助熔剂对绿色荧光粉Ba2 SiO4∶Eu2+ 颗粒形貌及发光性能的影响

采用高温固相法在弱还原气氛下合成了Ba₂SiO₄∶Eu²⁺绿色荧光粉,利用X射线衍射、扫描电子显微镜和荧光分光光度计考察了不同助熔剂对Ba₂SiO₄∶Eu²⁺荧光粉的结晶度、物相纯度、颗粒形貌和发光强度的影响,并详细讨论了不同助熔剂在荧光粉制备过程中的作用机理。结果表明:不加助熔剂时样品存在BaSi₂和SiO₂杂相;利用NH₄F、Na₂CO₃或H₃BO₃作为助熔剂时会抑制BaSi₂杂相的形成,而BaF₂作助熔剂可以得到纯的斜方晶系。与未加助熔剂合成的荧光粉相比,添加质量分数为2% 的BaF₂、NH₄F或Na₂CO₃后合成样品的发光强度分别提高了138%,81%和34%;而质量分数为2%的H₃BO₃作助熔剂时,荧光粉的发光强度反而降低了14%。BaF₂作助熔剂合成的荧光粉颗粒形貌接近球形,以NH₄F、Na₂CO₃或H₃BO₃作助熔剂合成的荧光粉颗粒形貌分别为不规则片状、纺锤体形和不规则多边形大颗粒。     

竖直微槽道内沸腾换热CHF实验研究

对于沸腾换热,一个主要的约束条件就是临界热流密度(Critical Heat Flux,简称CHF)。这个约束条件对沸腾换热量有一个最高值的限制。文中对矩形微槽道中的流动沸腾临界热流密度进行了实验研究。实验数据是在不同尺寸(0.15mm;0.4mm;1mm)微槽道中,在较大范围的面积质量流速和不同进口过冷度下,以去离子水为工质得到的。实验过程中发现,达到CHF时,靠近出口壁面温度会突然升高,此时传热效率迅速下降。实验数据分析结果表明:CHF随质量流量的增加而增加;进口过冷度对CHF没有明显影响;CHF随着出口干度的增加而降低。     

An extended rational thermodynamics model for surface excess fluxes

In this paper, we derive constitutive equations for the surface excess fluxes in multiphase systems, in the context of an extended rational thermodynamics formalism. This formalism allows us to derive Maxwell-Cattaneo type constitutive laws for the surface extra stress tensor, the surface thermal energy flux vector, and the surface mass flux vector, which incorporate a direct coupling to their corresponding bulk fluxes in the adjacent bulk phases. These constitutive laws also incorporate contributions to the time evolution of the surface excess fluxes from spatial inhomogeneities in these flux fields. These phenomenological equations can be used to model the dynamic behavior of complex viscoelastic interfaces in multiphase systems, in the small deformation limit.     

Photodetachment microscopy of a hydrogen negative ion in an electric field near a metal surface

According to the semi-classical theory, we study the photodetachment microscopy of H- in the electric field near a metal surface. During the photodetachment, the electron is photo-detached by a laser and the electron is drawn toward a position-sensitive detector. The electron flux distribution is measured as a function of position. Two classical paths lead the ion to any point in the classically allowed region on the detector, and waves traveling along these paths produce an interference pattern. If the metal surface perpendicular to the electric field is added, we find that the interference pattern is related not only to the electron energy and the electric-field strength, but also to the ion-surface distance. In addition, the laser polarization also has a great influence on the electron flux distribution. We present calculations predicting the interference pattern that may be seen in experiment. We hope that our study can provide a new understanding of the electron flux distribution of negative ions in an external field and surface, and can guide future experimental research on negative ion photo-detachment microscopy.     

Investigation on the undercooling and crystallization of Ni–Fe alloy coating melt by DSC

The undercooling of Ni–Fe alloy coating melt was in situ investigated by differential scanning calorimeter with flux processing technique. The highest undercooling of Ni–Fe alloy with 426 K was obtained as the thermal treatment temperature of the melt being 1904 K and the cooling rate being 50 K min^?1. When cooling rate is fixed, the undercooling depends on the melt processing temperature, and increases rapidly at the first stage. The effects of thermal treatment temperature and cooling rates on the undercooling were discussed.     

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介绍了EAST超导托卡马克偏滤器的最新冷却模块结构。利用FLUENT有限元软件对不同参数下的模型进行了稳态热分析,定量研究了冷却模块换热能力随各参数的变化规律。针对EAST装置给出了初步优化的参数范围,为水冷钨铜偏滤器的设计制造提供了参考。这对于托卡马克装置及聚变堆偏滤器研究具有十分重要的意义。     

Computational approaches to aspect-ratio-dependent upper bounds and heat flux in porous medium convection

Direct numerical simulation (DNS) has shown that Rayleigh–Bénard convection in a fluid-saturated porous medium self-organizes into narrowly spaced plumes at (ostensibly) asymptotically high values of the Rayleigh number Ra. In this Letter a combination of DNS and upper bound theory is used to investigate the dependence of the Nusselt number Nu on the domain aspect ratio L at large Ra  . A novel algorithm is introduced to solve the optimization problems arising from the upper bound analysis, allowing for the best available bounds to be extended up to Ra≈2.65×10⁴$\mathit{Ra}\approx 2.65\times {10}^4$. The dependence of the bounds on L(Ra)$L(\mathit{Ra})$ is explored and a “minimal flow unit” is identified.     

An Experimental Study of the Two-Phase Flow Adjacent to a Wire in Nucleate Boiling Under an Electric Field

An experimental study of the two-phase flow near a heated wire in nucleate pool boiling is presented. A nonuniform electric field of cylindrical geometry was imposed to the heater. The two-phase flow parameters were measured using specifically developed local phase-detection optical probes. The study includes experiments varying the vertical distance from the heater (z) and measurements of the void fraction and the impact rate in a plane perpendicular to the heater (y-z). The results show that the void fraction decreases with applied voltage and with z. The dependence of the void fraction and the impact rate with heat flux is complex, with a strong decrement in the impact rate and void fraction near the critical heat flux when voltage is applied. The maximum void fraction and impact rate move from the vertical centerline (z) to a position 3 heater radii away.     

Jet Impingement Cooling of a Hot Moving Steel Plate: An Experimental Study

In the current study, a hot moving steel plate of 6 mm thickness with an initial temperature of 900°C has been considered for jet impingement cooling. The experiment has been designed with the help of Design of Expert software to optimize the process parameters based on the highest cooling rate. The various subsurface transient temperature histories have been measured during the cooling process. The surface heat flux and surface temperature were calculated with the help of a commercial inverse heat transfer solver called INTEMP. The experimental result has been presented in terms of cooling rate and critical heat flux.     

ESTIMATED TEMPERATURE ON A MACHINED SURFACE USING AN INVERSE APPROACH

An experiment devoted to the heat flux estimation in a workpiece during a machining process by turning is presented. The method is based on temperature measurement from thermocouples embedded in the workpiece, close to the heated surface. A model that expresses the heat flux according to the temperature at the sensors is developed. The stationary and linearity assumptions are used in order to decompose the three-dimensional original problem into two bi-dimensional problems. This decomposition can be realized given the difference between the cutting speed and feed velocity in two orthogonal directions. The temperature on the machined surface is calculated from the estimated heat flux and the heat transfer model in the workpiece. The application concerns hard steel machining, using a CBN insert tool. Three parameters are placed into evidence from this application: the temperature magnitude on the machined surface, the thermal gradient in the workpiece, and the `thermal persistence' that represent the heating time of the machined surface. This study leads to a better understanding of the influence of temperature during a hard steel turning process.