Porous glass
Porous glasses are glasses with microscopic pores that play an important role in research into solids.
Porous glasses are made from phase- separated alkali borosilicate glasses by extraction processes. Due to their special properties and commercial availability, they are among the best investigated and characterized amorphous porous solids . Due to the possibility of modeling the microstructure , porous glasses have a high potential as a model system. They have a high chemical, thermal and mechanical stability, which results from a rigid and incompressible silicate network. They can be produced in reproducible quality in the pore size range from 1–1000 nm and thus cover the areas of micro (<2 nm), meso (2–50 nm) and macropores (> 50 nm). A light and varied functionalization of the inner surface opens up a wide range of applications for porous glasses.
Another essential advantage of porous glasses over other porous materials is that porous glasses are available not only as powder or granules, but also as molded bodies with in principle any shape and texture.
Historical development
In the first half of the 20th century, Turner and Winks found that glasses containing borosilicate are susceptible to acid leaching. In the years that followed, numerous works were carried out on this topic, which revealed that, in addition to chemical resistance, density , refractive index , thermal expansion and viscosity can also be influenced by heat treatment of borosilicate-containing glasses. In 1934 it was discovered by Nordberg and Hood that alkali borosilicate glasses separate into a soluble (sodium-rich borate phase) and an insoluble phase (silicate = matrix) when they are subjected to thermal treatment. The soluble phase can be removed by extraction with mineral acid, and a spongy, silicate-containing network remains. A subsequent sintering process resulted in a silicate glass whose properties can hardly be distinguished from quartz glass. The manufacture of such high-silica glasses has become known as the VYCOR process .
definition
In the scientific literature, the term porous glass is understood to mean about 96% silicate-containing porous materials that are produced by acidic or combined acidic and alkaline treatment of phase-separated alkali borosilicate glasses and have a three-dimensionally branched microstructure. In the real sense, these are porous silica glasses. Furthermore, the terms porous VYCOR glass (PVG) and Controlled Pore Glasses (CPG) are used for commercially available porous glasses. The pore structure is formed by an interconnected system of channels and has a specific surface of 40 to 300 m² / g. Porous glasses can be obtained by acid extraction of phase-separated alkali borosilicate glasses or by a sol-gel process . By controlling the production parameters, pore sizes of 0.4 to 1000 nm are possible for porous glasses, which have a narrow pore size distribution. You can produce them as a wide variety of moldings, to which z. B. spheres, plates, rods, fibers, capillaries, ultra-thin membranes and tubes.
Basics of the production of porous glasses
A prerequisite for the reproducible production of porous glasses is knowledge of the structure-determining and structure- directing parameters . The composition of the starting glass is a structure-determining parameter. The production of the starting glass, especially the cooling process, the temperature and duration of the thermal treatment, the extraction conditions and the post-treatment are among the structure-controlling parameters. The phase diagram for sodium borosilicate glass shows a miscibility gap for certain glass compositions.
The upper critical temperature is around 760 ° C and the lower around 500 ° C. The exact delimitation of the segregation area was examined for the first time by OS Moltschanowa. For a phase separation, the initial glass composition must lie within the miscibility gap of the ternary Na 2 O-B 2 O 3 -SiO 2 glass system. A thermal treatment creates a penetrating structure, which results from a spinodal separation of the sodium-rich borate phase and the silicate phase. This process is called primary segregation. With an initial glass composition that is on the anomaly line ( phase diagram ), maximum, almost stress-free segregation is obtained.
Since the two phases have different resistance to water, mineral acids and inorganic salt solutions, the sodium-rich borate phase, which is soluble in these media, can be removed by extraction. However, an optimal extraction is only possible if the initial glass composition (anomaly line) in connection with the thermal treatment is chosen so that composite and not droplet structures are created. The texture is therefore influenced by the composition of the initial glass, and the size and type of segregation areas are determined. In connection with porous glasses, texture is understood to mean properties such as specific pore volume, specific surface area, pore size and porosity. The resulting segregation areas in turn depend on the duration and temperature of the tempering process.
The concentration of the extractant and the ratio of liquid to solid also influence the texture of porous glasses. Furthermore, finely dispersed silicate dissolves in the sodium borate phase if the duration and the temperature of the thermal treatment are increased. This process, known as secondary segregation, has the result that the finely dispersed silicate is deposited in the pre-formed macropores during extraction and conceals the actual pore structure. Since the solubility of finely dispersed silicate in alkaline solutions is greater than that of network silicate, the finely dispersed silicate can be removed by an alkaline treatment.
Applications of porous glasses
Porous glasses are suitable for a variety of applications. Their high mechanical, thermal and chemical stability, the variably adjustable pore sizes with a narrow pore distribution and the variety of surface modifications open up a wide range of applications. The possibility of realizing different geometries is also an advantage for applications in industry, medicine, pharmaceutical research, biotechnology and sensor technology.
With their narrow pore size distributions, porous glasses are ideal materials for separating substances. For this reason, they are used in gas chromatography , thin layer chromatography and affinity chromatography . An adaptation of the stationary phase to a separation problem is possible through targeted modification of the surface of porous glasses.
In biotechnology , too, their advantageous properties make them suitable for purifying DNA and for immobilizing enzymes or microorganisms. They are also outstandingly suitable for the synthesis of oligonucleotides. Certain starter nucleotides are applied to the inner surface. The chain length of the resulting oligonucleotides is influenced, among other things, by the pore size of the CPG.
Furthermore, porous glasses are also used for the production of implants , in particular dental implants. Powders made from porous glass particles are processed with a plastic to form a composite, the particle and pore size having a positive effect on the elasticity of the composite and further improving the optical and mechanical properties of the surrounding tissue, e.g. B. the enamel , adjusts.
Since porous glasses can also be produced as membranes , membrane technology is another important area of application. The hyperfiltration of sea and brackish water and the ultrafiltration in the “downstream process” should be emphasized. In addition to being used as a separating material, porous glasses are also suitable as a carrier material in catalysis. The olefin methathesis was implemented, for example, in the metal-metal oxide / porous glass system.
Porous glasses can also be used as membrane reactors , since they have a high mechanical, but above all thermal and chemical stability. Membrane reactors can improve the conversion of equilibrium-restricted reactions by removing a reaction product through a selective membrane. For example, in the case of the decomposition of hydrogen sulfide on a catalyst in a porous glass capillary, the conversion of the reaction with a glass capillary in contrast to the reaction without a glass capillary was significantly higher.
literature
- WES Turner, F. Winks: The influence of boric oxide on properties of chemical and heat-resisting glasses . In: Journal of the Society of Glass Technology . tape 102 , 1926.
- F. Janowski, W. Heyer: Porous glasses - production, properties and applications . VEB German publishing house for basic industry, Leipzig 1982.
- F. Friedel: [No title given] . Halle 2001 (diploma thesis).
- F. Janowski: The application of porous glasses in process engineering offers great potential . In: machine market . tape 99 , 1993, pp. 28-33 .
- Werner Vogel: Glass chemistry . Springer-Verlag, Berlin 1992, ISBN 3-540-55171-9 .
- OS Moltschanowa: Area of the anomalous glasses in the Na 2 O-SiO 2 -B 2 O 3 system . In: glass and ceramics . tape 14 , 1957, pp. 5-7 .
- F. Wolf, W. Heyer: Modified porous glasses as carriers in gas chromatography . In: J. Chromatogr. tape 35 , 1968, pp. 489-496 , doi : 10.1016 / S0021-9673 (01) 82414-6 .
- Life Sciences - More than just porous glasses (user report) . In: Schuller GmbH (Ed.): LABO9 . 1999, p. 26-28 .
- SCHOTT information . tape 53 , 1990.
- M. Hermann: Process for the production of a porous glass and glass powder and glass material for carrying out the process . WO 098778, 2007.
- PW McMillan, CE Matthews: Microporous glasses for reverse osmosis . In: J. Mater. Sci. tape 11 , 1976, p. 1187-1199 .
- F. Janowski, A. Sophianos, F. Wolf: The role of acidity of MoO 3 -SiO 2 and WO 3 -SiO 2 catalysts . In: React. Kinet. Catal. Lett. tape 12 , 1979, pp. 443 .
- GR Gavalas, CE Megiris, SW Nam: Deposition of H2-permselective SiO2 films . In: Chem. Eng. Sci. tape 44 , no. 9 , 1989, pp. 1829-1835 .
Web links
VitraBio GmbH homepage - further information, view of production facilities
Individual evidence
- ^ OS Moltschanowa: Area of the anomalous glasses in the system Na 2 0-Si0 2 -B 2 0 3 . In: glass and ceramics . tape 14 , 1957, pp. 5-7 .