【作者】 张晓东；

【导师】 温广武；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 准一维SiC和Si₃N₄纳米材料普遍具有优异的力学性能、光学性能和电学性能,在复合材料增强体和纳米元器件等领域都有着广泛的应用前景。然而,现有的工艺方法难以实现结构控制与高产率制备,严重制约着它们的研究和应用。因此,准一维SiC和Si₃N₄纳米材料的可控生长与高产率制备,是亟待解决的瓶颈问题。本文从反应热力学与动力学出发,对准一维SiC和Si₃N₄纳米材料进行设计,首先合成出非晶态SiO₂/C纳米复合粉体作为前驱体原料,然后分别在Ar和N₂气氛中进行热处理,成功实现了具有不同结构的准一维SiC和Si₃N₄纳米材料的可控生长与高产率制备。针对一般原料转化率较低的问题,设计并合成出一种高效的新型原料。选用硅溶胶和蔗糖为原料,经过溶胶凝胶和热分解工艺获得黑色的非晶态SiO₂/C纳米复合粉体。其中,纳米复合粉体的颗粒为30⁵0nm,具有碳包覆二氧化硅的纳米结构。非晶态SiO₂/C纳米复合粉体在1109℃时即开始发生碳热还原反应,1290℃时反应放出大量的SiO和CO气体,热处理温度比热力学的计算值降低至少300℃。在不引入任何添加剂的情况下,将SiO₂/C粉末直接置于Ar气氛中进行热处理,热处理温度为1200℃时生长出SiC纳米线;将粉体直接在N₂气氛中热处理,1300℃时生长出Si₃N₄纳米纤维。SiO₂/C粉末具有较高的反应活性,是合成准一维SiC和Si₃N₄纳米材料的理想前驱体。在Ar气氛中对前驱体进行热处理制备准一维SiC纳米材料,系统地研究其生长规律,并对各种结构进行表征。当初始Ar压强0.1⁰.5MPa,在1200¹550℃温度区间内对非晶态SiO₂/C纳米复合粉体直接热处理,然后缓慢降温,即可获得平直的SiC纳米线。纳米线的生长具有如下规律:SiC纳米线的直径受Ar气压强的影响较大,气压越高纳米线的直径越小,直径可以在50¹000nm的范围内进行控制;SiC纳米线的长度受热处理温度和时间的影响较大,随着温度的升高和时间的延长纳米线的长度增加,长度可以在几微米至几厘米的范围进行控制;SiC纳米线的产率受到SiO₂/C粉末配比和热处理温度的影响较大,SiO₂/C粉末中C:Si比和温度的提高均有利于产率的提高。当初始Ar压强为0.3 MPa,在1550℃对C与Si的摩尔比为3:1的SiO₂/C粉末进行热处理并保温4h时,SiC纳米线可以形成无纺布,纳米线的平均直径为100¹20nm、长度可达几个厘米。SiC纳米线具有良好的发光性能,可在246 nm至595 nm的波长范围内发射荧光;纳米线的电导率为0.141 S·cm-1,明显优于碳化硅块体材料;单根纳米线的拉伸强度最高可达31GPa。当初始Ar压强0.5¹.0MPa,在1650¹800℃温度区间内对非晶态SiO₂/C纳米复合粉体直接热处理可获得具有串状结构的SiC纳米线。串状结构为β-SiC单晶,中心杆（或线）的直径50¹50nm,外围的串直径100⁵00nm,长度分布在几十微米至几个毫米范围内。在气氛浓度较大的环境中,气相生长出的SiC纳米线带有大量的层错缺陷,这些缺陷位置利于SiC晶体的外延生长,从而在纳米线上生长出SiC晶片,自组装生长成独特的串状结构。通过控制热处理温度和Ar压强,实现对串状结构SiC纳米线的生长进行控制。当初始Ar压强0.2⁰.5MPa,在1450¹600℃温度区间内对非晶态SiO₂/C纳米复合粉体直接热处理然后快速降温可获得伴有球状结构的SiC纳米线,呈现珍珠项链状的形貌,纳米线的直径80¹00nm、长度可达几个毫米,小球的直径主要分布在1¹0 m。当气相中存有较多未反应的SiO和CO气体情况下开始降温,SiO和CO气体容易在气氛压力和表面张力的共同作用下沉积到纳米线上并形成小球,从而自组装成为具有球状结构的SiC纳米线。通过调节热处理温度、反应时间、降温速率和Ar压强,可实现对伴生球状结构的SiC纳米线的可控生长。在N₂气氛中对前驱体进行热处理制备准一维Si₃N₄纳米材料,系统地研究其生长规律,并对各种结构进行表征。当初始N₂压强为0.2¹.0MPa,在1400¹600℃温度区间内对非晶态SiO₂/C纳米复合粉体直接热处理可获得Si₃N₄纳米纤维。Si₃N₄纳米纤维的生长,可以通过热处理温度、保温时间和N₂压强来进行控制。随着温度升高和保温时间延长,Si₃N₄纳米纤维由线状向带状过渡,且纳米带的尺寸逐渐增大;随着N₂压强的增大,Si₃N₄纳米纤维的尺寸也逐渐增大。当初始氮气压力为0.2MPa,对C与Si的摩尔比为4:1的SiO₂/C粉末进行热处理并保温4h时,可以得到高产率的无纺布状a-Si₃N₄纳米带,带宽150³00nm,带厚20⁹0nm。Si₃N₄纤维具有很好的发光性能,可发射波长范围246nm至595nm;电导率为10^-9 S·cm^-1,属于典型的半导体材料;具有优良的力学性能,纳米硬度和弹性模量分别为17.2GPa和180.8GPa。更多还原

【Abstract】 Quasi one-dimensional （quasi-1D） SiC and Si₃N₄ nanomaterials have promising applications for reinforcing composite materials and producing nanodevices, due to their excellent mechanical, optical and electrical properties. Although great efforts have been made to prepare quasi-1D SiC and Si₃N₄ nanomaterials, the difficulty of producing them on large scale with controllable structure is still a major hindrance limiting their further practical applications. Therefore, it is very necessary and important to realize the controllable growth and high-yield preparation of quasi-1D SiC and Si₃N₄ nanomaterials. In this paper, silica sol and sucrose were used to synthesize novel amorphous SiO₂/C nanocomposite powders through the process of sol-gel and thermal decomposition. Then the as-synthsized powders were annealed to fabricate quasi-1D SiC and Si₃N₄ nanomaterials under Ar and N₂ atmosphere, respectively. The influences of the raw materials, annealing temperature and gas pressure on the growth of quasi-1D SiC and Si₃N₄ nanomaterials were systematically investigated, and the optical, electrical and mechanical properties of the obtained 1D nanomaterials were also studied.The SiO₂/C nanocomposites are amorphous in nature and homogeneous with the microstructure of close packed SiO₂ and carbon at nanoscale. Carbothermal reduction reaction begins at 1109°C and becomes very severe at 1300°C during the annealing process of SiO₂/C nanocomposites. The SiO₂/C nanocomposites can be annealed to produce SiC and Si₃N₄ nanocrystals at 1200 and 1300°C, respectively. Owing to the high reactivity at high temperature, the SiO₂/C nanocomposites are ideal precursor for fabrication of 1D SiC and Si₃N₄ nanomaterials.SiC nanowires were obtained by directly annealing the amorphous SiO₂/C nanocomposites at 1200¹600°C under the initial Ar pressure arrange of 0.1¹.0 MPa. The average diameter of SiC nanowires showed a decreasing trend with the increase of the gas pressure. With the reaction temperature increased, the average diameter of SiC nanowires remained unchangable, however, the length of nanowires increased significantly. When the nanocomposites are annealed at 1550°C, the length of SiC nanowires can be up to several centimeters. A large number of stacking faults, resulting to the formation of nano-fins structure, were found in the SiC nanowires when the reaction temperatures were more than 1600°C. For increasing the holding time from 0.5h to 4h at 1550°C, the yield of SiC nanowires were obviously increased as well. The optimal synthetic parameters for the growth of SiC nanowires based on the results of experiments were proposed as follows: annealing the SiO₂/C nanocomposites at 1550°C for 4h under 0.2MPa Ar gas-pressure atmosphere, large quantities of SiC nanowires were obtained with the average diameter of 80¹00nm and the length of several centimeters. The photoluminescence, electrical, and mechanical properties of SiC nanowires were investigated.With regard to the string-like SiC nanowires achieved by annealing the amorphous SiO₂/C nanocomposites at 1650¹800℃under the initial Ar pressure arrange of 0.5¹.0 MP, the phase composition is mainlyβ-SiC single crystal with the average diameters of central nanowires and external string of 100nm and 200nm, respectively, and the length range of string-like SiC nanowires are from tens of microns to several millimeters. The controllable structure of string-like SiC nanowires can be fulfilled technically by adjusting the annealing temperatures and the initial Ar gas pressures.Necklace-like SiC nanowires were obtained by rapid cooling after annealing the amorphous SiO₂/C nanocomposites at 1450¹600℃under the initial Ar pressure arrange of 0.2¹.0 MPa. The formation mechanism of necklace-like SiC nanowires is proposed as following: Firstly, the straight SiC nanowires are formed; then, unreacted gaseous SiO and CO deposite into small balls on the SiC nanowires so as to form the novel structure of necklace-like SiC nanowires. By adjusting the annealing temperatures, holding times, cooling rates and Ar pressures, the growth of necklace-like SiC nanowires can be controlled.Quasi-1D Si₃N₄ nanomaterials, including nanowires and nanobelts, were obtained by directly annealing the amorphous SiO₂/C nanocomposites at 1200¹600°C under the initial N₂ pressure arrange of 0.2¹.0 MPa. The growth of Quasi-1D Si₃N₄ nanomaterials can be controlled by adjusting annealing temperature, holding time and the initial N₂ pressure. With increasing the annealing temperature and holding time, Si₃N₄ nanowires can turn into nanobelts. With the increase of initial N₂ gas pressure, the size of quasi-1D Si₃N₄ nanomaterials also increases. When the amorphous SiO₂/C nanocomposites （with molar ratio Si:C=1:3） were annealed at 1500 for 4 h under the initial nitrogen pressure of 0.2 MPa, the products were non-woven self-assembled by high yield Si₃N₄ nanobelts with a bandwidth of 150³00 nm and thickness of 20⁹0 nm. The properties of Si₃N₄ nanobelts were also investigated and results show the nanobelts exhibit excellent optical, electrical and mechanical properties.更多还原