Determining the oxygen content in partly oxidized silicon nitride

Conclusions The vacuum-extraction method is suitable for determining with adequate accuracy the content of oxygen (oxynitride) in partly oxidized silicon nitride and in silicon-carbide refractories with a nitride and silicon oxynitride bond.     

Ceramic made from SHS silicon nitride powder

A ceramic made from a powder of silicon nitride which is obtained by self-propagating high-temperature synthesis is investigated. Yttrium-aluminum garnet and mullite are used as sintering aids. The strength of the material obtained reaches 540 MPa. __________ Translated from Steklo i Keramika, No. 3, pp. 17–19, March, 2007.     

硅粉对刚玉-氮化硅复合材料表面氧化膜的影响

将T-60氧化铝粉、氮化硅粉和硅粉分别按50%、37.5%和12.5%的质量分数配料混合,经压制成型后,在空气中于1600℃保温2h烧成制得刚玉-氮化硅复合材料试样,借助于XRD、SEM、EDS等研究了加入硅粉对刚玉-氮化硅复合材料表面氧化膜组成和结构的影响。结果表明:在空气中烧成的刚玉-氮化硅复合材料表面氧化膜主要由富硅玻璃相、刚玉和莫来石组成;在刚玉-氮化硅复合材料中引入硅粉能减小氧化膜厚度,并能提高氧化膜的致密程度。     

Effect of oxygen on the strength of reaction-bonded silicon nitride

Conclusions When obtaining reaction-bonded Si₃N₄, drying of the silicon powder or the charge at 400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.Addition of small quantities of SiO₂ to the reaction-bonded Si₃N₄ improves the high-temperature strength to some extent although it affects the room temperature strength adversely.Translated from Ogneupory, No. 5, pp. 13–14, May, 1991.     

亚微米氮化硅粉体的制备及表征

以自蔓延高温合成法合成的氮化硅粉料为原料,用搅拌球磨的方法对其进行超细粉碎,测试不同球磨工艺条件下粉料的粒度、形貌和XRD谱图,探讨不同球磨工艺参数对粉料粒度、形貌及晶格类型的影响,最终确定合适的搅拌球磨工艺参数。优化工艺参数:粉料与溶剂的质量比为1∶1,粉料与磨介的质量比为1∶3,最佳研磨时间为8~10 h。在此条件下,可将中位径为5μm左右的氮化硅粉料用搅拌球磨的方法制备得到中位经为0.719μm的粉料。     

Densification of silicon nitride powder with various additives

Cold compaction behavior of a sub-micron size silicon nitride powder with additives has been studied at various loading rates up to about 450 MPa. Liquid lubricants are found to be more effective than the solid lubricants. A marked increase in relative density has been obtained with the aid of 10 weight percent polyethyleneglycol after surface treatment of the powder with a dispersant solution. The loading rate between 0.8 and 800 MPa per minute has no effect on the densification process. Results are analysed with various theoretical equations available in the literature. The Cooper and Eaton's equation and also its modified form are used to determine the mechanism of densification during cold compaction.     

Densification of silicon nitride powder by spark plasma extrusion

The simultaneous extrusion and sintering of ceramic materials using the spark plasma extrusion (SPE) technique is an alternative densification route for the powders examined in this study. SPE with Y₂O₃ and Al₂O₃ as the additives was employed for the concurrent densification and extrusion of pure α-Si₃N₄ commercial powder. Three compositions of α-Si₃N₄ powders, namely SN-4, SN-12, and SN-20, were studied to understand the effect of additives on the sinter-extrusion process. The number in the mixture name corresponded to the weight percentage of additives in the samples. A mechanical shaker was used to thoroughly mix the selected powder compositions in isopropanol for 24 h at 720 rpm. It was then dried in a furnace at 100 °C for 3 h. The sinter-extrusion treatment was conducted for a holding time of 5 min at a heating rate of 300 °C/min and a sintering temperature of 1500 °C with a pressure in the range of 12–63 MPa. The maximum degree of extrusion was attained when the total amount of additives was at 20 wt% (SN-20 series). As the total amount of additives increased, the capillary pressure gradients led to an inhomogeneous liquid distribution throughout the specimen, which in turn promoted further densification of the Si₃N₄ ceramics. Owing to the conical geometry of the graphite die used for the extrusion process in this study, the largest densification values observed in the extruded ceramics corresponded to the base and the middle sections of the SN-20 series (relative density ~92%). The latter could only take place once the α-to β-Si₃N₄ phase transformation occurred, leading to significant shrinkage of the material.     

Effects of amino groups on dispersibility of silicon nitride powder in aqueous media

Silicon nitride (Si₃N₄) powders were subjected to amination modification by grafting γ-aminopropyltriethoxysilane (APTES) via a direct blending method in solution. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the hydroxyl groups present on the surface of Si₃N₄ powder particles interacted with the silanols groups of APTES to combine through covalent bonding. Thermogravimetric analysis (TGA) suggested that the grafting of APTES on Si₃N₄ powder surface was successful with grafting content reaching up to 7%. Compared to native Si₃N₄, the surface hydrophilicity of amino Si₃N₄ powder was enhanced and dispersibility was improved. Overall, these findings indicated the promising aspects of amination modification and future potential use in environmental protection by using water instead of organic solvents during Si₃N₄ ceramic formation process.     

Ceramic membranes from reaction-sintered silicon nitride on nitride and oxide substrates

The gas permeability and pore sizes in membranes and porous materials from reaction-sintered silicon nitride (RSSN) are studied. The substrates for the RSSN membranes are highly porous (80% pores) silicon nitride with pores 50–80 μm in size and alumina-based ceramics with pores about 10 μm in diameter. Silicon powder with a specific surface of 1.4 g/cm² is deposited onto the substrates in the form of an aqueous dispersion by various methods including filtering and sintered in nitrogen at 1300–1400°C. The main difficulty of the process is in preventing the formation of defects at the places where the largest channel pores reach the surface of the substrate. Membranes with 0.5 -1 μm pores can be obtained on both types of substrate after a single deposition.     

Preparation of single-phase magnesium silicon nitride powder by a two-step process

Single-phase magnesium silicon nitride (MgSiN₂) powder was prepared by a two-step process: combustion synthesis followed by acid washing. The effects of starting material compositions, diluent addition and N₂ pressure on the phase compositions of the final products were studied, and the acid washing process was also discussed. It is difficult to synthesize single-phase MgSiN₂ by one-stage combustion reaction through regulating the process parameters because of the evaporative loss of Mg. Combustion synthesis of MgSiN₂ by using an excess of magnesium in the starting materials and then acid washing off the impurity was an effective way to prepare the single-phase MgSiN₂ powder.     

硅粉加入量对刚玉-氮化硅复合材料力学性能的影响

以刚玉、氮化硅、粘土和硅粉为主要原料,在空气气氛下于1550℃保温3h烧成制得刚玉-氮化硅复合材料,研究了硅粉加入量(w)分别为0、4%、8%、12%时对试样常温物理性能、高温抗折强度的影响,并采用XRD、SEM、EDS等手段分析了试样的物相组成和显微结构。结果表明:在硅粉加入量<8%时,Si主要填充在烧成制品基质的空隙中;随着硅粉加入量的增加,制品致密化程度提高,常温强度增大,同时由于硅粉原位生成了纤维状赛隆而使制品的高温强度增大;但在硅粉加入量>8%时,Si可能构成连续相,又导致制品致密度下降和高温力学性能恶化。     

Effect of the silicon content in the initial alloy on synthesis of silicon nitride in burning ferrosilicon in nitrogen

Nitriding of the industrial alloy FS 65 in conditions of self-propagated high-temperature synthesis was investigated. The effect of the phase composition of the initial alloy on the combustion process was established. The dependence of the degree of nitriding of the products of combustion on the nitrogen pressure, sample diameter, and dilution of the initial batch with inert and activating additives was investigated. Silicon nitride with a 0.07% residual iron content was obtained by the method of acid concentration.     

Sintered silicon nitride/nano-silicon carbide materials based on preceramic polymers and ceramic powder

A flexible method is presented, which enables the fabrication of porous as well as dense Si₃N₄/nano-SiC components by using Si₃N₄ powder and a preceramic polymer (polycarbosilazane) as alternative ceramic forming binder. The SiCN polymer benefits consolidation as well as shaping of the green body and partially fills the interstices between the Si₃N₄ particles. Cross-linking of the precursor at 300 °C increases the mechanical stability of the green bodies and facilitates near net shape machining. At first, pyrolysis leads to porous ceramic bodies. Finally, subsequent gas pressure sintering results in dense Si₃N₄/nano-SiC ceramics. Due to the high ceramic yield of the polycarbosilazane binder, the shrinkage during sintering is significantly reduced from 20 to 15 lin.%. Investigations of the sintered ceramics reveal, that the microstructure of the Si₃N₄ ceramic contains approx. 6 vol.% nano-scaled SiC segregations, which are located both at the grain boundaries and as inclusions in the Si₃N₄ grains.     

Removal of organic impurities from the silicon surface by oxygen and UV cleaning

We have studied the effect of oxygen and water vapor on the removal of organic impurities from the porous silicon surface under UV irradiation. Infrared spectrum observations of the treated surface suggest that decomposition of oxygen to produce ozone, atomic oxygen, and hydroxyl radical is a rate determination step for the overall cleaning process.     

Crystallization of an amorphous silicon nitride powder produced in a radiofrequency thermal plasma

Crystallization behavior of an amorphous silicon nitride powder produced in an RF thermal plasma by the vapor-phase reaction of silicon tetrachloride and ammonia has been investigated. Effects of annealing conditions such as temperature and duration of heat treatment on the properties of powders were studied. Changes in the chemical and phase compositions, as well as in the morphology of powders were measured and interpreted. Annealing of the amorphous silicon nitride powder at 1450°C for 120 min resulted in a powder of about 80% crystalline phase content with an /β ratio of about 6.5. ©     

碳化硅超细粉末中的总碳量测定

测定碳化硅中的总碳量,国标法采用燃烧－重量法,即用四氧化三铅作助熔剂,此助熔剂在加热过程中易溢出使结果受影响,并且与磁燃烧管熔在一起使燃烧管破损量增加,不能连续测定,本采用钨锡合金作助熔剂,分析结果表明这种助熔剂是可行的。     

Evaluation of Pd-based brazes to join silicon nitride to copper-clad-molybdenum

Si₃N₄ (SN-281)/Cu-clad-Mo joints, brazed using a soft (YS: 341 MPa) and ductile (43% elongation) Pd–Co braze were sound and exhibited an interaction zone comprised of Pd₆₆Mo₁₄Cu₁₀Co₆Si₃ and Pd₇₄Mo₁₁Co₈Cu₆Si. Similar joints made using a less ductile and stronger Pd–Ni braze led to cracking from large CTE mismatch-induced strain energy (64 mJ to 348 mJ).     

水溶性N-(2-羧基苯甲酰基)化壳聚糖等电点的测定

利用浊度法测定了水溶性N(2羧基苯甲酰基)化壳聚糖的等电点,结果表明N(2羧基苯甲酰基)化壳聚糖都有一等电点,并且都在7左右,等电点随着分子中氨基残余量的减小而逐渐减小。从理论上解释了浊度法测定聚电解质的原理。     

纳米氮化硅粉体的大分子改性剂表面修饰研究

从分子设计的角度合成了一种新型的大分子表面改性剂(LMPB-g-MAH):采用溶液聚合法将极性单体马来酸酐(MAH)接枝到低分子量的聚丁二烯液体橡胶(LMPB)分子长链中,并用其对纳米氮化硅粉体进行表面修饰;对合成的大分子表面改性剂、改性前后的纳米氮化硅粉体,运用FT-IR、TEM、TGA、粒径分析、沉降实验等方法进行了表征.实验结果表明:马来酸酐已经接枝到低分子量的聚丁二烯液体橡胶分子长链中;当大分子表面改性剂的接枝率为9%～11%、用量为10%～12%、反应温度为65℃、反应时间为3 h时,表面修饰后的纳米氮化硅粉体颗粒粒径减小,有效阻止了纳米颗粒的团聚.