提拉法生长钆镓石榴石的晶体缺陷

观察了提拉法生长钆镓石榴石的晶体缺陷。主要研究位错及其攀移、蜷线位错、位错环和包裹物。棱柱位错环和攀移位错环可存在同一晶体中。攀移位错环和蜷线位错可分两类:各对应于吸收间隙原子的攀移和微细沉淀粒子在位错处反复成核成长这两种攀移生长机制。     

Yb∶Ca_5(PO_4)_3F激光晶体的生长缺陷

采用提拉法生长了质量优异的Yb∶Ca5(PO4)3F(Yb∶FAP)晶体。运用化学腐蚀,光学显微镜、扫描电子显微镜以及能量散射光谱仪观察了该晶体中的生长条纹和包裹物等宏观缺陷,以及晶体的位错腐蚀形貌、位错密度及其分布情况,同时观察了晶体中亚晶界的形态。由晶体中位错的径向变化以及生长条纹可知:晶体在生长过程中为微凸界面生长。高温下CaF2的挥发造成了在晶体生长后期熔体中组分偏离化学计量比,出现组分过冷,形成包裹物,且位错密度显著增加。Yb∶FAP晶体的各向异性使得晶体在(10 10)面的位错蚀坑形状、大小以及深度不同,而(0001)面的位错蚀坑呈规则的六边形;这也是晶体中形成亚晶界结构的主要原因。讨论了减少晶体中缺陷的一些方法。     

Yb:Ca5(PO4)3F激光晶体的生长缺陷

采用提拉法生长了质量优异的Yb：Ca5（PO4）2F（Yb：FAP）晶体。运用化学腐蚀,光学显微镜、扫描电子显微镜以及能量散射光谱仪观察了该晶体中的生长条纹和包裹物等宏观缺陷,以及晶体的位错腐蚀形貌、位错密度及其分布情况,同时观察了晶体中亚晶界的形态。由晶体中位错的径向变化以及生长条纹可知：晶体在生长过程中为微凸界面生长。高温下CaF2的挥发造成了在晶体生长后期熔体中组分偏离化学计量比,出现组分过冷,形成包裹物。且位错密度显著增加。Yb：FAP晶体的各向异性使得晶体在（10 10）面的位错蚀坑形状、大小以及深度不同,而（0001）面的位错蚀坑呈规则的六边形;这也是晶体中形成亚晶界结构的主要原因。讨论了减少晶体中缺陷的一些方法。     

用激光光散射层貌术研究Nd:YAG晶体中的缺陷

用激光光散射层貌术研究Nd:YAG晶体中杂质缀饰的缺陷,从中清晰地观察到不同类型的缺陷及其相互作用。在晶体中存在两种类型的攀移蜷线位错,Ⅰ型蜷线位错是围绕包裹物形成的并与GGG晶体中的蜷线相似。Ⅱ型蜷线位错是围绕小晶面组成的内核而攀移成的,其立体结构可以通过逐层扫描照相显示出来,蜷线的环面直径为1.2mm,长度为40mm,这类蜷线位错还未见报道。     

掺钕钨酸钇钠晶体的缺陷

掺钕钨酸钇钠晶体Nd:NaY(WO4)2是一种性能优良的激光晶体.对该晶体中的包裹物、生长台阶等缺陷进行了观察,并分析了缺陷产生的原因.在晶体的不同部位取样进行扫描电镜和能谱分析,发现随着晶体生长过程的进行,晶体中Nd3 的浓度逐渐增加.研究了晶体中位错蚀坑的分布特点,发现位错蚀坑的密度与晶体生长的不同部位、Nd3 的掺杂浓度等因素有关.     

LBO晶体的生长缺陷研究

采用化学腐蚀光学显微法和同步辐射截面形貌术研究了三硼酸锂（ＬＢＯ）晶体的生长缺陷,实验结果表明,ＬＢＯ晶体中的主要缺陷是位错、包裹物和形界,讨论辽些缺陷形成的原因和降低缺陷的措施。     

YAG:Nd晶体中的自身缀饰位错

YAG:Nd晶体因具有急熔点及高掺杂等特点,容易发生位错的攀移运动及自身缀饰位错。我们用超显微术观察到晶体内各种形态的缀饰位错:包括直线、折线、螺线及封闭位错环。高温退火实验显示了位错被缀饰的过程。可以认为杂质及沉积粒子的存在是使位错增殖的重要原因。     

五磷酸钕晶体中缺陷的X射线形貌研究

用X射线衍射形貌法研究了NdP_5O_(14)晶体中的微观缺陷,主要有铁弹畴、反相畴界、生长区的界面、生长层和位错。铁弹畴的应变矢量是R=[100],反相畴界的位移矢量是R=[001]。位错有螺位错和刃位错。部分螺位错的Burgers矢量为b=[100]。分析了晶体缺陷和生长条件之间的关系,提出提高晶体完美性的措施。     

莫来石陶瓷中的位错及其对力学性能的影响

利用透射电镜对莫来石陶瓷中位错的组态进行观察，结果表明，莫来石陶瓷中的位错易被第二相钉扎，两列位错遇易发生反应，全位错易分解成偏位错，并扩展形成层错。这说明，即使是脆性材料，其位错在一定条件下仍然是可动的。本文认为，位错在常温和高温条件下莫来石陶瓷力学性能影响不同，并从位错的角度解释了莫来石陶瓷力学性能的特点，提出改进材料制备工艺的途径。     

激光晶体BeAl_2O_4:Cr~(3+)生长缺陷的电镜观察

本文报道提拉法c轴生长紫翠宝石(BeAl_2O_4·Cr~(3+))激光晶体中生长缺陷的电镜观察结果。晶体生长中出现的云层区中存在大量的气泡,包裹物、位错、小角晶界等缺陷。胞状结构的胞壁附近存在大量的包裹物。相邻胞体之间存在取向差。在垂直于晶体生长方向的(001)面上观察到可滑移位错。对这些缺陷的形成原因进行了初步的分析。     

Dislocations and the Tensile Strength of Magnesium Oxide

It has been shown with single crystals, bicrystals, and polycrystalline material that the strength of magnesium oxide depends on the availability of mobile dislocation sources. Mobile dislocations, introduced by mechanical contact at the surface, are responsible for the ductility normally observed in single crystals. When mobile dislocations are removed by a chemical polish, single crystals become extremely strong (>160,000 psi) and elastic. In the absence of mobile dislocations bicrystals are also extremely strong and elastic but in their presence bicrystals are relatively weak (10,000 psi) and brittle. The brittleness is due to the direct interaction of slip bands with grain boundaries to generate cracks. The strength of high-density polycrystalline magnesia is similarly sensitive to the presence of mobile dislocations. When care is taken to eliminate surface defects and to immobilize dislocations, tensile strengths of 30,000 psi can be attained, but when mobile dislocations are present the strength drops to 15,000 to 20,000 psi. Mobile dislocations can be introduced directly by mechanical contact or indirectly through the presence of pores.     

Dislocation Cores and Hardness Polarity of 4H-SiC

The hardness of opposite basal faces of 4H-SiC single crystals has been measured in the temperature range 25°–1200°C. A strong hardness anisotropy between the silicon-terminated (0001) and carbon-terminated (0001) faces of this polar crystal has been found. Transmission electron microscopy investigation of the dislocations in the plastic zone of the 1200°C indentations shows that they lie predominantly on the basal planes parallel to the indented face, and the extra-half planes of the nonscrew dislocations originate from the indented face. It is also found that, when the (0001) Si-terminated face is indented, the dislocations are either widely dissociated, with the width of the stacking fault ribbon much larger than the equilibrium value, or else they are single leading partials, with the corresponding trailing partials absent. In this case, all the leading partials are found to have a silicon core. On the other hand, the dislocations in the plastic zone of the carbon-terminated face are in the form of dissociated dislocations, with the width of the associated stacking fault ribbons appreciably less than the equilibrium value. Moreover, the leading partials of these dissociated dislocations have a carbon core. The results indicate that the hardness of the polar basal faces of 4H-SiC at elevated temperatures is partly determined by the nature of the dislocation cores nucleated by the indentation process. It is argued that this is due to the influence of the core on the generation and glide of the leading partial dislocations.     

Low-angle twist grain boundary in SrTiO3 fabricated by spark plasma sintering techniques

A SrTiO₃ bicrystal with a low-angle twist grain boundary was fabricated using the spark plasma sintering (SPS) instrument. The atomic and electronic structure of the grain-boundary core was characterized using scanning transmission electron microscopy techniques. It was determined that the boundary is comprised of 2 types of defects with distinct electronic structures: screw dislocations and dislocations with an [001] edge component. The dislocations with an [001] edge component dissociated into 2 partial dislocations, separated by a stacking fault consisting of 2 Ti–O layers. The screw dislocations are attributed to the twist component of the grain boundary, while dislocations with an [001] edge component are attributed to surface steps on the original (100) SrTiO₃ surfaces prior to diffusion bonding. The observed repeat distances between the dislocations with edge components along the grain-boundary plane are smaller than those discovered during traditional diffusion bonding experiments. The higher planar defect density observed in this study results partly from higher heating rates, lower processing temperatures, and shorter holding times during SPS processing.     

Morphology of undeformed and deformed polyethylene lamellar crystals

Morphology of undeformed polyethylene crystals obtained by high pressure crystallization was investigated by SEM. It was revealed by exposing the interior of the samples by microtoming followed by permanganic etching. The etching procedure was refined to reveal defected sites of lamellae in addition to differentiation of crystalline and amorphous phases. From 1 to 3 screw dislocations with large Burgers vector per 1 μm² of lamellae basal planes were detected. Lamellae, when viewed edge-on give an impression of a “blocky architecture”, while their real shape, as seen on SEM images of flat-on and oblique lamellae, resembles platelets with a few defects in the form of screw dislocations protruding a platelet.High pressure-crystallized polyethylene samples were deformed plastically by uniaxial compression and were studied by SEM and AFM. When the deformation is interrupted dislocations are arrested within the crystals. It was observed that in contrast to undeformed samples, the side faces of deformed lamellae were not any longer smooth and a large number of screw dislocations with low Burgers vectors crossing the lamellae thickness could be distinguished. These observations are in accordance with polymer crystal plasticity theory that relies on the rate controlled nucleation and propagation of screw dislocations across polymer crystals. An existence of numerous screw dislocations arrested in lamellae is a direct proof of action of fine crystallographic slips along the macromolecular chains in PE crystals during plastic deformation. The kinking of lamellae due to plastic deformation was also observed. Large sections of lamellae between kinks rotated towards the plane of compression while the chain stems in lamellae rotated in the opposite direction, away from the compression direction, which is a signature of the fine crystallographic slip.Plastically deformed polyethylene crystals are highly defected due to many dislocations incorporated within them - the density of dislocations was approximated as 10¹⁶ m⁻². However, deformed crystal melting temperature is nearly unaffected while the heat of melting is slightly reduced, yet only in thin crystals. It suggests that the arrested dislocations contribute more to the surface energy of lamellae basal planes rather than to a bulk energy of polyethylene crystals.     

Dislocations at Prismatic Fracture Surfaces in Sapphire

Single-crystal sapphire exhibits a highly anisotropic fracture behavior. The surfaces of specimens fractured along prismatic planes are wavy, with fractographic features appearing as small areas of contrast under an optical microscope. Optical microscopy, atomic force microscopy, X-ray topography, and confocal microscopy are used to demonstrate a correlation between the areas of contrast and dislocations penetrating the sample surface. The surface features are argued to be a consequence of the stress field surrounding the dislocations, which deflect the crack approximately 10 nm normal to the surface as the crack cuts the dislocations. The lateral extent of measurable surface deflection is of the order of 5 μm. These images can be compared directly to show the equivalence of the position of the dislocations observed by X-ray topography and the surface contrast observed optically.     

Thermal-Mechanical History and the Strength of Magnesium Oxide Single Crystals: II, Etch Pit and Electron Transmission Studies

Mechanical tests show that the room-temperature flow strength of magnesia single crystals varies with heat treatment. Flow strength depends on the size, density, and distribution of precipitate particles which inhibit the motion of dislocations. The present paper describes optical and electron transmission microscope evidence for a precipitation reaction in magnesium oxide. The observations correlate fairly well with mechanical test data. Mechanical tests also show that mobile dislocations are locked by heating above 600°C. The room-temperature yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Studies to determine the nature of this locking mechanism are described. Etch pit studies show that dislocations aged above 600°C etch at a slower rate than fresh dislocations. This corresponds precisely with the change in mechanical behavior and both effects are attributed to impurity diffusion to dislocation lines. Electron transmission studies indicate a redistribution of point defects, left by moving dislocations, above 600°C. Eventually these result in a change in dislocation configuration which is considered responsible for the strong locking observed above 1000°C.     

Dislocations in as-Grown CVD α-Silicon Nitride

CVD αSi₃N₄ has been studied by transmission electron microscopy, and dislocations are found to occur in as-grown material. Two-beam contrast analysis has been used to determine that these dislocations have Burgers vectors of (0001), 1/3(2110), and 1/3(2113). Because of their smaller Burgers vector, (0001) dislocations are dominant. Their dissciation into half partials has been observed.     

Theoretical investigation on the interaction of nitrogen with dislocations in single crystal CVD diamond

In recent experimental studies, evidence for an atmosphere of nitrogen around the cores of dislocations was found in irradiated single crystal CVD diamond. In this paper, we present a first-principles study on the interaction of nitrogen with dislocations in single crystal CVD diamond, where dislocations are observed as mixed-type 45° and 90° edge-type dislocations lying along <100>. We find a strong binding of nitrogen to the dislocation core for both types of dislocations and show that our results are in consistency with the experiment.     

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.     

Surface Structure in Corundum: I, Etching of Dislocations

Triangular etch pits, corresponding to the intersection of individual edge dislocations with the surface, have been observed in corundum single crystals after about 5 minutes of immersion in boiling phosphoric acid. Dislocations of both the (0001), (1120) and (1210), (1010) slip systems (where reference is made to the slip plane and slip direction, respectively) have been detected by this method. Edge dislocations of the (0001), (1120) system etched on surfaces close to and including the (1011) and {2021} planes. Dislocations of the {1210}, (1010) systems etched on the basal (0001) plane. The orientation dependence and technique of etching is described. The agreement with the Nye formula and with some of the expected properties of dislocations is cited as evidence that individual edge dislocations are actually detected. Typical photomicrographs of the etched surface in as-received, deformed, flame-polished, and polygonized crystals are shown. The dislocation structure produced in basally deformed crystals was found to depend critically on the constancy of the temperature during deformation. Crystals held at 2000°C. during bending exhibited dislocations arrayed in rows in the slip planes in good agreement with the Nye formula. Crystals bent while cooling (quench-bent) had about twice as many dislocations as predicted by the Nye formula and these were distributed randomly. The possibility of vacancy condensation as a method for dislocation generation in quench-bent crystals is discussed.