Synthesis of pure rod-like α-Si3N4 powder with in situ C/SBA-15 composite

Well crystallized pure rod-like α-Si₃N₄ powder (PRSN) without β-Si₃N₄ was successfully synthesized by carbothermal reduction-nitridation (CRN). In situ carbon/mesoporous silica composite (C/SBA-15) was used as a new kind of raw material. Due to in situ composited carbon, the CRN temperature was decreased and the phase transition from α to β-Si₃N₄ was hindered. The sintering temperature was lowered to 1380 °C and the soaking time of the optimal synthesis condition was reduced to 6 h. Moreover, the as-synthesized rod-like α-Si₃N₄, which is induced by SBA-15, was used to enhance the fracture toughness (K_Ic) of α-Si₃N₄ based ceramics, which was sintered by spark plasma sintering (SPS). Compared with the undoped ceramics (2.9 MPa m^1/2), α-Si₃N₄ ceramics doped with 10 vol% PRSN exhibited a higher K_Ic value (4.9 MPa m^1/2), and lower dielectric loss in MHz frequency range. The results demonstrated that the PRSN powder would be promising for toughening α-Si₃N₄ based ceramics.     

Processing of biomorphic Si3N4 ceramics by CVI-R technique with SiCl4/H2/N2 system

Biomorphic porous silicon nitride Si₃N₄ ceramics have been produced by chemical vapor infiltration (CVI) of carbonized paper preforms with silicon, followed by gas–solid chemical reaction (R) of nitrogen with the infiltrated silicon. The paper was first carbonized in inert atmosphere to obtain a biocarbon (C_b) template. In a second step, silicon tetrachloride in excess of hydrogen was used to infiltrate silicon into the pores of the C_b template and to deposit silicon onto the C_b fibers. Finally, a gas–solid chemical reaction between nitrogen and infiltrated silicon in a temperature range of 1300–1450 °C took place in N₂ or N₂/H₂ atmosphere to form reaction bonded silicon nitride (RBSN) ceramics. After nitridation, the samples consist mainly of α-Si₃N₄ phase for thermal treatment below the melting point of silicon (1410 °C) or of β-Si₃N₄ phase and β-Si₃N₄/SiC-mixed ceramics for treatment at temperatures above.The crystalline phases α- and β-Si₃N₄ were identified by X-ray diffraction (XRD) analysis and the microstructure of these samples was investigated by scanning electron microscopy (SEM). Energy-dispersive X-ray analysis (EDX) was used to detect the presence of silicon, nitrogen, carbon and oxygen, whereas Raman spectroscopy was applied to identify the presence of Si and SiC. Using thermal gravimetric analysis (TGA), residual carbon was determined. It was found, that addition of 10% H₂ to the nitridation gas at temperatures near the melting point of silicon allows to increase the conversion of Si as well as to control the exothermic nitridation reaction obtaining the preferable needle-like microstructure.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

The effect of fabrication parameters on the mechanical properties of sintered reaction bonded porous Si3N4 ceramics

Porous silicon nitride ceramics were prepared via sintered reaction bonded silicon nitride at 1680 °C. The grain size of nitrided Si₃N₄ and diameter of post-sintered β-Si₃N₄ are controlled by size of raw Si. Porosity of 42.14–46.54% and flexural strength from 141 MPa to 165 MPa were obtained. During post-sintering with nano Y₂O₃ as sintering additive, nano Y₂O₃ can promote the formation of small β-Si₃N₄ nuclei, but the large amount of β-Si₃N₄ (>20%) after nitridation also works as nuclei site for precipitation, in consequence the growth of fine β-Si₃N₄ grains is restrained, the length is shortened, and the improvement on flexural strength is minimized. The effect of nano SiC on the refinement of the β-Si₃N₄ grains is notable because of the pinning effect, while the effect of nano C on the refinement of the β-Si₃N₄ grains is not remarkable due to the carbothermal reaction and increase in viscosity of the liquid phase.     

Effect of glassy bonding materials on the properties of LAS/SiC porous ceramics with near zero thermal expansion

Near zero thermal expansion porous ceramics were fabricated by using SiC and LiAlSiO₄ as positive and negative thermal expansion materials, respectively, bonded by glassy material. The coefficient of thermal expansion value of a desired porous composite can be easily controlled by choosing the appropriate ratios of the different phases. It was shown that some of LiAlSiO₄ was decomposed to LiAlSi₂O₆ and LiAlO₂, some of LiAlSiO₄ reacted with SiO₂ to form LiAlSi₂O₆ during sintering. With increasing the content of glassy materials, the reaction between LiAlSiO₄ and SiO₂ was accelerated. The Young's modulus increased due to the neck growth between the SiC grains. The 52.5 vol% LiAlSiO₄ (LAS)/SiC ceramics with ∼36% porosity had a combination of near zero coefficient of thermal expansion ∼0.39 × 10⁻⁶ K⁻¹ at room temperature and relatively high Young's modulus ∼59 GPa.     

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.     

Direct coagulation casting of silicon nitride suspension via a dispersant reaction method

A direct coagulation casting method for silicon nitride suspension via dispersant reaction was reported. Tetramethylammonium hydroxide (TMAOH) was used as dispersant to prepare silicon nitride suspension with high solid loading and low viscosity. Influences of TMAOH and pH value on the dispersion of silicon nitride powder were investigated. Glycerol diacetate (GDA) was used to coagulate the silicon nitride suspension. Influences of the concentration of glycerol diacetate on the viscosity and pH value of the suspension were investigated. It was indicated that high viscosity sufficient to coagulate the suspension was achieved by adding 1.0–2.0 vol% glycerol diacetate at 40–70 °C. The coagulation mechanism was proposed that the silicon nitride suspension was destabilized by dispersant reacting with acetic acid which was hydrolyzed from glycerol diacetate at elevated temperature. Coagulated samples could be demolded without deformation by treating 50 vol% silicon nitride suspensions with 0.2 wt% tetramethylammonium hydroxide and 1.0–2.0 vol% glycerol diacetate at different temperatures. Dense silicon nitride ceramics with relative density above 98.8% had been prepared by this method using glycerol diacetate as coagulating agent sintered by different methods.     

Influence of in-situ synthesized Zr-Al-C on microstructure and toughening of ZrB2-SiC composite ceramics fabricated by spark plasma sintering

Zr-Al-C was in-situ synthesized as a toughening component in ZrB₂-SiC ceramics by spark plasma sintering (SPS) ball-milled ZrB₂-based composite powders with SiC and graphite powders. The phase composition of Zr-Al-C toughened ZrB₂-SiC (ZSA) composite ceramics fabricated through the two-step process (ball milling and SPS) did not change dramatically with varying content of Zr-Al-C which shows a major phase of Zr₃Al₄C₆. With increasing Zr-Al-C content, the fracture toughness of the ZSA ceramics initially increased and then decreased when the content reached 40 vol%. The ZSA ceramic with 30 vol% Zr-Al-C exhibited a maximum fracture toughness value of 5.96 ± 0.31 MPa m^1/2, about 22% higher than that of the ZSA ceramic with 10 vol% Zr-Al-C. When the Zr-Al-C content goes beyond 30 vol%, the higher open porosity and component agglomeration led to the relatively lower fracture toughness. Crack deflection and bridging resulted from the weak interface bonding between Zr-Al-C and matrix phases and the weak internal layers of Zr-Al-C crystals, leading to longer crack paths and, hence, the toughened ZSA composite ceramics. Compared to the one-step in-situ synthesis process of Zr-Al-C and the direct incorporation process of synthesized Zr-Al-C grains, the two-step in-situ synthesis process not only led to the more uniform distribution of different components but also resulted in a much larger size of Zr-Al-C grains with a large aspect ratio causing longer crack propagation path as the result of crack deflection and bridging. The larger Zr-Al-C grains combined with the more homogeneous microstructure achieve the most substantial toughening of the ZSA composite ceramics. This work points out a promising approach to control and optimize the microstructure and improve the fracture toughness of ZrB₂-SiC composite ceramics by selecting the incorporation process of compound reinforcement components.     

The Microstructure and thermal diffusivity of carbon nanostructures/Si3N4 composites processed using spark plasma sintering

Graphene nanoribbons (GNRs) were obtained by unzipping multiwall carbon nanotubes (MWCNTs). Three different silicon nitride-carbon nanostructures were prepared by spark plasma sintering (SPS): ceramic composites that contained 1 wt% carbon nanofibers (CNFs), 1 wt% MWCNTs and 1 wt% GNRs respectively. The α to β-Si₃N₄ transformation ratio and thermal diffusivity of GNR/Si₃N₄ composites were higher than both CNF/Si₃N₄ composites and MWCNT/Si₃N₄ composites. Furthermore, the higher thermal diffusivities of GNR/Si₃N₄ composites can primarily be attributed to the higher number of elongate β-Si₃N₄ grains.     

Thermal expansion of β-eucryptite in oxide-based ceramic composites

This paper presents an experimental investigation on the thermal expansion behaviour of β-eucryptite (E) when it is used for processing alumina/β-eucryptite (AE) or zirconia/β-eucryptite (ZE) composites. Composite materials were prepared from a β-eucryptite powder synthetised in our laboratory and commercially available alumina or zirconia nanopowders. In order to preserve the β-eucryptite crystalline phase in sintered materials, the pressureless sintered step was performed at relatively low temperatures (<1300 °C). It is experimentally shown that in well-densified oxide-based composites, the β-eucryptite lost its slightly negative thermal expansion coefficient. This behaviour could be related to compressive residual stresses applied on β-eucryptite grains due to the thermal expansion mismatch between alumina or zirconia and the β-eucryptite.     

Effect of seeding on the thermal conductivity of self-reinforced silicon nitride

Thermal conductivity of Si₃N₄ containing large β-Si₃N₄ particles as seeds for grain growth was investigated. Seeds addition promotes growth of β-Si₃N₄ grains during sintering to develop the duplex microstructure. The thermal conductivity of the material sintered at 1900 °C improved up to 106 W m⁻¹ K⁻¹, although that of unseeded material was 77 Wm⁻¹ K⁻¹. Seeds addition leads to reduction of the sintering temperature with developing the duplex microstructure and with improving the thermal conductivity, which benefits in terms of production cost of Si₃N₄ ceramics with thermal conductivity. ©     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Spark plasma sintered β-phase silicon nitride with Sr and Ca as a sintering aid for load bearing medical applications

Due to its inherent good physical and chemical properties silicon nitride has high potential to be used for load bearing implants. However, the standard sintering additives alumina and rare earth oxides are limiting the biocompatibility of the material. The aim of the current project is to exchange the additives for more biologically beneficial additives. Spark plasma sintered silicon nitride was manufactured with strontium or calcium as sintering aids. The ability of forming high strength β-phase microstructure silicon nitride was investigated. Powders were prepared with 10 and 30 wt.% glass phase and sintered at 1600, 1650, 1700 and 1750 °C. X-ray diffraction demonstrated compositions with 10 wt.% glass phase with strontium as sintering aid to yield larger amount of β-phase. The highest amount of β-phase (96% of the crystalline structure) was obtained using SPS for strontium-doped silicon nitride at sintering temperature 1750 °C, resulting in the highest fracture toughness, 4.2 MPa m^1/2.     

High-frequency welding of polypropylene using dielectric ceramic compounds in composite adhesive layers

In this study, high-frequency welding of polypropylene by melting composite adhesive layers containing dielectric ceramics was investigated. Various dielectric ceramics were mixed in a fixed ratio with polypropylene to make the composite adhesive layers, and the resulting dielectric properties were measured using an impedance analyser. The highest loss factor in the composite adhesive layer was found when 40 vol% silicon carbide (SiC) was used in the mixture. Dynamic viscoelasticity measurements showed that all composite adhesive layers softened at approximately 170 °C and melted (fluidised) at approximately 190 °C. Each composite adhesive layer was inserted between two polypropylene plates, and irradiated at a frequency of 40 MHz. The composite adhesive layers that included 20 vol% anatase-titanium oxide, 20 vol% or 40 vol% zinc oxide and 20 vol% or 40 vol% SiC melted in 40–70 s. The bond strength of the welded material obtained was high, and the adherend failure occurred by a tensile lap shear test. The heating efficiencies of the composite adhesive layers by high-frequency radiation were related to the tanδ/ε′ value of the composites.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Performance investigation of resin bonded ferro-silicon nitride-corundum refractories after creep at 1300 °C

Novel tempered resin bonded ferro-silicon nitride-corundum refractories containing 0 wt%, 15 wt% and 25 wt% ferro-silicon nitride were prepared respectively. Creep tests were performed under a load of 0.2 MPa at a temperature of 1300 °C for 50 h in air. The results showed that creep performance was significantly improved by the addition of ferro-silicon nitride. Ferro-silicon nitride-corundum containing 15 wt% ferro-silicon nitride initially presented a steady-state stage and was able to remain stable from the beginning of the holding time until 50 h of creep testing. All the specimens exhibited cold crushing strength more than 100 MPa both before and after creep testing. Phase composition and microstructure were analyzed following the creep experiments. The results showed that Si₂N₂O and O’-sialon crystals formed in situ during creep testing, in addition to the conversion of α-Si₃N₄ to β-Si₃N₄. Liquid Fe₃Si from the ferro-silicon nitride component accelerated the formation of the O’-sialon and prolonged the growth of β-Si₃N₄, which improved the creep performance significantly. Fe₃Si liquid migrated into the pores, and some Fe₃Si coexisted with residual carbon from the resin, which filled a part of pores and protected the specimens from severe oxidation.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

Fabrication of Si3N4-based composite containing needle-like TiN synthesized using NH3 nitridation of TiO2 nanofiber

A Si₃N₄ composite containing needle-like TiN particles (7 vol%) was fabricated. Needle-like TiN particles several micrometers long were synthesized using NH₃ nitridation of TiO₂ nanofiber, which was obtained using hydrothermal treatment. A mixed powder of α-Si₃N₄ and the needle-like TiN particles with additives was hot pressed at 24 MPa and 1850 °C for 1 h in N₂ atmosphere. Mechanical properties of the composite were compared with those of a composite containing rounded TiN particles and a monolithic β-Si₃N₄ ceramic. The Si₃N₄ matrix of the composites containing TiN was mainly a-phase, suggesting that the α–β phase transformation of Si₃N₄ was inhibited by the presence of TiN. Although fracture strength of the composites was lower, fracture toughness was comparable to that of monolithic β-Si₃N₄ ceramics. Hardness of the composites was about 19 GPa and was greater than that of the monolithic β-Si₃N₄ ceramic.     

Dispersion,slip casting and reaction nitridation of silicon–silicon carbide mixtures

The dispersing behaviour of silicon, silicon carbide and their mixtures in aqueous media were monitored by particle size, sedimentation, viscosity and zeta potential analyses as a function of pH of the slurry. The pH values for optimum dispersion were found to be 4 and 8 for silicon, 10 for SiC and 9 for Si+SiC mixtures. Optimum slips of Si+SiC mixtures were slip cast to obtain green compacts which were nitrided once at 1450°C for 2 or 4 h or successively and cumulatively for 8 (2+6) and 10 (4+6) h in a resistively heated graphite furnace. The binding phases in the nitrided products were found to be fibrous/needle like α-Si₃N₄, flaky grains of β-Si₃N₄ and Si₂ON₂. The products containing 19–47% of silicon nitride as bond/matrix possessed flexural strength (three-point bending) values of 50–85 MPa. ©     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.