Ordered mesoporous ceramics stable up to 1500 degrees C from diblock copolymer mesophases

In the present study, a poly(isoprene-block-dimethylamino ethyl methacrylate) diblock copolymer (PI-b-PDMAEMA) is used to structure-direct a polysilazane pre-ceramic polymer, commercially known as Ceraset. To the polymer was added a 2-fold excess in weight of the silazane oligomer (Ceraset). The resulting composite was cast into films, and after cooperative self-assembly of block copolymer and Ceraset, the structure was permanently set in the hexagonal columnar morphology, as evidenced by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Cross-linking of the silazane oligomer was achieved with a radical initiator at 120 degrees C. Upon heating of the composite to 1500 degrees C under nitrogen, the structure is preserved and a mesoporous ceramic material is obtained, as demonstrated by SAXS and TEM. The pores are open and accessible, as evidenced by nitrogen sorption/desorption measurements indicating a surface area of about 51 m2 g-1 and a pore diameter of 13 nm, consistent with TEM analysis. These results suggest that the use of block copolymer mesophases may provide a simple, easily controlled pathway for the preparation of various high-temperature ceramic mesostructures.     

Controlled/living radical polymerization of vinylcyclicsilazane by RAFT process and their block copolymers

High molecular weight poly(vinyl)silazane were synthesized successfully by reversible addition fragmentation chain transfer (RAFT) polymerization in toluene at 120 °C, using dithiocarbamate derivatives and 2,2′‐azobis‐isobutyrylnitrile (AIBN) as the RAFT agents and thermal initiator, respectively. The polymerization of a vinylcyclicsilazane oligomer with 82.5% conversion was readily controlled to increase the molecular weight from 1000 to 12,000 g/mol with a narrow polydispersity <1.5. The resulting polymer showed a high ceramic yield of 70 wt % at 1000 °C. Moreover, the approach was extended successfully to the synthesis of poly(vinyl)silazane‐block‐polystyrene as an inorganic–organic diblock copolymer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4594–4601, 2008     

硅氮烷添加剂的水解稳定性对硅橡胶热稳定性的影响

采用一种新的方法研究了硅氮烷水解反应的动力学规律 ,根据实验数据测出硅氮烷水解速率对硅氮烷的浓度符合一级动力学关系 .计算出了几种硅氮化合物与水蒸气反应的表观活化能 ,结果表明硅氮烷的结构与其水解反应的表观活化能有密切的关系 .硅原子或氮原子上带有较大空间位阻的基团后 ,其水解稳定性提高 .其中六苯基环三硅氮烷 4的水解表观活化能为 2 14kJ mol,而苯基硅氮聚合物 5的水解表观活化能更达到 2 91 3kJ mol.添加到硅橡胶中的硅氮化合物水解表观活化能越大 ,即水解稳定性越高 ,其改进硅橡胶热稳定性的效果越好 .将 4和 5添加到硅橡胶生胶中 ,35 0℃下老化 2 4h的热失重分别为 0 96 %和 0 6 % .     

硅氮化合物的结构与水解稳定性的关系

采用硅氮化合物本体与水蒸汽直接作用的方法,研究了一系列硅氮化合物的结构与水解稳定性之间的关系.结果表明,空间位阻越大则硅氮化合物水解稳定性越好,环张力对硅氮烷的水解稳定性有一定的影响,但空间位阻的作用更为显著.带有苯基的环二硅氮烷、六苯基环三硅氮烷和含硅氮键的高分子聚合物具有较大的空间位阻,因此水解稳定性非常好.     

Synthesis and characterization of block copolymer/ceramic precursor nanocomposites based on a polysilazane

The amphiphilic block copolymer poly(isoprene-block-ethylene oxide) was used as a structure-directing agent for a polysilazane preceramic polymer commercially known as Ceraset. Two block copolymers of different molecular weights and poly(ethylene oxide) weight fractions with body-centered cubic sphere and hexagonal cylinder morphologies were used. To both polymers, 50 wt % of the silazane oligomer (Ceraset) was added. The resulting composites were cast into films and characterized by small-angle X-ray scattering and transmission electron microscopy. The silazane was chemically compatible with the poly(ethylene oxide) microdomains of the block copolymer, and this resulted in a swelling of those domains. After the cooperative self-assembly of the block copolymer and Ceraset, for both systems the structure was permanently set in the lamellar morphology by the crosslinking of the silazane oligomer with a radical initiator at 120 °C. These results suggest that the use of block copolymer mesophases may provide a simple and easily controlled pathway for the preparation of various high-temperature SiCN-type ceramic mesostructures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3346–3350, 2003     

Synthesis of soluble and meltable pre‐ceramic polymers for Zr‐containing ceramic nanocomposites

Polymer‐derived methods are one of the most important tools for the synthesis of ceramics with a finely dispersed microstructure. In this study, a soluble and meltable ZrC/C pre‐ceramic polymer, P‐DACZ, (which would later exhibit a high ceramic yield of 71 wt%) was synthesized via radical polymerization. By adding low molecular weight polycarbosilane in any proportion during the radical polymerization process of P‐DACZ, a soluble and meltable ZrC/SiC/C pre‐ceramic precursor, PCS‐DACZ (which would later exhibit a high ceramic yield of >80 wt%) was synthesized. After annealing at 1400 °C under an argon flow, the precursors converted into bulk ZrC/C and ZrC/SiC/C ceramic nanocomposites. The ZrC nanoparticles could resist any grain growth when heat‐treated at temperatures above 1800 °C because the C or SiC matrix prevented long‐range atomic diffusion of zirconium. Such ceramic nanocomposites would be suitable for structural and (multi)functional applications at harsh environments with high temperatures.     

采用红外漫反射光谱研究纳米Si3N4的表面结构

采用付利叶红外漫反射光谱对激光法制取的具有不同化学组成的纳米Ｓｉ３Ｎ４粉（１５４－３０ｎｍ）的表面结构，室表面氧化及热稳定性进行了研究，结果表明新鲜的富氮粉体表面主要为硅胺基（Ｓｉ３－ｘＮＨｘ，ｘ＝１－３）结构，粉体暴露空气后硅胺基会与空气中的水分子反应形成硅醇基（ＳｉＯＨ）结构，具有不同组成的粉体随粉中氮含量的增大粒子表面硅胺基量也增加；富硅粉体表面硅胺基较少，其氧化主要为表面硅原子与空气中氧原     

复合镀膜在陶瓷颗粒透射电镜分析中的应用

本文介绍了一种对陶瓷颗粒进行透射电镜分析的样品制备方法。采用复合电沉积方法,将陶瓷颗粒均匀分散包埋在金属铜镀层中,得到复合镀层薄膜,再通过电解双喷将薄膜减薄,获得透射电镜样品,从而可以对陶瓷颗粒进行透射电镜观察分析,讨论了获得此种复合镀层薄膜所需的电流密度、镀液pH值、搅拌措施和电镀时间。透射电镜观察分析的结果显示此方法效果良好。     

Pyrolytic conversion of an Al?Si?N?C precursor prepared via hydrosilylation between [Me(H)SiNH]4 and [HAlN(allyl)]m[HAlN(ethyl)]n

An iminoalane‐silazane polymer (ISP), an Al? Si? N? C precursor, has been synthesized via Pt‐catalyzed hydrosilylation between poly(allyl iminoalane‐co‐ethyl iminoalane) {[HAlN(allyl)]_m[HAlN (ethyl)]_n, AE‐alane} and 1,3,5,7‐tetrahydro‐1,3,5,7‐tetramethylcyclotetrasilazane {[Me(H)SiNH]₄, TCS}. The IR and ¹H NMR spectra of ISP indicate that the relative amounts of the allyl groups decrease slightly in comparison with those of AE‐alane, suggesting that hydrosilylation occurs partially. TG analysis up to 900 °C reveals that the ceramic yield of ISP is 83.1 mass%. It is suggested that the high ceramic yield can be ascribed to cross‐linking reactions occurring during pyrolysis. Possible reactions during pyrolysis are hydrosilylation, polymerization of the C?C bonds in the allyl groups and dehydrocoupling among the SiH groups, NH groups and AlH groups in ISP. The pyrolyzed residue at 1700 °C contains crystalline AlN, 2H‐SiC, β‐SiC and β‐Si₃N₄ and amorphous carbon, as revealed by solid‐state nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy and X‐ray diffraction (XRD) analysis. Copyright © 2006 John Wiley & Sons, Ltd.     

Preparation of Si–C–N–Fe magnetic ceramics from iron‐containing polysilazane

A new type of hyperbranched polysilazane containing iron (PSZI) compound was synthesized by the polycondensation of silazane lithium salts with FeCl₃, and the structure of the PSZIs was investigated by IR, NMR and elemental analyses. The PSZIs were pyrolyzed under nitrogen, argon or NH₃, and magnetic ceramics could be obtained. The ceramic yields of the PSZIs were higher than those of their corresponding silazanes, and the PSZIs or silazanes with reactive groups containing Si? H, ? CH?CH₂ or higher branched structures had higher yields. The magnetism of the ceramics could be controlled by a pyrolytic atmosphere and temperature: the saturation magnetization M_s ranged from 20 to 100 emu g^?1 and coercivity H_c ranged from 463 to 50 Oe. The transformation of the magnetic loop of the PSZIs pyrolyzed at different temperatures under NH₃ was quite different from those under nitrogen. It was shown by X‐ray diffraction measurements that the magnetic crystalline form could exist as Fe₄N, Fe(0) or Fe₃N depending on temperature under NH₃, but under a nitrogen atmosphere Fe(0) was nearly the only magnetic crystalline form from 600 to 1100 °C. By dipping or spin‐coating of the PSZI solution, then through pyrolysis under nitrogen, argon or NH₃, thin uniform magnetic ceramic films could be fabricated on the substrates. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis and kinetics of non-isothermal degradation of acetylene terminated silazane

Novel acetylene terminated silazane compounds,with three types of substituent,were synthesized by the aminolysis of dichlorosilane with 3-aminophenylacetylene(3-APA).Thermal property of the compounds is studied by thermogravimetry analysis (TGA).It shows that the acetylene terminated silazane has high temperature resistance.The char yield at 1000℃is 77.6,81.9 and 68.7 wt%for methyl,vinyl,and phenyl substituted silazane,respectively.The pyrolysis kinetics of the silazane is investigated by non-isothermal thermogravimetric measurement.The pyrolysis undergoes three stages,which is resolved by PEAKFTT.The kinetic parameters are calculated by the Kissinger method.The role of functionalities on the thermal resistance is discussed.The vinyl-silazane exhibits higher thermal stability because of higher cross-linking density.     

Thiol‐ene photopolymerization of polymer‐derived ceramic precursors

The liquid, ceramic precursor monomer VL20 was copolymerized with a thiol monomer in a traditional radical thiol‐ene photopolymerization. Polymerization occurred via addition of the thiol functional group to the vinyl silazane functional group in a 1:1 ratio consistent with a step‐growth polymerization. Gelation occurred at a high conversion of functional groups (70%) consistent with an average molecular weight and functionality of 560 and 1.7, respectively, for VL20 monomers. Initiatorless photopolymerization of the thiol‐VL20 system also occurred upon irradiation at either 365 or 254 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1752–1757, 2004     

Sol-gel processing of inorganic membranes

Basic principles involved in sol-gel processing of ceramic membranes are described. This process has been applied to ceramic ultrafiltration membranes and is now investigated to prepare ceramic nanofilters. In this paper special emphasis is put on new developments concerning microporous zirconia membranes obtained by the polymeric route. A zirconium alkoxide precursor modified with an acetylacetone ligand has been used in order to control particle growth in the sols and pore size distribution in the membranes. N₂ adsorption and X-ray diffraction analysis have been performed on membrane materials showing the influence of process parameters (molar ratio r = acacH/Zr and sintering temperature T) on membrane structural evolution.     

纳米-微米复合孔泡沫陶瓷固定化脂肪酶

考察了泡沫陶瓷的孔径分布和表面性质对脂肪酶固定化的影响.研究表明,泡沫陶瓷的纳米孔孔径分布非常适合脂肪酶的固定化,对固定化酶的催化效率有决定性的影响.经1h的定化,泡沫陶瓷固定化酶的活性达商业化硅藻土固定化酶的1.33倍,体积活力为其2.63倍,蛋白载量为45.36mg/g陶瓷,比活为1215.39U/g,活力回收为41.2%.泡沫陶瓷固定化脂肪酶在有机相乙酸乙酯合成中表现优良,连续使用5次,每次反应3h,乙酸转化率均在93%左右.     

Fabrication and Characterization of Diatomite-supported Activated Carbon Functional Ceramic

Using porous diatomite ceramic as carrier and phenolic resin as carbon precursor, the activated carbon functional ceramic with the activated carbon fixed into porous ceramic was prepared by the impregnation load phenolic resin, carbonization and activation isolated air. The influences of impregnation, curing, carbonization, activation etc. on the material property were discussed. The iodine value, SEM, elemental analyzer, BET and spectrum analysis chart were used to characterize the microstructures and performance of material at different conditions. The results showed that the excellent comprehensive property of activated carbon functional ceramic was gained when it adsorbed phenolic resin in 4 h under vacuum condition at curing temperature of 150 ℃ for 0.5 h and carbonization temperature of 600 ℃ for 1.0 h, and then put into 25wt% KOH for 4.0 h at activation temperature of 700 ℃ for 1.5 h. The iodine value is 176.9 mg/g, the specific surface area can reach 86.3 m2/g and the yield of carbonization is 50.48%.     

Reaction of Free Methyl Cation with (Isobutylamino)trimethylsilane in the Gas Phase. A Radiochemical and Quantum-Chemical Study

The reaction of free methyl cation with (isobutylamino)trimethylsilane was studied by the radiochemical method. Like with organic amines and silazane, the reaction occurs by two channels: condensation and proton transfer. Proton transfer remains to be the major reaction channel, while its contribution is less that with amines and silazane. The radiochemical and quantum-chemical methods were used to show that the condensation and proton-transfer complexes decompose primarily via Si-N bond cleavage.     

The Effect of Microwave Sintering on the Microstructure and Properties of Calcium Phosphate Ceramic

Despite good ability to improved biological affinity and activity of calcium phosphate (CP) ceramic to the surrounding host tissue when implanted, CP ceramic is capable to encourage direct bond with bone as their chemical compositions are similar to the mineral phase of bone. However, the low mechanical property of CP ceramic restricts use in load-bearing applications. Therefore, the primary aim of this study was to fabricate dense CP ceramic via used of microwave sintering analyze the microstructure and mechanical properties comparison to conventional ceramic sintering. The sintered specimens were characterized by their surface microstructure, density measurement and hardness test. The result demonstrated that the density and hardness values of sintered CP ceramic specimens by microwave sintering were higher than conventional sintering. The microstructures of CP ceramic microwave sintered specimens show also better microstructures (analyzed by FESEM technique), with fines grain size and the present of apatite layer growth on the surface when examined with SBF solution. It can be concluded that the microwave sintering enhanced better microstructure and mechanical properties of CP ceramic.     

Internal motion in organosilicon polymers. I. Linear dimethylpolysilazane

A linear dimethylpolysilazane polymer has been prepared and its internal motion studied by the techniques of broadline nuclear magnetic resonance. Second moments and line widths are reported as a function of temperature from ?196 to 30°C. The results are compared to those for linear dimethylpolysiloxane, as well as to those for the crosslinked siloxane and silazane. The experimental low-temperature second moment of 7.25 ± 0.5 gauss² corresponds to a C₃ reorientation of methyl groups about the silicon–carbon bond. The motions occurring in the silazane are found to be the same as those in the corresponding siloxane. The transition to low second moments occurs approximately 30°C. higher in the linear silazane than in the linear siloxane. This is attributed to a somewhat greater resistance to motion in the silazane.     

Continuous Ink-Jet Printing Using Sol-Gel “Ceramic” Inks

Although sol-gel processing is under intense world-wide investigation, the vast majority of interest in this technique is concerned with synthesis of engineering ceramics. Sol-gel has not been widely applied to the preparation of materials for use in conventional ceramics, such as tiles and tableware. This paper describes a novel application of aqueous sol-gel processing in which sol-gel based inks are used to decorate ceramic articles by continuous ink-jet printing. These inks contain major components of the colour for ceramic decoration in the form of a colloidal aqueous sol which dries to a gel after printing and is then converted to the coloured decoration on firing. The ink can be printed directly onto the ceramic, or printed onto paper and applied as a gel transfer. The key properties in formulating the inks are addressed and experiments are described that demonstrate this process. Its potential advantages over conventional decorating methods are discussed.     

Hydridosilazanes hydrolysis-condensation reactions studied by <Superscript>1</Superscript>H and <Superscript>29</Superscript>Si liquid NMR spectroscopy

Hydridosilazane compounds containing Si–N and Si–H bonds can be used as precursors of SiO_x materials. The hydrolysis-condensation reactions of tetramethyldisilazane, as a polyhydridosilazane model compound, were investigated by ¹H and ²⁹Si liquid NMR spectroscopy. These reactions were carried out at room temperature for up to 120 min in presence of water. The identified products are short linear siloxane species (hydride terminated polydimethylsiloxanes M^HD_xM^H) and cyclosiloxanes. Silicon hydride persistence in the reactional mixture suggested that silazane group is more sensitive to hydrolysis reaction than silicon hydride group. Moreover, additional experiments evidenced that the low steric hindrance of the silicon hydride influences the silazane hydrolysis kinetic. Hence the presence of ammonia released during silazane hydrolysis reaction was demonstrated to be a catalyst of the silicon hydride hydrolysis reaction.