【作者】 李梦华；

【导师】 钱逸泰；

【作者基本信息】 山东大学 ， 无机化学， 2010， 博士

【摘要】 氮化硼作为一种重要的无机非金属材料,具有优异的物理化学性质,在很多方面有潜在的应用前景。常见的物相包括：立方相氮化硼,六方相氮化硼,菱形氮化硼(也称为三方相)和纤锌矿型氮化硼。其中,立方相氮化硼具有与金刚石相似的晶体结构,其硬度仅次于金刚石,而它的抗氧化温度(～1200℃)和“石墨化温度”(～1500℃)都高于金刚石(～600℃和～1400℃),立方相氮化硼对铁族元素在高温下仍具有很高的惰性,这些性质使其广泛应用于机械加工行业,成为高效、节能的切削和磨削的良好材料。同时,立方相氮化硼具有宽的带隙(Eg～6.0 eV)和良好的导热性,并且还可通过掺入特定的杂质使其具有独特的半导体性质,例如,添加铍(Be)可获得p型半导体材料,添加硫(S)、碳(C)、硅(Si)等则可以得到n型半导体材料,这些优良的特性使得立方相氮化硼在抗辐射电子学、光电子学、高温电子学和高频大功率器件领域具有良好的应用前景。此外,六方相氮化硼具有与石墨相似的层状晶体结构,二者具有相似的物理化学性质,如良好的润滑性、耐化学腐蚀性、高热导率等。与石墨不同的是,六方相氮化硼是一种宽带隙半导体或者绝缘材料,并且其抗氧化温度更高,具有更高的化学惰性。此外,它还能实现n型和p型掺杂,具有中子吸收能力,是很好的耐高温电子器件材料和很有前途的声光电材料。由于其优良的性质和广泛的应用价值,所以,目前对氮化硼纳米材料的制备、纳米结构的表征、纳米器件的组装以及光学性能、电学性能的测试成为当今氮化硼纳米材料研究领域的重要方向。目前,国内外已报道的制备氮化硼纳米材料的方法很多,主要有碳纳米管模板法、熔融盐法、激光熔融法、化学气相沉积法和电弧放电法等。在对氮化硼材料的合成、测试、表征和应用等方面的发展现状进行了充分调研的基础上,本论文采用高温退火法、金属还原法和热分解法,成功地制备了：(1)三方相氮化硼的三角片和六角片,(2)立方相氮化硼的纳米颗粒,(3)六方相氮化硼的纳米棒。通过对实验结果的分析和有关的文献调研,分别对它们的生长机理进行了探讨。论文中取得了一些创新性的成果,主要内容概括如下：1.发展了一种可实现三方相氮化硼由三角片转变为六角片的新方法。首先,通过NaBH4和NaNH2在550℃反应24小时的新路线,成功的制备了氮化硼的三角片,标注为样品1(Sample 1)。Χ-射线粉末衍射(XRD)显示制得的样品是三方相氮化硼,其晶格常数为：a=2.53 A和c=10.00 A,这与三方相氮化硼标准数据基本一致(JCPDS card no.45-1171：a=2.504 A and c=10.000 A)。场发射扫描电子显微镜(SEM)和透射电子显微镜(TEM)照片显示：样品1的主要形貌是三角片(直径大约分布在200～500 nm，平均粒径约为360 nm,平均厚度约为50 nm),还有一些碎片和团聚的颗粒。此外,我们通过粒径分析知道,极小的三角片(粒径小于150 nm)和极大的三角片(粒径大于700 nm)占的比例极小,分别约为～5%和～3%。为了进一步表征三方相氮化硼的微结构,我们在高分辨电镜下得到三角片的晶格条纹图。图中清晰显示的两个相邻条纹之间的平均距离为0.21 nm,这和三方相氮化硼(JCPDS card no.45-1171)的(101)晶面的晶格间距非常接近,这也与XRD的分析结果相吻合,说明产物主要有三方相氮化硼的纳米三角片组成。除此之外,我们还考虑了其它的硼源和氮源对形成三角片的影响。实验结果表明,在NaNH2过量的前提下,即便是NaBH4被其它硼源如B，KBH4，KBF4,H3B03,Na2B407，Mg(B02)2·H20所取代,同样能制备出氮化硼三角片。然后,将已经制备出的氮化硼三角片在氮气中煅烧8 h,三角片的顶点会逐渐脱落,转变为BN六角片,标注为样品2(Sample 2)。样品2的XRD数据分析显示制得的样品物相是r-BN,说明了在煅烧的过程中没有发生相转变。样品2的形貌主要是由约为70%的六角片(直径大约分布在200～500nm,平均粒径约为320 nm,平均厚度约为50 nm)和一些无规则的碎片所组成。由此可见,在三角片转化为六角片的过程中,二者的厚度几乎保持一致。此外,我们通过粒径分析知道,极小的六角片(粒径小于200 nm)和极大的六角片(粒径大于500 nm)占的比例极小。为了进——步表征三方相氮化硼的微结构,我们在高分辨电镜下得到六角片的晶格条纹图和电子衍射图。图中清晰显示两个相邻条纹之间的平均距离为0.20 nm,和三方相氮化硼(JCPDS card no.45-1171)的(101)晶面的晶格间距(0.211 nm)基本一致,这也说明在氮气中煅烧的过程中,只是样品的形貌发生了改变,物相没有发生改变。样品的区域电子衍射(SAED)花样进一步分析表明,样品具有单晶结构。通过分析不同煅烧时间下的SEM照片和相关的文献资料,提出了形貌转变的可能机理。最新文献报道显示,厚度为几个纳米的氮化硼薄片添加到聚合物内,由于氮化硼薄片具有较大的比表面积能与高分子化合物形成复合物,氮化硼片能有效控制高分子化合物分子链的运动和扩散,从而大大减低聚合物的弹性系数和热膨胀系数,增加聚合物的拉伸强度,因此,我们制备的的氮化硼纳米片为后期的氮化硼的剥离及性能研究提供了基础和依据。我们还研究了所制备的氮化硼三角片和六角片的光学性质,在200 nm的激发波长下,氮化硼的三角片和六角片分别在316 nm和297 nm处有强的发射带,说明片状结构的氮化硼有望成为一种优良的紫外发光材料。2.探索了一条温和条件下制备立方相氮化硼的新路线。我们选择金属Na为还原剂,BBr3和NH4Br作为反应物,一起装入20毫升的不锈钢反应釜中,置于高温炉内从室温开始,以每分钟10℃的升温速率升至450-600℃,然后在目标温度下恒温24 h。待冷却到室温,取出反应釜内的粗产品,将其用无水乙醇和水洗涤分别除去残余的金属钠和产生的溴化钠；然后分别用稀盐酸和高氯酸洗涤以除去氮化铬杂质；最后产物在蒸馏水和无水乙醇洗涤后经过离心分离,在60℃下真空干燥12 h,获得浅灰色的粉末样品。此外,探索研究了温度、时间及反应物用量等对产物结构和性能的影响及可能的反应机理。我们发现,如果设定的温度低于500℃(比如450℃),所得样品的XRD和FTIR显示,样品只有六方相氮化硼；在500℃时,样品主要还是六方相氮化硼,但是,已经获得少量的立方相氮化硼。如将反应温度升至650℃或者700℃,则观察到六方氮化硼逐渐减少,同时立方相氮化硼的含量也随之增加,但纯度降低。实验结果表明,在600℃时所获得的样品中立方相含量和样品纯度都相对较高,因此,我们选定600℃为最理想的合成立方相氮化硼的温度。反应原料BBr3和NH4Br的摩尔比也会影响立方相氮化硼在样品中的含量。当BBr3：NH4Br=2：1或者1.5：1时,产品中主要是六方相氮化硼和单质硼；当BBr3：NH4Br=1：1时,可以获得少量的立方相氮化硼；当BBr3：NH4Br=1：3时,立方相的含量明显增加。可能是因为当NH4Br的量较多时,其分解产生的气体使得釜内压力增加导致立方相的含量增加。为了研究不同的硼源和氮源对结果的影响,我们做了一系列的对比实验。当BBr3被B粉,B2O3，H3BO3，NaBF4，NaBH4,或者是NH4Br被NH4HCO3,(NH2)2CO所代替时,XRD检测显示没有立方相氮化硼的特征峰；当NH4X(X=F,Cl,I)取代NH4Br做氮源时,有极少量的立方相氮化硼。此外,为了避免Cr的引入,我们在釜体内添加铜或者铁的内衬管,实验结果表明,粗产品只有六方相氮化硼而没有立方相氮化硼。可见氮化铬或者铬的存在对于立方相氮化硼的形成有一定的催化作用。(3)成功合成了氮化硼一维纳米棒材料。BN一维材料是BN研究的重要方向,但相对于纳米线、纳米管、纳米带材料的研究,有关氮化硼纳米棒的制备与性质研究的报道却相对较少。本文中,我们用C24H20BNa，N2H4·H20,Zn和S粉在600℃,24 h下制备出BN纳米棒。在SEM图像中可以观测到产物由大量分散纳米棒组成,平均直径约为50-150 nm。最后研究了不同的合成条件对产物的影响及可能的形成机理。更多还原

【Abstract】 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.更多还原