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Thermal transformations of boehmite gel in controlled furnace atmospheres
Mat. Res. Bull., Vol. 27, pp. 397-404, 1992. Printed in the USA. 0025-5408/92 $5.00 + .00 (c) 1992 Pergamon Press Ltd.
THERMAL TRANSFORMATIONS OF BOEHMITE GEL IN CONTROLLED FURNACE ATMOSPHERES
Zdenek Hrab6, Olga M. Spaldon, Ladislav Pach, Jana Koz~,nkov~t Slovak Technical University, Faculty of Chemical Technology, Department of Ceramics, Glass and Cements, CS-812 37 Bratislava, Czecho-Slovakia (Received J a n u a r y 27, 1992; Refereed) ABSTRACT Boehmite derived alumina gel was annealed at intermediate temperatures for three hours in five different inner furnace atmospheres. Phase development, porosity, specific surface area, morphology, and changes in bulk density were compared. An atmosphere of water vapour has specific effects on the fired product, most likely by means of intensifying surface diffusion. The product has a different microstructure from those of the other atmospheres, characterized by a higher pore volume, larger average pore size, and smaller specific surface. MATERIALS INDEX : boehmite, gels, aluminas
INTRODUCTION Boehmite gel, which is often used in sol-gel processing, (1 to 6) undergoes a known series of thermal topotactic transformations (7 to 10) while undergoing heat-treatment and eventually becomes thermodynamically stable a-A1203. When the gel is seeded with a tiny amount of a-alumina, the nucleation and crystal growth of the product are markedly affected (6,7)through changes in nucleation frequency and growth rate. At relatively low temperatures, up to 1300 °C, seeded gel already develops nearly full density; under the same experimental conditions, unseeded gel forms a well-known microstructure of monocrystalline colonies that has a high pore volume (1,2,7). While the influence of the thermal treatment regime and the effects of chemical and phase dopants (seeds) are frequently described in literature, an important, but very often neglected, influence on the microstructure and properties of the final product is the type 397
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Vol. 27, No. 4
of inner furnace atmosphere. Various furnace atmospheres, such as water vapour, nitrogen, hydrogen, etc can produce different microstructures in the products, both singly and in combination with each other. In this paper, we present the results of an experimental study performed on boehmite gel fired in various furnace atmospheres: dry nitrogen, (chosen as the reference atmosphere), carbon dioxide, hydrogen, air and water vapour. The pure gases and water vapour were chosen because they are closely related to actual furnace atmospheres found in industry; they are frequent components of furnace environments. Fired samples were characterized for physical properties and phase composition. EXPERIMENTAL The boehmite (Disperal, Condea Chemie) sol and gel were formulated according to the procedure described (6,7). A boehmite powder dispersion (18 weight %) with 1% or without seeds of a-alumina ( < 0.5 #m ) was peptised by conc. HNO 3 (6% on a solid basis). When a high degree of a gel thickness was reached, long, narrow cylinders (approx.$ 6x50 mm) were extruded. After air and oven drying, the gel was calcined at 550 °C for 1 hour with a heating rate of 5 K.min -1. Each sample consists of 2 cylinders and 2.5g coarsely ground xerogel. Samples were fired in five atmospheres at two temperatures, 1050 oC and 1300 oC, and treated isothermally for three hours. The heat treatment was carried out at atmospheric pressure in through-flow furnace atmospheres. In addition to a set of fundamental experiments carried out in a one component gas/vapour atmosphere, similar experiments were carried out in two-component furnace atmospheres with p (n20)/p (total)ratios of 0.1, 0.2, and 0.4. After thermal treatment, samples were characterized for phase composition using XRD (Dron 2.0USSR), morphology using SEM (Tesla BS 300, CSFR), pore volume distribution by Hgintrusion and specific surface area by nitrogen adsorption (Porosimeter 1500, Sorptomatic 1800, both Carlo Erba, Italy). Double weighing in air and mercury as well as measurements of cylindrical samples were used simultaneously for bulk density determination (hereafter p'). Mean values are used throughout this paper. The reproducibility of bulk density and specific surface area results was better than 1 and 5%, respectively. RESULTS AND DISCUSSION Both seeded and unseeded boehmite derived gels go through transient phases to form a-alumina when heated, according to the literature (1,6,7,10). XRD patterns generated by samples specially prepared by annealing in different atmospheres at various temperatures to compare phase composition show that the content of the 0-alumina polymorph is higher for the samples annealed in water vapour than those annealed in other
Vol.
27,
No.
4
B O E H M I T E GEL
399
gases. This indicates that phase transformations, rather than sintering, are affected by the furnace atmosphere. According to the XRD patterns, all samples annealed at higher temperatures, 1050 °C and 1300 °C, are composed of pure a-alumina. The integral intensities of the corundum peaks differ, however. Peak intensities for the sample annealed in water vapour are 10 to 30% higher than for the samples annealed in other furnace atmospheres. Porosimetry and BET results, as well as the bulk density values of treated samples are summarized in Table 1. While nearly full density was achieved in samples fired at 1300 °C in the reference atmosphere, dry nitrogen, samples from a water vapour atmosphere show 25% porosity. From this it also follows that bulk density change is different: from up to 140% in dry nitrogen to 80% in water vapour. Because of the high degree of sintering in nitrogen, the specific surface area is lower than the detection limit of the method used. Pores in samples from a water vapour environment, however, are easily detected. Table 1 Properties of a-AIzO 3 samples,prepared from seeded boehmite-derived gel and fired at temperatures 1300 °C and 1050 °C for 3 hours in various furnace atmospheres with a total pressure of approx. 101kPa
Atmospheres
Itemp nitrogenxlorbonrwater4dri dI dried oC
+ / 100. PT/Pth
dioxide
vapour
air
hydrogen
1300 1050
> 99 64,9
96,9 61,3
74,9 55,1
98,9 64,5
99,9 64,8
Change in bulk density
1300
140
133
80
138
140
(PT-Po)/Po + 100.
1050
50
51
22
54
53
Surface area S/m2g-1- poros
1300 1050 1050
< 0,1 16,7 12,4
< 0,1 13,4 9,9
1,5 5,3 2,5
< 0,1 16,5 11,6
< 0,1 14,6 20,0
1300 1050 1300 1050 1300 1050
0,002 0,133 < 1 50,0
0,002 0,158 < 1 38,4
0,002 0,138 < 1 54,0
16
24
0,085 0,253 25 49,3 120 96
0,003 0,140 1,1 53,0 15 19
BET Vp/cm3g -1 (porosimetry) Porosity % Rmax/nm xx Loss of ignition, % x
xx q-
9 - 10
reference furnace atmosphere characteristic pore radius for maximum of d V / d R Pth, P0, PT - bulk density theoretical, of unfired and fired samples, resp.
17
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Samples fired at 1050 °C show a similar dependence of properties on the furnace atmosphere to what was seen at 1300 °C. The differences in specific surface area and total pore volume between samples fired in water vapour and those in a dry atmospheres are even more apparent at 1050 °C than at the higher temperature (see Table 1). The pores found in the samples fired in water vapour have a radius (R) larger than 100 nm and a total pore volume (Vp) two or three times larger than those of samples fired in dry atmospheres, in which R is approximately 10 to 20 nm. For example, in the reference atmosphere at 1050 oC, the maximum of the dVp/dR curve is found at 16 nm (=Rmax) and 98.5% of the total pore volume is found in the interval of 10 to 20 nm. In a water vapour atmosphere, Rmax is approximately 100 nm and there are no pores in the interval of 10 to 20 nm; 85% of all pores are found between 90 and 105 nm. In this atmosphere,small pores quickly disappear or move to grain boundaries, where they coalesce. The pore size distribution has a monodispersed character in all samples. This shows that the pore size and volume can be controlled by the type and composition of the furnace atmosphere. Sample morphology was observed on fracture surfaces by means of SEM. Samples sintered at 1300 °C were also polished, thermally, etched, and examined. The sample sintered in the reference atmosphere has the structure typical of polycrystalline corundum on its fracture surface (Figure 1) with a narrow size distribution (1 to 2 nm) of variously oriented trigonal crystals. The fracture runs along grain boundaries. The sample appears dense with separated closed pores of an average size of up to 200 nm. This microstructure, with small differences, is also characteristic of the samples fired in the other, "dry" atmospheres. The samples from the CO z atmosphere contain a larger fraction of close pores, samples from the H e atmosphere have a finer microstructure, crystal size between 0.3 and 1 nm. Samples fired in water vapour have a completely different microstructure. SEM micrographs show rounded a-alumina grains surrounded by interconnected pores of sizes ranging in the hundreds of nm; closed pores are not common. Samples fired at 1050 oC in "dry" atmospheres have a smaller fraction of extremely small pores that are interconnected and homogeneously distributed in the a-alumina. The crystal matrix is not fully developed. The size of the individual parts of the dense skeleton is smaller than approximately 50 nm. The microstructure that appears in water vapour has a more developed pore system and crystal matrix. Crystal size is up to 100 nm. At high magnifications, rounding and densification of a-alumina grains are apparent along with the development of pores that separate grains from the surrounding matrix. The developing porous skeleton is not a suitable precursor for obtaining a dense product with further firing. The experiments show that the influence of water vapour produces samples that are the most different of all of the experimental atmospheres. Water vapour affected samples at both temperatures (1300 oC and 1050 °C) and for that reason, it should be pointed out that the most significant influence is felt in the stage of polymorphic
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401
Fig.1 Microstructure (SEM) of t~-alumina annealed (1300 °C,3h) in : a,d - dry nitrogen; b,e - water vapor; c - air; f -hydrogen. Fracture surface (a,b,c), polished and etched surface (d,e,f).
z. HRAB/~ et al.
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transformations (below 1050 °C) and not during sintering (above 1050 °C). By comparison with the other experimental atmospheres (see Table 1 ), it is seen that the effects of water vapour do not depend on the oxygen partial pressure or the non-grey IR optical character, important for heat transmission. At the temperatures used in this experiment, the effects also cannot be explained by gas phase material transport mechanisms, and for that reason, their being influenced by a change in the type of furnace atmosphere (oxidizing or reducing, e.g. air or hydrogen) cannot bring forth a change in the process and the product properties. The smallest amount of shrinkage occurs in an atmosphere of water vapour (bulk density and its change in Table 1), so a different mechanism is utilized in that atmosphere compared to the mechanism in the dry atmospheres. For that reason, the effects of water vapour are considered specific and are explained by an intermediate interaction of water molecules from the atmosphere at the phase interface during the transformation of 7 to ~ and O, 0 to u-alumina, at which point the decisive period of the nucleation of the final product occurs. Table 2. Influence of partial pressure of water vapour in a furnace atmosphere of nitrogen on the densification of boehmite gel samples (a- seeded, b- unseeded); firing conditions are the same as in Table 1. Atmosphere
PH20/Ptot nitrogen 0
0,10
0,20
0,40
1,00
3,99
3,61
3,42
3,21
2,99
2,57
2,41
2,31
2,23
2,02
3,22
2,93
2,81
2,65
2,34
a. 1300°C/3 h PT/kg.m'3.103 a. 1050 °C/3 h
PT/kg.m'3.103 b. 1300 °C/3 h
PT/kg.m'3.103
Intermediate sorption of polar water molecules, for example, in the formation of hydrogen bridges on the surface of the rigid skeleton of the oxygen network in intermediate phases, can lower the activation energy and speed the transport of A1 ions. In the period on the sintering of u-alumina, the influence of oxygen diffusion along the grain boundaries is debatable. Chemisorption on active surface sites and on the phase interface during the production of intermediate phases (10) results in different rates of nucleation, growth,
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403
and surface diffusion. The last does not contribute to shrinkage (11), but significantly speeds the entire process through fast diffusion paths. Utilization of this mechanism in an atmosphere of water vapour depends on the fraction of active sites per unit area of solids along with the partial pressure of the water vapour. The relationship between bulk density and partial pressure is shown in Table 2. It is interesting to note that the dependence of 0' on (p(H20)/p(total)) 1/2 is linear, similar to, for example, the solubility of water vapour in oxide systems of glasses, as is described in the literature (12). A correlation between dilatometric measurements carried out at various partial pressures of water vapour and the properties of the resulting products would be one of several possible approaches, to make the conclusion more definite.
CONCLUSIONS The microstructure of u-alumina derived from the alumina gel depends on the type and composition of inner furnace atmospheres. Water vapour accelerates the growth of u-alumina and causes the development of less dense microstructure than those found in "dry" N 2, H 2, air and CO 2 atmospheres. The mechanism of specific effects of water vapour is expected to be the intensification of surface diffusion in the period of nucleation and growth of new phases of A120 3. Smaller differences were noticed in CO2-(slighty increased porosity), H2-(smaller average grain size) atmospheres. The effects of the type and/or composition of the furnace atmosphere during the microstructure-forming process may be one possible way of controlling the porosity of thermally treated materials, e.g. of sorbents, membranes, and catalyst supports.
REFERENCES
1. W.Yarborough and R.Roy, J.Mat.Res. 2, 949 (1987). 2. F.W.Dynys and J.W.Halloran, Ultrastructure Processing of Ceramics, Glasses, and Composites, p. 142-151. ed. L.L.Hench and D.R Ulrich. J.Wiley, New York (1984). 3. J.Brinker, and G.W. Scherer, Sol-gel Science: Physics and Chemistry of Sol-gel Processing. Academic Press, (1990). 4. G.W.Scherer, J.Am.Ceram.Soc. 73, 3 (1990). 5. D.S.Horn and G.L.Messing, J.Am.Ceram.Soc. 72, 1719 (1989). 6. R.A.Shellman, G.L.Messing, and M.Kumagi, J.Non-Cryst. Solids. 82, 277 (1986). 7. L.Pach, R.Roy, and S.Komareni, J.Mat.Res. 5,278 (1990).
404
Z. HRABE et al.
Vol. 27, No. 4
8. W.D.Callister, I.B.Cutler, and R.S.Gordon, J.Am.Cer.Soc. 49, 419 (1966). 9. E.Breval, G.Dodds, and C.G.Pantand, Mat.Res.Bul. 20, 1191 (1985). 10.S.J.Wilson and M.I-I.Stacey, J.Col.Int.Sci. 82, 507 (1981). 11.W.D.Kingery, H.K.Bowen, and D.R.Uhlmann, Introduction to Ceramics. 2nd ed., J.Wiley, New York 1976. 12.H.A.Schaffer, Nitrogen Ceramics, p. 103. ed. F.LRiley. Nijhoff Publ., Boston 1983.
THERMAL TRANSFORMATIONS OF BOEHMITE GEL IN CONTROLLED FURNACE ATMOSPHERES
Zdenek Hrab6, Olga M. Spaldon, Ladislav Pach, Jana Koz~,nkov~t Slovak Technical University, Faculty of Chemical Technology, Department of Ceramics, Glass and Cements, CS-812 37 Bratislava, Czecho-Slovakia (Received J a n u a r y 27, 1992; Refereed) ABSTRACT Boehmite derived alumina gel was annealed at intermediate temperatures for three hours in five different inner furnace atmospheres. Phase development, porosity, specific surface area, morphology, and changes in bulk density were compared. An atmosphere of water vapour has specific effects on the fired product, most likely by means of intensifying surface diffusion. The product has a different microstructure from those of the other atmospheres, characterized by a higher pore volume, larger average pore size, and smaller specific surface. MATERIALS INDEX : boehmite, gels, aluminas
INTRODUCTION Boehmite gel, which is often used in sol-gel processing, (1 to 6) undergoes a known series of thermal topotactic transformations (7 to 10) while undergoing heat-treatment and eventually becomes thermodynamically stable a-A1203. When the gel is seeded with a tiny amount of a-alumina, the nucleation and crystal growth of the product are markedly affected (6,7)through changes in nucleation frequency and growth rate. At relatively low temperatures, up to 1300 °C, seeded gel already develops nearly full density; under the same experimental conditions, unseeded gel forms a well-known microstructure of monocrystalline colonies that has a high pore volume (1,2,7). While the influence of the thermal treatment regime and the effects of chemical and phase dopants (seeds) are frequently described in literature, an important, but very often neglected, influence on the microstructure and properties of the final product is the type 397
398
Z. HRABE et al.
Vol. 27, No. 4
of inner furnace atmosphere. Various furnace atmospheres, such as water vapour, nitrogen, hydrogen, etc can produce different microstructures in the products, both singly and in combination with each other. In this paper, we present the results of an experimental study performed on boehmite gel fired in various furnace atmospheres: dry nitrogen, (chosen as the reference atmosphere), carbon dioxide, hydrogen, air and water vapour. The pure gases and water vapour were chosen because they are closely related to actual furnace atmospheres found in industry; they are frequent components of furnace environments. Fired samples were characterized for physical properties and phase composition. EXPERIMENTAL The boehmite (Disperal, Condea Chemie) sol and gel were formulated according to the procedure described (6,7). A boehmite powder dispersion (18 weight %) with 1% or without seeds of a-alumina ( < 0.5 #m ) was peptised by conc. HNO 3 (6% on a solid basis). When a high degree of a gel thickness was reached, long, narrow cylinders (approx.$ 6x50 mm) were extruded. After air and oven drying, the gel was calcined at 550 °C for 1 hour with a heating rate of 5 K.min -1. Each sample consists of 2 cylinders and 2.5g coarsely ground xerogel. Samples were fired in five atmospheres at two temperatures, 1050 oC and 1300 oC, and treated isothermally for three hours. The heat treatment was carried out at atmospheric pressure in through-flow furnace atmospheres. In addition to a set of fundamental experiments carried out in a one component gas/vapour atmosphere, similar experiments were carried out in two-component furnace atmospheres with p (n20)/p (total)ratios of 0.1, 0.2, and 0.4. After thermal treatment, samples were characterized for phase composition using XRD (Dron 2.0USSR), morphology using SEM (Tesla BS 300, CSFR), pore volume distribution by Hgintrusion and specific surface area by nitrogen adsorption (Porosimeter 1500, Sorptomatic 1800, both Carlo Erba, Italy). Double weighing in air and mercury as well as measurements of cylindrical samples were used simultaneously for bulk density determination (hereafter p'). Mean values are used throughout this paper. The reproducibility of bulk density and specific surface area results was better than 1 and 5%, respectively. RESULTS AND DISCUSSION Both seeded and unseeded boehmite derived gels go through transient phases to form a-alumina when heated, according to the literature (1,6,7,10). XRD patterns generated by samples specially prepared by annealing in different atmospheres at various temperatures to compare phase composition show that the content of the 0-alumina polymorph is higher for the samples annealed in water vapour than those annealed in other
Vol.
27,
No.
4
B O E H M I T E GEL
399
gases. This indicates that phase transformations, rather than sintering, are affected by the furnace atmosphere. According to the XRD patterns, all samples annealed at higher temperatures, 1050 °C and 1300 °C, are composed of pure a-alumina. The integral intensities of the corundum peaks differ, however. Peak intensities for the sample annealed in water vapour are 10 to 30% higher than for the samples annealed in other furnace atmospheres. Porosimetry and BET results, as well as the bulk density values of treated samples are summarized in Table 1. While nearly full density was achieved in samples fired at 1300 °C in the reference atmosphere, dry nitrogen, samples from a water vapour atmosphere show 25% porosity. From this it also follows that bulk density change is different: from up to 140% in dry nitrogen to 80% in water vapour. Because of the high degree of sintering in nitrogen, the specific surface area is lower than the detection limit of the method used. Pores in samples from a water vapour environment, however, are easily detected. Table 1 Properties of a-AIzO 3 samples,prepared from seeded boehmite-derived gel and fired at temperatures 1300 °C and 1050 °C for 3 hours in various furnace atmospheres with a total pressure of approx. 101kPa
Atmospheres
Itemp nitrogenxlorbonrwater4dri dI dried oC
+ / 100. PT/Pth
dioxide
vapour
air
hydrogen
1300 1050
> 99 64,9
96,9 61,3
74,9 55,1
98,9 64,5
99,9 64,8
Change in bulk density
1300
140
133
80
138
140
(PT-Po)/Po + 100.
1050
50
51
22
54
53
Surface area S/m2g-1- poros
1300 1050 1050
< 0,1 16,7 12,4
< 0,1 13,4 9,9
1,5 5,3 2,5
< 0,1 16,5 11,6
< 0,1 14,6 20,0
1300 1050 1300 1050 1300 1050
0,002 0,133 < 1 50,0
0,002 0,158 < 1 38,4
0,002 0,138 < 1 54,0
16
24
0,085 0,253 25 49,3 120 96
0,003 0,140 1,1 53,0 15 19
BET Vp/cm3g -1 (porosimetry) Porosity % Rmax/nm xx Loss of ignition, % x
xx q-
9 - 10
reference furnace atmosphere characteristic pore radius for maximum of d V / d R Pth, P0, PT - bulk density theoretical, of unfired and fired samples, resp.
17
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Z. HRABE et al.
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Samples fired at 1050 °C show a similar dependence of properties on the furnace atmosphere to what was seen at 1300 °C. The differences in specific surface area and total pore volume between samples fired in water vapour and those in a dry atmospheres are even more apparent at 1050 °C than at the higher temperature (see Table 1). The pores found in the samples fired in water vapour have a radius (R) larger than 100 nm and a total pore volume (Vp) two or three times larger than those of samples fired in dry atmospheres, in which R is approximately 10 to 20 nm. For example, in the reference atmosphere at 1050 oC, the maximum of the dVp/dR curve is found at 16 nm (=Rmax) and 98.5% of the total pore volume is found in the interval of 10 to 20 nm. In a water vapour atmosphere, Rmax is approximately 100 nm and there are no pores in the interval of 10 to 20 nm; 85% of all pores are found between 90 and 105 nm. In this atmosphere,small pores quickly disappear or move to grain boundaries, where they coalesce. The pore size distribution has a monodispersed character in all samples. This shows that the pore size and volume can be controlled by the type and composition of the furnace atmosphere. Sample morphology was observed on fracture surfaces by means of SEM. Samples sintered at 1300 °C were also polished, thermally, etched, and examined. The sample sintered in the reference atmosphere has the structure typical of polycrystalline corundum on its fracture surface (Figure 1) with a narrow size distribution (1 to 2 nm) of variously oriented trigonal crystals. The fracture runs along grain boundaries. The sample appears dense with separated closed pores of an average size of up to 200 nm. This microstructure, with small differences, is also characteristic of the samples fired in the other, "dry" atmospheres. The samples from the CO z atmosphere contain a larger fraction of close pores, samples from the H e atmosphere have a finer microstructure, crystal size between 0.3 and 1 nm. Samples fired in water vapour have a completely different microstructure. SEM micrographs show rounded a-alumina grains surrounded by interconnected pores of sizes ranging in the hundreds of nm; closed pores are not common. Samples fired at 1050 oC in "dry" atmospheres have a smaller fraction of extremely small pores that are interconnected and homogeneously distributed in the a-alumina. The crystal matrix is not fully developed. The size of the individual parts of the dense skeleton is smaller than approximately 50 nm. The microstructure that appears in water vapour has a more developed pore system and crystal matrix. Crystal size is up to 100 nm. At high magnifications, rounding and densification of a-alumina grains are apparent along with the development of pores that separate grains from the surrounding matrix. The developing porous skeleton is not a suitable precursor for obtaining a dense product with further firing. The experiments show that the influence of water vapour produces samples that are the most different of all of the experimental atmospheres. Water vapour affected samples at both temperatures (1300 oC and 1050 °C) and for that reason, it should be pointed out that the most significant influence is felt in the stage of polymorphic
Vol. 27, No. 4
BOEHMITE GEL
401
Fig.1 Microstructure (SEM) of t~-alumina annealed (1300 °C,3h) in : a,d - dry nitrogen; b,e - water vapor; c - air; f -hydrogen. Fracture surface (a,b,c), polished and etched surface (d,e,f).
z. HRAB/~ et al.
402
Vol. 27, No. 4
transformations (below 1050 °C) and not during sintering (above 1050 °C). By comparison with the other experimental atmospheres (see Table 1 ), it is seen that the effects of water vapour do not depend on the oxygen partial pressure or the non-grey IR optical character, important for heat transmission. At the temperatures used in this experiment, the effects also cannot be explained by gas phase material transport mechanisms, and for that reason, their being influenced by a change in the type of furnace atmosphere (oxidizing or reducing, e.g. air or hydrogen) cannot bring forth a change in the process and the product properties. The smallest amount of shrinkage occurs in an atmosphere of water vapour (bulk density and its change in Table 1), so a different mechanism is utilized in that atmosphere compared to the mechanism in the dry atmospheres. For that reason, the effects of water vapour are considered specific and are explained by an intermediate interaction of water molecules from the atmosphere at the phase interface during the transformation of 7 to ~ and O, 0 to u-alumina, at which point the decisive period of the nucleation of the final product occurs. Table 2. Influence of partial pressure of water vapour in a furnace atmosphere of nitrogen on the densification of boehmite gel samples (a- seeded, b- unseeded); firing conditions are the same as in Table 1. Atmosphere
PH20/Ptot nitrogen 0
0,10
0,20
0,40
1,00
3,99
3,61
3,42
3,21
2,99
2,57
2,41
2,31
2,23
2,02
3,22
2,93
2,81
2,65
2,34
a. 1300°C/3 h PT/kg.m'3.103 a. 1050 °C/3 h
PT/kg.m'3.103 b. 1300 °C/3 h
PT/kg.m'3.103
Intermediate sorption of polar water molecules, for example, in the formation of hydrogen bridges on the surface of the rigid skeleton of the oxygen network in intermediate phases, can lower the activation energy and speed the transport of A1 ions. In the period on the sintering of u-alumina, the influence of oxygen diffusion along the grain boundaries is debatable. Chemisorption on active surface sites and on the phase interface during the production of intermediate phases (10) results in different rates of nucleation, growth,
Vol. 27, No. 4
BOEHMITE GEL
403
and surface diffusion. The last does not contribute to shrinkage (11), but significantly speeds the entire process through fast diffusion paths. Utilization of this mechanism in an atmosphere of water vapour depends on the fraction of active sites per unit area of solids along with the partial pressure of the water vapour. The relationship between bulk density and partial pressure is shown in Table 2. It is interesting to note that the dependence of 0' on (p(H20)/p(total)) 1/2 is linear, similar to, for example, the solubility of water vapour in oxide systems of glasses, as is described in the literature (12). A correlation between dilatometric measurements carried out at various partial pressures of water vapour and the properties of the resulting products would be one of several possible approaches, to make the conclusion more definite.
CONCLUSIONS The microstructure of u-alumina derived from the alumina gel depends on the type and composition of inner furnace atmospheres. Water vapour accelerates the growth of u-alumina and causes the development of less dense microstructure than those found in "dry" N 2, H 2, air and CO 2 atmospheres. The mechanism of specific effects of water vapour is expected to be the intensification of surface diffusion in the period of nucleation and growth of new phases of A120 3. Smaller differences were noticed in CO2-(slighty increased porosity), H2-(smaller average grain size) atmospheres. The effects of the type and/or composition of the furnace atmosphere during the microstructure-forming process may be one possible way of controlling the porosity of thermally treated materials, e.g. of sorbents, membranes, and catalyst supports.
REFERENCES
1. W.Yarborough and R.Roy, J.Mat.Res. 2, 949 (1987). 2. F.W.Dynys and J.W.Halloran, Ultrastructure Processing of Ceramics, Glasses, and Composites, p. 142-151. ed. L.L.Hench and D.R Ulrich. J.Wiley, New York (1984). 3. J.Brinker, and G.W. Scherer, Sol-gel Science: Physics and Chemistry of Sol-gel Processing. Academic Press, (1990). 4. G.W.Scherer, J.Am.Ceram.Soc. 73, 3 (1990). 5. D.S.Horn and G.L.Messing, J.Am.Ceram.Soc. 72, 1719 (1989). 6. R.A.Shellman, G.L.Messing, and M.Kumagi, J.Non-Cryst. Solids. 82, 277 (1986). 7. L.Pach, R.Roy, and S.Komareni, J.Mat.Res. 5,278 (1990).
404
Z. HRABE et al.
Vol. 27, No. 4
8. W.D.Callister, I.B.Cutler, and R.S.Gordon, J.Am.Cer.Soc. 49, 419 (1966). 9. E.Breval, G.Dodds, and C.G.Pantand, Mat.Res.Bul. 20, 1191 (1985). 10.S.J.Wilson and M.I-I.Stacey, J.Col.Int.Sci. 82, 507 (1981). 11.W.D.Kingery, H.K.Bowen, and D.R.Uhlmann, Introduction to Ceramics. 2nd ed., J.Wiley, New York 1976. 12.H.A.Schaffer, Nitrogen Ceramics, p. 103. ed. F.LRiley. Nijhoff Publ., Boston 1983.