Anal. Chem. 1997, 69, 1485-1491
Thermal Decomposition of CaC2O4‚H2O Studied by Thermo-Raman Spectroscopy with TGA/DTA Hua Chang* and Pei Jane Huang
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
The advantage of Raman spectroscopy is that it can be used to identify structure. In this work, Raman spectra of CaC2O4‚H2O, a calibration standard for thermogravimetric analysis/differential thermal analysis (TGA/DTA), were taken or monitored continuously in a thermal process from 25 to 750 °C with a heating rate 4 °C min-1, similar to that in TGA/DTA. The variation in Raman spectra for three structural transformations was found to be consistent with those in TGA/DTA. Furthermore, minor changes in spectra indicated the phase transformations of CaC2O4‚H2O, CaC2O4, and CaCO3. Other features of the Raman spectra, the intensities of the representative bands, the derivative variation of the band intensities, the band positions, and even the background, showed some potential for application in the thermal analysis. Both the advantages and the disadvantages of thermo-Raman spectroscopy for thermal analysis were discussed in detail. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are important in studying the transformation of a solid sample in a thermal process.1-4 They are commonly used in studying adsorption, sintering, calcination, phase transition, decomposition, and many other processes for different materials. However, no direct information on the structural variation, physical and chemical, of the sample can be obtained during a thermal process. Raman spectroscopy has the advantage that it can identify the structure of the sample from its vibrational bands. It has been used with TGA/DTA to identify the structures of the samples at various temperatures5-13 and to study the decomposition of superconductors.8 The Raman spectra were also measured in a range of temperatures to illustrate the structural variation.9-13 Naturally, conjunction of these two techniques was suggested.6 Raman spectra and TGA were measured simultaneously on NH4NO3 for identifying the changes in structures.13 However, the (1) Wendlandt, W. W. Thermal Analysis; John Wiley: New York, 1986. (2) Kociba, K. J.; Gallagher, P. K. Thermochim. Acta 1996, 282/283, 277296. (3) Levy, L. W.; Laniepce, J. C. R. Hebd. Seances Acad. Sci. 1964, 259, 46854688. (4) Simons, E. L.; Newkirk, A. E. Talanta 1964, 11, 549-571. (5) Herman, R. G.; Bogdan, C. E.; Kumler, P. L.; Nuszkowski, D. M. Mater. Chem. Phys. 1993, 35, 233-239. (6) Haro-Poniatowski, E.; Rodriguez-Talavera, R.; Heredia, M. de la C.; CanoCorona, O.; Arroyo-Murillo, R. J. Mater. Res. 1994, 9, 2102-2108. (7) Dutta, P. K.; Gallagher, P. K.; Twu, J. Chem. Mater. 1992, 4, 847-851. (8) Chang, H.; Xiong, Q.; Xue, Y. Y.; Chu, C. W. Physica C 1995, 248, 15-21. (9) Lutz, H. D.; Steiner, H. J. Thermochim. Acta 1992, 211, 189-197. (10) Duval, D.; Condrate, Sr. R. A. Appl. Spectrosc. 1988, 42, 701-703. (11) Jiang, Y. J.; Zeng, L. Z.; Wang, R. P.; Zhu, Y.; Liu, Y. L. J. Raman ,Spectrosc. 1996, 27, 31-34. (12) Chang, H. Chin. J. Phys. 1996, 34, 310-314. (13) Harju, M. E. E. Appl. Spectrosc. 1993, 47, 1926-1930. S0003-2700(96)00881-5 CCC: $14.00
© 1997 American Chemical Society
application of Raman spectroscopy to thermal analysis has not been reported, to our knowledge. Calcium oxalate monohydrate CaC2O4‚H2O is a calibration standard for TGA. It has been studied in detail by varying many factors. Three weight losses were detected.1-4 These peaks in TGA/DTA had been clarified by the IR spectral studies.14-17 The Raman spectra of CaC2O4‚H2O and CaC2O4 were also reported.10,17-20 Furthermore, the Raman spectra during the dehydration of CaC2O4‚H2O were studied from 124 to 210 °C.10 In this work, its transformation was monitored by Raman spectroscopy from 25 to 750 °C in a thermal process similar to TGA/DTA. Three variations in spectra were found in the temperature intervals corresponding to the dehydration and decomposition of CaC2O4 and CaCO3 when compared with the known spectra. They were consistent with the variations in TGA/DTA. Furthermore, phase transformations or changes in crystal symmetry of I and II forms of CaC2O4‚H2O,10,16-20 R, β, and γ forms of CaC2O4,3,16,17 and two forms of CaCO321,22 were observed in this work by Raman spectroscopy which were not detectable in TGA/DTA. Thus, we illustrate in this work that Raman spectroscopy can be operated alone in thermal analysis to provide much direct and valuable information. This method, thermo-Raman spectroscopy, measured the Raman spectra in a thermal process. Different aspects of Raman spectra, such as band positions, band intensities, and background, indicated the transformation in the thermal process. They are analyzed and presented in this work in order to show the variety of information that can be obtained. The bandwidth should be valuable; however, the spectral resolution in this work was kept low in order to cover a large spectral range. Finally, the advantages and the disadvantages of thermo-Raman spectroscopy are discussed in detail relevant to this work. EXPERIMENTAL SECTION The TGA and DTA thermograms were obtained from a SEIKO I SSC 5000 thermal gravimetry and differential thermal analysis instrument. The temperature program was from 25 to 900 °C, with an increasing rate of 5 °C min-1 in a flow of air. The sample CaC2O4‚H2O used was from SEIKO, supplied as a calibration standard for TGA/DTA. (14) Freeberg, F. E.; Hartman, K. O.; Hisatsune, I. C.; Schempf, J. M. J. Phys. Chem. 1967, 71, 397-402. (15) Petrov, I.; Soptrajanov, B. Spectrochim. Acta 1975, 31A, 309-316. (16) White, R. L. Appl. Spectrosc. 1992, 46, 1508-1513. (17) White, R. L.; Ai, J. Appl. Spectrosc. 1992, 46, 93-99. (18) Edwards, H. G. M.; Farewell, D. W.; Jenkins R.; Seaward, M. R. D. J. Raman Spectrosc. 1992, 23, 185-189. (19) Duval, D.; Condrate, Sr. R. A. Phys. Stat. Sol. (b) 1985, 132, 83-92. (20) Shippey, T. A. J. Mol. Struct. 1980, 63, 157-166. (21) Rao, C. R. M.; Mehrotra, P. N. Can. J. Chem. 1978, 56, 32-35. (22) Nakamoto, K. Infrared application of group theory; Interscience Publishers: New York, 1964.
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Figure 1. TGA/TDS of CaC2O4‚H2O with a heating rate 5 °C min-1 in a flow of air.
Raman spectra were measured by excitation with a laser light of 20 mW at a wavelength of 514.5 nm from an argon ion laser (Coherent, Model Innova 100-15). The plasma lines were removed by a filter. The scattered light was collected (a camera lens), dispersed (Spex, 0.5 m spectrometer), and detected by a CCD camera (Princeton Instrument, 1024 × 1024 pixels). The Rayleigh scattered light was reduced by a Notch filter. Spectra were taken every 15 s, with an exposure time of the CCD camera of 14.1 s. The sample was placed in a shallow pit on a stainless steel plate in a home-made oven with a glass window. It was in open air under natural convection. The temperature was monitored by a thermocouple contacted with the sample holder and controlled by a programmable controller. The heating rate was 4 °C min-1, and the uncertainty in temperature measuring was about 2 °C. The positions of the spectral bands were calibrated with an argon spectral tube. The slit was set to 70 µm, and the resolution was low, about 8 cm-1. RESULTS Although the TGA/DTA and the Raman spectra of CaC2O4‚H2O were measured separately, a similar thermal process was set in order to take Raman spectra continuously at conditions similar to those used for TGA/DTA. TGA/DTA. Calcium oxalate monohydrate CaC2O4‚H2O is commonly used as a calibration standard for TGA. Figure 1 shows the TGA and DTA thermograms of CaC2O4‚H2O measured in a flow of air with a heating rate 5 °C min-1 from 25 to 900 °C in a platinum sample holder. The weight losses of 13, 19, and 30% happened at 168, 467, and 675 °C, respectively, as shown in TGA.1-4,14-17 It is known that H2O, CO, and CO2 were lost. Two endothermic (168 and 675 °C) peaks and one exothermic (467 °C) peak appeared in DTA. These transformation temperatures depended on several factors, such as the sample weight, heating rate, atmosphere, etc. Raman Spectroscopy. Raman spectra of a crystal such as CaC2O4‚H2O consist of two parts.18-20 One is the intramolecular vibrational modes due to the vibration of the atoms in the molecules or ions such as C2O42- and H2O. Their band positions are always higher than 200 cm-1. The other is the lattice modes, due to the translatory and rotatory motion of ions Ca2+ and groups such as C2O42- and H2O as a whole. Their band positions are always lower than 400 cm-1. In this work, two kinds of transformations were observed. Structural transformation, such as 1486 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
Figure 2. Eight typical Raman spectra observed in the heating process for the spectra (a) 5 (25 °C), (b) 50 (70 °C), (c) 170 (190 °C), (d) 250 (270 °C), (e) 400 (420 °C), (f) 590 (610 °C), (g) 610 (630 °C), and (h) 710 (730 °C).
dehydration and decomposition, caused changes in intramolecular vibrational modes and the whole spectrum. On the other hand, the phase transformation between two phases of CaC2O4‚H2O and among three phases of CaC2O4 caused a change in symmetry of the crystal lattice, and only minor changes in lattice modes could be found in the Raman spectrum. They might not be easy to distinguish because the spectra were similar. The thermal analysis of CaC2O4‚H2O is well studied. The species appearing in different temperature intervals are depicted and identified by X-ray diffraction,2,3 IR,2,16,17 and Raman spectroscopy.2,18-20 In this work, the species appearing in each temperature interval were identified by comparison with the known Raman spectra. In this work, a total of 729 Raman spectra were taken continuously (the first five spectra were taken at 25 °C) in the range from 25 to 750 °C, with a heating rate of 4 °C min-1. In other words, the temperature which one spectrum covered was 1 °C. The Raman spectra measured were within the range from 66 to 2366 cm-1. Typical Spectra. Eight typical spectra were found and are shown in Figure 2. The positions of the intense Raman bands are listed in Table 1. The assignment of Edwards et al. was followed.18 The spectra shown in Figure 2a,b were measured at 25 and 70 °C, respectively. A strong doublet at 1491 and 1460 cm-1 and many weak bands were observed. They were from phases I and II of CaC2O4‚H2O, with the symmetry of P21/n and I2/m, respectively.10,18-20 These spectra had only minor differences: the relative intensity of the doublet and the lattice mode at 921 cm-1. The spectra taken at 190, 270, and 420 °C are shown in Figure 2c-e, respectively. They were similar with a strong, single band around 1460 cm-1. Other bands shifted little; however, they were within the spectral resolution. Only the part below 400 cm-1 was
Table 1. Intense Raman Bands and Their Assignmentsa CaC2O4‚H2O I 30 °C
II 90 °C
1724 1624 1480 1454 1390 921 878 847 572 478 218 170 106 a
CaC2O4 R 190 °C
β 270 °C
γ 420 °C
1720 1623 1482 1454 1385
1723 1634 1465
1635 1458
1718 1634 1458
875 847 570 480 215 167 104
880
875
880
ν (CC)
573 473 228 157 132
563 473 220 157
563 475 220 157 119
F (CO2)asym
1396
1390
assignmentb ν (CO2)asym ν (CO2)sym ν (CO2)sym ω (CO2)
τ (CO2)
The resolution of the spectra was 8 cm-1. b From ref 18.
different. These spectra belonged to three phases of anhydrous CaC2O4: R, β, and γ forms.3,16,17 They should have similar structures. The next two spectra showed a strong band at 1070 cm-1 with weak bands around 695, 233, and 160 cm-1, measured at 600 and 630 °C, as shown in Figure 2f,g. They are for CaCO3, which has two forms, aragonite and calcite.21,22 These spectra were similar except for the weak band at 160 cm-1 and the strong Rayleigh scattering. No spectral band appears in the last spectrum measured at 730 °C, as shown in Figure 2h. At that temperature, the sample should decompose to CaO. CaO has only one very weak band at about 359 cm-1 which could not be detected here due to the short exposure time. Variation in Spectra. The variation of Raman spectra in different temperature intervals is shown in Figures 3-10. Figure 3 shows the variation from spectra 21 to 86 (41-106 °C). All spectra were similar with only minor differences: the relative intensities of the doublet and the presence of a weak band at 921 cm-1, which is attributed to the lattice mode. The transformation was around spectrum 50 (70 °C), but it was not well defined because the spectra were so similar. However, it should correspond to the phase transformation of CaC2O4‚H2O (I) to CaC2O4‚H2O (II).3,10,16-20 The transformation temperature was reported around 45-50 °C.10,18-20 The transformation is reversible and happens in a large temperature range. Both species were found to coexist in powder sample at room temperature.10,18 Of course, TGA/DTA did not give any information about this transformation. The next variation in the spectra was observed around spectrum 146, or 166 °C. The corresponding spectra from 121 to 166 are shown in Figure 4. The main change in the Raman spectrum was the transformation of the doublet to a singlet around spectrum 146 (166 °C). It was due to dehydration. CaC2O4‚H2O (II) transformed to R-CaC2O4.3,16-20 The other change was the replacement of the band at 110 cm-1 by two bands at 127 and 99 cm-1. The dehydration process was found to occur at 168 °C in TGA/DTA. Thus, the two methods were consistent.. The spectra from 221 (241 °C) to 266 (286 °C) and from 351 (371 °C) to 396 (416 °C) are shown in Figures 5 and 6, respectively. Variations were found around spectra 236 and 376. There were only minor changes: the raising in background in Figure 5 and the enhancement in Rayleigh scattering in Figure 6. These implied that the structures were the same and the crystal
Figure 3. Raman spectra 21 (41 °C) to 86 (106 °C). CaC2O4‚H2O (I) transformed to CaC2O4‚H2O (II).
Figure 4. Raman spectra 121 (141 °C) to 166 (186 °C). CaC2O4‚H2O (II) transformed to R-CaC2O4.
lattice changed. The intensity of the Rayleigh scattering increased during the phase transformation, as was also observed by Duval and Condrate in their study on CaC2O4‚H2O.19 This was caused by the new phase formed at the dislocations or defects in the crystal.19 These two variations should correspond to the phase transformations from R to β form and from β to γ form, respectively.4,16,17 There was neither weight loss nor thermal change; therefore, no peak could be observed in TGA/DTA. Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
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Figure 5. Raman spectra 221 (241 °C) to 266 (286 °C). R-CaC2O4 transformed to β-CaC2O4.
Figure 7. Raman spectra 431 (451 °C) to 491 (511 °C). γ-CaC2O4 dehydrated to CaCO3 (aragonite).
Figure 6. Raman spectra 351 (371 °C) to 396 (416 °C). β-CaC2O4 transformed to γ-CaC2O4.
Figure 8. Raman spectra 576 (596 °C) to 611 (631 °C). CaCO3 (aragonite) transformed to CaCO3 (calcite).
Figure 7 shows the spectra from 431 to 491 (451-511 °C). The bands of CaC2O4 gradually lost intensity. In the meantime, the band at 1070 cm-1 of CaCO3 appeared and gained intensity slowly. This was the decomposition of CaC2O4 to CaCO3.1-4 The transformation was around spectrum 476 (496 °C), a temperature slightly higher than the 467 °C observed by TGA/DTA. There are two forms of CaCO3, aragonite and calcite.21,22 The variations in spectra were the increase in intensity of the band at
1070 cm-1, the Rayleigh scattering, and the disappearance of the small band at 160 cm-1, as shown in Figure 8 from spectra 576 (596 °C) to 611 (631 °C). These indicated a phase transformation of CaCO3 from anagonite to calcite around spectrum 596 (616 °C), higher than the reported 513 °C.21 The reason might be the different experimental conditions and the property of this transformation. It is irreversible, reconstructive, and sluggish.21 Finally, CaCO3 decomposed to CaO,1-4 as indicated by the disappearance of the band at 1070 cm-1, as shown in Figure 9
1488 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
Figure 9. Raman spectra 671 (691 °C) to 706 (726 °C). CaCO3 (calcite) decomposed to CaO.
from spectra 671 (691 °C) to 706 (726 °C). It happened at spectrum 686 (706 °C). This decomposition temperature was close to that found by TGA/DTA (675 °C). These Raman spectra showed three structural transformations due to dehydration of CaC2O4‚H2O and decomposition of CaC2O4 and CaCO3 and four phase transformations of CaC2O4‚H2O, CaC2O4, and CaCO3. The application of Raman spectroscopy in studying the transformation was always to show the variation in spectra.5-13 However, this way does not give the transformation temperatures. They could be obtained by plotting the intensity variation. Variation in Intensities. The intensity (area) variation of the bands around (a) 1465 and (b) 1070 cm-1 is shown in Figure 10. They represented the relative amounts of (a) CaC2O4‚H2O or/ and CaC2O4 and (b) CaCO3, respectively. The intensity around 1465 cm-1 decreased in three regions, spectra 43-77, 140-150, and 434-495, as shown in curve a in Figure 10. They corresponded to the transformation of CaC2O4‚H2O form I to form II, CaC2O4‚H2O form II to R-CaC2O4, and γ-CaC2O4 to CaCO3, respectively. The phase transformation from R- to β-CaC2O4 and from β- to γ-CaC2O4 did not change the intensity of this band much, but a small variation in intensity revealed that there might be phase transformations in the regions where the intensity dropped from spectra 150 to 286 and the intensity increased from spectra 286 to 434. From Figures 5 and 6, the phase transformations should be around spectra 236 and 376. They were marked in the figures, although there was no trace for transformation. Note that the intensities of the Raman bands for pure materials may not be the same. The relative intensities for pure CaC2O4‚H2O (I), CaC2O4‚H2O (II), R-CaC2O4, β-CaC2O4, and γ-CaC2O4 might be about 1:0.86:0.5:0.36:0.43, respectively. Curve b in the same figure shows the intensity variation of the band at 1070 cm-1 or the amount of CaCO3. The band of CaCO3 appeared at spectrum 434 (454 °C), increased, and slowed
Figure 10. Intensities (areas) of the Raman bands at (a) 1465 and (b) 1070 cm-1, corresponding to the amounts of CaC2O4‚H2O or/ and CaC2O4 and CaCO3, respectively.
down at spectrum 495 (515 °C). It increased rapidly at spectrum 582 (602 °C) to a final intensity at spectrum 602 (622 °C) and decayed rapidly from spectra 675 (695 °C) to 705 (725 °C). The intensities of the bands of CaC2O4 reduced from spectrum 434 (454 °C) and vanished at spectrum 495 (515 °C), as curve a shows. It indicated the transformation of CaC2O4 to form CaCO3 (aragonite) in that region: the corresponding increase in intensity of the band at 1070 cm-1 was from spectra 434 to 495. The continuing increase in intensity might be caused by the growth of the CaCO3 (aragonite) crystals. The phase transformation to calcite might be between spectra 582 (602 °C) and 602 (622 °C). CaCO3 (calcite) decomposed to CaO in the range of spectra 676 (696 °C) to 705 (725 °C). It should correspond to the weight loss of CO2 in TGA and the endothermic peak in DTA at 675 °C. CO2 gas may change the decomposition temperature of carbonates.1-4 In this experiment, the intensity of the band at 1070 cm-1 decayed between spectra 675 (695 °C) and 705 (725 °C). The evolution of CO2 in the deep part might be the reason for the delay of the decomposition of CaCO3 on the surface to a higher temperature. These two curves showed the appearance and disappearance of the forms of CaC2O4‚H2O, CaC2O4, and CaCO3. However, they could not show the transformation temperatures as the peaks in the thermograms of TGA/DTA. Derivatives of the Band Intensities. The derivatives of the intensity variation of these two bands for (a) CaC2O4‚H2O or/and CaC2O4 and (b) CaCO3 could be plotted as shown in Figure 11. Supposedly, they should show variation similar to that in TGA/ DTA. The raising peaks and the falling pits represented the increase and the decrease in the amount of that phase, respectively. Curve a showed three dips at spectra 50 (70 °C), 147 (167 °C), and 475 (495 °C), corresponding to the phase transformation of CaC2O4‚H2O form I to form II, dehydration of CaC2O4‚H2O, and Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
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Figure 11. Derivative of the intensities of the Raman bands at (a) 1465 and (b) 1070 cm-1, respectively.
decomposition of CaC2O4, respectively. The phase transformation of CaC2O4‚H2O form I to form II did not show up in TGA/DTA. The other two pits were consistent with the peaks at 168 and 467 °C of TGA/DTA for dehydration and deprivation of CO. Other two phase transformations of CaC2O4 did not appear in this curve, either. Curve b showed two peaks at spectra 483 (503 °C) and 594 (614 °C) and one pit at spectrum 689 (709 °C). They corresponded to the product of CaCO3, phase transformation of aragonite to calcite, and the decomposition of CaCO3, respectively. The transformation temperatures, 503 and 709 °C, were little higher than those for the peaks at 467 and 675 °C for TGA/DTA. The positions of the decomposition of CaC2O4 were different in these two curves because the intensity of the band at 1070 cm-1 was still increasing in curve b. These positions are also shown in Figure 10 by crosses. These two curves should present the variation of these two components CaC2O4‚H2O or/and CaC2O4 and CaCO3. The peaks and pits should correspond to the peaks in the thermogram of TGA/DTA. The discrepancy might be caused by the different experimental conditions. Variation in Band Positions. The band positions are used to identify the phase or structure and should be sensitive to the transformation. The positions of the Raman bands were also measured and are plotted in Figure 12. The bands at 1631, 1492, 1459, 921, and 895 cm-1 belonged to CaC2O4‚H2O (I) and appeared at the beginning. The band at 921 cm-1 disappeared for the phase transformation of form I to form II of CaC2O4‚H2O. These bands shifted to 1643, 1477, and 904 cm-1 at spectrum 147 for the dehydration process from CaC2O4‚H2O (II) to R-CaCO3. The phase transformation of R to β and β to γ forms of CaC2O4 could not be observed, for the shifts of the bands were within the spectral resolution. These bands vanished at spectrum 495 due 1490 Analytical Chemistry, Vol. 69, No. 8, April 15, 1997
Figure 12. Band positions of some strong bands of CaC2O4‚H2O, CaC2O4, and CaCO3.
to its decomposition. The band at 1070 cm-1 of CaCO3 appeared at spectrum 457 and disappeared at spectrum 705. The transformation of aragonite to calcite was not observed. The variation in band positions clearly illustrates the phase transformation of CaC2O4‚H2O from form I to form II, dehydration of CaC2O4‚H2O (II), and decomposition of CaC2O4 and CaCO3. Variation in Background. The background was arbitrarily measured on a flat range from 1200 to 1100 cm-1 and is plotted in Figure 13. Similar variation was found in other region. Surprisingly, it varied dramatically. The background decreased at first under irradiation. It grew at spectrum 140. This was the beginning of the dehydration process. It leveled at spectrum 238, where the phase transformation occurred from the R to the β form of CaC2O4. After the transformation was completed, the background started to decay under irradiation, which was at spectrum 286. At spectrum 434, the background raised again to spectrum 495. This was due to the decomposition of CaC2O4. After that, it decayed slowly to spectrum 582 during the crystal growth and decayed rapidly to spectrum 602 for the phase transformation from aragonite to calcite. Apparently, it always grew up during the formation of a new phase and decayed after completion of the transformation. Similar behavior in background was found for TiO2 and CuSO4‚5H2O.23 The increase in background should not be attributable to the impurities. It should be due to the scattered light from the mixture of the two phases during the transformation from one phase to another. In other words, the rearrangement of the atoms during the transformation enhanced the background. This might give some clue to the formation of a new phase. This phenomenon is not understood completely yet. A similar increase in Rayleigh scattering was reported by Duval and Condrate.19 (23) Chang, H.; Huang, P. J.; Fu, Y. V. Unpublished work.
small, 50 µm in diameter. (7) Raman spectroscopy probes the surface within a penetration depth of the laser light, about 50 nm. The variation of a thin film on the surface can be monitored. However, thermo-Raman spectroscopy has the following disadvantages: (1) Laser radiation has a heating effect. That may introduce some error in temperature measurement. However, the rise in temperature should be small under weak irradiation on a white or transparent sample. (2) Many ionic binary compounds do not show any Raman band or shows only weak ones, such as CaO in this study. (3) Raman spectroscopy probes only the surface layer. The inner part of the sample could not be examined. (4) The gas evolved from the inner layer, CO2 in this case, may delay the reaction in the surface layer. The ability to identify the structure and phase and to monitor the transformation should be the main contribution of this thermoRaman spectroscopy. The determination of the transformation temperature may be the next. Furthermore, to measure and to identify the Raman spectra by comparing with the known ones should not be difficult. In all, the thermo-Raman spectroscopy should be valuable in thermal analysis.
Figure 13. Background of the Raman spectra.
DISCUSSION In this work, the Raman spectrum was taken continuously in a thermal process up to 750 °C. The structures and the phases can be identified directly from the Raman bands by comparing with those of known structures or phases. The intensities of the strong bands or representative bands gave some information about for the transformation. Their derivatives showed the transformation temperatures on the curves like those in thermograms of TGA/DTA. The peaks and pits showed the appearance and disappearance of one phase. The band positions also indicated the transformation. In all, the structural transformation, such as dehydration and decomposition, gave distinct variations in spectra. However, the phase transformation could be revealed by minor changes in spectra and had to be identified from the spectra. Surprisingly, the background or the Rayleigh scattering gave some clues to the formation of the new phase. From the above results, apparently, Raman spectroscopy can be used to identify the structure and phase and to monitor the transformation of a sample in thermal analysis. This is the thermo-Raman spectroscopy. In this work, advantages of this thermo-Raman spectroscopy for thermal analysis were found as follows: (1) The sample can be identified by its vibrational bands for the molecular structure or the crystalline phase; particularly, the lattice modes are important in identifying the phase. (2) The structural or phase transformation can be monitored on a time scale of seconds. (3) If the temperature rise is slow, the peaks in DTA may become very broad and weak. However, this would not affect the Raman spectra. This was the case for the phase transformation of CaC2O4‚H2O and CaC2O4 in this work. (4) Both the appearance and disappearance of one structure or one phase can be observed in Raman spectra. (5) The growth in background probably indicates the formation of a new compound or a new phase, such as the appearance of CaC2O4 and CaCO3. (6) Only a small amount of sample is needed, since the focusing spot of the laser light is
CONCLUSION In this work, Raman spectroscopy was used to identify the structure and phase and to monitor the transformation of CaC2O4‚H2O (calcium oxalate monohydrate), a calibration standard for TGA/DTA, in a thermal process similar to that for TGA/ DTA. The structural transformation for CaC2O4‚H2O consisted of dehydration of CaC2O4‚H2O to CaC2O4 and decomposition of CaC2O4 and CaCO3 to CaCO3 and CaO, respectively. These were indicated by the variations in Raman spectra. Furthermore, several phase transformations of CaC2O4‚H2O, CaC2O4, and CaCO3 were observed as some minor changes in Raman spectra. Not only the results were consistent with those from TGA/DTA but also more information can be provide than those TGA/DTA can. That thermo-Raman spectroscopy would be a valuable method for thermal analysis. Thermo-Raman spectroscopy not only gives the spectrum as the evidence for the identification of the structure and phase but also provides many other features for thermal analysis. In this work, (1) the intensity variation of the representative bands at 1645 and 1070 cm-1 indicated the amounts for CaC2O4‚H2O or/ and CaC2O4 and CaCO3; (2) the derivatives of their intensities vs temperature showed curves similar to those in the thermograms obtained in TGA/DTAsthe peaks and pits indicated the appearance and disappearance of one phase; (3) the changes in the band positions also indicated a transformation; and (4) even the annoying background provided some clues for the appearance of a new phase or a new compound. ACKNOWLEDGMENT TGA/DTA was run by Miss Ching Wen Tsai and Prof. Chiu Tih Yeh. The figures were prepared by Miss Yee Ven Fu. This work was supported by the National Science Council of the Republic of China (NSC-85-2113-M-007-010). Received for review August 30, 1996. Accepted January 31, 1997.X AC960881L X
Abstract published in Advance ACS Abstracts, March 1, 1997.
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