Henri Mongeot

Boron nitride matrices and coatings from boryl borazine molecular precursors

Thermal treatment of 2,4,6-[(NHPr i ) 2 B(NPr i )] 3 B 3 N 3 H 3 in vacuo led to a polyborazine in which the borazine rings are mainly linked by –N–B–N– bridges. Pyrolysis of up to 1400 °C resulted in the formation of high crystallinity hexagonal boron nitride which has been characterised by ESCA, powder X-ray diffraction and FTIR spectroscopy. Precursor has been used to prepare carbon fibres/BN microcomposites by dipping bundles of fibres into a solution of followed by pyrolysis up to 1200 °C. Optical micrographs and SEM analysis have shown that the BN matrices were compact and displayed good adherence properties onto graphite substrates. By contrast, BN coatings obtained from a polymer derived from B(NHPr i ) 3 displayed poor adherence properties. These results are related to the hydrocarbon chain ratio of each preceramic polymer. The effectiveness of the BN protective coatings in preventing oxidation has been measured by isothermal gravimetry in air. As expected, by considering the quality of the coatings, those obtained from showed the most interesting properties.

Conversion of B(NHCH3)3 into boron nitride and polyborazine fibres and tubular BN structures derived therefrom

The reaction of ammonia with tris(methylamino)borane B(NHCH 3 ) 3 1 or with a B(NHCH 3 ) 3 derived polyborazine 2 led to completely different processes. Compound 1 reacted spontaneously with NH 3 to give mainly the borazine (CH 3 NH) 3 B 3 N 3 H 3 and subsequently a BN prepolymer which was not processible. A polymer 2 obtained by heating 1 under argon could be spun into crude polyborazinic fibres. When 2 was heated under ammonia in order to obtain carbon free BN, it was partially dissociated into 1,3,5-trimethyl-2,4,6-tri(amino)borazine (NH 2 ) 3 B 3 N 3 (CH 3 ) 3 3. The borazine 3 was fully characterised by multinuclear NMR spectroscopy, FTIR and mass spectrometry. The structure of 3 has been confirmed by a single crystal X-ray diffraction analysis. An illustration of the formation of 3 was the formation of BN tubes from crude fibres. These results were explained by the TGA curves of the polymer 2 under ammonia or nitrogen.

Replacement of the nitrogen of [1-N2B10H9]− by amines or nitriles, a route to hydrophobic monoanions

Abstract Starting from closo-[B 10 H 10 ] 2− hydrophobic monoanions [R 1 R 2 R 3 N–B 10 H 9 ] − {R=H, C 6 H 5 CH 2 , C 6 H 5 , CH 3 , C 18 H 37 (CH 3 ) 2 } could be obtained by a multistep process in which the displacement of nitrogen from [1-N 2 B 10 H 9 ] − by amines was the key step. Attempts at direct synthesis employing bulky tertiary amines were unsuccessful: no reaction occurred at 120°C and at 150°C [1-N 2 B 10 H 9 ] − decomposed to [B 20 H 18 ] 2− . Pd{P(C 6 H 5 ) 3 } 2 Cl 2 used as a catalyst produced a favourable effect, but the [R 1 R 2 R 3 N–B 10 H 9 ] − ions were present in too low concentration to be isolated from the reaction mixtures. A more suitable route to monoanions carrying three bulky organic groups attached to the amino nitrogen consisted in preparing amino derivatives from the appropriate primary or secondary amines and reacting these intermediate products with alkylhalides in alkaline aqueous propanol solution. The displacement of N 2 by nitriles produced [1-RCNB 10 H 9 ] − monoanions {R=CH 3 , (C 6 H 5 ) 2 CH} which proved to be thermally stable, but were easily hydrolysed to [1-RCONH 2 B 10 H 9 ] − monoanions.

CONVERSION OF TRIS(ISOPROPYLAMINO)BORANE TO POLYBORAZINES. THERMAL DEGRADATION TO BORON NITRIDE

Abstract Thermolysis of tris(isopropylamino)borane (1) and its reaction with ammonia were investigated. Compound 1 exhibited good thermal stability resulting in almost complete evaporation during its thermogravimetric analysis under Ar up to 1000°C. Refluxing of 1 for 42 h at 250°C produced its conversion into the expected B,B′,B′-tris(isopropylamino)-N,N′,N″-triisopropylborazine (2), and into the new intermediate [(NHPri)2B]2NPri (3), both found in low concentrations. Reaction of 1 with ammonia at room temperature resulted in spontaneous transamination, cyclization and polycondensation reactions leading first to B,B′,B″-tris(isopropylamino)borazine (4), and subsequently to polyborazines. In spite of the high carbon content of 1, its pyrolysis at 1000°C under NH3 led to the formation of boron nitride containing less than 1% carbon.

Reaction of B11H14NR4 (REt, Bu) with M2O3 (MAs, Sb, Bi) in biphasic systems

Abstract The reaction of B 11 H 14 NR 4 (REt, Bu) with M 2 O 3 (MAs, Sb Bi) in biphasic systems H 2 O/CH 2 Cl 2 or NaOH/CH 2 Cl 2 , with C 6 H 5 NEt 3 Cl as phase transfer catalyst, was investigated as a new route to the already described icosahedral B 11 H 11 MNR 4 (MAs, Sb, Bi; REt, Bu). When the above reaction was performed in basic medium, besides the expected heteroboranes, a new anionic species could be isolated and characterized by its 1D 11 B, 1 H{ 11 B} and 2D 11 B 11 B NMR spectra. Consistently with these data the formula B 11 H 13 OB 11 H 13 2− ,2NEt 4 + is proposed, the anionic part consisting of two icosahedral fragments B 11 H 13 (eleven normal BH bonds and two bridge hydrogens in the open face) linked via a BOB bond; on both sides, the oxygen atom is attached, in endo position, to the unbridged boron atom of the open face. The same compound could be prepared by reaction of B 11 H 14 − with PO 4 HNa 2 , in NaOH/CH 2 Cl 2 mixtures. A possible path involving a mild oxidation of B 11 H 13 2− by M 2 O 3 (MAs, Sb, Bi) is suggested.

Si3N4BN composites obtained from aminoboranes as BN precursors and sintering aids

Abstract The Si 3 N 4 -BN composite ceramic has been elaborated by combining Si 3 N 4 fine powder with boron nitride provided by the thermolysis of a molecular precursor. The properties of this new type of sample has been compared with those of composite ceramics obtained by the classical hot-pressing method using Si 3 N 4 -BN powders and sintering aids (Y 2 O 3 , Al 2 O 3 ). Giving the best ceramic yield, tris (methylamino) borane (TMB) has been used as BN precursor and sintering aid. Boron nitride formed from TMB thermolysis was poorly crystallized when the hot-pressing was run up to a temperature lower than 1800 °C. The density of the composite ceramic samples obtained from molecular precursors was higher than expected, this could be related to the sintering activation properties of the precursor. Moreover the problems due to the BN platelets orientations were removed. The Vickers hardness was clearly improved for samples without BN platelets, however the bending strength was not increased. The precursor improved some properties of the composite and could be considered as a sintering activator avoiding oxide addition as sintering aid.

Study of the interaction of AlCl3 with the B10H102 − cage in the solid state

Abstract AlCl3 reacts with the hydroborates M2B10H10 (M = Et4N, Bu4N, K or Na) in the solid state, the reaction going to completion more rapidly with the tetraalkylammonium salts. The products obtained are dissociated by THF by complexation and dissolution of the AlCl3. The reaction perturbs the BH stretching vibrations and produces an increase of more than 100 cm−1 in these frequencies. According to the AlCl3 to B10H102− ratio, the IR spectra in the 300–600 cm−1 region and the 27Al NMR spectra of the solid resemble those of AlCl4− or Al2Cl7−. However the results are best interpreted by assuming the formation of [B10H10(AlCl3)2]2− with localization of a negative charge in the vicinity of each AlCl3. Subsequent addition of AlCl3 should produce [B10H10(Al2Cl6)2]2−. The same reaction performed in liquid SO2 after evaporation of the solvent yields solid products which are hazardous to handle.

Phase transition investigations of closo-hydroborates

Abstract Decahydrodecaborates M 2 Ba 10 H 10 (M  Men 4 , Et 4 N, Et 3 NH, n-Bu 4 N, Na, K) and dodecahydrododecaborates M 2 B 12 H 12 (M  Et 4 N, Li) have been studied in the solid state between 200 and 400 K using DSC, XRD and 11 B MAS NMR techniques. Reversible phase transitions were observed with (Et 4 N) 2 B 10 H 10 at 263–277 K (Δ H = 10.03 kJ mol −1 ), (Et 3 NH) 2 B 10 H 10 at 324 K (Δ H =15.32 kJ mol −1 ), Na 2 B 10 H 10 at 382 K (Δ H =9.81 kJ mol −1 ) and (Et 4 N) 2 B 12 H 12 between 200 and 220 K (Δ H = 3.99 kJ mol −1 ). The transitions were first order except for (Et 4 N) 2 B 12 H 12 . The well resolved signals in the MAS NMR spectra recorded at high temperature became wide quadrupolar signals at temperatures lower than the transition temperature. These results were interpreted as to be caused by changes in the motions of the hydroborate cages in the cation framework. The sharp lined MAS NMR spectra are caused by large motions of the cages which average the observed quadrupolar magnetic moments of the boron atoms. When these large motions are stopped or hindered due to a crystallographic or an order-disorder transition, the MAS NMR spectra are broadened. Attempts to determine the structure of tetraethylammonium salts probably failed because the positions of the anions in the solid are disordered even at temperatures lower than the transition temperature.

Catalytic curing agents

SummaryThe BY3 (Y=F, Cl, Br, I, NCS, H) adducts with the tertiary amines N,N-dimethyl-n-octylamine (DMOA) and N,N-dimethyl-benzylamine (DMOA) were prepared, characterized and tested as catalytic curing agents for epoxy (DGEBA) and epoxy-isocyanate resins.Evaluation of the storage stability and reactivity of the resins showed that the BBr3, BCl3, BI3 and B(NCS)3 adducts were good latent catalysts. No correlation could be found between the temperature of decomposition of the catalysts in an inert atmophere or in contact with air and the catalytic properties. Consequently the reaction medium was involved in the activation process.

Catalytic curing agents: A study of boron halide and boron isothiocyanate complexes with tertiary amines as catalytic curing agents for epoxide and epoxy-isocyanate mixtures

Summary The BY3 (Y= F, CI, Br, I, NCS, H) adducts with the tertiary amines N,N-dimethyl-n-o ctylamine (DMOA) and N,N-dimethyl-ben zylamine (DMOA) were prepared, characterized and tested as catalytic curing agents for epoxy (DGEBA) and epoxy-isocyanate resins. Evaluation of the storage stability and reactivity of the resins showed that the BBr3, BC13, BI3 and B(NCS)3 adducts were good latent catalysts. No correlation could be found between the temperature of decomposition of the catalysts in an inert atmosphere or in contact with air and the catalytic properties. Consequently the reaction medium was involved in the activation process.