【作者】 杨丽丽；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2009， 博士

【摘要】 纤维隔热材料由于具有低成本、低密度、高效隔热等特点,一直得到建筑、工业以及国防等领域的青睐。但是由于纤维隔热材料本身对辐射电磁波具有高透过、低反射的特点,从而大大降低了其高温隔热性能。本研究采用溶胶-凝胶和逐层吸附法在石英纤维表面原位制备掺银氧化铟锡(Ag-ITO)多层薄膜作为红外反射层以降低纤维隔热材料的辐射传热。影响Ag-ITO多层薄膜性能的因素主要有三个:薄膜的孔隙率、表面粗糙度以及Ag粒子在薄膜中的空间分布状态。本文中,通过扫描电镜(SEM)、X射线衍射(XRD)、掠入射小角X射线散射(GISAXS)、掠入射X射线反射技术(GIXR)等手段研究了热处理工艺中温度、升温速率和保温时间三个因素对ITO薄膜的孔隙率、表面粗糙度的影响,以及对多层薄膜中Ag层结构的影响;同时借助傅里叶红外(FT-IR)、紫外-可见-近红外(UV-VIS-NIR)等手段表征了薄膜材料的光学性能。本文主要研究内容与结论如下:通过溶胶-凝胶法在玻璃基底制备了ITO薄膜。结果显示,ITO薄膜结晶温度约450℃、表面富锡、薄膜中孔洞具有分形结构,且其形状多呈椭圆状,沿薄膜表面展开。沿着薄膜厚度方向,薄膜的结构大体呈现三个区域:接近基底区域致密性好,孔径小;中间区域孔隙率高,孔径大;近表面区域在热处理温度T≤800℃时孔隙率与孔径较低,而温度T>800℃时,孔隙率与孔径反而会升高。利用GISAXS、GIXR、XRD等手段研究了热处理工艺对ITO薄膜孔隙率和表面粗糙度的影响。结果表明,随着热处理温度的升高,薄膜的致密度提高,孔隙率从低温时的33.6%降到高温时的23.8%,其中低温时的孔隙率降低主要来源于大孔的收缩,高温时的孔隙率降低主要来源于小孔的消除,而薄膜的表面粗糙度略有增大;提高升温速率由于降低了晶体生长的时间而有利于薄膜的致密化,且薄膜的表面粗糙度降低,但是当升温速率过高时,表面粗糙度又大大增大。1000℃快速热处理中,延长保温时间增大了近表面的孔隙率以及表面粗糙度而提高了整体薄膜材料的孔隙率。结合薄膜孔隙率以及表面粗糙度两个影响因素,优化溶胶-凝胶ITO薄膜的热处理工艺过程为:溶剂挥发阶段慢速升温,然后快速升温到高温。通过SnCl2作为前驱体之一和银的还原剂,逐层吸附原位得到Ag-ITO多层膜。当ITO溶胶浓度为0.5M时,银溶胶的最佳浓度是0.1M。GISAXS结果显示,ITO层的形成符合扩散限制聚集(DLA)模型,然后通过金属诱导效应形核、长大;而Ag层开始由反应控制聚集(RLA)形成,然后在热激发下,与附近的原子交换位置发生交换机制,同时避免了Ag粒子间的聚结长大。随着热处理温度的升高,Ag相的形核密度存在一个“V”型变化,转折点在900℃。热处理过程中,在升温阶段,多层薄膜中的Ag粒子/团簇的形核、生长符合扩散-换位-再扩散-再换位生长模型;而在保温阶段,Ag粒子/团簇的生长主要发生再换位过程,导致关联距离增大,分形维数降低。以十二烷基苯磺酸钠和氨水的混合液作为表面活性剂,对纤维材料进行表面处理,一定程度上提高了纤维表面的负电荷性,另外通过每层吸附后用乙醇冲洗的方式解决“挂胶”问题。纤维表面沉积后得到薄膜材料致密、均匀,且具有比较好的热稳定性。ITO薄膜的孔隙率对其2.5-7.5μm区间的半球反射率有决定性作用,但在孔隙率相差不大时,薄膜表面粗糙度对其的影响才表现出来。随着热处理温度的升高,Ag-ITO多层薄膜在1-2.5μm区间其半球反射率呈现“N”型变化,而随着保温时间的延长,其半球反射率略有降低,并运用激活隧道理论解释了此过程。可见,800℃的热处理温度是最佳处理温度。利用压片法得到镀膜纤维的透过率,发现随着热处理温度的升高以及保温时间的延长,其透过率都会下降。通过Rossland平均理论以及光厚近似假设,计算得到纤维隔热材料在沉积Ag-ITO多层膜后其辐射传热可以降低30%左右。更多还原

【Abstract】 Fibrous thermal insulation materials are of great interest in architectural and national defence fields owing to their low cost, low density and high efficiency. However, the high temperature insulation performance is decayed a lot due to the materials’high transmittance of radiation wave. In this research, silver embedded indium tin oxide (Ag-ITO) multilayer films were in situ deposited by sol-gel and layer-by-layer adsorption methods onto fibres to be as infrared reflective coating to decrease the radiation heat transfer. There are three main factors to influence the reflectivity, i.e. porosity, surface roughness and the spatial distributional state of Ag. The effects of thermal treatments including temperature, heating rate and holding time on the porosity, surface roughness of ITO film and Ag distribution of multilayer film were studied through scanning electron microscopy (SEM), X-ray diffraction (XRD), grazing incidence small angle X-ray scattering (GISAXS), grazing incidence X-ray reflectivity (GIXR) et al. Furthermore, the optical performance of films is obtained from the UV-VIS-NIR and FT-IR spectra. The main contents and conclusions are listed as follows:ITO films are prepared by sol-gel dip-coating method. ITO film is stannum-rich on the surface with the crystalline temperature of about 450℃. Pores in the films show fractal structure and ellipsoid along the film. There are three basic regions along the thickness of film. The layer access to the substrate shows dense and small pores while the middle layer shows higher porosity and bigger pores. When the temperature is higher than 800℃, the near surface layer even shows higher porosity and bigger pores than next while it show opposite when the temperature is not.The effect of thermal annealing process on the porosity of ITO films is studied through GISAXS technique. The densification is improved with the increase of annealing temperature. The porosity is decreased due to the shrinkage of big pores as well as elimination of small pores in low temperature in low temperatures and just the latter one in high temperatures. The porosity is decreased with the increase of heating rate due to the shorter crystalline time. The porosity will be increased due to the increasing near surface porosity with the holding time at high temperature. Thus, thermal treatments of ITO film were optimized to be that slow heating during the volatity of solvent and then high heating.Ag-ITO composite films were prepared by layer-by-layer adsorption method with an in situ reducing agent (stannuous chloride). The favorite concentration of silver sols is 0.1M to keep the continuous,small and not aggregated. It is found that the ITO is diffused through diffusion-limited aggregate dynamic and metal introduced formation. The silver atoms are controlled by exchange-reaction limited aggregation mechanism and the coalescence of silver particles is inhibited. The nucleation density of silver phase shows“V”with the increase of thermal annealing temperatures of composite films. The knee point is at 900℃which means an active temperature of silver atoms. During the heating period in thermal annealing process, the formation and growth of silver particles according to the diffusion-exchange-diffusion-exchange model, and during the holding period, the exchange mechanism increases the correlation distance of silver particles and decreases the fractal dimension.Films were prepared on the surface of fibers through layer-by-layer adsorption method. In order to increase the electronegativity of fibers, fibers were treated by a solution including sodium dodecylbenzene sulfonate and ammonia. Films on the fibers show dense, homogenous and have thermal stability. The porosity and surface roughness both influence the hemisphere reflectivities of ITO films. The former factor is major and the latter is minor. While the hemisphere reflectance of Ag-ITO multilayer film during 1-2.5μm shows“N”curves. This phenomenon is explained by activation tunnel effect theory. The best thermal annealing temperature is proved to be 800℃. The transimittance of coated fibers are decreased with the increase of annealing temperature and holding time through pelleting technique. The reduction of radiation heat transfer could be up to 30% after the coating on the fibers through Rosseland mean theory.更多还原