CARBONIZATION AND GRAPHITIZATION OF POLYI~IDE FILM “NOVAX”

MICHIO INAGAKI Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo, 060 Japan

LI-JUN MENG, TSUTOMU IBUKI*, MOTOTSUGU SAKAI Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 441 Japan

and

YOSHIHIRO HISHIYAMA Musashi Institute of Technology, Tamazutsumi, Setagaya-ku, Tokyo 15X, Japan

(Received 15 January 1991: accepted it? final form 27 .March 1991)

Abst~et-Polyimide films “Novax” consisting of the imide part with the bridging part of a random mixture of two different structures, were carbonized at various temperatures up to 1100°C and graph- itized at 3000°C. The weight change occurred in two steps, as other polyimide films; large weight decrease in a narrow temperature range between 450” and 600°C. and a further decrease above 800°C. The former weight decrease could be explained by the evolution of large amount of CO and CO2 gases, and the latter corresponded to the evolution of N, gas and rapid increase in electrical conductivity up to 2fX) S/cm. The thinnest film gave a graphite film with a high degree of graphiti~ation, high value of magnetoresistance. and very low anisotropy ratios.

Key Words-Carbonization. graphitization, polyimide. film.

1. INTRODUCTION

Polyimide film is an attractive material as a precursor

for carbon films. as well as thermoresistant poiy- mers, from fundamental and practical points of view. “Kapton” is a representative commercialized polyi- mide film and has been reported to give a carbon film with a graphitizing nature[l]. The carbon film from Kapton results in a film with a high degree of graphitization and also a high degree of orientation of graphite layers along the film surface, by heat treatment at high temperatures above Z500°C[I-51.

The molecules of polyimides can be considered to consist of two structural parts, as shown by some representative examples in Fig. 1. In the present work, we may call them as an “imide part” and a “bridging part” connecting two imide parts. Kapton and another polyimide film, Upilex, were studied in our previous work on their carbonization and gra- phitization(2--71. The Kapton molecule has a flat structure except for a kink at the etheric oxygen in the bridging part[8]. and is known to give a carbon film with a highly graphitizing nature[2-61. The Up- ilex molecule, however, has twisting at the bi-phenyl bond in the imide part, in addition to the kink at the bridging one. A very low degree of graphitization was observed on the carbon film prepared from Up- ilex film[7]. From these experiments, three factors for getting graphitizing carbon films were pointed out. 1) flatness of starting polyimide molecules, 2) a high degree of orientation of molecules in the film and, 3) release of non-carbon atoms without distur-

“Present address: Mitsubishi Rayon Co., Ltd., Sunad- ahashi-4, Higashi-ku, Nagoya. 461 Japan.

bance of oriented texture in the film during thermal decomposition of polyimide.

In the present work, a commerciai~y available po- lyimide film named “Novax” (,~itsubishi Chemicals Ind. Ltd.) was used as a precursor for carbon films; its molecular structure is shown in Fig. l(c), the bridging part consisting of two different randomly mixed components.

2. EXPERIMENTAL

2.1 Polyirnide films used The polyimide Novax films used are listed in Table

1, having different thickness of the film and different ratios of two components ODA and OTD, which came from 4,4’-oxydianiline and o-tridine respec- tively, in the bridging part (see Fig. l(c)). The films with different thicknesses and with the same ODAi OTD ratio were used, because a remarkable effect of film thickness on graphitization behavior was ob- served on other polyimide films. Two films with dif- ferent ODAIOTD ratios, but with the same thickness of 50 pm were also used. The films were prepared as follows[9]. The viscous solution of po- lyamic acid with N,N’-dimethylformamide was dried on a glass plate as thin film at about lOO”C, and then imidized by heating to 200”-350°C under constraint in a metallic frame.

2.2 Carbonization The rectangular specimens with dimensions of

6 x 20 mm’ were cut from a large sheet (ca. 210 x 280 mm’) of Novax supplied and sandwiched between two alumina plates. Between the polyimide

1239

1240 M. ~NAGAKI ef al.

b) +N~+-o* IPlLEX

cf -f-N M n--s-+ NOVAX n

0 0

ODA 010

Fig. 1. Chemical structures of representative polyimide molecules.

film specimen and alumina plate, thin gold foil was

placed in order to avoid sticking of the film during carbonization. The carbonization of the film speci- mens was carried out at various temperatures rang- ing from 400” to 1100°C in a nitrogen flow with a rate of 10 mllmin. The specimen was heated by the rate of 400°C/h and held at each prescribed tem- perature for 1 hour.

Changes in weight and size were measured as a function of carbonization temperature. Measure- ment of electrical conductivity for the films carbon- ized at temperatures above 800°C was made at room temperature by a four-terminal method. Conductiv- ity was determined from the relation between cur- rents ranging from 0.1 to 20 mA and voltage, and the cross-sectional area was measured under an op- tical microscope.

The pyrolysis gas evolved from the film specimens was analyzed by gas chromatography in every 25°C of heating process up to 1000°C with a heating rate of 200”Clhour; carrier gas of helium with a flow rate of 20 ml/min, active carbon column kept at 75°C in a stainless steel tube, and a thermal conductivity detector. The volume of each evolved gas per 1 g of starting polymer and per 1°C increment was deter- mined by referring to standard gases.

2.3 ~ru~hitization The film carbonized at 900°C was graphitized at

3000°C for 1 hour in an argon atmosphere, by being sandwiched between polished isotropic high-density graphite blocks.

Table 1. Polyimide “Novax” films used

Sample Code ODAiOTD Ratio Original

Thickness (km)

ML-25 50150 2.5 ML-50 50150 50 ML-75 50150 75 MK-50 55145 50

X-ray powder pattern of the films thus graphitized was measured by using Cu Ka radiation. The inter- layer spacing dnL and the crystallite thickness L,. were determined from 002 and 004 diffraction lines, by referring to the internal standard of silicon. The magnetoresistance for the graphitized films was mea- sured at liquid nitrogen temperature as functions of strength and also orientation of magnetic field, the orientation being referred to the film surface. The anisotropy ratios r7 and rfL as measures of orienta- tion degree and scheme, respectively, were calcu- lated from the ratios of (Ap/p)~~,” and (AP/~)~~,~~” to (A pip),,,, the former two being measured by di- recting the magnetic field parallel or perpendicular to the current direction, respectively, and the latter by directing the field perpendicular to both film sur- face and current direction. The details of the magne- toresistance measurement are explained in the reference[lO]. The cross-section of the graphitized films was also observed under polarized light after embedding in the epoxy resin and polishing.

3. RESULTS AND DISCUSSION

3.1 Carbonization behavior The weight change and linear shrinkage along the

film surface with carbonization temperature are shown in Fig. 2 (a) and (b), respectively. Weight decreases for four samples are very similar with each other and occur in two steps, as observed on other polyimide films[6,7]; rapid decrease of weight in a rather narrow temperature range from 450” to 600°C and gradual decrease in a wide temperature range above 800°C. The thinnest film ML-25 shows rather clearly two steps in weight decrease and two films, ML-50 and MK-50, with the same thickness but with a little different com~sition in the bridging part cannot be differentiated from the weight changes. After carbonization at llOO”C, the thickest film ML- 75 gives the lowest weight loss of about 45% and the weight loss of the other three films are about the same, 55%.

The size changes (linear shrinkage) with carbon- ization on four films are also very similar with each other; shrinkage occurs gradually with the increase in temperature and becomes negligibly small above 800°C. The linear shrinkage at 1100°C is 22% for all four films.

Even such a remarkable weight loss and shrinkage with carbonization did not give any cracks in the resulting carbon films, as already pointed out on other polyimide filmsf6,7].

Figure 3 shows the results of the analysis of py- rolysis gas on the films ML-50. There is no re- markable difference in gases evolved. Up to 600°C the evolution of CO and CO, is very remarkable, corresponding to the rapid weight decrease in this temperature range (Fig. 2a). Above 6oo”C, CO and CO? tend to decrease with temperature, and CH, and Hz start to evolve, the former being much less than the latter. Until 800°C the gas evolution is al-

Polyimide film “Novax” 1241

-F GO-

.\e ,” --

P 0 i>

3 5;; 1

4o 400 I I I I I

o\s 600 800 1000

HTT ("C)

& b

Y c .- L

6

15- / .rn 0

n

10 - 0

/

5- J'

/p o- ' I I I I I I I

400 600 800 1000

HTT ("C)

Fig. 2. Weight change (a) and linear shrinkage along the film surface (b) with carbonization temperature.

most finished, except H,. Above 85o”C, Nz starts to evolve. These gas evolution behaviors agree with the weight change shown in Fig. 2; the first step of weight loss being mostly due to the conversion of four car- bony1 groups (C = 0) in the imide part to either CO or COz, and the second step corresponding to the NZ evolution. These decomposition schema were already pointed out on both Kapton and Upilex po- lyimides from the data on weight changes[6,7].

At temperatures below 800°C electrical conduc- tivity along the film surface was so low that it could not be measured by the conventional method em- ployed. Above 800°C however, it increases rapidly with the increase in carbonization temperature, as shown in Fig. 4 on two kinds of films with the same original thickness. The conductivity value reaches about 200 S/cm after carbonization up to 1100°C. The rapid increase in conductivity value with car- bonization and its absolute value at 1100°C are al- most the same as those of the polyimide Kapton film.

3.2 Graphitization behavior The data of X-ray diffraction and magnetoresist-

ante on the films graphitized at 3000°C are sum- marized in Table 2.

The thinnest film, ML-25, has the value of inter- layer spacing d,, close to that of natural graphite and the high value of maximum transverse magne- toresistance (Aplp),,,, which is much higher than that observed on the Kapton-derived graphite films[2,3]. These values reveal the highest degree of graphitization in the graphite materials prepared from organic precursors[ lo]. The galvanomagnetic properties of these well-graphitized films prepared from the present Novax ML-25 and from Kapton with 25pm thickness were measured and discussed in detail elsewhere[ll].

The magnetoresistance values, particularly the value of (ApIp),,,, are so sensitive to the crystal perfection that they scatter to a certain extent, de- pending on the location in a film. For the film ML-

1242 M. INAGAKI etal.

400 500 600 700 800 900 1000 HTT ("G)

Fig. 3. Chromatogram of gases evolved during carbonization of the film ML-50 up to 1000°C.

25, for example, we got the (A~/P)~,, value of about 700% by another graphitization run though its X- ray parameters are the same as those in Table 2.

The films ML-50 and -75 also show relatively high degrees of graphitization, giving rather high values of (A~/P),,,~~ and low values of d,,,,.

For the thinnest film, ML-25, very small values

i

3; JO2 )r

CI .-

> .-

5 2 E u

10'

/ 4

/

B

n

0

/ 51

I I I I

800 1000

HTT ("C)

of anisotropy ratios, rT and rT,., are observed, which indicate high degrees of plane orientation of hex- agonal carbon layers along the film surface. This high degree of orientation can be seen by the polarized light micrograph of the cross-section of the graphi- tized film, shown in Fig. 5. On the films ML-50 and -75, a little larger anisotropy ratios were observed, but they were still relatively small in comparison with graphitized carbon blocks. The lower degree of ori- entation in the films ML-50 and -75 was qualita- tively supported by optical observations of their cross-sections. The optical micrograph on the graph-

Fig. 4. Electrical conductivity at room temperature for the carbonized films prepared from ML-50 (W) and MK-50

(0) as a function of carbonization temperature. Fig. 5. Polarized light micrograph of the cross-section of

graphitized film from ML-25.

Polyimide film “Novax” 1243

Table 2. X-ray and magnetoresistancc parameters of the graphitized films

I X-ray Parameters

Sample d,,,> Code (nm)

Magnetoresistance at 77 K

(‘/o rl ril

ML-25 0.3355 > 100 > 100 1191 ML-50 0.3355 > 100 > 100 400.1 ML-75 0.3358 X7 62 39Y.Y MK-50 0.3357 96 80 55.38

itized film prepared from the film ML-75 showed thin cracks and misoriented parts.

It has to be mentioned that the film MK-50 does

not give high value of (A~/P)“,.,~, and has relatively high values of anisotropy ratios (0.2%0.27), in spite of the similarity to ML-50 in thickness and carbon- ization behavior. This could be explained by the presence of optically isotropic regions in the majority of anisotropic areas in the cross-section of the film MK-SO. It might be worthwhile to try carbonization of the polyimide having only OTD component in the bridging part of the molecule, though it is not avail- able commercially now. The flatness of the starting molecules is important to get high graphitizabihty.

A(,Xno~~/r~~emenrv-The authors thank Messrs. T. Sogabe and M. Okada of Toyo Tanso Co., Ltd. for their help in heat treating the films at 3000°C. The present work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas, New Functionality Materials, Design. Preparation and Control (No. 02205060). from the Ministry of Education. Scicncc and Culture, and also hy a Grant for International Joint Research Project from the NEDO. Japan.

I.

2.

3 _

3.

5 n:

7.

8.

Y.

10.

II.

9.2X 3.69 0.008 0.003 38.55 29.58 0.096 0.073 23.46 19.06 0.059 0.048 16.23 11.79 0.2Y3 0.267

REFERENCES

A. Buerger. E. Fitzer. M. Heim. and B. Terwiesch. Carbon 13. 149 (1975). Y. Hishiyama. S. Yasuda, A. Yoshida, and M. Inagaki, J. Mat. Sri. 23. 3722 (lY88). Y. Hishiyama, I. Natsume, Y. Ushijima. 0. Komada, and M. Inagaki. In MRS Symposium on Graphite In-

tercalution Compounds; Science and Applicutions. Ma- terial Research Society. Boston. Extended Abstracts p. 231 Fall (198X). C. Bourgerette. A. Oberlin. and M. Inagaki. J. ,Mat.

Rex. (in press). M. Inagaki and Y. Hishiyama. J. Mat. Res. (in press). M. Inagaki. S. Harada. T. Sato, T. Nakajima. Y. Hor- ino, and K. Morita, Carbon 27, 253 (IYXY). M. Inagaki. K. Sakamoto. and Y. Hishiyama. J. Mat.

Res.. 6. I IOH (199 I) S. Isoda. H. Shimada. M. Kochi. and H. Kambe. J.

Polym. Sci. Polym. fkys. 19. 1293 (19X1). T. Ohta. E. Tanaka, andT. Miyashita. Mit~uhki Kaser

R & D Review 2. 90 (198X). Y. Hishiyama. Y. Kahuragi. and M. Inagaki. C/W~. Pkys. Curhort (Edited by P. Thrower) Vol. 23. p. I- 68. Marcel Dekker. New York (1990). Y. Hishiyama and M. Inagaki. J. Mot. Re.s. submitted for publication.