Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

Novel load responsive multilayer insulation with high in-atmosphere and on-orbit thermal performance

Aerospace cryogenic systems require lightweight, high performance thermal insulation to preserve cryopropellants both pre-launch and on-orbit. Current technologies have difficulty meeting all requirements, and advances in insulation would benefit cryogenic upper stage launch vehicles, LH₂ fueled aircraft and ground vehicles, and provide capabilities for sub-cooled cryogens for space-borne instruments and orbital fuel depots. This paper reports the further development of load responsive multilayer insulation (LRMLI) that has a lightweight integrated vacuum shell and provides high thermal performance both in-air and on-orbit.LRMLI is being developed by Quest Product Development and Ball Aerospace under NASA contract, with prototypes designed, built, installed and successfully tested. A 3-layer LRMLI blanket (0.63 cm thick, 77 K cold, 295 K hot) had a measured heat leak of 6.6 W/m² in vacuum and 40.6 W/m² in air at one atmosphere. In-air LRMLI has an 18× advantage over Spray On Foam Insulation (SOFI) in heat leak per thickness and a 16× advantage over aerogel. On-orbit LRMLI has a 78× lower heat leak than SOFI per thickness and 6× lower heat leak than aerogel.The Phase II development of LRMLI is reported with a modular, flexible, thin vacuum shell and improved on-orbit performance. Structural and thermal analysis and testing results are presented. LRMLI mass and thermal performance is compared to SOFI, aerogel and MLI over SOFI.     

Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.     

Microstructures and thermal properties of A356/SiCp composites fabricated by liquid pressing method

A356/45vol.%SiCp composites with a uniform distribution of SiC particles have been fabricated by a liquid pressing method. Increasing the melt temperature, holding time and pre-treatment of SiCp by thermal oxidation improves the soundness of composites for the liquid pressing method. The sound composites exhibited low coefficient of thermal expansion (8 ppm/K) and high thermal conductivity (155 W/m K). The measured values for coefficient of thermal expansion agree well with the predicted values based on Turner’s model irrespective of porosity. The measured values for thermal conductivity decrease with porosity, and the effect of pore on the thermal conductivity has been evaluated based on the modified Hasselman–Johnson model.     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Processing of Al/SiC composites in continuous solid–liquid co-existent state by SPS and their thermal properties

Silicon carbide (SiC)-particle-dispersed-aluminum (Al) matrix composites were fabricated in a unique fabrication method, where the powder mixture of SiC, pure Al and Al–5mass% Si alloy was uniquely designed to form continuous solid–liquid co-existent state during spark plasma sintering (SPS) process. Composites fabricated in such a way can be well consolidated by heating during SPS processing in a temperature range between 798 K and 876 K for a heating duration of 1.56 ks. Microstructures of the composites thus fabricated were examined by scanning electron microscopy and no reaction was detected at the interface between the SiC particle and the Al matrix. The relative packing density of the Al–matrix composite containing SiC was higher than 99% in a volume fraction range of SiC between 40% and 55%. Thermal conductivity of the composite increased with increasing the SiC content in the composite at a SiC fraction range between 40 vol.% and 50 vol.%. The highest thermal conductivity was obtained for Al–50 vol.% SiC composite and reached 252 W/mK. The coefficient of thermal expansion of the composites falls in the upper line of Kerner’s model, indicating strong bonding between the SiC particle and the Al matrix in the composite.     

Thermal conductivity of Cu/diamond composites prepared by a new pretreatment of diamond powder

Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the surface pretreatment of the diamond powders, in which the diamond particles were mixed with copper powder and tungsten powder (carbide forming element W). The effects of the pretreatment temperature and the diamond particle size on the thermal conductivity of diamond/copper composites were investigated. It was found that when 300 μm diamond particles and Cu–5 wt.% W were mixed and preheated at 1313 K, the composites has a relatively higher density and its thermal conductivity approaches 672 W (m K)⁻¹.     

Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route

Silicon carbide (SiC) interphase was introduced by chemical vapor deposition (CVD) process to prevent carbon fiber degradation and improve fiber–matrix interface bonding of C/ZrC composite prepared via precursor infiltration and pyrolysis (PIP) process. Moderate thickness of SiC interphase in fiber bundles could increase the density of the composite, but when the thickness of SiC interphase was over 0.5 μm, more close pores formed and the density of the composite decreased. The SiC interphase could protect carbon fiber effectively from carbo-thermal reduction, but could not enhance the mechanical properties of C/ZrC composite. The flexural strength and fracture toughness of C/ZrC composites with 0.05 μm thickness SiC layer were 252 MPa and 13.6 MPa m^1/2, and for those with 0.5 μm thickness SiC layer 240 MPa and 12.8 MPa m^1/2, both close to the value of the composite without SiC interphase (254 MPa and 14.5 MPa m^1/2), while those with 0.7 μm thickness SiC layer were only 191 MPa and 10.8 MPa m^1/2, respectively. Moderate content of SiC interphase could improve the ablation property of C/ZrC composites; however excessive content of SiC interphase would decrease the ablation property.     

Enhanced thermal-conductive and anti-dripping properties of polyamide composites by 3D graphene structures at low filler content

In this work, 3D graphene structures constructed by graphene foam (GF) were introduced into polyamide-6 (PA6) matrix for the purpose of enhancing the thermal-conductive and anti-dripping properties of PA6 composites. The GF were prepared by one-step hydrothermal method. The PA6 composites were synthesized by in-situ thermal polycondensation method to realize PA6 chains covalently grafted onto the graphene sheets. The 3D interconnected graphene structure favored the formation of the consecutive thermal conductive paths or networks even at relatively low graphene loadings. As a result, the thermal conductivity was improved by 300% to 0.847 W·m⁻¹·K⁻¹ of PA6 composites at 2.0 wt% graphene loading from 0.210 W·m⁻¹·K⁻¹ of pure PA6 matrix. The presence of self-supported 3D structure alone with the covalently-grafted PA6 chains endowed the PA6 composites good anti-dripping properties.     

Effects of polymer derived SiC interphase on the properties of C/ZrC composites

Polymer derived silicon carbide (SiC) interphase was introduced by precursor infiltration and pyrolysis (PIP) to prevent carbon fiber erosion and to improve the fiber–matrix interface bonding of C/ZrC composites prepared by PIP. Introducing SiC interphase increased the density of the composites. The SiC interphase not only protected carbon fibers effectively from erosion by carbo-thermal reduction, but also enhanced the mechanical properties of C/ZrC composites by strengthening the interface bond. The flexural strength and fracture toughness of C/ZrC composites with SiC interphase prepared by two PIP cycles were 319 MPa and 18.8 MPa m^1/2 respectively. The ablation properties of C/ZrC composites were with rising content of SiC interphase but then decreased when excessive. The mass loss rate and the linear recession rate of the C/ZrC composites with SiC interphase prepared by one PIP cycle were 0.0079 g/s and 0.0084 mm/s, respectively.     

In situ joining of titanium to SiC/Al composites by low pressure infiltration

A method of in situ joining of titanium to SiC/Al composites by low pressure infiltration was proposed. The effect of infiltration temperature on microstructure and bending strength of in situ joining composites was investigated and the best infiltration temperature was confirmed to be 710 °C. The interfacial region of SiC/Al/Ti composites was consisted of Ti substrate, Al–Ti interfacial layer, Al layer and SiC/Al composite. The bending strength of SiC/Al composites kept nearly constant as the infiltration temperature changed while that of SiC/Al/Ti composites was influenced significantly by the infiltration temperature. The fracture occurred at the Al–Ti and Al–SiC/Al interfaces alternately as infiltrated at 670 °C. But as the infiltration temperature was increased to 710 °C, the fracture occurred only at the Al–SiC/Al interface which shows a great interfacial bonding at the Al–Ti interface. The formation of Al–Ti brittle intermetallics and the effect of crystallization and grain coarsening are two possible reasons which lead to the decrease of bending strength when the infiltration temperatures were increased from 710 °C to 730 °C.     

The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints

Silica carbide modified carbon cloth laminated C–C composites have been successfully joined to lithium–aluminum–silicate (LAS) glass–ceramics using magnesium–aluminum–silicate (MAS) glass–ceramics as interlayer by vacuum hot-press technique. The microstructure, mechanical properties and fracture mechanism of C–C/LAS composite joints were investigated. SiC coating modified the wettability between C–C composites and LAS glass–ceramics. Three continuous and homogenous interfaces (i.e. C–C/SiC, SiC/MAS and MAS/LAS) were formed by element interdiffusions and chemical reactions, which lead to a smooth transition from C–C composites to LAS glass–ceramics. The C–C/LAS joints have superior flexural property with a quasi-ductile behavior. The average flexural strength of C–C/LAS joints can be up to 140.26 MPa and 160.02 MPa at 25 °C and 800 °C, respectively. The average shear strength of C–C/LAS joints achieves 21.01 MPa and the joints are apt to fracture along the SiC/MAS interface. The high retention of mechanical properties at 800 °C makes the joints to be potentially used in a broad temperature range as structural components.     

Synergistic effect of hybrid graphene nanoplatelet and multi-walled carbon nanotube fillers on the thermal conductivity of polymer composites and theoretical modeling of the synergistic effect

We found that the thermal conductivity of polymer composites was synergistically improved by the simultaneous incorporation of graphene nanoplatelet (GNP) and multi-walled carbon nanotube (MWCNT) fillers into the polycarbonate matrix. The bulk thermal conductivity of composites with 20 wt% GNP filler was found to reach a maximum value of 1.13 W/m K and this thermal conductivity was synergistically enhanced to reach a maximum value of 1.39 W/m K as the relative proportion of MWCNT content was increased but the relative proportion of GNP content was decreased. The synergistic effect was theoretically estimated based on a modified micromechanics model where the different shapes of the nanofillers in the composite system could be taken into account. The waviness of the incorporated GNP and MWCNT fillers was found to be one of the most important physical factors determining the thermal conductivity of the composites and must be taken into consideration in theoretical calculations.     

The thermal and mechanical properties of graphite foam/epoxy resin composites

A series of epoxy resin (EP) filled graphite foam (GF) composites were prepared to explore a new material with good heat transfer property. The effects of the mass fraction of EP and the network structure of GFs on the thermal diffusivity and the compression strength of the composites were investigated. The thermal diffusivity of the GF/EP composite with EP mass fraction of 91.45% was raised to 6.541 mm²/s, which was 45.7 times higher than the pure EP. The thermal conductivity reached to 14.67 W/(m K), which was 43.1 times higher than the pure EP. The compression strength of the GF/EP increased 55% above the value of pure EP. In addition, the thermal diffusivity of GF/EP increased with the decrease of the mass fraction of EP. A model was formulated to calculate the pressure needed for a mass fraction of EP.     

The effect of zirconium addition on the microstructure and properties of chopped carbon fiber/carbon composites

Carbon/carbon composites containing zirconium were prepared using chopped carbon fiber, mesophase pitch and Zr powder by the traditional process including molding, carbonization, densification and graphitization. The influence of Zr on the microstructure and properties of the composites were investigated. Results show that Zr can improve the interface bonding, promote more perfect and larger crystallites and enhance the conductive/mechanical properties of the composites. The high in-plane thermal conductivity of 464 W/(m K) and excellent bending strength of 83.6 MPa was obtained for a Zr content of 13.9 wt% at heat treatment temperature(HTT) of 2500 °C. However the conductive/mechanical properties of the composites decrease dramatically for an higher HTT of 3000 °C. SEM micrograph of the fracture surface for the composites shows that lower disorder crystallite arrangement of fiber and carbon matrix come into being in the composites during HTT of 3000 °C, which should be responsible for the low properties. Correlation between the content of Zr and the microstructure and properties are discussed.     

Effect of a weak fiber interface coating in ZrB2 reinforced with long SiC fibers

This study reports the microstructural analysis and mechanical properties of a ZrB₂ ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB₂ materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m², compared with 20–30 J/m² of unreinforced ZrB₂ or ZrB₂ containing chopped fibers.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Preparation of SiC ceramics by aqueous gelcasting and pressureless sintering

Gelcasting is an attractive forming process to fabricate ceramic parts with complex shape. In the present work, aqueous gelcasting of SiC was studied. SiC slurry (50 vol.%) for gelcasting was prepared with sintering assistants, Al₂O₃ and Y₂O₃. The slurry was solidified in situ to green body with relative density of 55.9 ± 0.9% and flexural strength of 13.9 ± 0.7 MPa. SEM shows that ceramic powders in green body compact closely by the connection of polymer networks, and that the pores decrease greatly with the size less than 1 μm. SiC samples were also obtained by the process of gelcasting and pressureless sintering at 2000 °C for 1 h in Ar atmosphere. The relative density and flexural strength of SiC sintered body are 97.3 ± 0.4% and 637 ± 156 MPa, and the hardness and toughness are 20.68 ± 0.80 GPa and 3.85 ± 0.23 MPa m^1/2, respectively.