Anal. Chem. 2006, 78, 2366-2369
Application of Ion Attachment Mass Spectrometry to Evolved Gas Analysis for in Situ Monitoring of Porous Ceramic Processing Takahisa Tsugoshi,*,† Takaaki Nagaoka,† Megumi Nakamura,‡ Yoshiro Shiokawa,‡ and Koji Watari†
Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) and Canon ANELVA Technix Corporation, 2266-98 Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan
Ion attachment mass spectrometry was applied to evolved gas analysis-mass spectrometry (EGA-MS), generally known as thermogravimetry-mass spectrometry. Characteristic species arising from poly(vinyl alcohol) (PVA) and poly(methyl methacrylate) (PMMA) resin, used as a binder and a porogen, respectively, in the starting materials for porous ceramics, were detected in the mass spectra. The EGA curves of the characteristic mass peaks from PVA and PMMA, when plotted against the programmed temperature, successfully showed the individual pyrolysis behavior of each polymer during the firing process. Figure 1. Schematic of prototype EGA-MS apparatus using IAMS.
Sintering aids for ceramics are usually organic materials. For example, poly(vinyl alcohol) (PVA), latex, methylcellulose, and starch are used as binders, because ceramic starting powders do not themselves harden when mixed with water.1-5 The role of a binder is to provide green strength, so that a green body can be formed and will retain its desired shape before heating. Acrylic resin beads, carbon powder, and starch are widely used as porogens (pore-forming materials) for the production of porous ceramics.5,6 These organic additives are pyrolyzed during the firing and sintering processes. Their pyrolysis behavior is important in improving the conditions for the process and the properties of the final product. The pyrolysis behavior is also important from the viewpoint of environmental protection, because some organic materials emit harmful species during pyrolysis. The analysis of the gaseous species that are evolved during pyrolysis in order to elucidate the pyrolysis behavior usually requires evolved gas analysis-mass spectrometry (EGA-MS), better known as thermogravimetry-mass spectrometry. However, the usual method for EGA-MS has problems in relation to in situ monitoring of the pyrolysis process, because fragmentation resulting from ionization * To whom correspondence should be addressed. Tel: +81-52-736-7155. Fax: +81-52-736-7405. E-mail
[email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Canon ANELVA Technix Corp. (1) Benbow, J. J.; Oxley, E. W.; Bridgwater, J. Chem. Eng. Sci. 1987, 42, 21512162. (2) Kristoffersson, A.; Carlstro¨m, E. J. Eur. Ceram. Soc. 1997, 17, 289297. (3) Kristoffersson, A.; Roncari, E.; Galassi, C. J. Eur. Ceram. Soc. 1998, 18, 2123-2131. (4) Sawyer, C. B.; Reed J. S. J. Am. Ceram. Soc. 2001, 84, 1241-1249. (5) Lyckfeldt, O.; Ferreira, J. M. F. J. Eur. Ceram. Soc. 1998, 18, 131-140. (6) Lindqvist, K.; Linden, E. J. Eur. Ceram. Soc. 1997, 17, 359-366.
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complicates the mass spectrum; for example, different organic species can produce identical fragment ions. To avoid this obstacle to in situ monitoring, an ion attachment mass spectrometry (IAMS) can be used, because IAMS is definitely a soft-ionization technique. IAMS was introduced by Beauchamp et al.,7,8 who presented mass spectra consisting of quasi-molecular ions formed by addition of Li+ ions to sample molecules. Their study was extended to determine the ultimate sensitivity of Li+ ion CI mass spectrometry.9 Very recently, IAMS has been developed by Fujii et al.,10-12 who named the technique “ion attachment mass spectrometry for Li+ CI mass spectrometry”, IA ionization offers completely soft ionization of gaseous species, including radical species. These features of IAMS have permitted Nakamura et al.13 to successfully detect and monitor the products of the thermal decomposition of a key material in the metal oxide chemical vapor deposition process. Fujii et al.14 developed an apparatus for the analysis of ambient air at atmospheric pressure that could be coupled to a system for supercritical fluid chromatography. This apparatus offers gas sampling at atmospheric pressure through an aperture, so that Fujii mentioned an interest in connection with thermogravimetry. (7) Staley, R. H.; Beauchamp, J. L. J. Am. Chem. Soc. 1975, 97, 5920-5921. (8) Hodges, R. V.; Beauchamp, J. L. Anal. Chem. 1976, 48, 825-829. (9) Fujii, Y.; Ogura, M.; Jimba, H. Anal. Chem. 1989, 61, 1026-1029. (10) Fujii, T. Mass Spectrosc. Rev. 2000, 19, 111-138. (11) Fujii, T.; Arulmozhiraja, S.; Nakamura, M.; Shiokawa, Y. Anal. Chem. 2001, 73, 2937-2940. (12) Fujii, T.; Nakamura, M. J. Appl. Phys. 2001, 90, 2180-2184. (13) Nakamura, M.; Shiokawa, Y.; Fujii, T.; Takayanagi, M.; Nakata, M. J. Vac. Sci. Technol., A 2004, 22, 2347-2350. (14) Fujii, T. Anal. Chem. 1992, 64, 775-778. 10.1021/ac0518248 CCC: $33.50
© 2006 American Chemical Society Published on Web 02/24/2006
Figure 2. IAMS spectrum obtained from gaseous species evolved from PVA. The mass number of each peak includes an additional 7 amu of Li+ as the ionization source. The peaks at 51, 77, 103, and 113 amu (marked with asterisks) coincide with the molecular weights of acetaldehyde, crotonaldehyde, methylhexadiene, and benzaldehyde detected by Py-GC/MS.
Figure 3. IAMS spectrum obtained from gaseous species evolved from PMMA. The mass number of each peak includes an additional 7 amu of Li+ as the ionization source. The peak at 107 amu (marked with asterisk) coincides with the molecular weight of methyl methacrylate detected by Py-GC/MS. Table 1. Assignment of Main Peaks of PVA by Py-GC/MS15 peak assignment (mw)
relative intensity
acetaldehyde CH3CHO (44) C4H6O (70) crotonaldehyde CH3CHdCHCHO (70) CH3CHdCHCHdCHCHO (96) C7H12 methylhexadiene? (96) benzaldehyde C6H5CHO (106)
100 12.1 72.0 2.3 15.4 5.5
In this study, IAMS was applied to EGA-MS for in situ monitoring of the pyrolysis behavior of ceramic additives. The peaks in the IAMS spectrum were expected to be due solely to fragments produced by pyrolysis and not by ionization. Such a technique is required because although gas chromatography (GC) is an excellent technique for analyzing gaseous species, it cannot readily be applied to in situ monitoring inside a heated furnace. EXPERIMENTAL SECTION All the IAMS measurements were carried out with a prototype apparatus consisting of a gold image furnace (MR-39H/S; ULVACRIKO) and an IAMS instrument (L-240G-IA; ANELVA), as shown in Figure 1. The two devices were simply connected with 1/4-in.-
Table 2. Assignment of Main Peaks of PMMA by Py-GC/MS15-18 peak assignment (mw) methyl methacrylate CH2C(CH3)COOCH3 (100) CH2dC(CH3)-CH2-CH(COOCH3)-CH3 ? (142) CH2dC(CH3)-CH2-C(CH3) (COOCH3)-CH3 ? (156) CH2dC(CH3)-CH2-C(COOCH3)dCH2 ? (140) CH2dC(CH3)-CHdC(COOCH3)-CH3 ? (140) CH3-CH(CH3)-CH2-C(CH3) (COOCH3)-CH3 ? (158) CH2dC(COOCH3)-CH2-C(CH3) (COOCH3)-CH3 ? (200) CH3-C(COOCH3)dCH-CH (COOCH3)-CH3 ? (186) CH3-C(COOCH3)dCH-C(CH3) (COOCH3)-CH3 ? (200) C11H18O4 ? (214) CH3-C(COOCH3)dCH-C(CH3) (COOCH3)-CH2-C(CH3) (COOCH3)-CH3 (300)
rel intens 100 >0.1 >0.1 >0.1 >0.1 >0.1 0.1 >0.1 0.2 0.1 0.2
o.d. stainless steel tube (Swagelok). Nitrogen, which was used as the balance gas in the IA ionization chamber, was also used as the carrier gas flowing into the furnace during heating, which was performed at a rate of 20 °C‚min-1. Specimens were raw materials for porous alumina ceramics. Poly(vinyl alcohol) is generally used as a binder for alumina powder to provide green strength so that bodies can be formed and retained in the desired shapes before firing. Spheres of acrylic Analytical Chemistry, Vol. 78, No. 7, April 1, 2006
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Figure 4. EGA-IAMS curves obtained from PVA. Only the curve of Li+ ion is indicated on the right-hand logarithmic axis. The EGA peak of 113 amu due to PVA was obtained, but the EGA peak of 107 amu due to PMMA was not observed.
Figure 5. EGA-IAMS curves obtained from PMMA. Only the curve of Li+ ion is indicated on the right-hand logarithmic axis. The EGA peak of 107 amu due to PMMA was obtained, but the EGA peak of 113 amu due to PVA was not observed.
resin are generally used as porogens: in this case, monodispersed spheres (1.8-µm i.d.) of poly(methyl methacrylate) (PMMA) were used. The preparation of the samples was performed as follows. First, alumina powder (16.683 g) and PMMA spheres (3.317 g) were mixed. A 4-g portion of the mixture was blended with PVA (0.06 g) and water (1.5 g). The components were then mixed well and dried to give the analytical sample. RESULTS AND DISCUSSION Figure 2 shows the IAMS spectrum obtained from gaseous species evolved from PVA. The mass number of each peak in the IAMS spectra includes an additional 7 amu as a result of using Li+ as the ionization source. The saturated peak of 7 amu is based on the Li+ ion. The main peak of 113 amu is characteristic and coincides with the molecular weight (106) of benzaldehyde, which was also detected by Py-GC/MS, as shown in Table 1.15 The 25amu peak showed that H2O was evolved. From a comparison with the data from the Py-GC/MS technique, the peaks at 51, 77, and 103 amu (marked with asterisks), which had minor intensities in the IAMS spectrum, coincide with the molecular weights of (15) Tsuge, S.; Ohtani, H. Py-GC of polymers, 3rd ed.; Technosystem: Tokyo, 1995 (in Japanese).
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acetaldehyde, crotonaldehyde, and methylhexadiene detected by Py-GC/MS. The species that give rise to other detected peaks, e.g., those at 43 69, 87, 95, and 132, were not identified, even though the molecular weight of each fragment formed by pyrolysis could be determined from the IAMS spectrum. The species corresponding to these molecular weights were not detected by Py-GC/MS. The IAMS spectrum of species evolved from the PMMA spheres is shown in Figure 3. This result is almost consistent in terms of molecular weights with the results obtained by Py-GC/ MS, as shown in Table 2.15-18 The main peak at 107 amu in the IAMS spectrum is consistent with the molecular weight of methyl methacrylate plus the Li+ ion. The peak at 125 amu corresponds to the main peak ion plus H2O. The peaks at 25 and 35 amu were identified as H2O and CO, respectively. The other species identified by Py-GC/MS may be below the detection limit of the IAMS technique. (16) Haken, J. K.; McKay, T. R. Anal. Chem. 1973, 45, 1251-1257. (17) Ohtani, H.; Tanaka, M.; Tsuge, S. Bull. Chem. Soc. Jpn. 1990, 63, 11961200. (18) Ohtani, H.; Luo, Y. F.; Nakashima, Y.; Tsukahara, Y.; Tsuge, S. Anal. Chem. 1994, 66, 1438-1443.
Figure 6. EGA-MS curves obtained from starting material of porous alumina, i.e., a mixture of alumina powder, PVA as a binder, and PMMA spheres as a porogen.. Only the curve of Li+ ion is indicated on the right-hand logarithmic axis. The pyrolyses of PVA and PMMA started at ∼200 °C and at ∼260 °C, respectively.
Figures 4 and 5 show the EGA curves obtained from PVA and PMMA, respectively. The characteristic main peak of each polymer indicated the pyrolysis behavior individually. The EGA peak of 113 amu due to PVA was obtained, but the EGA peak of 107 amu due to PMMA was not observed in the case of PVA (Figure 4) and vice versa (Figure 5). In Figure 4, the decrease in the peak intensity at ∼290 °C was caused by a decrease in the concentration of Li+ ion as the ionization source (as indicated on the logarithmic axis), because the high concentrations of gaseous species resulted in consumption of the Li+ ion. Therefore, the EGA curve does not reflect a two-step behavior in PVA pyrolysis as previously reported.19,20 In the case of PMMA, as shown in Figure 5, it can be estimated that the total evolution of gaseous species was not as high as that for PVA, because the decrease in the Li+ ion intensity, indicated on the right-hand axis, was smaller than that observed for PVA. The EGA curves shown in Figure 6 were obtained for in situ monitoring of the pyrolysis behavior of porous alumina starting material, i.e., a mixture of alumina powder, PVA as a binder, and PMMA spheres as a porogen. The characteristic mass peaks of 103 amu for PVA and 107 amu for PMMA were monitored. The pyrolysis of PVA started at ∼200 °C. The intensity of the peak decreased between about 270 and 350 °C as a result of a reduction in the concentration of Li+ ion, the ionization source, through (19) Gilbert, J. B.; Kipling, J. J.; McEnaney, B.; Sherwood, J. N. Polymer 1962, 3, 1-10. (20) Tsuchiya, Y.; Sumi, K. J. Polym. Sci., Part A-1 1969, 7, 3151-3158.
greatly increased consumption by the evolved species. The pyrolysis of PMMA, as indicated by the evolved methyl methacrylate, began at ∼260 °C, a higher temperature than that for PVA. From these results, it is possible to confirm the firing model as follows. First, the PVA as a binder starts to pyrolyze. Then the alumina powder adheres so that a shell structure of alumina is temporarily formed. After that, the PMMA, as the porogen, is pyrolyzed and eliminated without breaking the shell structure of the alumina powder. Finally, porous alumina is obtained by sintering to retain the shell structure at higher temperatures than those encountered during the firing process. CONCLUSION EGA-MS using IAMS can be successfully applied to monitor pyrolysis behavior during the firing of ceramic materials. The pyrolysis behaviors of different additives such as PVA and PMMA can be monitored individually by using the characteristic evolved species, which depend on the nature of the additives, according to the soft ionization of IAMS technique. Also the existence was confirmed of an interaction between the organic additive used as the binder and that used as the porogen. Received for review October 12, 2005. Accepted January 16, 2006. AC0518248
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