Exact analytic solutions for an elliptic hole with asymmetric collinear cracks in a one-dimensional hexagonal quasi-crystal

Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric colfinear cracks in a one-dimensional hexagonal quasi-crystal is solved, and the exact analytic solutions of the stress intensity factors （SIFs） for mode Ⅲ problem are obtained. Under the limiting conditions, the present results reduce to the Griffith crack and many new results obtained as well, such as the circular hole with asymmetric collinear cracks, the elliptic hole with a straight crack, the mode T crack, the cross crack and so on. As far as the phonon field is concerned, these results, which play an important role in many practical and theoretical applications, are shown to be in good agreement with the classical results.

CONSTITUTION OF Al-BASE MULTI-COMPONENT QUASI-CRYSTALLINE ALLOYS

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Interfacial fracture analysis for a two-dimensional decagonal quasi-crystal coating layer structure

The interface crack problems in the two-dimensional(2D)decagonal quasicrystal(QC)coating are theoretically and numerically investigated with a displacement discontinuity method.The 2D general solution is obtained based on the potential theory.An analogy method is proposed based on the relationship between the general solutions for 2D decagonal and one-dimensional(1D)hexagonal QCs.According to the analogy method,the fundamental solutions of concentrated point phonon displacement discontinuities are obtained on the interface.By using the superposition principle,the hypersingular boundary integral-differential equations in terms of displacement discontinuities are determined for a line interface crack.Further,Green’s functions are found for uniform displacement discontinuities on a line element.The oscillatory singularity near a crack tip is eliminated by adopting the Gaussian distribution to approximate the delta function.The stress intensity factors(SIFs)with ordinary singularity and the energy release rate(ERR)are derived.Finally,a boundary element method is put forward to investigate the effects of different factors on the fracture.

Effect of Mg-Zn-Nd spherical quasi-crystals on microstructure and mechanical properties of ZK60 alloy

To improve the strength, toughness, heat-resistance and deformability of magnesium alloy, the microstructure and mechanical properties of ZK60 alloy strengthened by Mg-Zn-Nd spherical quasi-crystal phase (I-phase) particles were investigated. Mg40Zn55Nd5 (I-phase) particles in addition to α-Mg, MgZn phase and MgZn2 phases can be obtained in ZK60-based composites under normal casting condition by the addition of quasi-crystal containing Mg-Zn-Nd master alloy. The experimental results show that the introduction of Mg-Zn-Nd spherical quasi-crystal phase into ZK60 alloy makes a great contribution to the refinement of the matrix microstructures and the improvement of mechanical properties. While adding Mg-Zn-Nd spherical quasi-crystal master alloy of 4.0wt.%, the ultimate tensile strength and yield strength of ZK60-based composite at ambient temperature reach their peak values of 256.7 MPa and 150.4 MPa, which were about 17.8% and 24.1% higher respectively than those of the ZK60 alloy. The improved mechanical properties are mainly attributed to the pinning effect of the quasi-crystal particles (I-phase) at the grain boundaries. This research results provide a new way for strengthening and toughening of magnesium alloys as well as a new application of Mg-based spherical quasi-crystals.

Vortex quasi-crystals in mesoscopic superconducting samples

There seems to be a one to one correspondence between the phases of atomic and molecular matter（AMOM） and vortex matter（VM） in superfluids and superconductors. Crystals, liquids, and glasses have been experimentally observed in both AMOM and VM. Here, we propose a vortex quasi-crystal state which can be stabilized due to boundary and surface energy effects for samples of special shapes and sizes. For finite sized pentagonal samples, it is proposed that a phase transition between a vortex crystal and a vortex quasi-crystal occurs as a function of magnetic field and temperature as the sample size is reduced.

Effects of spherical quasi-crystal on microstructure and mechanical properties of ZA155 high zinc magnesium alloy

Effects of spherical quasi-crystal contained in Mg-Zn-Y-Mn master alloy on the microstructure and as-cast mechanical properties of ZA155 high zinc magnesium alloy have been investigated by means of optical microscopy,XRD,SEM,EDS,tensile test,impact test and hardness test.Experimental results show that the addition of spherical quasi-crystal contained in the Mg-Zn-Y-Mn master alloy into the ZA155 high zinc magnesium alloy resulted in grain refinement of the matrix,changing the morphologies of φ-Al2Mg5Zn2 phase and τ-Mg32(Al,Zn)49 phase from continuous net-like structures to discontinuous strip-like structure and blocky one,respectively.In the present research,the best comprehensive mechanical properties of reinforced ZA155 high zinc magnesium alloy has been obtained when 5.0wt% spherical quasi-crystal was introduced from the Mg-Zn-Y-Mn master alloy into the target alloy system.In such case,the room-temperature tensile strength reached 207 MPa,about 23% higher than that of the base alloy;the impact toughness peaked at 5.5 J/cm2,about 40% higher than that of the base alloy;and the elevated-temperature tensile strength reached 203 MPa,indicating improved heat resistance.

Fabrication of Mg-Zn-Y-based quasi-crystal by pouring melt into different cooling media

Mg-Zn-Y-based quasi-crystal (QC) alloys were fabricated by pouring melts into different cooling media. The effects of different cooling rates on the QC morphology, size, volume fraction and micro-hardness were studied. Multi-component Mg-Zn-Y-based QC alloys were synthesized based on amorphous design principle. QC morphology transformation and its influencing factors were analyzed. Micro/nano spherical quasi-crystals (QCs) were fabricated through a wedge-shaped copper mould and their forming mechanism were discussed by atoms cluster theory, optimum cooling rate theory and the known crucial criteria. The results of research show that with the cooling rate reduced, the solidified morphology of QC phase changes from near-spherical, micro petals (1 to 2 μm) to big petals (20 μm) and finally grows up to bulk pentagon or hexagon (200 to 400 μm). Multi-component micro-spherical QCs possess higher value of micro-hardness than petal-like QCs with the same components, and also higher than ternary micro-Mg-Zn-Y QCs. The fine master alloys containing micro-QCs (0.4 μm) and nano-QCs (about 300 nm and 40 nm) have been fabricated correspondingly on the middle and the tip of wedge-shaped castings. A morphology evolution schematic diagram of Mg-Zn-Y-based QCs is included in this paper.

Fabrication of quaternary Mg-Zn-Y-Ce quasi-crystal alloys and their strengthening effects on AZ91 magnesium alloys

Quaternary Mg-Zn-Y-Ce quasi-crystal-containing alloys were fabricated using a metal mold casting route. The microstructures of Mg-Zn-Y-Ce alloys and the distribution of the major elements were analyzed. The differences in morphology and micro-hardness between quaternary Mg-Zn-Y-Ce quasi-crystals and ternary Mg- Zn-Y quasi-crystals were discussed. The result showed that the micro-hardness of Mg-Zn-Y-Ce quasi-crystals reached peak value when the content of Ce was 0.8at.%. Subsequently, various contents of Mg-Zn-Y-0.8Ce alloys were used to strengthen AZ91 magnesium alloys. The microstructural modification and mechanical properties of reinforced AZ91 alloys with and without solution, as well as their aging treatments, were also studied. Further research showed that aging-treated AZ91-10wt.%(Mg-Zn-Y-0.8at.%Ce) alloys exhibited excellent mechanical properties: their Brinell hardness, tensile strength, and elongation were HV91, 252 MPa, and 4.1%, respectively. Through this study, the range of application of AZ91 alloys can further be extended.

Microstructure and phase transformation of as-cast and annealed Mg–4Zn–1Y alloy containing quasi-crystal phase

By means of optical microscope （OM）, X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM） analyses, the microstructures of as-cast and heat-treated Mg--4Zn-IY （wt%） alloy containing quasi-crystal phase were studied. The microstructure of the as-cast alloy consists of a-Mg solid solution grains, intermetallic particles and eutectic phases （W-phase and 1-phase）, and huge grains with seri- ous dendritic segregation are clearly observed. After heat treatment, phase transformation and dissolution occur in the alloy and many phases remain. When the alloy was treated above 410 ~C, the eutectic phases transform into spherical shape as the I-phase turns to W-phase. After heat treatment for long time, the alloy is over burnt and the W-phase decomposes to Mg-Y binary phase.

Guided Waves in the Multilayered One-Dimensional Hexagonal Quasi-crystal Plates

Guided waves in the multilayered one-dimensional quasi-crystal plates are,respectively,investigated in the context of the Bak and elasto-hydrodynamic models.Dispersion curves and phonon and phason displacements are calculated using the Legendre polynomial method.Wave characteristics in the context of these two models are analyzed in detail.Results show that the phonon-phason coupling effects on the first two layers are the same at low frequencies;but,they are more significant on the first layer than those on the second layer at high frequencies.These obtained results lay the theoretical basis of guided-wave nondestructive test on multilayered quasi-crystal plates.

Electromagnetic behavior of two-dimensional quasi-crystal left-handed materials with dendritic unit

We demonstrated a two-dimensional quasi-crystal dendritic structure model with negative permeability.The microwave transmission and reflection properties of periodic and quasi-crystal dendritic structure mediums with varied angle incidence were measured.The results showed that the resonant peaks of quasi-crystal negative permeability samples and left-handed samples had tiny shift with the altering of incident angle.Because of the influence of coupling between quasi-crystal dendritic cells,the resonance of the entire material was enhanced or weakened.The quasi-crystal dendritic structure model will provide an easy method to design isotropic left-handed materials(LHMs).

基于MgO:PPLN晶体的1554.8nm倍频器

为满足风云四号气象卫星搭载的闪电成像仪图像定位精度检验的光源需求,设计了一台基于两级主振荡功率放大器结构(MOPA)和掺氧化镁的周期极化铌酸锂(MgO:PPLN)晶体倍频技术,中心波长为777.4 nm,连续输出功率为1.5 W的激光器。采用反向940 nm off-peak泵浦方式较好地抑制了光放大器中自激放大辐射(ASE)和受激布里渊散射(SBS)效应,主放大器光-光转换效率达37.2%,获得了输出光功率为15 W,信噪比为59.6 dB,线宽为323.4 kHz,相对强度噪声为-145 dBc/Hz的单频窄线宽激光。在理论分析所用MgO:PPLN晶体倍频效率、温度调谐带宽以及可接收的波长带宽基础上,采用单通倍频方式获得了功率为1.5 W的777.4 nm连续光。

基于表面等离子体共振的光子准晶体光纤甲烷氢气传感器

设计了一种用于同时检测甲烷和氢气的基于表面等离子体共振(SPR)的新型光子准晶体光纤(PQF)传感器。在该传感器中,在银膜上分别沉积Pd-WO3和掺杂聚硅氧烷的笼型分子E薄膜作为氢气和甲烷的敏感材料。采用全矢量有限元方法对PQF-SPR传感器进行数值分析,结果证明该传感器具有良好的传感性能。在0%~3.5%的浓度范围内,氢气的最大检测灵敏度和平均灵敏度分别为0.8 nm/%和0.65 nm/%,甲烷的最大灵敏度和平均灵敏度分别为10 nm/%和8.81 nm/%。该传感器具有同时检测多种气体的能力,在设备小型化和远程监测方面具有很大的潜力。

一维六方准晶纳米板中Lamb波特性研究

准晶纳米结构在工程中经常承受以弹性波为代表的动态工作载荷,为深入研究其动态失效机制,研究了一维六方准晶纳米板中Lamb波的波动特性。基于修正的偶应力理论,推导出Bak模型下Lamb波的波动控制方程,使用勒让德正交多项式方法(LOPM)求解该动力学方程,计算得到其频散曲线和位移分布。研究了声子场和相位子场尺寸效应、声-相耦合效应对波动特性的影响。结果表明:尺寸效应使声子模态和相位子模态相速度增大;声-相耦合效应显著地增大了声子模态中的相位子位移振幅和相位子模态中的声子位移振幅。研究结果为准晶纳米结构的设计和无损检测奠定了一定的理论基础。

类球形草酸钴沉淀粉末的制备研究

草酸钴是制备钴粉的重要前驱体,其常见的沉淀产物呈长条棒状,而工业上急需球状或类球状的颗粒。本研究提出了一种新颖的方法,控制钴离子/草酸根的化学计量比远离1.0和温度在80℃以上时,能成功地将草酸钴沉淀形貌从晶须状改变成类球形。研究认为,在钴离子过量或草酸根离子过量的情况,会影响草酸钴成核基团及晶核表面的荷电状态,而80℃以上高温作用可能显著改变晶核表面液膜层内出现紊流传质现象,使得晶核各个晶面上生长速度差异变小,最终形成类球形颗粒草酸钴。本研究结果,将为草酸钴沉淀形貌调控及类球形草酸钴的生产提供重要指导。

高功率无线光通信波长转换系统性能研究

针对水下高速无线激光通信发射功率较低,通信距离受限的问题,该研究采用非线性光学中二次谐波理论,将1064 nm的高速外调制与放大技术和非线性晶体的准相位匹配技术相结合,通过光学设计和系统参数研究,设计了一种532 nm的高速大功率无线激光调制发射系统,并建立了基于温度和角度变换的波长转换效率模型。然后通过仿真分析的方法得到了不同温度下,最大转换效率对应的角度范围。最后对系统的性能进行了系统搭建和实验分析,验证了该系统在常温下可实现1 W大功率532 nm绿光无线输出,且频率大于1 GHz的模拟信号和速率大于500 Mbps的数字信号调制。实验结果表明:常温下,该系统输出的光斑光强分布均匀,光束质量显著好于调制后的基频光模式。系统输出的方波信号上升沿宽度和下降沿宽度会发生压缩,但随着脉冲宽度的减小压缩现象会发生收敛,上升沿宽度收敛于0.27,下降沿宽度收敛于0.06,系统输出的正弦信号随着频率的增加没有发生明显的畸变。研究结果可以为未来水下长距离高速无线光通信应用提供理论和技术支撑。

一维准周期类结构复合材料的透射性质

本文构造了一维声子晶体的3种准周期类结构,并比较和分析这3类具有代表性的准周期结构声子晶体在透射性质方面的异同点。计算结果表明:弹性波在这些材料中传播时,3种结构类型的声子晶体都有带隙产生,并且都存在共振模;其中共振模最为丰富的是广义斐波那契结构的声子晶体;随着介质层数的变化,共振模的透射率表现最为稳定的是斐波那契结构声子晶体。通过比较研究这3类声子晶体结构的透射性质,有利于全面理解此类复合结构材料中波的传播特性。

堆积硅料的阶跃分布孔隙率对准单晶铸锭过程籽晶熔化的影响

本文针对光伏太阳能用准单晶硅铸锭系统的硅料熔化过程进行了数值模拟研究,尤其是孔隙率阶跃分布的堆积硅料熔化过程对籽晶熔化的影响。研究结果表明:堆积硅料孔隙率呈轴向阶跃分布有利于降低籽晶的熔化比例;籽晶的熔化界面形状主要受下层孔隙率影响,在特定的平均孔隙率范围内,上下两层孔隙率差异较小时,孔隙率的轴向阶跃分布对籽晶的熔化界面形状影响较小;当籽晶的熔化比例相近时,平均孔隙率越小,籽晶的熔化界面形状越平缓,越有利于籽晶边缘区域的保留;当平均孔隙率一定时,下层孔隙率越小越有利于籽晶边缘区域的保留。堆积硅料区域孔隙率呈径向阶跃分布会使籽晶的熔化界面形状发生畸变,内层孔隙率的逐渐增大会使籽晶的熔化界面形状由“凸”逐渐转变为“凹”,外层孔隙率不大于内层孔隙率时籽晶可以得到有效保留;内外两层孔隙率差值越小,籽晶的熔化比例越小。籽晶的熔化比例分布在不同轴向阶跃分布孔隙率下呈现一定的中心对称性,而在不同径向阶跃分布孔隙率下呈现一定的周期性,孔隙率均匀分布时的籽晶熔化界面形状优于其他情况。在实际工况条件下,可以根据由不同孔隙率分布条件下获取的籽晶熔化状态数据绘制的等值线图对堆积硅料区域的孔隙率分布进行合理配置。

Y对普通凝固Mg-Zn-Y系合金组织及准晶相形成的影响

采用普通凝固技术在高镁、低锌和低钇合金中制备出镁合金稳定态准晶相,为制备准晶增强自生复合材料的可行性开辟途径。通过光学显微镜、X-ray衍射、扫描和透射电镜显微分析技术,确定了准晶的组织、相成分及结构。分析了由于合金成分不同,特别是稀土元素Y摩尔分数的不同导致准晶的分布、数量等差异。结果表明:Mg-Zn-Y三元合金在室温冷却过程中,准晶相直接从液相形核、长大;伴随Y摩尔分数的增加,准晶相的晶粒度、圆整度和分布均匀化程度均有提高。

准晶:奇特而又平凡的晶体——2011年诺贝尔化学奖简介

准晶是具有长程准周期性平移序和非晶体学旋转对称的固态有序相。它的出现导致了晶体学与凝聚态物质结构理论的一次革命,极大推动了相关学科的完善和发展。其发现者Daniel Shechtman因此获得2011年度诺贝尔化学奖。本文简介了准晶的发现、结构特征和发展现状,同时介绍了中国科学家在这一领域的独特贡献。