热解前驱体制备AlN粉末

以硝酸铝、尿素、蔗糖为原料低分解得到均匀分散的前驱身粉末,对前驱体粉末的碳热的过程研究表明：不存在γ－ＡＩ２Ｏ３→α－ＡＩ２Ｏ３晶型转变,因而ＡＩＮ粉末的合成温度降低合成的ＡＩＮ粉末粒径约８０～１２０ｎｍ,比表面积１９．２ｍ＾２／ｇ,脱碳后的ＡＩＮ粉末氮的质量分数为３２．３％,氧的质量分数为２．１％,碳的质量分数为０．２％。     

钨酸锆前驱体脱水反应动力学

采用水热法合成了Zr W2O8的前驱体Zr W2O7(OH)2(H2O)2,并应用热重分析技术研究了其脱水反应。升温速率β=10℃/min,实验失重率为8.382%,与理论失重率8.452%基本吻合,脱水反应温度在173℃左右。应用非等温方法计算了钨酸锆前驱体脱水反应的动力学参数:表观活化能E=232.54 k J/mol;阿伦尼乌斯指前因子ln A=58.06。前驱体脱水为二级反应,动力学方程可以描述为:dα/dt=1025.21exp(-27.97×103/T)(1-α)2。     

碳化锆陶瓷的氧化及其对导电性能的影响

采用高温自蔓延制备的碳化锆粉体作为原料,研究了碳化锆陶瓷在空气中的氧化机制和热压烧结块体的氧化动力学行为。结果显示：在空气中,碳化锆陶瓷在200℃时开始氧化;在200～450℃时,氧化产物为ZrCxO1–x (x=0～0.42);在500～600℃时,生成中间相Zr2O出现;当氧化温度升高到1000℃,氧化产物主要为ZrO2。对烧结体的氧化动力学行为分析发现,在200～450℃的氧化过程中烧结陶瓷表面形成ZrCxO1–x致密氧化层,氧化层会抑制氧原子的扩散。当表层氧化产物为 ZrC0.42O0.58时,氧化反应基本停止,达到一个稳定状态。继续升高温度,由于产物(Zr2O 和 ZrO2)的晶型结构发生较大的变化,烧结块体会开裂破坏。随着固溶氧的增加,ZrCxO1–x (x=0～0.42)的电阻从37μ??cm增加到690μ??cm。     

二硼化锆陶瓷前驱体纤维的制备及表征

利用八水合氧氯化锆、硼酸、蔗糖和柠檬酸为无机原料,聚乙烯醇为有机原料,采用有机、无机共混反应制得前驱体溶液,并利用干法纺丝制得前驱体纤维,通过红外光谱(IR)、差示扫描(DSC)、热失重(TG)分析表征发生的反应及产生的一系列变化,利用扫描电镜(SEM)观察纤维的形态结构。结果表明:无机成分与聚乙烯醇发生了反应并使聚乙烯醇变得稳定,制得了光滑致密的纤维,为下一步制得ZrB2陶瓷纤维做好了准备。     

超细硼化锆粉体制备过程中的影响因素探究

探究了溶胶-凝胶协同碳热还原反应制备超细ZrB2粉体中Zr4+与柠檬酸的配比r、溶胶-凝胶温度T、体系pH值和分散剂含量c对ZrB2前驱体及热解产物的影响.并采用X射线衍射仪、红外光谱仪、激光粒度仪、场发射扫描电镜和透射电镜对ZrB2前驱体及热解产物进行表征和分析.结果表明:r=2,pH=4为柠檬酸和Zr4+发生络合反应的最佳条件;当T=50℃时,产物基本为球形或类球形,且粒度分布均匀;聚乙二醇除分散作用外,还能够对溶胶中团簇的交联起到“导向”作用,使热解后的晶粒排列有序,当c=2％时,可获得分散性好、尺寸均匀的产物.     

采用有机前驱体制备Si3N4／SiC纳米复相陶瓷

本研究采用有机前驱体为主要原料，通过热解及烧结制备了两类Ｓｉ３Ｎ４／ＳｉＣ纳米复相陶瓷，研究了这些材料的微结构特点，讨论了材料强化的机制及力学性能与显微结构的关系。     

聚碳硅烷/二乙烯基苯陶瓷前驱体交联样品的制备

研究了聚碳硅烷交联样品的制备过程，着重讨论了用二乙烯基苯交联聚碳硅烷的条件。结果表明：选择熔点为200℃，摩尔质量为1500g/mol作为熔融纺丝，生产碳化硅纤维前驱体用的聚碳硅烷，以二乙烯基苯为交联剂，氯铂酸异丙醇溶液为催化剂，聚碳硅烷与二乙烯基苯的质量比不能低于1：0．2，二乙烯基苯的用量为20％－40％，交联反应温度120℃时，制成的聚碳硅烷／二乙烯基苯陶瓷前驱体交联样品性能良好。     

氧氯化锆前驱体氧化锆溶胶的制备与研究

探讨了以无机盐氧氯化锆为前驱物,以双氧水为水解促进剂,通过溶胶－凝胶法制备氧化锆溶胶的工艺条件。探讨了双氧水的作用机理,通过研究胶凝过程中双氧水加入量对溶胶粘度－时间关系的影响以及对粒径分布的影响,结合IR、XRD和TEM等测试,讨论了双氧水的作用效果并分析了双氧水加入后溶胶和凝胶的结构、晶型以及溶胶粒子形貌的变化。结果表明,双氧水对于本溶胶体系有效的催化作用,溶胶具有较好地稳定性,是制备氧化锆薄膜的一种经济实用的前驱溶胶。     

A meltable precursor for zirconium carbide ceramics and C/C-ZrC composites

For process simplification and rapid densification of ceramic composites, a meltable single-source ZrC precursor was prepared by condensing zirconium acetylacetonate (Zr(acac)₄) at 190?°C for 40–150?min. The preparation of ZrC precursor and the conversion from precursor to ceramics were investigated by using FTIR and NMR spectroscopies, GPC, DSC-TGA, XRD, SEM, EDS and TEM. The precursor had low viscosity (~ 10?mPa?s) and proper processing window (60?min) for precursor infiltration and pyrolysis (PIP). The ceramic yield at 1650?°C was 29.6%, and EDS revealed that the composition was (ZrC)_0.337(HfC)_0.0025(ZrO₂)_0.044C_0.1865. The ceramics were composed of 0.2–0.5?µm grains which aggregated to form a stacked structure surrounded by amorphous carbon. The preparation processes were designed, and C/C-ZrC composites with the density of 2.45?g/cm³ were successfully fabricated through 11 cycles of PIP with Zr(acac)₄. In conclusion, the synthetic method provides a simple and cheap route for precursors, and allows combined composite preparation with high efficiency.     

Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Development of precursor ceramics using organic silicon polymer

This paper relates to the Bridge Building Award, which was presented to the author (Toshihiro Ishikawa) by the American Ceramic Society on 27 January 2020. We have developed many types of functional ceramics using polycarbosilane as a raw material. Since 1983, several grades of SiC-based fibers have been produced from polycarbosilane by Ube Industries, Ltd. Of these grades, we developed the highest heat-resistant SiC-polycrystalline fiber (Tyranno SA), which can withstand up to 2000°C, using an organic silicon polymer (poly-aluminocarbosilane) containing a small amount of aluminum as a precursor material. By employing curing (in air) and firing (in nitrogen atmosphere at 1300°C) processes using the precursor fiber, an amorphous fiber (Si-Al-C-O fiber) containing a small amount of aluminum was obtained; subsequent heat treatment at higher temperatures (~2000°C) in argon atmosphere led to carbothermal reduction (SiO₂ + 3C SiC + 2CO(g)) and a sintering process, producing the abovementioned SiC-polycrystalline fiber (Tyranno SA). In the same year, using the same raw precursor fiber (Si-Al-C-O fiber), we also developed a new type of tough, thermally conductive SiC composite (SA-Tyrannohex) with high strength up to 1600°C in air. This ceramic consists of a highly ordered, close-packed structure of very fine hexagonal columnar SiC-polycrystalline fibers with a thin interfacial carbon layer between them. Further, by using the polycarbosilane as a starting material, we successfully developed a strong photocatalytic fiber (TiO₂/SiO₂ fiber) with a gradient surface layer composed of TiO₂-nanocrystals, making the best use of controlled phase separation (bleed-out) of additives (titanium (IV) tert-butoxide) contained in polycarbosilane. In this paper, the story of the development of these materials and the subsequent progress will be described along with the historical background.     

Synthesis and properties of a new, branched polyhydridocarbosilane as a precursor for silicon carbide

Polymerization of Cl₂Si(CH₃)CH₂Cl with Mg in THF, followed by reduction with LiAlH₄, gave a polycarbosilane with Si-H groups and branches at the Si atoms. The polymer could be cross-linked thermally at 150°C. Pyrolysis of the cross-linked material gave SiC with a yield of 70%.Presented at the XXVIth Silicon-Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.     

Synthesis and pyrolysis mechanism of a novel polymeric precursor for SiBN ternary ceramic fibers

A novel polymeric precursor polyborosilazane (PBSZ) for SiBN ternary ceramic fibers was successfully synthesized from trichlorosilane (HSiCl₃), boron trichloride (BCl₃) and hexamethyldisilazane (HDMZ) by a simple one step reaction process. The chemical structures and ceramic yield of the PBSZ precursors were investigated by NMR spectroscopy, FT-IR and TGA. The preparation of PBSZ fibers was conducted in a lab-scale melt-spinning equipment at a spinning speed of 130 m/min. SiBN ternary ceramic fibers were obtained after the non-fusible treatment and pyrolysis of PBSZ fibers in an NH₃ atmosphere. The pyrolysis mechanism, high-temperature behavior and morphologies of the SiBN ternary ceramic fibers were investigated by NMR, XRD, TEM and SEM. The obtained SiBN ternary ceramic fibers had good flexibility, and possessed a tensile strength of 0.84 GPa with a diameter of ∼18 μm. Furthermore, these SiBN ceramic fibers exhibited excellent thermal stability, and maintained the amorphous state up to 1600 °C.     

Up-scalable preparation of nano zirconium carbide powder in liquid polymeric precursor and its pyrolysis mechanism

Nano size ZrC powder was prepared by liquid polymeric precursor method. Zirconium n-butoxide (Zr(OⁿBu)₄) and benzoylacetone (BA) were mixed directly with different molar ratios to synthesize transparent liquid zirconium carbide single-source precursors. The carbon content in the precursor could be changed by adding different amount of BA. X-ray pure ZrC was obtained when the molar ratio of BA/Zr(OⁿBu)₄ was 4.6:1. The viscosity of the precursor was very low (<8 mPa s) without the addition of solvents. Zirconium carbide powders were fabricated by the pyrolysis at 800 °C in argon and subsequent heating at various temperatures in vacuum for carbothermal reduction reaction. The pyrolysis behavior, phase composition and transformation, and microstructure of the as-fabricated ZrC powders were analyzed. The gases of CH₄, CO and CO₂ released due to decomposition and evaporation of the organic component and transformation from ZrO₂ to ZrC during pyrolysis resulted in total 60–70% mass loss. The average grain size of the synthesized X-ray pure ZrC powders was less than 30 nm. Meanwhile, the pyrolysis mechanism of nano zirconium carbide powder was deduced.     

Preparation of zirconium carbide foam by direct foaming method

Ultra light, highly porous, closed-cell structured ZrC foam can be produced in two steps. First, pre-ceramic foam is prepared by direct foaming of zirconia sol and phenolic resin. In the next step, the foamed green body is converted into ZrC foam after carbothermal reduction at 1600 °C under argon atmosphere. The obtained ZrC foam has porosity of 85% and possesses uniform cells with an average size of about 40 μm. The foam also displays excellent thermal stability up to 2400 °C. Its compressive strength and thermal conductivity at room temperature are 0.4 MPa and 0.94 W/(m K), respectively.     

Making lightweight SiC(rGO,xMoSi2) bulk ceramics via polymeric precursor route

Unique properties of MoSi₂ open new opportunities for preparing bulk polymer-derived ceramics (PDCs) displaying favorable structural-functional capabilities. Herein, an ingenious production route via re-pyrolysis process of ball-milling-induced rigid SiC(rGO, xMoSi₂)_p fillers/flexible polycarbosilane-vinyltriethoxysilane-graphene oxide (PCS-VTES-GO, PVG) precursors blends is proposed to obtain in situ formed SiC(rGO, xMoSi₂) bulk PDCs. Interestingly, the possible dense β-SiC/SiOxCy/C_free(rGO, xMoSi₂) framework suffers load and tiny microsized pores relaxes stress, which is beneficial to providing optimized hardness and fracture toughness, ceramic yield, and linear shrinkage. Attractively, MoSi₂ prominently enhances thermal and electrical conductivities of the products owing to increased continuity and compactness. To the best of our knowledge, lightweight SiC(rGO, 20%MoSi₂) bulk PDCs own brilliant ceramic yield (92.13%), liner shrinkage (6.69%), hardness (10.34 GPa), fracture toughness (4.35 Mpa·m^1/2), and thermal conductivity (8.57 W·m^–1·K^–1), opening potential emerging uses in aerospace fields.     

Carbon vacancy ordering in zirconium carbide powder

Ordered carbon vacancies were detected in zirconium carbide (ZrC_x) powders that were synthesized by direct reaction. Zirconium hydride (ZrH₂) and carbon black were used as starting powders with the molar ratio of ZrH₂:C = 1:0.6. Powders were reacted at 1300°C or 2000°C. The major phase detected by x-ray diffraction (XRD) was ZrC_x. No excess carbon was observed by transmission electron microscopy (TEM) in powders synthesized at either temperature. Ordering of the carbon vacancies was identified by neutron powder diffraction (NPD) and further supported by selected area electron diffraction (SAED). The vacancies in carbon-deficient ZrC_x exhibited diamond cubic symmetry with a supercell that consisted of eight (2 × 2 × 2) ZrC_x unit cells with the rock-salt structure. Rietveld refinement of the neutron diffraction patterns revealed that the synthesis temperature did not have a significant effect on the degree of vacancy ordering in ZrC_x powders. Direct synthesis of ZrC_0.6 resulted in the partial ordering of carbon vacancies without the need for extended isothermal annealing as reported in previous experimental studies.     

前驱体转化法制备碳化硼粉体的研究进展

碳化硼材料具有优良的物理和化学性能,被广泛应用于各个领域。目前,传统的碳热还原方法生产碳化硼粉体存在温度高、产率低、环境污染重等问题。相比之下,前驱体转化法具有能耗低、工艺简便、原料易得、产品尺寸小等特点,在制备碳化硼粉体方面有明显的优势。详细介绍了前驱体转化法合成碳化硼粉体的制备过程,综述了使用不同碳源经前驱体转化法合成碳化硼粉体的最新研究进展,并展望了前驱体转化法合成碳化硼的研究方向。