高质量LiGaO2晶体的提拉法生长及表征

采用提拉法成功地生长了高质量的LiGaO2单晶体,生长过程中没有观察到挥发现象。通过四晶X射线衍射、化学腐蚀、光学显微、透过光谱以及原子力显微镜对晶体的质量进行了表征。结果表明：晶体中无包裹物及气泡,具有很高的质量,（001）面晶片的摇摆曲线半高宽仅为16.2arcsec,正交的（001）、（100）及（010）三个晶面具有不同的腐蚀形貌,其位错密度均低于10^4/cm^2;LiGaO2晶体的吸收边约为220nm;化学机械抛光后的晶片表面非常光滑,其均方根粗糙度仅为0.1nm（5×5μm^2）。

铝酸钇晶体中的生长小面

采用提拉法生长了掺Ce、掺Yb和掺Mn的铝酸钇(YAlO3,YAP)晶体,晶体均完整透明,无肉眼可见的气泡、散射和包裹物等宏观缺陷。通过化学腐蚀和同步辐射白光形貌实验检测了YAP晶体中的生长小面缺陷。结果表明:晶体生长过程中,由于凸向熔体的固-液界面,造成了小面生长现象。沿[101]方向生长的YAP晶体中出现的小面为(102),(201),(121)和(121)奇异面。X射线摇摆曲线表征的结果表明:生长小面的存在严重破坏了晶体的微观结构完整性和均匀性,并导致了小角度晶界缺陷的产生。

长脉冲激光作用下薄膜表面鼓包的形成机理

在光学薄膜的激光损伤实验中,可以在显微镜下观测到激光辐照引起的尺寸较大、形状对称性很好的鼓包。从杂质气化的角度，应用弹性力学的球壳受压膨胀模型，分析薄膜表面鼓包的形成机制，同时得到了相应情况下薄膜的损伤机制。结果表明，半径越大杂质在激光辐照下越容易引起薄膜破坏,填埋得越浅的杂质也越容易引起薄膜破坏。杂质诱导破坏机制下,薄膜的裂纹首先出现在杂质附近的薄膜基体中。在薄膜中出现裂纹的临界状态时，半径越大、填埋得越深的杂质所引起的鼓包尺寸越大、高度越高。这种热力耦合模型，弥补了现有理论的不足。进一步完善了现有理论

Two-dimensional network of dislocations and nanocavities in hydrogen-implanted and two-step annealed silicon

Conventional transmission electron microscopy and energy-filtering were used to study the dislocations and nanocavities in proton-implanted [001] silicon. A two-dimensional network of dislocations and nanocavities was found after a two-step annealing, while only isolated cavities were present in single-step annealed Si. In addition, two-step annealing increased materially the size and density of the nanocavities. The Burgers vector of the dislocations was mainly the 1/2[110] type. The gettering of oxygen at the nanocavities was demonstrated. (C) 1998 American Institute of Physics. [S0003-6951(98)00620-2].

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from -20 to 190 K, and the heat fluxes reach 140 W/cm(2). The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60 ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90 ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed.

Periodic bubble emission and appearance of an ordered bubble sequence (train) during condensation in a single microchannel

Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30 pm, a width of 800 mu m and a length of 5.0 mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble.

Study on the kinetics of hydrate formation in a bubble column

Gas hydrate formation experiments were performed using methane in the presence of tetrahydrofuran (THF) in aqueous solution in a transparent bubble column in which a single pipe or a sintered plate was used to produce bubbles. The mole fraction of THF in aqueous solution was fixed at 6%. The hydrate formation kinetic behaviors on the surface of the rising bubble, the mechanical stability of hydrate shell formed on the surface of the bubble, the interactions among the bubbles with hydrate shell were observed and investigated morphologically. The rise velocities of individual bubbles with hydrate shells of different thickness and the consumption rates of methane gas were measured. A kinetic model was developed to correlate the experimentally measured gas consumption rate data. It was found that the hydrate formation rate on the surface of the moving bubble was high, but the formed hydrate shell was not very easy to be broken up. The bubbles with hydrate shells tended to agglomerate rather than merge into bigger bubble. This kind of characteristic of hydrate shell hindered the further formation of hydrate and led to the lower consumption rate of methane. The consumption rate of methane was found to increase with the decrease of temperature or increase of pressure. The increase of gas flux led to a linear increase in consumption rate of methane. It was demonstrated that the developed kinetic model could be used to correlate the consumption rate satisfyingly.

PROTON-IMPLANTED NEW TYPE SILICON MATERIAL SUBJECTED TO 2-STEP FURNACE ANNEALING

A high-resistivity defect layer buried beneath the silicon surface layer by using proton implantation and two-step conventional furnace annealing is described. During the first annealing step (600-degrees-C), implanted hydrogen atoms move towards the damage region and then coalesce into hydrogen gas bubbles at the residual defect layer. During the second annealing step (1180-degrees-C) these bubbles do not move due to their large volume. Structural defects are formed around the bubbles at a depth of approximately 0.5-mu-m. The defect layer results in a high resistivity value. Experiments show that the quality of the surface layer has been improved because the surface Hall mobility increased by 20%. The sample was investigated by transmission electron microscopy.

A transmission electron microscopy study of microstructural defects in proton implanted silicon

The microstructure of silicon on defect layer, a new type of silicon-on-insulator material using proton implantation and two-step annealing to obtain a high resistivity buried layer beneath the silicon surface, has been investigated by transmission electron microscopy. Implantation induced a heavily damaged region containing two types of extended defects involving hydrogen: {001} platelets and {111} platelets. During the first step annealing, gas bubbles and {111} precipitates formed. After the second step annealing, {111} precipitates disappeared, while the bubble microstructure still remained and a buried layer consisting of bubbles and dislocations between the bubbles was left. This study shows that the dislocations pinning the bubbles plays an important role in stabilizing the bubbles and in the formation of the defect insulating layer. (C) 1996 American Institute of Physics.

Large-eddy simulation of flows past a flapping airfoil using immersed boundary method

The numerical simulation of flows past flapping foils at moderate Reynolds numbers presents two challenges to computational fluid dynamics: turbulent flows and moving boundaries. The direct forcing immersed boundary (IB) method has been developed to simulate laminar flows. However, its performance in simulating turbulent flows and transitional flows with moving boundaries has not been fully evaluated. In the present work, we use the IB method to simulate fully developed turbulent channel flows and transitional flows past a stationary/plunging SD7003 airfoil. To suppress the non-physical force oscillations in the plunging case, we use the smoothed discrete delta function for interpolation in the IB method. The results of the present work demonstrate that the IB method can be used to simulate turbulent flows and transitional flows with moving boundaries.

Experimental Investigation of Performance of a Miniature Direct Methanol Fuel Cell in Short-Term Microgravity

Experimental study of a liquid fed direct methanol fuel cell has been conducted in different gravity environments. A small single cell with 5 cm x 5 cm active area has single serpentine channel on the graphite cathode polar plate and 11 parallel straight channels on the graphite anode flow bed. Cell voltage and current have been measured and two-phase flow in anode channels has been in situ visually observed. The experimental results indicate that the effect of gravity on power performance of the direct methanol fuel cell is large when the concentration polarization governs fuel cells operation. Gravitational effect becomes larger at higher current density. Increasing methanol feeding molarity is conducive to weaken the influence of gravity on performance of liquid fed direct methanol fuel cells. Increasing feeding flow rate of methanol solution from 6 to 15 ml/min could reduce the size of carbon dioxide bubbles, while the influence of gravity still exist. Transport phenomena inside direct methanol fuel cells in microgravity is also analyzed and discussed.

Transition to Film Boiling in Microgravity: Influence of Subcooling

The transition process to film pool boiling in microgravity is studied experimentally aboard the Chinese recoverable satellite SJ-8. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Small, primary bubbles are formed and slid on the surface, which coalesce with each other to form a large coalesced bubble. Two ways are observed for the transition from nucleate to film boiling at different subcoolings. At high subcooling, the coalesced bubble with a smooth surface grows slowly. It is then difficult for the coalesced bubble to cover the whole heater surface, resulting in a special region of transition boiling in which nucleate boiling and local dry areas can coexist. In contrast, strong oscillation of the coalesced bubble surface at low subcooling may cause rewetting of local dry areas and activation of more nucleate sites, resulting in an abrupt transition to film boiling.

Bubble Dynamics and Heat Transfer in Microgravity Pool Boiling

Boiling is an extremely complicated and illusive process. Microgravity experiments offer a unique opportunity to study the complex interactions without external forces, such as buoyancy, which can affect the bubble dynamics and the related heat transfer. Furthermore, they can also provide a means to study the actual influence of gravity on the boiling. Two research projects on pool boiling in microgravity have been conducted aboard the Chinese recoverable satellites. Ground-based experiments both in normal gravity and in short-term microgravity in the Drop Tower Beijing and numerical simulations have also been performed. Steady boiling of R113 on thin platinum wires was studied with a temperature-controlled heating method, while quasi-steady boiling of FC-72 on a plane plate was investigated with an exponentially increasing heating voltage. It was found that the bubble dynamics in microgravity has a distinct difference from that in normal gravity, and that the heat transfer characteristic is depended upon the bubble dynamics. Lateral motions of bubbles on the heaters were observed before their departure in microgravity. The surface oscillation of the merged bubbles due to lateral coalescence between adjacent bubbles drove it to detach from the heaters. Slight enhancement of heat transfer on wires is observed in microgravity, while diminution is evident for high heat flux in the plate case.