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.     

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.     

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.     

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 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.     

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.     

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.     

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.     

Synthesis of rod-like β-Si3N4 seed crystals with tailored morphology

β-Si₃N₄ seed crystals were synthesized by sintering (α+β)-Si₃N₄ powders with Y₂O₃+MgO additives at 1800 °C. Full α- to β-phase transformation was achievable at 1800 °C for 1 h. The pre-existing β-Si₃N₄ particles acted as nuclei during a sintering process. The length and mean aspect ratio of β-Si₃N₄ seed grains could be tailored by careful control of α/β-Si₃N₄ ratio, which resulted in various nuclei and driving force. The sample A95B5 with 5% β-nuclei shows a bimodal size distribution containing large amount of abnormal elongated β-Si₃N₄ grains with remarkable large diameter. With increasing the β-phase content from 5 wt% to 100 wt%, the average diameter and aspect ratio of the β-Si₃N₄ single crystals decreased from 1.43 µm to 0.92 µm and from 4.36 to 2.79, respectively.     

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.     

β-Si3N4及添加β-Si3N4的α-Si3N4的气氛加压烧结

介绍了β－Ｓｉ３Ｎ４及添加β－Ｓｉ３Ｎ４的α－Ｓｉ３Ｎ４的气氛加压烧结,β－Ｓｉ３Ｎ４在ＧＰＳ中具有低于α－Ｓｉ３Ｎ４的烧结活性而且陶瓷显微结构更容易调节,由ＧＰＳβ－Ｓｉ３Ｎ４制备的陶瓷材料晶粒比较均匀,具有较高的力学性能,尤其是高的韦泊模数,添加于α－Ｓｉ３Ｎ４中的β－Ｓｉ３Ｎ４对陶瓷材料显微结构具有明显的调控作用。     

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.     

α-Si3N4与γ-Si3N4、α-Si3N4混合粉体超高压烧结的比较研究

以Y2O3-Al2O3-La2O3体系作烧结助剂,在5．4～5．7GPa、1620-1770K的高温高压条件下进行了α-Si3N4与γ-Si3N4、α-Si3N4粉体的烧结研究,并探讨了烧结温度及压力对烧结体性能的影响。实验结果表明：α-Si3N4、γ-Si3N4完全相变为β-Si3N4;在相同的烧结条件下,α-SigN4比γ-Si3N4、α-Si3N4混合粉体烧结试样的相对密度、维氏硬度高。α-Si3N4与γ-Si3N4、α-Si3N4混合粉体烧结试样的最高相对密度与维氏硬度分别为98．78％、21．87GPa和98．71％、21．76GPa。烧结体由相互交错的长柱状β—Si3N4晶粒组成．显微结构均匀。     

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.     

Preparation,Properties and Growth Mechanism of Low-Cost Porous Si3N4 Ceramics with High Levels of β-Si3N4 Powders

Silicon - Nowadays, porous Si3N4 ceramics are fabricated by high purity α-Si3N4 powders, resulting in a higher cost of production. To reduce cost and save energy, in this research, high levels...     

Effect of α-Si3N4 initial powder size on the microstructural evolution and phase transformation during sintering of Si3N4 ceramics

By using the Si₃N₄ ceramic specimens prepared with fine and coarse α-Si₃N₄ powders, respectively, the phase transformation from α- to β-Si₃N₄ and concurrent microstructural evolution during sintering were monitored. For the compact prepared with fine powder, the α/β transformation was completed much earlier than the coarse powder. The higher fraction of pre-existing β-grains in fine powder and its higher reactivity compared to those of coarse one are likely to cause a rapid phase transformation. The growth rate of β-Si₃N₄ grains at the expense of α-Si₃N₄ during phase transformation stage was quite significant while that after they impinge each other was very limited. As a result, the specimens prepared with coarse and fine initial α-Si₃N₄ powders resulted in coarse and fine grained β-Si₃N₄ ceramics, respectively. The specimen prepared with mixture of fine and coarse α-Si₃N₄ powders exhibited the microstructure containing a few elongated large grains and showed an increased value of fracture toughness.     

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.     

热压烧结透明β-Si3N4陶瓷

采用MgSiN2作为烧结助剂,在2000℃高温下热压26h,制备了透明β-Si3N4陶瓷.X射线衍射分析表明:透明β-Si3N4陶瓷由纯β-Si3N4相组成.透明-Si3N4陶瓷的透过率随波长增加而增加,当波长为2.5 μm时透过率达到最大值,为70％,波长在0.7～4.0 μm区间,透过率保持在60％以上,截止波长为5.0 μm.     

Effect of in-situ formed Y2O3 by metal hydride reduction reaction on thermal conductivity of β-Si3N4 ceramics

The effect of YH₂ on densification, microstructure, and thermal conductivity of Si₃N₄ ceramics were investigated by adjusting the amount of YH₂ in the range of 0–4 wt% using a two-step sintering method. Native SiO₂ was eliminated, and Y₂O₃ was in situ formed by a metal hydride reduction reaction, resulting in various Y₂O₃/SiO₂ ratios. Full densification of YH₂-doped samples could be achieved after sintering at 1900 °C for 4 h. The Y₂O₃/SiO₂ ratio had a significant influence on the composition of crystalline secondary phases. Besides, the increased Y₂O₃/SiO₂ ratio is conducive not only to the grain growth but also to the reduction of activity of SiO₂ in the liquid phase, resulting in enlarged purified grains, reduced volume fraction of intergranular phases and increased Si₃N₄-Si₃N₄ contiguity. Ultimately, the thermal conductivity increased by 29 % from 95.3 to 123.0 W m⁻¹ K⁻¹ after sintering at 1900 ℃ for 12 h by the substitution of Y₂O₃ with YH₂.     

β-Si3N4陶瓷热导率的研究现状

β-Si3N4陶瓷具有较高的热导率(200～320 W·m-1·K-1),在高速电路和大功率器件散热及封装材料等领域展现了良好的应用前景,并引起广泛关注.基于氮化硅陶瓷导热机理,本文阐述了影响β-Si3 N4陶瓷热导率的因素,并从原料的选取、烧结助剂的选择、晶种的引入和工艺控制四个方面,介绍了国内外提高其热导率的研究进展.