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.     

镍掺杂铝酸镧涂层的制备及其红外辐射性能

采用溶胶?凝胶法合成了Ni2+掺杂的LaAlO3基红外辐射材料LaAl0.6Ni0.4O2.89 (LANO),以其为辐射基料,利用喷涂工艺在氧化铝陶瓷片表面制备红外辐射涂层,考察了磷酸二氢铝、铝溶胶、硅溶胶和钠水玻璃4种粘结剂对涂层物相组成、热稳定性和红外辐射性能的影响. 结果表明,以LANO为辐射基料、铝溶胶为粘结剂时,涂层红外辐射性能最佳,3?5 ?m波段红外发射率达0.93;所制涂层具有良好的抗热震性能,50次热震后涂层未明显剥落失效;涂层具有显著的强化辐射传热效果,节能率达31.7%.     

Infrared radiation and thermal cyclic performance of a high-entropy rare-earth hexaaluminate coating prepared by atmospheric plasma spraying

In this study, a high-entropy RMgAl₁₁O₁₉ (HE-RMA, R = La, Pr, Nd, Sm, Gd) and LaMgAl₁₁O₁₉ (LMA) coatings were fabricated by atmospheric plasma spraying. The phase composition, microstructure, thermal stability, infrared emissivity performance and shock resistance were comparatively characterized. The results showed that doping multiple rare-earth cations could be conductive to enhance the infrared emissivity. The as-sprayed HE-RMA coating exhibited the highest infrared emissivity, which reached up to 0.971 at 1000 °C. The reason for the improvement of the infrared emissivity was attributed to introduced impurity energy level resulting from doping cations, which could reduce the forbidden bandwidth and increase probability of electronic transition. Meanwhile, HE-RMA coating exhibited better shock resistance at 1100 °C due to superior fracture toughness (1.84 ± 0.41 MPa·m^1/2) during thermal cycling test at 1100 °C. In addition, HE-RMA coating still exhibited high infrared emissivity (0.932 at 1000 °C) at 1100 °C annealing for 100 h with only a slight reduction.     

Ca-Mn co-doping LaCrO3 coating with high emissivity and good mechanical property for enhancing high-temperature radiant heat dissipation

Two techniques, including spray drying and electrostatic spray, were applied to produce feedstocks for preparing Ca-Mn co-doping LaCrO₃ ceramic coatings with two different structures on Ni-based alloy by the atmospheric plasma spraying method. The results show that coating from feedstocks produced by spray drying exhibits lower roughness and porosity than the coating from feedstocks produced by electrostatic spray due to the full melting of smaller feedstocks. Higher proportion of melting zones is beneficial to enhance the ratio of hardness to modulus to improve wear resistance. The emissivity of the coatings with roughness from 0.65 µm to 4.6 µm is all above 0.9 in the waveband of 1–14 µm at room temperature. What’s more, structure-dependent emissivity is affected by surface roughness and pore size due to the infrared scattering. The temperature-dependent thermal infrared emissivity at 1–14 µm decreases with the increasing temperature, and is still above 0.67 at 1200 °C.     

Probing the spectrally selective property of NbB2-based tandem absorber coating for concentrated solar power application

Solar selective coating with good thermal stability is the primary requirement for a concentrated solar power (CSP) plant to function with better photothermal efficiency. In recent years, ultra-high-temperature ceramic-based coatings have been explored as potential materials for solar selective coatings. In this context, NbB₂/Nb(BN)/Al₂O₃ tandem absorber coating was designed to be fabricated on a stainless-steel substrate by the radio frequency magnetron sputtering of spark plasma sintered ceramic target. In the bulk form, the NbB₂ ceramic exhibits high solar absorptance (α = 0.756) and thermal emissivity (ε = 0.43), whereas the amorphous single NbB₂ coating exhibits α/ε = 0.716/0.13. Reactive sputtering of NbB₂ in nitrogen produced a semi-transparent coating with an optical bandgap of ∼2.80 eV and was used as the secondary absorber layer. Raman and X-ray photoelectron spectroscopy analyses reveal mild oxygen incorporation in the absorber layers. The developed SS/NbB₂/Nb(BNO)/Al₂O₃ tandem absorber exhibits a good solar absorptance of 0.950 and a moderate thermal emissivity of 0.15 at room temperature. The coatings exhibited good thermal stability when heated in vacuum for 5 h up to 700°C, and the selectivity (α/ε) remains above 6. The present work shows the possibility of exploring NbB₂-based tandem absorber coatings for CSP applications.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO₃ perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO₃ perovskite powder is associated with Al³⁺-doping content, phase composition and particle morphology. The flaky particles SrZr_0.85Al_0.15O_2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr_0.85Al_0.15O_2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m^?1 K^?1 below 1400 °C. The Al-doped SrZrO₃ perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.     

High-entropy alloy: An alternative approach for high temperature microwave-infrared compatible stealth

To achieve microwave-infrared compatible stealth in high temperature conditions, high-entropy alloys (HEAs) thin films were deposited on Al₂O₃ matrix by magnetron sputtering technology. Films were annealed to investigate thermal stability at 500 °C, 600 °C and 700 °C, respectively. Results from X-ray diffract meter (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), and Fourier transform infrared spectrometer (FTIR) suggested that high-entropy alloy (HEA) film was seriously oxidized when the annealed temperature reached 700 °C for 6 h, causing a significant decrease of infrared reflectivity. Conversely, HEA films showed low infrared emissivity of 0.09 at 600 °C. Additionally, the films possessed excellent thermal stability at 500 °C for 20 h with low infrared emissivity of 0.11. Finally, a simple metamaterial design utilizing HEA films was proposed for infrared-microwave compatible stealth. With the ability of incorporating excellent thermal stability and durable high temperature stealth performance, the study shows great potential of introducing HEAs in the field of high temperature compatible stealth.     

An elevated temperature infrared emissivity ceramic coating formed on 2024 aluminium alloy by microarc oxidation

An infrared emissivity coating material containing γ-Al₂O₃ was prepared on 2024 aluminium alloy surface by the microarc oxidation (MAO) method. The microstructure of the coatings was analysed by SEM, XRD and EDS techniques. The infrared emissivity properties tested at 500 °C were investigated by an infrared radiometer based on a Fourier transform infrared spectrometer. The results show that the infrared emissivity values of coated Al samples depend on the phase composition and surface roughness of the coatings. Corresponding to increasing coatings thickness, the gradually increasing γ-Al₂O₃ content and some oxide compounds containing Si and P contribute to the higher infrared emissivity value (about 0.85) in the wavelength range of 8–20 μm. The increasing surface roughness leads to an obvious increase in emissivity from 0.2 to 0.4 at wavelength 3–5 μm.     

Influence of doping Mg2+ or Ti4+ captions on the microstructures,thermal radiation and thermal cycling behavior of plasma-sprayed Gd2Zr2O7 coatings

Gadolinium zirconate (Gd₂Zr₂O₇) coatings doped by the transition metal Ti and the alkaline earth metal Mg were expected to have improved thermal radiation performance, which could be combined with their excellent thermal barrier properties to comprehensively improve the thermal insulating performance. The results show that the parent Gd₂Zr₂O₇ powder as well as the Gd-site and Zr-site substituted powders crystallize as pyrochlore Gd₂Zr₂O₇ in Fd-3m space group, while all the as-sprayed coatings have the combination of fluorite and a little part of pyrochlore phase. Gd₂Zr₂O₇ ceramic has high mid-infrared emittance and the addition of Ti⁴⁺ into Gd₂Zr₂O₇ can enhance the infrared absorption/emittance in a specific wavenumber range, dominantly in the near-infrared (0.75–2.5 μm) band due to the enhancement of electron transition induced by the impurity energy levels linked to the widening of the conduction band. The normal spectral infrared emissivity of Gd₂Zr₂O₇-based coating was higher than 0.88 at 1073 K. The monolayered doped Gd₂Zr₂O₇ coatings present very low thermal cycling lifetime, similar with the parent coating, mainly related with their low fracture toughness, despite (Gd_1-xMg_x)₂Zr₂O₇ series display lower thermal conductivity than the parent one.     

Development and characterization of silicon dioxide clad silicon carbide optics for terrestrial and space applications

Silicon carbide (SiC), a non-oxide ceramic with superior thermo-mechanical stability, chemical and radiation resistive properties, finds extensive utilization in optical instruments for terrestrial and space applications. However, its inherent porous texture (α-HCP) becomes a deterrent for high-performance optical telescopes, although several techniques of surface alterations over sintered or reaction-bonded SiC are available. In the present work, the physical vapour deposition (PVD) technique is adopted to deposit a thick (～5 μm) Silicon dioxide (SiO₂) clad layer on a sintered and optically polished SiC (SSiC) substrate. SiO₂ clad layer coated SSiC (SDO-SSiC) substrate reduces the surface porosity of SSiC which is found to be suitable for optical mirror application. Finally, an Al based reflective and oxides protective coatings are deposited on SiO₂ clad layer to achieve reflective behaviour. The surface figure of 75 nm PV (peak-to-valley) and less than 2 nm surface micro-roughness values are achieved which meets the stringent optical telescope specifications for terrestrial and space applications. The structural and nano-mechanical properties of presently developed SiO₂ clad layer-based SiC telescopic mirror have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-Ray Analysis (EDX), atomic force microscopy (AFM), and nanoindentation techniques. The optical properties are investigated by optical profilometry and wavelength based spectrometric (both in visible and infrared ranges) techniques. Finally, space worthiness studies viz., thermo-vacuum, thermal storage, thermal shock and relative humidity tests have been carried out successfully. The process of cleaning, grinding and polishing at each substrate preparation stage and coatings are also reported comprehensively.     

The effect of Gd3+ and Ti4+ co-doping on the thermal radiation performance of (Sm1-xGdx)2(Hf1-xTix)2O7 (0 ≤ x ≤ 0.2) ceramic coatings

In this study, the (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ (0 ≤ x ≤ 0.2) ceramic coatings were fabricated by atmospheric plasma spraying. The chemical compositions, morphologies and thermo-optical properties of the samples were systemically investigated. It can be found that the infrared emissivity of (Sm_1-xGd_x)₂(Hf_1-xTi_x)₂O₇ ceramic coatings at the wavelength range of 0.76–15 μm increased with the increasing content of Gd³⁺ and Ti⁴⁺. The (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating exhibited the highest infrared emissivity among the coatings, which was 0.773 and 0.816 at room temperature and 1400 °C, respectively. The mechanism of the increasing infrared emissivity was attributed to the Gd³⁺ and Ti⁴⁺ co-doping can improve the free carrier concentration and the frequency and mode of the lattice vibration. Moreover, the (Sm_0.8Gd_0.2)₂(Hf_0.8Ti_0.2)₂O₇ ceramic coating possessed good thermal resistance, which did not show obvious change in the phase, surface morphology and infrared emissivity after 60 h calcination at 1400 °C.     

High‐performance infrared emissivity of micro‐arc oxidation coatings formed on titanium alloy for aerospace applications

Aerospace vehicles are subjected to high temperatures because of surrounding aerodynamic drag and the formation of large temperature gradients across the external structural parts of their airframe. To protect the vehicles, high‐infrared emissivity coatings that can radiate a large amount of heat into outer space are in demand. In this work, we describe the development and characterization of high emissivity ceramic coatings formed on a TC4 alloy surface by micro‐arc oxidation. We evaluate, in particular, the influence of NiSO₄ concentration on current‐time response, the thickness, surface roughness, morphologies, bonding strength, and emissivity of these coatings. The results indicate that by increasing the NiSO₄ concentration in electrolytes, the thickness and surface roughness of the coatings increase. The bonding strength becomes smaller with increasing concentration of NiSO₄, but is still maintains a value higher 30 MPa. The coatings possess good thermal shock resistance after being subjected to severe thermal shocks for 50 cycles, and no peeling of the coating is observed. A higher concentration of NiSO₄ in electrolytes also leads to an increasing percentage of the nickel components in the coating to form a NiO phase, which enhances the emissivity of the coatings in the wavelength range of 3‐8 μm.     

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.     

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.     

High-temperature interface stability of tri-layer thermal environmental barrier coatings

Thermal environmental barrier coatings (TEBCs) are capable of protecting ceramic matrix composites (CMCs) from hot gas and steam. In this paper, a tri-layer TEBC consisting of 16 mol% YO_1.5 stabilized HfO₂ (YSH16) as thermal barrier coating, ytterbium monosilicate (YbMS) as environmental barrier coating, and silicon as the bond coating was designed. Microstructure evolution, interface stability, and oxidation behavior of the tri-layer TEBC at 1300 °C were studied. The as-sprayed YSH16 coating was mainly comprised of cubic phase and ～3.4 vol% of monoclinic (M) phase. After 100 h of heat exposure, the volume fraction of the M phase increased to ～27%. The YSH16/YbMS interface was proved to be very stable because only slight diffusion of Yb to YSH16 was observed even after thermal exposure at 1300 °C for 100 h. At the YbMS/Si interface, a reaction zone including a Yb₂Si₂O₇ layer and a SiO₂ layer was generated. The SiO₂ grew at a rate of ～0.039 μm²/h in the first 10 h and a reduced rate of 0.014 μm²/h in the subsequent exposure.     

High-entropy La(Fe0.2Co0.2Ni0.2Cr0.2Mn0.2)O3 ceramic exhibiting high emissivity and low thermal conductivity

A novel, high-entropy, perovskite-structured, solid solution La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ ceramic was successfully synthesized via high-temperature solid-state reaction. The crystal structure, microstructure, infrared emissivity, and thermophysical properties were investigated. The experimental results indicated that La(Fe_0.2Co_0.2Ni_0.2Cr_0.2Mn_0.2)O₃ exhibited an infrared emissivity as high as .92 in the near-infrared region of .76–2.50 μm. The thermal conductivity was 1.38–1.72 W m⁻¹ K⁻¹ in the temperature range of 25–1200°C.     

Black ceramic coatings on Ti alloy with enhanced high absorptivity and high emissivity by plasma electrolytic oxidation

A black ceramic coating with high absorptivity and emissivity was successfully prepared on TA7 (Ti-5Al-2.5Sn) in a hybrid electrolyte solution by plasma electrolytic oxidation for improving the imaging precision of optical system. The influence of electrolyte components and technical parameters on the composition, structure, and optical properties was investigated. The results show the coatings with typically porous structure are mainly composed of O, P, Si, Ti, V, Fe, and Ni. The corresponding amorhous oxide in the outer layer endows the coating with strong absorption in the visible light and infrared areas, and the crystallized TiO₂ indwelling the inner layer contributes to the strong UV absorption property. In addition, the micropores of the coatings have different size ranges corresponding to the wavelengths, facilitating the increase of absorptivity and emissivity in some degree. The absorptivity and emissivity can be adjusted by electrolyte components and technical parameters. The coating presents the best absorptivity of 0.962 and emissivity of 0.950 in the electrolyte solution of 3 g/L NH₄VO₃, 5 g/L FeSO₄, and 5 g/L C₄H₆O₄Ni under 400 V for 10 minutes.     

Infrared radiative performance and anti-ablation behaviour of Sm2O3 modified ZrB2/SiC coatings

To improve the emissivity of ZrB₂/SiC coatings for serving in more serious environment, ZrB₂/SiC coatings with varying contents of high emissivity Sm₂O₃ were fabricated using atmospheric plasma spraying. The microstructure, infrared radiative performance and anti-ablation behaviour of the modified coatings were investigated. The results showed that as the content of Sm₂O₃ increased, the density of the coatings increased because of the low melting point of Sm₂O₃. When the content of Sm₂O₃ was 10 vol%, the coating had the highest emissivity in the 2.5–5 μm band at 1000 °C, up to 0.85, because of the oxygen vacancies promoting additional electronic transitions. Due to the high emissivity, the surface temperature of the coating modified with 10 vol% Sm₂O₃ decreased by 300 °C, which led to little volatilisation of the sealing phase. Further, the mass ablation ratio of the above coating was 3.19 × 10^?4 g/s, decreasing 31% compared to that of a ZrB₂/SiC coating. The formed dense surface structure of the coatings showed considerable oxygen obstructive effects. These findings indicate that the modified coatings show considerable anti-ablation performance, which provides effective anti-ablation protection for the C/C composite substrate.     

Fabrication and performances of preceramic polymer-based high-temperature High emissivity coating

High emissivity coatings on nickel-based superalloy, with good infrared radiating ability and good high temperature resistance, were prepared at room temperature, using preceramic polymer cured at room-temperature as coating former and CeO₂ and B₄C as passive high emissivity fillers. The influences of high temperature and wind channel test on the microstructure and thermal performance of the high emissivity coating were investigated in detail. The high emissivity coating has good thermal stability and no cracking and flaking after heating at 1100 K and the wind tunnel test. The emissivity of the coating reached 0.85−0.92 between room temperature and 1100 K. The high emissivity coating on the nickel-based alloy can make the back temperature of the nickel-based alloy decrease from 686 to 646 ℃.