Bimodal microstructure toughens plasma sprayed Al2O3-8YSZ-CNT coatings

Current work pursues generating controlled bimodal microstructure by plasma spraying of micrometer-sized Al₂O₃ and nanostructured spray-dried agglomerate with reinforcement of 20 wt% of 8 mol % yttria stabilized zirconia (8YSZ) and 4 wt% carbon nanotube (CNT) as potential thermal barrier coating (TBC) on the Inconel 718 substrate. Composite coatings exhibit bimodal microstructure of: (i) fully melted and resolidified microstructured region (MR), and (ii) partially melted and solid state sintered nanostructured regions (NR). Reinforcement with 8YSZ has led to an increase in hardness from ∼12.8 GPa (for μ-Al₂O₃) to ∼13.9 GPa in MR of reinforced Al₂O₃-YSZ composite. Further, with the addition of CNT in Al₂O₃-8YSZ reinforced composite, hardness of MR has remained similar ∼13.9 GPa (8YSZ reinforced) and ∼13.5 GPa (8YSZ-CNT reinforced), which is attributed to acquiescent nature and non-metallurgical bonding of CNT with MR. Indentation fracture toughness increased from 3.4 MPam^0.5 (for μ-Al₂O₃) to a maximum of 5.4 MPam^0.5 (8YSZ- CNT reinforced) showing ∼57.7% improvement, which is due to crack termination at NR, retention of t-ZrO₂ (∼3.3 vol%) crack bridging, and CNT pull-out toughening mechanisms. Modified fractal models affirmed that the introduction of bimodal microstructure (NR) i.e., nanometer-sized- Al₂O₃, nanostructured 8YSZ and CNTs in the μ-Al₂O₃ (MR) contributes ∼44.6% and ∼72% towards fracture toughness enhancement for A8Y and A8YC coatings. An enhanced contribution of nanostructured phases in toughening microstructured Al₂O₃ matrix (in plasma sprayed A8YC coating) is established via modified fractal model affirming crack deflection and termination for potential TBC applications.     

Enhanced wet-oxidation resistance of Y2O3 modified SiC ceramics by the formation of Y2Si2O7 protective layer

The poor wet-oxidation resistance limits the long-life service of SiC_f/SiC composites as the hot end components of aero-engines. The stability of SiC_f/SiC composites under high-temperature wet oxygen environment can be promoted by more robust SiC matrix. In this work, the effect of Y₂O₃ on the corrosion behaviors of SiC ceramics in flowing O₂/H₂O atmosphere at 1400 ℃ was studied. Duo to the continuous Y₂Si₂O₇ layer formed on the surface, SiC-Y₂O₃ ceramics exhibit much better wet-oxidation resistance than original SiC ceramics. During the oxidation process, Y₂O₃ dispersed in the ceramics migrates to the surface and reacts with SiO₂ to form β-Y₂Si₂O₇. Subsequently, the β-Y₂Si₂O₇ aggregates and grows to form a continuous Y₂Si₂O₇ layer, inhibiting the corrosion from oxidizing medium to the inner SiC matrix. This study is expected to provide important ideas for the design and structure regulation of wet-oxidation resistant SiC_f/SiC composites.     

Structure and electrochemical characterization of carbon films formed by unbalanced magnetron (UBM) sputtering method

We previously reported nanocarbon films formed by the electron cyclotron resonance (ECR) sputtering method. The films contain a nanocrystalline structure consisting of sp³ and sp² bonds with an extremely flat surface (Ra = 0.07 nm). The film also has a wider potential window than glassy carbon and superior electrochemical activity to boron doped diamond for certain species. However, ECR sputtering equipment is much more expensive than that used for conventional sputtering and requires a ring-shaped target. Therefore, it is difficult to use this method to develop new electrode materials such as metal-carbon hybrid film. Here, we describe a nanocarbon film electrode that we developed with a potential window and electrochemical activity equivalent to those of ECR nanocarbon films by using unbalanced magnetron (UBM) sputtering equipment. Our approach uses conventional equipment and has widely controllable sputtering conditions including a high sputtering rate, a large sputtering area and the capacity for co-sputtering multiple materials. The film can contain a maximum of 53% sp³ bonds by increasing the substrate bias voltage between the target and substrate, and also exhibits a potential window equivalent to that of the ECR nanocarbon film. However, the electrode surface is about one order of magnitude rougher than that of the ECR nanocarbon film due to the effect of reflected Ar⁺ ions caused by the fact that the target surface is facing the substrate surface. By employing transmission electron microscopy, we could observe nanocrystalline graphene structures in the UBM nanocarbon film, which are difficult to observe in conventional diamond-like carbon film. The electron transfer rate at the UBM nanocarbon film is similar to those of ECR nanocarbon film for Ru(NH₃)₆^3 + and Fe(CN)₆^4 −, suggesting that the nanocrystalline structure could contribute to a relatively fast electron transfer rate. The UBM nanocarbon films were successfully used for detecting kynurenic acid, which has a high oxidation potential and is difficult to detect with a conventional glassy carbon electrode.     

热喷涂制备 TiO2 光催化涂层研究进展

TiO2光催化剂因其利用可持续的太阳光进行光催化反应,在环境保护、医疗卫生等领域具有潜在的应用价值,近年来引起了研究者的广泛关注。目前,颗粒状TiO2催化剂获得了一定的实际应用,但在液相中使用后需要回收,不仅增加了工艺的复杂性,而且提高了设备等成本的投入。负载型催化剂和涂层技术是将TiO2固定在载体上,可有效避免颗粒状催化剂难回收的问题。在众多涂层制备技术中,热喷涂技术可快速高效大面积地制备TiO2光催化涂层,且涂层机械性能优异,喷涂成本低廉,因而使TiO2的工业化制备和应用更具前景。综述了近年来国内外制备TiO2涂层常用的传统热喷涂技术、改进后的液相热喷涂技术和冷喷涂技术,并论述了影响TiO2光催化性能的材料相组分、涂层结构和元素掺杂等因素,总结了相应的性能改进措施,指出了目前TiO2光催化涂层的应用研究存在的问题和研究方向。     

Spectroscopic and electrical characterization of some aniline oligomers and polyaniline

Aniline oligomers (n = 2 − 4) of different degrees of oxidation have been synthesized, and their structure determined by ¹H and ¹³C n.m.r. and Fournier transform infrared (FTIR) spectra. Comparative studies on electrical conductivity, electronic and FTIR spectra of proton acid- and iodine-doped oligomers and polyaniline (PAn) are presented. We found that PAn prepared by chemical oxidation in pH ⩽ 1 consists of almost alternating benzenoid and quinoid phenyl ring repeat units and protonation leads to the formation of an ion radical delocalized on the chain for oxidized oligomers as well as for polyaniline.     

The performance of hybridized nano-ceramics on the microstructure and corrosion of Mg alloy in an auto-engine cooling system

This current novel research focuses on developing a hybrid magnesium-based composite through the spark plasma sintering process with an interest in the improvement of mechanical properties and corrosion resistance of Mg Alloy in an auto-cooling system. AZ91D was selected as the primary Mg alloy and it was reinforced with hybridized AlN-VB at a sintering temperature of 500 °C, a heating rate of 75 °C/min, a pressure of 30 MPa, and a dwell time of 5 min. The microstructure of the sintered composite at varying weight constituents shows a homogenous dispersion of the reinforcing material and refinement of the inherent eutectic β-Mg₁₇Al₁₂ phase. The refinement of the phase yielded a small grain size of 16.6 ± 0.20 μm by the MgAZ91D-8wt%AlN-8wt%VB composite compared to the coarse grain size of 36.1 ± 0.9 μm by the unreinforced MgAZ91D. likewise, a higher microstrain value of 7.57E-2 was achieved by the composite with the highest reinforcing constituents. The load-displacement curve shows a gradual shift to the left following the inclusion of the reinforcing substances, this implies a higher resistance to penetration as a result of higher nanohardness and elastic modulus. The corrosion behavior of the sintered composite was examined in a simulated auto-engine cooling system using ethylene-glycol and corrosive water. The Unreinforced MgAZ91D shows a high corrosion rate (Cr) of 2.2217 mm/year, a higher degree of disorderliness (n) of 0.8829, and a low charge resistance transfer (Rct) of 4.6143 kΩcm². However, the corrosion rate decreases rapidly to 0.0475 mm/year and the Rct to 20.9543 kΩcm² at MgAZ91D-8wt%AlN-8wt%VB composite.     

Enhanced piezoelectricity and non-contact optical temperature sensing performance in Er3+ ion modified (K,Na)NbO3-based multifunctional ceramics

Rare earth ions doped ferroelectrics have attracted wide attentions due to their multifunction characteristics with both ferroelectric/piezoelectric properties and intriguing photoluminescence performance, which show great prospects for future multifunctional devices. In this work, a novel rare earth Er³⁺ ion modified potassium-sodium niobate (KNN) based ceramics were elaborately designed and prepared by the conventional solid-state reaction. The microstructure, phase structure, electric properties and photoluminescence performance of the Er³⁺ ion modified KNN-based ceramics were systematically investigated. Enhanced piezoelectricity (a considerable d₃₃ of exceeding 300 pC/N and a large d₃₃* up to 500 p.m./V) was realized through optimizing the substitution of BaZrO₃ by (Er_0.5,Na_0.5)ZrO₃. Both down-conversion and up-conversion photoluminescence emissions were detected in the optimal composition. The temperature-dependent upconversion emissions of the optimal Er³⁺ modified ceramic sample in the temperature range of 303–573K were verified to be applicable for non-contact optical temperature sensing with a maximum sensitivity S_a of 0.0028 K^-1 and a peak relative sensitivity S_r of 0.96% K⁻¹. Moreover, low-temperature sensing performance with a maximum S_r of 16.7% K⁻¹ in the temperature range of 80–280K was also presented based on the temperature-dependent down-conversion emissions. With both decent electrical properties and intriguing photoluminescence performance, the Er³⁺-modified KNN-based ferroelectrics exhibit good application potential in the future multifunctional optoelectronic devices.     

Mechanism of ceramic slurry light scattering affecting contour accuracy and method of projection plane correction

Digital light processing (DLP) is a relatively mature ceramic additive manufacturing technology widely applied in medical bone implantation, electronic communication, and other fields. However, the size error caused by the light scattering from the complex contour can seriously affect the precision and forming quality of the printed green body. This paper studied the scattering behavior of complex structure ceramics under different exposure energies through two structural parameters: exposure width and contour shape. A formula for excess cure width is presented. The formula will be then applied to simplify the machine learning algorithms. Finally, by inverse compensation for the complex contour, we optimized some pore structures and complex shapes to greatly improve the fidelity of the structure.     

Engineering Structural Janus MXene-nanofibrils Aerogels for Season-Adaptive Radiative Thermal Regulation

Aerogels have provided a significant platform for passive radiation-enabled thermal regulation, arousing extensive interest due to their capabilities of radiative cooling or heating. However, there still remains challenge of developing functionally integrated aerogels for sustainable thermal regulation in both hot and cold environment. Here, Janus structured MXene-nanofibrils aerogel (JMNA) is rationally designed via a facile and efficient way. The achieved aerogel presents the characteristic of high porosity (≈98.2%), good mechanical strength (tensile stress of ≈2 MPa, compressive stress of ≈115 kPa), and macroscopic shaping property. Based on the asymmetric structure, the JMNA with switchable functional layers can alternatively enable passive radiative heating and cooling in winter and summer, respectively. As a proof of concept, JMNA can function as a switchable thermal-regulated roof to effectively enable the inner house model to maintain >25 °C in winter and <30 °C in hot summer. This design of Janus structured aerogels with compatible and expandable capabilities is promising to widely benefit the low-energy thermal regulation in changeable climate.     

Synthesis of a high-entropy (TiVCrMo)3AlC2 MAX and its tribological properties in a wide temperature range

A novel high-entropy (TiVCrMo)₃AlC₂ MAX with a solid solution of Ti, V, Cr, and Mo atoms in its lattice is synthesized. The tribological properties as well as the wear mechanism of the high-entropy MAX against a Si₃N₄ counterpart in a wide temperature range from room temperature to 800 ºC in the air are systematically studied. Combined with the experimental observations and the theoretical simulations, the effects of high-entropy structure on the tribological behaviors of the (TiVCrMo)₃AlC₂ MAX are illuminated. Benefited from the high-entropy structure, the (TiVCrMo)₃AlC₂ MAX presents an improved hardness and oxidation resistance, which contributes to a dry-sliding tribological behavior in the temperature window of R.T.–600 °C. While further increasing the operating temperature to 800 °C, hydrodynamic lubrication emerges, which boosts the lubricity in the high-entropy (TiVCrMo)₃AlC₂ MAX.