VO2（M）控温包装薄膜制备的研究进展

二氧化钒(VO_2)纳米颗粒具有多种晶型结构,其单斜型半导体M相(VO_2(M))在接近室温时具有完全可逆的相转变性质,相变为四方金红石金属R相,从而引起光学性能的急剧变化,因而VO_2(M)可作为良好的阻热材料应用于控温包装薄膜领域。综述了VO_2(M)纳米颗粒和控温包装薄膜的制备方法,展望了VO_2(M)控温包装薄膜的研究方向。VO_2(M)纳米颗粒的传统制备方法有一步水热法和热分解法等,近年来也出现了新型制备方法,如籽晶诱导法和火焰燃烧法等;控温包装薄膜主要的制备方法为气相法和液相法。根据不同产品的温度需求,通过改变原料种类、温度及掺杂元素等工艺参数调节VO_2(M)纳米颗粒的相变温度,可制备出不同相变温度的VO_2控温包装薄膜。VO_2(M)控温包装薄膜作为一种高新科技产物,可以有效调控商品运输和储藏过程中包装内氛围,具有广阔的应用前景。     

W掺杂二氧化钒的水热晶化机理及其相变性能

以硫酸氧钒为钒源,采用沉淀-胶溶法制备VO_2溶胶。然后向溶胶中加入偏钒酸铵,利用溶胶水热晶化制备出W掺杂二氧化钒(W-VO_2,M相)粉体。通过XRD,FESEM和DSC对合成产物的物相组成、形貌和相变性能进行研究。结果表明:在280℃条件下水热处理4~48h,VO_2溶胶经过水热晶化生成长约1~2μm、直径约100~200nm棒状W-VO_2(B)晶体,伴随着B相向M相晶型转变,W-VO_2(B)逐渐消溶,而W-VO_2(M)逐渐长大,形貌由棒状转变为片状或雪花状;W-VO_2(M)相变温度随着W掺杂量增加而降低,当名义掺杂量为6.0%(原子分数)时,相变温度降低到28℃。根据水热晶化和形貌演变过程,提出了W-VO_2(M)可能的"形核-生长-转化-熟化"形成机理。     

二氧化钒薄膜的制备及光电性能研究进展

二氧化钒(M/R相)作为一种典型的热致相变材料,在诸多领域都有着广阔的应用。仅在68℃左右便可发生高温金属相-低温半导体相的完全可逆相变,且相变前后材料的光学、电学等特性均会发生明显变化。基于该特性,二氧化钒可应用于设计各种近红外和中红外调制器件,如“智能窗”、光学器件、军事防护器件等,并具有极高的实用价值。二氧化钒热致变色性能的优劣在很大程度上取决于薄膜的合成方法和制备过程中的参数调控,首先总结了关于二氧化钒相变机理的探索研究,其次重点概述了近几年二氧化钒薄膜制备方法的研究进展,包括磁控溅射法、脉冲激光沉积法、溶胶-凝胶法、分子束外延法和溶剂热/水热法等,并讨论了各种制备技术的优缺点。另外,在改善薄膜的热致变色性能方面,总结概述了掺杂和复合工艺对薄膜性能的影响。最后,对二氧化钒薄膜存在的问题及其未来的研究及应用方向进行了讨论与展望。     

微波辅热-甲醇还原法制备M相二氧化钒粉体

为获得具有良好相变特性的M相VO_2粉体,本文以V_2O_5为原料、甲醇作还原剂,采用微波辅热制备M相VO_2粉体.探讨了微波辅热功率、填充度、反应温度、反应时间、甲醇用量因素对制备VO_2的影响,采用X射线衍射(XRD)、差示扫描量热法(DSC)和扫描电子显微镜(SEM),对产物进行了物相、相变和形貌特征的表征.结果表明:在微波辅热功率为1 000 kW,填充度为0.7,反应温度为250℃时,反应时间为60 min,V_2O_5质量与甲醇体积比为5∶3时,得到B相的VO_2粉体;B相的VO_2再经过800℃高温热处理后完全转化为具有相变功能的M相,M相VO_2粉体纯度为99.20%,产率为92.30%,相变点温度为68.6℃,晶体形貌呈现较匀称的棒状结构,长度为1～2μm.     

晶格畸变对VO_2相变温度的影响

采用高功率脉冲磁控溅射在石英玻璃基片上成功制备了具有明显金属-绝缘体转变特性的多晶VO_2薄膜,其最低相变温度仅为32℃。X射线衍射结果表明沉积薄膜的晶体结构均为存在明显晶格畸变的VO_2(M),且薄膜(011)晶面间距越接近相变后VO_2(R)(110)晶面的晶面间距,相变温度越低。根据试验结果,利用从头算分子动力学分别对VO_2金属-绝缘体相变过程的晶体结构与态密度演化规律进行了研究。结果表明:随计算温度升高,不同(011)晶面间距绝缘态超胞的晶体结构均逐渐由VO_2(M)向VO_2(R)转变,同时伴随着禁带宽度的逐渐降低,最终转变为费米能级完全被电子占据的金属态;初始VO_2(M)超胞(011)面的晶面间距与相变后VO_2(R)(110)面的晶面间距之差越小,费米能级附近的禁带宽度也越小,这可能是导致VO_2金属-绝缘体转变温度降低的本质物理原因。     

热致变色二氧化钒薄膜的研究进展

二氧化钒薄膜具有优异的热致变色特性,已成为功能材料领域研究的热点.结合二氧化钒的结构分析了其热致变色特性;综述了二氧化钒薄膜的制备方法,着重评述了溅射法、化学气相沉积法及溶胶-凝胶法等几种常用方法;阐述了二氧化钒薄膜在智能窗、新兴光子晶体、伪装隐身技术方面的应用前景;最后指出了其今后的研究与发展方向.     

二氧化钒热致变色薄膜的制造策略与先进应用

智能包装是印刷与包装行业新的发展方向,实现了包装与消费者高效便捷的信息交互。智能包装材料的研发以实现不同功能的智能包装技术为基础,其中二氧化钒（VO2）是制备热致变色膜的主要材料之一,广泛应用于智能隔热控温材料领域。在VO2的多种晶形结构中,具有半导体特性的M相在68 ℃ 时可逆相变为金属R相,能够阻挡太阳光波中的红外波段以达到主动调控温度的目的。为此,对VO2热致变色薄膜的相变温度及光学性能的调控方法进行综述,调控方法主要以层状结构为研究基础,通过元素掺杂、不同材料复合及表面改性等方式改善VO2薄膜的性能,这种智能包装材料可以有效调控产品所处的环境温度,因此更加适用于日常生活,并为产品质量提供保证。同时,展望了VO2薄膜的应用发展趋势。     

掺钨二氧化钒薄膜的制备与分析

通过凋研国内外的各种制备方法,比较它们的优缺点后,选用磁控溅射法.在硅片上得到了电阻变化2个数量级的二氧化钒(VO2)薄膜.对薄膜进行电学性能的测试,结果表明:掺钨后二氧化钒薄膜的相变温度比纯的二氧化钒薄膜相变温度有所降低,掺钨后薄膜的近红外透射率也随之减小.通过X射线衍射和X射线光电子谱对薄膜的微观结构和组分进行了分析.     

MBE技术蓝宝石衬底上生长VO_2薄膜及其太赫兹和金属–绝缘体相变特性研究（英文）

采用分子束外延(MBE)技术在单晶蓝宝石衬底上生长了高质量化学计量比二氧化钒(VO_2)薄膜,通过该技术实现薄膜厚度15~60 nm精确控制。对于优化条件下VO_2薄膜,实现了电阻率变化超过4个数量级的优异金属–绝缘体相变,近似于之前报道高质量单晶VO_2相变特性。特别是通过太赫兹时域光谱分析了不同厚度的VO_2薄膜在太赫兹波段的光学特性。结果表明:VO_2薄膜的厚度对其在太赫兹波段的光学特性有很大影响。因此,为了获得更优的可靠性和重复性能,VO_2薄膜的厚度必须得到精确控制。本研究结果对于下一步VO_2基太赫兹器件研究具有重要意义。     

微波等离子体低温制备氮杂二氧化钒薄膜

选用V2O5为前驱物,通过在玻璃片上镀膜,采用微波等离子体增强法,在低温条件下,成功制备了氮杂二氧化钒薄膜.通过X射线衍射(XRD),FT-IR对样品进行表征,结果表明:合成的样品为多晶氮杂二氧化钒.相变温度测试结果表明:退火工艺可以降低相变温度,同时提高薄膜的结晶度;改变氮气流量,相变温度先降低后升高,当氮气流量为20ml/min时,相变温度可以降低至40℃.     

Oxygen pressure dependent VO2 crystal film preparation and the interfacial epitaxial growth study

High quality VO₂ crystal films have been prepared on sapphire substrates by pulsed laser deposition method and the effects of oxygen pressure on the crystal phase structure are investigated. Results indicate that the phases and microstructures of VO₂ films are strongly sensitive to oxygen pressure. High oxygen pressure tends to form coarse B-VO₂ nanocrystals while low pressure favors a flat M1-VO₂ film epitaxial growth. X-ray diffraction φ-scan patterns confirm the [020] epitaxial growth orientation of the M1-VO₂ film and the in-plane lattice epitaxial relationship at the interface is also examined. Raman spectra indicate that M1-VO₂ phase has much stronger Raman scattering modes than B-VO₂, and the clear phonon modes further confirm the idea stoichiometry of VO₂ crystal film. Infrared transmittance spectra as the function of temperature are recorded and the results show that M1-VO₂ crystal films undergo a distinct infrared transmittance variation across metal insulator transition boundary, while B-VO₂ exhibits negligible thermochromic switching properties in the temperature range concerned. The pronounced phase transition behavior of the M1-VO₂ crystal film makes it a promising candidate for optical filter/switch and smart window applications in the future.     

Synthesis,characterization, and thermodynamic parameters of vanadium dioxide

A novel process was developed for synthesizing pure thermochromic vanadium dioxide (VO₂) by thermal reduction of vanadium pentoxide (V₂O₅) in ammonia gas. The process of thermal reduction of V₂O₅ was optimized by both experiments and modeling of thermodynamic parameters. The product VO₂ was characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TG), and differential scanning calorimetry (DSC). The experimental results indicated that pure thermochromic VO₂ crystal particles were successfully synthesized. The phase transition temperature of the VO₂ is approximately 342.6 K and the enthalpy of phase transition is 44.90 J/g.     

Characteristics of CeOx–VO2 composite thin films synthesized by sol–gel process

Pure vanadium dioxide (VO₂) and CeOx–VO₂ (1.5 < x < 2) composite thin films were grown on muscovite substrate by inorganic sol–gel process using vanadium pentaoxide and cerium(III) nitrate hexahydrate powder as precursor. The crystalline structure, morphology and phase transition properties of the thin films were systematically investigated by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, FE-SEM and optical transmission measurements. High quality of the VO₂ and CeOx–VO₂ composite films were obtained, in which the relative fractions of +4 valence state vanadium were above 70 % though the concentrations of cerium reached 9.77 at %. However, much of cerium compounds were formed at the edge of grains and the addition of cerium resulted in more clearly defined grain boundaries as shown in SEM images. Meanwhile, the composite films exhibited excellent phase transition properties and the infrared transmittance decreased from about 70 to 10 % at λ = 4 μm bellow and above the metal–insulator phase transition temperature. The metal–insulator phase transition temperatures were quite similar with about 66 °C of the pure VO₂ and CeOx–VO₂ composite thin films. But hysteresis widths increased with more addition of cerium, due to the limiting effect of grain boundaries on the propagation of the phase transition. Particularly, the CeOx–VO₂ composite film with an addition of 7.82 at % Ce showed a largest hysteresis width with about 20.6 °C. In addition, the thermochromic performance of visible transmittance did not change obviously with more addition of cerium.     

Recent progress in VO2 smart coatings: Strategies to improve the thermochromic properties

Vanadium dioxide (VO₂) has attracted a great interest for smart coating applications because of its promising thermochromic properties. Thermochromic performance of VO₂ is closely related to the phase composition and the microstructure, which are largely dependent on the synthesis method and growth control. This review summarizes the recent progress in fabrication of VO₂ by gas deposition. Representative deposition techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), sol–gel and chemical solution methods and their relative merits are discussed. To be practically applicable, high-performance thermochromic VO₂ films are desired, often featured with a suitable phase transition temperature (T_c), high luminous transmittance (T_lum) and good modulation capability of solar energy (ΔT_sol). Focused on the strategies used to improve thermochromic properties, this review also covers topics such as multilayer construction, elemental doping, substrate selection, and structure modification. Some theoretical progresses in understanding thermochromic coatings, including phase transition mechanism and energy modeling are also provided. Although significant progress has been made in improving the thermochromic performance of VO₂ films, challenges are still present, particularly in commercial applications. Discussions on future trend and perspectives, as well as some important issues, of VO₂ films used as smart coatings will be given finally.     

Structural, electrical and optical properties of thermochromic VO2 thin films obtained by reactive electron beam evaporation

We present the structural and physical characterization of vanadium dioxide (VO₂) thin films prepared by reactive electron beam evaporation from a vanadium target under oxygen atmosphere. We correlate the experimental parameters (substrate temperature, oxygen flow) with the films structural properties under a radiofrequency incident power fixed to 50 W. Most of the obtained layers exhibit monocrystalline structures matching that of the monoclinic VO₂ phase. The temperature dependence of the electrical resistivity and optical transmission for the obtained films show that they present thermoelectric and thermochromic properties, with a phase transition temperature around 68 °C. The results show that for specific experimental conditions the VO₂ layers exhibit sharp changes in electrical and optical properties across the phase transition.     

VO2 thin films deposited on silicon substrates from V2O5 target: Limits in optical switching properties and modeling

Thermochromic VO₂ thin films presenting a phase change at T_c = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V₂O₅ targets. Depending on deposition conditions, a non-thermochromic metastable VO₂ phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at T_c: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO₂ film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C.     

Hydrothermal Synthesis of VO2 Polymorphs: Advantages,Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long‐standing issues that hindered its application in energy efficient windows: high τ_c, low luminous transmittance (T_lum), and undesirable solar modulation ability (ΔT_sol). Many approaches, including nano‐thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach—nano‐thermochromism—which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high‐quality VO₂ nanoparticles, and has its own advantages of large‐scale synthesis and precise phase control of VO₂. This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂(M), and discusses the advantages, challenges, and prospects of VO₂(M) in energy‐efficient smart windows application.     

Study on a terahertz modulator based on metamaterial with photoinduced vanadium dioxide film

Abstract

Applying the photoexcitation characteristics of vanadium dioxide (VO₂), a dynamic resonant terahertz (THz) modulation with the combination of a VO₂ film and a metamaterial was suggested to realize THz wave active manipulation. The designed metamaterial with structured copper rings arrays can realize a passband from 0.776 to 1.045 THz. When insulator–metal phase transition in VO₂ thin film, which is deposited on the other surface of the metamaterial substrate, is induced by optical pumping, the metamaterial/VO₂ film hybrid structure behaves as an absorber with absorption rates of 90% at 0.88 THz and the transmission energy decrease to less than 3%. Therefore, about 78% modulation depth and more than 250 GHz modulation bandwidth have been reached under the photoinducing. The simulation results illustrate the promise of using phase transition materials for efficient broadband fast response modulators for THz waves.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.