Glass is transparent and strong, but the melting temperature is high and the material is brittle. PhD research by Clarissa Justino de Lima shows that it is possible to improve its properties. She developed a glass from phosphate and fly ash with a low melting temperature and unusual strength.
Standard glass consists largely of silica or silicon oxide and has an incredible theoretical strength of up to 7000 MPa. But due to its brittleness, it 'only' achieves a practical strength of only 35 to 70 MPa. Fragility remains the Achilles heel, even in architectural applications, such as glass bricks. Are there no ways to improve that, De Lima wondered? She experimented in the laboratory with new glass compositions, starting with phosphorus pentoxide (P2O5) and a bit of fly ash and blast furnace slag - waste products from incinerators and blast furnaces.
Phosphate glass not new
Phosphate glass is not entirely new. Advanced glass has been developed since the 1950s with phosphate, which provides lower melting temperature and reduces viscosity. "A major problem is just that phosphate absorbs a lot of water," De Lima says of her experiment. "The glass produced turned out to be extremely hygroscopic." To test the quality, she immersed the produced workpiece in water for a month. Result: the glassware lost 100 per cent of its weight, all the phosphate had disappeared from it.
When she conducted an experiment with glass containing 85 per cent potassium phosphate, half the mass had disappeared after a month. De Lima: "It was no longer glass, it was a kind of gel." That changed when she reduced the potassium phosphate content to between 50 and 75 per cent and added at least 15 per cent alumina (Al2O3) to increase water resistance. She further jacked up the content of fly ash and blast furnace slag significantly. In this case, the loss of mass proved negligible.
Low melting temperature
Fly ash and blast furnace slag are widely used as aggregates in cement, but in this experiment were also found to be perfectly suitable for use in glass. Its transparency was found to be affected only at levels above 35 per cent. At a higher percentage, the fly ash causes crystallisation on cooling - and this is not beneficial for strength and transparency. When the composition of the aggregates changed, De Lima also saw the colours of the glass change. She produced colourless, as well as brown, blue, yellow and green glass.
The various mixtures of phosphate glass that rolled out of the tests already had one pleasant property: a low melting temperature. This was around 1200°C instead of 1500 to 1600°C like borosilicate and soda-lime glass. That makes a big difference in energy costs and environmental impact. Moreover, the price per kilo is roughly three times lower than the aforementioned types of glass. So potentially an interesting product, provided the mechanical properties are satisfactory.
De Lima conducted laboratory tests to determine those properties. These showed that the elastic modulus and hardness of the phosphate glass produced are lower than those of standard silicate glass, but the fracture resistance of some mixtures is higher. This may come in handy when using phosphate glass as a structural material. Thermal stability is also high, while expansion remains limited. Some blends could therefore be suitable for use in the 3d printer.
Anisotropic
The lab research yielded another interesting discovery. In certain compositions, the developed phosphate glass - unlike any other glass - can be anisotropic. "That is exceptional, because it means it could have a large deformation in one direction, while in other directions there is only a small deformation," De Lima explains. "So it could mean that it is much stronger in one direction than in the other." Remarkably, the anisotropic glass retained its transparency. In theory, this enables glass with extraordinary properties. A structure made from this type of phosphate glass could withstand specific loads due to smart orientation. Does this make it suitable for applications in construction, for example as glass building blocks or elements for building bridges? De Lima: "That could very well be the case. But that is a subject for follow-up research."
Source: TU Delft.
