激光熔池三维非稳态对流传热过程的数值模拟

建立了带有移动热源的激光熔池流体流动及传热过程三维非稳态数学模型。采用自适应网格技术离散求解动量方程，计算出了不同时刻激光熔池温度分布和速度分布。结果表明，激光熔池对流传热非稳态过程是一个预热过程，随着时间的推移，熔池最高温度不断升高，熔深和熔池半径不断增大。非稳态过程按时间先后次序分为3个阶段，初始阶段（加热熔化阶段）、准稳态阶段和快速升温阶段。准稳态阶段熔池形貌、温度分布和速度分布增加幅度不大，且持续时间比另两个阶段长，说明三维准稳态模型是三维非稳态模型的较好近似。计算结果与巳有的实验结果相比大体吻合。     

激光熔池三维非稳态对流传热过程的数值模拟

建立了带有移动热源的激光熔池流体流动及传热过程三维非稳态数学模型.采用自适应网格技术离散求解动量方程,计算出了不同时刻激光熔池温度分布和速度分布.结果表明,激光熔池对流传热非稳态过程是一个预热过程,随着时间的推移,熔池最高温度不断升高,熔深和熔池半径不断增大.非稳态过程按时间先后次序分为3个阶段初始阶段(加热熔化阶段)、准稳态阶段和快速升温阶段.准稳态阶段熔池形貌、温度分布和速度分布增加幅度不大,且持续时间比另两个阶段长,说明三维准稳态模型是三维非稳态模型的较好近似.计算结果与已有的实验结果相比大体吻合.     

SMT激光低温钎焊接合部热过程的数值模型

本文在实验与分析的基础上,建立了表面组装激光低温钎焊接合部温度场数值计算模型,考虑了钎料的热物理性质变化、激光的实际能量分布和在钎焊材料表面的反射损失以及表面散热对热过程的影响,试验验证了该模型的正确性。     

复合涂层激光熔池凝固过程的数值模拟

采用数值模拟技术研究了复合涂层激光熔池凝固过程。结果表明其凝固过程分为凝固准稳态阶段和凝固加速阶段两个阶段。在凝固准稳态阶段,熔池熔深和半径变化较小,但表面最高温度、熔池流速急剧减小;凝固加速阶段则反之。凝固准稳态阶段,由于上熔池的高度过热,下熔池产生后熔现象,形成后熔区。影响后熔的主要因素是熔覆层和过渡区凝固过程中释放的潜热和熔覆层的过热。     

D.C. characteristics of silicon p-n junctions at avalanche breakdown including selfheating

The computation of the d.c. characteristics of a silicon p-n junction at breakdown is described in this paper. In the analysis, the effect of the dissipated electric power on the junction temperature is taken into account. The influences of the thermal resistance, the ambient temperature and the low-voltage reverse current on the d.c. characteristics at breakdown are shown. A formula describing the differential resistance of a junction at breakdown is derived. The temperature coefficients of voltage and current for the d.c. characteristics under consideration are derived as well. The coordinates of the points at which these coefficients change the sign are calculated and the influence of the thermal resistance, the ambient temperature and the low-voltage reverse current on the coordinates of these points is discussed.     

Advanced thermal simulation of SiGe:C HBTs including back-end-of-line

Advanced 3-D thermal simulations of state-of-the-art SiGe:C HBTs are performed, which ensure improved accuracy with respect to conventional approaches. The whole back-end-of-line architecture is modeled so as to account for the cooling effect due to the upward heat flow. Moreover, a nonuniform power density is considered to describe the heat source, and thermal conductivity degradation effects due to germanium, doping profile, and phonon scattering in narrow layers are implemented. The numerical thermal resistances are compared with those experimentally evaluated by means of a robust technique relying on the temperature dependence of the base-emitter voltage.     

多峰负氢离子源全三维数值模拟研究

理论分析了负氢离子体积产生机制,采用自主开发的三维PIC?MCC模拟软件,对多峰磁场约束的负氢离子源进行数值模拟研究。模拟了引出磁场、放电初始能量、及放电位置对振动激发碰撞效率的影响,探索了多峰负氢离子源的优化手段。研究表明：如果进入引出区的快电子越多且-y方向漂移越剧烈,那么振动碰撞越频繁且相对碰撞率越高,即负氢离子体积产生效率越高。     

激光加工仿生蜂巢结构制备及其沸腾传热性能研究

池沸腾作为一种高效的能量转换方式,在提升能源系统效率及强化高功率电子设备热管理等领域具备重要作用.采用激光加工技术在铜片表面制备了仿生蜂巢结构(BHS),使用扫描电镜对BHS表面形貌进行观察,探究了不同加工参数对微结构形貌和沸腾传热性能的影响.结果表明,主次沟槽复合结构(BHS-C)对沸腾传热性能具有最佳强化作用.强化...     

D.C. numerical model for arbitrarily biased bipolar transistors in two dimensions

A method for the complete steady-state finite-difference solution of the most general form of the equations governing transistor action in two dimensions is proposed. Attention is drawn to certain computational difficulties and the means of overcoming them. Results of the method for a limited case are reported.     

A Monte-Carlo simulation of the electrodeposition process

Nucleation, grain growth, texture formation, and porosity formation in the electro-deposition process has been simulated with a Monte-Carlo method. In the simulation, a two-dimensional hexagonal lattice containing 12000 lattice sites are used to map the microstructure at the cross section of deposit. The criterion for grain growth is based on the energy state of the lattice site which is set by counting the interactions between the site and its nearest neighbors. This model can be used to obtain information of how the deposition rate affects the microstructure and porosity. The texture formation during the deposition process is modeled by taking surface energy anisotropy and deposition rate into account.     

同轴送粉喷嘴粉末汇聚特性三维数值模拟

在激光金属直接制造过程中,实际的粉末汇聚不是自由射流,而是受阻射流.为了获得受阻射流工艺参数对粉末汇聚特性的影响规律,提高激光直接制造精度,应用FLUENT软件中的离散相模型模拟了工艺参数对粉末汇聚浓度分布的影响.结果表明:在自由射流时,粉末汇聚浓度随着送粉量的增加而增大,随着载气速度的增大而明显降低,保护气速度的增大...     

CdS纳米粒子自组装体系室温单电子现象的计算机模拟

对形成室温单电子现象的典型串联双隧道结结构模型利用温克布 (WKB)三氏法求解薛定谔方程 ,计算了其隧穿电流与偏压的关系。利用该方法对 Cd S纳米粒子自组装体系在室温下的 I- V特性进行了计算机模拟 ,发现结果与实验符合得很好。该方法对于进一步指导纳米电子器件的实验及其原型化有重要意义。     

CCD芯片热电制冷系统的非稳态性能分析

随着电子设备不断向小型化、集成化发展,热电制冷技术作为一种有效的主动冷却方法被广泛用于重要部件的温度控制。为了获得最佳的制冷效果,文中针对散热受限条件下的CCD芯片热电制冷系统非稳态过程,建立了一个数值分析模型。分析结果表明:在散热受限时,热电制冷系统的传热过程长时间处于非稳态过程;在不超过最大制冷电流的条件下,增大制冷电流可以提高制冷效果,但是大的制冷电流可能出现温度回升的现象;虽然热端散热能力的提高可以改善制冷效果,但是存在一个极限值,这与热电制冷器(TEC)的优值系数有关;当系统载荷发生变化时,合理改变制冷电流和热端散热能力可以提高系统的温度稳定性,其中制冷电流对系统温度稳定性的影响更大。     

Application of experimental and numerical simulation methods for studies of the dry groove silicon etching process

A physical-chemical silicon etching model is described; an etching profile is calculated by the string method; and model adequacy is considered. The simulation results of the groove etching process as function of process optimization parameter variations are analyzed.     

碲锌镉晶体生长全局热传递模拟模型准确度研究

在碲锌镉晶体生长技术开发方面,数值模拟软件发挥着越来越重要的作用。以全局热传递模型为基础进行晶体生长模拟能够极大的提高模拟结果的实用性,缩短晶体生长设备及生长工艺的开发周期。但其前提是采用的全局热传递模型的准确度较高。因此,本文主要研究了几何模型、物性参数、边界控温条件等对模型准确度的影响,并根据模型计算值与炉体中心测温比较结果,修正了上述模型各参数,获得了在多种温度设定条件下,计算结果都能与实际过程很好的吻合的全局热传递模型。采用修正后的模型应用于碲锌镉晶体生长过程模拟,最终晶体生长模拟结果的温度与实际监测温度差距在2 ℃以内。     

A theoretical analysis of C.C.D. operation with square clock pulses

Currently published analyses of C.C.D. operation have dealt with the case where a fixed voltage is maintained on the gate above the emptying well during the charge transfer process. However, in contrast to this, most C.C.D.'s appear to be operated with square or trapezoidal clock pulses. The consequences of rapidly falling gate voltages are discussed and a simple mathematical model is developed to illustrate the situation. It is shown that if the falling edges of the clock pulses are too steep the operation of the C.C.D. will be degraded in that carriers may be lost by recombination and the overall transfer process will be slowed down due to the smaller electrostatic fields which are developed.     

A transient numerical scheme for the simulation of GaAs MESFETs and circuits

In this paper, an algorithm, Alternating Direction Scheme (ADS), for solving a two-dimensional time-dependent continuity equation is presented. By separating the continuity equation into two parts based on the direction (x or y) of the vectors and solving them alternatively, ADS deals with the two-dimensional equation in a one-dimensional way and therefore has the advantages of better stability, less calculation, faster convergence and wider time steps than the schemes previously used. The application of the algorithm not only greatly improves the efficiency of the intrinsic response simulation, but also makes the two dimensional GaAs DCFL inverter simulation possible and practical.     

数控仿真软件在机电数控教学中的运用

机电数控教学普遍存在教学硬件设备不足的情况,在实践训练的过程当中学生还需要排队等候才能进行设备操作,这样就给机电数控教学带来了非常大的困难,并且非常不利于此专业人才的培养。文章探讨了数控仿真软件在机电数控教学中的运用,有效解决上述问题的同时,还能够充分发挥出机电数控教学的优越性,在对老师教课提供便利的同时,还可以有效地提升学生的动手能力,培养学生的创新意识,这样就有利于提升数控教学的水平。     

Coupled simulation of gas flow and heat transfer in an RTP-system with rotating wafer

In this study the concept of a lamp heated RTP-system with rotating wafer is considered. Using the fluid-flow-simulation software Phoenics-CVD, we investigated the cooling of the wafer by a process gas flow which is injected at room temperature into the hot process chamber through an inlet pipe in the side wall. In a full 3D-simulation of the gas flow and of the heat transfer in the gas and in the wafer the Navier–Stokes equations and the energy equation are solved. The radiative power consumption and the energy loss of the wafer have been modeled by the Stefan–Boltzmann law. Simulations without wafer rotation show a strong drop in the temperature distribution at the wafer near the inlet pipe. In contrast to this, simulations with rotation show an axisymmetric temperature distribution with a considerably smaller temperature gradient over the wafer. Comparisons with oxidation experiments showed good agreement with the simulation results.     

A numerical simulation of scattering from one-dimensionalinhomogeneous dielectric random surfaces

An efficient numerical solution for the scattering problem of inhomogeneous dielectric rough surfaces is presented. The inhomogeneous dielectric random surface represents a bare soil surface and is considered to be comprised of a large number of randomly positioned dielectric humps of different sizes, shapes, and dielectric constants above an impedance surface. Clods with nonuniform moisture content and rocks are modeled by inhomogeneous dielectric humps and the underlying smooth wet soil surface is modeled by an impedance surface. In this technique, an efficient numerical solution for the constituent dielectric humps over an impedance surface is obtained using Green's function derived by the exact image theory in conjunction with the method of moments. The scattered field from a sample of the rough surface is obtained by summing the scattered fields from all the individual humps of the surface coherently ignoring the effect of multiple scattering between the humps. The statistical behavior of the scattering coefficient σ° is obtained from the calculation of scattered fields of many different realizations of the surface. Numerical results are presented for several different roughnesses and dielectric constants of the random surfaces. The numerical technique is verified by comparing the numerical solution with the solution based on the small perturbation method and the physical optics model for homogeneous rough surfaces. This technique can be used to study the behavior of scattering coefficient and phase difference statistics of rough soil surfaces for which no analytical solution exists