Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Electrical conductivity,dielectric and microwave absorption properties of graphene nanosheets/magnesia composites

Herein, a novel microwave absorbing material with Graphene nanosheets (GNSs) as microwave absorbing filler and magnesia (MgO) as matrix were prepared by hot-pressing sintering. The composites were highly dense with a homogeneous distribution of GNSs. Electrical conductivity, dielectric and microwave absorption properties in X-band were investigated. The results revealed that the electrical conductivity of the GNSs/MgO composites showed a typical percolation-type behavior with a percolation threshold of 3.34 vol%. With GNSs content increased to 3 vol%, the real permittivity, imaginary permittivity and dielectric loss tangent of the composites increased from ～9, ～0 and ～0 to 26–43, 23–28 and 0.55–0.96, respectively. By adjusting the GNSs content, thickness and frequency, the 2.5 vol% GNSs/MgO composite shows the minimum reflection loss of ?36.5 dB at 10.7 GHz and the reflection loss below ?10 dB (90% absorption) ranges from 9.4 to 11.4 GHz with 1.5 mm thickness, exhibiting excellent microwave absorption properties.     

Electrical and thermal properties of SiC-Zr2CN composites sintered with Y2O3-Sc2O3 additives

SiC-Zr₂CN composites were fabricated from β-SiC and ZrN powders with 2 vol% equimolar Y₂O₃-Sc₂O₃ additives via conventional hot pressing at 2000 °C for 3 h in a nitrogen atmosphere. The electrical and thermal properties of the SiC-Zr₂CN composites were investigated as a function of initial ZrN content. Relative densities above 98% were obtained for all samples. The electrical conductivity of Zr₂CN composites increased continuously from 3.8 × 10³ (Ωm)⁻¹ to 2.3 × 10⁵ (Ωm)⁻¹ with increasing ZrN content from 0 to 35 vol%. In contrast, the thermal conductivity of the composites decreased from 200 W/mK to 81 W/mK with increasing ZrN content from 0 to 35 vol%. Typical electrical and thermal conductivity values of the SiC-Zr₂CN composites fabricated from a SiC-10 vol% ZrN mixture were 2.6 × 10⁴ (Ωm)⁻¹ and 168 W/m K, respectively.     

Fabrication of toughened B4C composites with high electrical conductivity using MAX phase as a novel sintering aid

MAX phase Ti₃AlC₂ was chosen as a novel sintering aid to prepare electrically conductive B₄C composites with high strength and toughness. Dense B₄C composites can be obtained at a hot-pressing temperature as low as 1850 °C with 15 vol% Ti₃AlC₂. The enhanced sinterability was mainly ascribed to the in situ reactions between B₄C and Ti₃AlC₂ as well as the liquid phase decomposed from Ti₃AlC₂. Both the Vickers hardness and fracture toughness increase with increasing Ti₃AlC₂ amount, and high hardness and toughness values of 28.5 GPa and 7.02 MPa m^−1/2 respectively were achieved for B₄C composites sintered with 20 vol% Ti₃AlC₂ at 1900 °C. Crack deflection by homogenously distributed TiB₂ particles was identified as the main toughening mechanism. Besides, B₄C composites sintered with Ti₃AlC₂ show significantly improved electrical conductivity due to the percolation of highly conductive TiB₂ phase, which could enhance the machinability of B₄C composites largely by allowing electrical discharge machining.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

Using a non-covalent modification to prepare a high electromagnetic interference shielding performance graphene nanosheet/water-borne polyurethane composite

We prepared flexible, lightweight, and high electromagnetic interference (EMI) shielding performance graphene nanosheet (GNS)/water-borne polyurethane (WPU) composites. WPU, with sulfonate functional groups, was used as the polymer matrix. By adsorbing the cationic surfactant (stearyl trimethyl ammonium chloride) on the surface of the GNSs (S-GNSs), restacking and aggregation of the GNSs have been efficiently suppressed, which also attracted sulfonate groups from the WPU matrix. Because of the favorable interfacial interactions arising from electrostatic attraction, the S-GNS exhibited good compatibility with the WPU matrix. Such a homogeneous dispersion contributed to the construction of an electrical conductive network. The S-GNS/WPU composite exhibited a low electrical conductivity percolation threshold and an outstanding enhanced electrical conductivity of approximately 5.1 S/m. A high EMI shielding effectiveness of approximately 32 dB was obtained by the WPU composites with contents of 5 vol.% (approximately 7.7 wt.%) S-GNSs.     

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.     

Novel synthesis and thermal property analysis of MgO–Nd2Zr2O7 composite

MgO-Nd₂Zr₂O₇composites with ratios of 50–70 vol% MgO were produced via a one-pot combustion synthesis. A suite of characterization techniques, including X-ray diffraction, scanning and transmission electron microscopy were employed to investigate the structural properties while dilatometry, simultaneous thermal analysis and laser flash analysis were used to characterize the thermal properties of the composites. Dense pellets were produced after sintering at 1400 °C with grain sizes between 200 and 500 nm for both phases. The thermal properties of the composites are similar to those produced using standard methods. The composite with 70 vol% MgO was found to have the highest thermal conductivity below 1000 °C, while above this temperature the thermal conductivity was found to be similar and independent of MgO content. This novel synthesis route produces materials which show significant improvements in homogeneity with smaller particle sizes when compared to current standard synthesis techniques without significantly reducing thermal conductivity.     

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.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

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.     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08 µW m⁻¹ K⁻² and higher thermoelectric figure of merit 3.11×10⁻³, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407 eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (10¹⁴–10¹⁶ cm⁻³) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

Structural,optical and electrical properties of low temperature grown undoped and (Al,Ga) co-doped ZnO thin films by spray pyrolysis

Aluminum and gallium co-doped ZnO (AGZO) thin films were grown by simple, flexible and cost-effective spray pyrolysis method on glass substrates at a temperature of 230 °C. Effects of equal co-doping with aluminum (Al) and gallium (Ga) on structural, optical and electrical properties were investigated by X-ray diffraction (XRD), UV–vis–NIR spectrophotometry and Current–Voltage (I–V) measurements, respectively. XRD patterns showed a successful growth with high quality polycrystalline films on glass substrates. The predominant orientation of the films is (002) at dopant concentrations ≤2 at% and (101) at higher dopant concentrations. Incorporation of Al and Ga to the ZnO crystal structure decreased the crystallite size and increased residual stress of the thin films. All films were highly transparent in the visible region with average transmittance of 80%. Increasing doping concentrations increased the optical band gap, from 3.12 to 3.30 eV. A blue shift of the optical band gap was observed from 400 nm to 380 nm with increase in equal co-doping. Co-doping improved the electrical conductivity of ZnO thin films. It has been found from the electrical measurements that films with dopant concentration of 2 at% have lowest resistivity of 1.621×10⁻⁴ Ω cm.     

Ba0.7Sr0.3TiO3–glass–silver percolative composite

Ba_0.7Sr_0.3TiO₃–glass–silver (BST–glass–Ag) composites were prepared by the solid state ceramic route. Their percolation behavior and dielectric properties were examined. The pure BST had a percolation limit of 24 vol% of silver whereas an addition of 8 wt% of 50PbO–30B₂O₃–20SiO₂ (PBS) glass lowered the percolation limit to 14 vol% of Ag. Glass addition lowered the sintering temperature of BST from 1300 to 975 °C and addition of Ag further lowered the sintering temperature to 925 °C, minimizing the Ag loss during sintering. The relative permittivity increased from 2700 for pure BST to about five orders of magnitude in the BST–glass–Ag composites near the percolation threshold.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².