Insight into the evolution process from novel polyborazine precursor PPMAB to inorganic BN fiber

Understanding the elemental and structural evolution from green fibers to inorganic fibers is important for fabricating high-performance ceramic fibers via the polymer-derived ceramics (PDCs) route. In a previous study, we found a novel polyborazine precursor, poly[2-propylamino-4,6-bis(methylamino)borazine-co-tri(methylamino)borazine] (PPMAB), which possesses a high ceramic yield and outstanding spinnability. Herein, we provide an insight into the evolution process from PPMAB to inorganic BN fibers. During the curing process, NH₃ only reacted with the B–NHCH₃ groups in PPMAB to form B–NH₂ groups, which further reacted with B–NHCH₃ to form B–NH–B bridges. Therefore, crosslinked networks were formed in the cured fibers and the gel content increased from 52.3 to 92.1 wt%. In the nitridation process, NH₃ not only reacted with unreacted B–NHCH₃ but also reacted with B–NHCH₂CH₂CH₃ groups to form B–NH₂ groups, which reacted with N–CH₃ bridges in the adjacent PPMAB molecule to form –N(B₃N₃)₃- networks. Simultaneously, CH₃NH₂ and CH₃CH₂CH₂NH₂ escaped from the fibers, and the carbon content of the fibers was reduced from 20.588 wt% to 0.115 wt%. By controlling the curing and nitridation processes, the curing degree can be enhanced, whereas the pore defects resulting from gas molecular escape can be effectively reduced. Thus, dense inorganic BN fibers with excellent dielectric properties (2.33 < ε < 2.5, tan δ < 3.4ⅹ10^?3) and high tensile strength (1.6 GPa) were fabricated. The present study not only provides ideas for the design and synthesis of polyborazine precursors, but also for the fabrication of high-performance BN fibers.     

Q235钢表面渗氮化钛对腐蚀性能的影响

研究了Q235钢在经表面渗氮化钛处理后,其在10%的硫酸与富液中的耐蚀性能。对其腐蚀前后的宏观形貌进行对比,对渗后试样进行电化学腐蚀性能实验,对渗层的显微特征、结构形貌等进行了检测分析。结果表明,Q235钢在10%H2SO4中腐蚀速度是氮化钛渗层的78931倍,在富液中则是6768倍,说明在腐蚀液中,氮化钛渗层耐蚀性是相当好的,可以大大延长Q253钢在腐蚀介质中的使用寿命。     

高铝矾土-硅粉氮化合成SiAlON的过程研究

分别以w(Al2O3)为68.08%和45.56%的两种高铝矾土及硅粉为原料,按合成SiAlON的理论配比配料(Si粉过量5%),成型后在流动N2(流量为0.06~0.1m3.h-1)中进行热重分析,同时测定试样在不同温度(900~1500℃)保温6h后的质量变化,并分析氮化后试样的物相变化,从而探讨该试样的氮化过程及其机理。结果表明,高铝矾土-硅粉试样在流动N2中的氮化反应过程可大致分为3个阶段:1)Si粉氮化阶段(900~1200℃),Si粉氮化生成Si3N4和Si2N2O;2)SiAlON形成阶段(1300~1400℃),生成O’-SiAlON和β-SiAlON;3)β-SiAlON的生长发育阶段(1450~1500℃),部分O’-SiAlON转化为β-SiAlON,Al2O3在β-SiAlON中的固溶度增加。     

反应烧结温度对Si_3N_4-SiC材料性能的影响

以高纯绿SiC和Si粉为原料,用630 t摩擦压力机成型为600 mm ×400 mm×90 mm的坯体,经远红外干燥后,以城市煤气为燃料,在20 m3梭式窑中分别于1 420和1 500℃隔焰氮化烧成,对烧后Si3N4-SiC试样中心和边缘部位的理化性能(包括抗冰晶石侵蚀性能)和显微结构进行了检测和分析.结果表明:1)反应烧结Si3N4-SiC耐火材料中Si3N4的形成、分布和完全氮化等受诸多因素影响;要想使制品(尤其是尺寸较大的制品)的氮化比较完全,提高氮化烧成温度是非常有效的途经;2)制品中Si3N4分布不均匀:中心部位Si2N2O和游离si的含量均较边缘部位的高,显气孔率也比边缘部位高约1%～2%;3)随着温度的升高,不可避免地出现Si的迁移,从而导致最终制品中Si3N4的分布不均匀,但这是否会影响制品的使用性能,尚需进一步研究.     

Preparation of Si3N4 porous ceramics via foam-gelcasting and microwave-nitridation method

Si₃N₄ porous ceramics with improved mechanical strength were fabricated for the first time by a combined foam-gelcasting and microwave-nitridation method at 1273–1373?K. The Si₃N₄ porous samples prepared at 1373?K/20?min with the porosity of 68.9% had respectively flexural and compressive strength as high as 8.1 and 20.8?MPa, which values were comparable or even superior to those of Si₃N₄ porous ceramics prepared previously by the conventional heating technique at a much higher temperature of 1773–1973?K, indicating that present preparation strategy is feasible to prepare high quality Si₃N₄ porous ceramic at a much milder condition. Moreover, the thermal conductivity of as-prepared Si₃N₄ porous ceramics at 1073?K was as low as 1.697?W/(m?K), suggesting it could be a potentially good heat insulating material for aluminum electrolyte cells.     

Sialon结合刚玉砖的研制

在合适氮化制度下 ,研究了基质组成和不同加入物对Sialon结合刚玉砖力学性能的影响 ,并对试样进行结构和相分析。结果表明 :基质中加入适当的氧化铝微粉和添加剂A对试样的力学性能提高很大 ,试样组织结构致密 ,气孔少、烧结好 ,主晶相为刚玉 ,次晶相为 β Sialon ,杂质含量很少     

Influence of nitridation on the oxidation of a 304 steel at 800 °C

This paper presents a study on the oxide growth on a AISI 304 chromia-forming alloy, in air at 800 °C. After the nitridation treatment was performed on the steel surface, a γ_N solid solution is detected. In this case, no nitride formation in the alloy surface could be observed. In situ X-ray diffraction has been used to follow the oxides evolution at testing temperature. At the beginning of the oxidation test, CrN is formed together with Fe₂O₃. Nevertheless, Cr₂O₃ quickly appears and leads to a protective oxide scale formation growing according to a parabolic rate law. During oxidation in situ X-ray diffraction also shows that Fe₂O₃ is transformed into FeCr₂O₄. Our results show that nitridation increases the high temperature oxidation resistance of 304 steels at 800 °C.     

天然原料还原氮化合成β′-SiAlON反应动力学研究进展

介绍了β′SiAlON的合成方法及其合成原料,并将合成方法分成还原剂外加法和内加法(即有机插层法)两类加以介绍。将各动力学参数(原料组成、添加剂及杂质、反应温度及时间、反应气氛、反应物颗粒尺寸等)对天然原料碳热还原氮化合成β′SiAlON的反应结果的影响进行了详细阐述,叙述并讨论了对该合成反应的不同反应机理及动力学模型的研究进展,并针对该领域的研究现状提出了一些展望。     

Fe2O3对β-SiAlON结合刚玉材料性能的影响

以白刚玉、硅粉、铝粉和α-Al2O3微粉等为原料,在氮化炉中于1460℃原位合成的β-SiAlON-刚玉复合材料,其氮含量(w)为3%,显气孔率为11.8%,体积密度为3.22g.cm-3,耐压强度为106.5MPa。在此基础上,用2%的Fe2O3替代白刚玉,在相同条件下合成的复合材料,其氮含量下降为2.4%,显气孔率和体积密度分别为12.4%和3.21g.cm-3,耐压强度增大为174.5MPa。试样的XRD分析表明,这两个试样的主晶相均为刚玉和z=3的β-SiAlON;未加Fe2O3的试样杂质很少,而加Fe2O3的试样中杂质较多,杂质主要为莫来石,这说明加入的Fe2O3阻碍了材料的氮化。另外,其β-SiAlON相中除了有Si3Al3O3N5外,还有Si4.69Al1.31O1.31N6.69。     

Effects of Substrate Pretreatment Conditions on Quality of GaN Epilayer

The pure cubic GaN(c-GaN) has been grown on (001) GaAs substrates by ECR-PAMOCVD technique at low temperature using TMGa and high pure N2 as Ga and N sources,respectively.The effects of substrate pretreatment conditions on quality of cubic GaN epilayer are investigated by the measurements of TEM and XRD.It is found that hydrogen plasma cleaning,nitridation and buffer layer growth are very important for quality of cubic GaN epilayer.