Femtosecond laser ultrafast photothermal exsolution

Exsolution,as an effective approach to constructing particle-decorated interfaces,is still challenging to yield interfacial films rather than isolated particles.Inspired by in vivo near-infrared laser photothermal therapy,using 3 mol%Y_(2)O_(3)stabilized tetragonal zirconia polycrystals(3Y-TZP)as host oxide matrix and iron-oxide(Fe3O4/γ-Fe_(2)O_(3)/α-Fe_(2)O_(3))materials as photothermal modulator and exsolution resource,femtosecond laser ultrafast exsolution approach is presented enabling to conquer this challenge.The key is to trigger photothermal annealing behavior via femtosecond laser ablation to initialize phase transition from monoclinic zirconia(m-ZrO_(2))to tetragonal zirconia(t-ZrO_(2))and induce t-ZrO_(2)columnar crystal growth.Fe-ions rapidly segregate along grain boundaries and diffuse towards the outmost surface,and become‘frozen’,highlighting the potential to use photothermal materials and ultrafast heating/quenching behaviors of femtosecond laser ablation for interfacial exsolution.Triggering interfacial iron-oxide coloring exsolution is composition and concentration dependent.Photothermal materials themselves and corresponding photothermal transition capacity play a crucial role,initializing at 2 wt%,3 wt%,and 5 wt%for Fe3O4/γ-Fe_(2)O_(3)/α-Fe_(2)O_(3)doped 3Y-TZP samples.Due to different photothermal effects,exsolution states of ablated 5 wt%Fe_(3)O_(4)/γ-Fe_(2)O_(3)/α-Fe_(2)O_(3)-doped 3Y-TZP samples are totally different,with whole coverage,exhaustion(ablated away)and partial exsolution(rich in the grain boundaries in subsurface),respectively.Femtosecond laser ultrafast photothermal exsolution is uniquely featured by up to now the deepest microscale(10μm from 5 wt%-Fe_(3)O_(4)-3Y-TZP sample)Fe-elemental deficient layer for exsolution and the whole coverage of exsolved materials rather than the formation of isolated exsolved particles by other methods.It is believed that this novel exsolution method may pave a good way to modulate interfacial properties for extensive applications in the fields of biology,optics/photonics,energy,catalysis,environment,etc.

Residual Thermal Stresses of Laser Cladding of Intermetallic-Ceramic Composite Coatings

The stresses in laser cladding of Ni3Al-WC composite coating co and in heat affect zone (HAZ) σh have been induced based on considering the influences of laser processing parameters power P and beam traverse speed v.According to the calculated results, certain limits of P and v are necessary in order to obtain crack free coatings. It agrees well with the experimental results.

Single,composite,and ceramic Nd:YAG 946-nm lasers

Single, composite crystal and ceramic continuous wave （CW） 946-nm Nd：YAG lasers are demonstrated, respectively. The ceramic laser behaves better than the crystal laser. With 5-mm long ceramic, a CW output power of 1.46 W is generated with an optical conversion efficiency of 13.9%, while the slope efficiency is 17.9%. The optimal ceramic length for a 946- nm laser is also calculated.

Thermal Interaction in Semiconductor Laser Arrays

A novel theoretical model of thermal diffusion has been established to study thermal interaction between two neighboring diodes in semiconductor laser arrays. The main cause of the ocurrence of the thermal interaction between two neighboring diodes in array devices is the heat conduction through heat sink. We hold that as the devices must have heat sink to diffuse heat, this kind of interaction in the array would always exist. However, when the pitch between two neighboring diodes in the array is reasonably defined, this troublesome thermal interaction can be simply reduced by using our model. Based on the individual diodes with leaky waveguide structure, we experimentally succeeded in fabricating 2D 4 ×4 arrays. The thermal interaction between upper and lower diodes in the 2D array is also considered as well as the function of the heat sink. The measured results show that the pulse peak output powor of the 2D 4 ×4 array is high up to 11 W.

Thermal Diffusivity and Thermal Conductivity of Epoxy-Based Nanocomposites by the Laser Flash and Differential Scanning Calorimetry Techniques

Nanocomposites were fabricated by using a commercial two part epoxy as a matrix and multiwalled carbon nanotubes, graphite fibers and boron nitride platelets as filler materials. Multiwalled carbon nanotubes (MWCNTs) that were produced by chemical vapor deposition were found to produce nanocomposites with better thermal diffusivity and thermal conductivity than the MWCNTs that were produced by the combustion method. Compared to the MWCNTs produced by both methods and graphite fibers, boron nitride produced nanocomposites with the highest thermal conductivity. Specific heat capacity was measured by using differential scanning calorimetry and thermal diffusivity was measured by using the laser flash.

THE CHARACTERISTICS OF LASER GENERATED ULTRASOUND IN SOLIDS

The laser generated ultrasound in solids in a thermoelastic regime is studied by solving thermoelastic wave equations. Analytic expressions of two-dimensional far-field ultrasonic displacements including the effects of thermal diffusion and optical penetration were obtained by means of the integral transform method. The meaning of the expressions is discussed. The effects of optical absorption coefficient on the directivity of laser generated ultrasound in non-metallic solids are also discussed. The directivity patterns of both longitudinal waves and shear waves are presented.

Theoretical analysis and experimental research on thermal focal length of a YVO_4/Nd:YVO_4 composite crystal

This paper investigates the temperature field distribution and thermal focal length within a laser diode array （LDA） end-pumped YVO4/Nd：YVO4 rectangular composite crystal. A general expression of the temperature field distribution within the Nd：YVO4 rectangular crystal was obtained by analysing the characteristics of the Nd：YVO4 crystal and solving the Poisson equation with boundary conditions. The temperature field distributions in the Nd：YVO4 rectangular crystal for the YVO4/Nd：YVO4 composite crystal and the Nd：YVO4 single crystal are researched respectively. Calculating the thermal focal length within the Nd：YVO4 rectangular crystal was done by an analysis of the additional optical path differences （OPD） caused by heat, which was very identical with experimental results in this paper. Research results show that the maximum relative temperature on the rear face of the Nd：YVO4 crystal in the composite crystal is 150 K and the thermal focal length is 35.7mm when the output power of the LDA is 22 W. In the same circumstances, the experimental value of the thermal focal length is 37.4 mm. So the relative error between the theoretical analysis and the experimental result is only 4.5%. With the same conditions, the thermal focal length of the Nd：YVO4 single crystal is 18.5 mm. So the relative rate of the thermal focal length between the YVO4/Nd：YVO4 crystal and the Nd：YVO4 crystal is 93%. So, the thermal stability of the output power and the beam quality of the YVO4/Nd：YVO4 laser is more advantageous than the laser with Nd：YVO4 single crystal.

An Improved Thermal Conductivity Measurement Scheme for Macroscopic Graphitic Films Using the Laser Flash Method

Achieving efficient thermal management urges to exploit high-thermal-conductivity materials to satisfy the boosted demand of heat dissipation.It is critical to adopt standardized characterization protocols to evaluate the intrinsic thermal conductivity of thermal management materials.However,for the most representative laser flash method,the lack of standard measurement methodology and systematic description on the thermal diffusivity and influencing factors has led to significant deviations and confusion of the thermal conduction performance in the emerging thermal management application.Here,the measurement error factors of thermal diffusivity by the common laser flash analyzer(LFA)are discussed.Taking high-thermal-conductivity graphitic film(GF)as a typical case,the key factors are analyzed to guide the measurement protocol of related carbon-based thermal management materials.The basic principle of the LFA measurement,actual pre-processing conditions,instrument parameters setting,and data analysis are elaborated for accurate measurements.Furthermore,the graphene thick films and common isotropic materials are also extended to meet various thermal measurement requirements.Based on the existing practical problems,we propose a feasible test flow to achieve a unified and standardized thermal conductivity measurement,which is beneficial to the rapid development of carbon-based thermal management materials.

High speed laser cladding as a new approach to prepare ultra-high temperature ceramic coatings

Ultra-high temperature ceramic(UHTC)coatings are used to protect the hot-end components of hypervelocity aerocrafts from thermal ablation.This study provides a new approach to fabricate UHTC coatings with high speed laser cladding(HSLC)technology,and places more emphasis on investigating the formation mechanism,phase compositions,and mechanical properties of HSLC-UHTC coatings.Results show that a well-bonded interface between the coating and the tantalum alloy substrate can be formed.The coating is mainly composed of(Zr,Ta)C ceramic solid solution phase with a content of higher than 90% by volume and Ta(W)metal solid solution phase.At a relatively high powder feeding rate,the ZrC ceramic phase appears in the coating while a dense ZrC UHTC top layer with a thickness of up to~50μm is successfully fabricated.As for the mechanical properties of the HSLC coatings,the fracture toughness of the coating decreases with the increase of powder feeding rate.The increase of carbide solid solution phase can significantly improve the high temperature microhardness(552.7±1.8 HV0.5@1000℃).The innovative design of HSLC ZrC-based coatings on refractory alloys accomplishes continuous transitions on microstructure and properties from the substrate to the UHTC top layer,which is a very promising candidate scheme for thermal protection coating.

Anisotropy of the thermal and laser output properties in Yb,Nd:Sc_(2)SiO_(5) crystal

The anisotropy of thermal property in an Yb,Nd∶Sc_(2)SiO_(5) crystal is investigated from the temperature of 293 to 573 K. Based on the systematical study of thermal expansion, thermal diffusivity, and specific heat, the thermal conductivity in Yb,Nd∶Sc_(2)SiO_(5) crystals orientated at(100),(010),(001), and(406) is calculated to be 3.46, 2.60, 3.35, and 3.68 W=/(m·K),respectively. The laser output anisotropy of a continuous-wave(CW) and tunable laser is characterized, accordingly. A maximum output power of 6.09 W is achieved in the Yb,Nd∶Sc_(2)SiO_(5) crystal with(010) direction, corresponding to a slope efficiency of 48.56%. The tuning wavelength range in the Yb,Nd∶Sc_(2)SiO_(5) crystal orientated at(100),(010), and(001) is 68, 67,and 65 nm, separately. The effects of thermal properties on CW laser performance are discussed.

Fluorescence spectrum, thermal properties, and continuous-wave laser performance of the mixed crystal Nd:Lu_(0.99)La_(0.01)VO_4

The fluorescence spectrum and thermal properties of the mixed crystal Nd：Luo.gvLa~.o1VO4 are determined. The strongest emission peak located at 1065.6 nm had a full width at half maximum （FWHM） of 2.1 nm. Continuous-wave （CW） laser performance is demonstrated by a compact planar planar cavity that is end- pumped by a diode laser. The laser output characteristics are investigated by using output couplers with different transmissions. A maximum CW output power of 8.09 W was obtained at an incident pump power of 19.4 W, which corresponds to an optical-to-optical conversion efficiency of 41.7% and a slope efficiency of 54.6%. The dependence of optimum transmission on pump power is calculated theoretically and is found to be consistent with experimental results.

Crystallization Behavior of Nd_2O_3 Doped Na_2O–CaO–SiO_2 Laser Glass-ceramics

For the desirable laser optical property, transition metals or rare-earths are always doped into parent glasses as active ions, and this doping will affect the crystallization of the precursor glasses inevitably. In this work, crystallization behavior of NaO-CaO-SiO2 system glasses doped with Nd2O3 was investigated. The crystallization kinetic parameters including the crystallization apparent activation energy （E） and the Avrami parameter （n） were also measured. The results show that the NaO-CaO-SiO2 system glassceramics with the NaxCa2Si3O9 crystal as primary phase can be highly crystalized as above 90%. The Nd2O3 doping has a significant influence on the crystallization apparent activation energy and the Avrami parameter, which affect the crystallization behavior and morphology of the transparent glass-ceramics of this system.

Thermal distribution analysis of multi-core photonic crystal fiber laser

The thermal properties of photonic crystal fiber(PCF) laser with 18 circularly distributed cores are investigated by using full-vector finite element method(FEM).The results show that the 18-core PCF has a more effective thermal dispersion construction compared with the single core PCF and 19-core PCF.In addition,the temperature distribution of 18-core PCF laser with different thermal loads is simulated.The results show that the core temperature approaches the fiber drawing value of 1800 K approximately when the thermal load is above 80 W/m which corresponds to the pumping power of 600 W approximately,while the coating temperature approaches the damage value of about 550 K when the thermal load is above 15 W/m which corresponds to the pumping power of 110 W approximately.Therefore the fiber cooling is necessary to achieve power scaling.Compared with other different cooling systems,the copper cooling scheme is found to be an effective method to reduce the thermal effects.

Nd∶GdYAG激光晶体的光谱分析及热学性能研究

本文对所生长的Nd∶GdYAG晶体的吸收光谱和荧光光谱进行了测试,采用Judd-Ofelt理论对振子强度和线强、强度参数Ω_(t)、4 F_(3/2)→4 I_(J)能级间的跃迁概率、能级辐射寿命、荧光分支比等光谱参数进行了计算,相关数据与YAG、LuAG、YVO4等晶体进行了对比。Ωt计算得出Ω2=(0.813±0.200)×10^(-20)cm^(2),Ω4=(3.381±0.187)×10^(-20)cm^(2),Ω6=(5.135±0.278)×10^(-20)cm^(2),相比YAG、LuAG、YVO4等晶体,Nd∶GdYAG晶体的强度参数Ω2较大,表明Gd^(3+)的掺入增大了基质的无序度。计算并拟合了300~1200 nm的吸收截面和折射率,其中808 nm处的吸收截面为5.0×10^(-20)cm^(2),1064 nm处的折射率为1.80。实测4 F3/2能级的荧光寿命为228μs,与计算的能级辐射寿命240μs接近;1064 nm处的发射截面为1.23×10^(-19)cm^(2),接近Nd∶YAG晶体的发射截面。室温下Nd∶GdYAG晶体的热导率为5.992 W/(m·K),略小于Nd∶YAG和Nd∶GGG,但大于Nd∶YVO4晶体。研究表明,Nd∶GdYAG是一种有应用前景的新型激光晶体材料。

基于红外图像时域特征的沥青混合料细观裂缝评价研究

为优化沥青混合料中细观裂缝的快速评价方法与效率,首先通过室内试验设计沥青混合料细观裂缝制备及其评价方法,采用线激光对试件进行扫描并采集红外图像,通过对红外图像数据分析,建立基于时间序列的温度区间特征追踪方法,进而计算温度区间特征面积及其扩散速率与线激光位移的热特性变化关系,最后对试件中的细观裂缝位置和宽度进行定量分析。研究表明,基于温度区间热效应的扩散速率,可以增强试件中细观裂缝的空间分辨率,同时随着线激光逐渐靠近裂缝,温度特征面积扩散速率明显表现出先降低后增加的趋势,说明试件裂缝处的热扩散能量逐渐降低并趋近于零,当线激光远离裂缝时,试件表面的热扩散速率又逐渐增加,体现出很强的细观裂缝热阻隔现象。结果表明,在裂缝处温度区间B、C的特征面积始终位处于特征曲线的最低波谷位置,且温度区间B的裂缝定位误差小于5.61%;同时,在可靠度水平为95%条件下,通过亚像素算法计算的裂缝特征面积宽度与裂缝宽度的相关性达到0.9952,预测裂缝宽度的相对误差不大于4.13%。该评价方法显示了基于时序特征的温度区间对不同细观裂缝宽度均具有显著的敏感性,可为车载移动式热成像技术自动化检测沥青路面裂缝的应用提供理论基础和借鉴。

增材制造单晶镍基高温合金微观组织、热稳定性及耐磨性能

利用定向能量沉积的增材制造方法制备了单晶镍基高温合金SRR99,通过微观组织表征、硬度测量与纳米划痕实验,系统地研究激光增材制造单晶镍基高温合金在服役温度下的组织稳定性。结果表明:经过服役温度(650℃)退火100 h后,增材制造样品γ'强化相的形貌及尺寸(70 nm)依旧保持。相比于传统铸造样品,服役温度退火前后增材制造样品的硬度(6.6 GPa)均提高了20%以上。此外,退火前后增材制造样品的摩擦因数均具有数值低(0.19)、离散性小的特点。本研究证明定向能量沉积增材制造的单晶镍基高温合金样品拥有优异的组织热稳定性和耐磨性能,为激光增材制造技术应用于单晶镍基高温合金涡轮叶片的榫头修复与提性延寿提供了实验支撑。

热键合、凹端面及晶体棒直径对Er∶YSGG中红外激光性能的影响

对热键合、凹端面及不同直径Er∶YSGG晶体棒的中红外激光性能进行了系统对比研究。晶体经高温退火、定向切割、端面精密抛光后,测量得到其平面度和表面粗糙度分别小于0.1λ@633 nm和0.5 nm。通过精密加工实现了YSGG与Er∶YSGG室温光胶,然后进行了高温热键合,透过谱表明键合界面基本无损耗,达到了一体化。热焦距结果表明,热键合、凹端面能够有效改善和补偿高功率泵浦条件下晶体的热透镜效应,有利于提高晶体的激光性能。对比未键合、热键合及凹面热键合Er∶YSGG晶体在968 nm LD侧面泵浦条件下的激光性能发现,低频150 Hz以下,键合与凹面并未表现出优势,三种棒的最大输出功率和斜效率分别均在24 W和28%附近;但随重频增加,热效应逐渐加重,键合晶体良好散热能力和凹面热补偿效应使激光性能得到提高。在重频300 Hz下,未键合、热键合及凹面热键合三种棒的最大输出功率分别为15.54,17.85和18.33 W、斜效率分别为16.6%,18.3%和18.4%;重频600 Hz条件下,三种棒的最大输出功率分别为9.4,13.32和13.18 W、斜效率分别为6.7%,8.6%和9%。此外,对比研究了不同直径Er∶YSGG晶体的激光性能,在低于100 Hz的低频下,三种晶体的激光性能接近。但相比于3和4 mm键合凹面棒,直径2 mm棒有大的比表面积,拥有更好的热管理能力,使其在高重频条件下具有更好的激光性能。如在150 Hz条件下,2 mm凹面键合Er∶YSGG晶体棒获得了功率23.82 W、斜效率27.7%的中红外激光输出,大大高于直径3和4 mm棒的18 W和23%。在600 Hz,直径2 mm键合凹面棒中实现了功率13.18 W,斜效率9%的激光输出,相比于直径3和4 mm键合凹面棒的9 W,7%具有较大的改善。测量得到2 mm凹面键合Er∶YSGG晶体棒在150 Hz,200μs条件下的光束质量因子M_(x)^(2)/M_(y)^(2)=6.28/6.30,表明该激光具有良好的光束质量。综上,选择合适晶体棒直径,并将键合与凹面相结合是实现高性能2.79μm激光的有效方法。

激光二极管端泵方形Tm:YAG复合晶体的热效应

为了有效解决激光二极管端面泵浦激光晶体引起的热效应问题,引入复合晶体的概念,通过两种复合晶体模型(即单端键合和双端键合)来降低激光晶体的热效应。根据激光二极管端面泵浦激光晶体工作特点,建立端面泵浦方形Tm:YAG复合晶体热模型,利用热传导理论,用有限元分析法对复合晶体的温度场、热应力场和形变量进行了数值计算,分析了单端/双端键合方式、未掺杂晶体长度、增益晶体长度对方形复合晶体内部温度场及形变量的影响。结果表明,平衡状态下,激光二极管泵浦功率为30 W、泵浦光斑半径为400μm时,YAG晶体厚度c_(1)为1 mm,增益晶体厚度c_(2)为1.5 mm,方形单端键合和双端键合的Tm:YAG复合晶体内部最大温升分别为81.2、77.9℃;内部最大应力分别为146、104 MPa;热形变量为0.468、0.172μm。可见,复合晶体能有效缓解晶体的温升和热形变,且双端键合的方式降低晶体热效应的效果更好。当增益晶体厚度为2.6 mm以上时,两种键合方式对复合晶体内部最大温升的影响基本保持一致。该研究为方形Tm:YAG复合晶体的增益晶体厚度、未掺杂晶体厚度的选择提供了参考依据,也为实现Tm:YAG激光器高功率输出目标提供了理论指导。

Preparation of(Lu,Y)_(3)(Al,Sc,Cr)_(2)Al_(3)O_(12)phosphor ceramics with high thermal stability for near-infrared LED/LD

Near-infrared(NIR)phosphor-converted light-emitting diodes/laser diodes(LEDs/LDs)are prospective lighting sources for NIR spectroscopy.However,developing NIR phosphor materials with desired thermal robustness and high photoelectric efficiency is a crucial challenge for their applications.In this work,based on the cationic radius matching effect,a series of(Lu,Y)_(3)(Al,Sc,Cr)_(2)Al_(3)O_(12)NIR phosphor ceramics(LuYScCr NIR-PCs)were fabricated by vacuum sintering.Excellent thermal stability(95%@150℃)was obtained in the prepared NIR-PCs,owing to their weak electron-phonon coupling effect(small Huang-Rhys factor).Being excited at 460 nm,NIR-PCs realized a broadband emission(650-850 nm)with internal quantum efficiency(IQE)of 60.68%.Combining NIR-PCs with LED/LD chips,the maximum output power of the encapsulated LED prototype was 447 mW@300 mA with photoelectric efficiency of as high as 18.6%@180 mA,and the maximum output power of the LD prototype was 814 mW@2.5 A.The working temperatures of NIR-PCs were 70.8℃@300 mA(LED)and 102.8℃@3 A(LD).Finally,the prepared NIR-PCs applied in food detection were verified in this study,demonstrating their anticipated application prospects in the future.

基于熔池热历史的陶瓷增强金属基复材激光定向能量沉积质量实时监测方法

针对激光定向能量沉积(L-DED)制备陶瓷增强金属基复合材料(CRMMC)过程中成形质量不稳定的问题,提出一种基于熔池热历史的CRMMC质量监测方法。为实现在CPU硬件上的实时监测,构建了单路结构的轻量级全深度可分离卷积神经网络模型(FD-Net)。输入9个不同激光能量制备不同状态的CRMMC成形质量,使用红外热像仪同步采集熔池红外图像作为数据集训练和测试FD-Net,并与当前先进的轻量级卷积神经网络(CNN)模型进行性能对比。结果表明:FD-Net在Inter-CPU上以7.90ms/帧的推理时间实现了高精度监测,显著低于其他CNN模型,证明所提方法可在工业微型计算机上实现CRMMC质量状态的实时监测。