Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

Improved electromagnetic absorbing properties of Si3N4–SiC/SiO2 composite ceramics with multi-shell microstructure

Porous Si₃N₄–SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄–SiC grains during oxidation at 1100 °C in air. The as-received Si₃N₄–SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄–SiC/SiO₂ fabricated by oxidation of Si₃N₄–SiC for 10 h in air can achieve a reflection loss of ?30 dB (>99.9% absorption) at 8.7 GHz when the sample thickness is 3.8 mm. When the sample thickness is 3.5 mm, reflection loss of Si₃N₄–SiC/SiO₂ is lower than ?10 dB (>90% absorption) in the frequency range 8.3–12.4 GHz, the effective absorption bandwidth is 4.1 GHz.     

Reclamation of used SiC grinding powder and its sintering characteristics

A process of recycling used abrasive SiC powder after grinding Si wafer was proposed to raw powder for sintering. The used SiC powder could be successfully converted to composite powders consisting of SiC particle and Si₃N₄ whisker via a heat treatment in N₂ atmosphere, in which iron oxide acted as a catalyst in the vapor–liquid–solid (VLS) formation of Si₃N₄. With the addition of 3 mass% Al₂O₃ and 1 mass% Y₂O₃, the composite powders sintered at 1900 °C for 2 h exhibited a 3-point bending strength of 626 ± 48 MPa and a fracture toughness of 3.9 ± 0.1 MPa m^1/2, which were significantly enhanced as compared with those of using recovered powder merely composed of SiC particle. The strength and fracture toughness of the sintered material could be improved by optimization of chemical and heat treatment parameters and controlling the amount of sintering additives and hot pressing conditions.     

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.     

Synthesis and mechanical properties of (Mo0.94Nb0.06)(Si0.97Al0.03)2−x vol% SiC composites fabricated via SHS-HP

(Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂–SiC composites were consolidated via vacuum HP sintering from the mixture of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder and SiC nanoparticles. The (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂ powder was prepared by self propagating synthesis (SHS). The effects of SiC content on the ambient temperature mechanical properties and microstructure of the SHS-HP composites were investigated. The results show that the indentation fracture toughness, bending strength and Vickers hardness of (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−x vol% SiC composites increase gradually with the increase of SiC content. The grain size and relative density of the composites decrease gradually with increase of SiC content. (Mo_0.94Nb_0.06)(Si_0.97Al_0.03)₂−15 vol% SiC composite exhibits excellent mechanical properties: Vickers hardness 12.62 GPa, fracture toughness 5.01 MPa m^1/2, bending strength 472 MPa. With the addition of SiC, the fracture mode transforms from a mixed mode of transcrystalline and intercrystalline fracture to mainly transcrystalline fracture.     

Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

Phase evolution and crystallization in Si–B–C–N ceramics derived from a polyborosilazane precursor: microstructural characterization

Amorphous Si–B–C–N ceramic powder samples obtained by thermolysis of polyborosilazane {B[C₂H₄Si(H)NH]₃}_n were isothermally annealed at different temperatures (1400–1800 °C) and hold-times (3, 10, 30, 100 h). Scanning electron microscopy (SEM) of annealed powders as well as polished cross sections of large powder particles from selected samples were carried out to study surface morphology, crystallization and associated microstructural changes. Microstructural and phase evolution were additionally investigated using high-resolution transmission electron microscopy (HRTEM) and energy filtering TEM (EFTEM). Higher surface areas of the powders were found to promote vapor-phase decomposition reactions resulting in SiC whisker growth. In coarser powders the influence of surface is manifested as a skin-core effect, where the ‘skin’ has undergone a higher degree of decomposition accompanied by an increased SiC crystal growth, compared to the ‘core.’ Crystallization of SiC occurs already at 1400 °C, although Si₃N₄ crystallization occurs only at 1700 °C, after more than 3 h of annealing.     

The influence of lignosulphonate on the properties of single and mixed Si3N4 and ZrO2 suspensions

Lignosulphonate (LS), was evaluated as a dispersing agent for single and mixed powder slips, containing Si₃N₄ and ZrO₂ powders, respectively. The surface charge was investigated by measuring the electrophoretic mobility in dilute powder dispersions. The slip stability was determined by visual observations of the settling of the powder and the fluidity of the slip was monitored by measuring the viscosity. It was evident that the different powder dispersions were most successfully stabilized by a high charge induced at high pH-values. However, in order to produce a sufficient fluidity of the slip, a dispersing agent had to be introduced. The optimal concentration range of LS at pH = 10 was found to be 0·2 wt%. The stabilizing mechanism in alkaline dispersions seems to be electrostatic depletion stabilization. The flow properties were finally tested by recording the viscosity for a mixed (composite) powder containing 68·4 wt% Si₃N₄ and 31·6 wt% ZrO₂ at a total solids content of 29–37 vol%.     

Effect of magnesium titanate content on microstructures,mechanical performances and dielectric properties of Si3N4-based composite ceramics

Silicon nitride-based composite ceramics with different contents of magnesium titanate have been fabricated via gas pressure sintering method. The phase compositions, microstructure, mechanical performances and dielectric properties of the composite ceramics were investigated. The density of the Si₃N₄-based composite ceramics firstly increased with additive of magnesium titanate powder up to 5 wt% and then gently decreased, and the mechanical properties firstly increased and then declined. Besides, the dielectric constant and dielectric loss increased with the increase of magnesium titanate contents. For the Si₃N₄-based composite ceramics with 5 wt% magnesium titanate powders, the flexural strength, elastic modulus, dielectric constant and dielectric loss reached 451 MPa, 274 GPa, 7.65, 0.0056, respectively. These results suggested that the magnesium titanate was beneficial for the improvement of mechanical performances and dielectric constant of Si₃N₄-based composite ceramics.     

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti₃SiC₂ laminated composite is prepared by spark plasma sintering at 1300 °C under a pressure of 50 MPa. Al powder is used as sintering aid to assist the formation of Ti₃SiC₂. The fabricated composites were annealed at 800, 1000 and 1150 °C under vacuum for 5, 10, 20 and 40 h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo₂C layer, AlMoSi layer and Ti₅Si₃ solid solution layer are formed during sintering. Experimental results show that the Mo/Ti₃SiC₂ layered composite prepared in this study has good interfacial phase stability up to at least 1000 °C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150 °C for 10 h, cracks formed at the interface.     

A comparative study of SiAlON ceramics

Two different varieties of Si₃N₄ powders were used to prepare SiAlON ceramics. 100% β-Si₃N₄ was used from refractory grade powders (B1) and another purer 98% α-Si₃N₄ (50A) powder was used to prepare the SiAlON samples. Since SiC + SiAlON composites reportedly perform better, batches were prepared with 15% SiC addition to the refractory powders (B1) and 17.5% SiC was added to the other SiAlON composition (50A). The samples were gas pressure sintered at 1840 °C and at 22 bar with 1 h dwelling time. Thereby, we could achieve 97–98% theoretical density. The hardness was recorded 14–17 GPa while fracture toughness varied from 4.3 to 5 MPa m^1/2. Fretting experiments showed initial running-in period of 300 cycles for all the tribo-couples. After which, the steady state coefficient of frictions (COF) were achieved. Steel ball of 10 mm diameter, fretting against 50A composition, showed 0.6 average steady state COF while the same composition while fretting against alumina ball of the same diameter, showed 0.57 average steady state COF. Results have been compared with SiAlON composition derived from refractory powder (B1) and found that the 50A composition performs better under identical test conditions. Moreover, cytocompatibility study also suggests that the investigated 50A composition can be used as substrate to support cell adhesion and proliferation of L929 mouse fibroblast cell lines whereas B1 composition derived from refractory powders are toxic in nature.     

Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4

HfB₂ powders were synthesized via a borothermal reduction route from mechanically activated HfCl₄ and B powder blends. Mechanical activation of the powder blends was carried out for 1 h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100 °C for 1 h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B₂O₃) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO₂ phase. HfB₂ powders without any detectable HfO₂ were prepared by adding 20 wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM).     

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si₃N₄) ceramics were fabricated using a mixture of Si₃N₄ and silicon (Si) powders via conventional processing and sintering method. These Si₃N₄ ceramics with sintering additives of ZrO₂ + Gd₂O₃ + MgO were sintered at 1800 °C and 0.1 MPa in N₂ atmosphere for 2 h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si₃N₄ samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39 g/cm³ to 2.92 g/cm³ except for the sample without initial Si₃N₄ powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si₃N₄ ceramics with a good combination of mechanical and thermal properties.     

Novel microwave dielectric LTCCs based uponV2O5 doped M2+Cu2Nb2O8 compounds (M2+ = Zn,Co, Ni,Mg and Ca)

The copper-niobates, M²⁺Cu₂Nb₂O₈ (M²⁺ = Zn, Co, Ni, Mg or Ca) have good microwave dielectric properties when sintered between 985–1010 °C and 1110 °C for CaCu₂Nb₂O₈. Therefore, they would be potential dielectric LTCC materials if they could be made to sinter below 960 °C (melting point of silver). To this end, additions of 3 wt.% V₂O₅ were made to ZnCu₂Nb₂O₈, CoCu₂Nb₂O₈, NiCu₂Nb₂O₈, MgCu₂Nb₂O₈ and CaCu₂Nb₂O₈, and their sintering and dielectric behaviour was investigated for samples fired between 800 and 950 °C. Doping lowered sintering temperatures to below the 960 °C limit in all cases. Doping had the general effect of reducing ɛ_r, density, Qf and τ_f, although doped CaCu₂Nb₂O₈ had a Qf value of 9300 GHz, nearly four times that of the best undoped sample. Doped ZnCu₂Nb₂O₈ fired to 935 °C had Qf = 10,200 GHz, and for doped CoCu₂Nb₂O₈ fired to 885 °C Qf = 7500 GHz. When doped and undoped samples all fired to 935 °C were compared, all doped samples had greater ɛ_r and density, and all except ZnCu₂Nb₂O₈ had a smaller τ_f. All doped samples had a more linear relationship between frequency and temperature in the range 250–300 K.     

Effects of PMMA on porous structure of pollucite

The effects of PMMA as a pore-forming reagent and the powder for Cs-deficient pollucite, Cs₉Al_0.9Si_2.1O₆, calcined at 1073 K, on the microstructure of the porous body of Cs_0.9Al_0.9Si_2.1O₆ were investigated. The Cs_0.9Al_0.9Si_2.1O₆ porous bodies were fabricated by sintering the green compacts of the calcined powder and PMMA adding 35 mass% to the calcined powder. When the green compact was heated at 873 K in air for 20 h, pores <1 μm were observed in the porous body, suggesting that the PMMA previously dissolved in acetone was uniformly distributed in the calcined powder by the ball milling. The pore size of the obtained porous structure increased with increasing the size of the aggregated particles and the pore size distribution was significantly related to the size of Al₂O₃ balls and the time for the ball milling for mixing the calcined powders and PMMA.     

Sinterability,microstructures and electrical properties of Ni/Sm-doped ceria cermet processed with nanometer-sized particles

Ni/Sm-doped ceria (SDC) cermet was prepared from two types of NiO/SDC mixed powders: Type A—Mechanical mixing of NiO and SDC powders of micrometer-sized porous secondary particles containing loosely packed nanometer-sized primary particles. The starting powders were synthesized by calcining the oxalate precursor formed by adding the mixed nitrate solution of Ce and Sm or Ni nitrate solution into oxalic acid solution. Type B—Infiltration of Ni(NO₃)₂ solution into the SDC porous secondary particles subsequently freeze-dried. Type B powder gave denser NiO/SDC secondary particles with higher specific surface area than Type A powder. The above two types powders were sintered in air at 1100–1300 °C and annealed in the H₂/Ar or H₂/H₂O atmosphere at 400–700 °C. Increased NiO content reduced the sinterability of Type A powder but the bulk density of Type B powder compact showed a maximum at 34 vol.% NiO (25 vol.% Ni). Type B cermet was superior to Type A cermet in achieving fine-grained microstructure and a homogeneous distribution of Ni and SDC grains. The electrical resistance of the produced cermet decreased drastically at 15 vol.% Ni for Type B and at 20 vol.% Ni for Type A.     

Sol–emulsion–gel synthesis of cordierite ceramics for high-frequency multilayer chip inductors

Nano-Cordierite powders used for high frequency chip inductors (MLCIs) were prepared by sol–emulsion–gel method. Effects of precursor concentration and [H₂O]/[Si] molar ratio on this material were studied. The sol–emulsion–gel processing of Mg₂Al₄Si₅O₁₈ as well as its dielectric property were investigated. Owing to the better packing efficiency and therefore higher surface energy of the freestanding nano-powder, the pressed pellets made by cordierite powder showed 98.6% theoretical density at 900 °C for 2 h. The additive Bi₂O₃ was utilized to promote the crystallization or transformation to α-cordierite and sintering. The sol–emulsion–gel-derived cordierite ceramics have low dielectric constant (ε=3.0～4.0; 18 GHz) and low dielectric loss (tgδ<0.001; 18 GHz) and can be co-fired with high conductivity metals such as Au, Ag/Pd internal electrode at low temperature (900 °C), suggesting that it was an ideal dielectric material for high-frequency multilayer chip inductors.     

The preparation of silicon nitride ceramics by gelcasting and pressureless sintering

A new gelling system based on the polymerization of hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was successfully developed for fabricating silicon nitride (Si₃N₄) ceramics. The effects of pH value, the dispersant content, solid volume fraction and hydantion epoxy resin amount on the rheological properties of the Si₃N₄ slurries were investigated. The relative density of green body obtained from the solid loading of 52 vol% Si₃N₄ slurry reached up to 62.7%. As the concentration of hydantion epoxy resin increased from 5 wt% to 20 wt%, the flexural strength of Si₃N₄ green body enhanced from 5.3 MPa to 31.6 MPa. After pressureless sintering at 1780 °C for 80 min, the sintered samples exhibited the unique interlocking microstructure of elongated β-Si₃N₄ grains, which was beneficial to improve the mechanical properties of Si₃N₄ ceramics. The relative density, flexural strength and fracture toughness of Si₃N₄ ceramics reached 97.8%, 687 MPa and 6.5 MPa m^1/2, respectively.     

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 and fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as filler. The influence of the addition of GPLs on the microstructure development and on the fracture toughness of Si₃N₄ + GPLs composites was investigated. The GPLs with thickness from 5 nm to 50 nm are relatively homogeneously distributed in the matrix of all composites, however overlapping/bundle formation of GPLs was found, containing 2–4 platelets as well. The single GPLs and overlapped GPLs are located at the boundaries of Si₃N₄, and hinder the grain growth and change the shape of the grains. The fracture toughness was significantly higher for all composites in comparison to the monolithic Si₃N₄ with the highest value of 9.9 MPa m^0.5 for the composite containing the GPLs with smallest dimension. The main toughening mechanisms originated from the presence of graphene platelets, and responsible for the increase in the fracture toughness values are crack deflection, crack branching and crack bridging.