DEPOSITION AND RESTRUCTURING OF ADHESIVE PARTICLES IN UNIDIRECTIONAL FIELDS

The deposition of particles under the influence of gravity and adhesive interactions with the other deposited particles is examined using computer simulation. A parameter, s, that represents the probability of adhesion or sticking is used for studying the effects of variations in sticking on the resulting microstructure. A range of packing structures is generated between two limiting cases representing complete restructuring (s = 0) and no restructuring (s = 1). The bulk properties of the resulting structures, such as packing density, particle number density, and coordination number, are obtained. Structural correlations are examined through contact networks, radial and angular distribution functions, and diffraction patterns. The results show that local sticking rules have a significant influence on the long-range structure. Rhombic domains and significant long-range correlations are observed for the case of zero sticking probability. The range of correlations drops very sharply with increases in sticking probability, and large elongated voids, oriented in the direction of the external field, are formed with further increases in s. In the limit of perfect sticking, the deposit becomes columnar. The results have important implications to the structure of particle compacts and its influence on sintering of such compacts.     

溶液中钙离子对玻璃颗粒电泳沉积的影响

研究了玻璃颗粒在乙醇溶液中对Ca^2 的选择吸附，反转了玻璃颗粒在乙醇中的荷电性质，使玻璃颗粒能够在乙醇中与羟基磷灰石发生共沉积。讨论了不同离子浓度下电泳沉积的沉积量与沉积电流的变化情况，并以此对Ca^2 在电泳沉积中的作用进行了分析。实验结果证实：悬浮液中Ca^2 或吸附到玻璃颗粒上，改变其荷电性质；或留在溶液中，作为电泳沉积过程中电流的主要承载者。离子浓度对电泳沉积的效果有重要的影响，离子浓度太低或太高都易使涂层出现缺陷。分析了不同沉积条件下电泳沉积所得涂层的各种缺陷及其成因。     

SIMULATION OF PARTICLE TRANSPORT AND DEPOSITION IN A COMBUSTOR

A computational model for Lagrangian particle tracking for studying dispersion and deposition of particles in a combustor with swirling flow and chemical reaction is developed. The model accounts for the effect of thermophoretic force, as well as the drag and lift forces acting on particles, in addition to the Brownian motion and gravitational sedimentation effects. The mean turbulent gas flow, temperature fields and chemical species concentration in the combustor are evaluated using the stress transport turbulent model of the FLUENT code. The instantaneous fluctuation velocity field is generated by a Gaussian filtered white noise model.

The simulated axial, radial and tangential mean gas velocities are compared with the existing experimental data. Ensembles of particle trajectories are generated and statistically analyzed. The effects of size and initial distribution on particle dispersion and deposition are studied. The particle concentration at different sections are also evaluated and discussed. The results shows that the turbulence dispersion effect is quite important, while the thermophoresis effect is small.     

EXPERIMENTAL STUDIES OF DEPOSITION RATES IN THE PRESENCE OF ALKALI SULFATE VAPOR SCAVENGING BY SUBMICRON PARTICLES IN COMBUSTION GAS BOUNDARY LAYERS†

We examine the mass transfer consequences of the nonequilibrium scavenging of condensible vapor (Na₂SO₄) by suspended solid particles (MgO) within combustion gas boundary layers (BLs) using an extension of our 'flash-evaporation' technique (Rosner and Liang, 1986) and laser light scattering methods. In contrast to the result of introducing additives which form miscible condensates with the primary alkali sulfate on a sufficiently cold deposition target—a situation which leads to vapor deposition rate increases (Liang and Rosner, 1986), additives which form nonvolatile, submicron particles in the main-stream, capable of scavenging the alkali vapor(s), reduce total alkali deposition rates at all levels. This result indicates that the thermophoretic diffusivity of such 'getter' particles is smaller than the Fick diffusivity of the scavenged alkali sulfate vapor. Both nonequilibrium laminar BL-theory (Castillo and Rosner, 1987) and our flash evaporation-light scattering experiments reveal that the alkali mass transfer rate reduction factor correlates with the product N_pr_p²—a quantity proportional to the scavenging particle cloud external surface area per unit volume. These results also imply that, in practice, finely divided inorganic sulfur getters, present in the 'fire-side' boundary layer at sufficiently high number densities, would also suppress the total deposition rate of alkali compounds on heat exchanger surfaces.     

电泳沉积法制备碳化钛膜的研究

电泳沉积法(EPD)制备薄膜具有设备简单,成本低,成膜快,被镀件(用于沉积薄膜的基体)形状不受限制,薄膜厚度均匀,并且其厚度在较大范围内可控等优点。碳化钛熔点高、硬度高、化学稳定性好,在复合材料领域具有广阔的应用前景。本文利用电泳沉积法制备出了均匀的碳化钛膜,研究了碳化钛粉末在悬浮介质中的荷电机理;考察了碳化钛粉末在有机悬浮液中的分散性和稳定性;研究了电泳沉积的动力学规律,为今后制备含碳化钛的层状复合材料打下了基础。     

氧化铝粒子化学气相包覆氧化钛的过程研究

探讨了回转炉化学气相淀积（CVD）包覆新工艺,利用Ti（OC_4H_9）_4水解,在Al_2O_3粒子表面实现了TiO_2包覆,研究了包覆过程特征。结果表明,TiO_2在Al_2O_3粒子表面的包覆是成功的,并具有一定的致密性,包覆量为0.1%～10%;化学气相包覆过程中既存在成膜包覆,又存在成核包覆,通过控制反应物浓度等方法可以强化成膜包覆,提高包覆效果。     

EFFECTS OF SUBMICROMETER PARTICULATE SILICA ADDENDA ON THE ADHESION OF MICROMETER-SIZE PARTICLES TO A POLYESTER-COMPOSITE SUBSTRATE

The force needed to detach five sets of different size particles, having number-averaged diameters between 3.6 and 8.5 µm, from a composite substrate was measured using an ultracentrifuge. In addition to size variations, the asperity concentration for each size particle was adjusted by varying the silica concentration, adjusted so that the surface area concentration at each level was kept constant for the five sizes of particles. Due to the changing silica concentration and particle size, the charge per particle also varied. It was found that the detachment force appeared to be virtually independent of charge, with any correlation actually appearing slightly negative, if anything. However, the detachment force increased monotonically with increasing particle diameter and decreased monotonically with increasing silica concentration. Moreover, upon normalizing the detachment force to the particle diameter and the silica concentration to the surface area concentration of silica, it was found that the detachment force clustered into groups in which the force needed to separate the particle from the substrate depended only on the silica concentration. These results suggest that van der Waals interaction, rather than electrostatic forces, are the dominant mechanism controlling toner adhesion in this instance.     

NANOCAVITY EFFECTS ON N2O DECOMPOSITION ON DIFFERENT TYPES OF FE-ZEOLITES (Fe-FER,Fe-BEA,Fe-ZSM-5 AND Fe-FAU): A COMBINED THEORETICAL AND EXPERIMENTAL STUDY

Nitrous oxide decomposition on iron-exchanged zeolites (Fe-FER, Fe-ZSM-5, Fe-BEA, and Fe-FAU) has been studied both theoretically, by using the ONIOM (B3LYP/6-31G(d,p):UFF) method, and experimentally, by temperature programmed reaction, to determine the effect of different zeolitic nanostructured pore networks on the catalytic activity. The ONIOM quantum mechanical calculations show that the nitrous oxide molecule adsorbs with slightly stronger interactions energies on Fe-FER and Fe-ZSM-5 than on the larger pore Fe-BEA and Fe-FAU zeolites. In the transition state leading to the decomposition of nitrous oxide, the smallest pore ferrierite zeolite exerts the strongest van der Waals interactions on the reacting species and, thus, results in the lowest activation energy. Therefore, the predicted intrinsic activity trend is Fe-FER > Fe-BEA ～ Fe-ZSM-5 ～ Fe-FAU. On the other hand, the temperature programmed reaction on zeolites containing trace amounts of iron impurities shows an observed activity trend of Fe-FER > Fe-BEA > Fe-ZSM-5 > > Fe-FAU. The experimentally observed activity trend can be explained by the intrinsic activity of each zeolite except for Fe-FAU. Nitrous oxide decomposition in Fe-FAU could be limited by the mass transfer process and not governed by the intrinsic activity. It is known that cations are preferentially located on the six-membered ring in the sodalite cage of the faujasite, to which the reactants have a very limited access.