DYNAMICAL SIMULATION OF ORIENTATION TEXTURE EVOLUTION OF A POLYMERIC LIQUID CRYSTAL UNDER SIMPLE SHEAR FLOW

Abstract

Dynamical simulation of orientation texture evolution of a polymeric liquid crystal under simple shear flow has been carried out. The Leslie-Ericksen equation was numerically solved to obtain three dimensional orientation over the shear plane spanned by the flow (x) and thickenss (y) axes. The present study shows a cascade of complex orientation texture evolutions as a function of the Ericksen number (E) during shear flow starting from a near perfect monodomain condition. When E₂>E>E₁, directors in the middle gap region twist out of the shear plane in a uniform sense. When E₃>E>E₂, the twist sense is not of the same sign, but alternating along the flow axis. When E₄>E>E₃, the middle orientation wall of the alternating twist sense divides into a pair, which migrate towards the bounding plates, forming a series of tubular orientation walls. When E>E₄, the tubular orientation walls still form, but through a reentrant two-dimensional in-shear-plane state.     

石墨衬底上铝诱导法制备多晶硅薄膜

利用磁控溅射技术在石墨衬底上制备了石墨/a-Si/Al和石墨/Al/a-Si叠层结构,采用常规退火(CTA)和快速热退火(RTA)对样品进行退火,系统研究了不同退火条件对多晶硅薄膜制备的影响.利用X射线衍射(XRD),拉曼光谱(Raman)对制备的多晶硅薄膜进行表征,并利用谢乐公式计算了晶粒尺寸,结果表明制备的多晶硅薄膜具有高度(111)择优取向,结晶质量良好,利于后续外延制作多晶硅厚膜电池.基于实验结果,建立了铝诱导晶化模型,很好的解释了实验现象.     

加氮对直流电弧等离子体喷射金刚石膜生长、形貌和质量的影响

利用直流等离子体喷射化学气相沉积法制备掺氮的金刚石厚膜.本文研究了在甲烷/氩气/氢气中加入氮气对金刚石膜生长、形貌和质量的影响.反应气体的比例由质量流量计控制,在固定氢气(5000sccm)、氩气(3000sccm)、甲烷(100sccm)流量的情况下改变氮气的流量,即反应气体中氮原子和碳原子的变化比例(N/ C比)范围是从0.06到0.68.同时金刚石膜在固定的腔体压力(4kPa)和衬底温度(800℃)下生长.金刚石膜用扫描电镜(SEM)、拉曼谱和X射线衍射表征.结果表明,氮气在反应气体中的大量加入对直流等离子体喷射制备金刚石膜的形貌、生长速率、晶体取向、成核密度等有非常显著的影响.     

Domain structure of a nematic-liquid-crystal layer in low-frequency Couette flow

The spatially periodic structure arising in a homeotropic nematic-liquid-crystal (NLC) layer in low-frequency Couette flow is described theoretically. The analysis of this phenomenon is based on the hydrodynamic equations for NLCs, from which a self-consistent system of equations is selected for perturbations of hydrodynamic variables: the steady-state angle of the molecule rotation, the liquid flow, and the velocity of oscillating vortex flows. The formation of the periodic structure is explained by the phase delay of the velocities of the vortex oscillating flows forming in the deformed structure with respect to the shear velocity in the Couette flow. It is shown that at low frequencies this difference in the velocities is caused by the orientational waves near the layer boundaries. In the case of fixed orientation of molecules at the boundaries, the dependence of the threshold shear amplitude on the frequency and layer thickness is given by the relation u_th ～ (ωh²)^?1/4. The influence of the conditions for the molecule orientation at the layer boundaries on the above phenomenon is analyzed.     

Effect of gallium incorporation on the physical properties of ZnO films grown by spray pyrolysis

Gallium-doped zinc oxide thin films were deposited by the spray pyrolysis technique onto Corning 7059 glass substrates at a temperature of 350°C using a precursor solution of zinc acetate in isopropyl alcohol. The films were prepared using different gallium concentrations keeping the other deposition parameters such as air and solution flow rates and solution concentration constant. The variations of the structural, electrical and optical properties with the doping concentration were investigated. X-ray diffraction data showed that the films were polycrystalline with the (0 0 2) preferred orientation. The texture coefficient and grain size were evaluated for different doping concentrations. The films with 5.0 at% gallium had a resistivity of 1.5×10⁻³ Ω cm and a transmittance of 85% with an energy band gap of 3.35 eV.     

Aging and flow in a complex fluid

The comprehension of the physical mechanism governing the interaction between aging dynamics and shear flow is crucial to both elucidating the nature of slow dynamics in soft materials and controlling their complex rheological behavior. Dynamic light scattering is used to probe the relaxation of the intermediate scattering function of an aging colloidal suspension and the effect of shear on the non-equilibrium structural dynamics is investigated during various protocols of applied shear. The shear flow influences significantly the aging dynamics as soon as structural relaxation enters the timescale set by the inverse shear rate. Aging is strongly reduced in this shear dominated regime, while, for a fixed system age, the average structural relaxation time scales as the inverse shear rate. The fast component in the relaxation function, usually interpreted as the single particle diffusion, shows a cross-over behavior as well, suggesting a close link between slow and fast dynamics. Shear rejuvenation of old samples is also observed leading to a faster relaxation dynamics while the subsequent aging proceeds in an apparently new regime.     

Aspects of low heterostructure symmetry in (311)A (In,Ga)As/GaAs

We investigate the structural and optical properties of (In,Ga)As/GaAs heterostructures induced by the low symmetry of the [311]A orientation. High-resolution X-ray diffraction (HRDXD) measurements reveal the existence of a shear-strained unit cell. The onset of relaxation in heavily strained structures leads to pronounced anisotropies and to satellite splittings in the X-ray diffraction patterns. Optical investigations indicate that the impact of the piezoelectric fields on the ground state energy is compensated for by In segregation. Nevertheless these fields cause a pronounced reduction of the excitonic binding energy in the (311)A structures.     

MOCVD方法在Ti/Si(111)模板上生长ZnO薄膜的研究

本文采用常压MOCVD方法在Ti/Si（111）模板上生长了氧化锌（ZnO）薄膜，使用二乙基锌为Zn源，去离子水为O源。Si衬底上的Ti薄层采用电子束蒸发台蒸发，然后低温生长缓冲层并在高温下进行重结晶，接着在680％进行ZnO薄膜的生长。采用粉末衍射法、双晶X射线衍射及光致发光技术研究了材料的取向、结晶性能及发光性能。结果表明，本文制备了高度择优取向和良好发光性能的ZnO薄膜。     

“Bands” and “Torsads” Textures in Films and Threads of Hydroxypropyl Cellulose

The present paper deals with films and threads of a mesomorphic cholesteric polymer: the hydroxypropyl cellulose. The steady flow behaviour and the shear induced textures (observed by optical microscopy under crossed polars) of films of lyotropic solutions of this polymer have been studied. A detailed analysis of the banded structures, of either the sheared solutions or the dried films obtained by solvent evaporation, is reported. We point out the existence of transverse striated patterns—“torsads”—superposed on the main banded structures, which are associated with the rewinding of the cholesteric structure following the cessation of the shear stress. The torsads, also observed in some threads, originate from the same relaxation phenomena and are discussed in the last part of the paper.     

工艺参数对高功率MPCVD金刚石膜择优取向的影响研究

使用自行研制的椭球谐振腔式MPCVD装置,以H2-CH4为气源、沉积功率8 kW条件下,在不同CH4浓度、沉积温度和气体流量工艺条件下制备了大面积金刚石膜.使用X射线衍射仪对金刚石膜的择优取向的变化规律进行了研究.实验结果表明,高功率条件下工艺参数对金刚石膜的择优取向有不同程度的影响.在CH4浓度由0.5;上升到1.0;时,金刚石膜的择优取向由(220)转变为(111),由1.O;上升到2.5;时,则由(111)转变为(220)以及(311);在700 ～ 1050℃温度范围内,随着沉积温度的升高,金刚石膜(111)择优取向生长的倾向增高,当沉积温度高于1050℃时,金刚石膜改变了原先的以(111)择优取向生长的趋势,变为了以(100)择优取向生长;在气体流速为200～1000 sccm范围内时,随气体流量的增加,金刚石膜(111)择优取向的倾向增加.当气体流量大于1000sccm时,金刚石膜(111)择优取向的倾向又稍有降低.     

Influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells grown by MOCVD

We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces.     

Growth of oriented GaN nanowires by controlling nucleation conditions

GaN nanowires (NWs) were grown on GaN(0001) coated sapphire substrate with Ni/Au catalyst by using metalorganic chemical vapor deposition. Nucleation conditions were investigated for improving the growth orientation of NWs. With decreasing catalyst thickness from 5nm/5nm to 2nm/2nm, the NW orientation was improved and the NW morphology was changed from taper to cylindrical, due to the varying of growth mode. Vertical alignment of NWs can be further improved by inserting an additional high‐temperature (850 ℃) nucleation step with an optimum V/III flow ratio of 20.     

Surface and Magnetic Orientation in a Type II Nematic Lyomesophase

Abstract

A type II nematic lyomesophase formed by a quaternary system (K decanoate/water/decanol/KCI) has been studied by X-ray diffraction (XD) and optical microscopy (OM) at room temperature. Samples in capillaries of various thicknesses, materials and geometries have been studied under the influence of wall effects, residual magnetic orientation and orientation in presence of magnetic fields. Results evidence that surface orientation extend typically up to 0.5 mm. The planar diffracting units have a characteristic distance in the director direction of 36 Å. The analysis of the compromise between surface and magnetic orientation by XD allowed to determine a critical field of 200 G for a 0.5 mm thickness; the elastic constant is estimated as 10^?-⁷ dynes. Analised by OM, the sample is axially positive, but shows also a weak biaxiality.     

X-ray scattering from a material under an external load

The differential X-ray scattering cross section of a polycrystalline sample placed in a high-pressure chamber under an arbitrary load has been calculated. It is shown that the broadening and displacement of most diffraction lines under shear load are, generally speaking, of the same order of magnitude, except for the reflections on the vectors parallel to threefold and higher order axes (these reflections do not undergo broad-ening). The scattering line shape contains information about the components of the compressibility tensor; to derive it, one needs a diffractometer resolution satisfying the condition δθ/θ < τ/G (τ is the shear stress and G is the typical shear modulus of the material studied). An experiment under hydrostatic load is analyzed, and it is shown that the differential bulk compressibility modulus of a polycrystal under pressure, which are derived from X-ray data, coincides with the value given by the natural strain theory.     

Structure of azadirachtin-I, 11 β-H epimer

The 11 -H epimer of Azadirachtin-I, isolated from the seed kernels of Azadirachta indica A. Juss (Neem), was characterized by both NMR and X-ray crystallographic techniques. NMR data reveals that the H11 proton is in -orientation and the X-ray studies confirm this observation. The compound crystallized in space group P2₁ with the cell parameters a = 11.933(2) Å, b = 7.752(5) Å, c = 17.241(9) Å, = 106.80(3). Though the structural features are similar to the 11 -H epimer of Azadirachtin-H, the orientation of the acetoxy group at C3 is different. The dihedral angle C9–C8–C14–O13, which describes the relative orientation of the modified decalin and modified furan moities of the molecule, is 23.3(8).     

三坩埚提拉法生长铽镓石榴石单晶研究

采用三坩埚提拉法生长出高质量铽镓石榴石(Tb3Ga5O12,TGG)单晶,晶体尺寸为φ25 mm×40 mm.通过X射线衍射、双晶衍射分析讨论TGG单晶的晶体结构和单晶性,并采用He-Ne激光费尔德常数测试装置测定TGG单晶的费尔德常数.结果表明:采用三坩埚提拉法生长的TGG单晶具有＜111＞取向、强度高,双晶摇摆曲线半峰宽为17 s;晶体在632.8 nm处的费尔德常数为0.553 min/cm·Oe.     

氩氧流量比对CeOx-SnOx膜微观结构及光学性能的影响

采用磁控反应共溅射的方法,以金属Ce和Sn为金属源,成功地制备出CeOx-SnOx薄膜.利用扫描电子显微镜(SEM)、X射线衍射(XRD)、透射电子显微镜(TEM)和X射线光电子能谱(XPS)等测试手段对薄膜的结构、表面形貌及成分进行了分析和表征.结果表明薄膜以岛状模式生长,随氩氧比降低,结晶性增强,出现CeO2和SnO相.此外,利用紫外-可见分光光度计对薄膜的光学性能进行了研究,测试结果表明薄膜对紫外光有极强的吸收作用.当氩氧流量比为3∶1时,紫外光平均透过率仅为5.80;,而可见光平均透过率为81.48;.     

TMIn流量对GaN基蓝光LED外延薄膜的影响

以蓝宝石(Al2O3)为衬底,采用有机金属化学气相沉积(MOCVD)技术生长InGaN/GaN多量子阱结构.本文通过调整外延生长过程中三甲基铟(TMIn)流量,研究了TMIn流量对InGaN/GaN多量子阱结构的合金组分、晶体质量和光学性质的影响.本文采用高分辨X射线衍射(HRXRD)、原子力显微镜(AFM)和光致发光(PL)测试表征其结构和光学性质.HRXRD测试结果表明,随TMIn流量增加,"0"级峰与GaN峰之间角偏离增大,更多的In并入薄膜中.HRXRD与AFM表征结果表明:增大TMIn流量会导致外延薄膜中的位错密度增大,V形坑数量增加,晶体质量严重恶化;PL测试结果表明,随着TMIn流量增加,发光强度逐渐降低,半高宽增大,这是由于晶体质量恶化所导致.因此严格控制铟源流量对于改善量子阱薄膜的晶体质量与光学性质有着至关重要的作用.     

State of Order in Liquid Crystalline Elastomers

The orientation behavior of crosslinked liquid crystal-line side chain polymers is determined by IR-dichroism measurements. In stretched samples the conformation of the polymer main chain and the chemical constitution of the rodlike side chain determine the position of the director in relation to the axis of stress. The order parameter S has the same temperature dependence and is of the same magnitude as in the corresponding uncrosslinked liquid crystalline polymers. The order parameter and the nematic to isotropic phase transformation temperature are independent of the applied mechanical stress under the experimental conditions (elongation up to 50%).     

Influence of arsenic flow on the crystal structure of epitaxial GaAs grown at low temperatures on GaAs (100) and (111)<Emphasis Type="Italic">A</Emphasis> substrates

The influence of arsenic flow in a growth chamber on the crystal structure of GaAs grown by molecular-beam epitaxy at a temperature of 240°C on GaAs (100) and (111)A substrates has been investigated. The flow ratio γ of arsenic As4 and gallium was varied in the range from 16 to 50. GaAs films were either undoped, or homogeneously doped with silicon, or contained three equidistantly spaced silicon δ-layers. The structural quality of the annealed samples has been investigated by transmission electron microscopy. It is established for the first time that silicon δ-layers in “low-temperature” GaAs serve as formation centers of arsenic precipitates. Their average size, concentration, and spatial distribution are estimated. The dependence of the film structural quality on γ is analyzed. Regions 100–150 nm in size have been revealed in some samples and identified (by X-ray microanalysis) as pores. It is found that, in the entire range of γ under consideration, GaAs films on (111)A substrates have a poorer structural quality and become polycrystalline beginning with a thickness of 150–200 nm.