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1.
事务处理技术研究综述   总被引:14,自引:0,他引:14  
事务处理技术是保证信息一致性和可靠性的关键技术.首先阐述了事务处理技术的起源与发展,总结了并发与恢复两个核心技术的研究现状,探讨了平坦事务模型的各种表现形式,接着研究了各种扩展事务模型和事务工作流,概括了多库、面向对象数据库、实时数据库和移动数据库等现代数据库中的事务处理技术,分析了相关研究的进展,最后结合已有的研究成果,展望了其未来研究方向及面临的挑战.  相似文献
2.
一种基于拓扑连接图的三维模型检索方法   总被引:13,自引:1,他引:12  
潘翔  张三元  张引  叶修梓 《计算机学报》2004,27(9):1250-1255
提出了一种基于分割技术的拓扑连接图构造方法。并用于三维模型检索.首先构造多层邻域计算用于度量每个三角片平坦度的信号值。根据这些信号值采用一种改进的分水岭方法把模型分割成不同的曲面片.然后利用这些曲面片之间的连接关系构造拓扑连接图.最后通过比较不同模型的拓扑连接图相似性来得到它们的匹配度。根据匹配度进行三维模型检索.  相似文献
3.
基于曲面局平特性的散乱数据拓扑重建算法   总被引:11,自引:0,他引:11       下载免费PDF全文
谭建荣  李立新 《软件学报》2002,13(11):2121-2126
提出了一种基于曲面局平特性的,以散乱点集及其密度指标作为输入,以三角形分片线性曲面作为输出的拓扑重建算法.算法利用曲面的局平特性,从散乱点集三维Delaunay三角剖分的邻域结构中完成每个样点周围的局部拓扑重建,并从局部重建的并集中删除不相容的三角形,最终得到一个二维流形拓扑曲面集作为重建结果.该算法适应于包括单侧曲面在内的任意不自交的拓扑曲面集,并且重建结果是相对优化的曲面三角形剖分,可以应用于科学计算可视化、雕塑曲面造型和反求工程等领域.  相似文献
4.
Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d→3d, 5d→4f or 4f→4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y2SiO5, Y3Al5O12, Y2O3, and BaMgAl10O27 along with controlled amounts of the activators (Cr3+, Mn2+, Ce3+, Eu2+, Eu3+, Tb3+, Tm3+) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials.

1. Introduction

The visible-light-generating components of emissive, full color, flat panel displays are called phosphors. Phosphors are composed of an inert host lattice and an optically excited activator, typically a 3d or 4f electron metal. For application in the emerging full color, flat panel display industry, thermally stable, high luminous efficiency, radiation resistant, fine particle size powders are required. The demands of these newer technologies have produced a search for new materials and synthesis techniques to improve the performance of phosphors.Oxide phosphors were found to be optimal for field emission display (FED) and plasma panel display (PDP) devices. Compared with a cathode ray tube, an FED operates with lower energy (3–10 keV) but higher current density (1 mA/cm2) beams impinging on the phosphors. This requires more luminous efficient and thermally stable materials. Luminous efficiency is defined as the ratio of the energy out (lumens) to the input energy. Outgassing from the highly efficient sulfide based phosphors has been shown to degrade the cathode tips of the field emitter array and cause irreversible damage [1]. For PDPs, high energy photons (147 nm, 8.5 eV) impinge on the phosphor powders and cause a reduction in luminous efficiency of the display over time because of radiation damage induced in the material [2].Another requirement is on the particle size distribution: there is a maximum and minimum particle size limitation to the powders. For FED applications, about five particle layers are required to achieve optimal light output [3]. Large particles (>8 μm) require thicker layers, increasing the phosphor cost and also producing more light scattering. Additionally the pixel pitch (250 μm) places a maximum on particle size [4]. Alternatively, it was found that small particles (<0.2 μm) do not have high luminous efficiency arising from grain boundary effects [5]. The activator ion in the crystal is most efficient when located in the bulk material in a regular crystal field. Activators located on the surface or on the grain boundaries are thought to be non-luminescent or even luminescence quenching regions.For full color displays, three phosphor compositions are necessary to emit in the red (611–650 nm), green (530–580 nm) and blue (420–450 nm) regions of the visible spectrum. Some oxide based phosphors used in FEDs are the red-emitting (Y1−xEux)2O3, the green-emitting (Y1−xTbx)3Al5O12 and the blue-emitting (Y1−xCex)2SiO5 [6]. For some PDPs the red-emitting component is (Y1−xEux)2O3, the green-emitting is Zn1−xMnxSi2O5 and the blue-emitting is (Ba1−xEux)MgAl10O17 [7]. Sulfide phosphors are also used in FEDs but suffer from the aforementioned degradation problems.

2. Phosphor synthesis techniques

Synthesis of oxide phosphors has been achieved by a variety of routes: solid-state reactions [8 and 9], sol–gel techniques [10], hydroxide precipitation [11], hydrothermal synthesis [12 and 13] and combustion synthesis [14, 15, 16 and 17]. Solid-state reactions are performed at high temperatures, typically around 1600°C, because of the refractory nature of the oxide precursors. For multielement compositions, an incomplete reaction is often obtained with undesirable precursor products present in the final product. This technique requires several heating and grinding steps in order to achieve well-reacted, small particle size phosphors. For sol–gel and hydroxide precipitation methods, dilute solutions of metallorganics or metal salts are reacted and condensed into an amorphous or weakly crystalline mass. The advantage of these methods is that atomically mixed powders are obtained in the as-synthesized condition and problems associated with incomplete reactions are avoided. However, these as-synthesized materials must also be heat treated to high temperatures to crystallize the desired phase and to achieve particle sizes greater than 0.2 μm. Hydrothermal synthesis is a low temperature and high pressure decomposition technique that produces fine, well-crystallized powders [13]. These powders must also be heat treated to high temperature to extract the maximum luminous efficiency. Combustion synthesis is a novel technique that has been applied to phosphor synthesis in the past few years. This technique produces highly crystalline powders in the as-synthesized state and will be described in more detail in Section 3.

3. Combustion synthesis of oxide phosphors

Combustion synthesis involves the exothermic reaction between metal nitrates and a fuel. Combustion synthesis is an important powder processing technique generally used to produce complex oxide ceramics such as aluminates [18, 19, 20 and 21], ferrites [22, 23, 24 and 25], and chromites [26 and 27]. The process involves the exothermic reaction of an oxidizer such as metal nitrates, ammonium nitrate, and ammonium perchlorate [28], and an organic fuel, typically urea (CH4N2O), carbohydrazide (CH6N4O), or glycine (C2H5NO2).The combustion reaction is initiated in a muffle furnace or on a hot plate at temperatures of 500°C or less; much lower than the phase transition of the target material. In a typical reaction, the precursor mixture of water, metal nitrates, and fuel decomposes, dehydrates, and ruptures into a flame after about 3–5 min. The resultant product is a voluminous, foamy powder which occupies the entire volume of the reaction vessel. The chemical energy released from the exothermic reaction between the metal nitrates and fuel can rapidly heat the system to high temperatures (>1600°C) without an external heat source. Combustion synthesized powders are generally more homogeneous, have fewer impurities, and have higher surface areas than powders prepared by conventional solid-state methods [28].The mechanism of the combustion reaction is quite complex. The parameters that influence the reaction include: type of fuel, fuel to oxidizer ratio, use of excess oxidizer, ignition temperature, and water content of the precursor mixture. In general, a good fuel should react non-violently, produce non-toxic gases, and act as a complexant for metal cations [28]. Complexes increase the solubility of metal cations, thereby preventing preferential crystallization as the water in the precursor solution evaporates [29]. The adiabatic flame temperature, Tf, of the reaction is influenced by the type of fuel, fuel to oxidizer ratio, and the amount of water remaining in the precursor solution at the ignition temperature [27]. The flame temperature can be increased with the addition of excess oxidizer such as ammonium nitrate [28], or by increasing the fuel/oxidizer molar ratio. The following equation can be used to approximate the adiabatic flame temperature for a combustion reaction:
(1)
where ΔHr and ΔHp are the enthalpies of formation of the reactants and products, respectively, cp is the heat capacity of products at constant pressure, and T0 is 298 K. Measured flame temperatures are typically lower than calculated values of flame temperature as a result of heat loss. Table 1 lists the various phosphor compositions that were synthesized by combustion synthesis.
Table 1. Phosphor compositions obtained by combustion synthesis
An example of a stochiometric combustion reaction of yttrium, aluminum and terbium nitrate with carbohydrazide to form (Y1−xTbx)3Al5O12 is:
(2)
(1−x)Y(NO3)3+5Al(NO3)3+3xTb(NO3)3+15CH6N4O→ (Y1−xTbx)3Al5O12+42N2+15CO2+45H2O.
When complete combustion occurs, the only gaseous products obtained are N2, CO2, and H2O, making this an environmentally clean processing technique. The generation of gaseous products increases the surface area of the powders by creating micro- and nanoporous regions. For the earlier reaction, for every mole of solid produced, 102 mol of gas are produced.The difference in particle size with the use of different fuels depends upon the number of moles of gaseous products released during combustion. As more gases are liberated, the agglomerates are disintegrated and more heat is carried from the system thereby hindering particle growth. A greater number of moles of gas are produced in combustion reactions with carbohydrazide. If complete combustion is assumed, the gaseous product amounts liberated in combustion reactions with glycine, urea and carbohydrazide, are shown in Table 2. The reactions shown are for 2 mol of nitrate producing 1 mol of metal sesquioxide. If Y3Al5O12 (YAG) is produced, the reactions must be multiplied by four, as 8 mol of nitrate are used in the reaction.
Table 2. Number of moles of gas produced for different fuels per mole of metal sesquioxide formed
The BET surface area for the as-synthesized YAG phosphors made with glycine, urea and carbohydrazide was measured to be 19, 22 and 25 m2/g, respectively [16], which is consistent with the increase in number of moles of gas produced.Fig. 1 shows the efficiency in lumens per watt (lm/W) as a function of electron accelerating voltage for (Y1−xTbx)Al5O12 made by solid-state, hydrothermal synthesis and combustion synthesis. The combustion synthesized YAG produced in this work has low-voltage cathodoluminescence efficiencies that are comparable to powders produced by other techniques. The efficiencies for all three phosphors were essentially the same at voltages below 600 V. At these voltages, the penetration depth of the incident electron beam is low, (0.004 nm at 600 V) exciting the surface layer of the phosphor particles. In the higher voltage regime, (>600 V), the penetration depth of the electron beam is greater (0.03 nm at 1 kV). The efficiencies of solid-state and hydrothermal synthesized (Y1−xTbx)Al5O12 at these voltages were approximately 1.0 lm/W greater than combustion synthesized (Y1−xTbx)Al5O12. This is because of the smaller crystallite size of these powders (60 nm) compared with hydrothermal and solid-state synthesized powders (100 nm).
Fig. 1. Effect of synthetic route on the low-voltage cathodoluminescence efficiency of (Y1−xTbx)3Al5O12.  相似文献
5.
基于等高线建立DTM中平坦区域的一种处理方法   总被引:7,自引:0,他引:7  
对构筑三角形网格建立数字地形模型进行了研究,给出了基于散乱点和边建立三角形网格地形模型的生成算法和数据结构。在基于等高线生成的三角线网格地形模型中,平坦区域不能反映地表真实形状,文中给出了平坦三角形的修正方法。根据这些理论和方法,在AutoCAD R14 for Windows环境下,用Microsoft VisualC++4.2开发研制了应用程序。  相似文献
6.
基于颜色和形状的图像检索   总被引:7,自引:1,他引:6       下载免费PDF全文
在分析了基于图像颜色信息和空间信息进行图像捡索的基础上,提出了一种新的基于图像颜色和形状的图像检索算法。该算法采用HSI颜色空间,整幅图像首先被划分为具有固定尺寸的分块,对其中的每一分块,提取该分块的主色调作为该分块的颜色特征,对整幅图像采用主色调直方图作为其颜色特征;在对图像形状特征的提取上,针对图像1分量的每一分块,提出了分块平坦度和凹凸度的概念,并利用分块的这两个属性,将图像的分块划分为不同类别,同时采用不同类别分块的直方图作为图像的形状特征。试验表明利用该算法提取的图像的颜色特征和形状特征在进行图像检索时效果显著。  相似文献
7.
一种基于误差放大的快速BP学习算法   总被引:6,自引:0,他引:6  
针对目前使用梯度下降原则的BP学习算法,受饱和区域影响容易出现收敛速度趋缓的问题,提出一种新的基于误差放大的快速BP学习算法以消除饱和区域对后期训练的影响.该算法通过对权值修正函数中误差项的自适应放大,使权值的修正过程不会因饱和区域的影响而趋于停滞,从而使BP学习算法能很快地收敛到期望的精度值.对3-parity问题和Soybean分类问题的仿真实验表明,与目前常用的Delta-bar-Delta方法、加入动量项方法、Prime Offset等方法相比,该方法在不增加算法的复杂度和额外的CPU机时的情况下能更快地收敛到目标精度值.  相似文献
8.
A new convex-hull based approach to evaluating flatness tolerance   总被引:5,自引:0,他引:5  
Moon-Kyu Lee 《Computer》1997,29(12):861-868
In this paper, we consider a minimum-zone evaluation problem for flatness to determine the minimum tolerance zone enclosing all the measurement points obtained from a surface. For the problem, a new geometric approach called the ‘convex-hull edge method’ (CONHEM) is proposed. The method guarantees the minimum zone tolerance for a given set of measurement points. Several examples discussed by other researchers are illustrated to compare the method with some existing approaches.  相似文献
9.
自确认压力传感器结构设计   总被引:5,自引:1,他引:4       下载免费PDF全文
自确认传感器能够实现传感器故障的自诊断和自恢复,除了提供被测量的确认的测量值之外,还提供测量值的不确定度、测量值的状态等表征传感器输出可信度的参数,可以实时了解传感器的工作状态,及时采取相应的措施,大大提高监测系统的可靠性.本文针对自确认圆平膜片应变式压力传感器的设计,分析了圆平膜片的应力和应变分布,建立了圆平膜片上两种传统布片方式的数学模型,在分析圆平膜片压力传感器故障模式的基础上,对传统的布片方式进行了改进,建立了在圆平膜片上进行多组布片的方案,为实现自确认圆平膜片应变式压力传感器提供了坚实的基础.  相似文献
10.
基于Web服务的电子政务集成结构的研究   总被引:5,自引:0,他引:5  
电子政务是当前社会信息化的关键。本文阐述了我国电子政务发展中现存的问题,介绍了它的实施阶段。为解决现有问题,本文在分析了Web服务思想的基础上,构架了一个集成服务体系结构,并实现了一个基于集成服务的电子政务的实例。从而可以使各方面在一个政务平台上共享信息,互相交互,达到应用的集成。  相似文献
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