Which of the following statements describes why ion exchange of 99% SiO2 - 1% B2O3 in a salt bath of molten postassium chloride (KCl) is not an effective way to increase the surface strength of the glass sample?
a -The ion exchange works correctly, however both air-cooling and water-quenching are more efficient ways to induce a surface strengthening effect.
b -The glass is composed of only 1% B2O3, which is not a large enough proportion of B2O3 to undergo ion exchange, and therefore does not increase the surface strength of the sample effectively.
c -The potassium ion is smaller than B2O3, so when ion exchange takes place the surface is actually being weakened instead of strengthened.
d =The sample described above is composed of SiO2 and B2O3.
e -Neither of these components are network modifiers, so ion exchange does not take place to strengthen the surface of the glass.
f -The surfaces of the glass sample are not cooled quickly enough to create a high internal strain energy that leads to strong surfaces.
e -Neither of these components are network modifiers, so ion exchange does not take place to strengthen the surface of the glass.
The correct answer is c - The potassium ion is smaller than B2O3, so when ion exchange takes place, the surface is actually being weakened instead of strengthened.
To understand why this is the case, let's break down the process of ion exchange and its effect on the glass surface.
Ion exchange is a process where ions in a solid material are replaced by other ions from a surrounding solution. In this case, the SiO2-B2O3 glass is immersed in a salt bath of molten potassium chloride (KCl). The goal is to replace some of the ions in the glass (e.g., Si4+ or B3+) with potassium ions (K+).
In the context of increasing the surface strength, ion exchange is typically used to introduce larger ions into the glass surface. This is because larger ions create compressive stress, which strengthens the surface. However, in the given scenario, the potassium ion is smaller than both Si4+ and B3+ ions in the glass.
When ion exchange occurs between smaller and larger ions, the surface is actually weakened instead of strengthened. This is because the smaller ions easily replace the larger ions, leading to a less compressed surface. As a result, ion exchange with potassium ions in this case would weaken the glass surface rather than strengthen it.
Therefore, option c - The potassium ion is smaller than B2O3, so when ion exchange takes place, the surface is actually being weakened instead of strengthened - correctly describes why ion exchange of 99% SiO2 - 1% B2O3 in a salt bath of molten potassium chloride is not an effective way to increase the surface strength of the glass sample.