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The synthesis of nanosized nickel colloids is investigated by using nickel chloride as the precursor, hydrazine as the reducing agent and palladium as the nucleation agent. High purity nickel can be obtained when the reaction temperature is above 80°C. At a lower reaction temperature, nickel hydroxide is also present in the precipitate form. Starting concentration of nickel ion is a dominant factor in affecting colloidal sizes, while other parameters such as reaction temperature and quantity of poly(vinyl pyrrolidone) (PVP) exhibit only minor effects. Although PVP has some dispersion power, it is not capable of preventing nickel colloids from gradual agglomeration. Our study shows that the subsequent addition of poly(acrylic acid) (PAA) after synthesis can help to stabilize the nickel suspension. This stabilized suspension can then be de-stabilized by changing pH to lower than 4 and vice versa. This reversible stabilization–de-stabilization process is likely due to the behaviour of PVP–PAA complex in the aqueous solutions. [ABSTRACT FROM AUTHOR]

We investigate the morphology of and surface modification by titanium dioxide (TiO2) deposits on porous ceramic substrates placed in a horizontal, tubular, hot-wall, atmospheric pressure chemical vapor deposition (CVD) reactor with titanium tetraisoperoxide (TTIP) as the precursor. The TiO2 particles are produced from TTIP through two routes: first by thermal decomposition which then kicks off hydrolysis. The deposit characteristics is found to be location dependent. Those at the reactor entrance and exit are different from that within the reactor. Within the reactor, the deposit characteristics is further found to depend strongly on the deposition temperature (Td) and is almost independent of the flow-rate and concentration of the reactant. With a Td of about 380 °C, the deposited TiO2 forms a forest-like structure with poor adhesivity to the substrate and results in an insignificant pore size reduction for the substrate. If Td drops down to about 300 °C, the deposited TiO2 forms a rock-packing structure with good adhesivity to the substrate and can reduce the pore size of the substrate from about 1–2 μm down to about 64 nm. At both the reactor entrance and exit, the deposited TiO2 are loosely-packed spherical particles of average diameter of 140–400 nm. A theory, based on whether or not the reaction is gas-phase or surface dominated, is proposed to explain the dramatic effect of Td. [ABSTRACT FROM AUTHOR]

Ceramic/polyaniline composite porous membranes are successfully made by diffusing ammonium peroxydisulfate and aniline into inorganic membrane disks. The ceramic disks are fabricated by mixing CMC, water, kaolin and alumina followed by the processes of drying, milling, sieving, pressing at 4000 pounds, and firing. The pore size of the disk is approximately 1 μm. When the concentration of the oxidant is 0.25 M and that of the monomer is 1 M, the incorporation of polyaniline into the ceramic disks levels off after about 40 hours. The maximum incorporation percentage is approximately 4.86 wt% (0.18 g of polymer/3.7 g of disk). Characterizations of the ceramic disk and its composites include N2-flow tests, solubility tests, BET, SEM, OM and acid diffusion studies. Nitrogen-flow tests indicate that the incorporated polyaniline is structurally unstable. However, after applying N2 gas of 23 psig for about 40 min, no further degradation is observed in these composite membranes even under 40 psig of N2 gas. BET shows that the surface area of the composite is greater than that of the ceramic disk. SEM reveals that polyaniline is grown on the surface of the pores of the ceramic disks. Time constants, corresponding to the time when the pH value reaches 36.8% of the initial value, are estimated from the results of acid diffusion studies. The magnitude of the time constant is in the following order: ceramic/Peani base > ceramic/Peani salt > ceramic/Pani base > ceramic/Pani salt > plain ceramic. [ABSTRACT FROM AUTHOR]

In this article, we reported experimental results on the development of microstructure of the membrane layer deposited on a porous support. The changes in porosity and mean pore diameter were followed as a function of layer thickness. It was found that in addition to particle packing and sintering within the membrane layer, the porous structure of support surface also exhibited certain effects on membrane microstructure until a critical thickness was reached. After that, the porous characteristics of a membrane would be totally determined by the packing and sintering characteristics of particles. It was also shown that the surface roughness could be reduced from about 1.5 μm of the support, to 0.8 μm after coating with 0.5 μm titania particle and further reduced to about 0.55 μm after another coating with 30 nm titania colloids. With proper dispersing and coating procedures, we could reduce the mean pore diameter from about 1.5 μm of the support to 0.12 μm, while the gas permeability was only changed from 500 × 10−7 to 250 × 10−7 mol/m2 s Pa. [ABSTRACT FROM AUTHOR]

A 2-dimensional mathematical model was established to simulate the growth rate uniformity of CdTe epilayers which were grown in a horizontal MOCVD reactor. Both transport phenomena and the surface decomposition kinetics of diethyl telluride were considered. The calculated effects of reactor pressure and substrate temperature on growth rates along the flow axis were compared with experimental results. They agreed well with each other. These effects were then explained by the relative importance of mass transfer to reaction kinetics. In general, higher growth temperature and higher reactor pressure both favor a mass transfer controlled CVD growth situation. However, to obtain uniformly grown epilayers, one would wish to operate under reaction kinetics controlled region, i.e. low substrate temperature and low reactor pressure. [ABSTRACT FROM AUTHOR]