Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part I, Experimental Observations

Cylindrical buttuohead specimens of an advanced silicon nitride were tested in uniaxial tension at temperatures between 1422 and 1673 K. In the range 1477 to 1673 K, creep deformation was reliably measured using high-temperature contact probe extensometry. Extensive scanning and transmission electron microscopy has revealed the formation of lenticular cavities at two-grain junctions at all temperatures (1422–1673 K) and extensive triple-junction cavitation occurring at the higher temperatures (1644–1673 K). Cavitation is believed to be part of the net creep process. The stress rupture data show stratification of the Monkman–Grant lines with respect to temperature. Failure strain increased with increase in rupture time or temperature, or decrease in stress. Fractography showed that final failure occurred by subcritical crack growth in all specimens.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

数据安全治理现状研究与分析

近年来,国内外数据泄露事件频发,大量企业的商业利益、声誉受损。数据安全法律法规相继颁布,监管力度不断升级,企业逐渐意识到数据安全治理的重要性与紧迫性。通过对2021年开展的企业数据安全治理能力评估现状进行整理,总结企业数据安全治理工作在组织建设、人才培养、技术工具等方面的现状与趋势,提供能力提升思路,以供业界参考。     

Reverse cyclic fatigue of a hot isostatically pressed silicon nitride at 1370 °C

The uniaxial, reverse cyclic fatigue performance of a commercially available hot isostatically pressed silicon nitride was examined at 1370 °C in air and with a 1 Hz sinusoidal waveform using button-head tensile specimens. Specimens did not fail in less than 10⁶ cycles when the applied stress amplitude was less than 280 MPa. Slow crack growth occurred at stress amplitudes 280 MPa and failure always occurred during the tensile stroke of the waveform. Multi-grain junction cavities resulted (i.e., the accumulation of net tensile creep strain) as a consequence of the reverse cyclic loading even though the specimens endured half their life under tensile stresses and the other half under compressive stresses. The presence of multi-grain junction cavities was a consequence of the stress exponent of tensile creep strain being greater than the stress exponent of compressive creep strain. Lastly, it was observed that the static creep resistance of this material improved when it was first subjected to reverse cyclic loading at 1370°C for at least 10⁶ cycles at 1 Hz. Silicon nitride grain coarsening (which was a consequence of the completion of the to silicon nitride solution/reprecipitation process that occurred during the history of the reverse cyclic loading) lessened the capacity for grain boundary sliding resulting in an improved static creep resistance.     

Thermodiffusion‐Assisted Pyroelectrics—Enabling Rapid and Stable Heat and Radiation Sensing

Sensors for monitoring temperature, heat flux, and thermal radiation are essential for applications such as electronic skin. While pyroelectric and thermoelectric effects are suitable candidates as functional elements in such devices, both concepts show individual drawbacks in terms of zero equilibrium signals for pyroelectric materials and small or slow response of thermoelectric materials. Here, these drawbacks are overcome by introducing the concept of thermodiffusion‐assisted pyroelectrics, which combines and enhances the performance of pyroelectric and ionic thermoelectric materials. The presented integrated concept provides both rapid initial response upon heating and stable synergistically enhanced signals upon prolonged exposure to heat stimuli. Likewise, incorporation of plasmonic metasurfaces enables the concept to provide both rapid and stable signals for radiation‐induced heating. The performance of the concept and its working mechanism can be explained by ion–electron interactions at the interface between the pyroelectric and ionic thermoelectric materials.     

Effect of crystal structure on the luminescence properties of CaF2-PrF3 solid solutions

The structure and luminescent properties of lightly doped CaF₂:Pr crystals and Ca_0.65Pr_0.35F_2.35 solid solution have been studied. The results demonstrate that the evolution of the emission spectra of the crystals with increasing PrF₃ concentration can be understood in terms of the structural changes in the fluorite solid solutions, namely, the formation of defect clusters.     

CD317 功能研究进展

CD317 (Tetherin,BST-2或HM1.24)于1994年被发现并命名,是终末分化B细胞的特异性表面标志.2008年首次被鉴定为干扰素诱导型宿主抗病毒因子,此后越来越多的科学家加入到该领域的探索中.经过近十年的研究,目前已经阐述了CD317结构、抗病毒及免疫特性等问题,也陆续发现了一些诸如参与肿瘤进展、束缚外泌体释放等新功能,研究热度不减当年.因此,文章对近几年CD317功能的研究进展进行一个系统的总结,以期为病毒感染、肿瘤发病以及治疗等方面的理论进步和技术发展提供新的思路.     

Chemical reactivity of mineral aggregates in aqueous solution: relationship with bitumen emulsion breaking

This paper reports a study on chemical reactivity of gneiss, diorite and limestone aggregates in aqueous solution. The originality of this study is that it extended to very short times (less than 1440 min). Rise in pH tests was implemented and dissolution kinetics was analysed. The results showed that calcium was the major element released by the aggregates. It has also been found that dissolution had an influence on the final morphology of aggregates. Polyamine emulsifier adsorption onto aggregates has been assessed using electrophoresis. Finally, the rise in pH and electrophoretic tests were compared to the breaking test traditionally performed to characterise bitumen emulsions. It was found that breaking values were controlled by both the surface area and the surface charge of the particles. Results may be correlated to polyamine adsorption on aggregates. Adsorption seemed to be efficient for gneiss and diorite: at pH 2, their charge turned from slightly negative to highly positive. At this pH value, limestone particles were dissolved and polyamine adsorption must be less efficient than with gneiss and diorite, for which the emulsion breaking was facilitated by the high attraction of particles for the emulsifier, due to their negative surface charge.     

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n 1.3–1.4 and Q 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.     

ISO learning approximates a solution to the inverse-controller problem in an unsupervised behavioral paradigm

In "Isotropic Sequence Order Learning" (pp. 831-864 in this issue), we introduced a novel algorithm for temporal sequence learning (ISO learning). Here, we embed this algorithm into a formal nonevaluating (teacher free) environment, which establishes a sensor-motor feedback. The system is initially guided by a fixed reflex reaction, which has the objective disadvantage that it can react only after a disturbance has occurred. ISO learning eliminates this disadvantage by replacing the reflex-loop reactions with earlier anticipatory actions. In this article, we analytically demonstrate that this process can be understood in terms of control theory, showing that the system learns the inverse controller of its own reflex. Thereby, this system is able to learn a simple form of feedforward motor control.