Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si₃N₄) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm^?1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT < 0) for high GNP contents. A thin amorphous layer was identified at the Si₃N₄/GNPs interface that may affect the conduction for the parallel configuration.     

High thermal conductive diamond/Ag–Ti composites fabricated by low-cost cold pressing and vacuum liquid sintering techniques

Diamond/Ag–Ti composites were fabricated by a low-cost liquid sintering technique. The Ti addition can effectively improve wetting and promote penetration in composite pores during liquid sintering. The interface structure of the diamond/Ag–Ti composite was identified as Ag/TiC/Ag–Ti/diamond. A high thermal conductivity of 719 W/mK was obtained for the 50 vol.% diamond/Ag-1 at.% Ti composite. Using a bimodal mixture (60 vol.% 150 μm + 10 vol.% 50 μm diamond/Ag-2 at.% Ti composite), a low coefficient of thermal expansion of 6.3 × 10^− 6/K still with high thermal conductivity of 687 W/mK was achieved. These composites have potential applications for thermal management of high integration electronic devices.     

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.     

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.     

Fabrication and origin of high-k carbon nanotube/epoxy composites with low dielectric loss through layer-by-layer casting technique

The distribution of polarized space charges and their relaxation behavior in high dielectric constant electric conductor/polymer composites are main factors that determine the frequency-dependent dielectric constant and dielectric loss. However, few reports focus on this motif. We present here the dielectric performance and mechanism of a unique kind of composites with multi-layers (coded as [MWCNT/EP]_x, where x refers to the number of layers), fabricated by using layer-by-layer casting technique. Each composite layer with same thickness was composed of multi-walled carbon nanotubes (MWCNTs) and epoxy (EP) resin. When the loading of MWCNTs is 0.5 wt%, the four-layer [MWCNT0.5/EP]₄ material shows the highest dielectric constant (465 at 1 Hz) and low dielectric loss tangent (0.7 at 1 Hz), about 4 and 2.1 × 10⁻² times the values of traditional MWCNT0.5/EP composite, respectively. By investigating the space charge polarization (SCP), Debye polarization and dielectric moduli in [MWCNT/EP]_x materials, the complex relationships and the origin among dielectric constant, dielectric loss, frequency and the content of filler were clearly elucidated. The SCP within each layer is different from that between layers. The greatly improved dielectric properties of [MWCNT/EP]_x materials are believed to be the reinforced SCP and blocked transport of carriers between every two layers.     

Efficient CVD diamond film/alumina composite substrate for high density electronic packaging application

Due to its extreme hardness, chemical and mechanical stability, large band gap, low dielectric constant and highest thermal conductivity, diamond film is expected to be an excellent electronic packaging material for high frequency and high power devices. Under an alcohol concentration of 0.8% and a substrate temperature of 850 °C, high quality diamond films deposited on alumina are obtained by hot filament chemical vapor deposition (HFCVD) method using the optimum parameters determined by an infrared spectroscopic ellipsometer. Prior to the deposition of diamond film, carbon ions are implanted into alumina wafers to release the residual stress between interfaces. The measurement results indicate that dielectric properties and the thermal conductivity of diamond film/alumina composites are improved further with the increase of diamond coating. When the thickness of diamond coating is up to 100 μm, dielectric constant and dielectric loss of diamond film/alumina composite are 6.5 and 1.1 × 10^− 3, respectively. However, a thermal conductivity of 3.98 W/cm·K is obtained.     

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.%.     

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.     

The production of carbon nanotube/epoxy composites with a very high dielectric constant and low dielectric loss by microwave curing

Multiwalled carbon nanotube (MWCNT)/epoxy (EP) composites were developed using microwave curing (m-MWCNT/EP). They have a very high dielectric constant and low dielectric loss. For comparison, composites based on the same components were also prepared by thermal curing (t-MWCNT/EP). Results show that the two types of composites have greatly different dielectric properties. With the same content of MWCNTs, m-MWCNT/EP composites show a much higher dielectric constant and lower dielectric loss than t-MWCNT/EP composites. Specifically, the dielectric constant and loss at 100 Hz of m-MWCNT/EP composite with 0.04 vol% MWCNTs are about 2.5 and 0.05 times the corresponding value of t-MWCNT/EP composites, respectively, because of their different structures. Compared with t-MWCNT/EP composites, the nanotubes in m-MWCNT/EP composites not only have a better dispersion in the matrix, but also align in a direction. An equivalent circuit model was set up to evaluate the influence of dispersion and spatial distribution of MWCNTs on the dielectric properties. It shows that it is possible to control the dispersion and spatial distribution of carbon nanotubes using a different curing technique to obtain high performance composites with unexpected dielectric properties, especially those with very high dielectric constant and low dielectric loss.     

Improved dielectric properties for chemically functionalized exfoliated graphite nanoplates/syndiotactic polystyrene composites prepared by a solution-blending method

Exfoliated graphite nanoplates (xGNPs) were chemically functionalized by the 1,3 dipolar cycloaddition reaction of azomethine ylides. The introduction of organic groups on the external graphitic layers of xGNPs was confirmed by Fourier-transform infrared spectroscope, scanning electron microscope, transmission electron microscope, and thermal gravimetric analyzer. The resultant functionalized xGNPs (f-xGNPs) could be suspended in N-methyl-2-pyrrolidinone after ultrasonic treatment, which facilitated their homogeneous dispersion in syndiotactic polystyrene (sPS) matrix to form f-xGNPs/sPS composites by a solution-blending method. It was found that the existence of f-xGNPs increased the crystallization temperature but reduced the crystallization degree of sPS due to their strong interfacial interaction with sPS matrix. The dependences of electrical and dielectric properties of f-xGNPs/sPS composites on concentration of f-xGNPs and frequency were investigated. When the concentration of f-xGNPs is 10 wt.%, the electrical conductivity and the dielectric constant of f-xGNPs/sPS composites were 1.5 × 10⁻⁷ S/m and 36.4 at 1000 Hz, respectively, while the corresponding values of sPS were 3.8 × 10⁻¹⁰ S/m and 2.83, respectively. The enhanced and stable dielectric constant of f-xGNPs/sPS composites could be ascribed to the well dispersion of f-xGNPs in sPS matrix, resulting in the formation of a large number of microcapacitors.     

Dielectric properties of barium titanate–molybdenum composite

The barium titanate–molybdenum composites were prepared through solid state reaction method in argon atmosphere. The microstructure, resistivity, and dielectric properties of the composites were investigated. XRD results indicated that chemical reactions between barium titanate (BaTiO₃:BT) and molybdenum (Mo) have taken place during sintering, resulting in the formation of BaMoO₄ (BM) and BaTi₂O₅ (BT₂). The resistivity decreased with the increasing amount of Mo in the composites. The composites (when x = 5 and 20 wt.%) showed lower dielectric constant than pure BaTiO₃, especially, the dielectric constant (when x = 20 wt.%) reached a minimum value (<10⁴), while composites (when x = 10 and 15 wt.%) showed rather high dielectric constant at temperatures range from 25 °C to 160 °C. The dielectric constant of the composite gradually decreased with increase in frequency at the room temperature. The dielectric constant of composite (when x = 5 wt.%) comes up to 10⁴, and the T_c (Curie temperature) of the composite was relatively higher than that of BT (120 °C).     

Dielectric and ferroelectric hysteresis properties of 1–3 lead magnesium niobate–lead titanate ceramic/Portland cement composites

In this work, lead magnesium niobate–lead titanate (PMN–PT) ceramic was cut and filled with Portland cement (PC) to produce 1–3 connectivity PMN–PT/PC composites. Dielectric and ferroelectric hysteresis properties of these composites with PMN–PT ceramic volume content of 60% were investigated. Room temperature dielectric constant (?_r) at 1 kHz of the PMN–PT/PC composite was found to be ≈1500. At higher frequency (20 kHz), the dielectric constant was reduced to the value of ≈1300. Ferroelectric (polarization–electric field) hysteresis loops at 10–90 Hz and varying electric field were measured. The “instantaneous” remnant polarization (P_ir) at 50 Hz and at the electric field of 7 kV/cm of the PMN–PT/PC composite was found to be ≈10 μC/cm². These values of 1–3 composites therefore are promising when compared to previous results of composites at similar conditions.     

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.     

Two-layer materials of polyethylene and a carbon nanotube/cyanate ester composite with high dielectric constant and extremely low dielectric loss

Double-layer materials with one layer being a polyethylene (PE) film and the other layer a multi-wall carbon nanotube (MWCNT)/cyanate ester (CE) resin composite, PE-MWCNT/CE, were prepared. They have high dielectric constant and extremely low dielectric loss. For comparison, MWCNT/CE composites with different contents of MWCNTs were also prepared. Results show that the two kinds of materials have greatly different dielectric properties. With the same content of MWCNTs, the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/CE composite. More specifically, the dielectric constant and loss tangent at 10 Hz of the PE-MWCNT/CE material with 0.5 wt.% MWCNTs are respectively 168 and 0.006, about 1.4 and 2.5 × 10⁻⁵ times the values of the corresponding MWCNT/CE composite. The nature behinds these interesting data was detected from the space charge polarization effect and equivalent circuits. The mechanism for the unique dielectric behavior of the PE-MWCNT/CE materials is that the presence of PE film not only reinforces the space charge polarization, but also subdues the leakage current. On the other hand, based on the discussion on the Cole–Cole plots, an effective method is developed to accurately calculate the relaxation time of space charge polarization in electric conductor/polymer materials.     

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.     

Magneto-dielectric laminated Ba(Fe0.5Nb0.5)O3-Bi0.2Y2.8Fe5O12 composites with high dielectric constant and high permeability

Magneto-dielectric laminated ceramic composites of xBa(Fe_0.5Nb_0.5)O₃-(1-x)Bi_0.2Y_2.8Fe₅O₁₂(BFN-BYIG) with high volume fractions of the giant dielectric constant material BFN (x=10, 30, 50, 70 wt%) were fabricated by the solid-state sintering method. Microstructure, dielectric and magnetic properties of the composites were investigated. The composites possess stable dielectric properties in the frequency range from 100 Hz to 1 MHz with high dielectric constant and low dielectric loss. The maximum permeability of the magneto-dielectric laminated composites reaches up to about 25. And the magnetic behaviors are strongly dependent on the mass ratio of BYIG. The results indicate that such multilayer structures of BFN/BYIG can enhance the permeability and decrease the dielectric and magnetic loss efficiently.     

Room-temperature UV-ozone assisted solution process for zirconium oxide films with high dielectric properties

A facile, low-cost, and room-temperature UV-ozone (UVO) assisted solution process was employed to prepare zirconium oxide (ZrO_x) films with high dielectric properties. ZrO_x films were deposited by a simple spin-coating of zirconium acetylacetonate (ZrAcAc) precursor in the environment-friendly solvent of ethanol. The smooth and amorphous ZrO_x films by UVO exhibit average visible transmittances over 90% and energy bandgap of 5.7 eV. Low leakage current of 6.0 × 10⁻⁸ A/cm² at 3 MV/cm and high dielectric constant of 13 (100 Hz) were achieved for ZrO_x dielectrics at the nearly room temperature. Moreover, a fully room-temperature solution-processed oxide thin films transistor (TFT) with UVO assisted ZrO_x dielectric films achieved acceptable performances, such as a low operating voltage of 3 V, high carrier mobility of 1.65 cm² V⁻¹ s⁻¹, and on/off current ratio about 10⁴–10⁵. Our work indicates that simple room-temperature UVO is highly potential for low-temperature, solution-processed and high-performance oxide films and devices.     

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.     

Mechanical and functional properties of Al2O3–ZrO2–MWCNTs nanocomposites

Multi-walled carbon nanotubes (MWCNTs) are often reported as additives improving mechanical and functional properties of ceramic composites. However, despite tremendous efforts in the field in the past 20 years, the results are still inconclusive. This paper studies room temperature properties of the composites with polycrystalline alumina matrix reinforced with 0.5–2 vol.% MWCNTs (composites AC) and zirconia toughened alumina with 5 vol.% of yttria partially stabilised zirconia (3Y-PSZ) containing 0.5–2 vol.% of MWCNTs (composites AZC). Dense composites were prepared through wet mixing of the respective powders with functionalised MWCNTs, followed by freeze granulation, and hot-pressing of granulated powders. Room temperature bending strength, Young's modulus, indentation fracture toughness, thermal and electrical conductivity of the composites were studied, and related to their composition and microstructure. Slight increase of Young's modulus, indentation fracture toughness, bending strength, and thermal conductivity was observed at the MWCNTs contents ≤1 vol.%. At higher MWCNTs contents the properties were impaired by agglomeration of the MWCNTs. The DC electrical conductivity increased with increasing volume fraction of the MWCNTs.     

Ferroelectric hysteresis behavior and dielectric properties of 1–3 lead zirconate titanate–cement composites

Lead zirconate titanate (PZT) ceramic was mixed with Portland cement (PC) to form 1–3 connectivity PZT–PC composite using a dice-and-fill technique. Ferroelectric hysteresis behavior and dielectric properties of these composites were investigated using PZT volume content of 60%, 70% and 80%. The results showed that the dielectric constant of the composite materials increased with PZT content and the dielectric constant (?_r) value is 781 for 80% PZT composite at 1 kHz. The dielectric loss tangent (tan δ) was found to decrease with increasing PZT content and the tan δ value of 80% PZT composite is 0.06. Parallel and series models were also compared to the dielectric measurement results. For the hysteresis measurements, the ferroelectric hysteresis loops can be seen for all composites. The “instantaneous” remnant polarization (P_ir) was found to increase with increasing PZT content from 3.20 to 4.28 μC/cm² at 90 Hz when PZT volume content used was 60% and 80% respectively.