Electrical conductivity and infrared radiation performance of SiC-CNT composite ceramics

SiC ceramic is an excellent infrared source material that can be used in a wide range of fields, like infrared heating, night vision and communication, but its poor electrical properties limit it. In this work, carbon nanotubes (CNTs) were selected as conductive phase filler, and SiC-CNT composite ceramics were prepared by SPS method. The effects of CNT content on the microstructures, electrical properties and infrared radiation performance of the composites were studied. The introduction of CNT effectively reduced the height of Schottky barrier at grain boundary, thus weakening the grain boundary effect, reducing the grain boundary resistance, further weakening the nonlinear characteristics and bulk resistivity of the composite ceramics. When the content of CNT was 1 wt%, electrical percolation was achieved, and the bulk resistivity of SiC ceramics dropped by nearly 3 orders of magnitude. The preferred orientation distribution of CNT made the bulk resistivity perpendicular to the pressure direction R_⊥ always lower than that parallel to the pressure direction R_//. The sample with 5 wt% CNT assumed linear conductivity characteristics, with bulk resistivity in different direction of 16.5 Ω cm (R_//) and 11.8 Ω cm (R_⊥), respectively. CNT addition slightly increased the infrared radiation performance of SiC ceramics, and the sample with 5 wt% CNT possessed the highest total emissivity of 0.675. The excellent electrical conductivity and infrared radiation performance of SiC-CNT composite ceramic confirmed this class as a promising infrared source material.     

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.     

Conducting aluminum‐filled nylon 6 composites

This work is concerned with the preparation and characterization of composite materials prepared by compression molding of a mixture of aluminum flakes and nylon 6 powder. The electrical conductivity, density, hardness and morphology of composites were investigated. The electrical conductivity of the composites is < 10⁻¹¹ S/cm unless the metal content reached the percolation threshold, beyond which the conductivity increased markedly by as much as 10¹¹. The volume fraction of conductive filler at the percolation threshold was calculated from experimental data, by fits to functions predicted by the percolation theory. Decreasing the average particle diameter of filler leads to increased percolation threshold (it varies from 23 to 34 vol% for the three different fillers studied) and decreased maximal conductivity of composites. The density of the composites was measured and compared with values calculated assuming different void levels within the samples. Furthermore, it is shown that for certain sizes of particle filler, the hardness decreases initially with the increase of metal concentration, possibly because of poor surface contact with the nylon matrix, but, starting from a certain value, there is a hardness increase. For the smallest particle filler, the hardness of samples is not influenced by the presence of the filler.     

Designable air-stable and dendrite-free Li metal anode via the oligomer layer for in-situ gel polymer batteries

Lithium (Li) metal anode has been an indispensable electrode material in the development of the future battery system, especially in the pursuit of energy density of the solid state battery. However, the poor air stability and dendrite problems are stumbling blocks to the practical application of Li metal anode. Herein, we design the poly(methyl methacrylate) (PMMA) oligomer coating layer for the surface of Li metal anode (Li-PMMA). The Li-PMMA anode can obtain hydrophobic oxygen resistance ability and maintain air stability for 48h in the environment of 70% relative humidity. When applied to the liquid battery, the PMMA oligomer can open the double bond by the catalysis of Li⁺ and repolymerize into the PMMA gel polymer electrolyte (GPE). As a result, the cathode and anode are cohered together by GPE to become an in-situ GPE battery. The GPE displays high ionic conductivity (7.92 mS cm^-1) and perfect interfacial contact with various electrodes. The newly designed in-situ GPE is employed to Li||Li symmetric battery without separator, which can run up to 2000 h with the overpotential of only 2.8 mV. Applications of the in-situ GPE in Li-S battery and 5000mAh LiNi_0.5Co_0.2Mn_0.3O₂/Li pouch cell both obtain excellent electrochemical performance. Our strategy may provide a versatile and practical approach to promote the large-scale application for solid state Li metal batteries.     

Potential-current characteristics in SiC/ZrB2 composite ceramics

ZrB₂/SiC composite ceramics were fabricated to improve the electrical conductive properties of SiC matrix. The debinding and sintering temperatures were determined by computation of Gibbs free energy. As a result, all the samples have the relative density above 99%, and have excellent mechanical and electrical properties. The effects of ZrB₂ content on the microstructure, mechanical and electrical properties were systematically studied. With increasing ZrB₂ content, as-prepared composites show great improvement in their mechanical properties. Importantly, the introduction of ZrB₂ weakened varistor nonlinear characteristic of composite and reduced its resistivity. The reason is the evolution of grain boundary in conductive paths. The sharp decrease of resistivity indicates the formation of percolation paths. The percolation threshold at 1?mA?cm^?2 obtained via percolation model is 10.7963?vol% (19.7098?wt%) ZrB₂. This value is much less than conventional composites, because the percolation path originates from grain boundary breakdown other than continuous conductor chains.     

Electrical performance of orthotropic and isotropic 3YTZP composites with graphene fillers

3 mol% yttria tetragonal zirconia polycrystal (3YTZP) composites with orthotropic or isotropic microstructures were obtained incorporating few layer graphene (FLG) or exfoliated graphene nanoplatelets (e-GNP) as fillers. Electrical conductivity was studied in a wide range of contents in two configurations: perpendicular (σ_?) and parallel (σ_//) to the pressing axis during spark plasma sintering (SPS). Isotropic e-GNP composites presented excellent electrical conductivity for high e-GNP contents (σ_? ～ 3200 S/m and σ_// ～ 1900 S/m for 20 vol% e-GNP), consequence of their misoriented distribution throughout the matrix. Optimum electrical performance was achieved in the highly anisotropic FLG composites, with high electrical conductivity for low contents (σ_? ～ 680 S/m for 5 vol%), percolation threshold below 2.5 vol% FLG and outstanding electrical conductivity for high contents (σ_? ～ 4000 S/m for 20 vol%), result of the high aspect ratio and low thickness of FLG.     

Characterization of Ce0.9Gd0.1O1.95 powders synthesized by spray drying

Ce_0.9Gd_0.1O_1.95 powders were synthesized by spray drying and successive calcinations. The phase purity, BET surface area, and particle morphology of as-sprayed and calcined powders were characterized. After calcination above 300 °C, the powders were single phase and showed a BET surface area of 68 m²/g when calcined at 300 °C. The conductivity, in air, of sintered pellets was measured by electrochemical impedance spectroscopy (EIS) and it was found to be comparable with literature values. The activation energy for the total conductivity was around 0.83 eV. The powder calcined at lower temperature showed better sinterability and higher total conductivity due to an increased bulk conductivity.     

Phase-controlled solvothermal synthesis and morphology evolution of nickel sulfide and its pseudocapacitance performance

Phase-controlled solvothermal synthesis has been proposed for the synthesis of nickel sulfide of single phase including α-NiS, Ni₃S₄ and NiS₂ by tuning the reaction time and the addition of surfactant. The phase evolution of nickel sulfide proceeds with the increase of sulfur stoichiometry with longer reaction time in the presence of surfactant. With the addition of hexadecyl-trimethyl-ammonium bromide (CTAB), a higher sulfur stoichiometry NiS₂ phase with hollow sphere geometry was synthesized at 9 h, a much shorter time due to the enrichment of S²⁻ on CTAB micelle surface, followed by the transformation to single phase Ni₃S₄ finally due to dissipation of enriched sulfur to the bulk solution. The application of these three single phase materials in supercapacitors was investigated. The α-NiS electrode material outperformed the Ni₃S₄ and NiS₂ electrodes, exhibiting a much higher specific capacitance of 800 F g⁻¹ at 0.5 A g⁻¹, attributable to the small particle size, high electrical conductivity and the unique hexagonal crystal structure.     

Electrical conductivity measurements as a microprobe for structure transitions in polysiloxane derived Si–O–C ceramics

Polysiloxanes [RSiO_1.5]_n with R=CH₃ (PMS) and C₆H₅ (PPS), respectively, were transformed to Si–O–C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800–1500°C. The electrical conductivities of the Si–O–C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500°C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400°C (PPS)–700°C (PMS). At temperatures above 800°C (PPS) and 1400°C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8×10⁻⁴ (Ωcm)⁻¹) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers.     

S-doped crosslinked porous Si/SiO2 anode materials with excellent lithium storage performance synthesized via disproportionation

The volume expansion during cycling and low electrical conductivity of a Si anode limit its commercial development. Nanostructure can effectively alleviate the volume expansion and doping can increase the electrical conductivity of silicon. Hence, in this paper, uniformly S-doped crosslinked porous Si/SiO₂ (S-doped pSi/SiO₂) were prepared by the disproportionation reaction of SiO at a high temperature. As a bifunctional additive, sulphur can be used to prepare crosslinked porous silicon by a silicon-sulphur reaction. Furthermore, sulphur can improve the conductive properties of the bulk Si via doping. At the same time, residual SiO₂ can also be used as a buffer material. This strategy not only provides space for the volume expansion of silicon, but also enhances its electrical conductivity and improves charge transfer. Consequently, the S-doped pSi/SiO₂ anode exhibits superior cycling capacity and rate performance (1035 mAh·g^?1 at 1 A g^?1 after 300 cycles and an exceptional rate performance of 1233 mAh·g^?1 at 2 A g^?1). Moreover, the electrochemical performance of the S-doped pSi/SiO₂//LiFePO₄ full cell was also evaluated, which exhibits favourable lithium storage performance.     

Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies

In this study, the preparation and consolidation of nanocrystalline LaB₆ powders originating from powder blends of La₂O₃, B₂O₃ and Mg were reported. A consecutive route of mechanochemical synthesis (MCS) and purification was utilized for the achievement of nano-sized LaB₆ powders. As-synthesized powders were leached out from intermediate reaction products or impurities. Then, a sequential step of cold pressing (uniaxial pressure at 800 MPa) and pressureless sintering (at 1700 °C for 5 h under Ar gas flow) were utilized for the consolidation of the purified LaB₆ powders. The type of mill (vibratory and planetary high-energy ball mills) was employed as a MCS parameter to reveal its effect on the physical, microstructural and mechanical properties of the LaB₆ powders, and their bulk structures. Compositional, physical and microstructural properties of the products after powder processing were determined via X-ray diffractometer (XRD), particle size analyzer (PSA), differential scanning calorimeter (DSC), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) coupled with both SEM and TEM, and vibrating sample magnetometer (VSM). The bulk properties of the LaB₆ consolidated from nanocrystalline powders with a minimum 99.99% purity, and ∼62 nm (for vibratory ball mill) or ∼74 nm (for planetary ball mill) average particle size were compared according to various properties. LaB₆ powders were synthesized in planetary mill at an approximately six times longer duration than that of in vibratory mill. According to the results, density, surface area and mean particle size values of the vibratory ball-milled samples (containing paramagnetic powders) are better than those of planetary ball-milled (containing diamagnetic powders) ones. However, mechanical properties such as hardness, surface roughness, wear rate, friction coefficient, and also electrical conductivity were improved in the planetary ball-milled LaB₆ bulks.     

Symmetrical solid oxide fuel cells based on titanate nanocomposite electrodes

Nanocomposite electrodes of (Sr_0.7Pr_0.3)_0.95TiO_3±δ?Ce_0.9Gd_0.1O_1.95 are directly prepared by spray-pyrolysis deposition on Zr_0.82Y_0.16O_1.92 electrolytes and their properties are compared with those obtained by the traditional screen-printing powder method. The structural, microstructural and electrical characteristics are investigated for their potential use as both cathode and anode in Solid Oxide Fuel Cells. The nanocomposite electrodes with reduced particle size ～30 nm achieved a polarization resistance at 700 ºC of 0.50 and 0.46 Ω cm² in air and pure H₂, respectively, outperforming those obtained for the analogous screen-printed electrodes with particle size of 450 nm, i.e. 4.8 and 3.9 Ω cm², respectively. An electrolyte-supported cell with symmetrical electrodes reached a maximum and stable power density of 354 mW cm^-2 at 800 ºC. These results demonstrate that the performance of electrode materials with modest electrochemical properties but high phase stability, such as doped-SrTiO₃, can be highly improved by preparing nanocomposite electrodes directly on the electrolyte surface.     

Synthesis of Yb and Sc stabilized zirconia electrolyte (Yb0.12Sc0.08Zr0.8O2–δ) for intermediate temperature SOFCs: Microstructural and electrical properties

Ceramic electrolytes based on Yb and Sc stabilized zirconia enable efficient heat transfer and effective ionic conductivity. Here, the design and synthesis of Yb and Sc stabilized zirconia electrolyte is presented for intermediate temperature solid oxide fuel cells (SOFCs). Yb_0.12Sc_0.08Zr_0.8O_2–δ was synthesized using the sol-gel method, and a thorough characterization of the electrolyte properties was conducted including structural and electrical properties. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) confirmed the composition of the electrolyte. A single-phase cubic structure with a density of 6.7041 ± 0.0008 g cm⁻³ was obtained. The thermal expansion coefficient in the temperature range from 25 °C to 800 °C is equal to 1.17 × 10⁻⁶ K⁻¹. The activation energy of 1.06 eV and 1.15 eV was obtained for the bulk and grain boundary conductivity, respectively. The ionic conductivity of approx. 2.10 S m⁻¹ was achieved at 667 °C, thus it is suitable for efficient ionic conduction at intermediate temperatures.     

Microstructure–conductivity relationship of Na3Zr2(SiO4)2(PO4) ceramics

The ionic conductivity of solid electrolytes is dependent on synthesis and processing conditions, ie, powder properties, shaping parameters, sintering time (t_s), and sintering temperature (T_s). In this study, Na₃Zr₂(SiO₄)₂(PO₄) was sintered at 1200 and 1250°C for 0-10 hours and its microstructure and electrical performance were investigated by means of scanning electron microscopy and impedance spectroscopy. After sintering under all conditions, the sodium super-ionic conductor-type structure was formed along with ZrO₂ as a secondary phase. The microstructure investigation revealed a bimodal particle size distribution and grain growth at both T_s. The density of samples increased from 60% at 1200°C for 0 hours to 93% at 1250°C for 10 hours. The ionic conductivity of the samples increased with t_s due to densification and grain growth, ranging from 0.13 to 0.71 mS/cm, respectively. The corresponding equivalent circuit fitting for the impedance spectra revealed that grain boundary resistance is the prime factor contributing to the changing conductivity after sintering. The activation energy of the bulk conductivity (E_a,bulk) remained almost constant (0.26 eV) whereas the activation energy of the total conductivity (E_a) exhibited a decreasing trend from 0.37 to 0.30 eV for the samples with t_s = 0 and 10 hours, respectively—both sintered at 1250°C. In this study, the control of the grain boundaries improved the electrical conductivity by a factor of 6.     

Effect of Al2O3 particle size on the microstructure,phase transformation and mechanical properties of hot pressed Al2O3-CA6-ZrO2/Ni multi-phase composites

Al₂O₃-CA6-ZrO₂/Ni multi-phase composites were fabricated by vacuum hot pressing sintering at 1650 °C under the pressure of 30 MPa for 30 min. The microstructural evolution rule of the composites was investigated as a function of Al₂O₃ particle size. Upon increasing the Al₂O₃ particle size to 30 μm, the generated CA6 underwent a transformation from unfixed type to a plate-like pattern and to a combined CA6-Al₂O₃ matrix, whereas the fracture mode of m-ZrO₂ changed from an intergranular fracture to an intergranular and transgranular mixed type due to the improved interface binding energy. Additionally, satisfactory mechanical properties of the composites were achieved when the Al₂O₃ particle size was 30 μm. Under the synergistic effect of different strengthening and reinforcing phases, the inhomogeneous distribution caused by poor wettability between Al₂O₃ and Ni was effectively solved by the distributions of “intercrystalline type” and “intracrystalline type” for the Ni phase. The mechanisms of the microstructural evolution, phase transformation and improved mechanical properties are discussed in detail.     

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.     

Role of the effective electrical conductivity of nanosuspensions in the generation of TiO2 agglomerates with electrospray

Suspensions with varying volume fraction of TiO₂ nanoparticles and ionic strength were electrosprayed to obtain agglomerates of different characteristics, which were then deposited to produce films with tailored morphology, thickness, and porosity. The role of the nanoparticle volume fraction in both the effective electrical conductivity of TiO₂ nanosuspensions and the control of the size of agglomerates produced by electrospray was investigated. A simple modified equation for the effective electrical conductivity of TiO₂ nanoparticle suspensions was derived. The equation, which accounted for nanoparticles' diffuse ionic layer and their agglomeration in a liquid, showed that the effective electrical conductivity is not only a function of the liquid and particle conductivities, and the particle volume fraction but also a function of both the thickness of the adsorbed ionic layer on the particles and the particle size. Gradual increase of particle volume fraction resulted in an increase in the suspension's effective electrical conductivity, when the initial liquid conductivity was in the range of 10^?4–10^?3 S m^?1. When the liquid conductivity was in the range of 10^?3–10^?2 S m^?1; however, addition of particles did not have any significant effect on the effective electrical conductivity. Control over the size of the TiO₂ nanoparticle agglomerates was achieved by electrospraying suspensions with liquid electrical conductivity of the order of 10^?3 S m^?1 and by varying the particle volume fraction. Electrospray deposition of suspensions with TiO₂ volume fraction=0.04% resulted in a more compact film with lower porosity and showed better water-splitting performance.     

One step densification of SDC—Na2CO3 nano-composite electrolytes for SOFC applications by cold sintering process

Sm_0.2Ce_0.8O_1.9- 30% Na₂CO₃ (Sm doped ceria (SDC)-30N) nano-composite electrolytes were densified in a single step via cold sintering process (CSP). At 200°C and 450 MPa of uniaxial pressure, samples up to 97% of their theoretical density could be obtained. The effect of processing parameters, such as temperature, uniaxial pressure, processing duration, and moisture content, on the densification of the nano-composite electrolytes was investigated. The thermal, microstructural, and electrical properties of nano-composites were investigated by differential scanning calorimetry, X-ray diffractometer, scanning electron microscope, and EIS analysis. SDC crystallite sizes were found to be around 25 nm, barely coarsened after CSP by which the true nano nature of the nano-composite could be preserved. Because, by conventional processing high density values could not be attained and high processing temperatures in excess of 600°C had to be used, promoting particle coarsening. The highest total electrical conductivity was found to be 2.2 × 10⁻² S cm⁻¹ at 600°C, with an activation energy of 0.83 eV for SDC-30N nano-composites. The present investigation revealed that the implementation of cold sintering technique resulted in significant enhancements in the densification of nano-composite electrolytes, thereby rendering them suitable for efficient utilization in SOFC applications, as compared to the conventional production methods.     

Flexural strength and flaw distributions of SrFe0.2Co0.4Mo0.4O3−δ ceramic-supports for SOFCs at operating conditions

Solid-oxide fuel cells (SOFCs) have the potential to increase electricity generation efficiency, but traditional SOFCs supported by nickel cermets suffer from reliability challenges due to weaker mechanical strength caused by cracking after redox cycling. To solve this problem, a new ceramic anode material, SrFe_0.2Co_0.4Mo_0.4O_3−δ (SFCM) combined with Ce_0.9Gd_0.1O₂ (GDC), was evaluated for conductivity and mechanical strength at SOFC operating conditions and after redox cycling. Fracture toughness of SFCM was determined to be (0.124 ± 0.023) MPa√m at room temperature in air, increasing to (0.286 ± 0.038) MPa√m at 600°C. A mixture of SFCM:GDC showed fracture toughness between the two materials, following SFCM's trend with temperature. The SFCM-GDC anode supported half-cell strength increases by 31% from room temperature to 600°C as intrinsic stresses remaining from sintering are relaxed and thermal expansion pushes existing cracks closed. Exposure to reducing gasses decreases strength by 29% compared to ambient, due to oxygen vacancy formation and microstructural flaw changes. It is found that SFCM-GDC based cells tolerate cycling well because of phase stability but weaken from 34.3 to 22.4 MPa due to uniform growth of critical microstructural flaws.     

Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y₂O₃, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y₂O₃ composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y₂O₃-phase and grain-growth of the bulk AlN grains.