Evolution of aluminum hydroxides at the initial stage of aluminum nitride powder hydrolysis

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Evolution of aluminum hydroxides at the initial stage of aluminum nitride powder hydrolysis Enhui Wang a, Junhong Chen b, Xiaojun Hu a, Kuo-Chih Chou a, Xinmei Hou a,n a b

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, China

art ic l e i nf o

a b s t r a c t

Article history: Received 27 March 2016 Received in revised form 14 April 2016 Accepted 14 April 2016

Aiming at better understanding and controlling the hydrolysis process of aluminum nitride (AlN), the evolution of aluminum hydroxides in AlN powder suspensions (2 wt%) at 60 °C at initial stage, i.e. 0–2 h was systematically investigated. The hydrolysis exhibits three interdependent stages including incubation time (first stage), formation, growth and transformation of boehmite (second stage), and formation and growth of bayerite (third stage). The incubation stage is caused by the dissolution of aluminum hydroxide compound on the surface of AlN powder. The formation and growth of boehmite and bayerite are characterized using various techniques including XRD, FT-IR, Raman, SEM and TEM. Based on these results, a clear schematic conversion diagram of the hydrolysis process of AlN powder at initial stage is presented. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Suspensions B. Surfaces D. Nitrides Hydrolysis

1. Introduction Aluminum nitride (AlN) is of great interest as refractory and substrate material in microelectronics [1,2] owing to its excellent physical properties such as high thermal conductivity (3.2 W/ (cm K)), low coefficient of thermal expansion (4.03 6.0  10 6/K) and high electrical resistivity ( 44  108 Ω cm) [3,4]. Recent economic and environmental concerns related to the use of organicbased media have stimulated an increasing interest toward processing AlN powder from aqueous suspensions. Unfortunately, unprotected AlN powder prones to undergo hydrolysis when in contact with water. The hydrolysis of AlN is also the major problem when dealing with aluminum dross, which is a kind of scum formed on the surface of molten aluminum [5]. It consists of the compounds Al2O3, AlN, MgN, KCl, MgO, Fe3O4, Al2S3 and Al4C3 etc. Approximately 50% of the aluminum dross is now used as a fluxing agent for steelmaking and the rest is treated as industrial waste. The compounds of AlN, MgN, Al2S3 and Al4C3 react with water, e.g. rain to produce harmful gases such as NH3, CH4. Moreover these reactions are exothermic, they can be easily ignited. Aiming to solve the above problem, a variety of fundamental and practical research work have been carried out [6–16]. It is known that the hydrolysis of AlN gradually transforms into γAlOOH, Al(OH)3, or γ-Al2O3 as revealed by thermal analysis [6], X-ray photoelectron spectroscopy [7] and Fourier transform n

Corresponding author. E-mail address: [email protected] (X. Hou).

infrared spectroscopy [8] etc. Besides this, some researchers investigated AlN powder hydrolysis at room temperature (RT), focusing on the kinetics and mechanisms of AlN powder degradation in water [9]. The kinetics of the AlN hydrolysis was described by an un-reacted-core model. The chemical reaction at the productlayer/un-reacted-core interface was proposed to be the rate-controlling step during the initial stage of the reaction. In addition, solution calorimetry tests and calculations of the heats of formation for the AlN powder hydrolysis at RT were also performed [10]. The formation of the amorphous mono-hydroxide (AlOOH) is exothermic and so is the formation of bayerite (Al(OH)3). It is known that the hydrolysis of AlN powder is a dynamic process, where the pH and the temperature of the AlN powder suspension increases with time due to the formation of ammonia and the exothermic nature of the hydrolysis [12–14]. However, several discrepancies can be observed when comparing the above results, especially those studies dealing with the microcosmic evolution of AlN powder hydrolysis at initial stage. Some researchers noticed an incubation time prior to the start of the hydrolysis reactions [9,15,16], which was attributed to a thin, hydrated aluminum oxide layer on the surface of the AlN powder. Other researchers did not observe the layer of any kind on the surface of the AlN powder. Therefore, the existence of the protective layer needs to be confirmed and its dissolution process is also required to be clarified. In view of the evolution of aluminum hydroxides, 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 [14]. Unfortunately, the exact crystallization

http://dx.doi.org/10.1016/j.ceramint.2016.04.079 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: E. Wang, et al., Evolution of aluminum hydroxides at the initial stage of aluminum nitride powder hydrolysis, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.079i

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sequence during hydrolysis in the suspension at elevated temperature remains unclear. This hinders the application of AlN powder, for instance, the hydrolysis-assisted solidification (HAS) forming process where AlN powder can be used as a setting agent, the reaction is thermally activated and later proceeds at elevated temperatures, typically 60–80 °C [14]. In this work, the hydrolysis temperature was setted at 60 °C to clearly observe the morphology evolution of AlN powder during hydrolysis. Especially its morphological development at initial stage is systematically investigated using various techniques. Besides, the mechanism of aluminum hydroxide formation during hydrolysis in suspensions is further discussed. The corresponding schematic conversion diagram of the reaction process is obtained. This will lay important foundation for the practical application of AlN.

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3. Results and discussion 3.1. Hydrolysis behavior Fig. 1 shows the pH change versus time for 2 wt% AlN powder suspension at 60 °C. The starting pH of the deionized water is measured to be about 6.5 because CO2 from air is dissolved in the solution during stirring. According to Fig. 1, the hydrolysis process can be divided into three stages. In the first stage, the pH of AlN powder suspension remains unchanged for about 2 min. Based on the reported results [9,15,16], the initially stable stage is called the incubation time. In the second stage, the pH value starts to increase and it reaches up to 10.25 when the hydrolysis time is approximately 60 min. The pH value becomes stable in the third stage. The formation mechanism will be discussed in the later section.

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Fig. 1. pH change versus time for 2 wt% AlN powder suspension at 60 °C.

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1 AlN 2 AlOOH 1 1 3 Al(OH)3 3 1 1 1 2h

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AlN powder used in this study was commercial grade C ( 98 wt%, Advanced Technology & Materials Co., Ltd, Beijing, China) with an average particle size of 3.71 μm, a specific surface area of 2.190 m2/g and an oxygen content of 2 wt%. As for the hydrolysis test, water suspensions of 2 wt% of AlN powder were prepared. Before test, deionized water was preheated with an electric heater under constant stirring to the required temperature, i.e. 60 °C. AlN powder was added into the water. The time-dependent pH profile of the AlN powder suspensions was recorded using a combined glass-electrode/Pt (PHS3C) with a precision of 0.01. To characterize the hydrolysis evolution of AlN powder suspensions at initial stage, samples were taken out from the suspensions at different time within 2 h, filtered and washed with 2-propanol to remove the excess water. Then the resulting cakes were dried at 60 °C for 8 h and stored in the airtight plastic containers. The phase and morphology evolution of the hydrolysis products were characterized using X-ray diffraction (XRD; M21XVHF 22; MAC Science, Japan) analysis over the 2θ range from 10° to 90°, scanning electron microscopy (SEM; JSM-840A, JEOL, Japan) and transmission electron microscopy (TEM; Tecnai G2 F30 S-TWIN, FEI, America). Besides, the microstructure of the hydrolysis products was also investigated using fourier transformation infrared spectroscopy (FT-IR; Nicolet-Nexus 670, Germany) and Raman spectrum (Horiba LabRAM HR Evolution, France). All above experiments were performed within 2 days to exclude any possible further hydrolysis of the AlN powder with moisture in the air.

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2θ (degree) Fig. 2. XRD patterns of AlN powder hydrolyzed at 60 °C for different time.

3.2. Phase and microstructure evolution XRD patterns of the sample hydrolyzed at 60 °C for different time are shown in Fig. 2. For comparision, XRD pattern of AlN powder before hydrolysis is also investigated. It can be seen that AlN powder used in this work belongs to wurtzite type (JCPDS Card no. 25–1133). The characteristic peaks in XRD pattern almost do not change when hydrolysis for 2 min. With the hydrolysis time extending to 5 min, the relative intensity of the characteristic peaks of AlN decreases, indicating the occurrence of the hydrolysis reaction. AlOOH phase appears after the hydrolysis time extending to 30 min. The characteristic peaks of Al(OH)3 can be observed after 1 h. FT-IR analysis is carried out to learn the microstructure evolution during hydrolysis. As for AlN before hydrolysis (Fig. 3a), the wavenumber centered at 710, 1326 and 1385 cm 1 are assigned to the vibration of AlN bond [17]. The peak at 1635 cm 1 is characterized as the presence of physisorbed water [18]. The broad peak at around 3440 cm 1 is attributed to the O–H stretching vibration of AlOH that undergoes hydrogen bonding with neighboring hydroxyl groups [18,19]. Both the above two peaks verify the existence of aluminum hydroxides compound (AHC) on AlN powder surface. This leads to the incubation stage of AlN

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518 cm

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representing the γ-OH in Al(OH)3 can be observed.

3.3. Morphology evolution

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Raman Shift (cm-1) Fig. 3. FT-IR (a) and Raman patterns (b) of AlN powder hydrolyzed at 60 °C for different time.

hydrolysis. With the hydrolysis time prolonging to 5 min, the absorption peak positioned at 710 cm 1 gradually shifts toward lower wavenumber, suggesting the amount of aluminum hydroxides increasing[17]. After hydrolysis for 30 min, peaks at 1060 cm 1 and 1500 cm 1 assigned as the overtone of Al–O stretching fundamental of the growing oxyhydroxide phase (AlOOH) [7] both appear, indicating the appearance of AlOOH. With hydrolysis time prolonging to 1 h, some weak and narrow absorption peaks ranging from 3410 to 3656 cm 1 appear. In addition, a new peak at 980 cm 1 also emerges. These bands correspond to the characteristics of Al(OH)3 [7], confirming the existence of Al(OH)3. This is in agreement with XRD result (Fig. 2). The Raman patterns of AlN powder hydrolysis at 60 °C for different time are also investigated. From the Raman pattern of asreceived AlN powder (Fig. 3), five characteristic peaks, i.e. the A1(TO) mode at 609 cm 1, the A1(LO) mode at 898 cm 1, the E1(TO) mode at 665 cm 1, the E2l mode at 243 cm 1 and the E2h mode at 654 cm 1 correspond well to wurzite AlN [20]. In addition, the phonon frequency of E2h and A1(LO) increases with hydrolysis time extending. This should be resulted from the increasement of some microstructure defect in the material [21]. After hydrolysis for 30 min, the new frequencies of 284 and 350 cm 1 attributed to the vibrational mode of Al–O in AlOOH [22] appear. As hydrolysis time increases to 1 h, a new frequency at

The microstructure of the hydrolysis of AlN powder suspensions at initial stage is further verified using SEM analysis. It can be seen that AlN powder before hydrolysis is characterized by irregular shapes and the particle size varies from a few hundred nanometers to several micrometers (Fig. 4a). After reacting with water at 60 °C for 2 min, some corrugations appearing on the surface of the powders (Fig. 4b). Since the hydrolysis reaction does not take place during this period as shown in Fig. 1, these corrugations are possibly caused by the dissolution of the thin layer of AHC formed on the surface of AlN powders. With hydrolysis time extending to 5 min, some lamella structures in the nanometer scale appear on the surface of the powders as shown in Fig. 4c. EDS result (Fig. 4h) indicates that the lamella structure should be AlOOH. This is in agreement with the reported result [23]. The amount of AlOOH should be very little or poorly crystalline and therefore it can not be detected in XRD patterns (Fig. 2). Due to the formation of AlOOH, the pH value of the suspension increases quickly. With hydrolysis time prolonging, the amount of the crystalline AlOOH phase increases and thus almost all the surfaces of AlN powders are coated with the lamella structure as shown in Fig. 4c–e. When the hydrolysis time extends to 1 h, some phases with somatoid and rod structure come into being (Figs. 4f and g). EDS result (Fig. 4h) indicates that these structures should be Al(OH)3, suggesting the transformation from AlOOH to Al(OH)3 crystal. This is coincidence with the XRD analysis. Given longer hydrolysis time, pH becomes stable, indicating that the hydrolysis reaction can hardly proceed due to dynamic factors. Therefore, the transformation and growth of Al(OH)3 predominate at this stage and thus more Al(OH)3 with rod shape is formed (Fig. 4g). TEM analysis of AlN powder hydrolyzed at 60 °C for different time is performed to obtain more detail knowledge of the hydrolysis process. From Fig. 5a and b, it can be seen that the asreceived AlN powder possesses a homogenous single crystalline structure. In addition, AHC with irregular thickness is observed on the surface of AlN powder as shown Area 2 in Fig. 5b. The thickness of AHC decreases after hydrolysis for 2 min due to dissolution (Figs. 5c and d). The existence of AHC layer on the surface of AlN powder contributes to the phenomenon of incubation and the initially stability of PH value. Once the AHC layer dissolves, the hydrolysis reaction takes place. When the reaction time reaches 30 min, new phase with lamellar and polycrystallite structure appears (Fig. 5e). Combining with the XRD and SEM analysis, the lamellar structure should be AlOOH. After extending to 2 h, the lamellar structure is transformed into rod shape (Fig. 5f). SAED pattern indicates the rod structure is single crystalline. The phase is Al(OH)3 according to XRD and SEM analysis. 3.4. Hydrolysis mechanism From above experimental results, the time-dependent formation of various aluminum hydroxides can be divided into the following sequences as shown in Fig. 6. At the first stage, the thin layer of AHC gradually dissolves and AlN with defective surface is exposed directly to water during the incubation stage (shown in Figs. 5b and d). The pH value stays unchanged during this stage (Fig. 1). Once the AHC dissolves completely, the hydrolysis reaction takes place quickly to form NH3. At this stage, the amorphous boehmite nucleation is formed on the surface of AlN powder (Fig. 4c). It gradually forms polycrystallite lamella structures and covers the surface of AlN powders (Figs. 4d and e). Due to dynamics factors, the hydrolysis reaction rate of boehmite formation decreases and the pH value of solution becomes stable. The

Please cite this article as: E. Wang, et al., Evolution of aluminum hydroxides at the initial stage of aluminum nitride powder hydrolysis, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.079i

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Fig. 4. SEM micrographs and EDS results of AlN powder hydrolyzed at 60 °C for different time. (a) as-received AlN powder; (b) hydrolysis for 2 min; (c) hydrolysis for 5 min; (d) hydrolysis for 10 min; (e) hydrolysis for 30 min; (f) hydrolysis for 1 h; (g) hydrolysis for 2 h; (h) EDS analysis of AlN powder hydrolyzed for 30 min and 1 h.

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5

Fig. 5. TEM micrographs of AlN powder hydrolyzed at 60 °C for different time (a) TEM micrograph and corresponding SAED pattern of as-received AlN powder; (b) HRTEM of as-received AlN powder; (c) TEM micrograph and (d) HRTEM of AlN powder hydrolyzed for 2 min; TEM micrograph and corresponding SAED pattern of AlN powder hydrolyzed for 30 min (e) and 2 h (f).

transformation and growth of bayerite predominate. As a result, rod-shape crystalline bayerite developed from bayerite somatoids is being formed (Fig. 4g).

4. Conclusions The evolution of phase, microstructure and morphology during the hydrolysis of AlN powder suspensions (2 wt%) at 60 °C at initial stage were systematically investigated using PH meter, XRD,

Please cite this article as: E. Wang, et al., Evolution of aluminum hydroxides at the initial stage of aluminum nitride powder hydrolysis, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.04.079i

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6

Fig. 6. Schematic conversion diagram of the reaction process of 2 wt% AlN powder suspension at 60 °C.

SEM, TEM, FT-IR and Raman. There is an incubation time before the onset of AlN hydrolysis due to the existence of a thin layer of aluminium hydroxide compound. After 2-min hydrolysis, this layer is dissolved and the hydrolysis reaction starts, resulting in pH value increasing quickly soon after the complete dissolution of the barrier layer. The amorphous boehmite nucleation is firstly formed on the surface of AlN powder and then grows to polycrystalline lamella structures. With reaction time extending, the hydrolysis reaction rate slows down and the formation of bayerite dominates.

Acknowledgments This study was supported by the National Natural Science Foundation of China (No.51572019), the National Science Fund for Excellent Young Scholars of China (No. 51522402) and the Central Universities of Nos. FRF-TP-13-006A and FRF-TP-15-006C1.

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