Borazine oligomers

Borazine oligomers are formed by means of condensation reactions of the 6-membered ring compound borazine, B N H. These reactions occur very readily upon heating in a sealed vessel at temperatures between 60 and SO C [21]. Both molecular weight and viscosity increase as a function of polymerization time, allowing the flexibility of tailoring the melt viscosity to meet the needs of the application. 

Figure 2.2 Mass spectrum of borazine oligomer with average of 5000.

X-ray diffraction (XRD) of a borazine oligomer melt at 25 is shown in Figure 2.5. This specimen was formed by polymerization at VO C and then cooled to 5 C for 2 weeks. At 25 ""C, the oligomer shows a characteristic (002) peak at 3.72 A. A reversible transition to a more isotropic phase occurs upon heating to 70" C. Following heating to 100°C, this transition becomes irreversible and the peak is broadened. Presumably disorder becomes permanent in the isotropic melt as additional polymerization proceeds at the elevated temperatures. 

It is noteworthy that the borazine oligomer does not show a transition enthalpy in differential scanning calorimetry (DSC) measurements. This is also comparable to the carbonaceous mesophase in which a nematic to isotropic transition enthalpy has not been observed [24]. In both the... 

Both composition and physical treatments affect the degree of crystallinity in the borazine oligomer melt. For example, the propensity to exhibit anisotropic behavior is enhanced by cooling the oligomer at 5" C for extended periods of time [17]. This allows segregation and molecular orientation at very slow polymerization rates. 

In contrast when the gaseous products are not continuously removed and internal pressure continues to increase, the reaction proceeds more slowly (Figure 2.7b). Mass spectrometry of the product indicates that the structural evolution of the borazine oligomer is not affected by these changes. 

Figure 2.7 Average of borazine oligomer as a function of polymerization time for (a) pressure slightly above I atmosphere and (b) without degassing.

Boron nitride is formed when the borazine oligomer is heated above 1000 C. This is a particularly novel processing route because it of ers... 

One unique characteristic of the borazine oligomer is that it has a very high mass yield observed upon conversion to BN. Figure 2.11 shows a thermogravimetric analysis (TGA) plot of the borazine oligomer. This data reveals that the mass yield upon conversion to BN is approximately 85% which is very high for a ceramic precursor [27]. Additional examination has revealed evidence for viscoplastic flow in the crosslinked oligomer under the application of pressure. This may increase the effective BN mass yield even further for a bulk structure such that the volume yield is between 60 and 70%. 

To illustrate this point. Figures 2.12 and 2.13 compare the crystallinity as a function of processing temperature for two different specimens [25]. The first specimen (Figure 2.12), consisted of the borazine oligomer from the melt to which no external pressure has been applied. A turbostratic BN phase with an intermediate d(002) value of 3.41 A is formed only upon heating to 1200°C. Specimens like that shown in Figure 2.6c were also investigated. In these specimens, pressure was... 

Boron nitride derived from the borazine oligomer has been successfully used to replace the carbonaceous matrix in carbon fiber/carbon matrix composites (C/C) [15,18]. The carbon fiber/boron nitride matrix composites (C/BN) exhibit improved oxidation resistance over the C/C. Excellent mechanical strength and toughness have also been observed in the C/BN. As shown in Figure 2.16, both the flexural strength and modulus of pitch carbon fiber/BN specimens actually increase with extended heat treatment [18]. 

Transmission electron microscopy (TEM) of ceramic fiber/BN matrix composite has verified the preferential orientation of BN parallel to the interface (Figure 2.17). In this figure, the diffraction pattern in the BN matrix reveals a very well aligned crystalline structure. This evidence suggests that the liquid crystallinity of the borazine oligomer provides... 

In conclusion, the liquid crystalline character which has been observed in the inorganic borazine oligomers provides a mechanism for the development of an oriented structure in BN. It is now understood that the interlayer spacing must be close to that of theoretical BN to produce a stable structure. The borazine oligomer offers a convenient... 

---