Dielectric and mechanical properties of porous Si3N4 ceramics prepared via low temperature sintering

Borophosphosilicate bonded porous silicon nitride (Si₃N₄) ceramics were fabricated in air using a conventional ceramic process. The porous Si₃N₄ ceramics sintered at 1000–1200 °C shows a relatively high flexural strength and good dielectric properties. The influence of the sintering temperature and contents of additives on the flexural strength and dielectric properties of porous Si₃N₄ ceramics were investigated. Porous Si₃N₄ ceramics with a porosity of 30–55%, flexural strength of 40–130 MPa, as well as low dielectric constant of 3.5–4.6 were obtained.     

Highly porous corundum–mullite ceramics – Structure and properties

The aim of this study was to improve the mechanical properties of porous corundum ceramics by adding various types of SiO₂ source (SiO₂, SiC and Si₃N₄), but at the same time retaining high porosity (at least 55%). Ceramics were fabricated by slip casting. Pores were formed using aluminium's reaction with water. It was found that the bending strength of the material can be improved and relatively high porosity retained by producing corundum–mullite composites. Addition of 3.7 equivalent wt% of SiO₂ source increased the bending strength by up to 250% in comparison with unmodified corundum ceramics. The apparent porosity decreased by up to ca. 8%. If the amount of SiO₂ source was increased from 3.7 equivalent wt% to 7.3 equivalent wt%, the bending strength decreased. The best mechanical properties were achieved with samples that were modified with SiC and Si₃N₄ nanopowders. This is due to better dispersion in Al₂O₃ matrix.     

Paper derived SiC–Si3N4 ceramics for high temperature applications

Biomorphic Si₃N₄–SiC ceramics have been produced by chemical vapour infiltration and reaction technique (CVI-R) using paper preforms as template. The paper consisting mainly of cellulose fibres was first carbonized by pyrolysis in inert atmosphere to obtain carbon bio-template, which was infiltrated with methyltrichlorosilane (MTS) in excess of hydrogen depositing a silicon rich silicon carbide (Si/SiC) layer onto the carbon fibres. Finally, after thermal treatment of this Si/SiC precursor ceramic in nitrogen-containing atmosphere (N₂ or N₂/H₂), in the temperature range of 1300–1450 °C SiC–Si₃N₄ ceramics were obtained by reaction bonding silicon nitride (RBSN) process. They were mainly composed of SiC containing α-Si₃N₄ and/or β-Si₃N₄ phases depending on the nitridation conditions. The SiC–Si₃N₄ ceramics have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Raman spectroscopy. Thermal gravimetric analysis (TGA) was applied for the determination of the residual carbon as well as for the evaluation of the oxidation behaviour of the ceramics under cyclic conditions. The bending strength of the biomorphic ceramics was related to their different microstructures depending on the nitridation conditions.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

In situ preparation of Si3N4–TiN composite by pyrolysis of polytitanosilazane (PTSZ)

Si₃N₄–TiN composite powders were obtained by in situ pyrolysis of polytitanosilazane. Dense Si₃N₄–TiN composites were prepared by hot-pressing at 1800 °C under 20 MPa for 2 h without sintering additive. Crystallization of amorphous PTSZ powders occurred between 1400 and 1500 °C with major phases, α-Si₃N₄, β-Si₃N₄, and small amount of phase TiN. Mechanical properties and microstructure of Si₃N₄–TiN composites were characterized. The results showed that the mechanical strength was 620 MPa, the fracture toughness was 7.8 MPa m^1/2 and the Vickers hardness was 8.5 GPa. SEM analysis indicated that Si₃N₄–TiN composite possessed excellent fracture toughness because TiN grains produced by in situ pyrolysis were well dispersed in Si₃N₄ matrix.     

Effect of carboxymethyl cellulose addition on the properties of Si3N4 ceramic foams

The effect of carboxymethyl cellulose (CMC) addition on the preparation of Si₃N₄ ceramic foam by the direct foaming method was investigated. The addition of CMC in the foam slurry can reduce the surface tension, increase the viscoelasticity of foams, and improve their stability and fluidity. The foam ceramics show low shrinkage during drying owing to the CMC and the gelation of acrylamide monomers. The surface structure of dried foam is uniform, and there are no macropores and cracks on the surface. The sintered Si₃N₄ foam ceramics have very uniform pore distribution with average pore size of about 16 μm; the flexure strength is as high as 3.8–77.2 MPa, and the porosity is about 60.6–82.1%.     

Microstructure and reaction phases in Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was self-jointed using a filler alloy of Cu–Pd–Ti, and the microstructure of the joint was analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15 (at.%), a high quality Si₃N₄/Si₃N₄ joint was obtained by brazing at 1100–1200 °C for 30 min under a pressure of 2 × 10⁻³ MPa. The microstructure of the Si₃N₄/Si₃N₄ joint which was observed by EPMA, XRD and TEM, and the results indicated that a reaction layer of TiN existed at the interface between Si₃N₄ ceramic and filler alloy. The center of the joint was Cu base solid solution containing Pd, and some reaction phases of TiN, PdTiSi and Pd₂Si found in the Cu [Pd] solid solution.     

Simultaneous synthesis and consolidation of nanostructured TaSi2–Si3N4 composite by pulsed current activated combustion

Dense nanostructured 4TaSi₂–Si₃N₄ composite was synthesized by pulsed current activated combustion synthesis (PCACS) method within 3 min in one step from mechanically activated powders of TaN and Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense 4TaSi₂–Si₃N₄ composite with relative density of up to 98% was produced under simultaneous application of a 60 MPa pressure and the pulsed current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Microstructure evolution of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti + SiCp composite filler

The Si₃N₄ ceramic was brazed by Ag–Cu–Ti + SiCp composite filler (p = particle) prepared by mechanical mixing. Effects of the content of Ti and SiC particles on microstructure of the joint were investigated. A reliable Si₃N₄/Si₃N₄ joint was achieved by using Ag–Cu–Ti + SiCp composite filler at 1173 K for 10 min. A continuous and compact reaction layer, with a suitable thickness, forms at the Si₃N₄/braze interface. The SiC particles react with Ti in the brazing layers, forming Ti₃SiC₂ thin layers around the SiC particles themselves and Ti₅Si₃ small particles in the Ag[Cu] and Cu[Ag] based solid solution. The higher content of SiC particles in the filler (≥10 vol%) depresses interfacial bonding strength between the Si₃N₄ substrate and composite brazing layer due to the thinner reaction layer and the bad fluidity of the filler. The Ti₃SiC₂ → TiC + Ti₅Si₃ reaction occurs when Ti concentration around SiC particles in the filler increases.     

Fabrication and characterization of Si3N4 whisker-reinforced SiO2 ceramic for radome materials

Fused silica ceramic has become one of the most widely used radome materials in the world since the 1970s. But its poor mechanical properties restricted its application to some extent. To improve the mechanical properties of the fused silica ceramic and keep its characteristic for radome materials, silicon nitride (Si₃N₄) whisker-reinforced fused silica ceramics were prepared by a slip-casting method in the work. The influence of Si₃N₄ whisker contents on the properties of the slurry was studied, indicating that the preferable pH values of the slurry were 4–6 and whisker contents were 10 wt.%. The flexural strength of as-prepared Si₃N_4w/SiO₂ ceramic was about 74.35 MPa, exhibiting an increase of 7.75% over that of the pure silica sample. Its dielectric constant in the range from 8 to 12 GHz and tanδ under 10 GHz were, respectively, 3.37 and .0011. It is of great interest to find that Si₃N_4w/SiO₂ has excellent oxidization resistance and its mass maintains even at 1270°C.     

Effect of in situ synthesis of Si2N2O on microstructure and the mechanical properties of fused quartz ceramics

Si/SiO₂ composite billets were prepared using a low-toxicity gel system, and the resulting billets were sintered at high temperature in nitrogen to synthesize Si₂N₂O in the central position of the fused silica ceramic matrix. The influences of in situ synthesized Si₂N₂O on the microstructure and mechanical properties of fused silica ceramics were studied. The results show that Si/SiO₂ composite billets can be used to synthesize spike-like and fibrous Si₂N₂O in situ in nitrogen at 1450 °C. Si₂N₂O synthesized in situ can improve the mechanical properties and microstructure of quartz ceramics. When the Si/SiO₂ composite billet is sintered in nitrogen at 1450 °C for 2 h, the volume density and bending strength of the quartz ceramics can reach 2.36 g/cm³ and 114.37 MPa, respectively.     

In-situ reaction synthesis of porous Si2N2O-Si3N4 multiphase ceramics with low dielectric constant via silica poly-hollow microspheres

In the present work, a novel and facile process has been proposed to fabricate porous Si₂N₂O-Si₃N₄ multiphase ceramics with low dielectric constant (ε_r＜4.0). Since silica poly-hollow microspheres could serve as the source of SiO₂ and the pore-forming agent, they have been introduced into Si₃N₄ slurry through the gelcasting technique. This process is benefited from the liquid phase sintering reaction between SiO₂ and Si₃N₄ with the aid of sintering additives, leading to in-situ synthesis of Si₂N₂O phase and porous structure. The content of silica poly-hollow microspheres has great influence on the properties of the final products. It indicates that Si₂N₂O phase would become the major phase when the content of silica poly-hollow microspheres was above 25 wt%. Furthermore, the micromorphology results reveal that the content of pores with many smaller aggregate microspheres increases as microspheres amount rises. As a result, along with the addition of silica poly-hollow microspheres, the bulk density decreases to 1.32±0.01 g/cm³, and open porosity ranges from 28.4±0.4% to 52.0±0.5%. Porous Si₂N₂O-Si₃N₄ multiphase ceramics prepared with 25 wt% silica poly-hollow microspheres addition possess flexural strength of 42.3±3.8 MPa, low dielectric constant of 3.31 and loss tangent of 1.93×10⁻³. It turns out to be an effective method to fabricate porous Si₂N₂O-Si₃N₄ composites with excellent mechanical and dielectric properties, which could be applied to radome materials.     

Pyrolysis synthesis of Si–B–C–N ceramics and their thermal stability

Si–B–C–N ceramics were synthesized by co-pyrolyzing hybrid polymeric precursors of polycarbosilane (PCS) and polyborazine (PBN). The pyrolysis behavior and structural evolution of the hybrid precursor, the microstructure and composition of the prepared Si–B–C–N ceramics were fully investigated. It was found that the copyrolysis of hybrid polymeric precursors in Ar led to the release of CH₄, CH₃NH₂ and CH₃CN gases at temperatures ranging from 200 to 1100 °C, and finally resulted in the formation of amorphous Si–B–C–N ceramics. In particular, the Si–B–C–N ceramics formed from the hybrid precursor with PBN/PCS mass ratio of 1 could keep amorphous state up to the annealing temperature of 1800 °C with weight change of only 2.08%. But this amorphous ceramics would decompose to form crystalline SiC, BN and Si₃N₄ at 2000 °C. Additionally, compared with PCS-derived SiC ceramics, the Si–B–C–N ceramics showed improved anti-oxidation performance up to 1300 °C due to the formation of borosilicate layers covering the ceramics.     

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.     

A facile strategy for fabricating self-sealing dense coating grown in-situ on porous silicon nitride ceramics

This present work explores initially the feasibility of producing in-situ surface oxidized coating on porous silicon nitride (Si₃N₄) ceramics. Theoretical prediction identifies the applied conditions of self-sealing strategy and oxidation time required to form dense coating. Experimentally, the porous Si₃N₄ ceramics with different pore structures were selected to fabricate in-situ oxidized coatings. The phase compositions, microstructures and mechanical properties of the porous Si₃N₄ ceramics were investigated before and after oxidation. The results show that flat and dense coatings are prevailed in all samples, which consist of amorphous SiO₂ and its precipitates besides dominant Si₃N₄ phase. The strengthened substrate and strengthening effect of coating are the essential mechanisms associated with the improved mechanical properties. Self-sealing method seems to offer an inexpensive and efficient route to prepare coating on porous Si₃N₄ ceramics.     

Synthesis and microwave dielectric properties of CaSiO3 nanopowder by the sol–gel process

The synthesis and microwave dielectric properties of CaSiO₃ nanopowder by sol–gel method have been investigated in this paper. CaSiO₃ nanoparticles with an average grain size of 50–60 nm were obtained by calcining the CaO–SiO₂ xerogel that was prepared from Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and tetraethylortho silicate (TEOS). Calcining the CaO–SiO₂ xerogel at 1150 °C, the pseudowollastonite-CaSiO₃ phase was completely formed. However, the main phase is not CaSiO₃ or CaSi₂O₄ but SiO₂ when calcining the mixture of SiO₂ and CaCO₃ at 1150 °C. Comparing with CaO–SiO₂ ceramics prepared by solid-state process, the CaSiO₃ ceramics made from nanopowders calcined at 1000 °C achieved more compact structure at the sintering temperature of 1320 °C, and then had excellent microwave dielectric properties: ?_r = 6.69, Qf = 25398 GHz.     

Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Thermally activated sintering–coarsening–coalescence-polymerization of amorphous silica nanoparticles

An onset sintering–coarsening–coalescence-polymerization (SCCP) event of amorphous SiO₂ nanoparticles (ca. 40–100 nm in size) by isothermal firing in the 1150–1300 °C range in air was characterized by an N₂ adsorption–desorption hysteresis isotherm coupled with X-ray diffraction and vibrational spectroscopy. The apparent activation energy of such a rapid SCCP process was estimated as 177±32 kJ/mol, based on 30% reduction of a specific surface area with an accompanied change of medium range orders, i.e. forming Si₂O₅ while retaining the Si–2ndO yet losing the Si–2ndSi without appreciable crystallization. The minimum temperature of the SCCP process, as of concern to industrial silica applications and sedimentary/metamorphosed sandstone formation, is 1120 °C based on the extrapolation of steady specific surface area reduction rates to null.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.