THERMODYNAMIC ANALYSIS AND EXPERIMENTAL VERIFICATION FOR SYNTHESIZING SILICON NITRIDE NANOPARTICLES USING RF PLASMA CVD

Silicon nitride nanoparticles were synthesized by radio-frequency (RF) plasma chemical vapor deposition (PCVD) using silicon tetrachloride and ammonia as precursors, and argon as carrier gas. By assuming chemical thermodynamic equilibrium in the system, a computer program based on chemical thermodynamics was used to calculate the compositions of the system at different initial concentrations and final temperatures. At first, five elements and thirty-four species were considered. The effects of temperatures, and concentrations of ammonia, hydrogen and nitrogen on the equilibrium compositions were analyzed. It was found that the optimal reaction temperature range should be 1200 to 1500 K to obtain the highest conversion and yield of Si3N4. The inlet position of ammonia should be lower than that of silicon tetrachloride, and both should be located at the tail of the plasma torch. The best moleratio of ammonia to silicon tetrachloride was found to be about 6. Later, the influences of water (.and oxygen) were considered, and 17 additional species were included in the computations. It was found that oxygen or water content in the raw materials should be as low as possible in order to have high nitride content in the produced Si3N4. Nitrogen or hydrogen might be used to replace some or even all the argon to improve the yield of silicon nitride and reduce the cost. The ratio of ammonia to silicon tetrachloride should be high enough to obtain high conversion, but not excessively high to reduce the oxygen content due to the existence of water in ammonia. The simulated results were verified bv experiments.     

THERMODYNAMIC AND PARTICLE-DYNAMIC STUDY FOR COMBUSTION SYNTHESIS OF TITANIA NANOPARTICLES

Recent referential studies on combustion synthesis of titania nanoparticles were briefly reviewed. Computations based on the minimization of Gibbs free energy were conducted to find the equilibrium compositions, the optimal reaction temperature, the suitable mole ratio of oxygen to titanium tetrachloride, and the best inlet positions of titanium tetrachloride. The mean particle diameter was obtained from particle-dynamic simulation. A combustion apparatuswas setup to synthesize titania nanoparticles by the oxidation and hydrolysis of titanium tetrachloride at high temperatures. Experimental investigation verified some results obtained from thermodynamic and particle-dynamic computations.     

Synthesis of TiO2 nanoparticles by propane/air turbulent flame CVD process

Synthesis of TiO2 nanoparticles by the oxidation of titanium tetrachloride (TiCl4) in high-strength propane/air turbulent flame is investigated tentatively for mass production ofTiO2 nanoparticles. Effects of reactor heat flux varying from 247 to 627 kJ/m2 s, initial TiO2 number density from 2×1020> to 1 × 1021 m-3, and apparent residence time of TiO2 nanoparticles in reactor from 0.06 to 0.9 s, on particle morphology, phase composition, UV absorption and photoluminescence (PL) spectra are studied. The TiO2 nanoparti-cles synthesized, with mean size of 30-80 nm and rutile mass fraction from 0.155 up to 0.575, exhibited a strong PL signal at the wavelength of 370-450 nm, with a wide peak signal at 400-420 nm, reflecting significant oxygen vacancies on the surface of the TiO2 nanoparticles.     

Effect of nanoparticle concentration on zeta-potential measurement results and reproducibility

The effect of nanoparticle concentration on zeta-potential measurement results at dilute concentrations was evaluated.The values of the zeta-potential for four different types of nanoparticles,Ludox(silica),multi-walled carbon nanotubes(bamboo-shaped and hollow nanotubes)and gold,at various concentrations,were obtained using a laser Doppler electrophoresis instrument.The size of the nanoparticles on dilution was measured using dynamic light scattering(DLS).The results show that there is a concentration range within which the zeta-potential,and particle size,are not affected by nanoparticle concentration.The lower concentration limit for the system to produce consistent results was dependent on the nature of the sample under study and ranged between 10-2 and 10 4wt%.Below this concentration,there was an apparent shift in zeta-potential values to less negative values,which was accompanied by an increase in the particle size.The shift in zeta-potential was attributed to an increase in contribution of the signal from extraneous particulate matter.The increase in particle size was attributed to the nature of the homodyne optical configuration of the instrument.The aim of this study was to elucidate the range in nanopatticle concentration that allows for accurate and reliable measurement of the zeta-potential and DLS data.     

壳层形态对核/壳结构PS/SiO_2复合磨料抛光性能的影响

以聚苯乙烯(PS)微球为内核,通过控制正硅酸乙酯的水解过程制备具有不同壳层形态的核/壳结构PS/SiO2复合磨料,应用于二氧化硅介质层的化学机械抛光,借助AFM测量抛光表面的形貌、轮廓曲线及粗糙度.SEM和TEM结果显示:碱性水解条件下,复合磨料的壳层由SiO2纳米颗粒组成(非连续壳层);酸性条件下,复合磨料的壳层则呈无定型网状(连续壳层).抛光对比试验结果表明:复合磨料的PS弹性内核有利于降低表面粗糙度并减少机械损伤,SiO2壳层则有利于提高材料去除率,复合磨料的核/壳协同效应对于提高抛光质量具有主要影响.相对于非连续壳层复合磨料,具有连续壳层的PS/SiO2复合磨料能够得到更低的抛光表面粗糙度值(RMS=0.136 nm),且在抛光过程中表现出了更好的结构稳定性.然而,PS/SiO2复合磨料的壳层形态对抛光速率的影响则不明显.     

Influence of suspension stability on wet grinding for production of mineral nanoparticles

Grinding behavior of nanoparticles in an attritor mill and the minimum achievable particle size are strongly influenced by the suspension stability. In the present work, suspension stability (i.e. (-potential) of nanoparticles was studied by measuring pH as a function of grinding time in the wet milling process. It was found that after a certain time in an attritor mill, there is no further size reduction and the average product particle size increases monotonically. One of the reasons is that the production of submicron particles leads to more particle-particle interactions and consequently pH of the suspension decreases with grinding time. Usually pH value is related to suspension stability and it can be enhanced by addition of NaOH solution. The maximum negative (-potential of -51.2 mV was obtained at pH of 12 for silica. The higher the (-potential with the same polarity, higher will be the electrostatic repulsion between the particles. Hence, the maximum electrostatic repulsion force was maintained by the adjustment of pH value in wet milling. The experiments were conducted at different pH conditions which were maintained constant throughout the experiments and nanosized particles were obtained consequently.     

INTEGRAL COLLISION KERNEL FOR THE GROWTH OF AEROSOL PARTICLES

Integral collision kernel is elucidated using experimental results for titania, silica and alumina nanoparticles synthesized by FCVD process, and titania submicron particles synthesized in a tube furnace reactor. The integral collision kernel was obtained from a particle number balance equation by the integration of collision rates from the kinetic theory of dilute gases for the free-molecule regime, from the Smoluchowski theory for the continuum regime, and by a semi-empirical interpolation for the transition regime between the two limiting regimes. Comparisons have been made on particle size and the integral collision kernel, showing that the predicted integral collision kernel agreed well with the experimental results in Knudsen number range from about 1.5 to 20.     

Effects of laponite and silica nanoparticles on the cleaning performance of amylase towards starch soils

This work aims to understand the effect of nanoparticle-enzyme interactions and how such interactions affect starch based soil removal. Silica and laponite are used as the model nanoparticles, and s-amylase is employed as the model enzyme. The results show that, if the nanoparticles and enzyme are added simultaneously, laponite enhances the enzyme performance toward starch soil removal, whereas silica imposes a small effect on the enzymatic activity towards the same soil substrates. However, when nanoparticles are added first, the enzyme activity is not affected much by laponite but is hindered significantly by silica nanoparticles. Furthermore, sequential addition of the enzyme followed by silica nanoparticles improves soil removal. Electron microscopic analyses, measurements of the enzyme activity in suspen- sions of nanoparticles, and particle size characterisation suggest that dense coverage of soil surface by the silica nanoparticles be likely a mechanism for the experimentally observed hindrance of soil removal when silica nanoparticles are added before enzyme.     

Influence of suspension stability on wet grinding for production of mineral nanoparticles

Grinding behavior of nanoparticles in an attritor mill and the minimum achievable particle size are strongly influenced by the suspension stability. In the present work, suspension stability （i.e. ξ-potential） of nanoparticles was studied by measuring pH as a function of grinding time in the wet milling process. It was found that after a certain time in an attritor mill, there is no further size reduction and the average product particle size increases monotonically. One of the reasons is that the production of submicron particles leads to more particle-particle interactions and consequently pH of the suspension decreases with grinding time. Usually pH value is related to suspension stability and it can be enhanced by addition of NaOH solution. The maximum negative ξ-potential of -51.2 mV was obtained at pH of 12 for silica. The higher the ξ-potential with the same polarity, higher will be the electrostatic repulsion between the particles. Hence, the maximum electrostatic repulsion force was maintained by the adjustment ofpH value in wet milling. The experiments were conducted at different pH conditions which were maintained constant throughout the experiments and nanosized particles were obtained consequently.     

DISPERSION AND NUCLEATION FOR ULTRAFINE PARTICLES OF SILICA AND SILICATE IN POLY（ETHYLENE TEREPHTHALATE） BASED COMPOSITES

In this paper,the dispersion and nucleation behavior of ultrafine particles of silica and layered silicate (LS) in poly(ethylene terephthalate) (PET) matrix are investigated and characterized by Transmission Electron Microscopy (TEM),Wide Angle X-ray Diffraction (WAXD),Dynamic Scanning Calorimetry (DSC),and Atomic Force Microscopy (AFM).The solid precursors based on silica and LS are suggested originally for preparing nanocomposites with good dispersion morphology.Results show that the initial sub-micron (1000-500nm) LS particles are exfoliated or dispersed into nanometer-scale particles (30-70nm) during their polymerization with PET monomers.These dispersed nanoparticles form an ordered morphology in their nucleation and growth during annealing nanocomposites.DSC patterns reveal that the double melting peaks of annealed PET-LS nanocomposites disappear,while they have shrunken in PET-silica ones.These findings strongly demonstrate that the dispersed nanoparticles accelerate the crystallization of PET.The dispersed LS particles have higher percolation and nucleation performance than those of silica.The homogeneous distribution morphology of ultrafine particles is easily obtained by controlling the load of their corresponding precursors.Such a dispersion obviously improves PET properties in that its heat distortion temperature (HDT)increases from 76℃ to 103℃, and crystallization increases 2-4 times more than that of PET.Especially,the nanocomposite films keep themselves transparent when particle load is within 2 wt.% though there are 3 wt.% or so of agglomerated particles in the nanocomposites.     

Effect of fumed silica nanoparticles on the morphology and rheology of immiscible polymer blends

We examined the effect of interfacially active particles on the morphology and rheology of droplet/matrix blends of two immiscible homopolymers. Experiments were conducted on polybutadiene/polydimethylsiloxane (10/90) blend and the inverse system. The effects of fumed silica nanoparticles, at low particle loadings (0.1–2.0 wt%), were examined by direct flow visualization and by rheology. Fumed silica nanoparticles were found to significantly affect the morphology of polymer blends, inducing droplet cluster structure and decreasing the droplet size, regardless of which phase wets the particles preferentially. This is surprising in light of much past research that shows that particles are capable of bridging and thus induce droplet cluster structure in droplet/matrix systems only when they are preferentially wetted by the continuous phase. Therefore, there should exist other possible mechanisms responsible for these droplet cluster structures except for the bridging mechanism. We proposed a particle-flocculating mechanism based on the fact that fumed silica particles readily flocculate due to their high aspect ratio, fractal-like shape, or interparticle attractions. Optical microscopy also reveals that the clustering structure becomes more extensive, and the droplet sizes in the clusters become smaller when the particle loading is increased. Rheologically, the chief effect of particles is to change the flow behavior from a liquid-like rheology to gel-like behavior. This gel-like behavior can be attributed to droplet clustering. Moreover, it should be emphasized that such gel-like behavior can be seen in the blends regardless of which phase wets the particles preferentially, suggesting that, once again, bridging is not the only cause of droplet clustering.     

Modification of silica with PMMA via ultrasonic irradiation and its application for reinforcement of polyacrylates

Polymethyl methacrylate （PMMA） encapsulated silica nanocomposite particles were prepared by ultra- sonically induced in situ polymerization of methyl methacrylate （MMA） on the surface of silica sol. The nanoparticles were characterized by Fourier transform infrared spectroscopy （FFIR）, transmission electron microscopy （TEM）, thermogravimetry （TG）, scanning electron microscopy （SEM）. The results showed that core-shell structure nanocomposite particles with an average size of 36 nm were obtained, and the thickness of polymer encapsulating layer was about 8 nm. The pretreatment of silica sol with tert-butyl hydroperoxide （TBHP） and the addition of ~-methacryloxypropyl trimethoxysilane （MAPTS） significantly enhanced the encapsulation effect. Modified by the polymer layer, the silica particles could be well dispersed in matrices and utilized to improve the mechanical performance of polyacrylates.     

On the quenching of steel and zircaloy spheres in water-based nanofluids with alumina,silica and diamond nanoparticles

The quenching curves (temperature vs time) for small (∼1 cm) metallic spheres exposed to pure water and water-based nanofluids with alumina, silica and diamond nanoparticles at low concentrations (?0.1 vol%) were acquired experimentally. Both saturated (ΔT_sub = 0 °C) and highly subcooled (ΔT_sub = 70 °C) conditions were explored. The spheres were made of stainless steel and zircaloy, and were quenched from an initial temperature of ∼1000 °C. The results show that the quenching behavior in nanofluids is nearly identical to that in pure water. However, it was found that some nanoparticles accumulate on the sphere surface, which results in destabilization of the vapor film in subsequent tests with the same sphere, thus greatly accelerating the quenching process. The entire boiling curves were obtained from the quenching curves using the inverse heat transfer method, and revealed that alumina and silica nanoparticle deposition on the surface increases the critical heat flux and minimum heat flux temperature, while diamond nanoparticle deposition has a minimal effect on the boiling curve. The possible mechanisms by which the nanoparticles affect the quenching process were analyzed. It appears that surface roughness increase and wettability enhancement due to nanoparticle deposition may be responsible for the premature disruption of film boiling and the acceleration of quenching. The basic results were also confirmed by quench tests with rodlets.     

Validation of surface coating with nanoparticles to improve the flowability of fine cohesive powders

Fluidization of fine cohesive powders is seriously restricted by the strong interparticle cohesion. The rational combination of nanoparticles with fine cohesive powders is expected to obtain composite particles with improved flowability. In this work, we firstly reviewed the sandwich and three-point contact models regarding the fundamental principles of nano-additives in reducing cohesiveness. Based on these previous models, the effects of the size of nanoparticles, their agglomeration and coverage on the surface of cohesive powders in reducing interparticle forces were theoretically analyzed. To validate the theory effectiveness for the irregularly shaped cohesive powders, an extreme case of cubic powders coated with silica nanoparticles was fabricated, and the flowability of the composite particles was determined experimentally. Ultimately, based on force balance of a single particle, a semi-theoretical criterion for predicting the fluidization behavior of coated powders was developed to guide the practical applications of improving the flowability of cohesive powders through structural design and modulation.     

Nanoparticle coagulation in a planar jet via moment method

Large eddy simulations of nanoparticle coagulation in an incompressible pla- nar jet were performed.The particle is described using a moment method to approximate the particle general dynamics equations.The time-averaged results based on 3000 time steps for every case were obtained to explore the influence of the Schmidt number and the Damkohler number on the nanoparticle dynamics.The results show that the changes of Schmidt number have the influence on the number concentration of nanoparticles only when the particle diameter is less than 1 nm for the fixed gas parameters.The number concentration of particles for small particles decreases more rapidly along the flow di- rection,and the nanoparticles with larger Schmidt number have a narrower distribution along the transverse direction.The smaller nanoparticles coagulate and disperse easily, grow rapidly hence show a stronger polydispersity.The smaller coagulation time scale can enhance the particle collision and coagulation.Frequented collision and coagulation bring a great increase in particle size.The larger the Damkohler number is,the higher the particle polydispersity is.     

Micromechanics-based modelling of stiffness and yield stress for silica/polymer nanocomposites

Establishing structure–property relationships for nanoparticle/polymer composites is a fundamental task for a reliable design of such new systems. A micromechanical analytical model is proposed in the present work, in order to address the problem of stiffness and yield stress prediction in the case of nanocomposites consisting of silica nanoparticles embedded in a polymer matrix. It takes into account an interphase corresponding to a perturbed region of the polymer matrix around the nanoparticles. Its modulus is continuously graded from that of the silica nanoparticle to that of the polymer matrix. Considering the thickness of the third phase as a characteristic length scale, the influence of particle size on the overall nanocomposite behaviour is examined. The key role of the interphase on both the overall stiffness and yield stress is studied and the model output is compared to experimental data of various silica spherical nanoparticle/polymer composites extracted from the literature. The model is also used to examine the influence of interphase features on the overall nanocomposite behaviour. A finite element analysis is then achieved and the numerical results are validated using the analytical predictions. Local stress and strain distributions are analysed in order to understand the phenomena occurring at the nano-scale.     

TiN薄膜的应力状态对摩擦学性能的影响

用Ｘ射线衍射仪测定了在５２１００钢基体上离子束增强沉积ＴｉＮ膜、等离子体化学气相沉积ＴｉＮ膜和离子镀ＴｉＮ膜的应力状态，分析了不同工艺方法制取的ＴｉＮ薄膜的应力形成的影响因素，比较了３种薄膜在不同载荷和摩擦速度条件下的摩擦学性能，分析了膜－基界面两侧应力状态对膜－基结合力、薄膜的耐磨性能和磨损机理的影响．结果表明：３种ＴｉＮ／５２１００钢试样在薄膜内的应力均为压应力，但在界面附近基体一侧的应力状态是随着工艺方法的不同而不同，３种膜的硬度和膜－基结合力都依次下降，而其内应力与膜－基应力的差值则是依次增大，分别为２６９．０ＭＰａ，６６０．５ＭＰａ和１０６３．３ＭＰａ，因而前者显示出最高的膜－基结合力和最佳的摩擦学性能；而后２种膜则显示出依次渐差的膜－基结合力和摩擦学性能     

TiO2-modified nano-egg-shell Pd catalyst for selective hydrogenation of acetylene

Pd-based egg-shell nano-catalysts were prepared using porous hollow silica nanoparticles(PHSNs)as support,and the as-prepared catalysts were modified with TiO2 to promote their selectivity for hydrogenation of acetylene.Pd nanoparticles were loaded evenly on PHSNs and TiO2 was loaded on the active Pd particles.The effects of reduction time and temperature and the amount of TiO2 added on catalytic performances were investigated by using a fixed-bed micro-reactor.It was found that the catalysts showed better performance when reduced at 300 C than at 500 C,and if reduced for 1h than 3h.When the amount of Ti added was 6 times that of Pd,the catalyst showed the highest ethylene selectivity.     

Pore-Scale Monitoring of Wettability Alteration by Silica Nanoparticles During Polymer Flooding to Heavy Oil in a Five-Spot Glass Micromodel

It is well known that the oil recovery is affected by wettability of porous medium; however, the role of nanoparticles on wettability alteration of medium surfaces has remained a topic of debate in the literature. Furthermore, there is a little information of the way dispersed silica nanoparticles affect the oil recovery efficiency during polymer flooding, especially, when heavy oil is used. In this study, a series of injection experiments were performed in a five-spot glass micromodel after saturation with the heavy oil. Polyacrylamide solution and dispersed silica nanoparticles in polyacrylamide (DSNP) solution were used as injected fluids. The oil recovery as well as fluid distribution in the pores and throats was measured with analysis of continuously provided pictures during the experiments. Sessile drop method was used for measuring the contact angles of the glass surface at different states of wettability after coating by heavy oil, distilled water, dispersed silica nanoparticles in water (DSNW), polyacrylamide solution, and DSNP solution. The results showed that the silica nanoparticles caused enhanced oil recovery during polymer flooding by a factor of 10%. The distribution of DSNP solution during flooding tests in pores and throats showed strong water-wetting of the medium after flooding with this solution. The results of sessile drop experiments showed that coating with heavy oil, could make an oil-wet surface. Coating with distilled water and polymer solution could partially alter the wettability of surface to water-wet and coating with DSNW and DSNP could make a strongly water-wet surface.     

磁控溅射和脉冲直流化学气相沉积Ti-Si-N薄膜摩擦磨损性能对比研究

分别利用磁控溅射和脉冲直流化学气相沉积(PCVD)技术制备了Ti-Si-N薄膜，测定了2种Ti-Si-N薄膜的显微硬度，并采用球一盘式高温摩擦磨损试验机对比考察了其高温摩擦磨损性能．结果表明，当薄膜中Si含量(原子分数)约为10％时，2种薄膜的显微硬度达到最大值；2种Ti-Si-N薄膜的耐磨性能同其硬度之间不存在对应关系，其中采用PCVD方法制备的Ti-Si-N薄膜的高温抗磨性能较优；2种薄膜在高温下的摩擦系数均有所降低，这归因子高温下氧化膜的润滑作用。