Finite element modeling of laser-induced three-dimensional Transient Thermal Grating in two-layered systems

The three-dimensional (3D) temperature field of laser-induced transient thermal grating (TTG) for two-layered structures such as diamond/substrate is obtained by the finite element method (FEM). The numerical results indicate that the thickness of the diamond film has a significant influence on the temperature field. On the other hand, we further find that variation of the substrate significantly affects on the temperature distribution in the diamond film, which has been ascribed to the various thermal conductivities of the substrates.     

金刚石/ZnSe结构中瞬态热栅场的有限元法分析

利用有限元法分析计算了脉冲激光激发瞬态热栅在金刚石薄膜/ZnSe衬底双层结构中的三维温度场分布及变化。通过比较不同厚度金刚石薄膜样品的温度场分布,结果显示金刚石薄膜的厚度对样品中温度场的分布有较大影响,随着薄膜厚度的增大,峰值温度提高并且二次加热现象更加明显。     

Three-dimensional laser-induced thermal and stress analyses in diamond/ZnSe system

Thermal and stress responses of diamond/ZnSe system working as a IR window illuminated by a continue-wave (CW) laser have been investigated by means of finite element method (FEM). We find that the temperature and stress fields have a significant dependence on thickness of the diamond coating. If the increase of the diamond thickness, the laser-induced steady state temperature increases dramatically due to more laser power absorbed, which induces high stress in the interface between diamond and ZnSe. We further calculate the variation of the temperature and stress fields with changing the structural parameters in diamond/ZnSe system.     

类金刚石薄膜在硅基底上的沉积及其热导率

采用分子动力学方法模拟了碳在晶体硅基底上的沉积过程, 并分析计算了所沉积的类金刚石薄膜的面向及法向热导率. 对沉积过程的模拟表明, 薄膜密度及sp³杂化类型的碳原子所占比例均随沉积高度的增加而减小, 在碳原子以1 eV能量垂直入射的情况下, 在硅基底上沉积的薄膜密度约为2.8 g/cm³, sp³杂化类型的碳原子所占比例约为22%, 均低于碳在金刚石基底上沉积的情况. 采用Green-Kubo方法, 计算了所沉积类金刚石薄膜的热导率, 其面向热导率可以达到相同尺寸规则金刚石晶体的50%左右, 并且随着薄膜密度与sp³杂化类型碳原子所占比例的升高而升高.     

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

This paper presents the use of molecular dynamics simulation in the study of laser-induced thermal desorption (LITD) of water molecules adjacent to a laser-heated Au substrate. The local structure of the water molecules is investigated by considering the densities of the oxygen and hydrogen atoms, the average number of neighbors, n_NN, and the average number of H-bonds, n_HB. At an equilibrium temperature of 300 K, the simulation results show that three adsorption water layers are formed in the immediate vicinity of the Au surface, and that each four-fold hollow site on the uppermost Au(0 0 1) surface is occupied by a single water molecule. Following laser-induced heating of the Au substrate with a sub-picosecond laser pulse of 350 fs, the substrate temperature increases to 1000 K. This causes a gradual heating of the adjacent water film, which is accompanied by a decrease in the values of n_NN and n_HB. Hence, it can be concluded that an increase in the water film temperature destroys the hydrogen-bonding network throughout the water film. Although the maximum local temperature of the water film occurs in the region immediately adjacent to the Au substrate, it is determined that the attractive energy between the Au atoms and the water molecules in this region causes the water molecules to aggregate together to form three-dimensional water clusters. Furthermore, this energy prevents the hydrogen bonds in this region from breaking apart as violently as those within the phase explosion region. Finally, it is observed that the phase explosion phenomenon occurs in the region of the water film where the values of n_NN and n_HB are at a minimum.     

Heat resistance of the interface in the silicon-on-diamond structure at a temperature of 80 K

Heat propagation at liquid nitrogen temperature in a heterostructure consisting of a polycrystalline diamond film deposited from hydrocarbon plasma on an oriented silicon substrate is studied. A technique for measuring the cooling kinetics of a thin-film indium thermometer deposited on a diamond film after heating by nanosecond pulses of a nitrogen laser is used. The experimental data are compared with the results calculated within the theory of heat conduction for multilayer systems. The analysis performedmade it possible to simultaneously determine the thermal conductivity of the diamond film and the interfacial heat resistance of diamond/Si and In/diamond interfaces at liquid nitrogen temperature.     

热处理对纳米金刚石涂层场发射性能的影响

用旋涂法在金属钛衬底上涂敷纳米金刚石,经过适当的热处理形成金刚石涂层与金属钛衬底的化学键合,即形成衬底与涂层之间的过渡层,从而为纳米金刚石颗粒提供电子,使其成为有效的发射体。用扫描电镜、原子力显微镜、X射线衍射和拉曼散射等手段分析了温度对键合效果以及场发射性能的影响,温度过高或过低都不利于提高纳米金刚石涂层的场发射性能,只有在700℃左右对样品进行热处理,才能得到较好的键合状态。改变涂膜时旋涂的次数以获得不同涂层厚度的样品,对其在700℃的相同温度下进行热处理,发现涂层过厚或过薄都不利于样品发射性能的提高。旋涂9次并于700℃热处理的样品具有较好的场发射性能,其发射阈值场强可达4.6V/μm,而15.3V/μm场强下的电流密度为59.7μA/cm2。     

Thermal parameters of layers and interfaces in silicon-on-diamond structures

The heat propagation at room temperature was studied in a heterostructure consisting of a polycrystalline diamond film deposited from a hydrocarbon plasma on an oriented silicon substrate. The dynamics of cooling of a thin-film indium thermometer evaporated atop the diamond film was measured following its heating by nanosecond nitrogen laser pulses. The experimental data were compared with the values calculated in the framework of the theory of thermal conductivity for multilayer systems. This analysis permitted the determination of both the thermal conductivity of the diamond film and the thermal resistance of the diamond/Si and In/diamond interfaces.     

金刚石基底上氮化硼薄膜的场发射特性研究

 利用射频磁控溅射方法,在金刚石膜上沉积了氮化硼薄膜。红外光谱分析表明,氮化硼薄膜的结构为六角氮化硼。在超高真空系统中测量了样品的场发射特性,沉积在金刚石膜上的氮化硼薄膜的阈值电场为12 V/μm,最大发射电流密度为272 μA/cm²。并且沉积在金刚石膜上的氮化硼薄膜的场发射特性明显优于金刚石薄膜本身的场发射特性。这说明,氮化硼薄膜可以有效地改善金刚石膜的场发射特性。场发射Fowler-Nordheim（F-N）曲线表明,电子发射是通过遂穿表面势垒完成的。     

Formation of diffractive microrelief on diamond film surface

The fabrication of diamond-based optical elements for high-power CO₂ lasers is of particular interest because of low optical absorption, high thermal conductivity and weak temperature dependence of the refractive index of diamond. In earlier publications, we presented the results of the implementation of ion-chemical etching technology for generating a diffraction grating on the surface of a diamond film. The results on generating diamond diffractive optical elements and a sub-wavelength antireflection structure by plasma etching were also presented. In this paper, we study the generation of the grating on the diamond surface by ion-chemical etching. The choice of (Ar+O₂) mixture as a working gas and use of niobium as a masking layer are substantiated. The results of studying a diffraction microrelief generated on the diamond-film surface through ion-chemical etching are discussed.     

Laser-induced damage threshold at chemical vapor deposition-grown diamond surfaces for 200-ps CO(2) laser pulses

We report laser-induced damage thresholds (LIDTs) at chemical vapor deposition (CVD)-grown diamond surfaces for 200-ps CO(2) laser pulses, obtained with photoacoustic diagnostics. The results are compared with those at ZnSe and Ge surfaces under the same experimental condition. For 200-ps laser pulses, CVD diamond, ZnSe, and Ge were measured and found to have damage fluences of 1.2, 0.45, and 0.2 J/cm(2) , respectively, for a laser waist radius of 134 mum. Acoustic measurement indicated a relatively large variation in the LIDT of the CVD-grown diamond because of its polycrystalline structure.     

单层SiO2物理膜与化学膜激光损伤机理的对比研究

采用离子束溅射沉积技术和溶胶-凝胶技术在K9基片上镀制了厚度相近的SiO₂单层介质膜，用表面热透镜技术对两类膜层分别进行了热吸收及实时动态热畸变实验测试，结合散射光阈值测试及实验前后膜层的显微观测，对相同基底、相同膜层材料而采用不同方法镀制的光学膜层，发现化学膜的强激光损伤阈值远高于相应物理膜；从热力学响应及膜层特性差异的角度揭示了化学膜层的强激光损伤阈值远高于相应物理膜层的微观机理，即物理膜具有高吸收下的致密膜层快传导的基底热冲击效应，而化学膜则有低吸收下的疏松空隙填充慢传导的延缓效应，大量的实验数据及现象都证实了这一结论. 关键词： 强激光辐照损伤 损伤形貌 热冲击 热吸收     

Thermal relaxation of exchange bias field in an exchange coupled CoFe/IrMn bilayer

This paper reports that a CoFe/IrMn bilayer was deposited by high vacuum magnetron sputtering on silicon wafer substrate; the thermal relaxation of the CoFe/IrMn bilayer is investigated by means of holding the film in a negative saturation field at various temperatures. The exchange bias decreases with increasing period of time while holding the film in a negative saturation field at a given temperature. Increasing the temperature accelerates the decrease of exchange field. The results can be explained by the quantitative model of the nucleation and growth of antiferromagnetic domains suggested by Xi H W et al. [2007 Phys. Rev. B 75 014434], and it is believed that two energy barriers exist in the investigated temperature range.     

Thermal conductive performance of deposited amorphous carbon materials by molecular dynamics simulation

A series of diamond-like carbon (DLC) films with different microstructure were prepared by depositing carbon atoms on diamond surface with incident energy ranging from 1 to 100 eV. The thermal conductivity of the deposited films and the Kapitza resistance between the film and the diamond substrate were investigated. Results show that the average density, the average fraction of sp³ bonding and the thermal conductivity of the DLC films increase first, reaching a maximum around 20–40 eV before decreasing, while the Kapitza resistance decreases gradually with increased deposition energy. The analysis suggests that the thermal resistance of the interface layer is in the order of 10^?10 m²K/W, which is not ignorable when measuring the thermal conductivity of the deposited film especially when the thickness of the DLC film is not large enough. The fraction of sp³ bonding in the DLC film decreases gradually normal to the diamond surface. However, the thermal conductivity of the film in normal direction is not affected obviously by this kind of structural variation but depends linearly on the average fraction of sp³ bonding in the entire film. The dependence of the thermal conductivity on the fraction of sp³ bonding was analysed by the phonon theory.     

Temperature effects in the photolytic LCVD of platinum

The photolytic laser chemical vapor deposition (LCVD) rate of platinum from its bishexafluoroacetylacetonate precurser has been measured in situ and in real time. Optical transmission of the 350 nm photolysis light through the deposited platinum film and a transparent glass substrate is monitored and analysed in detail. From these measurements, as well as measurements of the reflected light, the fraction of the laser beam power absorbed in the metal film is found. The latter allows a simple estimate of the laser-induced temperature rise at the metal surface. It is shown that even rather small temperature increases of the order of several tens of degrees centigrade can completely change the photolysis mechanism and hence drastically influence the photolytic LCVD rate. A simple modification of Lax's model, in which a temperature dependent thermal conductivity of the substrate is introduced, is used to describe the laser-induced heating of a strongly absorbing thin metal film on a glass substrate.     

衬底温度对共蒸发法制备Cu2ZnSnSe4太阳电池的影响

采用共蒸发法在不同衬底温度下沉积Cu_2ZnSnSe_4(简称CZTSe)薄膜,分析了衬底温度对CZTSe材料性质及电池性能的影响。研究表明:当衬底温度较低时(380℃),CZTSe薄膜中含有SnSe_x使电池失效;随着衬底温度的升高,CZTSe薄膜的结晶质量明显提升,电池开路电压增加。但当衬底温度达到460℃时,电池的转换效率反而下降;结合CZTSe的生长机理及器件模型分析了电池效率下降可能的原因。最终在衬底温度420℃的条件下制备出效率为3.12%(有效面积0.34 cm~2)的CZTSe太阳电池。     

高压烧结镀Cr、Ti膜金刚石/铜复合材料热导率研究

 采用磁控溅射方法,在金刚石表面镀覆活性金属Cr膜和Ti膜,在高温高压下合成了镀活性金属膜金刚石/铜复合材料。实验发现,活性金属的加入增强了金刚石与铜界面间的结合强度,减少了界面热阻,提高了复合材料的热导率。复合材料热导率随着金刚石体积分数的增加而降低,随着金刚石的粒度增大而提高。这主要是由界面热阻引起的,可以通过增大金刚石粒度和改善界面状态来提高复合材料热导率。     

激光诱导热光栅光谱测温技术研究

报道了激光诱导热光栅光谱测温技术的研究. 通过两束相干交叉的脉冲抽运光, 在NO₂/N₂混合气中诱导出热光栅, 一束满足布拉格散射条件的连续探测光在交叉区域激励出相干的热光栅信号, 经过空间和光谱滤波的信号光由光电倍增管探测, 并由数字示波器显示和存储. 该信号携带了丰富的流场信息, 通过频域分析, 对气体的温度进行了测量, 热光栅光谱技术测量的温度与热电偶温度符合得很好. 同时还利用热光栅光谱技术进行了气体声速的直接测量, 在一定的温度范围内, 测量结果与理论曲线基本一致, 显示了该技术具有较高的测量精度与多参数同时测量的能力. 对影响信号波形的因素进行了分析, 结果表明, 热光栅光谱测温技术在高压强环境下应用具有独特的优势, 是一种应用前景广阔的激光燃烧诊断技术.     

532 nm激光辐照下ZnSe薄膜光学特性研究

采用电子束热蒸发技术制备了ZnSe薄膜,研究了532 nm波长的不同能量(2.0 mJ、2.5 mJ、3.0 mJ)、不同脉冲数(3、10、15)激光诱导前后,ZnSe薄膜的透射率、折射率、消光系数、损伤阈值(LIDT)的变迁。研究结果显示,在能量为2.0 mJ激光辐照后,ZnSe薄膜折射率提高,透射率下降。相比较能量为2.5 mJ、3.0 mJ激光辐照,在能量为2.0 mJ激光辐照后折射率提高最明显,由2.489 4提高到2.501 6。薄膜损伤阈值从0.99 J/cm2提高到1.39 J/cm2(10脉冲辐照);薄膜的损伤经过了无损伤到严重损伤突变的损伤演变过程。采用原子力显微镜对预处理后薄膜表面粗糙度进行检测,发现激光预处理后的薄膜表面粗糙度Ra有所下降,从0.563 nm降低到0.490 nm(15脉冲激光辐照)。     

气相生长金刚石膜对衬底金刚石性能的影响

 本文以高压合成金刚石为衬底材料，用热解CVD法在衬底金刚石表面生长了金刚石晶体。结果表明，生长金刚石后的衬底金刚石热性能有明显改善。