Gas transport and separation with ceramic membranes. Part II: Synthesis and separation properties of microporous membranes

Non-supported microporous silica (amorphous) and titania thin films were made by the polymeric gel route. The titania system consisted of particles smaller than 5 nm. Reproducible modification of supported γ-alumina films with silica demands a strict control of every modification step. Silica films of 30–60 nm thickness on top of and presumably partly inside the γ-alumina film were realised. The permeabilities of helium and hydrogen through this film are activated, while the propylene permeability was below the detection limit. Separation factors of a H2---C3H6 mixture are larger than 200 at 200 °C with a flux of the preferentially hydrogen of 1.6 × 10−6 mol/m2-sec-Pa. The pores must be of molecular dimensions to realise this (< 1 nm diameter). Preliminary research shows that changes in the synthesis parameters result in higher activation energies and improved separation properties. The relation between synthesis, resulting microstructure and gas separation properties, however, is not yet fully understood

Gas transport and separation with ceramic membranes. Part I: Multilayer diffusion and capillary condensation

Multilayer diffusion and capillary condensation of propylene on supported γ-alumina films greatly improved the permeability and selectivity. Multilayer diffusion, occurring at relative pressures of 0.4 to 0.8 strongly increased the permeability of 6 times the Knudsen permeability, yielding permeabilities of 3.2 × 10−5 mol/m2-sec-Pa. The occurrence of a maximum in the permeability coincides with blocking of the pore by adsorbate (capillary condensation). This point could be predicted, employing adsorption data and the slit shape form of the pore. Separation factors of 27 were obtained with a N2---N3H6 mixture and a supported γ-alumina film, with C3H6 the preferentially permeating component. This very effective separation is due to pore blocking by adsorbate. The separation factor increased to 85 after modification of the system with magnesia by the reservoir method. However, the permeability of propylene decreased by a factor of 20 to 1.6 × 10−6 mol/m2-sec-Pa

Gas and surface diffusion in modified γ-alumina systems

The transport of pure gases through a microporous membrane is described. The alumina-based membrane (pores 2.5-4 nm) is suitable for Knudsen diffusion separation. To improve the separation factor, interaction with and mobility on the pore wall of one of the gases of a mixture is necessary. To introduce surface diffusion of oxygen and hydrogen, a γ-alumina membrane was impregnated with silver. If temperature and atmosphere are controlled carefully, finely dispersed silver up to 17% by weight can be introduced. At higher loads and under oxidizing conditions, particle growth occurs. In adsorption experiments, little oxygen adsorption on the silver-modified γ-alumina could be detected. This is due to a decrease in accessible surface area of the silver because of particle growth of silver under oxygen. The mobility of hydrogen on the surface was tested by counterdiffusion experiments, of which the theory is given. Hydrogen shows a considerable mobility on the surface at 293 K. At low pressures the flux ratio of hydrogen to nitrogen improved from 3.8 to 8.8. Magnesia was introduced into the γ-alumina membrane to enhance the adsorption and mobility of CO2. It is known that 30% of the CO2 transport on non-modified γ-alumina is surface diffusion. The highest achievable magnesia load was 2.2% by weight. Introduction of magnesia into the γ-alumina surface gives more strong base sites and fewer weak base sites. This results in stronger bonding of CO2 on the surface, but the amount adsorbed is comparable with the amount of CO2 adsorbed on non-modified γ-alumina. The contribution of surface diffusion to the total transport decreases with the introduction of magnesia, as is shown by counterdiffusion. The more strongly bonded CO2 is less mobile, resulting in a smaller surface flux

Anomalous dielectric behaviour of La(III) substituted lead titanate ceramics

In La(III) substituted PbTiO3 ceramics the behaviour of the reciprocal dielectric constant in the paraelectric phase can be described by the relation [...] with the exponent 1 γ<2 depending on the composition. This γ-value is an intermediate of values for Curie-Weiss behaviour (γ = 1) and of values connected with diffuse phase transitions (γ = 2) and increases with increasing La(III) concentration.\ud \ud An excellent agreement between experimental and calculated permittivity curves in the paraelectric phase can be achieved using an alternative equation consisting of a linear and a quadratic term with coefficients varying systematically with the composition. The last equation can be derived and interpreted by means of a physical model assuming the existence of heterophase fluctuations

Ceramic nanostructure materials, membranes and composite layers

Synthesis methods to obtain nanoscale materials will be briefly discussed with a focus on sol-gel methods. Three types of nanoscale composites (powders, membranes and ion implanted layers) will be discussed and exemplified with recent original research results. Ceramic membranes with a thickness of 1–10 μm consist of a packing of elementary particles with a size of 3–7 nm. The mean pore size is about 2.5–3 nm. The preparation routes are based on sol and sol-gel technologies. The pores can be modified by liquid as well as by gas deposition techniques. This leads to modification of the chemical character and the effective pore size and gives rise to microstructure elements well below the size of the pores (3 nm). The modification of ceramic surface layers with a thickness of 0.05–0.5 μm by ion implantation and annealing procedures yields amorphous or strongly supersatured metastable solid solutions of e.g. Fe2O3 (or TiO2) in zirconia-yttria solid solutions or of very finely dispersed metal particles in the ceramic surface layers. Particle sizes are of the order of 2–4 nm. Both types of structures have interesting transport, catalytic and mechanical properties

Drying process in the formation of sol-gel-derived TiO2 ceramic membrane

Accurate drying data for thin titania gel layers dried at 40°C and 20% relative humidity (RH) are given. The drying rate versus free moisture content diagram should show three regions as predicted by the classical drying theory. They are the constant rate period, the first falling rate period and the second falling rate period. The second falling rate period was not observed in the present case, because at 40°C and 20% RH the equilibrium moisture content will be enough to provide a continuous fluid network in the gel. The total drying time in the falling rate period increases with layer thickness. The drying mechanism in the first falling rate period was identified as capillary flow

Permeation and separation studies on microporous sol-gel modified ceramic membranes

Permeation and separation experiments with H2, CO2, O2, N2, CH4 and isobutane with microporous sol-gel modified supported ceramic membranes were performed to determine the gas transport characteristics and the hydrogen separation performance of these membranes. It is found that the permeation is activated, and for defectfree membranes the apparent activation energies are in the ranges 13¿15 and 5¿6 kJ mol¿1 for H2 and CO2, respectively. Correction for the pressure drop over the support results in apparent activation energies for the silica top-layer on the order of 17¿22 and 10¿15 kJ mol¿1 for H2 and CO2 respectively. Due to the very thin top-layer, the permeation is relatively high, with representative values of 6·10¿7 and 20·10¿7 mol m¿2s¿1 Pa¿1 for H2 at 25 and 200°C, respectively. The H2 permeation is almost pressure-independent up to pressures of at least 5 bar. Typical separation factors for H2---CH4 and H2---isobutane are approximately ¿40 and ¿200, respectively, at 200°C for high-quality membranes. For moderate-quality membranes the H2---CH4 separation factor is around 10, while the H2---isobutane separation factor remains at a high value of around 100 at 200°C and 120 at 300°C

Collateralization of the pathways descending from the cerebral cortex to brain stem and spinal cord in cat and monkey

The present study deals with the collateralization of the descending pathways from the cerebral cortex to the brain stem and the spinal cord in cat and monkey. The distributions of the branching cortical neurons were studied using retrograde fluorescent tracers. In addition, a new retrograde fluorescent tracer is describe

Influence of particle size and structure of ZrO2 on microstructure development and dielectric constant of PbZr0.5Ti0.5O3

The synthesis of PbZr0.5Ti0.5O3 ceramics from the raw materials was reinvestigated in order to find relations in the characterizations for the products in various stages of the preparation procedure. Techniques used were particle size measurements, X-ray powder diffractometry, density and dielectric constant measurements and scanning electron microscopy. The results show, that the particle size and structure of ZrO2 determine the inhomogeneity, expressed as xt/xr, of the calcination product. An inhomogeneous calcination product sintered at relatively low temperatures results in an inhomogeneous ceramic of low density. Using high sintering temperatures and long periods of time an inhomogeneous calcination product converts into a dense (>97%) and homogeneous ceramic

Microstructural properties of non-supported microporous ceramic membrane top-layers obtained by the sol-gel process

Dried and calcined non-supported membrane top-layers of SiO2, SiO2/TiO2, SiO2/ZrO2 (10, 20 and 30 mol% TiO2 and ZrO2, respectively) and SiO2/Al2O3 (10 mol% AlO1.5) were prepared using acid catalyzed hydrolysis and condensation of alkoxides in ethanol. The microstructure was determined using nitrogen physisorption. The modified Horváth-Kawazoe model for nitrogen adsorption in cylindrical pores was used for pore size assessment. SiO2 non-supported membrane top layers were 100% microporous with an average porosity of 30¿37%, depending on drying conditions. The bimodal pore size distribution shows a maximum at an effective pore diameter of 0.5 nm, and a broader tail with a weaker maximum around 0.75 nm. Microporous non-supported binary systems can be prepared with porosities between 15 and 40%. The high reactivity of the Ti, Zr, Al-alkoxides requires carefully chosen conditions. Too much water results in dense materials. The pore size distributions (PSDs) of the binary systems resemble the PSDs for silica