低温预冷换热规律及表面改性影响研究

调研了低温管路预冷及两相换热的研究现状,阐述了低温预冷瞬态换热特性;介绍了金属表面改性对预冷规律的影响,仿真了低导热涂层管的预冷换热规律。研究发现:液氮预冷主要由膜态沸腾支配,且预冷耗时较长;液氢预冷未见膜态沸腾,预冷耗时更短。促进膜态沸腾向过渡沸腾的更早转化有利于预冷加速,且转化温度迁移可通过内壁表面结构改性实现,可采用的方法包括表面纳米微结构、低导热涂层、增加微肋结构等,讨论了结构改性对换热规律及预冷效率的影响。     

六方氮化硼薄膜对石墨烯-碳化硅结构近场热辐射的增强

六方氮化硼(hBN)具有跟石墨烯类似的层状结构和晶格参数,研究发现hBN薄膜具有良好的热传导、电绝缘、光学和力学等性能。本文从理论上研究了hBN薄膜对石墨烯-碳化硅(G/S)结构的近场热辐射的影响。研究发现在红外频段.hBN薄膜在低频率区和高频率区会增强G/S结构的近场热辐射,经计算在G/S结构中加入厚度为10 nm的hBN薄膜时获得的辐射热流是同物理条件下G/S结构的1.5倍;而在中频率区hBN薄膜的厚度阻碍了石墨烯表面等离激元和碳化硅表面声子极化激元的耦合,使得近场热辐射热流随hBN薄膜厚度增加而逐渐减弱。本研究的结果可为下一步实验与应用中对hBN薄膜厚度的选择提供理论基础。     

煤粉燃烧过程中灰膜形成比例实验研究

煤焦颗粒燃烧过程中,灰膜形成显著影响其燃烧特性。因此,本文借助高温沉降炉研究了61~75,75~90和90~125μm三种粒径黄陵烟煤在1273和1673 K温度下的燃烧特性与灰膜形成比例;借助扫描电镜(SEM)详细观测空心微珠颗粒内部结构,提出灰膜比例计算公式,并分析温度,粒径和碳转化率对灰膜比例的影响。结果表明,高温下大部分灰分在焦炭烧尽阶段以灰膜形式存在。灰膜比例随温度和碳转化率增加而增加,随煤粉粒径增大而减小。高温下灰分用于形成灰膜比例相对较高,这为煤焦燃尽阶段的低反应性提供了合理的解释。煤焦颗粒动态燃烧过程中灰膜形成比例随燃烧工况变化而变化。该研究为煤焦颗粒燃烧动力学模拟灰膜比例选择提供了关键数据支撑。     

Tb3+掺杂锂铝硅酸盐玻璃的光致发光和辐照致发光

采用熔融-淬冷法制备了Tb3+掺杂锂铝硅酸盐闪烁玻璃,用紫外激发光谱、发射光谱及荧光寿命表征了光致发光性能,用X射线和阴极射线激发测试了辐射致发光性能。研究结果表明：低Tb3+掺杂浓度时,随着其浓度增大,Tb3+间的交叉弛豫增加导致了5D₃→7F_j跃迁的能量逐渐向5D₄→7F_j迁移转变,5D₃激发态的荧光寿命和发射强度均明显下降,5D₄-7F_j发射强度逐渐增大。较高Tb3+浓度时,其浓度继续增加会提升非辐射比例,是荧光寿命降低和荧光猝灭的最主要原因。比较光致发光和辐照致发光性能,发现随着激发源的能量上升,会增加激发态5D₃能级向5D₄能级的能量转移,同时,由于玻璃的密度低会导致辐照致发光效率随激发源的能量上升而下降。     

Pulsed laser deposition of pure fluoroapatite film without OH groups

The relation between O₂ pressure and composition in the pulsed‐laser deposition of fluoroapatite was investigated using both on‐axis and off‐axis methods to determine the optimal conditions for obtaining a pure fluoroapatite film without OH groups. Through this, it was found that an O₂ pressure of 10 Pa, combined with an off‐axis method, results in P/Ca and F/Ca values (0.6 and 0.2, respectively) that match closely with a stoichiometric composition of Ca₁₀(PO₄)₆F₂. Fourier transform infrared spectroscopy analysis confirmed that this optimized film was almost pure fluoroapatite, with no evidence of any OH groups originating from hydroxyapatite. X‐ray diffraction also revealed that this fluoroapatite film crystallizes with a c‐axis orientation perpendicular to its surface.     

A ZnO/TiO2 composite nanorods photoanode with improved performance for dye‐sensitized solar cells

A dye‐sensitized solar cell (DSSC) based on ZnO/TiO₂ composite nanorods (NRs) photoanode is fabricated. The power conversion efficiency (PCE) of the ZnO/TiO₂ composite NRs film DSSC is 4.36%, which is obviously higher than that of DSSCs based on pure TiO₂ NRs (0.6%) and ZnO NRs (3.10%). The enhanced performance of ZnO/TiO₂ composite NRs film DSSC can be attributed to the combined effects of ZnO and TiO₂ NRs. In this architecture, the thick ZnO NRs overlayer offers a large surface area for enough dye absorption, while the thin TiO₂ NRs underlayer not only offers a direct and quick pathway for photoinjected electron transfer along the photoanode but also acts as a blocking layer, which effectively hinders the direct contact between the substrate and the electrolyte resulting in lower carrier recombination.     

Determination of Volatile Organic Compounds by a Novel Polymer Spin-Coated Thin Film and Surface Plasmon Resonance

Here, the synthesis, characterization, and volatile organic compound (VOCs) sensing of a 1,3-dimethyl polyphenylene vinylene polymer is reported. The synthesis was performed by a Witting condensation through the reaction of 1,4-terphthaldehyde with the phosphonium chloride of meta-xylene. The material was characterized by infrared spectroscopy, elemental analysis, and thermogravimetric analyses. Thin films of the polymer were prepared by spin coating at speeds from 1000 to 5000?rpm. Ultraviolet–visible spectroscopy and surface plasmon resonance were used to characterize the spin-coated films. The thicknesses of the films were estimated by fitting the curves and were between 4.5 and 24.5?nm depending on the speed. The refractive index of the new polymer was 1.72. The polymer spin-coated films were exposed to volatile organic vapors to characterize their sensing properties by surface plasmon resonance as a function of time. The results showed that the new material responded rapidly, sensitively, and reversibly to VOCs.     

Morphological change of crystalline polymer films by annealing: substrate‐ and heating/cooling‐rate‐dependent surface roughness

We study the morphological change of crystalline polymer films by annealing using atomic force microscope, X‐ray diffraction, and Fourier transform infrared spectroscopy techniques. As typical samples, we employ high‐density and low‐density polyethylene films prepared by the cast method. After annealing at 135 °C for 4 h, the surface roughness of polyethylene films by the atomic force microscope significantly increases, and the crystallite size by the X‐ray diffraction also shows some increase, while the Fourier transform infrared spectroscopy spectrum hardly exhibits any change. This can be well explained as a result of the growth of crystal structure by recrystallization during annealing. More interestingly, we find that the choice of the substrate and also the heating/cooling rates for annealing significantly influences the surface roughness of the films. Copyright © 2017 John Wiley & Sons, Ltd.     

A novel method for the determination of diffusion coefficients in amorphous poly(3-hydroxybutyrate)

A novel approach is proposed for the determination of the diffusion coefficient of certain drugs in amorphous poly(hydroxybutyrate) (PHB), which can be a reliable alternative to the conventional permeation based measurements. The method requires the preparation of PHB films with various concentrations of the drug and if the latter absorbs in the visible wavelength range, its concentration gradient in the polymer film as well as the time dependence of the latter can be analyzed quantitatively by following changes in color. Color can be converted into concentration with the help of adequate calibration and thus the dependence of additive concentration on space (x) and time (t), i.e. the c(x,t) function, can be determined relatively easily. The fitting of the numerical solution of Fick's second law onto the measured values provides directly the targeted diffusion coefficient. The comparison of diffusion coefficients obtained by the proposed approach to values published in the literature proved that the new method provides reliable results and requires reasonable time and effort at the same time.     

Separation of water-in-oil emulsion with microfiber glass coalescing bed

Water-in-oil emulsion separation through a fibrous media bed is a complex and challenging process in industries. In this article, we used a vertical column separator to investigate the effects of the height and porosity of the fiber bed, the structure arrangement (the mixed or the layered fibrous bed), the superficial velocity of the water-in-oil emulsion through the bed, and the influent water content of the emulsion on water removal. Four kinds of glass microfibers (GF1-GF4) with mean diameters of 0.6, 2.6, 4.6, and 8.0?µm, respectively, acted as the coalescence medium and composed the fibrous bed with different structure types. The separation efficiency could reach 97.1% with a relatively low pressure drop under the optimal bed structure and operational conditions. It also showed that the mixed bed had higher separation performance compared to the layered fibrous bed.