The Investigations On The Phase And Morphology Controlled Synthesis Of Boron Nitride

Boron nitride (BN) is an important inorganic non-metallic material because of its excellent physical and chemical properties.It has several different phases:such as cubic (c-BN), hexagonal (h-BN), wurtzite (w-BN),and rhombohedral (r-BN),et al. Among them, cubic boron nitride is the second hardest material only inferior to diamond, while its thermal stability is superior to that of diamond. The oxidation (1200℃) and graphitization temperatures(1500℃) of c-BN are higher than those of diamond (600℃and 1400℃),respectively. Moreover, c-BN is chemically inert against molten ferrous material, which makes it as the good material of cutting and grinding in the machining industry. In addition, it can also be doped for both n-and p-type conductivity. Hexagonal boron nitride has similar layered crystal structure with graphite, so they have similar physical and chemical properties, such as good lubrication, chemical resistance, and high thermal conductivity. Moreover, h-BN has superior properties than graphite in some respects.For example, h-BN is inert against metal (such as aluminum, copper, zinc, iron, etc.)and non-metallic (silicon, boron, glass, etc.).It has also high resistance to oxidation temperature, which shows that h-BN is a good chemical inert material.Based on excellent properties and wide applications,many researchers have engaged in the preparation and characterization of boron nitride. A number of preparation methods for BN nanomaterials have been reported at home and abroad, such as carbon nanotubes template method, molten salt method, laser melting method, chemical vapor deposition method and arc discharge method. In this paper, we have successfully synthesized the rhombohedral boron nitrde triangular nanoplates and hexagonal nanoplates,cubic boron nitride, and hexagonal boron nitride nanorods by the thermal-induced, metal reduction, thermal decomposition reaction, respectively. The main research contents of the dissertation are listed as follows: 1.We developed a new method for r-BN from uniform triangular to hexagonal nanoplates.Firstly, triangular r-BN nanoplates have been prepared through the reaction of NaBH4 and NaNH2 at 550℃for 24 h, which were labeled as "Sample 1", all the diffraction peaks in the range of-25-80°in the typical XRD pattern of Sample 1 can be indexed as rhombohedral BN.The high diffraction intensity character of these peaks indicates the high crystallinity of Sample 1.The calculated lattice constant is a=2.53 (?) and c=10.00 (?),which is close to the reported value (JCPDS card no.45-1171:a=2.504 (?) and c=10.000 (?))of rhombohedral BN. The morphology and structure of the as-prepared samples were further analyzed by SEM and TEM. The images indicates that Sample 1 is composed of a large quantity of triangular nanoplates with an average thickness of about 50 nm and the rest are irregular shaped BN nanoparticles.These triangular nanoplates have widths mainly ranging from 200 to 500 nm and with the average edge lengths of about 360 nm, however, a little amount of small nanoplates (-5%) with a width of about 150 nm and few of large nanoplates (-3%,～700 nm) were also observed, which were further confirmed by the size distribution pattern.Through statistical SEM and TEM observations, the triangular nanoplates were estimated to be -90% among the product and the clear fringes with an average spacing of 0.21 nm correspond to the (101)lattice spacings of a r-BN crystal.The possible formation mechanism of these triangular BN nanoplates may be related to its anisotropic structure. Besides, under the excessive dosage of NaNH2, if NaBH4 was substituted by other boron sources (such as B,KBH4, KBF4, H3BO3,Na2B4O7, or Mg (BO2)2·H2O), triangular r-BN nanoplates could also be obtained. Therefore, it is obvious that the high content of NaNH2 was beneficial to the fabrication of triangular nanoplates because the synthetic environment is abundant in hydrogen and nitrogen. Then, these triangular BN nanoplates could transform into hexagonal ones in large quantities during calcination process with floating nitrogen, which were labeled as "Sample 2", the typical XRD pattern of Sample 2 shows the similar characteristics compared with that of Sample 1.These results reveal that no phase transformation occurred after the calcination process.The typical SEM and TEM images of Sample 2 show that these nanoplates are mainly hexagonal in shape with an average thickness of 50 nm. It is worth noting that the average thickness value of triangular and hexagonal r-BN nanoplates is similar(～50 nm), implying that the(111)basal plane is not change during the shape conversion process.The average edge width of the hexagonal nanoplates is 320 nm and mainly ranging from 200 to 500 nm, the size distribution histogram of these nanoplates exhibited that the proportion of small (less than 200 nm) and large (more than 500 nm) nanoplates is small.The observation results of the SEM and TEM images indicate that these hexagonal nanoplates have smooth surfaces and with uniform shapes.The HRTEM images performed on the single hexagonal r-BN nanoplate shows that the average distance between the neighboring fringes is about 0.20 nm, which is consistent with the interplanar distance of 0.211 nm in bulk r-BN. HRTEM and SAED(Selected Area Electron Diffraction) examinations of other nanoplates show similar results, which unambiguously imply their single crystalline nature. A series of experiments were further carried out by changing the synthetic conditions to study the process of this shape transition from triangular to hexagonal nanoplates.Their average edge sizes are 360 and 320 nm, and their intense emission bands are centered at 316 and 297 nm (λex=200 nm), respectively. Therefore, the samples possessing interesting varied optical properties and might be used as promising materials for deep-blue and UV applications.2.Cubic boron nitride was successfully prepared by metal reduction method on the basis of preparing hexagonal boron nitride. In a typical process,NH4Br (0.03 mol), BBr3 (0.01 mol),and sodium (0.13 mol) were placed into a 20 ml stainless steel autoclave. The autoclave was sealed and heated from room temperature to 450-600℃at a rate of 10℃min-1,then maintained at the target temperatures for 24 h in an electrical furnace. After the autoclave was cooled to room temperature, the inner raw product was collected and washed with absolute ethanol, dilute hydrochloric acid, perchloric acid (HClO4), distilled water and absolute ethanol for several times to remove the impurities.Finally, the black powders were obtained after a drying process in a vacuum at 60℃for 12 h. In addition, the possible influence factors (the reaction temperature, the reaction time and dosage of the reactants) on the structure and properties of the product have also been discussed. First, the effects of reaction temperatures on the yield of the c-BN were studied. If the reaction temperature was set below 500℃(such as 450℃), only h-BN could be obtained;At 500℃,little amount of c-BN were observed besides the dominant h-BN. It is found that the higher reaction temperature usually lead to the higher yield of c-BN, and its yield was dramatically increased in the whole product if the target temperature was set at 600℃;If the reaction temperature was further raised, such as at 650℃or 700℃, the yield of cubic boron nitride increased slowly but impurity peaks were also appeared.Besides the reaction temperature, the molar ratio of BBr3 to NH4Br was found to play an important role on the formation of c-BN.For instance, amorphous boron would be produced along with h-BN when the molar ratio is larger than 1:1 (such as 1.5:1 or 2:1);Only little c-BN co-existed with h-BN when the ratio was approaching 1:1,and its yield increased along with the increased ratio.The optimal ratio for the highest yield production of c-BN was found to be 1:3.It is also observed that the longer reaction time usually led to the production of c-BN with higher crystallinity, but had no obvious influence on its yield augment. Finally, a series of contrast experiments were carried out to investigate whether the result was similar or not if the reagents were substituted by others.For example, the BBr3 was substituted by B powder, B2O3,H3BO3,NaBF4, NaBH4, or NH4HCO3,(NH2)2CO were used instead of NH4Br, almost no c-BN was obtained. However, a small quantity of c-BN could be gained using other ammonium salts NH4X (X=F, Cl,I) cover for NH4Br. However, in the contrast experiments, only h-BN could be generated if a Cu or Fe tube was put into the same autoclave and acted as a liner while keeping other conditions unchanged. Therefore, it is obvious that the production of CrN was favorable for the formation of c-BN.3.Preparation and characterization of rod-shaped BN crystal.The reports about the preparation and properties of BN nanorods are relatively fewer compared to the nanowires,nanotubes and nanobelts.In this paper, rod-shaped BN crystal were synthesized through the reaction of C24H2oBNa, N2H4·H2O, Zn powders and sulphur powders at 600℃for 24 h. From the SEM and TEM results, it could be seen that the BN nanorods have a length of 300-1700 nm and a diameter of 50-150 nm.The ratio of length to the diameter could reach more than 10:1.In the preparation process, the growth mechanism of the BN nanorods has also been investigated by varying the synthetic conditions.