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.     

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.     

The novel preparation method of thermochromic VO2 films with a low phase transition temperature by thermal oxidation of V–Mo cosputtered alloy films

Vanadium dioxide (VO₂) has been studied extensively for its unique insulator-metal transition characteristics and potential applications in thermochromic smart windows, switching devices, and infrared detectors. However, how to balance the metal-insulator transition temperature, luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) of VO₂ thin films remains a challenge. In this work, high-quality thermochromic VO₂ thin films were prepared by a two-step method of magnetron sputtering and thermal oxidation annealing. Metallic and alloyed V–Mo layers were first deposited by direct-current reactive magnetron sputtering, and then a thermal oxidation annealing process was used to obtain pure and Mo-doped VO₂ thin films. The Mo content in the films was regulated by changing the sputtering power of the vanadium target, and the effect of Mo doping on the crystallinity, microstructure, phase transition temperature and optical properties of VO₂ thin films was studied. The shift of the VO₂(011) peak to a lower 2θ angle in the XRD patterns showed that Mo was successfully diffused into vanadium dioxide films. The phase transition temperatures were decreased continuously from 57.4 to 32.7 °C by decreasing the sputtering power of vanadium. The thinner Mo-doped VO₂ thin films showed higher luminous transmittance and lower transition temperature. Our results were shown to be an innovative preparation method to fabricate thermochromic VO₂ films with a low phase transition temperature, balanced luminous transmittance and solar modulation ability by thermal oxidation of V–Mo cosputtered alloy films.     

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.     

Multistep synthesis of VO2 (M) nanoparticles and their application to thermochromic hybrid films for IR modulation

Vanadium dioxide (VO₂) is widely known as one of the excellent thermochromic materials based on a reversible insulator-to-metal phase transition upon temperature change. In this study, VO₂ (M) powder was initially synthesized through a hydrothermal method and a subsequent post-annealing treatment. Additionally, a particle size of the VO₂ (M) powder was reduced and uniformized by introducing a ball-milling process. The resultant VO₂ (M) nanoparticles (NPs) were dispersed in ethanol with the addition of polyvinylpyrrolidone (PVP). The ethanolic dispersion was then coated on a transparent heater used as a substrate by spin-coating to produce VO₂ (M)/PVP composite films. We have attained an exact temperature control of the films by applying voltages to the heater for the assessment of their thermochromic performance such as the solar and the infrared modulation ability. For example, the film temperature could be raised from room temperature to 85.5°C within 180 s at a low voltage of 11 V, which was enough for inducing the phase transition of the VO₂ (M) NPs showing the infrared modulation ability of 19.3%. The combination of the composite films and the heater was thus proved to be a promising way for realizing transparent thermochromic devices.     

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.     

Multifold stiffness and fracture toughness enhancement in W-doped VO2 microcrystals

Vanadium dioxide is popular for the metal-insulator phase transition at 68°C. Chemical doping is one of the effective ways adopted to tune the phase transition temperature, where tungsten is known to reduce the transition temperature of VO₂. This work investigates the effect of tungsten doping on the mechanical properties of VO₂ microcrystals and their polymer composites. Doping of VO₂ with W shows a systematic reduction in phase transition temperature up to 33°C for 4 wt% W-doped VO₂. For 3 wt% W-doped VO₂, the elastic modulus values enhance by 50%. The fracture toughness of 3 wt% W-doped VO₂ shows an enhancement of fourfold compared to the undoped VO₂. The dynamic compressive strength of 3 wt% W-doped VO₂–UHMWPE polymer composite at room temperature is found to be 7% higher than the undoped VO₂—composite.     

Low metal–insulator transition temperature of Ni-doped vanadium oxide films

Elemental doping is the main means to regulate the phase transition of vanadium oxide (VO₂); however, the effects of low valence elemental (<4+) doping on the phase transition of VO₂ are still controversial. In the present work, Ni-doped VO₂ films were prepared on quartz glass by direct current reactive magnetron sputtering and subsequent annealing. With the increase of the Ni doping content, the phase transition temperature of heating (T_H) of the VO₂ films decreased from 73.4 °C to 52.4 °C. The temperature required for the occurrence of phase transition (T_b) was lower than T_MIT. Different from the undoped VO₂ film, the Ni-doped VO₂ films had a T_b of around 30 °C. XRD and Raman results revealed that some rutile VO₂ microcrystals appeared in the vanadium oxide films because of the lattice distortion by incorporated Ni. Hence, rutile VO₂ micro-crystallinities significantly facilitated the phase transition of monoclinic VO₂ to rutile one.     

Mechanical properties study of VO2 micro-beam according to metal-insulator transition

Many attempts have been made to develop applications using the metal-insulator transition (MIT) phenomenon of VO₂. However, the difference in the densities of the two phases poses serious obstacle for those applications, as it can destroy or disable during the phase transformations. For microsized or nanosized devices, this aspect can be critical. We attempted to measure the mechanical properties when the two phases co-exist, as well as for an individual phase, via in-situ control of the temperature of plate-shaped VO₂. The lamella structure is formed during MIT. At this time, the stress is applied by the gradient of density, and the residual strain can easily occur at the interface of each phase. Therefore, the co-exist state was judged to be the most vulnerable during the MIT. The change in mechanical properties of VO₂ during phase transition was also simulated by finite element method.     

The influence of precursors and additives on the hydrothermal synthesis of VO2: A route for tuning the metal–insulator transition temperature

Thermochromic materials have attracted the attention of scientific and technological researchers due to their ability to change color depending on the temperature. Vanadium dioxide (VO₂) is capable of considerable polymorphs and has aroused interest mainly because its metal–insulator transition (MIT) presents a thermochromic characteristic at a relatively low temperature. This work aimed to obtain vanadium oxide nanostructures using hydrothermal synthesis to tune the MIT temperature. Ammonium metavanadate or vanadium pentoxide was used as a precursor of vanadium, oxalic acid as a reducing agent, and sodium molybdate as an additive. The starting materials were homogenized and inserted in a hydrothermal reactor at 180 °C. After 24 h of synthesis, part of the resulting product was heat-treated at 400 °C for 3 h. The powders obtained were characterized by their structure, morphology, and thermal properties. The results showed a fiber/rod-shaped VO₂ (M) morphology. Distinct strategies were used to obtain the crystalline phase of interest (VO₂(M)), and the presence of a reversible change occurring at ~68 °C was evaluated according to the parameters from the VO₂ phase transition. The addition of sodium molybdate favored a 22% reduction in the MIT temperature when the precursor used was vanadium pentoxide, indicating possible doping in the structure increased the effects of smaller crystallite size and the presence of crystalline phases. This work opens new perspectives for applications of the vanadium oxides obtained, such as in thermal sensors and/or intelligent materials.     

Revealing the high sensitivity in the metal toinsulator transition properties of the pulsed laser deposited VO2 thin films

Vanadium dioxide (VO₂) is known as a typical 3d-orbital transition metal oxide exhibiting the metal-to-insulator-transition (MIT) property near room temperature. However, their electronic applications have been challenged by the quality and uniformity of VO₂ thin films. In this work, we demonstrate the high sensitivity in the valence charge of vanadium and the MIT properties of the VO₂ thin films to the deposition temperature. This observation indicates the necessity to eliminate the inhomogeneity in the temperature distribution of substrate during the vacuum-deposition process of VO₂. In addition, a high thermoelectric power factor (PF, e.g., exceeding 1 μWcm⁻¹K⁻²) was achieved in the metallic phase of the VO₂ thin films and this value is comparable to typical organic or oxide thermoelectric materials. We believe this high PF enriches the potential functionality in thermoelectric energy conversions beyond the existing electronic applications of the current vacuum-grown VO₂ thin films.     

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.     

Thermally induced double‐positive temperature coefficients of electrical resistivity in combined conductive filler–doped polymer composites

Polymer composites of low‐density polyethylene/polypropylene/graphite/vanadium dioxide (LDPE_0.8/PP_0.2/Gr_0.4/VO₂) are prepared by classical melt‐mixing technology and show a notable double positive temperature coefficient of electric resistivity (PTC), which originates from the combined effect of highly conductive Gr and VO₂ with a thermal phase transition. When the weight ratio of VO₂ is 8 wt %, the positive temperature coefficient intensity (PTCI) for the composites reaches 3.85 orders of magnitude. The model system demonstrates the reason for the improvement in the PTC performance of the polymer composites by analyzing the construction of the conductive networks. Therefore, the addition of phase‐transition compounds may be a promising path to improving PTC materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44876.     

Optimization of temperature coefficient of resistance of Al-doped vanadium oxide thin film prepared by atomic layer deposition for uncooled microbolometer

Vanadium oxide (VO_X) is an excellent thermal sensitive candidate for uncooled microbolometers. However, undoped VO_X prepared by atomic layer deposition (ALD) has a temperature coefficient of resistance (TCR) of ca. ?2 ～ ?3%/K. For improving its TCR, our deposition approach based on the combination of ALD and rapid post-deposition annealing (RTA) is proposed. Besides, aluminum-doping into the VO_X films is performed via that approach, and the number of Al₂O₃ cycles is adjusted for varying the dopant loadings. Changes in physical, chemical, and electrical characteristics of the VO_X films due to Al-doping are discussed in detail. The advantage of introducing Al³⁺ dopants is to hinder the thermally activated phase transition of VO₂ phases, leading to an improvement in TCR of Al-doped VO_X up to ?4.2%/K, which remains stable over a wide temperature range of 298–328 K. However, the excessive Al doping also carries an adverse effect on TCR. The reason for that is discussed for further understandings of doping effects.     

Phase evolution and thermochromism of vanadium oxide thin films grown at low substrate temperatures during magnetron sputtering

Vanadium oxides (VO_x) have been studied extensively for applications in thermochromic materials, electrochomics, and infrared detectors due to their unique phase transition characteristics. However, various vanadium oxide phases usually occur under different deposition conditions due to their particularly complex vanadium-oxygen system. In this research, V₃O₇, VO₂(B), VO₂(M), and V₂O₅ thin films were obtained as pure or mixed phases by controlling the substrate temperatures between 250 °C and 400 °C during magnetron sputtering. The microstructure and phase composition of vanadium oxide thin films were characterized and analyzed using X-ray diffraction and Raman spectroscopy. The phase evolution was dependent on the substrate temperature and could be clarified. Metastable V₃O₇ and VO₂(B) phases were obtained at substrate temperatures of 250–300 °C, while stable VO₂ and V₂O₅ phases were obtained at 350–400 °C. The surface morphology and optical properties of vanadium oxide thin films with different substrate temperatures were investigated in detail. Our results provide methods for transforming vanadium oxide phases under well controlled substrate temperatures.     

First-principles study of phase-transition temperature and optical properties of alkaline earth metal (Be,Mg, Ca,Sr or Ba)-doped VO2

Vanadium dioxide (VO₂) is an attractive material for energy-saving smart windows due to its metal-to-insulator reversible phase transition near ambient temperature, accompanied by large changes in its optical properties. We conducted first-principles calculations to study the phase-transition temperature and optical properties of alkaline earth metal (Be, Mg, Ca, Sr or Ba)-doped VO₂. The results show that the Be atom prefers to locate at the octahedral interstitial site, while Mg, Ca, Sr and Ba atoms prefer to substitute for the V atom in VO₂. Be, Mg, Ca, Sr and Ba doping reduces the phase-transition temperature of VO₂ 0by 51.4, 59.7, 61.5, 58.4 and 58.3?K, respectively, when the doping concentration is set at one atomic percentage. In addition, the introduction of alkaline earth metal scales the band structures of VO₂, which enhances the ability to block the infrared light (in the order of Be > Mg > Ca > Sr > Ba) and promotes the transmission of visible light (in the order of Be > Mg ≈ Ca > Sr > Ba).     

Temperature dependence of field‐responsive mechanisms in lead zirconate titanate

An electric field loading stage was designed for use in a laboratory diffractometer that enables in situ investigations of the temperature dependence in the field response mechanisms of ferroelectric materials. The stage was demonstrated by measuring PbZr_1?xTi_xO₃ (PZT) based materials—a commercially available PZT and a 1% Nb‐doped PbZr_0.56Ti_0.44O₃ (PZT 56/44)—over a temperature range of 25°C to 250°C. The degree of non‐180° domain alignment (η₀₀₂) of the PZT as a function of temperature was quantified. η₀₀₂ of the commercially available PZT increases exponentially with temperature, and was analyzed as a thermally activated process as described by the Arrhenius law. The activation energy for thermally activated domain wall depinning process in PZT was found to be 0.47 eV. Additionally, a field‐induced rhombohedral to tetragonal phase transition was observed 5°C below the rhombohedral‐tetragonal transition in PZT 56/44 ceramic. The field‐induced tetragonal phase fraction was increased 41.8% after electrical cycling. A large amount of domain switching (η₀₀₂=0.45 at 1.75 kV/mm) was observed in the induced tetragonal phase.     

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     

Bandwidth controlled metal-insulator transition in Au–VO2 nanocomposite thin films

Recognizing and controlling the metal-insulator transition (MIT) in VO₂ transition-metal oxides is interesting for the future electronic devices. However, the effect of the electron correlation for the structure-coupled MIT in VO₂ is as yet an open question. In this study, we present for the first time direct spectroscopic evidence for the charge-transfer assistance bandwidth controlled MIT (BC-MIT) in Au–VO₂ nanocomposite thin films (NCTFs). A significantly enhancement of the MIT temperature (about 350 K) is realized in Au–VO₂ films with Au volume ratio of 1.1 mol%. However, by further increasing Au ratios, the MIT temperature in Au–VO₂ NCTFs is downward shifted by ~16 K and forward shifted 6 K. The V L-edge and O K-edge have been investigated. The basic electronic parameters such as the covalency (W) have been tuned. The relationship between bandwidth and the MIT temperature has been clearly elucidated a linear relationship. The experimental results demonstrate that MIT in VO₂ is BC-MIT which improved our understanding of the electron correlation effect in VO₂ systems.     

Facile hydrothermal synthesis of vanadium oxides nanobelts by ethanol reduction of peroxovanadium complexes

V₃O₇·H₂O and VO₂(B) nanobelts were successfully synthesized by a one-pot hydrothermal approach using peroxovanadium (V) complexes, ethanol and water as the starting materials. Some parameters, such as the ratio of ethanol/water, the reaction temperature and the reaction time, were briefly discussed to reveal the formation of vanadium oxides nanobelts. It was found that the ethanol was oxidized to aldehyde confirmed by the silver mirror reaction and gas chromatography. V₃O₇·H₂O and VO₂(B) nanobelts could be selectively synthesized by controlling the quantity of ethanol. The possible formation mechanism of the synthesis of vanadium oxides nanobelts was proposed. The electrochemical properties of V₃O₇·H₂O and VO₂(B) nanobelts were studied, and they exhibited a high initial discharge capacity of 350 mAh/g and 190 mAh/g, respectively. VO₂(M) nanobelts were prepared by the irreversible transformation of VO₂(B) nanobelts at 700 °C for 2 h under the inert atmosphere. The phase transition properties of VO₂(M) nanobelts were investigated by DSC and variable-temperature IR, which revealed that the as-obtained VO₂(M) nanobelts could be applied to the optical switching devices.