The Borate Anomaly

Question 1

What is the mixed alkali effect?

Answer 1

~ with increased alkali mixing, SiO4(4-) TETRAHEDRAL groups become MORE MOBILE under an external force (viscosity decreases)
~ the ALKALI ION becomes LESS mobile under applied electric field (electrical resistivity increases)

Question 2

What’s the effect of mixing alkalis on on viscosity and electrical resistivity?

Answer 2

~ decreased viscosity
~ increased electrical RESISTIVITY

Question 3

What is the borate anomaly?

Answer 3

~ with initial additions of Na2O to B2O3: CTE decrease, viscosity increase (opposite of Na2O-SiO2 glasses)

~ BO3(3-) triangles –> BO4(5-) tetrahedra

Question 4

What happens to CTE with borate anomaly?

Answer 4

~ tetrahedra network is more open, lower cte

Question 5

What happens to viscosity with borate anomaly?

Answer 5

~ tetrahedra w/ 4 bridging oxygens less likely to move under stress than 3 bridging oxygens = higher viscosity
~ BO3(3-) triangles –> BO4(5-) tetrahedra