**3. Results and discussion**

**Figure 1.** Schematic experimental setup used for field-assisted ion exchange

failure stress was calculated as:

142 Ion Exchange - Studies and Applications

load.

P\*

lated as: [23]

A four-point bending test was carried out to determine the mechanical strength, using inner (L1) and outer (L2) span equal to 10.3 and 40 mm, respectively. The test was carried out in lab air (temperature≈20°C, relative humidity ≈40%) with a constant loading rate of 1.1 MPa/s. The

> ( ) <sup>2</sup> 4 4 2 1

where a is (L2-L1)/2, r1 and r2 the internal and the external radius of the tube, Fmax the maximum

The presence of residual stresses on the surface layers was estimated by Vickers indentation method. The residual stress, σr, approximated as constant on the surface layers can be calcu‐

> \* 3 2

*c*

*r r* <sup>=</sup> - (2)

(3)

4 *max*

*F ra <sup>S</sup>* p

1 *<sup>c</sup> r r*

geometrical constants depending on the indenter and the crack shape, respectively.

p

s

*K P c K c*

ç ÷ = - <sup>W</sup> ç ÷

 c

æ ö

è ø

being the indentation load, c the crack length, Kc the fracture toughness, χr and Ω two

The potassium penetration profile was measured on the fracture surface of test tube fragments. The fragments were fixed on an aluminum disk with an adhesive conducting tape and coated with Au-Pd alloy. The microscopic observation was carried out by a scanning electron microscope (SEM) (JSM 5500, Jeol, Japan). A clean path was chosen for analyzing the potassium Kα on a certain length, around 70 μm long, by using energy dispersion x-ray spectroscopy

*b*

Figure 2 shows how the current density changes as a function of time during the EF-IOX tests. The current density limit was fixed at 20–25 A m-2 depending on the immersion depth of the test tube in the molten salt. For the samples subjected to fields with intensity lower than 500 V cm-1, the current density shows similar trend, steadily decreasing with process time; for more intense electrical fields the current reaches a saturation limit, corresponding to the imposed current density limit, and rapidly decreases. During the process, it is thought that small sodium ions with high mobility are replaced by larger less mobile potassium ions, this determine an overall decrease in the ions movement and, consequently, a reduction of the current density.

The evolution of current density with time under different fields (500 to 3000 V cm-1) in the condition of current limit = 16 mA cm-2 is plotted in Figure 3. All curves have a similar shape: the maximum current density is recorded at the initial application of the field and it is proportional to the applied field intensity. After 400 s, the current density decreases to 10% of the initial one.

**Figure 2.** Current density vs. time for different fields (a constant current density limit was set at ≈ 4 mA cm-2)

**Figure 3.** Current density vs. time for samples treated under different fields with current density limit of 16 mA cm-2

The molar ratio between alkaline oxides (K2O/(Na2O+ K2O)) as measured by EDXS is chosen as an indicative parameter of the surface chemical composition variation. The values for samples treated under fields of 500 and 2000 V cm-1 and current limit of 16 mA cm-2 are summarized in Table 2: It is clear that sodium ions are completely replaced by potassium in the outer surface, cathode side, while in the inner surface, anode side, sodium is detected.

By applying the electric field, potassium ions are moved into the glass in the direction of the field. Highly mobile sodium ions move faster in the glass than potassium ions and can reach the other side of the glass; as a result, one side of the glass is saturated with potassium and the other one has high amount of sodium. Some limited thermal ion exchange can occur during the process on the anode side and sodium ions are replaced by potassium.


**Table 2.** K2O/ (Na2O+ K2O) molar ratio of test tubes after the field-assisted ion exchange

The area beneath the current density versus time curves represents the amount of ions moved through the unit area of the sample during the process. According to Eq. 1, the exchanged layer has a constant concentration of potassium ions and the following equation was proposed to calculated the thickness of the exchanged layer, Δ, in soda-lime silicate glass [24]:

$$
\Delta \left( t\_{2'}, t\_1 \right) = \frac{Q \left( t\_{2'}, t\_1 \right)}{F C\_0} = \frac{\int\_{t\_1}^{t\_2} I dt}{F C\_0} \tag{4}
$$

F being Faraday's constant, 96458.34 C mol-1, C0 the concentration of mobile ions in the base glass (i.e., the sodium concentration in the base glass = 2.64×10-3 mol cm-3), and J the current density. By using Eq. 4, the depth of ion exchange varies between 8 to 120 μm depending on the intensity of the applied field. The depth of the exchanged layer is constant when the current density changes. Figure 4 shows how the current density varies with time for samples subjected to a field with intensity of 2000 V cm-1 and different current limits. By applying a current density limit, it assumes a constant value for a certain time and then decreases. After 200 s, all samples have similar behavior. The duration of the constant current step for the sample treated under a current limit of 8 mA cm-2 is longer; this makes the area underneath the curves and the amount of ions moved into the glass equal to the samples treated under current limit of 8 or 16 mA cm-2 and the amount of exchanged ions is the same for all samples after enough long time. Conversely, it is reported that the amount of exchanged ions depends on both the intensity of the electric field and the applied current limit for soda-lime silicate glass [24].

**Figure 3.** Current density vs. time for samples treated under different fields with current density limit of 16 mA cm-2

The molar ratio between alkaline oxides (K2O/(Na2O+ K2O)) as measured by EDXS is chosen as an indicative parameter of the surface chemical composition variation. The values for samples treated under fields of 500 and 2000 V cm-1 and current limit of 16 mA cm-2 are summarized in Table 2: It is clear that sodium ions are completely replaced by potassium in the outer surface, cathode side, while in the inner surface, anode side, sodium is detected.

By applying the electric field, potassium ions are moved into the glass in the direction of the field. Highly mobile sodium ions move faster in the glass than potassium ions and can reach the other side of the glass; as a result, one side of the glass is saturated with potassium and the other one has high amount of sodium. Some limited thermal ion exchange can occur during

> **Treatment conditions Alkali Ratio** As-received 0 500 V cm-1 - Inner surface 0.7 500 V cm-1 - Outer surface 1 2000 V cm-1 - Inner surface 0.7 2000 V cm-1 - Outer surface 1

the process on the anode side and sodium ions are replaced by potassium.

144 Ion Exchange - Studies and Applications

**Table 2.** K2O/ (Na2O+ K2O) molar ratio of test tubes after the field-assisted ion exchange

**Figure 4.** Current density vs. time for samples treated under field with intensity of 2000 V cm-1 and different current density

The potassium concentration profile near the surface of the samples subjected to the EF-IOX for 10 min are shown in Figure 5. In the outer surface a step in the concentration profile occurs for all conditions and the potassium penetration depth increases with the field intensity. The penetration depth is lower than that estimated by Eq. 4, according to mass balance and charge neutrality. During the process a local electric charge can be formed in the sample due to different mobility of sodium and potassium ions and, consequently, a local field is formed. Such local field can neutralize the external applied filed and prevent further exchange.

**Figure 5.** Potassium concentration profile in the external surface (cathode side) of tubes treated under different fields

Figure 6 shows the potassium concentration profile of glass tubes near the inner surface. Differently from Figure 5, similar profiles are recorded for different applied electrical fields and these resemble typical diffusion profiles obtained by conventional ion exchange process [7]. Probably, thermal ion exchange can be responsible for the observed diffusion concentration profile; nevertheless, more studies are required to reveal possible causes of such phenomenon.

All glass tubes subjected to the double (direct and inverse) field polarization—the initial one being out-to-in (Figure 7)—showed several surface cracks as in Figure 8; the cracks are on the inner surface and in some cases they reach the external one. Conversely, the samples treated first under a field with in-to-out polarization possess a structure similar to those subjected to conventional ion exchange. One possible reason for the formation of the observed cracks can be related to differential deformations between inner and outer surface upon the double ion exchange process. When potassium ions move into the outer surface of the tube first, the exchanged layer expands and compressive stress is formed due to the difference between the specific volume of the surface layer and the base glass. By exchanging the ions in the inner

The potassium concentration profile near the surface of the samples subjected to the EF-IOX for 10 min are shown in Figure 5. In the outer surface a step in the concentration profile occurs for all conditions and the potassium penetration depth increases with the field intensity. The penetration depth is lower than that estimated by Eq. 4, according to mass balance and charge neutrality. During the process a local electric charge can be formed in the sample due to different mobility of sodium and potassium ions and, consequently, a local field is formed. Such local field can neutralize the external applied filed and prevent further exchange.

146 Ion Exchange - Studies and Applications

**Figure 5.** Potassium concentration profile in the external surface (cathode side) of tubes treated under different fields

Figure 6 shows the potassium concentration profile of glass tubes near the inner surface. Differently from Figure 5, similar profiles are recorded for different applied electrical fields and these resemble typical diffusion profiles obtained by conventional ion exchange process [7]. Probably, thermal ion exchange can be responsible for the observed diffusion concentration profile; nevertheless, more studies are required to reveal possible causes of such phenomenon.

All glass tubes subjected to the double (direct and inverse) field polarization—the initial one being out-to-in (Figure 7)—showed several surface cracks as in Figure 8; the cracks are on the inner surface and in some cases they reach the external one. Conversely, the samples treated first under a field with in-to-out polarization possess a structure similar to those subjected to conventional ion exchange. One possible reason for the formation of the observed cracks can be related to differential deformations between inner and outer surface upon the double ion exchange process. When potassium ions move into the outer surface of the tube first, the exchanged layer expands and compressive stress is formed due to the difference between the specific volume of the surface layer and the base glass. By exchanging the ions in the inner

**Figure 6.** Potassium concentration profile in the internal surface (anode side) of tubes treated under different fields

surface during the second step, the expansion of the internal surface due to the geometry of the tube produces extra stresses in the interface of the exchanged layer and base glass. Conversely, by treating the inner surface first, the outer surface goes into tension because of the inner expansion and some cracks can be formed.

Figure 9 shows the bending strength of samples treated by different methods. The samples treated under the field on only one side are stronger than the as-received tubes but slightly weaker than the specimens subjected to conventional ion exchange processes. After the doublefield polarization process, the strength increases further although the scatter becomes much larger.

Optical microscopy photographs of typical indentations on treated samples are reported in Figure 10. If the indentation load is 40 N no cracks are generated; conversely, at 50 N welldeveloped cracks are generated; therefore, a threshold load for crack formation can be pointed out for the strengthened samples, which is much higher than that for as-received glass, estimated equal to about 4 N. The effect of the compressive residual stress is absolutely clear.

The residual stress is built up by local replacement of small ions by larger ions during the ion exchange process; in the samples treated under the electric filed one should expect that the compressive stress is substantially built up only on one surface of the glass tube, it being balanced by tensile stresses in the remaining of the thickness [24]. The residual stress can be calculated by Eq. (3) from the measurement of the indentation crack length [25] and the results are shown in Table 3. The residual stress built up by the electric field-assisted process is significantly higher than that obtained in conventionally treated samples; the stress intensity increases further after the double-reversed electrical field treatment. Stress build up by

**Figure 7.** Current density vs. time for samples treated under direct and inverse electric field. The initial field polariza‐ tion is (a) outside to inside, (b) inside to outside

exchanging ions is considered based on the volume change of glass during the process; the steep change of the potassium concentration profile produces higher residual stress compared to the gradual change in the thermally induced exchanged glass.


**Table 3.** Calculated residual stress in the glass tubes treated by conventional ion exchange and EF-IOX

**Figure 8.** Crack formation in the glass tube during the EF-IOX with "out-to-in" electrical field polarization

**Figure 9.** Four-point bending strength of glass tubes

exchanging ions is considered based on the volume change of glass during the process; the steep change of the potassium concentration profile produces higher residual stress compared

**Figure 7.** Current density vs. time for samples treated under direct and inverse electric field. The initial field polariza‐

**Treatment Condition Conventionally-IOX EF-IOX Double & reversed EF-IOX**

**Residual stress (MPa)** 26 49 91

**Table 3.** Calculated residual stress in the glass tubes treated by conventional ion exchange and EF-IOX

to the gradual change in the thermally induced exchanged glass.

tion is (a) outside to inside, (b) inside to outside

148 Ion Exchange - Studies and Applications

**Figure 10.** Vickers indentations on as-received samples subjected to (a) 3 N, (e) 5 N and samples subjected to 40 N indentation load, (b) ion exchanged (IOX), (c) EF-IOX, one side, (d) EF-IOX both sides, and 50 N indentation load, (f) ion exchange, (g) electric field-assisted ion exchange(EF-IOX), one side, and (h) EF-IOX both
