Borazine precursors

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

The preparation of high surface area boron nitride powders and foams from various molecular precursors has been studied. The highest specific areas were obtained using the poly-borazylenic precursor. However the difference with the measured surface area of the samples prepared from the polyhaloborazinic precursor was specially related to an enhanced micro-porosity of the samples prepared from the borazinic precursor. Curiously, the samples obtained from aminoborazinic precursors always exhibited a smaller specific surface area. This has been related to the ability of this precursor to melt and stay under a waxy form at... 

The identities of all molecular precursors presented in Table 2 have been confirmed by standard analytical techniques such as mass spectrometry and NMR. Some of the precursors having melting points above ambient temperature have been successfully crystallized, and their crystal structures have been determined by X-ray diffraction. This has been achieved for some of the species from Table 2 dimethylchlorosilyl-amino-dichloroborane (DADB) (Eq. 1) [22], (trimethylsilylamino)dichloroborane [23], the two borazine precursors TSMB and DSMB (Eq. 9) [41], the cyclodisilazane in Eq. (3) [25], and the dimethylsulfide adduct of DSDE [42]. The molecular structures are given in Fig. 3. 

A boron nitride fiber derived from a borazine precursor fiber has recently been reported [32]. As with boron nitride fibers from B2O3, the final heat treatment of boron nitride fibers from borazine must be carried out in a reactive atmosphere (also ammonia) to chemically complete the conversion of the precursor fiber. This nitridation step renders the reaction sequence a chemical vapor infiltration process. 

The conversion of borazine precursors to a-BN proceeds under smoother conditions than the reduction conversion processes originating from boron-oxygen species. The borazine derivatives are prepared by known procedures (for example, from Na[BH4] and [NH4]Cl BCI3 and [NH4]CI [46, 47]), and the B-Cl borazines formed may be cross-linked, for example, by reaction with [(CH3)3Si]2NH [48, 49] to give oligomeric gel precursors for a-BN. Resultant a-BN... 

X-Ray fluorescence is widely used in the analysis of modern refractories, including boron nitride, together with standard analytical methods [48]. Radiation-induced products in a-BN can be studied with scanning transmission electron microscopy (STEM) [49]. Transmission electron microscopy using bright field and dark field imaging and selected area electron diffraction was used to study a-BN ceramics formed by pyrolysis of organyl-substituted amino-borazine precursors. These ceramics are mainly amorphous with pockets of microfibrils or microcrystallites [50]. 

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

Recently we have also extended these low temperature polycondensation synthetic techniques to the preparation of borazine containing polyureas as precursors for BN/G C ceramics.- ... 

C. Polymeric Precursors Derived from tris(Borylamino)borazines 112... 

Poly[/i-(alkylamino)borazines] can be prepared through a multistep thermolysis process involving tris(monoalkylamino)boranes of the type B(NHR)3 (R = Me, Pr ). Heating these molecular precursors under reduced pressure or in an inert atmosphere... 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

By analogy with B-trialkylaminoborazi ne and polyborazine derived therefrom, the first route envisioned to poly(borylaminoborazine) was the thermal condensation of molecular precursors under a convenient atmosphere. As detailed earlier the innovative idea behind this procedure is to tailor the polymeric precursor structure by increasing the distance between the two borazinic rings. For that purpose, we explored... 

From published photoelectron data for these compounds we can infer that cations formed under these conditions will carry some excess internal energy. The photoelectron spectrum of borazine indicates no significant change in the structure of the cation relative to the precursor although a slight Jahn-Teller distortion is possible. The alternate cation structure... 

Renewed interest in borazine derivatives has resulted from their possible application as precursors to boron nitride ceramics. For example, the inorganic analogue of styrene, (H->C=CH)B3NjH5i has been polymerized and decomposed to produce BN.72... 

Borazines have the correct B-to-N ratio for the production of this ceramic and it% polymeric precursor may be used to deposit a uniform surface coaling. 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

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