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.     

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.     

Advances in chromogenic materials and devices

Chromogenic materials allow the transmittance of visible light and solar energy to be varied under the action of an external stimulus. This paper first discusses buildings related energy savings that can be accomplished by chromogenic technologies, and their beneficial effects on comfort issues. We then summarize recent work on thermochromic VO₂-based thin films with particular attention to multi-layers of VO₂ and TiO₂ and to new VO₂:Mg films for which the doping gives significantly lowered absorption of visible light. The final part covers electrochromic materials and devices with foci on coloration efficiency and on durability issues for foil-type constructions based on films of WO₃ and NiO.     

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.     

Thermochromic properties of vanadium oxide films prepared by dc reactive magnetron sputtering

A transparent vanadium oxide film has been one of the most studied electrochromic (EC) and Thermochromic (TC) materials. Vanadium oxide films were deposited at different substrate temperatures up to 400 °C and different ratios of the oxygen partial pressure (P_O2). SEM, AFM and X-ray diffraction's results show detail structure data of the films. IR mode assignments of the films measured by IR reflection-absorbance in NGIA (near grazing incidence angle) are given. It is found that the film has V₂O₅ and VO₂ combined structures. The films exhibit clear changes in transmittance when the environment temperature (T_e) is varied, especially in the 3600-4000 cm^− 1 range. Applying a T_e that is higher than a critical temperature (T_c) to the samples, the as-RT (room temperature) deposited film with 9% P_O2 has a transmittance variation of 30%, but the films that were deposited on a heated substrate of 400 °C have little variation. There is tendency of bigger variation in transmittance for the sample deposited at a larger P_O2, when it is applied by 200 °C T_e.     

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.     

Magneto-resistance and Excess Conductivity Studies of Tl(Ba2−y Mg y )Ca 2 Cu 3 O 10−δ Samples

The flux pinning characteristics of T l(B a _2?y M g _y )C a ₂ C u ₃ O _10?δ(0.0≤y≤ 1.5) samples have been studied under the influence of various dc magnetic fields in the light of thermally activated flux flow model. It was observed form the magneto resistivity measurements that the activation energy U has been increased in the sample with Mg incorporation of y=0.5, however, it is decreased in the samples with higher Mg doping. This shows that lower Mg doping concentration promotes enhancement, whereas its higher concentration suppresses the flux pinning characteristics of Tl(Ba _2?yMg _y )Ca ₂Cu ₃ O _10?δ (0.0 ≤y≤1.5) samples. The excess conductivity analysis of Tl(Ba _2?yMg _y )Ca ₂Cu ₃ O _10?δ (0.0 ≤ y ≤ 1.5) samples has shown that the coherence length along the c-axis ξ _c(0), the interlayer coupling J, the phase relaxation time of the carriers τ _? , and the Fermi velocity v _F of the carriers are suppressed with Mg doping. However, the values of B _c0(T), B _c1(T), and J _c(0) are enhanced which complement the magnetoresistivity analysis. These results showed that the flux pinning characteristics are enhanced for lower doping concentration of Mg at the charge reservoir layer sites.     

Influence of preparation conditions on superconducting properties of Bi-2223 thin films

We report electrical transport properties of Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting thin films fabricated by pulsed-laser deposition on SrTiO₃ substrate. The aim of the study was to investigate the influence of preparation conditions such as deposition temperature (T _S), annealing time (t _A) and deposition rate (r). A critical temperature (T _c) as high as 110 K and critical current density (J _c) of 6·2 × 10⁶ A/cm² at 20 K were obtained for T _S = 760°C, t _A = 4 h and r = 1·5 Å/s. We also investigated the effect of Li doping on Bi-2223 thin films. Li intercalation results in high resistive onset transition temperature and the resistivity shows broadening in magnetic field that increases with field. The large broadening of resistivity curve in magnetic field suggests that this phenomenon is directly related to the intrinsic superconducting properties of the copper oxide superconductors. The sudden drop in J _c at relatively low magnetic field (H < 0·5 tesla) is due to the effect of Josephson weak-links at the grain boundaries.     

Lattice Effects in Diborides

The lattice effects in diborides have been investigated by Raman spectroscopy using several atomic substitutions for Mg and B. The doping dependence of T _c can be associated with the gradual filling of the ??- and the ??-bands and the modifications in the Fermi surface. The data show that C doping for B induces a much faster reduction in T _c than Al for Mg, while the simple Li substitution has a minor effect. On the other hand, the double Li-C substitution reduces T _c faster than the simple C doping, which can be attributed to the different role of hole and electron doping for the ??- and ??-bands. Finally, Mn ions have a drastic effect on superconductivity apparently acting as magnetic pair breaking centers. In the Raman spectra of the mixed compounds, two modes of E_2g symmetry appear at ??600 cm^?1 and ??850 cm^?1 (defined as the soft and hard modes, respectively), which do not evolve one to the other with doping, but they coexist in a typical two-mode behavior for both Mg(B_1?x C _x )₂ and Mg_1?x Al _x B₂ sets of compounds. MgB₂ appears to be metastable with intrinsic structural and electronic phase competition, close to topological electronic transitions of the Fermi surface that can be tuned by doping or internal strain from the atomic substitutions. The analysis of the phonon frequency dependence on doping indicates that the behavior of the renormalized mode cannot explain the reduction of T _c in the phase coexistence level. It appears that other effects play also an important role in the superconductivity of diborides.     

Upper limit ofT c for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c. A new parameter, “intrinsic”T _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

Synthesis and Para-conductivity Analyses of TlBa2(Ca 3 − y Mg y )Cu4 O 1 2 − δ Superconductors

TlBa ₂(Ca _3?yMg _y)Cu ₄ O _12?δ (y=0,0.5,1.0, 1.5,2) superconductors are synthesized at normal pressure, and the influence of doped Mg atoms on the superconductivity parameters at the microscopic level is investigated by carrying out para-conductivity analyses of conductivity data. The samples have shown tetragonal structure, and the unit cell volume decreases with increased Mg doping. The onset temperature of superconductivity [ T _c(onset)] and zero resistivity critical temperature [ T _c(R=0)] decreases with Mg. Maximum magnitude of diamagnetism is observed in the samples with Mg of y=2. The apical oxygen mode of the type Tl-O _A-Cu(2) and CuO ₂ planar oxygen modes are softened as observed in fourier transform infrared spectrometer (FTIR) absorption measurements. The fluctuation-induced conductivity (FIC) analyses of conductivity data have shown the enhancement of inter-plane coupling and coherence length along the c-axis. These analyses have shown an increase in the coherence length along the c-axis and the inter-layer coupling J. The enhancement of inter-plane may possibly be arising due to a small decrease in the value of the order parameter of the carriers from ∣ψ∣ ²=1 in the CuO ₂ planes in Mg-doped samples. The values of B _c0(T), B _c1(T), J _c(0) are suppressed with the increased incorporation of Mg, which most likely arises due to the weak coupling of the grains induced by Mg doping.     

Hall effect in detwinned single crystals of YBa2(Cu1−x Zn x )3O7 −δin the mixed state

The effect of zinc doping on the mixed-state Hall effect in detwinned single crystals of YBa₂Cu₃O_{7 ?δ} has been studied. We observe a sign reveral of the Hall effect in crystals with a superconducting transition temperature (T _c) as low as 71 K. The Hall conductivity in the mixed state follows theC ₁/H+C ₂ H field dependence. The absolute value of the coefficientC ₁ increases faster than (1–T/T _c) with decreasing temperature.C ₂ increases rapidly asT falls belowT _c. The temperature dependence of the ratioC ₁/C ₂ scales approximately asT/T _c for different levels of zinc doping.     

High-temperature anomalies of dielectric constant in TiO2 thin films

Anatase and rutile TiO₂ thin films were prepared by chemical vapor deposition with precursors Ti(OPrⁱ)₄ and Ti(dpm)₂(OPrⁱ)₂ (dpm = 2,2,6,6-tetramethylheptane-3,5-dione and Prⁱ = isopropyl), respectively. The dielectric properties of TiO₂ thin films have been studied in 20-1100 K temperature range in air, in controlled Ar/O₂ atmospheres, and in vacuum with silicon-based metal-insulator-semiconductor Au/TiO₂/Si capacitors. High-temperature (T_c ∼ 980 K) anomalous behavior of dielectric constant was observed in both anatase and rutile TiO₂ thin films.     

Impact of Gd Doping on Morphology and Superconductivity of NbN Sputtered Thin Films

We report the effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique, i.e., deposition of Nb and Gd in presence of reactive N₂ gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis, and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX), and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum T _c ) is achieved with gas ratio of Ar:N₂ (80:20) for both pristine and Gd-doped films. The optimized NbN film possesses T _c (R=0) in zero and 140 kOe fields are at 14.8 K and 8.8 K, respectively. The Gd-doped NbN film showed T _c (R=0) in zero and 130 kOe fields at 11.2 K and 6.8 K, respectively. The upper critical field H _c2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.     

Lattice Effects Across the Phase Diagram of Pnictides

We have studied by Raman spectroscopy the effect of doping, temperature, and hydrostatic pressure on selected Fe pnictides of the 1111 series. Two sets of RFeAsO_1?x F _x compounds have been examined (R=Sm and Nd) with a varying amount of doping and transition temperature. The doping dependence of the Raman active modes reveals that the rare earth phonon is correlated with the transition temperature (T _c) and not with the amount of doping. As in the case of several other pnictides, the low temperature measurements indicate phonon modifications at much higher temperatures than T _c even in the superconducting compounds. The application of hydrostatic pressure indicates a nonlinear behavior of the rare earth phonon, which increases with doping and in the superconducting compounds correlates with modifications in T _c. The results are similar with those of the cuprates, where hydrostatic pressure has induced phonon and structure modifications at characteristic pressures where the T _c dependence on pressure is also modified. All results point to some role of the lattice for superconductivity in the pnictides.     

Superconductivity of Ba1 − x K x BiO3 (0.35 < x < 1) Synthesized by the High Pressure and High Temperature Technique

Ba_{1 ? x} K _x BiO₃ (BKBO) samples with 0.35 < x < 1 were synthesized by the high pressure and high temperature technique. XRD analysis showed that the BKBO samples were single phase for the whole range of the potassium doping concentration. The change of superconducting transition temperature, T _c, as well as lattice parameters have been investigated upon doping concentration. As the K doping concentration (x) increases from x = 0.37, T _c decreases from 30.4 K to almost zero at x = 0.74. However, in some BKBO samples without including any barium in the starting composition (x = 1), which is denoted as KBO samples, superconductivity is observed with T _c as high as 9 K with partial substitutions of Bi at the K site. Depending on the synthesis condition of the KBO samples, T _c and lattice parameters were different from sample to sample. Compared with other superconducting bismuthates, the evolution of T _c by potassium doping in the cubic BKBO system is discussed in terms of its electronic band structure.     

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.     

Measurements ofH c2(T) in Bi-Sr-Cu-O

H _c2(T) has been measured for thin BSCO films at temperatures down to 65 mK and pulsed fields up to 35 T.H _c2(T) diverged anomalously as the temperature decreased. At the lowest temperature, it was five times that expected for a conventional superconductor.     

Molybdenum-doped vanadium dioxide coatings on glass produced by the aqueous sol-gel method

Thin films of vanadium dioxide (VO₂) on glass substrates were produced by the aqueous sol-gel method. Various levels of doping were achieved by adding small quantities of a water-soluble molybdenum compound to the sol. After dip coating, the substrates were reduced by heat treatment in a low-pressure carbon monoxide/carbon dioxide (CO/CO₂) atmosphere. The change in electrical conductivity with temperature, and optical reflectance in the semiconductor and metallic phases were measured and compared to undoped VO₂ films. Doping the VO₂ films with molybdenum lowered the transition temperature of the semiconductor-to-metal phase change; at a doping level of 7 at.% the transition temperature was measured at 24 °C, as indicated by the electrical conductivity. All the films showed a substantial change in reflectance upon heating through the transition. The optical reflectance in the semiconductor state increased slightly with additional dopant, while the reflectance in the metallic state remained constant.     

Superconducting Properties of In Situ Mg1.05(B1−xCx)2 Doped with Melanin (C16H2O3N2) via Ultrasonication in Ethanol

In this paper, we have reported melanin (C₁₆H₂O₃N₂) as a dopant of MgB₂ for the first time. Here, the effects of melanin doping to the microstructures and superconducting properties of bulk MgB₂ are thoroughly studied from XRD, SEM, TEM, magnetization, and resistivity data. We have analyzed the critical current density (J _c), irreversibility field (H _irr), flux pinning, resistivity, lattice parameters, grain sizes, critical temperatures (T _c), and other microstructures of all the samples. We have varied the doping percentage according to the nominal atomic ratio of Mg_1.05(B_1?x C _x )₂, x=0,0.02,0.06,0.08,0.1. The J _c of all the melanin-doped samples are improved as compared to that of the undoped sample in high-field region (above 6 Tesla) at low temperature. The 8 and 10 % doped samples give the best results. The 8 % doped sample registers an enhancement of J _c by a factor of 3.6 at 7 T and 5 K as compared to that of the undoped one. But, in the low-field region, melanin doping reduces J _c. The H _irr shows remarkable enhancement at low temperatures below 20 K. The best value of H _irr was found for the 8 % doped sample. However, H _irr reduces at high temperatures above 20 K in all the melanin-doped samples. The volume pinning strength of all the doped samples is enhanced over the entire field range. Further improvement in superconducting properties can be achieved by further reducing the size of the melanin particles, increasing density, and improving the homogeneity of doping.