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1

Microwave-assisted phase separation of alkali-borosilicates in the production of nanoporous glasses

Kullmann J., Enke D., Hahn T., Bauerschaefer U., Ledig L., Gai S., Bochmann A.

Optica Applicata

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2012

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Vol. 42, nr 2

255-263

EN

The influence of a hybrid thermal processing on the spinodal decomposition of sodium borosilicate glasses was investigated. The pore diameter of the resulting porous glasses is a function of the parameters of the thermal treatment. One result of this study was the inversion of the phase separation under preservation of the external shape of glass monoliths. Furthermore by using microwaves in addition to thermal radiation it was possible to generate temperature gradients in the samples and so gradients in pore size too. The glass templates were heated with microwaves in a tube furnace consisting of a resonant rectangular waveguide (TE103). Additionally the tube wall was heated by thermal radiation. The heating control was realized by steering the input power and the frequency of a 200 W semiconductor source. The regulation was performed with a HOMER 3-stub tuner, which simultaneously measured the impedance by reflected wave analysis and the actual template temperature with an IR-sensor. Various temperature-time courses of the hybrid furnace led to different pore size gradients. This was discussed by analyzing specific properties like mean pore diameter, surface area and pore volume. Further effects like the pore orientation and properties of the pore walls were determined by scanning electron microscopy and focused ion beam (FIB).

2

Inverse gas chromatographic studies on porous glass

Rueckriem M., Hahn T., Enke D.

Optica Applicata

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2012

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Vol. 42, nr 2

295-306

EN

Porous glass is a well suitable system for the exploration of new characterization techniques due to adjustable texture properties and the possibility to "tailor-made" the surface chemistry. In this study porous glass beads were utilized as model system for the evaluation of inverse gas chromatography (IGC). Mesoporous glass beads with 10 and 20 nm pore size and macroporous glass beads with 50 nm pore size were synthesized and modified by thermal treatment and chemical reaction with hexamethyldisilazane (HMDS), ethyl-, propyl-, butyl- and octyldimethylchlorosilane. The correlations between the textural properties and the dispersive surface energy were investigated by means of the IGC technique. Furthermore dependences between surface chemistry and surface energy were pointed out on the basis of IGC measurements.

3

Hierarchically structured silica via combination of salt sintering process and phase separation

Reinhardt B., Enke D., Bienhaus G.

Optica Applicata

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2012

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Vol. 42, nr 2

265-270

EN

Hierarchically structured, porous glass monoliths have been prepared successfully via combination of a salt sintering process and phase separation of a sodium borosilicate glass. The materials were characterized by N2-adsorption, Hg-intrusion and scanning electron microscopy. Secondary pore sizes were adjusted by the use of different grain fractions of the filler, different primary pore sizes could be obtained by a variation of the annealing conditions. Thus, both pore systems could be adjusted independently. The resulting monoliths consist of a system of secondary pores between 20 and 150 ?m and primary pores within the walls of the open-pored sintered material between 1 and 70 nm. The hierarchical porous materials exhibit surface areas up to 420 m2g-1 and total porosities up to 74%. Various shapes, e.g., rods, plates and granules were obtained by using appropriate pressing tools.

4

Characterization of pore filling of mesoporous host systems by means of positronium annihilation lifetime spectroscopy (PALS)

Kullmann J., Enke D., Thraenert S., Krause-Rehberg R., Beiner M.

Optica Applicata

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2012

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Vol. 42, nr 2

281-286

EN

Positronium annihilation lifetime spectroscopy (PALS) is an alternative method for the determination of pore sizes and pore size distributions. It is a measuring methodology which shows no limitations in the lower nanometer region and works, besides, without destruction of the sample material. It can be used for the characterization of open and closed pores. Additionally, this technology offers the possibility to obtain pore filling ratios of gases, liquids and solids precisely. Polymorphous medicaments, like acetaminophen, show different crystallization behavior within a pore system, depending on the pore size. This property can be used to control the crystalline state of the medicament and to optimize therefore the pharmaceutical use of the active substance. In this study, acetaminophen (C8H9NO2), also known as Paracetamol, is incorporated into different porous systems. The filling of the pores is realized by an acetaminophen melt at 453 K. The silica membranes are dipped into the melt, subsequently removed and cooled. Information concerning the filling ratio of the pores with the pharmaceutical was received with the help of PALS. The extended Tao-Eldrup model forms the theoretical base, taking into account the pore size sensitive annihilation properties of the ortho-positronium in matter.