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Carbonization of boron containing polyimide films
Carbon 37 (1999) 887–895
Carbonization of boron containing polyimide films - B–N bond formation H. Konno*, H. Oka, K. Shiba, H. Tachikawa, M. Inagaki Graduate School of Engineering, Hokkaido University, Sapporo 060 -8628, Japan Received 1 June 1998; received in revised form 17 July 1998; accepted 21 July 1998
Abstract Two types of boron containing polyimide (Kapton type) film were prepared either by adding dihydroxyphenylborane (DPB) to polyamic acid [mixing method] or by using diaminodiphenyl ether having a boron containing functional group (DDE–B) to prepare polyamic acid [substitution method]. Formed polyimide films containing boron were carbonized at 600–12008C. By doping boron into the precursor polyimide, preferential formation of B–N bonds in carbonized films was observed and appreciable amounts of nitrogen were chemically trapped in the carbon film even after carbonization at 12008C. The polyimide prepared by the substitution method gave uniform carbon films in composition and structure, though DDE–B is not simply synthesized and didn’t polymerize to high degree. The polyimide prepared by the mixing method gave non-uniform carbon films, but the process was very simple and possible to increase boron content by using different boron compounds. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Doped carbons; B. Carbonization; C. X-ray photoelectron spectroscopy; D. Chemical structure
1. Introduction Carbon materials are usually made by pyrolysis and carbonization of organic precursors, and when the precursors contain foreign materials, the structure and properties of formed carbon materials are known to be strongly affected by them [1–3]. Aromatic polyimide films have aroused a great deal of interest as one of the attractive precursors for producing carbon and graphite films in recent years [4–12]. They have a wide range of molecular structure and various degrees of preferred orientation of imide molecules, which have been shown to affect strongly on the carbonization and graphitization process. We have reported on the Kapton type polyimide film that about 1 at.% of nitrogen remains at 12008C mainly as a form of tertiary nitrogen by substituting the carbon atom in the hexagonal layer [13], and such nitrogen atoms are not removed by the heat treatment up to 22008C [14]. As a consequence, the electromagnetic properties of the resultant carbon and graphite films show distinctive changes with heat treatment temperature [15]. *Corresponding author. Tel.: 181-11-706-7114; fax: 181-11706-7114. E-mail address: [email protected] (H. Konno)
The procedure for the preparation of a polyimide film is rather simple: first polyamic acid is synthesized by stirring a mixture of a dianhydride and a diamine in an appropriate solvent at room temperature under inert atmosphere, then it is casted on a substrate and imidized either thermally or chemically. Most of polyimide films can easily be carbonized by heat treatment under inert atmosphere, keeping the original shape even accompanied by large shrinkage. These simple processes for the preparation of precursor and carbonization inspired us with a process for the doping of foreign atoms into carbon to modify the properties. Boron is one of the few elements that can be substituted into the carbon structure of both sp 2 and sp 3 . It has been shown that substituted boron atoms accelerate the graphitization and suppress the oxidation of carbon materials [16–20]. Further, electromagnetic properties of graphite is reported to be strongly affected by a small amount of substituted boron [21]. Although the properties of boron doped carbons have been extensively studied, the doping method of boron into carbon is not sufficiently explored, and the conventional methods such as solid state reactions using diffusion have disadvantages. In the present work, boron-containing polyimide films were synthesized by two different methods, mixing a boron-containing organic compound with polyamic acid
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00203-6
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(mixing method) and partial substitution of boron-containing functional group into polyamic acid (substitution method). Their carbonization behavior was examined mainly by X-ray photoelectron spectroscopy (XPS).
2. Experimental
2.1. Sample preparation Preparation processes of polyimide films containing boron and the structure of related materials are shown in Fig. 1. The raw materials, pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (DDE), were of reagent grade and used after recrystallization, and the solvent was N,N9-dimethylacetamide (DMAc). As is well known, Kapton type polyimide film (symbol PI) is prepared by the process (a) in Fig. 1, without addition of dihydroxyphenylborane (DPB). In the present work, a DMAc solution of DPB was added to the intermediate, polyamic acid (PAA) solution. The mixtures with different amounts of DPB were casted onto a glass substrate and thermally imidized to form boron containing polyimide film (PI–B) [(a) mixing method]. The PI–B film thus synthesized was yellow and about 35 mm thick, and the calculated boron content was 0.5, 1 and 3 at.%. Alternately, DDE–B which was substituted with a boron containing functional group (R in Fig. 1) at the 2-position of DDE was used as an additional raw material. The PAA–B prepared from PMDA and DDE–B was hard to cast to film due to low degree of polymerization. This may be attributed to relatively low purity of DDE–B (¯90%). Therefore, the PAA–B solution was mixed with PAA, as shown in (b) in Fig. 1. The mixture was able to be casted to film on a glass substrate and imidized (PI–SB) [(b) substitution method]. Thickness of PI–SB films thus prepared was about 50 mm and the boron content was 0.8 at.%. Boron-containing aromatic polyimide films, PI–B and PI–SB, and polyimide films without addition of borane, PI, were cut into a square shape, mostly with a size of about 10310 mm 2 , and carbonized in between two alumina plates. Carbonization was carried out at temperatures between 600 and 12008C for 1 h in a flow of high-purity argon. Heating rate was 400 K h 21 .
2.2. Characterization of carbon films X-ray photoelectron spectroscopic measurements were carried out with a VG Scientific ESCALAB MkII and Mg Ka X-rays (15 kV, 20 mA). Binding energy, EB , was corrected by referring to the EB of Au 4f 7 / 2 electrons (84.0 eV) for gold particles sputtered on the sample films. The integral intensities were calculated by subtracting background by Shirley method. Peak separation was carried out by using a Gaussian / Lorentzian (G / L) product function
with the VGS 5250 software, where a full width at half maximum, FWHM, and the G / L ratio of each component were assumed to be the same. Scanning electron microscopic (SEM) observation was performed for a cross-section of carbonized films using JEOL JSM-6300F under acceleration voltage of 2 kV. The elemental analysis of carbonized film was carried out by a conventional combustion method: about 3 mg of sample was heated up to 9008C in a flow of oxygencontaining helium gas to analyze for H, C and N mass%. The total analysis for boron was not carried out due to the lack of suitable method for the low level in the present films. Theoretical calculation of chemical bonding states of boron and nitrogen in a graphitic layer was done by AM1-MO method.
3. Results and discussion
3.1. States of boron and nitrogen in carbon films The changes in N 1s spectra by XPS for PI with carbonization temperature are shown in Fig. 2. The peak of N 1s electrons for the original PI is located at 400.5 eV: the heat treatment produces mainly two types of nitrogen in the film, secondary nitrogen (pyridine type) which locates at 398.760.3 eV and tertiary nitrogen (pyrrole type) at 400.960.3 eV, there being small peaks at 402–406 eV due to nitrogen oxides and / or satellite peaks of the major ones. With increasing temperature relative intensity of secondary nitrogen decreases and that of the tertiary one becomes predominant. These results coincide with those reported for commercially available Kapton film which has the same molecular structure with the present PI [13]. In case of PI–B(3 at.% B) shown in Fig. 2, however, N 1s peak shape above 8008C is different from PI and the difference becomes pronounced at higher temperatures. Peak separation of N 1s for PI–B shown in Fig. 2 reveals that in addition to secondary and tertiary nitrogens a new component starts to appear from 8008C. The EB [N 1s] of the new peak was 398.860.2 eV. In parallel with the changes in N 1s peaks, B 1s peak observed for PI–B films shifts gradually from 193 eV for starting DPB to around 191.5 eV at 10008C with increasing temperature, as shown in Fig. 3. At 10008C and above, B 1s peak seems to be composed of two components at 189.660.2 and 191.860.2 eV (e.g. 12008C in Fig. 3). These results can be explained by the formation of B–N bonds in the cluster of hexagonal carbon structure during carbonization. The observed EB [N 1s]5398.8 eV is higher than the reported EB [N ls]5398.1 eV for boron nitride (BN) [22] and the observed EB [B 1s]5189.6 eV is lower than the reported EB [B 1s]5190.5 eV for boron nitride (BN) [22], but it is reasonable when other bonding atoms to B–N are all carbons. Another peak of E B [B 1s]5191.8 eV is too high for B–N type but much
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Fig. 1. Preparation processes of boron-containing polyimide film: (a) mixing method and (b) substitution method.
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Fig. 2. N 1s spectra by XPS for carbonized (a) PI, (b) PI–B, and (c) PI–SB.
lower than those for B 2 O 3 and H 3 BO 3 (both around 193 eV [22]). Accordingly, one of the bonds of this boron species may be still occupied by oxygen, since originally the boron in DPB is bonded to two –OH’s as shown in Fig. 1. It is likely that boron atoms bonding to carbon and / or nitrogen are terminated by an oxygen or hydroxyl group in the surface layer. It is worthwhile to note that the formation of boron carbides is unlikely because, for
example, the reported EB [B 1s] for B 4 C is 187.5 eV [23] and much lower. The appearance of the new component in N 1s spectrum and the change in EB [B 1s] are also observed for PI–SB films in the same manner as PI–B, as shown in Fig. 2 and Fig. 3. Accordingly, the doping of boron to polyimide causes the formation of B–N type bonds in carbon films, irrespective of the doping method.
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depending on the substituted position. The formation of B–N bond stabilized nitrogen in the polyimide films and increased nitrogen content in carbon films is described below.
3.2. Composition of carbon films
Fig. 2. (continued)
Formation of a B–N bond in a carbon film was also supported by AM1-MO calculations using a cluster model consisted of 14 rings shown in Fig. 4. When one B and one N were substituted for two C’s in a graphitic layer, the total energy of system became a minimum by the formation of a B–N bond (position a for B in Fig. 4) as tabulated in Fig. 4. When B and N were set apart (position b to f ), the energy increased by 162 kJ mol 21 or more,
The total intensity ratios of N 1s peak to C 1s peak by XPS, I[N 1s] / I[C 1s], for PI, PI–B (3 at.% B) and PI–SB(0.8 at.% B) are plotted against carbonization temperature in Fig. 5: points are an average of two or three experimental data sets. In the case of PI, the ratio, I[N 1s] / I[C 1s], decreases abruptly between 900 and 10008C. The changes in the ratio for PI–B and PI–SB are nearly the same with that of PI up to 8008C, and the ratios decrease monotonously up to 12008C. At 12008C, the relative intensities of nitrogen in carbon films for PI–B and PI–SB were more than twice that of PI. The results clearly indicate that nitrogen in polyimide is stabilized in carbon films by doping with boron. With a PI–B of 0.5 and 1 at.%, the effect of boron did not appear markedly. The changes in I[N 1s] / I[C 1s] for three nitrogen components, namely secondary nitrogen, tertiary nitrogen and B–N type, with carbonization temperature are shown in Fig. 6. With PI–B, the relative intensities of each component doesn’t show smooth change, whereas with PI–SB, the changes were monotonous. Especially, I[N 1s] / I[C 1s] for B–N type in carbonized PI–B reaches a maximum around 10008C and decreases at 12008C, which is in contrast to the gradual increment of B–N type in carbonized PI–SB. Similar behavior is observed for the changes in I[B 1s] / I[C 1s] with carbonization temperature as shown in Fig. 7. The changes in I[B 1s] / I[C 1s] are parallel to those in I[N 1s] / I[C 1s] of B–N type for both PI–B and PI–SB, indicating that most of boron atoms in the surface layer are present as the B–N type. Thus the boron doping into polyimide increases nitrogen content, consequently boron content, too, in the formed carbon films by forming B–N bonds. The irregular changes observed for PI–B in Figs. 6 and 7 disagree with its carbonization behavior by mass loss measurements, since the mass loss from 600 to 10008C was about 15% for PI–B and gradual (no step). Since the XPS data provides only the information on the surface (less than about 10 nm in depth), the results of elemental analysis of the films were compared with them as shown in Fig. 8. In the case of PI, the relation between I[N 1s] / I[C 1s] by XPS and N / C mole ratio in the matrix becomes a straight line reaching to the origin (dashed line in Fig. 8), indicating that the composition is uniform from the surface to the center of carbon films as observed on Kapton film [13]. The relation for PI–SB is slightly deviated, but very similar to PI, so that the carbon films derived from PI–SB are supposed to have fairly uniform distribution of nitrogen in the depth direction. Though the total analysis for boron was not carried out, the distribution of boron in depth of the film was expected to be similar to
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Fig. 3. B 1s spectra by XPS for carbonized (a) PI–B and (b) PI–SB.
that of nitrogen, since a large part of boron was present as B–N type as discussed above. In contrast, the behavior of PI–B(3 at.% B) deviates markedly from that of PI, showing that nitrogen is deficient in the surface layer. Considering this and the results for PI–B shown in Fig. 6 and Fig. 7, fairly large fragments containing B and N must be released at high temperatures. This was confirmed by observation of the cross section of carbonized films at 12008C by SEM (Fig. 9). The films derived from PI and PI–SB do not show any particular structure except for a thin surface layer which has been faced to the glass substrate during imidization (Fig. 9). In PI–B film, however, there are many forms as large as 1–5 mm (Fig. 9) and the size increases from substrate side to surface side. It is evident that the DPB-based fragments were released by high temperature carbonization. It is worthwhile to note that the total nitrogen content determined by chemical analysis in the carbon film derived from PI–B was about twice of that from PI–SB at 12008C
(Fig. 8). This may be due to the higher content of boron in the original polyimide film, but the mixing method is very simple compared with the synthesis of the boron containing raw materials for polyimide. There is the possibility of forming uniform carbon films from PI–B by contriving a heating process.
4. Conclusions In the present work, two types of boron containing polyimide film, PI–B and PI–SB, were prepared by adding dihydroxyphenylborane (DPB) to polyamic acid [mixing method] and by using diaminodiphenyl ether having a boron containing functional group (DDE–B) to prepare polyamic acid [substitution method], respectively. By doping boron into the precursor polyimide, nitrogen content in the carbon film increased markedly after carbonization at 12008C due to the formation of B–N bonds in a
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Fig. 4. The cluster model of graphite doped with N and B, and the calculated energies for different positions of B by AM1-MO method.
Fig. 6. Changes in the XPS peak intensity ratio, I[N 1s] / I[C 1s], of three types of nitrogen with carbonization temperature after peak separation (e.g. Fig. 2(b)): -앳- secondary nitrogen (pyridine type), -h- tertiary nitrogen (pyrrole type), and -d- B–N type.
Fig. 5. Changes in the total peak intensity ratio, I[N 1s] / I[C 1s], by XPS with carbonization temperature: -s- PI, -쏆- PI–B, and -j- PI–SB.
Fig. 7. Changes in the total peak intensity ratio, I[B 1s] / I[C 1s], by XPS with carbonization temperature: -d- PI–B and -s- PI– SB.
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Acknowledgements The authors wish to thank Prof. N. Miyaura of the Graduate School of Engineering, Hokkaido University, for synthesizing boron containing DDE-B. The present work was partly supported by the ‘‘Research for the Future’’ Program by the Japan Society for the Promotion of Science (JSPS-RFTF96R11701).
References [1] [2] [3] [4] Fig. 8. The relationship between the total peak intensity ratio, I[N 1s] / I[C 1s], by XPS (surface composition) and the mole ratio, N / C, in the film by elemental analysis (bulk composition) plotted with carbonization temperature as a parameter.
[5] [6] [7] [8]
carbon structure. The PI–SB prepared by the substitution method gave uniform carbon films in composition and structure, but DDE–B is not simply synthesized and didn’t polymerize to high degree mainly due to the purity. The PI–B prepared by the mixing method gave non-uniform carbon films, but the process is very simple and it is possible to increase boron content by surveying other boron compounds in addition to DPB. There are some points to be improved but the present processes are promising in the formation of B / C / N compounds.
[9] [10] [11] [12] [13] [14]
Oberlin A, Rouchy JP. Carbon 1971;9:39. Countney RL, Duliere SF. Carbon 1972;10:65. Sato Y. Carbon 1995;33:602. Burger A, Fitzer E, Heym M, Termiesch B. Carbon 1975;13:149. Hishiyama Y, Yasuda S, Yoshida A, Inagaki M. J Mater Sci 1988;23:3272. Inagaki M, Meng L-J, Ibuki T, Sakai M, Hishiyama Y. Carbon 1991;29:1239. Inagaki M, Sakamoto K, Hishiyama Y. J Mater Res 1991;6:1108. Inagaki M, Ibuki T, Takeichi T. J Appl Polym Sci 1992;44:521. Takeichi T, Takenoshita H, Ogura S, Inagaki M. J Appl Polym Sci 1994;54:361. Inagaki M, Hishiyama Y, Kaburagi Y. Carbon 1994;32:637. Takeichi T, Kaburagi Y, Hishiyama Y, Inagaki M. Carbon 1995;33:1621. Hatori H, Yamada Y, Shiraishi M, Nakata H, Yoshitomi S. Carbon 1992;30:305. Konno H, Nakahashi T, Inagaki M. Carbon 1997;35:669. Inagaki M, Tachikawa H, Nakahashi T, Konno H, Hishiyania Y. Carbon 1998;36:1021.
Fig. 9. The cross sections of carbonized film at 12008C observed by SEM: (a) PI, (b) PI–B and (c) PI–SB.
H. Konno et al. / Carbon 37 (1999) 887 – 895 [15] Hishiyama Y, Igarashi K, Kanaoka K, et al. Carbon 1997;35:657. [16] Lowell CE. J Am Ceram Soc 1967;50:142. [17] Hagio T, Nakamizo M, Kobayashi K. Carbon 1989;27:259. [18] Jones LE, Thrower PA. Carbon 1990;28:239. [19] William C, Thomas EP, Oariva G, Carlo GP. Carbon 1995;33:367.
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[20] Inagaki M, Nakahashi T, Konno H, Sogabe T. Carbon 1997;35:1194. [21] Hishiyama Y, Kaburagi Y, Kobayashi K, Inagaki M. JSPS Committee 117 Report. [117-241-AS, Tokyo, Japan, 1997. [22] Wagner CD. In: Briggs D, Seah MP, editors. Practical surface analysis, vol. 1. New York: Wiley, 1990:599. [23] Cermignani W, Paulson TE, Onneby C. Carbon 1995;33:367.
Carbonization of boron containing polyimide films - B–N bond formation H. Konno*, H. Oka, K. Shiba, H. Tachikawa, M. Inagaki Graduate School of Engineering, Hokkaido University, Sapporo 060 -8628, Japan Received 1 June 1998; received in revised form 17 July 1998; accepted 21 July 1998
Abstract Two types of boron containing polyimide (Kapton type) film were prepared either by adding dihydroxyphenylborane (DPB) to polyamic acid [mixing method] or by using diaminodiphenyl ether having a boron containing functional group (DDE–B) to prepare polyamic acid [substitution method]. Formed polyimide films containing boron were carbonized at 600–12008C. By doping boron into the precursor polyimide, preferential formation of B–N bonds in carbonized films was observed and appreciable amounts of nitrogen were chemically trapped in the carbon film even after carbonization at 12008C. The polyimide prepared by the substitution method gave uniform carbon films in composition and structure, though DDE–B is not simply synthesized and didn’t polymerize to high degree. The polyimide prepared by the mixing method gave non-uniform carbon films, but the process was very simple and possible to increase boron content by using different boron compounds. 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Doped carbons; B. Carbonization; C. X-ray photoelectron spectroscopy; D. Chemical structure
1. Introduction Carbon materials are usually made by pyrolysis and carbonization of organic precursors, and when the precursors contain foreign materials, the structure and properties of formed carbon materials are known to be strongly affected by them [1–3]. Aromatic polyimide films have aroused a great deal of interest as one of the attractive precursors for producing carbon and graphite films in recent years [4–12]. They have a wide range of molecular structure and various degrees of preferred orientation of imide molecules, which have been shown to affect strongly on the carbonization and graphitization process. We have reported on the Kapton type polyimide film that about 1 at.% of nitrogen remains at 12008C mainly as a form of tertiary nitrogen by substituting the carbon atom in the hexagonal layer [13], and such nitrogen atoms are not removed by the heat treatment up to 22008C [14]. As a consequence, the electromagnetic properties of the resultant carbon and graphite films show distinctive changes with heat treatment temperature [15]. *Corresponding author. Tel.: 181-11-706-7114; fax: 181-11706-7114. E-mail address: [email protected] (H. Konno)
The procedure for the preparation of a polyimide film is rather simple: first polyamic acid is synthesized by stirring a mixture of a dianhydride and a diamine in an appropriate solvent at room temperature under inert atmosphere, then it is casted on a substrate and imidized either thermally or chemically. Most of polyimide films can easily be carbonized by heat treatment under inert atmosphere, keeping the original shape even accompanied by large shrinkage. These simple processes for the preparation of precursor and carbonization inspired us with a process for the doping of foreign atoms into carbon to modify the properties. Boron is one of the few elements that can be substituted into the carbon structure of both sp 2 and sp 3 . It has been shown that substituted boron atoms accelerate the graphitization and suppress the oxidation of carbon materials [16–20]. Further, electromagnetic properties of graphite is reported to be strongly affected by a small amount of substituted boron [21]. Although the properties of boron doped carbons have been extensively studied, the doping method of boron into carbon is not sufficiently explored, and the conventional methods such as solid state reactions using diffusion have disadvantages. In the present work, boron-containing polyimide films were synthesized by two different methods, mixing a boron-containing organic compound with polyamic acid
0008-6223 / 99 / $ – see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 98 )00203-6
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(mixing method) and partial substitution of boron-containing functional group into polyamic acid (substitution method). Their carbonization behavior was examined mainly by X-ray photoelectron spectroscopy (XPS).
2. Experimental
2.1. Sample preparation Preparation processes of polyimide films containing boron and the structure of related materials are shown in Fig. 1. The raw materials, pyromellitic dianhydride (PMDA) and diaminodiphenyl ether (DDE), were of reagent grade and used after recrystallization, and the solvent was N,N9-dimethylacetamide (DMAc). As is well known, Kapton type polyimide film (symbol PI) is prepared by the process (a) in Fig. 1, without addition of dihydroxyphenylborane (DPB). In the present work, a DMAc solution of DPB was added to the intermediate, polyamic acid (PAA) solution. The mixtures with different amounts of DPB were casted onto a glass substrate and thermally imidized to form boron containing polyimide film (PI–B) [(a) mixing method]. The PI–B film thus synthesized was yellow and about 35 mm thick, and the calculated boron content was 0.5, 1 and 3 at.%. Alternately, DDE–B which was substituted with a boron containing functional group (R in Fig. 1) at the 2-position of DDE was used as an additional raw material. The PAA–B prepared from PMDA and DDE–B was hard to cast to film due to low degree of polymerization. This may be attributed to relatively low purity of DDE–B (¯90%). Therefore, the PAA–B solution was mixed with PAA, as shown in (b) in Fig. 1. The mixture was able to be casted to film on a glass substrate and imidized (PI–SB) [(b) substitution method]. Thickness of PI–SB films thus prepared was about 50 mm and the boron content was 0.8 at.%. Boron-containing aromatic polyimide films, PI–B and PI–SB, and polyimide films without addition of borane, PI, were cut into a square shape, mostly with a size of about 10310 mm 2 , and carbonized in between two alumina plates. Carbonization was carried out at temperatures between 600 and 12008C for 1 h in a flow of high-purity argon. Heating rate was 400 K h 21 .
2.2. Characterization of carbon films X-ray photoelectron spectroscopic measurements were carried out with a VG Scientific ESCALAB MkII and Mg Ka X-rays (15 kV, 20 mA). Binding energy, EB , was corrected by referring to the EB of Au 4f 7 / 2 electrons (84.0 eV) for gold particles sputtered on the sample films. The integral intensities were calculated by subtracting background by Shirley method. Peak separation was carried out by using a Gaussian / Lorentzian (G / L) product function
with the VGS 5250 software, where a full width at half maximum, FWHM, and the G / L ratio of each component were assumed to be the same. Scanning electron microscopic (SEM) observation was performed for a cross-section of carbonized films using JEOL JSM-6300F under acceleration voltage of 2 kV. The elemental analysis of carbonized film was carried out by a conventional combustion method: about 3 mg of sample was heated up to 9008C in a flow of oxygencontaining helium gas to analyze for H, C and N mass%. The total analysis for boron was not carried out due to the lack of suitable method for the low level in the present films. Theoretical calculation of chemical bonding states of boron and nitrogen in a graphitic layer was done by AM1-MO method.
3. Results and discussion
3.1. States of boron and nitrogen in carbon films The changes in N 1s spectra by XPS for PI with carbonization temperature are shown in Fig. 2. The peak of N 1s electrons for the original PI is located at 400.5 eV: the heat treatment produces mainly two types of nitrogen in the film, secondary nitrogen (pyridine type) which locates at 398.760.3 eV and tertiary nitrogen (pyrrole type) at 400.960.3 eV, there being small peaks at 402–406 eV due to nitrogen oxides and / or satellite peaks of the major ones. With increasing temperature relative intensity of secondary nitrogen decreases and that of the tertiary one becomes predominant. These results coincide with those reported for commercially available Kapton film which has the same molecular structure with the present PI [13]. In case of PI–B(3 at.% B) shown in Fig. 2, however, N 1s peak shape above 8008C is different from PI and the difference becomes pronounced at higher temperatures. Peak separation of N 1s for PI–B shown in Fig. 2 reveals that in addition to secondary and tertiary nitrogens a new component starts to appear from 8008C. The EB [N 1s] of the new peak was 398.860.2 eV. In parallel with the changes in N 1s peaks, B 1s peak observed for PI–B films shifts gradually from 193 eV for starting DPB to around 191.5 eV at 10008C with increasing temperature, as shown in Fig. 3. At 10008C and above, B 1s peak seems to be composed of two components at 189.660.2 and 191.860.2 eV (e.g. 12008C in Fig. 3). These results can be explained by the formation of B–N bonds in the cluster of hexagonal carbon structure during carbonization. The observed EB [N 1s]5398.8 eV is higher than the reported EB [N ls]5398.1 eV for boron nitride (BN) [22] and the observed EB [B 1s]5189.6 eV is lower than the reported EB [B 1s]5190.5 eV for boron nitride (BN) [22], but it is reasonable when other bonding atoms to B–N are all carbons. Another peak of E B [B 1s]5191.8 eV is too high for B–N type but much
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Fig. 1. Preparation processes of boron-containing polyimide film: (a) mixing method and (b) substitution method.
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Fig. 2. N 1s spectra by XPS for carbonized (a) PI, (b) PI–B, and (c) PI–SB.
lower than those for B 2 O 3 and H 3 BO 3 (both around 193 eV [22]). Accordingly, one of the bonds of this boron species may be still occupied by oxygen, since originally the boron in DPB is bonded to two –OH’s as shown in Fig. 1. It is likely that boron atoms bonding to carbon and / or nitrogen are terminated by an oxygen or hydroxyl group in the surface layer. It is worthwhile to note that the formation of boron carbides is unlikely because, for
example, the reported EB [B 1s] for B 4 C is 187.5 eV [23] and much lower. The appearance of the new component in N 1s spectrum and the change in EB [B 1s] are also observed for PI–SB films in the same manner as PI–B, as shown in Fig. 2 and Fig. 3. Accordingly, the doping of boron to polyimide causes the formation of B–N type bonds in carbon films, irrespective of the doping method.
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depending on the substituted position. The formation of B–N bond stabilized nitrogen in the polyimide films and increased nitrogen content in carbon films is described below.
3.2. Composition of carbon films
Fig. 2. (continued)
Formation of a B–N bond in a carbon film was also supported by AM1-MO calculations using a cluster model consisted of 14 rings shown in Fig. 4. When one B and one N were substituted for two C’s in a graphitic layer, the total energy of system became a minimum by the formation of a B–N bond (position a for B in Fig. 4) as tabulated in Fig. 4. When B and N were set apart (position b to f ), the energy increased by 162 kJ mol 21 or more,
The total intensity ratios of N 1s peak to C 1s peak by XPS, I[N 1s] / I[C 1s], for PI, PI–B (3 at.% B) and PI–SB(0.8 at.% B) are plotted against carbonization temperature in Fig. 5: points are an average of two or three experimental data sets. In the case of PI, the ratio, I[N 1s] / I[C 1s], decreases abruptly between 900 and 10008C. The changes in the ratio for PI–B and PI–SB are nearly the same with that of PI up to 8008C, and the ratios decrease monotonously up to 12008C. At 12008C, the relative intensities of nitrogen in carbon films for PI–B and PI–SB were more than twice that of PI. The results clearly indicate that nitrogen in polyimide is stabilized in carbon films by doping with boron. With a PI–B of 0.5 and 1 at.%, the effect of boron did not appear markedly. The changes in I[N 1s] / I[C 1s] for three nitrogen components, namely secondary nitrogen, tertiary nitrogen and B–N type, with carbonization temperature are shown in Fig. 6. With PI–B, the relative intensities of each component doesn’t show smooth change, whereas with PI–SB, the changes were monotonous. Especially, I[N 1s] / I[C 1s] for B–N type in carbonized PI–B reaches a maximum around 10008C and decreases at 12008C, which is in contrast to the gradual increment of B–N type in carbonized PI–SB. Similar behavior is observed for the changes in I[B 1s] / I[C 1s] with carbonization temperature as shown in Fig. 7. The changes in I[B 1s] / I[C 1s] are parallel to those in I[N 1s] / I[C 1s] of B–N type for both PI–B and PI–SB, indicating that most of boron atoms in the surface layer are present as the B–N type. Thus the boron doping into polyimide increases nitrogen content, consequently boron content, too, in the formed carbon films by forming B–N bonds. The irregular changes observed for PI–B in Figs. 6 and 7 disagree with its carbonization behavior by mass loss measurements, since the mass loss from 600 to 10008C was about 15% for PI–B and gradual (no step). Since the XPS data provides only the information on the surface (less than about 10 nm in depth), the results of elemental analysis of the films were compared with them as shown in Fig. 8. In the case of PI, the relation between I[N 1s] / I[C 1s] by XPS and N / C mole ratio in the matrix becomes a straight line reaching to the origin (dashed line in Fig. 8), indicating that the composition is uniform from the surface to the center of carbon films as observed on Kapton film [13]. The relation for PI–SB is slightly deviated, but very similar to PI, so that the carbon films derived from PI–SB are supposed to have fairly uniform distribution of nitrogen in the depth direction. Though the total analysis for boron was not carried out, the distribution of boron in depth of the film was expected to be similar to
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Fig. 3. B 1s spectra by XPS for carbonized (a) PI–B and (b) PI–SB.
that of nitrogen, since a large part of boron was present as B–N type as discussed above. In contrast, the behavior of PI–B(3 at.% B) deviates markedly from that of PI, showing that nitrogen is deficient in the surface layer. Considering this and the results for PI–B shown in Fig. 6 and Fig. 7, fairly large fragments containing B and N must be released at high temperatures. This was confirmed by observation of the cross section of carbonized films at 12008C by SEM (Fig. 9). The films derived from PI and PI–SB do not show any particular structure except for a thin surface layer which has been faced to the glass substrate during imidization (Fig. 9). In PI–B film, however, there are many forms as large as 1–5 mm (Fig. 9) and the size increases from substrate side to surface side. It is evident that the DPB-based fragments were released by high temperature carbonization. It is worthwhile to note that the total nitrogen content determined by chemical analysis in the carbon film derived from PI–B was about twice of that from PI–SB at 12008C
(Fig. 8). This may be due to the higher content of boron in the original polyimide film, but the mixing method is very simple compared with the synthesis of the boron containing raw materials for polyimide. There is the possibility of forming uniform carbon films from PI–B by contriving a heating process.
4. Conclusions In the present work, two types of boron containing polyimide film, PI–B and PI–SB, were prepared by adding dihydroxyphenylborane (DPB) to polyamic acid [mixing method] and by using diaminodiphenyl ether having a boron containing functional group (DDE–B) to prepare polyamic acid [substitution method], respectively. By doping boron into the precursor polyimide, nitrogen content in the carbon film increased markedly after carbonization at 12008C due to the formation of B–N bonds in a
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Fig. 4. The cluster model of graphite doped with N and B, and the calculated energies for different positions of B by AM1-MO method.
Fig. 6. Changes in the XPS peak intensity ratio, I[N 1s] / I[C 1s], of three types of nitrogen with carbonization temperature after peak separation (e.g. Fig. 2(b)): -앳- secondary nitrogen (pyridine type), -h- tertiary nitrogen (pyrrole type), and -d- B–N type.
Fig. 5. Changes in the total peak intensity ratio, I[N 1s] / I[C 1s], by XPS with carbonization temperature: -s- PI, -쏆- PI–B, and -j- PI–SB.
Fig. 7. Changes in the total peak intensity ratio, I[B 1s] / I[C 1s], by XPS with carbonization temperature: -d- PI–B and -s- PI– SB.
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Acknowledgements The authors wish to thank Prof. N. Miyaura of the Graduate School of Engineering, Hokkaido University, for synthesizing boron containing DDE-B. The present work was partly supported by the ‘‘Research for the Future’’ Program by the Japan Society for the Promotion of Science (JSPS-RFTF96R11701).
References [1] [2] [3] [4] Fig. 8. The relationship between the total peak intensity ratio, I[N 1s] / I[C 1s], by XPS (surface composition) and the mole ratio, N / C, in the film by elemental analysis (bulk composition) plotted with carbonization temperature as a parameter.
[5] [6] [7] [8]
carbon structure. The PI–SB prepared by the substitution method gave uniform carbon films in composition and structure, but DDE–B is not simply synthesized and didn’t polymerize to high degree mainly due to the purity. The PI–B prepared by the mixing method gave non-uniform carbon films, but the process is very simple and it is possible to increase boron content by surveying other boron compounds in addition to DPB. There are some points to be improved but the present processes are promising in the formation of B / C / N compounds.
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Fig. 9. The cross sections of carbonized film at 12008C observed by SEM: (a) PI, (b) PI–B and (c) PI–SB.
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