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Chem. Mater. 2006, 18, 4716-4718
Aerosol Synthesis of Hollow Spherical Morphology Boron Nitride Particles Gary L. Wood† and Robert T. Paine*,‡ Department of Chemistry and the UniVersity of New Mexico/National Science Foundation Center for Micro-Engineered Materials, UniVersity of New Mexico, Albuquerque, New Mexico 87131, and Department of Chemistry, Valdosta State UniVersity, Valdosta, Georgia 31698 ReceiVed February 2, 2006 ReVised Manuscript ReceiVed August 2, 2006
Commercially produced boron nitride powders are typically prepared in large-scale, metallurgical-type pyrolysis reactions that utilize simple starting materials such as boric acid and urea or melamine.1,2 The powders are often obtained as irregular agglomerates of primary particles that have a platelet morphology resulting from the hexagonal, graphite-like solid-state structure. These powders typically display low surface areas (2-40 m2/g), and they are nonporous. Small-scale chemical vapor deposition or polymer precursor preparations also produce platelet shaped primary particles (0.1-3 µm) or loose agglomerates with low surface areas and minimal porosity.1-3 The commercial powders find numerous applications in the fabrication of high-temperature crucibles, insulators, and mold release liners, as well as in lubricant and composite formulations found in microelectronic devices and cosmetics. Currently, there are several evolving interests in new solidstate materials for catalyst supports, membranes, sensors, and gas adsorbents that would benefit from the availability of high surface area/high porosity non-oxide ceramic powders. In this context, several examples of porous silicon nitride, silicon boron nitride, and gallium nitride materials have recently been reported by Bradley and co-workers4-10 and Kaskel and co-workers.8,11-15 A few reports of the synthesis of mesoporous or microporous boron nitride have also appeared. Pertinent to the current study, Lindquist et al.16 prepared aerogels by critical point drying of poly(2,4,6* Corresponding author: e-mail
[email protected]. † Valdosta State University ‡ University of New Mexico
(1) Paine, R. T.; Narula, C. K. Chem. ReV. 1990, 90, 73. (2) Haubner, R.; Wilhelm, M.; Weissenbacher, K.; Lux, B. Structure Bonding 2002, 102, 2. (3) Paine, R. T.; Sneddon, L. G. ACS Symp. Ser. 1994, 572, 358. (4) Dismukes, J. P.; Johnson, J. W.; Bradley, J. S.; Milar, J. M. Chem. Mater. 1997, 9, 699. (5) Bradley, J.S.; Vollmer, O. Rovai, R. AdV. Mater. 1998, 10, 938. (6) Rovai, R.; Lehmann, C. W.; Bradley, J. S. Angew. Chem., Int. Ed. Eng. 1999, 38, 2036. (7) Vollmer, O.; Lefebvre, F. Bradley, J. S. J. Mol. Catal. A 1999, 146, 87. (8) Farrusseng, D.; Schlichte, K.; Spliethoff, B.; Wingen A.; Kaskel, S.; Bradley, J.S.; Schuth, F. Angew. Chem. Int. Ed. 2001, 40, 4204. (9) Cheng, F.; Toury, B.; Lefebvre, F.; Bradley, J. S.; Lefebvre, F. Chem. Commun. 2003, 242. (10) Cheng, F.; Archibald, S. J.; Clark, S.; Toury, B.; Kelly, S. M.; Bradley, J. S. Lefebvre, F. Chem. Mater. 2003, 15, 4651. (11) Kaskel, S.; Farrusseng, D.; Schlichte, K. Chem. Commun. 2000, 2841. (12) Kaskel, S.; Schlichte, K. J. Catal. 2001, 201, 270. (13) Kaskel, S.; Schlichte, K.; Zibrowius, B. Phys. Chem. Chem. Phys. 2002, 4, 1675. (14) Chaplais, G.; Schlichte, K.; Stark, O.; Fischer, R. A.; Kaskel, S. Chem. Commun. 2003, 730. (15) Chaplais, G.; Kaskel, S. J. Mater. Chem. 2004, 14, 1017.
borazinylamine) gels. Subsequent pyrolysis produced low density, high surface area (∼400 m2/g) BN materials with most of the porosity appearing in mesopores as would be expected from an aerogel precursor. Vacuum pyrolysis (6001200 °C) of another, more compliant polymer precursor, poly(4,6-borazinylamine), gave BN materials with high surface areas (50-700 m2/g) with the porosity dominated (∼80%) by micropores.17 Additional studies with a larger array of poly(borazinylamine) precursors produced BN materials with narrower distributions of surface areas (400-700 m2/g).18 Gas absorption studies with these materials showed promise for selective gas separations (CO2/CH4) as well as gas storage.18 Recently, Auroux and co-workers19 have reported the formation of high surface area BN samples from borazinebased polymer precursors and the examination of these materials as catalyst supports. Dibandjo and co-workers20 have also described the formation of mesoporous BN materials by use of a mesoporous silica template infiltrated with a borazine precursor. Although the properties of these porous BN materials are quite interesting, the difficulty and cost of synthesis of borazine and poly(borazinylamine) precursors limits the utilization of these intriguing materials in most largescale applications. Therefore, the development of alternative low cost approaches to porous BN materials are of interest. During our recent development of aerosol-based routes to non-oxide ceramic powders,21-26 we have occasionally observed the formation of hollow, porous, spherical particles. Although hollow particles are normally considered to be a nuisance contaminant in aerosol produced powders, they could have applications if obtained in dominant amounts.27 As a result, we have explored several possible routes for the formation of hollow, porous BN powders, and we report here their facile formation, with a spherical primary particle morphology, from a simple two-stage aerosol process. Although the literature is nearly silent on the topic, it appears that boric acid is relatively soluble in N,N-dimethylformamide (DMF; 3.9 M at 25 °C28), but without formation of a stable complex.29,30 We observe that freshly prepared unsaturated and saturated H3BO3/DMF solutions readily form (16) Lindquist, D. A.; Kodas, T. T.; Smith, D. M.; Xiu, X.; Hietala, S. L.; Paine, R. T. J. Am. Chem. Soc. 1991, 74, 3126. (17) Borek, T. T.; Ackerman, W.; Hua, D. W.; Paine, R. T. Langmuir 1991, 7, 2844. (18) Janik, J. F.; Ackerman, W. C.; Paine, R. T.; Hua, D.-W.; Maskara, A.; Smith, D. M. Langmuir 1994, 10, 514. (19) Postole, G.; Caldararu, M.; Ionescu, N. I.; Bonnetot, B.; Auroux, A.; Guirnon, C. Thermochim. Acta 2005, 434, 150. (20) Dibandjo, P.; Bois, L.; Chassagneux, F.; Toury, B.; Cornu, D.; Babouneau, F.; Miele, P. Stud. Surf. Sci. Catal. 2005, 156, 279. (21) Pruss, E. A.; Wood, G. L.; Kroenke, W. J.; Paine, R. T. Chem. Mater. 2000, 12, 19. (22) Wood, G. L.; Janik, J. F.; Visi, M. Z.; Schubert, D. M.; Paine, R. T. Chem. Mater. 2005, 17, 1855. (23) Wood, G. L.; Pruss, E. A.; Paine, R. T. Chem. Mater. 2001, 13, 12. (24) Wood, G. L.; Janik, J. F.; Pruss, E. A.; Dreissig, D.; Kroenke, W. J.; Habereder, T.; No¨th, H.; Paine, R.T. Chem. Mater., submitted. (25) Paine, R. T.; Kroenke, W. J.; Pruss, E. A.; Wood, G. L.; Janik, J.F. U.S. Patent 6,824,753 B2, Nov. 30, 2004. (26) Paine, R. T.; Kroenke, W. J.; Pruss, E. A. U.S. Patent 6,348,179 B1, Feb. 19, 2002. (27) Messing, G. L.; Zhang, S.-C.; Jayanthi, G. V. J. Am. Ceram. Soc. 1993, 76, 2707.
10.1021/cm0602601 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/01/2006
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an aerosol mist at 23 °C under ultrasonic agitation without decomposition of solute or solvent. For convenience, in this study, the aerosol mist is ultrasonically generated from a 17.5% (w/w, ∼2.7 M)31 solution contained in a glass vessel as described in previous reports.21-26 The aerosol droplets are transported from the generator by a controlled flow of N2 carrier gas (0.5-2.0 L/min) into a heated (1000-1500 °C) mullite tube (3.5 in. × 60 in.) positioned inside a Lindberg three-zone horizontal tube furnace fitted with gastight end caps. Anhydrous ammonia (99.995%) is simultaneously injected into the furnace (0.5-2.0 L/min), and cream colored to white powders, designated here by the general formula BNxOyCz, are collected on an impact filter at the exit end of the reactor. The minimum powder collection rate under these conditions is ∼1.3 mg/min. The residence time in the reactor hot zone in this first stage of the process varies between ∼20-60 s depending upon temperature and gas flow rates. In stage two, the harvested BNxOyCz powders are transferred to an open geometry boron nitride crucible and placed into an alumina tube, outfitted with gastight connections, held in a Cabolite single zone furnace. Calcinations of BNxOyCz powders are performed under a slow ammonia purge (0.5 L/min) for 8 h (ramp rate 4 °C/min) at 1500 °C. The progress of the boric acid/DMF/NH3 aerosol chemical conversion process (stage one) was followed by oxygen elemental analysis of harvested BNxOyCz powders as a function of aerosol reactor temperature. It is observed that the oxygen contents decrease with increasing process temperature (1100-1500 °C), and the values are dramatically lower than those measured for BNxOy powders obtained from aerosol syntheses that employ H3BO3/H2O solutions.21,26 It is particularly noteworthy that the increase in oxygen content previously observed for BNxOy samples formed above 1300 °C from the H3BO3/H2O/NH3 system is not seen until 1500 °C in the H3BO3/DMF/NH3 system. In addition, the BNxOyCz material produced at 1500 °C still has a relatively low oxygen content (∼18 wt %). This is most likely a direct result of the large reduction of water in the H3BO3/DMF aerosol relative to the H3BO3/H2O aerosol.32 This behavior is clearly summarized in Figure 1: (O) H3BO3/DMF; (2) H3BO3/H2O. In addition, it is also observed that oxygen contents decrease, at fixed process temperatures, as the mole fraction of NH3 increases in the process gas stream. Oxygen content data for BNxOyCz powders obtained under identical conditions from the (MeO)3B/NH3 solvent-free system are also presented in Figure 1: (9) (MeO)3B.24 At process temperatures below 1300 °C, the oxygen contents for powders produced (28) Shvedova, L. V.; Krivtsova, G. E.; Khvatova, E. V.; Shormanov, V. A. IzV. Vyssh. Uchebn. ZaVed., Khim. Khim. Tekhnol. 1985, 28, 55. (29) Paal, T. Magy. Kem. Foly. 1977, 83, 527. (30) Kagawa, S.; Sugimoto, K.-I.; Funahashi, S. Inorg. Chim. Acta 1995, 231, 115. (31) Saturated solutions of H3BO3/DMF tend to deposit boric acid during extended runs because of evaporation of DMF. This is avoided by use of less than saturated solutions. (32) Increases in oxygen content in BNxOyCz or BNxOy stage one powders as a function of process temperature above 1500 °C or 1300 °C, respectively, probably result from back-reaction of the particles with water (steam). In the H3BO3/DMF system the only sources of water originate from in situ dehydration of the H3BO3 and high temperature nitridation of boron oxide and/or boron oxy nitride species. Powders formed in stage one of the H3BO3/H2O system encounter water from the same sources, but the dominate source is from solvent water.
Chem. Mater., Vol. 18, No. 20, 2006 4717
Figure 1. Variation in oxygen content (wt %) of BNxOyCz aerosol samples formed with gas flow rates of 0.5 L/min N2 and 0.5 L/min NH3 as a function of reactor temperature: (b) H3BO3/DMF/NH3; (9) (MeO)3B/NH3; and (2) H3BO3/H2O/NH3.
in the H3BO3/DMF/NH3 system are lower compared to the BNxOyCz powder formed in the (MeO)3B solvent-free aerosol system.24 However, above this temperature the back-reaction of in situ produced water appears to reintroduce oxygen into the BNxOyCz powders formed in H3BO3/DMF/NH3.32 This affect does not appear in the solventless (MeO)3B/NH3 system because the high-temperature reaction between these reagents does not produce water. Full elemental analyses33 indicate comparable levels of carbon and hydrogen impurities suggesting that DMF pyrolysis is not leading to significant introduction of impurities.34 Once formed, the stage one BNxOyCz powders are stable in moist air, showing no sign of decomposition or hydrolysis at 23 °C.35 Elemental analyses are unchanged after storage for 6 months. Diffuse reflectance infrared Fourier transform (DRIFT) (Bruker Tensor 27, Specor 19900 Series DRIFT attachment) for the BNxOyCz powders show a strong, broad absorption centered at ∼1410 cm-1 and a sharp, medium intensity absorption at 810 cm-1, typical of BNxOy and BNxOyCz powders with low oxygen contents.21,22 Powder X-ray diffraction (XRD) scans (Scintag Pad V) for stage one BNxOyCz powders all show relatively weak scattering patterns, but two peaks (d ) 2.15 Å and 3.07 Å), typical of turbostratic BN,21 are observed. Scanning electron micrographs (SEMs) of the BNxOyCz powders obtained at 1200 °C reveal the formation of unagglomerated spherical particles, and a typical micrograph is shown in Figure 2. The majority have a smooth surface structure although a small percentage of particles show dimpling that may result from inter-particle impacts during formation. Transmission electron micrographs (TEMs) for BNxOyCz powders clearly show that the spherical particles have an eggshell, hollow structure. In addition, grinding the powders prior to SEM sample preparation results in additional crush(33) Full (CHBNO) analysis of these samples has proven to be a considerable challenge in part because of their low density and electrical charging characteristics. In addition, the samples readily absorb water and carbon dioxide, a fact supported by TGA analyses. Typical analyses are represented by the following: BNxOyCz (1100 °C) C, 0.26; H, 0.90; B, 38.01; N, 44.48; O, 13.4 wt %; BNxOyCz (1400 °C) C, 0.15; H, 1.88; B, 33.22; N, 39.10; O, 11.3 wt %. Analyses for calcined powders are represented by the example as follows: C, 0.17; H, 0.30; B, 40.40; N, 53.42; O, 2.5. (34) The carbon and hydrogen contents determined by combustion elemental analyses for the BNxOyCz materials fall in the ranges C, 0.1-1.5 wt %, and H, 0.9-2.2 wt %, respectively. The calcined samples show ranges C, 0.5 to trace wt %, and H, 1.0 to trace wt %, respectively. (35) Some commercial experimental high surface area samples of BNxOyCz formed by different methods and samples formed by pyrolysis of preceramic polymers undergo noticeable hydrolysis at 23 °C as noted by the distinct odor of NH3 and appearance of BO-H stretching modes in IR spectra.
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Figure 2. SEM for BNxOyCz aerosol powder from the H3BO3/DMF/NH3 aerosol system.
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Figure 5. SEM for BN aerosol powder from stage two BNxOyCz powder nitridation at 1500 °C.
Figure 3. SEM for the cracked-open BNxOyCz aerosol particle showing edge feature.
Figure 6. TEM for BN aerosol powder from stage two BNxOyCz powder nitridation at 1500 °C.
Figure 4. TGA scan for BNxOyCz aerosol powders from the H3BO3/DMF/ NH3 aerosol system: N2 flow rate 40 L/min.
ing of some particles that in turn display the eggshell character of the spheres upon SEM analysis. A SEM of a typical edge feature is shown in Figure 3. The edge thickness for the majority of the particles falls in a range of 0.05-0.2 µm. Although some very small spheres are generated (
