Improvement of film boiling chemical vapor infiltration process for fabrication of large size C/C composite

An improved film boiling chemical vapor infiltration process was developed to fabricate a large size C/C composite with homogeneous density and microstructure. The C/C composite was prepared by processing a disc-shaped carbon felt preform, whose upper and lower sides were fixed and heated simultaneously by two flat surfaces of two heat sources, with kerosene as a precursor at 1050 °C for 3 h at an atmospheric pressure. The in-situ temperature distribution along the radial direction of the preform upper surface was analyzed to get better information and control of the process. Experimental results show that the average density of the composite of Φ 110 × 10 mm³ size is about 1.72 g/cm³ and its maximal difference along radial direction is 0.05 g/cm³. Polarized light microscopy (PLM) and scanning electron microscopy (SEM) reveal that the carbon fibers of the composite are surrounded by ring-shaped pyrocarbons with a thickness of ∼ 20 μm, and that pyrocarbons are delaminated to 4–6 layers. A schematic model is proposed to analyze the process by dividing the reactor into different regions associated with specific functions.     

Investigation on structure optimization of crashworthiness of fiber reinforced polymers materials

Composite materials, in most cases fiber reinforced polymers, are nowadays used in the aerospace and transportation, in which high specific energy absorption (SEA) and strength are critical issues. Aimed at the improvement of SEA and the peak impact load (P), the structure optimization of composite tape sinusoidal specimen and corresponding experiments are investigated in this paper. Firstly, the finite element model of composite tape sinusoidal specimen is constructed and is validated by experiments. Then, both the single-objective and multi-objective optimizations are performed for composite tape sinusoidal specimen under axial impact loading. At last, the optimal results are validated by experiments. The optimal results show that the SAE increases 67.8% (from 51.3666 kJ/kg to 88.887 kJ/kg) and the P decreases 42.9% (from 34.9936 kN to 20.178 kN). This work lays a foundation for structural design of crashworthiness using fiber reinforced polymers materials.     

High-precision CTE measurement of hybrid C/SiC composite for cryogenic space telescopes

This paper presents highly precise measurements of thermal expansion of a “hybrid” carbon-fiber reinforced silicon carbide composite, HB-Cesic® – a trademark of ECM, in the temperature region of ～310–10 K. Whilst C/SiC composites have been considered to be promising for the mirrors and other structures of space-borne cryogenic telescopes, the anisotropic thermal expansion has been a potential disadvantage of this material. HB-Cesic® is a newly developed composite using a mixture of different types of chopped, short carbon-fiber, in which one of the important aims of the development was to reduce the anisotropy. The measurements indicate that the anisotropy was much reduced down to 4% as a result of hybridization. The thermal expansion data obtained are presented as functions of temperature using eighth-order polynomials separately for the horizontal (XY-) and vertical (Z-) directions of the fabrication process. The average CTEs and their dispersion (1σ) in the range 293–10 K derived from the data for the XY- and Z-directions were 0.805 ± 0.003 × 10^?6 K^?1 and 0.837 ± 0.001 × 10^?6 K^?1, respectively. The absolute accuracy and the reproducibility of the present measurements are suggested to be better than 0.01 × 10^?6 K^?1 and 0.001 × 10^?6 K^?1, respectively. The residual anisotropy of the thermal expansion was consistent with our previous speculation regarding carbon-fiber, in which the residual anisotropy tended to lie mainly in the horizontal plane.     

Compressive behavior of C/SiC composite sandwich structure with stitched lattice core

C/SiC composite sandwich structure with stitched lattice core was fabricated by a technique that involved polymer impregnation and interweaving. The mechanical behaviors of C/SiC composite sandwich structure were investigated at room temperature. The out-of-plane compressive strength was 20.97 MPa while modulus was 1473.55 MPa. The microstructural evolution on compression fracture surfaces of the stitching yarns was investigated by scanning electron microscopy, and the damage pattern of fibers on compression fracture surface was presented and discussed. Under an in-plane compression loading, the C/SiC composite sandwich structure displayed a linear-elastic behavior until failure. The peak strength and average modulus are 165.61 MPa and 19.74 GPa, respectively. The failure of the specimen was dominated by the fracture of the facesheet.     

Design,preparation and properties of online-joints of C/SiC–C/SiC with pins

This work is aimed at providing a new joining technology for C/SiC composites and investigating the influence of drilling holes, hole distribution (including ratios of edge distance to diameter (E/D), width to diameter (W/D) and hole distance to diameter (H/D)) and the number of applied pins on the mechanical properties of C/SiC substrates and joints. The mechanical testing results show that drilling holes and hole distribution greatly affects the mechanical properties of C/SiC substrates but when adopting an optimized design principle (E/D ⩾ 3, W/D ⩾ 3 and H/D ⩾ 3) the effect could be neglected. 1D C/SiC pins with higher shearing strength (107.2 MPa) are more suitable to join the substrates. With the increase of pins (1, 2 and 4), the bearing loads of the joints increase almost linearly, and the reliability of joints is also improved in that the fracture mode changes from the interlayer damage to the substrate rupture. Besides, the joining process generates uniform and dense joining layer (composition of ZrC and SiC) and a strong bonding without obvious interface.     

In situ fabrication of carbon nanotube–MgAl2O4 nanocomposite powders through hydrogen-free CCVD

Carbon nanotube–MgAl₂O₄ composite powders were successfully prepared through solution combustion synthesis (SCS) followed by catalytic chemical vapor deposition (CCVD) of methane. Catalyst powders were synthesized starting with the stoichiometric ratios of metal nitrates and urea with a small amount of water and different Fe contents followed by subjecting the solution to heat. The obtained powders were placed in a silica tube to react with methane and form carbon nanotubes. It is noteworthy that no hydrogen was used throughout the whole process. Catalysts and composite powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The quality of products were evaluated by I_D/I_G ratio obtained from G and D bands intensities in Raman spectra of samples having 10 and 15 wt.% iron. The final product mostly comprised a mixture of single- and double-walled nanotubes on the catalyst containing 10 wt.% Fe, while no carbon product was formed on the catalyst with 5 wt.% Fe.     

Influence of cryogenic thermal cycling treatment on the thermophysical properties of carbon/carbon composites between room temperature and 1900 °C

Influence of cryogenic thermal cycling treatment (from ?120 °C to 120 °C at 1.3 × 10^?3 Pa) on the thermophysical properties including thermal conductivity (TC), thermal diffusivity (TD), specific heat (SH) and coefficient of thermal expansion (CTE) ranging from room temperature to 1900 °C of carbon/carbon (C/C) composites in x-y and z directions were studied. Test results showed that fiber/matrix interfacial debonding, fiber pull-out and microcracks occurred after the cryogenic thermal treatment and they increased significantly with the cycle number increasing, while cycled more than 30 times, the space of microdefects reduced obviously due to the accumulation of cyclic thermal stress. TC, TD, SH and CTE of the cryogenic thermal cycling treated C/C composites were first decreased and then increased in both directions (x-y and z directions) with the increase of thermal cycles. A model regarding the heat conduction in cryogenic thermal cycling treated C/C composites was proposed.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Simulations of deformation and damage processes of SiCp/Al composites during tension

The deformation, damage and failure behaviors of 17 vol.% SiCp/2009Al composite were studied by microscopic finite element (FE) models based on a representative volume element (RVE) and a unit cell. The RVE having a 3D realistic microstructure was constructed via computational modeling technique, in which an interface phase with an average thickness of 50 nm was generated for assessing the effects of interfacial properties. Modeling results showed that the RVE based FE model was more accurate than the unit cell based one. Based on the RVE, the predicted stress-strain curve and the fracture morphology agreed well with the experimental results. Furthermore, lower interface strength resulted in lower flow stress and ductile damage of interface phase, thereby leading to decreased elongation. It was revealed that the stress concentration factor of SiC was ～2.0: the average stress in SiC particles reached ～1200 MPa, while that of the composite reached ～600 MPa.     

Dramatic mechanical and thermal increments of thermoplastic composites by multi-scale synergetic reinforcement: Carbon fiber and graphene nanoplatelet

We report an easy and efficient approach to the development of advanced thermoplastic composites based on multi-scale carbon fiber (CF) and graphene nanoplatelet (GN) reinforcement. Poly (arylene ether nitrile) (PEN)/CF/GN composites, prepared by the twin-screw extrusion, exhibited excellent mechanical properties. For example, the flexural modulus of PEN/CF/GN composites was 18.6 GPa, which is 1.7, 4.5 and 6.4 times larger than those of PEN/CF composites, PEN/GN composites and PEN host, respectively. Based on the SEM image observation, such mechanical enhancements can be attributed to the synergetic effect of micro-scale CF and nano-scale GN in the PEN matrix (decreased matrix-rich and free-volume regions and enhanced interfacial interactions). For 5 wt.% GN-filled PEN/CF/GN composites, the T_d30% of PEN/CF/GN composites was 145 °C and 62.8 °C compared with those of PEN host and PEN/CF composites, respectively. This study has demonstrated that multi-scale CF and GN have an obvious synergetic reinforcing effect on the mechanical properties and thermal stabilities of thermoplastic composites, which provides an easy and effective way to design and improve the properties of composite materials.     

Fabrication and characterization of three-dimensional PMR polyimide composites reinforced with woven basalt fabric

A PMR polyimide composite reinforced with three-dimensional (3D) woven basalt fabric is fabricated for medium high temperature applications. The PMR polyimide matrix resin is derived from 4,4′-methylenediamine (MDA), diethyl ester of 3,3′,4,4′-oxydiphthalic (ODPE) and monoethyl ester of Cis-5-norbornene-endo-2,3-dicarboxylic acid (NE). The rheological properties of the PMR polyimide matrix resin are investigated. Based on the curing reaction of the PMR type polyimide and the rheological properties, an optimum two-step fabrication method is proposed. The three dimensional fabric preforms are impregnated with the polyimide resin in a vacuum oven at 70 °C for 1 h followed by removing the solvent and pre-imidization. The composites are then consolidated by an optimized molding procedure. Scanning electron microscopy analysis shows that needle shaped voids are generated in yarns and the void volume fraction is 4.27%. The decomposition temperature and the temperature at 5% weight loss of the composite post-cured at 320 °C for 24 h are 440 °C and 577 °C, respectively. The dielectric constant and the dielectric loss of the composite are measured by circular cavity method at 7–12 GHz. The tensile strength and the modulus in the warp direction of the composite are 436 MPa and 22.7 GPa. The composite shows a layer-by-layer fracture mode in three-point bending test. The flexure strength and modulus in the warp direction of the composite are 673 MPa and 27.1 GPa, respectively.     

Microstructure evolution and flow behavior of hot-rolled aluminum – 5% B4C composite

Differential strain rate compression tests were conducted to study flow behavior of hot rolled Al–5 wt% B₄C composite as a function of sample orientation (longitudinal and transverse) over the temperature and strain rate ranges of 25–500 °C and 10⁻⁴ to 1 s⁻¹, respectively. The longitudinal samples are found to show lower flow stress than that shown by the transverse samples in the temperature range of 25–200 °C. The reverse becomes true at higher temperatures of 300–500 °C. The values of stress exponent (n) and activation energy for deformation (Q), based on applied stress, ranged from 10 to 46 and 307–416 kJ/mol, respectively. However, by considering effective stress, these values were reduced to n = 8 and Q = 126–190 kJ/mol. This stress exponent ofn = 8 is further reduced to n = 5 by considering substructural evolution, which suggests the dislocation climb creep mechanism to be favorable for deformation.     

Using finite element and ultrasonic method to evaluate welding longitudinal residual stress through the thickness in austenitic stainless steel plates

This paper uses a 3D thermo-mechanical finite element analysis to evaluate welding residual stresses in austenitic stainless steel plates of AISI 304L. The finite element model has been verified by the hole drilling method. The validated finite element (FE) model is then compared with the ultrasonic stress measurement based on acoustoelasticity. This technique uses longitudinal critically refracted (L_CR) waves that travel parallel to the surface within an effective depth. The residual stresses through the thickness of plates are evaluated by four different series (1 MHz, 2 MHz, 4 MHz and 5 MHz) of transducers. By combining FE and L_CR method (known as FEL_CR method) a 3D distribution of residual stress for the entire of the welded plate is presented. To find the acoustoelastic constant of the heat affected zone (HAZ), a metallographic investigation is done to reproduce HAZ microstructure in a tensile test sample. It has been shown that the residual stresses through the thickness of stainless steel plates can be evaluated by FEL_CR method.     

Effect of manufacturing defects on mechanical properties and failure features of 3D orthogonal woven C/C composites

For high performance 3D orthogonal textile Carbon/Carbon (C/C) composites, a key issue is the manufacturing defects, such as micro-cracks and voids. Defects can be substantial perturbations of the ideal architecture of the materials which trigger the failure mechanisms and compromise strength. This study presents comprehensive investigations, including experimental mechanical tests, micron-resolution computed tomography (μCT) detection and finite element modeling of the defects in the C/C composite. Virtual C/C specimens with void defects were constructed based on μCT data and a new progressive damage model for the composite was proposed. According to the numerical approach, effects of voids on mechanical performance of the C/C composite were investigated. Failure predictions of the C/C virtual specimens under different void fraction and location were presented. Numerical simulation results showed that voids in fiber yarns had the greatest influences on performance of the C/C composite, especially on tensile strength.     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

Evaluation of wear characteristics of Al3Tip/Mg composite

The dry sliding wear tests were performed for a novel developed Al₃Ti_p/Mg composite under the ambient temperatures at 25–200 °C and the loads of 25–150 N. The wear rate of the composite increased with increasing the load, but reduced with increasing the ambient temperature. The Al₃Ti_p/Mg composite had relatively lower wear rates than AZ91D alloy under the loads of less than 100 N at 25 °C. At 200 °C, the Al₃Ti_p/Mg composite presented an absolutely higher wear resistance than AZ91D alloy, and the mild-severe wear transition was delayed. These were attributed to Al₃Ti particulates and the mechanical mixing layer formed on the worn surfaces, which hindered the plastic deformation and thermal softening of the matrix. The mechanical mixing layer contained MgO, Fe–Ti–O, Al₃Ti, Mg₁₇Al₁₂ and Mg and thickened with increasing the ambient temperature. The predominant wear mechanisms of the composite were oxidation wear and delamination wear.     

Mechanical properties and structure of silkworm cocoons: A comparative study of Bombyx mori,Antheraea assamensis,Antheraea pernyi and Antheraea mylitta silkworm cocoons

As a protective shell against environmental damage and attack by natural predators, the silkworm cocoon has outstanding mechanical properties. In particular, this multilayer non-woven composite structure can be exceptionally tough to enhance the chance of survival for silkworms while supporting their metabolic activity. Peel, out-of-plane compression and nano-indentation tests and micro-structure analysis were performed on four types of silkworm cocoon walls (domesticated Bombyx mori, semi-domesticated Antheraea assamensis and wild Antheraea pernyi and Antheraea mylitta silkworm cocoons) to understand the structure and mechanical property relationships. The wild silkworm cocoons were shown to be uniquely tough composite structures. The maximum work-of-fracture for the wild cocoons (A. pernyi and A. mylitta) was approximately 1000 J/m², which was almost 10 times the value for the domesticated cocoon (Bombyx mori) and 3 ~ 4 times the value for the semi-domesticated cocoon (A. assamensis). Calcium oxalate crystals were found to deposit on the outer surfaces of the semi-domesticated and wild cocoons. They did not show influence in enhancing the interlaminar adhesion between cocoon layers but exhibited much higher hardness than the cocoon pelades.     

Application of Taguchi experimental design in optimization of desalination using purified carbon nanotubes as adsorbent

Carbon nanotubes (CNTs) were synthesized by chemical vapor deposition of cyclohexanol and ferrocene in nitrogen atmosphere at 750 °C, and purified by thermal oxidation and acid treatment. Desalination using purified CNTs as adsorbent was performed. Effects of purification (treatment with HNO₃ (8 M), HNO₃/H₂SO₄ (1/1) and HNO₃/H₂SO₄ (1/3)), initial salt concentration (10,000, 20,000 and 30,000 mg/l), temperature (25, 35 and 45 °C) and initial pH (3, 7 and 11) on adsorption uptake (mg/g) of the purified CNTs in desalination process were investigated using Taguchi experimental design (L₉ orthogonal array). Optimum conditions were predicted, and confirmed by carrying out some new experiments. Relative importance of the effects of various factors was evaluated using analysis of variance (ANOVA). Langmuir and Freundlich isotherm models were applied to fit the experimental data.     

Microstructure,mechanical property and corrosion behaviors of interpenetrating C/Mg-Zn-Mn composite fabricated by suction casting

A novel interpenetrating C/Mg-Zn-Mn composite was fabricated by infiltrating Mg-Zn-Mn alloy into porous carbon using suction casting technique. The microstructure, mechanical properties and corrosion behaviors of the composite have been evaluated by means of SEM, XRD, mechanical testing and immersion test. It was shown that the composite had a compact structure and the interfacial bonding between Mg-Zn-Mn alloy and carbon scaffold was very well. The composite had an ultimate compressive strength of (195 ± 15) MPa, which is near with the natural bone (2–180 MPa) and about 150-fold higher than that of the original porous carbon scaffold, and it still retained half of the strength of the bulk Mg-Zn-Mn alloy. The corrosion test indicated that the mass loss percentage of the composite was 52.9% after 30 days′ immersion in simulated body fluid (SBF) at 37 ± 0.5 °C, and the corrosion rates were 0.043 mg/cm²h and 0.028 mg/cm²h after 3 and 7 days′ immersion, respectively. The corrosion products on the composite surface were mainly Mg(OH)₂ and hydroxyapatite (HA).     

Improving Prediction Accuracy of a Rate-Based Model of an MEA-Based Carbon Capture Process for Large-Scale Commercial Deployment

Carbon capture and storage (CCS) technology will play a critical role in reducing anthropogenic carbon dioxide (CO₂) emission from fossil-fired power plants and other energy-intensive processes. However, the increment of energy cost caused by equipping a carbon capture process is the main barrier to its commercial deployment. To reduce the capital and operating costs of carbon capture, great efforts have been made to achieve optimal design and operation through process modeling, simulation, and optimization. Accurate models form an essential foundation for this purpose. This paper presents a study on developing a more accurate rate-based model in Aspen Plus^® for the monoethanolamine (MEA)-based carbon capture process by multistage model validations. The modeling framework for this process was established first. The steady-state process model was then developed and validated at three stages, which included a thermodynamic model, physical properties calculations, and a process model at the pilot plant scale, covering a wide range of pressures, temperatures, and CO₂ loadings. The calculation correlations of liquid density and interfacial area were updated by coding Fortran subroutines in Aspen Plus^®. The validation results show that the correlation combination for the thermodynamic model used in this study has higher accuracy than those of three other key publications and the model prediction of the process model has a good agreement with the pilot plant experimental data. A case study was carried out for carbon capture from a 250 MW_e combined cycle gas turbine (CCGT) power plant. Shorter packing height and lower specific duty were achieved using this accurate model.