Degradation Adaptability Assessment of Semisolid Powder Molded Mg-Zn-Mn Alloys for Orthopedic Applications.

Semisolid powder molding was used to prepare the medical Mg-6Zn alloy; in order to further improve its degradation adaptability, 0.5 and 1 wt % Mn were added. Then, the effect of the forming temperature (540, 560, 580, and 600 °C) on the in vitro degradation behavior of the prepared Mg-6Zn-xMn (x = 0.5, 1 wt %) was analyzed, and the optimized alloy was obtained. Finally, the biocompatibility and in vivo degradation performance of the optimized and Mn-free alloys were evaluated. Importantly, single-photon emission tomographic imaging (SPECT/CT) was first applied to monitor the in vivo degradation process. The results show that the corrosion mechanism of the Mn-free alloy is microgalvanic corrosion control with corrosive pitting. After adding Mn, the in vitro degradation rate decreases by half (0.17 ± 0.01 mm/year) as the forming temperature increases to 600 °C, and Mg-6Zn-1Mn prepared at 600 °C is the optimized alloy. Mn addition improves the corrosion product film protection and discontinuous secondary phases, and thus, the corrosion mechanism is changed to corrosive pitting control. Additionally, semisolid powder molding is an easy method to prepare alloys with low average pore interconnectivity (<10%), which is helpful for slowing down the degradation rate. The Mn-containing alloy has better biocompatibility, with a cytotoxicity of grade 0-1, due to its lower degradation rate. The in vivo corrosion rate of the Mn-free alloy is 0.19 mm/year after 28 days of implantation, which was precisely detected by SPECT/CT in real-time. The long-term in vivo degradation adaptability of Mn-free and Mn-containing alloys was not correctly presented, which may be due to the unreasonable bone defect model causing implant displacement. However, both of these alloys cause no obvious inflammation and show good healing. In summary, semisolid powder molding is a potentially promising technique to prepare medical Mg alloys, and nuclear imaging is an effective in vivo degradation evaluation method.

Deep knowledge tracing with learning curves.

Knowledge tracing (KT) models students' mastery level of knowledge concepts based on their responses to the questions in the past and predicts the probability that they correctly answer subsequent questions in the future. Recent KT models are mostly developed with deep neural networks and have demonstrated superior performance over traditional approaches. However, they ignore the explicit modeling of the learning curve theory, which generally says that more practices on the same knowledge concept enhance one's mastery level of the concept. Based on this theory, we propose a Convolution-Augmented Knowledge Tracing (CAKT) model and a Capsule-Enhanced CAKT (CECAKT) model to enable learning curve modeling. In particular, when predicting a student's response to the next question associated with a specific knowledge concept, CAKT uses a module built with three-dimensional convolutional neural networks to learn the student's recent experience on that concept, and CECAKT improves CAKT by replacing the global average pooling layer with capsule networks to prevent information loss. Moreover, the two models employ LSTM networks to learn the overall knowledge state, which is fused with the feature learned by the convolutional/capsule module. As such, the two models can learn the student's overall knowledge state as well as the knowledge state of the concept in the next question. Experimental results on four real-life datasets show that CAKT and CECAKT both achieve better performance compared to existing deep KT models.

The Blow-Off Impulse Equivalence of Typical Missile Homogeneous Al-Alloy under Multienergy Composite Spectrum Electron Beam and Powerful Pulsed X-ray.

The electron beam, one of the most effective approaches to simulate the irradiation effects of powerful pulsed X-ray in the laboratory, plays an important role in simulating the thermodynamic effects of powerful pulsed X-ray. This paper studies the thermodynamics equivalence between multienergy composite spectrum electron beam and blackbody spectrum X-ray, which is helpful to quickly determine the experimental parameters in the simulation experiment. The experimental data of electron beam are extrapolated by numerical calculation, to increase the range of energy flux. Through calculating the blow-off impulse of blackbody spectrum X-ray irradiation, we obtained the curve of X-ray blow-off impulse varying with energy flux, and then found two categories of equivalent relations-equal-energy flux and equal-impulse-by analyzing the calculation results of electron beam and X-ray blow-off impulse. Based on such relations, we could directly or indirectly obtain the results of blackbody spectrum X-ray irradiation blow-off impulse via electron beam experiment.

The Activation of Ti-Zr-V-Hf Non-Evaporable Getter Films with Open-Cell Copper Metal Foam Substrates.

Secondary electron emission (SEE) inhibition and vacuum instability are two important issues in accelerators that may induce multiple effects in accelerators, such as power loss and beam lifetime reduction. In order to mitigate SEE and maintain high vacuum simultaneously, open-cell copper metal foam (OCMF) substrates with Ti-Zr-V-Hf non-evaporable getter (NEG) coatings are first proposed, and the properties of surface morphology, surface chemistry and secondary electron yield (SEY) were analyzed for the first time. According to the experimental results tested at 25 °C, the maximum SEY (δmax) of OCMF before and after Ti-Zr-V-Hf NEG film deposition were 1.25 and 1.22, respectively. The XPS spectra indicated chemical state changes of the metal elements (Ti, Zr, V and Hf) of the Ti-Zr-V-Hf NEG films after heating, suggesting that the NEG films can be activated after heating and used as getter pumps.