Effect of the addition of β-Si3N4 nuclei on the thermal conductivity of β-Si3N4 ceramics

Various microstructures of β-Si₃N₄ were fabricated, with or without the addition of β-Si₃N₄ seed particles to high-purity β-Si₃N₄ powder, using Yb₂O₃ and ZrO₂ as sintering additives, by gas-pressure sintering at 1950 °C for 16 h. The thermal conductivity of the specimen without seeds was 140 W·(m·K)⁻¹, and the specimen exhibited a bimodal microstructure with abnormally grown grains. The thermal conductivity of the specimen with 24 vol.% seed addition was 143 W·(m·K)⁻¹, and this specimen had the bimodal microstructure with finer grain size than that without the seeded material, but maintained the same amount of large grains (⩾2 μm in diameter) as in the specimen without the seeds. This finding indicates that the thermal conductivity of β-Si₃N₄ is controlled by the amount of reprecipitated large grains, rather than by the grain size of the β-Si₃N₄.     

Combustion synthesis of well-dispersed rod-like β-Si3N4 crystals by the addition of carbon

The well-dispersed rod-like β-Si₃N₄ crystals have been prepared by combustion synthesis with the addition of carbon. The added carbon helps to separate Si particles and remove the SiO₂ oxide layer, and thus reduces the agglomeration of β-Si₃N₄ crystals. By adding carbon, the reaction temperature and the width of β-Si₃N₄ crystals are decreased, but the aspect ratio of β-Si₃N₄ crystals is increased. The well-dispersed β-Si₃N₄ crystals with an average width of 0.84 μm and aspect ratio of 2.3 are produced by adding 2 wt% carbon. When 5 wt% carbon is added, the reaction temperature is too low and the nitridation of Si becomes incomplete, and at the same time much SiC occurs in the product.     

Influence of β-Si3N4 whiskers on sintering and crystallization of fused silica ceramics

Fused silica ceramics are widely applied for radome materials, crucibles, and vanes, but the mechanical properties were deteriorated due to the cristobalite crystallization. The fused silica ceramics added with by β-Si₃N₄ whiskers were prepared by a slip-casting method to retard the cristobalite crystallization. The influences of the sintering environments and the β-Si₃N₄ whiskers on the microstructure and phase structure were investigated. The silanol (Si-(OH)_n) and oxygen vacancies (V_O) in the fused silica in formed in different conditions were studied by Fourier Transform Infra-Red (FT-IR) and X-ray photoelectron spectroscopy (XPS), and the results indicated that the ball-milled produced a large amount of the silanol groups onto the surface of the fused silica particles. The fused silica heated in the vacuum created the maximum oxygen vacancies (24.2%) on the surfaces. Silanol groups reacted with the β-Si₃N₄ whiskers, and the O atoms in the silanol groups were fixed into the bulk materials. And the crystallization kinetics and the activation energy of Si₃N_4w/SiO₂ ceramics at the temperature ranging from 1200 to 1400°C were calculated based on the JMA(Johnson-Mehl-Avrami) model. The activation energy of the fused silica ceramics with the addition of the β-Si₃N₄ is 506.2 kJ/mol, increased by 23.6% than that of the pure fused silica ceramic.     

Low temperature heat capacity measurements of β-Si3N4 and γ-Si3N4: Determination of the equilibrium phase boundary between β-Si3N4 and γ-Si3N4

Isobaric heat capacities of β-Si₃N₄ and γ-Si₃N₄ were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of γ-Si₃N₄ are smaller than those of β-Si₃N₄ in this temperature range. Using these data, we determined the standard entropies of β-Si₃N₄ and γ-Si₃N₄ to be 62.30 J·mol⁻¹ K⁻¹ and 51.79 J·mol⁻¹ K⁻¹, respectively. The equilibrium phase boundary between β-Si₃N₄ and γ-Si₃N₄ was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which γ-Si₃N₄ was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K⁻¹, which is consistent with those of theoretical studies.     

Microstructural development of silicon nitride with aligned β-Si3N4 whiskers

Silicon nitride was sintered with 3 wt.% silicon nitride whiskers that were aligned using tape casting. Sintering was carried out at temperatures between 1550 and 1850°C for 1 h. The α to β phase transformation was complete at 1750°C. XRD results also showed that the amount of β-phase grains in the matrix increased faster than growth rate of the whisker grains at the early stage of sintering. The intensities of the peaks diffracted from the whisker grains increased faster than those diffracted from the matrix β-phase grains after the α to β phase transformation was complete. Both XRD results and the etched microstructures indicated that the whisker grains grew preferentially in the length direction.     

Effect of large β-Si3N4 particles on the thermal conductivity of β-Si3N4 ceramics

Mean-field micromechanics model, the rule of mixture is applied to the prediction of the thermal conductivity of sintered β-Si₃N₄, considering that the microstructure of β-Si₃N₄ is composed of a uniform matrix phase (which contains grain boundaries and small grains of Si₃N₄) and the purified large grains (⩾2 μm in diameter) of Si₃N₄. Experimental results and theoretical calculations showed that the thermal conductivity of Si₃N₄ is controlled by the amount of the purified large grains of Si₃N₄. The present study demonstrates that the high thermal conductivity of β-Si₃N₄ can be explained by the precipitation of high purity grains of β-Si₃N₄ from liquid phase.     

Synthesis of β-Si3N4 particles from α-Si3N4 particles

This report describes an investigation of the synthesis of β-Si₃N₄ particles from α-Si₃N₄ particles. The β fraction of Si₃N₄ particles was found to depend on temperature, heating time, and the type of crucibles in which the Si₃N₄ particles were heated. When Si₃N₄ particles were heated in a crucible made of carbon, most α-Si₃N₄ particles converted to β-Si₃N₄ after heating at 2000°C for 90 min in an atmosphere of N₂ of 9 kgf/cm². The morphology of the resulting β-Si₃N₄ particles appeared as a whisker shape. When Si₃N₄ particles were heated in a crucible made of boron nitride, most α-Si₃N₄ particles converted to β-Si₃N₄ after heating at 2000°C for 480min in an atmosphere of N₂ of 9kgf/cm². The resulting morphology was equiaxed. It is suspected that the transformation occurs via the gas phase and is affected by the partial pressure of oxygen in the atmosphere.     

Enhancement of thermal shock resistance in β-Si3N4 coating with in situ synthesized β-Si3N4 nanowires/nanobelts on porous Si3N4 ceramics

A dense β-Si₃N₄ coating toughened by β-Si₃N₄ nanowires/nanobelts was prepared by a combined technique involving chemical vapor deposition and reactive melt infiltration to protect porous Si₃N₄ ceramics in this work. A porous β-Si₃N₄ nanowires/nanobelts layer was synthesized in situ on porous Si₃N₄ ceramics by chemical vapor deposition, and then Y–Si–Al–O–N silicate liquid was infiltrated into the porous layer by reactive melt infiltration to form a dense composite coating. The coating consisted of well-dispersion β-Si₃N₄ nanowires/nanobelts, fine β-Si₃N₄ particles and small amount of silicate glass. The testing results revealed that as-prepared coating displayed a relatively high fracture toughness, which was up to 7.9 ± 0.05 MPa m^1/2, and it is of great significance to improve thermal shock resistance of the coating. After thermal cycling for 15 times at ΔT = 1200 °C, the coated porous Si₃N₄ ceramics still had a high residual strength ratio of 82.2%, and its water absorption increased only to 6.21% from 3.47%. The results will be a solid foundation for the application of the coating in long-period extreme high temperature environment.     

Effect of β-Si3N4 starting powder size on elongated grain growth in β-Si3N4 ceramics

The microstructural evolution of pressureless sintered silicon nitride ceramics prepared from different particle sizes of β-Si₃N₄ as starting powders, has been investigated. When the specimen prepared from as-received β-powder of 0.66 μm in average size, was sintered at 1850°C, equiaxed β-Si₃N₄ grains were observed. As the size of the initial β-powder went down to 0.26 μm, however, the growth of elongated grains was enhanced, which resulted in a whisker-like microstructure similar to that made from α-starting powder. When the sintering temperature was increased to 2000°C, the elongated grains were also developed even in the specimen made from 0.66 μm β-powder. The observed results were discussed with relation to the two dimensional nucleation and growth theory for faceted crystals. In addition, fracture toughness of the specimen consisting of elongated grains, which was prepared from finer powders, increased.     

Synthesis of α-Si3N4 whiskers or equiaxed particles from amorphous Si3N4 powders

Silicon nitride (Si₃N₄) particles with different morphologies have been used in many fields. In this work, α-Si₃N₄ whiskers and granular particles with high-phase purity were successfully tailored by the controllable crystallisation process of amorphous Si₃N₄ powders under different N₂ pressure. Impressively, α-Si₃N₄ whiskers were prepared by simply heat treating amorphous Si₃N₄ powders at 1550 °C for 2 h under the low N₂ pressure of 0.2 MPa, whereas equiaxed α-Si₃N₄ particles with uniform size of ~280 nm were obtained under an elevated N₂ pressure of 2.0 MPa. With the evaluated N₂ pressures and temperatures, large scale α-Si₃N₄ whiskers or equiaxed α-Si₃N₄ particles could be produced. The growth mechanisms of the α-Si₃N₄ particles with distinct morphologies were rationally proposed, and these consist of two main growth processes. First, amorphous Si₃N₄ powders decomposed into Si(g) and N₂(g) under high-temperature treatment. Subsequently, N₂(g) dominated the recombination of the evaporated chemical with the Si₃N₄ molecule. The initial N₂ concentration, which plays a key role in tailoring the shape and size of products, was controlled by the N₂ pressure.     

Coarse-grained β-Si3N4 powders prepared by combustion synthesis

Coarse-grained β-Si₃ N₄ powders were prepared by combustion synthesis under N₂ pressure of 6 MPa, with a low diluent content of not more than 10 wt.% and high reaction temperature of >1900°C. β-Si₃ N₄ was obtained as the major phase in the products, except for a small amount of residual Si. The addition of carbon black was effective to reduce the residual Si, but resulted in the formation of β-SiC when too much carbon black was used. The coarse-grained β-Si₃ N₄ powders consisted of β-Si₃ N₄ crystals with an average thickness of more than 10 µm, and some crystals were thicker than 20 µm. The growth mechanism of the coarse β-Si₃ N₄ crystals was discussed, associated with the particular reaction conditions in combustion synthesis.     

Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si₃N₄ were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb₂O₃ and ZrO₂ as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm⁻¹K⁻¹, respectively. The sintered material which possessed 110 Wm⁻¹K⁻¹ had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si₃N₄ grains. In the sintered material with 150 Wm⁻¹K⁻¹ neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si₃N₄ was attributable to the reduction of internal defects of the β-Si₃N₄ grains with sintering and annealing time as the grains grew.     

Origin of the existence of inter-granular glassy films in β-Si3N4

Inter-granular glassy films (IGFs) are ubiquitous in structural ceramics and they play a critical role in defining their properties. The detailed origin of IGFs has been debated for decades with no firm conclusion. Herein, we report the result of quantum mechanical modeling on a realistic IGF model in β-Si₃N₄ that unravels the fundamental reason for its development. We calculate the electronic structure, interatomic bonding, and mechanical properties using ab initio density functional theory with parallel calculations on crystalline β-Si₃N₄, α-Si₃N₄, γ-Si₃N₄, and Si₂N₂O. The total bond order density—a quantum mechanical metric characterizing internal cohesion—of the IGF model and crystalline β-Si₃N₄ are found to be identical. Detailed analysis shows that weakening of the bonds in the glassy film is compensated by strengthening of the interfacial bonds between the crystalline grain and the glassy layer. This provides a natural explanation for the ubiquitous existence of IGFs in silicon nitride and other structural ceramics. Moreover, the mechanical properties of this IGF model reveal its structural flexibility due to the presence of the less rigid glassy layer. This work demonstrates that high-level computational modeling can now explain some of the most intriguing phenomena in nanoscale ceramic materials.     

Effect of lattice impurities on the thermal conductivity of β-Si3N4

Effect of impurities in the crystal lattice and microstructure on the thermal conductivity of sintered Si₃N₄ was investigated by the use of high-purity β-Si₃N₄ powder. The sintered materials were fabricated by gas pressure sintering at 1900 °C for 8 and 48 h with addition of 8 wt.% Y₂O₃ and 1 wt.% HFO₂. A chemical analysis was performed on the loose Si₃N₄ grains taken from sintered materials after the chemical treatment. Aluminum was not removed from Si₃N₄ grains, which originated from the raw powder of Si₃N₄. The coarse grains had fewer impurities than the fine grains. Oxygen was the major impurity in the grains, and gradually decreased during grain growth. The thermal conductivity increased from 88 Wm⁻¹ K⁻¹ (8 h) to 120 Wm⁻¹ K⁻¹ (48 h) as the impurities in the crystal lattice decreased. Purification by grain growth thus improved the thermal conductivity, but changing grain boundary phases might also influence the thermal conductivity.     

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.     

Effect of the residual phases in β-Si3N4 seed on the mechanical properties of self-reinforced Si3N4 ceramics

In this work, the self-reinforced silicon nitride ceramics with crystal seed of β-Si₃N₄ particles were investigated. Firstly, the seeds were prepared by heating of α-Si₃N₄ powder with Yb₂O₃ and MgO, respectively. Then the self-reinforced silicon nitride ceramics were obtained by HP-sintering of α-Si₃N₄ powder, Yb₂O₃ and the as-prepared seeds which were not treated with acid and/or alkali solution. The results indicated that the introduction of seed with Yb₂O₃ could obviously increase the toughness and room temperature strength of the ceramics. Furthermore, its high temperature strength (1200 °C) could nearly keep higher value as the one of room temperature measured from unreinforced ceramic. However, the seed with MgO abruptly decrease the high temperature strength of the ceramics. The SEM and TEM characterization showed that the rod-like seed particle could favor the toughness and the presence of the Mg promote the formation of crystalline secondary phase.     

Preparation and properties of ultralight fibrous β-Si3N4 3D scaffolds with honeycomb-like structure by directional freeze-casting

The directional freezing of β-Si₃N₄ whiskers suspensions followed by high-temperature sintering was employed for fabricating novel highly porous fibrous Si₃N₄ three-dimensional (3D) scaffolds. A honeycomb-like structure was achieved, in which the directionally aligned lamellar walls were composed of the oriented fibrous Si₃N₄ grains and bridged by the transverse grains. Ultrahigh porosities ranging from 97.8% to 90.2% and rather low densities from 0.073 to 0.320 g cm^?3 could be obtained by controlling the Si₃N₄ contents from 1.5 to 7.5 vol%. The longitudinal compressive strength was superior to the transverse and increased obviously from 0.19 to 3.7 MPa as the porosity decreased. The superior compressive strength was due to the excellent resistance to bucking-induced elastic instability for the lamellar fibrous Si₃N₄ walls. Meanwhile, the dielectric constant and loss were decreased to 1.08 and 6.6 × 10^?4, respectively. This study provides a strategy for fabricating porous Si₃N₄ ceramics with ultrahigh porosities and improved strength.     

Simultaneous improvement in porosity and strength of porous β-Si3N4 ceramics by formation of ultrafine fibrous grains

Si₃N₄ ceramic with ultrafine fibrous grains are expected to exhibit remarkable mechanical properties. In this work, highly porous Si₃N₄ ceramic monoliths composed of ultrafine fibrous grains were developed via a novel vapor-solid carbothermal reduction nitridation (V-S CRN) reaction between SiO vapor and green bodies comprised of carbon nanotubes (CNTs), α-Si₃N₄ diluents and Y₂O₃ in a N₂ atmosphere. The unique fibrous grains-interconnected structure was developed through in-situ formation of Si₃N₄ and following liquid phase sintering. The porous Si₃N₄ monoliths with porosity of 61–78% was developed by controlling the contents of α-Si₃N₄ diluents and densities of the CNT green bodies. With increasing of the α-Si₃N₄ contents, Si₃N₄ fibrous grains with an aspect ratio of approximate or higher than 20 could be achieved, and the grains were gradually refined. For the samples with 40 wt% α-Si₃N₄, the minimum mean grain diameter and pore size of 164 nm and 0.79 μm were achieved, respectively, and the resultant porous Si₃N₄ monolith exhibited a flexural strength of as high as 73–102 MPa with the porosity of 61–73%, which is much higher than that of the reported in literature. The improvement of mechanical strength could be attributed to the densely interconnected bird's nests structure formed by the ultrafine fibrous grains. The effects of the α-Si₃N₄ diluents on the resulting porous Si₃N₄ monolith via this method were analyzed.     

Polymer-derived preparations of the blue-excited greenish-yellow-emitting calcium-containing β-Si3N4:Eu-like phosphors

Polymer-derived greenish-yellow emitting silicon (oxy)nitride phosphors excited by blue light were prepared by using polycarbonsilane, europium (Eu) and calcium (Ca) acetylacetonates as raw materials. The chemical compositions, crystal phases, particle morphologies and microstructures of the samples prepared with Eu, and/or Ca, and none of any metal ions were studied and are compared. It was found that the Ca-containing Eu-activated silicon (oxy)nitride sample prepared with the atomic ratios of Eu/Si and Ca/Si being 0.06 and 0.12, respectively, was composed mainly of single crystallized particles in the β-Si₃N₄-like phase with Eu^2+/3+ and Ca²⁺ ions locating at interstices of the β-Si₃N₄-like lattices. This sample demonstrated a substantial increment in the intensity of its broad and spike-less emission spectrum excited at 468 nm as compared to that for the Ca-free sample with its major emission peak being at 570 nm, and recorded a CIE coordinate (0.3386, 0.3474) near the equal energy white point with a Ra value of 71.4 when employed as the phosphor for a white light emitting diode device with blue chip of 460 nm. The emission mechanisms of Eu-activated for the Ca-free sample and the Ca-containing sample, the later res-shifted the major excitation band from the near-ultraviolet to blue, are discussed based on the experimental data.     

Synthesis and spark plasma sintering of the α-Si3N4 nanopowder

A two-step process (milling and then heat treatment) was used for the preparation of α-Si₃N₄ nanopowder. The influence of the milling time and heat treatment temperature as processing parameters were investigated on the formation of α-Si₃N₄. Silicon nitride ceramic was produced by spark plasma sintering at 1700 °C for 15 min, using MgSiN₂ additive. The optimum sample was produced in a 30 h milling time, heat treatment at 1300 °C, and a 22 °C/min heating rate conditions. X-ray fluorescence analysis showed that the purity of the final product is above 98%. Nanoindentation hardness and Young’s modulus of the SPS-ed sample were measured as 17±2.0 GPa and 290±11.0 GPa, respectively.