反应溅射制备TiN薄膜的靶动力学研究

从微观粒子的相互作用和输运出发，探讨靶溅射过程中的基元化学物理步骤，计算了溅射速率和离子能量分布，为深入研究溅射薄膜的生长速率和靶中毒奠定基础。     

Mechanical properties,crystallization characteristics,and foaming behavior of polytetrafluoroethylene‐reinforced poly(lactic acid) composites

In this study, poly(lactic acid) (PLA)/polytetrafluoroethylene (PTFE) composites containing different amounts of PTFE were prepared by melt blending. The mechanical, crystallization, and foaming properties of the prepared composites were investigated. Tensile test results indicated that the mechanical properties of the composite with PTFE showed significant reinforcement and toughening effects. The average elongation‐at‐break of the composite increased by 72% compared with pure PLA. Scanning electron microscopy (SEM) showed that the PTFE elongated into fibrils during blending and formed a physical network of entanglements in the melt. Differential scanning calorimetry (DSC) also showed that PTFE had a significant nucleation effect on polymer crystals and greatly increased the crystallinity of the PLA matrix. Moreover, PTFE dramatically enhanced the melt viscosity of PLA, which was investigated by rheological tests. The injection molding foaming experiments revealed that adding 1 wt% PTFE had the most notable heterogeneous nucleation effect on foamed cells, with the cell size decreasing from 81.5 μm for neat PLA to 25.2 μm, and the cell density increasing from 1.34 × 10⁸ cells/cm³ to 2.53 × 10⁹ cells/cm³. POLYM. ENG. SCI., 57:570–580, 2017. © 2016 Society of Plastics Engineers     

Microstructure and crystallography in microcellular injection‐molded polyamide‐6 nanocomposite and neat resin

The effects of adding nanoclay to polyamide‐6 (PA‐6) neat resin, and the effects of processing parameters on cell density and size in microcellular injection‐molded components were investigated. In addition, the crystal sizes, structures, and orientation were analyzed with the use of x‐ray diffraction (XRD) and a polarized optical microscope. The standard ASTM D 638‐02 tensile bars for the analyses were molded according to a fractional four‐factor, three‐level, L9 Taguchi design of experiment (DOE) with varying melt temperature, injection speed, supercritical fluid (SCF) concentration, and shot size. It was found that the presence of montmorillonite (MMT) nanoclay greatly reduced the size of the cells and crystals, but increased their density in comparison with neat resin processed under identical molding conditions. In addition, at the sprue section downstream of the machine nozzle, cell size gradually decreased from the part center toward the skin for both the neat resin and the nanocomposite. It was also found that shot size was the most important processing parameter for both the neat resin and nanocomposite in affecting cell density and size in microcellular injection molding components. Weakly preferred crystal orientations were observed on the surface of microcellular injection‐molded PA‐6/MMT tensile bars. Finally, the addition of nanoclay in PA‐6 neat resin facilitated the formation of γ‐phase crystals in the molded components. Polym. Eng. Sci. 45:52–61, 2005. © 2004 Society of Plastics Engineers.     

Effect of bond-coat pre-aluminization and pre-oxidation duplex pretreatment on the performance of ZrO2-8 wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

A modification to the conventional thermal-barrier coating (TBC) system was made. In this study, the low-pressure, plasma-sprayed Co-29Cr-6Al-1Y bond coat received a duplex pretreatment of bond-coat pre-aluminizing and pre-oxidation prior to overlaying the air-plasma sprayed ZrO₂-8 wt.% Y₂O₃ top coat. The effects of this treatment on the properties of the TBC system were evaluated by thermal-cyclic test at 1050° C in air. The results of cyclic-oxidation tests showed that the proposed processes could remarkably improve the performance of ZrO₂-8 wt.% Y₂O₃/Co-29Cr-6Al-1Y TBC. The improvement was attributed to a modification of bond-coat oxides and the associated reduction of the oxidation rate of the MCrAlY bond coat.     

Effect of pre-aluminization on the properties of ZrO2-8wt.% Y2O3/Co-29Cr-6Al-1Y thermal-barrier coatings

Specimens of investment-cast Mar-M247 superalloy were vacuum-plasma sprayed with Co-29Cr-6Al-1Y bond coat, and part of the specimens were further pre-aluminized at 980°C for 2, 4, 6, 8, and 10 hours. All the specimens were then deposited with ZrO₂-8 wt.% Y₂O₃ thermal-barrier coatings (TBCs) and thermally cycled at 1050°C to evaluate the effect of time of the prealuminizing treatment on the performance and failure mechanism of the modified system. Results showed that TBC specimens with pre-aluminized bond coatings exhibited lower oxidation rates and significantly higher cyclic life when compared with unaluminized specimens. The failure of bond-coat pre-aluminized TBC specimens was observed to propagate mainly along the lamellar splats of the top coat, whereas the failure of conventional TBC specimens occurred mainly along the top-coat/spinel oxides interface.     

The effect of nanoclay on the crystallization behavior,microcellular structure,and mechanical properties of thermoplastic polyurethane nanocomposite foams

The effects of nanoclay on the crystallization behavior, microcellular structure, and mechanical properties of thermoplastic polyurethane (TPU)/clay nanocomposite (TPUCN) foams were investigated using differential scanning calorimetry, rheometry, scanning electron microscope, transmission electron microscopy, and X‐ray diffraction. It was found that the nanoclay acted as an effective nucleating agent for both crystal nucleation and cell nucleation. As a result, it significantly enhanced the crystallization behavior of the hard segment (HS) domains in TPU while refining the foamed structure of the microcellular injection molded parts. In particular, the average cell diameter of TPUCN foams decreased from 45 µm for neat TPU to 27 µm for TPUCN5 (5 wt% clay) and 18 µm for TPUCN10 (10 wt% clay). Furthermore, the cell density increased from 0.7 × 10⁷ cell/cm³ for neat TPU to 1.4 × 10⁷ cell/cm³ and 3.1 × 10⁷ cell/cm³ for TPUCN5 and TPUCN10, respectively. In addition, the tensile strength also increased by 56.3% and 89.2% with 5 and 10 wt% clay content, respectively. By controlling the cell nucleation behavior through uniformly dispersed nanoclay, this study demonstrates that it is feasible to produce TPUCN foams via microcellular injection molding with desirable microcellular structures and improved mechanical properties. POLYM. ENG. SCI., 56:319–327, 2016. © 2015 Society of Plastics Engineers     

Effects of bond coat preoxidation on the properties of ZrO2-8wt.% Y2O3/Ni-22Cr-10Al-1Y thermal-barrier coatings

Plasma-sprayed thermal barrier coatings (TBCs) consist of an intermediate MCrAlY bond coat to protect the substrate superalloy from oxidation/hot corrosion and a thermal insulating zirconia-based ceramic top coat. This system is developed for advanced turbine-engine, hot-section components. In this study the as-sprayed Ni-22Cr-10Al-1Y bond coat was subjected to preoxidation treatment at 1000° C for 1, 50, 100, and 200 hr, also at 1100°C, 1200°C and 1300°C for 1 hr to form an oxide scale before subsequent deposition of a ZrO₂-8wt.% Y₂O₃ top coat. The oxidation kinetics were measured, and the phase constitution and morphology of pregrown oxides on the Ni-22Cr-1Y bond coat were analyzed by x-ray diffractometer and SEM/EDS to elucidate the improvement and degradation mechanisms of the new system. The results of the experiments in this study showed that the tentative ZrO₂-8wt.% Y₂O₃ TBC specimens with preoxidized Ni-22Cr-10Al-1Y bond coat, when properly processed, exhibited lower oxidation rates and generally longer lifetime compared with traditional TBC specimens.     

A floating point conversion algorithm for mixed precision computations

The floating point number is the most commonly used real number representation for digital computations due to its high precision characteristics. It is used on computers and on single chip applications such as DSP chips. Double precision (64-bit) representations allow for a wider range of real numbers to be denoted. However, single precision (32-bit) operations are more efficient. Recently, there has been an increasing interest in mixed precision computations which take advantage of single precision efficiency on 64-bit numbers. This calls for the ability to interchange between the two formats. In this paper, an algorithm that converts floating point numbers from 64- to 32-bit representations is presented. The algorithm was implemented as a Verilog code and tested on field programmable gate array (FPGA) using the Quartus II DE2 board and Agilent 16821A portable logic analyzer. Results indicate that the algorithm can perform the conversion reliably and accurately within a constant execution time of 25 ns with a 20 MHz clock frequency regardless of the number being converted.     

Modeling and prediction of morphology and crystallinity for cylindrical‐shaped crystals during polymer processing

To describe and predict the crystallization and morphology evolution of semicrystalline polymers, the shape of crystals is often modeled as spheres and/or cylinders. The Kolmogoroff–Avrami model, which is often used to model the crystallinity and spherulite growth of polymer materials under isothermal and nonisothermal conditions, has shortcomings in dealing with conditions when spherulites and cylindrical crystals form simultaneously. This study adopted the Monte Carlo method, a method based on the random theory, to model and predict the morphological evolution of crystallization and the degree of crystallinity for polymers that exhibit concurrently growing spherulites and cylindrical crystals. A case study on predicting the morphology and crystallinity of a solidifying polymer melt with the memory effect (self‐seeding) from prior stretching is presented. The effectiveness of this Monte Carlo approach vs. the original Kolmogoroff–Avrami model was demonstrated when compared with experimental results. In addition, for the ideal cylindrical growth of crystals, a modified Kolmogoroff–Avrami model based on the Monte Carlo method solutions is proposed to predict the crystallinity without intensive computation. The results showed that the modified expression performs better than the original Avrami prediction. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers