马氏体时效钢18Ni(350)热塑剪切变形的研究

18Ni(350)马氏体时效钢在时效处理状态下的切削法试验中有热塑剪切失稳现象,试验结果指出:热塑剪切失稳是一种热——力效应,切屑成锯齿形,齿间由热塑剪切带分隔,带的宽度与切削速度有关.     

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of β-Mg₁₇Al₁₂ phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.     

Adhesion properties of lactic acid based hot melt adhesives and their storage stability in different packaging applications

Adhesion properties and the stability during the use life of the package for two biodegradable hot melt adhesives were evaluated. Adhesives were based on poly (L-lactide) (PLLA) and poly(-caprolactone) (PCL) with molar ratio 81:19. One sample was stabilized by end-capping the terminal hydroxyl groups with acetic anhydride. The other sample was unmodified. Reference adhesive that was used in the studies, was a conventional non-biodegradable hot melt adhesive based on poly(ethylene-co-vinylacetate) (EVA).

Materials, which were bonded with these hot melt adhesives, were typical biodegradable packaging materials: a pigment-coated cardboard, a similar cardboard extrusion laminated with film type PLLA and an uncoated cardboard. The storage stabilities of the copolyester films and bonded structures were monitored during an eight week period at room (23°C) and at low (−18°C) temperatures.

Changes in molecular weight and crystallinity of the copolyesters were measured with gel permeation chromatography (GPC) and differential scanning calorimetry (DSC), respectively. Mechanical strength of the adhesive bonds was measured by a tensile testing instrument. Fracture surfaces of the adhesive bonds were subjected to microscopic studies.

Initial adhesion properties of the copolyesters were similar to those of EVA and better in case of PLLA-laminated cardboard. Both biodegradable copolyester samples degraded during the studied period; however, end-capping of the copolymer retarded this degradation rate.     

Crystallization and phase separation in blends of high stereoregular poly(lactide) with poly(ethylene glycol)

The effect of cooling rate on crystallization and subsequent aging of high stereoregular poly(lactide) (PLA) blended with poly(ethylene glycol) (PEG) was studied by thermal analysis and by direct observation of the solid state structure with atomic force microscopy (AFM). Blending with PEG accelerated crystallization of PLA. When a PLA/PEG 70/30 (wt/wt) blend was slowly cooled from the melt, PLA crystallized first as large spherulites followed by crystallization of PEG. The extent of PLA crystallization depended on the cooling rate, however, for a given blend composition the PEG crystallinity was proportional to PLA crystallinity. The partially crystallized blend obtained with a cooling rate of 30 °C min⁻¹ consisted of large spherulites dispersed in a homogeneous matrix. The blend was not stable at ambient temperature. With time, epitaxial crystallization of PEG on the edges of the spherulites depleted the surrounding region of PEG, which created a vitrified region surrounding the spherulites. Further from the spherulites, the homogeneous amorphous phase underwent phase separation with formation of a more rigid PLA-rich phase and a less-rigid PEG-rich phase. Decreasing the amount of PEG in the blend decreased the crystallization rate of PLA and increased the nucleation density. The amount of PLA crystallinity did not depend on blend composition, however, PEG crystallinity decreased to the extent that PEG did not crystallize in a PLA/PEG 90/10 (wt/wt) blend.     

Multi-month thermal aging of electro-optic polymer waveguides: Synthesis, fabrication, and relaxation modeling

The preparation of polyimides containing side-chain chromophores and the long-term aging performance of poled films are described. These materials were compared to guest-host polycarbonate films. Mach-Zehnder optical interferometers were fabricated from these polymers that contained CLD- and FTC-type chromophores. Changes in optical properties were monitored for months at four temperatures ranging from ambient to 110 °C. The isothermal relaxation data were modeled using both a stretched exponential equation and a power law in time equation. The temperature dependency of the time constants of these equations was modeled using a new activation-energy equation: ln(τ/τ_p) = E_R(1 + tanh[(T_c − T)/D])/2RT + E_p/RT where T_c is the central temperature of the transition zone, D is the breadth of the zone, and Es are the activation energies of rigid and pliable materials. Multi-year high-temperature stability of the poled guest-host and side-chain materials was predicted.     

医疗保险应对人口老龄化的几点思考

21世纪的中国将是一个老龄化社会。我国的医疗保障制度经过近20年的改革探索,目前社会医疗保险制度已初具规模。但制度还不够完善,保障范围和水平还很低,还没有达到“人人享有卫生保健”的基本目标。为应对人口老龄化给医疗保险制度带来的压力。分析了人口老龄化对医疗保险基金的影响,列举了社保中心应对人口老龄化的几点举措,提出人口老...     

Mechanical,Thermal, and Morphological Behavior of Silicone Rubber during Accelerated Aging

An accelerated aging study on silicone rubber exploring the effects of exposure to a functional oil (polyalkylene glycol) at elevated temperature (195°C) is reported in this paper. Variations in mechanical (tensile, tear, hardness) and thermal (conductivity, specific heat capacity) properties were monitored versus aging time while permanent deformation of the rubber was evaluated through creep and recovery measurements. Morphology and surface chemistry of the aged rubber were also investigated through scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy, respectively. Aging had a significant impact on the mechanical properties with the ultimate tensile strength and elongation at break decreasing from 7.4?MPa and 2250% in unaged samples to 1.5?MPa and 760% in 6-week aged samples, respectively. The tear strength and hardness exhibited an initial increase during the early stages of aging, followed by a decreasing trend. In contrast, the thermal properties did not change significantly and FTIR did not detect any changes in the surface chemistry of the rubber with aging. SEM however, provided evidence of an increase in brittle behavior from the morphology of the fractured surfaces.     

Modeling the pressure drop behavior of cleanable dust filters during pressure-controlled operation

Cleanable dust filter media are typically used in huge baghouse filter apparatuses. Thereby, the regeneration by back-pulsing from the clean gas side is done by either time-controlled or pressure-controlled operation, whereas the latter is more common. Hence, the need for a detailed knowledge of the clogging and filtration mechanisms during long time operation of a pressure-controlled filter aging arises.

A mathematical model describing the pressure drop evolution during time-controlled filter aging has been developed. The core of the developed model is the concept of dust masses that distribute themselves on a specific particle deposition area inside and on the surface of the filter medium. By altering this particle deposition area, various clogging mechanisms, occurring during an aging procedure, are covered by the model.

In this work, the model was adapted and coefficient parameters adjusted for pressure-controlled filter regeneration operation. A multitude of pressure-controlled test runs were performed in a specially designed filtration apparatus. From these tests, process-specific parameters were regressed and used to model the respective pressure drop curves. These model pressure drop curves show good accordance both quantitatively and qualitatively to experimental data and give a detail view on different clogging mechanisms.     

The stability of electrical characteristics of Ti/n-Si/Ag,Ti/n-Si/Cu and Ti/n-Si/AgCu diodes prepared under the same conditions with respect to increasing aging time

The purpose of this study is to fabricate Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu Schottky type diodes and to investigate the effects of aging time on the diode parameters such as ideality factor, barrier height, series resistance, interface state density and rectification ratio. High purity titanium (Ti) metal was deposited on the back side of the n-Si semiconductor and then the Ti/n-Si junction was annealed at 420 °C in nitrogen atmosphere. This junction showed ohmic behavior. To fabricate rectifier contacts, Ag, Cu metals and AgCu alloy have been evaporated on the other polished surface of n-Si with Ti ohmic contact. Ag and Cu ratios in the AgCu alloy which are used in the process of preparing the Schottky contact were taken in equal weights. Thus, Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu Schottky type diodes were prepared under the same conditions. The current-voltage (I-V) characterization of Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu diodes were immediately made at room temperature in dark conditions. To investigate the effect of aging time, the I-V measurements of the diodes have been repeated after 1, 7, 15, 30 and 90 days. Characteristic parameters of the diode were calculated from the I-V measurements which are taken with respect to aging time. The results were compared. From these results, it can clearly be seen that the electrical characteristics of diode which is made from AgCu alloy are more stable than other two diodes.     

Effect of Aging Treatment on Microstructural and Mechanical Characteristics of PEO Coatings on Mg-Al Alloy

Cast Mg-6 wt pct Al alloy solution-treated at 683 K for 16 h and aged at 498 K was coated by plasma electrolytic oxidation （PEO） method.The Mg-6 wt pct Al alloy aged for 16 h exhibited the highest microhardness and wear resistance.After PEO coating,however,the microhardness and wear resistance of coatings on Mg- 6 wt pct Al alloy showed a tendency to decrease with increasing aging time,which was in aggrement with the change of thickness with aging time.In addition,the coatings on solution-treated Mg-6 wt pct Al alloy had better microhardness and wear resistance than those on aged Mg-6 wt pct Al alloys.Consequently,it can be understood that the aging treatment has a deleterious influence on the mechanical properties of coatings on Mg-6 wt pct Al alloy.