Synthesis Of Cubic Boron Nitride Research Articles

Facilitated synthesis of cubic boron nitride by a mechanochemical effect

Abstract A superhard cubic boron nitride (c-BN), which is now an industrially important material used in preference to diamond especially for cutting and lapping steel products, has so far been synthesized by heating a starting powder of hexagonal boron nitride (h-BN) under a high pressure at a high temperature. In the present study we have mechanically milled h-BN starting material to introduce lattice defects prior to its conversion to c-BN. The synthesis of c-BN has then been facilitated prominently owing to a mechanochemical effect; under 7.7 GPa the formation of c-BN starts at 1250°C with the pre-milled h-BN powders, and at 1450°C with non-milled powders. High resolution transmission electron microscopy has clarified that a variety of defects form in h-BN during milling. The h-BN changes to w-BN by compression in the c direction, and a slight tilt at the interface between (0002)h and (0002)w partially compensates for the lattice misfit between the basal planes of both phases. Characteristic stacking ...

Facilitated synthesis of cubic boron nitride by a mechanochemical effect

Facilitated synthesis of cubic boron nitride by a mechanochemical effect

Pathways to c-BN from melts at atmospheric pressure

The industrial synthesis of cubic boron nitride is performed under high pressure and high temperature from the hexagonal phase with catalysts. Since it has been demonstrated that c-BN is the stable BN modification under standard conditions, there should also be a way to synthesize the superhard c-BN phase at normal or low pressure. To reach this goal growth conditions with high mobility for both the B and the N species have to be found. Melts containing Li/B/N could be a possible solvent to form c-BN analogous to the high-pressure synthesis, whereby Li 3BN 2 is acting as a solvent-catalyst. Under normal pressure possible interactions of two different c-BN samples with various phases from the ternary Li-B-N phase system were investigated at temperatures up to 950 °C. The following melts were applied: elemental lithium (Li), lithium fluoride (LiF), lithium nitride (Li 3N), lithium boride (Li 7B 6), lithium boronitride (Li 3BN 2) and boronoxide (B 2O 3). After these melt treatments morphological and other changes of the c-BN crystals were examined by SEM, Raman spectroscopy and XRD.

Synthesis of cubic boron nitride thin films

An equilibrium thermodynamic analysis of the Gibbs free energy in a non-hydrostatic stress field is applied to the formation of boron nitride thin films. The theory predicts a specific sequence of microstructures from non-oriented hexagonal boron nitride, through oriented hexagonal boron nitride and finally cubic boron nitride. It also predicts a threshold stress for cubic boron nitride formation. These predictions are in agreement with experimental observations reported here and made by others. The synthesis of cubic boron nitride in a new helicon wave plasma source is demonstrated. The operating parameters which produce the cubic phase are determined.

Characterization of cubic boron nitride films grown by mass separated ion beam deposition

We have prepared BN films by ion beam deposition of mass separated low energy 11B + and 14N + ions with energies between 100 and 500 eV. The silicon substrates were kept at room temperature or heated up to 350°C. The films were characterized with Auger electron spectroscopy and Rutherford backscattering. The influence of ion energy, ion flux and substrate temperature on the c-BN formation will be discussed. Successful synthesis of cubic boron nitride (c-BN) with a phase purity up to 80% is confirmed by infrared absorption spectroscopy, transmission electron microscopy (TEM) and transmission electron diffraction (TED). For a film with high c-BN content TEM revealed a growth sequence of an amorphous interface layer, followed by few nm of highly oriented hexagonal BN (h-BN) and a layer of nanocrystalline c-BN. We were also able to produce a textured h-BN layer in between two c-BN layers.

On the low-pressure synthesis of cubic boron nitride

The phase transformation of amorphous boron nitride (aBN) into the cubic modification (cBN) in the presence of magnesium boron nitride has been studied in a wide range of pressures and temperatures. The minimum pressure of cBN formation was found to be 2.5 GPa at 2100 K. This is the lowest pressure for cBN crystallization employing the catalyst-solvent process and is reported here for the first time.

Comparative aspects of c-BN and diamond CVD

Abstract Various key aspects of the low-pressure synthesis of cubic boron nitride and diamond are compared and discussed. Based on recent thermodynamic data for c-BN and h-BN (heats of formation, entropies, specific heats), c-BN should be the stable phase under standard and low-pressure conditions. The current problems in c-BN CVD synthesis can be explained by the empirical rules of Ostwald and Ostwald-Volmer, which show that the formation of the metastable and less dense phase, h-BN, is favoured. Thus, for a successful CVD synthesis of c-BN it is necessary to find a way to circumvent the natural tendencies of the system. Thermodynamic equilibrium calculations for c-BN and h-BN in different gas atmospheres suggested a phase-selective reaction y which h-BN could be preferentially etched. Thus a c-BN CVD process similar to that of diamond should be possible and impingement with energetic ions should not be required to grow c-BN. However, the atomic attachment kinetic during crystal growth caused by the anisotropy of the c-BN crystals (completely N- or B-terminated facets), different bonding energies, and differences of bond lengths of B-N, B-B and N-N bonds must be taken into consideration and could cause problems.

Synthesis of cubic boron nitride using Li 3BN 2, Sr 3B 2N 4 and Ca 3B 2N 4 as solvent-catalysts

Three kinds of metal boron nitride, namely Li 3BN 2, Sr 3B 2N 4 and Ca 3B 2N 4, were prepared at normal pressure, and cubic boron nitride (c-BN) was synthesized at high pressure and high temperature using these compounds as solvent-catalysts. The synthetic regions of c-BN in P-T space were U-shaped, having a clear minimum threshold pressure. The threshold pressures were found to lie between 4.6 and 5.0 GPa with no temperature dependence for all examined solvent-catalyst systems. It was found that the temperature limit of c-BN synthesis is lower than that reported previously. The c-BN grains grown in the Ca 3B 2N 4-containing system were about 500 μm maximum which was several times larger than that of the grain sizes using other solvent-catalysts. The formation mechanism of c-BN was discussed on the basis of the dissolution and precipitation of BN in the solvent-catalyst.

New scope of high pressure-high temperature synthesis of cubic boron nitride

The stability region of cubic boron nitride (c-BN) has been determined by high pressure-high temperature experiments. The P- T regions of c-BN formation by catalytic conversion and by direct transformation were reviewed and indicated that a temperature-independent threshold does exist between 4.6 and 5.0 GPa for most catalysts of alkali and alkaline earth boron nitrides. The corresponding solid state transformation from hexagonal BN (h-BN) to wurtzite BN showed a threshold at about 6.5 GPa. The threshold pressure of the formation region of c-BN is discussed based on the simple energy diagram for activation of sp 2 bonding of h-BN.

Synthesis of cubic boron nitride from hexagonal boron nitride with AlMg alloy solvent under high pressures and high temperatures

An AlMg metallic alloy solvent has been proved to be an effective solvent for the synthesis of cubic BN(cBN) from hexagonal BN(hBN) under high pressures and temperatures. The Mg content in the AlMg solvent ranged from 30 to 70% by weight. The conversion rate from hBN to cBN and morphology of the cBN crystals could be controlled by varying the amount of solvent in the hBN-solvent mixture and the Mg content in the solvent, respectively. The conversion rate increased as the amount of solvent in the hBN-solvent mixture or the Mg content in the solvent increased. The large blocky cBN crystals with well-developed crystallographic surfaces were synthesized by the solvent with high Mg content and the fine cBN crystals, with irregular shape, by the solvent with low Mg content.

Synthesis of cubic boron nitride using Mg and pure or M?-doped Li3N, Ca3N2 and Mg3N2 with M?=Al, B, Si, Ti

The growth pressure-temperature region of cubic boron nitride (cBN) in the systems Mg-BN and MxNy-BN (MxNy=Li3N, Ca3N2, Mg3N2) has been redetermined using well-crystallized hexagonal BN (hBN) with low oxygen content (0.2%) as the starting material. The data on the MxNy-BN systems are compatible with the existence of two growth regions: a high-temperature region where cBN grows from a liquid phase, and a low-temperature region where cBN forms from solid-solid reactions. Previous data are discussed according to this model and possible solid-state reactions are proposed on the basis of thermodynamic considerations. The results for the Mg-BN system confirm the effect of the O2 content of the starting BN on the cBN growth region. The systems (MxNy + M′)-BN (M′=Al, B, Si, Ti) have been shown to produce cBN crystals of increased size and improved morphology (more compact and perfect) compared to those obtained with the MxNy-BN systems. Their colour is dark or black and their size reaches 0.6 mm. The effect of the relative proportions of M′ and MxNy on the growth region and yield has been determined and is discussed on the basis of the chemical reactions likely to occur.

Synthesis of Cubic Boron Nitride by Decomposition of Magnesium Boron Nitride

Cubic boron nitride (c‐BN) was synthesized by the decomposition of Mg3BN3 under high pressure and high temperature. The minimum pressure for c‐BN synthesis was 4 GPa, which was 1 GPa lower than that of the conventional catalytic process. The decomposition of Mg3BN3 was observed only when H2O was added. Therefore, the reaction was as follows: Mg3BN3+ 3H2O = 3MgO +c‐BN + 2NH3. The c‐BN crystals obtained were tetrahedron in shape and about 10 μm in diameter.

Synthesis of cubic boron nitride with boron nitride powder formed from triammoniadecaborane

Cubic BN was synthesized under high temperature and pressure conditions from BN powder formed by reaction of triammoniadecaborane (TAD) with ammonia. The BN powder formed from TAD and ammonia had a low degree of ordering. The crystal lattice of the BN powder increased in regularity with increasing synthesis temperature and time for the reaction of TAD with ammonia. The conversion yield of cubic BN at 1300 °C and 6.5 GPa in the presence of AIN increased with decreasing of reaction temperature of TAD and ammonia from 1000–700 °C. Cubic BN decreased in yield with increasing reaction time of TAD and ammonia at 800 °C. BN powder pre-heat treated at 1550 °C had a crystallite size,Lc, of 22 nm, and was converted to cubic BN in a 43% yield at 1300 °C and 6.5 GPa for 10 min. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 97 kJ mol−1, when the starting BN was synthesized at 800 °C. The conversion yield of cubic BN from the disordered BN-20 mol% AIN was 100% after heat treatment at 1300 °C and 6.5 GPa for 20 min.

Synthesis of cubic boron nitride using magnesium as the catalyst

Abstract Cubic boron nitride (CBN) single crystals have been synthesized employing the BN-Mg system under high pressure and high temperature conditions. During the exploration of the thermodynamically stable CBN growth region it has been found that apart from the CBN phase the other crystalline phases formed during the chemical reaction are: MgO, Mg 3 (BO 3 ) 2 , Mg 3 N 2 , MgB 6 , or MgB 12 . Along with these catalytic phases the wurtzitic boron nitride (WBN) has also been detected and reported for the first time. The role of various phases formed during the reaction has been examined and a possible explanation for the formation of WBN has been suggested. The morphology of CBN crystals and their yield have been studied as a function of temperature and pressure. The effect of oxygen impurity present in the starting hexagonal boron nitride (HBN) on the conversion to CBN has also been discussed.

ChemInform Abstract: Synthesis of Cubic Boron Nitride from Boron Nitride Synthesized by Pressure Pyrolysis of Borazine.

AbstractBN powders are prepared by pyrolysis of borazine (B3N3H6) under various conditions (250‐700 °C; 25‐100 MPa).

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.

Synthesis of Cubic Boron Nitride from Boron Nitride Synthesized by Pressure Pyrolysis of Borazine

Cubic BN was synthesized under high‐temperature and ‐pressure conditions from BN powder formed by pressure pyrolysis of borazine below 700°C and 100 MPa. The conversion of BN powder to cubic BN was strongly influenced by the residual hydrogen identified by the BH/BN ratio of IR absorption band. The activation energy for cubic BN synthesis from BN powder‐20 mol% AIN was 46 kJ/mol, when the starting BN was synthesized at 250°C. A mixture of BN powder and cubic BN particles was converted to cubic BN in a 100% yield by heat treatment at 1800°C and 6.5 GPa without any catalyst. The presence of cubic BN particles does enhance the conversion to cubic BN from BN powder. The energy required for the transformation of starting BN to cubic BN in the presence of cubic BN seed was 355 kJ/mol.

High pressure synthesis of cubic boron nitride from amorphous state

Cubic-zincblende-type boron nitride was synthesized from amorphous state at pressures higher than 6 GPa and at temperatures higher than 800°C, without any planned addition of catalysts. Also, wurtzite type boron nitride was formed although the amount was small.

Effects of AIN Additions and Atmosphere on the Synthesis of Cubic Boron Nitride

Aluminum nitride was found to act as the catalyst for the synthesis of cubic BN by sealing the mixture of hexagonal BN and AIN in a pressure cell under the inert or reducing atmosphere. No conversion of hexagonal BN to cubic BN was observed under pressures below 7×109 Pa (7 GPa, 70 kbar) without AIN addition. All hexagonal BN could be completely converted to cubic BN under 6.5 GPa at 1600°C by the addition of 20 mol% AIN. The cubic BN thus synthesized was a typical tetrahedron (grain size ≅2 μm). The pressure‐temperature diagram for the synthesis of cubic BN was determined at >7 GPa and T<1700°C.

Phase equilibria pertinent to the growth of cubic boron nitride

The practical synthesis of cubic boron nitride (cubic BN) requires the simultaneous application of hig temperatures and pressures. Since the discovery of cubic BN more detailed understanding of the chemistry of the process has been acquired from equilibria or “approach-to-equilibria” studies in the Li-B-N system. This paper describes some of these results and considers their application to the growth of borazon.The principal compound that coexists with cubic BN at high pressures and temperatures is a high pressure modification of Li3BN2 (called Li3BN2(W)). It has been shown that cubic BN does not form when this compound melts or decomposes. When BN is added to Li3BN2 in excess of the composition 53 mole% BN: 47 mole% Li3N, then cubic BN precipitates from solution. At 55 kb, the system Li3BN2-BN has a eutectie at 6% BN and 1610°c and the upper limit of the cubic BN plus liquid region is at 1750°C. The solubility curve is a very steep function of temperatures and consequently the system is not too favourable for crystal growth by slow cooling.These new results can be compared with previous results by plotting a series of P-T curves defining the stability region for cubic BN under different conditions. Good agreement is found between the solidus determined in the Li3BN2-BN study and the lower limit of the catalyst growth area. In effect, this lower limit is directly related to the initial formation of liquid in the Li3BN2-BN portion of the system. The lower temperature limit of growth of cubic BN reported from Li-BN compositions is below that of the present study and the catalyst growth area previously defined. This result would have some fundamental basis in fact if a lower temperature reaction involving cubic BN occurred in the ternary system Li-B-N.