Kinetics of α- and β-Si3N4 formation from oxide-free high-purity Si powder

Available kinetic data for the nitridation of high-purity oxide-free Si powder are analysed. The analysis suggests that the - and -phases of Si₃N₄ are formed by separate and parallel reaction paths, and kinetic expressions for their formation are reported. The formation of the -phase follows first-order kinetics, while the -phase is formed by a phase-boundary-controlled rate law. These conclusions are consistent with other kinetic and micrographic analyses reported in the literature.     

Synthesis of monodisperse and high-purity α-Si3N4 powder by carbothermal reduction and nitridation

Carbothermal reduction-nitridation method is an effective means for synthesizing Si₃N₄ powder. Herein, spherical monodisperse silica was used as silicon source. The effects of reaction temperature, nitrogen flow rate and Si₃N₄ seeds content on the products were studied. It was found that high-purity α-Si₃N₄ (>99.0 wt%) was synthesized from C/SiO₂ = 3:1 at 1400 °C, reaction time of 6 h and nitrogen flow rate of 800 ml/min. The powder, with an average size of 0.5 μm, showed good dispersity and regular morphology because spherical monodisperse silica could be completely coated with carbon. The more contact sites between SiO₂ and C, the higher concentration of SiO(g) would be produced in the initial stage. It also indicated that the nucleation rate of α-Si₃N₄ increased, thereby inhibiting the formation of an agglomerate phase and suppressing the grain growth of α-Si₃N₄. Furthermore, higher nitriding temperature and Si₃N₄ seeds content both decreased the grain size and increased β-Si₃N₄ content. The forming mechanism of non-agglomerated and submicron-sized α-Si₃N₄ was clarified.     

Carbothermal nitridation synthesis of α-Si3N4 powder from pyrolysed rice hulls

The carbothermal nitridation synthesis of α-Si₃N₄ was studied using a high-temperature tube furnace to react a precursor, comprised of pyrolysed rice hulls (C/SiO₂) and additive “seed” Si₃N₄, with N₂. The experimental design for synthesis was a three-level factorial surface response design for determining the effect of temperature (1300–1380°C) and reaction time (1–5 h) on kinetics. In addition, all precursors were reacted at 1460, 1480 and 1500°G for 5 h in order to ensure high conversion suitable for product powder evaluation (composition and morphology). Following excess carbon removal, the product Si₃N₄ was >95% α-phase and had a surface area of 7.7 m²g^?1 with an oxygen content of 3.6 wt% O. The powder was comprised of a bimodal size distribution of submicrometre solid α-Si₃N₄ crystallites centred at 0.03 and 0.22 μm. No whiskers or high aspect ratio elongated crystallites were found in the powder. The addition of carbon black to the seeded pyrolysed rice hull C/SiO₂ mixture had no significant impact on the reaction rate or product powder properties. The reaction was modelled using a nuclei-growth rate expression as $$\begin{gathered} (kt)^{0.58} = - ln(1 --- X) \hfill \\ k = 1.09 \times 10^{10} exp (--- 50502/T) \hfill \\ \end{gathered} $$ k=1.09×10¹⁰ exp (?50502/T) where (1573 K<T<1653 K), (3600<t<18000 s), (0<X<1), andk=rate in s^?1.     

Analysis of Si3N4+β-Si3N4 whisker ceramics

Dense Si₃N₄+-Si₃N₄ whisker composite ceramics were fabricated by hot pressing powder-whisker mixtures. Addition of -Si₃N₄ whiskers had no significant influence on the densification behaviour for up to 20 wt% addition. Light microscopy and scanning and transmission electron microscopy were used to study their microstructure and fracture behaviour. An increase in fracture toughness was observed for -Si₃N₄ whisker additions of up to 10 %. The main toughening mechanisms observed were crack deflection, crack branching, whisker-matrix debonding and whisker pull-out.     

Phase quantification of β-Si3N4/β-SiC mixtures by X-ray powder diffraction analysis

X-ray powder diffraction methods of phase quantification were adapted and compared to mixtures of -Si₃N₄ and -SiC. Multiline mean-normalized-intensity methods and whole pattern analysis (Rietveld) both have advantages and disadvantages over each other. Satisfactory results (less than 3% absolute deviation) can be achieved in minimal time using intensity normalization methods. Phase quantification using the Rietveld method requires significantly longer measuring time, evaluation time and expertise to obtain the same results.     

The α- to β-Si3N4 transformation in the presence of liquid silicon

The compacts consisted of , -Si₃N₄ and free silicon are heat treated in the range 1650° C to 1750° C in an argon atmosphere in order to observe the following behaviours; the to phase transformation and variations of the microstructure during heat treatment in silicon nitride. For the microstructural observation of the heat treated specimens, the same grains in the polished surface were investigated before and after eliminating the retained silicon by etching. The to phase transformation, in this case, occurs via silicon melts irrespective of added -Si₃N₄. Both and phases are soluted and precipitated into molten silicon and their morphology are changed from an equiaxed shape to prismatic one. Although elongated grains are precipitated at low temperature or in the early stage of heat treatment, fine precipitated grains are mainly observed with increasing heat treating temperature.     

β-Si3N4单晶体的制备

在自增韧Si3N4陶瓷的烧结过程中,添加作为晶种的长柱状β-Si3N4单晶体对于改善陶瓷的强度和韧性是非常有效的。本研究旨在制备出长柱状β-Si3N4单晶体,并对其尺寸和形貌进行有效的控制。通过对87.3wt/α-SiN4 8.3wt/Y2O3 4.4wt/SiO2体系进行气压烧结,经去除掉玻璃相等漂洗工艺后,制得β-Si3N4单晶体,其直径为1－2μm、长度为4－6μm。同时对不同烧结工艺下制得的β-Si3N4单晶体的尺寸和工艺参数的关系进行了研究。     

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

β-Si3N4粉末烧结及其显微结构形成

由β－Ｓｉ_３Ｎ_４粉末通过一定的工艺条件得到致密的氨化硅陶瓷,试样的显微结构为短柱状和等轴状颗粒交织排列而成的均匀结构．材料的烧结过程分为重排、晶形转变、晶粒生长三个阶段,随烧结时间增加,烧结试样的显微结构开始阶段变化很明显,２ｈ后结构比较稳定．温度升高有利于柱状颗粒长径比的提高,添加剂量的增加使显微结构粗化．     

单晶α-Si3N4纳米线宏量制备研究

本研究提出了一种宏量制备单晶α-Si₃N₄纳米线的方法。以造粒硅粉为原料, 通过在N₂-H₂混合气氛中直接氮化, 得到具有核壳结构的氮化产物(Si₃N₄纳米线@多孔Si₃N₄微米粉体), 氮化产物经过破碎、研磨、分离后即可获得Si₃N₄纳米线。检测结果表明, 制备的Si₃N₄纳米线直径为80~150 nm, 长径比为20~50, 其中纳米线含量>95wt%, α相/β相比为17.6, 收率为3.1%。进一步研究表明, 原料中微量Fe元素在还原气氛下具有催化作用, 纳米线由典型的气-液-固(VLS)生长机制控制。实验中对原料硅粉造粒后再氮化具有三大优点: 数量级地增大了Si₃N₄纳米线生长空间; 纳米线生长分布集中, 有利于后续高效分离; 显著提高了氮化速率。     

高能球磨-盐辅助氮化低温合成α-Si3N4粉体

以硅粉为原料,NaCl-NaF复合盐为反应介质和稀释剂,采用高能球磨-盐辅助氮化法制备出α-Si_3N_4粉体。研究了氮化温度、保温时间、盐硅比及复合盐中NaF含量对合成α-Si_3N_4的影响。利用X射线衍射仪(XRD)和场发射扫描电子显微镜(FE-SEM)对产物的物相组成和显微结构进行了分析表征。结果表明:氮化温度为1 200℃、保温时间为4 h、盐硅比为2∶1、复合盐中NaF含量为10%时,硅粉完全氮化。合成的产物中存在大量的α-Si_3N_4晶须,晶须的直径为40～280 nm,长度为几微米到几十微米;晶须的生长机制为VC机制。     

自蔓延燃烧合成β-Si3N4棒晶

采用自蔓延高温合成（SHS）,在高压氮气中成功地合成了β-Si₃N₄棒晶,研究了添加不同量Y₂O₃对自蔓延燃烧合成β-Si₃N₄。棒晶长径比的影响.结果表明,Y₂O₃添加量有一个最佳范围,当Y₂O₃的添加量在2Wt％～5wt％时,棒晶生长均匀,长径比约为8.通过铜坩埚吸热淬火的方法,观察到β-Si₃N₄棒晶不同生长阶段的显微形貌,从而推测其生长机理为VLS和VS两种机理协同作用的结果.本文对β-Si₃N₄棒晶生长的反应历程也进行了阐述.     

Growth,morphology and slip system of α-Si3N4 single crystal

Under the conditions of growth temperatures 1500 to 1700° C and total gas pressure 10 to 50 Torr, -Si₃N₄ single crystals have been grown by chemical vapour deposition from a mixture of NH₃, SiCl₄ and H₂. The crystals were transparent and brownish-red to colourless. The effects of the growth conditions on the crystal morphology, growth habit and growth direction have been investigated. On the basal and prismatic planes, the variation in Knoop hardness with orientation of the indenter long-axis has been measured at temperatures up to 1500° C; maximum hardness values were obtained along the 1 0 ¯1 0 direction for the basal plane and along the [0 0 0 1] directions for the prismatic planes. Hardness anisotropy analysis suggests that the active slip systems of -Si₃ N₄ are {1 0 ¯1 0} [0 0 0 1] from room temperature to 1500° C.     

低纯度β-Si3N4粉末的无压烧结及其性能

为了降低氮化硅材料的成本,运用便宜的低纯度β-Si3N4粉末,通过无压烧结制备了氮化硅材料. 发现β-Si3N4粉末具有很好的烧结性能,得到的结构是由柱状颗粒和小球状颗粒形成的嵌套结构,结构组成均匀,没有晶粒的异常生长. 所得材料的抗弯强度为587MPa,韧性达到5.3MPa·m1/2,说明可在一般条件下使用.     

添加β-Si3N4棒晶对氮化硅陶瓷力学性能的影响

将由自蔓延燃烧合成法制备的β—Si₃N₄棒晶加入到α-Si₃N₄起始原料中,研究了热压烧结氮化硅陶瓷力学性能的变化.随棒晶添加量的增加,材料的韧性提高,抗弯曲强度下降.与不加棒晶相比,加入8wt％的β-Si₃N₄棒晶可使陶瓷的韧性从4.0MPa·m^1/2提高到6.7MPa·m^1/2.断口形貌和压痕裂纹的显微结构观察表明,韧性的提高源于长柱状晶粒的拔出和裂纹的偏转.     

Effect of grain size distribution on the strength of porous Si3N4 ceramics composed of elongated β-Si3N4 grains

The grain size distributions (diameter and aspect ratio) of porous Si₃N₄ ceramics composed of elongated -Si₃N₄ grains were evaluated statistically, and their effect on the pore size distribution and the flexural strength of the porous Si₃N₄ was investigated. Porous Si₃N₄ ceramics having porosities of 27 to 43% and median pore diameters of 0.56 to 0.96 m were used as specimens. The grain diameter distribution was well correlated to the pore size distribution of the porous Si₃N₄ ceramics. We concluded that the strength of the porous Si₃N₄ ceramics increased with increasing grain length of -Si₃N₄ as well as with decreasing porosity.     

长柱状β-Si3N4晶粒与SiC晶须在层状Si3N4/BN复合材料中的作用

用SiC晶须和长柱状β－Si3N4对Si3N4/BN层状材料的基体层和分隔层进行了强化,轧膜工艺对SiC晶须和原料中存在的少量β－Si3N4晶粒有一定的定向作用,而基体层中定向分布的SiC晶须和长柱状的β－Si3N4晶粒对基体层的强讳论作用类似于块体材料,其抗弯强度和断裂韧性的提高幅度都大于50％,分隔层中β－Si3N4的定向度较差,但这利利于形成网状结构,在分隔层中起到骨架的作用,同时还可以增加裂纹的扩展长度,改善分隔层与基体层的界面结合状态,最终得到性能最佳的材料的成分为Si3N4 20wt%SiC晶须（基体层）,BN＋15vol%β-SiN4（分隔层）,其断裂功可达4820J／m2,抗弯强度仍可保持在650Mpa。β     

Gas-phase synthesis of amorphous silicon nitride — reaction paths and powder characteristics

The gas phase reaction between SiCl₄ and NH₃ is investigated in the temperature range between 525 and 800°C at atmospheric pressure and at conditions typical for powder synthesis. By means of mass spectrometric in-situ measurements it was possible to detect the gaseous compounds H₂NSiCl₃, H₂NSiCl₂NH₂, Cl₃SiNHSiCl₃, NH₂Cl₂SiNHSiCl₃, (SiCl₂NH)₃ and Si₃(NH)₃Cl₅NH₂. The reactions taken place in the gas phase are very fast and result in the formation of a fine, chlorine containing product. Powders sampled at a reaction temperature of 800°C have an average molar ratio Si : N : Cl of 1 : 1, 33 : 0.28. Based on the proved gaseous intermediates and the composition of the powders reaction paths resulting in the formation of powders are derived. -Si₃N₄ powders with a high sintering activity are obtained after thermal dechlorination of the synthesis products in ammonia atmosphere followed by a crystallization process between 1200 and 1500°C.     

Mechanical properties of β-Si3N4 whisker reinforced α′-SiAlON ceramics

The effects of -Si₃N₄ whisker additions on the mechanical properties of -SiAlON ceramics were studied. The room temperature fracture toughness and fracture strength of the composites increased with increasing whisker content, and were 6.5 MPa m^1/2 and 900 MPa, respectively, for the addition of 30 vol% whiskers. Although creep resistance of the composites was not enhanced at 1200°C, the whisker additions were observed to be beneficial in reducing the oxidation induced slow crack growth of -SiAlON that occurred at 1300 °C, and thereby, improved the creep resistance of the composites at 1300°C.ORNL Postdoctoral Fellow, Oak Ridge Institute of Science and Technology, Oak Ridge Associated University.