重频激光作用下碳纤维/环氧树脂复合材料热损伤规律

 运用热化学分析、扫描电子显微技术等手段,分析了碳纤维增强环氧树脂基复合材料在ms量级重频激光辐照下的损伤形式,研究了峰值功率密度、辐照时间、重复频率和脉冲宽度等对复合材料烧蚀规律的影响。研究结果表明：在激光辐照过程中,复合材料树脂基体在300 ℃开始裂解;由于裂解气体的保护作用,碳纤维不发生氧化,而是在汽化点（3 300 ℃）汽化烧蚀;复合材料热烧蚀率随峰值功率密度和重复频率提高而增大,随辐照时间增加而减小,最终均趋于定值;增加脉冲宽度可以提高辐照区峰值温度,降低碳纤维损伤的功率密度阈值。     

Ablation properties of carbon/carbon composites with tungsten carbide

The ablation properties and morphologies of carbon/carbon (C/C) composites with tungsten carbide (WC) filaments were investigated by ablation test on an arc heater and scanning electron microscopy. And the results were compared with those without tungsten carbide (WC) filaments tested under the same conditions. It shows that there is a big difference between C/C composites with and without WC filaments on both macroscopic and microscopic ablation morphologies and the ablation rates of the former are higher than the latter. It is found that the ablation process of C/C composites with WC filaments includes oxidation of carbon fibers, carbon matrices and WC, melting of WC and WO₃, and denudation of WC, WO₃ and C/C composites. Oxidation and melting of WC leads to the formation of holes in z directional carbon fiber bundles, which increases the coarseness of the ablation surfaces of the composites, speeds up ablation and leads to the higher ablation rate. Moreover, it is further found that the molten WC and WO₃ cannot form a continuous film on the ablation surface to prevent further ablation of C/C composites.     

纤维增强复合材料激光烧蚀效应的数值模拟

考虑材料的热解、氧化、相变及辐射和内外对流换热等物理过程,给出了激光烧蚀纤维增强复合材料的物理模型及数学模型。以碳纤维/环氧树脂复合材料为例,编程计算了材料的激光烧蚀过程,计算结果与实验结果符合得较好。计算结果表明：考虑复合材料的内对流时得到的结果更准确;较强功率密度激光辐照时,氧化对烧蚀的贡献可以忽略;功率密度一定时,烧蚀质量随时间近似为线性变化,功率密度越高,烧蚀效率越高。以辐照结束时背表面温度及烧蚀质量为目标物理量,对烧蚀过程做了参数敏感性分析,结果表明：热容及热导率对背表面温度的影响较大;树脂含量对烧蚀质量的影响较大,但其相对敏感度随激光功率密度增加而下降;激光功率密度超过1 kW/cm2时,辐射系数对烧蚀质量影响较大,但其相对敏感度随激光功率密度增加而下降。     

碳/碳化硅复合材料静止环境下氧化行为模拟

碳/碳化硅（C/SiC）复合材料是应用于临近空间高超声速飞行器热防护的一种新型防热材料.国内外通过性能测试较多地研究了材料不同制备工艺对抗烧蚀性的影响,提出的抗烧蚀分析理论模型均基于液态氧化膜.而近期开展的C/SiC复合材料管式炉加热实验和试样微观形貌电镜表征显示:常压下,当温度低于1 696 K时,C/SiC复合材料氧化后表面形成了多孔的固态氧化膜.采用压汞法测试了氧化物孔隙.基于孔隙中的气体扩散行为,结合氧化反应动力学关系,建立了一种新的C/SiC复合材料惰性氧化模型.模型预测值与实验结果吻合良好,表明该惰性氧化模型对氧化膜厚度和质量损失具有较好的预测能力.      

Fabrication of Materials with Low Optical Reflectance Based on Laser-Microstructured Metal Surfaces

It has experimentally been shown that the specular reflectance of some metals can be reduced by one to two orders of magnitude by subjecting them to multipulse laser ablation in air. An increase in the coefficient of grayness has been implemented for copper, nickel, aluminum, and stainless steel. Multipulse ablation of the corresponding targets leads to the formation of a quasi-periodic microstructure with an amplitude relief ranging from 30 to 50 μm. The specular reflectance of duralumin has been reduced using anodic oxidation of the microstructures formed by laser ablation and filling the newly formed pores with carbon nanoparticles. The thus obtained surfaces are close to ideal black body in their optical characteristics.     

Texture structure and ablation behavior of TaC coating on carbon/carbon composites

TaC coatings with hybrid, (2 0 0) and (2 2 0) texture structure were prepared on carbon/carbon (C/C) composites by isothermal chemical vapor deposition with TaCl₅-Ar-C₃H₆ system. The residual stress, hardness and ablation behaviors of the different coatings were characterized by Raman spectra, nano-indentation and oxyacetylene flame ablation machine respectively. Results shown tensile stress exists in the TaC coatings and increases when texture orientation turns from hybrid to (2 2 0) and (2 0 0), while nano-indentation hardness of the coatings also obeys the same trend. The deposited coatings could improve the ablation-resistance properties of C/C composites effectively. The texture structure also had great effects on the ablation properties and ablation morphologies of the coatings. The mass ablation rate obviously decreases when the texture structure changes from hybrid orientation to (2 0 0) and (2 2 0) orientations. The hybrid orientation and (2 0 0) texture coatings exhibit coarse oxide morphologies with crater or some breakage existed; while the (2 2 0) texture coating shows dense, molten oxide morphology. The main ablation behaviors of the hybrid, (2 0 0) and (2 2 0) texture TaC coatings are oxidation and particle denudation and block denudation, oxidation and block denudation, oxidation and mechanical erosion and block denudation, respectively.     

XPS investigation on Upilex-S polyimide ablated by pulse TEA CO2 laser

Laser ablation of Upilex-S polyimide films 80 μm thick was performed in air using a pulse TEA CO₂ laser with wavelength 9.3 μm. A halo surrounding the hole was observed, which is covered with sub-micro particles. Pieces of ablation products protrude from the ablated surface, leading to considerable roughness of the ablated area. Chemical and structural changes of Upilex-S polyimide film surface irradiated by the pulse TEA CO₂ laser in air were analyzed by X-ray photoelectron spectroscopy (XPS). Relative C content in the ablated area was found to be higher, whilst both O and N contents were lower than in the untreated area. This means that TEA CO₂ laser ablation released both the O and N atoms. Also, the peak areas corresponding with carbonyl group (C=O) in the imide system were reduced much more and a new component at 287.0 eV assigned to the amide structure (N---C=O) was detected after laser ablation. These suggest that the pyrolysis of the Upilex-S polyimide was the decomposition of the imide ring between the nitrogen/aromatic carbon atom and carbonyl carbon atom. In addition, another new component arising from >C=O groups was also detected for higher fluence (7.83 J/cm²), and its peak areas is very small. This result indicates that the slight oxidation may take place with higher fluence during laser ablation in air. Based on above-mentioned experimental results, a possible thermally-induced decomposition path of Upilex-S polyimide ablated by TEA CO₂ laser is presented.     

Micro-patterning for polymer electrolyte fuel cells: Single pulse laser ablation of aluminum films from glassy carbon

Microfuel cells are a possible replacement for batteries as energy sources in portable devices. At PSI a micropolymer electrolyte fuel cell was developed, whose flow fields consist of micro-structured glassy carbon plates. Micro-structuring of glassy carbon is carried out in a multi-step process. A sputtered aluminum mask is selectively removed by single pulse laser ablation from glassy carbon thereby defining micro-channels subsequently etched by reactive ion etching.A pulsed XeCl excimer laser (308 nm) is used for the single pulse patterning of a metal mask on the glassy carbon. The influence of the excimer laser typical pulse to pulse energy fluctuations on the micro-structuring process must be known to minimize defects during RIE etching of the micro-channels. To obtain a better understanding of the processes occurring during ablation, ns-shadowgraphy was performed. The formation of a shockwave was observed, followed by a similar but much slower perturbation, and the subsequent release of fragments. The calculated velocities can be correlated with the energy release during ablation. The post-ablation examination of the samples by profilometry, optical microscopy, SEM and EDX is used to measure the amount of removed material, the quality of the aluminum mask edges and aluminum residues on the glassy carbon surface. Such criteria allow us to classify the laser irradiation as a function of laser fluence: no ablation, partial ablation, complete ablation, and over-ablation.     

SiC热氧化SiO_2层结构的光谱学表征

C原子的存在,不仅影响SiC热氧化SiO2层与SiC间的界面态,也直接影响SiO2层的结构和致密性。本文用红外光谱对SiC和Si热氧化生长SiO2层进行了研究,分析和讨论SiO2/SiC和SiO2/Si的红外反射光谱特征峰,以及不同的热氧化条件和退火过程对这些谱峰的影响,对SiC热氧化SiO2层质量的光谱学表征进行了初步探讨。     

Imaging-XPS/Raman investigation on the carbonization of polyimide after irradiation at 308 nm

The laser ablation of polyimide was studied using 308 nm laser irradiation 𙜡 mJ cm^-2. Confocal Raman microscopy revealed the deposition of carbon surrounding the ablation crater, which consists of amorphous carbon with some crystalline features. Inside the crater, graphitic material was detected on top of the cones, very similar to the material from cw-Ar⁺ ion laser irradiation. FT-Raman measurements reveal the presence of intermediates of the polyimide decomposition. Imaging-X-ray photoelectron spectroscopy confirmed the deposition of carbon material surrounding the ablation crater and showed that the oxygen and nitrogen contents of the remaining material decrease.     

玻璃纤维/环氧树脂复合材料热分解动力学参数的确定

采用热重分析仪对空气和氮气气氛中的玻璃纤维增强环氧树脂基复合材料进行热分析,得到该材料在空气气氛中的烧蚀热为3125～3440J/g,而在氮气气氛中并未出现明显的氧化放热峰。基于阿伦纽斯形式的多步分解模型和直接解法,计算了该材料在空气气氛中的热分解动力学参数。分析表明:阿伦纽斯形式的多步分解模型能够较好地描述该材料的热分解过程;直接解法适用于计算复合材料的热分解动力学参数;确立的热分解动力学参数是正确有效的。     

Optical studies on the deposition of carbon nitride films by laser ablation

2 radicals when the 355 and 1064 nm outputs of a Nd:YAG laser were applied. While for the 532 nm ablation, a relatively higher concentration of excited atomic carbon was obtained. Different Raman and FTIR spectral features were observed from the deposited films with different ablation wavelengths. The 532 nm laser ablation is proposed for the synthesis of high quality carbon nitride films. Received: 16 October 1996 / Accepted: 11 April 1997     

Dust mode of carbon pellet ablation in the W7-AS stellarator

A new type of carbon pellet ablation in the high-density ion-heated plasma of the Wendelstein 7-AS stellarator is studied. In contrast to conventional ablation in the form of atoms or clusters, this type of ablation features the emission of small (micrometer) carbon particles from the surface of the pellet. The emission angles and velocities of such microparticles are determined. Their sizes are estimated with two methods. The experimental data are compared with theoretical predictions.     

The influence of SiCp oxidized on the interface layer and thermal conductivity of SiCp/Al composites

In this work, oxidation of silicon carbide particles (SiCp) at elevated temperature and its influence on the interface layer and thermal conductivity of SiCp/ZL101 composites prepared using pressure infiltration process were investigated respectively. It is found that initial temperature for the oxidation of SiCp is about 850?°C, and that the oxidation increment of SiCp and the thickness of SiO₂ layer increase with the increase in pre-oxidation temperature and time, when the oxidized temperature exceeds 1100?°C, or the duration time exceeds 2?h at 1100?°C, a small amount of ablation will take place on the SiCp, as well as the oxidized layer has some loss. The formation of SiO₂ layer can provide certain interface reactions with interface layers (3.1–6.36?μm), and the higher the thickness of SiO₂ layer, the thicker the interface layer in SiCp/Al composites. However, the thickness of SiO₂ layer is more than 5.9?μm, which is not benefit for the formation of interface layer. With the increase in the thickness of interface layer, thermal conductivity declines, but is not linear.     

Involvement of small carbon clusters in the enhancement of high-order harmonic generation of ultrashort pulses in the plasmas produced during ablation of carbon-contained nanoparticles

Various carbon-based nanoparticles ablated at the conditions suitable for efficient harmonic generation during propagation of ultrashort pulses through the laser-produced plasmas were studied. The transmission electron microscopy of ablated debris and the time-of-flight mass-spectroscopy studies of plasmas are presented. The conditions of laser ablation of the carbon-contained nanoparticles (fullerenes, graphene, carbon nanotubes, carbon nanofibers, and diamond nanoparticles) were varied to define the impeding processes restricting the harmonic yield from such laser-produced plasmas. These studies show that the enhancement of harmonics during ablation of nanoparticle targets was related with the appearance of small carbon clusters at the moment of propagation of the ultrashort laser pulses though such plasmas.     

A mechanistic study on kinetic compensation effect during low-temperature oxidation of coal chars

This paper provides mechanistic insights into the low-temperature oxidation of a range of carbon materials (graphite, a sub-bituminous coal char, and a brown coal char). Kinetic analysis was carried out on oxidation of the chars, prepared from fast-heating pyrolysis, under chemical-reaction-controlled regime. FT-Raman spectroscopic analysis was adopted to provide direct structural information on the carbon structure of reacting carbon materials throughout oxidation. The results demonstrate the significance of selective oxidation under the conditions, and parallel to this, the kinetic compensation effect of carbon oxidation reaction throughout conversion for all samples. Supported by the results from FT-Raman spectroscopy, the kinetic compensation effect seems to be a result of the selective oxidation of these carbon materials with heterogeneous carbon structures. Oxidation of all samples, with or without catalysts, appears to be similar in terms of the ‘nature’ of carbon structural condensation during low-temperature oxidation, suggesting a similar increase in apparent active sites population with respect to increase of apparent energy barrier. Under the current experimental conditions, a general kinetic compensation effect correlation has been deduced for various materials, requiring only the initial char kinetic parameters. The inherent inorganic species in chars also seem to alter the ‘degree/extent’ of carbon structural condensation as results of selective oxidation. In this case, the use of the compensation effect correlation will require more information on the catalysis during oxidation, apart from the initial char kinetic parameters.     

Changes in the etch rate of photosensitive polymers as a function of the pulse number

The ablation rates of a polyimide and a triazene polymer were studied gravimetrically by a quartz micro balance for 248-nm and 308-nm irradiation. Special care was taken to examine the dependence of the ablation rate at constant fluences for single pulses and the influence of consecutive pulses at the same position. A clear trend was observed in these measurements, i.e., that the mass loss after the first pulse is always different from values for the following pulses. This implies that it is very difficult to determine true ablation rates, which are the foundation of most ablation models. The differences of the mass loss between the first pulse and the following pulses is most probably due to carbonization of the material, resulting in varying ablation rates for the following pulses. The ablation rates are thus not a real material property but a superposition of the material ablation rates with the ablation rates of carbon and carbonized material. PACS 52.38.Mf; 71.20.Rv; 07.10.Lw     

Catalytic soot oxidation in microscale experiments: Simulation of interactions between co-deposited graphitic nanoparticle agglomerates and platinum nanoparticles

Continuously regenerating catalytic soot traps are under development to reduce particulate emissions from diesel exhaust. A good understanding of the processes that take place during soot oxidation is needed to optimize diesel soot trap performance. To gain insight into these processes from the perspective of nanoparticle technology, the effects of catalyst particle size and the interparticle distance between soot and catalyst particles were measured. A model catalyst was prepared by depositing Pt nanoparticles on a SiO/SiO₂-coated transmission electron microscope (TEM) grid. A soot surrogate composed of graphitic nanoparticle agglomerates generated by laser ablation was deposited on the same surface. This system simulates, morphologically, catalytic soot traps used in practice. The reaction was carried out in a tubular flow reactor in which the gas phase simulated diesel exhaust gas, composed of a mixture of 10% O₂ and 1000 ppm NO with the remainder N₂. The progress of the carbon nanoparticle oxidation was monitored off-line by analysis of electron microscopy images of the agglomerates before and after reaction. This experimental method permitted the correlation of reaction rate with particle sizes and separation distances as well as catalyst surface area in the direct environs of the soot particles. The experimental results revealed no effect of Pt catalyst particle size in the range 7–31 nm on the rate of reaction. Also observed were a decrease in the rate of reaction with increasing distance between carbon agglomerates and catalyst particles and a linear dependence of the reaction rate on the fractional catalyst surface area coverage.     

激光诱导荧光技术在高焓空气氮原子测量中的应用

高超声速飞行器再入大气层时,受到激波的压缩和激波层内粘性阻滞作用,周围绕流流场的空气温度在4 000～15 000 K之间,使空气中的氧、氮分子发生离解,从而出现高温气体效应,形成高焓化学非平衡流。飞行器表面防热材料基本都含有大量的碳元素,通常情况下主要是氧原子与碳发生反应,但在焓值大于18 MJ·kg-1的情况下,氮原子与飞行器表面碳反应的无量纲烧蚀因子BC_N＞0.172 5,此时碳在高焓空气介质中的无量纲质量烧蚀因子BC_air＞0.345,因此,碳的氮化烧蚀变得非常剧烈,和氧化烧蚀相当;同时离解的氮原子也会在飞行器表面发生催化复合反应放出大量的热,使飞行器表面承受更多的热载荷。因此,分析高焓化学非平衡流流场中的氮原子具有现实意义。在地面模拟设备中建立高焓化学非平衡流场,对氮原子进行测量技术,可以很好的对其进行研究。双光子吸收激光诱导荧光（TALIF）技术作为一种非接触测量,在不干扰流场环境的情况下,可以直接获得氮原子的浓度分布。对流场氮原子激发荧光,通过布置在风洞试验段外与流场和激光形成的平面呈垂直方向的ICCD获取到二维氮原子的荧光信号。为确保荧光图像的清晰及视场合适,选择了Nikon f=50 mm F/1.4镜头作为前级收光设备。实验成像采用50次曝光的累计结果,以消除气流扰动及激光能量抖动造成的不确定性。实验中,在理论激发波长附件进行测试,优化选择出206.717 nm作为正式实验中脉冲激光的最佳激发波长。在最佳激光波长条件下,从小到大调整激发能量,获得了该环境下的氮原子激发的非饱和线性区为1.8 mJ以下。正式实验过程中激光能量为1.6 mJ,处于线性区。对所获取的荧光图像进行处理,提取激光中心线上的荧光强度进行分析,发现无论是亚声速还是超声速,荧光强度沿径向都呈驼峰状分布,与之前氧原子测试结果比较,发现流场中氮分子还未完全离解,这符合实验风洞流场特性。     

Numerical simulation of the ablation of thin molybdenum films under laser irradiation

Laser irradiation of a molybdenum film on a quartz substrate is numerically studied. The simulated results prove the experimental effect lying in a threefold decrease in the size of the ablation region in comparison with the focal spot. The numerical experiment proves the hypothesis on the two-stage ablation of metal film with the primary formation of oxide phase. It is demonstrated that oxidation leads to a selective decrease in the thermal resistance of the film along the vertical direction, so that the anisotropic character of the ablation is enhanced.