Nitrogen-doped-CNTs/Si3N4 nanocomposites with high electrical conductivity

Novel highly electrically conducting nanocomposites consisting of a silicon nitride (Si₃N₄) ceramic matrix containing up to 13.6 vol.% of nitrogen-doped multi-walled carbon nanotubes (CNx) were fabricated. As-synthesized CNx were treated with hydrogen peroxide in order to efficiently detach/isolate the nanotubes from bundles, then they were mixed with the ceramic powders and fully densified using the spark plasma sintering (SPS) technique. Composites containing 13.6 vol.% CNx reached an electrical conductivity of 2174 S m⁻¹ that is the highest value reported hitherto for carbon nanotubes/Si₃N₄ nanocomposites. The nitrogen doping also favored a strong mechanical interlocking between the nanotubes and the Si₃N₄ matrix; when compared to the undoped carbon nanotubes. These novel nanocomposites could be used in devices associated to power generation or telecommunications.     

Fabrication and properties of high performance PEEK/Si3N4 nanocomposites

The crystallization, morphology, microhardness, scratch hardness, dynamic modulus, and wear behavior of high performance poly(ether‐ether‐ketone) (PEEK) matrix nanocomposites reinforced with 0 to 30 wt % silicon nitride (Si₃N₄) nanoparticles were reported. The crystallinity of PEEK nanocomposites increases at 2.5 wt % Si₃N₄ but, thereafter it decreases with increasing Si₃N₄ content due to the hindrance to the ordering of PEEK chains. The crystallization peak temperature and crystallization onset temperature increases by 14°C for 10 wt % nanocomposite. The melting temperature does not vary significantly with Si₃N₄ content. SEM shows almost uniform distribution of Si₃N₄ in the PEEK matrix. The Vickers microhardness and scratch hardness increases significantly up to 10 wt % Si₃N₄ content.The dynamic modulus of nanocomposites increases below and above T_g of PEEK. The specific wear rate of nanocomposites with 2.5 wt % Si₃N₄ is reduced significantly and it is lowest at 10 wt % Si₃N₄. However, the coefficient of friction of nanocomposites is more than that of pure PEEK. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Cup-stacked carbon nanotubes hybridized Si3N4/Si3N4 composite ceramics for high-effciency microwave absorption with excellent thermal stability

Hybridization between carbon nanotubes (CNTs) and Si₃N₄ is a promising strategy for developing high-temperature microwave absorption (MA) materials for military application. Toward long-life services, it's important to achieve strong MA at a filler loading as low as possible on account of antioxidant protection against CNTs wastage. Herein, cup-stacked CNTs (CSCNTs) have been prepared in porous Si₃N₄ ceramics by chemical vapor deposition (CVD) and then CVD Si₃N₄ has been coated on them, forming CSCNT-Si₃N₄/Si₃N₄ composite ceramics. Results show that CSCNTs possess abundant exposed atomic edges on the outer surface and in the inner channel. Such unique defects not only benefit the impedance match but also cause considerable conductive loss, which helps CSCNT-Si₃N₄/Si₃N₄ with a filler content of only 0.79 wt% to achieve an effective absorption bandwidth (EAB) of 3.74 GHz in the X band at a thickness of 3.5 mm coupled with a minimum reflection loss of ?43.3 dB and an EAB covering the entire Ku band at a thickness of 2.25 mm. The ultralow filler loading generates a high efficiency of CVD Si₃N₄ in protecting CSCNTs against high-temperature oxidation, leading to a steady MA performance for CSCNT-Si₃N₄/Si₃N₄ during 23–1200 °C thermal shock tests in air.     

Effect of long-term oxidation on creep and failure of Si3N4 and Si3N4/SiC nanocomposites

The high-temperature mechanical behaviour of an Si₃N₄/SiC nanocomposite and its monolithic Si₃N₄ reference material was studied after long-term oxidation treatments intended to simulate future operating conditions in a severe environment. Creep and failure at elevated temperature were significantly affected, in the direction of increased brittleness. The transition stress between the ductile range present at low stresses and the brittle range existing at high stresses was shifted to distinctly lower values. The creep resistance in the low-stress range was increased by the oxidation treatment. The failure time under a given stress was drastically reduced; this was attributed to an increased sensitivity to subcritical crack growth. The failure stress for a given failure time was decreased by about half. The phenomena are explained in terms of a purification of the intergranular phase and by the formation of surface defects and of a uniformly distributed pore population.     

Modification of carbon nanotubes and its effect on properties of carbon nanotube/epoxy nanocomposites

We have studied an effect of three types of modifications of carbon nanotubes (CNTs) on dispersion and mechanical properties of final epoxy‐amine based nanocomposites. First approach includes end‐walled covalent chemical modification at the ends of nanotubes. The second one is side‐walled covalent chemical modification along the whole length of nanotubes. The third procedure is noncovalent, physical modification done by the CNT surface coating with polyaniline. The modification of nanotubes was determined by X‐ray photoelectron spectroscopy. The prepared epoxy‐amine nanocomposites were characterized by dynamic‐mechanical analysis, tensile testing, light microscopy, transmission electron microscopy, and thermogravimetry. We observed an improvement of the mechanical properties and the thermal stability by addition of the carbon nanotubes to the epoxy matrix. The strong interactions between the nanotube and the polymer matrix were discovered in the nanocomposites with physically modified nanotubes. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Preparation and mechanical properties of Si3N4 nanocomposites reinforced by Si3N4@rGO particles

A new type of reduced graphene oxide-encapsulated silicon nitride (Si₃N₄@rGO) particle was synthesized via an electrostatic interaction between amino-functionalized Si₃N₄ particles and graphene oxide (GO). Subsequently, the Si₃N₄@rGO particles were incorporated into a Si₃N₄ matrix as a reinforcing phase to prepare nanocomposites, and their influence on the microstructure and mechanical properties of the Si₃N₄ ceramics was investigated in detail. The microstructure analysis showed that the rGO sheets were uniformly distributed throughout the matrix and firmly bonded to the Si₃N₄ grains to form a three-dimensional carbon network structure. This unique structure effectively increased the contact area and load transfer efficiency between the rGO sheets and the matrix, which in turn had a significant impact on the mechanical properties of the nanocomposites. The results showed that the nanocomposites with 2.25 wt.% rGO sheets exhibited mechanical properties that were superior to monolithic Si₃N₄; the flexural strength increased by 83.5% and reached a maximum value of 1116.4 MPa, and the fracture toughness increased by 67.7% to 10.35 MPa·m^1/2.     

Fabrication and properties of CTBN/Si3N4/cyanate ester nanocomposites

Hybrid fillers of carboxyl‐terminated liquid butadiene‐acrylonitrile/silicon nitride (CTBN/Si₃N₄) are performed to regulate the mechanical properties, heat resistance properties, dielectric properties and thermal conductivities of the bisphenol A dicyanate ester (BADCY) resins. With the 10 wt% addition of CTBN/Si₃N₄ hybrid fillers, the maximum impact, and flexural strength of the CTBN/Si₃N₄/BADCY nanocomposite is increased to 15.4 kJ/m² and 144.3 MPa, respectively. And the corresponding thermal conductivity value of the CTBN/Si₃N₄/BADCY nanocomposite is increased to 0.622 W/mK with 18 wt% addition of CTBN/Si₃N₄ hybrid fillers. Moreover, with the addition of CTBN/Si₃N₄ hybrid fillers, the heat resistance properties of the CTBN/Si₃N₄/BADCY nanocomposites are increased, but the dielectric properties get worse slightly. POLYM. COMPOS., 37:2522–2526, 2016. © 2015 Society of Plastics Engineers     

Thermal and mechanical properties of carbon nanotube/epoxy nanocomposites reinforced with pristine and functionalized multiwalled carbon nanotubes

In this study, the effects of functionalization and weight fraction of mutliwalled carbon nanotubes (CNTs) were investigated on mechanical and thermomechanical properties of CNT/Epoxy composite. Epoxy resin was used as matrix material with pristine‐, COOH‐, and NH₂‐functionalized CNTs as reinforcements in weight fractions of 0.1, 0.5, and 1.0%. Varying (increasing) the weight fraction and changing type (pristine or functionalized) of CNTs caused increment in Young's modulus and tensile strength as observed during mechanical tests. CNT reinforcement improved thermal stability of the nanocomposites as observed by thermogravimetric analysis. Thermomechanical analysis showed a slight reduction in free volume of the polymer, that is a drop in coefficient of thermal expansion, prior to glass transition temperature (T_g) beside a slight increase in T_g value. Dynamic mechanical analysis indicated an increase in storage modulus and T_g owing to the strength addition of CNT to the matrix alongside the hardener. Scanning electron microscopy analysis of the fractured surface(s) revealed that CNTs were well dispersed with no agglomeration and resulted in reinforcing the matrix. POLYM. COMPOS., 36:1891–1898, 2015. © 2014 Society of Plastics Engineers     

Failure investigation of carbon nanotube/3Y-TZP nanocomposites

3Y-TZP matrix composites containing 0.1–1 wt.% of multi-wall (MWCNT) and single-wall (SWCNT) carbon nanotubes were fabricated by spark plasma sintering technique. The sintered composites reached full density. Hardness and fracture toughness were measured using Vickers indention method. The hardness of the composite decreased with increasing weight content of the MWNT. The fracture toughness was 5.52 MPa m^0.5 when the amount of MWCNTs was 0.5 wt.%, however it decreased to 4.5 MPa m^0.5 when the content was raised to 1.0 wt.%. The composite containing 0.5 wt.% SWCNTs showed similar fracture toughness as that of matrix. The incorporation of CNTs into 3Y-TZP matrix led to no prominent improvement on the mechanical properties. The failure mechanism was analyzed finally.     

Linking microstructure evolution and impedance behaviors in spark plasma sintered Si3N4/TiC and Si3N4/TiN ceramic nanocomposites

We proposed a novel approach to investigate the three-dimensional microstructures and sintering behaviors of Si₃N₄-based ceramic nanocomposites by electrochemical impedance spectroscopy. Si₃N₄/TiC and Si₃N₄/TiN with various weight percentages of conductive phases were prepared by spark plasma sintering (SPS) at different temperatures and dwell times. The incorporation of TiC and TiN into β-Si₃N₄ provides pulse current paths inside the ceramics due to their higher conductivity. These paths enable the localized Joule heating and mass transport, facilitating the densification and grain growth of ceramic compact. The electrochemical study of such nanocomposites has revealed three-dimensional information of the evolution of their microstructures, and the capacitive and resistive characteristics of the nanocomposites reflect the densification, grain growth, and element distribution in the compact. The impedance model presented in this work suggests isolated distribution of TiN in Si₃N₄ while Si₃N₄/TiC of the same amount of additives at the same sintering conditions formed conductive network. This impedance analysis further explained the differences in densification mechanism of SPS in Si₃N₄/TiN and Si₃N₄/TiC.     

Mechanical reinforcement and wear resistance of aligned carbon nanotube/epoxy nanocomposites from nanoscale investigation

The anisotropic mechanical reinforcement compression and scratch resistance of epoxy nanocomposites filled with well-aligned carbon nanotubes (ACNTs) sliding in different orientations were investigated by nanoindentation techniques. It was found the addition of ACNTs to epoxy very effectively improved the microscopic hardness, elastic modulus, and nanoscratch resistance of pure epoxy, and the nanocomposites in antiparallel orientation of ACNTs showed the highest enhancement. However, macroscopic compression test showed the normal orientation of ACNTs enhanced the compression strength the most. Based on nanoscale observations, new ACNTs related reinforcing and scratch resistance mechanisms were further proposed and discussed. This study will enhance the understanding of the anisotropic reinforcement effect of ACNTs with new characteristics, and will be helpful for the design and application of ACNT nanocomposite materials into precision instruments and related anisotropic nanomaterials.     

A Crystalline Si3N4/Amorphous Si3N4 Composite

A composite consisting of elongated α-Si₃N₄ crystallites (5–50 (Am in diameter) embedded in an amorphous Si₃N₄ matrix was synthesized by chemical vapor deposition. The hardness and indentation fracture toughness of the amorphous matrix and of the composite have been evaluated at temperatures from ambient to 1200°C. It was found that the crystallites have relatively little influence on the hardness and indentation fracture toughness when the surrounding matrix is amorphous. However, a 1400°C heat treatment of the material results in a matrix consisting of small crystals (100 nm in diameter) surrounded by carbon-containing regions which appear to be amorphous in the TEM; TEM and EELS in nearby triple points revealed the presence of amorphous carbon. After heat treatment, the indentation fracture toughness at ambient and at 1200°C is increased due to extensive microcracking. The Vickers hardness at 1200°C also increased significantly as a result of the heat treatment. The relationship between the mechanical properties and the microstructure is discussed.     

Microstructure and mechanical properties of Si3N4f/Si3N4 composites with different coatings

The Si₃N₄ coating and Si₃N₄ coating with Si₃N₄ whiskers as reinforcement (Si₃N_4w-Si₃N₄) were prepared by chemical vapor deposition (CVD) on two-dimensional silicon nitride fiber reinforced silicon nitride ceramic matrix composites (2D Si₃N_4f/Si₃N₄ composites). The effects of process parameters of as-prepared coating including the preparation temperature and volume fraction of Si₃N_4w on the microstructure and mechanical properties of the composites were investigated. Compared with Si₃N₄ coating, Si₃N_4w-Si₃N₄ coating shows more significant effect on the strength and toughness of the composites, and both strengthening and toughening mechanism were analyzed.     

Synthesis of carbon nanotube/epoxy composite films with a high nanotube loading by a mixed-curing-agent assisted layer-by-layer method and their electrical conductivity

A mixed-curing-agent assisted layer-by-layer method is reported to synthesize carbon nanotube (CNT)/epoxy composite films with a high CNT loading from ∼15 to ∼36 wt.%. The mixed-curing-agent consists of two types of agents, one of which is responsible for the partial initial curing at room temperature to avoid agglomeration of the CNTs, and the other for complete curing of epoxy resin at high temperature to synthesize epoxy composite films with good CNT dispersion. The electrical conductivity of the composites shows a value up to ∼12 S/m, which is much higher than that for CNT/epoxy composites with a low CNT loading prepared using conventional methods.     

流延法在Si3N4块体及Si3N4/BN层状材料制备中的应用

本采用流延法制备了Si3N4块体及Si3N4/BN层状材料,流延法已经在陶瓷的制备工艺中得到了广泛的应用,但是很少用于Si3N4体系,尤其是水基流延法。用流法制备Si3N4/BN层状材料时,可以较为容易地控制坯片的厚度,得到性能稳定的层状材料。     

Preparation of Si3N4 ceramics with high strength and high reliability via a processing strategy

In this study, the preparation of Si₃N₄ ceramics with high mechanical reliability is investigated. The influences of several processing steps on the bending strength and the Weibull modulus are reported including: (i) coating of the Si₃N₄ powder with its sintering aids, (ii) oxidation of the coated powder, (iii) cold isostatic pressing, (iv) gelcasting of the green bodies and (v) gas pressure sintering. It was found that all the aforementioned steps contribute to improvements of strength and reliability of Si₃N₄ ceramics. Via an optimised processing strategy, Si₃N₄ ceramics with a bending strength and a Weibull modulus as high as 944.7±29.5 MPa and 33.9, respectively, could be prepared. Additionally, it was also found that surface modifications, i.e. coating and oxidation of Si₃N₄ powder, increased the rheological properties of the powder suspension in aqueous media, which is favourable for in situ colloidal forming such as gelcasting.     

Flammability properties of polymer nanocomposites with single-walled carbon nanotubes: effects of nanotube dispersion and concentration

The effects of the dispersion and concentration of single walled carbon nanotube (SWNT) on the flammability of polymer/SWNT nanocomposites were investigated. The polymer matrix was poly (methyl methacrylate) (PMMA) and the SWNT were dispersed using a phase separation (‘coagulation’) method. Dispersion of SWNTs in these nanocomposites was characterized by optical microscopy on a micrometer scale. Flammability properties were measured with a cone calorimeter in air and a gasification device in a nitrogen atmosphere. In the case where the nanotubes were relatively well-dispersed, a nanotube containing network structured layer was formed without any major cracks or openings during the burning tests and covered the entire sample surface of the nanocomposite. However, nanocomposites having a poor nanotube dispersion or a low concentration of the nanotubes (0.2% by mass or less) formed numerous black discrete islands with vigorous bubbling occurring between these islands. Importantly, the peak heat release rate of the nanocomposite that formed the network layer is about a half of those, which formed the discrete islands. It is proposed that the formation of the discrete islands is due to localized accumulation of the nanotubes as a result of fluid convection accompanying bubble formation and rise of the bubbles to the surface through the molten sample layer and bursting of the bubbles at the surface. The network layer acts as a heat shield to slow the thermal degradation of PMMA.     

Biomimetic high toughness Si3N4 ceramics with inverse-bouligand structure

Inspired by the Bouligand structure in nature, a kind of Inverse-Bouligand structure was designed. Different from the bionic Bouligand structure of a one-dimensional material unit, the inverse-Bouligand structure is obtained by stacking and twisting the ceramic layer with a parallel groove structure. The grooves arranged in parallel are first etched on the ceramic green body and then sintered after stacking and twisting. After sintering, the groove structure can still be maintained, so the resulting laminated twisted groove structure in the bulk forms an inverse-Bouligand structure, which can promote crack deflection and improve the toughness of the material during the fracture process. After testing different twist angles, the results show that when the twist angle is 15°, the toughness of the material can reach 11.22 ± 1.77 MPa m^1/2, and high strength can be guaranteed. The synergistic effect of interlayer crack deflection and regional crack twist is the main mechanism for the improvement of material toughness.     

Polyaniline / carbon nanotube composites: starting with phenylamino functionalized carbon nanotubes

Summary Composites of carbon a nanotube with polymers are a developing and interesting area of research. The dispersion of the nanotube in polymer matrices is an important factor while making its nanocomposites. Even though in-situ polymerization approach offers a better approach for synthesizing homogeneous polymer nanotube composites, the dispersion of the nanotubes in the monomer solution is a problem. In this article we report a new chemical method for dispersing nanotubes in monomer and the preparation of uniform tubular composite of polyaniline (PANI) and multiwalled carbon nanotube (MWNT). For this the oxidized multiwalled nanotube (o-MWNT) was functionalized with p-phenylenediamine, which gave phenylamine functional groups on the surface. This functionalization helped to disperse the nanotubes in acidic solution. The in-situ polymerization of aniline in the presence of these well dispersed nanotubes gave a new tubular composite of carbon nanotube having an ordered uniform encapsulation of doped polyaniline. The phenylamine functional groups on the surface were grown into polyaniline chain so that the composite contains polyaniline functionalized CNT and they were no more an impurity in the final nanocomposite. The microscopic and structural properties of this composite were compared with that of a composite prepared under identical condition using o-MWNT.     

氮化硅及其微粉的制备

介绍了氮化硅的性能、应用范围以及其微粉的制备方法。