Analysis of carbon fiber structure based on dynamic laser Raman spectroscopy

The in-plane modulus and inter layer shear modulus are the intrinsic properties determining the carbon fiber's characterization, but could not be gotten for a single sample. With the addition of dynamic Raman spectra, the parameters are obtained from five carbon fibers individually, which interpret perfectly that the Young's modulus and conductivities are not consistent according to the normal views. The mesophase pitch-based graphitized carbon fiber has thermal conductivity of 635 Wm⁻¹ K⁻¹, much higher than 78 Wm⁻¹ K⁻¹ of the polyacrylonitrile graphitized carbon fiber, even the modulus of the latter is higher than the former. The Young's modulus of polyacrylonitrile carbon fiber and rayon-based graphitized carbon fiber are 260 and 140 GPa; however, their thermal conductivities are lower than 1.0 Wm⁻¹ K⁻¹. With these two intrinsic properties, the structure of a carbon fiber can be understood clearly, so that the properties can be enhanced accordingly.     

Mechanical anisotropy in oriented linear polyethylene of various crystallinities

We have studied the mechanical moduli of oriented linear polyethylene with crystallinities X varying from 0.44 to 0.63 and draw ratios λ = 1–9 by using a dynamic tensile method at 10 Hz and an ultrasonic technique at 10 MHz. Wide-angle X-ray diffraction and birefringence measurements reveal that the chains in the crystalline regions are fully aligned at λ > 4, but the degree of amorphous orientation increases steadily up to the highest draw ratio. From −180°C to the β relaxation region (near 0°C at 10 Hz) the mechanical behavior at all crystallinities is controlled by three factors: molecular orientation, weak c-shear deformation and stiffening effect of taut tie molecules. At low temperature the chain alignment in an oriented sample gives rise to an axial Young's modulus E₀ which is much larger than the transverse Young's modulus E₉₀, with the modulus for the undrawn material lying in-between. However, the results that E₄₅ < E₉₀ and C₄₄ (axial shear modulus) < C₆₆ (transverse shear modulus) imply that a weak c-shear process occurs even at low temperature. At the β relaxation where the amorphous regions are rubbery, the stiffening effect of taut tie molecules becomes prominent and leads to increases in all moduli upon drawing. For the polyethylene with the lowest cyrstallinity a strong c-shear process is activated at the α relaxation (about 50°C at 10 Hz), which gives rise to very low values of C₄₄ and E₄₅. This effect becomes weaker with increasing crystallinity and is hardly observable at X > 0.6.     

Elastic moduli of a liquid crystalline polymer and its in-situ composites

The elastic moduli of a liquid crystalline polyesteramide (LCP) and polycarbonate/LCP in-situ composites with 10 to 80 wt% of LCP have been measured as functions of draw ratio λ from 1 to 15 by an ultrasonic method. For the LCP, the sharp rise of the axial Young's modulus E₃ and the slight decreases of the transverse Young's modulus E₁ and the axial (C₄₄) and transverse (C₆₆) shear modulus with increasing λ result from the alignment of chains along the draw direction. E₁, C₄₄, and C₆₆ follow the lower bounds calculated using the series coupling scheme of the aggregate model. Although E₃ lies close to the lower bound at low λ, it follows the upper bound calculated according to the parallel coupling scheme at λ > 3. The elastic moduli of the composites have similar draw ratio dependences as those of the LCP. The strong increase in E₃ with increasing λ arises from the higher aspect ratio of the LCP domains in the composites and the improved molecular orientation within the domains. The reinforcement effect on the other moduli is much weaker, with E₁ and C₄₄ of the composites only 5 to 30% higher than those of polycarbonate at λ = 15. Since C₆₆ of the LCP decreases to a value below that of polycarbonate at λ > 2, there is a positive reinforcement effect at low λ but a negative effect at high λ.     

Elastic moduli and thermal conductivity of injection-molded short-fiber–reinforced thermoplastics

The nine independent stiffness constants of injection-molded tensile bars of poly(phenylene sulfide) reinforced with 30 and 40% by weight of carbon or glass fibers have been measured by ultrasonic techniques. The thermal conductivities along the three principal directions of these thermoplastic composites have also been determined by the laser-flash radiometry method. The elastic moduli (tensile and shear) and thermal conductivity increase with increasing fiber volume fraction, v_f, with the tensile modulus and thermal conductivity along the mold flow direction showing the greatest change. For a composite, containing 40 weight % of carbon fibers, the Young's modulus and thermal conductivity along this direction exceed those of the polymer matrix by a factor of 8. Using the known values of v_f and the observed aspect ratio and orientation factor of the fibers, the elastic moduli and thermal conductivity have been calculated on the basis of the laminate theory. The agreement between theoretical predictions and experimental data is better than 10% on the average.     

Thermal conductivity and thermal expansivity of in situ composites of a liquid crystalline polymer and polycarbonate

The thermal conductivity and thermal expansivity of extruded blends of a liquid crystalline polymer (LCP) and polycarbonate (PC) with volume fraction (V_f) of LCP between 0.09 and 0.8 have been measured as functions of draw ratios λ ranging from 1.3 to 15. At V_f < 0.3, the LCP domains are dispersed in a PC matrix and the aspect ratio of the domains increases with increasing λ. At V_f > 0.55, phase inversion has occurred and the LCP becomes the continuous phase. The axial thermal conductivity K_∥ increases while the axial expansivity α_∥ decreases sharply with increasing λ, as a result of the higher aspect ratio of the LCP fibrils and the improved molecular orientation within the fibrils. Since the transverse thermal conductivity and expansivity are little affected by drawing, the blends exhibit strong anisotropy in the thermal conduction and expansion behavior at high λ. At V_f < 0.3, the behavior of K_∥ is reasonably modeled by the Halpin-Tsai equation for short fiber composites. At high draw ratio (λ = 15), all the blends behave like unidirectional continuous fiber composites, so K_∥ and α_∥ follow the rule of mixtures and the Schapery equation, respectively.     

Unidirectional fiber–polymer composites: Swelling and modulus anisotropy

The solvent swelling of unidirectional rubber–fiber composites was studied. The amount of matrix swelling was constrained to the extent that would be predicted from the thermodynamic theories of elasticity and polymer–solvent interaction. The geometry of swelling was found to be orthotropic in nature. A simple trigonometric function was derived to relate linear deformation due to swelling to the angle which the direction of its measurement makes with the fiber direction. The validity of the derivation was demonstrated experimentally. Considering swelling to be the imposition of tensile forces of equal magnitude in all directions, and considering a swelling-induced linear deformation to be analogous to a tensile compliance, a simple set of relationships between elastic parameters and their direction of measurement was derived: where E_θ, G_θ, v_θ, and η_θ are Young's modulus, shear modulus, Poisson's ratio, and the shear coupling ratio measured in a longitudinal transverse plane at an angle with the fiber direction, respectively, and E_L, G_LT, and θ_LT are the longitudinal Young's modulus, the longitudinal transverse shear modulus, and the longitudinal transverse Poisson ratio, respectively. Further simplifying the case of combined transverse isotropy and special orthotropy was the conclusion that 1/G_LT = 1/E_T + (1 + 2v_LT)/E_L. The relationships for G and E were experimentally demonstrated.     

Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

The effect of aspect ratio of inclusions on the elastic properties of unidirectionally aligned composites

This paper examines the influence of aspect ratio α, from zero to infinity, on the effective elastic moduli of a transversely isotropic composite. The reinforcing inclusions, which could be flakes or short fibers, are assumed to be spheroidal and unidirectionally aligned. Of the five independent elastic constants, the longitudinal Young's modulus E₁₁ and in-plane shear modulus μ₁₂ appear to increase with increasing aspect ratio, while the transverse Young's modulus E₂₂, out-plane shear modulus μ₂₃, and plane-strain bulk modulus K₂₃, generally decrease. It is further noted that E₁₁ is more sensitive to α when α > 1 but the others are more so when α < 1. The present analysis was carried out by the combination of Eshelby's and Mori-Tanaka's theories of inclusions.     

Interaction of melt spinning and drawing variables on the crystalline morphology and mechanical properties of high-density and low-density polyethylene fiber

An experimental study of the spinnability and the variation in crystallinity and orientation in high-density and low-density polyethylene fibers with melt spinning and drawing conditions has been carried out. Three polymers (two high-density and one low-density) and eicosane (C₂₀H₄₂) were studied. The maximum spinnability was in the lower molecular weight high-density polyethylene. Hermans-Stein a, b, and c crystallographic axis orientation factors were computed from wide-angle x-ray scattering patterns. In the spun fiber, small take-up velocities cause the b axis to become perpendicular to the fiber axis in each fiber. The c axis increasingly orients itself parallel to the fiber axis as take-up velocity increases. The a axis orientation is different for each polymer. The results are interpreted in terms of modern theories of crystalline morphology, specifically the development of row structures. In the drawing experiments, the two high-density polyethylenes necked. A phenomenological theory of necking is discussed. The a, b, and c axis orientation factors were determined for different stages of drawing. In the necked regions and in completely drawn fibers, the c axis was parallel to the fiber axis and the a and b axes are perpendicular to the fiber axis. The tangent Young's modulus and tensile strength of the spun fibers increased with take-up velocity and in the drawn fibers were an order of magnitude higher than in the spun fiber. The mechanical properties of spun fiber may be correlated with the c axis (Hermans) orientation factor. The drawn fiber shows significant variations in Young's modulus and tensile strength at constant unit cell orientation.     

In-plane thermal expansion behavior of dense ceramic matrix composites containing SiBC matrix

In order to reveal the effect of matrix cracks resulted from thermal residual stresses (TRS) on the thermal expansion behavior of ceramic matrix composites, SiBC matrix was introduced into C_f/SiC and SiC_f/SiC by liquid silicon infiltration. The TRS in both two composites were enlarged with incorporating SiBC matrix which has higher coefficients of thermal expansion (CTEs) than SiC matrix. Due to the relatively high TRS, matrix cracks and fiber/matrix (f/m) debonding exist in C_f/SiC-SiBC, which would provide the space for the expansion of matrix with higher CTEs. For SiC_f/SiC, no matrix cracking and f/m debonding took place due to the close CTEs between fiber and matrix. Accordingly, with the incorporation of SiBC matrix, the in-plane CTE of C_f/SiC between room temperature to 1100 °C decreases from 3.65 × 10⁻⁶ to 3.19 × 10⁻⁶ K^-1, while the in-plane CTE of SiC_f/SiC between room temperature to 1100 °C increases slightly from 4.97 × 10⁻⁶ to 5.03 × 10⁻⁶ K^-1.     

SiC fiber reinforced geopolymer composites,part 2: Continuous SiC fiber

In this paper, unidirectional SiC fiber (SiC_f) reinforced geopolymer composites (SiC_f/geopolymer) were prepared and effects of fiber contents on the microstructure and mechanical properties of the composites in different directions were investigated. The XRD results showed that addition of SiC_f retarded geopolymerization process of geopolymer matrix by weakening the typical amorphous hump. SiC_f in all the composites were well infiltrated by geopolymer matrix, but microcracks which were perpendicular to the fiber axial direction were noted in the interface area due to the thermal shrinkage of matrix during the curing process. With the increases in fiber contents, although Young's modulus of the composites increased continuously, flexural strength, fracture toughness and work of fracture increased at first, reached their peak values and then decreased. And when fiber content was 20 vol%, the composites showed the highest flexural strength, fracture toughness and work of fracture, which were 14.2, 15.2 and 81.6 times as high as those of pristine geopolymer, respectively, indicating significant strengthening and toughening effects from SiC_f. Meanwhile, SiC_f/geopolymer composites failed in different failure modes in the different directions, i.e., tensile failure mode in the x direction (in-plane and perpendicular to the fiber axial direction) and shear failure mode in the z direction (laminate lay-up direction).     

Synergistic modification of properties in Ba[(Zn0.8Mg0.2)1/3Nb2/3]O3 ceramics through ordered domain engineering

The ordered domain engineering was investigated for Ba[(Zn_0.8Mg_0.2)_1/3Nb_2/3]O₃ microwave dielectric ceramics to synergistically modify the physical properties especially the temperature coefficient of resonant frequency τ_f and quality factor Q value together with the thermal conductivity. The ordered domain structure could be tailored and controlled by the post-densification annealing, and the fine ordered domain structures with high ordering degree and low-energy domain boundary were obtained in the present ceramics annealed around 1400°C for 24 h, where the Qf value was improved from 51 000 to 118 000 GHz, τ_f was suppressed from 30 to 25.5 ppm/°C. Moreover, the thermal conductivity at room temperature was increased from 3.79 to 4.30 W m⁻¹ K⁻¹, and the Young's modulus was improved from 98 to 214 GPa. The present work provided a promising approach for synergistic modification of physical properties in Ba-based complex perovskite microwave dielectric ceramics.     

Microstructure and mechanical properties of quasi-isotropic chopped fiber-reinforced PyC–SiC matrix composite prepared by novel nondamaging method

In this work, quasi-isotropic chopped carbon fiber-reinforced pyrolytic carbon and silicon carbide matrix (C_f/C–SiC) composites and chopped silicon carbide fiber-reinforced silicon carbide matrix (SiC_f/SiC) composites were prepared via novel nondamaging method, namely airlaid process combined with chemical vapor infiltration. Both composites exhibit random fiber distribution and homogeneous pore size. Young's modulus of highly textured pyrolytic carbon (PyC) matrix is 23.01 ± 1.43 GPa, and that of SiC matrix composed of columnar crystals is 305.8 ± 9.49 GPa in C_f/C–SiC composites. Tensile strength and interlaminar shear strength of C_f/C–SiC composites are 52.56 ± 4.81 and 98.16 ± 24.62 MPa, respectively, which are both higher than those of SiC_f/SiC composites because of appropriate interfacial shear strength and introduction of low-modulus and highly textured PyC matrix. Excellent mechanical properties of C_f/C–SiC composites, particularly regarding interlaminar shear strength, are due to their quasi-isotropic structure, interfacial debonding, interfacial sliding, and crack deflection. In addition to the occurrence of crack deflection at the fiber/matrix interface, crack deflection in C_f/C–SiC composites takes also place at the interface between PyC–SiC composite matrix and the interlamination of multilayered PyC matrix. Outstanding mechanical properties of as-prepared C_f/C–SiC composites render them potential candidates for application as thermal structure materials under complex stress conditions.     

Degradation of the mechanical properties of composite vulcanizates loaded with paraffin wax

A composite of Styrene Butadiene Rubber (SBR) and Natural Rubber (NR) loaded with 40 phr of High Abrasion Furnace (HAF) carbon black is loaded with different concentrations of paraffin wax. From the stress–strain curves, Young's modulus was found to decrease with increasing the amount of added paraffin wax. The modified Mooney-Revlin equation was used to calculate the parameters C₁ and C₂. A plot between the true tensile stress as a function of (λ² − λ⁻¹) was used to calculate two parameters σ₀ and G, and then both the average molecular weight between crosslinks and the number of effective plastic chains per unit volume were also calculated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2265–2270, 2001     

First-principles study of thermophysical properties of polymorphous YTaO4 ceramics

Yttrium tantalate ceramics with ferroelasticity are potential candidates for thermal barrier coating (TBC) ceramics. During the phase transition process, there are three main phases with monoclinic (I2/a), monoclinic-prime (P2/a), and tetragonal structures (I₄₁/a), and a comprehensive understanding of their thermophysical properties is required. In this study, the thermal and mechanical properties of polymorphous yttrium tantalate (YTaO₄) ceramics are systematically investigated under finite temperature by performing first-principles calculations combined with quasi-harmonic approximation. The first-principle study results show that the volume change from M' to T phase is 12.85 Å³ to 12.95 Å³ per atom, whereas the T to M is 12.95 Å³ to 12.84 Å³ per atom, and the change is less than 1%, showing that this process produces almost no volume change. However, the thermal expansion coefficients (TECs) and Young's modulus vary greatly, the TECs value of M YTaO₄ is about 11.13 × 10⁻⁶ K⁻¹, which is smaller than T YTaO₄ as the value 12.01 × 10⁻⁶ K⁻¹, and the Young's modulus values of M, M', and T phases are 140.34, 156.68, and 123.29 GPa, respectively. Lastly, the calculated O–Ta bond is stronger than the O–O and O–Y bonds according to the mean bond population and average bond length, resulting in a higher modulus. This work will not only expand the internal mechanism of the thermophysical properties of YTaO₄, but also provides support for the design and application of TBC systems.     

Elevated temperature strength and thermal shock behavior of hot-pressed carbon fiber reinforced TiC composites

In order to improve the elevated strength and thermal shock resistance of TiC materials, 20vol% short carbon fiber-reinforced TiC composite (C_f/TiC) was produced by hot pressing. With carbon fiber addition, the strength and fracture toughness of TiC is increased remarkably, and the elastic modulus and thermal expansion coefficient are decreased. The strength value of C_f/TiC composite is 593 MPa at room temperature and 439 MPa at 1400°C, and the fracture toughness value at room temperature is 6.87 MPa m^1/2. The thermal stress fracture resistance parameter, R, thermal stress damage resistance parameter, R^IV, and thermal stress crack stability parameter, R_st, are all increased. The residual strength decreases significantly when the thermal shock temperature difference, ΔT, is higher than 900°C, and the residual strength is 252 MPa when ΔT is 1400°C. Carbon fiber reinforced-TiC composite exhibits superior resistance to thermal shock damage compared with monolithic TiC. The catastrophic failure induced by severe thermal stresses can be prevented in C_f/TiC composite.     

Mechanical and thermal properties of light weight boron-mullite Al5BO9

Al₅BO₉ is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al₅BO₉ have not been carried out. Herein, we report the mechanical and thermal properties of bulk Al₅BO₉ prepared by spark plasma sintering of solid-state reaction synthesized Al₅BO₉ powders. The bulk (B), shear (G), and Young's (E) moduli are 148 GPa, 85 GPa, and 214 GPa, respectively, which are close to the theoretical values. The Pugh's ratio G/B is 0.574, indicating its intrinsic damage tolerance, which is also revealed by Hertzian contact test. The Vickers hardness (H_v) is 10.8 GPa, being lower than mullite. The flexural strength, compressive strength, and fracture toughness are, respectively, 277 ± 35 MPa, 814 ± 75 MPa, and 2.4 ± 0.3 MPa·m^1/2, which are close to those of mullite. Al₅BO₉ has anisotropic coefficient of thermal expansion (CTE) in three crystallographic directions, ie α_a = (4.40 ± 0.21) × 10⁻⁶ K⁻¹, α_b = (7.11 ± 0.18) × 10⁻⁶ K⁻¹, α_c = (6.70 ± 0.29) × 10⁻⁶ K⁻¹ from Debye temperature to 1473 K, which are underpinned by its structural feature, ie lower α_a is resulted from the edge-shared AlO₆ octahedron chains along the [100] direction. The average CTE is (6.05 ± 0.06) × 10⁻⁶ K⁻¹. The thermal conductivity declines with temperature as κ = 1336.39/T + 1.97, consisting with predicted trend from Slack's model. The low thermal conductivity and low density guarantee Al₅BO₉ a promising candidate as ceramic wafer in the seal structure for hypersonic vehicles.     

Processing and mechanical performance of 3D Cf/SiCN composites prepared by polymer impregnation and pyrolysis

The processing of 3D carbon fiber reinforced SiCN ceramic matrix composites prepared by polymer impregnation and pyrolysis (PIP) route was improved, and factors that determined the mechanical performance of the resulting composites were discussed. 3D C_f/SiCN composites with a relative density of ∼81% and uniform microstructure were obtained after 6 PIP cycles. The optimum bending strength, Young's modulus and fracture toughness of the composites were 75.2 MPa, 66.3 GPa and 1.65 MPa m^1/2, respectively. The residual strength retention rate of the as-pyrolyzed composites was 93.3% after thermal shock test at ΔT = 780 °C. It further degraded to 14.6% when the thermal shock temperature difference reached to 1180 °C. The bending strength of the composites was 35.6 MPa after annealing at 1000 °C in static air. The deterioration of the bending strength should be attributed to the strength degradation of carbon fibers and decomposition of interfacial structure.     

Crystal structure,mechanical and thermal properties of Yb4Al2O9: A combination of experimental and theoretical investigations

The key requirements for a successful thermal and environmental barrier coating (T/EBC) material include stability in high temperature water vapor, low Young's modulus, close thermal expansion coefficient (TEC) with mullite, low thermal conductivity and weak mechanical anisotropy. The current prime candidates for top coat are ytterbium silicates (Yb₂SiO₅ and Yb₂Si₂O₇). A major weakness of these two silicates is the severe anisotropy in mechanical properties and thermal expansion that would lead to cracking of the coating. Thus, searching for new materials with weak mechanical and thermal anisotropy is of signification. In this work, the crystal structure, mechanical and thermal properties of a promising T/EBC candidate, Yb₄Al₂O₉, are investigated theoretically and experimentally. Good ductility, low shear deformation resistance, low Young's modulus (151 GPa) and low thermal conductivity (0.78 W m⁻¹ K⁻¹) is underpinned by heterogeneous bonding characteristic and distortion of the structure. Close TEC (6.27 × 10⁻⁶ K⁻¹) with mullite and weak mechanical anisotropy highlight the suitability of Yb₄Al₂O₉ as a prospective T/EBC.