Bi-Te基薄膜热电材料的研究进展

热电材料是一种能够实现热能与电能直接转换的功能材料,由于无法有效降低块体热电材料的热导率,其性能研究进展缓慢.自上世纪90年代初Hicks等提出了低维化能够显著提高热电材料性能的理论后,薄膜热电材料开始受到广泛关注.低维化提高材料性能的原因主要是材料在低维化后能够产生量子限制效应,使得电子在被压缩维度的运动受到限制.首先,在费米能级附近,与Seebeck系数呈正相关的电子态密度会增大,导致低维热电材料的Seebeck系数相比块体材料显著增大.其次,与块体材料相比,薄膜材料存在更多能够散射声子的晶界,能有效降低晶格热导率.在这两种效应的共同作用下,材料的热电优值(ZT值)能够显著增大.低维热电材料的研究初期主要是通过数学模型和数值计算,从理论上证明量子效应会影响材料的Seebeck系数和电导率,且能实现二者的独立控制,从而提高材料的ZT值.后期的实验数据证明,通过合适的热处理工艺能够有效降低薄膜材料的缺陷,提高其综合性能.因此,热处理工艺的改进对性能的提升也非常重要.热电材料性能的提升离不开制备工艺的进步.为了获得低维化的热电材料,多种薄膜材料制备工艺被用于样品的制备,且不同的制备工艺各有优缺点.Bi-Te基合金不仅可用于低温发电还可用于低温制冷,是目前应用最广泛的低温热电材料,虽然其块体状态下的热电性能研究已趋于完善,但其薄膜状态下热电性能的理论研究还相差甚远,因此Bi-Te基低温薄膜热电材料成为研究热点.本文介绍了国内外采用不同制备工艺生长Bi-Te基热电薄膜材料的发展状况以及热电性能测试方法,提出了在目前发展薄膜热电材料时需要重点关注的方面,并对低维热电材料的发展方向进行了阐述.     

薄膜材料导热行为及其测试和预测

薄膜材料在集成电路,光电子技术,微结构传感器等微电子元件的应用日益广泛,其导热性能直接影响元器件的热噪声,进而对其可行一和使用性能产生明显影响,薄膜材料导热性能及其测试研究愈益受人瞩目,为此,本文对薄膜材料导热性能及各种测试方法进行了综述,并在分析薄膜微结构模型的基础上,对计算薄膜有效热导率的不同预测议程进行了评述,从而可为薄膜材料的制备工艺和性能变化提供技术判据。     

石墨烯导热材料研究进展

石墨烯作为一种具有超高热导率的二维纳米材料,在导热领域有着广阔的应用前景.本文综述了石墨烯导热材料的研究进展,介绍了石墨烯本征热导率及其层数、缺陷、边缘情况等对热导率的影响,分析了石墨烯纤维的研究现状及存在的问题,讨论了各类石墨烯导热薄膜(纯石墨烯薄膜/石墨烯杂化薄膜/石墨烯聚合物复合薄膜)热导率的影响因素,归纳总结了各类三维石墨烯导热材料(无规分散石墨烯三维复合材料和特定结构石墨烯三维复合材料)的结构、性能与研究现状,最后指出了目前几种导热材料研究存在的问题并展望了石墨烯未来导热领域的发展方向,尤其是在LED照明、智能手机等高功率、高度集成系统中,石墨烯导热材料有着良好的发展前景.     

有机-无机透明导电薄膜的研究进展

透明导电膜是一种重要的光电材料,应用广泛.以传统的无机氧化物和导电高分子材料制备的导电薄膜虽然电学性能优良,但综合性能不甚理想.通过有机-无机杂化的透明导电薄膜能够借助各组成材料特性的互补作用使复合材料具有出色的综合性能.介绍了透明导电薄膜的种类、用途和特性,综述了有机-无机杂化导电材料在块体材料和透明导电薄膜方面的应用,并展望了其发展前景.     

贵金属钌的制备、性能及应用研究进展

贵金属钌在现代工业中具有广泛的应用.本文首先介绍了钌块体的制备方法,对钌块体的塑性、机加工性能及其影响因素进行了分析,介绍了X衍射测定钌块体内应力的方法;其次,综述了钌薄膜的制备方法,介绍了钌薄膜在集成电路中作为铜的粘结层/扩散阻挡层、磁记录媒介中作为中间层/籽晶层、抗氧化保护层材料及电接触材料等方面的应用;最后,总结了目前钌研究的不足之处,并对其未来发展进行了展望.     

纳米复合材料力学性能的研究

本文从薄膜和块体材料两个方面介绍了近年来有关纳米复合材料力学性能的研究成果。综述了薄膜和块体纳米复合材料优异的力学性能及其产生机制 ,并指出了进一步研究方向     

纳米复合材料力学性能的研究

本文从薄膜和块体材料两个方面介绍了近年来有关纳米复合材料力学性能的研究成果。综述了薄膜和块体纳米复合材料优异的力学性能及其产生机制,并指出了进一步研究方向。     

薄膜热导率测量方法研究进展

薄膜材料对微纳器件制造不可或缺,而其热导率直接限制微纳器件的散热性能,从而影响器件的可靠性。因此,研究薄膜热物理性质对于半导体器件的制造以及集成电路的设计极为重要。为此,对薄膜材料热导率测量方法进行了综述,并在分析薄膜微结构模型的基础上,对热导率测量方法进行了可行性分析,从而为薄膜材料热性能测量提供技术参考。     

金刚石薄膜热导率的研究现状

分析了金刚石薄膜热导率的声子导热机理,从金刚石薄膜晶体结构的角度总结了影响热导率的因素,例如晶界、晶内杂质、缺陷、晶粒尺寸、晶粒取向、同位素;从金刚石薄膜合成工艺角度探讨了影响金刚石薄膜热导率的因素,例如基底材料及预处理、合成温度、气源及添加气体(氧气、氮气)、工作压强、功率等.     

Bi2Sr2CaCu2O8＋x单晶的热导率测量

介绍了有关高温超导材料的热导率性质、测试方法和测试结果。结合我们的测试经验，讨论了稳态热流法测量高温超导体热导率应注意的问题。给出了Ｂｉ２Ｓｒ２ＣａＣｕ２Ｏ８＋ｘ单晶沿ａｂ面热导率的测试结果。     

Experimental determination of the thermal conductivity of thin films

Two techniques have been developed to determine experimentally the thermal conductivity of thin solid films of thickness 500 Å or more at low and high temperatures. The first technique is a steady state and is suitable for measurements above room temperature. The method enables the thermal conductivity of eight film specimens to be measured simultaneously. The second technique is a transient one (an adaptation of Ioffe's method for bulk materials) and is suitable for measurements in the temperature range 100–260 K. The two techniques have been used to make measurements of thin films of copper and various crystalline and amorphous semiconductors. The values of the thermal conductivity for thick copper films by both techniques agree quite well with the bulk values.     

Thin-Film Thermal-Conductivity Measurement on Semi-Conducting Polymer Material Using the 3ω Technique

Organic solar cells have gained increasing interest in recent years due to their promising low-cost processing possibility and high throughput compared to inorganic solar cells. Since the efficiency of organic solar cells is still low, further optimization has to be done. Reliable simulation of solar cell layout and performance strongly depends on correct input data of the electrical and thermal transport properties of the applied film materials. In many cases these material properties are only available for bulk material if available at all. Owing to the given film thicknesses on the order of tenths to hundreds of nanometer and to the preparation methods, the properties of the used system can differ from the bulk material values. For determination of the thin-film thermal conductivity, only a few measurement methods are known to provide accurate results with one of them being the 3ω technique. It allows the determination of the thermal conductivity of bulk materials as well as thin films down to a thickness of around 50 nm. This study is part of an investigation on the influence of local hot spots, generated by defects in the active layer of organic solar cells, and on the charge carrier mobility as well as the propagation of the hot spot due to the thermal conduction of the material. Applying the 3ω technique, the effective thermal conductivity of solution-derived poly(3-hexylthiophene) thin films of different thickness on a common glass substrate was investigated.     

Thermal Diffusivity of Metallic Thin Films: Au, Sn, Mo, and Al/Ti Alloy

The thermal diffusivity of Au, Sn, Mo, and Al_0.97Ti_0.03 alloy thin films, which are commonly used in microelectromechanical (MEMs) system applications, is measured by two independent methods — the ac calorimetric and photothermal mirage methods. Both methods yield similar results of the thin-film thermal conductivity, but the uncertainty of the mirage technique is found to be relatively large because of the large temperature increase during the measurement. The measured thermal diffusivities of the thin films are generally lower than those of the same bulk material. Especially, the Al_0.97Ti_0.03 thin film shows a pronounced thermal conductivity drop compared with bulk Al, which is believed to be mainly due to impurity scattering. Comparison of the thermal conductivity with the electrical conductivity measured by the standard four-probe technique indicates that the relation of thermal and electrical conductivities follows the Wiedemann–Franz law for the case of Au and Sn thin films. However, the Lorentz number is significantly larger than the theoretical prediction for the case of Al_0.97Ti_0.03 and Mo thin films.     

Thermal Conductivity of AlN and SiC Thin Films

The thermal conductivity of AlN and SiC thin films sputtered on silicon substrates is measured employing the 3ω method. The thickness of the AlN sample is varied in the range from 200 to 2000 nm to analyze the size effect. The SiC thin films are prepared at two different temperatures, 20 and 500°C, and the effect of deposition temperature on thermal conductivity is examined. The results reveal that the thermal conductivity of the thin films is significantly smaller than that of the same material in bulk form. The thermal conductivity of the AlN thin film is strongly dependent on the film thickness. For the case of SiC thin films, however, increased deposition temperature results in negligible change in the thermal conductivity as the temperature is below the critical temperature for crystallization. To explain the thermal conduction in the thin films, the thermal conductivity and microstructure are compared using x-ray diffraction patterns.     

Thermal and Electrical Properties of a Suspended Nanoscale Thin Film

This paper reports on measurements of in-plane thermal conductivities, electrical conductivities, and Lorentz number of two microfabricated, suspended, nanosized thin films with a thickness of 28 nm. The effect of the film thickness on the in-plane thermal conductivity is examined by measuring other nanofilm samples with a thickness of 40 nm. The experimental results show that the electrical conductivity, resistance–temperature coefficient, and in-plane thermal conductivity of the nanofilms are much smaller than the corresponding bulk values from 77 to 330 K. However, the Lorentz number of the nanofilms is about two times that of the bulk value at room temperature, and even up to three times that of the bulk value at 77 K. These results indicate that the relation between the thermal conductivity and electrical conductivity of the nanofilms does not follow the Wiedemann–Franz law for bulk metallic materials.     

Theoretical Modeling of Obliquely Crossed Photothermal Deflection for Thermal Conductivity Measurements of Thin Films

A three-dimensional theoretical model has been developed to calculate the normal probe beam deflection of the obliquely crossed photothermal deflection configuration in samples which consist of thin films deposited on substrates. Utilizing the dependence of the normal component of probe beam deflection on the cross-point position of the excitation and probe beams, the thermal conductivity of the thin film can be extracted from the ratio of the two maxima of the normal deflection amplitude, which occurs when the cross-point is located near both surfaces of the sample. The effects of other parameters, including the intersect angle between the excitation and the probe beams in the sample, the modulation frequency of the excitation beam, the optical absorption and thickness of the thin films, and the thermal properties of substrates on the thermal conductivity measurement of the thin film, are discussed. The obliquely crossed photothermal deflection technique seems to be well suited for thermal conductivity measurements of thin films with a high thermal conductivity but a low optical absorption, such as diamond and diamond-like carbon, deposited on substrates with a relatively low thermal conductivity.     

Photoacoustic measurement of thermal properties of TiN thin films

Titanium nitride (TiN) thin films were prepared by reactive DC magnetron sputtering under different nitrogen flow rates and at constant substrate temperature as well as at constant nitrogen flow rate and at different substrate temperatures. Photoacoustic measurement of the thermal properties of the films revealed that the thermal diffusivity and thermal conductivity of the TiN thin films are significantly lower than the bulk values and that the grain size of the films has substantial influence on the thermal properties of TiN thin films. The thermal conductivity of the films decreases with increasing nitrogen flow rates and increases with increasing substrate temperature. The above opposing behaviour in the thermal properties is found to be related to the microstructure, especially, the grain size of the films.     

Surface and electrical studies of CuO:V2O5 thin films

Results from the studies of multicomponent CuO:V₂O₅ bulk material and thermally evaporated thin films of highly conducting bulk composition prepared at different substrate temperatures are thus compared and discussed. The electronic conductivity is enhanced on increase in the substrate temperature T_s and reaches a maximum value of 12.3 × 10⁻⁶Ω⁻¹ cm⁻¹ for T_s = 423 K. X-ray photoelectron spectroscopy studies indicate an increase in the reduced states of vanadium and copper ions in going from the bulk glass to the thin film. Dynamic secondary-ion mass spectroscopy studies on thin films over a depth of 3000 Å show a strong dependence of T_s on the Cu-to-V intensity ratio. Even though stoichiometric values for thin films are achievable by varying the T_s, the oxidation states of Cu in these films are predominantly monovalent. The electrical behaviors of these materials and their thin film counterparts are finally being discussed in relation to the surface analysis data.     

A Numerical Simulation to Propose a Flash Method for In Situ Detection of the Thermal Diffusivity of Anisotropic Thin Film Materials

Strong anisotropy of thermal diffusivity is frequently observed in thin film materials. We propose an in situ experimental method to remotely measure radial and axial components of the thermal diffusivity. The method is based on the traditional laser flash technique but is specialized to also highly challenging experimental situations such as sample manufacture and use phase when thin films may be exposed to very high pressures or temperatures and to high temperature gradients. The method requires laser pulses of very short duration and fast measurement of transient temperature excursions in only radial directions on the surface of the thin film samples. The accuracy of the method is checked by comparison with results from a finite element calculation for a graphite sheet with high anisotropic conductivity that simulates a thermo-physical experiment.