Effect of SiO_2 on the Preparation and Properties of Pure Carbon Reaction Bonded Silicon Carbide Ceramics

Effect of SiO 2 content and sintering process on the composition and properties of Pure Carbon Reaction Bonded Silicon Carbide (PCRBSC) ceramics prepared with C-SiO 2 green body by infiltrating silicon was presented.The infiltrating mechanism of C-SiO 2 preform was also explored.The experimental results indicate that the shaping pressure increases with the addition of SiO 2 to the preform,and the pore size of the body turned finer and distributed in a narrower range,which is beneficial to decreasing the residual silicon content in the sintered materials and to avoiding shock off,thus increasing the conversion rate of SiC.SiO 2 was deoxidized by carbon at a high temperature and the gaseous SiO and CO produced are the main reason to the crack of the body at an elevated temperature.If the green body is deposited at 1800℃ in vacuum before infiltration crack will not be produced in the preform and fully dense RBSC can be obtained.The ultimate material has the following properties:a density of 3.05-3.12g/cm3,a strength of 580±32MPa and a hardness of (HRA)91-92.3.

Application of Reaction-Bonded Silicon Carbide in Manufacturing of Spacecraft Combustion Chamber

Silicon carbide (SiC) ceramics is a good structural ceramics material, which have a lot of excellent properties such as superior high-temperature strength up to a temperature of 1 350 ℃, chemical stability, good resistance to thermal shock and high abrasion resistance. The silicon carbide ceramics material has so far been used widely for manufacturing various components such as heat exchangers, rolls, rockets combustion chamber. Sintering of ceramics structural parts have many technological method, the reaction-bonded is one of important sintering technology of ceramics structural parts. The preparation of reaction-bonded silicon carbide (RBSC) is based on a reaction sintering process, whereby a compacted body of α-SiC and carbon (graphite) powders is heated in contact with liquid silicon or gas silicon, which impregnates the body, converting the carbon (graphite) to β-SiC which bonds the original alpha grain. This process is characterized by low temperature and a short time sintering, and being appropriate to the preparation of large size and complex-shaped components, and so on. Besides, during compacting process of reaction sintering, it can maintain a stable dimension of ceramics parts. Therefore, the method of reaction-bonded silicon carbide ceramics has been identified as a technology suitable for producing complicated and highly exact dimensions’ ceramics parts. In this paper, the method of reaction-bonded silicon carbide was applied to the manufacturing of a complex-shaped spacecraft combustion chamber of SiC ceramics. SiC and carbon powder of 4～30 μm were chosen as the raw materials, green compacts containing appropriate wt.% carbon were formed using the mold press method, sintering was performed in a graphite electric furnace under an argon atmosphere. It was introduced in detail that the technological parameters and technological flow of reaction sintering silicon carbide ceramics. At the same time, physical and mechanical experiments such as bending strength, coefficient of thermal expansion, coefficient of thermal conductivity, gastight property, heat resisting property etc. have been carried out. The results demonstrated that spacecraft combustion chamber made from reaction sintering of silicon carbide ceramics is feasible and the results of experiment is satisfactory. The strength of high-temperature structural parts made by reaction sintered SiC varied with silicon content; Under the this article testing condition, the optimum silicon content is 10.5% for the part investigated. The method of reaction sintered SiC ceramics is suitable for manufacturing of complicated spacecraft parts with a working temperature of 1 500 ℃.

Properties of a reaction-bonded β-SiAlON ceramic doped with an FeMo alloy for application to molten aluminum environments

An FeMo-alloy-doped β-SiA1ON （FeMo/β-SiA1ON） composite was fabricated via a reaction-bonding method using raw materials of Si, Al2O3, A1N, FeMo, and Sm2O3. The effects of FeMo on the microstructure and mechanical properties of the composite were investi- gated. Some properties of the composite, including its bending strength at 700℃ and after oxidization at 700℃ for 24 h in air, thermal shock resistance and corrosion resistance to molten aluminum, were also evaluated. The results show that the density, toughness, bending strength, and thermal shock resistance of the composite are obviously improved with the addition of an FeMo alloy. In addition, other properties of the composite such as its high-temperature strength and oxidized strength are also improved by the addition of FeMo alloy, and its corrosion re- sistance to molten aluminum is maintained. These findings indicate that the developed FeMo/β-SiA1ON composite exhibits strong potential for application to molten aluminum environments.

Dielectric Properties of Porous Reaction-boned Si_3N_4 Ceramics with Controlled Porosity and Pore Size

This paper presents the microwave dielectric properties of reaction bonded porous silicon nitride ceramics with variant porosity and pore size, which were prepared by adding pore-forming agent grains into the silicon powders. The experimental results show that the dielectric constant and the dielectric loss of the samples reduce evidently with increasing porosity in the sample. When the porosity is constant, the dielectric constant and the dielectric loss of the ceramics decrease visibly as the pore size increases. Among all the obtained samples, the minimum dielectric constant is about 2.4.

A FACILE SYNTHESIS OF 2-TRIFLUOROMETHYLPYRROLE VIA 1,3-DIPOLAR CYCLOADDITION REACTION OF 2-TRIFLUOROMETHYLOXAZOLONE AND THE ACTIVATED CARBON-CARBON MULTIPLE BOND

The 1.3-dipolar cycloaddition reaction of 2-trifluoromethyl- oxazolone and the activated carbon-carbon multiple bond was studied and gave a convenient way to synthesize 2-trifluoromethylpyrrole derivatives.

Reshapeable,rehealable and recyclable sensor fabricated by direct ink writing of conductive composites based on covalent adaptable network polymers

Covalent adaptable network(CAN)polymers doped with conductive nanoparticles are an ideal candidate to create reshapeable,rehealable,and fully recyclable electronics.On the other hand,3D printing as a deterministic manufacturing method has a significant potential to fabricate electronics with low cost and high design freedom.In this paper,we incorporate a conductive composite consisting of polyimine CAN and multi-wall carbon nanotubes into direct-ink-writing 3D printing to create polymeric sensors with outstanding reshaping,repairing,and recycling capabilities.The developed printable ink exhibits good printability,conductivity,and recyclability.The conductivity of printed polyimine composites is investigated at different temperatures and deformation strain levels.Their shape-reforming and Joule heating-induced interfacial welding effects are demonstrated and characterized.Finally,a temperature sensor is 3D printed with defined patterns of conductive pathways,which can be easily mounted onto 3D surfaces,repaired after damage,and recycled using solvents.The sensing capability of printed sensors is maintained after the repairing and recycling.Overall,the 3D printed reshapeable,rehealable,and recyclable sensors possess complex geometry and extend service life,which assist in the development of polymer-based electronics toward broad and sustainable applications.

A reverse particle grading strategy for design and fabrication of porous SiC ceramic supports with improved strength

Porous ceramics usually require high mechanical strength and maximized porosity simultaneously,while for conventional particle grading strategies,it is highly challenging to meet both demands.To this end,a reverse particle grading strategy was developed based on the linear packing model by unusually introducing coarse particles(d_(50)=16μm)into a fine particle(d50=5μm)matrix.Following the extrusion and sintering process,tubular porous SiC ceramic supports with improved mechanical strength were successfully fabricated.The effects of coarse particles on the rheological properties of the ceramic paste and the macroscopic properties and microstructure of the SiC supports were systematically investigated.With an increase in the content of coarse SiC particles to 30 wt%,the pressure generated during extrusion decreased from 5.5±0.2 to 1.3±0.1 MPa.Notably,the bending strength of the tubular supports increased from 36.6±5.6 to 49.1±4.5 MPa when 20 wt%coarse powder was incorporated.The notably improved mechanical strength was attributed to the distribution of coarse particles that prolonged the route of crack deflection.Additionally,the optimized tubular supports had an average pore size of 1.2±0.1μm,an open porosity of 45.1%±1.6%,and a water permeability of 7163±150 L/(m^(2)·h·bar)as well as good alkali and acid corrosion resistance.Significantly,the strategy was proven to be feasible for the scale-up fabrication of 19-channel SiC tubular porous ceramic supports.

Catalytic Hydrogen Transfer Reactions

Our recently studies on three types of reactions with hydrogen transfer as a key step,including catalytic asymmetric proton transfer reactions using"chiral proton transfer shuttle",catalytic B—H bond insertion containing a hydrogen atom transfer,and iron-catalyzed hydrosilylation reactions containing hydride transfer were briefly introduced.

Mechanism and rate constants for complete series reactions of 19 fluorophenols with atomic H

Fluorine-containing halogenated fluorophenol may have effect as intermediate species involved in the formation of polyfluorinated dibenzo-p-dioxin/dibenzofurans （PFDDs/Fs）. The mechanism for the atomic H initiated reactions with complete series of nineteen fluorophenol congeners was studies using the density functional theory. At the MPWB1K,/6-31＋G（d,p） level, the geometries and frequencies of reactants, transition states, and products were obtained, and the accurate energetic values were acquired at the MPWB 1K/6-311 ＋G（3df,2p） level. The rate constants were evaluated by the canonical variational transition-state theory with the small curvature tunneling contribution over a wide temperature range of 600-1000 K. The study shows that the intramolecular hydrogen-bond in the ortho-substituted FPs as well as the inductive effect of the electron-withdrawing fluorine and steric repulsion of multiple substitutions may ultimately be responsible for the relative strength of the O-H bonds in FPs. The results can be used for further studies on PFDD/Fs formation mechanism.

Preparation of AlN-Al_2O_3 composite via reaction bonding

Recently, the study of nanocomposites has attracted much attention, but the powder isdifficult to be homogenized and nanoparticles are not well distributed when nanophase isintroduced into matrix via general mechanical mixing. The in situ formation of AlNsubmicron particles in Al2O3 matrix has been explored to prepare AlN-Al2O3 composite bymeans of reaction-bonding method.

Theoretical Study on the Kinetics of Electron Transfer for Bond-breaking Reaction

To test the theory of dissociative electron transfer, a simple model describing the kinetics of electron transfer bond breaking reactions was used. The Hamiltonian of the system was given. The homogeneous and heterogeneous kinetic data fit reasonably well with an activation driving force relationship derived from the Marcus quadratic theory. In the heterogeneous case, there is a good agreement between the theoretical calculation and the experimental result, while in the homogeneous case, a good agreement is only observed for the tertiary halides. This is due to the stability of tertiary radical resulting from the sterical effect.

Rationalization of regioselectivity of electrophilic substitution reaction for cyclic compounds in terms of D_(pb) values

Accepted theories predict that substitution reactions are controlled by the electronic nature of the attacked site for electrophilic aromatic substitution. Here it is shown that in addition the bond strength of the broken bond may also influence the regioselectivity of the substitution reaction, and that the Dpb is a good indicator of the strength of a chemical bond. The Dpb denotes the depth of the potential acting on one electron in a molecule at the bond center （bc）. In this letter, the values of Dpb along the C-H and N-H bonds have been investigated, and it is demonstrated that for aromatic compounds, the regioselectivity of the electrophilic substitution can well be rationalized in terms of Dpb values.

Influences of bracket bonding on mutans streptococcus in plaque detected by real time fluorescence-quantitative polymerase chain reaction

Background Enamel demineralization occurs frequently during orthodontic treatment. In this study, we evaluated the changes of the density of mutans streptococcus （MS） in plaque after bracket bonding and using fluoride adhesive on maxillary incisors by real time fluorescence-quantitative polymerase chain reaction （RT-FQ PCR）.Methods The study was designed as a self-paired test. Brackets were bonded with fluoride adhesive on the left side, while non-fluoride adhesive on the right side for each patient. Plaque samples were taken from the surfaces around the brackets of four maxillary incisors before brackets bonding and after the bonding 4 weeks later. The amount of MS was measured by RT-FQ PCR. The data obtained were analyzed statistically using the SPSS 11.5 version and the alpha level was set at 0. 05 （ 2-tailed）.Results The amount of MS in plaque increased significantly after bracket bonding （ P 〈 0.01 ）, whereas no significant differences were observed among four maxillary incisors both before and after brackets bonding （P 〉 0. 05 ）, and among the incisors using and not using fluoride adhesive （ P 〉 0. 05 ）.Conclusions The increase of the density of MS in plaque after bracket bonding is one of the etiological factors for enamel demineralization in orthodontic patients. The result of this study did not support what we observed clinically that the incidence of enamel demineralization for lateral incisors was higher than that for central incisors. Using fluoride adhesive for bonding did not affect the amount of MS in plaque in our study. Further study is needed.

A novel N-N bond cleavage reaction of 4-amino-1,2,4-triazole derivatives

5-Substituted-4-animo-3-thiol-l,2,4-triazoles (1a—b) react with ortho nitrochloro-benzene or para-nitrochlorobenzene to give N—N bond cleavage products 2a—d, one structure of which (2b) has been unambiguously confirmed by an X-ray structural analysis.

A spatiotemporally-nonlocal continuum field theory of polymer networks

A physically-based continuum theory that captures the microstructure-dependent and temporal effects of both permanent and transient polymer networks is still lacking,despite the fact that it is greatly needed for the analysis of polymeric microstructures.To fill in this gap,this work proposes a physically-based spatiotemporally nonlocal continuum field theory.A general frame-work is established that quantitatively connects microscopic descriptions of polymer networks(chain energetics,chain-length distribution,assembly structure of the interpenetrating network,and rate of bond exchange reactions)to key components in the spatiotemporally nonlocal constitutive relations(explicit form of the nonlocal kernel function,magnitude of nonlocal characteris-tic length,two-phase weighting factors,and explicit form of the relaxation function),based on three hypotheses on the continuum viewpoint of the underlying discrete network structure:the existence of a finite bottom bound of volume to define intensive quan-tities,uniformity of energy density field inside the representative volume of a polymer network,and the condition for initiation of chain stretch.Applying the general framework to a permanent 8-chain concentric network yields a concrete two-phase nonlocal elasticity constitutive relation,where the explicit form of the kernel function can be derived by simply assuming an implicit form.Application to a transient network with bond exchange reactions yields a spatiotemporally nonlocal constitutive relation.The spatiotemporally nonlocal continuum theory can be helpful for exploring transformative and subversive high-performance materials involving the specific spatial stacking and arrangement of functional units through artificial design.

Effective preparation of 5,5'-(arylmethylene)bis(4-hydroxythiazole-2(3H)-one) in an aqueous fluoroalcohol solvent system under ultrasound irradiation at room temperature

A green and convenient approach for the synthesis of a series of 5,5＇-（arylmethylene）bis（4-hydroxythiazole-2（3H）-one） by the reaction of aryl aldehydes,monochloroacetic acid and ammonium thiocyanate in water/trifluoroethanol（TFE）（1：1） under ultrasound irradiation at room temperature is described.This method provides several advantages such as environmental friendliness,shorter time,excellent yields,and simple work-up procedure.

Asymmetric allylation of aldehydes and alkylation using "carbon-centered" chiral organosilicon compounds

The effects of the chiral substituents attached to silicon on the stereoselectivity of the reactions of C-centered chiral silicon compounds wm examined. The investigation was focused on the asymmetric C—C bond formation reaction of chiral allylsilanes and α-silylallyl anions with aldehydes. The functionalities of the substituents on silicon can be manipulated to improve the stereoselectivities of the reactions remote from silicon atom.