Simultaneous tuning of the phase transition temperature and infrared optical properties of Mo-doped VO2 powders for intelligent infrared stealth materials

The development of an intelligent infrared camouflage material whose infrared emissivity can actively adapt to environmental changes is a key frontier in the field of infrared stealth. In this study, Mo-doped VO₂ powder was prepared via a hydrothermal method, which led to an intelligent infrared camouflage material whose infrared radiation characteristic can adaptively change with the environmental temperature. The samples were characterized by XRD, SEM, DSC, FTIR and infrared thermal imaging. Combined with the results of the first-principles calculation, the coupling effect mechanism of Mo⁶⁺ doping concentration on the phase transition temperature and infrared photoelectric properties of VO₂ material was systematically analyzed. The results showed that Mo⁶⁺ impurities had significant effects on the structure, morphology, composition, phase transition temperature and infrared reflectivity of VO₂ powder. The doping process effectively reduced the phase transition temperature of VO₂ and expanded the change range of infrared emissivity (△ε) before and after the metal-to-insulator (MIT) transition. With the increasing amount of Mo⁶⁺ doping, the infrared reflectance of VO₂(M) gradually decreased at low temperatures, while the infrared reflectance of VO₂(R) increased at high temperatures. The MIT transition temperature of Mo-doped VO₂ versus undoped VO₂ reduced to 31.5 °C, and the △ε increased by 153%, this is expected to meet the performance requirements of intelligent infrared stealth materials.     

Selective growth and texturing of VO2(B) thin films for high-temperature microbolometers

Vanadium oxides exhibit a broad spectrum of physical properties due to their ability to form various compounds and polymorphs. To utilise a particular property, it is essential to selectively synthesise a desired phase. Herein, we demonstrate a method to selectively and reproducibly grow (00l)-textured VO₂(B) thin films using an amorphous SrTiO₃ buffer layer by sputtering at <350 °C, which enables their direct integration with read-out-integrated-circuits (ROICs), glass, and polymer substrates. The VO₂(B) films exhibit high temperature-coefficient-of-resistances (TCRs) (>−3.5%/K at 25 °C and >−1.5%/K at 95 °C) and low electrical resistivities (∼5 × 10⁻¹ Ω cm at 25 °C and <1 × 10⁻¹ Ω cm at 95 °C), which are favourable for realising highly-sensitive, low-noise, and high-temperature microbolometers. A robust thermal stability of these VO₂(B) thin films at ambient pressure will provide new opportunities to incorporate thermal sensing functions to various electronics.     

Design criterion based on the cohesive energy and defect patterns of VO2 thermally induced phase transition materials

Vanadium dioxide (VO₂) is one of the most widely used thermally induced phase transition materials. However, the phase transition regulation mechanism and specific modulation relationship of VO₂ materials are still not entirely clear. Here, we propose a comprehensive and precise phase transition material design criterion based on the cohesive energy and defect patterns. The results revealed that the associated regulation mechanism, including the size, strain, vacancy as well as crystal plane and shape, of these materials can be determined via design criteria and first principles calculations. Moreover, the specific modulation relationship of the thickness-induced phase transition can also be confirmed by means of experiments. These findings show that our design criterion provides an effective approach for the design of VO₂ thermally induced phase transition materials.     

Boron doped M-phase VO2 nanoparticles with low metal-insulator phase transition temperature for smart windows

Vanadium dioxide (VO₂) is considered to be a promising candidate for energy-efficient smart windows because of its special reversible Metal-Insulator Transition (MIT) near the ambient temperature. However, its use is constrained by its high transition temperature (T_C) relative to the room temperature. In this paper, VO₂ doped by boron, could achieve an outstanding metal-insulator phase transition property with a low T_C (28.1 °C) close to the room temperature. This enhancement strongly contributes to the studies of the VO₂-based smart windows. A limit doping level of around 9.0 at% is observed for the boron-doped VO₂. Moreover, the particle size is getting smaller and more uniform and the particle distribution becomes more equal and compact with the continued increase in the doping content. Such uniform grain size and grain boundary conditions suppress the extension of the hysteresis loop (ΔT decreases from 25 °C to 7 °C). In addition, the T_C first declines with the increase in the boron content and it starts to increase after reaching its minima of 28.1 °C at 6.0 at% doping level. This feature is the consequence of the competition between the inhibition on the phase transition caused by the V⁵⁺ and the promotion on the phase transition caused by the heterogeneous defect-nucleation sites. VO₂ doped with 6.0 at% boron exhibits a favorable thermochromic performance with ΔT_sol of 12.5% and T_lum up to 54.3%, which is promising for the smart windows.     

Photoluminescence response of colloidal quantum dots on VO2 film across metal to insulator transition

We have proposed a method to probe metal to insulator transition in VO₂ measuring photoluminescence response of colloidal quantum dots deposited on the VO₂ film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO₂ across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data.

PACS

71.30. + h; 73.21.La; 78.47.jd     

Metal-to-insulator transition and its effective manipulation studied from investigations in V1-xNbxO2 bulks

Nb doping effects on crystalline structure, transport transition and magnetic properties are investigated for V_1-xNb_xO₂ (0 $\le$x$\le$ 5%) bulks. At room temperature, doping does not change structure of monoclinic phase but causes an increase in lattice parameters for the range of x$\le$ 3%, further doping leads to a structure transition to rutile tetragonal phase. Each sample simultaneously displays a transition to insulating behavior and a sharp decrease in susceptibility below an almost same critical temperature (T_c). The T_c is shown to shift from ~ 340 K at x = 0 to ~ 277 K at x = 5% at a rate of ~ ?13 K/at%. It is argued that the observed sharp decrease in susceptibility originates from the formation of V-V dimers with spins in anti-parallel, while the insulating behavior below T_c is due to the electron movement from one dimerized state to another with the help of Variable-Range-Hopping mechanism. Furthermore, it is proposed that doping by larger cations, which would suppress the formation of dimers, can be used to manipulate the metal-insulator transition to lower temperatures.     

VO2薄膜智能玻璃的研究进展

从建筑节能的角度,概述了几种节能玻璃的特点,提出了新型VO2镀膜玻璃的智能化特性,并阐述了近年来对VO2薄膜的主要研究工作以及今后的努力方向.     

Yb2O3—ZrO2系超细粉的制备

本文采用溶胶—凝胶法,以ZrOCl_2·8H_2O和Yb_2O_3作为起始原料,按事先确定的组成配成水溶液,与氨水溶液进行反应制得ZrO_2·XH_2O和Yb（OH）_3混合胶体。然后对湿凝胶用蒸馏水反复漂洗,经无水乙醇脱水处理、烘干,在700℃煅烧1小时,获得颗粒尺寸为0.02μm的Yb_2O_3—ZrO_2超细粉。所得粉末不仅颗粒尺寸小、分布狭窄,而且不含硬团聚体,为制备高强度、高韧性Yb—TZP材料提供了有利条件。     

(001)-oriented Cu2-ySe thin films with tunable thermoelectric performances grown by pulsed laser deposition

Highly (001)-oriented Cu_2-ySe thin films with tunable thermoelectric performances have been grown by pulsed laser deposition. By using targets with different Cu/Se ratios that further determines the copper deficiency of as-grown films, the carrier concentrations of as-grown films are tuned within a broad range from 10¹⁸ to 10²¹ cm⁻³. The optimum performance is observed at carrier concentration ~1.58×10²⁰ cm⁻³. The distinct properties of Cu_2-ySe thin films with nearly ideal chemical stoichiometric ratio are observed. In addition, a weak change in the electrical transport during the second-order phase transition was observed in the thin films due to the anisotropic structure of the Cu_2-ySe.     

Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (~68?°C) of M-VO₂ film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO₂ due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO₂ film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO₂ nanocrystals was employed to prepare VO₂ film. Interestingly, the obtained 0.4?at%?W-doped mesoporous VO₂ nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔT_sol = 11.4%), suitable luminous transmittance (T_lum = 61.6%) and low phase transition temperature around 43?°C, much lower than 65.3?°C of undoped VO₂. It was demonstrated that the lower phase transition temperature of VO₂ can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO₂ nanocrystals, enhance the luminous transmittance and guarantee enough VO₂ nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO₂ film (ΔT_sol, T_lum and T_c) and probably promote the application of VO₂ nanocrystals in the energy efficient windows.     

Semiconducting properties of cold sintered V2O5 ceramics and Co-sintered V2O5-PEDOT:PSS composites

Recently we established a sintering approach, namely Cold Sintering Process (CSP), to densify ceramics and ceramic-polymer composites at extraordinarily low temperatures. In this work, the microstructures and semiconducting properties of V₂O₅ ceramic and (1-x)V₂O₅-xPEDOT:PSS composites cold sintered at 120 °C were investigated. The electrical conductivity (25 °C), activation energy (25 °C), and Seebeck coefficient (50 °C) of V₂O₅ are 4.8 × 10⁻⁴ S/cm, 0.25 eV, and −990 μV/K, respectively. The conduction mechanism was studied using a hopping model. A reversible metal-insulator transition (MIT) was observed with V₂O₅ samples exposed to a N₂ atmosphere, whereas in a vacuum atmosphere, no obvious MIT could be detected. With the addition of 1–2 Vol% PEDOT:PSS, the electrical conductivity (50 °C) dramatically increases from 10⁻⁴ to 10⁻³ ∼ 10⁻² S/cm, and the Seebeck coefficient (50 °C) shifts from −990 to −(600 ∼ 250) μV/K. All the results indicate that CSP may offer a new processing route for the semiconductor electroceramic development without a compromise to the all-important electrical properties.     

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature,excellent thermochromic properties,and superior durability

In the last few decades, smart windows made from VO₂-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (T_lum), low solar modulation ability (ΔT_sol), high phase transition temperature (T_c), and poor durability. In this study, glass/HfO₂/VO₂/HfO₂ tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO₂ buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (?111) crystal planes, respectively. HfO₂ buffer layer not only reduced T_c of VO₂ film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO₂ films. More importantly, by covering films with HfO₂ ARL, T_lum and ΔT_sol of VO₂ film were greatly improved. In particular, when the thicknesses of HfO₂ buffer layer and ARL were 80 nm and 120 nm, the obtained HfO₂/VO₂/HfO₂ tri-layer film reached a balance between high T_lum (～47.2%), high ΔT_sol (～9.1%) and low T_c (～49.1 °C). In addition, after 216 h of boiling water treatment, T_lum and ΔT_sol of HfO₂/VO₂/HfO₂ film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO₂-based smart windows with high performance and good durability.     

二氧化钒薄膜制备研究的最新进展

VO2在68℃附近发生从高温金属相到低温半导体相的突变,且相变可逆。由于相变前后其电、磁、光性能有较大的变化,使得它在光电开关材料、存储介质、气敏传感器和智能玻璃等方面有着广泛的应用。然而由于VO2稳定存在的组分范围狭窄,使得制备高纯度VO2薄膜较为困难。为此人们做了很多工作来研究VO2薄膜的制备。本文综述了2000年以来VO2薄膜制备方法的研究情况,比较了各种制备方法对薄膜性能的影响,介绍了VO2薄膜研究的最新研究进展,并为扩大其应用领域而探讨了今后的研究方向。     

Modification characteristics of filamentary traces induced by loosely focused picosecond laser in sapphire

The ability of loosely focused picosecond laser to induce filamentation (4.8 times the Rayleigh length) in sapphire was confirmed. The morphology and microstructure of the filamentary traces under typical pulse energy were studied, in which the colorful birefringence phenomenon and damage defects were detected in sapphire. Irreversible ceramic-like polycrystalline microstructures were formed in filamentary traces, and the phase transition from α-Al₂O₃ to γ-Al₂O₃ directly reflected the thermal effect of picosecond laser filamentation. According to the filamentary characteristics, a continuous filamentary channel with a uniform diameter of 33 μm and a length of 7443 μm was obtained by gradually raising the focal position. The research provides first-hand information for the high throughput micro-cutting, micro drilling and thin slicing of sapphire based on ultrafast laser non-linear effects.     

The effect of particle size on structural,magnetic and transport properties of La0.7Sr0.3MnO3 nanoparticles

This paper reports the synthesis of different particle size La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles using non-aqueous sol gel synthesis route by calcination at temperatures 750 °C, 850 °C and 950 °C. In the present work, the effect of particle size of LSMO nanoparticles on its structural, magnetic and transport properties has been studied in detail. The X-ray diffraction analysis confirms the formation of LSMO nanoparticles having rhombohedral ($R\stackrel{?}{3}c$) structure with average particle size of 20 nm, 22.5 nm and 25.6 nm. An increase in magnetization and decrease in coercivity with increase in particle size is attributed to the magnetically disordered surface layer. The bifurcation in ZFC-FC magnetization indicates the possibility of spin glass like behavior of the LSMO nanoparticles. The effect of particle size on the resistivity and magnetoresistance were studied by using different conduction mechanism for different temperature regions. The upturn in the ρ-T curve at lower temperatures was explained by using Kondo-like transport mechanism. The maximum LFMR achieved was 32.3% at a field of 1 T at 10 K for 20 nm LSMO nanoparticle.     

EVA/LDPE/CB导电复合材料结构与性能的研究

以炭黑(CB)粒子为导电填料,乙烯-乙酸乙烯共聚物(EVA)和低密度聚乙烯(LDPE)为基体树脂,在HAAKE转矩流变仪中制备了EVA/LDPE/CB导电复合材料,研究了CB粒子的分散形态、共混体系相形态以及其与EVA/LDPE/CB共混体系导电性能的关系。通过DSC、DMA、SEM、溶剂溶解等方法考察了EVA/LDPE两相体系随着EVA含量的变化引起的相转变情况,同时也考察了CB在EVA/LDPE共混体系中的选择性分散情况。     

The thermochromic characteristics of Zn-doped VO2 that were prepared by the hydrothermal and post-annealing process and their polyurethane composite films

In this paper, Zn-doped VO₂ nanoparticles have been successfully fabricated by a two-step hydrothermal-annealing process, and the thermally induced visible light transmittance enhancement of Zn-doped VO₂ has been studied for the first time. It is found that Zn-doped VO₂ not only exhibits excellent solar modulation ability (ΔT_sol = 15.27%) but also can reduce the phase transition temperature and increase the visible light transmittance after the heat-induced phase transition (ΔT_lum=+5.78%). Moreover, with the increase of Zn doping concentration, the phase transition temperature (T_c) and phase transition hysteresis (ΔT) both decrease. It is shown that the Zn-doped VO₂-PU films not only have good solar light modulation ability and properties of improving visible light transmission after phase transition, but also have good durability. The research result is of great significance for improving the visible light transmittance after phase transition and realizing the practical application of VO₂ in the field of smart windows.     

Role of a low level of La2O3 dopant on the tetragonal-to-monoclinic phase transformation of ceria-yttria co-stabilized zirconia

Yttria, yttria-ceria and yttria-ceria-lanthana stabilized zirconia powders were prepared by coprecipitation. Their tetragonal-to-monoclinic phase (t→m) transformation was investigated by calcining the powders in a temperature range of 400–1400 °C for 2 h. The results show that after doping with 0.1 to 0.3 mol.% La₂O₃ and calcining at 1400 °C in air, unusual redox behaviours of cerium were detected in the 1.5 mol.% Y₂O₃+5.5 mol.% CeO₂ co-stabilized zirconia. Grain refinement and a sharp reduction in oxygen vacancy concentration were observed simultaneously. The t→m transition was not found in the 0.1 mol.% La₂O₃ doped zirconia but appeared in the cases with a higher dopant content. The changes are discussed with regards to the grain size, valence change of cerium, presence of oxygen vacancies, and segregation of the dopants at grain boundaries.     

Gram scale synthesis of monoclinic VO2 microcrystals by hydrothermal and argon annealing treatment

We report gram scale synthesis of 100% phase fraction of VO₂ (M) monoclinic in powder form with reversible phase transition by combining hydrothermal method and Ar annealing at high temperature. Optimization of single phase VO₂ (M) growth and its phase transition characteristics have been analysed systematically by varying growth parameters such as time, synthesis temperature and post growth annealing conditions. Argon annealing of hydrothermally grown VO₂ powders at 800?°C found to play key role in obtaining VO₂ (M) phase in gram scale with characteristic phase transition temperature of 68?°C. In-situ TEM has been performed to investigate the microstructure and phase change across the annealing temperature. Detailed characterizations have been carried out to correlate the phases, microstructure and transition temperature of VO₂ with respect to growth parameters.     

Effect of consolidation pressure up to 1.8 GPa on the sintering of nanocrystalline nc-Y2O3

Sintering of nanocrystalline (nc) monoclinic yttrium oxide (Y₂O₃) was studied in the homologue temperature range of 0.4–0.7T_M. Samples were isostatically consolidated at super high pressure (SHP), up to 1.8 GPa. The combined effects of consolidation pressure and sintering temperature on the properties and microstructure are explored. The physical properties of the samples, mainly elastic modulus follow in general the density changes, or the interparticle contact area, but are also affected by sintering temperature. The effect of compaction pressure and sample density on the phase transition is studied and discussed.