无微管缺陷六英寸SiC单晶的制备

SiC单晶衬底中的微管缺陷对SiC基器件是一种致命的缺陷,会严重影响SiC功率器件的成品率。基于物理气相传输(PVT)法,通过改进生长设计装配制备了绝对零微管缺陷6in的n型4H-SiC单晶。从结晶学和动力学原理对改进生长设计装配消除微管的机理进行分析,阐明了单晶生长过程中微管分解和闭合的机制。采用的优化生长设计方案不仅有利于提高SiC单晶生长的稳定性,更可以提高SiC单晶的结晶质量,达到快速降低微管缺陷目的。所制备的无微管缺陷、大尺寸6inn型4H-SiC单晶更加适合制作高压以及特高压功率器件。     

无微管缺陷六英寸SiC单晶的制备EI北大核心CSCD

SiC单晶衬底中的微管缺陷对SiC基器件是一种致命的缺陷,会严重影响SiC功率器件的成品率.基于物理气相传输(PVT)法,通过改进生长设计装配制备了绝对零微管缺陷6 in的n型4H-SiC单晶.从结晶学和动力学原理对改进生长设计装配消除微管的机理进行分析,阐明了单晶生长过程中微管分解和闭合的机制.采用的优化生长设计方案不仅有利于提高SiC单晶生长的稳定性,更可以提高SiC单晶的结晶质量,达到快速降低微管缺陷目的.所制备的无微管缺陷、大尺寸6 in n型4H-SiC单晶更加适合制作高压以及特高压功率器件.     

中子辐照6H-SiC晶体的退火特性及缺陷观测

利用X射线衍射（XRD）研究中子辐照6H-SiC晶体的退火特性,发现辐照后晶体的XRD峰的半高宽（full width atha lf maximum,FWHM）增大,之后又随退火温度的升高,在700～1230℃范围内呈线性规律的回复。以此规律为依据,可发展一种适合测量高温和复杂温度场温度的测温方法。采用添加了K2CO3的KOH为腐蚀剂,对辐照前、辐照后及辐照后退火的掺氮6H-SiC单晶进行位错腐蚀观察,发现经中子辐照的晶体中位错面积比随退火温度的变化趋势与FWHM随退火温度的变化趋势基本一致,由此认为经中子辐照所产生的位错可能是导致XRD峰的FWHM变化的一个重要因素。     

导模法生长的片状白宝石单晶中生长方向与晶界产生之间的关系

用导模法生长了白宝石片状单晶。采用偏光显微镜法、X射线背射劳埃法、X射线形貌法研究了片状晶体中的缺陷。发现晶体中晶界的产生与生长取向有明显的关系。在生长具有(1?02)板面的片晶时,若生长方向为[?101],晶体中存在有严重的晶界;而沿[0221]或[1120]方向生长的晶片,晶界则显著减少。 研究表明,白宝石中(0001)面沿〈11?0〉方向的滑移对晶界的形成起重要的作用。为了生长出满足SOS器件衬底材料所需要的完整性较好的片状白宝石,除应控制适当的生长速率和选用缺陷少的晶种外,必须注意选择有利的生长取向,以防止晶界的产生。     

温度梯度法生长BaY2F8晶体

应用温度梯度法生长了BaY2F8单晶.实验表明:温度梯度、降温速率、坩埚形状和尺寸是影响BaY2F8晶体生长的重要因素.温度梯度为6 ℃/mm,降温速率小于6 ℃/h,用相对较小的坩埚圆锥角度,可以获得质量较好的BaY2F8单晶体.X射线定向分析表明:温度梯度法生长的BaY2F8晶体的生长轴平行于[001]方向.结合晶体结构图分析,可认为晶体的[001]方向为结构的强键延展方向,是晶体生长的优势方向.     

小粒径α-Al2O3粉体的表征及其晶体生长机制

对自制的六角片状α—Al2O3颗粒进行了电子衍射分析和晶体学位向标定．并对含促进剂的氧化铝晶体的生长机制做了初步探讨。研究表明：合成的六角片状的α—Al2O3颗粒为单晶体．其基面为（0001）面．其棱面为（10 10）面;促进剂的加入导致了晶体的各向异性生长,使[10 10]方向的生长速率大于[0001]方向的生长速率,导致片状颗粒的形成。     

6H-SiC单晶的生长与缺陷

采用升华法,在一定的温度、气体压力和流量的条件下,生长了尺寸ф50．8mm的6H—SiC单晶。利用光学显微术观察了原生晶体的表面形貌,发现了微管在晶体表面的露头点具有明显的多个螺位错成核特征。采用透射模式对抛光晶片进行观察,发现了SiC晶体内的典型缺陷,如：负晶、微管、碳颗粒等,并对它们的形成机理进行了讨论。     

二硼化镁超导晶体的制备与形貌表征

利用镁助熔剂法制备出六方片状的二硼化镁（MgB2）单晶,并通过扫描电子显微镜和金相显微镜对MgB2晶体形貌进行了研究。结果发现,（111）、（101）和（011）晶面的生长速率远远大于（1-10）、（100）和（010）晶面生长速率,即晶体沿口或6轴的生长速率明显很大,而沿c轴的生长速率很小,甚至随着晶体生长并未发生变化。在MgB2单晶中首次观察到{0001}晶面层的解理与石墨相似,对MgB2单晶的生长机制进行了讨论。     

掺铌钨酸锌单晶的研究

本文研究了掺铌钨酸锌单晶的生长特性,该种晶体对生长方向、籽晶质量和扩肩速度很敏感,测定了铌的有效分凝系数K_(eff)=1.1。对掺铌钨酸锌单晶的去色机制进行了深入的探讨,提出由于氧空位导致ZnWO_4单晶中色心的存在,而氧空位的产生是由于W—O键较弱,高温下WO_(?)八面体容易失去氧离子所致。还测定了该晶体的能量分辨率和衰减时间,对漂移特性亦进行了初步的测定。已用提拉法生长出无色透明的大块掺铌钨酸锌单晶。     

分凝对0.80Na_(1/2)Bi_(1/2)TiO_3-0.20BaTiO_3铁电单晶电学性能的影响

采用坩埚下降法生长了尺寸为φ12mm×70mm的0.80Na1/2Bi1/2TiO3-0.20BaTiO3(0.80NBT-0.20BT)无铅铁电单晶.通过X射线荧光分析研究了晶体中的分凝现象.结果表明:该单晶沿其纵向生长方向由项部至底部,BaTiO3(BT)含量逐渐增加,晶体棒底部BT含量为32.15%(摩尔分数,下同),而顶部BT含量为14.26%.XRD分析表明:室温下晶体棒为四方相钙钛矿结构.随着BT含量增加,室温下晶体[001]方向的介电常数减小,去极化温度升高.位于晶体棒中间部位的晶体样品0.81NBT-0.19BT的压电性能最佳,室温下该样品在[001]方向的电学性能指标分别为:压电系数d33=158 pC/N,机电耦合系数kt=0.463.     

A STUDY ON THE INTERFACE CONTROL OF VERTICAL BRIDGMAN CRYSTAL GROWTH USING A TRANSPARENT MULTIZONE FURNACE

A transparent computer-controlled multizone furnace is used to study the interface control of the vertical Bridgman crystal growth. Sodium nitrate is chosen as a model material. The effects of heating profiles and growth speeds on the interface shape during steady-state growth are illustrated. It is observed that the growth front becomes more concave with the increasing growth speed and with decreasing axial temperature gradients near the growth front. When a suitable combination of the growth speed and the heating profile is used, a flat or slightly convex interface can be easily obtained. Numerical simulation is also conducted, and the calculated interface shape and position under various growth conditions are in good agreement with the observed ones.     

Real time observation of crystallization in polyethylene oxide with video rate atomic force microscopy

Video rate atomic force microscopy (VideoAFM), with a frame rate of 14 frames/s and a tip velocity of up to 15 cms⁻¹, is used to image polyethylene oxide films during crystal growth. The capabilities of VideoAFM when applied to semicrystalline polymer surfaces are explored. Image quality comparable to that found with conventional contact AFM is achieved but with a nearly 1000 times improvement in time resolution. By applying the technique to the real-time observation of crystal growth, different modes of rapid crystallization are followed in real time. Observation of the spherulite growth front allows measurement of growth rates at the lamellar scale, from which a factor of two difference in the rate of radial growth to the rate of tangential growth is observed, confirming that the elongated nature of spherulite lamellae is due to geometric constraints rather than an inherent fibrillar character. Measurements on screw dislocation growth, when large amounts of crystallizable material is trapped at the surface show that the terrace height does not influence the rate of crystal growth, confirming that under these conditions processes at the lamellar growth front control the rate of growth. When only a thin film of molten material is left on the surface of the already crystallized film dendritic growth is observed, implying a diffusion controlled process under these far from equilibrium conditions.     

Current status and advances in the growth of SiC

Recent achievements in crystal growth and homoepitaxy of SiC, mainly of the 4H polytype, have been discussed. Several growth techniques, such as seeded sublimation growth, high temperature chemical vapor deposition, sublimation epitaxy and liquid phase epitaxy have been utilized to develop technological procedures and understand the growth processes better. The advantages of either method have been stressed. The main target has been the reproducible growth of micropipe free substrate material and thick epitaxial layers for device applications. The purity of the layers has been of special interest. The results obtained are indicative for the massive progress that has been achieved in SiC crystal growth during recent years.     

Evolution of thermoelastic strain and dislocation density during sublimation growth of silicon carbide

The evolution of crystallization front and growth conditions during sublimation growth of SiC bulk crystal is studied using a coupled heat and mass transport two-dimensional model. It is shown, in particular, that movement of the inductor coil used for heating of the growth crucible modifies the temperature profile at the growth surface but can have no remarkable effect on the growth rate.Anisotropic elasticity theory and a semi-empirical model of dislocation generation are applied for a detailed analysis of thermoelastic strain and dislocation density evolution during SiC bulk crystal growth. An important effect of a method of SiC seed attachment to the holder is revealed by modeling. It is shown that under optimal attachment, the maximum dislocation density is concentrated near the crystallization front at the periphery of the crystal. The region of high dislocation density expands with enlargement of the crystal.     

鄂尔多斯盆地东部某区上古生界储层物性分析及其影响因素

储层物性纵向上差异较大,且纵向上孔隙度和渗透率间的正相关性较为明显,渗透率在纵向上的规律并不明显。测井解释孔隙度的误差较小,可用于储层孔、渗的平面分布图。山西组具有相对较大的渗透率差异,河道砂体展布和含气层段分布与高值区段也有较好的相关性,强烈表明了储层的非均质性。本溪组各井中储层孔隙度最大值为6.4%,最小为0.5%,平均值2.9%。本区控制储集砂体展布及储集砂体孔隙度和渗透率变化的沉积微相主要是三角洲前缘水下分流河道,其砂体比较发育。研究发现:孔隙度、渗透率的变化是与石英含量变化呈正相关的,同时,随着岩屑含量的增加,孔隙度和渗透率降低。本区孔、渗、饱的高值区即为石英砂岩的发育区。     

Surface Roughening Transition and Coarsening of NbC Grains in Liquid Cobalt-Rich Matrix

When NbC–30 wt% Co powder compact is sintered at various temperatures where NbC grains (with small amounts of Co) coexist with a liquid Co–NbC matrix, the NbC grains undergo a surface roughening transition with temperature increase and the grain growth changes from abnormal to normal growth. When sintered at 1400°C, the grains are polyhedral with sharp edges (and corners) and grow abnormally because their singular surfaces move by nucleation of surface steps. When sintered at 1600°C, the edges become round, indicating the surface roughening transition. The grains still grow abnormally, but their number density is larger than that at 1400°C because of the smaller surface step free energy. When sintered at 1820°C, the grains are nearly spherical, but the flat-surface segments still remain. The grain growth at this temperature is nearly normal because of very small surface step free energy. The surface roughening transition is reversed when a specimen initially sintered at 1820°C is heat-treated again at 1400°C, but some grains show transition shapes with nearly flat edges and slope discontinuities (shocks).     

Effect of Shear Work on the Heat Transfer Characteristics of Gaseous Flows in Microchannels

The effect of shear work at solid boundaries for parallel plates and a micropipe is considered to analyze the heat transfer characteristics in the slip flow region for gaseous flow. The fluid flow is assumed to be laminar, incompressible, steady, and hydrodynamically and thermally fully developed. The effects of second‐order velocity slip, temperature jump, shear work at the solid surface, and viscous dissipation are considered. The constant heat flux boundary condition is used at the surface of the parallel plates and of the micropipe. Closed‐form expressions are obtained for the temperature distribution and Nusselt number as a function of various modeling parameters for both geometries. The results show that neglecting the shear work underpredicts the Nusselt number.     

Morphological changes during oriented polymer crystallization

A theory of the stress-induced crystallization of polymeric networks is presented which takes into account 1) the free energy of fusion, 2) crystal surface energies and 3) entropic changes in the amorphous sections of crystallizing chains. It is assumed that the vector running from one end to the other of the crystallite is oriented in the direction of network extension, irrespective of crystal morphology, thus minimizing the free energy of crystallization. Assuming that the network assumes the crystal morphology which minimizes the free energy of the network at a given degree of crystallinity and that the crystallization proceeds along this lowest free energy path, it is predicted for simple network extension that growth of a perfectly oriented extended-chain crystal occurs initially, changing to a one-fold crystal oriented perpendicular to extension at low extension and to a two-fold crystal having nearly perfect orientation at High extension. The stress is predicted to decay initially and then to rise as the network chains switch from an extended-to a folded-chain morphology. Spatial factors which may trap chains in the ex tended-chain morphology or prematurely stopping the crystallization process can result in a mixed crystal morphology. At high extension, the structure is similar to that of the shish kebab.     

钛宝石晶体中的位错以及退火对位错的影响

用化学腐蚀剂腐蚀出样品的 (0 0 0 1)表面上的位错蚀坑 ,在原生态和退火样品的对应径向线上测量了位错腐蚀坑的密度分布 .由位错腐蚀坑的形成理论确定出原生态晶体中很可能有 3种位错类型 ,即Burgers矢量b =13〈112 0〉 ,13〈110 1〉和〈10 10〉的位错线 ,然而长时间在还原性高温气氛中的退火难以降低晶体中的总的位错密度 .在晶体的放肩至等径生长阶段 ,沿着晶体的生长方向 ,晶体棒中心的位错密度由高变低 ,这显示出 :在晶体的放肩至等径生长的转变过程中 ,生长界面发生了翻转 ,由凹形界面转变为凸形界面 ,位错线随之从晶体棒的中心向边缘发散 .