A Versatile Chemical Strategy for Ultrafine AlN and Al−O−N Powders

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J. Phys. Chem. B 2000, 104, 7895-7907

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A Versatile Chemical Strategy for Ultrafine AlN and Al-O-N Powders Jin Yong Kim,† Prashant N. Kumta,*,† Brian L. Phillips,‡ and Subhash H. Risbud‡ Department of Materials Science and Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213, and Department of Chemical Engineering and Materials Science, One Shields AVenue, UniVersity of California, DaVis, California 95616 ReceiVed: February 18, 2000; In Final Form: June 5, 2000

A versatile low-temperature chemical approach utilizing an alkoxide-based hydrazide process was developed for the synthesis of nanometer-size aluminum nitride, oxynitride, and composite powders. The process consists of reacting aluminum tri-sec-butoxide and anhydrous hydrazine in acetonitrile at 80 °C to yield solid precipitates, which, when dried and heated in argon, nitrogen, or ammonia, yielded nanosize powders of the desired chemistry. The precursors and products of the reactions were identified by gas chromatography (GC), chemical analyses, and X-ray diffraction (XRD), while the morphology and particle size of the powders were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). GC revealed the release of butyl alcohol due to the reaction of the alkoxide with anhydrous hydrazine, suggesting the partial replacement (56.5%) of alkoxy groups by hydrazide groups (i.e., formation of Al-NHNH2 species). Fourier transform infrared (FTIR) and 27Al magic-angle-spinning-nuclear-magnetic-resonance (MAS-NMR) spectroscopy provided structural insights regarding the changes in molecular linkages during heating of the precursor and the role of hydrazine in the subsequent nitridation reaction. Hydrazine’s critical function as a nitride former in the initial stages of formation of the precursor, as well as during heat treatment, was further confirmed by studying its reaction with the alkoxide in the presence of controlled amounts of deionized (DI) water. Hydrolysis of the alkoxide while limiting the hydrazide reaction was found to promote the formation of versatile precursors that lead to oxide, oxynitride, or composite powders containing nanoparticles of the nitride phase in an oxynitride matrix.

Introduction Burgeoning efforts directed toward innovative chemical routes for the preparation of ultrafine powders of a wide variety of inorganic substances has been an integrating theme in coupling chemical synthesis and materials processing research during the last two decades. Chemical mixing at the molecular level provides considerable advantages (e.g., chemical homogeneity) for synthesizing multiphase materials, often in the form of nanosize powders, in contrast to the cumbersome mixinggrinding-heating reaction methods which have been customarily used for centuries. Vigorous recent interest in the use of synthesis chemistry for nanophase materials has been driven by the demand for products (e.g., ultrafine powders, thin films) with a multitude of catalytic, electrochemical, and microelectronic or optical applications. For multicomponent oxide chemistries, the alkoxide-based solution-gelation synthesis strategies, popularly called solgel, have reached a fair degree of maturity after nearly 25 years of intense research and development. Thus, sol-gel synthesis methods can now routinely be used to produce fibers, powders, and thin films in a diverse set of oxide systems.1 Regrettably, similar successes in synthesizing non-oxide nanophase materials (e.g., in nitride or sulfide systems) have been slow in coming, especially because of the less-than-forgiving chemistry of the precursors and our limited understanding of the pathways involved in chemical reactions leading to the final product. One * Corresponding author. † Carnegie Mellon University. ‡ University of California, Davis.

major challenge in nitride synthesis, for example, is the control of the molecular structure and the inherent sensitivity of the precursors to -OH moieties, which render it quite difficult to generate metal-N bonds at low temperatures. An indirect conventional sol-gel methodology for making non-oxide powders is based on obtaining an oxide precursor, which is subsequently heated at a high temperature in a suitable gas (e.g., ammonia for nitridation) atmosphere.2,3 In addition to the indirect sol-gel approach, several chemical routes have been reported for synthesizing AlN using polymeric and organometallic precursors such as poly(ethyliminoalane)- and hydroxo(succinato)Al(III) complexes.4-6 While metal-alkoxide chemistry has been largely exploited to generate a variety of oxide phases, the susceptibility of metal centers in the alkoxides to nucleophilic attack (by H2S, RSH, and R2NH) makes them very attractive for synthesizing nonoxide phases such as nitrides and sulfides. This unique feature of the alkoxides was found to be useful in our previous work on the synthesis of two sulfides, TiS2 and NbS2.7,8 Further, we also reported preliminary success in the synthesis of AlN9 by using the same principle of nucleophilic attack of the alkoxide, but the reaction mechanisms and particularly the role of hydrazine in the formation of the nitride were not explicitly addressed. In the present study, we investigated the reaction of aluminum tri-sec-butoxide [Al(OC4H9)3] with hydrazine (N2H4) in acetonitrile solution (CH3CN) to elucidate the role of hydrazine and the mechanism of nitride formation. The reactions were monitored by controlled addition of water to study the influence of hydrazine on the formation of nitride and oxynitride phases.

10.1021/jp000671j CCC: $19.00 © 2000 American Chemical Society Published on Web 07/27/2000

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Figure 1. Flow sheet showing the procedure followed in processes I and II.

Hydrazine is a known weak inorganic base but a strong nucleophile and forms adducts with several metallo-organic compounds including Al, Ti, and Zr alkoxides.10 It is believed that the reaction of Al alkoxides with hydrazine in the presence of oxygenated donor solvents results in the formation of adducts such as Al(OR)3‚xN2H4 (1 e x e 2.5). However, a thorough investigation of the alkoxide reaction with hydrazine in a nonoxygen-containing aprotic polar solvent has not been undertaken. Furthermore, a study of the subsequent reactions leading to the formation of nitride and oxynitride nanophases is also lacking at the present time. Thus, by combining analytical and spectroscopic data, we have focused attention on the chemical reaction of Al alkoxide with hydrazine in solution to form a solid precursor with sufficient versatility that it can be converted by low-temperature processing to oxide, nitride, or composite oxynitride nanomaterials. Experimental Section I. Synthesis. Two different processes were used for synthesizing the solid precursors. The first process, process I, involved the reaction of aluminum tri-sec-butoxide with hydrazine in an acetonitrile solution in the absence of air and moisture. The second process, process II, involved conducting the same reactions with the addition of controlled amounts of water in the absence of air to understand the role of hydrazine as a nitriding agent. Figure 1 shows the procedures employed for synthesizing the precursors in both processes I and II. It must be noted that hydrazine is a very toxic chemical and its vapors could be explosive if brought in contact with air and, therefore, extreme care must be exercised in handling it. In process I, aluminum tri-sec-butoxide [Al(OC4H9)3, 97%, Aldrich] was first dissolved in anhydrous acetonitrile (CH3CN, Aldrich) and heated to 80 °C, followed by refluxing for 1 h. Anhydrous hydrazine (N2H4, 98%, Aldrich) was then carefully added to the solution to achieve a hydrazine-butoxide molar ratio of 10:1. After a few minutes, a turbid solution resulted,

owing to the precipitation of a solid. The reaction was allowed to continue at the same temperature for 12 h, and then the liquid was evaporated under flowing ultra-high-purity nitrogen (UHP N2). The precipitate was dried under vacuum at 120 °C for 12 h, while taking proper precaution against exposure to air and moisture. Hereafter, the precursor synthesized using process I is referred to as HZ in the entire discussion. In process II, water was added either with or without hydrazine to induce precipitation. Two kinds of precursors were generated, using an excess of pure deionized (DI) water and a mixture of anhydrous hydrazine and water, which are referred to as WT and HZ + WT precursors, respectively, in the subsequent discussion. Water was chosen in this approach to initiate hydrolysis and, thereby, to observe its effect on the nitridation reaction of hydrazine. A series of these reactions were selected mainly as control experiments to test the role of hydrazine as a nitriding agent in its reaction with the alkoxide and also to demonstrate the flexibility of this process to synthesize oxide, oxynitride, and nitride powders. Similarly to process I, the alkoxide was first dissolved in anhydrous acetonitrile and heated to 80 °C for 30 min. Hydrazine was then added, maintaining the same molar ratio of hydrazine to butoxide (10:1) that was used to obtain the HZ precursor in process I. Introducing a small amount of DI water to partially hydrolyze the alkoxide then generated the HZ + WT precursor. The molar ratio of water to butoxide was 1:5. The third type of precursor, WT, was synthesized by adding only excess water to the solution of alkoxide in acetonitrile, which initiates hydrolysis and condensation reactions and yields a solid alumina gel similarly to the well-known sol-gel process.11 A water-butoxide molar ratio of 10:1 was used to generate this precursor. In all the processes involving the addition of water, the reaction was allowed to continue at 80 °C for 12 h after the addition of water, followed by distillation of the liquid under flowing UHP N2. The precipitates were then dried under vacuum at 120 °C for 12 h.

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TABLE 1: Heat-Treatment Schedule sample

atm

temp (°C)

duration (h)

heating ratea (°C/min)

product (XRD)

HZ-Ar-800 HZ-N2-800 HZ-NH3-800 HZ-Ar-1000 HZ-N2-1000 HZ-NH3-1000 HZ-Ar-1200 HZ-N2-1200 HZ-NH3-1200 HZ-Ar-1300 HZ-N2-1300 HZ-NH3-1300 HZ + WT-NH3-800 HZ + WT-NH3-1000 HZ + WT-NH3-1300 WT-NH3-800 WT-NH3-1000 WT-NH3-1300

UHP Ar UHP N2 NH3 UHP Ar UHP N2 NH3 UHP Ar UHP N2 NH3 UHP Ar UHP N2 NH3 NH3 NH3 NH3 NH3 NH3 NH3

800 800 800 1000 1000 1000 1200 1200 1200 1300 1300 1300 800 1000 1300 800 1000 1300

10 10 10 10 10 10 10 10 10 15 15 15 10 10 15 10 10 15

2 2 2 2 2 2 5 up to 800 °C, 1 up to 1200 °C 5 up to 800 °C, 1 up to 1200 °C 5 up to 800 °C, 1 up to 1200 °C 5 up to 800 °C, 1 up to 1300 °C 5 up to 800 °C, 1 up to 1300 °C 5 up to 800 °C, 1 up to 1300 °C 2 2 5 up to 800 °C, 1 up to 1300 °C 2 2 5 up to 800 °C, 1 up to 1300 °C

amorphous amorphous AlN amorphous γ-AlON AlN AlN, R-Al2O3, γ-AlON AlN, R-Al2O3, γ-AlON AlN, γ-AlONb AlN, R-Al2O3, γ-AlON AlN, R-Al2O3, γ-AlON AlN γ-AlON γ-AlON, AlNb AlN, R-Al2O3, γ-AlONb γ-Al2O3 γ-Al2O3 R-Al2O3, γ-Al2O3b

a

All samples were furnace-cooled. b Minor phases.

II. Heat Treatments. The as-prepared dried precursors were subjected to various heat-treatment conditions in different atmospheres. Table 1 shows the full heat-treatment profile and the nomenclature followed for naming each heat-treated sample. The as-prepared precursors synthesized using process I were heat-treated in ultra-high-purity argon (UHP Ar, referred to as HZ-Ar), UHP N2 (referred to as HZ N2), and anhydrous ammonia (referred to as HZ-NH3). The heat treatments for each atmosphere were conducted at 800, 1000, 1200, and 1300 °C. The rightmost numbers in the sample nomenclature represent these temperatures. The two kinds of precursors generated using process II, HZ + WT and WT, were subjected only to heat treatments in flowing ammonia at various temperatures, referred to as HZ + WT-NH3 and WT-NH3, respectively, to be compared with the NH3-treated precursor derived using process I (HZ-NH3 samples). In a manner similar to process I (HZ-NH3 samples), NH3 treatments employing final temperatures of 800, 1000, and 1300 °C, respectively, were conducted on the precursors. III. Materials Characterization. Gas chromatography (Hewlett-Packard, 5830A) was conducted to quantify the amount of sec-butyl alcohol released from aluminum sec-butoxide after addition of hydrazine. Standard solutions were prepared using anhydrous sec-butyl alcohol (99.5%, Aldrich) and anhydrous acetonitrile (99.8%, Aldrich) having several molar ratios. Using these standard solutions, area fractions in the GC peaks corresponding to acetonitrile and sec-butyl alcohol are correlated to the molar ratio of acetonitrile to sec-butyl alcohol. To quantify the replacement of butoxy groups in aluminum sec-butoxide with hydrazine, samples were distilled from the solution of Al butoxide and acetonitrile before and after adding hydrazine. First, 18 mL (0.071 mol) of Al sec-butoxide (97%, Aldrich) was dissolved in 200 mL (3.829 mol) of anhydrous acetonitrile at 80 °C and was then stirred for 30 min. The liquid was distilled at this stage to identify the amount of butyl alcohol present before the addition of hydrazine arising from the residual alcohol in the alkoxide. A second distillate was prepared after adding anhydrous hydrazine (98%, Aldrich) to the solution of Al secbutoxide (18 mL) and acetonitrile (200 mL). Gas chromatography was conducted on these two distillates. All the heat-treated powders were studied for their phaseevolution and crystallization characteristics using X-ray diffraction, employing a Rigaku θ/θ diffractometer. X-raydiffraction analysis was not conducted on the as-prepared precursors because of their reactivity and potential hazards when

exposured to moisture. Using a Mattson Galaxy Series 5000 FTIR spectrometer, absorption infrared spectra were collected on the heat-treated precursors generated when using process I (HZ-Ar-800, HZ-N2-800, HZ-NH-800, HZ-Ar-1000, HZ-N21000, and HZ-NH3-1000) in the spectral range of 400-4000 cm-1, using the KBr pellet technique.27 Al magic-anglespinning-nuclear-magnetic-resonance (MAS-NMR) spectra (Chemagnetics CMX-400 spectrometer) were obtained for the heat-treated powders with the samples spinning at 18 kHz in a sample-probe assembly fitted for 4 mm (o.d.) rotors. Spectra were also obtained for the as-prepared precursor, HZ, which was sealed in a quartz-glass tube (Wilmad Glass Co.) and spun up to 12 kHz in 5 mm rotors. Peak positions were reported as peak maxima at 104.3 MHz, uncorrected for second-order quadrupolar shifts, and referenced to external Al(H2O)63+ in a 0.1 M Al(NO3)3 solution. Samples heat-treated in NH3 at 1000 °C (HZ-NH3-1000, HZ + WT-NH3-1000, and WT-NH3-1000) were chemically analyzed by Galbraith Laboratories (Knoxville, TN) for Al and N contents. Scanning electron microscopy (SEM), employing a CamScan scanning-electron microscope, was used to observe the morphology of all the as-prepared and heat-treated powders derived using process I. The NH3-treated powders obtained using process I were also observed under a transmission electron microscope (TEM, JEOL 1200CX) in order to assess the particle size and morphology of the crystalline phase. Results I. GC Analysis on the Distillates Collected before and after the Addition of Hydrazine in Process I. Figure 2 shows the results of GC analysis conducted on the distillates collected from the solution of aluminum butoxide and acetonitrile before and after the addition of hydrazine. GC analysis on the distillate collected before adding hydrazine shows a very small amount of butyl alcohol (0.070 mol % of butyl alcohol with respect to acetonitrile; refer to Figure 1a). The presence of a small amount of butyl alcohol in the solution even prior to the addition of hydrazine is probably due to either the presence of residual butyl alcohol in the aluminum butoxide or the reaction of the aluminum butoxide with residual water in the acetonitrile or moisture incorporated during handling. After the addition of hydrazine, the amount of butyl alcohol increases due to the reaction of the aluminum butoxide with the hydrazine (3.200 mol % of butyl alcohol). Therefore, a 3.130 mol % increase in

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Figure 2. Results of gas chromatography collected from distillates in process I (a) before addition of hydrazine and (b) after addition of hydrazine.

Figure 3. X-ray-diffraction traces of the precursors generated using process I, obtained after heat treatment in UHP Ar: (a) HZ-Ar-800, (b) HZ-Ar-1000, (c) HZ-Ar-1200, and (d) HZ-Ar-1300.

butyl alcohol content was detected due to the reaction of the aluminum butoxide with hydrazine. From the initial molar ratio of aluminum butoxide to acetonitrile (1.845 mol %), the maximum molar ratio of butyl alcohol to acetonitrile is 5.535 mol % when all the butoxy groups in aluminum butoxide are converted to butyl alcohol. Therefore, a 3.130 mol % increase in the butyl alcohol content implies that 56.5% of the butoxy groups are released into the solvent upon the addition of hydrazine. II. Phase Evolution of Heat-Treated Precursors Generated Using Process I. Figure 3 shows the XRD traces collected on the powder heat-treated in UHP Ar. The precursor heat-treated in UHP Ar at 800 °C for 10 h remains amorphous (Figure 3a) but exhibits the onset of crystallization after heat treatment in UHP Ar at 1000 °C for 10 h (Figure 3b). Heat treatment of the precursor in UHP Ar at 1200 °C for 10 h yields the crystalline phases AlN, R-Al2O3, and γ, which could be either γ-Al2O3 or γ-AlON (Figure 3c). Powders obtained after heat treating the as-prepared precursors in UHP Ar at 1300 °C for 15 h show increased intensity for peaks corresponding to AlN and R-Al2O3 relative to the phase denoted as γ (Figure 3d). This result

Kim et al.

Figure 4. X-ray-diffraction traces of the precursors generated using process I, obtained after heat treatment in UHP N2: (a) HZ-N2-800, (b) HZ-N2-1000, (c) HZ-N2-1200, and (d) HZ-N2-1300.

Figure 5. X-ray-diffraction traces of the precursors generated using process I, obtained after heat treatment in NH3: (a) HZ-NH3-800, (b) HZ-NH3-1000, (c) HZ-NH3-1200, and (d) HZ-NH3-1300.

indicates that the phase labeled as γ is most likely γ-AlON, which undergoes decomposition to AlN and R-Al2O3 at 1300 °C. XRD traces collected on the powders heat-treated in UHP N2 are shown in Figure 4. Similar to the Ar-treated samples, the powders treated in UHP N2 at 800 °C appear to be amorphous (Figure 4a). However, after heating at 1000 °C, the powder exhibits different features when compared to the sample heat-treated in UHP Ar at the same temperature (compare Figure 4b with Figure 3b). The N2-treated sample clearly shows the presence of a single phase of γ-AlON, whereas the Ar-treated powder shows broad peaks of an unidentifiable nanocrystalline or poorly crystalline phase. This result seems to be related to the reaction of the as-prepared precursor with N2. Heat treatment of the as-prepared precursor in N2 at 1200 °C for 10 h results in the formation of AlN and R-Al2O3, most likely from decomposition of the AlON phase (Figure 4c). At 1300 °C (Figure 4d), the intensities of peaks corresponding to AlN and R-Al2O3 increase due to further decomposition of the AlON phase, which is known to be unstable above 1000 °C. Heat treatment of the precursors in NH3, however, results in an entirely different phase-evolution behavior in comparison with the Ar- or N2-treated samples (Figure 5). The powder heattreated in NH3 at 800 °C for 10 h exhibits broad peaks

Ultrafine AlN and Al-O-N Powders

Figure 6. Comparison of infrared absorption spectra collected on the precursors heat-treated at 800 °C in UHP Ar and UHP N2: (a) HZAr-800 and (b) HZ-N2-800.

corresponding to AlN (Figure 5a), whereas the powders remained largely amorphous after heat treatment at this temperature in UHP Ar and UHP N2 (compare Figures 3a and 4a). The powder continues to show peaks characteristic of crystalline AlN after heat treatment in NH3 at 1000 °C for 10 h (Figure 5b). Heat treatment in NH3 at 1200 °C for 10 h shows the growth in the intensities of AlN peaks as expected, while small peaks of R-Al2O3 and crystalline γ-phase (which is probably γ-AlON; see Discussion) also appear (see Figure 5c). This result suggests that, although the precursors show AlN as the principal crystalline phase after heat treatment in NH3 at 800 and 1000 °C, it may still contain remnant single or multiple phases of amorphous oxide. This oxide phase crystallizes to form the γ-phase and R-Al2O3 at 1200 °C. These crystalline phases labeled as γ, as well as R-Al2O3, disappear after heat treatment in NH3 at 1300 °C for 15 h and single-phase AlN is obtained (refer to Figure 5d). On the other hand, similar heat treatments of the precursor at 1300 °C for 15 h in UHP Ar and UHP N2 show a mixture of AlN, R-Al2O3, and γ-AlON. This result indicates that heat treatment under NH3 removes nearly all of the oxygen from the sample. III. IR Analysis on Heat-Treated Precursors Generated Using Process I. Infrared absorption spectra obtained on the HZ-Ar-800 and HZ-N2-800 samples are shown in Figure 6. Overall, the IR spectra collected on both the HZ-Ar-800 and the HZ-N2-800 samples are similar. In both cases, an intense vibration at 2160 cm-1 is observed. This absorption is due to the stretching vibration of the CtN bond (νCtN),12,13 suggesting the presence of terminal nitrogen bonds on the carbon residue within the structure obtained after pyrolysis. There are also peaks at 1530 and 1330 cm-1 in both samples. The absorption at 1330 cm-1 corresponds to carbon vibrations in disordered graphite (indicated as D in Figure 6),14,15 and that at 1530 cm-1 corresponds to the carbon vibrations in a graphitic-carbon network modified by nitrogen atoms (indicated as G in Figure 6).15 The absorption peaks in the range between 750 and 500 cm-1 correspond to aluminum-related linkages. Both the HZAr-800 and HZ-N2-800 powders reveal broad peaks at 750 and 540 cm-1, a small peak at 700 cm-1, and a shoulder at 600 cm-1. The broad peak centered at 750 cm-1 corresponds to both Al-N and Al-O vibrations (Al-O vibration is observed at ≈750 cm-1 for γ-Al2O3 and γ-AlON),16,17 while the lowintensity peak centered at 700 cm-1 corresponds to the Al-N linkage.16 The shoulder shown at 600 cm-1 and the broad peak centered at 540 cm-1 correspond to Al-O and pseudo-γ-Al2O3, respectively.16 The presence of Al-N linkages in the powder obtained after heat treating the precursor in UHP Ar agrees with the XRD data, which show peaks for γ-AlON and AlN at higher temperature. This is an indication of the reaction of the alkoxide with hydrazine.

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Figure 7. Comparison of infrared absorption spectra collected on the precursors heat-treated at 1000 °C in UHP Ar and UHP N2: (a) HZAr-1000 and (b) HZ-N2-1000.

Figure 8. Infrared absorption spectra collected on the precursors heattreated in NH3: (a) HZ-NH3-1000, (b) HZ-NH3-800, and (c) the difference between these two spectra. Scale of the difference spectrum has been expanded by a factor of 5.

Figure 7 shows the IR absorption spectra obtained for the HZ-Ar-1000 and HZ-N2-1000 samples. Similar to the samples heat-treated at 800 °C, both the HZ-Ar-1000 and HZ-N2-1000 samples reveal an intense vibration at 2160 cm-1 corresponding to the stretching vibration of the CtN triple bond (νCtN). Furthermore, these samples also exhibit peaks at 1330 and 1530 cm-1 that correspond to carbon vibrations in disordered graphite and the carbon vibrations in a graphitic-carbon network, respectively. Some differences occur in the range of 750-500 cm-1 related to Al-N and Al-O linkages. Compared to the samples heat-treated at 800 °C, the powders heat-treated at 1000 °C in either UHP Ar or UHP N2 exhibit collapse of the peak centered at 540 cm-1 assigned to pseudo-γ-Al2O3. This result indicates that the absorption features related to γ-Al2O3 disappear during heat treatment at the higher temperature of 1000 °C. Another distinct feature exhibited by the samples heat-treated at 1000 °C is the presence of a shoulder at 600 cm-1 corresponding to Al-O linkages. The intensity of this peak observed in the N2-treated sample (HZ-N2-1000) is relatively small when compared to the precursor heat-treated in UHP Ar at 1000 °C (HZ-Ar-1000; refer to Figure 7a,b). The IR absorption spectra of the precursors heat-treated in NH3 at 800 and 1000 °C show different features when compared to the Ar- and N2-treated samples (Figure 8). The precursors heat-treated in NH3 at 800 and 1000 °C show N-H-stretching vibration (3440 cm-1)18-20 and stretching vibration of the CtN bond (νCtN, 2160 cm-1).12,13 In addition, the precursors show CdC stretching vibration (νCdC, 1625 cm-1),21 carbon vibrations representative of graphitic-carbon network (G, 1530 cm-1)15 and disordered graphite (D, 1330 cm-1),14,15 and Al-N stretching vibration (700 cm-1).16 IV. NMR Spectra of the As-Prepared and Heat-Treated Powders Generated Using Process I. The 27Al MAS-NMR spectrum of the as-prepared precursor HZ shows broad peaks centered near 36 and 12 ppm, a smaller peak at 62 ppm, plus

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Figure 10. SEM micrograph showing the morphology of the asprepared precursor powder.

Figure 9. 27Al MAS-NMR spectra of the as-prepared precursor (HZ) and its derivatives obtained by heat treatment: (a) as-prepared precursor, (b) HZ-Ar-800, (c) HZ-N2-800, (d) HZ-NH3-800, and (e) HZ-NH31300. Spectra of all heat-treated samples were obtained at 104.3 MHz with single-pulse excitation using 0.8-µs pulses (6-µs nonselective 90° pulse length), 0.5-s relaxation delay (1000-4000 acquisitions), and 18kHz spinning rate. Spectrum of as-prepared precursor was obtained using 2-µs pulses, 0.3-s relaxation delay, 12-kHz spinning rate (18 000 acquisitions). Features marked by an asterisk (*) are spinning sidebands.

a shoulder at about -20 ppm (Figure 9a). On the basis of chemical shifts previously reported for well-characterized oxide solids22-26 and aluminum alkoxide oligomers in solution,27 the main peaks at 62, 36, and 12 ppm can be assigned to Al in AlO4, AlO5, and AlO6 coordinations, respectively. The assignment of the peak at 36 to AlO5, rather than a mixed coordination Al(O,N) species, is supported further by the lack of a peak in this chemical-shift range for well-crystalline γ-AlON.9 The shoulder at -20 ppm is most likely due to fine structure from strong second-order nuclear quadrupole effects, although data at another frequency would be required to confirm this interpretation. The powders heated at 800 °C in Ar and N2 give nearly identical spectra (see Figures 9b,c), somewhat similar to that of the as-prepared precursor HZ. However, the peaks are broader and more intense for AlO4 and AlO5 (62 and 34 ppm, respectively), and a narrower, less-intense peak is seen for AlO6 environments (7 ppm). The increase of AlO4 intensity relative to the precursor could result from the formation of a poorly crystalline or amorphous spinel-type oxide or an oxynitride γ-phase. The XRD data show formation of a crystalline γ-phase upon heat treatment at 1000 °C, and the positions of

the AlO4 and AlO6 NMR peaks are similar to those reported previously for γ-Al2O3 and γ-AlON.22,28 All of the powders obtained by heat treating the as-prepared precursor HZ in NH3 show the characteristic peak for hexagonal AlN near 113 ppm.22,29 In addition, peaks due to Al-O environments are seen at about the same positions as those for the as-prepared precursor and the Ar- and N2-treated samples (refer to Figure 9d,e). With increasing annealing temperature from 800 to 1300 °C, the relative intensity of the AlN peak increases sharply, accompanied by corresponding decreases in the peaks due to Al-O environments (65, 36, and 7 ppm). In addition, the AlN peak shifts about 2 ppm and narrows with increased processing temperature, reflecting the increased crystallinity of the AlN that is also apparent from the XRD data. After heating in NH3 at 1300 °C, only a very small fraction of the intensity (0.5 µm in size (Figure 10). Heat treatments in UHP Ar and UHP N2 yield similar morphology, as shown in Figures 11 and 12. Both powders obtained after heat treatment in UHP Ar and UHP N2 at 800 °C show the conchoidal-fracture surfaces of the partially fused clumps of powders, characteristics of glassy materials. The powders heat-treated at 1200 °C and beyond, however, reveal crystallized particles of varying sizes on the conchoidal-fracture surfaces. The only noticeable difference in morphology is found for the powders heat-treated at 1000 °C. The SEM micrograph of the powder heat-treated at 1000 °C in UHP N2 (Figure 12b) clearly shows the crystallized particles, which are largely absent in the samples heat-treated in UHP Ar at the same temperature. The morphology of the powders obtained after heat treatment of the precursors in NH3 is shown in Figure 13. After heat treatment in NH3 at 800 (HZ-NH3-800) and 1000 °C (HZ-NH31000), the morphology of the powders remains essentially unchanged and is similar to the morphology of the as-prepared precursors shown in Figure 10, although the XRD results show growth of the AlN phase (Figure 5). On the other hand, the powders heat-treated in UHP Ar and UHP N2 exhibit different morphologies similar to amorphous glassy materials (Figures

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Figure 11. SEM micrographs representing the fracture surfaces of the precursors obtained after heat treatment in UHP Ar at different temperatures: (a) HZ-Ar-800, (b) HZ-Ar-1000, (c) HZ-Ar-1200, and (d) HZ-Ar-1300.

Figure 12. SEM micrographs representing the fracture surfaces of the powders heat-treated in UHP N2 at different temperatures: (a) HZ-N2-800, (b) HZ-N2 -1000, (c) HZ-N2-1200, and (d) HZ-N2-1300.

11a and 12a). An increase in the temperature of NH3 treatment appears to facilitate the growth of AlN particles and also to initiate some sintering between them. The sintered AlN particles appear to be in the size range of 0.5-1 µm when heat-treated at 1300 °C for 15 h (Figure 13d). All the powders heat-treated in NH3 showed the presence of AlN as identified by the XRD traces (refer to Figure 5). To observe the morphology and size of the crystalline AlN particles further, transmission electron microscopy (TEM) was conducted on these powders. Figure 14 shows the diffraction patterns and the corresponding dark-field images obtained from the precursors heat-treated in NH3 at 800 (HZ-NH3-800), 1000 (HZ-NH31000), and 1300 °C (HZ-NH3-1300). The powders heat-treated in NH3 at 800 and 1000 °C show the diffraction pattern consisting of rings corresponding to nanocrystalline AlN (Figure 14a,c). The dark-field image collected on the precursor heattreated in NH3 at 800 °C exhibits a homogeneous distribution

of nanocrystalline AlN particles