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.
引文
[1]Paderno Y B, Paderno V, Filippov V. Some crystal chemistry relationships in eutectic crystallization of d-and f-transition metalsborides[J]. J Alloy Compounds, 1995, (219):116-118
[2]Craciun V, Criciun D. Pulsed laser deposition of crystalline LaB6 thin films[J]. Applied Surface Science,2005, (247):384-389
[3]郑树起,闵光辉,于化顺等. LaB6功能陶瓷材料的研究现状[J].材料导报,2000,14(3):50-51
[4]朱炳金,陈泽祥,张强.六硼化镧薄膜的制备及发射特性的研究[J].真空电子技术,2007,05:44-47
[5]王小菊,蒋亚东,林祖伦等.单晶六硼化镧的制备及主要应用[J].材料导报网刊,2006,2:13-15
[6]Schelm S, Smith G B. Dilute LaB6 nanoparticles in polymer as optimized clear solar control glazing [J]. Applied Physics Letters,2003,24:4346-4348
[7]Lafferty J.M. [J].Appl Phys,1951,22:299-309.
[8]Nagao T.et al. [J].Surf Sci,1993,290:287-288.
[9]Structure and chemical bond characteristics of LaB6 Physica.[J].B404.4086-4089
[10]Paderno V N, Paderno YB,Pilyankevich A N,et al. The Micro2mechanical Properties of Melted Boride of Rare Earth Metals [J]. Journal of the Less 2 Common Metals,1979, (67):431-436.
[11]Takahashi Y, Nitobe K,Momose T, et al. Aluminum oxide thin film deposition by reactive ion plating using the cathode system composed of LaB6 disc and Ta pipe [J].Journal of Vacuum Science Technology,1993,11 (4):1491 - 1495.
[12]Mushiaki M,Akaishi K, Mori T,et al. LaB6 coating to reduce the outgasing rate of a vacuum wall [J]. Materials Science and Engineering,1993, (163):177-179.
[13]Futamoto M,Nakazawa M,Hosoki S, et al. Thermionic emission properties of a single2crystal LaB6 cathode[J]. J.Appl. Phys.,1980,51 (7):3869.
[14]Shimizu R,Onoda H, Hashimoto H. Oxygen2enhances thermionic emission pat tern of hemispherical single2crystal LaB6 cathode [J]. J. Appl. Phys.,1984, 55 (5):1379-1387.
[15]Furukawa Y, Yamabe M, Inagaki T. Emission character istics of single crystal LaB6 cathodes with large tip radius [J]. J. Vac. Sci. Technol,1983,1(3); 1516-1521.
[16]Furukawa Y, Yamabe M, Itoh A, et al. Emission char2 acteristics of single2crystal LaB6 cathodes with< 100>and< 110> orientations [J]. J. Vac. Sci. Technol.,1982,20 (2):199.
[17]Zheng shuqi, Min Guanghui, Yu Huashum,et al. Present status of research on LaB6 functional ceramic matereials [J]. Journal of Materials,2000,14:50.
[18]成建波等,六硼化镧阴极[M].四川:成都电讯工程学院出版社,1998.
[19]Lung K N,Pincosy P A, et al. [J].Rev Sci Instru,1986.57(7):165.
[20]Nakasuji M, Wade H. [J].Vac Sci Technol,1980,17(6):73.
[21]芫川二郎等,稀土的最新应用技术[M].北京:化学工业出版社,1993.
[22]Schelm S, Smith G B. Tuning the surface-plasmon resonance in nanoparticles for glazing applications. Appl.Phys,2005,97(12),4314(1-8).
[23]Takeda H, Kuno H, Adachi K. Solar control dispersions and coatings with rare-earth hexaboride nanoparticles. Am. Ceram. Soc.2008,91(9):2897-2902.
[24]Kenji A, Mitsunobu M, Tsuyoshi A. Absorption and scattering of near-infrared light by dispersedlanthanum hexaboride nanoparticles for solar control filters[J].Materials Research Society,2010,25(3):510-521.
[25]李申生.太阳常数与太阳辐射的光谱分布[J].太阳能,2003,4:5-6.
[26]樊慧庆,韩志江,史运则等.纳米金属粉体制备及其表面等离子共振应用[J].功能材料,2007,38:2015-2017.
[27]住友金属矿山株式会社.包含热辐射屏蔽组分的母料及其使用该母料的热辐射屏蔽透明树脂成形物和热辐射屏蔽透明层压材料.CN.1477150A,2003
[28]W. K费施尔,红外吸收性聚乙烯醇缩丁醛组合物,由此制成的片材以及包含此片材的层合材料.中国专利,CN92908-A,2001-11-14
[29]梁丽梅,LaB6纳米粉末及其复合薄膜的制备工艺及性能研究,硕士论文,2009.
[30]顾广新,武利民,魏勇,游波.一种透明隔热涂敷材料及其制备方法和应用. 中国专利,CN101550307-A,2009-5-14.
[30]姚晨,赵石林,缪国元,纳米透明隔热涂料的特性与应用,[J].涂料工业,2007,01.
[31]张永进,赵石林.聚氨酯纳米ATO透明隔热涂料的研制[J].化学建材,200420(6):67-71.
[32]陈飞霞.纳米氧化铟锡透明隔热涂料的制备及性能研究.南京工业大学硕士学位论文,2003.
[33]姚晨,赵石林,缪国元,纳米透明隔热涂料的特性与应用,[J].涂料工业,2007,01
[34]G. Savineau, Responsive glazing for solar control shading, International Conference "Passive and Low Energy Cooling for the Built Environment", May 2005, Santorini, Greece.
[35]江晴等人,灰色热发射涂料的降温极限与展望,[J].红外技术1998,(5):38.
[36]倪建刚,第一性原理研究BaTiO_3-ZnO异质结构的电子特性.电子科技大学硕士学位论文,2008.
[37]徐仁军,冰在高压下的相变及电子和光学性质研究.吉林大学硕士学位论文,2004.
[38]Xu G L, Chen J D, Xiao Y Z. First-Principles Calculations of Elastic and Thermal Properties of Lanthanum Hexaboride. Chin.Phys.Lett.2009,26(5): 056201(1-4).
[39]Segall M D, Philip Lindan J D, Probert M J, et al. First-principles simulation: Ideas, illustrations and the CASTEP code. Phys Rev Condense Matter,2002, 14(11):2717-2744.
[40]Perdew J P, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phy. Rev. Lett,1996,77:3865-3868.
[41]Zhang M F, Yuan L, Wang X Q, et al. A low-temperature route for the synthesis of nanocrystalline LaB6. Journal of Solid State Chemistry 2008,181:294-297.
[42]崔冬萌,谢泉,陈茜,等Ru2Si3电子结构及光学性质的第一性原理计算.中国科学G辑:物理学力学天文学2009,39(10):1431-1438.
[43]沈学础.半导体光谱和光学性质.第二版.北京:科学出版社,2002.76-94.
[44]连景宝,李星,肖冰,等La2O2S电子结构及光学性质的第一原理研究.材料导报,2008,22(8):116-119.
[45]Kino,H.;Aryasetiawan,F.;Terakura,K.;Miyake,T. Abnormal quasiparticle shifts in CaB6, Phys.Re.B 2002,66,121103 (R).
[46]Kino, H; Aryasetiawan, F; vanSchilfgaarde, M. GW quasiparticle band structure of CaB6 J.Phys.Chem.Solids 2002,63,1595-1597.
[47]ButlerJ W. Neutron-absorbing bricks made from CaB6. Nuclear Instruments and Methods,1960,7(2):201-203.
[48]王雷刚.聚乙烯醇缩丁醛的合成与应用.大连理工大学硕士学位论文,2008
[49]樊慧庆,韩志江,史运则等.纳米金属粉体制备及其表面等离子共振应用[J].功能材料,2007,38:2015-2017.
[50]殷澄.表面等离子体共振理论和实验研究.南京理工大学硕士学位论文,2007.