生长工艺对生长室内温度分布影响的数值模拟

为给实验研究晶体生长工艺提供必要的理论指导,以氢气和氧气的燃烧为基础,研究了焰熔法生长金红石单晶体过程中生长室内的温度分布特征,分析了H_2和O_2流量、喷嘴尺寸对温度分布的影响.研究表明:适合金红石单晶体生长的最佳燃烧器为内O_2、中H_2和外O_2的三管结构;随着H_2流量增加,生长室轴心线上和径向温度逐渐增大,H_2流量增加2 L/min,中心最高温度平均升高160℃,位置向下移动约2.5 mm;随着内、外O_2流量增加,生长室轴心线上和径向温度逐渐降低,与内O_2的影响相比,外O_2对中心温度影响较小,而对径向温度的影响较大;随着内O_2喷嘴孔径的增加,生长室轴心线上最高温度逐渐增大,而位置逐渐向喷嘴方向移动,而外O_2和H_2喷嘴孔径对轴心线上最高温度的影响非常小.     

焰熔法制备单晶体生长室内的燃烧特性

以氢气和氧气的燃烧为基础,研究了焰熔法生长单晶体过程中生长室内的燃烧特性。结果表明:在O2和H2的流量分别为6 L/min和20 L/min条件下炉膛中心最高温度为3504.3 K,根据这个结果可确定晶体生长的位置或确定在已知位置晶体生长所需要的最佳流量;随着H2流量的增大,生长室内中心和径向的温度都逐渐提高,而最高温度的位置随着O2流量的增大而逐渐向下移动,中心O2流量每增大1 L/min,最高温度的位置向下移动5 mm,在距喷嘴110 mm处的温度平均升高230℃左右;H2分布圆直径对生长室内中心与径向的温度分布、O2的冲击深度和开始燃烧位置的影响很小。     

温度及温度梯度对SiC单晶生长的影响

利用升华法在高温低压下生长大直径SiC单晶.通过实验发现在相同的轴向温度梯度下,SiC晶体平均生长速率随籽晶温度的升高而变大.通过减小轴向温度梯度,降低晶体生长界面的径向过饱和度分布,可以抑制多型的生长.通过优化温场的径向温度梯度,利用φ50mm的籽晶进行生长,得到了φ57mm的SiC单晶,实现了晶体的扩径生长.     

冷孔板直径对涡流管内部压力场的影响

以理想的CO_2气体为工作流体,使用标准Standard k-ε湍流模型来模拟涡流管的能量分离效应。当涡流管冷端出口压力是2. 5 MPa、喷嘴进口压力是6. 5 MPa和喷嘴进口温度是298. 15 K、喷嘴进口压力为6. 5 MPa时,进行了涡流管内轴向、径向压力分布的模拟研究。模拟结果表明:冷流率μ为0. 1,冷孔板直径在1. 5～3. 5 mm之间变化时,轴心线上的总压分布呈现先减小后逐渐增大的趋势,各冷孔板直径在轴向距离为0 mm处的径向总压随着径向距离的增大呈逐渐增大的趋势;当径向距离在0. 5～1. 0 mm之间变化时,轴线上总压随着涡流管轴向距离的增大而增大;当径向距离为1. 5 mm时,总压随着涡流管轴向距离的增大呈先增大后减小的趋势;当径向距离在2. 0～2. 5 mm之间变化时,总压随着涡流管轴向距离的增大呈逐渐减小的趋势;当轴向距离在0～100 mm之间变化时,各轴向位置上随着径向距离增大的总压分布均呈现逐渐增大后趋于稳定的趋势。     

聚乳酸和聚己内酯共混物多孔支架超临界流体技术制备

以二氯甲烷作为聚合物溶剂,利用超临界CO_2诱导相分离-循环干燥过程制备了质量比为1∶1的聚乳酸和聚己内酯混合物组织工程用多孔支架,同时考察了过程操作压力和温度、溶液浓度、CO_2流量及循环干燥时间等操作参数对孔径大小和形态的影响。结果表明,溶液浓度、操作压力和温度及CO_2流量对孔结构的影响较大。溶液浓度增大、操作温度升高均会引起混合物支架孔径的减小。操作压力升高,混合物多孔支架孔径逐渐增大,孔径分布越来越不均匀。随CO_2流量升高,混合物支架的孔径不断减小。循环干燥时间虽然对混合物支架的孔结构影响不大,但对超临界CO_2诱导相分离-循环干燥过程的可实现性有重要的影响。实验范围内,当混合物溶液浓度为25%(质量分数),循环干燥用CO_2流量为31kg/h,操作压力和温度分别为12 MPa和45℃的操作条件下,制备的混合物多孔支架更有利于组织工程的应用。     

轴向磁场下分离结晶熔体内热毛细对流的数值模拟

为研究轴向磁场对分离结晶Bridgman法生长CdZnTe晶体熔体热毛细对流的影响,采用有限差分法进行了三维数值模拟。结果表明轴向磁场能有效抑制熔体内的热毛细对流;轴向磁场对熔体内部温度分布也有较大的影响,能使等温线分布变得平缓;当磁场强度不变时,随着狭缝宽度的增大熔体内部的流动减弱。     

数控焰熔法生长金红石单晶体的气体参数优化试验研究

使用数控焰熔法晶体生长炉和三管燃烧器进行了金红石单晶体的生长工艺实验研究。研究表明,在3个气体参数中,内氧是基础。当内氧流量小于特征值O_(in)(min)=6.6 L/min或大于特征值O_(in)(max)=7.1 L/min时,晶体生长或易于发生熔体流淌,或不易扩肩,生长过程被迫中止,晶体很难生长成为大尺寸晶体;当内氧流量大于6.6 L/min而小于7.1 L/min时,生长过程稳定,能够生长成为大尺寸晶体。当内氧流量为7 L/min时生长出了直径30 mm、长度45 mm的金红石单晶体。通过能谱仪、X射线衍射仪(XRD)和拉曼光谱仪对晶体的组成、结构和键的结构进行表征,表明该晶体为典型的金红石单晶体,生长方向是[110]。     

水热法合成ZnO单晶生长工艺研究

采用水热温差法进行ZnO晶体的生长研究,分析在不同温度压力和矿化剂条件下分别合成几十纳米ZnO晶体和径向尺寸和高度到毫米级的ZnO晶体的生长工艺,探讨了晶体不同晶面生长速度和质量的一般规律及晶体生长机制.     

在横向磁场中用Bridgman法生长HgCdTe晶体

在横向磁场中用Ｂｒｉｄｇｍａｎ法生长的晶锭其轴向组分分布在中部和尾部具有相同的趋势,在头部有三种类型的分布。磁场通过对固液混合区对流的作用影响溶质的再分布和轴向组分分布。突然施加磁场和中断磁场都引起轴向组分分布的突变。当安瓿绕生长轴匀速旋转时,晶锭的径向组分分布既没有安瓿不旋转时的偏心特征,也没有常规Ｂｒｉｄｇｍａｎ法生长晶体的径向对称性,尾部呈现圆锥状的凸起,可能是旋转生长抑制胞状结构的证据。     

瓶式容器收口成形旋压力数值模拟研究

通过数值模拟方法对瓶式容器热旋压成形过程中不同因素对旋压力的影响进行研究。研究表明,瓶式容器热旋压成形中,随着温度的升高,旋压力逐渐降低,随着进给比的增大,旋压力逐渐升高;各道次内径向旋压力与总旋压力大小非常接近,切向旋压力轴向旋压力径向旋压力;随着旋压道次的增多,旋压力逐渐增大。     

温度梯度和生长速率对CdZnTe-VBM生长晶体的影响

计算模拟了半导体材料CdZnTe垂直布里奇曼法(CdZnTe-VBM)单晶体生长过程，分析了炉膛温度梯度和坩埚移动速率对结晶界面形态和晶体内组份偏析的影响。计算结果表明炉膛温度梯度和生长速率的变化明显影响固-液界面前沿对流场的形态和强度。界面凹陷深度随着炉膛温度梯度的增加和生长速率减小而减小。炉膛温度梯度的增加和生长速率的减小虽然均能有效的减小径向偏析，但却增加轴向偏析，减小轴向等浓度区的长度。     

Effective Increase of Crystal Area during Sublimation Growth of 6H-SiC by Using the Cone-shaped Baffle

A novel design of crucible is proposed in this paper for the growth of SiC crystals. The relation between grown crystal shape and temperature distribution in a growth chamber was discussed. It is pointed out that the crystal shape had a close relationship with temperature distribution. The calculations suggested that the radial temperature field of the growing crystal became homogenous by setting up the cone-shaped baffle in the growth chamber. By modifying the crucible design and temperature distribution in the growth chamber, it is possible to enhance the enlargement of crystal, and also possible to keep grown surface flat.     

ACRT—B法晶体生长过程的传质模型

基于对ＡＣＲＴ－Ｂ法晶体生长对时流及传质特性的认识，提出了ＡＣＲＴ－Ｂ法晶体生长过程的一维传质模型。得到了包括初始过渡区、稳态区及终端过渡区轴向成分分布的解析解。定量计算结果表明，与传统Ｂｒｉｄｇｍａｎ法相比，在生长参数不变的条件下，运用ＡＣＲＴ会失去部分轴向面分均匀区。在保护ＡＣＲＴ提高结晶及径向成分均匀性的前提下，适当提高生长速度，选择尽可能小的坩埚最大转速△ωｍａｘ或选用长径比更大的坩埚，是     

燃烧合成AlN-TiC陶瓷及致密化机理分析

采用ＳＨＳ工艺,在１００ＭＰａ高压氮气下,制备了致密的ＡＩＮ－ＴｉＣ陶瓷,相对密度达９５％,抗弯强度达２４０ＭＰａ．研究表明,ＡＩ－Ｎ_２－ＴｉＣ体系ＳＨＳ过程中,当反应温度升至６６０℃时,ＡＩ熔化．高温下熔融ＡＩ在ＴｉＣ表面发生漫流现象,在ＳＨＳ反应区发生液相烧结,从而使反应区孔隙率显著下降,当开气孔闭合时在气相等静压作用下形成致密产物．随着ＴｉＣ含量增加,液相烧结作用增强,产物致密度显著提高,抗弯强度及断裂韧性提高．ＡＩ－Ｎ_２－ＴｉＣ体系的ＳＨＳ致密化主要发生在燃烧波蔓延方向,具有明显的方向性．     

Numerical investigation of gas-particle hydrodynamics in a vortex chamber fluidized bed

Gas-particle hydrodynamic behaviour inside a vortex chamber fluidized bed is studied numerically with respect to different design and operating conditions. A three-dimensional computational fluid dynamics (CFD) model of a cylindrical vortex chamber is developed. Simulations are carried out with particles and without particles. In order to understand the gas-particle flow behavior velocity distribution, particle volume fraction distribution, radial pressure distribution and axial pressure distribution inside the vortex chamber are analyzed in detail. Particles of different diameters are used and its effect on the gas-particle flow behaviour is studied. Design parameters like the number of gas inlet slots and slot width are varied and their impact on the hydrodynamics of the vortex chamber is investigated. The numerical model is validated by comparing the numerical results with experimental results reported in literature.     

Oxygen ion beam assisted etching of single crystal diamond chips using reactive oxygen gas

The etching characteristics of single crystal diamond chips processed using an oxygen ion beam with reactive oxygen gas flux were investigated. The specific etching rate increased linearly with increasing ion energy in the range of 250 to 1000 eV. The specific etching rates processed in a 1000-eV oxygen ion beam with oxygen gas was approximately twice that processed only in a 1000-eV oxygen ion beam. The angular dependences of only a 500-eV oxygen ion beam (no assist), and a 500-eV argon ion beam with oxygen gas were quite different from that of the other conditions. The specific etching rates were almost constant as a function of ion incident angle in the range of 0 to 50°. Those for the other conditions first increased with increasing ion incident angle, and reached a maximum rate at an ion incident angle of 40° or 50°, and then decreased gradually with further increase in ion incident angle. The specific etching rates using an argon ion beam with oxygen gas first increased with increasing gas partial pressure and then reached a saturation level at a gas partial pressure above 0.015 Pa, whereas those for the other conditions increased linearly with increasing gas partial pressure in the range of 0 to 0.06 Pa. The specific etching rates using an oxygen ion beam increased linearly with increasing substrate temperature in the range of 100 to 500 °C. The specific surface roughness was almost constant as a function of the substrate temperature, in the range of 100 to 500 °C. The specific surface roughness after assisted etching using oxygen or hydrogen gases was approximately half that processed in only oxygen or argon ion beams (no assist). © 2001 Kluwer Academic Publishers     

Flow Characterization of a Diamond-Depositing DC Arcjet by Laser-Induced Fluorescence

Laser-induced fluorescence (LIF) measurements of seeded nitric oxide and naturally occurring species in a diamond-depositing dc arcjet of hydrogen/argon/methane (0.8:1.0:0.005) at 25 Torr are used to determine the temperature and velocity fields in a gas jet. LIF measurements are also used to demonstrate the importance of gas recirculation on the chemical composition of the arcjet plume. The gas flow in the arcjet plume is supersonic, with a maximum axial speed of 2.6 km/s at the center of the nozzle exit. This axial velocity decreases with radius with a parabolic distribution in the plume. There is no measurable radial velocity in the free stream of the arcjet plume, and the radial expansion of the plume is consistent with diffusion. The maximum temperature at the plume center is 2400 K and varies less than 15% with chamber pressures of 10-50 Torr. The substrate is placed in the arcjet plume normally to the directed velocity, producing a stagnation point. The gas temperature above this stagnation point is observed to rise abruptly as a consequence of the supersonic shock. The radial velocity near the stagnation point becomes significant, and a maximum radial velocity of 1300 m/s is determined.     

Luminescence tuning of amorphous Si quantum dots prepared by plasma-enhanced chemical vapor deposition

Amorphous Si (a-Si) quantum dots (QDs) embedded in a silicon nitride film were prepared by a plasma-enhanced chemical vapor deposition (PECVD) technique using gaseous mixtures of silane, hydrogen and nitrogen. We observed that the Si QDs had an amorphous structure from the Raman spectroscopy measurement. The Fourier transform infrared (FTIR) spectra showed that the relative transmittance of the SiH bands decreased, but that of the NH bands increased, with increasing nitrogen flow rate. During the deposition of SiNx, the number of dangling bonds of silicon acting as nucleation sites increased. As the hydrogen flow rate increased the growth rate decreased, due to the reduction in the hydrogen partial pressure. The hydrogen and nitrogen gas flow rates were found to be important parameters for determining the size of the a-Si QDs. In addition, we observed that the PL peak shifted toward a higher energy with increasing hydrogen and nitrogen gas flow rates, which was attributed to the increase in the quantum confinement effect in the a-Si QDs.     

Effects of ceramic processing parameters on the microstructure and dielectric properties of (Ba1-xCax)(Ti0.99-y′ ZryMn0.01)O3 sintered in a reducing atmosphere

The effect of A/B ratio, sintering temperature, and oxygen partial pressure on microstructure and ceramic dielectric properties of 1 mol % MnO₂ doped (Ba,Ca)(Ti,Zr,)O₃ sintered in a reducing atmosphere were investigated. Microstructure is found to be closely related to processing parameters. With decreasing A/B ratio, decreasing oxygen partial pressure and increasing sintering temperature, grain growth is enhanced. Concurrent with the grain size reduction, the crystal structure transformed from tetragonal to pseudocubic at room temperature and the dielectric constant, the dissipation factor and Curie point all decreased. However, the effects of grain size give a marked discrepancy on breakdown. The effect of A/B ratio and sintering temperature both suggest that breakdown voltage is decreased with increasing grain size. However, the breakdown voltage in relation to grain size by changing the oxygen partial pressure seems to show no significant difference.