Effects of Ca2+-Sr2+ doping on the infrared emissivity of LaCrO3

LaCrO₃ shows excellent thermal stability and good emissivity, and can be used as a potential thermal protection material for hypersonic vehicle. In this study, LaCrO₃ and Ca²⁺-Sr²⁺ doped LaCrO₃ were prepared by solid state reaction at 1400 °C for 2 h. The microstructures of the samples and effects of Ca²⁺-Sr²⁺ doping on the infrared emissivity of LaCrO₃ were studied by XRD, XPS, FT-IR, and UV–VIS–NIR spectrophotometer. The results show that after doping Ca²⁺ and Sr²⁺ ions, the infrared emissivity of all samples has significantly improved at 2.5–10 μm, from 0.61 (minimum value) to above 0.90. In the range of 10–14 μm, the emissivity of pure LaCrO₃ and La_0.8Ca_xSr_0.2-xCrO₃ samples shows a similar trend and all remains above 0.97. Therefore, doping Ca²⁺ and Sr²⁺ can significantly increase the emissivity of LaCrO₃ at 2.5–10 μm, which makes it have a wider application prospect in the field of high temperature thermal protection.     

Preparation of infrared high radiation coatings from modified spinel NiFe2O4 and its energy saving applications

The NiFe spinel material itself has good thermal stability and emissivity and can be prepared as an infrared high radiation coating for energy saving applications in industrial high temperature furnace applications. In this study, Cr³⁺ and Cu²⁺ doped spinel NiFe₂O₄ was prepared by solid phase reaction at 1250 °C for 3 h and the microstructure and physicochemical properties of the powder and coating were characterised by XRD, SEM, EDS and IR radiometry. The effect of Cr³⁺ and Cu²⁺ doping on the infrared emissivity of spinel NiFe₂O₄ was investigated and energy saving assessment was carried out in a resistance furnace. The results show that the doping of Cr³⁺ and Cu²⁺ can significantly affect the emissivity of spinel powders in the 2.5–10 μm band, and the coatings prepared from the four powders have an emissivity of up to 0.95 in the 2.5–10 μm band. using this high temperature infrared radiation energy saving coating in a resistance furnace resulted in significant energy savings compared to no coating. The furnace was tested for energy saving by holding the furnace for 2 h and 5 h, and the energy saving efficiency reached 20.7% and 17.0% respectively. The coating was subjected to 10 thermal shock tests from room temperature to 700 °C. The coating bonded well and had good thermal shock resistance. Therefore, the coating has wide application prospects for energy saving applications in the field of industrial high temperature furnaces.     

Temperature-dependent infrared emissivity property of Ce-doped ZnO nanoparticles

The low infrared emissivity materials with good high-temperature properties remain a challenge for the infrared stealth of hot targets in 3–5 μm waveband. To further decrease the infrared emissivity of ZnO, the Ce-doped ZnO nanoparticles were prepared by a facile sol-gel method and the infrared emissivity properties in 3~5 μm waveband in high temperature conditions were deeply investigated by doping different concentration of Ce in ZnO. The influences of Ce dopant concentration on the microstructure, morphology, conductivity, lattice vibration and high-temperature infrared emissivity properties of Ce-doped ZnO were systematically studied, as well as the detailed analysis of temperature-dependent infrared emissivity properties through the conductivity and lattice vibration based on the theory of solid state physics. When the Ce dopant concentration is 3%, the infrared emissivity of Ce-doped ZnO decreases dramatically from room temperature to 800 °C in comparison with undoped ZnO and reaches the lowest value of 0.329 at 500 °C. It is indicated that the excess doping of Ce would produce an impurity phase of CeO₂ in the crystal, therefore decreases the conductivity, and causes extra lattice vibration in infrared region, and results in the increase of infrared emissivity. The infrared emissivity versus temperature exhibits a “U” type curve, which is caused by the competition effects of the conductivity and lattice vibration at elevated temperature.     

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.     

Laser absorption and infrared stealth properties of Al/ATO composites

In this work, an Al powder was coated with antimony-doped tin oxide (ATO) to obtain an infrared-laser compatible stealth material. The composites are prepared via a coprecipitation method, characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and measured by ultraviolet spectrophotometer and dual band infrared emissometer. The morphology and microstructure show that the flaky Al powder was coated by the ATO nanoparticles and doped into the SnO₂ rutile structure by an Sb⁵⁺ ion_. The optimal Al content was 20%, and the optimized Sn/Sb molar ratio was 10: 1. Meanwhile, the reflectivity of the composites was 43.454%, and the infrared emissivity in 8–14 μm far infrared waveband range was 0.708. It may shed light on a new material design orientation to obtain high performance laser-infrared compatible stealth materials.     

The effect of Mg2+ doping on the infrared emissivity of LaCrO3 perovskites

The infrared high emissivity ceramic material plays an important role in thermal protection of hypersonic vehicles. LaCrO₃, characterized by excellent thermal stability and high emissivity, can be applied as infrared high emissivity material. LaCrO₃ and Mg²⁺ doped LaCrO₃ were prepared via solid state reaction method. XRD analyses showed that LaCr_1-xMg_xO₃ (x = 0, 0.1, 0.2, 0.25, 0.3) were single-phase solid solutions. The doping-effect of Mg²⁺ on the infrared emissivity was investigated. In the range of 2.5–8 μm, the infrared emissivity of all doped materials had significant improvement, the average emissivity of materials increased from 0.66 to 0.83. In the range above 8 μm, the emissivity of all materials had a similar trend and compared to LaCrO₃, the emissivity of Mg²⁺ doped LaCrO₃ had little decrease.     

Controlling metal-insulator transition in (010)-VO2/(0001)-Al2O3 epitaxial thin film through surface morphological engineering

Surface morphological control of the metal-insulator transition behaviors of VO₂ epitaxial thin films is achieved by annealing substrates of (0001)-Al₂O₃ single crystals. The well-defined terraces of the (0001)-Al₂O₃ substrates are formed by annealing in air at 1200 °C. Correspondingly, the surface roughness dramatically decreases in the VO₂ epitaxial thin films on the annealed substrates, compared with that on the unannealed substrates. The order of magnitude of the resistivity change ratio (~ 10²) of annealed samples across the metal-insulator transition (MIT) decreases by a factor of one, compared with that (~ 10³) in unannealed samples. This result is ascribed to grain size effect in the VO₂ epitaxial thin films. Moreover, the MIT temperature is reduced in the annealed samples with various thickness, compared with the unannealed ones. A reduction of 14.4 K of the MIT temperature is observed in the thinnest VO₂ films on the annealed substrates, compared with the unannealed samples. This behavior results from a compressive strain along the V-V atom chains in the annealed samples, which modifies the orbital occupancy of the V⁴⁺ ions. While increasing the film thickness, the MIT change ratio keeps on the order of magnitude 10², and the MIT temperatures of the VO₂ films on the annealed substrates becomes closer and closer to those of the unannealed samples due to the weakened substrate effect. This work suggests a promising approach to decrease the MIT temperature and still maintain a moderate change ratio for the MIT, potentially enabling room-temperature electronic devices based on VO₂ thin films.     

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.     

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.     

Electrical percolation and infrared emissivity of pressureless sintered SiC-MoSi2 composites tailored by sintering temperature

SiC-MoSi₂ composites with low electrical resistivity and high infrared emissivity were fabricated via pressureless sintering. The relationship between microstructure evolution and electrical behaviors along with infrared emission properties of the resulting composites is investigated at various sintering temperatures. The electrical resistivity undergoes two significant drops with increasing sintering temperature. Pore elimination bears responsible for the initial decrease in electrical resistivity. Transmission electron microscopy (TEM) observation manifests that the thinned amorphous layers at SiC/MoSi₂ interface decrease grain boundary resistivity and allow for electrical percolation to occur when sintering temperature further rises. Additionally, increasing sintering temperature leads to a higher infrared emissivity owing to the formation of Mo_4.8Si₃C_0.6 and the decreased boundaries. The lowest electrical resistivity of 7.2 Ω cm and the highest infrared emissivity of 0.721 are recorded for composite sintered at 2000 ℃. Overall, SiC-MoSi₂ composites exhibit a promising prospect as infrared source elements that must endure harsh environments.     

Photocatalytic Degradation of 4-Nitrophenol on Well Characterized Sol?CGel Molybdenum Doped Titania Semiconductors

Mesoporous TiO₂?CMo materials with different Mo contents (0?C5.0?wt%) were synthesized by the sol?Cgel method. In samples annealed at 500???C, specific surface areas ranging from 150 to 86?m²/g were obtained. X-ray diffraction spectra denoted that the titania anatase phase (~28?nm) was significantly strengthened by the presence of Mo. The UV-vis diffuse reflectance spectra showed that the band gap was shifted to lower energy levels in the samples with higher Mo contents. Fourier transform infrared analysis evidenced that the hydroxylated samples persisted after annealing at 500???C. X-ray photoelectron spectroscopy analyses showed the presence of Mo⁴⁺ and Mo⁶⁺ species, where Mo⁶⁺ can be transformed from this state to a lower oxidation state during the annealing treatments. The study of the photocatalytic degradation of 4-nitrophenol (4-NP) showed that the activity can be related to the c cell parameter of the anatase phase in the Mo-doped semiconductors. Total mineralization of 4-NP up to 98% (TOC) was obtained in the sample (1.0?wt% Mo) with the highest photoactivity.     

Significantly Enhanced Infrared Emissivity of LaAlO3 by Co‐Doping with Ca2+ and Cr3+ for Energy‐Saving Applications

In this study, Ca²⁺–Cr³⁺ co‐doped LaAlO₃, a novel energy‐saving material with significantly enhanced infrared emissivity, was synthesized by solid‐state reaction. The experimental results demonstrated that 20 mol% Ca²⁺ and 10 mol% Cr³⁺‐doped LaAlO₃, namely La_0.8Ca_0.2Al_0.9Cr_0.1O₃, had an infrared emissivity as high as 0.92 in the spectral region of 1–5 μm, which was 12 times higher than that of pure LaAlO₃. The first‐principles electronic structure calculations revealed that the Ca²⁺–Cr³⁺ co‐doping results in the occurrence of impurity energy levels in the forbidden band of LaAlO₃, which were mainly composed of the Cr 3d orbitals. Electrons partly occupied these impurity donor states and significantly reduced the energy bandgap, thus the infrared radiation property of LaAlO₃ was enhanced. This novel material with high infrared emissivity shows promising applications for energy‐saving in the field of thermal process equipment.     

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.     

Al0.97Y0.03PO4 thermal radiation material with enhanced infrared emissivity

Al_0.97Y_0.03PO₄ thermal radiation material was prepared by homogeneous precipitation method. The influences of Y-doping on crystallization behavior, infrared vibration absorption, surface chemical elemental composition, chemical environment, grain size and infrared emissivity property were analysed in detail by X-ray diffraction, X-ray photoelectron spectroscopy, Solid state NMR, Scanning electron micrograph and infrared emissivity spectrometer, respectively. It is found that the doping of Y greatly improves the infrared radiation property and reduces the grain size. Compared with non-doped AlPO₄, Al_0.97Y_0.03PO₄ thermal radiation material possesses a higher infrared emissivity of 0.931 ± 0.002, which suggests that it will have a promising application in the field of infrared heating and drying.     

铬酸酐掺杂V2O5制备高红外反射率氧化铬绿颜料

研究了铬酸酐热分解制备高性能红外反射氧化铬绿颜料,在优化现有铬酸酐热分解工艺的基础上,详细探讨了掺杂制备红外反射氧化铬绿颜料的工艺条件和相关机理。借助UV-vis-NIR、FT-IR、SEM、XRD和CIE-L^*a^*b^*等手段发现:在铬酸酐热分解过程中,不同的热分解温度导致粒径变化,从而影响红外反射率;优化的制备工艺条件(热分解温度1250℃、热分解时间0.5 h)下,氧化铬绿的红外反射率达到90%。在掺杂过程中,V₂O₅的添加可使氧化铬绿的最高红外反射率达到98%。随着V₂O₅添加量的增加,红外反射率先增加后减少;当V/Cr摩尔比为0.004时,红外反射率、电导率、介电常数都达到极值,三者呈现一致的规律性变化。初步机理探索表明,氧化铬本征导电类型为空穴导电,掺杂V₂O₅以后导电类型发生改变,伴随着电阻率的变化,氧化铬吸收和反射光子能力改变,从而影响红外反射性能。     

Ca2+‐Doped LaCrO3: A Novel Energy‐Saving Material with High Infrared Emissivity

The present study describes the successful synthesis of a Ca²⁺‐doped LaCrO₃ ceramic with high infrared (IR) emissivity, which is important for high‐temperature applications for significant energy saving. It is demonstrated that 20 mol% Ca²⁺‐doped LaCrO₃, i.e., La_0.8Ca_0.2CrO₃, exhibited an IR emissivity as high as 0.95 in the spectral region of 3–5 μm, which was 33.8% higher than that of LaCrO₃. By using La_0.8Ca_0.2CrO₃ as IR radiation agent in surface coating of heating unit, the radiative heat transfer could be enhanced significantly. The mechanism of the high IR emissivity of La_0.8Ca_0.2CrO₃ was attributed to the following aspects: Ca²⁺ doping introduced an impurity energy level of Cr⁴⁺ into LaCrO₃ and increased the hole carrier concentration, enhancing both impurity absorption and hole carrier absorption in the IR region; moreover, the doping caused lattice distortion enhanced the lattice vibration absorption. This novel high IR emissivity ceramic shows a promising future in high‐temperature applications for the purpose of energy‐saving.     

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     

Recent progress in two-dimensional nanomaterials of graphene and MXenes for thermal camouflage

Thermal camouflage is one of the most effective ways to guarantee the security and survival of military objects against infrared (IR) detectors. However, with the rapid development of IR detection technology, the IR thermal camouflage of objects and weapons has become more and more difficult, and advanced thermal camouflage materials are urgently needed to meet the demands of warfare. So far, two dimensional (2D) nanomaterials have demonstrated excellent performances with low IR emissivity, optoelectronic tunability, and compatible stealth capabilities. Among them, graphene and MXenes are the most attractive 2D materials used for thermal camouflage applications. Here we present a review of recent progress on the thermal camouflage properties of graphene and Ti₃C₂T_x MXene in the form of composite film, microcapsule and aerogel structures, a summary of current understanding of the working principle of thermal camouflage materials, and current limitations and future opportunities in graphene and Ti₃C₂T_x MXene research and development.     

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.     

Infrared radiation performance and calculation of B-site doped lanthanum aluminate from first principles

Pure LaAlO₃ and LaAl_1-xNi_xO₃ samples (x?=?0.05, 0.1, 0.15, 0.2, 0.25, 0.3) were prepared using a sol-gel technique. The samples were analyzed and characterized using XRD, SEM, FT-IR and XPS. The results showed that the infrared emissivity of LaAl_1-xNi_xO₃ powder prepared at 1500?℃ for 2?h increases with Ni²⁺ doping content. For x?=?0.25, the mean emissivity in the 3–5?µm infrared spectral region was 0.835. This was a 142% increase compared with that of pure LaAlO₃ (0.345). The doped Ni ions mainly exist with valences of +?2 and +?3 in the LaAlO₃ lattice. After doping, the concentration of electron holes and oxygen vacancies increased, leading to an enhancement of free carrier absorption in the system. It indicated that the Ni²⁺ doping would introduce an impurity energy level in the forbidden band of LaAlO₃ by first principles calculation, forming primarily by the hybridization of the 3d orbital electrons of the Ni ions and the 2p orbital electrons of the oxygen atoms. When x?=?0.25, the band gap decreased from 3.50?eV to 0.77?eV. The impurity energy level allows for a reduction in the energy required for the electrons transferring from the valence band to the conduction band, causing increased numbers of electron transitions between the band gaps, thus enhancing free carrier absorption and increasing the infrared emissivity of the material. The LaAl_1-xNi_xO₃ oxide materials prepared in this work had excellent infrared radiation properties. As a lining material at high temperature reacting furnace, the energy loss could be reduced, the heat utilization efficiency would be greatly improved, and the utility model could be used in the field of high-temperature thermal energy saving.