Permeation characteristics of electrolytes and neutral solutes through titania nanofiltration membranes at high temperatures.
Nanoporous titania membranes with controlled pore sizes ranging from 0.7 to 2.5 nm, which had molecular weight cutoffs (MWCO) ranging from 500 to 2000, were successfully prepared by sol-gel processing, and the transport characteristics were evaluated across a temperature range of 30-80 degrees C. With increasing temperature, the permeate flux increased 2- to 3-fold, depending on the pore size. The water permeation mechanism was found to be different from viscous flow and was explained by the state of the water (free water/bound water/nonfreezing water) inside confined pores. The rejection of neutral solutes such as raffinose, the separation mechanism of which is molecular sieving (steric hindrance), decreased with temperature whereas that of electrolytes (MgCl(2) and NaCl), the separation mechanism of which is the charge effect (Donnan exclusion), was approximately constant. The temperature dependence of neutral and electrolyte solutes was analyzed using the Spiegler-Kedem equation by combining the Arrhenius equations for diffusivity and viscosity, which we obtained DeltaE(m), the activation energy of diffusion, after eliminating the effect of viscosity. For large DeltaE(m), which corresponds to the rejection of neutral solutes on the basis of molecular sieving, rejection decreased with temperature but remained unchanged for small DeltaE(m), which corresponds to the rejection of electrolytes based on the charge effect.
Enhanced water permeation of reverse osmosis through MFI-type zeolite membranes with high aluminum contents
MFI-type zeolite membranes with different Si/Al ratios were synthesized by seeding and secondary growth. Several factors including alkalinity, synthesis time, and Si/Al ratio were varied to optimize the synthesis conditions. Ion separation of 0.10 M NaCl solution by zeolite membranes was investigated through a cross-flow reverse osmosis system. The aluminum content was found to play an important role in zeolite growth and reverse osmosis separation performance afterward. Membrane hydrophilicity and surface charge measured by the contact angle and particle ζ-potential were found to be strongly dependent on the aluminum composition of the membranes. Both water flux and ion rejection increased considerably as aluminum ions were incorporated into the zeolite framework with a water flux increase from 0.112 to 1.129 kg/m2·h and ion rejection improvement from 90.6% to 92.9%. The enhancement in ion separation and water flux for high aluminum ZSM-5 membranes was further analyzed by enhanced water affinity on the m...
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.