[acs symposium series] inorganic and organometallic polymers ii volume 572 (advanced materials and...

Chapter 28

Synthesis and Preceramic Applications of Poly(aminoborazinyls)

Yoshiharu Kimura and Yoshiteru Kubo

Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan

Soluble and thermoplastic derivatives of poly(aminoborazinyls) were used as ceramic precursors for fabricating hexagonal boron nitride (h-BN) ceramics. They were readily prepared by solution condensation of B,B,B-tri(methylamino)-borazine (MAB) and bulk co-condensation of MAB and laurylamime (LA). The structure analysis revealed that both the solution and bulk condensates consist of several borazinyl rings which have two- and three-point linkages with other rings as well as a one point-linkage. The bulk co-condensate of MAB-LA was melt-spun and pyrolyzed to fabricate BN fiber, while the solution condensate was used as a ceramic binder in the powder consolidation molding of h-BN and as a matrix material in the fabrication of fiber-ceramic composites (FCC). These examples demonstrated that these BN precursors are useful for developing well-designed super heat-resistant materials that may be essential for future technological innovation.

Among the newly developed high-tech ceramics, hexagonal boron nitride (h-BN) has been occupying an unique position (1), because of its excellent heat resistance, low density, and structural similarities with graphite. In the utilization of h-BN, however, there have been intrinsic problems related with its processability and mechanical properties. Since the processing of h-BN cannot usually be performed by conventional thermal sintering, a high-energy process like hot-pressing has been utilized. Therefore, fabrication of fibers and molding with complicated, fine shape are difficult. Furthermore, mono-lithic h-BN is likely to crack from a surface micro flaw, and reinforcement with ceramic fiber is preferable (2). For these purposes the so-called precursor method using an appropriate inorganic polymer as the ceramic precursor should be effective (5), since die polymeric precursor should be easily processed to a desired shape and then pyrolyzed into the desired ceramics by the ordinary thermal sintering process without using sinter-aiding agent (4, 5). In this method the structure and properties of the precursor have a close relationship with the performance of the ceramics obtained, and poly(aminoborazinyls) consisting of the unit components of h-BN have received much attention for use as the h-BN precursor (Z). In particular, Paciorek (6), Paine (7), and Sneddon (8) designed various derivatives of poly(aminoborazinyls) which can be readily converted to h-BN. Recently, we demonstrated that poly(aminoborazinyls) prepared from Β,Β,Β-

0097-6156/94/0572-0375508.00/0 © 1994 American Chemical Society

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In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

376 INORGANIC AND ORGANOMETALLIC POLYMERS II

triaminoborazine (TAB) and B,B,B-tri(methylamino)borazine (MAB) are potentially useful for the fabrication of BN fibers and molds (9-11). The further applications of this technique, bench-scale preparation of BN fiber, powder consolidation molding of BN ceramics, and fabrication of fiber-ceramic composite (FCC) have been studied in detail. This paper deals with the concepts of these methods as well as a preliminary feasibility study utilizing poly[(methylamino)borazinyl], i.e., the MAB condensate, as the precursor for making BN ceramics.

Synthesis of MAB

MAB was prepared in high yield by the following reaction scheme (12). It was isolated as white waxy material.

CI NHCH3 I I 3

BCI3 n o . c

H - N - B ^ N - H r.t. Η χ Ν χ Β ^ Ν ' Η

+ • ι 1 • I I NH4CI " H C 1 c i ^ N ^ C I Œ 3 N H 2 C H a N H ^ N ^ N H C H a

I I H H

MAB: IR (nujol) 3460 (NH), 1410 cm'1 (B3N3 ring), etc.; *HNMR (C6U6) δ

1.4 ppm (broad, 3H, M6NH), 2.4 (d, 9H, CH3), 2.6 (s , 3H, Ni/), n B NMR (QD6) δ 26 ppm (the line width at half height = 284 Hz).

Solution polymerization of MAB

MAB polymerizes in the bulk state when heated above 130°C to form a thermoplastic condensate. However, its thermoplasticity is likely to be lost by crosslinking during the bulk condensation (10). In order to control the degree of condensation, the thermal condensation of MAB was carried out in refluxing chlorobenzene solution. With extended reflux an insoluble crosslinked product precipitated. Therefore, the reaction was stopped upon observation of the first trace of the precipitate, and the MAB condensate was isolated from this solution as white solid material.

Table I shows a typical result of the solution condensation of MAB as compared with that of the bulk condensation. The yields of the condensate were almost identical, but the solution condensate was wholly soluble in organic solvents, while the bulk condensate contained some insoluble gel. The average molecular weight of the former was slightly lower than that of the soluble part of the bulk condensate. The softening point of the solution condensate was about 80° C, which was similar to that of the co-condensate of MAB and laurylamine (LA) (90/10 in wt; vide infra).

Table I, Thermal condensation of MAB Solvent Temp.

f Q Time (h)

Condensate (wt%)

Mn

chlorobenzene 130 150

2 2

85 87

500 720

The X H NMR spectrum of the condensate is shown in Figure 1(b). As compared with the spectrum of MAB (Figure 1(a)), the signals at δ 1.4 and 2.4 ppm were reasonably ascribed to the NH and methyl protons of the substituent

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28. KIMURA & KUBO Synthesis of Poly(aminohorazinyls) 377

Figure 1. X H NMR spectra of (a) MAB, (b) the solution condensate of MAB, and (c) the bulk co-condensate of MAB-LA (at 200 MHz, in QDo).

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methylamino groups, respectively. In contrast, the NH protons of the borazine ring appeared as three sets of signals around δ 2.6, 3.5, and 4.3 ppm. While the highest-field one is ascribed to the NH protons of the borazine ring that is bonded with only one borazinylamino group (one-point-linkage form). The middle- and the lowest-field signals are reasonably ascribed to those of the borazine rings that are bonded with two and three borazinylamino groups (two- and three-point-linkage forms), respectively. The splittings of the latter signals may be due to the asymmetry of the borazinyl substituents. With this signal assignment combined with the signal intensities and the molecular weight, the structure of the condensate could be represented by 1, for example, whose average degree of polymerization was five. These facts supported that the homo condensation of MAB is easily controlled in solution state.

Η CH3NHx Ν xNHCH 3

Β Β I I

HN^ ^NH CH Β

3 ^ N ^ NHCH3 NHCH3

Β Β Β H N ' X N H H N ' X N H H N ' ^NH

I I I I I I

Ο Η 3 Ν " · Β χ Ν ' Β ^ Ν ' Β ν Ν Η " Β ^ Ν ' Β ^ Ν ' Β % Ν Η Ο Η 3

B H CH 3 CH 3 Η HN" V NH

, B \ , B X

CH3NH Ν NHCH3

Η 1

Bulk Co-Condensation of MAB and Laurylamine

Another effective method for preparing thermoplastic poly(aminoborazinyl) is to make a co-condensate of MAB with such a higher alkylamine as LA. We have shown that the latter can be trapped in the condensate to decrease the crosslinks and work as plasticizer (9).

In the present study, a larger scale synthesis of the co-condensate was studied by heating about 20 g of MAB and 10 wt% of LA at 200-260°C for 1 h in a nitrogen flow. Since in this larger scale reaction the elimination of methylamine was slowed, a relatively higher reaction temperature was necessary to effect the condensation.

Table Π. Thermal Co-Condensation8 of MAB and LA Run No. LA

(wt%) Temp. rc)

Time (h)

Weight Loss (%)

Mn D

(dalton) Spinnability

1 10 200 1 10.1 _ good 2 10 220 i 13.4 good 3 10 240 1 17.1 470 excellent 4 10 260 1 18.6 - poor

a20 g of MAB and 2.0 g of LA were reacted. b By GPC relative to PPG standards.

As summarized in Table Π, the degree of condensation in terms of weight loss wassuccessfully controlled by changing the reaction temperature, and colorless thermoplastic products were formed when the weight loss due to the elimination of

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28. KIMURA & KUBO Synthesis ofPoly(aminoborazinyls) 3 7 9

MA was within 10-20 wt%. Mn of the soluble condensates were in the range of 400-800 as determined by GPC, which corresponds to that of pentamers - decamers of MAB. The typical Ή NMR spectrum of the condensate is shown in Figure 1(c). The characteristic signals due to the methyl and methylene protons of lauryl group were noted at δ 0.9,1.28, and 2.2 ppm, respectively. The methyl signal of NHCH3 group at δ 2.4 ppm became broader, and the two signals at δ 1.3 and 1.8 ppm may be ascribed to the pendant NH of the methylamino and laurylamino groups, respectively. The signals of the NH protons of the borazine ring were detected at the region where those of the borazine rings with one- and two-point linkages should appear. Therefore, it appears that the number of three-point-linkage was decreased. The U B NMR signal of this product was detected as a single peak at the same chemical shift (δ 26 ppm) as observed for MAB. Based on these spectroscopic data, the structure of the MAB-LA condensate is postulated as 2, in which LA units are incorporated as alkylimino and alkylamino units to interfere with the formation of crosslinks in the product.

R H N . ,NHCH 3

Β Β i I

HN^ ^NH R ^ N ^ B NHCH3 NHCH3

Β Β Β HN^ X N H H N ' X N H HN^ ^NH

I I I I I I Β Β Β B R

RHN' - N " -NH - N ' N N ' % H R Η 6H3 A Η

2 (R=C12H25)

The DTA of these condensates exhibited a broad endothermic peak around 85°C and an exothermic peak ranging from 200 to 430°C which are ascribed to their melting and thermal decomposition, respectively. A large weight loss was observed at 200-500 °C in TGA, and the final weight loss at 900°C was 38 %.

Fabrication of BN Fiber from Bulk Co-Condensate of MAB-LA

As discussed in the Introduction, the poly (aminoborazinyls) can be pyrolyzed into h-BN ceramics and have various potential applications for use as preceramics. In this article, some examples based on the different processabilities of the above MAB condensates are shown.

In the early 1970s, Economy et al invented a chemical conversion method of fabricating BN fiber, in which boron oxide fiber was melt-spun and converted to BN by solid phase nitridation in an ammonia atmosphere (13). In this process, however, quality control of the precursor fiber and the process control of the nitridation were not easy. Therefore, an alternative preceramic process was developed (14, 15), although the spinnability of the polymer precursors utilized were too poor to fabricate precursor fibers with excellent quality. Recently, the author reported a new method of fabricating high-performance h-BN fiber based on a thermoplastic derivative of poly[(methylamino)borazinyl] as the precursor polymer (9, 11). In this process the precursor fiber is spun by melt-spinning techniques, pyrolyzed directly up to 1000eC in an ammonia gas flow, and further sintered up to 1800°C in a nitrogen flow. The strength of the sintered BN fiber was found to reach 1 GPa. In the present study, the bench-scale preparation of BN fiber was investigated by a similar approach using the co-condensate of MAB-LA prepared above.

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Melt spinning. Using a micro screw extruder with a nozzle of 500 mm in diameter, the condensate was melt-spun at 160-170°C. The extrudate from the spinneret was taken up by a winder to give a continuous filament of about 50 mm in diameter. The best melt-flow of the extrudate was obtained for the condensate obtained at 240°C (Run No.3 in Table I). The good spinnability of the MAB-LA condensate in spite of its low molecular weight is attributed to the strong intermolecular aggregation between the planar borazine rings. This phenomenon is comparable to that of amorphous carbon pitch (16), which is utilized as the precursor to carbon fiber and has an excellent spinnability. The latter consists of aromatic rings in place of borazine networks with Mn of 800 - 1000. A typical SEM micrograph of the precursor fiber (see Figure 2) indicates that the cross-section of the fiber is circular and that the inner phase is homogeneous.

Pyrolysis of fiber. During the spinning, the fiber surface was slightly hydrolyzed, and the fiber became infusible. Therefore, it was directly pyrolyzed in an electric tube furnace without special thermosetting. The temperature of the furnace was raised to 600-1000°C at a rate of 100°C/h in a flow of ammonia gas (30 ml/min), held at the predetermined temperature for 1 h, and then cooled to room temperature. At this stage the sintered fiber was gray due to the carbonization of the organic residue. It was further sintered up to 1400 and 1800eC in a nitrogen flow. Then, its color turned white above 1400°C. Figure 2 shows the typical SEM photographs of the fibers pyrolyzed at various temperatures. The inner phase of the fibers pyrolyzed at 1000 and 1400eC was homogeneous, while that of the fibers sintered above 1600eC exhibited a slight grain growth.

Mechanical properties. We reported in the former paper (9) that the modulus of the BN fiber prepared by the precursor method increased with increasing pyrolysis temperature, while its tensile strength decreased around 1400°C and increased again at the higher temperature. The average tensile strength and modulus observed were 1 GPa and 78 GPa, respectively, which are lower than those of the common ceramics fibers reported thus far (3). Most of the latter fibers, however, are known to undergo serious deterioration above 1400eC because of the abrupt crystallization and coarse grain growth (17). The present BN fiber is characterized by the exceptional increase in strength at high temperature greater than 1400eC.

Table ΙΠ. Comparison of the properties of the BN fibers prepared by precursor and chemical conversion methods

Precursor Method Chemical Conversion Method3

Density Diameter Tensile strength Tensile modulus

2.05 g/cm3

10 mm 0.98 GPa 78 GPa

1.85-2.10 g/cm3

4-6 mm 0.3-1.3 GPa 35-67 GPa

Heat resistance

Thermal conductivity Electroresistivity

-25ÔÔ°C (in nitrogen)

900°C (in air) 0.0665cal/sec/cm2/0eC/cm (500°Q

ΙΟ 1 3 Ω-απ aSee ref. (33).

Table ΠΙ summarizes the properties of the present BN fiber as compared with those of the BN fiber prepared by the chemical conversion method (13). The mechanical properties of the both fibers are almost identical. We expect that further

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28. KIMURA & KUBO Synthesis of Poly(aminoborazinyls) 381

Figure 2. SEM micrographs of the precursor fiber and the fibers pyrolyzed at the temperatures noted.

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increase in strength would be realized by quality control of the precursor fiber, as well as by control of the coarse grain growth in the sintered fiber.

Fabrication of BN Disk with Solution Condensate as Ceramic Binder

For sintering BN powder which has no sintering ability, a high-temperature, high-pressure process of hot-pressing has generally been adopted in the presence of a sinter-aiding agent such as boron trioxide. When the polymer precursor is used as ceramic binder, powder consolidation may be allowed by the conventional sintering technique without loading pressure (18, i9). Here, the solution condensate of MAB was used as the ceramic binder in powder molding of BN ceramics, because it is soluble and contains no higher alkylamine which ought to create large pores and carbon contaminant in the pyrolysate.

A typical experimental procedure is shown in Scheme I. The MAB condensate obtained was dissolved in a least amount of chlorobenzene, and the solution was mixed with h-BN powder in a condensate to powder ratio of 25/75 by weight. After the mixture had been well agitated, chlorobenzene was evaporated. The powder mixture obtained was then compressed to a circular disk of 10.15 mm in diameter and 1.35 mm in thickness. It was first pyrolyzed up to 1000°C in an ammonia flow and was subsequently sintered up to 1800°C in a nitrogen flow. In each step, a hard, well-shaped disk was obtained, indicating the effectiveness of the MAB condensate as the binder. The size, weight, and bulk density of the disk obtained are summarized in Table IV. The disk was found to shrink slightly in diameter with constant thickness when it was sintered up to 600°C. Above 600eC, however, further shrinkage was not observed in spite of the decrease in weight. Therefore, the density of the disk decreased with increasing porosity, with the disk sintered up to 1800°C having a density of 1.58 g/cm3. Figure 3 shows the morphology change of the disk with sintering temperature, as observed by optical microscopy and SEM. SEM revealed that the BN particles adhered well with increasing temperature. In the disk sintered up to 1800°C the formation of flake like particles was observed. These data indicate that the solution condensate of MAB is an effective ceramic binder.

Table IV. Sintering of BN powder with MAB condensate as the binder3

Sintering Diameter Thickness Weight Bulk Density Temp, f C) (mm) (mm) (mg) (g/cm3)

- 10.15 1.35 195 1.79 600 10.00 1.35 181 1.71

1000 10.00 1.35 175 1.65 1400 10.00 1.35 173 1.63 1800 10.00 1.35 167 1.58

a A circular disk comprising 75 wt% of BN powder and 25 wt% of the MAB condensate was made by compression molding.

The disk sintered up to 1800eC was then subjected to an impregnation process to decrease porosity as illustrated in Scheme Π. The disk was first soaked in a 50 wt% solution of the MAB condensate in chlorobenzene. The whole system was evacuated so that the solution could penetrate small pores inside the disk. The disk was then taken out, thoroughly dried in vacuo, and pyrolyzed up to 1000eC in an ammonia flow. This cycle was repeated several times, and the disk was finally sintered up to 1800°C in a nitrogen flow. Figure 4 shows the increase in bulk density with the number of impregnation cycles. It suggests that the increase in density leveled off beyond the fourth impregnation cycle where the bulk density reached 1.74 g/cm3.

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BN powder MAB condensate

BN powder coated with MAB condensate

Cold presj) 20 MPa for 10 min

Scheme 1. Fabrication process of BN disk by powder consolidation.

BN disk sintered up to 1800eC

Impregnation of MAB condensate I (50 wt% chlorobenzene solution) J

( Drying and curing of the disk ) (QJ-TMS'mNH^ (4 times)

Green body

' Pyrolysis up to 1000'C) inNH^ J

Sintering up to 1800eC I inN2

BNbody

Scheme 2. Impregnation process.

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Figure 3. Optical and SEM micrographs of the BN disks pyrolyzed at the temperatures noted.

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28. KIMURA & KUBO Synthesis of Poly (aminoborazinyls) 385

When this impregnated disk was finally sintered up to 1800°C, the density became 1.72 g/cm3. This value is much higher than that of a commercially available BN mold fabricated by the ordinary powder sintering (1.4 g/cm3), but slightly lower than that of a hot-pressed mold (1.9 g/cm3). Figure 5 shows the SEM of the fracture surfaces of the BN disks before and after the impregnation process. The former has a number of cracks and pores, while in the latter pore sizes and porosity were decreased by impregnation.

Fabrication of Carbon Flber-BN Matrix Composites

Incorporation of continuous fibers into a ceramic matrix yields a composite material with superior properties (20). Here, a BN matrix composite reinforced with carbon cloth was fabricated by using the solution condensate of MAB as the matrix precursor. Scheme ΠΙ shows the procedure of making a carbon fiber-BN matrix composite. A thick film (thickness : 100 mm) of the MAB condensate was cast by hot-pressing at 70°C. Two sheets of this condensate film were piled up with a piece of carbon cloth in the center, covered with a parting paper from both sides, and hot-pressed at 70°C for impregnation. The prepreg thus obtained was cut into a prescribed size (100 χ 100 χ 0.25 mm), and twelve piles of this prepreg were laid up on a flat steel plate with a dam. The whole system was put into a Teflon bag and pressed at 70eC at 1.5 MPa by the conventional vacuum bag technique to form a green body. It was then transferred to an tubular furnace and pyrolyzed up to 1000°C in an ammonia flow. The pyrolysate was subjected to the aforementioned impregnation process, in which the soaking and pyrolysis were repeated several times. The composite obtained was finally sintered up to 1800°C in a nitrogen flow. From the weight of the composite, the final volume fraction of carbon fiber (Vf) was calculated to be 24%.

After the first pyrolysis of the green body up to 1000°C, the density of the composite was 0.74 g/cm3. So, the aforementioned impregnation cycle was repeated in order to increase the density of the composite. The increase in density with the number of impregnation cycle is shown in Figure 6. After the sixth impregnation cycle the density reached a maximum of 1.10 g/cm3. When this composite was finally sintered up to 1800°C in nitrogen flow, the density became 1.05 g/cm3, indicating that it still had a high porosity. Its mechanical properties were evaluated by the ordinary three point-flexural test. The maximum flexural strength and modulus of the composite was found to be 61 MPa and 23 GPa, respectively. These values are much higher than those of the compacted plate of h-BN ceramics (20-30 MPa in strength), although they should be improved by decreasing the porosity.

Conclusions

Soluble and thermoplastic derivatives of poly(aminoborazinyls) were prepared by solution condensation of MAB and bulk co-condensation of MAB and LA. Both of the condensates consist of several borazinyl rings which have two- and three-point linkages with other rings as well as a one point-linkage.

A bench-scale preparation of BN fiber was successfully carried out by the precursor method with the bulk condensate of MAB-LA. The results were as follows: 1. The precursor fiber could be spun by the conventional melt spinning of the MAB-

LA condensate. 2. Because the surface of the fiber was slightly hydrolyzed during the spinning, the

precursor fiber was directly pyrolyzed to 1000eC in an ammonia flow and further sintered up to 1800°C in nitrogen flow.

3. The mechanical properties of the BN fiber was found to increase above 1400°C. 4. Further increase in strength would be realized by quality control of the precursor

fiber, as well as by control of the coarse grain growth in the sintered fiber.

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1.9 Hot-pressed h-BN

Ο - - -

1.4 Thermally s i n t e r e d h-BN

PI P2 P3 P4

Number of impregnation c y c l e

Figure 4. Increase in bulk density of the BN disk with increasing number of impregnation cycle. Each impregnation cycle (shown by P) involves pyrolysis up to 1000°C in ammonia flow. The final sintering up to 1800eC in nitrogen flow is shown by S.

Figure 5. Optical micrographs of the fracture surface of the BN disks (a) before and (b) after the impregnation cycles.

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Film of MAB condensate Hot-press

> Ê£^7 Film stacking Cutting and

stacking (15ply)

•

Hot-press

Green body Pyrolysis in NI% (lOOO'C)

1 1 Y/S/S/S/j

φ 6 times Vacuum bag

technique

I Sintering

I in N2 (1800eC) composite

Impregnation and Pyrolysis in NH3

curing (1000eC)

Scheme 3. Fabrication process of carbon fiber-BN matrix composite.

1.21

PI P2 P3 P4 P5 P6 S

Number of impregnation

Figure 6. Increase in bulk density of the carbon fiber re-inforced BN matrix composite (Vf = 24%) with increasing number of impregnation cycle. See Figure 4 for the notations.

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The solution condensate of MAB was used as ceramic binder in molding BN ceramics from BN powders and as matrix material in fabricating carbon fiber-BN matrix composite. The following results were obtained : 1. With the MAB condensate as ceramic binder, BN powders could be sintered by

ordinary thermal sintering to give a well-shaped BN mold. 2. The bulk density of the mold was higher than that of commercially available BN

mold, but slightly lower than that of hot-pressed mold. 3. The impregnation procedure with MAB condensate was successful for

densification of the mold. 4. The mechanical properties of the composite were superior to those of the h-BN

ceramics, but inferior to the recently developed carbon-fiber reinforced composites such as carbon-carbon composites.

5. h-BN matrix composites may be promising as a high temperature structural material in terms of heat resistance.

Literature Cited

(1) Paine, R. T.; Narula, C. K. Chem. Rev. 1990, 90, 73-91 (2) Prewo, Κ. M. Am. Ceram. Soc. Bull. 1989, 68, 395-400 (3) Rice, R. W. Am. Ceram. Soc. Bull. 1983, 62, 889 (4) Schwab, S. T.; Graef, R. C.; Davidson, D. L.; Pan, Y. Polym. Prepr. (Am.

Chem Soc., Div. Polym. Chem.) 1991, 32 (3), 556-558 (5) Sato, K; Suzuki, N; Hunayama, T; Isoda, T. J. Ceram. Soc. Jap 1992, 100 (4),

444-447 (6) Paciorek, Κ. J. L.; Harris, D. H.; Kratzer, R. H. J. Polym. Sci., Polym. Chem.

Ed., 1986, 24, 173-185 (7) Narula, C. K.; Schaeffer, R.; Datye, A. K.; Borek, T. T.; Rapko, Β. M.; Paine, R.

T. Synthesis of Chem. Mater. 1990, 2 (4), 384-389 (8) Fazen, P. J.; Beck, J. S.; Lynch, A. T.; Remsen, Ε. E.; Sneddon, L. G. Chem.

Mater. 1990, 2 (2), 96-97 (9) Kimura, Y.; Hayashi, N.; Yamane, H.; Kitao, T. Proc. 1st. Japan International

SAMPE Symposium, Nov. 28-Dec. 1 1989, 1, 906 (10) Kimura, Y.; Kubo, Y.; Hayashi, N. J. Inorg. Organometal Polym. 1992, 2, 231-

242 (11) Kimura, Y.; Kubo, Y.; Hayashi, N. Composites Sci. Tech., in print (12) Brown, C. A. ; Laubengayer, A W. J. Chem. Soc. 1955, 77, 3699 (13) Economy, J.; Anderson, R. V. J. Polym. Sci. 1967, C19, 283 (14) Paciorek, K. J. L.; Krone-Schmidt, W.; Harris, D. H.; Kratzer, R. H.; Wynne, Κ.

J. In Inorganic and Organometallic Polymers, ACS Symposium Series 360; Zeldin, M.; Wynne, K. J.; Allcock, H. R. Ed. Am. Chem. Soc., Washington 1988, pp. 392-406,

(15) Lindquist, D. A.; Janik, J. F.; Datye, A. K.; Paine, R. T. Chem. Mater. 1992, 4, 17-19.

(16) Singer, L.S. Carbon 1978, 16, 408 (17) Douglas, J. P.; Kenneth, C. G.; Robert, S. H. J.; Richard, Ε. T. J. Am. Ceram.

Soc. 1989, 72 (2), 284-288 (18) Yajima, S.; Shishido, T.; Okamura, K. Ceram. Bull. 1977, 56, 1060 (19) Schwartz, K. B.; Rowcliffe, D. J. J. Am. Ceram. Soc. 1986, 69 (5), C-106-C-

108 (20) Nishikawa, N. Kagaku Kougyou 1989, 40 (9), 780, 803-809

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