Effect of number of graphene layers on mechanical and dielectric properties of graphene–epoxy nanocomposites

In this study, mechanical and dielectric properties of epoxy nanocomposites with two types of graphene, <?10 layer stacks (GEC10) and <?30 layer stacks (GEC30) were investigated. Results showed that the number of graphene layers remarkably affected the dielectric properties of epoxy nanocomposites. The real and imaginary parts of relative permittivity and loss tangent of GEC10 samples were noticeably enhanced and reached to 1.29, 20 and 15.6 times respectively for 1?wt-% graphene sample compared to GEC30 samples. Meanwhile, tensile tests showed a peak for tensile strength of GEC10 and GEC30 samples with 0.1?wt-% graphene, which improved by 13 and 7.9% with respect to pure epoxy respectively. In addition, flexural properties did not change significantly compared to the pure epoxy.     

Dielectric properties of Si3N4–SiCN composite ceramics in X-band

Si₃N₄–SiCN composite ceramics were successfully fabricated through precursor infiltration pyrolysis (PIP) method using polysilazane as precursor and porous Si₃N₄ as preform. After annealed at temperatures varying from 900 °C to 1400 °C, the phase composition of SiCN ceramics, electrical conductivity and dielectric properties of Si₃N₄–SiCN composite ceramics over the frequency range of 8.2–12.4 GHz (X-band) were investigated. With the increase of annealing temperature, the content of amorphous SiCN decreases and that of N-doped SiC nano-crystals increases, which leads to the increase of electrical conductivity. After annealed at 1400 °C, the average real and imaginary permittivities of Si₃N₄–SiCN composite ceramics are increased from 3.7 and 4.68 × 10^?3 to 8.9 and 1.8, respectively. The permittivities of Si₃N₄–SiCN composite ceramics show a typical ternary polarization relaxation, which are ascribed to the electric dipole and grain boundary relaxation of N-doped SiC nano-crystals, and dielectric polarization relaxation of the in situ formed graphite. The Si₃N₄–SiCN composite ceramics exhibit a promising prospect as microwave absorbing materials.     

Influence of rare-earth oxide additives on the oxidation resistance of Si3N4–SiC nanocomposites

The influence of different rare earth oxide additives (La₂O₃, Nd₂O₃, Sm₂O₃, Y₂O₃, Yb₂O₃ and Lu₂O₃) on the oxidation behaviour of carbon derived Si₃N₄–SiC micro-nanocomposites has been investigated. All investigated composites exhibited predominately parabolic oxidation behaviour indicated diffusion as the rate limiting mechanism. Except the Si₃N₄–SiC composite sintered with Lu₂O₃ the rate-limiting oxidation mechanism for all other materials was an outward diffusion of the additive cations along the grain boundary towards the surface. Such diffusion of cation has been strongly suppressed in the Lu-doped composite because of the beneficial effect of stable grain boundary phase and the presence of the SiC particles predominately located at the grain boundaries of Si₃N₄. Nanoparticles at the grain boundaries act as the obstacles for migration of cations of the additives resulting in superior oxidation resistance of Si₃N₄–SiC–Lu₂O₃ where the rate-limiting step is inward diffusion of oxygen through the oxide layer to the bulk ceramics.     

Fabrication of Si3N4–SiBC composite ceramic and its excellent electromagnetic properties

To obtain better electromagnetic wave absorbing property, it is vitally necessary to develop novel ceramics with not only high dielectric loss but also low dielectric constant. Si₃N₄–SiBC, a composite ceramic with such dielectric properties, was fabricated by infiltrating SiBC into porous Si₃N₄ ceramic via low pressure chemical vapor infiltration. The high dielectric loss and the low dielectric constant are attributed to the unique microstructure of SiBC, which also leads to a very excellent wave-absorbing property of Si₃N₄–SiBC ceramic, attaining a minimal reflection coefficient of ?28 dB. Besides, the Si₃N₄–SiBC ceramic also shows a high mechanical property. Therefore, the Si₃N₄–SiBC ceramic exhibits great potential as an excellent functional and structural ceramic.     

Mechanical properties and microstructure of porous BN–SiO2–Si3N4 composite ceramics

Porous silicon nitride (Si₃N₄) ceramics incorporated with hexagonal boron nitride (h-BN) and silica (SiO₂) nanoparticles were fabricated by pressureless-sintering at relatively low temperature, in which stearic acid was used as pore-making agent. Bending strength at room and high temperatures, thermal shock resistance, fracture toughness, elastic modulus, porosity and microstructure were investigated in detail. The mechanical properties and thermal shock resistance behavior of porous Si₃N₄ ceramics were greatly influenced by incorporation of BN and SiO₂ nanoparticles. Porous BN–SiO₂–Si₃N₄ composites were successfully obtained with good critical thermal shock temperature of 800 °C, high bending strength (130 MPa at room temperature and 60 MPa at 1000 °C) and high porosity.     

Preparation and structural properties of Fe3O4–polyimide hybrid nanocomposites

Polyimide films in which magnetic Fe₃O₄ nanoparticles are uniformly distributed are prepared. Before the preparation of the Fe₃O₄–polyimide composites, pure magnetite nanoparticles (Fe₃O₄) have been synthesized in water by co-precipitation (from ferric chlorides). Its surface was firstly modified with the 3-aminopropyl triethoxysilane. The prepared polyimide–Fe₃O₄ nanocomposite films were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy, scanning electron micrograph, X-ray diffraction, and thermal analysis (DTA/TGA/DSC) techniques.     

Room temperature multiferroic properties of BiFeO3–MnFe2O4 nanocomposites

The novel functionalities of multiferroic magneto-electric nanocomposites have spawned substantial scope for fast-paced memory devices and sensor applications. Following this, herein we report the development of nanocomposites with soft ferromagnetic MnFe₂O₄ and ferroelectric BiFeO₃ to fabricate a system with engineered multiferroic properties. A modified sol-gel route called Pechini method is demonstrated for the preparation of the (1-x) BiFeO₃-x MnFe₂O₄ (x = 10%, 30%, 50%, 70%) nanocomposites. The crystallographic phase, structure, and morphology are characterized by XRD, FESEM, and HRTEM. The accurate crystallite size and lattice strain are determined by Williamson-Hall plot method and a comparative study with Scherer's equation is carried out. TEM image evidences the interface between BiFeO₃ and MnFe₂O₄ nanoparticles in the composite. The room temperature magnetic response reveals the strong dependence of magnetic saturation, remanent magnetization, and coercivity of the nanocomposites on MnFe₂O₄ addition. The dielectric response and impedance analysis of the prepared nanocomposites are observed. The electrical performance of the composite is affected by grain, grain boundaries, and oxygen vacancies. The unsaturated P-E loops exhibit the leaky ferroelectric behavior for the nanocomposite. The intrinsic magnetoelectric coupling between ferroelectric BiFeO₃ and ferromagnetic MnFe₂O₄ has been determined by varying H_dc/H_ac and its maximum coupling coefficient (α) is found to be 25.39 mV/cmOe for 70% BiFeO₃ -30% MnFe₂O₄ nanocomposite. These distinctive and achievable characteristics of the nanocomposite would enable the designing of magnetic field sensors, spintronic devices, and multiferroic memory devices.     

Fabrication and contact resistivity of W–Si3N4/TiB2–Si3N4/p–SiGe thermoelectric joints

The design and fabrication of silicon germanium (SiGe) thermoelectric elements, typically including the selection of electrode and intermediate materials, the process of joining electrode and intermediate layer onto thermoelectric materials, are the major challenge for SiGe thermoelectric device technology. In this study, W–Si₃N₄ and TiB₂–Si₃N₄ composites are designed as the electrode and intermediate layer, respectively, and the W–i₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 joints are fabricated by a one-step spark plasma sintering process. The influences of the composition of TiB₂–Si₃N₄ intermediate layer on the interfacial structure, contact resistivity and interfacial thermal stability are investigated. The interfacial thermal stability is improved with increasing Si₃N₄ content in TiB₂–Si₃N₄ intermediate layer due to the reduced mismatch of coefficients of thermal expansion between the intermediate layer and SiGe. On the other hand, the contact resistivity increases with the rising of Si₃N₄ content due to the weakened TiB₂/SiGe ohmic contact, which degrades the device efficiency. As the balanced point, the intermediate layer with the composition of 80 vol% TiB₂+20 vol% Si₃N₄ provides good interfacial thermal stability and moderately small contact resistivity (~75 μΩ cm² after aging at 1000 °C for 120 h) simultaneously, which is an optimized intermediate layer composition for W–Si₃N₄/TiB₂–Si₃N₄/p–Si₈₀Ge₂₀B_0.6 thermoelectric element. The TiB₂–Si₃N₄ intermediate layer has excellent chemical stability to both W–Si₃N₄ electrode and SiGe thermoelectric material at high temperatures, which contributes to the sharp interface of the joint and effectively prevents the inter-diffusion between the electrode and the SiGe.     

Mechanical properties of Si3N4 ceramics from an in-situ synthesized a-Si3N4/β-Si3N4 composite powder

Sintered Si₃N₄ ceramics were prepared from an ɑ-Si₃N₄/β-Si₃N₄ whiskers composite powder in-situ synthesized via carbothermal reduction at 1400–1550 °C in a nitrogen atmosphere from SiO₂, C, Ni, and NaCl mixture. Reaction temperatures and holding time for the composite powder, and mechanical properties of sintered Si₃N₄ were investigated. In the synthesized composite powder, the in-situ β-Si₃N₄ whiskers displayed an aspect ratio of 20–40 and a diameter of 60–150 nm, which was mainly dependent on the synthesis temperature and holding time. The flexural strength, fracture toughness and hardness of the sintered Si₃N₄ material reached 794±136 MPa, 8.60±1.33 MPa m^1/2 and 19.00±0.87 GPa, respectively. The in-situ synthesized β-Si₃N₄ whiskers played a role in toughening and strengthening by whiskers pulling out and crack deflection.     

Microstructure evolution and mechanical properties of porous Si3N4 and dense Si3N4 joints bonded using CaO–Li2O-Al2O3–SiO2 glass-ceramic

A novel CaO-Li₂O-Al₂O₃-SiO₂ (CLAS) glass was developed for the joining of porous Si₃N₄ and dense Si₃N₄. A multiphase interlayer consisting of CaAl₂Si₂O₈, LiAlSi₂O₆ and CaSiO₃ phases was formed in joint, which possessed matched CTE with the Si₃N₄ substrates. In addition, the infiltrated layer with bilayer structure in the porous Si₃N₄ substrate was observed. The effects of joining temperature and cooling rate on microstructure, phase evolution and shear strength of joints were studied carefully. The results showed that the kinds of precipitated phases remained invariable with the joining temperature increased, but the crystallinity in the interlayer was improved remarkably as the cooling rate reduced. The maximum shear strength of 45 MPa was obtained when the joining temperature and cooling rate were 1100 °C and 5 °C/min, respectively. Moreover, fracture during the shear test occurred mainly within porous Si₃N₄ side, indicating superior joining of dense Si₃N₄/glass-ceramic/porous Si₃N₄.     

Thermomechanical and corrosion inhibition properties of graphene/epoxy ester–siloxane–urea hybrid polymer nanocomposites

The effect of graphene on the corrosion inhibition properties of a hybrid epoxy–ester–siloxane–urea polymer was investigated. The weight fraction of graphene was varied from 1 to 2 wt%. Direct current polarization (DCP) and electrochemical impedance spectroscopic (EIS) techniques were used to measure the polarization and coating resistance of the coated aluminum alloy substrate. The grapheme/hybrid polymer composite coatings showed much higher corrosion inhibition property when compared to the neat hybrid polymer coating. An increase in glass transition temperature and rubbery region modulus was also observed for composites containing 1–2 wt.% of graphene. A direct correlation between the rubbery plateau modulus of free standing composite thin films and corrosion resistance of the composite coatings was made, indicating that the corrosion protection mechanism is due to restriction of the polymer chain motion by graphene which causes a decrease in coating permeability.     

Thermal shock resistance of Si3N4 and Si3N4–SiC ceramics with rare-earth oxide sintering additives

The influence of various rare-earth oxide additives and the addition of SiC nanoparticles on the thermal shock resistance of the Si₃N₄ based materials was investigated. The location of SiC particles inside the Si₃N₄ grains contributed to a higher level of residual stresses, which caused a failure at the lower temperature difference compared to the composites with a preferential location of the SiC at the grain boundaries. A critical temperature difference increased with an increasing ionic radius of RE³⁺ for both the composites and the monoliths. The critical temperature difference for the composite (580 °C) and the monolith (680 °C) sintered with La₂O₃ was significantly higher compared to the composite and the monolith doped with Lu₂O₃ (430 °C). A good agreement was found between the results of the critical temperature difference estimated by the indentation quench test and that obtained by the strength retention method.     

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Mechanical properties of Si3N4–Al2O3 FGM joints with 15 layers for high-temperature applications

“Crack-free” alumina-silicon nitride joints, comprised of 15 layers of gradually differing compositions of Al₂O₃/Si₃N₄, have been fabricated using sialon polytypoids as functionally graded materials (FGM) bonding layers for high-temperature applications. Using flexural strength tests conducted both at room and at elevated temperatures, the average fracture strength at room temperature was found to be 437 MPa; significantly, this value was unchanged at temperatures up to 1000 °C. Scanning electron microscopy (SEM) observations of fracture surfaces indicated the absence of any glassy phase at the triple points. This result was quite contrary to the previously reported 20-layer Al₂O₃/Si₃N₄ FGM samples where three-point bend testing revealed a severe strength degradation at high temperatures. Consequently, we believe that the joining of alumina to silicon nitride using polytypoidally functional gradients can markedly improve the suitability of these joints for high-temperature applications.     

Impedance matching optimization of SiCf/Si3N4–SiOC composites for excellent microwave absorption properties

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) are considered to be one of the most promising materials in the electromagnetic (EM) stealth of aero-engines, which is expected to achieve strong absorption and broad-band performance. Multiscale structural design was applied to SiC_f/Si₃N₄–SiOC composites by construction of micro/nanoscale heterogeneous interfaces and macro double-layer impedance matching structure. SiC_f/Si₃N₄–SiOC composites were fabricated by using SiC fibers with different conductivities and SiOC–Si₃N₄ matrices with gradient impedance structures to improve impedance matching effectively. Owing to its unique structure, SiC_f/Si₃N₄–SiOC composites (A3-composites) achieved excellent EM wave absorption performance with a minimum reflection coefficient (RC_min) of ?25.1 dB at 2.45 mm and an effective absorption bandwidth (EAB) of 4.0 GHz at 2.85 mm in X-band. Moreover, double-layer SiC_f/Si₃N₄–SiOC with an improved impedance matching structure obtained an RC_min of ?56.9 dB and an EAB of 4.2 GHz at 3.00 mm, which means it can absorb more than 90% of the EM waves in the whole X-band. The RC is less than ?8 dB at 2.6–2.8 mm from RT to 600 °C in the whole X-band, displaying excellent high-temperature absorption performance. The results provide a new design opinion for broad-band EM absorbing SiC_f-CMCs at high temperatures.     

Microstructural and mechanical characterization of the Si3N4/Si3N4 joint brazed using Au–Ni–V filler alloys

In this study, two Au–Ni–V filler metals were used to braze Si₃N₄ ceramic in the form of foils. The effects of brazing temperature and V content in the filler alloy on microstructure and bonding strength of the joint were studied. The results reveal that a VN reaction layer with a thickness about 4 μm was formed at the interface between Si₃N₄ substrate and filler alloy. With increasing brazing temperature or V content the thickness of VN reaction layer increased. A maximum joint bending strength of 242 MPa was achieved when the joint was brazed at the temperature of 1423 K for 30 min using Au58.7Ni36.5V4.8 filler alloy. The bonding mechanism was discussed with reference to the discovered phases and brazing parameters.     

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel-base superalloy joints

Si₃N₄/nickel-base superalloy (Inconel-625) and Si₃N₄/Si₃N₄ joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti-active braze Cu-ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si₃N₄/Mo/W/Inconel and Si₃N₄/Mo/W/Si₃N₄. The joints exhibited Ti segregation at the Si₃N₄/Cu-ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si₃N₄/Si₃N₄ joints both with and without W and Mo layers was ∼142 MPa but the strength of Si₃N₄/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.     

Si3N4／Si3N4陶瓷连接的研究进展

连接技术是Ｓｉ３Ｎ４陶瓷实用过程中必须解决的难题之一。本文综述了Ｓｉ３Ｎ４／Ｓｉ３Ｎ４陶瓷连接的研究现状以及不同连接工艺对连接强度的影响。     

Si3N4／Si3N4陶瓷连接的研究进展

本文综述了Ｓｉ３Ｎ４／Ｓｉ３Ｎ４陶瓷连接的研究现状，论述不同连接工艺对接头强度的影响。     

The influence of Ti: Si3N4 ratio on the microstructure evolution of Ti–Si3N4 reaction in Cu melts

In this work, through the study of the obtained Cu–Ti–Si–N intermediate products by controlling the reaction degree of Ti and Si₃N₄ powder in Cu melts at 1250～1300 °C, the effects of different Ti: Si₃N₄ mass ratios on the microstructure evolution of Ti–Si₃N₄ reaction in Cu melts were verified. When the mass ratio of Ti: Si₃N₄ is higher, such as 3.09:1, TiN will cooperate with Ti₅Si₃ and depend on each other to nucleate and grow to form TiN/Ti₅Si₃ composites. The formed TiN are spherical and wetted by Ti₅Si₃ to uniformly disperse in Cu melts. As a result, the TiN–Ti₅Si₃ hybrid reinforced Cu matrix composites will be formed. However, when the mass ratio of Ti: Si₃N₄ is lower, such as 1.37:1, Ti and Si₃N₄ will firstly react to form TiN and Ti–Si liquid. The formed TiN are irregularly polygonal and connect with each other. The Ti–Si liquid will combine with Cu melts to form Cu–Ti–Si liquid and finally form δCu₄Si, ηCu₃Si and τ₁-CuSiTi phases during the colling stage. In this case, TiN are difficult to be wetted, and the Ti–Si₃N₄ compact will keep its original shape and not spread in Cu melts.