Adjusting the Oxygen Content of the Cured Polyaluminocarbosilane Fibers†

doi:10.7503/cjcu20150014

Abstract

Abstract:

The oxygen content of the SiAlCO fibers was a key for the transformation from SiAlCO fibers to dense stoichiometric SiC fibers after sintering at >1700 ℃, which was mainly introduced during curing process. The novel preoxidation-thermal curing method was applied to the conversion of polyaluminocarbosilane(PACS) fibers and the oxygen content of the SiAlCO fibers as-pyrolyzed was adjustable in 10%—13%(mass fraction). A minimal degree of preoxidation of the PACS green fibers, which performed at 190 ℃ for 4 h leading to 7.87%(mass fraction) oxygen pick-up, is necessary for their further thermal-curing. Then, the preoxidation fibers formed crosslinking structure after thermal-curing at 450 ℃ under N₂ flow. The curing process was investigated by gel permeation chromatography(GPC), infrared spectrum(IR) and thermogravi-metric analysis-mass spectra(TG-MS). The results show that the Si—OH bonds are formed after the Si—H bonds in the structure of PACS fibers reacting with oxygen molecular during preoxidation. Additionally, partial Si—OH bonds could react with each other continuously to form Si—O—Si structure, resulting in molecular increase. During the thermal-curing process, the residue of Si—OH further react with each other at lower than 300 ℃. The formation of Si—CH₂—Si structure resulted from the reaction between Si—H bond and Si—CH₃ bond happened in the range of 300—450 ℃.

Key words: Polyaluminocarbosilane fiber, Thermal curing, Oxygen content, SiC fiber

CLC Number:

O631

TrendMD:

YUAN Qin, SONG Yongcai, WANG Guodong. Adjusting the Oxygen Content of the Cured Polyaluminocarbosilane Fibers^†[J]. Chem. J. Chinese Universities, 2015, 36(6): 1213.

Figures/Tables 12

Table 1 Properties of L-PCS, PACS and PCS

Sample	Soften temperature/℃	$M ˉ n$	I_SiH/ $I SiC H 3$	Mass fraction(%)
Sample	Soften temperature/℃	$M ˉ n$	I_SiH/ $I SiC H 3$	Si	C	O	Al
L-PCS	145—156	1275	0.97	45.28	38.62	0.43
PACS	201—215	2923	0.68	43.60	37.68	3.23	0.67
PCS^[22]	215—225	1979	0.92	47.00	39.85	0.60

Table 1 Properties of L-PCS, PACS and PCS

Sample	Soften temperature/℃	$M ˉ n$	I_SiH/ $I SiC H 3$	Mass fraction(%)
Sample	Soften temperature/℃	$M ˉ n$	I_SiH/ $I SiC H 3$	Si	C	O	Al
L-PCS	145—156	1275	0.97	45.28	38.62	0.43
PACS	201—215	2923	0.68	43.60	37.68	3.23	0.67
PCS^[22]	215—225	1979	0.92	47.00	39.85	0.60

Fig.1 Change in ISiH/ISiCH3(a, b), gel content(a', b')(A) and oxygen content(B) of PCS(a, a') and PACS(b, b') fibers during oxidation curing process

Fig.2 Change in ISiH/ISiCH3 and oxygen content of preoxidation PACS fibers vs. curing time

Fig.3 Relationship between Mˉn and oxygen content of preoxidation PACS fibers

Fig.4 GPC curves of preoxidation PACS fibers in different conditions

Fig.5 Change in gel content of preoxidation PACS fibers vs. thermal-curing temperature

Fig.6 IR spectra of PACS fibers in different state PACS: green fibers; A-190-4: preoxidation fibers;T-190-4: thermal cured fibers.

Fig.7 Change in Ix/ICH and gel content of A-190-4 vs. thermal-curing temperature

Fig.8 TG(A) and MS(B) curves of A-190-4 heated at 10 ℃/min from room temperature to 500 ℃ in N2

Fig.9 TG curves of PACS fibersPACS: green fibers; A-190-4: preoxidation fibers; T-190-4: thermal cured fibers.

Table 2 Oxygen content after pyrolysis of different cured PACS fibers

Sample	T-190-4	T-190-5	T-200-3	T-200-4	T-200-5	T-240-2
Oxygen content after pyrolysis(%)	10.37	11.26	10.80	11.22	12.73	15.2

Fig.10 SEM images of SiAlCO fibers derived from T-190-4 surface(A) and cross-section(B)

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