The Microstructure and Dielectric Properties of SiBCN Ceramics Fabricated Via LPCVD/CVI

Low‐pressure chemical vapor deposition and infiltration was used for the first time to fabricate SiBCN ceramics with proper dielectric properties to lower the preparation temperature. SiBCN ceramics indicated by thermodynamic phase diagram with different phase compositions from CH₃SiCl₃–NH₃–BCl₃‐H₂ could be obtained through adjustable parameters. The as‐prepared SiBCN ceramics fabricated above 900°C showed proper dielectric properties with dielectric loss of about 0.1, which was due to the generation of amorphous carbon and SiC nanocrystals.     

A new synthetic route to 2-boratanaphthalenes

Double kaliation of 1-isopropenyl-2-methylbenzene (4) and subsequent ring closure with BCl₂(NR₂) (R=Me, iPr) provides a new and efficient access to 1,2-dihydro-2-boranaphthalenes 6′ and, after metalation with LDA in Et₂O or with LiN(SiMe₃)₂ in hexane, to lithium 2-boratanaphthalenes 7. The substituent at boron may be modified. Treatment of the B-dialkylamino compounds with BCl₃ affords the highly reactive β-chloro derivative 6c′, and subsequent methylation with ZnMe₂ in toluene affords the B-methyl compound 6d′. The lithium 2-boratanaphthalenes 7 readily form transition metal complexes such as Cp^*Fe(4-MeC₉H₆BNMe₂) (9) and Cp^*ZrCl₂(4-MeC₉H₆BMe) (10) with hexahapto coordination of the metal to the boron-containing ring.     

Unique and contrasting mixed-metal lithium–calcium and lithium–barium hexamethyldisilazide complexes

Addition of lithium hexamethyldisilazide to calcium or barium bis(hexamethyldisilazide) in THF resulted in the synthesis of two unique but very different mixed-metal complexes: X-ray crystallography shows these to be, respectively, the heterobimetallic complex [Ca{N(SiMe₃)₂}₃Li(THF)] (1), containing two calcium–lithium bridging amide ligands and the remarkable co-crystalline compound [Ba{N(SiMe₃)₂}₂(THF)₃][Li₂{N(SiMe₃)₂}₂(THF)₂] (2).     

Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

2,4-Bis(trimethylsilyl) substituted n-pentasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe(SiMe₃)SiMe₃ (1) has been prepared from Me₂SiCl₂ and (SiMe₃)₂SiMeK, and its solid state molecular structure has been determined by single crystal X-ray diffraction. By reaction of 1 with KO^tBu and subsequent treatment of the resulting silyl anion with (MeO)₂SO₂, the novel hexasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe₂SiMe₃ (2) has been synthesized. By again using the reaction sequence KO^tBu/(MeO)₂SO₂, known n-pentasilane Me₃SiSiMe₂SiMe₂SiMe₂SiMe₃ (3) has been prepared in a novel and very convenient way from 2. All silanes 1, 2, and 3 as well as the intermediate silyl anions obtained from 2 and 3 have been characterized by ²⁹Si and ¹H NMR spectroscopy, 1 and 2 by elemental analyses also. Raman spectra in the temperature range 210–370 K of the pure liquids 1, 2 and 3 as well as of solutions in toluene (1 and 2) or cyclopentane (3) reveal that all of the silanes exist as mixtures of conformers. The symmetric SiSi stretch observed in the range 341 to 363 cm^-1 and possessing by far the largest Raman intensity of all SiSi stretching vibrations splits into five bands for 3 and two bands for 1 and 3. Quantum chemical DFT B3LYP/6–311G(d) calculations located 15 conformational minima for 2 with relative energies up to 19.9 kJmol^-1, and five minima for 1 with relative energies up to 15.5 kJmol^-1. Calculated SiSiSiSi torsion angles vary between 26.2° (smallest value found) and 179.3°, demonstrating the flexibility of backbones composed of silicon.     

SiBCN聚合物前驱体聚合机理的研究

SiBCN陶瓷前驱体的合成对于制备非晶SiBCN陶瓷有着重要的意义,本文研究了SiBCN聚合物前驱体聚合过程中不同阶段的产物的结构变化,研究表明在合成前驱体的氨解过程中,单体聚合反应的机理是首先氨气与氯硅烷单体上的氯发生反应生成伯胺,然后伯胺与其他的氯硅烷单体上的氯反应聚合生成仲胺。最终产物中伯胺和仲胺的数量由氨气流量的大小决定。氨气流量较小时,产物主要是仲胺;氨气流量大时,以伯胺为主。在此过程中并不会发生伯胺与伯胺发生反应脱掉氨气生成仲胺的化学反应。因此,氨气流量过大会降低陶瓷前驱体的聚合度。     

Synthesis,Characterization, and Reactivity of Lanthanide Complexes with Bulky Silylallyl Ligands

The synthesis of new lanthanide allyl complexes of enhanced stability and solubility in saturated hydrocarbons based on silyl-substituted allyl ligands is reported. Thus the potassium salt K(CH₂CHCHSiMe₃) ( 1 ) reacts with YCl₃ in tetrahydrofuran to give the tris-allyl complex Y(CH₂CHCHSiMe₃)₃ ( 2 ), while K(CH₂CHCHSiMe₂^tBu) ( 3 ) affords Y(CH₂CHCHSiMe₂^tBu)₃(THF)_1.5 ( 4 ). Slow re-crystallization of 4 from light petroleum in the presence of tert-butylcyanide led to multiple insertion to give the sec-amido complex Y{NHC(^tBu)(CH)₃SiMe₂^tBu}₂{η₂-NHC(^tBu)CH=CHCH₂SiMe₂^tBu)CH(CHCHSiMe₂^tBu)C^tBuNH}(THF)·(CH₃CH(Me)(CH₂)₂CH₃) ( 5 ), which was crystallographically characterized. The reaction of ScCl₃(THF)₃ with two equivalents of Li{1,3-C₃H₃(SiMe₃)₂} in tetrahydrofuran gives the bis-allyl complex {1,3-C₃H₃(SiMe₃)₂}₂Sc(μ-Cl)₂Li(THF)₂ ( 6 ), while the analogous reaction of K{1,3-C₃H₃(SiMe₃)₂} ( 7 ) with either LaCl₃ or YCl₃ in tetrahydrofuran affords the bis-allyl complexes MCl{1,3-C₃H₃(SiMe₃)₂}₂(THF)_x (8, M = La, x = 1; 9, M = Y, x = 0). An attempt to prepare the similar neodymium complex gave the mono-allyl complex NdI₂{1,3-C₃H₃(SiMe₃)₂}(THF)_1.25 ( 10 ). The reactions of 8 and 9 with triisobutyl aluminum in benzene-d₆ show allyl exchange between lanthanide and aluminum. Complexes 8 , 9 , and 10 have been tested with a variety of activator systems as catalysts for the polymerization of 1,3-butadiene.     

Gas phase reaction kinetics in boron fibre production

In the production of boron fibres using the chemical vapor deposition (CVD) technique, boron deposition and dichloroborane formation reactions occurs simultaneously. Boron deposition reaction occurs at the surface, whereas the formation of dichloroborane is the result of both gas phase and surface reactions. A continuous stirred tank reactor (CSTR) type of reactor was designed to investigate the reaction kinetics and kinetic parameters in the gas phase reactions of boron trichloride and hydrogen. It was concluded that reaction rate of the product increased with an increase in the inlet concentration of both reactants (BCl₃ and H₂) and with an increase in the reactor temperature. While reaction order with respect to BCl₃ was almost constant at about 0.5 at each temperature, reaction order with respect to hydrogen increased significantly at temperatures lower than 350°C. A general rate expression was derived for BHCl₂ formation from BCl₃ and hydrogen. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Investigation of 1,3,5-tris(2-metoxypropane)benzene/BCl3 initiated living isobutylene polymerization by C13 and B11 NMR spectroscopy

Summary The living carbocationic polymerization of isobutylene initiated by 1,3,5-tris(2-methoxypropane) benzene (TriCumOMe)/BCl₃ system was investigated by C¹³ and B¹¹ NMR spectroscopy. The reaction between the TriCumOMe and BCl₃ at-30°C in CH₂Cl₂ after 15 mins reaction time resulted in 1,3,5-tris(2-chloropropane) benzene (TriCumCl) and methyl-dichloroboronite (BCl₂OMe). The same system in the presence of isobutylene yielded three-arm star, chlorine terminated telechelic polyisobutylene and BCl₂OMe. No counterions, i.e., BCl₃OMe, BCl ₄ , or neutral boron complexes, e.g., TriCumOMe, 3BCl₃ could be detected. The simultaneous measurement of static permittivity (direct monitoring method) showed different reaction rate patterns in the case of AMI method, and when the TriCumOMe+BCl₃ mixture was aged and the polymerization was started by isobutylene.     

Investigation of the mechanism of living cationic polymerization of isobutylene by B11 NMR spectroscopy

Summary The living carbocationic polymerization of isobutylene initiated by tri-cumyl-ether (1)/BCl₃ and tricumyl-acetate(2)/BCl₃ was investigated by B¹¹ NMR spectroscopy in the presence and absence of DMSO. With BCl₃, 1 yields tri-cumyl-chloride and BCl₂OMe due to fast exchange reaction, while 2 forms complexes. If the 1/BCl₃ mixture contains DMSO, well defined complexes can be detected, i.e., DMSO.BCl₃ and BCl₂OMe.DMSO. In the system 2/BCl₃/DMSO neutral complexes with broad NMR signals are formed. In the presence of isobutylene (real polymerization mixture) the same complexes can be detected.     

Influence of pressure on chemical vapor deposition of boron nitride from BCl3/NH3/H2 gas mixtures

Boron nitride (BN) was synthesized from BCl₃/NH₃/H₂ precursor mixtures via chemical vapor deposition, with a focus on the influence of the total partial pressure of BCl₃, NH₃, and H₂ (p_BCl3+NH3+H2) on the surface deposition rate. The surface deposition rate of BN initially increased and then decreased with increasing p_BCl3+NH3+H2, implying that the deposition process transitioned from surface reaction control to mass transfer control. BN deposition from BCl₃ and NH₃ was mainly attributable to several intermediate gaseous products containing B, N, Cl, and H, such as Cl₂BNH₂, ClB(NH₂)₂, and B(NH₂)₃. The microstructures of BN coatings deposited on SiC fibers were also analyzed. The BN coatings were uniformly and evenly deposited on the SiC fiber surfaces at low p_BCl3+NH3+H2 values, whereas excessive p_BCl3+NH3+H2 values afforded coatings containing large grains. The as-prepared BN coatings were stoichiometric but amorphous. Heat treatment substantially improved the texture and crystallinity to afford hexagonal BN.     

Polymerization of butadiene and copolymerization of butadiene with styrene using neodymium amide catalysts

The polymerization of butadiene was performed with catalysts based on the complex Nd{N(SiMe₃)₂}₃ (1). This amide complex in combination with methyaluminoxane or with a boron compound ([HNMe₂Ph][B(C₆F₅)₄], [CPh₃][B(C₆F₅)₄] or B(C₆F₅)₃) and Al(iBu)₃ showed high activity and stereospecificity in polymerization of butadiene. The cationic complex [Nd{N(SiMe₃)₂}₂(THF)₂][B(C₆F₅)₄] (2) was prepared by reaction of 1 and [HNMe₂Ph][B(C₆F₅)₄]. The catalyst 2/Al(iBu)₃ (ratio Al/Nd: 10/1) was highly active for butadiene polymerization. Copolymerization of butadiene and styrene was performed with the catalytic system Nd{N(SiMe₃)₂}₃/[HNMe₂Ph][B(C₆F₅)₄]/Al(iBu)₃ (3). Copyright © 2004 Society of Chemical Industry     

Synthesis and ceramic conversion of a novel processible polyboronsilazane precursor to SiBCN ceramic

Polyboronsilazane (PBSZ) precursors for SiBCN ceramics were prepared by using 9-borabicyclo-[1,3,3] nonane (9-BBN) and copolysilazanes (CPSZ) as starting materials, involving the hydroboration reaction between vinyl groups of PSZ and BH groups of 9-BBN under mild conditions. The as-synthesized PBSZ was obtained as a soluble liquid, which was characterized by FT IR and NMR. The polymer-to-ceramic conversion of PBSZ at a ceramic yield of 62.2–79.9% was investigated by means of FT IR and TGA. The crystallization behavior and microstructures of PBSZ-derived SiBCN ceramics were studied by XRD, SEM and HRTEM. The SiBCN ceramic began to crystallize at 1600 °C. Further heating at 1800 °C induced partial crystallization to give mixed XRD patterns for SiC, Si₃N₄, and BN(C). It is observed that the introduction of boron improves the thermal stability of SiBCN ceramics, especially under high temperatures of 1600–1800 °C. In addition, the introduction of boron significantly improves the ceramic density while inhibits the SiC crystallization.     

Synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient pressure using sodium silicate precursor

The present paper deals with the synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient pressure using low cost sodium silicate precursor. The hydrogels were prepared by sol–gel processing of sodium silicate precursor and acetic acid catalyzed water followed by vapour passing and solvent exchange with methanol. The mixture of MeOH:trimethylchlorosilane (TMCS):hexane was used for the end capping of the silanols present on the silica surface. The process was optimized by varying vapour passing time, gel ageing time, molar ratios of H₂O/Na₂SiO₃, CH₃COOH/Na₂SiO₃ and TMCS/Na₂SiO₃ and silylation period. The aerogels have been characterized by bulk density, % of volume shrinkage, porosity, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis and contact angle measurements. The best quality silica aerogels in terms of low density (0.073 g/cc), higher porosity (96%) and better hydrophobicity (θ = 146°) have been obtained with the molar ratio of Na₂SiO₃:H₂O:CH₃COOH:TMCS at 1:166.6:2.25:11.9.     

New polyisobutylene stars XI. Synthesis and characterization of allyl-telechelic octa-arm polyisobutylene stars

The synthesis and characterization of a novel star comprising eight allyl-terminated polyisobutylene (PIB) arms radiating from a calix[8]arene core is described. The synthesis was accomplished by a core-first method, by inducing the living polymerization of isobutylene (IB) by a suitably functionalized calix[8]arene initiator (1) in conjunction with BCl₃-TiCl₄ coinitiators, and terminating the growth of the living PIB^⊕ arms by allyltrimethylsilane. The relative concentrations of BCl₃ and TiCl₄ are critical for the synthesis of well-defined 8-arm stars. Characterization of the products (which included triple detector GPC studies and ¹H NMR spectroscopy) indicated quantitative allylation. A mechanism which summarizes the experimental observations is proposed. Received: 17 July 1997/Revised version: 11 September 1997/Accepted: 19 September 1997     

Synthesis and structure of sterically overloaded tetra-coordinated yttrium and lanthanum disiloxides

The synthesis, structures and reactivity of the spirocyclic yttrium and lanthanum disiloxides {[(CH₂R₂SiO)₂]₂M}H [M = Ln, Y; R = SiMe(SiMe₃)₂] 3 and 4 are reported. Compounds 3 and 4 were prepared from reactions of two equivalents of [CH₂(R)₂SiOH]₂ [R = Si(SiMe₃)₂Me] (1) with one equivalent of M[N(SiMe₃)₂]₂ (M = Y, La), respectively.     

Preparation of self-healing Cf/SiBCN(O) composite using a novel polyborosilazane

In this study, novel polyborosilazane-derived SiBCN(O) ceramic was used as self-healing component in self-healing C_f/SiBCN(O) composite, which was prepared by polymer infiltration and pyrolysis (PIP) process. Molecular-level structure design of boron-containing ceramic precursors was utilized to achieve uniform dispersion of boron-containing self-healing components in prepared composites. No elemental diffusion was observed at the interface of ceramic matrix and carbon fibers, which resulted in stable SiBCN(O) structure. In addition, boron was uniformly distributed in C_f/SiBCN(O) composite ceramic matrix, which was beneficial for self-healing of cracks. Cracks and indentations were able to heal at high temperatures in air. The best crack-healing behavior occurred in air atmosphere at 1000 °C, with nearly complete crack healing. This excellent self-healing behavior was achieved because silicon and boron atoms in SiBCN(O) ceramic reacted with available oxygen at high temperatures to form SiO₂(l), B₂O₃(l), and B₂O₃·xSiO₂ liquid phases, which effectively filled cracks. In general, as-prepared C_f/SiBCN(O) composite exhibited excellent self-healing properties and shows great application potential in high-temperature environment applications such as aviation, aerospace, and nuclear power.     

Effect of cobalt bis(dicarbollides) on electrochemical oxygen reduction on Pt electrode in methanol-acid solution

Electrochemical oxygen reduction on a platinum electrode has been investigated in methanol-0.05 M H₂SO₄ aqueous solutions in the presence of acids of cobalt bis(dicarbollide) (H⁺B⁻, H⁺[(1,2-C₂B₉H₁₁)₂-3-Co]⁽⁻⁾) or its hexachlorinated derivative (H⁺BCl⁻, H⁺ [(8,8′,9,9′,12,12′-Cl₆-(1,2-C₂B₉H₈)₂-3-Co)]⁽⁻⁾). Methanol oxidation was suppressed with H⁺BCl⁻ significantly, and selective oxygen reduction was achieved on a platinum electrode. The efficiency of oxygen reduction to H₂O, which depended on the electrode potential, was lower by 2% to 20% than that without H⁺BCl⁻. The mechanism was investigated by surface-enhanced infrared absorption spectroscopy (SEIRAS). H⁺BCl⁻ suppresses formation of both the catalytic poison, linear CO (ads.), from methanol and the formate, the intermediate of the non-CO path of methanol oxidation.     

Living polymerization of isobutylene initiated byp-dicumyl chloride/BCl3/n-Bu4NX systems

Summary The 1,4-di-(2-chloro-2-propyl)-benzene (pDCC)/BCl₃/IB system was investigated in the presence and absence ofn-Bu₄NX. The presence ofn-Bu₄NX (X=Cl or I) changes the mechanism increasing the living character of the polymerization of isobutylene in both cases. Whenn-Bu₄NCl is added, BCl₄ ^– is formed and as a common ion in excess it shifts the dissociation equilibrium toward the non-dissociated species. In the case of the addition ofn-Bu₄NI, an exchange reaction between the gegenions, i.e., BCl₄ ^– and BCl₃I^– has been recognised and a possible reaction mechanism is given.     

Hydrosilylation Synthesis of New Ferrocenylene Copolymers Containing Carbosilylene,Carbosiloxy and Cyclopentadienyliron Dicarbonyl Bridges

Bifunctional organometallic silicon precursor monomers and substrates FC(SiMe₂H)₂ (1) [FC = (η⁵-C₅H₄)Fe(η⁵-C₅H₄)]; FC(SiMe₂(CH₂)_xCH=CH₂)₂ [x = 0 (2), 1 (3)], [η⁵-C₅H₄-SiMe₂(CH₂)_xCH=CH₂)]Fe(CO)₂SiMe₂(CH₂)_xCH=CH₂ x = 0 (4), 1 (5) and (η⁵-C₅H₄-SiMe₂H)Fe(CO)₂SiMe₂H (6) have been used to make a series of new iron containing polymers via hydrosilylation reactions. In addition to the vinyl- and allyl-containing substrates 2, 3, 4 and 5 the organosilicon compounds [CH₂=CHSiMe₂]₂O, 1,4-(H₂C=CH-SiMe₂)₂C₆H₄ and (HC≡CH–SiMe₂)₂O were also used as substrates for the hydrosilylation reaction. The reactions between the various SiH and CH=CH₂ and C≡C functionalities were performed in the presence of Pt(0) catalyst and resulted in regioselective (β-isomer and β-(E) isomer) products as determined by NMR spectroscopy. Molecular weights of all the polymers were determined by Gel Permeation Chromatography, which revealed oligomeric materials with narrow polydispersity. Cyclic voltammetric studies of exhibited single reversible redox processes due to the Fe(II)/Fe(III) couple when present, and irreversible oxidation for the presence of any Fp Fe atom. This article is dedicated to Professor Astruc.     

Carbocationic polymerization in supercritical carbon dioxide

Polymerization of isobutylene (IB) in supercritical carbon dioxide (SC·CO₂) at 32.5°C and 140 bar by the use of 2-chloro-2,4,4-trimethyl-pentane (TMPCl) initiator in conjunction with a mixture of TiCl₄/BCl₃ leads to well-defined polyisobutylenes (PIB) capped by a t-Bu head group and a t-Cl tail group (tBu-PIB-Cl^t) of M_n1800 g/mole and M_w/M_n=1.3. The TiCl₄/BCl₃ mixture may be viewed a new Friedel-Crafts Acid that effects rapid initiation, essentially chaintransferless propagation and reversible termination. The mechanism of IB polymerization of TiCl₄/BCl₃ mixtures is discussed.