Research on the effects of surface modification of ceramic powder on cure performance during digital light processing (DLP)

Digital light processing (DLP) is one of the most important additive manufacture technologies to fabricate ceramic parts with complex geometries. Compared with pure photosensitive resin, the cure performance of ceramic suspensions is obviously different due to the optical property change after the addition of ceramic powders. In this paper, a unique oxidation process was used to modify the optical properties of nitride powders including AlN and Si₃N₄. The properties of oxidized ceramics were investigated and the cure performance of ceramic suspensions was then characterized. The effect of oxidation time on cure performance was evaluated. The results showed that for AlN, oxidation process leads to the smaller cure depth and smaller excess cure width as compared with non-oxidized AlN and for Si₃N₄, oxidation process leads to the larger cure depth and larger excess cure width as compared with non-oxidized Si₃N₄, indicating that both refractive index and light absorbance of ceramic powders have obvious effects on cure behaviors. Additionally, the cure behavior of oxidized ceramic suspension in this study shows that the relationship of cure depth vs. incident energy agrees well with Beer- Lambert model, but the excess cure width vs. incident energy is not consistent with quasi Beer-Lambert model due to the nature of digital micromirror device (DMD).     

Local microstructure of Ge layers buried in a silicon crystal studied by extended X-ray absorption fine structure

The local structure of Si-capped Ge quantum dots formed in Stranski–Krastanov growth mode on a (001) Si substrate was probed by X-ray absorption at the Ge K-edge. Analysis by extended X-ray absorption fine structure (EXAFS) was performed under the assumption of a solid solution model of the Si distribution inside the quantum dots (QDs) and taking into account polarization effects. It was found that QDs formed for 8 ML are strained and the Ge–Ge and Ge–Si bond lengths vary with direction. The Si content inside the QDs was found to be at a level of ∼25 at.% for strained (8 ML sample) QDs and ∼12 at.% for unstrained (10 ML sample) QDs. This work demonstrates the usefulness of EXAFS analysis for the determination of the local atomic structure in covered low-dimensional structures.     

Fabrication of piezoelectric nano-ceramics via stereolithography of low viscous and non-aqueous suspensions

A high-performance piezoelectric nano-ceramic was fabricated through stereolithography of low viscosity and high solid loading ceramic/polymer composite suspensions. Through the proper fitting calculation of experimental data, the maximum theoretical solid loading, rheological and curing behaviors of the suspension system were evaluated and lucubrated. The suspensions with a 40 vol% solid loading of the BaTiO₃ nanoparticles displayed shear thinning behavior and a relatively low viscosity of 232 mPa·s at 46.5 s⁻¹ shear rate. After post-process, the 3D printed ceramic specimens showed a nanometer-level grain size with a density of 5.69 g/cm³, which corresponds to about 95% of the theoretical density. The printed ceramics exhibit a piezoelectric constant of 163 pC/N and relative permittivity of 2762 respectively. The results achieved in this research indicate that the stereolithography process is a promising 3D printing technology to fabricate piezoelectric materials with complex geometries and exquisite features for the applications of ceramic components.     

Preparation of highly dispersive solid microspherical α-Al2O3 powder with a hydrophobic surface for stereolithography-based 3D printing technology

The present research demonstrates the feasibility of the preparation of a highly dispersive, solid microspherical α-Al₂O₃ powder with a hydrophobic surface using a simple, rapid, and low-cost method for stereolithography-based 3D printing technology, including the synthesis of a precursor based on the in situ hydrothermal reaction of aluminum sulfate and urea, heat treatment, and surface modification using sodium oleate. The dependence of the morphologies of the precursors and α-Al₂O₃ powders on the type of aluminum salt, reaction time, and temperature was studied. The formation conditions of the solid spherical structure were explored. The results indicate that the formation of a solid spherical structure strongly depends on the type of aluminum salt, reaction time, and temperature. A spherical structure is only obtained by mixing aluminum sulfate with urea. Very high reaction times and temperatures facilitate the transformation of the microstructure from a solid sphere to a hollow sphere. The optimum hydrothermal reaction conditions are as follows: aluminum sulfate concentration of 0.05 mol/L, urea concentration of 0.2 mol/L, reaction temperature of 120 °C, and reaction time of 2 h. The optimum surface modification is obtained by using 3 wt% of sodium oleate, which can be used for the preparation of a 50 vol% α-Al₂O₃ ceramic UV-curable suspension with a viscosity of 1.66 Pa s at 30 s^-1. An alumina part with a sintering density of 95.5% was fabricated using stereolithography-based 3D printing technology. The analysis of the formation mechanism of the solid spherical structure indicates that the high decomposition temperature of urea, homogeneous dissolution of urea in aluminium sulfate solution, slow OH^- release by urea in the hydrothermal system, and strong coordination ability of SO₄²⁻ effectively restrain the hydrolysis of Al³⁺ and facilitate the formation of the solid spherical precursor Al₄(OH)₁₀SO₄. The solid microspherical α-Al₂O₃ powder is prepared by heat treatment of the precursor.     

选区激光熔化成形Ti6Al4V合金微弧氧化生物活性膜层的制备、结构及性能

目的提升选区激光熔化成形(SLM)Ti6Al4V合金的生物活性。方法研究了不同电压对微弧氧化技术(MAO)在SLMTi6Al4V表面制备含钙磷生物陶瓷涂层的影响。通过扫描电子显微镜(SEM)、能量分散X射线光谱(EDS)和X射线衍射仪(XRD)等分析和研究了不同电压下微弧氧化涂层的显微结构、组织和成分等性能,并通过接触角测量和模拟体液浸泡实验及后续的红外光谱分析(FT-IR)等检验涂层的生物活性。结果经过微弧氧化处理,SLM Ti6Al4V表面含一定比例的钙磷且与基体结合良好的涂层,涂层的主要物相为锐钛矿,涂层厚度、钙/磷含量以及锐钛矿组织含量均随电压的升高而增加。300 V电压制备的膜层,表面均匀,钙、磷的原子数分数分别为7.04%、9.65%。涂层截面质量均一,厚度适宜,为3.19μm,且随着涂层增厚,基体元素Ti含量下降,Ca、P和O元素的含量增加。300V电压制备的膜层润湿性相比SLM Ti6Al4V基体的更好,膜层在SBF溶液中浸泡35天后,钙、磷比由0.73增加到1.2,并有羟基磷灰石生成。结论 SLM Ti6Al4V通过微弧氧化技术制备生物活性膜层的最优电压为300 V,经过微弧氧化后的钛合金表面生物活性得到提升。     

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Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Dense 8 mol% yttria-stabilized zirconia electrolyte by DLP stereolithography

Solid Oxide Fuel Cells (SOFCs) are environmentally efficient energy conversion devices, but are partially limited by the complicated fabrication procedure. In this work, dense 8 mol% yttria-stabilized zirconia (8YSZ) ceramics were successfully realized through a DLP (digital light processing) stereolithography method and the electrolyte self-supported fuel cell was also tested at 800 °C. The sintering behavior of the as-printed planar samples were investigated and a fully dense ceramic can be achieved at 1450 °C. The total conductivity of the sintered 8YSZ can reach 2.18 × 10⁻² S cm⁻¹ at a test temperature of 800 °C, which is acceptable for practical application. For the electrolyte self-supported fuel cell test, a power density of 114.3 mW cm⁻² can be achieved when Ni-8YSZ cermet and La_0.8Sr_0.2MnO₃ (LSM) were used as anode and cathode. It was demonstrated that 3D printing is a promising processing technique to build up electrolyte self-supported SOFCs with desired structure for the future development.     

热震对TiAlSiN涂层结构与性能的影响

目的探究TiAlSiN涂层经过不同热震次数后,其组织结构及性能的变化规律及机制。方法采取电弧离子镀技术在单晶硅和M2高速钢(W6Mo5Cr4V2)表面沉积TiAlSiN涂层,采用加热-水淬循环的方法进行热震试验。采用3D表面轮廓仪、扫描电子显微镜(SEM)表征涂层显微形貌,用金相显微镜测定膜/基结合力,用能谱仪(EDS)分析涂层元素含量变化,用X射线衍射仪(XRD)表征物相结构,用划痕仪和硬度计测量涂层力学性能,用摩擦磨损试验仪、光学显微镜探究涂层摩擦学性能及摩擦磨损机制。结果随着热震次数的增加,涂层表面产生的TiO颗粒尺寸增大,含量增多,粗糙度增加。XRD衍射峰向小角度发生偏移,但仍保持立方结构。涂层的力学性能变差,硬度值由2066HV_(0.025)下降至1447HV_(0.025),结合力由常温的71.8 N下降至33.9 N,结合力等级由常温的HF1降至HF4。此外,30、40、50次热震后,涂层展现出比常温下更优异的耐磨性能,摩擦系数由常温的0.571分别降低至0.427、0.389、0.273,磨损率由常温时的1.4×10~(-14) m~3/(N·m)分别降至1.01×10~(-14)、0.93×10~(-14)、0.71×10~(-14)m~3/(N·m),磨损类型主要为粘着磨损与氧化磨损。结论 TiAlSiN涂层在600℃下具备优异的抗热震性能,多次冷-热循环后仍为立方结构。随着热震次数的增加,TiAlSiN涂层表面质量及力学性能下降,但摩擦磨损试验中,由于涂层表面多次热震形成的氧化物起到润滑效果,有效减缓了涂层与摩擦球的剧烈接触,使TiAlSiN涂层的耐磨减摩性能提高。