复合材料激光辐照实验中的吸收特性研究

在复合材料激光烧蚀研究中,首要解决的问题是激光的吸收问题。利用积分球法测量了材料蒙皮层及材料整体结构对1.319μm红外激光的反射率和透过率,获得了蒙皮层材料激光吸收特性随厚度的变化规律,以及材料表面状态变化导致的反射与透射特性的变化。在此基础上研究了蜂窝夹层材料激光烧蚀前后的反射、透射性能变化,结果表明,材料表面碳化后对近红外激光的反射率由30%降为10%左右,透过率由40%降为0,平均吸收率由30%增加到90%,蜂窝夹层材料对激光的吸收大大增加。     

碳化烧蚀材料内部热响应的数值分析

为了揭示碳化烧蚀材料的详细热响应,运用有限元数值模拟方法建立了高温环境下碳化烧蚀材料性能分析的数值计算模型,并通过阿伦尼乌斯定律对材料的内部热解进行了分析。碳化烧蚀材料热响应计算过程中,建立了材料变物性参数计算的数学函数,并对某高硅氧酚醛复合材料的碳化烧蚀过程进行了数值模拟。结果表明:碳化烧蚀材料的热响应是多种因素综合作用的结果,随着材料内部热解,材料出现碳化层、原材料层的分层现象,材料内部发生质量损失。材料的热物理性质与碳化过程中材料温度和热解状态相关。研究方法对计算任意的碳化烧蚀具有适应性。     

玻璃微珠和ZrB2改性石英酚醛复合材料的耐烧蚀性能

在石英/酚醛防热复合材料中引入玻璃微珠和ZrB_2颗粒,旨在提高其耐冲刷、烧蚀性能。采用氧乙炔烧蚀试验测试所得石英/酚醛复合材料的耐烧蚀性能,对比分析了玻璃微珠和ZrB_2颗粒对低密度和全密度石英/酚醛材料烧蚀机理和烧蚀性能的影响。结果表明,在低密度石英酚醛复合材料中掺入适量的ZrB_2颗粒能使复合材料在烧蚀表面形成熔覆层,该熔覆层能有效保护碳化层及基体材料,降低线烧蚀率和质量烧蚀率。而表面熔覆层的形成与ZrO_2在硅系熔融物中产生的"钉锚效应"相关,同时也与B_2O_3降低硅系熔融物的表面能有关。在全密度石英酚醛复合材料中引入ZrB_2颗粒,可使其在烧蚀过程中形成多孔的ZrO_2层,有效地将碳化层和烧蚀环境隔离。然而,ZrO_2层没有熔融铺展,与碳化层结合力较弱,在烧蚀过程中容易剥落。     

激光烧蚀对石英/氰酸酯透波复合材料电性能的影响

研究激光烧蚀对石英/氰酸酯复合材料电性能的影响并揭示其影响机制,对极度恶劣热环境条件下石英/氰酸酯复合材料透波性能评估分析、热防护设计等具有重要意义。利用激光作为外热流加载手段,对石英/氰酸酯复合材料进行激光辐照烧蚀实验,对实验前后的介电常数进行了测试。为分析介电常数变化机制,对石英/氰酸酯复合材料激光烧蚀前后的表面产物进行了透射红外光谱、XRD测试,对实验后的石英/氰酸酯复合材料表面进行微观形貌观察,并对氰酸酯和石英纤维进行了热失重测试。结果表明：与初始状态相比,激光烧蚀后的石英/氰酸酯复合材料在7~18 GHz范围内的介电常数为6左右,增大近1倍。分析认为激光烧蚀对石英/氰酸酯复合材料电性能的影响机制为：在激光辐照作用下,材料吸收激光能量升温,使氰酸酯树脂基体发生热分解、裂解等变化,在表面原位生成具有导电能力和岛链状态的炭黑物质,致使发生烧蚀炭化石英/氰酸酯复合材料的介电常数增大,将增强对雷达波的吸收。同时烧蚀形成的粗糙表面状态和疏松状态对电磁波的反射、散射作用增强,可进一步削弱石英/氰酸酯复合材料的雷达波透射能。     

硅橡胶绝热材料气相环境烧蚀特性试验研究

采用试验发动机,在双基推进剂的气相燃气环境中对一种硅橡胶绝热材料进行烧蚀特性研究,分析了环境气流速度及压强对硅橡胶绝热材料碳化烧蚀率及炭化层宏观和微观结构形貌的影响。结果表明:在无粒子作用条件下,环境气流速度和压强通过对流热流、炭化层外部和内部气动力影响硅橡胶绝热材料的烧蚀特性。气流速度高,碳化烧蚀率大,炭化层薄且致密;环境压强高,碳化烧蚀率大,炭化层易分层鼓包。同时将硅橡胶绝热材料的烧蚀特性与相同实验条件下的三元乙丙橡胶绝热材料进行了对比,为进一步对绝热材料进行烧蚀机理分析和建模计算打下基础。     

伪装遮障的雷达散射截面新模型

散射型防雷达伪装遮障依靠其对微波的强散射和低透过作用实现对军事目标的伪装。在遮障材料几乎不吸收微波的情况下，强散射与低透过是相辅相成的。不过就军事目标的隐身而言，真正理想的遮障应该对微波具有低散射和低透过的特性，这意味着微波吸收性能对理想遮障的不可或缺。分析了遮障的散射、吸收和透射作用对其雷达散射截面的影响，讨论用现有吸波材料构造理想遮障的可行性。根据分析，遮障面所用材料的吸波性能、遮障本身结构形成的对入射微波的均匀散射以及遮障对透射微波的传播方式的影响都是理想遮障的必要条件。其中，在计算遮障的雷达散射截面时考虑其透射微波的分布是本文的特点。适当的遮障结构能够导致其透射微波在空间的弥散，从而大大降低目标的回波信号，因而十分有利于达成理想的伪装效果。     

碳/碳复合材料的激光烧蚀行为与机制

碳/碳(C/C)复合材料作为性能优良的耐烧蚀材料得到了广泛的应用,其作为抗激光烧蚀材料的潜力待被发掘。本文制备了不同密度的C/C复合材料,在无氧环境下以CO₂激光器为光源,探究了高能激光与C/C复合材料之间的作用机制,系统地分析了材料的特性和激光参数不同对烧蚀表现的影响。采用三维轮廓仪对线烧蚀率进行表征。结果表明,随着烧蚀时间或激光功率的变化,C/C复合材料的烧蚀表现均为非线性变化。C/C复合材料的本征特性决定了其热量载荷。密度越高的C/C复合材料,其热量载荷越高,烧蚀性能越好。热量载荷与激光热流密度之间的关系则决定了材料的烧蚀表现,当激光的热流密度大于材料的热流载荷时,烧蚀速率会呈阶跃式攀升。     

高性能二维碳/碳复合材料的制备与性能

为获得高性能热结构复合材料,以国产T300碳纤维为原料,通过碳布预浸料交替铺层热压及液相浸渍裂解工艺方法制备了一系列二维碳/碳复合材料,并对二维碳/碳复合材料的微观结构特征、力学性能及烧蚀性能进行了测试与分析。研究结果表明:碳布规格及制备工艺对二维碳/碳复合材料力学性能有较大影响,当碳布规格选用八枚缎纹、经过碳化预处理且高温处理温度达到2 300℃时,二维碳/碳复合材料表现出较好的综合性能,拉伸强度和层间剪切强度的最大值分别高达301 MPa和12.4 MPa,达到了国际先进水平;在模拟典型服役环境考核状态下,制备的不同规格二维碳/碳复合材料的烧蚀性能基本相当,均未出现由于层间强度偏低而发生的烧蚀揭层现象,表现出较好的烧蚀均匀性和结构可靠性。      

碳化导致水泥基材料微观结构演变的研究进展

水泥基材料的微观结构是表征其抗碳化性能的重要因素。首先总结了水泥基材料在加速碳化条件下碳化前后固相组成、孔相结构以及液相组成的演变,然后分析了温度、湿度、二氧化碳浓度等环境因素对水泥基材料碳化过程及微结构演变的影响,最后介绍了矿物掺合料在水泥基材料碳化方面的应用,并展望了碳化条件下微观结构演变的发展方向。     

微波辅助加热法合成LiFePO_4/C正极材料

以离子液体1-丁基-3甲基咪唑二腈胺盐为碳源,通过微波辅助加热法制备LiFePO_4/C正极材料。采用X射线衍射(XRD)表征了样品的物相,扫描电镜(SEM),透射电镜(TEM)考察了样品的微观形貌,元素分析检测了离子液体最终产物中C、H、N元素的含量。通过恒电流充放电测试研究了材料的电化学性能,主要研究了加热方式和Mg2+掺杂对材料性能的影响。结果表明,微波加热可以高效快速制备LiFePO_4/C,有利于离子液体碳化形成碳层。在加热时间较长的情况下,会造成颗粒的不规则长大。Mg2+的掺杂不会改变LiFePO_4/C颗粒的形貌,0.1C倍率下首次放电比容量达到146.3mAh/g,经过45次循环后容量没有明显衰减,可以有效地改善LiFePO_4/C的电化学性能。     

飞秒激光烧蚀Ag膜的研究

研究飞秒激光烧蚀Ag膜表面,采用扫描电镜(SEM)观测其表面烧蚀形貌,发现烧蚀点大致分为3个区域,烧蚀面积随着脉冲能量的增大而增大,脉冲数的增加而增大,但当脉冲数达到一定值后烧蚀面积变化不大.改变脉冲数在烧蚀点得到了周期为650和150nm的长短周期条纹结构.根据烧蚀区域面积与脉冲能量的关系算出单脉冲与多脉冲的烧蚀阈值...     

Microwave absorbing composite lattice grids

Stretching dominated lattice grids reinforced by glass fibers and carbon fibers filled with spongy materials were designed and manufactured to achieve multifunctional structures with microwave absorbing abilities. The reflectivity of the GFRC and CFRC grid panels was measured in the darkroom at normal and oblique incidences, respectively. GFRC grid panels of 20 mm thickness and CFRC panels of 18 mm thickness displayed excellent microwave absorbing abilities in the range from 4 GHz to 18 GHz. The reflectivity of the grid panels is poorer than that of the foam panels in the range from 4 GHz to 10 GHz, but is better in the range from 10 GHz to 18 GHz. That difference in the reflectivity enhancement is explained by the grating lobe mechanism. The studies show that the composite grid panels are structural absorbent with weight efficiency.     

Enhancement of aging resistance of glass fiber reinforced cement

Long-term weathering tests revealed that glass fiber-reinforced cement (GFRC) might exhibit tensile strength reduction and ductility loss with aging. The main goal of this research was to assess the effectiveness of polyvinyl alcohol (PVA) powder in enhancing the durability characteristics of GFRC sheets. Accelerated aging was achieved by using low pressure steam curing. The strength and ductility of GFRC sheets were measured by the direct tension test. The test results show that the incorporation of PVA powder into GFRC improves the mechanical behavior and changes the failure mode from brittle to ductile. To investigate the mechanism of the enhancement, the fiber-matrix interface was examined by polarizing optical microscopy and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). It was found that the addition of the PVA powder results in the deposition of a polymeric film on the fiber surface and thus prevented the accumulation of calcium hydroxide in the interfacial zone. The use of the PVA powder led to a more ductile interfacial microstructure and to better bonding between fiber and matrix, which is believed to be responsible for the tensile property enhancement.

     

Noncrystalline micromachining of amorphous alloys using femtosecond laser pulses

Femtosecond laser micromachining of a Zr-based amorphous alloy in air, including measuring the ablation threshold, micro-drilling and trenching, was investigated. The threshold of ablating this amorphous alloy was determined by experiment. Laser-induced ablation and associated damage were examined by means of optical microscope, scanning electron microscopy, transmission electron microscopy and electron diffraction diagram. The results show that conventional processing method induced defects in the vicinity of machined area, such as crystallization, molten trace and spatter, were absent in femtosecond laser ablation area with selected parameters. This indicates that femtosecond laser ablation is a promising method for micromachining amorphous alloys without crystallization.     

Synthesis of nanocrystalline NbAl3 by laser ablation technique

Synthesis of nanocrystalline material has been developed using laser ablation technique. NbAl₃ nanocrystalline powders have been synthesized by laser ablation and identified by transmission electron microscopy (TEM). The average particle size was measured about 5.0 nm. Production yield and ablation rate were obtained as a function of He background pressure and laser pulse energy by measuring the weight of produced powders and weight loss of the target material before and after experiment. The ablation rate appeared to be relatively constant over the range of the He gas pressure. Production rate varied along with both He gas pressure and laser pulse energy. Optimum condition for efficiently high production rate is at 1.0 Torr of He gas pressure and at 320 mJ of the laser pulse energy.     

Measurement of small-signal absorption coefficient and absorption cross section of collagen for 193-nm excimer laser light and the role of collagen in tissue ablation

A 193-nm ArF excimer laser transmission was measured at subablative fluence through varying strength solutions of dissolved collagen, yielding an absorption cross section of 1.14 x 10(-17) cm2 for the peptide bond, which accounts for 96% of the total collagen attenuation that is based on additional transmission measurements through solutions of isolated constituent amino acids. The measured absorption cross sections, in combination with typical corneal tissue composition, yield a predicted corneal tissue absorption coefficient of 16,000 cm(-1). In addition, dry collagen films were prepared and ablation-rate data were recorded as a function of laser fluence. Ablation rates were modeled by use of a Beer-Lambert blow-off model, incorporating a measured ablation threshold and an absorption coefficient that are based on the measured collagen absorption cross section and the film bond density. The measured ablation rates and those predicted by the model were in very good agreement. The experiments suggest that collagen-based absorption coefficients are consistent with predicted corneal tissue ablation rates and previously observed dynamic changes in tissue properties under ablative conditions.     

聚磷酸铵-三聚氰胺-三嗪成炭剂协同阻燃改性环氧树脂及玻璃纤维增强树脂复合材料

采用聚磷酸铵(APP)与不同比例三聚氰胺(MA)和三嗪成炭剂(CFA)复配对环氧树脂进行阻燃改性。系统研究了不同配比阻燃剂(总量保持40wt%)的加入对环氧树脂流变特性、固化行为、热机械性能、力学性能及阻燃性能的影响。将优化后的阻燃改性环氧树脂用于制备玻璃纤维增强环氧树脂复合材料(GFRC),对并其力学和阻燃性能进行了研究。结果表明,APP单独与MA或CFA复配改性环氧树脂并未表现出明显的协同阻燃效应,但它们组成的三元复配阻燃体系(30wt%APP-5wt%MA-5wt%CFA)具有良好的协同阻燃效应。相比未改性环氧树脂,APP-MA-CFA改性环氧树脂的极限氧指数(LOI)由18.0%提高到了50.2%,热释放峰值速率(PHRR)下降了84%,总热释放量(THR)下降了78%。树脂基体中加入阻燃剂后,GFRC的力学性能有所下降,尤其是层间剪切强度。同样地,基于APP-MA-CFA复配改性环氧树脂的GFRC表现出最佳阻燃性能,相比未改性的GFRC,其LOI值由22.8%提高到了66.0%,PHRR由354 kW/m2下降到93 kW/m2,THR由49.3 MJ/m2下降到22.8 MJ/m2。      

A finite element model to predict the ablation depth in pulsed laser ablation

This work presents development of a two-dimensional finite element model to predict temperature distribution and ablation depth in a laser ablation process. The model considers a number of aspects of the process, which hitherto have been considered independently in the literature. The aspects considered include: temperature dependent material properties of the target material, effect of plasma shielding on the incident laser flux, and temperature dependent absorptivity and absorption coefficient of the target. It was evident that these considerations have resulted in a significant improvement in the ability of the model to predict the ablation depth. Finally, the predicted ablation depth was found to match extremely well with experimental results at lower laser fluences, though at higher fluences there is a marginal overestimation.     

Dependence of gold nanoparticle production on pulse duration by laser ablation in liquid media

The dependence on laser fluence and laser pulse duration of size, size distribution and concentration of gold nanoparticles synthesized by laser ablation in liquid media was investigated. It was demonstrated that increasing laser energy from 1 to 5 mJ/pulse enhances the ablation rate by a factor of 100. The behavior of the ablation rate, hence of the nanoparticle concentration, as a function of pulse duration (varied from 40 fs to 200 ps) was found to strongly differ from that in air, which can be explained by photoionization and important losses of laser energy in the femtosecond regime. The optimal pulse duration for maximum ablation rate in liquid media was found to be equal to 2 ps.