www.ptcer.pl/mccm
W
ALDEMARP
YDA1*, A
LEKSANDRAK
ĘDZIERSKA2, J
ERZY
M
ORGIEL3, N
ORBERTM
OSKAŁA11AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 35,
30-059 Kraków, Poland
2Institute of High Pressure Physics PAN, Laboratory of Ceramics and Glass, ul. Sokołowska 29/37, 01-149 Warsaw,
Poland
2Warsaw University of Technology, Faculty of Chemistry, Division of Inorganic Technology and Ceramics,
ul. Noakowskiego 3, 00-664 Warsaw, Poland
3Polish Academy of Sciences, Institute of Metallurgy and Materials Science, Kraków, Poland
*e-mail: pyda@agh.edu.pl
Calcined aluminium oxide nanopowders of
boehmite origin and narrow particle size
distribution
Abstract
The synthesis of aluminium oxide powders composed of nanocrystalline particles of high temperature polymorphs that show extremely narrow size distributions is reported. Al(NO3)3∙9H2O was used as a reagent in two bottom-up synthesis processes: i) calcination of precursor
boehmite obtained through controlled dehydroxylation of aluminium hydroxide, and ii) calcination of a hydrothermally crystallized precursor boehmite. The former method is solely the solid state synthesis of the product, the latter one involves crystallization of the precursor in a liquid state. Both methods, creating different crystallization conditions for boehmite, deliver alumina particles of different mechanical strength and therefore different behaviour during comminution. The processes of alumina nanopowder preparation were accompanied by intensive attrition milling at crucial manufacturing stages. The infl uence of the applied ways of boehmite precursor preparation on the morphology and size distribution of alumina particle was studied. The nano-powder composed of α-Al2O3 and a mixture of θ-Al2O3 and
α-Al2O3 was produced in the case of the conventional calcination and hydrothermal method, respectively. The additional attrition milling of
the hydrothermal boehmite product heated for 1 h at 450 °C decreased a crystallization temperature of α-Al2O3 from 1297 °C to 1199 °C
when compared to the unmilled product. A crystallization temperature of 1185 °C was determined for the solely calcined alumina powder. The hydrothermal treatment followed by pre-calcination at 450 °C and attrition milling allowed obtaining mesoporous θ-Al2O3
nano-partic-les of elongated shapes and sizes in the range of 60-300 nm with a median of 150 nm when crystallized for 0.5 h at 1040 °C. When the same comminution conditions applied to the fi nally calcined precursor, the alumina powder originated from the precursor, which has not been treated hydrothermally, was composed of particles ranging from 35 nm to 3 μm with a median of 200 nm and the presence of some amount of the fraction of 20-180 μm. Thus, a signifi cant role of the hydrothermal treatment combined with low temperature pre-calcination and attrition milling has been proved for the manufacturing of narrow distribution Al2O3 nanopowders.
Keywords: Nanopowder, Boehmite, Calcination, Hydrothermal crystallization, Mesoporous particle
KALCYNOWANE NANOPROSZKI TLENKU GLINU O WĄSKIM ROZKŁADZIE CZĄSTEK POCHODZĄCE Z BEMITU Opisano syntezę proszków tlenku glinu, złożonych z nanocząstek odmian wysokotemperaturowych, które charakteryzują się wąskim rozkładem wielkości. Azotan glinu Al(NO3)3∙9H2O wykorzystano jako prekursor w dwóch procesach bottom-up: i) kalcynacja prekursora
bemitowego otrzymanego w drodze kontrolowanej dehydroksylacji wodorotlenku glinu, ii) kalcynacja prekursora bemitowego krystalizo-wanego hydrotermalnie. Pierwsza z metod jest wyłącznie syntezą w fazie stałej, druga obejmuje krystalizację prekursora w fazie ciekłej. Obydwie metody, tworząc różne warunki krystalizacji bemitu, dostarczają cząstek tlenku glinu o różnej wytrzymałości mechanicznej i dla-tego różniące się zachowaniem podczas rozdrabniania. Proces przygotowania nanoproszków obejmował również intensywne mielenie ścierne zastosowane w kluczowych etapach wytwarzania. Zbadano wpływ zastosowanych sposobów otrzymywania prekursora glinowego na morfologię i rozkład wielkości cząstek w nanoproszkach tlenku glinu. Nanoproszki złożone z α-Al2O3 lub mieszaniny θ-Al2O3 i α-Al2O3
wytworzono w przypadku odpowiednio metody kalcynacji i metody hydrotermalnej. Dodatkowe mielenie ścierne hydrotermalnie otrzyma-nego produktu bemitowego, który ogrzano przez 1 h 450 °C wpłynęło na obniżenie temperatury krystalizacji α-Al2O3 z 1297 °C do 1199
°C w porównaniu z produktem niemielonym. Temperaturę krystalizacji wynoszącą 1185 °C stwierdzono w przypadku proszku α-Al2O3
otrzymanego wyłącznie metodą kalcynacji. Obróbka hydrotermalna z następczą prekalcynacją w 450 °C i mieleniem ściernym pozwoliła na uzyskanie mezoporowatych nanocząstek θ-Al2O3 o wydłużonych kształtach i rozmiarach w zakresie 60-300 nm i medianie wynoszącej
150 nm, wtedy gdy krystalizowano je przez 0,5 h w 1040 °C. W przypadku zastosowania takich samych warunków rozdrabniania do fi nalnie kalcynowanego prekursora, nieotrzymywanego w procesie krystalizacji hydrotermalnej, proszek α-Al2O3 składał się z cząstek o
rozmia-rach od 35 nm do 3 μm z medianą przy 200 nm i zawierał również pewną ilość frakcji 20-180 μm. W ten sposób wykazano znaczącą rolę połączenia obróbki hydrotermalnej i niskotemperaturowej prekacynacji z następczym mieleniem ściernym na wytwarzanie nanoproszków Al2O3 o wąskim rozkładzie cząstek.
1. Introduction
The manufacturing of alumina nanoparticles with control-led morphology and size distribution is the important fi eld of current nanotechnology. Elongated, one-dimensional (1D) nanostructures of aluminium oxide, such as rods, wires, or tubes, exhibit very attractive properties, including their high chemical and thermal stability, and also high mechanical features particularly a high elastic modulus [1, 2]. That is why the 1D alumina nanoparticles can be effi cient reinforcements of metallic matrices [3, 4]. On the other hand, alumina nano-powders consisting of isometric particles are crucial for fabri-cation of the single phase nano-ceramics, ceramic and metal matrix composites, and also may be exploited as adsorbents [5]. Thus, signifi cant efforts have been made concerning the preparation of aluminium oxide nanopowders of different morphology, especially α-Al2O3. The detailed description of
very extensive literature on the alumina powder preparation methods goes beyond the confi nes of this paper, and only several methods are mentioned to illustrate the main goal.
The crystallization under hydrothermal conditions is very useful for the production of alumina powders of controlled morphology [6]. The precipitated precursors (e.g., aluminium oxide-hydroxides) transform to alumina at lower tempera-tures during hydrothermal treatment, when compared to calcination, which helps in suppressing aggregation of the powder. According to Suchanek [6], experimental conditions that yield the product of α-Al2O3 are as follows: a
tempe-rature between 380 °C and 450 °C, and a pressure lower than 15 MPa. Commercial boehmite or gibbsite powders, consisting of ca. 100 nm grains, were the initial materials. Mineralisers, e.g., sulphuric or phosphoric acid, infl uence size and shape of the resultant crystals.
Calcination of hydrothermally produced boehmites is ano-ther method for the production of alumina powders. Mishra et
al. [7] used Al(NO)3∙9H2O and urea to prepare boehmites with
varying amounts of water when hydrothermally treated in the temperature range of 160-220 °C. γ-Al2O3 was obtained by
subsequent calcination of the boehmite at 725 °C. Guangshe Li et al. [8] prepared single-phase boehmite nanocrystals that were free of nitrate contamination at supercritical hy-drothermal conditions at 400 °C and 35 MPa. At 725 °C, the boehmite powders were decomposed into 10 nm γ-Al2O3,
and further at 1250 °C to single phase α-Al2O3. Kaya et al. [9]
produced boehmite sols by dispersing aluminium acetate po-wder i deionised water, and adjusting the pH by ammonia or acetic acid. After hydrothermal processing in the temperature range of 200-300 °C, the morphology of boehmite particles changed from needle-shaped to plate-shaped following the pH change of the initial solution from acid to alkaline.
Calcination of hydrothermally produced ammonium alu-minium carbonate hydroxide (AACH, also called ammonium dawsonite) was applied by Zhu et al. [10] to prepare Al2O3
nanorods with hierarchically mesoporous structures. AACH nanorods with a length of 1 ~ 2 μm crystallized in the environ-ment of PEG2000 aqueous solution in an autoclave heated to 140 °C for 24 h. After thermal decomposition treatment for 2 h at 900 °C, the AACH nanorods were converted to γ-Al2O3 nanorods of 60 nm in diameter and 1-2 μm in length
without the morphology deformation. Calcination for 2 h at 1200 °C completely transformed the AACH precursor into
the α-Al2O3 phase. A plausible surfactant-induced nanorod
formation mechanism using the polyethylene glycols as the template agent for the nanorod assembly was proposed.
Conventional calcination of precipitated precursors in the as-received state is the most simple alumina synthesis route [e.g., 11]. A precursor (e.g., Al(NO3)3∙9H2O, Al(OH)3) is
produced and then calcined at an appropriate temperature. Volatile products of decomposition (i.e., water, nitric oxides) are released during heating between 300 °C and 600 °C, and nascent transition aluminium oxide passes a thermal transformation sequence until the crystallization of α-Al2O3
over 1100 °C [12].
Though many aspects of alumina powders preparation,
e.g., the thermal transformation sequence [12], the
hydro-thermal transformation of hydroxides into oxide-hydroxides [7, 13, 14], or the shape of alumina particles with relation to preparation conditions [9, 10, 15, 16] are well documented, literature appears to be somewhat scarce on effects of the morphology of alumina particles on their behaviour during comminution and a resultant particle size distribution which belongs to key attributes controlling powder sinterability. Li et
al. [11] studied the effect of milling media on agglomeration
strength of nanocrystalline α-Al2O3 powders, using either
anhydrous alcohol or deionised water. The powder was ag-glomerated and composed of isometric primary particles of 10 nm in diameter. The milling fl uid had a signifi cant effect on the strength of the agglomerates. The powder milled in anhydrous alcohol had an agglomeration strength of 76 MPa, while the one milled in deionised water had an agglomeration strength of 234 MPa. Particle size distributions for the studied powders have not been shown.
In the present work, we report on the synthesis of alumi-na polymorphs using Al(NO3)3∙9H2O as the starting reagent
aimed to obtain powders composed of nanocrystalline particles of the narrowest possible size distribution. Two bottom-up synthesis processes accompanied by intensive attrition milling at crucial manufacturing stages were selected: i) calcination of boehmite produced from aluminium hydroxide by controlled thermal dehydroxylation, and ii) calcination of boehmite produced from AACH by hydrothermal transforma-tion. The former method is solely the solid state synthesis of the product, the latter one involves crystallization of the precursor boehmite in a liquid state. As a result of extremely different conditions for crystallization, morphologically dif-ferent precursor boehmite powders are expected, showing different abilities to produce narrow particle size distribution powders by comminution. The infl uence of the applied ways of the precursor hydroxide preparation on the morphology and size distribution of alumina nanoparticles is presented and discussed. The studied alumina nanopowders are intended to have uses in sintering polycrystalline alumina ceramics and in reinforcing metal matrix composites.
2. Experimental procedures
2.1. Conventional route of calcination
Analytically pure aluminium nitrate and ammonia were used as starting chemicals. A solution of ammonia in water (6M) was added in portions to a rapidly stirred aqueous so-lution of Al(NO3)3∙9H2O (3M). A white, sticky gel of aluminium
hydroxide was formed, and fi ltrated with a Büchner funnel, and subsequently washed twice with deionised water to de-crease the concentration of by-product ammonium nitrate. Then, the gel was desiccated in air for over 12 h at 105 °C. The dried gel was pre-calcined at 300 °C for approximately 1 h to produce boehmite as indicated in Ref. [15, 17] and to facilitate decomposition of ammonium nitrate, being a by-pro-duct of the precipitation reaction. The pre-calcined gel was then coarsely comminuted to obtain a granulated product of a particle size below 100 μm in diameter, and fi nally calcined in air at the temperature range of 1000-1100 °C for time ranging from 0.5 h to 6 h. After cooling to room temperature, the powder was milled in an attrition ball mill for 6 h in water. The addition of nitric acid kept a pH value at ~4. Y-TZP balls of 2 mm in diameter were used as milling media through the entire experimental work.
2.2. Hydrothermal treatment followed by
calcination
Initial procedures and chemicals were similar to the above mentioned route. Aqueous solutions of analytically pure aluminium nitrate (1.5M) and ammonia with ammonium hydrogen carbonate (AHC) were used as starting materials. The solution of aluminium nitrate was added in portions into the vigorously stirred solution of ammonia and AHC, when a pH maintained at 9 or above. The obtained gel, composed of ammonium aluminium carbonate hydroxide of the dawsonite structure was rinsed several times until the ionic conductance values of fi ltrate and distilled water were similar to each other (< 10-2 mS/cm); the rinsing lasted
several days. The gel was subsequently hydrothermally treated in a laboratory autoclave for 4 h at 240 °C under a pressure of 3.2 MPa in a water environment of pH 6.5 to produce boehmite, and then pre-calcined at 450 °C for 1 h, referring to the γ-AlO(OH) → γ-Al2O3 transformation. The
pre-calcined product was coarsely ground to a particle size below 100 μm, and then fi ne milled in an attrition ball mill for 6 h in deionised water at a pH value adjusted to ~4 for all the time of milling. After separation of the milling media from the powder slurry, the slurry pH was set to 7 to facilitate coagulation of powder particles, and the slurry was dried at 105 °C in air. The next step was fi nal calcination for 0.5 h at 1040 °C in air; a heating rate of 10 °/min was used. After
the synthesis, the powder was again milled with the attritor for 6 h, using the same milling conditions as in the case of the conventional route of calcination.
2.3. Methods of measurement
Precipitates and powders were examined with differential scanning calorimetry and thermogravimetry conducted in air, using a Thermal Analyzer type NETZSCH STA 449 F3 Jupiter. A heating rate of 20 °C/min and a fi nal temperature of 1300 °C was used. Phase identifi cation was performed by XRD, using an X’Pert PRO (PANanalytical) diffractometer with CuKα1 radiation (λα1 = 0.1540598 nm). The scan step
was 0,008°. The crystallite sizes were determined by using the Scherrer equation and both the lattice parameters and phase contents were derived from Retvield refi nement. Specifi c surface area was determined by the multipoint BET method using nitrogen adsorption at 77.35 K, and a Nova 1200e (Quantachrome Instruments) apparatus. Particle size distributions were measured with a Mastersizer 2000 Particle Size Analyzer (Malvern Instruments). Morphology of particles was visualized with Tecnai G2 F20 TEM in standard and high resolution modes.
3. Results
3.1. XRD characteristics of precursor boehmites
When heat treated at 300 °C, the precursor of alumina in the conventional route was composed of very fi ne crystal-line particles as indicated by the large X-ray crystal-line broaden-ing (Fig. 1a). The Cmcm orth orhombic boehmite structure with cell parameters of a = 0.28267 nm, b = 1.21881 nm, and c = 0.36653 nm fi ts the measured X-ray diffraction pat-tern best. Boehmite crystallites had a size of 2.5 nm in the direction perpendicular to the (021) plane as determined from the peak broadening. In the case of the hydrothermal route, boehmite γ-AlO(OH) crystallized under hydrothermal conditions at 240 °C as the only phase as shown in Fig. 1b. Cell parameters of a = 0.28644 nm, b = 1.22152 nm, and c = 0.36891 nm) were measured for this phase. The hydro-thermal boehmite showed the d(021) crystallite size of 14.0 ± 0.5 nm, being about 6 times larger than in the case of the conventional calcination route.a) b)
Fig. 1. X-ray diffraction patterns of the conventional route precursor aluminium hydroxide dehydroxylized at 300 °C for 1 h (a) and ammonium aluminium carbonate hydroxide treated hydrothermally for 4 h at 240 °C (b).
3.2. DSC/TG characteristics of precursor
boehmites
The DSC/TG measurements allowed determining cal-cination temperatures that are relevant to the formation of θ-Al2O3 and α-Al2O3, being a matter of interest to us. Figs.
2 and 3 show DSC/TG curves for the precursor boehmites collected at selected stages of the alumina powder prepa-ration procedures for both the conventional route and the hydrothermal one. Fig. 2a corresponds to the by-product-free precursor aluminium hydroxide after the one-hour treatment at 300 °C in the case of the conventional route. Fig. 2b shows the thermal behaviour of the hydrothermal boehmite in the as-received state. Fig. 3a and Fig. 3b corresponds to the hy-drothermal boehmite after additional heating for 1 h at 450 °C before and after the six-our attrition milling, respectively.
When interpreting the DSC/TG results one must take into consideration that the formation of α-Al2O3 from
bo-ehmite (γ-AlO(OH)) occurs via the following sequence of polymorphic transformations on heating (adapted from Ref. [18] and [ 19]): 3 2 C 1200 1050 3 2 C 1000 930 3 2 C 800 780 3 2 C 470 400 O Al -α O Al -θ O Al -δ O Al -γ AlOOH -γ ⎯ ⎯ ⎯ ⎯ → ⎯ ⎯ ⎯ ⎯ ⎯ → ⎯ ⎯ ⎯ ⎯ ⎯ → ⎯ ⎯ ⎯ ⎯ ⎯ → ⎯ ° − ° − ° − ° −
All the DSC curves shown in Figs. 2 and 3 contain endo-thermic peaks in the temperature ranges of 50-150 °C and 400-550 °C. The fi rst peak is connected with the desorption of physically adsorbed water; the second one corresponds to
the dehydroxilation process, condensation of adjacent Al-OH groups and the formation of γ-Al2O3 [8, 11, 18, 19]. The DSC
curve of both the as-received hydrothermal boehmite (Fig. 2b) and the hydrothermal boehmite additionally heated at 450 °C and attrition milled in deionised water at the pH of ~4 (Fig. 3b) contains also a weak endothermic peak at 265 °C and 273 °C, respectively. It can be assigned to the strongly bound water associated with a transitional (amorphous) Al(OH)3 as indicated by Morgado et. al [20].
There is no exothermic effect coming from the decom-position of NH4NO3 at 320 °C [8, 11] in the DSC curve of
the precursor used in the conventional route (Fig. 2a) which suggests that ammonium nitrate was removed during the rinsing of the precipitated gel and its pre-calcination.
In the DSC graph in Fig. 2a, one can observe a weak exothermic effect at about 950 °C and a strong one at a temperature of 1185 °C; both the effects can be attributed to crystallization of θ-Al2O3 and α-Al2O3, respectively. The
original hydrothermal boehmite transformed to α-Al2O3 at the
temperature being higher than shown by the conventional route boehmite. The attrition milling of the hydrothermally cry-stallized precursor, which has been pre-calcined at 450 oC,
decreased signifi cantly the crystallization temperature from 1297 °C to 1199 °C (Fig. 3).
a) b)
Fig. 2. DSC/TG thermal analysis graphs of precursor boehmites: a) aluminium hydroxide precipitate pre-calcined at 300 °C for 1 h used in the conventional calcination route, b) dawsonite precipitate treated hydrothermally at 240 °C for 4 h used in the hydrothermal-calcination route.
a) b)
3.2. XRD characteristics of heat treated
boehmites
Effects of the calcination temperature on the phase composition of the conventional route boehmite powder are shown in Figs. 4a-4c. A mixture of 39 wt% α-Al2O3 and 61 wt%
θ-Al2O3 appeared after 1 h heating at 1000 °C. Crystallites
of the corundum phase had a size of 120 nm (Fig. 4a), and it was 8 times larger than the size of θ-Al2O3 crystallites
(15 nm). A single phase corundum nanopowder was obtained during 0.5 h heating at both 1050 °C and 1100 °C, showing a crystallite size of 80±10 nm and 250 nm, respectively (Figs. 4b and 4c).
Effects of the calcination temperature on the phase composition of the hydrothermal route boehmite powder are shown in Figs. 5a-5c. Pre-calcination of the powder for 2 h at 450 °C caused crystallization of strongly textured γ-Al2O3
in the amount of 55 wt% and with the crystallite size of 8 nm (Fig. 5a). The remaining boehmite crystallites showed the d(021) size of ~18 nm. The additional attrition milling of the pre-calcined powder in water at the pH of ~4 produced coarse crystalline gibbsite (γ-Al(OH)3), being the most probably
a result of the reaction between γ-Al2O3 and water (Fig. 5b).
The d(021) boehmite crystallite size had a value of ~20 nm, remaining unchanged within the range of measurement error. A mixture of α-Al2O3 and θ-Al2O3 appeared in the
pre-calcined milled powder when heat treated for 0.5 h at 1040 °C (Fig. 5c). Crystallites of the θ phase were 35 nm in size and larger than the α phase ones.
3.3. TEM observations of milled aluminas
The TEM investigations show signifi cant differences in the grain shape of the two examined powders. Moderately iso-metric particles of alumina powder were detected (Fig. 6a), when prepared from the precursor boehmite obtained by controlled thermal dehydroxylation of aluminium hydroxide. Two particle populations are visible: a population of extremely small crystallites of few nanometers in size and a population of large particles reaching even ~250 nm. A comparison of the largest alumina particle size to that determined from the X-ray line broadening (Fig. 4b; 80 nm) suggests the polycrystalline structure of original alumina particles before milling. Any internal porosity of alumina particles is detectable in Fig. 6a.
Fragments of elongated rod-shaped particles are ob-served in the case of alumina powder composed of the mixture of θ- and α-Al2O3, when prepared by calcination of
the hydrothermal boehmite (Fig. 6b). The fragments form a population of particles which is more uniform in sizes than the alumina particles of the conventional route origin. The largest fragments seems to occur as a result of only trans-verse fracturing of the rods and allows estimating transtrans-verse sizes of the rods before the milling to be 15-30 nm. The longest fragment measured from TEM images has a length of 280 nm.
Internal porosity of rod-shaped particles was registered by HRTEM as shown in Fig. 7. Mesopores have the Feret diameters ranging from 4 nm to 11 nm.
a) b)
c)
Fig. 4. X-ray diffraction patterns of the conventional route precursor boehmite calcined at: a) 1000 °C for 1 h, b) 1050 °C for 0.5 h, and c) 1100 °C for 0.5 h.
a) b)
c)
Fig. 5. XRD patterns of hydrothermal boehmite heated for 1 h at 450 °C: a) unmilled, b) 6 h attrition milled, c) 6 h attrition milled followed by calcination for 0.5 h at 1040 °C.
a) b)
Fig. 6. TEM images of Al2O3 powders after 6 h attrition milling: a) conventional route - calcination for 0.5 h at 1050 °C, b) hydrothermal
route - calcination for 0.5 h at 1040 °C.
3.4. Comminution characteristics
Particle size distributions of the fi nally calcined precursor boehmites in both the as-received state and after the 6 h attrition milling are shown in Fig. 8. The unmilled particles range from ~0.45 μm to ~200 μm for the both studied po-wders. The particles size distribution of the conventional calcination route powder is symmetrical and shows a mode of 10 μm being equal to the median particle size (d50; Table 1).
A value of specifi c surface area of 1.1 m2/g was measured for
this powder. Regarding the hydrothermal route powder, two modes of 3 μm and 25 μm are detectable, and the volume content of particles with sizes ranging between the modes varies only slightly. A median value of 10 μm averages an image of the powder.
The attrition milling shifted the particle size distribu-tions towards nanometric sizes, living a lot more of large particles as a secondary size population in the case of the
conventional calcination route powder. Simultaneously an increase in specifi c surface area of the powders appeared up to 13.4 m2/g and 26.0 m2/g for the conventional and
hy-drothermal route, respectively (Table 1).
The data presented in Table 1 and Fig. 8 prove nanome-tric scale of the alumina powders synthesized via the both methods. Nevertheless, the synthesis exploiting hydrother-mal pre-processing of alumina precursors gave the alumina nanopowder with much narrower particle size distribution (60-300 nm) than that of the powder obtained without treat-ment in autoclave (35 nm - 3 μm, including the secondary population in the size range of 20-180 μm).
4. Discussion
Two boehmite precursors were obtained involving two different mechanisms. The dehydroxilation of the crystalline hydroxide, most probably gibbsite γ-Al(OH)3, was
responsi-ble for the formation of boehmite used in the conventional calcination route and determined the formation of γ-Al2O3 by
continuous dehydroxylation at 400-550 °C. Further heating produced δ-Al2O3 and then θ-Al2O3 as a micrometer sized
powder [15]. This fi ne grained and undoubtedly agglomerated powder (no dispersant added to avoid agglomeration of the ultrafi ne hydrous alumina [e.g, 11]) changed its structure to α-Al2O3 at 1050 °C and remained as a powder composed of
aggregates with no or limited porosity, giving mechanically strong particles.
The usage of ammonium aluminium carbonate hydroxide (AACH) as a precursor of boehmite changes the mechanism
Fig. 7. HRTEM microphotograph of Al2O3 particle originated from the
precursor dawsonite treated hydrothermally and fi nally calcined for 0.5 hour at 1040 °C, which reveals internal voids.
a)
b)
Fig. 8. Particle size distributions of alumina powders originated from: a) conventional calcination route, b) hydrothermal route; N - before milling, M - after 6 h attrition milling.
Table 1. Specifi c surface area (Sw) and median particle size (d50) of alumina nanopowders as a function of preparation route.
Preparation route Temperature/time of calcination SW [m
2/g] d
50 [μm]
Before milling After milling Before milling After milling
Conventional 1050°C/0.5h 1.1 ± 0.2 13.4 ± 0.3 10.2 0.20
Hydrothermal 1040°C/0.5h 2.1 ± 0.3 26.0 ± 0.4 10.1 0.15
of its formation. Zhu et al. [10] proposed a surfactant-induced nanorod formation mechanism by using the poly ethylene glycols assemblies as the template agent for the alumina nanorod assembly with hierachically mesoporous structure. However, as indicated by the results shown in Figs. 6b and 7, the mesoporous θ-Al2O3 particles can be obtained from
the hydrothermal boehmite which originated from the AACH precursor prepared with no surfactant addition when the pH is mantained at 9 during precipitation and at 6.5 during hydrothermal treatment. This does not exclude a particle coagulation mechanism [9, 21] which indicates that elon-gated boehmite particles are built up of small sub-units. It is suggested that the process involves particle coagulation,
i.e., very small crystallite units nucleate from the solution
fi rst, and then coagulate into the fi nal fi bril structure obse-rved. According to this view, the balance of attractive and repulsive interaction forces between the sol particles would play a very important role in the process of boehmite particle growth. Affecting dispersion stability of ceramic sols, the pH affects the AACH and further boehmite particle morphology. It was shown by Zhu et al. [10] that AACH nanorods convert to γ-Al2O3 nanorods during thermal treatment with
no morphology deformation. This explains the mesoporous structure of the θ-Al2O3 particles obtained from the
hy-drothermal precursor boehmite. The presence of porosity decreased strength of the θ-Al2O3 particles and allowed the
narrow particle size distribution to be obtained in the attrition milled powder.
5. Conclusions
Precursor boehmites of different morphology were produ-ced from Al(NO3)3∙9H2O by using two methods: a simple
pre-cipitation/calcination method and a hydrothermal treatment of AACH at the pH of 6.5. Both methods created different conditions for the formation of boehmites and delivered alu-mina particles of different mechanical strength and therefore different behaviour during comminution.
An ultrafi ne powder of α-Al2O3 and a mixture of θ-Al2O3
and α-Al2O3 was prepared by calcination of the precursor
boehmites and subsequent attrition milling in the case of the conventional calcination and the hydrothermal method, respectively.
The additional attrition milling of the hydrothermal bo-ehmite product heated for 1 h at 450 °C was applied and decreased a crystallization temperature of α-Al2O3 from
1297 °C to 1199 °C, when compared to the unmilled product. A crystallization temperature of 1185 °C was determined for the solely calcined alumina powder.
The hydrothermal treatment of the precursor AACH fol-lowed by pre-calcination of the resultant boehmite at 450 °C and attrition milling allowed obtaining mesoporous θ-Al2O3
nano-particles of elongated shapes and sizes in the range of 60-300 nm with a median of 150 nm, when crystallized at for 0.5 h at 1040 °C. When the same comminution conditions have been applied to the fi nally calcined precursor but not treated hydrothermally, the alumina powder was composed of particles ranging from 35 nm to 3 μm with a median of 200 nm and the presence of some amount of the fraction of 20-180 μm.
The presence of 4-11 nm mesopores within the θ-Al2O3
particles of the hydrothermal boehmite origin was responsible for a decrease in their mechanical strength and for obtaining a very narrow nanoparticle size distribution.
Acknowledgements
The work was supported by the AGH University of Science and Technology, Faculty of Materials Science and Ceramics under the statutory grant nr 11.11.160.617. The assistance of Magdalena Szumera and Mirosław M. Bućko from AGH UST WIMiC in receiving the DSC/TG and XRD results, respectively, is greatly acknowledged.
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