Evolution of Aluminum Hydroxides in Diluted Aqueous Aluminum

Article pubs.acs.org/crystal

Evolution of Aluminum Hydroxides in Diluted Aqueous Aluminum Nitride Powder Suspensions Andraž Kocjan,* Aleš Dakskobler, and Tomaž Kosmač Engineering Ceramics Department, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia S Supporting Information *

ABSTRACT: The evolution of aluminum hydroxides in diluted aqueous aluminum nitride (AlN) powder suspensions in the temperature range 22−90 °C was studied in order to set up a general mechanistic model over a broad temperature range, uniting previously observed hydrolysis reactions at room temperature and at elevated temperatures into a single scheme. It is shown that dispersing the AlN powder in the water results in the temperature-dependent formation of various aluminum hydroxides in the following sequence: amorphous aluminum hydroxide gel, aluminum monohydroxide (boehmite), and aluminum trihydroxides (bayerite, nordstrandite, and gibbsite). The unique interdependency between the temperature and the pH of the hydrolyzing AlN powder suspension, governed by the ammonia’s solubility and the exothermic hydrolysis reactions producing Al(OH)4− species, is the driving force for several evolution paths of these aluminum hydroxides exhibiting numerous morphologies.

■

INTRODUCTION The degradation of aluminum nitride (AlN) powder in the presence of water involves the evolution of various aluminum hydroxides, mostly in the form of aluminum mono- and trihydroxides.1 The evolution path is similar to the one that occurs during the corrosion of aluminum in aqueous environments,2 where the mechanisms of aluminum hydroxide formation on the aluminum surface were studied in detail.3,4 In general, when the aluminum is exposed to an aqueous environment a thin layer of amorphous aluminum oxide will be formed on its surface by an electrochemical reaction. Longer exposure times will result in a subsequent hydrolysis of the asformed oxide layer, leading to the heterogeneous nucleation and growth of pseudoboehmite and bayerite on the oxide-layer surface.3,4 This evolution path represents the aluminum−water reaction taking place over broad temperature and pH ranges. On the other hand, even though there have been a number of investigations describing the formation of aluminum hydroxides during the hydrolysis of AlN powder in water,5−11 a detailed mechanistic model covering a broad temperature range is still missing. Several discrepancies can be observed when comparing results, especially those from studies dealing with hydrolysis at elevated temperatures. Some of these discrepancies originate from the disparate experimental conditions under which the studies were conducted. It is useful to mention that the hydrolysis of AlN powder is a dynamic process, where the pH and the temperature of the AlN powder suspension increase with time, because of the ammonia formation and because of the exothermic nature of the hydrolysis, respectively.12 As a consequence, the release of the Al3+ ions in the AlN powder © 2012 American Chemical Society

suspension varies with the hydrolysis time and with the starting temperature of the suspension.13 Thus, a series of aluminum hydroxides are formed during the hydrolysis of the AlN powder, depending on the hydrolysis conditions. Bowen et al.5 extensively studied the hydrolysis of AlN powder in water and proposed the following mechanistic model for the hydrolysis at room temperature (RT):

AlN + 2H2O → AlOOHamorph + NH3

(1)

NH3 + H2O ↔ NH 4 + + OH−

(2)

AlOOHamorph + H2O → Al(OH)3,xstal

(3)

According to the X-ray powder-diffraction (XRD) data on water-treated AlN powders collected after various times, the initial solid hydrolysis product was an amorphous aluminum monohydroxide (AlOOHamorph) formed by Reaction 1. This product possessed a stoichiometry close to boehmite, based on the Al/O ratio, which was evaluated using X-ray photoelectron spectroscopy (XPS). The amorphous aluminum monohydroxide subsequently recrystallized to aluminum trihydroxide, that is, bayerite (and to traces of nordstrandite), with longer hydrolysis times by the dissolution−recrystallization (D-R) process (Reaction 3). However, the authors could not discern whether the transformation of the quasi-amorphous monohydroxide to crystalline bayerite by Reaction 3 takes place via a Received: October 10, 2011 Revised: December 4, 2011 Published: January 6, 2012 1299

dx.doi.org/10.1021/cg201349s | Cryst. Growth Des. 2012, 12, 1299−1307

Crystal Growth & Design

■

solid-state transformation or via a D-R process. The occurrence of a D-R process was supported by the decrease in the specific surface area of the precipitates in the suspension according to the nitrogen BET surface-area measurements. It was anticipated that the D-R process during AlN hydrolysis is very similar to the process of aging of aluminum hydroxide gels.2,14−17 In addition to the work of Bowen et al.,5 several studies on the hydrolysis reaction products at elevated temperatures in diluted aqueous suspensions have been performed. Abid et al.6 investigated the thermal stability, that is, the oxidation resistance, of thermally and chemically pretreated AlN powders and thin films and reported that fully crystalline boehmite is formed when AlN powder is treated with boiling water. Fukumoto et al.7 investigated the hydrolysis behavior of spherical AlN powder in various diluted solutions at RT and at elevated temperatures up to 100 °C. According to these authors, the hydrolysis behavior changes at 78 °C: below this temperature, crystalline bayerite will be the predominant phase. However, in contrast to Bowen et al.,5 no amorphous monohydroxide was identified prior to the bayerite formation. On the other hand, above 78 °C crystalline boehmite will be formed by the dehydration of bayerite. This shift in the crystallization mechanism during the hydrolysis was ascribed to a change in the reaction kinetics and the thermodynamics, which were not studied in detail. Svedberg et al.8 studied the corrosion of AlN powder in aqueous solutions at various constant pH values (5, 8, 11, 14) when heated to 85 °C for 1 h. In all the pH regimes the (pseudo)boehmite and bayerite/ gibbsite phases in various proportions were detected with Fourier-transform infrared spectroscopy (FTIR) and XRD analyses. It is now generally accepted that bayerite is the main aluminum trihydroxide crystallizing at a lower hydrolysis temperature, while at higher hydrolysis temperatures boehmite is the predominant hydrolysis product. The exact crystallization sequence during the hydrolysis and additional aging of the precipitates in the suspension remains unclear. In our recent work,18 we have shown that the hydrolysis is made up of three interdependent stages, distinguishing between the formations of various aluminum hydroxides. During the first stage, that is, the induction period, which is the initial slow rate of the hydrolysis, a thin layer of amorphous aluminum hydroxide gel forms on the surface of the AlN particles in the temperature range between 22 and 70 °C. The second stage of the hydrolysis is accompanied by the growth of boehmite, whereas during the third stage it is bayerite that is predominantly being formed.18 The present study was conducted in order to set up a general mechanistic model for AlN powder hydrolysis in diluted aqueous suspensions covering a broad temperature range uniting the so-far distinctive hydrolysis mechanisms at room temperature and at elevated temperatures into a single scheme. The hydrolysis behavior of the 3 wt % AlN powder suspensions was monitored using pH/temperature and 27 Al NMR spectroscopy measurements. The evolution of the amorphous and crystalline aluminum hydroxides during the hydrolysis of the AlN powder suspensions, starting from various temperatures in the range from 22 to 90 °C, was followed by thermogravimetric/differential thermal analysis (TG/DTA), XRD, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses. Finally, the mechanisms of aluminum hydroxide formation during the hydrolysis and after an additional aging are also discussed.

Article

EXPERIMENTAL SECTION

AlN Powder. The AlN powder used in this study was AlN grade C (H.C. Starck, Berlin, Germany), synthesized by the direct nitridation of alumina, and according to the supplier it has a nominal particle size of 1.2 μm, a specific surface area of 3.2 m2/g, and an oxygen content of 2.2 wt %. The morphology of the powder is shown in the SEM micrograph in Figure 1. The powder consists of particle aggregates of

Figure 1. Scanning electron micrograph of the as-received AlN grade C powder. various shapes and sizes, where the particle size of the crystallites varies from a few hundred nanometers to several micrometers. Several AlN rods are also present in the powder. The surface of the AlN particles was free of any hydrated oxide or hydroxide layer, which was confirmed and shown in our previous study.18 Hydrolysis Tests. For the hydrolysis tests, aqueous dilute suspensions containing 3 wt % AlN powder were prepared in deionized water, where the water temperature was set to 22, 50, 60, 70, 80, and 90 °C. The time-dependent pH and temperature profiles of the AlN powder suspensions were recorded using a Metrohm 713 pH meter and a combined pH-glass electrode linked to a PC. The data were collected with VesuvTM software (Metrohm AG, Herisau, Switzerland). For the 27 Al NMR spectroscopy measurements (300 MHz Varian Unity INOVA spectrometer) of the solvent, after the hydrolysis of the AlN powder suspension, the suspension was filtered with a 0.2-μm PTFE syringe filter (Minisart, Sartorius, Goettingen, Germany). Characterization of the Powders. The powders for the characterization were prepared as follows: AlN aqueous suspensions at particular starting temperatures and times were filtered and thoroughly washed with 2-propanol to remove the excess water and to exclude any possible further hydrolysis. The cakes were dried at 80 °C for 24 h and then stored in plastic, airtight containers for subsequent analysis. The samples were labeled as 22C-11H, 50C-61M, or 90C-12D, etc., where the first number accompanied by the letter C stands for the starting temperatures of the AlN powder suspension in degrees Celsius, while the last number and the letter stand for the time period of the powder in the suspension (H - hours, M - minutes, and D - days). The TG/DTA analyses of the as-received AlN powder and the hydrolyzed AlN powders were performed at a heating rate of 10 °C/ min up to 1300 °C in flowing air/argon using a Jupiter 449 (Netzsch, Selb, Germany) instrument. On the basis of the TG analyses the mass ratio between the aluminum hydroxide and the AlN in the hydrolyzed powders was evaluated. At 800 °C, the solid reaction products, that is, aluminum hydroxides, were already dehydrated and transformed into alumina, since the mass of the sample was constant, whereas the remaining AlN in the powder started to oxidize at higher temperatures in the following reaction:19

4AlN + 3O2 → 2Al2O3 + 2N2 1300

(4)



Article

which resulted in a mass increment of the powder, due to the formation of alumina. The conversion of the AlN powder in water (XAlN) was calculated using the following equation:

XAlN = 1 −

exothermic temperature increments (ΔT): the first one (ΔT22 °Cα) in the form of a quasi-plateau, which was lower and was followed by a more exothermic one (ΔT22 °Cβ), coinciding with the additional abrupt increase in the pH value. The preheated suspensions possessed only single temperature increments. Moreover, the increase in temperature of the suspensions preheated to 80 °C (not shown) and to 90 °C was stopped and overlapped by the boiling of the suspension in an open beaker, which is manifested by a plateau. Additional data that were extracted from the hydrolysis tests are listed in Table 1.

m Al2O3 + AlN /mf mAlN /mf,AlN

(5)

where mAl2O3+AIN is the mass of dehydrated hydrolyzed sample at 800 °C containing only alumina (Al2O3) and AlN, mf is the final mass of the analyzed sample at 1300 °C, mAlN is the mass of the as-received AlN sample, and mf,AlN is the mass of the fully oxidized, as-received AlN sample after the TG analysis. The XRD data were collected on a PANanalytical X’Pert PRO diffractometer (Almelo, The Netherlands) using Co Kα radiation. The particle sizes of the boehmite phase in hydrolyzed powders were estimated from the XRD peak-broadening using Diffrac plus TOPAS software. SEM and TEM micrographs were collected on a FE-SEM Zeiss Supra 35LV (Supra 35LB, Carl Zeiss, Oberkochen, Germany) and JEM 2100 (Jeol, Tokyo, Japan) operating at 1 kV and 200 kV, respectively.

Table 1. Data Collected during the Hydrolysis Tests of 3 wt % AlN Powder Suspensions at Different Starting Temperatures

■

RESULTS Hydrolysis Tests. The hydrolysis tests of a 3 wt % AlN powder water suspension were performed in order to evaluate the environmental conditions under which the aluminum hydroxides form. The ammonia evolution (Reaction 1) and its dissociation (Reaction 2) together with the heat evolved during the hydrolysis reactions can be followed by simply monitoring the pH and the temperature change of the suspensions. The results of the hydrolysis tests performed at different starting temperatures of the suspensions are presented in Figure 2.

α

T [°C]

22

50

60

70

80

90

pHmax ΔT [°C] tΔT [min]

11.3 3.6α/6.2β 660α/1620β

10 17.9 61

9.8 16.5 30

9.6 17.1 15

9.3 15.3c 12

9.3 7.7c 10

1st exothermic temperature increment. β2nd exothermic temperature increment. cBoiling of the AlN powder suspension.

The inversely proportional solubility of the ammonia decreased with higher starting temperatures of the suspensions,20 and thus the pHmax reached the lowest value of 9.3 at 80 °C, indicating the temperature at which the minimum solubility of ammonia was obtained. The following ΔT values of the suspensions were recorded: the ΔT22 °Cα and ΔT22 °Cβ were 3.6 and 6.2 °C, whereas ΔT50 °C, ΔT60 °C, and ΔT70 °C, were all about 17 °C. The heat evolved during the exothermic hydrolysis reactions resulted in the boiling of the suspensions that were initially preheated to 80 and 90 °C, lowering their respective ΔT values. The hydrolysis rate was progressively higher with higher starting temperatures, which can be concluded from the ever shorter time needed for the onset of the cooling of the suspension temperature after obtaining the respective ΔT value (tΔT) (Table 1). Powder Characterization. The hydrolyzed powders for further characterization were selected and divided into three major groups, depending on whether the hydrolysis and/or aging processes were taking part in the AlN powder suspension. The first group of powders was taken out from the suspensions at tΔT22α for the 22 °C and at tΔT for the preheated suspensions (Table 1) in order to characterize the aluminum hydroxides formed during the first temperature increment at 22 °C and during the single temperature increment at elevated temperatures (Figure 2). The second group of powder samples was taken out of the suspension after 24 h to cover the second pH and the temperature increment of the hydrolysis at 22 °C (tΔT22β) and to analyze the aging phenomenon of the formed aluminum hydroxides in the initially preheated AlN powder suspensions. The third group of powder samples was characterized after 12 days in the suspension in order to observe the distinctive aging mechanisms in all the temperatures, after the AlN was completely consumed. The TG/DTA analysis proved to be a powerful tool for providing useful data on the dehydration of aluminum hydroxides into aluminas and to follow the conversion of the AlN powder in water (XAlN) during the hydrolysis. The oxidation of AlN takes place at about 800 °C according to Reaction 4 and the conversion was calculated based on the

Figure 2. pH and temperature change versus time for a 3 wt % AlN powder suspension in water at 22, 50, and 90 °C.

For reasons of clarity, only three suspensions are plotted versus time, that is, 22, 50, and 90 °C. After the AlN powder was added to the water, the pH started to increase immediately, indicating the onset of the hydrolysis reaction. Note that the pH value of the AlN powder suspension at 22 °C abruptly increased to 10.6 after 21 h of hydrolysis after almost settling at a value of 10.34. The maximum pH (pHmax) values of the suspensions were obtained sooner at higher starting temperatures, indicating a faster hydrolysis rate. In the lower part of the diagram, the three characteristic time-dependent temperature profiles of the AlN powder suspensions are plotted versus time. The AlN powder suspension at 22 °C exhibited two 1301



Article

trihydroxides must have taken place up to the point when tΔT was reached (Table 3), as in accordance with the DTA analysis of the 50C-61M sample (Table 2, Figure S1b). The conversion of the AlN powder in water (Table 2) showed that times longer than tΔT (Table 1) only resulted in the aging of the formed aluminum hydroxides in the suspensions that were initially preheated. Aging of the suspensions preheated to 50 °C, and to a minor extent to 60 °C, for 24 h and 12 days resulted in the slow dissolution of the boehmite and the crystallization to bayerite via the D-R process (Figure S2b,c), when the AlN was already depleted (Table 3). At 70 °C, on the other hand, irrespective of the aging time, the intensities of the boehmite remained unchanged, which resulted in a minimal increase in the bayerite and nordstrandite/gibbsite intensities from 24 h to 12 days (Figure S2d). Compared to the lower hydrolysis temperatures, the gibbsite was hardly detectable. At the two highest hydrolysis starting temperatures, that is, 80 and 90 °C, longer aging times resulted in increased peak intensities for the boehmite’s (020) and (021) crystallographic planes (Figure S2e,f), indicating the ripening of the boehmite crystallites after 24 h of aging. This process was more pronounced at 90 °C. While the intensity of the nordstrandite peaks decreased with an increasing hydrolysis temperature, at 90 °C gibbsite was not detected at all (Table 3). After 12 days of aging, the increment in the intensities of the boehmite peaks was negligible, indicating an arrest of the crystallites’ ripening (Figure S2e,f). The morphology of the AlN powders hydrolyzed at 22, 50, and 90 °C, taken from the suspensions at tΔT22 °Cα and tΔT (Table 1), is shown in Figure 3. According to the XRD results in Table 3, these powders were mainly composed of boehmite and the AlN phases. Sample 22C-11H shown in Figure 3a retained the original morphology of the as-received AlN powder (Figure 1) to some extent, except that the particles are covered with an X-ray amorphous layer of nanometric-sized boehmite nuclei, as shown in the inserted close-up of Figure 3a. This is due to the low conversion at that point, that is, only 23% (XAlN, Table 2). The powder hydrolyzed at 50 °C (sample 50C-61M, Figure 3b) has already lost the morphology of the starting powder, as it contains globules consisting of agglomerated bundles of corrugated sheets (inset of Figure 2b), that is, lamellas, typical for the boehmite phase.24 The powder hydrolyzed at 90 °C possesses a powder morphology similar to the case of 50 °C (Figure 3b), but the boehmite globules consist of fewer lamellas. They are also larger and better defined (sample 90C-10M, Figure 3c). Figure 4 shows the characteristic morphologies of the AlN powders aged for 12 days in the suspensions with starting temperatures of 22, 50, and 90 °C. Sample 22C-12D presented in Figure 4a contains bunches of cones and large rods, which are characteristic of the bayerite somatoids.2 However,

weight gains of the annealed (un)hydrolyzed powders using eq 5. The TG/DTA curves are shown in Figure S1 (Supporting Information), while the calculated XAlN are summarized in Table 2. The respective XAlN values of the 22C-11H and 22CTable 2. The Conversion of AlN Powder in Water (XAlN) in 3 wt % Suspensions at 22, 50, and 90 °C sample

22C11H

22C24H

50C61M

50C24H

90C10M

90C24H

XAlN

0.23

0.68a

0.93a

1a

0.91

1

a

The presence of Al(OH)3 according to the DTA analysis.

24H samples were 0.23 and 0.68, respectively (Table 2). Compared to 22 °C, the hydrolysis rates of the AlN powder in the suspension preheated to 50 and 90 °C were greatly accelerated; that is, after only 61 and 10 min, the hydrolyses were near to their completion, with the XAlN values being 0.93 and 0.91, respectively. The absence of any weight gain in the 50C-24H and 90C-24H samples (Figure S1a) indicates that the conversion of AlN was complete (Table 2), and only the aging processes of the formed aluminum hydroxides were already taking place in the suspension after 24 h. Also displayed in Table 2 is the potential presence of aluminum trihydroxides in hydrolyzed and aged powders (labeled with a) based on the endothermic DTA peaks positioned at 280 °C (Figure S1b), characteristic for the dehydroxylation of aluminum trihydroxides to transient aluminas.2,21,22 On the other hand, in samples 90C-10M and 90C-24H no endothermic peak typical for aluminum trihydroxides was found (Figure S1b), but both exhibited a progressive weight loss up to 450 and 460 °C, respectively (Figure S1a), which should be related to the completion of the boehmite dehydration resulting in the formation of γ-alumina.2,21,23 The evolution of crystalline aluminum hydroxides in the AlN powder suspensions in the temperature range 22−90 °C was monitored with the use of XRD analysis. The crystalline phases found in the hydrolyzed powders are listed in Table 3, while the XRD patterns can be found in Figure S2 (Supporting Information). The hydrolysis of the AlN powder suspension at 22 °C yielded crystalline reaction products in the form of aluminum trihydroxides, but not before 24 h (sample 22C24H; Table 3). At that point the temperature of the suspension was nearing the secondary maximum (Figure 2) and bayerite was the main crystalline phase, accompanied by the traces of gibbsite and nordstrandite (Table 3, Figure S2a). As a result, the growth of aluminum trihydroxides should correspond to the secondary increase in the pH and the temperature of the suspension (tΔT22 °Cβ; Figure 2). On the other hand, the poorly crystalline boehmite phase is present in all the hydrolyzed powders at elevated temperatures at tΔT (Tables 1 and 3). Furthermore, at 50 °C the nucleation and growth of

Table 3. Crystalline Phases Found in the Powders after the Hydrolysis and/or Aging of the AlN Powder Suspensiona 22 °C 11 h A

a

24 h A, B, G 70 °C

50 °C 12 days

61 min

A, B, G, N

A, Bo, B

24 h Bo, B, N, G 80 °C

60 °C 12 days

30 min

24 h

12 days

Bo, B, N, G

A, Bo

Bo, B, G, N 90 °C

Bo, B, G, N

15 min

24 h

12 days

12 min

24 h

12 days

10 min

24 h

12 days

A, Bo

Bo, N

Bo, N, G

A, Bo

Bo, B, N

Bo, B, N

A, Bo

Bo, B, N

Bo, B, N

A - AlN, B - bayerite, Bo - boehmite, G - gibbsite, N - nordstrandite. 1302



Article

Figure 3. Scanning electron micrographs of the AlN powders after hydrolysis in the 3 wt % suspensions at (a) 22 °C (sample 22C-11H), (b) 50 °C (sample 50C-61M), and (c) 90 °C (sample 90C-10M).

Figure 4. Scanning electron micrographs of the AlN powders after 12 days of aging in the 3 wt % suspensions at (a) 22 °C (sample 22C-12D), (b) 50 °C (sample 50C-12D), and (c) 90 °C (sample 90C-12D).

amorphous aluminum hydroxide gel, which is spontaneously formed on the surface of the AlN particles during the initial stage of the hydrolysis, the induction period,18 by the following reaction:

according to the XRD analysis, traces of nordstrandite and gibbsite should also be present in the powder (Figure S2a and Table 3), but they could not be discerned in the micrograph. The powder hydrolyzed and aged for 12 days at 50 °C (sample 50C-12D; Figure S2b and Table 3) is composed of large, ovoid, rod-like particles with attached leftovers of the agglomerated bundles of boehmite lamellas (close-up in Figure 4b). The morphology of the bayerite somatoids found in this sample differs from that observed in the sample 22C-12D (Figure 4a). The untransformed residues of the boehmite lamellas in the 50C-12D sample support the occurrence of the D-R process during the 12-day aging, resulting in decreased boehmite peak intensities in the XRD patterns of the powders hydrolyzed at 50−60 °C (Figure S2b,c). In contrast, 12 days of aging of the AlN powder hydrolyzed at the highest temperature, that is, 90 °C (sample 90C-12D), shown in Figure 4c, resulted in a ripening of the boehmite crystallites. Compared to the 90C10M sample, the globules of bundles of agglomerated lamellas are even larger and the lamellas are also thicker, which is in agreement with the observed increase in the intensity of the boehmite’s (020) and (021) reflections shown in Figure S2f.

AlN + 4H2O → Al(OH)3 (OH2)gel + NH3

(6)

The hydrolysis rate is slow during the so-called induction period. However, the overall rate increases with an increasing temperature, such that at temperatures higher than 70 °C the period was so short or nonexistent, and we were not able to detect it anymore.18 In general, the amorphous aluminum hydroxide gel is stable in a solution containing aluminum mononuclear species in the pH range 5−10 at 25 °C. This is the pH range were all existent mononuclear hydrated aluminum species are in the equilibrium, that is, [Al(H2O)6]3+, [Al(H2O)5(OH)]2+, [Al(H2O)4(OH)2]+, [Al(H2O)(OH)3]0, and [Al(OH)4]−.17,24,25 The neutral [Al(H2O)(OH)3]0 has a solubility limit at about 4.6 × 10−6 mol/ L,26 and when surpassed it results in the precipitation of an amorphous aluminum trihydroxide, also known as the aluminum (hydroxide) gel.2,21 The aforementioned pH range is also passed by the AlN powder suspension as a result of ammonia formation during the hydrolysis reaction (Figure 2, Table 1). In addition to the pH-dependent stability of the gel, the temperature also greatly affects its solubility. Yoldas,27 Pierre and Uhlmann28 were among those who extensively studied the formation of aluminum hydroxide gels prepared by the hydrolysis of aluminum alkoxides and their transformation to boehmite by peptization. They reported on the instability of these gels at temperatures higher than 80 °C, which is in agreement with our observations. The aluminum gel layer grows on the AlN surface/gel-layer interface, while it is being dissolved on its outer surface in contact with water. Previously, we merely detected it using TEM, but the exact duration of its presence was not yet

■

DISCUSSION The Aluminum Hydroxide Gel. The hydrolysis of AlN powder is a dynamic process, where the changes in the pH and the temperature of the AlN powder suspension are time- and temperature-related phenomena. This in turn influences the evolution of the crystalline aluminum hydroxides, as demonstrated in the powder-characterization section. Boehmite and bayerite are the most abundant aluminum mono- and trihydroxides. The crystallization of the latter was also found to be accompanied by the presence of small amounts of nordstrandite and gibbsite. However, the very first short-lived hydrolysis reaction product at temperatures lower than 70 °C will be the 1303



Article

investigated.18 Therefore, we analyzed the DTG curves obtained by TG analysis of the AlN powder and the hydrolyzed powders at a temperature of 22 °C (Figure S1a), since the amorphous aluminum hydroxide gel layer lasts the longest at this temperautre.18 In Figure S3 (Supporting Information) the DTG curves of these powders are presented. According to these results, the amorphous layer should be the thickest after the 2 h of hydrolysis at 22 °C, while after 6 h it is already dissolving. It should be pointed out that the formation of boehmite was also already underway after 3 h of hydrolysis.18 Between 9 and 11 h of hydrolysis the amorphous layer totally dissolved and nucleated into the nanometric-sized crystallites of boehmite, as observed in Figure 3a. At that point, the pH of the suspension surpassed the value 10.15 (Figure 2), which is the pH value just above the stability of the amorphous aluminum hydroxide gel.24,25 The boehmite phase is then the second reaction, but first crystalline product of the AlN powder hydrolysis according to the following reaction:

Al(OH)3 (OH2)gel → AlOOH + 2H2O

temperature and the pH value of the suspension (Table 1) is governed by the ammonia’s solubility and by the release of the Al3+, which should be the driving force for the evolution of aluminum hydroxides during the hydrolysis. At pH values of the suspension higher than 7 the Al3+ ions are released into the alkaline environment forming Al(OH)4− mononuclear species,25 resulting in the nucleation and growth of boehmite, irrespective of the starting temperature. The boehmite formation during the hydrolysis is relatively fast, compared to its growth during the aging of aluminum hydroxide gels.31 The boehmite synthesis conditions during the AlN powder hydrolysis should be similar to those of adding an aluminum salt to a base (8 ≤ pH ≥ 10), where the hydroxylation is immediate and complete, and the precipitation is very fast, resulting in nanometer-sized crystallites.24,32 Thus, the crystallite sizes for boehmite powders formed during the hydrolysis of AlN powder at tΔT, after their extensive growth was completed (Table 1), were evaluated. The powders hydrolyzed at elevated temperatures all exhibited a crystallite size of about 6 nm, irrespective of the starting temperature of the AlN powder suspension. However, as seen in Figure 3b,c, the morphologies of the lamellas in the hydrolyzed powder at 50 and 90 °C were quite different, even though the crystallites were of the same size. We assume that this morphological difference is due to the temperature-dependent growth of the boehmite lamellas. According to the literature, the boehmite lamellas may form by the aggregation of metastable, poorly crystalline hydrated nuclei, rather than by crystal growth.17 Penn et al.,33 who studied the growth of TiO2, FeOOH, and CoOOH nanoparticles, observed a similar phenomenon. They concluded that the dislocation-free nuclei tend to form nanocrystalline material by an irreversible, oriented-aggregation mechanism, resulting in slight misorientations of the freshly aggregated particles. On the basis of the TEM micrograph in Figure 5, we surmise that analogous growth mechanisms could

(7)

It remains unclear, however, whether this transformation could also occur via a dehydration-structuring mechanism that usually happens when boehmite forms from a fresh, highly hydrated, amorphous hydroxide gel by inter- and intraparticle condensation-aggregation.15 The Aluminum Monohydroxide (γ-AlOOH). Poorly crystalline boehmite (γ-AlOOH) was detected in all the AlN powder suspensions at tΔT (Table 1) preheated to elevated temperatures, as evidenced by the results of the XRD analysis (Table 3, Figure 2Sb−f). The exception was the AlN powder suspension at 22 °C, where 10-times longer irradiation times of the hydrolyzed powder prior to the start of the secondary temperature increment (Figure 2) are needed in order to obtain broad peaks representing the (020) and (021) crystallographic planes of poorly crystalline boehmite.18 Thus, the layer consisting of nanometric-sized particles found on the surface of the AlN particles, shown in Figure 3a, represents the onset of the growth of boehmite lamellas that are composed of nanometric crystallites and their growth can therefore be assigned to the first temperature increment (tΔT22 °Cα, Figure 2). The aluminum hydroxide gel was noticed in the 22−70 °C temperature range of the AlN powder suspensions, where it acts as a precursor to the boehmite formation according to Reaction 7. However, as soon as the gel is completely dissolved or in an instance when the AlN is dispersed in the water preheated to 80 and 90 °C, the growth of boehmite obeys the following reaction:

AlN + 2H2O → AlOOH + NH3

(8)

In the case of crystals that exhibit a variety of polymorphs, the least stable and most soluble phase usually precipitates first, since it is favored kinetically. 17 At RT boehmite is thermodynamically less stable than bayerite/gibbsite.29,30 The same holds for the AlN powder suspension in the temperature range 22−50 °C, where the ammonia solubility is high, resulting in higher pH values (Table 1). As a result, boehmite is slowly dissolving with aging times and recrystallizing to aluminum trihydroxides by the D-R process, as confirmed by the XRD results (Figure S2b). On the other hand, boehmite is thermodynamically stable only in the suspensions initially preheated to, at least, 70 °C (Figure S2e,f) and at a pH of 9.6 or lower, respectively. This unique interdependency between the

Figure 5. Transmission electron micrograph showing the upper part of the boehmite lamella (sample 90C-10M).

also be operating during the growth of boehmite lamellas, ultimately leading to imperfectly stacked elongated nanoblocks forming a single lamella. The rate of formation and the subsequent oriented aggregation of these nanoblocks should be temperature- and pH-dependent, resulting in the variety of lamellar morphologies seen in the SEM micrographs (Figure 3b,c). It is known that the crystallites of boehmite tend to form a lamellar structure 1304



Article

not yet occur at that point. The same should hold for the AlN powder hydrolysis at 70 °C. After 15 min of hydrolysis, unreacted AlN is still present in the powder and the intensity of the boehmite peaks did not change throughout the hydrolysis and aging up to 12 days (Figure S2d). This further supports the hypothesis that the trihydroxides are not solely formed by the D-R mechanism (Reaction 9). In order to further confirm this assumption, the following simple experiment was conducted. A 3 wt % AlN powder suspension preheated to 50 °C was filtered after 45 min, when boehmite must still have been forming, but was not yet dissolving. A clear filtrate thus obtained was additionally aged in a glass beaker for 24 h at 50 °C. After the aging, a white precipitate was observed on the surface of the glass beaker, indicating that heterogeneous precipitation must have occurred. As shown in Figure 6, bayerite somatoids with a

during their growth having only two growth dimensions, which makes them 2D particles.24 The Aluminum Trihydroxides (Al(OH)3). Aluminum trihydroxides are the third group of hydrolysis reaction products formed at the end of the hydrolysis reaction and/or most notably during the aging of the suspension. Among the trihydroxides formed during the hydrolysis of the diluted aqueous AlN powder suspension, bayerite is the predominant polymorph and is accompanied by traces of nordstrandite and gibbsite. The latter was identified only in those powders that were hydrolyzed and aged in the temperature range from 22 to 70 °C. The interplay of the hydrolysis-induced parameters in the suspensions governed the various morphologies of the precipitated trihydroxides, as shown in Figure 4a,b. The resulting difference in the somatoid morphology is due to the difference in the temperatures, the pH, and the supply of Al3+ ions. This is in accordance with the study of Lefevre et al.,34 who showed that bayerite somatoids in the form of rods, ovoid rods, or bunches of cones can be obtained, depending on the synthesis conditions, that is, the pH, the time of the synthesis, the presence of the boehmite phase, and the feeding rate of the Al3+ ions. It has been generally accepted that during the AlN powder hydrolysis the formation of bayerite occurs via the D-R process (Reaction 3), where bayerite forms at the expense of an amorphous or crystalline monohydroxide. The D-R was set as the sole mechanism for the bayerite formation during the hydrolysis at RT by Bowen et al.5 Our results presented in Figure S2b and Figure 4b indicate the transformation of crystalline boehmite into trihydroxides in the temperature range 22−50 °C, by the D-R process according to the following the reaction:

AlOOH + H2O → Al(OH)3

Figure 6. Precipitated bayerite somatoids on the surface of the glass beaker, where the filtrate of the 3 wt % AlN powder suspension hydrolyzed for 45 min at 50 °C was additionally aged for 24 h at the same temperature.

(9)

According to the literature,24 crystalline boehmite transforms into bayerite at temperatures lower than 40 °C and at pH values higher than 10. This is in line with the aging of the diluted aqueous AlN powder suspensions where the boundary temperature in which the D-R process will be operative was at 60 °C and where the suspension also exhibited a maximum pH value of 9.8 (Table 1). At still higher temperatures the D-R process was not observed anymore. It should be pointed out, however, that trihydroxides are not always necessarily formed by the D-R process, but their crystallization path should also obey the following reaction:

AlN + 3H2O → Al(OH)3 + NH3

similar morphology to that found in the sample 50C-12D (Figure 4b) were formed on the surface of the glass beaker. Since prior to aging the filtrate was clear, free of any colloidal boehmite particles, the necessary supersaturation of the [Al(OH)4]− species needed for the crystallization of bayerite (and gibbsite/nordstrandite) must have been provided by Reaction 10 and not by the dissolution of boehmite lamellas (D-R process), thereby confirming the initial assumption. Additional experimental support was obtained using the 27Al NMR spectroscopy analysis of the filtrate, where the presence of the [Al(OH)4]− species was confirmed (Figure S4). Mechanistic Model. We propose the following mechanistic model for the hydrolysis and aging of the AlN powder in diluted aqueous suspensions in the temperature range from 22 to 90 °C (Figure 7). Irrespective of the starting temperature of the AlN powder suspension, at least 4 out of 7 reaction mechanisms may occur in the suspension, depending on whether the hydrolysis of the AlN powder and/or the aging phenomena of the aluminum hydroxides take place in the suspension. At temperatures lower than 70 °C, a fewnanometers-thick layer of amorphous aluminum hydroxide gel will form on the surface of the AlN particles (Mechanism I) during the induction period.18 At the same time, ammonia, as a gaseous byproduct of the hydrolysis reactions, is being dissolved, thereby increasing the concentration of OH− ions in the suspension. As a consequence, the pH value of the suspension is increased (Mechanism II) until the maximum

(10)

Fukumoto et al.7 anticipated that bayerite is the first crystalline reaction product during the hydrolysis of a diluted aqueous AlN powder suspension at various temperatures. The assumption was based on thermodynamic calculations, but was not experimentally confirmed. It was further assumed that the crystalline boehmite will be formed by a two-step reaction above 78 °C, implying that Reaction 10 is followed by the dehydration of bayerite, which usually occurs at temperatures above 150 °C.35 Therefore, the validity of Reaction 10 was additionally checked. The results from the X-ray analysis indicate that the nucleation and growth of bayerite was initiated during the hydrolysis of AlN powder at 50 °C when AlN was still present in the powder (Figure S2b). The growth of the initially formed boehmite was not yet completed and the process of D-R did 1305



Article

Figure 7. Mechanistic model for the aluminum hydroxides formation during the hydrolysis and aging of the AlN powder in diluted aqueous suspensions in the temperature range 22−90 °C.

the ammonia solubility and the exothermic hydrolysis reactions producing Al(OH)4− species, is the driving force for the evolution of various amorphous and crystalline aluminum hydroxides. The impact of an additional aging of the suspensions after the end of the hydrolysis process on the asformed aluminum hydroxides was also investigated. We showed that by controlling the hydrolysis and aging processes in diluted aqueous AlN suspensions, it is possible to prepare powders consisting of the various aluminum hydroxides phases. Furthermore, several morphologies of the individual phase can be prepared by varying the suspension temperature and aging time.

solubility of ammonia at a given temperature is reached (Figure 2). The increase in the pH value and the temperature of the suspension result in the dissolution of the amorphous aluminum hydroxide gel layer and recrystallization to poorly crystalline boehmite (Mechanism III). When the dissolution of the amorphous hydroxide gel is completed, the OH− ions have a free path to the surface of the AlN particles. Thus, the formation of poorly crystalline boehmite can proceed according to Mechanism IV, speculatively by oriented aggregation.34 The crystallization of boehmite is followed by the extensive formation of aluminum trihydroxide (Mechanism V), where bayerite is the major phase. This occurs at pH values greater than 9.3 and at temperatures lower than 80 °C. In the AlN powder suspension preheated to 80 and 90 °C, however, a small amount of trihydroxides also forms after the pHmax value of the suspension is obtained (Table 1). The principle of the last two mechanisms, which correspond to the aging of the aluminum hydroxides in the alkaline suspensions, is governed by the equilibrium solubility of ammonia with respect to the temperature of the AlN powder suspension and of the concentration of the [Al(OH)]4− species. The transformation of boehmite into the thermodynamically more stable trihydroxides by the D-R process (Mechanism VI) is provoked when the temperature of the suspension is in the range 22−60 °C after the conversion of the AlN powder is almost complete (tΔT; Table 2) and is more explicit at lower temperatures. It is also clear that aluminum trihydroxides and boehmite are in equilibrium in the temperature range 60−80 °C, while the ripening of the latter takes place at temperatures higher than 80 °C (Mechanism VII), where the solubility of ammonia reaches its lower value (Table 1).

■

ASSOCIATED CONTENT

S Supporting Information *

The TG/DTA curves along with the analyzed XRD patterns of the hydrolyzed and/or aged powders, the DTG curves of the hydrolyzed powders and the spectra of the 27Al NMR spectroscopy analysis obtained from the filtrate of the AlN powder suspension are available in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +38614773940. Fax: +38614773171.

■

ACKNOWLEDGMENTS The authors are thankful to Dr. Darko Kocjan for performing the 27Al NMR spectroscopy analysis and to Aljaž Ivekovič for his assistance in the graphical modeling of figures. The support by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the National Research Program is gratefully acknowledged.

■

CONCLUSION The evolution of aluminum hydroxides during the hydrolysis of 3 wt % AlN powder in aqueous suspensions, in the temperature range 22−90 °C, was studied in detail. A general mechanistic model for AlN powder hydrolysis in diluted aqueous suspensions over a broad temperature range was proposed, uniting the so-far separate treatments of hydrolyses at room temperature and at elevated temperatures into a single scheme. The unique interdependency between the temperature and the pH of the hydrolyzing AlN powder suspension, governed by

■

REFERENCES

(1) Heslop, R. B.; Jones, K. In Inorganic Chemistry: A Guide to Advanced Study; Eslevier Scientific Publishing Co. 1976. (2) Wefers, K.; Misra, C. In Oxides and Hydroxides of Aluminum; Technical Paper No. 19, Alcoa, Pittsburgh, PA, revised 1987. 1306



Article

(3) Vedder, W.; Vermilyea, D. A. Trans. Faraday Soc. 1969, 65, 561− 584. (4) Alwitt, R. S. J. Electrochem. Soc. 1974, 121, 1322−1328. (5) Bowen, P.; Highfield, J. G.; Mocellin, A; Ring, T. A. J. Am. Ceram. Soc. 1990, 7, 724−728. (6) Abid, A.; Bensalem, R.; Sealy, J. J. Mater. Sci. 1986, 21, 1301− 1304. (7) Fukumoto, S.; Hookabe, T; Tsubakino, H. J. Mater. Sci. 2000, 35, 2743−2748. (8) Svedberg, L. M.; Arndt, K. C.; Cima, M. J. J. Am. Ceram. Soc. 2000, 83, 41−46. (9) Krnel, K.; Drazic, G.; Kosmac, T. J. Mater. Res. 2004, 19, 1157− 1163. (10) Kocjan, A.; Krnel, K.; Kosmac, T. J. Eur. Ceram. Soc. 2008, 28, 1003−1008. (11) Novak, S.; Kosmac, T.; Krnel, K.; Dražič, G. J. Eur. Ceram. Soc. 2002, 22, 289−295. (12) Mobley W. M. In Colloidal properties, processing and characterization of aluminum nitride suspensions. Ph.D. Thesis. Alfred University, Alfred, NY; 1996. (13) Takahashi, M.; Kataoka, Y.; Chieh, C.-S.; Oya, M.; Fuji, M. Key. Eng. Mater. 2003, 247, 45−50. (14) Pierre, A. C.; Uhlman, D. R. J. Am. Ceram. Soc. 1987, 70, 8−32. (15) Bye, G. C.; Robinson, J. G. Kolloid Z. 1964, 198, 53−60. (16) Aldcroft, D.; Bye, G. C.; Hughes, C. A. J. Appl. Chem. 1969, 19, 167−172. (17) Jolivet, J. P. In Metal Oxide Chemistry and SynthesisFrom Solution to Solid State; Wiley: Chichester, 2000. (18) Kocjan, A.; Dakskobler, A.; Krnel, K.; Kosmac, T. J. Eur. Ceram. Soc. 2011, 31, 815−823. (19) Yue, R.; Wang, Y.; Wang, Y.; Chen, C. Appl. Surf. Sci. 1999, 148, 73−78. (20) Perry, R. H.; Green, D. W. In Perry's Chemical Engineers' Handbook, 7th ed.; McGraw Hill: New York, 1997. (21) Gitzen, W. H. In Alumina as a Ceramic Material; American Ceramic Society: Westerville, OH, 1970. (22) Sarkar, D.; Mohapatra, D.; Ray, S.; Bhattacharyya, S.; Adak, S.; Mitra, N. Ceram. Int. 2007, 33, 1275−1282. (23) Bokhimi, X.; Toledo-Antonio, J. A.; Guzman-Castillo, M. L.; Mar-Mar, B.; Hernandez-Beltran, F.; Navarrete, J. J. Solid. State Chem. 2001, 161, 319−326. (24) Euzen, P. Raybaud, P. Krokidis, X. Toulhoat, H. Le Loarer, J. L. Jolivet, J. P. Froidefond, C. Alumina. In Handbook of Porous Solids; Schuth, F.; Sing., K. S. W.; Weitkamp, J. Wiley: Chichester, 2002; pp 1591−677. (25) Baes, C. F.; Mesmer, R. E. In The Hydrolysis of Cations; Robert E. Krieger Publishing Co.: Malabar, FL, 1986. (26) Xiao, F.; Zhang, B.; Lee, C. J. Environ. Sci. 2008, 20, 907−914. (27) Yoldas, B. E. J. Appl. Chem. 1973, 23, 803−809. (28) Pierre, A. C.; Uhlman, D. R. J. Am. Ceram. Soc. 1987, 70, 28−32. (29) Parks, G. A. Am. Mineral. 1972, 57, 1163−1189. (30) Vayssieres, L. Int. J. Nanotechnol. 2007, 4, 750−775. (31) Violante, A.; Huang, P. M. Clay Clay Minerals 1993, 41, 590− 597. (32) Henry, M.; Jolivet, J. P.; Livage, J. Struct. Bonding (Berlin) 1992, 77, 153−206. (33) Penn, R. L.; Oskan, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177−2182. (34) Lefevre, G.; Pichot, V.; Fedoroff, M. Chem. Mater. 2003, 15, 2584−2592. (35) Digne, M.; Sautet, P.; Raybaud, P.; Toulhoat, H.; Artacho, E. J. Phys. Chem. B 2002, 106, 5155−5162.

1307


Evolution of Aluminum Hydroxides in Diluted Aqueous Aluminum

Recommend Documents