafine ceramic powders.These researchers have reported that single- component powders, including Zr021-, and MgO,' and mul- tiple-component powders, including Y20,-Zr026 and MgO- ZI-O~,~can be precipitated from molten nitrites and nitrates below 600°C. However, there has been no report on the preparation of A120, powders from molten salts. Therefore it is necessary and interesting to carry out further research into the production of A120, powders by the molten salt method. The starting materials employed in this study were anhy- drous Al2(SO,), (BDH, LR) and AlCl, (BDH, LR). Other chemicals such as molten nitrites, nitrates and Lux-Flood bases, the experimental procedures, and the extraction and characterisation methods of the powders produced were the same as those described previ0usly.~3~ In normal furnace runs, anhydrous A12( was observed to react with the NaN0,-KNO, eutectic at temperatures above 100"C with the evolution of brown NO2.After melting of the eutectic (220 "C), the reaction became more violent with further evolution of gas and the formation of a white powder. Above about 400"C, hardly any gas formation or further precipitation of powders was observed. The TG graph [Fig. l(a)] shows three stages of mass loss: the first from ca. 100 to 210"C, before the melting of the NaN02-KN02 eutectic; the second with a very sharp peak from ca. 220 to 250 "C; and the third, which overlapped with the second peak, from ca.260 to 420°C. The overall mass loss of Al,(SO,), added (64.8 +0.3%) was close to that predicted on the basis 140~ 50 100 150 200 250 300 350 400 450 500 550 600 TI'C of the following reaction (66.6%): A1,(SO4),+6NO,-+A1203+3N02+3NO+3S042-(1) In a similar way to the case of nitrites, it was observed that anhydrous A12(S04)3 began to react with NaN03-KN03 at about 220"C, around the melting point (220°C) of the NaN03-KN03 eutectic, with the evolution of brown NO2 and the formation of a white powder. Above 400"C, the reaction became very violent and bubbled nearly to the top of the test tube. The reaction continued to temperatures above 500 "C. The TG diagram [Fig. l(b)] indicates that the reaction took place in two stages with the first at ca.22O-32O0C, and the second at 320-520°C. The overall mass loss was ca. 91.4 +0.4% of A12(SO,), added, which is close to that predicted on the basis of the following reaction (94.7%): A12(S04),+ 6N0,-+A1203 + 3s042-+ 6N02+502 (2) Similar reaction phenomena were observed for the reaction of Al,(SO,), in single-component NaNO, to those in NaN03-KN03. The reaction started at ca. 300 "C and was complete at ca. 530"C, higher than the starting (ca. 220°C) and completion (ca. 520 "C) temperatures in NaN0,-KN03. From these results, it can be seen that the reaction tempera- tures are lower in nitrites than in nitrates, and in the different nitrate systems they increase with increasing melting point.The differences of acidity of these melts are probably the reason for this., When using AlCl, as the starting material, the reaction was observed to occur at lower temperatures than when employing A12(S04), in a similar melt. For example, AlCl, began to react with NaN0,-KNO, at about 160 "C, almost 50 "C lower than when using A12(S04),, also with the evolution of brown NO, and the formation of a white powder. The TG graph of the reactions of A12(SO,), in the NaN03-KN03 system containing different Lux-Flood bases (Na20z and Na2C0,) is shown in Fig. 2. It can be seen that reaction temperatures were reduced by adding bases. This is because the Lux-Flood bases can give rise to oxide ions in the melt and thus can accelerate the reaction between A13+ and 02-to form A1203.The contributions of Na202 and Na2C03 to the reactions have been explained previ~usly.~ The powders precipitated by the reactions of A12(S04), and AlCl, with different melts under various conditions were examined by XRD. The results (Fig. 3) indicate that the ) 50 100 150 200 250 300 350 400 450 500 550 600 TI'C Fig. 1 TG diagram (differential mass loss vs. temperature) for the Fig. 2 TG diagram (differential mass loss vs. temperature) of A12(S04)3 reaction of A12(S04)3 (7.5 mol%) in the NaN02-KN02 (a) and (7.5 mol%) in NaN03-KN03 containing bases: (a)no base; (b)Na20, NaN0,-KNO, (b) systems (heating rate 5 "C min-') (22.5 mol%); (c) Na,CO, (45.0 mol%) (heating rate 5 "C min-') J. Muter. Chem., 1996, 6(7), 1239-1240 1239 4 0-(a1 0 040 ' 20 ' 40 I 60 I 80 2Bldegrees Fig.3 XRD patterns of powders precipitated by the reactions of Al,(SO,), with NaNO,-KNO, (a)and LiN0,-KN03 (b) Table 1 Crystallite sizes of Al,03 powders precipitated from different nitrate melts (450 "C; 90 min) crystallite size/nm LiN03-KN03 132 3.3 2.0 NaN03-KN03 220 3.8 2.3 NaN0, 307 4.7 2.8 "Starting matenal.precipitates were very fine or poorly crystalline A120, powders, in accord with JCPDS card 37-1462 for Al,03. From the line broadening of the XRD patterns, the crystallite sizes of the powders were estimated. The results are listed in Table 1. It is seen that the crystallite sizes of all the powders were <10 nm and the crystallite sizes produced from various nitrate melts under the same conditions (450 "C; 90 min) increased with the melting points of the melts.The XRD results also revealed that the structure of the product was the same but the crystallite sizes were smaller when employing AlCl, in place of Al,( SO4), under the same reaction conditions. The crystallite sizes of powders precipitated by the reaction of A12(S04)3 in NaN0,-KNO, (450 "C; 90 min), estimated to be ca. 1-2 nm, were even smaller than those in the nitrate melts. This is in agreement with the results discussed for the precipi- tation of ZrO,, and Mg0' powders. The XRD patterns of the powders produced by adding bases were different from those produced from the pure NaNO,-KNO, melt. There were changes in peak positions and the peaks became broader, corresponding to structural changes and smaller crystallite sizes. The very broad and overlapping XRD peaks also indicate that the Al,O, powders produced were very fine or poorly crystalline.Fig. 4 TEM image of Al,O, prepared from Al,(SO,), with NaN0,-KNO, (45 "C; 90 min) The total impurity levels of Na and K elements (measured by AAS) were all <0.5 mass% when no Na,O, was added to the melts. The addition of Na,02 to the NaNO,-KNO, melt as a Lux-Flood base increased the impurity levels of Na and K to 0.53 and 0.64 mass%, respectively. This might be because Na,O produced by the decomposition of Na202 in the melt could combine directly with Al,03. The TEM image obtained after using ultrasonic agitation and isopropyl alcohol in the final wash of the powders precipitated is shown in Fig.4, It indicates that the powders were well dispersed and that individual powders were uniform and spherical with soft agglomeration. The size of each individ- ual powder particle was as small as a few nanometers, which was close to the crystallite size measured by XRD. This seems to show that the elementary grain was nanocrystalline. References 1 D. H. Kerridge and J. Cancela Rey, J. Inorg. Nucl. Chem., 1977, 39,405. 2 H. Al-Raihani, B. Durand, F. Chassagneux, D. H. Kerridge and D. Inman, J. Muter. Chem., 1994,4, 1331. 3 Y. Du and D. Inman, J. Muter. Chem., 1995,5,1927. 4 Y. Du, P. Rogers and D. Inman, J. Muter. Sci., 1996, in press. 5 Y.Du and D. Inman, J. Muter. Sci., 1996, submitted. 6 B. Durand and M. Boubin, Muter. Sci. Forum, 1991,73-75,663. 7 Y.Du and D. Inman, Br. Ceram. Trans., 1996, in press. Communication 6/01 804A; Received 14th March, 1996 1240 J. Muter. Chem., 1996, 6(7), 1239-1240