page_title: Porous glass
section_title: Manufacturing
hierarchical_section_title: Porous glass / Manufacturing
caption_attribution_description: English: Dry porous glass, sample about 1 mm thick, made by phase separation in a thermal gradient (high temperature on the right) of a sodium borosilicate glass, followed by acid leaching
context_page_description: Porous glass is glass that includes pores, usually in the nanometre- or micrometre-range, commonly prepared by one of the following processes: through metastable phase separation in borosilicate glasses, followed by liquid extraction of one of the formed phases; through the sol-gel process; or simply by sintering glass powder.
  The specific properties and commercial availability of porous glass make it one of the most extensively researched and characterized amorphous solids. Due to the possibility of modeling the microstructure, porous glasses have a high potential as a model system. They show a high chemical, thermal and mechanical resistance, which results from a rigid and incompressible silica network. They can be produced in high quality and with pore sizes ranging from 1 nm up to any desired value. An easy functionalization of the inner surface opens a wide field of applications for porous glasses.
  A further special advantage of porous glasses compared to other porous materials, is that they can be made not only as powder or granulate, but also as larger pieces in almost any user defined shape and texture.
context_section_description: Precondition for repetitious manufacturing of porous glass is the knowledge about structure determining and structure controlling parameters. The composition of the initial glass is a structure controlling parameter. The manufacturing of the initial glass, mainly the cooling process, the temperature and time of thermal treatment, and the after treatment are structure determining parameters. The phase diagram for sodiumborosilica glass shows a miscibility gap for certain glass compositions.
  The upper critical temperature lies at about 760 °C and the lower one at about 500 °C. O.S. Moltschanova was the first person who exactly described the definition of the exsolution. For a phase separation the initial glass composition must lie in the miscibility gap of the ternary Na
  ₂O-B
  ₂O
  ₃-SiO
  ₂ glass system. By a thermal treatment, an interpenetration structure is generated, which results from a spinodal decomposition of the sodium-rich borate phase and the silica phase. This procedure is called primary decomposition. Using an initial glass composition, which lies on the line of anomaly, it is possible to attain a maximum decomposition, which is almost strainless.
  As both phases have a different resistances to water, mineral acids, and inorganic salt solutions, the sodium-rich borate phase in these mediums can be removed by extraction. Optimal extraction is possible only if the initial glass composition and thermal treatment are chosen such that combine structures form, and not droplet structures. The texture is influenced by the composition of the initial glass, which directs size and type of decomposition areas. In the context of porous glasses, "texture" implies properties like specific pore volume, specific surface, pore size, and porosity. The emerging areas of decomposition depend on time and temperature of the thermal treatment. Furthermore, the texture of porous glasses is influenced by the concentration of the extraction medium and the ratio of fluid to solid.
  Also, colloidal silica is solving in the sodium-rich borate phase, when time and temperature of thermal treatment are increased. This process is called secondary decomposition. The colloidal silica deposit in the macro pores during extraction and obscure the real pore structure. The solubility of colloidal silica in alkaline solutions is higher than network silica, and thus can be removed by an alkaline after-treatment.
caption: Same porous glass as above, but dry. The increased difference between the refractive indices glass/air as compared to glass/water is causing greater whiteness based on the Tyndall effect.
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