Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO₂-YTaO₄ (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO₄, while its effects on thermal/mechanical properties are elucidated from microstructures. Y³⁺ is substituted via Ce³⁺, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W?m^?1?K^?1, 900 °C) is realized in 9 mol% CeO₂-YTaO₄, and the thermal expansion coefficients are increased to 10.2 × 10^?6 K^?1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.     

High temperature anti-oxidative and tunable wave absorbing SiC/Fe3Si/CNTs composite ceramic derived from a novel polysilyacetylene

With the rapid development of high power electromagnetic (EM) equipment and high-speed aircraft, the powerful and high oxidation-resistance absorbers are fundamentally desirable for the EM field. Herein, a novel high temperature anti-oxidative SiC/Fe₃Si/CNTs composite is synthesized by a facile polymer derived ceramic (PDC) route from a Fe-containing polysilyacetylene (PSA). The microstructure of as-prepared SiC/Fe₃Si/CNTs composite absorber is featured by micro-sized SiC ceramic grains with spherical Fe₃Si nanoparticles and carbon nanotubes (CNTs) attached to. The vector network analyzer tests show a tunable wave-absorbing performance by adjusting the thickness of layer, and the effective bandwidth (the reflection loss < −10 dB) is 3.3–16.8 GHz for the sample S-1400 (heat treatment at 1400 °C in nitrogen flow). The minimal RL value is −41.2 dB at 10.5 GHz at a thickness of 2 mm and an effective bandwidth is nearly 4 GHz (12.9–16.9 GHz) at the thickness of only 1.5 mm. Moreover, after the oxidation treatment at 800 °C in the air, this absorber maintains the main structure and shows a good high temperature oxidation resistance. This absorber still remains excellent wave absorption property, in view of a minimal RL value of −40 dB at the thickness of 3 mm and a bandwidth of 4.8 GHz (10.4–15.2 GHz) at the thickness of 2.5 mm. The mechanism of high EM wave absorption performance is studied and attributed to the impendence matching, polarization, and the magnetic properties. Thus, the SiC/Fe₃Si/CNTs composite is a promising EM absorber for high-temperature EM wave-absorbing applications.     

铜铝/ 聚苯酯涂层500 ~ 700 ℃的长时氧化行为研究

铜铝/ 聚苯酯( CuAl/PHB ) 涂层是一种用于发动机气路封严的可磨耗涂层。以铜铝合金与铜铝/ 聚苯酯涂 层为研究对象,采用试验与基于密度泛函理论的第一性原理计算相结合的方法,研究了不同Al 元素含量、不同 氧化温度和不同氧化时间对铜铝/ 聚苯酯涂层的影响,并表征不同温度下最高氧化时间达1000h 的氧化产物,系 统研究了合金与涂层的抗氧化性。结果表明, Cu7Al/PHB 可磨耗封严涂层在620 ℃工况温度下可以达到抗氧 化级( HB 5258-2000 标准 ),但当温度达到650 ℃后抗氧化性已存在不足,建议应用于620 ℃以下的服役工况。     

6061铝合金搅拌摩擦焊工艺窗口的研究

目的提高6061铝合金搅拌摩擦焊接头的质量,确定合适的工艺参数范围。方法设计3种不同的搅拌头进行焊接,分析接头拉伸强度与组织性能,并根据试验结果建立工艺窗口,选择合适的轴肩尺寸及工艺参数范围。结果随着轴肩尺寸减小,焊缝宽度、金属流动性、热力影响区面积均减小,在较大的焊接速度及较小的搅拌头转速下,焊缝底部出现缺陷;采用轴肩直径小的搅拌头进行焊接,在一定焊接参数范围内,焊接接头的拉伸强度得到提高;随着轴肩直径减小,焊核区晶粒组织细化,材料变形程度减小;由建立的工艺窗口可知,当轴肩尺为9 mm时,可选择的参数范围最大。结论焊接时采用小尺寸轴肩,可以在搅拌头行走速度更低、转速更大的情况下,仍然可以保持合理产热量,使接头性能得到提高。     

超音速火焰喷涂 Fe 基非晶耐磨涂层的组织与性能

采用超音速火焰喷涂 (HVOF) 在 304 不锈钢基体上制备了 FeSiBP 非晶涂层, 利用扫描电子显微镜、 X 射 线衍射仪、 维氏显微硬度计、 摩擦磨损试验机、 三维光学轮廓仪等设备对涂层的组织结构、 摩擦性能和磨损机制 进行了深入分析。 结果表明： 涂层大部分为非晶态, 非晶含量随着热量输入的增大呈先增加后减少的趋势。 当喷 涂参数为煤油流量 5.5 GPH, 氧气流量 2000 SCFH, 送粉器转速 4 RPM, 喷涂距离 360 mm 时, 涂层非晶含量可 达 83.8%, 硬度 857 HV0.2, 耐磨性是 304 不锈钢基体的 2.52 倍。     

Charge compensation and electrical characteristics of Ta2O5–doped SnO2–CoO ceramics

We investigated the effect of pentavalent donor dopant Ta₂O₅ on microstructure development, electric and dielectric characteristics of SnO₂–CoO based ceramics. Already low additions of Ta₂O₅ (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO₂ growth rate. Rietveld analysis shows that the amount of Co₂SnO₄ spinel phase drops with the addition of Ta₂O₅ due to incorporation of Co²⁺ and Ta⁵⁺ into SnO₂ structure. With higher additions, however, Ta₂O₅ segregates to the grain boundaries and hinders SnO₂ grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta₂O₅ the Co:Ta ratio in SnO₂ grains is constant 1:2, which means that a twice lower amount of Ta⁵⁺ is incorporated in the SnO₂ structure compared to the Nb₂O₅-doped SnO₂–CoO system. Accordingly, the following charge compensation mechanism is proposed: 3 Sn(IV)_S^˟_{n (IV)} ⇋ Co(II)_Sn ̎_(IV) + 2 Ta(V)˙_{Sn (IV)}.     

加压内冷却方法在高温合金磨削中的应用

针对镍基高温合金在磨削加工中大量磨削热的冷却问题,提出采用加压内冷却与断续磨削结合的冷却方法,实现磨削过程中充分冷却磨削弧区高温的目的。设计制备磨粒有序排布的加压内冷却砂轮,采用Fluent有限元软件建立砂轮磨削GH4169高温合金的温度场模型,模拟分析砂轮转速和冷却液压力对砂轮散热性能的影响。开展加压内冷却砂轮磨削GH4169实验研究,分别对磨削温度、表面粗糙度以及表面微观形貌进行对比和分析。结果表明：在相同的磨削参数条件下,相对外冷却方式,内冷却方式能获得更优良的加工表面质量,磨削温度和表面粗糙度均明显降低;在其他磨削参数相同时,冷却液压力越大,磨削温度越低且表面粗糙度越小,表面形貌更加规则、完整。     

Sintering,thermal expansion,and thermal transport properties of A4Ta2O9 (A= Ca,Mg) tantalates

Searching for new oxides with low thermal conductivity and high thermal expansion coefficients (TECs) as thermal barrier coatings (TBCs) is vital for the development of highly efficient gas turbines and aeroengines. We report the densification sintering, high TECs, and low thermal conductivity of A₄Ta₂O₉ (A = Ca, Mg) tantalates. The best sintering temperature of dense A₄Ta₂O₉ ceramics was determined via an optical contact angle tester, and samples with a relative density of 99.8% were synthesized via spark plasma sintering (SPS). The hardness (9–10 GPa), Young's modulus (172.7–211.8 GPa) and fracture toughness (1.5–1.6 MPa m^1/2) of the A₄Ta₂O₉ ceramics are primarily affected by the bonding strength. Furthermore, we studied the thermal transport properties of A₄Ta₂O₉. The low thermal conductivity (1.78–1.93 W m^?1 K^?1 at 900 °C), extraordinary phase stability, and high TECs (11.4–11.8 × 10^?6 K^?1 at 1200 °C) of A₄Ta₂O₉ ceramics make them candidate TBCs with high operating temperatures.     

Oxidation resistance of SiCf/SiC composites with three-layer environmental barrier coatings up to 1360 °C in air atmosphere

Atmospheric plasma spraying (APS) was used to prepare three-layer environmental barrier coatings (EBCs) Si/Yb₂SiO₅/LaMgAl₁₁O₁₉ (LMA) on a SiC_f/SiC substrate. Isothermal aging test of the specimens were performed between 1000 and 1360 °C for 500 h. The flexural strength of the specimens after isothermal aging was investigated. Microcracks and holes were observed in the as-sprayed EBCs because of the shock cooling during the APS process, but reduced after isothermal aging, and the EBCs became denser. At least 80.07% of the flexural strength of the SiC_f/SiC substrate with EBCs was maintained after isothermal aging, but only 35.91% strength was maintained without EBCs. In particular, the retention ratio of flexural strength was 90.72% after isothermal aging at 1360 °C, despite a reaction between the layers of the EBCs. All the specimens with EBCs showed “pseudo-plastic” fracture, compared with the brittle fracture of specimens without EBCs.     

固溶温度对GH4720Li合金显微组织和力学性能的影响

通过光学显微镜、场发射电镜和力学性能测试,研究了固溶温度对GH4720Li合金显微组织(晶粒、γ′相)及力学性能的影响。结果表明:随着固溶温度的升高,一次γ′相含量减少,三次γ′相尺寸增大,晶粒长大的趋势也变得明显。当固溶温度超过1120 ℃后,一次γ′相回溶迅速,晶粒长大迅速,晶粒尺寸分布不均匀性增加。固溶温度与强度呈抛物线性关系,在1130 ℃强度出现峰值;固溶温度的升高,合金塑性下降,固溶温度超过1100 ℃时塑性下降得更快。680 ℃/830 MPa持久拉伸试验表明,随着固溶温度的提高,持久时间增加,当固溶温度超过1100 ℃持久时间增加明显,但超过1130 ℃持久时间基本不变;随着固溶温度的提高,持久塑性下降,但在1110 ℃之前下降缓慢,超过该温度塑性降低很快,甚至不达标。并讨论了固溶温度-组织-力学性能之间的关联性,该结果为GH4720Li合金盘件的固溶热处理工艺的选择提供了理论参考。