Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral （R3c） phase of BiFeO3 first transforms to monoclinic （C2/m） phase at 7 GPa, then to orthorhombic （Pnma） phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic （Pbam） phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.     

低气压条件下氢气潘宁放电的模拟分析

为更好地理解低气压、弱电离条件下潘宁离子源放电过程中离子和电子的动力学行为, 通过建立二维轴对称模型, 采用粒子模拟与蒙特卡罗相结合(PIC/MCC)的方法, 考虑了电子与氢气之间的弹性碰撞、激发、电离以及氢原子、离子之间的弹性碰撞和电荷交换等过程, 对微型氢气潘宁离子源放电和引出过程进行了数值研究. 考察了磁场位形、壁面二次电子发射系数、引出电压和充气压力对放电过程的影响, 得到了实验中难以诊断得到的放电腔内电子与离子数密度分布, 阳极电流、引出极离子电流、单原子氢离子比例和双原子氢离子比例等宏观参数与实验结果相一致. 通过仿真使得对氢气潘宁放电机制的研究从定性过渡到定量, 这对于潘宁离子源的设计和改进具有重要意义. 关键词： 潘宁放电 氢气 粒子模拟 蒙特卡罗     

成年口吃者流畅朗读中塞音的声学分析

研究对成年口吃者在流畅朗读过程中的塞音进行了声学分析,测量了嗓音起始时间并且计算了塞音爆破时刻的频谱矩,并将口吃者在言语矫治前后与非口吃者进行了对比。多因素方差分析结果显示,口吃者嗓音起始时间虽稍长于非口吃者但未达到统计意义上的显著性差异水平,而且受发音部位和韵母的影响程度较大。同时还观察到矫治前口吃者和非口吃者在塞音爆破段的频谱均值呈现出显著性差异,口吃者频谱均值低于非口吃者可能是由于口吃者舌与齿龈或软硬腭形成阻塞的部位在声道中偏后所导致,还发现口吃者韵母对塞音爆破段频谱的影响较小,此结果表明口吃者表现出相对较弱的协同发音现象。口吃者经过言语矫治后塞音的嗓音起始时间和爆破段频谱有向非口吃者逼近的趋势。     

Effects of effective attractive multi-body interaction on quantum phase and transport dynamics of a strongly correlated bosonic gas across the superfluid to Mott insulator transition

We theoretically investigate quantum phases and transport dynamics of ultracold atoms trapped in an optical lattice in the presence of effective multi-body interaction.When a harmonic external potential is added,several interesting phenomena are revealed,such as the broadening and the emergence of a central insulator plateau and the phase transition between superfluid and Mott insulator phase.We also study the transport of the system which runs across the superfluid–insulator transition after ramping up the lattice,and predict a slower relaxation which is attributed to the influence of the multi-body interaction on the mass transport.     

风速测量实验的设计与探究

基于电磁理论和空气动力学的相关知识,利用磁铁、金属棒和DISLAB传感器等器件设计并制作了风速测量系统,可准确地测量2-11级范围内的风速值.该装置还可作为演示电磁感应现象的设计型实验内容,实验过程中可加深学生对电磁感应现象的理解,并提高学生对matlab软件的应用能力.     

CaMoO4：Eu3+,Bi3+,Li+红色荧光粉的共沉淀制备与表征

采用共沉淀法合成了红色荧光粉Ca_0.75MoO₄：Eu_0.25³⁺、Ca_0.75MoO₄：Eu_0.25-x³⁺,Bi_x³⁺及Ca_0.5MoO₄：Eu_0.25-2x³⁺, Bi_x³⁺,Li_0.25+x⁺,并采用X射线衍射(XRD)、拉曼光谱,扫描电镜(SEM)和荧光光谱(PL)测定分析了其结构形貌特征及发光性能。结果表明：制备的CaMoO₄：Eu³⁺,Bi³⁺,Li⁺红色荧光粉为白钨矿结构,颗粒尺寸约为0.5~1 μm。掺杂Bi³⁺的Ca_0.75MoO₄：Eu_0.25-x³⁺,Bi_x³⁺的相对发光强度明显高于未掺Bi³⁺的Ca_0.75MoO₄：Eu_0.25³⁺荧光粉。Bi³⁺离子的掺杂将吸收来的能量传递给激活离子Eu³⁺,起到了能量传递的作用。当Bi³⁺掺杂量为x=0.005时,在395 nm激发下,主发射峰在616 nm处的相对发光强度最大,但掺杂浓度过高时会出现浓度猝灭现象。另外,电荷补偿剂的掺入能够解决材料中因同晶取代引起的电荷不平衡的问题,以Li⁺作电荷补偿剂、Eu³⁺和 Bi³⁺共掺合成的Ca_0.5MoO₄：Eu³⁺_0.23,Bi_0.01³⁺,Li⁺_0.26红色荧光粉的发光性能强于Ca_0.75MoO₄：Eu_0.25³⁺、Ca_0.5MoO₄：Eu_0.25³⁺, Li_0.25⁺及Ca_0.75MoO₄：Eu_0.24³⁺,Bi_0.01³⁺。     

Simulation and fabrication of carbon nanotubes field emission pressure sensors

A novel field emission pressure sensor has been achieved utilizing carbon nanotubes (CNTs) as the electron source. The sensor consists of the anode sensing film fabricated by wet etching process and multi-wall carbon nanotubes (MWNTs) cathode in the micro-vacuum chamber. MWNTs on the silicon substrate were grown by thermal CVD. The prototype pressure sensor has a measured sensitivity of about 0.17-0.77 nA/Pa (101-550 KPa). The work shows the potential use of CNTs-based field-emitter in microsensors, such as accelerometers and tactile sensors.     

Diode-pumped 593.5 nm cw yellow laser by type-I CPM LBO intracavity sum-frequency-mixing

A design of LD-pumped Nd:YVO₄ laser that generates simultaneous laser action at wavelengths 1064 and 1342 nm by optimizing film design is presented. An optimized continuous-wave (cw) yellow laser at 593.5 nm in room temperature is obtained for the first time. Using type-I critical phase-matching (CPM)LBO crystal, a yellow laser at 593.5 nm is obtained by 1064 and 1342 nm intracavity sum-frequency mixing. The maximum laser output power of 85 mW is obtained when an incident pump laser of 1.8 W is used. The optical-to-optical conversion is up to 4.7%, and the power stability in 24 h is better than ±2.8%.     

Thin strain-relaxed SiGe grown by ultrahigh vacuum chemical vapor deposition

Large-scale preparation of thin strain-relaxed SiGe is achieved by combining ion implantation and ultrahigh vacuum chemical vapor deposition. The resulting materials were analyzed by double crystal X-ray diffraction, micro-Raman spectroscopy, and tapping mode atomic force microscope. Results revealed that 100-nm-thick Si_0.7Ge_0.3 layers with the diameter of 125 mm and full strain relaxation are successfully prepared by pre-modifying the Si substrates using 50 keV Ar⁺ ions. The strain relaxation is also disclosed to change with both ion species and energy. However, post-modification of SiGe by ion implantation will cause serious damage to the crystal structures, and result in the formation of poly-crystal SiGe.     

Phase transition and pitting corrosion behaviour for Mo—implanted aluminium

Molybdenum ions are implanted into aluminium with high ion flux and high dose at elevated temperatures of 200℃, 400℃ and 500℃. Due to the high temperature and high flux of vacancies and interstitial atoms, the atom diffusion and chemical effects are enhanced during the ion implantation. The effects increase with increasing ion flux and dose, so that new phase formation and phase transition emerge noticeably. X-ray diffraction analysis shows that when the aluminium is implanted with Mo ions at a low ion flux (25μA/cm²), the Al₅Mo alloy is formed. The atomic ratio of Mo/Al of the Al₅Mo phase is close to 20%. When the aluminium is implanted with Mo ions at a high ion flux (50μA/cm²), the phase transition from Al₅Mo to Al₁₂Mo appears, and the latter is dominant, which is determined to be the final phase. The ratio of Mo/Al in Al₁₂Mo is 7.7%. Rutherford backscattering spectroscopy indicates also that the Mo/Al atom ratio is ～7% to ～8% in Mo-implanted aluminium. The atomic ratios of the constituents in Al₅Mo and Al₁₂Mo are of stoichiometric composition for these alloys. The thicknesses of the Al₁₂Mo alloy layers for Mo-implanted Al with ion doses of 3×10¹⁷/cm² and 1×10¹⁸/cm² are 550nm and 2000nm, respectively. The pitting corrosion potential V_p increases obviously. It is clear that due to the formation of Al₁₂Mo alloy layer, the pitting corrosion resistance is enhanced.