Carbon fiber reinforced carbon aerogel composites for thermal insulation prepared by soft reinforcement

To overcome the brittleness and the pyrolysis shrinkage of carbon aerogels, carbon fiber reinforced composites were prepared by copyrolysis of polyacrylonitrile fiber reinforced resorcinol-formaldehyde aerogel composites (PAN/RFs). The PAN/RFs were obtained by impregnating the PAN fiber felt with RF sol and then supercritical drying. Upon carbonization the PAN fiber shrinks with the RF aerogel, thus reducing the shrinkage differences between the fiber and the aerogel, and results in crack-free carbon fiber reinforced carbon aerogel composites, with a thermal conductivity of 0.073 W/m K at 25 °C in air. Our new method may greatly expand the usage of carbon aerogels in general applications.     

碳纤维增强SiO2气凝胶复合材料的制备及保温性能研究

以正硅酸乙酯（TEOS）为前驱体,无水乙醇和去离子水为溶剂,采用溶胶-凝胶法制备了SiO₂气凝胶,再以不同含量(0,1%,3%和5%（质量分数）)短切碳纤维为增强材料,在胶凝完成后,经过表面改性,采用常压干燥工艺,制备了碳纤维增强SiO₂气凝胶复合材料。采用XRD、SEM、FT-IR和孔径测试等方法对制备所得复合材料的微观结构、形貌、孔径分布和导热性能进行了测试分析。结果表明,碳纤维增强SiO₂气凝胶复合材料为典型的非晶态结构,属于毛细凝聚特征的介孔材料,碳纤维的掺杂并没有改变SiO₂气凝胶的晶态结构;未掺杂碳纤维的SiO₂气凝胶的颗粒相互堆搭,掺入碳纤维的SiO₂气凝胶颗粒的孔隙明显减小,孔洞结构较为完整,碳纤维的掺入填充了大尺寸孔隙,有助于气凝胶孔径分布区间的收窄,当碳纤维的含量为3%（质量分数）时,颗粒分布最佳;随着碳纤维含量的增加,复合材料的导热系数呈现出先降低后升高的趋势,当碳纤维的含量为3%（质量分数）时,样品的导热系数最低为0.019 W/（...     

二氧化硅气凝胶隔热复合材料的制备与应用

介绍了二氧化硅(SiO2)气凝胶的结构特点及隔热性能;对二氧化硅气凝胶隔热复合材料的制备方法及其应用前景进行总结并作了适当的评述;探讨了该领域今后的研究方向。     

Fragmentation of aramid fibres in single-fibre model composites

Raman spectroscopy has been used to monitor the state of axial stress along fragmented, high-modulus Kevlar 149 aramid fibres in an epoxy resin matrix by monitoring the peak position of the strain-sensitive 1610 cm^–1 aramid Raman band along individual fragments. It is shown that the interfacial shear stress along each fragment, derived from the strain distribution profiles, is not constant as assumed by conventional fragmentation analysis. The fragmentation process of as-received Kevlar 149 fibres is compared to that of irradiated Kevlar 149 fibres exposed to ultraviolet light where the tensile strength and modulus of the fibres have been reduced. It is found that the derived interfacial shear stress and interfacial shear strength values are higher for those fibres exposed to ultraviolet light compared with the as-received fibres. It is also clearly demonstrated that the values of interfacial shear strength calculated at high matrix strains from conventional fragmentation analysis are considerably lower than the maximum value of interfacial shear stress prior to fibre fracture that was found to be close to the shear yield stress of the resin matrix. Hence the determination of the interfacial shear strength following the saturation of the fragmentation process may give rise to misleading results.Nomenclature e _f Fibre strain - e _m Matrix strain - e _f ^max Maximum strain along each fragment - e _f ^* Failure strain of the fibre - E _f Fibre tensile modulus - l _c Critical fragment length - l _c Mean critical fragment length - l _f Fragment length - r Fibre radius - x Distance along the fibre - _f ^max Maximum stress along each fragment - _f ^* Fibre tensile strength - Interfacial shear stress - _s Interfacial shear strength     

Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance

Aramid fibers reinforced silica aerogel composites (AF/aerogels) for thermal insulation were prepared successfully under ambient pressure drying. The microstructure showed that the aramid fibers were inlaid in the aerogel matrix, acting as the supporting skeletons, to strengthen the aerogel matrix. FTIR revealed AF/aerogels was physical combination between aramid fibers and aerogel matrix without chemical bonds. The as prepared AF/aerogels possessed extremely low thermal conductivity of 0.0227 ± 0.0007 W m⁻¹ K⁻¹ with the fiber content ranging from 1.5% to 6.6%. Due to the softness, low density and remarkable mechanical strength of aramid fibers and the layered structure of the fiber distribution, the AF/aerogels presented nice elasticity and flexibility. TG–DSC indicated the thermal stability reaching approximately 290 °C, can meet the general usage conditions, which was mainly depended on the pure silica aerogels. From mentioned above, AF/aerogels present huge application prospects in heat preservation field, especially in piping insulation.     

Ambient-dried silica aerogel doped with TiO2 powder for thermal insulation

Two step-derived SiO₂ sol doped with TiO₂ powder dissolved in ethanol was prepared. After aging and washing of wet gel, surface modification with TMCS (trimethylchlorosilane) was followed to prevent additional condensation reaction during drying. Ambient-dried SiO₂-TiO₂ gel was heat-treated at various temperatures. The effects of the heat-treated temperature on the chemical bonding state of SiO₂-TiO₂ gel were investigated by means of FTIR and XRD analyses. After heat-treatment at 350 °C, crack-free SiO₂-TiO₂ gel was synthesized with density of 0.14 g/cm³, porosity of 94%, and specific surface area of 613 m²/g. Ambient-dried SiO₂-TiO₂ gel heat-treated at 350 °C exhibited thermal conductivities of 0.0136 W/m·K and 0.0284 W/m·K at room temperature and 400 °C, respectively.     

溶胶配比对碳纤维增强炭气凝胶隔热复合材料力学性能的影响

以异丙醇(I)为溶剂、 六次甲基四胺(H)为催化剂, 配制间苯二酚(R)-糠醛(F)的醇溶胶, 经浸渍纤维预制件、凝胶老化、超临界干燥和炭化制得碳纤维增强炭气凝胶隔热复合材料。研究了溶胶配比对碳纤维增强炭气凝胶隔热复合材料密度、微观结构和力学性能的影响规律。结果表明:随着异丙醇与间苯二酚物质的量之比增大, 碳纤维增强炭气凝胶隔热复合材料的密度逐渐降低, 基体炭气凝胶内及与碳纤维形成的界面内孔径增大, 大孔数量增多, 碳纤维增强炭气凝胶隔热复合材料的强度降低。当异丙醇与间苯二酚物质的量之比由18增加到28时, 压缩强度由2.498 MPa(应变10%)降至0.716 MPa(应变10%), 拉伸强度由2.019 MPa降至1.001 MPa, 弯曲强度由3.984 MPa降至1.818 MPa。随着六次甲基四胺与间苯二酚物质的量之比增大, 碳纤维增强炭气凝胶隔热复合材料的密度先增大后减小, 基体炭气凝胶内及与碳纤维形成的界面内孔径先减小后增大, 大孔数量先减少后增加, 碳纤维增强炭气凝胶隔热复合材料的强度先增大后减小。当六次甲基四胺与间苯二酚物质的量之比为0.008 5时, 碳纤维增强炭气凝胶隔热复合材料的密度最大, 强度最大, 其压缩强度为1.066 MPa(应变10%), 拉伸强度为1.256 MPa, 弯曲强度为3.556 MPa。      

Crystallite size evolution of aramid fibres aged in alkaline environments

Aramid fibres are high-performance materials proposed in geotextiles for alkaline ground reinforcement. To study their durability in such environments, accelerated ageing has been carried out at pH 9 and at pH 11 for up to one and a half years. First, the lateral and longitudinal crystallite sizes have been determined before and after ageing under these conditions by Wide-Angle X-ray diffraction. Next, the tensile fracture surfaces have been observed after different ageing times by scanning electron microscopy. Finally, molecular weight changes have been evaluated by size exclusion chromatography. This study highlights the dependence of structural changes on the ageing conditions. A structural degradation scheme is proposed.     

Robust,lightweight gelatin composite aerogel with outstanding thermal insulation

Journal of Materials Science - Thermal insulation materials with high mechanical strength and low thermal conductivity play an important role in the construction industry. In this study, a compound...     

Properties of cement composites reinforced with Kevlar fibres

The organic polyamide fibre, Kevlar, is promising as an efficient reinforcement for cementitious matrices. For cement boards, in which chopped fibres are distributed randomly in two dimensions, typical mechanical properties obtained with 1.9 vol% fibre addition are as follows: ultimate tensile strength (UTS) 16 MN m^–2; MOR 44 MIN m^–2; impact strength 17 kJ m^–2. The composite material can be produced by autoclaving if desired and at ambient temperatures they are expected to be durable in most environments. The relatively low decomposition point of Kevlar (as opposed to glass fibres or steel) is a disadvantage for its use in building components which may come into contact with high temperatures, as in a fire. It should be noted that a solvent which is used in the manufacture of the fibre and remains in the fibre in minute quantities has been found to produce cancer in rats. There is no evidence of it causing cancer in humans but the significance of this in terms of a possible health risk, if any, will need to be assessed by the appropriate medical authorities in relation to any applications.     

Fracture toughness of composites reinforced with discontinuous fibres

Measurements of the work of fracture of composites of polyester resin reinforced with chopped steel wires of various lengths are compared with the theory developed by Cooper. For composites containing aligned wires the results agree well with the model except where there is excessive resin cracking. The toughness of composites containing wires which are randomly distributed in the resin can be significantly greater than that of aligned composites with wires of similar length. This is probably due to the plastic shearing of wires not lying parallel or normal to the specimen axis.     

Dynamic mechanical and thermal analysis of vinylester-resin-matrix composites reinforced with untreated and alkali-treated jute fibres

Vinylester-resin-matrix composites reinforced with untreated and 5% NaOH treated jute fibres for 4 and 8 h with different fibre loading were subjected to dynamic mechanical and thermal analysis to determine their dynamic properties as a function of temperature. For all the composites the storage modulus, E′, decreased with increase in temperature, with a significant fall in the temperature range 110°–170 °C. For the treated composites, the rate of fall, dE′/dT, had an inverse relationship with the defect concentrations in the composites. The lowest defect concentrations in the 4 h treated composites corresponded to the highest rate of fall. The glass transition temperature, T_g, of the unreinforced resin, corresponding to the loss modulus peak, was 101.2 °C, whereas that of the composites increased by nearly 28 °C on account of the restricted mobility of the resin molecules in the presence of the fibres. In the case of the treated composites, the T_g value showed a decreasing trend (128 to 125 °C). Unlike the plain resin, a tiny hump was observed in the loss modulus, E″, curves of all the composites around 166 °C, which became broader and more prominent with increase in the jute fibre content. The very high tanδ value of the resin decreased in the composites, indicating that the addition of the fibres lowered the damping capacity of the composites.     

Electrospun nanofiber composites with micro-/nano-particles for thermal insulation

Polyacrylonitrile (PAN) nanofiber and silica aerogel (SAG) laminated composites were prepared via electrospinning for thermal insulation. Conventional single nozzle and co-axial electrospinning were used to increase the fraction of aerogel particles in the composite sheets while maintaining the mechanical strength of the sheet. When the core-shell electrospinning technique with co-axial nozzle was applied, the proportion of aerogel particles increased two fold without a deterioration of the mechanical properties. The average thermal conductivity of the laminated composite sheet was reduced by approximately 12.5% as compared to the nanofiber composite prepared using the single-nozzle electrospinning technique. For additional reduction in thermal conductivity, hollow glass microspheres (HGM) was inserted between the interlayer spacing of the electrospun sheets to increase the interlayer spacing. When HGM particles were inserted, it was observed that the thermal conductivity decreased by approximately 20% compared to that of the specimen without particles.     

The fracture toughness of composites reinforced with weakened fibres

Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite.Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.     

Aluminium based composites reinforced with alumina coated carbon fibres

Abstract

A fine Al₂0₃ coating could be obtained from alumina sols modified by chelator acetylacetone, with exact control of parameters. Coating with alumina by the sol–gel method on a carbon fibre surface was investigated in detail to improve the oxidation resistance of carbon fibre. Further study focused on making the alumina coated fibre reinforced aluminium composite prefabrication. X-ray diffraction and SEM methods were used to analyse the alumina gels and the carbon fibre/aluminium (CF/Al) preformed wire. After the coating treatment, oxidation resistance of carbon fibres is enhanced, the wettability between the fibres and melting aluminium is greatly improved, and the tensile strength of CF/Al preformed wire is increased.     

Epoxy composites reinforced with deacetylated Phormium tenax leaf fibres

Leaf fibres from Phormium tenax (harakeke, New Zealand flax) were treated with 1 wt.% aqueous NaOH at 30 °C to remove all of the acetyl groups, accounting for most of a 7% mass loss. Increasing the treatment time to 4 h showed no detectable increase in mass loss, but increasing the NaOH concentration to 5% doubled the mass loss by removing some of the xylans. Changes in fibre morphology were interpreted in terms of swelling of cell walls into lumens, followed by shrinkage on drying without reopening the lumens. Deacetylation retarded the rate of water uptake in unidirectional epoxy composites, but showed no detectable influence on the equilibrium moisture content, flexural modulus or strength. The results suggest that high levels of natural acetylation are not a useful feature of leaf fibres, at least in the context of reinforcement for epoxy composites.     

Toughness of aromatic polymer composites reinforced with carbon fibres

This experimental study focuses on the toughness of a thermoplastic composite, namely, poly ether-ether-ketone (PEEK) reinforced with 60% by volume of continous carbon fibres (APC 2). Toughness is assessed using both comparative and intrinsic techniques and a critical discussion of the two approaches is presented.The comparative toughness of cross-ply and quasi-isotropic sheets of APC 2 is studied using a damage tolerance test (compression after impact) and by using an instrumented falling weight impacr test over a range of temperatures. Intrinsic toughness is discussed by applying fracture mechanics techniques to unidirectional laminates. Double cantilever beam and three-point flexure tests are used, the latter being performed in six different crack directions. Fracture toughness results are presented for APC 2 and unreinforced PEEK.An ultrasonic C-scan on impacted specimens and scanning electron microscopy on fracture surfaces are used to explore further the mechanisms of fracture, e.g. delamination, fibre breakage and matrix cracking.     

Interfacial separation of fibres in fibre reinforced composites

An analytical model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding.

The analytical analysis is based on the principle of the compliance method in fracture mechanics with the presence of an interfacial crack at the fibre/matrix interface. The model is developed on the basis of the assumption that both the matrix and the fibre behave elastically and the matrix strain at a zone far from the matrix-fibre interface is equal to the composite strain. Furthermore, it is assumed that a complete bond exists between the fibre and the matrix and that the crack faces are traction free.

It is shown that the separation strain energy release rate for fibre-reinforced composites can be obtained for cases with and without the existence of an interfacial crack. Numerical examples are presented and compared with results obtained in the literature by finite element analyses and from experimental tests. The comparison demonstrates the accuracy and the convergence of the model.