Boron nitride precursors

In covalent azides, the pseudohalogen azide RN3 has an angular structure as in HN3. Triazidoborazine [H3N3B3(N3)3] and other boron azides, for example, salts of the tetraazidoborate ion B(N3 (4 have been considered as boron nitride precursors (see Boron-Nitrogen Compounds). The M(Ns)3 azido complexes of the other group 13 elements (Al, Ga, In, Tl) and their M(N3)4 tetraazido anions are all known. They and their derivatives are also used as precursors for the nitrides. 

It should be said that many boron nitride precursors also yield boron carbide if calcined in an inert gas atmosphere. 

One attractive molecular species upon which polymeric boron nitride precursors could be based is the ring-compound borazine, BaNsHe The compound is easily made and has both the correct boron/nitrogen ratio and a hexagonal structure. Indeed, Paine and Paciorek have each demonstrated the use of linked polyborazine gels or polymers as precursors for boron nitride. 

Poly(B-vinylborazine) shows excellent conversions to boron nitride, but because of it.s carbon backbone, it requires pyrolysis under an ammonia atmosphere in order to achieve low-carbon boron nitride. In an effort to develop new boron nitride precursors that give improved ceramic yields and/or do not require the use of ammonia during the ceramic conversion step, we initiated investigations of the syntheses of alternative types of borazinc based polymers. 

In addition to being of use as a boron nitride precursor, polyborazylene has proven to serve as a useful reagent for the formation of new composite metal-nitride/metal-boride materials that have improved properties over the individual pure phase metal boride or nitride. For example, titanium-boride/titanium-nitride composites have been made (eq 4) by dispersing titanium powder into polyborazylene... 

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. 

There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. 

Potential Non-Cvcllc Precursors of Preceramic Polymers. Boranes such as bis(trimethylsilyl(aminotrimethylsilylaminochloroboranes can be viewed as monomers for preceramic polymer and, ultimately, boron nitride production. Intermolecular dehydrohalogenation of this borane would be thus expected to yield either the dimer or the polymeric system. 

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... 

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. 

---