摘要
LaB6热电子发射阴极材料不仅具有很好的导热性和导电性等金属性能,而且具有高熔点、高硬度和高化学稳定性等陶瓷性能,这些优异性能使其具有广阔的应用前景。最近研究者发现具有自由电子的局域表面等离子体共振(LSPR)效应的六硼化镧(LaB6)纳米粉末对太阳光中的近红外辐射有着明显的吸收和散射,因而被认为是一种新型窗用隔热节能材料,使其成为继ATO、ITO之后光电研究领域又一热门的研究课题。在理论方面本文采用基于密度泛函理论的第一性原理的从头算分子动力学方法系统地计算了LaB6基态的电子结构、态密度和光学性质以及温度、外压下LaB6电子结构等性质进行了研究；另外,实验上采用微波固相合成法在较低温度下合成了六硼化镧(LaB6)纳米粉末,制备了LaB6纳米粉末PVB涂层,对其光学性能和隔热效果进行了探讨。主要研究内容及其结果如下：
1.理论计算结果表明：
①.利用密度泛函理论计算了LaB6的电子结构、态密度和光学性质。能带结构分析表明LaB6是一种导体,其价带主要由B的2p态电子构成,导带主要由La的5d6s态电子构成；静态介电常数ε1(0)=213.7,折射率n(0)=14.803,吸收系数在可见光范围内最小波谷为21585.2cm-1。
②.理论透光率在紫外和近红外几乎为零的情况下,LaB6对可见光有很高的透光率,呈“吊铃型”分布；理论透光率与温度呈非线性关系,在300K理论透光率最大,为57.6%。研究结果为LaB6光电材料的设计和应用提供了理论依据。
③.计算了外压调制下LaB6的电子结构和光学性质,结果表明：当外压增大到一定程度时LaB6的光学性质会发生突变。
④.LaB6和CaB6光学性质的本质区别取决于电子结构中La 5d和Ca 3d对其自身材料的贡献。
2.实验结果表明：采用涂覆法制备了LaB6/PVB透明隔热涂层。在LaB6含量为0.05%时,其在可见光区(380-780 nm)透射率可达79%,而近红外光区(780-1000 nm)的屏蔽率为54%。结果说明含纳米LaB6的PVB涂层对近红外光有明显的阻隔作用。在红外灯下照射60min后,透明隔热玻璃的箱体温度与空白玻璃之间的箱体温差可达2.5℃,隔热效果显著。
Lanthanum hexaboride (LaB6) as an excellent thermionic electron emission source combines unusual properties of both metals and ceramics. Like metals, it is a good thermal and electrical conductor. And like ceramics, it is elastically stiff, exhibits excellent chemical stability. The excellent properties mentioned above make it a technically important material. Recently, researches discovery that LaB6 nanopowder which has obvious absorbing and scattering effect to NIR due to the localized surface plasmon resonance (LSPR) of conduction electrons is believed to be a new energy-saving and heat-insulating material for use with windows. Many investigations have been focusing on it in the field of photoelectronics. The electronic structure, optical properties and electronic structures under temperature or press of LaB6 were investigated theoretically by using the first-principle density function theory ab initio molecular dynamics method based upon the Density Functional Theory (DFT) using the CASTEP package of in this thesis. In addition, the synthesis of LaB6 nanopowder by solid-state reaction at a low temperature through microwave heating was studied. Meanwhile, PVB coatings containing LaB6 nanopowder was prepared and the optical performance was investigated. The main contents are as the following:
1. The electronic structure and optical properties of LaB6 have been calculated using the first-principle density function theory.
1) The calculations of band structure show that CaB6 is a conductor material.The density of state is mainly composed of La 5d 6s and B 2p. The static dielectric functionε1(0) is 213.7; the reflectivity n(0) is 14.8, the minimum trough of absorption coefficient is 21585.2 cm-1 in the visible light region; Moreover, the complex dielectric functions, reflectivity, absorption, refractive index, energy loss function and extinction coefficient of LaB6 are analyzed in terms of calculated band structure and density of state.
2) Theoretical transmittance of LaB6 is nearly equal to zero in the UV and NIR, but visible light transmittance is very higher and its shape looks like "a hanging bell"; Theoretical transmittance of LaB6 is nonlinear with its temperature, the maximum value of theoretical transmittance is 57.6% at 300K. The results offer theoretical data for the design and application of the optoelectronic material LaB6.
3) The electronic structure and optical properties of LaB6 is calculated at press, the results show that optical properties of LaB6 changes abruptly when the external pressure increases to a certain extent.
4) The essential difference of optical properties of LaB6 and CaB6 depends on La 5d and Ca 3d of its own electronic structure which make contribution on themselves.
2. Polyvinyl butyral resin (PVB) transparent thermal insulation coatings containing LaB6 nanoparticles were obtained by coating method. When the ratio of LaB6 is 0.05%, the transmittance is about 79% at visible light zone (380-780 nm) and the shielding efficiency of near-infrared radiation (780-1000 nm) was 54%, thus the PVB coatings containing LaB6 nanoparticles can block near infrared radiation. The temperature of the chamberloaded with the insulating glass was lower than that of the one with the conventional glass for 2.5℃under the irradiation of iodine-tungsten lamp for 60 minutes. Thus LaB6 nanoparticles can significantly reduce solar heat gain.
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