Preparation by tape casting and hot pressing of copper carbon composites films

During this last decade, the use of metal matrix composites (MMCs) such as AlSiC or CuW for heat dissipation in microelectronic devices has been leading to the improvement of the reliability of electronic power modules. Today, the continuous increasing complexity, miniaturization and density of components in modern devices requires new heat dissipating films with high thermal conductivity, low coefficient of thermal expansion (CTE), and good machinability. This article presents the original use of copper carbon composites, made by tape casting and hot pressing, as heat dissipation materials. The tape casting process and the sintering have been adapted and optimised to obtain near fully dense, flat and homogeneous Cu/C composites.A good electrical contact between carbon fibres and copper matrix and a low porosity at matrix/copper interfaces allow obtaining a low electrical resistivity of 3.8 μΩ cm⁻¹ for 35 vol.% carbon fibre (electrical resistivity of copper = 1.7 μΩ cm⁻¹). The CTE and the thermal conductivity are strongly anisotropic due to the preferential orientation of carbon fibres in the plan of laminated sheets. Values in the parallel plan are, respectively, 9 × 10⁻⁶ °C⁻¹ and 160–210 W m⁻¹ K⁻¹ for 40 vol.% fibres. These CTE and thermal conductivity values are in agreement with the thermo-elastic Kerner's model and with the Hashin and Shtrikman model, respectively.     

A theoretical quantification of the possible improvement in the mechanical properties of carbon nanotube bundles by carbon ion irradiation

Improvement of single wall carbon nanotube (CNT) bundle mechanical properties through carbon ion irradiation is investigated using molecular dynamics. Increased inter-tube shear and toughness properties through formation of inter-tube cross-links is balanced against decreased tensile strength from induced defects. Bundles irradiated with carbon ions with energy 50–300 eV/ion, and fluence between 4 × 10¹³ cm⁻² and 2 × 10¹⁴ cm⁻², are mechanically tested. We find that with careful control of irradiation parameters, shear and toughness parameters increase by an order of magnitude, while tensile properties reduce by only 30–40%; in real CNT fibres with discontinuous CNT filaments the reduction would be much less. The nano-scale interface response resembles that of micro-scale composites, in which interstitial C atoms play a key role. This makes C ion deposition an attractive option over irradiation by electrons or other types of ions, since the extra C atoms can provide the required interstitial atoms. Within a certain cross-link density range, the interface shear modulus, shear stress at bonding onset, and frictional sliding stress after debonding are all linearly related to cross-link density making controlled design of fibre shear properties feasible. A possible post-treatment with very low energy irradiation is proposed for healing holes and partially restoring tensile strength.     

Synthesis and mechanical properties of novel composites of inorganic polymers (geopolymers) with unidirectional natural flax fibres (phormium tenax)

This study reports the synthesis and mechanical properties of new inorganic polymer (geopolymer) composites unidirectionally reinforced with 4–10 vol.% natural cellulose-based fibres (NZ flax, phormium tenax). The geopolymer matrix was derived from dehydroxylated kaolinite-type clay. The mechanical properties of the fibre-reinforced composites improve with increasing fibre content, achieving ultimate flexural strengths of about 70 MPa at 10 vol.% fibre content. This represents a significant improvement on the flexural strength of the unreinforced geopolymer matrix (about 5.8 MPa), and all the composites show graceful failure, unlike the brittle failure of the matrix. Scanning electron microscopy was used to study the morphology of the fibre-matrix regions and a combination of thermogravimetric analysis (TGA) and thermal shrinkage measurements of these composites suggests that despite the formation of microcracks due to water loss from the geopolymer matrix, the fibres are thermally protected by the matrix up to 400 °C. The flax fibres do not appear to be compromised by the alkaline environment of the matrix, suggesting new possible applications for these low-cost simply prepared construction materials.     

Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns

The electrical resistivity of CNT yarns of diameters 10–34 μm, spun from multi-walled carbon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T. The magnetoresistance is large and negative at low temperatures. The thermal conductivity also has been determined, by parallel thermal conductance, from 5 to 300 K. The room-temperature thermal conductivity of the 10 μm yarn is (60 ± 20) W m^?1 K^?1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Investigation of thermal expansion of 3D-stitched C–SiC composites

Carbon fiber reinforced silicon carbide (C–SiC) composites are promising materials for a severe thermo-erosive environment. 3D-stitched C–SiC composites were fabricated using liquid silicon infiltration. The infiltration was carried out at 1450–1650 °C for 10–120 min in vacuum. Coefficient of thermal expansion (CTE) of the composites was determined in in-plane and through-thickness directions in the temperature range from room temperature to 1050 °C. The in-plane CTE varies in the range (0.5–2) × 10^?6/°C, while that in the through-thickness direction, it varies in the range (1.5–4) × 10^?6/°C. The effect of siliconization conditions is higher in the through-thickness direction than in the in-plane direction. The CTE values are lower than the values reported for chemical vapor impregnation based 3D C–SiC composites. An extensive microstructure study was also carried out to understand the thermal expansion behavior of the composites. It was found out that CTE behavior is closely related to the composition of the composite which in turn depends upon siliconization conditions. The best conditions were 1650 °C and 120 min.     

Physical properties of carbon nanotube sheets drawn from nanotube arrays

We report mechanical, thermal, and electrical properties of novel sheet materials composed of multiwalled carbon nanotubes, drawn from a CNT array. At low loading there is some slippage of CNTs but at higher loading tensile strength σ₀ = 7.9 MPa and Young’s modulus E = 310 MPa. The room-temperature thermal conductivity of the CNT sheet was 2.5 ± 0.5 W m^?1 K^?1, giving a thermal conductivity to density ratio of κ/ρ = 65 W m^?1 K^?1 g^?1 cm³. The heat capacity shows 1D behavior for T > 40 K, and 2D or 3D behavior at lower temperatures. The room-temperature specific heat was 0.83 J g^?1 K^?1. The iV curves above 10 K have Ohmic behavior while the iV curve at T = 2 K is non-Ohmic, and a model to explain both ranges is presented. Negative magnetoresistance was found, increasing in magnitude with decreasing temperature (?15% at T = 2 K and B = 9 T). The tensile strength, Young’s modulus and electrical conductivity of the CNT sheet are low, in comparison with other CNT materials, likely due to defects. Thermal conductivity is dominantly phononic but interfacial resistance between MWCNTs prevents the thermal conductivity from being higher.     

Electrical conductivity of carbon nanotube/poly(vinylidene fluoride) composites prepared by high-speed mechanical mixing

Electrically conductive multi-walled nanotube (MWCNT)/poly(vinylidene fluoride) (PVDF) composites with a segregated structure were prepared by high-speed mechanical mixing method. It was found that MWCNTs were uniformly dispersed on polymer particle surfaces. At the MWCNTs composition of 0.1 vol.%, the composites exhibited a dramatic enhancement in electrical conductivity by 11 orders of magnitude. A low percolation threshold was achieved at the CNT concentration of 0.078 vol.%. The mechanical mixing method presented can be adapted to other CNT/polymer composites with a segregated structure.     

Electrical and thermal properties of low permittivity Sr2Al2SiO7 ceramic filled HDPE composites

The feasibility of low permittivity Sr₂Al₂SiO₇ (SAS) ceramic filled high density polyethylene (HDPE) composites for substrate and packaging applications has been investigated in this paper. The composites were prepared by the melt mixing and hot pressing techniques. Scanning electron microscopic images of SAS filled HDPE showed the increased connectivity with filler loading. The composites showed excellent relative density (>98%) with low bulk density (<2.40 g cm^?3) and very low moisture absorption (<0.10 wt%). The relative permittivity (ε_r) and the dielectric loss (tan δ) at 1 MHz and at 5 GHz were found to be low and found to increase with filler volume fraction (V_f). The experimentally observed relative permittivity at 5 GHz was correlated with the values proposed by different theoretical models. Among them, effective medium theory (EMT) gave better fit with experimental values except at the highest filler loading (0.50 V_f). Improvement in the thermal properties was also observed with filler content. The coefficient of linear thermal expansion (CTE) was found to decrease with filler content. Thermal conductivity (TC) of the composite was greatly enhanced as a function of filler volume fraction. The composite with 0.50 filler volume fraction showed balanced thermal and dielectric properties with ε_r=4.2, tan δ=3.9×10^?3, TC=2.2 W m^?1 K^?1 and CTE=101 ppm/°C.     

Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (K_eff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS₂) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (R_bd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R_bd of CNT–PEEK and WS₂–PEEK (0.0232–115.8 × 10⁻⁸ m²K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K_eff of a multi-phase composite are quantified. The simulation results show that K_eff of multiphase composites increases when the CNT concentration increases, and when the R_bd of CNT–PEEK and WS₂–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.     

Tailored thermal expansion and electrical properties of α-Cu2V2O7/Al

α-Cu₂V₂O₇/Al composites (with 5–80 wt% of Al) were prepared by a solid state method. Their structural stability, thermal expansion, hardness and electrical properties were studied in detail. The coefficient of thermal expansion (CTE) and hardness of α-Cu₂V₂O₇/Al composites sample can be tailored with the content of Al. The CTE is only 0.49×10⁻⁶ K⁻¹ (RT–780 K) when the Al content is 10 wt%, which is near-zero thermal expansion. The electrical conductivity of α-Cu₂V₂O₇/Al composites increases with increasing the content of Al. When the content of Al is larger than 40 wt%, the α-Cu₂V₂O₇/Al composites exhibit excellent electrical conductivity, which can be mainly attributed to the conductive percolation phenomena of Al in the α-Cu₂V₂O₇/Al composites.     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.     

Thermal fatigue and hypothermal atomic oxygen exposure behavior of carbon nanotube wire

In low earth orbit (LEO), components of space systems are exposed to damaging hypothermal atomic oxygen and thermal fatigue. Carbon nanotube (CNT) wires are candidate materials for different applications in space systems. Thirty-yarn CNT wire’s behavior was evaluated when exposed to hypothermal atomic oxygen and thermal fatigue. CNT wire specimens were exposed to a nominal fluence of hypothermal atomic oxygen of 2 × 10²⁰ atoms/cm². The erosion rate due to hypothermal collision between atomic oxygen and CNT wires was calculated to be 2.64 × 10⁻²⁵ cm³/atom, which is comparable to highly ordered pyrolytic graphite. The tensile strength of CNT wire was not affected by this exposure, and a minor reduction of electrical conductivity (2.5%) was found. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy analysis showed erosion of surface layer with depleted carbon and increased oxygen. Thermal fatigue excursion of 5000 cycles from 70 to −50 °C at a rate of 55 °C/min showed no loss in tensile strength; however a large decrease in conductivity (18%) was seen. SEM analysis showed that the thermal fatigue created buckling of yarn and fracture of individual CNTs bundles. These reduced the effective area and electrical conductivity of CNT wire.     

Graphene nanoplatelet paper as a light-weight composite with excellent electrical and thermal conductivity and good gas barrier properties

Paper forms (i.e. thin free-standing films) of carbon-based materials have received increasing attention. Here we present a novel approach to fabricating a binder free, self-standing flexible paper consisting of exfoliated graphite nanoplatelets (GNPs). It is found that the electrical conductivity of the GNP paper can be as high as 2200 S cm^?1 and the thermal conductivity reaches 313 W m^?1 K^?1. Both thermoset and thermoplastic matrices were used to impregnate the porous GNP paper and an extremely high tensile modulus was attained. Even with 30 vol.% polymer, the GNP paper composite can still exhibit ～700 S cm^?1 electrical conductivity thanks to the highly continuous GNP network formed in the paper making process. The impregnated GNP paper was also investigated as a component in carbon fiber composite. It is found that when inserted into a layered laminate composite construction, gas permeability can be severely reduced and electrical and thermal conductivity can be greatly enhanced.     

Oxidation resistance of SiC/SiC composites containing SiBC matrix fabricated by liquid silicon infiltration

The mechanical behavior and oxidation resistance of SiC/SiC-SiBC composites were studied in this work. According to the debonding criterion of He and Hutchinson, the debonding could occur at the BN interphase, which insures that the fibers can well play the strengthening and toughening performance. The oxidation resistance of SiC/SiC-SiBC composites consisting of SiC fibers with thermal expansion coefficients (CTE) of 5.1 × 10^?6 K^?1 and 4.0 × 10^?6 K^?1 was compared. The composites consisting of SiC fibers with higher CTE show slight weight changes at 800, 1000, and 1200 °C, and the corresponding strength retention ratios are 109.6%, 103.2% and 102.9%, exhibiting excellent oxidation resistance. The CTE of composites consisting of SiC fibers with higher CTE matches well with the CTE of SiC coating, so rarely no cracks can be formed in the coating, which inhibits the inward diffusion of oxidizing medium and leads to high strength retention ratios after oxidation tests.     

Preparation and characterization of BaCe0.95Tb0.05O3−α hollow fibre membranes for hydrogen permeation

BaCe_0.95Tb_0.05O_3?α (BCTb) perovskite hollow fibre membranes were fabricated by spinning the slurry mixture containing 66.67 wt% BCTb powder, 6.67 wt% polyethersulphone (PESf) and 26.67 wt% N-methyl-2-pyrrolidone (NMP) followed by sintering at elevated temperatures. The influence of sintering temperature on the membrane properties was investigated in terms of crystal phase, morphology, porosity and mechanical strength. In order to obtain gas-tight hollow fibres with sufficient mechanical strength, the sintering temperature should be controlled between 1350 and 1450 °C. Hydrogen permeation through the BCTb hollow fibre membranes was carried out between 700 and 1000 °C using 50% H₂–He mixture as feed on the shell side and N₂ as sweep gas in the fibre lumen. The measured hydrogen permeation flux through the BCTb hollow fibre membranes reached up to 0.422 μmol cm^?2 s^?1 at 1000 °C when the flow rates of the H₂–He feed and the nitrogen sweep were 40 mL min^?1 and 30 mL min^?1, respectively.     

Fabrication and thermal expansion properties of ZrW2O8/Zr2WP2O12 composites

ZrW₂O₈/Zr₂WP₂O₁₂ composites were fabricated by sintering ZrW₂O₈–Zr₂WP₂O₁₂ powder mixtures at 1473 K for 1 h, and their negative thermal expansion properties were investigated. The relative density of sintered pure-phase ZrW₂O₈ was 72.3%, while that of the sintered composites was 88.4–92.3%. In the composites, the observed hysteresis in the thermal expansion data was small because of the small difference between the CTEs of ZrW₂O₈ and Zr₂WP₂O₁₂. The CTE of the composites was negative and increased with the Zr₂WP₂O₁₂ content. When the Zr₂WP₂O₁₂ volume fraction in the composites was increased from 0 to 75 vol%, the CTEs of the composites increased from ?9.1 × 10^?6 to ?3.1 × 10^?6 K^?1 and from ?5.0 × 10^?6 to ?1.9 × 10^?6 K^?1 over the temperature ranges of 323–373 and 473–673 K, respectively. In composites with Zr₂WP₂O₁₂ volume fractions of 0–25 vol%, the experimentally obtained CTE values were in good agreement with the calculated values obtained by assuming mixed law behavior.     

Effect of processing technique on the transport and mechanical properties of vapour grown carbon nanofibre/rubbery epoxy composites for electronic packaging applications

Vapour grown carbon nanofibre (VGCNF)/rubbery epoxy (RE) composites were produced, by either mechanical mixing, three-roll milling (RM) or combined ultrasonication/mechanical mixing. Incorporation of VGCNFs resulted in significant enhancements in the thermal and electrical conductivities of the material. Appropriate selection of processing technique and parameters can help to maximise the potential of VGCNF additions by improving their dispersion in the matrix. The composites produced by RM have superior transport properties compared with those produced by other techniques. The thermal conductivity of such composites at 40 wt.% VGCNFs reached 1.845 W/m K, a 10-fold increase compared to RE alone. The thermal conductivity data of VGCNF/RE composites best fits to the Hatta–Taya model. The lowest electrical percolation threshold is at 2 wt.%, obtained for composites produced by RM. The thermal conductivity of VGCNF/glassy epoxy (GE) composites at 12 wt.% is 10% lower than the corresponding RE composite but its electrical conductivity is 2 orders of magnitude higher than the corresponding RE composite. VGCNFs at 40 wt.% increase the compressive strength of rubbery epoxy by ～5× but the compressive modulus of 40 wt.% VGCNF/RE composite is 12 times lower than that of 12 wt.% VGCNF/GE composite, demonstrating highly compliant nature of RE composites.     

Mechanical,electrical, and thermal properties of graphene nanosheet/aluminum nitride composites

Graphene nanosheet (GNS)/aluminum nitride (AlN) composites were prepared by hot-pressing and effects of GNSs on their microstructural, mechanical, thermal, and electrical properties were investigated. At 1.49 vol% GNSs content, the fracture toughness (5.09 MPa m^1/2) and flexural strength (441 MPa) of the composite were significantly increased by 30.17% and 17.28%, respectively, compared to monolithic AlN. The electrical conductivity of the composites was effectively enhanced with the addition of GNSs, and showed a typical percolation behavior with a low percolation threshold of 2.50±0.4 vol%. The thermal conductivity of the composites decreased with the addition of GNSs.